A study on the effectiveness of varying curative ratio to optimize performance of hydroxy terminated polybutadiene (HTPB) and hexamethylene diisocynate (HDI) based polyurethane elastomers

Abstract

Polyurethanes possess exceptional performance traits and are employed in almost every sphere of life. Extended pot life up to 6–8 h with acceptable performance to produce larger composite propellants is a pre-requisite. Aromatic diisocyanates being more reactive gives inadequate pot life (3–4 h) and aliphatic diisocyanates gives unrealistic pot life (20–24 h). Curative ratio (R = NCO/OH) is the most frequently used to tailor the performance properties of polyurethanes. In this research work, hexamethylene diisocyanate (HDI),an aliphatic diisocynate with symmetric structure and a moderate reactivity has been utilized as a curative to extend the pot life up to 6–8 h with acceptable performance traits by varying the curative ratio from 1.0 to 2.0. The synthesized PU elastomers showed a reasonable pot life of 437 min at NCO/OH = 1.0 that decreased to 305 min when curative ratio increased to 2.0. The effectiveness of curative ratio variation to fine tune physical, mechanical and thermal properties was gauged. With increasing curative ratio, the degree of hydrogen bonding and microphase separation increased. Moreover, for these polyurethanes, tensile strength increased from 0.519 MPa to 1.249 MPa, and elongation at break declined from 1068 % to 438 %. The structural integrity of composite propellants is dependent upon the mechanical properties and the curing ratio = 1.6 is considered as optimum because it provides a good balance between the soft segment and the hard segments and gives polyurethane elastomers a good combination of mechanical properties and hardness. Thermal stability in terms of onset degradation temperature (T₅) also increased from 345 to 364°C.