PTFE Sliding Bearings: Calculating Coefficient of Friction

PTFE is a preferred material in sliding bearings for three very specific reasons:

1. Load bearing capacity
2. Weather ability (due to its overall chemical inertness)
3. Low coefficient of friction

The first two factors are well accepted and easily tested. The vertical load on a bearing is simply tested by placing the bearing under a hydraulic press of suitable capacity, applying 1.25 times the rated load of the bearing and holding this load for a period of 1 hour to observe any adverse impact on the bearing material. In our own experience, it is very rare that pure PTFE would fail in this instance, since:

1. Pure PTFE genuinely does have a very high load capacity and even in the event of over-loading, would tend to deform rather than break down
2. The design load for most bearings incorporates a safety factor of up to 60% – implying that while PTFE may be able to withstand a load of up to 40Mpa, it is designed with a load of only 16Mpa and is thus well within its own capacity to take the load applied

Weather ability is difficult to test, as this is a long-term guarantee that the material can stay in outdoor conditions without experiencing any degradation in properties. However, most clients are happy to take this assurance at face value – as long as they can satisfy themselves that the material being used is in-fact pure PTFE. Its should be mentioned here that in the event that reprocessed or recycled PTFE is used in sliding bearings (a gross violation of quality norms, but one that may occur all the same, especially if the bearing manufacturer is buying their PTFE from a third-party and is therefore not directly in control of the quality), there will definitely be a failure of the bearing after installation.

As mentioned in earlier articles, we have witnessed many deviations from expected performance when presented with reprocessed PTFE. Amongst these is the tendency of the material to become brittle and even crumble after being kept outside for a prolonged period (usually over a few months). No doubt, a similar effect would be experience by a material used in a bearing that is installed on a bridge or flyover – with the result that the bearing may fail after only a year of service.

Coefficient of friction

The problem with the coefficient of friction is that most people are not fully familiar with what it implies. So let us start with defining it and then look at the misassumptions surrounding it.

The coefficient of friction between two planes is defined as the ratio of the force needed to move one plane over the other divided by the force pushing the two planes together.

So in the case of a block resting on a table, coefficient of friction between the block and table would simply be the force needed to slide the block across the table, dived by the weight of the block itself.

Since the coefficient is a ratio of two forces, it does not have any units. A common mistake clients make is to ask us to define the unit we have considered for the coefficient of friction.

In the case of the PTFE sliding bearing, the coefficient of friction being considered is that between PTFE and polished stainless steel. Here again, a mistake is often made asking what the coefficient of friction of PTFE is. There is no such thing as a stand-alone value for coefficient of friction for any material. The coefficient between PTFE and polished stainless steel will no doubt be much lower that between PTFE and concrete. In other words, it would take more force to move a slab of PTFE across a concrete surface than it would to move the same slab across a polished stainless steel surface. Thus, when we talk about a coefficient of 0.04 between PTFE and stainless steel (the commonly accepted value for bearing manufacturers), we are saying that for a 1Kg PTFE block to slide across a polished stainless steel surface, it would require only 40 Grams of horizontal force. (Note: we are aware here that Kgs and Grams are units of mass and not force, but seeing as these are ratios of force, the values in Kgs/Grams against the values in Newtons would yield the same results).

Measuring coefficient of friction

We have already described that the value for the coefficient is derived by dividing the horizontal force over the vertical force. However, in practice, this is less straightforward. We would like to look at some of the methods that are used around the world to check these values as pertaining specifically to PTFE bearings, before describing what we feel is the most straightforward and easily implemented method.

1. Two-press method
   It is not possible to test the bearings simply by applying a vertical load and seeing at what horizontal load the bearing slides. This is because the bearing plate on which the horizontal load is applied also has another surface, which would be in contact with the vertical press and therefore be subject to friction from the press itself (which is likely to be very high).
   Therefore, the accepted method is to place 2 bearings, back-to-back (shown below) and exert load on the centre.
   This process is technically sound, but practically not always feasible. For starters, the bearing shape itself may not lend itself to being places back to back. It may have welded guides attached to it or be of an unusual shape. In addition to this, the process in expensive – requiring two hydraulic presses.
2. Load indicator method (laboratory)
   In this method, the stainless steel plate is placed horizontally with the PTFE plate on top. The PTFE plate is connected to a steel wire which is in-turn connected to a load indicator. The load indicator has a motor, which causes it to move upwards very slowly. As the indicator moves up and the wire gets tight, the load reading starts to show the horizontal load being applied. Once the PTFE plate starts to move, the reading is recorded and divided by the weight of the PTFE plate to give the coefficient of friction.
   This method is possibly the most accurate, as load indicators can offer values in grams. However, it again suffers from the issue that if the bearing plates are not totally flat or are too big for the equipment, they cannot be accurately tested. Furthermore, it is debatable whether such accuracy is needed in the realm of sliding bearings
3. Poly Fluoro method (inclined plane)
   In an attempt to find a quick, repeatable, logical and universally applicable method to check the coefficient of friction, our method follows the rather simple process of gauging the angle of incline.
   For starters, we do not believe that getting an accurate value of the coefficient of friction would add any value to the product. If we can confirm that the plates slide at a coefficient of friction set to 0.04, then it does not matter whether the coefficient is in-fact 0.03 or 0.036, as the product has met its required specification.
   
