Rather than use the term preventive conservation when describing ways in which deterioration of polymeric substances can be slowed down, Shashoua (2008) prefers to use ‘inhibitive’ conservation. This is probably more accurate as it is generally accepted that once degradation has commenced in polymeric substances it cannot be stopped, merely ‘inhibited’ or slowed down. The type of preventive or inhibitive action that can be taken will be largely dependent on the following factors:
- the type of polymer;
- the current condition of the polymer; and
- the use to which the polymer is to be put.
Although there are no international guidelines regarding environmental conditions for polymers, knowing the polymer type and the likely causes of degradation will obviously assist in determining the optimum storage conditions. Precise specification of preservation guidelines is also complicated by the complex compositions of most polymers. Rarely are they just one component; rather they comprise a myriad of ingredients (co-polymers, fillers, flame retardants, plasticisers, dyes etc), many of which affect the stability and longevity of the object. Thus while some general guidelines may be given for polymeric substances, quite specific conditions must be maintained for those materials for which deterioration mechanisms are well known. Almost all polymers for example, benefit if light and UV radiation levels are limited and low temperatures are maintained. It is mainly polyester and ester-based polyurethanes however, that benefit from storage under low relative humidity conditions that limit hydrolytic degradation.
Conditions which slow the rate of deterioration of most plastics include:
- low temperatures;
- relative humidity levels of less than 50 per cent;
- low oxygen levels; and
- low light levels.
Storing most plastics and rubbers at low temperatures and low relative humidity (less than 50%) will retard the rate of chemical deterioration. Common wine refrigerators are quite good polymer storage units as they maintain low temperatures and relative humidity levels while allowing the condition of objects to be monitored visually. Note that nylon and casein formaldehyde polymers should be stored at about 60 % relative humidity to minimise shrinkage and brittleness.
As the rate of chemical reactions halve for every 10 °C drop in temperature, storage of most polymers in frost-free deep freezers or cold rooms will slow chemical deterioration. Caution is still needed however as some plastics may undergo stress cracking at very low temperatures and some materials such as painted animation cels (early cartoon materials) and laminates are at risk due to differential shrinkage rates of their components. In addition many polymers are brittle and therefore susceptible to physical damage if handled inappropriately while still very cold. Although research is continuing in this area, it is considered safe to store thin-walled polymers including cellulose nitrate, polystyrene, polyesters and acrylonitrile-butadiene-styrene copolymers in polythene bags in a freezer (Shashoua 2008). While thicker polymers can also be stored in freezers, they cannot simply be bagged and put directly into the freezer. To minimise the risk of damage, thicker polymers should be progressively cooled, moving from ambient conditions to cooler storage, then to a refrigerator before finally being moved into a freezer. This slow acclimatisation reduces the risk of water damage as the temperature is reduced. Conversely, thicker polymers should also be slowly acclimatised to warmer conditions should they be removed from a freezer (Shashoua 2014).
Self-indicating silica gel may be used to maintain a dry environment, but is not recommended for plasticised PVC as it has been shown to absorb phthalate plasticisers from the polymer, leading to accelerated deterioration (Shashoua 2001). As for all artefacts, it is wise to maintain stable conditions to minimise problems associated with adjustment to a different environment.
Oxygen exclusion will also enhance the longevity of most plastic and rubber artefacts. Achieve this by using an inert storage atmosphere (nitrogen or argon) or a chemical oxygen scavenger such as those described previously (see the chapter Mould and Insect Attack in Collections). There are obvious practical difficulties associated with building and maintaining a large, oxygen-free storage area. It is more practical to seal sensitive artefacts with oxygen scavengers in transparent, oxygen-impermeable envelopes. In 1991, British Museum staff sealed approximately 50 assorted polymeric objects in bags with oxygen scavengers. When these objects were examined about 10 years later, those objects still in sealed bags were essentially unchanged, unlike others that had been either inadvertently or deliberately opened (Dyer et al 2011). If this approach is adopted, monitor the activity of the oxygen scavengers and replace them when necessary.
Remember that placing polymers in a sealed container is not recommended for polymers like cellulose nitrate and cellulose acetates that produce damaging gases as they age and deteriorate. Enclosing these polymers in a sealed environment will accelerate their deterioration.
Transparency is an important feature of storage media as it allows objects to be monitored without having to open them. To adsorb degradation and other potentially damaging products in sealed containers, incorporate materials such as activated carbon, zeolites, potassium permanganate pellets, Artsorb® board, Microchamber™ papers and boards and buffered tissue paper in storage envelopes.
To minimise photochemical degradation, store objects in the dark or maintain light levels at 30 lux with a maximum UV exposure of 50 µwatts/lumen (1500 µW/m2).
