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, 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 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 grow, 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, coatings, and binders

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.

The recent spike in the price of PVDF has been almost entirely a result of the copious amount of this polymer needed to manufacture EV batteries. It is estimated that each EV battery requires about 6.5Kgs of PVDF. PVDF is used both as an electrode binder and as a coating material for the battery separators. The effectiveness of PVDF in this application has seen the demand for this polymer shoot up over the past year.

Case Study - Expanded PTFE (ePTFE) Tubing

Among the most challenging processes to master within the polymer space is that of manufacturing ePTFE (expanded PTFE) tubes. ePTFE tubes combine the complexity of making standard PTFE tubes with the complexity of making expanded PTFE. Both PTFE tube extrusion and expanded PTFE manufacture are challenging to make on their own thanks to the peculiarities of PTFE as a material. Combining them only compounds the difficulties.

PTFE tube manufacture

The process of paste extrusion involves mixing a PTFE fine powder with an extrusion aid (lubricant) and then passing it though a die to achieve the final shape. The issue here is that because PTFE does not melt (or more specifically, has no melt flow) it needs to be extruded at room temperature and then passed through a heating system to cure it into its final form. The challenge is working with a dry powder, which when subjected to the high pressures of the extrusion press, starts to behave more like a fluid, but can still not be controlled easily, meaning that dimensional variations, non-concentricity, and material properties can all change depending on various factors that cannot be controlled once the extrusion begins.

ePTFE manufacture

Standard mono-axial ePTFE manufacture also starts with extrusion. However, since the end product is usually a tape, the extrusion itself is not as challenging as making a PTFE tube. The extrudate is then passed through a stretching device, which adds heat and force to pull the tape into its final – marshmallow-like- form that allows it to be such an effective sealing element.

One key issue with mono-axial ePTFE tape is that it is prone to splitting. Since the extrusion force only acts in the longitudinal direction, laterally the tapes tend to be weak and spit easily when torn apart sideways. This is a property that can be addressed pre-stretching, but it involves a lot of mechanical manipulation of the material.

Expanded PTFE tube

The same process to make PTFE tubes forms the beginning of the ePTFE tube process. However, unlike ePTFE gasket tape – which has a solid form that can be easily handled – the tube profile is very weak. The slightest pressure on the tube in this raw form will cause it to collapse, after which the tube is effectively useless. Careful handling is needed to ensure that the tube in this ‘green state’ holds its form until the stretching process begins. However, the stretching is itself the bigger issue. Stretching ePTFE involves gripping the tape tightly so that it can be pulled through the starching machine. However, the extruded tube cannot be gripped at all, as even squeezing it lightly between one’s fingers will cause it to collapse.

At Poly Fluoro, we have devised a number of ways to mitigate this problem. Extensive R&D went into understanding what the extruded tube would be able to withstand mechanically and building the right equipment to ensure that the tube passes through the stretching process without collapsing. In this regard, the final properties of our expanded PTFE (ePTFE) tube were the following:

  1. Non-splitting – by creating the right kind of forces on the tube, the final product gains strength in the lateral direction and the tube no longer splits when torn sideways

  2. Porous – like all expanded PTFE, the tubes gain a special kind of porosity, making the tube walls impervious to liquids and dust, but permeable to gases and vapours

  3. Non-kinkingunlike regular PTFE tube, which is prone to kindling when the bending radius is breached, ePTFE tubes do not kink and will allow themselves to be bent and positioned as required

As a chemically inert, corrosion resistant material capable of taking high temperatures, PTFE tubes are highly sought after. However, with the addition of expansion, the tube takes on a whole new dimension and becomes invaluable in applications ranging from fluid control, to electrical insulation, to medical devices and grafts.

Read More

1. Case Study - Development of a 4-axis PEEK Valve

2. Over-moulding PTFE on to Stainless Steel

3. Case Study - Cross Directional Expanded PTFE Gasket Tape

Polymer Wear Plates - Grade Selection and Key Advantages

As automation and industrialisation has evolved, the key objective for any mechanised system has always been twofold: higher speed with higher precision. While sensors, Servo motors, and computed aided software have allowed for ever increasing levels of precision, the issue of speed has always brought with it one unavoidable problem: friction.

