Among the varied high-performance attributes of PTFE, its electrical properties are probably the least understood. PTFE’s insulative properties make it useful in various applications, particularly in the electronics and electrical industries. However, the electrical properties of PTFE can vary based on the grade and type of resin used, the filler content, and the final part being produced.  

PTFE comes in many grades, each suited to a specific end shape. For example, granular moulding resins are used for basic compression moulding, whereas fine powder resins are used in the extrusion of PTFE tubes, profiles, and in making expanded PTFE tapes and sheets. Presintered resins are also used to make rods and high wall thickness tubes and pipes.

Further to the type of resin, the material itself can either be used in its virgin form or with fillers that may enhance or reduce the properties.

Virgin PTFE

Virgin PTFE, also known as unfilled PTFE, is the purest form of the polymer. It exhibits the highest electrical properties among PTFE grades due to the absence of fillers. This grade is often used in applications where high dielectric strength and low dissipation factor are crucial, such as in high-frequency cables, capacitors, and insulators.

Filled PTFE

Filled PTFE is modified with various fillers such as glass, carbon, graphite, and bronze to enhance its mechanical properties. While the addition of fillers can improve attributes like wear resistance and thermal conductivity, it generally impacts the electrical properties. For instance, glass-filled PTFE maintains good dielectric strength but may have a higher dissipation factor compared to virgin PTFE. Carbon-filled PTFE, on the other hand, exhibits lower dielectric strength due to the conductive nature of carbon and are used to make anti-static PTFE. Special fillers such as mica can help enhance dielectric properties but need to be used in the right concentrations.

Several PTFE products are commonly used in insulation applications and their properties need to be better understood.

Thickness (Microns)	Dielectric Strength (kV/mm)
15	115-230
25	89-180
50	63-130
75	52-100
100	45-90
150	37-70
200	32-60
300	26-50
500	20-40

1. Skived PTFE tapes – skived PTFE tapes, or films are usually made in thicknesses between 25 and 75 microns. Skiving is a process specific to PTFE and involves “peeling” a layer of PTFE in a controlled manner to produce a continuous length of tape with a specific thickness. For the best quality tapes, high-purity granular resins need to be used with a low particle size. The purity of the resin ensures a higher breakdown voltage, while the particle size allows for a very consistent film that will not have any pinholes.

   Sintered PTFE tapes – as they are often called – can be used in a range of applications including cable wrapping, as battery separators, and can be wrapped around high-intensity electrical points for insulation.

   At these low thicknesses, the breakdown voltage can be as high as 250kV/mm for films less than 15 microns. However, one cannot assume this would sustain as the material gets thicker. As the thickness increases to 100+ microns, there would be a reduction in the breakdown voltage, going to as low as 20kV/mm for lower grades and thicknesses in excess of 3mm.

   PTFE Tubes – both thin and thick-walled tubes are used in insulation. Thin-walled tubes would typically have wall thicknesses within 2mm, with thick-walled tubes having between 2 and 5mm.

   Both thin and thick-walled tubes can be made via paste extrusion, which employs the use of “fine powders”. These powders need to be mixed with a volatile extrusion aid in order to allow for extrusion. Thick-walled tubes can also be made via ram extrusion, wherein a ram repeatedly compresses charges of “presintered resin” to create a continuous tube or pipe.

2. Expanded PTFE – made using special “fine powders”, ePTFE is also extruded, but then undergoes a process wherein it is rapidly stretched at high temperatures. The resulting material has microporosity but is also an excellent insulator. Like skived films, it can be used to wrap cables and to act as an insulating medium in electrical systems.

Electrical Properties of PTFE

The key electrical properties of PTFE that are critical for its applications in the electrical and electronics industries include dielectric strength, dielectric constant, dissipation factor, and volume resistivity.

