ARAMID FIBER FILTER PAPER AND ITS PREPARATION PROCESS

Abstract

TITLE: ARAMID FIBER FILTER PAPER AND ITS PREPARATION PROCESS The present invention relates specifically to a process for preparing aramid fiber filter paper, comprising the following steps: step (1): adding aramid pulp fibers in several fractions to a dispersion solution and preparing a dispersion to obtain homogenized aramid fibers with a size gradient; the dispersion solution comprising a dispersant, a strong alkali, and a solvent; step (2): mixing the homogenized aramid fibers with a size gradient with water, followed by high-speed mechanical agitation, to obtain a pulp of homogenized aramid fibers with a size gradient; step (3): processing and molding the pulp of homogenized aramid fibers with a size gradient to obtain the aramid fiber filter paper. The present invention also relates to aramid fiber filter paper.The aramid fiber filter paper of the present invention exhibits both high filtration accuracy and low filtration resistance, and can be used as a laminated composite material. Fig. 1.

Description

Title of the invention: ARAMID FIBER FILTER PAPER AND ITS PREPARATION METHOD technical field

[0001] The present invention relates to the field of filter paper technology, more particularly to an aramid fiber filter paper and its preparation process. PREVIOUS STATE OF THE ART

[0002] Currently, water filtration materials for the water circuit and auxiliary system of nuclear power plants are mainly manufactured from glass fibers used as raw material, generally produced using a wet forming process, and are characterized by a uniform fiber distribution, high impurity retention capacity, low resistance, and high robustness, making them an ideal water filtration material. However, glass fiber filter materials contain the element silicon, which can precipitate in the water circuit. The combination of the absence of silicon, high robustness, excellent radiation resistance, and the ability to be processed into smaller diameters makes aramid fibers an ideal substitute for glass fibers for the production of nuclear-grade filter materials.

[0003] Currently, conventional aramid pulp fibers are generally on the order of microns, resulting in low filtration accuracy when processed into paper. Aramid fibers dispersed using DMSO / KOH can be used to produce aramid membranes offering higher filtration accuracy, but these membranes exhibit considerable resistance to filtration, preventing their subsequent use in filter cartridges.

[0004] Therefore, improving filtration accuracy combined with reducing the filtration resistance of aramid fiber filter papers presents a challenge for existing technology. Summary of the invention

[0005] The present invention aims to provide an aramid fiber filter paper and a method for preparing it. The prepared aramid fiber filter paper exhibits both high filtration accuracy and low filtration resistance, and can therefore be used as a laminated composite material.

[0006] To achieve the above objective, the present invention proposes the following technical solutions:

[0007] A process for preparing aramid fiber filter paper, comprising the following steps:

[0008] step (1): adding, in several fractions, aramid paste fibers to a dispersion solution and preparing a dispersion, in order to obtain homogenized aramid fibers with a size gradient; the dispersion solution comprising a dispersant, a strong alkali and a solvent;

[0009] step (2): mixing of homogenized size gradient aramid fibers with water, followed by high-speed mechanical agitation, in order to obtain a paste of homogenized size gradient aramid fibers;

[0010] step (3): processing and molding of the homogenized aramid fiber paste with size gradient to obtain the aramid fiber filter paper.

[0011] In the present invention, in step (1), the diameter of the aramid paste fibers is preferably from 0.5 to 10 pm and the length of the aramid paste fibers is preferably from 30 to 300 pm.

[0012] In the present invention, in step (1), the aramid pulp fibers are preferably dried before use. The drying temperature is preferably from 120 to 160 °C, more preferably from 130 to 150 °C; and the drying time is preferably from 12 to 36 h, more preferably from 24 to 30 h.

