Patent ID: 7387700

Claim:
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.