Wet-laid nonwoven fabric separation membrane support having a gradient structure and method for producing the same

Abstract

The application provides a wet non-woven fabric separation membrane support with a gradient structure and a preparation method thereof. The wet non-woven fabric separation membrane support comprises a support layer and a dense smooth layer above the support layer; the dense smooth layer is composed of 50wt%-80wt% superfine polyester fibers and 20wt%-50wt% first adhesive hot melt fibers; the support layer is composed of 50wt%-70wt% polyester fibers and 30wt%-50wt% second adhesive hot melt fibers; the first adhesive hot melt fibers are polyolefin multi-fiber SWP, and the second adhesive hot melt fibers are un-stretched modified low-melting-point PET fibers. The upper and lower layers are both composed of two kinds of fibers, which not only meets the mechanical properties of the separation membrane support, but also has smaller pore size, higher porosity, better tortuosity and higher smoothness; the wet non-woven fabric separation membrane support can significantly reduce pinholes and the phenomenon of print-through, and provide a flat and uniform carrier support for the casting of a casting solution.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of liquid separation and filter material manufacturing, and particularly relates to a wet non-woven fabric separation membrane support body with a gradient structure and a preparation method thereof. BACKGROUND

[0002] Membrane technology as a new separation technology has been widely used in gas separation, material separation and water treatment fields, among which the demand for membrane products is the largest and the application is the most extensive in the field of water treatment. For example, microfiltration membranes and ultrafiltration membranes are applied to water treatment in water purification fields and treatment of industrial wastewater; reverse osmosis membranes are applied to preparation of fresh water from seawater, semiconductor manufacturing water, household water purifier water, medical water and test pure water.

[0003] However, the pure membrane structure is fragile and cannot withstand high operating pressure, so a support body is needed to support the membrane to obtain a composite membrane. Most of the composite membranes sold on the market are non-woven fabrics or woven fabrics as support bodies, and a separation function membrane is formed on the non-woven fabric or woven fabric by flow casting and solidification to realize the integration of the membrane and the support body. For reverse osmosis membranes, a solution of a high molecular polymer is first flow cast on a non-woven fabric or a woven fabric to form a base membrane, and then a separation membrane is formed on the base membrane to realize the integration by this method.

[0004] The structure and performance of the non-woven fabric support body have a significant influence on the structure and performance of the composite membrane. Initially, the support body is mostly made of ordinary low-melting-point fibers as adhesive fibers, and a high-strength wet non-woven support body is obtained by wet laying, hot pressing and adhesive solidification. However, ordinary low-melting-point fibers are prone to softening and deformation under high-temperature filtration conditions, which leads to failure of the support function. In view of the above problems, a homogeneous reinforced wet non-woven support body suitable for high-temperature resistance is developed by using the adhesive and crystallization properties of amorphous polyester fibers (UDPET), but this support body prepared by single-diameter distribution fibers needs to control the pore size of the reverse osmosis membrane non-woven fabric by increasing the basis weight and tightness to avoid the phenomenon of penetration of the casting solution, which to some extent affects the water flux and filtration performance of the membrane and reduces the separation efficiency of the membrane.

[0005] As a non-woven fabric support body for a separation membrane, it needs to have sufficient mechanical strength, moderate thickness and gram weight, small pore size, appropriate porosity and air permeability, smooth and flat surface, no deformation and fuzzing, no penetration and no pinhole defects. There are many single-layer non-woven fabric support bodies in the prior art. Although the single-layer support body is beneficial to reducing the thickness of the support body, in order to meet the mechanical performance requirements, relatively thick fiber raw materials need to be used, which leads to poor surface uniformity and low smoothness of the obtained support body, and the pore size and air permeability are too large, which easily leads to the leakage problem of the casting solution during the doctor blade coating.

[0006] In view of this, it is necessary to design an improved wet nonwoven separation membrane support with a gradient structure and its preparation method to solve the above problems. Summary of the Invention

[0007] The purpose of this invention is to provide a wet-laid nonwoven separation membrane support with a gradient structure and its preparation method. Both the upper and lower layers are made of two types of fibers, which not only meets the mechanical properties of the separation membrane support, but also has the advantages of smaller pore size, higher porosity, lower tortuosity, and higher smoothness. It can significantly reduce pinholes and through-printing, provide a flat and uniform carrier support for the casting solution, and effectively prevent excessive leakage of the casting solution.

