JPS603845B2 - Method for manufacturing dry cellulose acetate semipermeable membrane - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a novel reverse osmosis membrane and gas separation membrane made of cellulose acetate that can be handled in a dry state, and a method for producing the same.

In recent years, reverse osmosis treatment and ultrafiltration using semipermeable membranes have been attracting attention for seawater desalination, irrigation water desalination, industrial wastewater treatment, food industry, etc. What is important in these membrane separation techniques is the selective permselectivity of the semipermeable membrane. That is, a semipermeable membrane is required that blocks as much of the solute to be separated as possible while allowing a large amount of the solvent (generally water) to pass through. Cellulose acetate has long attracted attention as a material that satisfies these properties, and most reverse osmosis membranes currently on the market are made of cellulose acetate. The basic principle of its manufacturing method is disclosed in Japanese Patent Publication No. 42-2818, and many improved methods have been proposed since then. However, the semipermeable membranes obtained from these disclosed manufacturing methods must be stored in a moist state, and once the membrane dries out, its properties change dramatically and it is no longer usable. . Therefore, when assembling a reverse osmosis device or an ultrafiltration device using these semipermeable membranes, the semipermeable membrane must be kept moist at all times, and there may be problems such as adhesion between the semipermeable membrane and the device. This poses a major obstacle to handling. Special attention must also be paid to the transport and preservation of semi-transparent waistbands. As a result of intensive studies to eliminate these drawbacks, it was discovered that a new cellulose acetate semipermeable membrane that can be handled satisfactorily even in a dry state can be produced if a specific amount of non-solvent is added. Furthermore, the dry semipermeable membrane obtained from the present invention has a gas permeability several tens of times higher than that of conventional acetate films.
It was found that the moss was several tens of times larger than the moss and could be used industrially as a gas separation membrane. The present invention provides a method for obtaining a semipermeable membrane by casting a solution consisting of cellulose acetate with a degree of acetylation of 37.0 to 40.5% by weight, its solvent and a non-solvent (those that are soluble in the solvent).
○) As a non-solvent, its boiling point is 200° higher than the boiling point of the solvent.
(2) the amount b (wt%) of non-solvent in the casting solution is 10 or more, and (a-15) <b>
a is the amount (wt%) of the nonsolvent required to precipitate the polymer from solution or gel the solution at the polymer (cellulose acetate) concentration and temperature at which it is cast. This is a method for producing a dry cellulose acetate semipermeable membrane, which is characterized by evaporating 60% or more of the solvent and nonsolvent in the solution after rigid casting.

Generally, the phase diagram of a three-component system consisting of polymer, solvent, and non-solvent at a certain temperature is expressed using triangular coordinates as shown in FIG.

The curve AaD is a hyperbolic curve, and at a composition below this curve, the solution undergoes phase separation. When the concentration of the polymer in the solution is high, gelation may occur at a composition near this hyperbolic curve. In the region above the hyperbolic curve, the solution forms a homogeneous phase. When a non-solvent is added to a homogeneous solution consisting of a polymer-solvent system represented by the composition at point E, the composition of the solution changes along the line EA.
The top changes from B to A. When the nonsolvent is added until the composition of the solution reaches the composition represented by point a, the polymer solution begins to become cloudy and precipitation of the polymer occurs. Therefore, if such precipitation points are confirmed for solutions with various polymer concentrations, a hypergonal curve AaD can be drawn. One major feature of the present invention is that the composition of the casting solution is near the upper side of this hyperbolic curve.

In other words, a nonsolvent is added to a system consisting of a polymer solvent at an arbitrary concentration, and the amount of nonsolvent at which the polymer starts to precipitate or the solution becomes a gel is calculated as follows:
If a is % by weight, the amount b of the non-solvent in the casting solution is (
a-15) An excellent dry semipermeable membrane can only be obtained when the amount is greater than % by weight and less than awt%. In this case, the amount b of the nonsolvent used must be at least 10% by weight,
If the amount used is less than this, an excellent semipermeable membrane cannot be obtained. The value of a, which indicates the precipitation point or gelation point, varies depending on the polymer concentration, temperature, and type of nonsolvent, but can be determined by experiment, so it is possible to select a combination of solvent and nonsolvent that can be used in the present invention. It is.

