Unravelling Polymers

The Definitive Blog on Polymers by Poly Fluoro Ltd.

Metal Replacement with PEEK

There is virtually no industry that does not strive for reductions in weight of the final product. In some cases, higher weight implies higher costs of the product itself; in some, weight reduction leads to improved efficiencies of the equipment.

Advancements in polymer science over the past few decades have allowed us to develop formulations that come close to mimicking the strengths offered by metals. While it is fair to say that on average, the strength of a polymer lags significantly against the toughness of metals, there are several applications where the polymer offers more than adequate strength, whilst bringing a host of other benefits that make it a more viable option.

We were recently approached by a client inquiring whether PEEK parts could replace the existing metal parts being used. This led to some research as to the compositions that would be needed to augment the tensile and yield strengths of PEEK beyond what it offered in its natural (virgin) state.

PEEK is a crystalline material that ranks as having amongst the highest tensile strengths within the polymer space. It is also a light-weight polymer, which makes it highly sought after in applications where weight reduction is crucial. In addition to this, the introduction of certain fillers allows the strength of PEEK to be enhanced significantly.

  SS 304 Virgin PEEK Reinforced PEEK Units
Ultimate Tensile Strength 515 95 586 MPa
Specific Gravity 8 1.3 1.52  
Youngs Modulus 190 3.65 57 GPa
Elongation at Break 55 60 1.6 %
Service Temperature 900 300 300 °C
         
Cost Index (SS=1) 1 32 20  
         
Adjusted Cost over SS 5.2 3.8  

 

The table above shows the improvements that can be made when adding reinforcements to PEEK when compared to Stainless Steel 304.

  1. It is apparent that we can exceed the tensile strength of steel, although this comes at the cost of very low elongation.
  2. The Young’s Modulus (or Tensile Modulus) of stainless steel remains significantly higher than that of reinforced PEEK. When combined with the data on elongation, this tells us that steel still has a higher tolerance to loads and will deform when the loads increase. Reinforced PEEK, however, will most likely crack when pressures of over 57 Gpa are applied
  3. At elevated temperatures, PEEK would still lag stainless steel, or indeed most metals. However, with a temperature rating of 300°C, PEEK is still sufficiently capable of handling most industrial applications
  4. PEEK is over 30 times the cost of stainless steel. Even with reinforcement – which brings the price of PEEK lower as the fillers are not as expensive as virgin PEEK – the cost is 20 times that of steel. However, when you factor in the reduction in densities, the numbers fall to 5.2x and 3.8x respectively
    The implication of this however is that replacing steel with PEEK would bring a four-fold cost increase. Hence the decision to replace cannot be based on cost savings.
  5. In addition to the basic properties listed above, PEEK offers a range of other benefits such as lower coefficients of friction, lower thermal conductivity, higher resistances to chemicals and electrical insulation.

The above data tells us why, while PEEK can certainly match up to stainless steel in some regards, the replacement is done in only very specialised fields.

  • Aerospace

    PEEK is used extensively in this industry, replacing steel and even aluminium in many cases. Since the cost of replacement is one-time, but the benefits if weight reduction are continuous, PEEK is the preferred material of choice in aerospace.

PEEK is also a safer material to use, since its low conductivity allows for lower heat build-up. Furthermore, if a part does break off and/or release debris – the resulting damage is minimised as the material is not hard enough to jam other metallic moving parts.

  • Healthcare and Pharma

Being chemically inert, PEEK is preferred to metals in areas where chemicals may be present and the reaction to metals may not be fully understood or predictable.

Medical devices may also use PEEK as the equipment may benefit from being light weight. Certain scanning devices shun metallic parts, as they may affect the readings. Here again, PEEK is used to ensure the device is physically durable but does not have too much metal in its structure

  • Oil and Gas

PEEK is tough enough to withstand some of the harshest forces, while offering a resistance to corrosive fluids and high temperatures. In most cases, oil & gas equipment is placed deep underground, where the forces and chemicals are not fully predictable.  Oil and gas equipment frequently employs PEEK in seals and valve plates to ensure that there is no failure once the equipment is installed.

