JP4822657B2 - Rubber-modified styrene resin composition for food packaging materials - Google Patents
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Description
【０００１】
【発明の属する技術分野】
本発明はラジカル重合にて製造したスチレン２量体及びスチレン３量体の溶出率がきわめて少なく、かつ強度に優れる食品包装用スチレン系樹脂組成物及び該樹脂組成物を成形してなる多層シート及び該シートを成形してなる食品容器に関する。
【０００２】
【従来の技術】
スチレン系樹脂に耐衝撃性を付与させたゴム変性スチレン系樹脂組成物は軽量性、剛性、成形加工性、寸法安定性に優れており、大量に使用されている。中でも、ゴム状重合体にスチレン系重合体がグラフトしてなり分散相を形成するゴム状重合体粒子と、連続相を形成するスチレン系重合体で構成されるゴム変性スチレン系樹脂組成物は一般にラジカル重合によって安価に製造され、かつ耐衝撃性、剛性、成形加工性に優れていることから、食品包装材料においても幅広く使用されている。例えば、スチレン系樹脂発泡シートを成形した一部の容器においては、容器成形時の割れ、容器内の食品と容器内壁との接触、箸やフォークの突き刺しによる破損を防止するために、容器の内側にゴム変性スチレン系樹脂組成物が積層されている。しかし近年これらの食品包装容器を実用に供した場合、スチレン２量体やスチレン３量体が食品に移行する問題が指摘される様になった。これらの安全性を示す研究結果は既に公表されているものの、食品スープへの溶出量を抑制する社会的ニーズは依然として残っている。
【０００３】
スチレン２量体及びスチレン３量体の溶出量を減らすためには、容器中のスチレン２量体及び３量体の含有量そのものを減らすことが有効であることは以前より公知である。しかしラジカル重合法にて得られるゴム変性スチレン系樹脂組成物は重合製造時にスチレン２量体及びスチレン３量体の副生を伴い、またスチレン２量体及びスチレン３量体は揮発性が低く、結果としてゴム変性スチレン系樹脂中のスチレン２量体、スチレン３量体の含有量を減らすには限界があった。
【０００４】
ラジカル重合によるゴム変性スチレン系樹脂組成物の製造法としては主にバッチ式で重合する塊状−懸濁重合法、及び主に連続的に重合される塊状重合法などがあるが、塊状−懸濁重合法により製造することで、ゴム変性スチレン系樹脂中のスチレン２量体及びスチレン３量体の含有量を少なくなることも以前より公知である。しかし一方塊状−懸濁重合法は塊状重合法に比べコスト的に不利でかつ、分散剤等の不純物が混入し易いといった問題があり、現在は塊状重合法が主流となっている。そこで市場では、塊状重合法によるラジカル重合にて連続的に製造されるゴム変性スチレン系樹脂を使用しても、スチレン２量体及びスチレン３量体の溶出量の少ない食品包装材の開発が望まれていた。
【０００５】
スチレン２量体及び３量体の含有量を減らす方法としては、例えば、特開２００１−２０６４３６、特開２００２−１２８８４２にはアニオン重合法によって得られるスチレン２量体及びスチレン３量体の含有量の少ないスチレン系樹脂をラジカル重合法にて製造したゴム変性スチレン系樹脂と混合し、全体のスチレン２量体及びスチレン３量体の含有量を少なくしたゴム変性スチレン系樹脂組成物が示されている。しかし、アニオン重合法は一般に高価であるとともに、全体のスチレン２量体及びスチレン３量体の含有量を少なくするべく、アニオン重合にて製造したスチレン２量体及びスチレン３量体の少ないスチレン系樹脂の割合を増やし、ラジカル重合にて製造したスチレン２量体及びスチレン３量体の比較的多いゴム変性スチレン系樹脂の割合を減らすと、耐衝撃性が低下してしまうという問題があった。
【０００６】
また一方、特開２００１−２２０４７７ではゴム変性スチレン系樹脂組成物の代わりに、スチレン系樹脂とスチレン系ゴムとの混合物を用いた例が記載されている。しかし、ゴム変性スチレン系樹脂組成物に比べ、スチレン系ゴムは高価であるとともに、配合割合を増やすと、熱履歴によりスチレン系ゴムが架橋し、いわゆるゲル状物質が生成するという問題があった。また、スチレン系ゴムを混合すると、スチレン２量体及びスチレン３量体の含有量に対するスチレン２量体及びスチレン３量体の溶出率がゴム変性スチレン系樹脂組成物を使用した場合に比べて高くなり、その結果本来の目的である溶出量の低減が達成できないという問題があった。
【０００７】
【発明が解決しようとする課題】
本発明の解決しようとする課題は、スチレン２量体及びスチレン３量体の溶出量がきわめて少なく、かつ強度に優れた、ラジカル重合法、好ましくは塊状ラジカル重合法にて製造した食品包装材料用ゴム変性スチレン系樹脂組成物、及び該樹脂組成物を成形してなる多層シート及び該シートを成形してなる食品容器を提案しようとするものである。
【０００８】
【課題を解決するための手段】
本発明者らは上記課題を解決すべく鋭意検討した結果、特定の条件を満たすゴム変性スチレン系樹脂組成物からなる食品包装容器が、強度に優れ、かつスチレン２量体及びスチレン３量体の含有量に対するスチレン２量体及びスチレン３量体の溶出量、即ちスチレン２量体及びスチレン３量体の溶出率が極めて低くなり、結果として食品包装容器としてのスチレン２量体及びスチレン３量体の溶出量を低くすることができることを見いだし、本発明を完成するに至った。
【０００９】
すなわち本発明は、ゴム状重合体にスチレン系重合体がグラフトしてなり、分散相を形成するゴム状重合体粒子と、連続相を形成するスチレン系重合体で構成されるラジカル重合によって得られるゴム変性スチレン系樹脂組成物であって、概樹脂組成物中のスチレン２量体及びスチレン３量体の含有量の合計が２０００ｐｐｍ以下であり、かつ
（Ａ）該ゴム状重合体粒子のゲル分Ｇが１７．４〜２５％であり、
（Ｂ）該ゴム状重合体粒子の中位径ｄｖが０．３〜２．０μｍであり、
（Ｃ）該ゴム状重合体粒子中、２．５μｍ以上の粒径を有するゴム状重合体粒子の体積分率φが１０％未満であり、
（Ｄ）該ゴム状重合体粒子のゲル膨潤度ＳＩが９．０〜１２．０であり、
（Ｅ）ゲル分Ｇと中位径ｄｖとゲル膨潤度ＳＩの関係が式（１）において、１，５００≦Ｋ≦５，０００を満足することを特徴とする、食品包装容器に適したゴム変性スチレン系樹脂組成物に関する。
Ｋ＝ｅｘｐ（Ｇ／８）×ｅｘｐ（ｄｖ／２）×ＳＩ２ ・・・（１）
【００１０】
【発明の実施の形態】
以下に本発明を詳しく説明する。本発明のゴム変性スチレン系樹脂組成物は、ゴム状重合体にスチレン系重合体がグラフトしてなり、分散相を形成するゴム状重合体粒子と、連続相を形成するスチレン系重合体で構成されるが、基本的にはゴム状重合体の存在下、スチレン系単量体をラジカル重合して得られるゴム変性スチレン系樹脂、または該ゴム変性スチレン系樹脂とゴム成分を含有しないスチレン系樹脂との混合物を主成分としてなる。スチレン系単量体は、スチレン、α −メチルスチレン、ｏ−メチルスチレン、ｍ−メチルスチレン、ｐ−メチルスチレン等の公知のものが使用できるが、好ましくはスチレンである。スチレン系単量体は単独で使用することも混合物として使用することもできる。また、これらのスチレン系単量体と共重合可能なアクリロニトリル、（メタ）アクリル酸、（メタ）アクリル酸エステル等のスチレン系単量体以外の単量体も、ゴム変性スチレン系樹脂組成物の性能を損なわない程度、すなわちスチレン系単量体１００質量部に対し、５質量部以下なら添加して重合したものであっても良い。さらに本発明ではジビニルベンゼン等の架橋剤をスチレン系単量体１００質量部に対し、１質量部未満を添加して重合したものであっても差し支えない。
