vvEPA
United States
Environmental Protection
Agency
Office of Research and
Development
Washington DC 20460
EPA/625/R-93/015
February 1994
Guide to Cleaner
Technologies
Organic Coating Removal
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EPA/625/R-93/015
February 1994
GUIDE TO
CLEANER TECHNOLOGIES
ORGANIC COATING REMOVAL
Office of Research and Development
United States Environmental Protection Agency
Cincinnati, Ohio 45268
Printed on Recycled Paper
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NOTICE
This material has been funded wholly or in part by the United States Environmen-
tal Protection Agency under Contract No. 68-CO-0003, Work Assignment 3-49, to
Battelle. Mention of trade names or commercial products does not constitute
endorsement or recommendation for use.
This Guide to Cleaner Technologies: Organic Coating Removal has been
subjected to U.S. Environmental Protection Agency peer review and administra-
tive review and approved for publication. Approval does not signify that the
contents necessarily reflect the views and policies of the U.S. Environmental
Protection Agency.
This document identifies new approaches for pollution prevention in paint re-
moval. Site-specific selection of a technology will vary depending on shop and
manufacturing process applications. It is the responsibility of individual users to
make the appropriate application of these technologies. Compliance with
environmental and occupational safety and health laws is the responsibility of
each individual business and is not the focus of this document.
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FOREWORD
Today's rapidly developing and changing technologies and industrial products
and practices frequently carry with them the increased generation of materials
that, if improperly dealt with, can threaten both public health and the environ-
ment. The U.S. Environmental Protection Agency (EPA) is charged by Congress
with protecting the nation's land, air, and water resources. Under a mandate of
national environmental laws, the agency strives to formulate and implement
actions leading to a compatible balance between human activities and the ability
of natural systems to support and nurture life. These laws direct the U.S. EPA to
perform research to define our environmental problems, measure the impacts,
and search for solutions.
Reducing the utilization or generation of hazardous materials at the source or
recycling the wastes on site is one of EPA's primary pollution prevention goals.
Economic benefits to industry may also be realized by reducing disposal costs
and lowering the liabilities associated with hazardous waste disposal.
The series, Guides to Cleaner Technologies, summarizes information collected
from U.S. Environmental Protection Agency programs, peer-reviewed journals,
industry experts, vendor data, and other sources. The cleaner technologies are
categorized as commercially available or emerging. Emerging technologies are
technologies that are in various stages of development and are not immediately
available for purchase and installation. For each technology, the Guide ad-
dresses its pollution prevention benefits, operating features, application, and
limitations.
HI
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ACKNOWLEDGMENTS
This Guide was prepared under the direction and coordination of Douglas
Williams of the Center for Environmental Research Information (CERI) and Paul
Randall of the Risk Reduction Engineering Laboratory (RREL) of the U.S.
Environmental Protection Agency (EPA), Cincinnati, Ohio. Battelle compiled and
prepared the information used for this Guide.
The following people provided significant assistance in reviewing the Guide and
making suggestions: Ron Joseph, Ron Joseph & Associates, Inc., Saratoga,
California; David Thomas, Hazardous Waste Research and Information Center,
Champaign, Illinois; Thomas F. Stanczyk, Recra Environmental Inc., Amherst,
New York; and Charles Darvin, U.S. EPA, Organics Control Branch, Research
Triangle Park, North Carolina.
IV
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CONTENTS
Notice ii
Foreword iii
Acknowledgments iv
Section 1. Overview 1
What is Cleaner Technology? 1
Coating Remoyal Applications 1
Pollution Problem 1
Solution '. 3
What's In This Guide? 4
Other Questions Affecting Investment Decisions 4
Who Should Use This Guide? 4
Keywords 5
References 5
Section 2. Available Technologies 6
How To Use the Summary Tables 6
Plastic Media Blasting 12
Wheat Starch Blasting 16
Burnoff Coating Removal 18
Molten Salt Coating Removal 20
Sodium Bicarbonate Wet Blasting 22
Carbon Dioxide Pellet Cryogenic Blasting 25
High-Pressure Water Blasting 30
Medium-Pressure Water Blasting 31
Liquid Nitrogen Cryogenic Blasting 32
Section 3. Emerging Technologies 36
How To Use the Summary Tables 36
Laser Heating 38
Flashlamp Heating 39
Ice Crystal Blasting 39
Section 4. Pollution Prevention Strategy 41
Section 5. Cleaner Technofogy Transfer Considerations 43
Introduction 43
Coating Properties 44
Part and Substrate Properties 44
Process Concerns 44
References 45
Section 6. Information Sources 46
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FIGURES
Figure 1. Sodium bicarbonate system with wet blast head 23
Figure 2. Flow diagram of sodium bicarbonate wet blast system 24
Figure 3. A typical liquid CO2 blast system 28
Figure 4. Operation of cryogenic coating removal process equipment 34
TABLES
Table 1. Available Cleaner Technologies for Coating Removal:
Descriptive Aspects 7
Table 2. Available Cleaner Technologies for Coating Removal:
Operational Aspects 10
Table 3. Emerging Cleaner Technologies for Coating Removal:
Descriptive Aspects 37
Table 4. Emerging Cleaner Technologies for Coating Removal:
Operational Aspects 37
Table 5. Health, Physical, and Chemical Data for Methylene Chloride 42
Table 6. Trade Associations and Technology Areas 46
VI
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SECTION 1
OVERVIEW
What Is Cleaner Technology?
A cleaner technology is a source reduction or recycle
method applied to eliminate or significantly reduce
hazardous waste generation. Source reduction in-
cludes product changes and source control. Source
control can be further characterized as input material
changes, technology changes, or improved operating
practices.
Pollution prevention should emphasize source reduc-
tion technologies over recycling but, if source reduction
technologies are not available, recycling is a good
approach to reducing waste generation. Therefore,
recycling should be used where possible to minimize or
avoid waste treatment requirements when source
reduction options have been evaluated and/or imple-
mented.
The cleaner technology must, of course, reduce the
quantity, toxicity, or both of the waste produced. It is
also essential that the final product quality be reliably
controlled to acceptable standards. In addition, the cost
of applying the new technology relative to the cost of
similar technologies needs to be considered.
Coating Removal Applications
Paint and other coatings are applied to surfaces to
enhance corrosion resistance, improve appearance, or
both. Often the coatings need to be removed either as
part of the manufacturing operation or, later in the life of
the equipment, to enable maintenance or repair.
Examples among the many industries that need to
remove organic coatings include builders and main-
tainers of
Automobiles
Aircraft
Appliances
Defense material
Shipbuilding
Wood products.
These examples indicate some of cross-industry
applications for cleaner coating removal technologies.
Coating removal frequently is required as part of
rework operations on the production line. Even in the
best of operations, some parts will be improperly
covered. For all but the simplest and cheapest items,
stripping the defective coating and refinishing is more
economical than disposing of the poorly finished item.
Production line equipment also must be cleaned on a
routine basis. Racks, hangers, load bars, or spray
booth grates support parts during painting. The sup-
ports and other components in the painting line be-
come covered with overspray. A heavy buildup of paint
interferes with proper support of the product or can
flake off and contaminate the work surface. Even thin
buildup of paint residue reduces electrostatic ground-
ing, increases material loss, or increases the need for
touch-up painting. Therefore, excess paint must be
removed from supports and other paint line compo-
nents.
The need for paint removal also occurs later in equip-
ment life as the paint becomes soiled, worn, or dam-
aged with use. Touch-up or complete recovering can
renew the function of the paint for a few cycles, but
buildup eventually requires removing the old paint.
Also, particularly in the aircraft industry, a paint must be
removed to allow inspection of the underlying part.
Pollution Problem
Solvent strippers have been widely used for industrial
coating removal for many years. Solvent strippers can
be applied at room temperature to remove a wide
range of organic coatings without attacking metal
substrates. Solvent strippers consist mainly of methyl-
ene chloride which typically constitutes 60% to 65% of
the formulation. Other ingredients such as activators,
corrosion inhibitors, thickeners, and evaporation
retarders are used to supplement the methylene
chloride to improve coating removal performance.
Neutral solvent strippers typically supplement methy-
-------
lene chloride with cresylic acid, methanol, and
monoethanolamine. Acidic solvent strippers typically
include phenol, formic acid, and methanol mono-
ethanolamine in the formulation in addition to the
methylene chloride. Other additives may include
toluene, sodium chromate, ammonia, benton'rte,
metallic soaps, polyacrylate, esters, cellulose acetate,
ethyl cellulose, and waxes (Operowsky, 1993).
Activators include methanol, acids, alkalies, and
amines, which increase the rate of paint removal. For
example, formic and acetic acids remove epoxy resins
by hydrolyzing their ether linkages. Corrosion inhibitors,
such as propylene oxide and butylene oxide, scavenge
free acids such as HCI, which can form due to
decomposition of methylene for wipe-on application
methods and also may impart desirable characteristics
for immersion stripping systems. Most thickeners are
based on alkyl cellulose and work by forming hydro-
philic colloids. Evaporation retarders, such as paraffin
wax, are used to reduce vapor losses of volatile
solvents such as methylene chloride.
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Methylene chloride removes the coating mainly by
causing it to swell and then lift off the substrate.
Because of the small size of the molecule, methylene
chloride penetrates beneath the coating surface to the
substrate and causes the film to swell and thereby lifts
it off the substrate. The intermediate solvency of
methylene chloride prevents the coating from being
dissolved and redeposited. These properties produce a
characteristic wrinkling, bubbling, and blistering action,
which signifies that the film is ready for scraping or
flushing.
The solvent stripping chemical is wiped or spread onto
the coated substrate. The softened coating and solvent
sludge are then wiped, scraped, or flushed off. In many
applications, several repetitions are needed to give
satisfactory coating removal. A water rinse often is
used for final cleaning of the part.
Use of solvent strippers generates organic vapors,
sludge containing solvents and metals, and wastewater
containing solvents and metals. A wide range of
environmental concerns about these environmental
release paths are leading industries to seek cleaner
alternatives to coating removal. Both federal and state
programs are moving toward significant reduction of
release of volatile organic compounds (VOCs), partic-
ularly hazardous air pollutants (HAPs). Examples of
some of the major federal environmental regulations
favoring reduction of VOCs include the Clean Air Act
Amendments (CAAA), the Resource Conservation and
Recovery Act (RCRA), the Right to Know provisions of
the Superfund Amendment and Reauthorization Act
(SARA), and the Pollution Prevention Act with its
emphasis on eliminating pollution at the source.
Reducing use of solvent strippers is also driven by
increasing concerns for potential workplace health
hazards due to VOCs.
189
Title III of the CAAA is a comprehensive plan for
reducing emissions of hazardous air pollutants. An
initial list of 1 89 HAPs is given in the CAAA; other
HAPs may be added to the list. The EPA has, in
accordance with the CAAA, identified major source
categories for HAPs and is now defining Maximum
Achievable Control Technology (MACT) standards for
source categories. (Paint stripper use is identified as
one of the source categories requiring MACT standards
(57 FR 31592, July 16, 1992). A number of coating
operations also are identified in the initial list of cate-
gories of major and area sources of hazardous air
pollutants (57 FR 31 591, July 16, 1992). Since paint
removal may be required as part of the rework process,
paint removal MACT standards for paint stripping may
be developed for some of the coating industries as
well.
Solvent waste disposal procedures and requirements
of the RCRA increase waste management costs,
establish cradle-to-grave responsibility for wastes, and
require the waste generator to maintain a waste
minimization program.
Section 313 of Title III of the Superfund Amendments
and Reauthorization Act (SARA) establishes toxic
chemical release reporting requirements. Facilities with
Standard Industrial Classification (SIC) Codes in the
range of 20-39, meeting company size and chemical
quantity thresholds, must report discharge and recy-
-------
cling of toxic chemicals. The common stripping sol-
vents such as methylene chloride, phenol, and metha-
nol are among the more than 300 chemicals covered
by the toxic chemical reporting requirements.
In addition to the RCRA requirement for a waste
minimization program for all hazardous waste gen-
erators, the Pollution Prevention Act of 1990 establish-
es a priority on reducing use of hazardous materials. Of
specific interest to organic coating removal, methylene
chloride is one of the 17 priority toxic chemicals identi-
fied for voluntary reduction by the 33/50 Program (U.S.
EPA, 1991;1992).
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The organic solvents in cold solvent stripper formula-
tions result in sufficient vapor concentrations to cause
concern for workers in the area. In particular methylene
chloride has been identified by the National Institute for
Occupational Safety and Health (NIOSH) as a chemical
that should be treated as an occupational carcinogen.
NIOSH recommends that occupational exposure to
carcinogens be limited to the lowest feasible concentra-
tion. Of course, complete elimination of methylene
chloride gives the lowest reduction. If carcinogens must
be used, NIOSH recommends that only the most
reliable and protective respirators be used to ensure
maximum protection.
The removed coating materials may also cause envi-
ronmental concerns. Some pigments contain toxic
metals such as cadmium, lead, and chromate. The
removed coating debris may also contain unreacted
resins which can cause problems for the environment
or worker safety. Waste generation from lead paint
abatement is an area of particular concern due to the
toxicity of lead and the large surface area currently
coated with lead-containing paints. Most lead paint
removal is done by abrasive blasting and thus is not
covered specifically by this guide. However, the
technologies discussed in this guide can be applied to
minimize waste generation in lead paint abatement.
Other techniques such as abrasive media recycling,
which apply specifically to abrasive blasting waste
minimization, are not discussed in this guide but should
be explored for lead paint abatement waste minimiz-
ation.
Solution
The solution to pollution from paint removal operations
that should be explored first is to not paint the part and
thus avoid the need to strip it. Some airlines have tried
polished aluminum skins and report that the appear-
ance is acceptable and the life-cycle cost is lower than
painting with periodic removal to allow inspections
(Boeing, 1993). However, for most applications, the
coating improves appearance or performance or both
and must still be used.
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Cleaner technologies based on physical coating
removal are commercially available or are being
developed to replace solvents strippers. Physical
coating removal technologies take advantage of
differences in physical properties between the coating
and the substrate to destroy the bonding and/or abrade
the coating from the underlying substrate. Protecting
the underlying substrate from damage while achieving
good coating removal is a major concern.
pro&sss ft&eds,
Cleaner coating removal technologies use one or more
of four general types of physical mechanisms:
Abrasive technologies wear the coating off with
scouring action.
Impact technologies rely on particle impact to
crack the coating to remove it.
Cryogenic technologies use extreme cold to
make the coating more friable and induce
differential contraction to debond the coating.
Thermal technologies use heat input to oxidize,
pyrolyze, and/or vaporize the coating.
Many cleaner organic coating removal applications
combine these methods. The abrasion and impact
mechanisms typically occur together in technologies
emphasizing one mechanism over the other. For
example, sodium bicarbonate stripping relies mainly on
abrasion with some removal by impact. On the other
hand, plastic media blasting (PMB) relies mainly on
impact to crack and remove the coating but includes
some abrasive action. The cryogenic technologies use
a coolant, such as liquid nitrogen, to provide a cooling
mechanism supplemented with PMB or other tech-
nology using an impact removal mechanism. Thermal
technologies burn the organic coating to form an ash
but often are followed by ash or soot removal with a
technology providing an impact mechanism.
No one coating removal technology will replace solvent
strippers in all applications. Alternative methods are
available for effective, safe coating removal in specific
-------
applications. The important factors in reviewing the
applicability of a technology are discussed in Cleaner
Technology Transfer Considerations (Section 5).
What's In This Guide?
This application guide describes cleaner technologies
that can be used to reduce waste in coating removal
operations. The objectives of this application guide are
to help identify potentially viable cleaner technologies
to reduce waste by using alternative organic coating
removal methods and to provide resources for obtain-
ing more detailed engineering information about the
technologies. We address the following specific ques-
tions:
What alternative coating technologies are
available or emerging that could significantly
reduce or eliminate pollution being released from
current operations?
Under what circumstances might one or more of
these alternative coating systems be applicable
to your operations?
What pollution prevention, operating, and cost
benefits could be realized by adapting the
technology?
Other Questions Affecting Investment
Decisions
Other aspects affecting the decision to explore one or
more cleaner technologies include
Might new pollution problems arise when imple-
menting cleaner technologies?
Are tighter and more complex process controls
needed?
Will product quality and operating rates be
affected?
Will new operating or maintenance skills be
needed?
What are the overall capital and operating cost
implications?
Whenever possible, these questions are answered in
this guide. The cleaner coating removal systems
described in this guide are applicable under different
sets of product and operating conditions. If one or more
are sufficiently attractive for your operations, the next
step would be to contact vendors or users of the
technology to obtain detailed engineering data and
make an in-depth evaluation of its potential for your
plant.
Who Should Use This Guide?
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Keywords
References
Cleaner Technology
Pollution Prevention
Recycling
Source Reduction
Source Control
Blasting
Coating Removal
Debonding
Depainting
Paint Stripping
Plastic Media
Wheat Starch
Burnoff
Molten Salt
Sodium Bicarbonate
Carbon Dioxide Pellets
High-Pressure Water
Medium-Pressure
Water
Liquid Nitrogen
Cryogenic Coating
Removal
Laser
Flashlamp
Ice Crystal
Pyro lysis
Thermal
Boeing. 1993. "Paint Stripping." Structure Conference.
SE-ACM-3-1-93, Boeing Commercial Airplane
Group, Section 10, Service Engineering, Seattle,
Washington.
Dotson, R., and R. Ballard. 1992. "A Safe and Cost-
Efficient Method of Stripping Rejected Parts." Metal
Finishing, 90(4):29-32.
Operowsky, R. M. 1993. "Chemical Immersion Paint
Stripping." In: Metal Finishing Organic Finishing
Guidebook and Directory, Elsevier Publishing, New
York, New York.
U.S. Environmental Protection Agency. 1991. The 337
50 Program: Forging an Alliance for Pollution
Prevention (2nd ed.). Special Projects Office, Office
of Toxic Substances, Washington, D.C. July.
U.S. Environmental Protection Agency. 1992. EPA's33/
50 Program Second Progress Report. TS-792A,
Office of Pollution Prevention and Toxics, Washing-
ton, D.C.
