Unravelling Polymers

The Definitive Blog on Polymers by Poly Fluoro Ltd.

Polymers in Fluid Transfer Applications

The transfer of fluids can be a complicated affair. In most applications that involve fluid transfer, the system is simultaneously subject to one or all of the following conditions:

  1. Pressure

  2. Temperature

  3. Corrosion

Each of these conditions further compounds the effects of the other. For example, while a system may be equipped to handle high pressures, the added effect of corrosion can lead to ruptures or pinholes within the system that can cause failure. Hence, that any system that seeks to contain or transport fluids needs to ensure that all precautions are taken to accommodate the effects and minimise the risk of leakages.

Polymers are the preferred choice for fluid transfer applications for several reasons. First, there exist a huge range of choices that can be compared with the chemical properties of the fluid in question to ensure that the polymer does not react during functioning. A major issue with using metals is that even though they may not necessarily corrode, there is no guarantee that there will not be some reaction with the chemical fluids. Such reactions can alter the properties of the fluids themselves, which would be a problem. Polymers also operate at a wide range of temperatures and given that they are not as hard as metals, they invariably bring sealing properties that metals cannot match.

1. Polymer Seals
Seals can be machined from polymer stock shapes to match the tolerances of any system. Polymer seals are a vast area of application and can include everything from:

  • Ball valve seats

  • Spring-energised seals

  • Sealing rings

  • Chevron V-packings

  • Rotary seals

  • Linear sealing strips

There is no limit to the types of polymers that can be used in a sealing application. PTFE, PEEK, PPS (Ryton) and Polyimide seals are usually preferred in applications where there is a combination of high temperature and corrosive chemicals. In lower temperature applications (say, within 120°C), polymers such as POM (Delrin), PVC or even Polypropylene can be used.

The choice of polymer here is entirely application based. As always, it starts with the chemical compatibility and moves from there. For example, while PEEK is a very robust and machinable polymer, PPS is a preferred option in the paper and pulp industry. This is primarily because even though PEEK is chemically very inert, it does suffer some reaction to the chemicals used specifically in pulp and paper manufacture. With PTFE – which is easily the most chemically inert polymer – the issue is that of deformation. While PTFE can take high pressures – the combination of high pressure and temperatures can cause deformation in PTFE seals over time, leading to leakages.

2. Polymer valves

The function of a valve is to regulate the flow of fluids through a system. Not only does the valve need to ensure that there is a tight sealing around it (fluids should not be able to flow around the valve), but the valve needs to resist the fluids flowing through it and ensure that thermal expansion due to high temperatures does not hinder the smooth movement of the valve.

One application of PTFE valves is in the paints industry. Paint mixing machines use PTFE valves to regulate the flow of liquid paint. PTFE is the material of choice because paints are composed of myriad different chemicals. Each shade of paint would be a result of a specific combination of additives and it is therefore essential to have a material that does not react with what may be potentially thousands of different compounds.

PEEK valves are used extensively in coffee machines. The combination of high temperature liquids and food grade requirements means that PEEK – which is FDA approved – is a key material of choice. PEEK is also very thermally stable, which means that the valve does not expand (and therefore tighten) even when higher temperature liquids are passed through it.

3. PTFE bellows

Bellows are complex parts that need to be machined out of PTFE. The key function of a bellow is to accommodate excess pressures and ensure that the system – typically a pumping system – does not fail over a long period of time. PTFE is used primarily because it is soft and because it is chemically inert. The softness of the material allows for the bellow to expand and contract, rather than succumb to higher pressures.

Different bellows have different ratings for the number of cycles that they can accommodate. However, a range of 1-2 million cycles is standard in the industry.
ptfe bellows
The only issue with a PTFE bellow is that because PTFE cannot be injection moulded, the bellow needs to be machined out of a block. This results in a wastage of nearly 80% on the material, making PTFE an expensive choice. Nonetheless, there are applications where nothing other than PTFE will suffice, and hence it is a preferred material in high-end chemical pumps. 

4. ePTFE gaskets

Expanded PTFE, like regular PTFE, comes with a stellar ability to resist chemicals. The only drawback with regular PTFE is that it lacks the elasticity, or seal-ability, of some softer materials such as silicone or Viton rubber. However, these materials still cannot accommodate the high temperatures that PTFE can.

ePTFE arrests some of the issues seen with regular PTFE in that it is highly compressible (up to 65%) and offers an excellent sealing between harder surfaces including metals and glass. More importantly, ePTFE provides sealing in pressures of up to 100Bar, with minimal torque. This means that even more delicate assemblies can be fully sealed without having to put excess pressure on the bolted areas.

