Unraveling PTFE

PTFE machining considerations – tapping

2012-02-02T13:06:00.000+05:30

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Machining PTFE, as we have touched upon before, is never a straightforward process. Most machining handbooks will suggest that PTFE should be treated much like wood when it comes to machining, as this is the material it most closely behaves like when machined. And while this is a good starting point for tool selection and CNC programme settings, as we delve deeper into the aspects of machining PTFE, we see that it behaves much like it’s own material. So learning by doing becomes the only option – since PTFE is a niche product (when compared with other known polymers).

Recently, we faced an interesting issue when creating a rather complex part. The part is approximately 200 Grams in weight and machining it involved multiple operations including CNC turning, CNC milling, drilling and finally tapping. All in all, the drawing highlighted over 28 dimensions that needed to be within a strict tolerance and it took us the better part of a week to just get 10 prototypes ready.

We were pretty happy with the result: everything measured, as it should. We almost didn’t check the tapping – which called for an M3 tap in two places. The M3 taps used were brand new and the first tap was done on the VMC as part of the programme – so there was no way it could be an issue, we thought. But we were wrong.

The no-go gauge entered in the hole all too easily and we were pretty shocked to realise that even an M3 bolt was sitting loose in the hole. At first we though we had the wrong tap – which we didn’t. We then argued that the gauge would always enter – as it was designed mainly for harder materials and PTFE would yield all too easily, since it was much softer. To check this we used the same taps on a mild steel plate and confirmed that the no-go did not enter. But this still did not answer why the bolt itself was loose.

We searched extensively for an answer online, but there was very little information on tapping and even less on the issue we in particular were facing. We then decided to start experimenting with different combinations of taps and drill holes.

On the part, we had used a 2.2mm drill with all 3 taps. The first tap was done on our VMC, while the next 2 were done by hand. We tried the following combinations:

Drill Hole	Tap 1	Tap 2	Tap 3	Result	Remark
1.5	Y	Y	Y	Reject	Bolt loose
1.5	Y			Reject	Bolt loose
1.5			Y	Reject	Bolt tight
2	Y	Y	Y	Reject	Bolt tight
2			Y	Reject	Bolt loose
2.2	Y	Y	Y	Reject	Bolt loose
2.2			Y	Reject	Bolt loose

In a couple of cases – where we used only the 3^rd (finest) tap, the bolt was tight. However, none of the holes were answering to the no-go gauge, which passed equally easily in all the holes. We once again argued that this was a PTFE related issue and that as long as the bolt was tight, it should not be a problem. But many of the consultants and experts I spoke with said that they had come across parts in PTFE that answered to the no-go gauge, and hence there must be a way to machine such a part.

The problem was finally solved when an engineer in our client’s side suggested we use a “Form Tap”. I had never heard of a form tap and when I searched it, it seemed to apply mainly to tapping soft metals (such a aluminium). There was no mention of applications to PTFE. Nonetheless, it was our last shot, so we tried it and were pleasantly surprised.

We eventually went with a 2.0mm drill and an M3 form tap to get a result that was both functionally good and which answered to the gauge.

The reason the form tap works, is because unlike a regular tap, it does not bore into the PTFE, taking out material as it does. Instead, it merely forms the tap profile within the drilled hole and as PTFE is soft, it yields quite easily. The result is that the tapped hole is much fuller than when a normal M3 tap is used – making it tighter and ensuring the pitch profile does not yield to the no-go gauge.

Surprisingly, this does again strengthen the PTFE-Wood similarity in machining. Tapping is unheard of in wood; a screw can be passed through a drilled hole and sit tight forever! In many ways, a form tap is nothing more than passing a screw/bolt into the PTFE to imprint its profile within the hole. Only that the form tap is possibly more exact and can ensure that the resulting tap is accepted when inspected with the correct gauges!

UHMWPE - the unknown polymer

2012-01-13T09:57:00.002+05:30

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One of the few good things to happen due to the unprecedented escalation of PTFE prices globally was that it allowed us to look at alternate materials and seriously gauge the feasibility of manufacturing them.

In an earlier post, we looked at the various properties of PTFE and compared them to the other polymers. And although the key takeaway from that exercise was that PTFE was an immensely versatile material which was difficult to replace, we did make mention of possible alternatives, provided the user was willing to compromise on some parameters.

A key polymer which struck us then and continues to feature prominently in our product offering today is UHMWPE. We would like to take a more detailed look at UHMWPE for 2 reasons:

It does measure up against PTFE as a low-cost substitute (with certain limitations)
It’s properties do not seem to be as widely known to end-users, resulting in limited use in many applications where it would otherwise be ideal

What is UHMWPE?

Sometimes referred to as just “UHMW”, UHMWPE or Ultra High Molecular Weight Poly Ethylene is an off-white polymer that exhibits superior strength while being both light-weight and possessing a low coefficient of friction.

While it is not entirely accurate to refer to it as an “unknown” polymer – our own analysis of search terms within Google tells us that a total of ~62,000 searches per month are done globally for UHMWPE and/or UHMW. This is tiny in comparison to searches for PTFE/Teflon (1,300,000 per month) or for Nylon (5,500,000 per month).

Comparison with PTFE

So how does UHMWPE compare with PTFE? In our own opinion – it compares rather well. In fact, if you take all the applications involving PTFE and remove the ones that call for heat resistance, UHMWPE is a very workable substitute.

Although a full comparison chart is given at the end of this article, we would like to look at some specific properties more subjectively.

Temperature resistance
Let’s get this one out of the way, since we know that it is UHMWPE’s weakness. Having an operating temperature of only about 80°C compared with 260°C for PTFE, UHMWPE is automatically disqualified in a range of industrial applications where the temperatures surrounding the material are expected to be well in excess of it’s upper limit.
Wear resistance
Before we were familiar with UHMWPE, we were asked to advice a cement plant on whether they could use Lubring sheets (PTFE+Bronze) in a wear application. We were confident that it would work and when they mentioned that they had tried UHMWPE and it had failed, we did not think it was worth looking into. But when we did compare the materials, we realized that if UHMWPE had failed, there was little chance PTFE would work – since the gap between the two materials on this parameter is quite wide.
Keep in mind that PTFE+Bronze is the most wear resistance grade of PTFE available. So if we compare UHMWPE with plain PTFE, the rift is even wider.
Coefficient of friction
It is difficult to beat PTFE on this parameter, although UHMWPE comes fairly close. While it remains true that the coefficient of friction between PTFE and polished stainless steel is the lowest between two known solids (0.03-0.05), UHMWPE is able to reach a somewhat respectable 0.1-0.15 on this metric. While this does put it out of range for many applications where the recommended coefficient cannot exceed 0.1 (eg: sliding bearings) – it is a useful substitute in components where smooth movement between parts is the only requirement.
Dielectric strength
Both materials are pretty much neck and neck on dielectric strength. Where UHMWPE loses out is on its ability to be skived into thin tapes. While we regularly skive PTFE down to 0.04-0.05mm thicknesses, the same is more challenging with UHMWPE, since it lends a much higher wear on to the skiving blade, making it difficult to achieve long lengths of tape before the blade dulls out and breaks the tape. Nonetheless, thicknesses of 0.1mm and above are more than feasible, meaning that as an insulating pad or even a component used in high voltage applications, UHMWPE is more that suitable.
Chemical inertness
PTFE is well known for it’s inertness and this allows it to lend itself to applications ranging from biotechnology to medical devices and chemical linings. While UHMWPE does not have quite the same extreme inertness as PTFE, it does find use in medical applications (it is used in parts for joint replacements) and can easily be used in both biotech and chemical applications, provided the exact nature of chemicals is known and compared against it’s capabilities.
Weight
While weight has never been a consideration for PTFE in any of it’s applications, we would still like to highlight that UHMWPE is less than half the weight of PTFE (specific gravity of 0.95 vs. 2.15 for PTFE). The key difference this adds is in their respective cost cacluations. Not only is UHMWPE cheaper in resin form (roughly 1/4^th the cost per Kg), the fact that you consume only half the weight to get the same volume part implies that the effective cost is 1/8^th the cost of PTFE. This represents a significant saving.