   The diagrams  show that when the planes are inclined, the coefficient of friction takes the value of the tangent of the angle of inclination. This allows us to easily check the coefficient of friction, as we simply set the ratio of Y to X to equal 0.04 and check if the PTFE plate slides down the plane, when placed on the SS plate. Again – note here that only in the event that the PTFE plate does not slide, can we conclude that the coefficient is greater than 0.04 (and hence outside tolerance). Whether the plate slides slowly or very fast, is of no consequence – as either way it confirms that the coefficient is at least 0.04.
   
   In the event that the client specifies a higher or lower coefficient, the same method can be employed, by simply changing the value of Y. So we first assess what the value of Y should be by multiplying the length of the bearing plate by 0.04 and then use a calibrated slip gauge to prop up the bearing on one side (preferably on a flat bed) so that the angle is attained.
   The method employed here is useful to check PTFE bearings as it can be applied to any design and multiple bearings can be checked from one lot if needed, without the hassle of making separate fixtures and modifications. Furthermore, it can be checked even before the bearing is assembled so as to confirm that the PTFE material being employed meets the parameters.

Static vs Dynamic coefficients

It must be mentioned that both static and dynamic coefficients of friction are relevant metrics and that the inclined plane method only measures the static coefficient. However, this is of no concern for PTFE bearings as PTFE is known to exhibit nearly identical values for both static and dynamic coefficients (a property unique to PTFE and not universally applicable).

We believe that the method described above is a more effective way to check for a very important parameter that bearings are based on.

PEEK in India – A Growing Market with Many Challenges

At Poly Fluoro Ltd. we started our journey with PTFE and gradually expanded into other polymers. Initially, this was at the behest of existing customers, but over time our expertise in machining plastics meant that we were comfortable offering a variety of options to our clients, rather than try and force fit PTFE into their application.
We discovered the benefits of PEEK in one such exercise. Although we have already blogged extensively on the benefits and properties of PEEK, our own experience in dealing with this material serves to explain much of the commercial and technical queries surrounding this material.

PEEK in India is a small market in terms of volumes. The total consumption is only about 35 Tonnes. Of this, most of the material is imported as semi-finished rods and sheet, with only 12-15 Tonnes being processed from resin indigenously. Small as these numbers are, keep in mind that semi-finished PEEK sells at anywhere between US$275-US$400 per Kg – so in value terms, the market is not as small as the volumes suggest. Nonetheless, it is very much a niche market – even among speciality polymers.

Being present in the PEEK market as a processor poses many challenges. Some of these are technical in nature, while others relate to the commercial issue (PEEK is very expensive) and how clients respond to PEEK. Again, we have touched on some of these points in our earlier article – but as we have delved deeper into PEEK processing, many new findings have arisen.

Compression moulding PEEK not a simple affair

There are many challenges in compression moulding PEEK and most of these do not get explicitly highlighted in manuals and guidebooks. In most manuals, the process is outlined in 5-6 basic steps, which at first glance make PEEK appear a very friendly material to deal with.

In reality, the process is time-consuming, highly sensitive to the exact process needed and very specific in the type of tooling required.

The benefit of compression moulding PEEK over, say extrusion is that we are able to make customized dimensions based on the customer drawings. The stock piece for a part measuring 70mm in diameter can be moulded as 72mm, rather than using a 75mm rod. Over a 50mm length, this saves almost 25 Grams per part – which is significant when we consider the cost per Kg. Furthermore, if the part has an internal diameter the saving is even more, as the same cannot be attained in extrusion for large diameters.

However, against this saving, the time consumed to make a 50mm part would be many times what extrusion would take. Compression moulding is known for low productivity and even a large processor is only able to consume 20-25 Kgs per day of production. In India, however, where labour is inexpensive, this is not a huge cost factor – it only limits volumes. And since PEEK is still a low volume polymer – even processing 4-5 Kgs a day can be significant.

