An in-depth understanding of the development, preparation, and application of polytetrafluoroethylene microporous membranes in this article!

introduction

It has been 76 years since DuPont began producing polytetrafluoroethylene (PTFE) in 1945. As an important product of PTFE, polytetrafluoroethylene microporous film (PTFE film) is widely used in fields ranging from biotechnology to the clothing industry, from the mechanical industry to petrochemicals. In the environmental protection industry, it can not only be used in water treatment engineering, but also in the purification of air particles.

Performance of polytetrafluoroethylene membrane

one

Molecular structure characteristics

                                                                      Figure 1 PTFE molecular structure formula

It is characterized by a high content of fluorine, resulting in high-density polar C-F bonds, good stability of molecular structure, inactive polymer chain structure, and difficult chemical reaction. These characteristics of PTFE molecular structure formula result in its many advantages.

Due to the larger Van der Waals radius (0.136nm) of fluorine atoms, the repulsive force between fluorine atoms is larger, which makes the rotational barrier of PTFE macromolecular chain much larger than that of polyethylene, so it can be expected that the flexibility of PTFE chain is smaller than that of polyethylene chain. This gives PTFE a high melting point and high melting viscosity.

two

chemical stability

The two fluorine atoms connected by each carbon atom of PTFE are structurally symmetric, and the entire molecule has a high and stable bond energy without polar C-F bonds. The molecule is in a helical configuration, and the C-C main molecular chain is completely obscured by the F atom. Therefore, PTFE has extremely excellent chemical stability and is known as the "king of plastics". It has excellent performance for chemical corrosion. In addition to the Erosion of fluorine element and molten alkali metal under high temperature and pressure conditions, some halogenated amines and Aromatic hydrocarbon will cause slight expansion, many other substances, such as strong acid, strong base, strong oxidant, ketone, ether, alcohol, grease, will not affect it even under high temperature conditions.

three

Thermal performance

PTFE has a melting point of 327 ℃ and a decomposition temperature of 415 ℃. It can be used for a long time in the temperature range of -250 ℃ to 260 ℃, and can withstand high temperatures of 300 ℃ instantly. In short, it can withstand both low and high temperatures. However, one major drawback of PTFE is that it does not transition from a high elastic state to a viscous flow state above the melting point, and does not flow even when heated to the decomposition temperature, which makes it impossible to use the molding methods of general thermoplastic materials. PTFE has a low thermal conductivity and poor thermal conductivity.

four

mechanical property

Due to the non polarity of PTFE molecular chains, the mutual attraction between large molecules is small; Macromolecular chains are unbranched and have high rigidity, with little entanglement, resulting in poor macroscopic mechanical performance of PTFE. But PTFE has good ductility, and when the crystallinity is low, its ductility is better. The optimal rigidity of PTFE

corresponds to a crystallinity of 75-80%, above which the creep resistance of PTFE decreases with further increase in crystallinity. PTFE has excellent fatigue resistance, even after the material is damaged, it can still maintain physical integrity and will not

fracture.

five

Electrical performance

In PTFE macromolecular chains, fluorine atoms are symmetrically and uniformly distributed, so the molecules do not have polarity, making them excellent dielectric properties. The dielectric performance is basically unaffected by the frequency of the electric field and can remain unchanged over a wide temperature range. In addition, the presence of voids in PTFE can also reduce the corona resistance of the material.

six

Surface performance

The friction coefficient of PTFE is almost the lowest in polymers, and it is difficult for PTFE to be wetted by ordinary liquids. Its critical surface tension is 0.0185N/m, and its contact angle with water is 108 °. Therefore, PTFE has outstanding non stick properties and is an excellent anti stick material. On the other hand, this performance makes it extremely difficult to bond with other substances, limiting its application.

