CA1042166A

CA1042166A - Hollow fibers of acrylonitrile polymers for ultrafilter and method for producing the same

Info

Publication number: CA1042166A
Application number: CA169,757A
Authority: CA
Inventors: Yasuo Hashino; Masatsugu Yoshino; Hitoshi Sawabu; Tsugio Konno
Original assignee: Asahi Kasei Kogyo KK
Current assignee: Asahi Kasei Corp
Priority date: 1972-04-28
Filing date: 1973-04-27
Publication date: 1978-11-14
Also published as: DE2321460A1; DE2321460B2; IT982837B; US3871950A; FR2182202A1; FR2182202B1; GB1434055A; DE2321460C3

Abstract

ABSTRACT OF THE DISCLOSURE
A hollow fiber of acrylonitrile polymer for ultrafilter use, said acrylonitrile polymer containing at least 60 mol % of acrylonitrile units, having a structure consisting of (1) a reticulate porous region concentric with the central axis of the hollow fiber; the reticulate porous region including therewithin at least one succession of voids concentric with the central axis of the hollow fib??? and (2) a gradient-type porous region adjacent to the outer portion of the reticulate porous region or to each of the outer and inner portions of the reticulate porous region and con-centric with the reticulate porous region and the central axis of the hollow fiber; the average sized of pores in a gradient-type porous region decreasing from about 5µ-1µ adjacent the reticulate porous region to about 0.1µ-several .ANG. at the surface of the hollow fiber, is prepared by dissolving, at a temperature in the range of -5°C to 0°C, an acrylonitrile polymer containing at least 60 mol% of acrylonitrile units in a nitric acid aqueous solution having a concentration in the range of 65-95% by weight to prepare a spinning solution, having a polymer concentration in the range of 2-40% by weight, extruding the resultant solu-tion through spinnorettes having ? ring form orifices into an aqueous toagulation bath, having a concentration of nitric acid of 30% by weight or less, at a temperature of 4-50°C and con-comitantly feeding a liquid as an internal coagulant into the central part of the ring form orifices.

Description

104~
~escription of the Inventlon The present lnventlon relate6 to hollow flbers of acrylonltrlle polymers for ultrafilter use and a methot for produclng the same. More partlcularly, lt relates to hlgh performance hollow flbers of acrylonltrlle polymers for ultra-fllter use, havlng hlgh water-permeablllty, ln whlch pore slzes are extremely small and reasonably unlform, and the method for produclng the same.
An ob~ect of the present inventlon ls to provlde novel hollow flbers for ultrafllter use, havlng hlgh water-permeablllty and thus capable of malntalnlng a rapld flltration rate; relatlvely unsusceptible to pore clogglng; and hence capable of contlnuous filtratlon operation for extended periods of time. Another ob~ect of the present inventlon is to provlde hollow fibers for ultra-filter use, having hlgh mechanical strength, high chemlcal stabllity, and superlor resistance to microorganisms, i.e. microbially almost incorruptible.
For separatlng bacterla. proteins, vlruses, colloidal substances, etc. by filtration, collodion membranes, gel Cellophane @9 films, etc, have been heretofore used. Recently, however, in place of these conventional ultrafilters, cellulose acetate membranes, collagen membranes, dextran membranes, etc, have been utilized widely in such flelds as the food industry, the pharmaceutical in-dustry, the electronic industry, sanitation etc., ln addition to laboratory scale use. However, these conventional filters which have heretofore been used have various drawbacks, for example low water-permeability, a requirement for high pressure at the time of the filtration operation, frequent filter changes due to clogging, etc. Further, ultrafilters of cellulosic membranes are liable to undergo degradation due to hydrolysis as well as corruption due to tl/' -2-m1croblal actlon, an~ moreover have poor chemlcal reslstance.
Recently, methods for producl~g ultraflleers have been advanced, and methods for produclng film havlng pores of smaller sizes and higher water-permeability have been proposet.
One such method ls that in which solvent withln the sur-face of a film ls positlvely vaporlzed at the tlme of fllm-maklng in order to form a dense structure layer having a thickness of about 0.2~ on one side of the film, and a porous structure having a thickness of about 100~ on the other side of the fllm. Thus the alm of overcomlng a dlsadvantage of conventlonal film has been achieved to a certain extent by thls method, According to Baker Unlted States Patent No. 3,567,810 issued March 2, 1971, which uses the above-mentioned method, polysulfone, polyacrylonltrile or the like is dissolved in a mixed solvent of dimethylsulfoxide and acetone or N,N-dimethylacetamide and acetone and made into film. At the time of film making, the solvent within the surface of film is vaporized, for example by exposlng the film for a few seconds to an alr current having a temperature of 90 - 150C, and the resultlng film then dipped in a coagulation bath to obtain a film having a dense structure close -to the surface.
The water-permeability of the films thus prepared is - i~proved compared to conventional products, but is still low, e.g.

i~;! 0.31 - 0.086 ml/cm min-atm. Bechtold et al United States Patent No. 2,846,727, lssued August 12, 1958,4~e-dis-closes a similar method, and affords about the same water-perme-ability.
As the result of studies contucted by the inventors in an attempt to overcome the above-mentioned drawbacks of conven-tional ultrafllters, particularly to improve water-permeability, 104Z16~

~ -3- -. . .