【００１１】
本発明のゴム変性スチレン系樹脂組成物に使用するゴム状重合体としては、ポリブタジエン、スチレン−ブタジエンのランダムまたはブロック共重合体、ポリイソプレン、スチレン−イソプレンのランダムまたはブロック共重合体、エチレン−プロピレンゴム、エチレン−プロピレン−ジエンゴムなどが挙げられるが、特にポリブタジエン、スチレン−ブタジエンのランダムまたはブロック共重合体が好適に用いられる。また、これらは一部水素添加されていても差し支えない。
【００１２】
ゴム変性スチレン系樹脂の製造方法としては、有機系重合開始剤を用いたラジカル重合法が好ましく、またコスト面から連続的にラジカル重合する方法がより好ましい。このときに使用する有機系重合開始剤としては、１時間半減期温度が１０５〜１１５℃の範囲にある有機過酸化物重合開始剤をスチレン系単量体に対して８００〜１２００ｐｐｍ使用することが好ましい。これらの有機過酸化物重合開始剤としては、例えば、１，１−ビス（ｔ−ブチルパーオキシ）−３，３，５−トリメチルシクロヘキサン、１，１−ビス（ｔ−ブチルパーオキシ）−シクロヘキサン、１，１−ビス（ｔ−アミルパーオキシ）−３，３，５−トリメチルシクロヘキサン、１，１−ビス（ｔ−アミルパーオキシ）−シクロヘキサン、１，１−ビス（ｔ−ヘキシルパーオキシ）−３，３，５−トリメチルシクロヘキサン、１，１−ビス（ｔ−ヘキシルパーオキシ）−シクロヘキサン、２，２−ビス（４，４−ジ−ブチルパーオキシシクロヘキシル）プロパン、ｔ−ヘキシルパーオキシイソプロピルモノカーボネート、ｔ−アミルパーオキシイソノナノエート等が挙げられる。また、場合により連鎖移動剤等、例えばｔ−ドデシルメルカプタン、ｎ−ドデシルメルカプタン、α−メチルスチレンダイマー等を使用しても差し支えない。
【００１３】
ゴム変性スチレン系樹脂の重合工程においては、スチレン系単量体の重合転化率が少なくとも６０％に到達するまで、開始剤の残存量が１００ｐｐｍ以上を維持する温度条件下にて重合を進行させ、重合工程出口におけるスチレン系単量体の重合転化率が９０%以上かつ開始剤の残存量が１０ｐｐｍ以下となるまで、開始剤の１ 時間半減期温度に対して２０℃を超えない温度範囲にて重合を進行させることが好ましい。なお、開始剤の残存量は、下記の式（３）に、使用する開始剤の特性値（頻度因子、活性化エネルギー）、開始剤濃度、及び処理温度を代入することにより算出したｃ値と、重合工程における滞留時間から式（２）を用いて算出することができる。
［Ｉ］＝［Ｉ］０×ｅｘｐ（−ｃｔ） ・・・（２）
ｃ＝Ａ×ｅｘｐ（−Ｅａ／ＲＴ） ・・・（３）
【００１４】
但し、［Ｉ］は開始剤の残存量、［Ｉ］０は初期における開始剤濃度、ｔは時間、Ａは頻度因子、Ｅａは活性化エネルギー、Ｒは気体定数、Ｔは絶対温度である。
【００１５】
ゴム成分を含有しないスチレン系樹脂の製造方法としては、ラジカル重合法であれば特に限定されず、公知の方法により製造できるが、連続的なラジカル重合により製造する場合は、ゴム変性スチレン系樹脂と同様の条件にて製造することが好ましい。
【００１６】
本発明を構成するゴム状重合体粒子のゲル分Ｇは１０〜２５％であり、好ましくは１４〜２０％である。ゴム状重合体粒子のゲル分Ｇが１０％未満であると得られるゴム変性スチレン系樹脂組成物の強度が著しく低下する。また、ゴム状重合体粒子のゲル分Ｇが２５％を越えるとスチレン２量体及びスチレン３量体の含有量に対するスチレン２量体及びスチレン３量体の溶出量、即ちスチレン２量体及びスチレン３量体の溶出率が極めて増加し、結果としてスチレン２量体及びスチレン３量体の溶出量を増加させることとなる。ゴム状重合体粒子のゲル分Ｇは、質量Ｗのゴム変性スチレン系樹脂組成物をメチルエチルケトンに５％の割合で溶解し、その溶液を遠心分離して不溶分を沈降せしめ、デカンテーションにより上澄み液を除去して不溶分を得、７０℃で１５時間真空乾燥し、２０分間デシケーター中で冷却した後、乾燥した不溶分の質量Ｄを測定して次の式により求めた。
ゲル分Ｇ（％）＝（Ｄ／Ｗ）×１００
【００１７】
本発明を構成するゴム状重合体粒子の中位径ｄｖは０．３〜２．０μｍであり、好ましくは０．５〜１．８μｍ、さらに好ましくは０．８〜１．７μｍである。中位径ｄｖが０．３μｍ未満であると、得られるゴム変性スチレン系樹脂組成物の強度が著しく低下する。また、２．０μｍを超えると、得られる食品容器のスチレン２量体及びスチレン３量体の溶出率が極めて増加し、結果としてスチレン２量体及びスチレン３量体の溶出量を増加させることとなる。なお、中位径ｄｖはゴム変性スチレン系樹脂をジメチルホルムアミドに溶解させ、レーザー回折方式粒度分布測定装置（コールター社製レーザー回折方式粒子アナライザーＬＳ−２３０型）により測定して求め、体積基準の粒径分布曲線の中位粒子径をもって中位径ｄｖとした。
【００１８】
本発明を構成するゴム状重合体粒子中、２．５μｍ以上の粒径を有するゴム状重合体粒子の体積分率φは１０％未満である必要があり、好ましくは５％未満、さらに好ましくは３％未満である。ゴム状重合体粒子の体積分率φが１０％以上であると、得られる食品容器のスチレン２量体及びスチレン３量体の溶出率が極めて増加し、結果としてスチレン２量体及びスチレン３量体の溶出量を増加させることとなる。なお、２．０μｍ以上の粒径を有するゴム状重合体粒子の体積分率φは中位径ｄｖと同様の方法でレーザー回折方式粒度分布測定装置により測定して求めた。
【００１９】
本発明を構成するゴム状重合体粒子の構造としては特に限定されないが、ゴム相中にスチレン系重合体が粒子状に点在しているサラミ構造や、ゴム相内部に単一のスチレン系重合体が粒子状に存在しているコアシェル構造などが挙げられる。このような構造は樹脂組成物の薄切片の電子顕微鏡写真を撮影することによって観察することが出来る。また、未架橋のゴム状重合体は樹脂組成物１００質量部に対し、１質量部未満であることが好ましい。未架橋のゴム状重合体が１質量部以上あると、得られる食品容器のスチレン２量体及びスチレン３量体の溶出率が極めて増加し、結果としてスチレン２量体及びスチレン３量体の溶出量を増加させることとなる。なお、未架橋のゴム状重合体の含有量は樹脂組成物をトルエンに溶解し、その溶液を遠心分離して不溶分を沈降せしめ、その上澄み液を真空乾燥させた試料のゴム分を測定することで求めた。なお、ゴム分は試料をクロロホルムに溶解させ、一定量の一塩化ヨウ素／四塩化炭素溶液を加え暗所に約１時間放置後、ヨウ化カリウム溶液を加え、過剰の一塩化ヨウ素を０．１Ｎチオ硫酸ナトリウム／エタノール水溶液で滴定し、付加した一塩化ヨウ素量から求めた。
【００２０】
本発明を構成するゴム状重合体粒子のゲル膨潤度ＳＩは９．０〜１２．０であり、好ましくは９．５〜１１．０である。９より小さくなると得られる多層シート及び容器の強度が低下する。また１２．０より大きくなると得られる食品容器のスチレン２量体及びスチレン３量体の溶出率が極めて増加し、結果としてスチレン２量体及びスチレン３量体の溶出量を増加させることとなる。なお、ゴム状重合体粒子のゲル膨潤度ＳＩは、樹脂組成物をトルエンに溶解し、その溶液を遠心分離して不溶分を沈降せしめ、デカンテーションにより上澄み液を除去してトルエンで膨潤した不溶分の質量Ｓを測定した。次いでトルエンで膨潤した不溶分を７０℃で１５時間真空乾燥し、２０分間デシケーター中で冷却した後、不溶分の乾燥質量Ｄを測定して次の式により求めた。
膨潤度ＳＩ＝Ｓ／Ｄ
【００２１】
本発明を構成するゴム状重合体粒子のゲル分Ｇと中位径ｄｖとゲル膨潤度ＳＩの関係は式（１）において、７００≦Ｋ≦６０００、好ましくは１５００≦Ｋ≦５０００，さらに好ましくは２０００≦Ｋ≦４０００を満足することが必要である。
