Caring for Plastics

//Caring for Plastics
Caring for Plastics 2015-07-17T10:15:54+00:00

Caring for Plastics

General Care

Most, if not all, degradation processes in polymers are ongoing and irreversible so preventive care is very important. This should start before an object is acquired.

  • Is it in sound condition?
  • Is it showing tell-tale signs of deterioration?
  • Does it appear to have been exposed to a harmful environment in the past?

Finding an object in perfect condition may be impossible (or, perhaps prohibitively expensive), but it is better to acquire one that is physically damaged (e.g. with a crack or chip) rather than one that is chemically deteriorating.

Many people can spot deterioration in rubbers and it is just as easy to learn the tell-tale signs of degradation in other plastics. It is particularly important to be able to recognise when cellulose nitrate or cellulose acetate is degrading.

Do’s and Don’ts

  • Do keep polymers in the dark whenever possible.
  • Do ensure storage and display area is cool, dry, dust-free and with some ventilation.
  • Do inspect regularly for signs of deterioration (not less than yearly). Remember if you suspect an object is degrading it is best to isolate it from others and remove it from the collection until you are sure it is OK.
  • Do support soft or flexible objects in their normal shape.
  • Do handle objects carefully and wear clean cotton gloves if possible.
  • Do wrap objects in uncoloured tissue paper (preferably acid-free tissue) rather than newspaper.
  • Don’t expose objects to strong light.
  • Don’t keep polymers in damp or stuffy places.
  • Don’t store objects in completely sealed boxes, plastics bags or other wrapping which would restrict ventilation.
  • Don’t clean plastics with solvents or other household cleaners unless their long-term effect on polymers is known.
  • Don’t allow objects to contact each other.

Deterioration of plastics may be divided into two main causes

  • Physical
  • Chemical

Sings of Deterioration

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.

Cellulose nitrate handbag comb showing chemical degradation Cellulose nitrate setsquare showing chemical degradation Staining on wrapping tissue removed from degrading cellulose nitrate object Polyethylene lid of solder flux container degraded through contact with contents Vulcanite bracelet showing fading from exposure to light