Unravelling Polymers

The Definitive Blog on Polymers by Poly Fluoro Ltd.

Polymer Sealing Solutions – High Performance Seals, Valves, and Seats

As mechanical technologies continue to advance, there is an increasing need to push for higher speeds, loads, and outputs. The result is that moving parts within an equipment are not only subjected to more wear and tear, but the nature of the system’s construction calls for components that can operate for years without the need for external maintenance.

To keep up with these requirements, engineering polymers have also continued to advance in performance. Existing polymers have benefitted from an increased understanding on how to blend them with other materials to enhance their properties. At the same time, new polymers have entered the market, having hitherto been either in a purely development stage or else being manufactured at such low scales that there was no commercial viability to mass usage.

While a truly comprehensive list of all the polymers used in seals, valves, and seats would require a much longer write-up, the most popular polymers and their benefits are given below:

1. PTFE

The preference of PTFE as a sealing material is well known. Items such as seals, ball-valve seats, and sealing tapes are manufactured using PTFE. The material exhibits certain key properties that make it invaluable in sealing applications. PTFE has the lowest coefficient of friction among any commercially available materials. Against polished stainless steel, PTFE exhibits a ‘near rolling’ coefficient of as low as 0.03.

In addition to this, the ability of the material to withstand high temperatures and resist very corrosive chemicals makes it both durable and maintenance free.

PTFE can be used both in virgin form or compounded with fillers such as carbon, bronze, and glass to enhance properties for a given application.

Areas of application include:

  1. Ball valve seats

  2. Valves/conduits for semi-conductor manufacturing

  3. Gaskets and gasket tapes (ePTFE)

  4. Lip seals and rotary seals

  5. Chevron seals (V-packings)

  6. Hydraulic and pneumatic seals

  7. Bellows and manifolds for pumps

  8. PTFE ferrules

  9. Sealing guides and strips

  10. Spring-energised seals

2. Peek

PEEK is one of the most robust polymers available. It exhibits a tensile strength in excess of 100Mpa and is a hard material that is easily machinable to very close tolerances. Unlike PTFE, which is softer and can be prone to deformation under high loads and temperatures, PEEK has excellent dimensional stability across a huge range of temperatures. As a result, PEEK valves are used in high temperature fluid applications. PEEK is also chemically strong and only succumbs to sulfuric acid.

While PEEK is effective and versatile in its virgin form, the addition of carbon, glass, graphite, and PTFE micropowers can greatly enhance it and add self-lubricating properties to the base material.

Areas of application include:

  1. Ball valve seats
  2. Valves/conduits for high temperature fluids (eg: coffee machines)
  3. Chevron seals (V-packings)
  4. Hydraulic and pneumatic sealing rings
  5. Pump manifolds
  6. Rotary seals
  7. PEEK ferrules
  8. Spring-energised seals

3. POM/Polyacetal/Delrin

POM is a lightweight, durable polymer that lends itself very easily to machining. The ability to attain close tolerances and high finishes makes this a very popular choice for sealing applications. POM has excellent dimensional stability, but its inability to withstand temperatures above 150°C limits its use to applications where very high temperatures are not present.

Cost-wise POM is far cheaper than both PTFE and PEEK, which makes it preferred in automotive applications and high-volume applications. Further, POM can be both easily machined and easily injection moulded. In applications where volumes are huge, POM can be moulded rather than machined, allowing for superior savings while not compromising on dimensional parameters.

While POM is usually used in its virgin form, the addition of PTFE to this polymer can create a compound with superior lubricity.

Areas of application include:

1. Ball valve seats

2. Valves/conduits for electrical and mechanical applications

3. Chevron seals (V-packings)

4. Hydraulic and pneumatic sealing rings

5. Rotary seals

6. POM ferrules for cable end-fittings

4. Nylons/Polyamides

While nylon is a well know polymer, nylons or polyamides are actually a family of polymer grades, each with its own properties. The common grades of nylon include PA6, PA66, PA11, and PA12. PA6 and PA66 are mechanical grades, exhibiting higher tensile strengths and wear resistance. PA11 is primarily used in coatings while PA12 is used to make extruded tubes and profiles.

For most sealing applications, PA6 and PA66 are commonly used. Nylons are easily machinable and are soft polymers that create very effective seals when under pressure. The material is not prone to cracking, so the sealing is effective over the long term. One disadvantage of nylons is that they have high water absorption, causing the parts to swell. As a result, their use in humid/wet conditions must be limited.

Nylons work especially well when blended with MoS2. The addition of MoS2 greatly enhances both the strength and the wear resistance of the material.

Areas of application include:

1. Hydraulic and pneumatic sealing rings

2. Pump manifolds

3. Nylon bobbins

4. Sealing guides and strips

5. Polyurethane

Among all the polymers, polyurethane is unique in that it behaves most like an elastomer (rubber). The nature of PU is such that the material feels almost elastic and has a lot of ‘bounce back’. Despite this, the polymer machines easily and is among the most widely preferred sealing materials.

Like POM, PU also has a limitation on temperatures and can deform when subjected to very high loads. However, the ability of the material to regain its shape and to create seals against even uneven surfaces makes it a very special sealing material. Further, the option to injection mould PU means that sealing strips and elements can be made in large volumes and at a relatively low price when compared with other polymers

Areas of application include:

1. Hydraulic and pneumatic sealing rings

2. Sealing guides and strips

3. Chevron seals (V-packings)

4. Ball valve seats

5. Spring-energised seals

6. UHMWPE (Ultra-high Molecular Weight Polyethylene)

While UHMWPE is mostly used for its high wear resistance, there exist a few areas where wear must be combined with good seal-ability. Along with high abrasion resistance, UHMWPE is also known for a very low coefficient of friction (second only to PTFE on this list). These characteristics make it an ideal low-cost candidate for low temperature applications where movement is high and there is the need for a soft polymer seal.

UHMWPE is also the lightest polymer in this group, with a specific gravity of only 1. It is extremely useful as a rotary seal material where lubrication is difficult and where high movement requires a material that can withstand the same.

Areas of application include:

1. Rotary seals

2. Sealing elements

3. Sealing strips and guides


Read More

1. High Performance Polymers in Railway Applications

2. Polymer Sealing Solutions – High Performance Seals, Valves, and Seats

3. Polymer Wear Plates - Grade Selection and Key Advantages

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