Unravelling Polymers

The Definitive Blog on Polymers by Poly Fluoro Ltd.

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.

Demystifying Rulon

We have earlier looked at Turcite B* 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, Turcite* 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 Turcite*, 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 Turcite* 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*.

 

 

 

Product Description Filler Details Max Load (Mpa) Max. PV ((psi-fpm); Mpa-m/s) Properties Colour
Rulon LR PTFE+15% Glass 6.9 10,000; 0.35 High creep and abrasion resistance Maroon
Rulon AR PTFE+25% Glass 6.9 10,000; 0.35 Wear resistant, improved hardness, lower thermal expansion, lower deformation under load Maroon
Rulon 142 PTFE+Bronze (40-60%) 6.9 10,000; 0.35 High thermal conductivity; better creep resistance; linear bearing material Turquoise
Rulon 641 PTFE+15% Mineral 6.9 10,000; 0.35 Used mainly in food processing, FDA approved White
Rulon J PTFE+15% Polyimide 5.2 7500; 0.26 Good friction against soft metals Gold

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; Turcite is a brand name of Trelleborg Sealing Solutions