PLASTICS USED IN TELEPHONE PRODUCTION
The discoloration of plastic computer cases
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.
THE TELECOMMUNICATION JOURNAL OF AUSTRALIA
Each year the range of telecommunication equipment which utilise
moulded products in its construction increases so rapidly that some information concerning them and later or alternative products is opportune.
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
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.
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.
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.
Last revised: December 06, 2010