JPS60220109A - Cellulose acetate for semi-permeable membrane and refining method thereof - Google Patents

Abstract

PURPOSE:To obtain a semi-permeable membrane of cellulose acetate of high strength and durability by using cellulose acetate free from a component of low polymerization degree having a polymerization degree of less than 100. CONSTITUTION:Cellulose is refined by extracting and removing a component of low polymerization degree having a polymerization degree of less than 100 with the use of an organic solvent or an aqueous organic solvent solution. Acetone, acetic acid and non-solvents such as methanol and ethanol are available as solvents here. The non-solvent need not be made into an aqueous solution. However, the solvent should be used in the form of aqueous solution so that the component of low polymerization degree may be dissolved, because cellulose acetate of high polymerization degree would not be dissolved. The concentration of solvent and water is 30-70 in terms of weight ratio, the temperature of fluid is room temperature -50 deg.C and the extraction time 60-90min. The extraction is effective, if it is divided into two stages. The quantity of a component of low polymerization degree to be removed is approx. 5%.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention relates to reverse osmosis, which has been put into practical use as a method for treating various liquids, ultra-permeable membranes, and semipermeable membranes used for artificial organs. This invention relates to purified cellulose acetate as a raw material and a method for purifying the same.

Reverse osmosis, which is a concentration, separation, and purification method that utilizes the selective permselectivity of semipermeable membranes, ultra-permeability, and other methods have been put into practical use in almost all industries that handle liquids in recent years, improving quality. , contributing to streamlining processes and saving energy. Furthermore, in the medical field, most artificial organs, especially artificial kidneys, use semipermeable membranes, and it would be no exaggeration to say that the performance of this semipermeable membrane controls almost all of their functions. isn't it.

(Prior art and its drawbacks) There are various materials for semipermeable membranes used in the reverse osmosis method, the ultrafonic method, etc., and in artificial kidneys, as described above, but they are difficult to obtain due to their ease of availability, water permeability, and solute content. Cellulose acetate is well known as the most commonly used material due to its good balance of exclusion properties, chlorine resistance, biocompatibility, etc.

By the way, when these semipermeable membranes for separation are put into practical use,
In most cases, except for those for artificial kidneys, they undergo a history of several months to years under pressurization of several kg/C- to several tens of kg/cIil.

For this reason, the hydrous gel-like semipermeable membrane becomes thick and dense, and especially in reverse osmosis membranes used under pressure of several tens of kg/cJ, the membrane thickness shrinks to 60 to 70% of its initial thickness. ,
Accompanying this, the permeation ability also decreases approximately proportionally.

A possible solution to this problem is to modify the semipermeable membrane itself in some way to improve its compaction resistance.

As one of these methods, for example, Japanese Patent Application Laid-Open No. 53-127374
shows a method of acetalizing the surface layer of a cellulose-based semipermeable membrane. However, in this method, only the surface layer of a semipermeable membrane with a thickness of 100 to 150μ, that is, less than 1% of the total thickness, is crosslinked, and overall, the semipermeable membrane is modified. In fact, the compaction resistance has not been improved. In fact, a decrease in membrane performance was observed. Therefore, the present inventors developed a method capable of modifying not only the surface layer of a semipermeable membrane but also the entire surface thereof, and arrived at the present invention.

(Structure of the present invention) This cellulose acetate is a semi-synthetic product made by reacting acetic acid with cellulose, which is a natural product, so it is semi-synthetic. ! While it is an excellent material, it also has some drawbacks due to being a natural product.

One of the reasons is that the degree of polymerization of cellulose has a wide distribution.
It was discovered that when used as a semipermeable membrane for separation, components with a relatively low degree of polymerization adversely affect the strength, durability, etc. of the semipermeable membrane, and the present invention as described below was completed.

That is, the present invention provides (1) cellulose acetate for semipermeable membranes, characterized in that low polymerization degree components having a polymerization degree of 100 or less are extracted and removed using an organic solvent or an aqueous solution of an organic solvent.

(2) Polymerization degree of 1 using an organic solvent or an organic solvent aqueous solution
This is a method for purifying cellulose acetate for semipermeable membranes, which is characterized by extracting and removing components with a low polymerization degree of 0.00 or less.

The solvent used may be acetate, which is a solvent for cellulose acetate, acetic acid, and alcohols, such as methanol or ethanol, which are non-solvents.

As for the extraction conditions, if a non-solvent is used, it is not necessarily necessary to make an aqueous solution, but if a solvent is used, an aqueous solution is used so that cellulose acetate with a high degree of polymerization does not dissolve and the components with a low degree of polymerization dissolve. It turns out.

The concentration of the aqueous solution is a weight ratio of solvent/water = 30 to 70, preferably 40 to 60, and the liquid temperature during extraction is room temperature to 50°C.
The extraction time is 60 to 90 minutes, and the extraction efficiency is improved by performing the extraction in two stages.

The amount of low polymers to be removed is about 5%, and it is appropriate to remove components with a degree of polymerization of 100 or less, or components with an average degree of polymerization of 70 or less.

