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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The advent of mechanisation, automation, and the use of robotics in manufacturing shows no signs of slowing down. One of the critical requirements of such systems is continuity. Any automated system only works when each segment of the process functions smoothly and without any interruption. Considering the speed at which some of these systems move the smallest glitch can often push the entire manufacturing line to shut down.

Polymer scraper blades are primarily used to remove obstructions, clear surfaces, and remove sticky materials – such as glue or residual polymers from running systems. Their purpose ensures that debris and other materials are taken out of the process so that they may not cause jamming or scratch surfaces.

There are many advantages of polymer scraper blades in such applications. While metal blades were used earlier on, there is a lot of potential damage that can be caused by these. Polymers, in contrast are hard enough to be effective scrapers, but not so hard that they will damage other elements. Further, with polymers, a designer can choose the level of hardness needed, depending on the other materials the scraper will interact with. There are numerous polymers that can be thus employed.

PTFE (Teflon)

PTFE has multiple advantages as a scraper blade. The low coefficient of friction means that it can smoothly run over a system – such as a conveyor belt or glass surface – without putting any load on the other material. PTFE is also soft – so even delicate systems can benefit from PTFE scraper blades. However, the same softness places a limit on how sharp the blade edge can be made with PTFE. Although fillings of glass, carbon, and even stainless steel (see picture) can improve the stiffness of PTFE, its use is mainly beneficial where aggressive scraping is not needed.

UHMWPE

Like PTFE, UHMWPE has a low coefficient of friction. It is also lightweight and exhibits superior wear resistance. UHMWPE is also soft, so again, its use is limited in non-aggressive applications. Unlike PTFE, UHMWPE does not perform well in high temperatures. However, it is very cost effective, highly machinable, and does exceedingly well against rough materials, that would necessarily place a lot of wear load on the scraper blades.

Nylons

Both PA6 and PA6.6 perform well as scraper blades. The addition of Molybdenum di Sulphide further improves wear resistance while the inherent hardness of the material exceeds that of PTFE and UHMWPE. Nylons are light weight, but are also prone to moisture absorption, making them better suited to dry environments.

POM

One of the most versatile polymers, POM (Polyacetal, acetal, or Delrin) is an exceptional choice for scraper blades. Unlike PTFE, UHMWPE, or Nylons, POM is a harder and can be machined to a far finer and shaper blade edge, making it excellent for fine and even aggressive scraping. The addition of PTFE fillers to POM can help reduce the coefficient of friction further. Unlike PTFE, however, POM cannot work in temperatures above 150°C.

PEEK

PEEK combines all the best characteristics of the other polymers. It is very hard, making it possible to machine to a very sharp edge. The toughness of PEEK means that it can be used in very harsh environments – mechanically, chemically, and at temperatures in excess of 250°C. The blade will not dull easily and the addition of PTFE can help to make the material more smooth. However, such a swell of properties does come at a price. PEEK is at least 6-8 times more expensive than PTFE and about 20 time more expensive than POM. Hence, it’s use is limited in applications where nothing else can be used.

At Poly Fluoro, we have the capability to design, blend, mould, and machine the scraper blade that best suits the client’s application. The use of special blends can be incorporated, if the end-use calls for it, while the dimensions of the blade itself can be fine-tuned before bulk production commences.

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Within the polymer space, PEEK (Polyetheretherketone) is considered one of the most robust materials. Not only does PEEK exhibit tensile strengths in excess of 100Mpa, but it can withstand compressive loads of over 300Mpa, making it tough enough for high-load, high-wear, and high-RPM applications where mating materials of steel can be used without fear that they will wear out the PEEK component. These properties can be further enhanced with the addition of carbon and glass, both of which give a sizeable boost to the overall strength of the material, while also making it more thermally stable.

	Unfilled PEEK	PEEK+30% Carbon	PEEK+30% Glass	Unit	Test
Tensile Strength	97	201	158	Mpa	ASTM D638
Young's Modulus	3650	19700	10500	Mpa	ASTM D638
Flexural Modulus	3860	17500	10400	Mpa	ASTM D790
Flexural Strength	152	317	261	Mpa	ASTM D790
Coefficient of Linear Thermal Expansion	5 x 10-5	3 x 10-5	1.7 x 10-5	cm/cm/°C	ASTM E831
Deflection Temperature Under Load	162	315	315	°C	ASTM D648
Coefficient of Friction	0.35	-	-	-	ASTM D3702

 

PEEK seals and valves are commonly used in applications where high temperatures and loads are involved. While seals are relatively simple to machine, PEEK valves can prove challenging, especially if multiple ports of entry and exit are specified. It is likely that a turn-mill centre or a vertical milling centre are needed to make valves of consistent dimension and quality.

An even bigger challenge that the valve is the PEEK manifold, which is usually machined from a solid block, with each of the six faces of the block having its own set of holes. The manifold is essential in many fluid transfer applications. It is designed and machined is a way that allows it to sit within the system, and ensure not only that the fluid conduits all align perfectly, but that no leakages take place during operation. PEEK being a very chemically resistant material, the manifolds are highly durable across a range of fluids and substances and will hold their dimensions even with large variations in temperature.

Machining the PEEK manifold is a challenging task. For one, the PEEK stock shape for machining needs to be moulded into a block form. Most commercially available PEEK stock shapes are sold as round bars. This can prove highly wasteful when the final form is rectangular, especially as PEEK is an expensive material. Even technically speaking, machining a round bar into a rectangular shape places significant internal stress on the polymer. Considering PEEK has a tendency to build up stress the more it is machined, such a route will necessarily cause the final component to crack at some point during its operation.

In contrast, Poly Fluoro uses in-house moulding to make a rectangular block that is as close to the final dimension as possible, thereby minimising the excess machining, while lower the cost. The addition of glass or carbon is also vital. Again, commercially available grades would be made using Virgin PEEK. However, the increased thermal stability of PEEK when filled with glass or carbon makes it essential in applications where high variations in temperature may be expected.

Once the moulding is done, the key step is to machine. Again, considering tolerances can be as close as 10 microns, a 4-axis or 5-axis machine is essential to minimise the number of operations needed. However, more important than the dimensions themselves is the handling of the material. Even with reduced machining as compared with a round bar, there is every chance that as the block is machined, the internal stresses will build up. Hence, care must be taken in annealing the block, not just after moulding, but between operations during machining as well. 

As you can see, the end product, if done properly, can be rather pleasing. The PEEK manifold is a very challenging part. Getting it right is not something everyone can do. Moreover, our control over the entire process – from moulding, to blending, to machining – allows us to ensure that the final properties – both for the material as well as dimensionally – are always best-in-class.

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