Unlocking the secrets of high-water barrier stereocomplex polylactide blend extrusion films

Abstract

Consumers, companies, and governments are becoming more interested in compostable materials as the amount of waste generated from single-use petrochemical-based and non-biodegradable plastics has grown exponentially since 1960. A drawback to compostable flexible option films is the lack of equivalent moisture barrier performance to maintain a comparable shelf life of food products compared with traditional petrochemical-based versions. Stereocomplex PLA (SC-PLA), a combination of the stereoisomers L-PLA (PLLA) and D-PLA (PDLA), offers a potential solution to this problem. This study presents a viable and novel method to cast extrude SC-PLA films without using a masterbatch. For comparison, several blend compositions were produced, including 85/15, 70/30, 50/50, and 30/70 PLLA/PDLA and homocomplex PLLA and PDLA films. The samples were then annealed at 160 °C for 5, 15, and 30 min to understand the effect on the crystallinity (X_c), moisture vapor permeability coefficient (MVPC), and functional and mechanical performance. As expected, the X_c increased for all the samples upon annealing. We observed increased crystallization kinetics in the blended samples, which crystallized in 5 minutes, which can be ascribed to the nucleation effect, while the PLLA and PDLA started crystallizing between 15 and 30 minutes. PDLA acted as a nucleating agent at levels as low as 15% to induce crystallization in ≤5 minutes in the blended samples. We observed a correlation between the crystallization trend of various compositions and MVPC, with the blended annealed films having a significantly better moisture barrier than all the non-annealed films. The increased X_c reduces the strength properties due to higher brittleness associated with the higher X_c. Further optimization is needed to produce fully viable films for commercial applications; however, this work shows pathways to unlock the creation of PLA films with high barriers to water.