[0008] To achieve the above-mentioned objectives, the present invention provides a wet-laid nonwoven separation membrane support with a gradient structure, characterized in that it comprises a support layer and a dense, smooth layer located above the support layer; the dense, smooth layer is composed of 50wt%-80wt% of ultrafine polyester fibers and 20wt%-50wt% of a first adhesive hot-melt fiber; the support layer is composed of 50wt%-70wt% of polyester fibers and 30wt%-50wt% of a second adhesive hot-melt fiber; the first adhesive hot-melt fiber is polyolefin multifiber SWP, and the second adhesive hot-melt fiber is unstretched modified low-melting-point PET fiber.

[0009] As a further improvement of the present invention, the basis weight of the dense smooth layer is 20-40 g / m³. 2 The basis weight of the support layer is 40-60 g / m³. 2 .

[0010] As a further improvement of the present invention, in the dense and smooth layer, the diameter of the ultrafine polyester fiber is 0.1-0.3 dtex, the fiber length is 2-8 mm, and the melting point is 240-260℃; the first adhesive hot melt fiber has a fiber length of 1 mm and a melting point of 150-180℃.

[0011] As a further improvement of the present invention, in the support layer, the diameter of the polyester fiber is 0.3-1 dtex, the fiber length is 2-8 mm, and the melting point is 240-260℃; the diameter of the second adhesive hot melt fiber is 1-2 dtex, the fiber length is 2-8 mm, and the melting point is 160-200℃.

[0012] To achieve the above-mentioned objective, this invention provides a method for preparing the wet-laid nonwoven separation membrane support with a gradient structure as described above, comprising the following steps:

[0013] S1. 50wt%-80wt% of the ultrafine polyester fiber and 20wt%-50wt% of the first adhesive hot melt fiber are sequentially fed into a first pulper containing water, and the total concentration of the ultrafine polyester fiber and the first adhesive hot melt fiber is controlled to be 1wt%-2wt%, and the pulping is carried out for 10-15 minutes; then it is put into the first mixing tank, water and dispersant are added and stirred evenly to prepare a dense and smooth slurry of a preset concentration;

[0014] S2. 50wt%-70wt% of the polyester fiber and 30wt%-50wt% of the second adhesive hot melt fiber are sequentially fed into a second pulper containing water, and the total concentration of the polyester fiber and the second adhesive hot melt fiber is controlled to be 1wt%-2wt%, and the pulping is carried out for 10-15 minutes; then it is put into a second mixing tank, water and dispersant are added and stirred evenly to prepare a support layer slurry of a preset concentration;

[0015] S3. The dense smooth layer slurry from step S1 and the support layer slurry from step S2 are respectively fed into a double-layer inclined wire mesh wet forming machine to prepare a double-layer wet wire mesh.

[0016] S4. The double-layer wet-laid fiber web obtained in step S3 is subjected to pressing and reinforcement, drying in a drying cylinder, and hot rolling and calendering to obtain the wet-laid nonwoven separation membrane support with a gradient structure.

[0017] As a further improvement of the present invention, the basis weight of the wet nonwoven separation membrane support with a gradient structure is 60-100 g / m³. 2 .

[0018] As a further improvement of the present invention, the preset concentration of the dense smooth layer slurry is 1wt%-2wt%; the preset concentration of the support layer slurry is 1wt%-2wt%.

[0019] As a further improvement of the present invention, the weight of the dispersant accounts for 1%-3% of the total weight of the polyester fiber and the adhesive hot melt fiber; the dispersant is prepared by compounding PEO type polyoxyethylene ether and PAM type polyacrylamide at a mass ratio of 7-9:1-3, and has a molecular weight of 3.7-4.3 million.

[0020] As a further improvement of the present invention, in step S4, the pressing pressure for the pressing reinforcement is 2 bar-5 bar; the drying temperature for the drying cylinder is 100-130°C; the hot rolling and calendering is completed by a hot rolling and calendering device, which consists of a first rolling device and a second rolling device arranged in series. The first rolling device is a hot rolling device composed of two metal heating rollers; the second rolling device is a hot rolling device composed of a metal heating roller group and a composite material roller.

