Friday, April 4, 2014

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 PTFEbeing 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.
VirginFilled grades
StandardValue in standardStandardValue in standard
DensityASTM D 4894YesASTM D 4745Yes
Avg. Particle SizeASTM D 4894YesASTM D 4894Yes
Tensile StrengthASTM D 4894YesASTM D 4745Yes
Elongation at breakASTM D 4894YesASTM D 4745Yes
Shore D HardnessASTM D 2240NoASTM D 2240No
Linear Expansion Coefficient (-50°C to +15°C)ASTM E 831 / ASTM D 696Yes (696)ASTM E 831 / ASTM D 696No
E-modulus (tensile)ISO R 527NoISO R 527No
Wear resistance (with Taber abraser method)ASTM G 195-08NoASTM G 195-08No
Deformation Under Load – Total deformation after 24HASTM D 621 ANoASTM D 621 ANo
Static Coefficient of frictionASTM D 1894NoASTM D 1894No
Dielectric StrengthASTM D 149NoASTM D 149No
Dieelectric ConstantASTM D 150NoASTM D 150No
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.