JP3614810B2

JP3614810B2 - Polyethylene-based crosslinked shrink film

Info

Publication number: JP3614810B2
Application number: JP2001336612A
Authority: JP
Inventors: 和宏浜田; 敏勝大山; 文夫堀田
Original assignee: Kohjin Holdings Co Ltd
Current assignee: Kohjin Holdings Co Ltd
Priority date: 2001-11-01
Filing date: 2001-11-01
Publication date: 2005-01-26
Anticipated expiration: 2021-11-01
Also published as: JP2003136653A

Description

【０００１】
【発明の属する技術分野】
本発明は、蓋付きの弁当容器や惣菜容器、精肉や生鮮食品の蓋無しトレー等の、特に食品を主体とする被包装物の自動包装機によるシュリンク包装用として用いられ、その包装品が必要に応じて電子レンジで加熱使用される架橋シュリンクフィルムに関する。
【０００２】
【従来の技術】
肉類、魚介類、野菜、果物等の生鮮食品を始めとし、コンビニエンスストアーの拡大に伴い、惣菜、弁当など様々な食品がストレッチフィルムを用いて包装されている。これらのストレッチフィルムは、適度な粘着性、優れた透明性、柔らかい触感等の特性を有している可塑化ポリ塩化ビニルを素材としたものが最も多用されている。しかしながら、可塑化ポリ塩化ビニルを素材としたストレッチフィルムは、大量に添加されている可塑剤のために、水蒸気の透過量が多く、目減りや変質が起こりやすい等の欠点に加え、廃棄焼却時に有害な塩化水素ガスが発生する等の安全衛生、公害の問題を有している。
本発明者らは、先に、前記のストレッチフィルムが有する欠点を解決すべく、安全衛生、公害の問題の無いポリエチレンを素材とした検討を行い、ポリエチレン系ストレッチシュリンクフィルムを提案した（特開平３−２１５０３４号公報、特開平８−０９０７３７号公報）。この提案によって得られるフィルムは、前記の問題点を解決し、更に熱収縮性を有し、尚かつストレッチシュリンク方式の自動包装機での使用も可能であることから、良好な包装仕上がりが得られ、包装速度向上も可能とした。
【０００３】
【発明が解決しようとする課題】
一方、最近では、弁当容器、惣菜容器、蓋無しトレー等の被包装物の種類、大きさ、形状も多種多様となり、例えば、折り箱形、四角型、丸形、丼型、三角オニギリ型など様々である。このような種々の大きさの被包装物すべてに対応出来るように、自動包装機では、製袋する際の余裕率（被包装物の大きさに対する整袋するフィルムの大きさの度合い）を大きくしたり、大きな余裕率で密封、製袋された時の製袋中に含まれる空気の脱気用の小孔が付与される等の工夫がなされてきている。また、これら包装品の大半は、店頭でそのまま電子レンジ加熱で利用されているのが現状である。これらの状況に伴い、フィルムにも以下のような特性が要求されてきているが、これら特性に十分に応えきれていないのが実情である。
▲１▼大きな余裕率に対応出来る高い収縮率、
▲２▼高い収縮率が有効に発揮出来る為の耐熱性（耐熱性が不足すると、熱収縮性はあっても溶融劣化してしまう。）、
▲３▼大きな余裕率を収縮しても商品価値を損なわない透明性、
▲４▼小孔や突起（折り箱容器の角等）から、包装中や輸送中に破れないような強度、
▲５▼電子レンジ加熱時の容器変形が起きないような熱収縮力、
【０００４】
例えば、特開平９−２１６９５６号公報には耐熱性を付与した架橋されたポリエチレン系フィルムが提案されているが、突起に対して破れやすかったり、特開平６−１０６６６８号公報には高い引き裂き強さを有する多層架橋フィルムが提案されているが、大きな余裕率では透明性が不十分であった。
更に、特開平５−１６２２７０号公報には、基層と基層より密度が０．０１ｇ／ｃｍ^３以上大きい表面層を有する積層フィルムに、フィルムの厚さ方向にできるかぎり均一に照射し加熱延伸した架橋熱収縮性積層フィルムが開示されている。しかしながら、電子レンジ加熱時に容器変形を起こしやすい等の欠点があった。
【０００５】
【課題を解決するための手段】
本発明者等は、上記欠点を克服するため鋭意検討した結果、特定のエチレン系重合体組成物からなる３層フィルムであり、その厚み方向に均一に電子線照射された後、２軸延伸する事により、高い耐熱性、収縮率、引き裂き強度を有し、尚かつ電子レンジ加熱での容器変形が小さいフィルムが得られることを見いだし、本発明を完成するに至った。
すなわち本発明は、
（１）３層構成からなるポリエチレン系架橋シュリンクフィルムであって、（ａ）両表面層のエチレン系重合体混合物が０．９００〜０．９３０ｇ／ｃｍ^３の密度である線状低密度ポリエチレン９７〜９５重量％と０．８５０〜０．９００ｇ／ｃｍ^３の密度であるエチレン−α−オレフィン共重合体３〜５重量％からなる混合物であり、（ｂ）表面層と芯層のエチレン系重合体混合物のメルトインデックスの比（表面層／芯層）が０．６〜１．６であり、（ｃ）厚み方向において均一な吸収線量となるように３０〜１２０ｋＧｙの電子線を照射した後、縦方向及び横方向に２軸延伸加工する、ことを特徴とする３層構成からなるポリエチレン系架橋シュリンクフィルム、
（２）厚み方向において均一な照射線量となるように照射する方法が、相対する二つの電子線照射装置によって両表面から照射する方法である、上記（１）記載のポリエチレン系架橋シュリンクフィルム、
（３）２軸延伸加工した後に７０〜１２５℃の温度で熱処理する事を特徴とする上記（１）〜（２）記載のポリエチレン系架橋シュリンクフィルム、
（４）芯層のエチレン系重合体組成物が、０．９００〜０．９４０ｇ／ｃｍ^３の密度である線状低密度ポリエチレン４０〜９０重量％と０．９１０〜０．９４０ｇ／ｃｍ^３の密度である長鎖分岐を有する低密度ポリエチレン１０〜６０重量％の混合物である、上記（１）〜（３）記載のポリエチレン系架橋シュリンクフィルム、
