Unravelling Polymers

The Definitive Blog on Polymers by Poly Fluoro Ltd.

Injection Moulding of PEEK and PPS

Injection moulding is a widely used manufacturing process that involves the creation of parts and products by injecting molten material into a metal mould. This process is ideal for producing high-volume, complex components with great precision and repeatability. Because the process involves taking the polymer into a molten state and maintaining it there until it is injected into the mould, it requires careful consideration and equipment that is capable of managing the polymer in its liquid state. Well known polymers – such as polypropylene, polyethylene, and even nylons – are relatively easy to handle in this process. Not only do they melt at relatively low temperatures (anywhere between 150-175°C) but they are also fairly easy to handle when liquid, as they do not give off any corrosive gaseous effluents. They also have very predictable melt-flows and shrinkages, meaning that a part can be quickly developed using off-the-shelf metrics that can usually be provided by the raw material suppliers.

Two materials that are less commonly used in injection moulding are PEEK and PPS. However, considering the immense advantages of these plastics, it is worth exploring how and why they are excellent candidates for injection moulding.

PEEK, which stands for polyetheretherketone, is a high-performance engineering thermoplastic that is known for its exceptional strength, stiffness, and heat resistance. It has excellent chemical resistance, which makes it ideal for use in harsh chemical environments. PEEK is often used in aerospace, automotive, and medical applications due to its high strength-to-weight ratio, biocompatibility, and resistance to wear and tear.

PPS, which stands for polyphenylene sulfide, is another high-performance engineering thermoplastic that is known for its excellent mechanical properties, high heat resistance, and chemical resistance. It is often used in automotive, electrical, and electronic applications due to its excellent electrical insulation properties and resistance to corrosion.

There are many benefits to using injection moulded parts made from PEEK and PPS, some of which include:

High strength and stiffness: PEEK and PPS are both known for their exceptional strength and stiffness. Injection moulded parts made from these materials can withstand high loads and stresses without deforming or breaking, making them ideal for use in high-stress applications.

Resistance to heat and chemicals: PEEK and PPS both have excellent resistance to heat and chemicals, making them ideal for use in harsh environments. Injection moulded parts made from these materials can withstand high temperatures and exposure to corrosive chemicals without degrading, which can increase the longevity of the parts and reduce maintenance costs.

Precision and repeatability: Injection moulding allows for the creation of highly precise parts with excellent repeatability. This means that each part will be identical to the next, which is important in applications where consistency is critical.

Lightweight: PEEK and PPS are both lightweight materials, which can reduce the overall weight of the finished product. This can be especially beneficial in applications where weight is a concern, such as aerospace or automotive applications.

Biocompatibility: PEEK is biocompatible, meaning that it is compatible with human tissue and can be used in medical applications such as implants. This makes it an excellent choice for medical device manufacturers who need to create parts that are both strong and biocompatible.

Electrical insulation: PPS is an excellent electrical insulator, which makes it ideal for use in electrical and electronic applications where insulation is critical. Injection moulded parts made from PPS can provide excellent insulation properties, which can help to protect sensitive electronic components.

Resistance to wear and tear: PEEK and PPS both have excellent resistance to wear and tear, which can increase the longevity of parts and reduce maintenance costs. Injection moulded parts made from these materials can withstand high levels of wear and tear without degrading, which can be especially beneficial in applications where the parts are exposed to abrasive materials.

Despite the benefits of the end-products, the issue with both these polymers – as well as other high-temperature plastics such as PEI, PEK, and Polyimide (PI) – is that they are not straightforward to mould. For one, the melting points are far higher than those of regular polymers, meaning that the equipment and moulds need to be able to hold the polymer at a consistent temperature in excess of 400°C. The matter is further complicated by the effluent gases generated by these polymers when in a molten state. Some of these gases can be extremely corrosive, causing damage to the regular metal barrels within which they will be held before injection. Finally, the melt flow of these polymers, while consistent, needs to be understood properly before moulding. To compound the issues, polymers such as PEEK and PPS are expensive, costing anywhere from 20X to 50X the price of regular plastics. Hence, the room for trial-and-error is limited and the equipment itself needs to be made such that wastages are minimised.

