PEEK filled PTFE – A Useful Blend

The blending of polymers with additives is a common practice. There are very few materials that are used purely in their virgin form and PTFE is no exception here. Adding materials such a bronze, carbon and glass (to name but a few) have allowed us to augment the properties of PTFE to suit specific applications. In each case, we sacrifice some element of the original property of the PTFE, but enhance another. To take the case of bronze – the addition significantly increases the coefficient of friction of the PTFE and eliminates all electrical insulation properties. However, this is offset by a large and highly sought after increase in wear and hardness. Hence bronze filled PTFE is a preferred compound for a number of automotive and industrial applications.

In exploring what different additives do to the final properties of PTFE, we have found literature relating to materials such as those above, as well as less used additives such as molybdenum-di-sulphide, ekonol, stainless steel and graphite.

With the addition of PEEK, however, we find few sources with which to refer to on properties. While we do receive many requests for PEEK filled PTFE, the actual test data to support the compound is not easily obtained.

A 2006 paper titled: “A low friction and ultra-low wear rate PEEK/PTFE composite”, by David L. Burris, W. Gregory Sawyer, is all we have to refer to in this respect, but we will see that there are sufficient insights to help any OEM designer to assess the exact composition needed.

PTFE with PEEK fillers

We have come across a few applications where PEEK filled PTFE is the requested material. In most cases, what we receive is only a sample from the client. The light brown colour combined with the fact that the material “feels like”PTFE, is usually all we have to go with. Usually, the compound is used in sealing applications where high RPMs are involved.

Blending PEEK and PTFE

Unlike most other additives, PEEK blends with PTFE quite effortlessly. The lower particle size of PEEK (about 5microns against 25microns for PTFE) means that the grains of loose PEEK powder flow easily in between the PTFE grains and allow for a reasonably good blend. Further mixing is needed to ensure that the blend is uniform, but in our experience, it was less of a challenge to blend PEEK with PTFE than to blend pigments with PTFE.

Processing the material requires some minor fine tuning in the sintering cycle. However, when done properly, the resulting product is a very light brown that machines easily and offers some interesting properties.

Properties of PEEK filled PTFE

The paper by David L. Burris, W. Gregory Sawyer only looks into the wear and coefficient of friction of the blends of PEEK with PTFE. The paper looks at ratios (by weight) of 5%, 10%, 20%, 30%, 40%, 50% and 70%. The results obtained can be seen on the graphs below.

Coefficient of friction

Coefficient of Friction – PEEK filled PTFE

PTFE has a lower coefficient of friction than PEEK, so it would be reasonable to assume that the value keeps increasing with the addition of more PEEK. However, it is surprising to note that the coefficient is lowest at 50% of PEEK – at about 0.12.

It is important to mention than even at its lowest, the coefficient of friction is still much higher than for pure virgin PTFE(between 0.03-0.05). However, from a design standpoint, it is useful to know that adding a very small amount of PEEK is not the key to keeping the overall coefficient of friction as low as possible.

Wear resistance

Wear Resistance – PEEK filled PTFE

Again, given that virgin PEEK has better wear resistance when compared with virgin PTFE, we would assume that adding more PEEK keep improving this property. However, we again see that the best performing blend is PTFE+32% PEEK.

Conclusion

The above findings are useful from the point of view of grade selection. If an OEM wishes to design a seal using a combination that minimises the coefficient of friction and wear rates, they would be better off using a filler percentage close to 40%.

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 PTFE does 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 PEEK far 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.

PEEK in India – A Growing Market with Many Challenges

At Poly Fluoro Ltd. we started our journey with PTFE and gradually expanded into other polymers. Initially, this was at the behest of existing customers, but over time our expertise in machining plastics meant that we were comfortable offering a variety of options to our clients, rather than try and force fit PTFE into their application.
We discovered the benefits of PEEK in one such exercise. Although we have already blogged extensively on the benefits and properties of PEEK, our own experience in dealing with this material serves to explain much of the commercial and technical queries surrounding this material.

