Liquid purification or separation – Filter – Material
Reexamination Certificate
2001-01-10
2003-02-04
Kim, John (Department: 1723)
Liquid purification or separation
Filter
Material
C210S500100, C210S500210, C210S500270
Reexamination Certificate
active
06514409
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a hollow fiber membrane made of an ethylene-vinyl alcohol polymer (hereinafter the term “ethylene-vinyl alcohol” is simply referred to as “EVA,” and the term “hollow fiber membrane made of an EVA polymer” is simply referred to as “EVA hollow fiber membrane”), and a process for producing the hollow fiber membrane. More particularly, the present invention related to an EVA hollow fiber membrane used for hemopurificafion membranes such as hemodialysis membranes, hemodiafiltration membranes, hemofiltration membranes and continuous hemofiltration membranes, and a process for producing the same.
2. Discussion of the Related Art
Hollow fiber membranes made of an EVA polymer have been widely used in applications for various separation membranes for industrial, medical and other purposes since the hollow fiber membranes are excellent in hydrophilicity (see Japanese Patent Laid-Open No. Hei 5-42208). Especially, the hollow fiber membranes are excellent in biocompatibility and chemical stability, and have very little eluted substances, so that the hollow fiber membranes have been widely used for medical applications. Their representative uses include, for instance, hemodialysis filtration membranes.
Japanese Examined Patent Publication No. Sho 58-36602, Japanese Patent Laid-Open Nos. Sho 58-45239 and Hei 5-42208, and the like disclose membranes having an asymmetric structure comprising a dense layer in the inner surface predominantly imparting fractionalization and permeability, and a porous layer supporting the dense layer as an EVA hollow fiber membrane having both high permeability and high fractionalization.
Recently, it has been greatly desired to remove by dialysis not only low molecular weight substances (molecular weight: less than 1000) such as urea and creatinine but also moderate or high molecular weight substances (molecular weight: 1000 to 40000 or so) represented by, for instance, &bgr;
2
-microglobulin (molecular weight: 11800) (hereinafter simply referred to as “&bgr;
2
-MG”). The EVA hollow fiber membrane having the above-mentioned asymmetric structure has been developed mainly for the purpose of eliminating the low molecular weight substances such as urea and creatinine contained in blood. Accordingly, the EVA hollow fiber membrane does not sufficiently satisfy the elimination for moderate or high molecular weight substances.
An object of the present invention is to provide an EVA hollow fiber membrane being excellent in elimination of moderate or high molecular weight substances and having little loss of albumin.
Another object of the present invention is to provide a process for producing the hollow fiber membrane.
These and other objects of the present invention will be apparent from the following description.
SUMMARY OF THE INVENTION
According to the present invention, there is provided a hollow fiber membrane made of an ethylene-vinyl alcohol polymer, comprising a dense layer existing in the inner surface and a porous layer existing in the layer other than the dense layer, wherein the hollow fiber membrane has a porosity of 60 to 90%, an overall mass transfer coefficient for myoglobin in a water-based system of not less than 0.003 cm/min., and a rejection rate for albumin in a bovine blood system of not less than 97%.
In a preferred embodiment of the EVA hollow fiber membrane, the clearance for urea in a bovine blood system is not less than 175 mL/min, and the clearance for &bgr;
2
-microglobulin is not less than 35 mL/min per a membrane area of 1.6 m
2
at a blood flow rate of 200 mL/min and a dialysate flow rate of 500 mL/min.
Also, in another preferred embodiment of the EVA hollow fiber membrane, the hollow fiber membrane has a rate of pore area of not less than 30% at the depth of 1 &mgr;m from the inner surface, and a diameter of a nodule forming the dense layer of 5 to 50 nm.
