Abstract
This is a study of concept on the initial application for wood adhesives totally biosourced from the covalent reaction between soy protein isolate (SPI) and a commercial flavonoid tannin, namely quebracho tannin. The adhesive is composed exclusively of the two vegetable biomaterials mentioned and thus is totally biosourced and non-toxic, as tannin has been classified as being not at all toxic by the European Commission REACH program. The pre-reaction between the two yielded the best plywood bonding results when limited to a temperature of 40 °C, final cross-linking being achieved during the plywood higher temperature hot pressing procedure, as for any other thermosetting adhesive. Pre-reaction at higher temperatures, namely 60 °C and 80 °C, achieved extensive premature cross-linking that lost any activity to cross-link further when hot pressed for preparing plywood. The reaction was followed by thermomechanical analysis, by matrix assisted laser desorption ionization time of flight (MALDI ToF) mass spectrometry, and by plywood shear strength tested dry, after a 24 h cold water soak and 1 h in boiling water. The adhesive of this approach lends itself to be further reinforced by the multitude of approaches on soy resins already developed by several other research groups.
Highlights
Soy-based resins, being soy protein and flour biosourced, are of particular interest, for bonding wood panel adhesives
They show that pretreating the 50:50 soy protein isolate (SPI)–quebracho tannin adhesive at 40 ◦C gave better results than the SPI alone control, but as preheating passed at higher temperatures, the performance of the bonded plywood progressively decreased
The results indicated that the 40 ◦C pretreated SPI–tannin adhesive, while presenting only a moderate improvement in dry strength, presented much better 24 h cold water strength (30% improvement, from 0.62 for the control to 0.81 MPa for the SPI–tannin), and 1 h boil water strength (866% improvement, from 0.06 to 0.52 MPa)
Summary
Soy-based resins, being soy protein and flour biosourced, are of particular interest, for bonding wood panel adhesives. Vegetable condensed and hydrolysable tannins are one good choice to modify soy adhesives for wood panels, as they are classed as non-toxic, even for human consumption, and are classified as such by the relevant Registration, Evaluation, Authorization, and Restriction of Chemicals (REACH) classification of the European Commission [14] They are commercially obtained by water extraction from the bark or wood of several tree species [15,16] from several perennial shrubs [15] and from residual grape skins after grape juice extraction [17]. What is really sought is to prepare an adhesive based exclusively on a protein and on tannin, without any hardener of any type, and totally biosourced based on the covalent reaction between the two materials at higher temperature It is for this reason that the present work builds on previous research work for the bonding of interior grade plywood with just SPI and a tannin and nothing else. The present demonstration of the feasibility of this approach is done to render possible the reinforcing of this system by the superposition to it of other approaches using small additions of other materials, as already reported by a number of different authors [3–6,8–13,18–24,29]
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