Method for preparing filtering material
A method of manufacturing a filter medium that has a PF value that exceeds 32, the PF value calculated from the pressure loss and the collection efficiency of the filter medium according to the following formula: PF=[−log(permeability (%)/100)/(pressure loss (Pa)]×1000[Formula 1] in which permeability (%)=100−collection efficiency (%). In this method, 380 ml or more of a liquid lubricant at 20° C. is added per 1 kg of a PTFE fine powder, this compound is processed into a tape shape and elongated to obtain a PTFE porous film, and an air permeable support member is heat laminated to at least one surface of the PTFE porous film.
1. A method of producing a filter medium having a three layer structure comprised of a porous film comprised of polytetrafluoroethylene and air permeable support members comprised of a heat fusible non-woven fabric, wherein a PF value of the filter medium exceeds 32 and is computed in accordance with the following formula from a pressure loss measured when air permeates the filter medium at a flow rate of 5.3 cm/s and a collection efficiency measured by employing silica particles having diameters between 0.10 and 0.12 micrometers
PF=[−log(permeability (%)/100)/(pressure loss (Pa)]×1000
wherein permeability (%)=100−collection efficiency (%),
the method comprising:
a first step in which 380 ml or more of a liquid lubricant at 20° C. is mixed per 1 kg of a polytetrafluoroethylene fine powder to obtain a compound;
a second step in which the compound is rolled and the liquid lubricant therein removed to obtain a non-sintered tape;
a third step in which the non-sintered tape is elongated to obtain the porous film; and
a fourth step in which each of the air permeable support members is heat laminated to opposite surfaces of the porous film by their own weight and without the application of additional pressure or compression in a thickness direction thereby producing the three layer structure of the filter medium.
2. The method of producing the filter medium set forth in claim 1 , in which 406 ml or more of a liquid lubricant at 20° C. is mixed per each 1 kg of polytetrafluoroethylene fine powder in the first step thereof.
3. The method of producing the filter medium set forth in claim 1 , wherein in the third step the total area of the non-sintered tape is elongated to between 80 times and 800 times its original size by first elongating the non-sintered tape in the longitudinal direction between 3 times and 20 times its original length and then elongating the non-sintered tape in the lateral direction between 10 and 50 times its original width.