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Composite Systems Made of PTFE for Sealing and Engineered Solutions

Thanks to its outstanding properties, PTFE (poly-tetra-fluoro-ethylene), features a broad spectrum of applications for the process industry.

The wide range of innovative solutions enabled 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 temperature range from -260°C to +300°C, and its virtually universal chemical resistance. PTFE is suitable for powder-coating application to different material surfaces under pressure and temperature.

Semi-finished products are made through either compression moulding or extrusion. In compression-moulding processes, the PTFE powder is either compressed in one direction (automatic compression moulding) or compressed from all sides (isostatic compression moulding). Extrusion technology on the other hand, presses PTFE powder through a nozzle, followed by the sintering process. Once a semi-finished product has been made of PTFE in this way, machining like turning, milling, etc will produce a PTFE assembly or 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 joints, such as welding, gluing, vulcanizing, laminating
• Frictionally connected (force-locking) joints, for example, pressing, shrinking, clamping
• For-locking joints for example, flanging, folding
• These various joining techniques can be combined with each other. The following is a brief description 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 oradhesion-bonded to other materials after the respective bonding surfaces have been properly prepared. There are various techniques know 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 behaviour 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 or 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 (+/- 340°C) 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 E-module, 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 fibers, 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.

The hydraulic and the isostatic press methods to produce semi-finished products are processed into finished components on modern CNC machines. The emphasis is on the processing of application-related special PTFE compounds. FTL Technology and Elring are equipped to develop and manufacture compounds. So far we have developed and used 500 compounds to customer-specific requirements. We have also set new standards with our facilities for manufacturing semi-finished PTFE products over 3000mm in length and diameter.

MATERIALS

Products in PTFE are, because of their unique properties, used particularly in the chemical, pharmaceutical and semi-conductor industries. The exceptional electrical properties (extremely high resistance, low loss factor) made PTFE essential in electronics engineering. PTFE components are non-adhesive and therefore easy to clean; they have excellent anti-friction properties (seals), are temperature resistant up to 260°C in continuous operations, stress-corrosion cracking resistant and they do not become brittle nor age. They are hydrolysis resistant and have extremely low residual ion content. Further advantages of modified PTFE are a more closely packed polymer structure with fewer pores, very low permeability, smoother surfaces, better film quality and a reduced stretch void index.

LINING LAMINATES AND FILMS

Products made from PTFE are ideal for linings used in the chemical, pharmaceutical and semi-conductor industry. They protect the apparatus against the media and the media against contamination.

PURITY OF THE MATERIALS

The purity of the PTFE products surpasses that of other "high purity" materials. PTFE can only be polymerized under extremely clean conditions. Only in that way are the high degrees of polymerization, which characterize this material achievable. When the raw material is processed into a semi-finished product or finished components, it passes through a sintering process with temperaturesof approximately 380°C.

FOILS AND LAMINATES

For lined steel and GRP vessels, lining laminates and sheets are ideal. They can be heat-formed into complex components and welded. The laminates are provided on one side with an extremely expandable knitted fabric and therefore provide the basis for a durable bond with metals and other plastics.

RESISTANCE TO CHEMICALS

As perfluorated polymers, PTFE and modified PTFE have an excellent resistance to chemicals. This is due to the strong carbon-fluorine bond and the near-perfect steric screening of the carbon chain in the polymer through the surrounding fluorine atoms. Chemical attacks are therefore - if at all - only possible via the polymer end group.

They are also common within food applications. Additionally, they are inert to practically all media with the exception of extremely aggressive chemicals.

PRODUCTS

• Lining laminates
• Lining films
• Large-diameter branches and pipes
• Isostatically pressed linings and jackets
• Welding rod and films
• Semi-finished products and finished components to measure
• Cover strips

All products are made of PTFE/modified PTFE (trade name e.g Dyneon™ TFM™ ) available in pure and conducting quality.

DESIGN ELEMENTS

Inserted bases for Vessels and Reactors. The bases must generally absorb mechanical forces, so that the use of pure flouropolymer is generally not possible. Since the use of fillers in fluoropolymers leads to a reduction in chemical resistance, Elring have the possibility of manufacturing a large number of compounds. For example, a material of sufficient resistance can be supplied for almost every requirement. Apart from the various PTFE compounds, all fully fluorinated plastics,if necessary with reinforcement,are used.

