JP3582671B2

JP3582671B2 - Method for producing biaxially oriented polyester film

Info

Publication number: JP3582671B2
Application number: JP07827695A
Authority: JP
Inventors: 勝也豊田; 克俊宮川; 研二綱島
Original assignee: Toray Industries Inc
Current assignee: Toray Industries Inc
Priority date: 1995-03-08
Filing date: 1995-03-08
Publication date: 2004-10-27
Anticipated expiration: 2019-10-27
Also published as: JPH08244112A

Description

【０００１】
【産業上の利用分野】
本発明は、機械特性、熱寸法安定性に優れ、しかも厚み均一性に優れた、磁気記録媒体用、プリンターリボン用、コンデンサー用、包装用などとして好適な二軸配向ポリエステルフィルムの製造方法に関するものである。
【０００２】
【従来の技術】
ポリエステルフィルムは、その優れた機械特性、熱寸法安定性および電気特性から、磁気記録媒体用、プリンターリボン用、コンデンサー用、包装用など様々な工業用途で用いられ、需要量も増大してきている。ポリエステルの中でも、ポリエチレンテレフタレートやポリエチレンナフタレートは、その機械特性、熱特性が特に優れ、またポリエチレンテレフタレートは低価格であることから、広範囲に利用されている。
【０００３】
これらの利用分野の中の磁気記録媒体用途においては、記録時間の長時間化、テープを収納するカセットのコンパクト化に伴い、磁気テープの総厚みが年々薄くなる傾向にある。テープの厚みが薄くなると、腰が無くなり、テープの磁性面とビデオデッキの記録・再生ヘッドとの接触状態が不良となり、電磁変換特性が悪化するという問題が生じる。これを解決するためは、支持体であるベースフィルムの機械特性を上げる必要がある。また一方では、高密度記録化も進められており、トラックピッチが非常に狭くなってきていることから、従来では問題にならなかった小さい熱寸法変化が記録のずれを生じさせるために、一層の熱寸法安定性が要求されている。
【０００４】
また、プリンターリボン用途においても、年々印字速度の高速化が求められ、それにはベースフィルムの熱伝達を向上させる必要があり、このために薄膜化が求められている。磁気記録媒体用途同様、薄膜化するとフィルムの剛性が低下するために、ベースフィルムの一層の機械特性の向上が求められている。また高速印字化のために、印字ヘッドの温度も高温化し、これに接するベースフィルムに要求される熱寸法安定性も年々厳しくなってきている。
【０００５】
ポリエステルフィルムの機械特性を上げる手法として、縦方向、横方向に延伸した二軸延伸フィルムをさらに縦方向に延伸する再縦延伸法（例えば特公昭３４−５８８７号公報）や、再縦延伸後さらに再横延伸する方法（例えば特開昭５０−１３３２７６号公報、特開昭５５−２２９１５号公報）、あるいは縦・横方向に延伸した二軸延伸フィルムを縦・横両方向に同時に再延伸する方法（例えば特開昭５５−３７３０５号公報、特開昭５５−２７２１１号公報）が知られている。また最近では再縦・再横延伸を必要とせず、縦延伸を２段階に分けて延伸する方法（例えば特開昭６１−２４１１２８号公報、特開昭６１−２４２８２４号公報）が提案されている。しかし、これらの手法では、確かに機械特性の向上は図れるものの、もう一方の要求特性である熱寸法安定性が著しく悪化してしまうという問題がある。
【０００６】
また、ポリエステルフィルムの熱寸法安定性を向上させる手段としては、製膜工程の熱処理工程で、横方向または／および縦方向に弛緩させながら熱処理を施す方法、二軸延伸熱処理後のフィルムをインラインまたはオフラインで、横方向または／および縦方向に弛緩させながら再熱処理する方法が一般に知られている。しかるに、このような弛緩熱処理を施す方法は、確かに熱寸法安定性は向上するものの、もう一方の要求特性である機械特性の向上が図れないという欠点がある。さらに、過度の弛緩熱処理はフィルムの平面性をも悪化させてしまう。
【０００７】
このように、フィルムの機械特性と熱寸法安定性の向上は、各分野において年々厳しく要求される特性であるものの、互いに相反する特性であり、両者を満足する手法は今までに見い出されてはなかった。
【０００８】
また、厚み均一性を向上させる手段としては、溶融ポリマーを冷却固化する冷却ドラムの回転むらを抑える方法（例えば特開昭５５−９３４２０号公報）や、溶融ポリマーを冷却ドラム上に静電気力で密着させる際に、静電気力を受け易いようにポリマーを改質する方法（例えば特開昭５９−９１１２１号公報）が提案されているが、いずれも効果が充分でない。
【０００９】
【発明が解決しようとする課題】
本発明の目的は、上記課題を解決し、機械特性および熱寸法安定性の両方を高度に満足し、しかも厚み均一性に優れたポリエステルフィルムを製造する方法を提供することにある。
【００１０】
【課題を解決するための手段】
この目的に沿う本発明の二軸配向ポリエステルフィルムの製造方法は、縦方向の屈折率（ｎ_ＭＤ）から横方向の屈折率（ｎ_ＴＤ）を引いた値（ｎ_ＭＤ−ｎ_ＴＤ）が、０．１×１０^−３以上、１０×１０^−３以下の範囲にあるキャストフィルムを、縦方向に少なくとも２段階以上に分けて延伸した後、横方向に延伸することを特徴とする方法からなる。
【００１１】
本発明に適用されるポリエステルとは、ジオールとカルボン酸から縮重合により得られるエステル基を主鎖にもつポリマーであり、ジカルボン酸としては、テレフタル酸、イソフタル酸、ジフェン酸、フタル酸、ナフタレンジカルボン酸、アジピン酸、セバチン酸、ダイマー酸、エイコ酸、ドデカンジオン酸などで代表されるものでありまた、ジオールとは、エチレングリコール、トリメチレングリコール、テトラメチレングリコール、ビスフェノールなどで代表されるものである。具体的には、例えばポリエチレンテレフタレート、ポリテトラメチレンテレフタレート、ポリエチレン−ｐ−オキシベンゾエート、ポリ−１，４−シクロへキシレンジメチレンテレフタレート、ポリエチレン−２，６−ナフタレートなどが挙げられる。もちろん、これらのポリエステルは、ホモポリマーであってもコポリマーであってもよく、共重合成分としては、例えばジエチレングリコール、ネオペンチルグリコール、ポリアルキレングリコールなどのジオール成分、アジピン酸、セバチン酸、フタル酸、イソフタル酸、２，６−ナフタレンジカルボン酸などのジカルボン酸成分が挙げられる。本発明の場合、特にポリエチレンテレフタレート、ポリエチレン−２，６−ナフタレートが機械的強度、耐熱性、耐薬品性、耐久性、汎用性などの観点から好ましい。
【００１２】
また、本発明のポリエステルフィルムには、本発明の効果を阻害しない範囲であれば、公知の各種添加剤、例えば酸化防止剤、帯電防止剤、結晶化剤、無機粒子などが添加されていてもかまわない。
