Unravelling Polymers

The Definitive Blog on Polymers by Poly Fluoro Ltd.

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

What's the matter with PTFE prices?

Like most processors and users alike, we too have been asking this question, repeatedly.

When PTFE prices first increased in 2010, it seemed like an inevitable price correction which, if viewed objectively, was understandable. PTFE resin prices had been steadily declining up to early 2010 and only in retrospect do we see how low they actually were. So when the initial hike was announced, it seemed justified and we went about our work gauging the impact it would have on margins and preparing to educate customers as to what we believed would be only a 5-10% price correction for them.

But in the one year following the initial hike, the continued increase in prices has been so steep that for possibly the first time, processors are actually asking themselves if PTFE is a business they should even be in. On the other side of the table, clients are seriously considering whether any alternate to PTFE could be used in their products.

If ever there was a suitable application of the term – a paradigm shift – it is in the PTFE industry post April 2010.

To quantify the extent of the price increase, let’s look at some of the prices we were getting for our resins in 2010 and compare with the current rate (April 2011).

1. Virgin PTFE

April 2010 – US$7.25 per Kg (US$3.28 per pound)

April 2011 – US$20.55 per Kg (US$9.35 per pound)

% increase: 185%

2. 15% Glass Filled PTFE

April 2010 – US$12.9 per Kg (US$5.85 per pound)

April 2011 – US$21.1 per Kg (US$9.6 per pound)

% increase – 64%

3. 40% Bronze Filled PTFE

April 2010 – US$14.95 per Kg (US$6.8 per pound)

April 2011 – US$ 24.55 per Kg (US$11.15 per pound)

% increase – 64%

In most cases – at first attempt – the customer simply refuses to believe that any increase in pricing could be so drastic. The fact that there is little or no published data online regarding this makes it even more difficult to convince them!

After speaking with many industry insiders, suppliers and processors, we are only able to slowly piece together the effects causing this dramatic surge in PTFE prices. In most cases, we are inclined to believe what these sources say – although a more thorough study of the actual economics driving the PTFE pricing may be required to truly quantify the degree to which each factor influences the eventual rate.

To start with, let’s look at the changes globally:

1) Fluorspar

This is the most common reason that resin suppliers offer up as soon as anyone questions the price of PTFE. Its a bit of a black box – because PTFE processors have never really delved into the details of resin manufacture and suppliers in turn, remain secretive about the process – which suits them in times like this!

Fluorspar is a mineral which, apart from being a critical raw material input in PTFE resin manufacture, is also vital in the manufacture of refrigerants, steel and pharmaceuticals. We are told that China controls most of the global fluorspar reserves and that owing to higher realisations in areas other than PTFE, the availability of fluorspar for PTFE resin manufacture has been severely constrained. This supply-demand gap for fluorspar is set to have triggered off the initial price increases in PTFE.

I’m actually speaking with some fluorspar manufacturers later to gauge exactly how much impact this supply-demand gap may realistically have on the pricing. Will add more on that later.

2) Plant shutdowns

It is highly likely that some of these may be rumours, but we have heard that the largest resin manufacturer in China shut down their plant for maintenance. Following this, some of the larger manufacturers in Europe also had forced maintenance shut downs and more recently, the earthquake in Japan has forced their largest PTFE resin manufacturer to remain closed for the next few months. There are also rumours that DuPont may be coming out of the compression moulded resins business – putting further pressure on supplies. All in all – we are told that this has resulted in about 1000 tonnes of global shortage in PTFE resins.

3) Greed… and the ‘what else’ factor

One of my more honest suppliers of compounded resins (so virgin PTFE is a raw material for him as well) offered the only remaining explanation for the continuing increase in prices – “greed”. Another manufacturer of PTFE micro-powders seconded this and also suggested the “what else” implication of all this.

To explain this better – we need to look at PTFE as a polymer among a huge family of fluoroplastics and thermoplastics. As we have struggled to cope with the price increase we have also looked for alternatives to offer our customers – sometimes at their request, sometimes in desperation to keep the customer from going elsewhere.

In truth, there are only 2 polymers which somewhat compare across properties with PTFE. The first is UHMWPE – which, like PTFE has excellent wear resistance, dielectric strength and a low coefficient of friction. However, its inability to withstand high temperatures restricts its use in many industrial applications (PTFE can withstand up to 250 Degrees Celsius, whereas UHMWPE can only withstand up to 80 Degrees Celsius). UHMWPE is much cheaper than PTFE (especially now!) – but is still viewed as a sort of ‘poor cousin’ and despite our efforts, customers are not nearly as satisfied with it as they are with PTFE.

The other polymer is PEEK (better known by its trade name – Victrex). PEEK is again comparable to PTFE on all metrics (exceeding PTFE on properties such as tensile strength and temperature resistance) except coefficient of friction. However, PEEK resin currently sells at over 5 times the price of PTFE resin – making any discussion on substitution for PTFE pointless.

There are other polymers which may match up to PTFE on at least one or two of it’s properties – but all that we have looked at are still more expensive than PTFE – even after the price increase! (a more in depth look into the properties of PTFE and the comparable properties and price of substitutes will be dealt with in a separate piece)

In our own experience – many clients have (grudgingly) accepted the new pricing we have offered because – by their own admission – they have no alternative materials which would suit their application.

It is my belief that at some point in the last year, resin manufacturers realised that a material as versatile and irreplaceable as PTFE was not selling for as much as it should. While the initial hike in pricing was due to a fundamental shift in the demand-supply equation, the more recent bursts seem more likely due to pressure testing by resin manufacturers to see what level they can hold prices at in the long run.

In India there has been one extra factor which has influenced PTFE resin prices. In 2010 the local resin companies lobbied for an anti-dumping duty on all Chinese an Russian resins. The duty – which still stands – called for a flat charge of US$3.25 per Kg – which allowed the Indian resin manufacturers to raise their prices immediately from US$7.25 to US$10.5 per Kg. The continued rise beyond this price is explained by the global shifts which ‘coincidentally’ occurred within weeks of the duty being levied.

The question that we’re being forced to ask now is “what next?” In truth, there is still no visibility on when the PTFE resin prices will stabilise – which means processors are living from one day to the next, trying to convince clients in the hope that any orders they secure will hopefully be executed before the prices increase again. It is a very sticky situation and if rumours are to be believed, it will only stabilise by the end of 2011.

In case you wish to know more about PTFE, do visit us on: www.polyfluoroltd.com

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