In Figure 1, point a indicates the amount of non-solvent of about 42% by weight, c
Although the points indicate an amount of non-solvent of approximately 2% by weight, a cast solution having the composition of point c does not yield an excellent dry semipermeable membrane.

A dry semipermeable membrane with excellent performance can be obtained from a casting solution having a composition at point b containing a nonsolvent in an amount greater than (a-15) weight %, which is a requirement of the present invention. Note that when the concentration of the polymer is high and the amount of nonsolvent is large, the solution tends to gel. This gelation is sensitive to temperature; for example, even if the solution is uniformly dissolved at 60%, gelation occurs at room temperature and the entire system loses its fluidity. Therefore, such a casting solution must be heated to 6000℃ before casting. Casting of solutions can occur in various forms.

That is, in order to obtain a flat semipermeable membrane, it is possible to use, for example, a glass plate or a metal plate. It is also possible to cast onto a porous support by adjusting the viscosity of the solution. In addition, if an appropriate nozzle is used, it is possible to form the cellulose into a hollow fiber shape, and it can be handled in a dry state.
Acetate hollow fiber semipermeable membranes can be obtained. That is, casting here refers to an initial process for forming a solution into any form, and includes operations such as casting, spinning, and extrusion. Another major feature of the present invention is that (solvent + non-solvent) in the casting solution is evaporated from 60% to 100% after the solution is cast.

That is, in the conventional semipermeable membrane manufacturing method, after a very small amount of the solvent in the casting solution is evaporated, the casting solution is clarified in a coagulation chamber and gelled. In the present invention, the gelation of the casting solution is (
Completed by evaporation of solvent + non-solvent). That is, if 60% or more of (solvent + non-solvent) is evaporated, the casting solution of the present invention completes gelation. Since the structure of the gelled solution is fixed, you can either immerse it in a non-solvent for cellulose acetate and extract the remaining (solvent + non-solvent), or proceed with evaporation to obtain 100% It may be removed by evaporation. A dry semipermeable membrane can be obtained by drying the membrane immersed in a non-solvent as it is, or by washing it with water and drying it.

With conventional semipermeable membranes, this drying process causes the membrane to shrink and deform dramatically, and even when it is returned to water, it does not return to its original state, and at the same time, the performance of the semipermeable membrane is significantly reduced. The semipermeable membrane according to the present invention does not undergo any change in form even when dried from a wet state, and the performance of the semipermeable membrane does not change. For the purpose of obtaining a dry film, (solvent + non-solvent) in the casting solution is
It is easy to evaporate 00% in terms of process. The final feature of the present invention is the evaporation conditions for the Aya Z medium and non-solvent in the cast solution. In the phase diagram of Fig. 1, if we consider the case where a casting solution having the composition at point b is cast and the solvent and non-solvent are evaporated, if only the solvent selectively evaporates at the initial stage of evaporation, then The composition of the casting solution varies along the bF line in the F direction. On the other hand, if only the nonsolvent is selectively evaporated, the composition of the solution changes in the direction of E along the bE line. Generally, the proportion of both solvent and non-solvent is
The composition of the cast solution changes in the direction of the vector r because it evaporates with different values. Here, the feature of the present invention is that the solvent evaporation vector r is directed downward so that it intersects with the hypergonal curve. That is, a homogeneous casting solution causes phase separation or gelation by evaporating the solvent faster than the nonsolvent. As the evaporation progresses further, a translucent structure containing many microvoids is formed in the film. If the casting liquid has the composition at point c or if evaporation is performed in a direction where the direction of the vector r does not intersect with the hyperbolic curve, the cast solution will not undergo phase separation due to evaporation and a homogeneous transparent film will be obtained. , resulting in an excipient with poor performance as a semipermeable membrane. The evaporation of the solvent and nonsolvent varies in a complicated manner depending on the evaporation temperature, the boiling points of the solvent and the nonsolvent, the vapor pressures of both solvents in the evaporation atmosphere, and the like. As a result of various studies by the present inventors, if the boiling point of the non-solvent is 2000 or more higher than the boiling point of the solvent,
It has been discovered that, independent of other factors, the medium in the casting solution evaporates in a direction intersecting the hypergonal curve, resulting in a semi-transparent material with excellent performance. To prepare the casting solution, if the hyperbolic curve of the system is known in advance, cellulose
Acetate, solvent, and nonsolvent may be mixed and dissolved at the same time. The degree of acetylation of cellulose acetate used in the present invention is 37.0 to 40.5% by weight, and those commonly used for reverse osmosis membranes can be applied.

