Unravelling Polymers

The Definitive Blog on Polymers by Poly Fluoro Ltd.

PTFE tubing - one product, numerous applications

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 tube 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 250oC 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.

Property

Comments

Applications

Heat resistance

  • Working temperature range of -260 to +260oC
  • Melting point of 327oC
  • High temperature fluid transfer
  • Insulation of metal parts

 

Dielectric strength

 

  • Working range of 50-120 Kilo volts per mm

 

  • Insulation of electrical cables

 

Low friction

 

  • Coefficient of friction of 0.1
  • Almost identical static and dynamic coefficients

 

  • Catheters
  • Snares

 

Corrosion resistance

  • Water absorption at 0%
  • Chemically inert - affected only by molten alkali metals, fluorine and chlorine trifluoride at elevated temperatures and pressures

 

  • Chemical substances transfer
  • Protection of metal parts

Table 1: Key properties and applications of PTFE tubing

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).

 

Diameter (mm)

Wall thickness (mm)

Spaghetti tubing

0.2-8

0.1-0.5

Pressure hose

6-50

1-2

Pipe Liner

12-500

2-8

Table 2: Categories of PTFE tubing

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

Type

Description

Purpose

 

Multi-lumen

 

Single outer tube with multiple inner tubes

Each inner tube holds a different fluid/ wire - useful in medical applications

 

Split

 

Ridge on tube wall allowing it to be split longitudinally

Surgeon can remove a PTFE introducer from a patient while the primary device remains in place

 

Corrugated/ convoluted

 

Folds on outer wall

Gives higher bend-ability, reducing risk of kinks when tube is passed through tight angles

Heat shrinkable

Thin tubing which shrinks in diameter when hot air is applied to it

Used to sheath wires, glass tubes for insulation or protection

 

Filled

Chemical additive giving radiopaque properties

Used in medical inserts - to show up in X-rays

Table 3: Variants of PTFE tubing

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 tubing. After the cleaning the tubes are packed in polythene covers for dispatch.

Fig 1: Typical extrusion die

PTFE Tubing and Poly Fluoro Ltd. - FluoroTube™

Poly Fluoro Ltd. was established in 1985 – at a time when India was not yet fully aware of the properties of PTFE or its usefulness across so many industries.

The company has built its expertise in mainly industrial applications – making machine components, slideway bearing materials (Turcite/Lubring) and PTFE tapes – and become a reputable player in the industry.

More recently, Poly Fluoro Ltd. has embarked on a plan to strengthen its presence in medical applications. With this in mind, the company has invested heavily in developing laboratory wares, PTFE coated guidewires (used extensively in urology) and PTFE tubing.

FluoroTube™ marks the entry of Poly Fluoro Ltd. into the PTFE tubing segment. With this product, Poly Fluoro is looking to build a tubing brand, which assures the client the highest quality of PTFE tubing.

FluoroTube™ will also be the first PTFE tubing manufactured in India – giving the local market
PTFE tubing at a price point that would greatly improve their cost dynamics and allowing the full demand for PTFE tubing to be met in India.

The grades and sizes available make FluoroTube™ ideal for applications such as medical, chemical and automotives.

FluoroTube™ comes in sizes ranging from 1mm to 25mm diameters and is unique in many ways when compared to conventional polymer tubing. Table 3 shows the technical properties for FluoroTube™.

Fig 2: FluoroTube™

Table 3 : Technical specifications of FluoroTube

Property ASTM test Value

Physical properties

   
Specific gravity D792 2.15
Water absorption ( % ) D570 / 24 hrs 1/3" t < 0.00
Mold shrinkage ( cm / cm )   0.02 – 0.05
Contact angle ( degree ) Angle to level 110

Thermal properties

   

Thermal conductivity (cal/sec/cm2, o /cm )

C177

6 x 10-4

Coefficient of liner thermal expansion(1/oC) D696 / 23 - 60oC 10 x 10-5
Melting point (oC )   327 

Melt viscosity ( poise )

 

10^11–10^13

(340 -380oC)

