Polysulfone porous hollow fiber

A porous hollow fiber of polysulfone suitable for use in filtration such as microfiltration and having a prolonged life, the hollow fiber having an inner surface of a network structure such that approximately circular openings having a minor axial length of 0.1 to 10 .mu.m are present in proximity to each other at intervals of not more than the average minor axial length of the openings, thus providing a surface fraction of openings of 50 to 90%, and/or an outer surface of a structure such that the outer surface region has undergone fibrillation to a high degree to form fibrils oriented in the fiber axial direction at intervals of 0.2 to 3 .mu.m, thus providing a surface fraction of openings of at least 50%. The hollow fiber is prepared by a process characterized in that a polymer substance having an extremely high molecular weight such as polyethylene glycol and polyvinyl pyrrolidone is added to the spinning solution of polysulfone, thereby improving the spinnability.

BACKGROUND OF THE INVENTION 
The present invention relates to a porous hollow fiber of a polysulfone 
suitable for use in filtration, e.g. microfiltration. More particularly, 
the invention relates to a polysulfone porous hollow fiber which has a 
relatively large pore size, and the surface fraction of openings on the 
inner or outer surface of which is extremely increased by providing the 
inner surface with a structure such that approximately circular open pores 
(openings) are formed at intervals not exceeding the average diameter of 
the openings or by fibrillation of the outer surface portion to a high 
degree, whereby the clogging is prevented and the filtration life is 
prolonged. 
Polysulfone porous hollow fibers have been appreciated to have excellent 
heat resistance and chemical resistance and have been widely used for 
industrial microfiltration, ultrafiltration and other purposes. 
Conventional polysulfone porous hollow fibers have been prepared by 
so-called dry-wet spinning method which basically comprises extruding a 
homogeneous or approximately homogeneous raw spinning solution of 
polysulfone together with a core liquid into the air through a double ring 
nozzle and then immersing in a coagulation liquid. 
As the dry-wet spinning method, there are proposed many methods wherein the 
composition of the polysulfone solution, the composition of the core 
liquid, the composition of the coagulation liquid and the spinning 
conditions such as temperature and spinning speed are altered and 
combined. For example, as the polysulfone solution, there is used a 
solution of polysulfone dissolved in a mixed solvent of an aprotic good 
solvent such as dimethylformamide, dimethylacetoamide or 
N-methyl-2-pyrrolidone and a non-solvent such as propylene glycol, which 
causes a phase separation at a temperature below room temperature (as 
disclosed in Japanese Patent Publication Kokai No. 60-222112), a 
polysulfone solution to which polyethylene glycol having a molecular 
weight of 400 to 20,000 is added as an additive (as disclosed in Japanese 
Patent Publication Kokai Nos. 57-35906 and 58-114702), or a polysulfone 
solution to which polyvinyl pyrrolidone having a molecular weight of 
10,000 to 450,000 is added as an additive (as disclosed in Japanese Patent 
Publication Kokai Nos. 61-93801, 61-238306 and 63-97205). Hollow fibers 
having openings with a diameter on submicrometer order are obtained by 
extruding such a spinning solution with a core liquid, e.g. an aqueous 
solution of the above-mentioned good solvent, a mixed solvent of a good 
solvent and a non-solvent or mixtures of these core liquids and the 
above-mentioned additives such as polyethylene glycol and polyvinyl 
pyrrolidone, through a double ring nozzle at a temperature that the 
spinning solution is kept homogeneous or approximately homogeneous, 
running the extrudate several centimeters to several tens of centimeters 
in the air and coagulating it in water or an aqueous solution of a good 
solvent. 
In general, clogging of filtration membranes is considered to result from 
the mechanism that impermeable substances are deposited on the surface, 
and the mechanism that impermeable substances are caught and accumulate 
inside the filter medium. In the former case, as would be easily 
understood, the larger the surface fraction of openings on the surface of 
the filtration membrane, the harder the clogging occurs and the longer the 
life. In the latter case, too, it is apparent that a larger surface 
fraction of openings on the membrane surface is desirable so long as 
impermeable substances have a uniform size and the life owing to clogging 
does not depend on only the amount of impermeable substances caught and 
accumulated inside the membrane. Evaluating conventional hollow fibers 
produced by the above-mentioned method from such a point of view, the 
inner surface has a surface fraction of openings of about 10-40% and 
openings of circular shape or irregular shapes are formed, and the outer 
surface has a surface fraction of openings of at most several tens of % 
and openings of circular or irregular shapes are formed. It has been 
pointed out that conventional hollow fibers are insufficient in life in 
both the uses where the filtration is conducted in the direction toward 
the outer surface of the hollow fiber from the inner surface and the uses 
where the filtration is conducted in the direction toward the inner 
surface of the hollow fiber from the outer surface. 
It is expected that the clogging becomes hard to occur by increasing the 
surface fraction of openings on the inner and outer surfaces of the hollow 
fiber, thus the life could be prolonged. However, hollow fibers having 
both a sufficiently high surface fraction of openings and a sufficiently 
high strength cannot be obtained by a method according to conventional 
methods. For example, even if a known spinning solution is spun using a 
core liquid having a lower coagulating function, an inner surface having a 
structure which is composed of irregular openings and continuously 
distributed spherical polymer portion, is formed. The surface fraction of 
openings does not exceed 50%. 
