Filter material comprising a gradient structure nonwoven base layer and a nanofiber top layer

A pleatable filter material for use in particle filters, the filter material comprising:

• a nonwoven base layer having a media inlet surface and a media outlet surface;
• and
• a nanofiber top layer disposed on the media outlet surface of the base layer;
• wherein
• the nonwoven base layer is a gradient material whose cross-section comprises at least two sub-layers of different fiber structure in terms of different fibers, different fiber packing, or both;
• wherein fibers of each sub-layer extend into an adjacent sub-layer, such that the nonwoven base layer is an integral material with a gradual change in fiber structure at the interface between the at least two sub-layers of different fiber structure;
• and
• wherein the average linear mass density of the fibers in the sub-layer adjacent the media inlet surface of the nonwoven base layer is higher than the average linear mass density of the fibers in the sub-layer adjacent the media outlet surface of the nonwoven base layer.