Patent Description:
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.

<CIT> discloses a hot press cushioning material as in the preamble of claim <NUM>. hot press cushioning material disclosed in <CIT> (Patent Literature <NUM>) 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 <NUM> to <NUM> parts by mass of an acid acceptor and <NUM> to <NUM> parts by mass of other compounding agent that is added as necessary per a total of <NUM> parts by mass of raw fluororubber comprised of a vinylidene fluoride-hexafluoropropylene binary copolymer having a number average molecular weight of <NUM> × <NUM><NUM> to <NUM> × <NUM><NUM> and a polyol vulcanizing agent. The degree of vulcanization is <NUM>% to <NUM>% in gel fraction, and the durometer A hardness of the vulcanized fluororubber is A40 to A55.

A hot press cushioning material disclosed in <CIT> (Patent Literature <NUM>) 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 <NUM>/<NUM> to <NUM>/<NUM> and the void fraction of the composite is <NUM> to <NUM>%.

According to the hot press cushioning materials disclosed in Patent Literatures <NUM> and <NUM>, 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 <NUM> may not have satisfactory conformability to protrusions and recesses having a size of, e.g., <NUM> or more.

The hot press cushioning material of Patent Document <NUM> 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 <NUM> to <NUM>%, 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.

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, wherein a volume ratio of the fiber material to the rubber present in the voids between the fibers of the fiber material is <NUM>/<NUM> or more and less than <NUM>/<NUM> and the independent pores are independent air bubbles that are completely closed.

It is preferable that a porosity (void fraction) of the hot press cushioning material be <NUM> to <NUM>% 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 porosity (void fraction) of the hot press cushioning material be <NUM> to <NUM>% 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 <NUM>/<NUM> or more and <NUM>/<NUM> 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 completely closed; 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.

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.

A hot press cushioning material shown in <FIG> includes a fiber material comprised of a multiplicity of randomly oriented fibers <NUM>, rubber <NUM> present in the voids between the fibers <NUM> of the fiber material, and a multiplicity of independent pores <NUM> dispersedly present in the rubber <NUM>.

The fiber material comprised of the multiplicity of fibers <NUM> preferably contains one or more materials selected from the group consisting of glass, rock wool, carbon, polybenzazoles, polyimides, aromatic polyamides, and polyamides. The rubber <NUM> 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 <NUM> are pores formed by expansion of thermally expandable microcapsules. The average particle size of the microcapsules before expansion is about <NUM> to <NUM>, which is large enough for the microcapsules to easily pass through the voids between the fibers <NUM> of the fiber material. The average particle size of the microcapsules is more preferably <NUM> to <NUM>, and even more preferably <NUM> to <NUM>.

According to the invention in the structure shown in <FIG>, 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 <NUM> is <NUM>/<NUM> or more and less than <NUM>/<NUM>. When the volume ratio of the fiber material to the rubber <NUM> is less than <NUM>/<NUM>, 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 <NUM> is <NUM>/<NUM> 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 <NUM>/<NUM> or more and less than <NUM>/<NUM>.

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 <NUM> to <NUM>% based on volume. A hot press cushioning material having porosity of less than <NUM>% does not have sufficient conformability to unevenness. A hot press cushioning material having porosity of more than <NUM>% 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 <NUM>%, and the upper limit thereof is more preferably <NUM>%.

The hot press cushioning material shown in <FIG> is produced by the following process.

As shown in <FIG>, a fiber sheet <NUM> comprised of a multiplicity of randomly oriented fibers <NUM> and an unvulcanized rubber sheet <NUM> having thermally expandable microcapsules <NUM> dispersed therein are first prepared. In one embodiment, two fiber sheets <NUM> are prepared and are stacked on top of each other with the unvulcanized rubber sheet <NUM> interposed therebetween. The fiber sheet <NUM> 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 <NUM> 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 <NUM> and the microcapsules <NUM> to enter the voids between the fibers <NUM>, the fiber sheet <NUM> preferably has a void fraction of <NUM> to <NUM>%.

