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Universal problem solver

Flat seals based on modified PTFE
Universal problem solver

In the chemical, petrochemical and pharmaceutical industries great importance is placed on leakproof components or flanges with long-term sealing properties. The materials produced and utilised are often hazardous, harmful, toxic or flammable. The avoidance of leakages serves to protect not only employees but also equipment and the environment. The flat seals outlined below, which are based on modified PTFE, have diverse applications and maintain a permanent seal.

Dr. Michael Schlipf, Cyril X. Latty

Its numerous advantages – such as good resistance to chemicals, no adhesion to flanges, easy processability according to special methods and a wide temperature range from -250 to +250 °C – make polytetrafluoroethylene (PTFE) an ideal material for the requirements placed on seals in the chemical, petrochemical and pharmaceutical industries. However, the cold flow of the material – at room temperature and especially under pressure at higher temperatures – poses an enormous obstacle. The flow under pressure and at temperature may cause a reduction in the surface pressure of PTFE flange seals after a relatively short time period, which can lead to leakage. In order to counter this drawback, El-ringKlinger and Latty have jointly developed a special compound for flat seals based on modified PTFE (Fig. 1). The advantages of this modified PTFE compound in sealing systems are in particular:
  • Low deformation under load
  • Denser, low-pore polymer structure and lower permeability
  • Higher modulus of elasticity and reduced elongation under tensile load
  • Better elastic recovery, especially at higher temperatures
Several options for product optimisation are open to developers of new materials intended for the production of flange seals. The cold flow can be varied by choosing standard PTFE or modified PTFE, whereby the compressibility, pressure state strength and cold/hot deformation of the seal are altered. The permeation of gases and chemicals through the sealing material is also reduced up to 50 % with modified PTFE. The cold flow can be further reduced and the hardness of the material increased by adding fillers. In this case, however, it must be borne in mind that the potential spectrum of applications for the seal is reduced owing to the lower chemical resistance of all potentially suitable fillers in comparison to standard PTFE. Furthermore, fillers usually increase permeation through the sealing material. The hardness of the material additionally increases as a result of the filler, with an inevitable reduction in the adaptability and unevenness of the flange. The method used to produce compounds and the downstream processing steps have an effect on the homogeneity of the filler distribution and the sealing of the polymer compound.
Seal properties
The results of the optimisation process for manufacturing a flat seal material are described below. The permanent deformation of the material is shown in figure 2. The polymer matrix and filler content variables were changed. The dominant influence exerted by modified PTFE on the permanent deformation is clearly discernible. The compound variants are distinguished by a reduction in permanent deformation of up to two thirds, virtually independently of the filler content, which varied within the range from 15 to 30 % by weight. The material density of both PTFE variants decreases as the filler content increases. This is an indication that high filler contents cause the material bond to be loosened. But how do the polymer matrix variants and filler variations affect the mechanical properties of the material? Figure 3 shows the tensile strength at break. The best mechanical properties are obtained – as is to be expected – with the minimum filler load, as the polymer bond is least impacted by the filler in this case. The properties of the seal were analysed in a helium leakage test and compared to competitor products intended for identical applications. The test conditions were as follows:
  • Seal dimensions: 55 x 75 mm
  • Test medium: Helium, 40 bar
  • Surface pressure: 35 MPa
  • Test duration: 2 h
The combination of modified PTFE, which is characterised in particular by low permeation and reduced cold flow, with the minimum possible quantity of effective filler, facilitates a seal which cannot be matched by any of the tested rivals.
Empirical experience
Experience in the last few years has confirmed that the special flat seal Lattyflon 94L can be used in many industrial fields. An extensive package of approvals for demanding applications, for example conformity with the requirements of German clean air regulations (VDI 2440), approval in the presence of oxygen according to BAM, approval for food contact according to LNE (France) and FDA, and approval according to Air Liquide (oxygen), forms the basis for a broad range of applications. Several solutions to typical problems are listed below. Media such as formic acid, acetic acid or caustic lye of potash can be sealed up to 110 °C and 16 bar. Empirical experience up to 150 °C and 10 bar exists in the presence of ammonium hydroxide, ammonium nitrate and nitric acid. Sealing piston pumps with a combination of punched rings and packing rings enable failure-free operation up to 85 °C, pH 6 and 130 bar pressure of a hydrocarbon/mercury mixture. Fibre seals can be replaced in the food industry, with an associated reduction in the leakage rate and sealing variety.
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