Method for producing an ultra-thin uniform nonwoven fabric

Abstract

This invention relates to the field of nonwoven material preparation, specifically a method for preparing an ultrathin, uniform nonwoven fabric. The method includes: preparing a cellulose fiber precursor web with a basis weight of 5–30 g / m² and a basis weight variation coefficient (CV) not exceeding 5%; spraying, impregnating, or roller-coating the precursor fiber web with an alcohol solvent, ketone solvent, a mixture thereof, or an aqueous solution thereof, causing the fiber web to expand under low constraint; and drying and shaping the expanded, wet fiber web under the constraint of a planar substrate, release film, supporting screen, or stretching frame. This invention reduces the mass per unit area by expanding the precursor fiber web, achieving a basis weight of 1–5 g / m². 2 This method produces flat, uniform, and dimensionally stable ultrathin nonwoven fabrics. It is applicable to wood pulp fibers, viscose fibers, and other natural or regenerated cellulose fibers.

Description

Technical Field

[0001] This invention relates to the field of nonwoven material preparation technology, specifically to a method for preparing an ultrathin uniform nonwoven fabric. Background Technology

[0002] Nonwoven fabrics are sheet-like materials made by forming a fiber network through mechanical, physical, chemical, or thermal methods and then reinforcing it. They are characterized by high production efficiency, wide raw material adaptability, and strong structural design flexibility. Depending on the web-forming method, the manufacturing processes of nonwoven fabrics typically include wet web forming, dry web forming, air-jet web forming, carding web forming, spunbond web forming, meltblown web forming, and electrospinning web forming. With the development of fields such as electronic component packaging, lithium battery separator substrates, precision filtration, medical dressings, ultra-thin insulating materials, daily chemical mask base fabrics, and disposable hygiene products, nonwoven fabrics increasingly require properties such as extremely low basis weight, high uniformity, large width, and dimensional stability after drying.

[0003] Existing wet web forming processes typically disperse fibers in water, which are then web-formed, dehydrated, pressed, and dried to form fiber sheets, enabling the production of low-basis-weight paper sheets or wet-laid nonwoven materials. However, when the product basis weight is reduced to 5 g / m²... 2 At the following conditions, the fiber coverage density per unit area decreases significantly, and problems such as uneven fiber distribution, local fiber loss, voids, edge instability, and breakage are prone to occur during web formation. Meltblowing can form finer thermoplastic polymer fibers, but under extremely low basis weight conditions, it is also prone to increased voids and greater fluctuations in areal density, and its raw materials are usually limited to melt-processable thermoplastic polymers. Dry carding or air-blowing processes are constrained by factors such as fiber length, fiber fineness, mechanical carding ability, and reinforcement methods; the basis weight of conventional products is usually higher than that of wet-laid products. For cellulose fiber materials, low-basis-weight fiber webs obtained by conventional processes are also prone to warping, wrinkling, and dimensional changes during drying due to inter-fiber hydrogen bond recombination, inconsistent local shrinkage, and internal stress release.

[0004] Therefore, existing processes are unable to directly produce products with a basis weight of 1–5 g / m³ within the conventional and stable precursor web-forming basis weight range. 2 This results in a cellulose fiber nonwoven fabric with a low coefficient of variation in basis weight, no obvious holes, and a smooth and dimensionally stable surface after drying. To address the aforementioned issues, it is necessary to provide a preparation method that can reduce the mass per unit area by using a highly uniform precursor fiber web as a basis, through subsequent in-plane expansion, and lock the expanded dimensions using constrained drying. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a method for preparing ultrathin uniform nonwoven fabrics. This method employs a process route of precursor web formation, solvent-induced expansion, and planar constrained drying and shaping, which enables the precursor fiber web to undergo in-plane uniform expansion while maintaining the continuity of the overall fiber network, thereby reducing the mass per unit area. Furthermore, the expanded fiber web size is maintained through constrained drying.