Ｋ＝ｅｘｐ（Ｇ／８）×ｅｘｐ（ｄｖ／２）×ＳＩ２ ・・・（１）
【００２２】
本発明を構成するゴム状重合体粒子のゲル分Ｇ、中位径ｄｖ、ゲル膨潤度ＳＩと上式のＫ値との関係の例を図１〜図４に示すが、ゲル分Ｇ、中位径ｄｖ、ゲル膨潤度ＳＩがそれぞれの請求範囲内であっても、Ｋ値が請求の範囲を満足しない、つまり図中の斜線で記載した範囲を逸脱した場合、所望のゴム変性スチレン系樹脂組成物は得られない。すなわちＫ値が７００未満であると、得られるゴム変性スチレン系樹脂組成物の強度が著しく低下する。また、Ｋ値が６０００を越えると、得られる食品容器のスチレン２量体及びスチレン３量体の溶出率が極めて増加し、結果としてスチレン２量体及びスチレン３量体の溶出量を増加させることとなる。
【００２３】
【図１】
【図２】
【図３】
【図４】
本発明におけるゴム変性スチレン系樹脂組成物中のスチレン２量体及びスチレン３量体の含有量の合計は２５００ｐｐｍ以下、好ましくは２２００ｐｐｍ以下、さらに好ましくは２０００ｐｐｍ以下である。スチレン２量体及びスチレン３量体の含有量の合計が２５００ｐｐｍを越えると、得られる食品容器のスチレン２量体及びスチレン３量体の溶出率が極めて増加し、結果としてスチレン２量体及びスチレン３量体の溶出量を増加させることとなる。
【００２８】
なお、本発明におけるスチレン２量体とは１，３−ジフェニルプロパン、２，４−ジフェニル−１−ブテン、１，２−ジフェニルシクロブタンをいう。またスチレン３量体とは１−フェニル−４−（１’−フェニルエチル）テトラリン、２，４，６−トリフェニル−１−ヘキセンをいう。
【００２９】
本発明のゴム変性スチレン系樹脂組成物には、所望により高級脂肪酸、高級脂肪酸塩、高級脂肪酸アミド、鉱物油、酸化防止剤、耐侯剤、帯電防止剤、難燃剤、摺動剤等公知の添加剤を添加しても良い。
【００３０】
本発明のゴム変性スチレン系樹脂組成物は食品包装用多層シートの最外層の少なくとも１層に用いることが出来る。ゴム変性スチレン系樹脂組成物を主体としてなる層を最外に少なくとも１層有すれば、それ以外の層は特に制限はなく公知の層が採用できるが、スチレン系樹脂の発泡シートであることが最も好ましい。ゴム変性スチレン系樹脂組成物を主体としてなる層の形状は、発泡体であっても、フィルム状であっても差し支えないが、厚さ１０〜１０００μｍのフィルムであることが最も好ましい。また、多層シートの厚みは特に制限はないが、０．１〜１０ｍｍであることが好ましい。
【００３１】
食品包装用多層シートの製造方法は公知の手法が採用できる。例えばフィードブロックを付した多層シート押出機を用いて製造する方法や、シート押出機等により製造したシートにフィルムを融着させて得る方法等が採用できる。
【００３２】
食品包装用多層シートは、ゴム変性スチレン系樹脂組成物を主体としてなる層が容器の内側となる様に成形し食品包装容器を製造することができる。成形の手法は特に制限はなく、真空成形、圧空成形等が採用できる。
【００３３】
このようにして得られた食品包装容器を用いるとスチレン２量体及びスチレン３量体のｎ−ヘプタンへの溶出量の合計は５０ｐｐｂ以下、好ましくは４０ｐｐｂ以下、更に好ましくは３０ｐｐｂである。
【００３４】
【実施例】
以下、実施例を挙げて本発明を具体的に説明するが、本発明はこれら実施例に限定されるものではない。最初に本発明における評価法を以下に説明する。
【００３５】
▲１▼樹脂中のスチレン２量体とスチレン３量体の測定
樹脂２００ｍｇを２ｍＬの１，２−ジクロロメタンに溶解し、メタノールを２ｍＬ添加してゴム変性スチレン系樹脂を析出させ、静置させたのち、上澄み液について、ヒューレットパッカード社製ガスクロマトグラフィーＨＰ−５８９０を用いて測定した。なお詳細な条件を以下に記す。
（イ）カラム：ＤＢ−１（ｈｔ） ０．２５ｍｍ×３０ｍ 膜厚０．１μｍ
（ロ）インジェクション温度：２５０℃
（ハ）カラム温度：１００〜３００℃
（二）検出器温度：３００℃
（ホ）スプリット比：５０／１
（へ）内部標準物質：ｎ−エイコサン
【００３６】
▲２▼スチレン２量体及びスチレン３量体のｎ−ヘプタンへの溶出量の測定
多層シートを１７０℃のオーブンで２５秒間加熱し、口径１６０ｍｍ、底部直径１４０ｍｍ、深さ５５ｍｍの円錐台状容器をゴム変性スチレン系樹脂組成物が内側となる様に成形した。この容器にｎ−ヘプタンを５５０ｍＬ入れ、２５℃で６０分溶出を行った溶液についてＧＣ／ＭＳ−ＳＩＭで測定した。なお詳細な条件を以下に記す。
（イ）カラム：ＤＢ−５ ０．２５ｍｍ×３０ｍ 膜厚０．２５μｍ
（ロ）インジェクション温度：２５０℃
（ハ）カラム温度：１２０℃（５分）→１０℃／分→２５０℃（１２分）
（二）インターフェース：２００℃
（ホ）イオン化電圧：７０ｅＶ
（へ）スプリット比：３０
【００３７】
▲３▼強度：▲２▼で得た容器を底部からテンシロン測定機を用い４００ｍｍ／分のスピードで圧縮して、容器が座屈したときの強度で示した。
【００３８】
参考例１
ゴム変性スチレン系樹脂（ＨＩＰＳ−１）の重合
容積２５Ｌの攪拌翼付完全混合型反応器を第一反応器とし、容積４０Ｌの攪拌翼付塔型プラグフロー型反応器を第二反応器とし、容積５０Ｌの攪拌翼付塔型プラグフロー型反応器を第三反応器とし、容積５０Ｌのスタティックミキサー式プラグフロー型反応器を第四反応器とし、それぞれを直列に接続して重合工程を構成した。第一反応器での攪拌数は１１０ｒｐｍ、反応温度１０５℃に、第二反応器の攪拌数は１１０ｒｐｍ、反応温度は反応液の流れ方向に１０７℃〜１１０℃の温度勾配がつくように調節し、第三反応器の攪拌数は１１０ｒｐｍ、反応温度は反応液の流れ方向に１１２℃〜１１８℃の温度勾配がつくように調節し、第四反応器は流れ方向に１２０℃〜１２５℃の温度勾配がつくように調節した。
スチレン単量体７５．３質量部、エチルベンゼン１８．８質量部とゴム状重合体として旭化成社製ポリブタジエン「ジエン５５ＡＥ」を５．９質量部、重合開始剤として１，１−ビス（ｔ−ブチルパーオキシ）シクロヘキサン（一時間半減期温度 １１１℃）を０．０２６質量部添加して溶解した原料液を２３Ｌ／ｈｒの供給速度で第一反応器に連続的に供給し重合した後、第二反応器に連続的に装入して重合した。第一反応器の出口では、いまだゴム状重合体が分散粒子化（相反転）していない状態で、第二反応器の出口では重合液は分散粒子化が終了した状態となり、このときの重合転化率は２３％であった。次いで、第二反応器の出口からの重合液に対し、１，１−ビス（ｔ−ブチルパーオキシ）シクロヘキサンを０．０３８質量部添加したのち、第三反応器に上記重合液を連続的に装入した。第三反応器の出口での重合転化率は５７％であり、この重合液中に残存している重合開始剤量は重合初期のスチレン単量体に対し１５０ｐｐｍであった。さらに第四反応器にて重合転化率９２％になるまで重合を進行させた、この重合液中に残存している重合開始剤量は重合初期のスチレン単量体に対し５ｐｐｍであった。得られた重合液を３ベント付き二軸押出機において２３０℃の加熱処理を施しながら、減圧下で未反応スチレンと溶剤のエチルベンゼン等の揮発分などを除去した後、冷却し切断してペレット形状のゴム変性スチレン系樹脂（ＨＩＰＳ−１）を得た。得られたゴム変性スチレン系樹脂の物性を表１に示す。
【００３９】
参考例２
ゴム変性スチレン系樹脂（ＨＩＰＳ−２）の重合
第二反応器の攪拌数を１８０ｒｐｍとした以外は、全て参考例１と同様になるように製造し、ゴム変性スチレン系樹脂（ＨＩＰＳ−２）を得た。第二反応器、第三反応器及び第四反応器出口での重合転化率、及び第三反応器及び第四反応器出口での残存開始剤量、得られたゴム変性スチレン系樹脂の物性を表１に示す。
【００４０】
参考例３
ゴム変性スチレン系樹脂（ＨＩＰＳ−３）の重合