The actual process of compression moulding PEEK is also not straightforward. The 5-6 steps mentioned in the manuals each contain nuances that need to be fine tuned until you reach a process that most suits the equipment available. In our own experience, we have found that over 25-30 trials had to be taken, each using up between 250-800 Grams of resin. After each trial, some parameters were changed before taking another trial. Parameters such as pressure, peak temperature and soak time all need to be varied to control issue such as porosity, cracking, black spots and cold spots.

In addition to this, the selection of dies is critical. PEEK, in its molten form can be a very aggressive material and we have had many steel dies get corroded during moulding. Again – finding a balance between a strong die metal and the correct process is critical in obtaining a final process that is both economical and productive and which yields a high quality final product.

Variants and substitutes do exist for the price conscious

We have had some success in blending PTFE with PEEK ratios of 5%, 10% and 15% by weight (ie: PTFE+5/10/15% PEEK). Again – the process of blending is not straightforward and the PTFE itself needs to be processed slightly differently owing to the fact that PEEK melts at a higher temperature than PTFE. However, the final blend has proven to be useful in applications involving sealing and needing high wear resistance, with a low coefficient of friction.

Another alternative to PEEK is PEK. PEK is very similar to PEEK and is processed in much the same way. As far as properties go, some have even suggested it is slightly superior on some parameters. Commercially, it is roughly half the price of PEEK – which makes it a very tempting alternative. However, PEK is still being proven in OEM applications, whereas PEEK has been around long enough to give any OEM designer confidence in its properties.

Marketing PEEK is a challenge

In a market like India – which is highly price sensitive – PEEK is a difficult product to win customers over with.

PEEK is usually the last choice of any OEM due to its price, and if someone has not come across the material before, it takes some educating before they are convinced that any polymer exists at such a price. And while PEEK is well established in the West – in India, it is still very nascent in comparison and clients do not always see the long-term benefits of using it.

Furthermore, the relatively recent introduction of PEK into India is threatening to take some of the long-term market share from PEEK, as in a price sensitive market, people may be willing to make the gamble on a cheaper substitute.

Mapping the PTFE Price Increase - An Update

 We have been receiving many mails asking us to map or at least project the PTFE price trend going forward. Since our last post on PTFE pricing was about a year ago (Feb 2012), it is a valid question to ask whether there has been any further volatility in this market and what that implies for prices as a whole.

To lay any suspense to rest right away – we can firmly say that prices have indeed been stable this past year and it is due to this stability that our own interest in analyzing the prices has dimmed somewhat. However, that is not to say that things would continue along this vein indefinitely. PTFE is a complex material and the dynamics involving its manufacture and sale are constantly in flux – meaning that the next shock may just be around the corner at any time. So we would like to look at some of the buzz surrounding the industry in a hope to at least demystify the future to some extent.

Prices are expected to remain stable throughout 2013

This is the general consensus as of now and is due to two primary factors:

1. There is a general slowdown in global demand (much in line with most other industries) that makes it risky for resin manufactures to experiment with pricing like was done early in the price escalation of 2010-11
2. There was a significant over supply of resins when prices were high and this led to huge inventories which manufacturers are still offloading

In some areas it is believed that there is still some scope for prices to fall further. However, it is most widely accepted that the current rates are stable and should be for the foreseeable future

China still a key player – but not the only price maker

Back in 2010, it was largely a price war between manufacturers that led to very low PTFE prices. Much of this was driven by China – where the abundance of fluorspar and the support from their government allowed Chinese PTFE manufacturers to scale up very quickly.

Although China remains a key player still, a few factors are affecting their economics and scale right now:

1. The government support has reduced for PTFE resin manufacturers in China. With the clamping down of R22 within China (for environmental reasons), there is a subsidy of US$0.5 per Kg of R22, which is no longer being offered by the Chinese government.  Owing to this, Chinese manufacturers have had to pass on this cost increase of about US$2 per Kg on to PTFE processors.
   
2. Due to quality issues with Chinese resins, many OEMs have started specifying that their parts be made from resins such as DuPont, Daikin or AGC. Many semi-finished PTFE processors have also shifted away from Chinese resins due to the instability of the material. It was long believed that China reserved the good quality resin for their domestic manufacture and preferred to dump the off spec grades into other countries (one of the reasons why the anti-dumping duty on Chinese resins was upheld by the Indian government). This has hit the volumes of Chinese resin manufacturers
   
3. Another key issue is repro material. While India earlier had significant imports of Chinese semi-finished PTFE materials, most of this was repro material but was being passed off as 100% pure virgin PTFE in the local market. This high intake of semi-finished PTFE was affecting the local semi-finished manufacturers as well as local resin suppliers. However – owing to the major quality issues with repro and the lack of accountability and transparency in the percentage of repro being incorporated, many companies have had to stop procuring semi-finished PTFE from China and have started buying domestically – where it is easier to monitor quality and also return material if found defective. This has led to a resurgence in domestic PTFE semi-finished goods production and also an increased off-take from local resin manufacturers.

