Process for producing asymmetrical hollow filament membranes of polyamide

A process is disclosed for the production of asymmetrical hollow filament membranes suitable for ultrafiltration and/or microfiltration, in which a spinning solution composed of a polyamide or a mixture of polyamides and/or copolyamides, formic acid and a coagulating core liquid, is extruded into a coagulating setting bath liquid, and the hollow filaments are stretched after leaving the setting bath, in the wet state. The pH-value-difference between core liquid and setting bath liquid should be at least 3. The spinning solution contains, in particular, 15 to 25% by weight polyamide, 5 to 20% by weight polyethylene glycol, up to 10% by weight customary additive and formic acid. Preferred embodiments include a stretching ratio amounting to between 1:1.5 and 1:2.5, and having core liquid and setting bath liquid chosen from various combinations of caustic soda, glycol, formic acid or polyethylene glycol. Also disclosed are hollow filament membranes in which the hollow filament is of uniform, eccentric or profiled shape.

BACKGROUND OF THE INVENTION 
The invention concerns a process for the production of an asymmetrical 
hollow filament membrane suitable for ultrafiltration and/or 
microfiltration, through extrusion of a spinning solution of a polyamide 
or a mixture of polyamides and/or copolyamides with formic acid and a 
coagulating core liquid into a coagulating precipitation bath liquid 
different from the core liquid, and subsequent drying, and asymmetrical 
hollow filament membranes produced thereby. 
German Offenlegungsschrift DE-OS No. 26 06 244 describes a process for the 
production of symmetrical hollow filament membranes, whereby the spinning 
solution, for example composed of a 15 to 25% polyamide solution, is 
provided with a pore-forming substance, for example a metal salt, after 
which a second solvent is added, in which the pore-forming substance 
dissolves but which is a non-solvent for the polyamide. This spinning 
solution is spun through customary ring nozzles, whereby through a core 
nozzle a coagulating core liquid is provided, for example water or alcohol 
or ketone dissolved in water, into the hollow space or cavity of the spun 
solution. The spinning solution must then be spun into air and first 
enters into a precipitation bath after a determined stretch of air. The 
coagulating precipitation bath liquid is likewise composed of water or an 
alcohol or ketone dissolved in water. After the termination of 
coagulation, the metal salt with the second solvent must be washed out, in 
a second bath. 
It has now been discovered that it is possible, without addition of a 
pore-forming substance and through the choice of the core liquid and the 
precipitation bath liquid, to produce asymmetrical hollow filament 
membranes of polyamide. Due to the selection of a particular pore-forming 
substance, the second washing bath for washing out the pore-forming 
substance is superfluous. Through suitable choice of the coagulating core 
and precipitation bath liquids the pore gradient of the hollow filament 
membrane can be adjusted as desired. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to make available a 
simple and more favorable process for the production of a new hollow 
filament membrane from polyamide, asymmetrical and possessing a thin 
separating skin and a substantially thicker support layer. 
This object is attained according to the present invention by a process of 
the above-described type, which is thereby distinguished in that the 
pH-value-difference between core liquid and precipitation bath liquid is 
at least 3 and the hollow filaments are stretched in the wet state after 
leaving the precipitation bath. In advantageous manner, the degree of 
stretching lies between 1:1.5 and 1:2.5. 
Further particulars include that the spinning solution contain 15 to 25% by 
weight polyamide 5 to 20% by weight polyethylene glycol, up to 10% by 
weight customary additives and formic acid. 
When the core liquid has a higher pH-value than the precipitation bath 
liquid, the result is that the pore size decreases from the inner to the 
outer membrane wall. In contrast, when the precipitation bath liquid has a 
greater pH-value than the core liquid, the pore size increases from the 
inner to the outer membrane wall. In view of this, pore sizes can be 
obtained within the membrane wall which, for example, as a rule hold back 
albumen in the retentate up to 90%, and pore sizes which separate plasma 
from blood cells. Membranes containing the latter are particularly 
suitable for microfiltration. The stretching has a great influence on the 
efficiency of separation and the flow rate. 
For the choice of core liquid and precipitation bath liquid the following 
combinations have turned out to be particularly advantageous: 
The core liquid contains caustic soda and the precipitation bath liquid 
contains glycol. 
The core liquid is a 1 to 10% by weight NaOH-containing caustic soda, and 
the precipitation bath liquid is a 15 to 30% by weight glycol-containing 
solution in water. 
The core liquid is a 5 to 10% by weight NaOH-containing caustic soda, and 
the precipitation bath liquid is water. 
The core liquid contains caustic soda, and the precipitation bath liquid 
contains formic acid. 
