Filter medium

1. A filter medium comprising a substrate and a fine fiber layer on top of the substrate, wherein the substrate comprises

a first layer comprising first fibers having a first average diameter and a first maximum fiber length;

a second layer comprising second fibers having a second average diameter and a second maximum fiber length; and

a third layer comprising third fibers having a third average diameter and a third maximum fiber length;

wherein

a boundary area between the first and the second layer forms a first blended area comprising a first homogeneous mixture of first and second fibers; and

the boundary area between the second and the third layer forms a second blended area comprising a second homogeneous mixture of second and third fibers;

and wherein the first and the third average diameters are each larger than the second average diameter.

2. The filter medium according to claim 1 , wherein the ratio of the first and the second average diameters (d1st/d2nd) and the ratio of the third and the second average diameters (d3rd/d2nd) are each independently from each other of from 1.2 to 115, preferably of from 1.4 to 43.

3. The filter medium according to claim 1 , wherein the first and the third average diameters independently from each other are of from 6-45 μm, preferably of from 7-38 μm, most preferably of from 7-17 μm; and wherein the amount of fibers having a diameter of less than 5 μm within the first and the third layer is less than 3 wt. %, preferably less than 1 wt. % and most preferably 0 wt. % based on the total weight of each of the first or third layer and/or wherein the first and the third maximum fiber length independently from each other is up to 45 mm.

4. The filter medium according to claim 1 , wherein the second average diameter is of from 0.4-5 μm, preferably of from 0.6-2.6 μm; and/or wherein the second maximum fiber length is 18 mm, preferably 10 mm, more preferably 3 mm.

5. The filter medium according to claim 1 , wherein the first and the third fibers independently from each other are selected from the group consisting of polymeric fibers and inorganic fibers, wherein the polymeric fibers are preferably selected from polyester, polyethylene, polyethylene terephthalate, polyolefin, polybutylene terephthalate and/or polyamide, and wherein the inorganic fibers are preferably glass fiber strands.

6. The filter medium according to claim 1 , wherein the first fibers in the first layer are present in an amount of at least 65 wt. %, preferably at least 80 wt. % based on the total fiber weight of the first layer; and/or wherein the third fibers in the third layer are present in an amount of at least 65 wt. %, preferably at least 80 wt. % based on the total fiber weight of the third layer.

7. The filter medium according to claim 1 , wherein the first, the second and/or the third layer further contains up to 50 wt. %, preferably from 2-35 wt. %, more preferably 2-25 wt. % of binder fibers based on the total weight of each of the first, the second and/or third layer.

8. The filter medium according to claim 1 , wherein the second fibers are selected from the group consisting of synthetic fibers and microglass fibers.

9. The filter medium according to claim 1 , wherein the second fibers in the second layer are present in an amount of at least 10 wt. %, preferably at least 20 wt. %, more preferably at least 30 wt. % based on the total weight of the second layer.

10. The filter medium according to claim 1 , wherein the fine fiber layer on top of the substrate comprises fine fibers having an average fiber diameter of less than 3 μm.

11. The filter medium according to claim 1 , wherein the fine fiber layer on top of the substrate comprises sub-micronic fibers having an average fiber diameter of less than 1 μm; preferably of from 0.08 to 0.8 μm, most preferably of from 0.1 to 0.4 μm.

12. The filter medium according to claim 1 , wherein the fine fiber layer is deposited on top of the first or third layer.

13. The filter medium according to claim 1 , wherein the filter medium comprises an adhesive between the substrate and the fine fiber layer.

14. The filter medium according to claim 11 , wherein the sub-micronic fibers are polymeric fibers that are preferably selected from the group consisting of polyester, polypropylene, polyurethane, polycarbonate, polyamide and polyvinylidene fluoride sub-micronic fibers.

15. The filter medium according to claim 1 , comprising a protective layer on top of the fine fiber layer.

16. A process for the preparation of a filter medium according to claim 1 , the process comprising the steps of

a. providing first, second and third homogeneous slurries;

b. supplying the first slurry onto a dewatering screen to form a first deposit;

c. supplying the second slurry onto the first deposit to form a second deposit on top of the first deposit;

d. supplying the third slurry onto the second deposit to form a third deposit on top of the second deposit;

e. removing the water from the deposits to form a wet fibrous mat or sheet;

f. drying the wet fibrous mat or sheet while heating to form a substrate; and

g. applying a fine fiber layer on top of the substrate;

wherein

the first slurry comprises water and first fibers having a first fiber average diameter and a first maximum fiber length;

the second slurry comprises water and second fibers having a second fiber average diameter and a second maximum fiber length; and

the third slurry comprises water and third fibers having a third fiber average diameter and a third maximum fiber length;

and wherein the first and the third average diameters are each larger than the second average diameter.

17. A filter element including the filter medium according to claim 1 .