Effects of typical soybean meal type on the properties of soybean-based adhesive

doi:10.1016/j.ijadhadh.2019.01.010

International Journal of Adhesion and Adhesives

Volume 90, April 2019, Pages 15-21

https://doi.org/10.1016/j.ijadhadh.2019.01.010 Get rights and content

Abstract

In order to effectively apply soybean meal for the preparation of water-resistant soybean-based adhesives for plywood, the effects of three typical soybean meal products, namely, low-temperature soybean meal (LM), high-temperature soybean meal (HM), and physical soybean meal (PM), on the properties of soybean-based adhesive were investigated. The results indicated that the number of reactive groups in the three soybean meals followed the order LM > HM > PM, which in turn led to various crosslinking densities when these soybean meals were crosslinked by epichlorohydrin-modified polyamide (EMPA) during the curing process. The LM soybean adhesive had 6.6% higher soaking bond strength and 16.5% higher boiling-dry-boiling bond strength than the HM soybean adhesive, and 19% higher soaking bond strength and 33% higher boiling-dry-boiling bond strength than the PM soybean adhesive, respectively. These three soybean meals could be used to prepare soybean adhesives for interior-use plywood because all plywood panels bonded with their adhesives passed a water-soaking test at 63 °C for 3 h, but only the LM soybean adhesive achieved the desired water resistance for floor-base plywood. Among the three evaluated soybean meals, LM was the most promising raw material for the preparation of soybean-based adhesive because of a greater number of reactive groups, higher crosslinking density, and superior bond strength. Plywood panel bonded with HM soybean adhesive had a water resistance lower than, but very close to, the standard required value (>0.8 MPa) for floor-base plywood.

Introduction

Formaldehyde-based adhesives have been widely used in the preparation of wood-based panels because of their low price, high bonding strength, and desirable water resistance [1], [2]. However, these adhesives release harmful formaldehyde during the production process, storage, transportation, and use. In addition, these adhesives are mainly derived from non-renewable fossil resources. Therefore, the development of environmentally friendly adhesives derived from renewable biomass resources has become an important research topic due to growing environmental concerns and decreasing reserves of non-renewable fossil resources [3], [4].

Soybean protein, which is derived from soybean, is one of the most promising candidates for the development of water-resistant adhesives for wood composites, because it has the advantages of being derived from an abundant resource, renewable, and biodegradable. However, traditional soybean-protein-based adhesives have shortcomings such as low bond strength and poor water resistance. As a result, many researchers have attempted to improve the bonding strength and water resistance of soybean-protein-based adhesives with approaches including physical [5], chemical [6], and biological methods [7] in recent years. Among these, chemical modification, which includes methods such as acid and alkali treatment [8], graft copolymerization [9], [10], and crosslinking modification [11], [12], is the most effective and commonly used approach.

Chemical-crosslinking modification is one of the most widely used chemical modification methods for soybean protein, and involves introducing a curing agent with reactive groups to crosslink the soybean protein molecules into an insoluble three-dimensional network structure, and thus improve their bonding properties and water resistance. Cross-linked soybean-soluble polysaccharides [13], phosphorylated epoxidized soybean oil [14], polyethyleneglycol diacrylate (PEGDA) [15], commercial epoxy resin [16], polyethyleneimine (PEI) [11], and epichlorohydrin-modified polyamidoamine (EMPA) [4], have been widely employed as crosslinkers for soybean protein. EMPA is a water-soluble polymer that is widely applied in commercial soybean-protein-based adhesives for wood bonding due to its desired crosslinking efficiency, relatively low cost and good technological applicability [4], [17], [18]. After crosslinking, both the bond strength and water resistance of the soybean-protein-based adhesive were improved to some extent, and could be used to prepare Type II plywood for interior use according to the Chinese National Standard GB/T 9846-2015.

