Morphology, Mechanical Properties, and Biodegradability of Modified Thermoplastic Starch/PETG Blends with In Situ Generated Graft Copolymers

This paper reports on synthesis of modified thermoplastic starch (MTPS) and glycol-modified polyethylene terephthalate (PETG) blends in a twin-screw extruder. Scanning electron microscopy (SEM) images showed uniform, microdispersion of MTPS in PETG matrix, confirming compatibilization of the blend by graft copolymers generated in situ during the reactive extrusion process. Incorporating 30% by wt. MTPS in the blend gives a biobased carbon content of 22.8%, resulting in reduced carbon footprint by removal of 0.5 kg CO2 from the environment/kg resin relative to unmodified PETG. MTPS with 80% glycerol grafted onto starch was prepared by reactive extrusion in the twin-screw extruder. A total of 33% of added PETG was grafted on MTPS backbone as determined by soxhlet extraction with dichloromethane (DCM). The grafting was confirmed by presence of PETG peak in the TGA analysis of residue and appearance of carbonyl peak in FTIR spectra of the residue after Soxhlet extraction. The synthesized MTPS–PETG reactive blend had lower but acceptable mechanical properties. Even after a 15% reduction in the tensile stress and 40% reduction in the strain and impact strength obtained after adding 30% MTPS, this blend still had good mechanical properties and can be used in many applications requiring a balance of cost, mechanical properties, and biobased content. Aqueous biodegradability studies using ISO 14852 showed that the 30% starch component in the blend biodegraded rapidly within 80 days, whereas PETG remained as it was even after 150 days. Thus, this study categorically proves that addition of starch does not improve the biodegradability of nonbiodegradable polymers.