【００１３】
次に本発明のポリエステルフィルムの製造方法について詳細に説明する。
ポリエステル樹脂のペレットを充分乾燥脱水した後、公知の押出機に供給する。押出機内では、樹脂は、示差走査熱量測定法（ＤＳＣ）から求められる、融解時の吸熱ピークの終了温度（Ｔｍｅ）以上に加熱して溶融状態にする必要がある。温度が融解時の吸熱ピークの開始温度（Ｔｍｂ）よりも低いと樹脂は流動性がほとんどなく、押出できない。また温度がＴｍｂ以上でも、Ｔｍｅ未満であれば未溶融物が残るため、そのままではフィルターの目詰まり、成形後のフィルムの異物欠点等が生じるため好ましくない。従って樹脂の加熱溶融は未溶融物のない完全な溶融状態にするためにＴｍｅ以上、好ましくはＴｍｅ＋１０℃以上の温度で行う必要がある。ダイに送られた樹脂はダイで目的の形状に成形された後、押出され、回転する冷却ロール上で急冷固化されて、キャストフィルムとなる。
【００１４】
本発明のキャストフィルムは、縦方向の屈折率（ｎ_ＭＤ）から横方向の屈折率（ｎ_ＴＤ）を引いた値（ｎ_ＭＤ−ｎ_ＴＤ）の下限が、０．１×１０^−３、好ましくは０．３×１０^−３、さらに好ましくは０．５×１０^−３である必要がある。（ｎ_ＭＤ−ｎ_ＴＤ）の値が０．１×１０^−３未満であると、通常のキャストフィルムと差異がなくなり、本発明の機械特性と熱寸法安定性の両立効果が発現されないので、（ｎ_ＭＤ−ｎ_ＴＤ）の値は０．１×１０^−３以上であることが必要である。また（ｎ_ＭＤ−ｎ_ＴＤ）の値の上限は、１０×１０^−３、好ましくは７×１０^−３、さらに好ましくは５×１０^−３であるのが好ましい。（ｎ_ＭＤ−ｎ_ＴＤ）の値が１０×１０^−３よりも大きくなると、続く二軸延伸性が悪化してしまい、厚みむらの悪化、延伸破れを引き起し、かえって機械特性の向上したフィルムが得られないため、（ｎ_ＭＤ−ｎ_ＴＤ）の値は１０×１０^−３以下であることが必要である。
【００１５】
かかるキャストフィルムを得る手法としては、樹脂をダイでＴｍｅ未満、降温結晶化開始温度（Ｔｃｂ）を越える温度に冷却して押出す方法が、最も好ましく用いられる。通常のようにＴｍｅ以上の温度で押出し、キャストする方法では、得られたキャストフィルムは完全未配向となり、縦方向に配向したフィルムを得ることは困難である。しかるに、このように冷却、押出する方法によると容易に縦方向に配向したキャストフィルムを得ることができる。ポリエステルは結晶化速度が比較的遅く、溶融状態からＴｍｅ未満に冷却しても短時間では固化しない、いわゆる過冷却の液相状態を保つことができる。本発明ではポリエステルのもつこの特性を利用して、過冷却の液相状態で押出するため、ダイ内で樹脂を固化させない。そのため冷却は樹脂のＴｃｂを越える温度までに留める必要がある。Ｔｃｂ以下の低い温度では樹脂が結晶化し始め、押出されたフィルムの表面荒れ、押出異常、流れむらを生じたり、経時で固化し、もはや押出不可能となるため好ましくない。
【００１６】
冷却はダイのランド部で行われることが好ましい。もし冷却が樹脂がダイに入る以前に行なわれると、粘度の上昇、流動性の悪化が生じてしまい、その結果、押出異常や流れ異常が生じたり、押出機、フィルター、ギヤポンプに負荷をかけ、変形または寿命の低下を引き起こすので好ましくない。またダイ中でもランド部以前（ダイホッパー部）で冷却を行うことは、樹脂が目的の形に成形される過程であり、温度むら、流れ異常を生じる原因となるため、好ましくない。特にフラットダイは樹脂の流路長が幅方向で異なるため、冷却時間の違いから熱履歴が均一でなくなり、幅方向の温度むらが生じたりするため、成形性が悪化したり、厚みむらが悪くなる場合もあるため好ましくない。
【００１７】
これに対し、冷却をダイのランド部で行うことは、樹脂が幅方向に拡大され、押出される形状に成形された後での冷却となり、均一な冷却が可能となる。ランド部はダイ中の最も間隙の狭い部分であり、熱交換効率が優れており好適である。また樹脂は冷却後、すぐに押し出されるため、粘度上昇に伴う濾圧上昇、押出異常も最小限に抑えることができる。さらに、厚みの厚いキャストフィルムを得るためには、複数の幅方向に拡大され、冷却された樹脂を押出前に厚み方向に重ね合せることも好ましく行われる。
【００１８】
本発明では、樹脂は冷却過渡状態でダイランド部より押出することが好ましい。ここで冷却過渡状態とは、冷却過程で樹脂が定常温度に達していない状態をいう。冷却過渡状態にあることにより、厚みの厚いエッジ部が比較的温度が高い状態に残され、エッジ部からの固化を抑えられるため好ましい。
【００１９】
本発明におけるポリエステル樹脂の融解終了温度（Ｔｍｅ）、降温結晶化開始温度（Ｔｃｂ）はＤＳＣによって決定される。ＤＳＣとは熱分析で通常用いられる示差走査熱量測定法のことであり、物質の融解、結晶化、相転移、熱分解等の状態変化に伴う吸熱、発熱を測定する方法である。ＤＳＣによってポリエステル樹脂の昇温時の融解温度、降温時の結晶化速度を測定する場合、公知の方法を用いることをができるが、ここで注意する点は測定時の昇温、降温速度である。実際の押出条件を想定すると、好適な昇・降温速度としては、通常１０〜３０℃／分である。
【００２０】
ダイから押出されたポリエステルシートは、冷却ドラム上で急冷固化せしめることで、分子配向の主軸が縦方向であるキャストフィルムを容易に得ることができる。この際、静電気を印加してキャストする方法が好ましく用いられる。かかる手法で得られたキャストフィルムは、溶融状態から過冷却液相状態を経て、溶融粘度が高まった状態で押出され、ダイと冷却ドラム間でのドラフトのために、分子鎖が縦方向に無理なく整列されているため、二軸延伸後も分子鎖の歪みが少ない状態となり、機械特性、熱寸法安定性の両面から好ましい構造となる。
【００２１】
またポリマーが通常よりも、高粘度化されているために高剛性となり、回転する冷却ドラムの随伴気流が主な原因である、ダイと冷却ドラム間のポリマー膜振動が大幅に減少し、厚み均一性に優れるフィルムを得ることができる。かかる手法で得られたキャストフィルムは二軸延伸後も厚み均一性の優れたものとなる。
【００２２】
該キャストフィルムは、続いて、二軸延伸に供せられる。まず、縦方向の延伸を行う。縦方向の延伸は、周速差のある複数の加熱ロール群からなる通常のロール延伸機により行われる。縦方向の延伸は、少なくとも２段階以上に分けて行われることが必要である。本発明の場合、１段目の縦延伸温度の下限は、ポリエステルのガラス転移温度（Ｔｇ）＋２０℃、好ましくはＴｇ＋２５℃、上限はＴｇ＋５０℃、好ましくはＴｇ＋４０℃である。この温度範囲で１段目の縦延伸を行うことにより、機械特性および熱寸法安定性の向上を図ることができる。その理由は、この温度範囲で延伸を行うことにより、ポリエステルフィルムはマクロ的にはほとんど配向しないが、ミクロ的には分子鎖のベンゼン環が安定にスタックした、いわゆる微結晶核を多数生成することができるためである。この微結晶核は、いわば分子鎖の結節点の役目を果し、続く２段目以降の縦延伸により、フィルムを高度に配向させることができ、機械特性を高めることができる。１段目の縦延伸温度がＴｇ＋２０℃よりも低いと、その安定スタック構造の生成が不充分となり、機械特性の向上効果が少なくなるため好ましくない。またＴｇ＋５０℃よりも高い温度では、フィルムの剛性が少なくなりすぎて、フィルムの厚みむらが著しく悪化してしまうため好ましくない。また１段目の縦延伸の延伸倍率の下限は１．５倍、好ましくは１．７倍、上限は３．０倍、好ましくは２．５倍である。延伸倍率が１．５倍よりも低いと安定スタック構造の生成が不充分となり、機械特性の向上効果が少なくなるため好ましくない。また３．０倍よりも高いと、厚みむらの悪化が激しくなるため好ましくない。１段目の縦延伸は１段で行ってもよく、また多段で行ってもよい。多段で行う場合でも、温度範囲がＴｇ＋２０℃以上、Ｔｇ＋５０℃以下で行う延伸は全て含めて、１段目の延伸と見なす。この場合、各段の延伸倍率の積が、１．５〜３．０倍の範囲に入るようにすればよい。
