In addition to the two classic chain extenders recommended for the polyurethane systems derived from hydroxyl-terminated polybutadienes, two more aliphatic diols are identified through extensive testing for the same utility.
Chain extenders play a significant role in enhancing the final mechanical properties of polyurethanes derived from hydroxyl-terminated polybutadiene (HTPB) and 4,4´-methylenebis(phenyl isocyanate) (MDI). Conventionally, N,N-diisopropanol aniline (DIPA) and 2-ethyl-1,3-hexanediol (EHD) are recommended for their compatibility and physical property enhancement of the polyurethane elastomer. By comparison, 1,4-butanediol (BDO), widely employed in polyurethanes based on polyether or polyester polyols, is not a good choice owing to its poor compatibility with HTPB resins. A systematic screening of both aromatic and aliphatic diols as potential chain extenders in standard HTPB-based polyurethane formulations has been carried out. Although a good correlation could not be drawn between the diol structure and mechanical performance, two new aliphatic diols were notable for their superior performance.