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SECTION 2
AVAILABLE TECHNOLOGIES
How To Use the Summary Tables
Nine available cleaner coating removal technologies
are evaluated in this section, namely
Plastic media blasting
Wheat starch blasting
Burnoff coating removal
Molten salt coating removal
Sodium bicarbonate wet blasting
* Carbon dioxide pellet cryogenic blasting
High-pressure water blasting
Medium-pressure water blasting
Liquid nitrogen cryogenic blasting.
Tables 1 and 2 summarize descriptive and operational
aspects of these technologies. Readers are invited to
refer to the summary tables throughout this discussion
to compare and contrast technologies.
Descriptive Aspects
Table 1 shows the main Coating Removal Mechan-
ism(s) of each available technology. It next lists the
Pollution Prevention Benefits, Reported Applica-
tion, Benefits, and Limitations of each available
cleaner technology.
Operational Aspects
Table 2 shows the key operating characteristics for the
available technologies. These tables give users a
compact indication of the range of technologies cov-
ered to allow preliminary identification of technologies
that may be applicable to specific situations. Tables 1
and 2 contain evaluations or annotations describing
each available cleaner technology.
In Table 2, Process Complexity is qualitatively ranked
as "high," "medium," or "low" based on such factors as
the number of process steps involved and the number
of material transfers needed. Process Complexity is
an indication of how easily the new technology can be
integrated into existing plant operations. A large
number of process steps or input chemicals, or multiple
operations with complex sequencing, are examples of
characteristics that would lead to a high complexity
rating.
The Required Skill Level of equipment operators also
is ranked as "high," "medium," or "low." Required Skill
Level is an indication of the level of sophistication and
training required by staff to operate the new technology.
A technology that requires the operator to adjust critical
parameters would be rated as having a high skill
requirement. In some cases, the operator may be
insulated from the process by complex control equip-
ment. In such cases, the operator skill level is low but
the maintenance skill level is high.
Table 2 also lists the Waste Products and Emissions
from the available cleaner technologies to indicate
tradeoffs in potential pollutants, the waste reduction
potential of each, and compatibility with existing waste
recycling or treatment operations at the plant. The
Capital Cost and Energy Use columns provide a
preliminary measure of process economics. The
Capital Cost is a qualitative estimate of the initial cost
impact of the engineering, procurement, and instal-
lation of the process and support equipment compared
to current coating removal equipment.
Due to the diversity of cost data and the wide variation
in plant needs and conditions, it is not possible to give
specific cost comparisons. Cost analysis must be plant-
specific to adequately address factors such as the type
and age of existing equipment, space availability,
throughput, product type, customer specifications, and
cost of capital. Where possible, sources of cost data
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Table 1. Available Cleaner Technologies for Coating Removal: Descriptive Aspects
Technology/
Coating Removal
Mechanism
Plastic
Media
Blasting
Impact/
Abrasive
Pollution
Prevention
Benefits
Eliminates VOCs and
HAPs
Uses nontoxic media
Uses a dry process
Spent media are
cleaned and reused
several times for paint
stripping
Some spent thermoplastic
media are recyclable to
make plastic products
Reported
Application
Removes paint from a
variety of metal and
non-metal substrates
Strips aircraft compon-
ents and ground
support equipment
Cleans/strips commer-
cial and industrial parts
Removes powder
coatings from sensitive
substrates
Benefits
Provides high-throughput-
controlled coating removal
Can selectively remove
individual coating layers
Eliminates water use
When stripping is done with
. thermoplastic media, the
waste may be recyclable
Limitations
Spent plastic media contain paint chips and so may be
hazardous waste
Requires workers to wear respiratory and eye
protection equipment
Blasting generates high noise levels
May cause metal substrate damage
More aggressive media types damage fiberglass or
composite materials
Contaminants in media cause stress risers in the
substrate
Uses flammable media
Wheat
Starch
Blasting
Impact/
Abrasive
Burnoff
Coating
Removal
Thermal
Molten Salt
Coating
Removal
Thermal
Eliminates VOCs and
HAPs
Spent media are
cleaned and reused
several times for paint
stripping
Uses a nontoxic,
biodegradable medium
Uses a dry process
Eliminates VOCs and
HAPs
Eliminates VOCs and
HAPs
Gentle stripping action
suitable for abrasion
sensitive fillers and
composite materials
Gaining acceptance for
thin, soft aluminum in
commercial aircraft
skins
Removes thick coat-
ings from a variety of
coating line fixtures
and tools
Removes thick coat-
ings from a variety of
coating line fixtures
and tools
Provides controlled coating
removal
Can selectively remove
individual coating layers
Eliminates water use
Uses inexpensive media
Media are nontoxic and
biodegradable
Provides rapid removal of
thick coatings
Can process complex shapes '
Bumoff ovens can remove
uncured coating
Provides rapid removal of
thick coatings
Can process complex shapes
Salt bath ensures even
heating
Rinsewater waste is
compatible with conventional
water treatment systems
Spent starch media contain paint chips and so may be
hazardous waste
Dense contaminants in recycled media may damage
substrate
Stripping rate is generally slow to moderate
Requires workers to wear respiratory and eye
protection equipment
Blasting generates high noise levels
Media are moisture sensitive
Generates coating ash residue that may be hazardous
waste
Will damage heat-sensitive materials such as heat-
treated aluminum or magnets
Coatings containing halogens (PVC or PTFE) and/or
nitrogen will produce corrosive offgas
Must not be used for low-melting metals or alloys
Must not be used with pyrophoric metals
May require offgas treatment, depending on local air
permitting regulations
Potential for generation of products of incomplete
combustion
Presents possibility of fire
Generates by-product sludge that may be hazardous
waste
Will damage heat-sensitive materials such as heat-
treated aluminum or magnets
Must not be used for low-melting alloys
Must not be used with pyrophoric metals
May require offgas treatment, depending on local air
permitting regulations
Potential for generation of products of incomplete
combustion
(continued)
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Table 1. (Continued)
Technology/
Coating Removal
Mechanism
Pollution
Prevention
Benefits
Reported
Application
Benefits
Limitations
Sodium
Bicarbonate
Wet Blasting
Abrasive/
Impact
Eliminates VOCs and
HAPs
Uses nontoxic media
Carbon Dioxide Eliminates VOCs and
oo
Pellet
Cryogenic
Blasting
Cryogenic/
Abrasive/
Impact
HAPs
Uses a dry process so
no wastewater is
generated
Coating chips
collected dry with no
media
Uses natural or
industrial sources so
no net production of
carbon dioxide occurs
Removes paints from a
variety of metal
substrates
Depaints wood without
damaging the
substrate
Cleans and depaints.
brick walls
Removes heavy
accumulations of
grease and dirt from
mechanical equipment
Strips surfaces
needing high degree of
final cleanliness
Useful for equipment
where it is desirable to.
avoid disassembly
Useful when volume of
residue must be
minimized such as with
radioactive-
contaminated
components or
coatings containing
hazardous metals
(e.g., cadmium or lead)
Provides a controllable
process for coating removal
Can selectively remove
individual coating layers
Uses inexpensive stripping
media
In some cases, liquid waste
may be discharged to a
conventional wastewater
treatment plant
Use of water dissipates heat
generated by the abrasion
Eliminates need to prewash
surface
Sodium bicarbonate waste-
streams are generally
compatible with existing
water treatment systems
Generates low volume of dry
waste (none from the media)
Eliminates water use
Provides well-defined coating
removal pattern
Can selectively remove
individual coating layers
Requires limited pre- or
poststripping cleanup
No masking needed except
for delicate materials such as
soft clear plastics
Equipment can be stripped
without disassembly
No media separation/
recycling system needed
No media disposal cost
Pellets driven into interstitial
spaces vaporize, leaving no
residue
Uses nonrecydable stripping media
Generates wet sodium bicarbonate sludge containing
coating debris, which may be a hazardous waste
System must be available to collect and treat waste-
water containing sodium bicarbonate and paint chips
May require exhaust ventilation system to control
particulate
Requires workers to wear respiratory and eye
protection equipment
Blasting generates high noise levels
Media can be aggressive so potential for substrate
damage exists
Generates small volume of coating debris, which may
be a hazardous waste
Requires ventilation to avoid potentially dangerous CO2
concentrations
Generates airborne particulates that may contain metal
from the coatings
Requires workers to wear respiratory and eye
protection equipment
Requires workers to wear hearing protection
Possible worker exposure to extreme cold
Potential for worker injury from high-velocity CO2 pellet
impact
Rebounding pellets may carry coating debris and
contaminate the work area or workers
Nonautomated system fatigues workers quickly
Possible static electricity buildup on substrate if no
grounding provided
Some coating debris may redeposit on substrate
Low temperatures can cause condensation on
substrate
Large local temperature drops can occur in substrate
but confined mainly to the surface layer
May damage thermoset composite materials
Difficult to control coating removal on graphite-epoxy
composites
Slow coating removal rate
Equipment bulky and capital intensive
(continued)
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Table 1. (Continued)
15,000 psi) require
expensive robotic operation
Misapplied water jet will damage substrate
Blasting generates high noise levels
Water can enter cavities
Water can penetrate and/or damage joints, seals,
and bonded areas
Coating debris sludge may be hazardous waste
System is needed to collect and recycle stripping water
Requires workers to wear respiratory and eye
protection equipment
Blasting generates high noise levels
- Mechanized applications typical due to high reaction
forces
Misapplied water jet will damage substrate
Water can enter cavities
Water can penetrate and/or damage joints, seals, and
bonded areas
Generates some solid waste containing coating chips
and spent plastic media, which may be a hazardous
waste
May require ventilation system to prevent nitrogen
buildup in confined spaces
Requires worker protection from low temperatures
during unloading
Not effective on thin coating films
Less effective on epoxies and urethanes
Existing technology limits part size to less than 6 ft tall
and 38 in diameter weight less than 400 Ib per
stripping cycle
-------
Table 2. Available Cleaner Technologies for Coating Removal: Operational Aspects
Available
Technology
Type
Process
Complexity/
Required
Skill Level
Waste Products
and Emissions
Capital
Cost
Energy
Use
Operations Needed
After Stripping
References
Plastic
Media
Blasting
Medium/
Medium
Wheat Starch Medium/
o Blasting Medium
Bumoff
Coating
Removal
Molten Salt
Coating
Removal
Low/
Low for operation
High for
maintenance
Low/
Low for operation
High for
maintenance
Sodium Medium/
Bicarbonate Medium
Wet Blasting
Solid coating residue Medium
and spent media
waste
Airborne particulates
Noise
Solid coating residue Medium
and spent media
waste
Airborne particulates
Noise
Ash
Offgas
Medium
Salt/coating by-prod- Medium
uct sludge
Offgas
Rinse water
Liquid waste contain- Low
ing coating residue
and spent media
Some airborne parti-
culates
Noise
Compressed air to pro-
pel blasting media
Energy for media re-
covery and recycle,
dust collection, and
ventilation
Compressed air to pro-
pel blasting media
Energy for media re-
covery and recycle,
dust collection, and
ventilation
Electricity or gas sup-
ply for heating
Electricity or gas sup-
ply for heating
Compressed air and
water supply to propel
blasting media
Ventilation to control
p articulate
Continuous separation of media
from stripped coating particles and
spent media during stripping
Remove masking
Dispose of spent media and con-
tained coating residue waste.
Some spent thermoplastic media
(even with coating residue) can be
reused to make plastic parts
Continuous collection and reuse of
spent media during stripping
Remove masking
Dispose of spent media and con-
tained coating residue waste
Spent media can be treated by bio-
degradation
Cool down
Ash removal and collection
Cool down
Water rinse
Remove masking
Dispose of sodium bicarbonate
solution and coating residue waste
Abbott, 1992
Bailey, 1992
Baker, 1991
Bowers-Irons et al., 1991
Capron and Wells, 1990
Composition Materials Co., 1993
Cundiffetal., 1989
Dicaire, 1990
Galliher, 1989
Groshart, 1988
HazTECHNews. 1991
Lyons, 1990
Novak, 1990
Pauli, 1989
Roberts, 1989
U.S. DoD, 1988
U.S. Navy, 1987
U.S. Technology Corporation, 1993
Wasson and Pauli, 1993
Drake, 1993
Larson, 1990
Lenz, 1991
Oestreich and Porter, 1992
Oestreich and Waugh, 1993
Coberth and Ceyssons, 1993
Izzo, 1989
Mann, 1991
Metal Finishing, 1990
Whelan, 1993
Gatetal., 1993
Malloy, 1993
Metal Finishing, 1990
Chen and Olfenbuttel, 1993
Kline, 1991
Larson, 1990
Peebles staJ., 1990
Spears, 1989
Svejkovsky, 1991
Wasson and Haas, 1990
(continued)
-------
Table 2. (Continued)
Available
Technology
Type
Carbon
Dioxide
Pellet
Cryogenic
Blasting
High-
Pressure
Water
Blasting
Medium-
Pressure
Water
Blasting
Liquid
Nitrogen
Cryogenic
Blasting
Process
Complexity/
Required
Skill Level
Medium/
Medium
High/
Low for operation
High for
maintenance
Low/
High
Medium/
Low for operation
High for
maintenance
Waste Products Capital
and Emissions Cost
Solid coating Medium
residue waste
Airborne particulates
CO, gas
Noise
Sludge waste con- High
taining paint residue
Wastewater
Some airborne
particulates
Noise
Sludge waste con- Low
taining paint residue
(and in some sys-
tems solvent or
abrasive additives)
Wastewater
Some airborne
particulates
Noise
Solid coating Medium
residue and spent
media waste
Inert nitrogen gas
Energy
Use
Liquid carbon dioxide
supply
Compressed air supply
to propel blasting me-
dia
Electricity to drive
water pump
Electricity to drive
water pump
Liquid nitrogen supply
Operations Needed
After Stripping
Remove masking
Dispose of coating residue waste
Remove masking
Dispose of coating residue sludge
and wastewater
Remove masking
Dispose of coating residue sludge
and wastewater
If used, dispose of abrasive or
sorbent or other treatment medium
carrying solvent
Vent nitrogen gas from the strip-
ping cabinet
Allow parts to warm for 5 minutes
Dispose of coating residue waste
References
APCI, 1984b
Boyceetal., 1990
Burcham, 1993
Cheney and Kopf, 1990
Cold Jet, Inc., 1989
Cundiff and Matalis, 1990
Foster et al., 1992
Ivey, 1990
Kopf and Cheney, 1989
Larson, 1990
Schmitz, 1990
Svejkovsky, 1991
Wolff, 1984
Hofacker et al., 1993
Hewlett and Dupuy, 1993
Stone, 1993
U.S. Army, 1992
Bailey, 1992
Boeing, 1993
Hewlett and Dupuy, 1993
New Scientist, 1990
Petkas, 1993
APCI, 1982, 1984a, 1984b, 1985
Mathur, undated
Products Finishing, 1983
Stroup, 1991
Wolff, 1984
-------
are referenced in the discussions of each cleaner
technology.
Some additional inspection, hand cleaning, or other
operations may be needed to prepare the surface after
use of the cleaner technology for coating removal.
These are noted to indicate special considerations in
the application of the cleaner coating removal technol-
ogy.
Process Complexity, Required Skill Level, Waste
Products and Emissions, and Capital Cost serve to
qualitatively rank the cleaner technologies relative to
each other. The rankings are estimated based on the
descriptions and data in the literature. The text further
describes the operating information, applications,
benefits, known and potential limitations, technology
transfer, and the current state of development for each
technology. Technologies in earlier stages of develop-
ment are summarized to the extent possible in Section
3, Emerging Technologies.
The last column in Table 2 cites References to publica-
tions that will provide further information about each
available technology. These references are given in full
at the end of the respective technology sections.
Plastic Media Blasting
Pollution Prevention Benefits
The plastic media blasting (PMB) coating removal
process eliminates the use of solvent strippers. The
process uses nontoxic plastic media for coating re-
moval and does not generate volatile organic air
emissions. PMB is a completely dry stripping process,
thus eliminating generation of wastewater.
In most applications the plastic media are collected,
cleaned to remove coating debris, and reused. The
plastic particles do breakdown in use so they can not
be reused indefinitely. Once the particle size is smaller
than about 60 to 80 mesh, the stripping efficiency
drops. These small plastic fragments, mixed with
coating debris, must be discarded.
wt jkH&x& but my teqwr*
disposal,
The disposal of the spent media could be a problem.
Although the plastic media are not toxic, the spent
stripping medium will be contaminated with coating
chips. These coating residues may contain hazardous
metals or unreacted resins. The disposal options
available depend on the nature of the media used and
the coating stripped. If the spent media do require
disposal at a hazardous waste site, the cost will be
high.
A thermoplastic material has been developed to allow
recycling of spent blasting medium (Lyons, 1990). If
thermoplastic media are used, it is often possible to
recover the spent media for manufacture of plastic
parts even with the coating chip contamination.
Bioreactors are also under development to treat the
spent PMB waste (Baker, 1991). It may be possible to
degrade either the plastic media or the coating residue
(Bowers-Irons et al., 1991). Generally, however, the
spent PMB media are not recyclable or biodegradable,
so disposal is required.
How Does It Work?
The PMB process uses low-pressure air or centrifugal
wheels to project plastic media at a surface. The blast
particles have sufficient impact energy, coupled with
hardness and geometry, to chip away or erode the
coating. The sharp-faceted particles fracture on impact,
leaving new sharp edges to allow continued use for
stripping. After the coating has been removed, the part
can be prepared for recoating by air pressure and/or
vacuuming to remove plastic dust and coating debris.
The hardness of the plastic particles varies from 34 to
72 on the Barcol scale (3.0 to 4.0 on the Mohs' scale).
In general, the plastic media are selected to be harder
than the coating. Otherwise, a larger particle size must
be used to reach the necessary impact energy level.
In typical applications, the air pressure measured in the
pot ranges from 10 to 60 psi. The higher pressures
remove coaling faster but also are more likely to induce
substrate damage.
Operating Features
There are two basic types of PMB systems (1) cabinet
systems and (2) open-blast systems. Automated and
manual cabinet systems are available for stripping
smaller parts. Standard cabinet dimensions typically
are limited to about 8 feet. The cabinet systems provide
an controlled environment for media collection and
reuse. Automated cabinets use either air pressure or
rotating wheels to project the media toward the parts.