Storage and Display
Regardless of the storage medium it is essential to regularly monitor the condition of plastic artefacts, especially when they are stored in closed environments. In these situations, if degradation continues, deterioration may be accelerated by the build-up of a harmful microenvironment in the closed system. The deterioration of cellulose acetate for example is usually accompanied by the release of acetic acid vapours. These would build up in a closed system and increase the rate of degradation of an already degrading object. Acid detection (A-D) strips, which change colour as the acidity of an environment changes, can be used in a closed system to detect the build-up of acid vapours. They can also be used to determine when a zeolite or activated charcoal absorbing material has lost its activity and needs refreshing. Consult a conservator to receive the most recent advice on the use of these and other monitoring methods.
Isolate deteriorating plastics from other materials to prevent damage to unaffected objects. Do not store deteriorating objects in a closed environment; instead, store them in open metal cupboards, protected from dust and light by muslin (or similar) curtains or in ventilated polypropylene containers. This arrangement will permit maximum ventilation. Use buffered acid-free paper to scavenge free acids and acid-free card to line the metal shelving.
Acid-free paper, silicone release paper, polyester wadding and polyethylene foams are suitable for wrapping or supporting most polymers.
Make replicas of particularly valuable objects and use these for display. Store the originals under the best conditions possible (cold, dark and with both low relative humidity and oxygen levels).
Support flexible objects, such as rubber-based materials, at all times. As rubber degrades and plasticisers are lost from some plastic objects, artefacts made from these materials may harden over time. Support these objects in their correct shape using polyethylene foam or tightly crumpled acid-free tissue paper.
Do not allow objects which contain a high proportion of plasticisers, such as those made from cellulose acetates and PVC, to come into contact with each other or with other plastics. This will avoid the possibility of migration of plasticiser from one object to another, a process which could lead to either the softening of objects or of their ‘welding’ together.
Special mention needs to be made of objects composed of cellulose nitrate, cellulose acetate, plasticised polyvinyl chloride, polyurethane foams and rubber as these polymers are particularly susceptible to deterioration.
Cellulose nitrate-based materials were produced extensively during the period 1860 to 1930 and include imitation tortoise shell and ivory objects, cutlery handles, toys, films and negatives. As cellulose nitrate objects degrade there is an increased risk of fire. In addition, the release of acidic or oxidising gases by degrading cellulose nitrate objects increases the risk of continued damage to the object itself and damage occurring to metal and other objects that may be stored in close proximity. Camphor was used as a plasticiser in some cellulose nitrate objects. Loss of the volatile camphor from the polymer is usually indicated by a characteristic smell, increased brittleness and cracking of the object’s surface (Figure 4).
Segregate cellulose nitrate objects from other objects in a collection in a well ventilated, fire-proof area. Do not circulate vented air to other areas of the collection. Obviously isolation from heat and ignition sources is also essential.
Store cellulose nitrate motion picture film in perforated cans to allow gases to vent and store negatives in buffered paper envelopes. If stored in boxes, add activated charcoal cloth, charcoal paper, Microchamber™ papers and boards or other zeolites to absorb damaging gases that are given off as the polymer degrades.
Copy degrading films and negatives to ensure that the information that they contain is not lost. As specialist equipment is needed to reformat these materials, consult a conservator.
Careful monitoring is critical in these cases. Stages of degradation include progressive yellowing, the formation of bubbles or foam, embrittlement and shrinkage. Store these artefacts at 2 – 5 °C, at a relative humidity between 20 and 30 % and at maximum light and UV levels of 50 lux and 75 microwatts/lumen respectively (Shashoua 2008). Dark storage is strongly recommended.
As the gases given off by degrading cellulose nitrate objects are damaging to health, take appropriate precautions when working with these materials. Work in a well ventilated area and wear protective clothing including nitrile gloves.
Cellulose acetate objects including films, combs, toys and other moulded goods were mainly produced from the late 1920s to the 1970s. Whereas films and fibres were made primarily from cellulose triacetate, three-dimensional objects were usually made from cellulose diacetate.
Many cellulose acetate objects contain plasticisers (often phthalates) which can migrate from the polymer structure and acetate groups that can react to produce acetic acid. Migration of plasticisers to the polymer surface can produce an acidic environment and warping of the plastic whereas hydrolysis of the acetate groups produces acetic acid and a characteristic vinegar-like smell. If cellulose acetate butyrate is present in the polymer, it will smell more like rancid butter as it degrades. The presence of bubbles or crystals (plasticiser) on the surface of an object or a mild vinegary smell is an indication that active degradation has commenced.
