Analysis and Characterization of Polyurethane Elastomers

The formulations used in the manufacture of polyurethane elastomers are critical, as variations in the chemical composition of raw materials seriously affect properties. In particular, trace impurities can catalyze or inhibit the various reactions, changing network structures and hence final elastomer properties. For these reasons analytical tests to quantify the quality and reactivity of urethane intermediates have been developed and their use in urethane technology is essential. Additionally, characterization and identification of unknown polymers is frequently necessary. Finally, it is necessary to have procedures which characterize the structure and degree of crosslinking (where this applies) of polyurethane elastomers. 

A routine measurement of diisocyanate content is usually desirable because, as most of the isocyanates are prepared by the phosgenation process, free phosgene may be present if it has not been removed in the final purification step. This will be included in the hydrolyzable chloride test, and partly included in the acidity test, if hydrolysis takes place. Also, if the diisocyanate has been stored under adverse conditions, some isocyanate could be converted by moisture to insoluble urea. Alternatively some dimerization may occur. In both cases isocyanate is consumed and the result is a decrease in active content. Additionally, diisocyanates do show some batch to batch variations in activity hence it is convenient to measure their reactivity against a control, a polyester (or polyether) of previously known activity, by making a small quantity of the polyurethane and measuring its final reaction exotherm temperature, rate at which its exotherm is achieved, pot life and time to achieve gelation. If a more 

The isomer ratio in commercial toluene diisocyanate (that is, the proportion of the 2,4 and 2,6 isomers) affects its reactivity. This ratio may be determined by the infrared method described later. 

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