Polymerization:

Chloroform reacts with hydrofluoric acid to form trifluoromethyl chloroform, which is then heated to produce the monomer tetrafluoroethylene. This monomer is purified through distillation or drying to remove impurities. There are two main methods for polymerizing tetrafluoroethylene: suspension polymerization and emulsion polymerization.

• Suspension Polymerization: Tetrafluoroethylene is placed in water, along with an initiator and dispersing agent. It’s heated to form granular PTFE resin, usually ranging from 50 to 500 microns in size. The PTFE solid particles are then separated through filtration.

• Emulsion Polymerization: In this process, water is used as the medium with an emulsifier, resulting in a fine powder PTFE resin, typically less than 1 micron in size. The emulsion is broken, followed by coagulation, washing, and drying to produce PTFE powder resin.

Processing Methods:

Compression Molding:
The principle is to compress the powder into a preform and then sinter it at high temperatures to melt and bond the particles together, followed by cooling to set the shape.

• • Sintering: High temperatures cause PTFE particles to melt and diffuse, forming a continuous structure.

  • Melting: When the temperature exceeds 327°C, PTFE particles transition from a crystalline to an amorphous state, with the surface melting to form a “flow layer.”

  • Conventional Compression Molding: Powder is manually placed into a mold, then pressed, and the part is removed after the mold is opened.

  • Automatic Compression Molding: The process is fully automated and controlled by machinery.

  • Isostatic Molding: The raw material is placed in a flexible mold, and uniform pressure is applied using liquids like water or oil to create an equal-pressure environment.
    

                                 

This method is ideal for making simple, thick-walled products such as plates, rods, seals, and discs. The process is simple and cost-effective, suited for small batch production, but the size is limited by the mold.

Extrusion Molding:
The principle is to use an “extrusion aid + pressure” to drive PTFE powder to flow and shape continuously through a mold at room temperature, followed by degreasing, sintering, and curing.

           Degreasing: Removes the extrusion aid, which vaporizes at 60-120°C.

This method is typically used for making long, thin-walled products such as pipes, cable insulation, and thin rods. It is more efficient than compression molding.

Impregnation Molding:
This method uses the fluidity of PTFE dispersion liquid, which penetrates, adheres, and cures to bond with the substrate. Porous materials like fabric or metal mesh are soaked in the PTFE dispersion liquid, forming a continuous coating or composite structure on the surface or inside the pores.

It is commonly used for making corrosion-resistant filter fabrics, high-temperature conveyor belts, and composite sealing materials. However, controlling the uniformity of the coating thickness can be challenging.

Stretching Molding:
PTFE is stretched at a specific temperature to break the tight crystalline structure, creating a porous network structure (ePTFE) in either one direction (uniaxial) or both directions (biaxial).

This method is used for making artificial blood vessels, breathable membranes, sealing tapes, etc.

The main advantage is the ability to control the porosity and mechanical properties, with process parameters such as stretching speed and temperature having a significant effect on the product’s performance.

Machining:
After sintering, the PTFE preform can be precision machined using processes like turning, milling, and drilling to achieve the desired thickness and shape.

This method is used for producing precision sealing rings, custom gaskets, bearings, and more.

                                             

 

Summary:

• Compression Molding controls product size and mechanical properties well, making it ideal for high-strength parts with uniform density.Isostatic Molding creates a uniform density throughout the material, making it suitable for high-strength applications.

• Extrusion Molding is typically used for pipes and rods and is more efficient than compression molding.

• Impregnation Molding is flexible and suitable for small or irregularly shaped products, though coating thickness can be difficult to control.

• Stretching Molding enhances mechanical properties and allows precise control over porosity and surface quality.

Machining provides high precision for complex shapes and parts like custom gaskets and bearings.