Diakon: A New Material for Coloured Telephones
By C. A. R. PEARCE, M.Sc.
Taken from the POEEJ, Vol 30, Jan 1938

The Author gives a brief description of the manufacture and properties of Diakon, a new moulding powder which is being used for coloured telephones.

For some time past it has been apparent that the moulding materials in use for coloured telephones are unsatisfactory. Trouble in service due to cracks and fading have been frequent and manufacturers have experienced serious difficulty in meeting their commitments from their inability to control the colour shades of the finished mouldings or to produce parts free from flaws. Yet so persistent has been the public demand that, despite the unsatisfactory position, it has been necessary to maintain relatively large supplies while at the same time searching for an alternative material. Recently a new moulding powder, Diakon, has appeared, which is available in a wide range of colours and which promises to be free from the difficulties associated with the use of the Thio-Urea and Urea Formaldehyde powders which have been used up to the present. In justice it should be stated that there are many occasions when the Urea class of plastics can be used with success but they are, in the writer's opinion, generally unsuited to designs incorporating relatively large metal insets or including thick sections.

The Manufacture of Diakon
The raw material from which Diakon is produced is supplied by the organic chemist. It is produced from coal, air and water but as received by the manufacturer it is a liquid very similar to water in appearance and about equal in density. This liquid is first filtered to remove dirt, etc., and is then mixed with the catalyst necessary for the subsequent chemical reaction. The mixture is then warmed and maintained at a steady temperature for a period during which it changes to a solid, Diakon. The transformation which occurs is not the creation of a new chemical compound, since the resultant Diakon still consists of the same carbon and hydrogen atoms which constituted the original liquid, and the catalyst (as its name implies) merely assists the change but does not itself undergo modification. The metamorphosis which occurs is one which the chemist terms "polymerisation," and is briefly the linking together into chains of the molecules which in the original substance were more or less dissociated. With Diakon the molecules may reach a length of up to 300 of those constituting the original liquid. The process occurs accompanied by the evolution of much heat and precautions have to be taken to prevent the reaction from getting "out of hand." To avoid the contamination which would occur if attempts were made to break up large blocks of material into a form suitable for use in the moulding press a device is introduced whereby the liquid solidifies into small spheres instead of into a complete block. After the removal of the catalyst, etc., and thorough washing and drying the final powder emerges.

Mould for Telephone No. 232

When Diakon granules, prepared as described above, are moulded, the resultant mouldings are completely transparent. For telephones, however, the powder is now mixed with carefully adjusted quantities of Ivory, Red, or Green colouring matter to give either of the three standard Post Office shades. At this stage the preparation of the "powder" is complete and it is purchased by the moulder for manufacturing into telephone parts, etc. It is estimated that over 60 tons of  "powder" will be used in the preparation of coloured telephones for next year.

diakon2.gif (40532 bytes)Unlike the Bakelite class of materials used for black telephones, Diakon is not a thermosetting material, i.e., heat and pressure do not harden it. In working Bakelite a steel mould heated to about 170' C. is filled with "powder" and closed under a pressure of the order of one or two tons per square inch; this condition is maintained for about three minutes during which time the material is "cured"; at the end of this period the mould is opened and the finished part removed hot and hard and an exact reverse of the mould. With Diakon, and incidentally with cellulose acetate, another thermal plastic from which telephone cradles are made, the process is briefly as follows:-

The mould is first closed under pressure and the Diakon is forced into the cold mould at a pressure of up to 15 tons/sq. in. from a cylinder in which it has When heated and to which the mould is connected. The Diakon must be at about 200' C. before injection may take place and at this temperature, although plastic, it is far from being fluid. Narrow vents are left to permit the exit of the air which originally fills the mould. When injection is completed the material remains for a short time under pressure until it has hardened sufficiently due to the falling temperature for the mould to be opened and the moulding removed. This whole process may be completed in about 90 seconds as against the three minutes necessary with Bakelite and ten minutes for the coloured Urea powders. An injection type press suitable for moulding telephone cradles is shown right.

