Section4-PLASTICS |
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In this sub-section, you'll find out why plastic cases tend to discolour over time, tips on cleaning and more major repairs... |
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THE DISCOLOURATION OF PLASTIC COMPUTER CASES by Dr. Edward Then Senior Conservator (New Materials) Science Museum, London |
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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.
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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." |
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PLASTIC CASES |
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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!
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| Cleaning fine details | Plastic Polish | |
| Repainting bakelite | Repairing other plastics | Seized bakelite parts |
| Putting a gloss on other plastics | Bakelite - keeping the gloss | |
| 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! | ||
| Bakelite - keeping the gloss | ||
| Everyone agrees, there are few finer sights than a gleaming
article made of bakelite. Bakelite, named after Baekeland, its Belgian inventor,
has been described as one of the most gorgeously tactile materials ever made;
it is hard and glossy yet capable of being moulded into fine detail. When
new it carries a lustrous sheen which is difficult to better, even in black,
which is otherwise a pretty lifeless colour. In fact there are few man-made
objects more functionally attractive than a gleaming black bakelite telephone,
although it doesn't really matter whether it's a radio, telephone or a TV
receiver... there's just something immensely appealing in the fine details
and hard lustre of that plastic. It's clear that bakelite was made in several
different grades; that used around 1950 for interior components of motor
cars (dashboards and steering wheels) seems to age and discolour very little.
On the other hand, if you compare a Bush TV12 television with the later TV22
model, it is all too clear that the older set had the better finish. The
bakelite of some telephones was even worse and soon came to look more like
pumice stone. Quite possibly the better variants contained more resin and
less filler material. No doubt it was cost-cutting that reduced the resin
content. What a shame then that the lustre vanishes all too fast. So many
artefacts made of bakelite seem to lose their gloss and end up brown and
porous.
It doesn't seem to have anything to do with the age of the item, so why does this happen and what can be done about it? First, let's look into the technology. Bakelite is a thermoset plastic, that is a plastic which starts molten as a liquid but once solidified, does not revert to its liquid state when heated. In crude terms it consists of a resin (which has the glossy appearance) plus a bulk filler material, usually wood flour. The shiny surface you see and admire is the top layer of resin but this is often very thin. Once rubbed away through atmospheric action, over-enthusiastic polishing or by scorching in the sun's rays, it is lost and nothing will bring it back. You are then left with a pitted mixture of resin and wood flour (or asbestos powder) and wood flour, being very fine sawdust, is not a particularly glossy material. It is this wood flour that looks brown and porous once exposed. Cleaning should precede any physical restoration of plastic mouldings. De-grease first, if necessary, with methylated spirits and a hard cloth. The restoration techniques you use will depend on whether the top layer of resin is intact; let's assume first that it is. Bakelite and other plastics which are only lightly soiled should be cleaned with Paste Polishing No. 5. This is a waxy polish compound, with a mild abrasive, and extremely effective. It will remove any film of 'gunge' and leave a smooth, glossy surface. Everyone who sees this cannot believe how good it is, far better than the metal polish that people used to use to clean bakelite! PP No. 5 is the stuff the GPO formulated for refurbishing old bakelite phones to new condition, by the way, and you can buy it either in bulk from the manufacturer or in small quantities as Baykobrite from Radiophile Publications, Larkhill, Newport Road, Woodseaves, STAFFORD, ST20 0NP. The paste should be rubbed on with a hard cloth, then removed and buffed up with a soft cloth. Alternatively you can use Solvol Autosol or similar paste (remove with white spirit) to rub down to the clean plastic underneath, but this always leaves a white powdery deposit which is difficult to remove from cracks and mouldings. If the surface is already rough and porous the technique described will not help. The remedy then is to cut away the discoloured layer with a hard brush and grinding paste ('soap') if you have a buffing machine. Otherwise use an abrasive polish such as T-Cut or Color-Bak (car paint restorers) or Bluebell (metal polish). Then let this dry, wipe off the residue and inspect the bare material exposed. If it has retained its pigment, polishing with a liquid real wax polish will suffice. Buff it up to a high gloss and be prepared to repeat this exercise every twelve months or so. If the bare material is discoloured, you will need to add new pigment. For black there is an intense black stain in car accessory shops called Back to Black - use it but try not to get it on your clothes. Otherwise you can use shoe polish (work it well into the pores of the bakelite) and buff it off several hours later. Several applications may be required. Then finish off with liquid wax polish. There are some people who prefer to avoid the hard work and paint low-gloss tung oil varnish on badly dulled bakelite (this usually takes about three coats and gives a very fine finish, not as shiny as lacquer). You need to use a lint-free cloth to apply the varnish and for details and crevices, use a tape recorder head cleaning swab. - these have foam rubber heads instead of spun cotton). This method, which aims to restore the finish by filling the microscopic pits with a clear substance, is valid if the bakelite has not lost its colour. If it has started to go brown, on the other hand, sterner methods are necessary. Small cracks and chips in the bakelite can be filled with soft furniture restoration wax, boot polish (my favourite) or car body fillers (cellulose paste or fibreglass resin types - you can buy these which are already tinted black, intended for filling cracks in black plastic car bumpers). Clean breaks can be joined with Superglue (cyanoacrylate) or Araldite (epoxy resin), though care is needed to prevent smears of glue showing. These techniques work. I know they do and they work very well. There may be other products and methods, so let us know if you can help.
