Unravelling Polymers

The Definitive Blog on Polymers by Poly Fluoro Ltd.

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 tube 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.

Precision Machining Polymers - The Challenges Are Plenty

Over the past decade we have seen a rapid shift from conventional machining to CNC machining. While CNC machined parts used to be required only in the most critical of applications earlier, they are now the mainstay, with even simple items like washers being churned out in this fashion, rather than depending on the perceived unpredictability of a manual system.

In the polymer space, while the shift to CNC has also been essential, there have been several complications that have arisen. We look at these here, in a bid to better understand the nuances of precision polymer machining and show that it is not always as straightforward as machining metals.

  1. Grades and varieties

    The first thing to realize is that the term “polymer” is both broad and vague. As a company deep rooted in PTFE (Teflon) as our core product, our experience into other polymers taught us that the differences in each make the process of CNC machining that much more unique. Let’s take a look at how some of the high-performance plastics behave:

    1. PA 6/ PA 66 (Nylon or Polyamide) – Nylon machines easily, but due to its low melting point, the feed rate and RPM need to be optimized to ensure that burrs do not melt and stick to the part. Furthermore, the high moisture absorption of Nylon implies that coolants can rarely be used, as these would ‘swell’ the component, causing dimensional deviations
    2. UHMWPE – like nylons, UHMWPE also suffers from having a very low melting point. Furthermore, as UHMWPE needs to be compression moulded, the orientation of the molecules within the part are not always predictable. Achieving high tolerances on UHMWPE is not always possible as a result
    3. PEEKPEI (Ultem)PI (Kapton) – these polymers are able to withstand high-temperatures and can therefore be run at higher speeds. However, due to the crystalline nature of the internal structures, the more stress applied during machining, the higher chance that the parts will crack. PEEK especially requires a special annealing process before it can be machined. In the event that multiple operations are required on a PEEK part, the part may be re-annealed between operations to ensure that the stress build up does not cause the part to crack later on.

      PTFE Radomes for Radar Applications

      Bellows – Virgin PTFE

      PTFE Radomes for High Precison Radar Applications

      PTFE Bobbins – Virgin PTFE – Tolerance of 0.04mm

      PEEK Adaptors for Aerospace

      PEEK Piston – Tolerance of 0.025mm

      PEEK Back Up Rings – 15% Carbon Filled

      PEEK Adaptors for Aerospace

      Nylon 66 Bobbins for Aerospace

      The above examples are just a few of the peculiarities that each polymer brings. With polymers such as PTFE (Teflon)DelrinPVDF (Kynar) and PVC, we have found the machining to be more straightforward. However, as the complexity of the part increases and the tolerances become tighter, the level of care needed increases, along with an increased need to understand the internal structure of the material.

  1. Tolerances and dimensions

    We are often approached by other companies also involved in some form of polymer machining, requesting whether we have any excess demand that they can support us with. Our first question is always “what tolerances are you able to achieve”?”. The answer is usually between 0.05mm and 0.1mm.

    From our perspective this is not adequate. While it is true that polymers do not lend themselves to the dimensional stability of metals (where tolerances of up to 1 micron are sometimes demanded), we have found that with the proper programming and handling, polymers can be machined to achieve a consistent tolerance of within 10-20 microns.

    It is in this endeavour that we have put a lot of our focus and effort. It is also why having CNC machines is alone not enough to ensure the parts would be of the highest possible precision. Knowing the material and understanding how the part needs to be handled – both during and after the machining process is complete, is critical to be able to get that extra 30-40 microns in tolerance.

    The other complexity on dimensions relates to the strength of the material. The longer the component, the tougher it becomes to attain close tolerances at the end – as the material starts to bend slightly, throwing the dimensions off. Again, knowing what the polymer is capable of and machining in a way that minimizes the deflection that the material would experience is key to ensuring a consistently machined component.

  2. Volumes

    While polymer machined parts have certainly found their foothold across industries, the volumes remain tiny when compared with metals, or even some injection molded polymer components.

    One of our concerns when shifting to CNC machining, was whether we could justify the expense against the low volumes of parts required. Keep in mind that apart from the machine cost itself, there are the added expenses of labour and special tooling.

    Getting high-volume parts that also demand the criticality that we offer remains a crucial challenge.

    Overall, the intricacies of polymer machining make it a rewarding experience. To be able to attain industry leading levels of tolerance across a whole range of polymers is something we are very proud of. So while CNC machining technologies certainly helped us move ahead, what set us apart was the ability to take the precision machining of polymers up a notch.

PEEK: The Superman of Polymers

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:

  1. The resin is 5-8 times more expensive than PTFE
  2. Processing PEEK is time consuming and expensive in comparison to PTFE
  3. 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:

  1. 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.
  2. 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.
  3. 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 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.

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