In the earliest phones, wood was the most common construction material. It was easy to work and a reasonable insulator, and finished well. Parts that would take a lot of wear were usually constructed from nickel-plated brass, as it cast easily, was a soft metal to finish, and the nickel plate gave it a tough corrosion-resistant finish. Other metals were tried with various degrees of success. Cast white metal gearwheels were fairly common, and pressed, stamped or folded steel or even cast iron made an appearance in items like switchhooks. Bell Telephone Manufacturing Company in Antwerp started using a comparatively new metal, aluminium, in pillars and cradles in the late 1890s. All these materials had their problems, particularly in the area of cost. They were either costly to produce (like aluminium) or costly to finish (like brass). Shortages of some items began to occur. In the 1890s it was necessary for L M Ericssons to import walnut from the U.S. The cost of shellac lacquer, produced from the secretions of a south east Asian beetle, doubled in the 1890s due to the increased demand for its use for French polishing and electrical insulation. Gutta Percha, a tree sap that formed a rubber-like compound, became a temporary substitute but it , too, ran into shortage.
EBONITE and VULCANITE
The discovery that rubber could be hardened by reacting it with chemicals under heat produced two useful compounds that could be readily moulded into electrical parts. Ebonite (rubber cured with mercury) was patented in Britain by Thomas Hancock in 1844, although it was probably invented slightly before Charles Goodyear's U.S. invention of vulcanite (rubber cured with sulphur) patented in 1839. Although adequate for their purpose, these substances were also flammable, weathered to an unattractive colour, and were not always dimensionally stable. In some cases it was necessary to machine them to an accurate finish, increasing the cost again. These two compounds were mainly used for terminal blocks, receiver caps, and other minor non-electrical or low-voltage parts. Rubber underwent a tremendous surge in demand, particularly for insulating electrical cables. Existing cast-moulding techniques were ideally suited for these compounds, but the extended heating and curing period needed in the moulds made production rather slow.
Left: Western Electric mouthpiece in vulcanite. Some of vulcanite's defects are evident - the edges must be made thicker to allow for the compound's brittleness (in spite of this, it is chipped at the edge) and the vulcanite is weathering to a greyish colour.
It was becoming increasingly obvious to the world's manufacturers that a new compound, synthetically produced rather than derived from natural ingredients, was needed. It should have good electrical insulation properties, it should be able to be coated onto wires, and it should be able to be moulded to reduce the expensive working and finishing needed for wooden items.
There was already a compound that almost fitted the need - cellulose nitrate. It was invented by Alexander Parkes in Britain in 1862. It was marketed as Parkesine, Celluloid, or Xylonite. When cotton was treated with nitric acid, the result was a fibrous mass that could be moulded into quite detailed shapes or rolled into sheets.. Some firms including Ericssons used this compound to form the mouthpieces of their telephone transmitters, and in moulded protection plates to prevent the handset rubbing on the polished woodwork of their phones. It had one severe drawback - it easily caught fire, and in the wrong circumstances became explosive. These are not assets in electrical equipment. Its dangerous nature meant that it could only be casting moulded, not pressure moulded. This limited its use somewhat, although sheet product could be press moulded in steam-heated presses. After a few decades the compounds decomposed into a white powder.
Left: Beautiful Ericsson mouthpiece in tortoiseshell celluloid
The science of moulding cellulose nitrate (and its later more stable cousin, cellulose acetate) was, however, well developed. Cellulose acetate was invented by Eichengrun and Becker in Germany in 1903, and was less flammable than the nitrate. It was eventually made available in a powder form from which it was readily injection moulded. Eichengrun developed the first injection moulding press in 1919.
A Belgian chemist named Leo Baekland was working on the problem in the U.S. He had already made a fortune by inventing a photographic paper that could be developed with artificial light, and he then turned his skills to a replacement for shellac. An understanding was developing about the formation of long molecules that made compounds that were strong, yet could be formed and moulded under heat. Baekland investigated compounds of phenol, a product derived from coal tar, and its combination with formaldehyde. Under heat the compounds combined to produce a thick, sticky substance that showed promise as an insulating coating. With further heating the compounds reacted and produced bubbles of gas that broke up the compound and made it useless. Other chemists had given up on it at this point, but Baekland allowed the reaction to continue under pressure to around 170 degrees Celsius. This stopped the bubbles forming, and when the compound cooled he had a solid mass that was an accurate copy of the vessel that contained it. He had invented the first all-synthetic thermoplastic, which he named Bakelite.
A thermoplastic resin is one that can be moulded when it softens under heat.