[0021] As a further improvement of the present invention, the calendering temperature of the first calendering equipment is 160-220℃ and the roll pressure is 200N / cm-1000N / cm; the calendering temperature of the second calendering equipment is 160-220℃ and the roll pressure is 200N / cm-1000N / cm.

[0022] The beneficial effects of this invention are:

[0023] (1) The method for preparing a wet-laid nonwoven separation membrane support with a gradient structure provided by the present invention involves a dense smooth layer and a support layer composed of two types of fibers. By controlling the diameter of the fibers in different layers and using a specific dispersant, the two types of fibers in different layers are first dispersed more uniformly in the slurry, and the fibers intertwine with each other to form a dense smooth layer and a support layer with a special structure and pore size gradient. Then, the dense smooth layer slurry and the support layer slurry are prepared into a double-layer wet-laid fiber web. Finally, the hot rolling and calendering process is controlled to make the interior of the dense smooth layer, the interior of the support layer, and the contact points of the two layers intertwine, resulting in a wet-laid nonwoven separation membrane support with a special structure and gradient structure. This production method is simple, highly controllable, and can meet the needs of industrial production, thus having high practical application value.

[0024] The dense and smooth layer constructed with ultrafine fibers not only satisfies the mechanical properties of the separation membrane support, but also has the advantages of smaller pore size, higher porosity, lower tortuosity, and higher smoothness. It can significantly reduce pinholes and through-printing, provide a flat and uniform carrier support for the casting solution, and effectively prevent excessive leakage of the casting solution.

[0025] (2) The method for preparing a wet-laid nonwoven membrane support with a gradient structure provided by this invention achieves effective entanglement of the membrane support material in both the thickness and width directions, solving the problems of easy edge curling, poor morphological stability, and easy delamination of existing wet-laid materials, and greatly improving the physical and mechanical properties and structural stability of the reverse osmosis membrane support. While ensuring the good physical and mechanical properties and structural stability of the membrane support material, the smoothness and flatness of the dense and smooth layer surface structure are achieved through wet web forming, shaping, pressing and drying, and high-temperature hot rolling and calendering.

[0026] (3) The unique double-layer structure of the wet nonwoven separation membrane support with gradient structure provided by the present invention can macroscopically control the dual diffusion rate of solvent and non-solvent in the phase transformation preparation process of separation membrane, and balance the membrane formation. The lower PET support structure is relatively loose and porous, which reduces the obstruction for non-solvent to pass through the support. At the same time, the fine and porous structure of the upper ultrafine island fiber sheet can prevent a large amount of leakage of casting liquid, which has a positive impact on the water flux and retention rate of the separation membrane. Attached Figure Description

[0027] Figure 1This is a schematic diagram of the wet nonwoven separation membrane support with a gradient structure according to the present invention.

[0028] Figure 2 This is a process flow diagram of the wet nonwoven separation membrane support with a gradient structure according to the present invention. Detailed Implementation

[0029] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0030] It should also be noted that, in order to avoid obscuring the present invention with unnecessary details, only the structures and / or processing steps closely related to the present invention are shown in the accompanying drawings, while other details that are not closely related to the present invention are omitted.

[0031] Additionally, it should be noted that the terms “comprising,” “including,” or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such process, method, article, or apparatus.

[0032] Please see Figure 1 and Figure 2 As shown, the present invention provides a wet-laid nonwoven separation membrane support with a gradient structure, comprising a support layer and a dense, smooth layer located above the support layer. The dense, smooth layer is composed of 50wt%-80wt% of ultrafine polyester fibers and 20wt%-50wt% of a first adhesive hot-melt fiber; the support layer is composed of 50wt%-70wt% of polyester fibers and 30wt%-50wt% of a second adhesive hot-melt fiber; the first adhesive hot-melt fiber is a polyolefin multifiber (SWP), and the second adhesive hot-melt fiber is unstretched modified low-melting-point PET fiber or low-melting-point bicomponent polyester fiber.