[0006] To achieve the above objectives, the present invention provides the following technical solution: a method for preparing an ultrathin uniform nonwoven fabric, comprising the following steps:

[0007] S1. Using precursor fiber web as raw material, the precursor fiber web is made of cellulose fibers, and the basis weight of the precursor fiber web is 5-30 g / m². 2 The coefficient of variation (CV) for weight is no greater than 5%.

[0008] S2. Spraying, impregnating or roller coating the precursor fiber web with an expansion liquid, so that the expansion liquid penetrates between the fibers, and the precursor fiber web expands uniformly in the plane without applying tension that restricts the in-plane expansion of the fiber web. The area of ​​the expanded fiber web is 1.2 to 5 times the area before expansion.

[0009] S3. The expanded wet fiber web is placed under planar constraint conditions for drying and shaping, so that the fiber web is dried in the expanded planar dimensional state to obtain an ultra-thin uniform nonwoven fabric.

[0010] The swelling liquid is an alcohol or ketone solvent; the resulting ultrathin uniform nonwoven fabric has a basis weight of 1–5 g / m². 2 .

[0011] Preferably, the cellulose fiber is wood pulp fiber or viscose fiber, and the precursor fiber web is made of wood pulp fiber, viscose fiber, or a mixture of wood pulp fiber and viscose fiber.

[0012] Preferably, the fineness of the cellulose fibers is no greater than 1.5 dtex; the target basis weight of the resulting ultrathin uniform nonwoven fabric is 3 g / m². 2 The fineness of the cellulose fibers is no greater than 1 dtex; the target basis weight of the resulting ultrathin uniform nonwoven fabric is 1 g / m². 2 In the following cases, the fineness of the cellulose fibers is no greater than 0.5 dtex.

[0013] Preferably, when the precursor fiber web is prepared using a wet web-forming process, the wet web-forming process specifically comprises: preparing a fiber suspension with a mass concentration of 0.5% to 2.0% of cellulose fibers; subjecting the fiber suspension to pulping, dispersion, and web-forming treatment; and then sequentially subjecting it to gravity dehydration, vacuum dehydration, and initial drying to obtain a basis weight of 5 to 20 g / m³.2 The wet-process precursor fiber web; wherein the fiber suspension has a pH of 4.5 to 7.0, a freeness of 20 to 45°SR, and a vacuum degree of 0.01 to 0.08 MPa for vacuum suction dehydration.

[0014] Preferably, when the precursor fiber web is prepared using a dry web-forming process, the dry web-forming process includes opening, carding, and air-forming of the fibers. The airflow velocity during air-forming is 15–30 m / s, resulting in a basis weight of 10–30 g / m². 2 The dry precursor fiber web; when the dry precursor fiber web is reinforced by hydroentangling, the pre-wetting hydroentangling pressure is 1.5-4 MPa, the main hydroentangling pressure is 4-8 MPa, and the hydroentangling speed is 20-60 m / min.

[0015] Preferably, the alcohol solvent is one or more of ethanol, methanol, isopropanol, and n-propanol, and the ketone solvent is one or more of acetone and butanone; when the swelling liquid is an aqueous ethanol solution, the volume concentration of ethanol is 30% to 100%.

[0016] Preferably, the mass ratio of the precursor fiber web to the expansion liquid is 1:(5-20); the immersion time of the expansion liquid in the precursor fiber web is 0.5-10 min, and the immersion temperature is room temperature; after the precursor fiber web is fully immersed in the expansion liquid, its mass per unit area decreases proportionally according to the fiber web area expansion factor.

[0017] Preferably, the planar constraint condition is any one of the following:

[0018] The expanded wet fiber mesh is attached to the surface of a glass plate, stainless steel plate, or flat substrate covered with a release film.

[0019] The expanded wet fiber web, along with the supporting screen, is transferred to the tensioning frame, and the frame clamps the periphery of the fiber web.

[0020] When the precursor fiber web is wet, an expansion liquid is applied to the original forming web or conveyor belt to expand the fiber web in situ and then dry it together with the original forming web or conveyor belt.

