Radiation Effects on Polymers - American Chemical Society

Chapter 15

Electron-Beam Curing of Aramid-FiberReinforced Composites 1

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1

1

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C. B. Saunders , A. Singh , V. J. Lopata , S. Seier , G. D. Boyer , W. Kremers , and V. A. Mason 1

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1

Whiteshell Laboratories, AECL Research, Pinawa, Manitoba, Canada Chalk River Laboratories, AECL Research, Chalk River, Ontario, Canada

Downloaded by FUDAN UNIV on March 23, 2017 | http://pubs.acs.org Publication Date: November 12, 1991 | doi: 10.1021/bk-1991-0475.ch015

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Electron-beam (EB) processing of aramid fiber-reinforced composites uses ionizing radiation, specifically high-energy electrons or X-rays produced by an electron accelerator, to initiate polymerization and cross-linking reactions in suitable matrices, curing the polymer and enhancing its specific physical and chemical properties. Many benefits have been identified for using EB processing for composites rather than thermal curing, including ambient curing temperatures, reduced curing times, and improved material handling and resin stability. EB-curable matrices have been developed with properties comparable to thermally cured epoxies and polyesters. This paper examines the effect of irradiation dose and dose rate on the curing of acrylated epoxies and discusses selected properties of aramid fabric-acrylated epoxy laminates.

Aramid or aromatic polyamide fibers are used to reinforce several polymer matrices, producing composite materials with high tensile strengths and moduli, fatigue resistance, damage tolerance and thermal stability (1). Applications for aramid fiber-reinforced composites are found in several industries, including aircraft and aerospace, military, marine and sporting goods (2). Electron beam (EB) processing of composites involves using electrons as ionizing radiation to initiate polymerization or cross-linking reactions in suitable substrates, thereby enhancing specific physical and chemical properties. Industrial EB processing of plastics and composites has been an established industry for 45 years (3). Currently, there are about 600 EB accelerators being used worldwide to process plastics (4).

0097-6156/91/0475-0251S06.00/0 © 1991 American Chemical Society

Clough and Shalaby; Radiation Effects on Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

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RADIATION EFFECTS ON POLYMERS


The benefits of EB curing fiber-reinforced products, include (5): 1. Curing at ambient temperature. Tooling/mandrel materials with appropriate thermal expansion coefficients must be used in composite processing methods to produce a component to strict dimensions while minimizing internal stresses. The tool expands and contracts during the thermal curing cycle, often at a different rate than the molded composite. These movements can alter the dimensions and produce excessive internal stresses in the cured product, which can decrease its strain-to-failure and fracture toughness (6). EB curing at ambient temperatures eliminates thermal expansion in both the tool and the product, thus reducing dimensional changes and internal stresses in the final product. 2. Reduced curing times for individual components. A typical EBcurable composite can be cured with a maximum dose of about 50 kGy (50 kJ/kg). The product throughput for an accelerator is a function of the machine's power. A 50-kW electron accelerator can provide this dose to about 1.8 Mg/h of material, assuming that at least 50% of the energy provided by the accelerator is actually absorbed by the product being irradiated. This production speed is several times higher than for thermal curing with a typical autoclave or oven, even though the products are cured one at a time rather than in large batches. 3. Improved material handling. EB processing is a continuous operation and components can be EB-treated immediately after they are produced. This continuous operation makes production scheduling and inventory control easier and reduces the number of identical molds needed to manufacture products economically, as compared to using a batch process such as thermal curing with autoclaves or ovens. EB processing is also better suited for short production runs because parts are cured one at a time. 4. Improved resin stability at ambient temperature. The shelf-life of EB-curable resins can be much longer than the shelf-life of formulations for thermal curing because EB-curable formulations do not normally auto-cure at room temperature, making low-temperature storage unnecessary. 5. Reducing the amount of volatiles produced. Thermal curing of composites often produces volatile degradation products that can be hazardous and require proper control procedures (6). EB processing eliminates the production of thermal degradation products, though very small amounts of gases such as hydrogen and carbon dioxide may be produced (7). The disposal of these gases would not cause problems


15. SAUNDERS ET AL.

Aramid-Fiber-Reinforced Composites

253

and would require much less effort than that required to deal with the volatiles from thermal curing.


