Toluene diisocyanate elastomers, diamine chain-extenders

The principal of formation of this type of polyurethane elastomer medium, based on a cationic urethane latex, is where an isocyanate-terminated prepolymer derived from either a polyester or polyether diol and toluene diisocyanate is first chain-extended with an alkyl diethanolamine to yield a relatively low molecular weight urethane capable of further chain-extending reactions. Emulsification occurs when the partially extended urethane is added with high-speed mixing to 3% aqueous acetic acid. Curing of the latex takes place either by reaction of water with the terminal isocyanate groups or by reaction with water-soluble diamines. 

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

A number of different hindered diamines have been investigated as a substitute for MOCA (1). In addition to diamine curing agents, which are used most frequently with elastomers based on polyether polyols and toluene diisocyanate (TDI), prepolymers based on polyether or polyester polyols and 4,4 -diphenylmethane diisocyanate (MDI), can be cured with diols to yield elastomers with similar properties to those of diamine-cured polyester-TDI elastomers. The most common chain extender is butanediol. However, to achieve improved mechanical properties, especially at elevated temperatures, aromatic diols are often used. The most common one is hydroquinone di-(beta-hydroxy-ethyl) ether (HEQ). 

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... 

---