[0021] Preferably, the drying and shaping are carried out by natural drying at room temperature or hot air drying at 40-80℃; the longitudinal shrinkage rate and transverse shrinkage rate of the dried ultrathin uniform nonwoven fabric are both no more than 2%, the maximum warping height of a 500mm×500mm sample placed on a plane is no more than 3mm, and there are no holes or pores on the surface of the nonwoven fabric.

[0022] Preferably, the ultrathin uniform nonwoven fabric comprises an interwoven and dried cellulose fiber network with a basis weight of 1–5 g / m². 2The weight variation coefficient (CV) is no greater than 5%, the thickness is 0.01–0.2 mm, and the in-plane dimensions remain flat in the dry state.

[0023] This invention provides a method for preparing an ultrathin, uniform nonwoven fabric. It has the following beneficial effects:

[0024] 1. This invention first prepares a precursor fiber web using conventional wet or dry web-forming processes, then uses an expanding liquid to cause in-plane expansion of the precursor fiber web. This treatment causes the original fiber distribution and interlacing relationship in the precursor to expand synchronously with the fiber web area, and the mass per unit area decreases accordingly with the area expansion factor. This avoids the local fiber deficiency, voids, uneven thickness, and unstable web formation caused by insufficient fiber coverage per unit area when directly preparing ultra-low basis weight fiber webs. When using cellulose fibers with a fineness of no more than 1.5 dtex as raw materials, the expanded fiber web can still maintain a relatively continuous fiber coverage structure, and the resulting product basis weight can reach 1-5 g / m². 2 The weight variation coefficient (CV) is no greater than 5%, and the product has an appearance quality without obvious holes or pores.

[0025] 2. This invention regulates the original bonding state between fibers by allowing an expanding liquid to penetrate into the fiber network, enabling the wet fiber web to expand uniformly in-plane under low-constraint conditions. By controlling the type of expanding liquid, solvent concentration, feed-to-liquid ratio, and wetting time, the area expansion ratio and final basis weight of the fiber web can be adjusted. This process can obtain ultrathin products of different basis weight grades based on conventionally and stably prepared precursor basis weights, with a clearly defined basis weight control method.

[0026] 3. This invention involves expanding the fiber web, then attaching the wet fiber web to a glass plate, stainless steel plate, or a planar substrate covered with a release film, or clamping it together with a supporting screen onto a tensioning frame, followed by natural drying at room temperature or hot air drying at 40–80°C for shaping. The planar constraint limits edge shrinkage, localized shrinkage, and uneven stress release during the drying process, allowing the fibers to dry and recombine in their relative expanded positions, thus maintaining the predetermined length, width, and flatness. The resulting product exhibits longitudinal and transverse shrinkage rates both below 2%, and the maximum warpage height of a 500mm × 500mm sample is no greater than 3mm. This method can also be implemented using in-situ expansion, support screen transfer, or continuous conveying drying methods, reducing the risk of breakage of the ultra-thin wet fiber web during transfer and facilitating its use with wet web forming, airflow web forming, and continuous drying equipment. Attached Figure Description

[0027] Figure 1 This is a schematic diagram of the preparation process of the nonwoven fabric of the present invention;

[0028] Figure 2This is a schematic diagram of the expansion of the precursor fiber web of the present invention;

[0029] Figure 3 This is a schematic diagram of three planar constraint conditions of the present invention. Detailed Implementation

[0030] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0031] like Figures 1-3 As shown, this invention provides a method for preparing an ultrathin uniform nonwoven fabric, the specific steps of which are as follows:

[0032] Wood pulp fiber was selected as the raw material, with an average fineness of 0.8 dtex. The wood pulp fiber was added to deionized water for pulping to obtain a fiber suspension with a mass concentration of 1.0%. Polyethylene oxide was added to the fiber suspension as a dispersant, with the amount of polyethylene oxide being 0.15% of the oven-dry wood pulp fiber mass.

