PTFE (Teflon) Tubing

PTFE Tubes are known for their versatility and durability.

As a material, it comes with the numerous properties of PTFE (Teflon), including temperature resistance, chemical resistance, electrical insulation and high-strength.

The high ratio of strength (both tensile and electrical) to weight gives us the option of using a much smaller tube of PTFE to do the same task that might require a lot more of a less capable material.

A high burst pressure means that both pneumatic and hydraulic systems would benefit tremendously from the use of PTFE tube. Similarly, a dielectric breakdown strength in the region of 150KV/mm implies that even a wall thickness as thin as 0.5mm allows for a breakdown resistance of 75KV.

Chemical lines, which often require a material that does not in any way react with the chemical inside it, benefit from the inert nature of PTFE. This means that even in the event that some unknown chemicals are present in the fluid passing through the tube, there is no risk that the tube will corrode or in any way impart its own reaction with the chemical.

PTFE tubes are known by many names across the industry:

1. PTFE Tubing
2. Teflon Tubing
3. PTFE Hose
4. PTFE Pipe
5. PTFE Sleeve
6. PTFE Sheath
7. PTFE  Liners

They are all part of the same process and extruded in the same manner.

PTFE (Teflon) Tubing finds application in nearly every industry

Please visit our website for more details or email us: enquries@polyfluoroltd.com

PTFE (Teflon®) Tubes as Insulators

The benefits of PTFE as an electrical insulator are well known. The high dielectric strength and breakdown voltage of PTFE allows it to be used in applications where standard insulation materials would fail.

In our experience, we have seen PTFE skived tapes used as wrapping around high-voltage junctions and circuits, PTFE components used as transducer covers and high-voltage casings and PTFE pads used to shield metal bodies from one another in electrical and thermal applications.

PTFE (Teflon®) tubes find similar uses as electrical insulators, although one really needs to dig deep to see where exactly they are used. Ever since we started manufacturing PTFE (Teflon®) Tubes, we have begun exploring its application and approaching various clients currently making insulation assemblies to understand whether there is a possibility to use PTFE in their products.

PTFE (Teflon®) Tubes in Cables

The requirement of cables to have a single outer casing that houses a variety of conductors necessitates the use of an insulating medium. PTFE tubes are used to shield each conductor from the next. In this application, it is essential that the tube is both flexible and free of any cracks/inclusions that would affect the insulating properties.

Additionally, since the requirement may be for a cable that is continuous and without joints, the PTFE tube would itself need to be continuous. It is therefore imperative to be able to manufacture a continuous tube that would be free from any defects for a length of at least 150 to 300 meters.

It has taken extensive research and multiple trials to attain a level of extrusion that guarantees a flawless, continuous length of tube.

PTFE Tube forms the insulating medium around the cable core

PTFE (Teflon) Tubes in Short Neutral Sections

While the tubes used inside cables are usually thin-walled tubes (within 1mm wall thickness), there are applications where thick-walled tubes form the outer casing of insulating assemblies.

The Short Neutral Section (SNS), is an insulating assembly used in overhead lines for the railways. Typically, the pantograph will run along a high-voltage wire above the train, allowing current to be supplied to the train. The current is drawn from a sub-station and when switching from one sub-station to the next, the pantograph will pass over the neutral section. The assembly ensures that the wires from two separate sub-stations do not make contact. Hence, it is important for the insulating medium to be effective. Furthermore, since the pantograph runs over the neutral section at high speed, it requires an insulator that is also capable of high wear resistance.

We were approached by the railways to develop this assembly. The core of the product is the thick-walled PTFE tube, which is combined with additives to improve wear resistance, while maintaining the dielectric properties.

PTFE Tube used as an outer sheath for electrical and wear properties

Conductive or Anti-Static PTFE Tube

Because PTFE has such a high resistance to current, there are areas where this becomes a problem. With no way to pass through PTFE, there is the possibility that excess static discharge simply collects on the surface of the material. Once a critical mass of discharge is reached, there is a high possibility of sparking.

