Effects of Conditioning Exposure on the pH Distribution Near Adhesive-Wood Bond Lines
Keywords:pH distribution, phenol formaldehyde, melamine formaldehyde, melamine-urea formaldehyde, adhesives, bond line, accelerated aging, chemical change, FTIR ATR
AbstractThe pH distribution near the adhesive-wood bond line in black spruce (Picea mariana) and Douglas-fir (Pseudotsuga menziesii) bonded with various acidic and alkaline adhesives was investigated. For alkaline adhesives, exposure to moderate to high RH conditions and to accelerated aging treatments such as vacuum-pressure-dry or water soaking resulted in diffusion of hydroxyl ions (OH-) away from the bond line and decreased alkalinity. A moderate increase of pH in the bond line was also observed for acidic adhesives, and under very wet conditions, hydrogen ions (H+), also diffused away from the bond line. Spruce and Douglas-fir samples exposed to strongly acidic and alkaline buffered solutions for 3 mon did not show appreciable changes in chemical composition by wet chemical analysis. FTIR attenuated total reflectance spectra of samples bonded with an alkaline adhesive and exposed to either dry or wet conditions showed dissociation of carboxylic acid groups in hemicelluloses (decreased absorbance at 1735 cm -1) to a distance of 150-300 μm from the center of adhesive bond lines. No effects on wood chemistry were observed around acidic adhesive bond lines. In summary, wood bonded with high-pH adhesives and exposed to wet service conditions rapidly reached moderate pH conditions in the bond line because of the high rate of OH- diffusion in wood and the acidic buffering capacity of wood. This mitigated the effects of high pH in these alkaline adhesives. A similar but less effective process occurred with acidic adhesives.
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