[0033] The fiber suspension was pulped using a disc mill, with the freeness controlled at 32°SR. After pulping, the pH of the fiber suspension was adjusted to 6.2. The pulp was then fed into the headbox of a wet web forming apparatus and formed into a web at a web speed of 20 m / min. The web was then dewatered by gravity and vacuum suction, with a vacuum level of 0.04 MPa.

[0034] The wet fiber web was initially dried under hot air at 80℃, with the moisture content controlled at 8%. The resulting wet-processed precursor fiber web had a basis weight of 10.1 g / m³. 2 The thickness is 0.16 mm, and the coefficient of variation (CV) of the weight was measured to be 3.8% after sampling at 20 points.

[0035] Prepare an ethanol-water solution with a volume concentration of 75%. Cut the precursor fiber web into 500mm × 500mm samples, and denote the initial area of ​​the precursor fiber web as A0. The mass ratio of the sample to the ethanol-water solution is controlled at 1:10. Place the sample flat on a polyester sieve and immerse it completely in the ethanol-water solution for 3 minutes. During the immersion process, the fiber web expands freely in-plane under the support of the sieve.

[0036] After impregnation, the screen carrying the fiber web was gently removed, allowing excess liquid to drip naturally for 30 seconds. Then, a negative pressure adsorption transfer method was used to transfer the wet fiber web onto a glass plate coated with a polyester release film. During the transfer, the fiber web was kept flat, and no localized tension was applied. Measurements showed that the area of ​​the expanded fiber web was approximately 2.0 times that before expansion, or 2A0.

[0037] The glass plate, along with the wet fiber web, was placed in an environment of 25°C and 50% relative humidity for natural drying for 12 hours. After drying, the product was peeled off from the release film surface. The resulting ultrathin, uniform nonwoven fabric had a basis weight of 5.0 g / m². 2 The thickness is 0.09 mm, the coefficient of variation (CV) is 4.1%, the longitudinal shrinkage rate is 1.4%, the transverse shrinkage rate is 1.2%, and the maximum warpage height of the 500 mm × 500 mm sample is 2.5 mm. Visual and translucent inspection revealed no obvious holes or pores in the product.

[0038] Example 1:

[0039] The embodiments of the present invention verify the effect of different ethanol concentrations on the area swelling effect, as detailed below:

[0040] Using the same wet-process wood pulp precursor fiber web as described above, the precursor basis weight was controlled at 10.0 ± 0.3 g / m³. 2 The initial sample size was 500mm × 500mm. Each sample was treated with ethanol-water solutions of different concentrations, with a material-to-liquid ratio of 1:10 and a soaking time of 3 minutes. After soaking, the fiber mesh was attached to a glass plate covered with a release film and allowed to air dry at 25℃.

[0041] When the ethanol volume concentration is 30%, the fiber web area expands to approximately 1.2 times its original area, and the product weight is approximately 8.3 g / m². 2 The resulting product remained intact, with good flatness and no obvious holes.

[0042] When the ethanol volume concentration is 50%, the fiber web area expands to approximately 1.5 times its original area, and the product weight is approximately 6.7 g / m². 2 After drying, the product surface was smooth, and no obvious warping was observed.

[0043] When the ethanol volume concentration is 75%, the fiber web area expands to approximately 2.0 times its original area, and the product weight is approximately 5.0 g / m². 2 After drying under planar constraints, the product's shrinkage rate in both the longitudinal and transverse directions is no greater than 2%, and there are no obvious holes on the surface.

[0044] When the ethanol volume concentration is 95%, the fiber web area expands to approximately 2.5 times its original area, and the product weight is approximately 4.0 g / m². 2When drying with a glass plate attached, the product can remain intact and flat; if drying without planar constraint is not performed, the edges of the product will show a certain degree of shrinkage.

[0045] When treated with anhydrous ethanol, the area expansion factor of the fiber web is 3.0 to 3.5 times, and the basis weight of the resulting product is 2.9 to 3.3 g / m². 2 Due to the significant expansion, the use of a mesh frame or a flat substrate attachment method during the drying process helps maintain the dimensional stability of the product.

