Foaming behavior of poly(lactic acid) with different D-isomer content based on supercritical CO₂-induced crystallization

Abstract

Using supercritical carbon dioxide (sc-CO2) as a physical foaming agent, the effect of sc-CO2 on the formation of crystalline domains and subsequently on the foaming behaviors of the two grades of PLA with different D-isomer content were investigated in a wide foaming temperature range. The PLA’s final crystallinity is significantly increased with decreasing annealing temperature and by reducing the D-isomer content. Cellular structure results show that not only the crystallinity but also the crystalline morphology play an important role in cellular structure. A novel spherulite morphology including ringless and ring-banded morphology in the same spherulite was formed at lower foaming temperature, as a result, some entities were nonuniformly distributed in the PLA foams. Uniform and closed cellular structure were obtained when only the ring-banded spherulites were formed. An opened and interconnected cellular structure is tended to be formed because of the synergistic effect of high temperature and plasticization of CO2. Based on the crystallinity and morphology, a suitable foaming window as a function of temperature is proposed. It is found that PLA with 4.1% D-isomer content had much broader suitable foaming window range to produce homogeneous cellular structure.

Forefront Research of Foaming Strategies on Biodegradable Polymers and Their Composites by Thermal or Melt-Based Processing Technologies: Advances and Perspectives

“…In addition, low melt strength is also responsible for the loss of gas during foam expansion, resulting in foam shrinkage. Several strategies were developed to improve the PLA melt strength, including the use of chain extenders to create a branched molecular structure [107,[243][244][245][246], monomer composition [247][248][249], the blending with other polymers [250,251], and the addition of additives or fillers [92,252,253]. Hou et al [254] published a feature article that reviews the recent advances to improve the performance and the cell structure of PLA foams.…”

Progress in the Preparation, Properties, and Applications of PLA and Its Composite Microporous Materials by Supercritical CO2: A Review from 2020 to 2022

“…Compared with PLA with high D-lactic acid content, PLA with low D-lactic acid content is more conducive to forming a good pore structure at higher foaming temperatures due to its high crystallinity under the same other foaming conditions. [ 10 , 11 , 12 , 13 ]. It is one of the most widely used biodegradable polymers.…”