JP2004352760A

JP2004352760A - Foamed rubber composition and its manufacturing method

Info

Publication number: JP2004352760A
Application number: JP2003149071A
Authority: JP
Inventors: Yoichi Nishimuro; 陽一西室; Yoneji Kobayashi; 米次小林
Original assignee: Bridgestone Corp
Current assignee: Bridgestone Corp
Priority date: 2003-05-27
Filing date: 2003-05-27
Publication date: 2004-12-16

Abstract

<P>PROBLEM TO BE SOLVED: To provide a foamed rubber composition having pores or cells having a superfine pore structure uniformly dispersed in a rubber component, and its manufacturing method. <P>SOLUTION: The foamed rubber composition is obtained by contacting and introducing a supercritical fluid or a subcritical fluid into a rubber composition having a polyolefin resin finely dispersed in a rubber component to foam it. The above polyolefin resin is preferably at least one kind of polyethylene, polypropylene, polybutene and a modified product thereof. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

【０００１】
【発明の属する技術分野】
本発明は発泡ゴム組成物及びその製造方法に関し、詳しくは、ポリオレフィン系樹脂を微分散させたゴム組成物に超臨界流体もしくは亜臨界流体を接触、導入させて発泡した発泡ゴム組成物及びその製造方法に関する。
【０００２】
【従来の技術】
従来、発泡体を得る方法としては、加圧下で均一濃度のガスを発生させて発泡加工すべき高分子材料を予備飽和し、その後に断熱膨張させる技法（例えば、特許文献１参照）等が公知である。
最近では、超臨界流体をポリマー或いはプラスチック材料に連続的に導入して微細に発泡させる技術（例えば、特許文献２〜５参照）等が知られるようになり、熱可塑性エラストマーや加硫ゴム発泡体についての応用例（例えば、特許文献６〜１１参照）等も開示されている。
【０００３】
また最近では、超臨界流体を用いて微細で均一に気泡が分布した加硫ゴム発泡体及びそれを用いたタイヤが開示され、強度を低下させずに比重を低下させ、且つその微細構造に起因して、断熱性や吸音性、遮音性等を有するゴム発泡体が得られるという記載もある（例えば、特許文献１２参照）。
しかしながら、熱可塑性樹脂、特に汎用性の高いポリオレフィン系樹脂を含む複合ゴム組成物を、超臨界流体を用いて微細に発泡させる技法及び発泡体については、未だ知られていない。
【０００４】
【特許文献１】
米国特許第３７９６７７９号明細書
【特許文献２】
特許第２６２５５７６号公報
【特許文献３】
米国特許第４４７３６６５号明細書
【特許文献４】
米国特許第５１５８９８６号明細書
【特許文献５】
米国特許第５３３４３５６号明細書
【特許文献６】
特開平１１−２９３０２２号公報
【特許文献７】
特開平１１−３１０６５６号公報
【特許文献８】
特開２０００−１６９８０３号公報
【特許文献９】
特開２０００−２２６４６５号公報
【特許文献１０】
特開２００１−２６１８７４号公報
【特許文献１１】
特開２００１−３４８４５２号公報
【特許文献１２】
特開平１１−２９３０２２号公報
【０００５】
【発明が解決しようとする課題】
本発明は、熱可塑性樹脂、特にポリオレフィン系樹脂を微分散させたゴム組成物を、未加硫ゴムの混練工程又は／及び加硫工程において、超臨界状態もしくは亜臨界状態の流体を用いて発泡させることにより、超微細構造の気孔が均一に分散された発泡ゴム組成物及びその製造方法を得ることを目的とする。また本発明では、該発泡ゴム組成物の超微細発泡構造を利用することにより、強度を低下させずに比重を大巾に低下させることができ、且つ断熱性や吸音性、遮音性等の特性を有していることを応用して、上記発泡ゴム組成物を適用したタイヤ及びベルトや防振ゴム等の工業用ゴム製品を得ることも目的とする。
【０００６】
【課題を解決するための手段】
上記の目的は、下記に示す本発明の発泡ゴム組成物及びその製造方法によって達成される。
＜１＞ゴム成分中にポリオレフィン系樹脂を微分散させたゴム組成物に、超臨界流体もしくは亜臨界流体を接触、導入させて発泡したことを特徴とする発泡ゴム組成物。
＜２＞前記ポリオレフィン系樹脂が、ポリエチレン、ポリプロピレン、ポリブテン、及びその変性物の少なくとも１種を含むことを特徴とする上記＜１＞に記載の発泡ゴム組成物。
＜３＞前記ゴム成分が、天然ゴム（ＮＲ）、イソプレンゴム（ＩＲ）、ブタジエンゴム（ＢＲ）、スチレンブタジエンゴム（ＳＢＲ）、ニトリル−ブタジエンゴム（ＮＢＲ）、クロロプレンゴム（ＣＲ）、イソブチレン−イソプレン共重合体ゴム（ＩＩＲ）、エチレン−プロピレン−ジエン系三元共重合体ゴム（ＥＰＤＭ）、エチレン−プロピレン共重合体ゴム（ＥＰＭ）、フッ素ゴム、シリコンゴム、ウレタンゴム、アクリルゴム、及びその変性物の少なくとも１種を含むことを特徴とする上記＜１＞又は＜２＞に記載の発泡ゴム組成物。