[0046] Example 2:

[0047] The embodiments of this invention verify the effect of different types of expanding liquid on the area expansion effect, as detailed below:

[0048] Select a weight of 10g / m 2 A wet-process wood pulp precursor fiber web with a basis weight variation coefficient (CV) of 4.0% was used as the sample. Methanol, isopropanol, n-propanol, and acetone were each prepared into 75% aqueous solutions. The material-to-liquid ratio for each sample group was controlled at 1:10, the wetting time was 3 minutes, and the wetting temperature was 25℃.

[0049] When treated with a 75% methanol aqueous solution, the fiber web area expands to 1.8 to 2.0 times its original area. The expanded fiber web is then attached to the surface of a glass plate with a release liner and allowed to air dry. The resulting product is smooth and free of obvious holes. Methanol is toxic; therefore, the operation should be carried out under ventilated conditions, and protective gloves, safety glasses, and organic vapor protection measures should be in place.

[0050] When treated with a 75% isopropanol aqueous solution, the fiber web area expands to 1.5 to 1.8 times its original area, and the product surface is relatively smooth after drying under planar constraint.

[0051] When treated with a 75% n-propanol aqueous solution, the fiber web area expands to 1.4 to 1.7 times its original area. After attachment and drying, the resulting product has a continuous structure without obvious pores.

[0052] When treated with a 75% acetone aqueous solution, the fiber web area expands to approximately 1.5 times its original area. Acetone is volatile and flammable; therefore, soaking and drying operations should be carried out away from open flames and high-temperature heat sources, in an environment with solvent recovery or ventilation facilities. The product remains intact after drying under planar constraints.

[0053] Example 3:

[0054] like Figure 3 As shown in the figure, the embodiments of the present invention verify the influence of different drying and setting methods on the area expansion effect, as follows:

[0055] Select a weight of 10g / m2 A wet-process wood pulp precursor fiber web with a basis weight variation coefficient (CV) of 4.0% was used as the sample, and a 75% methanol aqueous solution was used as the expansion liquid.

[0056] When dried using an unconstrained method, after the fiber web expands to approximately twice its original area, it is placed directly on the polypropylene mesh surface to dry naturally without attaching glass plates, clamping with a mesh stretching frame, or fixing with a support mesh. After drying, the product edges exhibit irregular shrinkage, and localized wrinkles appear on the surface. This indicates that applying planar constraints during the drying process of the expanded, wet fiber web helps improve the flatness and dimensional stability of the product.

[0057] When drying using a flat plate attachment method, the fiber web expands to approximately twice its original area before being attached to a glass plate, stainless steel plate, or a flat substrate covered with a release film for natural drying. After drying, the product exhibits irregular shrinkage at the edges, noticeable surface warping, and localized wrinkles. This indicates that applying planar constraints during the drying process of the expanded wet fiber web helps improve the flatness and dimensional stability of the product.

[0058] When using a flat-plate drying method with hot air drying, the expanded fiber web is held on a continuous support belt as it enters a 60℃ hot air drying zone. The drying zone is 12m long, and the fiber web remains in the drying zone for 8 minutes. After drying, the product is peeled off the support belt and wound up using a peeling roller. Sampling and testing at 20 points per square meter of the resulting product revealed no obvious areas of fiber deficiency. The maximum warpage height of the product is less than 3mm, the shrinkage rate is less than 2%, and the product has a smooth surface, allowing for continuous winding.

[0059] When using a stretched mesh frame for drying, the sample is immersed and then freely spread on a stainless steel screen. After the area expansion stabilizes, it is transferred along with the screen to a stretched mesh frame with adjustable dimensions. By adjusting the four-sided clamping mechanism of the stretched mesh frame, the fiber web is kept in its original expanded state. The stretched mesh frame containing the fiber web is placed in a 60℃ hot air drying oven for 40 minutes, followed by equilibration at room temperature for 30 minutes. After drying, the product is removed from the frame. The resulting ultra-thin, uniform nonwoven fabric has a maximum warpage height of less than 2 mm, a shrinkage rate of less than 1%, and light transmission inspection shows no through-holes and good surface uniformity.

