Process for producing a semi-permeable membrane by extrusion

A process for the production of a porous organic material, in particular a semi-permeable membrane, comprising a plurality of separate passageways, including dissolving a polymer in a solvent, using a drawplate having needles of a shape conjugate with those of the passageways and of external dimensions between 0.7 and 1.2 times those of the passageways and an extrusion orifice of a shape conjugate those of the extrudate and of internal dimensions comprising between 0.8 and 1.2 times those of the extrudate, injecting around the needles a solution having a viscosity greater than 500 mPa.s and introducing a centering fluid into the interior of these needles, recovering the extrudate at the output of the drawplate in a precipitating medium in such a manner that the extrudate travels in the medium a distance such that 800.times. e<de<3 m, (e representing the maximum distance of travel for a precipitaing fluid for obtaining complete precipitation of the extrudate), and taking up the precipitated extrudate with a linear take-up speed Ve such that Ve/Vs<1.2 (Wv representing the average speed of the extrusion).

This invention relates to a process for the production of a porous organic 
material, in particular an organic semi-permeable membrane comprising a 
plurality of separate longitudinal passageways, the process being of the 
type comprising dissolving a polymer in a solvent, extruding the solution 
obtained through an extrusion die provided with a plurality of separate 
conduits on the interior of which is introduced a centering fluid, and on 
the exterior of which the solution flows, and finally precipitating the 
extrudate obtained. 
BACKGROUND AND OBJECTS OF THE INVENTION 
Organic semi-permeable membranes provided with a plurality of separate 
longitudinal passageways have several advantages with respect to 
conventional hollow fibers. In effect, they have a high mechanical 
strength which notably simplifies problems of handling. Moreover, the 
speeds of production are accelerated due to the fact that a lesser length 
of membrane is necessary for the production of a bundle. Finally, the 
provision of filtration modules is greatly simplified. Such advantages 
have led to a the development of use of these membranes of which the 
production techniques or the applications are particularly described in 
the following patents: DE-A-3 022 313; WO-A-8102750; FR A-2,445,163 and JP 
5,982,906. However, known techniques and particularly those described in 
these patents do not permit mastering in a rational manner all of the 
parameters of production of these membranes and confering thereon 
particular predefined structures or mastering in a very precise manner the 
external dimensions of these membranes as well as their passageways. 
The present invention seeks to overcome these deficiencies and has as its 
principal object to provide a process permitting production of membranes 
having particularly well defined structures and of which the dimensions, 
as well as those of their passageways, are precisely defined. 
DESCRIPTION OF THE INVENTION 
Thus, the invention comprises a process characterized in: 
using a drawplate or extrusion plate comprising needles of shapes conjugate 
to those of the passageways and of external dimensions comprising between 
0.7 and 1.2 times those of the passageways and an extrusion orifice of a 
shape conjugate to that of the extrudate and having internal dimensions 
between 0.8 and 1.2 times those of said extrudate, 
arranging the drawplate in such a manner as to extrude the solution 
essentially vertically, 
injecting around the needles a solution having a viscosity greater than 500 
millipascal seconds (as measured with a "Contraves" Rheomat 115, rate of 
shearing of 28 s.sup.-1) with a flow rate adapted in such a manner as to 
obtain an average speed of extrusion Vs at the output of the extrusion 
plate, 
recovering the extrudate from the output of the drawplate in a medium which 
is a non-solvent with respect to the polymer and able to precipitate the 
polymeric solution, in such a manner that the extrudate travels in the 
medium a distance d.sub.e such that 800.times.e&lt;d.sub.e &lt;3 m where e 
represents the maximum distance of travel by the precipitating fluid on 
the interior of the extrudate for obtaining the complete precipitation of 
the extrudate, 
and taking up the precipitated extrudate with a linear take-up speed Ve 
such that Ve/Vs&lt;1.2. 
This process permits production of a membrane in which the polymeric 
material is composed of an active layer on the surface of the membrane and 
of an intermediate thickness of a porosity greater than that of the active 
layer. 
The interest in such membranes resides in the fact that an active layer 
constitutes a filtration screen avoiding that the filtration would take 
place in the depth of the polymeric material and at the same time the 
membranes do not become irreversibly saturated. 
Further, all of the operative conditions carried out in this process are 
adapted to obtain the complete precipitation of the membrane before it can 
encounter an obstacle which would tend to deform it, while precisely 
controlling the stretching phenomena of the extrudate which conditions the 
external dimensions of this membrane as well as those of the passageways. 
According to a first preferred embodiment, a centering fluid which is 
non-precipitating with respect to the polymeric solution is introduced 
into the conduits of the drawplate. The membrane produced thus has a 
single active layer on its external surface. 
This process is especially very advantageous when the membranes produced 
are planar as the latter may then replace a portion of the stack in the 
modules of a filter press. 