[0013] In the present invention, in step (1), the dispersant in the dispersion solution is preferably chosen from dimethylformamide, dimethyl sulfoxide and diphenyl sulfone, alone or in combination; and the ratio of the mass of the aramid paste fibers to the volume of the dispersant in the dispersion solution is preferably 2.5 g : (100 to 1,500) ml, more preferably 2.5 g : (200 to 1,000) ml, and ideally 2.5 g : (250 to 500) ml.

[0014] The alkali in the dispersion solution is preferably a strong alkali, preferably further selected from potassium hydroxide, sodium hydroxide, calcium hydroxide and barium hydroxide, alone or in combination; and the mass ratio of aramid paste fibers to the alkali in the dispersion solution is preferably (0.1 to 25): 1, preferably further (1 to 20): 1, and ideally (5 to 15): 1.

[0015] The solvent in the dispersion solution is preferably chosen from methanol, ethanol and water, alone or in combination; and the ratio of the mass of the aramid paste fibers to the volume of the solvent in the dispersion solution is preferably 2.5 g : (0.1 to 100) ml, preferably again 2.5 g : (1 to 50) ml, and ideally 2.5 g : (10 to 30) ml.

[0016] In the present invention, in step (1), the aramid paste fibers are preferably added in 2 to 5 fractions, more preferably in 3 to 4 fractions; and the dispersion time after the addition of each fraction of the aramid paste fibers to the The dispersion solution is preferably 5 to 150 min, preferably still 10 to 120 min, and ideally 30 to 60 min.

[0017] In the present invention, in step (1), the dispersion is preferably carried out under agitation at a speed of 300 to 500 rpm, preferably even more so at 400 rpm; and, after the dispersion, the dispersed product is preferably washed and filtered in order to obtain the homogenized size-gradient aramid fibers.

[0018] In the present invention, in step (2), the rotation speed of the high-speed mechanical stirring is preferably from 2,000 to 20,000 rpm, more preferably from 12,000 to 18,000 rpm; and the duration of the high-speed mechanical stirring is preferably from 1 to 30 min, more preferably from 5 to 25 min.

[0019] In the present invention, the mass ratio of aramid paste fibers in step (1) to water in step (2) is preferably 1: (90 to 110), and even more preferably 1:100.

[0020] The present invention further proposes an aramid fiber filter paper prepared according to the above process, composed of aramid fibers having variable diameter distributions.

[0021] Advantageous technical effects of the present invention:

[0022] For the aramid fiber filter paper and its preparation process according to the present invention, aramid pulp fibers are first added in several fractions to a dispersion solution under stirring. Controlling the process parameters, such as the composition of the dispersion solution, the number of fractions added, and the amount added, allows the aramid pulp fibers to be dispersed for varying durations, thus producing homogenized aramid fibers with a size gradient exhibiting variable diameter distributions. During subsequent processing and forming, the homogenized aramid fibers with a size gradient are randomly layered and interwoven to form a loose, porous structure capable of ensuring excellent filtration accuracy combined with low filtration resistance.Experiments show that the aramid fiber filter paper of the present invention achieves a filtration efficiency of 99.95% for particles of 0.45 pm, with a self-pressure loss between 15 and 25 kPa. Brief description of the drawings

[0023] [Fig.1] is a full-view SEM micrograph of the aramid fibre filter paper prepared using the process of Example 1;

[0024] [Fig.2] is a detailed SEM micrograph at the micrometer scale of the paper- aramid fibre filter prepared using the process of Example 1;

[0025] [Fig.3] is an annotated SEM micrograph of the nanofiber dimensions of the aramid fibre filter paper prepared using the process of Example 1;

[0026] [Fig.4] is a SEM micrograph of the intertwined region of fibers micrometric and nanofibers in aramid fiber filter paper prepared using the process of Example 1;

[0027] [Fig. 5] is a full-view SEM micrograph of the fibre filter paper aramid prepared using the process of Example 2;

[0028] [Fig.6] is a detailed SEM micrograph at the micrometer scale of the paper- aramid fiber filter prepared using the process of Example 2;

