Patent Publication Number: US-2021171730-A1

Title: Hot press cushioning material and production method thereof

Description:
TECHNICAL FIELD 
     The present invention relates to hot press cushioning materials and production methods thereof. More particularly, the present invention relates to hot press cushioning materials that are used when press forming or thermocompression bonding an intended product in the process of producing a laminate that is a high precision machine component such as a printed circuit board like a flexible printed circuit board or a rigid flexible board, an IC card, a liquid crystal display panel, or a battery, and production methods thereof. 
     BACKGROUND ART 
     When producing a laminate such as a printed circuit board, a laminate material, which is an object to be pressed, is placed between heating platens, which are heating and pressing means, and a certain pressure and heat are applied to the laminate material in a press forming or thermocompression bonding process. In order to produce an accurate formed product, it is necessary to uniformly heat and press the entire surface of the laminate material in hot press. For this purpose, hot press is performed with a flat plate-like cushioning material being interposed between the heating platen and the laminate material. 
     When hot pressing a laminate material (e.g., a flexible printed circuit board) having an uneven surface with a cushioning material being in direct contact with the laminate material or with a release film interposed between the laminate material and the cushioning material, it is necessary that the cushioning material be in contact with the entire uneven surface of the laminate material and conform to protrusions and recesses of the uneven surface and it is also necessary to uniformly transmit a pressure and heat to the entire uneven surface including the protrusions and the recesses, in order to produce an accurate formed product. 
     A hot press cushioning material disclosed in Japanese Patent No. 4316937 (Patent Literature 1) includes a layer of polyol vulcanization type vulcanized fluororubber and the vulcanized fluororubber has the following properties in order for the hot press cushioning material to have improved conformability to unevenness of an object to be pressed. The vulcanized fluororubber is produced by vulcanizing a composition prepared by mixing 1 to 10 parts by mass of an acid acceptor and 0 to 5 parts by mass of other compounding agent that is added as necessary per a total of 100 parts by mass of raw fluororubber comprised of a vinylidene fluoride-hexafluoropropylene binary copolymer having a number average molecular weight of 3.5×10 4  to 2.0×10 5  and a polyol vulcanizing agent. The degree of vulcanization is 90.0% to 98.8% in gel fraction, and the durometer A hardness of the vulcanized fluororubber is A40 to A55. 
     A hot press cushioning material disclosed in Japanese Patent No. 4843396 (Patent Literature 2) has the following characteristics in order for the hot press cushioning material to be able to be used repeatedly for multiple hot presses and to have excellent cushioning properties, excellent in-plane uniformity, and excellent heat transfer capability. That is, this hot press cushioning material is a composite of paper comprised of a fiber material and rubber with which the paper has been impregnated, and the volume ratio of the fiber material to the rubber is 1/1.5 to 1/7.5 and the void fraction of the composite is 60 to 90%. 
     CITATION LIST 
     Patent Literatures 
     Patent Literature 1: Japanese Patent No. 4316937 
     Patent Literature 2: Japanese Patent No. 4843396 
     SUMMARY OF INVENTION 
     Technical Problem 
     According to the hot press cushioning materials disclosed in Patent Literatures 1 and 2, conformability to unevenness is improved to some extent. The present invention provides a cushioning material having satisfactory conformability even to larger protrusions and recesses. 
     The hot press cushioning material of Patent Literature 1 may not have satisfactory conformability to protrusions and recesses having a size of, e.g., 50 μm or more. 
     The hot press cushioning material of Patent Document 2 has voids therein, but these voids are continuous pores. Since the void fraction of the voids that are continuous pores is as large as about 60 to 90%, this hot press cushioning material is somewhat disadvantageous in terms of conformability to unevenness and resilience after press. 
     It is an object of the present invention to provide a hot press cushioning material that has excellent conformability to unevenness and has excellent resilience and thus can be used repeatedly and a production method thereof. 
     Solution to Problem 
     A hot press cushioning material according to the present invention includes: a fiber material comprised of a multiplicity of randomly oriented fibers; rubber present in voids between the fibers of the fiber material; and independent pores dispersedly present in the rubber. 
     It is preferable that a volume ratio of the fiber material to the rubber be 1/15 or more and less than 1/7.5, and porosity (void fraction) of the hot press cushioning material be 15 to 70% based on volume. 
     The independent pores are formed by, e.g., expansion of thermally expandable microcapsules dispersed in the rubber. 
     The fiber material preferably contains one or more materials selected from the group consisting of glass, rock wool, carbon, polybenzazoles, polyimides, aromatic polyamides, and polyamides. 
     The rubber preferably contains one or more materials selected from the group consisting of fluororubber, EPM, EPDM, hydrogenated nitrile rubber, silicone rubber, acrylic rubber, and butyl rubber. 
     In one embodiment, the rubber includes one-side rubber layer located on one of upper and lower sides of a layer of the fiber material, and the one-side rubber layer has independent pores dispersedly present therein. It is preferable that the rubber include the other-side rubber layer located on the other of the upper and lower sides of the layer of the fiber material, and the other-side rubber layer have independent pores dispersedly present therein. 
     In the above preferred embodiment, it is preferable that a volume ratio of the fiber material to the rubber having the independent pores dispersed therein be 1/75 or more and less than 1/15 and porosity (void fraction) of the hot press cushioning material be 15 to 70% based on volume. It is more preferable that a ratio of a thickness of the layer of the fiber material with the rubber being present in the voids between the fibers to a total thickness of the upper and lower rubber layers be 1/7 or more and 1/1.5 or less. 
     A method for producing a hot press cushioning material according to the present invention includes the steps of: stacking a fiber material sheet comprised of a multiplicity of randomly oriented fibers and an unvulcanized rubber sheet having thermally expandable microcapsules dispersed therein on top of each other; pressing the stack of the fiber material sheet and the unvulcanized rubber sheet to cause unvulcanized rubber and the microcapsules in the unvulcanized rubber sheet to enter voids between the fibers of the fiber material sheet to form a composite; heating the composite to cause expansion of the microcapsules to form independent pores; and heating the composite to vulcanize the unvulcanized rubber in the composite. The steps may be performed separately, or two or more of the steps may be performed simultaneously or successively. 
     In one embodiment, the stacking step includes sandwiching the unvulcanized rubber sheet between two of the fiber material sheets. 