It is an object of the present invention to provide a polysulfone porous 
hollow fiber having an extremely prolonged life owing to clogging of pores 
of hollow fiber as compared with conventional polysulfone hollow fibers. 
Another object of the present invention is to provide a polysulfone hollow 
fiber which has a large surface fraction of openings and can be used in 
filtration such as microfiltration or ultrafiltration for a long term. 
These and other objects of the present invention will become apparent from 
the description hereinafter. 
SUMMARY OF THE INVENTION 
Investigation has been made by the present inventor in order to increase 
the opening percentage of the inner and outer surfaces of a polysulfone 
hollow fiber. 
In order to further increase the surface fraction of openings in the inner 
surface, it is necessary to form the inner surface into a network 
structure that the wall surface-constituting portion (framework) continues 
two-dimensionally in the face direction and moreover the proportion of the 
area occupied by the framework region is small. In order to form such a 
structure, there must be formed a island-sea structure where a 
polymer-concentrated phase in the spinning solution forms a sea continuous 
two-dimensionally in the face direction and a polymer-dilute phase forms a 
large number of islands, when the spinning solution causes a phase 
separation by contact with the core liquid. For achieving this structure, 
it is necessary that polymer molecules in the concentrated phase have a 
strong attractive interaction to each other and the concentrated phase 
keeps the continuity until coagulation occurs. However, the molecular 
weight of polysulfone synthesized by condensation polymerization is too 
small to have such an interaction. Even if such a continuous state could 
be achieved by specially raising the polymer concentration, the number of 
islands of the dilute phase which form pores and openings is decreased, 
thus the surface fraction of openings does not increase. It is considered 
that if the substances whose molecular weight is sufficiently high enough 
to retain the interaction and which are soluble in solvents for 
polysulfone to form a homogeneous or approximately homogeneous solution, 
are present, these high molecular weight substances can maintain the 
continuous state while causing polysulfone exist in proximity to them in 
the concentrated phase until the polysulfone coagulates, thus revealing 
the network structure mentioned above. Further, if these high molecular 
weight substances could be removed by cleaning, a hollow fiber of 
polysulfone having an inner surface of the above-mentioned network 
structure would be formed. 
With respect to the outer surface of the hollow fiber, it is considered 
that in conventional methods openings are formed on the outer surface by 
occurrence of phase separation of the spinning solution during the dry 
step. In the conventional methods, the surface fraction of openings on the 
outer surface is at most several tens of % since the dry step is left to 
follow its own thermodynamic course without applying a special procedure. 
It is considered that the surface fraction of openings on the outer 
surface could be increased by applying any procedure such as drawing 
without leaving the stage from occurrence of phase separation to 
coagulation to follow its own thermodynamic course. However, according to 
conventional methods, hollow fibers are cut off if such a procedure is 
applied to the above-mentioned entire stage from the moment immediately 
after extruding the spinning solution through a nozzle, since the spinning 
solution of polysulfone is poor in spinnability. In particular, after the 
complete coagulation, application of dynamical procedure such as drawing 
invariably cut off the hollow fiber before the surface fraction of 
openings is changed by the dynamical procedure. Therefore, it is expected 
that if a drawing procedure could be applied immediately after occurrence 
of phase separation by using a spinning solution formulated to have an 
increased drawing ability, it would be possible to further increase the 
surface fraction of openings on the outer surface of the hollow fiber. The 
spinnability of a spinning solution is increased with increasing the 
molecular weight of a polymer dissolved therein. However, it is difficult 
to increase the molecular weight of polysulfone as stated above, and in 
practice, the degree of polymerization of commercially available 
polysulfones is low. However, it is possible to improve the spinnability 
of the spinning solution by adding thereto other polymers having a high 
molecular weight which can be dissolved homogeneously with polysulfone. 
The present inventor has continued investigation in order to find suitable 
high molecular weight substances under the above-mentioned consideration. 
It has now been found that a polysulfone hollow fiber the inner surface of 
which has a network structure having a large surface fraction of openings 
is obtained by adding to the spinning solution a high molecular weight 
polyethylene or a high molecular weight polyvinyl pyrrolidone which 
imparts specific solution characteristics to the spinning solution and 
which has not been used in any conventional methods. It has been also 
found that when the above proposed spinning solution is extruded and 
subjected to drawing to a high degree immediately after the phase 
separation which occurs during the stage between the dry step and the 
moment just after entering into the coagulation liquid, the outer surface 
undergoes a high degree of fibrillation, thus producing a polysulfone 
hollow fiber having an extremely increased surface fraction of openings on 
its outer surface as compared with conventional hollow fibers. 
In accordance with the present invention, there is provided a porous hollow 
fiber of polysulfone, the inner surface of which has a structure that 
approximately circular openings having a minor axial length of 0.1 to 10 
.mu.m are present in the inner surface at intervals of not more than the 
average minor axial length of the openings. 
There is also provided a porous hollow fiber of polysulfone, the outer 
surface of which has a structure which is in the state of a high degree of 
fibrillation. 
The hollow fiber according to the present invention may have both 
above-mentioned inner and outer surface structures.

DETAILED DESCRIPTION 
The hollow fiber of the present invention useful as a filter medium is made 
from a polysulfone. Any of known polysulfone resins can be used in the 
present invention. Representative examples of the polysulfone which 
constitutes the hollow fiber of the present invention are a polysulfone 
having a repeating unit of the formula (1): 
##STR1## 
and a polysulfone having a repeating unit of the formula (2): 
##STR2## 
The polysulfone (1) is preferred, since it provides a hollow fiber having 
a particularly large elongation at breaking. 