PEEK in India is a small market in terms of volumes. The total consumption is only about 35 Tonnes. Of this, most of the material is imported as semi-finished rods and sheet, with only 12-15 Tonnes being processed from resin indigenously. Small as these numbers are, keep in mind that semi-finished PEEK sells at anywhere between US$275-US$400 per Kg – so in value terms, the market is not as small as the volumes suggest. Nonetheless, it is very much a niche market – even among speciality polymers.

Being present in the PEEK market as a processor poses many challenges. Some of these are technical in nature, while others relate to the commercial issue (PEEK is very expensive) and how clients respond to PEEK. Again, we have touched on some of these points in our earlier article – but as we have delved deeper into PEEK processing, many new findings have arisen.

Compression moulding PEEK not a simple affair

There are many challenges in compression moulding PEEK and most of these do not get explicitly highlighted in manuals and guidebooks. In most manuals, the process is outlined in 5-6 basic steps, which at first glance make PEEK appear a very friendly material to deal with.

In reality, the process is time-consuming, highly sensitive to the exact process needed and very specific in the type of tooling required.

The benefit of compression moulding PEEK over, say extrusion is that we are able to make customized dimensions based on the customer drawings. The stock piece for a part measuring 70mm in diameter can be moulded as 72mm, rather than using a 75mm rod. Over a 50mm length, this saves almost 25 Grams per part – which is significant when we consider the cost per Kg. Furthermore, if the part has an internal diameter the saving is even more, as the same cannot be attained in extrusion for large diameters.

However, against this saving, the time consumed to make a 50mm part would be many times what extrusion would take. Compression moulding is known for low productivity and even a large processor is only able to consume 20-25 Kgs per day of production. In India, however, where labour is inexpensive, this is not a huge cost factor – it only limits volumes. And since PEEK is still a low volume polymer – even processing 4-5 Kgs a day can be significant.

The actual process of compression moulding PEEK is also not straightforward. The 5-6 steps mentioned in the manuals each contain nuances that need to be fine tuned until you reach a process that most suits the equipment available. In our own experience, we have found that over 25-30 trials had to be taken, each using up between 250-800 Grams of resin. After each trial, some parameters were changed before taking another trial. Parameters such as pressure, peak temperature and soak time all need to be varied to control issue such as porosity, cracking, black spots and cold spots.

In addition to this, the selection of dies is critical. PEEK, in its molten form can be a very aggressive material and we have had many steel dies get corroded during moulding. Again – finding a balance between a strong die metal and the correct process is critical in obtaining a final process that is both economical and productive and which yields a high quality final product.

Variants and substitutes do exist for the price conscious

We have had some success in blending PTFE with PEEK ratios of 5%, 10% and 15% by weight (ie: PTFE+5/10/15% PEEK). Again – the process of blending is not straightforward and the PTFE itself needs to be processed slightly differently owing to the fact that PEEK melts at a higher temperature than PTFE. However, the final blend has proven to be useful in applications involving sealing and needing high wear resistance, with a low coefficient of friction.

Another alternative to PEEK is PEK. PEK is very similar to PEEK and is processed in much the same way. As far as properties go, some have even suggested it is slightly superior on some parameters. Commercially, it is roughly half the price of PEEK – which makes it a very tempting alternative. However, PEK is still being proven in OEM applications, whereas PEEK has been around long enough to give any OEM designer confidence in its properties.

Marketing PEEK is a challenge

In a market like India – which is highly price sensitive – PEEK is a difficult product to win customers over with.

PEEK is usually the last choice of any OEM due to its price, and if someone has not come across the material before, it takes some educating before they are convinced that any polymer exists at such a price. And while PEEK is well established in the West – in India, it is still very nascent in comparison and clients do not always see the long-term benefits of using it.

Furthermore, the relatively recent introduction of PEK into India is threatening to take some of the long-term market share from PEEK, as in a price sensitive market, people may be willing to make the gamble on a cheaper substitute.