Electrospinning first appeared in the early 20th century. In 1917, Zeleny J explained the principle of electrospinning. In 1934, Formhals applied for a patent for an electrospinning device for preparing polymer ultrafine fibers; in 1966, Simons applied for a patent for the preparation of ultra-thin and ultra-fine nonwoven films by electrospinning; in 1981, Larrondo et al. Conducted research on melt electrospinning with polypropylene; in 1995, the Reneker research group began to study electrospinning, and electrospinning developed rapidly; in 1999, Fong et al. studied the beading phenomenon and microstructure of electrospinning nanofibers Research; In 2000, Spivak et al. used fluid dynamics to describe the electrospinning process for the first time, and proposed the process parameters of electrospinning; in 2004, the nanofiber electrostatic produced by the Czech Technical University of Liberec and Almaco The spinning machine came out.

One of the most extensive uses of nanofibers in the future is for filter materials. The electrospinning method can be used to obtain nano-scale fibers with a diameter of tens or hundreds of nanometers. The formed fiber mat is light in weight, good in permeability, large surface area, high porosity, and good internal pore connectivity, which is very suitable for filtration material. After laying the nanofiber layer composite on the base fabric, the filtration efficiency of the base fabric can be significantly improved. The pore size of the nanofiber layer is about two orders of magnitude smaller than that of the base fabric, and the pore size distribution of the nanofiber layer is uniform and the dispersion is small.

As people realize that nanofibers can significantly improve the filtration performance of various filter materials, the research and development of nanofibers has made great progress. Nanofiber (superfine fibers with a diameter of less than 1 micron) technology has been around for many years. The most commonly used nanofiber production process is electrospinning.

Electrospinning is the most commonly used nanofiber production process. The process uses syringes, nozzles, capillaries or movable emitters. The polymer liquid provided by these devices is attracted to a collection area under the action of a high-voltage electrostatic field. When the polymer solution is pulled from the emitter, it is accelerated through the electrostatic zone, and finally the fiber is formed by the solvent volatilization.

Initially, for industrial production, the production efficiency of electrospinning was very low, which increased the production cost of nanofibers. Electrospinning will also produce a large number of two-dimensional planar structure fibers, lacking depth or Z directionality. This structure is suitable for surface filtration, but not for depth filtration. Electrospun nanofibers are usually fragile and easily break or fall off the surface of the substrate. Therefore, there is a need for an improved nanofiber to overcome the shortcomings of the current electrospinning nanofiber.

There are many factors affecting the preparation of nanofibers by electrospinning. These factors can be divided into solution properties, such as: viscosity, elasticity, conductivity and surface tension; control variables, such as: static voltage in the capillary, the potential of the capillary port and the capillary port The distance between the collector and the environmental parameters, such as the solution temperature, the air humidity and temperature in the spinning environment, and the airflow velocity. The main influencing factors include:

(1) The concentration of polymer solution. The higher the concentration of the polymer solution, the greater the viscosity and the greater the surface tension. After leaving the nozzle, the droplet splitting ability decreases with the increase of surface tension. Usually when other conditions are constant, as the concentration increases, the fiber diameter increases.

(2) Electric field intensity. With the increase of the electric field intensity, the jet of the polymer electrospinning solution has a greater surface charge density and therefore a greater electrostatic repulsion. At the same time, the higher electric field strength enables the jet to obtain greater acceleration. Both of these factors can cause the jet and the formed fiber to have a greater tensile stress, resulting in a higher tensile strain rate, which is conducive to the production of finer fibers.

(3) The distance between the capillary tube and the collector. After the polymer droplets are ejected through the capillary orifice, the solvent is volatilized in the air, and the polymer is concentrated and solidified into fibers, which are finally received by the receiver. As the distance between the two increases, the diameter becomes smaller.

 (4), the flow rate of the electrospinning fluid. When the spinneret aperture is fixed, the average jet velocity is obviously proportional to the fiber diameter.

(5) The state of the collector is different, and the state of the nanofibers produced is also different. When a fixed collector is used, the nanofibers show random irregularities, and when a rotating disk collector is used, the nanofibers show a regular arrangement in parallel.

New nanofiber technology

Compared with the traditional electrospinning process, a newly developed solvent-free nanofiber coating technology has greater flexibility, controllability and durability.

This new nano-coating is usually made of fibers with a diameter of 0.3-0.5 microns, but sometimes the fiber size can reach up to 1 micron. The fiber diameter distribution and coating thickness can be flexibly changed according to different application requirements. This nanofiber technology can significantly improve the filtration performance of the filter material.

The thickness of the new nanofiber layer is between 15-30 microns and can be directly coated on the filter substrate. This nanofiber coating can be coated on various non-woven fabric substrates, such as glass fiber, wood pulp paper or synthetic fibers, while electrospun fibers need to rely on resin to bond. Nanofiber technology can prepare single-layer or double-layer nano-coating on the substrate, and the second layer can be made of the same or different polymers.

The composition of the filter substrate is determined by the specific application of the filter material. The required structural properties such as hardness, strength, pleating and high temperature resistance can be achieved by using substrates of different structures.

