Applied Surface Science, Vol.478, 441-450, 2019
Bio-inspired cellulose nanofiber-reinforced soy protein resin adhesives with dopamine-induced codeposition of "water-resistant" interphases
Poor interface strength and water resistance are significant challenges in taking full advantage of the excellent performances of natural fiber-reinforced biopolymer composites. Herein, a mussel-inspired co-deposition process was developed to synthesize a nanostructured layer on the surface of cellulose nanofiber (CNF) with target water-resistant property and high adhesion strength of soy protein isolate (SPI)/CNF composite adhesives. Renewable resource cardanol was selected as an environmentally sustainable interphase constituent. First, hydrolyzable alkoxy silane cardanol (ASC) was synthesized via an epoxide ring opening reaction of the cardanolbased glycidyl ether with aminopropyltriethoxysilane. When assembled on the CNFs, the ASC/dopamine system conformally coated every fiber, yielding a nanostructured polydopamine (pDA)/ASC hybrid coating that sub-sequently acted as the resin/fiber interphases during the adhesive preparation. The nanohybrid coatings contained versatile catechol functional groups as "bonding sites" and long aliphatic chains as a "flexible barrier". The integrated advantages of the CNF network and the "bonding site and flexible barrier" structures on the outside surface contributed to the strong cohesive interaction between the soy protein and nanofibers, and hence the reactive water-resistant barrier was formed between the two substrates. Thanks to the tailored interphase, the resins achieved impressive adhesion properties with a wet shear strength up to 1.27 MPa, which was 189% and 95% higher than that of the neat resin and the adhesive prepared with pristine fibers, respectively. The findings demonstrate that the sustainable cardanol holds potential for the development of water-resistant bio-based composites.