Taking PTFE sealing technology a step further

The wide range of sealing options provided by PTFE can be expanded even further by combining it with other materials, such as metal, elastomers or high-performance plastics

Thanks to its outstanding properties, PTFE (polytetrafluoroethylene), also known under such trademarks as Teflon, Hostalflon, Fluon and others, spans a broad spectrum of applications, ranging from the temperature-resistant, non-stick coating of standard household frying pans all the way through to high end functional components for automotive safety and security systems.

Available from FTL Seals Technology is a comprehensive range of sealing and engineered solutions including PTFE compounds.

Tony Snowdon, Marketing Director, FTL Seals Technology says: "The wide range of innovative solutions encompassed by PTFE can be expanded even further by combining PTFE with other materials, such as metal, elastomers or high-performance plastics, or by individually adapting PTFE to specific requirements by adding appropriate fillers.

For this reason, the use of PTFE compound requires extensive compound and material know how." Among the best known characteristics of PTFE are its excellent sliding properties, its extremely wide range of temperature - from - 260deg to +300deg, and its virtually universal chemical resistance.

PTFE is suitable for powder-coating application to different material surfaces under pressure and temperature.

Either compression moulding or extrusion makes semi-finished products.

In compression-moulding processes, the PTFE powder is either compressed in one direction (automatic compression moulding) or pressed from all sides (isostatic compression molding).

Extrusion technology on the other hand, presses PTFE powder through a nozzle, followed by the sintering process.

Once a semi-finished PTFE product has been made of in this way, respective processing, like turning, milling, etc will create a PTFE assembly component.

These components can later be finished into composites with other compounds or materials, like duroplastics, thermoplastics, elastomers and metals.

To join PTFE with PTFE or other compounds, various joining techniques may be used: Flush jointing, such as welding, gluing, vulcanizing, laminating; Frictionally connected (force-locking) joints, for example, pressing, shrinking, clamping; or Form-locking joints for example, flanging, folding.

These various joining techniques can be combined with each other.

The following is are brief descriptions of the major joining techniques for PTFE: Gluing: Due to its extremely high resistance to solvents as well as its anti-adhesive properties, PTFE can only be glued or adhesion-bonded to other materials after the respective bonding surfaces have been properly prepared.

There are various techniques known for this purpose.

In addition to wet-chemical activation with sodium and ammonia solutions, plasma treatments of PTFE surfaces have been successfully used.

Clamping: Considering the cold-flow behavior of PTFE, a sealing lip may be inserted into a metal housing and subsequently extruded.

The cold-flow behaviour of PTFE is counter-acted by additional insertion of an elastomer disk.

Among others, this joining technique is used for radial lip seals with PTFE sealing lips.

Welding: Due to its high melting viscosity, PTFE will not produce a liquid smelt when heated.

For this reason, flush joints of PTFE require a special process.

The PTFE-component is heated up to sintering temperature (340C) and joined under pressure and temperature.

To join the two PTFE surfaces, a hot-melt adhesive made of a PTFE-like fluorothermoplastic compound (PFA, FEP) is used.

The fact that this process is performed in the melting range of PTFE requires complex tools.

Shrinking: Roller covers made of hot-shrinkable PTFE tubes can be applied to all types of rollers and rolls quickly and cost efficiently.

The tubes are expanded after manufacture, pushed over the roller and then hot-shrunk by using a standard industrial hot air gun.

This technique takes advantage of the memory effect of PTFE, which means that, while being heated, the expanded tube will "remember" its dimension prior to expansion.

Roller covers of any size will shrink by 20 to 25% of their diameter for a taut and reliable fit.

This process requires no adhesives or other special preparations.

Vulcanizing: If PTFE and elastomers are to be joined, the PTFE-surface must be chemically activated and primed.

Thereafter, non-cross-linked elastomers (e.g FPM, NBR) can be scorched under pressure and temperature.

Laminating: Laminating means that base materials, e.g metal or plastic fabrics are imbedded into PTFE under application of pressure and temperature.

Certain properties of PTFE, like modulus of elasticity, dimensional stability, thermal conductivity, pressure resistance, etc.

are increased.

Properties like cold flow, permeation and elongation can be reduced.

Even the properties of the composite are enhanced by the use of PTFE.

Chemical resistance, anti-stick characteristics, electric insulation and corrosion resistance are enhanced as well.

Properties like dielectric characteristics can be reduced.

Pure PTFE has a relatively low wear resistance.

This is due to the fact that the PTFE particles are not bonded in a real smelt but are more or less mechanically connected in a sintering process.

A significant improvement in wear resistance is achieved by means of fillers such as carbon, graphite, glass fibres, bronze or organic fillers (thermoplastics).

Since no PTFE compound is capable of meeting all requirements, the type of PTFE best suited to the needs of a particular application must be investigated.

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