JPH0751553A - Cellulose acetate membrane with improved heat resistance - Google Patents

Abstract

PURPOSE:To provide a cellulose acetate membrane with improved heat resistance by blending cellulose triacetate and cellulose diacetate. CONSTITUTION:The cellulose acetate membrane is made from a mixture in which cellulose triacetate and cellulose diacetate are blended in a specified ratio. Sterilization with a high pressure steam is applicable to the hollow fibers made from this mixture when they are used for clarifying blood; water permeability and a screening coefficient of myoglobin are also improved by the sterilization.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a separation membrane having improved heat resistance, and more particularly to a cellulose acetate membrane capable of high-pressure steam bacteria.

[0002]

2. Description of the Related Art Cellulose acetate, which has an excellent balance of hydrophilicity and hydrophobicity, is used not only for industrial membranes such as gas separation membranes, reverse osmosis membranes, ultrafiltration membranes and microfiltration membranes, but also artificial kidneys as blood purification membranes. Widely used in. However, for example, when using cellulose triacetate, cellulose diacetate membrane as a blood purification hollow fiber membrane,
When high-pressure steam sterilization (for example, 120 ° C., 20 minutes) is performed, the membrane structure is changed due to crystallization and oriented crystallization, respectively, and water permeability and solute permeability are significantly reduced.

[0003]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a cellulose acetate membrane with thermal stability that can withstand high pressure steam sterilization.

[0004]

Means for Solving the Problems As a result of intensive studies for solving the problems of the prior art, the present invention has been achieved. That is, the present invention is a mixture of cellulose triacetate and cellulose diacetate, substantially does not have a melting peak of cellulose diacetate by differential thermal analysis, and substantially has a melting peak of cellulose triacetate, thermal A cellulose acetate membrane having improved stability and a mixture of cellulose triacetate (CTA) and cellulose diacetate (CA), which has substantially no melting peak of cellulose diacetate by differential thermal analysis, and A cellulose acetate membrane having a thermal melting point of cellulose triacetate and having improved thermal stability. The water permeability ratio before and after high-pressure steam treatment (water permeability AC
After) / (before water permeability AC) is 1.1 or more, and the sieving coefficient ratio of myoglobin (after sieving coefficient AC) / (before sieving coefficient AC) is 1.1 or more, which is a cellulose acetate membrane with improved heat resistance. is there.

The term "cellulose acetate" as used by the present inventor means a mixture of cellulose triacetate having an acetylation degree of about 60 to 62% and cellulose diacetate having an acetylation degree of about 54 to 56%. The acetylation degree is calculated by the following formula. Acetylation degree (%) = (mass in terms of acetyl group acetic acid / mass of cellulose acetate) × 100

The conventional cellulose triacetate and cellulose diacetate membranes have glass transition temperatures of 110 to 120 ° C. and 90 to 100 ° C., respectively, which are lower than those under high pressure steam sterilization conditions in a wet state. Therefore, when high-pressure steam sterilization is performed on each of the cellulose triacetate film and the cellulose diacetate film, crystallization and oriented crystallization occur because they correspond to heat treatment near the glass transition temperature in a wet state. As a result, the membrane structure is changed and the water permeability and the permeability are significantly reduced. That is, the cellulose triacetate hollow fiber membrane is contracted in the radial direction and the yarn length direction due to crystallization, and the cellulose diacetate hollow fiber membrane is contracted in the radial direction due to the oriented crystallization and remarkably elongated in the yarn length direction, resulting in membrane performance. It will drop significantly.

Therefore, paying attention to the compatibility of cellulose triacetate and cellulose diacetate, it was found that the two properties are blended at an appropriate ratio by utilizing their contradictory properties, and microphase separation formation is carried out. It was found that the cellulose triacetate membrane has a water permeability and solute permeability retention effect.

When the ratio of cellulose triacetate / cellulose diacetate is 95/5 to 50/50, the water permeability and the sieving coefficient of myoglobin after the high pressure steam sterilization increase 1.1 times or more as compared with those before the treatment. It has been found that performance can be improved by steam sterilization. If the mixing ratio of cellulose diacetate is 5% or less, no effect is observed, and if it exceeds 50%, the water permeability and the sieving coefficient of protein (myoglobin) decrease again.

The water permeability and the sieving coefficient here are defined as follows. Water permeability = V / (A · P · T) where V: Water permeability (m1) A: Membrane area (m ² ) P: Pressure (mmHg) T: Time (hr) Sieving coefficient = 2Cf / (Ci + Co) where Cf: protein concentration in filtrate Ci: protein concentration before filtration Co: protein concentration after filtration

X-ray diffraction measurements were performed on hollow fiber membranes composed of cellulose triacetate, cellulose diacetate and mixtures thereof, which had been subjected to high-pressure steam sterilization. As a result, all had a CTA II type crystal structure. It is difficult to distinguish each from.
However, the differential thermal analysis result of each cellulose acetate hollow fiber membrane mixed with a freeze-pulverized product is only observed as a superposition of those peaks, but the one prepared by preliminarily mixing each is cellulose As the mixing ratio of diacetate increases, the cold crystallization peak of cellulose triacetate shifts to the high temperature side, the melting peak of cellulose triacetate does not change its position,
The melting peak of cellulose diacetate disappeared, and the thermal characteristics were improved.

