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May-2018

Pushing the limits of ptfe-based sealing elements

In sealing technology, PTFE has become an established material in the low-pressure range.

Inga Olliges-Stadler and Norbert Feistel
Burckhardt Compression
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Article Summary
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. However, these high-temperature polymers exhibit lower chemical resistance and require harder counterpart materials.

The newly developed Persisto® 870 pushes the limits of PTFE based sealing elements to higher pressures, resulting in a number of benefits. PTFE-based materials offer outstanding chemical resistance and are much cheaper than those made from high-temperature polymers such as PEEK. 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.

Introduction
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. Plastics suitable for this purpose have almost completely replaced dry-running materials based on carbon or graphite.

The majority of modern oil-lubricated compressors also use plastic sealing elements. Metals such as grey cast iron or bronze are only used at very high pressures. The dominance of plastics over metals is not just due to the tribological features for such applications. 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. This allows for lower operating risks and a significantly reduced lubrication rate compared to metal rings.

The best-known plastic for use in sealing elements is polytetrafluoroethylene (PTFE), commonly known as Teflon. It offers a range of positive characteristics for this application.

Characteristics of ptfe and how they are influenced1
PTFE is a semi-crystalline polymer consisting of carbon (C) and fluorine (F) with extremely long linear molecule chains (n to 106). The carbon chains in PTFE are completely surrounded by a layer of fluorine. 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 low surface tension leads to very low adhesion to other materials, which is why PTFE is often used as a non-stick material. But this can cause problems related to wetting, adhering or welding PTFE. 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. This leads to unwanted cold flow behaviour (creep).

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. 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. Fillers for use in sealing materials must be selected carefully, taking external factors such as gas, pressure, dry-running/lubricated and counterparts into account. 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. Ideally, different fillers can be combined to create a sealing material that offers the right mechanical and tribological properties for the respective application.

One major challenge when improving the creep resistance of PTFE is connecting the filler to the matrix. Due to PTFE‘s low surface tension, this usually takes the form of mechanical anchoring. The connection between the fillers and the PTFE matrix can be improved through either physical anchoring or chemical bonding agents. Fibres or polymer fillers with a rough surface, for example, adhere better to the PTFE matrix. Chemical bonding agents are used with glass fibres, which are usually functionalised with a corresponding bonding agent for various polymer based materials. However, chemical bonding agents can cause unwanted reactions when used as a sealing material in piston compressors. The corresponding rings are no longer suitable for universal use, e.g. for operation in oxygen. Roughening the filler surfaces to achieve better adhesion in the matrix is also not always sufficient for parts such as sealing elements that are subjected to high tribological stress.

The limits to using filled ptfe in piston compressors
In dry-running, the use of filled PTFE materials is limited above all by a high wear rate and/or high temperatures caused by high friction. In oil-lubricated applications, by contrast, wear and friction are reduced to low values, assuming that the parts are lubricated correctly. Use is still limited here, however, mostly due to the aforementioned creep behaviour under pressure and thermal load. As a result, sealing elements made from PTFE materials are usually only used at pressures well below 200 bar. The significant influence of temperature on creep resistance is another reason why permitted usage limits are difficult to define. It is not possible to make precise statements on the average temperature of individual sealing elements during operation. If these limits are exceeded, ring material can flow into the gap between the piston and the cylinder or between the packing cup and the piston rod. In extreme cases, this can lead to a complete failure of the sealing system. Design measures, such as large cross sections and support rings made from metal or high-temperature polymer, aim to counter the flow of filled PTFE sealing elements. Despite this, sealing elements made from filled PTFE are unable to cover the pressure ranges present in the piston compressor.

High-temperature polymers as an alternative
High-temperature polymers are usually used when filled PTFE materials cannot provide the necessary reliability. These materials, such as polyimides, polyaryletherketones or polyphenylene sulphides, offer better high-temperature strength than PTFE. Polyetheretherketone (PEEK) is a plastic compound whose bearing-grade variant is commonly used for sealing elements in piston compressors. Bearing-grade PEEK often uses PTFE, graphite and carbon fibres as a filler, commonly with a mass fraction of 10% of each. A PEEK material modified in this way offers impressive mechanical properties compared to a PTFE material filled with carbon or graphite. Its tensile strength of 18 MPa at 250°C, for example, is higher than that of carbon graphite filled PTFE material at room temperature (just 10 MPa2). In industry, sealing elements made from these materials are used in oil-lubricated compressors at pressures of over 500 bar.

However, the high elastic modulus and low ductility of bearing grade PEEK has a negative effect in sealing element assembly, dynamic pressure loading and wear compensation. Rings made from this material are usually very stiff, which prevents them from lying evenly and completely on the sealing surfaces. This can increase leakage rates. Despite its general suitability for dry-running with soft counterparts like grey cast iron, this material can also cause damage due to abrasive wear. Some sealing element manufacturers therefore recommend a minimum hardness of 40 to 50 HRC for the counterparts.

When it comes to chemical resistance, the fluorine polymers far outperform the high-temperature polymers. PTFE displays a virtually universal chemical resistance, making it suitable for use in almost all process gases except for coolants containing fluorinated hydrocarbons. By contrast, the chemical resistance of high-temperature polymers like PEEK, PI or PPS in the respective process gas must be checked precisely.
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