Expanded porous polytetrafluoroethylene film having elastic recovery property in thickness-wise direction of the film, production process thereof, and use of the porous film

Abstract

An expanded porous polytetrafluoroethylene film having residual strain of at most 11.0% as measured after a load required to indent a rod, which is in a columnar form that its outer diameter is at least 2 mm and at least 1.9 times as much as the thickness of the film, and has a smooth plane perpendicular to its axis at a free end surface thereof and a modulus of longitudinal elasticity of at least 1.0×10 4 kgf/mm 2 , up to 20% of the film thickness at a strain rate of 100%/min from the free end surface is applied repeatedly 20 times, and a production process of the porous film, in which a step of compressing an expanded porous polytetrafluoroethylene film having a high draw ratio is provided.

1 . An expanded porous polytetrafluoroethylene film having a microstructure composed of fine fibrils and nodes connected by the fibrils and elastic recovery property in its thickness-wise direction, wherein the film has residual strain of at most 11.0% as measured after a load required to indent a rod, which is in a columnar form that its outer diameter is at least 2 mm and at least 1.9 times as much as the thickness of the film, and has a smooth plane perpendicular to its axis at a free end surface thereof and a modulus of longitudinal elasticity of at least 1.0×10 4  kgf/mm 2 , up to 20% of the film thickness at a strain rate of 100%/min from the free end surface is applied repeatedly 20 times.  
   
   
       2 . The expanded porous polytetrafluoroethylene film according to  claim 1 , wherein a variation of tangent modulus is at most 10.0%.  
   
   
       3 . The expanded porous polytetrafluoroethylene film according to  claim 1 , wherein residual strain is at most 10.5, and a variation of tangent modulus is at most 7.0%.  
   
   
       4 . The expanded porous polytetrafluoroethylene film according to  claim 1 , wherein residual strain is at most 6.5, and a variation of tangent modulus is at most 7.0%.  
   
   
       5 . A process for producing an expanded porous polytetrafluoroethylene film having a microstructure composed of fine fibrils and nodes connected by the fibrils, the process comprising the following steps 1 to 6: 
 (1) an extrusion step 1 of extruding a mixture of unsintered polytetrafluoroethylene powder and a lubricant to prepare an extrudate in the form of a sheet or rod;    (2) a rolling step 2 of rolling the extrudate to prepare a rolled sheet;    (3) a stretching step 3 of biaxially stretching the rolled sheet in lengthwise and crosswise directions at a total draw ratio exceeding 12 times to prepare an expanded porous polytetrafluoroethylene film (A);    (4) a sintering step 4 of heating the expanded porous polytetrafluoroethylene film (A) to a temperature not lower than the melting point of polytetrafluoroethylene in a state fixed so as not to shrink the film to sinter the film;    (5) a cooling step 5 of cooling the sintered expanded porous polytetrafluoroethylene film (A); and    (6) a compression step 6 of compressing the cooled expanded porous polytetrafluoroethylene film (A) in a thickness-wise direction of the film, thereby obtaining an expanded porous polytetrafluoroethylene film (B) having elastic recovery property in the thickness-wise direction of the film.    
   
   
       6 . The production process according to  claim 5 , wherein in the rolling step 2, a sheet-like extrudate is rolled to a rolling ratio of at least 1.3 times.  
   
   
       7 . The production process according to  claim 5 , wherein in the stretching step 3, the rolled sheet is biaxially stretched in such a manner that the total draw ratio is at least 20 times.  
   
   
       8 . The production process according to  claim 5 , wherein in the sintering step 4, an expanded porous polytetrafluoroethylene film (A) having a porosity of at least 66% is prepared.  
   
   
       9 . The production process according to  claim 5 , wherein in the cooling step 5, the sintered expanded porous polytetrafluoroethylene film (A) is air-cooled at ambient temperature or quenched by blowing a cooling medium against the film.  
   
   
       10 . The production process according to  claim 5 , wherein in the compression step 6, the expanded porous polytetrafluoroethylene film (A) is compressed at a compression ratio of 1.1 to 4.0.  
   
   
       11 . The production process according to  claim 5 , wherein after the compression step 6, a expanded porous polytetrafluoroethylene film (B) having a porosity of 40 to 75% is obtained.  
   
