Unravelling Polymers

The Definitive Blog on Polymers by Poly Fluoro Ltd.

ePTFE Membranes - Application in high-end face masks

With the recent advent of Covid-19, there is a significant strain on resources pertaining to either the prevention or containment of the virus. 

One of the key shortages highlighted has been around face masks, where some experts have suggested that if the pandemic continues to wreak having, the US presently has only 1% of the face masks it would need.

This shortfall has led to opportunistic behavior, such that the price of face masks has more than quadrupled in the past three weeks. As a result, good quality masks are not only scarce, but are being sold at an unimaginable premium.

Enter ePTFE membranes.

While ePTFE itself is not a low-cost material, its application in face masks could be revolutionary and allow for a very effective product in fighting the current crisis. The reasons for this are as follows:

  1. Permeability – ePTFE has a unique property of being impervious to liquids, but permeable to gases. Since Covid-19 is known to be spread by droplets in the air, ePTFE forms an ideal medium to arrest the passage of droplets

  2. FDA approved – as PTFE is an FDA approved material, it poses no risks to being used in face masks. Indeed, PTFE is one of the few materials that is approved for insertion into the human body, making it completely safe for such an application

  3. Hydrophobic – not only is ePTFE resistant to droplets, it is hydrophobic in nature, meaning that droplets that do reach it are immediately repelled. Hence, there is less risk that an infected droplet would remain on the surface of the material. This allows the face mask to have a longer lift, as there is less chance that the infection stays on the surface of the material.

  4. Low cost – as mentioned, ePTFE is itself an expensive material. However, when used in a small quantity – such as would be needed in a face mask – the price is negligible when compared with standard face masks

With the current crisis in play, it is essential that newer and more effective materials are brought into action. ePTFE is one such material, as it can be manufactured in bulk and embedded into a standard non-woven polypropylene mask to give a manifold improvement in protection, while adding very little in terms of cost.

By our estimate, a simple ePTFE lined face mask should cost nothing more than US$0.2 per piece – which is a far cry from the US$2-3 we are currently seeing for high-end masks on the market.

Demystifying IGLIDUR

We have earlier looked at Turcite B* and Rulon and explained how they are the result of branding exercises that were set in place at a time when the polymer space was more obscure. In both their cases, we find that even today, older drawings received from OEMs will specify the brand and grade to be used and it usually takes some convincing and possibly development and trials on the part of the OEM to shift to an alternative.

Both Turcite and Rulon, as we illustrated in earlier articles, are PTFE based materials. In some cases, specific pigments have been added to the material to enhance wear properties and offer a visual uniqueness to the grade that other processors might struggle to match. It should be mentioned that especially in the case of Turcite, the distinct turquoise pigment used brings certain synergies with the base PTFE material, causing the wear properties to increase significantly in comparison with other pigments. However, there is no restriction as to who can either procure, compound, or process these pigments with PTFE to ensure the same properties are met by other manufacturers.

What is IGLIDUR?

More recently, as we have ventured further down the path of precision machined components, we have had many OEMs asking us for IGLIDUR material. An initial glance through IGLIDUR’s properties told us that here too, a very significant branding push had been given to re-market generic polymer materials. We also realized that IGLIDUR – manufactured by IGUS in Germany – was priced at several multiples of the cost for a comparable grade bought locally.

While there may no doubt be certain base properties or processing techniques used in the manufacture of IGLIDUR that enhance the properties over a generic substitute, the sheer cost difference makes for a compelling case for OEMs to look beyond the brand and evaluate whether an alternative will suit the application.

Below, we list a few of the most common grades of IGLIDUR. Mapping four key properties, we are able to identify – with some certainty – the generic polymer base for the grade. 

 

 

Colour

Specific Gravity

Max Service Temperature (°C)

Tensile Strength (Mpa)

Comparable Material

Iglidur J

Yellow

1.49

120

73

POM / Acetal / Delrin

Iglidur X

Black

1.44

250

170

Carbon PEEK

Iglidur G

Grey

1.46

130

210

PA66 40GF

Igludur P

Black

1.58

130

120

Carbon Filled POM/PA66

Igludur K

Yellow/Beige

1.52

170

80

PES/PESU

For the most part, IGLIDUR grades appear to use either Nylons of Acetal for the base material. In one case – IGLIDUR X, the high service temperature gives away the fact that it must either be PEEK or Polyimide. Similarly, IGLIDUR K, has a high service temperature, but relatively low tensile properties. However, the yellow/beige colour suggests that Polyethersulfone might be the most possible base polymer for this grade.

It should be said that there may certainly be property enhancing additives used in these grades to improve overall performance. However, as mentioned above, any product today can be tested to uncover the true composition of the same. With the composition no longer a mystery, any premium paid would be unjustifiable.

Armed with this knowledge, an OEM can at least begin the process of identifying an alternative. In most of these cases, the grades are easily available from generic stock shape manufacturers. Hence, a proto batch of 20-30 components would be easily developed and can be put under testing without the need for expensive tooling or R&D costs.

Considering the above, there appears a lot of room for exploration for OEMs that are using expensive components because the brand is obscure. IGUS does not easily share the base material used in its products, which might leave many end-users thinking it would be safer to pay the premium and get the right part. However, with the information available today, there is no reason for an OEM to pay many multiples on the cost.

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