Plastics degradation is potentially a huge problem for any museum that has these materials in its collection. Most objects have had a past life in unrecorded environmental conditions that will have had an impact on the object. Some objects acquired from new still show signs of degradation over time, possibly due to additives within the material.
The particular plastics materials most likely to degrade are cellulose acetate, cellulose nitrate, polyvinyl chloride and polyurethane, especially polyurethane foam, as follows:
Cellulose acetate
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.
Cellulose nitrate
Light and moisture cause the loss of nitrate as nitrogen oxides. Water and oxygen then turn this into acids, accelerating the process of deterioration and 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.
Polyvinyl chloride
Light causes yellowing and darkening and can lead to the giving off of hydrochloric acid. Oxygen is also problematic. Plasticisers have a tendency to migrate to the surface causing bloom and tackiness which 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 become more rigid.
Polyurethane foam
Oxidation causes discolouration and loss of strength. The result can be catastrophic loss of structure, leading to collapse.
Preserving and storing
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°C and relative humidity at 30% to 50%. Sharp fluctuations of heat and humidity are especially damaging.
Ultraviolet 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 or 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 (a measure of light intensity).
The degradation products from objects (for example 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.
Objects should be stored and displayed in and on inert materials, and in such a way as to minimise handling. Polypropylene is a good material to use as trays on which to store objects.
Avoid materials that could produce 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 these should also 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.
MoDiP offers free advice and facilitates loans of its Identifying Plastics Toolkit to help museum professionals learn to identify plastics materials.
MoDiP is undertaking an ACE-funded project, Confronting Plastics Preservation, through the plastics SSN. This aims to build workforce skills and share excellence and best practice in collections knowledge relating to the preservation of plastics objects across the museum sector.
The particular plastics materials most likely to degrade are cellulose acetate, cellulose nitrate, polyvinyl chloride and polyurethane, especially polyurethane foam, as follows:
Cellulose acetate
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.
Cellulose nitrate
Light and moisture cause the loss of nitrate as nitrogen oxides. Water and oxygen then turn this into acids, accelerating the process of deterioration and 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.
Polyvinyl chloride
Light causes yellowing and darkening and can lead to the giving off of hydrochloric acid. Oxygen is also problematic. Plasticisers have a tendency to migrate to the surface causing bloom and tackiness which 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 become more rigid.
Polyurethane foam
Oxidation causes discolouration and loss of strength. The result can be catastrophic loss of structure, leading to collapse.
Preserving and storing
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°C and relative humidity at 30% to 50%. Sharp fluctuations of heat and humidity are especially damaging.
Ultraviolet 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 or 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 (a measure of light intensity).
The degradation products from objects (for example 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.
Objects should be stored and displayed in and on inert materials, and in such a way as to minimise handling. Polypropylene is a good material to use as trays on which to store objects.
Avoid materials that could produce 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 these should also 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.
MoDiP offers free advice and facilitates loans of its Identifying Plastics Toolkit to help museum professionals learn to identify plastics materials.
MoDiP is undertaking an ACE-funded project, Confronting Plastics Preservation, through the plastics SSN. This aims to build workforce skills and share excellence and best practice in collections knowledge relating to the preservation of plastics objects across the museum sector.
