FINAL REPORT
PRESENT AND EMERGING TECHNOLOGY FOR
MANUFACTURING PVC AND ITS IMPACT ON
VINYL CHLORIDE MONOMER EMISSIONS
prepared by
ARTHUR D. LITTLE, INC.
CAMBRIDGE, MASSACHUSETTS O214O
CONTRACT NO. 68-O2-1332
TASK ORDER NO. 13
PROJECT OFFICER: LESLIE B. EVANS
prepared for
ENVIRONMENTAL PROTECTION AGENCY
CONTROL SYSTEMS LABORATORY
DURHAM, NORTH CAROLINA 27711
APRIL 1976
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95OR76O08
FINAL REPORT
PRESENT AND EMERGING TECHNOLOGY FOR MANUFACTURING PVC
AND ITS IMPACT ON VINYL CHLORIDE MONOMER EMISSIONS
Prepared by
Arthur D. Little, Inc.
Cambridge, Massachusetts 02140
Contract No. 68-02-1332, Task Order No. 13
Project Officer: Leslie B. Evans
Prepared for
Environmental Protection Agency
Control Systems Laboratory
Durham, North Carolina 27711
April 1976
ADL Reference 76086-32
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TABLE OF CONTENTS
Page No.
SUMMARY 1
INTRODUCTION 3
PRESENT POLYMERIZATION TECHNOLOGY 4
Suspension Polymerization 4
The Polymerization Recipe 6
Polymerization Process 7
Stripping 9
Recovery of Resin 10
Reactor Clean-up 11
Emulsion Polymerization 12
Polymerization Recipe 14
Polymer Recovery 15
The Wacker Process 16
Bulk Polymerization 17
Solution Polymerization 21
Continuous Polymerization 22
Suspension Polymerization 22
Emulsion Polymerization 23
EMERGING TECHNOLOGY " 26
The Present Status of VCM Emission Standards
Throughout the World 26
United States 27
Germany. 27
Holland 28
United Kingdom 28
France 28
Belgium 29
Sweden 29
Italy 29
Japan 29
E.E.C. (European Economic Community 29
Large Reactors 30
Reactor Clean-up 32
Stripping 35
Suspension Systems 37
Emulsion Resin 39
Other Technology 40
Fluid-bed system 40
In-Kettle Compounding 41
CONCLUSIONS 43
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TABLE OF CONTENTS (Continued)
Page No.
Figure 1. Suspension Polymerization 5
Figure 2. Emulsion Polymerization 13
Figure 3. Bulk Polymerization 18
Table 1. Large Reaction Vessel Technology 36
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SUMMARY
PVC is currently manufactured by four processes: suspension,
emulsion, bulk, and solution polymerization. A vapor polymerization
process has been investigated in the laboratory but will probably
not attain commercialization. From industry's point of view, the
bulk polymerization process represents the last real breakthrough in
PVC manufacturing technology.
During the past several years the most significant advance in
PVC polymerization has been the trend to very large reactors. In the
past, PVC reactors were typically 3,000-5,000 gal. in size, but with
the advance in computer control technology the Japanese and Germans
began to construct reactors 35,000-52,000 gal. in size. Today, most
new PVC capacity in this country is based on large reactors.
Large reactors can significantly reduce vinyl chloride monomer
(VCM) emissions in two ways: (1) large reactors yield more PVC
with less losses of VCM, (2) large reactors require "clean wall"
systems that eliminate hand scraping and fugitive emissions—
systems such as automatic high-pressure water for cleanup or defouling
agents in the polymerization recipe or sprayed on the walls of the
reactor. Large reactor technology and the introduction of VCM emission
standards in 1974 have advanced PVC reactor cleanup technology worldwide
not only for large reactors but for conventional size reactors as well.
But the introduction of VCM emission standards also has started
a worldwide scramble for improvements in stripping technology. Although
two approaches are being considered, most emphasis is on improvements
in the mechanics of stripping—new hardware, continuous vs. batch processing,
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and modification of stripping conditions. Resin producers are
also investigating the modification of the polymerization recipe to
increase the porosity of the resin particles and thus improve the
efficiency of the stripping step, but this approach has not been as
rewarding as the former one.
In spite of the advances that have been made in reducing emissions,
the PVC resin producers are most concerned about their ability to strip
the emulsion resins. The reduction of VCM emissions in the manu-
facture of emulsion resins continues to challenge the industry. Most
of this effort is still in the laboratory or pilot plant stage.
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INTRODUCTION
The Office of Air Quality Planning and Standards of the U.S.
Environmental Protection Agency (EPA) has recently proposed emission
standards for all polyvinyl chloride (PVC) resin plants with respect
to emissions of vinyl chloride. Of the proposed regulations, one of
the key ones is concerned with the emissions from all plant sources
downstream of the stripper that is used to recover and recycle
unreacted monomer. Recognizing that the efficiency of stripping
varies with different PVC polymers, the EPA has proposed that suspension
polymers should be stripped to 400 ppm, on a dry solids basis and
emulsion polymers to 2,000 ppm on a dry solids basis.
While the standard is still at the proposal stage, in the
interest of gaining a better understanding of the present and emerging
technology for manufacturing PVC, EPA has requested Arthur D. Little, Inc.
to review on a worldwide basis (1) the current technology for
polymerizing vinyl chloride; and (2) new and emerging technology
especially as it can affect vinyl chloride emissions.
To carry out this investigation we surveyed the literature and met -with
key PVC producers in the United States and Europe. We also met with the
technical committee for vinyl chloride of.Jthe. European.. Council of Chemical
Manufacturers' Federations (CEFIC) in Brussels. Because details of new
processes were considered proprietary, our study was limited primarily
to a review of the effect of emerging technology on vinyl chloride
emissions.
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PRESENT POLYMERIZATION TECHNOLOGY
There are four commercial processes currently employed for the
manufacture of PVC resin: suspension, emulsion, bulk and solution
polymerization. All four processes are based on free-radical
polymerization of vinyl chloride monomer. The choice of polymerization
method depends upon the end-use application of the resulting resin
and economics.
In the U.S.A., suspension polymerization dominates. It accounts
for about 80% of PVC production. Emulsion polymerization accounts for
another 11%, bulk polymerization 6%, and solution polymerization 3%.
In Europe, the production of suspension and emulsion resins vary
country by country. In Sweden, the ratio of suspension to emulsion
resin is 70/30, in the U.K. and Belgium it is 75/25, and in Italy, it
is 87/13. In Holland, only the suspension resin in made. Bulk resin
is made in Germany, France, Spain, U.S.S.R, India, and Japan.
Suspension Polymerization
A simplified flow chart for this process is shown in Figure 1
The vinyl chloride is usually stored in steel storage tanks. The
number of storage tanks depends upon plant size, tank size, and
availability of vinyl chloride. Because of the high purity of vinyl
chloride, it can be stored without inhibitors or refrigeration. The
lines and vessels in the rest of the system are commonly glass-lined,
resin-lined or stainless steel.
