ELECTRON-BEAM CURABLE EPOXY COMPOSITIONS

Abstract: A process for high-speed curing of low reactivity epoxy resins to produce products having high strength, high temperature resistance, and high storage modulus is described. A composition comprising a low reactivity epoxy resin is also described. The process and the composition both utilize onium salts and are cured utilizing ionizing radiation, specifically gamma irradiation.

Description: ELECTRON-BEAM CURABLE EPOXY COMPOSITIONS This invention relates to epoxy compositions curable by means of electron- beam radiation and more particularly relates to the curmg of common inexpensive low-reactive epichlorohydrin-based epoxide resms m the presence of omum salts by electron-beam radiation. BACKGROUND OF THE INVENTION In recent times, electron-beam curmg of resm compositions has become an important technique for the application and cure of coatings, mks, adhesives, castings, roams, compositions, and the like. High-power electron-beam curmg has a high rate of through put and a relatively low energy requirement versus thermal curmg. It is also essentially pollution free and lower cost equipment has now become available. High energy electron-beam curing is also desirable tor use m reinforced composite applications because of the great penetration of electron- beam radiation which allows it to pass through considerable thicknesses or many different materials in direct proportion to the mverse of the material densitv Electron-oeam curmg can be employed for the cure of high reactivity organic resms m the presence of optically opaque reinforcing agents such as fibers and fillers. Further, electron-beam curmg for composites is very rapid, requirmg seconds to minutes whereas conventional thermal curmg requires hours. The prior art describes the UV and electron beam cationic curmg of high reactivity cycloaliphatic epoxy resms and the free radical UV electron-beam curmg of acrylated and other unsaturated resms. While electron-beam curmg proceeds homogeneously throughout the cross-section of the composite, thermal curmg proceeds from the outside mward, producing a considerable temperature gradient The resultmg thermally cured composites are typically highly stressed and as a result, possess reduced mechanical properties. In contrast, electron-beam cured composites possess considerably less residual stress. While there are many advantages to curing composites by electron-beam irradiation, there are currently few materials which are suitable for this purpose. Either the monomers such as acrylate and methacrylate themselves are toxic and have poor mechanical properties which render them unsuitable for composite applications, or they are not responsive to curing by electron-beam irradiation. In this latter class are epoxy resins. Epoxy resins are widely employed by the aerospace and high-performance sporting goods industries for use in composite fabrication. Unfortunately, these resins being any multifunctional glycidyl ether- epichlorohydrin based epoxides of low reactivity do not undergo efficient curing under UV radiation. Certain complex cycloaliphatic based epoxides of high reactivity including some silicone-containing cycloaliphatic (e.g., APR-I and Union Carbide Cyracure UVR-6110) epoxides have been curable by electron beam induced cationic polymerization, but heretofore simpler epichlorohydrin or ethylene oxide based epoxides of low UV reactivity have not been suitable for such curing method. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 illustrates a chart showing cationic polymerization of two test samples. Figure 2 illustrates a chart showing cationic polymerization of three test samples. DESCRIPTION OF THE PREFERRED EMBODIMENT(S) Certam types of simple, inexpensive epichlorohydrin-based epoxy resins of low UV reactivity which are freely available in commercial quantities have been found to undergo rapid electron-beam mduced curmg m the presence of certam onium salts. Such resms include bisphenol-A, diglycidyl ether, novolac epoxy resms and any other glycidyl (epichlorohydrin or ethylene oxide based) multifunctional epoxy resins. Glycidyl ether resms alone or in combmation with each other and with silicon-containing epoxy resms have been found to be electron-beam curable when cured using specific onium salt photoinitiators The resms which can be employed in this invention consist of the reaction products of bisphenol-A with epichlorohydrin, commonly known as bisphenol- A epoxy resins Commercially available examples mclude Epon 826, Epon 828 from the Shell Chemical Company and Tactix 123 from the Dow Chemical Company or bisphenol-A molecular weight extended epoxy resms such as Dow Chemical Company s DER-317, DER-337, Shell Company's Epon 830, Epon 834 or Ciba Geigy s Araldite 6020. Simple resms of low reactivity may be extended bv reaction with bisphenol-A to provide epoxy termmated resms of various molecular weights. An additional class of related monomers and resins are those based on bisphenol-F (4,4'-dιhydroxydιphenylmethane) commercially available as Epon 862 from the Shell Company. Still another class of simple low reactivity resins which may be employed are epoxy phenol novolac resms such as Ciba Geigy s Araldite EPN-1138 or EPN-1139 or Dow Chemical Company s DEN 431, DEN 438 and Tactix 742 or cresol formaldehyde resms (epoxy cresol novolac resms) such as Shell Chemical Company s Epon 164 as well as other multifunctional bisphenol-A novolac resms including Shell Chemical Company's Epon SU-2.5 and Epon SW-8. Also common additives