Cyclic carbonates as green reactants for improving thermoplastic properties of lignocellulosic materials

Abstract

Lignocellulosic materials are attractive raw materials for producing thermoplastics with more sustainable manufacturing. They come from a renewable source that can reduce the dependency on conventional fossil-based feedstock and has good tensile properties. However, their polymeric chains have poor mobility because of the multiple hydrogen bonds of their hydroxyl groups, which is an essential obstacle for thermoplastic processing. To improve their thermoplasticity, their hydroxyl groups can be converted by chemical modifications that introduce the side groups that can increase the flowability of their chains. In this study, unbleached softwood kraft pulp was oxyalkylated with cyclic carbonates (propylene carbonate and ethylene carbonate), acting as a reactant and medium. These two reactants create low environmental impacts because of their biodegradability and low toxicity. In addition, they are also safer compounds from their high boiling point, flash point, and vapor pressure. The influence of temperature, catalysts, and reaction time were investigated. The molecular structures, purity, and thermal properties of the modified products were also evaluated. The chemical modification with ethylene carbonate provides the highest yields and appears to be the most effective pathway to substitute hydroxyl groups with the alkyl side chains. In addition, the products from the chemical modifications with ethylene carbonate have a higher purity and are easier to separate than the products from the chemical modifications with propylene carbonate. Increasing the temperature and amount of catalyst promotes the substitutions on the hydroxyl group. Finally, the modified pulp from chemical modification with ethylene carbonate at a higher temperature and amount of catalyst has better thermal properties than the unmodified pulp. The glass-transition temperature (Tg) of the modified pulp can be detected at approximately 180 °C while the Tg of its raw material is above 220 °C, so the polymeric chains of modified pulp become more flowable.