Influence of Hydroxyl-Terminated Polybutadiene Variants on the Wettability of Ammonium Perchlorate

Increased adhesion is important for many material science and manufacturing processes, including solid propellants. Improved adhesion affects mechanical properties and minimizes unwanted flame spread in rocket motors. This study investigated the surface energy interactions between ammonium perchlorate (AP) and hydroxyl-terminated polybutadiene (HTPB) polymers. Analytical techniques were used to determine the energy of the three-phase system. The pendant drop method was employed to determine the surface tension of the commercial polymers, with results showing no variation besides PolyBD R45. Static contact angle measurements show that molecular weight, OH-group content, and viscous properties affect the wettability of commercial variants of HTPB with polytetrafluoroethylene (PTFE) and AP solid substrates. Results show commercial variants of HTPB forming contact angles on AP ranging from 27.0 to 40.1°. The contact angle of HTPB on PTFE ranged from 86.0 to 88.0°. The wettability of AP varied greatly with which crystal face formed the liquid–solid interface. Nanoindentation of the ⟨001⟩ and ⟨210⟩ crystal faces provided a first-order AP surface free energy measurement. The ⟨210⟩ crystal face was found to have higher energy than the ⟨001⟩ crystal face, indicating better adhesion. Limitations of the measurement and implications for AP propellant performance are discussed.