United States
Environmental Protection
Agency
National Risk Management
Research Laboratory
Cincinnati, OH 45268
Research and Development
EPA/600/SR-95/138 August 1995
&EPA Project Summary
Advanced Composites
Technology Case Study at NASA
Lang ley Research Center
Kenneth R. Stone and Johnny Springer Jr.
This report summarizes work con-
ducted at the National Aeronautics and
Space Administration's Langley Re-
search Center (NASA-LaRC) in Hamp-
ton, VA, under the U.S. Environmental
Protection Agency's (EPA) Waste Re-
duction Evaluations at Federal Sites
(WREAFS) Program. Support for this
study was provided by the Strategic
Environmental Research and Develop-
ment Program (SERDP). SERDP is a
cooperative effort between DoD, DOE
and EPA to develop environmental so-
lutions that enhance mission readiness
in defense operations.
The purposes of the WREAFS Pro-
gram are to identify new technologies
and techniques for reducing wastes
from process operations and other ac-
tivities at Federal sites, and to enhance
the implementation of pollution preven-
tion/waste minimization through tech-
nology transfer. New techniques and
technologies for reducing waste gen-
eration are identified through waste
minimization opportunity assessments
and may be further evaluated through
joint research, development, and dem-
onstration projects.
Under the Chesapeake Bay Agree-
ment, NASA-LaRC is a member of the
Tidewater Interagency Pollution Preven-
tion Program (TIPPP). At NASA-LaRC,
a technique for producing advanced
composite materials without the use of
solvents has been developed. This as-
sessment was focused on the produc-
tion of non-refractory composite
materials and aircraft structures made
from those materials.
This Project Summary was developed
by EPA's National Risk Management
Research Laboratory, Cincinnati, OH,
to announce key findings of the re-
search project that is fully documented
in a separate report of the same title
(see Project Report ordering informa-
tion at back).
Introduction
To produce composites, fiber tow
bundles are impregnated with a polymer
resin-a process called "prepregging"-in
order to produce a composite towpreg
which can then be fabricated into com-
posite material products. There are sev-
eral technologies available to do this,
solution prepregging being among the most
common. In solution prepregging, the poly-
mer resin is placed in a solvent carrier
and applied to the tow. The liquid polymer
has a limited shelf life and must be refrig-
erated.
At the NASA LaRC, the Polymeric Labo-
ratory has developed a dry powder
prepregging process in which the fiber
strands are separated in a small air cham-
ber and a finely-powdered polymer resin
is "dusted" onto them. The polymer dust
fully impregnates the fibers just before
being passed through a furnace. NASA
refers to this process as "dry powder
towpregging." Later, the towpreg can be
formed into a laminate. Two goals of the
dry powder towpreg process are to elimi-
nate the use of solvents, and reduce en-
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ergy consumption because, in dry powder
form, polymer resins do not require refrig-
eration.
For the purposes of this case study,
EPA assessed the attributes of dry pow-
der towpregging with those of solution
prepregging. For comparison, NASA-LaRC
provided information on the consumption
of methyl ethyl ketone to manufacture ther-
moset composites and on the usage of n-
methyl-pyrrolidone for thermoplastic
materials.
Project Description
Plastic composites exhibit properties that
make them attractive alternatives for other
materials and metal alloys in a variety of
applications both public and commercial.
Weight and advantageous mechanical
properties at elevated temperatures make
advanced composites desirable for many
aerospace uses. The cost of manufactur-
ing is a major concern, as the process
tends to be costly and laborious. The high
viscosity of polymer melts and solubility
limitations of polymers in solvent solutions,
along with storage limitations of prepregs,
have limited the use of both the "hot melt"
and "solution" prepregging processes.
To evaluate the NASA-LaRC process,
a direct comparison with conventional so-
lution prepregging was made. A brief de-
scription of each process is provided
below.
Solution Prepregging
In the form of solution prepregging
tested, the continuous fiber tow is spread
and rolled through a bath of polymer resin
suspended in a volatile solvent carrier.
The tow is pressed and passed through
ovens to extract most of the solvent and
bond the polymer to the tow fibers, gener-
ating VOC emissions. In order to keep
equipment clean and keep product quality
consistent, wax paper is used to prevent
the impregnated tow from sticking to the
rollers. This process is illustrated in Fig-
ure 1.
This process produces a composite tow
"ribbon," which has to be refrigerated to
prevent degradation of the polymer mate-
rials. To finish the process, the laboratory
conducts "B-staging," wherein the ribbon
reels are taken out of refrigeration, unrav-
eled and passed through a final oven in
order to bake out the residual solvent.
