The Conservation of Plastics
How a plastic object is looked after (combined with its chemical make-up about which nothing can be done) is the factor that has the most impact on its life expectancy. It is unfortunate that objects may have had adverse experiences that are not visible at the time they enter your care but may impact on their future. But good care will slow down most forms of degradation.
Plastics differ from each other in their care needs. The exact recipe of each plastic, including its range of additives, influences how it will age. Even the pigment used to colour an otherwise identical object can cause objects to age differently. That said, most plastics are relatively stable if looked after appropriately but degradation, when it does occur, is irreversible making good care in the first place vital.
There are four plastics that are especially problematic. These are cellulose acetate, cellulose nitrate, polyvinyl chloride and polyurethane. Objects made of these materials should be identified and managed separately, according to their special needs.
Gloves of some inert material, such as latex, should always be worn. Cotton gloves are not recommended as they may leave specks of lint on plastics that have become tacky. Otherwise good practice is the same as for other objects: always use both hands and hold the object in a manner that puts as little strain on any part of it as possible. Do not hold objects by their handles.
Barrier coatings as often applied to objects and adhesive tapes are not appropriate for plastics as they may react adversely with the surface. Rubber bands should also be avoided. The options are:
- labels tied on with cotton tape or
- writing directly on the plastic surface with a soft pencil, ideally inside the object.
Which method is the most appropriate will depend on the characteristics of the object.
Environmental conditions impact dramatically on the life expectancy of plastic objects. Appropriate environmental conditions are therefore vital. What follows is good practice for the majority of plastics.
Plastics should be kept in a dark, cool, dry room. Whether in store or on display the temperature should be restricted to 20 degrees centigrade and relative humidity (RH) to 30 to 50%. Sharp fluctuations of heat and RH are especially damaging.
UV should be filtered out from any light to which plastics are exposed. However even UV-filtered light is bad for plastics so when stored plastics must be kept in the dark and when on display light should be limited. Plastic objects should not be on permanent display. The damage is cumulative and dependent on the overall amount of light whether a short blast of very bright light or a very low light for a long time. It is for each curator/conservator to decide what is appropriate for any particular object at any particular time. Recommended good practice varies from a maximum of 50 to 150 lux.
Storage and display guidelines
Objects should be stored and displayed in and on inert materials, in such a way as to minimise handling. Polypropylene is a good material to use as trays on which to store objects. Avoid especially materials that could off-gas organic vapours, such as painted materials, wood and MDF. Plinths should be left for at least 72 hours for paint to dry completely before covering. Plasticisers are drawn out by contact with absorbent materials so they too should be avoided.
Objects should never touch each other and air should be able to flow freely around them. Ideally the space should be dust free, but it must not be airtight. Ideally each type of plastic should be stored separately.
Try to store objects supported, as you would wish them to be on display. Should degradation take place this will enable them to be displayed without leading to further degradation as they are opened, unfolded or otherwise handled in order to make them appropriate for display. This is especially important for objects made of polyvinyl chloride and polyurethane foam.
The onset of degradation is unpredictable and rapid. It can manifest itself in an advanced state apparently almost overnight. It is irreversible and in most cases, once started, unstoppable. The best that can be achieved is to slow down the process.
Degradation products from objects (e.g. acidic vapours) can contaminate other objects in the vicinity. Collections should be checked regularly, ideally at least once a year, and any object showing signs of degradation should be separated from the rest of the collection.
Causes and effects of degradation:
- light, leading to darkening, loss of flexibility and embrittlement of the plastic and fading of pigments.
- excessive humidity can lead to chemical breakdown of certain plastics.
- fluctuating temperature and humidity, leading to shrinkage and expansion, which in turn result in crazing and cracks.
- migration and loss of plasticizers, leading to surface bloom and /or surface tackiness and then to loss of flexibility and embrittlement.
- pollutants and exhaustion of stabilisers leading to chemical break down of the material’s structure and, ultimately, collapse
- bad handling, leading to chips, cracks and breaks.
