Series editors' preface - Wood Tools

158 Conservation of Furniture
surface energy of 22 mJ/m 2 . In order for a liquid
adhesive to spread and wet the adherend
surface, there must be a greater attraction of its
constituent molecules for the substrate than for
each other. Low energy liquids will therefore
be attracted to high energy surfaces. A good
example of the effect of relative surface energy
is to use two materials that can be either liquid
or solid (and thus can be either adhesive or
adherend) that are relatively close in surface
energy. Liquid epoxy resin (surface energy
43 mJ/m 2 ) applied to solid polyethylene (surface
energy 31 mJ/m 2 ) will wet the surface
poorly and produce a comparatively weak
adhesive bond. Liquid polyethylene applied to
solid epoxy will wet the surface well and produce
a much stronger adhesive bond (Blomquist
et al., 1983). Similarly, whilst low surface
energy Teflon (18 mJ/m 2 ) will adhere to high
surface energy metal, few materials will adhere
to Teflon (hence the success of non-stick cookware).
A high energy surface contaminated with
low energy material (e.g. a metal surface contaminated
with oil) will behave as though it
were a low energy surface until the contaminants
are removed. Similarly some low energy
surfaces present on polymeric materials will
either require selection of an adhesive with an
even lower surface energy or some form of
pre-treatment to raise the surface energy of the
adherend.
Another way to visualize surface wetting is
by the contact angle formed between the
surface of a drop of liquid on a solid and the
tangent
tangent
(a) (b)
Figure 4.9 One way to quantify the surface wetting
characteristics of a liquid is to measure the contact
angle of a drop of liquid placed on a surface. The
contact angle () is the angle formed by the solid/liquid
interface and the liquid/vapour interface measured from
the side of the liquid. If the contact angle is less than
90° the liquid will wet the solid well (a). A zero
contact angle represents complete wetting. If the contact
angle is greater than 90° the liquid will wet the surface
poorly (b)
surface of the solid. A drop of water on a piece
of wax, for example, beads up into a spherical
shape having a large contact angle. A drop of
water that can spread and flatten on a wettable
surface has a small contact angle (Figure 4.9).
Observation of this phenomenon can be a useful
guide in judging the appropriateness of an
adhesive, or of the condition of the adherend
surface which may be too oily to be wet with
a water based adhesive.
There are various other practical aspects of
adhesive bonding which can be explained by
physical phenomena.
Glue line thickness, adhesive failure
The thickness of the glue layer between the
adherend surfaces can affect the strength of a
joint in several ways. First, an excessively thick
glue line composed of a thermoplastic adhesive
will tend to ‘creep’ or gradually change
shape and fail under a prolonged loading
stress. In some situations the rigid adhered surfaces
will restrain movement of the plastic
adhesive (plastic constraint) but only where
the adhesive line is thin enough that its internal
flow characteristics (rheological properties)
will not be the dominating factor.
Secondly, adhesives may be hard and brittle
due to their inherent characteristics (such as
low molecular weight or highly crosslinked
structures), low temperatures (below the glass
transition temperature at which they show fragile
glass-like characteristics) or due to excessive
desiccation (as with hide glues). With such
brittle adhesives, a thick glue line may make it
possible for flaws and fissures at the edges to
serve as ‘stress raisers’ and propagate the
cracks throughout the adhesive causing failure.
Closely mating surfaces and thin glue lines
are generally desirable but even this rule has
limits. If excessive clamping pressure is exerted
on adherends in order to create a thin glue
line, high spots on the surfaces are forced into
compression and can then exert a continuous
stress on the glue line after the clamps are
removed. This stress can cause ‘strain’ resulting
in a change of shape and failure due to either
plastic flow or fracture. There is a theoretically
ideal glue line thickness which achieves a balance
between failure due to excessively thick
glue lines on the one hand and failure due to
starved joints on the other. Whether this can be
achieved in practice is another matter. Ideal