【発明の属する技術分野】
本発明は、残留発泡剤ガスの無機ガスへの置換を容易にしたオレフィン系樹脂発泡体及びその製造方法に関するもので、本発明の発泡体は工業製品等の緩衝包装材、住宅等の断熱材、ビート板やボディボート等のスポーツ用具芯材、フロート等の浮き材などに使用される。
【従来の技術】
プラスチック発泡体を製造するにあたり、発泡剤が使用されるが、近年オゾン層破壊や地球温暖化の見地から、その発泡剤がフロン系発泡剤から炭化水素系のブタンを主とした発泡剤に転換されつつある。ブタンを発泡剤として使用した熱可塑性樹脂押出発泡体の製造方法としては、特許第２１３７７２１号、米国特許第６３２３２４５号等が挙げられる。しかしながら、これら特許記載の方法では、発泡剤に主としてイソブタンを使用しており、イソブタンはノルマルブタンと比べ樹脂に対するガス透過性が低いことから、発泡体内部に長期に渡り残留する。ブタンは可燃性のガスであることから、火源が近くにある場合、発泡体が着火・燃焼する可能性が高まる。
この問題を解決する方法として、高温下で発泡体を長期間保存し、樹脂発泡体内部の発泡剤濃度を安全な濃度まで低下させる方法が一般的にとられているが、それでも数ヶ月あるいはそれ以上の保存期間を必要とするのであまり効率的ではない。
そこで、発泡体中に残存する可燃性発泡剤を空気等の不燃性の無機ガスと置換する時間を短縮する方法として、発泡体を製造後にその表面から穿孔を施す方法（特表平６−５０７１２９号）がある。
しかしながら、特表平６−５０７１２９号の方法では穿孔のタイミングが早すぎる場合、発泡剤の急激な逸散によって発泡体の体積収縮が大きくなってしまい、充分な体積回復が得られなくなるという問題があった。
【発明が解決しようとする課題】
本発明は、発泡剤としてブタンを使用した押出発泡による発泡体の製造方法において、イソブタンを発泡剤の主成分として使用する製造方法や、穿孔を施さない製造方法に比べ、短期間で可燃性発泡ガスを空気等の無機ガスに置換することができ、かつ、発泡後に発泡体への穿孔を施しても充分に体積回復が可能な熱可塑性樹脂発泡体およびその製造方法を提供することを目的とする。
【課題を解決するための手段】
本発明者等は、ノルマルブタンを主成分とし、かつ発泡後の発泡体中心部温度が添加されているガス透過調整剤の結晶化温度以下に降下した後、発泡体の表面から厚み方向に穿孔を施すことにより、上記課題が解決されることを見出し、本発明をなすに至った。
すなわち、本発明は、下記の通りである。
１．発泡剤としてブタンを用いた押出発泡による発泡体の製造方法において、発泡剤中のノルマルブタンの占める比率が６５〜１００重量％であることを特徴とする、熱可塑性樹脂発泡体の製造方法。
２．発泡剤としてブタンを用いた押出発泡による発泡体の製造方法において、ブタン１００重量部に対しプロパンを０．３〜２．０重量部添加することを特徴とする、１．記載の熱可塑性樹脂発泡体の製造方法。
３．発泡剤としてブタンを用いた押出発泡による発泡体の製造方法において、発泡後の発泡体中心部温度が、添加されているガス透過調整剤の結晶化温度以下に降下した後、発泡体の表面から厚み方向に穿孔を施すことを特徴とする、１．または２．記載の熱可塑性樹脂発泡体の製造方法。
４．熱可塑性樹脂発泡体への穿孔が、多数の針を有する剣山状の針集合具を用いて実施されることを特徴とする、３．記載のオレフィン系樹脂発泡体の製造方法。
５．熱可塑性樹脂発泡体への穿孔の深さが、該発泡体の厚み−５ｍｍ以上であることを特徴とする、３．記載のオレフィン系樹脂発泡体の製造方法。
６．熱可塑性樹脂発泡体の表面への穿孔の間隔が５ｍｍ以上、２０ｍｍ以下であることを特徴とする、３．記載のオレフィン系樹脂発泡体の製造方法。
７．穿孔処理後の熱可塑性樹脂発泡体を３０℃以上、添加されているガス透過調整剤の融点未満の温度で保管することを特徴とする、１．〜６．のいずれかに記載のオレフィン系樹脂発泡体の製造方法。
８．ノルマルブタンが６５〜１００重量％からなるブタンを発泡剤として用いた熱可塑性樹脂押出発泡体であって、発泡後の発泡体中心部温度が、添加されているガス透過調整剤の結晶化温度以下に降下した後、発泡体の表面から厚み方向に穿孔を施すことにより残存ブタンガスが無機ガスへ置換されたことを特徴とする熱可塑性樹脂発泡体。
【発明の実施の形態】
以下、本発明について、特にその好ましい実施態様を中心に、具体的に説明する。
本発明における熱可塑性樹脂としては、高密度ポリエチレン、中密度ポリエチレン、低密度ポリエチレン、直鎖状低密度ポリエチレン等のポリエチレン単独重合体、ポリプロピレン単独重合体、ポリブテン単独重合体、エチレン−酢酸ビニル共重合体、エチレン−プロピレン共重合体、エチレン−ブテン共重合体、エチレン−ブテン−プロピレン共重合体、エチレン−アクリル酸共重合体等のオレフィン系樹脂や、スチレン系単独重合体、α−メチルスチレン、アクリルニトリル、ブタジエン、メチルメタアクリレート等のスチレンと共重合し得る単量体とスチレンの共重合体、更には一般的に耐衝撃ポリスチレンと呼ばれるポリスチレンを主体とするゴム系ポリマーとの共重合体等のポリスチレン系樹脂等が挙げられる。これらの樹脂は単独で用いるほか、適宜混合して用いることもできる。
本発明における発泡剤はノルマルブタンとイソブタンからなるブタンであり、ブタンに占めるノルマルブタンの比率は６５〜１００重量％、好ましくは７０〜１００重量％である。ノルマルブタンの比率が６５〜１００重量％であると、発泡体からの発泡剤の逸散が速く、早期に燃焼範囲下限濃度（１．８ｖｏｌ％）未満にまでガス濃度が低下し、発泡体が着火・燃焼する可能性を低減できる。