Maximum temperature for continuous use (oC / oF)   260 / 500

Mechanical properties

   
Tensile strength ( kgf / cm2 )

D638 / 23oC

140 - 350
Elongation ( % ) D638 / 23oC 200 - 400
Compression strength ( kgf / cm2) D695 / 1 % deformation, 25oC 50 - 60
Tensile modulus ( kgf / cm2 ) D638 / 23oC 4,000
Flexural modulus ( kgf / cm2 ) D790 / 23oC 5,000 – 6,000
Impact strength ( ft - lb / in ) D256 / 23oC, Izod 3
Hardness (Shore) Durometer D50 - D65
Deformation under load ( % ) D621 / 100oC, 70 kgf / cm2, 24 hrs 5
  D621 / 25oC, 140 kgf / cm2, 24 hrs 7
Static friction coefficient Coated - steel surface 0.02

Electrical properties

   

Dielectric constant

D150 / 103Hz

2.1

Dielectric dissipation factor D150 / 106 Hz 2.1
  D150 / 103 Hz < 1 x 10-5
Dielectric break down strength (V / mil) D149 / Short time,1/ 8 in 480
Volume resistivity( ohm - cm ) D257 > 1018
Chemical resistance   Excellent
Weather ability   Excellent
Combustibility ( % ) D2863 / Oxygen concentration index > 95

 

Polymers in Fluid Transfer Applications

The transfer of fluids can be a complicated affair. In most applications that involve fluid transfer, the system is simultaneously subject to one or all of the following conditions:

  1. Pressure

  2. Temperature

  3. Corrosion

Each of these conditions further compounds the effects of the other. For example, while a system may be equipped to handle high pressures, the added effect of corrosion can lead to ruptures or pinholes within the system that can cause failure. Hence, that any system that seeks to contain or transport fluids needs to ensure that all precautions are taken to accommodate the effects and minimise the risk of leakages.

Polymers are the preferred choice for fluid transfer applications for several reasons. First, there exist a huge range of choices that can be compared with the chemical properties of the fluid in question to ensure that the polymer does not react during functioning. A major issue with using metals is that even though they may not necessarily corrode, there is no guarantee that there will not be some reaction with the chemical fluids. Such reactions can alter the properties of the fluids themselves, which would be a problem. Polymers also operate at a wide range of temperatures and given that they are not as hard as metals, they invariably bring sealing properties that metals cannot match.

1. Polymer Seals
Seals can be machined from polymer stock shapes to match the tolerances of any system. Polymer seals are a vast area of application and can include everything from:

  • Ball valve seats

  • Spring-energised seals

  • Sealing rings

  • Chevron V-packings

  • Rotary seals

  • Linear sealing strips

There is no limit to the types of polymers that can be used in a sealing application. PTFE, PEEK, PPS (Ryton) and Polyimide seals are usually preferred in applications where there is a combination of high temperature and corrosive chemicals. In lower temperature applications (say, within 120°C), polymers such as POM (Delrin), PVC or even Polypropylene can be used.

The choice of polymer here is entirely application based. As always, it starts with the chemical compatibility and moves from there. For example, while PEEK is a very robust and machinable polymer, PPS is a preferred option in the paper and pulp industry. This is primarily because even though PEEK is chemically very inert, it does suffer some reaction to the chemicals used specifically in pulp and paper manufacture. With PTFE – which is easily the most chemically inert polymer – the issue is that of deformation. While PTFE can take high pressures – the combination of high pressure and temperatures can cause deformation in PTFE seals over time, leading to leakages.

2. Polymer valves

The function of a valve is to regulate the flow of fluids through a system. Not only does the valve need to ensure that there is a tight sealing around it (fluids should not be able to flow around the valve), but the valve needs to resist the fluids flowing through it and ensure that thermal expansion due to high temperatures does not hinder the smooth movement of the valve.

One application of PTFE valves is in the paints industry. Paint mixing machines use PTFE valves to regulate the flow of liquid paint. PTFE is the material of choice because paints are composed of myriad different chemicals. Each shade of paint would be a result of a specific combination of additives and it is therefore essential to have a material that does not react with what may be potentially thousands of different compounds.

