Unravelling Polymers

The Definitive Blog on Polymers by Poly Fluoro Ltd.

The effects of Carbon Nanotubes on PTFE Properties

It has long been known that the properties of PTFE, while diverse and highly sought after, can be further enhanced with the addition of fillers. Given the high-melting point of PTFE (>350°C), most such fillers need to be stable at high temperatures in order to maintain their integrity during the sintering (curing) process that takes PTFE from a raw state into its final, workable form.

Over the years, fillers such as glass, bronze, stainless steel and carbon have become commonly used. Each of these fillers – used anywhere between 5% and 60% by volume - adds something to the property of the PTFE, while also taking away. For example, the addition of bronze greatly enhances the wear resistance of the material but increases the coefficient of friction and also renders the PTFE unsuitable for any electrical applications.

Over the past few months, we have been working extensively with carbon filled PTFE, in an attempt to improve the anti-static properties of the material. Being an excellent electrical insulator, PTFE tends to allow static charges to build up on its surface, rather than allow any vent for the charge to dissipate. While this property is useful in applications where high voltage loads need to be contained, it poses some problems in areas where a spark – caused when the static charge reaches some critical mass - can cause other materials around the PTFE to ignite. In order to dissipate the static charge through the material, carbon fillers are usually added. The resulting material retains much of the dielectric properties of PTFE but gives just enough conductivity within the material to allow static charges to dissipate. Such anti-static materials have become popular, calling for both machined components and PTFE tubes to be made using the compound.

The quantum and variants of carbon used in such compounds can offer differing values for the surface resistivity of the material. The standard grade of PTFE+Carbon employs a 25% content of the filler by volume. By most accounts, this is rather high. In addition to being a little too conductive, the compound also tends to “shed” some quantum of carbon in the form of microparticles. This can cause big problems in precision equipment, where clean-room levels of non-contamination are called for both within and around the apparatus. As a result, compounds have been developed using Vulcan.

Vulcan is a form of specialty carbon black which can be used in percentages as low as 2% to induce conductivity in PTFE. The resulting material is a lot softer and smoother than PTFE+25% Carbon – which, apart from being less ‘clean’, is also tougher to the machine – and is ideal for electrical applications calling for anti-static properties in sanitized environments.

Carbon nanotubes

We recently used Vulcan to manufacture anti-static tapes but were informed that the tests were unsuccessful. The client’s application called for a surface resistivity of 1 x 104 Ohms, whereas our material was only able to offer 1 x 103 ohms. The failure led us down a path of discovery to see what other materials could be added to further improve the properties as required.

Many new studies have pointed to the addition of carbon nanotubes to achieve the surface resistivity required.

Carbon nanotubes have been around for a while and its property as a high-strength to weight material has made it the stuff of the future. In addition to its impressive load bearing properties, the material also acts as a semiconductor. Although the applications into PTFE are relatively new, studies have prescribed its addition as a substitute for carbon black. According to these studies, the addition of as little as 1% of carbon nanotubes into PTFE can offer a surface resistivity of 1 x 104 Ohms, as required by the application.

In addition to the electrical properties, fillers of 7-10% have been shown to improve the wear properties of the material significantly. Another study looks at a composition of PTFE+20% Ekonol+1% CNT, claiming that at this composition, the tensile properties and elongation reach an optimum.

The mixture of carbon nanotubes to PTFE as a dispersion allow for enhanced corrosion resistance and deliver a superhydrophobic layer.

In the wider scheme of things, both PTFE and carbon nanotubes are relatively new materials. We are still discovering properties of each and there is no doubt we will experiment extensively with different compositions to see what effects arise.

PTFE Bellows - Machining Challenges & Durability Tests

Bellows are among the most complex components that can be machined from PTFE (Teflon). Owing to the nature of PTFE, there exist several limitations to making the bellow by conventional, melt-processable means. However, the properties of PTFE make it invaluable in certain applications, requiring the bellows to be machined from solid PTFE.

For the most part, high-performance bellows are made using rubber or equivalent elastomers. The limitations of rubber include high temperatures and resistance to corrosion. It is likely that PTFE is only used in very specialised circumstances both because of cost and complexity.

The challenges for machining PTFE bellows include:

  1. The bellow often needs to be machined out of a solid rod or bush. The process can result in wastage of up to 80% in some cases
     
  2. The process of creating the convolutions needed to form the bellow requires special tooling and high-precision CNC machining. The smallest deviation in the machining process can result in a complete loss of the whole component. In cases where the final part is large, this represents a high risk of loss due to wastage and rejection
     
  3. The raw material needed for making bellows needs to be of the highest purity and the final rod or bush that is moulded would need to be completely free from any micro-faults, such as cracks, inclusions or discoloration. For the best quality bellows, grades such as DuPont NXT or 3M TFM are used. More recently, Inoflon has come out with a resin grade – M690, which also works well in making bellows. These grades are modified grades, having a high purity and capable of more flexibility
     
  4. The final produce needs to have total uniformity and be completely free from any irregularities in colour or dimension. Even the smallest deviation in wall thickness or the slightest micro-crack in the walls of the bellow can result in complete failure during operation.
     