The membrane of the present invention, which is necessary to impart heat resistance, further improves the characteristics of cellulose triacetate itself. A film composed of a mixture of cellulose triacetate and cellulose diacetate has a structure having cellulose triacetate as a main skeleton, and the addition of cellulose diacetate suppresses its crystallization, and the mixing ratio of cellulose triacetate and cellulose diacetate is 95/5. At -50/50, the permeability and sieving coefficient are greatly improved by controlling the interaction of crystallization and oriented crystallization.

The mixture of cellulose acetate referred to in the present invention requires that cellulose triacetate and cellulose diacetate are compatible with each other. Therefore, it is desirable that the cellulose triacetate and the cellulose diacetate are once dissolved in a solvent and then molded.
When the hollow fiber membrane is molded, a boiling point of 150 ° C. is obtained by adding a non-solvent so that the concentration of cellulose triacetate and cellulose diacetate is between 10 and 40% by weight.
The dope dissolved in the above non-polar solvent is passed through a double tube nozzle and extruded into a coagulation bath consisting of an aqueous solution of the above solvent and a non-solvent to form a film. Mixing ratio of solvent / non-solvent is 5
0/50 to 90/10 is suitable. Extrusion of the dope from the nozzle to the coagulation bath is carried out by a wet spinning method in which the dope is extruded directly into the coagulation bath, an air gap is provided between the nozzle and the surface of the coagulation bath, and the dope is extruded from the nozzle into the air to the coagulation bath. Any spinning method can be applied.

Further, when the dope is discharged from the nozzle, it is possible to spin a liquid such as liquid paraffin inside the hollow fiber, but in the present invention, spinning is performed while an inert gas is flown into the hollow fiber at an appropriate flow rate. Membrane is desirable.

It is preferable to use an aprotic polar solvent as the solvent, and N-methylpyrrolidone, dimethylformamide, dimethylacetamide, dimethylsulfoxide, etc. are used as the solvent, and ethylene glycol, triethylene glycol, polyethylene glycol, glycerin,
Polyhydric alcohols such as polypropylene glycol and alcohols such as methanol and ethanol can be used. Further, this hollow fiber is hydrophilized in a hydrophilization bath through a coagulation bath and a washing bath. As the hydrophilizing agent, polyhydric alcohols such as glycerin and polyethylene glycol, and alcohols such as methanol and ethanol can be used. Then, the hollow fiber membrane is wound through a drying process.

The cellulose acetate mixture of the present invention having improved heat resistance can be used not only for hollow fiber membranes such as blood purification membranes and desalination reverse osmosis membranes, but also for forming films and the like. The present invention will be described in detail below with reference to Examples and Comparative Examples.

[0016]

【Example】

Example 1 Cellulose triacetate having an oxidation degree of 60.8% / cellulose diacetate having an acetylation degree of 55.0% were used in a weight ratio of 8/2,
A dope was prepared by mixing a polymer concentration of 19%, N-methylpyrrolidone 64% and ethylene glycol 16%. Using a double ring nozzle, N-
Methylpyrrolidone 32%, Triethylene glycol 8%
The mixture was discharged into a coagulating liquid composed of an aqueous solution, and a hollow fiber membrane was obtained through a washing bath and a glycerin bath. The discharge temperature and the coagulation bath temperature were 120 ° C and 35 ° C, respectively. The obtained hollow fiber membrane was a perfect circle having an outer diameter of about 250 μm and a film thickness of about 20 μm. Approximately 5 cm of this hollow fiber membrane, bundled with 100, isopropyl alcohol / cyclohexane mixed solvent 100/0, 7
The solution was subjected to solvent substitution under ice cooling at 5/25, 50/50, 25/75, 0/100, and dried under reduced pressure overnight. About 10 mg of this was packed in an aluminum pan for DSC measurement, and a Perkin Elmer DSC-7 was used under a nitrogen atmosphere at a temperature rising rate of 20 ° C./min for 30 times.
Differential thermal analysis was performed in the temperature range of ˜350 ° C. The results are shown in Table 1 and FIG. The following procedure was used to evaluate the dialysis performance. 120 hollow fiber membranes, 25
The test pieces were bundled in a length of cm and bonded to an acrylic pipe with urethane to prepare a mini module for testing. This mini-module was autoclaved at 121 ° C. for 20 minutes. The water permeability is 3 on the blood side using the mini module.
Flow pure water kept at 7 ° C, stop the flow, and apply a pressure of 150 mmHg according to the method of Klein et al. did. Regarding the sieving coefficient of myoglobin, a pressure of 100 mmHg was applied to the myoglobin aqueous solution kept in a box kept at 37 ° C., and 20 ml / min was applied to the blood side of the mini-module. The sieving coefficient was determined by measuring the myoglobin concentration in the outlet / inlet and the filtrate. Table 2 shows the results of the water permeability before and after high-pressure steam sterilization and the sieving coefficient of myoglobin. Table 2 shows that the water permeability after sterilization under high pressure and the sieving coefficient of myoglobin are improved.