＜４＞ゴム成分１００質量部に対して、１〜３０質量部の前記ポリオレフィン系樹脂を配合したことを特徴とする上記＜１＞〜＜３＞のいずれかに記載の発泡ゴム組成物。
＜５＞前記発泡ゴム組成物の発泡率が、３〜１００％であることを特徴とする上記＜１＞〜＜４＞のいずれかに記載の発泡ゴム組成物。
＜６＞前記発泡ゴム組成物が、タイヤのトレッド、アンダートレッド、サイド補強層、ビードフィラー、及びベルト、防振ゴム、シーリング材用であることを特徴とする上記＜１＞〜＜５＞のいずれかに記載の発泡ゴム組成物。
＜７＞未加硫の前記ゴム組成物に超臨界状態もしくは亜臨界状態の流体を接触、導入させ、次いで該状態から逸脱させた後に加硫することを特徴とする上記＜１＞〜＜６＞のいずれかに記載の発泡ゴム組成物の製造方法。
＜８＞未加硫の前記ゴム組成物に超臨界状態もしくは亜臨界状態の流体を接触、導入させ、且つ引き続き該状態を保持したまま加硫することを特徴とする上記＜１＞〜＜６＞のいずれかに記載の発泡ゴム組成物の製造方法。
＜９＞未加硫の前記ゴム組成物の加硫中に超臨界状態もしくは亜臨界状態の流体を接触、導入させて加硫することを特徴とする上記＜１＞〜＜６＞のいずれかに記載の発泡ゴム組成物の製造方法。
＜１０＞未加硫の前記ゴム組成物に押出機内で超臨界状態もしくは亜臨界状態の流体を接触、導入させ、次いで該状態から逸脱させた後に加硫することを特徴とする上記＜１＞〜＜６＞のいずれかに記載の発泡ゴム組成物の製造方法。
＜１１＞未加硫の前記ゴム組成物を押出機で加硫機内に注入した後、これに加硫機内で超臨界状態もしくは亜臨界状態の流体を接触、導入させて加硫したことを特徴とする上記＜１＞〜＜６＞のいずれかに記載の発泡ゴム組成物の製造方法。
＜１２＞未加硫の前記ゴム組成物に押出機内で超臨界状態もしくは亜臨界状態の流体を接触、導入させ、且つ引き続き該状態を維持したまま加硫機内で加硫することを特徴とする上記＜１＞〜＜６＞のいずれかに記載の発泡ゴム組成物の製造方法。
＜１３＞前記超臨界流体もしくは亜臨界流体が、ゴムの加硫温度以下の超臨界温度もしくは亜臨界温度を有する流体であることを特徴とする上記＜７＞〜＜１２＞のいずれかに記載の発泡ゴム組成物の製造方法。
＜１４＞前記超臨界流体もしくは亜臨界流体が二酸化炭素であることを特徴とする上記＜７＞〜＜１２＞のいずれかに記載の発泡ゴム組成物の製造方法。
【０００７】
【発明の実施の形態】
本発明の発泡ゴム組成物、ゴム成分中にポリオレフィン系樹脂を微分散させたゴム組成物に、超臨界流体もしくは亜臨界流体を接触、導入させて発泡したことを特徴とする。ここで、超臨界流体もしくは亜臨界流体（本明細書では、両者を含めて「超臨界流体」と称することがある。）は、上記のゴム組成物において発泡剤として作用する。
【０００８】
上記の超臨界流体もしくは亜臨界流体とは、気体と液体が共存できる限界の温度及び圧力（臨界点）を超えた状態にあり、通常の気体や液体とは異なる性質を示す特殊な流体である。例として、二酸化炭素の状態図を用いて、該流体の超臨界域もしくは亜臨界域を示すと、添付の図１の通りである。本発明で用いる超臨界流体もしくは亜臨界流体としては、ゴム組成物の加硫温度以下の超臨界温度もしくは亜臨界温度を有する所定の流体（例えば、二酸化炭素、エタン、エチレン等の超臨界もしくは亜臨界流体等）が好適に使用される。中でも特に、二酸化炭素は３１℃を超える温度及び７．３８ＭＰａを超える圧力で超臨界状態となるので、本発明に用いる超臨界流体もしくは亜臨界流体として最も好ましい。
【０００９】
ゴム組成物に、比較的低温及び比較的高圧で製造された比較的高密度の超臨界流体を接触させることにより、ゴム組成物内に該超臨界流体が速やかに導入され、該ゴム組成物系の温度を特に上昇させる必要もなく、該ゴム組成物系を飽和させることができる。この速やかに飽和に到達する機構は、詳細が完全に解明されている訳ではないが、例えば、ゴム組成物中の超臨界流体の濃度が適度なレベル（例えば、数十パーセント）に達するまでは、超臨界流体（溶質として）は最初にゴム組成物材料（溶媒として）に溶解していると考えられている。数パーセントのレベルでは、超臨界流体は溶媒として作用し、ゴム組成物は溶質として作用すると考えられている。しかしながら、超臨界流体及びゴム組成物が溶媒として又は溶質として作用しようとも、超臨界流体をゴム組成物に接触し導入して暫くすると、極めて速やかに該超臨界液体及びゴム組成物系の完全飽和溶液ができると推定される。
【００１０】
好適に選択した温度及び圧力の下で、超臨界流体／ゴム組成物系がその系内に十分量の超臨界流体を含む場合、該超臨界液体／ゴム組成物系の温度及び／又は圧力を急激に変化させる（一般には、大気状態に戻す）と熱力学的な不安定性が触発され、ゴムマトリックスに微分散したポリオレフィン樹脂を核として、微細孔を有する本発明の発泡ゴム組成物が製造される。この様にして得られた発泡ゴム材料は、平均気泡又は気泡サイズが１．０μｍ未満、時には０．５μｍ未満となり得る。更に本発明においては、この様なゴム材料の発泡は、時には環境温度（室温）条件下で達成し得る。
【００１１】
本発明の発泡ゴム組成物は、未加硫ゴム組成物の混練工程又は／及び加硫工程において、該ゴム組成物に超臨界状態もしくは亜臨界状態の流体を接触し浸透ないし溶解させ、次いで上記超臨界状態を解放する（一般には、大気状態に戻す）時に、ゴム組成物内に浸透ないし溶解していた超臨界流体（もしくは亜臨界流体）が流体から気体に変化して、その一部はゴム組成物内から散逸して超微細な気孔を残し、また、大部分はゴム組成物中に超微細な気泡となって残留することによって、超微細構造のゴム発泡体が得られると推定される。
