The properties of liquid hydroxyl-terminated polybutadiene (HTPB) stocks crosslinked with isocyanates of different functionalities were intensively investigated. The stocks were modified by inclusion of small proportions of various low molecular weight diols to increase the urethane concentration. The following studies were carried out: (1) the system functionality was varied at a low and relatively constant urethane concentration; (2) the system functionality was varied with the urethane concentration constant but at a higher level; (3) at a constant crosslink density, the urethane concentration was varied by adding low molecular weight diols; and (4) the structure of the diols used in the previous experiments was varied.
A correlation between the per cent functional groups reacted, the average functionality of the system, and the mechanical properties of simple HTPB stocks was attained. Furthermore, a correlation was found between mechanical properties and urethane concentration at constant crosslink density. The properties of the stocks could be varied enormously by addition of low molecular weight diols in relatively small proportions by weight. The structure and concentrations of these diols caused significant variations in the elongation-at-break and tensile strength values.