  • Precision Instrumentation

PEEK offers an elevated level of structural rigidity, while providing electrical insulation. Connector covers, transducer covers, and cable channels are made using PEEK because it does not interfere with the electrical signals being passed through the wires within the device. Again, as weight reduction is a key criterion in this industry, PEEK is preferred even though it is expensive.

  • Food processing

Several factors make PEEK the material of choice in food processing. For one, the material does not react with or impart any flavour to the food. For example – coffee machines that traditionally used aluminium valves replaced them with PEEK, because the aluminium imparts a metallic taste to the coffee. In addition to this, at high RPMs, PEEK offers minimal heat build-up. In food processing appliances, where the body may be made with a low-temperature plastic and the designs tend to be compact, it is essential that moving parts do not become so hot as to warp/damage the body of the equipment. Finally, since liquids are often present in food preparation, PEEK is preferred as it is less likely to corrode and/or cause an electrical short circuit within the device.

It is apparent that PEEK is not a low-cost alternative to metals. However, applications that look for efficiency and performance enhancements can benefit tremendously from using PEEK and not worry that its strength would not be sufficient. Since the replacement of metal with PEEK is a one-time expense in most cases, the consequent savings from the improved efficiency of the system would need to be weighed in to get a true measure of the overall cost savings.

Expanded PTFE (ePTFE) Tapes in Aerospace

Some of the most complex engineering done today revolves around the aerospace industry. Keeping a plane, satellite, or rocket airborne is a challenge involving thousands of variables, all of which keep changing during the course of operations. The equipment needs to be robust and capable of maintaining accurate, predictable performance over a long period of time. In addition to this, efficiency calls for the need to keep things light-weight.

This demanding combination of low-weight and high-strength has led to the invention of some of the most exceptional materials in recent times. Polymers such as PEEK and Polyimides have been used with reinforcement to offer metal-like levels of tensile properties, while reducing the weight by as much as 85%.

Among the most useful such materials is expanded PTFE (ePTFE).

While PTFE alone is a useful polymer in aerospace – finding applications in radar (radomes), fluid transfer (tubes, bobbins), and insulation (tapes, films) – expanded PTFE offers a range of new properties that augment the usefulness of this material further.

We look here at some of the areas of application of ePTFE tapes within the aerospace domain:

  1. Sealing function

    ePTFE has a soft, foam-like structure that easily adopts the shape of the material around it. As such, it can be sandwiched between any two harder elements to create a perfect seal instantly. ePTFE is so pliable that the seal can be created with minimal torque. The seal is able to perform even at high-pressures, meaning there is little risk that a good seal at ground level, will suddenly fail when pressures start increasing.

    In addition to the seal-ability of the material, it is also resistant to both high and low temperatures (-250°C to +250°C) and chemically inert. Again – the idea that the seal will cease to perform at extreme environmental conditions is completely mitigated.

    ePTFE tapes are usually used in areas where a permanent seal is needed and where the assembly is not likely to be disturbed frequently. It is a vital part of both build and maintenance operations, since it is available as a roll and can be very easily used on-site.

     

     

  2. Anti-squeak / anti-chafe function

    Another frequent problem with aerospace is that even with the most modern design techniques, it is impossible to fully predict which components will vibrate during operation. Vibrating parts can rub against one another causing wear outs and noise. Expanded PTFE tapes are an ideal medium to use between rubbing parts. Not only does the tape squeeze down to accommodate the exact gap between the parts, but it’s low coefficient of friction eliminates any damage that the parts can do to one another and any noise that might be created.

    ePTFE is also light-weight (the density can be as low as 0.35g/cm3), so it can be used without any worry that you would be adding load to the system.

     

     

  3. Anti-corrosion function

    In areas with heavy exposure to fuels and aviation oils, ePTFE can be used as an effective protector. Being oleophobic, ePTFE repel oils and can therefore form an effective layer over areas that would otherwise be corroded by repeated contact with oils and fuels that may splash on to it.

    The material is particularly useful in access panels and fairings where hydraulic or engine oil contamination could occur.