So where can we use UHMWPE?

There are a range of applications where UHMWPE could and should be used. In many cases, we have tried to suggest to the end-user that we can offer them UHMWPE in stead of PTFE, but due to restrictions on standards and because changing specifications can be time consuming, very few have opted for the change.

Strangely, in many cases, clients have opted for suppliers offering reprocessed PTFE, but not UHMWPE. Given the highly diminished properties of reprocessed PTFE, this is functionally not a great trade-off in the medium to long term.

Automotives

Most automotive applications use PTFE in high temperature environments, so UHMWPE does not fit the requirement. However, there are a number of applications where the parts operate at room temperature eg: car doors, seats, hand levers etc. and here UHMWPE can find a lot of use. We are aware that the wear strip used inside car doors employs UHMWPE.

In general, UHMWPE wear strips offer a low cost and effective alternative to PTFE wear strips.

Valves and seals

Typically, valves and seals require a low coefficient of friction with a good wear resistance. UHMWPE is an excellent replacement for PTFE in these areas.

Medical

UHMWPE is widely used in joint replacements due to its chemical inertness and light-weight.

Infrastructure

Although regulatory restrictions prevent materials other than PTFE to be used POT bearings, there are many sliding bearing applications which do not fall under the government codes and are therefore potential areas where UHMWPE can be used. UHMWPE could be employed successfully in sliding bearings and as plain sliding pads.

Electronics

Many components used in electronics have traditionally employed PTFE components for insulation. In a number of cases, we have successfully tested UHMWPE for these applications and convinced the client to shift.

Overall, there continues to be a resistance to employ a material like UHMWPE. Part of this is regulatory – drawings and specifications that call for PTFE cannot be changed over night. But mostly there is a genuine dearth of awareness about the material – which is equally difficult to change. While it is true that UHMWPE is a substitute for PTFE – we see it as more of a partner in application – allowing many end-users to find a competitive, low-cost solution where they would otherwise be unable to proceed with their development or manufacturing.

Comparison chart between PTFE and UHMWPE

	UHMWPE	PTFE	Units
Colour	Off-white	White
Specific Gravity, 73°F	0.944	2.25
Tensile Strength @ Yield, 73°F	3250	4000	psi
Tensile Modulus of Elasticity, 73°F	155,900	150,000	psi
Tensile Elongation (at break), 73°F	330	350	%
Flexural Modulus of Elasticity	107,900	145,000	psi
Compressive Strength at 2% deformation	400	1650	psi
Compressive Strength 10% Deformation	1200	2200	psi
Deformation Under Load	6-8%	2.5-5%	%
Compressive Modulus of Elasticity, 73°F	69,650	79,750	psi
Hardness, Durometer (Shore "D" scale)	69	55-65
Izod Impact, Notched @ 73°F	30	161	ft.lbs./in. of notch
Coefficient of Friction (Dry vs Steel) Static	0.17	.06-0.12
Coefficient of Friction (Dry vs Steel) Dynamic	0.14	0.12
Sand Wheel Wear/Abrasion Test	95	90	UHMW=100
Coefficient of Linear Thermal Expansion	11	6-7.2	in/in/°F x 10^-5
Melting Point (Crystalline Peak)	135-145	380	°C
Maximum Service Temperature	80	260	°C
Volume Resistivity	>10¹⁵	NA	ohm-cm
Surface Resistivity	>10¹⁵	NA	ohm-cm
Water Absorption, Immersion 24 Hours	Nil	Nil	%
Water Absorption, Immersion Saturation	Nil	Nil	%
Machine-ability Rating	5	3	1 = easy, 10 = difficult

Unraveling PTFE has shifted

2012-01-12T07:52:00.002+05:30

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PTFE Pricing Again – is there another price hike in the offing?

2011-12-03T14:24:00.002+05:30

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The adage “Only the paranoid survive” has held very true in the PTFE industry these past 20 odd months. You see, while it is difficult to imagine worst case scenarios and constantly plan along their likelihood, it is usually the only way to make sure that one is not blind-sided by bad news when it does arrive. And since bad news has been arriving thick and fast, being mentally and commercially prepared for the price hikes in PTFE resins are what have allowed many PTFE processors to survive this period of turbulence.

So we choose to re-look at pricing again because despite the fact that PTFE prices have been stable since July 2011, the last thing we can afford to do is assume all will be well from now on and be rudely shocked if and when another price hike does come around.

But rather than subject ourselves to speculation, we have been looking at trends, hearing out rumors and gleaning information from various sources to gauge what might actually be the future of PTFE pricing in the near term. As always, there are various factors at play, but together they do suggest that another price hike is unlikely and that there may even be some easing out of prices in the offing.

1. Rate contracts

Our sources in Europe tell us that there is an increasing push by PTFE resin suppliers to enter into rate contracts for the coming quarter. In an environment where the suppliers have enjoyed increasing prices month-on-month, rate contracts suggest that the scenario may be changing and that an easing out of prices is expected.

Furthermore, many companies are rejecting the rate contracts, since there is a general feeling that prices will reduce in the near term.

2. China and Russia back in the game

In an earlier article we mentioned how both Chinese and Russian resin suppliers were experiencing capacity constraints due to a number of reasons ranging from Fluorspar reallocation to maintenance shutdowns and internal restructuring of capacities.

Now our sources tell us that China and Russia are once again making supplies into Europe and that the pricing is highly competitive in comparison to other suppliers.

Even locally, the marketing push by Chinese companies to try and sell resins into India has accelerated. We receive more mails every day from China and have even been approached by some sourcing agents, asking if we would be interested in entering an agreement to buy resins.

