TA No.81 Thermal Analysis of Polylactic Acid －Crystallinity and Heat Resistance－ 2007.9

TA no.81 Thermal analysis of polylactic acid －Crystallinity and heat resistance－ 2007.9

1. Introduction 10mg samples were heated from 20 °C to 200 °C Recent views on waste disposal and environmental at 10 °C /min. in a nitrogen atmosphere. For the conservation have increased the focus on biode- TG measurements, 10-mg samples were heated from gradable plastic, which can be produced from re- room temperature to 400 °C at rates of 10, 5, 2 and newable raw materials and decomposes in the natural 1 °C / min. in a nitrogen atmosphere. environment. Polylactic acid (PLA) is a biodegrad- 3. Results able plastic derived from plants that is widely used in packing materials, fibers and medical materials. Figure 1 shows the DSC results when the samples Crystallinity is an important consideration for the were melted and then cooled at 0.1 °C / min. strength, impact resistance, and transparency re- Figure 2 shows the DSC results when the samples quirements of these materials and influences biode- were quenched rapidly. gradability. Furthermore, lactic acid, the PLA As seen in Figure 1, glass transition (Tg) occurred monomer, has asymmetrical carbon atoms and thus around 60 °C for all samples. Melting occurred with has optical isomers. The isomeric ratio and molecu- endothermic peaks at 150 °C and 170 °C for lar weight of polymers influence crystallinity and heat Sample b and Sample c/ c', respectively. These resistance, so their roles must be considered during results show that the higher the L ratio, the easier it the formation process. is for crystallization to occur. The crystallinity of In this brief, DSC and TG are used to measure the Sample was so low that crystallization did not occur, crystallinity and heat resistance of PLA plastic with even at a cooling rate of 0.1 °C/ min. The melting different isomeric ratios and molecular weights. temperature and heat of fusion for Sample c and c' were roughly the same so these results did not show 2. Measurement any clear differences in crystallinity due to molecular Four samples were measured. PLA plastic a, b, and weight. c had roughly the same molecular weight but differ- While Sample b has a melting peak in Figure 1, ent levo-rotatory(L)/ dextro-rotatory(D) ratios (L there is no melting peak in Figure 2. This result ratio: apolymer molding weight. DSC samples were heated to 200 °C and manufacturing. Furthermore, while there are no then cooled at various rates (quench cooling, 10, 5, differences between c and c' in Figure 1, there are 1, 0.5 and 0.1 °C/ min). No heating or cooling was differences at cold crystallization around 140 °C used for the TG measurements. and at the melting peak in Figure 2. The size dif- The measurements were performed using the ference of the peaks (heat of crystallization, heat of DSC6220 Differential Scanning Calorimeter and the fusion) indicates that when the L ratio is the same, TG/DTA6200 Thermo-Gravimetric/Differential the sample with the lower molecular weight has Thermal Analyzer. For the DSC measurements, higher crystallinity.

0 60.1℃ 0 ａ 53.9℃

-5 62.0℃ 16.2mJ/mg -2 ｂａ -10 147.7℃ -4 56.3℃ 62.1℃ 52.4mJ/mg ｃｂ -15 -6 57.2℃ 138.5℃ 1.85mJ/mg 62.0℃ 53.4mJ/mg

DSC ／ mW -1.79mJ/mg ｃ’ ｃ 167.3℃ -20 DSC ／ mW -8 57.3℃ 167.5℃ 137.1℃ 4.71mJ/mg -25 -10 ｃ’ -4.41mJ/mg 167.3℃ 167.5℃ -30 -12 40 60 80 100 120 140 160 180 40 60 80 100 120 140 160 180 Temp ／ ℃ Temp ／ ℃ Figure 1. DSC results after cooling at 0.1 °C/ min Figure 2. DSC results after quench cooling

10 10

53.9℃ 56.3℃ 5 5 55.1℃ 57.2℃ 55.3℃ 0 0 57.6℃ 56.8℃ 59.0℃ -5 -5 0.29mJ/mg 57.7℃ 151.1℃ 60.0℃ 0.79mJ/mg DSC ／ mW

-10 DSC ／-10 mW 60.1℃ 62.0℃ 151.2℃ 16.2mJ/mg -15 -15 147.7℃

-20 -20 40 60 80 100 120 140 160 180 40 60 80 100 120 140 160 180 Temp ／ ℃ Temp ／ ℃