for resin toughening such as multifunctional epoxy additives such as Shell Chemical Company's Heloxy H505 and viscosity thickeners such as thermoplastic polymers such as polyethersulfone or poly(2,6-dimethylphenylene oxide) for the fabrication of prepreg tapes can be used to formulate higher viscosity composite resins. Examples of such low reactivity glycidyl ether-epichlorohydrin based resins include the following: TYPICAL LOW UV REACTIVITY GLYCIDYL ETHER / EPICHLOROHYDRIN BASED EPOXY RESINS THAT ARE E-BEAM CURABLE WITH INITIATORS OTHER TYPICAL COMPATIBLE REACTIVE EPOXIES The key feature of this invention is that certain onium salt photoinitiators can be employed along with the above low reactivity epoxy resins to "sensitize" them to electron-beam irradiation. Without such onium salts, chemical hardeners or very high electron-beam doses are required to cure the epoxides. Thus, in the absence of such onium salts, the cure of low UV reactivity epoxy resins by electron-beam irradiation is not a practical or useful process. Such onium salts include diaryliodonium salts and triarylsulfonium salts containing the SbF 6 (antimony hexafluoride) and perfluoro terra phenyl borate (C 6 F 5 ) 4 B " counterions. Examples of these onium salts include the following: The above salts are well known in the art and are cited in J.V. Crivello and K. Dietliker, In Chemistry & Technology of UV & EB Formulation for Coatings, Inks & Paints, Vol. 3, 1991, p. 329. Unexpectedly, the simple low reactivity resin compositions of this invention undergo very rapid cure on exposure to electron-beam irradiation in the presence of the above onium salt types. Among these compositions containing antimony hexafluoride or fluorinated phenyl borate counterions, diaryliodonium salts are preferred for rapid cure. The amount of the onium salt which may be included in an electron-beam curable composition is from 0.5 - 10% by weight. Several sources of electron irradiation to accomplish curing may be employed. The most practical and widely used instrument for this purpose is a high-energy electron-beam accelerator. These may be two or more types: a Van de Graff accelerator or a klystron-driven instrument. A low intensity instrument called an "electrocurtain apparatus" in which electrons produced by a hot cathode are accelerated by a magnetic field can be used to cure thin films or composites. Alternatively, gamma-irradiation from a 60 Co source can be used for curing. In this technique, fast electrons are generated by interaction of the gamma- irradiation with the monomer. Still another technique for generating x-rays consists of focusing a high-energy electron beam onto a heavy metal conversion target. This process produces highly penetrating x-rays which can be used to achieve the desired cure. Electron-beam doses which are useful for curing composites are from 5 to 125 Mrads (50 - 1250 KiloGreys (kGy)) and most particularly effective from 30 to 90 Mrads. Various composite articles may be prepared using this invention applying many well-known composite fabrication techniques. Such composites include traditional reinforced composite laminates consisting of layers of woven fibers saturated with the epoxy resin compositions of this invention. Such laminates may be prepared with the aid of pressure or the use of "vacuum bagging" techniques. In addition, these formulations can also be employed in resin transfer molding (RTM) in which a fiber preform is infused with the liquid resin formulation under vacuum. Further well-known composite techniques which can be employed are filament winding and pultrusion. In all cases, after fabrication, the composites are cured by exposure to electron-beam irradiation as described above. Electron-beam cures of the compositions can conveniently be carried out in ambient air. The nature of these polymerizations can be classified as proceeding by a cationic mechanism. This means that the traditional amine, anhydride, thiol, amide or phenol hardeners do not have to be utilized. Although an inert atmosphere is not required for cure, nitrogen may be used to avoid deleterious effects of oxygen-induced oxidation in certain resins. Typically, useful composites can be produced immediately after only several seconds of exposure to electron-beam irradiation. This speed of curing stands in contrast to conventional thermal cures of epoxy resins in which the composite must be heated for several hours to achieve cure. Since electron-beam curing occurs by a cationic polymerization, it is non-terminating. That is, curing will continue via chain reaction until most of the epoxy groups have reacted, even after electron-beam radiation exposure has ceased. Hence, the properties of certain composites may be optionally enhanced by a brief thermal cure after electron-beam irradiation. The compositions disclosed herein may not only include the monomer or oligomers specified above, but also a wide assortment of photosensitizers, fillers, flow control agents and other additives, essential to impart thixotropy, flatting, impact modifying and reinforcement characteristics to the finished article. In particular, such formulations can be combined with a variety of reinforcing fillers including glass, carbon, boron, alumina, boron nitride, polyamide, polybenzimidazole and polyimide fibers. Additionally, particulate fillers such as mica, talc, silica, carbon whiskers or platelets can be used. The amount of such reinforcements may be up to 80% by volume of the formulation. A wide variety of impact modifiers such as carboxyl terminated