The various forms of Sliding Bearings

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The application of PTFE in load bearings is not new. Amongst its many other attributes, PTFE also has an excellent compressive strength, allowing it to absorb pressures of up to 200 Kgf/cm2 (2900 psi). This is approximately double the compressive strength of neoprene (the material used in most elastomeric bearings) meaning that PTFE bearing pads can be much smaller and manage the same load.

In addition to the load bearing capacity, PTFE also exhibits a low coefficient of friction (the lowest of any known solid) – which only goes lower with the addition of more pressure and is exceptionally low when PTFE slides against polished stainless steel (the lowest between any two known solids).

This combination of load bearing strength and low-friction makes PTFE the preferred material for sliding bearings – where both load bearing and sliding movement are required to create an effective bearing assembly.

The use of sliding bearings is fairly widespread. Some of the areas we have supplied to are:

1. Oil and gas pipelines
2. Waterways and water pipelines
3. Conveyor systems (both indoors and outdoors)
4. Boiler plants
5. Minor bridges
6. Power plants

The compact size and overall effectiveness of the bearing makes it an ideal choice in lower load applications (under 100 Tonnes). Furthermore, the simplicity of slide bearing design ensures that as long as the basic design specifications are adhered to, there exists a lot of latitude as to the exact dimensions and form of the bearing. This is useful for clients, who would prefer to design their structures independently and have the bearing modified to suit their overall design.

It must be pointed out that in India, there is no official rulebook for the design of sliding bearings. For the most part one refers to standards such as BS:5400 and AASHTO – taking care to cross check against the IRC:83 (the Indian code book for POT-PTFE bearings) to ensure that the material specifications match.

As a manufacturer of these bearings, this does add a lot of flavour to the task of design. Very rarely do two separate projects look for the same bearing design – there are always nuances and specific constraints against which the bearing must be altered to accommodate the client’s requirements. And although the constraints may be somewhat common – the method of accommodating them can vary significantly.

Movement

In many cases, the bearing requires a sliding movement in only one direction. This results in the requirement of guides. Our experience with guides is that as long as there is negligible horizontal load on the bearing (under 2 tonnes), any of the two following guiding elements can be used.

- Bracketed guides – these are normally two guide plates welded/ bolted to the side of the top or bottom plate

- Dowel guides – guide pins can be used either at the center of the plate or on the sides

In case the load is higher than 5 Tonnes, a centre dowel guide is always preferable. Some designs may also specify a guide that is monolithic with the top plate. While this is the definitely better from a load bearing stand point – it is often expensive, as the plate needs to be either cast or machined out of a much thicker plate.

In any case, as the horizontal load increases beyond 10-15 Tonnes, it becomes viable – both technically and commercially – to look at POT-PTFE bearings.

Rotation (lateral)

Rotation along the horizontal axis (perpendicular to the direction of the vertical load) is not a common requirement.

It is most easily achieved by employing a circular dowel pin at the centre of the bearing around which the top plate can rotate.

In case the load is high, you could also look at a hybrid POT bearing – where a PTFE disc is used in place of the elastomer and a polished stainless steel sheet is affixed on the piston to allow for rotational sliding movement.

Rotation (vertical)

Vertical rotation (around the direction of the vertical load) is most easily achieved by employing an elastomeric pad along with PTFE. In most design specifications, there is a stainless steel sheet required in between the PTFE and the elastomer.

In more heavy-duty applications, a fully reinforced elastomeric bearing may be employed. The bearing is affixed (either by bonding or during vulcanizing) to the base plate housing the PTFE.

However – as discussed earlier – the lower compressive strength of elastomeric bearing material (such as neoprene) would require the size of the PTFE bearing to be defined by the size of the elastomeric bearing required. In some cases, where space is a constraint, designers opt for spherical bearings to accommodate the vertical rotation.

The benefit of a spherical bearing is that it can be compact and that the radius can be changed to match the extent of the rotation required. In contrast, to accommodate higher rotation in an elastomeric bearing, the thickness of the bearing would need to be increased – making it more expensive and bulky.

On the other hand, the smoothness of the rotation provided by an elastomeric bearing (which is effectively using it’s elasticity to accommodate the rotation) is compromised in a spherical bearing. Although in most spherical bearings a PTFE-SS match is created to allow for smooth rotation – it will perform slightly less effectively than an elastomeric bearing. Ultimately, this is a trade-off that the designer will need to assess depending on the requirement of the project.

