Unravelling Polymers

The Definitive Blog on Polymers by Poly Fluoro Ltd.

PEEK Seals - Numerous Applications, Many Choices

As a polymer, PEEK is most often compared with PTFE. The two have multiple similarities including good temperature resistance, chemical inertness and dielectric strength. When it comes to pure physical strength however, PEEK moves ahead on two counts.

First – the absolute strength of the material is much higher. With a higher tensile strength and hardness, PEEK is preferred to PTFE in applications where dimensional stability over prolonged physical strain is required. Although PTFEdoes have fillers, such as glass and carbon, which allow for increased stiffness, it still does not compare with PEEK on this metric.

Second – PEEK has a lower specific gravity (1.35 against 2.25 for PTFE). As a result, in applications where the overall weight of the assembly needs to be minimized, PEEK emerges a winner.

One such application where PEEK is highly sought after is in the seals industry. Seals themselves include a huge range of polymers, elastomers and metals, each of which rely on the specific characteristics of the material being used to achieve effectiveness in its application.

Types of PEEK seals

Piston Ring Seals

Piston rings are used primarily to aid wear absorption on the outer diameter of the piston shaft. PEEK is hard enough to withstand the extensive wear induced within the piston, but not hard enough to damage the metal components themselves. The rings are usually machined from a PEEK bush and have different types of cuts, which aid in installation and performance.

Ball Valve Seats

Ball valve seats show a predominant preference for PTFE, as they require a soft material that yields easily to the shape of the ball valve. However, there are a significant number of PEEK seats being used in high-performance valves, where both the PTFE and the metal are machined to ensure a proper fit. Typically, we see these being used in valves employed on oil-rigs or power plants, where the high temperatures indicate a requirement for a polymer slightly tougher than PTFE.

Rotary Shaft Seals

We have developed compounded grades of PTFE with PEEK to cater to the rotary shaft seals market. The combination of PTFE and PEEK is a powerful one. The PTFE provides a boost to the self-lubrication properties, while the PEEK adds strength. Although they work well together, specific applications do call for pure PEEK. The purpose is similar to that of the piston ring, except here the shaft moves radially. PEEK again serves the purpose of being able to withstand wear at high RPMs, while being soft enough not to damage the metal in the event of misalignment or seal failure.

Ball and Butterfly Valve Seats

A number of different materials are used in this application, including PTFE, Delrin and UHMWPE. PEEK finds acceptance specifically in applications with high pressures and temperatures. Butterfly valves are an integral part of any fluid regulatory system, including hydroelectric power plants, oil and gas refineries and shipping.

Manufacturing process

PEEK seals and seats are made primarily via machining. It is possible to injection mould the components directly, but this involves extensive tooling. Furthermore, the precision needed on the part’s dimensions would dictate the need for further machining. Hence, unless the volumes are vast, it is most likely machined from a bush.

The bush itself may be either extruded or compression moulded. Extrusion offers higher productivity and longer length parts, but is again dependent on the correct type of tooling being available. Compression moulding is cost effective and allows the dies to be modified easily, so that the moulded part is made with minimal excess material (a very key criterion when dealing with an expensive material like PEEK). The issue with compression moulding is that it is a slow process with very limited productivity.

So looking at the trade-off between productivity and tooling cost, an OEM can accordingly decide which method to adopt, depending on the volumes.

Variants in PEEK

While most specifications call for pure, virgin (unfilled) PEEK, there are requirements for filled variants also. Most commonly, PEEK is used with a 30% Glass or Carbon filler to aid properties such as creep, dimensional stability and flexural strength.

As mentioned above, PEEK also does well with PTFE. More specifically, compression moulding best-practices sometimes recommend the addition of 5% PTFE into the PEEK mould, as this allows for better self lubrication of the material, while letting it maintain its superior strength.

Another polymer well suited to blending with PEEK is Polyimide. Although the blend is not nearly as proven as the regular filled variants, initial studies show that the addition of Polyimide allows PEEK to maintain its flexural modulus over a much high temperature range as against unfilled PEEK.

It is difficult to combine too many other polymers with PEEK, simply because the temperatures needed to process PEEKfar exceed the melting points of most of these polymers.

A word on PEK

PEK or PAEK has recently emerged as a competitor to PEEK. Industry experts have observed that while PEK does match PEEK on most metrics, it’s long-term effectiveness in maintaining its properties is still being tested.

We recently received a failed seal from an OEM, asking us to analyse whether it was PEEK. After testing it in a lab, it was found that the part was made using PEK. The end-user claimed that the part had only survived a few months in his valve assembly, before failing. This may have been a one-off incident, or could also point to the improper processing of the PEK part. However, it is useful to keep in mind.

Conclusion

PEEK is well known as a versatile polymer. Seals and seats are one more application where this material finds application. The product, however, requires precise dimensional tolerances that not all processors are able to offer. In addition to this, the availability of variants both within PEEK and amongst competing polymers makes the choice of material an exercise that the OEM must take very seriously, before committing one way or another.

Charting the standards used in defining PTFE properties

It was recently brought to our notice by an astute client that the data quoted in many of the generic online sources did not give a complete picture of the values and correct test methods needed in checking the properties of PTFE.