Apart from the areas cited about, ePTFE finds plenty of uses across both the maintenance and manufacture of the equipment. Its unique properties combine seal-ability, temperature and chemical resistance, abrasion resistance and a high strength-to-weight ratio, making it one of the most versatile materials to be used in the field of aerospace.

PTFE in Semiconductor Manufacturing

The importance of semiconductors in our daily lives cannot be overstated. Whether we realize it or not, the devices that constantly run in the background, allowing us to carry on with our regular routines are packed with electronics that rely on the semiconductor to ensure they perform properly.

As we try and scale down our devices, making them even more compact, the semiconductor too needs to reduce in size, while not compromising on any of the functionality. Manufacturing these semiconductors is an industry unto itself. Some of the world’s largest companies – such as Intel, Samsung and Qualcomm, have made semiconductor manufacturing their mainstay. In the race to stay ever relevant in a field where 18-24 months is seen as the average product lifecycle, there is an increasing need for materials that support the manufacturing process.

The impact of PTFE (Teflon) in semiconductor manufacturing

Of the many variables that exist in the manufacturing process for semiconductors, one factor that is of utmost importance is purity. The entire manufacturing line, and indeed the final product, rests on the ability to maintain absolute purity throughout the process.

The use of PTFE in this industry has been a revolution in recent times. While PTFE has many properties, which we have outlined across other articles, the one that stands out with regards to semiconductors is its chemical inertness. This same property allows PTFE to also be used in medical devices and in laboratory analysers. Because PTFE is only affected by molten alkalis at elevated temperatures, it allows chip manufacturers to apply it in nearly every aspect of the manufacturing process, safe in the knowledge that it will not impart any contaminants nor react with any of the myriad chemicals that it would come into contact with during the process.

Some of the key areas that PTFE is used would include:

1. Piping and Tubing
For deionized water and chemicals, piping of highly pure PTFE allows the passage of fluids without the risk of reacting with them. Pipes and tubes can be welded or joint by flaring to ensure that the entire line is pure PTFE.
PTFE tubes are also resistant to temperatures (both high and low) and offer a burst pressure unparalleled within the polymer space.

2. Fluid-Handling Components
Valves and fittings made with pure PTFE can be used to regulate the flow of fluids through the system. PTFE can be machined to ensure that the required valve design is met. Similarly, PTFE seals can be employed to arrest any leakages.
Traditionally, semiconductor manufacturers used quartz components in such applications. PTFE offers the added advantage of being more durable and resistant to cracking. This allows for less maintenance and lower down times.

3. Baths and Sinks
Any chemical bath benefits from being lined with PTFE. Vessels and columns for reactions, distillations, absorptions and other processes can be made of steel or reinforced plastic lined with PTFE. The inside of the vessel can be either coated with PTFE or lined using a PTFE sheet. The sheet is then either welded or jointed at the edges to ensure that any fluid within the bath does not come in contact with the steel. This process requires careful fabrication, as PTFE is a difficult material to weld. Care needs to be taken to ensure that the weld or joint does not offer any chance for leakage, even in the long term

4. Wafer Carriers
PTFE can be moulded to form carriers that take the wafers forward after wet processing. As it is imperative that the wafers remain 100% free from any contaminants, PTFE is the ideal material for this application.
Wafer carriers and dippers come in many variants and require a high degree of machining capability. Since PTFE cannot be injection moulded, the parts are machined from a block and adhere to strict parameters on both dimensional tolerance and material purity.

5. Sensors
Capacitive-type sensors shielded in dip tubes of high-purity PTFE are used to monitor fluid levels in tanks. Other designs can sense through walls of tanks or pipe made from PTFE. Connecting electrical cables are jacketed with PTFE for additional protection against contamination.

6. Filters
Filters made from PTFE and/or ePTFE are ideal for removing particulates from liquids and gases and as venting membranes in chemical chambers. Effective filtration allows the system to remain more efficient, as the regular removal of contaminants ensures the line does not need to stop for constant maintenance.

While we have outlined only the main applications of PTFE in semiconductor manufacturing, there are no doubt many more areas where the material can find use. Fundamentally, the property of chemical inertness makes it an invaluable material to insert into any process where purity is important.

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