Unravelling Polymers

The Definitive Blog on Polymers by Poly Fluoro Ltd.

Expanded PTFE (ePTFE) Tapes vs Thread Sealant Tapes - What's the Difference?

When you manufacture a specialised product, one of the biggest challenges lies in ensuring the end-user recognises the technical advantages of the same. This is especially true when there are cheaper substitutes that compare with your product visually, implying the end-user might have doubts on whether you are selling him anything special, or whether you are simply charging a premium over something commoditised.

Expanded PTFE (ePTFE) Tapes are one of the most specialised variants of PTFE. Their uses go from simple gasket applications, all the way to intricate membranes for use in high-end filtration and medical membranes. However, the similarity between ePTFE Tapes and Thread Sealant Tapes (also called: Plumber’s Tape), can sometimes confuse clients, who might question whether they are not one and the same.

Thread Sealant Tape is a highly commoditised PTFE tape that is cheaply available in nearly any hardware store. As its name suggests, it is mainly used in plumbing, where it is wrapped around the threads of a pipe, before a mating pipe is tightened over it. The ability of the tape to easily take the shape of the existing threads means that it creates a tight fit, thereby preventing water leakage. Plumber’s Tape is a highly useful material in its designated application. However, it has grave limitations when compared with expanded PTFE (ePTFE) Tape.

Differences in Production

There exist many key differences in the production process. Thread Sealant Tape is made by extruding a bead of PTFE, which is then passed through various calendaring, slitting and spooling operations. The end result is an unsintered PTFE tape with a thickness of ~0.075mm (75µm).

Expanded PTFE (ePTFE) Tape is also made by first extruding the tape. However, this tape is then put through a drying process, after which it is passed through a stretching machine at elevated temperatures. The stretching process needs to be CNC controlled in order to ensure the stretch rate, speed and temperature are maintained as per strict parameters. The resulting tape would usually have a thickness ranging from 0.25mm to 15mm

The fact that Thread Sealant Tape is calendared, unsintered tape, while ePTFE Tape goes through a stretching and heating process is the reason the tapes exhibit such different properties.

Differences in End Properties

As mentioned above, the key purpose of Plumber’s Tape is to seal leaks in piping. The unsintered PTFE material is soft and easily takes the shape of the threads that it is wrapped around.

In contrast, ePTFE Tapes exhibit a range of properties, in addition to sealing, which make them vital across a number of industries. To start with, as a sealing material, ePTFE is used in areas where you not only require a seal, but where the seal needs to be capable of taking high pressures (up to 100Bar), high temperatures (up to 250°C), and be resistant to a range of corrosive chemicals. Trying to use simple Thread Sealant Tape in such demanding environments will cause the tape to degrade almost instantly, as it lacks the mechanical strength to withstand the same.

Expanded PTFE (ePTFE) also has high dielectric properties. The tape can resist immense levels of voltage and is used in high-end cable wrapping to improve efficiency and insulation. Again, some cable manufacturers do try and use Thread Sealant Tape to wrap around cables, after which the cables are sintered in order to fuse the tape. However, the resulting cable has a much lower insulative capacity and may tend to fail in higher intensity applications.

Finally, expanded PTFE (ePTFE) exhibits micro porosity. The calendaring process used in making Thread Sealant Tapes ensures that there are no pores in the material. However, because ePTFE is stretched under high temperatures, it attains a level of porosity. Most notably, ePTFE exhibits hydrophobic and oleophobic properties, meaning it repels water and oil respectively. At the same time, the material will allow the passage of vapours. This unique characteristic makes it an invaluable material in venting and filtration applications. It also allows ePTFE Membranes to find use as grafts and stents for use in the medical industry.

The combination of these properties ensures that ePTFE is demanded not only in fluid sealing systems, but in filtration, medical, heavy electricals, chemical plants and even aerospace applications.

When you consider the above points, it is easy to see that ePTFE Tape and Thread Sealant tapes are worlds apart in terms of their effectiveness and their breadth of application. Nonetheless, it should be noted that many applications are basic enough that using Thread Sealant Tapes might suffice. Commercially speaking, ePTFE would cost many multiples of what a simple Thread Sealant Tape costs. Hence, the decision to use ePTFE rests on the end application itself and on whether the required properties of the material need to extend to as high as what expanded PTFE offers.