【００１２】
ここで、上記の様な方法によって得られた加硫ゴム発泡体は、その超微細構造の気孔及び気泡が均一に分布したゴム発泡体となるため、その強度を殆ど低下させないで比重を低下させることができるので、これをタイヤ及び各種産業用ゴム製品のゴム部材に適用することにより、構造強度を低下させないで軽量化が図れるという利点を有し、また、該微細構造に起因する断熱性や吸音性、遮音性、絶縁性、制動性（スキッド特性）等を利用して、広汎な用途部材に適用が可能である。これらの利用に供する加硫ゴム発泡体としては、その発泡率を例えば３〜１００％であるものから適宜に選ばれる。
【００１３】
（ポリオレフィン系樹脂）
本発明の発泡ゴム組成物に用いられるポリオレフィン系樹脂としては、特に制限はなく、結晶性高分子或いは非晶性高分子等の中から、目的とする製品特性に応じて適宜に選択することができる。中でも、微細に均一に分散させ且つ所望の発泡構造に容易に制御するために、相転移現象を示しある温度で粘度変化が急激に起こり且つ粘度制御が容易な観点より、結晶性高分子を含むポリオレフィン系樹脂からなるものが好ましく、結晶性高分子のみからなるポリオレフィン系樹脂がより好ましい。
【００１４】
上記の結晶性ポリオレフィン系樹脂の具体例としては、例えば、ポリエチレン（ＰＥ）、ポリプロピレン（ＰＰ）、ポリブチレン（ＰＢ）、ポリブチレンサクシネート、ポリエチレンサクシネート、シンジオタクティック−１，２−ポリブタジエン（ＳＰＢ）、ポリビニルアルコール（ＰＶＡ）等の単一組成の重合体や、これらの共重合体及びブレンド物等により融点を適切な範囲に制御したものが挙げられ、更にこれらに充填剤や添加剤或いは可塑剤等を加えたものも好適に使用できる。これらの樹脂は、１種を単独で使用してもよく、２種以上を併用してもよい。
上記の結晶性ポリオレフィン系樹脂の中でも、溶融温度及び原料供給性の観点より、ポリエチレン（ＰＥ）、ポリプロピレン（ＰＰ）、ポリブテン（ＰＢ）がより好ましく、特に、融点が低く取扱いが容易で価格が安い観点より、ポリエチレン（ＰＥ）が最も好ましい。
【００１５】
また、非晶性ポリオレフィン系樹脂の具体例としては、例えば、ポリメチルメタクリレート（ＰＭＭＡ）、アクリロニトリルブタジエンスチレン共重合体（ＡＢＳ）、ポリスチレン（ＰＳ）、ポリ塩化ビニル（ＰＶＣ）、ポリアクリロニトリル、及びこれらの共重合体又はブレンド物等が挙げられる。これらも、１種を単独で使用してもよいし、２種以上を併用してもよい。
【００１６】
上述したポリオレフィン系樹脂には、本発明の効果を損なわない範囲において、必要に応じて公知の充填剤や各種添加剤を配合することができる。
【００１７】
本発明のポリオレフィン系樹脂の分子量としては、該樹脂の化学組成及び分子鎖の分岐度態等によって異なり一概に規定することはできないが、一般的に言って、該ポリオレフィン系樹脂は、同じモノマーで形成されていてもその分子量が高い程、ある一定の温度における粘度（溶融粘度）は高くなる。
【００１８】
本発明においては、発泡ゴム組成物におけるポリオレフィン系樹脂の含有量としては、該ゴム成分１００質量部に対して、１〜３０質量部が好ましく、１．５〜２５質量部がより好ましく、２〜２０質量部が最も好ましい。
上記含有量が、１質量部未満であると、該ゴム組成物を加硫して得られる加硫ゴム中に微分散される発泡の核となるポリオレフィン系樹脂の量が少なく、発泡の気泡径が大きくなることがあり、一方、上記含有量が３０質量部を超えると、該樹脂のゴム組成物への混練が難しくなり分散性及び作業性が悪化し、また機械的強度の低下を引き起こすことがある。一方、該含有量が上記の好ましい数値範囲内にあると、その様なことがない点で好ましい。
【００１９】
（発泡ゴム組成物の製造方法）
次に、本発明の発泡ゴム組成物の製造方法について説明する。
本発明の発泡ゴム組成物は、その一態様として、未加硫ゴム組成物の混練り工程において、超臨界状態もしくは亜臨界状態の流体を接触し導入させ、次いで加硫前に一端、該状態から逸脱させた後に通常の加硫条件下で加硫する方法によって得られるものがある。
また他の態様として、未加硫ゴム組成物の混練り工程を通常の条件下で行い、その後の加硫操作中に超臨界状態もしくは亜臨界状態の流体を接触し導入させ、次いで加硫後に該状態から逸脱させる方法によって得られるものがある。
更に他の態様として、未加硫ゴム組成物の混練工程及び次工程の加硫操作中に、超臨界状態もしくは亜臨界状態の流体を接触し導入させ、その加硫後に該状態から逸脱させる方法によって得られるものもある。
【００２０】
本発明の発泡ゴム組成物は、具体的には、上記未加硫ゴム組成物の混練に使用する押出機内でのみ、この未加硫ゴム組成物に超臨界状態もしくは亜臨界状態の流体を接触し導入させる工程を経ることによって、また、加硫機内でのみ、混練後の未加硫ゴム組成物に超臨界状態もしくは亜臨界状態の流体を接触し導入させることによって、或いはまた、上記押出機及び加硫機の両機内で、超臨界状態もしくは亜臨界状態の流体を接触し導入させることによって、所望の加硫ゴム発泡体を得ることができる。
【００２１】
（発泡ゴム組成物）
本発明の発泡ゴム組成物に用いられるゴム成分としては、天然ゴム（ＮＲ）、イソプレンゴム（ＩＲ）、ブタジエンゴム（ＢＲ）、スチレンブタジエンゴム（ＳＢＲ）、ニトリル−ブタジエンゴム（ＮＢＲ）、クロロプレンゴム（ＣＲ）、イソブチレン−イソプレン共重合体ゴム（ＩＩＲ）、エチレン−プロピレン−ジエン系三元共重合体ゴム（ＥＰＤＭ）、エチレン−プロピレン共重合体ゴム（ＥＰＭ）、フッ素ゴム、シリコンゴム、ウレタンゴム、アクリルゴム、及びその変性物の１種或いはこれらのゴム混合物が好適に使用される。
【００２２】