<FIG> is an image of the surface of glass paper that is a preferred example of the fiber sheet <NUM>. This glass paper has a void fraction of <NUM>%. As can be seen from the figure, glass fibers are randomly oriented in the in-plane direction in this glass paper.

<FIG> is an enlarged image of <FIG>. As can be seen from <FIG>, this glass paper has a high void fraction. Accordingly, the unvulcanized rubber <NUM> and the microcapsules <NUM> can be caused to enter the voids between the fibers <NUM> by stacking the glass paper and the unvulcanized rubber sheet <NUM> on top of each other and pressing the stack of the glass paper and the unvulcanized rubber sheet <NUM>.

The fiber sheets <NUM> and the unvulcanized rubber sheet <NUM> stacked on top of each other as shown in <FIG> 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 <NUM> and is, e.g., about <NUM> to <NUM>. The pressing force is, e.g., about <NUM> MPa and the pressing time is, e.g., about <NUM> minutes at high temperatures (about <NUM>) and then about <NUM> minutes at room temperature.

By this pressing, the unvulcanized rubber <NUM> in the unvulcanized rubber sheet <NUM> enters the voids between the multiplicity of fibers <NUM> in the fiber sheet <NUM>, as shown in <FIG>. Similarly, the microcapsules <NUM> dispersed in the unvulcanized rubber <NUM> also enter the voids between the multiplicity of fibers <NUM>. The microcapsules <NUM> have not expanded in this state.

The fiber-rubber composite sheet shown in <FIG> is heated to a second temperature higher than the first temperature to cause expansion of the thermally expandable microcapsules <NUM> to form the independent pores <NUM>. The independent pores <NUM> 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 <NUM> expand and is, e.g., about <NUM> to <NUM>.

The fiber-rubber composite sheet is then further heated to a third temperature higher than the second temperature to vulcanize the unvulcanized rubber <NUM>. The hot press cushioning material shown in <FIG> is thus produced. The third temperature is, e.g., a temperature equal to or higher than <NUM>. Expansion of the thermally expandable microcapsules may be caused when the temperature passes the second temperature during heating to the third temperature. The rubber <NUM> has been vulcanized in the state of <FIG>.

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 <NUM> to <NUM>. Specific examples of the commercially available thermally expandable microcapsules include "Expancel" (average particle size: <NUM> to <NUM>) made by Japan Fillite Co. , "Matsumoto Microsphere" (average particle size: <NUM> to <NUM>) made by Matsumoto Yushi-Seiyaku Co. , and "KUREHA Microsphere" (average particle size: <NUM> to <NUM>) made by KUREHA CORPORATION. The thermally expandable microcapsules used in the example samples were "Expancel <NUM>-DU40" made by Japan Fillite Co. , whose average particle size was <NUM> to <NUM>.

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-<NUM>" made by ORIBEST CO. The glass paper had a thickness of <NUM>, a basis weight of <NUM>/m<NUM>, and a void fraction of <NUM>%.

Thermally expandable microcapsules were also prepared. The thermally expandable microcapsules used were "Expancel <NUM>-DU40" made by Japan Fillite Co. <NUM> parts by mass of the thermally expandable microcapsules were mixed per <NUM> parts by mass of fluororubber, and the mixture was kneaded to prepare an unvulcanized fluororubber sheet with a thickness of <NUM>. The unvulcanized fluororubber sheet had the thermally expandable microcapsules dispersed therein.

As shown in <FIG>, an unvulcanized fluororubber sheet <NUM> containing thermally expandable microcapsules and having a thickness of <NUM> was sandwiched between two sheets <NUM> of glass paper, and the stack of the unvulcanized fluororubber sheet <NUM> and the two sheets <NUM> of glass paper was hot pressed to form a composite sheet.

This pressing was performed under the following conditions.