VESSELS AND CONTAINERS

The isostatic pressing method is used for the seamless pressing of the containers, which are as a result freefrom stress and distortion. The internal surfaces are not machined. Due to the extremely high resistance of PTFE to chemicals, all normally used acids, lyes and solvents can be stored in the vessels and containers, and this at temperatures from -200°C to +260°C. In this connection, geometric stability and complete drainability are important, which are also ensured by the fact that there is no welding. The products also do not absorb any water. The anti-adhesive property of PTFE is ideal particularly for containers and vessels.

SPHERES

Solid spheres can be manufactured, hollow spheres with a metal core. The hollow spheres are used, for example, as floats in etching baths. Solid spheres and spheres with a metal core are used as valve spheres in chemical pumps or in corrosion resistant check valves. FTL Technology also manufacture the appropriate ball valve seats for the spheres.

WAFER CARRIERS

The extremely high degree of purity of the raw material, the very smooth surfaces of the finished parts and the good weldability of the modified PTFE allow new design and manufacturing possibilities of large wafer carriers for 300mm wafers or for the manufacture of carriers for large flat-picture screens. At FTL Technology extensive machining facilities provide the basis for the manufacture even of complex structures.

BELLOWS

Bellows fail either because of the high number of stress cycles in the corrugated area or at the flange connection due to the stresses existing there. In both cases, FTL Technology have decisively improved the bellow design. Using modified PTFE, it was possible to increase the number of stress cycles further and PTFE compounds in the connection area of the bellows increased the service life many times over. Customer-specific parts of up to 3000mm can be manufactured.

LAMINATES AND FILMS FOR EXPANSION JOINT CONSTRUCTION

Expansion joints are flexible components in pipework and connection points for apparatus and plant construction. They are used particularly in plants with gaseous media as well as acid-bearing or alkaline gases. Expansion joints must be chemical and temperature resistant, gas-tight, flexible and tear-resistant. Only modern composite materials can meet these requirements. For special customers needs, too, effective solutions can be found quickly, offering expansion joint manufacturers suitable laminates and films.

COMPOSITE MATERIALS FOR SPECIAL AND NEW REQUIREMENTS

In plant construction the use of traditional material is for difficult tasks often expensive and unsatisfactory. A composite material which is specially matched to the application can produce increased system safety and a reduction in overall costs. With our experience, we can in this sector react quickly to customer enquiries.

HONEYCOMB STRUCTURE IN DESULPHURISATION PLANTS

Honeycombs in PTFE are, due to their chemical and temperature resistance, ideal as fillers for chemical reactors; they can be easily cleaned. These honeycombs are used in gas and air preheaters of flue gas desulphurization plants. The patented honeycombs are manufactured as monoblocks and can if required be welded together into large elements.

POROUS POLYTETRAFLUOROETHYLENE

Diaphragms, plates, fillers and diaphragms are made from porous PTFE. The porous PTFE filters can filter highly corrosive vapours, gases and liquids up to high temperatures. After that they are easy to clean, even after filtering sticky media. Gas feed pipes, which dip into water or water-like liquids, do not become dirty and clogged.Here again, the unique resistance to chemicals of the PTFE comes into its own. Water and electrolyte cannot enter the pores. The gas throughput is nevertheless high and uniform. It is similar with the diaphragms. They are impermeable to water and electrolyte up to high hydrostatic pressures. At the same time they offer a high gas and steam permeability.

SEMI-FINISHED PRODUCTS

Hydraulic pressing and sintering produces semi-finished products with diameters up to 3000mm. With the isostatic press method complicated shaped parts can be manufactured, including hollow parts and linings which do not require any appreciable finishing.The products are used for the production of finished components on CNC machines. The emphasis lies in the manufacture and processing of application-related PTFE special compounds. Sheets, bars, rods, rings, films and laminates of various sizes can be manufactured. The following products can be supplied:

• Bars
• Sheets up to 1500 x 1500mm and 3000 x 1000mm
• Rings up to 3000mm diameter
• Films up to 1600mm width
• Laminates up to 2600mm width

FTL Technology work in conjunction with Elring Kunststoff Technik of Bietigheim in Germany, one of the leading formulators of PTFE materials and manufacturers of sealing solutions in PTFE.

END

Further Information
For further information relating to any of the above products, please contact Tony Snowdon on 0113 252 1061 or e-mail tonys@ftltech.co.uk