Series editors' preface - Wood Tools

8.10 Adhesives
In practice, adhesives and adherends often fail
long before theoretical calculations of bond
strengths predict that they should. In theory,
failure can occur within the adherend, along
the adhesive interface or within the adhesive.
In practice it will occur at the weakest point
in the system. This normally involves cohesive
failure in the adhesive layer (Kinloch, 1987).
In the case of surface coatings with a layer
structure (support, ground, paint, varnish etc.)
each component of the laminate may potentially
fail adhesively or cohesively.
It has been argued that all joint failure is
actually cohesive and that even where failure
has apparently occurred at the interface there
is still a very thin layer of adhesive on the
surface of the adherend (or, conversely, a thin
layer of adherend on the surface of the
adhesive). This suggests that the bulk properties
of an adhesive can be different from those
at the boundary layer of the interface and that
it is the formation of a weak boundary layer
which leads to adhesive failure. The theory of
the weak boundary layer suggests that if a
region of low cohesive strength exists at the
interface between a substrate and a hardened
adhesive, failure will occur at a lower stress
level than predicted. High surface energy
solids attract low surface energy contaminants
and these may prevent a strong adhesive bond
from forming if they are not displaced before
joint formation. Weak boundary layers may
also form in situ after the adhesive joint has
been assembled.
Delollis (1973) has argued that this is an
oversimplified generalization and has proposed
an alternative mechanism of desorption
of adhesive caused by contamination of the
interface by water. This desorption may occur
either through absorption of water by the
adhesive followed by migration to the interface
or by water diffusing along the adhesiveadherend
interface.
Other factors which may contribute to
adhesive joint failure include inherent differences
between adhesive and adherend (e.g.
density, coefficient of linear thermal expansion,
modulus of elasticity etc.) air bubbles
trapped at the interface or in the adhesive,
swelling in response to exposure to polar
molecules such as water, stress concentrations
Deterioration of other materials and structures 345
(e.g. caused by the geometry of the joint),
contamination of the adherend surfaces,
improper preparation of the adhesive (e.g.
inadequate mixing of two pack adhesives or
overcooking of animal glue) and deterioration
of the adhesive in storage. The effects of
inherent differences between materials are
most likely to be concentrated at the interface
and to produce stress concentrations in the
boundary layer that may predispose to failure
in this region.
8.11 Deterioration of specific
materials
8.11.1 Oils and fats
The high susceptibility of fatty acids containing
two or more double bonds to oxidation is
the basis of their use as drying oils, the
gradual conversion of liquid oil through soft
gel to rubbery solid occurring as a result of
free radical chain reactions (Mills and White,
1987). Oxidation, which at first is necessary for
drying continues throughout the life of the film
which becomes more oxidized and crosslinked
and consequently more brittle and less soluble
with time, especially under the influence of
UV radiation. At the same time, oxidation
reactions may occur that are degradative in
nature leading to the formation of small
molecules such as carboxylic acids. The influence
of pigments (and other additions) on the
behaviour of linseed oil films is very profound.
As a clear exterior finish, linseed oil would fail
under the influence of UV radiation in a matter
of weeks but when used in pigmented finishes
it is very satisfactory. Besides light, the influence
of metal ions and the availability of
oxygen, as determined by the thickness of the
film, are of great importance. Yellowing is
most marked in oils containing appreciable
amounts of linolenic acid (e.g. linseed oil).
The ester groups of fatty acid esters may be
broken down by saponification or hydrolysis
to yield glycerol and free fatty acids. Under
appropriate conditions this form of degradation
of fats can be achieved by micro-organisms
such as Staphylococcus spp. and Proteus
spp. The probable mechanisms involved in
these reactions are outlined by Mills and White
(1987). The behaviour of different oils and the