PEEK valves are used extensively in coffee machines. The combination of high temperature liquids and food grade requirements means that PEEK – which is FDA approved – is a key material of choice. PEEK is also very thermally stable, which means that the valve does not expand (and therefore tighten) even when higher temperature liquids are passed through it.

3. PTFE bellows

Bellows are complex parts that need to be machined out of PTFE. The key function of a bellow is to accommodate excess pressures and ensure that the system – typically a pumping system – does not fail over a long period of time. PTFE is used primarily because it is soft and because it is chemically inert. The softness of the material allows for the bellow to expand and contract, rather than succumb to higher pressures.

Different bellows have different ratings for the number of cycles that they can accommodate. However, a range of 1-2 million cycles is standard in the industry.
ptfe bellows
The only issue with a PTFE bellow is that because PTFE cannot be injection moulded, the bellow needs to be machined out of a block. This results in a wastage of nearly 80% on the material, making PTFE an expensive choice. Nonetheless, there are applications where nothing other than PTFE will suffice, and hence it is a preferred material in high-end chemical pumps. 

4. ePTFE gaskets

Expanded PTFE, like regular PTFE, comes with a stellar ability to resist chemicals. The only drawback with regular PTFE is that it lacks the elasticity, or seal-ability, of some softer materials such as silicone or Viton rubber. However, these materials still cannot accommodate the high temperatures that PTFE can.

ePTFE arrests some of the issues seen with regular PTFE in that it is highly compressible (up to 65%) and offers an excellent sealing between harder surfaces including metals and glass. More importantly, ePTFE provides sealing in pressures of up to 100Bar, with minimal torque. This means that even more delicate assemblies can be fully sealed without having to put excess pressure on the bolted areas.

Both ePTFE cut gaskets and ePTFE gasket tapes are being increasingly adopted across fluid sealing applications. A combination of its ability to withstand high-temperature and high corrosion while offering high sealing makes it a material of choice.

5. Polymer bobbins

Some fluid sealing systems have a combination of metals and elastomers. For example, airline fluid systems use neoprene rubber for the transfer of fluids. In the event of fire or excess heat, it is essential that these rubber tubes be kept safe and away from metals – which can heat up quickly and melt the rubber easily.

Polymer bobbins are used a medium to shield the rubber from the metals. Metal clamps – rather than being fitted directly on the rubber hose – will clamp around the bobbin which will then come in contact with the hose. This arrangement ensures that the bobbin, which can withstand higher temperatures and will not transfer heat, will keep the neoprene hose safe in the event that the clamp heats up.

6. PTFE (Teflon) Tubes

PTFE tubes are one of the most sought after for fluid transfer. Tubes are resistant to chemicals and high temperatures. At the same time, the wall thickness of a PTFE tube can be enhanced to allow it to accommodate high pressures. Further, with stainless steel braiding, these pressures can be even higher. 

PTFE tubes find application across industries such food processing, chemicals, electrical, and pneumatic lines, to name only a few.

A key example of the application of PTFE tubes is in analyser equipment. The equipment is used to evaluate the chemical composition of gases that reach it though several tubes. Since the gas cannot be allowed change its chemical structure in any way, it is essential that it travels through a medium that will not react with it. PTFE is an ideal medium, as there is no chance that the gas running through it will react with it and this ensures the purity of the system.

The above are but a few examples of polymers and their applications in fluid sealing systems. For the most part, OEMs will design their own systems and then look for polymer solutions that specifically match their application. As such, there are literally thousands of different areas where polymers are used to ensure fluid systems are kept robust and free from leakages.

PEEK Components and Bearings - Durable, lightweight, and dependable

Developing a new polymer is challenging and needs to take into account what end applications are being catered to. Often, specific polymers will be identified for a certain application, but their limitations would be highlighted. Working off these limitations, a new material can be developed that seeks to fill gaps in performance. Selecting the best material is a critical step in the development of a polymer and this selection process will determine the quality of the final product. Strength, durability, malleability and many other factors are considered when a material is being selected and each of these factors holds a significant importance. 

