Chemical Modification of Biomass Okara Using Poly(acrylic acid) through Free-radical Graft Polymerization

Abstract

Okara (Ok), or soybean residue, is produced as a by-product from the soybean milk and soybean curd industries world widely, most of which is disposed or burned as waste. It is important to explore the possibilities to convert okara to useful materials, since okara is a naturally renewable bioresource. Here we report the chemical modification of okara by grafting poly(acrylic acid) (PAA) onto the backbones of okara in water medium, and the characterization of the Ok-PAA graft copolymers. It was found that the received okara mainly contained insoluble contents in water. The insoluble okara component Ok(Ins) was suspended in water and activated with ammonium persulfate as an initiator, followed by grafting PAA through a free radical polymerization. After the graft polymerization, the product (Ok-PAA) was separated into precipitate and supernatant, which were dried to give Ok-PAA(pre) and Ok-PAA(sup), respectively. It was found that PAA was grafted on Ok backbones and co-precipitated with the insoluble Ok. In addition, Ok-PAA(sup) was found to be translucent due to the grafting of PAA. Further, the successful grafting of PAA onto okara backbones was proved by FTIR, TGA, and microscopic measurements. Ok-PAA(sup) dispersed in water formed nanoparticles with average diameter of 420 nm, while Ok-PAA(pre) were clustered coarse particles in water. The rheological data including the storage modulus, loss modulus, and viscosity indicated that the Ok-PAA product was a viscoelastic gel-like material with potential for agricultural and environmental applications.