                                                                            Table 1 Performance of PTFE

Development of polytetrafluoroethylene microporous membranes

In the 1960s, DuPont Company in the United States first used unidirectional stretching to prepare 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 maturity of the industrial application of polytetrafluoroethylene microporous films in technology. 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 industrial use. The biaxial stretching process of polytetrafluoroethylene film enables it to have 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.

The research on polytetrafluoroethylene microporous films in China was carried out relatively late, and foreign manufacturers have automated these processes. However, there is still a certain gap in the process equipment in China.

At present, the production processes of polytetrafluoroethylene microporous films include rolling film method, turning film method, and stretching film method. The stretching membrane method can be divided into unidirectional stretching and biaxial stretching. Through structural analysis and actual measurement, only biaxial stretching membranes have good microporous structures. The production process of biaxially stretched polytetrafluoroethylene microporous film is shown in the figure.

                     Figure 2 Production process flow of biaxially stretched polytetrafluoroethylene microporous film

Preparation of polytetrafluoroethylene microporous membrane

The raw material of 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 a small number of domestic manufacturers also manufacture it. Foreign manufacturers are represented by DuPont Corporation in the United States and Daikin Corporation in Japan.

The conventional production process of PTFE microporous membrane is to mix polytetrafluoroethylene dispersion resin with liquid additives, make the mixture into thin sheets through rolling method, and then use a machine to biaxially stretch the thin sheets to obtain PTFE microporous membrane. The process flow is: 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, the following factors should be taken into account:

Selection of raw materials: PTFE resin should choose a brand 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 brand 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 for continuous production, and the product strength is also high. Excessive compression ratio makes the extruded material too hard, which is not conducive to normal production in 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 forming process of PTFE microporous film is 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 ℃.

picture

Application of polytetrafluoroethylene microporous membrane

one

PTFE filtration microporous membrane for dust separation

The most widely used type of film is the PTFE membrane composite filter material used for air pollution control, which is made into a bag filter as the core technology. It has high separation efficiency and low running resistance, and can be used for the separation of high temperature dusty gas and dust. Its base cloth is made of PPS, P84, Nomex, glass fiber and other materials. It also has acid resistance, alkali resistance, high temperature resistance and other properties, and can be used for Incineration, triple supply gas purification, and coal to oil process tail gas dust purification. The substrate made of pure PTFE fiber has more prominent advantages. The performance parameters are shown in Table 2.

                       Table 2 Main Technical Performance Indicators of Microporous Membrane Composite Filter Media

two

PTFE microporous film for Water purification

This type of film requires special treatment, and its hydrophobicity is different from that of ordinary PTFE microporous membranes. It can allow water to pass through, while the suspended particles in the water are separated.

three

PTFE microporous film for clothing

This kind of film is used in the Clothing industry, and has a special clothing that is completely different from the 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) emitted by the human body. People wearing this type of clothing can also walk on rainy days, and it can be used for firefighting clothing to showcase their skills.

From the data in Table 2, it can be seen that the current research on ultra-high temperature and heat-resistant explosives mainly uses single substance explosives with a melting point below 350 ℃, such as PYX, PCS, and other mixed explosives as raw materials. Its high-temperature resistance is relatively good below 250 ℃, but when the temperature exceeds 300 ℃, some of the properties of the mixed explosive are affected.

four

Application in Medical Materials

In the field of sterile filtration in medicine and bioengineering, PTFE microporous membranes have been widely used in medical materials due to their advantages of no phase change, high efficiency, energy conservation, simple process, and no secondary pollution, such as surgical gowns, surgical towels, wound dressings, and packaging materials for disinfection instruments.

According to the data, after conducting experiments on 500 surgical gowns, it was found that in terms of blood permeability resistance, the blood permeability infection rate of single-layer spunlaced nonwoven surgical gowns was 9%, that of enhanced nonwoven surgical gowns was 5%, and that of surgical gowns using polytetrafluoroethylene microporous film composite was 2%. In terms of comfort, surgical gowns made of polytetrafluoroethylene microporous film are as comfortable to wear as regular clothing.