However, when manufacturing hollow fibers or shaping a solution with a relatively low polymer concentration, acetate with a high degree of polymerization for fibers is desirable. The solvent preferably has a low boiling point, and acetone, methyl acetate, tetrahydrofuran, dioxane, etc. are used, but acetone is most preferred.

The nonsolvent may be one that has a boiling point 20 oo or more higher than the boiling point of the solvent and that mixes with the solvent.

When acetone is used as a solvent, the non-solvent is water, which has a boiling point of 76. Alcohols, ketones, esters, ethers, etc. having 0 or more can be used. It is undesirable to make the boiling point of the nonsolvent too high, considering the evaporation process of the solvent and nonsolvent in the cast solution.
Alcohols include propyl alcohol, butyl alcohol, bentyl alcohol, hexyl alcohol and their isomers; ketones include 3-bentanone,
Cyclohexanone, 4-methyl-2-pentanone, esters such as propyl acetate, inpropyl acetate, butyl acetate, ethyl propionate, and the like, and ethers such as propyl ether, butyl ethyl ether, butyl ether, and the like. The polymer concentration in the casting solution can be arbitrarily varied depending on the excipient form.

In order to obtain a hollow fiber semipermeable membrane, a highly viscous solution is required, and the concentration of the polymer is preferably 20 to 35 wt%. On the other hand, in order to obtain a composite membrane by coating on a porous support, a lower viscosity is preferred, and a polymer concentration of 20 to 5 wt% is desirable. Evaporation of the solvent and non-solvent in the cast solution is easily achieved by blowing hot air onto the surface of the solution. The temperature of the hot air can be appropriately selected depending on the volatility of the solvent and nonsolvent and the film forming process. In order to obtain a dry semipermeable membrane, it is necessary to evaporate 60% or more of (solvent + nonsolvent), so a high temperature is desirable as the hot air temperature in order to shorten the membrane forming time. When acetone is used as a solvent, the temperature is preferably in the range of 50 to 130°C. If 100% of the solvent is evaporated during the evaporation process of the solvent and non-solvent, the obtained semipermeable film can be used as it is as a dry film.
On the other hand, when evaporating in the range of 60% to 100% and extracting the remaining medium by immersing it in a liquid, the liquid must not dissolve the polymer and must be completely soluble in the solvent and nonsolvent.

When acetone is used as the solvent and alcohol is used as the non-solvent, water is preferably used as the extraction liquid for the purpose of cleaning the membrane.
To make a dry film, after washing with water, dry it in a storm or at a temperature that does not deform the waist.

In conventional membrane manufacturing methods, after gelling the membrane by wet coagulation, heat treatment is performed in warm water in order to increase the permselectivity of the membrane, but in the manufacturing method of the present invention, this heat treatment step is not necessarily required. When an aqueous solution with an osmotic pressure m is subjected to reverse osmosis treatment at an operating pressure P across a semipermeable membrane, the flux (similar to n) Jv of water permeating the unit area of the semipermeable membrane per unit time is generally J
V ni K (P Itchiku) (1}).

K is a membrane constant, and it can be said that a membrane with a larger value of K is a semipermeable membrane with better performance. K generally depends on the material forming the film, but when the same material is used, it is inversely proportional to the film thickness.
Therefore, it is advantageous to make the film as thin as possible because of the water flux, but there is a limit to how thin the film can be, and if it is too thin, it cannot withstand mechanical strength and cannot be used. . The solution to this conflicting effect is the so-called asymmetric membrane, which has a thin dense layer on the surface and has a porous structure in most of the membrane that does not show resistance to the movement of water.
Observation of the dried semipermeable membrane according to the present invention using an electron microscope revealed that the membrane had such an asymmetric structure.

In addition to direct observation using an electron microscope, such an asymmetric membrane structure can also be revealed by examining the water flux Jv in a reverse osmosis experiment. That is, by producing a film that is clearly composed of only a dense layer, and measuring and comparing the Jv thereof, the asymmetry of the dry film obtained from the present invention as well as the thickness of the dense layer can be indirectly calculated. Next, the present invention will be explained in more detail using Examples.

