Irradiation of Polymers - American Chemical Society

Chapter 15

Electron Processing of Carbon-FiberReinforced Advanced Composites: A Status Report

Downloaded by FUDAN UNIV on March 23, 2017 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/bk-1996-0620.ch015

Ajit Singh, Chris B. Saunders, Vince J. Lopata, Walter Kremers, Tom E. McDougall, Miyoko Tateishi, and Minda Chung Research Chemistry Branch, AECL Research, Whiteshell Laboratories, Pinawa, Manitoba R0E 1L0, Canada An emerging application of industrial electron accelerators is the pro duction of advanced composites for the aerospace and other industries. Our work on the production of these advanced composites, using electron-curable matrices, is briefly reviewed. The dose required for curing the matrix can vary with the dose rate, and, as our results show, this dose rate dependence can vary from resin to resin. Our re sults on the reduction of the void content, the use of fiber-matrix coupling agent, and fabrication and curing of thick composites are also discussed. Industrial radiation processing involves the use of natural and man-made sources of high-energy radiation (γ-radiation, electron beams), to manufacture a wide range of products (1-3). Radiation processing is a growing industry; in 1990, the value added to products by radiation processing was estimated to be in the billions of dollars (4). Both cobalt-60 sources and electron accelerators are used for radiation processing. Gamma radiation and X-rays are much more penetrating than electrons of the same energy (5,6). Therefore, thick products are typically irradiated with gamma radiation (and some with X-rays), and thin products are typically irradiated with electrons, e.g., 10-MeV electrons allow uniform irradiation of up to 9 cm of unit density material by two sided-irradiation. Gamma irradiation is almost ex clusively used for sterilization of medical disposables and for food irradiation (7), though other applications, e.g., sewage irradiation, are under investigation (8,9). Electron accelerators are primarily being used for processing plastics or products de rived from them (3); their other present or potential uses (8,10) include irradiation of sewage sludge (11), wastewater treatment (12,13), sterilization of medical devices

and disposables (14-18), and food irradiation (19-22). An emerging application of industrial electron accelerators is the production of advanced composites for the aerospace and other industries. Traditionally, the carbon-, aramid- and glass-fiber-reinforced composites with epoxy matrices are pro duced by thermal curing (23). However, equivalent composites with acrylated-

0097-6156/96A)620-0197$12.00/0 Published 1996 American Chemical Society Clough and Shalaby; Irradiation of Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

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epoxy matrices can be produced by electron processing (24-26). In this paper we present a status report on our work on electron processing of carbonfiber-reinforced advanced composites, which has been in progress for almost a decade.


Advanced Composites Advanced composites, specifically carbon-fiber-reinforced epoxies, are being used for many applications, primarily because of their high strength-to-weight and stiffness-to-weight ratios (23). Applications for these thermosetting composites are found in the aircraft, aerospace, sporting goods, transportation and automotive industries (23). Electron processing or curing of composites involves using electrons as ionizing radiation to initiate polymerization or cross-linking reactions in suitable matrix resins (e.g., acrylated epoxy oligomers) in place of the traditional thermal initiation of the polymerization and cross-linking reactions in epoxy formulations (24,25,27,28). Advantages of Electron Processing. Many advantages have been identified for using electron processing (25,29) rather than thermal curing of advanced composites, as outlined below. 1. Curing at Ambient Temperature: The thermal curing cycle can change the dimensions of the product and create internal stresses, which can decrease its strain to failure and the fracture toughness (29). Electron processing at ambient temperature can reduce these dimensional changes in both the tool and the product, thus reducing the internal stresses in the final product. 2. Reduced Curing Times: The production speed for a 50 kW accelerator to cure composites would be up to 6 times higher than thermal curing with a typical autoclave. This is so, even though the products are cured one at a time during electron processing, compared to large batches in autoclaves. 3. Improved Resin Stability: Most electron-curable matrix resins do not auto-cure at room temperature, making low-temperature storage unnecessary. 4. Reduction in the Amount of Volatiles Produced: Electron processing eliminates the production of toxic thermal degradation products, though very small amounts of gases such as hydrogen and methane may be produced. 5. Better Material Handling: Two of the factors that contribute to more efficient material handling during electron processing are: (i) the ability to handle the resins at room temperature makes it easier to prepare prepregs, and to fabricate components from them; and (ii) the ability to electron process each item as it gets fabricated, reduces the space requirements for storage of the uncured items. In the case of electron processing, components with different resins requiring different doses can be processed one after the other. However, in thermal curing, all the contents of the autoclave need to have the same thermal curing cycle. 6. Reducing the Operating Costs, Primarily Energy Costs: The energy required for electron processing could be lower by a factor of 5 or more (30) with overall savings of the order of 30%, compared to thermal curing.