It is therefore important that injection moulding presses that are capable of handling these high-performance plastics are specially constructed. Metal housings and other components must be designed to withstand the corrosion, stress, and temperatures of the polymers in their molten state.

At Poly Fluoro, we have harnessed our existing knowhow on PEEK compression moulding and our experience with injection moulding polymers such as POM, Nylon, and polypropylene to develop a new equipment only for high-temperature polymers such as PEEK and PPS.

Overall, injection moulded parts made from PEEK and PPS offer many benefits over other materials. They are highly durable, resistant to heat and chemicals, and can be used to create highly precise parts with excellent repeatability. They are also lightweight, biocompatible, and offer excellent electrical insulation properties. There is a world of applications and we at Poly Fluoro, as always, are at the forefront.

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PTFE Seals in Food Processing

The properties of PTFE as a food grade material that can be machined to close tolerances is often exploited in applications where any kind of chemical contamination needs to be avoided at all costs. 

Recently, we were asked to develop a set of chevron v-seals made earlier with PTFE infused fabric, specifically to be used in the dairy industry. Our study into the existing seals being used by the client told us that the PTFE infused fabric was no longer the best option for what the client required. Fabric based components were first developed over thirty years ago and were a mainstay for many OEMs. Over time, as PTFE compounding techniques improved and the uniformity of the blend became consistent, these seals were replaced by either virgin PTFE seals or PTFE with fillers of glass, Ekonol (aromatic polyester), or even PEEK.

One of the primary reasons PTFE seals are used in the food industry is because of their resistance to chemical and biological contamination. PTFE is non-reactive to most chemicals and food ingredients, making it an ideal material for seals that come into contact with food products. Additionally, PTFE is naturally hydrophobic, which means that it repels water and other liquids, making it resistant to mould, bacteria, and other biological contaminants. This is especially important in the food industry, where contamination can lead to serious health risks for consumers.

Another important characteristic of PTFE seals is their durability and longevity. PTFE is a highly stable material that is resistant to most forms of physical and chemical degradation, including high temperatures, corrosive chemicals, and UV radiation. This makes PTFE seals ideal for use in food processing equipment that is subjected to harsh conditions, such as high-pressure washdowns, heat, or exposure to corrosive substances.

In addition to their resistance to contamination and durability, PTFE seals are also an ideal choice for the food industry because of their low friction coefficient. This means that they can easily slide against other materials without sticking or causing wear, which is important in applications where high-speed movement is required, such as in conveyor systems or packaging equipment. PTFE seals can also operate at a wide range of temperatures, from -200°C to 260°C, which makes them suitable for use in both cold storage and high-temperature cooking applications.

There are several applications in the food industry where PTFE seals are commonly used. One of the most common applications is in the sealing of rotary valves, which are used in the transfer of bulk materials such as powders, grains, and liquids. PTFE rotary seals are ideal for this application because they can withstand the abrasive and corrosive nature of these materials, while also maintaining their sealing properties over time.

PTFE seals are also used in other food processing equipment such as pumps, mixers, and blenders. These seals help to prevent the leakage of fluids or the ingress of contaminants into the equipment, which can compromise the quality and safety of the food products being processed. In addition, PTFE seals are often used in high-speed conveyor systems, where their low friction coefficient helps to minimize wear and ensure smooth movement of the food products. As a soft polymer, PTFE is unique in that it can take a tremendous amount of load (both mechanical and environmental), but that it will ultimately succumb to any major misalignment of mating metal parts. This means that in the event of failure, the seal will allow itself to be destroyed rather than damage the otherwise expensive equipment that it functions within.

The effectiveness of PTFE seals in ensuring the quality and safety of food products is unmatched. With their excellent performance characteristics and versatility, PTFE seals will continue to play an important role in the food industry for years to come.

