Process for preparing reverse-osmosis membrane, and membrane obtained with the process

The present invention relates to a process for preparing polypiperazinamide-based reverse-osmosis membranes comprising: PA1 (a) preparing a solution of polypiperazinamide in a polar organic solvent; PA1 (b) applying the solution on a planar support so as to form a planar body; PA1 (c) evaporating the solvent under the action of a flow of air, said air flow having, with respect to said planar support, such a speed as to produce Reynolds numbers between 5 and 100; PA1 (d) gelling the planar body by phase reversal by passing through a coagulation bath to form the membrane; and PA1 (e) washing the membrane to extract the solvent contained therein. The entire process is carried out at a temperature lower than about 30.degree. C. The invention allows the production of reverse-osmosis membranes with high flow and high mechanical characteristics.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a process for preparing reverse-osmosis 
membranes and to the membranes produced by said process. In particular, 
the field of the present invention relates to a process which is 
particularly suitable for forming planar membranes which are asymmetrical 
with respect to the plane which cuts the membrane halfway across its 
thickness. The field of the present invention further relates to 
polypiperazinamide-based membranes, which are defined as all 
polycondensation products of piperazine or of piperazine with alkyl 
substitutions in the nucleus, possibly in mixture with other diamines, 
with anhydrides or dichlorides of saturated or unsaturated, aromatic or 
heterocyclic aliphatic di-carboxylic acids, such as fumaric acid, 
mesaconic acid, adipic acid, phthalic acid, isophthalic acid, phthalic 
acids with substituted aromatic nucleus or heterocyclic acids derived from 
furan, thio-furan, pyridine, thiophene and the like, either alone or in 
mixture with one another. Polypiperazinamides belonging to the field of 
the present invention are described in the Italian patent application No. 
22179 A/82 filed by Montedison S.p.A. which related to the production of 
ultrafiltration and reverse-osmosis membranes. 
In particular, Italian patent application No. 22179 A/82 discloses a 
procedure for manufacturing asymmetrical planar supported membranes based 
on polypiperazinamides, wherein the polypiperazinamide is dissolved in a 
polar organic solvent, the obtained solution is applied on a planar or 
tubular support, the body thus formed being then gelled by being passed 
through a coagulation bath. 
The above is summarily the main field of industrial use of the invention; 
said field however does not limit the scope of the present invention, 
since the process according to the invention, in particular as described 
and claimed hereinafter, can be advantageously used in any other 
equivalent field in which reverse-osmosis membranes are prepared by means 
of a solubilization step and a phase-reversal step. 
2. Prior Art 
In practice, however, it has been observed that the process illustrated in 
the Italian patent application No. 22179 A/82 is not free from 
disadvantages. In particular planar membranes thus obtained have scarce 
mechanical resistance, which prevents or at least severely complicates the 
formation of coiled-spiral modules, so that the membranes can be used only 
in the planar or tubular form in which they were originally formed. This 
constitutes a limitation in practical use, since the formation of 
coiled-spiral modules is in itself particularly useful and practical for 
the final use of the membranes. 
The aim of the present invention is therefore to eliminate the above 
described disadvantages by providing a polypiperazinamide-based 
asymmetrical planar membrane having considerable mechanical resistance 
such as to allow the working of said planar membrane by folding to form 
coiled-spiral membrane modules, without the overall properties of the 
membrane suffering from any appreciable degradation. 
An object of the invention is to form the membrane on a non-woven-fabric 
support in a planar configuration which can subsequently be folded to form 
coiled-spiral modules. 
SUMMARY OF THE INVENTION 
This aim, this object and others are achieved by the present invention, 
which is characterized in that it comprises the following succession of 
steps: 
(a) preparing a solution of polypiperazinamide in a polar organic solvent; 
(b) applying said solution on a planar support so as to form a planar body; 
(c) evaporating the solvent under the action of a flow of air; 
(d) gelling said planar body by phase reversal by passing through a 
coagulation bath to form the membrane; a 
(e) washing the membrane to extract the solvent contained therein. 
The above described aim and objects are further achieved by a 
polypiperazinamide-based membrane adapted to produce a coiled-spiral 
membrane module according to the invention, characterized in that the 
thickness of said membrane is smaller than 100 microns and preferably 
greater than 50. 
