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Preventive conservation of plastics
Plastic is a relatively young material and its preservation in museums still has its challenges. It is well known that some of the early plastics from the end of the 19th and beginning of the 20th century are unstable. Some younger plastics also turn out to be less indestructible than originally thought. Modern plastics are sometimes designed to degrade rapidly, for instance biologically degradable disposables.
The ageing and degradation of plastic objects depends on several internal and external factors, such as composition, production, construction, environment and handling
Internal factors
Plastics of the same type may have the same polymer at its basis, but the addition of plasticisers, anti-oxidants, pigments and fillers, changes their properties and degradation processes. Like all organic materials the base polymer ages by chemical reactions with the environment, for instance with oxygen (oxidation) and moisture (hydrolysis). Physical processes such as diffusion of components from the object and deformation by temperature or pressure also play an important role. When the plastic is compressed into a mould to make an object, stress develops in certain areas which increases sensitivity to chemical and physical degradation.
Rubber is sensitive to oxidation; first it becomes tacky, then hard and brittle and it cracks. Soft PVC contains plasticiser which migrates to the surface making it tacky and stiffer. Plasticisers can condense on other objects and cause damage. Ink on paper may dissolve in plasticiser from PVC sleeves, causing the letters to stick to the sleeve. Upon degradation of the early plastics CN and CA acids are released which accelerate further degradation. This so-called auto-catalytic process also forms a risk for objects in the vicinity.
Because artists often experiment with materials, plastics may be processed and treated in unusual ways, for example by applying paint, heat, fire or tension. Components may be mixed in ratios other than recommended which changes properties and degradation processes. These internal factors are difficult to control. Usually the only way to slow down degradation is to lower the temperature in order to reduce the rate of chemical reactions.
External factors
Ultraviolet radiation contains enough energy to break polymer chains which subsequently can cross-link; processes that often cause yellowing of plastics. Light (visible radiation) has less energy but can still cause fading of dyes and pigments. Oxygen in the air can react with plastics, in the presence of ozone these oxidation reaction go even faster. Dust on a tacky plastic surface is hard to remove and can interfere with the visual experience of the object. Handling, vibration, and shock during transport or wear and tear during use, can cause cracks, tears and loss of material in susceptible objects. Suitable preventive conservation can slow down degradation by these external factors.
Twelve commandments carved in plastic
It is impossible to determine the precise composition of a plastic with the PIT, but in most cases identification of the base polymer provides sufficient information to take preventive measures and slow down degradation. As a starting point, there are a number of general guidelines to support preservation of plastics in museum collections.
1. Avoid exposure to UV-radiation. Indoors this can be achieved by choosing no/low-UV light sources and applying UV filters to windows, display cases, or frames. Outdoors a coating with UV-absorber can provide protection.
>> see online publication ‘Agent of Deterioration: Light, Ultraviolet and Infrared’
2. Store plastics in the dark. Adjust exhibition light levels to the light sensitivity of the object and the period in which a particular colour change is acceptable.
3. Keep the temperature below the point at which it softens.
>> see online publication ‘Caring for plastics and rubbers, chapter Temperature and physical properties’
4. The lower the temperature, the slower the chemical reaction.
Keep the temperature as low as practically possible but avoid it dropping below the glass transition temperature (Tg) when the object needs to be handled (see the information page for glass transition temperature of each plastics). At that temperature flexible plastics become stiffer and more brittle which makes them more susceptible to breaking on handling. In storage the temperature can be kept somewhat lower, for example by heating less in winter. Ensure that the RH does not get too high in that case (see point 7). In exhibition the temperature is usually higher for human comfort. Keep objects away from heat sources such as radiators, incandescent lamps and warm equipment.
5. Store plastics that undergo auto-catalytic degradation (CA, CN) as cool (refrigerator 5-7°C) or cold (freezer, ca. -20°C) as possible, lowering the temperature is the only way to slow down this degradation process.