- , ' ultraf~lter~ have been prepared from acrylonltrlle polymer8, whiCh exhlblt excellent performance as flltratlon materlals, l.e. having much higher water-permeablllty than the conventlonal protuct8, hlgher mechanlcal strength, a lower rate of clogglng and higher capablllty of contlnuous flltratlon operatlon for extended perlods of tlme, hlgh chemlcal stablllty and superlor resistance to mlcro-organisms.
The present inventors, however, to improve on these results, have continued their studles wlth regard to a manner of convertlng the above-mentloned hlgh performance fllms into a hollow flber shape. For low-cost commerclal ultraflltration, a hollow fiber shap- ls preferable because the effective membrane area which can be contained in a unit space is remarkably increasel.
The present inventors succeeded in preparing a gel-like ultrafilter of polyacrylonitrile in the form of hollow fibers having a water-permeability rate ten to several ten tlmes hlgher than those which ~ æ
i ~ ~ been known (Belgian Patent No, 740,927 and M. Bier, Membrane Proces~es in Industry and Biomedicine, Plenum press, 1971).
Conventional mlcrofilters have been prepared by ~ f~P-aLl.,~ as many uniform pores as possible through a base material;the passage of solvent in the microfilters thereby being limited to through the pores only. Accordingly, in order to increase the filtratlon rate, it is necessary to enlarge the diameters of the pores a8 much as possible without permitting the passage of partlcles. However, even when the diameters of the pores are enlarged, clogging of the pores is still liable to occur, since both the particle8 and the tiameter8 of the pores have 80me 81ze overlap.
It then occurred to the inventors that a microfilter exhibiting a high filtration rate without the accompanying sus-~042166 B ~ 4_ , . .

-- lV~ ti~j ceptlblllty to clogglng mlgllt be obtslned by a structure formed ~ ~o ~
basically from a h~rop~roL~ materlàl, ln which the average pore diameter is acceptably small but in-whlch the number of pores 18 increased. This structure ls quite different from the conventional one in which filtration is carried out only through the pores of ~ y~r~Q~
the basic IlO~ h~ro~or~u~ material.
~)se, As a means of employing such an idea one may ~r~-e-~fllm havlng a water-containing gel-like structure. However polymers having a gel-like structure generally form a so-called ~elly, whose strength is insufficlently high for use as a membrane which coult be usable as a filter.
Thus, it is a general phenomenon than any increase in water-permeability is offset by a reduction in mechanical strength and vice versa. However, we have succeeded in making two incon-sistent characterlstic properties consistent as hereinafter des-crlbed.
Polymers having a gel-like structure, that is water-soluble polymers, their copolymers or their cross-linked polymers are known, but are mechanically weak in the presence of water ant cannot be used as a filter even when shaped into a film. Thus, the inventors have searched among hydrophobic high molecular weight polymers for materials hsving a water-containing, gel-like structure, and a method for formlng the same. This may appear to be incon-~istent in the search for a film having a water-containlng, gel-llke structure.
In order to produce a water-containing, gel-like structure, it 18 necessary that the principal chain or the side chain of the polymers has an affinity with water, ant when the afflnlty of the molecular chain wlth water 18 6trong, water molecules are co-ordlnated with the chaln over the whole length of the ~olecular .~ ~ -- . .
.
: , :
' - ' ' - ' .

104Z16~
chain to form a gel structure having a high water-content. How~
ever, if its affinity for water is too strong, the polymer is completely dissolved in water, or even when undissolved, it swells producing a film having a low mechanical strength, as a~ove mentioned. Thus, in order to obtain a microfilter having a high water-content, e.g, high water-permeability, and also high mechanical strength, it is important to select a material having an optimum hydrophilic or hydrophobic property.
As for a parameter for expresslng the affinity between substances, SP value (solubility parameter) has often been employ-ed. Table 1, which follows, shows SP values of various types of insoluble polymers. The SP yalue of water is as high as 23.41.

Table 1 SP values of various kinds of water-insoluble polymers ;~ Polytetrafluoroet~ylene 6.2 Polydimet~ylsiloxane ~, Csilicone rubber~ 7.3 , Butyl rubber 7.7 Polypropylene 7.9 2Q Polyethylene 7.9 ~atural rubber 8.0 ', Polyisobutylene 8. a - Polybutadiene 8.5 Polybutyl acrylate 8.8 Polystyrene 9.1 Polysulfide ¢thiokok rubber) g,2 Polymethyl methacrylate 9.2 ~eoprene 9.3 . Polybutadiene-acrylonitrile ¢5:25) 9.4 Polyvinyl acetate 9.4 `~ cb/
, ~,~. . -.
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~ , - -: ~ :

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Polyethyl acrylate 9.4 Polyvinyl chloride 9.5 Polyurethane 10.0 Epoxy resin 10.1 Ethylcellulose 10.3 - Polyethylene terephthalate 10.7 Cellulose diacetate 10.9 Cellulose dinitrate 11.0 Polymethylene oxide 11.0 Phenol resin 11.0 Polyvinylidene chloride 12.2 ~ylon 13.0 Polymethacrylonitrile lS.0 Polyacrylonitrile 15.4 As seen in Table 1, among water-insoluble polymers, the SP value of polyacrylonitrile, i.e., 15.4 is highest and the closest to the value of water, 23.41. Thus, it can be concluded that poly-acrylonitrile is most suitable as the base material for preparing a microfilter having high water-content and high mechanical strength.
With regard to sheets or films consisting mainly of poly-;' acrylonitrile, it has been known that they generally e~hibit ex-tremely low gas-, steam- and water-permeabilities when comparet to those of other polymers. By utilizing polyacrylonitrile of such low permeabllities, various studies have been made with regard to the development of these materials as packaging materials for preserving fragrance or moisture. Also, on account of the characteristics of such low gas-, steam- and water-permeabilities, production of bottles shaped from acrylonitrile polymers, such as those used for beer or soft drinks, is now being investigated in earnest, and it is thought . 30 that if the problem of discarded bottles can be resolved, then .. ~,~

