JP4422985B2

JP4422985B2 - Styrene resin foam sheet, molded container, and method for producing the same

Info

Publication number: JP4422985B2
Application number: JP2003201962A
Authority: JP
Inventors: 茂小林; 輝岩井
Original assignee: Sekisui Kasei Co Ltd
Current assignee: Sekisui Kasei Co Ltd
Priority date: 2003-07-25
Filing date: 2003-07-25
Publication date: 2010-03-03
Anticipated expiration: 2023-07-25
Also published as: JP2005041516A

Description

【０００１】
【発明の属する技術分野】
本発明は薄肉で表面性が優れた美麗なスチレン系樹脂発泡シート成形容器とその経済的に有利な製法、および二次発泡シートに関する。
【０００２】
【従来の技術】
スチレン系樹脂発泡シート成形容器は安価で、強度があり、清潔感、断熱性がある事等より、各種食品のトレー、容器として大量に使用されていた。しかし、容器の肉厚みが厚い為に、容器が嵩張る、シャープな形状容器が得られない、表面が粗くギラギラした光沢がある等の問題があり、総菜容器等の分野でスチレン樹脂シートやオレフィン樹脂シートからの成形容器に替えられている。この為に、スチレン系樹脂発泡シート成形容器の低発泡化による薄肉化が進められている。容器を薄肉化した場合、容器の嵩張りや形状の出方は改善されるが、容器の透明性が増し、中身が透けて見える等の問題の他に、発泡シートを構成するセル自身が見えて容器表面が汚く見える問題が生じている。この課題を解決するために樹脂に無機充填剤を添加して樹脂自身を不透明化する方法が挙げられるが、充填剤の凝集物が発生し、押出発泡シート化時のダイス詰まりや発泡シートの穴あきが多発する等の生産上の問題が多い。又、発泡シートのセル径を細かくして、厚み方向にセルを多数個積み重ねて透明性を抑える方法があるが、汎用のブタン等の炭化水素を発泡剤とする場合、造核剤の添加量を増やしても厚み方向１mm当たり１５個程度以下のセル積み重ねしか実現できず、透明性を抑える効果は少ない。炭酸ガスや水等の特殊な発泡剤を使用すると厚み方向１mm当たり３０個以上のセル積み重ねが可能となるが、製造設備上に問題がある他に、スチレン系発泡シートの発泡倍率は２〜３倍程度しか得られず、更に、加熱による二次発泡性が乏しく、強度が弱く、断熱性の劣る容器しか得られない。
【０００３】
発泡シートの透明性を下げる手段として特開平２００１−３０１１０３の如く押出時の発泡シート表面を急冷し、成形時の加熱で微細セルを発生させる方法がある。この方法では発泡シートの透明性は低下し、表面性も改善されるが、薄肉容器の透明性を改善するには不十分である。又、特公平５−４９７０１にはセル径０．２５mm以下のセルとセル径０．４〜１mmのセルよりなる複合セルの押出発泡体が提案されている。本提案は断熱発泡ボードに関する物で、成形容器に使用されるスチレン系樹脂発泡シートを対象としていない。そのためにスチレン系樹脂発泡シートに比較して、発泡体の厚みが厚く、セル径が大きすぎる。又、押出発泡体に存在するセルは成形時の加熱で肥大して表面性を低下させるばかりか、成形性を低下させて好ましくない。
【０００４】
【特許文献１】
特開平２００１−３０１１０３
【０００５】
【特許文献２】
特公平５−４９７０１
【０００６】
【解決しようとする課題】
本発明は、スチレン系樹脂発泡シートの厚みを薄くして成形容器の嵩高性等を改善するに当たり、発泡シートの厚みを薄くする事により新たに発生する容器の透明性等の問題点を改善し、成形性、型決まり性が良好で、美麗な容器を得る事、並びに、特別な設備を使用する事無く汎用の押出発泡設備を使用し、製造時の穴開き等の収率低下等を起こさず、経済的に有利な薄肉のスチレン系樹脂発泡シートを作成し、該容器を得る事を目的とする。
【０００７】
【問題を解決するための手段】
本発明は、発泡シート全体のセル径を小さくして厚み方向の積み重ねセル数を多くしなくても、大きなセルとセルの間に微細なセルを発泡シートの中央部まで全体に発生させて複合セル構造にすると発泡容器の透明性を抑えられると着目すると共に、成形性への影響を考慮して、成形時の加熱で始めて発泡シートの中央部まで全体に微細セルが発生する方法の開発に取り組み、本発明に到達したものである。