[0029] [Fig.7] is an annotated SEM micrograph of the nanofiber dimensions of the aramid fibre filter paper prepared using the process of Example 2;

[0030] [Fig.8] is a SEM micrograph of the interlaced region of fibers micrometric and nanofibers in aramid fiber filter paper prepared using the process of Example 2;

[0031] [Fig.9] is a full-view SEM micrograph of the fibre filter paper aramid prepared using the process of Example 3;

[0032] [Fig. 10] is a micrometer-scale detailed SEM micrograph of the aramid fibre filter paper prepared using the process of Example 3;

[0033] [Fig. 11] is an annotated SEM micrograph of the nanofiber dimensions of the aramid fiber filter paper prepared using the process of Example 3; and

[0034] [Fig. 12] is an SEM micrograph of the interlaced region of micrometric fibers and nanofibers in the aramid fiber filter paper prepared using the process of Example 3. DETAILED DESCRIPTION OF IMPLEMENTATION METHODS

[0035] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as that commonly understood by a person skilled in the art to which this application relates; the terms used in the description of this application are intended solely to describe specific embodiments and not to limit this application; the terms "include" and "possess" and all their variants used in the description, claims and brief description of drawings in this application are intended to encompass a non-exclusive inclusion.

[0036] The term “example” as used herein means that the specific features, structures, or characteristics described in relation to the examples may be included in at least one example of this application. The appearance of this term in different places in the description does not necessarily refer to the same example, nor does it refer to independent or mutually exclusive alternative examples. A person versed in The art will understand explicitly and implicitly that the examples described herein may be combined with other examples.

[0037] The present invention proposes a method for preparing a filter paper made of aramid fibers, comprising the following steps:

[0038] step (1): adding, in several fractions, aramid paste fibers to a dispersion solution and carrying out a dispersion, in order to obtain homogenized aramid fibers with a size gradient; the dispersion solution comprising a dispersant, a strong alkali and a solvent;

[0039] step (2): mixing of homogenized size gradient aramid fibers with water, followed by high-speed mechanical agitation, in order to obtain a paste of homogenized size gradient aramid fibers;

[0040] step (3): processing and molding of the homogenized size gradient aramid fiber paste to obtain the aramid fiber filter paper.

[0041] Unless otherwise indicated, the sources of the various raw materials for the preparation of the aramid fibre filter paper are not specifically limited in the present invention, and commercially available products known to a person versed in the art may be used.

[0042] In the present invention, in step (1), the diameter of the aramid paste fibers is preferably from 0.5 to 10 µm and the length of the aramid paste fibers is preferably from 30 to 300 µm. Unless otherwise specified, the source of the aramid paste fibers in the present invention is not specifically limited, and commercially available products having diameters and lengths in the above ranges, as known to a person skilled in the art, may be used.

[0043] In the present invention, in step (1), the aramid paste fibers are preferably aramid paste fibers produced by DuPont.

[0044] In the present invention, in step (1), the aramid pulp fibers are preferably dried before use; the drying temperature is preferably 120 to 160 °C, more preferably 130 to 150 °C; and the drying time is preferably 12 to 36 h, more preferably 24 to 30 h. In the present invention, drying allows the removal of moisture from the aramid pulp fibers so as not to affect the weighing operations.

[0045] In the present invention, in step (1), the dispersant in the dispersion solution is preferably chosen from dimethylformamide, dimethyl sulfoxide and diphenyl sulfone, alone or in combination.

[0046] In the present invention, during the dispersion of the aramid paste fibers, the amide groups of the aramid fibers are deprived of hydrogen in order to form negatively charged polyanions; and in the initial deprotonation phase, the Negative charges gradually accumulate on the molecular chains of aramid fibers, and the resulting electrostatic repulsion progressively splits the macroscopic aramid paste fibers into bundles of PPTA microfibers with an average diameter of 1 to 2 µm. As the degree of deprotonation increases, the electrostatic repulsion between the polymer chains also gradually strengthens, thus providing the energy necessary to break the intermolecular hydrogen bonds within the polymer chains, ultimately yielding aramid fibers with a high aspect ratio. Aramid fibers can disperse stably in the dispersant at the equilibrium of electrostatic repulsion, van der Waals forces, and ir-ir stacking.

