Graded particle-size retention filter medium for fluid filtration unit with improved edge seal

A filter unit comprising:

• a first composite filter medium;
• a second composite filter medium, each of the first composite filter medium and the second composite filter medium having at least first and second filter material zones, each of the at least first and second filter material zones comprising fibers and filter aids and each having a different PRC;
• wherein the first composite filter medium and the second composite filter medium are each disk-shaped, and wherein the first composite filter medium is separated from the second composite filter medium by a non-filtering disk-shaped separator;
• wherein the first filter material zone is capable of retaining smaller particle sizes than the second filter material zone, wherein the first filter material zone is configured to be located downstream of the second filter material zone and proximal to the separator;
• wherein the PRC of the filter medium zones increases in the downstream direction so that contaminants that are desired to be removed are progressively retained throughout the filter medium thickness as a function of the filtered particle size and proximity from the separator;
• wherein the first and second composite filter medium are (i) substantially the same in fabrication, composition, dimension, and charge, and (ii) positioned in the same manner relative to the separator;
• wherein the first and second disk-shaped composite filter medium each comprise a perimeter edge and are affixed to one another along their perimeter edges by a thermoplastic edge seal;
• wherein the thermoplastic edge seal is formed from a material having a tensile modulus of elasticity of about 600 psi to about 35,000 psi at about 73° F. and of about 300 psi to about 10,000 psi at about 212° F.