Filter medium having excellent heat resistance

1. A filter medium comprising at least one wet-laid fiber layer, wherein the at least one wet-laid fiber layer comprises:

(i) synthetic polymer fibers having a melting point of greater than 155° C.; and

(ii) binder fibers having a core-sheath structure, wherein a core polymer material constituting the core of the binder fibers is at least partially surrounded by a sheath polymer material constituting the sheath of the binder fibers;

wherein both the core polymer material and the sheath polymer material have a melting point of greater than 155° C.; and

wherein the melting point of the sheath polymer material is in the range of 155° C. to 180° C.

2. The filter medium according to claim 1 , wherein the synthetic polymer fibers comprise polymer materials selected from polyesters (such as polyalkylene terephthalates like polyethylene terephthalate (PET), polybutylene terephthalate (PBT), etc.); polyalkylenes (such as polyethylene polypropylene, etc.); polyacrylonitriles (PAN); polyamides (nylon such as nylon-6, nylon-6.6, nylon-6.12, etc.); or mixtures thereof.

3. The filter medium according to claim 1 , wherein the synthetic polymer fibers comprise polyethylene terephthalate.

4. The filter medium according to claim 1 , wherein the mean pore size of the at least one wet-laid fiber layer is in the range of 20 μm to 200 μm as measured according to ASTM 316-03.

5. The filter medium according to claim 1 , wherein the core polymer material comprises polyethylene terephthalate and the sheath polymer material comprises a co-polyester of polyethylene terephthalate, 1,4-butanediol, polytetramethylene glycol and adipic acid.

6. The filter medium according to claim 1 , wherein the core polymer material has a melting point greater than 200° C.

7. The filter medium according to claim 1 , wherein the sheath polymer material is characterized by having a degree of crystallization in the range of 20% to 80%, as determined by X-ray diffraction.

8. The filter medium according to claim 1 , wherein the onset temperature Tm Onset of the melting of the sheath polymer material is at least 130° C., as determined using DSC.

9. The filter medium according to claim 1 , wherein the end temperature Tm end of the melting of the sheath polymer material is at least 160° C., as determined using DSC.

10. The filter medium according to claim 1 , wherein the difference between the onset temperature of the melting Tm onset and the end temperature of the melting Tm end of the sheath polymer material is at least 30° C., as determined using DSC.

11. The filter medium according to claim 1 , wherein the sheath of the binder fibers is not completely amorphous.

12. The filter medium according to claim 1 , wherein the synthetic fibers and the binder fibers are staple fibers.

13. The filter medium according to claim 1 , wherein

(i) the amount of the synthetic polymer fibers is in the range of 10 to 90 wt.-%, based on the total amount of fibers in the at least one fiber layer; or

(ii) the amount of binder fibers is in the range of 10 to 90 wt.-%, based on the total amount of fibers in the at least one fiber layer.

14. The filter medium according to claim 1 , wherein

(i) the synthetic polymer fibers have an average fiber length in the range of 3 to 15 mm or a linear mass density in the range of 0.1 to 1.0 g/9000 m, as measured by ASTM D 1577; or

(ii) the binder fibers have an average fiber length in the range of 4 to 8 mm or a linear mass density in the range of 1 to 5 g/9000 m, as measured by ASTM D 1577.

15. The filter medium according to claim 1 , wherein

(i) the synthetic polymer fibers have an average fiber diameter in the range of 3.3 to 10 μm, as measured by microscope analysis; or

(ii) the binder fibers have an average fiber diameter in the range of 10 to 23 μm, as measured by microscope analysis.

16. The filter medium according to claim 1 which is obtainable by wet laying a mixture of the synthetic polymer fibers and the binder fibers and heating the wet-laid mixture to a temperature of at least the melting temperature of the sheath polymer material but lower than the melting temperature of the core polymer material.

17. The filter medium according to claim 1 , which is characterized by a basis weight in the range of 120 to 250 g/m 2 , as measured by DIN EN 29073-1 (nonwoven).

18. The filter medium according to claim 1 , which is characterized by an air permeability of at least 320 l/m 2 s, as measured by DIN EN ISO 9237.

19. The filter medium according to claim 1 , which is characterized by a heat shrinkage of 5% or less after a 24 hour storage under an air temperature of 150° C.

20. The filter medium according to claim 1 , which is characterized by an oil filtration efficiency against 20 μm sized particles of 50% or more, as measured by ISO 4548-12.

21. The filter medium according to claim 1 , which is characterized by a stiffness of 1800 to 5500 mgs as determined by ASTM 6125-97.

22. The filter medium according to claim 1 , wherein the maximum pore size of the at least one wet-laid fiber layer is in the range of 20 μm to 200 μm, as measured according to ASTM 316-03.

23. A filter element comprising the filter medium according to claim 1 .

24. The filter element according to claim 23 , wherein the filter medium is pleated and optionally comprises a wire mesh as supporting layer co-pleated with the filter medium.

25. The filter element according to claim 23 , which is an oil filter for a combustion engine.

26. A filter medium comprising at least one wet-laid fiber layer, wherein the at least one wet-laid fiber layer comprises:

(i) synthetic polymer fibers having a melting point of greater than 155° C.; and

(ii) binder fibers having a core-sheath structure, wherein a core polymer material constituting the core of the binder fibers is at least partially surrounded by a sheath polymer material constituting the sheath of the binder fibers;

wherein both the core polymer material and the sheath polymer material have a melting point of greater than 155° C.; and

wherein the melting point of the sheath polymer material is in the range of 155° C. to 180° C.,

which is characterized by a burst strength of greater than 4 bar, as measured by EN ISO 2758.

27. A process for preparing the filter medium comprising at least one wet-laid fiber layer, wherein the at least one wet-laid fiber layer comprises:

(i) synthetic polymer fibers having a melting point of greater than 155° C.; and

(ii) binder fibers having a core-sheath structure, wherein a core polymer material constituting the core of the binder fibers is at least partially surrounded by a sheath polymer material constituting the sheath of the binder fibers;

wherein both the core polymer material and the sheath polymer material have a melting point of greater than 155° C.; and

wherein the melting point of the sheath polymer material is in the range of 155° C. to 180° C.,

the process comprising the steps of

providing a homogeneous slurry of the synthetic fibers and the binder fibers in water;

supplying the slurry onto a dewatering screen to form a deposit;

removing the water from the deposit to form a wet fibrous mat or sheet;

drying the wet fibrous mat or sheet while heating to a temperature which at least corresponds to the melting point of the binder fibers' sheath polymer material, but is lower than the melting point of the binder fibers' core polymer material to form a substrate.

28. The process according to claim 27 , wherein the drying is performed in a temperature range of 100° C. to 200° C.