Post on 17-Dec-2015
Caring for Plastics in Collections
A brief overview
Cordelia Rogerson
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Deterioration of plastics
Undesirable chemical and physical changes to plastic materials
Influenced by light (especially UV), atmospheric oxygen (oxidation), moisture (hydrolysis)
Chain scission – shortening of polymer chains, crumbling
Cross-linking - joining of polymer chains, embrittlement
Effect of additives, plasticisers, UV stabilisers and others
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Chemical deterioration - the most susceptible, problematic plastics
So called malignant plastics because they will off gas and could harm other collection items
•Cellulose nitrate (cellulose ester)
•Cellulose acetate (cellulose ester)
•Polyurethane (especially foam)
•PVC (plasticised)
• Fully vulcanised hard rubber (Vulcanite, Ebonite)
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Light, UV (RH) moisture emitting
Cellulose nitrate yellow, brittle hydrolysis acetic acid, plasticiser
Cellulose acetate yellow, brittle hydrolysis acidic & oxidising nitrogen oxide, plasticiser
Polyurethane yellow, brittle, yellow, brittle, nitrogenous organic gases & sticky, crumbles sticky, crumbles liquids
Poly(vinyl chloride) yellow, brittle, resistant oily plasticiser liquids, hydrochloric gas in extreme moisture & light
Rubber brittle, discoloured, hydrogen sulphide, hydrogen sulphide, sulphuric matte sulphuric acid on surface acid on surface
Adapted from Scott Williams, Care of Plastics: Malignant plastics
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Light, UV (RH) moisture emitting
Acrylics shows resistance shows resistance less danger
(PMMA)
Nylon yellow, brittle hydrolysis less danger
in extreme conditions
Phenolics discoloured, matte discoloured, matte phenol & formaldehyde with severe degradation
Polyolefin yellow, brittle, shows resistance less danger
Polystyrene yellow, brittle, shows resistance less danger
Adapted from Scott Williams, Care of Plastics: Malignant plastics
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Some signs of deterioration:
Smells
Yellowing
Crazing
Cracking
Crumbling
Embrittlement
Shrinking
Warping
Weeping
Blistering
Blooming
Sticky surface
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Some characteristic smells
•Acrid smell – cellulose nitrate
•Vinegar syndrome, acetic acid from cellulose acetate
•Vomit – cellulose butyrate, cellulose acetate butyrate
•New car smell, plasticiser - PVC
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Cracking, crazing
May occur for different reasons
Buckle, acrylic bangle
Stress cracking in PMMA
Deterioration seen as cracking in cellulose nitrate, which is also causing the metal bar to corrode badly
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Vulcanised rubber emitting sulphur compounds, seen as yellowing on the surrounding packaging materials
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Yellowing, discolouring
Nylon – discoloured from the original white colour
PVC (including plasticised varieties) showing yellowing
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Cockling, shrinking – plasticiser loss
‘Shrinking’ cellulose acetate bangle
Cockled plasticised PVC sheet
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Sticky surfaces – migrating plasticisers
Camera case with a sticky surface causing it to tear and lose areas where it has been stuck to and then removed from surrounding items
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Beware of mechanical damage –
Plastic objects are not just susceptible to chemical deterioration
Far reaching effects on the appearance and construction of objects
Surfaces can be fragile and vulnerable
Constructions can be surprisingly fragile
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Mechanical damage –poly (methyl methacrylate) – Perspex, acrylic, Lucite, Plexiglas
Surface scratches on Perspex
Mechanical failure of a Perspex construction
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Mechanical damage – polyester resin
Mechanical break in polyester resin bangle
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Preventive conservation
Controlling the storage environment is vitally important to slow the rate of deterioration
Should be as stable as possible
Light sensitive – eliminate light in storage, particularly UV, regulate on display & eliminate UV
Temperature - below room temperature if possible – cool 10C, ideal to slow deterioration
Broken glass gramophone record with a yellowed lacquer coating that has crazed and cracked. Damage could be prevented through appropriate handling/storage whilst the deterioration of the lacquer could be slowed.
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Relative Humidity
•RH% - generally between 30-50%
•never above 65% RH
•Plastics degrading via hydrolysis (cellulose esters) lower is appropriate, 30% RH
•30% RH or below may prevent dissipation of static, polyester film (Melinex)
•Hygroscopic plastics, Casein, some polyesters, are better with around 60%
Metal and lacquer gramophone records stuck together and with corrosion products, partly as a result of an uncontrolled storage environment
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Storage: •Monitor items regularly – every 3-6 months ideally
•A-D Test strips – cellulose acetate, detects emitted acetic acid (available Image Permanence Institute, Preservation Equipment)
•Group items together for ease of monitoring
•Isolate malignant items from other materials to prevent off gassing affecting other items
•Good ventilation is essential for malignant plastics, do not place these in closed environments or allow circulated air to reach other collection items
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Isolate vulnerable materials from one another
– whenever possible
Imprint of degraded rubber balloon transferred onto paper
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More advanced methods of storage:
Cold storage, slows the rate of deterioration particularly for cellulose nitrate film, 5C
Freezing suggested as suitable for thin cellulose nitrate, polystyrene, polyesters, ABS
BUT NOT plasticised PVC/ vinyl, degraded cellulose nitrate
(Shashoua, 2006)
Anoxic storage – suitable for plastics that degrade by oxidation not hydrolysis, and all rubbers - Escal bags™, Ageless™
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Preventing mechanical damage
•Handle items to ensure their surfaces are not damaged
•Provide a cushioned surface – polyethylene foam (Plastazote™)
•For plasticised items, sticky surfaces use non-stick interleaving layers, silicon release paper, Teflon coated paper
•Ensure items cannot move around becoming damaged
•Wear gloves to protect yourself and the objects, finger marks can be harmful
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Materials for storage/display: always archival quality
Materials generally suitable in contact with plastic objects:
Polyester sheet (Melinex), nylon, Polyethylene, polypropylene
Materials considered suitable to be near plastic objects:
Poly(Methyl methacrylate) Perspex, polycarbonate, polystyrenes
Not untreated plywood or MDF or non-archival quality materials
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Cleaning the surface of plastics
Only when absolutely necessary
Use a dry lint free, soft cloth
If further cleaning is necessary:
Do not use cleaning agents
Avoid solvents
Use water with caution – a barely damp lint free cloth or swab, dry thoroughly afterwards with a dry cloth
No water on items with gelatine, film, sequins
Avoid degraded areas
If in doubt don’t!
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Conservation treatment
Generally limited treatment is possible with chemically deteriorated objects
Mechanically damaged objects may present more possibilities but will depend on the object, its construction and circumstances
Nylon filament bangle, shown before and after treatment, successfully conserved to regain original shape