JP3670913B2 - Polylactic acid-based shrink film or sheet - Google Patents

Description

（式中、Ｒ^１およびＲ^２は、炭素数２〜１０のアルキレン基またはシクロアルキレン基である。ｎは、重量平均分子量が２万〜３０万となるのに必要な重合度である。ｎ個のＲ^１またはＲ^２は、それぞれ同一でも異なっていてもよい。また、式中には、エステル結合残基に代えて、ウレタン結合残基および／またはカーボネート結合残基を重量平均分子量の５％まで含有することができる。）
また、第２の要旨として、当該フィルムは延伸温度７０〜９５℃で、少なくとも一軸方向に延伸倍率２．５〜６．０で延伸されたフィルムまたはシートであることで上記課題を解決した。
【００１２】
加えて当該フィルムまたはシートのヘーズが１０％以下であることが重要となる。
【００１３】
【発明の実施の形態】
以下、この発明の実施形態を説明する。
【００１４】
この発明にかかる包装用ポリ乳酸系フィルムまたはシート（以下「シート状物」とすることもある）は、ポリ乳酸系重合体と生分解性脂肪族ポリエステルを主成分とする生分解性脂肪族ポリエステルを主成分とする生分解性樹脂組成物から成形されるシート状物である。
【００１５】
上記生分解性樹脂組成物は、生分解性、すなわち最終的に微生物によって分解される重合体をいう。
【００１６】
上記ポリ乳酸系重合体とは、乳酸、具体的にはＤ−乳酸又はＬ−乳酸の単独重合体又はそれらの共重合体をいう。すなわち、構成単位がＬ−乳酸であるポリ（Ｌ−乳酸）、構造単位がＤ−乳酸であるポリ（Ｄ−乳酸）さらにはＬ−乳酸とＤ−乳酸の共重合体であるポリ（ＤＬ−乳酸）がある。また、これらの混合体も含まれる。
【００１７】
上記ポリ乳酸系重合体は、縮重合法、開環重合法等、公知の方法で製造することができる。例えば、縮重合法では、Ｄ−乳酸、Ｌ−乳酸又はこれらの混合物を直接脱水宿重合して任意の組成を持つポリ乳酸が得られる。また、開環重合法では、乳酸の環状二量体であるラクチドを、必要に応じて重合調整剤等を用いながら、所定の触媒の存在下で開環重合して任意の組成を持つポリ乳酸が得られる。上記ラクチドには、Ｌ−乳酸の二量体であるＬーラクチド、Ｄ−乳酸の二量体であるＤ−ラクチド、Ｄ−乳酸とＬ−乳酸の二量体であるＤＬ−ラクチドがあり、これらを必要に応じて混合して重合することにより任意の組成、結晶性を持つポリ乳酸系重合体を得ることができる。
【００１８】
上記生分解性脂肪族ポリエステルは、上記ポリ乳酸系重合体を除く、生分解性を有する脂肪族ポリエステルである。この生分解性脂肪族ポリエステルは、ガラス転移点（以下、「Ｔｇ」と略する。）が０℃以下であることが好ましい。
【００１９】
Ｔｇが高すぎる場合は耐破断性が低く、シート状物の引張り試験を行っても、伸びが１０％を越えることはない。耐破断性に優れるということは、衝撃を受けても容易に破断しないことであり、これはシート状物の引張り試験での伸びで代用できる。伸びが１０％以上、好ましくは５０％以上、より好ましくは伸びがおよそ１００％以上あることが重要で、これを実現させるためには配合する脂肪族ポリエステルの（配合量にもよるが）Ｔｇが０℃以下、好ましくは−１０℃以下、より好ましくは−２０℃以下であり、混合する量もポリ乳酸系重合体１００重量部に１０重量部〜１００重量部、好ましくは２０〜６０重量部である。
【００２０】
一方、混合する脂肪族ポリエステルが多くなると、室温下で使用者が意図せずフィルムが徐々に収縮する、いわゆる自然収縮がおこり、使用前シート状物の平面性が失われることがあるので好ましくない。さらに、シート状物の透明性も低下する。特に配合量が５０重量部をこえると収縮ラベル用としての透明性が確保できなくなる。
【００２１】
また、シート状物の透明性を左右する重要な要素が他にある。すなわち、可能なかぎりの透明な脂肪族ポリエステルを使用して、ポリ乳酸系重合体に混合することである。あてはまる脂肪族ポリエステルとしては、脂肪族ジカルボン酸と脂肪族ジオールを縮合して得られる脂肪族ポリエステル、環状ラクトン類を開環重合した脂肪族ポリエステル、合成系脂肪族ポリエステル、菌体内で生合成される脂肪族ポリエステル等がある。この内、最も透明性に優れるものは脂肪族ジカルボン酸および脂肪族ジオールを縮合して得られる脂肪族ポリエステルである。
【００２２】
上記脂肪族ジカルボン酸としては、コハク酸、アジピン酸、スベリン酸、セバシン酸、ドデカン二酸等が例としてあげられ、また、上記脂肪族ジオールとしてはエチレングリコール、１，４−ブタンジオール、１，４−シクロヘキサンジメタノール等があげられる。これらの任意の脂肪族ジカルボン酸と脂肪族ジオールとをエステル化することにより、上記脂肪族ポリエステルが製造される。ここで、上記ジカルボン酸とジオールの選択は重要である。Ｔｇが０℃以下の脂肪族ポリエステルでは結晶化がおこり、特に高結晶性になると球晶となり光の乱反射を起こして失透する。シート状物を透明にするためには結晶を低めることであり、その目安は脂肪族ポリエステルを昇温したときの融解熱量ΔＨｍが５５Ｊ／ｇ以下であることである。ΔＨｍが５５Ｊ／ｇを越えると脂肪族ポリエステルは不透明となり、ポリ乳酸に混合しても、ラベルとして使用可能な程度の透明感は得られない。上記ジカルボン酸とジオールの好ましい組み合わせは、ジカルボン酸成分にコハク酸およびアジピン酸を使用し、エチレングリコールまたは１，４−ブタンジオールと縮重合したものである。特に、結晶性ならびにＴｇを低めたい場合は後者にすることがより好ましい。
【００２３】
上記、各種重合工程において、分子量増大を目的として少量の鎖延長剤、例えば、ジイソシアネート化合物、エポキシ化合物、ジフェニル化合物、酸無水物などを使用できる。上記のポリ乳酸系重合体又は生分解性脂肪族ポリエステルの重量平均分子量の好ましい範囲としては６万〜１００万であり、６万を下回ると得られたシート状物の物性は低く、１００万を越えると成形加工性に劣る。
【００２４】
ここで、重量平均分子量に依存する溶融粘度について言及する。溶融粘度とは樹脂を加熱して溶融させ、流動状態であるときの粘度のことを言う。溶融粘度の測定法にはいくつかあるが、ここでは（ＪＩＳ Ｋ ７１９９）に記載されている「熱可塑性プラスチックのキャピラリーレオメーターによる流れ特性試験方法」によって求められる「見かけのせん断粘度」で表す。これは、溶融樹脂を細い円管内をとおす時のせん断速度とせん断応力から算出される数値で、一般的にはせん断速度との対比で表される。本発明では、混合するポリ乳酸系重合体の溶融粘度（見かけのせん断粘度）と脂肪族ポリエステルの溶融粘度（見かけのせん断粘度）の差異によってシート状物の透明性が著しく異なることを見出し、透明なシート状物を得る時の関係を導き、完成させた。
【００２５】
すなわち、見かけのせん断速度が１０^２（１／sec）のときのポリ乳酸系重合体および脂肪族ポリエステルの見かけの溶融粘度をそれぞれＳＶａ、ＳＶｂとすると、その関係が
１≦（ＳＶａ／ＳＶｂ）≦２