cb/

~`'-, ~ . - ' ' - ' - ' , --: . ' 104'~16f~
they will be substituted for glass bottles. In su~mary, poly-acrylonitrile articles shaped according to conventional processes have low gas-, steam- and water-permeabilities as well as low water-absorptivity, and are excellent from tEle point of overall resistance to water. Further, they also haye superior resistance to micro-organisms and to various chemicals.
We have found that in spite of the superior resistance to water, polyacrylonitrile has a small contact angle Cwhich is a parameter for expressing wettability to water, i.e. the so-call~d wettable property2 compared to most hydrophobic high molecular weight polymer materials, as shown in Table 2.
Table 2 Contact angles of various polvm-ers Polyacrylonitrile 49 Nylon-6 52 N-methoxymethyl-polyamide 52 Polymethyl acrylate 52 - Cellulose diacetate 53 Polycarbonate 56 Polyvinyl acetate 57 Melamine-coated plate 58 Vinylon~ ~aldehyde-treated PVA) 61 Polymethyl methacrylate 62 Phenol formaldehyde resin 63 Cellulose triacetate 67 Polyyinyl chloride 68 Chlorinated rubber 68 Polyethylene terephthalate 69 Polytrifluorochloroethylene 72 Nf:oprene 73 Low pressure process polyethylene 73 cb/ - 8 -10~
High pressure process poly-ethylene 81 Polystyrene 84 Silicone rubber 9 Polypropylene 91 Polytetrafluoroethylene 104 Note: The value for polyacrylonitrile was observed. The other values were taken from "~andbook of Materials and Their Water Contents" edited by Kobunshi Gakkl, Japan, Kyoritsu ~ublishing Co. Ltd., 1968.
With the view that, if a filtration film can be prepared from such a water-wettable material, the resulting film would have excellent stability, the inventors have investigated a method for producing a fine-structured body which gives high water-permeability and have finally succeeded in preparing an ultrafilter from a starting material of polyacrylonitrile~ having high water-permeabil-ity and a uniform distribution of pore diameters. As a method for s 20 preparing an ultrafilter from copolymers of acrylonitrile as a raw material, reference has already been made to Baker United States Patent No. 3,567,810, issued March 2, 1971. Even when a dense structure i8 formed on the surface part of the film by using an organlc solvent and promoting the vaporization of the solvent on the surface part of film, and then dipping the resultant film in a coagulation bath, thus controlling the diameters of the pores by the resulting dense layer, only a product having low water-:~
I permeability can be obtained due to the presence of the dense layer.

As the result of attempts to discover a method for pre-paring films having a uniform gel structure, but without forming a cb/

- :

s ; :~5,~. - .-- , 1(~4;~
particularly dense layer as mentioned aboye, and yet retaining fu]l control o~ the diameters of the pores, an ultrafilter has been prepared using nitric acid as an inorganic solvent, whose water-permeability is of an order greater than those of the films shaped by the use of an organic solvent, and yet whose distribution of pore diameters ls narrow.
A method for shaping a semi-permeable membrane in the form of hollow fibers in order to increase the effective area of the membrane per unit space has been well known, e.g. in the pro-cess for preparing a semi-permeable membrane for desalting by a counter-impregnation process. The techniques of shaping such hollow fibers are extremely exacting and thus there are few examples of successful methods.
A gel-like ultrafilter of the polyacrylonitrile group in the form of hollow fibers, having a water-permeability ten or more times larger than those of heretofore known hollow fibers of acrylonitrile polymer, has now been prepared according to the present invention. The term "gel-like" referred to herein means a state containing at least 50~ water by volume.
In drawings illustrating aspects of the present invention:
Figure 1 schematically represents apparatus for produc-ing the novel polymeric fibers.
Figure 2 is a cross,section of a spinnerette nozzle useful for extrusion of the novel polymeric fibers.
Figure 3 shows a filtration module utilizing the novel polymeric fibers.
Figure 4 sho~s enlarged cut end surfaces of the novel polymeric fibers positioned wit~in the module of Figure 3.
- Figure 5(a~ to S(k) are photomicrographs of polymeric ~ 30 fibers according to the present invention.

cb/ - 10 -~ .

1(~4;~
~ Figure 6 shows an alternate type of flltratlon module to that of Figure 2, llkewlse utlllzlng polymerlc flbers accorting - to the invention.
Figure 7 is a flowsheet representlng a heavy metals-contalning waste water treatment system utilizing a polymerlc fiber ultrafilter, A filter of polyacrylonitrile hollow fibers has already been disclosed in Belgian Patent No. 740,927, but the product has low water-permeabllity and its fine structure is basically differ-ent from the filter of hollow fibers according to the present in-vention.
The difference of the structure of the hollow fibers of the present invention from those disclosed in the above-mentioned Belgian Patent, will be clear by comparing Figures 5a - 5k, which lllustrate the hollow fibers of the present invention, with the specification of said Belgian Patent, Namely, the hollow fibers : of the present invention have a great number of voids which are absent from the hollow fibers recited in said Belgian Patent. Also, - the novel hollow fibers have no skin layer. The water-permeability (which is the most important characterlstic property of hollow fibers for ultrafilter use) of the present invention is ten or more times larger than that disclosed in said Belgian Patent, due to the above-mentioned specific hollow fiber structure of the present lnvention.
A filter of hollow fibers according to the present in-vention not only exhlbits high water-permeability but also~ ag another important property, has sufficiently small pore diameters to carry out ultraflltratlon.
An ultrafilter made of the hollow fibers accorting to . ., the present invention, as shown in Figures 5a - 5k, has the ~_J

~f , , '' . , ' ' , "

.

