Series editors' preface - Wood Tools

Plastics and polymers, coatings and binding media, adhesives and consolidants 157
sive is a result of the type of stresses experienced,
the bonding area, the overall load and
whether this is intermittent or continuous. If
the adhesive is visible, for example as a glue
line, then colour and other optical considerations
may be important. The choice of an adhesive
is affected by many additional practical
considerations discussed below.
Adhesives may hold materials together
simply by invading tiny pores and undercuts in
the adherends thereby locking them together
mechanically (called mechanical adhesion), or
by molecular attraction of the adhesive and
adherend – the same inter-molecular forces
that generate cohesion. This attraction that
molecules of one surface have to those of
another is termed ‘specific adhesion’ and is
greatest between materials that are chemically
similar. As everyone knows, oil and water do
not mix, and a polar water-based adhesive is
unlikely to adhere well to an oily non-polar
substrate. However, specific adhesion may also
be high when hydrogen bonds are capable of
being formed between dissimilar materials
where one is a proton donor and the other a
proton acceptor.
The forces of attraction depend on very
close proximity of molecules. A non-porous
solid such as steel will attract itself if the mating
surfaces are so perfectly polished that air is
excluded and very close contact is made, but
such close contact is not possible between
most solid surfaces due to roughness, or
because they are porous and consist mostly of
voids (e.g. wood). This dictates the use of a
liquid adhesive which can flow out onto a
rough and void filled surface ‘wetting’ it intimately
and serving as an intermediate between
the solid surfaces. When the adhesive itself
becomes solid by cooling (heat set), chemical
reaction (polymerization or thermosetting) or
solvent loss, the adherends are firmly stuck
together. Pressure sensitive adhesives are
exceptions to this type of adhesion, being soft
enough in solid form to conform very closely
to surfaces.
The better a substrate is ‘wet’ by an adhesive,
the better the bond will be because the
degree of wetting is itself dictated by the attraction
generated between the substrate and
adhesive (specific adhesion). Wetting is dependent
on the surface tension, or energy, of the
liquid and the solid and on the viscosity of the
liquid. Surface tension is a direct measure of
intermolecular forces. The tension at the surface
of a liquid or solid material is the result of
the attraction for the bulk of the material to the
surface layer. A molecule in the bulk of the
material is attracted equally to the molecules
that surround it whereas a molecule at the surface
is attracted from below but not from
above (Figure 4.8). This attraction tends to
reduce the number of molecules in the surface
region and causes an increase in the distance
between molecules at the surface. In order to
keep these molecules at the surface, energy is
transferred from the body of the material to its
surface (i.e. work is done). Surface energy is a
measure of the energy necessary for the body
of the material to hold the surface to itself (i.e.
of the work being done). Thus materials whose
bulk and surface are strongly bound together,
such as metals, ceramics or diamond, will
exhibit high melting points and hardness and
will have a high surface energy (500–
5000 mJ/m 2 ). Materials which are held together
only by secondary molecular bonding (e.g.
hydrogen bonding or Van der Waals forces)
will have a low surface energy (less than
100 mJ/m 2 ). Water, for example, has a surface
energy of 72 mJ/m 2 whilst paraffin wax has a
Figure 4.8 Surface tension/energy. Diagrammatic
representation of the intermolecular forces in a liquid,
with arrows indicating the attraction between molecules.
The molecules at the surface of the liquid are more
widely separated than adjacent molecules in the bulk
liquid. Surface tension (or surface energy) is a measure
of the intermolecular forces between the body of the
material and its surface