Method for preparing a membrane from fibril cellulose and fibril, cellulose membrane

1. A method for preparing a nanofibrillar cellulose membrane, the method comprising:

supplying a nanofibrillar cellulose dispersion on a filter layer;

draining liquid from the nanofibrillar cellulose dispersion by the effect of reduced pressure through the filter layer that is impermeable to fibrils of the nanofibrillar cellulose but permeable to the liquid to form a membrane sheet on the filter layer;

continuing draining the liquid from the nanofibrillar cellulose while applying heat to a side of the membrane sheet opposite the filter layer to promote the removal of the liquid in liquid state while continuing draining of the liquid through the filter layer by pressure difference over the filter layer; and

subsequent to the heat application, removing the membrane sheet from the filter layer as the nanofibrillar cellulose membrane, the nanofibrillar cellulose membrane being freestanding.

2. The method according to claim 1 , wherein heat applied on the opposite side of the membrane sheet to the membrane sheet is accomplished by contact of the surface of the membrane sheet with a heated surface.

3. The method according to claim 2 , wherein pressure is also applied by the heated surface to the membrane sheet, said pressure causing at least partly the pressure difference over the filter layer.

4. The method according to claim 3 , wherein liquid is drained from the membrane sheet through the filter layer by the effect of reduced pressure while pressure is being applied by the heated surface to the membrane sheet, said reduced pressure and the pressure applied by the heated surface causing together the pressure difference over the filter layer.

5. The method according to claim 4 , wherein the membrane sheet is dried to a freestanding membrane in a sheet mold where the nanofibrillar cellulose dispersion was supplied.

6. The method according to claim 3 , wherein liquid is drained from the membrane sheet through the filter layer to at least one adsorbent sheet while pressure is being applied by the heated surface to the membrane sheet, said pressure applied by the heated surface causing the pressure difference over the filter layer.

7. The method according to claim 6 , wherein the membrane sheet is removed together with the filter layer from a sheet mold where the nanofibrillar cellulose dispersion was supplied and placed in a press where the membrane sheet is dried to a freestanding membrane.

8. The method according to claim 6 , wherein the nanofibrillar cellulose dispersion is supplied to a moving filter layer as a continuous layer and a continuous membrane is produced by carrying the continuous layer by the moving filter layer through different processing steps, whereafter the membrane is separated from the filter layer.

9. The method according to claim 1 , wherein heat applied on the opposite side of the membrane sheet is accomplished by contact of a heated surface with a layer interposed between the heated surface and the membrane sheet.

10. The method according to claim 9 , wherein pressure is also applied by the heated surface to the membrane sheet, said pressure causing at least partly the pressure difference over the filter layer.

11. The method according to claim 1 , wherein liquid is drained from the membrane sheet through the filter layer by the effect of reduced pressure, while heat applied on the opposite side of the membrane sheet to the membrane sheet is accomplished by radiation heat to the membrane sheet, said reduced pressure causing the pressure difference over the filter layer.

12. The method according to claim 11 , wherein the membrane sheet is dried to a freestanding membrane in a sheet mold where the nanofibrillar cellulose dispersion was supplied.

13. The method according to claim 1 , wherein a first nanofibrillar cellulose dispersion is first supplied on the filter layer and liquid is drained from it to form a nanofibrillar network, whereafter a second nanofibrillar cellulose dispersion where the size of the fibrils is smaller than the size of the fibrils of the first nanofibrillar cellulose dispersion is supplied on said nanofibrillar network and liquid is drained through said nanofibrillar network and the filter layer from the second nanofibrillar cellulose dispersion.

14. The method according to claim 13 , wherein the fibrils of the second nanofibrillar dispersion are of such size that they are capable of penetrating through the filter layer if the second nanofibrillar dispersion is supplied directly to the filter layer.

15. The method according to claim 1 , wherein the cellulose of the nanofibrillar cellulose dispersion includes anionically charged groups, the nanofibrillar cellulose dispersion being pretreated by lowering its pH, whereafter the pretreated nanofibrillar cellulose dispersion is supplied at the lowered pH on the filter layer.

16. The method according to claim 1 , wherein the nanofibrillar cellulose dispersion is supplied to the filter layer at a concentration of 0.1-10.0%.

17. The method according to claim 1 , wherein the temperature of the membrane sheet is kept under 100° C. by the heat applied to the membrane sheet.

18. The method according to claim 1 , wherein the liquid is drained from the membrane sheet by the effect of reduced pressure to opposite directions through both surfaces of the membrane sheet.

19. The method according to claim 18 , wherein heat and pressure are applied to opposite sides of the membrane sheet.

20. The method of claim 1 , wherein the fibrils of the nanofibrillar cellulose are the main constituent of the membrane.

21. The method of claim 1 , wherein the nanofibrillar cellulose is chemically modified cellulose containing anionically charged groups.

22. The method according to claim 1 , wherein the acts of draining liquid and applying heat are performed for less than ten minutes.

23. The method of claim 1 , further comprising filtering the nanofibrillar cellulose dispersion through a nonwoven sheet or paper, the nonwoven sheet or paper remaining as a structural part, of the formed membrane sheet.

24. The method of claim 1 , wherein the thickness of the membrane is in the range of 5-50 μm.

25. The method of claim 1 , wherein the nanofibrillar cellulose comprises fibrils having a length exceeding one micrometer and a number-average diameter below 200 nm.

26. The method of claim 1 , wherein the nanofibrillar cellulose comprises fibrils having a diameter of 50 nm or less.

27. A method of forming a fibril cellulose membrane laminate, the method comprising:

supplying a nanofibrillar cellulose dispersion on a filter layer;

draining liquid from the nanofibrillar cellulose dispersion by the effect of reduced pressure through the filter layer that is impermeable to fibrils of the nanofibrillar cellulose but permeable to the liquid to form a membrane sheet on the filter layer;

continuing draining the liquid from the nanofibrillar cellulose while applying heat to a side of the membrane sheet opposite the filter layer by contact of a heated surface with a layer interposed between the heated surface and the membrane sheet to promote the removal of the liquid in liquid state while continuing draining of the liquid through the filter layer by pressure difference over the filter layer, the layer interposed between the heated surface and the membrane sheet being a filter fabric or a structural layer;

subsequent to the heat application, removing the membrane sheet from the filter layer as the nanofibrillar cellulose membrane, the nanofibrillar cellulose membrane being freestanding.

28. A method of forming a fibril cellulose membrane laminate, the method comprising:

supplying a nanofibrillar cellulose dispersion on a filter layer;

draining liquid from the nanofibrillar cellulose dispersion by the effect of reduced pressure through the filter layer that is impermeable to fibrils of the nanofibrillar cellulose but permeable to the liquid to form a membrane sheet on the filter layer;

continuing draining the liquid from the nanofibrillar cellulose while applying heat to a side of the membrane sheet opposite the filter layer to promote the removal of the liquid in liquid state while continuing draining of the liquid through the filter layer by pressure difference over the filter layer;

subsequent to the heat application, removing the membrane sheet from the filter layer as the nanofibrillar cellulose membrane, the nanofibrillar cellulose membrane being freestanding; and

laminating a structural material to the membrane sheet, the structural material including plastic films, cardboard, or paper.

29. The method of claim 28 , wherein the structural material is laminated to the membrane sheet by interposing the structural material layer between a heated surface and the membrane sheet and applying pressure for dewatering.