Abstract

Poly(lactic acid) (PLA) is a well known bio-based and biodegradable thermoplastic polyester with promise for replacing fossil-derived materials. Importantly, PLA is produced on an industrial scale from corn and sugar beets and it is becoming cost-competitive with standard packaging plastics. However, PLA has low thermal stability, impact resistance and flexibility with limited gas barrier properties, so blends with polymers which can compensate for these disadvantages are sought. Polyamide-12 (PA-12) is a petroleum-derived material with high impact resistance, thermal stability and dimensional stability. PA-12 is commercially produced at high volume, but property improvements it imparts to other commodity polymers must be justified by its higher cost. Renewable polyamides such as PA-11 (from castor oil) and PA-6, 10 (from fermentation byproducts) are becoming available and thus would further improve the environmental profile of the blends explored here. In this work, reactive blending and high shear are explored as means of compatibilizing PLA/PA-12 and PLA/PA-11 blends with interfacial block copolymers using transesterification catalysts. Two different catalyst systems are employed to improve blend compatibility: titanium isopropoxide (TTIP), and p-toluenesulfonic aicd (TsOH). The catalysts are added to reduce the ester-amide trans-reaction time and increase the extent of chain exchange at the interface between PLA and PA. The ability of high shear to increase the interfacial area and thus the fraction of copolymer in the blend is explored through the use of standard batch mixing and high-shear twin-screw extrusion. The compatibility and properties resulting from low (hundreds of RPM) to high (thousands of RPM) shear are studied. The thermal and physical properties are examined using intrinsic viscosity, differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and tensile testing. The morphology and extent of reaction are characterized using scanning electron microscope (SEM), and solubility testing. The results demonstrate that while transreaction takes place, molecular weight degradation due to high shear and catalyst addition must be controlled. This research will result in improvements to the physical properties of biobased plastics for durable goods applications and industrial scale up, and could also result in potential applications in pharmaceuticals and medical devices.