Thermoplastic resins
Thermoplastic resin consolidants can be introduced into wood as
monomers or prepolymers and polymerized in situ, using either irradia-
tion or a combination of heat and a catalyst to initiate the polymeriza-
tion reaction. Commercial production of wood-polymer composites
uses vinyl-type monomers such as styrene, methyl methacrylate, vinyl
acetate, or acrylonitrile, but methyl methacrylate is considered to be best
suited for industrial products (Meyer 1989). Unless cross-linking agents
were introduced, the resin may still be soluble after polymerization,
but in practical terms very little chance of removal remains. Unger
and coworkers found that surface films and crusts remaining after they
treated old pistol grips with mixtures of methyl methacrylate, styrene,
and polyester could only be removed with considerable difficulty (Unger,
Reichelt, and Nissel 1981). Schaudy has made extensive studies of a wide
variety of consolidants that can be cured by irradiation. Some of these
findings have been summarized recently: only certain resins tested were
found suitable, but many types of objects, including polychrome wood,
have been treated successfully (Schaudy 1990). The advantage of curing
in situ lies in the low viscosity of monomers or prepolymers, which
assures good penetration and good resulting strength. However, it is
not likely to be the method of choice for use on panel paintings because
of difficulties in ensuring that the ground and paint layers will remain
unaffected by the treatment.
Alternatively, thermoplastic polymers can be introduced into
deteriorated wood in solution form. Commonly used polymers for this
purpose are polyvinyl acetate (PVA), polyvinyl butyral, acrylics, and
soluble nylon (Grattan 1980; Unger 1988). Of these, soluble nylon is no
longer used because it has poor durability and loses its solubility very
quickly due to cross-linking (Bockhoffet al. 1984). The advantages of the
other three types of resins are that they are reversible at least in principle;
theycan be applied by a variety of methods; and, in the case of PVA and
acrylics, they have a record of stability extending over a period of more
than sixty years. Disadvantages are that some solvents may cause the
wood to swell during treatment, and that the strengthening effect is not
as great as that which can be achieved with epoxy resins and other materi-
als. With regard to PVA and acrylics, of particular interest is their use in
picture varnishes: should these resins be used as consolidants for wooden
supports of paintings varnished with such products, a degree of compati-
bility could be assured.Consolidants must be in either gaseous or liquid form if they are to be
applied to deteriorated wood. There is one method of applying gas-
phase consolidant that uses Union Carbide Corporation’s Parylene poly-
mers (Humphrey 1986). However, this process does not appear to
penetrate sufficiently for effective wood consolidation, and the conse-
quent thin films achieved would provide very little strengthening. In liq-
uid form the consolidants may be in the molten state (e.g., waxes); they
may be liquid monomers that are then polymerized in situ (e.g., methyl
methacrylate); or they may be thermoplastic polymers in solution (e.g.,
PVA in acetone).Application of
Consolidants92 Schniewind