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Conservation of Plastics Collections

By Peter Winsor and Stephen Ball

Plastics: Truth and Fiction

There is a widespread popular view that plastics are inert, everlasting materials invented in the post-war era and best suited for use in cheap or disposable objects. No museum can afford to adopt this view; it is almost entirely false.

Plastics are only suitable for cheap, disposable objects . . .

False - This is perhaps a dying prejudice, because durable and high-quality plastic goods are now familiar in most homes and workplaces. However, there is a huge volume of disposable plastic packaging in everyday use that serves to reinforce public concern about 'everlasting' plastic in the environment.

Plastics are a postwar invention . . .

False - The first semi-synthetic plastics appeared in the 1860s, and plastics made out of natural polymers have been used for centuries. Other plastic materials followed in the twentieth century, and some were in mass production well before the Second World War. Many early plastic objects are now showing clear signs of degradation, and all plastics represent a challenge to museum care and conservation practices.

Plastics are inert and last for ever . . .

False - Plastics are inherently unstable in the long term. The life of plastic objects in the museum can be prolonged only through conservation practices that are at least as well-planned and intensive as the care expended on any other class of vulnerable objects. Light, an enemy of so many museum objects, is especially harmful to plastics.

The conservation challenge

There are so many different types of plastics that a conservation strategy designed for one group of plastic objects may be harmful to others in the vicinity. Many plastics, particularly older types or those from early phases of production, also display great variability in manufacture and composition, which further complicates conservation planning.

The first step in a conservation programme is to identify the base polymer used to make the plastic object. Polymers are the long chain molecules that give all plastics their characteristic qualities. Some identification guidance appears below, but for all significant objects you should call in a conservator for both identification and conservation guidance.

Plastics in the Museum

Museums contain an ever-growing number of plastic objects, or objects with plastic components. For example, themed displays representing nineteenth and twentieth century life - an Edwardian shop or wartime interior, say - are likely to contain plastic objects, as are displays
including electrical and electronic items. Some of these plastic objects may be small items like bottle tops or combs, or inconspicuous or concealed components such as coil formers inside radio sets.

A second important group of plastic objects comprises archival materials from the industrial and post-industrial eras. Photographs and films, and audio, video and computer storage media from the first celluloid negatives to contemporary CDs use plastic materials as a base. (See the separate fact sheets on Photographs and Film and on Magnetic Media.)

An understanding of plastic materials is also important for the safe storage of all classes of museum objects. Plastic boxes and sleeves are commonplace in stores, and shelves and racking are frequently faced with plastic coatings or laminates. It is important to choose the right plastic materials for museum storage; unsuitable plastics will eventually damage the objects they are supposed to protect, and some may even create harmful conditions for other objects stored in the general vicinity. (Note: rubbers, laminates, adhesives and foams are outside the scope of this fact sheet.)

Storage and Display

The dangers

Plastics are composed of natural, modified natural, or completely synthetic polymers - long-chain molecules that determine the properties of the resulting materials. Various additives are added to the base polymer to produce other desirable qualities such as colour, bulk or improved longevity. There are two main types of plastics: the relatively soft and pliable thermoplastics can soften and flow when reheated, whereas the normally harder and more brittle thermosetting plastics do not.

Mechanical or thermal stress can cause physical damage to a plastic material, and latent stresses can be induced at manufacture and released at a later date. The result may be shrinkage, crazing or the appearance of surface deposits, sometimes because of the loss or migration of additives. More serious are chemical changes, usually caused by excess light and other unsuitable environmental conditions, poor manufacturing, or harmful materials or contaminants such as cleaning solvents. Symptoms include colour change, brittleness, weakness, softness, surface 'blooms', and the release of gaseous breakdown products.

Combinations of materials (such as metals in close contact) and circumstances can accelerate chemical degradation. The harmful effects of ultraviolet radiation in light are substantially increased under high moisture or heat levels, for example. The removal of harmful materials and conditions does not always stop the degradation of the plastic, creating problems for conservators.

The remedies

As for all museum objects, plastic items will benefit from stable temperature and relative humidity conditions, but light of all wavelengths should be kept at the lowest possible level consistent with staff and public safety and access. Ultraviolet (UV) light is most damaging, and may be kept in check using standard UV filters and screens; these need regular checking and periodic replacement.

Keep strong cleaning materials and solvents (including their vapours) well away from stored or displayed plastic items. Some solvents can be used to clean certain types of plastic, but only if you can identify the base polymer - white spirit can damage polystyrene, for example. It may be possible to wrap metal components with PTFE tape to separate them from plastic surfaces - take advice from a conservator.

Inspect all plastic objects on a regular basis for signs of degradation, perhaps as part of the routine cleaning programme. Isolate any suspect item and refer it to a specialist conservator as soon as possible.