（５）芯層のエチレン系重合体組成物が、０．９００〜０．９２５ｇ／ｃｍ^３の密度である線状低密度ポリエチレン６０〜９７重量％と０．９４０〜０．９７０ｇ／ｃｍ^３の密度である高密度ポリエチレン３〜４０重量％の混合物である、上記（１）〜（３）記載のポリエチレン系架橋シュリンクフィルム、
（６）表面層と芯層のエチレン系重合体組成物のメルトインデックスの比（表面層／芯層）が０．９〜１．５である、上記（１）〜（５）記載のポリエチレン系架橋シュリンクフィルム、
（７）ＡＳＴＭＤ１９２２で測定した３０〜５０％収縮後の引き裂き強度が縦及び横方向共に１００ｍＮ以上である、上記（１）〜（６）記載のポリエチレン系架橋シュリンクフィルム、
（８）ＡＳＴＭＤ１９２２で測定した１０〜５０％収縮後の引き裂き強度が縦及び横方向共に１００ｍＮ以上である、上記（１）〜（７）記載のポリエチレン系架橋シュリンクフィルム、
を提供するものである。
【０００６】
以下、本発明をさらに詳細に説明する。
本発明において、両表面層に用いられる組成物は、０．９００〜０．９３０ｇ／ｃｍ^３の密度である線状低密度ポリエチレン９７〜９５重量％と０．８５０〜０．９００ｇ／ｃｍ^３の密度であるエチレン−α−オレフィン共重合体３〜５重量％の混合物からなる組成物である。組成物がこの範囲を外れると、自動包装機で要求される低温ヒートシール性が劣り、かつ、エチレン−α−オレフィン共重合体が５重量％を超えると、自動包装機との滑りが劣り、好ましくない。両表面層に用いられる線状低密度ポリエチレンとしては、エチレンとプロピレン、ブテン−１、ペンテン−１、ヘキセン−１、４−メチルペンテン−１、オクテン−１、デセン−１を含む炭素数３〜２０個、好ましくは炭素数が４〜８個の１種または２種以上のα−オレフィンとの共重合体が、エチレン−α−オレフィン共重合体としては、非晶性或いは示差走査熱量計（以下、ＤＳＣと記す。）にて測定される溶融ピーク温度（ＪＩＳＫ７１２１記載の方法に従って測定）が５０〜１００℃である低結晶性の共重合体、例えば、エチレンとプロピレン、ブテン−１、ペンテン−１、ヘキセン−１、４−メチルペンテン−１、オクテン−１、デセン−１のα−オレフィン或いはこれらの混合物との共重合体が挙げられ、好ましくはブテン−１との共重合体が用いられる。
【０００７】
芯層に用いられるエチレン系重合体は、密度が０．８５０〜０．９７０ｇ／ｃｍ^３のものから選ばれる一種以上であり、例えば、上述した線状低密度ポリエチレン、低結晶性のエチレン−α−オレフィン共重合体、あるいは超低密度ポリエチレン、長鎖分岐を有する低密度ポリエチレン（高圧法ポリエチレン）、高密度ポリエチレン等が例示される。
これらの中では、０．９００〜０．９４０ｇ／ｃｍ^３の密度である線状低密度ポリエチレン４０〜９０重量％と０．９１０〜０．９４０ｇ／ｃｍ^３の密度である長鎖分岐を有する低密度ポリエチレン１０〜６０重量％の混合物、或いは０．９００〜０．９２５ｇ／ｃｍ^３の密度である線状低密度ポリエチレン６０〜９７重量％と０．９４０〜０．９７０ｇ／ｃｍ^３の密度である高密度ポリエチレン３〜４０重量％の混合物、からなる組成物が、延伸バブルを安定させやすい点で好ましい。
【０００８】
表面層及び芯層に用いられるエチレン系重合体のメルトインデックス（１９０℃、２１６０ｇ荷重下、以下ＭＩと記す。）は、特に限定しないが、成形加工し易い点で０．１〜２０ｇ／１０分の範囲のものが用いられる。但し、表面層及び芯層の原料のＭＩ（原料が２成分以上の場合、組成比に応じた平均ＭＩ）の比、つまり表面層原料のＭＩを芯層原料のＭＩで除したものが、０．６〜１．６の範囲である事が、より好ましくは０．９〜１．５の範囲である事が、必要である。０．６〜１．６の範囲から外れると、フィルム自体の透明性が劣り、それが収縮包装後に更に劣る傾向にある。この透明性低下の理由は、延伸加工時の層間のズレによるものと推察され、前述のＭＩの比の範囲は、層間のズレが生じないように各層の溶融張力を近づけているものと推定している。各層の溶融張力は、原料のＭＩのみによって決まるものではなく、分子量分布、分岐の種類や量や付き方、また延伸温度が低い時には融点等の結晶性など種々の因子によっても変わり得るが、ポリエチレン系架橋シュリンクフィルムの透明性に対しては、ＭＩ値がもっとも強く影響する。
【０００９】
本発明では、３層構成からなるフィルムに電子線を照射し各層のエチレン系重合体組成物を架橋せしめ、分子量を増大させ、耐熱性を付与するが、この電子線照射で架橋される事により更に各層原料の溶融張力が高く変化するので、前述した透明性の観点から、厚み方向における照射の度合い、つまり吸収線量は均一である事が必要になる。
厚み方向で不均一であると、各層の溶融張力の差が大きくなることになり、透明性が低下する。
通常、ある片方の表面から電子線を照射すると、そのエネルギー（吸収線量）は物質を通過する毎に減衰し、その減衰の程度は、物質の比重、厚みや電子線照射する際の加速電圧によって変わる。従って、厚み方向で均一な吸収線量にするには、被照射物の両面から照射し、被照射物の厚みに応じて加速電圧を設定する必要がある。両面から照射する方法としては、１台の電子線照射装置で、被照射物のフィルムを一旦照射後、反転させて反対面からもう一度照射する方法、相対する２台の電子線照射装置で両面照射する方法があるが、フィルムの通紙作業が簡略化出来る等の点から、後者の２台の装置で両面照射する方法が好ましい。
なお、本発明でいう、厚み方向における均一とは、被照射物の厚み及び比重に対して、使用する電子線照射装置及び加速電圧での相対線量と電子線の透過深さの関係（ｄｅｐｔｈ−ｄｏｓｅ曲線）から、厚み方向の各位置での相対線量を算出し、その相対線量の範囲が２５％以内のものをいう。
【００１０】
厚み方向で均一な吸収線量は３０〜１２０ｋＧｙであり、３０ｋＧｙ未満であると、耐熱性が不足し、良好な収縮仕上がりが得られにくく、１２０ｋＧｙを越えると自動包装機で付与される小孔やトレー角のような突起から破れ易くなる。先に述べた電子線の照射方法や加速電圧の設定が不十分で、厚み方向で均一な吸収線量になっていない状態であると、透明性を得るために吸収線量を高くしなければならず、その結果、破れ易くなってしまう。破れ易さは被包装物の形状や余裕率にもよるが、１２０℃で１０〜５０％収縮させた後の引き裂き強度が縦及び横方向共に１００ｍＮ以上未満になると、包装品の輸送中等に小孔やトレー角の突起から破れやすい。