The hollow fiber of the present invention can be designed to have optimum 
inner diameter depending on average diameter of openings or pores, 
effective length, liquids to be filtered and the like. The inner diameter 
of the hollow fiber is usually from 200 to 2,000 .mu.m. The wall thickness 
of the hollow fiber is usually selected from 30 to 300 .mu.m from the 
viewpoints of strength and resistance to pressure. 
The hollow fiber of the present invention has an inner surface of a 
structure (hereinafter referred to as specific inner surface structure) 
that approximately circular openings having a minor axial length of 0.1 to 
10 .mu.m are present in close proximity to each other on the inner surface 
at intervals of not more than the average minor axial length of the 
openings and may have an outer surface of any structure, or the hollow 
fiber has an outer surface of a structure (hereinafter referred to as 
specific outer surface structure) which is in the state of a high degree 
of fibrillation and may have an inner surface of any structure. Also, the 
hollow fiber of the present invention can have both the specific inner 
surface structure and the specific outer surface structure. The hollow 
fiber having the specific inner surface structure according to the present 
invention is particularly useful for purposes where the filtration is 
conducted from the inner surface toward the outer surface. The hollow 
fiber having the specific outer surface structure according to the present 
invention is particularly useful for purposes where the filtration is 
conducted from the outer surface toward the inner surface. The hollow 
fiber having the specific inner and outer surface structures is useful for 
both purposes. The wall of the hollow fiber is porous, and the openings of 
pores on the inner surface communicate to the outer surface. 
In case of the hollow fiber having the specific inner surface structure, 
since approximately circular openings having a minor axial length of 0.1 
to 10 .mu.m, especially 0.5 to 5 .mu.m, are present in close proximity to 
each other in the inner surface at intervals of not more than the average 
minor axial length of the openings, the surface fraction of openings is 
much higher than that of conventional hollow fibers. In particular, the 
hollow fiber is hard to cause clogging in uses where impermeable 
substances are easy to accumulate on the inner surface, thus the life is 
further prolonged. 
The term "approximately circular opening" as used herein means an opening 
(opening of a pore in the inner surface) having a major axis/minor axis 
ratio of not more than about 2/1. Such openings are present in close 
proximity to each other in the surface. In other words, the inner surface 
of the hollow fiber of the present invention has a network structure that 
a polysulfone framework extends two-dimensionally and continuously in the 
face direction to form the inner surface having openings and moreover the 
area occupied by the polysulfone framework region is small. Accordingly, 
the surface fraction of openings in the inner surface is from 50 to 90%, 
especially reaches up to 70 to 90%. 
When the minor axial length of the approximately circular opening in the 
inner surface exceeds 10 .mu.m, the strength of the hollow fiber is 
decreased. When the minor axial length is less than 0.1 .mu.m, the 
velocity of water permeation is decreased. The spacing between adjacent 
approximately circular openings in the inner surface (in other words, the 
width of the polysulfone framework) is not more than the average minor 
axial length of the openings. From the viewpoint of balance between the 
surface fraction of openings and the fiber strength, it is preferable that 
the spacing at the narrowest portion is from 0.05 to 8 .mu.m, especially 
from 0.1 to 5 .mu.m. 
The term "surface fraction of openings" as used herein means the proportion 
of the area of openings on the surface of the hollow fiber. The surface 
fraction of openings is determined by using an electron microphotograph of 
the surface of the hollow fiber. For example, it can be obtained by 
cutting the opening portion of the surface out of the electron 
microphotograph and representing it in terms of the weight fraction of the 
opening portion. 
The structure or state of the outer surface of the hollow fiber having the 
specific inner surface structure is not particularly limited. In general, 
the average diameter of openings in the outer surface is from 0.1 to 5 
.mu.m, preferably 0.5 to 2 .mu.m, and the surface fraction of openings is 
from 20 to 90%, preferably 50 to 90%. The outer surface having the average 
opening size and the surface fraction of openings within the above ranges 
can be obtained by a process described after. The hollow fiber having the 
specific inner surface structure may of course have the specific outer 
surface structure as explained below in detail. 
In case of the hollow fiber having the specific outer surface structure, 
since the outer surface has a structure that the outer surface region has 
undergone fibrillation to a high degree, the surface fraction of openings 
in the outer surface is extremely increased as compared with that of 
conventional hollow fibers. In particular, the hollow fiber is hard to 
cause clogging in uses where impermeable substances are easy to accumulate 
on the outer surface, thus the life is very prolonged. 
The term "having undergone fibrillation to a high degree" or "being in the 
state of a high degree of fibrillation" as used herein means that the 
portion excepting the space portion or open portion in the outer surface 
of the hollow fiber is composed mainly of fibrils. The both ends of the 
fibril are in a prous lamella-like state, and the fibril means a fibrous 
portion having a thickness of 0.1 to 0.5 .mu.m and a length of 1 to 7 
.mu.m. The term "high degree" in fibrillation means that the area of the 
fibril region of the outer surface excepting the porous lamella-like 
region present at both ends of fibril is at least 50%, preferably at least 
60%, of the entire outer surface. The porous lamella-like region is a 
portion having circular or elliptical fine openings, which is considered 
to be in a prior state for fibrillation and has not undergone 
fibrillation. It is preferable that the size of each porous lamella-like 
region is from 1 to 5 .mu.m.sup.2. Most of the fibrils are oriented in the 
direction of the axis of hollow fiber, and it is preferable that the 
spacing between the fibrils in the surface (which corresponds to the minor 
axial length of the opening in the outer surface) is from 0.2 to 3 .mu.m. 