【００２３】
かくして得られた１段目縦延伸フィルムは、引続き、同方向に２段目以降の延伸に供せられる。２段目以降の縦延伸は、ほとんど配向していない１段目縦延伸フィルムに高度な配向を与えるのが、その役目である。２段目以降の縦延伸においては、１段目で生成させた安定スタック構造を緩和させず、機械特性を向上させるために、１段目の縦延伸温度よりも低い温度で行うことが好ましい。２段目以降の縦延伸はもちろん１段で行ってもよく、また多段で行ってもよい。
【００２４】
１段目の縦延伸と２段目の縦延伸に至る温度過程は、単調に低温化するのが好ましい。こうすることにより、厚いエッジ部が高温のまま残り、延伸白化などのトラブルを防止することができる。
【００２５】
また１段目の縦延伸、２段目以降の縦延伸の各延伸倍率の積で表すことができる全縦延伸倍率の下限は４．０倍、好ましくは４．５倍、上限は７．０倍、好ましくは６．５倍の範囲から選択する。全縦延伸倍率が４．０倍よりも低いと、得られる二軸配向フィルムの機械特性が向上しないので好ましくない。逆に７．０倍よりも高いと、フィルムの横延伸性が悪化し、厚みむらの悪化を招くばかりか、フィルム破れを生じ、安定して製品が得られないため好ましくない。
【００２６】
かくして得られた一軸延伸フィルムは、フィルムの両端をクリップで把持しながらテンターに導かれ、Ｔｇ以上、Ｔｇ＋６０℃以下の温度範囲で横方向に３．０以上６．０倍以下の範囲で延伸される。横延伸は１段で行ってもよく、多段で行ってもよい。多段で行う場合には、後段の横延伸ほど高温に設定することが、安定製膜のために好ましい。引続き、かくして得られたフィルムは、ポリエステルの融解開始温度（Ｔｍｂ）−６０℃以上、Ｔｍｂ−５℃以下の温度範囲で、０．５秒以上熱処理を施される。熱処理は横延伸と同一のテンターで行ってもよく、またフィルム端部を一旦解放し、再把持して別のテンターで行ってもよい。また熱処理は、緊張下で行ってもよく、熱寸法安定性を向上させるために、横方向に弛緩させながら行ってもよい。しかしながら、過度の弛緩は二軸配向フィルムの機械特性を著しく悪化させるため、弛緩率の上限は８％、好ましくは５％に留めておくのが好ましい。
【００２７】
二軸配向フィルムの機械特性をさらに高めるために、上記横延伸と熱処理の間に、再縦延伸および／または再横延伸を施すことも好ましく行われる。再縦延伸は、特に限定されないが、横延伸温度よりも高い温度で行うことが安定製膜の点から好ましい。さらに再横延伸を行う場合には、延伸温度は、再縦延伸を行わない場合は前工程の横延伸温度よりも高い温度、再縦延伸を行う場合には再縦延伸温度よりも高い温度、好ましくは１８０℃以上、Ｔｍｂ−５℃以下の高温で行うと、破れ等のトラブルなしに、かつ横方向強度を上げるために有効である。
【００２８】
なお、本発明の二軸配向フィルムの厚みの上限は２０μｍ、好ましくは１５μｍ、より好ましくは１０μｍという薄いフィルムの場合に、本発明の効果である機械特性、熱寸法安定性の向上、厚み均一性の向上という効果が一層顕著化し、磁気記録媒体用、プリンターリボン用、コンデンサー用、包装用などとして好適なフィルムとなるため好ましい。また本発明の二軸配向フィルムの厚みの下限は０．１μｍ、好ましくは０．３μｍ、より好ましくは０．５μｍであるのが、破れなく安定に製膜でき、得られた二軸配向フィルムの後加工などの際のハンドリングも容易であるという点から好ましい。
【００２９】
本発明の二軸配向ポリエステルフィルムの、縦方向と横方向のヤング率の和の下限は１２００ｋｇ／ｍｍ^２、好ましくは１３００ｋｇ／ｍｍ^２、より好ましくは１５００ｋｇ／ｍｍ^２である場合に、薄膜化した際の機械特性向上効果が著しいため好ましい。また本発明による二軸配向ポリエステルフィルムは、従来では困難であった特に縦方向の熱寸法安定性向上に著しい効果があり、縦方向の熱収縮率の上限が１．５％、好ましくは１．２％、より好ましくは１．０％である場合に、磁気記録媒体用、プリンターリボン用などに供した場合に好適である。また厚み均一性に関しては、薄膜化した場合ほど要求特性が厳しくなり、厚みむらの上限は３．０％、好ましくは２．０％、より好ましくは１．５％である場合に、磁気記録媒体用に供した場合の電磁変換特性の悪化、プリンターリボン用として用いた場合の印字ミスなどを招かずに好ましい。
【００３０】
［物性の測定方法］
（１）屈折率
アタゴ社製アッベ屈折率計により、マウント液としてヨウ化メチレンを用いて、２３℃、６５％ＲＨの雰囲気下で、フィルムの縦方向屈折率ｎ_ＭＤ、横方向屈折率ｎ_ＴＤを求めた。
【００３１】
（２）熱特性
マックサイエンス社製示差走査熱量計ＤＳＣ３１００を用いて、サンプル５ｍｇを３００℃で５分間溶融保持し、液体窒素で急冷固化した後、室温から昇温速度２０℃／分で昇温した。この時観測される結晶融解吸熱ピークの開始温度をＴｍｂ、ピーク温度をＴｍ、ピーク終了温度をＴｍｅとした。また、サンプル５ｍｇを３００℃で５分間溶融保持した後、降温速度２０℃／分で降温した。この時観測される降温結晶化発熱ピークの開始温度をＴｃｂ、ピーク温度をＴｃ、ピーク終了温度をＴｃｅとした。
【００３２】
（３）フィルムのヤング率
オリエンテック社製テンシロン型引張試験機に幅１０ｍｍのサンプルフィルムをチャック間長さ１００ｍｍとなるようにセットし、２３℃、６５％ＲＨの雰囲気下で、引張速度２００ｍｍ／分で引張試験を行い求めた。
【００３３】
（４）熱収縮率
１０ｍｍ幅に切出したフィルムに、測定長約２００ｍｍとなるように２本のラインを引き、この２本のライン間距離を正確に測定し、これをＬ０とする。このサンプルを１５０℃に加熱されたオーブン中に３０分間、無荷重で放置後、再び２本のライン間距離を正確に測定し、これをＬ１とする。次式により、熱収縮率を求めた。
熱収縮率（％）＝｛（Ｌ０−Ｌ１）／Ｌ０｝×１００
【００３４】
（５）フィルムの厚みむら
アンリツ社製フィルムシックネステスターＫＧ６０１Ａおよび電子マイクロメーターＫ３０６Ｃを用い、縦方向に３００ｍｍ幅、１０ｍ長にサンプリングしたフィルムを連続的に厚みを測定する。１０ｍ長での厚み最大値Ｔｍａｘ（μｍ）、最小値Ｔｍｉｎ（μｍ）から、
Ｒ＝Ｔｍａｘ−Ｔｍｉｎ
を求め、Ｒと１０ｍ長の平均厚みＴａｖｅ（μｍ）から、
