Patent Application
20230321611
The present invention relates to a nonwoven fabric for a separation membrane and a method of producing the same. A nonwoven fabric for a separation membrane of the present invention is used as a support (It is also referred to as a substrate.) for coating of a resin solution for producing a separation membrane such as a microfiltration membrane (MF membrane), an ultrafiltration membrane (UF membrane), a nanofiltration membrane (NF membrane), or a reverse osmosis membrane (RO membrane).
Separation membranes such as MF membranes, UF membranes, NF membranes, and RO membranes are used in various applications such as food, medical treatment, and wastewater treatment, and in particular, in recent years, separation membranes have come to be often used in the field of drinking water production, for example, in the field of clean water treatment, clean water production by seawater desalination, and lower waste water treatment such as activated sludge process. In the production of these separation membranes, a process is used in which a resin solution is applied to a nonwoven fabric and solidified. At that time, the nonwoven fabric used as a support affects the performance of phase separation of a resin layer, and as a result, has a large influence on a blocking rate of the resulting separation membrane and liquid permeation performance. Furthermore, a step of applying the resin solution also affects the cost and treatment performance of the separation membrane as a product. For example, a nonwoven fabric that can obtain sufficient tensile strength even if it is thin makes it possible to reduce the weight of the separation membrane module and increase the membrane area per unit weight. Furthermore, when the separation membrane is produced by applying a small amount of resin, the raw material cost can be reduced. For this reason, research on performance enhancement of nonwoven fabrics has been conducted in various fields.
Some of the conventional nonwoven fabrics for a separation membrane are designed based on a concept that a two-layer or multilayer structure is used, and a resin solution is applied to a surface having a low density to allow a coating resin to permeate therethrough and prevent a bleed-through of the resin solution on the surface having a high density. In this case, a penetration speed of the resin solution having high viscosity is reduced to prevent the bleed-through. Although the density is not necessarily described in Patent Literature 1, it is proposed to prevent the bleed-through of the resin solution (cast liquid) by mixing thick fibers on a surface layer (coating surface of the resin solution) side, forming a two-layer structure using thin fibers on a back side, and densifying the back side.
In the production of an asymmetric membrane, a porous membrane is produced by applying a resin solution and then immersing the resin solution in a coagulation bath, but when a rate of penetration of the resin solution is high, it is necessary to increase a conveyance speed of the nonwoven fabric and immerse the nonwoven fabric in the coagulation bath before the applied resin reaches the back surface. Furthermore, a low-viscosity resin solution having a high penetration rate cannot be used, that is, there is a limit to the coating of a low-molecular-weight resin or a low-concentration resin solution. If most of the resin solution penetrates into the nonwoven fabric, it becomes difficult to control the pore size in the non-solvent-induced phase separation. Moreover, the resin solution that has reached the back surface tends to form a dense layer, which causes a decrease in the permeation flux of the solution as a separation membrane.
On the other hand, when the nonwoven fabric was densified by compression, the resin solution that penetrated into the nonwoven fabric did not cause ideal phase separation, and the pore size tended to decrease. That is, it inhibited an increase in the pore diameter in the phase separation of the resin solution that penetrated the surface having a high density, resulting in a decrease in the permeation flux of the separation membrane.
In the support of Patent Literature 1, since the permeability of the coating resin in the surface layer mixed with thick fibers is high, it is necessary to increase the coating thickness of the resin, and this may also cause a decrease in the permeation flux of the separation membrane.
Nonwoven fabrics have already been used as substrates for various separation membranes, and nonwoven fabrics and nonwoven fabrics having many pinholes in which penetration of a resin solution is significantly poor have not been put into practical use. The nonwoven fabric obtained by the conventional dry web has many pinholes, and there are cases where the resin solution bleeds through and poor defoaming, which limits the production conditions of the separation membrane. Furthermore, a substrate obtained by subjecting the surface of a nonwoven fabric to fluorine processing for the purpose of suppressing bleed-through is also used, but bleed-through is not completely prevented, and uneven application of a resin solution is likely to occur. These problems still limit the conditions of use of the resin solution.
In order to solve the problem of pinholes in nonwoven fabrics obtained in dry webs, a method of combining wet webs has been devised. For example, Patent Literature 2 proposes to produce a two-layer nonwoven fabric by producing a high-density nonwoven fabric by a wet web using relatively thin fibers, winding the nonwoven fabric into a roll shape, forming a low-density nonwoven fabric on the nonwoven fabric by a dry web using relatively thick fibers, and finally subjecting the nonwoven fabric to a thermal calendering treatment. Although the number of pinholes is reduced by combining the wet webs, the weight per unit area is increased by superposition. Furthermore, since nonwoven fabrics having different strengths form layers, a tensile strength in the lamination direction is weak, and thinning becomes difficult. Moreover, in a case where the integration of the upper and lower layers is not sufficient, delamination occurs depending on conditions. Note that, in Patent Literature 2, the distinction between the back surface and the surface is not described.
On the other hand, Patent Literature 3 relates to a separation membrane having a separation-functional membrane, and it is described that a nonwoven fabric having a surface layer having a high density and a back layer having a low density is suitable as a support thereof. This is an idea that when liquid flows from a surface layer having a high density to a back layer having a low density, the speed is rapidly reduced, so that the resin solution does not reach the back surface, and it is important to form recesses on the back surface by embossing or the like. In some cases, a method of reducing thermal compression of the back surface by controlling the upper and lower roll temperatures in calendering is adopted, but information on the internal structure of the nonwoven fabric is not shown other than density. That is, Patent Literature 3 has no information on the size and pore diameter of the fibers inside the nonwoven fabric, and does not disclose a technique for controlling pores.
