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Electron-Beam Curing of Polymeric Composites
As An Enabling Technology
for Advanced Manufacturing
E-BEAM Services, Inc.
32 Melrich Road
Cranbury, NJ 08512
as
presented at:
The International Composites Expo
’99 (ICE’99)
Cincinnati, OH
May 10-13, 1999
ABSTRACT
Electron-beam (EB) composite curing of adhesives and resins is an emerging technology which has the potential to provide significant advantages for rapid manufacturing of a variety of composite structures and components for aerospace, automotive, and consumer applications. Traditional thermal curing methods necessitate long cure times, involve high energy consumption and volatile toxic by-products, create residual stresses in the materials, and require expensive tooling capable of withstanding high autoclave temperatures. E-beam curing is a fast non-thermal process which utilizes highly energetic electrons at controlled doses to polymerize and cross-link polymeric materials. EB curing of composites can take place at both ambient and sub-ambient temperatures. The advantages of this technology over the conventional autoclave curing process include significantly reduced curing time leading to order-of-magnitude improvements in throughput, ability to utilize low-cost tooling materials, reduced volatiles emissions, reduced energy consumption, and control over curing energy-absorption profile. EB cured materials have been shown to posses excellent mechanical properties, with low molded-in stresses, high glass transition temperatures (Tg) and low void content. These materials demonstrate excellent property retention after thermal or cryogenic cycling. This paper examines the recent applications of e-beam curing technology to several fabrication processes, including conventional pre-preg lay-up, resin transfer molding (RTM), vacuum-assisted resin transfer molding (VARTM), and filament winding. Developments in the area of EB-curable resin formulations and adhesives are also discussed.
INTRODUCTION
Polymer
matrix composite (PMC) materials have demonstrated clear-cut advantages
in advanced performance at decreased weight over more conventional
metallic materials in a number of demanding aerospace, automotive,
infrastructure, and consumer applications [1,2]. A major cost issue in
manufacturing of PMC structures
and parts is the conventional thermal cure process [3]. Additionally, the
autoclave-based thermal cure processing requires long cure times,
involves high energy consumption and volatile toxic by-products, creates
residual stresses in the materials, and necessitates the use of expensive
tooling capable of withstanding high autoclave temperatures [4]. It has
been suggested that decreasing the manufacturing costs is a key step in
increasing the overall usage of PMC’s [3,5].
Electron-beam
(EB) curing represents a significant technological advancement with
respect to the thermal cure deficiencies mentioned above. EB curing is a
rapid non-autoclave process which utilizes high-energy electrons
generated by an industrial accelerator at controlled doses to initiate
polymerization and crosslinking reactions in suitable material
substrates [6]. A number of benefits have been identified for EB curing
of PMC’s, as compared to thermal
curing [7].
The
purpose of this paper is to provide a brief introduction to EB
processing, as well as to highlight selected recent applications of EB
curing technology to several established composite fabrication processes.
ELECTRON
BEAM PROCESSING
Ionizing
radiation is a distinct and efficient means of bringing about controlled
beneficial changes in polymeric systems. These beneficial changes include
increases in modulus, tensile and impact strength, hardness, deflection
and service temperature, stress-crack resistance, abrasion resistance,
creep and fatigue resistance, and barrier properties [8]. It has been
observed that radiation processing requires a lower expenditure of energy
than the conventional thermochemical processes [9].
In the case of
electron-beam irradiation, highly energetic electrons strike at or near
the carbon-hydrogen bonds in target molecules, and give up enough energy
to the molecules to break some of the bonds, releasing hydrogen, and
leaving the molecules with excited carbon atoms (free radicals). When
this process occurs at two adjacent molecules or nearby sites, excited
carbon atoms can release excitation energy forming a chemical bond, known
as a cross-link, between them. The amount of electron beam radiation
absorbed by the target is referred to as the dose, which is typically
defined in terms of kiloGrays (where 1 kGy=1000 J/kg) or MegaRads (where
1 MRad=1,000,000 erg/g) [10]. The degree or efficiency of crosslinking
depends on certain complex secondary chemistries involving, among others,
polymer additives, including radiation promoters, initiators,
stabilizers, etc.