  5. The design of the bellow itself needs to accommodate the properties of PTFE. For instance, it may not be able to have a wall thickness that goes below 0.5mm. As a result, the maximum compressibility of the bellow is limited when compared with an elastomeric bellow. Not only are elastomers more elastic, but their melt processability allows for much thinner cross sections, meaning more compressibility

Despite these limitations, when applications require high corrosion or temperature resistance, the only option is to use PTFE (Teflon). Hence, designers work around the limitations of the material to create a bellow that would work in their conditions.

If properly manufactured, a PTFE (Teflon) bellow can withstand up to 2,000,000 cycles. Since most requirements call for anywhere between 500,000 and 1,000,000 cycles for the life of the bellow, this metric allows for a sufficient cushion.

Once manufactured, a bellow is subjected to a series of tests to confirm its long-term durability during application.

  1. Burst pressure test
    The burst pressure of a PTFE bellows shall be at least four times the design pressure given by the manufacturer, after it has been subjected to 2000 cycles at 10 cycles per minute between its maximum axial extensions. The pressure and related temperature at which cycling is carried out shall be selected from the pressure/temperature graph supplied by the manufacturer. The pressure to produce failure shall be applied uniformly at such a rate that failure occurs within 5 minutes. As a minimum two tests shall be carried out; one at ambient temperature and the other at 180 °C.
     
  2. Cycle testing
    No failure shall occur when a PTFE bellows is subjected to 100 000 cycles at 10 cycles per minute between its maximum axial extension, or a combination of axial and lateral extension, at a pressure and temperature selected from the pressure/temperature graph supplied by the manufacturer.
     
  3. Temperature test
    The bellows shall be bolted to a mating flange and held at a temperature of 260 °C for 2 hours. After cooling, the PTFE flange face shall be examined to ensure no deformation or damage.
     
  4. Dimensional inspection
    PTFE bellows shall be subjected to dimensional inspection, visual examination, liquid penetrant examination, holiday testing (if necessary) and a hydrostatic pressure test. A burst pressure test and a pressure shock test shall be applied if the bellows ordered are to be used for critical applications (this to be specified by the Principal).

Because of the extensive testing and care needed during manufacture, the cost of a PTFE (Teflon) Bellow does tend to be many times that of its elastomeric counterpart. As a result, these bellows are only used sparingly, in applications where there is no option but to use PTFE.

 

Over-moulding PTFE on to Stainless Steel

As polymers go, PTFE is not the easiest material to deal with. It behaves contrary to nearly every other plastic and requires special processes to create even the simplest of forms. Whether we look at the extrusion of PTFE tubes or the forming of PTFE films (both rather straightforward when we consider melt-processable polymers), the methods we need to employ for PTFE are a practically standalone and need to be understood and developed from first principles.

Similarly, over moulding a plastic onto a metal part is not a very complex task when we look at injection moulding. In such a process, the metal part is inserted within the injection moulding die and molten polymer is injected and cools around it.

However, there is limited scope to follow this process when trying to over-mould PTFE onto a metal object. The limitations are the following:

  1. As PTFE cannot be melted, there is no way to form the polymer around the metal part in any way that would be uniform and consistent using heat alone
     
  2. Since PTFE can only be compression moulded, the metal part would need to be kept within a compression moulding die and the dry PTFE powder would need to be packed around it. However, as PTFE is very sensitive to the amount of pressure being applied during compression, it is essential that the metal part does not have too many contours, as this would lead to an irregularity in the compression.

    It is possible, in the event of a more complex metal part, that isostatic moulding is used to ensure there is even pressure on the PTFE powder during compression. However, as isostatic moulding requires a lot of die and mould costs, such a process could only be justified if the volumes are significant.
     
  3. Even if we do manage to pack the PTFE around the metal uniformly, the final issue remains concerning the heat cycle.

    PTFE is sintered (cured) at temperatures of around 375°C over a period of anywhere between 15 and 100 hours – depending on the size of the part. This heat cycle means that within the oven, both the PTFE and the metal are subject to high temperatures. Here the challenge is to match the thermal expansion and contraction of the PTFE and the metal, so that mismatches in the rates of expansion do no cause the PTFE to crack.
     

Overcoming these challenges required a lot of R&D. Different combinations of pressures and heat cycles were employed until we were able to consistency achieve a part that would not crack. The moulded part also needed to be machined, which meant the part needed to be free from any internal irregularities as well.

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