Although it has been described that there are problems of tensile strength and delamination in the two-layer structure as in Patent Literature 2, Patent Literature 4 proposes to improve an adhesive force between the nonwoven fabric and the applied resin by using thicker fibers than before. Patent Literature 4 is intended to solve the problem of the separation membrane support described in Patent Literature 1, and for the purpose of preventing peeling of the support (nonwoven fabric) and the separation functional layer (resin layer) in long-term use, heteromorphic cross-sectional fibers having a non-circular cross section are mixed into the surface layer. As a result, although the improvement in adhesive strength can be achieved, a problem that pinholes of the coating membrane (separation membrane) are generated by raising of fibers having a heteromorphic cross section, and defects are generated on the surface of the separation membrane may also occur. That is, it is still unsatisfactory from the viewpoint of stable production of the separation membrane.
In Patent Literature 5, a multilayer structure is formed by using long fibers, and studies for achieving both a bleed-through prevention property and water permeability are conducted. A nonwoven fabric obtained by papermaking using long fibers is excellent in strength, but has a poor formation (fiber density constituting the nonwoven fabric is likely to be unevenly distributed), and is likely to have variations in the bleed-through preventing property and water permeability, and there are many problems from the viewpoint of production stability. Furthermore, when long fibers are used to prevent strike-through, it is necessary to use a large amount of fibers, which is also disadvantageous in terms of cost.
The present invention is intended to solve the above-mentioned problems in the prior art, and it is an object of the present invention to provide a nonwoven fabric for a separation membrane which is compatible with both of the bleed-through prevention property of a coating resin (resin solution for forming a separation membrane applied to a nonwoven fabric) and the high water permeability of a separation membrane to be produced, which have been in a trade-off relationship so far. Furthermore, another object of the present invention is to provide a method of producing such a nonwoven fabric for a separation membrane. Although there has been a concept of changing a structure of the nonwoven fabric on the coating surface and the non-coating surface of the resin solution, there have been problems such as densification of the interface due to bonding processing of a plurality of layers having different structures, deterioration of the water permeability of the separation membrane due to thermal compression in the process of producing the nonwoven fabric, and inhibition of phase separation of the applied resin due to densification of the nonwoven fabric. Furthermore, a method of using thick fibers for improving the adhesive strength between the coating resin and the nonwoven fabric has also been adopted, but the homogeneity of the surface of the nonwoven fabric (coating surface of the resin solution) is lowered, and the coatability of the coating resin by raising of thick resin is adversely affected, which causes formation of pinholes in the separation membrane.
In order to solve such problems, the present invention has the following configuration.
One aspect of the nonwoven fabric substrate for a separation membrane of the present invention is a nonwoven fabric substrate for a separation membrane including a nonwoven fabric for a separation membrane composed of two or more layers, the nonwoven fabric substrate for a separation membrane including: a surface layer; a back surface layer; and an optional intermediate layer, wherein a coating surface of a coating solution during membrane formation is a surface of the surface layer, and, when the nonwoven fabric substrate is impregnated with the coating solution for membrane formation, the surface layer has a large Laplace force and the back surface layer and the optional intermediate layer have a small Laplace force.
Another aspect of the nonwoven fabric substrate for a separation membrane of the present invention is a nonwoven fabric substrate for a separation membrane including a nonwoven fabric for a separation membrane composed of two or more layers, the nonwoven fabric substrate for a separation membrane including: a surface layer having a coating surface of a coating solution for membrane formation; a back surface layer; and an optional intermediate layer, wherein the surface layer has an average pore diameter smaller than that of the back surface layer or the optional intermediate layer located under the surface layer, and a difference between the average pore diameter of the surface layer and the average pore diameter of the back surface layer or the optional intermediate layer is 0.5 µm or more.
In the nonwoven fabric substrate for a separation membrane of the present invention, the surface layer may be composed of one or more kinds of fine fibers having a small fiber diameter and one or more kinds of thick fibers having a larger fiber diameter than the fine fibers, and the back surface layer and the optional intermediate layer may be configured to include a portion consisting substantially of the thick fibers.
In the nonwoven fabric substrate for a separation membrane of the present invention, a fiber diameter of the fine fibers is in a range of 0.01 dtex or more and 0.5 dtex or less, and a fiber diameter of the thick fibers is in a range of more than 0.5 dtex and 10 dtex or less.
In the nonwoven fabric substrate for a separation membrane of the present invention, a fiber diameter of the fine fibers is in a range of 0.05 dtex or more and 0.5 dtex or less, and a fiber diameter of the thick fibers is in a range of more than 0.5 dtex and 3.5 dtex or less.
In the nonwoven fabric substrate for a separation membrane of the present invention, a thickness of the nonwoven fabric may be in a range of 30 to 300 µm, and a composition ratio in a thickness direction of the surface layer, the back surface layer, and the optional intermediate layer (a ratio of a thickness of the surface layer to a thickness of the back surface layer and the optional intermediate layer) may be 1:9 to 9:1.
The nonwoven fabric substrate for a separation membrane of the present invention may include further comprising a portion in which fibers constituting the surface layer, the back surface layer, and the optional intermediate layer are continuously entangled between the layers.
In the nonwoven fabric substrate for a separation membrane of the present invention, a material of the nonwoven fabric may be one or more materials selected from the group consisting of polyethylene terephthalate (PET), polyethylene (PE), polypropylene (PP), a composite material of polypropylene and polyethylene (PP/PE), polyphenylene sulfide (PPS), and mixtures thereof. Here, the materials of the surface layer, the back surface layer, and the optional intermediate layer may be the same or different from each other.
