Hollow fiber membrane and method for manufacturing thereof
The present invention concerns a semipermeable hollow fiber membrane having an outer wall surface, an inner wall surface and an interior lumen extending along the length thereof and having the selective layer on the outer wall surface with a surface roughness below 10 nm. According to the invention the membrane has the smallest pore size on the outer wall surface, and has an outer wall surface which is smooth, continuous and homogeneous in a nanoscopic scale and four to five distinct layers of different pore size and density. Further the present invention concerns a process for manufacturing thereof and the use thereof.
1. A semipermeable hollow fiber membrane comprising:
an outer wall surface, an inner wall surface, and an interior lumen extending along the length thereof and having a selective layer on the outer wall surface, characterized by having a smallest pore size on the outer wall surface, and wherein the outer wall surface is smooth, continuous and homogeneous in a nanoscopic scale, being virtually devoid of roughness with roughness parameters R a and R q of not more than 10 nm, a roughness being measured with an atomic force microscope (AFM), and calculating the roughness parameters R a and R q using the following equations:
R
a
=
1
N
∑
i
=
1
N
Z
i
R
q
=
1
N
∑
i
=
1
N
Z
i
2
with N being the total number of data points and Z i being the height of a data point above an average picture level, and wherein a membrane wall between the outer wall surface and the inner wall surface has at least four layers with different pore sizes and mass densities, and wherein a layer positioned closest to a middle of the membrane wall has a smaller pore size and a higher mass density than the two layers directly adjacent on each side of said layer positioned closest to the middle of the membrane wall.
2. A hollow fiber membrane according to claim 1 , wherein the membrane wall between the outer wall surface and the inner wall surface has four layers with different pore sizes and mass densities, and wherein a first layer, at the outer wall surface, has the smallest pore size and the highest mass density, wherein a second layer, adjacent to the first layer, has a larger pore size and lower mass density than the first layer, wherein a third layer, adjacent to the second layer, has a smaller pore size and a higher mass density than the second layer, but a larger pore size and a lower mass density than the first layer, a fourth layer, at the inner wall surface and adjacent to the third layer, has a larger pore size and a lower mass density than the first, second, and third layers.
3. A hollow fiber membrane according to claim 1 , wherein the membrane wall has five layers with different pore sizes and mass densities between the outer wall surface and the inner wall surface, wherein a first layer, at the outer wall surface, has the smallest pore size and the highest mass density, wherein a second layer, adjacent to the first layer, has a larger pore size and a lower mass density than the first layer, wherein a third layer, adjacent to the second layer, has a smaller pore size and a higher mass density than the second layer, but a larger pore size and a lower mass density than the first layer, a fourth layer, adjacent to the third layer, has a larger pore size and a lower mass density than the first, second and third layers, and a fifth layer, at the inner wall surface and adjacent to the fourth layer, has a larger pore size and a lower mass density than the first, second, third, and fourth layers.
4. A hollow fiber membrane according to claim 1 , wherein the membrane wall has five layers with different pore sizes and mass densities between the outer wail surface and the inner wall surface, wherein a first layer, at the outer wall surface, has the smallest pore size and the highest mass density, wherein a second layer, adjacent to the first layer, has a larger pore size and a lower mass density than the first layer, wherein a third layer, adjacent to the second layer, has a smaller pore size and a higher mass density than the second layer, but a larger pore size and a lower mass density than the first layer, a fourth layer, adjacent to the third layer, has a larger pore size and a lower mass density than the first, second and third layers, and a fifth layer, at the inner wall surface and adjacent to the fourth layer, has a smaller pore size and a higher mass density than the fourth layer.
5. A hollow fiber membrane according to claim 1 , wherein the membrane has a hydraulic permeability within the range of 1×10 −4 to 100×10 −4 [cm 3 /cm 2 ×bar×s].
6. A hollow fiber membrane according to claim 1 , wherein the membrane comprises a polymer composition comprising polysulphone (PSU), polyethersulphone (PES) or polyarylethersulphone (PAES); and polyvinylpyrrolidone (PVP).
7. A hollow fiber membrane according to claim 6 , wherein the polyvinylpyrrolidone (PVP) in the membrane comprises a blend of at least two homo-polymers of polyvinylpyrrolidone (PVP), and wherein one of the homo-polymers has an average relative molecular weight within the range of 10.000 g/mole to 100.000 g/mole, and another one of the homo-polymers has an average relative molecular weight within the range of 500.000 g/mole to 2.000.000 g/mole.
8. A hollow fiber membrane according to claim 1 , wherein the membrane has an inner diameter within the range of 50 to 2000 μm.
9. A hollow fiber membrane according to claim 1 , wherein the membrane has a wall thickness within the range of 10 to 200 μm.
10. A hollow fiber membrane according to claim 1 , wherein the membrane has an effective diffusion coefficient through the membrane for urea (60 g/mole) of 4×10 −6 to 15×10 −6 cm 2 /s.