Improvement of Crystallization and Mechanical Properties of PLA by Means of Clay Nanocomposite
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Improvement of Crystallization and Mechanical Properties of PLA by Means of Clay Nanocomposite Hirotaka Okamoto1, Mitsuru Nakano1, Makoto Ouchi1, Arimitsu Usuki1 and Yuji Kageyama2 1 Toyota Central R&D Labs., Inc., Nagakute, Aichi, 480-1192, Japan 2 Toyota Motor Corporation, 1200, Mishuku, Susono, Shizuoka, 410-1193, Japan ABSTRACT Some composite materials of poly (lactic acid) (PLA) with organophilic clays and an organic crystal accelerator (OCA) were prepared by melt compounding method to improve the rate of crystallization and mechanical properties, especially heat distortion temperature (HDT). The PLA/clay nanocomposite with 2.9wt% of finely dispersed organophilic clay and 1wt% of the OCA showed an exothermic peak corresponding to its crystallization at 1.9 minutes in the isothermal measurement of DSC at 100oC from its molten state, although PLA itself did not show a clear peak in the same procedure. Crystallized specimens of the nanocomposite were well obtained by injection molding when the hold time was set at more than 90 seconds. The HDT of the specimens that were held for more than 120 seconds in the mold exceeded 120oC. It is about 65oC higher than amorphous PLA. The crystallized specimen with 1wt% of the clay showed higher tensile modulus and Izod impact strength than the amorphous one. INTRODUCTION In view point of environmental conservation, such as reduction of petroleum resource consumption and carbon dioxide emission, wide use of renewable plant-derived material is expected as alternative to conventional petrochemical plastics in recent years [1]. In this direction, poly (lactic acid) (PLA) is one of the most promising candidates because it is made from agricultural products such as sugar and corn [2, 3]. PLA is usually used as amorphous material in molded products because of its low rate of crystallization, though it is semi-crystalline polymer. About 57oC of the glass transition temperature, Tg, of PLA leads to low heat distortion temperature (HDT) of PLA products, which is about 58oC with the load of 0.45MPa. It is necessary to increase the degree of crystallinity for use of PLA as automotive parts with high HDT. Recently, we succeeded in synthesizing a PLA/clay nanocomposite and showed that the nanocomposite improved modulus at higher temperature than Tg and the rate of crystallization [3]. In the present study, some composite materials of PLA with organophilic clays and an organic crystal accelerator (OCA) were prepared by melt compounding method in order to improve the rate of crystallization and mechanical properties, especially the HDT. EXPERIMENTAL Two kinds of PLA produced by Toyota Motor Corporation were used in this study. Their contents of the monomer unit of L-lactic acid and weight-average molecular weights are listed in Table I. Several organophilic clays were supplied by Nanocor or Southern Clay Products, or prepared in our laboratory from Na+-montmorillonite (Kunipia F; Kunimine Ind. Co.) according to the ion-exchanging method reported in the literature [4-6]. Their featur
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