The nonwoven fabric substrate for a separation membrane of the present invention may be subjected to a surface treatment for controlling the wettability of the nonwoven fabric.
A method of producing a nonwoven fabric substrate for a separation membrane of the present invention includes sequentially papermaking a fiber dispersion liquid for a surface layer composed of one or more kinds of fine fibers having a small fiber diameter and one or more kinds of thick fibers having a larger fiber diameter than the fine fibers, a fiber dispersion liquid for an optional intermediate layer consisting of the thick fibers, and a fiber dispersion liquid for a back surface layer consisting of the thick fibers using a wet papermaking method.
In the method of producing a nonwoven fabric substrate for a separation membrane of the present invention, the fiber dispersion liquid for the surface layer may be obtained by dispersing 1 to 50 wt% of fine fibers having a fiber diameter in a range of 0.01 dtex or more and 0.5 dtex or less and 50 to 99 wt% of thick fibers having a fiber diameter in a range of more than 0.5 dtex and 10 dtex or less in water, the fiber dispersion liquid for the back surface layer and the fiber dispersion liquid for the optional intermediate layer may be obtained by dispersing 100 wt% of thick fibers having a fiber diameter in a range of more than 0.5 dtex and 10 dtex or less in water, and the fine fibers and the thick fibers may have a fiber length in a range of 1 to 10 mm.
In the method of producing a nonwoven fabric substrate for a separation membrane of the present invention, the fiber dispersion liquid for the surface layer may be obtained by dispersing 5 to 50 wt% of fine fibers having a fiber diameter in a range of 0.05 dtex or more and 0.5 dtex or less and 50 to 95 wt% of thick fibers having a fiber diameter in a range of more than 0.5 dtex and 3.5 dtex or less in water, and the fiber dispersion liquid for the back surface layer and the fiber dispersion liquid for the optional intermediate layer may be obtained by dispersing 100 wt% of thick fibers having a fiber diameter in a range of more than 0.5 dtex and 3.5 dtex or less in water.
The method of producing a nonwoven fabric substrate for a separation membrane of the present invention may further include subjecting a nonwoven fabric obtained by the papermaking to a surface treatment to control wettability of the nonwoven fabric.
According to the nonwoven fabric substrate for a separation membrane according to an aspect of the present invention, in a nonwoven fabric for a separation membrane composed of two or more layers, by having a surface layer, a back surface layer, and an optional intermediate layer, setting a coating surface of the coating solution during membrane formation as a surface of the surface layer, and, when the nonwoven fabric substrate is impregnated with the coating solution for membrane formation, making the surface layer exhibits a large Laplace force and making the back surface layer and the optional intermediate layer exhibit a small Laplace force, it is possible to achieve both of the prevention of bleed-through of a coating resin and high water permeability of a separation membrane to be produced, which have been in a trade-off relationship heretofore.
Furthermore, according to the nonwoven fabric substrate for a separation membrane according to another aspect of the present invention, in a nonwoven fabric for a separation membrane composed of two or more layers, the nonwoven fabric substrate for a separation membrane includes: a surface layer having a coating surface of a coating solution for membrane formation; a back surface layer; and an optional intermediate layer, the surface layer has an average pore diameter smaller than that of the back surface layer or the optional intermediate layer located under the surface layer, and a difference between the average pore diameter of the surface layer and the average pore diameter of the back surface layer or the optional intermediate layer is 0.5 µm or more, and thereby, it is possible to achieve both of the prevention of bleed-through of a coating resin and the high water permeability of a separation membrane to be produced, which have been in a trade-off relationship so far.
More specifically, in the nonwoven fabric substrate for a separation membrane of the present invention, since one or more kinds of fine fibers having a small fiber diameter are incorporated in the surface layer of which surface is a coating surface of a resin solution, the surface layer exerts a large Laplace force, and an average pore diameter of the surface layer is smaller by 0.5 µm or more than an average pore diameter of the back surface layer or the optional intermediate layer located under the surface layer, and the coated resin substantially penetrates only into a surface layer region, so that the resin solution can be left on the coated surface (on a surface of the surface layer), solvent exchange by non-solvent induced phase separation is facilitated, and a high-performance separation membrane can be produced. Furthermore, since the back surface layer and the optional intermediate layer are configured to include a portion consisting substantially of one or more kinds of the thick fibers having a larger fiber diameter than the fine fibers, it is possible to prevent the bleed-through of the resin solution, and it is possible to suppress occurrence of defects such as pinholes in the separation membrane in a step of applying the resin solution.
Furthermore, according to the method of producing a nonwoven fabric substrate for a separation membrane of the present invention, since a surface (outermost surface) of the surface layer to be a surface coated with a coating solution during membrane formation of the separation membrane is covered with relatively thin fibers, even if raising occurs in the papermaking process, the raising can be suppressed by subsequent heat treatment. Therefore, the quality of the separation membrane prepared using the nonwoven fabric substrate for a separation membrane of the present invention is improved.
Moreover, according to the nonwoven fabric substrate for a separation membrane of the present invention, the separation membrane can be produced with a small amount of coating solution as compared with a conventional nonwoven fabric, and cost reduction due to reduction of the coating resin and improvement of liquid permeation performance per unit weight can be achieved.
In addition, according to the nonwoven fabric substrate for a separation membrane of the present invention, since the thick fibers are mixed in the surface layer of which surface is the coating surface of the resin solution, the anchor effect when the coating solution is solidified is enhanced, and the nonwoven fabric substrate can withstand the use environment of a backwashing step.
