Unravelling Polymers

The Definitive Blog on Polymers by Poly Fluoro Ltd.

PTFE - Myths Busted!

Ever since its discovery in 1938, PTFE has constantly found new uses, becoming an invaluable part of myriad applications. However, despite being around for over 70 years, many misnomers exist around PTFE, with many assuming to be just another polymer and expecting it to behave and be processed in the same way.

As a processor, this often poses problems, with clients unable to understand why PTFE components should be so expensive (even before the price increase), why certain shapes are not possible to make and also why the scrap value has not been factored back into the pricing.

As awesome as PTFE is given its various properties, it is just as difficult to process, handle, machine and even dispose of! I want to look at some of the aspects of PTFE processing and compare them to the myths that I sometimes come across in the industry.

1. Moulding

Unlike other thermoplastics, PTFE can only be cold-moulded. That is, you cannot melt PTFE and inject it into a mould to give a desired end-shape. The main reason for this is that PTFE does not flow when heated above its melting point. It attains what is referred to as a “gel state” – where the material goes from being opaque-white to transparent, but retains its shape even in this state. While in gel state, PTFE is soft, but still not completely pliable – making it very difficult to handle.

Given the absence of injection moulding – many conventional and otherwise obvious shapes which other polymers are available in become more complex when applied to PTFE.

All in all, 4 conventional methods exist to mould/ extrude PTFE.

– Compression moulding: PTFE resin (powder) is filled into a die cavity – usually with a simple shape (eg: inner-outer diameter, basic profile, length and width) – and the powder is compressed using a hydraulic press. Pressures would range from 300-400 Kg per square cm. Due to the high bulk density of PTFE, the resin is compressed to a third of the volume it occupies in the die. So if you were looking for a 100mm height, you would need to fill the die to 300mm. Once compressed, the PTFE is left to “dwell” for anywhere between a few hours to a day (depending on the size), before being loaded into a sintering over where the heat finally reaches the melting point of PTFE (about 375 Deg. C). At this point the granules melt and fuse to one another to form the final product.

Given compression moulding can only be done with very basic shapes, the cost of a component/ part gets amplified due to the wastage factor. A simple bowl, for example, would require a block of PTFE to be moulded and the cavity to be scooped out – making for a very expensive affair, especially given that the PTFE raw material is more expensive than most other plastics. Similarly, a film of PTFE requires a special process called “skiving” – where a cylinder of PTFE is rotated perpendicular to a blade, which “peels” a thin layer of film. Again the wastage involved here is high – thereby inflating the cost and final price accordingly.

– Ram and paste extrusion: Usually this is used to make PTFE rods, tubes, pipes and profiles. Extrusion is also used to make thread sealant tapes and expanded PTFE tapes and sheets. Here the resin is blended with an extrusion aid (normally naphtha or Isopar) and pushed through a die at high pressures to give the final shape. Again – the process is cold, with heat only being applied at the final stage to take the PTFE from its “green” (raw) state to its sintered state. Sintering for large tubes can be done in a conventional sintering oven (as described above) while for thinner tubes, a conveyor system can be used, as the tube can be in lengths of up to 500 meters.

Wastage here is again high. A single extrusion requiring a charge of 5-8Kgs would have a 500-800 Gram “end cone” wastage – or about 10%. For long tubes, there is invariably some wastage during sintering as well.

– Isostatic moulding: It is possible to achieve some degree of complexity in shape using this process. The PTFE is filled into a rubber mould which has the desired shape on the inside. The mould is then fitted into a chamber which is then filled with fluid (usually oil). Pressure is applied to the chamber, so that the mould is compressed – thereby compressing the PTFE into its final shape. Isostatic moulding is however not wildly popular because it is an expensive equipment and although it offers savings on material consumption, the payback was not considered fast enough – given that PTFE prices were consistently falling up to mid-2010. It remains to be seen – with the consequent rise in PTFE resin prices – whether this process catches on again, as processors try to control costs.

Even with isostastic moulding, there is a degree of inaccuracy in final dimensions due to the tendency of PTFE to shrink during sintering. It is therefore common practice to keep sufficient stock on the component – which can then be machined to attain the desired final dimensions. Again – wastage is increased here, over and above the degree experienced in injection moulding.