Arc bearing

Arc bearings are normally used in pipelines, as the bearing needs to take the curved shape of the pipe. The most common arc type bearings we have come across employ two sets of PTFE-Neoprene pads, which have been heated and bent to form the required radius needed to match the pipe. One set of PTFE-Neoprene is bonded with the pipe, such that the PTFE layer faces downwards. The second set is bonded to the concrete base, such that the PTFE surface faces upwards. When the pipe is lowered on to the concrete base, the PTFE layers mate, such that there is sliding along the length of the pipe. Also, due to the neoprene layers – there is rotation allowable.

This bearing can also be made using stainless steel to replace one of the PTFE layers. However, bending the stainless steel to match the radius of the pipe is more expensive than bending PTFE (which can be done using heat and a cheap metal die). Furthermore, it is likely that there would be slight variations on-site in the radii of the pipe and the concrete support. In this case, the stainless steel may develop kinks/ irregularities on the surface once the load is applied whereas PTFE, being much more pliable, will accommodate the same quite easily.

Two-way bearing

Our experience with this type of bearing has been mainly in the erection of conveyor systems. Often, along with the vertical load exerted on the bearing, there is some amount of horizontal load (along with restricted horizontal sliding movement in one direction) and some upward load. Usually, these loads are very small – within 2 Tonnes – so a complex or heavy-duty solution becomes wasteful

The concept of a low-cost, but effective bearing has let us to consider 2 alternate designs as shown below.

The simple design would employ side guides to form a bracket around the lower plate – allowing sliding movement in one direction and ensuring any uplift is contained. However, as the guides are welded, their strength is limited to within 2 Tonnes at most.

In case the uplift load is higher than 2-3 Tonnes, one would need to look at the second design – where a bolting arrangement allows the total load to go much higher. The second arrangement is altogether more elegant and compact – but comes at a much higher cost, owing to the extensive fabrication required and the extra thickness on the top plate needed to accommodate the guide-cum-anchor pin.

Rocker bearing

Although rocker bearings are usually stand-alone metal bearings, we have seen them used along with a PTFE sliding arrangement to give a rocking-cum-sliding arrangement.

The base plate housing the PTFE is usually the top plate of the rocker bearing.

Conclusion

We have described here only some of the bearing types and features that can be designed, based on the requirement of a specific project. Considering that projects take many forms and the constraints they may present could be very unpredictable, the above list could only be a fraction of the complete set of sliding bearings that can be envisaged. However, our experience in this field suggests that these are the primary features which are required of a given bearing and that ultimately, most bearings would be a combination of the above design forms.

PTFE and the “Repro” Conundrum

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In recent times, the landscape of the PTFE industry has been significantly altered by the ascent of PTFE recycling. The combining of recycled PTFE (known technically as “Reprocessed” or “Repro” PTFE) with pure PTFE has become so widespread and unchecked that more often than not the material that customers are buying does not even remotely adhere to the quality standards required – due the abnormally high levels of repro being mixed in an attempt to keep costs low for the processor.

More alarming – processors and dealers alike are choosing not to offer the transparency to most clients on the proportion of recycled material being used (or that it is being used at all). This misleads the client into assuming he is receiving a material which is superior in performance – but which will most likely fail in any long run application. Additionally – processors who supply pure PTFE are forced to compete on price with a material that is not truly a substitute.

We would like to look at the issue of Reprocessed PTFE – both from the technical standpoint as well as a commercial standpoint. We believe the issue is critical to the understanding of the PTFE industry and as a technical tool for those looking to incorporate PTFE in their applications.

Pricing irregularity in PTFE

By 2010, the price for PTFE resins globally had reached some level of stability. Those in the industry will know that this was short-lived as one year on, we continue to work in oblivion to what price fluctuations may occur in the next week or month. However, it would be fair to say that even historically – the prices availed during the first half of 2010 may be the lowest that PTFE prices have ever sunk. Nonetheless, the competitiveness of pure PTFE processors was still not great.

In the few years leading up to 2010 (just before the current price escalation began) we began observing an obvious disconnect in India between the price of PTFE resins and the price of semi-finished articles (rods and sheets) being imported from China by traders.