Figure 3. DSC results for Sample a Figure 4. DSC results for Sample b

0 0 57.2℃ 57.3℃ 138.5℃ 1.85mJ/mg 137.1℃ 4.71mJ/mg 58.6℃ -1.79mJ/mg 167.3℃ 138.1℃ 1.96mJ/mg -4.41mJ/mg 167.3℃ 58.1℃ -5 168.2℃ -5 136.3℃ 4.56mJ/mg 58.8℃ -1.82mJ/mg 133.7℃ 3.09mJ/mg -4.16mJ/mg 168.3℃ 58.4℃ 135.8℃ 59.7℃ -2.60mJ/mg 167.8℃ 7.44mJ/mg 128.1℃ 13.2mJ/mg -10 -10 -5.67mJ/mg 60.2℃ 128.8℃ 167.9℃ 60.9℃ -4.42mJ/mg 18.7mJ/mg 33.4mJ/mg 168.3℃ -4.12mJ/mg 60.2℃ -15 62.1℃ -15 37.8mJ/mg 52.4mJ/mg 168.9℃ 62.0℃ 53.4mJ/mg DSC ／ mW -20 168.5℃ DSC ／-20 mW

169.8℃ -25 -25

167.5℃ -30 -30 167.5℃ 40 60 80 100 120 140 160 180 40 60 80 100 120 140 160 180 Temp ／ ℃ Temp ／ ℃ Figure 5. DSC results for Sample c Figure 6. DSC results for Sample c’

After quench cooling After cooling at 10 °C/ min After cooling at 5 °C/ min After cooling at 1 °C/ min After cooling at 0.5 °C/ min After cooling at 0.1 °C/ min

Figures 3 to 6 show the DSC results for Sample a, b, Table 1. Comparison of relative crystallinity* by cooling rate (°C / min) c and c' at different cooling rates. Sample a did not Rate 0.1 0.5 1 5 10 Quench show a melting peak under any cooling condition and cooling was considered nearly amorphous, regardless of the Sample a ------cooling rate. Sample b showed a melting peak when b 0.303 0.015 0.005 0 0 0 the cooling rate was 1 °C / min or less. This result c 0.981 0.625 0.164 0.009 0.003 0.001 c' 1 0.708 0.273 0.033 0.007 0.006 shows that to increase the crystallinity, the cooling * The heat of fusion of Sample c’ after cooling at 0.1 °C/ min rate must be at this rate or slower. For Sample c and was set as 1. c', there was no difference in crystallinity at a cool- 20 ing rate of 0.1 °C/ min. However, when the cooling 0 rate was 0.5 °C / min or greater, Sample c' had a large heat of fusion. This shows that the crystallinity -20 of Sample c' is higher. -40

-60 By setting the heat of fusion of Sample c' after TG ／ % ｂｃ -80 1℃/min cooling at 0.1 °C/ min to one, the relative crys- 2℃/min 5℃/min tallinity was calculated from heat of fusion. The -100 10℃/min results in Table 1 show that the higher the L ratio, -120 the higher the crystallinity. The results for Sample c 200 250 300 350 400 and c' show the influence of molecular weight. While Temp ／ ℃ there was little difference in the crystallinity after Figure 7. TG results for Sample b and c cooling at 0.1 °C/ min, these results confirm that molecular weight has an influence on crystallinity that is dependant on the cooling rate. 1.2 Figure 7 shows the TG/DTA results for Sample b 1.0 and c at different heating rates. The TG results show 0.8 little difference in thermal decomposition so there 0.6 were no clear differences in heat resistance. Next, 0.4 kinetics analysis was performed using the Ozawa 0.2 method with the TG results for Sample b, c and c' at Log B ／ ℃/min different heating rates. Figure 8 shows the results of 0.0 the kinetics analysis for Sample c. Table 2 shows the -0.2 activation energy results. These results show that 1.50 1.55 1.60 1.65 1.70 1.75 1.80 1000/T ／ 1/K Sample c', which has a low molecular weight, reacts Figure 8. Arrhenius plot results for Sample c quickly. When the molecular weight is the same, samples with a lower L ratio have a faster reaction. These results confirm that heat resistance differs by molecular weight and L ratio.

Table 2. Calculation results for activation energy Sample ΔE [kJ/mol] Constant Temperature Degradation Time (Life Time) (hr.) b 144 15.4 c 155 21.6 c’ 136 10.9 * 230 °C, 10% reaction

4. Summary In this brief, DSC and TG/DTA were used to measure PLA, a biodegradable plastic. Two important factors during the polymer molding manufacturing of PLA plastic are crystallinity and heat resistance. The DSC results showed the relation between the cooling rate during polymer molding manufacturing and the crystallization rate, which revealed the relevance of crystallinity and crystallization conditions. The TG/DTA measurements made it possible to evaluate the heat resistance at the formation temperature.