Series editors' preface - Wood Tools

Plastics and polymers, coatings and binding media, adhesives and consolidants 181
late/methyl acrylate 70/30 co-polymer), and
Paraloid B44 a material similar to, but slightly
harder than, B72 (Figure 4.15d). The Paraloid
series was previously known in the United
States under the name Acryloid. The names
Acryloid and Paraloid are both trade marks of
Röhm and Haas. Acrylics are mostly easily prepared
for use, are clear, and have solubility
parameters that are often different from an
extant coating making it possible to remove
them without causing harm. Shellac will mostly
adhere adequately to the acrylics making it
possible to use them as an interface between a
shellac and an original surface. The more
apparent draw backs of the acrylics are possibly
inadequate adherent properties due to high
molecular weight/long-chain structure and different
optical qualities to natural resin coatings
being matched. Acrylics are usually soluble in
aromatic hydrocarbon solvents. For further
information on the acrylics see Kolesky (1995).
Figure 4.15(d) (i) Paraloid B72: the monomer units
are arranged randomly in the polymer chain; (ii)
Paraloid B67 monomer
Cyclohexanone resins
This class of low molecular weight resins has
somewhat variable properties depending on
starting materials and processing. Generally,
they are hard but somewhat brittle, optically
similar to natural resins and with similar solubility
characteristics to them, initially more stable
than natural resins but tending to require
more polar solvents for removal as they age.
AW2 (BASF) a co-polymer of cyclohexanone
and methyl cyclohexanone, was one of the first
resins of this type to be used in conservation
as a picture varnish. It was later replaced by
Ketone Resin N, also called Laropal K80
(BASF), a product based on cyclohexanone
alone. MS2, another product resembling AW2,
was based on methyl cyclohexane. MS2
formed the starting point for a reduced derivative,
MS2A, in which all the carbonyl groups
were converted to hydroxyl groups by catalytic
hydrogenation (Figure 4.15e). MS2A is much
more stable to light-induced oxygenation and
less sensitive to water vapour with less tendency
to bloom. It has therefore remained soluble
in non-polar solvents for much longer and
has enjoyed considerable popularity in the UK
as a final varnish on cleaned painted surfaces.
However, at some point in the history of manufacture
of this material (c.1963) the starting
material was changed from methyl cyclohexanones
to cyclohexanone itself. MS2A prepared
from this starting material was found to be too
brittle for satisfactory use as a coating. Another
reduced cyclohexanone material MS2B prepared
from AW2 had similar good properties to
Figure 4.15(e) MS2A (Routledge, 2000): MS2A is the
product of a complex mixture formed by reactions
involving methyl cyclohexanone, methanol and their
derivatives. Routledge (2000) states that ‘typical’ MS2A
has a weight average molecular weight of 1800, a
number average molecular weight of 800, and will
contain around eight ring units per molecule