本発明においては、ブタン１００重量部に対しプロパンを０．３〜２．０重量部添加することが好ましい。プロパンはブタンより発泡効率が高く、少量の添加で発泡倍率を高める効果が得られる。従って、プロパンを少量添加するとブタンの使用量を減らすことができ、結果的に発泡体に残存する可燃性ガス量を低減できる。プロパンの添加量が０．３重量部以上であると発泡倍率を高める効果が得られ、添加量が２．０重量部以下であると、プロパンの逸散による発泡体の初期収縮が抑制できる。
また、これらの発泡剤の添加量を調節することで得られる発泡体の密度を任意に制御することができる。
本発明におけるガス透過調整剤としては、公知のガス透過調整剤、例えば、パルチミン酸グリセリド、ステアリン酸グリセリド等の脂肪酸グリセリド、オレイン酸アミド、エルカ酸アミド等の脂肪酸アミド、ステアリルステアリン酸アミド等のアルキル脂肪酸アミドが挙げられる。これらのガス透過調整剤は単独で用いるほか、適宜混合して用いることができる。
本発明においては、必要に応じて一般に使用されている気泡核形成剤を用いてもよい。この気泡核形成剤としては、例えば、タルクのような無機物質、ステアリン酸亜鉛のような脂肪酸の金属塩、あるいは押出機の温度で分解して分解ガスを発生するような化学発泡剤、またはその温度で反応して炭酸ガスを発生する酸とアルカリの混合物のようなものである。これらの気泡核形成剤を使用することで得られる発泡体のセルサイズの大きさを任意に制御することができる。
さらに、本発明においては必要に応じて、混合樹脂に対し帯電防止剤、酸化防止剤、紫外線吸収剤、着色剤等の添加剤も添加することもできる。
本発明の発泡体の製造方法は、押出機内で樹脂と発泡剤及びガス透過調整剤、必要に応じて気泡核形成剤等の添加剤を加圧下で溶融混練した後、適正な発泡温度まで冷却して得られた発泡性溶融混合物を押出機先端に取り付けたダイスを通して大気圧下に押し出して発泡させることによるものである。
本発明の発泡体への穿孔は、発泡体中心部の温度が添加されているガス透過調整剤の結晶化温度以下に降下した後に実施される。発泡体中心部とは該発泡体の外周から同発泡体の厚み以上内部に入った部位の厚み中央部分のことを指す。発泡体中心部の温度が添加されているガス透過調整剤の結晶化温度以下で穿孔を施すことで、ガス透過調整剤のガスバリアー効果発現によって急激な発泡剤の逸散が抑止され、良好な発泡体の体積回復が可能となる。また、一般的に上記ガス透過調整剤の結晶化温度は発泡体の基材樹脂の結晶化温度よりも低いので、穿孔の段階では基材樹脂は完全に固化しており、ガス逸散による気泡の座屈も抑制できる。
本発明の発泡体への穿孔は、好ましくは多数の針を有する剣山状の針集合具を用いて実施される。剣山状の針集合具とは、発泡体への多数の穿孔が１回で実施できるように針が配列されているものを指す。針は、発泡体の幅方向に平行な一列当たりが少なくとも８本以上から成り、その針列が少なくとも９０列以上並んでいるものが好ましく、その配置は例えば、千鳥格子状あるいは正方格子状である。剣山状の針集合具を用いることで、発泡体への穿孔を同時に実施することができ、発泡体内からのガスの逸散が均一になるため、ガス逸散に起因する発泡体の変形を抑制しやすくなる。また穿孔処理能力が高まるので、相対的に穿孔速度を低下させることが可能となり、針引き抜き時の発泡体表面の毛羽立ちが無い表面外観が良好な発泡体が得られる。
本発明の発泡体への穿孔の深さは、該発泡体の厚み−５ｍｍ以上、即ち、完全に貫通するか非貫通代が５ｍｍ以下であることが好ましい。穿孔の深さが該発泡体の厚み−５ｍｍ以上であると、発泡体中の残留可燃性ガスと空気等の無機ガスとの置換が速やかに行われるとともに、発泡体の両表面からのガスの放散が均一に進行するため、ガス逸散に起因する発泡体内の残留応力分布が小さくなり、発泡体加工時の変形を抑止できる。また穿孔の深さが該発泡体の厚み未満、すなわち非貫通である場合は、穿孔面のみを加熱溶融等の熱処理を施すことで非貫通面において表面外観が良好で吸水性、透水性の低い発泡体が得られる。
本発明の発泡体表面への穿孔の間隔とは、隣接する孔同士の孔の中心間距離で最短のものを指し、好ましくは、５ｍｍ以上、２０ｍｍ以下、さらに好ましくは、１０ｍｍ以上、１５ｍｍ以下である。穿孔の間隔が５ｍｍ以上であると、急激な可燃性発泡ガス逸散が抑制しやすくなり、発泡体の体積回復が良好となる。また穿孔の間隔が２０ｍｍ以下であると、発泡体中の残留可燃性発泡剤と空気等の無機ガスとの置換が速やかに行われる。穿孔の間隔が１０ｍｍ以上、１５ｍｍ以下であると、発泡体の体積回復性とガス置換促進効果のバランスに優れる。
本発明の発泡体への穿孔の径は０．５ｍｍ以上、２．０ｍｍ以下であることが好ましい。穿孔の径０．５ｍｍ以上であると、発泡体中の残留可燃性ガスと空気等の無機ガスとの置換が速やかに行われる。また、穿孔の径が２．０ｍｍ以下であると表面外観が良好な発泡体が得られると同時に、水と接触した場合に孔への水の浸入を抑制しやすくなる。
本発明で更に発泡体中の残留可燃性ガスと空気等の無機ガスとの置換を促進するためには、穿孔処理後に３０℃以上、添加されているガス透過調整剤の融点未満の温度で保存することが好ましい。３０℃以上であれば、残留可燃性ガスと空気等の無機ガスとの置換が速やかに行われる。また、添加されているガス透過調整剤の融点未満の温度であれば、ガス透過調整剤の結晶が融解しないのでガスバリアー性が維持でき、発泡剤の逸散による発泡体の収縮を抑制しやすくなる。
本発明で得られる発泡体厚みは２０ｍｍ以上８０ｍｍ以下が好ましく、その厚み構成は単層または熱融着等による積層のいずれでも構わない。
本発明の発泡体の密度は、好ましくは１０〜１００ｋｇ／ｍ３、さらに好ましくは２０〜７０ｋｇ／ｍ３である。密度が１０〜１００ｋｇ／ｍ３であると、断熱材、浮き材、緩衝包装材用途として使用できる。
さらに、本発明で得られる発泡体の独立気泡率は、穿孔による孔の部分を除いて、好ましくは８０〜１００％、より好ましくは９０〜１００％である。独立気泡率が８０％以上であると、緩衝包装材として充分な緩衝性能を発揮することができるとともに、発泡体内へ水が浸入しにくいため吸水率を低くすることができる。