[0047] In the present invention, in step (1), the alkali in the dispersion solution is preferably a strong alkali, preferably selected from potassium hydroxide, sodium hydroxide, calcium hydroxide, and barium hydroxide, alone or in combination. In the present invention, the alkali acts as a dispersing agent, working primarily in synergy with the dimethyl sulfoxide to accelerate the dispersion of the aramid and to deprotonate the aramid, in other words, to rapidly remove hydrogen from the aramid molecules in order to form polyanions on the surface of the aramid.

[0048] In the present invention, in step (1), the solvent in the dispersion solution is preferably chosen from methanol, ethanol, and water, alone or in combination. The solvent selected in the present invention can facilitate the rapid removal of protons from the aramid fibers by the dispersion solution, thereby promoting the deprotonation process and increasing the dispersion rate of the aramid fibers.

[0049] In the present invention, in step (1), the mass ratio of aramid paste fibers to alkali in the dispersion solution is preferably between (0.1 to 25):1, preferably even more (there 20):1, and ideally (5 to 15):1. The present invention makes it possible to accelerate the dispersion of aramid paste fibers and to reduce the dispersion time by defining the mass ratio of aramid paste fibers to alkali in the above range.

[0050] In the present invention, in step (1), the ratio of the mass of the aramid paste fibers to the volume of the dispersant in the dispersion solution is preferably 2.5 g : (100 to 1500) ml, more preferably 2.5 g : (200 to 1000) ml, and ideally 2.5 g : (250 to 500) ml. The present invention allows for sufficient and stable dispersion of the aramid paste fibers by defining the ratio of the mass of the aramid paste fibers to the volume of the dispersant within the above range.

[0051] In the present invention, in step (1), the ratio of the mass of the aramid paste fibers to the volume of the solvent in the dispersion solution is preferably 2.5 g : (0.1 to 100) ml, more preferably 2.5 g : (1 to 50) ml, and ideally 2.5 g: (10 to 30) ml. The present invention makes it possible to improve the dispersion rate of aramid paste fibers and to reduce the dispersion time by defining the ratio of the mass of aramid paste fibers to the volume of solvent in the above range.

[0052] In the present invention, in step (1), the aramid paste fibers are preferably added in 2 to 5 fractions, and even more preferably in 3 to 4 fractions. In the present invention, adding the aramid paste fibers to the dispersion in several fractions allows the aramid paste fibers to disperse for varying durations, thus producing homogenized aramid fibers with a size gradient exhibiting variable diameter distributions. During subsequent processing and forming, the homogenized aramid fibers with a size gradient are randomly layered and intertwined to form a loose, porous structure capable of providing excellent filtration accuracy combined with low filtration resistance.By defining the number of aramid pulp fiber fractions added within the above range, the present invention makes it possible to adjust the distribution of aramid fibers in homogenized size-gradient aramid fibers in order to further improve the filtration accuracy of the aramid fiber filter paper and to further reduce the filtration resistance of the aramid fiber filter paper.

[0053] In the present invention, when the aramid pulp fibers are added in 3 fractions, the mass ratio of the added aramid pulp fiber fractions is preferably (0.1 to 1): (0.5 to 2): (0.5 to 2), and even more preferably (0.1 to 0.5): (0.5 to 1.5): (0.5 to 1.5), and ideally 0.5:1:1. By defining the mass ratio of the added aramid pulp fiber fractions in the above range, the present invention makes it possible to obtain a suitable gradient size distribution in the homogenized size-gradient aramid fibers obtained after dispersion, in order to further improve the filtration accuracy of the filter paper and to further reduce filtration resistance.

