HK40066520A - A method for preparing a composite filter medium and the composite filter medium obtained with this method - Google Patents

Description

A method for preparing composite filter media and the composite filter media obtained by the method

Background of the Invention

This invention relates to a method for preparing composite filter media. The invention also extends to composite filter media obtained using this method.

The field of this invention is the field of composite filter media, particularly composite filter media for preventing the intrusion of dust particles and repelling liquids (generally such as water and oil) in order to ensure high air permeability, i.e., low acoustic impedance, for optimal sound transmission; for example, in consumer electronic devices, especially in the electroacoustic components of mobile phones.

Known composite filter media are formed by a combination of at least one nanofiber layer supported by a weft and warp base fabric, wherein the nanofiber layer is deposited on the base fabric via an electrospinning process, and wherein a plasma coating layer is applied to both the base fabric and the nanofibers. This method produces composite filter media in which the nanofiber layer adheres to the base fabric.

To ensure the desired performance of the plasma coating, it is crucial that the monomers injected into the plasma system chamber polymerize on the surfaces of the substrate fabric and nanofibers under optimal conditions. However, these polymerization conditions depend on the process parameters set for the plasma treatment, such as the power supply, the sealing pressure in the vacuum chamber, the duration of fiber exposure to plasma treatment, the distance between the matrix and the electrodes, and other process parameters.

During the plasma processing described above, the pressure in the vacuum chamber may vary relative to the set value, and in particular, the pressure may increase due to gases released by the material being processed within the vacuum chamber. During plasma processes used to form coatings on the surfaces of substrate fabrics and nanofibers, the pressure increase within the chamber can primarily be attributed to the moisture content of the material placed within the vacuum chamber. Indeed, during this process, water molecules leave the fiber material to be coated, leading to increased pressure, mixing with the coating plasma feed gas, and thus contaminating it. This becomes even more critical when working on rolls of materials with large diameters and heavy weights, i.e., in industrial production processes.

Such increased pressure inevitably alters the polymerization conditions of the materials forming the substrate fabric and the coating layer of nanofibers, leading to incomplete polymerization of the coating layer. This, in turn, prevents the reduction of the surface energy of the nanofibers and, therefore, the desired water and oil repellency cannot be achieved in the final filter medium.

The polymerization reaction is altered by contamination of the plasma feed gas by water molecules released from the fabric, resulting in a coating with chemical-physical properties that are less than expected for water- and oil-repellent coatings, and failing to ensure adequate adhesion of the polymerized coating to the matrix.

Invention Overview

The main objective of this invention is to provide a composite filter medium and its manufacturing process, which, compared to known filter media of this type, ensures optimal polymerization of coatings deposited on the surface of monofilaments forming the substrate fabric and on the surface of nanofibers.

The present invention also aims to provide a process for manufacturing a filter medium having a coating layer on the surface of a monofilament that strongly adheres to the substrate fabric and the surface of a nanofiber.

These and other objectives are achieved, respectively, by the method of claim 1 and the filter medium of claim 10. The preferred embodiments of the invention will become apparent from the remaining claims.

Compared to known filter media, the filter media of the present invention offers the advantage of maintaining a desired level of repellency against water and oil due to the formation of a fully polymerized coating layer that strongly adheres to the surface of the substrate fabric and nanofibers.

The composite filter medium of the present invention (in which individual nanofibers and individual threads of fabric are covered with a thin, highly hydrophobic and oleophobic coating) also has the ability to expel dust and, in particular, liquids, not only water (high surface tension, 72 mN/m) but also liquids such as oil with low surface tension (30 mN/m-40 mN/m). This property of the filter medium of the present invention is particularly useful in its application as a protective screen for electroacoustic components, especially electroacoustic components of mobile phones. In fact, the filter medium of the present invention is composed of nanofibers, which provide very high air permeability (and very low acoustic impedance), thereby ensuring effective prevention of particle intrusion. Furthermore, due to the specific coating of the composite filter medium of the present invention, the composite filter medium of the present invention prevents the penetration of water, oil, and other types of liquids. In fact, the filter medium of the present invention not only prevents the penetration of these liquids but is also easier to clean due to its water repellency.