6. Better control of the energy-absorption profile for a component, allowing greater design flexibility. In conventional processing, materials with vastly different thermal curing cycles cannot be combined in a single product. Since electron beams can be easily manipulated, different EBcurable materials can be used together in the same product and each component can be given a different dose, if required. Many traditional fabrication methods, including filament/tape winding, resin transfer molding, pultrusion and hand lay-up, can be combined with EB processing. Filament/tape winding is particularly well-suited because it requires limited or no external pressure (high pressure requirements complicate the EB process) and high winding rates can make efficient use of the available energy. The constraint of EB processing for manufacturing composite structures is that EB-curable (free-radical-initiated) matrix systems are required, but none have as yet been widely qualified for highperformance aerospace or aircraft applications. Although the polyester systems commonly used for winding consumer products can be cured via free-radical mechanisms, the epoxy formulations currently being used in advanced composites for aircraft, aerospace, and naval applications are generally unsuitable for EB curing (8). For many products, epoxies provide an almost unbeatable combination of handling characteristics, processing flexibility, composite mechanical properties, and acceptable cost (1). Most epoxies polymerize by a cationic mechanism under the influence of high-energy radiation. This process is inhibited by trace amounts of water (8). However, equivalent EB-curable polymer systems are being designed using appropriate resin formulations; for example, by acrylating the terminal epoxy groups of the epoxy resin (8). Research in EB-curable composites over the past 15 years, together with the recent availability of industrial, high-energy, high-power accelerators, has made possible the industrial production of filamentwound, fiber-reinforced motor cases for aerospace applications (9). Research has been conducted on a number of topics, including matrix development, sizings and interface properties, EB effects on fiber properties, process development, process control, and radiation safety and dosimetry. This paper describes our research program to study EBcurable aramid fiber-reinforced composites. The program objective is to design and manufacture EB-curable composites that meet mechanical and physical property specifications for selected applications.


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Materials and Sample Preparation Table I gives the common forms of acrylated epoxies, polyesters and methanes for composite manufacturing (9). The resin system selected for our initial experiments was an epoxy diacrylate. Size exclusion chromatography (SEC) showed that the resin was 60% dimer with a molecular weight of 908 g/mol, with 36% trimer and higher oligomers, 3% monomer and 1% acrylic acid and hydroquinone, an ultraviolet stabilizer. Table Π gives the range of properties presently available for both epoxy and EB-curable acrylated epoxy polymers (10). The glass transition temperature (T ) of the difunctional EB-curable acrylated epoxy is up to 150°C., while the tetrafunctional forms have a T up to 200°C. Aramid fibers, specifically Kevlar fibers, are poly (p-phenyleneterephthalamide) (PPD-T). These fibers were selected for composite preparation because of their radiation stability, a consequence of the highly aromatic structure (11), and because Kevlar is the fiber of choice for many of the products that are suitable for EB processing (2). The suitablity of a product depends on the product size and shape, and the required curing conditions, such as pressure and temperature. A plain-weave Kevlar 68 fabric was selected for our initial experiments. Table ΙΠ lists some of the fabric/fiber specifications (1). A solvent process, using acetone, was employed to impregnate the aramid fabric to a resin loading of 40% (by mass). The fibers were not chemically treated prior to impregnation. Chemical treatment increases the expected adhesion between the selected polymer and the fibers (12). Laminates containing 10-plys, all in the same fiber orientation (±1°), were prepared using standard hand lay-up methods, including vacuumbagging to remove volatiles produced during EB treatment and to eliminate oxygen, a free radical scavenger. Table IV gives the experimental curing conditions. A laboratory-scale pressure chamber can be used to EB cure the laminates. The pressure limit for our chamber is 5 MPa at 100°C and the maximum sample thickness is 15 mm. Samples of the neat resin were also EB treated under vacuum, at various dose rates, for gel fraction analyses.