のときシート状物の透明性が向上する。特に収縮ラベルとして一般的に使用されるに最も厚い方である１００μｍでさえシート状物のヘーズを１０％以下にすることができる。収縮ラベルの好ましいヘーズは１０％以下であり、好ましくは７％以下、さらに好ましくは５％以下である。かかる範囲で鋭意検討すれば、このような範囲に収めることができる。
【００２６】
上記の（ＳＶａ／ＳＶｂ）が１を下回っても、あるいは２を上回っても透明なフィルムを得ることはできなくなる。透明なシート状物を達成するためには使用するポリ乳酸系重合体に応じて、上記式を満足するような溶融粘度をもつ脂肪族ポリエステルを選択しなければならない。
【００２７】
さらに本発明の最も重要な点は、上記シート状物の片面もしくは両表面上に、上記シート状物の構成成分であるポリ乳酸系重合体のスキン層を設けたことにある。上記シート状物を構成するポリ乳酸系重合体と脂肪族ポリエステルは延伸時の変形挙動が異なるので、表面あれを起こす。ヘーズは、下記▲２▼式
【数１】
ヘーズ（％）＝（拡散透過率（％）／全光線透過率（％））×１００ …▲２▼
で求められる。含まれる脂肪族ポリエステルの量が多いほど表面あれは起りやすく、透過光の拡散が起るためヘーズが上昇し、透明感が低下する。
【００２８】
このシート状物表面上の透過光の拡散を抑制する方法は、表面あれを抑えることであり、透明性の高いポリ乳酸系重合体を積層することにより解決され、積層しないときのシート状物に比較して、よりいっそうの透明性をシート状物に与えることになる。
【００２９】
スキン層は少なくとも表面あれの凸凹の大きさよりも上回る程度の厚みを設けることが必要であり、具体的には１μｍ以上、好ましくは２μｍ以上設ける。上限は特に限定しないが、シート状物の厚み構成比上スキン層が厚くなりすぎると脂肪族ポリエステルを混合した効果が薄れることになり、耐破断性が低下する。５０μｍ厚のシート状物ではスキン層の厚みを２〜１０μｍ程度に設定することが好ましい。スキン層はシート状物の片面のみに設けてもシート状物全体のヘーズを低下させる効果があるが、好ましくは両表層に設ける方がよい。
【００３０】
上記シート状物とは、シート又はフィルムをいう。ＪＩＳにおける定義上、シートとは、薄く、一般にその厚さが長さと幅の割りには小さな平らな製品をいい、フィルムとは、長さ及び幅に比べて厚さが極めて小さく、最大厚さが任意に限定されている薄い平らな製品で、通例、ロールの形で供給されるものをいう（ＪＩＳ Ｋ ６９００）。したがって、シートの中でも厚さの特に薄いものがフィルムであるといえる。しかし、シートとフィルムの境界は定かでなく、明確に区別しにくいので、本願においては、上記のとおり、シートとフィルムの両方を含んだ概念として「シート状物」の用語を使用する。
【００３１】
上記ポリ乳酸系重合体のＬ乳酸とＤ乳酸の構成割合が１００：０〜９２：８または０：１００〜８：９２がよく、９８：２〜９４：６または６：９４〜２：９８が好ましい。上記の範囲を外れると、延伸時に厚みのバラツキが生じやすく、奇麗に印刷できなかったりする。また、シート状物の延伸倍率は、少なくとも一軸方向の延伸倍率２．５〜６．０に延伸することがよく、３．０〜５．０倍が好ましい。この範囲外ではフィルムの厚みムラが生じやすくなる。
【００３２】
得られるシート状物の収縮率は、８０℃、１０秒間において、１０％以上であることがよく、２０〜１００％が好ましい。収縮率が１０％未満であると、収縮包装や収縮結束包装に使用するためには、不十分となりやすいからである。一般的に、収縮包装や収縮結束包装には、上記ポリ乳酸系収縮シート状物の収縮率は、１０％程度でよく、ペットボトル等のラベル等の場合には、３０％以上の収縮率がよい。
【００３３】
次に、この発明にかかる包装用ポリ乳酸系収縮シート状物の製膜方法について説明する。ポリ乳酸系重合体と脂肪族ポリエステルの混合は、同一の押出機にそれぞれの原料を投入して行う。そのまま口金より押出して直接シート状物を作製する方法、あるいはストランド形状に押し出してペレットを作製し、再度押出機にてシート状物を製造する方法がある。いずれも、分解による分子量の低下を考慮しなければならないが、均一に混合させるには後者を選択する方がよい。ポリ乳酸系重合体および脂肪族ポリエステルを充分に乾燥し、水分を除去した後押出機で溶融する。ポリ乳酸系重合体は、Ｌ−乳酸構造とＤ−乳酸構造の組成比によって融点が変化することや、脂肪族ポリエステルの融点と混合の割合を考慮して、適宜溶融押出温度を選択する。実際には１００〜２５０℃の温度範囲が通常選ばれる。
【００３４】
これらの混合物には、諸物性を調整する目的で、熱安定剤、光安定剤、光吸収剤、滑剤、可塑剤、無機充填材、着色剤、顔料等を添加することもできる。
【００３５】
積層方法としては、通常に用いられる方法を採用することができる。例えば複数の押出機からフィードブロック式あるいはマルチマニホールド式にひとつの口金に連結するいわゆる共押出をする方法、巻き出した混合フィルムの表面上に別種のシート状物をロールやプレス板を用いて加熱圧着する方法がある。
【００３６】
シート状に溶融成形された生分解性樹脂組成物は、回転するキャスティングドラム（冷却ドラム）に接触させて急冷するのが好ましい。混合するポリマーの性質と割合もよるがキャスティングドラムの温度は６０℃以下が適当である。これより高いとポリマーがキャティングドラムに粘着し、引き取れない場合が生じる。また、ポリ乳酸部分の結晶化が促進されて、延伸できなくなるため、６０℃以下に設定して急冷し、ポリ乳酸部分を実質上非晶性にすることが好ましい。
【００３７】
得られたシート状物は少なくとも一方向に延伸される。シート状物の延伸倍率は、例えば、横（幅）方向の延伸倍率は２．５〜６倍の範囲で、縦（長手）方向の延伸倍率を１〜３倍の範囲で、延伸温度は７０〜９５℃の範囲で適宜選択することができる。延伸工程はシート状物を周速差のある２個のロール間で延伸するロール延伸および／または、テンターを用いクリップでシート状物を把持しながらクリップ列の列間隔を拡大させて延伸するテンター延伸によって行われる。二軸延伸する場合においては、特に限定されるものではなく、同時あるいは逐次延伸法、どちらでも構わない。
【００３８】
この発明によって得られるポリ乳酸系収縮シート状物は、包装材や収縮ラベル材として使用することができる。この包装材や収縮ラベル材が使用される被包装物としては、容器、生鮮食品等の食品等があげられる。上記容器としては、ガラス瓶、ガラス容器、硬質プラスチック容器等の硬度の高い容器、又は、紙や、ポリスチレン、ポリエチレン、ポリエチレンテレフタレート等のプラスチックから成形される容器等があげられる。これらの容器は食品用、飲料用、薬品用等任意の用途に使用されるものである。
【００３９】
上記被包装物は、上記包装材によって、収縮包装又は収縮結束包装される。このとき、上記包装材がポリ乳酸系収縮シート状物が充分な収縮率を有し、延伸時の厚みムラがないので、収縮包装したときの仕上りがよく、包装されて状態において見栄えがよい。さらに、包装後に、加熱処理を行っても、包装材が互いに融着しないので、取り扱いが容易となる。また、上記ポリ乳酸系収縮シート状物は、透明性に優れ、シート状物の裏面に印刷した後、表面から読み取る際に印刷文字・図柄の鮮明さに優れている。被包装材に収縮させて密着させることにより、ラベルとして効果よく使用することができる。
