Radial Variation in Bonding Performance of Preservative-treated Wood within Pinus elliottii Trees and Its Relationships with Wood Density and Adhesive Penetration

Pinus elliottii wood from different radial locations within trees was used to prepare two-layer preservative-treated wood joints with alkaline copper quaternary preservative and emulsion polymer isocyanates (EPI) or resorcinol-phenol-formaldehyde (RPF) adhesives. The radial variations in shear strength and wood failure percentage were analyzed. Radial variations in wood density and average penetration depth (AP) of adhesives were investigated to establish relationships between shear strengths and characteristics of bonded joints. The shear strengths of EPI- and RPF-bonded joints showed similar increasing trends from pith to bark, and they varied in ranges of 2.92 to 8.13 MPa and 2.03 to 7.12 MPa, respectively. The wood failure percentage of EPI joints (93% to 100%) had no significant differences in all the radial locations, but that of RPF joints (60% to 100%) showed a clear decreasing trend from pith to bark. High positive linear correlations were found between the shear strengths and wood density, and Pearson’s correlation coefficients for EPI and RPF were 0.934 (p < 0.01) and 0.931 (p < 0.01), respectively. Wood density is a very important factor influencing radial variations of bonding strengths. The correlation between the AP and shear strength was not significant at the 0.05 level.