Because of the varied methods to make different end-products, standards exist to define the dielectric strengths accordingly. While ASTM D4894 covers the basic properties for granular resins, the following standards are a few that cover the more specific values expected:

ASTM Standard	Item Covered	Breakdown Voltage (kV/mm)	Remarks
D3293	PTFE Sheet	4-24	Value increases with grade purity
D3294	Basic PTFE Shapes	12-24	Value increases with grade purity
D3295	Thin-walled PTFE Tubes	35-55	Value reduces as wall-thickness increases
D3308	PTFE Skived Tape and Film	20-250	Value reduces as thickness increases
D1710	Thick-walled PTFE Tubes	10-30	Applies to tubes made using granular resins

Dielectric Strength

Dielectric strength is the maximum electric field that a material can withstand without breaking down. Virgin PTFE typically exhibits a high dielectric strength, often exceeding 30 kV/mm. However, as the table above shows, for skived PTFE films of extremely high purity, very thin films can easily exceed 200 kV/mm, if properly processed. The dielectric strength of filled PTFE varies depending on the type and concentration of fillers. For example, glass-filled PTFE may have slightly lower dielectric strength compared to virgin PTFE but still maintains good insulating properties. However, for the most part, ASTM standards do not have specific values for various fillers.

Dielectric Constant

The dielectric constant (relative permittivity) of PTFE is low, typically around 2.1. This property remains relatively stable across a wide range of frequencies and temperatures, making PTFE an excellent material for high-frequency and microwave applications. Filled PTFE grades may exhibit slightly higher dielectric constants, but the values generally remain low enough to be suitable for most electrical applications.

Dissipation Factor

The dissipation factor (loss tangent) of PTFE indicates the energy loss as heat in an insulating material when subjected to an alternating electric field. Virgin PTFE has an exceptionally low dissipation factor, typically in the range of 0.0002 to 0.0004, making it ideal for high-frequency applications where minimal signal loss is critical. Filled PTFE grades usually have higher dissipation factors due to the presence of fillers that can introduce additional energy losses.

Volume Resistivity

Volume resistivity is a measure of a material's resistance to electrical conduction through its volume. PTFE exhibits extremely high volume resistivity, often exceeding 10^18 ohm-cm. This property ensures excellent insulation and minimal leakage currents. Even filled PTFE grades retain relatively high volume resistivity, making them suitable for various electrical insulation applications.

Applications of PTFE Based on Electrical Properties

The unique combination of electrical properties in PTFE makes it an indispensable material in numerous applications:

• High-Frequency Cables and Connectors: Virgin PTFE's low dielectric constant and dissipation factor are crucial for maintaining signal integrity in high-frequency and microwave cables and connectors.

• Insulators and Capacitors: The high dielectric strength of PTFE ensures reliable performance in insulators and capacitors used in high-voltage and high-frequency applications.

• Printed Circuit Boards (PCBs): PTFE laminates are used in PCBs for high-frequency and microwave circuits due to their stable dielectric properties and low signal loss.

• Seals and Gaskets: ePTFE's combination of electrical insulation and mechanical flexibility makes it ideal for seals and gaskets in electrical and electronic equipment.

• Antennae and Radomes: PTFE's low dielectric constant and excellent weather resistance make it suitable for antennae and radomes, ensuring minimal signal distortion and protection against environmental factors.

Conclusion

PTFE's remarkable electrical properties, including high dielectric strength, low dielectric constant, low dissipation factor, and high-volume resistivity, make it a preferred material for a wide range of electrical and electronic applications. While the electrical properties can vary depending on the grade and type of resin used, PTFE remains an exceptional choice for applications demanding reliable insulation, minimal signal loss, and high-frequency performance. Understanding the nuances of different PTFE grades and resin types allows engineers and designers to select the optimal material for their specific needs, ensuring enhanced performance and longevity of electrical components and systems.

Finally, most applications that have electrical requirements would consequently need the PTFE being used to be tested. It is important to note that fillers can play an important role here, as they can improve the properties in some cases. However, the choice of filler and the concentration can only be arrived at by trial and error. Further, it is vital that the right material is tested. Testing a moulded sheet can produce a very different result when compared with testing an extruded tube. It would be wise to ensure that the grade, filler, and process are all being checked so that the end-product conforms to the requirements of the application.

---

Read More

1. Understanding PTFE Impellers: Revolutionizing Fluid Dynamics

2. Tensile Testing of High-Performance Plastics

3. Expanded PTFE (ePTFE) Gasket Tapes - Challenges in Testing

Among the various properties of PTFE, that of its chemical resistance is one that industries are increasingly leaning on for newer applications. PTFE is resistant to almost all known chemicals, with the exception of some alkali metals (molten or in solution) and rare fluorinated compounds at high temperatures and/or pressures. This uniqueness means that PTFE is one of the few materials that can be left inside the human body indefinitely with no adverse effects.

