Series editors' preface - Wood Tools

(a)
(b)
Figure 8.13 Degradation of polyurethanes: (a)
degradation by hydrolysis; (b) degradation by oxidation
carboxyl groups are formed in the polymer.
Over a period of time, crosslinking continues
and the polymer becomes weak and brittle.
Mechanical stress, such as stretching and
flexing, then causes the material to break
down completely (Figure 8.14).
(a)
(b)
Deterioration of other materials and structures 357
PU objects should be kept in a cool, dry,
dark, dust-free environment in conditions of
stable RH and temperature with RH in the
range 40–55%. The use of dust covers made
from Tyvek ® , a non-woven, bonded polyethylene
sheet material, is recommended.
Removal of the antistatic finish from Tyvek ®
before use with artefacts containing metal
components is also recommended (Tímár-
Balázsy and Eastop, 1998). The progress of
oxidation of PU could be delayed by storage
in low oxygen environments (see Chapter 6).
Polyvinyl chloride (PVC)
The degradation of PVC is caused by both
crosslinking and chain scission. Degradation
may be initiated by heat or light energy
leading to thermal or photo oxidation. Colour
changes are due to photo oxidation. A chain
unzipping reaction (Figure 8.15a) that leads to
sequential loss of hydrogen chloride (HCl)
molecules along the backbone of the polymer
is accompanied by discoloration and by
unfavourable changes in mechanical and
electrical properties (Figure 8.15b). The HCl
produced is auto-catalytic and once
dehydrochlorination is initiated the reaction
will proceed quite quickly if the HCl liberated
remains in the vicinity of the object. The
mechanism of this reaction and the use of
Zeolite pellets as adsorbents to remove the
HCl produced from storage and display
environments are discussed by Shashoua
Figure 8.14 Sacco (bean bag or bean sack chair), designed by Design
Studi, Turin, Italy, 1969: (a) in storage; (b) breakdown of polymer structure
in the polyurethane textile of Sacco