また、本発明の発泡体のセルサイズは、好ましくは０．３ｍｍ〜３．０ｍｍ、より好ましくは０．５ｍｍ〜２．５ｍｍである。断熱材用途においては、セルサイズは小さいほど断熱性能はよく、また、セルサイズが大きいと水と接触した場合にセル開口面への水の浸入が発生し、吸水率の増加につながる。
【実施例】
以下、本発明を実施例に基づいて説明するが、本発明の内容をこれらの実施例に限定するものではない。実施例に示された値は次の方法により測定したものである。なお、実施例中、部及び％は特に断りのない限り、重量基準である。また、各種の評価、測定は下記の方法に拠った。
（１）体積回復性
押出発泡１分後に樹脂発泡体の長さを１０００ｍｍに切断し、当該発泡体の幅、厚み、長さを正確に測定し、それぞれの寸法を乗じて算出した発泡体の体積をＶｏとする。同樹脂発泡体を発泡１時間後から一定の温度で保存した後、発泡１週間後に再び各寸法を測定、それぞれの寸法を乗じて算出した体積をＶとする。体積維持率（Ｒ）を次式によって算出し、その値を基に体積回復性を以下の基準で評価した。
Ｒ（％）＝Ｖ／Ｖｏ×１００
◎：Ｒ≧９５（体積回復性が非常に優れ、製造時の寸法維持が非常に容易）
○：９０≦Ｒ＜９５（体積回復性が優れ、製造時の寸法維持が容易）
×：Ｒ＜９０（体積回復性に乏しく、製造時の寸法維持が困難）
（２）ガス置換促進効果
発泡１時間後から一定の温度で保存した樹脂発泡体を内径１６ｍｍのコルクボーラーを使用して厚み方向に円柱状に抜き出して試験片とする。この試験片の体積と重量をすばやく測定後、直ちに、内容積を測定したヘッドスペースボトル（ジーエルサイエンス社製）に入れ密封し、１７０℃で１．５時間加熱溶融させる。その後、ボトルを室温まで自然冷却後、該ボトル内のガスを分取、ガスクロマトグラフ（島津製作所製：ＧＣ−１４Ｂ）にて分析した。予め既知のガス濃度測定より作成しておいた検量線と、試験片の体積、試験片の樹脂部分容積、ボトル内容積を用いて発泡体中の残存発泡剤濃度をｎ＝３の平均で算出した。発泡体内残存発泡剤濃度と無機ガスとの置換促進効果は以下の基準で評価した。
◎：残存ブタン濃度が発泡後１週間の時点で燃焼範囲下限未満に到達
○：残存ブタン濃度が発泡後２週間の時点で燃焼範囲下限未満に到達
×：残存ブタン濃度が発泡後２週間の時点で燃焼範囲下限以上
（３）ガス透過調整剤の結晶化温度及び融解温度
示差熱量分析計（パーキンエルマー社製ＤＳＣ−７）を用いて、３０℃で０．５分間保持後、１０℃／分で１２０℃まで昇温して１分間保持、その後再び３０℃まで１０℃／分で降温した際の最も高いピークの温度を結晶化温度とした。その後、３０℃で１分間保持し、１０℃／分で１２０℃まで昇温した際の最も低いピークの温度を融解温度とした。ガス透過調整剤を２成分以上混合して用いる場合については、単独で測定した結晶化温度が最も高い成分の結晶化温度をガス透過調整剤の結晶化温度、単独で測定した融解温度が最も低い成分の融解温度をガス透過調整剤の融解温度とした。
（４）発泡体の中心部温度
挿入時に発泡体の温度低下を防止する目的で予め発泡温度まで暖めた棒状の接触式熱電対（温度感知部の直径３ｍｍ）を、発泡直後の板状発泡体の外周部から該発泡体の厚み以上内部に入った部位において発泡体厚みの２分の１の深さまで垂直に挿入して温度を測定、発泡体の中心部温度とした。
（５）発泡体の穿孔径
穿孔樹脂発泡体を発泡１時間後から一定の温度で保存し、発泡１週間後に、０．０５ｍｍ刻みで作製したピンゲージを用いて孔の径を測定した。ピンゲージは発泡体厚みの２分の１の深さまで挿入し、その際に抵抗がない最大のピンゲージ径を発泡体の穿孔径とした。
（６）発泡体の独立気泡率
ＡＳＴＭ−Ｄ２８５６に記載されているエアーピクノメーター法（東京サイエンス（株）製、空気比較式比重計１０００型使用）により測定し、ｎ＝５の平均で算出した。
（７）発泡体のセルサイズ
発泡体の中央部から試験片をカットし、カット面に発泡体の押出方向、幅方向、厚み方向に沿ってＬ（ｍｍ）の直線を引き、これらの直線に接触している気泡の数を数え、自式により押出方向、幅方向、厚み方向のセルサイズを算出し、更に３方向の平均値をセルサイズとした（グリッドライン法）。
セルサイズ（ｍｍ）＝１．６２６×Ｌ／気泡数
【実施例１】
１５０ｍｍのバレル内径を有するスクリュー型押出機の供給領域に９００ｋｇ／時間の速度で、低密度ポリエチレン（密度０．９２１ｇ／ｃｍ３、ＭＩ＝２．９ｇ／１０分）をこの樹脂１００重量部に対し１．５重量部の気泡調整剤（タルク）と０．５重量部のガス透過調整剤（ステアリン酸モノグリセリド：結晶化温度６０．５℃、融解温度７２．８℃）とともに供給した。押出機のバレル温度を１９０℃〜２１０℃に調整し、押出機の先端に取り付けた発泡剤注入口からノルマルブタン６５重量％、イソブタン３５重量％（燃焼範囲下限値：１．８ｖｏｌ％）からなる発泡剤をこの樹脂１００重量部に対し６．７重量部を圧入し、当該溶融樹脂組成物と混合して発泡性溶融混合物とした。この発泡性溶融混合物を押出機の出口に取り付けた冷却装置で１０８℃まで冷却した後、約３．４ｍｍの平均厚みと約２１５ｍｍ幅の開口部形状を有するオリフィスプレートより、常温、大気圧下の雰囲気中に連続的に押し出して発泡させ、樹脂発泡体の引き取り速度を調整しながら成形して、厚み６２ｍｍ、幅６００ｍｍ、長さ１０００ｍｍ、セルサイズ１．１ｍｍ、密度３８ｋｇ／ｍ３、独立気泡率９５％の板状樹脂発泡体を得た。この発泡体の中心部温度が５０℃まで降下した直後に該樹脂発泡体の上面から、一辺１０．０ｍｍの正三角形から構成される千鳥格子状に針を配列した剣山状の針集合具（穿孔の間隔：１０．０ｍｍ）を用い、貫通穿孔処理を実施して穿孔樹脂発泡体を得た。この穿孔樹脂発泡体を発泡１時間後から４０℃の環境下で保存し、穿孔径、体積回復性及びガス置換促進効果の評価を行なった。その結果を表１に示す。