Early signs of degradation can include
- bloom, a white powder of the surface.
- corrosion of metal parts or surrounding objects.
- crazing and cracking.
- discoloured or even shredded packaging materials.
- distortion of the shape of the object.
- smells: mothballs (camphor), sweetness vinegar, vomit, and rancid butter.
- Surface stickiness.
- Haze, wet acidic deposit on the surface.
If you think an object may be degrading wrap it in charcoal cloth and store it away from other objects.
What signs of deterioration can you see?
The following signs of deterioration are associated with the materials listed:
This takes the form of a white powder that can be wiped off or a pale mistiness. Cellulose acetate Cellulose nitrate Polyvinyl chloride
Cracks and splits
Casein formaldehyde Cellulose nitrate Phenol formaldehyde Polycarbonate Polystyrene Polyvinyl chloride Shellac Urea formaldehyde
Casein formaldehyde (surface crazing) Cellulose nitrate (internal crazing) Gutta percha (network of small cracks on surface) Polymethyl methacrylate Polystyrene Urea formaldehyde (an orange peel effect)
Gutta percha Polyurethane foam
Fading and discolouration
Pigments can fade independently, leading to complete changes of colour. Phenol formaldehyde, also dulls Polyamide, tendency to yellow Polymethyl methacrylate, sometimes discolours in light Polyvinyl chloride: yellows and goes brown Polyurethane: yellows Urea formaldehyde, also dulls Vulcanite, often has a yellowish brown tinge
Physical distortion, warping
The most likely plastics to suffer chemical degradation are cellulose acetate, cellulose nitrate, polyvinyl chloride and polyurethane. Try to identify objects made of these materials before deterioration is visible and store them separately according to the guidelines given below. If degradation has begun you cannot reverse it or stop it. If however you move it into storage as outlined below you will slow down its progress.
Deterioration: Moisture causes the loss of acetate groups and the subsequent production of acetic acid. The presence of acetic acid accelerates the process of deterioration. As this happens a smell of vinegar is given off. Plasticisers can also migrate to the surface leaving a white powdery deposit and resulting in shrinkage which itself often causes distortion and further stress. As degradation proceeds, crazing and cracking may occur. The acetic acid fumes from deterioration corrode metals.
Storage guidelines: Temperature ideally 2- 5 centigrade. RH ideally 20 to 30 %. Do not wrap. Keep away from absorbent materials. Isolate from metals and other materials if possible. Use air filtration or vapour scavengers. Use indicators.
Deterioration:Light and moisture cause the loss of nitrate as nitrogen oxides. Water and oxygen then turn this into acids and that accelerates the process of deterioration making the object brittle and prone to crazing and cracking, as well as forming sticky droplets on the surface. The emanations from deterioration corrode metals.
Storage guidelines: Temperature 2- 5 centigrade. RH 20 to 30 %. Do not wrap. Keep away from absorbent materials. Isolate from metals and other materials if possible. Use air filtration or vapour scavengers. Good ventilation desirable. Use indicators (for more information got to Useful material and products).
Deterioration: Light causes yellowing and darkening and can lead to the giving off of hydrochloric acid. Oxygen is also harmful. Plasticisers have a tendency to migrate to the surface causing bloom and tackiness that attracts dirt. The weeping is accompanied by a sweet smell. The loss of plasticiser causes the plastic to shrink and thus to warp and also to become more rigid.
Storage guidelines: Temperature 5 degrees centigrade. RH 20 to 30%. Enclose in non-absorbent material such as glass or polyester bags to prevent loss of plasticiser Ideally oxygen free, using products such as oxygen scavengers. Do not wrap. Keep away from absorbent materials. Store with future display requirements in mind.
Deterioration: Oxidation causes discolouration and loss of strength. The result can be catastrophic loss of structure leading to collapse.