butadiene, butadiene- acrylonitrile copolymers, or polysulfone polymers may be used. These also include core-shell polymers and particulate rubbers and microballoons. Example I A formulation consisting of 50 parts Shell Epon 862 (BF), 50 parts of epoxy novolac Dow Chemical Company DEN 431 and containing 1% by weight diaryliodonium initiator IV was subjected to electron-beam irradiation. These irradiations were conducted using the Atomic Energy Commission of Canada Ltd (AECL) 1-10/1 Electron Linear Accelerator in Pinawa, Manitoba operating at 10 MeV and 1 kW. The scanning 10 MeV pulsed electron beam was mounted over a conveyor on which the samples were placed. The sample received a total irradiation dose of 75 kGy. Mechanical measurements on the samples gave a storage modulus of 1.0 X 109 Pa at room temperature and a glass transition temperature of 200 degrees C. Example 2 Using diaryliodonium salt IV as an initiator, 1% by weight mixtures with the following epoxy resins were made APR- 1 illustrated below, bisphenol-A diglycidyl ether (BA), bisphenol-F diglycidyl ether (BF) and 3,4- epoxycyclohexylmethyl 3',4'-epoxycyclohexanecarboxylate (CY) also shown below. The samples were placed into vials and irradiated under the same conditions as described in the previous example. Excellent polymerization response was observed in all cases, however, the APR- 1 resin had the highest radiation sensitivity. Given in the following table is a comparison for the peak doses for all the samples. Resin /Monomer Peak Dose (Gv) APR- l 1000 BA 4000 BF 4800 CY 5000 From these results as seen in Figure 2, it can be concluded that both BA and BF re have unexpectedly high response to gamma-irradiation in the presence of diaryliodonium catalyst. Example 3 The following formulated epoxy resins were made. All are given in parts. The samples were poured into 0.25 oz vials, sealed and a thermocouple was attached to the outside of the vial to record the temperature. The vials were then placed in a 60 Co gamma cell and exposed to gamma-irradiation at a dose rate of 98 Gy/min. The temperature was continuously recorded as a function of time to follow the polymerization. The following results were obtained. From these results, it can be concluded that while formulation 1 containing the triarysulfonium initiator undergoes excellent polymerization, the anhydride-based formulation essentially does not undergo any substantial polymerization under these conditions. Example 4 To two samples of the siloxane resin APR-l there were added 1% by weight of triarysulfonium I and diaryliodonium rV initiators. The samples were poured into 0.25 oz. vials, sealed and a thermocouple was attached to the outside of the vial to record the temperature. The vials were then placed in a 60 Co gamma cell and exposed to gamma-irradiation at a dose rate of 98 Gy/min. The temperature was continuously recorded as a function of time to follow the polymerization. Both samples exhibited exothermic polymerization during irradiation as shown in Figure 1. However, there was a large and unexpected difference in the two photoinitiators. The polymerization mixture containing the iodonium salt, diaryliodonium initiator IV has an exceptionally low onset temperature of approximately 750 Gy (0.075 Mrad) and a dose at the peak of 1000 Gy (0.10 Mrad). In addition, the rate of polymerization of this sample as indicated by the slope of the curve is much higher than for the corresponding triarysulfonium initiator I containing sample which exhibits a very gradual onset of polymerization. The peak dose in this latter sample was approximately 3000 Gy (0.30 Mrad). It should also be noted that the exothermic reached by the two samples during polymerization are very different. When diaryliodonium initiator IV is used, the temperature of the sample rises to 100 degrees C whereas, in the case where diaryliodonium is used a temperature of only 65° C is attained. Clearly, there is a very large and unexpected response difference when the diaryliodonium initiator is used in contrast to the triarysulfonium initiator. Example 5 A formulation consisting of 11 parts of Shell Epon 862 (BF), 88 parts of APR-l epoxy resin and containing 0.5 % part diaryliodonium initiator TV was subjected to electron-beam irradiation. These irradiations were conducted using the 1-10/1 Electron Linear Accelerator operating at 10 MeV and 1 kW. The scanning 10 MeV pulsed electron beam was mounted over a conveyor on which the samples were placed. The sample received a total irradiation dose of 75 kGy. Mechanical measurements on the samples gave a modulus of 0.9 X 1010 Pa (RSA-2, E') at room temperature and a glass transition temperature of 200° C. Example 6 A formulation consisting of Shell Epon 862 (BF) together with 1 part of diaryliodonium initiator IV was subjected to electron-beam irradiation as described above. There were obtained a cured specimen with a storage modulus of 1.5 X 109 Pa (RSA-2, E) and a glass transition temperature of 170° C. The experiments shown above demonstrate that conventional glycidyl ether epoxides and other epoxides can be cured either by themselves or in admixtures with silicon containing epoxides by simple electron-beam irradiation in the presence of a diaryliodonium salt or triarylsulfonium salt initiator. Although the present invention has been described with reference to particular embodiments, it will be apparent to those skilled in the art that variations and modifications can be substituted therefor without departing from the principles and spirit of the invention.