Expanded PTFE (ePTFE) - Processing Challenges

The toughest part of being a pioneer in any field is that there is usually no one to turn to for technical help. When Poly Fluoro installed its ePTFE (expanded PTFE) gasket tape line in 2016, we never expected how peculiar and intricate the manufacturing process would be. Although there are a handful of manufacturers across the world, we knew we were the first company in India to start making this product. Furthermore, as all other manufacturers would be our competitors, we realised that we only had a few consultants, our equipment manufacturers and our raw material suppliers to turn to for assistance on processing techniques.

However, despite support from these areas, the fact that PTFE behaves differently to other materials based on the environment meant that a lot of learning had to be done in-house. As we went deeper into the process, we realised that there are many different parameters that need to be measured and monitored in order to achieve the final properties required. Here we touch upon these parameters. We cannot go into too much detail, as many of our findings remain proprietary. However, we would like to give a gist of the complexity involved and the engineering that goes into the development and manufacture of such a product.

1. Resin – the resin grade is important. ePTFE involves extrusion, followed by stretching. The resin needs to be a fine powder, capable of taking a good extrusion load. There are certain properties of the resin – such as extrusion pressure and particle size – that need to fall within a specified range. Else, the powder will not work.

2. Blending – the resin is blended with an extrusion aid (usually, naphtha), which will allow it to be extruded. Once the blending process is done, the resin can be extruded into the required profile (either rectangular, circular or even irregular). The blend time and the quantum of extrusion aid are critical. Too much or too little extrusion aid can mean a very soft or very hard extrudate respectively. It is very important to get this right.

3. Extrusion – extrusion is done at a steady rate at a pressure that will ensure the material is suitable for stretching. It is essential that the extrusion pressure sits within an acceptable range. If the pressure is too low, it would mean a weak extrudate, that might break during stretching. A high pressure is good, but too high might mean that the extrudate is too hard to stretch.

4. Drying – the drying process is needed in order to remove the naphtha from the material. The only purpose of the naphtha is to aid extrusion, so once that it done, it need not be present in the material. Furthermore, as the material would soon be heated to a high temperature, it is imperative that all traces of the naphtha are removed for the purpose of safety.

5. Stretching – the stretching process involves three sub-parameters, all of which combine to ensure the final product is as required.

a. Temperature – the temperature needs to be set to ensure the material is heated, but not over-heated. Over-heating would mean that the PTFE gets sintered, which we do not want. ePTFE is a semi-sintered material, so the heat must be just enough to ensure that the ePTFE stays soft.

b. Stretch ratio – the rate at which the profile is stretched will define the density of the final product. Lower densities would call for a higher-stretch rate. However, care needs to be taken at the extrusion stage to ensure that material is capable of taking higher stretch rates. Typically, an ePTFE profile would have a specific gravity of between 0.55 and 0.75. However, in the event that the product needs a lower density, a much higher stretch would need to be given.

c. Speed – the speed of the system feeds back to the temperature. A slow speed may be needed for higher cross-section profiles, but this would also mean the material spends more time in the heat and can get over-cured. In general, speed needs to be adjusted after setting the rest of the parameters, such that the final product achieves the right properties

6. Final curing – even after the stretching process, ePTFE Tapes have a habit of ‘breathing in’. Expanded PTFE tapes – especially those that have been stretched at high rates – will try and pull back into themselves. This is mainly a factor when the material is still cooling and results in both a shortening of length and an increase in density. Therefore, special spooling techniques need to be incorporated in order to ensure that the material holds its properties. Typically, once the material has cooled down sufficiently, it will stabilise.
 

There are other nuances, apart from the ones mentioned above. While the specific technical parameters are kept intentionally vague, we hope that this serves to illustrate the complexities involved in the manufacture of ePTFE (expanded PTFE) gasket tapes.

Dimensional Stability in PTFE Machined Components

The advent of precision CNC technology has allowed us to machine components to ever closer levels of tolerance. Metal parts, in particular, will lend themselves to tolerances as close as 1µm (1 micron), or 0.001 millimeters. Not only can such tolerances be attained, but on a piece of stable, well-maintained equipment, their repeatability is guaranteed.

In contrast to metals, polymers do not conform to such close tolerances. Depending on the polymer in question, a variety of factors, including the heat build-up during machining, stress in the underlying material, and the impact of moisture can all play havoc on dimensions. Hence, when we see tolerances of less than 20µm on a customer drawing, we know that the part designer comes from a background in metals and does not appreciate the near impossibly of maintaining the repeatability of such a dimension on a polymer.

