In the realm of high-performance polymers, PEEK (Polyether Ether Ketone) and Polyimide stand out as two exceptional materials. Both are undoubtably among the toughest polymers, exhibiting tensile and flexural strengths far higher than even their nearest competitors. Given this, it is easy to see them as substitutes for one another and for an engineer to be confused over which one to choose in a given application. 

Comparing chemical structure:

PEEK is a semi-crystalline thermoplastic known for its excellent mechanical properties, chemical resistance, and high-temperature stability. Its molecular structure imparts exceptional resistance to chemicals, abrasion, and wear. 

On the other hand, Polyimide (often known by its brand names of Vespel or Kapton) is a high-performance polymer with a unique imide linkage in its molecular structure. This arrangement contributes to outstanding thermal stability, excellent dielectric properties, and exceptional resistance to radiation and chemicals. 

Mechanical Properties:

When it comes to mechanical properties, PEEK and polyimide both display distinct characteristics. PEEK offers a combination of high strength, stiffness, and toughness. Its tensile strength and modulus are comparable to some metals, making it a preferred choice in structural applications where mechanical integrity is crucial. PEEK's inherent toughness allows it to withstand repeated loading and impact without sacrificing performance. PEEK can also be enhanced with the addition of Glass, Carbon, and Graphite (Carbon-Graphite reinforced PEEK, also called HPV PEEK, is among the toughest polymer compounds known), which adds to PEEK’s versatility.

Polyimide, while not as stiff as PEEK, excels in maintaining its mechanical properties at elevated temperatures. Its ability to withstand prolonged exposure to high temperatures without significant degradation makes polyimide suitable for aerospace, electronics, and automotive applications where a combination of thermal stability at elevated loads is paramount.

When it comes to wear resistance, polyimide take the edge, as it exhibits a slightly lower coefficient of friction. While PEEK can be improved with the addition of PTFE, the base wear rate of polyimide is both low and constant over a range of loads. This means that in dry-running applications it is a better choice.

Thermal Stability:

Thermal stability is a key consideration in many high-performance applications, and both PEEK and polyimide offer exceptional heat resistance. PEEK is known for its thermal stability up to 260°C, making it suitable for applications in aerospace, automotive, and oil and gas industries. However, polyimide surpasses PEEK in terms of thermal stability, with some formulations capable of withstanding temperatures exceeding 300°C. This makes Polyimide the material of choice in extreme temperature environments such as electronics and aerospace applications.

PEEK boasts a high glass transition temperature (Tg) of around 143°C, making it suitable for applications in demanding thermal environments. However, polyimides typically exhibit a higher glass transition temperature than PEEK, often exceeding 250°C, making them more ideal for applications demanding extreme temperature resistance.

Chemical Resistance:

Chemical resistance is another critical factor in material selection, especially in harsh operating conditions. PEEK exhibits excellent resistance to a wide range of chemicals, including acids, bases, and hydrocarbons. It is also biocompatible. This makes it a preferred choice in chemical processing, medical, and oil and gas applications. Polyimide, with its unique molecular structure, provides outstanding chemical resistance, particularly against solvents, acids, and radiation. This property makes Polyimide suitable for applications in the aerospace, electronics, and semiconductor industries.

Cost and processing considerations

When it comes to cost, however, there is little need to compare the two. Polyimide’s key drawback is that it is prohibitively expensive. Polyimide is roughly 3-4X the cost of PEEK, which is significant considering that PEEK itself is about 20-25X the cost of more basic polymers such as POM or Nylons. Hence, most parts made from polyimides tend to be smaller and used sparingly in applications where PEEK does not make the cut.

Processing-wise, the other drawback of polyimide is that it cannot be injection moulded. It can only be compression moulded or extruded as a rod. This limits the complexity of parts that can be made when compared with PEEK. However, when we speak of compression moulding specifically, polyimide is more versatile and comparatively easier to process. While PEEK only lends itself to hot compression moulding (where the pressure and temperatures must act simultaneously), polyimide can also be cold compression moulded in a manner similar to PTFE. This allows for a much higher productivity, wherein material can be compressed and placed in an oven in batches, rather than moulded one at a time the way PEEK needs to be.

Conclusion

While it is fair to compare PEEK and Polyimide, the key consideration of cost means that PEEK usually wins out. However, there are certain applications where only Polyimide can be used and where cost may not be the biggest concern. It is likely that as polyimide gets cheaper (as polymers invariably do over time), the use cases will rise, and PEEK will have a worthy competitor.

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1. PTFE Extrusion - Ram vs Paste Extruded - A comparison of features

2. Exploring the Versatile World of PVDF

3. Air Permeability Testing and Water Entry Pressure Testing in Expanded PTFE Membranes

The versatility of PVDF as an engineering plastic is well known. Indeed, we have covered the advantages, properties, and applications of PVDF in an earlier article. Properties such as extreme chemical resistance, UV resistance, thermal stability, and piezoelectricity all make PVDF an intriguing polymer that finds application across a host of different industries.

Our own experience with PVDF has thus far stayed within the realm of machining. PVDF rods and sheets are available and can be easily machined to make a final component. The polymer itself throws up no surprises with regards to how it behaves dimensionally, post-machining.

Recently, however, we have encountered a challenging prospect. The part we were asked to develop was far too large to be machined from a rod or a sheet. With an outer diameter of about 200mm and an inner diameter of 130mm, the part would have caused far too much waste were it to be cut from a sheet or a rod. The option, therefore, would be to compression mould it.

