Unravelling Polymers

The Definitive Blog on Polymers by Poly Fluoro Ltd.

Polymers Are The Future For Electric Vehicles

The decline in sales for the auto industry has been pronounced and unprecedented.  While many point to short and medium factors, such as government policies and the non-availability of financing, the truth remains that most auto manufacturers remain woefully unprepared for the paradigm shift that is in the offing.

Electric vehicles are an inevitable mainstay of the future auto market both because of their economic and environmental impact. Thus far, fossil fuel-run vehicles have enjoyed the economic advantage, because EVs were both expensive to buy and had limited range and power. In addition to this, the limited infrastructure surrounding EVs meant that it was a hassle to own one, unless one was very inclined to shun fossil fuels. But as the technology has advanced, both these factors seem to be becoming less pronounced. Thanks to increased scale and large bets taken by the leaders in the EV space, the upfront costs of owning an EV have lowered significantly. In addition to this, continual improvements in the battery management systems have allowed the range to be increased to the point where a single charge may last over a week for someone doing only 30-40 kilometres a day. Further, government support for the industry has meant that the infrastructure has also moved ahead at a good pace. Many buildings – even in India – have mandatory EV charging points in all the parking spaces. Convenience-wise, this is even better than having to go to a fuel station once a week to fill up your tank with petrol or diesel!

Much of the technology of electric vehicles depends on high efficiency and a good strength to weight ratio. In such an endeavour, lightweight materials become essential. Polymers have long been known to provide long term performance and efficiency gains to any system. A rule of thumb in the auto industry has been that for a 10% reduction in weight, the fuel efficiency of the vehicle improves by 5%. For this reason, the quantum of polymers has increased from around 8Kgs to over 150Kgs over the last 40-50 years. 

The effectiveness of polymers in automotive applications has always been known. As polymer science has evolved, the range of application has also broadened. Polymers such as PEEK, PTFE, PEI (Ultem) and PI (Kapton) have exhibited tremendous resistance to heat, such that there seems a little argument for using metals (which would be at least 2-3 times heavier) in areas where these polymers can be used.

As electric vehicles gain in importance, we look at some of the areas in which polymers are especially useful in EVs.

1. Sensor shields and enclosures

The use of sensors is essential in ensuring safety. As autonomous vehicles see a rise in adoption, sensors will become possibly the single most important component set within a vehicle.

Polymer shields and connectors are important because unlike metals, they remain neutral to the signals and waves being sent and received by the sensors. PTFE and PEEK are already used extensively as Radomes in antennae. As the number of sensors in the vehicle grows, it is even more essential to ensure that there is no disruption to performance, in the event that all sensors are working at once. Polymers are unique in being able to offer protection from weather, heat, and additionally, do not interfere in any way with the signals.

2. Brackets

Brackets made from polymers are useful as they hold together other components and ensure that they do not get damaged during operation. Some of these components may generate heat, so the polymer would need to withstand this as well. Brackets made from Nylon have been used as replacements for metal even in conventional vehicles, as they offer a significant weight reduction and can be moulded to suit the exact shape of the component set that they are housing. Further, in the event that a component does come slightly loose, the potential noise from the rattling is minimised significantly when a polymer is involved.

3. Insulation

Much in an electric vehicle rides on the efficiency of the battery and the use of stored power. Anything that helps minimise the leakage of current from the system aids in improving the battery life and consequently the distance that can be traversed on a single charge. Materials like PTFE and Polyimide have proven highly effective as insulators in high-voltage-high-temperature applications.

4. EV charging stations

Electric Vehicles are gaining traction over traditional fuel powered vehicles. As their demand and prevalence grows, so too would the infrastructure needed to ensure that they can function smoothly. Investments in EV charging stations have increased significantly and new housing developments are increasingly required to ensure that there are charging stations for all parking slots.

As a superior insulation material, PTFE has been found effective in EV charging stations. PTFE insulation blocks can be used to improve the charging efficiency and ensure that there is minimal leakage of current.

5. Battery separators

One of the key factors with electric vehicles is that battery storage needs to be both ample and efficient. Both PTFE and PE (polyethylene) are seen as effective battery separators. These separators provide internal insulation to the battery, preventing the batteries from discharging when idle. Although PE separators are effective in most application, high-voltage applications need PTFE films, which possess higher breakdown voltage strengths and can remain effective over a much longer time period.

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 PTFE Tape is a highly commoditized 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). Sintering is the process by which PTFE is cured at high temperatures to let it attain its final properties. Unsintered PTFE tape is basically still ‘raw material’ which has been drawn and flattened into a tape form.

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. In truth, the rapid speed at which ePTFE tape is stretched and heated means that it too is not what one might call ‘fully sintered’. However, the stretching process intentionally does not cure the PTFE above its melting point in order to ensure that certain properties are preserved.

Differences in End Properties
As mentioned above, the key purpose of Plumber’s Tape (PTFE thread seal 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 to 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 calendering 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.