104;~i6 following micro-structure:
The hollo~ fibers have on the outer side, a layer having a thickness of about 20~ and containing no Yoids; at least one supporting layer close to the above-mentioned layer in which voids having diameters of 10~ or more are regularly disposed, and, on the inner side, a layer haYing a thickness of about 20~ and con-taining no voids as in the outer side. Illustrated are several examples of filters of hollow fibers according to the present invention. Basically, any structure having a layer or layers haying no voids on the outer or the inner or both the sides of the fibers, combined with a layer or layers close to the aboye-mentioned layer or layers and having voids, falls within the range of the present invention.
Further, the micro-structures of each layer thereof are specific as explained hereunder:
The outer side layer which has no voids, i.e. vacuoles of lQ~ or more, has a porous structure, and the average diameter of pores in this layer becomes smaller gradually as the pores approach the outer surface, as seen in Figure 5d. This layer has no distinct skin layer as those heretofore known haye had.
Further, a supporting layer which contains voids, and which is disposed close to the layer which does not contain any yoids~ is formed of a porous substance having a network structure, as shown in Figure 5c.
The inner side layer which has no voids has a similar micro-structure to that of the outer side layer which does not contain voids, as shown in Figures 5e and 5f.
The gradient type porous substance layer referred to herein has the following structure:
Referring toFigures 5d, 5e and 5f illustrating the cb/ - 12 -104;~16~i examples of this type of layer, said layer is ~ound on the outer-or inner- or bot~ of the sides of the hollow fibers, and has a thickness of 100 - 1~. When a number of cylindrical surfaces having their centers on the central axes of the hollow fibers is assumed, then eacR of the cylindrical surfaces would have a number of pores each having nearly unifor~ pore-diameters. The average pore-diameter of pores (in respect of the pores on the cylindrical surfaces whose central axes are on the central axes of said hollow flbers) contained in each of the cylindrical surfaces, becomes 10 continuously smaller, in a linear relationship, with distance of each of the cylindrical surfaces from the imaginary datum cylin-drical surface lying in the middle of the zone between the outer surface and the inner surface of the hollow fibers, i.e. the cylindrical surface lying at the same distance from the outer and inner surfaces. In other words, the remoter a surface from the middle datum cylindrical surface or the closer to the outer and inner surfaces, the smaller the average pore diameter of pores contained in the surface. The pore-diameter of pores contained in the gradient type porous layer is 5~ or less, preferably about . 20 1~, on the side closest to the imaginary middle datum cylindrical surface; and is 0.1~ or less or as small as several ~, on the outer or lnner surface. The advantage of the hollow fibers of the present invention is that clogging does not occur due to the presence of this gradient type porous layer.
The network porous construction referred to herein is the one shown in Figure 5c. The size of the network is not limited to the one shown in this flgure, since it may range - between about 500 ~ and 5~.
The voids contained in the hollow fibers are portions 30 where the hollow fiber-forming polymer is absent, and water is cb/ - 13 -:-- ~ '' - . ' .
;
- ' ': ' ; ' lO~Zlf~
contalned thereln ln lts llquld state, wlth alr in lts dry state.
The volds have ~ cyllndrical shape or a revolvlng elllpsold shape having ~heir long axes polnting towards the central axes of the hollow fibers. The cross-sectional surfaces perpentlcular to the long axes of the volds are nearly clrcular, and the lengths In the dlrection of the long axes are twice or more the dlameters of the above-mentioned circular cross-sectional surfaces. The slzes of the voids are almJst uniform in the same layer, and the voids are regularly disposed as seen in Figures 5a and 5g. If the diameter exceeds 50~, the mechanical strength of the hollow fibers is undesirably reduced.
Further, the pore-diameters of hollow fibers described in Belgian Patent ~o. 740,927 are comparatively large, i.e. as large as 0.6~ (bubble point: 1.5 bar), but nevertheless their water-permeability is comparatively low, i.e. 0.1 ml/cm2 min atm or less.
Conversely, the pore-diameters of the pores in the hollow fibers according to the present inventlon are very small e.g. the limit of filtration, by molecular we~ght, is as small as 45,000 (about 30 A)-The most significant characteristic feature of the hollow fibers of the present invention is that the water-permeability is very high, i.e. 0.2 ml/cm2-min-atm or more and even greater than 1 ml/cm min-atm in most cases, despite the extremely small pore-diameters.
One embodiment of the method for producing the hollow fibers for ultrafilter use will be described in detail hereunder.
The starting polymers for the hollow fibers of the present invention must contain at least 60 mol X of acrylonitrile unlt~ in their molecular chain, and preferably contain 84 mol X or more of scrylonitrile unit~, ~f the amount of the acrylonitrile : .
' :. :

104~1tjtj component ls less than 84 mol %, the solublllty of the polymers ln nitrlc acid is reduced and the viscoslty of the resultlng solutlons ls lncreased. Thus,it becomes dlfficult to produce hollow fibers havlng uniform properties. The water-permeability i8 increased with an increase in the content of comonomer. As for the comonomer, the following can be used:
Olefins such as isobutene, l-hexene, etc.; ethers such as ethyl vinylether, butyl vinylether, etc.; halogenated olefins such as vinylidene chloride, vinyl chloride, etc.; dienes such as butadiene, isoprene, etc.; esters such as methyl acrylate, ethyl acrylate, methyl methacrylate, vinyl acetate, etc.; aromatic com-pounds such as styrene, ~-methylstyrene, etc.; nitriles such as methacrylonitrile, vinylidene cyanide, etc.; or the like.
Multi-component copolymers containing combinations of the above-mentioned comonomers can be also used. As will be seen from the data in the following Examples, the water-permeability i8 improved with an increase in the amount of comonomer units, but the mechanical strength of the hollow fibers begins to decrease when the amount of the comonomer units is about 14 mol %. If the amount of the comonomer units exceeds 16 mol Z, the resultant pro-duct is unsuitable for the hollow fibers of the present invention.
As for the molecular weight of usable polymers, those capable of main;aining mechanical strength and falling wlthin the spinnable range (i.e. intrinsic viscosities of 0.4 - 3.0 as measured ln N,N-dimethylformamide at 35C), are sufficient.
As solvent to be used for the spinning solution for the hollow fibers, nitric acid is most desirable. The nitric acid referred to herein includes the total range of concentrations in whlch polyacrylonitrile is soluble.
Solvents capable of dissolving polyacrylonitrile, also B ~ -15-, ~ .

