Water-Resistant Zein-Based Adhesives

Protein-based adhesives have gained considerable interest, due to their unique ecofriendliness and abundant functional groups. However, the presence of polar groups results in poor water resistance and unsatisfactory bonding strength of protein-based adhesives, limiting their practical applications. In addition, the complicated preparation process also made the production of protein-based adhesives time-consuming and costly. In this work, a hydrophobic protein zein, which lacks polar groups, was chosen as a basic ingredient for the preparation of protein-based adhesive. Sodium dodecyl sulfate (SDS) with a concentration of 200 mmol/L was added to promote the dissolution of zein and also render it negatively charged. The cations in metal chloride solution effectively cross-linked with SDS modified zein molecular chains, resulting in the formation of final zein-based adhesives. Results showed that a zein-based adhesive formed after treatment with 5 wt % FeCl₃ aqueous solution (Fe­(III)@zein/SDS adhesive) showed the best performance. It indicates that Fe­(III)@zein/SDS adhesive could bond a wide range of materials. Four common substrates were chosen to test the adhesive properties of Fe­(III)@zein/SDS adhesive at 25 °C, including one inorganic material glass, one metal material copper, and two organic polymer materials polyvinyl chloride (PVC) and polyimide (PI). The adhesive strength of Fe­(III)@zein/SDS adhesive was found from 125 kPa (PI) to 586 kPa (copper) in dry conditions, while from 12 kPa (copper) to 33 kPa (PI) in the wet state, displaying a promising adhesive strength in both dry and wet conditions. Meanwhile, the Fe­(III)@zein/SDS adhesive can be easily removed from the attached surfaces without nonchemical contamination by immersing in 70% ethanol aqueous solution. Therefore, such an environmentally friendly protein-based adhesive has great potential for practical use in various fields.