【００４０】
この積層延伸フィルムの製造方法としては、Ｔダイ、Ｉダイ、丸ダイ等から押し出ししたシート状物または円筒状物を冷却キャストロールや水、圧空等により急冷し非結晶に近い状態で固化させた後、ロール法、テンター法、チューブラー法等により１軸もしくは２軸に延伸する方法が挙げられる。通常１軸延伸フィルムの製造においてはテンター法が、２軸延伸フィルムの製造においては縦延伸をロール法で、横延伸をテンター法で行う逐次２軸延伸法、また縦横同時にテンターで延伸する同時２軸延伸法が一般的である
【００４１】
【実施例】
評価方法
・見かけの溶融粘度（ＳＶａ、ＳＶｂ）
（ＪＩＳ Ｋ７１９９）に基づいて測定した。測定装置内にサンプルを挿入し、測定温度２００℃で加熱して溶融させた。このときの加熱時間は５分間とした。
【００４２】
・結晶化融解熱量△Ｈｍ
パーキンエルマー製ＤＳＣ−７を用い、（ＪＩＳ Ｋ ７１２２）に基づいて、融解熱を測定した。すなわち、フィルムからの試験片１０ｍｇを、標準状態で状態調節を行った後、窒素ガス流量２５ｍｌ／分、加熱温度１０℃／分で２００℃まで昇温する間に描かれるＤＳＣ曲線から、吸熱ピーク面積を読みとり△Ｈｍ（Ｊ／ｇ）とした。
【００４３】
・ヘーズ
（ＪＩＳ Ｋ ７１０５）に基づいて、全光線透過率および拡散透過率を求め、以下の▲２▼式で算出した。
【００４４】
【数１】
ヘーズ（％）＝（拡散透過率（％）／全光線透過率（％））×１００ …▲２▼
・耐破断性
（ＪＩＳ Ｋ ７１２７）に基づいて、引張り試験を行い、そのときのシート状物の伸びを測定し、耐破断性の代用評価とした。試験条件は２号試験片、引張り速度２００ｍｍ／ｍｉｎで５回測定し、その平均値を求めた。シート状物の伸びが低いものは耐破断性が低く、伸びが高ければ耐破断性が高いことを示す。伸びが５０％以上のものは良好な結果で○と表記した。また、これ未満で１０％以上のものはやや良好で△、１０％未満のものは不適として×と表記した。
【００４５】
なお「伸び」は、以下▲３▼式で表される。
【００４６】
【数２】

・収縮率
シート状サンプルを、試験方向を長手として長さ１４０ｍｍ、幅は１０ｍｍに切り出し、その試験方向に長さ１００ｍｍ間の評線を入れ、８０℃の温水バスに１０秒間浸漬した後、その評線間の寸法を計り、▲４▼式にしたがって熱収縮率を算出した。
【００４７】
【数３】

・厚みのバラツキ
得られたシート状物を幅方向に、等間隔で１０点、長さ方向には５００ｍｍ間隔で２０点の合計２００点の厚みをダイヤルゲージで測定し、その厚みの平均値（Ｘ）と標準偏差（σ）を求め、（3σ／Ｘ）×１００（％）を求めた。この値が１５％を未満のものは良好な厚みをもつものとして（○）と表記し、１５％以上のものは（×）と表記した。
【００４８】
実施例１
Ｌ−乳酸：Ｄ−乳酸＝９９．５：０．５の構造単位を持ち、ガラス転移点（Ｔｇ）５８℃、融点１７５℃のポリ乳酸重合体１００重量部と、ガラス転移点（Ｔｇ）が−４５℃で、融点９４℃の生分解性脂肪族ポリエステルであるポリブチレンサクシネート／アジペート１０重量部を各々乾燥した後、混合して溶融押出しにてペレット形状にした。得られたペレットから４０ｍｍφ単軸押出機にて、２１０℃でマルチマニホールド式の口金より中間層として押出し、また、上記ポリ乳酸系重合体１００重量部に乾燥した平均粒径１．４μｍの粒状シリカ０．１重量部混合して２５ｍｍφの同方向二軸押出機にて上記口金より表裏層として２１０℃で押出した。この共押出シートを約４３℃のキャスティングロールにて急冷し、未延伸シートを得た。続いて長手方向に６５℃で１．０２倍のロール延伸、次いで、幅方向にテンターで延伸した。テンターでの熱処理ゾーンの温度は５０℃と一定にし、実質熱固定しないシート状物を製造した。シート状物の延伸温度、延伸倍率は表２のとおりで、実質幅方向に延伸した一軸延伸フィルムである。フィルムの厚みはおおよそ平均が５０μｍとなるように押出機からの溶融樹脂の吐出量とライン速度を調整した。ライン速度はおよそ６〜１７ｍ／ｍｉｎ、得られたフィルムの幅も幅方向の延伸倍率によって異なりフィルムの両耳約１００ｍｍずつ落として３００ｍｍ幅から１２００ｍｍ程度までのフィルムを得た。
【００４９】
実施例２〜５、比較例３〜７
表２に示すポリ乳酸系重合体及び生分解性脂肪族ポリエステルを用いた以外は、実施例１と同様にしてポリ乳酸系積層収縮シート状物を得た。シート状物の厚みはおおよそ平均が４０〜６０μｍとなるように押出機からの溶融樹脂の吐出量とライン速度を調整した。使用した樹脂の詳細については表１に示す。なお、実施例５で使用したポリ乳酸を無乾燥状態で２２０℃で溶融させ、押出しペレット化することで分子量を低下させることで得たポリ乳酸を使用している。脂肪族ポリエステルとの混合は、乾燥状態で実施例１と同様に行った。
【００５０】
比較例１、２
実施例２において、脂肪族ポリエステルを配合しない単層シート状物、また配合した場合の単層シート状物について比較例１、２に記す。単層用の口金を使用し、４０ｍｍφ単軸押出機のみを使用して、あとは実施例２と同様にしてシート状物を作製した。
【００５１】
得られたポリ乳酸系収縮シート状物の透明性、耐衝撃性(耐破断性）、厚みバラツキを測定した。その結果を表２に示す。
【００５２】
【発明の効果】
本発明により、十分な収縮率を有し、耐衝撃性(耐破断性）そして特に透明性に優れ、延伸時の厚みムラのない生分解可能な包装用ポリ乳酸系収縮シート状物を提供することができる。
【００５３】
【表１】

【００５４】
【表２】

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a shrinkable film or sheet mainly composed of polylactic acid.
[0002]
[Prior art]
Sheets and films of polyvinyl chloride, styrene-butadiene copolymer, polyethylene terephthalate, etc. are known as heat shrinkable sheets or films used for shrink wrapping, shrink tying wrapping, shrink labels, etc. Widely used and consumed. However, when these sheets and films are discarded in the natural environment, they remain without being decomposed due to their stability, causing problems such as damage to the landscape and contamination of the living environment such as fish and wild birds.
[0003]
