Characterization of several HTPB binder samples by NMR, GPC and OH-number

Hydroxyl terminated polybutadiene (HTPB) continues to be the first choice as pre-polymer for an elastomer binder for high performance solid rocket propellants and plastic bonded high explosive charges. Cured with isocyanates to polyurethane elastomers, so-called HTPB binders have unique properties: high strain capacity with suitable values of young modulus, excel-lent low temperature properties with a glass-rubber transition temperature in the range of -75°C to -85°C (HTPB binders without plasticizer) determined using DMA at 0.1Hz, low hygro-scopicity, and good ageing behaviour, when protected by adapted antioxidant functionality. Further on, curing behaviour is well controllable and pot life is adjustable to the needs. In detail the achievable property spectrum depends on the structure of the chains, the cis-trans-vinyl ratio of C=C double bonds, the mean molar mass or mean chain length and the mean number of OH groups per molecule and their positions along the chains. Several HTPB samples were characterised with three analytical methods: (1) NMR (nuclear magnetic resonance) spectroscopy in 1H and 13C mode to retrieve several information such as mean molecular mass of the HTPB, ratio of cis, trans and vinyl isomers and their molecular environment, and with assumptions, also the mean number of OH groups per molecule; (2) GPC (gel permeation chromatography) to determine the molar mass distribution and the mean molar masses Mn, Mw, Mz as well as polydispersity D (relative to polystyrene standards); (3) the so-called OH number (OH-n) or hydroxyl value and the equivalent mass Eqm are obtained by acetylation of OH groups and back-titration of surplus acetic acid with potassium hydroxide solution (KOH). Discussion of the results in terms of suitability of and differences between the individual HTPB lots is included