PLASTICS USED IN TELEPHONE PRODUCTION


Bakelite and Phenolics

The discoloration of plastic computer cases
by Dr Edward Then


"Have your grey and ivory 706 telephones started to go brown and brittle?" If so, you are not alone! Just like telephone collectors, the people who collect early personal computers have also discovered the unattractive
discoloration which affects even modern plastics. This article,  found on the Internet, gives some expert advice. 



THE DISCOLOURATION OF PLASTIC COMPUTER CASES

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.


Dr. Edward Then
Senior Conservator (New Materials)
Science Museum, London


THE TELECOMMUNICATION JOURNAL OF AUSTRALIA
MOULDINGS FOR TELEPHONE EQUIPMENT 

C. Faragher, A.M.I.E. (Aust.) - June, 1938

Each year the range of telecommunication equipment which utilise phenolic moulded products in its construction increases so rapidly that some information concerning them and later or alternative products is opportune.
A brief history may be of interest. In 1908, Dr. Baekeland, a Belgian chemist in U.S.A., patented a method of controlling important chemical reactions between phenol (carbolic acid) and formaldehyde (the result of a destructive distillation of wood), the product being a yellow-brown fusible and soluble synthetic resin in solid form, which melts at about 120 degrees F. In modern practice this basic material is available as a fusible soluble material (liquid or solid) and used in. the liquid form as a varnish and for building up laminated sheets.

The title Bakelite has been applied generally but incorrectly to phenolic moulded products.

They are properly described as phenolic synthetic resin mouldings Bakelite -is a product of the Bakelite Corporation, U.S.A., or allied organizations. The equivalent product of other firms is marketed under the trade names of Elo, Nestorite, Moulderite, Rockite, etc.

Phenolic moulded products almost comply with the specification of the ideal material. They are attractive in appearance, light, strong, free from deterioration or odour, are cheap when produced in numbers, can be made with great accuracy, high finish and of intricate form, fitted with metal inserts, provided with moulded tie reads, or machined, are fire-resisting, possess high surface and volume resistivity, and are unaffected by water and most chemicals.

For mouldings, the basic material used is in powder form. As a moulding of pure material is somewhat brittle, a filler - generally wood flour is added to the powder. For special purposes other fillers are used, for example, asbestos for heat resistance, mica for greater dielectric strength, canvas or paper for mechanical reasons. The powder is placed in a hardened steel mould consisting of a punch and die fixed in a steam or electrically heated press, and heat and pressure are applied. The powder softens and becomes semi-plastic as the temperature rises and under pressure from the punch it flows into the interstices of the die. With the heat and pressure still applied, a surprising change takes place. The plastic changes into a solid and further heating or pressure leaves it unchanged, i.e., it is now infusible and insoluble, and the production process cannot be repeated or reversed, a new chemical compound having been produced. The temperature, the pressure, and the curing time varies according to the dimensions of the moulded product. The mould temperature varies from 320 deg. F. to 400 deg. F. and pressures vary from 500 to 3000 pounds per square inch. The time for production varies from one to five minutes. The specific gravity of the moulding is about 1.5.

The steel moulds are expensive, one may cost from £30 to £300 depending on complexity. For economic reasons it is usual to mould a number of pieces simultaneously, e.g., a dozen earpieces may be moulded at once on the plate of the press - one per punch and die provided. Moulds for expensive parts of high finish leave extremely hard surfaces and are usually chromium plated. Those surfaces of mouldings which appear parallel, on inspection will usually be found to he tapered slightly to facilitate withdrawal from the mould. Machines are available to press the powder into pellets, and so avoid thee time lost in measuring powder for each mould cavity. Pellets of the correct number and volume are fed into the cavities by the operator. The, volume of the powder is appreciably greater than the finished moulding (bulk factor is 3 to 1), but there is no loss of weight. As the punch enters the die, the excess plastic material is forced out and is called the “flash”. The aim is to ensure a complete moulding with minimum waste and good cut-off of the flash. After removal of the moulding, the flash is broken off and any fins huffed away. Most phenolic moulded telephone products are black or shades of brown, as the basic material is yellow-brown. Black or brown mouldings are fast to light. Laminated sheet is usually called S.R.V.P. board (synthetic-resin varnished paper). S.R.V.P. board is the yellow brown sheet material used as relay spring insulation. and for general purposes in modern automatic exchange plant. It is made by impregnating sheets of paper with phenolic varnish and curing it in a hot press. These sheets can be punched readily, have good insulating properties, particularly on uncut surfaces, are not
easily scratched, are cheaper and harder than ebonite, and mechanically superior to it. Sheared or cut surfaces having a lower insulation resistance than the natural polished surfaces may be improved by varnishing.

The foregoing applies particularly to phenolic mouldings. Other moulding materials used are of interest and are referred to below.

