a homogeneous mixture of a base and a binder material or fiber that is compressed to form a mat or sheet of selected porosity . the binder material has at least a surface with a melting temperature lower than that of the base fiber . the sheet is formed into a selected geometric shape and thermally fused to bind the base fiber into a porous filter element . the preferred shape is a helically wound tube of plural sheets , each sheet being self - overlapped and compressed to overlap another sheet . each sheet preferably heated and compressed individually and the sheets may be selected to have different porosities and densities . the binder fiber is selected from the group consisting of thermoplastic and resin , and the base fiber is selected from the group consisting of thermoplastic and natural .