Claims: We claim: 1. A process for high-speed curing of low reactivity epoxy resins to produce products having high strength, high temperature resistance, and high storage modulus, as compared with products produced from known high reactivity epoxy resins, which process comprises the steps of: mixing said low reactivity epoxy resin with 0.5 to 10 wt% of an onium salt; exposing said mixture to a source of ionizing radiation; and curing said epoxy resin and onium salt mixture by cationic polymerization. 2. The process of Claim 1 wherein said onium salt is selected from the group consisting of all diaryliodonium and triarylsulfonium salts that contain an antimony hexafluoro counterion. 3. The process of Claim 1 wherein said epoxy resin is selected from the group consisting of bisphenol-A epoxy resins, bisphenol-F epoxy resins, epoxy phenol novolac resins, epoxy cresol novolac resins, epoxy bisphenol-A novolac resins and any other multifunctional epoxy resin comprising a functional group selected from the group consisting of oxirane and glycidyl ether structures. 4. The process of Claim 1 wherein said source of irradiation comprises an electron beam. 5. The process of Claim 1 wherein said source of ionizing radiation comprises gamma ionizing radiation. 6. The process of Claim 1 further including the step of: adding filler materials selected from the group consisting of organic and inorganic fillers and organic and inorganic reinforcing fibers to said mixture before the step of exposing said mixture to a source of ionizing radiation. 7. The process of Claim 1 further including the step of: providing an inert atmosphere around said mixture before exposing said mixture to irradiation. 8. A composition comprising a low reactivity epoxy resin and 0.5 to 10 wt% of onium salts, said composition cured by exposure to ionizing radiation. 9. The composition of Claim 8 wherein said onium salt is selected from the group consisting of any diaryliodonium and triarysulfonium salt containing antimony hexafluoro or perfluoro tetraphenol borate counterions. 10. The composition of Claim 8 wherein said epoxy resin is selected from the group consisting of bisphenol-A epoxy resins, bisphenol-F epoxy resins, epoxy phenol novolac resins, epoxy cresol novolac resins, epoxy bisphenol-A novolac resins and any other multifunctional epoxy resin comprising a functional group selected from the group consisting of oxirane and glycidyl ether structures. 11. The composition of Claim 8 wherein said source of irradiation comprises an electron beam. 12. The composition of Claim 8 wherein said ionizing radiation comprises gamma irradiation. 13. The process of Claim 1 wherein said onium salt is selected from the group consisting of all diaryliodonium and triarylsulfonium salts that contain a perfluoro tetraphenyl borate counterion. 14. The process of Claim 3 wherein said source of ionizing radiation comprises X-ray irradiation. 15. The composition of Claim 8 wherein said ionizing radiation comprises X-ray irradiation.

Inventors: CRIVELLO JAMES V, WALTON THOMAS C, CRIVELLO, JAMES, V., WALTON, THOMAS, C.
Applicants: AEROPLAS CORP INT, CRIVELLO JAMES V, WALTON THOMAS C, AEROPLAS CORPORATION INTERNATIONAL, CRIVELLO, JAMES, V., WALTON, THOMAS, C.
Application year: 1997
Main classification: C08F2
IPC codes: B41M7, C08F2, C08G59, G03F7