二軸押出機において２１０℃の加熱処理を施した以外は、全て参考例１と同様になるように製造し、ゴム変性スチレン系樹脂（ＨＩＰＳ−３）を得た。第二反応器、第三反応器及び第四反応器出口での重合転化率、及び第三反応器及び第四反応器出口での残存開始剤量、得られたゴム変性スチレン系樹脂の物性を表１に示す。
【００４１】
参考例４
ゴム変性スチレン系樹脂（ＨＩＰＳ−４）の重合
第二反応器の攪拌数を１００ｒｐｍとし、原料液に添加する１，１−ビス（ｔ−ブチルパーオキシ）シクロヘキサンを０．０３８質量部とした以外は全て参考例１と同様に製造し、ゴム変性スチレン系樹脂（ＨＩＰＳ−４）を得た。第二反応器、第三反応器及び第四反応器出口での重合転化率、及び第三反応器及び第四反応器出口での残存開始剤量、得られたゴム変性スチレン系樹脂の物性を表１に示す。
【００４２】
参考例５
ゴム変性スチレン系樹脂（ＨＩＰＳ−５）の重合
第二反応器の攪拌数を８０ｒｐｍとした以外は、全て参考例１と同様になるように製造し、ゴム変性スチレン系樹脂（ＨＩＰＳ−５）を得た。第二反応器、第三反応器及び第四反応器出口での重合転化率、及び第三反応器及び第四反応器出口での残存開始剤量、得られたゴム変性スチレン系樹脂の物性を表１に示す。
【００４３】
参考例６
ゴム変性スチレン系樹脂（ＨＩＰＳ−６）の重合
第二反応器を容積４０Ｌの完全混合型反応器とし、反応温度を１０８℃、攪拌数を１３０ｒｐｍとした以外は参考例１と同様にして重合工程を構成、重合条件を調節し、第二反応器にてゴム状重合体の分散粒子化を行い、他の条件は参考例１と同様にして、ゴム変性スチレン系樹脂（ＨＩＰＳ−６）を得た。第二反応器、第三反応器及び第四反応器出口での重合転化率、及び第三反応器及び第四反応器出口での残存開始剤量、得られたゴム変性スチレン系樹脂の物性を表１に示す。
【００４４】
参考例７
ゴム変性スチレン系樹脂（ＨＩＰＳ−７）の重合
二軸押出機において２５０℃の加熱処理を施した以外は、全て参考例１と同様になるように製造し、ゴム変性スチレン系樹脂（ＨＩＰＳ−７）を得た。第二反応器、第三反応器及び第四反応器出口での重合転化率、及び第三反応器及び第四反応器出口での残存開始剤量、得られたゴム変性スチレン系樹脂の物性を表１に示す。
【００４５】
参考例８
ゴム変性スチレン系樹脂（ＨＩＰＳ−８）の重合
二軸押出機において１９０℃の加熱処理を施した以外は、全て参考例１と同様になるように製造し、ゴム変性スチレン系樹脂（ＨＩＰＳ−８）を得た。第二反応器、第三反応器及び第四反応器出口での重合転化率、及び第三反応器及び第四反応器出口での残存開始剤量、得られたゴム変性スチレン系樹脂の物性を表１に示す。
【００４６】
参考例９
ゴム変性スチレン系樹脂（ＨＩＰＳ−９）の重合
第四反応器の反応温度を反応液の流れ方向に１５０℃〜１５５℃の温度勾配がつくように調節した以外は、全て参考例１と同様になるように製造し、ゴム変性スチレン系樹脂（ＨＩＰＳ−９）を得た。第二反応器、第三反応器及び第四反応器出口での重合転化率、及び第三反応器及び第四反応器出口での残存開始剤量、得られたゴム変性スチレン系樹脂の物性を表１に示す。
【００４７】
参考例１０
スチレン系樹脂（ＧＰＰＳ−１）の重合
参考例６と同様の重合工程を構成し、第一反応器での反応温度を１０１℃に、第二反応器の反応温度１０３℃に、第三反応器の反応温度は反応液の流れ方向に１０３℃〜１０５℃の温度勾配がつくように調節し、第四反応器は流れ方向に１０７℃〜１１６℃の温度勾配がつくように調節した。
スチレン単量体８０質量部、エチルベンゼン２０質量部と、重合開始剤として１，１−ビス（ｔ−ブチルパーオキシ）シクロヘキサンを０．０８質量部添加して溶解した原料液を２３Ｌ／ｈｒの供給速度で第一反応器に連続的に供給し重合した後、第二反応器、第三反応器、第四反応器にて連続的に重合を行い、重合転化率９２％になるまで重合を進行させた。得られた重合液を参考例６と同様の方法にてスチレン系樹脂（ＧＰＰＳ−１）を得た。得られたスチレン系樹脂中のスチレン２量体及びスチレン３量体の含有量の合計は１５００ｐｐｍであった。
【００４８】
参考例１１
スチレン系樹脂（ＧＰＰＳ−２）として、東洋スチレン社製ポリスチレン「トーヨースチロールＧＰ ＨＲＭ５６」（スチレン２量体及びスチレン３量体の含有量の合計：９５０ｐｐｍ）を使用した。
【００４９】
実施例１〜実施例６
東洋スチレン社製ポリスチレン「トーヨースチロールＧＰ ＨＲＭ２６」を１００質量部に対して核剤としてタルクを０．０５質量部、発泡剤としてブタンガスを３質量部使用し、サーキュレーターダイを付したタンデム型押出機にて押し出すことにより厚み２ｍｍ、発泡倍率１０倍の発泡シートを得た。次に参考例に示したゴム変性スチレン系樹脂（ＨＩＰＳ−１〜ＨＩＰＳ−４）とゴム成分を含有しないスチレン系樹脂として、参考例に示したＧＰＰＳ−１またはＧＰＰＳ−２を表２に示す割合でブレンドした後、Ｔダイを付した押出機で溶融混練し、ゴム変性スチレン系樹脂組成物からなる厚さ７５μｍのフィルムを押し出しながら上記発泡シートに積層し、多層シートを得た。得られた多層シートの評価結果を表２に示す。またゴム変性スチレン系樹脂組成物中の未架橋のゴム状重合体の含有量の合計を測定したところ、全ての実施例において０．１質量％未満であった。
【００５０】
比較例１〜比較例９および比較例１１
実施例と同様の方法にて、参考例に示したゴム変性スチレン系樹脂（ＨＩＰＳ−１〜ＨＩＰＳ−９）と参考例に示したＧＰＰＳ−１を表２に示す割合でブレンドした後多層シートを得た。得られた多層シートの評価結果を表３に示す。またゴム変性スチレン系樹脂組成物中の未架橋のゴム状重合体の含有量を測定したところ、比較例１〜比較例９および比較例１１の全てにおいて０．１質量％未満であった。
【００５１】
比較例１０
比較例２と同様の割合でゴム変性スチレン系樹脂とスチレン系樹脂をブレンドし、さら、スチレン系ゴムとしてスチレン−ブタジエンブロック共重合体（旭化成社製：タフプレンＡ）２質量部を添加し、以下実施例及び他の比較例と同様の方法にて押出機で溶融混練し、厚さ７５μｍのフィルムを押し出しながら上記発泡シートに積層し、多層シートを得た。得られた多層シートの評価結果を表３に示す。またゴム変性スチレン系樹脂組成物中の未架橋のゴム状重合体の含有量を測定したところ、２．９質量％であった。
【００５２】
【表１】
【表２】
【表３】
表２，３より、比較例で得られた多層シート及び容器は、スチレン２量体及びスチレン３量体の含有量の合計に対するスチレン２量体及びスチレン３量体のｎ−ヘプタンへの溶出量の合計の比率が著しく大きいものであったり、強度が劣るものであったりしていることが分かる。
【００５６】
【発明の効果】
以上の通り、本発明のゴム変性スチレン系樹脂組成物はスチレン２量体及びスチレン３量体の溶出量がきわめて少なく、かつ強度に優れており、食品包装材料向けに最適である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a styrene-based resin composition for food packaging, in which the elution rate of styrene dimer and styrene trimer produced by radical polymerization is extremely small, and excellent in strength.TheA multilayer sheet formed by molding a resin composition, andTheThe present invention relates to a food container formed by forming a sheet.
