In the world of engineering polymers, plastics capable of withstanding temperatures above 150°C come at a price. While Polyamides, POM (Delrin®), and PVDF (Kynar®) are all well suited to temperatures within this barrier, when we look beyond we find the options become rather expensive.

Polymers that can accommodate higher temperatures, such as PTFE, PEEK, and Polyimides tend to be in the range of 3x-20x the price of lesser plastics. As a result, the cost implications of designing a system using high-temperature polymers are significant.

What do we mean by high-temperature polymer?

While the phrase seems fairly self-explanatory, high-temperature polymers need to be further evaluated to understand exactly how they behave. Usually, an OEM or product designer will look for the continuous service temperature to assure themselves that a part made using the polymer can withstand the conditions it will be subjected to.

However, for the component manufacturer, the service temperature is less relevant than the melting point and the glass transition temperature of the material. This is because these are the temperatures that directly impact the production of the component – both in molding as well as machining. We will be focusing here on glass transition temperatures and trying to understand how this metric needs to be used in component design and manufacture.

What is glass transition?

Put simply, a material moves from crystalline to amorphous states beyond its glass transition temperature. All polymers, when in a crystalline state, have internal stresses that keep it dimensionally stable. These stresses are a culmination of the inherent molecular arrangement of the molded shape and further stresses lent to it during the machining stage. Heating the part above the glass transition point causes the molecules to realign, thereby relieving the stresses and causing dimensional changes to the part. As stress due to machining can be significant, most polymers are subjected to an annealing cycle prior to machining, to ensure that the stress build-up does not cause the part to crack during the process. Polymers such as PEEK will crack under so little as a simple turning operation of not annealed beforehand.

The stresses are very relevant for machined components, as it ensures that machined parts subjected to temperatures within their glass transition point will not deviate dimensionally. However, it is equally true that in the event of higher temperatures, the deviation may result in part failure. This is typically the case for highly machined components.

Consideration for Dimensional Stability

Our experience with dimensional stability rests around the use of PTFE and PEEK. Both polymers behave very differently both during machining and after. We shall look at them one by one.

PTFE

Among high-temperature polymers, PTFE is unique in that it has a glass transition temperature under 0°C. The implication of this is that PTFE is generally amorphous even at room temperature and therefore does not suffer the internal stresses that other polymers do. As a result, PTFE typically does not require annealing, although it is still done as a means to improve the hardness of the material. No internal stresses mean that the material undergoes minimal duress during machining and any cracking of the machined part is avoided.

The flip side of this property is that PTFE has very weak dimensional stability when subject to applications where a high range in temperatures may be present. While PTFE can easily withstand high temperatures, close tolerances would need to be abandoned when subjecting it to these conditions as the material itself experiences an up to 3% deviation in linear dimensions between 0 and 100°C.

So although PTFE is capable of surviving the harshest of environments, a PTFE part machined with close tolerances is usually employed only in areas where the temperature, while high, must remain range-bound within +/-15°C.

PEEK

In contrast to PTFE, achieving close dimensional tolerances in PEEK and difficult due to the constant build-up of stress during machining. In our own experience, PEEK parts may sometimes need to be annealed multiple times to ensure that after each stage of machining, the internal stresses are adequately relieved so that the part does not crack/deform after the next stage.

Unlike PTFE, which constantly gives off heat as it is applied to it, PEEK needs external help in cooling it down. As a result, the use of coolant is common in PEEK machining and helps reduce the extent of stress-induced in the part.

Finally, while close tolerances of up to +/-0.01mm have been achieved on PEEK parts, there is no guarantee these tolerances will be retained should the part be subjected to a temperature above its glass transition point during application. In such an event, stresses induced during the final operation of machining will relieve themselves and cause the molecules within the PEEK material to realign slightly, causing dimensional deviations in the part.

So given the above hazards, why are PTFE and PEEK still so widely used? One reason is that there exist very few applications where strict dimensional stability in temperatures above 200°C are a co-requisite. Hence, we have applications of high temperature where the dimensional tolerances tend to be very lax and we have applications with tight machining tolerances, where the part may experience a maximum temperature of only 150-160°C.

Starting in 2015, Poly Fluoro Ltd. will be among the few companies globally, with the capability to manufacture expanded PTFE (ePTFE).

While the uses of ePTFE are numerous, it falls upon a select few manufacturers to produce this material. As we have covered in an earlier article, the production of ePTFE is as diverse as its applications. Some of the variants that exist include:

1. Mono-axially stretched ePTFE tape
2. Bi-axially stretched ePTFE tape and sheet
3. ePTFE membranes
4. ePTFE tubes and rods
5. ePTFE gland packing

Each of these products comes with unique production methods and specific nuances. However, the most commonly used variant at this point is the mono-axially stretched ePTFE tape.

ePTFE tape – also referred to as PTFE joint sealant or PTFE Gasket Tape, is widely used for creating a sealing joint between pipes and other mating metal parts. Specifically, as a chemically inert material, this tape finds application in chemical plants, biotech plants, and oil and gas pipelines. In fact, any pipe-lining application, where two pipes are connected using flanges, would benefit from using ePTFE tape, as the pliable nature of the material ensures that no gaps are left unfilled.