The parts may be in rotating baskets or can be moved
through the cabinet on tracks or conveyor belts if high
throughput with low labor use is needed. Manual
systems involve an operator manipulating an air-
powered blast nozzle. The open-blast systems are
applicable for parts too large to fit into the cabinets, for
example, automobiles, white goods, and aircraft. In the
open systems, the operator uses a nozzle to project the
air-driven blast media at the surface.
12
-------
A wte fange cf PMB eqttqmeflt is &va%-
abfe awtbfa$ting conditions c&ft be
ssfectetfto suftttm coating amisubstoatfa.
PMB stripping equipment may range from simple
single-nozzle systems to complex multinozzle com-
puter-controlled systems (Capron and Wells, 1990).
The electronic control systems provide not only for
remote control of the operating parameters, such as
blasting pressure and media flow rate, but also for fully
automated motion and process control, such as robotic
operations (Dicaire, 1990).
The parameters that affect the performance of the PMB
process include
Blasting pressure10 to 60 psi with an optimum
range of 20 to 40 psi
Angle of impingement30ฐ to 80ฐ
Media flow rate250 to 500 Ib/hr with a 1/2-in
nozzle
Blasting standoff distance6 to 30 in
Stripping rate0.5 to 5 ft2/min
Type of coating to be removed
Nature of substrate material and its thickness
Media type and size
Nozzle size
Masking requirements
Types and capabilities of commercially available
PMB systems (Abbott, 1992; Lyons, 1990).
During normal operations, a PMB operator will have a
set of predetermined parameters to be applied to a
given substrate. In the case of a complex workpiece
containing parts made of several types of materials or
with filled areas, the operators will adopt a blast plan
with each substrate marked as to type prior to blasting.
Problems may arise when higher air pressures are
used for blasting, including metal removal, reduced
resistance to metal fatigue, the hiding and causing of
surface cracks, and buckling. These problems have
caused some controversies in the aerospace industry
where materials such as aluminum and high-strength
composites are required to carry dynamic or fatiguing
loads.
The U.S. Bureau of Mines conducted a study of the
explosibility and ignrtabilrty of plastic abrasive media for
the Naval Civil Engineering Laboratory (NCEL) (U.S.
Navy, 1987). The study concluded that recycled media
in the size range of 12 to 80 mesh would not explode,
but that participate from degraded media had explosive
potential (for example, less than 40 mesh with Type V
media). The possibility for explosive condition is
greatest in portions of the media recycling system
where the concentration of fines is highest, for ex-
ample, a baghouse filtration system. The report sug-
gests locating such equipment away from occupied
areas, outside if possible, and providing overpressure
relief vents.
Ptastte &astmedi8-G&R b& selected io
adj&stthe stripping rate and aggressive-
ness.
Six thermoset and thermoplastic blast media have
been promulgated and/or approved for use by the U.S.
Department of Defense (Lyons, 1990; U.S. DoD, 1988).
Specifications for a biodegradable, nonpetroleum
polymer also were introduced later (Lyons, 1990). The
blast media are classified by type and hardness (Barcol
and/or Mohs1 scale), as follows:
Type Ipolyester (thermoset), 34 - 42 Barcol,
3.0 Mohs
Type IIurea formaldehyde (thermoset), 54-
62 Barcol, 3.5 Mohs
Type IIImelamine formaldehyde (thermoset),
64-72 Barcol, 4.0 Mohs
Type IVphenol formaldehyde (thermoset),
3.5 Mohs
Type Vacrylic (thermoplastic), 46-54 Barcol,
3.5 Mohs
Type VIpolycarbonate (allyl diglycol carbonate)
(thermoset), 20-30 Barcol, 3.0 Mohs
Type VIIa nonpetroleum amylaceous polymer
(biodegradable), 2.8 Mohs.
targer( hafder media 0ve more aggressive
stripping,
The order of media aggressiveness from mild to
aggressive is Type I, Type VI, Type V, Type II, and Type
III. Type I is soft abrasive that would be selected for
topcoat or primer removal from soft metals or fiber-
glass. Type VI is intended for low-air-pressure applica-
tion to removing coating from fiberglass or other
composites. Higher air pressure increases the break-
down rate of Type VI media, so the application pres-
sure is limited to about 20 psi. Type V is a durable
medium for general stripping of coatings from metal
sheeting. Type II, like Type V, is applied for general
stripping. Type II gives faster stripping rates but is less
more likely to damage the substrate if the operator
deviates from stripping parameters. Type IV is similar
to Type II in aggressiveness but breaks down faster
and has not found much market acceptance. Type III is
an aggressive, fast-acting medium for removal of
topcoats and primers from hard substrates such as
engine parts (Groshart, 1988; Bailey, 1992; Compos-
ition Materials Co., 1993); U.S. Technology Cor-
poration, 1993).
13
-------
Size is the second major factor controlling the aggres-
siveness of PMB media. Larger particles generate
more aggressive stripping action. The various types of
media typically are available in about five mesh size
ranges. The largest standard size available is 12 to 16
mesh and the smallest is 40 to 60 mesh. The material
type and particle size can be selected to optimize the
PMB system to the cutting speed and gentleness
required for particular application.
Plastic madia ean ba cteanatfandm0se&
Systems to recover and reuse the plastic media have
been developed. Media recovery is facilitated if the
parts are small enough to allow the use of a blasting
cabinet. Media reuse systems separate contaminants,
such as coating chips and undersized media frag-
ments, from the intact media. Separation can be done
by cyclone separators, vibrating screens, magnetic
separators, or similar equipment. The media reclama-
tion systems typically employ a combination of these
equipment types to separate contaminants and clean
the spent media for reuse (Wasson and Pauli, 1993).
The number of reuse cycles that can be achieved is
variable. Generally large media and lower operating
pressures allow more reuse cycles. Granulated plastic
pellets used at pressures below 50 psi are reported to
be durable with an average breakdown rate of less
than 10%.
Eash appttedtton requlm$ $pectifc ctts&m
don wcBvidvafpfOducffotl
; a generic *ฎtettem&nt of
The energy requirement is determined by the complex-
ity of each PMB system. Compressed air is required to
operate the blasting system at different blasting pres-
sures and nozzle sizes. For example, the air use is 8
SCFM for a 1/8-inch nozzle at 30 psi and 230 SCFM
for a 1/2-inch nozzle at 60 psi (Dotson and Ballard,
1992). Energy is required to operate a spent media
recovery subsystem that includes a pneumatic trans-
port vacuum hose, an induced draft fan, a rotary screw
conveyor, and a subfloor piping or mechanical convey-
ing system. Energy is also consumed by the media
recycling subsystem that includes a cyclone, an
airwash, a vibrating screen, a rotary airlock, and
pneumatic or mechanical conveyance devices. Other
subsystems, such as a dense particle separator, dust
collector, and ventilation system, also consume energy.
As seen in Table 2, PMB operation requires a medium
skill level. Effective use of PMB requires an initial
training period to familiarize the operator with the
required stripping media supply pressure and the
nozzle-to-surface distance and angle. With appropriate
training, operators should be able to perform the job
without much difficulty.
Application
The PMB process has been widely used by the military
and commercial sectors:
Types of coatings removed include powder
coatings, urethanes, military chemical agent
resistant coatings, epoxies, high solids,
polyamid, acrylic lacquers, polysulfide sealants,
fluorocarbon films
Cleaning/stripping of machinery, equipment,
engines, injection molds, etc.
Cleaning/stripping of aluminum, stainless and
mild steel, fiberglass and plastic totes, and tanks
and containers
Cleaning/stripping of commercial/industrial parts,
components, and structures fabricated of metal,
engineered plastics, fiberglass, and advanced
composites
Stripping of marine vessels and related compo-
nents and assemblies
Exterior airframe stripping
Stripping of aircraft ground equipment
Stripping aircraft components (e.g., wheels,
brakes, landing gear, engine parts, and compos-
ite parts) (Lyons, 1990; Novak, 1990; Pauli,
1989).
*
Current ^ppScaffym $how tftat PMB c&rt
PMB stripping of a C-5 aircraft (32,000 ft2) was studied
in detail at a large new Air Force installation designed
for PMB stripping of B-52 and C-5 aircraft using Type V
PMB media. The study indicated that PMB offers
significant economic advantages over solvent stripping.
The total working time for supervision, masking,
blasting, demasking, sanding, vacuum and blow-off,
and housecleaning was 3,010 hours. This reported to
be a savings of 2,000 hours over solvent stripping of
the same aircraft. The reported stripping rate, waste
generation rate, and unit cost were 1 .35 ft2/min, 0.22 Ib/
ft2, and $4.70/ft2. The costs include electrical, labor,
media use, hazardous waste disposal, and consuma-
bles. The PMB process is expected to save $4,800,00-
0/year and eliminate 72,000 gallons/year of methylene
chloride stripper (Wasson and Pauli, 1993).
The major factors controlling costs of operating a PMB
system are
Hourly cost of direct labor
Labor productivity rates, typically 75%
14
-------
Cost of blast media, ranging from $1.50/lb to
more than $2.00/lb (1991 prices)
Energy costs
Overhead costs
Waste disposal costs, ranging from inconsequen-
tial to up to $4/blast-hr if hazardous waste is
generated (assuming a 1/2-in nozzle at 30 psi)
Removal rate, typically ranging from 0.5 to more
than 4 ft2/min (assuming a 1/2-in nozzle at 30 psi)
Efficiency of the media reclamation system.
Under typical operating conditions, the variable operat-
ing costs are reported to range from $45 to $65/blast-
hr, and the cost of removal can range from $0.20 to
$2.15/ft2 (Lyons, 1990). The process can provide a high
throughput rate, but the capital investment and start up
costs for new system with state-of-the art media
recycling equipment can be high. In most cases the
PMB systems are not compatible wrtlji existing stripping
facilities, so facility modifications are required.
Benefits
Some of the major beneficial aspects of PMB include
High stripping rate
Eliminates water use
Can selectively remove individual coating layers
(e.g., remove topcoat leaving primer)
Often done with recyclable thermoplastic media
Fully automated robotic systems available
Fully developed systems available
No size limitations on parts to be stripped.
Limitations
Potential hazards and limitations of PMB include
Spent media contain coating chips and may be a
hazardous waste.
Operators should wear respiratory and eye
protection equipment for protection from re-
bounding media and airborne particulates.
Operators should wear hearing protection due to
high noise levels from blasting equipment.
PMB may cause metal substrate damage such
as reducing resistance to metal fatigue, hiding
and causing of surface cracks, and buckling.
PMB may cause crack closure.
More aggressive media types damage composite
materials.
Contaminants in media may damage substrate.
PMB has potential for high disposal costs if spent
media are hazardous and cannot be recycled.
PMB uses flammable media.
The technology has somewhat high capital and
startup costs.
PMB requires complex subsystems for media
recovery and recycling and dust collection and
control.
There is a possible explosive hazard from dust.
References
Abbott, K. E. 1992. "Plastic Media Blasting State of
the Technology." Materials Performance, 37(2):38-
39.
Bailey, J. 1992. "Strip It Off." Industrial Finishing,
68(2) :24-27.
Baker, S. 1991. "Bugs Help Strip Old Paint in Bio-
Reactor System." Skywriter, November 22, p. 22.
Bowers-Irons, G., R. Pryor, and C. Miller. 1991. 'The
Biostripping of Polyurethane Paint." In: Proceedings
of the 1991 DOD/lndustry Advanced Coatings
Removal Conference, San Diego, California, pp. 40-
48.
Capron, S., and M. Wells. 1990. "Performance Testing
of Plastic Media Blasting Equipment." In: Proceed-
ings of the 1990 DOD/lndustry Advanced Coatings
Removal Conference, Atlanta, Georgia.
Composition Materials Company, Inc. 1993. Plati-Grit.
Composition Materials Company, Fairfield, Con-
necticut.
Cundiff, C. H., O. L. Deel, and R. E. O'Sullivan. 1989.
"Plastic Media EvaluationA Comparative Study of
Performance Capabilities of Several Plastic Media."
In: Proceedings of the 1989 DOD/lndustry Ad-
vanced Coatings Removal Conference, Ft. Walton
Beach, Florida.
Dicaire, P. 1990. "Dust-Free Robotic PMB Paint-
Stripping for Transport-Size Aircraft." In: Proceed-
ings of the 1990 DOD/lndustry Advanced Coatings
Removal Conference, Atlanta, Georgia.
Galliher, R. D. 1989. "Surface Preparation and Paint
Adhesion on Aluminum Substrate after Blasting with
Plastic Abrasive." In: Proceedings of the 1989 DOD/
Industry Advanced Coatings Removal Conference,
Ft. Walton Beach, Florida.
Groshart, E. 1988. 'The New World of Finishing." Metal
Finishing, October, pp. 33-34.
HazTECHNews. 1991. HazTECH News, 6(8).
Lyons, P. J. 1990. "Plastic Media Comes of Age." Metal
Finishing, July, pp. 43-46.
15
-------
Novak, H. L. 1990. "Development and Testing of a
Fluorescent Plastic Grit Blast Media." In: Proceed-
ings of the 1990 DOD/lndustry Advance Coatings
Removal Conference, Atlanta, Georgia.
Pauli, R. 1989. "Plastic Media Blast Dry Stripping Four
Years Later." In: Proceedings of the 1989 DOD/
Industry Advanced Coatings Removal Conference,
Ft. Walton Beach, Florida.
Roberts, B. 1989. "PMB, Then, Now and the Future".
In: Proceedings of the 1989 DOD/lndustry Ad-
vanced Coatings Removal Conference, Ft. Walton
Beach, Florida.
U.S. Department of Defense (DoD). 1988. "Plastic
Media, for Removal of Organic Coatings." Military
Specification, MH-P-85891A, 0386 DD, May 6.
U.S. Navy. 1987. Explosibility and Ignitability of Plastic
Abrasive Media. CR 87.011, Naval Civil Engineering
Laboratory, Port Hueneme, California.
U.S. Technology Corporation. 1993. The Magic is the
Media. U.S. Technology Corporation, Canton, Ohio.
Wasson, N. E., Jr., and R. Pauli. 1993. "Dry Stripping
the C-5 and B-52 in the World's Largest Dry Strip-
ping Installation." In: Proceedings of the 1993 DOD/
Industry Advanced Coatings Removal Conference,
Phoenix, Arizona, pp. 530-540.
Wheat Starch Blasting
Pollution Prevention Benefits
The wheat starch blasting coating removal process
eliminates the use of solvent strippers. The process
uses nontoxic, biodegradable media for coating re-
moval and does not generate volatile organic air
emissions. The wheat starch blasting media are made
from renewable agricultural products rather than from
petroleum, which helps reduce resource consumption.
Wheat starch blasting is a completely dry stripping
process, and thus eliminates the generation of wastew-
ater.
The starch media can be collected and reused for
several blasting cycles. The wheat starch particles do
break down in use, so they cannot be reused indefi-
nitely. Fine dustlike particles are not effectively pro-
pelled for stripping. The starch media are processed in
equipment similar to that used for processing PMB
media. Small starch fragments, mixed with coating
debris, are separated and discarded.
The disposal of the spent media could be a problem.
Although the media are not toxic, the spent stripping
media will be contaminated with coating chips. These
coating residues may contain hazardous metals or
unreacted resins. The disposal options available
depend on the volume of the media used and the
coating stripped. The wheat starch blasting media are
100% carbohydrate, so proper aerobic biodegradation
can reduce the waste volume substantially. The media
are digested to produce a liquid that can be separated
from coating debris prior to disposal. Biodegradation is
most likely to be economical when spent media vol-
umes are on the order of 50,000 to 100,000 pounds
{Oestreich and Waugh, 1993).
How Does It Work?
Wheat starch blasting uses low-pressure air to propel
particles at the painted surface. The coating is stripped
away by a combination of impact and abrasion. Al-
though wheat starch blasting uses generally similar
equipment and techniques to PMB, the process has
somewhat different operating characteristics and
stripping action (Drake, 1993).
Operating Features
In wheat starch coating removal, particles of wheat
starch are propelled at a surface by a flow of air to
abrade and fracture the coating. The natural wheat
starch has the benefits of being nontoxic, biodegrad-
able, and made from a renewable resource (Lenz,
1991). The media are clear white granules in the size
range of 12 to 30 mesh with a density of 1.45 g/cm3
and a Shore D hardness of 85.
Testing determined that when the propelling air pres-
sure is above 30 psi (200 kPa), the starch particles
fracture. The fracturing occurs as the starch removes
coating material, resulting in smaller particles and more
edges per pound of medium to be recycled as stripping
proceeds. The wheat starch thus becomes more
effective as it is used until the particles become so
small that suspended starch dust obscures the opera-
tor's view of the surface. The used starch media are
collected and processed. Small starch particles and the
removed coating are collected for disposal, and the
larger particles are reused for blasting. Because the
media are reused continuously for coating removal, the
potential arises for contamination of the media with
harder coating particles. The coating particles could
impact the substrate and cause stress risers.
16
-------
Wtmat stetch Wasting uses similar Mptfi-
m&rjt Qftdt&ptmiques t& PMB but the
fnedfa properties restitiipt a gefttia, reflafcfe
stripping action.'
Particle fracturing reduces the sensitivity of wheat
starch coating removal to operating conditions. An
increase in air pressure increases particle flow rate but
does not cause the stripping action to become more
aggressive.
As with the plastic media, new or clean recycled wheat
starch media do not present explosive hazards. Dust
generation from the wheat starch raises the potential
for generating an explosive dust mixture. Testing
performed for a wheat starch media vendor indicates
that undried dust must be smaller than 120 mesh for
explosion to be a hazard. As with PMB dust, the
explosive hazard from wheat starch blasting media
dust is small and is limited to process areas where high
concentrations of dust may accumulate. Precautions
for handling wheat starch blasting media dust should
be similar to those mentioned for PMB dust handling.
The reported typical blasting conditions for coating
removal from composites are (Oestreich and Porter,
1992)
Blasting pressure20 to 25 psi
Angle of impingement20ฐ to 40ฐ
Media flow rate420 to 720 Ib/hr with a 3/8-inch
extended Venturi or double-Venturi nozzle
Blasting standoff distance6 to 8 in.
The reported typical blasting conditions for coating
removal from clad aluminum are (Oestreich and Porter,
1992)
Blasting pressure 25 to 30 psi
Angle of impingement 40ฐ to 70ฐ
Media flow rate 900 to 1,200 Ib/hr with a 1/2-inch
extended Venturi or double-Venturi nozzle
Blasting standoff distance 8 to 12 in
Stripping rate 0.9 ft2/min.
