(531b) Renewable Elastomers Based On Plasticized Starch

Most recent developments in polymers from renewable resources have focused on thermoplastics, whereas there have been no comparable developments of plastics with elastomeric properties. Here we evaluate the possibility of developing renewable elastomers based on starch. Dry granular starch, by itself, cannot be processed like a plastic; however, it can be blended with small polar molecules, giving a thermoplastic material generally called thermoplastic starch. Thermoplastic starch is a renewable, biodegradable, and economical thermoplastic that can be processed similarly to synthetic thermoplastics using standard polymer processing equipment. We studied blends of glycerol plasticized potato starch that was melt-blended with small quantities of maleated polypropylene. The maleic anhydride groups of the polypropylene react with the hydroxy groups of starch under melt blending conditions and crosslink the starch polymer chains, creating a crosslinked network of thermoplastic starch and polypropylene.

The resulting blends of plasticized starch and maleated polypropylene were characterized by mechanical testing, scanning electron microscopy, dynamic mechanical analysis and differential scanning calorimetry. The morphology of the blends, along with solubility tests indicate that the blends are two-phase materials in which the continuous phase plasticized starch is crosslinked by polypropylene domains. The materials display rubbery properties as judged by a low glass transition temperature which is independent of polypropylene content, and a wide rubbery plateau. Agreeing with the thermomechanical data, the tensile behavior of the blends suggests that they may be suited for elastomeric applications. However, slow aging due to starch crystallization, and extraction of glycerol upon water exposure remain two challenges that must be overcome before the materials can be used as practical elastomers.