Kynar PVDF (property of Arkema) is a high purity polymer that combines extreme-temperature performance, easy manufacturability, and durability in some of the harshest environments.
What is PVDF?
PVDF (polyvinylidene difluoride or polyvinyl fluoride) is a fluorinated thermoplastic resin that is classified as a specialty polymer whose brand names include Kynar (Arkema), KF (Kureha), and Solef or Hylar (Solvay). This engineering polymer can often be found in environments that involve high purity, hot acid, extremely high temperatures, and/or radiation.
It may come as a surprise to some but geckos are not, in fact sticky! Gecko’s can cling to glass and climb up walls, but geckos are not inherently adhesive. In fact, there are certain surfaces geckos can not cling to at all- mainly PTFE.
In this week’s blog post we will go over exactly how the gecko gets its Spiderman like abilities, and why exactly they can not seem to climb on PTFE.
A Sticky Situation
With certain types of geckos, their feet contain thousands of tiny, hair-like, hierarchical fibrils called setae, that end in even more, microscopic hair-like structures, so tiny they are not much larger than the wavelength of visible light.
These setae are also ultra-flexible, so when a gecko jumps to another surface, they are able to absorb an incredible amount of energy and redirect it, allowing the gecko to quickly cling from surface to surface.
There are two prevailing theories as to how this process works. One is known as van der Waals forces, or molecular attractions that operate over very small distances. The other, proposed by Yale research Hadi Izadi is that geckos use static electricity which allows them to cling to most surfaces.
Most surfaces except, it seems, Teflon.
Teflon – The Bane of Geckos?
Did you know that PTFE was engineering specifically to resist adhesion by van der Waals forces? PTFE is composed of carbon and fluorine atoms. Of all the elements known to date, fluorine has the highest electronegativity. This causes PTFE to repel other atoms that come near it. More specifically, it works against van der Waals forces.
Furthermore, the molecular structure of Teflon is such that the fluorine atoms surround the carbon atoms. It repels any atoms that try to come near the carbon atoms, giving PTFE its outstanding chemical inertness.
Researchers at the University of Akron, in an effort to further understand, and hopefully replicate, gecko stickability, decided to see what kind of surfaces geckos can cling to, and Teflon was one of the materials tested.
Because of its ability to resist adhesion by van der Waals forces- geckos, who potentially use van der Waal forces to cling to other materials, cannot cling to dry PTFE surfaces.
So, it would seem that the very mechanisms that prevent geckos from walking up dry PTFE provide its most attractive characteristics: extremely low friction and high chemical resistivity. So, when you are looking for a low-friction option for a bearing or seal, don’t forget the bane of gecko’s everywhere: PTFE.
There are three leading materials used for most back-up ring (BUR) applications: PTFE, Nylon, and PEEK. Each of these materials has specific benefits that it can bring to your application, starting with their stiffness and compressive strength.
Why Back-up Rings Are Important
Seal extrusion is one of the most common causes of polymer seal failure. Whether the cause of extrusion is a large gap between the mating surfaces, high temperatures, or extreme pressures, back-up rings can help. The right choice of a BUR can prevent seal extrusion, lengthen the useful life of the seal, and reduce the chances of a catastrophic failure.
PTFE Back-Up Rings
PTFE is well-known for its extremely low friction, dry running capabilities, and outstanding chemical resistance. Filled PTFE (either glass, carbon, graphite, or bronze filled) can handle operating pressures up to 5,800 psi, making it an excellent choice for both medium and high-pressure applications. Virgin PTFE has a much lower maximum operating temperature (around 3,600 psi) and is limited to low-pressure situations. PTFE also has a maximum operating temperature of 575°F, and that combined with chemical compatibility and the high-pressure capabilities of filled PTFE mean that it is an excellent option for harsh condition environments.
Nylon Back-Up Rings
Nylon 6,6 (sometimes written Nylon 6/6 or Nylon 66) is a polyamide material commonly used for back-up rings. It can handle high pressure very well but is limited to temperatures below 186°F. It possesses excellent rigidity, good compressive strength, and thermal stability, all of which are key to effective backup rings.
When used for back-up rings, Nylon is typically filled with Molybdenum Disulfide (MoS2) to achieve an even lower coefficient of friction. It is not recommended for use in wet or humid environments because it does absorb water unless fillers such as glass are added to offset the absorption effects.