Therefore, a material made of a degradable polymer that does not cause these problems is required, and many researches and developments are actually conducted. One example is polylactic acid. It is known that polylactic acid is naturally hydrolyzed in soil, does not leave its original form in the soil, and then becomes a harmless decomposition product by microorganisms.
[0004]
[Problems to be solved by the invention]
However, since polylactic acid is inherently brittle, even if it is made into a sheet or film, sufficient strength cannot be obtained and it is difficult to put it to practical use. In particular, in producing a uniaxially shrinkable film from polylactic acid by stretching substantially uniaxially, the brittleness in the non-stretching direction is not improved, so that it is easy to tear when subjected to an impact in that direction.
[0005]
JP-A-5-212790 discloses a heat shrink film for labels made of polylactic acid. However, this heat-shrinkable film is a high-temperature shrinkable film that has a high shrinkage temperature of 140 to 150 ° C. and can be used as a label for a glass bottle or the like. For this reason, when shrink-wrapping or shrink-bundling wrapping that tends to be thermally deformed, such as fresh food, paper boxes, or various containers containing foods and medicines, low-temperature shrinkage processing at about 60 to 120 ° C. is performed. It is necessary and it does not have sufficient heat shrinkability under these conditions. Further, even a high-temperature shrinkable label has a poor shrinkage finish, and a portion that floats without contacting the object to be shrunk can be formed, so that sufficient performance may not be exhibited. Furthermore, if the crystallinity of polylactic acid is not taken into consideration, stretching at a temperature as described in the publication becomes very difficult. That is, the film is crystallized and whitened during stretching, or polylactic acid having a low melting point is liable to cause problems such as melting and breaking of the film during stretching, and it is difficult to produce it substantially stably.
[0006]
Japanese Patent Application Laid-Open No. 7-256653 discloses a low-temperature shrinkable heat-shrinkable film made of a polylactic acid polymer that satisfies predetermined requirements. However, this heat-shrinkable film has a poor shrinkage finish, and there may be a case where a floating portion is formed without contacting the object to be shrunk.
[0007]
By the way, the present inventors disclosed a stretched film made of a mixture of a polylactic acid-based polymer and other aliphatic polyesters in JP-A-9-157408. This stretched film is a heat-fixed stretched film of a mixed resin of a predetermined polylactic acid-based polymer and a predetermined biodegradable aliphatic polyester, and has slipperiness, heat seal performance, fusing seal performance and thermal dimensional stability. It is shown that it is excellent in. However, this stretched film is disclosed to improve thermal dimensional stability, etc. by heat setting, but for stretched film before heat setting, shrinkage, shrink finish, fusion property Such properties as a heat shrink film are not disclosed.