Industrially, EB processing
is performed using medium to high energy (2.5 to 10 MeV), high power (50+ kW)
commercially-available accelerators, which are equipped with a variety of
material handling systems, and are capable of significant throughput
[11]. A typical direct-current accelerator consists of the voltage
generator, the electron gun, the
accelerator tube, the scan horn, and the control system [10]. This
accelerator creates a beam of electrons approximately 1 inch in diameter and energizes
it to near light speed. The beam passes through a scan horn, where a
magnet scans it back and forth at ca. 200 Hz, creating a curtain of
electrons 4-6 feet wide [10]. Target materials are passed under the scan
horn using conveyors, carts, reel-to-reel equipment, or other specialized
handling means. Worldwide, there are approximately 700-800 electron beam
accelerators in industrial use today [12].
With
respect to autoclave processing, EB curing of PMC’s is a considerably more energy efficient process. As
illustrated in Table I, a commercially-available 50 kW accelerator
delivering a cure dose of 100 kGy at 70% beam utilization uses roughly
one-tenth of the energy of a typical autoclave or oven running a 4 hour
thermal cure cycle [13].
TABLE I – RELATIVE ENERGY
EFFICIENCY OF EB CURING
|
Equipment Type
|
Equipment Specification
|
Total Energy (kW-hr)
|
Capacity
(kg/hr)
|
Energy Efficiency
(kW-hr/kg)
|
|
Autoclave
|
12.2 m length
2.4 m diameter
|
480
|
273
|
1.76
|
|
Autoclave
|
15.2 m length
7.6 m diameter
|
7660
|
2730
|
2.81
|
|
10 MeV EB
|
50 kW
|
400
|
1800
|
0.22
|
EB-CURABLE
RESINS
BACKGROUND
Historically,
a number of resin systems have been EB cured with mixed results. For
example, oligomers containing acrylate and methacrylate groups, as well
as those containing double bonds, such as dienes and vinyls have all been
reported as being EB-curable. However, the performance of these materials
has been characterized by low glass transition/service temperatures, low
modulus, high moisture absorption, and high shrinkage upon curing [4].
A
1992 European patent described a process for EB curing of bismaleimides
at room temperature, and claimed glass transition temperatures (Tg) of
over 300 oC [14]. Subsequently, a U.S. company had developed a
series of acrylated epoxies and acrylated bismaleimides, which while
demonstrating better properties than those of the equivalent European materials, still failed to meet the
end-user requirements [4]. Another patented curing system features
certain highly reactive cycloaliphatic silicone epoxies which are
EB-curable with the aid of certain cationic initiators, and have reported
Tg of about 190 oC [16].
RECENT DEVELOPMENTS
A considerable materials
development effort was undertaken from 1994 to 1997 as part of a
Department of Energy (DOE) Defense Programs Cooperative Research and
Development Agreement (CRADA), which involved the contributions from
two DOE national laboratories and
ten industrial participants, including three advanced materials companies
and three EB processing organizations. While the greater stated purpose
of the CRADA was to advance general understanding and utilization of EB
curing for the manufacture of PMC parts, its major achievement proved to
be the successful development of cationic epoxy resin formulations
suitable to EB curing that could also meet the demanding requirements of
high-performance composite structures [4].
The
CRADA participants successfully developed and evaluated hundreds of new,
toughened and non-toughened EB-curable epoxy resin systems. Carbon fiber
composites manufactured using these systems were found to match favorably
the performance of conventional thermally-cured composites and were
typically characterized by low void contents, low water absorption
values, high glass transition temperatures, and superior property
retention upon cryogenic and thermal cycling [15]. The typical properties
of selected EB-curable epoxy formulations are presented in Table II [4,7].