In any system of moving parts, friction is the number one culprit keeping things from moving indefinitely and with minimal damage. Hence, methods to reduce friction have always played a vital role in ensuring the durability of mechanical systems.

One of the most common uses of polymers lies in the realm of friction reduction. The low thermal conductivity of polymers allows for better heat dissipation and lower wear outs. However, as this blog has always maintained – not all polymers were created alike. While most polymers can form an effective medium between moving metal parts, the issue of longevity narrows the list down significantly. Typically, a good wear material should have some or all of the following properties:

  1. Self-lubrication – especially needed for systems where oil cannot be used or where oils and other synthetic lubricants cannot easily be replaced

  2. Low coefficient of friction – this is paramount to ensure the smooth sliding materials that come in contact with the wear material

  3. Heat resistance – although not essential, some polymers are needed in environments where high temperatures would build up, even without excessive friction

  4. Robust - metals are many factors harder than even the strongest of polymers. A tough material is needed to last long term in an environment where metals are moving at high speeds.

In this regard, most of the common polymers – such as HDPE, LDPE, or PP – would be unable to take high wear loads for an extended period of time. PVC – which is stronger and has admirable strength – has the key drawback that the presence of chlorine in the material can corrode metal parts over time. Based on our experience, the following materials are the best suited among polymers for wear applications:

  1. POM (Delring/Polyacetal) – POM is not an immediate choice as a wear resistant material. However, it is inexpensive, mechanically strong, and capable of taking temperatures of up to 120°C. This makes it a comfortable choice in applications where the loads and RPMs are not very high, but where a spacer or bush between two metallic moving parts will allow for lower heat build-up.
    Further – the addition of PTFE fillers to POM (Delrin AF), allows for even lower coefficients of friction and higher loads as a result

  2. PA66 (Nylon 66, Nylon 6.6) – like POM, nylon is on the lower end of both tensile strength and coefficient of friction. However, PA66 bobbins and ferrules are excellent are taking friction in rotary applications. Again – the loads, temperatures, and RPMs cannot be very high, but the material is effective if used within the parameters specified.

  3. UHMWPE – although less known than some of the other polymers, UHMWPE rates one of the highest on out-and-out wear resistance. It would probably be on the top of any list of wear applications, but for the fact that it has a very low temperature rating. With the ability to only withstand 80-100°C continuously, UHMWPE is restricted to application in ambient temperatures. Still, it is very useful as a wear plate, or sliding plate, especially in railway applications, where the heat build up is limited. UHMWPE also forms a very effective medium as a bush or collar in low speed, high friction applications.

  4. PEEK – while usually too expensive to use in any regular applications the effectiveness of PEEK can be unparalleled when it is used properly. Specifically, HPV PEEK, which is PEEK with a blend of carbon, graphite, and PTFE, can endure high loads at high RPMs and do so in temperatures of up to 300°C.
    PEEK’s only drawback is that it is prohibitively expensive- costing 15-30X of any of the above polymers. But in situations where cost is not a factor, there are few polymers that can compare.

  5. PTFE (Teflon) – the most well know and versatile of wear materials, PTFE’s absurdly low coefficient of friction (as little as 0.03 against polished stainless steel) combines with an equally low thermal conductivity to offer a material that simply does not heat up even at high RPMs and loads. With a service temperature of 250°C, there is virtually no wear application where PTFE does not find consideration. While the material is relatively soft and therefore easily deformed, the addition of glass, carbon, and bronze fillers adds hardness, making the material suited to such diverse applications as:

    1. Piston bands in shock absorber struts

    2. Railway sliding plates

    3. Linear slideway bearing strips (Turcite/Lubring)

    4. Automotive wear plates

    5. DU Bearings/bushings

It should be mentioned that while PTFE certainly is the most popular choice, the wear resistance of UHMWPE still rates higher. Designers and engineers would do well to understand that if temperature is not an issue, UHMWPE is an excellent choice for any wear application. However, as most industrial and automotive systems operate at elevated temperatures, PTFE and PEEK are the most accepted and effective choices in wear applications.

Read More

1. PTFE Amplatz Sheaths - Specialised Tubing for Medical Applications

2. PTFE Wear Plates: Misconceptions and Applications for Heavy Equipments

3. PCTFE vs PTFE - A Comparison of Two Very Similar Polymers