[0008]
On the other hand, Japanese Patent Application Laid-Open No. 9-169896 discloses a shrink film made of the composition of polylactic acid and aliphatic polyester. The purpose is to smooth the shrinkage curve by mixing aliphatic polyester with polylactic acid, and to suppress wrinkles generated during shrinkage. However, a film based on the method and examples disclosed in the present invention does not necessarily provide a film having excellent transparency. If the aliphatic polyester to be mixed is examined and selected, the transparency of the film is remarkably lowered, which causes a practical problem. In particular, applications such as shrink labels are printed on the back side of the film and read characters and designs through the film. Therefore, if the film is cloudy or the glossiness is low, the beauty of the label is impaired.
[0009]
Accordingly, the present invention provides a biodegradable polylactic acid-based shrinkable sheet for packaging that has a sufficient shrinkage ratio, is excellent in impact resistance (breaking resistance), and is particularly transparent, and has no thickness variation during stretching. The purpose is to provide.
[0010]
[Means for Solving the Problems]
The first gist of the present invention is as follows with respect to 100 parts by weight of the polylactic acid polymer (A) in which the constituent ratio of D lactic acid and L lactic acid is 100: 0 to 92: 8 or 0: 100 to 8:92. (1) A mixture of 10 to 60 parts by weight of an aliphatic polyester (B) represented by the formula:
(1) The amount of heat of crystal melting ΔHm of the aliphatic polyester is 55 J / g or less,
(2) When the apparent melt viscosities of the polymers A and B when the apparent shear rate is 10 ² (1 / sec) are SVa and SVb, respectively, the relationship is 1 ≦ (SVa / SVb) ≦ 2.
In a film or sheet formed by molding a plastic material,
(3) providing a skin layer mainly composed of the polylactic acid polymer (A),
(4) The above problems are solved by a heat-shrinkable film or sheet having a heat shrinkage rate of at least 10% in warm water at 80 ° C./10 seconds.
[0011]
[Chemical 1]

(In the formula, R ¹ and R ² are an alkylene group or a cycloalkylene group having 2 to 10 carbon atoms. N is a degree of polymerization necessary for the weight average molecular weight to be 20,000 to 300,000. N. Each R ¹ or R ² may be the same or different from each other, and in the formula, a urethane bond residue and / or a carbonate bond residue is substituted with an ester bond residue and a weight average molecular weight of 5 % Can be contained.)
Moreover, the said subject was solved by the said 2nd summary being the film or sheet | seat by which the said film was extended | stretched by the draw ratio of 2.5-6.0 at a uniaxial direction at a draw temperature of 70-95 degreeC.
[0012]
In addition, it is important that the haze of the film or sheet is 10% or less.
[0013]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below.
[0014]
The packaging polylactic acid film or sheet according to the present invention (hereinafter sometimes referred to as “sheet-like product”) is a biodegradable aliphatic polyester comprising a polylactic acid polymer and a biodegradable aliphatic polyester as main components. It is a sheet-like thing shape | molded from the biodegradable resin composition which has as a main component.
[0015]
The biodegradable resin composition refers to a polymer that is biodegradable, that is, finally decomposed by microorganisms.
[0016]
The said polylactic acid-type polymer means the homopolymer of lactic acid, specifically D-lactic acid or L-lactic acid, or those copolymers. That is, poly (L-lactic acid) whose structural unit is L-lactic acid, poly (D-lactic acid) whose structural unit is D-lactic acid, and poly (DL-) which is a copolymer of L-lactic acid and D-lactic acid. Lactic acid). A mixture of these is also included.
[0017]
The polylactic acid polymer can be produced by a known method such as a condensation polymerization method or a ring-opening polymerization method. For example, in the condensation polymerization method, polylactic acid having an arbitrary composition is obtained by direct dehydration polymerization of D-lactic acid, L-lactic acid or a mixture thereof. In the ring-opening polymerization method, polylactic acid having an arbitrary composition is obtained by ring-opening polymerization of lactide, which is a cyclic dimer of lactic acid, in the presence of a predetermined catalyst, using a polymerization regulator as necessary. Is obtained. The lactide includes L-lactide, which is a dimer of L-lactic acid, D-lactide, which is a dimer of D-lactic acid, and DL-lactide, which is a dimer of D-lactic acid and L-lactic acid. If necessary, a polylactic acid polymer having an arbitrary composition and crystallinity can be obtained by mixing and polymerizing.
[0018]
The biodegradable aliphatic polyester is an aliphatic polyester having biodegradability excluding the polylactic acid polymer. The biodegradable aliphatic polyester preferably has a glass transition point (hereinafter abbreviated as “Tg”) of 0 ° C. or less.
[0019]
When Tg is too high, the fracture resistance is low, and the elongation does not exceed 10% even when a tensile test is performed on the sheet-like material. The fact that it is excellent in fracture resistance means that it does not break easily even when subjected to an impact, and this can be replaced by elongation in a tensile test of a sheet-like material. It is important that the elongation is 10% or more, preferably 50% or more, more preferably about 100% or more. In order to realize this, the Tg of the aliphatic polyester to be blended (depending on the blending amount) is 0 ° C. or less, preferably −10 ° C. or less, more preferably −20 ° C. or less, and the amount to be mixed is 10 to 100 parts by weight, preferably 20 to 60 parts by weight with respect to 100 parts by weight of the polylactic acid polymer. is there.
[0020]
On the other hand, when the amount of the aliphatic polyester to be mixed increases, the film is gradually contracted unintentionally at room temperature, so-called spontaneous shrinkage occurs, and the flatness of the sheet-like material before use may be lost, which is not preferable. . Furthermore, the transparency of the sheet-like material is also lowered. In particular, when the blending amount exceeds 50 parts by weight, it becomes impossible to ensure transparency for a shrink label.