With China on the back foot due to the reasons listed above, it may be safe to say that price manipulation and/or competition is for the time being not a threat – since it is usually with China that most of these issue do arise. Hence, the current view is that of stability – and we should enjoy that while it lasts.

Delrin – the machinist’s best friend

In a world of specialized plastics requiring immense tensile strength or high wear resistance or minimal coefficient of friction, Delrin holds its own against the more versatile polymers such as PEEK and PTFE.

Our own experience with Delrin began with the PTFE price increases in 2010-2011, as we scrambled to find substitutes for PTFE to offer clients, without compromising too much on properties. As we have already stated in earlier articles – finding a true substitute for PTFE was futile. However, despite our attempts to push UHMWPE and PA66 as replacements (materials we were more familiar with), it was ultimately Delrin which clients were most comfortable in adopting.

What is Delrin?

Delrin (brand name of DuPont) is also commonly referred to as POM (Polyoxymethylene), polyacetal, or simply acetal. The names all refer to a polymer that is characterized by a high tensile strength, high stiffness, low coefficient of friction and excellent dimensional stability. In addition to its properties – Delrin is a relatively inexpensive material compared to PTFE and even PA66. This makes it a sought after choice in machined component development – as the parts are dimensionally very stable and significant trials can be done without being too expensive.

Properties of Delrin

• Delrin is characterized by its high strength, hardness and rigidity to ~40 °C
• In its natural form, it is a white (opaque) plastic, although it is easily pigmented and often available in a variety of colors
• Delrin has a specific gravity of 1.410-1.420 g/cm3
• As a homopolymer it is 75-85% crystalline with a melting point of 175°C, while as a copolymer has a slightly lower melting point of 165–175°C
• It has a relatively low coefficient of friction of 0.2 – much higher than PTFE, but still suitable for a wide number of engineering applications
• Delrin is resistant to a wide variety of chemicals including alcohols, aldehydes, esters, ethers, hydrocarbons, agricultural chemicals, and many weak acids and bases. This ability is even more impressive when we consider that even under harsh chemical environments, Delrin does not lose its dimensional stability
• Electrically, Delrin rates slightly below PTFE, but is nonetheless a very useful substitute. Its dielectric constant (~3.5) is only slightly higher than PTFE (~2)

Advantages of Delrin:

• High mechanical strength and rigidity
• Toughness and high resistance to repeated impacts
• Long-term fatigue endurance
• Excellent resistance to moisture, gasoline, solvents, and many other neutral chemicals
• Excellent dimensional stability
• Good resilience and resistance to creep
• Natural lubricity
• Wide end-use temperature range
• Good electrical insulating characteristics

Due to its versatility, Delrin finds uses in a number of applications including:

• Automotives
• Industrial equipments
• Consumer goods
• Medical equipments
• Electrical equipments

Machining Delrin

As a machined item, Delrin is particularly easy to work with. With PTFE, we need to consider the softness of the material and also its sensitivity to temperatures, with nylons and UHMWPE, we need to be careful of the part melting during machining, with PEEK, the tool itself can break, if we do not control the RPM. However, Delrin is surprisingly accommodating as the part retains its stiffness, but is still soft enough that the tool is able to work through the plastic. In addition, the dimensional stability post machining is also excellent. While we have had instances of PTFE parts being under tolerance when shipped to colder climates, the same is not an issue with Delrin parts.

Applications and Considerations for PTFE Seals

As a sealing element, PTFE has proven itself many times over. PTFE is used in seals because it encompasses all the properties essential for a good sealing element, mainly:

• High wear resistance
• Low coefficient of friction
• Moderate hardness (allowing for better overall mating with metal parts)
• Durability – both with temperature as well as corrosion

While these properties are not new to us, every material has a limit to how much it can withstand. Furthermore, every grade of PTFE offers something different to the sealing application. Understanding these limits and differences gives us a better understanding into choosing and applying PTFE seals to best suit the requirement.