It was always our belief that prices may never come down, since the value growth due to pricing has more than compensated the volume reduction. However, we also know that China has always preferred higher volumes rather than higher margins (a strange strategy, but one that has allowed them to aggressively expand and build scale). So it is unlikely that they will join the rest of the world’s resin suppliers in keeping prices stable and highly probable that they will induce a price war of some sort – forcing prices to reduce.

A quick look at the global price benchmarks we have obtained show that while the rest of the world (India, USA, Europe) had stabilized around a price of US$25-27 per Kg, this rate was sustainable only as long as China and Russia were not supplying globally.

3. Anti-dumping duty no longer effective

When the anti-dumping duty on Chinese and Russian resins was first imposed in India, it effectively increased PTFE resin prices by US$3.3 per Kg. Given the local price of PTFE resins was US$7-8 per Kg at the time, this acted as a serious deterrent for processors buying from China and Russia. At US$20-25 per Kg, the US$3.3 duty ceases to be effective, as the landed cost of the resin would still be below the local rates being availed in India.

It does remain to be seen whether local resin manufacturers are able to bring about a further increase in the duty amount, but even this will take time, so it is likely China and Russia will re-enter India and put pressure on prices.

4. Re-opening of Fluorspar mines

An obvious trend – looking at the past year, may be that China is only able to supply PTFE resins now because there would be an easing out of domestic demand for R22 in refrigeration. We had in an earlier article suggested that in the medium term, as winter approached, there would be an increase in R22 available for PTFE resins and this would ease out prices. However, there is no guarantee that the same pattern would not repeat next year – with supply constraints forcing prices up again.

In the mean time, there are reports that mines in Mexico and South Africa have been re-opened, although it would take at least another 12-18 months for them to be operational. This suggests that prices may again increase during summer 2012 – although if processors stock up on raw materials prior to this, it would not allow the prices to escalate in the same manner as they did in 2011.

Up until last week, the local buzz was that PTFE resin prices were being hiked by up to 30% in January 2012. This has coincided with other news that points to the contrary – rate contracts, China and Russia, capacity expansion. It could be that we have missed out some key information and as a result, our own analysis is wrong. It remains to be seen whether there will be a price hike, but for now we’re staying paranoid – because it seems the safer option currently.

PTFE Membranes – Variants and Typical Uses

2011-11-08T12:48:00.001+05:30

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Membranes involving PTFE have gained prominence over the past decade. When we are approached for this product, however, it usually involves a lot of discussion and deliberation, as OEM clients are aware that they require PTFE membranes, but are not fully sure which type of membrane they require.

In our own experience, there are four variants of PTFE membranes. There may be many more – but these are the variants we most frequently encounter and together they encompass most of the properties that a membrane would need.

Before we delve into the variants, we need to first understand that both pure PTFE and expanded PTFE are used in membranes. We have earlier posted a piece on expanded PTFE, but for the sake of brevity, we will say that it involves a processing technique which effectively pushes air into PTFE, making it softer and lighter than pure PTFE and giving it a spongy texture.

We also need to understand that with membranes, 2 properties define the product itself and need to be looked at during product development and manufacture.

Pore size: this is the size (or range of sizes) of the individual pores or holes within the material. As we will see, controlling for pore size is an integral part of the process of making a membrane
Porosity: this is the overall extent to which the PTFE is permeated by the pores. Typically, this is easy to control and calculate, as the final weight of the membrane compared with the weight for pure PTFE of the same volume will tell us to what extent the membrane is porous

Variant 1: Pure PTFE Membrane

In truth, this should be called a “filter” rather than a membrane, but it is referred to as both. This is the simplest form of membrane, comprising a PTFE sheet of 0.5mm – 5mm thickness (maybe more) into which holes are drilled/ punched. The process for making the sheet is the same as for any PTFE sheet: ie: skiving or moulding. The size and quantity of the holes can be altered based on the client requirement.

Typical uses of this membrane would be in separating large particles/ lumps from a liquid suspension. It finds uses in biotech, chemicals and even food processing – where the food grade and inert nature of PTFE makes it a suitable material to come in contact with chemicals/ food products and not react/ affect the materials passing through it.

Both porosity and pore size are easily controlled and measure here – as it is a machined item and the pore size is defined by the holes being drilled/ punched and the porosity is defined by the number of holes.

Variant 2: Porous PTFE membrane

Porous PTFE is made in the same way as pure PTFE ie: the material is molded or skived. The difference is that the resin is compounded with a substance, which would sublimate (move directly from solid to gas) at the temperatures at which PTFE is sintered. Thus, the material – which is molded along with the PTFE, is evacuated during sintering, leaving behind voids in the PTFE. The material would also make fissures within the PTFE as the sublimated gas charts a path through the PTFE during its exit

Porous PTFE is the most inexact of the membranes as it involves a foreign substance whose behavior cannot be predicted entirely. For one, the compounding process is unlikely to be 100% uniform – so you may have some amount of agglomeration of the substance implying that the porosity (and pore size) in one section of the PTFE, may be more than in another. Secondly, while pore size can be somewhat controlled by ensuring that the particles of the foreign substance are all within a fixed range (say 1-2 microns) – the fissures themselves are not possible to control, so 2 fissures may joint at some point to create a larger pore size than required. Overall porosity is controlled by limiting the ratio of PTFE to the substance – but as mentioned before, there will be some variance in porosity within the membrane due to the non-uniformity of compounding.Porous PTFE membranes do not have a huge demand in comparison to the other variants. Its typical uses are in automotives and chemical plants, where the particle sizes are in the range of 30-100 microns.

Variant 3: Plain expanded PTFE membrane

Expanded PTFE is used in cases where a much finer filtration is required. Pore sizes here can be as low as 0.1 micron – since the pores are formed by effectively incorporating air into PTFE and can thus be controlled by limiting the force and volume of air being used. Similarly, limiting the ratio of air to PTFE during the process also easily controls porosity.

The key feature of an expanded PTFE membrane is the property of “breathability”. This means that it is possible to control the pore size to an extent where air is able to pass through the membrane, but liquid vapors are not.

Such membranes find uses in medical equipments and also apparels – where many applications require the material to only allow the passage of air and not other substances.

Variant 4: Laminated expanded PTFE membrane

This is the most popular variant as per our experience. The drawback of plain EPTFE membranes is that due to its spongy texture, it does have a tendency to absorb some amount of moisture over time. Furthermore, EPTFE is very soft and light and thin membranes tend to cling to themselves, making handling difficult.

The lamination of the membranes is usually done with polypropylene or polyethylene. The benefit is that the membrane is easier to handle and also limits the long-term seepage of moisture. The limitation is that the laminate would not be nearly as effective as PTFE in withstanding harsh chemicals (although this is easily remedied by ensuring that the side facing the chemicals is the pure PTFE side). Furthermore, the membrane will not be able to withstand high temperatures.

We see a lot of applications of this membrane in filters for medical devices. There is also some use in the automotive segment – where the membrane acts as a filter to evacuate air from oil. The breathability ensures that only air is sucked through the filter and not oil.

