PTFE(Teflon) Machining: Properties, Fabrication, Application

Polytetrafluoroethylene or Teflon is one of the most important high-performance engineering plastics. It is well known for its excellent combination of properties, including high temperature resistance and strong chemical corrosion resistance. This article provides an introduction to the basic properties of PTFE, including its properties, processing methods and its important applications.

Introduction to PTFE Plastic

Polytetrafluoroethylene (PTFE), better known as Teflon, is a high-molecular-weight compound produced by polymerizing tetrafluoroethylene. Its structural formula is [CF₂-CF₂]ₙ. It is an important chemical material. PTFE offers excellent resistance to high temperature, corrosion, and aging. It is also water-resistant, non-stick, and self-lubricating, with outstanding dielectric properties and an extremely low coefficient of friction. It is commonly used as an engineering plastic or coating and can be made into PTFE tubes, rods, tapes, sheets, films, and other products.

Properties of PTFE

Mechanical and Physical Properties

PTFE has a relatively high density, ranging from 2.14 to 2.20. It hardly absorbs water, with an equilibrium water absorption rate of less than 0.01%. PTFE is tough but has no resilience. The intermolecular attraction between its large molecules is small, so it has only moderate tensile strength and low hardness. It will deform under long-term stress, but it has a high elongation at break.

Temperature has a significant effect on the stress-strain behavior of PTFE. As temperature increases, the elastic limit value E decreases. When PTFE is instantly compressed by an external force, the compressive deformation increases with increasing stress. Under sustained load, as time passes, the strain of PTFE continues to increase and then tends to stabilize. This increasing strain is called the cold flow property of PTFE, which is also caused by the small intermolecular attraction between PTFE molecules.

The creep of PTFE varies with compressive stress, temperature, and crystallinity. The higher the temperature, the greater the creep. When the crystallinity of PTFE is between 55% and 80%, the creep amount does not exceed 2%. When the crystallinity is below 55% or above 80%, the creep amount increases rapidly.

Because the intermolecular attraction of PTFE is small, the attraction of surface molecules to other molecules is also very small. Therefore, PTFE has a very low coefficient of friction and exhibits excellent lubricity. Due to the small intermolecular attraction, PTFE has low hardness and is easily worn by other materials. However, as long as the mating material has an appropriate surface roughness, the wear of PTFE can be significantly reduced. Usually, when the surface unevenness is between 0.1 and 0.4 μm, the wear of PTFE is minimized. The essence of this phenomenon is that during rotation or sliding, the surface of the mating material becomes coated with a thin PTFE film due to wear, turning the friction between two different materials into friction between PTFE and itself. Because of PTFE’s excellent self-lubricating property and extremely low coefficient of friction, the wear amount is very small.

Relative Wear Amount of PTFE Against Different Mating Materials (Taking carbon steel wear as 1)

Thermal Properties

The thermal stability of PTFE is extremely outstanding among all engineering plastics. This is because the carbon-fluorine bond energy in PTFE macromolecules is very high, and the carbon-carbon chain is surrounded by fluorine atoms, making it difficult for other atoms (such as oxygen) to attack. Although trace amounts of decomposition products begin to appear at 200°C, the decomposition rate is extremely slow from 200°C up to the melting point, and the amount of decomposition is very small. After heating at 200°C for one month, the decomposition amount is less than 2 parts per million, which is almost negligible. Significant decomposition only occurs above 400°C, with a weight loss of about 0.01% per hour.

At 250°C, the tensile strength of PTFE is still about 5 MPa, which is approximately 1/5 of its value at room temperature. Below 0°C, as the temperature continues to decrease, the tensile strength of PTFE keeps increasing while the elongation keeps decreasing. Below -75°C, the elongation reaches a minimum value of about 3% and remains at that value down to -250°C. Therefore, PTFE does not become brittle even at ultra-low temperatures of -250°C and still maintains a certain degree of flexibility.

PTFE has a very wide service temperature range and can be used continuously from -250°C to 260°C.

Electrical Properties

PTFE is a highly nonpolar material with excellent dielectric properties. Its outstanding feature is that above 0°C, its dielectric properties do not change with frequency or temperature, and are not affected by humidity or corrosive gases.

The volume resistivity of PTFE is greater than 10¹⁷ Ω·cm, and the surface resistivity is greater than 10¹⁶ Ω, which is the highest among all engineering plastics. Because PTFE does not absorb water, even after long-term immersion in water, its volume resistivity shows no obvious decrease. In air with 100% relative humidity, its surface resistivity also remains unchanged.