     It is preferable that the step of pressing the fiber material sheet and the unvulcanized rubber sheet to form the composite be performed at a first temperature, the step of forming the independent pores be performed at a second temperature higher than the first temperature, and the vulcanizing step be performed at a third temperature higher than the second temperature. The step of forming the independent pores and the vulcanizing step may be performed simultaneously or successively by continuously increasing the temperature from the second temperature to the third temperature. 
     In one embodiment, the stacking step includes sandwiching the fiber material sheet between two of the unvulcanized rubber sheets. In this case, it is preferable that the step of forming the composite includes causing a part of each of the unvulcanized rubber sheets containing the microcapsules to enter the voids between the fibers of the fiber material sheet and causing the remaining part of each of the unvulcanized rubber sheets to be located outside the fiber material sheet. 
     Advantageous Effects of Invention 
     The present invention having the above configuration provides a hot press cushioning material that has excellent conformability to unevenness and has excellent resilience and thus can be used repeatedly. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is an illustrative sectional view of a hot press cushioning material according to an embodiment of the present invention. 
         FIG. 2  is an illustration of a method for producing a hot press cushioning material according to an embodiment of the present invention. 
         FIG. 3  is an illustrative sectional view of a composite obtained by the pressing step shown in  FIG. 2 . 
         FIG. 4  is an illustrative sectional view of a fluororubber sheet containing thermally expandable microcapsules and two sheets of glass paper before being pressed into a composite. 
         FIG. 5  is an illustrative sectional view of a multilayered structure of Example Sample 1. 
         FIG. 6  is an illustrative sectional view of a multilayered structure of Comparative Example Sample 1. 
         FIG. 7  is an illustrative sectional view of a multilayered structure of Comparative Example Sample 2. 
         FIG. 8  is an illustrative sectional view of a multilayered structure of Comparative Example Sample 4. 
         FIG. 9  is an illustrative sectional view of Comparative Example Sample 5. 
         FIG. 10  is an illustrative sectional view illustrating the configuration of a press that is used to evaluate samples&#39; conformability to unevenness. 
         FIG. 11  shows images of a polyimide film in a stepped portion after single press of samples to be evaluated. 
         FIG. 12  shows images of the polyimide film in the stepped portion after 50 presses of the samples to be evaluated. 
         FIG. 13  is a sectional image of Example Sample 1 having the structure shown in  FIG. 5 . 
         FIG. 14  is an enlarged sectional image of a fiber-rubber composite sheet shown in  FIG. 13  in the thickness direction. 
         FIG. 15  is an enlarged sectional image of the fiber-rubber composite sheet shown in  FIG. 13  in the in-plane direction. 
         FIG. 16  is an enlarged image of a fiber-rubber composite sheet of Comparative Example Sample 3 in the in-plane direction. 
         FIG. 17  is an image of the surface of glass paper as a preferred example of a fiber sheet. 
         FIG. 18  is an enlarged image of  FIG. 17 . 
         FIG. 19  is an illustrative sectional view of a hot press cushioning material according to another embodiment of the present invention. 
         FIG. 20  is an illustration of a method for producing a hot press cushioning material shown in  FIG. 19 . 
         FIG. 21  is a sectional image of Example Sample 5 having the structure shown in  FIG. 19 . 
         FIG. 22  shows images of a polyimide film in a stepped portion after press of the samples to be evaluated, showing comparison between Example Sample 1 having the structure shown in  FIG. 5  and Example Sample 5 having the structure shown in  FIG. 19   
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     A hot press cushioning material shown in  FIG. 1  includes a fiber material comprised of a multiplicity of randomly oriented fibres  1 , rubber  2  present in the voids between the fibers  1  of the fiber material, and a multiplicity of independent pores  3  dispersedly present in the rubber  2 . 
     The fiber material comprised of the multiplicity of fibers  1  preferably contains one or more materials selected from the group consisting of glass, rock wool, carbon, polybenzazoles, polyimides, aromatic polyamides, and polyamides. The rubber  2  preferably contains one or more materials selected from the group consisting of fluororubber, EPM, EPDM, hydrogenated nitrile rubber, silicone rubber, acrylic rubber, and butyl rubber. 
     The independent pores  3  are preferably pores formed by expansion of thermally expandable microcapsules. The average particle size of the microcapsules before expansion is about 5 to 50 μm, which is large enough for the microcapsules to easily pass through the voids between the fibers  1  of the fiber material. The average particle size of the microcapsules is more preferably 5 to 40 μm, and even more preferably 10 to 40 μm. 
     In the embodiment having the structure shown in  FIG. 1 , in order to maintain satisfactory conformability of the hot press cushioning material to unevenness even after repeated use, the volume ratio of the fiber material to the rubber  2  is 1/15 or more and less than 1/7.5. When the volume ratio of the fiber material to the rubber  2  is less than 1/15, the hot press cushioning material has lower shape retention capability and stretching or tearing of the hot press cushioning material may occur with repeated use. When the volume ratio of the fiber material to the rubber  2  is 1/7.5 or more, the hot press cushioning material does not have sufficient conformability to unevenness. The volume ratio of the fiber material to the rubber is more preferably 1/10 or more and less than 1/7.5. 
     It is desirable that the porosity (void fraction) of the hot press cushioning material that is a composite of a fiber material and rubber be 15 to 70% based on volume. A hot press cushioning material having porosity of less than 15% does not have sufficient conformability to unevenness. A hot press cushioning material having porosity of more than 70% is not preferable because it has sufficient conformability to unevenness but its cushioning properties are significantly degraded with repeated use. The lower limit of the porosity of the hot press cushioning material is more preferably 20%, and the upper limit thereof is more preferably 60%. 
     The hot press cushioning material shown in  FIG. 1  is produced by the following process. 
     As shown in  FIG. 2 , a fiber sheet  10  comprised of a multiplicity of randomly oriented fibers  1  and an unvulcanized rubber sheet  20  having thermally expandable microcapsules  30  dispersed therein are first prepared. In one embodiment, two fiber sheets  10  are prepared and are stacked on top of each other with the unvulcanized rubber sheet  20  interposed therebetween. The fiber sheet  10  can be in the form of nonwoven fabric or paper but is preferably in the form of paper produced especially by a wet papermaking process. The fiber sheet  10  in the form of paper has in-plane uniformity as fibers are randomly oriented in the in-plane direction in paper. In order for unvulcanized rubber  21  and the microcapsules  30  to enter the voids between the fibres  1 , the fiber sheet  10  preferably has a void fraction of 84 to 99%. 
       FIG. 17  is an image of the surface of glass paper that is a preferred example of the fiber sheet  10 . This glass paper has a void fraction of 95%. As can be seen from the figure, glass fibers are randomly oriented in the in-plane direction in this glass paper. 