The wheat starch can absorb moisture causing clump-
ing of the media during blasting. In humid conditions, it
may be necessary to dry the blasting air to avoid
moisture pickup by the media.
Application
Wheat starch blasting is known mainly for its gentle
stripping action. Therefore most of the testing and
application has been on sensitive substrates such as
Thin aluminum, particularly soft alloys or anod-
ized surfaces (e.g., commercial aircraft skins)
Sensitive composites (e.g., automobile fiberglass
or plastic or aircraft radomes).
The wheat starch technology has been tested for
stripping a variety of epoxy, urethane, zinc chromate
primer, and alkyd enamel coatings such as MIL-P-
23377, MIL-C-83286, and TT-E-489 (Larson, 1990).
Test substrates have included aluminum, plated ferrous
alloys, and composites.
Benefits
Some of the major beneficial aspects of wheat starch
blasting include
Recent developments indicate that moderate
stripping rates can be achieved while maintaining
a gentle stripping action
Safe on soft clad aluminum and composites
Eliminates water use
Can selectively remove individual coating layers
(e.g., remove topcoat leaving primer)
Uses inexpensive stripping media
Media are nontoxic and biodegradable
Fully developed systems available
No size limitations on parts to be stripped.
Limitations
Potential hazards and limitations of wheat starch
blasting include
Spent media contain coating chips and may be a
hazardous waste
Generally slow to moderate stripping rate
Dense contaminants in recycled media may
damage substrate
Operators should wear respiratory and eye
protection equipment for protection from re-
bounding media and airborne paniculate
Operators should wear hearing protection due to
high noise levels from blasting equipment
Media are moisture sensitive and can require an
air dryer in humid atmospheres
Potential for high disposal costs if spent media
are hazardous and cannot be recycled or treated
by biodegradation
Somewhat high capital and startup costs
Requires complex subsystems for media recov-
ery and recycling and dust collection and control
Explosive hazard from dust.
17
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References
Drake, C. 1993. "Envirostrip Starch Media Dry Stripping
Process Update." In: Proceedings of the 1993 DOD/
Industry Advanced Coatings Removal Conference,
Phoenix, Arizona, pp. 427-431.
Larson, N. 1990. "LowToxicrty Paint Stripping of
Aluminum and Composite Substrates." In: First
Annual International Workshop on Solvent Substitu-
tion. DE-AC07-76ID01570, U.S. Department of
Energy and U.S. Air Force. Phoenix, Arizona, pp.
53-60.
Lenz, R. P. 1991. "Performance Characteristics of
Starch Blast Media." In: Proceedings of the 1991
DOD/lndustry Advanced Coatings Removal Confer-
ence, San Diego, California, pp. 147-188.
Oestreich, J., and T. Porter. 1992. "Starch Media
Blasting for Aerospace Finishing Applications." In:
28th Annual Aerospace/Airline Plating and Metal
Finishing Forum and Exposition, San Diego, Califor-
nia.
Oestreich, J., and R. D. Waugh. 1993. "Bioremediation
of Starch Media Waste." In: Proceedings of the 1993
DOD/lndustry Advanced Coatings Removal Con-
ference, Phoenix, Arizona, pp. 432-443.
Burnoff Coating Removal
Pollution Prevention Benefits
Burnoff coating removal technologies use a combina-
tion of volatilization, pyrolysis, and oxidation to remove
organic coating materials. Thermal methods completely
avoid the use of solvents for coating removal but
generate potentially contaminated offgas and wastewa-
ter streams. In a well-designed unit, the organic
materials will be almost completely converted to carbon
dioxide and water. However, traces of more complex
organic compounds may appear in the offgas. In
addition, coatings containing halogens or nitrogen
compounds will produce volatile, corrosive compounds
such as hydrogen chloride. Inorganic pigments will not
volatilize and thus remain as a residue on the part after
the organic coating burns off. Water may be needed to
scrub the pyrolysis stripping unit offgas stream and, in
some systems, is used to flush inorganic residue from
the stripped part.
How Does It Work?
Burnoff systems use temperatures of 370ฐC (700ฐF) or
higher to volatilize and/or burn the organic coating
material. A few metals such as mercury or arsenic will
volatilize at the operating temperature of burnoff ovens.
However, toxic volatile metals are not used in current
paint formulations. Inorganic materials such as pig-
ments remaining on the substrate must be removed by
mechanical cleaning such as low-energy shot blast,
manual cleaning, or water rinse.
Operational Features
Burnoff systems remove coating materials rapidly. Even
difficult coatings such as heavy layers of powder
coating can be removed. However, the substrate is
exposed to a harsh, high-temperature environment so
pyrolysis coating removal is generally suitable only to
noncrrtical items. Burnoff coating removal is not gener-
ally acceptable for parts that will be used in a product.
However, the functioning of part support equipment
usually is not impaired by many cycles of heating, so
burnoff coating removal can be used for hooks, racks,
and overspray collectors.
Burnoff coating removal can be accomplished by a
variety of methods including direct burnoff, heating in
an abrasive fluidized bed, or pyrolysis. In all cases a
high-temperature energy source is used to remove
organics followed by a cleaning process to remove
inorganic residues. Inorganic residue removal can be
accomplished by mechanical or manual brushing or
blast cleaning with water or airborne media (Izzo,
1989). Offgas treatment including an afterburner,
scrubber, and filter Jtypically is supplied to control air
pollution. To ensure safety, the system must be de-
signed to control the intense heat resulting from the
rapidly burning organic coating.
For direct burnoff, the coating is ignited to burn off the
organic material at an operating temperature of 540ฐC
to 650ฐC (1000ฐF to 1200ฐF). Direct burnoff is suitable
to continuous operation in which a conveyor carries the
racks through the burnoff oven and then through a
cleaning system to remove inorganic residue. As the
parts pass through the burnoff unit, ceramic nozzles
direct high-temperature flue gas onto the parts at high
velocity to ignite the coating. Complete combustion
typically occurs within the unit to ensure acceptable
coating removal and suitable air pollution control at the
source. With proper line speed and operating tempera-
ture, complete combustion can be obtained, but some
units include an afterburner to further ensure that
organic materials are fully converted to carbon dioxide
and water. Burnoff also can be done in batches in a
closed oven.
18
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In an abrasive fluid bed, the coating is thermally
degraded by a combination of pyrolysis and partial
oxidation at a temperature of 480ฐC to 510ฐC (900ฐF to
950ฐF). To maintain a fluidized state, airflows up
through a bed of abrasive media such as silica sand or
aluminum oxide (Coberth and Ceyssons, 1993). The
hot abrasive media transfer heat to the coating to
pyrolyze and remove organic constituents. After the
part is removed from the fluidized bed, inorganic
residues must still be removed. Heat is supplied to the
abrasive media by an electrical resistance heating unit.
The organic materials are not fully oxidized in the
fluidized bed, so an afterburner operating at 790ฐC to
870ฐC (1450ฐF to 1 600ฐF) is required to oxidize the
intermediate organic products.
proc&ss, ".
In the pyrolysis process, the coating is volatilized to
produce fumes rich in organic compounds (Whelan,
1993). The combustible materials on the substrate
volatilize to form an organic-rich vapor but do not burn
in the pyrolysis unit. The unit, therefore, operates with
low or no oxygen and at a lower temperature (370ฐC to
500ฐC (700ฐF to 930ฐF)) with no flame present in the
unit.
Some coatings, notably epoxies, contain oxygen
molecules bound in the coating. The oxygen in the
coating can support combustion which would cause
excessive temperature rise. Water vapor cloud injection
controls the temperature in the pyrolysis unit to ensure
no combustion takes place and to minimize damage to
the substrate. Typically pyrolysis units can only process
cured coating materials. The solvent and other volatiles
in uncured coatings will evaporate rapidly in the
pyrolysis unit. The rapid input of reacting materials will
cause temperatures in the unit to rise before the control
system can respond. The resulting uncontrolled
temperature rise causes the pyrolysis unit to shut down
to prevent excessive temperatures. Advanced control
systems are being developed and tested to allow
pyrolysis to be applied to uncured coatings (Mann,
1991).
Because of the need to control oxygen levels, pyrolysis
units typically are batch ovens. The organic fumes from
the pyrolysis unit are treated in an afterburner to
convert hydrocarbons to carbon dioxide and water.
Following removal from the pyrolysis unit, inorganic
residues must be removed from the part.
Burfxtffgives fapktmmovatQf ttotek
castings.
Heat is the principle removal mechanism for coating
removal. Although all of the thermal systems require a
follow-up cleaning step to remove inorganic residues,
no solvents or alkalis are used to soften and remove
the coating. Despite the heating value of the organic
material in the coating, heat input is needed to initiate,
maintain, and complete combustion. Heat for direct
burnoff or pyrolysis units usually is supplied by
combustion of a fossil fuel, typically gas, in the coating
removal unit. The fluidized bed units normally use
electrical heating. The afterburner in all units typically
uses gas or another fossil fuel to supply the required
energy.
The control systems for a burnoff coating removal
system are complex. Accurate temperature control is
needed to ensure that complete removal of the coating
and destruction of organics in the offgas is reliably
achieved. The control systems for the thermal units and
afterburner may be unlike equipment normally found in
coating shops, so new maintenance skills are needed.
Actual operation of a burnoff coating removal unit
involves only mechanical or manual loading and
unloading of parts. The units are typically designed to
operate automatically during the coating removal cycle,
so no operator attention is needed during a normal
cycle. The required skill level is, therefore, lower than
the level for solvent stripping units that require the
operator to handle potentially hazardous chemicals.
Application
Burnoff coating removal is commonly used for high-
volume, noncritical parts such as the hooks, racks,
overspray collectors, or other similar parts. Burnoff
methods can be used to remove both conventional and
powder coatings (Mann, 1991).
It also may be possible to use the burnoff coating
technology to strip parts with poor coatings, but some
limitations apply. Metals with a melting point below
900ฐF generally are not suitable for burnoff coating
removal. Magnesium will burn violently if ignited, so
magnesium and its alloys should not be stripped in a
burnoff oven. Iron, steel, and nontempered aluminum
generally are amenable to burnoff stripping. However,
testing must be performed to determine if heating
deforms, removes tempering, or otherwise damages
the part.
19
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Benefits
Molten Salt Coating Removal
Some of the major beneficial aspects of burnoff coating
removal include
Allows rapid removal of heavy coating accumula-
tion with a minimum of handling.
Can process parts with complex shapes.
Direct-burn ovens can remove wet, uncured
coatings.
Large ovens are available to process large items,
but the maximum size is limited by the oven size.
Limitations
Potential hazards and limitations of burnoff coating
removal include
Generates coating ash residue that may be
hazardous waste.
Will damage heat-sensitive materials such as
heat-treated aluminum or magnets.
Coatings containing halogens (polyvinyl chloride
[PVC] or polytetrafluoroethylene [PTFE]) and/or
nitrogen will produce corrosive offgas.
Must not be used for low-melting alloys such as
zinc-bearing materials.
Must not be used for magnesium or its alloys, or
for pyrophoric metals.
May require offgas treatment, such as scrubbers
and air filters, depending on local air permitting
requirements
May generate products of incomplete combustion
Presents possibility of fire.
References
Coberth, D., and L. Ceyssons. 1993. "Stripping Organic
Finishes with Fluidized Beds." Metal Finishing
Organic Finishing Guide Book and Directory Issue,
97(5A):234-236.
Izzo, C. 1989. "PF Coatings Clinic." Products Finishing
53(5) :14.
Mann, C. 1991. "Paint Waste Minimization by Means of
Recyclable Overspray Collectors." Product Finish-
ing, June, pp. 22-24.
Metal Finishing. 1990. "Product Showcase: Metal and
Paint Strippers." Metal Finishing, July, pp. 36-40.
Whelan, K. 1993. "Burn-Off Ovens for Stripping." Metal
Finishing Organic Finishing Guide Book and Direc-
tory Issue, 97(5A):225-227.
Pollution Prevention Benefits
The molten salt stripping process replaces solvent
strippers. The molten salt process produces a coating
pigment salt by-product residue, wastewater, and
offgas streams.
During molten salt stripping, by-products of the reaction
of the salt and the coating accumulate in the bath.
Even when the bath is saturated with by-products,
stripping will continue. Additional by-products develop
as more coating is removed from a separate phase in
the bath. The by-product sludge phase can be removed
for disposal. The organic content of the coating will be
oxidized by reaction with the salt bath. The by-product
sludge is a small volume containing mostly metal salts
formed by reaction of pigments with the salt bath
materials. Depending on the salts used in the bath and
the metals in the pigments, the sludge may have RCRA
hazardous characteristics.
The wastewater results from water used to cool and
rinse the part after it leaves the molten salt bath. The
salt in the coating removal bath usually is formulated
from alkaline materials, so for most installations the
rinsewater will have a pH of about 11 to 12. The
rinsewater will require neutralization to a pH range of
6 to 9 prior to discharge. For plants with a central
wastewater treatment plant, it may be possible to use
the alkaline rinse water to help neutralize acidic waste-
water from other metal-finishing operations. The
rinsewater also may contain metals from the coating
pigments. Analysis for potential metals should be
performed prior to discharge, and treatment for metal
removal may be required depending on the plant
discharge permits.
Tfie moften 38tttwtmol0gycbefWGaffy
As with the burnoff coating removal systems, molten
salt coating removal works by combusting the coating
organics. The result for hydrocarbon coatings should
be mainly the formation of CO2 and H2O. However,
products of incomplete combustion and entrained salt
particulates and pigments can enter the offgas stream.
A well-designed molten salt stripping system will
include provisions to control and treat the offgas.
How Does It Work?
The molten salt stripping process relies on chemical
oxidation of the coating by a specially formulated
molten salt bath. The process uses mixtures of inor-
ganic salts formulated to react with the coating mate-
rial. Carbon and hydrogen in the coating are oxidized to
20
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CO2 and H2O. Most metals are retained in the molten
salt bath. In sodium carbonate-based and similar
molten salt formulations, halogens combine with the
molten salt to form halides and to release CO2 from the
carbonate salts. Metals from the coating pigments
generally are retained in the molten salt and enter the
offgas stream only in small amounts.
The main functions of the molten salt are a heat
transfer medium and catalyst to oxidize the organics in
the coating. The salt bath provides thermal inertia and
effective heat transfer to avoid hot spots or temperature
excursions. The molten salt also acts as an in-place
scrubber which retains the nonvolatile by-products (Gat
etal., 1993).
Operational Features
Molten salt stripping uses simple and straightforward
processing steps. The items to be stripped are loaded
into baskets or supported on hooks. The items then are
lowered into the salt bath at a controlled rate. The
required heating time in the bath depends on a number
of variables including
Chemistry and temperature of the bath
Shape, size, and material of the item
Thickness and type of coating being stripped.
The typical dwell time ranges from seconds for thin
coatings to minutes for thick coatings (Malloy, 1993).
Following immersion, the items are removed from the
salt bath and rinsed with water for cooling and removal
of residual salt. The rinsed items are dried by an air
knife or other compressed air blow drying operation.
The process allows rapid and complete coating re-
moval with a minimum of hand work.
Molten salt stripping baths are formulated from inor-
ganic salts such as sodium carbonate. The exact
mixture of salts is tailored to the required operating
temperature, chemical reactivity, and performance. The
operating temperature for the salt bath varies, depend-
ing on the salt formulation. Formulations are available
with operating temperatures from 550ฐF to 900ฐF. The
lower temperature formulations usually are applied to
salvage materials with blemished coatings or for
maintenance stripping. Higher temperature formula-
tions strip heavy coating accumulations from hooks,
racks, and paint line fixtures.
Application
Molten salt stripping typically is targeted to the same
applications as bumoff technologies. Although the
molten salt process achieves coating oxidation by a
different mechanism, the process provides the same
basic features, that is, rapid destruction of thick coat-
ings. The items most often stripped with molten salt
baths are paint line supports and fixtures. Molten salt
baths can remove a variety of organic coatings includ-
ing nylon, polyester, and epoxies. Due to the chemistry
of the bath, molten salt systems also can be applied to
strip coatings containing halides, e.g., PVC and PTFE
(Malloy, 1993).
Benefits
Some of the major beneficial aspects of molten salt
coating removal include
Allows rapid removal of heavy coating accumula-
tion with a minimum of handling.
Can process parts with complex shapes.
Provides rapid, well-controlled, uniform heating.
Wastewater stream is compatible with conven-
tional wastewater treatment plants available to
many installations.
Salt baths are available to process moderate-
sized items, but the maximum size is limited by
the bath size.
Limitations
Potential hazards and limitations of molten salt coating
removal include
Generates a by-product salt sludge that may be
a hazardous waste.
Will damage heat-sensitive materials such as
heat-treated aluminum or magnets.
Must not be used for low-melting alloys such as
zinc-bearing materials.
Must not be used for magnesium, its alloys, or
pyrophoric metals.
May require offgas treatment, such as scrubbers
and air filters, depending on local air permitting
requirements.
May generate products of incomplete combus-
tion.
Wastewater and dissolved salt disposal require-
ments will depend on the toxicity of the coating
and pigments being removed.
References
Gat, U., S. M. Crosley, and R. L. Gay. 1993. "Waste
Treatment for Removed Protective Coatings." In:
Proceedings of the 1993 DOD/tndustry Advanced
Coatings Removal Conference, Phoenix, Arizona.
Malloy, J. C. 1993. "Molten Salt Bath Stripping of
Organic Coatings." Metal Finishing Organic Finish-
ing Guide Book and Directory Issue, 97(5A):234-
236.
21
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Metal Finishing. 1990. "Product Showcase: Metal and
Paint Strippers." Metal Finishing, July, pp. 36-40.