Given our expertise in PTFE and PEEK moulding, we assumed that PVDF – whose melting temperature is far less than either PTFE or PEEK – would be simple enough to mould.  We knew from discussions with our suppliers that the equipment we use for PEEK moulding could be easily used for PVDF, provided the processing parameters were adjusted accordingly.

Our first experience with the moulding convinced us that this was a fairly simple affair. We moulded a small rod of 100mm diameter and 50mm thickness and found the part to be uniformly coloured (PVDF should be milky white), with no signs of any blowholes. Skived sections taken from the rod confirmed that the tensile properties were in line with what was expected, while the specific gravity was also in the range of 1.8, as it should have been.

Moving from the test sample to the part we needed to mould showed us that we may have underestimated the material. A few challenges were immediately apparent:

1. The material was extremely sensitive to temperature. While the 100mm sample appeared to have formed easily under a temperature of 210°C, the larger part was getting discoloured and turning brown.

2. The melt flow of the material was challenging to control. If the temperature was held for too long, the viscosity of the material would reduce and cause it to leak from the mould. If the temperature was not held long enough, the part would come out with blowholes, having not been sufficiently melted throughout.

3. Similarly, too much pressure would cause material leakage, while too little pressure would not allow all the air to be expelled, resulting in blowholes.

In effect, moulding PVDF turned into a very precise give and take between temperature, dwell time, and pressure. Furthermore, although the material could be re-melted, doing so would discolour the polymer, rendering it useless. This meant that all parameters needed to be precise and that a cycle could be run only once else the material would be lost. (Incidentally, we are not new to this conundrum. PTFE behaves in much the same way, only we have decades of experience with PTFE and know how to get it right every time!).

Once we had moulded the part, the time came to machining it. Again, although our experience with machining PVDF had always been smooth, here too we observed that compression moulded PVDF behaves slightly differently post machining. Stresses in the material tend to relax overnight, causing slight deviations in dimension. Hence, adjustments needed to be made to the machining process to allow for the same.

There is a reason that engineering polymers are a niche space and that so few have the expertise to consistently manufacture certain high-performance plastics. We pride ourselves in being able to understand our polymers and to investing the time it takes to develop them.

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1. Case Study - PEEK in Coffee Machines

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In the ever-evolving world of coffee culture, enthusiasts and professionals alike constantly seek innovations to enhance the brewing experience. One such technological marvel making waves in the coffee industry is the use of PEEK (polyether ether ketone) valves in coffee machines. These valves, though small in size, play a significant role in ensuring a superior and consistent cup of coffee.

Our first introduction to this unusual application of PEEK happened about ten years ago. A manufacturer of high-end coffee equipment came to us with a dilemma. They had been using aluminium valves in their equipment for a while and had never faced any problems. However, as an increasing number of Indians travelled abroad and experienced the flavours of western-brewed coffee, the complaints had started to come in. The issue: the coffee tasted metallic.

The client had done their own research and found out that the Italian coffee machines had replaced aluminium with PEEK.

PEEK is a high-performance thermoplastic known for its exceptional mechanical and chemical properties. Its use in coffee machines brings a range of benefits that contribute to the overall efficiency and quality of the brewing process.

First and foremost, PEEK valves excel in temperature resistance, making them ideal for the hot and demanding environment of coffee machines. Unlike traditional materials that may degrade or lose integrity under high temperatures, PEEK valves maintain their structural integrity, ensuring a reliable and durable component in the coffee brewing system. This resistance to heat is crucial for consistent coffee extraction and flavour preservation. PEEK has a service temperature of 275°C and is therefore more than capable of withstanding the heat within the equipment.

Another notable feature of PEEK valves is their resistance to chemicals and corrosion. Coffee machines often come into contact with various substances, including minerals in water and coffee residues. PEEK's resistance to corrosion ensures that the valves remain unaffected by these elements, leading to a longer lifespan for the coffee machine and reduced maintenance requirements. This not only benefits coffee enthusiasts by providing a more reliable machine but also contributes to sustainability by reducing the need for frequent replacements.

Precision is paramount in the world of specialty coffee, where every parameter matters. PEEK valves offer a high level of precision in controlling the flow of water and steam in coffee machines. This precision allows for fine-tuning of the brewing process, enabling baristas and coffee enthusiasts to achieve the desired extraction profiles. The ability to control water flow with accuracy contributes to the consistency of flavour and aroma in each cup of coffee, a key factor in the pursuit of brewing excellence.

In addition to their mechanical properties, PEEK valves are preferred for their biocompatibility. This characteristic is particularly important in the food and beverage industry, where materials that come into contact with consumables must meet stringent safety standards. PEEK's biocompatibility ensures that it poses no risk of contaminating the coffee with harmful substances, meeting the highest hygiene and safety standards.

The incorporation of PEEK valves revolutionised our client’s coffee machines and allowed them to even build their export business. It should be mentioned that in shifting from Aluminium to PEEK, the client saw the part cost shoot up by a factor of 10 (PEEK is an expensive polymer!). The fact that they still chose to use the PEEK component tells us how vital the material was in ensuring the end product was exactly as needed.

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1. Polymers in Low Friction Applications

2. PTFE Extrusion - Ram vs Paste Extruded - A comparison of features

3. Exploring the Versatile World of PVDF