Left: Baekland's first "Bakelizer", now in the Smithsonian Institute
Further development was needed. Bakelite is a dark brown brittle compound in its raw form, but adding "filler" substances such as talc or wood flour before the final moulding made it stronger. It could be coloured with any solid coloring to produce a deep, glossy finish. Baekland patented Bakelite in 1907 after about three years of research, and publicly revealed it to a meeting of the American Chemical Society in 1909. He had taken the step of producing a number of demonstration objects in bakelite to show its tremendous range of colors and its ability to mould accurately, and it was an instant success.
So successful, in fact, that a number of firms started producing it without the expense of buying the rights from Baekland first. At this point Leo Baekland carried out a bold but brilliant move. Instead of taking each of the companies to court and fighting protracted legal battles, he invited them all to a conference. They joined together to form the Bakelite Corporation, with Baekland as its President. In Britain he formed Bakelite Limited to cover the European market. Conventional injection moulding techniques worked well for bakelite, although the higher heat and pressure still slowed down production times. In spite of this an item could still be produced after three to five minutes in the mould.
Early experiments in consumer goods like radios showed the efficiency and low cost of bakelite, and in 1914 Western Electric started testing production of telephone handsets in bakelite. The phone bodies were still cast in metal for some time. Other telephone companies quickly followed, and before long the old-style wood and metal telephones began to disappear from production. There was one problem. When colouring bakelite, it was found that some of the compounds used could weaken the bakelite slightly. This was not a problem with static items like radio cases, but it was a definite weakness with well-handled items like telephones. There were also problems in accurately controlling the colours, as the final bakelite colour varied between batches according to the exact heat, amount of pigment, and period under pressure. In the U.S., black was the most stable, so black it was until after World War 2. In 1941 Western Electric finally dropped the cast-metal phone bodies in favour of bakelite. The zinc used for the body castings was now needed for the war effort. At this time they added nine cast-in colours to their range rather than painting the bodies as before.
Left: Injection moulding press
Right:Mould for the BPO 162 telephone. Note the massive steel dies to withstand the high pressures.
In Europe the situation was a little different. Experiments with Bakelite continued and the British particularly developed the telephone into a more attractive range of designs. One advance was the discovery that two different colors could be introduced to the final moulding stage to produce a mottled walnut effect. Although a little difficult to produce accurately, some British firms mastered the technique and produced very attractive telephones in this style.
General Electric in particular seemed fond of this effect. Researchers also worked on stable colorings that would not weaken the bakelite. They experimented with organic colorings, as well as the mineral powders used in the U.S. These worked fairly well and were produced in a small range of colors, mainly red, green and ivory/white. Unfortunately these colors did tend to fade in strong sunlight, as Australian phone collectors have found out. After a couple of decades the bakelite surface could decompose slightly, leaving the surface dull as the filler was exposed.
Left: British Post Office 162 in walnut bakelite. Photo by Lawrence Rudolf, from Bob Freshwater's Telephone File website at http://web.ukonline.co.uk/freshwater/siemensb/sb162.htm
Work continued on other polymer compounds to overcome these weaknesses. In 1923 Fritz Pollack developed a thiourea formaldehyde resin, and a series of polymers sprang from this. These showed a superior hardness to bakelite, and they were colorless. This made it posible to tint them to produce colors like white. Although chemically different, they are usually called bakelite. Their first appearance in telephones appears to have been in Siemens' Neophone, where the white model particularly shows the deep translucence of the compound. They also proved useful as adhesives and insulating varnishes.
Left: Neophone in white
Much of the moulding work in Britain was contracted to a firm called Birkbys. They started as a chemical works and tannery in 1867 and gradually moved into making insulated parts for the electrical industry.
In 1907 they heard of bakelite, and one of the brothers, Freddie, travelled to the U.S. to get the rights to the new process and to learn how to produce it. They started production, and also began researching bakelite and its properties. They worked on new fillers to strengthen the product and to improve its electrical properties, and were able to gain their first patents on the improved formulas in 1920. The move into bakelite was fairly easy for them as they already had the moulding and chemical preparation equipment needed.
As bakelite telephones went into wider production, Birkbys contracted the case moulding work for many firms. A company poster shows many familiar telephones from companies like Siemens, Antwerp Bell, and Ericssons. They used the trade name ELO for their bakelite products. It may be possible to identify a Birkby's case by the moulding marks, but as far as I am aware this information is not available.
Production of thermoplastic bakelite telephone cases only ceased in 1973, and the firm still exists (although now owned by Marubeni). Their place in the history of British (and Australian) phones is mostly unappreciated.