TABLE II – PROPERTIES OF SELECTED
EB-CURABLE RESINS
|
Property
|
8H
|
10H
|
9H
|
3K
|
1K
|
|
Resin Shrinkage (%)
|
3.5
|
4.0
|
3.4
|
3.0
|
3.3
|
|
Tg (oC)
|
396
|
393
|
232
|
212
|
212
|
|
Water Uptake (%)
|
4.90
|
2.8
|
2.0
|
2.90
|
1.88
|
|
Density (kg/m3)
|
1260
|
1237
|
1225
|
1224
|
1228
|
APPLICATIONS
PRE-PREG LAYUP
Several toughened EB-curable resin formulations
have been evaluated for carbon fiber PMC applications. Table III
summarizes the data collected for unidirectional test panels utilizing
IM7-GP-12K carbon fiber unidirectional pre-preg which was fabricated by a
melt process at ca. 70 oC. The panels were prepared using
conventional lay-up techniques and then processed in multiple passes at a
10 MeV EB facility under vacuum bag pressure to total doses ranging from
150 to 250 kGy, at a dose rate of 50 kGy per pass [4]. Commercially
available thermally-curable thermoplastic toughened aerospace epoxy resin
977-3 specifications are presented for comparison purposes.
Table III – COMPARATIVE PROPERTIES OF
EB-CURED AND THERMAL CURED UNIDIRECTIONAL LAMINATES
Resin System
|
977-3
|
8H
|
10H
|
9H
|
3K
|
|
Cure
Conditions
|
3 hrs. at
355 oF/85psi
|
250 kGy
|
150 kGy
|
150 kGy
|
150 kGy
|
|
Void Volume (%)
|
Not Reported
|
1.77
|
0.72
|
1.24
|
0.64
|
|
Tg (oC, Tan Delta)
|
190/240
|
396
|
392
|
232
|
212
|
|
0o Flexural Strength (MPa)
|
1765
|
1986
|
2006
|
1793
|
1765
|
|
0o Flexural Modulus (GPa)
|
150
|
196
|
163
|
163
|
154
|
|
0o Interlaminar Shear Strength (MPa)
|
127
|
77
|
79
|
79
|
89
|
FILAMENT
WINDING
Similarly, a number of epoxy resin-initiator formulations
with low ambient viscosities have been evaluated for suitability
in filament winding applications. IM7-GP-12K carbon fiber
was used to produce hoop wound composite cylinders with
a diameter of 15.24 cm and thickness of 0.3175 cm at room
temperature. Two commercially available thermally-curable
epoxy resins were used as controls. Representative specimens
were also cryogenically and thermally cycled a total of
three times as per the following regime: -194 oC
for 30 min., ambient for 30 min, 121 oC for 30
min., ambient for 30 min. The comparative property data
is presented in Table IV [4].
Table IV – COMPARATIVE PROPERTIES OF
EB-CURED AND
THERMAL CURED FILAMENT WOUND LAMINATES
Resin System
|
ERL-2258
|
Tactix-123
|
34B
|
|
Cure
Conditions
|
3 hrs. @ 121 oC
3 hrs. @ 150 oC
4 hrs. @ 177 oC
|
3 hrs. @ 121 oC
3 hrs. @ 150 oC
4 hrs. @ 177 oC
|
150 kGy
|
|
Tg (oC, Tan Delta)
|
218
|
165
|
192
|
|
0o Tensile Strength (MPa)
|
1986
|
1538
|
2358
|
|
0o Tensile Strength Cycled (MPa)
|
2379
|
1565
|
2337
|
|
90o Flexural Strength (MPa)
|
68.3
|
57.2
|
70.3
|
|
90o Flexural Strength Cycled (GPa)
|
77.2
|
64.1
|
77.9
|
|
0o Interlaminar Shear Strength (MPa)
|
57.2
|
47.6
|
73.8
|
|
0o Interlaminar Shear Strength
Cycled (MPa)
|
56.5
|
46.9
|
82.7
|
RTM/VARTM
RTM
can reduce costs as compared to labor-intensive hand lay-up. However,
since it still requires high-temperature thermal curing, tooling for RTM
is often made from expensive metal alloys [1]. Utilization of a
non-thermal curing technology, such as EB, has recently been demonstrated
to allow the use of inexpensive tooling and bagging, such as low cost
thermoplastics, foams, wood, etc. [1]. A year ago, in separate papers
presented at the 43rd International SAMPE Symposium, Vastava
and Roylance, et al described efforts to combine EB curing and RTM/VARTM
technologies in order to enable cost effective production of large
assemblies and structures for the aerospace industry [1,3]. In both
cases, the efforts were successful and served to clearly demonstrate that
the use of EB curing to produce large military or aerospace components
was not limited by any inherent material or process limitations.