【実施例２】
発泡剤としてノルマルブタン７０重量％、イソブタン３０重量％からなるブタンを使用し、このブタンを樹脂１００重量部に対し６．４重量部圧入した他は、実施例１と同様の方法で厚み６２ｍｍ、幅６００ｍｍ、長さ１０００ｍｍ、セルサイズ１．１ｍｍ、密度３８ｋｇ／ｍ３、独立気泡率９５％の板状樹脂発泡体を得た。その後、一辺１１．０ｍｍの正三角形から構成される千鳥格子状に針を配列した剣山状の針集合具（穿孔の間隔：１１．０ｍｍ）を用い、穿孔深さが６１ｍｍとなるように穿孔処理を実施した他は、実施例１と同様の方法で穿孔樹脂発泡体を得た。この穿孔樹脂発泡体を発泡１時間後から４０℃の環境下で保存し、穿孔径、体積回復性及びガス置換促進効果の評価を行なった。その結果を表１に示す。
【実施例３】
発泡剤としてブタン１００重量部に対し１．３重量部のプロパンを添加し、ブタンを樹脂１００重量部に対し５．９重量部圧入した他は、実施例２と同様の方法で厚み６２ｍｍ、幅６００ｍｍ、長さ１０００ｍｍ、セルサイズ１．１ｍｍ、密度３７ｋｇ／ｍ３、独立気泡率９６％の板状樹脂発泡体を得た。その後、実施例２と同様の方法で穿孔処理を実施、穿孔樹脂発泡体を得た。この穿孔樹脂発泡体を発泡１時間後から４０℃の環境下で保存し、穿孔径、体積回復性及びガス置換促進効果の評価を行なった。その結果を表１に示す。
【実施例４】
発泡剤としてノルマルブタン８０重量％、イソブタン２０重量％からなるブタンを使用し、このブタンを樹脂１００重量部に対し６．５重量部圧入した他は、実施例１と同様の方法で厚み６２ｍｍ、幅６００ｍｍ、長さ１０００ｍｍ、セルサイズ１．１ｍｍ、密度３８ｋｇ／ｍ３、独立気泡率９５％の板状樹脂発泡体を得た。その後、一辺１５．０ｍｍの正三角形から構成される千鳥格子状に針を配列した剣山状の針集合具（穿孔の間隔：１５．０ｍｍ）を用いた他は実施例２と同様の方法で穿孔処理を実施、穿孔樹脂発泡体を得た。この穿孔樹脂発泡体を発泡１時間後から４０℃の環境下で保存し、穿孔径、体積回復性及びガス置換促進効果の評価を行なった。その結果を表１に示す。
【比較例１】
発泡剤としてノルマルブタン５０重量％、イソブタン５０重量％からなるブタンを使用し、このブタンを樹脂１００重量部に対し６．２重量部圧入した他は、実施例１と同様の方法で厚み６２ｍｍ、幅６００ｍｍ、長さ１０００ｍｍ、セルサイズ１．１ｍｍ、密度３８ｋｇ／ｍ３、独立気泡率９４％の板状樹脂発泡体を得た。その後、一辺１０．０ｍｍの正三角形から構成される千鳥格子状に針を配列した剣山状の針集合具（穿孔の間隔：１０．０ｍｍ）を用いた他は、実施例２と同様の方法で穿孔樹脂発泡体を得た。この穿孔樹脂発泡体を発泡１時間後から４０℃の環境下で保存し、穿孔径、体積回復性及びガス置換促進効果の評価を行なった。その結果を表１に示す。
【比較例２】
発泡体の中心部温度が７５℃まで降下した直後に穿孔処理を実施した他は、比較例１と同様の方法で穿孔樹脂発泡体を得た。この穿孔樹脂発泡体を発泡１時間後から４０℃の環境下で保存し、穿孔径、体積回復性及びガス置換促進効果の評価を行なった。その結果を表１に示す。
【比較例３】
発泡剤としてノルマルブタン３０重量％、イソブタン７０重量％からなるブタンを使用し、このブタンを樹脂１００重量部に対し６．０重量部圧入した他は、実施例１と同様の方法で厚み６２ｍｍ、幅６００ｍｍ、長さ１０００ｍｍ、セルサイズ１．１ｍｍ、密度３８ｋｇ／ｍ３、独立気泡率９４％の板状樹脂発泡体を得た。その後、比較例１と同様の方法で穿孔処理を実施、穿孔樹脂発泡体を得た。この穿孔樹脂発泡体を発泡１時間後から７５℃の環境下で保存し、穿孔径、体積回復性及びガス置換促進効果の評価を行なった。その結果を表１に示す。
【比較例４】
穿孔処理を実施しない他は、比較例３と同様の方法で穿孔樹脂発泡体を得た。この穿孔樹脂発泡体を発泡１時間後から４０℃の環境下で保存し、穿孔径、体積回復性及びガス置換促進効果の評価を行なった。結果を表１に示す。
【表１】
【発明の効果】
本発明の熱可塑性樹脂発泡体の製造方法は、発泡剤としてブタンを使用した押出発泡による発泡体の製造方法において、イソブタンを発泡剤の主成分として使用する製造方法や、穿孔を施さない製造方法に比べ、短期間で可燃性発泡ガスを空気等の無機ガスに置換することができ、かつ、発泡後に発泡体への穿孔を施しても充分に体積回復が可能な製造方法である。
【図面の簡単な説明】
【図１】本発明実施例で使用した千鳥格子状に針を配列した剣山状の針集合具の針配列図である。
【符号の説明】
ａ：穿孔の間隔TECHNICAL FIELD OF THE INVENTION
TECHNICAL FIELD The present invention relates to an olefin-based resin foam which facilitates replacement of a residual foaming agent gas with an inorganic gas, and a method for producing the same. The foam of the present invention is used as a buffer packaging material for industrial products and the like, and a heat insulating material for houses and the like. It is used as a core material for sports equipment such as a beat board and a body boat, and a floating material such as a float.