       FIG. 18  is an enlarged image of  FIG. 17 . As can be seen from  FIG. 18 , this glass paper has a high void fraction. Accordingly, the unvulcanized rubber  21  and the microcapsules  30  can be caused to enter the voids between the fibers  1  by stacking the glass paper and the unvulcanized rubber sheet  20  on top of each other and pressing the stack of the glass paper and the unvulcanized rubber sheet  20 . 
     The fiber sheets  10  and the unvulcanized rubber sheet  20  stacked on top of each other as shown in  FIG. 2  are pressed to from a composite sheet. This pressing is performed by heating to a first temperature. The first temperature is a temperature that is not high enough to cause expansion of the microcapsules  30  and is, e.g., about 100 to 115° C. The pressing force is, e.g., about 4 MPa and the pressing time is, e.g., about 60 minutes at high temperatures (about 110° C.) and then about 40 minutes at room temperature. 
     By this pressing, the unvulcanized rubber  21  in the unvulcanized rubber sheet  20  enters the voids between the multiplicity of fibers  1  in the fiber sheet  10 , as shown in  FIG. 3 . Similarly, the microcapsules  30  dispersed in the unvulcanized rubber  21  also enter the voids between the multiplicity of fibers  1 . The microcapsules  30  have not expanded in this state. 
     The fiber-rubber composite sheet shown in  FIG. 3  is heated to a second temperature higher than the first temperature to cause expansion of the thermally expandable microcapsules  30  to form the independent pores  3 . The independent pores  3  are independent air bubbles that are completely closed, and are distinguished from continuous pores (continuous air bubbles) having a void communicating with the outside. The second temperature is a temperature at which the thermally expandable microcapsules  30  expand and is, e.g., about 120 to 140° C. 
     The fiber-rubber composite sheet is then further heated to a third temperature higher than the second temperature to vulcanize the unvulcanized rubber  21 . The hot press cushioning material shown in  FIG. 1  is thus produced. The third temperature is, e.g., a temperature equal to or higher than 150° C. Expansion of the thermally expandable microcapsules may be caused when the temperature passes the second temperature during heating to the third temperature. The rubber  2  has been vulcanized in the state of  FIG. 1 . 
     The inventors produced several kinds of example samples and several kinds of comparative example samples and compared and evaluated their structures and properties such as conformability to unevenness. The average particle size of commercially available thermally expandable microcapsules (before expansion) is about 5 to 50 μm. Specific examples of the commercially available thermally expandable microcapsules include “Expancel” (average particle size: 10 to 40 μm) made by Japan Fillite Co., Ltd., “Matsumoto Microsphere” (average particle size: 5 to 50 μm) made by Matsumoto Yushi-Seiyaku Co., Ltd., and “KUREHA Microsphere” (average particle size: 10 to 50 μm) made by KUREHA CORPORATION. The thermally expandable microcapsules used in the example samples were “Expancel 920-DU40” made by Japan Fillite Co., Ltd., whose average particle size was 10 to 16 μm. 
     Example Sample 1 
     A. Preparation of Materials 
     Two fiber material sheets (two sheets of glass paper) comprised of a multiplicity of randomly oriented glass fibers were prepared. The glass paper used was “GRABESTOS SYS-80” made by ORIBEST CO., LTD. The glass paper had a thickness of 0.57 mm, a basis weight of 80 g/m 2 , and a void fraction of 95%. 
     Thermally expandable microcapsules were also prepared. The thermally expandable microcapsules used were “Expancel 920-DU40” made by Japan Fillite Co., Ltd. 5 parts by mass of the thermally expandable microcapsules were mixed per 100 parts by mass of fluororubber, and the mixture was kneaded to prepare an unvulcanized fluororubber sheet with a thickness of 0.6 mm. The unvulcanized fluororubber sheet had the thermally expandable microcapsules dispersed therein. 
     B. Primary Press 
     As shown in  FIG. 4 , an unvulcanized fluororubber sheet  40  containing thermally expandable microcapsules and having a thickness of 0.6 mm was sandwiched between two sheets  41  of glass paper, and the stack of the unvulcanized fluororubber sheet  40  and the two sheets  41  of glass paper was hot pressed to form a composite sheet. 
     This pressing was performed under the following conditions. 
     Temperature: 110° C. 
     Pressure: 4 MPa 
     Pressing Time: 60 minutes at high temperatures and 40 minutes at room temperature 
     C. Expansion of Microcapsules and Vulcanization of Rubber 
     The resultant fiber-rubber composite sheet  50  ( FIG. 5 ) was heated in a heating oven and held therein for a predetermined time to cause expansion of the thermally expandable microcapsule and to vulcanize the fluororubber and bake the fiber-rubber composite sheet  50 . The temperature and the holding time in the heating oven were 230° C. and 5 hours. The thermally expandable microcapsules expanded during heating to form a multiplicity of independent air bubbles in the fiber-rubber composite sheet  50 . The volume ratio of the fiber material to the rubber in the fiber-rubber composite sheet  50  was 1/9.5. 
     D. Secondary Press 
     As shown in  FIG. 5 , a fluorine film (thickness: 12 μm)  52  was bonded to both surfaces of the fiber-rubber composite sheet  50  with a fluororubber sheet  51  (thickness: 50 μm) interposed therebetween by hot press. 
     Example sample 1 was thus produced. 
     Example Sample 2 
     5 parts by mass of the thermally expandable microcapsules used in Example Sample 1 were mixed per 100 parts by mass of fluororubber to prepare an unvulcanized fluororubber sheet with a thickness of 0.5 mm. 
     The other conditions (glass paper, primary press, thermal expansions of capsules, vulcanization, and secondary press) are the same as those in Example Sample 1. The volume ratio of the fiber material to the rubber in the fiber-rubber composite sheet  50  was 1/7.9. 
     Example Sample 3 
     3 parts by mass of the thermally expandable microcapsules used in Example Sample 1 were mixed per 100 parts by mass of fluororubber to prepare an unvulcanized fluororubber sheet with a thickness of 0.6 mm. 
     The other conditions (glass paper, primary press, thermal expansions of capsules, vulcanization, and secondary press) are the same as those in Example Sample 1. The volume ratio of the fiber material to the rubber in the fiber-rubber composite sheet  50  was 1/9.5. 
     Example Sample 4 
     7 parts by mass of the thermally expandable microcapsules used in Example Sample 1 were mixed per 100 parts by mass of fluororubber to prepare an unvulcanized fluororubber sheet with a thickness of 0.6 mm. 
     The other conditions (glass paper, primary press, thermal expansions of capsules, vulcanization, and secondary press) are the same as those in Example Sample 1. The volume ratio of the fiber material to the rubber in the fiber-rubber composite sheet  50  was 1/9.5. 
     [Comparison of Mixing Ratio and Thickness among Example Samples 1 to 4] 
     The amount of thermally expandable microcapsules, the thickness of the unvulcanized fluororubber sheet, the thickness after primary press, the thickness after expansion and vulcanization, and the thickness after secondary press of Example Samples 1 to 4 are shown in Table 1 below. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                 Specifications of Material for Primary Press 
                 Product Thickness 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 Amount of Thermally 
                 Thickness of 
                 Thickness 
                 Thickness 
                 Thickness 
               