Furthermore, according to the method of producing a nonwoven fabric substrate for a separation membrane of the present invention, by using a “papermaking method” in a process for producing a wet nonwoven fabric, in a nonwoven fabric to be obtained, there is a portion in which fibers constituting the surface layer, the back surface layer, and the optional intermediate layer are continuously entangled between the layers, and the entanglement of the fibers inside the nonwoven fabric improves the adhesiveness between the layers. As a result, welding can be performed at a low compression pressure in the production process of the nonwoven fabric, and the nonwoven fabric is excellent in tensile strength and is hardly peeled off. Therefore, according to the nonwoven fabric substrate for a separation membrane of the present invention, it is possible to produce a separation membrane excellent in liquid permeation performance by non-solvent-induced phase separation.
Conventionally, a part of a nonwoven fabric has been densified in order to prevent bleed-through of a coating solution. The dense layer crushed by thermal compression or the like applied in the process of producing the nonwoven fabric can delay the penetration rate of the coating solution into the nonwoven fabric. This makes the formation of a barrier layer a design guideline for nonwoven fabrics.
On the other hand, in the present invention, a concept of nonwoven fabric design is not necessarily to form a layer having a high density, but to form a layer having a large Laplace force when impregnated with a coating solution. It is not always necessary to form a dense layer or a layer having a high density.
First, a schematic conceptual diagram of the structure of a nonwoven fabric of the present invention is shown in
The surface layer 11 includes one or more kinds of fine fibers FF having a small fiber diameter and one or more kinds of thick fibers TF having a larger fiber diameter than the fine fibers FF. The back surface layer 12 includes a portion substantially composed of only thick fibers TF. When the nonwoven fabric has the optional intermediate layer, the intermediate layer is configured to include a portion substantially composed only of thick fibers.
In other words, in the nonwoven fabric 10, the fine fibers FF are mixed in the surface layer 11, and the thick fibers TF are present on the entire surface layer 11 and the entire back surface layer 12. In particular, there are not layers having different densities, and the density distribution may be uniform. The fine fibers FF are not necessarily fibers having the same diameter, and a plurality of fine fibers having different diameters may be mixed. Further, the thick fibers TF do not have to have the same diameter, and thick fibers having a plurality of different diameters may be mixed.
In the nonwoven fabric 10 of the present invention, the surface layer 11 includes a portion (hereinafter, fine fiber mixture portion) A1 in which fine fibers FF are mixed, and the back surface layer 12 includes a portion (Hereinafter, it is also referred to as a thick fiber portion.) A2 substantially composed only of thick fibers TF. There is no clear boundary between the fine fiber mixture portion A1 and the thick fiber portion A2, and mainly the thick fibers TF are continuously entangled. In order to schematically show this, in the conceptual diagram of
That is, in the nonwoven fabric 10 of the present invention, there is no large difference in the degree of entanglement of the thick fibers TF constituting the nonwoven fabric between the surface layer 11 and the back surface layer 12 and the intermediate portion thereof, and when viewed as the entire nonwoven fabric 10, the fine fibers FF are simply mixed in the surface layer 11.
The fine fiber FF referred to in the present invention is a fiber having a fiber diameter in the range of 0.01 dtex or more and 0.5 dtex or less. The thick fiber TF referred to in the present invention is a fiber having a fiber diameter in the range of more than 0.5 dtex and 10 dtex or less. The fiber diameter of the thick fibers TF is preferably in a range of more than 0.5 dtex and 6.5 dtex or less, more preferably in a range of more than 0.5 dtex and 5.0 dtex or less, and still more preferably in a range of more than 0.5 dtex and 3.5 dtex or less. When the fiber diameter of the thick fibers TF is within a narrower range, it is easy to control the average pore diameter of the surface layer 11 of the nonwoven fabric 10 and the back surface layer 12 (or optional intermediate layer) located thereunder, and it is easy to obtain a difference in Laplace force between the surface layer 11 and the back surface layer 12 when impregnated with a coating solution and/or a difference in average pore diameter between the surface layer 11 and the back surface layer 12 (or optional intermediate layer).
The fiber lengths of the fine fibers FF and the thick fibers TF are preferably in the range of 1 to 10 mm. When the fiber lengths of the fine fibers FF and the thick fibers TF constituting the nonwoven fabric 10 are within this range, an intended effect is easily obtained, and it is also advantageous in terms of producing cost of the nonwoven fabric 10.
The total thickness of the nonwoven fabric 10 is in the range of 30 to 300 µm, more preferably in the range of 60 to 200 µm, and still more preferably in the range of 80 to 140 µm. The nonwoven fabric 10 of the present invention satisfying such a thickness range is suitable as a nonwoven fabric substrate for a separation membrane, and is excellent in handleability.
Furthermore, it is preferable that the fine fibers FF are mixed in the surface layer 11 at a ratio of 1 to 50 wt%, and the thickness of the surface layer 11 is in a range of 10 to 90% of the total thickness of the nonwoven fabric 10. More preferably, fine fibers FF are mixed in the surface layer 11 at a ratio of 5 to 20 wt%, and the thickness of the surface layer 11 is preferably in a range of 50 to 80% of the total thickness of the nonwoven fabric 10. In other words, in the nonwoven fabric 10, the composition ratio (ratio of the thickness of the surface layer 11 to the thickness of the back surface layer 12) in the thickness direction (the vertical direction of the paper surface of
Here, the composition ratio in the thickness direction of the surface layer 11 and the back surface layer 12 in the nonwoven fabric 10 may be calculated on the basis of X-ray CT scan, scanning electron microscope (SEM) observation, or optical microscope observation of the nonwoven fabric 10. When the production conditions of the nonwoven fabric 10 are known, the composition ratio in the thickness direction of the surface layer 11 and the back surface layer 12 may be estimated using the production conditions. For example, when the nonwoven fabric has a two-layer structure of a surface layer and a back surface layer made of the same material and is produced by sequentially performing papermaking using a wet papermaking method as in Examples described later, if the basis weights of the surface layer and the back surface layer in the papermaking step are Xg/m2 and Yg/m2, respectively, the composition ratio of the surface layer and the back surface layer in the thickness direction in the nonwoven fabric can be estimated to be X:Y. However, in a case where the materials of the surface layer and the back surface layer are different, the degree of compression applied to the surface layer and the back surface layer in the producing process is different, and thus it may not be easy to estimate the thicknesses of the surface layer and the back surface layer as described above. In such a case, it is preferable to estimate the thicknesses of the surface layer and the back surface layer by evaluating the distribution of the fine fibers from the X-ray CT scan image.
