Anomalous Postprocessing Dimensional Change of Injection-Molded Products Composed of Poly(lactic acid) and Poly(vinyl alcohol)

Abstract

The present study revealed that an injection-molded product comprising poly(lactic acid) (PLA) and poly(vinyl alcohol) (PVAL) exhibited an anomalous dimensional change, i.e., expansion in the flow direction, after postprocessing annealing (around 2.9% expansion by annealing at 80−100 °C for 3 h). Although the crystallization of the PLA during cooling was not accelerated by the addition of PVAL, the PLA showed transcrystallization from the dispersed PVAL droplets, which were deformed in the flow direction, during postprocessing annealing. Therefore, after annealing, the injection-molded products exhibited a high degree of molecular orientation, in contrast to the original unannealed products, which were not oriented. This anomalous behavior should be noted as a means of controlling dimensional change after processing.

Growth of molecular orientation during post‐process annealing of poly(lactic acid) containing a fibrous nucleating agent

“…Up to now, only a few studies have been reported on the orientation growth during annealing to the best of our knowledge. 48,49 A full-wave plate was inserted into the POM, which added 530 nm to the optical pathlength, to confirm the molecular orientation in the PLA/EBHS strands, as shown in Figure 13. The POM images revealed that the color of the strand before annealing was blue.…”

“…[42][43][44][45][46][47] Although this is a severe problem in industry, almost no study has been performed to control the structure and dimensional change during annealing from a scientific point of view, except for a few studies reported recently. 48,49 It was found that PLA with an appropriate shear history showed chain orientation during post-process annealing, 48 that is, molecular orientation increased, not relaxed. Moreover, injectionmolded products of PLA containing a low-molecular-weight poly(vinyl alcohol) (PVA) 49 exhibited anomalous dimensional changes, that is, expansion in the flow direction and increased molecular orientation during post-process annealing.…”

“…48,49 It was found that PLA with an appropriate shear history showed chain orientation during post-process annealing, 48 that is, molecular orientation increased, not relaxed. Moreover, injectionmolded products of PLA containing a low-molecular-weight poly(vinyl alcohol) (PVA) 49 exhibited anomalous dimensional changes, that is, expansion in the flow direction and increased molecular orientation during post-process annealing. This behavior is notable considering that shrinkage during annealing is a severe problem in industry.…”

“…However, annealing causes dimensional changes because of orientation relaxation, that is, decreased molecular orientation, and the volume decrease induced by crystallization 42–47 . Although this is a severe problem in industry, almost no study has been performed to control the structure and dimensional change during annealing from a scientific point of view, except for a few studies reported recently 48,49 . It was found that PLA with an appropriate shear history showed chain orientation during post‐process annealing, 48 that is, molecular orientation increased, not relaxed.…”

Superior heat‐resistant polylactide/poly(butylene succinate) blend fibers via in‐situ reactive compatibilization

“…Poly(lactic acid) (PLA), derived from renewable resources, has attracted widespread attention 1,2 . As an industry front‐running biodegradable polymer, PLA has been extensively applied in various fields, such as packaging films, 3 molded products, 4 3D‐printing, 5 and textiles 1 . Especially, PLA fibers have the potential as an alternative to the used petroleum‐based fibers 6,7 .…”