THERMO-MECHANICAL PROPERTIES AND THERMAL DEGRADATION CONTROL OF POLY (LACTIC ACID) AND POLY (LACTIC ACID)/CLAY/WOOD NANOCOMPOSITES

摘要

Poly (lactic acid) (PLA) is an important biopolymer with suitable mechanical properties for construction and packaging applications. However, there are also problems associated with cost, brittleness, distortion temperature, gas permeability and melt viscosity. The mechanical and thermal properties of PLA can be tailored by compounding reinforcing fillers. Wood flour is a renewable material of low density and cost, high resistance to breakage and less abrasion to processing equipment. Nanoclay is a layered structure silicate material of high aspect ratio. PLA nanocomposites based on nanoclay and/or wood flour were prepared by melt extrusion of PLA. The clay particles (Cloisite 20A) exhibited an intercalated structure in the PLA. The tensile and flexural moduli of PLA/clay/wood nanocomposites containing 5 wt% nanoclay and 30 wt% wood flour increase from 3.75 to 7.08 GPa and from 3.83 to 6.01 GPa, respectively, compared to neat PLA. Clay particles improve the thermal decomposition temperature (T_d) of PLA/clay/wood nanocomposites by about 10°C compared to that of PLA/wood composites. Three compatibilizers -maleated polyethylene (MAPE), maleated polypropylene (MAPP) and maleated poly (lactic acid) (MAPLA) were used in PLA/clay/wood nanocomposites. Scanning electron microscopy (SEM) images confirm that MAPLA improves the interfacial adhesion between the PLA matrix and wood flour compared to the MAPE and MAPP compatibilizers. The tensile and flexural strengths of the (MAPLA) nanocomposites decrease with increasing clay loading. However, 2-4 wt% MAPE or MAPP give rise to higher tensile and flexural strengths in the case of PLA/clay/wood nanocomposites with 1 wt% clay. All three compatibilizers decrease the maximum thermal decomposition rate of the nanocomposites. Four (4) wt% MAPLA in PLA/5 wt% clay nanocomposites also increases the steady shear viscosity compared to nanocomposites without MAPLA. Approaches to control PLA thermal degradation during melt extrusion were examined. Residual catalyst deactivation and molecular weight control by chain extenders were believed to be the most effective and feasible methods. Details of these two methods as well as the effect of nanoclay on gas permeability will be discussed.