by Sara McCaslin, PhD Sara McCaslin, PhD No Comments

The Basics of PVDF

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. 

Where is PVDF Used?

PVDF is used extensively in a wide range of industries. Semiconductor manufacturing makes use of PVDF’s ultra-pure status and its ability to perform in harsh environments that may involve extreme temperatures and aggressive chemicals. Electronics and electricity applications depend on PVDF’s outstanding low smoke emission and fire-resistant properties along with electrical properties for use as wiring insulation.

PVDF’s ability to handle radiation makes it an excellent choice for nuclear waste handling, and its high-temperature performance and chemical compatibility lends itself readily to the oil and gas industry. Because PVDF has excellent high-temperature performance, high purity, and low permeability, excellent strength, and chemical compatibility, it is used extensively in chemical processing.

Purity and FDA approval have made it a popular choice in food and beverage packaging and processing as well as pharmaceutical processing. It is often used in connection with water and wastewater management for similar reasons. PVDF is also used extensively in the medical market and healthcare industry where it is used as a biomaterial for medical textiles, such as hernia meshes, as well as for medical sutures.

The transportation and energy market has begun using PVDF as a binder for cathodes and anodes in HEV/EVs (Hybrid Electric Vehicle/Electric Vehicle). Its chemical compatibility and anti-corrosion properties make it useful as a barrier liner for fuel lines and tanker trailer lines. Aviation also makes ample use of PVDF for wiring harnesses and general coatings

How is PVDF Used?

PVDF is commonly used for several specific types of applications across industries:

  • Pump assemblies
  • Heat exchangers
  • Tanks and vessels
  • Nozzles
  • Sensors and actuators
  • Fittings, pipes, tubing, and valves
  • Membranes, including microfiltration membranes
  • Filters and filter housings
  • Liners and films
  • Cable jacketing and harnessing
  • Biocompatible materials

Key Properties of PVDF

As alluded to in previous sections, PVDF possesses several features of interest to engineers:

  • Extremely high purity with low permeability
  • FDA compliant and non-toxic
  • Excellent heat resistance and thermal stability 
  • Good mechanical properties
  • Resistant to a wide range of aggressive chemicals
  • Resistant to UV exposure, ozone oxidation reactions, and radiation
  • Resistant to the growth of microorganisms
  • Excellent burn characteristics 
  • Good manufacturability
  • One of the lowest melting points of commercial fluoropolymers
  • Excellent electrical properties
  • Excellent abrasion resistance
  • Low density (1.78 gm/cm3)
  • Recyclable

In addition, PVDF offers excellent abrasion resistance, is lightweight, and can be recycled. Also, note that there are additives available for PVDF to enhance its properties and its melt processability.

Purity and FDA Compliance

In addition to being an extremely high purity polymer, PVDF is both FDA compliant and non-toxic while exhibiting very low gas and liquid permeability.

Heat Resistant and Thermal Stability

One of the outstanding features of PVDF lies in its excellent performance, chemical stability, and dimensional stability in high-temperature environments with a service temperature rating of up to 300 F.

Mechanical Properties

Among the outstanding mechanical properties possessed by PVDF are good deflection, tension, compression, and torsion when compared to other fluorinated polymers. In addition, its low rate of water absorption (0.4%) means that it will remain dimensionally stable (not swell) when in a moisture-rich environment. In addition, PVDF has excellent impact strength.

Chemical Compatibility

PVDF is known for its excellent chemical compatibility that includes weak and strong acids (including mineral and organic); alcohols; aromatic and aliphatic solvents; weak bases; hydrocarbons; halogenated compounds; ionic and salt solutions; and oxidants. Its primary weaknesses are caustics, esters, strong bases, and ketones. 

Microorganism Growth

The surface of PVDF is highly resistant to the growth of microorganisms, including bacteria, fungi, and mold. It is also resistant to weathering, grime, and even graffiti (which is why it is often used in the architectural industry).

Burn Characteristics

PVDF has excellent flame and smoke properties, including UL 94 V-0 rating indicating it is both non-flammable and self-extinguishing along, or more specifically “Burning stops within 10 seconds on a vertical specimen; drips of particles allowed as long as they are not inflamed.” In addition, certain grades of PVDF also possess an excellent flame spread/smoke developed rating of 25/50 (when tested in accordance with ASTM E 84).

Manufacturability

PVDF is also highly manufacturable and melt-processable, lending itself to precision machining, rotomolding, compression molding, injection molding, and extrusion as well as subsequent welding and fabrication. Its ability to be used in molding is primarily due to its low melting point of 352 F, compared to PTFE at 621 F or FEP at 517 F.

Electrical Properties

In addition to electrochemical stability, PVDF also possesses a very high dielectric constant (280 volts per meter) and a high piezoelectric constant. In fact, it possesses both piezoelectric and pyroelectric properties.

Conclusion

One of the polymers we work with here at Advanced EMC is PVDF Kynar made by Arkema. If you are interested in Kynar, have questions about its usage and processing, or need a quote, feel free to contact us and we will have one of our experts respond right away.

by Jackie Johnson Jackie Johnson No Comments

Why Geckos Can’t Cling to PTFE

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.

The answer?

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.

In Conclusion

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.

by Sara McCaslin, PhD Sara McCaslin, PhD No Comments

The 3 Leading Materials for Effective Back-up Rings

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

Conclusion

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. 

by Sara McCaslin, PhD Sara McCaslin, PhD No Comments

Top Five Polymer Bearing Materials

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

Spring Energized Teflon SealsPAI 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. 

Conclusion

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.