Sintering

Sintering PTFE is again a difficult task, which many processors initially struggle with. Getting the temperature, and equally importantly the timing of the temperature changes correct is essential to ensure the moulded parts do not crack. A cracked piece is virtually useless – as PTFE cannot be recycled (more on this later)

Timing is crucial – as stated above. After moulding, the PTFE item is kept to dwell for anywhere from a few hours to a full day (depending on the size). The purpose of the dwell time is to ensure any air trapped in the moulded piece can escape, as it would otherwise cause the piece to crack in the oven. Many processors – in an attempt to increase the rate of output – compromise on dwell time, with the effect of having inferior products that will often break during machining.

The timing of the temperatures – or the sinter cycle – is also crucial. While the actual cycle curve is a proprietary technology for each processor, the total cycle time is fairly common across the industry. Small articles (under 100mm in diameter) require only 14 hours in the oven. Slightly larger articles need 24 hours, while very large items need up to 52 hours.

So unlike conventional polymers, PTFE processing is time consuming. A sheet requiring to be skived from a large billet would need about 5 days end-to-end, as moulding would take 3-4 hours, the dwell time would be about 24 hours and sintering would take over 2 days.

Machining

While PTFE machining is not very different from other polymers – as far as tool selection, feed rate and RPM is concerned, the material does behave differently during and after machining. A few of the anomalies we have observed are:

1. Tolerance: we often get drawings from the customer specifying tolerances in the range of +/-0.01mm for virgin PTFE. Usually, the designer is someone used to dealing with metal parts (where such tolerances and common and easily attainable) and assumes the same holds good for PTFE. While we have been able to machine glass filled PTFE (which is stiffer than virgin PTFE) to within 0.015mm – virgin PTFE, being much softer, does not allow itself to be machined to tolerances closer than 0.04mm. Again – it may be possible to achieve closer tolerances even on virgin PTFE. But it would require fine tuning of the machining process, and possibly some extra tooling – all of which would increase the machining cost.

2. On smaller pieces, the softness of the material causes it to bend during machining, resulting in ovality and poor finish.To counter this, very small parts often need to be done in more than one operation. This again puts pressure on the tolerance (with CNC machining, a single operation is always preferable in achieving close tolerances) and also increases the cost of the part.

3. Shrinkage: this is one of the toughest attributes to account for during machining. Especially for parts being exported to colder countries, we have received reports of components being out of spec. dimensionally. PTFE is known to change dimension by up to 3% between 0 and 100 Degrees Celsius. So a part with a 20mm outer diameter in India (at about 30 Degrees Celsius) could shrink by 0.2mm (1%) in going to, say, Canada (where 0 Degrees is not uncommon). So with a tolerance of 0.04-0.05mm – the part would definitely fall out of tolerance. It is normally not possible to apply a formula for shrinkage and expect that the part will be dimensionally correct when it reaches the other side. The best bet would be to take a range of 1-1.5% for shrinkage and machine 3-4 sets of samples with varying dimensions and see which one best works for the client.

4. While virgin PTFE is soft and puts very little wear load on the tool, some of the compounded grades are not so easy on the tooling. Our experience has shown that PTFE+Carbon+Graphite, for example, puts nearly as much load on a tool as when machining metal components. So when machining compounded grades – one needs to account for the tool wear out – as it does account for a significant portion of the costing.

Scrap Recovery

As mentioned earlier – clients do sometimes ask whether the scrap value has been factored back in to the costing. It usually takes some time to convince them that there is no scrap value as such when we look at PTFE.

Usually, thermoplastics lend themselves easily to scrap recovery. The scrap is either ground back into granules and can be re-melted and used in injection moulding, or it has some basic scrap value, eg: road builders sometimes add plastic waste scrap into the tar mixture where it melts and adds some strength to the tar.

Since PTFE does not melt, it does not lend itself to either of these processes. In fact, the only known way of recycling PTFE scrap is to convert it into micro-powders – which is an expensive process and done by very few companies globally – and is only applicable to virgin PTFE. So filled grade scrap is worthless, while virgin scrap does sell, but for a fraction of the resin price – making it’s impact on costing negligible.