Both ePTFE cut gaskets and ePTFE gasket tapes are being increasingly adopted across fluid sealing applications. A combination of its ability to withstand high-temperature and high corrosion while offering high sealing makes it a material of choice.

5. Polymer bobbins

Some fluid sealing systems have a combination of metals and elastomers. For example, airline fluid systems use neoprene rubber for the transfer of fluids. In the event of fire or excess heat, it is essential that these rubber tubes be kept safe and away from metals – which can heat up quickly and melt the rubber easily.

Polymer bobbins are used a medium to shield the rubber from the metals. Metal clamps – rather than being fitted directly on the rubber hose – will clamp around the bobbin which will then come in contact with the hose. This arrangement ensures that the bobbin, which can withstand higher temperatures and will not transfer heat, will keep the neoprene hose safe in the event that the clamp heats up.

6. PTFE (Teflon) Tubes

PTFE tubes are one of the most sought after for fluid transfer. Tubes are resistant to chemicals and high temperatures. At the same time, the wall thickness of a PTFE tube can be enhanced to allow it to accommodate high pressures. Further, with stainless steel braiding, these pressures can be even higher. 

PTFE tubes find application across industries such food processing, chemicals, electrical, and pneumatic lines, to name only a few.

A key example of the application of PTFE tubes is in analyser equipment. The equipment is used to evaluate the chemical composition of gases that reach it though several tubes. Since the gas cannot be allowed change its chemical structure in any way, it is essential that it travels through a medium that will not react with it. PTFE is an ideal medium, as there is no chance that the gas running through it will react with it and this ensures the purity of the system.

The above are but a few examples of polymers and their applications in fluid sealing systems. For the most part, OEMs will design their own systems and then look for polymer solutions that specifically match their application. As such, there are literally thousands of different areas where polymers are used to ensure fluid systems are kept robust and free from leakages.

PEEK Components and Bearings - Durable, lightweight, and dependable

Developing a new polymer is challenging and needs to take into account what end applications are being catered to. Often, specific polymers will be identified for a certain application, but their limitations would be highlighted. Working off these limitations, a new material can be developed that seeks to fill gaps in performance. Selecting the best material is a critical step in the development of a polymer and this selection process will determine the quality of the final product. Strength, durability, malleability and many other factors are considered when a material is being selected and each of these factors holds a significant importance. 

The development of PEEK, however, was surprising because suddenly, we had one polymer that exhibited properties at the far end of nearly every parameter. Whether we talk of tensile strength, strength-to-weight ratio, wear resistance, machinability or temperature resistance, PEEK truly checks all the boxes and leaves little doubt as to its effectiveness any time a debate on material selection is had.

Suffice to say, that the invention of PEEK revolutionized the plastic component manufacturing industry

PEEK was invented in the UK in 1978 and received a US patent in 1982. The processing conditions implemented to mould the material influence its crystallinity, or structure, and this results in the superb mechanical and chemical resistance properties of PEEK. PEEK is also highly resistant to degradation under high temperatures and resilient in organic and aqueous conditions. These properties make PEEK ideal for a wide array of components in diverse industries. Some of the major fields where this dynamic plastic material has been most effective are aerospace, the medical space, food & beverage and also the manufacture of PEEK plastic bearings.

PEEK in aerospace components

PEEK is optimal in aerospace due to its lightweight properties (it has a specific gravity that is half of aluminium), chemical inertness and toughness. Light-weight materials reduce the overall load of a vessel and, thus, reduce fuel costs and helps preserve the environment. Because PEEK is a chemically inert, the material is stable in extreme conditions and can withstand exposure to atmospheric particles. The fact that PEEK can be machined to the kind of close tolerances needed in aerospace make it an ideal material of choice.

PEEK in medical equipment

In the medical field, hygiene, durability and biocompatibility are all important factors when considering what materials to incorporate. PEEK offers flexible and durable properties and it also has no adverse effects on the skin or internal organs, which makes it an ideal material for implants. Biocompatible grades of PEEK can be used as sensor covers and transducers in medical equipment. Such grades ensure that even in the event of bodily contact, there would no adverse effects. Its resistance to extreme temperatures and low moisture absorption properties also make it a preferred choice for pumping mechanisms.