The tape has a soft texture and is easily compressed. Typically, the compression set of the tape is one of the parameters that define the material. The tape needs to be soft enough to compress under minimal load and ensure that it decompresses just enough to guarantee a complete sealing. At the same time, the tape needs to have adequate tensile and compressive strengths to allow for heavy loads to work upon it, without fatigue. In addition to its excellent chemical resistance, ePTFE tape also has a high temperature resistance, which allows it to be employed in applications involving a high temperature fluid transfer.

ePTFE tape comes in 2 variants: adhesive and non-adhesive. The application of the adhesive is not highly complicated, but for on-site convenience, it is preferred as else the installation of the tape is difficult.

The application of the tape is very simple. As the visual below shows, the tape is laid along the flange and allowed to overlap at the ends. The soft texture of the tape means that at the point of overlap, the tape does not bulge once compressed, by simply compressed further to make a uniform seal.

ePTFE joint sealant tape is among the most sought after materials for all sealing applications.

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PTFE Tube extrusion is among the most difficult processes within the polymer space. All polymers have their peculiarities and these certainly play a part in both their processing and machining. But PTFE tube comes with a set of so many different process parameters, that finding a combination that works consistently is something that not every tube manufacturer is able to discover. We have undertaken so many trials on tubes, each time assuming that we have looked into all the aspect. However, even after years of manufacturing, a new parameter may present itself that had hitherto gone unnoticed.

We would like to take a look at some of these parameters and their effect on the end-product:

1. Handling: Handling resin is among the most easily overlooked aspects of PTFE processing. While many resin manufacturers specifically layout guidelines for limiting the shear on the resin before processing, these become even more important where tubing is concerned. Due to the structure of PTFE tubing, the fibrils that form during extrusion are paramount to the strength of the final tubing. Excessive shearing of the resin before extrusion can cause a poor formation of fibrils and seriously hinder the achievement of good final properties
2. Blending: The parameters within blending include the type of extrusion aid used (the surface tension of the aid needs to be less than that of PTFE, while also not having a volatility and/or flash point that can cause fires during sintering), the amount of extrusion aid used, the RPM of the blending process and the post blending storage of the fine power mixture. Since our unit is in India, we need to follow a slightly different process to that in colder countries. For starters, we need to artificially cool the resin to allow of a more easy mixture of the PTFE with the extrusion aid. Such nuances are only learnt through extensive trial and error. But unless the blending is done in the correct manner, the final extrudate will be either too soft or too dry. Furthermore, unless the blend is uniform, the preform billet will have uneven densities, causing issues during extrusion.
3. Preforming: Preforming is done purely as a means to create a shape that can be fitted into the extruder. Preforming has two functions: first, it gives shape and second, it removes any air pockets from within the material. The process needs to be done keeping in mind that too little pressure will not allow for an adequate venting of the air within the material. Air pockets result in bursts during the extrusion, which damage the tubing and render it unusable. Too much pressure and the extrusion aid may get squeezed out of the preform, causing the extrudate to be too dry and increasing the extrusion pressure required to form the tube.
4. Extrusion: While extrusion is understandably the most important step, by the time the preform billet is loaded into the extruder, the preceding processes have already defined a lot of the tube’s final characteristics. Nonetheless, extrusion offers the tube it’s the final shape and this process needs to maintain both adequate pressures on the billet while ensuring the concentricity of the final tube. If the pressure is too high or too low, the tube will experience either too much shear, or too little pressure to form a proper end-product respectively. Concentricity is dependent not only on the tooling within the extruder (which needs to be precise and offer the correct extrusion angles depending on the size of the tube being drawn) but also on the uniformity of the billet’s density (discussed above). Finally, the extruder itself needs to be capable of offering a uniform load, so as to ensure the billet is under constant and non-erratic pressure throughout the extrusion run.
5. SinteringWhen heating the tube, the temperature needs to account for both a drying section as well as a sintering section. The drying section needs to be warm enough to evaporate all traces of vapour from the tube. At the same time, if it is too warm, there is a risk of the vapours igniting. Sintering needs to account for the fact that if the tube is heated too quickly, there is a chance of over-sintering. Also, although PTFE does not melt, it may under its own weight, elongate during sintering, causing dimensional deviations. Therefore the temperature has to be sent to ensure that the PTFE reaches its ‘gel state’ just before it leaves the sintering chamber, so it can cool down at room temperature.

Aside from the above-mentioned parameters, PTFE tube also undergoes pigmentation, the addition of anti-static fillers and extrusion of specific profiles. Each of these needs to re-look at all of the above processes and understand how they need to be modified to allow for a proper end-result.