[0060] When using the in-situ expansion drying method with a conveyor belt, the precursor fiber web expands in situ on the original forming web under the action of the expanding liquid, with the fiber web area expanding to approximately twice its original area. After expansion, the fiber web is not transferred separately; instead, the expanded wet fiber web, along with the original forming web, enters the hot air drying zone with the conveyor belt. The conveyor belt uses a continuously running flat support belt to keep the fiber web flat during the drying process. The drying zone temperature is 60℃, the drying zone length is 12m, the conveyor belt speed is 1.5m / min, and the residence time of the fiber web in the drying zone is 8min. After drying, the resulting ultra-thin uniform nonwoven fabric is peeled off from the original forming web and wound up. Testing showed that the longitudinal shrinkage rate and transverse shrinkage rate of the obtained product were both less than 2%, and the maximum warpage height when placed on a flat surface was less than 3mm. The product surface is smooth, with no obvious localized areas of fiber deficiency, holes, or through-holes, meeting the requirements for continuous production and winding.

[0061] Example 4:

[0062] The embodiments of this invention verify the influence of precursor basis weight on area expansion effect and target basis weight, as detailed below:

[0063] Using wood pulp fiber precursor fiber webs, fiber webs with a basis weight of 5 g / m were prepared. 2 8g / m 2 and 15g / m 2 The wet-process precursors were prepared by treating each precursor with a 75% (v / v) ethanol aqueous solution and drying them using a mesh frame.

[0064] For a weight of 5g / m 2 Precursors with a fiber fineness of less than 0.5 dtex were impregnated with a 75% ethanol aqueous solution for 3 minutes at a feed-to-liquid ratio of 1:10. The fiber web area expanded to approximately 4.5 to 5 times its original area, and the resulting product had a basis weight of 1.0 to 1.1 g / m². 2 This product is suitable for filter support layers, ultra-thin insulating substrates, or electronic packaging liners where ultra-low weight requirements are needed.

[0065] For a weight of 8g / m 2 Precursors with a fiber fineness of less than 1 dtex were impregnated with a 75% ethanol aqueous solution for 3 minutes, resulting in a fiber web area expansion to approximately 2.6 to 2.8 times the original area, and the resulting product had a basis weight of 2.9 to 3.1 g / m². 2 The product has good integrity and operational strength.

[0066] For a weight of 15g / m 2 Precursors with a fiber fineness of less than 1.5 dtex were impregnated with a 75% ethanol aqueous solution for 3 minutes, resulting in a fiber web area that expanded to approximately three times its original area. The resulting product had a basis weight of approximately 5 g / m².2 This product is suitable for ultra-thin dressings, mask substrates, precision packaging liners, or substrates for subsequent impregnation processing.

[0067] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for preparing an ultrathin uniform nonwoven fabric, characterized in that, Includes the following steps: S1. Using precursor fiber web as raw material, the precursor fiber web is made of cellulose fibers, and the basis weight of the precursor fiber web is 5-30 g / m². 2 The coefficient of variation (CV) for weight is no greater than 5%. S2. Spraying, impregnating or roller coating the precursor fiber web with an expansion liquid, so that the expansion liquid penetrates between the fibers, and the precursor fiber web expands uniformly in the plane without applying tension that restricts the in-plane expansion of the fiber web. The area of ​​the expanded fiber web is 1.2 to 5 times the area before expansion. S3. The expanded wet fiber web is placed under planar constraint conditions for drying and shaping, so that the fiber web is dried in the expanded planar dimensional state to obtain an ultra-thin uniform nonwoven fabric. The swelling liquid is an alcohol or ketone solvent; the resulting ultrathin uniform nonwoven fabric has a basis weight of 1–5 g / m². 2 .

2. The method for preparing an ultrathin uniform nonwoven fabric according to claim 1, characterized in that, The cellulose fiber is wood pulp fiber or viscose fiber, and the precursor fiber web is made of wood pulp fiber, viscose fiber, or a mixture of wood pulp fiber and viscose fiber.

