Microporous polyvinyl fluoride planar membrane and production thereof

Patent metadata

Field	Value
Publication number	US-2017312703-A1
Application number	US-201515520221-A
Country	US
Kind code	A1
Filing date	Nov 2, 2015
Priority date	Nov 3, 2014
Publication date	Nov 2, 2017
Grant date	—

Abstract

Hydrophobic flat membrane made from a vinylidene fluoride polymer with a wall, a first surface, and a second surface. The membrane has on its first surface a network structure with open pores and on its second surface a continuous skin in which pores are formed, and adjacent to the skin of the second surface a supporting layer with an isotropic pore structure across the wall thickness, wherein the supporting layer extends over at least 80% of the wall thickness and wherein the pores of the supporting layer have an average diameter of less than 1 μm. The weight average of the molecular weight M W of the vinylidene fluoride polymer lies in the range from 300 000 to 500 000 daltons, and the polydispersivity M W /M N is greater than 5.5.
The pores in the skin of the second surface have a closed perimeter in the plane of the skin and an average ratio of the extension in the direction of the longest axis thereof to the extension in the direction of the shortest axis thereof of at most 5. The pores in the first surface and second surface have an essentially isotropic distribution of their orientation. The porosity of the membrane lies in the range from 50 to 90 vol. % and the wall thickness in the range from 50 to 300 μm. The membrane has a maximum separating pore diameter d max in the range from 0.05 to 1.5 μm.

First claim

1 . Hydrophobic membrane comprising a flat membrane made from a vinylidene fluoride polymer, wherein the membrane has a wall with a wall thickness, a first surface, and a second surface,
wherein the membrane has a network structure with open pores on the first surface thereof and a continuous skin on the second surface thereof, in which pores are formed, wherein the membrane, adjacent to the skin of the second surface, has a supporting layer having an open-pored, microporous, and sponge-like pore structure that is substantially isotropic across the wall thickness, the supporting layer extending over at least 80% of the wall thickness and comprising pores having an average diameter of less than 1 μm, and
wherein
the vinylidene fluoride polymer forming the membrane has a weight-average molecular weight MW in the range from 300 000 to 500 000 daltons and a polydispersivity MW/MN, given by the ratio of the weight-average molecular weight MW and the number average MN of the molecular weight, that is greater than 5.5,
the pores in the skin of the second surface have a closed perimeter in the plane of the skin, the pores in the skin of the second surface having an average ratio of the extension in the direction of the longest axis thereof to the extension in the direction of the shortest axis thereof of at most 5, and the pores in the first surface and second surface having an essentially isotropic distribution of their orientation when viewed perpendicular to the surface,
the porosity of the membrane lies in the range from 50 to 90 vol. % and the wall thickness in the range from 50 to 300 μm, and
the membrane has a maximum separating pore diameter d max in the range from 0.05 to 1.5 μm determined according to the bubble point method.

2 . The membrane according to claim 1 , wherein the membrane has a transmembrane flow for isopropyl alcohol in the range from 3 to 15 ml/(cm 2 ·min·bar), measured at 25° C.

3 . The membrane according to claim 1 , wherein the membrane has a transmembrane flow for water vapor of at least 35 l/(m 2 ·h), determined by means of a flat membrane module with a membrane area of 40 cm 2 at a salt water circuit temperature of 80° C. and a distillate circuit temperature of 30° C., a volume flow in the circuits of 200 l/h, a pressure level in the circuits of 500 mbar at the inlet to the flat membrane module, and a salt concentration in the salt circuit of 36 g/l.

4 . The membrane according to claim 1 , wherein the membrane has an elongation at break of at least 50% at room temperature.

5 . The membrane according to claim 1 , wherein the membrane has a breaking strength of at least 200 cN/mm 2 at room temperature.

6 . The membrane according to claim 1 , wherein the membrane has a volume porosity in the range from 70 to 85 vol. %.

7 . The membrane according to claim 1 , wherein the membrane has a wall thickness in the range from 60 to 150 μm.

8 . The membrane according to claim 1 , wherein the membrane has a maximum separating pore diameter d max in the range from 0.1 to 1.0 μm.

9 . A method for producing the membrane according to claim 1 from a vinylidene fluoride homopolymer or copolymer, comprising at least the following steps:
a) preparing a homogeneous casting solution of 20-30 wt. % of a polymer component made from at least one vinylidene fluoride polymer in 80-70 wt. % of a solvent system, wherein the casting solution of the polymer component and solvent system has on cooling a critical demixing temperature and a solidification temperature, and a miscibility gap below the critical demixing temperature in the liquid state of aggregation, and wherein the solvent system contains a compound A and a compound B, which are liquid and can be mixed homogeneously with each other at the dissolving temperature, and wherein a solvent for the polymer component is selected for compound A and compound B is a non-solvent for the polymer component,
b) forming of the casting solution into a film with a first surface and a second surface in a forming tool, which has a tool temperature above the critical demixing temperature, and
c) placing the first side of the film onto a conditionable carrier, which is conditioned to a cooling temperature below the solidification temperature, resulting in cooling of the film via the conditionable carrier at such a rate that a thermodynamic non-equilibrium liquid-liquid phase separation into a polymer-rich and a polymer-poor phase takes place, and subsequently, on passing below the solidification temperature, solidification of the polymer-rich phase takes place, forming a membrane structure; at the same time
d) bringing the second surface of the film into contact with a gaseous atmosphere,
e) drawing the film with the formed membrane structure from the carrier,
f) removing at least part of the solvent system from the film to obtain the flat membrane, and
wherein the polymer component has a weight-average molecular weight MW in the range from 300 000 to 500 000 daltons and a polydispersivity MW/MN, given by the ratio of the weight-average molecular weight MW and the number average MN of the molecular weight, that is greater than 5.5.

10 . The method according to claim 9 , wherein glyceryl triacetate, glyceryl diacetate, 2-(2-butoxyethoxy-)ethyl acetate, dibutyl phthalate, adipic acid diethyl ester, adipic acid dibutyl ether, butyl diglycol acetate, butyl glycol acetate, glycol diacetate, propylene carbonate, butyrolactone, or ε-caprolactam, or a mixture of the compounds mentioned, is used as compound A.

11 . The method according to claim 9 , wherein dioctyl adipate, glyceryl monoacetate, glycerol, glycol, diglycol, or castor oil, or a mixture thereof, is used as compound B.

12 . The method according to claim 9 , wherein the carrier has a temperature in the range from 30 to 80° C.

13 . The method according to claim 9 , wherein the gaseous atmosphere has a temperature in the range from 20 to 25° C.

14 . The method according to claim 9 , wherein the conditionable carrier is a conditionable and rotating casting roller, which with a part of its perimeter on the bottom is immersed in a bath filled with a liquid cooling medium.

15 . The method according to claim 14 , wherein the liquid cooling medium comprises a solvent and a non-solvent for the polymer component, wherein the cooling medium acts as a non-solvent for the polymer component at the cooling temperature.

Assignees

• 3M Innovative Properties Co

Inventors

• Huang Quan
• Schuster Oliver
• Duong Phung Ngoc
• Ansorge Wolfgang

Classifications

• B01D67/0018

  Thermally induced processes [TIPS] · CPC title

• B01D69/02

  characterised by their properties · CPC title

• B01D2325/026

  Sponge structure · CPC title

• B01D2325/04

  Characteristic thickness · CPC title

• B01D2325/24

  Mechanical properties, e.g. strength · CPC title