Clough and Shalaby; Irradiation of Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

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Constraints for Electron Processing. Electron processing of composites also faces some constraints, as follows: 1. Availability of Electron-Curable Matrix Resins: The epoxy formulations current ly being used in the aerospace industry are not appropriate for electron curing. Acrylated epoxy resins that can be cured by irradiation are now commercially available, but in a much smaller selection. 2. Qualification Procedures: Extensive testing is required to develop electronprocessed matrices for advanced composites that are truly equivalent to the con ventional, thermally cured, matrices (31,32).


Our Work.

Our advanced composites work was initiated with various formulations derived from five commercially available electron-curable resins as the matrix mate rials (33), and Hercules' AS4 carbon fabric for reinforcement. Most of the work has been done with electron irradiation using the A E C L I-10/1 electron accelerator at Whiteshell (30,34). Some of the work has also been done using X-rays produced from the I-10/1 electron beam and gamma radiation from an A E C L Gammacell 220. The glass transition temperature and relevant mechanical properties for conven tional epoxies and for acrylated epoxies fall in similar ranges (28.35). The mechan ical properties obtained for a 14-ply electron processed laminate compare very well with the properties required by the aerospace industry. Recent data also show that the internal stress is lower in electron processed composites, compared to thermally cured composites (30). In principle, similar advanced composites using different fibers (aramid- or glass-fiber) can also be made using the electron-processing tech nology. Recently, we have focussed on the use of fiber-matrix coupling agents, reduction of void content, use of adhesives to join pre-cured composites, and pre paration of thick samples (up to 900 plies) (36). We, have used seven commercially available formulations and five formulations of our own as adhesives for advanced composites (37). The results show that electron-cured adhesives can give lap shear strengths similar to, and in some cases better than, the thermally-cured adhesives.

Dose-Rate Effect.

We have examined the radiation curing characteristics of several matrix resins. Some resins cure at a lower total dose on electron irradiation, while others require a lower dose on gamma irradiation. This is brought out by the data summarized in Table I, which shows the doses required to obtain a 90% gel frac tion for various resins on electron and gamma (or X-ray) irradiation. It is therefore important to determine the curing characteristics of the resin of interest.

Void Content.

Most of the acrylated epoxies shrink by up to 12% by volume on electron curing. In the fiber-reinforced composites containing 35% matrix resins (65% fiber, by weight), the void content in the product can be up to 8%. However, the void content has been reduced to -4%, in the composites with 45% resin content, by appropriate compaction methods.


200


Resin'

Table I. Dose Required for 90% Gel Fraction e' Reference r kGy kGy 25" 40^ 26


CN-104 C3000, Epoxy diacrylate S297, 1,3-Butylene glycol dimethacrylate S604, Polypropylene glycol monomethacrylate CN964, Urethane acrylate oligomer 2

8

60

3

Lopata et al.

5

28

8

Lopata et al.

5

40

6

Lopata et al.

5

2

1

Lopata et al.

5

1

X-rays, dose rate, 4 kGy.h" . Dose rate, 10 kGy.h . . Dose rate 5400 kGy. h* . Lopata, V.J., Kremers, W., McDougall, T.E., Tateishi, M , Saunders, C.B., Singh, A., unpublished results (1993).

