Development, preparation, and application of polytetrafluoroethylene microporous membranes!

The development of polytetrafluoroethylene microporous membranes
In the 1960s, DuPont Company in the United States first used unidirectional stretching to produce polytetrafluoroethylene microporous films, but the size, porosity, and membrane strength of the micropores were not ideal. In 1973, Gore Company in the United States successfully developed polytetrafluoroethylene microporous films using biaxial stretching technology, marking the industrial application of polytetrafluoroethylene microporous films in technology has matured. After more than 30 years of development, polytetrafluoroethylene microporous film has been widely used as a new type of membrane material in the fields of clothing and industry. The biaxial stretching process of polytetrafluoroethylene film endows it with good pore characteristics, but at the same time, it also endows the mechanical properties of the film with anisotropic properties. This anisotropic property will directly affect the composite of thin films and the performance of composite materials.
At present, the production processes of polytetrafluoroethylene microporous films include rolling film method, turning film method, and stretching film method. The stretching film method can be divided into unidirectional stretching and biaxial stretching. Through structural analysis and actual measurement, only biaxial stretching films have a good microporous structure. The production process flow of biaxially stretched polytetrafluoroethylene microporous film is shown in the figure.

                               The production process of biaxially stretched polytetrafluoroethylene microporous film
Preparation of polytetrafluoroethylene microporous membrane
The raw material for PTFE film is granular PTFE resin, which is a homopolymer of tetrafluoroethylene and cannot be processed using ordinary thermoplastic molding methods. It is processed by the principle similar to powder metallurgy, such as suspension method, dispersion method, i.e. lotion method and other polymerization methods. There are multiple foreign products for PTFE resin powder, and there are also a small number of domestic manufacturers manufacturing it. Foreign manufacturers are represented by companies such as DuPont in the United States and Daikin in Japan.
The conventional production process of PTFE microporous membrane is to mix polytetrafluoroethylene dispersed resin with liquid additives, make thin sheets of the mixture through rolling method, and then use a machine to biaxially stretch the thin sheets to obtain PTFE microporous membrane. The process flow is as follows: PTFE dispersed resin, booster (material selection) - mixing - rolling - biaxial stretching - coiling.
Preparation of base film: A good base film must have uniform thickness, reasonable crystallinity and density. The quality of the base film directly affects the performance indicators of the finished product. When preparing the base film, attention should be paid to the following factors:
Selection of raw materials: PTFE resin should choose a grade with a suitable molecular weight, which can withstand high-speed strain under high temperature conditions during stretching without fracture; The selection of extrusion aids should make the resin moist, non-toxic, with a high boiling point, easy to remove and no residual substances.
Proportioning: According to the resin and extrusion aid grade used, prepare in an appropriate proportion, with the extrusion aid generally ranging from 12% to 28%.
Compression ratio: Compression ratio is also an important parameter that affects product performance indicators. The compression ratio is high, the longitudinal fibrosis strength is high, and it is not easy to break during stretching, making it easy to produce continuously. Its product strength is also high. Excessive compression ratio makes the extruded material too hard, which is not conducive to the normal production of subsequent processes. Therefore, the compression ratio should be reasonably selected based on the resin grade, equipment, and production process.
Stretching temperature and stretching rate: The PTFE microporous film forming process is more commonly used in foreign countries using the biaxial stretching or stretching method. PTFE can be stretched between room temperature and 327 ℃, meaning that stretching below the melting point stage is carried out in a high elastic state. Low temperature stretching can cause the film to rupture, making stretching impossible. However, above 327 ℃, the crystalline state between PTFE molecules changes into an amorphous state, making it difficult to obtain a network structure. Therefore, the general stretching temperature is between 40 ℃ and 327 ℃.

                                           The application of polytetrafluoroethylene microporous membrane
one
PTFE filtration microporous membrane for dust separation
The most widely used film of this kind is the PTFE membrane composite filter material for air pollution control, which is used to make bag filter as the core technology of bag filter. It has high separation efficiency, low operating resistance, and can be used for the separation of high-temperature dusty gas and dust. Its base fabric is made of materials such as PPS, P84, Nomex, and fiberglass, and it also has properties such as acid resistance, alkali resistance, and high temperature resistance. It can be used for garbage incineration, triple supply gas purification, and dust purification of coal to oil process exhaust gas. The base fabric made of pure PTFE fibers has more prominent advantages. Its performance parameters are shown in Table 2.

two
PTFE microporous film for water purification
This type of film requires special treatment and has a different hydrophobicity compared to ordinary PTFE microporous membranes. It can allow water to pass through and separate the suspended particles in the water.
three
PTFE microporous film for clothing
This film is used in the clothing industry and has a special purpose clothing that is completely different from general clothing. Clothing made of this material has the function of being impermeable and only breathable. Due to the fact that the water vapor emitted by the human body is in the gas phase, it can emit sweat (liquid water) excreted by the human body. People wearing this kind of clothing can also walk on rainy days, and it can be used as a firefighting suit to show off their skills.
four
Application in medical materials
In the field of aseptic filtration in medicine and biotechnology, PTFE microporous membranes have been widely used in medical materials due to their advantages of no phase change, high efficiency, energy saving, simple process, and no secondary pollution, such as surgical gowns, surgical drapes, wound dressings, disinfectant instrument wrapping materials, etc.
According to the data, after conducting experiments on 500 surgical gowns, it was found that in terms of blood penetration resistance, the blood penetration infection rate of single-layer spunlace non-woven surgical gowns was 9%, enhanced non-woven surgical gowns was 5%, and surgical gowns made of polytetrafluoroethylene microporous film composite were 2%. In terms of comfort, surgical gowns made of polytetrafluoroethylene microporous films are as comfortable to wear as regular clothing.