Polytetrafluoroethylene (PTFE) impellers have become an essential component in various industrial applications, especially in fluid dynamics, due to their unique properties and versatile applications. Known for their exceptional chemical resistance, non-stick surface, and durability, PTFE impellers offer significant advantages over traditional materials.

Why PTFE Impellers?

Impellers are rotating components designed to move fluids by mechanical means in pumps and other fluid-handling equipment. The choice of material for impellers is crucial, as they are often exposed to harsh conditions. PTFE impellers stand out due to several key characteristics:

• Chemical Resistance: PTFE is virtually inert to almost all chemicals, making PTFE impellers suitable for handling aggressive substances, including acids, bases, and solvents. This resistance ensures long-lasting performance and reduces the risk of corrosion.

• Non-Stick Properties: The non-stick nature of PTFE prevents the build-up of materials on the impeller surface. This is particularly beneficial in processes involving sticky or viscous fluids, as it minimizes maintenance and cleaning requirements.

• Thermal Stability: PTFE can withstand a wide temperature range, from -200°C to 260°C. This makes PTFE impellers ideal for applications that involve extreme temperatures, ensuring consistent performance without degradation.

• Low Friction: PTFE has one of the lowest coefficients of friction of any solid material. This property reduces energy consumption and wear, leading to improved efficiency and longer service life of the impeller.

Applications of PTFE Impellers

PTFE impellers are used across various industries due to their robust characteristics. Some notable applications include:

• Chemical Processing: In chemical plants, PTFE impellers are used in pumps that handle corrosive chemicals. Their resistance to chemical attack ensures the integrity and reliability of the pumping system.

• Pharmaceutical Manufacturing: The non-reactive and non-stick properties of PTFE impellers are ideal for pharmaceutical processes, where contamination must be avoided. They help maintain the purity of products and prevent cross-contamination.

• Food and Beverage Industry: PTFE impellers are used in food processing equipment to handle ingredients and products that may be sticky or prone to contamination. Their easy-to-clean nature ensures hygiene and compliance with food safety standards.

• Water Treatment: In water treatment plants, PTFE impellers are employed in pumps that handle various chemicals used in water purification. Their durability and resistance to chemical wear enhance the reliability of the water treatment process.

Advantages Over Traditional Materials

Traditional impeller materials such as metals or ceramics may suffer from corrosion, wear, or chemical attack, leading to frequent replacements and maintenance. PTFE impellers, on the other hand, offer a longer lifespan and reduced downtime. This translates into cost savings and increased operational efficiency.

Furthermore, PTFE impellers contribute to energy savings due to their low friction properties. In high-speed applications, this can result in significant reductions in power consumption and operational costs.

Challenges in Development

One of the notable drawbacks of PTFE is that it cannot be moulded using traditional methods. As a result, the blades of a PTFE impeller need to be made by moulding a block of PTFE and machining the impeller from this. This poses two problems. The first is that it is very time consuming. A single impeller might take anywhere from 3-4 hours to machine, making it expensive. However, this is not a huge problem since these are fairly specialised items and the need for using PTFE negates any commercial hurdles that might present. More problematic, however, is the shape itself. The impeller blade has specific angles, and these can often not be machined using simple equipment. Usually, a 5-axis machine is needed to adequately make the final shape, with special CAM programming to allow the tool to finish the part to the desired dimensions. Even if the equipment is available, PTFE is extremely difficult to machine and it requires both a machining knowledge and an in-depth understanding of the polymer to get the right result.

Conclusion

PTFE impellers represent a significant advancement in the field of fluid dynamics, offering unmatched chemical resistance, non-stick properties, thermal stability, and low friction. Their application across various industries underscores their versatility and effectiveness. As technology advances, the use of PTFE impellers is likely to expand, driving improvements in efficiency and reliability in fluid-handling systems. Embracing PTFE impellers can lead to enhanced performance, reduced maintenance, and long-term cost savings, making them a valuable investment for any industry dealing with challenging fluid environments.

As 5-axis capabilities develop – as it the case at Poly Fluoro – these vital components can be handled with ease and with the precision needed.

