Investigation into the network structure of plasticized rocket propellant

Abstract

To improve the dimensional stability of a solid rocket propellant and to allow it to deform under stress a rubbery polymeric network is incorporated into the composition. Energetic plasticizers are also added to the propellant to improve its performance. However, migration of the plasticizer has been observed. This can lead to irregularities in the propellants ballistic properties and may sometimes lead to premature detonation. It is important therefore to be able to understand the effect the plasticizer has on the network structure of the polymer in solid rocket propellant. In order to look at this problem compositions containing polycaprolactone crosslinked with excess isocyanate were formulated. Varying amounts of plasticizer [glycerol triacetate (CH 3COO) 3C 3H 5)] were incorporated into the compositions. The network structure of polycaprolactone crosslinked with an isocyanate was found to be dependent on the quantity of plasticizer present during curing. A reduction in plasticizer results in a reduced mobility of the free isocyanate molecules, thus leading to a tighter polymeric network. The average number of polycaprolactone chains between each crosslink unit increases from 14 to 77 as the amount of plasticizer is increased. This leads to the polymeric network having a more open structure. On crystallization the looser networks were able to form structures with higher degrees of crystallinity. On total removal of the plasticizer by extraction, the network structure collapsed and formed structures with a high crystalline content. The networks were able to regain their integrity by reintroducing the samples back into plasticizer above the crystalline melting temperature. The more open networks could absorb more plasticizer. When allowing the samples to crystallize again the same amount of plasticizer was expelled, thus suggesting that plasticized network structures have a memory.