[Prior art]
Foaming agents are used in the production of plastic foams, but in recent years, from the perspective of depletion of the ozone layer and global warming, the foaming agents have been changed from fluorocarbon-based blowing agents to hydrocarbon-based butane-based blowing agents. Is being done. Examples of a method for producing a thermoplastic resin extruded foam using butane as a foaming agent include Japanese Patent No. 2137721 and US Pat. No. 6,323,245. However, in the methods described in these patents, isobutane is mainly used as a foaming agent, and isobutane remains in the foam for a long time because gas permeability to resin is lower than that of normal butane. Since butane is a combustible gas, the possibility of ignition and combustion of the foam increases when a fire source is nearby.
As a method for solving this problem, a method of storing the foam at a high temperature for a long period of time and lowering the concentration of the foaming agent inside the resin foam to a safe concentration is generally adopted, but it is still several months or even longer. It is not very efficient because it requires the above storage period.
Therefore, as a method of shortening the time for replacing the combustible foaming agent remaining in the foam with a nonflammable inorganic gas such as air, a method of perforating the foam after manufacturing the foam (Japanese Patent Application Laid-Open No. 6-507129). No.).
However, in the method disclosed in JP-T-Hei 6-507129, if the timing of the perforation is too early, the volume of the foam is significantly reduced due to the rapid escape of the foaming agent, and a sufficient volume recovery cannot be obtained. there were.
[Problems to be solved by the invention]
The present invention relates to a method for producing a foam by extrusion foaming using butane as a foaming agent, in which flammable foam is produced in a shorter time than a production method using isobutane as a main component of a foaming agent or a production method without perforation. It is an object of the present invention to provide a thermoplastic resin foam capable of replacing a gas with an inorganic gas such as air, and capable of sufficiently recovering a volume even after perforating the foam after foaming, and a method for producing the same. I do.
[Means for Solving the Problems]
The present inventors perforated in the thickness direction from the surface of the foam after the temperature of the foam center, which contains normal butane as the main component, and dropped after the foam core temperature dropped below the crystallization temperature of the added gas permeation modifier. It has been found that the above-mentioned problems can be solved by applying the method, and the present invention has been accomplished.
That is, the present invention is as follows.
1. A method for producing a foam by extrusion foaming using butane as a foaming agent, wherein the proportion of normal butane in the foaming agent is 65 to 100% by weight.
2. A method for producing a foam by extrusion foaming using butane as a foaming agent, wherein 0.3 to 2.0 parts by weight of propane is added to 100 parts by weight of butane. A method for producing the thermoplastic resin foam according to the above.
3. In the method for producing a foam by extrusion foaming using butane as a foaming agent, after the foam center temperature after foaming falls below the crystallization temperature of the added gas permeation modifier, from the surface of the foam It is characterized by perforating in the thickness direction. Or 2. A method for producing the thermoplastic resin foam according to the above.
4. 2. The perforation of the thermoplastic resin foam is performed using a needle-shaped needle assembly having a large number of needles. A method for producing the olefin resin foam according to the above.
5. 2. The depth of perforations in the thermoplastic resin foam is not less than -5 mm in thickness of the foam. A method for producing the olefin resin foam according to the above.
6. 2. The interval between perforations in the surface of the thermoplastic resin foam is 5 mm or more and 20 mm or less. A method for producing the olefin resin foam according to the above.
7. The perforated thermoplastic resin foam is stored at a temperature of 30 ° C. or higher and lower than the melting point of the added gas permeation modifier. ~ 6. The method for producing an olefin resin foam according to any one of the above.
8. A thermoplastic resin extruded foam using butane containing 65 to 100% by weight of normal butane as a foaming agent, wherein the foam center temperature after foaming is equal to or lower than the crystallization temperature of the added gas permeation modifier. A thermoplastic resin foam, characterized in that the remaining butane gas is replaced with an inorganic gas by perforating the foam from its surface in the thickness direction.
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the present invention will be specifically described with particular emphasis on its preferred embodiments.
As the thermoplastic resin in the present invention, polyethylene homopolymer such as high-density polyethylene, medium-density polyethylene, low-density polyethylene, linear low-density polyethylene, polypropylene homopolymer, polybutene homopolymer, ethylene-vinyl acetate copolymer Coalesce, ethylene-propylene copolymer, ethylene-butene copolymer, ethylene-butene-propylene copolymer, olefin-based resin such as ethylene-acrylic acid copolymer, styrene-based homopolymer, α-methylstyrene, A copolymer of styrene with a monomer copolymerizable with styrene such as acrylonitrile, butadiene, methyl methacrylate, and a copolymer of a rubber-based polymer mainly composed of polystyrene generally called high-impact polystyrene. And the like. These resins can be used alone or in combination as appropriate.
The blowing agent in the present invention is butane composed of normal butane and isobutane, and the ratio of normal butane to butane is 65 to 100% by weight, preferably 70 to 100% by weight. When the ratio of normal butane is 65 to 100% by weight, the escape of the blowing agent from the foam is fast, and the gas concentration is reduced to less than the lower limit of the combustion range (1.8 vol%) at an early stage. The possibility of ignition and combustion can be reduced.
