Cast Phenolic Resins
Martin Apley

Cast resins form a most interesting section of the phenolics field, with many outstandingly colourful, attractive and collectable products. The Author spent his early working life in this area with the famous specialist company Catalin Ltd. Apart from more conventional applications, he also reveals intriguing contributions made towards victory in the Second World War.

The process of casting dates back to prehistoric times, first with copper and bronze; then later with precious metals, as with gold and silver in ancient Egypt; iron for weapons and armour; and, more recently, steel. It is also more familiar as applied to wax and jelly. All these processes involve pouring a hot liquid into a container where it solidifies as it cools. And all these materials can be regarded as thermoplastic because when the solid castings are reheated they melt and reliquefy.

James Swinburne found a way of taming the reaction of phenol and formaldehyde to produce resins that formed tough, hard coatings on brass and other metals. But it was Leo Baekeland who invented and developed moulded products from these chemicals, which were produced under heat and pressure but could not be melted on reheating - i.e. they were thermosetting. While such mouldings increasingly found markets in the electrical, automotive and other industries, ways were being sought of avoiding the heavy equipment needed for the process. Chemists in Austria succeeded with a controlled reaction of phenol with higher ratios of formaldehyde to obtain liquid resins that were cast into moulds without pressure and hardened by prolonged heating at temperatures lower than those used in the moulding process. These were also thermosetting.

The Catalin Corporation was formed in New jersey in 1928 to exploit this process and made rapid headway, largely because the cast products were in an attractive variety of colours: whites, ivories, pastel shades, vivid reds, yellows and blues, etc., in strong contrast to the dull browns and blacks of the moulded phenolics.

To exploit the market in Europe and the Commonwealth, Catalin Ltd. was set up in England in 1937, not as a subsidiary of the American concern but autonomous although using the latter's technology. Dr Riesenfeld, who had acted as consultant in America and was one of the inventors of the process, became technical director. (There were at this time two other companies producing cast phenolics: Marblette in the USA and Raschig in Germany, with processes differing in some respects from that of Catalin.) The British company was installed in the large building in Waltham Abbey, Essex, that had housed the Nobel company in the First World War which made munitions. The building provided ample space to accommodate a battery of six nickel reaction vessels (called 'kettles' in the American style) and six large circulating hot-air ovens for curing (i.e. hardening) the cast resins in their moulds, together with ancillary pumps, boiler, workshops, laboratories and offices.

The process
Phenol, a colourless crystalline solid with a melting point just over 40'C, was stored warm in tanks to facilitate metering and pumping. Formaldehyde is a gas but was transported and used as a solution in water with a 40% gas content. Metered amounts of phenol and formaldehyde were charged into the kettle to dissolve on heating and an alkaline catalyst added to speed up the exothermic reaction, rapidly coming to the boil. Spillage was prevented by cooling the kettle to which the condensed vapours were returned. After a specified time, when the reaction had calmed, acid was added to neutralise the alkali. To get rid of the large amount of water in the formaldehyde solution, the kettle was connected to a vacuum line. Because evaporation by vacuum would cool the contents steam was applied to the jacket to maintain the temperature at WC until reaching the endpoint at a particular viscosity something like that of cold treacle. The hot resin was then run out through a valve at the base of the kettle and filtered into the moulds.

It was extremely important to ensure that the quantities of the reactants and process temperatures and timings were accurately maintained to ensure products with consistent properties. On a painfully memorable occasion - the only one in 16 years - a works chemist added ten times too much caustic soda catalyst Oust a simple slip of a decimal point!) resulting in a volcanic eruption, completely out of control. Masked operators directed jets of cold water from fire hoses on to the kettle. The whole area had to be evacuated and the kettle dismantled, and it took three days to remove the rockhard bubbled mass with pneumatic drills.