PEEK in food and beverages

The high-temperature resistance of PEEK makes it a favourable option in the production of powders and other ingredients in the food & beverage industry. PEEK is an effective alternative to PET and holds up well under the extreme temperatures for the drying of ingredients in food as well as feed for animals. The high wear resistance of PEEK also makes it an ideal choice for scrappers, where other polymers would either chip or deform. PEEK valves are also critical in coffee machines, where metallic valves have been known to impart an unfamiliar taste to the beverage. Since the last 5-10 years, high-end coffee machines around the world use PEEK valves in their equipment.

PEEK as a bearing material

One field that has some of the most diverse applications of PEEK is the manufacturing PEEK bearings. PEEK makes an ideal material for bearings for many reasons. PEEK bearings can be made self-lubricating (usually with the addition of PTFE at a concentration of 15-25%). Abrasion causes microscopic bits to function as lubrication with inconsequential alteration to the bearings themselves. The durability and high-temperature resistance of PEEK bearings allows them to be thinner and lighter than traditional material bearings. Their light-weight characteristics make them more fuel-efficient and they don’t require the constant grease and maintenance that metallic bearings demand. As a result, PEEK can reduce bearing production and maintenance costs significantly. PEEK bearings can also run for long periods of time without freezing during operation. The low thermal expansion coefficient of PEEK ensures that it retains dimensional integrity over a very wide range of temperatures – both high and low. The chemical resistance of PEEK bearings allows them to withstand conditions that would otherwise cause damage to metal bearings. All these attributes make PEEK bearings an exceptional mechanical component for an endless number of products and functions

As we can see from these examples, PEEK is effective in a range of industries and PEEK bearings have become a favoured option as a mechanical component. PEEK bearings can be found in many different types of products and even in motors that go into the refrigerators and air conditioners in our homes. PEEK bearings will continue to be implemented in more and more products in the future as well. Keep your eyes open for PEEK plastic materials and PEEK bearings in the machines and products around you.

Thermal stability of precision machined polymers

Polymer machining is challenging and throws up many problems that require a deep understanding of the plastic in question to solve. Unlike metals, which lend themselves to tolerances of within a few microns, polymers can rarely be expected to offer anything under 50 microns (0.05mm) on dimensional tolerances. In addition to this, there is unpredictability, both in the post-machining behavior of the part and in the long-term dimensional stability during operation.

For the most part – any post-machining dimensional issues can be easily resolved by observing the polymer for a few days and then working backwards to create the buffers needed such that once the part settles down, it conforms to the drawing requirements. Provided the polymer is processed correctly, there will be a degree of predictability on, say, the shrinkage of the part after machining.

Recently we machined an 8” PTFE Seal, which exhibited a shrinkage of 0.2-0.3mm within 24 hours of machining. When observed for another 24 hours, there was no additional shrinkage seen. In this case, we had to go back to the moulding and sintering cycle (heat cycle for curing the polymer) and make the necessary adjustments such that this shrinkage could be avoided. As an added measure, we ensured that the diameter we machined was on the very far end of the tolerance spectrum, such that even if a 0.1mm shrinkage did occur, the part – which had an overall tolerance of +/-0.25mm, would still be well within the criteria for acceptance.

Performance on the field is another issue altogether. While it is tough to ensure any level of dimensional stability on highly mechanical applications, where the polymer may be surrounded by metals and hence subject to loads and stresses from much harder materials, we do need to understand the reaction of the plastic to heat.

Understanding glass transition temperature

In an engineering application, a plastic will be analysed for load, wear, and temperature resistance. Usually, this third parameter will focus on the maximum service temperature as a measure for whether the polymer can survive in the given application. However, when we look at precision machined components, a lot of thought must be given to the glass transition temperature.

Put simply, the glass transition temperature is the point at which the polymer moves from a crystalline state, to an amorphous state. The resulting shift can be highly significant for a polymer, since the molecules will try and realign to the new state and this can result in dimensional changes to the part itself. In a crystalline state, one would expect the links between molecules to be more rigid. As such, any machining done on the part would be both predictable and allow for a high level of dimensional accuracy at low tolerances. However, when amorphous, the unpredictability sets in and the part is likely to ‘resettle’ causing tolerances to relax.