In addition, the present invention provides a process for producing a hollow fiber membrane made of an ethylene-vinyl alcohol polymer, comprising the steps of extruding a dope for forming a membrane comprising an ethylene-vinyl alcohol polymer and a solvent from a double annular nozzle, with pouring a hollow forming agent in the internal side of the double annular nozzle, the dope being transparent and homogeneous at a high temperature but inducing phase separation when its temperature is lowered, passing the hollow fiber membrane extruded from the double annular nozzle through the air, and thereafter introducing the hollow fiber membrane into a water bath, wherein a temperature (° C.) of the dope for forming a membrane (T
D
), a temperature (° C.) for phase separation (LST) and a temperature (° C.) of the air through which the hollow fiber membrane extruded from the double annular nozzle is passed (T
A
) satisfy the following relationships:
5≦LST≦40,
T
D
≦LST+20, and
T
A
≦LST.
DETAILED DESCRIPTION OF THE INVENTION
In the EVA hollow fiber membrane of the present invention, the porous layer existing in the layer other than the dense layer has relatively large pores. The shape of the pores can be arbitrarily chosen, so long as its influence to the permeability is small. The shape can be any of network structure, macro-voids, and the like.
Since the porosity of the EVA hollow fiber membrane of the present invention is not less than 60%, the permeability of the moderate or high molecular weight substances is high. Also, since the porosity is not more than 90%, its mechanical strength is high. It is preferable that the porosity is 65 to 85%.
The porosity is calculated by the equation:
Porosity (%)=(
W
W
−W
D
)/&rgr;
W
/W
D
/&rgr;
E
+(
W
W
−W
D
)/&rgr;
W
×100,
wherein W
W
is the weight of a water-containing membrane; W
D
is the weight of a dry membrane; &rgr;
W
is specific gravity of water; and &rgr;
E
is specific gravity of EVA.
The overall mass transfer coefficient for myoglobin of the EVA hollow fiber membrane in a water-based system is not less than 0.003 cm/min, preferably not less than 0.005 cm/min, from the viewpoint of increasing the removing efficiency of the moderate or high molecular weight substances.
The rejection rate for albumin of the EVA hollow fiber membrane in a bovine blood system is not less than 97%, preferably not less than 98%, from the viewpoint of suppressing the leakage of proteins useful for human bodies to a minimal level.
In addition, the clearance for urea in a bovine blood system is preferably not less than 175 mL/min, more preferably not less than 180 mL/min, still more preferably not less than 185 mL/min per membrane area of 1.6 m
2
at a blood flow rate of 200 mL/min and a dialysate flow rate of 500 mL/min, from the viewpoint of increasing the removing efficiency of the low molecular weight substances.
The clearance for &bgr;
2
-MG in a bovine blood system is preferably not less than 35 mL/min, more preferably not less than 40 mL/min, still more preferably not less than 45 mL/min per membrane area of 1.6 m
2
at a blood flow rate of 200 mL/min and a dialysate flow rate of 500 mL/min, from the viewpoint of increasing the removing efficiency of the moderate or high molecular weight substances.
In the present invention, the overall mass transfer coefficient for myoglobin, the rejection rate for albumin, the clearance for urea and the clearance for &bgr;
2
-MG can be determined in accordance with the evaluation criteria for dialyzer property [authored by T. Satoh et al.:
Kakushu no Ketsuekijokaho no Kino to Tekio
-
Ketsuekijokaki no Seinohyokaho to Kinobunrui
(
Functions and Application of Various Methods for Hemopurification
-
Performance Evaluation Methods and Function Classifications of Hemopurification Devices
), Tosekikaishi, published by Shadanhojin Nippon Toseki Igakukai, 29(8), 1231 -1245, 1996].
The overall mass transfer coefficient for myoglobin is calculated from the clearance determined in a water-based system (filtration flow rate Q
F
′=0 mL/min/m
2
) in accordance with the equation:
K
(
Kakiuchi Tomoki
Matsumoto Yoichi
Nakaji Shuhei
Sekiguchi Koji
Suehiro Takeshi
Kim John
Kuraray Co. Ltd.
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