Among the most challenging processes to master within the polymer space is that of manufacturing ePTFE (expanded PTFE) tubes. ePTFE tubes combine the complexity of making standard PTFE tubes with the complexity of making expanded PTFE. Both PTFE tube extrusion and expanded PTFE manufacture are challenging to make on their own thanks to the peculiarities of PTFE as a material. Combining them only compounds the difficulties.

PTFE tube manufacture

The process of paste extrusion involves mixing a PTFE fine powder with an extrusion aid (lubricant) and then passing it though a die to achieve the final shape. The issue here is that because PTFE does not melt (or more specifically, has no melt flow) it needs to be extruded at room temperature and then passed through a heating system to cure it into its final form. The challenge is working with a dry powder, which when subjected to the high pressures of the extrusion press, starts to behave more like a fluid, but can still not be controlled easily, meaning that dimensional variations, non-concentricity, and material properties can all change depending on various factors that cannot be controlled once the extrusion begins.

ePTFE manufacture

Standard mono-axial ePTFE manufacture also starts with extrusion. However, since the end product is usually a tape, the extrusion itself is not as challenging as making a PTFE tube. The extrudate is then passed through a stretching device, which adds heat and force to pull the tape into its final – marshmallow-like- form that allows it to be such an effective sealing element.

One key issue with mono-axial ePTFE tape is that it is prone to splitting. Since the extrusion force only acts in the longitudinal direction, laterally the tapes tend to be weak and spit easily when torn apart sideways. This is a property that can be addressed pre-stretching, but it involves a lot of mechanical manipulation of the material.

Expanded PTFE tube

The same process to make PTFE tubes forms the beginning of the ePTFE tube process. However, unlike ePTFE gasket tape – which has a solid form that can be easily handled – the tube profile is very weak. The slightest pressure on the tube in this raw form will cause it to collapse, after which the tube is effectively useless. Careful handling is needed to ensure that the tube in this ‘green state’ holds its form until the stretching process begins. However, the stretching is itself the bigger issue. Stretching ePTFE involves gripping the tape tightly so that it can be pulled through the starching machine. However, the extruded tube cannot be gripped at all, as even squeezing it lightly between one’s fingers will cause it to collapse.

At Poly Fluoro, we have devised a number of ways to mitigate this problem. Extensive R&D went into understanding what the extruded tube would be able to withstand mechanically and building the right equipment to ensure that the tube passes through the stretching process without collapsing. In this regard, the final properties of our expanded PTFE (ePTFE) tube were the following:

1. Non-splitting – by creating the right kind of forces on the tube, the final product gains strength in the lateral direction and the tube no longer splits when torn sideways

2. Porous – like all expanded PTFE, the tubes gain a special kind of porosity, making the tube walls impervious to liquids and dust, but permeable to gases and vapours

3. Non-kinking – unlike regular PTFE tube, which is prone to kindling when the bending radius is breached, ePTFE tubes do not kink and will allow themselves to be bent and positioned as required

As a chemically inert, corrosion resistant material capable of taking high temperatures, PTFE tubes are highly sought after. However, with the addition of expansion, the tube takes on a whole new dimension and becomes invaluable in applications ranging from fluid control, to electrical insulation, to medical devices and grafts.

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As automation and industrialisation has evolved, the key objective for any mechanised system has always been twofold: higher speed with higher precision. While sensors, Servo motors, and computed aided software have allowed for ever increasing levels of precision, the issue of speed has always brought with it one unavoidable problem: friction.

In any system of moving parts, friction is the number one culprit keeping things from moving indefinitely and with minimal damage. Hence, methods to reduce friction have always played a vital role in ensuring the durability of mechanical systems.

One of the most common uses of polymers lies in the realm of friction reduction. The low thermal conductivity of polymers allows for better heat dissipation and lower wear outs. However, as this blog has always maintained – not all polymers were created alike. While most polymers can form an effective medium between moving metal parts, the issue of longevity narrows the list down significantly. Typically, a good wear material should have some or all of the following properties:

1. Self-lubrication – especially needed for systems where oil cannot be used or where oils and other synthetic lubricants cannot easily be replaced

2. Low coefficient of friction – this is paramount to ensure the smooth sliding materials that come in contact with the wear material

3. Heat resistance – although not essential, some polymers are needed in environments where high temperatures would build up, even without excessive friction

4. Robust - metals are many factors harder than even the strongest of polymers. A tough material is needed to last long term in an environment where metals are moving at high speeds.

In this regard, most of the common polymers – such as HDPE, LDPE, or PP – would be unable to take high wear loads for an extended period of time. PVC – which is stronger and has admirable strength – has the key drawback that the presence of chlorine in the material can corrode metal parts over time. Based on our experience, the following materials are the best suited among polymers for wear applications:

1. POM (Delring/Polyacetal) – POM is not an immediate choice as a wear resistant material. However, it is inexpensive, mechanically strong, and capable of taking temperatures of up to 120°C. This makes it a comfortable choice in applications where the loads and RPMs are not very high, but where a spacer or bush between two metallic moving parts will allow for lower heat build-up.
   Further – the addition of PTFE fillers to POM (Delrin AF), allows for even lower coefficients of friction and higher loads as a result

2. PA66 (Nylon 66, Nylon 6.6) – like POM, nylon is on the lower end of both tensile strength and coefficient of friction. However, PA66 bobbins and ferrules are excellent are taking friction in rotary applications. Again – the loads, temperatures, and RPMs cannot be very high, but the material is effective if used within the parameters specified.