The price for virgin PTFE resin was about 8-9 US$ per Kg (3.6-4.1 US$ per pound), whereas the price for Chinese semi finished articles was 10-11 US$ per Kg (4.5-5 US$ per pound).

Given that the processing cost for PTFE is about 4-5 US$ per Kg (1.8-2.3 US$ per pound) – it seemed there was no way that manufacturers in India could compete with traders on price. Obviously, clients were equally surprised, as they should have been; you would expect manufacturers to be far more competitive than dealers, but this was not the case.

It seemed impossible that the price could be so low, considering it would need to include the price of resin in China plus the cost of shipping, plus the customs duties on Indian imports, plus the trader’s overheads and finally the trader’s margin.

To study this pricing abnormality, we placed a large enquiry to Chinese resin suppliers to gauge the local price in China and were offered a rate of 5.5 US$ per Kg (ex-works). If we used this as our base price (as we assume a large Chinese processor would avail such a price) and assumed the same costs of processing (not unlikely as India and China have similar wage structures and power costs), the cost structure for semi-finished PTFE could be built up as follows:

It turned out that the key difference between the prices was that Chinese suppliers are selling reprocessed PTFE – which allows the prices to be maintained at a much lower rate than if they used pure PTFE.

As you can see – the difference between the Implied Price and the Actual Price could be as high as 30%: the effect of using recycled material for processing semi-finished articles.

Of course, the figures above may not be fully accurate (customs could be as low as 11% if the trader is allowed to pass on excise duties), but it still points to a 12-15% gap, which can only be explained by the use of repro material.

Our trader contacts corroborated this – giving us figures ranging from 15% to 30% for the percentage of reprocessed PTFE used in making semi-finished articles. The estimations we came across for the price of repro were in the range of ~2.5-3 US$ per Kg – which could lower the raw material price by up to 15% - tying in with the overall price gap we estimated.

What is repro?

There are possibly a number of ways in which PTFE can be recycled for being used back into moulding. The most common way is to grind PTFE scrap (otherwise useless and therefore very cheap) into a fine powder and blend this powder with pure PTFE to be used either in compression moulding or ram extrusion.

Before grinding, the scrap is usually first heated to above its melting point to remove any organic contaminants. Once ground, it is treated with acid to dissolve inorganics after which it is washed and re-heated – to vapourise any volatiles.

However, since ground scrap is effectively sintered PTFE – during processing it will not form bonds with surrounding PTFE material the same way that un-sintered PTFE does (much the same way you cannot weld two PTFE articles to one another using heat alone). Therefore, it is essential to maintain a proportion of reprocessed PTFE that allows enough bonding of pure PTFE molecules during sintering to ensure the overall stability of the sintered product.

The right proportion to be used is as such not documented (there exists very little technical data on reprocessed PTFE as it is relatively “unorganized” in its application) – but one might like to think of one grain of repro PTFE needing at least 4 grains of pure PTFE surrounding it to ensure the bond strength is sufficient. So a ratio of 1:4 or 20% as an upper limit may not be off by much.

However, as the price of PTFE continues to increase, this rule of thumb has been stretched considerably. Recent reports suggest up to 45-50% of reprocessed PTFE being used in an attempt to keep the semi-finished price from escalating. The move has not been altogether successful as (1) the price of PTFE scrap has increased as well – making repro more expensive (though still cheaper than pure PTFE), and (2) the rejection rate has increased – which has increased costs and impacted price.

Aside from the commercial impact however, most end users remain unaware of the technical issues.

Issues with using reprocessed PTFE

Like any other material – recycling erodes the properties that the material originally had. In the case of PTFE, many of the core properties are so good, that reducing them by a small amount to keep costs low can be a feasible trade-off. So from the point of view of application, a 5-10% repro ratio would still allow the material to pass off as pure PTFE for most applications (although it would still be ethical to inform the client of the composition). As the ratio is increased, the degradation in core properties would continue to the point where the material is totally unsuitable for any regular application.

The table below illustrates how key properties we have observed change as the percentage of reprocessed PTFE increases.

One of the main issues with reprocessed PTFE is that it introduces porosity into the material, which then causes issues with water absorption and dielectric strength. Furthermore, weaker bonds between the molecules adversely impacts tensile strength and invariably causes crack lines within the material, which may not be visible, but will become apparent during machining and/or result in a failure of the component during long term usage. Although the chemical inertness remains good (as it is still 100% PTFE), the higher water absorption makes the material suspect for applications where the weather-ability and hydrophobic properties make pure PTFE such a sought after material.

