Curing, composite resins photoinitiators

Y.J. Park, K.H. Chai, H.R. Rawls, Development of a new photoinitiation system for dental light-cure composite resins. Dent. Mater. 15 (1999) 120-127. 

By contrast, most modem composites cure by a light-activated process. The typical photoinitiator system used is based on camphorquinone (Fig. 3.4) with an aromatic tertiary amine as accelerator [13]. These are sensitive to blue light at 468 nm, and irradiation causes homolytic decomposition of the camphorquinone to form free radicals, which then initiate the polymerization and associated hardening of the composite resin. 

Photopolymerizable epoxies using onium salt photoinitiators also show considerable promise for use in high performance composite applications. Traditional thermally cured epoxy resins are already well entrenched in these applications however, the use of the recently developed photocurable epoxy materials offers considerable potential for rapid fabrication of fiber-reinforced composites without the need for cumbersome ovens and long cure times. Photopolymerized epoxy laminates and filament wound pipe have already been demonstrated in our laboratory. 

FP is not currently in commercial use, although there have been several patents issued related to it. The first is for an In depth curing of resins induced by UV radiation, which used a combination of UV hght to start free-radical FP [63]. In 2001, Gregory patented Ultraviolet curable resin compositions having enhanced shadow cure properties in which the frontal curing of cycloaliphatic epoxides was achieved with a cationic photoinitiator in tandem with a peroxide [64]. Pojman was awarded two patents on functionally gradient materials prepared by FP [65, 66]. The maker of chemical anchors, Hilti, has two patents on FP but has not commercialized either of them [67, 68]. 

Gregory patented Ultraviolet Curable Resin Compositions Having Enhanced Shadow Curing Properties in 2001. This patent has the same idea as Dixon s patent, that is, use photopolymerization at the surface of a filled resin to trigger a thermal front. Gregory went beyond using peroxide-cured vinyl resins. He used dialkyl iodonium salts, sensitized by a-hydroxy ketones, that produced Lewis acids upon UV irradiation. The Lewis acid triggered cationic polymerization of epoxy resins and vinyl ethers. The heat from the photoinitiated process decomposes peroxides into radicals that react with the iodonium salts to produce Lewis acids. 

On-aircraft repairs of composite using a rapid-cure resin system of composite component with UV light irradiation based on TRI patenP have been developed by the U.S. Air Force Research Laboratory. Alternating layers of the acrylate-based resin system and woven fiberglass (the widely used wet la)mp procedure) are applied to fill the hole and form a UV curable composition. The width of the patch can be up to 2 ft (0.6 m) and the depth as much as 0.2 in. (5 mm). The cure time using a 400 W UVA lamp is reported to be 20 min. Although it is essentially a depot repair, it can be done field when necessary to return an aircraft to service. Because of the necessity to cure relatively thick repair patches, Us-acylphosphine oxide was used as a photoinitiator. An example of the patented UV curable resin system used for the repairs is in Table 11.1. 

The compositions of materials photocrosslinkable by cationic mechanism consist of mixtures of various vinyl ethers, or epoxides, or both. Difunctional cycloaliphatic epoxides have been used extensively in some UV curable systems, often as diluents for the various epoxy resins described in Chapter 6. Use of various divinyl ethers is also extensive. Because some cationic photoinitiators also generate free radicals, some compositions may contain mixtures of both types of materials, those that cure by cationic and those that cure by free-radical mechanisms. 

The papers presented in the following chapters represent advances in pressure sensitive adhesives (ultraviolet light activated acrylate monomer - low Tg polyether formulations) photoinitiated cationic polymerization (light activated aryliodonium and arylsulfonium salts of lewis acids in epoxy resin formulations) polymer and formulation design criteria for radiation curable adhesives radiation curable composites (dynamic thermal analysis characterization of electron beam cured... 

Multi-layers laminate composites made of epoxy resin and glass fibers were produced by UV curing in the presence of various photoinitiators, in order to be used in bullet proof vests [181]. For comparison, a vinyl ester was used. Sandwich strucmres were produced by lay up process. It was proven that epoxy resin cured rapidly under UV radiation, in opposition to vinyl ester which required 2 days to fully cure, and a 3 min exposure was enough to reach maximum penetration and composite hardness. 

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