In summary, one must point out that PTFE membranes are expensive due to the lengthy process involved in making them and the cost of the material itself. Hence they are sparingly used only in applications where only PTFE will suffice. Nonetheless, the range of options they offer – inertness, food grade, temperature resistance and breathability – make them unmatched by any other material in the area of membranes.

The Effect of Low Temperatures on PTFE Component Dimensions

2011-09-11T20:17:00.003+05:30

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One of the most challenging elements of machining PTFE components for export markets is factoring the effects of temperature on the material when it moves into colder climates.

Since we do all our machining in Bangalore, India, where the room temperature varies between 22-32 Degrees Celsius (on average), we need to be constantly mindful of the dimensional shrinkage that would happen when machined parts are shipped to colder countries.

In many cases, this problem is not a huge one – since the tolerance on the part may be high enough that even after shrinkage it would still fall within the acceptable band for that dimension. A tolerance of +/-0.25mm, for example, could be machined with a plus side bias of 0.15-0.2mm – which is easily maintained on a CNC machine. After shrinkage, even if the dimension reduces by 0.2-0.3mm (not unheard of as we will later demonstrate), the part would still be acceptable when inspected at the client’s works prior to assembly.

The real challenge lies in accommodating much closer tolerances. In our own experience, we encourage customers to design the part keeping in mind a tolerance of +/-0.05mm at most. Often, as the customer may have dealt mainly with metal parts, they expect that PTFE would also conform to the same dimensional yardsticks as metal (which can be machined to tolerances as fine as 1 micron). In reality – PTFE is a much softer material, which undergoes the following changes during machining, affecting it’s ability to be attain very close tolerances:

Stress build up in material due to tool hardness
Deformation of material due to heat from machining
Burrs forming on part which may need to be manually removed, affecting tolerance

The closest tolerance we have managed to maintain on PTFE has been +/-0.012mm – which was done on a component made from PTFE+15% glass fiber, having an outer diameter of 19.04mm. We did this by experimenting with different combinations of tools, RPM, feed rates and programs, until the dimension was consistently within the tolerance range. However, when the part was shipped to the client in Canada, the trial lots failed due to being undersize. Eventually, through trial and error, it was found that a dimension of 19.08-19.10 was needed in India, in order for the part to be within tolerance in Canada.

While this worked out well eventually, in many cases, customers are not willing to experiment with trial lots – especially if their requirement is urgent. This has led us to seriously consider the practical implications of shrinkage and how we can make an educated guess on dimensions so as to avoid rejection/ rework and/or minimize trials.

The study

Virgin PTFE theoretically experiences a 1.3% variation in dimension between 0 and 100 Degrees Celsius. Plotting this as a linear progression around the dimension of 19.04 would give us a chart like this:

In other words, given the room temperature in Canada being ~10-15 degrees cooler than in India, a shrinkage of 0.03mm could be expected when the parts reached Canada. We expected that for glass filled PTFE, this may not be quite as high – as glass itself may not be as susceptible to dimensional deviation based on temperature.

In order to check this, we machined identical components from different grades of PTFE, having the outer diameter of 19.07mm at room temperature and cooled them down to well below 0 degrees Celsius. We measured this dimension when the pieces were taken out of the sub-zero environment and then allowed them to sit at room temperature, measuring the outer diameters and temperatures at regular intervals to plot a curve of dimension versus temperature.

The grades we used were the following:

For each grade – 5 identical parts were machined and their dimensions were measured along the same point. The dimension considered was 19.07 +/- 0.02mm. The parts were first measured at room temperature (about 25 degrees Celsius) and then put into a sub-zero environment for 4-5 hours. Each part was then taken out individually and measured again at fixed intervals of 10 minutes. The aim was two –fold: (1) to observe the extent of shrinkage due to the cold and assess the rate of expansion as the part warmed up at room temperature (2) To gauge whether the part, when left at room temperature overnight, regained it’s original dimension.

The charts below show the results for each grade:

It was observed that the virgin material experienced the highest shrinkage (0.9% over a temperature range of 40 degrees Celsius). Both Glass and Bronze filled PTFE experienced lower shrinkage (0.5% over a temperature range of 30-32 degrees Celsius). It is also interesting to note that virgin PTFE reached a much lower minimum temperature. While the filled grades were recorded as having temperatures of -4 to -5.3 degrees at their lowest, the virgin material was recorded with a minimum of -11 degrees - despite being subjected to the same sub-zero environment prior to measurement.

Additionally, all three grades reverted to within 0.01mm of their original dimensions when left overnight to warm under room temperature. This suggests that the dimensional change is linked purely to the ambient temperature and that there is no observable stress build-up in the material due to the cold which causes it's original dimension to alter permanently (at least, not in the range of -15 to +30 degrees Celsius).

All three materials lend themselves to a high R-squared straight-line graph. While we accept that this is a fairly simplistic relation to assume, the R-squared changes only marginally when we try to introduce more complex equations. Furthermore, while the relation between the dimension and the temperature may not be a strictly linear one – for the purpose of practicality, we believe that it serves quite well. In other words, assuming a 0.2% shrinkage for every 10 degrees in temperature (for virgin PTFE) would imply that a 20mm dimension would need to accommodate a plus side tolerance of 0.4mm. This is in line with our own trial and error conclusions thus far, when exporting to colder countries.

Finally – it could be pointed out that the same experiment could be carried out at higher temperatures to gauge whether PTFE continues to expand the same way in the other direction. However, as the bulk of our export destinations are in fact colder countries, we have not looked at this right now. Perhaps the same could be taken up in a separate post.

Mapping the PTFE Price Increase

2011-08-15T18:47:00.000+05:30

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While much has been said about the causes and implications of the PTFE price escalation, we felt it necessary to go through our archives and chart out the exact extent to which the prices have changed.

The chart below shows the price per Kg in US$ for three standard grades – Virgin PTFE, Glass Filled PTFE (15%) and Bronze Filled PTFE (40%). In addition, we have included a table showing the total and monthly growth in prices.

Needless to day, the growth has been unprecedented. In Virgin PTFE, a nearly 8% increase in prices every month has put the industry in a state where there is no breathing time between processors getting new pricing information and passing on that information to the customers.

Most processors are well aware of the effect this has had on their businesses. The main issue has been convincing customers regarding the price increase and furthermore making them aware that the trend may be expected to continue. In addition to this, there is the impact on repeat business, as clients withhold contracts which would have otherwise spanned their requirements over a full year - since processors are unable to commit to prices for more than a one month horizon.

PTFE pricing revisited – inevitabilities in long term supply and demand

2011-08-01T11:31:00.002+05:30

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When we started this blog, our aim was two-fold:

1. To inform and educate readers about PTFE, it’s applications and derived products
2. To serve as a platform for clients and other end-users to understand PTFE better and make informed decisions regarding their applications

However, it seems to have been the articles on pricing which have brought most of our traffic as regardless of how important the applications of PTFE are, it is – understandably – on pricing that most questions currently centre.