Historically, the reactors have generally been glass lined to
reduce polymer buildup on the walls. The sizes of the reactors in
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Recovered vinyl chloride
Vinyl
chloride
storoge
tank
Deionized
and
deoeraled
water
Condenser
Charge bomb
Condenser
—i (X)
Reactor (X) y
Distillation
column
Centrifuge
Impurities
Blend
tank
T""— ~~Jl Liquid
I * discharge
Rotory dryer.
Air
Reboiler
.Wet
air
Heater
PVC to storage
Figure 1. Suspension Polymerization
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the U.S. and elsewhere have varied from 2,000 to 6,000 gallons (larger
kettles are of more recent vintage). The reactors are fitted with
impellers and baffles. Agitation is an important variable in this
reaction. The extent of agitation in a suspension reaction establishes
the average size of the vinyl chloride droplets, the porosity of the
final PVC resin, and the heat-transfer coefficient.
The Polymerization Recipe. Suspension polymerization recipes differ
widely—a typical one is given below.
Ingredient Parts By Weight
Vinyl Chloride 100
Deionized Water 200
Lauroyl Peroxide 0.04
Polyvinyl Alcohol (PVA) 0.01
PVA serves as the protective colloid that stabilizes the monomer
droplets and affects .their size, which eventually affects the size of
the polymer particle. These colloids also affect the porosity of the final
resin product. Increasing concentrations of suspending agents lead to
decreasing polymer particle sizes. The desired particle size is 50 to
150 microns. Other popular suspension agents include gelatin and
methyl cellulose. In some cases additional emulsifying agents are
added, usually in an amount approximately 1/10 that of the polyvinyl
alcohol, to obtain a more porous resin. Increasing the porosity of
these particles increases their ability to absorb plasticizer and
facilitates the removal of unreacted vinyl chloride monomer in the
stripping step. These secondary emulsifiers include sulfonated
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compounds and ethylene oxide condensates.
Initiators are employed in concentrations of 0.03 to 0.1 parts per
hundred parts of monomer. In addition to lauroyl peroxide, other initiators
include isopropyl peroxycarbonate (IPP) and azobisisobutyronitrile (AIBN).
Frequently two initiators are used in commercial operations. The initiators
differ with respect to their effectiveness at different temperatures.
IPP is the most active of the three initiators cited above. Rates of
conversion of monomer to polymer at 50°C varies from 8% per hour for lauroyl
peroxide to 15% per hour for IPP.
Ratios of water to vinyl chloride range from 1.5 to 1 to 4 to 1. Low
ratios allow higher monomer charges for a given reactor, while high
water contents facilitate better temperature control and thus permit
higher conversions. The latter approach also yields a more porous resin.
Polymerization Process. The charge for suspension polymerization
is often added to the reactor at ambient temperature although some
companies charge the kettle with preheated water. Heated water added
initially to the vessel reduces the time necessary to reach operating
temperature. It may also change the properties of the polymer obtained.
The desired quantity of monomer is measured in the weigh tank
and transferred into the reactor which contains the proper amount of
water. Initiator, suspending agent and buffering agents are charged into the
reactor via the charge bomb. One hour may be needed to raise the
reaction mixture to polymerization temperature by discharge of steam
into the reactor jacket. In general, the reaction temperature is
45° to 55°C at a pressure of 75 to 100 psig. At temperatures
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greater than 55° C the molecular weight of the polymer is seriously
reduced.
The molecular weight and, therefore, the physical properties
of PVC is highly sensitive to temperature changes during polymerization
and also slightly dependent on initiator concentration. Proper
temperature control throughout polymerization is essential and must
be obtained by sensitive instrumentation and an efficient heat transfer
system tp__remove the heat generated by the reaction.
A low molecular polymer can be made by operating at higher temperatures;
alternatively, chain-transfer agents can be used. Typically, these are
halogenated hydrocarbons, such as trichloroethylene, _aj.though isobutylene
also has been used. In either case, these compounds are added in concentrations
as low as 1.0-1.5 percent.
The production of high molecular PVC is carried out at lower temperatures.
In this case, the initiator is IPP rather than lauroyl peroxide.
On the average the polymerization reaction requires about six
hours to reach the desired conversion of monomer to polymer from 85 to
90%. Up until about 70% conversion both monomer and polymer phases are
present in the suspended droplets. At about this conversion the monomer
phase is depleted and is retained in the PVC polymer droplet or particle.
Some unreacted vinyl chloride remains in the vapor space of the reactor
as the polymerization continues. Toward the end of the polymerization
cycle the pressure in the system begins to drop, followed very shortly
by a peak in the polymerization rate. Beyond this peak, the rate drops
sharply and the polymer in the form of beads becomes less porous. The
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PVC manufacturer must determine the specific point at which polymerization
is terminated to give high quality polymer and acceptable economy of
production. Beyond 90% conversion polymerization is slow and polymer
degradation may occur.
Stripping. After the reaction is completed, the polymer slurry is
discharged into a dump tank which also serves as a stripper. This vessel
is similar to the reactor and is fitted with an agitator. As a minumum,
the stripper is 30% larger than the reactor (because of foaming), but
many companies use a stripper tank that is large enough to hold three
batches from a reactor.
In the stripper, vinyl chloride is stripped from the slurry by
applying heat and/or vacuum for 15 to 60 minutes. Some manufacturers
perform the stripping step in the reactor. The rate at which monomer
is removed at this stage depends upon the capacity of the vacuum pump
and on the temperature in the tank. Steam is often injected directly
into the suspension. Generally low molecular weight homopolymers and
vinyl acetate vinyl chloride copolymers are more difficult to strip.
Apparently these resins form granules that are less porous than those
formed during the manufacture of homopolymer. Consequently, it is
more difficult for the vinyl chloride to escape from the particle.
Historically, the industry has carried out this stripping step
in order to recover the unreacted monomer and to operate economically.
The efficiencies in the past were such that usually about 105 Ibs
of vinyl chloride monomer were used to produce 100 Ibs of PVC. The
unreacted monomer which is stripped from the slurry is condensed after
processing and recycled back into the system. A typical recycling
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system, which is shown in Figure 1, includes a water-separation tank,
gas hold tank, a compressor, storage tanks, and a fractionating still.
In this system the vinyl chloride and some water (carried over with
the Stripping) is compressed to a sufficiently high pressure
(80-90 psig.) so that condensation may be effected by normal cooling
water. The water collected in this system is separated by extraction
and discarded. The resulting vinyl chloride is relatively impure;
consequently, most recycled vinyl chloride must be purified, and this
is usually done by distillation. In the manufacture of vinyl acetate-
vinyl chloride copolymers both monomers are recovered and recycled.
When stripping is complete, the batch is transferred to slurry
or blend tanks, where various batches are blended together to form a
more uniform product. These tanks usually have a capacity of 15,000 gal.,
are made of steel, and are generally open to the atmosphere.
Recovery of Resin. Beyond this stage the operation may be continuous.
