Unravelling Polymers

The Definitive Blog on Polymers by Poly Fluoro Ltd.

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 realise 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 ever 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 Qualcom, 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 deionised 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.

The Effectiveness of PEEK Compressor Valve Platessss

The use of reciprocating compressors has become a mainstay for most industries. They form not only a vital part of any pneumatic or gas line, but also feature in areas like refrigeration as the key part of the equipment or appliance.

Like most equipment, we seek constantly to limit the failure and downtime. While the break-down of a compressor can seriously hinder operations within an industrial outfit, failure within a consumer appliance can be very problematic, as we usually do not expect the compressor in a refrigerator or air conditioner to fail during the life of the product. Furthermore, replacement of the compressor is expensive and time consuming.

Analysis into the durability this equipment yielded the failure of the compressor valve plate as one of the main causes for breakdown.

The compressor valve plate sits at the heart of the reciprocating arrangement and is responsible for taking a lot of the loads associated with the compressor function. In addition to this, it is in direct contact with the air, gas or fluids running through the compressor and is subject to both corrosive forces as well as high pressures and temperatures.

Materials used in compressor valve plates

Traditionally, metals were used in this application. While metals are inexpensive, can withstand high impacts over a sustained period and are not affected by pressure or temperature, there were always concerns about corrosion. As newer fluids were developed, it became imperative to use materials that would not react in any way to a variety of chemicals that may pass through the compressor. Corrosion of the metallic compressor plates could also lead to chipping and fracture. While a fractured plate could be replaced ad hoc, any metal debris coming off the plate could find itself elsewhere within the equipment and cause additional damage.

Metal plates also do not perform well with ingress. A piece of dirt or grit – often unavoidable in industrial applications – can become lodged within the equipment, as a metal plate will not yield or absorb the particles into itself.

Polymers have been adopted as compressor valve plates from the 1960s. However, the hunt for the right polymer took nearly two decades more. The advantages of polymers became apparent once nylon was introduced. The key benefits were:

  1. Adequate impact strength
  2. Resistant to chemicals and temperatures up to 120°C
  3. Lightweight and inexpensive
  4. Better lubrication
  5. Easier to manage debris that may fall into equipment
  6. Ingress often absorbed into the material

While these advantages were understood, there were also other issues that prevailed, leading manufacturers to constantly look for better replacements.

Nylon

Initially, glass reinforced nylon was introduced. The key drawbacks here were temperature resistance and moisture absorption. Even with glass reinforcement, nylon still has a moisture absorption of close to 1%. This clearly causes issues as the valve plate swells and deviates dimensionally when exposed to moisture over a prolonged period.

At elevated temperatures nylon begins to degrade and can be attacked by acids and bases. Nylon also becomes brittle in oxygen rich environments and as a result does not perform well in hot, moist gas service.

Polyetherimide (Ultem)

While Ultem clearly has the tensile strength, flexural modulus and temperature resistance needed, it suffers from issues with dimensional stability. Fundamentally, Ultem is an amorphous material – meaning its molecules are not locked into each other and can therefore move or slide over one another when under stress. This property means that over the life of the equipment, Ultem was unable to maintain serviceability.

Polyamideimide (Torlon)

Like Ultem, Torlon too checked a lot of the main boxes needed for this application, such as strength and overall durability. However, like Ultem, Tolron is also amorphous and like Nylon, it has a relatively high water absorption. The crystalline properties of Tolron can be increased by special processing, such that the molecules lock more rigidly. However, under heat, any absorbed moisture tends to swell the material from the inside, causing dimensional deviations in the long term.

PEEK – the ultimate choice for compressor valve plates

By the 1980s, PEEK had been around long enough and tested across enough applications to confirm that it was a potentially great candidate for compressor valve plates.

While PEEK exhibits a tensile strength at par with the toughest of polymers (reaching, in some compositions, to nearly metal-like levels of toughness), its crystalline structure ensures a level of dimensional stability even after years of operation.

PEEK is resistant to temperatures of up to 300°C, which goes well beyond what might be needed in this application. It is also resistant to a wide variety of chemicals and is affected only by concentrated nitric and sulphuric acids, which are highly unlikely to be present within a compressor.

PEEK’s water absorption, at 0.06%, is also far less than that of any of the above polymers.

The addition of fillers such as glass and carbon only enhance the properties of PEEK further, making it the right material for this application.

Overall, the introduction of PEEK into this application has seen a vast improvement in efficiency and durability, leading to reductions in maintenance costs and machine down time.

Strength to weight ratio

 

Impact Ratio

 

Water strength ratio