Urethane cross-links, formation

The addition polymerization of diisocyanates with macroglycols to produce urethane polymers was pioneered in 1937 (1). The rapid formation of high molecular weight urethane polymers from Hquid monomers, which occurs even at ambient temperature, is a unique feature of the polyaddition process, yielding products that range from cross-linked networks to linear fibers and elastomers. The enormous versatility of the polyaddition process allowed the manufacture of a myriad of products for a wide variety of appHcations. 

The water reaction evolves carbon dioxide and is to be avoided with solid elastomers but is important in the manufacture of foams. These reactions cause chain extension and by the formation of urea and urethane linkages they provide sites for cross-linking, since these groups can react with free isocyanate or terminal isocyanate groups to form biuret or allophanate linkages respectively (Figure 27.5). 

Network Formation and Degradation in Urethane and Melamine-Formaldehyde Cross-Linked Coatings... 

Other reactions which lead to branching and cross-linking are the formation of allophanate and biuret linkages. The allophanate linkages occurs when the hydrogen on the nitrogen atom of the urethane group reacts with an isocyanate ... 

The exceptional stability of the urethane bond results from thermodynamic determinants of bond strength, from the crystallisation that results from the structure of both the polyol and the strongly hydrogen bonding urethane and from cross-links that result from the formation of hard chain segments (Evans and Greene, 1982). 

Such multifunctional esters are used to promote the formation of a rigid cross-linked structure on polymerization. Quantitation of these esters is normally based on GC analysis using an internal standard. Either narrow or wide bore capillary columns may be used. In the case of ultraviolet cure-initiated formulations, the use of higher-molecular mass urethane-methacrylate resins such as Scheme 7 is involved. 

Polyurethanes are generally synthesized by addition polymerization between a polyalcohol and a poly-isocyanate. This is an exothermic reaction caused by the release of a proton from the alcohol group followed by a general molecular rearrangement by the formation of the urethane bond [23], If both reagents are bi-functional linear polyurethanes are obtained, while if functionalities are increased some cross-linked chains are formed, with the formation of reticulated structures. In summary, one of the most common s mthesis routes for TPUs consists basically of the reaction of three main components. 

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