Sealing is vital to almost any mechanical assembly. It serves to both retain fluid within the assembly and allows the assembly to function freely. A good sealing material – such as PTFE, needs to be elastic enough to close gaps and assist with the fluid retention while strong enough to take the wear load applied on in by (usually) metal mating parts. Still, within any assembly, there is likely to be some trade off between fluid retention and durability, and this is where the choice of grade becomes important. Typically, the following metrics needs to be studied:

Surface Finish

PTFE wears off in layers, and will usually deposit a coating on the mating surface. In general, it is easy to attain a surface finish of as high as Ra < 0.4 on a virgin PTFE part. However, once we introduce other materials such as glass, carbon, graphite or bronze into the mix, there is a huge drop in the finish. We have successfully attained a finish of Ra < 1.2 on PTFE+15% Glass seals – but going below this is always a challenge.

For the mating part, the surface finish is somewhat more important – as it is usually a metal can wear the PTFE out significantly faster if not properly finished. When the metal surface is rough, more wear occurs until the crevices and valleys within the metal are filled with PTFE. PTFE will wear in direct proportion to the surface finish. Testing shows that the life of the seal is doubled when the finish is improved from 16rms to 8rms.

Surface finish also affects the sealing ability of PTFE. A rough finish creates a microscopic “line of sight” channels allowing a flow path through mating parts. Hence, when sealing gases with small molecules, such as, hydrogen, helium, or oxygen, a 2-4 RMS is highly recommended.

Hardness

When the mating part is hardened (via heat treatment or plating), there is a significant improvement in the life of the seal. Typically, when a hard and soft surface are in contact, there is an exchange of ions, which can lead to adhesion. This reduces the effectiveness of the seal. Improving the surface hardness of the metal part can control the adhesion.

PV Value

PV is an often-quoted metric for all PTFE grades. It offers a trade-off between the pressure that the PTFE can take, against the speed at which the mating part is sliding against the PTFE.  Understanding PV is key to understating whether the PTFE grade being considered at would be able to withstand the combination of load and RPM involved.

Disregarding PV values would almost certainly lead to a failure in the seal to perform. We have received many requests to look into the replacement of standard phosphor-bronze bushings, bearings and seal with PTFE grades. In most cases, PTFE looks to be a perfect substitute along most metrics. However, when we look at the pressure it can withstand under high RPMs, PTFE is not always suitable.

Types of seals

Given the diversity in automotive and mechanical applications, a number of different PTFE seal dimensions have been developed – each with it’s own unique property. When we cross these dimensions with the different PTFE grades, we end up with potentially hundreds of seals. Thus, choosing the right seal is important and a lot of thought needs to go into the same, before a decision is made.

The spring-energised seal is a sealing device consisting of a PTFE ‘energized’ by a corrosion resistant metal spring.  Put simply – as PTFE is a soft material, it can be easily deformed by the metal parts surrounding it. The spring acts as a strengthening medium – allowing the PTFE to take loads while also applying force on the sealing surfaces to create a tighter fit and ensure no leakages. The spring also provides resiliency to compensate for seal wear, gland misalignment or eccentricity.

Types of such spring energised seals include:

Finger Spring:

This is mainly used in dynamic applications, has good sealing and a low coefficient of friction. It is recommended for surface speeds up to 250ft/min.

Coil Spring:

This is designed for more static or slow dynamic applications. It is not as flexible as the finger design – owing to the fact that the spring is coiled and more rigid as a result. However, it is significantly better than the finger design in sealing – due to the uniform pressure applied on all sides by the coil spring.

Double Coil Spring:

A more augmented version of the single coil – this is designed for purely static applications, such as cryogenics. The increased load applied by the double coil significantly improves sealing ability.

O-ring Energised:

This can be used in both static and dynamic applications and offers a good balance between the seal-ability of the coil energised seal and the flexibility of the finger spring. It is typically incorporated in areas where metallic springs cannot be used due to compatibility issues.

Rotary Lip Seals

Lip seals are used primarily to seal rotary elements such as shafts and bores. They provide a self-lubricating medium between (usually) two metal elements – allowing for both smooth rotation and good sealing. Common examples include strut seals, hydraulic pump seals, axle seals, power steering seals, and valve stem seals.

Lip seals may be designed with or without springs – depending on the application.

The examples shown above are merely indicative of the basic designs available in PTFE seals. In truth, each of the above types of seals may be expanded into many variants, depending on the exact requirements of the mating elements involved in the OEM designs. Furthermore, each may be provided in any of a number of grades of PTFE compounds available.

Choosing a PTFE compound for your PTFE seals

The grade of PTFE is a critical choice in the design of the seal. We have touched elsewhere on the variants and properties offered by the commonly used fillers in PTFE. In a nutshell – glass offers stiffness and creep resistance; bronze and molybdenum di sulphide offer wear resistance, but increase the coefficient of friction; carbon and graphite offer wear resistance and dimensional stability.