すなわち、本発明は
（１）大きなセルとセルの間に、大きなセルの平均セル径の１／３以下の微細セルが存在する複合セル構造を有するスチレン系樹脂発泡シートからなると共に、上記大きなセルの平均セル径が０．２〜０．５ｍｍ、微細セルのセル径が０．１ｍｍ以下であり、上記微細セルが上記スチレン系樹脂発泡シートの表面から中央部まで発泡層全体にわたり存在していることを特徴とするスチレン系樹脂発泡シート成形容器（請求項１）
（２）スチレン系樹脂に造核剤としてタルク、低級炭化水素発泡剤としてブタンおよび重曹−クエン酸系の化学発泡剤を添加し、ダイスを通して押出発泡させてスチレン系樹脂発泡シートを得、養生後、ヒーター加熱で二次発泡させ、平均セル径が０．２〜０．５ｍｍの大きなセルとセルの間に、上記大きなセルの平均セル径の１／３以下で且つセル径が０．１ｍｍ以下の微細セルがスチレン系樹脂発泡シートの表面から中央部まで発泡層全体にわたり存在する複合セル構造を有するスチレン系樹脂発泡シートとし、これを金型にて成形することをスチレン系樹脂発泡シート成形容器の製造方法（請求項２）
（３）大きなセルとセルの間に、大きなセルの平均セル径の１／３以下の微細セルが存在する複合セル構造を有するスチレン系樹脂発泡シートからなると共に、上記大きなセルの平均セル径が０．２〜０．５ｍｍ、微細セルのセル径が０．１ｍｍ以下であり、上記微細セルが上記スチレン系樹脂発泡シートの表面から中央部まで発泡層全体にわたり存在していることを特徴とするスチレン系樹脂発泡シート（請求項３）。
に関する。
【０００８】
【発明の実施の形態】
スチレン系樹脂発泡シートからの成形容器であって、発泡シート層が大きなセル間に微細セルが存在する複合セル状態になっている例（図３）、並びに、成形時の加熱で発泡シートの中央部まで、大きなセルとセルの間に微細セルが発生する状況を図１、２で示す。
【０００９】
図１：本発明の製法に従う押出発泡シートを、押出後２週間養生した時のセル写真の複写である。表面に０．１mm径程度の小さなセルが１〜２層存在するが、その他の部分は０．２〜０．４mm径のセルで充たされている。
【００１０】
図２：この発泡シートをヒーターで加熱して２．２５倍まで二次発泡させたシートのセル写真の複写である。発泡シートの表面から中央部まで二次発泡シート全体にわたり０．２〜０．５mm径の大きなセルの間に０．１mm径以下の微細なセルが多量に発生している。
【００１１】
図３：この二次発泡したシートを金型で成形した容器の底部のセル写真の複写である。成形化によりセルが扁平になった以外図２と同様で、０．２〜０．５mm（平均０．３５mm）径の大きなセルの間に０．１mm径以下の微細セルが発泡シートの表面から中央部まで発泡層全体にわたり多量に存在している。
【００１２】
押出発泡シートに見られたセルが二次発泡で厚み方向に大きくなり、他方、セルとセルの界面、及び、発泡シートの表面に、押出発泡シートでは見られなかった微細セルが新しく発生している状況が判る。ここで述べる押出発泡シートでは見られなかったとは、少なくとも５０倍の拡大観察で見られなかった事をさす。
【００１３】
押出発泡シートから見られる大きなセルのセル径は、小さい方が発泡シートの表面が綺麗になり、透明性を低下させる上で好ましい。しかし、セルを小さくするために造核剤を増量する等の処置によりシート穴開き、ダイス詰まり等の製造上の問題が多発する事、及び、微細セルを発生させると発泡シートの表面が美麗になると共に透明性も低下する事から特に小さくする必要がなく、平均セル径は０．２〜０．５mmであれば良い。尚、ここで示す平均セル径とは、シートの流れ方向（押出し方向：ＭＤ方向）に沿って切断した断面の顕微鏡写真を撮影し、０．１ｍｍ以下の微細セルを除く全セルの最大セル径を測定し、平均化したものである。
【００１４】
発生する微細セルは、押出発泡シートのセル径、発泡剤の種類・量、押出発泡シート化時のシート冷却条件等により、発生する量並びにセル径が変化するが、成形時の加熱条件の影響が特に顕著である。成形時の加熱で加熱前の発泡シート厚みに対し、１．７倍以上、好ましくは２．０倍以上の厚みに発泡させる。１．７倍未満の発泡加熱では微細セルが発生し難い。発生する微細セルのセル径は、押出発泡シートからの大きなセルの平均セル径の１／３以下で、０．１mm以下が好ましい。セル径が小さく、厚み方向に多層に発生する方が望ましいが、押出発泡シートからのセルの平均セル径の１／３以下であれば成形容器の表面性を向上させる上で効果があり、０．１mm以下であると透明性低下効果も発揮される。