The resultant fiber-rubber composite sheet <NUM> (<FIG>) 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 <NUM>. The temperature and the holding time in the heating oven were <NUM> and <NUM> hours. The thermally expandable microcapsules expanded during heating to form a multiplicity of independent air bubbles in the fiber-rubber composite sheet <NUM>. The volume ratio of the fiber material to the rubber in the fiber-rubber composite sheet <NUM> was <NUM>/<NUM>.

As shown in <FIG>, a fluorine film (thickness: <NUM>) <NUM> was bonded to both surfaces of the fiber-rubber composite sheet <NUM> with a fluororubber sheet <NUM> (thickness: <NUM>) interposed therebetween by hot press. Example sample <NUM> was thus produced.

<NUM> parts by mass of the thermally expandable microcapsules used in Example Sample <NUM> were mixed per <NUM> parts by mass of fluororubber to prepare an unvulcanized fluororubber sheet with a thickness of <NUM>.

The other conditions (glass paper, primary press, thermal expansions of capsules, vulcanization, and secondary press) are the same as those in Example Sample <NUM>. The volume ratio of the fiber material to the rubber in the fiber-rubber composite sheet <NUM> was <NUM>/<NUM>.

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 <NUM> to <NUM> are shown in Table <NUM> below.

Comparative Example Sample <NUM> shown in <FIG> was produced under the same conditions as those of Example Sample <NUM> except that no thermally expandable microcapsules were mixed with fluororubber. No independent pores were formed in a fiber-rubber composite sheet <NUM>. The volume ratio of the fiber material to the rubber in the fiber-rubber composite sheet <NUM> was <NUM>/<NUM>.

As shown in <FIG>, Comparative Example Sample <NUM> is a fiber-rubber composite sheet <NUM> having fluorine films <NUM> (thickness: <NUM>) bonded to its both surfaces. The fiber-rubber composite sheet <NUM> is a composite sheet of aramid cloth <NUM> comprised of aromatic polyamide fibers serving as a reinforcing material and fluororubber (thickness: <NUM>, durometer A hardness: <NUM>°) <NUM> on both surfaces of the aramid cloth <NUM>. The fiber-rubber composite sheet <NUM> has no independent pores formed therein.

Comparative Example Sample <NUM> is a fiber-rubber composite sheet produced by impregnating glass paper "GRABESTOS SYS-<NUM>" made by ORIBEST CO. with fluororubber and has voids (continuous pores) therein. This fiber-rubber composite sheet is described in <CIT>. The void fraction of the fiber-rubber composite sheet was <NUM>%, the volume ratio of the fiber material to the rubber was <NUM>/<NUM>, and the thickness was <NUM>.

As shown in <FIG>, Comparative Example Sample <NUM> was produced by bonding a thermoplastic film <NUM> to one surface of a stack <NUM> of three sheets of craft paper having a basis weight of <NUM>/m<NUM>.

Comparative Example Sample <NUM> is a fluororubber sheet <NUM> having a multiplicity of independent pores <NUM> therein. The fluororubber sheet <NUM> contains no fibers. The thickness of the fluororubber sheet <NUM> was <NUM> and the hardness (durometer A hardness) thereof was <NUM>°.

Each sample's conformability to unevenness was evaluated by a press test. <FIG> illustrates the configuration of a press.

A base <NUM> and a pressing unit <NUM> of the press contain a heater. A cushioning material <NUM>, a stainless steel sheet <NUM>, a spacer <NUM> with a thickness of <NUM>, a release film <NUM> made of fluororesin and having a thickness of <NUM>, a polyimide film <NUM> with adhesive, a sample <NUM> to be evaluated, a stainless steel sheet <NUM>, and a cushioning material <NUM> were placed in this order from bottom to top between the base <NUM> and the pressing unit <NUM>. The cushioning materials <NUM>, <NUM> were stacks of five sheets of craft paper having a basis weight of <NUM>/m<NUM>.