g

g

Process Considerations The electron beam penetration limit must be considered when determining the applicability of EB processing for composites. Electron penetration is a function of beam energy and product density. A 10-MeV beam can penetrate about 40 mm in unit-density material, or 40 kg/m with one-sided irradiation and 90 mm (90 kg/m ) with twosided irradiation, with the top and bottom surface doses being equal (13). The penetration limit is inversely proportional to density. The EB must also penetrate any vacuum bag and/or pressure vessel if these are required for manufacturing the component. This penetration limit makes it necessary to design the product and the pressure vessel so that the 2

2



Acrylated Urethanes

Acrylated Polyesters

Tetrafunctional Acrylated Epoxy

Difunctional Acrylate Epoxy

Resin Type

//I

2

2

R

R where R may be

4

6

4

2

I

r

3

W

I

2

2

2

2

4

6

3

Ο

\\

3

2

6

I

W

Ο

ο

\\

2

2

2

2

W

Ο

Ο

2

C H = CH-C-0-CH -CH -0-C-NH-(C H -CH )-NH-C-0-CH -CH -O.C-CH = C H

W

W

Ο

// Ο

6

Ο Ο

//

2

2

OH

- C H = C-COO-CH -C = C H

CH

I

I

and Ζ may be aromatic or cycloaliphatic

\

/ Ν - Ζ - Ν

OH

6

C H = CH-C-0-(CH ) -[0-C-(CH ) -C-0-(CH ) ]-0-C-CH = C H

R

/

R \

Ο

2

3

C H = C H - C - 0 - [ C H C H - C H -(C H )- C -(C H )- C H - C H - C H O - C - C H = C H 2

CH

Chemical Formula

Table L Chemical Forms of EB-Curable Resins [9]


2

fcs>

I f 1

> r

w

S3

w

Ο

256


Table Π Typical Properties of Epoxies and EB-Curable Acrylated Epoxies [10]


Property

Epoxies Acrylated Epïticies

Ultimate Strength (MPa): Tensile Compressive

28-90 103-172

50-82 ΝΑ»

Modulus (GPa): Tensile Compressive

2.4-4.1 2.8-4.1

2.3-4.2 ΝΑ

3 -6

1.5-8.2

Coef. of Linear Thermal Expansion 0im/(m.°C))

45 - 65

ΝΑ

Glass Transition Temperature (°C)

100-175

53-200

Elongation at Break (%)

•Not available from the supplier.

Table HI. Kevlar 68 Fabric Specifications/Fiber Properties [1]

Specification/Property

Value

Weave Type Fabric Thickness 10-ply Fiber Tensile Strength Modulus Fiber Density

Plain 1.6 mm 3600 MPa 110 GPa 1440 kg/m

3

product can be fully penetrated and cured by an EB. Products with specific densities greater than 40 kg/m can also be irradiated by modifying the 10-MeV electron accelerator to produce X-rays. Figure 1 gives the penetration of both 10-MeV electrons and X-rays in a typical aramid fiber composite (density = 1500 kg/m ). The absorbed 10-MeV electron dose initially increases because of an electron scattering effect, reaching a maximum at a penetration depth of about 25 kg/m . 2

3

2


15. SAUNDERS ET AL.


257

Table IV. EB Processing Conditions


Dose Range Dose Rate Beam Energy Beam Power Beam Scan Width Curing Pressure Curing Temperature

0 to 100 kGy 1 to 1500 kGy/h 10 MeV 1 kW 0.6 m 600 kPa 25°C

Normalized Dose, % 10-MeV EB X-Rays

80 (-

40 μ

20

0

\

j

j

L

i Î

\ ,

,

,

10

I I '

1

I J

I 100

200

Electron Penetration, k g / m

2

Figure 1. Penetration Limit of 10-MeV Electrons and X-Rays In Aramid Fiber Composites (Density = 1500 kg/m ) 3

Figure 2 gives the penetration limit for a 10-MeV EB in aramid composites (density = 1500 kg/m ) as a function of the pressure required during processing. This limit was calculated from the energy lost by the beam penetrating a titanium window thick enough to withstand the pressure and 100 mm of air at the specified pressure before hitting the product. The beam energy is effectively reduced from 10 to 8.4 MeV at the required pressure of 1 MPa. The penetration limit for the beam is therefore reduced from 27 to 24 mm for one-sided EB treatment, and from 60 to 50 mm for two-sided treatment. Thermal energy is added to material during EB processing by radiation-induced exothermic reactions and by the net energy of the 3