 Note that we say, ‘near impossibility’ and not an impossibility. This is because, with experience and a lot of trial and error, polymers too can be machined to as little as 10µm tolerances. It is not easy and requires a significant amount of care and caution during machining, but it is possible.

Machining PTFE

PTFE (Teflon) exhibits such a range of properties that it finds use in nearly every industry. As a component – either a valve, seal, seat or a ring – it can be machined either from its raw (virgin) form, or it can be combined with special fillers that add other properties to the material, but which also complicate its machinability. Here are some of the key factors to consider when machining PTFE.

1. Filler
The filler changes the composition of the PTFE material and as a result, it affects the behaviour both during and after machining. A filler such a glass, offers higher dimensional stability and the ability to machine down to as little as a 10µm tolerance. Fillers such as carbon and graphite cause the PTFE to become very coarse at a molecular level, resulting in extensive damage to the tool tip. As a result of this damage, the tool itself loses effectiveness and can result in the parts going out of spec.

Fillers are usually employed because they alter the properties of the base PTFE material. Glass is used to improve hardness and reduce creep, while carbon and bronze are used to improve wear resistance. 

2. Sinter cycle
The sinter cycle is a vital part of how PTFE is processed. Depending on the size of the part, a cycle of anywhere from 14 hours to 72 hours can be employed. The sintering cycle heats the PTFE up to its melting point, allowing the particles to fuse together. Then the cycle cools the part steadily to ensure that that parts do not crack. A cracked part cannot be re-used, and it is therefore vital from a cost perspective to get the sinter cycle right. Certain blends of PTFE may require a high or lower peak temperature and also a gradual or sharper cool down rate. Once the part comes out of the oven, the easiest thing to check for would be the tensile strength and elongation. If these are matching specifications, the material can be considered acceptable.

However, apart from curing the part, the sinter cycle is also vital in how the part behaves both during and post-machining. For one, a part may exhibit good tensile properties, but may still suffer from over-shrinkage after machining. Our own experience has been that parts that exceed 200mm (8”) in diameter need to be observed after machining for shrinkage. In case the part exhibits excessive shrinkage – say over 0.2mm – then attention needs to be given to the sintering as the part may need to be run on a longer cycle to arrest this issue.

In addition to dimensional stability, the part may also exhibit ovality post sintering. This is especially true for larger parts. The issue with ovality is that the part may not clear during machining. Hence, a post sintering re-shaping may be needed to ensure the part is evenly round. However, too much reshaping can also cause problems. Because PTFE has memory, it is likely to try and come close to its original shape over time or when exposed to high heat. In our experience, a temperature of even 150°C for one hour (well below the 250°C service temperature of PTFE) can cause the part to try and regain its post-sintering shape. Hence, care needs to be taken to minimise the ovality at the sintering stage itself or ensure that any re-shaping does not try and alter the part so much that it loses dimensional stability when on the field.

3. Tooling and environment

The type of tooling, machining process (RPM, feed rate, program) and post machining storage all lend themselves to the repeatability of critical dimensions. Very often, we believe that hard, durable tool tips will give us the best and closest finish on the part. However, in many cases we have seen a simple HSS tool tip offer superior results to the more expensive carbide tips.

Similarly, the RPM and feed rates both work towards improving the part dimension and finish. A high RPM may offer a better finish, but may also result in heat build-up, which may show up only post-machining. Hence, each part needs to be treated specially to ensure all the parameters work together for the best possible result.

In addition to tooling, note that PTFE is highly sensitive to temperature. Very often, shrinkage can be attributed to nothing more that a change in the climate. Since PTFE can experience a dimensional change of up to 3% between 0 and 100°C, it is not uncommon to see large parts exhibit shrinkage of a few tenths of a millimetre, when the weather becomes cooler or warmer over a few days. The only way to address this is to ensure that the part is inspected under temperature-controlled conditions.

There are other aspects of machining PTFE that we do not delve into here. Needless to say, as a material, it is as versatile in its processing techniques as it is in its end-properties. However, as a valuable and integral part of so many systems, it makes sense to understand and appreciate these processes so that a consistent and high-quality end-product can be manufactured.