Novel PLA-based materials with improved thermomechanical properties and processability through control of morphology and stereochemistry. A study in improving toughness and processability of PLA by blending with biodegradable polymers and the two PLA enantiomers PLLA and PDLA to accelerate crystallinity and heat resistance

Abstract

Polylactic acid (PLA) is an aliphatic polyester, derived from sustainable natural sources that is biodegradable and can be industrially composted. This material has been in the spotlight recently due to its sustainability and properties. However it has been invented in 1932 by Carothers and then patented by DuPont in 1954 (Standau et al. 2019). The properties of this material though limit its use for applications mainly in the medical sector and in some cases single use packaging. In this research, PLA based blends with improved rheological and thermomechanical properties are investigated. The focus is based in proposing strategies in improving these properties based on commercial methods and processing techniques. In this work, commercial grade PLA has been blended with polycaprolactone (PCL) and polybutylene succinate (PBS) in binary and ternary formulations via twin screw extrusion. PCL has been known to act as an impact modifier for PLA, but to cause a corresponding reduction in strength. Results showed that the binary PLA blends containing PBS and PCL, had reduced viscosity, elastic modulus and strength, but increased strain at break and impact strength. Morphological and thermal analysis showed that the immiscibility of these additives with PLA caused these modifications. Incorporation of a small loading of PBS had a synergistic effect on the PLA-PCL blend properties. Miscibility was improved and enhanced mechanical properties were observed for a ternary blend containing 5wt% of both PBS and PCL compared to binary blends containing 10% of each additive. To increase heat resistance of PLA, the material’s crystallinity has to be increased. However PLA has a relatively slow crystallisation rate making it difficult and expensive to be used in commercial applications where heat resistance is needed. For this reason the chiral nature of PLA has been used to investigate the effect of stereochemistry of PLA in crystallisation. Optically pure PDLA was added to its enantiomer in small amounts (up to 15%) and the properties and crystallisation mechanism of these blends was investigated. Results showed that the addition of PDLA accelerated crystallinity and developed a stucture that increased heat resistance, melt strength and stiffness. Finally, a processing model of developing a fully stereocomplex PLA part based in commercial techniques is proposed. Injection moulded PLA showed even higher heat resistance without the need of further processing the product (increasing crystallinity).