As mentioned above, the supporting substrate or the underlying material can be changed according to the target application. For medium-sized air filtration, gas turbine, automobile air filtration and pulse cleaning applications, wood pulp paper or synthetic fiber-wood pulp paper mixed substrates are mostly used. In HVAC, liquid filtration, car cabin air filtration and HEPA filtration, the supporting substrate can be wood pulp paper, glass fiber, synthetic fiber, spunbond and meltblown non-woven fabric.

Different nanofiber layers can be coated on different positions of the filter material according to application requirements. For example, in steam turbine or heavy-duty air filtration applications, a nanofiber layer can be placed on the windward side of the filter substrate to improve surface filtration performance. The nanofiber layer can also be placed on the air outlet surface of the filter substrate to improve the depth filtration performance and capture the particles inside the medium.

The application of nanofiber coating filter material

Nano fiber can improve the filtration performance of the filter material. This performance improvement can be reflected in automotive air intake filtration, computer hard drive vent filtration and high efficiency filtration applications. For car cabin air filters, the removal of particles is related to the comfort and health of passengers. Nanofibers can improve filtration performance in mobile and stationary engines and industrial filtration applications.

For engines, gas turbines and combustion furnaces, it is very important to remove particles from the air stream, because these particles can cause major damage to internal components. In addition, in other occasions, such as gas production or automobile exhaust and industrial exhaust gas may contain destructive particulate matter. Removal of these particles can protect downstream equipment and minimize waste pollution to the environment.

This new type of durable nano-coating can also be used in self-cleaning or pulse cleaning filters. The ash cake formed on the windward side of the filter material can be removed by the counter-pulse airflow, thereby regenerating the filter material. When the pulse backlash causes the surface of the filter material to be strongly impacted, after the vibration wave passes through the substrate from the inside of the filter to the nanofiber layer, the nanofibers that are not strongly bonded to the substrate may peel off. The new nanofiber filter material has good adhesion to the substrate, and the nanofiber coating itself has long-lasting structural stability, higher filtration efficiency and lower pressure drop, making it useful in pulse cleaning applications Longer service life and higher efficiency.

In heavy-duty air filters, dust collectors and clean equipment, nanofiber layers are used on the windward side of the filter material. In the filtration of car intake, cabin air filter, fuel and lubricating oil, the nanofiber layer is placed on the air outlet surface of the filter material to trap particles and confine them in the air filter medium, which greatly improves the filtration efficiency , While obtaining higher efficiency and dust holding capacity.

The results show that the nanofiber coated material has better cleanability than standard wood pulp paper. Self-cleaning filters have two main functions: to ensure that particles will not penetrate or penetrate the filter material and very high filtration efficiency.

For gas turbines, high-efficiency filter materials can prevent turbine blades from being damaged by dust particles. For industrial filtration with high dust concentration or gas turbines in a windy and sandy environment, the high efficiency depth filter is not the best solution because the air passage in the filter will be cut off by the dust. Surface filtration can gather the intercepted dust on the surface of the filter and form a uniform ash cake, so that the pressure drop rises slowly.

The nanofiber layer on the windward side of the filter material can prevent dust from entering the inside of the filter material. From the beginning of filtration, almost all particles are trapped on the surface of the filter material to form a sticky ash cake with a uniform structure, thereby obtaining a stable Flow resistance. Ideally, the ash cake should be removed in one piece so that no particles will enter the filter material. Otherwise, the ash cake will be torn during the cleaning process, and the remaining particles cannot be effectively removed when entering the filter material, which will cause the pressure drop to increase.

membrane and non-woven filters are widely used in the industry to separate solids from liquids or gases. Among them are conventional filters, such as simple dust filters for the air intake, and important filters, such as dialysis filters to prevent kidney failure.

The choice of filter media is usually considered based on factors such as cost, convenience of processing and performance requirements. Many different materials are available, but in the occasions where the filters need to be used repeatedly, no matter how harsh the environment they often work in, including the cleaning process, it is necessary to ensure that the pore size of the filter medium is throughout the life of the product. Always maintain unity and consistency.

In order to achieve this degree of reliability, polymers with high resistance and rigidity have traditionally been used to make reusable filter media, such as polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE). However, these materials are quite expensive and require complex chemical treatments to obtain more performance advantages. Therefore, it is more ideal to use a cheaper, easy-to-shape filter medium, and then apply other processes to change its surface properties to obtain the desired liquid repellency.

Non-woven filter materials are gradually replacing traditional materials to become the mainstream of filter materials, and nanofiber filter materials have superior filtration performance. Electrospun nanofibers have a large specific surface area due to their small diameter. There are many micropores in the formed mesh, so they have strong adsorption and good filtering performance. However, the strength of nanofiber felt is low and the mechanical properties are poor, so it is necessary to increase the base cloth to improve the strength. At the same time, there are many other problems to be solved. For example, the structure control and performance stability of nanofiber filter materials, the development and utilization of new technologies, etc. Therefore, it is necessary to further study the relationship between the aggregate microstructure of the electrospun nanofiber mat and its performance. The characteristics of the simple and fast manufacturing of nanofibers by electrospinning are combined with other research fields to achieve complementary performance and improve the performance of electrospinning nanofiber mats. The application value of fiber filter materials, to achieve the industrialization of research results.