               
                   
                 Expandable Microcapsules 
                 Unvulcanized 
                 After 
                 After 
                 After 
               
               
                   
                 (Per 100 Parts of 
                 Fluororubber 
                 Primary 
                 Expansion and 
                 Secondary 
               
               
                   
                 Fluororubber) 
                 Sheet 
                 Press 
                 Vulcanization 
                 Press 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Example 
                 5 parts 
                 0.6 mm 
                 0.61 mm 
                 2.41 mm 
                 1.32 mm 
               
               
                 Sample 1 
               
               
                 Example 
                 5 parts 
                 0.5 mm 
                 0.51 mm 
                 2.30 mm 
                 1.20 mm 
               
               
                 Sample 2 
               
               
                 Example 
                 3 parts 
                 0.6 mm 
                 0.61 mm 
                 2.12 mm 
                 1.14 mm 
               
               
                 Sample 3 
               
               
                 Example 
                 7 parts 
                 0.6 mm 
                 0.61 mm 
                 2.81 mm 
                 1.60 mm 
               
               
                 Sample 4 
               
               
                   
               
            
           
         
       
     
     Comparative Example Sample 1 
     Comparative Example Sample 1 shown in  FIG. 6  was produced under the same conditions as those of Example Sample 1 except that no thermally expandable microcapsules were mixed with fluororubber. No independent pores were formed in a fiber-rubber composite sheet  60 . The volume ratio of the fiber material to the rubber in the fiber-rubber composite sheet  60  was 1/9.5. 
     Comparative Example Sample 2 
     As shown in  FIG. 7 , Comparative Example Sample 2 is a fiber-rubber composite sheet  70  having fluorine films  73  (thickness: 25 μm) bonded to its both surfaces. The fiber-rubber composite sheet  70  is a composite sheet of aramid cloth  71  comprised of aromatic polyamide fibers serving as a reinforcing material and fluororubber (thickness: 0.5 mm, durometer A hardness:)60° 72 on both surfaces of the aramid cloth  71 . The fiber-rubber composite sheet  70  has no independent pores formed therein. 
     Comparative Example Sample 3 
     Comparative Example Sample 3 is a fiber-rubber composite sheet produced by impregnating glass paper “GRABESTOS SYS-80” made by ORIBEST CO., LTD. with fluororubber and has voids (continuous pores) therein. This fiber-rubber composite sheet is described in Japanese Patent No. 4843396. The void fraction of the fiber-rubber composite sheet was 85%, the volume ratio of the fiber material to the rubber was 1/2.2, and the thickness was 0.64 mm. 
     Comparative Example Sample 4 
     As shown in  FIG. 8 , Comparative Example Sample 4 was produced by bonding a thermoplastic film  81  to one surface of a stack  80  of three sheets of craft paper having a basis weight of 190 g/m 2 . 
     Comparative Example Sample 5 
     Comparative Example Sample 5 is a fluororubber sheet  90  having a multiplicity of independent pores  91  therein. The fluororubber sheet  90  contains no fibers. The thickness of the fluororubber sheet  90  was 1 mm and the hardness (durometer A hardness) thereof was 22°. 
     [Evaluation of Conformability to Unevenness] 
     Each sample&#39;s conformability to unevenness was evaluated by a press test.  FIG. 10  illustrates the configuration of a press. 
     Abase  100  and a pressing unit  101  of the press contain a heater. A cushioning material  102 , a stainless steel sheet  103 , a spacer  104  with a thickness of 0.1 mm, a release film  105  made of fluororesin and having a thickness of 12 μm, a polyimide film  106  with adhesive, a sample  107  to be evaluated, a stainless steel sheet  108 , and a cushioning material  109  were placed in this order from bottom to top between the base  100  and the pressing unit  101 . The cushioning materials  102 ,  109  were stacks of five sheets of craft paper having a basis weight of 190 g/m 2 . 
     The spacer  104  with a thickness of 0.1 mm has a slit with a width of 0.5 mm. A step (unevenness) with a depth of 0.1 mm is therefore formed by the lower stainless steel sheet  103  and the spacer  104  placed thereon. The sample  107  to be evaluated is pressed down and heated by the pressing unit  101  to cure the adhesive on the polyimide film  106 . Conformability to unevenness was evaluated from formation of a void in the stepped portion by the adhesive on the polyimide film  106 . 
     The pressing for evaluation was performed under the following conditions. 
     Temperature: 200° C. 
     Pressure: 2 MPa 
     Pressing Time: 60 minutes at high temperatures and 15 minutes at room temperature 
       FIG. 11  shows images of the polyimide film  106  in the stepped portion after single press. 
     The images of  FIG. 11  show that, in Example Samples 1 to 4, the sample  107  entered the stepped portion (recess) and an appropriate pressure was applied to the stainless steel sheet  103  located under the slit of the spacer  104 . In other words, Example Samples 1 to 4 had satisfactory conformability to unevenness. 
     The images of  FIG. 11  also show that, in Comparative Example 
     Samples 1 to 3, a void was formed in the stepped portion (recess) and an appropriate pressure was not applied. In other words, Comparative Example Samples 1 to 3 do not have sufficient conformability to unevenness. In Comparative Example Samples 4 and 5, no void was formed after single press. Comparative Example Samples 4 and 5 had satisfactory conformability to unevenness after single press. 
       FIG. 12  show images of the polyimide film  106  in the stepped portion after 50 presses. The images of  FIG. 12  show that, in Example Samples 1 to 4, an appropriate pressure was applied to the stainless steel sheet  103  located under the slit of the spacer  104  even after 50 presses. In other words, Example Samples 1 to 4 had satisfactory conformability to unevenness even after 50 presses. 
     Pressing was not able to be performed 50 times on Comparative Example Samples 4 and 5 that had satisfactory conformability to unevenness after the first press. Specifically, Comparative Example Sample 4 was plastically deformed by the first press and became unusable. Comparative Example Sample 5 had satisfactory conformability to unevenness after the second press but became unusable due to torn rubber. 
     [Structure and Conformability to Unevenness of Samples to be Evaluated] 
     The structure of the samples to be evaluated and the evaluation results of their conformability to unevenness are shown in Table 2 below. 
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                 Samples to be Evaluated 
                 Initial 
                 Single Press 
                 10 Presses 
                 50 Presses 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Example 1 
                 Conformability to Unevenness 
                   
