Step polymerization polyurethane adhesives

A typical polyurethane adhesive may contain, in addition to the urethane linkages, aliphatic and aromatic hydrocarbons, esters, ethers, amides, urea, and allophanate groups. Polyurethanes are formed by the addition reaction of diisocyanates or polyisocyanates with polyols (Figure 3.12) through a step-growth polymerization... 

A polyurethane adhesive is produced by reacting together two basic raw materials - a polyol and an isocyanate (see Step polymerization). The two most common polyols are based on polyether and polyester, and the two principal isocyanates used are toluene diisocyanate (TDI) and diphenyl methane diisocyanate (MDI), both of these latter products being extremely reactive. 

The cure mechanisms of structural adhesives depends on the resin chemistry and can be based on either Chain polymerization or Step polymerization reactions. The condensation cure mechanism is the most important for phenolic adhesives. Epoxide adhesives and Polyurethane adhesives, which require stoichiometric or balanced combination ratios as well as thorough mixing for maximum performance. Off-ratio mixing or incomplete mixing can alter significantly the final adhesive structure and effect the resulting properties. 

Condensation polymers, which are also known as step growth polymers, are historically the oldest class of common synthetic polymers. Although superseded in terms of gross output by addition polymers, condensation polymers are still commonly used in a wide variety of applications examples include polyamides (nylons), polycarbonates, polyurethanes, and epoxy adhesives. Figure 1.9 outlines the basic reaction scheme for condensation polymerization. One or more different monomers can be incorporated into a condensation polymer. 

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