[0002]
[Prior art]
A rubber-modified styrene resin composition obtained by imparting impact resistance to a styrene resin is excellent in light weight, rigidity, molding processability, and dimensional stability, and is used in a large amount. In particular, rubber-modified styrene resin compositions composed of rubber-like polymer particles formed by grafting a styrene-based polymer on a rubber-like polymer to form a dispersed phase and a styrene-based polymer that forms a continuous phase are generally used. Since it is produced at low cost by radical polymerization and is excellent in impact resistance, rigidity and moldability, it is widely used in food packaging materials. For example, in some containers molded with styrenic resin foam sheets, to prevent breakage during container molding, contact between food in the container and the inner wall of the container, and damage due to piercing of chopsticks and forks, etc. A rubber-modified styrenic resin composition is laminated. However, in recent years, when these food packaging containers are put into practical use, a problem has been pointed out that styrene dimers and styrene trimers migrate to food. Although research results showing these safety have already been published, there is still a social need to control the amount of elution into food soup.
[0003]
In order to reduce the elution amount of styrene dimer and styrene trimer, it has been known for some time that it is effective to reduce the content of styrene dimer and trimer in the container. However, the rubber-modified styrene resin composition obtained by the radical polymerization method is accompanied by by-production of styrene dimer and styrene trimer at the time of polymerization production, and styrene dimer and styrene trimer have low volatility, As a result, there was a limit in reducing the content of styrene dimer and styrene trimer in the rubber-modified styrene resin.
[0004]
Production methods of rubber-modified styrene resin compositions by radical polymerization include bulk-suspension polymerization methods that are mainly polymerized batchwise, and bulk polymerization methods that are mainly polymerized continuously. It has also been known for some time that the content of styrene dimers and styrene trimers in the rubber-modified styrenic resin is reduced by the production by the polymerization method. However, the bulk-suspension polymerization method is disadvantageous in cost as compared with the bulk polymerization method and has a problem that impurities such as a dispersant are easily mixed, and the bulk polymerization method is currently mainstream. Therefore, in the market, even if rubber-modified styrene resin continuously produced by radical polymerization by a bulk polymerization method is used, development of a food packaging material with a small amount of elution of styrene dimer and styrene trimer is desired. It was rare.
[0005]
As a method for reducing the content of styrene dimer and trimer, for example, JP 2001-206436 and JP 2002-128842 include the content of styrene dimer and styrene trimer obtained by anionic polymerization. A rubber-modified styrene resin composition in which the content of styrene dimer and styrene trimer is reduced by mixing a low-styrene styrene resin with a rubber-modified styrene resin produced by a radical polymerization method is shown. Yes. However, the anionic polymerization method is generally expensive, and in order to reduce the content of the entire styrene dimer and styrene trimer, the styrene dimer produced by anionic polymerization and the styrene type having a small amount of styrene trimer. When the proportion of the resin is increased and the proportion of the rubber-modified styrene resin having a relatively large amount of styrene dimer and styrene trimer produced by radical polymerization is reduced, there is a problem that impact resistance is lowered.
[0006]
On the other hand, JP-A-2001-220477 describes an example using a mixture of a styrene resin and a styrene rubber instead of the rubber-modified styrene resin composition. However, the styrene rubber is more expensive than the rubber-modified styrene resin composition, and when the blending ratio is increased, there is a problem that the styrene rubber is cross-linked by a thermal history and a so-called gel-like substance is generated. In addition, when styrene rubber is mixed, the elution rate of styrene dimer and styrene trimer with respect to the content of styrene dimer and styrene trimer is higher than when the rubber-modified styrene resin composition is used. As a result, there is a problem that the reduction of the elution amount, which is the original purpose, cannot be achieved.
[0007]
[Problems to be solved by the invention]
The problem to be solved by the present invention is for a food packaging material produced by a radical polymerization method, preferably a bulk radical polymerization method, in which the elution amount of styrene dimer and styrene trimer is extremely small and excellent in strength. Rubber-modified styrenic resin composition, andTheA multilayer sheet formed by molding a resin composition, andTheIt intends to propose a food container formed by molding a sheet.
[0008]
[Means for Solving the Problems]
As a result of intensive studies to solve the above-mentioned problems, the present inventors have found that a food packaging container made of a rubber-modified styrene resin composition that satisfies a specific condition is excellent in strength and has a styrene dimer and a styrene trimer. Elution amount of styrene dimer and styrene trimer with respect to content, that is, elution rate of styrene dimer and styrene trimer becomes extremely low, and as a result, styrene dimer and styrene trimer as food packaging containers It was found that the elution amount of can be reduced, and the present invention has been completed.
[0009]
That is, the present invention is obtained by radical polymerization comprising a rubber-like polymer grafted with a styrenic polymer and comprising a rubber-like polymer particle forming a dispersed phase and a styrenic polymer forming a continuous phase. A rubber-modified styrene-based resin composition, wherein the total content of styrene dimer and styrene trimer in the approximate resin composition is2000ppm or less, and
(A)TheThe gel content G of the rubber-like polymer particles is17.4~ 25%,
(B)TheThe median diameter dv of the rubber-like polymer particles is 0.3 to 2.0 μm,
(C)TheIn the rubber-like polymer particles, the volume fraction φ of the rubber-like polymer particles having a particle diameter of 2.5 μm or more is less than 10%,
(D)TheThe gel swelling degree SI of the rubber-like polymer particles is 9.0 to 12.0,
(E) The relationship between the gel content G, the median diameter dv, and the gel swelling degree SI is represented by the formula (1):1,500≦ K ≦5,000The present invention relates to a rubber-modified styrenic resin composition suitable for a food packaging container.
K = exp (G / 8) × exp (dv / 2) × SI2 ... (1)
[0010]
DETAILED DESCRIPTION OF THE INVENTION
The present invention is described in detail below. The rubber-modified styrenic resin composition of the present invention is composed of rubber-like polymer particles formed by grafting a styrenic polymer on a rubber-like polymer to form a dispersed phase, and a styrenic polymer that forms a continuous phase. Basically, a rubber-modified styrene resin obtained by radical polymerization of a styrene monomer in the presence of a rubbery polymer, orTheA main component is a mixture of a rubber-modified styrene resin and a styrene resin not containing a rubber component. As the styrene monomer, known monomers such as styrene, α-methylstyrene, o-methylstyrene, m-methylstyrene, p-methylstyrene, and the like can be used, and styrene is preferable. Styrenic monomers can be used alone or as a mixture. In addition, monomers other than styrenic monomers such as acrylonitrile, (meth) acrylic acid, (meth) acrylic acid ester and the like that can be copolymerized with these styrenic monomers are also included in the rubber-modified styrenic resin composition. It may be added and polymerized so long as the performance is not impaired, that is, 5 parts by mass or less with respect to 100 parts by mass of the styrene monomer. Further, in the present invention, a crosslinking agent such as divinylbenzene may be polymerized by adding less than 1 part by mass to 100 parts by mass of the styrene monomer.
[0011]
The rubbery polymer used in the rubber-modified styrene resin composition of the present invention includes polybutadiene, styrene-butadiene random or block copolymer, polyisoprene, styrene-isoprene random or block copolymer, ethylene-propylene. Examples thereof include rubber, ethylene-propylene-diene rubber and the like, and in particular, random or block copolymers of polybutadiene and styrene-butadiene are preferably used. These may be partially hydrogenated.
[0012]
As a method for producing the rubber-modified styrene resin, a radical polymerization method using an organic polymerization initiator is preferable, and a method of continuous radical polymerization is more preferable from the viewpoint of cost. As the organic polymerization initiator used at this time, it is possible to use 800 to 1200 ppm of an organic peroxide polymerization initiator having a one-hour half-life temperature in the range of 105 to 115 ° C. with respect to the styrene monomer. preferable. Examples of these organic peroxide polymerization initiators include 1,1-bis (t-butylperoxy) -3,3,5-trimethylcyclohexane and 1,1-bis (t-butylperoxy) -cyclohexane. 1,1-bis (t-amylperoxy) -3,3,5-trimethylcyclohexane, 1,1-bis (t-amylperoxy) -cyclohexane, 1,1-bis (t-hexylperoxy) -3,3,5-trimethylcyclohexane, 1,1-bis (t-hexylperoxy) -cyclohexane, 2,2-bis (4,4-di-butylperoxycyclohexyl) propane, t-hexylperoxyisopropyl Examples thereof include monocarbonate and t-amyl peroxyisononanoate. In some cases, a chain transfer agent, for example, t-dodecyl mercaptan, n-dodecyl mercaptan, α-methylstyrene dimer, or the like may be used.
[0013]
In the polymerization process of rubber-modified styrene resin, polymerization of styrene monomerConversionPolymerization is allowed to proceed under temperature conditions where the residual amount of initiator is maintained at 100 ppm or more until at least 60% is reached, and polymerization of the styrene monomer at the exit of the polymerization processConversionUntil the residual amount of the initiator reaches 10 ppm or less, the polymerization is preferably allowed to proceed in a temperature range not exceeding 20 ° C. with respect to the one-hour half-life temperature of the initiator. The residual amount of initiator is calculated by substituting the characteristic value (frequency factor, activation energy), initiator concentration, and processing temperature of the initiator to be used in the following formula (3): From the residence time in the polymerization step, it can be calculated using the formula (2).