Just before World War 2 a new range of plastics was developed by Otto Rohm, based on polymethylmethacrylate. This compound was synthesised from coal gas, air and water. It was a thermosetting resin - that is, the final heat and pressure stage of moulding finished the chemical process and left it rigid, although it softened a little more readily than bakelite under further heat. Its main advantages were that it was more dimensionally stable (especially when metal inserts were moulded in), and it was much tougher and scratch-resistant. Its hardness was similar to aluminium. It was introduced in Britain by ICI in 1934. The British Post Office found a use for it in 1938 in telephone case mouldings. The developed version was trademarked Diakon by ICI, and marketed by the Lucite Corporation which was eventually detached from ICI into a separate company. It is now owned by du Pont in the U.S. Diakon soon gained another use - for aircraft cockpit canopies and gun turrets, under the trade name Perspex (ICI in Britain) or Plexiglas (in the U.S. ). The Plexiglas name was originally used by Rohm and Haas for their production, but was appropriated for the U.S. production when German trademarks ceased to carry any legal weight at the outbreak of war. It could be moulded in conventional equipment, although at a higher pressure (up to 15 tonnes), and a higher temperature (up to 300 degrees Celsius but usually about 200 degrees). A completed moulding could be taken from the press in about 90 seconds, a considerable increase in production over bakelite.
In its raw state as either a liquid or a powder, Diakon could be readily colored with dyes or pigments. It was used to produce some of the first transparent display telephones in the 300 series. There were problems with the powdered form with uneven color distribution, and eventually most telephone manufacturers except TMC used the liquid form. It still suffered slightly from strong sunlight, which could lead to some fading or cloudiness, but without the wood pulp fillers it polished up more easily. Diakon also looked more attractive with its slight translucent finish. In the late 1940s Western Electric adopted it for their new 500 series of telephones.
Left: 300 magneto phone in black (Australian Post Office photo)
Right: 300 phone in Jade Green
The first silicones and epoxy and polyester resins were also developed about this time. The early epoxy resins found a use as a pourable, self-hardening encapsulating agent for electrical components. They found a place in the telephone industry that had previously been filled by shellac and gutta percha, and became particularly useful in encapsulating wire joints in cables.
The next step for most telephone manufacturers came in the 1960s with the introduction of the ABS family of plastics (Acrylonitrile Butadiene Styrene), generically known as Acrylics. These plastics are part of the range of polystyrene plastics, and can be manufactured for hardness, color depth, electrical properties or just about any other characteristic desired.
In Britain it was marketed as Bextrene by the comprehensively named Bakelite Xylonite Ltd company, among others. In the U.S. Dow Chemicals produced the aerated version that became famous as Styrofoam.
ABS was the plastic chosen for the Australian Post Office 800 series telephone, and almost solved the fading problem. It was also more resistant to scratching, and to most cleaning chemicals. Birkbys in Britain supplied the bulk of Britain's ABS cases , both to the BPO and its associated manufacturers.
Left: STC advertising brochure for the Australian Post Office 800 series, about 1966.
Polyvinyl chloride, a soft synthetic plastic, came into wide use as a substitute for rubber in grommets, telephone feet, and vibration-damping mountings. It was also used on the keypads of Telecom's T200. Vinyl chloride was noticed in the 1870s and patented by Bauman in 1912, but required some further decades of sporadic development, mostly by Goodrich in the United States, before its potential was realised. The problem was to find a suitable compound, caused a plasticiser, that would keep it soft or liquid until it was moulded. The plasticiser would then evaporate and the solid PVC casting was left. Nylon and Polyethylene are closely related. It is now pretty much the standard wire insulating compound worldwide, although in high temperature and high voltage applications other plastics are superseding it.
The market has now fragmented to the point that many manufacturers use custom plastics in a variety suited to the particular telephone being made. Some phones contain five or more plastics. In recent years the growth in mobile phones has forced new developments in reinforced plastics. They are now combined with fibres of glass, carbon, or other plastics to make an almost unbreakable case. Surface coating is a fast-growing area. How do you make a phone look like metal without embedding radio-wave blocking metal particles in it? How do you coat or surface-modify a plastic so it is (finally) immune to fading from ultraviolet light?
Just making a telephone case nowadays uses technologies that could not even be considered a century ago. For instance, the case will include high-tensile plastic clips to hold it together, or the plastic shells will simply be bonded together in a sealed unit (such as a handset). It will have the numbers cast into the case or buttons in a different colour at the moulding stage, rather than painted on later. Items like a mobile phone aerial or the cover over the screen will be moulded into the assembly during production, and will probably be made from a completely different plastic. The lense for the built-in camera will be a perfectly transparent, optically-accurate plastic. And the finished product will be durable, largely waterproof, and almost unaffected by sunlight.
For this article, I searched in vain for an example of a modern telephone with details of the range of plastics used to build it. I suspect that the information is not available simply because there is an incredible range of plastics available off the shelf, and the exact types are no longer important. In spite of the early struggles and intense research to solve the problems, plastics are now taken for granted.