In applications involving flammable vapours, such sparking can be very dangerous. To mitigate this issue, fillers such as carbon are added to PTFE to allow for some static discharge to take place through the material. The addition of carbon reduces the insulating properties of PTFE to some extent, but the end properties of the material are still far above any regular insulators.

There are multiple other uses for PTFE (Teflon®) Tubes across industries. As old designs are upgraded, it is important for product designers to keep in mind that PTFE has properties that make many of the older insulating materials obsolete.

Note: Teflon® in the registered trademark of Chemours™

End Properties and Characteristics of PTFE (Teflon) Tubing

The development of a new process or product is usually accompanied by a steep learning curve.

Some of the findings are obvious, and may even be accessible in the public domain. Others are less easily understood and might be specific to the manufacturer due to the nature of the set-up, the environment and the materials used.

Our recent installation of a continuous line PTFE Paste Extruder has thrown up many such findings. At each stage, we have needed to evaluate whether the finding impacts the properties of the final product. Given the fact that globally, very few companies manufacture PTFE tubes, our access to external information is limited. Thus, trial and error has been the key to fine tuning the extrusion process and derive a product of consistently good quality.

Our journey in developing the product confirms that much of what is learnt needs to be kept proprietary, as it is part of a rich process technology not easily obtained. However, in doing so, we have also studied the final characteristics of PTFE tube and tried to make sense of what properties go in to define a tube of good quality.

Properties of PTFE Tube

When clients approach us with enquiries for PTFE Tubing, they are primarily concerned with 3 factors. Each of these factors plays back into how the PTFE tube is processed and has relevance to the end-application in questions

1. Dimensional stability
   The outer and inner diameters of the tube (OD and ID) are of utmost importance. In almost all cases, the tube will be used in an assembly, where fittings have been designed to accommodate the tube in question. Although minor variations in dimension may be accommodated, the tube needs to adhere to the fitments used with it.
   We have observed that when high quality resins are used, the dimensional stability during extrusion is highly predictable and easily maintained within a tolerance of 50 microns (0.05mm). Certain Chinese resins, when extruded, do not maintain this integrity. So, a tube with a required OD of 6mm may sometimes measure at 5.9mm and at other times measure to 6.2mm, despite all other parameters remaining unchanged.
2. Burst Pressure
   In applications involving high pressures, it is important that the tube does not yield during service.
   An easy formula to calculate the burst pressure is as follows:
   
   There are two critical parameters here that define the effectiveness of the formula.
   The first is the tensile strength – which is denoted by “T”. In most cases, we are told to take a tensile strength of 25Mpa for this value. Our own testing indicates a tensile strength of 28-31Mpa on our tubes, meaning that the value of 25Mpa is safe to use. However, tube that is not properly processed can often have a tensile strength of less than 20Mpa. This means that while a manufacturer may use the value of 25Mpa for calculation, the actual burst pressure is at least 20% lower.
   The other factor – that does not even feature in this formula is the concentricity of the tube. PTFE tube that is non-concentric will show a higher wall thickness on one side as compare to the other side. It will not have the same burst pressure of good quality tubes, even though the ID and OD may be the same. This issue also results in problems when we try and braid the PTFE Tube using stainless steel. The unevenness in wall thickness causes one side of the tube to collapse during braiding due to the pressure applied by the stainless steel.
   
   As a rule, we try and maintain a concentricity exceeding 95%.
   Calculating concentricity is quite simple. It is the ratio of the minimum wall thickness to the maximum wall thickness of the PTFE Tube. So a 6mm x 4mm PTFE tube, which has a wall thickness of 1mm, would need to have a tolerance of +/-0.025 to attain such a degree of concentricity.
   We have observed many tubes where the concentricity varies by up to 0.1mm on the wall thickness (implying a concentricity of only 81% on a 6mm x 4mm tube). While some applications may be fine with this level, it is up to the manufacturer to inform the client regarding the same, as the client may not always be aware of how critical this parameter is in the final application.
   Both concentricity and tensile strength are end properties derived from how the tube is processed during extrusion. Factors such as blending, extrusion pressure and sintering all lend themselves to arriving at a tensile strength acceptable by global standards. Similarly, extrusion speed, alignment and the blending process all play a part in ensuring concentricity exceeds 95%.
3. Visual
   Good quality PTFE tube will have a smooth even surface without any pitting, waviness or discolouration. Visually, concentricity also plays a part, as a tube that is significantly off-centre will usually raise concerns from the client.
   We have already looked at how concentricity is influenced by the extrusion process. Similarly, factors such as quantity of extrusion aid, extrusion speed and pressure, finish on the die and sintering temperatures all weight in on how the tube appears.
   Invariably, visual factors such as pitting, waviness and discolouration will give clues as to the fundamental properties such as tensile strength, elongation and dielectric strength. Hence, these need to be evaluated no just from a cosmetic point of view, but also in terms of what characteristics of the final product are being diminished due to the appearance of visual indicators.

