As plastics not only form part of collections but are often used to store and conserve artefacts, it is important to be aware of the agents that contribute to the deterioration of these materials. Although many polymers are considered to be imperishable and potentially a worldwide pollution problem, in reality they are not. Collectors on the other hand invariably wish that these materials were less prone to deterioration. Polymeric substances that are most prone to degradation include cellulose nitrate, cellulose acetate, polyurethane foams, PVC and rubber (Shashoua, 2008).
Degradation may be reflected by changes in the chemical composition and the physical and chemical properties of a polymeric object. These changes will affect the form and possibly the function of an object, therefore altering its significance and possibly its ability to be interpreted as part of a collection or a museum display.
Signs of deterioration may include:
- distinctive odours (e.g. camphor from degrading cellulose nitrate);
- changes in colour, shape or surface appearance (blooms, cracks or crazing);
- the corrosion of metal components; and
- the discolouration of associated packing materials
Deterioration of plastics may be due to chemical, physical or biological activity. As with most organic materials, polymeric objects will be affected by elements such as light (especially ultra violet radiation), heat, moisture, oxygen and pollutants. Unfortunately it is not easy to recognise deterioration in its very early stage (the ‘induction’ period). This is problematic because once started, deterioration can be autocatalytic, with the degradation products stimulating further attacks on their own polymer chains.
Light and heat can provide sufficient energy to break weak bonds in a polymer chain, leading to depolymerisation and subsequent alteration of its properties. As the rate of chemical reactions increases with increasing temperature, the rate and extent of degradation of polymers likewise increases as the temperature rises. As well as stimulating chemical change, heat can cause physical changes in thermoplastics, the distortion and buckling of vinyl records being a classic example.
In addition, oxidation reactions are enhanced by the presence of heat, light, metal ions and elevated relative humidity levels. Light and UV radiation in particular may cause colour changes via either alteration of the polymer itself or of the pigments and dyes present in the matrix. Discolouration of clear polymers is often an indication that oxidation has occurred. Low density polyethylene, polyamides (e.g. nylon 6) and acrylics (e.g. polymethyl methacrylate) all yellow when exposed to UV radiation and light, with the former two polymers also becoming more brittle on prolonged exposure.
Certain atmospheric pollutants, such as oxides of nitrogen and sulphur, are acidic and under conditions of high relative humidity can cause considerable damage to plastics. Polymers containing ester linkages such as cellulose acetate, polyesters and polyester-based polyurethanes are more prone to hydrolytic cleavage than are simpler molecules like polyethylene. Ozone is particularly damaging to rubber and to polymers containing unsaturated linkages in the polymer chains.
Inappropriate handling, cleaning or repairs can also accelerate deterioration due to the associated physical stresses and the impacts of some solvents. Areas of physical stress are more prone to chemical attack and bond-breaking. Solvents may not only dissolve a plastic but may also leach out plasticisers or cause the plastic to swell, thereby making the object more susceptible to chemical attack. These latter effects of solvent damage are often not immediately visible, with changes only becoming evident over a prolonged period.
Biological attack may occur, but this mode of deterioration is usually confined to semi-synthetic polymers derived from natural sources such as cellulose and casein and to some acrylic polymers (Shashoua 2008). This type of attack is favoured by high relative humidity levels.
Physical ageing is a less well known phenomenon in which polymeric materials (thermoplastics) continue to alter for long periods after their manufacture. Physical ageing is due to changes in the arrangement of the polymer chains with respect to one another. These changes are reflected in properties such as brittleness, stiffness, solubility and optical characteristics. A typical example is the slow hardening of natural rubber. The effects of physical ageing would be reversible were it not for the complications introduced by associated chemical degradation.
It is rare for only one mechanism of degradation to operate within a polymeric material. Usually changes in the chemical make-up of polymeric materials are reflected in corresponding changes in properties and appearance. For instance, evaporation or migration of plasticisers from a plastic will produce a material with reduced flexibility and increased brittleness. Shown below (Figure 3) is a cellulose acetate doll damaged by both the migration of plasticisers from the polymer and autocatalytic degradation caused by acetic acid released following hydrolysis of acetate groups in the polymer.
Chemical attack on the polymer chain or on the cross-links between chains is likely to produce a weaker material with an altered resilience and changed stress relaxation. On the other hand, chemical changes which introduce cross-links into a plastic will produce a material with a reduced ability to stretch, reduced tear resistance and with an increased chance of suffering fatigue cracking. These sort of changes may be brought about by heat and oxidising conditions.
It is difficult to predict the precise behaviour of a particular object to a set of conditions. The formulations of many of the early polymers varied considerably, even for materials which were meant to be the same and the conditions of use and exposure of many polymeric objects will not be known. The use of impure materials in manufacturing processes also contributed to the variation in properties of early polymers. Thus with age, rubber may become hard, brittle and cracked or it may soften, becoming sticky and even liquefy. Plastics may shrink as they age and the surfaces of rubbers and plastics may craze or become chalky. More information regarding terms commonly used to describe polymer degradation, examples of these and summaries of the causes of degradation is provided elsewhere (Shashoua 2008).
It is likely that many recently produced polymers will have a greater resistance to deterioration due to the use of improved stabilisers and purer starting materials. Despite this, collection managers and conservators need to face the reality that some polymeric objects are inherently unstable and may simply have to be left to their inevitable fate.