The polypiperazinamide solutions are prepared, for example, in 
appropriately constructed and preset thermoagitators so that the 
temperature of the solution does not exceed 35.degree. C. and is not lower 
than 10.degree. C. The step of preparing the solution sequentially 
comprises the loading of the solvent, the addition of approximately 50% of 
the polymer, the beginning of the agitation, and the successive gradual 
addition of the remaining polymer. The agitation time is preferably 
shorter than 10 hours. 
The solution is filtered with a filtering gauze having a mesh of less than 
5 microns and preferably of 2-3 microns and under a head of nitrogen. The 
filtering solution is allowed to degas until the gases it contains are 
eliminated. The filtration and degassing of the solution are fundamentally 
important for subsequently preparing membranes which are uniform, 
mechanically resistant and stable over time. 
The characteristics of the polymer must be such as to ensure an intrinsic 
viscosity at 20.degree. C. of between 2.0 and 2.9 dl/g in 
tetrachloroethane or between 1.6 and 2.0 dl/g in formic acid. The 
characteristics of intrinsic viscosity, and therefore of relative 
molecular mass, of the polymer are extremely important in preparing 
membranes which are mechanically resistant and adapted to produce 
coiled-spiral modules. The humidity content of the polymer is lower than 
0.5%. This fact the phase reversal kinetics and is extremely important for 
preparing high-flow membranes. The solutions are preferably prepared using 
a polar organic solvent chosen from Class S of the classification of H. 
Burrel. Said solvent is preferably formic acid. The concentration of the 
polymer in the polymeric solution thus obtained is preferably between 16% 
and 19% by weight. The degassed solution is continuously applied in 
industrial quantities on a support of non-woven fabric in polyester or 
polypropylene with a thickness between 50 and 300 microns. The thickness 
of the film of solution applied on the support varies between 50 and 100 
microns according to the characteristics of the required membranes. The 
membrane being formed travels for a certain distance in a controlled 
environment where the solvent partially evaporates. The evaporation time 
is between 60 and 250 seconds; the application speed of the membrane 
varies between 0.3 m/min and 2 m/min; the temperature of the support is 
between 15.degree. and 28.degree. C.; and the relative humidity of the 
evaporation region is preferably higher than 80%. The temperature of the 
air of the evaporation region is preferably between 15.degree. and 
28.degree. C. The flow of the air overlying the planar support of the 
membrane must be such as to produce speeds resulting in Reynolds numbers 
between 5 and 100 and preferably between 10 and 20. The Reynolds numbers 
are expressed by Re=cvd/u, where v is the relative speed of the air with 
respect to the planar body of the membrane being formed, c is the density, 
d is the equivalent diameter and n is the viscosity. After passing through 
the evaporation region, the membrane being formed is immersed in a 
coagulation bath, where the phase reversal is completed. The bath contains 
demineralized water with a conductivity between 5 and 20 microsiemens/cm 
at 20.degree. C. The temperature of the coagulation bath is lower than 
15.degree. C., preferably lower than 5.degree. C. and more preferably 
lower than 3.degree. C. The angle of immersion of the film being formed in 
the coagulation bath must be between 80.degree. and 90.degree.. The 
immersion time is between 15 minutes and 25 minutes. The pH of the bath is 
between 2 and 4. The obtainment of mechanically resistant asymmetrical 
supported membranes with intermediate rejection according to the 
invention, so as to be usable for preparing coiled spiral modules, is 
controlled by the above described parameters and by the indicated values 
of the respective ranges of variability. 
Another operation which is decisive in obtaining membranes usable in coiled 
spiral modules is the washing of the membrane immediately after it has 
been formed. The membrane must be washed with mains water or with basic 
solutions at a temperature higher than 15.degree. C. but lower than 
50.degree. C. and preferably lower than 30.degree. C. continuously until 
the bath in which the manufactured membrane is immersed reaches a neutral 
pH. The membranes thus manufactured need no thermal treatment, their flow 
is between 30 and 60 lt/h/sq.m and their saline rejection is between 30% 
and 95% (at 25.degree. C., 2000 ppm of NaCl, 30 Atm); their mechanical 
resistance is such as to be usable to prepare coiled-spiral modules; the 
chemical resistance to contact with acids (up to pH=1.5) and bases (up to 
pH=12) is excellent; and the resistance to oxidating agents and in 
particular to chloride (up to a concentration of 50 ppm) is optimum. 