【００１１】
次いで架橋された原反フィルムは２軸延伸加工される。２軸延伸加工は、公知の延伸方法によって行うことが出来る。例えば、チューブラー同時２軸延伸、テンター同時２軸延伸、テンター逐次２軸延伸法等である。ここでは、チューブラー同時２軸延伸法を例にとって本発明の製造方法を述べるが、これに限定されるものではない。
まず溶融樹脂を環状ダイスより押し出し、一旦急冷固化したチューブ状の未延伸原反に、厚み方向で均一な吸収線量となるように加速電圧を設定した相対する２台の電子線照射装置により、原反の両面から照射し、架橋させる。その後、再加熱し、延伸倍率は縦、横それぞれ４〜８倍、好ましくは５〜８倍で延伸配向せしめる。延伸倍率が４倍未満であると、熱収縮率が小さくなり、８倍を越えるとフィルムが破れ易くなるので好ましくない。その時の延伸温度は、延伸ムラが発生せず、延伸バブルが安定した形状を維持出来、熱収縮力が付与される範囲でれば特に限定しないが、主として、要求される熱収縮力によって選択される。
電子レンジ加熱による容器変形の抑制が必要な場合、あるいは、自動包装機で熱収縮させる際に小孔から裂けやすい等の懸念がある場合は、延伸温度を高くし、低い熱収縮力を得るように調整する。特に本発明では架橋により耐熱性を向上させているので、エチレン系重合体の融点以上の延伸温度であっても、延伸バブルの形状を維持出来る。また、例えば集積包装のような包装フィルムによる強い結束力が必要な場合は、低温で延伸し、高い熱収縮応力になるように調整する。
【００１２】
原料の種類によって、延伸バブルが安定する延伸温度範囲が狭い場合には、２軸延伸後に、熱処理を行い、熱収縮力の調整を行う事が出来る。その際の熱処理方法としては、公知の方法が用いられ、例えば、熱ロール、テンター、バブル熱処理等が用いられる。熱処理温度は、要求される熱収縮力の程度に応じて選択すればよいが、７０〜１２５℃の範囲で通常使用される。
得られたフイルムは、必要に応じて、エージング、コーティング等の後処理を行うことができる。
【００１３】
また、本発明の目的に支障をきたさない範囲であれば、防曇剤、滑剤、アンチブロッキング剤、帯電防止剤等の添加剤を、それぞれ有効な作用を具備させる目的で適宜使用することは当然である。
【００１４】
【実施例】
以下実施例により、本発明を更に具体的に説明するが、本発明は、これらの実施例に限定されるものではない。
実施例及び比較例において用いた原料樹脂を下記に示す。
Ａ１：線状低密度ポリエチレン（マルチサイト触媒、密度＝０．９２０ｇ／ｃｍ^３、ＭＩ＝０．５ｇ／１０分、融解ピーク温度＝１２２℃）
Ａ２：線状低密度ポリエチレン（マルチサイト触媒、密度＝０．９１６ｇ／ｃｍ^３、ＭＩ＝１．２ｇ／１０分、融解ピーク温度＝１２０℃）
Ａ３：線状低密度ポリエチレン（マルチサイト触媒、密度＝０．９２６ｇ／ｃｍ^３、ＭＩ＝２．０ｇ／１０分、融解ピーク温度＝１２０℃）
Ａ４：線状低密度ポリエチレン（マルチサイト触媒、密度＝０．９２０ｇ／ｃｍ^３、ＭＩ＝１．０ｇ／１０分、融解ピーク温度＝１２３℃）
Ａ５：線状低密度ポリエチレン（シングルサイト触媒、密度＝０．９２０ｇ／ｃｍ^３、ＭＩ＝０．８５ｇ／１０分、融解ピーク温度＝１２４℃）
Ａ６：高圧法ポリエチレン（密度＝０．９２４ｇ／ｃｍ^３、ＭＩ＝０．７ｇ／１０分、融解ピーク温度＝１１３℃）
Ａ７：高密度ポリエチレン（密度＝０．９６８ｇ／ｃｍ^３、ＭＩ＝５．２ｇ／１０分、融解ピーク温度＝１３６℃）
Ａ８：エチレン−ブテン−１共重合体（密度＝０．８８ｇ／ｃｍ^３、ＭＩ＝３．６ｇ／１０分、融解ピーク温度＝７０℃）
【００１５】
また、本実施例の中で示した各物性測定は以下の方法により求めた。
（１）ＭＩ：ＡＳＴＭＤ−１２３８に準拠し、１９０℃、２１６０ｇ荷重下で測定した。
（２）ヘーズ：ＪＩＳ−Ｋ７１０５に準拠し、測定した。
（３）熱風収縮率：ＡＳＴＭＤ−１２０４に準拠し、１６０℃で測定した。
（４）熱収縮応力：ＡＳＴＭＤ−２８３８に準拠し、１２０℃測定した。測定値は、加熱開始から１０秒後の値を読んだ。測定値の縦及び横とは、フィルムロールの巻き長さ方向が縦、これと直角の方向が横である。
（５）引き裂き強度：ＡＳＴＭＤ−１００３に準拠し、測定した。１２０℃で１０％収縮させるとは、２００ｍｍ角のフィルムを、１８０ｍｍ角の木枠にフィルム四方の角を合わせ、フィルムをたるませた状態で固定した後、炉内の温度が１２０℃である収縮トンネル（ケーユーシステム株式会社ＭＳ−８４４１形）を５秒通過させてフィルムの縦及び横方向に１０％収縮させる事ことをいう。５０％収縮させるとは、前述の１０％収縮させる場合と同様に、木枠にフィルムを縦及び横方向とも５０％たるませた状態で固定した後、収縮トンネルを５秒通過させ収縮させる事をいう。
（６）包装評価
市販の横型ピローシュリンク包装機を使用して包装評価を行った。被包装物としては角の部分が突起となる蓋の付いた折り箱型弁当容器を用い、余裕率は縦方向が３０％、横方向が１０％とし、空気抜きの小孔は被包装物の上面に針状の突起により付与した。またこの時、収縮包装仕上がりと小孔や容器の角からの破れ難さについて、以下の基準で評価を行った。
包装評価▲１▼（収縮包装仕上がり）
○：目立つようなしわが無く被包装物に包装フィルムが密着して緊張しており、透明性が良好であるもの
×：被包装物にしわや十分に収縮していない角やフィルムの透明性低下が目立つもの
但し、破れ易いものについては、破れなかった包装体についてのみ評価した。
包装評価▲２▼（破れ難さ）
○：小孔やトレー角から破れないもの
×：包装中に、空気抜き用の小孔や弁当容器の角から破れるもの
（７）電子レンジ適性
上記の包装評価（６）と同様にして得た弁当の包装体を、１５００Ｗ４５秒間、電子レンジで加熱し、被包装物の変形の程度を評価した。
○：被包装物の変形がほとんんどみられないもの
×：被包装物が明らかに変形しており、商品価値を損なうと推察されるもの
【００１６】
実施例１
表１に示す配合比の原料樹脂に、非イオン系界面活性剤ジグリセリンオレイン酸酸エステル２重量％を添加した表層用組成物及び芯層用組成物を、３台の押出機で、それぞれを１７０℃〜２４０℃にて溶融混練し、延伸後の厚みが１０μｍ、各層の厚み比が表面層：内部層：表面層＝１：５：１になるように各押出機の押出量を調節して、２４０℃に保った３層環状ダイスのスリットより下向きに押し出した。
