Liquid purification or separation – Filter – Material
Reexamination Certificate
2000-04-05
2002-04-16
Fortuna, Ana (Department: 1723)
Liquid purification or separation
Filter
Material
C210S500290, C210S500300, C210S500310, C210S500320, C210S500330, C210S500270, C264S041000, C264S045100
Reexamination Certificate
active
06372136
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a cellulose acetate semipermeable membrane used for separation and concentration treatment in various fields besides water treatments such as drinking water treatment, sewage treatment and waste water treatment, and a process for producing the cellulose acetate semipermeable membrane.
PRIOR ART
Heretofore, a cellulose acetate membrane has been used as a material of separation membrane for various kinds of water treatment and for medical use in blood dialysis etc., for the reason of hydrophilicity and a little decrease in filtration rate, and widely used at present particularly in the form of a reverse osmosis membrane. For demand in practical use, the reverse osmosis membrane made of cellulose acetate is often constructed such that a separation active layer in the membrane is made very dense with a membrane pore diameter of 0.001 &mgr;m or less, but there arise various problems attributable to the membrane structure.
For example, JP-B 58-24164 discloses a reverse osmosis membrane made of cellulose acetate having a dense layer. However, since this reverse osmosis membrane has a dense layer, the operating pressure should be kept at high pressure as high as 1 MPa or more to increase the filtration rate, which however leads to problems including not only an increase in energy cost but also a decrease in water permeation rate and mechanical breakage of the membrane due to the compaction and densification of the membrane during filtrating operation. For application of this membrane t o a blood dialysis membrane, the membrane thickness has been rendered thin to raise the plasma separation rate, but this results in the general problem of easy breakage at low pressure.
The reverse osmosis membrane made of cellulose acetate has another problem that pinholes are generated due to the presence of micro-voids. JP-B 60-43442 discloses a method for suppressing the generation of pinholes and for improving membrane strength. However, this prior art does not solve the problem of a decrease in filtration rate during practical use, because the membrane has a substantially uniform structure containing no void layer.
JP-A 6-343842 and JP-A 8-108053 disclose a cellulose acetate hollow fiber separation membrane having a three-dimensional network-like part and a void part.
Another requirement of such a membrane material for various kinds of water treatment is that it is hardly degraded by microorganisms in order to suppress a decrease in its filtration ability and to increase the longevity of the membrane thus preventing an increase in operating cost.
The object of the present invention is to provide a cellulose acetate semipermeable membrane which has a high filtration rate at a low pressure, has high mechanical strength and is hardly degraded by microorganisms, and a process for producing the cellulose acetate semipermeable membrane.
Further, JP-A10-305220discloses a hollow fiber membrane fabricated by discharging a membrane-forming solution through a double pipe type spinning orifice while discharging an inside coagulating solution from a central pipe of the spinning orifice.
DISCLOSURE OF THE INVENTION
As a result of their eager study on the structure of cellulose acetate serving as the starting material for production of semipermeable membranes, the present inventor have found that 3 elements relating to the structure described above, which are specified by connecting them with one another, can act synergistically to achieve the object described above, thus completing the present invention.
That is, the present invention provides a cellulose acetate semipermeable comprising a cellulose acetate produced from cellulose having an &agr;-cellulose content of not less than 99% by weight, and having a 6 wt/vol % viscosity at 25±1° C. of 20 to 220 mPa·s and an acetylation degree of 58 to 62%. Also, it provides use of the cellulose acetate as a semipermeable membrane or a hollow fiber membrane.
Preferably, the semipermeable membrane is a hollow fiber membrane.
Further, the present invention provides a cellulose acetate hollow fiber membrane, wherein the thickness of the hollow fiber membrane is 100 to 400 &mgr;m, the cross-section of the hollow fiber membrane is composed of a three-dimensional network-like part and a void part, the void part is positioned inside 10 &mgr;m or more from both internal and external surfaces of the membrane, the area occupied by the void part is in the range of 5 to 60% of the total cross-sectional area of the membrane, a dense layer having a surface average pore size of 0.001 to 0.05 &mgr;m exists on each of the internal and external surfaces of the hollow fiber, and a crack-like muscular pattern (slit structure) is observed on the internal surface of the hollow fiber with an electron microscope at a magnification of ×20, 000.
Preferably, the cellulose acetate is produced from cellulose having an &agr;-cellulose content of not less than 99% by weight. Preferably, the 6% viscosity thereof is 50 to 200 mPa·s, and the acetylation degree thereof is 60.5 to 61.5%.
Further preferably, the pure water permeation rate is not less than 500 l/(m
2
·h), and the tensile strength at break is not less than 4 MPa.
Further, the present invention provides a process for producing a cellulose acetate semipermeable membrane, wherein a solution of the above-described cellulose acetate dissolved in a water-soluble, organic polar solvent, for example, dimethyl sulfoxide, N-methyl-2-pyrrolidone or dimethylacetamide, is used to produce the membrane.
Preferably, the solution is formed into a hollow fiber membrane by applying a wet or dry wet spinning process using a double pipe type spinning orifice.
Preferably, at least one selected from compounds containing metallic elements of the I to III groups of the periodic table, ethylene glycol and polyethylene glycol are dissolved with cellulose acetate.
Further, the present invention provides a process for producing a cellulose acetate hollow fiber membrane wherein the thickness of the membrane is 100 to 400 &mgr;m, the cross-section of the hollow fiber membrane is composed of a three-dimensional network-like part and a void part, the void part is positioned inside 10 &mgr;m or more from both internal and external surfaces of the membrane, the area occupied by the void part is in the range of 5 to 60% of the total cross-sectional area of the membrane, a dense layer having a surface average pore diameter of 0.001 to 0.05 &mgr;m exists on each of the internal and external surfaces of the follow fiber, and a crack-like muscular pattern (slit structure) is observed on the internal surface of the hollow fiber with an electron microscope at a magnification of ×20,000, comprising dissolving a cellulose acetate in a water-soluble, organic polar solvent, and then discharging the resulting membrane-forming solution from a double pipe nozzle while discharging an inside coagulating solution from the inner pipe of the double pipe to coagulate the membrane-forming solution in a coagulation bath.
Preferably, the linear velocity of the inside coagulating solution discharged is 8 times or more as high as the linear velocity of the membrane-forming solution discharged.
Preferably, the membrane-forming solution contains a compound containing metallic elements of the I to III groups of the periodic table in an amount of 0.2 to 1.5% by weight therein.
In addition, the present invention provides use, as water treatment or separation and concentration treatment, of a cellulose acetate hollow fiber membrane in which the thickness of the hollow fiber membrane is 100 to 400 &mgr;m, a cross-section of the hollow fiber membrane consists of a three-dimensional network-like part and a void part, the void part is positioned 10 &mgr;m or more inside from both internal and external surfaces of the membrane, the area occupied by the void part is in the range of 5 to 60% of the total cross-sectional area of the membrane, each of the internal and external surfaces of the follow fiber has a dense layer having a surface
Daicel Chemical Industries Ltd.
Flynn ,Thiel, Boutell & Tanis, P.C.
Fortuna Ana
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