Sodium Bicarbonate Wet Blasting
Pollution Prevention Benefits
The sodium bicarbonate technology eliminates solvent
use in coating removal. The sodium bicarbonate
stripping medium is not regulated under the OSHA
hazard communication standard 19 CFR 1910.1200 or
the SARA Title III reporting requirements. The stripping
medium is mixed with water, which controls dust and
substrate heating. Water also is used to rinse the
substrate after stripping is complete. As a result, an
aqueous waste stream is generated. Although the
medium is nontoxic, many coatings contain metals or
unreacted resins that are toxic. The spent media will
contain coating residue, so the aqueous waste must be
tested to determine if it will meet local discharge limits
for wastewater disposal. Testing should include quanti-
fying pH, total suspended solids (TSS), oil and grease,
and metal concentrations. If desired, the media can be
dissolved in excess water and the solid coating residue
can be removed by filtration. Even if waste treatment or
landfill disposal is needed, the total solid waste volume
generated by the sodium bicarbonate technology
typically would be less than for methods using solvents-
How Does It Work?
The sodium bicarbonate (baking soda) is delivered by a
wet blast system to remove coating in this way: Com-
pressed air moves the sodium bicarbonate medium
from a pressure pot to a nozzle where the medium
mixes with a stream of water. The blast medium/water
mixture, accelerated to several hundred miles per hour,
impacts the coated surface and shatters into a very fine
paniculate. The water prevents heat buildup in the
substrate and helps control the dust generated when
the media impact on the coating.
Operational Features
The sodium bicarbonate coating removal technology
operates mainly by abrasive action. The wet blast
system delivers a mixture of blast medium and water
through a hand-held, hand-actuated blast nozzle,
shown in Figure 1. The flow diagram (Figure 2) illus-
trates a typical configuration of the system.
The exact operating conditions are specific to the type
of coating and the substrate type and configuration.
The typical range for bicarbonate stripping applications
is (Spears, 1989):
Blasting pressure20 to 70 psi
Angle of impingement45 to 90ฐ
Media flow rate 2 to 4 Ib/min with a -in nozzle
Water flow rate 0.5 gpm
Blasting standoff distance 12 to 24 in
Stripping rate 0.25 to 2.5 ft2/min.
Btemtodnate fefasf eofuffitons <
selected to suit the coating mtt&ub$tf&8,
Other important parameters in bicarbonate coating
removal system operation are
Type of coating to be removed
Nature of substrate material and its thickness
Media type and size
Nozzle size
Masking requirements
Types and capabilities of commercially available
systems.
Various bicarbonate Media are available to
Typically a nontoxic flow agent is included in the
bicarbonate media to minimize caking in the blast pot.
The media come in six formulas to provide different
mesh sizes for different applications:
Composite formula
* Maintenance formula""
Maintenance formula XL
* Profile formula
Aviation formula
Electronics formula.
The wet blast system uses a pressurized nozzle
designed to allow a low propellant pressure while
maintaining a positive abrasive flow. The low pressure
of the air propellant minimizes damage to aluminum,
plastic composites, and other sensitive materials.
Operators can adjust the blast pressure to remove one
layer of coating at a time. The pressure of the water
can vary between 10 and 500 psi. The air requirement
is determined by blasting pressure and nozzle size. For
example, when blasting at 60 psi for a 1/2-in nozzle,
265 cfm of air is required; that, in turn, requires a
minimum of 66 HP electric compressor.
As seen in Table 2, sodium bicarbonate blasting
requires a medium skill level. Abrasive media blasting
requires an initial training period to familiarize the
operator with the required stripping media supply
pressure and the nozzle-to-surface distance and angle.
Maintenance formula and Maintenance formula XL are available with
SupraKleen Rinse Accelerator to improve removal of surface contaminants
or heavy coaling, if needed.
22
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Water Line
Source: Schmidt Manufacturing, Inc.
Figure 1. Sodium bicarbonate system with wet blast head.
With appropriate training, operators should be able to
perform the job without much difficulty.
Bfriptfient for sodium toicaftooriate wet
Sodium bicarbonate blasting uses modified sand-
blasting equipment and is less expensive than equip-
ment for PMB, wheat starch blasting, or carbon dioxide
pellet blasting.
The sodium bicarbonate medium costs more than
traditional abrasive media such as sandblasting, but is
relatively inexpensive compared to PMB. Startup costs
may include facility revamping to allow installation of
the wet blast system. An exhaust ventilation system
with cyclone separator and intake piping must be
added to control blast media overspray if the sodium
bicarbonate coating removal system is used indoors.
The sodium bicarbonate process often can be applied
in existing solvent stripping facilities, which also saves
investment in facility revisions.
Application
The technology has been applied for removal of both
friable and elastomer organic coatings. Substrate
materials include thin and thick metal parts, machinery,
and building surfaces. Sodium bicarbonate or similar
23
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Pot Blast
Pressure Pressure
Gauge ^w Gauge
Pot
Pressure
Regulator
Pot
Pressure
On/Off
Blast Air Blast Air
Regulator On/Off
Blast
Pot
Armex
Under
Pressure
Differential
Pressure
Gauge
To Blast
Nozzle
Strainer
Water Pump
Source: Schmidt Manufacturing, Inc.
Figure 2. Flow diagram of sodium bicarbonate wet blast system.
water-soluble abrasive technology has been tested or
applied to the following:
Remove failing topcoat over a tight red lead
primer on structural steel
Clean and decoat surfaces of historical buildings,
including 19th century buildings in Manchester,
England; the Parliament building and the Opera
House in Vienna, Austria; the Statue of Liberty in
New York; and the Mormon Church in Salt Lake
City, without damaging sensitive surfaces
Clean, in situ, disbonded coating from paper mill
roller bearings
Remove grease buildup from drive unit of paper
machine dryer
Remove graffiti from sandstone wall and factory-
finished metal siding
Clean railcar wheels prior to magnetic particle
inspection
Decoat diesel locomotive sheet metal door
(sandblasting had warped the panels)
Clean valving with thick coating buildup on
natural gas vaporizing tank
Clean dirt and coating residue from aircraft parts
without disassembling (Kline, 1991).
The NASA Johnson Space Center (JSC) Aircraft
Operation Division has used sodium bicarbonate to
strip the surface of aircraft wheels prior to inspecting for
cracks and structural defects. Prior to use of the
sodium bicarbonate product, NASA used a phenolic-
based stripper and another earlier chemical stripper
containing methylene chloride and other organic
solvents. Both stripping formulations required repetitive
soaking, and the costs for disposal of the solid and
liquid wastes they generated were high (Chen and
Olfenbuttel, 1993).
Tennessee Eastman has used the sodium bicarbonate
stripping to remove coating from equipment during
operation. The paper and pulp industry also has used
the technology for cleaning paper production equip-
ment in place.
Benefits
Some of the major beneficial aspects of sodium
bicarbonate wet blasting include
High stripping rate
Can selectively remove individual coating layers
(e.g., remove topcoat leaving primer)
24
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In some applications, bicarbonate stripping can
reduce prewashing and masking of the surface
needed prior to stripping
Use of water dissipates the heat generated by
the abrasive process and reduces the amount of
dust in the air
Wastewater stream is compatible with con-
ventional wastewater treatment plants available
to many installations
Low-cost stripping media and simple stripping
equipment
No size limitations on the parts to be stripped.
Limitations
Sodium bicarbonate coating stripping has several
potential hazards and limitations:
The sodium bicarbonate medium cannot be
recycled for stripping.
Operators should wear respiratory and eye
protection equipment for protection from re-
bounding media and airborne particulate.
The sodium bicarbonate blasting medium does
not pose a health risk, but the coating chips
being removed may. Airborne participates
generated during coating stripping may contain
toxic elements from the coating being removed.
An exhaust ventilation system should be used
during sodium bicarbonate coating removal to
remove the particulate cloud that forms as the
blast medium strikes the surface.
Operators should wear hearing protection due to
high noise levels from blasting equipment.
Uninhibited sodium bicarbonate and water
residue can corrode substrates; however, current
testing indicates that the corrosion potential of
uninhibited formulations is similar to that of
organic solvent strippers.
Wastewater and bicarbonate residue disposal
requirements will depend on the toxictty of the
coating and pigments being removed.
Slug discharge of bicarbonate (over about 3,000
ppm) can adversely affect the operation of an
anaerobic digester.
References
Chen, A. S. C., and R. F. Olfenbuttel. 1993. "Evaluating
Bicarbonate of Soda Blasting Technology: Its
Potential in Pollution Prevention and Waste Reduc-
tion." In: American Institute of Chemical Engineers
4th Pollution Prevention Topical Conference.
Kline, E. S. 1991. "Sodium Bicarbonate Blasting."
Painting & Wallcovering Contractor, July-August,
pp. 85-86, 95.
Larson, N. 1990. "Low Toxicity Paint Stripping of
Aluminum and Composite Substrates." First Annual
International Workshop on Solvent Substitution.
DE-AC07-76ID01570, U.S. Department of Energy
and U.S. Air Force. Phoenix, Arizona, pp. 53-60.
Peebles, H. C., N. A. Creager, and D. E. Peebles.
1990. "Surface Cleaning by Laser Ablation." First
Annual International Workshop on Solvent Substitu-
tion. DE-AC07-76ID01570, U.S. Department of
Energy and U.S. Air Force. Phoenix, Arizona.
Spears, W. E. 1989. "Mechanical Test Program." Paper
presented at Bicarbonate of Soda Stripping Techni-
cal Interchange Meeting. Southwest Research
Institute, San Antonio, Texas, June 21.
Steiner, R. 1993. "Carbon Dioxide's Expanding Role".
Chemical Engineering, 100(3): 114-119.
Svejkovsky, D. 1991. "Carbon Dioxide Paint Removal
Status." In: Proceedings of the 1991 DOD/lndustry
Advanced Coatings Removal Conference, San
Diego, California, pp. 344-393.
Wasson, N. E., Jr., and M. N. Haas. 1990. "Sodium
Bicarbonate Blasting for Paint Stripping." First
Annual International Workshop on Solvent Substitu-
tion. DE-AC07-76ID01570, U.S. Department of
Energy and U.S. Air Force. Phoenix, Arizona.
Carbon Dioxide Pellet Cryogenic Blasting
Pollution Prevention Benefits
Carbon dioxide (CO2) stripping generates a smaller
amount of waste than all of the available technologies
and some of the emerging thermal technologies. Upon
impacting the surface being cleaned or decoated, the
CO2 pellets disintegrate and sublime, that is, they pass
directly from solid to gaseous state without appearing
in the liquid state. Because the CO2 pellets return to the
gaseous state after use, the process does not generate
a spent media residue. The coating residue is collected
dry, without extraneous plastic beads, grit, or other
impacting material. Thus, no recycling or separation of
the media from the coating residue is required.
The carbon dioxide pellets are produced from liquid
CO2. The liquid CO2 is prepared industrially as a by-
product of ammonia manufacturing (35%), alcohol and
other chemical production (22%), oil and gas refining
(20%), or by collecting and purifying CO2 gas from
natural gas vents (20%) or combustion process offgas
(3%) (Steiner, 1993). The purified CO2 is compressed
and liquefied. The CO2 from these sources would enter
the atmosphere if it were not captured for industrial
25
-------
use, so carbon dioxide pellet blasting makes no net
addition of new C02.
The coating surface need not be washed before or
after CO2 blasting. The process removes dirt, oil, and
grease while stripping coatings, so these surface
contaminants do not interfere with the stripping action.
Because the media are not recycled, there is no need
for concern about dirt, oil, or grease contaminating the
media. In addition, because the pellets sublime, no
media remain behind to contaminate the substrate, so
no poststripping rinse is needed. As a result, CO2 pellet
blasting does not produce a wastewater stream and
thus eliminates the need for wastewater treatment.
How Does It Work?
The carbon dioxide blasting systems have a refriger-
ated liquid CO2 supply and a system for converting the
liquid to the solid media used in coating removal.
Compressed liquid is allowed to expand in a pressure-
controlled chamber where the temperature drops from
about -37ฐC (-35ฐF) to about -78ฐC (-109ฐF). The
temperature drop on expansion causes a mixture of
CO2 vapor and solid CO2 snow to form. The snow is
collected, compressed, and extruded through a die to
produce well-defined pellets of a selected size and
hardness as needed for the specific coating removal
operation.
CO2 pellet blasting applies a blast medium much the
same way as does PMB. Compressed air or liquid
nitrogen thrusts small CO2 pellets at a coated surface.
Because the CO2 reverts to a gas, the stripping media
do not contaminate the substrate (Ivey, 1990). A
system for centrifugal acceleration of the pellets also is
under development (Foster et al., 1992).
The actual mechanism for coating removal is, however,
different in CO2 pellet blasting. The CO2 pellets remove
the coating by a combination of impact, embrittlement,
thermal contraction, and gas expansion. The impor-
tance of each of these mechanisms in achieving
coating removal is not yet defined.
CO2 blasting uses a combtnaSQif 0fimpact
at#m$m!aim
-------
fctesflrtgi gives a stow;
With regard to a frequently mentioned limitation,
slowness, the reported coating removal rate for manual
CO2 pellet blasting ranges from 1 .5 ft2/min to 0.1 ft2/
min, depending on the substrate being stripped and the
coating color (Ivey, 1990; Cundiff and Matalis, 1990).
The net average strip rate on an F-1 6 aircraft was
0.189 ft2/min per minute of nozzle time (0.13 ft2/min
with worker effectiveness factored in) (Ivey, 1990). The
strip rate increased as the nozzle was widened. The
Alclad surfaces pulled the net average down. The
tested F-1 6 has 20% Alclad surfaces; other U.S. Air
Force aircraft have up to 80% Alclad surfaces. Thus,
strip rates will slow considerably on equipment with a
higher percentage of Alclad surfaces.
In fact, the process as tested cannoitemove all the
coating from Alclad surfaces. The.Alclad surface
left by CO2 pellet blasting must be removed by another
process to provide an adequate surface for recoating.
Held at chest level, the blast nozzle 'and hose weigh
about 20 Ib. When blasting underneath the aircraft,
another 10 Ib of thrust is added. In tests, workers
traded off the duty to other workers every 15 min. The
newer automated systems are easier to work with, strip
faster, and are safer on sensitive materials because the
optimal pressure and impingement angle can be
maintained.
SpecM techniques can imrmsa the
The preliminary results indicate that combining CO2
pellet blasting with other technologies may improve
CO2 pellet blasting performance in certain applications.
Combinations considered in the literature are
CO2 pellets + flashlamp vaporization to enhance
effectiveness of the flashlamp process alone and
to get where the flashlamp cannot reach (Bur-
cham, 1993)
CO2 pellets + chemical softener (i.e., benzyl
alcohol) to speed up the stripping rate
CO2 pellets + laser vaporization to enhance
effectiveness of the laser alone.
Preliminary studies indicate that none of these com-
bined technologies comes close to the overall desir-
ability of using CO2 pellets alone. However, further
testing may reveal that one or more of these combined
technologies has a specialized application or may be
useful on thin-skin materials. The combinations may
make CO2 more cost-effective to use. As currently
defined, CO2 pellets used alone damage unsupported
aluminum alloys that are less than 0.032 in thick. The
peening damage caused by the pressures required for
effective stripping could prevent use of CO2 on up to
20% of cargo aircraft skins (Ivey, 1990).
Chemical softeners applied before CO2 pellet blasting
would allow less blasting pressure and thus decrease
damage to thin-skin aluminum. Chemical softeners also
would provide more thorough and faster stripping at
well over 1 ft2/min but could require aircraft preprocess-
ing tasks such as degreasing and masking and would
generate more disposable waste. Softeners could
damage aircraft materials (Ivey, 1990).
Combined with flashlamp vaporization, CO2 pellet
cryogenic blasting may be useful on thin-skin materials.
The CO2 + flashlamp combination may increase
stripping speed to 3 ft2/min and promises to reduce the
aggressiveness of the CO2 pellets used alone (Ivey,
1990).
tmprewm$nt$ have bem made fa msk&
GG., Gryogeote bfaMng $y&taim easferto
handte.
Assistive devices have been developed to make the
blast nozzle and hose less bulky. These save on the
time needed for stripping and improve stripping quality.
The robotic system at least doubles the stripping rate.
The improved technology using either robotics or
manipulator arms could provide the precision needed
to avoid peening damage on thin aluminum skins (Ivey,
1990).
The material requirements include
Tank of liquid CO2 (supply ranging from 181 kg/hr
to 658 kg/hr)
Skid-mounted unit (compressor with maximum
output of 430 psi, pelletizing unit, propellant
system, etc.)
Variety of nozzle assemblies
Minimum 30 ft2 of work area
Optional robot or manipulator arms
Breathing apparatus if CO2 levels rise above
1.5% for 10 min or 0.5% for continuous use, or
dust masks if breathing apparatus not needed
Electricity to freeze the CO2 pellets and acceler-
ate them (@ $ .10/lb or $40/hr in 1984 $).
Figure 3 shows a set of typical CO2 blast system
components.
With the nonautomated nozzles and hoses, operators
require strength and stamina. The evolving automated
equipment will require a medium level of skill to control
27
-------
Source: Alpheus, 1990
Figure 3. A typical liquid CO, blast system.
the pressure relative to distance. Operators must take
care not to damage the thinner substrates.
Potential operator exposure to high levels of CO2 also
is a concern with CO2 pellet blasting. The greatest
health concern presented by CO2 is the risk of suffo-
cation (Steiner, 1993). An atmosphere with at least
19.5% oxygen generally is considered safe. At 19.5%
oxygen, if the remainder is CO2, the atmosphere is
unsafe. For continuous exposure during an 8-hour
workshift, OSHA sets an acceptable maximum concen-
tration of 1% CO2 in air.
Depending on the degree of automation adopted, CO2
pellet blasting could involve high capital cost and
relatively low labor cost. The capital cost is greater than
for PMB. To strip a large part within a reasonable
amount of time would require multiple nozzles. A high
continuous throughput application would be needed to
justify the capital cost of a CO2 system.
CO2 eliminates many of the costs associated with
chemical processes and with some of the other cleaner
coating removal technologies. By eliminating such
costs as pre- and poststripping cleanup, media dispos-
al, media separation/recycling, and aqueous waste
disposal costs, the overall system cost for CO2 strip-
ping can be competitive. For example CO2 stripping is
reported to have an average cost of $5/ft2 for typical
applications. The CO2 cost compares favorably to the
reported cost of $19+/ft2 for available chemical pro-
cesses (Schmitz, 1990).
Application
CO2 pellet cryogenic blasting has undergone field
testing on F-16 aircraft frames. It is not considered
aggressive enough to remove polyurethane topcoat
(Kopf and Cheney, 1989).