Read More

1. Tensile Testing of High-Performance Plastics

2. Expanded PTFE (ePTFE) Gasket Tapes - Challenges in Testing 

3. Polymers in Metal Replacement

A tensile test is a relatively straightforward process for the most part. The purpose of the test is to apply a tensile, or pulling, load to a specimen of the material and observe both the load at which the material breaks and the elongation of the specimen at the point of breaking. A slightly more advanced testing equipment will also give you the Young’s Modulus – also known as the Modulus of Elasticity – which takes into account the extent to which incremental loads create incremental elongations in the material.

Most mechanical labs will have the equipment needed to test for tensile properties. However, while materials such as steel and rubber have well documented and easily accessed standards on how the test must be performed, polymers – most specifically high-performance plastics – can be a different story altogether.

We have observed, on more than one occasion, that the values obtained from a lab differ significantly from those on the material datasheet. Since the materials we buy are from reputed suppliers with their own in-house labs, this creates some confusion as to whether the properties are indeed not meeting the specifications, or whether there is a way to test the materials that the external labs may not always be aware of.

A few factors are at play here and need to be properly understood before sending any polymer to a lab for testing:

1. Shape to be tested

Most standards will specify a certain set of shapes that need to be made for testing. Usually, these are dumbbell in shape, with a thickness of 3mm to 5mm. The dumbbell shape is wider at the ends, where the test sample will be gripped by the machine. The cross-section area of the thin section in the middle is where the load is applied when the dumbbell is stretched lengthwise.

While most standards offer a general suggestion on how to make the dumbbell shape, a lot of thought needs to be put on the exact shape as well as the thickness. Care also needs to be given on deciding how the part should be cut to make the dumbbell, as we shall see below

2. Method of processing – polymers such as PEEK, PTFE, Polyimide, and even PVDF can be processed in a number of ways. PTFE, for example, lends itself to compression moulding, ram extrusion, isostatic moulding, and paste extrusion. Similarly, PEEK can be compression moulded, extruded, or injection moulded. The varying process methods would accordingly cause the internal grain structure of the polymer to be similarly varied.

In the case of PTFE, a tensile test can be performed on a compression moulded part that has then been skived into a 3mm thick tape. A dumbbell can then be punched out of this tape. However, because of the nature of compression moulding, the tensile strength will vary depending on whether the dumbbell is cut along the length of the tape or across it. For this reason, many datasheets for PTFE will give different values for LD (longitudinal direction) and TD (transverse direction). If a lab offers to punch the material into the dumbbell, the manufacturer will still need to specify which the longitudinal direction is so that the best result can be obtained. Similarly, ram extruded tubes would generally exhibit lower tensile properties when compared with paste extruded tubes. This is because ram extrusion is an additive process, whereas paste extrusion is continuous. Hence, there are weak spots in a ram extruded tube that will bring the value of the tensile strength down.

With PEEK, extruded rods would similarly exhibit better tensile properties than compression moulded materials. An added issue with PEEK is that machining the dumbbell shape places a lot of internal stress on the material and the dumbbells themselves need to be properly annealed before any tests are done.

3. Testing speeds

Even if the above precautions are taken, polymers are extremely sensitive to testing speeds. Again, standards will prescribe a range of anywhere from 1mm per minute to 50mm per minute for the speed of the testing, but it is the manufacturer that needs to often take trials to determine which speeds offer values that match the materials base properties and then use this as the benchmark. PTFE generally benefits from being pulled faster, so we test PTFE at 5mm/minute. PEEK, on the other hand, does not do well under high speeds and will break very easily if tested at 5mm/minute. Hence, 1mm/minute is the maximum speed at which PEEK should be tested.

Beyond testing, it is also useful to note that some parts simply cannot be tested. Machined components, for example, cannot be subjected to a tensile test as prescribed in the standards, since the components are shaped in such a way that a dumbbell shape cannot be extracted from it. In such cases, it is up to the manufacture and OEM to decide between them what tests can be done and what tests – for example a test on a moulded part that acts as a proxy to the component – would suffice as representative of the final component.

---

Read More

1. Expanded PTFE (ePTFE) Gasket Tapes - Challenges in Testing 

2. Polymers in Metal Replacement

3. Advancements in Aerospace - High-Performance Plastics Take Flight