Evaluation of alternative vegetable proteins as wood adhesives

Adhesives industry is increasingly interested in products coming from natural and renewable resources, because of the limited reserve of oil, its prices variability and its negative impact on both environment and human health. However, soy crops are mainly concentrated in the Americas, and are not so widespread in Europe. Thus, it is interesting to evaluate if other vegetable proteins more common in Europe are compatible for their use as wood adhesives. In this study, zein, a protein component of maize (Zea mays L.), the pea (Pisum sativum L.) protein, and two products based on soy (Glycine max L.) proteins, one treated with alkali and the other not, were compared in order to verify their utilisation as wood adhesives for indoor applications. Dissolution tests in both water and ethanol/water mixture at different pH values, and FT-IR analysis in the same conditions were carried out. Also the effect of the addition of denaturing agents such us urea and guanidine hydrochloride was evaluated. Solubility results showed that, as expected, water soluble fraction of these proteins increased significantly by increasing pH: both the 2 soy proteins and the pea protein evidenced soluble fraction higher than 80% at pH 10, while for zein, this parameter was always lower than 30%. Moreover, water seemed to be a better solvent than the ethanol/water mixture. In fact, in one case a solubility decrease was observed when ethanol was added to water. Standard bonding tests on wood-to-wood joints were carried out according to EN 205, both in dry and in wet conditions, this latter after 4 days of immersion in water. Shear strength values were above the minimum threshold limit of 10 MPa required by the standard for both the 2 soy and the pea proteins. In contrast, all proteins were not suitable for wet conditions. Pea protein seemed to be a valid alternative to soy proteins, which is realistic considering that the amino acid profiles of pea and soy proteins are similar. However, like for the soy protein, it is essential to improve the water resistance with an additive. In contrast, zein did not seem suitable as wood adhesive, because of its low solubility in all the considered conditions.