The spacer <NUM> with a thickness of <NUM> has a slit with a width of <NUM>. A step (unevenness) with a depth of <NUM> is therefore formed by the lower stainless steel sheet <NUM> and the spacer <NUM> placed thereon. The sample <NUM> to be evaluated is pressed down and heated by the pressing unit <NUM> to cure the adhesive on the polyimide film <NUM>. Conformability to unevenness was evaluated from formation of a void in the stepped portion by the adhesive on the polyimide film <NUM>.

The pressing for evaluation was performed under the following conditions.

<FIG> shows images of the polyimide film <NUM> in the stepped portion after single press.

The images of <FIG> show that, in Example Samples <NUM> to <NUM>, the sample <NUM> entered the stepped portion (recess) and an appropriate pressure was applied to the stainless steel sheet <NUM> located under the slit of the spacer <NUM>. In other words, Example Samples <NUM> to <NUM> had satisfactory conformability to unevenness.

The images of <FIG> also show that, in Comparative Example Samples <NUM> to <NUM>, a void was formed in the stepped portion (recess) and an appropriate pressure was not applied. In other words, Comparative Example Samples <NUM> to <NUM> do not have sufficient conformability to unevenness. In Comparative Example Samples <NUM> and <NUM>, no void was formed after single press. Comparative Example Samples <NUM> and <NUM> had satisfactory conformability to unevenness after single press.

<FIG> show images of the polyimide film <NUM> in the stepped portion after <NUM> presses. The images of <FIG> show that, in Example Samples <NUM> to <NUM>, an appropriate pressure was applied to the stainless steel sheet <NUM> located under the slit of the spacer <NUM> even after <NUM> presses. In other words, Example Samples <NUM> to <NUM> had satisfactory conformability to unevenness even after <NUM> presses.

Pressing was not able to be performed <NUM> times on Comparative Example Samples <NUM> and <NUM> that had satisfactory conformability to unevenness after the first press. Specifically, Comparative Example Sample <NUM> was plastically deformed by the first press and became unusable. Comparative Example Sample <NUM> had satisfactory conformability to unevenness after the second press but became unusable due to torn rubber.

The structure of the samples to be evaluated and the evaluation results of their conformability to unevenness are shown in Table <NUM> below.

<FIG> is a sectional image of Example Sample <NUM> having the structure shown in <FIG>. Example Sample <NUM> is a fiber-rubber composite sheet having a fluorine film bonded to its both surfaces with a <NUM> thick fluororubber layer interposed therebetween.

<FIG> is an enlarged sectional image of the fiber-rubber composite sheet, which is a core layer, of Example Sample <NUM> in the thickness direction. <FIG> 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 <NUM> to <NUM>.

<FIG> is an enlarged image of the fiber-rubber composite sheet of Comparative Example Sample <NUM> 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 <NUM>% and the volume ratio of the fiber material to the rubber is <NUM>/<NUM>.

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 fibers <NUM>, rubber <NUM> present in the voids between the fibers <NUM>, and independent pores <NUM> dispersedly present in the rubber <NUM>, as shown in <FIG>.

Other possible forms include the fiber-rubber composite sheet with the structure of <FIG> having fluorine films bonded to its both surfaces with very thin fluororubber interposed therebetween, the fiber-rubber composite sheet with the structure of <FIG> 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> having a nonwoven fabric layer, a woven fabric layer, a rubber layer, etc. interposed therebetween.

<FIG> is an illustrative sectional view of a heat press cushioning material according to another embodiment of the present invention. A hot press cushioning material <NUM> shown in the figure includes a fiber-rubber composite layer <NUM> having the same structure as that shown in <FIG>, an upper rubber layer <NUM> located over the fiber-rubber composite layer <NUM>, a lower rubber layer <NUM> located under the fiber-rubber composite layer <NUM>, an upper fluorine film <NUM> placed on the front surface of the upper rubber layer <NUM>, a lower fluorine film <NUM> placed on the back surface of the lower rubber layer <NUM>.

