Series editors' preface - Wood Tools

16.2 Plastics
16.2.1 Introduction to plastics
Plastic is a generic term incorporating all types
of synthetic polymeric materials. These range
from the early semi-synthetic materials such as
cellulose nitrate to the latest fibre-reinforced
composite. Plastic may be used in furniture as
a decorative inlay or, in modern objects, it may
be the primary material. Different types of plastics
have diverse requirements in terms of care
and conservation, so it is important to have
some idea of the class of material you are dealing
with. In some cases knowledge of the date
of the plastic, its appearance and properties
may be sufficient to allow identification whilst
in other cases analytical methods may be
required. Techniques for the identification of
plastics have been described by Blank (1990),
Braun (1982), Morgan (1991), Mossman (1988),
van Oosten (1999) and Williamson (1999),
whilst Coxon (1993) has discussed the limitations
of simple identification methods. An historical
overview of the development of plastics
may be found in Katz (1986) and Mossman and
Morris (1994). Brydson (1995) provides an
overview of the properties of historical and
modern plastics. Blank (1988, 1990), Grattan
(1993), Quye and Williamson (1999) and Sale
(1988) provide a useful introduction to the
conservation of plastics.
Some plastics, such as cellulose nitrate and
poly(vinyl chloride) (PVC), produce acidic offgassing
and/or surface droplets as a side effect
of their degradation process. Many polymers,
especially those used to produce flexible plastics,
may have a sticky surface. This may be a
result of degradation (e.g. cellulose nitrate) or
oxidation (e.g. rubber). Plasticizers, added to
regulate flexibility, may migrate to the surface
of the material causing it to become sticky and
attract dust and dirt, for example poly(vinyl
chloride).
Conservation of plastic can be divided into
preventive treatments, designed to control
agents of degradation in order to retard the
deterioration rate, and interventive conservation,
designed to stabilize an object to allow
storage or display. Preventive treatments have
focused on retarding the rate of degradation.
Oxidation and the build up of damaging gases
have been identified as the primary mechan-
Conserving other materials II 719
Figure 16.3 Detail of red and white polyurethane
upholstery from ‘Armadillo’ designed by Designers
Associated Milan, Italy, 1969. Oxidation and loss of
plasticizer have caused cracking and peeling of the
plastic
isms of degradation of polymers in museum
collections. Oxygen scavengers such as
Ageless ® have been used in closed systems,
whilst ventilated storage combined with activated
charcoal cloth has been used to address
the problem of auto-catalytic reactions as a
result of off-gassing (Shashoua and Thomsen,
1993; Ward and Shashoua, 1999).
The conservation of plastics presents many
ethical dilemmas as conservators try to balance
the requirements for long-term stability or display
against removal of original material or
reversibility. Treatment may be complicated by
accurate identification, complexity of formulation
(e.g. co-polymers, plasticizers and other
additives), as well as the degree and effects of
degradation (Figure 16.3). As a result all treatments
should be tested cautiously before use.
Fenn (1993) has considered the problem of
applying acquisition numbers to plastic objects.
16.2.2 Cleaning
Accretions of dirt and dust on plastics may be
complicated by the presence of plasticizers or
acidic degradation products. Although mechanical
cleaning is often the least damaging alternative
for non-sticky plastics, many plastics are
vulnerable to scratching and abrasion, and
those with a Tg around room temperature may
be vulnerable to indentation. Vigorous polishing
may result in a localized temperature rise
above Tg and cause softening and damage.