Unravelling Polymers

The Definitive Blog on Polymers by Poly Fluoro Ltd.

PEEK - Robust Enough for Nuclear Applications

Among the most rigours manmade environments ever created, the inside of a nuclear reactor would probably rate high on the list. The process that allows the reactor to generate the kind of energy we need is not only complex in theory but poses huge practical hurdles when it comes to containment and durability.

Apart from the energy release, nuclear reactions also give off radiation – most notably gamma radiation – which most standard materials are unable to withstand. Gamma radiation causes other materials to get ionised, which in turn makes them hazardous for human exposure. It also accelerates the degradation of materials, causing brittleness and fractures that can increase the likelihood of leaks and failures.

PEEK has long been known as a polymer with unparalleled strength. We have compared earlier the tensile strengths, tensile modulus, heat resistance and chemical resistance of PEEK. In addition to this, the material can reach strengths comparable with metals, when used with the right kinds of fillers. Adding to this list is the capability of PEEK to dwell within high-radiation environments without suffering any significant losses in properties.

Studies have shown that when exposed to the harsh environments seen within the nuclear reactor chamber, PEEK is able to survive over an extended period, where other materials would easily fail. Most notably, the effect of gamma radiation on PEEK is low, meaning that PEEK becomes the material of choice for a host of applications within the nuclear reactor.

  1. PEEK Seals
    The performance of seals to contain liquids that flow within the reactor is of utmost importance. Most materials would suffer from brittleness and deformation when subjected to radiation and heat from this system. PEEK seals have been shown to hold their shape over time and tests show that the hardness and crystallinity of the material is not severely affected by the conditions.

  2. PEEK linings
    Vats and chambers within the nuclear reactor have relied traditionally on using linings made from heavy metals such as lead and titanium. This is both expensive and makes the process of holding and disposing of nuclear waster very cumbersome. PEEK linings have been shown to be very effective in containing radiation. It is possible to make thinner walled tanks and have them lined or coated with a thin layer of PEEK.

  3. PEEK Valves, Plates and Nozzles
    Given the quantum of fluids within the reactor, PEEK is now replacing many of the standard valves, plates and nozzles to ensure that key components within the system do not fail. PEEK is both highly machinable and injection mouldable. Further, PEEK can be 3D printed to create specialised shapes that fit perfectly within an existing system. This versatility lends itself both to precision components needed within the system during construction of the nuclear reactor as well as specialised parts that may be required for maintenance, reinforcement and/or damage control once the reactor is already operational.

As the field expands, new applications of PEEK within the nuclear space are constantly being discovered. Given we know that PEEK is able to survive within the environment without suffering adversely, it remains to be see where else this polymer will find uses.

Identifying Virgin Plastics

Despite ample data on the properties of various polymers, it is easy to understand that most end-users rely on the word of the supplier that the polymer they are paying for is the polymer they are getting.

Because many of the properties are inherently difficult to test, one would need to send the material to a lab for identification. And because the methods of identification are sometimes complex and require many types of tests, this can end up being an expensive affair. A client may be willing to undertake this expense one time, but if a component is supplied regularly, it would be cumbersome to test the materials each time a new lot is received. Hence, for the most part, clients accept the material test certificates (MTC) as provided by the supplier and trust that the parts supplied are from the lot corresponding with the MTC.

In our own experience, we have come across many instances of clients claiming they are using a certain polymer when in reality, the material they are being supplied is a cheaper variant of the polymer they think they are using.

A few examples of these are highlighted in the table below:

Polymer Substitute Price difference
PEEK PEK, PAEK 2X
PTFE Polypropylene, Polyethylene 4-5X
UHMWPE LDPE, HDPE 2-3X
PCTFE PTFE, Polypropylene 10-20X
FEP Polypropylene, Polyethylene 40-50X
PA66 PA6 1.5-2X
PFA Polypropylene, Polyethylene 40-50X

 

In most cases, the criteria for this substitution is clearly price. We receive many samples from potential clients claiming to be either PEEK or PTFE. In some cases, a lab test is not even needed as it is visually obvious they are using another polymer.

In a few cases, the non-availability or the non-processability of the polymer leads to suppliers opting for substitutes. For example, the inability of UHMWPE to be easily injection moulded leads some processors to use LDPE or HDPE instead. Visually, it is difficult to tell these polymers apart, so the client accepts the alternate material without question.

Obviously, the performance of these materials cannot match up to the polymer originally chosen for the application.

In one case, we received samples from a client claiming they were PEEK and enquiring as to why they should have failed in his application. The part was a ball valve seat, procured from another vendor and had deformed after only a few months of performance. When we explained that the part was PEK, the client insisted that his supplier was giving him PEEK. When the part was sent to the lab and it was confirmed that the part was PEK, the client asked us to supply him the same part, but with virgin PEEK. When we explained that the price would be nearly double, they refused to accept, asking us to match the price they were already getting. Eventually, they returned to their original supplier, even though they knew the material being supplied was inferior. The commercial impact of using virgin PEEK was too high for them and they preferred using the cheaper variant and dealing with the rejections that came with this.

In an attempt to make the identification of certain polymers more transparent, we created the above infographic. Using this, basic tests can be performed to ensure that the polymer is as committed.

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.