Removal of water from the slurry is accomplished by the use of a
continuous centrifuge. The resulting wet solids from this operation
contain ca. 75% PVC. Frequently the wet solids are washed in the
centrifuge with deionized water to remove soluble electrolytes.
This washing is particularly applicable in the production of electrical-
grade resins.
Final drying of the wet cake may be by rotary dryer, or
one and two-stage flash dryers, or combinations of these. A rotary
dryer is most commonly used. At this stage, most of the vinyl
chloride remaining as resin is released. Drying is a very critical
phase, because above about 65°C (150°F) the polymer can begin to
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degrade and discolor. Air entering the dryer at 300°F will give a
polymer surface temperature of about 120°F, and as air and solids pass
through the dryer, their temperatures approach 146°F and 136°F
respectively. In general,careful temperature control must be maintained
to produce quality resin. Typically the moisture level is reduced below
0.25% in the dryer.
Finally, the dried solid resin is collected in bank collectors or
cyclones, which also remove very coarse particles and fines. Bag
filters are used to clean the exit air stream.
Solid PVC resin, recovered from the cyclone separators and the
bag filters, are sized by screening, and the screened particles are
air-conveyed to storage bins or silos.
Reactor Clean-up. Of prime importance in the production of high
quality PVC is the cleanliness of the polymerization vessel. Normally,
polymer build-up occurs on the reactor walls, especially at the vapor-
liquid interface. Copolymers tend to form more build-up than homopolymers.
If the build-up is allowed to remain in the reactor, it becomes
hard, dense, and nonabsorbent and may give rise to imperfections and
so-called "fish eyes" in the finished product. These imperfections
are particularly troublesome in the fabrication of film and sheet
products.
Historically, the material along the walls of the reactor were
removed by hand scraping by personnel who entered the reactor vessel
at the end of the polymerization cycle. In more recent years the
removal has been accomplished by the use of solvents or the use of
extremely high pressure water systems that often consist of spray
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nozzles that are Installed at the top of the reactor. Normally,
several batches can be processed before the reactor needs clean-up.
(See Section on Emerging Technology.)
Emulsion Polymerization
A simplified flow diagram for a typical emulsion or dispersion
polymerization plant is shown in Figure 2. Emulsion polymerization is
the oldest technique for manufacturing PVC. This process is very
similar to the suspension process described above except that soap or
another emulsifying agent is added to the slurry of monomer and water
to stabilize the monomer droplets. Usually, emulsion resins are
polymerized more rapidly and at lower temperatures than suspension
resins. Another distinguishing feature of the emulsion system, in contrast
to the suspension system, is that only water-soluble initiators can be
used. The emulsion process usually produces polymers of considerably
higher molecular weight than those made by the suspension process.
A simple description of the emulsion polymerization process is
as follows. The addition of an emulsifying agent to the two-phase
monomer-water system decreases the surface tension between the
two phases. As more emulsifying agent is added to the system a critical
concentration is reached. Above this concentration the surface tension
increases less rapidly. This concentration is called the critical
micellar concentration (CMC). At this concentration the emulsifier
that was previously distributed uniformly begins to agglomerate into
groups or micelles. Micelles usually consist of a group of 20-30
emulsifier molecules.
Polymerization begins when a free radical from the initiator enters
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0-r
Figure 2. Emulsion Polymerization
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a monomer-swollen micelle. As the polymerization proceeds, additional
monomer is supplied to the growing latex particle by diffusion through
the aqueous phase from the monomer droplets. The emulsifier collects at
the suface of the polymer as it forms. The stability of the resulting
latex is maintained by adsorption of additional emulsifying agent on the
remaining micelles. In some cases, the emulsifier micelles disappear
at 12%-20% conversion of the monomer, and all of the emulsifier is
located at the surface of the polymer particle. At higher concentrations
(ca. 60%) all of monomer is in the polymer phase.
The number of polymer particles formed and hence their size is
affected by the concentration of emulsifier in the system, because
the emulsifier concentration affects the number of micelles. The
higher the concentration of emulsifier the more stable the emulsion
and the smaller the size of the resulting PVC latex particle.
To better control particle size, to produce particles with a
larger size, and to minimize the use of emulsifying agent that usually
contaminates the final resin, the industry early learned to modify the
emulsion technique by using a "seeding" technique. In this approach
a heel of latex is added to the reactor in conjunction with a slightly
lower concentration of emulsifier. Polymerization then proceeds on
the latex particles rather than in micelles.
Polymerization Recipe. A wide variety of emulsifier and
initiator systems are reported in the literature. Preferred emulsifying
agents are detergents such as alkyl sulphates, alkane sulfonates, and
fatty acid soaps. Widely used initiators are hydrogen peroxide, organic
peroxides, peroxy disulphates, and Redox systems. In addition to these
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ingredients, modifiers may be present such as colloidal protective
agents, gelatin or carboxy methyl cellulose, chelating compounds,
plasticizers, stabilizers, and chain-transfer, agents.
A typical recipe for emulsion polymerization is shown below.
Ingredient Parts By Weight
Vinyl Chloride - 100
Deionized Water 100-200
Sodium Persulfate 0.4
Sodium Dodecyl Benzene Sulfonate 0.5-2.0
Emulsion polymerizations are generally, carried out to somewhat
higher conversions than suspension polymerizations. Some can go even
as high as 95% conversion.
Polymer Recovery. Emulsion resins like suspension resins are
also stripped to recover unreacted monomer. However, emulsion resins
are considerably more difficult to strip, because they are more
sensitive to heat and shear stresses that occur during stripping.
These polymers tend to discolor more easily than the suspension
resins. Furthermore, the emulsion can coagulate during stripping and
consequently destroy the resulting PVC product.
In the United States, most emulsion polymers are recovered by
spray drying, which maintains the small particle size. Larger
particle size emulsion resins, which contain less residual soap, are
produced in Europe. These polymers are typically recovered by
coagulating the latex with salts followed by filtering, washing,
drying, and sieving. The European emulsion resins are used in
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calendering and extrusion, in contrast to the U.S. resins that are
used in plastisol compounds. Thus, contrary to suspension resins,
emulsion polymers are usually contaminated with more residual detergents
or soaps. These contaminants have an adverse effect on product clarity,
compatibility with some compounding ingredients, electrical resistance,
water absorption, and weathering of the final product.
Emulsion polymerization of vinyl chloride is the commercial route
in the U.S.A. to paste resins used to make plastisols or organosols.
Emulsion PVC resins are particularly well suited to this application
because they have a smaller particle size than that of normal suspension
PVC. Emulsion resins vary from 0.2 to 5 microns in particle size, and
those used in plastisols are typically about 1 micron in diameter. The
small particle size is necessary to obtain good fusion in fabricating
the plastisol. Furthermore, the emulsion polymers are relatively
impervious to plasticizer adsorption, because they are coated with
emulsifier.