3. UHMWPE – although less known than some of the other polymers, UHMWPE rates one of the highest on out-and-out wear resistance. It would probably be on the top of any list of wear applications, but for the fact that it has a very low temperature rating. With the ability to only withstand 80-100°C continuously, UHMWPE is restricted to application in ambient temperatures. Still, it is very useful as a wear plate, or sliding plate, especially in railway applications, where the heat build up is limited. UHMWPE also forms a very effective medium as a bush or collar in low speed, high friction applications.

4. PEEK – while usually too expensive to use in any regular applications the effectiveness of PEEK can be unparalleled when it is used properly. Specifically, HPV PEEK, which is PEEK with a blend of carbon, graphite, and PTFE, can endure high loads at high RPMs and do so in temperatures of up to 300°C.
   PEEK’s only drawback is that it is prohibitively expensive- costing 15-30X of any of the above polymers. But in situations where cost is not a factor, there are few polymers that can compare.

5. PTFE (Teflon) – the most well know and versatile of wear materials, PTFE’s absurdly low coefficient of friction (as little as 0.03 against polished stainless steel) combines with an equally low thermal conductivity to offer a material that simply does not heat up even at high RPMs and loads. With a service temperature of 250°C, there is virtually no wear application where PTFE does not find consideration. While the material is relatively soft and therefore easily deformed, the addition of glass, carbon, and bronze fillers adds hardness, making the material suited to such diverse applications as:

   1. Piston bands in shock absorber struts

   2. Railway sliding plates

   3. Linear slideway bearing strips (Turcite/Lubring)

   4. Automotive wear plates

   5. DU Bearings/bushings

It should be mentioned that while PTFE certainly is the most popular choice, the wear resistance of UHMWPE still rates higher. Designers and engineers would do well to understand that if temperature is not an issue, UHMWPE is an excellent choice for any wear application. However, as most industrial and automotive systems operate at elevated temperatures, PTFE and PEEK are the most accepted and effective choices in wear applications.

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The versatility of PTFE as a material has been explored many times. The applications of this miracle fluoroplastic range from bearings to seals to automotive components to heavy insulation. However, it is PTFE’s use in medical devices and equipment that is truly obscure and worthy of exploration.

PTFE properties for Medical Use

It is commonly known that PTFE is highly inert. This property allows PTFE to be one of the few materials that can remain in the human body for extended periods of time with no adverse effects. In addition to the inert properties of the final material, PTFE resins are, by default, FDA approved – meaning that if processed correctly, PTFE can be used in food and medical applications without too much additional documentation. Obviously, a medical device manufacturer would need to preform their own tests and validations to the material before employing it in their application, but the likelihood of PTFE failing any such tests is low. Finally, PTFE is processed at such a high temperature that, if handled properly, it is completely sterilized once fully processed.

Despite this, medical tubing usually calls for a clean room set up in order to ensure that the final products are suitable for medical use.

PTFE Amplatz Sheaths

Once such specialised application of PTFE in the medical space is the use of amplatz sheaths. These sheaths are used in medical devices to allow the smooth passage of surgical instruments into the body. The sheaths are designed to be probed into the body and form a channel through which instruments can be passed. In applications such as urology, a polyurethane pipe will pass through the sheath and a guidewire will the pass through the polyurethane pipe. This telescopic arrangement allows the medical guidewire to move into the body with precision and without causing any abrasions within the body.

In other instances, balloons can be fitted to the end of the sheath (which usually has a tapered tip), and these can be blown up once in position by passing air through the sheath.

Most notably, this product has the following properties, beyond what PTFE would normally have:

1. Radiopacity – a filling of barium sulphate or bismuth trioxide makes the tube visible to x-rays. PTFE is naturally invisible to x-rays so the filler arrests this property and allows the sheath to be detected as it is inserted into the body

2. Stiffness – while most PTFE tubes are flexible and come in coils, amplatz sheaths are made stiffer and straighter to suit their application. The added stiffness also makes the PTFE tube kink-resistant, meaning it is safer to use

3. Pigments – while pigmented tubes are not uncommon, amplatz sheaths are usually taken in dark-grey or blue pigments. The addition of pigments is useful for colour coding and also contributes to the stiffness of the material

4. Tapered end – the tubes can be either tapered by thermoforming or simply cut at an angle, allowing them to better glide into the body

The development of amplatz sheaths has allowed Poly Fluoro to become the only company in India with the capability to manufacture these items indigenously. With a fully equipped set-up for extruding high-precision PTFE tubes, we are able to customize for size, colour, and radiopacity, giving us a unique position in an ever-growing, ever-evolving medical device market within which India is poised to become a vital player.

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