【００１５】
本発明の透明性を低下させる効果を発揮するには薄肉の容器が好ましく、又、成形容器の嵩高性を下げるには側壁厚みが薄い方が良い。しかし、厚みが薄すぎると成形性確保が難しく、容器強度が低下するため、成形容器の側壁厚みは０．７〜１．５mmが好ましい。この側壁厚みの容器を得るためには、成形加熱前の押出発泡シート厚みは０．６〜１．３mmが適正である。又、発泡倍率は成形容器の強度を確保する上で高い方が好ましいが、成形容器の厚みを抑えるには倍率が低い方が良く、２．５〜５倍が好ましい。
【００１６】
成形時の加熱でセルとセルの間に微細セルが発生するスチレン系樹脂発泡シートは、ポリスチレン単独重合樹脂、スチレンを主成分とする（メタ）アクリル酸、無水マレイン酸、アクリロニトリル、アクリル（メタ）アクリレート等のビニル系モノマー及びブタジエン、イソプレン等のジエン系モノマーとの共重合樹脂、及びこれらの樹脂間、並びに、これら樹脂とポリフェニレンオキサイド等との混合樹脂よりなるポリスチレン系樹脂と、必要に応じて流動パラフィン等の可塑剤、エチレンビスステアリルアミド等の滑剤等を混合した樹脂を汎用の押出発泡設備を使用して押出発泡シート化した物、及び、必要に応じてこの押出発泡シートの片面又は両面にスチレン系樹脂フィルム、オレフィン系樹脂フィルム、ポリエステル系樹脂フィルム、酢酸ビニル系樹脂フィルム等の各種フィルムを直接又は接着剤層等を介して積層した物を指す。
【００１７】
押出発泡シート化するに当たり、スチレン系樹脂に造核剤として多孔質無機粉末として、例えば、炭酸カルシウム、硫酸バリウム、シリカ、酸化チタン、クレー、酸化アルミニウム、ベントナイト、ケイソウ土、タルク等が使用できるが、このうち特に好ましくは、タルクを選択し、これと共に化学発泡剤、例えば無機系二酸化炭素発生剤と弱酸を組み合わせたものや、アゾジカルボンアミド、アゾビスホルムアミド、アゾビスイソブチロニトリル、Ｎ，Ｎ’−ジニトロペンタテトラミン、重曹−クエン酸系の化学発泡剤などから、特に好ましくは、重曹−クエン酸系の化学発泡剤を選択して樹脂に混合し、押出機内で混練、樹脂を溶融させる。この後低級炭化水素、特に好ましくはブタンを発泡剤として圧入した後、発泡適正温度まで冷却してダイスを通して押出発泡させる。この過程での化学発泡剤の作用効果は明らかではないが、押出発泡時の造核剤としては殆ど作用せず押出発泡シートに微細セルを形成させない条件が容易に選択できる。
【００１８】
ダイスを通し押出発泡させたシートの内外両表面は、ダイス出直後に２０〜４０℃の冷却エアーを吹き付けて急冷する。冷却度合いは吐出樹脂Kg当たり０．７〜１．４ｍ³のエアーを吹き付ける事が好ましい。表面を冷却する事により、加熱による二次発泡で二次発泡シートの表面に微細セルが発生するようになり、発泡シートの表面から中央部まで均等に微細セルを発生させる上で効果がある。エアリング量が吐出樹脂Kg当たり０．７ｍ³未満では、表面に発生する微細セルの量が少ないし、１．４ｍ³を越えて吹き付けた場合、シートの引き取り抵抗が強く、マンドレル面との擦れ等による粉や筋が発生して問題になることがある。
【００１９】
この様にして得られたスチレン系樹脂発泡シートは、１週間程度以上室温で養生した後に汎用のシート成形機を用いてヒーターで加熱し、加熱前のシート厚みに対して１．７倍以上、好ましくは２．０倍以上に発泡させ、雌雄嵌合金型によるマッチモールド成形、及び、雌又は雄型の一方金型による真空成形又は圧空成形で成形される。
【００２０】
この様にして得られた成形容器は嵩が低く、シャープな形状の容器が得られると共に、不透明性の高い、美麗な容器が得られる。又、強度、脆性も優れた容器が得られる。
【００２１】
【実施例】
以下に実施例を掲げて本発明を具体的に説明するが、本発明はこれらに限定されるものではない。
（実施例１，２）
市販Ａ＆Ｍ社製スチレン樹脂６８５に造核剤としてステアリン酸マグネシウムを１０％混合したタルク（松村産業社製）、及び、重曹−クエン酸系の化学発泡剤（大日精化社製ダイブローＮｏ．２）を表１記載の如く混合して、汎用の発泡シート押出機（東芝機械社製）に投入、押出機内で混練・溶融した後、イソブタン８５％／ノルマルブタン１５％の混合ブタンを圧入、その後発泡適正温度まで冷却後、サーキュラーダイスより２４０Ｋｇ／Ｈの吐出量で押し出した。ダイスから押し出したシートの内側表面には２５℃のエアーを４ｍ³／Ｍｉｎ、外側表面には２５℃のエアーを３ｍ³／Ｍｉｎで吹き付けて表面を急冷した。発泡シートの押出中にはシート穴開きや凝集物等によるダイス詰まり等なく、収率ロスはなかった。
【００２２】