Raw materials such as soybean protein isolate, soybean protein concentrate and soybean meal are commonly applied to prepare soybean-protein-based adhesives [20]. However, soybean protein isolate and soybean protein concentrate increase the adhesive cost compared to soybean meal. Soybean meal is a by-product of soybean oil processing, and is classified as low-temperature soybean meal (LM), high-temperature soybean meal (HM), and physical soybean meal (PM) depending on the oil processing technology by which it was generated. Each type of soybean meal exhibits different properties [19], [20], [21], and only low-temperature soybean meal (also known as white flake or defatted soybean flake) has the desired protein dispersibility for preparing commercial soybean meal based adhesives [21], [22]. To the best of our knowledge, no study has been carried out to investigate the effect of different soybean meals on the properties of soybean-based adhesives.

LM is obtained by a low-temperature desolventizing treatment, and has a low degree of protein denaturation, high nitrogen solubility index, and high protein dispersion index. HM is obtained by a high-temperature desolventizing treatment, leading to a high extent of protein component denaturation and a decrease in the solubility of the protein. PM is obtained directly from the process of mechanical soybean oil pressing, and has a high oil residue content and some degree of protein degradation occurs due to the heat treatment and high pressure treatment applied.

Due to its environmentally safe and formaldehyde-free nature, soybean meal adhesive crosslinked by EMPA has increasingly been used to manufacture wood composites including plywood, chipboard, particleboard, and fiberboard in China in recent years [4]. However, adhesion failure or undesired bonding properties sometimes occur in the wood composites due to variations among the different types of soybean meal flours used for soybean adhesives [23]. The manufacturers were not sufficiently aware of the effect of the type of soybean meal on the bond properties of the resulting soybean adhesive, and thus did not take this factor into account.

Therefore, in this work, the structure-property relationships of three typical soybean meals and the resulting adhesives (formulated with LM, HM, PM and the crosslinker EMPA) were evaluated. Their structures were characterized by their acetaldehyde value, boiling-water-insoluble content, and FTIR, ¹³C-NMR, XPS, and XRD analysis. Their bonding properties and water resistance were evaluated using three-ply plywood fabricated with the corresponding soybean adhesives. The aim of this study was to investigate the structure-property effects of three typical soybean meals on the resulting soybean-based adhesives, thus providing practical guidance for preparing desirable soybean adhesives using the proper type of soybean meal.

Section snippets

Materials

LM, HM, and PM soybean meals were obtained from Laihe Oil Pressing Factory, Shangdong, China; the meals were obtained from the same batch of soybean seeds but using various oil extracting processes. The meals were then milled to 160 mesh powders. Epichlorohydrin-modified polyamide (EMPA) was synthesized in our lab using diethylenetriamine, adipic acid, and epichlorohydrin, and had a solids content of 13.4%, a pH value of 3.2, and a viscosity of 68 MPa·s (25 °C). Birch veneers (420 mm × 420 mm ×

Main chemical structure and properties of three soybean meals

The acetaldehyde value represented the number of active amino groups within the soybean meal, as acetaldehyde could react with the amino groups of the soybean meal by the Maillard reaction [17]. The amino and carboxyl groups in soybean protein or soybean meal are the predominant reactive groups that react with the crosslinker EMPA to obtain the crosslinking density. Because the protein component within LM underwent little denaturation during the lower-temperature soybean oil extraction process,

Conclusions

The structure-property relationships of three typical soybean meals, LM, HM, and PM, were investigated. Their production processes, in which various temperatures and pressures were applied, led to protein denaturation and degradation to some extent, which had important effects on the bond strength and water resistance of the resultant soybean adhesives. Their number of reactive groups, bond strength, and water resistance all followed the order LM > HM > PM, because the increased number of

Acknowledgements

This work was supported by the National Natural Science Foundation of China (grant number 31870542) and the Fundamental Research Funds for the Central Universities (2572018CP01) and the National Undergraduate Training Programs for Innovations (grant number 201810225063).

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¹: These two authors contributed equally to this work and considered as co-first authors

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