More than 50 systems have been custom-configured
for direct or contractor applications worldwide for the
automotive, military aircraft, and food processing
industries.
The CO2 cryogenic technology can be applied near
moving parts without interrupting the power source. It
can be used on sensitive electronic components that
would be damaged by other cleaning technologies
(Cold Jet, Inc., 1989).
CO2 pellet blasting has been investigated for possible
use as an aircraft coatings removal process (Ivey,
1990). This technology has successfully cleaned up
mould tools, coating jigs, extruder screws, and general
grease and oil contamination.
Benefits
Some of the major beneficial aspects of CO2 pellet
cryogenic blasting include
CO2 media vaporize, leaving only a small volume
of dry coating residue waste
Eliminates water use
Has a clean and well-defined coating removal
pattern (Cundiff and Matalis, 1990)
Can selectively remove individual coating layers
(e.g., remove topcoat leaving primer)
Pre- or poststripping cleanup requirements
typically are minimal
No masking is needed except for delicate materi-
als such as soft, clear plastics
Equipment can be stripped without requiring
disassembly
No separation/recycling system needed.
No media disposal costs incurred
Pellets driven into interstitial spaces vaporize,
leaving no residue
Benign to most substrates; surface damage
minimal for a clad or bare surface (Cundiff and
Matalis, 1990)
No size limitations on parts to be stripped.
Limitations
Hazards and limitations of CO2 pellet blasting include
Generates solid waste containing coating chips,
which may be hazardous; however, media do not
add to the volume
28
-------
Ventilation required to avoid potentially danger-
ous CO2 concentrations in the coating-stripping
area (>1.5% short-term or > 0.5% 8-hr average)
Possible worker exposure to extreme cold
Operators should wear respiratory and eye
protection equipment for protection from re-
bounding media and airborne partiallates
Operators should wear hearing protection due to
high noise levels from blasting equipment
Rebounding pellets may carry coating debris and
contaminate the work area or workers
Large local temperature drops can occur but are
confined mainly to the surface layer
There is potential hazard from compressed air
and/or high-velocity CO2 pellets
Static energy can build up if no grounding is
provided (Cundiff and Matalis, 1990)
Some coating debris may redeposrt on substrate
Low temperature can cause condensation on
substrate
A slight quantity of coating particles are emitted
to the air, requiring a standard air filtration
system
May damage thermoset composite materials
unless close attention is paid to dwell time and
stand-off distance
Difficult to control coating removal on graphite-
epoxy composites, perhaps because of brittle-
ness (Cheney and Kopf, 1990)
Slight reduction of fatigue life of metal substrates
(Cundiff and Matalis, 1990)
Peens and damages soft aluminum less than
0.020 in thick (Larson, 1990)
Particularly slow on Alclad-coated aluminum
skins and thermoset composites
Nonautomated system fatigues workers quickly
because of cold, weight, and thrust of blast
nozzles (Ivey, 1990; Wolff, 1984).
References
Alpheus. 1990. CO2 Cleanblast. Alpheus Cleaning
Technologies, Rancho Cucamonga, California.
APCI (Air Products and Chemicals, Inc.). 1984. Coat-
ings Removal Competitive Technology Review.
Allentown, Pennsylvania. Rev. 1, November 19.
Boyce, G., L. Archibald, and P. Andrew. 1990. "Cryo-
genic Blasting as a Tool Cleaning Process." In:
Proceedings of the 1990 DOD/lndustry Advanced
Coatings Removal Conference, Atlanta, Georgia.
pp. 261-303.
Burcham, D. H. 1993. "FLASHJET Coatings Removal
Process." In: Proceedings of the 1993 DOD/lndustry
Advanced Coatings Removal Conference, Phoenix,
Arizona, pp. 316-333.
Cheney, J., and P. Kopf. 1990. "Paint Removal and
Protective Coating Development." In: Proceedings
of the 1990 DOD/lndustry Advanced Coatings
Removal Conference, Atlanta, Georgia, pp. 372-
395.
Cold Jet, Inc. 1989. The Force of Nature. Videotape
presentation, Loveland, Ohio.
Cundiff, C., andT. Matalis. 1990. "A Preliminary
Evaluation of Paint Removal by a Carbon Dioxide
Pellet Blast System." In: Proceedings of the 1990
DOD/lndustry Advanced Coatings Removal Confer-
ence, Atlanta, Georgia, pp. 304-329.
Foster, C. S., P. W. Fisher, C. C. Tsai, and D. E.
Schecter. 1992. "A Centrifuge Accelerator CO2
Pellet Cleaning System." In: Third Annual Interna-
tional Workshop on Solvent Substitution, U.S.
Department of Energy and U.S. Air Force, Phoenix,
Arizona.
Ivey, R. B. 1990. "Carbon Dioxide Pellet Blasting Paint
Removal for Potential Application of Warner Robins
Managed Air Force Aircraft." First Annual
International Workshop on Solvent Substitution. DE-
AC07-76ID01570, U.S. Department of Energy and
U.S. Air Force, Phoenix, Arizona, pp. 91-93.
Kopf, P. W., and J. Cheney. 1989. "Paint Removal from
Composite Substrates: Advances, Abrasives,
Cryogenics, and Eximer [sic] Lasers." In: Pro-
ceedings of the 1989 DOD/lndustry Advanced
Coatings Removal Conference, Ft. Walton Beach,
Florida, pp. 122-169.
Larson, N. 1990. "Low Toxicity Paint Stripping of
Aluminum and Composite Substrates." First Annual
International Workshop on Solvent Substitution. DE-
AC07-76ID01570, U.S. Department of Energy and
U.S. Air Force, Phoenix, Arizona, pp. 53-60.
Schmitz, W. N. 1990. "CO2 Pellet Blasting for Paint
Stripping/Coatings Removal." First Annual Interna-
tional Workshop on Solvent Substitution. DE-AC07-
76ID01570, U.S. Department of Energy and U.S. Air
Force, Phoenix, Arizona, pp. 11-13.
Svejkovsky, D. 1991. "Carbon Dioxide Paint Removal
Status." In: Proceedings of the 1991 DOD/lndustry
Advanced Coatings Removal Conference, San
Diego, California, pp. 344-393.
Wolff, E. B. 1984. The Impact of EPA and OSHA
Regulations on Paint Stripping. Air Products and
Chemicals, Inc., Allentown, Pennsylvania.
29
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High-Pressure Water Blasting
Pollution Prevention Benefits
High-pressure water blasting eliminates the use of
chemical strippers containing HAPs. However, waste-
water is generated that contains paint debris. The
stripping water can be recycled to reduce waste
volume. The spent stripping water must be collected
and then either processed for reuse in the stripper or
treated for disposal. Unlike dry stripping processes,
water stripping does not generate dust.
How Does It Work?
High-pressure waterjet stripping removes coating with
a stream of water projected from specially designed
nozzles at pressures of 15,000 psi to 30,000 psi or
more. High-pressure pumps supply water to a system
of rotating nozzles that spray the water stream onto the
coated surface. The coating is removed by the kinetic
impact of the water stream.
Operational Features
High-pressure water stripping will be performed by
robotically manipulated equipment to control nozzle
movement over the surface to be stripped. Process
development testing for stripping polyurethane topcoat
from primed and clad 2024-T3 aluminum alloy used a
nozzle travel rate of 1.25 in/sec and rotation speed of
400 rpm. Additional operating conditions included
(Stone, 1993):
Blasting pressure maximum 24,000 psi
Blasting standoff distance 1.3 in
Stripping rate 1.25 to 1.7 fl2/min.
Application
The U.S. Air Force currently is supporting development
of a Large Aircraft Robotic Paint Stripping facility
(Hofacker et al., 1993). The facility is designed to use
high-pressure water blasting in a fully automated
system. Aircraft to be stripped include B-52, C-135, E-
3, andB-1.
The U.S. Navy is developing a high-pressure water/
garnet abrasive slurry stripping system for paint
removal and surface preparation on ship surfaces.
Testing of a manual system was completed, and a
semi-automatic system has been designed and as-
sembled. The system operates with a water pressure
higher than 35,000 psi. The blasting slurry is dis-
charged through a rotating blasting head with four
nozzles. The reported stripping rate is about 2.5 ft2/min.
The estimated cost of the high-pressure blasting
system is $150,000, and the unit cost of the garnet
abrasive is $300/ton (U.S. Army, 1092).
Benefits
Some of the major beneficial aspects of high-pressure
water blasting include
The technology has a high stripping rate.
Stripping water is recycled.
Wastewater stream is compatible with conven-
tional wastewater treatment plants available to
many installations.
There are no size limitations on parts to be
stripped.
Limitations
Potential hazards and limitations of high-pressure
water blasting include
Coating debris sludge may be a hazardous
waste.
Wastewater and residue disposal requirements
will depend on the toxicity of the coating and
pigments being removed.
A system must be available to collect, filter, and
recycle stripping water containing coating debris
(and in some systems abrasives).
Workers must be protected from direct impinge-
ment of water jet due to extreme danger from
>15,000 psiwatetjet.
Robotic applications are required due to high
reaction forces and high hazard from water jet.
There is a high capital cost for the robotic sys-
tem.
A misapplied water jet will damage the substrate.
The blasting operation generates high noise
levels.
Water can enter cavities.
Water can penetrate and/or damage joints, seals,
and bonded areas.
References
Hofacker, S. A., D. W. See, and W. A. Cain. 1993. 'The
Large Aircraft Robotic Paint Stripping (LARPS)
System." In:
Proceedings of the 1993 DOD/lndustry Advanced
Coatings Removal Conference, Phoenix, Arizona.
pp. 476-489.
Hewlett Jr., J. J., and R. Dupuy. 1993. "Ultrahigh-
Pressure Water Jetting for Deposit Removal and
Surface Preparation." Materials Performance,
32(1):38-43.
30
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Stone, A. 1993. "An Evaluation of High Pressure
Waterjet as an Alternative Paint Removal Method:
Summary of the Large Aircraft Robotic Paint Strip-
ping (LARPS) Process Validation." In: Proceedings
of the 1993 DOD/lndustry Advanced Coatings
Removal Conference, Phoenix, Arizona.
pp. 400-515.
U.S. Army. 1992. Installation Restoration and Hazard-
ous Waste Control Technologies. CETHA-TS-CR-
92053, U.S. Army Corps of Engineers, Toxic and
Hazardous Materials Agency, Aberdeen Proving
Ground, Maryland.
Medium-Pressure Water Blasting
Pollution Prevention Benefits
Medium-pressure water blasting eliminates the use of
chemical strippers containing HAPs. '(However, waste-
water is generated containing paint debris. The strip-
ping water can be recycled to reduce waste volume.
The spent stripping water must be collected and then
either processed for reuse in the stripper or treated for
disposal. In some applications the water may contain
small quantities of alcohol or a similar organic solvent.
Unlike dry stripping processes, water stripping does not
generate dust.
How Does It Work?
Medium-pressure water blasting removes coatings with
a stream of water projected from specially designed
nozzles at pressures of 3,000 psi to 15,000 psi. Heavy-
duty pumps, typically in the 15 to 600 hp range, supply
water at high pressure. The water is sprayed through a
nozzle or system of rotating nozzles onto the coated
surface. The coating is removed by the kinetic impact
of the water stream. The stripping action often is
supplemented by presoftening with an alcohol solvent
or by including soft or hard abrasives in the water
stream.
Operational Features
Water-jet blasting has been used on an industrial scale
for many years to clean a variety of corrosion, grease,
or other deposits from metal surfaces (Hewlett and
Dupuy, 1993). Several implementations of medium-
pressure water blasting for coating removal are being
developed and have reached varying stages of matu-
rity. Variations include water blasting alone, water
blasting with a solvent presoak, and water propelling
abrasive media such as sodium bicarbonate. Systems
using water only or water with a solvent presoak
typically use a rotating nozzle. Systems with abrasive
propelled by water typically use a fan nozzle.
Portable water-blasting stripping systems are in use for
stripping floor gratings in paint booths. Two or four
nozzles are carried on a rotating fixture. Several of the
rotating nozzle assemblies are mounted in an enclo-
sure. The enclosure has wheels so it can be moved
over the booth floor. The rotating nozzles spray a high-
pressure water stream onto the booth floor. The
enclosure protects the operator and prevents the
spread of water spray and paint debris (Bailey, 1992).
In response to a West German governmental directive
to minimize VOC emissions in industrial processes, the
German airline Lufthansa developed the Aquastripping
process to strip old coating from aircraft (New Scientist,
1990). Water stripping is preceded by a 3-hour dwell
time presoak with an alcohol softener. The water
stripping is performed by manually controlled mecha-
nized arms, each carrying rotating nozzles for the bulk
of the aircraft surface. One recent implementation uses
six nozzles on one stripping head (Boeing, 1993). The
nozzle rotation speed is 3,500 rpm. The undersides of
the wings are stripped with a counterbalanced hand-
manipulated stripping nozzle. Operating conditions
include
Blasting pressure maximum 7,350 psi typical
5, 100 psi
Water flow rate 50 gpm
Blasting standoff distance 1 to 4 in
Stripping rate 5 ft2/min.
M8tiitm~fsF&s$ufe waieftoJ&sting systems
combined wtih atoras/vas such as sodium
Medium-pressure water blasting systems are also
being developed using abrasive additives. One system
being tested uses bicarbonate as the abrasive (Petkas,
1993). The system differs from the low-pressure
bicarbonate blasting system in that the operating
pressure is higher, resulting in much lower abrasive
use rate. Reported test conditions for stripping clad and
bare 2024-T3 aluminum are
Blasting pressure 3,000 psi
Angle of impingement 45ฐ
Media flow rate 1 .0 to 1 .75 Ib/min
Stripping rate 0.56 ft2/min to 0.69 ft2/min.
Application
The automotive industry has found medium-pressure
water stripping to be very efficient for cleaning floor
grates, overhead conveyers, rails, and part support
hooks in water wall spray paint booths (Bailey, 1992).
Portable water-spray units provide removal rates in the
range of 15 to 30 ft2/min. No abrasive is used and the
stripping water is not recycled. The paint booth water
31
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collection and treatment system is used to handle the
stripping water.
A prototype test facility for medium-pressure water
blasting with sodium bicarbonate abrasive is planned
for installation at Warner Bobbins Air Force Base in
early 1994.
Benefits
Some of the major beneficial aspects of medium-
pressure water-jet stripping include
Low implementation cost using simple, robust
equipment
High stripping rates
Wastewater stream is compatible with con-
ventional wastewater treatment plants available
to many installations
No size limitations on parts to be stripped.
Limitations
Potential hazards and limitations of medium-pressure
water-jet stripping include
Coating debris sludge may be a hazardous
waste.
Wastewater and residue disposal requirements
will depend on the toxicity of the coating and
pigments being removed.
A system must be available to collect, filter, and
recycle stripping water containing coating debris
(and in some systems abrasives or alcohol
softener).
Workers must be protected from direct impinge-
ment of water jet.
Operators should wear respiratory and eye
protection equipment for protection from water
spray and airborne paniculate.
Operators should wear hearing protection due to
high noise levels from blasting equipment.
Mechanized applications are typical due to high
reaction forces and high hazard from water jet.
A misapplied water jet will damage the substrate.
Water can enter cavities.
Water can penetrate and/or damage joints, seals,
and bonded areas.
References
Bailey, J. 1992. "Strip It Off." Industrial Finishing
68(2) :24-27.
Boeing. 1993. "Paint Stripping." 1993 Structure Confer-
ence. SE-ACM-3-1-93, Boeing Commercial Airplane
Group, Section 10, Service Engineering.
Hewlett Jr., J. J., and R. Dupuy. 1993. "Ultrahigh-
Pressure Water Jetting for Deposit Removal and
Surface Preparation." Materials Performance
32(1):38-43.
New Scientist. 1990. "Cold-Water Treatment for
Painted Planes." New Scientist, August, p. 35.
Petkas, P. J. 1993. "AWhiteMetal Inc. Innovation
The BOSS System That Really Works Technically
and Cost Effectively." In: Proceedings of the 1993
DOD/lndustry Advanced Coatings Removal Con-
ference, Phoenix, Arizona, pp. 466-475.
Liquid Nitrogen Cryogenic Blasting
Pollution Prevention Benefits
The liquid nitrogen cryogenic coating stripping process
eliminates solvent use and results in no ash or residual
to clean. No fumes, smoke, or chemicals are released.
A small volume of coating residue and spent plastic
blasting media are collected dry for disposal. If hazard-
ous metals or unreacted resins are present, the residue
may be a hazardous waste. Liquid nitrogen is used to
cool the part and to help propel plastic bead blasting
media. The process does not use air to propel the
media, so neither dust nor wastewater is generated
(Stroup, 1991).
How Does It Work?
Unlike the classical solvent technologies that address
the chemical properties of coatings, cryogenic coating
removal addresses their physical properties, i.e., the
coefficient of thermal contraction and the cryogenic
brittle transition temperature. The cryogenic technology
takes advantage of extreme cold to embrittle and shrink
the coating. The part to be stripped is cooled by a
readily available cryogenic fluid, liquid nitrogen. Nitro-
gen is inert, colorless, odorless, noncorrosive, and
noncombustible.
The liquid nitrogen is sprayed on items to be stripped
as they rotate on a spindle within a stainless steel
cryogenic chamber. The liquid nitrogen chills the
coating, causing greater thermal contraction of the
coating than of the substrate. Tensile stresses thus
develop within the coating and make it brittle. High-
velocity, nonabrasive plastic pellets (media) are then
blasted by centrifugal throw wheels to make the coating
crack, debond, and break away from the substrate.
The fixtures emerge from the chamber clean. The dry
coating residue and plastic media are collected and
separated so that the media can be reused. Mean-
while, the liquid nitrogen warms and evaporates,
32
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changing back to a gas. The harmless nitrogen gas is
vented out to the atmosphere where it originally came
from(APCI, 1984a).
Operational Features
Liquid nitrogen cryogenic coating removal works well
for removal of heavy coating buildups such as those
that accumulate on coating line fixtures. The heavy
coating buildups can interfere with parts loading and
maintenance service to these fixtures. For electrostatic
coating application systems, the supports must be
clean to promote good electrical contact between the
part and the holder. Typical coating line fixtures include
Coating hangers
Conveyor racks
Masks
Grates (APCI, 1982; Products Finishing, 1983).
