Choice of solvent and concentration for
consolidant solutionsSince each synthetic resin has its own particular requirements, the choice
of solvent is immediately limited to those that can provide solutions of
compatible concentration and viscosity for the chosen resin. In wood con-
solidation, the choice between polar and nonpolar solvents is significant.
Although polar solvents have an affinity for wood, they tend to penetrate
poorly compared to nonpolar solvents, because polar molecules may be
adsorbed on the internal wood surfaces, and such adsorption would
reduce their mobility (Nicholas 1972).
As an organic solvent’s degree of polarity increases, so does its
tendency to swell wood. For example, among commonly used solvents, the
virtually nonpolar toluene swells wood a mere 1.6% as compared to the
swelling by water. Meanwhile, the polar acetone, ethanol, and methanol
produce swellings of 63%, 83%, and 95%, respectively (Stamm and Harris
1953). In deteriorated Douglas-fir samples, vacuum impregnation with 15%
solutions (weight basis) ofconsolidants produced values of swelling in the
tangential direction measured immediately after treatment as shown in
Table 1. When the nonpolar toluene was the solvent, swelling was less than
0.1% with two different resins, whereas Butvar B98 in methanol produced a
swelling of3.31%. This swelling was not permanent, though, and after four
weeks most of the swelling had been recovered (Schniewind 1990b). With
panel paintings, however, even temporary swelling could prove objection-
able, as this might lead to undesirable stresses in the paint layers.
Another point to consider is that solvents with low boiling points
are usually preferred over those with high ones, so that residual vapors
persisting after treatment can be avoided. Residual solvents have other
effects that will be discussed later.
When solution concentrations are chosen, it is necessary to bal-
ance the desire for good penetration—which can be achieved by keeping
the concentration and hence the viscosity low—against the need to obtain
a reasonable level of resin loading (the resin content after treatment).
Given equal penetration, loading can be increased by increasing consoli-
dant concentration. Thus, low concentration tends to yield good penetra-
tion but poor loading, whereas high concentration conversely results in
poor penetration but good loading. Resin loading is important because the
effectiveness of consolidation treatments largely depends on the amount
ofresin that can be added. The maximum possible loading can be calcu-
lated from the porosity of the wood. This is given by (Kellogg 1989):V 512 r 1 (1/rw 1 Mb/rb 1 Mf/rf) (4)where: V 5 fractional pore volume; r 15 relative density (specific gravity)
of (porous) wood, based on oven-dry weight and current moisture con-
tent; rw 5 relative density of cell-wall substance; Mb 5 content of bound
water; rb 5 relative density of bound (adsorbed) water; Mf 5 content of
free water; and rf 5 relative density of free water.
For instance, let us take wood with a relative density of 0.5 and a
total moisture content of 12% and treat this to saturation (i.e., filling allC W P 93Table 1 Swelling of wood samples immedi-
ately after consolidation treatment
Solvent and consolidant Swelling (%)
Acryloid B72 in toluene 0.06
AYATin toluene 0.07
Acryloid B72 in acetone 1.03
AYATin acetone 2.17
Butvar B98 in methanol 3.31