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Forestry 485
Lecture 2-2: Wood as an Adherend
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
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.
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.
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.
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
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
Low, medium, and high density oak
density
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
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
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
If adhesive cannot wet the
non-bondable
surface, the substrate cannot be
adhesively bonded.
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