Removal of acrylic, alkyd, lacquer, polyester, and vinyl
coatings has been successful; removal of epoxy and
urethane coatings has been less successful (APCI,
1984a; Mathur, undated). However, Products Finishing
(1983) reports successful stripping of urethane coat-
ings and adds phenolics to the list. According to APCI
(1984a) and Mathur (undated), coating thicknesses
ranging from 0.01 0 to 0.500 in give the best results;
coatings thinner than 0.010 in do not readily debond.
Products Finishing (1983) reports success on tests on
film as thin as 0.005 in, depending on the formulation.
rmtavsti of thtokcoMngz, particularly tram
Parts to be stripped (except hooks) are placed on a
loading tree and lifted onto a rotating spindle at the top
of the cryogenic chamber. The refrigerant liquid nitro-
gen coats the surface so that the impact of the plastic
pellets debonds and knocks off the cracked coating
chips. The residue of coating and plastic media collects
at the bottom of the chamber and is conveyed to a
separator. The separator divides the residue into
oversized chips, media, and undersized chips. The
media are returned by conveyor back to the throw
wheels.
Most coatings become brittle if subjected to tempera-
tures that are below -73ฐC (-1 00ฐF). Cryogenic coating
removal relies on the boiling temperature of nitrogen (-
196ฐC [-320ฐF]) to embrittle and shrink the coating so
that the high-velocity, nonabrasive plastic pellets can
knock off the coating particles without damaging the
fixture. The process requires a chamber and compo-
nents specially designed and built for low-temperature
use. Figure 4 shows the chamber, rotating spindle, and
separation areas.
The media sm tescb recesses &tftt
shaded su rfac&s.
Operators manage and optimize the decoating process
from a control panel. They can program temperature,
blast time, wheel speed, and media flow for automatic
operation, or they can control these variables manually.
Interlock systems must be provided to protect opera-
tors from the intense cold and the rotating equipment.
The outer door can be opened between cycles, but the
inner door prevents entry to the cryogenic chamber.
Guards and covers shield all moving parts.
One cycle lasts 5 to 15 min (typically 10 min). Although
the cryogenic technology works best when all parts are
readily exposed to the two throw wheels, the high-
velocity, turbulent plastic media cloud can reach
recesses and shaded surfaces on repeated rebounds
at such high velocity. Hooks touching adjacent hooks
have been effectively cleaned around the entire
circumference (APCI, 1984a).
Enefgy requimm&m for tiquitfN,, cryo-
g&nfcbfastfng am tow.
A typical cryogenic system includes a stainless steel
cryogenic chamber with double doors, a liquid nitrogen
delivery system, a rotating fixture support, two cen-
trifugal throw wheels, conveyors, and a media-coating
cyclone separator with a hopper for recycled media.
The total system occupies a length, width, and height
of 16ft x 15ft x 12 ft.
The cryogenic technology requires no heat. Energy
requirements, therefore, are low. The compressed air
requirement is 1 cfm @ 90 psig. A small amount of
electricity (10 kW) is used to condense air into liquid
nitrogen and to operate the throw wheels and convey-
ors (APCI, 1982,1984a).
The actual operation of cryogenic stripping is relatively
simple, once the appropriate operating cycle is estab-
lished for the specific parts and coatings to be stripped.
The required skill level for routine loading and stripping
is, therefore, lower than for solvent stripping units,
which require the operator to handle potentially hazard-
ous chemicals.
The cryogenic stripping equipment is, however, more
complex than stripping tanks. The equipment for the
cryogenic system may be unlike other equipment in a
typical small- or medium-sized coating shop, so new
maintenance skills may need to be learned.
33
-------
Liquid
Nitrogen
fed in
Rotating
spindle
Vibratory
separator
Figure 4. Operation of cryogenic coating removal process equipment.
Dry paint
screened out
Application
The equipment used in cryogenic coating stripping is
costly. In 1984, the system cost $130,000 plus a royalty
fee of $10,000. The operating cycle cost of $5 to $15
(APCI, 1984b) combines with the high throughput to
bring the cost down when frequent use justifies the
capital outlay. Because of its speed, labor costs are
reduced.
When throughput requirements are high enough to
justify the equipment purchase, cryogenic coating
stripping can reduce the costs of:
Hazardous solvent sludge disposal
Cleaning after stripping
Facility damage from fire or explosion
Measures to ensure worker safety (APCI, 1984a;
Mathur, undated).
Payback within 1 ye&r & poss&te #$% #ป .
liquid Nz
A major auto manufacturer reported payback within 1
year on a prototype cryogenic coating removal system
(Products Finishing, 1983).
Factors that influence the operating cost are
Average loading of the system
Mass and surface area of the fixtures
Coating type: and thickness
Tradeoffs of cycle time vs. cooling
Unload-reload interval.
One appliance maker uses a cryogenic coating re-
moval system to strip its inventory of more than 13,000
coating hangers and racks. The cryogenic system
removes the baked-on overspray acrylic coating in 10-
min cycles. Loads range from 12 to 60 hangers,
averaging 375 hangers/day. Stripping costs averaged
about $0.54 each in 1985 dollars. The dry coating chips
are collected in a drum for disposal.
The company reports no damage to its fixtures from the
cold. Workers can handle the parts without gloves 5 to
10 min after removal from the cryogenic chamber
(APCI, 1985).
The high capital cost of cryogenic coating stripping
equipment has limited the breadth of industrial appli-
cation. However, the technology is used to rapidly
34
-------
remove heavy layers of coating in applications requir-
ing high throughput.
Benefits
Some of the major beneficial aspects of liquid nitrogen
cryogenic blasting include
No ash residue
Low waste volume
Coating waste chemically unaltered
Eliminates water rinse
Very fast cycle time (5 to 15 min)
High throughput rate
Works well on thick coating buildups.
Limitations
Potential hazards and limitations of liquid nitrogen
cryogenic stripping include
Generates small volume of coating chips and
spent plastic media which may be hazardous due
to coating constituents.
May require ventilation system to avoid poten-
tially dangerous nitrogen concentrations in the
coating-stripping area.
Requires workers to wear long sleeves and
gloves during unloading.
Not effective on thin coatings (those of <0.010
in).
Less effective on epoxies and urethanes.
Existing technology limits part size to less than 6
ft tall and 38 inches in diameter.
Part weight limited to less than 400 Ib per cycle.
References
APCI (Air Products and Chemicals, Inc.). 1982. Cryo-
Strip Cryogenic Coating Removal System. Air
Products and Chemicals, Inc., Allentown, Pennsyl-
vania.
APCI (Air Products and Chemicals, Inc.). 1984a.
Freezing Paint for Removal. Reprinted from May
1984 issue of Industrial Finishing. Hitchcock Pub-
lishing Company.
APCI (Air Products and Chemicals, Inc.). 1984b.
Coatings Removal Competitive Technology Review,
Rev. 1. Air Products and Chemicals, Inc., Allentown,
Pennsylvania.
APCI (Air Products and Chemicals, Inc.). 1985. "Strip-
ping Paint Hangers Cryogenically." Reprinted from
January 1985 issue of Industrial Finishing. Hitch-
cock Publishing Company.
Mathur, A. Undated. Untitled paper on cryogenic
coating removal. Air Products and Chemicals, Inc.,
Allentown, Pennsylvania.
Products Finishing. 1983. "Cryogenic Paint Stripping."
PF Special New Product Report. Reprinted from
December 1982 issue of Products Finishing.
Gardner Publications, Cincinnati, Ohio.
Stroup, G. D. 1991. Letter from Stroup of Air Products
and Chemicals, Inc. (APCI), Allentown, Pennsylva-
nia, to Lawrence Smith of Battelle, Columbus, Ohio,
about the APCI patented CCR (Cryogenic Coating
Removal) process. September 27.
Wolff, E. B. 1984. The Impact of EPA and OSHA
Regulations on Paint Stripping. Air Products and
Chemicals, Inc., Allentown, Pennsylvania.
35
-------
SECTION 3
EMERGING TECHNOLOGIES
How To Use the Summary Tables
Three emerging cleaner coating removal technologies
are evaluated in this section, namely
Laser heating
Flashlamp heating
Ice crystal blasting.
Tables 3 and 4 summarize descriptive and operational
aspects of these technologies. Readers are invited to
refer to the summary tables throughout this discussion
to compare and contrast technologies.
Descriptive Aspects
Table 3 shows the main Coating Removal Mechan-
ism (s) of each emerging technology. It lists the Pollu-
tion Prevention Benefits, Reported Application,
Benefits, and Limitations of each emerging cleaner
technology.
Operational Aspects
Table 4 shows the key operating characteristics for the
emerging technologies. This table and Table 3 give
users a concise indication of the range of technologies
covered to allow preliminary identification of technol-
ogies that may be applicable to specific situations.
Tables 3 and 4 contain evaluations or annotations
describing each emerging cleaner technology.
characteristics that would lead to a high complexity
rating.
possible cartcflt&te cleaner tecbOQtogte$:>
In Table 4, Process Complexity is qualitatively ranked
as "high," "medium," or "low" based on such factors as
the number of process steps involved and the number
of material transfers needed. Process Complexity is
an indication of how easily the new technology can be
integrated into existing plant operations. A large
number of process steps or input chemicals, or multiple
operations with complex sequencing, are examples of
references, and ftQmifx&fstry mdtrade
gftffcp* feฃtf ###
-------
Table 3. Emerging Cleaner Technologies for Coaling Removal: Descriptive Aspects
Technology/
Coating
Removal
Mechanism
Pollution
Prevention
Benefits
Reported
Application
Benefits
Limitations
Laser Replaces solvents
Heating Produces a volume of
ash residue smaller
Thermal than the original coat-
ing volume
Provides coating removal
and/or cleaning
Often used in conjunction
with CO2 pellet blasting
Best when used with robotic
systems
Results in a very small volume of waste
Requires minimal training
Allows topcoat to be stripped without remov-
ing primer
No substrate damage detected under a vari-
ety of conditions
Does not damage electronic components or
change metallurgical properties
Can remove coating between clamped to
gather surfaces to a depth of 1.32 in
Does not damage composites
Control systems can be minimal
Particulates easy to collect for disposal
Requires offgas collection and filtration for
p articulates
Requires laser barrier wall to protect work-
ers from lethal energy laser beam
Requires air flow or other collection
mechanism to prevent ash redeposition on
the substrate
Can generate products of incomplete
combustion
Flashlamp
Heating
Thermal
Ice
Crystal
ฃฐ, Blasting
Impact/
Abrasive
Replaces solvents
Produces a small
volume of ash waste
Replaces solvents
Media are nontoxic
Produces a small
volume of coating chip
waste
Provides coating removal
and/or cleaning
Often used in conjunction
with CO2 pellet blasting
Good for use in confined
space such as submarine
interior
Useful on aluminum and
composite substrates
Results in a very small volume of waste Requires offgas collection and filtration for
particulates
Can generate products of incomplete
combustion
Leaves oily residue on substrate
Generates low volume of dry waste (none Generates small volume of coating chips,
from the media) which may be a hazardous waste
No media separation/recycling system need- Potential for worker injury from high-
ed velocity ice pellet impact
Requires workers to wear respiratory and
eye protection equipment
Requires workers to wear hearing
protection
Table 4. Emerging Cleaner Technologies for Coating Removal: Operational Aspects
Emerging
Technology
Type
Laser
Heating
Flashlamp
Heating
Ice
Crystal
Blasting
Process
Complexity/
Required Skill Level
High/
Low for operation
High for maintenance
High/
Low for operation
High for maintenance
Medium/Medium
Waste Products Capital
and Emissions Cost
*
Solid ash consisting High
primarily of pigment
Solid ash consisting High
primarily of pigment
Solid coating residue Medium
waste
Airborne particulates
Energy Use
Electricity supply to
laser
Ventilation to control
participate
Electricity supply to
flashlamp
Ventilation to control
partial late
Compressed air to
propel blasting media
Refrigeration to
prepare ice crystals
Operations Needed
After Stripping
Offgas collection of
particulates during
stripping
Removal of ash residual
Offgas collection of
particulates during
stripping
Removal of ash residual
Remove masking
Dispose of coating residue
waste
References
Head, 1990
Hill, 1993
Toohey, 1993
Larson, 1990
Apple and Jahn-Keith,
1993
Larson, 1990
Pauli, 1993
-------
Some additional inspection, hand cleaning, or other
operations may be needed to prepare the surface after
use of the cleaner technology for coating removal.
These are noted to indicate special considerations in
the application of the cleaner coating removal technol-
ogy.
Process Complexity, Required Skill Level, Waste
Products and Emissions, and Capital Cost serve to
qualitatively rank the cleaner technologies relative to
each other. The rankings are estimated based on the
descriptions and data in the literature.
The text further describes the operating information,
applications, benefits, and limitations, as known for
each emerging technology. More highly developed
technologies are discussed in Section 2, Available
Technologies.
The last column in Table 4 cites References to publica-
tions that will provide further information about each
emerging technology. These references are given in full
at the end of the respective technology sections.
Laser Heating
Removal of coatings using laser energy involves
heating the coating with laser radiation to vaporize thin
layers of material. The coating is removed by sweeping
the laser beam over the surface to be stripped. The
coating material absorbs energy from the laser. The
rapid heating action oxidizes organic in the coating to
CO2 and H2O. Production of organic products of
incomplete combustion has not been quantified.
Metals and other nonvolatile portions of the coating
form paniculate ash. A vacuum air removal system,
possibly supplemented by compressed air blowoff,
collects the ablated paniculate to prevent redeposition
or escape into the surrounding air space. The offgas
can be passed through filters to remove the paniculate.
Laser heating has the potential to reduce the final
disposal volume to less than the original volume of
coating material due to combustion of the organic
elements in the coating.
Laser coating removal relies on heating the coating by
absorption of light energy. Coatings with low light
absorbance, typically light-colored or glossy surtaces,
are less amenable to removal by laser systems.
Uncontrollable variations in the thickness of the coating
being stripped make optimization of the laser coating
removal difficult.
A commercial vendor has both used a 10-W pulsed
laser to develop data on laser cleaning of aircraft
materials and developed higher-powered modular
systems for full-scale applications. The new modular
systems have a laser beam generator and a manually
operated beam delivery arm. Pulse frequency varies
directly with power in the higher-powered systems.
Pulsed laser stripping of coating requires offgas
collection in filtration bags to remove the particulates,
which are primarily the coating pigments. In tests,
these particulates either did not deposit on the filter
housing or they plugged and shortened the life of the
filters. The coating particulates are collected on the
filter for disposal or recycling (Head, 1990).
Laser paint removal has been tested for removal of
topcoat and primer from smooth aluminum, smooth
steel, textured iron, fiberglass, and carbon fiber/resin
composite. The testing indicated that laser pulse
duration, timing, and energy density could be selected
and controlled to remove coating without substrate
damage (Hill, 1993).
laser systems c&n &fv& ra$s$, fxwfaottetf
A Laser Automated Decoating System (LADS) is being
developed for the U.S. Air Force Ogden Air Logistics
Center in cooperation with the Aeronautical Systems
Center RAMTIP office. The system will be designed to
remove coating from F-16 radomes. The planned
system consists of the following subsystems (Toohey,
1993):
Beam Delivery Subsystem (including turning
mirrors, a beam director head, and the beam
enclosures)
Material Handling Subsystem (including a
holding fixture, lathe head, and lathe bed)
Vision Imaging Subsystem
Waste Collection Subsystem
Control Subsystem
Pulsed Laser Subsystem.
Laser coating removal has several potential advan-
tages:
The waste is small (ash from noncombustible
coating materials).
Combination of robotic control with visual
overcheck can remove coating in a well-con-
trolled manner.
No substrate damage has been detected under a
variety of conditions, including coating removal
from composite materials.
38
-------
Laser-cleaned substrates show good coating
adhesion and corrosion resistance.
Laser stripping does not damage electronic
components or change metallurgical properties
(Head, 1990).
Some of the limitations of laser coating removal
systems include
The laser systems have a high capital cost and
are best used with robotic controls.
It is difficult to focus and control the laser beam
to allow stripping of curved or complex surfaces.
The use of high-power lasers for coating removal
requires the use of a Class 1 laser enclosure to
ensure worker protection.
Coating removal efficiency is affected by coating
color and gloss
The potential for production of products of
incomplete combustion has not been quantified.
References
Head, J. D. 1990. "Pulsed Laser Cleaning." In: Pro-
ceedings of the 1990 DOD/lndustry Advanced
Coatings Removal Conference, Atlanta, Georgia.
pp. 256-260.
Hill, A. E. 1993. "Physical Requirements and Methodol-
ogy Necessary to Achieve Controllable, Damage
Free Coatings Removal Using A High Energy
Pulsed Laser." In: Proceedings of the 1993 DOD/
Industry Advanced Coatings Removal Conference,
Phoenix, Arizona, pp. 304-316.
Toohey, J. A. 1993. "Laser Automated Decoating
System (LADS)." In: Proceedings of the 1993 DOD/
Industry Advanced Coatings Removal Conference,
Phoenix, Arizona, pp. 278-303.
Flash I amp Heating
Flashlamp coating removal is similar to laser coating
removal but with the thermal energy input from a xenon
flashlamp rather than from a laser. A high-intensity
flash from the lamp is focused on the surface to heat
and vaporize the coating. A special lens must be used
to focus the light for each different part of the configura-
tion to be stripped.
A review of the literature indicated that flashlamp
heating has the following characteristics (Larson,
1990). Dark, low-gloss coatings could be removed at a
rate of 1.0 ft2/min. However, as with laser coating
removal systems, light or glossy surfaces were difficult
to strip. Metal surfaces reflected the light and therefore
were not damaged, but composites absorbed light and
were removed in layers analogous to coating.
Flashlamp stripping leaves an oily layer on the stripped
surface, so final cleaning with CO2 pellet blasting or a
similar process is needed. A system combining xenon
flashlamp and CO2 pellet blasting in a single pass is
under development as discussed in the section on CO2
pellet cryogenic blasting in Available Technologies
(Section 2). Acutely toxic gases are released if polyu-
rethane coatings are vaporized in an inadequate
oxygen flow.
