Extension of Unsymmetrical Diisocyanates

Chain extender choice influences elastomer properties considerably. When a diamine is employed as extender, a higher level of physical properties usually results than if a diol were used, probably due to the introduction of urea linkages which enter into strong hydrogen-bonded interactions. A diamine is usually chosen as the chain extender when a relatively unsymmetrical diisocyanate is employed this is particularly true of polymers made by the prepolymer route and applies especially to the use of mixed toluene diisocyanates and to methylene diisocyanates whose bulky or hindered structure and, to some extent, their stereo configurations (see Fig. 3.5), limit the linearity in the polymer chain which is an essential feature of strength and elasticity in all rubber materials. 

However it is also established that the commonly used diphenylmethane and naphthalene diisocyanate-based elastomers are frequently chain-extended with glycols. Table 3.12 contrasts the effects of diamine and glycol chain extenders with a toluene diisocyanate/polyether-based prepolymer, Adiprene LI00. With the unsymmetrical diisocyanates, such as TDI and MDI, polyol chain extenders give elastomers of poorer properties than diamines, and it is usually desirable to introduce additional crosslinking into diol-extender elastomers by the use of a proportion of a triol such as 

However to keep the properties of polyol chain-extended urethane elastomers in perspective it should be noted that with symmetrical diisocyanates (described later), such as CHDI and PPDI, chain extension 

DIAMINE CHAIN-EXTENDER CONTROL OF THE PHYSICAL PROPERTIES OF TOLUENE DIISOCYANATE-BASED ELASTOMERS 

It can be seen from Table 3.12 that the use of a binary amine blend does not affect the elastomer properties much, as compared with the use of a single amine curing agent and this is true of a wide range of amines. However, tertiary amine blends do reduce the modulus and hardness in a manner consistent with the introduction of sufficient irregularity to interfere with good interchain interactions. 

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