Dry Powder Prepregging
The dry powder process developed by
NASA-LaRC, is shown in Figure 2. NASA-
LaRC applies dry powder resin particles
to the fiber tow by means of a gravity feed
via a screw-type auger drawing out the
polymer material from the hopper. The
tow fibers are spread by passage through
an air chamber just before being "dusted"
by the gravity feed. Coated with the pow-
der the fiber tow is directed into an oven
by horizontal rollers that also help to
spread the resin across the tow. The tow
is passed through the oven, flipped over
and directed back by a vertical roller. The
underside is coated by a second gravity
feed, and the tow enters the oven for a
second heating cycle. The tow is collected
on a take-up spool and can then be stored
at room temperature until needed to make
a laminate.
Because solvents are not added to the
material, B-staging and refrigeration steps
are unnecessary. Wax paper usage is
eliminated for prepregging. The process
has the potential to eliminate VOC emis-
sions, reduce energy usage, and reduce
solid waste.
Project Assessment
In order to conduct pilot-scale compos-
ites research, NASA-LaRC constructed
both a solution prepregging process line
and the dry powder towpregging line
onsite. NASA-LaRC provided information
and experience from running these pilot-
scale production lines to EPA for this study.
Estimates of environmental and energy
impacts data were included. The study
includes an estimate of solid waste in the
form of waxed release paper and waste
composite tow. Table 1 provides a sum-
mary of estimated average operating con-
ditions during the test runs.
A flow chart comparison of solution
prepregging with dry powder towpregging
is illustrated in Figure 3. Each process
begins with the prepared polymer resins
and fiber tow, continuing through to the
fabrication of a composite ribbon lami-
nate. Information on the fabrication of fi-
ber tow and polymer material were
excluded from the study, because identi-
cal fibers and polymers were used in both
processes. Also, the disposal of laminate
was excluded.
The study also included economic data
as an additional determinate of the feasi-
bility of the process. Please note that all
tables and results are based on a pro-
jected yearly production rate of 7,700,000
lin ft of 3 1/2" width composite ribbon for
the dry powder prepregging line. The pro-
duction rate estimate used for the solution
line is 240,000 lin ft of 3 1/2" width com-
posite ribbon. It is important to note that
the dry process line speed is 70 ft/min,
while the solution process speed is only 2
ft/min. The production rates used repre-
sent the maximum capacity of each NASA-
LaRC production line to produce a com-
parable product.
Environmental Impacts
VOC emissions would be eliminated by
the dry powder towpregging, because no
VOC-generating materials are used in the
process. The reduction of VOC emissions
for the epoxy would be much greater than
the thermoplastic, given the fact that MEK
is significantly more volatile than NMP.
With MEK as a common solvent in many
prepregging operations, this level of re-
duction can generate significant cost sav-
ings in terms of environmental control
equipment and maintenance.
Solid waste in dry powder towpregging
is virtually eliminated. Again, this is be-
cause the waxed paper, heavily used in
solution prepregging, is not required when
solvents are eliminated. NASA-LaRC en-
gineers indicated that, in their solution
prepregging line, waxed release paper con-
tributes 112.5 Ib of solid waste for every
1000 ft of processed tow. Another consid-
eration is that the release paper could
become contaminated with organic sol-
vents, complicating their proper disposal.
Energy Impacts
Energy consumption by the dry powder
process runs at about 2/3 the rate of con-
sumption by the solution process. Dry pow-
der towpregging was calculated to
consume 40,000 KwH/yr, while the solu-
tion process would consume 60,000 KwH/
yr. As noted in Table 1, power consump-
tion of the dry process is expected to be
less than one-fifth that of the solution pro-
cess. However, in order to meet the pro-
jected yearly production rate noted above,
the bench line would have to be scaled up
to handle 15 tows simultaneously. The
scale-up was calculated to raise energy
consumption to 2/3 that of the solution
prepregging line.
In order to maintain conservative esti-
mates, energy consumption by the refrig-
erators in the solution prepreg process
was not included in the study. Size and
efficiency of such units could vary widely
and it is expected that some manufactur-
ers might use such equipment for a vari-
ety of purposes beyond prepregging.
However, for a producer equipped to pro-
cess dry towpreg, it may be assumed that
energy consumption would fall below 60%
of the rate consumed by a comparable
solution process.