The obtained membrane has a thickness of less than 100 microns and 
preferably of more than 50 microns. The membrane has a dense surface layer 
and a spongy underlying layer, as illustrated in the accompanying 
photographs.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
EXAMPLE 1 
Preparation of a reverse-osmosis membrane and related spiral module with 
saline rejection equal to 95%. A 100-g polymeric solution is prepared with 
82 g of formic acid (titer 99/100%) and 18 g of polymer Poly-Trans 2.5, 
dimethylpiperazinethiofurazanamide, characterized by an intrinsic 
viscosity in tetrachloroethane higher than 2.3 dl/g. The solution is 
agitated for 10 hours at 20.degree. C. and then filtered with an AISI 316 
2-micron steel mesh and degassed under a vacuum. The solution is applied 
on a support of non-woven polyester fabric and a speed of 0.3 m/min, at a 
temperature of 20.degree. C., with a relative humidity of 80% in the 
solvent evaporation region, and such an air speed as to define 
fluidodynamic conditions characterized by a Reynolds number of 15. The 
thickness of the polymeric film is 100 microns. The coagulation bath is 
water, characterized by a conductivity of 20 microsiemens/cm at 20.degree. 
C. and by a pH of 3. The temperature of the bath is 0.5.degree. C. and the 
time of permanence in the bath is 18 minutes. The formed membrane is 
washed for 5 minutes in water at 15.degree. C. and stored for 24 hours in 
demineralized water. 
Five samples of the planar membrane are characterized with a planar-cell 
system fed by an aqueous solution of NaCl at 2000 ppm, 25.degree. C. and 
30 atm and with a flow-rate of 400 lt/h. The results are listed in Table 
1. 
Table 1. Flow and saline rejection values of the planar membranes of 
Example 1. 
TABLE 1 
______________________________________ 
flow saline rejection 
membrane 
(lt/h/sq.m) 
(%) test time (hours) 
______________________________________ 
ST74E11 50 95 20 
ST74E12 39 95 20 
______________________________________ 
A type 495T spiral module was coiled using the manufactured membrane and 
yielded the following values during characterization: flow 45 lt/h/sq.m, 
saline rejection 95%. Characterization was performed with an aqueous 
solution of NaCl at 2000 ppm, 25.degree. C., 30 atm, an axial flow rate of 
2000 it/h and a pH of 6.7. 
Microphotographs of the cross section of the membranes obtained are 
illustrated in FIGS. 1a, 1b, 2a, 2b and 2c. 
EXAMPLE 2 
Preparation of a reverse-osmosis membrane and of a related spiral module 
with 85% saline rejection. 
A 100-g polymeric solution is prepared with 82 g of formic acid (titer 
99/100%) and 18 g of polymer as in Example 1, except that the solution is 
applied with an application speed of 0.7 m/minute and an air speed 
corresponding to a Reynolds number of 13. The produced planar membrane is 
characterized as in Example 1 and the results are listed in Table 2. 
Table 2. Flow and saline rejection values of the planar membranes of 
Example 2. 
______________________________________ 
flow saline rejection 
membrane 
(lt/h/sq.m) 
(%) test time (hours) 
______________________________________ 
ST74E6 42 85 20 
ST74E7 38 88 20 
______________________________________ 
A type 470T spiral module was coiled with the produced membrane and yielded 
the following values during characterization in the conditions listed in 
Example 1: a flow of 40 lt/h/sq.m, and saline rejection of 85%. 
Microphotographs of the cross section of the produced membranes are 
illustrated in FIGS. 3a, 3b, 4a, 4b and 4c. 
EXAMPLE 3 
Preparation of a reverse-osmosis membrane and of the related spiral module 
with a saline rejection of less than 55%. 
The polymeric solution and the planar membrane are prepared as in Example 2 
except that the application speed is equal to 1 m/minute and the Reynolds 
number is 20. The planar membrane is characterized as in Examples 1 and 2 
and the results are listed in Table 3. Table 3. Flow and saline rejection 
value of the planar membranes of Example 3. 
______________________________________ 
flow saline rejection 
membrane 
(lt/h/sq.m) 
(%) test time (hours) 
______________________________________ 
SM 52 98 50 24 
SM 52 95 51 24 
SM 52 100 54 24 
SM 52 92 54 24 
SM 52 87 53 24 
SM 52 85 49.7 24 
______________________________________ 
A type 400T spiral module was coiled using the produced membrane and 
yielded the following values during characterization at the conditions of 
Examples 1 and 2: a flow of =90 lt/sq.m, and saline rejection of 50%. 