Storage guidelines: Temperature 20 degrees centigrade RH at the low end of 20 – 30%. Ideally oxygen free, using products such as oxygen scavengers. Store with future display requirements in mind.
Specialist subject areas
The key to the care of all plastics is identification of the specific material and then looking after it appropriately. Plastics that are essentially the same can manifest themselves in unexpectedly different ways. Under the subject areas that follow are a few pointers to get you started.
- Architectural drawings and other archival material
- Composite objects
- Film and photography
- Packaging and containers
- Textiles and fashion
Architectural drawings and other archival material Translucent synthetic supports made from cellulose acetate and cellulose nitrate film were introduced in the 1940s and became wide spread in the 1950s. Coated with light-sensitive compounds they were also used to make photo-reproductions. The care of such film is the same as that for other objects made of these materials.
There was, during the 1960s, a fashion for protecting architectural drawings and other large paper objects with a cellulose butyrate or cellulose acetate butyrate varnish. Overtime, this has had a tendency to darken and obscure the image. A by-product of the degradation process, which will help you recognise it, is the smell of vomit caused by the emission of acidic gases. The varnish may also have become acidic itself and thus be weakening the paper. The environmental requirements for such documents are the same as those for objects made of cellulose nitrate and acetate. A treatment strategy depends on the composition of the paper, drawing process, coating and degree of deterioration. It might be possible to remove the coating in a solvent bath but again this will depend on the degree of deterioration of the coating and the composition of any inks that are present because these could be removed. It might also be the case that removing the coating only removes one source of deterioration and thus, it may not actually be worth putting the object through such intensive treatment. Only a professional conservator should attempt this and only after some deliberation.
Polyester film as a draughtsman’s support and as means of reproducing drawings was introduced in 1955. Polyester is an inert material good also for encapsulating drawings or as use as a barrier to prevent cross-contamination between drawings. For more information see Useful materials and products below. Polyester film requires the same care as outlined for all but problem plastics.
Composite objects These are objects made from more than one material. Ideally plastics should be kept separately from other materials to prevent cross-contamination. However greater damage can result to an object by taking it apart. The care of composite objects is therefore likely to be a compromise between maintaining the integrity of the object and looking after its different components. Particular attention should be paid to plastic and metal objects as each can cause the other to degrade. They are the priority for segregation.
Film and film-based photographic negatives All film (motion and still) was made of cellulose nitrate until1923 when cellulose acetate was introduced. In 1937 this was replaced by cellulose diacetate, which was in turn replaced in 1947 by cellulose triacetate. However cellulose nitrate film continued to be manufactured into the early 1950s.
Life expectancy is affected by precise composition at manufacture and storage conditions since. Nitrate and acetate have long been recognised as problematic and relatively recently it has been realised that the diacetate and triacetate adaptations are too. All benefit from even more stringent conditions than those recommended in Problem materials. Temperatures of at or below 0 degrees centigrade in a moisture free environment will extend their life in good condition by factors of ten or more. For more information refer to Canadian Council of Archives, Basic conservation of archival materials, 2003, chap 6, pp. 59-60 available at www.cdncouncilarchives.ca/RB and the Cellulose Acetate Project: www.nla.gov.au/anica/natstratnew.html.
Cellulose nitrate film is extremely flammable and once on fire very difficult to quench.
Storage or transport of such film is extremely risky and the best course of action is to have it transferred by a licensed laboratory to safety film. For more information on the associated hazards, recommended actions and useful contacts with regards to cellulose nitrate please go to www.hse.gov.uk/pubns/cellulose.pdf.
