Unravelling Polymers

The Definitive Blog on Polymers by Poly Fluoro Ltd.

PTFE Lip Seals - Applications, Material Choice & Advantages

The effectiveness of PTFE as a sealing material has been explored before. Whether as a gasket material – such as ePTFE – or as a machined component designed for a specific OEM requirement, PTFE combines an ability to self-lubricate with a capacity for higher temperatures, pressures, and corrosive chemicals to be a highly effective sealing element in nearly all conditions.

While traditional lip seals have used either elastomers or even polyurethane (PU), PTFE lip seals are invaluable in areas where harsher environments call for a more robust polymer. Essentially, PTFE lip seals were designed to bridge the gap between conventional elastomer lip seals and mechanical carbon face seals, as they can operate at higher pressures and velocities when compared to most elastomer lip seals, they are an excellent alternative.

That said, there are many similarities between elastomeric rubber lip seals and PTFE lip seals. When we consider basic construction, both seals use metal casing for structure, a press-fit into a stationary housing gland, and a wear lip material to rub on the rotating shaft.  However, when compared with elastomer lip seals, PTFE lip seals use a far wider lip to shaft contact pattern. PTFE lip seals also make use of a lighter unit loading, but a wider footprint. Their design has taken this direction to address the wear rate, and these changes were made to lower the unit loading, which is also known as PV.

In terms of manufacturing, while elastomeric lips seals allow themselves to be directly moulded onto the metal, PTFE – being a non-injection mouldable material – is machined separately and then press-fitted onto the metal housing. This not only makes it critical to get the dimensions spot-on but also means a more expensive end-product, since PTFE is more expensive than most elastomers and because machining is usually far more expensive than moulding.

The specific application for PTFE lip seals is found in sealing rotating shafts, in particular those with high speed. They offer an excellent alternative to elastomer rubber lip seals if the conditions are challenging and exceed their capabilities.

A good example of PTFE’s superior functionality is in industrial air compressors, where they are configured to provide over 40,000 hours of maintenance-free service.

Other benefits of PTFE lip seals include:

  • Tight sealing, even under high pressure in excess of 35 BAR

  • Ability to run at temperatures far above or below elastomer rubber lip seals (with typical temperature ranges from -53 °C to 232 °C)

  • Elastomer coatings on the seal’s outer diameter make for easy installation without damaging mating hardware

  • Available in custom designs and a wide range of sizes and materials

  • Inert to most chemicals

  • Withstands high speed in excess of 35 m/s

  • Low friction and ability to address rotating equipment and vibration for longer life

  • Compatible with most lubricants and able to run in dry or abrasive media

Dynamic rotary sealing applications for PTFE seals include:

  • Industrial applications - PTFE lip seals replace elastomer and carbon face seals is in the air compressor market

  • Screw Compressors - PTFE lip seals offer excellent leak control and the ability to run at 1,000 to 6,000 RPM with a huge range of lubricants and over extended periods (15,000 hours) to reduce warranty claims

  • Aerospace – PTFE lip seals are used in auxiliary power units (APUs), turbine engines, starters, alternators and generators, fuel pumps, Ram Air Turbines (RATs) and flap actuators, which is one of the largest markets for lip seals

  • Automotive – PTFE lip seals are put to work in some of the most challenging applications in crankshafts, distributors, fuel pumps and cam seals, which are used in the racing industry where naturally, engines are frequently pushed to their limits

  • Turbochargers – PTFE lip seals run at high speeds of 4,000 to 36,000 RPM and can cope with extreme temperature ranges from -40 °F to 350 °F (-40 °C to 177 °C), with limited lubrication over an extended seal life

  • Pumps – PTFE lip seals address dynamic sealing environments, extreme speeds, pressures, and temperatures present in vacuum pumps, along with blowers, chemical pumps, encoders, alternators, drilling and tapping spindles, hydraulic motors and pumps, and air conditioning recovery pumps, among many more applications

Enhancing Performance of PTFE with Fillers

While PTFE has been used liberally across this article, the truth is that PTFE is never just one material. The addition of performance-enhancing fillers allows for augmentation in certain base properties. The selection of these fillers is highly dependent on the application in question.

Several other fillers are used in combination with PTFE. 