The Wacker Process. In the mid-601s, Wacker Chemie introduced a
modified process for manufacturing the emulsion resin. This process
is used in Germany by Wacker and in the United States by Tenneco. The
Wacker process can be used to make both paste resins and calendering and
extrusion resins, but Tenneco uses this process only to manufacture
paste resin.
In the Wacker process, the polymerization is carried out in a
tower reactor rather than a polymerization kettle. This is a batch
process and the tower reactor is not fitted with an agitator. The
recipe is somewhat different than the conventional emulsion process.
Different soaps are used, and the polymerization reaction is carried
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out very close to precipitation conditions. However, because different
soaps are used, the resulting emulsion polymer can be filtered, .washed and
recovered by drum drying. In this process, most of the soaps are
removed in the washing step. Consequently, the resulting polymer has
superior properties because it has fewer contaminants. It is used in
coating and film applications.
Bulk Polymerization
The polymerization of vinyl chloride in bulk or in mass has been
known in practice, at least in the laboratory, for many years. Commercial
utilization of this technique, however, has been fairly recent. So
far as is known, only one bulk process, that developed by Pechiney-St.
Gobain,is used to manufacture PVC on a commercial scale.
The commercial process is a two-stage one, which is shown in a
simplified flow diagram in Figure 3. The first-stage reactor or pre-
polymerizer is a 2,000-gallon stainless-steel vertical autoclave equipped
with a flat-blade turbine and baffles to give turbulent agitation.
Usually, about half of the total monomer to be polymerized is fed
to the prepolymerizer which has been freed of oxygen. The monomer
contains approximately 0.015 wt. percent of a monomer-soluble initiator.
In this first-stage reactor, the vinyl chloride and initiator are
heated to 40 to 70°C at a pressure of 75 to 175 psi, and the
polymerization begins. Some of the heat of polymerization is removed
by the refluxing of the vinyl chloride monomer. Because the resulting
polymer is insoluble in the monomer, it precipitates immediately as it
forms,yielding granules about 0.1 micron in diameter. Once formed, the
number of granules does not change, but their average diameter increases
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Figure 3. Bulk Polymerization
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to 1 micron at 1% conversion.
As the polymerization reaction proceeds, the granules agglomerate
into beads of about 50 microns in diameter. This occurs at about 2-3%
conversion. Polymerization in the first-stage reactor is continued
to 7-15% conversion to allow completion of the bead-forming process.
Beyond this point, the mixture becomes too viscous to stir. Prepolymeriza-
tion occurs within less than one hour.
The polymer beads that form in the first-stage reactor serve.as
seed for continued polymerization in the second step. The structure
of these beads greatly influences the properties of the final product.
To achieve a product with a narrow particle-size distribution, very
turbulent agitation is required during the bead-formation phase.
The greater the turbulence, the smaller the particles. Also, during
the first stage, one can control the compactness of the beads to obtain
PVC resins with various bulk densities.
The prepolymer beads and additional monomer and initiator are next
transferred to the second-stage reactor. This reactor is normally
a 4,000-gallon stainless-steel, horizontal autoclave, that is stirred
with a frame-type agitator that turns slowly at about 9 rpm. The
agitator blades are incurvated to prevent jerks when penetrating the
polymer powder. These blades rotate with minimum autoclave wall
clearance.
As the polymerization is continued, monomer conversion is followed
by determining the heat evolved during the reaction. The heat of the
second-stage reaction is removed by the autoclave jacket, the agitator
shaft cooling, as well as the refluxing vinyl chloride. As
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the reaction proceeds, the beads grow larger and the mixture takes on
the appearance of a dry powder. The polymerization is continued to a
conversion of 80 to 90%. Depending upon the grade of PVC being made,
this second stage requires an average of 5 to 9 hours.
Temperature control of this highly exothermic reaction is critical, because
it determines the molecular weight of the product. The inherently poor
heat transfer properties of the thick slurries and particulate solids
that are encountered in the reactors make this a challenging problem.
The keys to successful operation appear to be control of particle size,
efficient agitation, and minimum fouling of heat-transfer surfaces.
Generally, the second-stage reactor must be cleaned after every batch,
whereas the first stage reactor does not require cleaning as frequently.
When the reaction is halted, unreacted monomer is removed by
vacuum distillation and recovered by vapor compression and condensation
in the recycle condenser. Because no further drying is required in
this process after the monomer is removed, the product is screened to
remove large agglomerates and, finally, packaged or transferred to
storage. According to Goodrich, these polymers are relatively
difficult to strip.
PVC manufactured by this process is very pure for it contains no
contaminants such as soaps that are used in the other processes described
above. The particles are transparent, have a narrow size distribution,
and are relatively porous. Particle shape and size are uniform; the
granules are 0.5 to 1 micron in diameter. These properties lead to
better fusion, impact strength, heat and light stability, and improved
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electrical properties.
Solution Polymerization
Solution polymerization, as practiced in most commercial processes,
is not true solution polymerization, because the polymer precipitates from
the solution as it forms. Consequently, the term "precipitation"
polymerization is sometimes applied to this process. In several respects,
solution polymerization is related to bulk polymerization. In the USA
only Union Carbide Corp. manufactures PVC by solution polymerization.
Most resins produced by this process are copolymers based on vinyl
chloride and vinyl acetate.
In solution polymerization the solubility of the resulting polymer
in the mixture of solvent and monomer depends upon the solvent, the
concentration of vinyl chloride and vinyl acetate in the solution and
the molecular weight of the copolymer. The basic process that Union
Carbide is thought to be using.is probably described in two early
patents (U.S. Patent 2,075,429 and U.S. Patent 1,935,577). The process
described in these patents uses n-butane as the solvent.
Typically, the polymerization is carried out in a mechanically
stirred autoclave that is maintained at approximately 40° C. The solvent-
monomer system contains about 80% solvent. The usual free-radical
initiators described above are probably also used in this process. As
the polymerization proceeds, the polymer precipitates and a slurry forms
that is continuously drawn off, as the solvent and monomers are
continuously charged to the reactor. The process is continuous. The
slurry is pumped to a filter press to remove the precipitated PVC, and
the monomers and solvent are recovered and returned to the system. The
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filter cake is dried by flash vaporization, and the monomers and
solvent from this stage of the operation are also returned for recycling.
The PVC resin made by this process is remarkably pure, as no
emulsifier or suspending agents are used. Furthermore, because water
is not used in this process, recovery of the final product is simplified.
Generally, the unreacted vinyl chloride and solvent are easy to strip;
therefore, the emissions of vinyl chloride from this system are relatively
low. The products made by this process have relatively low molecular
Weights, because the solvent tends to act as a chain-transfer agent.
Continuous Polymerization
Although some of the U.S. PVC resin producers, such as Goodrich
Chemical and the General Tire and Rubber Company, have studied the
possibility of manufacturing suspension and emulsion PVC by a continuous
process for many years, no U.S. Manufacturer uses a continuous process
commercially. However, the Europeans have operated continuous polymeriza-
ion facilities for 20 to 25 years. In Germany, Huels, Hoechst, and
BASF operate both continuous and batch processes that produce emulsion
PVC. In Italy, Montedison has operated a continuous emulsion-PVC
process for about 25 years.
