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Thermal and Mechanical Properties of WF/Talc-filled PLA Composites 211 Most of the polymer composites are generally subjected to a thermal degradation at elevated temperatures. It is important to understand the effects of the processing temperature on the thermal degradation process, since there is constant thermal stress during the manufacturing of filler reinforced composites. Fundamental information regarding the thermal stability of natural fiber reinforced composites has been obtained from thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) [28]. Additional information on the thermal and mechanical properties can shed more light on the manufacture of the composite materials. The objective of this study was to investigate the effects of WF and talc loading and silane treatment on thermal, mechanical and morphological properties of the PLA-based composites. TGA, DSC, scanning electron microscopy (SEM), and stress–strain behavior were used to evaluate the thermal degradation, thermal transition, morphological, and mechanical properties of the composites. Materials MATERIALS AND METHODS Polylactic acid (2100D, M w ¼ 180,000–210,000 g/mol and MI ¼ 5–15 g/ 10 min) was obtained from the Natureworks Õ Co. (Minnetonka, U.S). Wood flour (Lignocel C120, particle size of 100–120 mesh per 2.54 cm 2 ) was supplied by J. Retenmaier & Sohne Co. (Rosenberg, Germany) and manufactured from European softwood. Talc powder supplied from Seobu Enterprise Co. (Goyang-City, Republic of Korea) has a chemical structure of Mg 3 Si 4 O 10 (OH) 2 and melting point of 900 10008C. Average particle size and density of talc were 2 mm and 2.5–2.8 g/cm 3 , respectively. Organo-silane (S-6020, H 2 N(CH 2 ) 2 NH 2 (CH 2 ) 3 Si(OCH 3 ) 3 ) was purchased from the Dow Corning Co. (Midland, Michigan, U.S.A.). The molecular weight and a density of the silane at 258C are 222 g/mol and 1.03 g/cm 3 , respectively. Melt Compounding A 19 mm co-rotating twin-screw extruder with 40 L/D ratio (Bautek Co., Uijungbu-City, Korea) a pelletizer (Bautek Co., Uijungbu-city, Korea) was used to compound the blends. Table 1 shows blend formulations of the composites used in this study. Silane was simply integral-blended with other components without any additional treatment. The compounding temperature was 1808C with screw speeds in the range of 100–150 rpm. The extrudated strands were air-cooled and pelletized with a pelletizer (Uijungbu-city, Bautek Co., Korea). The hybrid pellets were dried at 908C 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.
212 S.-Y. LEE ET AL. Table 1. Material formulations of the hybrids. PLA (wt%) WF (wt%) Talc (wt%) Silane (wt%) 100 0 – – 90 10 – – 80 20 – – 70 30 – – 60 40 – – 80 10 10 – 70 20 10 – 60 30 10 – 50 40 10 – 79 and 77 10 10 1 and 3 69 and 67 20 10 1 and 3 59 and 57 30 10 1 and 3 49 and 47 40 10 1 and 3 for 24 h in a vacuum oven to remove the absorbed moisture and cooled to room temperature, and then injection-molded at 1908C to form test samples. Thermal Analysis The thermal decomposition behavior of the composites was measured with a SDT Q600 Thermogravimetric analyzer (TA Instrument Inc., USA). Tests were done under nitrogen at a heating rate of 108C/min over a temperature range of 30–6008C. A sample of 5–10 mg was used for each run. The weight change was recorded as a function of heating temperature. Differential peak temperature (DT p ) was defined as the temperature of the maximum derivative of the weight change over time. DSC experiments were performed in a Q10 differential scanning calorimeter (TA Instrument Inc., USA). Each sample was heated and cooled at a heating rate of 108C/min under nitrogen atmosphere. Each test sample of 5–10 mg was placed in an aluminum pan and heated from 30 to 2008C and then cooled down to 308C after keeping at 2008C for 3 min. The glass transition temperature (T g ), melting temperature (T m ), melting enthalpy (H m ), and crystallinity (X c ) were determined from the first heating scan, while the crystallization temperature (T c ) and crystallization enthalpy (H c ) were obtained from the first cooling scan. T m is defined as the maximum of the endothermic melting peak and T g as the deflection of the baseline temperature from the first heating scan. The X c was obtained by the following expression: X c ð%Þ ¼ H m H mðcrysÞ 100 ð1Þ 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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