また、上記ゴム成分に配合される加硫剤としては、一般的なゴム加硫剤（架橋剤）を用いることができる。具体的には、イオウ系加硫剤としては粉末イオウ、沈降性イオウ、高分散性イオウ、表面処理イオウ、不溶性イオウ、ジモルフォリンジサルファイド、アルキルフェノールジサルファイド等を挙げることができ、例えば、ゴム成分１００質量部当り０．５〜４質量部程度を用いることができる。
また、有機過酸化物系の加硫剤としては、ベンゾイルパーオキサイド、ｔ−ブチルヒドロパーオキサイド、２，４−ビクロロベンゾイルパーオキサイド、２，５−ジメチル−２，５−ジ（ｔ−ブチルパーオキシ）ヘキサン、２，５−ジメチルヘキサン−２，５−ジ（パーオキシルベンゾエート）等が例示され、例えば、１〜２０質量部程度を用いることができる。
【００２３】
更に、フェノール樹脂系の加硫剤としては、アルキルフェノール樹脂の臭素化物や、塩化スズ、クロロプレン等のハロゲンドナーとアルキルフェノール樹脂とを含有する混合架橋系等が例示でき、例えば、１〜２０質量部程度を用いることができる。その他として、亜鉛華（５質量部程度）、酸化マグネシウム（４質量部程度）、リサージ（１０〜２０質量部程度）、ｐ−キノンジオキシム、ｐ−ジベンゾイルキノンジオキシム、テトラクロロ−ｐ−ベンゾキノン、ポリ−ｐ−ジニトロソベンゼン（２〜１０質量部程度）、メチレンジアニリン（０．２〜１０質量部程度）が例示できる。また、上記加硫剤には必要に応じて、加硫促進剤を添加してもよい。該加硫促進剤としては、アルデヒド・アンモニア系、グアニジン系、チアゾール系、スルフェンアミド系、チウラム系、ジチオ酸塩系、チオウレア系等の一般的な加硫促進剤を、例えば、０．５〜２質量部程度を用いることができる。
【００２４】
また、本発明の発泡ゴム組成物には、通常、ゴム組成物に配合される補強剤としてのカーボンブラック及び／又はシリカを配合することができる。更に、従来より公知の各種オイル、老化防止剤、酸化防止剤、充填剤、可塑化剤、軟化剤、加硫剤、加硫促進剤、加硫助剤、その他ゴム用に一般的に配合されている各種配合薬品を適宜に配合することができる。これら添加剤の配合量も、本発明の目的に反しない限り、従来の一般的な配合量とすることができる。
【００２５】
前述した種々の方法で得られる本発明の発泡ゴム組成物は、例えば、これをタイヤ及び産業用ゴム製品のいずれかのゴム部材に適用することにより、当該分野で公知の製法により、軽量で且つ他の物性の点では遜色のないゴム製品を得ることができる。
また、本発明の発泡ゴム組成物は、予め予備成型したグリーンタイヤ（生タイヤ）或いは防振ゴム等を、金型内で超臨界状態もしくは亜臨界状態の流体を接触し導入させ、次いでこれを加硫成型することにより、少なくともタイヤ或いは防振ゴム等の表層ゴムを発泡させてなるゴム製品を得ることもできる。
更に、本発明の発泡ゴム組成物は、加硫済ゴム組成物発泡体を未加硫もしくは加硫済の他の部材と貼り合わせた後に、一体化（該一体化は、物理的嵌合や接着処理、加硫接着等のいずれの手法を用いてもよい。）してタイヤ等のゴム製品を得ることができ、また、該一体化したゴム製品を金型内で超臨界状態もしくは亜臨界状態の流体と接触し導入させた後に、発泡及び加硫を行ってタイヤ等のゴム製品を得ることもできる。
【００２６】
【実施例】
以下、実施例に依って本発明を具体的に説明するが、本発明をこれらの実施例に限定されるものでない。尚、本実施例中の「部数」及び「％」は、全て「質量部」及び「質量％」を表す。
【００２７】
［実施例１〜５及び比較例１〜３］
下記の表１の配合組成欄に示す処方に従って、実施例１〜５及び比較例１〜３のゴム組成物を調合し、ラボプラストミルを使用して混練を行った。この混練は２ステージからなり、第１ステージでは加硫剤、加硫促進剤、加硫促進助剤等の高温でゴム成分の架橋に大きな影響を与える配合剤以外の成分及びゴムとカーボンブラック、ポリオレフィン系樹脂を添加し１５５℃で混練した。その後の第２ステージでは、第１ステージにおいて添加しなかった配合剤を添加し、第１ステージより低い温度（１００℃）にて混練した。
【００２８】
上記において、本実施例で用いたポリオレフィン系樹脂は、下記に示す仕様（スペック）の物である。
・オレフィン樹脂（Ａ）………直鎖状低密度ポリエチレン（融点＝１２５℃、ＭＦＲ（ＪＩＳ−Ｋ７２１０、温度１９０℃、荷重２．１６ｋｇｆ）＝５ｇ／１０分、密度＝０．９２１ｇ／ｃｍ^２）
・オレフィン樹脂（Ｂ）………低密度ポリエチレン（融点＝１０８℃、ＭＦＲ（ＪＩＳ−Ｋ７２１０、温度１９０℃、荷重２．１６ｋｇｆ）＝２０ｇ／１０分、密度＝０．９２０ｇ／ｃｍ^２）
・オレフィン樹脂（Ｃ）………高密度ポリエチレン（融点＝１３６℃、ＭＦＲ（ＪＩＳ−Ｋ７２１０、温度１９０℃、荷重２．１６ｋｇｆ）＝２ｇ／１０分、密度＝０．９６４ｇ／ｃｍ^２）
【００２９】
比較例の発泡体は、ポリオレフィン系樹脂を配合しなかったもの、従来の発泡剤／発泡助剤系で発泡したもの、及びそれらの組合せである。ここで、表中の発泡剤（ＡＤＣＡ）はアゾジカルボンアミドである。
【００３０】
次いで、上記で得られたゴム組成物に、二酸化炭素の超臨界流体を温度５０℃、圧力３０ＭＰａで接触させ浸透、溶解させた後、該超臨界状態を逸脱させて得た発泡ゴム組成物を温度１７０℃で加硫して、目的とする加硫ゴム発泡体を得た。この加硫ゴム発泡体のサンプルにつき、走査型顕微鏡（×５０〜×１００００倍）を用いてその発泡状態を観察し、その結果を下記の表１の下段に示した。
【００３１】
［実施例６］
実施例１の発泡ゴム組成物をタイヤのトレッド部に適用して、常法に従ってタイヤを作製した。該タイヤのトレッド部からゴムサンプルを切り出して、上記と同様に走査型顕微鏡（×５０〜×１００００倍）を用いてその発泡状態を観察したところ、平均孔径が１０μｍ以下の超微細発泡構造が見出された。また、該サンプルの圧縮強度及び耐久性とも、ソリッドゴム（非発泡品）対比で遜色のない物性を示し、軽量性という利点を有するタイヤを得ることができた。
【００３２】
【表１】

