Microcellular Foaming of Poly(lactic acid)/Silica Nanocomposites in Compressed CO₂: Critical Influence of Crystallite Size on Cell Morphology and Foam Expansion

Abstract

During the solid state foaming, the CO 2 saturated poly(lactic acid) (PLA) sample at 5 MPa and 20 °C has a high crystallinity of 23.2%, and the prepared PLA foams exhibits low foam expansion and nonuniform cell structure. This study presents an interesting effect of nanosilica addition on the cell morphology and expansion ratio of PLA foams. It was found that the presence of nanosilica increased the induced crystallinity of PLA up to 29.7% at 5 MPa. The resultant PLA/silica foams exhibited significant and concurrent increase in cell structure uniformity and cell density: the cell density increased about 5−10 times, the expansion ratio increased 1.4−2.1 times, and the crystallinity of foams increased 1.3 times, compared to pure PLA foams. Further investigation suggested that the formation of the tiny crystallite size and the well dispersed nanosilica aggregates were thought as the main reasons to explain the interesting effect of nanosilica addition on the foaming behavior of PLA.

Solid State Foaming of Poly(lactic acid) Blown with Compressed CO2: Influences of Long Chain Branching and Induced Crystallization on Foam Expansion and Cell Morphology

“…23,25,28,36,37 Figure 1 shows the DSC thermograms of L-PLA and B-PLA at AP and various CO 2 pressures. At AP, both L-PLA and B-PLA had a broad cold crystallization peak at around 124°C and a melting peak at around 150°C.…”

Non-isothermal crystallization behaviors of poly(lactic acid)/cellulose nanofiber composites in the presence of CO2

“…The crystallization kinetics of PLA also influences foam processes. Formed crystals during foam processing can affect cell nucleation through local stress variation and gas supersaturation [82][83][84][85]. In addition, entanglement of polymer molecules through these crystals could increase PLA's melt strength, thus improving its ability to expand and suppress cell coalescence and coarsening [15,17,81,82].…”

Poly (lactic acid)/Epoxidized Natural Rubber Blends Foams with Nanofillers Cellular Compressed Foaming: Effect of Nanofillers Blends Foam on Physiology, Compressive Strength, Thermal Stability and Morphology

“…In addition, the acceleration of the nanoparticles content might be agglomerated and affect the decrease in the compressive properties and elongation. Furthermore, more than 0.5 percent of the fillers could not significantly maintain the value of the mechanical properties as shown in the PLA/ENR/1.5 percent of SiO2 and 1.5 percent of TiO2 that exhibited 34.70±0.86 N/10cm 2 and 22.07±7.44 N/10cm 2 [14] . The step of the nanoparticles could indicate the limit of packing a filler's content.…”

“…In addition, they are non-toxic [ 13] and are used with a foam preparation in order to control the porous size. Ji et al ( 2013) investigated the PLA/silica nanocomposite to the morphology and cell density. After that, the silica nanoparticles could disperse in the average cell size whereas the incorporation of excess nanosilica indicated the aggregates of the nanosilica [14].…”