Example 1 Cellulose acetate (intrinsic viscosity at 30o0 in acetone is 1.5 μm and degree of acetylation is 39.4%, hereinafter referred to as C
．． A. ), acetone as a solvent (boiling point 56.
o), n-butanol (boiling point 117.7
A three-component solution of 0) was prepared.

The polymer concentration was fixed at 1 Sakaki t%, and at this concentration the amount of nonsolvent a at the precipitation point of this system at room temperature was 43 Wt%. The polymer concentration was fixed at rawt%, and three types of solutions containing different amounts of butanol were prepared in a flask while increasing the number of the solutions. The uniformly dissolved solution was cast onto a glass plate at room temperature using a doctor blade, and immediately placed in an 80° hot air dryer to completely evaporate acetone and butanol. All of the obtained films were opaque. For comparison with the above film, a fine t% acetone solution of the above cellulose acetate was prepared and a dense film of acetate was prepared using the vertical glass plate method. These membranes were subjected to reverse osmosis experiments using 0.5% saline at an operating pressure of 4 pressures, and the flux of permeated water and salt removal rate were measured. The salt removal rate R is calculated as follows: Cf is the salt concentration in the saline water before permeation through the semipermeable membrane, Cp is the salt concentration in the permeated water, and R = (1-harm)X.
.

. More calculated.

The results are shown in Table 1. Table 1 Flux units are expressed in gallons of permeate per day (gfd) passing through one square foot of membrane area.

From the results in Table 1, it can be seen that the salt removal properties of the dry semipermeable membrane are comparable to those of the dense film. Therefore, considering the flux, the film thickness of membrane A is as thick as 84.2 μm, but it is found that most of the film has a porous structure and has a dense layer of about 40 μm. As the amount of nonsolvent added decreases, the thickness of the semipermeable membrane obtained becomes thinner, but the amount of flux decreases, and when the amount is less than (a-15)% by weight, the membrane becomes difficult to use practically.

Membrane C in this example has a thick dense layer and a small flux because the amount of butanol in the casting solution is less than the above-mentioned limit value (2% by weight), and is therefore excluded from the present invention. Example 2 C. in Example 1. Using A, dry semipermeable membranes were prepared using acetone as a solvent and various non-solvents with different boiling points.

The polymer concentration was fixed at 8% by weight in each case, and the casting solution was cast on a glass plate at room temperature and then dried in a hot air dryer to completely evaporate the solvent and nonsolvent. The obtained dry semipermeable membrane was subjected to a reverse osmosis experiment in the same manner as in Example 1, and the flux of the permeate and the salt removal rate were determined. The results are shown in Table 2. Table 2 In the results shown in Table 2, when methanol was used as a non-solvent, a reverse osmosis experiment was conducted for several hours at an operating pressure of 4 pressures, but no permeate was obtained.

In other words, when using methanol, even if the amount of non-solvent is near the precipitation point, selective evaporation of the solvent is not achieved in the evaporation process because its boiling point is close to the boiling point of the solvent, and only a good solvent or medium is used. A semipermeable membrane is formed with a thick dense layer similar to that used when using this method. When the boiling point difference between the solvent and the non-solvent becomes large, and it exceeds 20 qo as in the case of ethanol, the flux becomes large, and a semipermeable membrane that is industrially valuable can be obtained. Comparative example
*x Example 1
Using the casting solution used to create membrane A in step 1, the solution was cast onto a glass plate, and the evaporation time of (solvent + non-solvent) was controlled in a hot air dryer at 50 qo to control the amount of evaporation. A membrane was obtained.

At that time, except for the completely evaporated membranes, after drying, the membranes were immersed in 40 oo hot water for a certain period of time to extract the remaining acetone and butanol, and after washing with water, the obtained semipermeable membranes were dried at room temperature. At that time, the shrinkage rate of the semipermeable membrane before and after drying was determined from the change in diameter of the semipermeable membrane cut into circular shapes. The reverse osmosis properties of the semipermeable membrane thus dried were measured in the same manner as in Example 1. Table 3 shows the results.
It was shown to. Table 3 In Table 3, the amount of residual (solvent + non-solvent) refers to the amount of (solvent + non-solvent) contained in the casting solution.
It is a percentage when it is set to 00.