押し出された３層構成溶融チューブ状フイルムを、ダイス直下に取付た内外部に約２０℃の冷却水を流している円筒形マンドレルの外表面を摺動させながら、外側は水槽を通すことにより水冷して室温まで冷却して引取り、厚さ４４０μｍのチューブ状未延伸フイルムを得た。
このチューブ状未延伸フィルムの片面から加速電圧２８０ｋＶ、吸収線量８０ｋＧｙに設定した電子線照射装置（定格３００ｋＶ）により電子線を照射し、次いで反対側から同様の加速電圧及び吸収線量に設定したもう一台の電子線照射装置（定格３００ｋＶ）により電子線を照射した。
このチューブ状未延伸フイルムを、チューブラー二軸延伸装置に導き、膨張延伸を行った。この時、予熱用の環状赤外線ヒーターの電圧、電流を調節し、フイルムの予熱温度を調節し、主熱用の環状赤外線ヒーターの電圧、電流を調節してフイルムの延伸温度まで加熱した。また主熱用の環状赤外線ヒーターの下方よりバブルに沿って流れる空気を供給する中で、低速ニップロール、高速ニップロールの間の管状フイルムに加圧空気を送り込んで該空気と低速、高速ニップロールの周速比によって縦７．３倍、横６倍にバブル延伸し、その後、加熱ロールにて熱処理を行い、約１０μｍの延伸フイルムを得た。
延伸性は良好であり、延伸点の上下動や延伸バブルの揺動もなく、また、ネッキングなどの不均一延伸状態も観察されなかった。
得られたフイルムは、表１に示すように、透明性は良好で、熱収縮率は高いが、熱収縮力は低いものであった。また１０〜５０％収縮後の引き裂き強度も大きいものであった。
得られたフィルムを用い、ピローシュリンク包装機にて、包装評価を行ったところ、耐熱性が良好でトンネル温度を高くすることが可能なので、収縮仕上がりも綺麗であり実用上十分であった。更に、弁当の包装体を電子レンジで加熱したが、大きな容器変形は見られなかった。
【００１７】
実施例２、３
表１に示すように、原料の組成比及び製造条件を変えた以外は実施例１と同様にして、延伸フィルムを得た。
延伸性はいずれも良好であり、延伸点の上下動や延伸バブルの揺動もなく、また、ネッキングなどの不均一延伸状態も観察されなかった。
得られたフイルムは、表１に示すように、透明性は良好で、熱収縮率は高いが、熱収縮力は低いものであった。また１０〜５０％収縮後の引き裂き強度も大きいものであった。
得られたフィルムを用い、ピローシュリンク包装機にて、包装評価を行ったところ、耐熱性が良好でトンネル温度を高くすることが可能なので、収縮仕上がりも綺麗であり実用上十分であった。更に、弁当の包装体を電子レンジで加熱したが、大きな容器変形は見られなかった。
【００１８】
実施例４
表１に示すように、原料の組成比及び製造条件を変えた以外は実施例１と同様にして、延伸フィルムを得た。
延伸性は良好であり、延伸点の上下動や延伸バブルの揺動もなく、また、ネッキングなどの不均一延伸状態も観察されなかった。
得られたフイルムは、表１に示すように、透明性は良好で、熱収縮率は高いが、熱収縮力は低いものであった。但し、１０〜５０％収縮後の引き裂き強度はやや低いものであった。
得られたフィルムを用い、ピローシュリンク包装機にて、包装評価を行ったところ、耐熱性が良好でトンネル温度を高くすることが可能なので、収縮仕上がりも綺麗であり実用上十分であった。更に、弁当の包装体を電子レンジで加熱したが、大きな容器変形は見られなかった。
【００１９】
比較例１、２
表２に示すように、原料の組成比及び製造条件を変えた以外は実施例１と同様にして、延伸フィルムを得た。
延伸性はいずれも良好であり、延伸点の上下動や延伸バブルの揺動もなく、また、ネッキングなどの不均一延伸状態も観察されなかった。
得られたフイルムは、表２に示すように、熱収縮率は高く、熱収縮力は低いものであったが、ヘーズ値がやや高めであった。１０〜５０％収縮後の引き裂き強度は大きいものであった。
得られたフィルムを用い、ピローシュリンク包装機にて、包装評価を行ったところ、フィルムの透明性低下が大きく低下していた。更に、弁当の包装体を電子レンジで加熱したが、大きな容器変形は見られなかった。
【００２０】
比較例３
表２に示すように、原料の組成比及び製造条件を変えた以外は実施例１と同様にして、延伸フィルムを得た。
延伸性は良好であり、延伸点の上下動や延伸バブルの揺動もなく、また、ネッキングなどの不均一延伸状態も観察されなかった。
得られたフイルムは、表２に示すように、透明性も良好で、熱収縮率は高く、熱収縮力は低いものであった。１０〜５０％収縮後の引き裂き強度は大きいものであった。
得られたフィルムを用い、ピローシュリンク包装機にて、包装評価を行ったところ、フィルムの耐熱性が不十分で、熱風トンネル温度をあまり上げる事が出来ず、しわを十分に無くすことが出来なかった。更に、弁当の包装体を電子レンジで加熱したが、大きな容器変形は見られなかった。
【００２１】
比較例４
表２に示すように、原料の組成比及び製造条件を変えた以外は実施例１と同様にして、延伸フィルムを得た。
延伸性は良好であり、延伸点の上下動や延伸バブルの揺動もなく、また、ネッキングなどの不均一延伸状態も観察されなかった。
得られたフイルムは、表２に示すように、透明性も良好で、熱収縮率は高く、熱収縮力は低いものであった。１０〜５０％収縮後の引き裂き強度はやや低いものであった。
得られたフィルムを用い、ピローシュリンク包装機にて、包装評価を行ったところ、弁当容器の角からフィルムが破れ易いものであった。更に、弁当の包装体を電子レンジで加熱したが、大きな容器変形は見られなかった。
【００２２】
比較例５
表２に示すように、原料の組成比及び製造条件を変えた以外は実施例１と同様にして、延伸フィルムを得た。
延伸性は良好であり、延伸点の上下動や延伸バブルの揺動もなく、また、ネッキングなどの不均一延伸状態も観察されなかった。
得られたフイルムは、表２に示すように、透明性も良好で、熱収縮率は高く、熱収縮力は低いものであった。１０〜５０％収縮後の引き裂き強度は高いものであった。
得られたフィルムを用い、ピローシュリンク包装機にて、包装評価を行ったが、ヒートシール部が開いてしまい、収縮包装仕上がりや破れ難さ、電子レンジ適性等を評価するに至らなかった。
【００２３】
【表１】