Cleaning and Handling

Few plastics patinate well or otherwise improve with constant handling, so valuable or significant plastic objects are not suitable for 'touch me' displays. Dust and dirt should be removed with a soft brush or cloth where possible. Avoid rubbing or scrubbing. More persistent staining can be removed by washing the object in warm water containing a few drops of non-ionic detergent (read the detergent bottle label).

Severe soiling can be removed with white spirit or isopropyl alcohol, but only where the polymer has been identified as safe for this treatment. Use a soft cloth moistened with a minimal amount of solvent, and once again do not rub or stress the object.

Polishing rarely helps, though a light application of a non-spray hard wax polish may enhance the appearance of Bakelite surfaces. There are proprietary polishes designed to remove scratches from acrylics such as Perspex (see below). Some collectors also use these on cellulosic plastics such as Celluloid.

Remember that scratch-removing and colour-restoring polishes are abrasive, as are many everyday polishes such as domestic 'cream cleaners'. If you are in any doubt at all, don't polish a plastic. However, you may decide that surface quality or colour must be restored before an object is worthy of display, or where an object is seriously affected by persistent stains, adhesive residues or similar. If so, consult a conservator before embarking on any cleaning treatment.

How to Identify the Main Groups of Plastics

The following list of identification points is necessarily brief. It provides only very general guidelines on the main groups of polymers and any related conservation issues. Some proprietary names are listed because these are often more familiar than the polymer names. Where odour is listed as a useful distinguishing guide, this can be tested in small objects by first washing and then warming them in cupped hands to retain the sometimes fugitive smell. Keeping the object in a sealed jar for a few hours may also help. The so-called 'hot pin' test carries the odour test a stage further by heating a pinpoint area to the point of decomposition. This is a risky test, and may set off a more general degradation. It is safer to use emery paper on an inconspicuous part of the object to collect a few grains of material for heating in a test tube. Litmus paper held in the tube mouth will show whether the fumes are acid or alkaline. Wait until the heat is removed and the fumes have subsided before odour testing. For more information on the tests and polymer groups, see The Conservation of Plastics, cited below.

Natural polymers

The earliest plastics were made out of naturally occurring polymers such as animal horn and latex. Powdered horn was moulded into buttons and other small or personal objects. Gutta percha and rubber products are latex derivatives; few of the former are likely to have survived except in the form of early golf balls. Hard vulcanised rubber mouldings were made into fountain pen barrels, combs, boxes and other similar objects, and many of these survive in good condition.

Shellac, an insect secretion, was used with wood and other fillers from the 1850s to form moulded objects that include the cases for Daguerreotypes and Ambrotypes. With a slate filler, shellac formed the basis of the 78 rpm gramophone records of the pre-vinyl era.

Proprietary names: Vulcanite and Ebonite (hard vulcanised rubber).

Identification: Rubber has a sulphurous odour, and shellac that of sealing wax.

Environmental guidelines: Horn: 60% RH, not above 20°C; vulcanised rubber: not above RH 50% or 20°C for extended periods; shellac plastic: 60% RH and up to 18°C, softens over 30°C.

Celluloid and other semi-synthetics

More details of the particular problems of cellulose-based plastics appear in the fact sheet on Photographs and Film, though materials for plastics manufacture were generally more stable than those used to make photographic film. The main plastics in this group appeared from the 1850s onwards as cellulose nitrate and, later, as the similar but slightly more stable cellulose acetate.

Towards the end of the nineteenth century, casein plastics based on milk proteins started to appear in the form of clothing fasteners, decorative objects, and artificial tortoiseshell etc.

Proprietary and alternative names: Parkesine, Xylonite (UK) and Celluloid (USA) for cellulose nitrate; 'CA' and 'acetate' for cellulose acetate; Galalith and Erinoid for casein plastics.

Identification: Heat tests produce odour of camphor (cellulose nitrate), vinegar (cellulose acetate) and burnt milk (casein). Warning: cellulose nitrate burns violently.

Environmental guidelines: Cellulose nitrate: not above 40% RH or 18°C, and keep well ventilated (consult a conservator); cellulose acetate: cool and dry as for cellulose nitrate, but in less well ventilated conditions; casein plastics: treat as wood, prevent drying out, and avoid excessive handling.

Bakelite and the early synthetics

This group of thermosetting plastics is based on formaldehyde. The first synthetics employed phenolic formaldehyde resins in cold moulding processes, but the most familiar form is Bakelite, a hot-moulded plastic with added wood dust as its most common filler. Bakelite was used for a wide variety of household and electrical items, and many objects survive in good condition. Comparisons of polished with unpolished items suggest that hard wax-based polishes have some conservation value, preserving surface finish and offering some degree of UV protection. The aminoplastics were a later group of formaldehyde-based synthetics developed in the 1920s and 1930s. The best known
of these is Melamine (melamine formaldehyde), whose main uses are plastic surface laminates and 'unbreakable' crockery.