[0033] Polyolefin multifiber SWP is a multifiber-like chemical fiber with a multibranched structure, produced by spray molding of polyolefins. It features a branched structure with branched portions having a diameter of 2 μm, resulting in a significantly larger specific surface area compared to typical resin fibers. After hydrophilic treatment, SWP exhibits excellent dispersibility in water and can be uniformly mixed with various synthetic fiber slurries. It also boasts high standard freeness and good water filtration during the molding process. The branched structure of SWP allows for more uniform mixing and interweaving with ultrafine polyester fibers, forming unique pore and interwoven structures to achieve a dense, smooth layer with adjustable pore size.

[0034] Specifically, the basis weight of the dense, smooth layer is 20-40 g / m³. 2 In the dense and smooth layer, the diameter of the ultrafine polyester fiber is 0.1-0.3 dtex, the fiber length is 2-8 mm, and the melting point is 240-260℃; the length of the first adhesive hot melt fiber is 0.5-2 mm, and the melting point is 150-180℃.

[0035] The basis weight of the support layer is 40-60 g / m² 2 In the support layer, the polyester fiber has a diameter of 0.3-1 dtex, a fiber length of 2-8 mm, and a melting point of 240-260℃; the second adhesive hot melt fiber has a diameter of 1-2 dtex, a fiber length of 2-8 mm, and a melting point of 160-200℃.

[0036] The present invention also provides a method for preparing the above-mentioned wet nonwoven separation membrane support with a gradient structure, comprising the following steps:

[0037] S1. Preparation of a dense, smooth slurry layer:

[0038] 50wt%-80wt% of ultrafine polyester fiber and 20wt%-50wt% of first adhesive hot melt fiber are sequentially fed into a first pulper containing water, and the total concentration of ultrafine polyester fiber and first adhesive hot melt fiber is controlled at 1wt%-2wt%, and the pulping is carried out for 10-15 minutes; then it is put into the first mixing tank, water and dispersant are added and stirred evenly to prepare a dense and smooth slurry with a concentration of 1wt%-2wt%.

[0039] The weight of the dispersant accounts for 1%-3% of the total weight of the ultrafine polyester fiber and the first adhesive hot melt fiber; the dispersant is a compound of PEO type polyoxyethylene ether and PAM type polyacrylamide in a mass ratio of 7-9:1-3, with a molecular weight of 3.7-4.3 million.

[0040] S2. Preparation of the support layer slurry:

[0041] 50wt%-70wt% of polyester fiber and 30wt%-50wt% of second adhesive hot melt fiber are sequentially fed into a second pulper that already contains water, and the total concentration of polyester fiber and second adhesive hot melt fiber is controlled at 1wt%-2wt%, and the pulping is carried out for 10-15 minutes; then it is put into a second mixing tank, water and dispersant are added and stirred evenly to prepare a support layer slurry with a concentration of 1wt%-2wt%.

[0042] The weight of the dispersant accounts for 1%-3% of the total weight of the super ester fiber and the second adhesive hot melt fiber; the dispersant is a compound of PEO type polyoxyethylene ether and PAM type polyacrylamide in a mass ratio of 7-9:1-3, with a molecular weight of 3.7-4.3 million.

[0043] S3. Preparation of a double-layer wet-laid fiber web:

[0044] The dense and smooth layer slurry from step S1 and the support layer slurry from step S2 are respectively fed into a double-layer inclined wire mesh wet forming machine to prepare a double-layer wet wire mesh.

[0045] S4. Preparation of wet-process nonwoven fabric separation membrane support:

[0046] The double-layer wet-laid fiber web from step S3 is reinforced by pressing, dried in a drying cylinder, and hot-rolled and calendered to obtain a wet-laid nonwoven separation membrane support with a gradient structure. The resulting wet-laid nonwoven separation membrane support with a gradient structure has a basis weight of 60-100 g / m³. 2 .

[0047] The pressing pressure for the pressing and strengthening process is 2-5 bar; the drying temperature for the drying cylinder is 100-130℃; the hot rolling and calendering process is completed by a hot rolling and calendering equipment, which consists of a first calendering equipment and a second calendering equipment arranged in series. The first calendering equipment is a hot rolling equipment composed of two metal heating rollers arranged vertically; the second calendering equipment is a hot rolling equipment composed of a metal heating roller group and a composite material roller (i.e., an unheated elastic roller).