PTFE has excellent arc resistance. When high-voltage surface discharge occurs, it does not cause short circuits due to carbonization or residual conductive substances. Instead, it only decomposes into low-molecular-weight fluorocarbons that volatilize, so it still maintains good electrical insulation and arc resistance.

Chemical Resistance

PTFE has extremely excellent chemical stability. This is because in the PTFE molecule, the carbon chain backbone that is easily attacked chemically is tightly surrounded by a layer of strongly bonded fluorine atoms, making the polymer’s main chain almost immune to erosion by any chemical substances. Many highly corrosive and strongly oxidizing chemicals, such as concentrated hydrochloric acid, hydrofluoric acid, sulfuric acid, nitric acid, chlorine, sulfur trioxide, sodium hydroxide, and organic acids, have almost no effect on it. Only molten alkali metals can remove fluorine atoms from PTFE molecules to form fluorides, turning the surface dark brown.

Changes in PTFE Properties After Immersion in Chemicals

Other Properties

In atmospheric environments, because there are no photosensitive groups in PTFE molecules and ozone cannot react with it, its resistance to atmospheric aging is extremely outstanding. Even after long-term exposure to the atmosphere, the surface remains unchanged. When processing PTFE into products, no anti-aging agents or stabilizers need to be added.

The flame retardancy of PTFE is also very outstanding, with an oxygen index as high as 95%. Even without any flame retardants, its flame retardancy can reach the UL94 V-0 level.

Non-stickiness is another important characteristic of PTFE. The surface free energy of PTFE is very low, only 0.019 N/m, which is the lowest among known solid materials. Therefore, almost all solid materials cannot adhere to its surface. Only liquids with surface tension below 0.02 N/m (such as ether, hexane, and petroleum ether) can completely wet it.

How to Process PTFE into Components

The main forming methods for PTFE include compression molding, extrusion molding and CNC machining.

Compression Molding

The free sintering method involves uniformly filling PTFE powder into a mold, preforming it at room temperature under a pressure of 10–100 MPa in a press, then placing the preform into a heating furnace. The temperature is raised to 360–380°C at a certain rate for sintering. After the entire preform is uniformly sintered, the furnace temperature is lowered to room temperature at a controlled rate to obtain the finished product. This method applies no constraint to the sintered material during sintering, so it is called the free sintering method. The holding time for free sintering is generally from a few minutes to tens of minutes, with complete elimination of voids as the standard. The amount of voids can be checked by measuring the density of the product. The higher the density, the smaller the void content. Higher pressure and higher temperature result in higher density.

The sintering heating rate is generally controlled at 25–60°C per hour. The larger the preform, the slower the heating rate should be. Sintering time is determined by when the product becomes transparent or translucent, and varies with product size. Generally, for a product thickness of 1 mm, the sintering time is 5–8 minutes. Cooling rate directly affects the crystallinity and physical-mechanical properties of the product. For small products, the cooling rate can be controlled between 50 and 150°C/hour. For large products, the cooling rate should not exceed 50°C/hour.

Compared with free sintering, the hot compression molding method requires secondary pressing in a second mold during sintering after preforming. Secondary pressing should be performed as soon as possible before the preform cools to the melting point. The expansion rate of the preform in the pressing direction is about 25%, and 6%–10% in the vertical direction. Therefore, the secondary mold must be designed slightly larger than the primary mold.

Secondary pressing involves applying pressure while cooling, so it has a faster cooling rate than free sintering. The product has lower crystallinity, better bending fatigue strength and toughness, but higher residual stress. Therefore, post-treatment at 120–125°C is generally required.

The performance of products formed by hot compression molding mainly depends on the secondary pressure. Higher secondary pressure gives better performance, but excessively high pressure increases flash and causes material loss. It is generally controlled at 10–20 MPa.

Extrusion Molding

PTFE extrusion molding is different from the extrusion of ordinary thermoplastics. The machine barrel is not heated. The screw only serves to convey and push the raw material. The material passes through the head of a single-screw extruder with a double-thread, constant pitch, and constant depth, then enters the die for sintering and cooling. With the help of back pressure provided by a counter-pressure device, PTFE is formed into thick-walled tubes, rods, and other special profiles.

Teflon Coating/PTFE Coating

PTFE can be applied by spraying to form coatings and linings. The spraying process mainly includes substrate treatment, spraying, and sintering. The main substrate materials are steel, cast iron, aluminum, aluminum alloys, ceramics, and glass. Copper and copper alloys are more difficult to spray, while tin, zinc, and lead cannot be sprayed. To ensure uniform spraying and strong coating, all right-angle areas on the substrate must be chamfered, with a chamfer radius of 3–10 mm recommended. Before spraying, the substrate surface must also be degreased and roughened. Degreasing can be done by high-temperature degreasing at 380°C or high-pressure steam spraying. Roughening is generally done by sandblasting.