Brief description of the attached diagram

These and other objectives, advantages and features will become apparent from the following description of preferred embodiments of the method and filter media according to the invention, which are illustrated by way of non-limiting example in the accompanying drawings.

In these attached figures:

Figure 1 is a cross-sectional schematic diagram of an example of the composite filter medium of the present invention;

Figure 2 shows a detailed diagram of nanofibers deposited on corresponding lines of a substrate fabric by electrospinning, wherein both the nanofibers and the lines of the substrate fabric are coated with a nanolayer of a water- and oil-repellent polymer applied by plasma treatment.

Figure 3 illustrates an electrospinning method for forming nanofiber layers in the filter media of the present invention;

Figure 4 schematically illustrates the plasma treatment of the filter medium of the present invention, which is obtained by depositing a nanofiber layer made by an electrospinning process on a substrate fabric.

Figure 5 illustrates the relationship between flow rate and pressure measured across the entire filter medium for both dry and wet samples.

Figure 6 illustrates the relationship between the venting pressure and the corresponding pressure drop for the declogging test performed on two different samples.

Description of preferred implementation scheme

The composite filter medium of the present invention, generally indicated by the number 1 in Figure 1, comprises a support formed of a base fabric 2 (preferably a monofilament fabric) of the warp and weft yarn type, on which nanofibers 4 are deposited by electrospinning onto the surface of the base fabric. The monofilaments 3 suitable for the present invention are made from monofilaments of polyester, polyamide, polypropylene, polyethersulfone, polyimide, polyamide-imide, polyphenylene sulfide, polyetheretherketone, polyvinylidene fluoride, polytetrafluoroethylene, or aromatic polyamide, wherein the mesh opening of the base fabric 2 is in the range of 2500 micrometers to 5 micrometers.

The base fabric used to prepare the composite filter media of the present invention is selected from a wide range of synthetic monofilament fabrics that differ in the chemical properties of the monofilaments used for weaving, such as polyester, polyamide, polypropylene, polyethersulfone, polyimide, polyamide-imide, polyphenylene sulfide, polyetheretherketone, polyvinylidene fluoride, polytetrafluoroethylene, and aromatic polyamides. Also suitable for the present invention are base fabrics having a weave structure of 4-300 threads/cm, a thread diameter of 10-500 micrometers, a weight of ^15-300 g/ ^m² , and a thickness of 18-1000 micrometers. For finishing and further surface treatment, in addition to metallization, washed and heat-set “white” fabrics, colored fabrics, plasma-treated fabrics, hydrophobic fabrics, hydrophilic fabrics, antibacterial fabrics, antistatic fabrics, and similar fabrics can be used. The preferred material of the present invention is a polyester monofilament fabric with 48 threads per centimeter and a diameter of 55 micrometers, and the mesh opening of the base fabric is 153 micrometers.

Suitable for use in this invention are nanofibers of polyester, polyurethane, polyamide, polyimide, polypropylene, polysulfone, polyethersulfone, polyamide-imide, polyphenylene sulfide, polyetheretherketone, polyvinylidene fluoride, polytetrafluoroethylene, alginate, polycarbonate, PVA (polyvinyl alcohol), PLA (polylactic acid), PAN (polyacrylonitrile), PEVA (polyvinyl acetate), PMMA (polymethyl methacrylate), PEO (polyethylene oxide), PE (polyethylene), PVC, PEI, PUR, and polystyrene. These nanofibers can have a diameter between 50 nm and 700 nm. PVDF (polyvinylidene fluoride) nanofibers with a diameter range from 75 nm to 200 nm are preferred.