Reference
Larson, N. 1990. "LowToxicity Paint Stripping of
Aluminum and Composite Substrates." First Annual
International Workshop on Solvent Substitution. DE-
AC07-76ID01570, U.S. Department of Energy and
U.S. Air Force, Phoenix, Arizona, pp. 53-60.
Ice Crystal Blasting
The Canadian Navy studied the use of ice crystals to
remove coatings from the interiors of submarines
(Larson, 1990). Preliminary testing on aircraft indicate
very low stripping rates (Pauli, 1993). No active devel-
opment programs were identified by this literature
review. The ice crystals are used as an air-propelled
blasting medium to remove coating. Because the ice
crystals melt and evaporate, separation of the coating
residue is simple and the waste volume is small. Ice
crystal coating removal is reported to be compatible
with aluminum and composite substrates.
Ice blasting uses ice crystals formed from tap water as
the coating removal media. Ice crystal production
equipment forms crystals of controlled size and density
to ensure reliable coating removal. Atypical hand-held
blasting system uses about 280 scfm supply air at 200
psig to propel the ice particles. Ice crystal use rate is
about 200 Ib per hour.
Ice crystal blasting has been employed commercially in
the United States to clean metals (stainless steels,
aluminum, lead), rubber, concrete, and plastic surfaces
in the nuclear industry. Oak Ridge National Laboratory
and Martin Marietta have used ice blasting to decon-
taminate hand tools, equipment, lead bricks, hot cell
walls, and other surfaces in different Oak Ridge
facilities. Similar applications have been reported at the
Wolf Creek and Oconee nuclear reactor plants (Apple
and Jahn-Keith, 1993).
39
-------
References International Workshop on Solvent Substitution. DE-
AC07-76ID01570, U.S. Department of Energy and
Apple, F. C., and L. Jahn-Ke'rth. 1993. "Ice Blasting U.S. Air Force, Phoenix, Arizona, pp. 53-60.
Flushes as it Scrubs." Nuclear Engineering Interna-
tional, August. Pauli, R. 1993. "Dry Media Paint Stripping Eight
Years Later." In: Proceedings of the 1993 DOD/
Larson, N. 1990. "Low Toxicity Paint Stripping of Industry Advanced Coatings Removal Conference,
Aluminum and Composite Substrates." First Annual Phoenix, Arizona, pp. 220-248.
40
-------
SECTION 4
POLLUTION PREVENTION STRATEGY
Using solvent-based methods for coating removal
causes release to all three environmental media: air,
land, and water. Both the solvent and the removed
coating materials may cause environmental concerns.
The solvents used in conventional stripper formulations
are hazardous and toxic. The removed coating debris
also may contain resins or pigments that can cause
problems for the environment or worker safety. Volatile
organic vapors will enter the air due to the volatility of
the stripper components. Removing the stripper and
softened coating generates sludge containing a mixture
of solvents and coating solids including resins and
inorganic pigments. Often the stripped part is rinsed
with water to remove remaining traces of stripper and
coating. The poststripping rinse generates wastewater
containing solvents and coating solids.
methyfem chtotkte use, ffmit emissions,
wtft&fatฎ rnqtostif* &w$tk$f$ to mityt-
taw,
health concerns amis weak economy.
These multimedia releases are monitored and con-
trolled under a diversity of laws and regulations. The
main federal environmental regulations influencing
selection of cleaner coating removal technology
decisions are the Clean Air Act Amendments (CAAA),
the Resource Conservation and Recovery Act (RCRA),
the Right to Know provisions of the Superfund Amend-
ment and Reauthorization Act (SARA), and the empha-
sis on eliminating pollution at the source in the Pollution
Prevention Act. Solvent strippers also increase the
potential workplace exposures to VOCs regulated
under OSHA. There are a wide variety of state and
local limits on VOC, hazardous, and aqueous wastes
that also are of concern.
Title III of the CAAA requires adoption of Maximum
Achievable Control Technologies (MACT) for control of
189 HAPs. Both paint stripper users and coating
industries are considered major sources of HAPs and
are subject to MACT standards. The coating industries
may use paint removal technologies either to remove
unsatisfactory coatings or to clean paint line equip-
ment. MACT standards may be developed for coating
removal specific to the needs and attributes of the
specific industry.
The requirements for cradle-to-grave management for
solvent waste established by RCRA create several
incentives to seek solvent-free alternatives. Disposal of
RCRA wastes is costly and carries continued liability.
RCRA also requires the waste generator to maintain a
waste minimization program. Converting all possible
plant applications to a coating removal technology that
eliminates or reduces solvent use helps to demonstrate
an effort to minimize hazardous waste.
Since 1988, manufacturing facilities have been report-
ing emissions of more than 300 chemicals or chemical
categories. The reporting requirements are established
under Title III of SARA. The toxic chemical release
reporting is usually referred to as the Toxics Release
Inventory (TRI). The reporting rule requires annual data
on direct releases to all environmental media. Facilities
meeting the following conditions must file TRI data:
An SIC code in the range of 20 to 39
10 or more employees
Manufacture or processing of more than 25,000
pounds or use of more than 10,000 pounds of a
chemical on the TRI list.
required expanded reporting of waste
The reporting requirements were expanded to include
data on recycling as required by the Pollution Preven-
tion Act. The effort required to track and report chemi-
cal usage is significant. For plants that meet the
reporting threshold, reducing chemical use below the
threshold eliminates the requirement to prepare a
report for the chemical. Methylene chloride is one of
the TRI chemicals, so reducing or eliminating its use
41
-------
will eliminate the need to complete one TRI reporting
form.
The TRI data also form the basis for tracking the
voluntary reduction of 17 priority toxic chemicals
identified in the 33/50 Program of voluntary pollutant
reductions. Methylene chloride is one of the priority
toxic chemicals identified by the EPA Administrator in
the 33/50 Program. Switching from solvent stripping to
a cleaner stripping technology will assist in meeting the
reduction goal.
The organic solvents in stripper formulations result in
sufficient vapor concentrations to cause concern for
workers in the area. Some of the health- and safety-
related data for methylene chloride are shown in Table
5. NIOSH recommends that occupational exposure to
carcinogens be limited to the lowest feasible concentra-
tion.
References
American Conference of Governmental Industrial
Hygienists. 1992. 1992-1993 Threshold Limit Values
for Chemical Substances and Physical Agents and
Biological Exposure Indices. ACGIH, Cincinnati,
Ohio.
National Institute for Occupational Safety and Health
1992. NIOSH Pocket Guide to Chemical Hazards.
U.S. Department of Health and Human Services,
NIOSH, Washington, D.C.
Table 5.
Health, Physical, and Chemical Data for Methylene Chloride
Property
Value
CAS Number
OSHA PEL
OSHA Ceiling
NIOSH REL
NIOSH IDLH
NIOSH Occupational Carcinogen
ACGIH TLV
ACGIH Designation
Molecular Formula
Molecular Weight
Boiling Temperature
Freezing Temperature
Solubility in Water
Specific Gravity (@ 68ฐF)
Viscosity (@ 68ฐF)
Vapor Pressure (@ 68ฐF)
Flash Point
Flammability Limit in Air: UEL
Flammability Limit in Air: LEL
Kauri Butanol Value
75-09-2
500 ppm">
1000 ppm (2000 ppm with 5-minute. maximum peak in any 2-hour period)
'Reduce exposure to lowest feasible concentration"
5000 ppm
Yes
50 ppm
A2 (suspected human carcinogen)
84.9 g/mol
104ฐF
-139ฐF
2%
1.33g/cc
0.44 cp
350 mmHg
None (designated combustible liquid)
22%
14%
132
Sources: ACGIH, 1992; NIOSH, 1990.
Note: CAS = Chemical Abstract Services; PEL = permissible exposure limit; REL = Recommended Exposure Limit; IDLH :
dangerous to life and health; TLV = threshold limit value; UEL = upper explosive limit; LEL = lower explosive limit.
(a) OSHA is expected to lower the PEL to 25 ppm or lower according to a proposed rule.
immediately
42
-------
SECTION 5
CLEANER TECHNOLOGY TRANSFER CONSIDERATIONS
Introduction
The conventional solvent stripping technologies are
becoming increasingly unfavorable due to environmen-
tal pressure. The solvent technologies were popular
because they were proven for removjng essentially all
paints and similar organic coatings from most metal
and composite surfaces. Individual cleaner technolo-
gies cannot encompass the full range of coating and
substrates that the solvent strippers processed. How-
ever with the variety of technologies available, there
usually are one or more cleaner alternatives that will
effectively fit a particular application.
the next several years in response to
regulatory pf4$$vr0$. 0$HA'$ prQp$$a&
PEL revision couM require use of th& more
tiorn.
Preliminary indications are that the MACT standards for
coating removal will limit application of HAP-containing
solvents. Most of the paint stripping MACT standards
are in the early stages of development. The aerospace
industry MACT standard is maturing, however. The
expected proposed standard will require virtual elimina-
tion of hazardous air pollutants for aircraft depainting.
An exemption of about 20 gallons of HAP solvent for
removable parts, such as composite radar domes, will
be allowed (Boothe, 1993). The elimination of HAPs
may be achieved by substitution of a chemical stripper
that does not contain HAPs or by switching to a tech-
nology that eliminates use of solvents containing HAPs.
As described above, a number of cleaner technologies
for coating removal are rapidly entering commercializa-
tion. There are, however, many questions to answer
before transferring an innovative technology to a
specific plant. Many promising innovative technologies
are convincingly demonstrated in the laboratory but
require site-specific evaluation and testing to gain
acceptance in commercial practice. The hurdles
between laboratory concept and field application are
(Schmitz, 1992):
Demonstrating feasibility
Ensuring environmental compliance
Ensuring worker safety
Obtaining certification/warranty approvals from
the maker of the equipment being stripped
(aircraft) or the end user of the product (commer-
cial goods)
Obtaining capital approval
Analyzing life-cycle costs
Integrating the new system into the existing
processes
Getting worker acceptance.
and fteld testing*
Finding the right stripping technology and transferring
to shop use can be a challenge. The characteristics of
an ideal stripping method include (Bell, 1993):
Effectively removes the required coating
Complies with VOC regulations and MACT
standards
Economically viable
Minimizes capital outlay
Is compatible with existing facilities
Minimizes waste/effluent
Minimizes worker hazards
Is simple to operate
Gives controlled stripping action
Does not damage substrate.
No single one of the cleaner technologies will fit the
requirements of all applications. However, many of the
technologies are suitable substitutes for solvent
stripping in specific applications. This document alerts
the user to potential alternatives and helps to perform a
preliminary evaluation. A few major items are dis-
cussed below to help organize the search for candi-
dates and plan the on-site tests. However, when
43
-------
selecting a coating removal technology, there is no
substitute for site-specific knowledge and on-site study
and testing.
Coating Properties
Coating type: Tough epoxy or elastomeric coatings
require aggressive abrasive action. Aggressive PMB
and bicarbonate blasting systems are possible ap-
proaches for tough coatings.
Coating thickness: Coatings range from less than 1
mil to thermoplastic powder coatings several mils thick.
Even thicker coatings are encountered on painting line
apparatus. Thick coatings generally are difficult to
remove with impact/abrasive systems such as PMB or
wheat starch blasting. However, medium-pressure
water systems, with or without bicarbonate addition,
have been successful in removing thick, multilayer
coatings. Burnoff ovens, molten salt baths, fluidized
beds, and cryogenic N2 generally provide rapid removal
of thick coatings.
Chlorocarbon and fluorocarbon films: Burnoff
technologies will generate hydrochloric or hydrofluoric
acid when used to remove chlorocarbon or fluorocar-
bon coatings. PVC can be stripped by thermal systems
if special offgas treatment systems are provided to
scrub out the HCI. Fluorinated films (Teflon) should
not be stripped with burnoff technologies.
Part and Substrate Properties
Area to be stripped: If the area to be stripped is large,
a faster removal system is desirable. The cleaner
technologies with higher removal rate potential and the
ability to handle large parts include PMB, bicarbonate
blasting, and medium- and high-pressure water blast-
ing. Also, a large volume of large-area parts may
support the capital investment required for a robotic or
mechanized system.
Size of part: Small parts may be stripped in cabinet or
confined systems. These systems include PMB cabinet
blasters, cryogenic N2 systems, burnoff ovens, molten
salt baths, and fluidized beds. For parts with one
dimension over about 10 feet, open-area systems such
as the various media blasting systems typically are
more applicable.
Substrate abrasion resistance: For less sensitive
substrates, more aggressive stripping media are
acceptable. Harder, more aggressive plastic media,
high- or medium-pressure water blasting, or aggressive
sodium bicarbonate blasting can give good removal
rates. For more sensitive substrates, options to con-
sider are softer plastic media, wheat starch, CO2, high-
or medium-pressure water blasting, or sodium bicar-
bonate blasting.
Thickness of substrate: With a thin, soft sheeting
(e.g., skin of commercial aircraft), substrate deforma-
tion can be a problem. The more aggressive bead-
blasting technologies may deform thinner materials.
Development is continuing to determine the best
condition for acceptable strip rates for various methods
with minimal substrate damage or deformation.
Testing oft smslt atom rftay tm d&ft& te
d&termtne pofaftfial etemagre. Pmssyms -
c&ft &e lowered to reduce ffie p&ssibifity of
mtfuce stripping effectiveness*
Poststripping inspection requirements: In some
applications, particularly aircraft parts, coating removal
is performed to allow inspection of the substrate. One
major concern is hairline fatigue cracks. The more
aggressive bead blasting technologies may peen
cracks closed, thus making detection difficult. Develop-
ment is continuing to determine the effects of cleaner
coating removal technologies on crack closure.
Presence and condition of substrate surface finish
(primer, filler, chemical film): In some maintenance
stripping applications, only the topcoat is to be re-
moved. Any primers, fillers, or chemical films are left
intact, if possible. Controlled layer-by-layer stripping
action is possible with less aggressive plastic media,
wheat starch, CO2, or certain types of sodium bicarbon-
ate blasting.
Heat sensitivity: High-temperature stripping systems
are unsuitable for temperature-sensitive parts. For
example low-melting metals such as zinc and its alloys,
parts with heat tempering such as springs, or parts
where critical dimensions must be maintained are not
suitable for high-temperature coating removal. Pyro-
phoric metals such as magnesium must not be treated
by thermal processes.
Process Concerns
Bs&i v&rfas Irtdivtfu&l G&rteฃm$ zrxt
priorities.
Pollution prevention: All of the cleaner technologies
eliminate use of HAP-containing solvent strippers. They
produce a variety of waste streams. Technology
selection depends on plant requirements and the
support equipment available.
Required throughput rates: The cleaner technology
must be able to achieve required throughput rates to be
44
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acceptable. Where many small parts need to be
stripped rapidly, technologies such PMB cabinet
blasters, cryogenic N2 systems, burnoff ovens, fluidized
beds, or molten salt baths should be considered.
Systems with the potential for high throughput with
larger items include PMB, bicarbonate blasting, and
medium- and high-pressure water blasting.
Coating removal quality standards: Cleaner tech-
nologies are all capable of removing coatings. The
desired end point is plant-specific and must be judged
based on site-specific needs.
Capital costs: Obtaining the necessary capital for
addition of a new system can be difficult, even if there
is an eventual payback in reduced operating expenses.
The most capital-intensive cleaner coating removal
technologies include high-pressure water blasting and
laser paint stripping due to the need for mechanized
controls.
Floor space available: In many cases the cleaner
technology will be considered as a replacement for an
existing solvent stripping system. In retrofit applica-
tions, it is always desirable to avoid major facility
modifications. Cleaner paint removal systems with
modest space and utility support requirements include
sodium bicarbonate blasting, bumoff systems, molten
salt baths, and medium-pressure water-blasting
systems.
Utilities available: The availability of compatible
support systems can be a factor in selecting a technol-
ogy. If water treatment systems are available on site,
systems such as bicarbonate blasting or medium- and
high-pressure water blasting can be attractive. For
example, high-pressure water spray is ideal for remov-
ing paint buildup from floor grates of a water-wall spray
booth.
References
Bell, B. 1993. "A/C Paint Stripping The Future." In:
Proceedings of the 1993 DOD/lndustry Advanced
Coatings Removal Conference, Phoenix, Arizona.
pp. 249-277.
Boothe, V. 1993. (919) 541-0164. U.S. Environmental
Protection Agency. Personal communication with
Lawrence Smith, Battelle Memorial Institute.
Schmitz, W. N. 1992. "New Technologies Hurdles to
Implementation." In: Third Annual International
Workshop on Solvent Substitution, U.S. Department
of Energy and U.S. Air Force, Phoenix, Arizona.
45
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SECTION 6
INFORMATION SOURCES
This section provides the trade associations affiliated with the technologies that have been discussed. Table 6
shows the trade associations and the technology areas they cover. Readers may contact these trade associations
and request their assistance in identifying one or more companies that could provide the desired technological
capabilities.
Table 6. Trade Associations and Technology Areas
Trade Association Technology Areas Covered
Contact
Association for Finishing
Processes of the Society of
Manufacturing Engineers
Federated Societies for
Coating Technology
International Air Transport
Association
Industrial finishing operations
Decorative and protective paints
Aircraft transportation issues including depainting
International Organization for Technical Committee 20/Working Group 8 is
Standardization working on aircraft depainting standards
National Paint & Coatings
Association
Powder Coating Institute
Radtech International
SAE
Paints and chemical coatings, related raw
materials, and equipment
Powder coating materials and equipment
Radiation-curable paints and coatings
Aeronautical Materials Specification Committee J is
working on implementing SAE AMS documents as
replacements for military specifications replacing
HAPs
P.O. Box 930, One SME Drive
Dearborn, Ml 48121
tel. (313)271-1500
492 Norristown Road / Bluebell, PA 19422
tel. (215) 940-0777
IATA Building / 2000 Peel Street
.Montreal, PQ / Canada H3A 2R4
(514)844-6311
1, rue de Varembe / Case Postale 56
CH-1121 Geneva 20 / SWITZERLAND
tel. 227490111
1500 Rhode Island Avenue, N.W.
Washington, DC 20005
tel. (202) 462-6272
1800 Diagonal Rd., Ste. 370
Alexandria, VA 22314
tel. (703)684-1770
60 Revere Drive, Ste. 500
Northbrook, IL 60062
tel. (708) 480-9576
400 Commonwealth Drive
Warrendale, PA 15096-0001
(412) 776-4841
46
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