In this case, the centering fluid used is preferably a solution miscible 
with the precipitating fluid and able to have a liquid state above a 
predetermined temperature tg, and pass to a gel state below said 
temperature. This solution is heated to a temperature greater than tg in 
such a manner as to be introduced in the liquid state into the conduits of 
the drawplate, then the temperature is lowered at the output of the 
drawplate in such a manner as to gel the liquid. 
The utilization of such a solution as well as the centering fluid permits 
rigidifying the extrudate at the output of the drawplate and thus assuring 
a better support thereof and assuring obtaining precisely formed 
passageways. 
In addition, one may also preferably cause the extrudate to travel a 
distance less than 0.3 m in a gaseous atmosphere which is 
non-precipitating with respect to the polymeric solution, before recovery 
in the precipitating medium. 
This passage in a non-precipitating gaseous atmosphere presents two 
advantages: first, it permits stretching however slight the extrudate and 
consequently causing a variation in a controlled manner the external 
dimensions of the membrane. Secondly, it permits obtaining a rearrangement 
of the polymeric material forming the extrudate before precipitating the 
latter, and especially eliminating the inflation to which this polymeric 
material is generally subjected at the output of the drawplate. 
According to a second preferred embodiment, the centering fluid introduced 
into the conduits of the drawplate is a non-solvent fluid with respect to 
the polymer and is able to precipitate the polymeric solution. 
The membrane produced has not only an active layer on its external surface 
but also active layers on the surface of each passageway. 
The advantages of such membranes are of three types: in the first place, 
they may function with a liquid circulating in the passageways or 
circulating on the exterior of the membrane and are therefor very easily 
washable. Further, they have a very good mechanical strength. Finally, 
such membranes permit preventing, in the case of microfiltration or 
ultrafiltration, the proliferation of bacteria on the interior of the 
polymeric material which may cause the saturation of the membrane or even 
destroy the same if the latter is biodegradable. 
Further, as for the first preferred embodiment, and in the same goal, the 
extrudate may be advantageously caused to travel a distance of less than 
500 c in a gaseous atmosphere before the recovery in the precipitating 
medium, c representing the minimum distance between the surface of the 
passageways and the external surface of the extrudate. 
Finally, a third embodiment may comprise introducing into the different 
conduits of the drawplate centering fluids of different natures in such a 
manner as to obtain passageways with different surface porosities.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
The polymeric solution obtained by dissolving a polymeric material in a 
solvent is initially stored in a reservoir 1. In a parallel manner, a 
centering fluid is stored in a reservoir 2. 
Each of these reservoirs 1, 2 is connected to a drawplate or extrusion 
plate 3 by means of feed conduits 4, 5 arranged so that the centering 
fluid flows through the interior of needles of this drawplate 3 and the 
polymeric solution on the exterior of these needles. 
The drawplate 3 comprises needles of a shape conjugate with that of the 
passageways of the membrane to be produced and of external dimensions 
comprising between 0.7 and 1.2 times that of said passageways. They 
comprise further an extrusion orifice of a shape conjugate with that of 
the extrudate, that is, of a cylindrical shape for the production of a 
cylindrical membrane or of a rectangular shape of lesser length than width 
for the production of a planar membrane. The internal dimensions of this 
extrusion orifice comprise between 0.8 and 1.2 times that of the membrane 
to be produced. 
Further, each of these passageways is provided with a circulation pump 6, a 
filter 7 and a heat exchanger 8 adapted to maintain constant the 
temperature of the fluid and the solution. 
Directly below this drawplate 3, and arranged in such a manner as to 
extrude the solution essentially vertically, is a recovery tank filled 
with a liquid which is a non-solvent with respect to the polymer and able 
to precipitate the polymeric solution. The recovery tank 9 is positioned a 
distance from the drawplate 3, such that the extrudate travels a distance 
da before reaching the surface of the liquid. 
In the bottom of this tank 9 is arranged a return pulley 10 permitting 
guiding the membrane through another reversing pulley 11 toward a take-up 
spool 12. The linear take-up speed Ve of this spool 12 is controlled as a 
function of the average extrusion speed Vs at the output of the drawplate 
3 in such a manner that Ve/Vs&lt;1.2. 
The first return pulley 10 is itself arranged in the recovery tank 9 at a 
depth such that the extrudate travels vertically for a distance in the 
liquid before coming into contact with this return pulley. 
Three examples of carrying out the process for the production of a planar 
membrane having seven passageways are described below. 
As shown schematically in FIG. 2, the drawplate 3 used has an essentially 
rectangular cross-section of which the extrusion orifice comprises a 
linear length of 12.8 mm and a width of 2 mm. On the interior of this 
drawplate 3 are arranged seven needles 13 of an external diameter of 0.81 
mm and of an internal diameter of 0.51 mm. The distance separating two 
needles 13 is 0.89 mm. 
EXAMPLE 1 
An example of the production of a membrane comprising an active layer on 
the external surface. 
