Liquid purification or separation – Filter – Material
Reexamination Certificate
1999-02-12
2001-09-04
Fortuna, Ana (Department: 1723)
Liquid purification or separation
Filter
Material
C210S500270, C210S500230
Reexamination Certificate
active
06284137
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an asymmetric polysulfone porous membrane which is a porous membrane capable of being effectively used in ultra filtration and precision filtration of fluid, and the membrane is comprising a dense layer functioning as a separation layer, and a supporting layer having a graded pore size distribution wherein the size of the existing pores therein gradually increases from the dense layer side to the opposite surface, and to a method of manufacturing the same. The present invention relates, especially, to an asymmetric polysulfone porous membrane capable of being used from a dry condition without any treatment to impart hydrophilicity, and the membrane having an excellent resistance to moist heat, having an excellent re-percolation characteristic (a permeability of a membrane when water is percolated at a second time, after the membrane is percolated with water and dried), and having an especially excellent re-percolation characteristic to hot water, and to a method of manufacturing the same.
BACKGROUND OF THE INVENTION
Various development efforts to have porous separation membrane having a good separation property and being capable of allowing hot water to permeate have been made, and a polysulfone porous membrane has especially received much attention. The invention of a thermal resistant polysulfone porous membrane using polysulfone according to the general formula of (1) and polyethersulfone according to the general formula of (2) are described in Japanese Laid-Open Patent Publication Nos. Sho63-99325 and Hei5-137982.
Among the above-mentioned inventions, the polysulfone porous membrane using the polysulfone of the formula (1) has a fairly good permeability, but it has a drawback wherein the membrane ruptures or becomes unsuitable to be used as a separation membrane due to its morphological change when it is subjected to a treatment of moist heat under a high temperature and a high pressure.
In addition, although the polyethersulfone porous membrane using the polyethersulfone of the formula (2) is characterized by having a relatively good re-percolation characteristic and resistance to moist heat, its performance is not sufficient as a ultra filtration membrane or as a precision filtration membrane because its water flux is extremely low by being at not more than 2 L/m
2
.hr.mmHg.
An invention directed to a polyallylsulfone porous membrane having a chemical formula of (3) is described in Japanese Laid-Open Patent Publication No. Hei1-184001. This polyarylsulfone porous membrane has an extremely high resistance to moist heat, however, it has a drawback that its water flux is low at not more than 2 L/m
2
.hr.mmHg and it has a low hydrophilicity.
The inventions directed to a porous membrane comprising a mixture of polysulfone and polyethersulfone are described in Japanese Laid-Open Patent Publication Nos. Sho54-144456 and Sho62-221402. The polysulfone porous membrane of this type has a relatively high water flux, but has a drawback that its resistance to moist heat is not sufficient and its water flux decreases rapidly when a hot water having a temperature not less than 80° C. is percolated for an extended period of time, and while its mechanical strength is not sufficient and its ease of handling is not satisfactory.
In addition, in case of a porous membrane obtained from materials of polysulfone and polyethersulfone, because the material itself is hydrophobic, during the manufacture of a membrane by a wet forming method, a process of imparting hydrophilicity to the membrane has been employed, wherein pores are formed by a solidification process while adding a large quantity of hydrophilic additives such as polyethylene glycol and polyvinylpyrrolidone to the dope, or then after these hydrophilic additives are cross-linked, or a hydrophilic polymer is coated on the membrane surface after the formation of pores followed by maintaining the hydrophilic polymer on the membrane surface.
Generally, to provide a sufficient hydrophilicity to the membrane, it is necessary to have the equilibrium rate of moisture content of the membrane at more than 2% by weight by employing the above mentioned treatment.
There was a problem where the hydrophilic additive or the hydrophilic polymer of these types dissolves during the use of the membrane from the membrane thus obtained by these manufacturing methods, because a large quantity of hydrophilic additives or hydrophilic polymer and the like are retained inside of the membrane or on the surface thereof.
Furthermore, in case of a hollow fiber membrane obtained by the conventional wet forming methods to increase the water flux, it was necessary to reduce the membrane thickness or to increase the porosity of the membrane. However, when such operation is employed, a drawback may be encountered wherein the ease of handling of the membrane, and thus the ease of handling during a production process, decreases because the mechanical properties (breaking strength and breaking extension) decrease.
DISCLOSURE OF THE INVENTION
In view of these facts, the inventors of the present invention carried out various tests to obtain polysulfone porous membrane having a high water flux, an excellent resistance characteristic to moist heat, no requirement for a treatment to make the membrane hydrophilic, a good re-percolation property, and an excellent mechanical strength, and then established the present invention after having found that a desired polysulfone porous membrane can be manufactured from a polymer mixture obtained by mixing polyarylsulfone of the formula (3) and polyethersulfone of the formula (2) at a specific weight ratio.
The gist of the present invention relates to a polysufone porous membrane which comprises a mixed polymer having polyarylsulfone and polyethersulfone at a ratio of 9/1-1/9 (by weight), the membrane being a graded porous membrane forming a cross-linked polymer network, wherein a number of micropores being interconnected from one surface to the other surface, and the membrane comprising two layers wherein one layer being a dense layer having a number of pores with pore sizes of 0.01-1 &mgr;m and thereby having a separation function, and the other layer being a graded type supporting layer supporting said dense layer and having a pore size distribution wherein the pores existing therein and having sizes of 1-100 &mgr;m continuously increase from the dense layer side toward the other surface side, wherein the separation particle size being not larger than 0.5&mgr;m, and a method of manufacturing the same.
BEST EMBODIMENT OF THE INVENTION
With the polysulfone porous membrane of the present invention, by maintaining of the ratio of composition of polyarylsulfone (hereinafter referred to as PAS) to polyethersulfone (hereinafter referred to as PES) at PAS/PES=9/1-1/9 by weight, or preferably at 7/3-3/7 by weight, the water flux thereof can be especially set at a high value of being not less than 7L/m
2
.hr.mmHg, and the resistance to moist heat can be good whereby a good permeability can be maintained even when hot water having a temperature of not less than 80° C. is percolated for an extended period of time. It also has a large advantage that the permeation pressure required when a membrane in a dry condition is percolated can be set at an extremely low pressure of being not more than 0.5 Kg/cm
2
.
The polysulfone porous membrane made from a polysulfone mixture having a mixing ratio of PAS and PES by weight being below 1/9 is not preferable, because it has a poor resistance to moist heat and its permeation performance decreases drastically when hot water having a temperature of not less than 80° C. is percolated for an extended period of time.
On the other hand, polysulfone membrane made from a polysulfone mixture having a mixing ratio of PAS/PES by weight being more than 9/1 does not show a sufficient permeability, and it shows a tendency to require a higher permeation pressure especially when a membrane in a dry condition is percolated with water.
Any component other than th
Hajikano Akira
Kamo Jun
Murase Kei
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Fortuna Ana
Mitsubishi Rayon Co. Ltd.
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