We therefore want to look at pricing again, just to see if any new information gleaned over the past couple of months helps us understand the situation any better than we did earlier.

Since our last blog on the impact of Fluorspar on PTFE prices, we have – like all other processors – been praying for stability. We haven’t been praying for a reduction in prices – that would be optimism to the point of pure irrationality. However, if prices could simply stabilize – even for a few months, it would give us some time to re-group, re-assess and possibly resume normal operations.

However, the pricing fluctuations have been coupled with lesser-known events in the background and together, these effects are causing the stabilization process to take much longer than earlier assumed.

We would like to take a look at some of the news floating in the market at present. We can vouch that since these are from rather reliable sources, we are inclined to believe them and therefore base our outlook on their implications:

1) Fluorspar shortage is no longer an issue

A supplier who regularly sources semi-finished PTFE from a Chinese manufacturer told us this anecdote: The supplier approached the manufacturer with the offer to supply R22. The proposed arrangement was that the PTFE manufacturer could then supply the resin manufacturer with R22 (assumed to be in very short supply) and in return procure resin at a discounted price. The supplier was shocked to hear that the resin manufacturer declined – saying that they had ample R22 to meet their production needs. This does lead us to believe that although the Fluorspar story may have started the PTFE price frenzy, it is now not playing as significant a part.

2) European resin manufacturers have re-allocated resources away from PTFE

This was partly confirmed by a representative from DuPont, who stated that their company was slowly coming out of PTFE resin manufacture, as long-term competition against Chinese suppliers was not feasible for them. The resulting effect, we hear, was that most of the European resin manufacturers have sub-contracted their PTFE business to Chinese resin suppliers. Since the realization for PTFE resins in Europe is much higher – the European price has become the new acting price across the global market.

This impact does throw some light on why the PTFE prices have increased so drastically all over the world. On the one hand, we have a supply constraint, as European manufacturers no longer compete in the market. At the same time, you have a huge supply-demand mismatch as European demand for resins stays the same and this drives up prices.

3) Russian suppliers are in a state of flux

From what we have heard, Russia has two main companies who manufacture PTFE resins, one of which acquired the other. The combined company is said to be undergoing some transition issues and management is also contemplating moving away from PTFE and into ETFE. The result has again been to constrain supply, impacting prices in the process.

4) Pricing set to stabilize within the next 2-3 months

Obviously, many are hoping that things will settle down sooner than this, but considering the extent of changes occurring across the market, one might expect that it would take no less than a few months to stabilise.

Our own local the supplier – who increased prices by another 30% in the month of July 2011, assured us that this would be the last-but-one, if not the last, price revision from their side. The current price we are getting is US$26.5 per Kilo for virgin PTFE resin. From what information we have from our European counterparts, it appears that local rates there are around the same price – so it does look like some sort of balance has been reached.

For those thinking about the long term implications of all this, we can infer the following from what data we have already collected:

High prices are here to stay. If there is one thing that all this has shown us, it is that the demand has stayed strong enough despite the price escalation. This has justified the price hike for resin manufacturers from a business standpoint
Long term, quality will improve. Although it looks like Chinese companies will be doing most of the manufacturing of PTFE resins, if they are supplying through companies like DuPont and 3M, the quality controls will most probably be more stringent.
Volumes in PTFE will shrink. Although we have not seen a significant amount of substitution away from PTFE, there are murmurs of new materials and possible replacement materials in some areas. For the most part, we continue to believe that as a material, the extensive spectrum of properties offered by PTFE makes it a difficult material to shift out from. However, we do expect that at least 15-20% of the volumes in PTFE would slowly shift to other polymers such as PA66 and UHMWPE. Nonetheless, we can take comfort in the fact that a 15-20% fall in volumes when combined with a 100-200% increase in prices still implies an overall growth in the industry in value terms.
Repro is here to stay. Not that anyone though that reprocessed PTFE would go away, but we do believe that the acceptance of recycled material in many applications (due to the price implications) would bring about some regularization in the market, with manufactures offering transparency on the extent to which reprocessed PTFE is used and possibly on the properties it could be expected to exhibit. Again - this would be a good thing from a quality standpoint, as buyers of semi-finished PTFE would at least know exactly what they were getting.

To conclude – we have, like most other processors, been trying to make the best out of a situation that has been completely out of our hands. We have faced a rather torrid 15-18 months, so if the end were 2-3 months away, we would look forward to that. In the mean time we would recommend planning one day at a time, because there is no telling what might happen during the next week or month.

Cantilever Load Considerations for PTFE Sliding Bearings

2011-07-04T13:44:00.001+05:30

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As a manufacturer of sliding bearings in India, one of the challenges we face is that there does not exist an official book of guidelines specifically for this type of bearing. The closest we have is the IRC:83 – which is the code book for POT-PTFE bearings, and which details the specifications of the materials to be used and also the testing parameters for POT bearings. As such the IRC:83 does provide some guidelines – but does not address the finer design requirements for sliding bearings. Consequently, during design and testing of the bearings, customers rely on the Quality Assurance Plan (QAP) we provide them, with the option to question, refute and even modify the requirements as they see fit.

Although the BS:5400 and AASHTO standards do make specific recommendations for PTFE sliding bearings, customers are not always willing to accept their parameters as they may sometimes vary from those specified in the IRC – in places where the codes overlap. This can result in technical stand-offs, between the customer and manufacturer, as each tries to convince the other regarding a certain process or parameter, without any official rule book, to back up their view point.

Among the more interesting technical debates we have had recently has been regarding the thickness of the top plate (upper sole plate) in bearings with high movement requirements.

Given a specified vertical load, a PTFE sliding bearing will be required to have a PTFE pad with side determined by the compressive strength of PTFE (usually taken at 100-200 Kgs per sq. cm). This side in turn determines the side of the lower sole plate. Give these two dimensions, the movement of the bearing (specified usually by the client/ contractor) will give us the side of the Stainless Steel sliding element, which in turn will give us the side of the upper sole plate.

Our designing of PTFE sliding bearings began with taking standard designs for specified loads and movements (such as C&P) and recommending them to clients. As we became better versed with PTFE sliding bearings, we started designing from scratch – using the parameters of movement, load and rotation to design customized PTFE bearings for specific project requirements. As many of the original standard drawings recommended sole plates of 12-15mm thickness for bearings up to loads of 50-65 Tonnes, we too continued with this recommendation. After all, the constraint of a bearing in taking a vertical load depends purely on the PTFE – as when compared with steel, PTFE has a much lower compressive strength (100-200 Kg per sq. cm versus steel’s 14000-15000).

It came as a surprise to us therefore when a client expressed concern over the top plate thickness – citing that the bending moment caused by the vertical load would be in excess of what the top plate could accommodate, given the overhang of the top plate over the bottom. This led us to revisit a lot of the standard designs – only to find that for similar load-movement parameters, other bearing designs did not recommend a thicker sole plate that our customer was insisting on.

