Filtering material for filter, filter pack and air filter unit using the same, and method for preparing filtering material

A method of producing a filter medium comprised of a porous film comprised of polytetrafluoroethylene and an air permeable support member that is heat laminated onto at least one surface of the porous film, 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 [Formula 16]
• 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 an air permeable support member is heat laminated to at least one surface of the porous film.

A method of producing a filter medium comprised of a multi-layer porous film having at least two layers and an air permeable support member that is heat laminated to at least one surface of the multi-layer porous film, the multi-layer porous film including a first porous film that is formed from homo-polytetrafluoroethylene and a second porous film that is formed from a modified polytetrafluoroethylene;

• the method comprising:
• a first step in which a first mixture is made by mixing a homo-polytetrafluoroethylene fine powder with a liquid lubricant, a second mixture is made by mixing a modified polytetrafluoroethylene fine power with a liquid lubricant, and the first and second mixtures are placed separately inside a cylinder of an extrusion mold;
• a second step in -which the first and the second mixtures are unitarily paste extruded;
• a third step in which the first and the second mixtures from the second step are rolled together to obtain a multi-layer formation;
• a fourth step in which the liquid lubricant in the multi-layer formation is removed therefrom to obtain a non-sintered tape;
• a fifth step in which the non-sintered tape is elongated first in the longitudinal direction and then in the lateral direction to obtain a multi-layer porous film;
• and
• a sixth step in which an air permeable support member is heat laminated to at least one surface of the multi-layer porous film.

A method of producing a filter medium comprised of a multi-layer porous film having at least two layers and an air permeable support member that is heat laminated to at least one surface of the multi-layer porous film, the multi-layer porous film including a first porous film that is formed from homo-polytetrafluoroethylene and a second porous film that is formed from a modified polytetrafluoroethylene;

• the method comprising:
• a first step in which a first mixture is made by mixing a homo-polytetrafluoroethylene fine power with a liquid lubricant, a second mixture is made by mixing a modified polytetrafluoroethylene fine power with a liquid lubricant, and the first and second mixtures are each paste extruded;
• a second step in which the first mixture from the first step is compressed to obtain a first formation, and the second mixture from the first step is compressed to obtain a second formation;
• a third step in which the liquid lubricants are removed from both the first and the second mixtures;
• a fourth step in which the first and the second formations from the third step are placed on top of each other to obtain a non-sintered tape;
• a fifth step in which the non-sintered tape is elongated first in the longitudinal direction and then in the lateral direction to obtain a multi-layer porous film;
• and
• a sixth step in which an air permeable support member is heat laminated to at least one surface of the multi-layer porous film.

A method of producing a filter medium comprised of a multi-layer porous film having at least two layers and an air permeable support member that is heat laminated to at least one surface of the multi-layer porous film, the multi-layer porous film including a first porous film that is formed from homo-polytetrafluoroethylene and a second porous film that is formed from a modified polytetrafluoroethylene;

• the method comprising:
• a first step in which a first mixture is made by mixing a homo-polytetrafluoroethylene fine power with a liquid lubricant, a second mixture is made by mixing a modified polytetrafluoroethylene fine power with a liquid lubricant, and the first and second mixtures are each paste extruded;
• a second step in which the first mixture from the first step is compressed to obtain a first formation, and the second mixture from the first step is compressed to obtain a second formation;
• a third step in which the liquid lubricants in the first and the second mixtures are removed therefrom to obtain two non-sintered tapes;
• a fourth step in which the two non-sintered tapes are each elongated in the longitudinal direction;
• a fifth step in which the two non-sintered tapes elongated in the longitudinal direction are placed on top of each other, and then are elongated in the lateral direction to obtain a multi-layer porous film;
• and
• a sixth step in which an air permeable support member is heat laminated to at least one surface of the multi-layer porous film.