-include sol~ents such as dimethylsuLfoxide, N,N-dimethylacetamlde, etc., but, as will be seen from the comparisons in the Examples, it is difficult to obtain hollow fibers for ultrafilter use, having a high water-permeability, if solvents other than nitric acid are used.
For the coagulation bath, water alone is preferable, and a content of nitric acid of 30% or less is desirable. As the con-centration of nitric acid in the coagulation bath is increaset, it becomes difficult to obtain hollow fibers having high water-permeability.
The polymer concentration of the spinning solution shouldbe ad~usted to a concentration of 2-40% by weight, preferably 5-30%
by weight, as will be seen from the data in the following Examples.
The relationship between the polymer concentration and the percent-age of water-permeability is shown in Table 5 of Example 3. If the concentration exceeds 40~ by weight, the water-permeability becomes undesirably low. Table 5 also shows the relationship between the polymer concentration and the mechanical strength of the hollow fibers. The mechanical strength is relatively low in 8 polymer solution of about 5% by weight, and if the concentration is 2% by weight or less, the mechanical strength is unsuitable for ultrafilter use. Further, in view of spinnability, if the concen-- tration is 2~ by weight or less, the viscosity of the solution is too low, while if it exceeds 40~ by weight, the viscosity of the solution is too high. Thus, hollow fibers of good quality cannot be obtained outside the concentration range of 2-40% by weight.
Further it is necessary to carry out the dissolution of polymers in nitric acid at a temperature of 0 C- -5 C, and maintain the temperature during filtration and defoaming. If the ; 30 temperature of the solution is elevated above 0 C during the pro-cbl - 16 -~0~
cesses of dissolution, ~iltration and dcfoaming, a considerable a~ount of hydrolysis occurs, and hollow fibers prepared from the resulting solution have a dense structùre of extremely reduced water-permeability.
Various conditions for spinning hollow fibers of the present invention are summarized as follows.

Best Preferable Operable Condition Range Range Concentration of 68% 65-70% 65-g5%
HN03 as a solvent Internal coagulant Water Liquid Liquid Concentration of Aqueous Aqueous - coagulation bath Water Solution Solution 0-20% Q-30%
~03 ~3 Concentration of 2-40%
Spinning solution 10-20% 5-30% by weight Temperature of - coagulation bath 20 C 4-50 C 4-50C
20 Stretching 0 0-1.5 times 0-1.5 times Take-up speed 20 m/min 10-30 mlmin 1-100 m/min In the process of spinning the hollow fibers according to the present invention, it is not preferable to stretch the fibers. In general, when hollow fibers for clothing are prepared by wet process spinning, their mechanical strength is not sufficient unless they are stretch to twice or more their original length in order to promote orientation. Thus, the spinning of hollow fibers of acrylonitrile polymers heretofore known always included a stretch-ing process. Howe~er, the water-permeability of the hvllow fibers prepared according to such conventional processes is substantially zero and hence the fibers cannot be used as ultrafilters. One of the characteristics of the present invention lies in obtaining . a filter having high water-permeability,through a process containing :

cb/ - 17 -lO~Z166 no stretching step.
It is preferable to carry out spinning using a wet pro-!cess as shown in Figure 1, through hollow fiber producing nozzles as shown in Figure 2.
IAfter filtration and defoaming the spinning solution is '-introduced into nozzle 3 by means of a gear pump 2. The nozzle 3 has a structure as shown in Figure 2 in which water i9 introduced into a hollow portion 6 while the spinning solution is introduced into the outer side portion 7. The fibers extruded from nozzle 3 are coagulated i~ a coagulation bath 4, and taken up on a take-up roll 5. The liquid infused into the hollow portion is not limited to water, but organic solvents stable against nitric acid such as n-heptane, tetrachloroethylene, cyclohexane, kerosene, etc. or nit-ric acid itself can be used. The specific properties of the ultra-;filter of hollow fibers, obtained according to the present lnvention, were measured by a module for testing the specific properties, as shown in Figure 3. In Figure 3, numeral 8 is a bundle of hollow fibers ~'numeral 9 is a part subjected to adhesion, and numeral 10 is an in-let for infusing a testing liquid. Figure 4 shows enlarged cut end surfaces of hollow fibers of the module for testing the specific pro-perties. Next, the parameters used for expressing the specific properties of the ultrafilter of hollow fibers will be described.
Water-permeability: ml/cm .min-atm !For the measurement of water permeability, a glven number of hollow fibers, whose outer and inner diameters have been measur~d in advance, are subjected to adhesion at one end thereof; a pressure difference of one atmosphere is applied between the infusion side and the flow-out side; and then the permeated volume of distilled water per unit time is measured. To measure the effective area of film, the surface area of the inner wall of hollow fibers is calculated.
The permeated amount of water by volume per cb/ - 18 -' ' ' ' ' . - , .

