Series editors' preface - Wood Tools

356 Conservation of Furniture
1993). This is a process in which ozone reacts
with non-stretched rubber by adding to double
bonds to form ozonides. When all the surface
double bonds are consumed, the reaction
stops but when mechanical stresses are
applied and more unsaturated molecules are
exposed, the process continues. Cracks propagate
and eventually cause complete failure
(see Figure 8.11).
Rubber is best stored in cool, dry, dark, well
ventilated, dust-free environments. Rubber
could be stored to advantage in a low oxygen
environment (see Chapter 6) or failing this, in
well ventilated conditions.
Polyurethanes
Polyurethanes (PUs) are extremely versatile. It
is possible to make a variety of different PUs
with different physical and mechanical properties
suitable for different applications. They are
generally synthesized by a two-step reaction
(Figure 8.12). The starting material is usually
a short chain polyester or polyether with
hydroxyl groups at each end (a polyol) which
is reacted with an excess of a diisocyanate to
form a prepolymer. In the second stage the
prepolymer units are linked into a long chain
polymer by reaction with a chain extender,
usually a diol or diamine (Hepburn, 1982; Kerr
and Batcheller, 1993). When the chain extender
is a diol the polyurethane polymer
contains urethane links; when an amine is
used, urea links are formed. If carbon dioxide
is released during the reaction a foam is
produced.
Polyurethanes are degraded by exposure to
light and heat and are inherently susceptible
to hydrolytic degradation (Crighton, 1988).
Photo-degradation by UV radiation and visible
light, the main cause of deterioration, may be
accelerated by moisture and ammonia and
produces a variety of breakdown products. It
is possible to stabilize PU to light degradation
and provide some resistance to UV by incorporating
antioxidants. Polyurethanes do not
usually tear or crumble in service but they
soften and loose height and the shape distorts,
especially initially. The coated fabrics and
imitation leathers made with PUs commonly
crack, peel away from the substrate or become
tacky so that folds stick to each other. The
yellowing of PU is an inherent problem particularly
with foams made from aromatic diiso-
(a)
(b)
(c)
Figure 8.12 Polyurethanes
(a) A urethane group
(b) The urethane linkages in a polyurethane
(c) Polyurethanes are synthesized in a two-step reaction.
For convenience, the functional groups of the generic
monomer types are shown, whilst the parts of the
molecules that do not participate in polymerization are
indicated with wavy lines (di-alcohol, or diol
components) or rectangular boxes (diisocyanate
components) (Hepburn, 1982)
cyanates. The weak points on the molecule
are the various reactive bonds in the main
polymer chain such as urethane, ester, ether,
amide urea and biuret, though these bonds
vary in their resistance to hydrolysis and
oxidation (Figure 8.13) (Kerr and Batcheller,
1993). PUs based on polyesters are much more
susceptible to hydrolysis than those based on
polyethers. Catalysts, such as tertiary amines
which remain from the polymerization of PU
foams, appear to accelerate degradation by
light causing both chain scission and crosslinking
to occur. When scission of the urethane
link occurs new, polar, amino groups and