Bond, Link biuret

Some cross-linking takes place in the form of side reactions such as biuret and allophanate bonds. Deliberate introduction of covalent cross-linking can be carried out by the introduction of multifunctional agents (mainly triols) into either the prepolymer or chain extension system. 

A variety of polymeric subunits is used to make polyurethanes. These include polyesters and polyethers. The major interchain linkages are molecular forces such as hydrogen bonding and the London force. Depending on the type of chain extender and processing temperature, there also may be biuret or allophanate cross-links. 

Another opportunity for bond break down is the cleavage of urethane and/or urea compounds and their subsequent products, such as allophanates or biurets from reverse reaction or hydrolysis. Figure 5.111. In consequence of this damage process, the rigid phase and the links between rigid and soft phases are particularly weakened. Cleavages in urethane and urea units are in particular facilitated by the presence of catalysts in the foam [702]. 

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