厚みむら（％）＝Ｒ／Ｔａｖｅ×１００
として求めた。
【００３５】
（６）ポリマー温度
ダイ内のポリマー温度は、測定したい箇所に棒状の熱電対を挿入する孔を開けて、熱電対を挿入し、ポリマーの漏れを防ぐシールを施して測定した。また、ダイのランド部出口の温度は、押出されるポリマーの温度をダイ直下で熱電対により測定した。
【００３６】
【実施例】
本発明を実施例に基づいて説明する。
実施例１〜６
ポリエステルとして、固有粘度０．６５のポリエチレンテレフタレート（滑り材として直径０．２５μｍの真円球であるコロイダルシリカを０．３５重量％添加）を用いた。ＤＳＣを用いて熱特性を測定したところ、Ｔｇ：８０℃，Ｔｍｂ：２４０℃，Ｔｍ：２５５℃，Ｔｍｅ：２６８℃，Ｔｃｂ：２０３℃，Ｔｃ：１８８℃，Ｔｃｅ：１７４℃であった。該チップを１８０℃で３時間真空乾燥して押出機（口径９０ｍｍ）に供給し、２９０℃で溶融状態とし、成形用ダイに供給した。ダイはリップ間隙１ｍｍ、幅４００ｍｍ、ランド長１００ｍｍのマニホールドダイを用いた。本ダイのランド部には、幅方向に直径７ｍｍの空孔を複数あけ、ここに冷媒を通すことにより冷却可能な構造としてある。ダイホッパー部の温度は２９０℃とし、冷媒として２５℃の冷却水を用い、通水量を変えてポリマー温度を変化させた。この状態でポリマーを押出し、ダイから押出された該フィルムに静電気を印加させながら、表面温度２５℃の冷却ドラム上で密着冷却固化せしめ、キャストフィルムを得た。該キャストフィルムを、ロール縦延伸機に導入し、縦方向に１１０℃で２．０倍、１段目の縦延伸を施した。引続き、フィルムを単調に降温させ、８０℃で３．０倍、２段目の縦延伸を施した。該縦延伸フィルムの端部をクリップで把持しながら、ステンターに導入し、８５℃で３．５倍、横延伸を施し、同一のステンターにて横方向に３％弛緩しながら、２１０℃で熱処理を行い、厚さ７μｍの二軸配向フィルムを得た。キャストフィルム、二軸配向フィルムの物性を表１に示す。得られた二軸配向フィルムは、機械特性と熱寸法安定性を高度に満足し、しかも厚み均一性に優れたものであった。
【００３７】
比較例１
実施例１〜６と同一のポリエチレンテレフタレート、ダイ、ロール縦延伸機、ステンターを用い、ダイランド部に冷却水を通さずに、押出ポリマー温度を、ダイマニホールド部と同じ２９０℃になるようにする以外は、実施例１〜６と全く同様にして厚さ７μｍの二軸配向フィルムを得た。キャストフィルム、二軸配向フィルムの物性を表１に示す。得られたキャストフィルムは全く配向しておらず、二軸配向フィルムは機械特性、熱寸法安定性とも不充分であり、しかも厚み均一も劣ったものであった。
【００３９】
比較例２
実施例１〜６と同一のポリエチレンテレフタレート、ダイを用い、通水量を変えて押出ポリマー温度を２００℃にする以外は、実施例１〜６と全く同様にして製膜を試みた。しかるに、ポリマーが急激に固化し始め、キャストフィルムを得ることはできなかった。
【００４０】
比較例３
実施例１〜６と同一のポリエチレンテレフタレート、ダイ、ロール縦延伸機、ステンターを用い、押出ポリマー温度を２４０℃にし、（ｎ_MD−ｎ_TD）の値が１．０×１０^-3のキャストフィルムを得た。該キャストフィルムを、ロール縦延伸機に導入し、縦方向に８０℃で３．２倍、１段階で縦延伸を施した。該縦延伸フィルムの端部をクリップで把持しながら、ステンターに導入し、８５℃で３．５倍、横延伸を施し、同一のステンターにて横方向に３％弛緩しながら、２１０℃で熱処理を行い、厚さ７μｍの二軸配向フィルムを得た。二軸配向フィルムの物性を表１に示す。得られた二軸配向フィルムは、機械特性と熱寸法安定性が不充分であり、厚み均一も劣ったものであった。
【００４１】
比較例４
比較例３のうち、縦延伸倍率を３．４倍にする以外は全く同様にして、製膜を試みたが、ステンターで破れを生じ、二軸配向フィルムを得ることはできなかった。
【００４２】
比較例５
実施例１〜６と同一のポリエチレンテレフタレート、ダイ、ロール縦延伸機、ステンターを用い、押出ポリマー温度を２４０℃にし、（ｎ_MD−ｎ_TD）の値が１．０×１０^-3のキャストフィルムを得た。該キャストフィルムを、ロール縦延伸機に導入し、縦方向に１１０℃で３．８倍、１段階で縦延伸を施した。該縦延伸フィルムの端部をクリップで把持しながら、ステンターに導入し、８５℃で３．５倍、横延伸を施し、同一のステンターにて横方向に３％弛緩しながら、２１０℃で熱処理を行い、厚さ７μｍの二軸配向フィルムを得た。二軸配向フィルムの物性を表１に示す。得られた二軸配向フィルムは、機械特性が全く向上しておらず、厚み均一も劣ったものであった。
【００４３】
比較例６
比較例５のうち、縦延伸倍率を４．０倍にする以外は全く同様にして、製膜を試みたが、ステンターで破れを生じ、二軸配向フィルムを得ることはできなかった。
【００４４】
実施例７
実施例３の二軸延伸フィルムを、熱処理前に、１６０℃で１．２倍の再縦延伸、さらに２００℃で１．５倍の再横延伸を施し、実施例１〜６と全く同様の条件で熱処理を施し、厚さ４μｍの二軸配向フィルムを得た。得られた二軸配向フィルムは機械特性が大きく向上し、その割には熱寸法安定が悪化せず、また、厚み均一性に非常に優れたものであった。
【００４５】
比較例７
比較例１の二軸延伸フィルムを、熱処理前に、１６０℃で１．２倍の再縦延伸、さらに２００℃で１．５倍の再横延伸を施し、実施例１〜６と全く同様の条件で熱処理を施し、厚さ４μｍの二軸配向フィルムを得た。得られた二軸配向フィルムは機械特性、熱寸法安定性、厚み均一性が実施例７よりも劣ったものであった。
【００４６】
【表１】

【００４７】
【発明の効果】
本発明は縦方向の屈折率（ｎ_ＭＤ）から横方向の屈折率（ｎ_ＴＤ）を差し引いた値（ｎ_ＭＤ−ｎ_ＴＤ）が、０．１×１０^−３以上、１０×１０^−３以下の範囲にあるキャストフィルムを、縦方向に少なくとも２段階以上に分けて延伸した後、横方向に延伸することを特徴とする二軸配向ポリエステルフィルムの製造方法であり、上記のような製造条件をとることによって、機械特性と熱寸法安定性を同時に満足したフィルムを供給することができる。また該フィルムは、厚み均一性にも優れたものとなる。本製造方法により得られるポリエステルフィルムは、磁気記録媒体用、プリンターリボン用、コンデンサー用、包装用など様々な工業材料分野に利用可能である。[0001]
[Industrial applications]
The present invention relates to a method for producing a biaxially oriented polyester film having excellent mechanical properties, thermal dimensional stability, and excellent thickness uniformity, and suitable for magnetic recording media, printer ribbons, capacitors, packaging, and the like. It is.