It should be noted that the above characteristics cover only the very basic aspects of PTFE tubing. Products such as anti-static tubing, ePTFE tubing and convoluted tubing will each bring a new set of challenges that will need to be studied from first principles.

For the time being, we are satisfied to have attained global quality standards on characteristics that drive a majority of the demand for PTFE tubes.

PTFE Tubing: Process Parameters And Their Impact

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 lay out 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 final shape and this process needs to maintain both adequate pressure 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. Sintering
   When 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, 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.

The Many Chemical Applications of PTFE

PTFE is known to be among the most chemically resistant materials known to man. While this property is well known and often quoted in manuals and our own blog articles, we would like to touch upon some of the common applications that this property leads to.

PTFE Labwares

PTFE has been a mainstay in lab-ware items for a number of years. Lab-ware items include stopcocks, beakers, pipes, test tubes, stirrers, petri dishes and stands. In most labs, glass is the commonly used material for these items, but as we know, glass has a tendency to break. Furthermore, when dealing with harsh chemicals at elevated temperatures and pressures, PTFE becomes a viable option for a number of reasons.

1. PTFE is chemically inert – barring certain alkalis at elevated temperatures
2. PTFE does not break easily: Under loads, virgin PTFE would tend to deform rather than break. This makes it useful in applications involving high centrifugal forces – where a glass test tube might break under extreme loads
3. PTFE is stable across temperature fluctuations: Even toughened glass would have a limit on how suddenly it can be cooled down when under high temperatures. PTFE is able to cool down rapidly from elevated temperatures without cracking
4. PTFE is a great sealing material: Especially for stopcocks, PTFE forms a much better seal than glass equipment owing to the fact that it is soft and will easily seal gaps which may arise due to minor variations in taper between the pipe and the stopcock.
5. PTFE is flexible: PTFE tubes can be used in place of glass and be bent to accommodate the layout of the apparatus
6. PTFE can be moulded with a magnetic core: PTFE stirrers are used because they can be moulded with a magnet at the core and used in magnetic mixers

PTFE Stirrers and Shafts

Stirrers and shafts are used primarily in highly corrosive applications including biotech, pharma and refineries. The ability of the PTFE to be constantly immersed in a chemical and neither modify nor be modified by the chemical makes it an invaluable component in many mixing arrangement.

More often than not, the shaft or stirrer needs to be custom moulded and machined to suit the mixing assembly. This makes it an expensive component and therefore only sparingly used. In some cases, a stainless steel core can be used over which the PTFE is moulded/ lined. In other cases, the stainless steel shaft can simply be coated with PTFE. However, this latter case only works where there is little or no abrasion expected on the shaft – since PTFE coating will peel off if the shaft is subjected to wear.

PTFE umbilical cords

Although the name sounds strange – the umbilical cord is a well-known arrangement of PTFE tubes used in the refinery industry. The purpose is simple – the refinery process yields a number of different gasses, which need to be analysed in a lab to gauge whether the right chemical reactions are taking place in the chamber. Taking these gasses to the lab (which needs to be a minimum of 200-250 meters from the reaction chamber) is a difficult process, as the gasses are corrosive and highly reactive – which may mean that they change composition during transit if not kept in a chemically inert environment.

An assembly of 12-15 tubes is bunched together using a PVC coating and each tube has a length of 250 meters and transports a single gas to the lab, where it is collected and analysed.

