Filter and process for the production thereof

Claim 1

A filter having an inlet side and an outlet side, comprising at least two different types of fibers having a denier of 0.001 to 15 bonded to each other with a powdery binder of 10 to 80% by weight based on the weight of webs, said binder being hot-adhered to the webs, said filter having a fiber density of 0.01 to 0.1 at the inlet side and a fiber density of 0.05 to 0.5 at the outlet side, said filter having a continuous fiber density gradient from the inlet side to the outlet side owing to a continuous distribution gradient of the powdery binder which bonds to the fibers while having a uniform distribution of the different types of fiber so that dust collection efficiency varies continuously from low efficiency at the inlet side to high efficiency toward the outlet side, thereby preventing formation of a dust cake at the inlet side and promoting uniform collection of dust throughout the thickness of the filter between the inlet and outlet sides, said fiber density gradient corresponding to an exponential curve defined by the following equation: ##EQU2## wherein α is the fiber density at a distance: l, α i is the fiber density at the fluid inlet side, a is a constant: 0.35-0.64, lo is the thickness of the filter, and l is the distance from a face at the fluid inlet side in the direction of the filter thickness.