Unravelling Polymers

The Definitive Blog on Polymers by Poly Fluoro Ltd.

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.

PTFE Pricing - Is volatility making a comeback?

An introduction to PTFE Pricing

Being a niche industry, PTFE doesn’t exactly get a lot of press even when large shock waves surge through it, disrupting the operations of manufacturers and end-users alike.

In 2010-11, the price of virgin PTFE began climbing, after having dipped consistently over the preceding 5 years. What started out as an understandable correction soon turned into an all-out crisis, as the price nearly quintupled over the next 8-10 months.

Imagine that scenario in any other industry. If the steel price to see a 5x increase in less than a year, what horrors would that unleash into the broader market? That the PTFE industry survived is a testament to the incredible properties of the material, that make it so difficult to substitute using other polymers such as PVC. Both PVC & PTFE are thermoplastics and The PVC prices are much lesser compared to PTFE.

To make matters worse, since the industry is small, there was no one to really make sense of the economic factors driving the price increases.

We stepped in at that time, partly driven by our own need to analyse the situation, but also because the information we collected seemed like it would be useful to other industry players.

Many articles were released between 2011 and 2013 that charted the various drivers of PTFE pricing and made sense of the driver that would play out going forward.

April 2011 – What’s the matter with PTFE prices?

May 2011 – The mysterious relationship between Fluorspar and PTFE prices

August 2011 – Mapping the PTFE Price Increase

December 2011 – PTFE Pricing Again – is there another price hike in the offing?

March 2012 – PTFE Prices – taking a step back to leap forward?

February 2013 – Mapping the PTFE Price Increase – An Update

 

Over the past few months, there have been murmurs of a return to the high pricing seen in 2011. Resin suppliers have offered ample warning that a price revision was imminent. So far, this has translated into 2 price corrections to the extent of 10-15% and 8-10% in the months of April and May respectively.

We need to compare this with the escalations seen in 2011-12 and understand that the rise is not quite so sharp. Nonetheless, we also need to compare the cost drivers and ensure that the same pattern is not repeating.

Key Drivers of PTFE Price

Unlike other polymers – such as polythene, polypropylene and polycarbonates, PTFE does not result as a by-product of the oil refining process. This comes as a shock to many, who assume that our fates are tied to oil prices and that we should therefore be benchmarking our final rates accordingly.

PTFE is made from the polymerisation of R22 gas – a refrigerant that finds its use mainly in the air conditioning and refrigerator industries. As a result, PTFE prices are driven by the demand and supply of R22. Many OEMs are working really hard to make sure R22 is available at competitive prices.

When prices started increasing in 2011, the key driver was thought to be Fluorspar – a mineral that is critical to the manufacture of R22. In conjunction with this, a spike in demand from the AC and refrigerator markets in China along with a general crackdown on R22 usage (as it is not an environmentally friendly gas) caused prices to shoot up.

Today, the scenario is slightly different.

For one – we’re over halfway through the summer, so we’re not likely to see any shocks due to AC and refrigerator demand. Furthermore, Fluorspar supply has also regularised. A key effect of the price increase in 2011 was that idle Fluorspar mines in South Africa were reopened, offering an easing of supply to the market that was otherwise dominated by China.

If industry insiders are to be believed, the key driver this time around is environmental. China has been actively seeking to clamp down on R22 usage and has, in the process, shut down two PTFE plants that were not adhering to the standards. It is from here than the supply constraint has originated, driving up PTFE raw material prices within China. Obviously, as the China price is usually the floor price for most goods in the world, this has allowed resin manufacturers around the world to increase rates accordingly.

Where the prices go from here depend on three factors:

  1. Whether the shut-down plants will be coming back on line after making the necessary changesIt is yet unclear if the plants have been permanently shut down or are only undergoing an overhaul to make them compliant with the environmental codes. If they do come back online within the next 6-8 months, we would see a return to lower prices
  2. Whether the move to phase out R22 will go as scheduledR22 is being phased out not only in China, but all over the world. India has also committed to stop using the gas completely by 2032. If this phase out continues, it could result in a supply surplus to the PTFE industry, driving down prices. However, if the use of R22 in PTFE is itself restricted, then it would require a shift to alternatives of R22, which would be expensive.Currently, as there is no talk of restricting R22 in PTFE specifically, it is likely that the former would result
  3. Price inelasticity of PTFEIn 2011, one thing that was made clear was that even at 5x multiples, there was still demand for PTFE resins. This allowed the resin manufacturers to continue increasing prices well above the rates that would have resulted from purely economic factors.

Commodity resins are often inexpensive and easy to process plastics. They are used in all kinds of applications, including toys, packaging, and consumer products.

Ultimately, it was due to competition from China that forced prices back to normal levels.

In the short term, it is likely that resin suppliers will again try and test the market to see what levels they can sustain at. With the threat of China temporality removed, it remains to be seen how far they will push the market.

We only have snippets of information at this point to make sense of the situation. Clearly, with PTFE being used in so many fields, this poses some concern to many industries. We will be keeping our ears to the ground to see if anything else come up.