Economic Feasibility
The total capital cost of a dry powder
line was estimated at $402,700, which
can be compared to the reported $650,000
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Air & solvent
vapor to exhaust
or recovery
Air & solvent
vapor to exhaust
or recovery
Top Top
paper paper
on off
1Cf Product
Product
Reverse roll
coaler
Air or
inert gas
Bottom
paper on
Figurel. Solution prepegging system.
Powder
curtain
feeder
Oven
Take-up
spool
Figure 2. NASA Langley dry powder towpregging system.
Powder
curtain
feeder
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Solution Process
Dry Powder Towpreg
Liquid Polymer
(Refrigeration)
Carbon |
Fiber Prepregging
Sizing (Solvent Removed)
Refrigeration
I
B-Staging
(Final Solvent Removed
Composite Ribbon Product
Dry Polymer
Carbon f
Fiber Towpregging
(Unsized) I
Composite Ribbon Product
Figure 3. Process flow diagram.
Table 1. Estimated Average Operating Conditions
Polymer
Polymer Type
& Process
Epoxy, Dry
Epoxy, MEK
Polyamide, Dry
Polyamide, NMP
Tow
Speed
ft/min
40.0
1.3
70.0
2.0
Total
Tow
ft
5080
2535
8050
3375
Oven
Temp.
OC
300
205
190
71
Feed
Rate
gr/min
3.0
4.4
3.3
4.8
Paper
Usage
ft2
0
670
0
880
Power
Usage
Kw
5
27
5
27
cost of the solution prepregging equip-
ment at NASA-LaRC. Even if we grant a
50% error in the estimate, the cost of the
dry process will be less than the cost of
the solution line. Also, due to the higher
line speed, the dry powder line will pro-
duce approximately 32 times more prod-
uct annually.
Table 2 shows the cost of a 3 1/2 in.
ribbon by the solution process to be $1.647
ft while the dry process cost is $.31/ft.
These calculations are based on assump-
tion, that the dry process could be engi-
neered to run 15 tows simultaneously to
produce the same composite ribbon as
the solution process. Again, the produc-
tion rate for the dry process is dramati-
cally higher due to the line speed.
Conclusions
When compared to solution prepregging,
the NASA-LaRC dry powder prepregging
process appears to eliminate VOC emis-
sions and significantly reduce solid waste
in prepregging operations. It is also pro-
jected to be capable of providing signifi-
cant reductions in energy consumption at
the operational level. Continued study will
be necessary to determine how well a dry
powder derived laminate performs in com-
parison to a solution derived laminate.
Performance qualities in such areas as
strength of coated fibers, impact resis-
tance, and shear strength will have to be
evaluated in order to determine the
product's suitability as a substitute for the
commercial product.
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Table 2. Production Cost Estimate
Dry Powder Process
Raw Materials
Fiber Tow (12K)($25/lb, = 1750 ft)
Epoxy Powder (AMD0036)($95/lb)
Labor
1 Operator ($20/hr x 2000 hr)
1 Assistant ($14/hrx 2000 hr)
Utilities (40,000 KwH/yrx$. 10/KwH)
Rent (1500 ft2 x$12/ft2/yr)
Depreciation (3yrlife, $402.7K Capital Cost)
Total Annual Cost
Cost/ft (7,700,000 ft/yr) = $.31
Est. Cost
$1,800,000
380,000
40,000
28,000
4,000
18,000
134,000
$2,404,000
Solution Prepregging Process
Raw Materials
Fiber Tow AS-4 (12K)($25/lb, = 1750ft)
Epoxy Powder (AMD0036)($95/lb)
Methyl Ethyl Ketone ($9.70/gal, 6.81 Ib)
Waxed Release Paper
Labor
1 Operator ($20/hrx2000 hr)
1 Assistant ($14/hrx 2000 hr)
Utilities (60,000 KwH/yrx$. 10/KwH)
Rent (2000 ff x$12/ft2/yr)
Depreciation (Syrlife, $650.0K Capital Cost)
Total Annual Cost
Cost/ft (240,000 ft/yr) = $1.64
Est. Cost
51,400
78,200
800
36,000
40,000
28,000
6,000
24,000
130,000
$394,000
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Kenneth R. Stone and Johnny Springer Jr. are with the U.S. EPA National Risk
Management Research Laboratory, Cincinnati, OH 45268
The complete report, entitled "Advanced Composites Technology Case Study
at NASA Langley Research Center," (OrderNo. PB95-264172; Cost:
$17.50, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
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