Hydrophilic film and process for producing the same

Details Hydrophilic film and process for producing the same Hydrophilic film and process for producing the same

1997-12-23

2001-04-10

Page, Thurman K. (Department: 1615)

Drug, bio-affecting and body treating compositions

Preparations characterized by special physical form

Matrices

C424S402000, C424S078100, C424S078080, C424S486000

Reexamination Certificate

active

06214382

ABSTRACT:

TECHNICAL FIELD
This invention relates to a hydrophilic membrane which can be obtained by making a hydrophobic membrane of an aromatic polymer hydrophilic and a method for producing it, and especially it relates to a hydrophilic membrane which is suitable for use in filtration such as a micro- filtration membrane, an ultra filtration membrane, a membrane for medical uses, etc. where any leaching from membranes is required to be limited especially small.
BACKGROUND ART
The method, where a hydrophobic membrane is made hydrophilic by adhering a hydrophilic polymer on a hydrophobic membrane through a process of first impregnating a hydrophobic membrane with a hydrophilic polymer solution, second eliminating this solvent by drying, is well known for a long time, and it may be said that this method is tried for almost all known hydrophilic polymers. Also, it is well known that cellulose derivatives can be used as a hydrophilic polymer. For examples, Examined Japanese Patent (JPB-56-16187(1981)) disclosed uses of methyl cellulose, ethyl cellulose, hydroxy ethyl cellulose and hydroxy propyl cellulose, and Unexamined Japanese Patent (JPA-62-176508(1987)) disclosed a use of hydroxy propyl cellulose of the molecular weight of not less than 10000.
Prior arts have put emphasis on points of making the hydrophobic membranes hydrophilic or making the hydrophilic polymer insoluble, and the arts to prevent leaching of even a quite small amount of the hydrophilic polymer have not sufficiently been studied. However, in fields of medical applications, electronics, etc., since even a quite small quantity of leaching substances from the membranes causes an important problem, it quite often happens that even a quite small quantity of leaching substances from the membranes is avoided essentially, and there are many cases of the prior arts cannot respond sufficiently to these requirements.
The cellulose derivatives used for making the hydrophobic membrane hydrophilic are produced by a chemical modification of a powder-like or flake-like cellulose. Therefore, some cellulose does not completely dissolve and stays in a dispersed state, and, thus, it may result that there is some chemically unmodified cellulose. In the case where the hydrophobic membrane is impregnated with a solution which contains such insufficiently modified cellulose derivatives, it may happen that not only a part of cellulose derivatives which does not dissolve completely clogs pores of the membrane, but the cellulose derivatives may leach out under such a severe condition as a sterilization by autoclaving, since the chemically insufficiently modified cellulose does not adhere molecularly and irreversibly to the surface of the hydrophobic membrane after drying (which means that the molecules once adhered do not detach from the hydrophobic membrane by dissolving again in water).
The subject of this invention is not only to make a hydrophobic membrane hydrophilic but also to provide a hydrophilic membrane from which a leaching substance is reduced as little as possible and which can be used suitably in such fields as medical applications, electronics, etc. where even a quite small quantity of a leaching substance from membranes is prohibited.
DISCLOSURE OF THE INVENTION
This invention is, in a first aspect, to provide a hydrophilic membrane wherein a hydrophilic cellulose derivative of a number average molecular weight between 2000~8000 is adsorbed irreversibly to a hydrophobic membrane of an aromatic polymer.
This invention is, in a second aspect, to provide a method of producing a hydrophilic membrane wherein a hydrophobic membrane is first impregnated with a solution of a hydrophilic cellulose derivative of a number average molecular weight between 2000~8000 and then the impregnated membrane is washed.
BEST MODE FOR CARRYING OUT THE INVENTION
This invention is to produce the hydrophilic membrane, which has a quite small amount of a leaching substance from the membrane, by adsorbing irreversibly to the hydrophobic membrane a hydrophilic cellulose derivative from which components which do not dissolve completely in a solvent are beforehand removed of by the fractionation. Though a filtration of a hydrophilic cellulose derivative solution is considered to be used as a usual method for removing insoluble parts of the cellulose derivative, this method is not an effective method, since the clogging of a filter does occur unexpectedly soon and the insoluble parts smaller than the pore of a filter remain in the filtrate.
Therefore, for removing of insoluble parts, this invention uses the fractionation where a poor solvent is used, which is usually used in the fractionation of polymers by a molecular weight (hereafter, called as the fractionation). According to this method, since insoluble parts precipitate as a large aggregate with higher molecular weight components, the insoluble parts can be surely removed.
As the hydrophilic cellulose derivatives, methyl cellulose, carboxy methyl cellulose, low substituted hydroxy propyl cellulose or hydroxy propyl methyl cellulose are preferable, and these are used individually or in combination of two or more.
These compounds are easy to handle, since these hydrophilic cellulose derivatives dissolve in water or in an aqueous alkali solution. On the solvents used in the fractionation for removing insoluble parts from these derivatives, there are no special limits and, for examples, either of water, ethanol, an aqueous solution of ethanol or ethanollan aqueous alkali solution can be used preferably. Further, it is well known that a molecular weight of a fraction can be adjusted by changing a solubility of the solvent. A large aggregate precipitated by a fractionation can be filtered off by using a filtration paper, but the components, which do not contain insoluble parts, can be also obtained by taking a supernatant in the case that large aggregates precipitate. The number average molecular weight of cellulose derivatives, which can be commercially available, is about 10,000~300,000 (See, for example, item on hydroxy propyl methyl cellulose in Japan Pharmacopoeia). Since the insoluble parts increase as the molecular weight increases, it is preferable to use a cellulose derivative of a molecular weight as small as possible as a starting material before the fractionation. A cellulose derivative of a molecular weight between 2,000 and 8,000 can be obtained by removing insoluble parts from such a starting material through use of the above mentioned fractionation.
The hydrophobic membranes used in this invention are composed of an individual or a combination of more than two of aromatic polymers such as aromatic polyethers, aromatic polyesters, aromatic polyamides or aromatic polysulfones. However, aromatic polysulfones such as a polyethersulfone or a polyarylethersulfone is preferable from points that the polysulfones are superior in the essential properties such as anti-chemicals, mechanical strength, heat resistance and filtration.
The Unexamined Japanese Patent (JPA-62-176508(1987)) mentions that a hydroxy propyl cellulose of an average molecular weight not less than 10,000 must be used because the hydroxy propyl group is an effective hydrophobic bond group, but-, since, according to studies of present invention, a methyl cellulose, a hydroxy propyl methyl cellulose and a low substituted hydroxy propyl cellulose, which contains about {fraction (1/10)} of hydroxy propyl group compared with the usual hydroxy propyl cellulose, can be adsorbed more strongly to a hydrophobic membrane than the usual hydroxy propyl cellulose, the hydrophobic group bond to aromatic groups can be considered to be a glucose group itself which constitutes a skeleton of cellulose. Though the hydroxy propyl group gives a hydrophilic property as well as the methoxy group, it may be considered that the hydroxy propyl group may prevent the hydrophobic bond to the polysulfone.
Therefore, in this invention, it is preferable to use a cellulose derivative, the substitution range of which is not more than 40% for giving a strong irreversible

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