The reason why the sample with an evaporation time of 0 does not show 100% is considered to be because some of the solvent evaporated during the dilution time. The shrinkage rate % was determined by {(Do-D)/Do}×100, where Do is the diameter of the semipermeable membrane before drying and O is the diameter after drying.
In Experiment No. 1 and the commercially available cellulose 1 acetate reverse osmosis membrane, the membrane contracted and deformed considerably upon drying, making it impossible to perform the reverse osmosis experiment. On the other hand, No. Films Nos. 2 and 3 underwent incomplete evaporation, so gelation progressed in hot water, and as a result, the films were considerably deformed upon drying.

In addition, regarding membrane performance, although the flux was large, the salt removal performance was low, and a semipermeable membrane with high permselectivity could not be obtained. Example 3 A hollow fiber semipermeable membrane was obtained by dry spinning a cast solution.

Using the cellulose acetate of Example 1, C.I. A:2
Touch%, acetone: 37.8wt%, n-butanol:
A mixture containing 34.2% was placed in a sealed barrel container containing 65%
Apothecology was dissolved at °C. This solution gelled and lost fluidity at 30 qo. The obtained solution was extruded at a stock solution temperature of 590 through a nozzle having an outer diameter of 0.6 and an inner diameter of 0.4 as shown in FIG. 2 into an 80 qo hot air spinning tube to evaporate acetone and butanol. The amount of residual (non-solvent) contained in the yarn at the exit of the spinning tube was 10.5 M%.
The yarn that came out of the weaving tube was washed with water and then air-dried. When the cross section of the air-dried fiber was observed with an optical microscope, it was found to be a nearly perfectly circular hollow fiber with an outer diameter of 290A and an inner diameter of 83R. The obtained hollow fibers were cut to a length of 30 mm, bundled into 100 fibers, and installed in a reverse osmosis apparatus shown in FIG. 3 for a test. As in Example 1, a 0.5% saline solution was pressurized from the outside of the hollow fiber at 4 pressure, and the permeate was taken out from the inside of the hollow fiber, and its flux and salt removal rate were measured. When the flux is calculated based on the area based on the outer diameter of the hollow fiber, 2. The fd and salt removal rate were 94.6%. Example 4 Using the membrane B of Example 1, the permeability of oxygen gas and nitrogen gas was investigated.

Separately, C. of Example 1. A was cast onto a glass plate from a solution using only acetone as a solvent, the acetone was evaporated, a dense film was obtained, and its permeability was investigated. The permeation flow rate of gas per unit area and unit time is generally expressed as .=Px.

ΔP is the gas pressure difference on both sides of the film, and i is the thickness of the film. Further, P is a transmission coefficient. Table 4 shows the results of determining P for each gas according to the above formula. Table 4 From the results in Table 4, it can be seen that the gas permeation selectivity of semipermeable membrane B is the same as that of the dense film, but the permeability coefficient is about 7" louder.

[Brief explanation of the drawing]

FIG. 1 shows a phase diagram of a three-component system consisting of a polymer, a solvent, and a nonsolvent, and the curve AaD is a hypergonal curve. FIG. 2 shows a cross section of a nozzle used for the hollow fiber grade yarn, where ■ indicates the nozzle, and ■ indicates the missing ridges. Figure 3 shows a reverse osmosis test device for hollow fibers, where ■ is the cock, ■ is the adhesive part between the hollow fiber and the container, ■ is the east of the hollow fiber, ■ is the saline solution, ■ is the pressure gauge, and ■ is the pressure gauge. show. ur ′ figure 2 figure 3 figure

Claims (1)

[Claims] 1. A method for obtaining a semipermeable membrane by casting a solution consisting of cellulose acetate having a degree of acetylation of 37.0 to 40.5 wt%, its solvent, and a nonsolvent (mixed with the solvent). (1) Use a nonsolvent whose boiling point is 20°C or more higher than the boiling point of the solvent, (2) use a nonsolvent in which the amount b (wt%) of the nonsolvent in the casting solution is 10 or more, and (a-1
5) <b<a, where a is the amount of non-solvent required to precipitate the polymer from the solution or gel the solution at the polymer (cellulose acetate) concentration and temperature during casting. (% by weight).) (3) Dry cellulose acetate semi-solvent, which is characterized in that after casting, the solvent and non-solvent in the solution are evaporated to make it substantially free of solvent and non-solvent. Method for manufacturing permeable membranes.