Pentaerythritol and Glycerol Ester-Based Rosin-Modified Hydroxyl-Terminated Polybutadiene (HTPB)

Hydroxyl-terminated polybutadiene (HTPB) has widespread applications such as in adhesives, coatings, and solid propellants due to its durability and excellent mechanical strength when cross-linked, which can be maintained at low temperatures. However, many of the additives used to modify the properties of HTPB are not sustainably sourced nor have the functionality such that tailoring of HTPB properties can be achieved. Here, we describe the use of the pentaerythritol ester of rosin (PER) and glycerol ester of rosin (GER), sourced from gum rosin (pine trees), to modify the rheology and mechanical properties of uncross-linked and cross-linked HTPB. Both rosin esters are compatible with HTPB resulting in a change in the glass transition temperature (T _g) of HTPB, which is concentration-dependent. The inclusion of PER increased the viscosity of uncross-linked HTPB more than GER due to the additional abietic functionality per molecule. The rosin esters also compete with the terminal hydroxyl groups of HTPB during cross-linking with toluene diisocyanate (HTPB-PU). Consequently, the cross-link density of HTPB-PU decreases and the molecular mass between cross-links increases with increasing PER/GER content. This competition results in a decrease in Young's modulus and tensile strength of HTPB but a significant increase in elongation at break (+153%) and tensile toughness (+76%) of HTPB.