[0048] The calendering temperature of the first calendering equipment is 160-220℃, and the roll pressure is 200N / cm-1000N / cm; the calendering temperature of the second calendering equipment is 160-220℃, and the roll pressure is 200N / cm-1000N / cm.

[0049] In the specific preparation process, when the double-layer wet-laid fiber web passes through the first calendering equipment, the side in contact with the upper heating roller is a dense and smooth layer, and the side in contact with the lower heating roller is a support layer; when passing through the second calendering equipment, the side in contact with the heating roller is a dense and smooth layer, and the side in contact with the unheated elastic roller is a support layer.

[0050] The present invention will be described below through specific embodiments:

[0051] Example 1

[0052] A method for preparing a wet-laid nonwoven separation membrane support with a gradient structure includes the following steps:

[0053] S1. Preparation of a dense, smooth slurry layer:

[0054] 70 wt% of ultrafine PET fiber and 30 wt% of polyolefin multifiber SWP were sequentially added to the first pulper containing water, and the total concentration of ultrafine PET fiber and polyolefin multifiber SWP was controlled at 1.5 wt%. The pulping was carried out for 12 minutes. Then, the pulp was put into the first mixing tank, water and dispersant were added and stirred evenly to prepare a dense and smooth slurry with a concentration of 1.5 wt%.

[0055] The diameter of the ultrafine PET fiber is 0.2 dtex and the fiber length is 3 mm; the length of the polyolefin multifiber SWP is 1 mm.

[0056] The weight of the dispersant accounts for 2% of the total weight of the ultrafine PET fiber and polyolefin multifiber SWP; the dispersant is a compound of PEO type polyoxyethylene ether and PAM type polyacrylamide at a mass ratio of 8:2, with a molecular weight of 4 million.

[0057] S2. Preparation of the support layer slurry:

[0058] 60 wt% of PET fiber and 40 wt% of unstretched modified low-melting-point PET fiber were sequentially fed into a second pulper containing water, and the total concentration of PET fiber and unstretched modified low-melting-point PET fiber was controlled to be 1.5 wt%. The pulping was carried out for 10 minutes. Then, the pulp was placed into a second mixing tank, and water and dispersant were added and stirred evenly to prepare a support layer slurry with a concentration of 1.0 wt%.

[0059] The diameter of the PET fiber is 0.6 dtex and the fiber length is 5 mm; the diameter of the unstretched modified low melting point PET fiber is 1.2 dtex and the fiber length is 5 mm.

[0060] The weight of the dispersant accounts for 2% of the total weight of PET fibers and unstretched modified low-melting-point PET fibers; the dispersant is a compound of PEO type polyoxyethylene ether and PAM type polyacrylamide at a mass ratio of 8:2, with a molecular weight of 4 million.

[0061] S3. Preparation of a double-layer wet-laid fiber web:

[0062] The dense and smooth layer slurry from step S1 and the support layer slurry from step S2 are respectively fed to a double-layer inclined wire mesh wet forming machine by different slurry supply systems to prepare a double-layer wet wire mesh.

[0063] S4. Preparation of wet-process nonwoven fabric separation membrane support:

[0064] The double-layer wet-laid fiber web from step S3 is subjected to pressing reinforcement, drying in a drying cylinder, and hot rolling calendering to obtain a wet-laid nonwoven separation membrane support with a gradient structure. The resulting wet-laid nonwoven separation membrane support with a gradient structure has a basis weight of 75 g / m³. 2 The basis weight of the dense, smooth layer is 20 g / m³. 2 The basis weight of the support layer is 55 g / m². 2 .

[0065] The pressing pressure for the pressing reinforcement is 3 bar; the drying temperature for the drying cylinder is 120℃; the hot rolling and calendering is completed by a hot rolling and calendering equipment, which consists of a first rolling equipment and a second rolling equipment configured in series. The first rolling equipment is a hot rolling equipment composed of two metal heating rollers; the second rolling equipment is a hot rolling equipment composed of a metal heating roller group and a composite material roller.

