Unravelling Polymers

The Definitive Blog on Polymers by Poly Fluoro Ltd.

High Performance Polymers in Railway Applications

The growth of the railways sector has been as pronounced in the developed world as it has been in emerging markets. While emerging markets have focussed on expanding their networks and increasing electrification, developed countries have invested significantly on technological advancements, which offer higher speeds, higher load carrying ability, and a more comfortable ride.

As anyone who is familiar with engineering can guess, these improvements in efficiency often come with the incorporation of new materials such as high-performance polymers. Polymers such as PTFE, Nylon, UHMWPE, and Polyethylene are finding higher levels of penetration in a growing array of applications within the railways space. These applications make use of a myriad of properties including, but not limited to, wear resistance, electrical insulation, coefficient of friction, and compressive strength.

UHMWPE-Part

Poly Fluoro – with an already strong portfolio in the polymer space – has had the opportunity to not only engage in the manufacture of such key materials, but to also work on R&D for new areas and reverse engineer existing products to introduce improvements and enhance cost-effectiveness.

Gliding Plates

Also known as skid plates, sliding plates, slide plates, and Gleitplatte (German), these flat plates are used in vibration dampening and to allow for the smooth sliding of metal plates without causing excess heat and/or friction. The plates are made from either PTFE, Nylon, UHMWPE, or HDPE. These polymers all have high self-lubricity and are capable of taking heavy loads without deformation. In the case of PTFE – which tends to be heavier than the other polymers mentioned, but also more robust – the plate can also withstand temperatures of up to 250°C.

Gliding plates have certain features, which are machined into the plates:

  1. Oil grooves – to allow for even smoother sliding, the plates have grooves machined into the surface for oil to rest in

  2. Dimples – some plates have dimples machined into the surface. This again forms pockets for oil, grease, or any other lubricant to sit within

  3. Holes – plates need to be machined with counter sunk holes to allow them to be bolted

  4. Flatness – the flatness of the guide plate is essential, as it allows for maximum surface-to-surface mating with the other sliding parts

  5. Chemical treatment – mainly for PTFE, the plates may sometimes need to be chemically etched so that they can be bonded to other substrates

Rail guides

Alignment is crucial in the railway industry. Efficient function depends on parts aligning exactly, even over considerably long distances. Components such as guide rails, conduits, and guiding ledges are essential in railways systems. They employ a variety of polymers such as Nylon 6.6 (PA66), Nylon 12 (PA12), POM (both virgin and with glass fillers), and Polypropylene. These can be either machined or injection moulded, or both. Accuracy is paramount, since tight tolerances are needed to ensure that parts stay aligned. The presence of vibration in railway systems means that parts need to be robust enough to withstand mechanical loads, while also being able to accommodate smaller vibrations without cracking.

Rail guides can be used to channel elements such as cables and sliding members. They can also be moulded as conduits or clamps that can be used to hold elements in place.

Pneumatic Tubes for Pantographs

PTFE tubes are challenging to make and there exist only a handful of manufacturers worldwide that can effectively extrude high-quality tubes.

The pantograph is a very vital equipment on all railway systems, as it takes electric current from overhead lines into the train itself. As such, the materials involved need to be both mechanically strong and highly resistance to heavy voltages and currents. PTFE thick-walled tubes are used in these arrangements as they meet both these criteria. Tubes are made using high-purity fine powder resins and are blended with fillers – including pigments – to allow for colour coding.

Thick-walled tubes have service pressures of up to 35Bar, with burst pressures in excess of 100Bar. In addition to this, the high dielectric strength of PTFE allows for breakdown voltages in excess of 50Kv/mm. 

As the demand for electrification increases across the developing world, pantograph demand is expected to skyrocket. High-quality PTFE tubes would be essential to support this demand.

Short Neutral Sections

Power to overhead electrical lines are provided by substations. These substations are often located at intervals of about 100Km along railway lines. It is also likely that their phases are different and hence essential that their currents are kept insulated from each other. Short neutral sections are insulating members that connect the lines between two sub-stations. The pantographs feeding off the overhead wires for current will pass over the neutral section in order to switch from one substation’s power to the next.

The short neutral section – or SNS – is required to be mechanically strong (the tension in the overhead wires can be significant), electrically resistance, and capable of taking high wear loads, as the pantographs will repeatedly rub over this element, causing a gradual wearing off.

PTFE with special wear increasing fillers is used in this application, which also incorporates metal elements, including copper and stainless steel. The importance of this assembly cannot be overstated, as it ensures the smooth supply of current to the railway engine and constantly endures many different forces – including the weather.

Again – PTFE thick-walled tubes are needed in this application. They are challenging to manufacture and go through multiple tests before they can be approved for usage on the field.

Above are only a few applications of high-performance polymers in railway applications. In truth, there are literally hundreds of different products used today that incorporate the unique and long lasting properties of polymer materials.


Read More

1. Polymers in Ventilators

2. Polymers in Fluid Transfer Applications

3. Polymers in Renewables - The Rising Role of High-Performance Plastics in Renewable Energy

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