Finally – there is the visual impact. In a given article, the percentage of black inclusions (normally due to foreign matter being mixed with the repro PTFE during the grinding process) could be as high as 40%. Usually, these are within the material – so it only becomes apparent after machining – which is doubly wasteful as the time spent machining is not recovered. In addition to this, too much repro will adversely impact the finish of the product to the point where the finish is rough to the touch and a white powdery discharge is seen on the surface of the machined part. Needless to say – these are all unacceptable for most clients.

To tie in the commercial and technical points we can say this: before it became apparent that the price gap was driven by the use of reprocessed PTFE, this gap was easily exploited by clients, who would compare our prices with the prices of traders and use it as a bargaining tool. However, as the use of repro has escalated, many clients have come back citing quality issues and inconsistency of properties (as would be expected). Even clients who had tested the repro material knowingly and found it to be in line with their requirements have found that in the long term, many of the initial properties have eroded. As a result, manufacturers are slowing gaining back favour – provided they are supplying pure PTFE and can support it with the appropriate test methods.

To conclude – reprocessed PTFE will always have inferior properties to PTFE and cannot be as consistent over time. The exact extent of this deviation in specifications is not easy to document. Therefore, it is always better to go in knowing what to expect and in case the core properties can be compromised on, it is better to experiment with reprocessed PTFE in your particular application to gauge the level with which you are comfortable.

We continue to get requests from clients who state in their enquiry that they are comfortable with recycled PTFE. This is because they are confident that their application does not require such high properties and that the trade off with better costing is worth their while. However, there comes a point where the material simply cannot be called PTFE anymore – and we have yet to come across a client that sees the feasibility in this!

The mysterious relationship between Fluorspar and PTFE prices

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It is strange that despite the excessive and unprecedented hike in PTFE prices, so many processors and end-users remain considerably in the dark with regards to where the problem originates. Even the more technically inclined processors with whom I have interacted have more or less thrown their hands in the air and decided to just take things as they come.

When we embarked on our own journey to understand the factors driving higher PTFE prices, we too felt fairly defeated by the complete lack of transparency into the workings of the PTFE industry higher up the value chain. All we had to go with was one word, which has been tossed around from the beginning as a sort of cover-all explanation for the predicament we are in.

“Fluorspar”

Understanding the Fluorspar situation is essential to answering the question of why PTFE prices have reached such highs.

We are going to look at the fluorspar issue in the following steps:

1. What is fluorspar?
2. How much fluorspar is used in PTFE resin manufacturing?
3. How has the price of fluorspar changed over the past year?
4. What is the supply side scenario?
5. How have pricing and supply combined to influence the current situation?
6. What are the implications – short and long term – based on what we know now?

What is Fluorspar?

We have looked at Fluorspar earlier. It is a naturally occurring mineral, used in a multitude of industries ranging from metallurgy, ceramics, glass, aluminium and yes – fluoropolymers.

Fluorspar (also commonly called Fluorite) is divided into 2 principal grades based on the concentration of Calcium Fluoride (CaF2) in the material.

· Metspar: Metallurgical grade fluorspar, which contains less than 97% CaF2

· Acidspar: acid grade fluorspar, which contains more than 97% CaF2

Acidspar – which comprises about 60% of the total – is the raw material for hydrofluoric acid (HF) and by extension for all fluorochemicals. A significant amount of acidspar is used in the aluminium industry, with about 55-60% used for fluoro chemicals. We are told that of this 55-60%, a bulk of the material is used in PTFE manufacture.

From here on when we refer to fluorspar, we will be referring to this grade only and the economics surrounding it, which have played such a huge role in the PTFE industry.

Converting Fluorspar to PTFE

While the practical conversion of fluorspar to PTFE is a proprietary technology (and by no means easy to fit into one blog article!) – we have looked at the theoretical formulae and corroborated this with available information to arrive at some basic ratios. These ratios can be used to calculate the impact of a rise in fluorspar prices on the cost of manufacturing PTFE.

