Abstract:

Objective Against the background of global climate and environmental deterioration caused by excessive CO₂ emission and other factors, the development of a low-carbon recycling economy has become a global consensus. As a biodegradable green material, polylactic acid (PLA) is one of the important materials that can be adopted to address the environmental pollution. However, the performance of PLA varies greatly among different brands. This study compares the properties of PLA produced by China and America companies.

Method PLA masterbatch (PLA FY from a domestic manufacturer, PLA-NW from an oversea manufacturer) was put into a beaker containing deionized water and washed by ultrasonic cleaning for 30 min, and then put into an oven for drying at 45 ℃ for 6 h. The dried PLA masterbatch was put into a beaker and sealed and stored for use. Then, the differences between the molecular weight, surface morphology and optical purity, crystallinity, melt rheological properties and thermal stability of the granules provided by the two manufacturers were evaluated by gel permeation chromatography, optical microscope, automatic polarimeter, X-ray diffractometer, viscometer, rheometer, thermogravimetric analyzer, and melt indexer.

Results From the gel chromatographic analysis, the molecular mass distribution curves of the two PLA masterbatches were generaly similar, and both existed as single peaks, and the values of number-average molecular weight (M_n) (41 755 g/mol), weight-average molecular weight (M_w) (105 887 g/mol), Z-average molecular weight (M_z) (183 764 g/mol), and peak molecular weight (M_p) (99 581 g/mol) of PLA-NW were higher than those of PLA-FY. The molecular weight distribution index of PLA-NW (M_w/M_n=2.535 9) is lower than that of PLA-FY (M_w/M_n=2.658 4), which indicates that the molecular mass distribution of PLA-FY is wider. The surface of PLA-NW is relatively much rougher. The multi-sample statistics show that the diameter of PLA-FY is (4.477 2±0.102 5) mm, and that of PLA-NW is (5.100 6±0.098 5) mm, in the long direction. PLA-FY has an optical purity of 97.85%, whereas PLA-NW has an optical purity of 105.6%. In addition, both PLA-NW and PLA-FY are typical type-α crystals, indicating the typical orthorhombic crystal structure of PLA. The characteristic viscosity of PLA-NW (140.898 mL/g) is higher than that of PLA-FY (112.749 mL/g). The decomposition temperatures of PLA-NW at 5% and 50% weight loss were higher than that of PLA-FY, while the temperature T_t (387.30 ℃) of PLA-NW at the termination of decomposition was higher than that of PLA-FY at the termination of decomposition (378.23 ℃). Its temperature at the maximum decomposition rate (T_max=371.79 ℃) was also higher than that of PLA-FY (T_max=363.73 ℃). PLA-NW showed better thermal stability. In addition, the melt index of PLA-FY was always greater than that of PLA-NW. In general, the higher the melt index, the lower the molecular weight, the lower the temperature resistance, and the less difficulty the processing. This further confirms that the temperature resistance of PLA-NW is better than that of PLA-FY.

Conclusion This paper selects two types of spinning-grade PLA masterbatch manufactured domestically and abroad, and analyzes the performance of the two types of PLA granules through a series of characterization tests. The molecular weight of PLA-NW is slightly higher than that of PLA-FY, and the molecular weight distribution of PLA-NW is more uniform. Also, the crystallinity, optical purity, thermal stability and rheological properties of PLA-NW are better than that of PLA-FY.

Key words: polylactic acid, crytalline structure, masterbatch, degree of crystallization, rheology property, thermal stability