If you wish to keep the packaging of plastic goods or plastic packaging of non-plastic goods the same guidelines should be followed as is given for the materials concerned. Those most often encountered in this use are:
|Polyethylene terephthalate||Fizzy drinks and water bottles|
|Polypropylene||Petrol cans; microwaveable meal trays; margarine tubs|
|Polystyrene||CD cases; yogurt pots|
|Polystyrene foam||Food trays, hamburger and egg boxes; protective packaging especially for electronic goods|
|Polythene (high density)||Milk and washing-up liquid bottles|
|Polythene (low density)||Carrier bags|
|Polyvinyl chloride||Sandwich boxes; blood bags|
Contents of containers, for example juice, shampoo, sweets or food should be removed unless, of course, it is the contents rather than the container you are collecting. Ideally
non-liquid contents should also be stored separately but whether this is feasible will depend on the availability of storage space.
Textiles and fashion
The plastics most commonly used in the manufacture of fabrics are polyamide, polyester, polypropylene, polyvinyl chloride and polyurethane. Trade names are Crimplene, Lycra, Nylon, Rayon, Terylene, and Viscose.
The best way to store semi-synthetic and synthetic materials is dependent on the size of the garment or textile. Large objects should be rolled to avoid creases. Normal size garments may be stored on hangers with proper support for the shoulders, inside Tyvek™ covers. Bear in mind that it is a good idea to remove as many separate materials for examples belts as you can and to isolate others such as buttons and foam shoulder pads by placing a barrier between them and the fabric (Melinex™ can be used for this purpose) to prevent cross-contamination.
Some semi-synthetic or synthetic fabrics or garments can be washed, depending on the composition of the fibres and the finish of the textile. Conservation advice should be sort before washing such materials.
Cleaning tends to cause both chemical and mechanical damage so keep a balance between the risk of damage and your wish for the object to look pristine.
The best way to clean plastic objects is with cotton swabs and lint-free cloths, ideally of microfibre, a mix of polyester and polyamide. If more in depth cleaning is essential dampened but never wet cloth using deionised water can be used but the dampness should be kept to a minimum and make sure that the object is completely dry after treatment. Water is especially bad for casein formaldehyde, cellulose acetate and cellulose nitrate. Never immerse a plastic object in water. Do not use solvents: severe damage that could ensue may not show immediately.
Once an object needs treatment for anything other than mechanical damage it is likely to be too late. The best conservation treatment for plastics is preventive conservation as outlined in Environmental requirements and Storage and display. Storing plastics at low temperatures and relative humidities and keeping them away from harmful substances and vapours will however slow down the rate of harmful reactions.
There are no standard interventive processes for plastic materials. Interventive treatments present risks of further damage to objects due to potential reaction between treatment and object. More damage than good can be done by interventive conservation of plastics. Do not consider mending plastics or doing other interventive work without the advice of a specialist conservator. Suitable conservators can be found at www.conervationregister.com.
Useful materials and products
Materials that are safe to use in direct contact with all plastics are:
- acid free paper for wrapping.
- acrylic (polymethyl methacrylate)is an acceptable material to use for display stands.
- charcoal cloth for wrapping objects that have the potential to offgas acidic fumes, e.g. cellulose acetate and cellulose nitrate objects
- microfibre, a blend of polyester and polyamide, useful as cloth for cleaning.
- Tyvek™, a form of polythene, useful for protecting objects from dust.
- Plastazote™, a form of polythene foam, useful for securing objects within storage spaces. It is easy to cut and carve to shape.
- Melinex™, a form of polyethylene terephthalate, an inert material to lay over degraded objects to protect them from dust and to put between them to discourage cross contamination. Also good as a buffer on painted surfaces.
- Polyester wadding useful for providing padding for example on hangers for costumes.
- polypropylene, appropriate for trays to hold objects.
- silicon release paper, useful for objects with potentially tacky surfaces.
Products that help you maintain a good environment are:
- Ageless oxygen scavengers for an oxygen free environment. This is suitable for preventing the crumbling of polyurethane foams.
- Silica gel, as a buffering agent moderating the effects of change in relative humidity.
- Scavengers such as charcoal cloth and molecular sieves to remove polluting vapours.
Products that help you detect trouble are indicator strips and chemical – impregnated string which change colour in the presence of acidic gasses.