As the material of the nonwoven fabric 10, fibers such as polyester-based fibers such as polyethylene terephthalate (PET), polybutylene terephthalate and polytrimethylene terephthalate, polyamide-based fibers such as nylon 6, nylon 66, nylon 610 and nylon 612, polyolefin-based fibers such as polypropylene (PP), polyethylene (PE) and a composite material of polypropylene and polyethylene (PP/PE), engineering plastic fibers such as polyphenylene sulfide (PPS), natural pulp and rayon fibers, and copolymers or mixtures mainly containing these resins are preferably used. Among them, polyester-based fibers are preferably used because they have high strength and dimensional stability. Furthermore, the material of the surface layer 11 and the material of the back surface layer 12 may be the same or different. The same applies when the nonwoven fabric includes an optional intermediate layer.
Now, the flow of the resin solution in the pores of the nonwoven fabric having the structure as schematically shown in
Here, the driving force with which the resin solution penetrates into the nonwoven fabric is capillary pressure (ΔP). The capillary pressure is inversely proportional to the pore radius. That is, as the pore radius decreases, the capillary pressure as the driving force increases in inverse proportion. Therefore, the flux per unit area with the capillary pressure as the driving force is proportional to the pore radius (R), and the arrival time to the back surface is inversely proportional to the pore radius (R). In the case of a porous body such as a nonwoven fabric, pores are bent in various ways, and various models considering tortuosity are considered (See, e.g., R. Kondo, M. Daimon, S. Ohsawa, Gypsum & Lime, No. 112, 14, 1971.).
In an actual nonwoven fabric, not only a simple flow path in the thickness direction as in the schematic diagram of
The Laplace force may be considered as a pressure difference defined by the Young Laplace equation (Formula 1), and is synonymous with capillary pressure of capillary phenomenon. ΔP = 2γ/R (Formula 1)
This Laplace force is very large in pores of about 5 µm formed in the nonwoven fabric. For example, N-methylpyrrolidone (NMP) often used as a solvent of a resin solution has a large surface tension (γ) of 41 mN/m at 25° C. In the Young Laplace equation (Formula 1), the capillary pressure (Laplace force) is obtained from the surface tension (γ) and the pore radius (R).
Introducing the above surface tension and 5 µm pore size (2.5 µm pore radius) into Equation 1, the pressure difference (ΔP) becomes 32.8 × 106 mN/m, which is equivalent to 32.8 kPa. For a pore size of 5.5 µm, the pressure difference (ΔP) is 29.8 kPa. The difference between vacuum and atmospheric pressure is 100 kPa, and the differential pressure of capillary forces in pores of 5 µm and 5.5 µm (3 kPa) reaches 3% of this difference. That is, as described above, when the surface layer 11 is in a state where the pore diameter is slightly smaller than that of the back surface layer 12 (in this case, 0.5 µm), it can be seen that a significant difference occurs in Laplace force between the surface layer 11 and the back surface layer 12.
The average pore diameter of the nonwoven fabric 10 of the present invention is desirably 20 µm or less, and more desirably 10 µm or less. Furthermore, it is desirable that the surface layer 11 has an average pore diameter smaller than that of the back surface layer 12 (in the case of having an optional intermediate layer, the intermediate layer) located thereunder, the difference between the average pore diameter of the surface layer 11 and the average pore diameter of the back surface layer 12 (or the optional intermediate layer) is 0.5 µm or more, and it is more desirable that the difference between the average pore diameters is 1.0 µm or more. In the nonwoven fabric 10 of the present invention having such a difference in average pore diameter between the surface layer 11 and the back surface layer 12 (or an optional intermediate layer), a difference in Laplace force between the surface layer 11 and the back surface 12 (or an optional intermediate layer) is more reliably generated, which is suitable as a nonwoven fabric substrate for a separation membrane. Since the Laplace force is determined not only by the average pore diameter of the nonwoven fabric (layer constituting the nonwoven fabric) but also by the surface tension of the solvent of the resin solution, theoretically, the difference in average pore diameter between the surface layer 11 and the back surface layer 12 (or optional intermediate layer) may be smaller than 0.5 µm as the surface tension of the solvent of the resin solution increases, but in the present invention, the difference in average pore diameter is preferably 0.5 µm or more in view of the flow of the resin solution in the pores of the nonwoven fabric described with reference to
Even when the composition ratio in the thickness direction of the surface layer 11 and the back surface layer 12 in the nonwoven fabric 10 can be calculated or estimated as described above, it is practically difficult to measure the average pore diameters of the surface layer 11 and the back surface layer 12 and calculate the difference between the average pore diameters for two-layer or multi-layer porous bodies having a thickness in the order of micrometers like the nonwoven fabric 10 by the analysis technique at the time of filing of the present application. However, when the production conditions of the target porous body (nonwoven fabric in the case of the present application) are known and a plurality of (preferably three or more) test bodies can be prepared, it is possible to estimate the difference in average pore diameter between the surface layer and the back surface layer as in Examples described later. As a method of measuring the average pore diameter, a palm porometer is generally used, and a bubble point method or a mercury intrusion method can be used. In the case of a nonwoven fabric, in particular, a bubble point method is suitably used, whereby it can also be effective to provide a test condition under which it can be confirmed that the difference in average pore diameter between the surface layer and the back surface layer is 0.5 µm or more.
