Process flow of PTFE microporous film
The research of PTFE microporous film in China is later than that in Europe, America, Japan and other developed countries. At present, the production technology of polytetrafluoroethylene microporous film includes calendering film, turning film and stretching film. The method of stretch film can be divided into two parts: one is unidirectional stretch and the other is biaxial stretch. The production process of biaxially stretched polytetrafluoroethylene microporous film is shown in the figure.
Microstructure of polytetrafluoroethylene microporous film
1. Pre process of PTFE microporous membrane
The raw material of PTFE membrane is granular PTFE resin, which is a homopolymer of tetrafluoroethylene. It can not be processed by ordinary thermoplastics molding method. However, it is processed by the principle similar to powder metallurgy, such as suspension method, dispersion method or liquid polymerization method. There are many products of PTFE resin powder in foreign countries and a small number of manufacturers in China. Foreign manufacturers are represented by DuPont company of the United States and Daikin company of Japan.
2. Preparation process
The conventional production process is to mix polytetrafluoroethylene dispersion resin and liquid assistant, and then make the mixture into thin sheet by calendering, and then use the machine to biaxially stretch the thin sheet to make PTFE microporous membrane. The technological process is as follows: PTFE resin, extrusion aid (material selection), mixing, rolling, biaxial stretching and coiling.
3. Manufacturing process
(1) Preparation of basement membrane
Good basement membrane must be uniform in thickness and reasonable in crystallinity and density. The quality of the base film directly affects the performance of the finished product. When preparing the basement membrane, the following factors should be paid attention to:
■ selection of raw materials: PTFE resin should be of a brand with appropriate molecular weight, and its performance can withstand the high-speed strain under high temperature conditions without fracture; extrusion aid should be selected to make the resin moist, non-toxic, high boiling point, easy to remove without residual substances.
■ proportioning: according to the resin used and the brand of extrusion aid, the proportioning is carried out according to the appropriate proportion, and the general range of extrusion aid is
Between 12% and 28%.
■ compression ratio: compression ratio is also an important parameter affecting product performance index. High compression ratio, high longitudinal fibrosis strength, not easy to break when stretching, easy to continuous production, and its product strength is also large. Too large compression ratio makes the extrudate too hard, which is not conducive to the normal production of subsequent processes. Therefore, the compression ratio should be selected reasonably according to the resin brand, equipment and production process.
(2) Tensile temperature and elongation
In the process of forming PTFE microporous membrane, biaxially stretching or extending is widely used abroad. PTFE can be stretched from room temperature to 327 ℃, that is to say, the stretch below melting point is carried out in high elastic state. When the temperature is higher than 327 ℃, the crystalline state of PTFE molecules changes into amorphous, and the network structure can be well obtained. Therefore, the tensile temperature is generally between 40 ℃ and 327 ℃.
In production, the tensile temperature is related to the tensile rate and speed, and also depends on the strength and size of the product. High temperature, soft and easy to stretch, good dimensional stability, not easy to shrink. However, if the temperature is too high, the strength of the product will be reduced, which can also lead to fracture. The higher the tensile rate is, the higher the temperature is required. Because the lower the tensile temperature is, the higher the tensile force is. When the tensile force exceeds the strength that the product can bear, it will break. On the contrary, the lower the tensile rate is, the lower the temperature is.
The different temperature also directly affects the performance index of the products. Therefore, it is necessary to find the best range suitable for the specific process and equipment in the production. The strength, pore size and void ratio of the products are important indexes to determine the quality of the products. For different products with different use environment and conditions, the corresponding production process is selected to produce the corresponding PTFE films with small pore size, uniform distribution, high void ratio and small nodule.
Effect of operation parameters on properties of PTFE film
The influence factors of PTFE microporous film on the properties of the film in the process of biaxial drawing and its preceding process are complex and various. In addition to the performance of processing equipment, drawing temperature and drawing rate, other factors are: operating temperature, operating humidity.
1. Operating temperature
PTFE film is very sensitive to temperature, different operating temperature can form different properties of the film. This is not only in the process of drawing, but also in the process of PTFE resin processing and intermediate technology, temperature is a crucial operating parameter.
2. Operating humidity
Humidity is related to temperature, leaving temperature, humidity from the beginning. Environmental humidity also has a certain impact on the film.
Application advantages of TPFE microporous film
█ from "deep filtration" to "surface filtration";
█ high filtering accuracy, good response in the future PM2.5 Problems;
█ the running resistance is low, and the resistance on both sides of the filter bag is 300 ~ 500pa;
The initial resistance of the port is 200 ~ 400pa high
● low resistance to normal operation of the port
█ easy to remove dust;
█ can cope with some "wet conditions";
█ it can be cleaned under extreme conditions.
Polytetrafluoroethylene microporous membrane has many characteristics, such as: no water absorption, high melting point 327 ℃, temperature range 200 ℃ - 260 ℃, i.e. thermal stability, low friction coefficient, resistance to many high corrosive media, weather resistance and electrical resistance, which determine its application in many fields. We briefly introduce the application of polytetrafluoroethylene microporous membrane in environmental protection, clothing and biomedicine Application.
1.Application of polytetrafluoroethylene microporous membrane in the field of environmental protection
In the field of environmental protection, polytetrafluoroethylene microporous membrane is mainly used in filter materials, and it is also used in bag type dust remover, which is highly successful in the prevention and control of air pollution caused by dust and ultrafine dust.
1）2. Application of polytetrafluoroethylene microporous membrane in the
field of clothing
2）Polytetrafluoroethylene microporous membrane is a kind of compact and porous membrane which is made by mechanical stretching fiber expansion under certain technological conditions. The microporous membrane can achieve billions of micropores per square inch, and excellent wind, water and humidity resistance can be achieved by using this structure. It can filter bacteria and has protective function. Therefore, it can be made into clothing, shoes and hats, tents, etc.
3.Application of polytetrafluoroethylene microporous membrane in biomedical field
In the field of medicine and Bioengineering, PTFE microporous membrane has been widely used because of its advantages of no phase change, high efficiency, energy saving, simple process and no secondary pollution.
Mainly made into membrane separation equipment.
Equipment for separation and filtration of aluminum membrane
At present, most of the protective masks on the market are made of melt blown non-woven fabric. The mechanism of filtering PM2.5 and virus is electrostatic adsorption. Once the masks are stained with water, the filtering function will fail. Therefore, they need to be replaced every 4 hours, with short service life. As the filter material of mask, the effect of nano membrane material is better than that of melt blown cloth, and it can be used repeatedly. After wearing the mask, just soak it in warm water of 65-68 ℃ (no rubbing) for 10 minutes, or evenly smear it with 75% alcohol on the surface of the mask, and then wait for the mask to dry naturally, then use it again.