Example 2 A hollow fiber membrane was produced under the same conditions as in Example 1 with the same cellulose triacetate / cellulose diacetate as in Example 1 in a weight ratio of 7/3, and differential thermal analysis was performed. The results are shown in Table 1 and FIG. Table 2 shows the results of the water permeability before and after high-pressure steam sterilization and the sieving coefficient of myoglobin. Table 2 shows that the water permeability after sterilization under high pressure and the sieving coefficient of myoglobin are improved.

Example 3 A hollow fiber membrane was produced under the same conditions as in Example 1 with the same cellulose triacetate / cellulose diacetate as in Example 1 in a weight ratio of 6/4, and differential thermal analysis was conducted. The results are shown in Table 1 and FIG. Table 2 shows the results of the water permeability before and after high-pressure steam sterilization and the sieving coefficient of myoglobin. Table 2 shows that the water permeability after sterilization under high pressure and the sieving coefficient of myoglobin are improved.

Example 4 A hollow fiber membrane was produced under the same conditions as in Example 1 with the same cellulose triacetate / cellulose diacetate as in Example 1 in a weight ratio of 5/5, and differential thermal analysis was conducted. The results are shown in Table 1 and FIG. Table 2 shows the results of the water permeability before and after high-pressure steam sterilization and the sieving coefficient of myoglobin. Table 2 shows that the water permeability after sterilization under high pressure and the sieving coefficient of myoglobin are improved.

Comparative Example 1 A hollow fiber membrane was produced under the same conditions as in Example 1 with the same cellulose triacetate / cellulose diacetate as in Example 1 in a weight ratio of 95/5, and differential thermal analysis was conducted. Table 2 shows the results of the water permeability before and after high-pressure steam sterilization and the sieving coefficient of myoglobin. According to Table 2, the water permeability and the sieving coefficient of myoglobin markedly deteriorate the performance after high-pressure steam sterilization as compared with the initial performance, and cannot be put to practical use.

Comparative Example 2 The differential thermal analysis was carried out on the cellulose triacetate membrane alone. The results are shown in Table 1 and FIG. Table 2 shows the results of the water permeability before and after high-pressure steam sterilization and the sieving coefficient of myoglobin. According to Table 2, the water permeability and the sieving coefficient of myoglobin markedly deteriorate the performance after high-pressure steam sterilization as compared with the initial performance, and cannot be put to practical use.

Comparative Example 3 The differential thermal analysis was carried out on the cellulose diacetate membrane alone. The results are shown in Table 1 and FIG. Table 2 shows the results of the water permeability before and after high-pressure steam sterilization and the sieving coefficient of myoglobin. According to Table 2, the water permeability and the sieving coefficient of myoglobin markedly deteriorate the performance after high-pressure steam sterilization as compared with the initial performance, and cannot be put to practical use.

Comparative Example 4 Differential thermal analysis was carried out on a mixture of cellulose triacetate / cellulose diacetate membranes at a weight ratio of 1/1. When they are simply mixed, they are observed as a superposition of peaks in the differential thermal analysis. The results are shown in Table 1 and FIG.

[0024]

[Effects of the Invention] By mixing cellulose triacetate and cellulose diacetate in an appropriate ratio to control the respective crystallization and oriented crystallization by high-pressure steam sterilization, water permeability after high-pressure steam sterilization and myoglobin A cellulose acetate membrane capable of increasing the sieving coefficient can be provided.

[0025]

[Table 1]

[0026]

[Table 2]

[Brief description of drawings]

FIG. 1 is a chart showing the results of differential thermal analysis of a cellulose acetate film.

Claims (2)

[Claims] 1. A mixture of cellulose triacetate and cellulose diacetate, which has substantially no melting peak of cellulose diacetate by differential thermal analysis,
A cellulose acetate film having substantially the melting peak of cellulose triacetate and improved thermal stability. 2. A mixture of cellulose triacetate (CTA) and cellulose diacetate (CA),
A cellulose acetate membrane having substantially no melting peak of cellulose diacetate by differential thermal analysis and substantially having a melting peak of cellulose triacetate, which has improved thermal stability, before and after high-pressure steam treatment (AC). Permeability ratio (after water permeability AC) / (before water permeability AC) is 1.1
Above, the sieving coefficient ratio of myoglobin (after sieving coefficient AC) /
A cellulose acetate film having improved heat resistance (before sieving coefficient AC) of 1.1 or more.