258

RADIATION EFFECTS ON POLYMERS Penetration, mm one-sided

40

two-sided

rr

j

60

One-sided


Two-sided

35

55

30 -

50

20

1

0

200

400

Processing

600

800

40 1000

Pressure, kPa

Figure 2. Penetration limit of 10-MeV Electrons in Aramid Fiber Composites as a Function of Gauge Curing Pressure (Density = 1500 kg/m ) 3

absorbed radiation (14). The final temperature of an irradiated object depends on its shape, specific heat, thermal conductivity and the electron dose and dose rate. In composites thicker than 2 cm, local heating can occur, leading to composite damage because the energy absorption properties of the reinforcements are different than those of the matrices (14). Excessive heating in composites can be avoided by proper selection of the resin formulation to minimize the thermal energy released by exothermic reactions, reducing the required curing dose by using cross-linking promoters and fractional irradiation techniques (15). Converting to low-dose-rate X-rays also reduces or eliminates heating problems. Multiple passes are normally required to deliver the needed energy to any large component, which allows the product to be maintained at an acceptably low temperature (< 50°C) (9). An electron accelerator is a source of penetrating radiation, and must therefore be operated in a shielded room. Shielding is usually supplied by concrete with an earth berm often supplementing the concrete. The penetrating radiation of concern is not the electrons, but the Bremsstrahlung (X-rays) and neutrons produced when the electrons impinge in materials. The efficiency for Bremsstrahlung production increases with the atomic number (Z) of the irradiated material. Clough and Shalaby; Radiation Effects on Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

15. SAUNDERS ET AL.


259

Gel Fraction, % 100

•

1 kGy/h

1*

10 k G y / h

Δ

1500 k G y / h

80


60

40 h

20

_j

0.1

10

1

ι :

ι ι 11

100

Dose, kGy

Figure 3. Effect of Dose Rate on the EB Curing of Epoxy Diacrylate Polymer Facilities designed to shield against Bremsstrahlung using concrete are generally more than adequate to shield against neutrons. Results and Discussion Figure 3 plots the gel fraction of our EB treated polymer matrix as a function of dose at three dose rates, 1 kGy/h (X-rays), 10 kGy/h (Xrays), and 1500 kGy/h (EB). Gelation begins at a dose of about 0.1 kGy over the entire dose rate range. At any specific dose up to 100 kGy, the gel fraction in the cured polymer increases with increasing dose rate. The final properties of a cured composite are a function of the degree of cross-linking in the matrix polymer, and are therefore affected by any large changes in dose rate, such as when going from EB to X-ray treatment to obtain the necessary beam penetration. The magnitude of these gel fraction changes must be known so the required dose can be delivered at each selected dose rate to ensure uniform properties throughout an entire product. Table V gives some of the mechanical properties of our 10-ply lam inates, EB-cured at 600 kPa to a dose of 50 kGy in the absence of ox ygen, and comparable thermally cured aramid fabric-epoxy laminates (1). As expected, the tensile properties of the EB-cured laminate compare favourably with the thermally cured material. The tensile properties are primarily fiber-dependent and EB treatment (50 kGy) does Clough and Shalaby; Radiation Effects on Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

260


Table V. Typical Tensile and Compression Properties of Kevlar 68 Fabric-Epoxy Laminâtes» [1]

Cure Type EB-Cure Thermal Epoxy Epoxy


Properly

Tensile Strength (MPa) Tensile Modulus (GPa) Compressive Strength (MPa) Compressive Modulus (GPa)

