Toluene diisocyanate polyether prepolymers

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). 

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... 

Following the early developments using NDI, it was found that by using TDI instead, a far more stable prepolymer could be made. Stable prepolymers are normally made using either polyesters or polyethers that have been reacted with a slight excess of a diisocyanate such as toluene diisocyanate (TDI) or methylene diisocyanate (MDI). Provided the storage is moisture free, the stable prepolymer may be kept for months before use. The polyurethane is prepared by chain extension with diols or diamines. 

Prepolymers are formed by terminating the polyether chain with toluene diisocyanate, resulting in free isocyanate groups at the ends of the chain. These chains are then joined by the chain-extender (e.g. MOCA) and the amount of chain-extender used greatly influences the final polymer properties. Theoretically the chain extender added should be equivalent to the isocyanate content of the prepolymer, but in order to promote... 

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. 

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