Natural Tannins as New Cross-Linking Materials for Soy-Based Adhesives

Abstract

Human health problems and formaldehyde emission from wood-based composites are some of the major drawbacks of the traditional synthetic adhesives such as urea formaldehyde resins. There have been many attempts to decrease formaldehyde emission and replace urea formaldehyde resins with bio-based adhesives for wood-based composites. Because of some weakness in soy-based adhesive, chemicals have been used as modifiers. Modified soy-based adhesives without any formaldehyde have been successfully used to prepare wood panels. To achieve this, different synthetic cross-linking chemicals such as phenol formaldehyde resins and polyamidoamine-epichlorohydrin were used. However, in reality, what we need are totally green adhesives that use natural materials. In our previous research work, the use of tannins in combination with soy-based adhesives to make wood composites was investigated. Thus, in this research work, the feasibility of using three types of natural tannins (quebracho, mimosa and chestnut tannins) as cross-linking materials for soy adhesive was studied. The chemical bond formation and adhesion behaviors of tannin-modified soy adhesives were also investigated by Matrix-Assisted Laser Desorption/Ionization Time-of-Flight Mass Spectrometry (MALDI-ToF-MS) and thermo-mechanical analysis (TMA). The results showed that at ambient temperature, both ionic and covalent bonds formed between tannin constituents and amino acids; however, at higher temperature, covalent bonds are largely predominate. Based on the results obtained from the thermo-mechanical analysis, the modulus of elasticity (MOE) of soy adhesive is increased by adding tannins to its formulation. In addition, the chemical bond formation was proved by MALDI-ToF-MS.

Optimization of the Hot-Pressing Regime in the Production of Eco-Friendly Fibreboards Bonded with Hydrolysis Lignin

“…formaldehyde emissions equivalent to the natural wood levels [19,20]. The use of natural, bio-based adhesives as partial or complete substitutes of the traditional thermosetting resins is a perspective approach to achieve this goal [21][22][23][24][25][26][27][28][29]. The use of natural binders has another environmental effect, namely the utilization of residual natural resources from other industries [30][31][32][33][34][35][36][37][38][39].…”

A Study of Concept to Prepare Totally Biosourced Wood Adhesives from Only Soy Protein and Tannin

“…This was expected, as the protein content of soy flour is 50% and it is only the protein in the soy flour that reacts with the tannin; the rest of the flour probably not only did not participate in bond formation, but moreover also increased diffusion hindrance. It is known that at ambient temperature, only ionic bonds and secondary interactions occur between a tannin and a protein, while covalent bonds progressively form as the temperature increases [23,24]. The only covalent bonds that could form between the protein and a tannin flavonoid unit are those between amino acids side chains presenting a -NH 2 [27,28,33,34] or a -COOH group.…”

Evaluation of Starch as an Environmental-Friendly Bioresource for the Development of Wood Bioadhesives

“…The alternative based on renewable raw materials for the development of bioadhesives has been considered as an option of special interest. These include the use of soy [10][11][12], tannins [13][14][15], lignin [16][17][18], wood fibers [19,20], plant polymers [21,22] and starch [23][24][25]. In particular, starch is the second most abundant lignocellulosic polymer in nature [26].…”