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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Read More

1. ePTFE Applications in Cable Manufacturing

2. Expanded PTFE (ePTFE) Tapes in Aerospace

3. ePTFE Gland Packing - An Effective Sealing Element

PTFE sliding bearing, sliding plate bearing, ptfe slide bearing plates, sliding bearings, ptfe slide bearings, teflon sliding plate, sliding bearing design, ptfe sliding plate, teflon slide bearing

PTFE sliding bearings are an essential part of any load bearing structure that is likely to experience either thermal or mechanical movement. Despite this, its design and construction remain obscure, with many consultants and civil engineers preferring to leave the bearing’s exact composition and measurements in the hands of the bearing manufacturer.

On the face of it, a PTFE sliding bearing - also called a sliding plate bearing, a bridge bearing, a bearing pad or a Teflon sliding plate - is a simple assembly. Primarily, it consists of only 2 layers – a PTFE plate and a polished stainless-steel plate. These two materials are known to have the lowest coefficient of friction between any two solids and slide over each other effortlessly, especially when subjected to high pressures. In order to keep the PTFE in place, a mild steel plate is normally used as a backer. Since PTFE does not bond easily to other materials, this mild steel plate is often recessed, with the PTFE being bonded within the recessed pocket. This ensures the PTFE plate stays in place, even under high shear loads. This mild steel-PTFE combination is called the lower element. Similarly, as polished stainless-steel is expensive and since only a layer of about 2mm is needed anyway, another mild steel plate is used as a backer for the stainless steel. In this case, the stainless steel is stitch welded to the mild steel to ensure that it stays in position over the long term. This mild steel-stainless-steel combination called the upper element.

Having thus defined the basic form of the bearing, the other aspects of design can now begin to take shape. Often, these parameters will be defined by the client, leaving it to the manufacturer to both design the sliding bearing accordingly and prove – using calculations based on material properties – that the specified parameters can indeed be accommodated. These include:

1. Vertical load – possibly the most relevant parameter. A PTFE sliding bearing needs to be able to comfortably hold the compressive load being placed on it. Since steel has a far higher compressive strength than PTFE, the design focusses on the area of PTFE to be used, considering PTFE’s own compressive strength. It should be noted that while PTFE is capable of taking loads as high as 400 Bar (40Mpa), designers would do well to take a safety factor of 50-60% against and consider a compressive strength of 150-200 Bar when making calculations. Once the quantity in square centimetres is established, the exact length and width can be altered to suit the size of the portal plate on which the bearing is to be installed.

2. Longitudinal movement – The whole purpose of a PTFE  sliding bearing is to accommodate movement while taking vertical loads. In the case of certain structures – such as pipelines - the linear thermal expansion of the system can be so high that between morning and noon, the bearing may be required to slide over 100mm in either direction. Movement dictates the extent to which the stainless-steel sheet would be required to extend beyond the PTFE plate.

While on its own, movement seems like a simple matter of adding the value of the total expected movement to the size of the PTFE plate, movement also creates an issue of cantilever loads. The further the upper element extends beyond the lower, the more chances there are of bending. Thus, care also needs to be taken to ensure that the thickness of the mild-steel plate in the upper element is high enough that bending will be avoided

3. Lateral movement – while some sliding bearings are free to slide in all directions (aptly called: free sliding bearings), for the most part, a bearing only needs to slide longitudinally. This means that in the lateral direction, movement would be restricted. To ensure this, either guide plates can be used along the side of the bearings or dowel pins can be incorporated on to the lower element, which would sit inside longitudinal slots on the upper element to prevent lateral movement.

The key consideration here is the extent of lateral load expected. Based on the same, the dowel pin can be designed such that it will not bend.

4. Uplift loads – many structures may experience uplift loads due to either heavy winds or some mechanical characteristics of the system. This can cause a misalignment in the bearing or – in the worst case – even cause the upper element to slip off the lower element completely, causing major structural damage.

Such uplift loads can be accommodated by the use of brackets or a T-shaped dowel pin. Care needs to be taken that the load on the pin does not exceed the tensile strength of the pin itself. To ensure this, we usually employ pins using stainless steel, where the tensile properties allow for higher loads on the same size pin.

In addition to the strength of the dowel pin and/or side guides, it is important to note that a sliding bearing is all about reduced friction. It means little that the PTFE and stainless-steel would slide over one another if there is friction between the guiding elements themselves. Hence, special care needs to be taken to ensure that the gap between the pins and the slots in the bearing are sufficient to allow for free movement. In addition to this, PTFE would need to be used between the slot and the pin to ensure that even if the pin came into contact with the slots, sliding movement would still take place. This is another reason that stainless steel is used for the dowel pin, as it can be polished to ensure a minimal coefficient of friction.

5. Rotation – while most sliding bearings require very minimal rotation (fractions of a degree), there are some assemblies where the flatness of the system could be compromised, causing the upper and lower elements to lose some contact. Employing an elastomer – like neoprene or even silicone – allows the bearing to compensate for this to some extent. Given the nature of the elastomer, higher rotation can only be accommodated by increasing the thickness of the said elastomer, which in turn can cause issues with stability. In such a situation, a spherical bearing arrangement could also be designed in to increase the allowable rotation.

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Our own experience with PTFE sliding bearings has shown us that oftentimes, the bearing is the last thing to be designed. In many cases, we have heard that the project has been in the last stages of completion and the bearing was either forgotten or it was otherwise assumed that it was an off-the-shelf item that could be supplied ex-stock! The result of this is that the bearing manufacturer needs to work around constrains such as the size of the portal plate and/or the available gap between the sub and superstructures within which the bearing needs to fit. Sometimes there are even restrictions on welding or bolting, meaning the bearing manufacturer has to design something that can be installed on site with minimal fitment.

The result of all this is that a PTFE sliding bearing is usually a premium product and that a good manufacturer needs to understand all the parameters such that an effective solution can be supplied, often with very little lead time. It is rare that we have even supplied the same bearing twice, because with each new project, the bearing needs to evolve to meet the project’s peculiarities!

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Read More

1. Shear Load Considerations for PTFE Sliding Bearings

2. PTFE Sliding Bearings: Calculating Coefficient of Friction

3. Cantilever Load Considerations for PTFE Sliding Bearings