Acetic acid will break down the polymer chain and can also attack any susceptible objects like metals, textiles and paper that are in the same vicinity. It is important therefore to segregate cellulose acetate objects from other sensitive objects.
As well as storing cellulose acetate films and negatives in appropriate conditions, copy them so that their original information is preserved. If there is no option but to store cellulose acetate films in sealed metal or plastic cans, incorporate zeolite packages in the containers to absorb damaging moisture and acetic acid vapours (Shashoua 2008).
Store cellulose acetate objects in a ventilated area, at 2 – 5 °C, at a relative humidity between 20 and 30 % and at maximum light and UV levels of 50 lux and 75 microwatts/lumen respectively (Shashoua 2008). Again dark storage is also strongly recommended.
Plasticised polyvinyl chloride objects, including toys, waterproof clothing and electrical cabling were initially produced in the 1940s and are still made today. Pure PVC materials are hard and rigid, with plasticisers added to give them flexibility and softness.
Discolouration (yellowing and darkening) and warping are often the first signs of degradation. Other signs include cracking and an acidic and/or ‘plastic’ smell from the breakdown of the polyvinyl chloride or migration and loss of the phthalate plasticisers respectively. As a PVC object degrades, hydrochloric acid may be given off. This may be reflected in the corrosion of metal components of an object or brittleness of paper wrappings, signs that degradation has started.
The migration of plasticisers is a major problem as the PVC objects lose flexibility, shape and integrity and related degradation is accelerated. Phthalate plasticisers can form a ‘bloom’ on the surface of objects. This should not be removed unless absolutely necessary as it will merely encourage the migration of more plasticiser to the surface. Isolate objects from direct contact with other objects but exercise caution if wrapping is the only isolation alternative as there is a risk of damage to soft, deteriorating surfaces.
Store plasticised PVC objects in cool, dark conditions, enclosed in non-absorbent materials like glass and polyester envelopes. The use of non-absorbent materials minimises plasticiser loss from the polymer. Low density polyethylene is not a good storage medium because it readily absorbs plasticisers (Shashoua 2008). Ideally these objects should also be stored in an oxygen-free environment.
It may be necessary to support some PVC objects in their preferred shape as they age and become more brittle.
Polyurethane foams have been used in the production of toys, packaging, textiles (including fake leather) and cushioning from the 1940s to the present.
Polyether-based polyurethanes are susceptible to oxidative degradation, particularly in the presence of light, while polyester-based polyurethanes, although more resistant to oxidation, are more vulnerable to hydrolysis in moist conditions. Degradation is usually indicated by a pungent odour, discolouration and/or loss of mechanical properties. Degradation can be quite rapid because of the large surface area of these foams and can result in the complete crumbling of the whole object.
If possible separate deteriorating foam objects from others and unless opting for low oxygen storage, provide good ventilation.
Store polyurethane foam objects in cool, dark, low oxygen conditions and polyester-based polyurethanes in low relative humidity conditions (20 – 30 %, see the chapter Preventive Conservation: Agents of Decay). The addition of activated charcoal or zeolites to any sealed storage system will help to minimise the build up of potentially damaging degradation products.
Synthetic rubber products have been available from the late 1830s when sulphur was introduced to cross-link the natural rubber chains. The hardness and elasticity of the products was determined by the amount of incorporated sulphur, with the hard rubbers like ebonite and vulcanite containing up to 30 % sulphur. As rubbers were also prone to oxidation, stabilisers were often added to rubber products. The off-gassing of sulphur-based vapours and the migration of volatile stabilisers are major problems of degrading rubber.
Because of the different initial properties of assorted rubber objects, their deterioration may be indicated in number of different ways. They may discolour, become harder or softer, less elastic, more brittle, stickier or even develop a more powdery surface. Hydrogen sulphide gases are emitted and sulphuric acid may be deposited on the surfaces of degrading objects. Paper or plastics in contact with rubber objects may become stained as coloured stabilisers migrate from the rubber. Silver is particularly susceptible to tarnishing in the presence of rubber-based objects.
As rubber objects emit damaging gases, separate them from other objects, preferably in a well-ventilated area. If ventilation is not possible, seal them in enclosures with activated charcoal or zeolites.
As oxidation is the major degradation pathway for rubber objects, store them at low relative humidity levels, low temperatures and in low oxygen environments. Although low temperatures will result in an alteration of the crystallinity of the rubber, causing it to lose its elasticity, this change is reversible on warming (Smithsonian Institute). Low temperature storage is acceptable for rubber objects in either good or degraded condition. If rubber objects are sealed in low oxygen environments, make sure that acid scavengers (such as activated charcoal or zeolites) are also included with the oxygen scavengers to avoid the build-up of damaging microenvironments.
Regular monitoring of the condition of objects is essential.