Unravelling Polymers

The Definitive Blog on Polymers by Poly Fluoro Ltd.

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.

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

As the world takes a conscious and collective stand against the rising use of fossil fuels, it also disrupts a status quo set in place during the dawn of the industrial revolution. Since fossil fuels began to power our homes, factories and cars, most technological developments have focussed merely on efficiency gains. The improved productivity of coal-fired power plants, combined with better refining techniques and higher efficiency engines have allowed us to do more with less. At the same time, demand has moved steadily upwards, implying that even with these productivity gains, costs have always been rising.

The biggest hurdle to renewable energy was always the fixed costs involved. Consider a solar, or wind-powered plant. Not only is there no option to run these plants 24x7, but the cost per unit of power produced, when added to the cost of the infrastructure itself, always lagged the numbers showcased by their traditional, fossil fuel counterparts. However, efficiency gains have been made in renewables as well. In fact, one might argue that while we have somewhat plateaued with the gains in fossil fuel productivity, the gains in renewables are only just getting started. As a result, the earliest solar plants are now starting to show net positive gains when compared with coal power plants. The same can be said for wind and hydroelectric power plants.

Assisting in this wave of continual improvements in efficiency and productivity are polymer materials that lend their properties to this endeavor. Here we look at but a few ways in which high-performance polymers are contributing to the field of renewable energy.

1. Solar Tracker Bearings

The effectiveness of a solar panel in maximizing the energy it can harvest is improved by ensuring it receives the highest intensity of sunlight possible. Solar tracker systems are designed to rotate the solar panels in order to keep them facing the sun at an optimal angle. However, the energy expended in rotating the panels cannot exceed the efficiency gains from the rotation itself. Thus, it is critical to have a low-cost, low-friction process that limits the energy consumption of the tracker.

Solar tracker bearings are made using high-performance, UV resistant polymers that are both light-weight, low in cost and easy to install. In addition to this, we have developed solar tracker bearings that employ fiction reducing additives, leading to higher efficiency gains in the system. Cost-wise, the bearing assembly can be made at roughly 1/10th the cost of a traditional metallic bearing. 

solar tracker bearings

2. Bearings for wind turbines

Like solar tracker systems, wind turbines too have a tracking system which maximizes the angle of the turbine to ensure it receives the most energy. In addition to this, the process of harnessing wind energy is a rotary process, implying higher RPMs and a need for lower friction. Any friction in the system immediately results in a net energy loss. Hence, polymers such as PTFE find use in these applications. Although PTFE can be an expensive material, the coefficients of friction it offers can be as low as 0.03 when moving against polished stainless steel. This allows a simple PTFE bearing to take on loads and prove effective in applications where otherwise heavy and more expensive metal assemblies would be needed. Furthermore, as PTFE is a self-lubricating material, the need for grease and/or constant maintenance is significantly reduced. This is especially useful for wind turbines, which are usually located in remote areas and benefit from needing less maintenance.

3. 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.

4. Battery separators
One of the key factors with renewable energy is that storage needs to be both ample and efficient. While traditional power plants feed directly into the grid, solar and wind plants will only generate power in bursts. As a result, a lot of effort has gone into battery technologies. 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.

5. Plastic to diesel

While polymers are certainly effective in improving the efficiency of renewable energy systems, the irony is that most polymers are manufactured from fossil fuels themselves. However, recent advancements in plastic-to-fuel technologies have allowed waste polymers to be converted back into combustible fuels. While this does not exactly fall in the same category as renewables, it does allow for a lot of waste plastics to be re-used, rather than end up in landfills.

The above are but a few applications where polymers are finding use. In truth, there are myriad different areas when the properties of high-performance plastics can be utilized to improve, enhance and augment existing systems of renewable energy.