  • Modified Virgin PTFE – the same basic properties as virgin, but with increased wear and creep resistance and lower gas permeability

  • Carbon-Graphite Filled - Carbon reduces creep, increases hardness, and elevates thermal conductivity of PTFE. Carbon-graphite compounds have good wear resistance and perform well in non-lubricated applications

  • Carbon Fibre Filled - Carbon fibre lowers creep, increases flex and compressive modulus, and raises hardness. The coefficient of thermal expansion is lowered, and thermal conductivity is higher for compounds of carbon fibre filled PTFE. Ideal for automotive applications in shock absorbers and water pumps

  • Aromatic Polyester Filled - Aromatic polyester is excellent for high temperatures and has excellent wear resistance against soft, dynamic surfaces. Not recommended for sealing applications involving steam

  • Molybdenum Disulphide and Fiberglass Filled - Molybdenum disulphide increases the hardness of the sealing surface while decreasing friction. It is normally used in small proportions combined with other fillers such as glass. MoS2 is also inert towards most chemicals

  • Fiberglass Filled – Glass fibre has a positive impact on the creep performance of PTFE. It also adds wear resistance and offers good compression strength

  • Graphite Filled – Since graphite is often used as a lubricant, it does not significantly increase the coefficient of friction of PTFE when used as a filler. The low friction allows the compound to be used when both shaft speed and pressure are high. Graphite also is chemically inert which enables its use in corrosive media

  • Mineral Filled – Mineral is ideal for improved upper temperatures and offers low abrasion to soft surfaces. PTFE with this filler can easily be qualified to FDA and other food-grade specifications

  • Stainless Steel Filled - Although stainless steel filler is very abrasive, this compound has excellent extrusion and high-temperature resistance in static and slow dynamic applications

  • Other fillers – there is virtually no filler that cannot be combined with PTFE to improve or add to the properties of the material. As long as the filler can to withstand the higher temperatures that PTFE needs to be sintered (cured) at, it can be used. 

Features of Other Machinable Plastics

While PTFE is the most commonly used high-performance plastics for lip seals, there remain other polymers that certainly merit further understanding. Some of these include:

UHMW Polyethylene

  • Excellent wear and abrasive resistance

  • Good lubricity in water

  • Excellent sealing of light gases at low pressures

  • Excellent high-pressure extrusion resistance

  • Moderate abrasion to soft hardware

  • Excellent wear resistance in reciprocating applications

Thermoplastic (TPE) Elastomer

  • Excellent wear and extrusion resistance

  • Excellent sealing of light gases at low pressures

  • Excellent high-pressure extrusion resistance

  • Low abrasion to soft dynamic hardware material

  • Minimum dynamic surface hardness 25 Rc

  • Excellent wear resistance in reciprocating applications

  • Good wear resistance in rotary applications

Polychlorotriflouroethylene (PCTFE)

  • Excellent electrical properties

  • Stable for continuous usage until 205°C

  • Low creep at room temperature

Polyetheretherketone (PEEK)

  • Chemically inert

  • Very strong and rigid

  • Temperature range -60 to 300°C

  • Excellent abrasion resistance

Read More

1. Can Indian Manufacturing Survive COVID?

2. The Challenge of Injection Moulded High-Performance Plastics

3. Polymer Prices Go to the Moon

4. Applications and Considerations for PTFE Seals

Case Study - Cross Directional Expanded PTFE Gasket Tape

The challenges thrown by PTFE as a material are myriad. As we have already illustrated in various articles, the processing techniques needed for PTFE differ from those of other polymers so dramatically, that nearly every end-product requires a special-purpose machine that can be used for no other plastic. The main reason for this is that PTFE is not melt-processable. As a result, heating the polymer into a liquid state to then either mould or extrude it is not possible. Hence, we end up with processes such as paste extrusion (PTFE Tubes), skiving (PTFE tapes), and isostatic moulding (moulded PTFE parts).

Even within the space of PTFE, expanded PTFE (ePTFE) remains an anomaly in terms of processing complexity. We have covered in earlier articles that ePTFE depends on a minimum of nine different parameters, including the grade of material, the nature of the lubricant, the extrusion process, and the stretching process – to name only a few. Each of these parameters needs to be kept in range, with any deviations only revealed once the final product is out – at which point it is too late to make any changes.