In our experience, a mixture of glass and molybdenum di sulphide offers the ideal sealing properties for most applications. However – the exact grade is usually a choice made by the OEM, based on what information we are able to provide.

The Many Chemical Applications of PTFE

PTFE is known to be among the most chemically resistant materials known to man. While this property is well known and often quoted in manuals and our own blog articles, we would like to touch upon some of the common applications that this property leads to.

PTFE Labwares

PTFE has been a mainstay in lab-ware items for a number of years. Lab-ware items include stopcocks, beakers, pipes, test tubes, stirrers, petri dishes and stands. In most labs, glass is the commonly used material for these items, but as we know, glass has a tendency to break. Furthermore, when dealing with harsh chemicals at elevated temperatures and pressures, PTFE becomes a viable option for a number of reasons.

1. PTFE is chemically inert – barring certain alkalis at elevated temperatures
2. PTFE does not break easily: Under loads, virgin PTFE would tend to deform rather than break. This makes it useful in applications involving high centrifugal forces – where a glass test tube might break under extreme loads
3. PTFE is stable across temperature fluctuations: Even toughened glass would have a limit on how suddenly it can be cooled down when under high temperatures. PTFE is able to cool down rapidly from elevated temperatures without cracking
4. PTFE is a great sealing material: Especially for stopcocks, PTFE forms a much better seal than glass equipment owing to the fact that it is soft and will easily seal gaps which may arise due to minor variations in taper between the pipe and the stopcock.
5. PTFE is flexible: PTFE tubes can be used in place of glass and be bent to accommodate the layout of the apparatus
6. PTFE can be moulded with a magnetic core: PTFE stirrers are used because they can be moulded with a magnet at the core and used in magnetic mixers

PTFE Stirrers and Shafts

Stirrers and shafts are used primarily in highly corrosive applications including biotech, pharma and refineries. The ability of the PTFE to be constantly immersed in a chemical and neither modify nor be modified by the chemical makes it an invaluable component in many mixing arrangement.

More often than not, the shaft or stirrer needs to be custom moulded and machined to suit the mixing assembly. This makes it an expensive component and therefore only sparingly used. In some cases, a stainless steel core can be used over which the PTFE is moulded/ lined. In other cases, the stainless steel shaft can simply be coated with PTFE. However, this latter case only works where there is little or no abrasion expected on the shaft – since PTFE coating will peel off if the shaft is subjected to wear.

PTFE umbilical cords

Although the name sounds strange – the umbilical cord is a well-known arrangement of PTFE tubes used in the refinery industry. The purpose is simple – the refinery process yields a number of different gasses, which need to be analysed in a lab to gauge whether the right chemical reactions are taking place in the chamber. Taking these gasses to the lab (which needs to be a minimum of 200-250 meters from the reaction chamber) is a difficult process, as the gasses are corrosive and highly reactive – which may mean that they change composition during transit if not kept in a chemically inert environment.

An assembly of 12-15 tubes is bunched together using a PVC coating and each tube has a length of 250 meters and transports a single gas to the lab, where it is collected and analysed.

The complication in this design is that the PTFE tube needs to be continuous for the entire length of 250 meters. Any bonding or jointing leads to a foreign chemical in the tube and this affects the gas passing through it. After extensive trials, we find that using a welded joint comprised of PTFE is able to create an effective solution for the tubes.

Filtration

Many chemical applications involve multiple substances, which often need to be separated from one another. In such cases, PTFE becomes the preferred medium for filtration.

PTFE is used in 2 forms here:

1. Porous PTFE sheet: This is a standard PTFE sheet skived from a porous PTFE billet. The billet is made porous by adding certain substances in the PTFE compound, which sublimate during the sinter cycle, leaving voids. These voids form the pores which aid in filtration. This type of membrane is not used extensively due to the inexact nature of the pores. However, the membrane can be made as thick as 5mm – which makes it useful in corrosive applications where a liquid needs to be separated from large solid particles.
2. Expanded PTFE membrane: This is also called breathable PTFE membrane owing to the fact that you can pass gas through it, but not liquids. Expanded PTFE is more commonly used that porous PTFE as the pore size can be easily defined to within a few microns. It finds multiple applications in automotives, pharma and biotech.

PTFE Valves and Ball Valve Seats

Although valves and ball valves form an industry unto themselves and use a variety of materials other than PTFE, certain applications involving the flow of chemicals need PTFE valves to withstand the corrosion otherwise caused to non-PTFE valves.

Our own experience with PTFE valves sees it being used in piping systems in chemical plants and in equipments such as paint dispersion machines.