[0066] The first calendering equipment has a calendering temperature of 180℃ and a roll pressure of 400N / cm; the second calendering equipment has a calendering temperature of 170℃ and a roll pressure of 1000N / cm.

[0067] In the specific preparation process, when the double-layer wet-laid fiber web passes through the first calendering equipment, the side in contact with the upper heating roller is a dense and smooth layer, and the side in contact with the lower heating roller is a support layer; when passing through the second calendering equipment, the side in contact with the heating roller is a dense and smooth layer, and the side in contact with the unheated elastic roller is a support layer.

[0068] Comparative Example 1

[0069] A method for preparing a single-layer wet-laid nonwoven fabric separation membrane support differs from Example 1 in that a dense, smooth layer slurry is not prepared. In step S3, the support layer slurry is fed to a single-layer wet-laid molding machine to form a single-layer nonwoven fabric support. After pressing, drying, and hot-pressing for reinforcement, the resulting single-layer nonwoven fabric support has a basis weight of 76 g / m³. 2 .

[0070] Table 1. Performance of the nonwoven fabric supports obtained in Example 1 and Comparative Example 1

[0071]

[0072] As shown in Table 1, the wet nonwoven separation membrane support with a double-layer gradient structure has higher tensile strength, smaller pore size, higher porosity, better tortuosity, and significantly improved smoothness, making it better suited to the requirements of reverse osmosis membrane coating.

[0073] Examples 2-3 and Comparative Examples 2-3

[0074] A method for preparing a wet nonwoven separation membrane support with a gradient structure differs from Example 1 in that the ratio of ultrafine PET fibers and polyolefin multifibers (SWP) in step S1 is different. The rest is roughly the same as in Example 1 and will not be repeated here.

[0075] Table 2. Ratio of microfiber PET and polyolefin multifiber SWP

[0076] Example Superfine PET fiber (%) Polyolefin multi-fiber SWP (%) Example 1 70 30 Example 2 50 50 Example 3 80 20 Comparative Example 2 90 10 Comparative Example 3 100 0

[0077] Table 3. Performance of the nonwoven fabric supports obtained in Examples 1-3 and Comparative Examples 2-3

[0078]

[0079] As shown in Table 3, by changing the ratio of ultrafine PET fibers and polyolefin multifiber SWP in the dense smooth layer, the smoothness and pore size of the obtained nonwoven fabric support changed. This is mainly because as the ratio of ultrafine PET fibers and polyolefin multifiber SWP changes, the intertwining and bonding of the two fibers are different, resulting in different structures and properties after bonding, but the overall performance is better.

[0080] As the content of polyolefin multifiber SWP decreases, the performance of the resulting nonwoven support is significantly reduced.

[0081] Examples 4-5 and Comparative Example 4

[0082] A method for preparing a wet nonwoven separation membrane support with a gradient structure differs from Example 1 in that the ratio of PET fiber to unstretched modified low-melting-point PET fiber in step S2 is different. The rest is roughly the same as in Example 1 and will not be repeated here.

[0083] Table 4. Ratio of PET fiber to unstretched modified low-melting-point PET fiber

[0084] Example PET fiber (%) Unstretched modified low-melting PET fiber (%) Example 1 60 40 Example 4 70 30 Example 5 50 50 Comparative Example 4 80 20

[0085] Table 5. Performance of the nonwoven fabric supports obtained in Examples 4-5 and Comparative Example 4.

[0086]

[0087] Table 5 shows that adjusting the ratio of PET fibers to unstretched modified low-melting-point PET fibers affects the basis weight, thickness, and other properties of the wet-laid nonwoven membrane support. In actual production, controlling the fiber ratio can better meet the coating quality requirements of the reverse osmosis membrane.

[0088] Example 6 and Comparative Example 4

[0089] A method for preparing a wet nonwoven separation membrane support with a gradient structure differs from Example 1 in that the slurry concentration and the amount of dispersant are different in steps S1 and S2. The other steps are roughly the same as in Example 1 and will not be repeated here.

[0090] Table 6 Slurry Concentration and Dispersant Dosage

[0091] Example Slurry concentration (wt%) Amount of dispersant (%) Example 1 1.5 2 Example 6 0.8 1 Example 7 2 3 Comparative Example 5 3 4 Comparative Example 6 1.5 0

[0092] Table 7 shows the performance of the nonwoven fabric supports obtained in Examples 6-7 and Comparative Example 5.