While we have mastered the process of making mono-axial ePTFE gasket tapes – the standard ePTFE tape used in industrial applications – cross-directional tapes are another issue altogether. In this article, we look at the development of the same and what advantages it affords.

Difference between mono-axial and cross-directional gasket tape

The main issue with mono-axial ePTFE gasket tapes is that due to the orientation of the fibrils being in only one direction, the material is susceptible to tearing/splitting when tension is applied in the transverse direction. On the other hand, the material exhibits immense compressibility, allowing it to form a perfect seal with minimal pressure and retain its form over a range of temperatures. As such, the tape works brilliantly in applications such as flange-to-flange connections, where the assembly is not intended to be dismantled frequently, once installed.

However, in applications where the frequent opening of the system occurs, the mono-axial tapes fail over the medium-long term. For example, using mono-axial tape along the rim of a vessel that is opened multiple times a day would not work, since the constant application and relaxation of pressure would cause the fibrils to come loose and the tape to slowly disintegrate. Furthermore, with very poor recovery – mono-axial tape is soft but lacks elasticity – the seal becomes less effective each time the lid is closed and opened again.

To remedy this, we set out to create a cross-directional tape that would allow for strength in the transverse direction and be ideal in applications where recovery was needed. Within the industry, there are already ePTFE sheets that are made using multi-directional ePTFE layers. However, these sheets are prohibitively expensive, come in standard sizes of only 1.5mt x 1.5mt, and do not incorporate any fillers. The aim was to:

  1. Develop a low-cost tape to match the properties of multi-directional ePTFE sheets

  2. Create a highly customisable process in-house so that key properties such as compressibility, density, and thickness can be matched to suit the exact OEM requirement

  3. Incorporate fillers such as silica and barium sulphate to reduce creep and add stability to the material


Mono-axial ePTFE Tape

Cross-directional ePTFE Tape



Specific Gravity




AMS 3255A

Tensile Strength




AMS 3255A











The result

As the table above shows, the results were rather impressive. While the exact process remains proprietary, the use of special mixing techniques along with calendaring, drying, layering, and curing resulted in an enhancement of properties. The tensile strength was high in both lateral and longitudinal directions. Compressibility was understandably reduced, but the improvement in recovery meant that the material could be used in areas where pressures were erratic. Most importantly, with strength in both directions, there is no chance of any disintegration in the longer term.

By modifying the process slightly, we can influence the compressibility, thickness, and density. Further, the addition of fillers greatly arrests the cold flow of the material, meaning that long term application is more stable, as there is no risk of excess deformation slowly affecting the sealing effectiveness of a gasket.

Read More

1. Case Study - Development of a 4-axis PEEK Valve

2. Over-moulding PTFE on to Stainless Steel

Can Indian Manufacturing Survive COVID?

To say that the last 12 months have been very tricky to navigate would be a gross understatement. The combined impacts of COVID, the resulting lockdown, the migrant worker crisis, and the highly mercurial post-lockdown business climate have made each passing month a challenge to work through. And yet, most clients and vendors we have spoken with will agree that post-June 2020, things did somehow settle down on the business front. Demand picked up, companies slowly returned to a new kind of normalcy, and work started to get done.

Throughout history, India’s numbers have always been staggering. We swelled with pride as our population numbers were in no way reflected in the caseloads we saw post-September 2020. We beat our chests at the news that we were now supplying the rest of the world with vaccines.

The idea that within 6 short weeks, India went from a COVID success story to staring down the abyss of a potential apocalypse is stunning. The blame game aside – we all know the levers that helped nudge the numbers up so that there was enough critical mass to begin the avalanche of cases – the fact remains that as the developed world starts leaning towards a post-COVID environment, India is still mired in the depths of a crisis that does not seem to be letting up in any way. 

Most manufacturers have been terrified of a second lockdown. It was tough enough to make it through the first one, but companies somehow survived. A second shutdown would be devastating, possibly beyond recovery, mainly because this time, things are very different.