The complication in this design is that the PTFE tube needs to be continuous for the entire length of 250 meters. Any bonding or jointing leads to a foreign chemical in the tube and this affects the gas passing through it. After extensive trials, we find that using a welded joint comprised of PTFE is able to create an effective solution for the tubes.

Filtration

Many chemical applications involve multiple substances, which often need to be separated from one another. In such cases, PTFE becomes the preferred medium for filtration.

PTFE is used in 2 forms here:

1. Porous PTFE sheet: This is a standard PTFE sheet skived from a porous PTFE billet. The billet is made porous by adding certain substances in the PTFE compound, which sublimate during the sinter cycle, leaving voids. These voids form the pores which aid in filtration. This type of membrane is not used extensively due to the inexact nature of the pores. However, the membrane can be made as thick as 5mm – which makes it useful in corrosive applications where a liquid needs to be separated from large solid particles.
2. Expanded PTFE membrane: This is also called breathable PTFE membrane owing to the fact that you can pass gas through it, but not liquids. Expanded PTFE is more commonly used that porous PTFE as the pore size can be easily defined to within a few microns. It finds multiple applications in automotives, pharma and biotech.

PTFE Valves and Ball Valve Seats

Although valves and ball valves form an industry unto themselves and use a variety of materials other than PTFE, certain applications involving the flow of chemicals need PTFE valves to withstand the corrosion otherwise caused to non-PTFE valves.

Our own experience with PTFE valves sees it being used in piping systems in chemical plants and in equipments such as paint dispersion machines.

In paint dispersion, the equipment is used by retailers to mix different colours of paint to form a batch of a new colour as chosen by the customer. The paint passing through the PTFE valve needs to remain un-changed. Any reaction due to, say, using a nylon or PVC valve can alter the colour to the extent that the colour being chosen by the customer does not match the actual colour of the final paint. Thus PTFE is an invaluable component within this assembly.

Reprocessed PTFE and chemical applications

We had earlier done an article on reprocessed PTFE and the various issues it presents with regards to the base properties of the material. One of the issues we have observed is that when using reprocessed PTFE, the scrap is seen to change colour when using a coolant during machining. This came as a huge shock to us – as common opinion suggests that it is only in mechanical properties that the material suffers when reprocessed.

The finding leads us to believe that a number of things may be happening to cause this:

1. There may be foreign substances used in making the reprocessed PTFE. Titanium Di-Oxide for example is a known additive in making PTFE appear whiter. Similarly – cheaper, un-tested additives may be added to improve appearance, which may not have the chemical inertness that PTFE has.
2. Micro-impurities may be present in the material that cannot be seen by the naked eye. These may be reacting with the coolant causing the colour change
3. The basic chemical structure may be altered on a microscopic level. PTFE is chemically inert because of its molecular structure, which involves a carbon atom, shielded by 2 atoms of fluorine. When we chemically etch PTFE, we effectively remove 1 flourine atom and expose the carbon atom, making it bondable. A similar transformation may be happening in parts of the material due to reprocessing – which cause a degradation in the chemical resistance of the material.

PTFE tubing - one product, numerous applications

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The evolution of Polytetrafluoroethylene (PTFE) - more commonly known as Teflon^® - from a niche product used only in high-value applications to a mainstream requirement has been very gradual.

However, over the past two decades PTFE usage seems to have crossed a critical mass, allowing it to become commercially viable in over 200 industrial, consumer and medical applications. And while sheets, rods, coatings and components corner the bulk of the market for PTFE products, PTFE tubing is now emerging as the key growth area.

PTFE tubing applications

The use of PTFE tubing has spread across various applications including automotive, chemical, electrical and medical. Table 1 shows the key properties which outline the versatility of PTFE tubing, while Fig 1 shows its uses in various fields.

