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Wood Handbook--Chapter 9--Adhesive Bonding of Wood Materials
91
Chapter
9
Adhesive Bonding of
Wood Materials
Charles B. Vick
Contents
Adhesion to Wood 91
Surface Properties of Wood Adherends 92
Extractives on Surfaces 93
Knife- and Abrasive-Planed Surfaces 93
Veneer Surfaces 94
Surfaces of Wood and Nonwood Composite Products 95
Physical Properties of Wood Adherends 96
Density and Porosity 96
Moisture Content and Dimensional Changes 97
Adhesives 99
Composition 99
Health and Safety 910
Strength and Durability 910
Adhesive Selection 912
Bonding Process 915
Moisture Content Control 915
Surface Preparation 916
Adhesive Spreading 916
Assembly and Pressing 916
Post-Cure Conditioning 918
Bonded Joints 918
Edge-Grain Joints 918
End-Grain Joints 918
End-to-Edge-Grain Joints 919
Construction Joints 919
Testing and Performance 920
Analytic Chemical and Mechanical Testing
of Polymers 920
Mechanical Testing of Bonded Assemblies 920
Short- and Long-Term Performance 921
Product Quality Assurance 922
References 923
dhesive bonding of wood components has played
an essential role in the development and growth of
the forest products industry and has been a key
factor in the efficient utilization of our timber resource. The
largest use of adhesives is in the construction industry. By
far, the largest amounts of adhesives are used to manufacture
building materials, such as plywood, structural flakeboards,
particleboards, fiberboards, structural framing and timbers,
architectural doors, windows and frames, factory-laminated
wood products, and glass fiber insulation. Adhesives are
used in smaller amounts to assemble building materials in
residential and industrial construction, particularly in panel-
ized floor and wall systems. Significant amounts are also
used in nonstructural applications, such as floor coverings,
countertops, ceiling and wall tile, trim, and accessories.
Adhesives can effectively transfer and distribute stresses,
thereby increasing the strength and stiffness of the composite.
Effective transfer of stress from one member to another de-
pends on the strength of the links in an imaginary chain of an
adhesive-bonded joint. Thus, performance of the bonded
joint depends on how well we understand and control the
complexity of factors that constitute the individual links
wood, adhesive, and the interphasing regions between
which ultimately determine the strength of the chain.
Adhesion to Wood
The American Society for Testing and Materials (ASTM)
defines an adhesive as a substance capable of holding
materials together by surface attachment. An adherend is a
substrate held to another substrate by an adhesive. Adhesion
is the state in which two surfaces are held together by interfa-
cial forces, which may be valence forces, interlocking action,
or both. Valence forces are forces of attraction produced by
the interactions of atoms, ions, and molecules that exist
within and at the surfaces of both adhesive and adherend.
Interlocking action, also called mechanical bonding, means
surfaces are held together by an adhesive that has penetrated
the porous surface while it is liquid, then anchored itself
during solidification. The extent to which valence forces and
interlocking action develop between adhesive polymers and
wood adherends is uncertain, but both are generally acknowl-
edged as essential for the most effective bonding. Bonding to
porous surfaces, such as wood, paper, and textiles, was 92
thought to be primarily mechanical, but now there is
evidence supporting bonding by primary valence forces. In
contrast, bonding to hard metal surfaces was believed to
involve only valence forces, but this is no longer the ac-
cepted view. Metal surfaces roughened by chemical etching
or made microscopically porous with a layer of oxide are
capable of mechanical interlocking with an adhesive to pro-
duce exceptionally strong and durable bonds.
Mechanical interlocking is probably the primary mechanism
by which adhesives adhere to porous structures, such as
wood. Effective mechanical interlocking takes place when
adhesives penetrate beyond the surface debris and damaged
fibers into sound wood two to six cells deep. Deeper penetra-
tion into the fine microstructure increases the surface area of
contact between adhesive and wood for more effective me-
chanical interlocking. The most durable structural bonds to
wood are believed to develop not only when an adhesive
penetrates deeply into cell cavities, but also when an adhe-
sive diffuses into cell walls to make molecular-level contact
with the hemicellulosics and cellulosics of wood. If an adhe-
sive penetrates deeply enough into sound wood and becomes
rigid enough upon curing, the strength of the bond can be
expected to exceed the strength of the wood.