Last year, the world was united in its fight against COVID. Companies in every country came to a standstill and those of us lucky enough to have cooperative clients learnt that orders were not being cancelled – only pushed out by a month or so. Even those in industries where existing orders would not roll over and accumulate over the following months were relieved to know that India’s total lockdown was seen somewhat favourably by the rest of the world and demand resumed once doors reopened. At the same time, China was reeling under a lot of ill-sentiment thanks to COVID, so the tendency for export clients to shun Indian suppliers in favour of their Chinese counterparts was limited. Indeed, India’s problems with China in mid-2020 brought a lot of local demand back to local suppliers as the rush of patriotism temporarily eclipsed the lure of better margins.

It is not an exaggeration to say that none of these factors are very much at play anymore. For one, India-China relations, while still somewhat icy, are not strained enough to push Indian buyers away from China’s deliciously low prices. Even global sentiment towards China is somewhat muddled between still blaming them for COVID, holding them accountable for human rights abuses, and an inability to fully detach themselves economically. China continues to be the world’s factory and its supposedly excellent handling of the COVID crisis locally has allowed it to project a ‘back to business’ image while India still struggles with record-breaking caseloads each day.

Finally, the fact remains that the US and Europe – with a huge ramp-up in vaccinations – have started returning to normal and expect their businesses to function as such. It would not be feasible for a company to rest easy knowing that part of its supply chain depended on an Indian manufacturer. This year was supposed to be the time for everyone to get back on their feet and India’s inability to keep pace with the rest of the world has severely hurt both our image as well as the stability of our contracts with clients in other countries. Even the most generous of customers cannot be expected to indulge an Indian supplier marred by COVID cases and lockdowns when the client’s own business is being jeopardised by the resulting delivery delays.

It is possibly due to this reason that lockdowns, when imposed, have given leeway to manufacturers to stay open. However, manufacturers themselves need to juggle workload with the very real possibility that their employees remain at risk if the proper precautions are not taken. It is a very fine balance and Indian manufacturers need to tread it carefully for the next 3-4 months at least until the crisis is brought under some kind of control.

In this regard, there are certain things manufacturers can do to manage the situation:

  1. Plan ahead: considering we are consistently exceeding our expectations of worst-case scenarios, this is not always easy. However, where possible we need to stock inventories of raw materials, so we are not exposed to supply-side shocks ourselves. With a disruption in everything from logistics to production schedules to worker movement, the risks remain high that just-in-time manufacturing will not work in the present climate

  2. Prioritise: with resources scarce – especially around the workforce – we need to focus on clients and orders where timelines cannot be compromised. Indian clients may be more willing to understand delays, as many of them would similarly be dealing with demand and supply shocks. Export clients whose lines are depending on materials being delivered on time would need to take priority to ensure business relationships are not eroded.

  3. Communicate: a client may have ordered 15 different parts but might suffer a potential line stoppage only for 3-4 parts. Open communication with the client would help their work to continue smoothly while allowing the manufacturer the leeway to push some parts forward. Our experience with Indian vendors has been that they tend to stay quiet when things don’t go as planned. Most clients can adjust their schedules, provided they are given some advance warning. Avoiding last-minute surprises would help ensure the client retains confidence in the supplier.

  4. Train: Especially for a small enterprise, a key component of survival is building skills among its employees. Staggered working and the inevitable quarantine period of an employee with COVID means that work can come to a standstill if a key staff member is not present. Encouraging staff to gain a basic understanding of cross functions would be critical in making sure that work gets done even without full strength.

  5. Motivate: This is a time when many tend to feel hopeless. The toll that COVID has placed on us all is starting to wear individuals down and the fact that there is no end in sight can be demoralising. Keeping staff motivated is critical in a time like this. Employees need to know that they are working towards keeping the business alive and that their efforts will ensure that there is still a business around once the pandemic eases off!

As we said in the beginning – this has been a very tricky time to steer a business forward. The necessity for a manufacturing plant to operate in person has only made that tougher since not everyone has the privilege to work from home. At the same time, without manufacturing, the economy, in general, starts to stumble and could collapse in a manner that causes long-lasting damage. Only by carefully navigating through the current climate can businesses plot a course for sustainability beyond the pandemic.

Read More

1. The Challenge of Injection Moulded High-Performance Plastics

2. Polymer Prices Go to the Moon

3. PEEK - The Impact of Carbon Fibre Fillers on HPV Bearing Grades