In paint dispersion, the equipment is used by retailers to mix different colours of paint to form a batch of a new colour as chosen by the customer. The paint passing through the PTFE valve needs to remain un-changed. Any reaction due to, say, using a nylon or PVC valve can alter the colour to the extent that the colour being chosen by the customer does not match the actual colour of the final paint. Thus PTFE is an invaluable component within this assembly.

Reprocessed PTFE and chemical applications

We had earlier done an article on reprocessed PTFE and the various issues it presents with regards to the base properties of the material. One of the issues we have observed is that when using reprocessed PTFE, the scrap is seen to change colour when using a coolant during machining. This came as a huge shock to us – as common opinion suggests that it is only in mechanical properties that the material suffers when reprocessed.

The finding leads us to believe that a number of things may be happening to cause this:

1. There may be foreign substances used in making the reprocessed PTFE. Titanium Di-Oxide for example is a known additive in making PTFE appear whiter. Similarly – cheaper, un-tested additives may be added to improve appearance, which may not have the chemical inertness that PTFE has.
2. Micro-impurities may be present in the material that cannot be seen by the naked eye. These may be reacting with the coolant causing the colour change
3. The basic chemical structure may be altered on a microscopic level. PTFE is chemically inert because of its molecular structure, which involves a carbon atom, shielded by 2 atoms of fluorine. When we chemically etch PTFE, we effectively remove 1 flourine atom and expose the carbon atom, making it bondable. A similar transformation may be happening in parts of the material due to reprocessing – which cause a degradation in the chemical resistance of the material.

PEEK: The Superman of Polymers

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If you deal in polymers and have not come across PEEK – it’s probably because its one of those materials which does not surface unless really needed. When it is needed – there’s little else that can be used in it’s place and this often confuses OEMs; because even among expensive, high-end engineering polymers PEEK sits at a price point that causes the client no small amount of shock.

It is important to talk about the price of PEEK before all it’s other characteristics, as this is usually the first thing the client want to discuss. Invariably, they come knowing that they need this polymer (PEEK), but knowing little else. They expect the price to be similar to Polyacetal or, at the very worst PTFE. When they find out that it is close to 10 times the price of PTFE, it comes as a huge surprise.

Why PEEK is expensive is not fully known. Perhaps it is because it has not yet reached the global scale of manufacture of more commoditized polymers, or perhaps the technology is so unique that it allows resin suppliers to charge a huge premium – knowing that alternatives are not available. As processors, we know only so much:

• The resin is 5-8 times more expensive than PTFE
• Processing PEEK is time consuming and expensive in comparison to PTFE
• Machining PEEK is tricky in comparison to other polymers

Since the resin prices are not in our control, we would like to look at points 2 and 3 and discuss them in more depth. But first, let’s get a better idea of what PEEK offers.

High tensile strength

In the polymer space, it would be tough to find something tougher than PEEK. It is so strong, in fact, that machining guidelines for PEEK need to follow the same as those for metals.

This strength allows PEEK to be used in applications such as gasketing and auto components – especially where metals cannot be used, but a metal-like durability is required

High temperature resistance

PEEK melts at about 400 Degrees Celsius and is capable of running in environments of 300-325 Degrees without deforming. While PTFE can withstand up to 250 Degrees, any pressure/ load on PTFE at this temperature will invariably cause deformation. In the case of PEEK, its hardness allows it to be in a high-load-high-temperature environment without loss of dimensional properties.

High wear resistance

Again, while both PTFE and UHMWPE can take a significant amount of wear, PEEK exhibits a high PV value and can withstand wear effects even under harsh physical and chemical conditions.

Chemical resistance

While not on the same level as PTFE for pure chemical inertness, PEEK exhibits resistance to many harsh chemicals, allowing it to be used in corrosive environments, under heavy loads

In a nutshell, PEEK’s ability to stay dimensionally stable under harsh environments makes it a highly sought after polymer. OEMs who use PEEK do so knowing well that for the properties offered, PEEK is unique and therefore expensive.

Processing PEEK

We will not delve very deep into the processing of PEEK (as this is a proprietary process unique to each processor), but we will point out the key differences between PEEK and PTFE processing (which has been looked at earlier). It should be noted that here we are referring only to compression moulding, and not injection moulding.