Proprietary and alternative names. Bakelite and Catalin (phenol formaldehydes); Melamine (melamine formaldehyde).

Identification: Bakelite is often instantly recognisable because of its dark, frequently brown, colour and polished 'walnut' appearance, and Melamine through its limited but well-known uses.

Environmental guidelines: The early synthetics are reasonably stable materials. High light exposure is still a risk, however. Avoid long periods of high humidity. Boiling water may discolour melamine, as many households can testify.

Thermoplastics: modern 'poly' plastics

The second half of the twentieth century saw many new thermoplastic materials come into use in a wide variety of applications. It is these materials that are the basis of our present-day 'plastics' era. There are too many 'poly' plastics to describe individually in this Fact Sheet, but among the most important are:

  • polyvinyl chloride (PVC)
  • polystyrene (including high impact grades and ABS)
  • polymethyl methacrylate (acrylic)
  • polyethylene (polythene)
  • polypropylene
  • polyamide (nylon)
  • polyesters
  • polycarbonate

Polystyrene production increased dramatically in the postwar years. So-called high-impact grades included a rubber additive to counter brittleness, and uses ranged from household items to model construction kits, but early styrene plastics have been supplanted by ABS compounds. Keep polystyrene objects away from strong light, which will rapidly degrade them, and avoid contact with alcohols, paraffins, white spirit and fats. These and other solvents can accelerate environmental stress cracking.

Familiar acrylics such as Perspex find applications wherever strong and transparent or translucent items are required. Acrylics are stable and show good resistance to moisture and light, though coloured acrylics may fade in strong light. The polishes designed to remove scratches from acrylics are abrasive, so cannot be recommended on strict conservation grounds.

Polythene (1930s) and the later and similar polypropylene (1950s) are soft and relatively flexible low-density plastics. Their good resistance to water makes these polymers especially suitable for water pipes, household objects, food storage etc. Light, especially the UV component, is a potential danger because it can induce photo-oxidation, an accelerating form of degradation that first appears as brown discoloration.

The various nylons are also prone to photo-oxidation, which is likely to be the biggest single risk factor and is largely responsible for the familiar yellowing of white nylon fabrics and net curtains. Protect these oxidation-prone materials from light, especially UV, and keep RH levels stable for nylon.

Polycarbonate is an extremely strong material that should remain in good condition, at least indoors, if light and humidity levels are stable. Most polyesters will remain sound under similar conditions.

Beware of PVC

PVC was among the first of the 'poly' plastics, and is perhaps the least stable. Stabilizers and other additives must be incorporated to make it usable for more than a few years, though the ubiquitous UPVC window frames and doors show that modern forms can survive outdoors.

PVC is unsuitable for museum storage materials such as sleeves and boxes. PVC objects should be kept below 20°C, away from other plastics and from metals such as iron, zinc and copper, and fully protected from sources of UV light. Inspect PVC items regularly for signs of degradation, which should show up as a brownish or purplish tinge. A further risk to other museum objects is the acidic gas (hydrogen chloride) given off by degrading PVC.

Choosing Plastic Storage Materials

If PVC is not a suitable material for films and boxes, how should museums decide which of the many other available plastic materials are suitable for safe storage?

There are many complications, partly because new materials are always under development and different plastics are being combined into laminates and compounds, and partly because no single plastic material can meet every museum need.

In general, however, the most stable plastics - hence the ones most suitable for museum use - are those composed of only carbon, hydrogen and oxygen. This means polyester, polyethylene and polypropylene are the best choices. The more reactive plastics, such as PVC, contain other elements - especially chlorine, fluorine or sulphur - and should not be used for museum storage.

Acknowledgements

This fact sheet draws extensively on information in John Morgan, Conservation of Plastics: An Introduction (Museums & Galleries Commission/Plastics Historical Society, 1991), which should be consulted for fuller details.

References and Sources of Information

Plastics Historical Society
2 Park Avenue
Radlett
Herts WD7 7EA
Tel: 01923 854588

The Society publishes a journal, Plastiquarian.

For more on plastic storage materials and their qualities, see:

R. S. Williams, A. T. Brooks, S. L. Williams and R. L. Hinrichs, Guide to the Identification of Common Clear Plastic Films (SPNHC Leaflet No. 3, Fall 1998).

For more information about private conservation work please contact:

Conservation Register
Tel: 020 7721 8246

Conservation Register (Scotland)
Tel: 0131 668 8668

Copies of this fact sheet can be provided in alternative formats. Please contact Viola Lewis, Information Officer at MLA for further information.

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