The discolouration of plastic computer cases by Dr. Edward Then
Senior Conservator (New Materials) Science Museum, London
The yellowing or discoloration of computer cases is an extremely common phenomenon. The problem is not unique to cases made by one manufacturer, nor is it restricted to computer casings. This chemical process is comparable to the discoloration of an apple skin, and is similarly irreversible. Fortunately, in most instances the damage associated with discolouring affects only the surface of the artefact.
Let's start with some background on polymers and ageing. The polymer most commonly used in casings and housings for electrical equipment and computers is ABS. The acronym is derived from the initial letters of the three main monomers used for its manufacture - acrylonitrile, butadiene and styrene. ABS polymer was first made available in the early 1950s and, since then, has become one of the most widely used industrial polymers. It is valued by producers for its excellent mechanical properties (impact resistance, stiffness, surface quality), thermal properties (good dimensional stability at high temperature) and electrical resistance. It also offers significant resistance to chemical and stress cracking. Polymers, including ABS, can be described as large molecules made up of simple repeating units; the word polymer is derived from the Greek words poly and mer meaning 'many' and 'part' respectively. Many types of polymers can be created by varying the molecular composition of the repeating unit. The total number of repeat units in a polymer chain, often referred to as the degree of polymerisation, may typically be hundreds or more. During degradation, different chemical reactions occur along the polymer chain. These can result in the breaking and rearranging of chemical bonds, causing (among other things!) discoloration of the polymer. Degradation may be initiated or accelerated by numerous factors including ultraviolet light (UV), visible light, ozone and other extraneous pollutants, intrinsic manufacturing impurities, oxygen, and heat. In the case of our computer housing, I think UV and light are the main causes of deterioration. The rate of deterioration is thought to be approximately proportional to the light intensity. Deterioration-fighting chemicals are commonly added to polymers during manufacturing; these may include antioxidants, antiozonations, light stabilisers, UV stabilisers and fire retardants. The type of additive used will be determined by the composition and application of the finished product. As the polymer ages, most of these additives are consumed while they hold back the degradation process; once the stabilisers are used up, the polymer is often left unprotected and will deteriorate very rapidly. Attempts have been made to restabilise polymers, but it is not known how well these will work, and the topic demands considerable exploration. The Science Museum in collaboration with other institutions is currently sponsoring research in this area.
What should we do now and how can we extend the life span? The best advice is, perhaps, to do nothing. Personally I would advise that discoloured surfaces should be left untreated. Maybe, one day, the discoloration will be seen as desirable or inevitable, like the patina on metals! In any case, each example must be evaluated individually, preferably by a conservator who deals with plastics. Dirt and grime are a separate problem, and may be cleaned with distilled or deionised water. Stubborn stains can be removed with a non-ionic detergent. The cleaned surface must then be dried immediately. A word of caution: When cleaning with water, use a cloth or cotton wool that is only slightly damp, and avoid making contact with metal parts - which may corrode - and with the electronics. Avoid using solvent; some solvents may appear harmless on contact, but will react with the plastic over time, crazing or cracking the object later. Use only soft cloth or cotton wool to dry the case to avoid abrasion or scratching. Until there is a solution to this problem, the only prudent strategy is preventive conservation. Try to keep the computer away from strong light, especially direct sunlight and other strong UV, and from any heat source.
Also keep it covered when it is not being used, to forestall the build-up of dust.
This next article is 'Copyright 1998 Andrew Emmerson' who says, "You may find that it wraps up all that needs to be said on the subject of repairs and restoration."
And he's probably right !
PLASTIC CASES
Generally these are made of bakelite, moulded celluloid or ABS (a tough styrene), and this applies whether we are talking of radios, televisions, telephones or any other communication or entertainment device. Some highly decorative pre-war American radio sets were made from a moulded resin known as Catalin, whilst some older coloured radios and telephones were made of Beetle or similar formaldehyde plastics. Finally, some utility items (such as German field telephone cases from World War II) are found in a very low-grade plastic made by soaking rags in resin and moulding these under pressure. Bakelite and ABS tend to be dimensionally stable, whereas the cellulose acetate plastics are not and can both distort and deliquesce (dribble away under the influence of atmospheric moisture). There's not much you can do about advanced cellulose acetate decay except look for another set with a better cabinet. Bakelite can be cleaned and repaired as described below, whilst cellulose acetate and ABS should be treated as for dial glasses made of the same materials. You can use harsh paint strippers and solvents in bakelite but not on ABS and other styrenes; cellulose paints also attack styrene plastics - you have been warned!
Discoloration and degradation
It needs to be noted that certain plastic materials degrade with time, a process which generally cannot be reversed. Certain self-coloured plastics (ABS in particular) discolour as well.