• In automotive applications, the ability of PTFE to withstand temperatures in excess of 250^oC makes it an ideal candidate for high temperature fluid transfer.
• In medical applications, PTFE tubing is in huge demand due to its lubricity and chemical inertness. Catheters employing PTFE tubing can be inserted into the human body without fear of reaction or abrasion with any body parts.
• In chemical applications - including laboratories - PTFE is an ideal replacement for glass due to its inertness and durability.
• In electrical applications, the excellent dielectric properties of virgin PTFE make it well suited for insulating high voltage cables.

Table 1: Key properties and applications of PTFE tube

Types of PTFE tubing

Depending on the application, PTFE tubing is divided into three broad categories - each defined by the tube's diameter and the wall thickness (see Table 2).

Even within categories, PTFE tubing lends itself to different variations, each allowing for a different application (see Table 3):




PTFE tubing in the medical device market
In general, small diameter spaghetti tubing is used in medical applications. The use of PTFE in this area centres on two key properties: lubricity and biocompatibility. Fluoropolymers exhibit very good lubricity compared with other plastics. PTFE is the most lubricious polymer available, with a coefficient of friction of 0.1, followed by fluorinated ethylene propylene (FEP), with 0.2. These two polymers represent the vast majority of all fluoropolymer tubing used in medical devices.

The biocompatibility of any polymer used in a medical device is an obvious concern. PTFE excels in this area and has a long history of in vivo use. Medical-grade fluoropolymers should meet USP Class VI and ISO 10993 testing requirements. Of course, processing cleanliness is also an important factor.

PTFE tubing - processing techniques
The uniqueness of PTFE tubing rests in the complexity of PTFE as a polymer. While most polymers lend themselves easily to injection moulding - allowing them to be made into complex shapes, PTFE due to its high melting point and melt viscosity can only be compression moulded. The high melting point of PTFE also means that extrusion - as conventionally practiced - cannot be applied to it. PTFE paste extrusion has therefore become a process which is increasingly sought after - given the growing demand for PTFE tubing.

Extruded grades of PTFE were first used in the wire and cable industry in the 1950s, where the good dielectric properties of the material proved critical to the developing electronics market. The first tubing was made by extruding PTFE over a wire and then removing it-a labour-intensive process. In the 1960s, technology emerged that could perform the extrusion of PTFE without a wire core. This process enables PTFE tubing to be economically produced in long continuous lengths.

PTFE paste extrusion follows 6 broad steps as illustrated below:

1. Mixing: The resin comes in a powder form with an average particle size of about 0.2µm. The powder is waxy and prone to bruising and mechanical shear fibrillation. Hence handling must be careful and done typically at a temperature of around 20°C. While standard compression moulding only requires that the powder be sieved thoroughly and then compressed, in paste extrusion the powder must be first mixed with a hydrocarbon extrusion aid or mineral spirits. The powder-spirit mixture is left in a sealed container before it is used in the next process
2. Pre-form: The pre-form is a billet made by compressing the mixture in a hydraulic press. A standard 30Kg billet would take approximately 2 hours to mould, following which a dwell time is necessary to ensure any excess air pockets get released
3. Extruding: the pre-form is loaded into the extruder - the key equipment in the process - and a die and mandrel are clamped in place above it. The die is a critical tool and its design defines the strength of the tube and its final dimensions. As the extrusion process starts, the extruder presses the pre-form against the die and mandrel, forcing the resin to extrude into the desired shape. The tubing in this stage is referred to as 'green' and can be easily crushed.
4. Pre-sintering: the green tubing is passed through an oven where it is heated at a very low temperature. The idea here is to evaporate the spirit in the tube and care must be taken so that the flash point of the spirit is not reached, causing it to ignite.
5. Sintering: the PTFE tubing is sintered at 350-400°C. The sinter cycle will depend on the thickness of the tubing and can last up to 24 hours for thick walled tubing
6. Cleaning and packaging: the tube is first cut into he desired lengths. In the case of medical tubing, the ends of the tube must be plugged as soon as the material comes out of the oven. The plugging ensures that the inside of the tubing - which has seen temperatures well in excess of 300°C - remains clean. For further cleaning an ISO Grade VI clean room is the minimum requirement for PTFE tube. After the cleaning the tubes are packed in polythene covers for dispatch.

Table 3 : Technical specifications of FluoroTube