Series editors' preface - Wood Tools

Plastics and polymers, coatings and binding media, adhesives and consolidants 139
markedly affected. The higher molecular
weight PVAC and acrylic polymers tend to give
semi-matt finishes whereas dammar, mastic
and AW2 tend to produce high gloss surfaces.
The nature of the surface therefore depends
partly on the ability of the film to level itself.
This in turn depends on the nature of the solvents
used to formulate the coating. Using solvents
that evaporate slowly will reduce the rate
at which viscosity rises and allow more time
for levelling of the coating before drying.
However, it will also allow more time for the
coating to pick up dust. Considerable experience
is needed for the proper selection of solvents
for coatings, particularly when using
materials of high viscosity grade. Selection of
solvents for coatings may, however, be
restricted by the potential effect of the solvent
on the underlying paint (Tsang and Erhardt,
1990). A slow evaporating solvent will remain
in contact with the paint for longer and there
is therefore greater risk of solvent action on the
paint. Provided that a polymer material used as
a coating is capable of high gloss then it is possible
to obtain a full range of effects using different
solvents, application techniques and
matting agents.
The ability of a coating to wet the surface is
important in saturating the surface and allowing
colours to be seen, particularly so with
porous surfaces. Penetration into porous surfaces
is more easily achieved with polymers of
low viscosity grade. An appropriate degree of
adhesion to the surface is also important.
Failure in this respect leads to light being
reflected by cracks and fissures. This is often
noted with polyvinyl alcohols which generally
have low adhesion on most kinds of paint. The
interaction of a coating with the wood on a
cellular level is difficult to ascertain. Microscopy
may show coatings that sit on the top of
the wood exhibiting no evidence of saturation.
Other coatings may be found to penetrate
many cells into the wood tissue. The adhesive
properties of surface coatings are similar to
those of the same polymers used as adhesives.
Some components of a coating such as oils can
be drawn into the wood structure selectively
due to the heterogenous nature of wood and
this can cause uneven ageing characteristics in
the coating. Coatings that are water or alcohol
based are capable of swelling the wood tissue
and thereby penetrating through cell walls.
This is one reason why alcohol-based coatings,
or ‘spirit varnishes’ are known for excellent
mechanical bonding with wood.
The refractive index of a coating can affect
the appearance of the underlying surface.
Refractive index measures the extent that light
is bent in travelling from one medium to
another (strictly speaking, from a vacuum but
air gives a close approximation). As the refractive
index of a coating increases, more light is
reflected from the top surface, less light
escapes from the coating into the air again and
less light is reflected at the coating/paint interface.
The refractive index therefore affects the
success with which the surface is revealed.
However, in practice the effect of variations in
refractive index is often less apparent than differences
in gloss due to the relative smoothness
of the upper surface of the coating and
the ability of the coating to penetrate and wet
out the surface. This point is demonstrated by
polyvinyl alcohols, cellulose ethers, and soluble
nylon which are, or have been, used as
consolidants of loose dry pigment. They have
a minimal darkening effect because of their
poor adhesive and wetting properties.
Very few polymers absorb radiation in the
visible spectrum that is roughly between 380
and 760 nm. Thus, most polymers are colourless.
However, some thermosets (including
phenol formaldehyde, epoxies, and
polyurethanes) absorb slightly more strongly at
the blue end of the spectrum and thus appear
yellowish or brownish in transmitted or
reflected light. These substances contain alternating
double and single bonds or aromatic
rings which act as chromophores, absorbing
light at frequencies corresponding to the excitation
energies of bonding electrons. Since the
eye is not sensitive to wavelengths of less than
about 400 nm it is not necessary for coatings to
transmit light of shorter wavelength. Indeed, it
is a definite advantage if they do not since this
will protect the underlying surface against
damage from UV. Fresh dammar resin films
absorb nearly all radiation below 280 nm and
some of that between 280 and 380 nm. When
aged, films of dammar and mastic absorb more
of the UV component and some of the shorter
wavelength visible radiation. They therefore
appear yellow and later even brown thus distorting
the true colours of the surface under the
coating.