The development of PEEK, however, was surprising because suddenly, we had one polymer that exhibited properties at the far end of nearly every parameter. Whether we talk of tensile strength, strength-to-weight ratio, wear resistance, machinability or temperature resistance, PEEK truly checks all the boxes and leaves little doubt as to its effectiveness any time a debate on material selection is had.

Suffice to say, that the invention of PEEK revolutionized the plastic component manufacturing industry

PEEK was invented in the UK in 1978 and received a US patent in 1982. The processing conditions implemented to mould the material influence its crystallinity, or structure, and this results in the superb mechanical and chemical resistance properties of PEEK. PEEK is also highly resistant to degradation under high temperatures and resilient in organic and aqueous conditions. These properties make PEEK ideal for a wide array of components in diverse industries. Some of the major fields where this dynamic plastic material has been most effective are aerospace, the medical space, food & beverage and also the manufacture of PEEK plastic bearings.

PEEK in aerospace components

PEEK is optimal in aerospace due to its lightweight properties (it has a specific gravity that is half of aluminium), chemical inertness and toughness. Light-weight materials reduce the overall load of a vessel and, thus, reduce fuel costs and helps preserve the environment. Because PEEK is a chemically inert, the material is stable in extreme conditions and can withstand exposure to atmospheric particles. The fact that PEEK can be machined to the kind of close tolerances needed in aerospace make it an ideal material of choice.

PEEK in medical equipment

In the medical field, hygiene, durability and biocompatibility are all important factors when considering what materials to incorporate. PEEK offers flexible and durable properties and it also has no adverse effects on the skin or internal organs, which makes it an ideal material for implants. Biocompatible grades of PEEK can be used as sensor covers and transducers in medical equipment. Such grades ensure that even in the event of bodily contact, there would no adverse effects. Its resistance to extreme temperatures and low moisture absorption properties also make it a preferred choice for pumping mechanisms.

PEEK in food and beverages

The high-temperature resistance of PEEK makes it a favourable option in the production of powders and other ingredients in the food & beverage industry. PEEK is an effective alternative to PET and holds up well under the extreme temperatures for the drying of ingredients in food as well as feed for animals. The high wear resistance of PEEK also makes it an ideal choice for scrappers, where other polymers would either chip or deform. PEEK valves are also critical in coffee machines, where metallic valves have been known to impart an unfamiliar taste to the beverage. Since the last 5-10 years, high-end coffee machines around the world use PEEK valves in their equipment.

PEEK as a bearing material

One field that has some of the most diverse applications of PEEK is the manufacturing PEEK bearings. PEEK makes an ideal material for bearings for many reasons. PEEK bearings can be made self-lubricating (usually with the addition of PTFE at a concentration of 15-25%). Abrasion causes microscopic bits to function as lubrication with inconsequential alteration to the bearings themselves. The durability and high-temperature resistance of PEEK bearings allows them to be thinner and lighter than traditional material bearings. Their light-weight characteristics make them more fuel-efficient and they don’t require the constant grease and maintenance that metallic bearings demand. As a result, PEEK can reduce bearing production and maintenance costs significantly. PEEK bearings can also run for long periods of time without freezing during operation. The low thermal expansion coefficient of PEEK ensures that it retains dimensional integrity over a very wide range of temperatures – both high and low. The chemical resistance of PEEK bearings allows them to withstand conditions that would otherwise cause damage to metal bearings. All these attributes make PEEK bearings an exceptional mechanical component for an endless number of products and functions

As we can see from these examples, PEEK is effective in a range of industries and PEEK bearings have become a favoured option as a mechanical component. PEEK bearings can be found in many different types of products and even in motors that go into the refrigerators and air conditioners in our homes. PEEK bearings will continue to be implemented in more and more products in the future as well. Keep your eyes open for PEEK plastic materials and PEEK bearings in the machines and products around you.