In the first process, fluorspar is reacted with Hydrogen Sulphide to give Hydrogen Fluoride (HF). HF is then reacted with Chloroform to give Chlorodifluoromethane (more commonly known as HCFC 22 or R22). R22 is a well-known refrigerant – used not only in PTFE manufacture, but in refrigeration and air conditioning as well (although it is being phased out slowly in these industries – we will look into that later). Finally, R22 undergoes polymerization to give the TFE chains, which become PTFE during the sintering process.

Theoretically (as per the molecular weights of each substance), it would require 1.95 Kgs of fluorspar to manufacture 1 Kg of HF. Practically, we are told this ratio is more like 2.25:1 – implying an inefficiency factor of about 15% in conversion.

Similarly, a theoretical calculation would suggest 0.8 Kgs of HF required for 1Kg of TFE. If we apply a more strict inefficiency factor of 40%, it suggests a ratio of 1.15 Kgs of HF for 1 Kg of TFE.

So putting this together gives us a ratio of 2.6 Kgs of fluorspar as the input for 1 Kg of TFE.

In other words, a $1/Kg increase in the price of fluorspar increases the cost of TFE by $2.6/Kg.

We will come back to this ratio later – as it is critical in assessing the scenario at present.

The price of fluorspar

It would not be inaccurate to say that there has indeed been a significant price increase in fluorspar. The graph below shows the price increases in China and in Mexico. Although Mexico is cheaper, China’s volumes are much higher – implying the global price more closely follows their price trend.

China’s high price is driven largely by a 15% export duty on fluorspar – which was put in place to ensure that the domestic market supply is adequate. It is not clear exactly why China has put this duty into place. Some feel it is a China dominance story as China tries to take over the PTFE industry, but our own sources estimate that close to 30% of the domestic PTFE processors in China have shut-down due to the price increases in PTFE resin. Hence, the China dominance story does not add up. We believe that it may be driven more by a nearly insatiable domestic demand for R22 as a refrigerant (fueled by China’s ever growing consumer base) coupled with China’s own moves to restrict R22 supply for environmental reasons. It does remain to be seen whether the passing of the summer months eases the domestic demand for R22 and gives some respite to fluorspar prices.

Whatever the reason, it does point to a price increase of about 53% in fluorspar over the past year – corroborating what many resin suppliers have indeed claimed.

It is interesting to note that the prices in Mexico have indeed fallen in the past 2 months, while reports indicate that there has been no movement in price in China between April and May. Nonetheless, industry experts do not see any significant easing of fluorspar prices within the next year.

Supply side dynamics of fluorspar

With regards to manufacturing fluorspar, China outstrips all other countries, as shown below:

However, when we look at global reserves, China’s dominance is clearly unsustainable.

In fact – unless China finds new reserves of fluorspar, at their current rate of extraction, they would run out of domestic supply within 7 years.

The recent crisis in fluorspar price and availability has led many countries to look into re-opening old mines, which had earlier shut down. We will look at the impact of this shortly.

Defragmenting our current predicament

Going by the price data given above, we can see there has been an increase of US$160/tonne – or US$0.16 per Kg - in the price of fluorspar from China. Taking our ratio of 2.6 – this would translate into a US$ 0.42 per Kg increase in the input cost to PTFE.

In an earlier article, we had outlined that the price for PTFE virgin resin had increased by 185% in the past year – or by about US$13.3 per Kg.

Clearly, the difference between these two figures cannot be explained by the price of fluorspar alone.

If we dig deeper – we would need to realize that it is not so much the price of fluorspar as the lack of availability that is driving the PTFE prices higher. PTFE resin manufacturing is a capital-intensive field, requiring a lot of infrastructure. With a restriction in their supply of fluorspar, resin manufacturers are constrained to supply quantities much below their installed capacities – meaning higher average costs per Kg and consequently a higher price for PTFE resin.

Hence, while it is not wrong to say that PTFE prices have been influenced by higher prices of fluorspar – it is on the supply of fluorspar that we need to focus to understand how and when this situation will get resolved.

Looking ahead to the short and long term

A few global trends have caught our attention with regards to the short and long term implications for fluorspar (and by extension – for PTFE).

In the short term, the only easing out that can be expected is if the passing of the summer months reduces China’s domestic appetite for refrigerants. If this is the case, we may see some price stability post June 2011.

In the medium to long term, there may be several factors, which may positively influence the PTFE industry.

For one, the scaling back of R22, as a refrigerant due to environmental reasons would bring focus back to fluoro polymers as the primary consumer of R22, boosting supplies of R22 to the PTFE resin manufacturers.

