Inhibition of CMDB Propellants

The combustion temperatures are much higher in CMDB propellants than that of DB and composite propellants and therefore, inhibition systems for CMDB propellants should have better BS and ablative properties. As major ingredients of CMDB propellants are NC and NG similar to DB propellants, it is logical to use polyesters which have already proved their potential for DB propellants for inhibition of CMDB propellants. However, the unsaturated polyesters which function 

Polyesterification between a diol and dibasic acid is usually done by (i) a 1-step process, (ii) a 2-step process and (iii) a 3-step process [163]. The commercially available unsaturated polyesters are generally manufactured by 1-step polyesterification process and as a result, possess poor flexibility and inferior heat resistance. 

At the same time, higher temperature of reaction in the 1-step process converts maleic configuration to fumaric configuration. On the contrary, unsaturated polyesters and chloropolyesters for inhibition of DB and CMDB propellants are synthesized by 2-step polyesterification process which imparts more regular structure, better flexibility and greater heat resistance to the resulting polyesters. 

The effect of method of polyesterification on various properties is depicted in Table. 4.16. 

Where G - Mexdiol Moiety AA- Adipic acid Moiety IP- Isophthalic acid Moiety MA- Maleic anhydride moiety FA- Fumaric acid Moiety 

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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 PU system based on HTPB-MDI-TMP and filled with carbon black and antimony trioxide was also evaluated for inhibition of CMDB propellants. Due to superior mechanical properties, thermal properties and low NG migration, this PU system holds a potential for inhibition of CMDB propellants [357]. 

The results obtained by the investigation enabled authors to select the best UPRs for inhibition of CMDB propellants. 

EP-4 developed by ERDL is a very flexible polyester based on polyethylene glycol with molecular weight-200 (PEG-200), isophthalic acid (IPA) and maleic anhydride (MAn). Before its use, it is blended with styrene monomer (1 1) and cured at room temperature using cobalt naphthenate (as an accelerator) and methyl ethyl ketone (MEK) peroxide (as a catalyst). This meets the requirements of the main inhibitor and is used for inhibition of DB and CMDB propellants after the application of a barrier coat (generally a rigid polyester such as PR-3). However, it is observed during manufacture of EP-4 that there is a lot of batch-to-batch variation in properties in spite of the strict quality control measures adopted during its manufacture. 

It is evident from the data that the batch-to-batch variation is within the specification limits. It is, therefore, considered a better substitute of EP-4 and is likely to substitute ERDL, EP-4 for inhibition of DB and CMDB rocket propellants in future. 

K.S., and Agawane, N.T. (1986) Development of Inhibition System for CMDB Propellants. ERDL Report No. 6/86. 

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