As illustrated in Figure 3, the electrospinning process for forming nanofibers 4 and subsequently depositing them onto a substrate fabric 2 includes injecting a material dissolved in a suitable solvent for forming the nanofibers 4 through a nozzle 5, so as to spread the material onto an electrode 6. Due to the potential difference between the nozzle 5 and the electrode 6, and due to the electric field and the stretching of the polymer deposited on the electrode by means of the nozzle, the nanofibers 4 are formed by the evaporation of the solvent. The nanofibers thus formed are then stretched and subsequently deposited onto the substrate fabric 2.

The composite filter media obtained in this way then undergoes a surface treatment, which is carried out by plasma deposition of a polymer layer 7 of nanometer thickness on the exposed surfaces of the fabric 2 and the nanofiber layer 4, thereby completely covering the outer surfaces of the monofilament 3 of the substrate fabric 2 and the aforementioned nanofiber 4 (Figure 2).

As shown in Figure 4, in the presence of the gas that forms the aforementioned coating layer 7, the composite filter medium 8 obtained from the previous electrospinning process in Figure 3 is arranged in the plasma treatment chamber 9 to cover the composite filter medium 1 of the present invention.

The present invention preferably utilizes gases based on fluoroacrylates, particularly heptadecafluorodecyl acrylate, perfluorooctyl acrylate, and the like. Due to the water- and oil-repellent properties of fluoroacrylates, gases formed by plasma treatment of fluoroacrylate deposits are advantageous for the present invention.

In the plasma processing described above, a carrier gas, such as the type described in WO2011089009A1, is also used.

The aforementioned plasma treatment includes generating a vacuum of 10-50 mTorr, an electrode power of 150-350 W, and an exposure time of 0.5-6 minutes.

Coatings deposited using plasma technology can have thicknesses up to 500 nm and, due to the specific techniques used, possess a continuous film structure, even capable of coating 3D surfaces such as fabrics. Depending on the compounds used, the aforementioned coatings can possess a variety of unique characteristics, such as hydrophobicity, oleophobicity, hydrophilicity, and antistatic properties.

The preferred coatings of this invention are obtained from the following compounds in the starting gas:

1H,1H,2H,2H-Heptafluorodecyl acrylate ( _CAS #27905-45-9, _{H₂C ＝ CHCO₂CH₂CH₂} ( _CF₂ ) _{₇CF₃ )}

1H,1H,2H,2H-Perfluorooctyl acrylate (CAS# _17527-29-6 , _{H₂C ＝ CHCO₂CH₂CH₂} ( _CF₂ ) _{₅CF₃ )} .

The coating layer 7 has a thickness of 15nm-60nm, which is suitable for preventing the coating layer from excessively narrowing the pores formed by the composite filter medium 1 in both the fabric 2 and the nanofiber 4. Excessive narrowing of the pores would hinder the free passage of sound.

The composite filter medium 8 obtained from the electrospinning process shown in Figure 3 was tested and compared with a similar composite filter medium 1 that underwent subsequent plasma treatment as shown in Figure 4.

Specifically, the aforementioned filter medium 8 is formed by a weft and warp fabric made of synthetic monofilament 3 (e.g., polyester monofilament), on which nanofibers 4, also made of synthetic materials (e.g., polyester), have been deposited to obtain an acoustic impedance of 25 MKS Rayls, which is measured using a Textest instrument or a similar instrument for measuring acoustic impedance/air permeability.

After plasma treatment of filter medium 8, it can be observed that the acoustic impedance remains unchanged at a value of 25 MKS Rayls on the composite filter medium 1 of the present invention. At a pressure of 200 Pa, the air permeability is 5,200 l/ ^m²s , and the filtration efficiency also remains unchanged.

On the other hand, a considerable increase was observed in the contact angle with water (from 50° to 130°) and the contact angle with oil (from 50° to 120° for corn oil with a surface tension of 32 mN/m), where the contact angle was measured using a sessile method (droplet deposition and contact angle measurement by high-resolution camera) based on a drop of water or oil with nanofibers.

Clear blockage test

To provide evidence for the observations described above, a testing method was developed to digitally quantify the energy required to remove oil deposited on the surface of the composite filter media of the present invention.