[0021]
There are other important factors that affect the transparency of the sheet-like material. That is, it is to mix the polylactic acid polymer with as much transparent aliphatic polyester as possible. Examples of aliphatic polyesters that can be used include aliphatic polyesters obtained by condensing aliphatic dicarboxylic acids and aliphatic diols, aliphatic polyesters obtained by ring-opening polymerization of cyclic lactones, synthetic aliphatic polyesters, and biosynthesis in cells. Examples include aliphatic polyester. Among these, those having the highest transparency are aliphatic polyesters obtained by condensing aliphatic dicarboxylic acids and aliphatic diols.
[0022]
Examples of the aliphatic dicarboxylic acid include succinic acid, adipic acid, suberic acid, sebacic acid, dodecanedioic acid and the like. Examples of the aliphatic diol include ethylene glycol, 1,4-butanediol, 1, 4-cyclohexanedimethanol and the like. The aliphatic polyester is produced by esterifying any of these aliphatic dicarboxylic acids and aliphatic diols. Here, the selection of the dicarboxylic acid and the diol is important. Aliphatic polyesters having a Tg of 0 ° C. or lower crystallize. In particular, when they become highly crystalline, they become spherulites, causing irregular reflection of light and devitrification. In order to make the sheet-like material transparent, the crystal is lowered, and the standard is that the heat of fusion ΔHm when the temperature of the aliphatic polyester is raised is 55 J / g or less. When ΔHm exceeds 55 J / g, the aliphatic polyester becomes opaque, and even when mixed with polylactic acid, a transparency that can be used as a label cannot be obtained. A preferable combination of the dicarboxylic acid and the diol is obtained by condensation polymerization with ethylene glycol or 1,4-butanediol using succinic acid and adipic acid as the dicarboxylic acid component. In particular, when the crystallinity and Tg are desired to be lowered, the latter is more preferable.
[0023]
In the above various polymerization processes, a small amount of chain extender, for example, a diisocyanate compound, an epoxy compound, a diphenyl compound, an acid anhydride, etc. can be used for the purpose of increasing the molecular weight. The preferable range of the weight average molecular weight of the polylactic acid polymer or the biodegradable aliphatic polyester is 60,000 to 1,000,000. If the weight average molecular weight is less than 60,000, the physical properties of the obtained sheet-like material are low. If it exceeds, molding processability is inferior.
[0024]
Here, reference is made to melt viscosity depending on the weight average molecular weight. The melt viscosity means the viscosity when the resin is heated and melted and in a fluid state. There are several methods for measuring melt viscosity, but here, it is expressed as “apparent shear viscosity” determined by “Method for testing flow properties of thermoplastics with capillary rheometer” described in (JIS K 7199). This is a numerical value calculated from the shear rate and the shear stress when the molten resin is passed through a thin circular tube, and is generally expressed as a comparison with the shear rate. In the present invention, it has been found that the transparency of the sheet-like material is significantly different depending on the difference between the melt viscosity (apparent shear viscosity) of the polylactic acid polymer to be mixed and the melt viscosity (apparent shear viscosity) of the aliphatic polyester. The relationship when obtaining a sheet-like material was derived and completed.
[0025]
That is, assuming that the apparent melt viscosities of the polylactic acid polymer and the aliphatic polyester when the apparent shear rate is 10 ² (1 / sec) are SVa and SVb, respectively, the relationship is 1 ≦ (SVa / SVb) ≦ 2
In this case, the transparency of the sheet is improved. In particular, the haze of the sheet-like material can be reduced to 10% or less even at 100 μm, which is the thickest one generally used as a shrink label. The preferred haze of the shrink label is 10% or less, preferably 7% or less, more preferably 5% or less. If an earnest study is made within such a range, it can be within such a range.
[0026]
Even if the above (SVa / SVb) is less than 1 or more than 2, a transparent film cannot be obtained. In order to achieve a transparent sheet, an aliphatic polyester having a melt viscosity satisfying the above formula must be selected according to the polylactic acid polymer used.
[0027]
Furthermore, the most important point of the present invention is that a skin layer of a polylactic acid polymer which is a constituent component of the sheet-like material is provided on one or both surfaces of the sheet-like material. Since the polylactic acid polymer and the aliphatic polyester constituting the sheet-like material have different deformation behaviors during stretching, surface roughness occurs. Haze is the following formula (2):
Haze (%) = (diffuse transmittance (%) / total light transmittance (%)) × 100 (2)
Is required. As the amount of the aliphatic polyester contained is larger, surface roughness is more likely to occur, diffusion of transmitted light occurs, haze increases, and transparency is lowered.
[0028]
The method for suppressing the diffusion of transmitted light on the surface of the sheet-like material is to suppress the surface roughness, which can be solved by laminating a highly transparent polylactic acid polymer, and to the sheet-like material when not laminated. In comparison, more transparency is given to the sheet.
[0029]
The skin layer needs to have a thickness that is at least larger than the size of the irregularities on the surface. Specifically, the skin layer is provided with a thickness of 1 μm or more, preferably 2 μm or more. The upper limit is not particularly limited, but if the skin layer is too thick in terms of the thickness composition ratio of the sheet-like material, the effect of mixing the aliphatic polyester is diminished and the fracture resistance is lowered. In the case of a sheet-like material having a thickness of 50 μm, the thickness of the skin layer is preferably set to about 2 to 10 μm. Even if the skin layer is provided only on one side of the sheet-like material, it has an effect of reducing the haze of the entire sheet-like material, but it is preferably provided on both surface layers.
[0030]
The sheet-like material refers to a sheet or a film. By JIS definition, a sheet is a flat product that is thin and generally has a thickness that is small relative to the length and width, and a film is extremely small compared to the length and width and has a maximum thickness. Is a thin flat product that is optionally limited and is typically supplied in the form of a roll (JIS K 6900). Therefore, it can be said that a particularly thin sheet is a film. However, since the boundary between the sheet and the film is not clear and is difficult to distinguish clearly, in the present application, as described above, the term “sheet-like material” is used as a concept including both the sheet and the film.
[0031]
The composition ratio of L lactic acid and D lactic acid in the polylactic acid-based polymer is preferably 100: 0 to 92: 8 or 0: 100 to 8:92, and 98: 2 to 94: 6 or 6:94 to 2:98. preferable. If it is out of the above range, variations in thickness are likely to occur during stretching, and printing may not be beautiful. Moreover, it is good to extend | stretch the draw ratio of a sheet-like material to the draw ratio 2.5-6.0 of a uniaxial direction at least, and 3.0-5.0 times are preferable. Outside this range, film thickness unevenness tends to occur.
