Hydroxyl-terminated polybutadiene is an oligomer of butadiene terminated at each end with a hydroxylfunctional group. It reacts with isocyanates to form polyurethane polymers. HTPB is a translucent liquid with a color similar to wax paper and a viscosity similar to corn syrup. The properties vary because HTPB is a mixture rather than a pure compound, and it is manufactured to meet customers' specific requirements. A typical HTPB is R-45HTLO. This product consists of oligomeric units typically containing 40–50 butadiene molecules bonded together, with each end of the chain terminated with a hydroxyl group: R-45HTLO has a functionality of 2.4-2.6, which means that there is one additional hydroxyl group located along the chain for every two oligomeric units. This provides side-to-side linkage for a stronger cured product. HTPB is usually cured by an addition reaction with di- or poly-isocyanate compounds.
Uses
Polyurethanes prepared from HTPB can be engineered for specific physical properties; polyurethanes may be highly elastic or tough and rigid. Some products include: rigid foam insulation panels; durable elastomeric wheels and tires ; automotive suspensionbushings; electrical potting compounds; high-performance adhesives; surface coatings and surface sealants; synthetic fibers ; carpet underlay; hard-plastic parts. An important application of HTPB is in solid rocket propellant. It binds the oxidizing agent, fuel and other ingredients into a solid but elastic mass in most composite propellant systems. The cured polyurethane acts as a fuel in such mixtures. For example, HTPB is used in all 3/4 stages of the Japanese M-5 rocket satellite launchers and PSLV rocket developed by ISRO for satellite launches. JAXA describes the propellant as "HTPB/AP/Al=12/68/20", which means, proportioned by mass, HTPB plus curative 12%, ammonium perchlorate 68%, and aluminium powder 20%. Similar propellants, often referred to as APCP are used in larger model rockets. A typical APCP produces 2–3 times the specific impulse of the black powder propellant used in most smaller rocket motors. HTPB is also used as a hybrid rocket fuel. With N2O as the oxidizer, it is used to power the SpaceShipTwohybrid rocket motor developed by SpaceDev. It will also be oxidized by high-test peroxide in the land speed record attempt Bloodhound SSC.