In the present invention, it is preferable to add 0.3 to 2.0 parts by weight of propane to 100 parts by weight of butane. Propane has a higher foaming efficiency than butane, and the effect of increasing the foaming ratio can be obtained by adding a small amount. Therefore, when a small amount of propane is added, the amount of butane used can be reduced, and as a result, the amount of combustible gas remaining in the foam can be reduced. When the added amount of propane is 0.3 parts by weight or more, the effect of increasing the expansion ratio is obtained, and when the added amount is 2.0 parts by weight or less, the initial shrinkage of the foam due to the escape of propane can be suppressed.
Further, the density of the foam obtained can be arbitrarily controlled by adjusting the addition amount of these foaming agents.
As the gas permeation regulator in the present invention, known gas permeation regulators, for example, fatty acid glycerides such as glyceride palmitate and glyceride stearic acid, fatty acid amides such as oleic acid amide and erucamide, and alkyls such as stearyl stearic acid amide Fatty acid amides; These gas permeation modifiers can be used alone or in combination as appropriate.
In the present invention, a generally used cell nucleating agent may be used as necessary. As the bubble nucleating agent, for example, an inorganic substance such as talc, a metal salt of a fatty acid such as zinc stearate, or a chemical foaming agent that decomposes at the temperature of the extruder to generate a decomposition gas, or the like. It is like a mixture of acid and alkali that reacts at temperature to generate carbon dioxide. The cell size of the foam obtained by using these cell nucleating agents can be arbitrarily controlled.
Further, in the present invention, if necessary, additives such as an antistatic agent, an antioxidant, an ultraviolet absorber, and a coloring agent can be added to the mixed resin.
In the method for producing a foam of the present invention, a resin, a foaming agent, a gas permeation modifier, and, if necessary, additives such as a foam nucleating agent are melt-kneaded under pressure in an extruder, and then cooled to an appropriate foaming temperature. The resulting foamable molten mixture is extruded through a die attached to the tip of an extruder under atmospheric pressure to foam.
The perforation of the foam of the present invention is carried out after the temperature at the center of the foam falls below the crystallization temperature of the added gas permeation modifier. The central part of the foam refers to the central part of the thickness of a part which enters the inside of the foam by a thickness equal to or more than the thickness of the foam. By performing the perforation at a temperature lower than the crystallization temperature of the gas permeation modifier to which the temperature of the foam center is added, the gas barrier effect of the gas permeation modifier suppresses the rapid escape of the foaming agent due to the development of the gas barrier effect. The volume of the foam can be recovered. In addition, since the crystallization temperature of the gas permeation modifier is generally lower than the crystallization temperature of the base resin of the foam, the base resin is completely solidified at the stage of perforation, and bubbles due to gas escape are formed. Buckling can also be suppressed.
The perforation of the foam according to the invention is preferably carried out using a sword-shaped needle assembly having a large number of needles. A sword-shaped needle assembly refers to one in which the needles are arranged so that a large number of perforations in the foam can be performed at one time. The needles preferably have at least eight or more needles per row parallel to the width direction of the foam, and the needle rows are preferably arranged in at least 90 rows. The arrangement is, for example, a staggered or square lattice. is there. By using a sword-shaped needle assembly, it is possible to pierce the foam at the same time, and the gas escape from the foam becomes uniform, thus suppressing the deformation of the foam due to gas escape. Easier to do. In addition, since the perforation processing ability is increased, the perforation speed can be relatively reduced, and a foam having a good surface appearance without fluffing of the foam surface at the time of needle withdrawal can be obtained.
The depth of the perforations in the foam of the present invention is preferably not less than the thickness of the foam minus 5 mm, that is, the penetration depth is not more than 5 mm. When the depth of the perforations is equal to or greater than the thickness of the foam −5 mm, the replacement of the combustible gas remaining in the foam with an inorganic gas such as air is quickly performed, and the gas from both surfaces of the foam is removed. Since the dissipation proceeds uniformly, the residual stress distribution in the foam due to the gas dissipation is reduced, and deformation during foam processing can be suppressed. When the depth of the perforations is less than the thickness of the foam, that is, when the foam is non-penetrating, only the perforated surface is subjected to a heat treatment such as heating and melting, so that the non-penetrating surface has a good surface appearance, water absorption, and low water permeability. A foam is obtained.
The interval of perforations on the foam surface of the present invention refers to the shortest distance between the centers of adjacent holes, preferably 5 mm or more, 20 mm or less, more preferably 10 mm or more, 15 mm or less. is there. When the interval between the perforations is 5 mm or more, rapid escape of the combustible foaming gas is easily suppressed, and the volume recovery of the foam becomes good. If the interval between the perforations is 20 mm or less, the replacement of the residual combustible foaming agent in the foam with an inorganic gas such as air is performed promptly. When the interval between the perforations is 10 mm or more and 15 mm or less, the balance between the volume recovery of the foam and the gas replacement promoting effect is excellent.
The diameter of the perforations in the foam of the present invention is preferably 0.5 mm or more and 2.0 mm or less. When the diameter of the perforations is 0.5 mm or more, the replacement of the residual combustible gas in the foam with an inorganic gas such as air is performed promptly. Further, when the diameter of the perforations is 2.0 mm or less, a foam having a good surface appearance can be obtained, and at the same time, when it comes into contact with water, it is easy to suppress intrusion of water into the holes.
In order to further promote the replacement of the residual flammable gas in the foam with an inorganic gas such as air in the present invention, the material is stored at a temperature of 30 ° C. or more after the perforation treatment and less than the melting point of the added gas permeation modifier. Is preferred. When the temperature is 30 ° C. or higher, the replacement of the residual flammable gas with an inorganic gas such as air is performed promptly. In addition, if the temperature is lower than the melting point of the added gas permeation modifier, the gas barrier property can be maintained because the crystals of the gas permeation modifier do not melt, and the shrinkage of the foam due to the escape of the foaming agent is easily suppressed. Become.
The thickness of the foam obtained by the present invention is preferably 20 mm or more and 80 mm or less, and the thickness configuration thereof may be either a single layer or a laminate by heat fusion or the like.
The density of the foam of the present invention is preferably 10 to 100 kg / m 3 , more preferably 20 to 70 kg / m 3 . When the density is 10 to 100 kg / m 3, it can be used as a heat insulating material, a floating material, and a buffer packaging material.