The moulds into which the resin was poured were made by dipping a cold mandrel, most simply a polished steel rod, into a large vessel of molten lead for a few seconds to form a thin solid shell. The mandrel was then plunged into a cold water bath and the shell tapped off on to a rack, to be conveyed with a number of others to the base of the kettle to be filled with resin. The rack was then taken by forklift to one of the large ovens where it remained for four to five days to harden the resin completely. After removal and cooling, the hard cast resin rod was punched out of the mould by a pneumatic hammer, and the empty and now distorted mould returned to the lead pot for remelting and re-use. The attractive colours were obtained by adding selected dyestuffs and pigments to the liquid resin in the kettle before pouring into the moulds.

Catalin produced three basic types of cast resin: opaque in white and ivory, mainly as sheet and rod for industrial applications; and translucent and transparent materials in a full range of colours. The transparent range was made with a different catalyst and included an alkyd plasticizer. It lent itself to delicately tinted shades and special effects such as fluorescent colours, products with glittering tinsel flakes and, for one customer, flakes of genuine gold leaf. Mottled effects were quite simple to effect. While pouring the main colour into the moulds, a resin of different colour was dribbled into the main flow in a thinner stream: a vein of a third colour could be added in the same way. Because the resins were so viscous during the hardening stage, the colours remained largely distinct. Thus a Brazilian onyx effect would consist of a pale green base with a white mottle plus a thinner red streak. It was not, of course possible to produce absolutely identical patterns - but then neither are those in the natural stones.

Products
Castings were delivered to customers who carried out their own fabricating or finishing operations - cutting, slicing, drilling, grinding, sanding and final polishing. They were mainly manufacturers of cutlery, brushes, buckles, dress jewellery, buttons, stationery goods and other domestic ware. The most prestigious order was for 2000 toilet seats and covers for the Queen Elizabeth, then being fitted out. These were in attractive pearlwhite, beautifully polished and, apart from being completely stainproof, were reputed to have a warm luxurious feel. As the liner was immediately pressed into service for the armed forces at the beginning of the Second World War, 1 don't know if they were actually fitted. 1 like to think that they were and gave some pleasure and comfort to their users.

With the advent of the war much of the production for domestic ware declined, except for items such as Addis toothbrush handles. Instead there was now demand for material, preferably in finished state, for gears, gear knobs, handles, brackets, instrument panels, electrical connectors, etc. for the services. One order was for control knobs for aircraft flying at night without instrument panel lighting: the pilots were to distinguish the knobs - square, circular, oval, etc. - by feel. A set of such handmachined knobs were rushed to De Havilland, but the ministry inspector failed them as differing by twelve-thousandths of an inch from the approved drawing. This of course made no difference to the pilot as they were only for recognition by touch, and after messages to higher authority they were accepted.

After the war, when demand for domestic ware resurfaced, a severe shortage of phenol coincided with a clamour from old customers and new entrepreneurs. If they needed half-inch diameter black rod that was not available they would accept one-quarter inch white sheet and stocks virtually vanished. Where all this material ended up is problematical: some is known to have appeared as counters, discs, buttons, dice, chessmen and draughtsmen, and toys. An unusual order was received from China at this time for one million chopsticks. It was never repeated - nothing wrong with the product, but the order had been placed by the Chiang Kai-Shek administration just prior to his exile by the Mao communist regime.

Another application was billiard balls, because of the attractive depth of colour that could be achieved. For this, it was necessary to increase density to give the right weight. Most available fillers lowered the strength of cast resins which owe their inherent strength to their colloidal structure and the absence of crystalline wedges. A very fine filler, precipitated barium sulphate, proved to give the required density with only marginal decreases in strength. The day came when the sales director displayed them to the prospective customer by dropping them from some forty-five feet on to a concrete pathway: they chipped (as even steel balls would) but did not disintegrate. A few years later a brilliant Hungarian engineer who joined the company devised a novel way of making lead moulds for spherical products that eliminated the wasteful method of machining them from round rod. This led to useful business in supplying bowls balls.

