Radiation-curable adhesives

In this paper/ we shall describe the development of adhesives during the last several years and the trends for further advances. In general/ traditionally used / solvent-borne systems (4) will be gradually replaced by waterborne systems/ hot-meltS/ nonvolatile solid (or liquid) systems/ two-part adhesiveS/ radiation-curable adhesiveS/ and powder and reactive liquid systems (Table 1). 

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... 

K. C. Stueben is a Senior Research Scientist at Union Carbide s Technical Center in Bound Brook, New Jersey. He received his Ph.D. from the Polytechnic Institute of New York in 1960. His research experience has encompassed a number of areas, including micellar catalysis, synthetic paper, polymer flammability, exploratory adhesives, radiation curable pressure sensitive adhesives and both laboratory and plant scale preparation of condensation polymers, latexes and industrial chemicals. He is the author of numerous papers, chapters and patents in these fields. Dr. Stueben is currently Director of the Adhesion Society s course on Adhesion. 

Fig. 10. Generalized formulation design outline for radiation-curable coatings and adhesive systems. The cross-linker is a multifimctional unsaturated cross-linking agent or oligomer, rj = viscosity CR = cure rate S = shrinl ge H = hardness F = flexibility A = adhesion 7 = surface energy ...

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. 

While the chemistry of radiation curable hot melt adhesives is the same as that used in liquid (syrup) adhesives and coatings discussed elsewhere in this volume, there is a fundamental difference between the objectives of reaction in the two types of systems. Syrups consist largely or entirely of reactive monomeric and/or oligomeric materials. Radiation is used to initiate the polymerization of virtually the entire mass. In contrast, hot melts generally contain polymers initially, and these polymers are capable of reaction via radiation to produce chain extension and... 

One key consideration in developing radiation curable adhesive systems is the thermal stability and volatility of any photoinitiators used. These chemicals are designed for liquid systems where these issues do not arise. Few of the commercial photoinitiators have adequate thermal stability at the highest hot melt temperatures (180-200°C) and many are too volatile. Reduced application temperatures and special antioxidant packages are often required. 

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

NR, styrene-butadiene mbber (SBR), polybutadiene rubber, nitrile mbber, acrylic copolymer, ethylene-vinyl acetate (EVA) copolymer, and A-B-A type block copolymer with conjugated dienes have been used to prepare pressure-sensitive adhesives by EB radiation [116-126]. It is not necessary to heat up the sample to join the elastomeric joints. This has only been possible due to cross-linking procedure by EB irradiation [127]. Polyfunctional acrylates, tackifier resin, and other additives have also been used to improve adhesive properties. Sasaki et al. [128] have studied the EB radiation-curable pressure-sensitive adhesives from dimer acid-based polyester urethane diacrylate with various methacrylate monomers. Acrylamide has been polymerized in the intercalation space of montmorillonite using an EB. The polymerization condition has been studied using a statistical method. The product shows a good water adsorption and retention capacity [129]. 

Since the optical fibers are placed info locations fhat are difficult to access, the coatings are expected to last over 20 years. Tests indicated that UV cured protective coatings would be mechanically stable up to 100 years under ambient conditions. Radiation curable adhesives can also be used for end-to-end splicing, termination of bundles, construction of optical sensors, and other areas in the optical field. ... 

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. 

Market penetration is expected to increase further due to stricter environmental regulations and the availability of a greater variety of products. The first radiation curable adhesives were limited to acrylate and epoxy resins. Today, many different types of radiation curable adhesive systems are commercially available. 

The chemistry and curing mechanisms of these systems are outlined in the following sections. Further information can be found in several textbooks on radiant cured adhesives.2,3,4 A significant amount of information can also be acquired through RadTech, a professional society focusing on radiation curable products.5... 

Practical radiation curable materials are composed of more than a single reactive monomer. Commercial adhesives normally contain the following ... 

The two most prominent liquid radiation curable adhesives are free radical polymerization epoxy acrylates and cationic polymerization epoxies. Such adhesives are generally used as polymerizable syrups. A wide range of prepolymers can be acrylated including epoxies, urethanes, polyesters, polyethers, and rubbers. Elastomer-tackifying resin blends are often used in these formulations. 

Epoxy acrylates are dominant oligomers in the radiation curable adhesives market. A bisphenol A epoxy resin is reacted with acrylic acid or methacrylate acid to provide unsaturated terminal reactive groups. The acrylic acid-epoxy reaction to make bisphenol A diacrylate destroys any free ingredients such as epichlorohydrin used to make the DGEBA epoxy starting raw material. 

Monomers are primarily used to lower the viscosity of the uncured material to facilitate application. The monomer must be matched with the resin to give the desired set of properties with respect to adhesion to the substrate and bulk properties such as flexibility, stiffness, cure behavior, and durability. Early radiation curable monomers had problems associated with toxicity and skin sensitivity newly developed monomers have been significantly improved in this respect. 

Pigments generally inhibit uv curing to some degree since the pigments absorb and/or scatter uv radiation. This interferes with the ability of the photoinitiator to absorb the light energy required to initiate the polymerization reactions. Thus, the majority of commercial radiation curable adhesives are clear or contain silica. 

Formulation details are then presented in Chapters 11 through 14 for the various possible forms of epoxy adhesive systems room temperature and elevated-temperature curing liquids, pastes, and solids. The more or less unconventional forms of epoxy adhesives are also identified and discussed, since these are now achieving prominence in industry. These include uv and electron beam radiation curable, waterborne systems, and epoxy adhesives capable of curing via the indirect application of heat or energy. 

R. P. Eckberg, "Chemistry and Technology of Radiation Curable Silicone Release Coatings," in D. Satas, ed.,. Advances in Pressure Sensitive Adhesives, Satas Associates, Warwick, R.I., 1992. 

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