The membrane covered filter material used in SFFILTECH bag type dust collection industry can not only achieve near zero emissions, but also because the film is not sticky and the friction coefficient is small, so powder will fall off automatically, ensuring long-term stability of equipment resistance. Therefore, it gives full play to the advantages of bag type dust collector, and is an ideal choice of filter material.
A new type of filter material is made by compounding a layer of polytetrafluoroethylene (PTFE) film on the surface of ordinary filter material, which is PTFE membrane covered filter material. PTFE film acts as a “primary dust layer”. The material exchange is carried out on the surface of the membrane, and effective filtration can be carried out at the beginning of use. The unique three-dimensional network structure of the film makes it impossible for dust to pass through, without the risk of pore blockage. This filtering method is called “surface filtering”.
At present, the main PTFE coating processes on the market are: adhesive method and high temperature hot pressing process.
The method is mainly to stick the film and the substrate together through the adhesion of the adhesive. The process is easy to operate and the cost of the film covering machine is low. It is a film covering process widely used at present.
Difficulties: Polytetrafluoroethylene (PTFE) has excellent characteristics such as high temperature resistance, weather resistance, chemical corrosion resistance, and low surface tension. However, because PTFE contains perfectly symmetrical linear polymers composed of extremely strong C-F bonds and C-C bonds strengthened by atoms, the surface tension of PTFE and its films is very low, the surface is not easy to be wetted, and it is difficult to be compatible with other substances.
Methods: At present, the adhesives that can directly bond low surface energy fluorocarbon materials mainly include solvent adhesives with organic amine complexes as initiators, hot-melt adhesives containing thermoplastic fluoropolymer and water-based adhesives. Through different gluing processes, a layer of adhesive is coated on the base cloth to bond the PTFE film with the base cloth.
Disadvantages: In the process of film covering, there will be problems such as whether the adhesive on the base cloth is uniform and whether the adhesive is easy to carbonize at high temperature. Because the adhesive will block the PTFE membrane microporous structure, it will cause the permeability of the membrane covered filter material to decline. In addition, under the high temperature service conditions, the adhesive will melt or even carbonize, further leading to membrane microporous plugging and filter material removal, which will affect the service life of the membrane covered filter material.
The high temperature hot pressing coating process is to first treat the substrate or PTFE film with a special surface treatment, and then cover the film and the substrate together under a certain pressure and temperature.
Advantages: This process can reduce the damage to PTFE film under high temperature hot pressing due to the high precision control of the equipment. The air permeability and service life of the product are better than those of the membrane covered filter material obtained by the adhesive method.
Methods: In the first step of the high temperature hot press film covering process, the PTFE film must be modified or the substrate must be specially finished to ensure the film covering effect of the high temperature hot press film covering process. At present, the surface modification methods of PTFE film mainly include chemical solution modification and plasma modification; The finishing of base cloth is mainly the surface finishing of PTFE lotion.
(1) Although the chemical modification method will have a good modification effect on the surface of PTFE film, the uniformity of the treatment effect cannot be accurately controlled during the modification process of PTFE film, and this method requires a large number of highly corrosive chemical solutions, which will inevitably produce a large amount of waste liquid in the treatment process, which is not conducive to the industrial application of the coating process.
(2) Plasma can enhance the reaction activity of the film surface, so that the adhesion between the film and the substrate is no longer a physical bond, and may also introduce chemical bond bonding, which is conducive to improving the adhesion strength of PTFE film and coated substrate. However, due to the fact that plasma treatment has not yet been industrialized, its treatment efficiency is still at a low level, and it is not suitable for online modification of large and wide substrates in the film covering process, which limits the production efficiency of the film covering process to a certain extent.
(3) PTFE lotion finishing uses PTFE dispersion to prepare foaming coating agent, uses foam finishing process to treat the filter material, forms a layer of foaming coating on its surface, and then combines with PTFE film by hot pressing. The film covering fastness of the film covered filter material is greater than that of the film covered filter material made by adhesive method, and its fastness is almost unchanged after 16 h temperature resistance test. The film will not fall off under high temperature. The high temperature hot pressure during film covering is much higher than the actual use temperature under working conditions.
At present, SFFILTECH uses PTFE lotion to finish and treat the filter material. Using the high temperature hot pressing process, combined with the heating electromagnetic roller, and taking advantage of its advantages of uniform temperature, SFFILTECH produces a membrane covered filter material with uniform ventilation, sufficient film covering fastness, and still not easy to fall off at high temperature.