As Polyurethane Elastomer Without or With Added Polyisocyanates

As Polyurethane Elastomer Without or With Added Polyisocyanates 

This has become the most important type of isocyanate-based adhesive system. In this method a preformed, fully reacted, high molecular weight polymer is employed as a vehicle in the adhesive formulation. The strength of the vehicle holds adherend members in exact position after assembly 

In the practice of this method the high molecular weight polyurethane gum or thermoplastic resin is dissolved in a suitable dry, isocyanate-inert solvent. In the case of the polyurethane gums free di- or polyisocyanate is added to the solution. This is also sometimes done in the case of the thermoplastic polyurethane resins but frequently it is unnecessary, in which case the use of dry, isocyanate-inert solvents is also unnecessary. 

Such cements are applied to adherend and allowed to dry to the desired degree of tackiness before contacting the adherend coated surfaces. Or, as in the case of the crystallizing thermoplastic polyurethane resins, the adhesive coating may be allowed to fully dry and crystallize. Subsequent heat activation of the coated adherend surfaces, then in contact with each other, rapidly produces a strong bond. 

So-called anaerobic polyurethane adhesives also belong to this category. This system combines urethane poly-addition chemistry with free-radical initiated addition polymerization. A polymerizable alcohol such as j -hydroxyethyl methacrylate is reacted with an equivalent amount of a diisocyanate such as TDI or with an isocyanate-terminated urethane prepolymer. An organic hydroperoxide is then added to such intermediates 

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