Microfiltration Membrane With Improved Filtration Properties

Abstract

Integrally asymmetric flat membrane for microfiltration comprising at least 40 wt. % of a hydrophobic first sulfone polymer and a hydrophilic second polymer with a pore size distribution over the membrane wall, having a separating layer in the wall's interior, and also having, in the direction from this separating layer, pore sizes increasing towards the surfaces, the second surface having pores with a mean diameter of at least 1 μm. The membrane comprises the hydrophilic second polymer in a concentration of 0.1-10 wt. %. The separating layer is located in an region facing the first surface, and the pore size passes through a maximum in the direction from the asymmetrical region towards the second surface. Method for producing this membrane from a casting solution comprising the hydrophobic first sulfone polymer and the hydrophilic second polymer in a solvent system, the method comprising the steps of pouring the casting solution, conditioned to a molding temperature, onto a carrier to form a film, which carrier has a temperature that is higher in comparison to the molding temperature, conveying the film through a climate-controlled zone, initiating the coagulation in a coagulation bath for the formation of a membrane structure, withdrawing the membrane structure from the carrier with a speed that is increased in comparison to the carrier speed, stabilizing, extracting, and subsequently drying the membrane.

1 . Integrally asymmetric membrane in the form of a flat sheet, in particular for microfiltration, based on a film-forming hydrophobic first polymer from the group of aromatic sulfone polymers, the membrane having a membrane wall with a first and a second porous surface and an interior situated between the surfaces, possessing a porous structure with a pore size distribution over the membrane wall, and having a separating layer in the wall's interior with a minimal pore size, and also having, in the direction from this separating layer towards the first surface, a first asymmetrical region, and, towards the second surface, a second asymmetrical region, with pore sizes increasing towards the surfaces, and the second surface having pores with a mean diameter of at least 1 μm, characterized in that the membrane comprises at least 40 wt. % of the film-forming hydrophobic first polymer and also comprises a hydrophilic second polymer, the concentration of the hydrophilic second polymer being 0.1-10 wt. % relative to the weight of the membrane, the separating layer with minimal pore size is located in an region of the membrane wall facing the first surface, and the pore size passes through a maximum in the direction from the second asymmetrical region towards the second surface. 
     
     
         2 . Membrane according to  claim 1 , characterized in that the maximum of the pore size is located in an essentially isotropic region adjacent to the second asymmetrical region, in which isotropic region the pore size is essentially constant, whereby the isotropic region extends over 15 to 70% of the membrane wall. 
     
     
         3 . Membrane according to  claim 1 , characterized in that the aromatic sulfone polymer is a polysulfone or a polyethersulfone. 
     
     
         4 . Membrane according to  claim 1 , characterized in that the hydrophilic second polymer has an average molecular weight MW of more than 10 000 daltons. 
     
     
         5 . Membrane according to  claim 4 , characterized in that the hydrophilic second polymer is polyvinylpyrrolidone, polyethylene glycol, polyvinyl alcohol, polyglycol monoester, polysorbitate, carboxymethylcellulose, polyacrylic acid, polyacrylate, or a modification or a copolymer of these polymers. 
     
     
         6 . Membrane according to  claim 1 , characterized in that it further contains a hydrophilic third polymer, which is different from the hydrophilic second polymer, whereby the hydrophilic third polymer is a hydrophilically modified aromatic sulfone polymer. 
     
     
         7 . Membrane according to  claim 6 , characterized in that the hydrophilically modified aromatic sulfone polymer is present in a concentration of 1 to 50 wt. % relative to the weight of the membrane. 
     
     
         8 . Membrane according to  claim 6 , characterized in that the hydrophilically modified aromatic sulfone polymer is based on the hydrophobic first aromatic sulfone polymer. 
     
     
         9 . Membrane according to  claim 6 , characterized in that the hydrophilically modified aromatic sulfone polymer is a sulphonated sulphone polymer. 
     
     
         10 . Membrane according to  claim 1 , characterized in that the ratio of the average size of the pores in the first surface to the average size of the pores in the second surface is at least 5. 
     
     
         11 . Membrane according to  claim 1 , characterized in that it has a transmembrane flow TMF of at least 10 000 l/(m 2 ·h·bar) and the transmembrane flow at the same time fulfills the condition:
     TMF≧ 85 000 ·d   max   2      whereby d max  is the diameter of the maximum separating pore determined by means of the bubble point method.   
     
     
         12 . Membrane according to  claim 11 , characterized in that the transmembrane flow TMF is at least 15 000 l/(m 2 ·h·bar). 
     
