Tuesday, March 1, 2016

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 PTFEgasket 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.
  1. 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 realized that although there are many ASTM standards for regular PTFE, forePTFE 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:

PropertyValue/ResultUnit
Specific Gravity0.4-1.2
Tensile Strength3.44Mpa
Tensile Strength (Fluid/Thermal Stability)8.27Mpa
Low Temperature FlexibilityNo evidence of cracking
Liquid SealabilityNo fluid leakage or loss of pressurization
ReparabilityNo 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.