Reason # 1 Low Friction
PTFE, also known as Polytetrafluoroethylene or by its trademarked name Teflon, has one of the lowest coefficient of friction of any solid known to mankind, even when in contact with metal. It’s usually estimated at 0.05 to 0.10 for virginPTFE. Because of this extremely low friction, in many rotary applications there will be significantly less frictional drag when compared to options, such as conventional rubber shaft seals.
Reason # 2 Film Transfer
When PTFE rubs against a hard surface, it transfers a microscopic layer to that counter surface. This contributes to its extremely low coefficient of friction. Here’s how engineers and scientist describe the process:
- The molecules on the contacting surface of PTFE are deformed due to applied forces acting at that surface.
- The molecules begin to stretch along the direction of the sliding motion.
- Because of the weak intermolecular forces, PTFE molecules are transferred to the counter surface in a film between 50 to 200 Angstroms thick.
- As a result, PTFE is now sliding on PTFE instead of directly on the counter surface.
Reason # 3 Dry Running
Another awesome feature of PTFE is its ability to perform without additional lubrication. PTFE is actually classified as a self-lubricating polymer. It offers reduced stiction and breakout friction, which can be key for many rotary applications.
Reason # 4 Chemical Resistivity
Another facet of PTFE that makes it popular as a shaft sealing material is its chemical resistivity. The list of chemicals that can attack PTFE is short, and primarily consists of alkali metals, such as elemental sodium, and rare fluorinated solutions. This chemical inertness is said to be a result of carbon-fluorine bonds in PTFE.
Another interesting facet of PTFE is its hydrophobic behavior, whereby it repels water and water-containing substances. The hydrophobic behavior aids in its chemical inertness, and also makes it easier to keep clean. This behavior due in part to the high electronegativity of fluorine.
Of course, there are more reasons behind the popularity of PTFE for rotary shaft seals in high speed applications. PTFE has an extended shelf life, can perform at both cryogenic and extremely high temperatures, and is known for its easy installation. However, the main reasons are its low friction, film transfer, dry running, and chemical resistivity.
So, the next time you need a rotary sealing solution that involves high speeds and low friction, check out PTFE rotary shaft seals. They might be just the solution you are looking for.
The impressive growth of the medical device, biomedical and healthcare industries over the past 15 years continues unabated even as materials such as fluoropolymers are meeting the increasingly challenging demand for new products and procedures. In general, fluoropolymers are progressively replacing other plastics in medical applications owing to their ability to meet the physical and biocompatibility requirements of the next generation drugs and devices. Fluoropolymers meet a unique set of performance criteria in such applications. These include biocompatibility, lubricity, sterilization, chemical inertness, a wide temperature use range, low binding to process equipment, high-purity with low extractables, dielectric properties, and USP Class VI certification.
The family of available fluoropolymers that meet the above needs include grades of PTFE, FEP, PFA and PVDF resin from various resin producers. PTFE(teflon) has a well-established implant history of soft tissue replacement due to its biocompatibility and inertness. Resin and downstream product manufacturers continue to respond to new performance requirements by making appropriate modifications to the chemistry of the resin and surface configuration of the products, respectively.
The biocompatibility of any polymer is a principal requirement in any medical device such as catheters, bio-containment vessels, syringes and sutures. PTFE, FEP and PVDF are well established biocompatible materials, and their lubricity and chemical resistance make them the material of choice for products such as multi lumen tubing and others that are used in minimally invasive procedures. Multi lumen fluoropolymer catheter tubing for example allows surgeons to perform multiple procedures using the same catheter.
Fluoropolymers, especially PTFE(teflon)and PVDF are widely used in microporous membranes. Membranes, containing billions of pores ranging in size from 0.01 to 10 microns act as filters for particles and bacteria in critical fluids. PTFE and PVDF are among the few polymers used for filter membranes. The surfaces of the polymers membranes can be modified to deliver specific filtration properties and can be hydrophobic (water repelling) and oleophobic (oil, solvent, low surface tension fluid repelling). PVDF membranes can be also be surface modified to be hydrophilic (water loving) for removal of viral particles in the manufacture of therapeutic proteins and monoclonal antibodies. PVDF blotting membranes are particularly well suited for low background immunoblotting (western blot analysis), as well as for amino acid analysis and protein sequencing.
Accessory equipment, such as pumps, tubing, fittings used in conjunction with medical devices must meet similar performance and specification parameters. Accessories in direct contact with fluids are constructed of fluoropolymers such as PTFE and PVDF. For example, metering pumps such as diaphragm pumps are required for precise and repeatable flow, sometimes for chemically aggressive fluids. Both PTFE and PVDF tubing are used in the construction of these pumps for this reason.
PVDF, although having a lower use temperature limit than PTFE(teflon), has a relatively high tensile strength and excellent permeation resistance to many fluids. It has a lower density (1.78 g/cc) than other fluoropolymers (approximately 2.18 g/cc). Due to a lower melting temperature than other fluoropolymers, it is more easily processible into products such as pipes, tubes, injection molded parts and films. It offers excellent dimensional and UV stability and is therefore finding new uses in aerospace, sensors, biotechnology and robotics markets.