                 Satisfactory 
                 Satisfactory 
                 Satisfactory 
               
               
                   
                 (Step: 0.1 mm) 
               
               
                   
                 State 
                   
                 Satisfactory 
                 Satisfactory 
                 Satisfactory 
               
               
                   
                 Fiber/Rubber Volume Ratio 
                 1/9.5 
                 1/9.5 
                 1/9.5 
                 1/9.5 
               
               
                   
                 Void Fraction 
                 50% 
                 45% 
                 41% 
                 32% 
               
               
                   
                 Thickness 
                 1.32 mm 
                 1.21 mm 
                 1.13 mm 
                 0.98 mm 
               
               
                 Example 2 
                 Conformability to Unevenness 
                   
                 Satisfactory 
                 Satisfactory 
                 Satisfactory 
               
               
                   
                 (Step: 0.1 mm) 
               
               
                   
                 State 
                   
                 Satisfactory 
                 Satisfactory 
                 Satisfactory 
               
               
                   
                 Fiber/Rubber Volume Ratio 
                 1/7.9 
                 1/7.9 
                 1/7.9 
                 1/7.9 
               
               
                   
                 Void Fraction 
                 53% 
                 49% 
                 45% 
                 37% 
               
               
                   
                 Thickness 
                 1.20 mm 
                 1.10 mm 
                 1.03 mm 
                 0.91 mm 
               
               
                 Example 3 
                 Conformability to Unevenness 
                   
                 Satisfactory 
                 Satisfactory 
                 Satisfactory 
               
               
                   
                 (Step: 0.1 mm) 
               
               
                   
                 State 
                   
                 Satisfactory 
                 Satisfactory 
                 Satisfactory 
               
               
                   
                 Fiber/Rubber Volume Ratio 
                 1/9.5 
                 1/9.5 
                 1/9.5 
                 1/9.5 
               
               
                   
                 Void Fraction 
                 42% 
                 39% 
                 34% 
                 26% 
               
               
                   
                 Thickness 
                 1.14 mm 
                 1.08 mm 
                 1.00 mm 
                 0.90 mm 
               
               
                 Example 4 
                 Conformability to Unevenness 
                   
                 Satisfactory 
                 Satisfactory 
                 Satisfactory 
               
               
                   
                 (Step: 0.1 mm) 
               
               
                   
                 State 
                   
                 Satisfactory 
                 Satisfactory 
                 Satisfactory 
               
               
                   
                 Fiber/Rubber Volume Ratio 
                 1/9.5 
                 1/9.5 
                 1/9.5 
                 1/9.5 
               
               
                   
                 Void Fraction 
                 59% 
                 54% 
                 40% 
                 31% 
               
               
                   
                 Thickness 
                 1.60 mm 
                 1.45 mm 
                 1.10 mm 
                 0.96 mm 
               
               
                 Comparative 
                 Conformability to Unevenness 
                   
                 Void Formed 
                 — 
                 — 
               
               
                 Example 1 
                 (Step: 0.1 mm) 
               
               
                   
                 State 
                   
                 — 
                 — 
                 — 
               
               
                   
                 Fiber/Rubber Volume Ratio 
                 1/9.5 
                 — 
                 — 
                 — 
               
               
                   
                 Void Fraction 
                  0% 
                 — 
                 — 
                 — 
               
               
                   
                 Thickness 
                 0.61 mm 
                 — 
                 — 
                 — 
               
               
                 Comparative 
                 Conformability to Unevenness 
                   
                 Void Formed 
                 — 
                 — 
               
               
                 Example 2 
                 (Step: 0.1 mm) 
               
               
                   
                 State 
                   
                 — 
                 — 
                 — 
               
               
                 Comparative 
                 Conformability to Unevenness 
                   
                 Void Formed 
                 — 
                 — 
               
               
                 Example 3 
                 (Step: 0.1 mm) 
               
               
                   
                 State 
                   
                 — 
                 — 
                 — 
               
               
                 Comparative 
                 Conformability to Unevenness 
                   
                 Satisfactory 
                 — 
                 — 
               
               
                 Example 4 
                 (Step: 0.1 mm) 
               
               
                   
                 State 
                   
                 Became unusable 
                 — 
                 — 
               
               
                   
                   
                   
                 due to plastic 
               
               
                   
                   
                   
                 deformation 
               
               
                 Comparative 
                 Conformability to Unevenness 
                   
                 Satisfactory 
                 — 
                 — 
               
               
                 Example 5 
                 (Step: 0.1 mm) 
               
               
                   
                 State 
                   
                 Significant 
                 — 
                 — 
               
               
                   
                   
                   
                 change over time 
               
               
                   
                   
                   
                 stretched out and 
               
               
                   
                   
                   
                 torn became 
               
               
                   
                   
                   
                 unusable 
               
               
                   
               
            
           
         
       
     
