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%.

Charting ePTFE (expanded PTFE) Specifications as per Global Standards

One of the toughest things about being the first in a given field is that there is so little data available for testing against.

As the only Indian company manufacturing ePTFE (expanded PTFE) gasket tapes, we are constantly met with questions regarding how the properties of our material hold up against those of competing brands operating in Europe and the USA. However, since the material is so new, there do not exist any established testing standards locally for us to check the product.

Basic Initial Data

To counter this, we initially took up the task of importing tapes from other manufacturers and testing the tensile properties and specific gravity against the same. Initially, we were trying to answer only 2 questions:

1. How soft should our tape be?
   Since the extent to which we expand the PTFE can be adjusted, it directly impacts the specific gravity of the end product. For a like-to-like comparison, we were hoping to match this with global brands. We eventually found that the standard density of ePTFE Gasket Tapes is 0.6-0.65g/cm3
   It should also be mentioned that some clients have specifically come to us asking whether they can get the tapes even softer, as their application is such that not much force can be applied to the tape. We have obliged – getting the density down to as little as 0.3g/cm3 in some cases.

2. How strong should our tape be?
   ePTFE tape looks great coming out of the machine. It is pure white, soft to touch and very smooth. However, two tapes that look exactly the same, could give completely contrasting values when tested for tensile strength. We found that global brands offered tensile strengths in the range of 5Mpa to 10Mpa.
   Once we standardised our production process, our own tapes showed a tensile strength of 12Mpa, so we were satisfied with the result.

Looking for global standards

Although we were happy with the properties of our material, there were still gaps in our understanding. Most notably, what were the other properties we should be testing? And rather than compare between brands, should there not be a global standard that specified the values we needed to obtain?

Again, going through competitor data provided very little information on this front. Expanded PTFE is a very niche market and from our own experience of getting the product right, we know that not much information can be divulged with regards to the behaviour of the material.

We looked around for global standards and realised that although there are many ASTM standards for regular PTFE, for ePTFE there were none. A few competitors had put up data on compressibility (ASTM F 36) and creep relaxation (ASTM F 38), but these were only comparing values to “leading brands” and not referring to any standard for the values. Others simply quoted the values, but did not elaborate the specifications against which these values would hold up.

We also went through the certifications that competitor brands were providing. These included:

1. DVGW VP 403 – The German standard for checking ePTFE Tapes
2. TUV MUC-KSP-A066 – The TUV Standard for ePTFE
3. BAM – For use in Oxygen rich environments

We contacted each of these organisations and were given estimates on how much the testing would cost. However, at no point are any values discussed. These remain guarded by the certification bodies. Our worry was that if we sent our material to these bodies without adequately testing them ourselves first – there was a risk that we may have overlooked a certain property and due to this, the product may not pass, resulting in an expensive mistake.

Eventually, we came upon the one standard that dealt specifically with ePTFE Tapes and was willing to offer values for us to compare against – the AMS 3255A.

The AMS is globally recognised as a leading authority for aerospace related materials. As such, we felt confident that their values would be stringent and thereby an effective standard to hold ourselves to.

The AMS 3255A prescribes many types of ePTFE Tape. Our basic tape falls under Class 2, Type 1, which requires the following properties to be met:

Property	Value/Result	Unit
Specific Gravity	0.4-1.2
Tensile Strength	3.44	Mpa
Tensile Strength (Fluid/Thermal Stability)	8.27	Mpa
Low Temperature Flexibility	No evidence of cracking
Liquid Sealability	No fluid leakage or loss of pressurization
Reparability	No fluid leakage or loss of pressurization

Armed with the above data and the testing procedures prescribed by the AMS 3255A, we were able to test our material in-house to confirm that the properties we were observing were as per the requirements of the standards.

In addition to this, we were also able to test the material with local certification bodies, to confirm the properties.

To the best of our knowledge, the AMS 3255A remains the only globally recognised standard to offer any values and/or testing procedures to verify the properties of ePTFE Gasket Tapes.

Expanded PTFE (ePTFE) Tapes – Properties and Installation Techniques

ePTFE Tapes are an ideal form-in-place gasket material made from 100% pure virgin PTFE that has been expanded to achieve a foamy structure.

While we have covered the applications of this earlier and also touched upon some of the variants, we would like to re-explore some of the finer aspects of the material as well as look at the installation in more detail.

ePTFE Structure

A number of variables contribute to what we would define as a suitable final product. While the nuances of the production technique are proprietary information, what we can reveal is that the end product, while seemingly uniform, can be anything but.