3. The method for preparing an ultrathin uniform nonwoven fabric according to claim 1, characterized in that, The fineness of the cellulose fibers is no greater than 1.5 dtex; the target basis weight of the resulting ultrathin uniform nonwoven fabric is 3 g / m². 2 The fineness of the cellulose fibers is no greater than 1 dtex; the target basis weight of the resulting ultrathin uniform nonwoven fabric is 1 g / m². 2 In the following cases, the fineness of the cellulose fibers is no greater than 0.5 dtex.

4. The method for preparing an ultrathin uniform nonwoven fabric according to claim 1, characterized in that, When the precursor fiber web is prepared using a wet web-forming process, the wet web-forming process specifically involves: preparing a fiber suspension with a mass concentration of 0.5% to 2.0% from cellulose fibers; subjecting the fiber suspension to pulping, dispersion, and web-forming treatment; and then sequentially performing gravity dehydration, vacuum dehydration, and initial drying to obtain a basis weight of 5 to 20 g / m³. 2 The wet-process precursor fiber web; wherein the fiber suspension has a pH of 4.5 to 7.0, a freeness of 20 to 45°SR, and a vacuum degree of 0.01 to 0.08 MPa for vacuum suction dehydration.

5. The method for preparing an ultrathin uniform nonwoven fabric according to claim 1, characterized in that, When the precursor fiber web is prepared using a dry web-forming process, the dry web-forming process includes opening, carding, and air-forming of the fibers. The airflow velocity during air-forming is 15–30 m / s, resulting in a basis weight of 10–30 g / m. 2 The dry precursor fiber web; when the dry precursor fiber web is reinforced by hydroentangling, the pre-wetting hydroentangling pressure is 1.5-4 MPa, the main hydroentangling pressure is 4-8 MPa, and the hydroentangling speed is 20-60 m / min.

6. The method for preparing an ultrathin uniform nonwoven fabric according to claim 1, characterized in that, The alcohol solvent is one or more of ethanol, methanol, isopropanol, and n-propanol; the ketone solvent is one or more of acetone and butanone; when the swelling liquid is an aqueous ethanol solution, the volume concentration of ethanol is 30% to 100%.

7. The method for preparing an ultrathin uniform nonwoven fabric according to claim 1, characterized in that, The mass ratio of the precursor fiber web to the expansion liquid is 1:(5-20); the immersion time of the expansion liquid in the precursor fiber web is 0.5-10 min, and the immersion temperature is room temperature; after the precursor fiber web is fully immersed in the expansion liquid, its mass per unit area decreases proportionally according to the fiber web area expansion factor.

8. The method for preparing an ultrathin uniform nonwoven fabric according to claim 1, characterized in that, The planar constraint condition is any one of the following: The expanded wet fiber mesh is attached to the surface of a glass plate, stainless steel plate, or flat substrate covered with a release film. The expanded wet fiber web, along with the supporting screen, is transferred to the tensioning frame, and the frame clamps the periphery of the fiber web. When the precursor fiber web is wet, an expansion liquid is applied to the original forming web or conveyor belt to expand the fiber web in situ and then dry it together with the original forming web or conveyor belt.

9. The method for preparing an ultrathin uniform nonwoven fabric according to claim 1, characterized in that, The drying and shaping process adopts room temperature natural drying or hot air drying at 40-80℃; the longitudinal shrinkage rate and transverse shrinkage rate of the dried ultra-thin uniform nonwoven fabric are both no more than 2%, the maximum warping height of a 500mm×500mm sample placed on a plane is no more than 3mm, and there are no holes or pores on the surface of the nonwoven fabric.

10. The ultrathin uniform nonwoven fabric prepared by the method according to any one of claims 1-9, characterized in that, The ultrathin uniform nonwoven fabric comprises an interwoven and dried cellulose fiber network with a basis weight of 1–5 g / m². 2 The weight variation coefficient (CV) is no greater than 5%, the thickness is 0.01–0.2 mm, and the in-plane dimensions remain flat in the dry state.

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