The core liquid is a 1 to 10% by weight NaOH-containing caustic soda, and 
the precipitation bath liquid contains 20 to 40% by weight formic acid. 
The core liquid contains polyethylene glycol and the precipitation bath 
liquid contains formic acid. 
The core liquid contains 15 to 30% by weight polyethylene glycol in water, 
and the precipitation bath liquid contains 20 to 40% by weight formic 
acid. 
The core liquid contains polyethylene glycol, and the precipitation bath 
liquid contains NaOH. 
The core liquid contains 15 to 30% by weight polyethylene glycol in water, 
and the precipitation bath liquid is a 1 to 10% by weight NaOH-containing 
caustic soda. 
The novel features which are considered characteristic for the invention 
are set forth in particular in the appended claims. The invention itself, 
however, both as to its construction and its method of operation, together 
with additional objects and advantages thereof, will be best understood 
from the following description of specific embodiments.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A substantially homogeneous mixture of polyamide, polyethylene glycol and 
formic acid is used as spinning solution. The spinning solution is spun 
with a core liquid from a customary hollow filament nozzle, whereby the 
exit orifice of the hollow filament nozzle lies below the precipitation 
bath surface. The discharge of the hollow filaments is effected at an 
angle less than 45.degree. from the vertical, whereby the filaments first 
become picked up by a first take-up roller after about 1.5 m. After 
leaving the precipitation bath, the filaments are washed with water, 
stretched and dried. The hollow filament membranes produced in this manner 
have a lumen between 300 and 1,200.mu. and a wall thickness from 100 up to 
300.mu.. 
The process parameters and the characteristics of different hollow filament 
membranes produced according to the process of the present invention are 
summarized in the following Table. Example 1 is a comparison example. In 
comparison with Example 2, it is clear that the stretching of the hollow 
filament membranes has a great influence on the ultrafiltration 
efficiency. 
The ultrafiltraiton efficiency is determined in the following manner: 
Several hollow filaments are embedded into plastic at both ends. The 
embedding mass at both ends is then cut so that the hollow filament ends 
are open, exposing the orifices. At one end a burette, and at the other 
end a cut-off, e.g. a valve, are connected. The hollow filaments are 
filled with water and de-aerated. Thereupon the system is provided with a 
pressure of 0.2 bar, and the removal of water into the burette is 
determined. 
The hollow filaments have centrical, eccentrical or profiled form, 
according to the spinning nozzle used. 
By centric form is to be understood that the wall thickness remains the 
same along the circumference of the hollow filaments, whereas with the 
eccentric form the wall thickness along the circumference of the hollow 
filaments increases at least once continuously up to a maximal thickness 
and decreases to a minimum thickness. 
By profiled form is to be understood that the cross section is formed 
profiled perpendicular to the filament axis, i.e. provided with rib-like 
thickenings, circular, elliptical or polygonal. 
TABLE 
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Ultrafiltration 
Exam- Setting Efficiency 
Cutoff 
ple Spinning Solution 
Core Liquid 
Bath Stretching 
1/m.sup.2 h .multidot. bar 
Dalton 
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1 100 g PA 6 (Mn = 33,800) 
water 2% formic acid 
-- 100 
390 g formic acid (80%) 
30 g PEG 600 
2 100 g PA 6 (Mn = 33,800) 
water 2% formic acid 
1:1.5 300 100,000 
390 g formic acid (80%) 
30 g PEG 600 
3 100 g PA 6 (Mn = 20,000) 
1% caustic 
water 1:1.5 48 50,000 
400 g formic acid (80%) 
soda 
30 g PEG 600 
4 100 g PA 6 (Mn = 20,000) 
1% caustic 
20% formic acid 
1:1.5 500 500,000 
400 g formic acid (80%) 
soda 
30 g PEG 
5 185 g PA 6 (Mn = 20,000) 
40% PEG 600 
1% caustic soda 
1:2 1800 1,000,000 
815 g formic acid (80%) 
in water - 
30 g PEG 600 
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It will be understood that each of the elements described above, or two or 
more together, may also find a useful application in other types of 
filaments or membranes differing from the types described above. 
While the invention has been illustrated and described as embodied in 
asymmetrical hollow filament membranes of polyamide and processes for 
their production, it is not intended to be limited to the details shown, 
since various modifications and structural changes may be made without 
departing in any way from the spirit of the present invention. 
Without further analysis, the foregoing will so fully reveal the gist of 
the present invention that others can, by applying current knowledge, 
readily adapt it for various applications without omitting features that, 
from the standpoint of prior art, fairly constitute essential 
characteristics of the generic or specific aspects of this invention.