Diisocyanate-Linked Polymers. III. Relationships between the Composition and Ultimate Tensile Properties of Some Polyurethane Elastomers

Abstract

Abstract The tensile strength and elongation at rupture of elastomers vary markedly with the experimental conditions used in measurement. For example, when measured at a fixed rate of extension, the tensile strength may increase by a factor of 100 or more as the temperature is decreased, and the ultimate elongation may increase concomitantly by a factor of 10 or more and then decrease to a few per cent. The ultimate properties also depend on the chemical nature of the network chains, the degree of crosslinking, and on the regularity of spacing of the crosslinking sites. In addition, those elastomers which crystallize during extension (e.g., vulcanized natural rubber) normally exhibit higher tensile strengths and ultimate elongations than those which do not crystallize (e.g., SBR rubbers). In seeking relationships between the structure and the ultimate properties of elastomers, these various factors which affect ultimate properties must be carefully considered. Previously a study was made of the tensile strength and ultimate elongation of several series of polyether-polyurethan elastomers prepared from polyoxypropylene glycol 2025 (PPG), trimethylolpropane (TMP), and either toluene 2,4-diisocyanate (TDI) or hexamethylene 1,6-diisocyanate (HDI). The structure of these elastomers was characterized by (1) the number of network chains per unit volume ν and (2) the concentration of urethan groups [U]. These parameters could be varied independently over certain ranges by making appropriate changes in the average molecular weight of PPG 2025 through blending it with dipropylene glycol (DPG). The glass temperature Tg of the elastomers increased linearly with [U], the rate of increase being considerably greater for the TDI-linked than for the HDI-linked elastomers. These elastomers apparently did not crystallize upon extension, and their ultimate properties, measured at a fixed extension rate, were found to depend on both [U] and ν. However, when compared in corresponding temperature states, i.e., at equal values of T−Tg, the ultimate properties over a wide temperature range were found to be independent of [U], or approximately so, when [U] was less than about 1.85 moles/kg. Also, the ultimate elongation was observed to be inversely proportional to ν, although the proportionality constant was temperature-dependent. It thus appears that certain elastomers which do not crystallize have ultimate properties which depend not only on ν and Tg but also on the chemical nature of the network chains.