Suspension Polymerization. However, no manufacturer produces
suspension PVC commercially by a continuous process, although examples
have been cited in the literature. For example, a recent patent
(U.S. Patent 2,537,337) to ICI in England described a continuous suspension
process for PVC, and the General Tire and Rubber Company also has
a patent (U.S. Patent 3,125,553) related to a continous flow reactor for
the suspension polymerization of PVC.
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According to the latter patent, three stirred tank reactors are
operated in parallel. Two are used to convert vinyl chloride to PVC
to a conversion level of ca. 70%. This is the region for maximum
conversion rate. The exit product of these reactors is then fed to
the third vessel where the reaction is essentially completed, (90 - 95%
conversion). The claim is made that the quality of the product of this
system is good, but line-plugging problems have remained unsolved.
General Tire never commercialized this process; not only because of
the plugging of the various transfer lines, but, in spite of the claims,,
the product quality was inadequate due to "fish eyes" or"polymer skin".
A major problem with a continuous suspension system, in addition
to the problem of maintaining product quality, is that a way would have
to be devised to change the properties of the resulting PVC product as
needed by the marketplace. Even if this approach were successful, this
would lead to a problem in manufacturing or switching from one product
to another without making off-specification materials.
Emulsion Polymerization Although continuous emulsion polymerization
plants were built many years ago overseas, no new ones are being considered
today. According to the literature, the German plants use two arrange-
ments of reactors. One arrangement consists of multiple reactors with
parallel flow; the other arrangement consists of reactors that are
connected in series. In the latter case, a 3600-gallon reactor is
connected to a smaller unit. In the first reactor the conversion to
polymer proceeds to about 88%, and in the second reactor an additional
4% of the monomer is reacted. The large reactor is glass lined and is jacketed
for cooling with brine or cold water; it is fitted with a simple blade, or
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paddle agitator, near the top of the vessel, the agitator is
operated at relatively moderate speeds, up to 50 rpm. This vessel
is described as being 23 feet tall and about 5 feet in diameter.
Water containing dissolved emulsifier, initiator, and buffer is fed
continuously at the top of the reactor through one feed line and vinyl
chloride is metered to the reactor through another. Apparently, a
'.4
maximum degree of shear is needed in the emulsion near the feed point.
Little agitation is provided in the bottom of the reactor. As PVC
particles form, they become heavier than the vinyl chloride droplets
in the emulsion, because of the greater density of PVC as compared to
vinyl chloride. Normally, the emulsions that are formed in this
system are very stable.
Temperature was reported as the controlling factor relative to
the capacity of the reaction system. Depending upon the specific PVC
polymer desired, temperatures varied from 39 to 50°C. Uniform temperatures
are desired throughout the reactor, but with the laminar flows that
undoubtedly occur in the bottom of the reactor, the overall heat
transfer coefficients tend to be low. Some convections currents occur,
because of temperature gradients in the emulsion from the heat of
polymerization.
Starting up the reactor for the continuous process requires several
hours in order to minimize the production of off-grade polymer. The
reactor is first filled with cool water, emulsifier solution, initiator
and vinyl chloride. Warm water is used in the jacket of the reactor
until the reaction material is sufficiently heated to start polymerization.
Gradually, cooling water is substituted in the jacket, and the density of
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the emulsion is measured frequently, until it reaches the desired
level. At this time, the continuous flow of vinyl chloride and water
solution are started up.
Montedison apparently uses a similar system. They use a line of six
kettles. The conversion to polymer is 85-90% .
Montedison claims that when the facility is operating these
kettles must be cleaned each day, in contrast to the kettles
used in batch operations. Montedison noted no real difference in the
vinyl chloride emissions between the batch and continuous processes,
although the continuous process probably has fewer vents than the batch
process. The Germans made a similar claim. Besides, continuous
polymerization still requires a stripping step.
Montedison PVC made by the continuous process is used to make
paste resin for plastisols and resin for rigid compounds. Apparently,
this PVC is not satisfactory for compounding flexible products, due to
its reduced ability to take up plasticizer to the same degree that the
batch polymers do.
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EMERGING TECHNOLOGY
Only recently has the PVC industry become interested in manufacturing
technology to reduce vinyl chloride monomer (VCM) emissions. This
interest arose in 1974, when VCM was recognized as a health hazard.
When the Occupational Safety and Health Administration (OSHA) first
began to consider new standards for worker exposure to VCM, the PVC
resin producers scrambled to modify their manufacturing processes to
reduce fugitive VCM emissions inside the plant and consequently the
outside atmosphere.
This effort, which is continuing, involved such mechanical improve-
ments as replacing leaking valves and pumps, employing double seal pumps
when warranted, improving ventilation systems, reducing the number of
emission sources, and generally tightening up the manufacturing process
to reduce monomer leakage. In particular, new plants have been and
are carefully designed to limit leakage. For example, Georgia Pacific's
new plant in Plaquemine, Louisiana claims to be in compliance with the
more stringent OSHA standards that will become effective in April 1976
(1 ppm time-weighted-average over an 8-hour period) without requiring
air masks. Furthermore, to better control fugitive emissions, most
resin manufacturers are developing new techniques for maintaining a
clean reactor so that production workers will no longer have to hand
scrape reactors after every few runs (see discussion below). This
general tightening up of the PVC manufacturing process is taking place
not only in the United States but worldwide.
The Present Status of VCM Emission Standards Throughout the World
Because VCM emission standards with respect to the workplace
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and the atmosphere, both proposed and promulgated, have been responsible
for the current search for new PVC technology, it is appropriate that
we first review the status of these standards throughout the world.
United States. In the United States, where OSHA presently limits
exposure to VCM to 25 ppm (time-weighted-average for 8 hours),
industry is attempting to comply, although OSHA has cited a number of
companies for violations. The United States will have a 1 ppm limit in
April 1976. According to Goodrich Chemical, most of its PVC plants
are currently (summer 1975) operating at about 6 ppm or less (time-weighted-
average for 8 hours). Union Carbide says VCM levels in its resin plant
averages about 5 ppm although excursions to 25 ppm occur occasionally.
The U.S. resin producers also are actively involved in programs
to reduce the residual VCM in their resins. Tenneco claims that the
maximum residual monomer in all of its homopolymers, dispersion grades,
and blending resins is 10 ppm, but Tenneco will not guarantee this low
level for its copolymers, which are more difficult to strip, as noted
above. Robintech also claims that the residual monomer content of its
polymers are at the 10 ppm level, and Goodrich says residual VCM has
been reduced to 10 ppm for 90% of its polymers. Moreover, most compounds
used for food packaging have 1 ppm or less of residual monomer.