[0093]

[0094]

[0095] As shown in Table 7, the performance of the obtained nonwoven fabric support fluctuates within a certain range as the slurry concentration and the energy used for dispersant change, but the overall performance is better.

[0096] When the slurry concentration is too high, the fibers in the slurry are not evenly dispersed, and the fibers are prone to interweaving and flocculation, resulting in poor uniformity of the fiber web formed during papermaking, which in turn directly affects the strength and surface properties of the final nonwoven fabric product.

[0097] When no dispersant is added, because the fiber density is slightly greater than the water density, it is easy to combine with small air bubbles in the water during dispersion and flocculate together, causing small clumps or fiber entanglement problems during papermaking, resulting in a decrease in the quality of the final nonwoven fabric and poor surface smoothness.

[0098] Examples 8-9 and Comparative Examples 7-9

[0099] A method for preparing a wet nonwoven separation membrane support with a gradient structure differs from Example 1 in that the temperature of the calendering equipment and the pressure of the pressure roller are different in step S4. The rest is roughly the same as in Example 1 and will not be described again here.

[0100] Table 8 Temperature and pressure of the first and second calendering units

[0101]

[0102] Table 9. Performance of the nonwoven fabric supports obtained in Examples 8-9 and Comparative Examples 7-9

[0103]

[0104]

[0105] As shown in Table 9, the hot pressing process in step S4 has a significant impact on the performance of the obtained nonwoven separation membrane support. This is mainly because the bonding and entanglement between fibers in each layer and between different fiber layers change with the temperature and pressure of the first and second calendering equipment during the hot pressing process. When the temperature and pressure are low, good bonding and entanglement cannot be achieved, resulting in reduced tensile strength, larger pore size, and poorer smoothness, which will affect the coating quality of the reverse osmosis membrane. When the temperature and pressure are too high, the fiber structure will be damaged.

[0106] Comparative Example 10

[0107] A method for preparing a wet nonwoven separation membrane support with a gradient structure differs from Example 1 in that ordinary adhesive fibers are used instead of polyolefin multifiber SWP in the dense smooth layer. Otherwise, it is largely the same as Example 1 and will not be described in detail here.

[0108] Table 10 shows the performance of the nonwoven fabric support obtained in Comparative Example 10.

[0109]

[0110]

[0111] As shown in Table 10, the use of polyolefin multifiber SWP can reduce and control the pore size of wet nonwoven fabric, while improving surface smoothness and enhancing the performance of the support.

[0112] In summary, the gradient-structured wet-laid nonwoven membrane support and its preparation method provided by this invention, with a dense and smooth layer constructed from ultrafine fibers, not only satisfies the mechanical properties of the membrane support but also offers advantages such as smaller pore size, higher porosity, lower tortuosity, and higher smoothness. It significantly reduces pinholes and through-printing, providing a smooth and uniform carrier support for the casting solution and effectively preventing excessive leakage of the casting solution. This production method is simple, highly controllable, and meets the needs of industrial production, possessing high practical application value.

[0113] The above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention.

Claims

1. A wet-laid nonwoven fabric separation membrane support with a gradient structure, characterized in that, It includes a support layer and a dense, smooth layer located above the support layer; the dense, smooth layer is composed of 50wt%-80wt% of ultrafine polyester fibers and 20wt%-50wt% of a first adhesive hot-melt fiber; the support layer is composed of 50wt%-70wt% of polyester fibers and 30wt%-50wt% of a second adhesive hot-melt fiber; the first adhesive hot-melt fiber is polyolefin multifiber SWP, and the second adhesive hot-melt fiber is unstretched modified low-melting-point PET fiber.

2. The wet-laid nonwoven separation membrane support with a gradient structure according to claim 1, characterized in that, The basis weight of the dense, smooth layer is 20-40 g / m³. 2 The basis weight of the support layer is 40-60 g / m³. 2 .

3. The wet-laid nonwoven separation membrane support with a gradient structure according to claim 1, characterized in that, In the dense and smooth layer, the ultrafine polyester fiber has a diameter of 0.1-0.3 dtex, a fiber length of 2-8 mm, and a melting point of 240-260°C; the first adhesive hot melt fiber has a length of 0.5-2 mm and a melting point of 150-180°C.