The outer surface which has undergone fibrillation to a high degree has a 
high surface fraction of openings as explained above and, therefore, it is 
difficult to cause clogging. The surface fraction of openings of the outer 
surface is usually at least 50%, especially from 70 to 90%. 
The inner surface structure for the hollow fiber having the specific outer 
surface structure explained above is not particularly limited. The inner 
surface of this hollow fiber is usually those having an average diameter 
of openings from the ultrafiltration level to several micrometers, 
preferably from 0.1 to 5 .mu.m, and having a surface fraction of openings 
of 20 to 90%, preferably 50 to 90%. The hollow fiber having the specific 
outer surface structure can of course have the specific inner surface 
structure mentioned before. 
It is desirable that the section of the hollow fiber of the present 
invention has an isotropic network structure in the great part of the 
sectional region in order to avoid the anisotropy in strength, and the 
diameter of the pores is from about 0.1 to about 0.6 .mu.m. It is also 
desirable that the section has a structure such that in the neighborhood 
of each of the inner and outer surfaces, the pore size substantially 
continuously changes toward the size of openings in each of the inner and 
outer surfaces. When the large portion of the section has an isotropic 
network structure, the tensile strength, elongation and pressure 
resistance of the hollow fiber are equal to or higher than those of 
conventional hollow fibers even if the surface fraction of openings in the 
inner surface is very large. Also, in case that the hollow fiber has the 
specific outer surface structure, it is preferable that the fibril 
structure in the outer surface is present in the outer surface region 
corresponding to 10% of the wall thickness, since it is possible to 
provide the hollow fiber with strength against tension and pressure which 
is equal to or higher than those of conventional hollow fibers despite 
that substantial surface fraction of openings is much higher. Also, the 
average diameter of pores present in the inside region in section of the 
hollow fiber may continuously increase or decrease in the direction from 
the outer surface toward the inner surface. 
The velocity of water permeation of the hollow fiber according to the 
present invention is from 1,000 to 30,000 1/m.sup.2.hr.kg/cm.sup.2, 
especially from 3,000 to 20,000 1/m.sup.2.hr.kg/cm.sup.2. The strength of 
the hollow fiber is measured, for example, by a universal tension tester 
(trade mark "Autograph AG-200", product of Shimadzu Corporation). The 
hollow fiber of the present invention has a maximum tensile strength of 50 
to 100 kg/cm.sup.2, a maximum elongation of 30 to 100%, preferably 50 to 
100%. The pressure resisting strength that the hollow fiber is not 
collapsed by a pressure is from 2 to 10 kg/cm.sup.2, preferably 4 to 8 
kg/cm.sup.2. 
The hollow fibers of the present invention having the features as mentioned 
above are useful particularly for, when they have the specific inner 
surface structure, general industrial microfiltration and filtration of 
blood where liquids flow from the inside to the outside of the hollow 
fiber, and are useful particularly for, when they have the specific outer 
surface structure, general industrial microfiltration and a water purifier 
where liquids flow from the outside to the inside of the hollow fiber. 
The hollow fibers having the specific inner surface structure according to 
the present invention are prepared by a process mentioned below. 
The hollow fibers having an inner structure that a large number of openings 
are formed in the surface in close proximity to each other at intervals of 
not more than the average minor axial length of the openings and the 
surface fraction of openings in the inner wall surface is as very high as 
50 to 90%, are obtained by adding a small amount of a high molecular 
weight substance to a solution of polysulfone to adjust the viscosity to 
5,000 to 50,000 cps, extruding the thus prepared spinning solution which 
has a normal stress effect (Weissenberg effect), into the air through a 
double ring nozzle together with a core liquid which has a small 
coagulation action, and then introducing the extrudate into a coagulation 
liquid. When the viscosity of the spinning solution is less than 5,000 
cps, no desirable Weissenberg effect is obtained, and when the viscosity 
is more than 50,000 cps, the spinning may become unstable. 
A large number of high molecular weight substances are applicable to the 
preparation of the spinning solution so long as they can impart a 
sufficiently high viscosity, but it is desirable that these substances 
remain as least as possible in the finally obtained hollow fiber in order 
to maintain excellent characteristics of polysulfone. Accordingly, high 
molecular weight substances soluble in the coagulation liquid or water 
which is the most economical cleaning liquid, are preferable. 
Representative examples of such high molecular weight substances to be 
added to the spinning solution are, for instance, polyethylene glycol and 
polyvinyl pyrrolidone. In conventional methods of the preparation of 
polysulfone hollow fibers, it is proposed to use polyethylene glycol or 
polyvinyl pyrrolidone, but the molecular weight of these polymers used in 
the conventional methods is too small to give a spinning solution having a 
high viscosity, thus hollow fibers as intended by the present invention 
are not obtained. 
