Series editors' preface - Wood Tools

inter-chain interactions are the main features
that determine mechanical properties.
Moisture, heat, light, oxygen and chemical
pollutants are the main sources of chemical
actions from which changes result. Variations
in polymer morphology can also arise from
purely physical causes (Crighton, 1988). The
deterioration of many of the synthetic materials
in early use in upholstery is clearly apparent
in artefacts already too fragile to display.
The most problematic deterioration is usually
the collapse of the structure. Latex foam is
frequently found to be so embrittled as to
crumble at the touch so that a grip on an edge
is sufficient to crush it. Polyurethane foam
becomes soft and tacky so that a gripped edge
collapses, sticks to itself and does not regain
its shape (Gill and Eastop, 2001). Chemical
degradation of polymers is affected by such
factors as conformational stress, points of
adhesion, storage and climatic history; that is,
physical factors may make chemical degradation
critical in one object over another. This is
further discussed below. This section, based
mainly on the work of Griffith (1997) will
explore the deterioration of rubber,
polyurethane, PVC and cellulose nitrate.
Rubber
Rubber components are relatively common in
furniture especially in foam fillings of the
1950s and 1960s. Natural rubber contains 95%
cis-1,4-polyisoprene and 5% resins, gums and
protein. Latex foam rubber contains in
addition organic accelerators, antioxidants,
loading materials and softeners. The most
basic mechanism of degradation of rubber is
by oxidation, a free radical chain reaction
which causes embrittlement, loss of strength
and discoloration (Kelen, 1983). The effects of
this process can be seen in Figure 8.11. This
is greatly accelerated by elevated temperature
and by exposure to short wavelength radiation
but relative humidity does not appear to be
an important factor at room temperature
(Shashoua and Thomsen, 1993). In upholstered
furniture, light damage has been
observed on the undersurface of rubber
webbing caused by reflected light from a
polished floor or through a textile covering.
These materials should therefore be protected
from light even during production. Ozone,
sulphur dioxide and nitrogen dioxide air
Deterioration of other materials and structures 355
Figure 8.11 Detail of degraded latex rubber cushion
for ‘Mambo Chair’ designed by Michael Inchbald in
1954
pollutants are also damaging. Stress cracks
caused by flexing of these materials allows air
to penetrate deeper into the material allowing
oxidation to occur beyond the surface.
Latex rubber foams used in upholstery are
susceptible to mechanical deterioration caused
by stress and tension imposed by the weight
load of the sitter and by mechanical abrasion.
These effects are much more noticeable in
oxidized material. In the Lady Chair of 1951,
designed by Zanuso, the structural upholstery
is of foamed rubber laid over a support of
Pirelli rubber webbing (Gill, 1990). Chemically
the upholstery and webbing were very similar
but the way the material was expected to
perform is different. The foam is under
compression below a taut top covering, the
webbing is under tension when applied and
subject to further tension in use. When
degraded, the foam loses its ability to recover
after compression and the webbing loses the
ability to stretch further when weight is
applied. The chair then loses its ability to
perform its designed functions because of the
physical consequences of chemical degradation
(see also Gill and Eastop, 2001).
Two important types of ageing of rubbers
are shelf ageing and atmospheric cracking.
Shelf ageing occurs at ambient temperatures in
the dark and is due to autoxidation.
Atmospheric cracking is the term given to the
reaction which occurs when rubber artefacts
are exposed to UV. In addition to the oxidation
mechanism, attack by ozone (ozonolysis)
may take place (Shashoua and Thomsen,