【００３３】
上記の表１の結果より、本発明に従う実施例１〜６の発泡ゴム組成物及びタイヤは、比較例１〜３のものに比べて、微細な気泡が均一に存在するゴム発泡体であり、物性にも優れていることが判明した。
【００３４】
【発明の効果】
本発明に依れば、超臨界流体を利用した発泡により、均一に分散した超微細発泡構造を有する発泡ゴム組成物及びその製造方法を提供することができ、これらはタイヤ及びベルトや防振ゴム等の産業用ゴム製品の部材として有用である。
【図面の簡単な説明】
【図１】二酸化炭素の超臨界域および亜臨界域を示す状態図である。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a foamed rubber composition and a method for producing the same, and more particularly, to a foamed rubber composition foamed by contacting and introducing a supercritical fluid or a subcritical fluid into a rubber composition in which a polyolefin resin is finely dispersed, and production thereof. About the method.
[0002]
[Prior art]
Conventionally, as a method for obtaining a foam, a technique of generating a gas having a uniform concentration under pressure to pre-saturate a polymer material to be foamed and then performing adiabatic expansion (for example, see Patent Document 1) is known. It is.
Recently, a technique of continuously introducing a supercritical fluid into a polymer or plastic material and finely foaming the foam (for example, see Patent Documents 2 to 5) has become known, and thermoplastic elastomers and vulcanized rubber foams have been known. Application examples (for example, see Patent Documents 6 to 11) are also disclosed.
[0003]
Also, recently, a vulcanized rubber foam in which bubbles are distributed finely and uniformly using a supercritical fluid and a tire using the same have been disclosed, the specific gravity has been reduced without reducing the strength, and due to its microstructure. There is also a description that a rubber foam having heat insulation, sound absorption, sound insulation and the like can be obtained (for example, see Patent Document 12).
However, a technique and a foam for finely foaming a composite rubber composition containing a thermoplastic resin, particularly a highly versatile polyolefin resin, using a supercritical fluid have not yet been known.
[0004]
[Patent Document 1]
US Pat. No. 3,796,779 [Patent Document 2]
Japanese Patent No. 2625576 [Patent Document 3]
US Pat. No. 4,473,665 [Patent Document 4]
US Pat. No. 5,158,986 [Patent Document 5]
US Patent No. 5,334,356 [Patent Document 6]
JP-A-11-293022 [Patent Document 7]
JP-A-11-310656 [Patent Document 8]
Japanese Patent Application Laid-Open No. 2000-169803 [Patent Document 9]
JP 2000-226465 A [Patent Document 10]
JP 2001-261874 A [Patent Document 11]
JP 2001-348452 A [Patent Document 12]
JP-A-11-293022 [0005]
[Problems to be solved by the invention]
The present invention provides a method for foaming a rubber composition in which a thermoplastic resin, particularly a polyolefin resin is finely dispersed, using a fluid in a supercritical state or a subcritical state in a kneading step or / and a vulcanizing step of an unvulcanized rubber. An object of the present invention is to obtain a foamed rubber composition in which pores having an ultrafine structure are uniformly dispersed, and a method for producing the same. Further, in the present invention, by utilizing the ultra-fine foamed structure of the foamed rubber composition, the specific gravity can be greatly reduced without lowering the strength, and properties such as heat insulation, sound absorption, sound insulation and the like can be obtained. It is another object of the present invention to obtain an industrial rubber product such as a tire and a belt or an anti-vibration rubber to which the foamed rubber composition is applied, by applying the above.
[0006]
[Means for Solving the Problems]
The above object is achieved by the following foamed rubber composition of the present invention and a method for producing the same.
<1> A foamed rubber composition characterized in that a supercritical fluid or a subcritical fluid is brought into contact with and introduced into a rubber composition in which a polyolefin-based resin is finely dispersed in a rubber component, and foamed.
<2> The foamed rubber composition according to <1>, wherein the polyolefin-based resin includes at least one of polyethylene, polypropylene, polybutene, and a modified product thereof.
<3> The rubber component includes natural rubber (NR), isoprene rubber (IR), butadiene rubber (BR), styrene butadiene rubber (SBR), nitrile-butadiene rubber (NBR), chloroprene rubber (CR), and isobutylene-isoprene. Copolymer rubber (IIR), ethylene-propylene-diene terpolymer rubber (EPDM), ethylene-propylene copolymer rubber (EPM), fluoro rubber, silicone rubber, urethane rubber, acrylic rubber, and modifications thereof The foamed rubber composition according to the above <1> or <2>, comprising at least one of the following.
<4> The foamed rubber composition according to any one of <1> to <3>, wherein 1 to 30 parts by mass of the polyolefin resin is mixed with 100 parts by mass of the rubber component.
<5> The foamed rubber composition according to any one of <1> to <4>, wherein the foamed rubber composition has a foaming rate of 3 to 100%.
<6> The foam rubber composition according to any one of <1> to <5>, wherein the foamed rubber composition is used for a tread, an undertread, a side reinforcing layer, a bead filler, a belt, an anti-vibration rubber, and a sealing material of a tire. The foamed rubber composition according to any one of the above.
<7> The above <1> to <6, wherein a supercritical or subcritical fluid is brought into contact with and introduced into the unvulcanized rubber composition, and then deviated from the state, followed by vulcanization. > The method for producing a foamed rubber composition according to any one of the above items.
<8> The above-mentioned <1> to <6, wherein a supercritical fluid or a subcritical fluid is brought into contact with and introduced into the unvulcanized rubber composition, and vulcanization is performed while maintaining the state. > The method for producing a foamed rubber composition according to any one of the above items.
<9> Any one of the above <1> to <6>, wherein a supercritical or subcritical fluid is brought into contact with and introduced during vulcanization of the unvulcanized rubber composition. The method for producing a foamed rubber composition according to the above.
<10> A supercritical or subcritical fluid is brought into contact with and introduced into the unvulcanized rubber composition in an extruder, and then vulcanized after deviating from the state. A method for producing the foamed rubber composition according to any one of <6> to <6>.
<11> The unvulcanized rubber composition is injected into a vulcanizer by an extruder, and then a supercritical or subcritical fluid is contacted and introduced into the vulcanizer to vulcanize the rubber composition. The method for producing a foamed rubber composition according to any one of the above <1> to <6>.
<12> A supercritical or subcritical fluid is brought into contact with and introduced into the unvulcanized rubber composition in an extruder, and then vulcanized in a vulcanizer while maintaining the state. The method for producing a foamed rubber composition according to any one of the above <1> to <6>.
<13> The supercritical fluid or the subcritical fluid is a fluid having a supercritical temperature or a subcritical temperature equal to or lower than the vulcanization temperature of rubber, and is a fluid according to any one of the above items <7> to <12>. A method for producing a foamed rubber composition.
<14> The method for producing a foamed rubber composition according to any one of <7> to <12>, wherein the supercritical fluid or the subcritical fluid is carbon dioxide.
[0007]
BEST MODE FOR CARRYING OUT THE INVENTION
The foamed rubber composition of the present invention is characterized in that a supercritical fluid or a subcritical fluid is brought into contact with and introduced into a rubber composition in which a polyolefin resin is finely dispersed in a rubber component, and foamed. Here, the supercritical fluid or the subcritical fluid (in this specification, sometimes referred to as “supercritical fluid”) acts as a foaming agent in the rubber composition.
[0008]