-.

il~421t~
unlt tlme ls dlvided by the calculated value of the surface area and the pres6ure employed to glve water-permeabllity (ml/cm .mln~atm).
Pore-diameter:
Pore-dlameters are too small to allow direct calculation.
Thus, aqueous ~olutions of various kinds of 6pherical proteln~
having d$fferent sizes were filtered, and the resulting filtrates were analyzed to give standards of pore-dlameters.
- A list of spherical proteins used in the Examples is shown ln Table 3.
Table 3 Spherical proteins for measuring Pore-diameters Molecular weight y-Globulin 160,000 Human serum albumen 67,000 Egg albumen 45.000 Pepsin 35.000 a-Chymotrypsin 24,500 Myoglobin 17,800 a-Lactoalbumen 16,000 Cytochrome-C 13,000 Insulin 5.700 y-Bacitracin 1,400 Flltration limit molecular weight:
the smallest molecular weight of partlcles ~ completely (100%) unable to pass through - the ultrafilter of hollow fibers Example 1 Polyacrylonitrlle havlng an intrinslc vlscoslty of 1.2 as measured in N,N-dlmethylformamlde was dlssolved in an aqueous B

.

104;~
solution of nitric acid ~65~) maineained at -5 C to give a solutlon having a polymer concentration of 15 g/100 ml, and was then filtered and defoamed ~hile ~eing maintained at -5C. The resulting solution was introduced into nozzle 3 by means of a gear pump 2 as shown in Figure 1. The inner diameter of the capillary at the central part of the nozzle Ccapillary diameter~ was 0.6 mm, and the inner dia-meter of the nozzle from which the polymer solution was extruded ~nozzle diameter) was 2.0 mm. Water was introduced into the nozzle by means of another gear pump 1. The nozzle has a structure as shown in Figure 2. Water was introduced into the hollow part 6, while the spinning solution was introduced into the outer side part 7. The feeding velocity of water was 1.5 mltmin, and that of the polymer solution was 4.5 ml/min. The fibers extruded from the nozzle were coagulated in a coagulation bath 4 filled with water and taken up on a take-up roll 5. The coagulation bath was maintained at 30C, and its total length was 10 m.
Take-up velocity of take-up roll 5 was lO nlmin. The hollow fibers thus obtained were washed, and their inner and outer diameters were measured as 0.4 mm and 0.8 mm, respectiYely.
Using these hollow fibers, a filtration module of hollow fibers as shown in Figure 3 was prepared. The water-permeability was measured by applying water under a pressure of two atmospheres.
Water was permeated at a rate of 4200 ml/hr per 1 m of one filament.
The water-permeabllity per unit area (calculated based on the sur-face area of the Inner wall) calculated from these data, was 2.8 ml/cm .min-atm, and the filtration limit molecular weight was 45,000.
With this structure, 100~ of pepsin CMW: 35,000) permeated, while no egg albumen (MW; 45,000) permeated.
A microscopic photograph of the cross-sectional surface of the hollow fibers is shown in Figure 5a. Figure 5b shows a cb/ - 20 -. . ~ . , .

lO~
microscopic photograph of a surface obtained by cutting the hollow fibers of Figure 5a in the direction A - A of tile Figure and observed in the direction B - B of the Figure.
Figure 5c is a photograph, ~y means of a scanning type electron microscope, of a wall surface which in part defines a void. Figure 5d shows an electron microscope photograph of the lateral cross-sectional surface of the outer side part of the hollow fibers. Figures Se and 5f show electron microscope photo-graphs of the lateral cross-sectional surface of the inner side 10 part thereof.
These hollow fibers, as shown in Figure 5a, have a grad-~ ient type porous layer having a thickness of about 40~ on the j outer side and another gradient type porous layer having a thick-i ness of about 10~ on the inner side, and an intermediate layer ~ containing voids having lengths of 20 - 50~ and diameters of 5 -3 15~
~ ~ext, with acrylonitrile copolymers haYing methyl q acrylate as comonomer and having various compositi~ns ~intrinsic viscosity as measured in N,N-dimethylformamide at 35C: 2.5 - 0.4), 20 hollo~ fibers were prepared by same method and the respective J specific properties measured.
The results of the measurement of specific properties of the ultrafilters of hollow fibers obtained using 65% aqueous ? solution of nitric acid as the solvent of the spinning solution for hollow fibers, are shown in Table 4.

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., , lV4;~ 6 As can be seen from these result.s, water-permeability is improved with an increase in content of comonomer units, but the mechanical strength begins to be reduced at a content of about 14 mol % of comonomer units.
Example 2 An acrylonitrile copolymer containing 1 mol ~ of metha-crylonitrile units and having an intrinsic viscosity of 1.1 as measured in N,N-dimethylformamide was spun in the same manner as in Example 1 to give hollow fibers having an inner diameter of 0.4 mm and an outer diameter of 0.7 mm. The water-permeability was measured by applying water under a pressure of two atmospheres, and the amount of water permeated was 5,750 ml/hr per 1 m of one fila-ment. The water-permeability was 3.8 ml/cm .min.atm, and the limit of molecular weight for filtration was 45,000.
Example 3 An acrylonitrile copolymer containing 8 mol % of methyl acrylate units and having an intrinsic viscosity of 1.7 as measured in N,N-dimethylformamide was spun in the same manner as in Example 1, and the relationship between the polymer concentration of the spinning solution and the specific properties of the fllter was investigated. The hollow fibers spun from a spinning solution having a concentration of 10% by weight in this Example had an outer diameter of 1.2 mm and an inner diameter of 0.6 mm. The microscopic photo of the cross-sectional surface of the hollow fibers is shown in Fig. 5 g.
The hollow fibers have a gradient type porous layer havi~
a thickness of about 15~ on the outer side and another having a thickness of about 10~ on the inner side, and an intermediate layer 29 having voids of lengths of 40 - 150~ and diameters of 10 - 30~ .