In the case of VARTM
of a carbon-fiber composite flat-braided intersection, the vacuum-bagged
part was infiltrated immediately prior to EB exposure. Resin system 34B
was used. The part was then cured utilizing a commercial 10 MeV
accelerator to a total dose of 150 kGy in increments of 25 kGy per pass.
The Tg of the finished part was determined to be 160 oC. It
was estimated that the overall cost to manufacture a flat intersected
braided demonstration part was nearly 100 times less utilizing
inexpensive polyurethane foam tooling and EB curing, as compared to
conventional thermal curing which necessitates the use of two-part metal
tooling for closed mold RTM [1].
CONCLUSION
EB
curing of PMC’s is an emerging technology which has the potential to
provide significant advantages for rapid and cost-effective manufacturing
of a variety of structures and components for aerospace, automotive, and
consumer applications. As detailed in the text above, considerable recent
efforts have been expended to increase the understanding and utilization
of EB curing for the production of PMC parts. A number of EB-curable
resins have been developed and evaluated. Application of EB curing to a
number of widely used fabrication methods has been attempted with a high
degree of success, and at considerable economic advantage. Thus, it can
be suggested that with continuing rapid advances in this enabling
technology, we can expect to arrive in the future at a point when the use
of EB curing would become more common than thermal curing, resulting in
lower production costs and increased utilization of composites in
general.
REFERENCES
1.
Roylance,
M.E., C.J. Janke, and J.M. Tuss; 43rd
International SAMPE Symposium, 43:1660-1671 (1998).
2.
Goodman,
D.L. and C.A. Byrne; 43rd
International SAMPE Symposium, 43:1691-1701 (1998).
3.
Vastava,
R.B., et al.; 43rd
International SAMPE Symposium, 43:1681-1690 (1998).
4.
Janke,
C.J., et al.; May 1997. “Electron Beam Curing of Polymer Matrix
Composites,“ Final CRADA Report, Lockheed Martin Energy Systems for the
U.S. Department of Energy, Oak Ridge, TN.
5.
Anonymous;
High-Performance Composites,
6:41 (1998).
6.
Singh,
A. and C.B. Saunders; “Radiation Processing of Carbon Fiber-Acrylated
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Oxford University Press (1992).
7.
Saunders,
C.B., V.J. Lopata, and W. Kremers; September 1997. “Electron Curing of
Composite Structures for Space Applications” at Electron Beam Curing of Composites Workshop. Oak Ridge, TN.
8.
Gehring,
J and A. Zyball; Radiation Physics
and Chemistry, 46:931-936 (1995).
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Ivanov,
V.S.; Radiation Chemistry of
Polymers. Utrecht, The Netherlands: VSP BV (1992).
10.
Bly,
J.H.; Electron Beam Processing.
Yardley, PA: International Information Associates (1988).
11.
Minbiole,
P.R.; Radiation Physics and
Chemistry, 46:421-428 (1995).
12.
Machi,
S. Radiation Physics and Chemistry,
46:399-410 (1995).
13.
Knobel,
T.M.; September 1996. “Electron Beam Facilities Operation: Contract and
Dedicated Composite Curing” at Electron
Beam Curing of Composites Workshop. Oak Ridge, TN.
14.
Beziers,
et al.; Eur. Pat. Appl. Publ. No. 499,542A1 (1992).
15.
Farmer,
J.D., C.J. Janke, and V.J. Lopata;
43rd International SAMPE Symposium, 43:1639-1646 (1998).
16.
Crivello,
J.V. U.S. Patent 5,260,349 (1993).
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