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