Evaluating fundamental position-dependent differences in wood cell wall adhesion using nanoindentation

Area of Science:

• Materials Science
• Wood Science
• Adhesion Science

Background:

• Understanding adhesive-wood interactions is crucial for wood product durability.
• Cell wall structure and chemistry influence adhesive bonding performance.
• Previous studies often lack detailed, position-specific adhesion data.

Purpose of the Study:

• To compare the cell wall adhesion of one-component polyurethane (PUR) and urea-formaldehyde (UF) adhesives on spruce wood.
• To investigate adhesion at two distinct cell wall locations: the inner S3 layer and the freshly cut S2 layer.
• To elucidate the relationship between surface chemistry and adhesive performance.

Main Methods:

• Utilized nanoindentation as a micromechanical assay to quantify adhesive-wood cell wall adhesion.
• Examined adhesion at the interface between adhesive and spruce wood cell wall layers (S2 and S3).
• Employed Atomic Force Microscopy (AFM) to characterize the surface polarity of cell wall layers.

Main Results:

• Urea-formaldehyde (UF) exhibited excellent adhesion to freshly cut S2 cell walls but poor adhesion to the inner S3 surface.
• One-component polyurethane (PUR) showed the opposite trend, with better adhesion to the inner S3 surface and weaker adhesion to S2.
• AFM analysis indicated that the inner S3 cell wall surface is less polar than the freshly cut S2 surface.

Conclusions:

• Adhesive performance is highly dependent on the specific wood cell wall surface chemistry and polarity.
• The polarity differences between UF and PUR adhesives likely explain their distinct adhesion behaviors on S2 versus S3 surfaces.
• This study provides novel insights into position-resolved adhesive interactions within the wood cell wall structure.