Furthermore, the closed cell rate of the foam obtained by the present invention is preferably 80 to 100%, more preferably 90 to 100%, excluding the portion of the hole formed by perforation. When the closed cell rate is 80% or more, a sufficient cushioning performance can be exhibited as a buffer packaging material, and the water absorption rate can be reduced because water hardly penetrates into the foam.
Moreover, the cell size of the foam of the present invention is preferably 0.3 mm to 3.0 mm, and more preferably 0.5 mm to 2.5 mm. In the heat insulating material application, the smaller the cell size, the better the heat insulating performance. If the cell size is large, when the cell comes into contact with water, water infiltrates into the cell opening surface, leading to an increase in water absorption.
【Example】
Hereinafter, the present invention will be described based on examples, but the contents of the present invention are not limited to these examples. The values shown in the examples were measured by the following methods. In the examples, parts and% are by weight unless otherwise specified. Various evaluations and measurements were performed according to the following methods.
(1) Volume recovery extrusion One minute after extrusion foaming, the length of the resin foam is cut to 1000 mm, the width, thickness, and length of the foam are accurately measured, and the foam is calculated by multiplying each dimension. Let the volume be Vo. After one hour after foaming, the resin foam was stored at a constant temperature. One week after foaming, the dimensions were measured again, and the volume calculated by multiplying each dimension was defined as V. The volume maintenance ratio (R) was calculated by the following formula, and based on the value, the volume recovery was evaluated according to the following criteria.
R (%) = V / Vo × 100
◎: R ≧ 95 (excellent volume recovery, very easy to maintain dimensions during manufacturing)
：: 90 ≦ R <95 (excellent volume recovery, easy to maintain dimensions during manufacturing)
×: R <90 (poor volume recovery, difficult to maintain dimensions during manufacturing)
(2) Gas replacement promoting effect One hour after foaming, the resin foam stored at a constant temperature is drawn out in a columnar shape in the thickness direction using a cork borer having an inner diameter of 16 mm to obtain a test piece. After quickly measuring the volume and weight of the test piece, it is immediately placed in a headspace bottle (manufactured by GL Sciences) whose internal volume has been measured, sealed, and heated and melted at 170 ° C. for 1.5 hours. Thereafter, the bottle was naturally cooled to room temperature, and then the gas in the bottle was separated and analyzed by a gas chromatograph (GC-14B, manufactured by Shimadzu Corporation). Using the calibration curve prepared in advance from the known gas concentration measurement, the volume of the test piece, the partial resin volume of the test piece, and the volume in the bottle, the residual foaming agent concentration in the foam is calculated as an average of n = 3. did. The concentration of the residual foaming agent in the foam and the effect of promoting the replacement with the inorganic gas were evaluated according to the following criteria.
：: Residual butane concentration reached below the lower limit of combustion range at one week after foaming. ：: Residual butane concentration reached below the lower limit of combustion range at two weeks after foaming. X: Residual butane concentration reached at two weeks after foaming. (3) Using a crystallization temperature and a melting temperature differential calorimeter (Perkin Elmer Co., Ltd., DSC-7) of the gas permeation modifier at 30 ° C. for 0.5 minute, and then at 10 ° C./min. The temperature was raised to 120 ° C. and held for 1 minute, and then the temperature of the highest peak when the temperature was lowered again to 30 ° C. at 10 ° C./min was defined as the crystallization temperature. Thereafter, the temperature was maintained at 30 ° C. for 1 minute, and the temperature of the lowest peak when the temperature was raised to 120 ° C. at 10 ° C./min was defined as the melting temperature. When a mixture of two or more gas permeation modifiers is used, the crystallization temperature of the component having the highest crystallization temperature measured alone is the crystallization temperature of the gas permeation modifier, and the melting temperature measured alone is the lowest. The melting temperature of the component was taken as the melting temperature of the gas permeation modifier.
(4) A rod-shaped contact-type thermocouple (3 mm in diameter of the temperature sensing portion) previously heated to the foaming temperature in order to prevent the temperature of the foam from dropping when the temperature of the center of the foam is inserted into the plate-like foam immediately after foaming Was vertically inserted to a depth of one half of the thickness of the foam at a portion which entered the inside of the foam from the outer peripheral portion to the thickness of the foam or more, and the temperature was measured to be the central temperature of the foam.
(5) Perforated Diameter of Foam The perforated resin foam was stored at a constant temperature one hour after foaming, and one week after foaming, the pore diameter was measured using a pin gauge prepared in 0.05 mm increments. The pin gauge was inserted to a half of the thickness of the foam, and the maximum diameter of the pin gauge having no resistance at that time was defined as the perforated diameter of the foam.
(6) The closed cell ratio of the foam was measured by an air pycnometer method described in ASTM-D2856 (manufactured by Tokyo Science Co., Ltd., using an air-comparison hydrometer 1000 type), and the average was calculated by n = 5. .
(7) Cell Size of Foam A test piece is cut from the center of the foam, and L (mm) straight lines are drawn on the cut surface along the extrusion direction, width direction, and thickness direction of the foam. The number of bubbles in contact was counted, the cell size in the extrusion direction, width direction, and thickness direction was calculated by the self-calculation method, and the average value in three directions was defined as the cell size (grid line method).