6. Store plastics that are highly susceptible to oxidation (natural rubber, latex) in a low-oxygen atmosphere if possible.
>> download brochure
7. For most plastics a relative humidity (RH) with small or slow fluctuations in a bandwidth between 40-60% is acceptable. At an RH above 70% mould growth may occur. Plastics that are sensitive to hydrolysis are better kept at a lower RH between 30-45%. (See Shashoua, Y., 2008, Conservation of Plastics). Hygroscopic plastics that easily absorb and desorb moisture, resulting in swelling and shrinking, should be kept at constant RH to avoid crazing and cracking.
>> see publication: ‘Conservation of Plastics: Materials Science, Degradation and Preservation’
8. Keep plastics that emit harmful vapours in a well ventilated area. Do not store plastics that undergo auto-catalytic degradation (CA, CN) in sealed packing.
9. Support objects properly to avoid deformation under pressure.
10. Handle plastics objects with gloves or clean hands to avoid leaving behind grease and dirt. Conversely, gloves provide protection against harmful components on the object.
11. Select materials for packing and construction of storage and display areas that do not emit harmful compounds. (See Tétreault: CCI Technical Bulletin 32 - Products Used in Preventive Conservation)
>> see online publication ‘Products Used in Preventive Conservation’
12. Monitor the environment and the objects to verify that the preventive measures perform as desired.
Problem plastics
These guidelines for preventive conservation apply to most plastics. However, there are five ‘problem plastics’ that require extra attention and a close watch.
Cellulose acetate (CA) and cellulose nitrate (CN)
Keep CA and CN dark, cool and dry and do not seal them in their packing. Isolate the object and ensure ventilation to prevent the emitted acids affecting other objects in the area. If these plastics emit an acidic smell or when droplets acid appear on their surface they are actively degrading (vinegar syndrome). Use pH-paper or A-D strips to test if acid is released.
>> download publication ‘User'’s guide for AD Strips’
Natural rubber or latex (NR)
NR or latex is highly sensitive to oxidation. Store under anoxic or hypoxic conditions if the type of object and other materials allow.
Polyurethane ether and ester - especially soft foams PUR)
PUR ether foam is sensitive to oxidation. Store in the dark and do not expose unnecessarily to light and UV-radiation. Degraded PUR ether foam can be consolidated with a special treatment, but prevention is better than cure. (See literature: van Oosten, T., PUR facts, 2011)
>> see website ‘POPART’
PUR ester foam is sensitive to hydrolysis and is best kept at low RH, if necessary packed with silica gel or another moisture buffer.
Plasticised polyvinyl chloride (PVC-P)
PVC-P is best kept in a closed environment, for example sealed in polyester film or in a glass container, to avoid complete loss of the plasticiser. Make sure to avoid contact between packing and object.
Susceptibility and recommendations for preventive conservation
The table ‘Susceptibility’ (tabel ‘Gevoeligheid’ ) gives an overview of all plastics and their susceptibility for different external factors. A green cell means that the plastic has a low susceptibility for that factor, yellow is average and red indicates a high susceptibility.
The table distinguishes between rigid plastics and foams since foams often have a higher susceptibility due to their open structure. When specific conditions apply, they are mentioned in the table. In all other instances the colours correspond with those of the general recommendations in the table ‘Recommendations for preventive conservation’.
How to deal with composite objects
Artworks are often composed of different materials which makes it hard to determine the appropriate conditions. Each object is unique and has its own characteristics. Therefore, it is important to assess each object individually and determine the most suitable conditions based on the guidelines. Deferring from these guidelines should be done carefully and preferably in consultation with a conservator.
Possible strategies for dealing with composite objects are:
- compromise: find the middle ground between recommendations
- survival of the weakest: chose the recommendations for the most susceptible material
- majority: the most occurring material determines the conditions
- value-based: the character determining material determines the conditions
More references can be found at references.