【００２４】
【表２】

【００２５】
【発明の効果】
本発明の包装フィルムは特定の原料からなり、各層均一に電子線照射されたフィルムであり、高い耐熱性と熱収縮率を有しているため、自動包装機での収縮仕上がりや包装後の透明性が良く、また突起を有する被包装物に対して破れにくいため、シュリンク包装フィルムとして好適に用いる事が出来る。また、熱収縮力を調整する事で、電子レンジ加熱による容器変形も少ない包装フィルムも得る事が出来、食品用包装の分野にも好適である。[0001]
BACKGROUND OF THE INVENTION
INDUSTRIAL APPLICABILITY The present invention is used for shrink wrapping by an automatic packaging machine for food-based packages such as lunch boxes and prepared food containers with lids, trays without lids for meat and fresh food, etc. The present invention relates to a crosslinked shrink film used by heating in a microwave oven.
[0002]
[Prior art]
In addition to fresh foods such as meat, seafood, vegetables, and fruits, various foods such as sugar beet and bento are packaged using stretch films as convenience stores expand. Most of these stretch films are made of plasticized polyvinyl chloride having properties such as moderate tackiness, excellent transparency, and soft touch. However, the stretch film made of plasticized polyvinyl chloride is harmful when discarded by incineration, due to the plasticizer added in large quantities, which has a large amount of water vapor permeation. Has problems of health and safety and pollution such as the generation of hydrogen chloride gas.
In order to solve the drawbacks of the stretch film, the present inventors previously made a study using polyethylene having no problem of safety and health and pollution, and proposed a polyethylene-based stretch shrink film (Japanese Patent Laid-Open No. Hei 3). No.-215034, JP-A-8-090737). The film obtained by this proposal solves the above-mentioned problems, has heat shrinkability, and can also be used in a stretch shrink type automatic packaging machine, so that a good packaging finish can be obtained. The packaging speed can be improved.
[0003]
[Problems to be solved by the invention]
On the other hand, recently, the types, sizes, and shapes of packed items such as lunch boxes, side dish containers, and trays without lids have been varied, such as folding box, square, round, bowl, and triangular onigiri. It is. In order to be able to cope with all the various types of packages, the automatic packaging machine increases the margin ratio (the degree of the size of the film to be packaged with respect to the size of the packages) when making bags. Or a small hole for deaeration of air contained in the bag making when the bag is sealed and made with a large margin has been devised. In addition, most of these packaged products are currently used in the microwave oven heating as they are in stores. Along with these circumstances, the following characteristics have been demanded for films, but the reality is that these characteristics are not fully met.
(1) High shrinkage rate that can accommodate a large margin rate
(2) Heat resistance so that a high shrinkage rate can be exhibited effectively (If heat resistance is insufficient, the heat shrinkability may be melted and deteriorated).
(3) Transparency that does not impair the product value even if the large margin is shrunk.
(4) Strength from small holes and protrusions (corner box corners, etc.) that does not break during packaging or transportation,
(5) Thermal contraction force that does not cause container deformation when heating in a microwave oven,
[0004]
For example, Japanese Patent Application Laid-Open No. 9-216956 proposes a crosslinked polyethylene film imparted with heat resistance, but it is easy to break against protrusions, or Japanese Patent Application Laid-Open No. 6-106668 discloses high tear strength. Although a multilayer cross-linked film having a surface area has been proposed, the transparency was insufficient with a large margin.
Further, JP-A-5-162270 discloses a density of 0.01 g / cm from the base layer and the base layer. ³ A crosslinked heat-shrinkable laminated film is disclosed in which a laminated film having a large surface layer is irradiated as uniformly as possible in the thickness direction of the film and heated and stretched. However, there are drawbacks such as easy deformation of the container during microwave heating.
[0005]
[Means for Solving the Problems]
As a result of intensive studies to overcome the above-mentioned drawbacks, the present inventors are a three-layer film made of a specific ethylene polymer composition, and are biaxially stretched after being uniformly irradiated with an electron beam in its thickness direction. As a result, it was found that a film having high heat resistance, shrinkage rate, tear strength and small container deformation by microwave heating was obtained, and the present invention was completed.
That is, the present invention
(1) A polyethylene-based crosslinked shrink film having a three-layer structure, wherein (a) the ethylene-based polymer mixture of both surface layers is 0.900 to 0.930 g / cm ³ The density of the linear low density polyethylene is 97 to 95% by weight and 0.850 to 0.900 g / cm. ³ (B) The ratio of the melt index of the ethylene-based polymer mixture of the surface layer and the core layer (surface layer / core layer) is a mixture consisting of 3 to 5% by weight of an ethylene-α-olefin copolymer having a density of 0.6 to 1.6, and (c) a 30 to 120 kGy electron beam is irradiated so as to obtain a uniform absorbed dose in the thickness direction, and then biaxially stretched in the longitudinal and lateral directions. A polyethylene-based crosslinked shrink film comprising a three-layer structure,
(2) The polyethylene-based crosslinked shrink film according to the above (1), wherein the method of irradiating so as to obtain a uniform irradiation dose in the thickness direction is a method of irradiating from both surfaces by two opposing electron beam irradiation devices
(3) The polyethylene-based crosslinked shrink film according to the above (1) to (2), which is heat-treated at a temperature of 70 to 125 ° C. after biaxial stretching.
(4) The ethylene polymer composition of the core layer is 0.900 to 0.940 g / cm. ³ The density of the linear low density polyethylene is 40 to 90% by weight and 0.910 to 0.940 g / cm. ³ The polyethylene-based crosslinked shrink film according to the above (1) to (3), which is a mixture of 10 to 60% by weight of low-density polyethylene having long-chain branches having a density of
(5) The ethylene-based polymer composition of the core layer is 0.900 to 0.925 g / cm. ³ The density of the linear low density polyethylene is 60 to 97% by weight and 0.940 to 0.970 g / cm. ³ A polyethylene-based crosslinked shrink film according to the above (1) to (3), which is a mixture of 3 to 40% by weight of high-density polyethylene having a density of
(6) The polyethylene system according to (1) to (5) above, wherein the melt index ratio (surface layer / core layer) of the ethylene-based polymer composition of the surface layer and the core layer is 0.9 to 1.5. Cross-linked shrink film,
(7) The polyethylene-based crosslinked shrink film according to the above (1) to (6), wherein the tear strength after shrinkage of 30 to 50% measured by ASTM D1922 is 100 mN or more in both the longitudinal and lateral directions,
(8) The polyethylene-based crosslinked shrink film according to the above (1) to (7), wherein the tear strength after shrinkage of 10 to 50% measured by ASTM D1922 is 100 mN or more in both the longitudinal and lateral directions,
Is to provide.
[0006]
Hereinafter, the present invention will be described in more detail.
In the present invention, the composition used for both surface layers is 0.900 to 0.930 g / cm. ³ The density of the linear low density polyethylene is 97 to 95% by weight and 0.850 to 0.900 g / cm. ³ It is a composition which consists of a 3-5 weight% mixture of ethylene-alpha-olefin copolymer which is the density of this. When the composition is out of this range, the low temperature heat sealability required for the automatic packaging machine is poor, and when the ethylene-α-olefin copolymer exceeds 5% by weight, the slipping with the automatic packaging machine is poor. It is not preferable. The linear low density polyethylene used for both surface layers has 3 to 3 carbon atoms including ethylene and propylene, butene-1, pentene-1, hexene-1, 4-methylpentene-1, octene-1 and decene-1. A copolymer of 20 or preferably 4 to 8 carbon atoms with one or more α-olefins may be used as the ethylene-α-olefin copolymer, which is amorphous or a differential scanning calorimeter ( Hereinafter, a low crystalline copolymer having a melting peak temperature (measured according to the method described in JIS K7121) measured in DSC) of 50 to 100 ° C., for example, ethylene and propylene, butene-1, pentene -1, hexene-1, 4-methylpentene-1, octene-1, decene-1 α-olefin or a mixture thereof, preferably butene-1 Are used.
[0007]
The ethylene-based polymer used for the core layer has a density of 0.850-0.970 g / cm. ³ For example, the above-mentioned linear low density polyethylene, low crystalline ethylene-α-olefin copolymer, ultra low density polyethylene, low density polyethylene having long chain branching (high pressure method) Polyethylene), high density polyethylene and the like.
Among these, 0.900 to 0.940 g / cm ³ The density of the linear low density polyethylene is 40 to 90% by weight and 0.910 to 0.940 g / cm. ³ A mixture of 10 to 60% by weight of low density polyethylene with long chain branching, or 0.900 to 0.925 g / cm. ³ The density of the linear low density polyethylene is 60 to 97% by weight and 0.940 to 0.970 g / cm. ³ A composition comprising 3 to 40% by weight of a high-density polyethylene having a density of 1 to 5 is preferred in terms of easy stabilization of stretched bubbles.
[0008]
