Solubility and phase behavior of PEP binders in supercritical carbon dioxide

Solubility and phase behavior data to temperatures of 300°C and pressures to 3000 bar are presented for various propellant, explosive, and pyrotechnic (PEP) binder polymers in supercritical (SC) carbon dioxide (CO2) with and without modifiers. The binder polymers investigated in this study are polyethylene (PE), oxidized PE, poly(urethane), cellulose acetate butyrate (CAB), poly(vinylidene fluoride-co-hexafluoropropylene) with 20mol% hexafluoropropylene (Fluorel™ and Viton™), and poly(chlorotrifluoroethylene-co-vinylidene fluoride) with 24 mol% (Kel-F™ 800) and 69 mol% (Kel-F™ 3700) vinlyidene fluoride. CO2 has the characteristics of a weak polar solvent which does not dissolve polyethylene although it is can dissolve low molecular weight nonpolar waxes. However, CO2 is not polar enough to dissolve very polar polymers, such as oxidized PE, polyurethane, Kel-F™ 3700, and CAB. Moderately polar poly(vinylidene fluoride-co-hexafluoropropylene), which is a partially fluorinated polyolefin, and Kel-F™ 800 are soluble in CO2 due to the polar contributions of the vinylidene fluoride repeat units. The solubility of these two copolymers can be further enhanced using acetone as a modifier with CO2. Additionally, oxidized PE is soluble in CO2 with acetone and ethanol as modifiers, but temperatures greater than 200°C are needed. The fact that many of these polymers are soluble in neat CO2 suggests the potential for a substantial impact upon environmentally sound processing technologies for polymer-based PEP materials.