The fiber-rubber composite layer <NUM> includes a fiber material comprised of a multiplicity of randomly oriented fibers <NUM>, rubber <NUM> present in the voids between the fibers <NUM> of the fiber material, and independent pores <NUM> dispersedly present in the rubber <NUM>. The upper rubber layer <NUM> is located over the layer of the fiber material and includes independent pores <NUM> dispersedly present therein. The lower rubber layer <NUM> is located under the layer of the fiber material and includes independent pores <NUM> dispersedly present therein.

The upper fluorine film <NUM> and the lower fluorine film <NUM> 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>, the volume ratio of the fiber material to the entire rubber <NUM> having the independent pores <NUM> dispersed therein is preferably <NUM>/<NUM> or more and less than <NUM>/<NUM> 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 <NUM> is less than <NUM>/<NUM>, the hot press cushioning material <NUM> has insufficient reinforcement capability, and the hot press cushioning material <NUM> may be stretched out, torn, etc. with repeated use. When the volume ratio of the fiber material to the rubber <NUM> is <NUM>/<NUM> or more, the hot press cushioning material <NUM> has insufficient conformability to unevenness. In the case of the structure shown in <FIG>, the volume ratio of the fibrous material to the rubber is more preferably <NUM>/<NUM> or more and less than <NUM>/<NUM>.

In the case of the hot press cushioning material <NUM> having the structure shown in <FIG>, it is desirable that the porosity (void fraction) of the hot press cushioning material be <NUM> to <NUM>% based on volume. When the porosity is less than <NUM>%, the hot press cushioning material <NUM> has insufficient conformability to unevenness. When the porosity is higher than <NUM>%, the hot press cushioning material <NUM> has sufficient conformability to unevenness but has poor resilience. The hot press cushioning material <NUM> therefore significantly changes over time with repeated use, and distinguishing marks of protrusions and recesses are left on the hot press cushioning material <NUM>. Moreover, since the upper and lower rubber layers (rubber-independent pores) <NUM>, <NUM> have lower strength, the hot press cushioning material <NUM> may have surface cracks, may be torn, etc. In the case of the structure shown in <FIG>, the lower limit of the porosity of the hot press cushioning material is more preferably <NUM>%, and the upper limit of the porosity thereof is more preferably <NUM>%.

Regarding the component ratio of the fiber-rubber composite layer (fibers-rubber-independent pores) <NUM>, the volume ratio of the fiber material to the rubber is preferably <NUM>/<NUM> or more and less than <NUM>/<NUM>, and more preferably <NUM>/<NUM> or more and less than <NUM>/<NUM>. When the volume ratio is less than <NUM>/<NUM>, 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 <NUM>/<NUM> or more, the hot press cushioning material has insufficient conformability to unevenness.

Regarding the ratio of the thickness between the fiber-rubber composite layer <NUM> and the upper and lower rubber layers <NUM>, <NUM>, the ratio of the thickness of the fiber-rubber composite layer <NUM> to the total thickness of the upper and lower rubber layers <NUM>, <NUM> is preferably <NUM>/<NUM> to <NUM>/<NUM>, and more preferably <NUM>/<NUM> to <NUM>/<NUM>. When this thickness ratio is less than <NUM>/<NUM>, 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 <NUM>/<NUM>, the hot press cushioning material has insufficient conformability to unevenness.

The hot press cushioning material <NUM> having the structure shown in <FIG> is produced by the following process.

As shown in <FIG>, a fiber sheet <NUM> comprised of a multiplicity of randomly oriented fibers <NUM>, an unvulcanized rubber sheet <NUM> having thermally expandable microcapsule <NUM> dispersed therein, and two fluorine films <NUM>, <NUM> are first prepared. In one embodiment, two unvulcanized rubber sheets <NUM> were prepared, and the two unvulcanized rubber sheets <NUM> are stacked on top of each other with the fiber sheet <NUM> interposed therebetween. The fiber sheet <NUM> 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 <NUM> 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 <NUM> and the microcapsules <NUM> to enter the voids between the fibers <NUM>, the fiber sheet <NUM> preferably has a void fraction of <NUM> to <NUM>%.