The test was conducted using a porosimeter (PMI 1200, manufactured by PMI), which employs capillary flow porometry to determine the bubble point, minimum pore size, and pore size distribution of the tested sample. Capillary flow porometry, or simply porosimetry, is based on a very simple principle: measuring the pressure of the gas required to force the wetting fluid through the pores of a material. The pressure required to expel the pores is inversely proportional to the size of the pores themselves. Larger pores require lower pressures, while smaller pores require higher pressures.

The test involves cutting the sample to be analyzed and placing it in the test chamber. The sample is then held in place using an O-ring to ensure no lateral air leakage. Once the chamber is closed, the air permeability of the filter media is measured, yielding a curve that correlates the airflow through the sample with the pressure drop measured across the filter media (the drying curve in Figure 5). Once the drying curve is obtained, the test chamber is opened, and the sample is left in place, its surface covered with a test liquid having low surface tension (typically <20 mN/m). The test chamber is then closed, and the air permeability of the material is measured again. As the material becomes clogged with the test liquid, the pressure increases, but no airflow is measured downstream until the pressure is high enough to force the liquid through the pores. From this point on, as the pressure increases, the pores, which have decreased in size, are emptied until the sample (previously wet) is completely dry, and the two curves in Figure 5 overlap. Without delving into analytical details, at a qualitative level, the difference between the two curves can determine the bubble point value (maximum pore size), the minimum pore size, and the pore size distribution.

In a specific case, this test was conducted to determine the oil repellency/removal capability, but corn oil (surface tension 32 mN/m) was used instead of the test liquid.

The graphs in Figure 6 show the venting pressure and the corresponding pressure drop (energy required for venting). The samples considered in the graphs of Figure 6 are filter media 8 (curve 10) treated with electrospinning and filter media 1 of the present invention (curve 11). It can be seen that, compared with composite filter media 8 that has not undergone plasma treatment, filter media 1 of the present invention can remove oil at a significantly lower pressure, or remove a significantly larger amount of oil at the same pressure.

According to the present invention, it has now been surprisingly discovered that by adding a preliminary step of degassing the materials of the monofilaments 3 and nanofibers 4 forming the composite filter medium 8 to be treated in a vacuum chamber before the step of forming the coating layer 7, and subsequent plasma treatment, complete polymerization and strong adhesion of the coating layer subsequently deposited on the monofilaments and nanofibers forming the substrate fabric are achieved.

Specifically, according to the invention, prior to the step of forming the plasma coating layer 7, a degassing step is performed in chamber 9 of the filter medium 8 obtained in the previous electrospinning process, so that the pressure in chamber 9 reaches a value of 5 mTorr to 250 mTorr. For this purpose, a degassing step is provided, depending on the size, weight, and hygroscopicity of the material to be treated, which typically exposes the material for an exposure time in the range of 5 seconds to 5 minutes. Of course, once the appropriate exposure time that allows for complete drying of the medium, i.e., the time to ensure a stable vacuum in the subsequent coating step, is determined, the correct speed for the degassing step should be set depending on the exposure area in the chamber. Such an area is determined by the distance between the unwinding cylinder and the winding cylinder, as well as the electrode size. In particular, if the material is packaged in rolls, depending on the moisture content of the material, it will be continuously unwound and rewound in chamber 9 at a speed between 0.1 m/min and 50 m/min. An opening, appropriately controlled by a valve system, will be provided in chamber 9 so that the gas to be eliminated can be discharged.

According to the present invention, the preliminary check of the aforementioned pressure value will allow the moisture contained in the material being treated in chamber 9 to be completely removed, so as to allow the desired polymerization pressure to be achieved on the surface of the coating layer 7 on the substrate fabric and nanofibers in subsequent steps of forming the coating layer.

