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HTPB has also been used before. Literature suggests that it is more suited for high-G flights where case adhesion is paramount. HTPB used as a liner should be around 50% solids for strength, with carbon being the primary additive, with TiO2, fiber, zinc borate, and other additives potentially useful. A plasticizer is also strongly suggested for HTPB based liners to improve casting performance. Chemical etching can significantly improve liner adhesion to the aluminum case, as can other binding agents. A suggested etchant is Chemloc. Use IPDI as a curative to allow for a slower reaction and provide time for the propellant to cure into the liner matrix. The propellant is slightly mobile in the liner.

Literature

http://dtic.mil/dtic/tr/fulltext/u2/a242297.pdf

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