Acryloid B72 in toluene, where bending strength increased significantly
between two weeks and one year, but not thereafter. This may well have
been a case where retained solvent did reduce the short-term strengthen-
ing effect of the consolidant.Consolidant effects on wood-moisture relations
Synthetic polymers introduced into wood can affect the amount and rate
of water absorption, as well as the shrinking and swelling. The extent of
this effect depends greatly on whether the polymer has entered into cell
walls or is contained within the cell lumina. Major reductions in hygro-
scopicity can be obtained only by polymer occupying sorption sites within
the cell wall, but depositions in the cell lumina will affect the rate of mois-
ture sorption while reducing shrinking and swelling by as much as 20%
(Schneider 1994). Although vinyl monomers do not swell wood and, there-
fore, do not enter the cell wall, Simunkovà and coworkers did obtain large
reductions in hygroscopicity, water absorption, and swelling with methyl
methacrylate polymerized in situ by irradiation (Simunkovà, Smejkalovà,
and Zelinger 1983). Butyl methacrylate was less effective. The extent of
the changes was proportional to polymer loading, which ranged up to
about 60% polymer. By contrast, at a loading of 24%, it was not possible
to detect an effect on hygroscopicity by a treatment of Acryloid B72 in
acetone (Schniewind 1990b). This was not unexpected because a molecular
weight of3000 is about the maximum that can enter the cell wall—even
in the presence of a swelling solvent like acetone—and Acryloid B72 is
believed to have asignificantly greater molecular weight. Another con-
tributing factor may be that Acryloid B72 introduced in solution does not
form a uniform film over the internal lumen surfaces but tends to concen-
trate heavily in some cells, leaving others with little or no resin in them
(Schniewind and Eastman 1994).Thermosetting synthetic polymers are not soluble in neutral organic sol-
vents and cannot be softened by heat, making treatments with something
like epoxy resins irreversible. In contrast, treatments with thermoplastic
synthetic polymers are reversible, at least in principle. Grattan and
Williams have questioned whether the reversing of consolidation treat-
ments can ever actually be successfully executed—the argument being that
if an object is frail enough to require consolidation, it will be too frail to
withstand the stresses ofhaving the consolidant extracted again (Grattan
1980; Williams 1988).
According to the principles of thermodynamics, all real processes
are irreversible—even the simple act of placing a drop of water on a
smooth but uncoated wood surface can result in minute irreversible
changes (Schniewind 1987). Horie proposed four standards of reversibility,
ranging from clearly irre versible—through a return either to original
appearance or to a state that does not interfere with subsequent treat-
ments—to a state where no trace of the original treatment remains (Horie
1983). Thus, in practical terms, it is useful to know if some or most (if not
all) consolidant can be extracted again if necessary. Thermosetting resins
are clearly irreversible and thus could never be removed if used as consoli-
dants. Thermoplastic resins polymerized in situ are also not likely to be
readily removed: Unger and coworkers found it difficult to remove evenReversibility
100 Schniewind