Enhancing foamability and flame-retardancy of polylactic acid bead foams through inter-beads bulk polymerization and continuous phase dispersion of LDHs

Poly (lactic acid) (PLA) bead foam has a promising application because of its renewable and naturally degradable nature. However, its processing is greatly limited by inherent shortcomings such as the complex polycrystals-inducing strategy. Herein, we developed a strategy of bulk polymerization reaction of polyvinyl acetate (PVAc) to sinter PLA beads to prepare EPLA foams accompanied by supercritical CO₂ foaming technology. In order to enhance the sintering behavior and flame-retardancy of EPLA foams, two-dimensional nanolayered double hydroxides (LDHs) were introduced and dispersed in the continuous phase of the sintering layers. The formation of unique dispersion of LDHs and sintering structure of PVAc generated substantial increase in the crystallinity, melt elasticity, and sintering strength of EPLA foams, which facilitated the growth and stabilization of cells. Thus, the cell-density and expansion ratio could be increased to 8.62 × 10⁶ cell/cm³ to 9.31-fold, respectively. Moreover, the mechanical properties of the EPLA foams were improved. The tensile strength and the compression strength increased to 2.96 and 62.5 MPa. Additionally, with adding 7 wt.% LDH, the EPLA foam reached UL-94 V-0 rating with high limiting oxygen index value of 29.1% and char residue of 20.4%. This study provides a novel strategy for the preparation of flame-retardant EPLA foams with low density, three dimensional complex shapes, as well as excellent mechanical properties.