Assessment of phenol-formaldehyde adhesive distribution on the strand surface by light microscopy

This study reports on a new approach for the detection of phenol-formaldehyde (PF) resin distribution on the strands surface after air-spray application in a glue drum using reflecting light microscopy (LM). The method allows visualization and quantification of PF adhesive distribution on wood strands by using either waterproof paper or 200 µm thick microtomed kiri (Paulownia tomentosa) veneer slices, which were clipped with staples on conventional strands typically used for the production of oriented strand boards (OSB). These strands were mixed with other conventional OSB strands and two types of molecular weight PF prepolymers were sprayed on the strand mixture. While the total area of adhesive coverage was similar for both adhesives, the low molecular weight (LMW) PF prepolymer formed a higher number of adhesive droplets than the high molecular weight (HMW) PF prepolymer. The study offers a new, easy applicable approach for a qualitative investigation of adhesive distribution on the strand surface during gluing on a laboratory and pilot plant scale.