[0002]
[Prior art]
Due to their excellent mechanical properties, thermal dimensional stability, and electrical properties, polyester films are used in various industrial applications such as magnetic recording media, printer ribbons, capacitors, and packaging, and the demand is increasing. Among polyesters, polyethylene terephthalate and polyethylene naphthalate are widely used because of their excellent mechanical properties and thermal properties, and because polyethylene terephthalate is inexpensive.
[0003]
In the use of magnetic recording media in these fields of use, the total thickness of magnetic tapes tends to decrease year by year with the increase in recording time and the compactness of cassettes for accommodating tapes. When the thickness of the tape is reduced, the tape loses its rigidity, the contact state between the magnetic surface of the tape and the recording / reproducing head of the VCR becomes poor, and there arises a problem that the electromagnetic conversion characteristics deteriorate. In order to solve this, it is necessary to improve the mechanical properties of the base film as the support. On the other hand, high-density recording has also been promoted, and the track pitch has become extremely narrow. Therefore, a small change in thermal dimension, which has not been a problem in the past, causes a shift in recording. Thermal dimensional stability is required.
[0004]
Further, in printer ribbon applications, a higher printing speed is required year by year, and it is necessary to improve the heat transfer of the base film. For this purpose, a thinner film is required. As in the case of magnetic recording media, when the thickness is reduced, the rigidity of the film is reduced. Therefore, further improvement in mechanical properties of the base film is required. Further, the temperature of the print head has been increased to achieve high-speed printing, and the thermal dimensional stability required for the base film in contact with the print head has been becoming stricter year by year.
[0005]
As a technique for improving the mechanical properties of the polyester film, a re-longitudinal stretching method in which a biaxially stretched film stretched in the machine direction and the transverse direction is further stretched in the machine direction (for example, JP-B-34-5887), A method of re-horizontal stretching (for example, JP-A-50-133276, JP-A-55-22915), or a method of simultaneously re-stretching a biaxially stretched film stretched in the vertical and horizontal directions in both the vertical and horizontal directions ( For example, JP-A-55-37305 and JP-A-55-27211 are known. Recently, there has been proposed a method in which longitudinal stretching and stretching in two stages are performed without re-longitudinal / re-lateral stretching (for example, JP-A-61-241128 and JP-A-61-242824). . However, although these techniques can certainly improve the mechanical properties, there is a problem that the other required property, that is, the thermal dimensional stability, is significantly deteriorated.
[0006]
Means for improving the thermal dimensional stability of the polyester film include a method of performing a heat treatment while relaxing in a horizontal direction or / and a vertical direction in a heat treatment step of a film forming step, or in-line or in-line the film after the biaxial stretching heat treatment. It is generally known to re-heat-treat off-line while relaxing in the horizontal and / or vertical directions. However, although the method of performing such a relaxation heat treatment certainly improves the thermal dimensional stability, it has a drawback that the other required characteristic, mechanical characteristics, cannot be improved. Furthermore, excessive relaxation heat treatment also deteriorates the flatness of the film.
[0007]
As described above, the improvement of the mechanical properties and thermal dimensional stability of the film is a property that is strictly required every year in each field, but they are mutually contradictory properties, and a method that satisfies both has not been found so far. Did not.
[0008]
Means for improving the thickness uniformity include a method of suppressing the rotation unevenness of the cooling drum for cooling and solidifying the molten polymer (for example, Japanese Patent Application Laid-Open No. 55-93420), and a method of bringing the molten polymer into close contact with the cooling drum by electrostatic force. At this time, a method of modifying a polymer so as to be easily affected by electrostatic force (for example, JP-A-59-91121) has been proposed, but none of them has sufficient effects.
[0009]
[Problems to be solved by the invention]
An object of the present invention is to solve the above-mentioned problems, and to provide a method for producing a polyester film which satisfies both mechanical properties and thermal dimensional stability at a high level and has excellent thickness uniformity.
[0010]
[Means for Solving the Problems]
According to the method for producing a biaxially oriented polyester film of the present invention for this purpose, the value obtained by subtracting the refractive index in the horizontal direction (n _TD ) from the refractive index in the vertical direction (n _MD ) (n _MD −n _TD ) is 0. The method is characterized in that a cast film in the range of 1 × 10 ⁻³ or more and 10 × 10 ⁻³ or less is stretched in at least two stages in the longitudinal direction and then stretched in the transverse direction.
[0011]
The polyester applied to the present invention is a polymer having an ester group obtained by condensation polymerization from a diol and a carboxylic acid in the main chain. Examples of the dicarboxylic acid include terephthalic acid, isophthalic acid, diphenic acid, phthalic acid, and naphthalenedicarboxylic acid. Acids, adipic acid, sebacic acid, dimer acid, eicoic acid, dodecanedioic acid, etc., and diols are represented by ethylene glycol, trimethylene glycol, tetramethylene glycol, bisphenol, etc. is there. Specific examples include polyethylene terephthalate, polytetramethylene terephthalate, polyethylene-p-oxybenzoate, poly-1,4-cyclohexylene dimethylene terephthalate, and polyethylene-2,6-naphthalate. Of course, these polyesters may be a homopolymer or a copolymer, and as the copolymerization component, for example, diol components such as diethylene glycol, neopentyl glycol, and polyalkylene glycol, adipic acid, sebacic acid, phthalic acid, And dicarboxylic acid components such as isophthalic acid and 2,6-naphthalenedicarboxylic acid. In the case of the present invention, polyethylene terephthalate and polyethylene-2,6-naphthalate are particularly preferred from the viewpoint of mechanical strength, heat resistance, chemical resistance, durability, versatility and the like.
[0012]
Further, the polyester film of the present invention may include various known additives such as an antioxidant, an antistatic agent, a crystallization agent, and inorganic particles as long as the effects of the present invention are not impaired. I don't care.
[0013]
Next, the method for producing the polyester film of the present invention will be described in detail.
After the polyester resin pellets are sufficiently dried and dehydrated, they are supplied to a known extruder. In the extruder, the resin must be heated to a temperature higher than the end temperature (Tme) of the endothermic peak at the time of melting, which is determined by differential scanning calorimetry (DSC). If the temperature is lower than the onset temperature (Tmb) of the endothermic peak at the time of melting, the resin has little fluidity and cannot be extruded. Further, even if the temperature is equal to or higher than Tmb, if the temperature is lower than Tme, an unmelted substance remains, which is not preferable because clogging of the filter and defects of foreign matters in the formed film occur as it is. Therefore, the resin must be heated and melted at a temperature equal to or higher than Tme, preferably equal to or higher than Tme + 10 ° C. in order to obtain a completely molten state without unmelted material. The resin sent to the die is formed into a desired shape by the die, extruded, and quenched and solidified on a rotating cooling roll to form a cast film.