     [Sectional Images of Example Sample 1 and Comparative Example Sample 3] 
       FIG. 13  is a sectional image of Example Sample 1 having the structure shown in  FIG. 5 . Example Sample 1 is a fiber-rubber composite sheet having a fluorine film bonded to its both surfaces with a 50 μm thick fluororubber layer interposed therebetween. 
       FIG. 14  is an enlarged sectional image of the fiber-rubber composite sheet, which is a core layer, of Example Sample 1 in the thickness direction. 
       FIG. 15  is an enlarged sectional image thereof in the in-plane direction. As can be seen from these sectional images, the fiber-rubber composite sheet has fluororubber in the voids between a multiplicity of randomly oriented glass fibers and has a multiplicity of independent pores formed in the fluororubber. The size (maximum diameter) of the independent pores is 15 to 115 μm. 
       FIG. 16  is an enlarged image of the fiber-rubber composite sheet of Comparative Example Sample 3 in the in-plane direction. This enlarged image shows that the fiber-rubber composite sheet has fluororubber between a multiplicity of glass fibers and has continuous pores formed therein. The void fraction of this composite sheet is 85% and the volume ratio of the fiber material to the rubber is 1/2.2. 
     [Various Forms of Hot Press Cushioning Material] 
     The hot press cushioning material according to the invention can be in various forms. One form is a fiber-rubber composite sheet including a fiber material comprised of a multiplicity of fibres  1 , rubber  2  present in the voids between the fibres  1 , and independent pores  3  dispersedly present in the rubber  2 , as shown in  FIG. 1 . 
     Other possible forms include the fiber-rubber composite sheet with the structure of  FIG. 1  having fluorine films bonded to its both surfaces with very thin fluororubber interposed therebetween, the fiber-rubber composite sheet with the structure of  FIG. 1  having a surface layer material bonded to its one or both surfaces, a plurality of the fiber-rubber composite sheets with the structure of  FIG. 1  having a nonwoven fabric layer, a woven fabric layer, a rubber layer, etc. interposed therebetween. 
       FIG. 19  is an illustrative sectional view of a heat press cushioning material according to another embodiment of the present invention. A hot press cushioning material  120  shown in the figure includes a fiber-rubber composite layer  121  having the same structure as that shown in  FIG. 1 , an upper rubber layer  122  located over the fiber-rubber composite layer  121 , a lower rubber layer  123  located under the fiber-rubber composite layer  121 , an upper fluorine film  124  placed on the front surface of the upper rubber layer  121 , a lower fluorine film  125  placed on the back surface of the lower rubber layer  123 . 
     The fiber-rubber composite layer  121  includes a fiber material comprised of a multiplicity of randomly oriented fibres  1 , rubber  2  present in the voids between the fibers  1  of the fiber material, and independent pores  3  dispersedly present in the rubber  2 . The upper rubber layer  122  is located over the layer of the fiber material and includes independent pores  3  dispersedly present therein. The lower rubber layer  123  is located under the layer of the fiber material and includes independent pores  3  dispersedly present therein. 
     The upper fluorine film  124  and the lower fluorine film  125  form a front surface layer and a back surface layer of the hot press cushioning material and have heat resistance. Although a fluorine film is used as a heat resistant film in the illustrated embodiment, heat resistant films made of other materials may be used. 
     In the embodiment having the structure shown in  FIG. 19 , the volume ratio of the fiber material to the entire rubber  2  having the independent pores  3  dispersed therein is preferably 1/75 or more and less than 1/15 in order to maintain satisfactory conformability of the hot press cushioning material to unevenness even with repeated use. When the volume ratio of the fiber material to the rubber  2  is less than 1/75, the hot press cushioning material  120  has insufficient reinforcement capability, and the hot press cushioning material  120  may be stretched out, torn, etc. with repeated use. When the volume ratio of the fiber material to the rubber  2  is 1/15 or more, the hot press cushioning material  120  has insufficient conformability to unevenness. In the case of the structure shown in  FIG. 19 , the volume ratio of the fibrous material to the rubber is more preferably 1/60 or more and less than 1/20. 
     In the case of the hot press cushioning material  120  having the structure shown in  FIG. 19 , it is desirable that the porosity (void fraction) of the hot press cushioning material be 15 to 70% based on volume. When the porosity is less than 15%, the hot press cushioning material  120  has insufficient conformability to unevenness. When the porosity is higher than 70%, the hot press cushioning material  120  has sufficient conformability to unevenness but has poor resilience. The hot press cushioning material  120  therefore significantly changes over time with repeated use, and distinguishing marks of protrusions and recesses are left on the hot press cushioning material  120 . Moreover, since the upper and lower rubber layers (rubber-independent pores)  122 ,  123  have lower strength, the hot press cushioning material  120  may have surface cracks, may be torn, etc. In the case of the structure shown in  FIG. 19 , the lower limit of the porosity of the hot press cushioning material is more preferably 20%, and the upper limit of the porosity thereof is more preferably 60%. 
     Regarding the component ratio of the fiber-rubber composite layer (fibers-rubber-independent pores)  121 , the volume ratio of the fiber material to the rubber is preferably 1/15 or more and less than 1/2, and more preferably 1/13 or more and less than 1/3. When the volume ratio is less than 1/15, the hot press cushioning material has insufficient reinforcement capability, and the hot press cushioning material may be stretched out, torn, etc. When the volume ratio is 1/2 or more, the hot press cushioning material has insufficient conformability to unevenness. 
     Regarding the ratio of the thickness between the fiber-rubber composite layer  121  and the upper and lower rubber layers  122 ,  123 , the ratio of the thickness of the fiber-rubber composite layer  121  to the total thickness of the upper and lower rubber layers  122 ,  123  is preferably 1/7 to 1/1.5, and more preferably 1/6 to 1/2. When this thickness ratio is less than 1/7, the hot press cushioning material has insufficient reinforcement capability, and the hot press cushioning material may be stretched out, torn, etc. When the thickness ratio is higher than 1/1.5, the hot press cushioning material has insufficient conformability to unevenness. 
     The hot press cushioning material  120  having the structure shown in  FIG. 19  is produced by the following process. 
     As shown in  FIG. 20 , a fiber sheet  10  comprised of a multiplicity of randomly oriented fibres  1 , an unvulcanized rubber sheet  20  having thermally expandable microcapsule  30  dispersed therein, and two fluorine films  124 ,  125  are first prepared. In one embodiment, two unvulcanized rubber sheets  20  were prepared, and the two unvulcanized rubber sheets  20  are stacked on top of each other with the fiber sheet  10  interposed therebetween. The fiber sheet  10  can be in the form of nonwoven fabric or paper but is preferably in the form of paper produced especially by a wet papermaking process. The fiber sheet  10  in the form of paper has in-plane uniformity as fibers are randomly oriented in the in-plane direction in paper. In order for unvulcanized rubber  21  and the microcapsules  30  to enter the voids between the fibres  1 , the fiber sheet  10  preferably has a void fraction of 84 to 99%. 
     One fluorine film  124  is placed on the front surface of the upper unvulcanized rubber sheet  20 , and the other fluorine film  125  is placed on the back surface of the lower unvulcanized rubber sheet  20 . 