3

4

1

1

5

To reduce the void content, the sizing was removed by treating the fiber with hot chromic acid, washing with distilled water and drying in an oven at 150°C for 24 hours. The resins used were FW3 (Applied Poleramics) and CN104 (Sartomer). An additional step was added to the normal fabrication of the composite. The vacuum-bagged composite was heated to 80°C for 4 hours to reduce the viscosity of the resin. At 80°C the viscosity for FW3 resin is 600 cps compared with 900,000 cps at room temperature. The excess resin was allowed to bleed into a breather cloth. The void content for these composites was much lower, as determined by the C-scan method (38) and confirmed by mercury intrusion measurements using a Micromeretics Auto-pore II porosimeter. The void content in these samples was ~4%, as compared to the previous samples which showed void content up to 8%. Fiber-Matrix Coupling Agents. We have used a number of different fiber-matrix bonding agents, with sized and unsized AS4 carbon fiber from Hercules. Surface treatment of the carbon fiber can play an important role in achieving acceptable bond strengths. The treatment of the fiber surface can take many forms, e.g., a coupling agent may be used to enhance chemical bonding between the fiber and the matrix resin. A proprietary coupling agent was applied by a solvent method at a 1% loading. A heating step was used to chemically bond the isocyanate coupling agent and the fibers. The acrylated matrix resin was then applied and the fabricated composite cured with a dose of 50 kGy. Figures la and b show Scanning Electron Microscopy (SEM) micrographs for the untreated and treated fiber surfaces of the fractured composite. The fibers with no coupling agent are clean, showing no adhesion between the fibers and the matrix resin (Figure la). Figure lb shows that the fibers treated with the coupling agent


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Figure 1. SEM photographs of the fiber surfaces of thefracturedcomposite: (a) untreatedfibers;(b) treatedfibers.(Reprintedfromref. 37 with permissionfromthe authors and the Society for the Advancement of Materials and Process Engineering.)



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have the matrix resin adhered to them, due to bonding between the fiber and the matrix resin. The use of a coupling agent greatly increases the mechanical properties of the composite. Table II shows a comparison of the mechanical properties of the com posite samples fabricated from the sized and unsized AS4 carbon fiber with CN104, an epoxy acrylate resin, as the matrix. As can be seen from the data, the use of the coupling agent with the sized fiber improves the mechanical properties of the com posite. It was expected that the composite fabricated from die unsized fiber (with the coupling agent) would also give a similar improvement in the mechanical pro perties, but it did not. The reason for the difference in the behaviour of the sized and unsized fiber may be attributed to the fate of active sites on the fiber with which the coupling agent can react. The sizing on the fiber most likely protects these active sites. In the case of the unsized fiber, the exposure of the fiber to the environment, following removal of the sizing, may lead to their inactivation over time, removing possible bonding sites for the coupling agent. Although the appli cation of the coupling agent on the sized fiber gives better mechanical properties, the dose required to cure the composite is higher, by a factor of as much as 3. Our results suggest that if the coupling agent were to be applied as the sizing agent dur ing the manufacture of the fiber, it could reduce the curing dose for the resulting composite. Table II. Mechanical Properties of Electron-Cured Carbon-Fiber Reinforced Composites': Use of a Coupling Agent

" Property Compression Modulus, GPa Strength, MPa

Sized Fiber (Hercules AS4) No Coupling Agent With Coupling Agent

Flexural Modulus, GPa Strength, MPa

56 156

60 320

48 349

56 643

'Matrix, CN104 epoxy acrylate. An acrylated isocyanate.

b

CONCLUSION

Our previous work has demonstrated the feasibility of using electron curing to pro duce carbon fiber-reinforced advanced composites (24-28) with lower internal stresses as compared to thermally cured composites (30). The use of electron curing of adhesives for composites (37), and the production of up to 900 ply thick composites (36) has also been demonstrated. Our recent work shows that (i) the optimum curing dose for the matrix resins varies from resin to resin and also depends on the dose rate; (ii) the void content of the electron-cured composites can be reduced to ~4%; and (iii) the use of a fiber-matrix coupling agent improves the mechanical properties of the composites produced.