[I] = [I]0Xexp (-ct) (2)
c = A × exp (−Ea / RT) (3)
[0014]
However, [I] is the remaining amount of initiator, [I]0Is the initial initiator concentration, t is time, A is the frequency factor, Ea is the activation energy, R is the gas constant, and T is the absolute temperature.
[0015]
The method for producing a styrene resin not containing a rubber component is not particularly limited as long as it is a radical polymerization method, and can be produced by a known method, but when produced by continuous radical polymerization, It is preferable to manufacture under the same conditions.
[0016]
The gel content G of the rubber-like polymer particles constituting the present invention is 10 to 25%, preferably 14 to 20%. If the gel content G of the rubber-like polymer particles is less than 10%, the strength of the resulting rubber-modified styrene resin composition is significantly reduced. When the gel content G of the rubber-like polymer particles exceeds 25%, the elution amount of the styrene dimer and the styrene trimer with respect to the content of the styrene dimer and the styrene trimer, that is, the styrene dimer and the styrene. The elution rate of the trimer is extremely increased, and as a result, the elution amount of the styrene dimer and the styrene trimer is increased. The gel content G of the rubber-like polymer particles is obtained by dissolving a rubber-modified styrene resin composition having a mass of W in methyl ethyl ketone at a ratio of 5%, and centrifuging the solution to settle the insoluble matter. To obtain an insoluble matter, vacuum dried at 70 ° C. for 15 hours, cooled in a desiccator for 20 minutes, and then measured the mass D of the dried insoluble matter to determineBy formulaSeekingThe.
Gel content G (%) = (D / W) × 100
[0017]
The median diameter dv of the rubber-like polymer particles constituting the present invention is 0.3 to 2.0 μm, preferably 0.5 to 1.8 μm, more preferably 0.8 to 1.7 μm. When the median diameter dv is less than 0.3 μm, the strength of the resulting rubber-modified styrenic resin composition is significantly reduced. On the other hand, if it exceeds 2.0 μm, the elution rate of styrene dimer and styrene trimer in the resulting food container is extremely increased, and as a result, the elution rate of styrene dimer and styrene trimer is increased. Become. The median diameter dv is obtained by dissolving a rubber-modified styrene resin in dimethylformamide and measuring it with a laser diffraction particle size distribution analyzer (Laser diffraction particle analyzer LS-230, manufactured by Coulter).TheWith the median particle diameter of the volume-based particle size distribution curve, the median diameter dvdid.
[0018]
Of the rubber-like polymer particles constituting the present invention, the volume fraction φ of rubber-like polymer particles having a particle size of 2.5 μm or more needs to be less than 10%, preferably less than 5%, more preferably Less than 3%. When the volume fraction φ of the rubber-like polymer particles is 10% or more, the elution rate of the styrene dimer and styrene trimer in the resulting food container is extremely increased. As a result, the amount of styrene dimer and styrene trimer is increased. This will increase the amount of body elution. The volume fraction φ of rubber-like polymer particles having a particle size of 2.0 μm or more is obtained by measuring with a laser diffraction particle size distribution analyzer in the same manner as the median diameter dv.The.
[0019]
The structure of the rubber-like polymer particles constituting the present invention is not particularly limited, but a salami structure in which styrene-based polymers are scattered in the rubber phase or a single styrene-based polymer in the rubber phase. Examples thereof include a core-shell structure in which the coalescence is present in the form of particles. Such a structure can be observed by taking an electron micrograph of a thin slice of the resin composition. Moreover, it is preferable that an uncrosslinked rubber-like polymer is less than 1 mass part with respect to 100 mass parts of resin compositions. When the amount of the uncrosslinked rubber-like polymer is 1 part by mass or more, the elution rate of styrene dimer and styrene trimer in the obtained food container is extremely increased, and as a result, elution of styrene dimer and styrene trimer The amount will be increased. The content of the uncrosslinked rubber-like polymer is determined by dissolving the resin composition in toluene, centrifuging the solution to settle the insoluble matter, and measuring the rubber content of the sample obtained by vacuum drying the supernatant. Asked byThe. For rubber, dissolve the sample in chloroform, add a certain amount of iodine monochloride / carbon tetrachloride solution, leave it in the dark for about 1 hour, add potassium iodide solution, and add 0.1 I of excess iodine monochloride. Titrate with sodium thiosulfate / ethanol aqueous solution and obtain from the amount of added iodine monochlorideThe.
[0020]
The gel swelling degree SI of the rubber-like polymer particles constituting the present invention is 9.0 to 12.0, preferably 9.5 to 11.0. If it is less than 9, the strength of the resulting multilayer sheet and container will be reduced. On the other hand, if it exceeds 12.0, the elution rate of the styrene dimer and styrene trimer in the food container to be obtained is extremely increased. As a result, the elution amount of the styrene dimer and styrene trimer is increased. The gel swelling degree SI of the rubber-like polymer particles is determined by dissolving the resin composition in toluene, centrifuging the solution to settle the insoluble matter, removing the supernatant by decantation, and swelling the toluene with toluene. Measure the mass S of the minutedid. Next, the insoluble matter swollen with toluene was vacuum-dried at 70 ° C. for 15 hours, cooled in a desiccator for 20 minutes, and then the dry mass D of the insoluble matter was measured to determineBy formulaSeekingThe.
Swelling degree SI = S / D
[0021]
The relationship between the gel content G, the median diameter dv, and the gel swelling degree SI of the rubber-like polymer particles constituting the present invention is 700 ≦ K ≦ 6000, preferably 1500 ≦ K ≦ 5000, more preferably in the formula (1). It is necessary to satisfy 2000 ≦ K ≦ 4000.
K = exp (G / 8) × exp (dv / 2) × SI2 ... (1)
[0022]
Examples of the relationship between the gel content G of the rubber-like polymer particles constituting the present invention, the median diameter dv, the gel swelling degree SI, and the K value of the above formula are shown in FIGS. Even if the unit diameter dv and the gel swelling degree SI are within the respective claims, if the K value does not satisfy the claims, that is, if it deviates from the range indicated by the oblique lines in the figure, the desired rubber-modified styrenic resin A composition cannot be obtained. That is, when the K value is less than 700, the strength of the resulting rubber-modified styrenic resin composition is significantly reduced. Moreover, when K value exceeds 6000, the elution rate of the styrene dimer and the styrene trimer of the food container obtained increases extremely, and as a result, the elution amount of the styrene dimer and the styrene trimer increases. It becomes.
[0023]
[Figure 1]
[Figure 2]
[Fig. 3]
[Fig. 4]
The total content of the styrene dimer and styrene trimer in the rubber-modified styrene resin composition in the present invention is 2500 ppm or less, preferably 2200 ppm or less, more preferably 2000 ppm or less. When the total content of styrene dimer and styrene trimer exceeds 2500 ppm, the elution rate of styrene dimer and styrene trimer in the resulting food container is extremely increased, resulting in styrene dimer and styrene. The amount of trimer elution will be increased.
[0028]
In the present invention, the styrene dimer refers to 1,3-diphenylpropane, 2,4-diphenyl-1-butene, and 1,2-diphenylcyclobutane. The styrene trimer refers to 1-phenyl-4- (1'-phenylethyl) tetralin and 2,4,6-triphenyl-1-hexene.
[0029]
The rubber-modified styrenic resin composition of the present invention may contain known additions such as higher fatty acids, higher fatty acid salts, higher fatty acid amides, mineral oils, antioxidants, antifungal agents, antistatic agents, flame retardants, and sliding agents as desired. An agent may be added.
[0030]
The rubber-modified styrenic resin composition of the present invention can be used in at least one of the outermost layers of a multilayer sheet for food packaging. If there is at least one outermost layer mainly composed of the rubber-modified styrenic resin composition, the other layers are not particularly limited and a known layer can be used. Most preferred. The shape of the layer mainly composed of the rubber-modified styrenic resin composition may be a foam or a film, but is most preferably a film having a thickness of 10 to 1000 μm. The thickness of the multilayer sheet is not particularly limited, but is preferably 0.1 to 10 mm.
[0031]
A known method can be adopted as a method for producing the multilayer sheet for food packaging. For example, a method of manufacturing using a multilayer sheet extruder provided with a feed block, a method of fusing a film to a sheet manufactured by a sheet extruder or the like can be employed.
[0032]
The multi-layer sheet for food packaging can be molded so that the layer mainly composed of the rubber-modified styrene resin composition is inside the container to produce a food packaging container. The molding method is not particularly limited, and vacuum molding, pressure molding, or the like can be employed.
