Radiation-Curing Adhesives

Gamma irradiation curing of epoxy resins for structural adhesives — Radiation cure polymerization of commercial diglycidyl ether of bisphenol F epoxy resin has been achieved using Co-60 irradiation source, compounding the monomer with and onium salt catalyst [9],... 

Chem. Descrip. Neopentyl glycol linear adipate dioi Uses Polyol for PU coatings, adhesives, radiation-curing appiics. Properties Vise. 2800 cps (40 C) hyd. no. 110-114 Storage Hygroscopic store in tightly closed containers Fomrez 55-225 [Uniroyai]... 

W.C. Perkins, "Radiation Curable Pressure-Sensitive Adhesives," Radiation Curing, T, 4 (August 1980). 

Higher alkyl acrylates and alkyl-functional esters are important in copolymer products, in conventional emulsion appHcations for coatings and adhesives, and as reactants in radiation-cured coatings and inks. In general, they are produced in direct or transesterification batch processes (17,101,102) because of their relatively low volume. 

The vast majority of reactive hot melts are moisture-curing urethane adhesives. Radiation (UV/EB) curable adhesives have been explored in the laboratory since the mid-1970s, but are only recently beginning to gain significant market penetration, particularly for PSA applications. The formulation and properties of these two classes of adhesives are discussed below. 

Huber, H.F., Radiation cured adhesives. Beta-Gamma, 1, 3 (1992). 

Woods, J.G., Radiation-curable adhesives. In Pappas, S.P. (Ed.), Radiation Curing, Science and Technology. Plenum Press, New York, 1992. 

This technique is used for the production of radiation-cured coatings, adhesives, and inks. The process is not accompanied by the release of heat, which is particularly important in the case of heat-sensitive materials, e.g., wood, cardboard, paper, plastics, etc. Various radiation sources are used for this technique, among which UV and EB are most useful. 

EB radiation-cured PVC film surface induces a big improvement in tensile strength and excellent printing ink adhesion and adhesive receptance [302]. PVC foams containing plasticizers, thermally activated blowing agents, cross-linkers, and heat stabilizers have been manufactured by EB radiation technology [303]. 

A polymerizable carbazole composition, (II), was prepared by Kanno et al. (2) that was radiation cured and used as an adhesive. 

IV), and related monomers, (V), useful as radiation curing agents were prepared by Brand et al. (4) and used in adhesives. 

Single-component epoxy adhesive formulations are the largest type of epoxy adhesives sold, with about 55 percent of the consumption, while two-component formulations account for another 44 percent of the volume. Radiation cure formulations represent the remainder of the market. Epoxy adhesives can also take many forms including solids, solvent-free liquids, solvent-borne systems, and waterborne systems. 

There are basically two types of epoxy acrylate resins used in formulating adhesive systems. One is a vinyl ester resin that is used in two-component adhesive formulations much as a DGEB A epoxy or a polyester resin is. The other is a special type of resin that is used in radiation cure processes. This latter type of epoxy acrylate does not have any free epoxy groups, but reacts through its unsaturation. 

Epoxy acrylates are also commonly used as oligomers in radiation-curing coatings and adhesives. However, their name often leads to confusion. In most cases, these epoxy acrylates have no free epoxy groups left but react through their unsaturation. These resins are formulated with photoinitiators to cure via uv or electron beam (EB) radiation. The reaction mechanism is generally initiated by free radicals or by cations in a cationic photoinitiated system. The uv/EB cured epoxy formulations are discussed in Chap. 14. 

Epoxy adhesives that cure via radiation (ultraviolet light or electron beam energy)... 

Radiation cured epoxy adhesives significantly reduce the long cure times needed for conventional adhesives. These adhesives develop excellent physical and mechanical properties in a nonthermal cure process that requires a cycle time of only seconds to minutes, versus hours for conventional thermal curing. 

In 1995, U.S. consumption of radiation cured products was 77 million lb, valued at 450 million.1 Growth is forecast to average about 7 percent per year—a rate about twice that of conventional thermal cured products. Although radiation curable adhesives comprise a relatively small segment of the overall adhesive market (13 percent) and epoxy adhesives represent an even smaller component, epoxy systems are a fast-growing part of the market. 

Radiation is used to crosslink (Fig. 14.2) or cure organic resins into durable coatings or adhesives having excellent physical properties with high chemical and temperature resistance. Radiation curing technology involves at least four considerations type of radiation... 

Radiation curing adhesives are generally applied as solvent-free liquids. High-solids EB and uv curing liquid adhesives have been formulated from a variety of resins and elastomers. They include epoxy acrylates, epoxies, other acrylates, polyesters, blends of acrylate monomers with elastomers, and other compositions. 

Other additives used to improve the processing and performance of radiation cured adhesives are similar to those that might be found in more conventional formulations. These include antioxidants, defoamers, flow and wetting agents, and slip aids. 

The yields of radiation-induced polymerizations can be very high. No additives are required, which makes it possible to synthesize very pure polymers. The initiation step is temperature independent giving rise to an easily controlled process at any desired temperature. These features account for the commercial interest in radiation polymerization. The very high speeds attainable within the layers of monomers subjected to powerful electron beams explain the wide use of this technique in radiation curing of adhesives, inks and coatings. The corresponding formulations are "solvent-free" and involve pre-polymers and monomers as reactive diluents. 

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