Alloyed or Modified Two-Polymer Adhesives

These adhesives are important as structural adhesives, especially in metal bonding. They are comprised of a thermosetting and a thermoplastic polymer, including certain elastomers. Although each component has adhesive properties by itself, on the whole, the conjoint system forms a stronger and more versatile adhesive. The two-polymer systems have been particularly successful as film and tape adhesives, with applications ranging from building construction to aerospace.  

The physical properties of each component polymer are modified by the addition of the other, possibly increasing heat resistance of one component, while reducing that of the other. Similarly, the toughness of one may be increased by sacrificing the flexibility of the other. Therefore, it is possible to formulate a variety of adhesives with a wide range of characteristics by simply varying the ratio of one polymer to the oflier. 

In most widely used two-polymer adhesives, the thermosetting component is phenolic. Phenolic resins are generally compatible, although not easily miscible, with a number of thermoplastic polymers. Particularly good compatibility is demonstrated between conventional alcohol-soluble phenolic resins and polyvinyl esters and acetals. Epoxies are important in two-polymer adhesive systems. The most important thermoplastic components are the polyvinyl acetals (polyvinyl formal and butyral) and synthetic rubber, particularly nitrile rubber. Soluble nylons are also an important class.  

Five of the most important two-polymer adhesives used in films and tapes include vinyl-phenolics, epoxy-phenolics, nitrile-phenoUcs, nylon-epoxies, and elastomer-epoxies. Neoprene-phenolics are available in organic solutions and in supported and unsupported 

Epoxy polysulfides are available as two-part liquids or pastes that cure at room temperature or higher to rubbery solids that provide bonds with excellent flexibility and chemical resistance. These adhesives bond well to a number of substrates. Shear strengths and elevated-temperature properties are low, but resistance to peel and low-temperature properties are quite high. 

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In rubber-modified polymers like high impact polystyrene or acrylonitrile-butadiene-styrene (ABS) resins, the toughening effect of the dispersed rubber particles appears only in the presence of block or graft copolymers. These copolymers regulate the particle size of the rubber dispersion and achieve adhesion of the two phases. Hence, graft copolymers are of practical importance in polymer alloys. 

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