In addition, this method can reduce the degree of acetylation of cellulose acetate from 40% to 60%.
Applicable to everything up to %.

Further, the cellulose acetate of the present invention can be formed into a semipermeable membrane using a conventionally known method, and the shape thereof can be any of a flat membrane, a cylindrical membrane, and a hollow fiber membrane.

(Effects of the present invention) When a semipermeable membrane is created using cellulose acetate from which low polymerization degree substances, that is, heterogeneous components have been extracted and removed, by the method of the present invention, each of the active layer and the porous structure supporting it is It is thought that the intertwining between the formed cellulose acetates became homogeneous and dense, improving the mechanical strength of the semipermeable membrane.

When the semipermeable membrane according to the present invention is put into practical use, it has the following advantages compared to a semipermeable membrane made from unpurified cellulose acetate.

b) The thickening resistance of the semipermeable membrane is improved and the membrane life is extended.

(1) The tensile strength of the semipermeable membrane is improved, improving workability during modularization.

Next, the effects of the present invention will be explained using Examples and Comparative Examples.

In addition, in the examples, the water permeation rate and the solvent removal rate are defined by the following formula.

Example-1 a) Purification of cellulose acetate b) Properties of raw material cellulose acetate flakes used: Degree of acetylation: 55.18% Degree of polymerization: 185 0) Solvent and aqueous solution concentration used for extraction: Acetone Acetone/water = 1 /1 c) Raw material cellulose acetate/solvent aqueous solution: 5o1768y 2) First extraction time, liquid temperature: 60 minutes, 35°C E) Second extraction time, liquid temperature: 15 minutes, to 35°C) After extraction Cellulose acetate weight: 47.8. ! 9 g) Low polymerization fabric extraction rate:
4.4% h) Degree of polymerization of cellulose acetate after extraction: 192. 95% b) Production of semipermeable membrane Semipermeable membrane i was prepared using the cellulose acetate obtained in a) under the following composition and conditions.

Purified cellulose acetate: 22 parts by weight 1.4-dioxane: 20 Acetone: 47 parts by weight Magnesium perchlorate-11 Evaporation time: 30 seconds Casting thickness Near 0 μm Heat treatment temperature Near 3°C Heat treatment time: 15 minutes Coagulation bath, Temperature: Water, 2℃ C) Semipermeable membrane performance: 35kg/c111 using 1800 ppm saline solution
Measurements were made under pressure, and the following performance was obtained.

Initial water permeation rate: 0.91 rn'/m', day Solute removal rate: 93.2% Water permeation rate after 24 hours: 0.9 Off+'/rn',
[1〃〃 Solute removal rate: 94.0% d) Semipermeable membrane strength The breaking strength was measured using a Tensilon tensile strength tester, and a value of 1.59 kg/cm was obtained.

Comparative Example 1 The same procedure as Example 1 was carried out except that cellulose acetate was used before extraction of the low degree of polymerization component, and the following results were obtained.

a) Semipermeable membrane performance Initial water permeation rate: 1.08rn'/rn", day Solute removal rate: 93.8% After 24 hours, water production rate: 0.88m'/rn", day//
7/ Solute removal rate: 89° 8% b) Semipermeable membrane strength: 1.36 kg/cm Example-2 The solvent aqueous solution used for purification had the following composition, and the obtained cellulose acetate and extracted cellulose acetate properties of,
The same procedure as in Example 1 was carried out except that the extraction amount was changed as shown below, and the following results were obtained.

a) Purification of cellulose acetate a) Concentration of solvent and aqueous solution used for extraction: acetic acid/water - 45155 0) Raw material cellulose acetate/solvent aqueous solution: 67 g for the first time
/ 1000ml, 2nd 671115 oynt
c) 1st extraction time, liquid s2 60 minutes, 30°C 2) 2nd extraction time, liquid temperature: 60 minutes, 30°C e) Cellulose acetate weight after extraction: 64.09) Low polymerized fabric Extraction rate = 4
4% G) Degree of polymerization of cellulose acetate after extraction: 190 H) Degree of acetylation: 55.18% S) Degree of polymerization of extracted low polymer: 56 N) Degree of acetylation of cellulose acetate: 53.04% b) Semipermeable membrane performance Initial water permeation rate: 1.02 m'/m', day 〃 Solute removal rate: 93.0% After 24 hours, hour rear water velocity, 97 moromi/momi, day i 〃 Solute removal rate: 93. 44 C) Semipermeable membrane strength: 1.54 kg/cm Based on the above results, semipermeable membranes made from cellulose acetate from which low polymerization degree components were extracted and removed were made from cellulose acetate that still contained low polymerization components. This shows that the method of the present invention is effective, with less deterioration in performance than semipermeable membranes.

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Claims (2)

[Claims] (1) Cellulose acetate Q for semipermeable membranes, characterized in that low polymerization degree components with a polymerization degree of 100 or less are extracted and removed using an organic solvent or an aqueous solution of an organic solvent. (2) Polymerization degree of 1 using organic solvent or organic solvent main solution
A method for purifying cellulose acetate for semipermeable membranes, which comprises extracting and removing components with a low polymerization degree of 0.00 or less.