The above-mentioned supercritical fluid or subcritical fluid is a special fluid that is in a state exceeding the limit temperature and pressure (critical point) at which gas and liquid can coexist and shows properties different from ordinary gases and liquids. . As an example, FIG. 1 shows the supercritical region or the subcritical region of the fluid using a phase diagram of carbon dioxide. As the supercritical fluid or subcritical fluid used in the present invention, a predetermined fluid having a supercritical temperature or subcritical temperature equal to or lower than the vulcanization temperature of the rubber composition (for example, a supercritical or subcritical fluid such as carbon dioxide, ethane, ethylene, etc.) Critical fluid) is preferably used. In particular, carbon dioxide is in a supercritical state at a temperature exceeding 31 ° C. and a pressure exceeding 7.38 MPa, and is therefore most preferable as the supercritical fluid or subcritical fluid used in the present invention.
[0009]
By contacting the rubber composition with a relatively high-density supercritical fluid produced at a relatively low temperature and a relatively high pressure, the supercritical fluid is rapidly introduced into the rubber composition, The rubber composition system can be saturated without the need to particularly raise the temperature of the rubber composition. The mechanism by which this rapid saturation is reached has not been fully elucidated, but, for example, until the concentration of the supercritical fluid in the rubber composition reaches a moderate level (for example, tens of percent). It is believed that the supercritical fluid (as a solute) is initially dissolved in the rubber composition material (as a solvent). At a few percent level, it is believed that the supercritical fluid acts as a solvent and the rubber composition acts as a solute. However, even if the supercritical fluid and the rubber composition act as a solvent or a solute, if the supercritical fluid is brought into contact with the rubber composition and introduced for a while, the supercritical fluid and the rubber composition system will be completely saturated very quickly. It is estimated that a solution is formed.
[0010]
Under a suitably selected temperature and pressure, if the supercritical fluid / rubber composition system contains a sufficient amount of supercritical fluid in the system, the temperature and / or pressure of the supercritical liquid / rubber composition system is increased. Abruptly changing (generally returning to the atmospheric state) triggers thermodynamic instability, and the foamed rubber composition of the present invention having micropores is produced using polyolefin resin finely dispersed in a rubber matrix as a core. You. The foamed rubber material thus obtained can have an average cell or cell size of less than 1.0 μm, and sometimes less than 0.5 μm. Further, in the present invention, such foaming of the rubber material can sometimes be achieved under environmental temperature (room temperature) conditions.
[0011]
In the kneading step and / or the vulcanizing step of the unvulcanized rubber composition, the foamed rubber composition of the present invention is brought into contact with a supercritical or subcritical fluid to permeate or dissolve the rubber composition. When the supercritical state is released (in general, returned to the atmospheric state), the supercritical fluid (or subcritical fluid) that has permeated or dissolved in the rubber composition changes from a fluid to a gas, and a part of the supercritical fluid changes. It is presumed that ultrafine pores are scattered from the rubber composition to leave ultrafine pores, and most of them remain as ultrafine bubbles in the rubber composition, whereby a rubber foam having an ultrafine structure is obtained. You.
[0012]
Here, the vulcanized rubber foam obtained by the method as described above is a rubber foam in which the pores and bubbles of the ultrafine structure are uniformly distributed, so that the specific gravity is reduced without substantially reducing its strength. Since this can be applied to tires and rubber members of various industrial rubber products, there is an advantage that the weight can be reduced without lowering the structural strength. It can be applied to a wide range of application members by utilizing sound absorbing properties, sound insulating properties, insulating properties, braking properties (skid characteristics), and the like. The vulcanized rubber foam to be used for these uses is appropriately selected from those having a foaming ratio of, for example, 3 to 100%.
[0013]
(Polyolefin resin)
The polyolefin resin used in the foamed rubber composition of the present invention is not particularly limited, and may be appropriately selected from crystalline polymers or amorphous polymers according to the desired product characteristics. it can. Among them, in order to easily disperse finely and uniformly and easily control to a desired foamed structure, a phase change phenomenon occurs, a viscosity change occurs at a certain temperature, and a viscosity control occurs easily, and from the viewpoint of easy viscosity control, a crystalline polymer is included. A resin composed of a polyolefin resin is preferable, and a polyolefin resin composed of only a crystalline polymer is more preferable.
[0014]
Specific examples of the above-mentioned crystalline polyolefin resin include, for example, polyethylene (PE), polypropylene (PP), polybutylene (PB), polybutylene succinate, polyethylene succinate, syndiotactic-1,2-polybutadiene (SPB). ), Polymers having a single composition such as polyvinyl alcohol (PVA), and those in which the melting point is controlled to an appropriate range by a copolymer or a blend of these, and the like. Those to which an agent or the like is added can also be suitably used. One of these resins may be used alone, or two or more thereof may be used in combination.
Among the above-mentioned crystalline polyolefin-based resins, polyethylene (PE), polypropylene (PP), and polybutene (PB) are more preferable from the viewpoints of melting temperature and raw material supply properties. In particular, the melting point is low, the handling is easy, and the price is low. From the viewpoint, polyethylene (PE) is most preferable.
[0015]
Specific examples of the amorphous polyolefin resin include, for example, polymethyl methacrylate (PMMA), acrylonitrile butadiene styrene copolymer (ABS), polystyrene (PS), polyvinyl chloride (PVC), polyacrylonitrile, and the like. Or a blend thereof. One of these may be used alone, or two or more thereof may be used in combination.
[0016]
Known fillers and various additives can be added to the above-mentioned polyolefin-based resin, if necessary, as long as the effects of the present invention are not impaired.
[0017]
The molecular weight of the polyolefin resin of the present invention varies depending on the chemical composition of the resin, the degree of branching of the molecular chain, and the like, and cannot be specified unconditionally. However, generally speaking, the polyolefin resin is the same monomer. Even if formed, the higher the molecular weight, the higher the viscosity (melt viscosity) at a certain temperature.
[0018]
In the present invention, the content of the polyolefin-based resin in the foamed rubber composition is preferably from 1 to 30 parts by mass, more preferably from 1.5 to 25 parts by mass, based on 100 parts by mass of the rubber component. 20 parts by weight are most preferred.
When the content is less than 1 part by mass, the amount of the polyolefin resin serving as a foam nucleus finely dispersed in a vulcanized rubber obtained by vulcanizing the rubber composition is small, and the foam cell diameter is reduced. On the other hand, if the content exceeds 30 parts by mass, kneading of the resin into the rubber composition becomes difficult, dispersibility and workability deteriorate, and mechanical strength is reduced. There is. On the other hand, it is preferable that the content be within the above preferable numerical range in that such a case does not occur.
[0019]
(Production method of foamed rubber composition)
Next, a method for producing the foamed rubber composition of the present invention will be described.
As one embodiment, the foamed rubber composition of the present invention, in a kneading step of an unvulcanized rubber composition, is brought into contact with a fluid in a supercritical state or a subcritical state, and then one end before vulcanization, And vulcanization under ordinary vulcanization conditions.
