Series editors' preface - Wood Tools

~CH 2—CH—CH 2~
|
O
Alkoxy radical
Ketone
~CH2—C—CH2~ + H
||
O
~CH 2 + CH—CH 2~
||
O
resins. All true resins are insoluble in water
but their solubility in organic solvents may
change on ageing due to crosslinking, degradation
(chain scission) or the development of
polar groups, accompanied by the absorption
of oxygen. These changes are more fully
explained by Copestake (1992) as follows.
Degradation of films of natural resins
involves photochemically initiated autoxidation
reactions followed by non-oxidative thermal
processes leading to the familiar degradation
phenomena such as yellowing, cracking,
hazing, loss of gloss, change in solubility and
fluorescence. The process involves homolytic
bond cleavage (the breaking of chemical bonds
so that neutral atoms or radicals are formed)
during which free radicals are produced and a
subsequent chain mechanism which leads to
new free radicals (i.e. the autoxidation process
is auto-catalytic). A variety of secondary autoxidation
reactions can occur leading to products
containing hydroxyl, carbonyl and carboxylic
acid groups and carbon–carbon double bonds
with consequent increase in polarity. Alkoxy
radicals, for example, can produce ketones or
undergo scission to form aldehydes which
easily oxidize further to carboxylic acids
(Figure 8.10). Certain functional groups (e.g.
carbonyls, ether oxygens, carbon–carbon
double bonds and tertiary carbon atoms) are
more susceptible to autoxidation reactions than
others and are subject to scission reactions
under UV.
The chemical constituents of natural resins
have high levels of unsaturation and contain
many susceptible functional groups. The
presence of hydro-peroxides also plays an
important part in photo-oxidation and these
are a large source of free radicals. Photooxidation
leads to the production of acid
groups, to the destruction of the original
O 2
Deterioration of other materials and structures 347
HO—C—CH 2~
||
O
Aldehyde Carboxylic acid
Figure 8.10 Degradation of natural
resins: alkoxy radicals may produce
ketones or undergo scission to form
aldehydes, which easily oxidize further to
carboxylic acids
carbonyl species (probably the groups primarily
responsible for absorption of UV radiation)
present in the resin, to a considerable change
in solubility and to increase in molecular
weight in some cases. These changes occur to
a much lesser extent under non-oxidative
thermal ageing alone.
Attempts to slow down the degradation
process should therefore focus on the beginning
of the degradation chain. Reductions in
light levels and elimination of UV radiation are
thus primary concerns. Stabilizing additives
such as the very powerful HALs (hindered
amine light stabilizers) and UV absorbers are
also useful in reducing the effects of degradation
although due to the abundance of reactive
functional groups it is not so easy to stabilize
natural resins as it is to stabilize synthetic
resins.
The polarity of dammar increases on
exposure but no crosslinking or degradation
seems to occur. Dammar films hold on to
solvent and dry relatively slowly which may
help to explain the formation of surface
wrinkles during drying. Also, they may form
an ammonium sulphate bloom if dried in
humid environments. Dammar becomes more
brittle and yellow with age though the yellowing
and the polarity change can be reduced
by the incorporation of an antioxidant (Horie,
1987). Mastic also shows changes in polarity,
yellowing and embrittlement. It is more brittle
than dammar and the effects of the changes
in polarity are more profound. This led to its
replacement by dammar as the picture varnish
of choice prior to the adoption of newer
synthetic materials. However, mastic remains
easy to remove when this becomes necessary.
Rosin oxidizes rapidly becoming yellow and
increasingly sensitive to water and losing its
initial high gloss.