[0054] In the present invention, at step (1), the dispersion time after the addition of each fraction of aramid pulp fibers is preferably 5 to 150 min, more preferably 10 to 120 min, and ideally 30 to 60 min. In the present invention, the dispersion is preferably carried out under agitation at a speed of 300 to 500 rpm, more preferably 400 rpm. By defining the dispersion time after the addition of each fraction of aramid pulp fibers and the agitation speed within the above ranges, the present invention allows the dispersed aramid fibers to retain their morphology and to have appropriate diameters and lengths, thereby further improving the filtration accuracy of the filter paper and further reducing filtration resistance.

[0055] In the present invention, in step (1), after dispersion, the dispersed product is preferably washed and filtered to obtain homogenized size-gradient aramid fibers. The washing and filtration operations are not specifically limited in the present invention, and washing and filtration processes known to a person skilled in the art may be used.

[0056] In the present invention, in step (2), high-speed mechanical agitation is preferably carried out at a rotational speed of 2,000 to 20,000 rpm, more preferably from 12,000 to 18,000 rpm; and the duration of the high-speed mechanical agitation is preferably from 1 to 30 min, more preferably from 5 to 25 min. The present invention allows for a more thorough dispersion of homogenized size-gradient aramid fibers by defining the rotational speed and the duration of the high-speed mechanical agitation within the above ranges.

[0057] In the present invention, the mass ratio of aramid paste fibers in step (1) to water in step (2) is preferably 1: (90 to 110), and even more preferably 1:100. By setting the mass ratio of aramid paste fibers in step (1) to water in step (2) in the above range, the present invention allows for further dispersion of homogenized size-gradient aramid fibers in water, which is advantageous for the subsequent processing and formation of homogenized size-gradient aramid fiber paste.

[0058] The processing and forming operations of the homogenized size-gradient aramid fiber pulp are not specifically limited in the present invention, and filter paper processing and forming operations known to a person skilled in the art may be used. In the present invention, the processing and forming method mentioned in step (3) is preferably as follows: subjecting the homogenized size-gradient aramid fiber pulp to papermaking, a first hot pressing, sizing, a second hot pressing, and cooling sequentially, in order to obtain the aramid fiber filter paper.

[0059] The papermaking operation is not specifically limited in the present invention, and papermaking operations known to a person versed in the art may be used.

[0060] In the present invention, the temperature for the first hot pressing of the homogenized size-gradient aramid fiber paste is preferably 80 to 90 °C, more preferably 85 °C; the duration of the first hot pressing of the homogenized size-gradient aramid fiber paste is preferably 20 to 40 min, more preferably 30 min; and the pressure of the first hot pressing of the homogenized size-gradient aramid fiber paste is preferably 1 to 5 kPa, more preferably 2 to 4 kPa. It is preferable to carry out the first hot pressing of homogenized aramid fiber pulp with size gradient to dry the paper and flatten the paper surface.

[0061] In the present invention, a gluing liquid for gluing is preferably chosen from polyamide-epichlorohydrin, polyacrylamide and polyurethane, alone or in combination.

[0062] In the present invention, the amount of sizing is preferably 1 to 3% by mass of the paper after the first hot pressing, and even more preferably 2% by mass of the paper after the first hot pressing. In the present invention, the sizing imparts rigidity and hardness to the paper, thereby improving its durability. The present invention makes it possible to obtain greater rigidity and hardness of the paper without compromising its filtration performance by setting the amount of sizing within the above range.

[0063] In the present invention, the temperature for the second hot pressing is preferably 80 to 90 °C, more preferably 85 °C; the duration of the second hot pressing is preferably 20 to 40 minutes, more preferably 30 minutes; and the pressure of the second hot pressing is preferably 1 to 5 kPa, more preferably 2 to 4 kPa. In the present invention, the second hot pressing serves to dry the paper and flatten its surface.

