Hydroxyl-terminated polybutadiene

Hydroxyl-terminated polybutadiene (HTPB) is an oligomer of butadiene terminated at each end with a hydroxyl functional 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.^[1] This product consists of oligomeric units typically containing 40–50 butadiene molecules bonded together, with each end of the chain terminated with a hydroxyl [OH] group:

R-45HTLO has a functionality of 2.4-2.6, which means that there is (approximately) 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

Materials production

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 (used for roller coasters, escalators, skateboards, etc.); automotive suspension bushings; electrical potting compounds; high-performance adhesives; surface coatings and surface sealants; synthetic fibers (e.g., Spandex); carpet underlay; hard-plastic parts (e.g., for electronic instruments).

Rocket propellant

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 launch vehicles and in 1/3 stages of Indian PSLV launch vehicle. JAXA describes the propellant as "HTPB/AP/Al=12/68/20", which means, proportioned by mass, HTPB plus curative 12% (binder and fuel), ammonium perchlorate 68% (oxidizer), and aluminum powder 20% (fuel).

Similar propellants, often referred to as APCP (ammonium perchlorate composite propellant) are used in larger model rockets. A typical APCP propellant mixture 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.^[2] With N₂O (nitrous oxide, or "laughing gas") as the oxidizer, it is used to power the SpaceShipTwo hybrid rocket motor developed by SpaceDev.^[3] The land speed record attempt Bloodhound SSC was to have used HTPB with a high-test peroxide oxidizer, but that plan was altered in 2017.

References

^ http://www.crayvalley.com/docs/TDS/poly-bd-r-45htlo.pdf Archived 2016-02-16 at the Wayback Machine ^{[bare URL PDF]}
^ Sutton, George Paul; Biblarz, Oscar (2010). Rocket propulsion elements (8th ed.). Hoboken, N.J: Wiley. pp. 595–599. ISBN 978-0-470-08024-5.
^ "SpaceDev Hybrid Propulsion". SpaceDev. Archived from the original on 2007-11-05. Retrieved 2008-07-24.

External links

US Patent 5159123 description of synthesis and other details.

[1] ttp://www.crayvalley.com/docs/TDS/poly-bd-r-45htlo.pdf Archived 2016-02-16 at the Wayback Machine ^{[bare URL PDF]}

[2] Sutton, George Paul; Biblarz, Oscar (2010). Rocket propulsion elements (8th ed.). Hoboken, N.J: Wiley. pp. 595–599. ISBN 978-0-470-08024-5.

[3] "SpaceDev Hybrid Propulsion". SpaceDev. Archived from the original on 2007-11-05. Retrieved 2008-07-24.

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