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.

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.

We have earlier looked at Lubring and explained how it is a result of a very successful branding exercise that has stood the test of time. In truth, as we now know, Lubring is a PTFE based composition and has been successfully substituted in many applications with equivalent PTFE formulations.

Another very successful branding venture has been that of Rulon*. Although we do not see the demand for Rulon* being as high as that of Lubring, there has been a very conscious and well thought out strategy which has kept the compositions of this brand ambiguous, to the point that clients find it very tough to accept any alternatives.

In addition to this, the unique pigmenting of each Rulon* grade offers further ambiguity. Visually, a client is unable to reconcile with a substitute when the colors do not match. It should be mentioned here that in many cases, we have seen that pigments help alter the properties of PTFE in a very measurable and positive way. For example, the green-blue pigment of Lubring has been proven to offer better PV values than the same composition in, say brown color. Therefore, while the pigmenting of Rulon* does help the branding considerably, we would assume the pigments themselves were not chosen randomly, but by testing different variants and choosing the one that had maximum impact on the properties required.

We have done some research to try and lay out the compositions of the most popular Rulon* grades, in the hope that it will make the choice a little easier for an OEM or manufacturer. In most cases, these appear to be regular PTFE grades that have been made unique using pigments. In some cases, such as Rulon J*, the grade is not regular, but can be easily blended as long as one knows the composition. The table below shows the various compositions and attributes of the most common grades of Rulon*.

We would like to point out a few things pertaining to the values of this table:

1. The Load values of Rulon* across grades seem to be considerably lower than those of comparable regular grades of PTFE. For example, PTFE+15% Glass has a tensile strength of >20 Mpa when tested in-house – which is almost 3 times what Rulon* offers. The reason for this lowering of load metrics is not quite known. Most likely the addition of pigments causes some sacrificing of load values
2. The PV values are comparable with regular grades of PTFE, however not so vastly different that it makes Rulon* superior in any obvious way. For example, Rulon LR* offers a PV of 10000, whereas PTFE+15% Glass offers only 7500. However, Rulon AR* also offers a PV of 10,000, whereas PTFE+25% Glass offers 12,000.

In a nutshell, we do not believe that the uniqueness of Rulon* pertains to any significant improvement in properties, but to a branding push given when PTFE was still an ambiguous material for many buyers. In recent times, many clients have adopted substitutes as they rightly feel the premium attached to Rulon* material is unjustified. Although rigorous testing is first done to prove that the substitute matches up with Rulon*, we have found that regular materials are more than equal to the task.

* Rulon is a brand name of Saint-Gobain Plastics.

PTFE is a preferred material in sliding bearings for three very specific reasons:

1. Load bearing capacity
2. Weather ability (due to its overall chemical inertness)
3. Low coefficient of friction

The first two factors are well accepted and easily tested. The vertical load on a bearing is simply tested by placing the bearing under a hydraulic press of suitable capacity, applying 1.25 times the rated load of the bearing and holding this load for a period of 1 hour to observe any adverse impact on the bearing material. In our own experience, it is very rare that pure PTFE would fail in this instance, since:

1. Pure PTFE genuinely does have a very high load capacity and even in the event of over-loading, would tend to deform rather than break down
2. The design load for most bearings incorporates a safety factor of up to 60% – implying that while PTFE may be able to withstand a load of up to 40Mpa, it is designed with a load of only 16Mpa and is thus well within its own capacity to take the load applied

Weather ability is difficult to test, as this is a long-term guarantee that the material can stay in outdoor conditions without experiencing any degradation in properties. However, most clients are happy to take this assurance at face value – as long as they can satisfy themselves that the material being used is in-fact pure PTFE. Its should be mentioned here that in the event that reprocessed or recycled PTFE is used in sliding bearings (a gross violation of quality norms, but one that may occur all the same, especially if the bearing manufacturer is buying their PTFE from a third-party and is therefore not directly in control of the quality), there will definitely be a failure of the bearing after installation.

As mentioned in earlier articles, we have witnessed many deviations from expected performance when presented with reprocessed PTFE. Amongst these is the tendency of the material to become brittle and even crumble after being kept outside for a prolonged period (usually over a few months). No doubt, a similar effect would be experienced by a material used in a bearing that is installed on a bridge or flyover – with the result that the bearing may fail after only a year of service.

Coefficient of friction

The problem with the coefficient of friction is that most people are not fully familiar with what it implies. So let us start with defining it and then look at the misassumptions surrounding it.

The coefficient of friction between two planes is defined as the ratio of the force needed to move one plane over the other divided by the force pushing the two planes together.

So in the case of a block resting on a table, coefficient of friction between the block and table would simply be the force needed to slide the block across the table, dived by the weight of the block itself.