590 34 89 33

525 27 180 27

•A 10-ply laminate; 600-kPa curing pressure; 60% fiber mass. not affect the properties of the fibers. The compression properties of the EB-cured laminate are lower than the thermally cured material because they depend largely on the polymer properties and the adhesion between the fibers and the matrix. The fibers used to prepare the EBcured material had no coupling agents applied to them to increase the fiber-matrix bonding (6). Research is continuing to study the properties of EB-curable resin formulations and the interface chemistry to select sizings/coupling agents to improve the compression properties of EB cured laminates. X-ray diffraction analysis of the EB-cured polymers confirmed that the polymers were amorphous (16). The volatile content in each EBcured polymer was below 2%. Gas chromatography and mass spectroscopy showed that the volatiles released during EB curing were primarily low-molecular-weight hydrocarbons and some carbon dioxide (16). Carbon dioxide was released from samples irradiated under either oxygen or nitrogen, suggesting that atmospheric oxygen takes little or no part in the formation of C0 . The C 0 is a radiolytic degradation product. 2

2

Summary Advanced composites, including aramid fiber-reinforced composites, are used extensively for a variety of demanding structural applications. The benefits of using EB-curing rather than thermal curing for aramid fiberreinforced composites include curing at ambient temperature, reduced curing times for individual components and fewer volatiles. The gel point of the EB-cured epoxy diacrylate polymer was about 0.1 kGy and was unaffected by dose rate. The gel fraction in the cured polymer increases with increasing dose rate at any dose selected up to 100 kGy. The final properties of a cured composite are therefore affected by any large changes in dose rate, such as when going from EB to X-ray treatment to obtain the necessary beam penetration, and the delivered dose must be increased to compensate for this change. The tensile Clough and Shalaby; Radiation Effects on Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

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properties of a 10-ply, EB-eured, aramid fiber laminate were comparable to the properties of thermally cured aramid fiber laminates. Literature Cited 1. 2.


3. 4.

5.

6. 7. 8. 9. 10. 11. 12. 13.

14. 15.

Composites; Dostal, C.A., Ed.; ASM International: Metals Park, Ohio; 1987. Advanced Thermoset Composites; Margolis, J.M., Ed.; Van Nostrand Reinhold Co.: New York, N.Y., 1986. Menezes, T.J.; Nablo, S.V.Radiat.Curing. 1985, 12, 2, pp. 2-9. Saunders, C.B. Radiation Processing in the Plastics Industry: Current Commercial Applications. Atomic Energy of Canada Limited Report, AECL-9569, 1988. Saunders, C.B.; Singh, A. The Advantages of Electron Beam Curing of Fibre-Reinforced Composites. Atomic Energy of Canada Limited Research Company Report, RC-264, 1989. Unpublished report available from SDDO, AECL Research, Chalk River Laboratories, Chalk River, Ontario K0J 0J0. Weeton, J.W.; Peters, D.M.; Thomas, K.L. Engineer's Guide to Composite Materials; ASM International: Metals Park, Ohio, 1987. Bradley, R. Radiation Technology Handbook; Marcel Dekker: New York, N.Y., 1984. Dickson, L.W.; Singh, A.Radiat.Phys. Chem. 1988, 31, 4-6, pp. 587593. Beziers, D. Electron Beam Curing of Composites; Proc. of 35 International SAMPE Symposium, Anaheim, CA, 1990. Modern Plastics Encyclopedia; Juran, R., Ed.; McGraw Hill: New York, N.Y., 1990, pp. 516-517. Bly, J.H. In Modern Plastics Encyclopedia; Juran, R., Ed.; McGraw-Hill: New York, N.Y., 1990. Chen, M.L.; Ueta, S.; Takayanagi, M. Polym. J. 1988, 20, 8, pp. 673-680. Dickson, L.W.; McKeown, J. Radiation Interactions with Linac Beams; in Proc. of Working Meeting on Radiation Interactions, Leipzig, 1987. Zagorski, Z.P.Radiat.Phys. Chem. 1985, 25, pp. 291-293. Saunders, C.B.; Carmichael, A.A.; Lopata, V.J.; Singh, A. Physical and Mechanical Characterization of Radiation-Curable Carbon Fibr Composites; Proc. of 9 Annual CNS Conference, pp. 480-486. Canadian Nuclear Society: Toronto, Ontario, 1988. Pigliacampi, J.J.; Riewald, P.G. In Modern Plastics Encyclopedia; Juran, R., Ed.; McGraw-Hill: New York, N.Y., 1990. TH

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R E C E I V E D February 4, 1991


Radiation Effects on Polymers - American Chemical Society

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