Therefore, it is not only important to select a polymer that is capable of a certain service temperature, but to also ensure that during operation, the temperatures do not fluctuate in a manner that would cause the part to keep moving over the glass transition threshold. The table and charts below show the glass transition and service temperatures for some high-performance polymers.

If we take the example of PEEK, we can see that the glass transition temperature is 140°C. While this may be well above the service temperature of other polymers, it also causes some problems. PEEK parts that are used at room temperature (25-30°C), but where the application can call for temperatures in excess of 200°C, may experience some issues. So, even though PEEK can withstand the service temperature comfortably, the increase of temperatures beyond 140°C will cause the part to experience changes at the molecular level and may push its dimensions out of tolerance, causing a misalignment in the assembly.

In contrast to PEEK, we have PI (Polyimide). Here we see that the glass transition temperature is 340°C – very close to the maximum service temperature of 360°C. As a result, for high temperature applications, PI ensures that you can operate close to its limit and not sacrifice any dimensional integrity.

Other polymers, such as PTFE, LDPE, and HDPE, have glass transition temperatures well below zero. This implies that for any non-sub-zero application, the polymers are always amorphous. Increasing the crystallinity of these polymers is challenging and as a result the typical tolerances one can expect on these plastics would rarely be below 50-100 microns. PTFE, with fillers of glass, can be machined to a tolerance of as low as 10 microns (0.01mm), but this is only in special cases.

In general, polymers which have a smaller difference between their glass transition and service temperatures would offer higher predictability on dimensions in applications where the temperature would fluctuate.

 

Glass Transition Temperature

Max Service Temperature

 

Polymer Name

Value (°C)

Value (°C)

Diff.

ABS - Acrylonitrile Butadiene Styrene

102

89

-13

CPVC - Chlorinated Polyvinyl Chloride

110

100

-10

HDPE - High Density Polyethylene

-110

120

230

HIPS - High Impact Polystyrene

92

80

-12

LDPE - Low Density Polyethylene

-110

100

210

PA 11, Rigid

45

120

75

PA 12, Rigid

45

120

75

PA 6 - Polyamide 6

60

120

60

PA 66 - Polyamide 6-6

58

140

82

PAI - Polyamide-Imide

275

280

5

PC - Polycarbonate, high heat

200

140

-60

PEEK - Polyetheretherketone

145

280

135

PEI - Polyetherimide

215

170

-45

PESU - Polyethersulfone

230

180

-50

PET - Polyethylene Terephthalate

78

140

62

PFA - Perfluoroalkoxy

90

260

170

PI - Polyimide

340

360

20

PMMA - Polymethylmethacrylate/Acrylic

110

90

-20

POM - Polyoxymethylene (Acetal)

-50

105

155

PP (Polypropylene) Homopolymer

-10

130

140

PPS - Polyphenylene Sulfide

93

220

127

PPSU - Polyphenylene Sulfone

220

210

-10

PSU - Polysulfone

190

180

-10

PTFE (Teflon)

-110

250

360

PVC Rigid

100

80

-20

PVDF - Polyvinylidene Fluoride

-25

150

175

UHMWPE

-150

130

280

 

Processes to limit dimensional deviation

Coming back to the issue with PEEK, the fact remains that PEEK is used in many high-temperature applications. The method to ensure its stability is tricky and involves a lot of trial and error. Annealing the part at above its glass transition temperature and allowing it to cool gradually can help immensely in ensuring that the molecular realignment is minimised. In other words, by letting the plastic become amorphous and then re-crystallise, we limit the extent to which dimensional deviations can occur during operation. Annealing is highly critical to PEEK, since non-annealed parts will easily crack not only during operation, but during machining itself. PEEK is somewhat unique in that it may even require annealing between machining operations to ensure that the structure of the polymer is being allowed to relax each time stresses are placed on it. This repeated annealing needs to be done using trial and error – as each component would behave differently and react in its own way to a specific machining operation.

Ultimately, the choice of polymer would depend on several factors other than temperature. However, it is very important to note that when making the choice for a polymer in a high-temperature application, thought needs to be given not only to the maximum service temperature, but also to the fluctuations in temperature during operation. Only by comparing this with the glass transition temperature can one be assured that the polymer fits the application.