     The multilayered structure of the upper and lower fluorine films  124 ,  125 , the upper and lower unvulcanized rubber sheets  20 , and the fiber sheet  10  which are stacked on top of each other as shown in  FIG. 20  is pressed from above and below to form a composite sheet. This pressing is performed with this entire multilayered structure heated to an appropriate temperature. In one embodiment, this entire multilayered structure is pressed with a pressure of 0.7 MPa at 110° C. for 30 minutes, is then heated to 190° C. and pressed with a pressure of 0.7 MPa at 190° C. for 50 minutes, and thereafter is cooled for 10 minutes with the pressure being maintained at 0.7 MPa. 
     The above heating and pressing process causes a part of each of the upper and lower unvulcanized rubber sheets  20  to enter the voids between the multiplicity of fibers  1  in the fiber sheet  10 , as shown in  FIG. 19 . Similarly, the above heating and pressing process also causes a part of the microcapsules  30  dispersed in the upper and lower unvulcanized rubber sheets  20  to enter the voids between the multiplicity of fibers  1  in the fiber sheet  10 . The microcapsules  30  have not expanded at low temperatures. 
     The pressing is performed so that the remaining part of each of the upper and lower unvulcanized rubber sheets  20  does not penetrate the fiber sheet  10  but is located over and under the fiber sheet  10 . 
     As the temperature is raised, the thermally expandable microcapsules  30  expand to form independent pores  3 . The independent pores  3  are present in the fiber-rubber composite layer  121 , the upper rubber layer  122 , and the lower rubber layer  123 . 
     As the temperature is further raised, the unvulcanized rubber  21  is vulcanized into vulcanized rubber  2 . 
     Example Sample 5 
     A. Preparation of Materials 
     A fiber material sheet (glass paper) comprised of a multiplicity of randomly oriented glass fibers was prepared. The glass paper used was “GRABESTOS SYS-80” made by ORIBEST CO., LTD. The glass paper had a thickness of 0.57 mm, a basis weight of 80 g/m 2 , and a void fraction of 95%. 
     Thermally expandable microcapsules were also prepared. The thermally expandable microcapsules used were “Expancel 920-DU120” made by Japan Fillite Co., Ltd. The thermally expandable microcapsules used in Example Sample 1 were “Expancel 920-DU40.” These two types of thermally expandable microcapsules have the following differences. 
     (a) Expancel 920-DU40 
     Particle size (before expansion): 10 to 16 μm 
     Thermal expansion start temperature: 123 to 133° C. 
     Maximum thermal expansion temperature: 168 to 178° C. 
     (b) Expancel 920-DU120 
     Particle size (before expansion): 28 to 38 μm 
     Thermal expansion start temperature: 123 to 133° C. 
     Maximum thermal expansion temperature: 194 to 206° C. 
     5 parts by mass of the thermally expandable microcapsules (Expancel 920-DU120) were mixed per 100 parts by mass of fluororubber, and the mixture was kneaded to prepare two unvulcanized fluororubber sheets with a thickness of 0.75 mm. Each of the unvulcanized fluororubber sheets  20  had the thermally expandable microcapsules  30  dispersed therein. 
     Two fluorine films with a thickness of 12 μm were prepared. 
     As shown in  FIG. 20 , the fiber sheet  10  made of glass paper was sandwiched between the two unvulcanized rubber sheets  20  containing thermally expandable microcapsules. One fluorine film  124  was placed on the front surface of one unvulcanized rubber sheet  20 , and the other fluorine film  125  was placed on the back surface of the other unvulcanized rubber sheet  20 . This multilayered structure was hot pressed to form a composite sheet. 
     The pressing was performed under the following conditions. First, the multilayered structure was heated to 110° C. and was pressed with a pressure of 0.7 MPa at 110° C. for 30 minutes. Next, the multilayered structure was heated to 190° C. over a period of 20 minutes with the pressure being maintained at 0.7 MPa and was maintained at 190° C. for 50 minutes. The multilayered structure was then cooled to room temperature over a period of 10 minutes with the pressure being maintained at 0.7 MPa. 
     During the pressing at 110° C., a part of the unvulcanized rubber  21  and a part of the thermally expandable microcapsules  30  in each unvulcanized rubber sheet  20  containing the thermally expandable microcapsules  30  entered the voids between the multiplicity of fibers  1  in the fiber sheet  10 . The pressing was performed so that the remaining part of each of the upper and lower unvulcanized rubber sheets  20  did not penetrate the fiber sheet  10  but was located over and under the fiber sheet  10 . 
     The thermally expandable microcapsules expand during the heating from 110° C. to 190° C. to form independent pores in the fiber-rubber composite layer  121  and the upper and lower rubber layers  122 ,  123 . 
     The unvulcanized fluororubber was vulcanized while the multilayered structure was maintained at 190° C. Thereafter, the multilayered structure was baked at 230° C. for 5 hours in order to improve the properties of the fluororubber. 
     The hot press cushioning material of Example Sample 5 produced as described above had the structure shown in  FIG. 19 . The thickness of Example Sample 5 was 2.34 mm. The volume ratio of the fiber material to the entire rubber  2  having the independent pores  3  dispersed therein was 1/48, and the overall porosity of the cushioning material was 35% based on volume. 
     [Image of Example Sample 5] 
       FIG. 21  is a sectional image of Example Sample 5 having the structure shown in  FIG. 19 . As can be seen from the image, independent pores are present in the fiber-rubber composite layer (rubber+fibers+independent pores) located in the middle in the thickness direction. Independent pores are also present in the upper and lower rubber layers (rubber+independent pores). 
     [Comparison Between Example Sample 5 and Example Sample 1] 
     Example Sample 5 is different from Example Sample 1 in the following points. 
     (a) In Example Sample 1, no independent pores are present in the fluororubber layers  51  located over and under the fiber-rubber composite layer (fiber-rubber composite sheet)  50 . In Example Sample 5, on the other hand, independent pores are present in the rubber layers  122 ,  123  located over and under the fiber-rubber composite layer  121 . 
     (b) In the production process, in Sample Example 1, the fluororubber sheets  51  having a thickness of 50 μm are bonded to the front and back surfaces of the fiber-rubber composite layer (fiber-rubber composite sheet)  50 . In Example Sample 5, on the other hand, a part of each of the upper and lower fluororubber sheets  20  enters the voids between the fibers of the fiber material sheet (glass paper)  10  to form the fiber-rubber composite layer  121 , and the remaining part of each of the upper and lower fluororubber sheets  20  forms the upper and lower rubber layers  122 ,  123 . 
     (c) The particle size before expansion of the thermally expandable microcapsules “Expancel 920-DU40” used in Example Sample 1 is 10 to 16 μm, while the particle size before expansion of the thermally expandable microcapsules “Expancel 920-DU120” used in Example Sample 5 is 28 to 38 μm. The size of the independent pores in Example Sample 5 is therefore larger than that of the independent pores in Example Sample 1. 
     The thickness reduction rate with repeated use is lower in Example Sample 5 than in Example Sample 1. Example Sample 5 has better conformability to unevenness than that of Example Sample 1. 
     [Comparison of Thickness Reduction Rate, Decrease in Void Fraction, and Conformability to Unevenness Between Example Sample 1 and Example Sample 5] 
     A press test was performed on Example Sample 1 and Example Sample 5 to measure a change in thickness and a change in void fraction with repeated press and evaluate conformability to unevenness. The configuration of a press used in the test is substantially the same as that shown in  FIG. 10  except for the thickness of the spacer  104 . The spacer  104  used in this test has a thickness of 0.2 mm and has a slit with a width of 0.5 mm. 
     The pressing for evaluation was performed under the following conditions. 
     Temperature: 200° C. 
     Pressure: 2 MPa 
     Pressing time: 60 minutes at high temperatures and 15 minutes at room temperature 
     The measurement and evaluation results for Example Sample 1 and Example Sample 5 are shown in Table 3 below. 
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 3 
               
