Series editors' preface - Wood Tools

Plastics and polymers, coatings and binding media, adhesives and consolidants 185
Alkyds
Alkyds are a type of oil-modified polyester
resin. They are produced from polyhydric alcohol,
polybasic acid and a fatty monobasic acid
or triglyceride. The term alkyd comes from ‘al’
for alcohol and ‘kyd’ for acid structure
(Martens, 1968). The polymeric networking is
commonly a function of oxygen and carboncarbon
double bond reactions. Alkyds are
often formulated in conjunction with a drying
oil and classified according to the use of short,
medium or long oils. High solid content alkyds
with low viscosity are available to limit the
amount of solvent released in the drying
process. Metallic driers may be used to accelerate
the crosslinking process but are not usually
required for alkyds because they are
pre-polymerized to relatively high molecular
weights and require relatively little further
polymerization to gel. Alkyd coatings are
highly resistant to moisture and solvents,
durable, and have greater initial transparency
than some other common thermosetting polymers.
However, they tend to discolour, particularly
in the presence of vegetable oil
modifiers.
Thermosetting acrylics are very similar to
thermoplastic acrylics except that the polymer
includes reactive sites such as carboxyl and
hydroxyl functional groups. Acrylics can be
reacted with epoxies, amino acids and isocyanates.
They are good performers on an
industrial level lacking any glaring disadvantages
and are used extensively in enamels and
emulsion paints (Friel, 1995). Thermosetting
acrylics are found in high gloss ‘lacquer’ fin-
(i)
R
(ii)
N
H
O
C
O
O
R
O C N N C O CH2 CH2 H H
Urethane units
Figure 4.15(k) (i) A urethane monomer; (ii) a
polyurethane
O
ishes on some furniture. They are highly
durable but difficult to remove.
Epoxies
Epoxies are a chemically and physically diverse
family of thermosetting compounds which have
in common the presence of the epoxide functional
group, a three-membered ring composed
of two carbon atoms and one oxygen (Figure
4.15l). Adhesives and casting resins normally
consist of two components; a resin and a hardener
which when mixed together react to form
a rigid crosslinked polymer. Epoxy resins are
more expensive than the previously discussed
thermosetting resins, but they have important
advantages. A large number of epoxy resins
and adhesives are on the market permitting a
wide choice of properties and potential applications.
Epoxies are solvent-free systems which
shrink very little on curing and do not off-gas
toxic or reactive compounds once cured. This
makes them good gap fillers which can be used
with very low clamping pressure. Their chemical
reactivity is low after curing and while they
are prone to darken with age they do not lose
adhesive or cohesive strength to any significant
degree as a result of ageing. Epoxies are highly
polar adhesives, with excellent adhesive and
cohesive strength, which bond very well to a
variety of substrates including wood, metal,
glass and stone. Conversely, they do not bond
well to non-polar oily and resinous materials. A
wide range of resins and hardeners is used in
epoxy formulations but a few generalizations
can be made. Quick-setting epoxies are more
viscous, initially darker in colour and more
prone to darken with age. They build higher
temperatures in curing because there is less
time for heat-of-reaction to dissipate. The
reverse is true of slower setting epoxies which
are available in clear (‘water white’), highly
fluid grades. Hardness varies from hard and
brittle to rubbery depending on formulation,
creep is very low even for soft varieties. Open
time is highly variable depending on formulation.
Epoxies are waterproof and their optical
saturation of substrates is high. Uncured resins
and hardeners are toxic, cured films are not.
Reversibility is very limited on porous substrates
but good on hard and non-porous substrates
such as porcelain and glass although it
requires use of chlorinated solvents to swell the
polymer matrix. Examples of epoxy resins