The melt index (190 ° C., 2160 g load, hereinafter referred to as MI) of the ethylene-based polymer used for the surface layer and the core layer is not particularly limited, but is 0.1 to 20 g / 10 min in terms of easy molding. The thing of the range of is used. However, the ratio of the surface layer and core layer raw material MI (average MI according to the composition ratio when the raw material is 2 or more), that is, the surface layer raw material MI divided by the core layer raw material MI is 0. It is necessary that it is in the range of .6 to 1.6, more preferably in the range of 0.9 to 1.5. If it is out of the range of 0.6 to 1.6, the transparency of the film itself is inferior, and it tends to be inferior after shrink wrapping. The reason for this decrease in transparency is presumed to be due to misalignment between layers during stretching, and the above-mentioned MI ratio range is estimated to be close to the melt tension of each layer so as not to cause misalignment between layers. ing. The melt tension of each layer is not determined solely by the MI of the raw material, but may vary depending on various factors such as molecular weight distribution, type and amount of branching, attachment, and crystallinity such as melting point when the stretching temperature is low. The MI value has the strongest influence on the transparency of the system cross-linked shrink film.
[0009]
In the present invention, a film composed of three layers is irradiated with an electron beam to crosslink the ethylene polymer composition of each layer, thereby increasing the molecular weight and imparting heat resistance. Furthermore, since the melt tension of each layer raw material changes to a high level, it is necessary that the degree of irradiation in the thickness direction, that is, the absorbed dose, be uniform from the viewpoint of transparency described above.
If it is non-uniform in the thickness direction, the difference in melt tension between the layers will increase, and the transparency will decrease.
Normally, when an electron beam is irradiated from one surface, the energy (absorbed dose) is attenuated each time it passes through the substance, and the degree of attenuation depends on the specific gravity of the substance, the thickness, and the acceleration voltage at the time of electron beam irradiation. change. Therefore, in order to obtain a uniform absorbed dose in the thickness direction, it is necessary to irradiate from both sides of the object to be irradiated and to set an acceleration voltage according to the thickness of the object to be irradiated. As a method of irradiating from both sides, a single electron beam irradiation device is used to irradiate the film to be irradiated once, and then irradiate again from the opposite surface, or double-side irradiation using two opposing electron beam irradiation devices. However, the method of performing double-sided irradiation with the latter two apparatuses is preferable from the viewpoint that the film passing operation can be simplified.
In the present invention, the uniformity in the thickness direction means the relationship between the electron beam irradiation apparatus used and the relative dose at the acceleration voltage and the transmission depth of the electron beam (depth−) with respect to the thickness and specific gravity of the irradiated object. The relative dose at each position in the thickness direction is calculated from the dose curve), and the range of the relative dose is within 25%.
[0010]
The uniform absorbed dose in the thickness direction is 30 to 120 kGy, and if it is less than 30 kGy, heat resistance is insufficient, and it is difficult to obtain a good shrink finish. If it exceeds 120 kGy, small holes and trays provided by an automatic packaging machine It becomes easy to tear from protrusions such as corners. If the electron beam irradiation method and acceleration voltage setting described above are insufficient and the absorbed dose is not uniform in the thickness direction, the absorbed dose must be increased to obtain transparency. As a result, it becomes easy to break. The ease of tearing depends on the shape of the package and the margin, but if the tear strength after shrinking 10-50% at 120 ° C is less than 100 mN in both the vertical and horizontal directions, it will be small during transportation of the package. Easily torn from holes and tray corner protrusions.
[0011]
Next, the cross-linked raw film is biaxially stretched. Biaxial stretching can be performed by a known stretching method. For example, tubular simultaneous biaxial stretching, tenter simultaneous biaxial stretching, tenter sequential biaxial stretching, and the like. Here, although the tubular simultaneous biaxial stretching method is taken as an example, the production method of the present invention will be described, but the present invention is not limited to this.
First, the molten resin is extruded from an annular die, and the tube-shaped unstretched raw material that has been rapidly cooled and solidified is then applied to two original electron beam irradiation devices in which an acceleration voltage is set so as to obtain a uniform absorbed dose in the thickness direction. Irradiate from opposite sides and crosslink. Thereafter, the film is reheated and stretched and oriented at a stretching ratio of 4 to 8 times, preferably 5 to 8 times in the longitudinal and lateral directions. If the draw ratio is less than 4 times, the heat shrinkage ratio is decreased, and if it exceeds 8 times, the film is easily broken, which is not preferable. The stretching temperature at that time is not particularly limited as long as stretching unevenness does not occur, the stretched bubble can maintain a stable shape, and heat shrink force is applied, but it is mainly selected depending on the required heat shrink force. The
If it is necessary to suppress container deformation by microwave heating, or if there is a concern such as easy tearing from small holes when heat shrinking with an automatic packaging machine, increase the stretching temperature and obtain a low heat shrinkage force. Adjust to. In particular, since the heat resistance is improved by crosslinking in the present invention, the shape of the stretched bubble can be maintained even at a stretching temperature equal to or higher than the melting point of the ethylene polymer. Further, when a strong binding force by a packaging film such as integrated packaging is required, the film is stretched at a low temperature and adjusted to have a high heat shrinkage stress.
[0012]
If the stretching temperature range in which stretched bubbles are stable is narrow depending on the type of raw material, heat contraction can be performed after biaxial stretching to adjust the heat shrinkage force. As a heat treatment method at that time, a known method is used, and for example, a heat roll, a tenter, a bubble heat treatment, or the like is used. The heat treatment temperature may be selected according to the required degree of heat shrinkage force, but is usually used in the range of 70 to 125 ° C.
The obtained film can be subjected to post-treatment such as aging and coating, if necessary.
[0013]
Moreover, it is a matter of course that additives such as an antifogging agent, a lubricant, an anti-blocking agent, and an antistatic agent are appropriately used for the purpose of having an effective action as long as they do not interfere with the object of the present invention. It is.
[0014]
【Example】
EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples, but the present invention is not limited to these examples.
The raw material resins used in the examples and comparative examples are shown below.
A1: Linear low density polyethylene (multisite catalyst, density = 0.920 g / cm ³ MI = 0.5 g / 10 min, melting peak temperature = 122 ° C.)
A2: Linear low density polyethylene (multisite catalyst, density = 0.916 g / cm ³ MI = 1.2 g / 10 min, melting peak temperature = 120 ° C.)
A3: Linear low density polyethylene (multisite catalyst, density = 0.926 g / cm ³ MI = 2.0 g / 10 min, melting peak temperature = 120 ° C.)
A4: Linear low density polyethylene (multisite catalyst, density = 0.920 g / cm ³ MI = 1.0 g / 10 min, melting peak temperature = 123 ° C.)
A5: Linear low density polyethylene (single site catalyst, density = 0.920 g / cm ³ MI = 0.85 g / 10 min, melting peak temperature = 124 ° C.)
A6: High-pressure polyethylene (density = 0.924 g / cm ³ MI = 0.7 g / 10 min, melting peak temperature = 113 ° C.)
A7: High density polyethylene (density = 0.968 g / cm ³ MI = 5.2 g / 10 min, melting peak temperature = 136 ° C.)
A8: ethylene-butene-1 copolymer (density = 0.88 g / cm ³ MI = 3.6 g / 10 min, melting peak temperature = 70 ° C.)
[0015]
Moreover, each physical property measurement shown in the present Example was calculated | required with the following method.
(1) MI: Based on ASTM D-1238, measured at 190 ° C. under a load of 2160 g.
(2) Haze: Measured according to JIS-K7105.
(3) Hot air shrinkage rate: Measured at 160 ° C. according to ASTM D-1204.
(4) Thermal shrinkage stress: measured at 120 ° C. according to ASTM D-2838. The measured value was read 10 seconds after the start of heating. The length and width of the measured value are the lengthwise direction of the roll length of the film roll, and the direction perpendicular thereto is the width.
(5) Tear strength: measured in accordance with ASTM D-1003. Shrinkage of 10% at 120 ° C means that a 200mm square film is aligned with a 180mm square wooden frame and fixed in a state where the film is slackened, and then the temperature inside the furnace is 120 ° C. This refers to shrinking 10% in the vertical and horizontal directions of the film by passing through a tunnel (Kyu System Co., Ltd. MS-8441 type) for 5 seconds. Shrinking 50% means that the film is fixed to the wooden frame with 50% slack in both the vertical and horizontal directions, and then shrunk by passing through the shrinking tunnel for 5 seconds. Say.
(6) Packaging evaluation