[0064] In the present invention, aramid paste fibers are first added in several fractions to a dispersion solution under stirring. Controlling the process parameters, such as the composition of the dispersion solution, the number of fractions added, and the amount added, allows the aramid paste fibers to be dispersed for varying durations, thus producing homogenized aramid fibers with a size gradient exhibiting variable diameter distributions. During subsequent processing and forming, the homogenized aramid fibers with a size gradient are randomly layered and intertwined to form a loose, porous structure capable of providing excellent filtration accuracy combined with low filtration resistance.

[0065] The present invention further proposes an aramid fiber filter paper prepared according to the above process, composed of aramid fibers having variable diameter distributions.

[0066] In the present invention, each square meter of the aramid fiber filter paper is preferably composed of 70 g of aramid fibers with a diameter of 10 to 10,000 nm, the volume percentages of aramid fibers with a diameter of 10 to 100 nm, of aramid fibers with a diameter greater than 100 nm and less than 500 nm, of aramid fibers with a diameter of 500 to 1,000 nm and of aramid fibers with a diameter greater than 1 pm and less than or equal to 10 pm being preferably (10 to 20)%: (30 to 40)%: (30 to 40)%: (10 to 20)%.

[0067] The aramid fiber filter paper proposed by the present invention offers high filtration accuracy, low filtration resistance and high robustness.

[0068] The present invention further proposes a use of aramid fiber filter paper as a laminated composite material for filtration.

[0069] The operation of using aramid fiber filter paper as a laminated composite material for filtration is not specifically limited in the present invention, and the operations of using filter papers for filtration well known to a person versed in the art may be employed.

[0070] The technical solution of the present invention will be described below in a clear and complete manner with reference to the attached drawings and specific examples.

[0071] Example 1

[0072] The present example proposed a process for preparing a filter paper made of aramid fibers, comprising the following steps:

[0073] step (1): adding, in three fractions, aramid paste fibers to a dispersion solution and carrying out a dispersion, in order to obtain homogenized aramid fibers with a size gradient; the dispersion solution comprising a dispersant, a strong alkali and a solvent;

[0074] step (2): mixing of homogenized size gradient aramid fibers with water, followed by high-speed mechanical agitation, in order to obtain a paste of homogenized size gradient aramid fibers;

[0075] step (3): processing and molding of the homogenized size gradient aramid fiber paste to obtain the aramid fiber filter paper.

[0076] In the present example, in step (1), the aramid pulp fibers were aramid pulp fibers with a diameter of 0.5 to 10 µm and a length of 30 to 300 µm, produced by DuPont.

[0077] The dispersion solution was composed of 0.5 g of potassium hydroxide, 10 ml of ethanol, 10 ml of deionized water and 250 ml of dimethyl sulfoxide, the mass ratio of aramid paste fibers to potassium hydroxide being 5:1, the ratio of the mass of aramid paste fibers to the volume of dimethyl sulfoxide being 2.5 g: 250 ml and the ratio of the mass of aramid paste fibers to the total volume of ethanol and deionized water being 2.5 g: 20 ml.

[0078] The aramid paste fibers were dried at 150 °C for 24 hours before being added to the dispersion solution and carrying out a dispersion.

[0079] Aramid paste fibers were added in three fractions to the dispersion solution to achieve dispersion and obtain a dispersed product. In the first fraction, 0.5 g of aramid paste fibers were added to the dispersion solution before stirring with a paddle at 400 rpm for 30 min; in the second fraction, 1 g of aramid paste fibers were added to the solution of dispersion before stirring using a stirring paddle at 400 rpm for 30 min; in the third fraction, 1 g of aramid paste fibers was added to the dispersion solution before stirring using a stirring paddle at 400 rpm for 5 min.

[0080] The dispersed product was washed and filtered three times with deionized water to obtain homogenized size-gradient aramid fibers.