Cleaning fine details
The modern biological detergents are very useful for cleaning fine details, such as the grooves and mouldings in some plastic knobs. A soak in a biological clothes stain remover such as Biotex for instance will be found to shift almost everything. Assistance with an old toothbrush may also help; never throw these away!
Repairing other plastics
A lot depends on where the break occurs, how visible the damage is and how much room there is to create a repair. One proven method is to use glass fibre cloth and epoxy resin to add strength behind cracks. Special packs can be had at car parts stockists and the repair is made on the inside of the cabinet where it's mostly invisible.
With Perspex, ABS and styrene cabinets, joints can be remade by applying the appropriate solvent with a thin artist's brush. A good place to buy bottles of solvent and the brushes is good hobby shops - the sort which sell plastic construction kits and plastic sheet and profiles - whilst a specialist plastic sheet distributor would be another good choice. Because these are thermoplastics, not thermosets, you can also dissolve shavings of the plastic in the solvent and make this into a kind of sloppy putty to fill cracks and gaps. The solvent will evaporate and the filling can be polished with very fine wet-and-dry paper and then polished with plastic polish. Minor cracks can be prevented from spreading by 'stop drilling'. This involves finding the end of the crack with a magnifying glass and drilling a hole (maybe 5/32" diameter) just beyond the end of the crack. This will relieve any stress that may be present and the crack will not travel further. It's worth loosening chassis mounting bolts a little; this will lessen the risk of cracks due to expansion when things get hot. Hairline cracks can be repaired by running superglue into the crack (from the inside of the cabinet). Be sure to flex the cabinet a little to ensure the superglue does find its way into the crack.
Putting a gloss on other plastics
Minor scratches and haziness can be polished out of most plastics with suitable abrasives. The Plastic Polish made by Greygate Chemical Company has these in a liquid suspension smelling strongly of camphor and I have found this superb on plastics of the Perspex, Diakon and ABS variety. Others swear by a repair kit called Micro Mesh, available from Antique Electronic Supply. The latter consists of several grades of abrasive paper and cloth that are used wet to gradually remove all of the surface blemishes. Both of these are industrial products, used commercially in the airline industry to clean and polish aircraft windows. Both are messy, and take some time, but they will take care of deep scratching and leave you with a surface looking like new. Experimenters who would like to do a bit of kitchen chemistry can try equal amounts of toothpaste and baking soda, mixed together into a paste, and then applied in a thin coat with a piece of soft cloth (such as T-shirt material). Results are said to be slow but rewarding.
Plastic Polish
Despite the boring-sounding name, Paste Polishing No. 5, is known to connoisseurs as the ideal material for cleaning bakelite and other plastics (even plastic baths!). Unlike Brasso and other liquid polishes, it leaves no active residue, and as it also contains a waxy agent, it also gives a gloss finish. Paste Polishing No. 5 is the stuff the Post Office used to polish up their old bakelite phones and is marvellous stuff - ask any user! Unfortunately the demand for it is reduced nowadays (BT doesn't need it now!), so it is only manufactured at intervals. You can buy it in small quantities under the name Baykobrite from The Radiophile, 'Larkhill', Newport Road, Woodseaves, Stafford, ST20 0NP at £1.95 a tube plus postage. Larger quantities and trade supplies from Greygate Chemical Company, Fir Tree Lane, Groby, Leicester, LE6 0FH (0116-287 7777). They also make special liquid polish for Perspex-type plastics, see below.
Plastic Polish
For cleaning and polishing telephones some special preparations are available. Since they were formulated specifically for the purpose, they will give far better results than normal household products.
Choose from:
- Plastic Polish in a bottle, ideal for thermoplastics (all modern telephones from the Tele. 706 onwards, also coloured 200 and 300 types). Great for removing scratches.
- Paste Polishing No. 5 in a tube, for black Bakelite phones (332 and older), radio knobs, Bakelite ornaments and other hard plastics. Despite the boring-sounding name, Paste Polishing No. 5, is known to connoisseurs as the ideal material for cleaning bakelite and other plastics (even plastic baths!). Unlike Brasso and other liquid polishes, it leaves no active residue, and as it also contains a waxy agent, it also gives a gloss finish. Paste Polishing No. 5 is the stuff the Post Office used to polish up their old bakelite phones and is marvellous stuff - ask any user!
Greygate Chemical Products Limited
Brunel Way
Stephenson Industrial Estate
Coalville
Leicestershire
LE67 3HF
Tel: 01530 839 222
Still manufacturing Plastic Polish and Paste Polishing No. 5
www.greygate.com
Sylmasta
We distribute Micro-Mesh which is used to polish telephone cases. We supply a number of restorers who have suggested we contact you.
Our site is:
www.sylmasta.com
And the link to the page is:
www.sylmasta.com/acatalog/Online_Catalogue_Micro_Mesh_Abrasives_87.html
Tel: + 44 (0)1444 415 027
Fax: + 44 (0)1444 458 606 |