We therefore went back to theory to gauge exactly how much of a bending moment would be caused by our given load and assess whether there was a case for changing our top plate design.

The formula for calculating the bending moment is as follows:

M= (P x L²)/2

Where:

P = Pressure (Kg/cm2)
L = Length of overhang (cm)
M = Maximum Bending Moment (Kg)

Once we know M, we divide by the Maximum Allowable Bending Stress (S) of the material (1650Kg/cm for steel) – to derive the thickness of the steel plate required.

In the example in question, the value of M obtained was 3941 Kgs, which when divided by S gave a thickness of 3.78cm – or 38mm.

This was a shocking revelation, as our recommendation was for 15mm – clearly insufficient for the load in question. Nonetheless, we wanted to dig deeper to find out how most standard bearing designs only called for a 12-15mm thickness – when the theory clearly showed that this was inadequate.

In most of our discussions with consultants and industry experts, the value of 38mm was ratified and we were told that this was in fact the thickness needed. They were unable to explain, however, why the standard designs did not tally with the theoretical calculations. Our question was finally answered when we spoke with a contractor who studied the drawings and design details and confirmed that while a thickness of 38mm was indeed required, the thickness of the insert plate also needed to be taken in to account.

The insert plate is installed at site and is simply a 25-30mm thick steel plate grouted/ cast along with the concrete substructure and/or superstructure. The bearing is welded or bolted to this plate and the load on the bearing is transferred through the plate as well. Thus, a bending moment will act through the layers of both the upper sole plates and the insert plates – meaning that even with a sole plate thickness of 15mm, the total thickness through which the load acts is 40mm – which is more than sufficient in our example.

We went back to our client with this, but were informed that they were not using an insert plate in this particular project and that the sole plate thickness needed to be at least 40mm as per our calculations.

The exercise was an eye-opener, because we had never before been questioned on the bending moment for sliding bearings, nor had we come across any design calculations for this in the various codebooks. However it is a critical point in the design of a PTFE sliding bearing – as there is always an overhang of top plate over the bottom. As a rule, one needs to verify that an insert plate is being used and of what thickness it is. Adding this thickness to the sole plate thickness, the designer needs to verify whether the bending moment is being accommodated. If not, the thickness of the upper sole plate must be re-worked.

The wonder that is Lubring (Turcite®)

2011-06-14T12:54:00.011+05:30

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It is only rarely that a marketing push succeeds so thoroughly that the brand name becomes synonymous with the product itself. When Turcite B ® was first introduced, PTFE itself had been around for quite a while. Yet, so effective was the branding of this variant of PTFE that everything from the properties, to the composition to the colour merged under a single umbrella and became known simply as “Turcite ®”. The material itself has become a mainstay in the industrial goods market with machine tool builders, re-conditioners and bearing manufacturers demanding it for their applications.

As the market expanded, new manufacturers developed their own variants under different brand names (ours being Lubring), which all succeeded in their own way. However, to this day, clients will initially demand “Turcite ®” – following which there will be a brief discussion about the fact that our material is equivalent to Turcite ®, but that we brand it under a different name. Usually the client is happy as long as the properties match and that the colour of the material matches the turquoise-green shade developed specifically for Turcite ®.

We want to take a closer look at this material, because despite it’s widespread usage, there are always questions from clients regarding the application and installation of this material. We will look at the following aspects:

What is Turcite ® (Lubring)
Where can it be used? What are the possibilities and limitations?
How must it be installed?

What is Turcite ® (Lubring)?

Quite simply – Turcite ® is PTFE impregnated with fillers and additives that serve to enhance the wear properties of the material. It is used, most often in a sheet form, in thicknesses ranging from 0.5mm (0.02”) to 4mm (0.16”), although in some applications, it is also used as a bush and in more rare applications it is used as a thick plate.

Being based on PTFE, the material cannot be extruded like a normal plastic sheet and instead needs to be “skived” – the process most commonly used to make thin PTFE sheets (See: PTFE – Myths Busted!). Also, the material will not easily adhere to other surfaces – another feature resulting from its PTFE base. Therefore a chemical etching is required on one surface of the material, so the sheet can be bonded to other articles.

In a broad sense, Turcite ® (Lubring) offers the following key advantages:

Very low friction for reduced power loss
No stick-slip for positional accuracy / control
Good specific bearing loads
Low wear for long life
Excellent chemical resistance / fluid compatibility
Unlimited shelf life
High temperature resistance
Absorbs vibration during machining

Applications of Turcite ® (Lubring)

Most commonly, Turcite ® (Lubring) has been used in the machine tool industry where it serves to either replace or reinforce standard phosphor-bronze LM guideways. The material was earlier used primarily to recondition old machines in which the guideways had worn out. However, increasingly it is incorporated in new machines as well – owing to the higher life and lower maintenance required in comparison to metal guideways.

As mentioned above, Turcite ® is also used as a bush – which needs to be specially moulded and machined as per the customer’s requirements. These bushes are usually replacements for metal bushes – especially in areas where the lubrication of the metallic bush is as issue. Turcite ® – and in fact all PTFE grades – has self lubricating properties which means it can function deep within a sub-assembly taking enormous wear loads and does not need to be lubricated constantly to avoid damage.

However, the material is not an out-and-out replacement for metal. Being PTFE based, the material has a compressive strength limited to 150 Kgs per square cm (2,200 psi). This means that a single square foot of Turcite ® can accommodate a load of up to 150 Tonnes – which is more than sufficient for most applications. However, it is also a soft material (Shore D Hardness: 50-60) – meaning that point loads and excessive squeezing of the material can cause deformation.

Another limitation is with regards to insulation. Although the material has excellent temperature resistance (up to 260 Degrees Celsius/ 500 Fahrenheit), it does not have any electrical insulation.

In all, the industries for which we have supplied Turcite ® (Lubring) include:

Automotive
Machine tool
Infrastructure
Nuclear power
Casting and forging
Textiles
Pumps and valves
Pipe liners

Installation guidelines for Turcite ® (Lubring)

Preparation: The metal surface to be mounted with Lubring can be prepared by the normal machining methods such as, grinding, milling, shaping, and planning. The surface roughness of all forms of preparation should be preferably between Ra = 1.6 µm and Ra = 3µm and not more than Ra = 6µm. Once roughened the surfaces can be cleaned with Trichloroethylene, Perchloroethylene or Acetone.

Bonding: For bonding of Lubring the following resin adhesive can be used: Ciba Geigy's Araldite - Hardener - HV 953U; Araldite AW106. The Araldite should be applied both to metal and Slideway and be spread as uniformly as possible by means of a serrated spatula. To obtain the best dispersion of the adhesive, when spreading on the surface brush in the longitudinal direction; when spreading on the metal, brush in the transverse direction. The total quantity of bonding should be approximately 200gm per sq. mt.