PEEK Back-Up Rings
Another commonly used back-up ring material is PEEK, which can handle temperatures of up to 500°F and pressures up to 20,000 psi. Like PTFE, it is low friction, dry running, and resistant to a wide variety of aggressive chemicals. It is also available with fillers to enhance properties such as compressive strength and stiffness. However, PEEK is much harder than PTFE: PTFE has a Shore hardness of D50 while PEEK has a significantly greater hardness of D85. For these reasons, PEEK back-up rings are often used in aggressive environments, such as those found in the oil and gas industry.
If you are having issues with extrusion-related seal failure, polymer back-up rings are a cost-effective solution that can extend the life of your seals. When it comes to polymer back-up ring materials, the top three choices are PTFE, Nylon, and PEEK. While each has its own pros and cons, they are excellent options for solving the problem of seal extrusion. PEEK works best for high pressure, high temperature environments that can involve exposure to corrosive materials. PTFE can also handle high temperatures and corrosive environments, but its maximum operating pressure is lower than that of PEEK. Nylon is also an excellent choice with excellent hardness and thermal stability, with its main limitations being high temperatures and exposure to humidity and moisture.
Polymer bearings can be found in almost any industry and environment, and this includes the clean rooms of electronics to the harsh conditions of the oil and gas industry. And this is no surprise considering the host of benefits that polymer bearings provide, including their resistance to corrosive chemicals, low maintenance, lightweight, and low friction.
So, what exactly are the top five materials used in polymer bearings? The top five polymer bearing materials include Torlon PAI, Bearing Grade PEEK, Bearing Grade PPS, Lubricated PET, and Lubricated Nylon.
1. Torlon PAI
PAI stands for Polyamide-imide and it is the highest performing polymer that is melt-processable. It offers excellent wear resistance, has an extremely low coefficient of friction, and can handle operating temperatures up to 500°F. The primary drawback of Torlon PAI lies in its relatively high level of moisture absorption. On the other hand, it has a low coefficient of thermal expansion and a high level of creep resistance, both of which are key characteristics for an effective bearing. Torlon PAI is often used in bushings, bearings, and wear rings.
2. Bearing Grade PEEK
Bearing grade PEEK is known for its excellent wear characteristics, good abrasion resistance, extremely low coefficient of friction, and outstanding chemical resistance. It can handle environmental operating temperatures up to 500°F and performs well even when continuously exposed to hot water and steam. Bearing grade PEEK is also easy to machine, has low moisture absorption, and possesses a high PV rating.
3. Bearing Grade PPS
PPS (polyphenylene sulfide), like the other bearing grade polymers discussed so far, has excellent wear resistance and a low coefficient of friction. However, it also offers very good wear resistance and dimensional stability even at elevated temperatures. Bearing grade PPS has a rated operating temperature of 425°F and offers outstanding chemical resistance. In addition, bearings can be made to extremely high tolerances when PPS is used.
4. Lubricated PET
Lubricated PET combines the stiffness, wear resistance, and dimensional stability of PET with the low friction demands of bearing applications. It offers extremely low water absorption, good abrasion resistance, and can be machined to very tight tolerances. It is internally lubricated using a dispersed solid and is dry running (needing no additional lubrication). The internal lubrication is released during operation, further reducing the naturally low coefficient of friction that PET possesses.
The primary drawback of PET lies in its limitations with regard to temperature: its continuous service temperature is 210°F, which makes it unsuitable for extreme temperature service conditions.
5. Lubricated Nylon
Nylon does an excellent job of balancing toughness and strength while combining good abrasion resistance with the ability to be extruded, cast, or machined. Lubricated Nylon, much like lubricated PET, includes a solid dispersal of lubricants that greatly reduces the standard coefficient of friction of virgin Nylon and allows it to be used in dry running applications. One of the more common lubricants used is MDS or Molybdenum Disulfide.
The primary issue with Nylon is its ability to absorb up to 7% of its weight water, which can affect its dimensions. However, it does have an extremely high limiting PV rating and excellent wear characteristics.
The top five polymer bearing materials–Torlon PAI, bearing grade PEEK, bearing grade PPS, lubricated PET, and lubricated Nylon–are commonly used to replace metal bearings in a variety of applications. They offer the wear resistance, high PV ratings, low friction, and chemical resistance that are required. If you are in the market for new or replacement bearings, be sure to consider polymer bearings and bushings, also.