Abstract
The effect of mold conditions was investigated in terms of mold temperature (30oC and 90oC) and cooling time (30 s and 60 s) on the heat resistance of injection-molded bars for stereocomplex polylactide-b-polyethylene glycol-b-polylactide (scPLA-PEG-PLA). Comparative study was performed for poly(L-lactide) (PLLA) and PLLA-b-PEG-b-PLLA (PLLA-PEG-PLLA). scPLA-PEG-PLA was 90/10 (w/w) PLLA-PEG-PLLA/poly(D-lactide) blend. scPLA-PEG-PLA exhibited the easiest crystallization upon cooling scan as shown by differential scanning calorimetry (DSC). Higher mold-temperature and longer cooling-time induced higher degree of crystallinity as assessed by X-ray diffractometry (XRD) except for PLLA bars. The heat resistance of both PLLA-PEG-PLLA and scPLA-PEG-PLA bars was improved with increased mold-temperature and cooling-time as shown by dynamic mechanical analysis (DMA), vicat softening temperature (VST) and heat distortion-resistance tests except for PLLA bars. In conclusion, the heat resistance of injection-molded bars prepared at 90˚C mold temperature was in the order scPLA-PEG-PLA ] PLLA-PEG-PLLA ] PLLA. The results suggested that flexible PLLA-PEG-PLLA and scPLA-PEG-PLA with high degrees of crystallinity were successfully obtained by injection molding for use as good heat-resistant bioplastic products.
Highlights
Poly(L-lactic acid) or poly(L-lactide) (PLLA) is an important bio-based plastic because it features non-toxicity, biodegradability, bio-renewability and good processability [1−3]
The effects of mold temperature and cooling time on heat resistance of injection-molded bars were investigated for stereocomplex polylactides (scPLA)-PEG-PLA compared with PLLA3052D and PLLA-PEG-PLLA
From X-ray diffractometry (XRD), both the PLLA3052D and PLLA-PEG-PLLA bars prepared at 30oC mold temperature were completely amorphous in character whereas the scPLA-PEG-PLA bars had both hc and sc-crytsallites
Summary
Poly(L-lactic acid) or poly(L-lactide) (PLLA) is an important bio-based plastic because it features non-toxicity, biodegradability, bio-renewability and good processability [1−3]. Low flexibility, poor crystallizability and low heat-resistance were the problems in practical use of PLLA [7−9]. Poly(L-lactide)-b-polyethylene glycol-b-poly(L-lactide) triblock copolymers (PLLA-PEG-PLLA) showed greater flexibility and faster crystallization-rate than PLLA due to flexibility of PEG middleblocks causing an enhanced plasticizing effect [10,11]. The heat resistance of PLLA-PEGPLLA was still poor [12]. The heat resistance of PLLA and PLLA-PEG-PLLA is directly related to degree of crystallinity [13−15]. This indicates that crystallizability of PLLA-PEG-PLLA is not enough to improve its heat resistance. Nucleating agents and heat treatment have been used to increase the degree of crystallinity of PLLA and so improve its heat-resistance [13−15]
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