Secondly, as more countries look inward for fluorspar mining, it is likely that there will be an easing of supply. We have already shown that the bulk of the reserves are not with China. With Mexico showing initiative in increasing their supply of fluorspar and South Africa expected to follow suit, we may see a significant increase in global production – possibly in the next 2 years.

Finally, there are technological advancements looking into production of Hydrogen Fluoride without the use of Fluorspar. If this is successfully commercialized, the dependence on fluorspar for PTFE manufacturing will reduce considerably.

Right now, all we can be assured of is that with a 53% price hike, Fluorspar is certainly a very commercially attractive mineral to produce and countries with reserves would be putting efforts to turn them into profits. And since we know that the fluorspar price only marginally dictates the price of PTFE resin, once supplies ease out, it should bring stability back to PTFE prices.

PTFE tubing - one product, numerous applications

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The evolution of Polytetrafluoroethylene (PTFE) - more commonly known as Teflon^® - from a niche product used only in high-value applications to a mainstream requirement has been very gradual.

However, over the past two decades PTFE usage seems to have crossed a critical mass, allowing it to become commercially viable in over 200 industrial, consumer and medical applications. And while sheets, rods, coatings and components corner the bulk of the market for PTFE products, PTFE tubing is now emerging as the key growth area.

PTFE tubing applications

The use of PTFE tubing has spread across various applications including automotive, chemical, electrical and medical. Table 1 shows the key properties which outline the versatility of PTFE tubing, while Fig 1 shows its uses in various fields.

• In automotive applications, the ability of PTFE to withstand temperatures in excess of 250^oC makes it an ideal candidate for high temperature fluid transfer.
• In medical applications, PTFE tubing is in huge demand due to its lubricity and chemical inertness. Catheters employing PTFE tubing can be inserted into the human body without fear of reaction or abrasion with any body parts.
• In chemical applications - including laboratories - PTFE is an ideal replacement for glass due to its inertness and durability.
• In electrical applications, the excellent dielectric properties of virgin PTFE make it well suited for insulating high voltage cables.

Table 1: Key properties and applications of PTFE tube

Types of PTFE tubing

Depending on the application, PTFE tubing is divided into three broad categories - each defined by the tube's diameter and the wall thickness (see Table 2).

Even within categories, PTFE tubing lends itself to different variations, each allowing for a different application (see Table 3):




PTFE tubing in the medical device market
In general, small diameter spaghetti tubing is used in medical applications. The use of PTFE in this area centres on two key properties: lubricity and biocompatibility. Fluoropolymers exhibit very good lubricity compared with other plastics. PTFE is the most lubricious polymer available, with a coefficient of friction of 0.1, followed by fluorinated ethylene propylene (FEP), with 0.2. These two polymers represent the vast majority of all fluoropolymer tubing used in medical devices.

The biocompatibility of any polymer used in a medical device is an obvious concern. PTFE excels in this area and has a long history of in vivo use. Medical-grade fluoropolymers should meet USP Class VI and ISO 10993 testing requirements. Of course, processing cleanliness is also an important factor.

PTFE tubing - processing techniques
The uniqueness of PTFE tubing rests in the complexity of PTFE as a polymer. While most polymers lend themselves easily to injection moulding - allowing them to be made into complex shapes, PTFE due to its high melting point and melt viscosity can only be compression moulded. The high melting point of PTFE also means that extrusion - as conventionally practiced - cannot be applied to it. PTFE paste extrusion has therefore become a process which is increasingly sought after - given the growing demand for PTFE tubing.

Extruded grades of PTFE were first used in the wire and cable industry in the 1950s, where the good dielectric properties of the material proved critical to the developing electronics market. The first tubing was made by extruding PTFE over a wire and then removing it-a labour-intensive process. In the 1960s, technology emerged that could perform the extrusion of PTFE without a wire core. This process enables PTFE tubing to be economically produced in long continuous lengths.

