Unravelling Polymers

The Definitive Blog on Polymers by Poly Fluoro Ltd.

PTFE Compounds and their effects

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!

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

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

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

1. Moulding

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

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

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

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

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

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

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

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

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

Sintering

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

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

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

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

Machining

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

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

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

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

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

Scrap Recovery

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

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

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

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

What you need to know about PTFE structural bearings

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

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

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

 

 

 

 

 

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

PTFE (Teflon) in Bearing Uses in Large Structures

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

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

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

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

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

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

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

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

PTFE 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 (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.

The prescribed loads on the PTFE (Teflon) material are more or less uniform across the different codes. Designing the bearing requires an understanding of what the PTFE material can withstand and calculating the sizes and thicknesses of the steel members around this. 

Ultimately however, the bearing manufacturer undertakes the responsibility to design the bearings based on customer specifications. In our own experience, it is rare that the same sliding bearing arrangement has ever been used in two separate projects. This illustrates the uniqueness of the PTFE bearing and places a huge onus on the manufacturer to ensure that they capture all elements of the material’s capabilities when designing a new bearing.