Although the injection process is the economical method of producing Diakon mouldings it is possible to compression-mould the material by water-cooling the mould before removing the moulding, but only at the expense of speed of production. There is little difference between the results of the two methods as regards the appearance or the mechanical or electrical properties of the finished article. The appearance of coloured telephones in Diakon is undoubtedly superior to those in any other moulding material at present available but the Post Office is, at this stage, primarily interested in their behaviour in service and, although it is as yet too early to predict complete satisfaction, there is every indication that the new powder will obviate most of the difficulties hitherto attendant upon the manufacture of coloured telephones and the trouble which has previously been associated with their use.

Mechanical Properties
An indication of the working properties of Diakon is best obtained by comparison with those of Bakelite as used for black telephones and the table below details some values obtainable.

Property   Diakon   Bakelite
Specific Gravity   1.18 to 1.20   1.35
Tensile Strength lbs. per sq. in.   6,500 to 8,000   6,500 to 7,500
Cross Breaking Strength lbs. per sq. in. (B.S.S.)   10,000 to 15,000   10,000 to 11,000
Impact Strength cm. kg/sq. cm.   2.5 to 4.0   1.4 to 1,6
Electrical breakdown at 20' C volts per .001"   480 to 500   300 to 350

There are several grades of Diakon available but their chief differences are in respect of their moulding properties. These differences do not result in any material change to the electrical properties of the finished moulding, and with the exception of the resistance and deformation when heated, only introduce minor alterations to the mechanical properties. The above figures include the whole range of values which will generally be encountered.

As already stated, Diakon, unlike Bakelite, softens under heat and is therefore unsuitable for use at raised temperatures, but it is not anticipated that this will give rise to difficulty in this country.

Although some telephones in the new material have been assembled and a number will have already found their way into service it must necessarily be some little time before a complete changeover can be effected, but it is anticipated that during the next three months Diakon instruments will be appearing in relatively large quantities. They may easily be distinguished by their fine translucent colouring.

In conclusion the author wishes to tender his grateful acknowledgements to Messrs. Imperial Chemical Industries for their very material assistance in the preparation of the article and to Messrs. The Telephone Manufacturing Company Ltd., and Messrs. John Shaw Ltd., for the illustrations.

Comments from Andy Emmerson

No manufacturer would be able to make anything in red or green from Bakelite.  It is effectively impossible to do so as the colouring pigments would not have sufficient 'oomph' to mask the colour of the wood flour that gave Bakelite its body. Only black or dark brown pigments can do this.

Coloured variants of the International Standard Electric Company (STC in Britain) 'Antwerp' telephone were made in Bakelite but these were made of black Bakelite that had been spray-painted.

What some people mistakenly call 'coloured Bakelite' was in fact Beetle/Beetleware/Betal (trade names), alias Urea Formaldehyde (technical name).  The surface appearance is similar to Bakelite, although without the high-gloss finish of Bakelite and is often wrongly called Bakelite.  Urea Formaldehyde can be mixed with white and other light colours (such as ivory, red and green) and has similar characteristics to Bakelite.

The chief fault of Urea Formaldehyde is that it cracks easily under stress, which is why you often see visible stress lines on telephone (1) on the main case around the places where brass inserts were placed (to take screws from the base) and (2) along the hand grip part of the handset.

Brown muck often accumulates in these cracks but can be removed with bleach or, better, hydrogen peroxide and an old toothbrush. The other fault appears on the ivory phones, when the ochre and white pigments separate out, looking like toffee ripple ice cream.

Urea Formaldehyde was a heavy plastic, which was a minor disadvantage for telephone parts but not a problem for the jewellery boxes and toothbrush mugs made in this material, where its weight was an advantage.  Melamine Formaldehyde (trade names Melamine, Melmex, Beetle) was a later (post-war, I think) replacement for urea formaldehyde, very similar but with a slightly shinier finish. For a while Melamine was widely used for kitchenware and picnic plates.

Once the lighter and glossier acrylic plastics became available after the war, the GPO dropped the use of Urea Formadehyde.  The transparent Polymethyl Methacrylate (trade names Perspex, Lucite) was used for clear telephone cases, whilst the coloured Diakon was used to mould the cases of telephones.  Perspex and Diakon were widely used for domestic radio parts and all manner of other things.


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