PTFE paste extrusion follows 6 broad steps as illustrated below:

1. Mixing: The resin comes in a powder form with an average particle size of about 0.2µm. The powder is waxy and prone to bruising and mechanical shear fibrillation. Hence handling must be careful and done typically at a temperature of around 20°C. While standard compression moulding only requires that the powder be sieved thoroughly and then compressed, in paste extrusion the powder must be first mixed with a hydrocarbon extrusion aid or mineral spirits. The powder-spirit mixture is left in a sealed container before it is used in the next process
2. Pre-form: The pre-form is a billet made by compressing the mixture in a hydraulic press. A standard 30Kg billet would take approximately 2 hours to mould, following which a dwell time is necessary to ensure any excess air pockets get released
3. Extruding: the pre-form is loaded into the extruder - the key equipment in the process - and a die and mandrel are clamped in place above it. The die is a critical tool and its design defines the strength of the tube and its final dimensions. As the extrusion process starts, the extruder presses the pre-form against the die and mandrel, forcing the resin to extrude into the desired shape. The tubing in this stage is referred to as 'green' and can be easily crushed.
4. Pre-sintering: the green tubing is passed through an oven where it is heated at a very low temperature. The idea here is to evaporate the spirit in the tube and care must be taken so that the flash point of the spirit is not reached, causing it to ignite.
5. Sintering: the PTFE tubing is sintered at 350-400°C. The sinter cycle will depend on the thickness of the tubing and can last up to 24 hours for thick walled tubing
6. Cleaning and packaging: the tube is first cut into he desired lengths. In the case of medical tubing, the ends of the tube must be plugged as soon as the material comes out of the oven. The plugging ensures that the inside of the tubing - which has seen temperatures well in excess of 300°C - remains clean. For further cleaning an ISO Grade VI clean room is the minimum requirement for PTFE tube. After the cleaning the tubes are packed in polythene covers for dispatch.

Table 3 : Technical specifications of FluoroTube

PTFE Compounds and their effects

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We have spent a significant amount of time looking at PTFE as a material, comparing it to other materials and analyzing its uses based on the various properties it exhibits.

However, our focus has been purely on “virgin” PTFE – namely, PTFE in its pure form. In this form, PTFE takes on an opaque-white hue, is best describes as a soft-waxy material and is smooth to the touch.

Compounding PTFE refers to the mixture of PTFE with additives, which would both add and remove certain characteristics from virgin PTFE. It is essentially a mixture of PTFE with other substances – done for the purpose of enhancing one or more of the characteristics of PTFE, so that the compounded material would be a better overall fit to a given application.

Compounding Process

Before we delve into the various compounds, let’s look briefly at the process behind PTFE compounds.

In truth – most of the large resin manufacturers (DuPont, Daikin, Solvay etc.) have focused on manufacturing virgin resins and left the compounding to smaller companies – who buy the virgin resin and use it in making their compounds. Although the compounder is very much the owner of the product’s quality – it must be mentioned that the input resin does have a huge impact on the final quality of the compounded grade.

For example – we had procured a large quantity of PTFE+Bronze resin from a Chinese company, only to find that when we moulded large pieces from the resin (in excess of 15-20 Kgs per piece) – the pieces would crack during sintering. When we took it up with the supplier, it became apparent that the base resin was of a poor quality, and unsuitable for large pieces.

The compounding process is usually a proprietary technology of the compounder. However, technical literature will point to one of two ways to compound resin:

1. - Physical blending – a physical process, done in an industrial blender where PTFE and the additives are added in the required proportion. The blended powder is then sifted through a mesh to separate the mixture from ‘lumps’ of PTFE that tend to form during blending. The process is repeated until all lumps are suitable removed.

Blending like this is a tedious process, and requires much iteration. Even when care has been taken, small lumps may still remain which will result in patches of white (assuming the blend is pigmented) on the final product.

- Chemical blending – this is more expensive, but also ensures fewer iterations and more uniformity in the blend. A range of chemicals is available for this process – but the basic principal is to have a liquid aid with a lower surface energy than PTFE. This will allow the pigment to flow in between the PTFE molecules so that even the lumps are suitably coated with the pigment.

However, the finer aspects of compounding are usually learnt only through experience and remain a technology that compounders would not part with easily (understandably!).

Properties of PTFE

We have looked at these before from a theoretical standpoint. Some of the properties remain unaffected (or hindered) by the addition of fillers, while others are impacted positively.

