New Process for Moulding Teflon

The name polytetrafluoroethylene or PTFE probably means very little to most of us, whereas we are more likely to be familiar with its trade name Teflon®. Invented by DuPont in 1938, this plastic was used in simple products such as films, pipes, and seals, and especially for coating frying pans to prevent food from sticking and burning. PTFE is created by the polymerisation of tetrafluoroethylene: when single molecules are joined together into larger molecule chains, a hard white mass is formed. Standard PTFE (Teflon®) molecule chains are very long. If you wanted to make a scale model of a PTFE chain where the basic element tetrafluoroethylene was a centimeter long, the chain would stretch out to a length of about a kilometer. PTFE shows extreme resistance to chemical influences and withstands temperatures of up to 325oC, but is far harder to process than most other plastics.

Tough molecule rods

The material‘s excellent thermic and chemical resistance are the result of the very strong covalent carbon-fluorine bonds in the material itself, which create great strength within the molecule chains. PTFE molecules look like smooth interlinked rods. They have low friction on account of their slipperiness, but are inclined to deform under prolonged mechanical loading. Their high viscosity is a disadvantage in industrial processing: because of the length of the molecules and their tendency to knot together, the material remains firm and can only be processed when cold, i.e. mechanically, unlike most other commercial plastics. For the past 60 years scientists the world over have been racking their brains without success to find a way to mould PTFE warm so that it can be used in a wider range of industrial products.

All this could now change with a discovery made at ETH Zurich, the Swiss University of Technology. Scientists working with Paul Smith, Professor of Polymer Technology, have been testing a wide number of commercial grades of PTFE with different molecular weights and viscosities for their meltability and their mechanical properties. As a starting point they concentrated mainly on PTFE micropowders with low molecular weight and low viscosity, such as are used today for inks and coatings. 'We were able to make products from these by the usual melt process, but they broke up like candle wax as soon as we extruded them, i.e. pressed them through nozzles in ductile form, or removed them from the press moulds“, says Scientist Theo Tervoort, who carried out the work in conjunction with Jeroen Visjager and Brian Graf, both working towards their doctorates. 'This brittleness results from the shortness of the molecule chains in these crystalline polymers: if the material is to have mechanical strength then the chains must be longer than the crystal is thick. Only then can the macromolecules form covalent bonds between the crystals“. Standard PTFE powders had too high a softening point, and were found to consist of gritty, slow-melting particles which took considerable time to become mechanically strong.

Refined mix for complex components

By cleverly combining both types of PTFE powder, the scientists succeeded in producing a new, highly dense PTFE which could be press or injection moulded when warm by an inexpensive process, and which moreover produced mechanically strong products. 'Previously it was thought that high molecular weights were the deciding factor for achieving the desired properties“, says Theo Tervoort. 'We have proven that the opposite is true.“ Paul Smith describes it as a quantum leap: 'The new PTFE opens up a wide spectrum of applications such as coatings, fibres, rods and above all complex components which cannot be produced in any other way. It also gives rise to the possibility of new types of PTFE adhesives and heat welds, as well as polymer granulates and mixes and reinforced PTFE.“ The team hope to develop the latter to counter the problem of eventual deformation under prolonged mechanical load, as yet unsolved. Reinforcement with glass or carbon fibres should increase the resistance of the material and should not be difficult to achieve with its new-found workability.

The ETH scientists have already set themselves a new challenge: can they produce the desired material with the necessary low molecular weight directly, rather than by mixing PTFE polymers of different molecular weights? The future will tell. For now the team has lodged a wide-ranging patent application, as there is much at stake. Paul Smith estimates the total annual global turnover in fluoropolymers to be in the region of 2 billion US dollars, the majority of which is generated by PTFE. 'The new product innovations that will become possible with the new types of PTFE could develop into a market worth an additional 400 million US dollars“.