Furthermore, according to the present invention, after the degassing treatment described above and before the step of forming the coating layer 7, the surfaces of the monofilaments 3 and nanofibers 4 forming the substrate fabric 2 are reactivated in chamber 9 by plasma treatment. This plasma treatment is carried out in chamber 9 at a pressure maintained between 10 mTorr and 400 mTorr, using an electrode power in the range of 100 W to 2000 W and an exposure time in the range of 5 seconds to 5 minutes, using a carrier gas preferably selected from nitrogen, helium, argon, and oxygen. Depending on the gas used, the exposure time, and the power, a more or less noticeable etching effect is obtained, resulting in a nanometer/micrometer roughness on the surface to be treated.

In this step, since there are no polymer monomers, no coating layer is formed on the treated surface. Instead, ions from the carrier gas, appropriately excited by plasma, bombard the surface of the matrix with a certain energy, which creates nanogrooves and thus nanoroughness. This facilitates the gripping and adhesion of the polymer coating layer 7 to the surfaces of the monofilaments 3 and nanofibers 4, significantly contributing to the repulsion of aqueous and oily liquids by the filter media.

The results provided by the filter media produced using the process of the present invention are shown in the table below. The values are measured for filter media having a layer 7 of polymer material, which is obtained by plasma treatment for forming the layer 7 of polymer material after the following steps:

- A degassing step, which is carried out by holding the material to be treated in chamber 9 for a period of 30 seconds suitable for ensuring a stable pressure of 25 mTorr in subsequent treatments.

-And subsequently, the plasma treatment of the material to be coated, which is carried out in the presence of helium as a carrier gas, using a vacuum of 150 mTorr, an electrode power of 600 W, and an exposure time of 1 minute:

These results demonstrate how the polymer coating 7 formed in the vacuum chamber 9, following the degassing step and preliminary plasma treatment, ensures a very high contact angle (>110°) between the filter media of the present invention and the oil, as well as a much higher level of adhesion to the matrix than the minimum required.

In the invention as described above and illustrated in the figures in the accompanying drawings, modifications may be made to produce variations that still fall within the scope of the appended claims.

In particular, when the filter media is made from a slightly hygroscopic material and undergoes a plasma deposition process, a reactivation step can be performed separately using plasma treatment and a carrier gas selected again from nitrogen, helium, argon, and oxygen. In fact, for this type of slightly hygroscopic material, the preliminary degassing step described above can be omitted.

Claims (18)