Packaging evaluation was performed using a commercially available horizontal pillow shrink wrapping machine. As a packaged item, a folded box lunch box with a lid with protrusions at the corners is used. The margin is 30% in the vertical direction and 10% in the horizontal direction. It was given by a needle-like protrusion. At this time, the shrinkage packaging finish and the difficulty of tearing from small holes and container corners were evaluated according to the following criteria.
Packaging evaluation (1) (Shrink packaging finished)
○: There is no noticeable wrinkle, the packaging film is in close contact with the package, and it is in tension, and the transparency is good.
X: Wrinkles on the packaged items or corners that are not sufficiently shrunken
However, only those packages that were not torn were evaluated for those that were easily torn.
Packaging evaluation (2) (difficult to tear)
○: Cannot be broken from small holes or tray angle
×: Breaking from small holes for venting or corners of lunch boxes during packaging
(7) Microwave suitability
The boxed lunch package obtained in the same manner as in the above-described packaging evaluation (6) was heated in a microwave oven for 1500 W for 45 seconds to evaluate the degree of deformation of the package.
○: The deformation of the packaged item is hardly seen
X: The packaged item is clearly deformed, and it is assumed that the commercial value is impaired.
[0016]
Example 1
The composition for the surface layer and the composition for the core layer obtained by adding 2% by weight of the nonionic surfactant diglycerin oleate to the raw material resin having the blending ratio shown in Table 1 are each used with three extruders. Melting and kneading at 170 ° C. to 240 ° C., adjusting the extrusion amount of each extruder so that the thickness after stretching is 10 μm and the thickness ratio of each layer is surface layer: inner layer: surface layer = 1: 5: 1 And extruded downward from the slit of the three-layer annular die maintained at 240 ° C.
The extruded three-layer molten tubular film is cooled by passing the outside through a water tank while sliding the outer surface of a cylindrical mandrel that flows cooling water of about 20 ° C. inside and outside attached directly under the die. And it cooled to room temperature and picked up and obtained the tube-shaped unstretched film of thickness 440 micrometers.
One side of this tubular unstretched film was irradiated with an electron beam by an electron beam irradiation device (rated at 300 kV) set to an acceleration voltage of 280 kV and an absorbed dose of 80 kGy, and then another side set to the same acceleration voltage and absorbed dose from the opposite side. The electron beam was irradiated by a stand electron beam irradiation device (rated at 300 kV).
This tubular unstretched film was introduced into a tubular biaxial stretching apparatus and expanded and stretched. At this time, the voltage and current of the annular infrared heater for preheating were adjusted, the preheating temperature of the film was adjusted, and the voltage and current of the annular infrared heater for main heating were adjusted to heat to the stretching temperature of the film. Also, while supplying air flowing along the bubble from the lower side of the main infrared heater, the pressurized air is fed into the tubular film between the low-speed nip roll and the high-speed nip roll, and the air and the peripheral speed of the low-speed and high-speed nip roll. Depending on the ratio, the bubble was stretched 7.3 times in length and 6 times in width, and then heat-treated with a heating roll to obtain a stretched film of about 10 μm.
The drawability was good, there was no up-and-down movement of the drawing point, no fluctuation of the drawing bubble, and no non-uniform drawing state such as necking was observed.
As shown in Table 1, the obtained film had good transparency and high heat shrinkage, but low heat shrinkage. Further, the tear strength after shrinkage of 10 to 50% was also large.
When the obtained film was used for packaging evaluation with a pillow shrink wrapping machine, the heat resistance was good and the tunnel temperature could be increased, so the shrinkage finish was also beautiful and practically sufficient. Further, the package of the lunch box was heated with a microwave oven, but no significant container deformation was observed.
[0017]
Examples 2 and 3
As shown in Table 1, a stretched film was obtained in the same manner as in Example 1 except that the composition ratio of the raw materials and the production conditions were changed.
The drawability was good, there was no vertical movement of the drawing point, no fluctuation of the drawing bubble, and no non-uniform drawing state such as necking was observed.
As shown in Table 1, the obtained film had good transparency and high heat shrinkage, but low heat shrinkage. Further, the tear strength after shrinkage of 10 to 50% was also large.
When the obtained film was used for packaging evaluation with a pillow shrink wrapping machine, the heat resistance was good and the tunnel temperature could be increased, so the shrinkage finish was also beautiful and practically sufficient. Further, the package of the lunch box was heated with a microwave oven, but no significant container deformation was observed.
[0018]
Example 4
As shown in Table 1, a stretched film was obtained in the same manner as in Example 1 except that the composition ratio of the raw materials and the production conditions were changed.
The drawability was good, there was no up-and-down movement of the drawing point, no fluctuation of the drawing bubble, and no non-uniform drawing state such as necking was observed.
As shown in Table 1, the obtained film had good transparency and high heat shrinkage, but low heat shrinkage. However, the tear strength after shrinkage of 10 to 50% was slightly low.
When the obtained film was used for packaging evaluation with a pillow shrink wrapping machine, the heat resistance was good and the tunnel temperature could be increased, so the shrinkage finish was also beautiful and practically sufficient. Further, the package of the lunch box was heated with a microwave oven, but no significant container deformation was observed.
[0019]
Comparative Examples 1 and 2
As shown in Table 2, a stretched film was obtained in the same manner as in Example 1 except that the composition ratio of the raw materials and the production conditions were changed.
The drawability was good, there was no vertical movement of the drawing point, no fluctuation of the drawing bubble, and no non-uniform drawing state such as necking was observed.
As shown in Table 2, the obtained film had a high heat shrinkage rate and a low heat shrinkage force, but had a slightly higher haze value. The tear strength after 10-50% shrinkage was high.
When the packaging evaluation was performed with the pillow shrink wrapping machine using the obtained film, the transparency of the film was greatly reduced. Further, the package of the lunch box was heated with a microwave oven, but no significant container deformation was observed.
[0020]
Comparative Example 3
As shown in Table 2, a stretched film was obtained in the same manner as in Example 1 except that the composition ratio of the raw materials and the production conditions were changed.
The drawability was good, there was no up-and-down movement of the drawing point, no fluctuation of the drawing bubble, and no non-uniform drawing state such as necking was observed.
As shown in Table 2, the obtained film had good transparency, a high heat shrinkage rate, and a low heat shrinkage force. The tear strength after 10-50% shrinkage was high.
When the packaging evaluation was performed with a pillow shrink wrapping machine using the obtained film, the heat resistance of the film was insufficient, the hot air tunnel temperature could not be raised too much, and wrinkles could not be eliminated sufficiently. It was. Further, the package of the lunch box was heated with a microwave oven, but no significant container deformation was observed.
[0021]
Comparative Example 4
As shown in Table 2, a stretched film was obtained in the same manner as in Example 1 except that the composition ratio of the raw materials and the production conditions were changed.
The drawability was good, there was no up-and-down movement of the drawing point, no fluctuation of the drawing bubble, and no non-uniform drawing state such as necking was observed.
As shown in Table 2, the obtained film had good transparency, a high heat shrinkage rate, and a low heat shrinkage force. The tear strength after 10-50% shrinkage was somewhat low.
When the packaging evaluation was performed with the pillow shrink wrapping machine using the obtained film, the film was easily torn from the corner of the lunch box. Further, the package of the lunch box was heated with a microwave oven, but no significant container deformation was observed.
[0022]
Comparative Example 5
As shown in Table 2, a stretched film was obtained in the same manner as in Example 1 except that the composition ratio of the raw materials and the production conditions were changed.
The drawability was good, there was no up-and-down movement of the drawing point, no fluctuation of the drawing bubble, and no non-uniform drawing state such as necking was observed.
As shown in Table 2, the obtained film had good transparency, a high heat shrinkage rate, and a low heat shrinkage force. The tear strength after 10-50% shrinkage was high.
Using the obtained film, packaging evaluation was performed with a pillow shrink wrapping machine. However, the heat seal part was opened, and it was not possible to evaluate shrinkage packaging finish, difficulty in tearing, suitability for microwave oven, and the like.
[0023]
[Table 1]