Since the coefficient is a ratio of two forces, it does not have any units. A common mistake clients make is to ask us to define the unit we have considered for the coefficient of friction.

In the case of the PTFE sliding bearing, the coefficient of friction being considered is that between PTFE and polished stainless steel. Here again, a mistake is often made asking what the coefficient of friction of PTFE is. There is no such thing as a stand-alone value for coefficient of friction for any material. The coefficient between PTFE and polished stainless steel will no doubt be much lower that between PTFE and concrete. In other words, it would take more force to move a slab of PTFE across a concrete surface than it would to move the same slab across a polished stainless steel surface. Thus, when we talk about a coefficient of 0.04 between PTFE and stainless steel (the commonly accepted value for bearing manufacturers), we are saying that for a 1Kg PTFE block to slide across a polished stainless steel surface, it would require only 40 Grams of horizontal force. (Note: we are aware here that Kgs and Grams are units of mass and not force, but seeing as these are ratios of force, the values in Kgs/Grams against the values in Newtons would yield the same results).

Measuring coefficient of friction

We have already described that the value for the coefficient is derived by dividing the horizontal force over the vertical force. However, in practice, this is less straightforward. We would like to look at some of the methods that are used around the world to check these values as pertaining specifically to PTFE bearings, before describing what we feel is the most straightforward and easily implemented method.

1. Two-press methodIt is not possible to test the bearings simply by applying a vertical load and seeing at what horizontal load the bearing slides. This is because the bearing plate on which the horizontal load is applied also has another surface, which would be in contact with the vertical press and therefore be subject to friction from the press itself (which is likely to be very high).Therefore, the accepted method is to place 2 bearings, back-to-back and exert load on the centre.This process is technically sound, but practically not always feasible. For starters, the bearing shape itself may not lend itself to being placed back to back. It may have welded guides attached to it or be of an unusual shape. In addition to this, the process in expensive – requiring two hydraulic presses.
2. Load indicator method (laboratory)In this method, the stainless steel plate is placed horizontally with the PTFE plate on top. The PTFE plate is connected to a steel wire which is in-turn connected to a load indicator. The load indicator has a motor, which causes it to move upwards very slowly. As the indicator moves up and the wire gets tight, the load reading starts to show the horizontal load being applied. Once the PTFE platestarts to move, the reading is recorded and divided by the weight of the PTFE plate to give the coefficient of friction.This method is possibly the most accurate, as load indicators can offer values in milligrams, if required. However, it again suffers from the issue that if the bearing plates are not totally flat or are too big for the equipment, they cannot be accurately tested. Furthermore, it is debatable whether such accuracy is needed in the realm of sliding bearings
3. Poly Fluoro method (inclined plane)In an attempt to find a quick, repeatable, logical and universally applicable method to check the coefficient of friction, our method follows the rather simple process of gauging the angle of inclination. For starters, we do not believe that getting an accurate value of the coefficient of friction would add any value to the product. If we can confirm that the plates slide at a coefficient of friction set to 0.04, then it does not matter whether the coefficient is in-fact 0.032 or 0.036, as the product has met its required specification.

   The diagrams  show that when the planes are inclined, the coefficient of friction takes the value of the tangent of the angle of inclination. This allows us to easily check the coefficient of friction, as we simply set the ratio of Y to X to equal 0.04 and check if the PTFE plate slides down the plane, when placed on the SS plate. Again – note here that only in the event that the PTFE plate does not slide, can we conclude that the coefficient is greater than 0.04 (and hence outside tolerance). Whether the plate slides slowly or very fast, is of no consequence – as either way it confirms that the coefficient is at least 0.04.

   In the event that the client specifies a higher or lower coefficient, the same method can be employed, by simply changing the value of Y. So we first assess what the value of Y should be by multiplying the length of the bearing plate by 0.04 and then use a calibrated slip gauge to prop up the bearing on one side (preferably on a flat bed) so that the angle is attained.

   The method employed here is useful to check PTFE bearings as it can be applied to any design and multiple bearings can be checked from one lot if needed, without the hassle of making separate fixtures and modifications. Furthermore, it can be checked even before the bearing is assembled so as to confirm that the PTFE material being employed meets the parameters.

Static vs Dynamic coefficients

It must be mentioned that both static and dynamic coefficients of friction are relevant metrics and that the inclined plane method only measures the static coefficient. However, this is of no concern for PTFE bearings as PTFE is known to exhibit nearly identical values for both static and dynamic coefficients (a property unique to PTFE and not universally applicable).

We believe that the method described above is a more effective way to check for a very important parameter that bearings are based on.