Germany. One State, North Rhein/Westphalia, is responsible for
about 70% of Germany's PVC production, and this State has strict
regulations concerning emissions. In terms of worker exposure to VCM,
the limit for existing plants is an average of 10 ppm with maximum
excursions to 30 ppm. New plants will be limited to an average of 5 ppm
in the workplace with maximum excusions to 15 ppm. North Rhein/Westphalia
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also regulates emissions of VCM to the outside. The concentration
of VCM in the air in the next habitated place to the resin producer's
facilities is limited to 0.1 ppm (99% of the year).
In March, 1975 many German PVC plants were operating with emission
levels of 100 ppm of VCM in the workplace. On the other hand, Huels
believes that they now meet the present U.S. OSHA standards.
The residual VCM levels in the German PVC polymers are, on the
average for emulsion grades, less than 20 ppm; and for suspension
grades, less than 100 ppm.
Holland. In Holland, the emission standards for VCM in the
workplace are limited to a maximum of 10 ppm and an average of 1 ppm.
Outside of the plant, the emissions are limited to 1/100 of the emissions
inside the plant. This regulation has been in effect since October 1974.
According to a spokesman for the Dutch PVC industry, the PVC industry is
complying with this standard.
United Kingdom. In the United Kingdom, the current regulations
limit the worker exposure to 25 ppm (time-weighted-average for 8-hour
period) with maximum exposure limited to 50 ppm. This regulation is a
so-called coded practice that has been agreed upon by both the Govern-
ment and the trade unions. Most plants are now complying with this
practice but excursions still occur that require respirators.
France. This country has no regulations concerning VCM exposure.
Regulations are now under discussion, and the industry is hoping for a
10 ppm level in the workplace. In the meantime, the industry is
gradually moving in this direction. France has no emission standards
with respect to emissions of VCM to the atmosphere.
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Belgium. Similarly, this country has no regulations concerning
exposure to VCM, but does have a code of good practice. On the
average, the emission of VCM in Belgian PVC plants is about 10 ppm.
Sweden. This country has a regulation since October, 1974 that
limits VCM exposure in the workplace to 1 ppm. Kemanord, which is the
only PVC producer in Sweden, claims that its plants now control workplace
emissions of VCM to 1 - 2 ppm. Kemanord1s PVC compounds contain 10 to
20 ppm residual monomer, although a few products have as little as
3 ppm.
Italy. There are no regulations in Italy concerning VCM exposure
or VCM emissions; however, the Government has made recommendations. At
the present time, the recommendation states that exposure to VCM in the
plant should be limited to 50 ppm. In the past, VCM exposure has
averaged about 200 ppm. Outside of the plant, the recommendations
state that the emissions should be limited to l/20th to l/30th of VCM
exposure inside the plant.
Montedison, which is the major PVC resin producer in Italy, claims
that they now operate at VCM levels between 2 ppm and 10 ppm in the
plant.
Japan. The Ministry of Health and Welfare has set a provisional
2 ppm limit inside monomer plants, but as yet no standard has been set
for PVC resin producers. The standard is expected to issue by the end
of 1975.
E.E.C. (European Economic Community). E.E.C. is drafting a
directive to be applied to all E.E.C. countries that would likely limit
VCM exposure in PVC plants to 10 ppm. The measurement time has not
been decided but is likely to be longer than 8 hours. The monitoring
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system would be defined in the directive. The E.E.C. approach would
be designed to Standardize regulations and eliminate unfair competition.
Large Reactors
A major step forward in PVC technology was made in the 1970*s
when both Shinetsu in Japan and Heuls in Germany began to use very
large reaction vessels—35,000 and 52,000 gallon reactors, respectively,
for suspension polymerization. This move to large reaction vessels reduced
vinyl chloride monomer.emissions; because, simply stated, these large
reactors have fewer connections per unit of PVC production and fewer
potential leaks. Consequently, more PVC can be made with fewer losses
of VCM. Thus, the large-kettle technology can reduce fugitive emissions
of VCM in the plant and the atmosphere.
In the United States, at present, about 30% of the PVC industry's
capacity is said to be in reactors smaller than 2,500 gallons, and 70%
in reactors smaller than 5,000 gallons. However, key representatives
of the PVC industry have indicated that as new capacity is needed, the
U.S. industry will no longer construct PVC reaction vessels as small
as 5,000 gallons. Nevertheless, not every PVC resin producer will be
able to utilize the capacity of the larger kettles, depending upon
the particular markets served by the company and the grades of compounds.
The large kettles are especially attractive to PVC manufacturers who
supply the construction industry that requires only a few grades of
PVC.
A list of the present manufacturers with large kettles appears in
Table 1. The kettles vary in size from 17,000 to 52,000 gallons. Many believe
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that the 52,000-gallon reaction vessel is probably the limit, because
of the problems of heat transfer. Today, six companies in the United
States have large-kettle capacity and others are considering them.
Overseas^large kettle capacity is in place in Germany, Japan, and Italy.
Note that in the United States, Georgia Pacific uses Wacker-
Chemie's process and both Tenneco and Robintech (Shintech) use
Shinetsu technology. Huels is marketing their technology in the United
States through Fluor Corporation. Others in the United States are
using technology developed within their own organization, although
apparently Borden has picked up Monsanto technology in the construction
of their 20,000 gallon kettles.
This trend to larger kettles also has sparked a trend to stainless-
steel kettles rather than glass-lined ones. In the past, most of the
reaction vessels used by the PVC industry were glass lined, but today
the newer vessels, especially the larger ones, are made from stainless-
steel.
In the United States only suspension resins are made in these
large kettles, but apparently Huels also makes emulsion resins in their
large reactor. Generally, in selecting products for manufacture, in the
large reactors the manufacturers have shied away from making compounds
for electrical wiring, flooring, or records, because these products
must be free of non-porous "fish eyes". These perhaps form more
easily in the large reactors. Perhaps, in the future, as the industry
gains confidence in these new reaction vessels these compounds too will
be made in the large kettles.
Historically, the PVC manufacturers turned to the large reactor
technology because of economics. These systems are considerably less
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labor Intensive though they require very sophisticated computer control
systems. Shinetsu also claims improvement in product quality, outstand-
ing homogeneity and porosity distribution that can be controlled over
a region twice as wide as that for conventional PVC, but not everyone
agrees. Large reactor technology is still "on the learning curve".
Because of the safety requirements of very large reactors, these
systems usually are strengthened with various backup systems such
as dual computer systems. For example, the Wacker-Chemie technology
utilizes a standby computer that is activated in case of prime computer
failure. In addition, in the case of power failure, standby generators
provide sufficient power to run the reactor agitators and a chilled
water pump (to prevent overpressure). A 200,000-gallon insulated
refrigerated water storage reservoir is maintained for a reactor cooling,
in case standby generator service is required.
Although the Americans are very enthusiastic about the large
kettle technology, some of the Europeans have questioned this approach.
For example, in the United Kingdom, as a result of the Flixborough
disaster, which involved the destruction of a caprolactam facility, the
British are very cautious about going to larger reactors and consequently
have no plans. In Belgium, on the other hand, the market situation is
such that only small reactors are economic. Nevertheless, based on
our review of technology worldwide, we expect that in future years
most new capacity will utilize large-reaction vessel technology.
