The tensile shear strength of veneer lap joints was characterised. The joints were produced with an Automated Bonding Evaluation System (ABES) using urea-formaldehyde (UF) as well as melamine-urea-formaldehyde (MUF) adhesive formulated for particleboard production. At a fixed heating temperature of 1108C, a systematic increase in bond strength was observed for both adhesives with increasing cure time. The absolute bond strength was significantly higher for MUF compared to UF. Nanoindentation experiments with the same specimens used for ABES revealed a very hard, stiff and brittle character of the UF resin, whereas the MUF proved significantly less hard and stiff, and less brittle. Wood cell walls in contact with adhesive, i.e., where adhesive penetration into the cell wall was assumed, showed significantly altered mechanical properties. Such cell walls were harder, stiffer and more brittle than unaffected reference cell walls. These effects were slightly more pronounced for UF than for MUF. Comparing UF and MUF, the micromechanical properties of cured adhesive and interphase cell walls confirm earlier observations that tougher adhesives can lead to higher macroscopic bond strength. In strong contrast to that, no obvious correlation was found between micromechanical properties and the strong cure time dependence of macroscopic bond strength.

Micromechanical properties of cured polymeric diphenylmethane diisocyanate (pMDI) and urea formaldehyde (UF) adhesive and wood cell walls (beech) in adhesive contact compared with cell walls without adhesive contact were measured in situ by means of nanoindentation. Using UV-microphotometry obtained absorbance spectra of micromechanical investigated cell wall regions gave a strong indicator for the presence of pMDI compounds in wood cell walls. Nanoindentation results reveal that both pure UF and UF-penetrated cell walls show a very brittle character. In contrast, pMDI adhesive is very tough and soft at the same time, and when diffused in cell walls, it does not mechanically embrittle the cell structure.

The mechanical properties of cured woodadhesive films were tested in dry and wet states by means of nanoindentation. A fluid cell was used to monitor possible property changes as a function of changing moisture content. Under wet conditions, the elastic modulus and hardness of all tested adhesives (i.e., urea formaldehyde, melamine urea formaldehyde, phenol resorcinol formaldehyde, and onecomponent polyurethane) were reduced to about half of their values in the dry state. After renewed drying, all adhesives largely recovered their initial mechanical properties.

A new procedure to detect urea-formaldehyde adhesive in industrial particle board is presented. The method uses thin sections stained with a visible dye (gentian violet) and a fluorescent dye (brilliant sulphaflavine), respectively, in a two-step procedure. Microscope images of a selected area of interest acquired in visible and fluorescence modes are combined to obtain sufficient contrast, enabling semi-automated detection of adhesive by means of image analysis. No addition of dye prior to particle board production is necessary.