   
       12 . The production process according to  claim 5 , wherein after the compression step 6, an expanded porous polytetrafluoroethylene film (B) having residual strain of at most 11.0% as measured after a load required to indent a rod, which is in a columnar form that its outer diameter is at least 2 mm and at least 1.9 times as much as the thickness of the film, and has a smooth plane perpendicular to its axis at a free end surface thereof and a modulus of longitudinal elasticity of at least 1.0×10 4  kgf/mm 2 , up to 20% of the film thickness at a strain rate of 100%/min from the free end surface is applied repeatedly 20 times is obtained.  
   
   
       13 . A process for producing an expanded porous polytetrafluoroethylene film having a microstructure composed of fine fibrils and nodes connected by the fibrils, the process comprising the following steps I to VII: 
 (1) an extrusion step I of extruding a mixture of unsintered polytetrafluoroethylene powder and a lubricant to prepare an extrudate in the form of a sheet or rod;    (2) a rolling step II of rolling the extrudate to prepare a rolled sheet;    (3) a stretching step III of biaxially stretching the rolled sheet in lengthwise and crosswise directions at a total draw ratio exceeding 12 times to prepare an expanded porous polytetrafluoroethylene film (A);    (4) a multi-layer film-forming step IV of laminating at least two expanded porous polytetrafluoroethylene films (A) to prepare a multi-layer film (A1);    (5) a sintering step V of heating the multi-layer film (A1) to a temperature not lower than the melting point of polytetrafluoroethylene in a state fixed so as not to shrink all the layers to sinter the film, and at the same time integrally fusion-bond the respective layers to each other to prepare an expanded porous polytetrafluoroethylene film (A2);    (6) a cooling step VI of cooling the sintered expanded porous polytetrafluoroethylene film (A2); and    (7) a compression step VII of compressing the cooled expanded porous polytetrafluoroethylene film (A2) in a thickness-wise direction of the film, thereby obtaining an expanded porous polytetrafluoroethylene film (B1) having elastic recovery property in the thickness-wise direction of the film.    
   
   
       14 . The production process according to  claim 13 , wherein in the rolling step II, a sheet-like extrudate is rolled to a rolling ratio of at least 1.3 times.  
   
   
       15 . The production process according to  claim 13 , wherein in the stretching step III, the rolled sheet is biaxially stretched in such a manner that the total draw ratio is at least 20 times.  
   
   
       16 . The production process according to  claim 13 , wherein in the sintering step V, an expanded porous polytetrafluoroethylene film (A2) having a porosity of at least 66% is prepared.  
   
   
       17 . The production process according to  claim 13 , wherein in the cooling step VI, the sintered expanded porous polytetrafluoroethylene film (A2) is air-cooled at ambient temperature or quenched by blowing a cooling medium against the film.  
   
   
       18 . The production process according to  claim 13 , wherein in the compression step VII, the expanded porous polytetrafluoroethylene film (A2) is compressed at a compression ratio of 1.1 to 4.0.  
   
   
       19 . The production process according to  claim 13 , wherein after the compression step VII, an expanded porous polytetrafluoroethylene film (B1) having a porosity of 40 to 75% is obtained.  
   
   
       20 . The production process according to  claim 13 , wherein after the compression step VII, an expanded porous polytetrafluoroethylene film (B1) having residual strain of at most 11.0% as measured after a load required to indent a rod, which is in a columnar form that its outer diameter is at least 2 mm and at least 1.9 times as much as the thickness of the film, and has a smooth plane perpendicular to its axis at a free end surface thereof and a modulus of longitudinal elasticity of at least 1.0×10 4  kgf/mm 2 , up to 20% of the film thickness at a strain rate of 100%/min from the free end surface is applied repeatedly 20 times is obtained.  
   
   
       21 . Anisotropically conductive film having a structure that a plurality of through-holes are formed in the expanded porous polytetrafluoroethylene film according to any one of  claims 1  to  4 , and a conductive metal is applied to wall surfaces of the respective through-holes.  
   
   
       22 . A cushioning material comprising the expanded porous polytetrafluoroethylene film according to any one of  claims 1  to  4 .  
   
   
       23 . A sealing material comprising the expanded porous polytetrafluoroethylene film according to any one of  claims 1  to  4 .  
   
   
       24 . An intracorporeally implanting material comprising the expanded porous polytetrafluoroethylene film according to any one of  claims 1  to  4 .