Fractionation of Polymer Binders for Solid Propellants

Supercritical fractionation was investigated for the special-purpose polymers of both the diol and triol of a glycidyl azide polymer and a hydroxy-terminated polybutadiene. Hydroxy polybutadiene is used on a large scale as an ingredient in plastic bonded explosive (PBX) propellant formulations the hydroxy functionality of the polybutadiene reacts with an isocyanate functionality of another prepolymer to form a urethane. In the polysiloxanes section we referred to functionally terminated polymers as macromonomers in the terminology of the urethane industry, however, isocyanate-terminated polyester polymers are normally referred to as prepolymers in their reaction to form the urethane, even though the prepolymers are typically between 1,000 and 10,000 molecular weight. 

The heterogeneous nature of polymeric propellant binders has long been suspected of contributing to the significant variability encountered in the cure and combustion properties of solid propellants. The feasibility of purifying and fractionating propellant binders with supercritical fluids to remove those 

A commercially available HTPB (R-45M), was studied for the experiments described in this section. The manufacturer (Atochem North America) lists the number average molecular weight for R45-M as 2,800 and gives a hydroxyl value of 0.70 mgq/g although the mgq/g value can vary from 0.68 to 0.80. Both carbon dioxide and propane were tested with HTPB. Carbon dioxide dissolved only about 12% of the parent HTPB at pressure levels up to 587 bar. Propane dissolves virtually all of the parent material at moderate pressure, therefore, propane was used for the fractionation experiments. 

The polymer was fractionated by the conventional means using an isothermal increasing pressure profile. In one test HTPB was separated into ten fractions in propane at 130°C. The first nine fractions were obtained at sequential pressure levels between 34 and 338 bar. The final fraction was dissolved at —538 bar. A summary of the fractionation conditions, fraction weights, and weight fraction w, of each cut is given in table 9.21. 

The increased functionality in the last three fractions is indicative of branching in the high molecular weight material. It is believed that the mechanical properties of the final cured resin are determined almost exclusively by the high molecular weight, highly functional portion of the polymer (Stephens et al., 1978). 

---