Preventive conservation of plastics
Plastic is a relatively young material and its preservation in museums still has its challenges. It is well known that some of the early plastics from the end of the 19th and beginning of the 20th century are unstable. Some younger plastics also turn out to be less indestructible than originally thought. Modern plastics are sometimes designed to degrade rapidly, for instance biologically degradable disposables.
The ageing and degradation of plastic objects depends on several internal and external factors, such as composition, production, construction, environment and handling
Internal factors
Plastics of the same type may have the same polymer at its basis, but the addition of plasticisers, anti-oxidants, pigments and fillers, changes their properties and degradation processes. Like all organic materials the base polymer ages by chemical reactions with the environment, for instance with oxygen (oxidation) and moisture (hydrolysis). Physical processes such as diffusion of components from the object and deformation by temperature or pressure also play an important role. When the plastic is compressed into a mould to make an object, stress develops in certain areas which increases sensitivity to chemical and physical degradation.
Rubber is sensitive to oxidation; first it becomes tacky, then hard and brittle and it cracks. Soft PVC contains plasticiser which migrates to the surface making it tacky and stiffer. Plasticisers can condense on other objects and cause damage. Ink on paper may dissolve in plasticiser from PVC sleeves, causing the letters to stick to the sleeve. Upon degradation of the early plastics CN and CA acids are released which accelerate further degradation. This so-called auto-catalytic process also forms a risk for objects in the vicinity.
Because artists often experiment with materials, plastics may be processed and treated in unusual ways, for example by applying paint, heat, fire or tension. Components may be mixed in ratios other than recommended which changes properties and degradation processes. These internal factors are difficult to control. Usually the only way to slow down degradation is to lower the temperature in order to reduce the rate of chemical reactions.
External factors
Ultraviolet radiation contains enough energy to break polymer chains which subsequently can cross-link; processes that often cause yellowing of plastics. Light (visible radiation) has less energy but can still cause fading of dyes and pigments. Oxygen in the air can react with plastics, in the presence of ozone these oxidation reaction go even faster. Dust on a tacky plastic surface is hard to remove and can interfere with the visual experience of the object. Handling, vibration, and shock during transport or wear and tear during use, can cause cracks, tears and loss of material in susceptible objects. Suitable preventive conservation can slow down degradation by these external factors.
Twelve commandments carved in plastic
It is impossible to determine the precise composition of a plastic with the PIT, but in most cases identification of the base polymer provides sufficient information to take preventive measures and slow down degradation. As a starting point, there are a number of general guidelines to support preservation of plastics in museum collections.
1. Avoid exposure to UV-radiation. Indoors this can be achieved by choosing no/low-UV light sources and applying UV filters to windows, display cases, or frames. Outdoors a coating with UV-absorber can provide protection.
>> see online publication ‘Agent of Deterioration: Light, Ultraviolet and Infrared’
2. Store plastics in the dark. Adjust exhibition light levels to the light sensitivity of the object and the period in which a particular colour change is acceptable.
3. Keep the temperature below the point at which it softens.
>> see online publication ‘Caring for plastics and rubbers, chapter Temperature and physical properties’
4. The lower the temperature, the slower the chemical reaction.
Keep the temperature as low as practically possible but avoid it dropping below the glass transition temperature (Tg) when the object needs to be handled (see the information page for glass transition temperature of each plastics). At that temperature flexible plastics become stiffer and more brittle which makes them more susceptible to breaking on handling. In storage the temperature can be kept somewhat lower, for example by heating less in winter. Ensure that the RH does not get too high in that case (see point 7). In exhibition the temperature is usually higher for human comfort. Keep objects away from heat sources such as radiators, incandescent lamps and warm equipment.
5. Store plastics that undergo auto-catalytic degradation (CA, CN) as cool (refrigerator 5-7°C) or cold (freezer, ca. -20°C) as possible, lowering the temperature is the only way to slow down this degradation process.