Reactor Clean-up
The trend toward large reactors has also affected the technology
of reactor clean-up, which is another source of VCM emissions. As
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indicated in the previous section, the polymerization of vinyl
chloride is usually attendant with a build-up of polymer on the
walls of the reactor. In the past, this build-up was removed by hand
scraping. This process is not only unsafe to the worker involved, but
also increases manufacturing costs. At cleaninp time, a PVC reactor
can contain from 5 to 50 pounds of polymer, and the vessels are cleaned
after anywhere from three to five batches.
To overcome these problems the industry has developed various
automatic cleaning systems. Automatic cleaning offers a number of
advantages, in addition to reducing fugitive emissions, because the
reactor need not be opened very frequently. Automatic cleaning can in-
crease plant production. Whereas in old installations it would take one
worker about four to five hours to clean a 5,000-gallon reactor,
in the new plants with large reactors total time to clean the reactor
is about 45 minutes. Also, frequent and thorough cleaning can improve
product quality by eliminating one cause of "fish eyes" in the finished
polymer. These fish eyes are caused by the residual polymer on the
reactor walls that can serve as reaction sites for generating high-
molecular-weight material.
In the past, primarily two systems were available to the resin
producer for automatic cleaning: solvent systems, and high-pressure (H.P.)
water systems. Solvent systems were developed by DuPont, GAF Corp. and
Japan's Nippon Carbide Industries. DuPont's Tetra-Solve system is
}'
based on tetrahydrofuran (THF); the GAF system uses N-methyl-2-Jyrrolidone.
In the GAF system the solvent is sprayed into a reactor until it reaches
the agitator level, solvent is then recirculated until all of the polymer
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is dissolved. In DuPont's system the reactor is filled with THF, heated
to near its boiling point of 66°C, and recirculated. Both systems use
flash evaporation and high-viscosity, wiped-film evaporation (Artisan
Industries, Waltham, Massachusetts) for solvent recovery. Monsanto and
Quaker Oats also have THF cleaning systems.
Because of the high prices for these solvents and the problems of
pollution, few PVC manufacturers use these systems today, but a number of
manufacturers use high-pressure water systems, and it is becoming even
more popular. Some of these systems are operated manually and others are
very sophisticated with computer control. For example, Georgia Pacific's
new facility has a spray-nozzle system that is built directly into the
reactors, and the reactors are cleaned automatically using a computer-
programmed system.
Goodrich Chemical developed a stationary high-pressure pump and
portable mechanical cleaner that operates on city water. This is a portable
cleaning device, which can be installed in a few minutes. Typically, it
will clean a reactor in about 20 minutes and can be disconnected in another
five minutes. The cleaner, which operates at about 6,000 psi, automatically
cleans from four different positions to reach all areas of the reactor.
Polymer and water are removed from the bottom nozzle and discarded.
The automatic high-pressure water system is also used by Diamond
Shamrbck in the U.S.A. and Wacker Chemie in Germany. Some of these systems
use water at pressures up to 10,000 psi. The high-pressure water system
is used widely in Europe to clean-up conventional-size kettles and, in
Germany, in particular, there is a definite trend toward the use of the
high-pressure water system. Because the high pressure system is relatively
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new, some manufacturers still open the reaction vessels at the end of
polymerization reaction for inspection, but manual cleaning is very
infrequent once the automatic cleaning systems have been installed.
For example, Tenneco claims that no manual cleaning is required at their
new Pasadena plant.
But the newest approach toward kettle clean-up, which has been
touched off by the trend toward large reaction kettles, is the use of
defouling agents. In this approach the defouling agent, which
has not been identified, is usually sprayed onto the reaction vessel
walls to achieve clean-wall polymerization. However, we believe that in
some instances the defouling agents are incorporated into the polymeriza-
tion recipe.
This is very new technology and is under study by many companies
in the world, consequently we were unable to determine how successful
these defouling agents are with respect to maintaining a clean kettle.
We believe that companies using this technique are pleased but still
learning. As shown in Table 1. the Shinetsu technology which is also
used by Robintech and Tenneco, uses the defouling agent. Huels in
Germany and Kamanord in Sweden also use a defouling agent. These
agents are under study by Montedison and PVC manufacturers in France
and Belgium.
Stripping
To reduce the residual monomer content of the PVC resin, the
producer can either attempt to increase the conversion of monomer to
polymer or he can improve the effectiveness of the stripping step that
removes the residual monomer from the polymer. Both approaches have
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TABLE 1
MANUFACTURER
KETTLE SIZE
(gal)
LARGE REACTION VESSEL TECHNOLOGY
NO. OF KETTLES
TECHNOLOGY
KETTLE CLEAN-UP PROCESS
BORDEN
CONOCO
DIAMOND SHAMROCK
£ GEORGIA PACIFIC
HUELS
MONTEDISON
D
SHINETSU
TENNECO
WACKER CHEMIE
20,000
20,000
20,000
22,000
52,000
17,000
N.A.
ROBINTECH (SHINTECH) 35,000
35,000
35,000
22,000
N.A.
N.A.
Monsanto
Conoco
Diamond
Wacker Chemie
Huels
Montedison
Shinetsu
Shinetsu
Shinetsu
Wacker Chemie
N.A.
N.A.
Automatic H.P. Water
Automatic H.P. Water
Defouling Agent
N.A.
Defouling Agent
Defouling Agent
Defouling Agent
Automatic H.P. Water
N.A. = Not Available
'Source :r Indus trfr* Contacts.
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their limitations. For example, in suspension polymerization
conversion is limited to about 90%. The use of more effective catalysts
does not overcome this limitation. As noted in the previous section,
after 60 to 70% conversion, the PVC droplet is swollen with monomer. At
this point, the monomer diffuses into the active polymerization site
with difficulty. Agitation does not help this problem. Moreover, as the
polymerization approaches very high conversion, there is a tendency to
form low-molecular-weight polymer.
On the other hand, improvements in stripping technology appear to be
a more effective approach. All PVC resin producers are presently focusing
their efforts in this area of research. As mentioned above, it is more
difficult to strip PVC resin made by the emulsion and bulk processes, than
resin made by the suspension process and it is more difficult to strin
copolymers than homopolymers, and low-molecular-weight polymers than high-
molecular-weight ones. But the stripping studies are paying off, especially
in the case of suspension polymers. The industry is learning how to increase
the porosity of the PVC particles made by the suspension process so that the
monomer is released more effectively. However, the required modifications
of the polymerization recipe and reaction conditions can grossly affect the
properties of the finished resin. Therefore, these constraints limit
this approach to the manufacture of certain PVC grades. The PVC
manufacturers are also investigating the use of improved heat stabilizers
that are more effective in maintaining the properties of the polymers and
avoiding discoloration during the stripping step.
