Effect of open assembly time and equilibrium moisture content on the penetration of polyurethane adhesive into thermally modified wood

The effect of wood moisture content and open assembly time on penetration of polyurethane (PU) adhesive into thermally treated Scots pine (195 and 210°C) was investigated according to effective penetration (EP) and maximum penetration (MP) measurements using fluorescence microscopy. For samples treated at 195°C, a higher EP was noted at 8.6% equilibrium moisture content (EMC) after both assembly times (15 and 30 min) while for samples treated at 210°C, increasing wood moisture content resulted in a significant decrease in EP at 12.5% EMC after 15 min assembly time. Extending open assembly time was found to increase the EP of PU adhesive only in the case of samples treated at 195°C and with 8.6% EMC. For samples treated at both treatment temperatures and after shorter open assembly time, the highest MP was observed at moderate EMC levels of 8.6 and 8.2% and the lowest at the higher EMC levels of 13.2 and 12.5%.