[0024]
[Table 2]

[0025]
【The invention's effect】
The packaging film of the present invention is made of a specific raw material, and is a film irradiated with an electron beam uniformly in each layer, and has high heat resistance and heat shrinkage, so that the shrink finish in an automatic packaging machine and transparency after packaging Since it has good properties and is difficult to break against an object to be packaged having protrusions, it can be suitably used as a shrink wrapping film. Further, by adjusting the heat shrinkage force, it is possible to obtain a packaging film with less container deformation due to microwave heating, which is suitable for the field of food packaging.

Claims

A polyethylene-based cross-linked shrink film having a three-layer structure, (a) 97 to 95 weights of linear low-density polyethylene having a density of 0.900 to 0.930 g / cm ³ in the ethylene polymer mixture of both surface layers % and 0.850~0.900g / cm ³ is the density mixture of ethylene -α- olefin copolymer 3-5 wt% which is, of the ethylene-based polymer (b) is reacted with the surface layer and the core layer The ratio of melt index (surface layer / core layer) is 0.6 to 1.6, and (c) after irradiation with an electron beam of 30 to 120 kGy so as to obtain a uniform absorbed dose in the thickness direction, A polyethylene-based cross-linked shrink film comprising a three-layer structure characterized by biaxially stretching in the transverse direction.

2. The polyethylene-based crosslinked shrink film according to claim 1, wherein the method of irradiating so as to obtain a uniform irradiation dose in the thickness direction is a method of irradiating from both surfaces by two opposing electron beam irradiation apparatuses.

The polyethylene-based crosslinked shrink film according to claim 1 or 2, wherein the polyethylene-based crosslinked shrink film is heat-treated at a temperature of 70 to 125 ° C after being biaxially stretched.

Ethylene polymer composition of the core layer is the density of 0.900~0.940g / cm linear is the density of the ^third low density polyethylene 40 to 90 wt% and 0.910~0.940g / cm ³ The polyethylene type | system | group crosslinked shrink film of Claims 1-3 which is a mixture of 10-60 weight% of low density polyethylene which has a long chain branch.

Ethylene polymer composition of the core layer is the density of 0.900~0.925g / cm linear is the density of the ^third low density polyethylene 60 to 97 wt% and 0.940 to 0.970 g / cm ³ The polyethylene type crosslinked shrink film according to claim 1, which is a mixture of 3 to 40% by weight of high density polyethylene.

The polyethylene type crosslinked shrink film according to claim 1, wherein a ratio of the melt index of the ethylene polymer composition of the surface layer to the core layer (surface layer / core layer) is 0.9 to 1.5.

The polyethylene type | system | group crosslinked shrink film of Claims 1-6 whose tear strength after 30 to 50% shrinkage | contraction measured by ASTM D1922 is 100 mN or more in both the vertical and horizontal directions.

The polyethylene type | system | group crosslinked shrink film of Claims 1-7 whose tear strength after 10 to 50% shrinkage | contraction measured by ASTM D1922 is 100 mN or more in both the vertical and horizontal directions.