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Introduction Biodegradable plastic has gained attention in recent years due to concerns about waste disposal and environmental conservation. Polylactic acid (PLA) is a biodegradable plastic derived from plants and continues to be widely used in packing, fibers and medical materials. Crystallinity is an important consideration for the strength, impact resistance, and transparency requirements of these products and also influences biodegradability. Furthermore, lactic acid, the PLA monomer, has asymmetrical carbon and thus optical isomers. The isomer ratio and molecular weight of polymers influence crystallinity and heat resistance, so they are factors in the molding process. In this paper, the crystallinity and heat resistance of polylactic acid are evaluated using DSC and TG. Three samples had roughly the same molecular weight but different optical isomer L-form/D-form ratios and two samples had the same L-form/D-form ratio but different molecular weights. Thermal characterization of Polylactic Acid by Thermal Analysis Yoshikazu.Nishiyama, Yuichi Kasai, Nobuaki.Okubo and Yutaka.Ichimura Analytical Application Engineering Department, SII NanoTechnology Inc. Conclusion Crystallinity and heat resistance are important factors in PLA plastic molding. This study shows that DSC can be used to learn about crystallinity and the validity of crystallization conditions. Furthermore, thermal decomposition measurements TGA were able to evaluate heat resistance at molding temperatures. Experiment Sample 1: Polylactic acid ; a, b, c and c’ L-form ratio ; a < b < c = c’ Molecular weight ; a = b = c > c’ Instrument : DSC6220 and TG/DTA6200 (SII NanoTechnology Inc.) Measurement Condition : DSC measurements Samples were heated to 200℃ and then cooled at various rates (quench, 10, 5, 1, 0.5 and 0.1℃/min). The samples were then heated from 20℃ to 200℃ at 10℃/min in N2 atmosphere. TG measurements Samples were prepared without any thermal treatments, but simply heated from room temperature to 400℃ at rates of 10, 5, 2 or 1℃/ min. C H COOH H3C OH L-lactic acid 10.9 136 c’ 21.6 155 c 15.4 144 b Lifetime * ［hr］ ΔE［kJ/mol］ Sample Log B / ℃/min 1.0 0.8 0.6 0.4 0.2 0.0 -0.2 1.2 1/T / 1000/K 1.50 1.55 1.60 1.65 1.70 1.75 1.80 Arrhenius plot for sample c *:230℃, Reaction fraction 10% 0.708 0.625 0.015 － 0.5 0.273 0.164 0.005 － 1 0.033 0.009 0 － 5 0.007 0.003 0 － 10 － － a 0.006 1 c’ 0.001 0.981 c 0 0.303 b quench 0.1 cooling rate [℃/min] Sample After 10℃/min cooling After quench After 5℃/min cooling After 1℃/min cooling After 0.5℃/min cooling After 0.1℃/min cooling Comparison of the relative crystallinity* * : Sample c’ melting heat ( after 0.1℃/min cooling ) is set as 1. DSC curves after 0.1℃/min cooling Temperature / ℃ 180 160 140 120 100 80 60 40 DSC / mW 0 -5 -10 -15 -20 -25 -30 60.1℃ 16.2mJ/mg 62.0℃ 167.5℃ 167.5℃ 62.1℃ 53.4mJ/mg b c c’ 62.0℃ a 52.4mJ/mg 147.7℃ DSC curves after quench 167.3℃ Temperature / ℃ 180 160 140 120 100 80 60 40 DSC / mW 0 -2 -4 -6 -8 -10 -12 167.3℃ 137.1℃ 4.71mJ/mg -4.41mJ/mg 1.85mJ/mg -1.79mJ/mg 138.5℃ 57.3℃ 53.9℃ 56.3℃ 57.2℃ b c c’ a Calculation results of Activation Energy Temperature / ℃ 180 160 140 120 100 80 60 40 DSC / mW 10 5 0 -5 -10 -15 -20 55.3℃ 56.8℃ 57.7℃ 53.9℃ 60.1℃ 55.1℃ DSC curves for sample a 1.85mJ/mg Temperature / ℃ 180 160 140 120 100 80 60 40 DSC / mW 0 -5 -10 -15 -20 -25 -30 168.3℃ 13.2mJ/mg -4.42mJ/mg 59.7℃ 128.1℃ 167.3℃ 57.2℃ -1.79mJ/mg 138.5℃ 167.5℃ 62.1℃ 52.4mJ/mg 168.5℃ 33.4mJ/mg 60.9℃ 167.8℃ -2.60mJ/mg 3.09mJ/mg 58.8℃ 133.7℃ 58.6℃ 168.2℃ 138.1℃ -1.82mJ/mg 1.96mJ/mg DSC curves for sample c Melting peak is not observed DSC curves for sample b Temperature / ℃ 180 160 140 120 100 80 60 40 DSC / mW 10 5 0 -5 -10 -15 -20 62.0℃ 147.7℃ 16.2mJ/mg 57.6℃ 59.0℃ 151.1℃ 0.29mJ/mg 60.0℃ 151.2℃ 0.79mJ/mg 56.3℃ 57.2℃ 18.7mJ/mg 168.9℃ Temperature / ℃ 180 160 140 120 100 80 60 40 DSC / mW 0 -5 -10 -15 -20 -25 -30 167.5℃ 57.3℃ 4.71mJ/mg 137.1℃ -4.41mJ/mg 60.2℃ 128.8℃ -4.12mJ/mg 169.8℃ 60.2℃ 37.8mJ/mg 167.9℃ 58.4℃ 135.8℃ -5.67mJ/mg 7.44mJ/mg 168.3℃ 58.1℃ -4.16mJ/mg 4.56mJ/mg 136.3℃ 167.3℃ 53.4mJ/mg 62.0℃ DSC curves for sample c’ Tendency to increase crystallinity at cooling rate less than 1℃/min Smaller molecular weight c’ has the higher crystallinity Temperature / ℃ 400 350 300 250 200 TG / % 20 0 -20 -40 -60 -80 -100 -120 2℃/min 1℃/min 5℃/min 10℃/min b c TG curves for sample b and c Small difference in the thermal decomposition on TG curves Smaller molecular weight c’ reacts quickly Smaller L-form ratio b reacts quickly No difference in crystallinity caused by the different molecular weight ot sample c and c’ Compared with c, c’ likely has the higher crystallinity because of the higher melting heat The higher the L-form ratio is, so is the crystallinity. Cooling rate dependent TG measurements DSC measurements after thermal treatment 19 th MRS-J Academic Symposium @ Yokohama
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