1. A method for preparing a composite filter medium (1), comprising the steps of forming a first filter medium (8) by depositing nanofibers (4) on a substrate fabric (2) by means of an electrospinning process and covering the filter medium (1) by plasma deposition of a coating layer (7) on the first filter medium (8) in a vacuum chamber (9), characterized in that, after the electrospinning process and before the plasma deposition of the coating layer (7), the method provides a degassing step of the substrate fabric (2) and the nanofibers (4) forming the aforementioned first filter medium (8) in the same chamber (9). 2. The method according to claim 1, characterized in that, during the degassing step, the aforementioned chamber (9) is brought to an internal pressure value between 5 mTorr and 250 mTorr. 3. The method according to claim 1, characterized in that, during the degassing step, the material is ensured to be exposed in the chamber for a period of 5 seconds to 5 minutes. 4. The method according to claim 1, characterized in that, after the aforementioned degassing step and before the plasma deposition of the coating layer (7), the method further provides a step of forming irregular portions on the surface of the substrate fabric (2) and the aforementioned nanofibers (4) by plasma treatment of the first filter medium (8) obtained in the previous degassing step in the presence of a carrier gas and without any polymer-containing gas in the chamber (9). 5. The method according to claim 4, wherein the aforementioned carrier gas is selected from nitrogen, helium, argon or oxygen. 6. The method according to claim 5, characterized in that the aforementioned plasma treatment is performed in the chamber (9) at a pressure of 10 mTorr to 400 mTorr, using an electrode power of 100 W to 2000 W, and with an exposure time between 5 seconds and 5 minutes. 7. The method according to claim 1, wherein the electrospinning process comprises extruding a polymer dissolved in a suitable solvent by means of a nozzle (5), and subsequently stretching the fibers between the nozzle itself and an electrode to obtain the deposition of nanofibers on a substrate fabric suitably placed between the nozzle and the electrode, wherein the filter medium (8) obtained therefrom subsequently undergoes a surface treatment by plasma deposition of a polymer layer (7) of nanometer thickness on the exposed surfaces of the substrate fabric (2) and the exposed surfaces of the nanofiber layer (4) to obtain the aforementioned composite filter medium (1), wherein the outer surfaces of the monofilaments (3) of the substrate fabric (2) and the outer surfaces of the aforementioned nanofibers (4) are covered with the polymer layer (7). 8. The method according to claim 7, wherein the aforementioned plasma deposition process comprises generating a vacuum of 10 mTorr to 50 mTorr, an electrode power of 150 W to 350 W, and an exposure time of 0.5 min to 6 min. 9. A method for preparing a composite filter medium (1), comprising the steps of forming a first filter medium (8) by depositing nanofibers (4) on a substrate fabric (2) by means of an electrospinning process and covering the filter medium (1) by plasma deposition of a coating layer (7) on the first filter medium (8) in a vacuum chamber (9), characterized in that, after the electrospinning process and before the plasma deposition of the coating layer (7), the method provides the step of forming irregular portions on the surfaces of the substrate fabric (2) and the nanofibers (4) by plasma treatment of the first filter medium (8) in the chamber (9) in the presence of a carrier gas and without any polymer-containing gas. 10. A composite filter medium comprising a substrate fabric (2) on which nanofibers (4) are deposited, characterized in that the substrate fabric and the aforementioned nanofibers are covered with a nanocoating layer (7) applied by means of a plasma process, the substrate fabric (2) and the nanofibers (4) having nanogrooves obtained by plasma treatment in the presence of a carrier gas and without any polymer-containing gas. 11. The filter medium according to claim 10, wherein the aforementioned coating layer (7) is formed of a membrane having a thickness of up to 500 nm, preferably having a thickness of 15 nm to 60 nm. 12. The filter medium according to claim 10, wherein the aforementioned coating layer (7) is a coating layer based on fluorocarbon acrylate having water-repellent and oil-repellent properties. 13. The filter medium according to claim 10, wherein the monofilament (3) is made from monofilaments of polyester, polyamide, polypropylene, polyethersulfone, polyimide, polyamide-imide, polyphenylene sulfide, polyether ether ketone, polyvinylidene fluoride, polytetrafluoroethylene, or aromatic polyamide. 14. The filter medium according to claim 10, wherein the aforementioned substrate fabric (2) has a mesh opening of 2500 micrometers to 5 micrometers. 15. The filter medium according to claim 10, characterized in that the aforementioned base fabric (2) has a textile structure of 4 threads/cm to 300 threads/cm, a thread diameter of 10 micrometers to 500 micrometers, a weight of 15 g/ ^m² to 300 g/ ^m² , and a thickness of 18 micrometers to 1000 micrometers. 16. The filter medium according to claim 10, wherein the aforementioned nanofibers (4) are nanofibers of polyester, polyurethane, polyamide, polyimide, polypropylene, polysulfone, polyethersulfone, polyamide-imide, polyphenylene sulfide, polyether ether ketone, polyvinylidene fluoride, polytetrafluoroethylene, alginate, polycarbonate, PVA (polyvinyl alcohol), PLA (polylactic acid), PAN (polyacrylonitrile), PEVA (polyvinyl acetate), PMMA (polymethyl methacrylate), PEO (polyethylene oxide), PE (polyethylene), PVC, PI or polystyrene. 17. The filter medium according to claim 10, characterized in that the nanofiber (4) has a diameter between 50 nm and 700 nm, preferably, the nanofiber (4) is a PVDF (polyvinylidene fluoride) nanofiber having a diameter in the range of 75 nm to 200 nm. 18. The use of the filter medium according to one or more of the preceding claims for protecting electroacoustic components in a mobile phone.