General care

Signs of deterioration

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• Physical
• Chemical

Physical causes are associated with loss or migration of plasticisers or other additives, with absorption of liquids or vapours, with crazing due to stress or fatigue, with mechanical damage (including wear and tear) or, with excessive heat or cold.

Apart from obvious mechanical damage the main effects observed with physical deterioration are:

• crazing due to stress, often in combination with a particular environment such as solvent vapour or moisture - so called environmental stress crazing
• changes in flexibility due to plasticiser leaching or migration
• an oily bloom on the surface of the material, occasionally a solid bloom - due to plasticiser or other additive migrating to the surface
• distortion due to uneven loss of plasticiser

Environmental stress crazing is common with rigid transparent thermoplastics such as polystyrene and poly(methyl methacrylate). Whilst it may spoil the appeaeance of an object and cannot be remedied, it is generally of little consequence.

Flexible PVC and cellulose acetate are the main plastics containing significant quantities of plasticiser and are the ones most likely to show effects of plasticiser migration. Loss of plasticiser causes the material to stiffen but not generally so much as to cause brittleness - flexible materials should therefore be supported in their natural shape so that they do not become 'set' in a distorted condition.

Uneven loss of plasticiser from cellulose acetate plastic is responsible for distortion and is quite common with objects made from this material.
It is often, however, a sign that the cellulose acetate is also chemically deteriorating; the plasticiser becomes less compatible with the degrading polymer and migrates to the surface.

Chemical causes are due to chemical reactions occurring to the polymer, or occasionally to additives. Chemical causes are more serious because they are nearly always ongoing and irreversible.

The main factors responsible for causing chemical changes to polymers are:

• light
• heat
• oxygen
• moisture
• ozone or other atmospheric contaminants
• contact with chemical agents in use, cleaning, repair or by accident
• biological attack

Stress accelerates many chemical degradation processes.
Limiting exposure to the above factors is the only way of minimising chemical deterioration, nevertheless it can be extremely effective in prolonging the life of an object, but is less effective with objects which have already suffered some deterioration.

Signs associated with chemical degradation may be:

• discolouration
• embrittlement
• severe crazing
• crumbling (especially of foams)
• blistering
• bloom
• weeping
• odours
• surface acidity

Other signs that might indicate problems are:

• corrosion of metal fittings
• discolouration or disintegration of wrapping paper

There are four plastics materials that may be regarded as the most vulnerable to severe degradation. They are cellulose nitrate (Celluloid), cellulose acetate, PVC and polyurethane.