Abstract
In this study, a bio-based soy protein adhesive derived from environmentally friendly and renewable enzymatic hydrolysis lignin (EHL), epoxidized soybean oil (ESO), and soy protein isolate (SPI), was successfully prepared. A novel biopolymer (EHL-ESO), as a multifunctional crosslinker, was firstly synthesized from modified EHL and ESO, and then crosslinked with soy protein isolate to obtain a bio-based soy protein adhesive. The structure, thermal properties, and adhesion performance of the obtained soy protein adhesives were determined by Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), scanning electron microscopy (SEM) and wet shear strength. The maximum degradation temperature of SPI/EHL-ESO adhesives (332–343 °C) was higher than that of the pristine SPI adhesive (302 °C). Moreover, plywood bonded by the modified adhesive reached a maximum wet shear strength value of 1.07 MPa, a significant increase of 101.8% from the plywood bonded by pristine SPI adhesive. The enhancements in the thermal stability and wet shear strength were attributed to the formation of a dense crosslinking network structure. This work not only highlights the potential to replace petroleum-based polymers, but also presents a green approach to fabricate fully bio-based soy protein adhesive for preparing all-biomass wood composite materials.
Highlights
In recent decades, interest in developing environmentally friendly and renewable adhesives has grown in order to offer alternatives to formaldehyde-based resins, including phenol formaldehyde (PF)based and urea formaldehyde (UF)-based resins, as they are derived from non-renewable petrochemicals
A er the Fenton oxidation and reductive amination of enzymatic hydrolysis lignin (EHL), three new signals appeared at 8.86, 8.49, and 7.97 ppm (H3), which was assigned to the protons of –NH– in different acetamide (RNHOCCH3) units that formed in the acetylation of the amino groups in enzymatic hydrolysis lignin amine (EHLA).[18]
This indicated that EHLA was successfully produced via Fenton oxidation followed by reductive amination, which was consistent with above Fourier transform infrared spectroscopy (FTIR) results
Summary
Interest in developing environmentally friendly and renewable adhesives has grown in order to offer alternatives to formaldehyde-based resins, including phenol formaldehyde (PF)based and urea formaldehyde (UF)-based resins, as they are derived from non-renewable petrochemicals. The C]O stretching (1658 cmÀ1) of amide I shi ed to the lower wavenumber of 1650 cmÀ1 These results were possibly due to a hydrogen bonding interaction between EHLA and SPI.[17] For SPI/ESO adhesive, new peaks appeared at 1741 (C]O in COOH or COOR), 827 (epoxy group), and 722 cmÀ1 ((CH2)n, HC]CH),[26] the peak intensity of –CH vibrations belonging to CH2 and CH3 at 2929, 2854, and 1461 cmÀ1 increased,[21,24] suggesting that ESO was successfully incorporated into the SPI matrix. According to the above results, an interpenetrating crosslinked network structure was formed by the internal physiochemical reaction between EHL-ESO and soy protein molecules, which could increase the water resistance of soy protein-based adhesives (Scheme 1)
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