Moisture and surface quality sensing of Douglas-fir (Pseudotsuga menziesii var. menziesii) veneer products

Abstract

The potential of near-infrared spectroscopy (NIRS) to estimate moisture content (MC) and surface inactivation parameters of Douglas-fir (Pseudotsuga menziesii var. menziesii) veneer products was assessed. The best prediction model for MC was produced for the lower range of MC (0%–50%) of Douglas-fir veneers. Exposure at 180°C produced surface colour changes and the CIE-L*a*b* colour parameters measuring colour changes were better estimated using the 400 nm to 900 nm spectral data than the 1100 nm to 2400 nm spectral data. Increased exposure time resulted in lower wettability and hence increasing contact angles, especially when ethylene glycol and formamide were used as solvents. NIRS-based predictions of contact angles were better when the angles were measured using formamide than when they were measured using ethylene glycol. Lap shear tensile strengths of bonds made with phenol formaldehyde (PF) resin decreased with exposure times. NIRS-based predictions of tensile strengths were also estimated and we found strong negative relationships between contact angle and tensile strength, whatever the probe solvent used (water, glycerol, ethylene glycol and formamide). It is apparent that NIRS can differentiate veneers samples that had undergone high temperature exposure, which resulted in lower wetting properties and somewhat lower adhesion bond strength.