Research on EB curing began in the late 1970s. The goal was to obtain high-performance thermosetting resins for aerospace. Saunders et al.'s research on EB curing of vinyl ester, acrylate, and methacrylate systems showed that these systems were difficult to be used in high-tech fields such as aerospace due to their high internal stress, large void content, poor mechanical properties, high water absorption, and low glass transition temperature (Tg).
Materials scientists then turned their attention to epoxy resins. Studies have shown that EB-cured epoxy resins can overcome the above shortcomings and obtain high-performance structural materials, and have been used in solid engine casing materials and other fields.
1. Characteristics of EB-cured epoxy resin
The main advantage of EB-cured epoxy resin is that it can achieve rapid curing of the resin at room temperature. Compared with thermal curing, the energy required for electron beam curing is only 1/10 to 1/20 of that, while the curing speed is 10 times that of thermal curing.
At the same time, since EB curing is carried out at room temperature, the stress concentration and residual stress caused by thermal shrinkage are greatly reduced, and the mechanical properties of the cured resin are improved.
The research of Janke et al. shows that the glass transition temperature of some EB-cured epoxy resins can be as high as 390°C, and its Tg has exceeded that of some polyimides, which can be used to prepare high heat-resistant materials.
Farmer et al. found through research that EB-cured epoxy resins can be formed by various methods such as lamination, fiber winding, resin transfer molding (RTM) and vacuum resin transfer molding (VARTM), and its processing technology is diverse.
Iverson et al. found that the mass fraction of volatile matter in EB-cured epoxy resins is generally less than 0.1%, which is much smaller than the emission during thermal curing, so that the impact of solvent volatilization on the environment and operators is minimized.
At present, there are still some differences in some aspects regarding the use of EB to cure epoxy resins. Zhang Zuoguang et al. found that if certain bisphenol A epoxy resins (such as Shell Epon 828) are only cured by EB, the resulting resin has a low degree of cure and requires heat treatment.
Heat treatment of EB-cured epoxy resins near their glass transition temperature can greatly improve their physical properties such as degree of cure and high-temperature modulus.
2. Reaction mechanism of EB-cured epoxy resin
EB curing refers to the cross-linking reaction between monomers or oligomers under the action of high-energy electron beams. Its reaction mechanism is similar to cationic polymerization.
Because the reaction system produces intermediates such as cations, anions, and free radicals under the action of EB, the curing mechanism of different systems is different. For epoxy resin systems, its curing mechanism is mainly cationic polymerization, and the initiator is usually diaryl iodonium salt or triaryl sulfonium salt.
Lappin et al. proposed a cationic curing mechanism that generates anions through free radical reactions and then initiates the curing reaction.
First, benzophenone is excited under the action of electron beams, then reacts with isopropanol to generate free radicals, and finally the free radicals react with iodine salts to produce proton acids to initiate cationic ring-opening polymerization. Its reaction mechanism is as follows:
Sui Gang et al. used GS, IR, ESR and other methods to study the reaction process and reaction mechanism of electron beam-cured epoxy resin.
According to the experimental results, it is deduced that the curing reaction of some epoxy resins (such as 828 epoxy resin) is carried out according to the cationic reaction mechanism. First, the iodine salt decomposes under the action of the electron beam and takes hydrogen atoms from the monomer or hydrogen-containing impurities to produce proton acid to initiate cationic ring-opening polymerization. The reaction mechanism is as follows: If the reaction system contains epoxy resins with more than two functional groups, it will react according to this mechanism to form a spatial network structure to achieve material curing.
3. Application prospects
Since France first realized EB curing of solid engine casing materials in 1990, the application field of this technology has expanded rapidly.
U.S. companies such as Aeroplas and Northrop have conducted extensive and in-depth research on the electron beam curing of large-scale monolithic structural materials and structural materials of aerospace vehicles with epoxy resin as the substrate, and obtained satisfactory results.
With the deepening of people's research on EB curing reaction, the preparation of high-performance composite materials by EB curing is or will be widely used in the following fields.
1) Aerospace field is used to manufacture structural and casing materials for military or civil aircraft;
2) Transportation field is used to prepare structural materials for vehicles such as automobiles, ships, and rail cars;
3) Construction and infrastructure field is used to prepare building materials with special requirements for weight and corrosion resistance. Such as telephone booths, oil pipelines, offshore drilling platforms, etc.;
4) Sports and leisure field is used for the manufacture of sports goods such as golf clubs, skis, tennis rackets, etc.
5) Other fields such as the use of EB-cured composite materials to prepare printed circuit boards, bulletproof equipment, lightweight protective devices and submarine shells.