Multi-phase filter medium

1. An article, comprising:

a first phase comprising a first plurality of fibers and a second plurality of fibers different from the first plurality of fibers;

a second phase comprising a third plurality of fibers and a fourth plurality of fibers different from the third plurality of fibers, the third plurality of fibers being the same as or different from the first or second plurality of fibers and the fourth plurality of fibers being the same as or different from the first or second plurality of fibers; and

a transition phase between the first and second phases, wherein the transition phase comprises at least a portion of the first plurality of fibers, at least a portion of the second plurality of fibers, at least a portion of the third plurality of fibers, and at least a portion of the fourth plurality of fibers, and wherein at least a portion of the first plurality of fibers, at least a portion of the second plurality of fibers, at least a portion of the third plurality of fibers, and at least a portion of the fourth plurality of fibers are intermingled with each other,

wherein the air permeability of the first plurality of fibers is higher than the air permeability of the second plurality of fibers, the air permeability of the third plurality of fibers is higher than the air permeability of the fourth plurality of fibers,

wherein the first plurality of fibers are hardwood or softwood fibers,

wherein a weight ratio of the first plurality of fibers to the second plurality of fibers is from about 50:50 to about 97:3,

wherein the basis weight ratio of the first phase to the second phase is from about 30:70 to about 70:30,

wherein the second phase is over the first phase, and

wherein the first and second phases form a wet laid, structure that is configured as a filter medium, wherein the first, second, third and fourth pluralities of fibers are wet laid.

2. The article of claim 1 , wherein the air permeability of the first or third plurality of fibers is from about 50 CFM to about 350 CFM.

3. The article of claim 2 , wherein the air permeability of the first or third plurality of fibers is from about 100 CFM to about 200 CFM.

4. The article of claim 1 , wherein the air permeability of the second or fourth plurality of fibers is from about 5 CFM to about 50 CFM.

5. The article of claim 4 , wherein the air permeability of the second or fourth plurality of fibers is from about 15 CFM to about 25 CFM.

6. The article of claim 1 , wherein a weight ratio of the first and second pluralities of fibers is from about 50:50 to about 90:10, and a weight ratio of the third and fourth pluralities of fibers is from about 10:90 to about 50:50.

7. The article of claim 6 , wherein a weight ratio of the first and second pluralities of fibers is from about 50:50 to about 70:30, and a weight ratio of the third and fourth pluralities of fibers is from about 25:75 to about 50:50.

8. The article of claim 1 , wherein the first or third plurality of fibers have an average fiber length from about 1.5 mm to about 6 mm.

9. The article of claim 1 , wherein the second or fourth plurality of fibers have an average fiber length from about 0.5 mm to about 2 mm.

10. The article of claim 1 , wherein the third plurality of fibers comprise softwood fibers, cotton fibers, polyester fibers, polyvinyl alcohol binder fibers, or rayon fibers, and the second or fourth plurality of fibers comprise hardwood fibers, polyethylene fibers, or polypropylene fibers.

11. The article of claim 10 , wherein the first or third plurality of fibers comprise softwood fibers, and the second or fourth plurality of fibers comprise hardwood fibers.

12. The article of claim 1 , wherein the second phase has a higher air permeability than the first phase.

13. The article of claim 1 , wherein a basis weight ratio of the first and second phases is from about 40:60 to about 60:40.

14. The article of claim 1 , wherein the article further comprises a binder.

15. The article of claim 14 , wherein the binder is at least about 2% of the basis weight of the article.

16. The article of claim 14 , wherein the binder is at most about 35% of the basis weight of the article.

17. The article of claim 1 , wherein the article has an average pore size from about 3 microns to about 1,000 microns.

18. The article of claim 17 , wherein the article has an average pore size from about 25 microns to about 125 microns.

19. The article of claim 1 , wherein the article has a dust holding capacity from about 0.3 g to about 3 g measured according to a Palas flat sheet test.

20. The article of claim 19 , wherein the article has a dust holding capacity from about 1 g to about 3 g measured according to a Palas flat sheet test.

21. The article of claim 1 , wherein the article has an initial dust capture efficiency from about 25% to about 99.5%.

22. The article of claim 21 , wherein the article has an initial dust capture efficiency from about 60% to about 99.5%.

23. The article of claim 1 , wherein each of the first, second, third, and fourth pluralities of fibers is made from an organic polymeric material.

24. The article of claim 1 , wherein the interface between the first and second phases is substantially non-linear.

25. The article of claim 1 , wherein the interface between the first and second phases is substantially free of an adhesive.

26. The article of claim 1 , wherein the article is formed by:

disposing through a wet laid process a first dispersion containing the first and second pluralities of fibers in a first solvent onto a wire to form the first phase;

while the first and second pluralities of fibers are on the wire, disposing a second dispersion containing the third and fourth pluralities of fibers in a second solvent onto the first and second pluralities of fibers to form the second phase; and

at least partially removing the first and second solvents, thereby resulting in the filter medium containing the first and second phases.

27. A filter element, comprising one or more articles of claim 1 .

28. The article of claim 1 , wherein each phase of the article comprises hardwood fibers and/or softwood fibers.

29. The article of claim 1 , wherein the fourth plurality of fibers are hardwood or softwood fibers.

30. The article of claim 29 , wherein the article has an air permeability between about 2 CFM and about 160 CFM.