Abstract
To lay a theoretical basis for the preparation of peanut protein-based adhesives and promote the sustainable development of the adhesive industry, properties of peanut protein isolate (PPI), arachin and conarachin-based adhesives modified by urea and epichlorohydrin (ECH) were investigated under different urea concentrations. When the urea concentration was 2 mol l−1, the wet shear strength of the PPI-based adhesive was 1.24 MPa with the best water resistance. With the increase of urea concentration from 0 to 4 mol l−1, the apparent viscosity of the PPI-based adhesive increased from 3.87 to 136.80 Pa s and the solid content increased from 18.11% to 31.11%. Compared with conarachin-based adhesive, the properties of arachin-based adhesive were improved more obviously during the combined modification. Scanning electron microscopy images illustrated that when the urea concentration was 2 mol l−1, the surface of the PPI-based adhesive was more compact and smoother, which was beneficial to the improvement of water resistance and related to the structure changes of arachin and conarachin components. Fourier-transform infrared spectroscopy results indicated that different urea concentrations caused the change of ester and ether bonds in the PPI-based adhesive, which was mainly related to arachin component. Thermogravimetry results suggested that when the urea concentration was 2 mol l−1, the decomposition temperature of protein skeleton in the PPI-based adhesive reached a maximum of 314°C exhibiting the highest thermal stability. The improvement of the thermal stability of conarachin was greater than that of arachin during the combined modification.
Highlights
Synthetic formaldehyde adhesives, for instance, phenol-formaldehyde, urea-formaldehyde and melamine-formaldehyde are widely used in the wood synthesis industry due to their excellent bonding performance and low cost [1]
With the addition of urea further increasing, the wet shear strength of adhesive I4 decreased to 0.55 MPa, which was unqualified for the demand of Chinese National Standard GB/T 9846-2015
During the combined modification of urea and ECH, different urea concentration led to changes in the structure and performance of the peanut protein isolate (PPI)-based adhesive, which was attributed to changes in its arachin and conarachin components
Summary
Synthetic formaldehyde adhesives, for instance, phenol-formaldehyde, urea-formaldehyde and melamine-formaldehyde are widely used in the wood synthesis industry due to their excellent bonding performance and low cost [1]. The development of environment-friendly biomass adhesives with excellent properties, has exceptional economic value, and has considerable environmental and social significance [3]. Considering the supply of biomass resources and economic benefits, researchers are increasingly interested in plant protein-based adhesives [5,6]. The poor water resistance of natural plant protein-based adhesives is the main reason for limiting its wide application in industrial production [7]. Liu et al [9] found that undecylenic acid with hydrophobic carbon chain and active carboxyl group was an efficient modifier to enhance the wet shear strength of soya protein isolate-based adhesives. Wei et al [10] discovered that after adding zinc oxide, the wet shear strengths of the sesame protein-based adhesives were improved due to the chelation reaction between sesame protein and Zn2+
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