It is preferable that polyethylene glycol used in the present invention has 
a number average molecular weight of 150,000 to 2000,000, especially 
200,000 to 500,000. When the molecular weight is less than 150,000, a 
large amount of polyethylene glycol must be added for providing the normal 
stress effect to the spinning solution. The inner surface of the hollow 
fiber prepared from such a spinning solution has a network structure 
having a very thin framework, thus the elongation and strength of the 
hollow fiber are decreased. On the other hand, when the molecular weight 
is more than 2000,000, the solubility of polyethylene glycol is small, the 
obtained hollow fiber has a cellular structure in section thereof, but not 
a network structure, and the filtration velocity is decreased. Similarly, 
it is preferable that polyvinyl pyrrolidone has a number average molecular 
weight of at least about 500,000, especially 7000,000 to 1000,000. In 
conventional methods, even if mere modification to simply increase the 
amount of polyethylene glycol or polyvinyl pyrrolidone is applied, hollow 
fibers having low elongation and low strength are only obtained. 
Solvents used for preparing the spinning solution are solvents common to 
polysulfone and polyethylene glycol or polyvinyl pyrrolidone. Preferable 
solvents are, for instance, dimethylsulfoxide, dimethylformamide, 
dimethylacetoamide, N-methyl-2-pyrrolidone, and mixtures thereof. Mixed 
solvents of these good solvents and a small amount of water or a low 
molecular weight polyhydric alcohol such as propylene glycol can also be 
used as the solvent for the spinning solution. N-methyl-2-pyrrolidone is 
particularly preferred as the above-mentioned good solvent, since it 
provides a definite normal stress effect when the high molecular weight 
additive is dissolved. 
The concentration of polysulfone in the spinning solution is from 14 to 30% 
by weight, preferably 15 to 25% by weight. When the concentration of 
polysulfone is less than 14% by weight, the strength of the obtained 
hollow fiber is low, and when the concentration is more than 30% by 
weight, the viscosity becomes too large and the spinning is difficult. 
The concentration of the high molecular weight additive such as 
polyethylene glycol in the spinning solution is preferably from 1 to 15% 
by weight, especially 2 to 10% by weight, from the viewpoint of high 
spinnability and normal stress effect. When the concentration of the high 
molecular weight additive is less than 1% by weight, the filtration 
velocity of the obtained hollow fiber is low, and when the concentration 
is more than 15% by weight, the obtained hollow fiber is poor in 
elongation and strength. 
The spinning solution at the time when it is discharged from a nozzle is 
required to be maintained at a temperature which is close to the phase 
separation temperature and at which the spinning solution is in the state 
of a homogeneous or approximately homogeneous solution. This temperature 
is usually from 40.degree. to 80.degree. C., preferably from 50.degree. to 
60.degree. C. When the temperature of the spinning solution discharged is 
less than 40.degree. C., there is a case where the viscosity exceeds 
50,000 cps, and when the temperature is more than 80.degree. C., there is 
a case where the viscosity does not reach 5,000 cps. 
The core liquid discharged from a double ring nozzle together with the 
spinning solution is a mixture of a good solvent for polysulfone and a 
non-solvent for polysulfone. 
The structure of the inner surface is also greatly influenced by the 
composition of the core liquid. That is to say, the size and number of 
openings in the inner surface increase with decreasing the coagulation 
action of the core liquid to the spinning solution, thus the inner surface 
widely changes as follows: From the state that the circular openings whose 
surroundings are smooth are scattered, to the state that the number of 
openings increase to show the state that an pore is adjacent to another 
pores, and with further increase of the number of pores, the inner surface 
appearance changes up to a network form where pores are two-dimensionally 
in contact with each other. 
The temperature of the core liquid is from 20.degree. to 80.degree. C., 
preferably 30.degree. to 60.degree. C. 
When the length of the dry step (length of a dry space into which the 
spinning solution is extruded) is made long, or when a non-solvent is 
added to the spinning solution to make it easy to coagulate, the size and 
number of openings in the outer surface are increased. 
Further, if the extruded fiber is drawn several times at the stage between 
the dry step and the moment just after the fiber enters into the 
coagulation liquid, the specific outer surface structure mentioned above 
is formed. 
A great part of the section of the hollow fiber prepared by the 
above-mentioned process has an approximately isotropic network structure. 
The average size of pores in the section of the hollow fiber almost 
continuously increases or decreases, in regions near the inner and outer 
surfaces, toward the size of openings present in each of the inner and 
outer surfaces. 
From the viewpoint of productivity, it is desirable that the velocity of 
spinning is high as much as possible. The spinning velocity is usually 
from 40 to 100 m/minute. When the velocity is more than 100 m/minute, 
cutting off of the hollow fiber may occur due to resistance of the 
coagulation liquid. 
The coagulation liquid is selected mainly from the viewpoint of cleaning 
effect of the hollow fiber. As easily understood, hot water is preferable 
as the coagulation liquid. Hot water having a temperature of 50.degree. to 
70.degree. C. is particularly preferred from the operational point of 
view. 
The process for the preparation of the hollow fiber having the specific 
outer surface structure according to the present invention is explained 
below. 
