Materials for Inhibition

The choice of material for use as inhibitor depends mainly upon the type of propellant, that is, DB, CMDB, Composite and Fuel-rich and also on the ingredients in their formulations. For double-base propellants, cellulosic materials such a cellulose acetate, ethyl cellulose and different filled or unfilled flexible polyesters are used while fuel or binder material filled with inert substances such as asbestos, mica, silica, etc. in fine powder form is used for composite propellants. Since nitroglycerine is present in CMDB propellants also as in DB propellants, the materials used for DB propellants may also be used with minor modifications for the inhibition of CMDB propellants. 

The major ingredients of composite propellants are ammonium perchlorate (AP-68%), metal powder (-16%) and polymeric binders like CTPB, HTPB and Thiokol. As total solids loading in composite propellants is -85-88%, polyesters which are commonly used for inhibition of DB propellants, have a relatively weak bond with composite propellants. This problem is further aggravated because of higher shrinkage in the case of polyesters. 

Similar to composite propellants, flexibilized epoxy or novolac epoxy resins reinforced with fillers or fibers are used for inhibition of fuel-rich propellants. 

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The patent literature contains a large range of chemicals which have been recommended as antifoamers, ranging from simple compounds, such as octyl alcohol, to more sophisticated mixtures of antifoamers, which are sometimes required in special applications. From a universal viewpoint, polydimethylsiloxanes are probably the most expensive, yet the most effective materials for inhibiting foam. Such compounds are in volatile and have extremely low surface tensions and low interfacial tensions against water. The surface tensions of liquid hydrocarbons are usually between 25 and 30 mN/m and since the silicones have lower surface tensions, they then can easily enter the hydrocarbon substrate. In addition, they are chemically inert, insoluble in water. 

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