Microphotographs of the cross section of the produced membranes are 
illustrated in FIGS. 5a, 5b and 5c. 
EXAMPLE 4 
Preparation of a reverse-osmosis membrane and of the related spiral module: 
influence of evaporation temperature. 
A polymeric solution and the planar membrane are prepared as in Example 1 
except that the solvent evaporation temperature is 40.degree. C. Then a 
comparison membrane is applied with an evaporation temperature of 
15.degree. C. The membrane is characterized as in Examples 1 and 2 and the 
results are listed in Table 4. Table 4. Flow and saline rejection values 
of the planar membranes of Example 4. 
______________________________________ 
flow saline rejection 
membrane 
(lt/h/sq.m) 
(%) test time (hours) 
______________________________________ 
SM 450A 21.8 81.5 24 
SM 450B 24.6 82.8 24 
T = 15.degree. C. 
SM 448A 33 94.1 24 
SM 448B 32 93.8 24 
SM 448C 29 93.6 24 
SM 448D 32 96.7 24 
SM 448E 35 95 24 
______________________________________ 
Two type 400T spiral modules were coiled using the produced membrane and 
yielded the following results during characterization at the conditions of 
Example 1: with membrane at T=40.degree. C.: flow=20 l/h/sq.m; rejection 
82% with membrane at T=15.degree. C.: flow=35 l/h/sq.m; rejection 95% 
EXAMPLE 5 
Influence of the filtration of the polymeric solution on the 
characteristics of the membrane and of the related modules. 
The polymeric solution and the planar membrane are prepared as in Example 1 
except that part of the solution is not filtered. 
The membranes produced with filtered and non-filtered solutions are 
characterized as in Examples 1 and 2 and the results are listed in Table 
5. Table 5. Flow and saline rejection values of the planar membranes of 
Example 5 
______________________________________ 
flow saline rejection 
membrane 
(lt/h/sq.m) 
(%) test time (hours) 
______________________________________ 
filtered 
solution 
SM 323A 85 87 24 
SM 323B 73 91.2 24 
SM 323C 79 91.2 24 
SM 323D 57 92.3 24 
SM 323E 69 90.6 24 
SM 323F 76 91 24 
non-filtered 
solution 
SM 325A 214 43.4 24 
SM 325B 142 49.4 24 
SM 325C 167 54 24 
SM 325D 209 47 24 
SM 325E 167 50.3 24 
SM 325F 146 56.2 24 
______________________________________ 
Two type 400T spiral modules were coiled using the produced membrane and 
yielded the following results during characterization in the conditions of 
Example 1: 
______________________________________ 
non-filtered 
Flow 174 l/h/sq.m 
Rejection 50% 
solution 
filtered Flow 73 l/h/sq.m 
solution 
______________________________________ 
EXAMPLE 6 
Influence of the thickness of the planar membrane and of the viscosity of 
the polymer on the characteristics of the membranes and of the related 
modules. 
The polymeric solution is prepared as in Example 1 except that the 
intrinsic viscosity of the polymer is equal to 1.33 dl/g. A planar 
membrane having a thickness of less than 50 microns and one having a 
thickness of 100 microns are prepared. The membranes obtained with the 
different two thicknesses are characterized as in Example 1 and 2 and the 
results are listed in Table 6. Table 6. Flow and saline rejection values 
of planar membranes of Example 6 
______________________________________ 
flow saline rejection 
membrane 
(lt/h/sq.m) 
(%) test time (hours) 
______________________________________ 
50 microns 
thickness 
MS 11A 1 
19 59 24 
MS 11B 1 
12 64 24 
MS 11C 1 
15 58 24 
MS 11D 1 
18 56 24 
100 microns 
thickness 
MS 11A 2 
51 91.6 24 
MS 11B 2 
49 92 24 
MS 11C 2 
51 91.8 24 
MS 11D 2 
56 90 24 
______________________________________ 
Two type 400T spiral modules were coiled using the produced membrane and 
yielded the following results during characterization in the conditions of 
Examples 1 and 2 yielded the following results; as the membrane was 
furthermore produced with a polymer having viscosity=1.33 dl/g, a 
comparison is made with a module produced with a membrane obtained in 
identical conditions but with polymer viscosity=2.4 dl/g. 
______________________________________ 
Flow Rejection 
Membrane Viscosity l/h/sq.m (%) 
______________________________________ 
50 microns low 26 49 
100 microns low 60 81 
100 microns high 36 94 
______________________________________