I hope the above piece has been somewhat useful in outlining some of the nuances of PTFE processing and clearing some of the commonly held beliefs about the same. In case you wish to know more, do contact us via our website: www.polyfluoroltd.com

What you need to know about PTFE structural bearings

Bridges typically consist of two components: the superstructure and the substructure. The superstructure is subject to various dimensional deformations due to the nature of loads placed upon it. These deformations could include:

  • Thermal expansion/contraction
  • Elastic deformation under live load
  • Seismic forces
  • Creep and shrinkage of concrete
  • Settlement of supports
  • Longitudinal forces – tractive/ breaking
  • Wind loads

The nature of these forces makes it necessary to have a device in between the substructure (base) and the superstructure which allows for the required movement, while also giving stability and having the capacity to bear the loads placed on the bridge. The device most popularly used, is a bridge bearing which assumes the functionality of a bridge by allowing translation and rotation to occur while supporting the vertical loads.

Thus, a bridge bearing is an element of the superstructure which provides a vital interface between the superstructure and substructure.

PTFE in Structural Bearings

The use of PTFE in such bearings has been steadily increasing, although its application does not extend to all variants of bridge and structural bearings.

PTFE has an exceptionally low coefficient of friction and high self-lubricating characteristics, resistance to attack by almost any chemical, and an ability to operate under a wide temperature range.

Furthermore, while unmodified PTFE can be used to a PV value of only 1,000, PTFE filled with glass fibre, graphite, or other inert materials, can be used at PV values up to 10,000 or more. In general, higher PV values can be used with PTFE bearings at low speeds where its coefficient of friction may be as low as 0.05 to 0.1.

The low coefficient of friction exhibited by PTFE is unique for two primary reasons:

 

  1. PTFE against stainless steel exhibits an even lower coefficient of friction that PTFE against PTFE. In fact, the coefficients of PTFE against steel have been found to be the lowest between any two solid materials
  2. The coefficient reduces with increased pressure – allowing for coefficients as low as 0.03 (See table 1)

 

To summarise, the following properties have driven the increased application of filled grade PTFE:

  • PTFE against steel has one of the lowest coefficients of friction
  • The load bearing capacity of the PTFE sheet is in the range of 130-140Kg/cm2
  • The PV values are found to be in excess of 10,000
  • Service temperatures of -250 to +250°C are possible


PTFE is most commonly used in two types of structural bearings:

Sliding bearings: A system of two plates, one sliding over the other makes one of the simplest types of bearings. These bearings permit translation in longitudinal and transverse directions, unless specifically restrained in any of these directions. No rotation is permitted unless specially provided in the form of articulation and only vertical loads are resisted / transmitted by these bearings.

 

Generally, plain sliding bearings are provided where span is less than 30m, because the movement capacity of these bearings is usually small.

The bearing is composed of two thick sheets of steel (preferably high-density carbon steel). Between the sheets are one layer of PTFE (with suitable fillers) and one layer of polished stainless steel. The stainless steel is welded to one of the bearing plates while the PTFE is bonded to the other plate. To provide for better bonding, a recess is created on the bearing plate into which the PTFE is fitted.

Their regular maintenance is very important, to keep a tab on friction otherwise the value of horizontal force transmitted to sub-structure will increase tremendously. Therefore, the frequency of lubrication has been prescribed as once in three years.

POT-PTFE Bearings: These consist of a circular non-reinforced rubber-pad (elastomer) fully enclosed in a steel pot. The rubber is prevented from bulging by the pot walls and it acts similar to a fluid under high pressure.

While the bearings were initially created without PTFE, the necessity of horizontal movement in addition to load bearing capacity made it necessary to incorporate PTFE on the piston. The rotation, therefore, is provided by the elastomer due to differential compression and translation by steel and PTFE.

POT bearings offers a much higher degree of movement than standard sliding bearings, although it is tougher to manufacture due to the extended recess needed for the POT as well as the sealing elements needed to contain the elastomer within the POT. These seals must be metallic. The PTFE plate must be recessed into the piston and requires ‘dimples’ into which additional lubricants are placed during time of installation.

 

Typical working conditions for standard POT-PTFE bearings include:

  • Provisions apply for temperature ranges of -10°C to +50°C
  • POT bearing of diameter up to 1500 mm are within scope of these specifications
  • Rotation up to 0.025 radians only considered
  • PTFE can withstand bearing pressures in excess of 40MPa – depending on the filler used(See table 2)

 

Designing PTFE structural bearings

Various design codes exist for structural bearings and most of them prescribe similar materials to be used and have similar requirements regarding the grade and strength of the materials. In India the code book for POT-PTFE bearings is the IRC:83 (Part II) while for sliding bearings, there is no system as yet established in India (although the IRC:83 does refer to the sliding arrangement requirements for POT-PTFE bearings, which can be adopted for all sliding bearings as well). Globally, standards such as the BS:5400 and AASHTO exist for sliding bearings and POT bearings.

