This study investigates the effect of annealing heat treatment on polymer composites composed entirely of biodegradable components. Poly(lactic acid) (PLA) was used as the matrix material paired with three different types of commercial lignocellulose fibers of varying sizes. Composites containing 10 wt.% fibers were processed through extrusion followed by injection molding. The amorphous (unannealed) and semi-crystalline (annealed) samples were characterized for their morphological, thermal, mechanical, and water absorption properties. Scanning electron microscopic analysis revealed a homogenous distribution of cellulose fibers within the PLA matrix, even though the composite with the smallest fiber size exhibited slight agglomeration. Differential scanning calorimetric measurements indicated that the annealing heat treatment successfully induced crystallization, with the filler particles capable of increasing the extent of crystallinity formed during the annealing heat treatment from 28% to 36%. Based on the tensile tests, as a result of annealing heat treatment, the composites' strength increased from 48–50 to 53–56 MPa, while their Young's modulus increased from 3.1 GPa to 3.3-3.5 GPa. The Charpy impact tests also revealed an enhanced toughness for the samples exposed to the annealing heat treatment. In terms of water absorption, annealing enhanced the hydrophobic nature of PLA. In addition, the semi-crystalline structure formed during the heat treatment also inhibited the highly hydrophilic cellulose fibers from absorbing as much moisture as they did when incorporated inside amorphous PLA; cellulose fibers embedded in the semi-crystalline PLA matrix consequently exhibited less moisture absorption than the ones in amorphous PLA. Highlights: Through annealing, crystallinity was developed in PLA/lignocellulose composites. Lignocellulose fibers facilitated crystallization by acting as a nucleating agent. Crystallized biocomposites exhibited superior mechanical properties Crystalline segments hindered the water absorption of embedded lignocellulose.

Wood flour-paired poly(lactic acid) (PLA)-based biocomposites were fabricated with filler contents of 0, 2.5, 5, 10, and 20 wt.%. The samples were processed through extrusion followed by injection molding. The injection-molded specimens were subjected to tensile tests, impact tests, and water absorption tests. Based on the results, increasing the amount of lignocellulose fibers effectively improved the modulus of PLA from 2.8 to 3.6 GPa at 20 wt.% wood flour loading; however, this came at the cost of strength, which dropped from 55.0 to 48.8 MPa. Additionally, the incorporation of lignocellulose increased the hydrophilicity of the composites, resulting in a threefold increase in water absorption at 10 wt.% filler content. These results provide insight into the effects of embedding lignocellulose fibers into PLA.

This study evaluated the changes in the molecular structure, as well as the thermal and mechanical properties of poly-lactic-acid (PLA), during successive closed-loop mechanical recycling through injection molding. Gel permeation chromatographic (GPC) analyses revealed a gradual decrease in molar mass (MM); after ten recycling steps, the MM of PLA was reduced to one-third of its initial value. According to differential scanning calorimetric (DSC) measurements, both the glass transition temperature and the cold crystallization temperature of PLA decreased, while the crystallization was facilitated by the successive recycling steps. This was ascribed to the improved chain mobility resulting from the reduced MM. Quasi-static mechanical properties were evaluated using tensile tests, while impact toughness was assessed through Charpy tests. The tensile strength of PLA was not significantly affected by recycling until the fifth recycling step, where it suffered a slight (∼5%) reduction. Meanwhile, Young's modulus, elongation at yield, and impact strength remained approximately the same as the freshly injected sample at that point. However, after ten recycling steps, all analyzed properties declined significantly: Young's modulus by 11%, tensile strength by 68%, elongation at yield by 63%, and impact strength by 30%.