Constructed of thermally-bonded polypropylene microfibers to ensure high efficiency. The filter incorporates a rigid polypropylene center core for increased collapse strength and durability. This thermal bonded micro-fiber construction has minimal fiber release, consistent flow rate and superior filtration performance. It also is not brittle or prone to breakage problems like resin-bonded cartridges. Case quantities available 24 in 9 3/4" case, 25 in 20" case, 15 in 30" and 40" case.