On the other hand, as shown in
For this reason, the nonwoven fabric 10 of the present invention in which the fine fibers FF are mixed in the surface layer 11 can delay the penetration of the resin solution R. As described above, the magnitude of the Laplace force is related to the surface tension of the solvent of the resin solution (coating solution for film formation), and the material of the nonwoven fabric needs to have a certain degree of wettability for expression of the Laplace force. Therefore, the nonwoven fabric 10 of the present invention may be subjected to a surface treatment for controlling the wettability of the nonwoven fabric. Examples of such a surface treatment include a hydrophilization treatment (plasma treatment or the like), and the surface treatment is typically applied to the surface layer 11 of the nonwoven fabric 10, but may also be applied to the back surface layer 12.
Nonwoven fabric 10 for a separation membrane shown in
As described above, the method of producing the nonwoven fabric 10 for a separation membrane of the present invention includes sequentially papermaking a fiber dispersion liquid for the surface layer 11 including fine fibers FF having a small fiber diameter and one or more kinds of thick fibers TF having a fiber diameter larger than that of the fine fibers FF, al fiber dispersion liquid for an optional intermediate layer including only the thick fibers TF, and a fiber dispersion liquid for the back surface layer 12 including only the thick fibers TF using a wet papermaking method.
The fine fibers FF and the thick fibers TF used in the method of producing the nonwoven fabric 10 of the present invention are as described above for the nonwoven fabric 10, and thus the detailed description thereof will be omitted.
More specifically, in the method of producing the nonwoven fabric 10 of the present invention, the fiber dispersion liquid for the surface layer 11 is obtained by dispersing 1 to 50 wt% of fine fibers FF having a fiber diameter in a range of 0.01 dtex or more and 0.5 dtex or less and 50 to 99 wt% of thick fibers TF having a fiber diameter in a range of more than 0.5 dtex and 10 dtex or less in water, and the fiber dispersion liquid for the back surface layer 12 and the fiber dispersion liquid for an optional intermediate layer are preferably obtained by dispersing 100 wt% of thick fibers TF having a fiber diameter in a range of more than 0.5 dtex and 10 dtex or less in water. By sequentially performing papermaking using such a fiber dispersion using a wet papermaking method, it is possible to efficiently obtain the nonwoven fabric 10 having the surface layer 11 and the back surface layer 12 (and optional intermediate layer), wherein, when the nonwoven fabric 10 is impregnated with a coating solution, the surface layer 11 has a large Laplace force and the back surface layer 12 (and the optional intermediate layer) has a small Laplace force. Furthermore, it is possible to more reliably obtain the nonwoven fabric 10 having a structure in which the average pore diameter of the surface layer 11 is smaller than the average pore diameter of the back surface layer 12 (or optional intermediate layer) by 0.5 µm or more.
Furthermore, when the fiber dispersion liquid for the surface layer 11 is obtained by dispersing 5 to 50 wt% of the fine fibers FF having a fiber diameter in a range of 0.05 dtex or more and 0.5 dtex or less and 50 to 95 wt% of the thick fibers TF having a fiber diameter in a range of more than 0.5 dtex and 3.5 dtex or less in water, and the fiber dispersion liquid for the back surface layer 12 and the fiber dispersion liquid for an optional intermediate layer are obtained by dispersing 100 wt% of the thick fibers TF having a fiber diameter in a range of more than 0.5 dtex and 3.5 dtex or less in water, the nonwoven fabric 10 having the surface layer 11 and the back surface layer 12 (and the optional intermediate layer), wherein, when the nonwoven fabric 10 is impregnated with the coating solution, the surface layer 11 has a large Laplace force and the back surface layer 12 (and the optional intermediate layer) has a small Laplace force, can be more reliably obtained, which is preferable. Furthermore, the nonwoven fabric 10 having a structure in which the average pore diameter of the surface layer 11 is smaller than the average pore diameter of the back surface layer 12 (or optional intermediate layer) by 0.5 µm or more can be more reliably obtained.
In the aspect shown in
It should be noted that the device configuration schematically shown in
As shown in
Examples of the present invention will be described below, but the present invention is not limited to the scope of the following Examples.
As schematically shown in
In Examples of the present invention, fibers of 0.1 dtex are referred to as fine fibers, and fibers of 0.6 dtex and 1.2 dtex are referred to as thick fibers. When it is necessary to distinguish the latter, fibers of 0.6 dtex are referred to as medium-thick fibers, and fibers of 1.2 dtex are referred to as extra-thick fibers. In the obtained nonwoven fabric, a portion where thick fibers (middle thick fibers or extra thick fibers) and fine fibers are mixed is referred to as a fine fiber mixture portion, and a portion substantially composed of thick fibers (middle thick fibers or extra thick fibers) is referred to as a thick fiber portion.