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As can be 8een from tih~g4; r~e~u;ts, water-permeabllltY

ls reduced as concentration ls increased, and at a concentratlon above 40% the high water-permeability, whlch ls characterl~tic of the hollow fibers of the present invention, 18 lost. Al~o, the mechanical strength i8 significantly reduced at a concentration of about 5%, and is unsultable for filtration at a concentration of 2Z.
Example 4 An acrylonitrile copolymer containing 8 mol % of methyl acrylate units (intrinsic viscosity: 1.7) was dissolved in an aqueous solution of nitric acid in the same manner as in Example l.
Spinning was carried out varying only the concentration of nitric acid in the coagulation bath. The specific properties of the hollow fibers thus obtained are shown in Table 6.

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Water-permeabillty ls reduced as the concentratlon of nitrlc acid 18 lncreased, and ls undeslrably reduced at a concen-tratlon of 30% by welght or ~ore.

Comparatlve Exsmples Hollow fibers were prepared in the same manner as in Example 1 except that the solvent for the spinnlng solutlon was dlmethylsulfoxlde. The results of measurement of the specific pro-- 9 pertles of the resultlng products are shown in Table 7.

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Hollow fibers spun by uslng dimethylsufoxlde aa the solvent had neither the gradient type porous layers nor vo~ds, and their water-permeabilities were low.
Further, spinning was carried out in the same manner as in Example 1 wlth N,N-dimethylacetamide used as solvent. The results of measurement of the specific properties of the resulting products 7 are shown in Table 8.

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i~ollow flbers spun utlllzing N,N-dlmethylacetamlde as solvent also had nelther the gradlent type porous layers nor volds, and their water-permeabillties were low.
The photograph of the cross-sectional surface of hollow fibers which were spun in the same manner as ln Example 1> uslng N,N-dimethylformamlde as solvent, is shownin Fig. 5(h). No voids are contained.
As shown in the above-mentioned comparative Examples, in the cases of the ultrafilters of hollow fibers prepared according 10 to the dimethylsulfoxide process ant the N,N-dimethylacetamide pro-cess, the water-permeability is reduced as the content of como-nomer is increased. Conversely, the hollow fibers prepared accor-ding to the nitric acid process show an entirely contrary tendency in that the water-permeability is increased as the content of como-nomer is increased. Further, the water-permeability of the products prepared according to the nitric acid process are 10 or more times greater than those obtained by the dimethylsulfoxide process or the N,N-dimethylacetamide process.
Example 5 A filtration module having a construction as shown in Fig. 6 was prepared by using 2000 filaments of hollow fibers (having a length of 1 m) of acrylonitrile homopolymer prepared by Example 1.
In Fig. 6> numeral 11 is an inlet for waste water> numeral 12 is a unit case, numeral 13 is a portion of the hollow fibers sub~ected to adhesion with an adhesive, the hollow parts of the - hollow fibers being opened only on the surface of the right end of the above-mentioned part, as shown in Fig. 4, numeral 16 is a porous gl/~ - 31 -B

104;~1f~fc;
plate, numeral 17 ~s ~n 0-rlng, numeral 15 is a snap rlng, nun~e~al 18 ls an exlt of original liquid, and numeral 14 i8 at~ exlt of dlscharge water flltrate which permeated the hollow flbers ant 19 freed of heavy r~etals.
A heavy metals-containlng waste water treatment system was set up u~ing the module as shown ln the flowsheet of Plg. 7.
To an aqueous solution containing 5ppm of mercuric chloride, was added sodium sulfide 80 as to form a concentration of two mols, whereby mercuric ions were converted to mercuric sulfide. Further, an aqueous solutlon of ferrous sulfate was added so as to give a concentration of 0.25 ppm, followed by stirring.
The resulting colloid dispersion is lntroduced by means of a feet pump 20. Numeral 21 shows a circulating pump by which coarse large particles formet in the ultrafilter of hollow fibers 22 are fet into a thickner 23. The colloid dispersion pressurized by the feed pump is imFregnated into the walls of the hollow fibers. At this time, fine particle colloids are forcibly brought into close proxi-mity to each other to form coarse large particles. The colloid particles converted into coarse large particles are separated by 20 means of the thickner 23, and taken out from a valve 25. Filtrate tischarget from exit 14 was analyzed and the mercury concentration - was found to be 0.0007 ppm.
Continuous filtration was carriet out for 30 tays, and no reduction in permeation veloclty due to clogging in the hollow fibers was observed. The filtration was carried out unter a pressu-re of 1 kg/cm2 and at a temperature of 38C, wlth a treatment capa-city of 2a ton/day.

. . .

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. . , E~smples 6 - 15 1~4~1~6 Various klnds of acrylonitrile copolymers were spun in the same manner as in Example 1 to give hollow fibers.
4 The speciflc properties thereof are shown in Table 9.

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r~xamoles 16- 20 1~4~
An acrylonltrile copolymer contalnlng 8 mol X of methyl acrylate units (lntrinslc vlscoslty as measuret ln N,N-dlmethylformamide: 1.6) was spun in the same manner ng ln Example 1 to obtaln hollow fibers havlng different sizes. Thc speclflc propertles of the hollow flbers sre shown in Table lO.
It can be seen that products having similar specific properties 8 can be obtained despite variations in thickneRs.

