Unravelling Polymers

The Definitive Blog on Polymers by Poly Fluoro Ltd.

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™

PTFE Tube - End Properties and Characteristics

Our recent installation of a continuous line PTFE Paste Extruder has thrown up many findings. At each stage, we have needed to evaluate whether the finding impacts the properties of the final product. Given the fact that globally, There are very few PTFE tube manufacturers, 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 material PTFE.

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 that can withstand high temperature and pressure.

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 / Teflon tube of different sizes, 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:

    ptfe tube burst pressure

    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. Non-concentric tube 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 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 PTFE tube Suppliers / 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 weigh 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 not 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.

Charting ePTFE (expanded PTFE) Specifications as per Global Standards

One of the toughest things about being the first in a given field is that there is so little data available for testing against.

As the only Indian company manufacturing ePTFE (expanded PTFEgasket tapes, we are constantly met with questions regarding how the properties of our material hold up against those of competing brands operating in Europe and the USA. However, since the material is so new, there do not exist any established testing standards locally for us to check the product.

Basic Initial Data

To counter this, we initially took up the task of importing tapes from other manufacturers and testing the tensile properties and specific gravity against the same. Initially, we were trying to answer only 2 questions:

  1. How soft should our tape be? 

    Since the extent to which we expand the PTFE can be adjusted, it directly impacts the specific gravity of the end product. For a like-to-like comparison, we were hoping to match this with global brands. We eventually found that the standard density of ePTFE Gasket Tapes is 0.6-0.65g/cm3 

    It should also be mentioned that some clients have specifically come to us asking whether they can get the tapes even softer, as their application is such that not much force can be applied to the tape. We have obliged – getting the density down to as little as 0.3g/cm3 in some cases.

  1. How strong should our tape be? 

    ePTFE tape looks great coming out of the machine. It is pure white, soft to touch and very smooth. However, two tapes that look exactly the same, could give completely contrasting values when tested for tensile strength. We found that global brands offered tensile strengths in the range of 5Mpa to 10Mpa.Once we standardised our production process, our own tapes showed a tensile strength of 12Mpa, so we were satisfied with the result.

Looking for global standards

Although we were happy with the properties of our material, there were still gaps in our understanding. Most notably, what were the other properties we should be testing? And rather than compare between brands, should there not be a global standard that specified the values we needed to obtain?

Again, going through competitor data provided very little information on this front. Expanded PTFE is a very niche market and from our own experience of getting the product right, we know that not much information can be divulged with regards to the behavior of the material.

We looked around for global standards and realized that although there are many ASTM standards for regular PTFE, for ePTFE there were none. A few competitors had put up data on compressibility (ASTM F 36) and creep relaxation (ASTM F 38), but these were only comparing values to “leading brands” and not referring to any standard for the values. Others simply quoted the values, but did not elaborate the specifications against which these values would hold up.

We also went through the certifications that competitor brands were providing. These included:

  1. DVGW VP 403 – The German standard for checking ePTFE Tapes
  2. TUV MUC-KSP-A066 – The TUV Standard for ePTFE
  3. BAM – For use in Oxygen rich environments

We contacted each of these organisations and were given estimates on how much the testing would cost. However, at no point are any values discussed. These remain guarded by the certification bodies. Our worry was that if we sent our material to these bodies without adequately testing them ourselves first – there was a risk that we may have overlooked a certain property and due to this, the product may not pass, resulting in an expensive mistake.

Eventually, we came upon the one standard that dealt specifically with ePTFE Tapes and was willing to offer values for us to compare against – the AMS 3255A.

The AMS is globally recognised as a leading authority for aerospace related materials. As such, we felt confident that their values would be stringent and thereby an effective standard to hold ourselves to.

The AMS 3255A prescribes many types of ePTFE Tape. Our basic tape falls under Class 2, Type 1, which requires the following properties to be met:

 

Property Value/Result Unit
Specific Gravity 0.4-1.2  
Tensile Strength 3.44 Mpa
Tensile Strength (Fluid/Thermal Stability) 8.27 Mpa
Low Temperature Flexibility No evidence of cracking  
Liquid Sealability No fluid leakage or loss of pressurization  
Reparability No fluid leakage or loss of pressurization  

Armed with the above data and the testing procedures prescribed by the AMS 3255A, we were able to test our material in-house to confirm that the properties we were observing were as per the requirements of the standards.

In addition to this, we were also able to test the material with local certification bodies, to confirm the properties.

To the best of our knowledge, the AMS 3255A remains the only globally recognised standard to offer any values and/or testing procedures to verify the properties of ePTFE Gasket Tapes.