![]() ![]() One major challenge when improving the creep resistance of PTFE is connecting the filler to the matrix. Ideally, different fillers can be combined to create a sealing material that offers the right mechanical and tribological properties for the respective application. Glass fibres, for example, can cause significant damage through abrasive wear if coupled with soft counterparts in a dry-running system, while graphite loses its lubricating effect in very dry gases. ![]() Fillers for use in sealing materials must be selected carefully, taking external factors such as gas, pressure, dry-running/lubricated and counterparts into account. Inexpensive glass fibres are often used here, along with carbon or aramid fibres and other fillers such as carbon, graphite, molybdenum disulphide, bronze particles, etc. To reduce cold flow and improve the mechanical properties, PTFE is not usually used in its pure form in mechanically stressed parts but is instead reinforced with fillers. This leads to unwanted cold flow behaviour (creep). The long molecule chains in PTFE display only very low intermolecular interactions, allowing them to slide over each other very easily in the crystalline areas. But this can cause problems related to wetting, adhering or welding PTFE. The low surface tension leads to very low adhesion to other materials, which is why PTFE is often used as a non-stick material. This molecular structure gives PTFE its characteristic properties such as a high melting point, excellent chemical resistance and low surface tension, and allows for very low friction coefficients. The carbon chains in PTFE are completely surrounded by a layer of fluorine. ![]() PTFE is a semi-crystalline polymer consisting of carbon (C) and fluorine (F) with extremely long linear molecule chains (n to 106). It offers a range of positive characteristics for this application.Ĭharacteristics of ptfe and how they are influenced 1 The best-known plastic for use in sealing elements is polytetrafluoroethylene (PTFE), commonly known as Teflon. This allows for lower operating risks and a significantly reduced lubrication rate compared to metal rings. Another important advantage of using plastics as a sealing element material is the fact that, in the event of insufficient lubrication, the sealing elements are worn while the counterpart surface remains free of damage. The dominance of plastics over metals is not just due to the tribological features for such applications. Metals such as grey cast iron or bronze are only used at very high pressures. ![]() The majority of modern oil-lubricated compressors also use plastic sealing elements. Plastics suitable for this purpose have almost completely replaced dry-running materials based on carbon or graphite. In a piston compressor, seal and rider rings made from dry-running plastic compounds operating within permitted limits enable gas compression without the need for additional lubricants such as oil or grease. Furthermore, PTFE-based sealing elements can be used in combination with low-cost counterpart materials such as grey cast iron instead of nitrided or tungsten carbide-coated steels. PTFE-based materials offer outstanding chemical resistance and are much cheaper than those made from high-temperature polymers such as PEEK. The newly developed Persisto® 870 pushes the limits of PTFE based sealing elements to higher pressures, resulting in a number of benefits. However, these high-temperature polymers exhibit lower chemical resistance and require harder counterpart materials. This pressure range is limited due to the lack of mechanical stability, which is why manufacturers often have to resort to costlier high-temperature polymers for medium and high pressures. ![]()
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