Composite filter and method of making the same

1. A composite filter for filtering a stream of ambient air comprising at least one non-prebonded upstream tier and one non-prebonded downstream tier, wherein

the ratio of absolute pore volume of upstream tier to downstream tier RAPV>2, and

the absolute projected fiber coverage of upstream tier and of downstream tier APFC>95%.

2. The composite filter of claim 1 , wherein the ratio of average pore diameter of upstream to downstream tier RPD is 4<RPD<10.

3. The composite filter of claim 2 , wherein the average pore diameter of the upstream tier PDU>60 μm.

4. The composite filter of claim 1 , wherein the upstream tier comprises a relative pore volume RPVU>94%, an apparent density ADU<0.05 g/cm 3 , and a thickness D in the range of 0.5 mm<D<2.5 mm.

5. The composite filter of claim 4 , wherein the downstream tier comprises a relative pore volume RPVD being smaller than RPVU, an apparent density ADD in the range of 0.07 g/cm 3 <ADD<0.14 g/cm 3 , and a thickness D in the range of 0.1 mm<D<0.4 mm.

6. The composite filter of claim 1 , wherein the upstream tier comprises fibers having a length in the range of 0.1 mm to 3.0 mm.

7. The composite filter of claim 6 , wherein the orientation of the fibers in flow direction in the upstream tier is higher than in the downstream tier.

8. The composite filter of claim 1 , wherein the upstream tier comprises a dust retention DR with respect to dust particles with a diameter corresponding to the average pore diameter of the downstream tier of DR>99%.

9. The composite filter of claim 1 , wherein the upstream tier comprises dry-laid, thermally bondable fusing, bicomponent or monocomponent polymer fibers and the downstream tier comprises meltblown fibers.

10. The composite filter of claim 9 , wherein the upstream tier has a composition selected from the group consisting of 100 wt % bicomponent polymer fibers, a blend of at least about 10 wt % bicomponent polymer fibers with a complementary amount of natural fibers, staple fibers or a mixture thereof, and a blend of at least about 10 wt % monocomponent polymer thermally bondable fusing fibers with a complementary amount of fluff pulp fibers, staple fibers or a mixture thereof.

11. The composite filter of claim 10 , wherein the bicomponent polymer fibers have a sheath of one polymer and a core of a different polymer having a melting point higher than the one polymer.

12. The composite filter of claim 11 , wherein the core is polypropylene and the sheath is polyethylene.

13. The composite filter of claim 12 , wherein the core is disposed eccentric relative to the sheath.

14. The composite filter of claim 9 , wherein the upstream tier further comprises fibers selected from at least one of uncharged split film fibers, charged split film fibers and mixed electrostatic fibers.

15. A vacuum cleaner bag comprising a composite filter in accordance with claim 1 .

16. A method of making a composite filter in accordance with claim 1 , comprising the steps of

(a) laying down a filtration material onto a support to form the upstream non-prebonded tier,

(b) depositing onto the upstream tier the downstream non-prebonded tier, and

(c) bonding the tiers to form a composite filter having a unitary stratified structure.

17. A composite filter produced by the method of claim 16 .

18. A vacuum cleaner bag comprising a composite filter produced by the method of claim 16 .

19. The composite filter of claim 2 , wherein the average pore diameter of the upstream tier PDU is >80 μm and > 200 μm.

20. The composite filter of claim 1 , wherein the upstream tier comprises a relative pore volume RPVU>96%.