               
                   
               
               
                 Samples to be Evaluated 
                 Initial 
                 Single Press 
                 10 Presses 
                 50 Presses 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Example 1 
                 Conformability to Unevenness 
                   
                 Void Formed 
                 — 
                 — 
               
               
                   
                 (Step: 0.2 mm) 
               
               
                   
                 State 
                   
                 Satisfactory 
                 — 
                 — 
               
               
                   
                 Fiber/Rubber Volume Ratio 
                 1/9.5 
                 1/9.5 
                 1/9.5 
                 1/9.5 
               
               
                   
                 Void Fraction 
                 50% 
                 45% 
                 41% 
                 32% 
               
               
                   
                 Thickness 
                 1.32 mm 
                 1.21 mm 
                 1.13 mm 
                 0.98 mm 
               
               
                 Example 5 
                 Conformability to Unevenness 
                   
                 Satisfactory 
                 Satisfactory 
                 Satisfactory 
               
               
                   
                 (Step: 0.2 mm) 
               
               
                   
                 State 
                   
                 Satisfactory 
                 Satisfactory 
                 Satisfactory 
               
               
                   
                 Fiber/Rubber Volume Ratio 
                 1/48  
                 1/48 
                 1/48  
                 1/48 
               
               
                   
                 Void Fraction 
                 35% 
                 34% 
                 32% 
                 30% 
               
               
                   
                 Thickness 
                 2.34 mm 
                 2.31 mm 
                 2.25 mm 
                 2.18 mm 
               
               
                   
                 LAYER OF FIBERS, RUBBER, 
                 1/7.6 
                 1/7.6 
                 1/7.6 
                 1/7.6 
               
               
                   
                 AND INDEPENDENT PORES 
               
               
                   
                 Fiber/Rubber Volume Ratio 
               
               
                   
                 LAYER OF RUBBER AND 
                 1/3.7 
               
               
                   
                 INDEPENDENT PORES/ 
               
               
                   
                 LAYER OF FIBERS, RUBBER, 
               
               
                   
                 AND INDEPENDENT PORES 
               
               
                   
                 Thickness Ratio 
               
               
                   
               
            
           
         
       
     
     In Example Sample 1, the initial thickness (the number of times pressing was performed: 0) was 1.32 mm, and the thickness after 50 presses was 0.98 mm. In Example Sample 5, the initial thickness was 2.34 mm, and the thickness after 50 presses was 2.18 mm. 
     The thickness reduction rate ([initial thickness−thickness after 50 presses]/initial thickness) was about 26% in Example Sample 1 and about 7% in Example Sample 5. 
     In Example Sample 1, the initial void fraction was 50%, and the void fraction after 50 presses was 32%. The void fraction thus decreased by 18%. In Example Sample 5, the initial void fraction was 35%, and the void fraction after 50 presses was 30%. The void fraction thus decreased by 5%. 
       FIG. 22  shows images of the polyimide film  106  of  FIG. 10  in the stepped portion. Since the thickness of the spacer  104  is 0.2 mm, the height of the step is 0.2 mm. As can be seen from the images after single press, a void was observed in Example Sample 1. On the other hand, no void was observed in Example Sample 5 even after 50 presses. The results show that Example Sample 5 has better conformability to unevenness to a large step (e.g., 0.2 mm) than Example Sample 1. 
     Although the embodiments of the present invention are described above with reference to the drawings, the present invention is not limited to the illustrated embodiments. Various modifications and variations can be made to the illustrated embodiments without departing from the spirit and scope of the invention. 
     INDUSTRIAL APPLICABILITY 
     The invention can be advantageously used as a hot press cushioning material that has satisfactory conformability to unevenness over long-term use. 
     REFERENCE SIGNS LIST 
       1  Fiber,  2  Rubber,  3  Independent Pore,  10  Fiber Material Sheet,  20  Unvulcanized Rubber Sheet,  21  Unvulcanized Rubber,  30  Thermally Expandable Microcapsule,  40  Unvulcanized Fluororubber Sheet Containing Thermally Expandable Microcapsules,  41  Glass Paper,  50  Fiber-Rubber Composite Sheet,  51  Fluororubber Sheet,  52  Fluorine Film,  60  Fiber-Rubber Composite Sheet,  61  Fluorine Rubber Sheet,  62  Fluorine Film,  70  Fiber-Rubber Composite Sheet,  71  Aramid Cloth,  72  Fluorine Rubber,  73  Fluorine Film,  80  stack of sheets of craft paper,  81  Thermoplastic Film,  90  Fluororubber Sheet,  91  Independent Pores,  100  Base,  101  Pressing Unit,  102  Cushioning Material,  103  Stainless Steel Sheet,  104  Spacer,  105  Release Film,  106  Polyimide Film With Adhesive,  107  Sample to be Evaluated,  108  Stainless Steel Sheet,  109  Cushioning Material,  120  Hot Press Cushioning Material,  121  Fiber-Rubber Composite Layer,  122  Upper Rubber Layer,  123  Lower Rubber Layer,  124  Upper Fluorine Film,  125  Lower Fluorine Film.