Cresol formaldehyde is a powder somewhat cheaper than phenol formaldehyde. It has slightly poorer mechanical and electrical properties and finish, but is suitable for certain moulded products used by the Department. A slight remnant odour makes cresol base mouldings unsuitable for drinking vessels. The Australian made four-part plugs and sockets for portable telephones and the four-way terminal strips No. 1 for telephones are mouldings of cresol base.
A most important moulding material is urea formaldehyde. The basic material is colourless. It has a high resistance to flow and requires pressures up to 5000 pounds per square inch. Urea base mouldings can be distinguished from those of phenol base by their translucency, phenolic base mouldings being opaque. Urea base material can be coloured to produce mouldings in all the colours of the rainbow, and the depth of colour gives them great beauty. All coloured specimens we have tested to date have faded on exposure to light, unfortunately. Coloured telephones are of urea base, as phenol base mouldings can be produced only in darker shades. Samples at present being tested for fastness to light are red, green or ivory. It is interesting to note that in nature urea is produced by the kidneys. In 1828 a German chemist, Woehler, startled the scientific world of that day by producing urea in the laboratory. Prior to this it was considered as fundamental that compounds produced by the hiving organism could not be synthesized in the laboratory, and his discovery stimulated the chemical world to other equally important - discoveries concerning organic compounds.  Some of the trade names of urea based powders are Beetle, Pollopas, Scarab, Mouldrite and Plaskon.

Cellulose acetate is a modern plastic of importance. Handset telephones in Australia are at present provided with cradles and plungers of phenolic base material. To diminish breakages of these parts, the British Post Office is now using cellulose acetate mouldings which have relatively great tensile strength. One form of this chemical compound is used extensively in the Department as artificial silk threads for the insulation of the tinsel conductor of instruments and switchboard cords. After being liquefied in a volatile solvent it is supplied as a liquid to the surface of protector carbons, and with evaporation of the solvent it hardens to form the insulating separation of the protector carbons now standard. It is a true thermo-plastic, i.e., as the temperature rises the solid becomes plastic and can be moulded to any shape, and resumes the solid condition on cooling and can be re-used. In this respect it differs from the phenol, cresol and urea base mouldings, which are thermosetting, not thermo-plastic. Cellulose is a carbohydrate. Although it is principally produced from cotton, many other plants serve for the production of cellulose. The cellulose is treated with acetic acid and the final product is cellulose acetate. It is non-inflammable, and is available as a liquid under controlled conditions and as a solid in sheets, rods, tubes and shapes. Before the application of stains or pigments it is transparent. Cellulose acetate cradles and plungers for handset telephones are on order for trial purposes, as a question still to be determined is its ability to withstand Australian sun temperatures without becoming plastic with consequent indentation. It might be of interest to mention that the striated and mottled coloured composition coverings now to be seen as a sheathing over the steel core of steering wheels of motor cars is cellulose acetate. Switchboard plugs were originally built up from machined parts of metal and insulation. Plugs having moulded insulations are now largely used by the Department. In manufacturing these, the metal parts are properly spaced in a jig and a plastic form of cellulose acetate is forced into the interstices and solidifies to secure the parts permanently. Some of the trade names under which cellulose acetate is marketed are: Cellomold, Gelastoid, Rhodoid, Trolitul and Lansil.

A compound marketed recently by Imperial Chemical Industries and named Diakon is the most promising material available at, present for coloured telephones. Chemically it is a synthetic resin the basis of which is understood to be methylmethacrylate. It is a true thermo-plastic and therefore clean scrap can be re-used. Grade F, which has a high softening temperature, is that specified for coloured telephones. Like phenol base material, it has many of the characteristics of the ideal material. It is available as a liquid or a solid as the base for transparent, translucent and opaque articles in many tints and shades. As a liquid it is used for cementing and in granular or powder form for compression  injection moulding. The specific gravity is 1.19.

An interesting difference in moulding technique is that whereas phenol, cresol, and urea base mouldings harden in the mould and can be removed hot, Diakon (and cellulose acetate) mouldings must be cooled somewhat to solidify them before removal. Mould temperatures may range between 260 deg. F. and 400 deg. F. and mouldings are produced at intervals of about two minutes.

Cellulose acetate base and Diakon articles can best be produced in what is termed an injection moulding press, which is not suitable for producing mouldings from the thermo-setting powders referred to earlier. In the injection moulding process the material is made plastic by preheating before entering the mould, and is then injected under pressure to fill the moulding cavity completely, after which slight cooling causes it to solidify, and it is then withdrawn or ejected as the final shape. Under this process articles are produced at high speed.

Celluloid, the base of which is cellulose, is used to a small extent such as a protective covering for trunking charts and designation strips in exchanges. Its inflammable properties and gradual loss of transparency are the chief objections to its use.
Casein is a by-product of the dried milk industry and is available as sheet, rods or tubes, dyed or pigmented in various colours. It is used as insets for jack number plates and as designation plugs in switchboard jack fields. Some of the trade names under which it is marketed are: Erinoid, Galaliath and Lactoid.

In concluding, a reference might be made to Cellophane. It is allied to the above compounds and although not used by the Department directly, includes ,in its manufacture a most interesting process; Fundamentally it is regenerated cellulose, and when it is in the liquid form and, ready for production as sheets it is floated out on a water surface, where it solidifies into an extremely thin sheet.

Diakon: A New Material for Coloured Telephones, C, R. Pearce, M.Sc. (Eng.), D.1C., “Post Office Electrical Engineers Journal” January, 1938.

 
 
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Last revised: December 06, 2010

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