Soybean meal-based wood adhesive enhanced by ethylene glycol diglycidyl ether and diethylenetriamine
Graphical abstract
Introduction
Formaldehyde-based adhesives, such as urea-, melamine-, and phenol-formaldehyde resins, play a dominate role in the plywood fabrication industry because of their high water resistance and durability. However, the formaldehyde-based adhesives are non-renewable and the urea-formaldehyde resins produce the formaldehyde emission issue, so that, the adhesives from renewable materials have become a desirable alternative for the plywood adhesive (Chen et al., 2014, Jang et al., 2011). Researchers focused on developing formaldehyde free soy protein-based adhesives in recent years, but the low water resistance limited their application (Amaral-Labat et al., 2008, Huang and Li, 2008). Chemical modifications have been used to solve this fatal disadvantage and develop some environmentally friendly soy protein-based adhesive formulations (Chen et al., 2013b, Qi et al., 2013).
The major chemical modifications can be classified into two categories: the first one is the protein denaturing agent modification. Alkali (Nordqvist et al., 2010), urea (Li et al., 2014), and sodium dodecyl sulfate (SDS) (Wang et al., 2005) are used to break the structure of soy protein molecules and expose inner hydrophobic groups to improve the water resistance of the resultant adhesive, however, the wet shear strength of the plywood bonded by the adhesive does not meet the interior use plywood requirement (≥0.7 MPa), since the soy protein molecules also contain many hydrophilic groups, such as NH2, COOH, and OH. The second one is the cross-linking agent. Researchers have used guanidine hydrochloride (Liu and Li, 2004) and maleic anhydride (Liu and Li, 2007) to enhance the soy protein-based adhesives, however, the water resistance of the resultant adhesives hardly meets the requirements of industrial production. Also, researchers have used the phenol-formaldehyde resins (Zhong and Sun, 2007), melamine-urea-formaldehyde resins (Gao et al., 2012b), and polyamidoamine epichlorohydrin resins (Li et al., 2004) blending with soy protein products to develop high water resistant adhesives, which are proofed an effective way to enhance the soy protein-based adhesives. But the formaldehyde emission issue caused by adding these formaldehyde-based reinforcers and low dry bonding strength issue of using polyamidoamine epichlorohydrin resins restrict their industrial application.
From our early researches, the bonding mechanisms of the soy protein-based adhesives were summarized into three aspects (Gao, 2012): the first one was the intermolecular hydrogen bond interaction. In the curing process of the adhesive, the functional groups (such as OH, NH2, COOH, and peptide bond) interacted with each other to form a large number of hydrogen bonds and produced mechanical properties. However, hydrogen bonds could only increase the dry bonding strength of the resultant plywood, since they were easily broken in the wet state. The second one was the intermolecular cross-linking reaction. The functional groups in soy protein molecules, such as SH (cysteine), Ph–OH (tyrosine), and CC (tryptophan), reacted with the active groups in additives and formed lots of water resistant chemical bonds during curing process, which generated net structures to prevent water intruding and thus improved the water resistance of the resultant adhesive (Luo et al., 2015). The last one was an interpenetrating network formation. Additives cross-linked themselves and combined with the protein molecules forming an interpenetrating network, which further improved the water resistance of the resultant adhesive.
In the study, soybean meal flour (SM) and sodium dodecyl sulfate (SDS) was used to develop the soybean meal-based adhesive. The ethylene glycol diglycidyl ether (EGDE) was firstly reacted with a diethylenetriamine (DETA) and then used to modify the soybean meal-based adhesive. Three-ply plywood was fabricated to measure the water resistance of the adhesive. The functional groups, cross section, crystallinity, and thermal behavior of the cured adhesives were evaluated to explain why the water resistance of the adhesive enhanced.
Section snippets
Materials
Soybean meal flour (SM) (46% soy protein content, milled 200 meshes flour using a laboratory grinder) was obtained from Xiangchi Grain and Oil Company in Shandong [S3] Province, China. Sodium dodecyl sulfate (SDS) was obtained from Tianjin Chemical Reagent, China. Poplar veneer (40 × 40 × 1.5 cm, 8% of moisture content) was provided from Hebei Province of China. The ethylene glycol diglycidyl ether (EGDE) was obtained from Guotai Chemical Reagent, China. DETA was obtained from Xiya Reagent Research
FTIR analysis
The FTIR spectrum of EGDE and EGDE/DETA are shown in Fig. 1. The characteristic absorption double bands of the primary amine groups (the symmetric and asymmetric stretching vibration bands) appeared between 3300 and 3500 cm−1 were almost disappeared (Cheng et al., 2011) and only a single peak appeared at 3291 cm−1, which responded to the characteristic absorption bands of the secondary amine in EGDE/DETA. It indicated that the reaction between DETA and EGDE was occurred. The reaction increased
Conclusions
- 1.
DETA reacted with EGDE to form a long chain structure with epoxy groups, which cross-linked the soy protein molecules to form a denser cured adhesive layer and combine with the soy protein molecules to form an interpenetrating network. Both of them improved the water resistance of the resultant adhesive.
- 2.
The wet shear strength of the plywood bonded by the SM/SDS/EGDE/DETA adhesive was 1.15 MPa and the adhesive viscosity was 22400 mPa s, which made the resultant adhesive practical for plywood
Acknowledgments
The authors are grateful for Special Fund for Forest Scientific Research in the Public Welfare (201404501) and Beijing Natural Science Foundation (2151003).
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