     
         13 . Membrane according to  claim 1 , characterized in that it has a volume porosity of at least 75 vol. %. 
     
     
         14 . Membrane according to  claim 1 , characterized in that it has a filtrate flow rate for an aqueous BSA solution of at least 750 l/h·m 2 , whereby the filtrate flow rate is determined 15 minutes after the initiation of a filtration of an aqueous BSA solution with a BSA concentration of 2 g/l and a pH value of 5 at a transmembrane pressure of 0.4 bar. 
     
     
         15 . Membrane according to  claim 14 , characterized in that the filtrate flow rate is at least 1 000 l/h·m 2 . 
     
     
         16 . Membrane according to  claim 1 , characterized in that it has a residual filtrate flow rate of at least 35%, whereby the residual filtrate flow rate is defined as the ratio of the filtrate flow rate after 120 minutes to the filtrate flow rate after 5 minutes during a filtration of an aqueous BSA solution with a BSA concentration of 2 g/l and a pH value of 5 at a transmembrane pressure of 0.4 bar. 
     
     
         17 . Membrane according to  claim 16 , characterized in that the filtrate flow rate constant is at least 45%. 
     
     
         18 . Membrane according to  claim 14 , characterized in that it has a nominal pore of 0.2 μm. 
     
     
         19 . A method for producing an integrally asymmetric membrane in the form of a flat sheet, whereby the method comprises the following steps:
 a. producing a homogeneous casting solution from a polymer component and a solvent system, the polymer component consisting of 10-25 wt. %, relative to the weight of the solution, of a hydrophobic first polymer from the group of aromatic sulfone polymers and 2-20 wt. %, relative to the weight of the solution, of a hydrophilic second polymer, and the solvent system consisting of 5-80 wt. %, relative to the weight of the solvent system, of a solvent for the polymer component, 0-80 wt. %, relative to the weight of the solvent system, of a latent solvent for the polymer component, as well as 0-70 wt. %, relative to the weight of the solvent system, of a non-solvent for the polymer component,   b. conditioning the homogeneous casting solution to a molding temperature.   c. pouring the homogeneous casting solution onto a carrier to form a film, which carrier can be temperature controlled and has a temperature that is at least 15° C. higher than the molding temperature of the casting solution, and which carrier has a speed v 1 ,   d. conveying the film located on the carrier through a climate-controlled zone having a temperature in the range between 35 and 55° C. and a relative humidity in the range from 40 to 75%,   e. introducing the film located on the carrier into a coagulation medium and initiating the coagulation of the film for the formation of a membrane structure,   f. withdrawing the membrane structure from the carrier within the coagulation medium by means of withdrawal device moving with a speed of v 2 , the speed v 2  being greater than the speed v 1  of the carrier, by which means the membrane structure is drawn,   g. stabilizing the membrane structure in the coagulation medium,   h. extracting the resulting membrane and subsequently drying the membrane.   
     
     
         20 . Method according to  claim 19 , characterized in that the aromatic sulfone polymer is a polysulfone or a polyethersulfone. 
     
     
         21 . Method according to  claim 20 , characterized in that the hydrophilic second polymer has an average molecular weight MW of more than 10 000 daltons. 
     
     
         22 . Method according to  claim 21 , characterized in that polyvinylpyrrolidone, polyethylene glycol, polyvinyl alcohol, polyglycol monoester, polysorbitate, carboxymethylcellulose, polyacrylic acid, polyacrylate, or a modification or a copolymer of these polymers is used as the hydrophilic second polymer. 
     
     
         23 . Method according to  claim 19 , characterized in that the viscosity of the homogeneous casting solution is set to a viscosity below 10 Pa s, determined at 40° C. 
     
     
         24 . Method according to  claim 19 , characterized in that the casting solution further contains 0.2-20 wt. %, relative to the weight of the casting solution, of a hydrophilic third polymer, which is different from the hydrophilic second polymer, the hydrophilic third polymer being a hydrophilically modified aromatic sulfone polymer. 
     
     
         25 . Method according to  claim 24 , characterized in that the hydrophilically modified aromatic sulfone polymer is a sulphonated sulphone polymer. 
     
     
         26 . Method according to  claim 19 , characterized in that the ratio of the speed v 2  of the withdrawal device to the speed v 1  of the carrier lies in the range between 1.05:1 and 1.2:1. 
     
     
         27 . Method according to  claim 19 , characterized in that a polar, aprotic solvent or a protic solvent is used as the solvent. 
     
     
         28 . (canceled)