EE05785B1 - Method for making filter material - Google Patents

Abstract

The present invention relates to a method for producing a fibrous filter material by an electric spinning method, wherein the fibers additionally have a triboelectric charge in addition to a stable electret charge. In order to carry out the disclosed method, the fibers are formed in an electrostatic field from two polymer solutions in which the polymers acquire a triboelectric charge upon contact. Due to the coalescence of the fibers on the collection electrode and the simultaneous friction between the fibers, these fibers acquire an electrically charged charge in addition to the electrode charge in a conventional electric spin, i.e. a triboelectric charge corresponding to a charge signal obtained at different ends of two polymers. Acquiring tribolelectric charge increases the electric field inside the resulting material and therefore improves the filtering properties of the material

Description

Technical field

The invention relates to the production of nonwoven synthetic filter materials, more specifically, the invention relates to a method for producing filter materials for filtering finely dispersed substances, especially for producing fine-fiber and microfiber nonwoven filter materials used in personal protective equipment.

State of the art

There is a known invention (CN102121137, Dalian University of Technology, published on 13.07.2011) which relates to a bipolar electrospinning method with a dual nozzle to produce a nanofibrous material. In the case of electrospinning, nanofibers obtained from nozzles with opposite charges are interwoven with each other, whereby the fibers acquire charges of different polarity under the influence of the attraction force, and in this way a bifilar interwoven nonwoven material is formed. The disadvantage of this solution is that when the fibers are firmly attached to each other, the electric field generated by the differently charged components of the dual fiber forms a dipole electric field and a conventional electret charge is obtained by conventional electrospinning.

A triboelectric air filter is known (US20040177759, Tsai P.P, Chang Liang-Yu, published 16.09.2004), which comprises a mixture of polyolefin and melamine fibres, the polyolefin being preferably made from a compound selected from the group consisting of polyethylene, polypropylene, polybutylene and mixtures thereof. The resulting mixture of the two fibres is carded using a carding machine to form a triboelectric charge on the fibres. The polyolefin fibres acquire a negative charge which is different from the charge of the polyamide fibres. The disadvantage of this solution is that the resulting fabric acquires a triboelectric charge through friction and the fibres remain charged as a result of the subsequent mechanical action of the fabric.

A method for producing an electrostatically charged filter material is known (US6811594, Hollingsworth & Vose Air Filter, published 02.11.2004), which comprises mixing polyester fibers with at least one other electrostatically charged fiber to produce the filter material. The disadvantage of this solution is that the mixing of the fibers and the formation of the material do not occur at the same time.

The known invention (US5726107A, Hoechst AG, 10.03.1998) relates to nonwoven materials containing at least two types of electret fibers, where the fibers are produced by a dry or wet spinning process. The fibers of the material acquire a triboelectric charge only after mechanical processing of the resulting material. The disadvantage of this solution is that the invention relates to improving the production of electret fibers and the stability of the electret charge.

The closest to the invention in terms of technical nature is the method for producing a filter material (EP0829293 B1, Esfil Limited, 18.03.1998.), which presents a method for producing a fine-fiber filter material from fibers with a permanent electret charge, where the fibers are formed in an electrostatic field from a solution with a dynamic viscosity of 0.1-3.5 Pa x s, an electrical conductivity of 10-8 - 10-4 ohm-1 x cm-1, a potential difference of 25 -150 kV and a volumetric flow rate of the solution of 10-3 - 2 x 10-2cm3/s per fiber-forming element. In this case, the fibers are additionally treated in the zone of their formation with supersaturated vapors of the solvent. The disadvantage of this solution is that the electret charge field formed on the fibers acts over smaller distances than the field of differently charged fibers, where the fibers are located 4-5 fiber diameters apart.

The essence of the invention

According to the invention, a method is provided for the production of filter material from fibers with a stable electret charge, where in the electrospinning process these fibers acquire an additional triboelectric charge. A modification of electrospinning from two solutions is also provided, which differ in the sign of the charge of the electric potential given to the spinning nozzle, where the sign of the charge corresponds to the position of the polymer being spun in the triboelectric series. In this case, a charge of the required sign is created on the fibers before the fibers are mixed to form a filtering medium, due to which subsequent mechanical treatment can be omitted, because in this case the filtering effect is often greater than in the case of subsequent mechanical treatment alone.

To implement the presented method, fibers are formed in an electrostatic field from such two polymer solutions, where the polymers acquire an electrostatic charge upon contact. During the electrospinning process, the electret fibers fall together onto a collecting electrode and simultaneously with their falling together, mutual friction of these fibers occurs, due to which the electret fibers additionally acquire a charge of opposite signs during the electrospinning process, i.e. a triboelectric charge, which corresponds to the sign of the charges formed during the friction/separation of the two polymers used in this electrospinning, and these two polymers are located at different ends of the triboelectric line. The acquisition of a triboelectric charge increases the electric field inside the resulting material and therefore the filtering properties of the material improve. In order to increase the triboelectric charge already present in the material, the resulting material is processed by subsequent mechanical action, for example, the resulting material is passed through the rollers of a winding machine or the material is corrugated, then the mutual friction of the fibers increases their charge. If bipolar spinning is used in this case, where the spinning nozzle, which sends polymer solutions to the spinning zone, is given special electric potentials, and the sign of the charge, which is given to the fiber by means of a high-voltage device, corresponds to the sign of the charge, which is formed as a result of the subsequent mixing, then the charging process proceeds more stably and the filtering properties of the material are additionally improved. In this case, to stabilize the spinning, additional treatment with saturated solvent vapors is carried out in the fiber formation zone.

