Structure–Property Behavior of Hydroxyl-Terminated Polybutadiene-Based Urethanes Additionally Cross-Linked Using Sustainable Biosourced Rosin Esters

Hydroxyl-terminated polybutadiene (HTPB) particularly when cross-linked with a diisocyanate is a very versatile elastomer having excellent mechanical and low temperature properties suitable for applications as diverse as binders in rocket propellants to surface coatings. These properties can be tailored further by the inclusion of a plasticizer, e.g., octadecyl adipate, but there are many technical challenges remaining around the use of such plasticizers, including migration from and miscibility with HTPB, together with the problem that such plasticizers are synthesized from non-renewable feedstocks. To address these limitations, rosin and functional rosin esters, sourced from pine trees, were blended with HTPB at loadings up to 20 wt % prior to cross-linking with toluene diisocyanate. All rosin esters studied were shown to be fully miscible with HTPB; a single glass transition temperature (T_g) was measured for all HTPB/rosin ester blends slightly above the T_g (−79 °C) of HTPB and well below that of the rosin esters (38–58 °C). Simultaneous wide-angle X-ray scattering (WAXS) and small-angle X-ray scattering (SAXS) measurements confirmed that there was no phase separation between the HTPB and rosin esters when blended. From increases in interdomain sizes, measured from X-ray scattering experiments on postcured samples, only the functional rosin ester (T₃) takes part in the cross-linking reaction. Consequently, for the HTPB modified with T₃ at 10 wt %, the elongation at break (ε) increased from 275% for unmodified HTPB to 600% and critically without a decrease in ultimate tensile strength (σ). For 20 wt % T₃, ε increased to 1200%, and the material displayed strain-hardening behavior. The mechanical properties of HTPB can be tailored using functional rosin esters to alter the diisocyanate cross-linking reaction of the rubber.