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Literature Cited (1) Silverman, J., Radiation Processing - The Industrial Applications of RadiationChemistry; J. Chem.Educ.,1981, 58, 168-173. (2) Singh, A.; Silverman, J., Radiation Processing: An Overview. In Radiation Processing of Polymers; Singh, A.; Silverman, J., Eds.; Hanser Publishers: Munich, 1992, pp 1-14. (3) Saunders, C.B., Radiation Processing in the Plastics Industry: Current Commercial Applications; Atomic Energy of Canada Limited Report, AECL-9569, 1988. (4) Cook, P.M., Impact and Benefit of Radiation Technology; Radiat. Phys. Chem., 1990, 35 , 7-8. (5) Holm, W.N.; Berry, R.J., Manual on Radiation Dosimetry: Marcel Dekker, Inc., New York, NY, 1970. (6) Rogers, D.W.O.; Bielajew, A.F., Monte Carlo Techniques of Electron and Photon Transport for Radiation Dosimetry; In The Dosimetry of Ionizing Radiation; Kase, K.R.; Bjarngard, B.E.; Attix, F.H., Eds.; Academic Press, Inc.: New York, NY, 1990, pp 427-539. (7) Kunstadt, P.; Steeves, C.; Beaulieu, D., Economics of Food Irradiation; Radiat. Phys. Chem., 1993, 42, 259-268. (8) Huang, Q.; Wu, J.; Takehisa, M.; Miller, A., Proc. 8th Intl Meeting Radiat. Processing, Beijing, China, 1992, Radiat. Phys. Chem., 1993, 42 (1-6), 1-1053. (9) Swinwood, J.F.; Fraser, F.M., Environmental Application of Gamma Tech nology: Update on the Canadian Sludge Irradiator; Radiat. Phys. Chem., 1993, 42, 683-687. (10) Leemhorst, J.G.; Miller, A., Proc. 7th Intl. Meeting Radiat. Processing, Noordwijkerhout, The Netherlands, 1989, Radiat. Phys. Chem., 1990, 35 (1-6), 1-878. (11) Bennett, G.S.; Saunders, C.B.; Singh, A., Radiation Disinfection of Sewage; Atomic Energy of Canada Limited Research Company Report, RC-94, 1988. Available from Scientific Document Distribution Office (SDDO), Atomic Energy of Canada Limited, Research Company, Chalk River, Ontario K0J 1JO. (12) Singh, A.; Sagert, N.H.; Borsa, J.; Singh, H.; Bennett, G.S., The Use of HighEnergy Radiation for the Treatment of Wastewater: A Review; In Proc. 8th Symp. on Wastewater Treatment, Montreal, 1985, Environment Canada, Ottawa, 1985, pp 191-209. (13) Removal of Phenol from Aqueous Solutions Using High Energy Electron Beam Irradiation; United States Environmental Protection Agency, Emerging Technology Bulletin EPA/540/F-93/509, 1993. (14) Sadat, T.; Morisseau, D.; Ross, A., Electron Beam Sterilization of Heterogeneous Medical Devices; Radiat. Phys. Chem., 1993, 42, 491-494. (15) Mehta, K., Process Qualification for Electron-Beam Sterilization; Med. Dev. Diagnost. lnd., 1993, 14 (6), 122-134. (16) Sato, Y.; Takahashi, T.; Saito, T.; Sato, T., Takehisa, M., Sterilization of Health Care Products by 5 MeV Bremsstrahlung (X-ray); Radiat. Phys. Chem., 1993, 42, 621-624.



