Filter medium

A method for the manufacture of a multilayered filter medium whose air permeability is at least 500 l/m2 sec comprising the measures:

• a) Forming a textile fabric comprising carrier and melt binder fibres wherein the carrier fibres consist of a polyester and the melt binder fibres consist of a polymer whose melting point is at least 5° C. below the melting point of the carrier fibres and the carrier and melt binder fibres have a titre between 1 to 2 dtex,
• b) Forming a further textile fabric on a textile surface formed in step a) comprising carrier and melt binder fibres wherein the carrier fibres consist of a polyester and the melt binder fibres consist of a polymer whose melting point is at least 5° C. below the melting point of the carrier fibres and the carrier and melt binder fibres have a titre between 2 to 4 dtex,
• c) Forming a further textile fabric on a textile surface formed in step b) comprising carrier and melt binder fibres wherein the carrier fibres consist of a polyester and the melt binder fibres consist of a polymer whose melting point is at least 5° C. below the melting point of the carrier fibres and the carrier and melt binder fibres have a titre between 4 to 12 dtex, wherein:
• d) The portion of the textile fabric formed in step a) is 20 to 60% by weight, the portion of the textile fabric formed in step b) is 10 to 40% by weight and the portion of the textile fabric formed in step c) is 10 to 40% by weight,
• e) The portion of the melt binder fibres in textile fabric formed in accordance with steps a) to c) is 5 to 40% by weight, and
• f) The weight per unit area of the textile fabric formed in accordance with steps a) to c) is 50 to 400 g/m2,
• g) Pre-solidifying the multilayered textile fabric formed in accordance with steps a) to c) by means of a heated roller whose surface temperature is at least 70° C.,
• h) Calendering the pre-solidified multilayered textile fabric formed in accordance with step g) by means of a calender whose surface temperature is at least 10° C. below the melting temperature of the melt binder fibres and a contact pressure or line pressure of least 20 daN,
• i) Introducing the multilayered textile fabric calendered in accordance with step h) into a hot-air oven whose minimum temperature is equal to or above the melting temperature of the melt binder fibres and whose maximum temperature is at least 10° C. below the melting temperature of the carrier fibres wherein the introduced textile fabric expands and the thickness of the textile fabric increases by at least 30% with respect to the introduced textile fabric, and
• j) Cooling a melt binder-solidified multilayered filter medium formed in step i).