[0014]
In the cast film of the present invention, the lower limit of the value obtained by subtracting the refractive index in the horizontal direction (n _TD ) from the refractive index in the vertical direction (n _MD ) (n _MD −n _TD ) is preferably 0.1 × 10 ⁻³ , Should be 0.3 × 10 ⁻³ , more preferably 0.5 × 10 ⁻³ . When the value of (n _MD -n _TD ) is less than 0.1 × 10 ⁻³ , there is no difference from a normal cast film, and the effect of balancing the mechanical properties and thermal dimensional stability of the present invention is not exhibited. n _MD −n _TD ) needs to be 0.1 × 10 ⁻³ or more. Further, the upper limit of the value of (n _MD -n _TD ) is preferably 10 × 10 ⁻³ , preferably 7 × 10 ⁻³ , and more preferably 5 × 10 ⁻³ . When the value of (n _MD -n _TD ) is larger than 10 × 10 ⁻³ , the subsequent biaxial stretchability is deteriorated, the thickness unevenness is deteriorated, and the stretching is broken, and the film with improved mechanical properties is obtained. Is not obtained, the value of (n _MD −n _TD ) needs to be 10 × 10 ⁻³ or less.
[0015]
As a method for obtaining such a cast film, a method of extruding the resin by cooling it to a temperature below Tme and exceeding a temperature-falling crystallization start temperature (Tcb) with a die is most preferably used. In the usual method of extruding and casting at a temperature equal to or higher than Tme, the cast film obtained is completely unoriented, and it is difficult to obtain a film oriented in the longitudinal direction. However, according to the cooling and extruding methods, a cast film oriented in the longitudinal direction can be easily obtained. Polyester has a relatively low crystallization speed, and can maintain a so-called supercooled liquid phase state in which it does not solidify in a short time even when cooled from a molten state to less than Tme. In the present invention, the resin is extruded in a supercooled liquid phase utilizing this property of the polyester, so that the resin is not solidified in the die. Therefore, it is necessary to keep cooling to a temperature exceeding Tcb of the resin. If the temperature is lower than Tcb, the resin starts to crystallize, and the surface of the extruded film becomes rough, extrusion abnormalities, uneven flow occurs, or solidifies with the lapse of time.
[0016]
Cooling is preferably performed at the land of the die. If the cooling is performed before the resin enters the die, the viscosity will increase and the fluidity will deteriorate, resulting in extrusion or flow abnormalities, or load on the extruder, filter, gear pump, It is not preferable because it causes deformation or shortened life. Also, cooling in the die before the land portion (die hopper portion) is a process in which the resin is molded into a desired shape, and it is not preferable because it causes uneven temperature and abnormal flow. In particular, since the flow path length of the flat die is different in the width direction, the heat history is not uniform due to the difference in cooling time, and the temperature in the width direction is uneven, so that the moldability is deteriorated and the thickness unevenness is deteriorated. In some cases, this is not preferable.
[0017]
On the other hand, when cooling is performed at the land portion of the die, the resin is expanded after being expanded in the width direction and formed into an extruded shape, so that uniform cooling can be achieved. The land portion is the narrowest portion in the die, and is preferable because it has excellent heat exchange efficiency. In addition, since the resin is extruded immediately after cooling, an increase in filtration pressure and an abnormal extrusion due to an increase in viscosity can be minimized. Further, in order to obtain a thick cast film, it is also preferable to overlap the cooled and expanded resin in the thickness direction before extrusion.
[0018]
In the present invention, it is preferable that the resin is extruded from the die land in a cooling transient state. Here, the cooling transient state refers to a state in which the resin has not reached a steady temperature during the cooling process. The transition to the cooling state is preferable because the thick edge portion remains in a relatively high temperature state and the solidification from the edge portion can be suppressed.
[0019]
In the present invention, the melting end temperature (Tme) and the cooling crystallization start temperature (Tcb) of the polyester resin are determined by DSC. DSC is a differential scanning calorimetry method usually used in thermal analysis, and is a method for measuring endothermic and exothermic accompanying a state change such as melting, crystallization, phase transition, and thermal decomposition of a substance. When the melting temperature of the polyester resin at the time of temperature rise and the crystallization rate at the time of temperature decrease are measured by DSC, a known method can be used, but the points to be noted here are the temperature rise and the temperature decrease during the measurement. . Assuming actual extrusion conditions, a suitable heating / cooling rate is usually 10 to 30 ° C./min.
[0020]
By quenching and solidifying the polyester sheet extruded from the die on a cooling drum, a cast film having a main axis of molecular orientation in the vertical direction can be easily obtained. At this time, a method of casting by applying static electricity is preferably used. The cast film obtained by such a method is extruded from a molten state through a supercooled liquid phase state and in a state where the melt viscosity is increased, and due to the draft between the die and the cooling drum, the molecular chains are forced in the vertical direction. Therefore, the molecular chains have little distortion even after biaxial stretching, and a preferable structure is obtained in terms of both mechanical properties and thermal dimensional stability.
[0021]
In addition, the polymer has a higher viscosity than usual and has high rigidity, and the polymer film vibration between the die and the cooling drum, which is mainly caused by the accompanying air flow of the rotating cooling drum, is greatly reduced, and the thickness is uniform. A film having excellent properties can be obtained. The cast film obtained by such a method has excellent thickness uniformity even after biaxial stretching.
[0022]
The cast film is subsequently subjected to biaxial stretching. First, longitudinal stretching is performed. The stretching in the longitudinal direction is performed by a general roll stretching machine including a plurality of heating roll groups having a difference in peripheral speed. The stretching in the longitudinal direction needs to be performed in at least two or more stages. In the case of the present invention, the lower limit of the first-stage longitudinal stretching temperature is the glass transition temperature (Tg) of the polyester + 20 ° C, preferably Tg + 25 ° C, and the upper limit is Tg + 50 ° C, preferably Tg + 40 ° C. By performing the first-stage longitudinal stretching in this temperature range, it is possible to improve mechanical properties and thermal dimensional stability. The reason is that by stretching in this temperature range, the polyester film hardly orients macroscopically, but microscopically generates a large number of so-called microcrystal nuclei in which the benzene rings of the molecular chains are stably stacked. This is because The microcrystal nucleus serves as a so-called molecular chain node, and the film can be highly oriented by the subsequent longitudinal stretching in the second and subsequent steps, and mechanical properties can be enhanced. If the first-stage longitudinal stretching temperature is lower than Tg + 20 ° C., the formation of a stable stack structure becomes insufficient, and the effect of improving mechanical properties is reduced, which is not preferable. On the other hand, if the temperature is higher than Tg + 50 ° C., the rigidity of the film becomes too low, and the thickness unevenness of the film is remarkably deteriorated. The lower limit of the draw ratio of the first-stage longitudinal stretching is 1.5 times, preferably 1.7 times, and the upper limit is 3.0 times, preferably 2.5 times. When the stretching ratio is lower than 1.5 times, the formation of a stable stack structure becomes insufficient, and the effect of improving the mechanical properties decreases, which is not preferable. On the other hand, if it is higher than 3.0 times, the unevenness of thickness becomes worse, which is not preferable. The first-stage longitudinal stretching may be performed in one stage, or may be performed in multiple stages. Even in the case of performing multi-stage, the stretching in the temperature range of Tg + 20 ° C. or more and Tg + 50 ° C. or less is considered as the first-stage stretching. In this case, the product of the draw ratio of each stage may be set in the range of 1.5 to 3.0 times.
[0023]
The first-stage longitudinally stretched film thus obtained is subsequently subjected to stretching in the same direction in the second and subsequent stages. The role of the longitudinal stretching in the second and subsequent stages is to give a high degree of orientation to the first-stage longitudinally stretched film that is hardly oriented. The longitudinal stretching in the second and subsequent stages is preferably performed at a temperature lower than the longitudinal stretching temperature in the first stage in order to improve the mechanical properties without relaxing the stable stack structure generated in the first stage. The longitudinal stretching in the second and subsequent steps may be performed in one step or may be performed in multiple steps.
