Title: Superabsorbent biopolymer matrices from agro-polymers: Rheology, processing, and applications

Abstract

Superabsorbent polymers (SAP) based on acrylic derivatives exhibit remarkable properties, as they can absorb an impressive amount of water (i.e. from 10 to 1000 times their weight, in g/g). Unfortunately, these polymers are challenged by a poor biodegradability, as well as potential toxicity, and low cost-effectiveness. Consequently, there is a pressing need for alternative materials to address these concerns.

Protein-based SAPs, which usually require a plasticizer for their manufacture, are gel matrices that can absorb and retain more than 10 times their own initial weight of water. A thorough understanding of the rheological properties of protein-plasticizer mixtures is crucial for successfully processing them into SAP gel matrices. This process can be accomplished through various polymer processing techniques, including extrusion, compression molding, injection molding, or 3D printing. The processing conditions play a significant role in achieving the desired properties of different protein-based SAP matrices.

Furthermore, it is also essential to consider the role of composition (such as protein and plasticizer content, as well as pH). These variables influence the balance between different interactions, including hydrogen bonds, electrostatic interactions, hydrophobic interactions, covalent bonds. Such a balance can lead to protein unfolding, promoting the exposure of hydrophobic groups that can induce relevant modifications, ultimately resulting in the formation of a protein network. In some cases, the integrity of the material can be compromised after water absorption. To overcome this problem, the addition of some natural crosslinkers (e.g., citric acid, genipin, tannic acid or cellulose) have been also considered. These crosslinkers can enhance the structural stability and water retention capacity of SAPs, making them more effective and durable. On the other hand, they can also reduce their water uptake capacity. This trade-off between improved mechanical properties and water absorption capacity must be carefully balanced to tailor SAPs for specific applications.

On these grounds, materials obtained from by-products and wastes of the agri-food industry have been studied, resulting in much lower biodegradability and suitable water absorption capacity, and overpassing the superabsorbent threshold.