In another embodiment, the kneading step of the unvulcanized rubber composition is performed under normal conditions, and a supercritical or subcritical fluid is brought into contact during the subsequent vulcanization operation, and then, after vulcanization, Some can be obtained by methods that deviate from this state.
In still another embodiment, a method of bringing a supercritical or subcritical fluid into contact during the kneading step of the unvulcanized rubber composition and the vulcanizing operation of the next step, and deviating from the state after vulcanization. Some can be obtained by:
[0020]
The foamed rubber composition of the present invention specifically contacts a fluid in a supercritical state or a subcritical state with the unvulcanized rubber composition only in an extruder used for kneading the unvulcanized rubber composition. Through the step of introducing and introducing a supercritical or subcritical fluid to the kneaded unvulcanized rubber composition only in the vulcanizer, or alternatively, the extruder A desired vulcanized rubber foam can be obtained by contacting and introducing a fluid in a supercritical or subcritical state in both the vulcanizer and the vulcanizer.
[0021]
(Foam rubber composition)
The rubber component used in the foamed rubber composition of the present invention includes natural rubber (NR), isoprene rubber (IR), butadiene rubber (BR), styrene butadiene rubber (SBR), nitrile-butadiene rubber (NBR), and chloroprene rubber. (CR), isobutylene-isoprene copolymer rubber (IIR), ethylene-propylene-diene terpolymer rubber (EPDM), ethylene-propylene copolymer rubber (EPM), fluorine rubber, silicon rubber, urethane rubber , Acrylic rubber, and one of modified products thereof or a rubber mixture thereof are preferably used.
[0022]
As the vulcanizing agent to be mixed with the rubber component, a general rubber vulcanizing agent (crosslinking agent) can be used. Specifically, examples of the sulfur vulcanizing agent include powdered sulfur, precipitated sulfur, highly dispersible sulfur, surface-treated sulfur, insoluble sulfur, dimorpholine disulfide, and alkylphenol disulfide. About 0.5 to 4 parts by mass per 100 parts by mass can be used.
Examples of the organic peroxide vulcanizing agent include benzoyl peroxide, t-butyl hydroperoxide, 2,4-bichlorobenzoyl peroxide, and 2,5-dimethyl-2,5-di (t-butyl). Examples thereof include peroxy) hexane and 2,5-dimethylhexane-2,5-di (peroxyl benzoate), and for example, about 1 to 20 parts by mass can be used.
[0023]
Further, examples of the phenolic resin-based vulcanizing agent include brominated alkylphenol resins, and mixed cross-linking systems containing a halogen donor such as tin chloride and chloroprene and an alkylphenol resin, and for example, about 1 to 20 parts by mass. Can be used. In addition, zinc white (about 5 parts by weight), magnesium oxide (about 4 parts by weight), litharge (about 10 to 20 parts by weight), p-quinonedioxime, p-dibenzoylquinonedioxime, tetrachloro-p- Examples thereof include benzoquinone, poly-p-dinitrosobenzene (about 2 to 10 parts by mass), and methylene dianiline (about 0.2 to 10 parts by mass). If necessary, a vulcanization accelerator may be added to the vulcanizing agent. Examples of the vulcanization accelerator include common vulcanization accelerators such as aldehyde / ammonia, guanidine, thiazole, sulfenamide, thiuram, dithioate, and thiourea. About 2 parts by mass can be used.
[0024]
In addition, the foamed rubber composition of the present invention may contain carbon black and / or silica as a reinforcing agent usually added to the rubber composition. Furthermore, it is generally blended with various oils, antioxidants, antioxidants, fillers, plasticizers, softeners, vulcanizing agents, vulcanization accelerators, vulcanization accelerators, and other known rubbers. Various compounding chemicals can be appropriately compounded. The amounts of these additives may be conventional general amounts as long as they do not contradict the purpose of the present invention.
[0025]
The foamed rubber composition of the present invention obtained by the various methods described above is, for example, a lightweight and lightweight material obtained by applying this method to a rubber member of a tire or an industrial rubber product by a method known in the art. A rubber product comparable in other physical properties can be obtained.
Further, the foamed rubber composition of the present invention is prepared by contacting a pre-formed green tire (raw tire) or a vibration-proof rubber or the like with a supercritical or subcritical fluid in a mold and then introducing the same. By vulcanization molding, it is also possible to obtain a rubber product obtained by foaming at least a surface rubber such as a tire or a vibration-proof rubber.
Furthermore, after the foamed rubber composition of the present invention is bonded to an unvulcanized or vulcanized other member, the vulcanized rubber composition foam is integrated (the integration is performed by physical fitting or bonding). Any method such as bonding treatment or vulcanization bonding may be used to obtain a rubber product such as a tire, and the integrated rubber product is supercritical or subcritical in a mold. After contacting and introducing the fluid in the state, foaming and vulcanization may be performed to obtain a rubber product such as a tire.
[0026]
【Example】
Hereinafter, the present invention will be described specifically with reference to Examples, but the present invention is not limited to these Examples. In this example, “parts” and “%” all represent “parts by mass” and “% by mass”.
[0027]
[Examples 1 to 5 and Comparative Examples 1 to 3]
The rubber compositions of Examples 1 to 5 and Comparative Examples 1 to 3 were blended according to the formulation shown in the composition column of Table 1 below, and kneaded using a Labo Plastomill. This kneading consists of two stages. In the first stage, vulcanizing agents, vulcanization accelerators, vulcanization accelerating additives and other components other than compounding agents that greatly affect the crosslinking of rubber components at high temperatures, rubber and carbon black, A polyolefin resin was added and kneaded at 155 ° C. In the subsequent second stage, a compounding agent not added in the first stage was added and kneaded at a lower temperature (100 ° C.) than in the first stage.
[0028]
In the above description, the polyolefin-based resin used in this example has the following specifications (specs).
-Olefin resin (A): Linear low-density polyethylene (melting point = 125 ° C, MFR (JIS-K7210, temperature 190 ° C, load 2.16 kgf) = 5 g / 10 min, density = 0.921 g / cm ² )
・ Olefin resin (B): low density polyethylene (melting point = 108 ° C., MFR (JIS-K7210, temperature 190 ° C., load 2.16 kgf) = 20 g / 10 min, density = 0.920 g / cm ² )
・ Olefin resin (C): high density polyethylene (melting point = 136 ° C., MFR (JIS-K7210, temperature 190 ° C., load 2.16 kgf) = 2 g / 10 min, density = 0.964 g / cm ² )
[0029]
The foams of the comparative examples are those in which no polyolefin resin is blended, those foamed with a conventional blowing agent / foaming aid system, and combinations thereof. Here, the blowing agent (ADCA) in the table is azodicarbonamide.
[0030]
Next, a supercritical fluid of carbon dioxide is brought into contact with the rubber composition obtained above at a temperature of 50 ° C. and a pressure of 30 MPa to penetrate and dissolve, and then the foamed rubber composition obtained by deviating from the supercritical state is obtained. It was vulcanized at a temperature of 170 ° C. to obtain a desired vulcanized rubber foam. The sample of this vulcanized rubber foam was observed for its foaming state using a scanning microscope (× 50 to × 10000), and the results are shown in the lower part of Table 1 below.
[0031]
[Example 6]
The foamed rubber composition of Example 1 was applied to a tread portion of a tire, and a tire was manufactured according to a conventional method. A rubber sample was cut out from the tread portion of the tire, and its foaming state was observed using a scanning microscope (× 50 to × 10000 times) in the same manner as described above. Was issued. In addition, the sample exhibited physical properties comparable to those of the solid rubber (non-foamed product) in both compressive strength and durability, and a tire having an advantage of light weight was obtained.
[0032]
[Table 1]

