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A composite filtration medium web of fibers containing a controlled dispersion of a mixture of sub-micron and greater than sub-micron diameter polymeric fibers is described. The filtration medium is made by a two dimensional array of cells, each of which produces a single high velocity two-phase sol

A composite filtration medium web of fibers containing a controlled dispersion of a mixture of sub-micron and greater than sub-micron diameter polymeric fibers is described. The filtration medium is made by a two dimensional array of cells, each of which produces a single high velocity two-phase solids-gas jet of discontinuous fibers entrained in air. The cells are arranged so that the individual jets are induced to collide in flight with neighboring jets in their region of fiber formation, to cause the individual nascent fibers of adjacent jets to deform and become entangled with and partially wrap around each other at high velocity and in a localized fine scale manner before they have had an opportunity to cool to a relatively rigid state. The cells are individually adjusted to control the mean diameters, lengths and trajectories of the fibers they produce. Certain cells are adjusted to generate a significant percentage of fibers having diameters less than one micron diameter, and which are relatively shorter in length and certain other cells are adjusted to generate a significant percentage of structure-forming reinforcing fibers having diameters greater than one micron diameter which are relatively longer in length. By employing appropriate close positioning and orientation of the cells in the array, the sub-micron fibers are caused to promptly entangle with and partially wrap around the larger reinforcing fibers. The larger fibers thereby trap and immobilize the sub-micron diameter fibers in the region of formation, to minimize the tendency of sub-micron diameter fibers to clump, agglomerate, or rope together in flight. Also, the larger fibers in flight are made to form a protective curtain to prevent the sub-micron fibers from being carried off by stray air currents.

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[ What is claimed is:] [1.] A method for making a composite fibrous web of fibers containing a significant quantity of discontinuous sub-micron diameter fibers, said method comprising:providing a two dimensional array of individually controlled fiber producing cells, said array having closely spaced

[ What is claimed is:] [1.] A method for making a composite fibrous web of fibers containing a significant quantity of discontinuous sub-micron diameter fibers, said method comprising:providing a two dimensional array of individually controlled fiber producing cells, said array having closely spaced, generally parallel rows of cells, said cells in the array defining a first plurality of cells and a remainder plurality of cells;producing a single jet spray of fluid containing entrained fibers from each cell in the array;controlling said first plurality of the cells in the array to produce discontinuous fibers containing a relatively high percentage of sub-micron diameter fibers;controlling the remainder plurality of cells in the array to produce sprays of fibers containing a relatively high percentage of fibers which have diameters greater than one micron;positioning the cells in the array so as to cause the fibers in neighboring sprays to collide in flight in a fiber formation region and to promote a fine scale entanglement of the sub-micron diameter fibers with the fibers which are larger than 1 micron in diameter; andcollecting the collided fibers in a fibrous web on a collection surface. [27.] An apparatus for producing a compsite fibrous web of fibers containing a significant quantity of discontinuous sub-micron diameter fibers, said apparatus comprising:a two dimensional array of individually controlled fiber producing cells, said array having closely spaced, generally parrallel rows of cells, said cells in the array defining a first plurality of cells and a remainder plurality of cells, wherein each cell includes means for producing a single jet spray of fluid containing entrained fibers;a fiber formation region adjacent said array of cells;wherein said first plurality of cells are adjusted to produce discontinuous fibers containing a relatively high percentage of sub-micron diameter fibers in said fiber formation region;wherein said remainder plurality of cells in the array are adjusted to produce sprays of fibers containing a relatively high percentage of fibers which have diameters greater than one micron in said fiber formation region; andwherein said cells are positioned so as to cause the fibers in neighboring sprays to collide in flight in the fiber formation region and to promote a fine scale entanglement of the sub-micron diameter fibers with the fibers which are larger than 1 micron in diameter; anda collecting surface for collecting the collided fibers in a fibrous web.