A quick online search of a given property of PTFE churns out a number of data sheets from various supplier websites. And although the values and standards more or less match across these sources, our own study has revealed the following:

  1. Some of the standards quoted are incorrect
  2. The values quoted do not have any reference as many of the standards only specify the test method and not the value reference

As a result, with an obscure polymer like PTFE, we find that information has been carried forward from older data sheets and passed on until no one is very sure what the “correct” value is anymore. We ourselves have reached a dead-end on a number of metrics, but we have done our best to fill the gaps using verifiable data.

Let’s look at point (1) above. The most commonly quoted standard for PTFE is ASTM D 1457. We see this standard in a number of places and only after trying to buy a copy online were we informed that ASTM D 4894 had replaced the ASTM D 1457 in 2001.

Clients who – due to the effect of legacy – still refer to the ASTM D 1457 sometimes get upset when we send them test reports quoting ASTM D 4894 and it requires some discussion with their QA team before the new standard is accepted.

However, even the ASTM D 4894 only applies to virgin PTFE. For filled grades of PTFE, we refer to the ASTM D 4745<. This again requires a discussion with the client as is especially problematic when the client orders a very specialized grade. Since the ASTM D 4745 only covers the more general filled grades of PTFE, clients who order an irregular grade feel frustrated that there is no standard pertaining to their requirement.

Both the standards, however, do provide some basic values of tensile strength, elongation and specific gravity, which help in checking whether the properties attained after testing are in line with the requirements.

However, as the table below shows, very few of the standards actually give any values. For a whole list of properties, the standards only tell you how to check the value, but do not make any recommendations on what those values should be. Furthermore, due to PTFE being a niche polymer, some of the standards – such as ASTM D 2240 – actually pertain to other materials and the test method is simply employed for PTFE.

  Virgin Filled grades
  Standard Value in standard Standard Value in standard
Density ASTM D 4894 Yes ASTM D 4745 Yes
Avg. Particle Size ASTM D 4894 Yes ASTM D 4894 Yes
Tensile Strength ASTM D 4894 Yes ASTM D 4745 Yes
Elongation at break ASTM D 4894 Yes ASTM D 4745 Yes
Shore D Hardness ASTM D 2240 No ASTM D 2240 No
Linear Expansion Coefficient (-50°C to +15°C) ASTM E 831 / ASTM D 696 Yes (696) ASTM E 831 / ASTM D 696 No
E-modulus (tensile) ISO R 527 No ISO R 527 No
Wear resistance (with Taber abraser method) ASTM G 195-08 No ASTM G 195-08 No
Deformation Under Load – Total deformation after 24H ASTM D 621 A No ASTM D 621 A No
Static Coefficient of friction ASTM D 1894 No ASTM D 1894 No
Dielectric Strength ASTM D 149 No ASTM D 149 No
Dieelectric Constant ASTM D 150 No ASTM D 150 No

As mentioned earlier, my client was curious to know what benchmarks were being followed when we quoted the values expected for each metric. However, we were unable to find any organisation that actively published data on PTFE and its filled grades.

In trying to trace back the values seen across so many data sheets (they are all in the same range, so we assumed they have some common source), we were able to find references old manuals released by DuPont, Dyneon and Daikin from where these values were obtained. Obviously, once we referenced DuPont, the client was satisfied and we were able to neatly define both the correct standard and the value with the proper reference.

It is however interesting to note that as widely used and accepted as PTFE is, there still does not exist any up-to-date properties standard that can be used by manufacturers as reference. The DuPont website does have values of virgin PTFE – but they reference the ASTM D 1457 – which suggests that maybe the information is dated. Not to say that the values would have changed significantly, but QA is a continuous process and something published within the last decade might offer a lot of support to both manufacturers and OEMs alike.

PTFE vs PEEK - A Comparison of Properties

Although both PTFE and PEEK are well established within their respective fields, there are frequently questions around which would better suit a given application. OEMs typically have to make a choice based on technical suitability and hence need to be better informed as to how these materials match up against each other.

Below is a short comparison on properties between these two polymers and can be used a guide to aid new product development.

Parameter PTFE PEEK Preferred material
Price Moderately expensive Very expensive PTFE
Tensile Strength 25-35 Mpa 90-100 Mpa PEEK
Elongation 350-400% 30-40% PTFE
Compressive Strength 30-40 Mpa 140 Mpa PEEK
Flexural Modulus 495 Mpa 3900 Mpa PEEK
Coefficient of Friction 0.03-0.05 0.35-0.45 PTFE
Temperature resistance Up to 250°C Up to 250°C NA
Dielectric strength 50-150 Kv/mm 50 Kv/mm PTFE
Chemical resistance Virtually inert Affected by Sulphuric acid PTFE
Coefficient of linear thermal expansion 14 x 10-5/K 5 x 10-5/K PEEK
Machine-ability Good Very good PEEK

In a nutshell, applications requiring strength and low levels of deformation would usually employ PEEK, whereas those requiring resistance to voltage or chemicals utilize PTFE. PTFE also rates highly in that it is self-lubricating. This makes it a preferred choice in high wear applications.

The biggest disadvantage of PEEK remains the price. It is roughly 10 times the price of PTFE and as a result has remained a niche polymer, used only in applications where it is absolutely necessary.

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