Cell size (mm) = 1.626 × L / number of bubbles [Example 1]
Low-density polyethylene (density 0.921 g / cm 3 , MI = 2.9 g / 10 min) was fed into the feed area of a screw-type extruder having a barrel inner diameter of 150 mm at a rate of 900 kg / hour based on 100 parts by weight of the resin. It was supplied together with 1.5 parts by weight of a cell regulator (talc) and 0.5 parts by weight of a gas permeation modifier (stearic acid monoglyceride: crystallization temperature 60.5 ° C., melting temperature 72.8 ° C.). The barrel temperature of the extruder was adjusted to 190 ° C. to 210 ° C., and 65% by weight of normal butane and 35% by weight of isobutane (lower limit of combustion range: 1.8 vol.%) From a foaming agent inlet attached to the tip of the extruder. 6.7 parts by weight of a foaming agent was injected into 100 parts by weight of the resin and mixed with the molten resin composition to obtain a foamable molten mixture. After cooling this foamable molten mixture to 108 ° C. with a cooling device attached to the outlet of the extruder, the orifice plate having an average thickness of about 3.4 mm and an opening having a width of about 215 mm is supplied from an orifice plate at room temperature and atmospheric pressure. The resin foam is continuously extruded and foamed in an atmosphere, and molded while adjusting the take-up speed of the resin foam, and has a thickness of 62 mm, a width of 600 mm, a length of 1000 mm, a cell size of 1.1 mm, a density of 38 kg / m 3 , and a closed cell rate. A 95% plate-like resin foam was obtained. Immediately after the temperature of the center of the foam drops to 50 ° C., from the top surface of the resin foam, a sword-shaped needle assembly tool in which the needles are arranged in a zigzag pattern composed of equilateral triangles having a side of 10.0 mm ( Using a perforation interval of 10.0 mm), a perforation process was performed to obtain a perforated resin foam. This perforated resin foam was stored in an environment of 40 ° C. from one hour after foaming, and the perforated diameter, volume recovery and gas displacement promoting effect were evaluated. Table 1 shows the results.
Embodiment 2
A butane consisting of 70% by weight of normal butane and 30% by weight of isobutane was used as a foaming agent, and 6.4 parts by weight of butane was press-fitted to 100 parts by weight of the resin. A plate-like resin foam having a width of 600 mm, a length of 1000 mm, a cell size of 1.1 mm, a density of 38 kg / m 3 and a closed cell ratio of 95% was obtained. Thereafter, using a sword-like needle assembly (interval of perforations: 11.0 mm) in which needles are arranged in a zigzag pattern composed of equilateral triangles each having a side of 11.0 mm, perforation is performed so that the perforation depth is 61 mm. A perforated resin foam was obtained in the same manner as in Example 1 except that the treatment was performed. This perforated resin foam was stored in an environment of 40 ° C. from one hour after foaming, and the perforated diameter, volume recovery and gas displacement promoting effect were evaluated. Table 1 shows the results.
Embodiment 3
As a foaming agent, 1.3 parts by weight of propane was added to 100 parts by weight of butane, and 5.9 parts by weight of butane was press-fitted to 100 parts by weight of resin. A plate-like resin foam having a size of 600 mm, a length of 1000 mm, a cell size of 1.1 mm, a density of 37 kg / m 3 and a closed cell ratio of 96% was obtained. Thereafter, a perforation process was performed in the same manner as in Example 2 to obtain a perforated resin foam. This perforated resin foam was stored in an environment of 40 ° C. from one hour after foaming, and the perforated diameter, volume recovery and gas displacement promoting effect were evaluated. Table 1 shows the results.
Embodiment 4
A butane composed of 80% by weight of normal butane and 20% by weight of isobutane was used as a foaming agent, and 6.5 parts by weight of butane was pressed into 100 parts by weight of the resin. A plate-like resin foam having a width of 600 mm, a length of 1000 mm, a cell size of 1.1 mm, a density of 38 kg / m 3 and a closed cell ratio of 95% was obtained. Thereafter, a method similar to that of Example 2 was used, except that a sword-like needle assembly (interval of perforations: 15.0 mm) in which needles were arranged in a staggered lattice pattern composed of equilateral triangles each having a side of 15.0 mm was used. A perforation process was performed to obtain a perforated resin foam. This perforated resin foam was stored in an environment of 40 ° C. from one hour after foaming, and the perforated diameter, volume recovery and gas displacement promoting effect were evaluated. Table 1 shows the results.
[Comparative Example 1]
A butane consisting of 50% by weight of normal butane and 50% by weight of isobutane was used as a foaming agent, and 6.2 parts by weight of butane was pressed into 100 parts by weight of a resin. A plate-like resin foam having a width of 600 mm, a length of 1000 mm, a cell size of 1.1 mm, a density of 38 kg / m 3 and a closed cell ratio of 94% was obtained. Thereafter, a method similar to that of Example 2 was used, except that a sword-like needle assembly (interval of perforations: 10.0 mm) in which needles were arranged in a staggered pattern composed of equilateral triangles having a side of 10.0 mm was used. To obtain a perforated resin foam. This perforated resin foam was stored in an environment of 40 ° C. from one hour after foaming, and the perforated diameter, volume recovery and gas displacement promoting effect were evaluated. Table 1 shows the results.
[Comparative Example 2]
A perforated resin foam was obtained in the same manner as in Comparative Example 1, except that the perforation treatment was performed immediately after the center temperature of the foam dropped to 75 ° C. This perforated resin foam was stored in an environment of 40 ° C. from one hour after foaming, and the perforated diameter, volume recovery and gas displacement promoting effect were evaluated. Table 1 shows the results.
[Comparative Example 3]
A butane composed of 30% by weight of normal butane and 70% by weight of isobutane was used as a foaming agent, and a pressure of 6.0 parts by weight with respect to 100 parts by weight of the resin was used. A plate-like resin foam having a width of 600 mm, a length of 1000 mm, a cell size of 1.1 mm, a density of 38 kg / m 3 and a closed cell ratio of 94% was obtained. Thereafter, a perforation treatment was performed in the same manner as in Comparative Example 1 to obtain a perforated resin foam. The perforated resin foam was stored in an environment of 75 ° C. one hour after foaming, and the perforated diameter, volume recovery, and gas replacement promoting effect were evaluated. Table 1 shows the results.
[Comparative Example 4]
A perforated resin foam was obtained in the same manner as in Comparative Example 3 except that the perforation treatment was not performed. This perforated resin foam was stored in an environment of 40 ° C. from one hour after foaming, and the perforated diameter, volume recovery and gas displacement promoting effect were evaluated. Table 1 shows the results.
[Table 1]
【The invention's effect】
The method for producing a thermoplastic resin foam of the present invention is a method for producing a foam by extrusion foaming using butane as a foaming agent, wherein a production method using isobutane as a main component of the foaming agent and a production method without perforation This is a production method in which the combustible foaming gas can be replaced with an inorganic gas such as air in a short period of time, and the volume can be sufficiently recovered even if the foam is perforated after foaming.
[Brief description of the drawings]
FIG. 1 is a needle arrangement diagram of a sword-shaped needle assembly in which needles are arranged in a houndstooth check pattern used in an embodiment of the present invention.
[Explanation of symbols]
a: Interval of perforation