6. Store plastics that are highly susceptible to oxidation (natural rubber, latex) in a low-oxygen atmosphere if possible.
>> download brochure
7. For most plastics a relative humidity (RH) with small or slow fluctuations in a bandwidth between 40-60% is acceptable. At an RH above 70% mould growth may occur. Plastics that are sensitive to hydrolysis are better kept at a lower RH between 30-45%. (See Shashoua, Y., 2008, Conservation of Plastics). Hygroscopic plastics that easily absorb and desorb moisture, resulting in swelling and shrinking, should be kept at constant RH to avoid crazing and cracking.
>> see publication: ‘Conservation of Plastics: Materials Science, Degradation and Preservation’
8. Keep plastics that emit harmful vapours in a well ventilated area. Do not store plastics that undergo auto-catalytic degradation (CA, CN) in sealed packing.
9. Support objects properly to avoid deformation under pressure.
10. Handle plastics objects with gloves or clean hands to avoid leaving behind grease and dirt. Conversely, gloves provide protection against harmful components on the object.
11. Select materials for packing and construction of storage and display areas that do not emit harmful compounds. (See Tétreault: CCI Technical Bulletin 32 - Products Used in Preventive Conservation)
>> see online publication ‘Products Used in Preventive Conservation’
12. Monitor the environment and the objects to verify that the preventive measures perform as desired.
Problem plastics
These guidelines for preventive conservation apply to most plastics. However, there are five ‘problem plastics’ that require extra attention and a close watch.
Cellulose acetate (CA) and cellulose nitrate (CN)
Keep CA and CN dark, cool and dry and do not seal them in their packing. Isolate the object and ensure ventilation to prevent the emitted acids affecting other objects in the area. If these plastics emit an acidic smell or when droplets acid appear on their surface they are actively degrading (vinegar syndrome). Use pH-paper or A-D strips to test if acid is released.
>> download publication ‘User'’s guide for AD Strips’
Natural rubber or latex (NR)
NR or latex is highly sensitive to oxidation. Store under anoxic or hypoxic conditions if the type of object and other materials allow.
Polyurethane ether and ester - especially soft foams PUR)
PUR ether foam is sensitive to oxidation. Store in the dark and do not expose unnecessarily to light and UV-radiation. Degraded PUR ether foam can be consolidated with a special treatment, but prevention is better than cure. (See literature: van Oosten, T., PUR facts, 2011)
>> see website ‘POPART’
PUR ester foam is sensitive to hydrolysis and is best kept at low RH, if necessary packed with silica gel or another moisture buffer.
Plasticised polyvinyl chloride (PVC-P)
PVC-P is best kept in a closed environment, for example sealed in polyester film or in a glass container, to avoid complete loss of the plasticiser. Make sure to avoid contact between packing and object.
Susceptibility and recommendations for preventive conservation
The table ‘Susceptibility’ (tabel ‘Gevoeligheid’ ) gives an overview of all plastics and their susceptibility for different external factors. A green cell means that the plastic has a low susceptibility for that factor, yellow is average and red indicates a high susceptibility.
The table distinguishes between rigid plastics and foams since foams often have a higher susceptibility due to their open structure. When specific conditions apply, they are mentioned in the table. In all other instances the colours correspond with those of the general recommendations in the table ‘Recommendations for preventive conservation’.
How to deal with composite objects
Artworks are often composed of different materials which makes it hard to determine the appropriate conditions. Each object is unique and has its own characteristics. Therefore, it is important to assess each object individually and determine the most suitable conditions based on the guidelines. Deferring from these guidelines should be done carefully and preferably in consultation with a conservator.
Possible strategies for dealing with composite objects are:
- compromise: find the middle ground between recommendations
- survival of the weakest: chose the recommendations for the most susceptible material
- majority: the most occurring material determines the conditions
- value-based: the character determining material determines the conditions
More references can be found at references.