Unravelling Polymers

The Definitive Blog on Polymers by Poly Fluoro Ltd.

Polymers in Food Processing

Across the world, food processing is understandably one of the most critical industries. Along with the medical industry, food processing calls for factors of hygiene that would otherwise be overlooked in areas such as automotive, chemical or oil & gas. With the advent of automation, food processing is increasingly seeing the need for materials that are FDA approved and that will not in any way degrade during operation. In addition to this, food processing usually involves heat, which means the materials used cannot deform or melt during operation.

Whether we look at large-scale food processing or kitchen top processing, high-performance polymers have found a way into nearly all aspects of this industry. Not only are polymers food-safe and non-contaminating, they also allow for a significant reduction in noise and weight – both of which are paramount, especially when dealing with home level food processing equipment. We take a look here at some of the key areas in which high-performance polymers find application.

  1. PEEK valves for Coffee Machines
    High-end coffee machines are built keeping the final taste of the finished brew as the most critical parameter. Traditional machines used valves made of aluminium, which routed the high-temperature liquid within the machine. However, as consumer palates became more discerning, the manufacturers realized that aluminium valves caused a faint metallic taste to be left in the mouth.

    In the hunt for a high-temperature material that is compatible with coffee and FDA approved, PEEK was used as a replacement for the aluminium. It should be said that PEEK being an expensive material, the price of a PEEK valve is many multiples of what the aluminium component costs. However, with the PEEK Valve, the taste of the coffee is preserved. PEEK Valves are now a mainstay of any high-end coffee machine
     

  2. PTFE and Acetal Rotary Seals and Shafts
    Most food processing equipment involves some rotary motion. Whether it is the gentle kneading of dough or the high RPM mixing and grinding of spices, all rotary motion causes some amount of friction and thereby, heat.

    PTFE and Acetal (POM/Delrin) seals and shafts are preferred in such applications. Not only are these materials light in comparison to metals, but they are also self-lubricating, implying no need for external lubrication and minimal noise creation.

    PTFE+Molybdenum-di-sulphide is possibly the most preferred material in rotary applications, as it possesses a low coefficient of friction, while also having superior wear resistance. In addition to this, parts can be machined to close tolerances, allowing a good fit between the seal and the moving parts that minimize vibration.

  3. PTFE Wiper Seals
    Many food processing applications involve food that are sticky and may not easily separate. The equipment may process the food and place it on a non-stick pan; however, an additional member is needed to move the food either out of the equipment or to another part of the apparatus for further processing. In such cases, PTFE wiper seals are used to push or move the food around. Since PTFE is non-stick, the wiper seals do not themselves allow any food particles to adhere to themselves. This is beneficial not only because the food can be moved without deforming or affecting its shape in any way, but also because food particles that get stuck pose a hygiene issue.

    PTFE is also unique because it can withstand up to 250°C of temperature. This means that even if the food is hot, there is no chance it will affect the PTFE seal.
     

  4. PTFE Tubes for liquids
    PTFE tubes are both chemically inert and have a service temperature of 250°C. The transfer of hot fluids is often essential for food processing. In addition to being able to take the temperature, the tube needs to ensure that it does not react in any way with the food. In some cases, liquids may collect within the tube for long periods, if the equipment is not used often. PTFE not only stays non-reactive over long periods of time, but its non-stick nature ensures that once the equipment is re-started, there is little chance that any residual fluids will remain stuck within the tube.
     

  5. PTFE and PEEK Stirrers and Impellers
    Much of food processing involves the mixing of various ingredients. While most stirrers are made using metals coated with a non-stick material, some applications do call for the stirrers themselves to be made of inert materials. This may be needed in applications where the material being mixed may be abrasive and cause the non-stick coating on the metals to chip. Since both PTFE and PEEK are FDA approved and will not chip or degrade when in contact with foods, they are preferred as stirrers and impellers in many mixing operations.
     

Fundamentally, the combination of chemical inertness, food grade, non-stick and high-temperature capabilities means that there are many more applications within food processing where high-performance polymers could find use. As food processing moves further into the realm of robotics and automation, the devices created will need an increasing number of polymers to be incorporated to ensure both hygiene and long-term performance can be guaranteed.
 

 

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.