[0081] In the present example, in step (2), the homogenized size-gradient aramid fibers were mixed with 250 g of deionized water and stirred at 15,000 rpm for 20 min to obtain a size-gradient mixed fiber paste.

[0082] In the present example, in step (3), the homogenized size-gradient aramid fiber pulp was transformed into filter paper with a diameter of 21.5 cm using a paper machine, which was then hot-pressed at 85 °C and 3 kPa for 30 min. Polyamide-epichlorohydrin was then added at a rate of 2% by weight of the paper, before a further hot pressing at 85 °C and 3 kPa for 30 min and cooling, in order to obtain the aramid fiber filter paper.

[0083] The aramid fiber filter paper prepared using the process of the example was composed of aramid fibers having variable diameter distributions, the volume percentages of aramid fibers with a diameter of 10 to 100 nm, aramid fibers with a diameter greater than 100 nm and less than 500 nm, aramid fibers with a diameter of 500 to 1000 nm and aramid fibers with a diameter of 1 to 10 pm being 10%: 30%: 40%: 20%.

[0084] Example 2

[0085] The present example was identical to Example 1, except that in step (1), stirring was carried out for 60 min after the addition of the first fraction of aramid paste fibers to the dispersion solution, and stirring was carried out for 60 min after the addition of the second fraction of aramid paste fibers to the dispersion solution.

[0086] The aramid fiber filter paper prepared using the process of the example was composed of aramid fibers having variable diameter distributions, the volume percentages of aramid fibers with a diameter of 10 to 100 nm, aramid fibers with a diameter greater than 100 nm and less than 500 nm, aramid fibers with a diameter of 500 to 1000 nm and aramid fibers with a diameter of 1 to 10 pm being 15%:35%:35%:15%.

[0087] Example 3

[0088] The present example is identical to Example 1, except that in step (1), stirring was carried out for 150 min after the addition of the first fraction of aramid paste fibers to the dispersion solution, and stirring was carried out for 15 min after the addition of the second fraction of aramid paste fibers to the dispersion solution.

[0089] The aramid fiber filter paper prepared using the process in the example was composed of aramid fibers with varying diameter distributions, the volume percentages of aramid fibers with a diameter of 10 to 100 nm, aramid fibers with a diameter greater than 100 nm and less than 500 nm, aramid fibers with a diameter of 500 to 1000 nm and aramid fibers with a diameter of 1 to 10 pm being 20%: 40%: 30%: 10%.

[0090] The aramid fiber filter paper prepared in Example#la was observed using a scanning electron microscope (SEM) to obtain the SEM micrographs at different magnifications illustrated respectively in Figures#1 to#4.

[0091] The aramid fiber filter paper prepared in Example#! was observed using a scanning electron microscope to obtain the SEM micrographs at different magnifications illustrated respectively in Figures#5 to#8, [Fig.5] being presented at a magnification of 100, [Fig.6] being presented at a magnification of 500, [Fig.7] being presented at a magnification of 30,000 and [Fig.8] being presented at a magnification of 10,000.

[0092] The aramid fiber filter paper prepared in Example #3 was observed using a scanning electron microscope to obtain the SEM micrographs at different magnifications illustrated respectively in Figures #9 to #12, [Fig.9] being presented at a magnification of 50, [Fig.10] being presented at a magnification of 3000, [Fig.11] being presented at a magnification of 4950 and [Fig.12] being presented at a magnification of 100.

[0093] It appears from Figures #1 to #12 that each of the aramid fiber filter papers prepared in Examples #1 to #3 has a porous structure with aramid fibers of varying diameters.

[0094] The aramid fibre filter papers prepared in Examples #1 to #3 were evaluated with regard to their filtration efficiency with respect to particles of different sizes, their own pressure drop and their ability to retain impurities, and the results are presented in Table #1.