The main difference is that while PTFE is cold compression moulded and then loaded in batches into a sintering oven, PEEK needs to be sintered during compression itself.  Furthermore, post sintering, PEEK needs to go through an annealing process, which is time consuming. This leads to a few complications:

• Batch processing is difficult. Since the total heating cycle for a single piece can take up to 8 hours, and since heaters are expensive, PEEK is normally moulded a few pieces at a time. So unlike PTFE, where a batch of 8-10 large pieces can be moulded in series and then put in the oven for a single cycle, PEEK will offer only a few pieces in the same amount of time
• Since PEEK is heated under pressure, issues of flash can arise as the resin becomes molten, but has pressure being applied on it. Furthermore, the pressure and temperature have to be balanced very carefully, since the temperature makes the PEEK molten, allowing it to reach its desired shape, but the pressure is responsible for vacating air bubbles from the material, so that there is no porosity.
• Batch processing the PEEK parts for annealing is possible, but takes about 24 hours

So overall, the productivity in moulding PEEK is far below that of PTFE. This does answer, in part, the question of why the price of the finished material is so expensive.

Machining PEEK

As discussed above PEEK machines more like a metal than like a polymer. It is hard and has a significant impact on the tool. The same tool that might churn out 3000-4000 PTFE parts may struggle to churn out a few hundred PEEK parts. Again – this adds to the cost of the finished product significantly.

More importantly for machining though is that if the PEEK is not annealed properly, the part will behave erratically during machining as different areas within the material react differently to the stress being placed by the tool. Thus, cracks can develop during machining and the dimensional stability across a batch of components can vary significantly.

As a result, PEEK machining is a difficult process and there are few who are willing to take on the risks of machining such an expensive item, knowing that the rate of rejection could be very high.

In conclusion – PEEK has remained a largely niche polymer mainly due to its prohibitively high price. If it were cheaper – say around the price of PTFE – there are chances that it could steal a significant chunk of the PTFE market. PTFE still rates much higher than PEEK on characteristics like coefficient of friction and dielectric strength, but where it is a question of sheer strength, PEEK stands unmatched amongst polymers.

PTFE Prices – taking a step back to leap forward?

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So we’re back to pricing – because until they fully stabilize, we need to be on our guard. Considering the data below, one might be allowed to assume that things are finally easing out and that the sector is slowing reaching an equilibrium of sorts, coming off the highs seen in mid-2011 to rest at about US$24/Kg. But we would rather still be wary.

Since prices spiked in July 2011, there has been a decrease of about 13% in prices – which has been gradual. There are a number of reasons one can point to for this decrease – most of which we have already touched upon in our last article on pricing. To list them out:

1. Re-entering of China and Russia into European and Indian markets at competitive rates
2. Easing out of Fluorspar supplies due to opening of new mines and reduction in China’s domestic consumption

However we remain wary for 2 very specific reasons:

1. China’s summer approaches.
   
   In our very first article on pricing, we specifically highlighted the impact that the Chinese summer was having on PTFE prices. Summer months spike domestic demand for refrigerators and air conditioning and consequently cause R22 to be diverted from PTFE and into these products. This creates the shortage in R22 and was one of the root causes for the price escalations seen last year. However, we also postulated that once summer passes, the prices would ease out – which they have. But what now? Summer is about 2 months away and there is nothing to suggest that the rest of the world’s fluorspar mines can support the industry as yet. Our own sources indicated that it would be at least 2 years before the re-opening of mines in Mexico and South America eased the supply side constraints on fluorspar.
   
2. The PTFE industry is far from efficient
   In finance, we always assume that if an event (like China’s summer) is imminent and the effects of that event are known – then the prices of goods linked to the event should already reflect this information. In other words, if processors were aware that prices are going to spike during the Chinese summer, they would already have stockpiled raw materials to avoid against it, implying that there would be less demand during the summer and prices would not escalate again. However, this is unlikely to have happened since, (1) there are mixed opinions on whether the prices will go up or keep going down and (2) processors have already had to triple their working capital in order to keep up with the price increase in raw materials and it is unlikely that too many would have funds to stockpile materials for a full quarter. Therefore we remain nearly as exposed as we were last year.

But the news may not be all that bad. For one, China has been seriously implementing the R22 phase-out and as of August 2011 was even awarded a grant to speed up the efforts. Whether this phase-out sees any immediate impact on domestic demand remains to be seen and would possibly define the price of PTFE for the next few years. Secondly, there would be good reason to believe that PTFE resin manufacturers have hedged against such a scenario – even if the processors themselves are unable to do so. In India, our local producer has not only augmented PTFE resin capacities, but also become one of the foremost global producers of R22.

In a nutshell, the state of the future depends largely on the balancing of the Chinese summer against the precautions taken by resin manufacturers to safeguard against a further spike. I do not believe processors have any real part to play in all this – we remain, for the most part, price takers. If there is a fluctuation in prices, we would need to absorb it much the same as we did last year. 

PTFE machining considerations – tapping

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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!