Alkaline hydrolysis of biomass as an alternative green method for bioplastics preparation: In situ cellulose nanofibrillation

Mild acid hydrolysis of various plant residues has been proposed in recent years as a novel way of transforming biomass into bioplastics. However, the alkaline hydrolysis of such residues has not yet been studied for this purpose. In this work, an in-depth comparative study is carried out for the first time on the physicochemical, thermal, mechanical, and morphological aspects of the bioplastics produced by acid and alkaline hydrolysis starting from two different plant residues: spinach stems (SS) and peanut shells (PS). The chemical treatments followed here, produced self-standing SS bioplastics and hydrolyzed PS powders that were incorporated as fillers in a thermoplastic starch (TPS) matrix to obtain composites. The alkaline hydrolysis led to bioplastics with superior mechanical and barrier properties than those obtained from acid hydrolyzed biomass. The Young's modulus (YM) of SS-bioplastics produced upon alkaline hydrolysis, tripled, their tensile strength (TS) almost doubled, and their water vapor permeability (WVP) was reduced by 15%, compared to SS-bioplastics produced upon acidic hydrolysis. TPS-alkali hydrolyzed PS composites showed increments of 22% in YM, 10% in TS, and a reduction of about 30% in their WVP compared to the respective acid hydrolyzed composites. The physicochemical, thermal, and morphological analysis confirmed that the main cause of these improvements was cellulose nanofibrillation, which was favored by the greater efficiency of the alkaline medium to hydrolyze the pectin, hemicellulose, and lignin polymers. This research represents a step ahead in understanding the processes of transforming non-edible vegetable wastes into sustainable bioplastics and comes in a critical moment when an urgent transition towards a circular economy is need, and industrial processes are expected to reduce their carbon footprint and generate zero waste.