4. The wet-laid nonwoven separation membrane support with a gradient structure according to claim 1, characterized in that, In the support layer, the polyester fiber has a diameter of 0.3-1 dtex, a fiber length of 2-8 mm, and a melting point of 240-260°C; the second adhesive hot melt fiber has a diameter of 1-2 dtex, a fiber length of 2-8 mm, and a melting point of 160-200°C.

5. A method for preparing a wet-laid nonwoven separation membrane support with a gradient structure as described in any one of claims 1 to 4, characterized in that, Includes the following steps: S1. 50wt%-80wt% of the ultrafine polyester fiber and 20wt%-50wt% of the first adhesive hot melt fiber are sequentially fed into a first pulper containing water, and the total concentration of the ultrafine polyester fiber and the first adhesive hot melt fiber is controlled to be 1wt%-2wt%, and the pulping is carried out for 10-15 minutes; then it is put into a first mixing tank, water and dispersant are added and stirred evenly to prepare a dense and smooth slurry of a preset concentration; S2. 50wt%-70wt% of the polyester fiber and 30wt%-50wt% of the second adhesive hot melt fiber are sequentially fed into a second pulper containing water, and the total concentration of the polyester fiber and the second adhesive hot melt fiber is controlled to be 1wt%-2wt%, and the pulping is carried out for 10-15 minutes; then it is put into a second mixing tank, water and dispersant are added and stirred evenly to prepare a support layer slurry of a preset concentration; S3. The dense smooth layer slurry from step S1 and the support layer slurry from step S2 are respectively fed into a double-layer inclined wire mesh wet forming machine to prepare a double-layer wet wire mesh. S4. The double-layer wet-laid fiber web obtained in step S3 is subjected to pressing and reinforcement, drying in a drying cylinder, and hot rolling and calendering to obtain the wet-laid nonwoven separation membrane support with a gradient structure.

6. The method for preparing a wet-laid nonwoven separation membrane support with a gradient structure according to claim 5, characterized in that, The basis weight of the wet-laid nonwoven separation membrane support with a gradient structure is 60-100 g / m³. 2 .

7. The method for preparing a wet-laid nonwoven separation membrane support with a gradient structure according to claim 5, characterized in that, The preset concentration of the dense and smooth layer slurry is 1wt%-2wt%; the preset concentration of the support layer slurry is 1wt%-2wt%.

8. The method for preparing a wet-laid nonwoven separation membrane support with a gradient structure according to claim 5, characterized in that, In step S1, the weight of the dispersant accounts for 1%-3% of the total weight of the ultrafine polyester fiber and the first adhesive hot melt fiber. The dispersant is a compound of PEO type polyoxyethylene ether and PAM type polyacrylamide in a mass ratio of 7-9:1-3, with a molecular weight of 3.7-4.3 million. In step S2, the weight of the dispersant accounts for 1%-3% of the total weight of the polyester fiber and the second adhesive hot melt fiber. The dispersant is a compound of PEO type polyoxyethylene ether and PAM type polyacrylamide in a mass ratio of 7-9:1-3, with a molecular weight of 3.7-4.3 million.

9. The method for preparing a wet-laid nonwoven separation membrane support with a gradient structure according to claim 5, characterized in that, In step S4, the pressing pressure for the pressing reinforcement is 2-5 bar; the drying temperature for the drying cylinder is 100-130°C; the hot rolling and calendering is completed by a hot rolling and calendering equipment, which consists of a first calendering equipment and a second calendering equipment arranged in series. The first calendering equipment is a hot rolling equipment composed of two metal heating rollers; the second calendering equipment is a hot rolling equipment composed of a metal heating roller group and a composite material roller.

10. The method for preparing a wet-laid nonwoven separation membrane support with a gradient structure according to claim 9, characterized in that, The first calendering equipment has a calendering temperature of 160-220℃ and a roll pressure of 200N / cm-1000N / cm; the second calendering equipment has a calendering temperature of 160-220℃ and a roll pressure of 200N / cm-1000N / cm.

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