Tags:teflon, Fluoropolymers,medical device
There are a few things about Teflon to start with that by no means tell the full picture, but are certainly the list of things that caused me to immediately abandon ship on all things teflon.
- Teflon is the trademarked name for the chemical Polytetrafluoroethylene (PTFE).
- Perfluorooctanoic acid (PFOA) used in the manufacture of Teflonproducts and banned by 2015, breaks down indefinitely in the environment. So think about everything in the list below that you’ve ever owned. Just one person. So scary to think about those particles never breaking down and ending up in oceans and rivers and US!
- The off gasses from Teflon products kill birds. It’s really well known in vet circles. This usually comes from Teflon in certain light bulbs or cookware.
- The women working in the Dupont factory where Teflon is produced had high incidence of birth defects and abnormalities, which DuPont was forced to pay millions in damages once exposed. The worst part? DuPont had known for 20 years that the PFOAs in Teflon technology was harmful to people, yet they remained silent.
- It takes a pan about 3.5 minutes to reach 738F on an electric stove top with your average non stick pan. Teflon off gases 6 toxic gases from reaching about 680F, including carcinogens, global pollutants and a lethal chemical MFA.
- At 1000F we get into warfare gases and a WWII nerve gas. Still hungry? Unbelievable isn’t it. It seems so often that big business simply does not have our best interests at heart. It ‘s soooo time to break up with them.
- There are elevated levels of cancers, including some very rare, in and around the village where the DuPont factory is located.
- The most common symptom in every day Teflon use, is with a high temperature cold and flu. Rarely are the two connected by doctors.
We largely know Teflon for its non stick qualities. So where else is it hiding? You’ll be surprised!
- Frying pans, woks and saucepans– opt for stainless, cast iron or enamel when cooking. I use the De Buyer brand of brushed stainless, that you have to season similarly to a wok before using. SolidTeknics is awesome too and Australian made. Both are non stick, especially with a good bit of coconut oil, ghee or butter and I like to oil the pans after cooking and wiping clean to keep building a home-made non stick coating. Other options are ceramic coated, high quality brands like Le Chasseur or Le Creuset. Don’t baulk at the price – I’ve still got my grandmother’s roasting dish from 1972 and mum’s frying pans from the 70s too. It lasts literally forever!
- Dental Floss. Something I discovered a few months ago.
- Microwave popcorn bags– Best to pop in a saucepan.
- Irons.The Sunbeam Aerosteam or the Tefal auto clean 400 are the only high quality ones that I’ve found, without the Teflon coating.
- Hair straighteners, curling wandsetc. Read the fine print on the packet and make sure it’s ceramic, stainless or don’t go near it!
- Baking gear, including most parchment/grease proof paper!Opt for Stainless and do your good old fashioned butter and flour dusting to naturally ‘non stick’ your muffin, Madeleine or cake tins and opt for a parchment paper that is greener such as the fabulous If You Care soy wax for hot or silicon for cold coatings. Google ebay or etsy for stainless bakeware options and place an amazon order to your place you’re staying if traveling overseas so you can stock up on stainless bakeware without the postage fortune.
- Carpets and sofas. Repeat after me: I will never tick ‘yes’ to free scotch guarding ever again! Any kind of sales pitch for stain repellent, liquid repellent technology, RUN A MILE and take your babies with you!
- In a sports shoe shop. Step away from the scotch guard spray up-sell and take a pair of socks instead! If you have leather shoes, beeswax will be a fab water repellent and is all natural!
- Waterproof mascara. Not in all of them, but Yes. It’s true. Check your brand.I buy this beautiful one. Just like my old favourite performance wise, minus all the extra weirdness!
- Ironing board covers(not all. Check your brand / manufacturer)
- Some light bulbs. Look for the words PTFEs or non stick to see whether your bulb is safe to buy or not. If you have pet birds or chickens, this is super crucial.
- Toasted Sandwich makers, waffle makers, rice cookers and many plug in slow cookers and woks. Slow cook in large cast iron or enamel pans and put the lid on and dish into a low 130 oven. Easy. For rice? learn the absorption method. For toasted sandwiches? Place your sandwich / wrap onto unbleached If We Care / other natural, parchment paper. Then, place another sheet on top and press as normal in the sandwich press. You can thank Mr Stuart for that genius tip, as he makes his way around a toxic free lunch in an office environment! Not easy…
- Waterproof clothes, namely raincoats.Use umbrellas where possible.
- Outdoor deck waterproofing sealants.There are natural ones available so it’s a matter of playing detective if you’ve got a renovation in the works.
- In children’s uniforms!It will have a label and seems to be restricted to boy’s pants and shorts as far as I can see right now, which means you can find an equivalent colour, non teflon treated and avoid it. In large department stores or online, you can usually find cotton school-like colours. We’ve had success with it so far.
1938: Fiddling around in the lab one day, Roy Plunkett accidentally discovers polytetrafluoroethylene, soon to be known as Teflon, a slippery substance that will have practical applications in everything from nonstick cookware to a presidential nickname.