[0032]
The shrinkage of the obtained sheet-like material is preferably 10% or more at 80 ° C. for 10 seconds, and preferably 20 to 100%. This is because if the shrinkage rate is less than 10%, it tends to be insufficient for use in shrink wrapping or shrink-bound wrapping. In general, the shrinkage rate of the above-mentioned polylactic acid-based shrinkable sheet material may be about 10% for shrink wrapping or shrink-bound packaging, and in the case of a label such as a PET bottle, the shrinkage rate is 30% or more. Good.
[0033]
Next, a film forming method for a packaging polylactic acid-based shrinkable sheet according to the present invention will be described. The mixing of the polylactic acid polymer and the aliphatic polyester is carried out by putting the respective raw materials into the same extruder. There is a method of directly extruding from a die as it is to directly produce a sheet-like material, or a method of extruding into a strand shape to produce pellets and then producing the sheet-like material again with an extruder. In any case, a decrease in molecular weight due to decomposition must be taken into account, but the latter is better selected for uniform mixing. The polylactic acid polymer and the aliphatic polyester are sufficiently dried to remove moisture, and then melted in an extruder. The melting point of the polylactic acid polymer is appropriately selected in consideration of the melting point changing depending on the composition ratio of the L-lactic acid structure and the D-lactic acid structure, and the melting point of the aliphatic polyester and the mixing ratio. In practice, a temperature range of 100 to 250 ° C. is usually selected.
[0034]
In order to adjust various physical properties, a heat stabilizer, a light stabilizer, a light absorber, a lubricant, a plasticizer, an inorganic filler, a colorant, a pigment, and the like can be added to these mixtures.
[0035]
As a laminating method, a commonly used method can be adopted. For example, a co-extrusion method that connects to a single die from a plurality of extruders in a feed block type or a multi-manifold type, and heats another sheet-like material on the surface of the unrolled mixed film using a roll or a press plate. There is a method of crimping.
[0036]
The biodegradable resin composition melt-molded into a sheet is preferably rapidly cooled by being brought into contact with a rotating casting drum (cooling drum). Depending on the nature and proportion of the polymer to be mixed, the temperature of the casting drum is suitably 60 ° C. or less. If it is higher than this, the polymer may stick to the casting drum and cannot be pulled off. Further, since crystallization of the polylactic acid part is promoted and stretching becomes impossible, it is preferable to set the temperature to 60 ° C. or lower and quench to make the polylactic acid part substantially amorphous.
[0037]
The obtained sheet-like material is stretched in at least one direction. As for the draw ratio of the sheet-like material, for example, the draw ratio in the transverse (width) direction is in the range of 2.5 to 6 times, the draw ratio in the longitudinal (longitudinal) direction is in the range of 1 to 3 times, and the draw temperature is 70. It can select suitably in the range of -95 degreeC. In the stretching process, roll stretching for stretching a sheet-like material between two rolls having a difference in peripheral speed and / or a tenter for stretching the row of clip rows while holding the sheet-like material with a clip using a tenter. This is done by stretching. In the case of biaxial stretching, there is no particular limitation, and simultaneous or sequential stretching may be used.
[0038]
The polylactic acid-based shrinkable sheet obtained by this invention can be used as a packaging material or a shrinkable label material. Examples of packages to be packaged and shrinkable label materials include containers, foods such as fresh food, and the like. As said container, containers with high hardness, such as a glass bottle, a glass container, and a rigid plastic container, or a container shape | molded from paper, plastics, such as a polystyrene, polyethylene, a polyethylene terephthalate, etc. are mention | raise | lifted. These containers are used for arbitrary uses such as foods, beverages, and medicines.
[0039]
The packaged object is shrink-wrapped or shrink-bound with the packaging material. At this time, since the polylactic acid-based shrinkable sheet material has a sufficient shrinkage rate and there is no thickness unevenness at the time of stretching, the finish when shrink-wrapped is good, and the packaged material looks good in the packaged state. Furthermore, even if heat treatment is performed after packaging, the packaging materials are not fused to each other, so that handling becomes easy. The polylactic acid-based shrinkable sheet-like material is excellent in transparency, and is excellent in sharpness of printed characters / designs when printed on the back surface of the sheet-like material and then read from the front surface. It can be effectively used as a label by being contracted and adhered to the packaging material.
[0040]
As a method for producing this laminated stretched film, a sheet or cylindrical material extruded from a T die, I die, round die or the like was rapidly cooled with a cooling cast roll, water, compressed air, etc., and solidified in a state close to an amorphous state. Thereafter, a method of stretching uniaxially or biaxially by a roll method, a tenter method, a tubular method or the like can be mentioned. Usually, in the production of uniaxially stretched film, in the production of biaxially stretched film, the longitudinal stretching is carried out by the roll method, the sequential biaxial stretching method in which the transverse stretching is carried out by the tenter method, and the simultaneous stretching in the longitudinal and transverse directions by the tenter. Axial stretching is common. [0041]
【Example】
Evaluation method / Apparent melt viscosity (SVa, SVb)
It measured based on (JIS K7199). A sample was inserted into the measuring device and melted by heating at a measurement temperature of 200 ° C. The heating time at this time was 5 minutes.
[0042]
-Heat of crystallization melting ΔHm
The heat of fusion was measured based on (JIS K 7122) using a Perkin Elmer DSC-7. That is, after adjusting the state of a test piece of 10 mg from a film in a standard state, an endothermic peak is obtained from a DSC curve drawn while the temperature is raised to 200 ° C. at a nitrogen gas flow rate of 25 ml / min and a heating temperature of 10 ° C./min. The area was read and taken as ΔHm (J / g).
[0043]
-Based on haze (JIS K 7105), total light transmittance and diffuse transmittance were obtained and calculated by the following equation (2).