Hardening: After mounting the Slideway a clamping pressure of between 30-35 Kg/cm2 is recommended. It is important to keep the pressure constant during the hardening process. Due to the differences in the thermal expansion coefficient of the materials, maximum curing temperature should not exceed 40°C. The hardening time for various temperatures is: 20°C min 15 hours; 25°C min 12 hours; 40°C min 5 hours.

Finishing: After curing of the adhesive, the PTFE can be machined by conventional means – if required. The choice depends on the machinery available viz.: grinding; grindstone.

Grinding: For grinding of Lubring use the same speed as grinding cast iron, taking care that sufficient cooling is used with an ‘open ’stone. The grindstone should be preferably silicon carbide based with rubber or polyurethane binding; grain size 80-30. Alternatively aluminum oxide with rubber bonding may also be used for soft, fine grinding action, pre-polishing and pre-mating treatment.

Oil Grooves: Lubring pads can be machined with oil grooves using the same methods and cutting data as used for cast iron. The form and depth of the oil grooves are optional. However, the oil grooves should never pierce through the Lubring Slideway. Oil grooves should be away from the edges by 6mm.

Metal Mating Surface: The metallic mating surface running against the PTFE pad should be preferably Stainless Steel (SS) 304 with a grade #8 mirror finish. Against this material, Lubring will have a coefficient of friction of between 0.1-0.12.

Conclusion

The recent surge in PTFE prices has obviously had a substantial impact on the price of Turcite ® (Lubring). However, owing to the ambiguity surrounding the material’s composition (many clients know it simply as “Turcite ®” and are unaware that it is PTFE based), there has been genuine confusion as to why the price has increased recently. While it take times and patience to convince clients that the price of Turcite ® is governed by the price of PTFE, it does serve as yet another reminder of how an effective branding campaign can truly give a product it’s own identity.

Turcite® is the registered trademark of Trelleborg Sealing Solutions

PTFE Wear Plates: Misconceptions and Applications for Heavy Equipments

2011-06-05T20:57:00.002+05:30

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Although PTFE is used extensively for its wear resistant properties in a range of different products, its application as a wear resistant plate remains restricted and not widely known in certain areas where it would be ideal.

In most cases, the preferred composition for PTFE wear resistant material is PTFE with bronze (along with some friction reducing additives). As discussed in our earlier article (see: PTFE Compounds and their effects), this composition improves the PV value and wear rate for the material and although the coefficient of friction does increase, over-all it performs superbly as a replacement to metal bearing parts that require frequent lubrication.

Currently, some of the main applications for PTFE as a wear resistant material for which we supply include:

Slideway bearings: Commonly referred to by the brand name “Turcite” (Poly Fluoro brand name: Lubring), PTFE slideway bearings are used widely in the machine tool industry, where they serve to either replace or reinforce standard phosphor-bronze LM guideways. The material was earlier used primarily to recondition old machines in which the guideways had worn out. However, increasingly it is incorporated in new machines as well – owing to the higher life and lower maintenance required in comparison to metal guideways
Wear strips: PTFE wear strips are used either in running lengths or punched into flat components, which are used in sub-assemblies, like shock absorber struts and pistons. Usually the tolerance on thickness for such wear strips is very low – implying the requirement of a high precision skiving machine. In most case, where we supply these items, a tolerance of +/-0.02mm is maintained on thickness.
Piston rings: Here, thin bands of PTFE wear material are machined and fitted on the piston shaft to absorb the wear resulting from a constant back-forth movement. As this process is wasteful (and therefore expensive) due to the machining involved, sometimes customers prefer to buy wear strips and bond them around the shaft. However, bonding PTFE is usually only recommended when there is minimal shear force being applied on the item – so this method is usually unsuccessful.
Bushings: PTFE can either be machined into a solid bush, or be used as a layer on a metallic bushing (commonly called DU bushings). Again – the idea here is to create a self-lubricating bush, which can be installed within a sub-assembly and allowed to run without the constant need for lubricants.
Wear plates: Used in more heavy duty applications, wear plates are usually employed in thicknesses exceeding 10mm and often require milling on the surface to create oil-grooves and holes for bolting. In most cases, their function is similar to that of a slideway bearing, however we have noticed that many OEMs remain apprehensive to employ PTFE wear plates into their equipments. In an attempt to clarify certain points regarding PTFE wear plates, we are going to be looking at 2 aspects of their usage:

1) The common pitfalls clients experience when using these bearings and misinformation regarding the same

2) Our own experience in the Die Casting Industry, where the success of these plates has led us to aggressively recommend it to OEMs

Issues hindering the adoption of PTFE wear plates

Installation: We find that most people adopting PTFE wear plates do so because they have some prior experience with installing slideway bearings. Consequently, they assume the installation methods would also be the same. However, as slideway bearings are much thinner (going up to no more than 5-6mm) and because following installation, they remain subjected to very little shear loads, they can be bonded to the metal bed and this bond is likely to survive over a long period of time.

In the case of wear plates, bonding is not an option as it is likely that there is some shear load which will get applied which, when coupled with the thickness of the plate would weaken the bonding and cause the plate to come loose in the medium to long term.

The correct method of installation is bolting – although valid apprehensions exist with regards to this. For one: the plate needs to be milled with a stepped hole to allow the bolt to rest within the piece. Care needs to be taken to ensure that the bolt head does not rest above the surface of the PTFE plate. As an added measure, PTFE discs can be bonded to the head of the bolt to ensure that in the event that any extra pressure squeezes the PTFE plate, the contact between the bolt and the moving plate is not damaging. Furthermore, tightening the bolt too much can cause the PTFE plate to get crushed (a common reason cited by OEMs for not using a soft material like PTFE). Hence the correct method would be to use a metal bush to ensure the bolt is not tightened beyond a point (see below).

The purpose of the bolt is to ensure the PTFE wear plate does not slide away during operation. As long as this is ensured, the plate will perform properly.
Load bearing

A common misconception relating to the load bearing capacity of PTFE leads many machine tool builders to write-off PTFE as a wear pad material. The assumption is that phosphor-bronze, being a metallic material, is the only option strong enough to take the load of heavy moving parts.

In truth – PTFE has a compressive strength of at least 135-140Kg per square cm. This implies that a 100mm x 100mm plate would be able to withstand 13.5-14 Tonnes of vertical load. In most heavy-duty equipments, maximum loads of 5-6 Tonnes are present, meaning that the load bearing is not an issue at all. Furthermore, the coefficient of friction of PTFE against another surface only reduces with the application of pressure – implying that apart from taking the load, the effectiveness of the wear plate in ensuring a smooth functioning of parts is greatly enhances.
Machining

Clients who are looking to convert to PTFE wear pads frequently express two concerns pertaining to machining.

The first is on tolerance: as the thickness on phosphor-bronze wear pads can be grond to within a few microns. In the case of PTFE – a maximum tolerance of 50 micros is possible – which we have found is acceptable in most industries.