When the SEM image (cross-sectional image) of
In the SEM image of
As is clear from
As understood from the production process described above, in the present Example, 40 g/m2 layers are made into paper as a surface layer and a back surface layer, respectively, and the fiber dispersion for the back surface layer does not contain fine fibers. Therefore, as shown in
A nonwoven fabric of Example 2 was obtained by the same production process as in Example 1 except that using a producing apparatus having the same configuration as in Example 1, fibers were scooped by a wire conveyor from a first net paper machine at a basis weight of 50 g/m2 to make a paper sheet, and then fibers were scooped by a wire conveyor from a second net paper machine at a basis weight of 30 g/m2 to make a paper sheet of a back surface layer. This is referred to as a nonwoven fabric (2). Physical properties of the nonwoven fabric (2) were a basis weight of 80 g/m2, a thickness of 120 µm, a density of 0.68 g/cm3, a tensile strength (MD) of 97.3 N/15 mm, a tensile strength (CD) of 63.0 N/15 mm, an air permeability of 1.85 cm3/cm2 • s, and an average pore size of 5.79 µm.
A nonwoven fabric of Example 3 was obtained by the same production process as in Example 1 except that using a producing apparatus having the same configuration as in Example 1, fibers were scooped by a wire conveyor from a first net paper machine at a basis weight of 60 g/m2 to make a paper sheet of a surface layer, and then fibers were scooped by a wire conveyor from a second net paper machine at a basis weight of 20 g/m2 to make a paper sheet of a back surface layer. This is referred to as a nonwoven fabric (3). Physical properties of the nonwoven fabric (3) included a basis weight of 80 g/m2, a thickness of 126 µm, a density of 0.69 g/cm3, a tensile strength (MD) of 94.5 N/15 mm, a tensile strength (CD) of 72.0 N/15 mm, an air permeability of 1.64 cm3/cm2 • s, and an average pore size of 5.56 µm.
In the nonwoven fabrics (1) to (3) in Examples 1 to 3, the ratios (% by weight) of each stretched fiber in the fiber dispersion liquid in the first cylinder paper machine and the second cylinder paper machine and the ratio (% by weight) of the stretched fiber in the produced nonwoven fabric were summarized in Table 1. In the three nonwoven fabrics, the compositions of the two kinds of fiber dispersion liquids used for production are the same, but only the basis weights of the nonwoven fabrics (the surface layer and the back surface layer) transferred from the first cylinder paper machine and the second cylinder paper machine are different. Therefore, in the nonwoven fabrics (1) to (3), it can be considered that only the thicknesses of the surface layer and the back surface layer are different. Thus, the proportions (% by weight) of the fine fibers in the nonwoven fabrics (1) to (3) are 5.0%, 6.25%, and 7.5%, respectively.
Table 2 summarizes the physical properties of nonwoven fabrics (1) to (3) in Examples 1 to 3. Compared with the nonwoven fabric (1), the nonwoven fabric (3) has a thickness increased by about 10%. The reason why the fine fibers are less likely to be crushed in the heat treatment step (heating/compression process) even though the ratio of the thick fibers (More specifically, medium thick fibers) is reduced as compared with the fine fibers may be that the fine fibers slightly inhibit thermal compression of the thick fibers. However, in the nonwoven fabrics (1) to (3), since the amount of change in density is small, there is a high possibility that the influence of the surface roughness affects the thickness measurement. On the other hand, the average pore size of the nonwoven fabric (3) is about 10% smaller than that of the nonwoven fabric (1), and the average pore size of the thick nonwoven fabric (3) is the smallest, so that the contribution of fine fibers can be clearly seen.
Each of the nonwoven fabrics (1) to (3) prepared in Examples 1 to 3 was cut to A4 size, and polyethersulfone (PES) was applied to one surface (Surface of surface layer or surface of back surface layer) of the fabric, and the fabric was immersed in water to perform non-solvent induced phase separation, thereby preparing an asymmetric membrane (In this case, an ultrafiltration membrane).
A high molecular weight PES having a viscosity number of 82 g/cm3 (Measured according to ISO 1628 using a 1: 1 solution of 0.01 g/mL phenol/1,2 dichlorobenzene) was used, and a 20 wt% NMP solution was applied with a 150 µm wire coater and immersed in a coagulation bath at 23° C. to perform non-solvent induced phase separation.
When PES is applied to the surface of the surface layer, as the fine fiber content (That is, the ratio of the fine fiber mixture portion to the thickness of the entire nonwoven fabric) increases, the layer of PES that is not immersed in the nonwoven fabric becomes thicker (
In Example 4, a high concentration (20 wt%) PES solution was used, and the pore size of the surface of the obtained ultrafiltration membrane was small because of the low coagulation bath temperature (23° C.). Therefore, the flux (L/m2 h) of water under a reduced pressure of 80 kPa was measured for 6 ultrafiltration membranes, and it was 15.9 L/m2 h even in the case of the highest performance ultrafiltration membrane (4-3-1). However, it was confirmed that the ultrafiltration membranes (4-1-1), (4-2-1), and (4-3-1) in which the PES was applied to the surface of the surface layer had a larger flux than the ultrafiltration membranes (4-1-2), (4-2-2), and (4-3-2) applied to the surface of the back surface layer of the same nonwoven fabric (Data not shown.).
In Example 5, an asymmetric membrane (In this case, an ultrafiltration membrane) was produced by applying PES to the surface of the surface layer using the nonwoven fabric (3) produced in Example 3. The nonwoven fabric (3) used had a width of 50 cm and a length of 200 m.
As a PES solution, the same 20 wt% NMP solution as in Example 4 was used, and applied with a thickness of 120 µm using a casting knife. The temperature of the coating solution was set to 25° C., and the temperature of the coagulation bath was set to 40° C.
In Example 6, in order to analyze the characteristics of the nonwoven fabric of the present invention, the nonwoven fabric (1) prepared in Example 1 was subjected to a water repellent treatment, and a water suction filtration test was performed.
