Moulding of Water-Free Thermoplastic Starch Blends

Abstract

Thermoplastic starch (TPS) is one of a number of biodegradable polymers which have become increasingly attractive in recent times as substitutes for petrochemicals. Starch is seen as particularly useful as both a low-cost filler and a promoter of biodegradation processes. A novel water-free TPS blend suitable for rotational moulding and other conventional thermoplastic manufacturing processes is presented here. A method for producing water-free TPS blends in a single extrusion with commodity polymer processing equipment has been developed. Selected mechanical properties of blends with a rotomoulding-grade polyethylene (PE) have been characterized, using different total TPS content, plasticizer type and amount. Extruded, injection moulded and rotomoulded samples were tested, both fresh and after aging, and compatibilizing additives were investigated. Completely biodegradable blends of TPS, poly(lactic acid) (PLA) and polybutylene succinate (PBS) were also demonstrated as rotomouldable and characterized in terms of mechanical properties. The ultimate tensile strength (UTS) of TPS/PE blends was found to be relatively consistent across TPS formulations and manufacturing methods, and not as high as PE alone. Tensile modulus was found to be more variable and could be raised or lowered with relation to PE. Compatibilizers had noticable but small effects. An increase in UTS was found to coincide with a TPS/PE modulus matching that of the PE alone, and a decrease in elongation to break. The impact strength of rotomoulded TPS/PE was low compared to the PE reference. Water-free blends have also been investigated in terms of electrical conductivity and oxygen barrier properties. Compression-moulded TPS/PE films were found to be an improvement on PE, and the TPS alone was demonstrated as an excellent oxygen barrier even when highly plasticized. The electrical conductivity of TPS was found to improve with the addition of either salts or dispersed conductive particles, despite the lack of water, indicating potential as a cheap electroactive polymer.