Elastomer Gum Types

Such adhesives can be applied to adherend surfaces from ketone solution and after drying may be heat- or solvent-activated at 50-70°C. They are especially popular for vinyl and wood bonding. In this, solvent mixtures of a urethane elastomer prepolymer are mixed with a solution of the diisocyanate. This reactive mixture is applied to a substrate where it dries 

A stable, blocked di- or polyisocyanate in suspension, including aqueous suspension or solution is applied alone in combination with a conventional plastic or rubber type, in combination with a di- or polyhydroxy material or in combination with a preformed polyurethane vehicle to adherend surfaces. The coating is dried, the coated adherend surfaces contacted and the assembly heated to decompose the blocked isocynate. This generates free di- or polyisocyanate which proceeds to bond the adherend directly and through reaction with the vehicle which may be a pre- or in-situ formed polyurethane. The cleaved blocking agent diffuses into the surrounding matrix or escapes into the air. 

The formation and thermal cleavage of blocked isocyanates can be represented as in Fig. 8.8, where n is usually 2, 3 and sometimes more. Cleavage temperatures range from c. 60 to 200°C depending upon the compound. 

The stability of blocked isocyanates toward active hydrogen substances enables their formulation and application in reactive media, including water and alcohols. But usually the reactive medium must be removed by evaporation before the blocked isocyanate is heated to its decomposition temperature, otherwise the generated isocyanate would be consumed by reaction with the medium before undergoing the intended reactions with adherend and reactive vehicle (if present). This may not be necessary if the isocyanate is more reactive with the adherend or vehicle than it is with the medium. 

Mondur SH, thought to be a cresol blocked TDI trimer (Mobay) 

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