Ultimately however, the bearing manufacturer undertakes the responsibility to design the bearings based on customer specifications.

The seven sides of PTFE (or, why PTFE is way cooler than most realize)

Considering we sell PTFE for a living, you may be skeptical when we shower praises on it’s versatility as a polymer. But it’s possibly this versatility that makes PTFE such a viable choice when we focus on a polymer that we would like to sell.

Since it’s discovery, the number of applications in which PTFE is used has grown consistently. Even today, we have clients who come to us unsure about whether PTFE might be suitable for their applications, only to realize that it’s everything and more that they were looking for. In most cases, PTFE outclasses other polymers by such a long stretch, that it not only becomes a suitable material for any given application, but rather the only viable option in the long term. Furthermore, the immense breadth of properties exhibited by PTFE allows the OEM user to augment the capability of his equipment (higher temperatures/ rpm/ wear rates etc.) without having to worry about whether the PTFE component within the equipment will be able to handle the increased load being applied on it.

 

In our earlier piece, we had also said we would look into the comparison between virgin PTFE and other polymers to gauge what level of substitution might be possible given the steadily increasing price of PTFE. In outlining the properties of PTFE, we are able to factually compare it with other polymers, so as to allow an educated analysis of possible substitutes for a given application.

 

1. Awesome dielectric strength

 

PTFE is an excellent insulator. One might use PVC tape to mend a wire around the house, but when presented with heavy duty currents and a risk of electrocution – no risks can be taken. PTFE rates highly on dielectric strength (effectively an indication of how much voltage can be passed through a film of a material before it ‘breaks’ and allows the current through. As the table shows – PTFE does have a few substitutes if we were to look at this metric alone, namely PFA, FEP and UHMWPE. However, when we look more closely, we realize that both PFA and FEP are 4-5 times the price of PTFE, while all three polymers have a lower melting point (discussed later) – making them unsuitable for applications where high-voltage coexists with high temperatures (as is often the case).

An additional issue with UHMWPE is that like PTFE, UHMWPE cannot be melt processed. Therefore, the only option to make tape from both UHMWPE and PTFE is a process called skiving (a peeling process where a thin film is drawn out from a billet of the polymer using a blade). While virgin PTFE allows itself to skive easily to thicknesses as low as 0.04mm, UHMWPE is much tougher to skive and cannot achieve such low thicknesses easily. Thus, thin tapes from UHMWPE are not so easily manufactured. However, for those with low temperature applications with high voltages, UHMWPE would be a suitable alternative to PTFE, provided thickness in excess of 0.2mm are needed.

 

2. Unbelievable temperature resistance

 

It’s obvious to see here why PTFE rates so highly rated in any application where the general operating temperatures are expected to go in excess of 100-120 Degrees Celsius. Not only does PTFE not succumb to high temperatures, but it’s heat retention is so low, that only a sustained temperature at levels in excess of 300 Deg. C can cause it to deform. The stubbornness of PTFE to heat was truly experienced by us for the first time when attempting to weld PTFE. The operation requires a concentration of heat (using a hot-air gun or a heating element) to bring the PTFE up to it’s gel state (the closestPTFE comes to melting is a transparent state at which point it is soft and will deform under pressure, but will still not flow or be easily adhered to any other surface – including itself). If the heat is removed – even for a second – the PTFE immediately “freezes” back into it’s opaque waxy state.

As mentioned above, most industrial applications – be they electrical, chemical or automotive – do experience temperatures in excess of 80-100 Deg. C – making most other polymers unsuitable. Furthermore, most applications involving high temperature transfer of fluids cannot do without PTFE tube – which are highly sought after in many chemical industries.

 

Again – PEEK, PFA and FEP – although equally proficient at handling high temperatures, are selling at many times the price ofPTFE; so the cost effectiveness of PTFE keeps it a preferred choice.

 

3. Shockingly low coefficient of friction

 

So low, in fact, that PTFE is the only material on which a gecko cannot stick itself! PTFE’s low coefficient of friction (also known as the non-stick effect) is well known thanks to the frying pans that tout its virtues. However, that’s not the only area in whichPTFE outshines in this respect.

