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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Even though PTFE remains a niche polymer among more generic materials such as PP (Polypropylene), PVC, PE (Polyethylenes, such as HDPE and LDPE), and even Nylons, within the engineering space it is now quite common. Most applications involving high temperature, corrosive chemicals, high voltages, or high wear/friction now look to PTFE automatically as a solution.

Despite this, there do exist applications where PTFE does not fit the bill and a compromise must be made. For example, applications where high dimensional stability is needed across a wide temperature range, PTFE tends to fall short. The high linear thermal expansion coefficient of PTFE means that it cannot hold its dimensions as temperatures vary. In our own experience, a PTFE can exhibit linear dimensional changes of up to 3% when the temperature moves from 0 to 100 Deg C.

In such a situation, we have seen PEEK being adopted. While PEEK does do the trick, it is also 10X the cost of PTFE. Similarly, certain applications where cost is a constraint need to make do with POM (Delrin), or even PVC, where PTFE cannot be used. In such a scenario, we possibly forego some of PTFE’s key properties.

Over the years a variety of new polymers have been developed to fill the performance and commercial gaps between PEEK and PTFE. These include PFA, FEP, PEK, PPS (Ryton), and PCTFE.

What is PCTFE?

Although not well known, PCTFE (Polychlorotrifluoroethylene) forms an ideal substitute for PTFE in certain applications where PTFE is unable to perform adequately. The table below is meant to offer a snapshot comparison of the two, such that any application engineer can evaluate the key differences.

		Unit	PTFE	PCTFE	Remarks
Properties	Tensile Strength	Mpa	20-30	30-35	With a marginally higher tensile strength, PCTFE rates higher than PTFE on this metric
	Elongation	%	200-350	100-250	PCTFE is stiffer than PTFE, which means it lacks some of the softness of PTFE when it comes to sealing, but that it also holds its dimensions more easily
	Melting Point	Deg. C	350-380	200-220	PTFE is still preferred on outright high-temperature applications
	Dielectric Breakdown Voltage	KV/mm	50-100	20-40	PTFE rates higher on outright dielectric strength
	Coefficient of Friction		0.03-0.05	0.25-0.35	PTFE rates higher as a non-stick material
Processing	Injection Moulding		No	Yes	PCTFE has more versatility when processing, allowing for more complex parts
	Compression Moulding		Yes	Yes
Characteristics	Chemical Resistance		Extreme	Very Good	PTFE is still unmatched in chemical resistance
	Thermal Stability		OK	Very Good	PCTFE rates higher than PTFE when it is a question of stability over a wide range of temperatures
	Price		Med	High	PCTFE is more expensive than PTFE, and is therefore used in specific applications only

 

As you can see from the above chart, PCTFE and PTFE each have unique advantages and disadvantages when compared with one another. Like all polymers, the application needs to be properly understood and the commercials need to be weighed in before any decision can be made.

In recent times, the enquiries for PCTFE - both as a rod and as a finished component - has increased significantly. With more cryogenic applications (fuelled in no small way by the boom in the medical industry due to COVID), PCTFE is being recognised more and more as an invaluable material for low temperature use.

While the PTFE vs PCTFE debate will always have two sides, it is fair to say that when dimensional stability across a temperature range is a must, PCTFE is growing to become a most effective substitute to PTFE.

Datasheet for PCTFE:

Property	Value	Units	Method
MECHANICAL PROPERTIES
Tensile Strength	4860 - 5710 34 - 39	psi MPa	D 638
Elongation	100 - 250	%	D 638
Flexural Strength, 73°F	9570 - 10300 66 - 71	psi MPa
Flex Modulus	200 – 243 x 103 1.4 – 1.7	psi MPa
Impact Strength, Izod, 23 deg C	2.5 – 3.5	ft-lb/in	D 256
Compressive Stress at 1% deformation,	1570 – 1860 11 - 13	psi MPa	D 695
Density	2.10 to 2.17	gm/cu.cm
THERMAL PROPERTIES
Coefficient of Linear Expansion	7 x 10-5	K-1
Melting Point	410 -414 210 - 212	deg F deg C
Thermal Conductivity	1.45 0.84	Btu·in/h·ft2·°F W/m·K	ASTM C 177
Specific Heat	0.22 0.92	Btu/lb/deg F kJ/Kg/deg K
Heat Distortion Temperature, 66 lb/sq.in (0.455 MPa)	259 126	deg F deg C	D 648
Processing Temperature	620 327	deg F deg C
ELECTRICAL PROPERTIES
Dielectric Strength, short time, 0.004”	3000	Volt/mil	D 149
Arc-Resistance	360	sec	D 495
Volume Resistivity, @ 50% RH	2 x 1017	ohm-cm	D 257
Surface Resistivity, @ 100% RH	1 x 1015	Ohm sq-1	D 257
Dielectric Constant, 1 kHz	2.6	ε	D150-81
Dissipation Factor, @ 1 kHz	0.02		D150-81
OTHER PROPERTIES
Water Absorption	0.00	% increase in weight	D570-81
Flame Rating+	Non-flammable		D 635
Coefficient of friction (Dynamic)		0.25-0.35	D 1894
Specific Gravity	2.10 to 2.17		D792
Moisture Permeability Constant	0.2	g/m, 24 hours
O2 Permeability	1.5 x 10-10	Cc, cm/sq.cm, sec, atm
N2 Permeability	0.18 x 10-10	Cc, cm/sq.cm, sec, atm
CO2 Permeability	2.9 x 10-10	Cc, cm/sq.cm, sec, atm
H2 Permeability	56.4 x 10-10	Cc, cm/sq.cm, sec, atm

Manufacturing ePTFE is challenging. The nature of the material throws up various peculiarities and sensitivities that need to be understood from first principles if one is to obtain a consistent and high-quality end-product. Minor changes in the raw material or even the climate can be the difference between an accepted and a rejected final product.

How is ePTFE made?

While there exist many proprietary nuances and technologies in the manufacture of ePTFE, at the basic level the process consists of the following steps:

1. Blending – where PTFE fine powder resin is mixed with an extrusion aid

2. Extrusion – where the powder is compressed and passed under high pressure through a die which defines its profile

3. Stretching – where the extruded material is stretched lengthwise under heat to yield a soft, marshmallow-like material

While this is obviously a gross simplification, the factors that can influence the process are many. These may include:

1. The properties of the resin itself. High crystallinity is needed to ensure the stretching process is effective.

2. The handling of the resin – over handling will cause sharing which can lead to the resin being unusable

3. The type of extrusion aid used

4. The extrusion pressure and post extrusion handling

5. The stretching parameters – stretch rate, speed, and temperature all combine in very precise ways to give the final product

Once the extrudate is stretched, it undergoes a process called “amorphous locking”, which allows it to take on two principal characteristics: texture and porosity.

Applications of ePTFE

While texture is the primary focus of anyone looking to use ePTFE as a gasket material, porosity is the focus of membranes and filter applications. Both these characteristics can be modified based on the stretching parameters such that the specific gravity of ePTFE tapes can vary from as little as 0.3 all the way up to 1.5. Similarly, porosity is a function of how much the material is stretched and changing the stretch ratio will influence this.

The porosity of ePTFE is unique in that it allows vapours and gases to pass through, while preventing liquids from doing so. This has very important implications in venting and high-end filtration, wherein a system may need to be leak-proof, while also allowing excess gases to escape rather than cause any pressure build up. Similarly, ePTFE membrane vents are also used in enclosures for electronic circuits, as they ensure that water cannot enter the enclosure, but that any accidental moisture build-up is evacuated rather than being allowed to condense around the circuits.

Advantages of Expanded PTFE (ePTFE) Tubes

In some sense, the use of an expanded PTFE (ePTFE) tube seeks to combine the properties of both texture and porosity. The ePTFE tube is made in a way similar to what is described above, with the exception being that the profile extruded is that of a tube. Since an extruded tube is delicate, a lot of care must be taken to ensure that it does not collapse during stretching. However, once stretched, the ePTFE tube is a very unique product.

For one, the tube is highly flexible. Normal PTFE tubes are prone to ‘kinking’ if the bend radius of the tube is exceeded. In contrast, ePTFE tubes will allow themselves to be bent, be folded, and even be wrapped while still resisting kinking. 

In addition to this, the ePTFE tube also has porosity. It is ideal for a system wherein a liquid must be passed through the tube but where vapours must be expelled. 

ePTFE tubes are invaluable in both chemical as well as medical applications. While standard ePTFE tubes are used for automotive applications, the development of specialised ePTFE tubes can be used for surgical grafts and stents.

While the challenges in making ePTFE tubes are numerous, overcoming these hurdles yields a product that is both versatile and highly effective in a multitude of applications.

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