PTFE CNC Machining

Polytetrafluoroethylene has low strength, low hardness, poor thermal conductivity, and high thermal expansion coefficient. When CNC machining PTFE, appropriate tools should be selected, reasonable cutting parameters determined, appropriate machining allowance considered, and sufficient cooling (usually compressed air cooling) provided according to the material properties, processing conditions, and quality requirements. This way, high processing quality and accuracy can be achieved.

PTFE CNC Turning

When CNC turning PTFE, the outer diameter dimensions, inner and outer round ovality, and inner and outer taper changes are relatively large and irregular. Therefore, during turning, a larger rake angle should be used in roughing to reduce cutting force and prevent elastic deformation of the workpiece. In finishing, a smaller rake angle can be used to improve tool heat dissipation and reduce cutting temperature.

PTFE CNC Drilling

The biggest problem when CNC drilling PTFE is how to remove chips in time. The following measures are usually taken:

• Use drills with small helix angles and chip flutes to facilitate chip removal.
• Reduce the drill speed as much as possible. Select a lower feed rate while ensuring drilling efficiency, and use multiple retractions to ensure timely chip removal, thereby improving surface quality and machining accuracy.

Generally, after sharpening, the helix angle of the drill is about 10°–15°, the clearance angle is 9°–20°, and the point angle is 60°–120°. The drilling speed is not only related to the material being drilled, but also to the hole size and depth. For hand drills, 900 r/min gives the best effect, while for fixed bench drills, 2100 r/min with a feed rate of 1.3 mm/s gives the best results.

PTFE CNC Milling

When CNC milling PTFE, the rake angle of the milling cutter should be larger than that used for milling metal, generally greater than 6° (6°–15°). The clearance angle should also be larger than that for CNC milling of metal, generally greater than 10° (15°–30°). The cutting speed during milling should be controlled at 180–300 mm/min, the feed per revolution should be 0.05–0.13 mm/r, and the depth of cut should be about 0.1 mm/s. Surface roughness decreases as the feed rate decreases. In production, side milling is recommended to avoid burning the workpiece. For roughing, straight-tooth back milling cutters with high strength and simple manufacturing should be selected. For finishing, broken-line back milling cutters with lower strength but larger chip space should be selected.

Applications of PTFE Machined Components

5G Communications

FR4 copper-clad laminates commonly used in the communications industry use epoxy resin as the substrate material, but they have high loss and are not suitable for high-frequency communications.

5G requires high-frequency copper-clad laminates with low dielectric constant and low dielectric loss factor. PTFE resin is currently the polymer material with the lowest dielectric constant. Its dielectric properties and dielectric loss can meet the requirements of 5G communication base stations. Therefore, PTFE is gradually being used in high-frequency communications such as 5G, aerospace, and military applications. Copper-clad laminates made from it are called high-frequency copper-clad laminates. In the 5G field, PTFE is also often used to make semi-flexible coaxial cables, RF coaxial cables, and radar antenna boards.

Hydrogen Energy

In the hydrogen energy field, PTFE is mainly used for sealing alkaline electrolyzers and as a reinforcement for proton exchange membranes in PEM fuel cells and water electrolysis.

In alkaline electrolyzers, sealing gaskets serve as the main component and provide both sealing and insulation functions. Leakage is one of the important factors affecting the service life and safety of alkaline electrolyzers. The compression resilience and creep relaxation of sealing gaskets are important indicators for measuring gasket performance. Domestic alkaline electrolyzer sealing materials have undergone multiple iterations: asbestos rubber sheets → cloth-diaphragm gaskets → PTFE-filled gaskets. At present, the commonly used electrolyzer sealing gaskets in China are mainly PTFE-filled gaskets. PTFE is filled and modified with reinforcing fillers such as glass fiber, alumina, and graphite, then molded and sintered to form sealing gaskets.

Medical

The low friction of PTFE makes it less prone to wear during long-term use in the body, thereby reducing complications caused by wear. At the same time, it makes it difficult for blood and other body fluids to form deposits on its surface, reducing the risk of thrombosis. This makes it a popular coating material for various medical devices, such as catheters and stents. This coating can reduce friction when the device moves inside the body and lower the risk of tissue damage.

PTFE is soft, easy to shape, and provides natural implantation results, making it suitable for plastic surgery. Its effects are superior to traditional silicone rubber. Using PTFE can achieve good postoperative results that are radiopaque, stable, and natural. The most common applications are implants for the nose, jawbone, or other soft tissue reconstruction. It can also be used for facial wrinkle removal, lip augmentation, repair of damaged soft tissue, or as a support material.

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