The Birth of PTFE
Polytetrafluoroethylene (PTFE) has been around for some time and has its own advantages and disadvantages. It was discovered accidentally, not unlike Alexander Fleming’s discovery that a green fungus was eating the bacteria he was trying to grow in a petri dish; Penicillin went on to change the world as we know it. PTFE was discovered by Roy Plunkett, an employee of DuPont back in 1938. Apparently the old boy was attempting to make a few million more for the already wealthy DuPont family by trying to come up with a new chlorofluorocarbon refrigerant when the tetrafluoroethylene in his test bottle stopped flowing. Much like Penicillin, Roy’s discovery had surprisingly positive and useful side effects; he noticed a coating inside the bottle he was using, sawed it open and found a particularly slippery substance within. Abracadabra, presto change-o: polytetrafluorethylene, or PTFE for short, was discovered. Three years later, the material was patented by a subsidiary of DuPont and given the trade name Teflon. The patent has since run out and there are now many manufacturers of PTFE throughout the world. For the sake of this blog, and more importantly, to avoid patent violations (yeah, they’re supposedly expired, but it IS DuPont) we will be referring to this material as PTFE. Interestingly enough, the byproduct of the production of PTFE, the stuff Roy most likely threw out, is perfluorelastomer that is used so often in critical o-ring applications in the semiconductor industry. That, however, is a whole other blog post.
Although recently methods to mold PTFE have been developed, the material comes in sheet form, is a common component in artificial joints, and is also useful for making internal repairs in the medical field, the primary focus of this post will be on seals and backup rings machined from billets of material.
Machining PTFE Seals
Most of the PTFE seals on the market today are machined from cylindrical billets. The process of making the billet is very similar to the process of sintering metal. The raw material in powder form is at times pelletized in order to use an automated process to pour the beaded PTFE material into the cylindrical, steel cast. For some very critical applications though, the powder must be put into the mold by hand which requires more labor at usually a higher cost. The material is then compressed into the mold using a large press. The resulting material is not unlike kiln-dried clay and as such is very fragile. In the next step the billet is carefully placed into ovens; PTFE has a very high melt point (one of the reasons it is so effective in hot applications); it is heated to 342 C (648 F). The end product produces a billet that is recognizable and is now ready to be machined into the end part that will be used.
PTFE Advantages and Disadvantages
Virgin PTFE is white and has the advantages of a very high temperature rating, extremely high lubricity, and being inert to most caustic fluids. A disadvantage is that it’s also very soft. PTFE manufacturers, therefore, add a variety of components into the blend like carbon fiber, glass, bronze, and many others, in order to strengthen weak characteristics of the material and improve performance like extrusion resistance and hardness. Since PTFE in its virgin form has a tendency to cold flow or take on the shape of its housing, filling the material with other compounds makes the end material more resistant to cold flow.
Due to its lubricity, PTFE is excellent in dry, oil-free environments. PTFE seals can function in static fashion and dynamic reciprocating, and make ideal high-speed rotary seals. There are a few things to consider when deciding to use a PTFE seal, though. PTFE is a ‘dead’ material, in other words, it has very, very slow memory; when compressed it tends to remain compressed. Seal manufacturers make up for this by loading the seal with an energizing device like an o-ring, or, more likely, a spring. In the case of a lip seal, these energizing devices lend their inherent mechanical energy to the seal and allow it to continue to do its job particularly when in a low pressure state, when the sealing media is not engaging the lips.
PTFE also is fairly fragile and it’s of utmost importance to make sure that the surfaces are smooth and relatively hard. For rotary motion for example, Rockwell hardness should be between 55C and 65C with a surface finish of 2 and 16 RMS. Most manufacturers will have data based on the design of seal and the material that is very important to take into consideration when designing the hardware for a PTFE seal. Elastomeric seals will ride on a thin coating of fluid that inhabit the crevasses of the hardware, PTFE however is a self-lubricating material that actually coats the shaft or bore (depending on your application) with a fine film of itself; so often lubricity and sealing improves with use.
PTFE Seal Installation
Another consideration to think about when making the decision to switch to a PTFE-style seal is installation. Due to the nature of both the sealing material and oftentimes the spring, it is not recommended that you ‘kidney bean’ the seal into the housing like you might with an o-ring or elastomeric lip seal. The PTFE and/or the spring can be damaged, rendering the seal useless. It’s important to pick the manufacturer-recommended groove design for the style of seal that you decide on. Most of the manufacturers will have detailed information of the installation of the seal. Some of the PTFE shaft seals have very delicate sealing lips which are shipped by the manufacturer on waxed tubes or rods of UHMW in order to keep the lips in the proper position; it takes only one fold of the lip while placing it on the shaft to create a permanent leak path. Often the manufacturer will have recommended assembly tools for these.
When to Choose PTFE for Sealing Applications
In conclusion, PTFE is inert with a high level of lubricity, great in dry applications or applications where the fluids are particularly aggressive, but has broad temperature that may be too hot for standard materials like polyurethane and nitrile. It is used often and is suitable for medical, pneumatic, and hydraulic applications. It is a common and useful backup material when higher pressures are called for in standard o-ring applications. PTFE is not the right choice for every occasion, but when the combinations of heat, pressure, and caustic fluids rule out elastomers, PTFE can oftentimes fill the gap