One fluorine film <NUM> is placed on the front surface of the upper unvulcanized rubber sheet <NUM>, and the other fluorine film <NUM> is placed on the back surface of the lower unvulcanized rubber sheet <NUM>.

The multilayered structure of the upper and lower fluorine films <NUM>, <NUM>, the upper and lower unvulcanized rubber sheets <NUM>, and the fiber sheet <NUM> which are stacked on top of each other as shown in <FIG> 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 <NUM> MPa at <NUM> for <NUM> minutes, is then heated to <NUM> and pressed with a pressure of <NUM> MPa at <NUM> for <NUM> minutes, and thereafter is cooled for <NUM> minutes with the pressure being maintained at <NUM> MPa.

The above heating and pressing process causes a part of each of the upper and lower unvulcanized rubber sheets <NUM> to enter the voids between the multiplicity of fibers <NUM> in the fiber sheet <NUM>, as shown in <FIG>. Similarly, the above heating and pressing process also causes a part of the microcapsules <NUM> dispersed in the upper and lower unvulcanized rubber sheets <NUM> to enter the voids between the multiplicity of fibers <NUM> in the fiber sheet <NUM>. The microcapsules <NUM> 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 <NUM> does not penetrate the fiber sheet <NUM> but is located over and under the fiber sheet <NUM>.

As the temperature is raised, the thermally expandable microcapsules <NUM> expand to form independent pores <NUM>. The independent pores <NUM> are present in the fiber-rubber composite layer <NUM>, the upper rubber layer <NUM>, and the lower rubber layer <NUM>.

As the temperature is further raised, the unvulcanized rubber <NUM> is vulcanized into vulcanized rubber <NUM>.

A fiber material sheet (glass paper) comprised of a multiplicity of randomly oriented glass fibers was prepared. The glass paper used was "GRABESTOS SYS-<NUM>" made by ORIBEST CO. The glass paper had a thickness of <NUM>, a basis weight of <NUM>/m<NUM>, and a void fraction of <NUM>%.

Thermally expandable microcapsules were also prepared. The thermally expandable microcapsules used were "Expancel <NUM>-DU120" made by Japan Fillite Co. The thermally expandable microcapsules used in Example Sample <NUM> were "Expancel <NUM>-DU40. " These two types of thermally expandable microcapsules have the following differences.

<NUM> parts by mass of the thermally expandable microcapsules (Expancel <NUM>-DU120) were mixed per <NUM> parts by mass of fluororubber, and the mixture was kneaded to prepare two unvulcanized fluororubber sheets with a thickness of <NUM>. Each of the unvulcanized fluororubber sheets <NUM> had the thermally expandable microcapsules <NUM> dispersed therein.

Two fluorine films with a thickness of <NUM> were prepared.

As shown in <FIG>, the fiber sheet <NUM> made of glass paper was sandwiched between the two unvulcanized rubber sheets <NUM> containing thermally expandable microcapsules. One fluorine film <NUM> was placed on the front surface of one unvulcanized rubber sheet <NUM>, and the other fluorine film <NUM> was placed on the back surface of the other unvulcanized rubber sheet <NUM>. 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 <NUM> and was pressed with a pressure of <NUM> MPa at <NUM> for <NUM> minutes. Next, the multilayered structure was heated to <NUM> over a period of <NUM> minutes with the pressure being maintained at <NUM> MPa and was maintained at <NUM> for <NUM> minutes. The multilayered structure was then cooled to room temperature over a period of <NUM> minutes with the pressure being maintained at <NUM> MPa.