[0024]
It is preferred that the temperature process leading to the first-stage longitudinal stretching and the second-stage longitudinal stretching is monotonously lowered. By doing so, the thick edge portion remains at a high temperature, and troubles such as whitening of the stretch can be prevented.
[0025]
The lower limit of the total longitudinal stretching ratio, which can be represented by the product of the respective stretching ratios of the first stage longitudinal stretching and the second and subsequent stages, is 4.0 times, preferably 4.5 times, and the upper limit is 7.0. Times, preferably 6.5 times. When the total longitudinal stretching ratio is lower than 4.0 times, the mechanical properties of the obtained biaxially oriented film are not improved, which is not preferable. On the other hand, when the ratio is higher than 7.0 times, the lateral stretchability of the film is deteriorated, and not only the thickness unevenness is deteriorated, but also the film is broken and a product cannot be obtained stably, which is not preferable.
[0026]
The uniaxially stretched film thus obtained is guided to a tenter while gripping both ends of the film with clips, and is stretched in a transverse direction at a temperature range of Tg or higher and Tg + 60 ° C or lower within a range of 3.0 to 6.0 times. You. Transverse stretching may be performed in one step or in multiple steps. In the case of performing in multiple stages, it is preferable to set the temperature to be higher as the horizontal stretching in the later stage is performed for stable film formation. Subsequently, the film thus obtained is subjected to a heat treatment at a temperature in the range of −60 ° C. or higher and Tmb−5 ° C. or lower for the polyester for 0.5 second or longer. The heat treatment may be performed in the same tenter as in the transverse stretching, or may be performed in another tenter by once releasing the film end and re-gripping. The heat treatment may be performed under tension, or may be performed while relaxing in the lateral direction to improve thermal dimensional stability. However, since excessive relaxation significantly deteriorates the mechanical properties of the biaxially oriented film, the upper limit of the relaxation rate is preferably kept at 8%, preferably 5%.
[0027]
In order to further enhance the mechanical properties of the biaxially oriented film, it is also preferable to perform re-longitudinal stretching and / or re-lateral stretching between the above-mentioned transverse stretching and heat treatment. The re-longitudinal stretching is not particularly limited, but is preferably performed at a temperature higher than the transverse stretching temperature from the viewpoint of stable film formation. Furthermore, when performing re-lateral stretching, the stretching temperature is a temperature higher than the horizontal stretching temperature of the previous step when not performing re-vertical stretching, a temperature higher than the re-vertical stretching temperature when performing re-vertical stretching, Preferably, it is performed at a high temperature of 180 ° C. or more and Tmb-5 ° C. or less, which is effective for increasing the strength in the lateral direction without any trouble such as breakage.
[0028]
In addition, the upper limit of the thickness of the biaxially oriented film of the present invention is 20 μm, preferably 15 μm, and more preferably 10 μm. This effect is more remarkable, and a film suitable for a magnetic recording medium, a printer ribbon, a condenser, a package, or the like is obtained, which is preferable. Further, the lower limit of the thickness of the biaxially oriented film of the present invention is 0.1 μm, preferably 0.3 μm, more preferably 0.5 μm, can be formed stably without tearing, of the obtained biaxially oriented film It is preferable because handling at the time of post-processing or the like is easy.
[0029]
When the lower limit of the sum of the Young's modulus in the longitudinal direction and the transverse direction of the biaxially oriented polyester film of the present invention is 1200 kg / mm ² , preferably 1300 kg / mm ² , more preferably 1500 kg / mm ² , In this case, the effect of improving the mechanical properties is remarkable. Further, the biaxially oriented polyester film according to the present invention has a remarkable effect on improving the thermal dimensional stability in the longitudinal direction, which was difficult in the past, and the upper limit of the thermal shrinkage in the longitudinal direction is 1.5%, preferably 1. When the content is 2%, more preferably 1.0%, it is suitable when used for magnetic recording media, printer ribbons and the like. Regarding the thickness uniformity, the required characteristics become stricter as the thickness becomes thinner. When the upper limit of the thickness unevenness is 3.0%, preferably 2.0%, more preferably 1.5%, It is preferable because it does not cause deterioration of electromagnetic conversion characteristics when used for printing and printing mistakes when used for printer ribbons.
[0030]
[Measurement method of physical properties]
(1) Refractive index Using a Abbe refractometer manufactured by Atago Co., Ltd., using methylene iodide as a mounting liquid under an atmosphere of 23 ° C. and 65% RH, the longitudinal refractive index n _MD and the transverse refractive index n _TD of the film. I asked.
[0031]
(2) Thermal characteristics Using a differential scanning calorimeter DSC3100 manufactured by Mac Science, 5 mg of a sample was melted and held at 300 ° C. for 5 minutes, rapidly cooled and solidified with liquid nitrogen, and then heated from room temperature at a heating rate of 20 ° C./minute. did. The start temperature of the crystal melting endothermic peak observed at this time was Tmb, the peak temperature was Tm, and the peak end temperature was Tme. After 5 mg of the sample was melted and held at 300 ° C. for 5 minutes, the temperature was lowered at a temperature lowering rate of 20 ° C./min. At this time, the start temperature of the exothermic peak of cooling crystallization observed was Tcb, the peak temperature was Tc, and the peak end temperature was Tce.
[0032]
(3) Young's modulus of film A sample film having a width of 10 mm was set on a Tensilon-type tensile tester manufactured by Orientec Co., Ltd. so as to have a chuck-to-chuck length of 100 mm, and was subjected to a pulling speed of 200 mm / at an atmosphere of 23 ° C. and 65% RH. The tensile test was performed in minutes.
[0033]
(4) Two lines are drawn on the film cut to a width of 10 mm in heat shrinkage so as to have a measured length of about 200 mm, and the distance between the two lines is accurately measured, and this is defined as L0. After leaving this sample in an oven heated to 150 ° C. for 30 minutes without load, the distance between the two lines is accurately measured again, and this is defined as L1. The heat shrinkage was determined by the following equation.
Heat shrinkage (%) = {(L0−L1) / L0} × 100
[0034]
(5) Unevenness of Film Thickness The thickness of a film sampled to a width of 300 mm and a length of 10 m in the longitudinal direction is continuously measured using a film thickness tester KG601A manufactured by Anritsu Corporation and an electronic micrometer K306C. From the maximum value Tmax (μm) and the minimum value Tmin (μm) at a length of 10 m,
R = Tmax-Tmin
Is determined from R and an average thickness Tave (μm) having a length of 10 m.
Uneven thickness (%) = R / Tave × 100
Asked.
[0035]
(6) Polymer Temperature The polymer temperature in the die was measured by making a hole for inserting a rod-shaped thermocouple at a position to be measured, inserting a thermocouple, and applying a seal to prevent polymer leakage. The temperature at the exit of the land of the die was measured with a thermocouple immediately below the die at the temperature of the extruded polymer.
[0036]
【Example】
The present invention will be described based on examples.