Thus, the disadvantage of the solution closest to the invention EP0829293 is eliminated, where the fibers obtained by the method according to EP0829293 have only an electret charge, remaining generally electrically neutral, with this charge being of a shorter duration (dipole type) compared to the electrostatic (excess) charge. Since the efficiency of entrapping aerosol particles by the fibers increases with increasing long-range effect, the filtering properties of the material made of such fibers are improved.

The disadvantages of the methods presented in application US20040177759 and patent US6811594 have also been eliminated by the presented solution, because the electrospinning process simultaneously includes both the mixing of fibers and the collapsing of fibers onto the collection electrode, i.e. the formation of the filter material in a single operation.

Examples of carrying out the invention

The present invention differs from existing methods of manufacturing triboelectrically charged filters in the method of imparting a triboelectric charge to the fiber.

In the conventional method, two types of staple fibers are produced separately, which differ in the polymer used and, as a rule, the necessary operation is washing out the oiler. The process of mixing these two types of fibers constitutes a separate operation. The final operation is the formation of the filter material, for example by the needle-punching method. During these two operations, a triboelectric charge is applied to the fibers.

In the presented method, the fiber manufacturing operations, i.e. fiber mixing and filter material formation, are combined into a single process, but in the case of bipolar electrospinning, the charging of the fibers is improved.

In unipolar spinning, a rotating cylinder (drum) passes between two rows of spinning elements. Each row is fed with its own polymer solution. Spinning elements are arranged along a conveyor on which the filter material is formed. Even-numbered elements are fed with one polymer solution, and odd-numbered elements are fed with another polymer solution. In this case, the mixing of the fibers and the formation of the material occur simultaneously during the electrospinning process, along with the formation of a triboelectric charge on the fibers. The triboelectric charge of the fibers in unipolar electrospinning, where the same-sign electric potentials are applied to the spinning nozzle that sends the polymer solution to the spinning zone, can be increased by subsequent mechanical action on the resulting material, either in the process of further processing the material or by rolling the material between rollers.

Example 1: In the case of unipolar electrospinning, solutions of polystyrene and highly chlorinated polyethylene are in an organic solvent to which tetramethylammonium iodide has been added. The viscosity of both solutions is 0.6 Pa x s, the electrical conductivity is 7x 10-5 ohm-1cm-1. Spinning was carried out on rotating rollers, the voltage of both solutions was -25 kV. The oil aerosol permeability coefficient of the resulting material at a speed of 6.8 cm/s without mechanical action was 0.09%, in the case of mechanical action, for example, after the resulting material passed through rollers and/or after corrugation, it was 0.0025%. Thus, the filtering properties of the material improved more than 30 times when using the subsequent mechanical action.

In bipolar electrospinning, voltage is applied to the spinning elements from two high-voltage sources of 20 to 120 kV with a polarity that corresponds to the position of the polymer being spun in the triboelectric series, and the mixing of the fibers and the charging of the fibers with different polarity occurs during the electrospinning process, i.e. the fibers are charged already during electrospinning and the charging process proceeds more stably and in addition, the filtering properties of the material are further improved, as can be seen in Example 2.

Example 2: a polystyrene solution is in an organic solvent with a negative polarity, and a polymethyl methacrylate solution is in an organic solvent with a positive polarity. The viscosity of the solutions is 0.6 Pa x s, the electrical conductivity is 7 x 10-5 ohm-1cm-1. The resulting materials have an oil aerosol permeability coefficient at a speed of 3 cm/s: for unipolar spinning it is 0.014%; for bipolar spinning - 0.00002%, i.e., for bipolar electrospinning, the filtering properties of the material improved by 700 times. If the resulting material is then subjected to mechanical action, for example, the material is passed between rollers or the material is corrugated, then the oil aerosol permeability coefficient of the material is 0.00015%) and the efficiency of the material increases by 1.3 times compared to bipolar electrospinning without mechanical action.

Claims (5)

1. A method for producing a filter material from fibers with a stable electret charge, which are formed from a polymer solution in a solvent in an electrostatic field, where the solution is characterized by viscosity and electrical conductivity, and the fibers are additionally treated with saturated solvent vapors in a fiber formation zone, characterized in that in the method for producing a filter material from two polymer solutions, an electrospinning is used and the polymer solution is prepared from polymers susceptible to triboelectric charging, wherein in the electrospinning process, the electret fibers fall together onto a collecting electrode and simultaneously with the falling together of these electret fibers, mutual friction of these fibers occurs and due to mutual friction, an additional triboelectric charge is acquired by the electret fibers during the electrospinning process in one operation, which corresponds to the sign of the charge obtained by mixing two polymers located at different ends of the triboelectric line. 2. The method according to claim 1, characterized in that the method applies an electrostatic field with a potential difference of 20 to 120 kV, the polymer solution in the solvent has a viscosity of 0.05 to 5 Pa x s and the electrical conductivity of the solution is 1x 10-6 to 1x 10-3 ohm-1cm-1. 3. The method according to claim 1, characterized in that the electrospinning is bipolar electrospinning and in the case of bipolar electrospinning, a voltage of 20 to 120 kV with a corresponding polarity is applied to two different polymer solutions prepared from polymers susceptible to triboelectric charging, and at the sign of the charge imparted to the fiber by means of a high-voltage device with a polarity corresponding to the position of the polymer to be spun in the triboelectric row, and wherein the mixing of the fibers and the special polarity charging of the fibers occur in a single operation during electrospinning. 4. The method according to claims 1 to 3, characterized in that the resulting material is subjected to a subsequent mechanical action to increase the triboelectric charge of the fibers. 5. The method according to claim 1, characterized in that the electrospinning is unipolar electrospinning, where the mixing of fibers and the formation of fibers occur in a single operation in the electrospinning process, and the triboelectric charge of the fibers is increased by subsequent mechanical action on the resulting material.