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Expanded PTFE (ePTFE) Gasket Tapes - Applications in Electrolysers

The onset of the green energy revolution has led to a burst of technologies around the generation and storage of clean electricity. A key advantage of erstwhile coal and gas-powered plants over their renewable energy counterparts is that power generation could be started and stopped with a flip of the switch. This also ensured that storage was not a key concern since electricity is available ‘on tap’ as it were.

With both solar and wind power, in contrast, power generation depends on the elements and the intensity with which they choose to act. While wind energy can vary erratically depending on the force and direction of the winds, solar power is available mainly during certain peak hours of the day, although devices like solar trackers allow us to maximise the energy we harvest during these hours.

The other issue, of course, is storage. With renewable energy, the ability to store becomes critical to ensuring that the supply to the grid does not suffer the same vagaries as the energies received. 

One of the main methods to store energy uses green hydrogen.

Green hydrogen involves using the harvested energy to power an electrolyser, which in turn converts fresh water into hydrogen and oxygen. This hydrogen is then stored in tanks and later burned (the by product being water) to create power for the grid. Across the world, companies are scrambling to set up green hydrogen plants, as they form a critical link to allow renewable energy to become the mainstay for future power needs.

In this endeavour, the efficiency of the electrolyser becomes paramount in ensuring that minimal energy is lost in the overall process. 

An electrolyser is a device capable of splitting water molecules into their constituent oxygen and hydrogen atoms. It consists of a conductive electrode stack separated by a membrane to which a high voltage and current is applied. This causes an electric current in the water which causes it to break down into hydrogen and oxygen. 

At present, there are different types of electrolysers depending on their size and function. The most commonly used are:

Alkaline electrolysers

They use a liquid electrolyte solution, such as potassium hydroxide or sodium hydroxide, and water. Hydrogen is produced in a cell consisting of an anode, a cathode and a membrane. The cells are usually assembled in series to produce more hydrogen and oxygen at the same time. When current is applied to the electrolysis cell stack, hydroxide ions move through the electrolyte from the cathode to the anode of each cell, generating bubbles of hydrogen gas on the cathode side of the electrolyser and oxygen gas at the anode. 

Proton exchange membrane (PEM) electrolyser

PEM electrolysers use a proton exchange membrane and a solid polymer electrolyte. When current is applied to the battery, water splits into hydrogen and oxygen and the hydrogen protons pass through the membrane to form hydrogen gas on the cathode side. They are the most popular because they produce high-purity hydrogen and are easy to cool. They are best suited to match the variability of renewable energies, are compact and produce high-purity hydrogen. On the other hand, they are somewhat more expensive because they use precious metals as catalysts.

Expanded PTFE (ePTFE) in electrolysers

Given the presence of liquids and chemicals, it is imperative that proper sealing exists between the stacks of the electrolysers. In this regard ePTFE tapes are used between the stacks to provide superior sealing. Expanded PTFE not only has a compressibility of up to 60% - allowing it to make a very robust seal even at low torques – but is also weatherable, resistant to chemicals, and highly effective even in extreme pressures. The exact dimensions of the ePTFE tape can vary from project to project, depending on the construction of the electrolyser. However, a thickness of 1.5-2.5mm is typically used with a width of 25-50mm. The tape is easily applied and can even be layered on to itself, eliminating the use of a standard cut gasket. This is relevant because the diameters of the electrolysers can be as high as 2 meters, meaning that a standard cut gasket would be very wasteful. Considering that a 5 MW alkaline electrolyser requires around 500 seals, this saving is particularly vital.

Over the past few years, a number of green hydrogen projects have shifted to ePTFE for the sealing of the electrolysers. While other materials such as EPDM, Low-hardness FKM, and butyl have all been tested and found reasonably effective, the efficacy and ease of use of ePTFE has proven unparalleled. It is likely that in the coming years, ePTFE gasket tapes will be a mainstay of any electrolyser plant.

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