Early development of an energetic biodegradable thermoset elastomer

In the last years plastics have gained a large fame as fascinating materials available for a large number of applications, from biomedical to military [1-3]. However, the expected depletion of oil resources and a greater awareness for the environmental impact of plastic products have created a strong interest towards polymers that are not only biodegradable but also obtained from renewable resources. Various natural and synthetic biodegradable polymers have been developed in order to satisfy specific mechanical and physical requirements and different techniques have been tried such as copolymerization, polymer blending, composite formulations and cross-linking networks in order to improve the product properties [4].;Biodegradable polyurethanes based on lysine have been reported in the literature to transform into non-toxic decomposition products [5, 6]. For those reasons polyurethane elastomers can be attractive materials for the preparation of composite solid propellants, in which the elastomer can act as a rubbery phase intimately mixed, and it can be used as a binder for other materials like ammonium perchlorate or other oxidizers, aluminum or other metallic fuels.;In the last years the researchers have developed new biodegradable materials which contain hetero-atoms in their backbone. Ether bonds, amide bonds or ester bonds can easily undergo a hydrolytic attack: in this way they are able to release low molecular weight by-products which are easily biodegradable in the soil or in the compost. Aliphatic polyester, like poly (epsilon-caprolactone), poly (L-lactide) are well known for their biodegradability, but they do not stand the comparison in terms of physical and mechanical properties.;The main objective of this work has been the design of a biodegradable and energetic co-poly (ester/ether-urethane) network, in order to be used as propellant or gas generator. The impact of the azide group on the biodegradability of the polyurethane is compared with a non-energetic polyurethane.;In these interesting surroundings a new trend has emerged in the production of energetic polymers for specialized civil and military applications. They represent the new generation of energetic binders with the double advantage of improved performance and lower vulnerability (LOVA). The explosive energy they release during the decomposition is due to the presence of an azido or nitro group in the side chain.;The first step has been the synthesis of a co-poly(ester/ether) from polyepichlorohydrin (PECH) and sebacoyl chloride (SC) using pyridine as catalyst and HCl acceptor. Consequently, the chlorinated co-polymer was azidified with NaN3 in dimethyl sulfoxide solutions to produce an energetic co-polymer. The success of the reaction was confirmed by H-NMR, 13C-NMR and FTIR techniques. The azide groups replaced 80% of the initially present chlorine atoms. Both non-azidified (non-energetic) and azidified (energetic) co-polymers were cross-linked in the presence of polycaprolactone triol (PCL) and L-lysine diisocyanate (LDI) as a non toxic coupling agent to form a soft thermoset polyurethane network. Subsequently, the chemical and mechanical properties of the produced polyurethanes along with the water uptake and biodegradation in compost of these materials were characterized. The non-energetic polyurethane showed a glass transition temperature at -14°C whereas the energetic one at -23°C. The weight loss of the polyurethanes after composting was traced and was shown to increase almost linearly with time for both materials: after 20 days the non-energetic binders had lost about 50% of their mass due to biodegradation; whereas, the energetic binders had lost only about 25% of their initial mass after 25 days. The experimental results revealed that the azide pendant group in the soft segment is the main factor that controls the physical, mechanical, and degradation properties of these polyurethane networks.