Catacol
Going back to 1939, when war looked inevitable, a development of cast resin proved to have good potential. It arose from an observation that, when the liquid at the pouring stage was coated on to a wooden panel and hardened in the oven, it emerged as a hard clear coating that was tenaciously bonded to the wood. So a sandwich of two wooden panels was made with a thin liquid resin layer in between. When this emerged from the heat treatment it was impossible to separate the panels without pulling out wooden fibres. The glue was stronger than the wood even after prolonged immersion in boiling water. A strong glue that takes days to harden was obviously of no commercial use: how to shorten the setting time? Strong common acids were found to work, but although they reduced the setting time to a practical level they considerably weakened the glue. Eventually an acid was found - para-toluene sulphonic acid (PTSA) - that when mixed with the resin at room temperature set within a few hours and completely hardened overnight: the product was very strong and tough. At higher temperatures of 80-100'C it would harden within a few minutes -just right for making high-performance plywood.

To comply with the British Standard for such glues, authentic testing was necessary but in the absence of a tensometer we had to resort to the classical British method of 'string and sealing wax'. With the help of our machine shop we used a steel girder with clamps for the glued specimen at one end and a large bucket at the other with a steel rod as fulcrum. Lead shot was tipped into the bucket until the test piece broke. It was always the wood that broke, not the glue. Many such tests confirmed the high strength after immersing test pieces in boiling water for several hours. To get official approval to use the glue in important wartime applications it was necessary to press a number of plywood boards. These were made at the Avro company on the Isle of Wight as Catalin did not have a hot press. They were tested by the Forest Products Laboratory and fully approved.

Catacol was tried out by an enterprising furniture- maker, Merron of Bow, to make a complete dinghy with the usual double curvature in plywood that withstood the attack of water. It was described in a trade journal as 'The Boat without a Nail'. Catacol was also to prove of value in the construction of the Mosquito aircraft, the wings and fuselage of which were of plywood. The orig
inal amino glue had been successful for service in Europe but high ambient temperatures and humidity in the Far East war zone caused the plywood to delaminate. Catacol solved this problem. A large furniture factory in Hertfordshire was converted to make Mosquito wings and fuselages, and the success of this enterprise brought a knighthood to the company's chairman Harris Lebus.

Catafil
A further development based on Catacol was a cast resin with a very fine filler, a special china clay similar to that used to make delicate porcelain. The filled resin mixed with PTSA acid could be cast at room temperature on to a plaster master held in a box, the surfaces being coated with parting lacquer to prevent adhesion. The casting could be hardened overnight at 0-60'C and the cast tool separated from the master ready for use. Such tools were used to shape and form by pressure thin alloy and cellulose acetate sheets into three-dimensional shapes used by the armed forces. They replaced metal tools that took weeks to make, and were much cheaper. One such tool, with rib recesses, slots and double curvature, was twelve feet long by three feet wide and weighed 800 pounds - the largest cast phenolic item to be made in the UK.

The special attribute of cast phenolics is their high strength achieved without the use of reinforcing fibrous material used in moulded products. The reason is the colloidal nature of the product with no particles to weaken the material when stressed. A few ultra-fine fillers could be tolerated without impairing strength: thus Catafil had a compressive strength of 12 000 lb/in'.

Catalex
Another development on the Catacol theme was Catalex, a cast phenolic solid expanded foam . As with Catafil it could be cast in solid shapes. It was produced by incorporating a special dyestuff in the resin which generated small bubbles of nitrogen when mixed with the PTSA acid and warmed at WC. The solid cast foam was produced within a few hours.

This product proved to have an unusual application in the'bouncing bomb'. The film The Dam Busters reveals the ingenuity and meticulous calculations directed to the delivery of the bombs by their creator, Barnes Wallis. One of the problems not there discussed was that the forces created by the speed at which the bomb first impacted on the water dented the original steel casing and diverted the trajectory from its calculated path. The use of a thicker steel shell would have added too much weight to the bomb. What was needed was a strong, reasonably lightweight material within the shell to hold it rigid at the point of impact. Castings of Catalex were made by Catalin in a series of decreasing densities for testing at the Royal Aircraft Establishment, one being found to have the required combination of low density and high strength. Each of the steel bomb castings received the appropriate weight of Catalex resin direct from the kettle, conveyed to the warm oven where it expanded to fill the entire shell except for the compartment needed to hold the explosive. How effectively these bombs were used by the RAF in the attack on the Ruhr dams is a matter of history.