得られた押出発泡シートの特性は表の如くで、ブタン圧入量の微調整で厚み１mm強、発泡倍率が約２．８倍の押出発泡シートを得た。実施例１に基づき得られた押出発泡シートの２週間養生後のセル状態を図１（顕微鏡写真の複写）に示す。表面に０．１mm径程度の小さなセルが僅かに存在するが、その他の部分は０．２〜０．４mm（平均０．３２mm）径のセルで充たされており、セルとセルの間の部分には微細セルがみられなかった。実施例２のセル状態は、セルが全体に小さくなり、数が増えている以外の実施例１と同様であった。
【００２３】
これらの押出発泡シートを市販のセンバ鉄工社製単発成形機（チヤレンジャーＶＡＳ）にて、雰囲気温度１５０℃のヒーター加熱炉内で２４秒間加熱して約２．３倍に二次発泡した。実施例１の二次発泡シートのセル状態を示したのが図２（顕微鏡写真の複写）である（注：図１，３，４と拡大倍率が異なる）。図示の如く、発泡シート表層から中央部まで、押出発泡時に出来た大きなセルとセルの間に微細セルが多数発生していた。
【００２４】
加熱二次発泡したシートは直ちに雌雄嵌合金型で仕切付き弁当容器（１８５mm×２３０mm×２５mmＨ）に成形した。実施例１、２のシートとも仕切部の成形伸びも問題なく成形性は良好で、シャープな形状の表面が緻密で、美麗な容器が得られた。又、容器の側壁は特に金型で規制していないが肉厚みは両シートとも約１．３mmであり、容器の積み重ね高さは市販のポリスチレン発泡シート（サンポリマー社製Ｍ−１７０）を使用して同一金型で成形した容器の１／２であった。この弁当容器の底部の発泡シート層のセル状態を図３（実施例１）、図４（実施例２）に示す。平均セル径０．３５mm（図３）、０．３２mm（図４）の大きなセルの間に０．１mm以下のセルが発泡シート層の中央部まで発生している事が判る。
これら実施例１，２の押出発泡シート、二次発泡シート、及び、成形品の底部の透明性を東洋精機製TRANSMITTANCE METERで測定した。表１記載の如く、微細セルが発生していない押出発泡シート段階では透明度が高いが、微細セルが発生した二次発泡反及び成形品は押し出し発泡反の１／２以下の透明度になり、透けて見える事が少なくなっている事が判る。
（比較例−１）
表１に記載の如く、化学発泡剤を添加しない点を除き実施例２と同一条件で比較例１を行い、実施例と同様の評価を行った結果を表１に併記する。
発泡シートの押出持の収率ロス、及び、成形伸び不良等の問題は実施例と同様に特に見られなかった。
【００２５】
成形容器の発泡層のセル写真を図５に示す。容器の表層に若干の０．１mm以下の微細セルが見られ、且つ、表層部を中心に実施例２よりもセルが小さいが、発泡シート層の中央部には微細セルが認められなかった。得られた容器の表面は実施例２よりもやや粗く、表面にギラギラ輝く光沢があり、透明感もあり好ましくない。
【００２６】
シートの透明度はセル径がやや小さい影響で押出発泡シート段階では実施例よりも低いが、加熱二次発泡しても微細セルが殆ど発生しないために透明度の低下は低く、実施例よりも明らかに透明度が高い結果であった。
【００２７】
【表１】

【図面の簡単な説明】
【図１】第1図は、実施例１で得られた押出発泡シートの２週間養生後のシート流れ方向断面セル状態を示す顕微鏡写真（電子写真）の複写である。
【図２】第２図は、実施例１で得られた押出発泡シートを加熱二次発泡した二次発泡シート流れ方向断面セル状態を示す顕微鏡写真（電子写真）の複写である。
【図３】第３図は、実施例１で得られた押出発泡シートから成形した弁当容器の底部の発泡シート層流れ方向断面セル状態を示す顕微鏡写真の複写である。
【図４】第４図は、実施例２で得られた押出発泡シートから成形した弁当容器の底部の発泡シート層流れ方向断面セル状態を示す顕微鏡写真の複写である。
【図５】第５図は、比較例１の押出発泡シートから成形した弁当容器の底部の発泡シート層流れ方向断面セル状態を示す顕微鏡写真の複写である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a beautiful styrene resin foamed sheet molded container having a thin wall and excellent surface properties, an economically advantageous production method thereof, and a secondary foamed sheet.
[0002]
[Prior art]