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| Repainting bakelite | ||
| Finally, some bakelite products were painted another colour
and we may need to restore these paint finishes. Bakelite itself is generally
either brown or black, simply because the moulding powders for bakelite had
to be dark colours to mask the wood flour and asbestos dust filler material
used. What many people call coloured bakelite isn't bakelite at all; these
coloured plastics were often too expensive to use for utilitarian products.
Paint was often used for bright colours, this being a lot cheaper than
choosing self-coloured plastics of these colours (even if not quite so
attractive). Certain brown bakelite radio cabinets were spray painted to
provide a contrasting colour (e.g. primrose panels on brown bakelite); equally
some bakelite telephones, such as the STC 'Antwerp' phone, were given paint
finishes over black bakelite to provide additional colourways. For repainting
bakelite an enamel finish adheres best, preferably an eggshell paint which
is not too glossy. A hardware shop will have some spray colours which may
match, otherwise you will need to go to a specialist paint supplier (see
previous chapter). Vintage TV and Radio Supply (USA) has two products, Barrier
Coat and coloured lacquer enamel. The former is a basic primer for rigid
plastics and bakelite; it ensures good adhesion and provides a barrier between
the plastic and finish coatings. The coloured lacquer enamel is fast drying
and has a superior hiding power. It comes in just two colours, gloss black
and antique ivory. In practice, most cellulose or enamel spray paints could
substitute for this product.
Advanced bakelite repairs It is possible to repair broken bakelite in a way that is very hard to detect. The first thing is to make solid joints and if you are lucky, you can piece the bits together and glue them in place with superglue (cyanoacrylate). Doing this slowly and carefully is the main secret; it may take quite a bit of 'dry runs' to get the three-dimensional jig-saw to go together properly. Plasticine will help keep the pieces together while the glue cures. For pieces under mechanical stress, superglue will probably not work; in this case you will need to use a modeller's drill to make holes for inserting short pieces of wire to pin the joint in several places. A traditional slow-setting epoxy glue such as Araldite is used to make the joints and this can be built up behind the joint out of sight to give added strength. When nearly set, surplus Araldite can be trimmed away with a sharp blade. An alternative method which also has a lot to commend it is the use of glass-fibre cloth and resin (bonding paste), as used for patching car bodies. The materials are used on the inside of the case to be mended and the trick is to use a file or grinding tool to thoroughly roughen the surface. Using a putty knife or wooden spatula, the roughened area is then covered with a thin layer of resin (about 1/16th inch thick). A piece of glass-fibre cloth is then pressed on, smoothed out and left to cure. Once fully set another thin layer of the bonding paste is spread over the cloth for extra strength. This can be sanded for a smoother finish if desired when cured. Yet another possibility is Loktite BlackMAX adhesive, which is a cyanoacrylate that has rubber dust loaded into it. This helps the impact resistance and also gives a black colour. Some hairline cracks will remain and these can be filled with the self-curing black resin body-filler paste available at auto accessory shops. Once cured you can use very fine wet-and-dry paper (as used for car body work) to smooth the joints until they can no longer be seen or felt. Finally polish with Paste Polishing No. 5 (Baykobrite). For brown bakelite you could mix artist's powder paint with clear epoxy glue but be sure to mix in the powder extremely thoroughly.
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| 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.
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| Seized bakelite parts | ||
| A recurring question is how do you unscrew the earpiece cover on the handset of an old bakelite phone? With great care is the facile answer. One method is to dripping some penetrating oil into the minute gap around the cover (the oil won't harm the bakelite), leave for 24 hours and then get a better grip on the cap, either by wrapping several large rubber bands around it or better by using one of those thin, flexible neoprene mats specially made for getting a grip on obstinate objects. There are also special tools sold in kitchen shops for unscrewing lids on jam jars and at car accessory shops for removing oil filters; the tools work in similar ways and may be of use. Failing this heat can help; you place the offending object in a bowl of scalding-hot water (and take great care!). On bakelite handsets (of the British pattern) it's not always clear how the mouthpiece is released. Actually this is very easy; there's a small hole in the rim of the mouthpiece, behind which is a metal detent. Forcing a thin metal point (such as a thumbtack or drawing pin) into this hole will release the mouthpiece. | ||
| 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.
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| 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. |
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Buy the Book ! |
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Is published by Newnes at £19.99. ISBN 0-7506-3788-9, paperback, 413 pages, illustrated. Available through any good bookshop.
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Copyright © 1998-2008 THG and contributors |
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