Series editors' preface - Wood Tools

Plastics and polymers, coatings and binding media, adhesives and consolidants 179
that this does not normally occur to the extent
that removal of an old shellac coating becomes
unacceptably difficult. Shellac readily wets
onto existing shellac and many other spiritsoluble
coatings. Shellacs are tinted and may
therefore cause some colour change to substrates
to which they are applied. This drawback
makes them inappropriate as a coating
for an existing painted surface. Shellac can be
applied by brush, spray, or pad. French polishing
and other application techniques are
described in Chapter 13. Further information
on shellac is given by Gardner (1938), Hicks
(1962), Parry (1935), Williams (1988) and in the
Bulletin of the London Shellac Research
Bureau.
4.7.6 Synthetic materials
Synthetic resins are polymers which generally
mimic or improve upon the properties of natural
resins. Synthetic resins or ‘plastics’ have
been and continue to be used as coatings and
media, adhesives and consolidants and for
moulding and casting applications. Virtually all
of the tremendous variety now available have
had some use, however brief, in the wood
working industries. Synthetic resins vary
greatly in chemical make-up in structure, properties
and uses.
Thermoplastics
Resins dissolved in various solvents have seen
extensive use as coatings, but have also been
used by conservators as consolidants for
degraded wood and poorly bound or adhered
coatings. Solvent type adhesives based on cellulose-nitrate
resin (H.M.G., Duco etc.) and
emulsions based on acrylics and PVACs have
also seen extensive use in conservation.
Poly(vinyl acetate) PVAC
Poly(vinyl acetate) (Figure 4.15a) was one of
the earliest synthetic materials available to conservators
and has been used as a consolidant,
adhesive and coating. Although never popular
as a transparent coating on wood it is liked by
some people for retouching and to make an
occasionally used varnish that is soluble in
toluene or in ethanol/water mixtures. Commercially
available poly(vinyl acetate) resins
include the Rhodopas series by Rhône
Poulenc, the Mowilith series by Hoechst, and
the Vinylite series by Union Carbide. PVA
emulsions used in conservation include the
Vinamul series by Vinyl Products, the Mowilith
series by Hoechst and Jade 834-403N by
Aabbitt.
Figure 4.15(a) Structure of some common polymeric
materials. (i) Vinyl acetate monomer; (ii) poly(vinyl
acetate) (PVAC)
PVAC resins are water clear and relatively
stable have little tendency for crosslinking and
are relatively unaffected by light. They are soluble
in a wide range of solvents including
toluene, acetone, and methanol but insoluble
in petroleum type solvents, hexane, water and
butanol. They are only partially soluble in
ethanol and xylene. PVACs whiten in contact
with water, have generally low glass transition
temperatures and have different optical properties
to natural resins including a generally
lower refractive index.
Poly(vinyl acetate) emulsions were the first
resin emulsions to be used as wood adhesives
and are familiar to most people as white glue.
They bond well to wood and a variety of other
substrates, are easy to use and clean up, and
are readily available. This combination of factors
has made them the most commonly used
non-industrial woodworking adhesives and the
choice of DIY enthusiasts everywhere. While
emulsions are initially dispersible in water,
they dry to tough films that are only swellable
in water but have greater solubility in organic
solvents such as ketones. Owing to their high
molecular weights they are also not completely
resoluble in organic solvents as are the parent
resins.
‘Yellow glues’ are emulsion-type adhesives
consisting principally of poly(vinyl acetate) and
ethylene vinyl acetate co-polymers. They are
generally more viscous than white glues and as
a result do not squeeze out as much under
clamping pressure. They set very quickly, are