Specifically, an aqueous solution containing 1.33 wt% of a fluorine-based water-and-oil repellent agent (Asahi Guard AG-E082, produced by AGC Inc.) and 1.67 wt% of a blocked isocyanate crosslinking agent was prepared, impregnated with the nonwoven fabric (1), squeezed to a predetermined squeezing rate with a mangle, and then dried at 110° C. for 30 minutes with a dryer. This is referred to as a nonwoven fabric (4).
For comparison with Example 6, a nonwoven fabric of Comparative Example 1 was obtained by using the same producing apparatus as in Example 1, scooping fibers with a wire conveyor from a first cylinder paper machine at a basis weight of 80 g/m2 to perform papermaking, and without using a second cylinder paper machine. A fiber dispersion containing 0.6 dtex in an amount of 40 wt% and 1.2 dtex in an amount of 60 wt% was placed in the first cylinder paper machine. That is, in the production of the nonwoven fabric of Comparative Example 1, 0.6 dtex fibers (medium thick fibers) and 1.2 dtex fibers (extra-thick fibers) were used, and 0.1 dtex fibers (fine fibers) were not used.
The obtained nonwoven fabric of Comparative Example 1 was subjected to a water repellent treatment according to the same procedure as in Example 6. This is referred to as a nonwoven fabric (5).
In the same manner as the test using the nonwoven fabric (4) of Example 6, a water suction filtration test was performed using the nonwoven fabric (5) (
Furthermore, the effect of including fine fibers having a small fiber diameter in the nonwoven fabric can be estimated as follows. The nonwoven fabric (4) is obtained by subjecting a nonwoven fabric (1) including fibers (fine fibers) of 0.1 dtex in a surface layer to a water repellent treatment using a fluorine-based water and oil repellent agent-containing aqueous solution. That is, since the nonwoven fabric (1) has a surface layer having a large Laplace force and a back surface layer having a small Laplace force when impregnated with a coating solution for forming a separation membrane which is a main application of the nonwoven fabric (1), and the surface layer has an average pore diameter smaller than that of the back surface layer located below the surface layer by 0.5 µm or more, in a state after being hydrophobized, a stronger suction resistance force acts on water in the surface layer having a small average pore diameter, and water resistance higher than that of the back surface layer is exhibited, and as a result, water resistance as a whole of the nonwoven fabric is considered to be improved. Here, as can be seen from the composition of the fiber dispersion described above, the ratio of the fine fibers in the fiber dispersion for the surface layer used for producing the nonwoven fabric (1) is 10 wt%. As described above, it is worth noting that a significant difference occurs in Laplace force between the surface layer and the back surface layer even under a condition where the content ratio of the fine fibers in the fiber dispersion is relatively low. Of course, the content ratio of the fine fibers is not limited to this condition, and can be appropriately adjusted, whereby the difference in Laplace force between the portion containing the fine fibers and the portion substantially not containing the fine fibers (the portion substantially composed only of the thick fibers) in the obtained nonwoven fabric can be controlled within a desired range.
As suggested from the test result using the nonwoven fabric (4) of Example 6, for the purpose of improving water resistance, the layer containing fine fibers (surface layer of nonwoven fabric) is preferably disposed on the side not in contact with water (liquid phase), and disposed such that the surface of the layer (back surface layer of nonwoven fabric) composed of a portion substantially composed of only thick fibers is in contact with water (liquid phase). In this arrangement, the pressure difference applied to the nonwoven fabric can be received by the back surface layer, that is, the layer on the side in contact with water being crushed in the thickness direction, and the water resistance of the nonwoven fabric as a whole is improved accordingly. On the other hand, when the surface of the layer containing fine fibers (the surface layer of the nonwoven fabric) is disposed so as to be in contact with water (liquid phase), the pressure difference applied to the nonwoven fabric is received only by the surface layer depending on the composition ratio in the thickness direction of the surface layer and the back surface layer, and thus the back surface layer that is not in contact with water and disposed on the gas phase side may not work effectively.
In other words, from the viewpoint of using the nonwoven fabric as a water-resistant film, fine fibers having a small fiber diameter may be mixed in the surface layer of the nonwoven fabric at the time of preparing the nonwoven fabric, and are not necessarily mixed in the entire nonwoven fabric. At the time of use, the back surface layer may be disposed so that the surface of the back surface layer configured to include a portion including only thick fibers substantially not including fine fibers and having a substantially large fiber diameter is in contact with water (liquid phase).
The nonwoven fabric substrate for a separation membrane according to the present invention is used as a support for applying a resin solution to a separation membrane such as a microfiltration membrane (MF membrane), an ultrafiltration membrane (UF membrane), a nanofiltration membrane (NF membrane), or a reverse osmosis membrane (RO membrane).
The nonwoven fabric substrate for a separation membrane according to the present invention has a surface layer and a back surface layer in which the degree and density of entanglement of fibers constituting the nonwoven fabric are generally uniform (macroscopically), but the average pore diameter is partially different (microscopically), and the average pore diameter of the surface layer is smaller than the average pore diameter of the back surface layer. Furthermore, the fibers are entangled also at the boundary portion between the surface layer and the back surface layer constituting the nonwoven fabric, so that the mechanical strength is high, the nonwoven fabric can be thinned, and the coating thickness of the resin solution can be reduced, so that it is possible to prepare a high-performance separation membrane that is lightweight at low cost and has a large practical membrane area. Furthermore, the durability of the nonwoven fabric is improved particularly by the entanglement of the thick fibers at the entangled portion. Furthermore, it is possible to apply a polymer that has not been applied due to a bleed-through of a resin solution, and it is possible to select a polymer according to various applications.
In particular, when the nonwoven fabric of the present invention is used as a substrate for RO membrane production, energy saving in seawater desalination treatment and the like is expected.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2021-027053 | Feb 2021 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2022/003206 | 1/28/2022 | WO |