Temperature Resistance

It would be sufficient to say that compounded grades have to have at least, if not higher temperature resistance than PTFE – as else they would not survive the sintering process – which happens at 350-400 °C

Dielectric Strength

In most cases, this is only reduced by the addition of fillers – as PTFE in its virgin form shows exceptional electrical resistance. We have yet to come across an application where a filled grade of PTFE is used for purely insulation purposes

Hardness

Being a soft material, the addition of fillers can greatly increase the hardness. This is especially sought after in PTFE components – where the softness of the material can lead to deformation in the long run, affecting the overall assembly within which the component is used.

Coefficient of Friction

Like with dielectric strength, the dynamic coefficient is usually hindered with the addition of fillers. However, because virgin PTFE exhibits significant creep – there is a case for filled grades in applications with minimum movement where a low static coefficient of friction is required.

PV Value

The PV value of a compound is the product of the unit load P (MPa) on the projected area and the surface velocity V. The PV of PTFE is usually enhanced by the addition of fillers.

Wear

In general, the addition of fillers to PTFE resins improves wear resistance but reduces abrasive resistance by providing discontinuities in the PTFE resin which can be entered by sharp practices that may tear the material.

Moisture Absorption

Unfilled PTFE does not absorb water. Filled PTFE compounds absorb small amounts of moisture. Since PTFE resin and fillers are not hygroscopic, any moisture picked up simply fills the voids. Extent of pickup is so small that the dimensional stability is essentially unaltered.

Chemical Resistance

Again – given PTFE is unmatched amongst other materials in its ability to remain intern to chemicals, adding fillers can only reduce this property. However, it does depend ultimately on the application and whether there is a requirement for such a high level of inertness. Typically however, for applications needing this property (medical, labwares etc.) – virgin PTFE remains the preferred choice.

Standard Compounds

Now that we have looked at each of the properties, let’s look at some of the standard compounds and see how each compound alters the characteristics.

Glass Fiber

This is the most universally used PTFE filler and is normally mixed in either 15% or 25% ratios. Glass is itself highly resistant to chemicals and also exhibits very good dielectric properties; add to this the added mechanical properties and creep resistance that it provides and it’s not difficult to see why it is so sought after.

Glass fiber also offers improved wear resistance, but reduces the coefficient of friction. Furthermore, it imposes a higher wear rate on the tools while machining– making it a slightly more expensive material to machine. For the same reason, it is very difficult to ‘skive’ glass filled PTFE tapes to thicknesses of under 0.25mm – as the wear induced on the skiving blade renders the blade dull before a significant length can be skived.

Carbon-Graphite

Graphite is generally used in compounds destined for chemical and mechanical service. Graphite reduces initial wear and provides general strengthening characteristics to the composition. Also, graphite compounds generally display high load carrying capabilities in high-speed rubbing contact applications and exhibits the highest hardness of any of the compounds.

Of all the compounds, we have found Carbon-Graphite to wear out tools the fastest. The same tool that might give 200-300 components if done in virgin PTFE, will only give 15-20 components in Carbon-Graphite.

Bronze

Bronze is usually mixed in a 40% or 60% ratio. Bronze compounds have higher hardness, lower wear, higher comprehensive strength, better dimensional stability, higher thermal conductivity, lower creep and cold flow than most other compounds.

However, test data shows that bronze compounds are not suited to many electrical applications or to those that involve corrosive service environments.

Molybdenum Disulfide

MoS2 adds substantially to the hardness, stiffness and wear resistance of PTFE resins. It reduces starting friction and has little effect on PTFE 's electrical and chemical properties. Generally, only small amounts of molybdenum disulfide are used, most often in conjunction with complementary fillers (usually bronze or glass).

In addition to the above fillers, we have used fillers of ceramic, stainless steel and ekonol. Many branded compounds of PTFE continue to exist (eg: Rulon, Turcite etc) – but a comparison of properties shows that there is little difference between the branded compounds and one of the regular grades.

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Ultimately, choosing a compounded grade is a question of application – asking which property needs to be enhanced and which can be foregone (or compromised on). In most mechanical applications, it becomes a trade-off between higher mechanical properties (hardness, wear resistance, creep) and lower coefficient of friction.

It should be noted than very rarely does cost play a huge decider in choosing a compounded grade. While historically, bronze has been most expensive, followed by glass and then carbon-graphite (virgin PTFE has usually been priced around the same level as carbon-graphite) – their properties are so different that the end user rarely sees them as substitutes.

To know more, please view our site: www.polyfluoroltd.com; or view our PTFE (Teflon) Manual