Examples 1 to 6
As the polyester, polyethylene terephthalate having an intrinsic viscosity of 0.65 (adding 0.35% by weight of colloidal silica which is a perfect sphere having a diameter of 0.25 μm as a sliding material) was used. The thermal characteristics were measured by using a DSC. As a result, Tg was 80 ° C., Tmb was 240 ° C., Tm was 255 ° C., Tme was 268 ° C., Tcb was 203 ° C., Tc was 188 ° C., and Tce was 174 ° C. The chips were vacuum-dried at 180 ° C. for 3 hours and supplied to an extruder (diameter: 90 mm), melted at 290 ° C., and supplied to a molding die. The die used was a manifold die having a lip gap of 1 mm, a width of 400 mm, and a land length of 100 mm. A plurality of holes having a diameter of 7 mm are formed in the land portion of the die in the width direction, and the die can be cooled by passing a coolant through the holes. The temperature of the die hopper was 290 ° C., cooling water of 25 ° C. was used as a refrigerant, and the amount of water passed was changed to change the polymer temperature. In this state, the polymer was extruded and, while applying static electricity to the film extruded from the die, was tightly cooled and solidified on a cooling drum having a surface temperature of 25 ° C. to obtain a cast film. The cast film was introduced into a roll longitudinal stretching machine, and subjected to a first-stage longitudinal stretching at 110 ° C. 2.0 times in the longitudinal direction. Subsequently, the temperature of the film was monotonously lowered, and the film was subjected to a second-stage longitudinal stretching at 80 ° C. 3.0 times. The longitudinally stretched film is introduced into a stenter while being gripped with a clip, and is stretched 3.5 times at 85 ° C., and is heat-treated at 210 ° C. while relaxing 3% in the transverse direction with the same stenter. Was carried out to obtain a biaxially oriented film having a thickness of 7 μm. Table 1 shows the physical properties of the cast film and the biaxially oriented film. The obtained biaxially oriented film was highly satisfactory in mechanical properties and thermal dimensional stability, and was excellent in thickness uniformity.
[0037]
Comparative Example 1
Using the same polyethylene terephthalate, die, roll longitudinal stretching machine and stenter as in Examples 1 to 6, except that the extruded polymer temperature is kept at 290 ° C., the same as that of the die manifold portion, without passing cooling water through the die land portion. In the same manner as in Examples 1 to 6, a biaxially oriented film having a thickness of 7 μm was obtained. Table 1 shows the physical properties of the cast film and the biaxially oriented film. The obtained cast film was not oriented at all, and the biaxially oriented film had insufficient mechanical properties and thermal dimensional stability, and was inferior in thickness uniformity.
[0039]
Comparative Example 2
Using the same polyethylene terephthalate and die as in Examples 1 to 6, a film formation was attempted in exactly the same manner as in Examples 1 to 6, except that the extruded polymer temperature was changed to 200 ° C. by changing the water flow rate. However, the polymer began to solidify rapidly and a cast film could not be obtained.
[0040]
Comparative Example 3
Using the same polyethylene terephthalate, die, roll longitudinal stretching machine, and stenter as in Examples 1 to 6, the extruded polymer temperature was set to 240 ° C., and the value of (n _MD −n _TD ) was 1.0 × 10 ⁻³ . Got. The cast film was introduced into a roll longitudinal stretching machine and longitudinally stretched in a longitudinal direction at 80 ° C. 3.2 times in one step. The longitudinally stretched film is introduced into a stenter while being gripped with a clip, and is stretched 3.5 times at 85 ° C., and is heat-treated at 210 ° C. while relaxing 3% in the transverse direction with the same stenter. Was carried out to obtain a biaxially oriented film having a thickness of 7 μm. Table 1 shows the physical properties of the biaxially oriented film. The obtained biaxially oriented film had insufficient mechanical properties and thermal dimensional stability, and was inferior in thickness uniformity.
[0041]
Comparative Example 4
In Comparative Example 3 , film formation was attempted in exactly the same manner except that the longitudinal stretching ratio was set to 3.4 times, but the film was broken by a stenter, and a biaxially oriented film could not be obtained.
[0042]
Comparative Example 5
Using the same polyethylene terephthalate, die, roll longitudinal stretching machine, and stenter as in Examples 1 to 6, the extruded polymer temperature was set to 240 ° C., and the value of (n _MD −n _TD ) was 1.0 × 10 ⁻³ . Got. The cast film was introduced into a roll longitudinal stretching machine and longitudinally stretched at 110 ° C. in the longitudinal direction by 3.8 times in one step. The longitudinally stretched film is introduced into a stenter while being gripped with a clip, and is stretched 3.5 times at 85 ° C., and is heat-treated at 210 ° C. while relaxing 3% in the transverse direction with the same stenter. Was carried out to obtain a biaxially oriented film having a thickness of 7 μm. Table 1 shows the physical properties of the biaxially oriented film. The obtained biaxially oriented film had no improvement in mechanical properties at all and was inferior in thickness uniformity.
[0043]
Comparative Example 6
In Comparative Example 5 , film formation was attempted in exactly the same manner except that the longitudinal stretching ratio was set to 4.0 times, but the film was broken by a stenter, and a biaxially oriented film could not be obtained.
[0044]
Example 7
Before the heat treatment, the biaxially stretched film of Example 3 was subjected to 1.2-fold re-longitudinal stretching at 160 ° C and 1.5-fold re-transverse stretching at 200 ° C, and was completely the same as Examples 1 to 6. Heat treatment was performed under the conditions to obtain a biaxially oriented film having a thickness of 4 μm. The obtained biaxially oriented film had greatly improved mechanical properties, did not degrade the thermal dimensional stability, and was very excellent in thickness uniformity.
[0045]
Comparative Example 7
Before the heat treatment, the biaxially stretched film of Comparative Example 1 was subjected to a 1.2-fold re-longitudinal stretching at 160 ° C and a 1.5-fold re-lateral stretching at 200 ° C, and was completely the same as Examples 1 to 6. Heat treatment was performed under the conditions to obtain a biaxially oriented film having a thickness of 4 μm. The obtained biaxially oriented film was inferior to Example 7 in mechanical properties, thermal dimensional stability, and thickness uniformity.
[0046]
[Table 1]

[0047]
【The invention's effect】
The present invention is a refractive index of the vertical direction _{(n MD)} transverse direction refractive index from _{(n TD)} obtained by subtracting the value _(n MD _{-n TD)} is, 0.1 × ^{10 -3} or more, 10 × ^{10 -3} or less Is a method for producing a biaxially oriented polyester film, characterized by stretching in at least two stages in the longitudinal direction and then stretching in the transverse direction. With this, it is possible to supply a film that satisfies both mechanical properties and thermal dimensional stability. The film also has excellent thickness uniformity. The polyester film obtained by this production method can be used in various industrial material fields such as magnetic recording media, printer ribbons, capacitors, and packaging.

Claims

The value (n _MD −n _TD ) obtained by subtracting the horizontal refractive index (n _TD ) from the vertical refractive index (n _MD ) is in the range of 0.1 × 10 ⁻³ or more and 10 × 10 ⁻³ or less. A method for producing a biaxially oriented polyester film, comprising: stretching a cast film in at least two stages in a longitudinal direction and then stretching in a transverse direction.

2. The cast film according to claim 1, wherein the cast film is extruded at a temperature of a polyester resin extruded from a die at a temperature lower than a melting end temperature (Tme) of the polyester resin and higher than a temperature-falling crystallization start temperature (Tcb). A method for producing an axially oriented polyester film.

The first-stage longitudinal stretching is performed in a temperature range of glass transition temperature (Tg) of the cast film + 20 ° C. or more and Tg + 50 ° C. or less, and the second-stage longitudinal stretching is performed at a lower temperature than the first-stage longitudinal stretching. A method for producing the biaxially oriented polyester film according to claim 1.

The method for producing a biaxially oriented polyester film according to any one of claims 1 to 3, wherein the total longitudinal stretching ratio in which the film is stretched in at least two stages in the machine direction is 4.0 times or more and 7.0 times or less.

The biaxially oriented polyester film according to any one of claims 1 to 4, wherein, after stretching in at least two stages in the machine direction, the film stretched in the transverse direction is stretched again and / or stretched again. Production method.