[0033]
When the food packaging container thus obtained is used, the total elution amount of styrene dimer and styrene trimer into n-heptane is 50 ppb or less, preferably 40 ppb or less, and more preferably 30 ppb.
[0034]
【Example】
EXAMPLES Hereinafter, although an Example is given and this invention is demonstrated concretely, this invention is not limited to these Examples. First, the evaluation method in the present invention will be described below.
[0035]
(1) Measurement of styrene dimer and styrene trimer in resin
200 mg of the resin is dissolved in 2 mL of 1,2-dichloromethane, 2 mL of methanol is added to precipitate the rubber-modified styrenic resin and allowed to stand, and then the supernatant is subjected to gas chromatography HP-5890 manufactured by Hewlett-Packard Company. And measured. Detailed conditions are described below.
(A) Column: DB-1 (ht) 0.25 mm × 30 m Film thickness 0.1 μm
(B) Injection temperature: 250 ° C
(C) Column temperature: 100 to 300 ° C
(2) Detector temperature: 300 ° C
(E) Split ratio: 50/1
(F) Internal standard substance: n-eicosane
[0036]
(2) Measurement of elution amount of styrene dimer and styrene trimer into n-heptane
The multilayer sheet was heated in an oven at 170 ° C. for 25 seconds, and a truncated cone-shaped container having a diameter of 160 mm, a bottom diameter of 140 mm, and a depth of 55 mm was molded so that the rubber-modified styrene resin composition was inside. 550 mL of n-heptane was put in this container, and the solution obtained by elution at 25 ° C. for 60 minutes was measured by GC / MS-SIM. Detailed conditions are described below.
(A) Column: DB-5 0.25 mm × 30 m, film thickness 0.25 μm
(B) Injection temperature: 250 ° C
(C) Column temperature: 120 ° C. (5 minutes) → 10 ° C./minute→250° C. (12 minutes)
(2) Interface: 200 ° C
(E) Ionization voltage: 70 eV
(F) Split ratio: 30
[0037]
(3) Strength: The container obtained in (2) was compressed from the bottom using a Tensilon measuring machine at a speed of 400 mm / min and indicated by the strength when the container was buckled.
[0038]
Reference example 1
Polymerization of rubber-modified styrene resin (HIPS-1)
A fully mixed reactor with a stirring blade with a volume of 25L is used as the first reactor, a tower-type plug flow reactor with a stirring blade with a volume of 40L is used as a second reactor, and a tower-type plug flow reaction with a stirring blade with a volume of 50L. The reactor was the third reactor, the static mixer type plug flow reactor of 50 L capacity was the fourth reactor, and each was connected in series to constitute the polymerization step. The number of stirring in the first reactor is 110 rpm and the reaction temperature is 105 ° C., the number of stirring in the second reactor is 110 rpm, and the reaction temperature is adjusted so that a temperature gradient of 107 ° C. to 110 ° C. is formed in the flow direction of the reaction solution. The number of stirring in the third reactor is 110 rpm, the reaction temperature is adjusted so that a temperature gradient of 112 ° C. to 118 ° C. is created in the flow direction of the reaction solution, and the temperature of the fourth reactor is 120 ° C. to 125 ° C. in the flow direction The gradient was adjusted.
75.3 parts by mass of styrene monomer, 18.8 parts by mass of ethylbenzene, 5.9 parts by mass of polybutadiene “diene 55AE” manufactured by Asahi Kasei Co., Ltd. as a rubbery polymer, and 1,1-bis (t-butyl) as a polymerization initiator Peroxy) cyclohexane (one-hour half-life temperature 111 ° C.) 0.026 parts by mass was added and dissolved in the first reactor at a supply rate of 23 L / hr for polymerization, followed by polymerization. The reactor was continuously charged and polymerized. At the outlet of the first reactor, the rubber-like polymer has not yet been dispersed into particles (phase inversion), and at the outlet of the second reactor, the polymerization liquid has been dispersed into particles. The conversion was 23%. Next, 0.038 parts by mass of 1,1-bis (t-butylperoxy) cyclohexane is added to the polymerization solution from the outlet of the second reactor, and then the polymerization solution is continuously added to the third reactor. I was charged. The polymerization conversion rate at the outlet of the third reactor was 57%, and the amount of the polymerization initiator remaining in this polymerization solution was 150 ppm with respect to the styrene monomer at the initial stage of polymerization. Further, the polymerization was allowed to proceed in the fourth reactor until the polymerization conversion rate reached 92%, and the amount of the polymerization initiator remaining in this polymerization solution was 5 ppm relative to the styrene monomer at the initial stage of polymerization. The resulting polymer solution is heated at 230 ° C. in a 3-bent twin screw extruder while removing volatile components such as unreacted styrene and the solvent ethylbenzene under reduced pressure, then cooled and cut to form a pellet. The rubber-modified styrene resin (HIPS-1) was obtained. Table 1 shows the physical properties of the rubber-modified styrenic resin obtained.
[0039]
Reference example 2
Polymerization of rubber-modified styrene resin (HIPS-2)
Except that the number of stirrings in the second reactor was 180 rpm, everything was produced in the same manner as in Reference Example 1 to obtain a rubber-modified styrene resin (HIPS-2). The polymerization conversion rate at the outlets of the second reactor, the third reactor and the fourth reactor, the residual initiator amount at the outlets of the third reactor and the fourth reactor, and the physical properties of the obtained rubber-modified styrene resin Table 1 shows.
[0040]
Reference example 3
Polymerization of rubber-modified styrene resin (HIPS-3)
Except that the heat treatment at 210 ° C. was performed in a twin-screw extruder, everything was produced in the same manner as in Reference Example 1 to obtain a rubber-modified styrene resin (HIPS-3). The polymerization conversion rate at the outlets of the second reactor, the third reactor and the fourth reactor, the residual initiator amount at the outlets of the third reactor and the fourth reactor, and the physical properties of the obtained rubber-modified styrene resin Table 1 shows.
[0041]
Reference example 4
Polymerization of rubber-modified styrene resin (HIPS-4)
The rubber was produced in the same manner as in Reference Example 1 except that the number of stirring in the second reactor was 100 rpm and 0.038 parts by mass of 1,1-bis (t-butylperoxy) cyclohexane added to the raw material liquid was used. A modified styrene resin (HIPS-4) was obtained. The polymerization conversion rate at the outlets of the second reactor, the third reactor and the fourth reactor, the residual initiator amount at the outlets of the third reactor and the fourth reactor, and the physical properties of the obtained rubber-modified styrene resin Table 1 shows.
[0042]
Reference Example 5
Polymerization of rubber-modified styrene resin (HIPS-5)
Except that the number of stirrings in the second reactor was 80 rpm, production was carried out in the same manner as in Reference Example 1 to obtain a rubber-modified styrene resin (HIPS-5). The polymerization conversion rate at the outlets of the second reactor, the third reactor and the fourth reactor, the residual initiator amount at the outlets of the third reactor and the fourth reactor, and the physical properties of the obtained rubber-modified styrene resin Table 1 shows.
[0043]
Reference Example 6
Polymerization of rubber-modified styrene resin (HIPS-6)
The second reactor was a fully mixed reactor with a volume of 40 L, the reaction temperature was set to 108 ° C., and the number of stirrings was set to 130 rpm. A rubber-like polymer was dispersed into particles in a vessel, and the other conditions were the same as in Reference Example 1 to obtain a rubber-modified styrene resin (HIPS-6). The polymerization conversion rate at the outlets of the second reactor, the third reactor and the fourth reactor, the residual initiator amount at the outlets of the third reactor and the fourth reactor, and the physical properties of the obtained rubber-modified styrene resin Table 1 shows.
[0044]
Reference Example 7
Polymerization of rubber-modified styrene resin (HIPS-7)
Except that the heat treatment at 250 ° C. was performed in a twin-screw extruder, all were manufactured in the same manner as in Reference Example 1 to obtain a rubber-modified styrene resin (HIPS-7). The polymerization conversion rate at the outlets of the second reactor, the third reactor and the fourth reactor, the residual initiator amount at the outlets of the third reactor and the fourth reactor, and the physical properties of the obtained rubber-modified styrene resin Table 1 shows.
[0045]
Reference Example 8
Polymerization of rubber-modified styrene resin (HIPS-8)
Except that the heat treatment at 190 ° C. was performed in a twin-screw extruder, all were manufactured in the same manner as in Reference Example 1 to obtain a rubber-modified styrene resin (HIPS-8). The polymerization conversion rate at the outlets of the second reactor, the third reactor and the fourth reactor, the residual initiator amount at the outlets of the third reactor and the fourth reactor, and the physical properties of the obtained rubber-modified styrene resin Table 1 shows.