Physical forces of attraction composed of three intermolecular
attraction forces are believed to be important to the formation
of bonds between adhesive polymers and molecular structures
of wood. Generally called van der Waals forces, these in-
clude dipoledipole forces, which are positively and nega-
tively charged polar molecules that have strong attractions for
other polar molecules; London forces, which include the
weaker forces of attraction that nonpolar molecules have for
each other; and hydrogen bonding, a special type of dipole
dipole force that accounts for strong attractions between
positively charged hydrogen atoms of one polar molecule and
the electronegative atom of another molecule. Hydrogen
bonding forces are important in the interfacial attraction of
polar adhesive polymers for the hemicellulosics and cellu-
losics, which are rich with polar hydroxyl groups. These
physical forces of attraction, sometimes referred to as specific
adhesion, are particularly important in wetting of water
carriers and adsorption of adhesive polymers onto the mo-
lecular structures of wood.
Covalent chemical bonds form when atoms of nonmetals
interact by sharing electrons to form molecules. The simplest
example of a purely covalent bond is the sharing of electrons
by two hydrogen atoms to form hydrogen. These covalent
bonds are the strongest of chemical bonds; they are more
than 11 times the strength of the hydrogen bond. Even
though covalent chemical bonds between adhesive polymer
and the molecular structure of wood seem a possibility, there
is no clear evidence that such bonds constitute an important
mechanism in adhesive bonding to wood.
For two wood adherends to be held together with maximum
strength, a liquid adhesive must wet and spread freely to
make intimate contact with both surfaces. Molecules of the
adhesive must diffuse over and into each surface to make
contact with the molecular structure of wood so that
intermolecular forces of attraction between adhesive and wood
can become effective. As will be discussed later, wood adher-
ends, as well as other materials, differ widely in their attrac-
tive energies, bulk properties, surface roughness, and surface
chemistry. Wood surfaces may appear to be smooth and flat,
but on microscopic examination, they become peaks, val-
leys, and crevices, littered with loose fibers and other debris.
Such surface conditions cause gas pockets and blockages that
prevent complete wetting by the adhesive and introduce
stress concentrations when the adhesive has cured. Thus, the
liquid adhesive must have high wettability, coupled with a
viscosity that will produce good capillary flow to penetrate
sound wood structure, while displacing and absorbing air,
water, and contaminants at the surface. Pressure is normally
used to enhance wetting by forcing liquid adhesive to flow
over the surfaces, displace air blockages, and penetrate to
sound wood.
Wetting of a surface occurs when the contact angle (the angle
between the edge of a drop of adhesive and the surface of
wood) approaches zero. The contact angle approaches zero
when the surface has high attractive energy, the adhesive has
an affinity for the adherend, and the surface tension of the
adhesive is low. If a drop of adhesive spreads to a thin film
approaching zero contact angle, the adhesive has spread well
and made intimate contact with the surface. The differences in
wettabilities of various wood surfaces are illustrated by a
simple water drop test in Figure 91.
The process of adhesion is essentially completed after transi-
tion of the adhesive from liquid to solid form. After the
viscosity of a liquid adhesive has increased and the adhesive
has solidified to the point where the film effectively resists
shear and tensile forces tending to separate the surfaces, the
surfaces are effectively bonded. An adhesive film changes
from liquid to solid form by one of three mechanisms, al-
though two may be involved in some curing mechanisms.
This transition can be a physical change as in thermoplastic
adhesives or it can be a chemical change as in thermosetting
adhesives. In thermoplastics, the physical change to solid
form may occur by either (a) loss of solvent from the adhe-
sive through evaporation and diffusion into the wood, or
(b) cooling of molten adhesive on a cooler surface. In ther-
mosets, the solid form occurs through chemical polymeriza-
tion into cross-linked structures that resist softening on
heating. Most thermosetting wood adhesives contain water
as a carrier; therefore, water also must be evaporated and
absorbed by the wood so that the adhesive can cure
completely.
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