1. Lecture 2-2: Wood as an Adherend
Forestry 485
Lecture 2-2: Wood as an Adherend

2. Adhesion
Adhesive – a substance capable of holding materials together by surface attachment
Adherend – a substrate held to another substrate by an adhesive
Adhesives bond substrates together via chemical interactions and/or mechanical interlocking

3. Requirements for Satisfactory Adhesion
Good adhesion requires transfer of resin to substrate, wetting of substrate surface, controlled penetration into the substrate, flow of resin, gelling of the resin in (ideally) a continuous film, cure of resin in place (i.e., little shrinkage as it cures), and cure durability of the adhesive.

4. Wood Adherend Variables
A number of wood characteristics can affect the performance of an adhesive. These characteristics may be called “wood adherend variables.”
The “top five” wood adherend variables are described in the following slides, but not necessarily in priority order.

5. Wood Adherend Variables 1. Porosity
Wood is a porous cellular solid
Flow of resin into pores, followed by curing, improves the “mechanical interlocking” mechanism of adhesion.
Note that porosity varies in the three principal planes of wood structure.

6. Wood Adherend Variables 2. Surface Roughness
Surface roughness is related to porosity and to surface preparation (machining)
Smoother surfaces are generally better for bonding, e.g., knife planed vs. sawn
Smooth surface allows for more intimate contact of adjacent substrates via a thin glue line
Roughness: Macroscopic and Microscopic

7. Wood Adherend Variables 3. Wood Density
Lower density woods are generally easier to bond due to their greater degree of porosity
% wood failure (a measure of adhesive bond quality) decreases with increasing wood density
Low, medium, and high density oak

8. Wood Adherend Variables 4. Wood Moisture Content
Wood is hygroscopic!
MC too low or too high will impede adhesion
A MC range of 6-14% is optimal for many wood adhesives

9. Wood Adherend Variables 5. Surface Chemistry
Oxidation by exposure to air or heating (drying or friction from machining) results in poor wetting
Contamination by airborne particles, dirt, grease, etc. has a negative effect on surface properties

10. Wood Adherend Variables 5. Surface Chemistry, continued
Extractives may be driven to surface by drying; many are hydrophobic, may inhibit reactions of adhesives, or block micropores
In extreme cases, woods with high extractives content (>20%) may be non-bondable
If adhesive cannot wet the surface, the substrate cannot be adhesively bonded.

11. More illustrative slides….
See slideshow courtesy of Dr. Fred Kamke, Oregon State University (formerly of Virginia Tech):
Shortcut to Forestry 485 Lecture 2-2 supplement, Kamke Penetration&Dist