[0044]
[Expression 1]
Haze (%) = (diffuse transmittance (%) / total light transmittance (%)) × 100 (2)
-Based on the fracture resistance (JIS K 7127), a tensile test was performed, and the elongation of the sheet-like material at that time was measured to make a substitute evaluation of the fracture resistance. The test conditions were a test piece No. 2, measured 5 times at a pulling speed of 200 mm / min, and the average value was obtained. A sheet-like material having a low elongation has a low fracture resistance, and a high elongation indicates a high fracture resistance. Those with an elongation of 50% or more were marked with good results. In addition, less than 10% or more is slightly good, and less than 10% is unsuitable and indicated as x.
[0045]
“Elongation” is represented by the following formula (3).
[0046]
[Expression 2]

・ Shrinkage rate sheet-like sample was cut into a length of 140 mm and a width of 10 mm with the test direction as the longitudinal direction, a rating line of 100 mm in length was put in the test direction, and the sample was immersed in a hot water bath at 80 ° C. for 10 seconds. The dimension between the evaluation lines was measured, and the heat shrinkage rate was calculated according to the formula (4).
[0047]
[Equation 3]

・ Thickness variation The thickness of the obtained sheet-like material was measured with a dial gauge at 10 points at equal intervals in the width direction and 20 points at intervals of 500 mm in the length direction. X) and standard deviation (σ) were determined, and (3σ / X) × 100 (%) was determined. When this value was less than 15%, it was expressed as (◯) as having a good thickness, and when it was 15% or more, it was expressed as (×).
[0048]
Example 1
It has a structural unit of L-lactic acid: D-lactic acid = 99.5: 0.5, has a glass transition point (Tg) of 58 ° C., a melting point of 175 ° C., 100 parts by weight of a polylactic acid polymer, and a glass transition point (Tg). After drying 10 parts by weight of polybutylene succinate / adipate, which is a biodegradable aliphatic polyester having a melting point of 94 ° C., at −45 ° C., they were mixed and melt-extruded to form pellets. The obtained pellet was extruded as an intermediate layer from a multi-manifold die at 210 ° C. with a 40 mmφ single-screw extruder, and dried to 100 parts by weight of the polylactic acid polymer. The mixture was mixed at 0.1 parts by weight and extruded at 210 ° C. as a front and back layer from the die with a 25 mmφ co-directional twin screw extruder. This coextruded sheet was quenched with a casting roll at about 43 ° C. to obtain an unstretched sheet. Subsequently, the film was stretched 1.05 times in the longitudinal direction at 65 ° C., and then stretched in the width direction with a tenter. The temperature of the heat treatment zone in the tenter was kept constant at 50 ° C. to produce a sheet-like material that was not substantially heat-set. The stretching temperature and stretching ratio of the sheet-like material are as shown in Table 2, and are uniaxially stretched films stretched in the substantial width direction. The amount of molten resin discharged from the extruder and the line speed were adjusted so that the average thickness of the film was about 50 μm. The line speed was about 6 to 17 m / min, and the width of the obtained film was different depending on the stretching ratio in the width direction, and about 100 mm at both ears of the film was dropped to obtain a film from about 300 mm to about 1200 mm.
[0049]
Examples 2-5, Comparative Examples 3-7
A polylactic acid-based laminated shrinkable sheet was obtained in the same manner as in Example 1 except that the polylactic acid-based polymer and the biodegradable aliphatic polyester shown in Table 2 were used. The discharge amount of molten resin from the extruder and the line speed were adjusted so that the average thickness of the sheet-like material was 40 to 60 μm. Details of the resins used are shown in Table 1. In addition, the polylactic acid used in Example 5 is melted at 220 ° C. in a non-dried state and extruded to form a molecular weight by reducing the molecular weight. Mixing with the aliphatic polyester was carried out in the same manner as in Example 1 in a dry state.
[0050]
Comparative Examples 1 and 2
In Example 2, Comparative Examples 1 and 2 describe a single-layer sheet material that does not contain an aliphatic polyester and a single-layer sheet material that contains an aliphatic polyester. A single-layer die was used, and only a 40 mmφ single-screw extruder was used. Thereafter, a sheet-like material was produced in the same manner as in Example 2.
[0051]
The transparency, impact resistance (breaking resistance), and thickness variation of the obtained polylactic acid-based shrinkable sheet were measured. The results are shown in Table 2.
[0052]
【The invention's effect】
According to the present invention, there is provided a biodegradable polylactic acid-based shrinkable sheet for packaging that has a sufficient shrinkage ratio, is excellent in impact resistance (breaking resistance), and is particularly transparent, and has no thickness unevenness during stretching. be able to.
[0053]
[Table 1]

[0054]
[Table 2]

Claims (3)

An aliphatic compound represented by the following general formula <1> with respect to 100 parts by weight of the polylactic acid polymer (A) in which the constituent ratio of D lactic acid and L lactic acid is 100: 0 to 92: 8 or 0: 100 to 8:92 It is a mixture of 10 to 60 parts by weight of polyester (B) ,
And (1) the heat of crystal fusion ΔHm of the aliphatic polyester (B) is 55 J / g or less ,
(2) When the apparent melt viscosities of the polymers A and B when the apparent shear rate is 10 ² (1 / sec) are SVa and SVb, respectively, the relationship is 1 ≦ (SVa / SVb) ≦ 2.
A film or sheet formed by molding a plastic material ,
(3) A skin layer mainly composed of the polylactic acid polymer (A) is provided on both surface layers of the film or sheet ,
(4) at least state, and are more than 10% in warm water at a uniaxial direction of the heat shrinkage 80 ° C. / 10 seconds,
(5) A heat shrinkable film or sheet having a haze of 10% or less .

(In the formula, R ¹ and R ² are an alkylene group or a cycloalkylene group having 2 to 10 carbon atoms. N is a degree of polymerization necessary for the weight average molecular weight to be 20,000 to 300,000. N. Each R ¹ or R ² may be the same or different from each other, and in the formula, a urethane bond residue and / or a carbonate bond residue is substituted with an ester bond residue and a weight average molecular weight of 5 % Can be contained.) The heat- shrinkable film or sheet according to claim 1, wherein the heat- shrinkable film or sheet is stretched at a stretching temperature of 70 to 95 ° C. and at a stretching ratio of 2.5 to 6.0 in at least a uniaxial direction. The heat- shrinkable film or sheet according to claim 1 or 2, wherein the polylactic acid polymer (A) has a constituent ratio of D lactic acid and L lactic acid of 98: 2 to 94: 6 or 2:98 to 6:94 .