Epoxy resin second-generation

A second generation of phenolic dispersions, patented by J. S. Fry (33). involved the post dispersion of phenolic resins in a mixture of water and water-miscible solvents. To conform with air pollution regulations, the solvent was held to 20 volume %, or less, of the volatiles. A heat-reactive phenolic resin dispersion (34) and a phenolic-epoxy codispersion have become commercially available based on the above technology. Supplied at 40-45% solids, these products, which have a small particle size (0.75-1.0 ym), are better film formers than the earlier dispersions. Used alone or in blends with other waterborne materials, corrosion-resistant baking coatings may be formulated for coil coating primers, dip primers, spray primer-surfacers, and chemically resistant one-coat systems. Products of this type are also tackifiers for acrylic latexes, and such systems have been employed as contact, heat seal, and laminating adhesives for diverse substrates. 

The second generation of polymers was introduced during 1950—65 and includes a number of engineering plastics such as high-density polyethylene, isotactic polypropylene, polycarbonates, polyurethanes, epoxy resins, polysulphones and aromatic polyesters, also used for films and fibres. New rubber materials, acrylic fibres made of polyacrylonitrile and latex paint were also introduced. 

The reaction between an epoxy resin and an anhydride is rather sluggish and it is common practice to increase the rate by the addition of a catalyst, usually a tertiary amine. In this case, the tertiary amine appears to react preferentially with the anhydride to generate a carboxyl anion. This anion opens an epoxy ring to give an alkoxide ion which forms another carboxyl anion from a second anhydride molecule and so on, e.g. 

Door panels have a sandwich structure consisting of aluminum or stainless steel surface plates and aluminum or paper honeycomb core bonded with two-part epoxy adhesives. On inspection lids, switchboard covers, and seat side panels, decorated aluminum sheets with melamine resin are bonded with second generation acrylic adhesives (SGAs) to aluminum frames or stiffeners. O Figure 47.9 shows the cross section of a railcar with adhesively bonded parts (Suzuki 2007). 

Radiation curable adhesives can be based on raw materials that are not greatly different from those used in acrylic adhesives (first, second, and third generations), epoxy adhesives, urethane adhesives, and anaerobic adhesives. The formulation approach, however, is somewhat different. Very important is that no solvents are used to thin the resins a monomer reactive diluent is used instead. The monomer must be matched with the resin to give the desired set of properties with respect to adhesion, substrate, flexibility or stiffness, cure behavior, and durability of the cured product. Most of the radiation curable adhesives have an acrylate (or methacrylate) basis many of the acrylics are modified. 

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