Series editors' preface - Wood Tools

12
Principles of consolidation, aesthetic
reintegration and coatings
12.1 Basic principles
This chapter considers consolidation, aesthetic
reintegration (filling and retouching) and the
application of coatings. Each section begins
with a discussion of principles and then considers
materials in common use and the properties
that influence selection for a given
application. The structure, material properties
and deterioration of the materials discussed
below are considered in Chapters 4 and 8.
Adhesives, consolidants and coatings often
utilize the same classes of polymeric materials.
Paraloid B72, for example, may be used as an
adhesive, a consolidant or a coating, but the
method of application and solution concentration
is adapted to the treatment objectives. Thus
as an adhesive, it may be used up to 50% w/v
concentration, with the solvent chosen to promote
or inhibit penetration of the substrate. As
a consolidant, it may be used in concentrations
as low as 0.5% in a slow-evaporating solvent to
prevent reverse migration (see section 12.2.2).
As a coating, it may be applied in 10–15% concentration
in a solvent that promotes or reduces
gloss.
It is important to understand the basic principles
of adhesion in order to be able to analyse
the cause of a given problem and formulate an
appropriate conservation treatment. It may be
useful to refer to Chapter 4. The adhesive bond
formed by an adhesive, consolidant or coating
will depend on the ability of the applied material
to achieve intimate interfacial contact by
efficiently wetting the surface. The ability to
wet is dependent on the compatibility of the
intermolecular forces of the applied material
and surface, and the viscosity of the applied
solution.
560
Materials whose bulk and surface are
strongly bound together, such as metals,
oxides, ceramics and glass have a high surface
energy between 500 and 5000 mJ/m 2 . Liquids
(excluding liquid metals), most solid organic
polymers and most organic compounds have a
comparatively low surface energy of less than
100 mJ/m 2 . Within this relatively narrow range
of less than 100 mJ/m 2 , higher energy surfaces
may be considered polar and lower energy surfaces
less polar. Lower energy liquids will be
attracted to, and spread easily on, higher
energy surfaces. If a liquid adhesive has a
higher surface energy than the substrate to
which it is applied will not wet or adhere to it
(Figure 12.1). Water has a surface energy of
72 mJ/m 2 , wax of around 20 mJ/m 2 and as a
result a water-based adhesive will not wet or
adhere well to a surface contaminated by wax.
(a)
(b)
Adhesive
Contaminated
surface
Low surface
energy adhesive
Higher surface
energy substrate
Figure 12.1 Surface energy and wetting
(a) If the surface is contaminated with low surface
energy material, such as wax or greasy dirt, the
adhesive will not flow out. Even if the adhesive is
spread mechanically, any adhesive bond that forms will
be very weak
(b) When the surface energy (sometimes called surface
tension) of the adhesive is lower than that of the
substrate, the adhesive will flow out onto the surface,
wetting it thoroughly