Fibrillation of the outer surface of the hollow fiber to a high degree can 
be achieved by drawing the fiber to a high degree at the stage between the 
dry step and the moment just after the fiber enters into the coagulation 
liquid. For achieving the purpose, it is necessary to select a spinning 
solution having a high spinnability. In general, the spinnability of a 
solution of a polymer increases with increase of the molecular weight of 
the polymer. Therefore, the spinnability of a solution of polysulfone can 
be raised by adding other high molecular substances having a high 
molecular weight (hereinafter referred to as polymer additive) which can 
be dissolved in a common solvent with polysulfone to form a homogeneous or 
approximately homogeneous solution. The viscosity of the spinning solution 
is increased by the addition of the polymer additive, but it is preferable 
that the viscosity of the spinning solution for preparing the hollow fiber 
having the specific outer surface structure is adjusted within the range 
of about 5,000 to about 50,000 cps, especially about 7,000 to about 30,000 
cps. When the viscosity is less than 5,000 cps, the spinnability is low, 
and when the viscosity is more than 50,000 cps, a stable spinning may be 
difficult. The thus prepared spinning solution has a so-called normal 
stress effect that the extruded spinning solution expands in its radial 
direction when extruded from the nozzle and, therefore, the drawing effect 
is accelerated. 
Like in the case of the production of the hollow fiber having the specific 
inner surface structure, materials which are easily soluble in water used 
as the coagulation liquid or as the cleaning liquid and which remain in 
the end product as least as possible are preferred as the polymer additive 
to be added to the polysulfone solution, and the same materials as those 
used in the production of the hollow fiber having the specific inner 
surface structure are applicable to the production of the hollow fiber 
having the specific outer surface structure. That is to say, as the 
polymer additive there are mentioned polyethylene glycol having a number 
average molecular weight of 150,000 to 2000,000, preferably 200,000 to 
500,000, and polyvinyl pyrrolidone having a number average molecular 
weight of at least about 500,000, preferably 700,000 to 1000,000. 
The solvents for spinning solution and the concentrations of polysulfone 
and polymer additive in the spinning solution in the production of the 
hollow fiber having the specific outer surface structure are selected in 
the same manner as in the production of the hollow fiber having the 
specific inner surface structure. 
The drawing of the spinning solution extruded from the nozzle must be 
conducted at the stage between the dry step and the moment just after 
entering into the coagulation liquid while the coagulation does not 
completely proceed. Moreover, if phase separation has not occurred at 
least in the outer surface region of the extruded spinning solution at 
this stage, fibril structure cannot be formed in the outer surface. 
Therefore, the spinning solution is required to have a composition capable 
of causing a phase separation at a certain temperature, and moreover it is 
necessary to maintain the spinning solution located in the nozzle at a 
temperature which is close to the phase separation-occurring temperature 
but can keep the spinning solution in the state of a homogeneous or 
approximately homogeneous solution. This temperature of the spinning 
solution at the nozzle is from 40.degree. to 80.degree. C., preferably 
50.degree. to 60.degree. C., like in the case of producing the hollow 
fiber having the specific inner surface structure. 
The core liquids discharged from the double ring nozzle together with the 
spinning solution in the production of the hollow fiber having the 
specific inner surface structure are also applicable to the production of 
the hollow fiber having the specific outer surface structure. That is to 
say, mixtures of good solvents for polysulfone and non-solvents for 
polysulfone are used as the core liquid. 
The inner diameter of a ring-like discharge port of the double ring nozzle 
for extruding the spinning solution is preferably from 1.2 to 2.0 times 
the inner diameter of the finally obtained hollow fiber. When the inner 
diameter of the ring nozzle is less than 1.2 times, the hollow fiber 
having the desired inner diameter is lacking in draw ratio and, therefore, 
there is a possibility that fibrils are not sufficiently formed in the 
outer surface of the hollow fiber. When the inner diameter of the ring 
nozzle is more than 2.0 times, the hollow fiber may be cut off in the 
drawing step because of too large draw ratio. The width of the ring 
(spacing of slit of the ring nozzle) is preferably from 1.5 to 3.5 times 
the wall thickness of the final hollow fiber. When the width of the ring 
is outside the above range, a problem that the discharged liquid is not 
stabilized may be encountered. Since the spinning solution sent out of the 
nozzle expands by normal stress effect, substantial draw ratio reaches 2 
to 5 times the nozzle size. The draw ratio is represented by the ratio of 
the outer diameter of the extruded spinning solution, which has expanded 
to the maximum in the vicinity of the discharging port of the nozzle, to 
the outer diameter of the final hollow fiber. 
It is preferable that the length of the dry step is relatively long in 
providing the drawing effect. The length of the dry step is usually from 5 
to 30 cm, preferably from 10 to 20 cm. The dry step exceeding 30 cm is 
unnecessary. 
The spinning velocity is the same as in the case of the hollow fiber having 
the specific inner surface structure, i.e. usually from 40 to 100 
m/minute. 
The spinning solution extruded from the nozzle is subjected to drawing at 
the location between the dry step and the position just after entering 
into the coagulation liquid. From the fact that the hollow fibers having 
an outer surface which has undergone fibrillation to a high degree are 
obtained even if a coagulation liquid having a high coagulating action 
such as water is used, it is considered that the drawing effect is almost 
provided to the fiber in the drying step. Therefore, the coagulation 
liquid is selected mainly from the viewpoint of cleaning effect, and hot 
water is preferred as easily understood. Hot water of 50.degree. to 
70.degree. C. is more preferable from the operational point of view. 
The present invention is more specifically described and explained by means 
of the following Examples, in which all % and parts are by weight unless 
otherwise noted. 