During the pressing at <NUM>, a part of the unvulcanized rubber <NUM> and a part of the thermally expandable microcapsules <NUM> in each unvulcanized rubber sheet <NUM> containing the thermally expandable microcapsules <NUM> entered the voids between the multiplicity of fibers <NUM> in the fiber sheet <NUM>. The pressing was performed so that the remaining part of each of the upper and lower unvulcanized rubber sheets <NUM> did not penetrate the fiber sheet <NUM> but was located over and under the fiber sheet <NUM>.

The thermally expandable microcapsules expand during the heating from <NUM> to <NUM> to form independent pores in the fiber-rubber composite layer <NUM> and the upper and lower rubber layers <NUM>, <NUM>.

The unvulcanized fluororubber was vulcanized while the multilayered structure was maintained at <NUM>. Thereafter, the multilayered structure was baked at <NUM> for <NUM> hours in order to improve the properties of the fluororubber.

The hot press cushioning material of Example Sample <NUM> produced as described above had the structure shown in <FIG>. The thickness of Example Sample <NUM> was <NUM>. The volume ratio of the fiber material to the entire rubber <NUM> having the independent pores <NUM> dispersed therein was <NUM>/<NUM>, and the overall porosity of the cushioning material was <NUM>% based on volume.

<FIG> is a sectional image of Example Sample <NUM> having the structure shown in <FIG>. 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).

Example Sample <NUM> is different from Example Sample <NUM> in the following points.

The thickness reduction rate with repeated use is lower in Example Sample <NUM> than in Example Sample <NUM>. Example Sample <NUM> has better conformability to unevenness than that of Example Sample <NUM>.

A press test was performed on Example Sample <NUM> and Example Sample <NUM> 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> except for the thickness of the spacer <NUM>. The spacer <NUM> used in this test has a thickness of <NUM> and has a slit with a width of <NUM>.

The measurement and evaluation results for Example Sample <NUM> and Example Sample <NUM> are shown in Table <NUM> below.

In Example Sample <NUM>, the initial thickness (the number of times pressing was performed: <NUM>) was <NUM>, and the thickness after <NUM> presses was <NUM>. In Example Sample <NUM>, the initial thickness was <NUM>, and the thickness after <NUM> presses was <NUM>.

The thickness reduction rate ([initial thickness - thickness after <NUM> presses]/initial thickness) was about <NUM>% in Example Sample <NUM> and about <NUM>% in Example Sample <NUM>.

In Example Sample <NUM>, the initial void fraction was <NUM>%, and the void fraction after <NUM> presses was <NUM>%. The void fraction thus decreased by <NUM>%. In Example Sample <NUM>, the initial void fraction was <NUM>%, and the void fraction after <NUM> presses was <NUM>%. The void fraction thus decreased by <NUM>%.

<FIG> shows images of the polyimide film <NUM> of <FIG> in the stepped portion. Since the thickness of the spacer <NUM> is <NUM>, the height of the step is <NUM>. As can be seen from the images after single press, a void was observed in Example Sample <NUM>. On the other hand, no void was observed in Example Sample <NUM> even after <NUM> presses. The results show that Example Sample <NUM> has better conformability to unevenness to a large step (e.g., <NUM>) than Example Sample <NUM>.

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.

The invention can be advantageously used as a hot press cushioning material that has satisfactory conformability to unevenness over long-term use.

Claim 1:
A hot press cushioning material (<NUM>), comprising:
a fiber material comprised of a multiplicity of randomly oriented fibers (<NUM>);
rubber (<NUM>) present in voids between the fibers (<NUM>) of the fiber material; and
independent pores (<NUM>) dispersedly present in the rubber (<NUM>), characterized in that a volume ratio of the fiber material to the rubber (<NUM>) present in the voids between the fibers (<NUM>) of the fiber material is <NUM>/<NUM> or more and less than <NUM>/<NUM> and
the independent pores (<NUM>) are independent air bubbles that are completely closed.