For one – PTFE is unique in that the static and dynamic coefficients of friction are pretty much the same. The practical implication of this is that you could have a PTFE component within an assembly which hasn’t been used in months, and when the parts start moving – PTFE behaves exactly as if it’s been used all along. This is especially useful in applications such as bridge bearings and ball valves – where a high static coefficient of friction could put undue stress on the other moving parts.

 

Another facet to this already burgeoning properties list, is that with PTFE – increasing the load reduces the coefficient of friction. Again – this is particularly useful in bridge bearings – where PTFE is an invaluable component of the bearing due to its compressive strength (discussed below) and low friction.


While UHMWPE also has a fairly low coefficient of friction – it does exhibit significant creep – making it unsuitable for bridge bearings. However, in applications requiring self lubricity and constant moving parts – UHMWPE would a a very cost effective solution to PTFE.

 

4. Impressive compressive strength

 

PTFE is capable of bearing high loads without deforming. It is however, not toally unique in this aspect. While Delrin and PEEK would not be cost effective versus PTFE, both PVC and Nylon have higher compressive strengths and are cheaper in comparison with PTFE. However, as discussed above, most of PTFE’s load bearing applications involve their use in bridge bearings – where the low coefficient of friction and chemical inertness also play a part in its preference.

5. Astounding chemical inertness

 

Barring a few exceptions, PTFE is largely inert. The only other plastics which share its range of inertness to different substances ate UHMWPE, FEP and PFA. All three materials, along with PTFE are used in the medical industry for both laboratory wares as well as in implants for patients. While PTFE, PFA and FEP tubes are used extensively for catheters and urological stents,UHMWPE has become the mainstay in medical implants such as joint replacements.

 

Another fast growing application of PTFE tubes is in the manufacture of umbilical cords. The cord (completely non-biological) is used in the oil & gas industry to takes vital gases from the refineries to the labs – where they are analysed to make sure that the reactions within the refinery are happening properly. The use of PTFE is vital, as its inertness ensures that the gases are not modified in any way during transit within the tube – as that would lead to a spurious analysis.

 

6. Excellent wear resistance

 

While PTFE has very decent wear resistance in comparison with weaker polymers, it is easily surpassed on this metric. It must be said, however, that PTFE with bronze fillers does exhibit improved wear resistance compared with virgin PTFE (bringing the value closer to that of UHMWPE), while its self-lubricity (due to its low coefficient of friction) gives it a huge boost in the use ofPTFE+Bronze as wear pads and slideway bearings (a material commonly known as Turcite B). Still – PTFE+Bronze has a 1:20 ratio on price when compared with UHMWPE – making UHMWPE the more cost effective solution. In an attempt to counteract the rising prices of PTFE, we have been recommending that clients shift to UHMWPE. However UHMWPE still suffers the disadvantage of low temperature resistance – making many continue with PTFE. Also – while PTFE can be bonded to a metal substrate by chemically treating (etching) the PTFE surface, UHMWPE requires corona treatment – which is only partially effective in making the material bond able.

7. Highly hydrophobic

 

PTFE has an incredibly low water absorption level which, when combined with its chemical resistance, makes it a clear winner in both outdoor applications and applications in a wet environment.

 

However – when dealing with wet environments, it must be noted that both UHMWPE and HDPE are just as effective – and much cheaper. PTFE continues to be used in applications such as sealants (especially thread sealant or plumbers tapes), while expanded PTFE is used extensively due to its added advantage of a spongy texture.

In conclusion: the main purpose of this piece was to illustrate the properties of PTFE in a quantified manner and subsequently compare it to other polymers in its class to gauge whether any substitution is possible. As we have seen, while some polymers compare on some metrics, there is no clear substitute across characteristics. In addition to this, is the cost comparison – with only UHMWPE, HDPE, PVC and Nylon having clear cost advantages when compared with PTFE. And even though the cheaper polymers do match up to PTFE on some metrics, to replace PTFE outright would be very difficult as the use of PTFE in many applications combines it efficacy across 2-3 metrics.

 

It has been our experience that most clients – even after being presented with alternatives, have continued with PTFE – even at the higher price. Given the facts we have presented above, it is not difficult to see why this would be so.

 

In case you wish to explore the properties of PTFE further, do visit us at: www.polyfluoroltd.com

 

Note: Above values are indicative an intended mainly for comparison between polymers