EXAMPLE 1 
In a mixed solvent of 66 parts of N-methyl-2-pyrrolidone and 9 parts of 
propylene glycol were dissolved 5 parts of polyethylene glycol having a 
number average molecular weight of 300,000 and 20 parts of a polysulfone 
(commercially available under the trade mark "Udel Polysulfone P-3500" 
made by Amoco). The thus prepared spinning solution had a lower critical 
solubility temperature of 62.degree. C. and a viscosity of 24,000 cps at 
60.degree. C. 
A mixture of 70 parts of N-methyl-2-pyrrolidone and 30 parts of propylene 
glycol was used as the core liquid. 
Through a double ring nozzle having a ring-like discharge port with an 
inner diameter of 0.8 mm and an outer diameter of 1.3 mm and a core liquid 
discharge port with a diameter of 0.5 mm, the spinning solution maintained 
at 60.degree. C. was extruded out of the ring-like discharge port at a 
rate of 25 g/minute, while extruding the core liquid at a rate of 10 
g/minute. The extruded spinning solution was introduced into 60.degree. C. 
hot water located below the nozzle at a distance of 15 cm to coagulate the 
spinning solution, and was wound up at a spinning velocity of 50 m/minute 
to give a hollow fiber having an inner diameter of 570 .mu.m and an outer 
diameter of 870 .mu.m. The spinning solution extruded from the nozzle 
expanded to a thickness of about 3 mm just below the nozzle, thus the draw 
ratio was about 3.5. 
Electron microphotographs of the inner surface and the outer surface of the 
obtained hollow fiber are shown in FIG. 1 (1,000 magnifications) and FIG. 
2 (5,000 magnifications), respectively. 
As seen in FIG. 1, a large number of approximately circular openings having 
a minor axial length of 5 to 7 .mu.m are distributed in the inner surface 
in proximity to each other at intervals of 2 to 5 .mu.m. The surface 
fraction of openings in the inner surface was about 70%. 
On the other hand, as seen in FIG. 2, fibrils having a thickness of 0.2 to 
0.5 .mu.m and a length of 2 to 3 .mu.m are formed in the outer surface. 
They are arranged in the fiber axial direction at intervals of 0.2 to 1.2 
.mu.m. Also, the both ends of fibrils are connected to fine and porous 
lamella-like regions having fine openings, which are considered to be in a 
prior state for fibrillation. The fraction of the area of the fibril 
region except the lamella-like region was about 80%. The surface fraction 
of openings in the outer surface was about 70%. 
The section of the obtained hollow fiber had a network structure where the 
pore size almost continuously increased from the outer surface toward the 
inner surface. 
The hollow fiber had a maximum tensile strength of 62 kg/cm.sup.2 and a 
maximum elongation of 55%. Also, the permeation velocity of water passed 
from the inner surface side was 11,000 liters/m.sup.2.hr.kg/cm.sup.2, and 
the permeation velocity of water passed from the outer surface side was 
13,000 liters/m.sup.2.hr.kg/cm.sup.2. 
EXAMPLE 2 
In a mixed solvent of 67.5 parts of N-methyl-2-pyrrolidone and 11.5 parts 
of propylene glycol were dissolved 5 parts of polyethylene glycol having a 
number average molecular weight of 300,000 and 16 parts of polysulfone 
(Udel Polysulfone P-3500). The thus prepared spinning solution had a lower 
critical solubility temperature of 60.degree. C. and a viscosity of 8,400 
cps at 60.degree. C. 
A mixture of 70 parts of N-methyl-2-pyrrolidone and 30 parts of propylene 
glycol was used as the core liquid. 
A double ring nozzle having a ring-like discharge port with an inner 
diameter of 0.5 mm and an outer diameter of 0.8 mm and a core liquid 
discharge port with a diameter of 0.3 mm was used. The spinning solution 
maintained at 60.degree. C. was extruded through the ring-like discharge 
port at a rate of 6.2 g/minute, while extruding the core liquid through 
the inside discharge port at a rate of 2.4 g/minute. The extruded spinning 
solution was introduced into 60.degree. C. hot water located below the 
nozzle at a distance of 15 cm to coagulate the spinning solution, and was 
wound up at a spinning velocity of 50 m/minute to give a hollow fiber 
having an inner diameter of 270 .mu.m and an outer diameter of 450 .mu.m. 
The spinning solution extruded from the nozzle expanded to a thickness of 
about 2 mm just below the nozzle, thus the draw ratio was about 4.5. 
Electron microphotographs of the inner surface and the outer surface of the 
obtained hollow fiber are shown in FIG. 3 (5,000 magnifications) and FIG. 
4 (5,000 magnifications), respectively. 
As seen in FIG. 3, the inner surface had a network structure composed of a 
framework having a thickness of 0.5 to 1 .mu.m and a length of 1 to 5 
.mu.m where a large number of openings having a minor axial length of 0.5 
to 3 .mu.m are formed, and the surface fraction of openings was about 90%. 
On the other hand, as seen in FIG. 4, fibrils having a thickness of 0.1 to 
0.5 .mu.m and a length of 2 to 4 .mu.m are formed and arranged in the 
almost entire outer surface through fine lamella-like regions in the fiber 
axial direction at intervals of 0.2 to 2 .mu.m. The fraction of the area 
of the fibril region was about 90%. The surface fraction of openings in 
the outer surface was about 90%. 
The section of the hollow fiber had a network structure where pores having 
approximately uniform size as a whole were present though the pore size in 
the vicinity of the inner and outer surfaces were slightly larger. 