Styrenic resin foam sheet molded containers have been used in large quantities as trays and containers for various foods due to their low cost, strength, cleanliness, and heat insulation. However, due to the thick thickness of the container, the container is bulky, a sharply shaped container cannot be obtained, and the surface is rough and glaring and shiny. It has been replaced with a molded container from the sheet. For this reason, thinning of the styrene resin foamed sheet molded container has been promoted by reducing the foaming. When the container is made thinner, the bulkiness and shape of the container will be improved, but the transparency of the container will increase and the contents of the cell will be visible in addition to problems such as the transparency of the contents. As a result, the container surface appears dirty. In order to solve this problem, there is a method in which an inorganic filler is added to the resin to make the resin itself opaque. However, aggregates of the filler are generated, and die clogging or foaming hole in the expanded foam sheet is produced. There are many production problems such as frequent occurrences of perforations. In addition, there is a method to reduce the cell diameter of the foamed sheet and suppress the transparency by stacking a number of cells in the thickness direction, but when using hydrocarbons such as general-purpose butane as the foaming agent, the amount of nucleating agent added Even if the number is increased, only about 15 cells per 1 mm in the thickness direction can be stacked, and the effect of suppressing transparency is small. When a special foaming agent such as carbon dioxide or water is used, it is possible to stack 30 or more cells per 1 mm in the thickness direction. In addition to problems in manufacturing equipment, the foaming ratio of the styrene foam sheet is 2 to 3 In addition, only a container having poor heat insulation and poor secondary foaming property due to heating, poor strength, and heat insulation can be obtained.
