Dr. Farkhondeh Hemmati Shirayeh, the assistant professor of Caspian Technical Faculty of Engineering of University of Tehran (UT), pointed to the necessity of using green composites and added, “In recent years, due to environmental concerns about the increase in plastic waste and global warming, biodegradable polymers have been introduced and produced as a suitable alternative to petroleum-based polymers. Biodegradable composites containing natural fibers and biopolymers obtained from renewable sources such as plants are very suitable for use in construction, packaging and medical industries.”
The polymer researcher explained more about poly(lactic acid), which is the basis of the composite designed in the research conducted at the UT, saying that "poly(lactic acid) is a biodegradable polymer with suitable properties such as transparency, defensible melt processability and high strength, which It can be synthesized from primary sources of renewable energy such as grains and potatoes, but it comes with weaknesses such as inherent brittleness, high price and low crystallization speed. Therefore, many researchers have tried to improve the properties and processability of poly(lactic acid) by using additives such as fillers and plant fibers. One of the available and cheap sources to strengthen and improve the properties of this polymer is lignocellulosic fibers obtained from agricultural residues.
One of the most suitable methods to chemically modify and soften plant fibers and, subsequentially, improve the properties of green poly(lactic acid)/rice straw biocomposites, is the environmentally friendly method of acetylation, in which the hydroxyl functional groups of lignocellulosic materials are replaced by hydrophobic chemical groups.
Acetylation is a chemical reaction that is called ethanoylation in the IUPAC nomenclature. It describes a reaction that introduces an acetyl functional group into a chemical compound. The opposite chemical reaction is called deacetylation – it is the removal of the acetyl group.
“In this research, a lignocellulosic filler, rice straw (RS), was thermoplasticized by acetylation modification process and compounded with plasticized poly(lactic acid) (PLA) through twin-screw extrusion process. The biocomposite films were prepared continuously by using a slit die for PLA/RS and PLA/acetylated RS (ARS) compounds. By the chemical reaction, thermoset microstructure of RS was changed to a more flexible one,” she continued to explain.
“Rice straw lignocellulosic filler was acetylated and found a core-shell structure with a thermoplastic shell. Biocomposite films of poly(lactic acid)/rice straw modified by using poly(ethylene glycol) plasticizer were prepared in a melt extrusion process by flat film method. These green biocomposite films were prepared at different loading percentages of modified rice straw reinforcement and different softener content, and their chemical, structural, mechanical, thermal, thermo-mechanical and rheological properties were studied,” she continued,” the UT researcher further elaborated on.
The findings of this study show that through the reaction, the interfacial adhesion of lignocellulosic filler/PLA matrix, filler hydrophobicity and PLA wettability was enhanced considerably. The miscibility of partially phase-separated PLA/poly(ethylene glycol) (PEG) mixtures was improved by adding RS-based microfillers, particularly by the acetylated RS. Compared with PLA/unmodified RS films, PLA/ARS biocomposites show better thermal stability, toughness, Young modulus and softening point, especially at certain filler loadings.