Suspension Systems. Historically, the manufacture of suspension
resins required 105 pounds of vinyl chloride monomer to make 100 pounds
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of PVC. Today, a new facility with improved stripping technology
can make 100 pounds of PVC from 102 pounds of monomer. A Montedison
representative indicated that eventually they might be able to improve
the efficiency of stripping and recycling so that only 100.5 pounds
of monomer would be required to make 100 pounds of PVC.
In this country, one of the resin producers indicated that in the
past the suspension polymer still had 5,000 ppm (dry basis) of monomer
after stripping. Today, with improvements in their stripping process
they can reduce the residual monomer in the slurry to 1,000 ppm (dry
basis). With further improvements, they hope to obtain even lower
values.
A number of the PVC resin producers have developed new stripping
processes for the suspension-system. Montedison, for example, uses
a high-speed blender with a vacuum takeoff for monomer removal. Air
Products and Chemicals also has an improved but proprietary stripping
process .that, reportedly, provides rapid and complete monomer removal at
high temperatures without resin discoloration.
Recently, Goodrich Chemical Company installed a continuous stripper
in their Long Beach, California plant. Goodrich plans to have continuous
strippers in all of their plants by the end of 1975. In the Goodrich
process the resin slurry is fed into the stripping column countercurrent
to steam that is fed to the bottom of the column. The steam rises
to the top of the column and picks up VCM on the way. This VCM is
then recovered for reprocessing, and the stripped slurry is removed
at the bottom of the column. Goodrich1s process is limited to
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suspension resins and apparently it does not work very well
with copolymer resins, due to the lower softening point of these
materials. As with other stripping processes, the effectiveness
depends on the porosity of the polymer particles.
The continuous process is still in development, though the results
to date have been very promising. Stripping time has been reduced from
hours to minutes, and the continuous process avoids both shear degradation
and agglomeration of the resin particles. Goodrich claims that with
the continuous stripper VCM emissions during drying will be at a low
leve]. Emissions are expected to be in compliance with the forthcoming
EPA regulations.
Furthermore, this continuous stripping process reduces the residual
monomer content of the -final resin product to such low levels that
processors of .this resin may not require regulated areas for handling
these resins as defined by the OSHA standards that cover worker
exposure to VCM. The Goodrich stripping technology is being offered to
other PVC producers under licensing arrangements.
Emulsion Resin. Stripping emulsion polymer is another matter.
In the past, a typical latex after stripping (but before drying) contained
as much as 25,000 to 27,000 ppm of monomer (dry basis). Today, though
the stripping system has been improved, the residual monomer content
of these particular emulsion polymers after stripping is still high. Only
two or three manufacturers today (1975) can strip the polymers below 2000 ppm.
At this time many resin producers are concerned whether they will be
able to meet the proposed EPA regulations. Tenneco has mentioned that
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they have a pilot plant stripper for emulsion polymer that looks promising,
and others are evaluating new hardware that will hopefully improve the
effectiveness of the stripping of emulsion polymer. For example,
Kenics Corporation has a proprietary mixer that may be useful for the
stripping step. This mixer is unique; it has no moving parts and the
pressure drop across the mixer is very low and it has very good heat
transfer characteristics. A few PVC resin producers are evaluating
this and similar hardware. The European PVC resin producers are also
diligently investigating new and improved techniques for stripping the
slurry from suspension resins and the latex from emulsion resins. At this
time (June, 1975), the Europeans claim that they are unable to meet
the proposed standards for emissions of VCM in the manufacture of
emulsion resins.
Other Technology.
Fluid-bed System. In recent years industry and academia have
investigated polymerizing vinyl chloride in the gaseous phase. For
example, the Technical Institute for Chemistry in Munich recently
reported (National Meeting of the American Chemical Society, Spring
1975) that they were able to polymerize gaseous vinyl chloride by
means of a solid carrier that contains free-radical initiators. The
carrier is prepared by bringing the initiator on the surface of
powdery PVC, and the polymerization is conducted in a stirred reactor
containing these "seeded" particles and gaseous vinyl chloride. Some
results indicate that this polymerization occurs within the polymer
phase, which contains dissolved monomer and initiator. Industry also
has been exploring this approach. Both Goodrich Chemical and Montedison
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have set up dry fluid-bed systems based on seeded PVC and gaseous VCM.
At this time, neither company is planning to commercialize this
system. They believe the system is extremely complicated and, furthermore,
the resulting PVC particle is actually not as porous as the PVC
made by present suspension technology and therefore is expected to
be more difficult to strip. Consequently, the fluid bed approach is not
one that can have a significant effect in reducing VCM emissions.
In-Kettle Compounding. Normally, PVC resins are compounded (blended
with other ingredients) after the resin has been made. However, recently,
because of the favorable economics, Robintech began manufacturing
rigid compound for pipe fabrication using in-kettle compounding (IKC).
In this process, stabilizers, lubricants, and at least one other
additive (pigments and/or polymer modifiers) are added to the aqueous media
that contains the suspending agent, and vinyl chloride is added and
polymerized. This technology was originally developed by Allied
Chemical (U.S. patent 3,862,066). Though this technology, as presently
practiced, does not reduce emissions of VCM, it does reduce the costs
of compliance with the OSHA standards and the proposed EPA standards.
According to Robintech, this approach has reduced their compliance costs
by one third, or one fourth. Typically, this IKC resin has a residual
monomer content below 10 ppm.
In about twelve months Robintech expects to install equipment
in their Painsville plant that will further reduce the residual monomer
of the stripped slurry from the suspension polymerization used to
make the IKC resin by a factor of five or ten. The process has been
demonstrated on a production scale, but it is only applicable to IKC resins.
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Though the present IKC compounds usually contain 92% PVC, research
is now underway at Roblntech to make a variety of other rigid compounds
and perhaps even flexible compounds. But, based on our survey of PVC
resin producers, few have any interest in the IKC process, because it
has limited utility industry wide. Robintech is in a special position,
because it is integrated forward to PVC pipe manufacture.
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CONCLUSIONS
Based oh our review of worldwide PVC technology, it appears that
the Pechiney process of bulk polymerization was the last breakthrough.
Since then, advances have been slow and have been prompted primarily by
an interest in lowering manufacturing costs. This has prompted the
present trend toward very large reaction kettles. However, large
kettles are not always practical or economical, depending upon the
manufacturer's marketing position, plant layout, and operations.
i
But we expect new advances in PVC technology in the near future, as
a result of the present intensive research effort by the industry to
develop superior processes for stripping monomer from the water slurries
or emulsions after polymerization. Manufacturers are now investigating
(1) ways of modifying polymerization recipes, (2) conditions to increase
the porosity of the resin particles, and (3) the effectiveness of
stripping. But, it appears that they are having greatest success in
developing improved mechanical processes for removing residual monomer.
In general, most of the PVC resin producers believe that they will
be able to meet the proposed emission standards for suspension resins,
but they question their ability to meet the standards for all emulsion
polymers. Hopefully, within the next two years, they will be able to
advance stripping technology for emulsion polymers to meet the proposed
EPA standard.
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