Effect of furfurylation on shear strength of bond line and screw withdrawal resistance of beech and fir

The aim of this study was to investigate the effects of furfurylation on shear strength of bond line and screw withdrawal resistance of beech (Fagus orientalis) and fir (Abies alba) species. In this regard, specimens with two different values of furfurylation i.e. low (fir 14% and beech 20%) and high (fir 38% and beech 65%) levels were prepared and evaluated in comparison with untreated specimens. The furfurylation was performed after impregnating the specimens by furfuryl alcohol under vacuum condition followed by heating duration to complete polymerization and crosslinking. Specimens were prepared according to standard ASTM D 905 in order to assess shear strength of bond line formed with three different types of adhesives, single-component polyurethane, polyvinyl acetate as well as urea-formaldehyde. Also screw withdrawal resistance of wood polymer specimens was measured perpendicular to grain and parallel to grain. The results indicated that the shear strength of the bond line decreased and screw withdrawal resistance increased by furfurylation level. Examination of adhesion mechanism demonstrated that formation of an appropriate interface between water-soluble adhesives and wood-polymer involves effective penetration of adhesive into cell wall. Hence, the interface of adhesives and material would be the determining factor in developing strength of adhesive joints. Since the wood-furfuryl alcohol is a cell-wall wood-polymer, the possibility of creating an appropriate interface by increasing of furfurylation level was impaired. On the other hand, the adhesion mechanism of polyurethane glue compared to water-soluble adhesives such as polyvinyl acetate and urea-formaldehyde is significantly different, therefore exhibited more strength. The withdrawal screw resistance of furfurylated wood was increased by furfurylation level due to changing nature of wood and increasing shear strength of wood-polymer. This gain was also more evident in fir than beech. The results showed that screw withdrawal resistance perpendicular to grain was higher than parallel to grain.