In this case and with a drawplate such as described above, the maximum 
distance of travel by the precipitating fluid is on the order of 1 mm. 
In this example, the recovery tank 9 is filled with water and has been 
placed in such a manner that the distance da is 0.5 cm, while the return 
pulley 10 is arranged in this tank 9 at a depth such that de=102 cm. 
The polymeric solution has the following composition (in mass percent): 
______________________________________ 
AMOCO "UDEL 3500" Polysulfone 
28% 
TRITON surfactant 100 30% 
(NMP) 42% 
______________________________________ 
At an extrusion temperature of 25.degree. C., this solution has a viscosity 
of 127,000 m.Pa.s with a Contrave Rheomat 115 at a shear rate of 28 
s.sup.-1. Further, it is extruded with a flow rate Qs=50 cm.sup.3 /mn. 
The non-precipitating centering fluid has the following composition in 
percent by mass: 
______________________________________ 
NMP 95% 
Water 5% 
______________________________________ 
It is introduced into the needles 13 with a flow rate Qf=40 cm.sup.3 /mn. 
Finally, the take-up spool 12 is controlled in such a manner that the 
take-up speed Ve is 2.4 m/mn. 
Under these conditions, the product obtained is a membrane comprising seven 
passageways not having active layers on their surface, but having an 
active layer on the external surface and an intermediate layer having a 
porosity greater than than of the active layer. 
This membrane has a width of 12.9 mm and a thickness of 2.2 mm, the 
internal diameter of each of the passageways being 0.85 mm. Finally, the 
value of the hydraulic permeability, measured with a fluid flowing on the 
exterior of the membrane, is 1.8.times.10.sup.-10 m/s.Pa for water at 
25.degree. C. and a P of 0.1.times.10.sup.5 Pa to 1.times.10.sup.5 Pa. 
EXAMPLE 2 
An example of the production of a membrane comprising an active layer on 
the external surface. 
The drawplate used is the same as in Example 1. The composition of the 
polymeric solution is also the same. However, it is extruded at a 
temperature of 70.degree. C. at which it has a viscosity of 9,000 m.Pa.s 
measured with a Contraves Rheomat 115, at a shear rate of 28 s.sup.-1. 
The other parameters are as follows: 
da=0.5 cm 
de=102 cm 
Qs=50 cm.sup.3 /mn 
Qf=40 cm.sup.3 /mn 
Vc=2.4 m/mn. 
The centering fluid comprises polyethylene glycol of an average molar mass 
of 10,000, of which the fusion point is 60.degree. to 62.degree. C. At the 
extrusion temperature of 70.degree. C. this fluid is therefor a liquid, 
while it solidifies rapidly in air and in water which is at a temperature 
of 30.degree. C. 
It should be noted that this centering fluid is eliminated after the 
extrusion, by rinsing with water at a temperature greater than the 
temperature tg, on the order of 80.degree. C. 
The membrane obtained has, as in Example 1, an active layer on its external 
surface. Its dimensions are 12.9 mm by 2.2 mm, and the diameter of the 
passageways is 0.81 mm. Finally, the permeability of this membrane, 
measured with a fluid flowing on the exterior thereof is 
1.6.times.10.sup.-10 m/s.Pa for water at 25.degree. C. and a P of 
0.1.times.10.sup.5 Pa to 1.times.10.sup.5 Pa. 
EXAMPLE 3 
An example of the production of a membrane comprising an active layer on 
the surface of each passageway, and an active layer on its external 
surface. 
In this case, and again with the same drawplate, the maximum distance 
travelled by the precipitating fluid is on the order of 0.5 mm as the 
centering fluid is itself precipitating. 
The other parameters are: 
da=2.8 cm 
de=82 cm 
Ve=1.6 m/mn 
Qs=33.5 cm.sup.3 /mn at 40.degree. C. 
Qf=38.9 cm.sup.3 /mn 
Vc=2.4 m/mn. 
The polymeric solution has the following composition in mass percent: 
______________________________________ 
cellulose diacetate (EASTMAN E398-10) 
20% 
ethylene glycol 30% 
N-methylpyrrolidone 50% 
______________________________________ 
Its viscosity is 370,000 m Pa.s at the extrusion temperature of 40.degree. 
C. 
The precipitating centering fluid is water. 
Under these conditions, the product obtained is a membrane comprising seven 
passageways and comprising an active layer on the surface of each 
passageway, an active layer on its surface, and an intermediate thickness 
of a porosity greater than that of said active layers. 
This membrane has a width of 12.9 mm, a thickness of 2.4 mm, the internal 
diameter of the passageways being 1.05 mm. Finally, the value of the 
hydraulic permeability measured with a fluid flowing in the passageways is 
3.6.times.10.sup.-10 m/s.Pa for water at 25.degree. C. and a P of 
0.1.times.10.sup.5 to 1.times.10.sup.5 Pa.