Series editors' preface - Wood Tools

182 Conservation of Furniture
the earlier type of MS2A. However the MS2B
prepared from Ketone N (when Ketone N
replaced AW2) was again found to have unsatisfactory
properties. For further information on
the structure of these resins see Mills and
White (1987) and Routledge (2000).
Cellulose nitrates
Two important classes of polymers derived
from cellulose are the cellulose esters and the
cellulose ethers. Of the various esters of cellulose
that have been manufactured, cellulose
nitrate and cellulose acetate are the most
important to the conservator (Figure 4.15f ).
Cellulose nitrate is produced through the conversion
of cellulose to nitrate esters by reacting
it with sulfuric and nitric acids. As a coating,
cellulose nitrate has been used extensively in
the furniture industry but is not commonly
employed in conservation. It has a high Tg
which is reduced by the addition of plasticizers.
The composition of cellulose nitrate products
varies significantly depending on the
blend with an assortment of plasticizers, solvents,
catalysts and other additives. They cure
initially by the rapid loss of solvent followed
by crosslinking and eventual decomposition
releasing acid forming nitrogen dioxide.
Cellulose nitrates are quick drying, and provide
initially durable surface resistant to moisture
and many solvents, though they remain largely
(i)
H
H
CH 2
OR
OR
OR
CH 2
–O C C
H H
C O O–
C
OR
H
H
C C
H
OR H
C
(ii)
C O O C C H
O
C CH3 H
OR
Figure 4.15(f) (i) Two monomer units of a cellulose
ester (Horie, 1992). In cellulose nitrate, about 2.1 of the
R groups (12% by weight) are ⎯NO 2. In cellulose
acetate, about 2.4 of the R groups are acetate groups
(ii)
removable even after significant ageing. In the
long term they show poor ageing properties,
with problems occurring due to migration of
plasticizers out of the coating and lack of saturation
of substrate. Cellulose nitrate coatings
show both crosslinking and reduction of molecular
weight through ageing. Typically cellulose
nitrate coatings require fairly aggressive,
toxic and flammable solvents both for application
and removal. They tend to dry too fast for
brush application therefore requiring spray
application to achieve a good result. HMG ® (H.
Marcel Guest) is a quick drying, clear, cellulose
nitrate based adhesive of moderate strength
suitable for a wide variety of substrates that has
been widely used in conservation.
Cellulose ethers (Figure 4.15g) form a large
class of materials used in conservation principally
for the consolidation of pigment and as
(i)
CH3 CH3 O
CH3 OH O CH2 OH O
OH C C
H H
C O O C C H
C
H H
C C
H
H
C C
H
H
C
O
(ii)
H
C O O C C H C O OH
CH 2
O
CH 3
OH
OH
CH 3
O
CH 2
O
CH 3
n–2
CH 2
O
CH 3
OH
C O O C C H C O OH
CH2 OH O CH2 O
CH3CHCH2 O
OH
CH 3
O
CH 2 CHCH 3
O
CH 3
CH 3
OH
OH C C
H H
C O O C C H
C
H H
C C
H
C C
H
H
H
C
n–2
Figure 4.15(g) Cellulose ethers: (i) methyl cellulose;
(ii) hydroxypropylmethyl cellulose (from Dow Methocel
Cellulose Ethers product literature)