The other concern is around specific grooves and the exact positioning of holes. As PTFE can be milled (we use a CNC vertical milling centre) – any groove pattern and hole dimensions can be machined on to the surface of the wear plate.
Environment

Finally – we have heard concerns over the conditions in which the equipment is used and whether PTFE will be able to withstand the same in the long term.

Firstly – PTFE has the ability to withstand temperatures of up to 250 Degrees Celsius. In most industries we know, the actual heat generation never causes the surrounding temperature to go about 80 Degrees, so clearly there is no issue in using PTFE.

The other concern is on the build up of dirt and whether grit and other hard particles will damage the surface of the PTFE plate. While the recommended option here would be to make a seal around the PTFE to ensure that dirt does not get accumulated between the PTFE and the other moving plate, it should also be noted that in case a particle does get lodged between the plates, PTFE has the unique ability to absorb the same so that it does not hinder the movement of the assembly.

Case Study: PTFE wear plates in the Die-casting industry

A client who was consulting on technical metrics with various companies engaged in aluminum die-casting approached us, a while back. The problem they were facing was that the wear plates that had been installed on as bearings between the platens was wearing out every 2-3 months, with the result that there was significant down time on the machines every time these plates needed to be replaced.

The plates being used were a fiber enforced resin plates and it was easy to see that a few months of usage had significantly worn out the plates leading to deformation and even cracks.

We offered them PTFE wear plates and these were installed on a few machines as a trial. The machines were run normally for a period of 3 months and the PTFE plates were analyzed with the following results:

Wear out was minimal: In fact, the PTFE wear plates were much the same dimension as when they were installed. The customer felt that the load of 2.5 Tonnes being applied on the plate would compress the plate and lead to a deformation on thickness – but this was not the case.
Lubricity was greatly enhanced overall: The plates had become completely smooth due to the constant sliding across its surface and this smoothness translated into the more efficient operation of the equipment. The customer also reported that while earlier there was some amount of “jerkiness” in the motion of the platens was no longer an issue.
Improved cycle time: Apart from the fact that the down-time of the machine was no longer an issue as the plates were not worn out, the overall cycle time of the machine during production was also improved. This was mainly because there was no longer a need to continuously monitor the level of lubrication on the wear plate.

Following the successful trial of the PTFE wear plates, the material was adopted in all machines of the client and we are now working with a number of clients in the die-casting industry to replace their resin plates with PTFE plates.

The various forms of Sliding Bearings

2011-05-24T16:15:00.016+05:30

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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:

Oil and gas pipelines
Waterways and water pipelines
Conveyor systems (both indoors and outdoors)
Boiler plants
Minor bridges
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

2011-05-20T09:42:00.005+05:30

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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

2011-05-11T16:47:00.012+05:30

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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:

What is fluorspar?
How much fluorspar is used in PTFE resin manufacturing?
How has the price of fluorspar changed over the past year?
What is the supply side scenario?
How have pricing and supply combined to influence the current situation?
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

2011-05-10T07:54:00.000+05:30

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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 tubing

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:

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
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
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.
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.
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
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 tubing. After the cleaning the tubes are packed in polythene covers for dispatch.

Table 3 : Technical specifications of FluoroTube

PTFE Compounds and their effects

2011-05-09T11:23:00.003+05:30

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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

PTFE - Myths Busted!

2011-04-26T08:51:00.003+05:30

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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

2011-04-25T06:48:00.002+05:30

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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:

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
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)

2011-04-18T02:30:00.010+05:30

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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 closest PTFE 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 tubes - 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 of PTFE; 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 which PTFE 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 of PTFE+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?

2011-04-10T19:16:00.008+05:30

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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

Expanded PTFE

2009-08-10T16:53:00.000+05:30

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Expanded PTFE (EPTFE) is one of the more innovative variations in the processing and application of PTFE in recent times.

Discovered in the 1970s, the EPTFE process is unique in that it does not require the use of soluble fillers, foaming agents or chemical additives. The product itself is chemically identical to PTFE - except that it constitutes billions of small pores within the structure of an article of PTFE. This gives it new mechanical properties and results in significant material savings.

EPTFE is used to make lightweight, waterproof and breathable fabrics, micro-porous membranes, medical tubes and implants, microwave carriers, industrial sealants and high-tensile fabrics and cords.

The material exhibits excellent dielectric properties and also a drastic reduction in creep which is a weakness of standard PTFE.

Process

The expansion process begins with paste extrusion of fine powder PTFE using typical lubricants or mineral spirits. The lubricant is completely removed by heat - similar unsintered PTFE tape or PTFE tubing. The lubricant-free extrudate, which can be in the shape of a rod, tube, or tape, is the feed for the expansion process.

The expansion process requires heating the unsintered PTFE anywhere from 35-320°C while keeping it restrained in a device capable of stretching it at high rates. The stretch rate can vary from 10% per second to 40,000% per second.

The stretched part is heated to a temperature above 330°C while being held in a restraining device to prevent its shrinkage. After this heat treatment, called "amorphous locking", for a period of time the expanded part is cooled and removed. The optimum temperature for this process is 350-370°C for anywhere between a few seconds to an hour.

Higher stretch rates and temperatures produce a more uniform matrix.

Expansion can be done by uniaxial or biaxial stretching of PTFE. The fibrils are reported to be wide and thin in cross section with a maximum width of 0.1 μm and a minimum width of one or two molecular diameter in the range of 0.005-0.01 μm. The nodes vary in size from less than a micron to 400 μm, depending on the conditions of the expansion.

A sintered PTFE part has a density of about 2.15 g/cm3 and an unsintered unexpanded part has a density of 1.5 g/cm3. The density of an expanded part can be as low as <0.1 g/cm3, with a porosity of 96%. Density and porosity have a linear relationship. Pore size is quite small, less than 1 μm, up to 90% porosity; larger size pores (1-6 μm) contribute to driving the porosity above 95%.

The heat-treated expanded web has a higher permeability than normally sintered PTFE to gases and liquids due to its porous structure. It can also act as semi-permeable membrane by allowing the wetting liquids through while being impermeable to the non-wetting fluids. For example, a gas-saturated membrane in contact with the gas and water will allow gas through and keep water out as long as the pressure of water does not exceed the water entry pressure. Research indicates that above 90% web porosity, air permeability increases drastically while water entry pressure of the web decreases to a fairly small value. There is a range where porosity can be selected to balance the air permeability and water impermeability, which is useful to applications such as clothing.

The table presents a comparison of the properties of expanded and full density (unexpanded) EPTFE. Crystallinity of the amorphously locked PTFE is about 95%, which is significantly above the highest commercially attainable value with unexpanded parts. The most striking improvement is in the tensile strength, which is orders of magnitude over the full density material and has opened new applications for PTFE. Tensile strength of expanded PTFE is calculated for the matrix by multiplying the measured value of tensile strength by the ratio of the densities of the full to expanded PTFE. Flex life and maximum service temperature of expanded PTFE are both higher than those of the full density material.