Plunkett, a chemist at DuPont's Jackson research lab in New Jersey, made his discovery in the time-honored scientific way: as the result of a mistake, and with an assistant's help.
Plunkett and his assistant, Jack Rebok, were testing the chemical reactions of tetrafluoroethylene, a gas used in refrigeration. The gas was contained in some pressurized canisters, one of which failed to discharge properly when its valve was opened.
Rebok picked up the canister, only to find that it was heavier than an empty canister would be. He suggested cutting it open to see what had happened and, despite the risk of blowing the lab to kingdom come, Plunkett agreed.
Of course, it was heavy: The gas hadn't accidentally escaped. It had solidified into a smooth, slippery white powder as the result of its molecules bonding, a process known as polymerization.
This new polymer was different from similar solids like graphite: It was lubricated better and extremely heat-resistant, due to the presence of dense fluorine atoms that shielded the compound's string of carbon atoms.
Setting other work aside, Plunkett began testing the possibilities of polytetrafluoroethylene, eventually figuring out how to reproduce the polymerization process that had occurred accidentally the first time.
DuPont patented the polymer in 1941, registering it under the trade name Teflon in 1944. The first products — most having military and industrial applications — came to market after World War II. It wouldn't be until the early 1960s that Teflon became a household word when it was used to produce the most effective, heat-resistant cookware yet seen.
The word gained a certain pop-culture notoriety in the 1980s when the media began referring to Ronald Reagan as the Teflon president, a reference to his infuriating ability to avoid being tarnished by the various scandals plaguing his administration.
Teflon cookware, however, remained as steadfast and reliable as ever.
Teflon is found virtually everywhere today, coating metals and fabrics, from the aerospace industry to clothing to pharmaceuticals.
For his discovery, Plunkett, who retired from DuPont in 1975, was enshrined in the National Inventors Hall of Fame.
Source: About.com, Wikipedia
Teflon-coated cooking tools like this muffin tin and baking tray have eased setup and cleanup in millions of kitchens.
For people who are concerned they might have been exposed to high levels of PFOA, blood levels can be measured, but this is not a routine test that can be done in a doctor’s office. Even if the test is done, it’s not clear what the results might mean in terms of possible health effects.
One of the primary uses of bellows is to absorb dimensional changes due to thermal effects, which is very useful when used high temperature flows such as steam. Bellows also serve to dampen vibration in the system caused by rotating components, protect sensitive and brittle processing equipment, and to absorb shock loadings.
Why is PTFE a popular choice for bellows?
PTFE (also known by its trade name Teflon) is a popular choice for the bellows material. It is ideal for use in highly corrosive environmentssuch as those involving strong oxidizing and reducing accents, salts, high concentrations of acid, and chemically active organic compounds. It has an extremely long flex life (how many flexing cycles it can handle before it fails), and a very low spring rate (amount of force needed to flex the bellows) – which means that it can reliably handle the challenge of fluctuating and vibrational loadings.
What types of movements can bellows be used to absorb?
There are three types of movement that bellow expansion joints can absorb: axial deflection, lateral deflection, and angular deflection. Axial deflection includes compression and extension affects along the longitudinal axis of the bellows. Lateral deflection occurs when the end joints of the bellows displace relative to each other. Also known as parallel misalignment, this type of deflection can also be absorbed by a bellows expansion joint. Angular deflection can be described as a rotational displacement, or twisting displacement.
How does the number of convolutions affect bellow performance?
Recall that a convolution is the smallest flexible unit in a bellows. The general heuristic for bellow convolutions is this: fewer convolutions will give you better pressure and temperature ratings, BUT the amount of movement it can handle is more limited than bellows with more convolutions. More convolutions, on the other hand, can absorb more movement but at a cost in pressure/temperature ratings.
Are there other polymers used for bellows?
Yes, another polymer option for bellows is UHMW PE, ultra-high molecular weight polyethylene. While not as chemically resistant as PTFE, it currently has the highest impact strength of any polymer on the market today. If the bellows are used in connection with abrasive materials, UHMW PE would be a valid alternative to PTFE because it has better abrasion resistance.
PFA, or Perfluoroalkoxy or Teflon PFA, is similar to PTFE(teflon) in many ways and is someone chosen in place of PTFE because it offers higher strength at extreme temperatures, even in the presence of extremely aggressive chemicals.
TFM, or PTFE-TFM, is a second-generation PTFE that has better fatigue properties than PTFE and offers better stress recovery. It is well adapted for situations that involve high temperatures and vacuum pressures.
Bellows serve a variety of purposes – form absorbing displacement and shock to preventing sensitive equipment of a brittle nature. They can absorb axial, lateral, and angular displacements. The number of convolutions in a bellow is related to both its strength and pressure rating as well as the maximum amount of displacement it can absorb. Finally, polymers such as PTFE(teflon), UHMW PE, PFA, and TFM are popular choices for bellows materials, although PTFE seems to remain the first choice for many engineers.