The hollow fiber had a tensile strength of 59 kg/cm.sup.2 and a maximum 
elongation of 90%. The permeation velocity of water passed from the inner 
surface side of the hollow fiber was 14,000 liters/m.sup.2.hr.kg/cm.sup.2, 
and the permeation velocity of water passed from the outer surface side 
was 17,000 liters/m.sup.2.hr.kg/cm.sup.2. 
EXAMPLE 3 AND COMATIVE EXAMPLES 1 TO 4 
A module whose effective total outer surface area of hollow fibers was 
1,900 cm.sup.2 was prepared using the hollow fiber obtained in Example 2. 
With respect to turbidity filtration ability (according to Water Purifier 
Test Standards, item 8.4.2 provided that the ability is represented by 
total amount of water filtered until the filtration rate of the dispersion 
of kaolin powder in water reaches 1.3 liters/minute.kg/cm.sup.2), this 
module was compared with hollow fiber filters having approximately the 
same effective area of household water purifiers commercially available 
from companies A, B, C and D. 
The turbidity filtration abilities were 1,600 liters for the filter using 
the hollow fiber of the present invention, 660 liters for company A 
filter, 700 liters for company B filter, 570 liters for company C filter 
and 530 liters for company D filter. In the filtration of Kobe city water, 
these filters showed filtration abilities roughly proportional to the 
above values. These results show that the filter using the hollow fiber of 
the present invention is hard to cause clogging and is extremely superior 
in the life of use. The hollow fibers used in the filters of companies A 
to D have circular openings or irregular shape openings in their outer 
surfaces, and the surface fraction of openings in the outer surface of 
these fibers was from about 20 to 40%. 
EXAMPLE 4 
In a mixed solvent of 68 parts of N-methyl-2-pyrrolidone and 11 parts of 
propylene glycol were dissolved 5 parts of polyethylene glycol having a 
number average molecular weight of 300,000 and 16 parts of polysulfone 
(Udel Polysulfone P-3500). The thus prepared spinning solution had a lower 
critical solubility temperature of 65.degree. C. and a viscosity of 8,200 
cps at 60.degree. C. 
A hollow fiber was prepared in the same manner as in Example 2 by extruding 
the thus prepared spinning solution maintained at 60.degree. C. except 
that the discharge rates of the spinning solution and the core liquid were 
set 6.1 g/minute and 4.1 g/minute, respectively. The obtained hollow fiber 
had an inner diameter of 310 .mu.m and an outer diameter of 440 .mu.m, and 
had almost the same inner, outer and sectional structures as those of the 
hollow fiber obtained in Example 2. 
A module having an effective length of 17 cm was prepared by bundling 240 
hollow fibers. A fresh bovine blood maintained at 36.degree. C. was 
filtered by passing through the inner hollow portion of the hollow fiber 
at a rate of 5 ml/minute. Even if the filtration velocity and filtration 
pressure reached 2.5 ml/minute and 250 mmHg, no hemolysis was observed. 
COMATIVE EXAMPLE 5 
In a mixed solvent of 61.5 parts of N-methyl-2-pyrrolidone and 25.5 parts 
of propylene glycol was dissolved 13.0 parts of polysulfone (Udel 
Polysulfone P-3500). The thus prepared spinning solution had a upper 
critical solubility temperature of 70.degree. C. and a viscosity of 150 
cps at 85.degree. C. 
A 70% aqueous solution of N-methyl-2-pyrrolidone was used as the core 
liquid. 
A double ring nozzle having an inner diameter of 400 .mu.m, an outer 
diameter of 600 .mu.m and a core liquid discharge port diameter of 250 
.mu.m was used. The spinning solution maintained at 85.degree. C. was 
extruded through the nozzle at a rate of 6.5 g/minute, while extruding the 
core liquid at a rate of 4.5 g/minute. The extruded spinning solution was 
introduced into 60.degree. C. hot water located below the nozzle at a 
distance of 15 cm to coagulate the spinning solution, and was wound up at 
a spinning velocity of 50 m/minute to give a hollow fiber having an inner 
diameter of 340 .mu.m and an outer diameter of 440 .mu.m. The diameter of 
the spinning solution being extruded from the nozzle became smaller than 
the outer diameter of the nozzle at just below the nozzle, and expansion 
as observed in the preparation of the hollow fiber of the present 
invention was not observed. 
In the inner surface of the obtained hollow fiber were present openings of 
irregular shape having a diameter in inscribed circle of 0.1 to 4 .mu.m, 
and the surface fraction of openings was about 20%. In the outer surface 
of the hollow fiber were present approximately circular openings having a 
diameter of 0.1 to 1.2 .mu.m, and the surface fraction of openings was 
about 20%. The section of the hollow fiber had an almost uniform network. 
The permeation velocity of water passed from the inner surface side of the 
hollow fiber was 20,000 liters/m.sup.2.hr.kg/cm.sup.2. 
The filtration test of bovine blood was carried out in the same manner as 
in Example 4 by using the hollow fiber obtained above. The filtrate was 
tinged with red, namely hemolysis was observed, when the filtration 
velocity and filtration pressure reached 1.9 ml/minute and 80 mmHg. 
From the test results in the above Examples and Comparative Examples, it 
would be understood that the hollow fiber of the present invention is 
remarkably superior in the life against clogging. 
In addition to the ingredients used in the Examples, other ingredients can 
be used in the Examples as set forth in the specification to obtain 
substantially the same result.