[0033]
From the results in Table 1 above, the foamed rubber compositions and tires of Examples 1 to 6 according to the present invention are rubber foams in which fine bubbles are uniformly present as compared with those of Comparative Examples 1 to 3, It turned out that it was also excellent in physical properties.
[0034]
【The invention's effect】
According to the present invention, it is possible to provide a foamed rubber composition having a uniformly dispersed ultrafine foamed structure and a method for producing the same by foaming using a supercritical fluid, and these can be used for tires, belts, and vibration-proof rubbers. It is useful as a member of industrial rubber products.
[Brief description of the drawings]
FIG. 1 is a phase diagram showing a supercritical region and a subcritical region of carbon dioxide.

Claims

A foamed rubber composition characterized in that a supercritical fluid or a subcritical fluid is brought into contact with and introduced into a rubber composition in which a polyolefin resin is finely dispersed in a rubber component, and foamed.

The foamed rubber composition according to claim 1, wherein the polyolefin-based resin contains at least one of polyethylene, polypropylene, polybutene, and a modified product thereof.

The rubber component is natural rubber (NR), isoprene rubber (IR), butadiene rubber (BR), styrene butadiene rubber (SBR), nitrile-butadiene rubber (NBR), chloroprene rubber (CR), isobutylene-isoprene copolymer Rubber (IIR), ethylene-propylene-diene terpolymer rubber (EPDM), ethylene-propylene copolymer rubber (EPM), fluorine rubber, silicon rubber, urethane rubber, acrylic rubber, and modified products thereof The foamed rubber composition according to claim 1, comprising one kind.

The foamed rubber composition according to any one of claims 1 to 3, wherein 1 to 30 parts by mass of the polyolefin resin is blended with respect to 100 parts by mass of the rubber component.

The foamed rubber composition according to any one of claims 1 to 4, wherein the foamed rubber composition has a foaming ratio of 3 to 100%.

The foam according to any one of claims 1 to 5, wherein the foamed rubber composition is used for a tire tread, an undertread, a side reinforcing layer, a bead filler, a belt, an anti-vibration rubber, and a sealing material. Rubber composition.

The supercritical or subcritical fluid is brought into contact with and introduced into the unvulcanized rubber composition, and then vulcanization is performed after deviating from the state. A method for producing a foamed rubber composition.

The fluid in a supercritical state or a subcritical state is brought into contact with and introduced into the unvulcanized rubber composition, and vulcanization is continuously performed while maintaining the state. A method for producing a foamed rubber composition.

The foamed rubber composition according to any one of claims 1 to 6, wherein during the vulcanization of the unvulcanized rubber composition, a fluid in a supercritical state or a subcritical state is contacted and introduced to vulcanize. Method of manufacturing a product.

A supercritical or subcritical fluid is brought into contact with and introduced into the unvulcanized rubber composition in an extruder, and then vulcanized after deviating from the state. A method for producing a foamed rubber composition according to any one of the above.

After injecting the unvulcanized rubber composition into a vulcanizer with an extruder, a supercritical or subcritical fluid is brought into contact with and introduced into the vulcanizer in the vulcanizer, and vulcanized. Item 7. The method for producing a foamed rubber composition according to any one of Items 1 to 6.

2. A supercritical or subcritical fluid is brought into contact with and introduced into the unvulcanized rubber composition in an extruder, and vulcanization is performed in the vulcanizer while maintaining the state. 7. The method for producing a foamed rubber composition according to any one of items 1 to 6.

The production of a foamed rubber composition according to any one of claims 7 to 12, wherein the supercritical fluid or the subcritical fluid is a fluid having a supercritical temperature or a subcritical temperature equal to or lower than a vulcanization temperature of rubber. Method.

The method for producing a foamed rubber composition according to any one of claims 7 to 12, wherein the supercritical fluid or the subcritical fluid is carbon dioxide.