[0095] Table#1 shows the filtration efficiency with respect to particles of different sizes, self-pressure loss and impurity retention capacity of each of the aramid fiber filter papers prepared in Examples#1 to#3.

[0096] [Tables!] Filtration efficiency Pressure drop (kPa) Impurity retention capacity (g / m²) 0.2 pm 0.3 pm 0.45 pm 0.5 pm 0.6 pm 0.8 pm Example 1 98.30 99.47 99.95 99.98 99.99 99.98 20 97.2 Example 2 81.67 99.29 99.95 99.97 99.97 99.95 25 83.3 Example 3 72.49 74.76 93.47 99.01 99.96 99.96 15 101.2

[0097] It appears from Table 1 that each of the aramid fiber filter papers prepared in Examples 1 to 3 exhibits high filtration efficiency for particles with a diameter of 0.45 µm and low filtration resistance. The aramid fiber filter papers supplied in Examples 1 to 3 exhibit high filtration accuracy, low filtration resistance, and high robustness.

[0098] The examples presented above have only illustrated several embodiments of the present invention, described in a relatively specific and detailed manner, and should not be interpreted as limiting the scope of the present invention. It should be noted that a person skilled in the art relevant to this application will understand that it is possible to make various modifications and improvements without departing from the spirit of the present invention, and that these all fall within the scope of the present invention. Therefore, the scope of the present invention will be defined by the appended claims.

Claims

Demands

1. A process for preparing an aramid fiber filter paper, comprising the following steps: step (1): adding aramid pulp fibers in several fractions to a dispersion solution and preparing a dispersion, in order to obtain homogenized aramid fibers with a size gradient; the dispersion solution comprising a dispersant, a strong alkali and a solvent; step (2): mixing the homogenized aramid fibers with a size gradient with water, followed by high-speed mechanical agitation, in order to obtain a pulp of homogenized aramid fibers with a size gradient; step (3): processing and molding the pulp of homogenized aramid fibers with a size gradient to obtain the aramid fiber filter paper.

2. A method according to claim 1, wherein, in step (1), the diameter of the aramid paste fibers is 0.5 to 10 µm and the length of the aramid paste fibers is 30 to 300 µm.

3. A process according to claim 1, wherein, in step (1), the aramid pulp fibers are dried at a drying temperature of 120 to 160 °C for 12 to 36 h before use.

4. A process according to claim 1, wherein, in step (1), the dispersant in the dispersion solution is selected from dimethylformamide, dimethyl sulfoxide and diphenyl sulfone, alone or in combination; and the ratio of the mass of aramid paste fibers to the volume of the dispersant in the dispersion solution is 2.5 g : (100 to 1500) ml; the alkali in the dispersion solution is a strong alkali; and the mass ratio of aramid paste fibers to the alkali in the dispersion solution is (0.1 to 25) : 1; the solvent in the dispersion solution is selected from methanol, ethanol and water, alone or in combination; and the ratio of the mass of aramid paste fibers to the volume of the solvent in the dispersion solution is 2.5 g : (0.1 to 100) ml.

5. A method according to claim 1, wherein, in step (1), the aramid pulp fibers are added in 2 to 5 fractions; and the duration The dispersion time after adding each fraction of aramid paste fibers to the dispersion solution is 5 to 150 min.

6. A method according to claim 1, wherein, in step (1), the dispersion is carried out under agitation at a speed of 300 to 500 rpm; and, after the dispersion, the dispersed product is washed and filtered, in order to obtain homogenized size-gradient aramid fibers.

7. A method according to claim 1, wherein, in step (2), high-speed mechanical agitation is carried out at a rotational speed of 2,000 to 20,000 rpm for 1 to 30 min.

8. A process according to claim 1, wherein the mass ratio of aramid paste fibers in step (1) to water in step (2) is 1: (90 to 110).

9. Aramid fibre filter paper prepared by means of the process according to any one of claims 1 to 8, composed of aramid fibres having variable diameter distributions.

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