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Thermal and Mechanical Properties of WF/Talc-filled PLA Composites 213 where H m is the enthalpy of the sample and H m(crys) is the enthalpy of 100% crystalline PLA taken as 93.7 J/g [5]. The H m , H c , and X c were converted by the mass fraction of the pure PLA in the composites. Mechanical Properties The tensile test for the composites was performed according to ASTM D638 using a Zwick an Universal Testing Machine (Zwick Testing Machine Ltd., Leomister, United Kingdom). Test specimens were molded to a size of 3.18 mm (width) 9.53 mm (length) 3.00 mm (thickness) with a gauge length of 12.5 mm. For each treatment level, five replicates were tested and the results were presented as the average of the tested samples. Tensile deformation was determined using an extensometer at a crosshead speed of 10 mm/min. Morphological Properties Morphology of the fractured composites after tensile testing was observed using a scanning electron microscope (SEM) (JEOL-6700F) with a field emission gun and an accelerating voltage of 5 kV. A gold layer of a few nanometers in thickness was coated onto the fracture surfaces. The samples were scanned perpendicular to the fractured surface. SEM micrographs were taken at the magnification level of 200. RESULTS AND DISCUSSION Thermal Properties TGA DATA TGA results of PLA, WF, and their composites are shown in Figure 1(a). As the WF content increased, the decomposition temperature of the PLA/WF composites decreased slightly. This may be due to the decomposition of wood components in a temperature range between 200 and 3808C. Cellulose and hemicellulose components of WF are two major contributors to the decomposition between 2008C and 3808C, whereas lignin is responsible for the char formation over 3808C [29]. The lower decomposition temperatures of PLA/WF composites indicated that the composites were less thermally stable than the neat PLA matrix. Poor interfacial adhesion between the PLA matrix and WF due to incompatibility of hydrophobic PLA with hydrophilic WF might contribute to this behavior. Tar and ash content after thermal degradation over 3808C increased with an increase in the WF loading level. The differential peak temperatures (DT p ), as a function of WF loading for various systems are shown in Figure 1(b). The DT p of neat PLA was Downloaded from http://jtc.sagepub.com at KAIST GRADUATE SCHOOL OF MGMT on April 27, 2008 © 2008 SAGE Publications. All rights reserved. Not for commercial use or unauthorized distribution.
214 S.-Y. LEE ET AL. (a) 100 80 Weight change (%) 60 40 PLA PLA90/WF10 20 PLA80/WF20 PLA70/WF30 PLA60/WF40 WF 0 200 300 400 500 600 (b) DT p (˚C) 370 350 330 Temperature (°C) PLA/WF PLA/WF/Talc10% PLA/WF/Talc10%/Silane1% PLA/WF/Talc10%/Silane3% 310 0 10 20 30 40 Wood flour (%) Figure 1. Thermogravimetric properties of the composites: (a) TGA thermograms of PLA/wood flour composites; and (b) Differential peak temperature of PLA/wood flour/talc/ silane composites. 358.28C, and the DT p of the PLA/WF composites ranged from 8 to 248C lower than that of neat PLA. The addition of talc to PLA/WF composites showed no significant effect on the thermal decomposition process. The loading of talc at 10 wt% level caused a slight shift of TGA curves. The DT p values of composites with talc ranged from 0 to 7.58C lower than those of composites without talc. For example, the DT p values of PLA80/WF20 Downloaded from http://jtc.sagepub.com at KAIST GRADUATE SCHOOL OF MGMT on April 27, 2008 © 2008 SAGE Publications. All rights reserved. Not for commercial use or unauthorized distribution.
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