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~1 /dh Examples_21 - 26 ~V4~
An acrylonltrile copolymer contalnlng 8 mol % of methyl acrylate unlts (lntrlnsic vlscosity as measured ln N,N-dlme-thylformamldes: 1.6) was spun ln the same manner as ln Example 1 except that the klnd of llquld fed into the hollow portion was varled. The speciflc propertles of the hollow flbers thus obtalned 7 are shown in Table 11.

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, When a liquld whlc~ does not coaxulate the polymer solutlon was lnfused lnto the hollow portlon, hollow flbers having a gradient type porous layer on the outer ~lde only were obtalnet.
As a representatlve example, photographs having a flne structure of the hollow fibers of Example 22 are ghown ln Figs. 5i, 5~ and 5k. Fig. 5~ shows a photograph of the whole of the cross-sectlonal surface; Fig. 5~ shows an electron microscope photograph of the outer side portion of the lateral cross~section and Fig. 5k shows that of the inner side portion thereof.

Examples 27 - 30 An acrylonitrile copolymer containing 8 mol Z of methyl acrylate units (intrinsic ~iscosity: 1.6) was spun in the same manner as in Example 1 except that the concentration of nitric acid used in the spinning solution was varled; The specific properties of the hollow fibers thus obtained are shown in Table 12.

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The strength under pressure (burstlng strength) shown ln thls Table refers to the pre~ure exhlblted at the tlme when hollow flbers ln a module set out as ln Flg. 3 are cru~hed and broken by the pressure applled through numeral 10 ln Fig. 3.
The strength showed the hlghest value at a nltrlc acld concentra-tlon of 68%. Thus lt can be seen that this concentratlon 18, 68X, 7 18 the optimum for the preparation of the novel hollow fibers.

gl/~ - 41 -~, .

... . .

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY
OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A hollow fiber of acrylonitrile polymer for ultrafilter use, said acrylonitrile polymer containing at least 60 mol % of acrylonitrile units, having a structure consisting of (1) a reticulate porous region concentric with the central axis of said hollow fiber, the pores of said reticulate porous region being essentially of the same size; said reticulate porous region including therewithin at least one succession of voids concentric with the central axis of said hollow fiber, each void defining a closed cylindrical or revolving ellipsoid shape having a diameter in the direction of its short axis of 10 to 50µ and a long axis that points essentially toward the central axis of said hollow fibers and a cross-section perpendi-cular to the long axis that is nearly circular; the length of the long axis of said voids being at least two times the diameters of their cross-sections and the diameters of their cross-sections being almost equal on the cylindrical surfaces whose central axes are the same as the central axis of said hollow fibers; and (2) a gradient-type porous region adjacent to the outer portion of said reticulate porous region or to each of the outer and inner portions of said reticulate porous region and concentric with the reticulate porous region and the central axis of said hollow fiber, the size of pores in the gradient-type porous region having a gradient in that the average size of pores con-tained in a gradient-type porous region becomes smaller toward the outer surface of said hollow fibers and the average size of pores contained in a gradient-type porous region adjacent the inner portion of said reticulate porous region becomes smaller toward the inner surface of said hollow fibers; the average size of pores in a gradient-type porous region decreasing from about 5µ-1µ adjacent the reticulate porous region to about 0.1µ-several .ANG. at the surface of said hollow fiber.

2. A hollow fiber of acrylonitrile polymer for ultrafilter use, having a water-permeability greater than 0.2 m?/cm2 min.
atm; said acrylonitrile polymer containing at least 60 mol% of acrylonitrile units; said hollow fiber having a structure consist-ing of (1) a reticulate porous region concentric with the central axis of said hollow fiber, the pores of said reticulate porous region being essentially of the same size; said reticulate porous region including therewithin at least one succession of voids concentric with the central axis of said hollow fiber, each void defining a closed cylindrical or revolving ellipsoid shape having a diameter in the direction of its short axis of 10 to 50µ and a long axis that points essentially toward the central axis of said hollow fibers and a cross-section perpendicular to the long axis that is nearly circular; the length of the long axis of said voids being at least two times the diameters of their cross-sections and the diameter of their cross sections being almost equal on the cylindrical surfaces whose central axes are the same as the central axis of said hollow fibers; and (2) a gradient-type porous region adjacent to the outer portion of said reticulate porous region or to each of the outer and inner portions of said reticulate porous region and the central axis of said hollow fiber, the size of pores in the gradient-type porous region having a gradient in that the average size of pores contained in a gradient-type porous region becomes smaller toward the outer surface of said hollow fibers and the average size of pores contained in a gradient-type porous region adjacent the inner portion of said reticulate porous region becomes smaller toward the inner surface of said hollow fibers; the average size of pores in a gradient-type porous region decreasing from about 5µ-1µ adjacent the reticulate porous region to about 0.1µ-several .ANG. at a surface of said hollow fiber.

3. The hollow fiber of claim 2 wherein the thickness of said gradient-type porous region is from about 1µ to about 100µ.

4. The hollow fiber of claim 2 wherein the thickness of said gradient-type porous region is from about 10µ to about 30µ.

5. A method for producing hollow fibers of acrylonitrile polymers for ultrafilter use, which comprises dissolving, at a temperature in the range of -5°C to 0°C, an acrylonitrile polymer containing at least 60 mol% of acrylonitrile units in a nitric acid aqueous solution having a concentration in the range of 65-95% by weight to prepare a spinning solution having a polymer concentration in the range of 2.40% by weight, extruding the resultant solution through spinnerettes having ring form orifices into an aqueous coagulation bath, haying a concentration of nitric acid of 30% by weight or less, at a temperature of 4-50°C and concomitantly feeding a liquid as an internal coagulant into the central part of the ring form orifices.