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(17) Barnard, J.B., E-Beam Processing in the Medical Device Industry, Med. Dev. Tech., 1991, 1 (5), 34-41. (18) Saunders, C.; Lucht, L.; McDougall, T., Radiation Effects on Microorganisms and Polymers for Medical Products; Med. Dev. Diagnost. Ind., 1993, 2 (5), 88-82. (19) Singh, H., Prevention of Food Spoilage by Radiation Processing; Can. Home Econom. J., 1987, 37 (1), 5-10. (20) Singh, H., Dose Rate Effect in Food Irradiation: A Review; Atomic Energy of Canada Limited Report , AECL-10343, 1991. (21) Sadat, T., Progress Report on Linear Accelerators; Radiat. Phys. Chem., 1990, 35, 616-618. (22) Borsa, J.; Iverson, S.L., The Cost and Benefits of Grain Disinfestation and Poultry and Frozen Shrimp Decontamination Using 10-MeV Electron Accel erators; Proc. Int. Symp. on Cost-Benefit Aspects of Food Irradiation Processing, France, 1993; IAEA-SM-328/75, pp 223-231. (23) Margolis, J.M., Ed., Advanced Thermoset Composites, Van Nostrand reinhold, New York, NY, 1986. (24) Singh, A.; Saunders, C.B., In Radiation Processing of Polymers; Radiation Processing of Carbon-Fiber Acrylated Epoxy Composites; Singh, A.; Silverman, J., Eds.; Hanser Publishers: Munich, 1992, pp 187-203. (25) Saunders, C.B.; Singh, A., The Advantages of Electron-Beam Curing of FibreReinforced Composites; Atomic Energy of Canada Limited Report, RC-264, 1989. Available from Scientific Document Distribution Office (SDDO), Atomic Energy of Canada Limited, Research Company, Chalk River, ON K0J 1J0. (26) Singh, A.; Lopata, V.J.; Kremers, W.; McDougall, T.E.; Tateishi, M.; Saunders, C.B., Electron-Cured Fibre-Reinforced Advanced Composites; Proc. CANCOM'93; Wallace, W.; Gauvin, R.; Hoa, S.V., Eds.; Ottawa, 1993, pp 277-289. (27) Dickson, L.W.; Singh, A., Radiation Curing of Epoxies; Radiat. Phys. Chem.; 1987, 31, 587-593. (28) Saunders, C.B.; Singh, A.; Czvikovszky, T., Radiation Processing of FibreReinforced Composites; Proc. 12th Annual Can. Nucl. Soc. Meeting, 1991, pp 60-64. (29) Weeton, J.W.; Peters, D.M.; Thomas, K.L., Engineers' Guide to Composite Materials; American Society of Metals, Metals Park, OH, 1987. (30) Saunders, C.B.; Lopata, V.J.; Kremers, W.; McDougall, T.E.; Tateishi, M.; Singh, A., Electron Curing of Fiber-Reinforced Composites; Recent Develop ments; Proc. 38th SAMPE Conf., 1993, pp 1681-1691. (31) McCarty, J.E., In Engineered Materials Handbook, Vol. 1; Johnson, J.H.; Kiepura, T.; Humphries, D.A., Eds.; ASM International, Metals Park, OH, 1987, pp 346-351. (32) Fila, J.A.; Fews, R.C., Civil Certification Methodology for Composite Mater ials in Primary Aircraft Structure; Presented at CANCOM '93, 2nd Canadian Int. Conf. on Composites, Ottawa, 1993.



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(33) Saunders, C.B.; Dickson, L.W.; Singh, A.; Carmichael, A.A.; Lopata, V.J., Radiation-Curable Carbon Fiber Prepreg Composites; Polym. Comp., 1988, 9, 389-394. (34) Barnard, J.W.; Stanley, F.W., Startup of the Whiteshell Irradiation Facility; Nucl. Instrum. Methods Phys. Res., B40/41, 1989, 1158-1161. (35) Beziers, D.; Capdepuy, B., Electron Beam Curing of Composites; Proc. 35th SAMPE Conf., 1990, pp 1221-1232. (36) Saunders, C.B.; Lopata, V.J.; Kremers, W.; McDougall, T.E.; Chung, M.; Barnard, J.W., Electron and X-ray Curing of Thick Composite Structures; Proc. 39th SAMPE Conf., 1994, pp. 486-496. (37) Lopata, V.J.; Chung, M.; McDougall, T.E.; Weinberg, V.A., Electron-Curable Adhesives for High-Performance Structures; Proc. 39th SAMPE Conf., 1994, pp. 514-520. (38) Henneke II, E. G. In ASM Engineered Materials Handbook; Dostal C.A., M; ASM International: Metals Park, OH, 1987; pp 774-778. RECEIVED August 17, 1995


Irradiation of Polymers - American Chemical Society

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