Expanded PTFE (ePTFE) Gasket Tapes - Challenges in Testing

ePTFE (expanded PTFE) is one of the most versatile sub-products of PTFE. Like so many products in the high-performance polymer space, ePTFE needs to be processed in its own special way. Unlike regular PTFE – which itself is very painstaking to manufacture due to its unique properties – ePTFE is made via a combination of paste extrusion, followed by stretching on special equipment. The resulting polymer structure is useful for a number of reasons:

1. The softness of ePTFE allows it to be used as a gasket material, creating very effective seals even with minimal torque

2. The insulative properties of ePTFE allow thin tapes to be used in cable wrapping, offering high dielectric properties.

3. The porous structure of ePTFE is highly unique, as is allows gases and vapour to pass through, but not liquids and dust, making it an idea material for vents

4. ePTFE is also safe for the human body, meaning medical devices frequently employ the material in stents and grafts that can be left in the body indefinitely without causing any harm.

Despite all this, there exist very few actual standards that define the properties of ePTFE and outline testing parameters. Because a lot of ePTFE applications are so specialised, companies that develop solutions in this material usually treat the testing parameters and methods as proprietary. So, while the end product – such as a medical device, for example – may be approved by the device manufacturer, the process and testing metrics are held on to by the manufacturer.

As a result, developing an ePTFE solution for an OEM can be tricky, since the OEM has no yardstick against which to judge the material supplied. In many cases, the OEM may use the material in the end-assembly and decide that it works well. However, they would still need the ePTFE manufacturer to provide a set of testing criteria to be applied to every batch such that the OEM is guaranteed a consistent product. In this endeavour, and in the absence of any suitable standards, the decision needs to be taken between the OEM and the manufacturer on what tests and what values count as acceptable.

An interesting case presented itself when an OEM approved our tapes for a very specialised application but felt that there was inconsistency in some of the batches. We wanted a quantifiable way to assess the same so that both parties could agree on the results. The key requirements were the following:

1. The tape needed to have a consistent density and softness throughout

2. The specific gravity of the tape needed to be as close to 0.4 as possible

3. The tape should not have any excess fibrils and ‘gatoring’

Before we get into the above points, it should be mentioned that there does exist one useful standard for ePTFE tapes. The AMS3255A is an aerospace standard but offers enough by way of density and tensile properties to be a good starting point.

Going back to the OEM, they were trying to find ways to determine whether the properties were as they needed, but the results were all over the place.

For starters, they were using a durometer to measure the hardness of the tape. While Shore A and Shore D durometers are frequently used for elastomers and polymers respectively, in the case of ePTFE the durometer does not work. Why? Well, for one, ePTFE is porous. The tip of the durometer can easily pierce the surface of the tape and since the pressure is applied by a human hand, there may be areas where the tip pierces through and other areas where it does not. The other issue is that unlike elastomers, ePTFE has no elasticity. Once compressed, the material stays as such until (in the case of a tape), you pull it lengthwise, at which point it retains its original dimension. Both these issues mean that a Shore A durometer will give a wildly inconsistent and inaccurate reading.

The other issue was around something we call ‘breath back’. ePTFE tape is made via a stretching process, wherein the tape is rapidly stretched at a high temperature. The resulting tape is a smooth, marshmallow-like material that is easily compressed by hand. However, unless the tape is sufficiently restrained after stretching, it will slowing begin to shrink lengthwise. The extent of this shrinkage could be as much as 10-20%. So, even if we were able to achieve the specific gravity requirement of 0.4, once the material reached the OEM, there was every chance that it could have gone up to 0.5. It should also be stated that most ePTFE tapes available for industrial applications have a specific gravity of around 0.75. By attempting 0.4, we were already stretching the tape to the maximum, meaning the tendency to breathe back would be even more pronounced.

In the absence of any global testing standards, we had to adopt customized methods to check the consistency of the tapes.

1. We ran the tape through a calendaring machine to compress it and then checked to see whether the readings were consistent across the length. If there were lumps or weak spots in the tape, the result of this exercise would be a tape with varying thickness

2. We cut the tape into even lengths and checked each lengths for specific gravity

3. We built a fixture wherein the tape would sit under a v-shaped weight that would compress it. Again, if the thickness of the compressed tape against a fixed weight was uniform across different points on the tape, the tape could be deemed consistent

4. Finally, we cut lengths of the tape and performed a simple tensile test to check tensile strength and elongation. Variations in tensile strength and/or sudden breaks in the tape during load application would suggest weak spots within the tape

5. To address the issue of ‘breathe back’, the tape was re-stretched at 150 Degrees C before spooling and then spooled tight before shipping. While this certainly helped keep the tape soft during transit, the client needed to be informed that once the spool was opened, the shrinkage was inevitable. However, even a manual pull on the tape lengthwise would bring the tape back to its earlier softness, so it was something the client would need to do prior to assembly.

Because the stretching process is mechanical and because the specific gravity required was so low, this tape was especially tricky. It would therefore be impossible to avoid micro-variations in density across the full length of tape. However, we were able to produce a product that was consistent enough to answer all the tests we conducted, and would therefore be deemed suitable to this specific end-application.

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