Flame Retardant Properties and Thermal Stability of Polylactic Acid Filled with Layered Double Hydroxide and Rice Husk Ash Silica

Polylactic acid (PLA) samples containing a layered double hydroxide modified with sodium dodecyl sulfate (PKL_DS), silica from rice husk ash (SiRHA) and a combination of the two nanofillers were prepared by both solvent casting and a more practical approach involving dry blending of constituents and their melting in a mixing tank followed by compression molding. The morphology of the nanocomposite films was investigated using transmission electron microscopy (TEM) and X-ray diffraction (XRD), and it was confirmed that the nanocomposite structures were intercalated. Additionally, SEM pictures of surfaces of nanocomposite films suggest that PKL_DS was well dispersed throughout the PLA matrix, and the films were essentially featureless except at the highest concentration examined. In the case of SiRHA, there was filler aggregation at low concentration, while holes and cavities were clearly visible at higher concentrations. The addition of PKL_DS to films containing SiRHA ameliorated these effects, and ultimately a dense structure was obtained. From TGA results, it was determined that only the addition of SiRHA significantly improved the thermal stability of PLA. The flammability performance of PLA nanocomposites was investigated using limiting oxygen index (LOI) and UL-94V type measurements. A maximum LOI value of 32.8% could be achieved for PLA nanocomposites containing 10 wt% PKL_DS and 5 wt% SiRHA. This was 78% higher than the LOI of neat PLA alone. Furthermore, PLA containing 10 wt% PKL_DS and 3–5 wt% SiRHA attained a V-0 rating on UL-94 type tests with anti-dripping behavior. It was found that there was synergism between PKL_DS and SiRHA, and this was very effective in improving the flame retardant performance of PLA. That this performance came about without having to completely exfoliate the nanofillers is a remarkably useful result.