1. Specific Gravity
   In achieving a final product that matches the properties of other ePTFE Gasket Tapes in the market, one of the key properties explored was the density of the material. By varying the process, this parameter can be altered to produce different results. While most commercially available ePTFE Tapes have a specific gravity of 0.6-0.7, we were able to bring this down to 0.3, making the material much softer and more malleable.
   This reduction in density is not always preferred. In applications where the ePTFE is sandwiched between steel elements, you would prefer a higher specific gravity. However, in delicate applications, such as electronics and medicine, we may need to use a gasket or sealing element upon which large pressures cannot be applied as the equipment themselves are fragile. In such cases, a low density tape would be ideal as it would take the shape required with minimal pressure
2. Thickness
   Standard ePTFE tapes come in thicknesses starting from 1.5mm. However, certain applications such as cable wrapping and filtration require tapes as low as 0.1mm in thickness. While achieving was not easy, it did allow us to explore the properties of tapes under 1mm in thickness and gauge what made them so different.
   ePTFE Tapes fall somewhere between sintered PTFE tapes (made from skiving a fully sintered PTFE billet) and thread sealant tape (made from calendaring unsintered PTFE strips). As such, they imbibe the electrical properties of skived tapes, while retaining the malleable structure of thread sealant tape. Furthermore, the foamy structure allows for better thermal insulation as compared to both the other variants
3. Fillers
   Fillers are commonly used in PTFE to attain variations in final properties. We have experimented with fillers of PEEK and Carbon to reveal variations that significantly improve what the ePTFE tapes are capable of. Carbon allows us to make anti-static tapes, which are used extensively in the manufacture of co-axial cables. PEEK, meanwhile allows for vastly improved wear properties, while not violating any dielectric parameters or FDA parameters.
   In addition to this, we will be looking at fillers of glass and graphite. Each will bring its own unique alterations to the material.

Sizes available

FluoroFoam ePTFE gasket tape is an ideal solution for flange connections, container rims and any other metal to metal application requiring a compressible, chemically resistant seal.

The tape comes with one-side adhesive that aids in installation by allowing an exact placement of the gasket lining.

	Standard Spool Lengths (others on request)
Size (mm)	5m	10m	25m	50m
1.5 x 3
2 x 5
2.5 x 7
3 x 10
4 x 12
5 x 14
6 x 17
7 x 20
5 x 25
5 x 28

Installation guidelines:

Completely clean the sealing area and remove any dirt, corrosion, oil or leftover from old gasket material.

Cut one ending of the sealing tape and remove just a little of the protecting paper. Place the tape at the nearest possible position next to the bolts, starting next to a bolt hole. Fit the gasket around the entire flange circumference and across the endings as shown in figure 1.

Assembled in fragile flanges apply techniques as shown in figure 2. Skive the endings as shown in fig. 3 and overlap according to the recommended overlap length. Cut off the excess, tapering to the end, leaving a total thickness of approx. 120 %.

At least 4 progressive torque sequences with a torque wrench, in a star of 180° (fig. 1), should follow the first torque by hand.

Lastly perform a circular torque to check and ensure a tight and long-lasting seal.

Photos of typical applications:

The ePTFE Tape is allowed to overlap at the ends to ensure complete sealing.
Due to the softness of the tape, this overlap is accommodated during compression causing no variation in thickness

The one-side adhesive backing allows installation even on vertical surfaces, eliminating the need for grooves, clamps or bolting arrangements.

Metal-to-metal flange connections benefit greatly from the use of FluoroFoam ePTFE.
The ePTFE takes the exact shape of the gap between the two metal members ensuring a perfect seal with minimal effort

Form-in-place ePTFE gasket tape can effectively seal flanges on large shell-and-tube exchangers
Available in running lengths – the tape saves big on cost in areas where large, custom made gaskets would be too expensive

The versatility and texture of the material ensures that there is no shape that cannot be attained.
This adds to the effectiveness as well and reduces costs significantly compared with custom made gasket and sealing solutions

Advantages of FluoroFoam ePTFE Gasket Tape:

• Quick and simple installation: adhesive strip makes installation easier while the shape and versatility of the material means minimal cutting and sizing
• Reduced down time: standard sizes are immediately available ex-stock
• Reduced stock: a few spools of different sizes cover most applications within a plant
• No risk: the texture of FluoroFoam ensures the material accommodates the shape of the mating member, so there is no chance of the equipment getting damaged by the ePTFE
• Safe: ePTFE is chemically inert and can therefore be used even in the harshest environments without risk of reacting with the surrounding substances
• No waste: FluoroFoam comes in a spool, so no material gets wasted
• Cost effective – FluoroFoam can be used to replace custom made gaskets that are expensive and made-to-order. This is results in a huge cost saving in larger diameter pipes and vessels. 

Technical Details:

• Temperature range: – 240°C up to +260°C, for short periods up to +310°C
• Chemical resistance: resistant against all chemicals from pH 0-14 – except molten alkali metals and elemental fluorine at high temperature and pressure
• Pressure resistance: vacuum up to 200 bar
• Density: 0,65 g/cm³, +/- 0,1g/cm³ (for rectangular cross sections only)
• Aging resistance: FluoroFoam itself does not age and is UV-resistant. However, the adhesive backing may lose its effectiveness if kept unused for too long
• Colour: white (other pigments available on demand)
• Fillers: FluoroFoam can be offered in virgin and carbon filled variants
• Others: FluoroFoam is physiologically harmless. It has no smell or taste. It is neither contaminating nor toxic. It is made using FDA approved raw materials