[0046]
Reference Example 9
Polymerization of rubber-modified styrene resin (HIPS-9)
Except that the reaction temperature of the fourth reactor was adjusted so that a temperature gradient of 150 ° C. to 155 ° C. was established in the flow direction of the reaction solution, the reaction was conducted in the same manner as in Reference Example 1, and a rubber-modified styrene resin ( HIPS-9) was obtained. The polymerization conversion rate at the outlets of the second reactor, the third reactor and the fourth reactor, the residual initiator amount at the outlets of the third reactor and the fourth reactor, and the physical properties of the obtained rubber-modified styrene resin Table 1 shows.
[0047]
Reference Example 10
Polymerization of styrene resin (GPPS-1)
The same polymerization step as in Reference Example 6 was configured, the reaction temperature in the first reactor was 101 ° C, the reaction temperature in the second reactor was 103 ° C, and the reaction temperature in the third reactor was in the direction of the reaction liquid flow. The temperature was adjusted so that a temperature gradient of 103 ° C. to 105 ° C. was obtained, and the fourth reactor was adjusted so as to have a temperature gradient of 107 ° C. to 116 ° C. in the flow direction.
Supply a raw material solution prepared by adding 80 parts by mass of a styrene monomer, 20 parts by mass of ethylbenzene, and 0.08 parts by mass of 1,1-bis (t-butylperoxy) cyclohexane as a polymerization initiator to supply 23 L / hr. After continuously feeding and polymerizing to the first reactor at a speed, polymerization is continuously performed in the second reactor, the third reactor, and the fourth reactor, and the polymerization proceeds until the polymerization conversion rate becomes 92%. I let you. A styrene resin (GPPS-1) was obtained from the obtained polymerization solution in the same manner as in Reference Example 6. The total content of styrene dimer and styrene trimer in the obtained styrene-based resin was 1500 ppm.
[0048]
Reference Example 11
As the styrene-based resin (GPPS-2), polystyrene “Toyostyrene GP HRM56” (total content of styrene dimer and styrene trimer: 950 ppm) manufactured by Toyo Styrene Co., Ltd. was used.
[0049]
Example 1 to Example6
To a tandem type extruder equipped with a circulator die, Toyo Styrene Co., Ltd. uses polystyrene “Toyostyrene GP HRM26” as a nucleating agent for 0.05 parts by mass of talc and 3 parts by mass of butane gas as a blowing agent. To obtain a foamed sheet having a thickness of 2 mm and a foaming ratio of 10 times. Next, as a rubber-modified styrene resin (HIPS-1 to HIPS-4) shown in Reference Example and a styrene resin not containing a rubber component, GPPS-1 or GPPS-2 shown in Reference Example is shown in Table 2. After blending, the mixture was melt-kneaded with an extruder equipped with a T-die and laminated on the foamed sheet while extruding a 75 μm-thick film made of a rubber-modified styrenic resin composition to obtain a multilayer sheet. Table 2 shows the evaluation results of the obtained multilayer sheet. Moreover, when the total content of the uncrosslinked rubber-like polymer in the rubber-modified styrenic resin composition was measured, it was less than 0.1% by mass in all Examples.
[0050]
Comparative Example 1 to Comparative Example 9And Comparative Example 11
In the same manner as in the examples, the rubber-modified styrenic resins (HIPS-1 to HIPS-9) shown in the reference example and GPPS-1 shown in the reference example were blended in the proportions shown in Table 2, and then the multilayer sheet was formed. Obtained. Table 3 shows the evaluation results of the obtained multilayer sheet. Further, when the content of the uncrosslinked rubber-like polymer in the rubber-modified styrenic resin composition was measured, Comparative Examples 1 to 9And Comparative Example 11In all, the amount was less than 0.1% by mass.
[0051]
Comparative Example 10
A rubber-modified styrene-based resin and a styrene-based resin are blended in the same proportion as in Comparative Example 2, and 2 parts by mass of a styrene-butadiene block copolymer (Tafprene A manufactured by Asahi Kasei Co., Ltd.) is added as a styrene rubber. It melt-kneaded with the extruder by the method similar to an Example and another comparative example, and it laminated | stacked on the said foamed sheet, extruding the film of thickness 75 micrometers, and obtained the multilayer sheet. Table 3 shows the evaluation results of the obtained multilayer sheet. The content of the uncrosslinked rubber-like polymer in the rubber-modified styrene resin composition was measured and found to be 2.9% by mass.
[0052]
[Table 1]
[Table 2]
[Table 3]
From Tables 2 and 3, the multilayer sheets and containers obtained in the comparative examples show the amount of styrene dimer and styrene trimer eluted into n-heptane with respect to the total content of styrene dimer and styrene trimer. It can be seen that the ratio of the total is significantly large or the strength is inferior.
[0056]
【The invention's effect】
As described above, the rubber-modified styrene resin composition of the present invention has an extremely small elution amount of styrene dimer and styrene trimer and is excellent in strength, and is optimal for food packaging materials.
Claims (7)
（Ａ）ゴム状重合体粒子のゲル分Ｇが１７．４〜２５％であり、かつ
（Ｂ）該ゴム状重合体粒子の中位径ｄｖが０．３〜２．０μｍであり、
（Ｃ）該ゴム状重合体粒子中、２．５μｍ以上の粒径を有するゴム状重合体粒子の体積分率φが１０％未満であり、
（Ｄ）該ゴム状重合体粒子のゲル膨潤度ＳＩが９．０〜１２．０であり、
（Ｅ）ゲル分Ｇと中位径ｄｖとゲル膨潤度ＳＩの関係が式（１）において、１，５００≦Ｋ≦５，０００を満足することを特徴とする、食品包装材料に適したゴム変性スチレン系樹脂組成物
Ｋ＝ｅｘｐ（Ｇ／８）×ｅｘｐ（ｄｖ／２）×ＳＩ ２ ・・・（１）A rubber-modified styrene resin composition composed of rubber-like polymer particles formed by grafting polystyrene onto polybutadiene and forming a dispersed phase, and polystyrene forming a continuous phase, and obtained by radical polymerization, wherein the resin composition And the total content of styrene dimer and styrene trimer is 2000 ppm or less, and (A) the gel content G of the rubber-like polymer particles is 17.4 to 25%, and (B) the The median diameter dv of the rubber-like polymer particles is 0.3 to 2.0 μm,
(C) In the rubbery polymer particles, the volume fraction φ of the rubbery polymer particles having a particle diameter of 2.5 μm or more is less than 10%,
(D) The gel swelling degree SI of the rubbery polymer particles is 9.0 to 12.0,
(E) A rubber suitable for a food packaging material, wherein the relationship between the gel content G, the median diameter dv, and the gel swelling degree SI satisfies 1,500 ≦ K ≦ 5,000 in the formula (1) Modified styrenic resin composition K = exp (G / 8) × exp (dv / 2) × SI 2 (1)
（Ｆ）１時間半減期温度が１０５〜１１５℃の範囲にある有機過酸化物重合開始剤をスチレン系単量体に対して８００〜１２００ｐｐｍ使用し、
（Ｇ）スチレン系単量体の重合転化率が少なくとも６０％に到達するまで、開始剤の残存量が１００ｐｐｍ以上を維持する温度条件下にて重合を進行させ、
（Ｈ）重合工程出口におけるスチレン系単量体の重合転化率が９０％以上かつ開始剤の残存量が１０ｐｐｍ以下となるまで、開始剤の１時間半減期温度に対して２０℃を越えない重合温度範囲にて重合を進行させ得られることを特徴とする、食品包装材料に適したゴム変性スチレン系樹脂組成物。A rubber-modified styrenic resin obtained by continuous radical polymerization of the rubber-modified styrenic resin composition according to claim 1, or a styrene-based resin not containing a rubber component obtained by radical polymerization and the rubber-modified styrenic resin. The rubber-modified styrenic resin, which is mainly composed of a mixture,
(F) An organic peroxide polymerization initiator having a one-hour half-life temperature in the range of 105 to 115 ° C. is used in an amount of 800 to 1200 ppm based on the styrene monomer,
(G) until the polymerization conversion of the styrene monomer reaches at least 60%, the polymerization is allowed to proceed under a temperature condition in which the residual amount of the initiator is maintained at 100 ppm or more,
(H) Polymerization not exceeding 20 ° C. with respect to the one-hour half-life temperature of the initiator until the polymerization conversion rate of the styrene monomer at the outlet of the polymerization step is 90% or more and the residual amount of the initiator is 10 ppm or less. A rubber-modified styrenic resin composition suitable for food packaging materials, wherein the rubber-modified styrenic resin composition is suitable for food packaging materials.
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