Polyurethanes as Binders

Polyols and polyisocyanates are major ingredients for polyurethane (PU)-based composite propellants in addition to chain extenders and catalysts etc. A brief description on these materials is considered necessary in order to understand the chemistry of composite propellants in a better way. Polyols are the largest group of raw materials for PUs. The properties of PUs/PU products are largely determined by their chemical composition and molecular mass. They may be broadly classified [87] as  

The details are already available in the literature. However, some important aspects of popular polyols i.e. castor polyols and HTPB, diisocyanates/ polyisocyanates, other ingredients and preparative methods of PUs are given in this section. 

Oil % volatile (moisture) Hydroxyl value/mg of KOH g 1 Acid value/mg ofKOHg 1 

Castor oil [CO Structure (4.3)] is a triglyceride of ricinoleic (12-hydroxyoleic) acid about 90% of the fatty acid portion of the molecule consists of ricinoleic acid and 10% in the form of non-hydroxy acids consisting largely of oleic and linoleic acids. Small amounts of stearic and dihydroxystearic acids are also found in some industrial grades. 

The three secondary hydroxyl groups in the 12-posihon are somewhat slow to react with diisocyanates and generally require heat for the completion of reaction. This can be of advantage or disadvantage depending upon the specific application. Modifications of CO and particularly of ricinoleic acid through esterification results in products that are of considerable interest as polyols for various urethane applications [88, 89]. 

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