[0003]
As a means for lowering the transparency of the foam sheet, there is a method of rapidly cooling the foam sheet surface during extrusion and generating fine cells by heating during molding as disclosed in JP-A-2001-301103. This method reduces the transparency of the foam sheet and improves the surface properties, but is insufficient to improve the transparency of the thin container. Japanese Patent Publication No. 5-49701 proposes an extruded foam of a composite cell comprising a cell having a cell diameter of 0.25 mm or less and a cell having a cell diameter of 0.4 to 1 mm. This proposal relates to a heat-insulating foam board and does not target styrene resin foam sheets used in molded containers. Therefore, compared with a styrene resin foam sheet, the thickness of a foam is thick and a cell diameter is too large. In addition, the cells present in the extruded foam are not preferable because they not only enlarge due to heating during molding and lower the surface property, but also lower the moldability.
[0004]
[Patent Document 1]
JP 2001-301103 A
[0005]
[Patent Document 2]
Japanese Patent Publication 5-49701
[0006]
[Problems to be solved]
In the present invention, when the thickness of the styrene resin foam sheet is reduced to improve the bulkiness of the molded container, problems such as the transparency of the container newly generated by reducing the thickness of the foam sheet are improved. , Good moldability, good moldability, obtain beautiful containers, and use general-purpose extrusion foaming equipment without using special equipment. Therefore, an object is to obtain a thin-walled styrene-based resin foam sheet that is economically advantageous and obtain the container.
[0007]
[Means for solving problems]
The present invention reduces the cell diameter of the entire foamed sheet and increases the number of stacked cells in the thickness direction, generating fine cells between large cells to the center of the foamed sheet. Focusing on the fact that the cell structure can reduce the transparency of the foam container, and taking into consideration the effect on moldability, the development of a method that starts with heating during molding and generates fine cells throughout the center of the foam sheet. Efforts have been made to the present invention.
That is, the present invention comprises (1) a styrene resin foam sheet having a composite cell structure in which fine cells having an average cell diameter of 1/3 or less of large cells are present between large cells, and the large cells The average cell diameter is 0.2 to 0.5 mm, the cell diameter of the fine cells is 0.1 mm or less, and the fine cells are present throughout the foam layer from the surface to the center of the styrene resin foam sheet. Styrenic resin foam sheet molded container characterized in that (Claim 1)
(2) Add talc as a nucleating agent to styrene resin, butane and baking soda-citric acid chemical foaming agent as lower hydrocarbon foaming agent, and extrude foam through a die to obtain a styrene resin foam sheet, after curing The secondary cell is foamed by heating, and the average cell diameter is between 1 and 3 mm or less of the average cell diameter of the large cell, and the cell diameter is 0.1 mm or less. Styrene resin foam sheet having a composite cell structure in which the fine cells of the styrene resin foam sheet exist from the surface to the center of the styrene resin foam sheet, and is molded with a mold. Manufacturing method (claim 2)
(3) It is composed of a styrene resin foam sheet having a composite cell structure in which fine cells having a size equal to or less than 1/3 of the average cell diameter of the large cells are present between the large cells, and the average cell diameter of the large cells is 0.2 to 0.5 mm, the cell diameter of the fine cell is 0.1 mm or less, and the fine cell exists over the entire foamed layer from the surface to the center of the styrene resin foam sheet. Styrenic resin foam sheet (Claim 3).
About.
[0008]
DETAILED DESCRIPTION OF THE INVENTION
An example of a molded container made of a styrene-based resin foam sheet in which the foam sheet layer is in a composite cell state in which fine cells exist between large cells (FIG. 3), and the center of the foam sheet by heating during molding 1 and 2 show the situation in which fine cells are generated between large cells.
[0009]
FIG. 1 is a copy of a cell photograph of an extruded foam sheet according to the production method of the present invention, which is cured for 2 weeks after extrusion. One or two layers of small cells having a diameter of about 0.1 mm are present on the surface, but the other portions are filled with cells having a diameter of 0.2 to 0.4 mm.
[0010]
FIG. 2: A reproduction of a cell photo of a sheet obtained by heating the foamed sheet with a heater and secondarily expanding to 2.25 times. A large number of fine cells having a diameter of 0.1 mm or less are generated between large cells having a diameter of 0.2 to 0.5 mm over the entire secondary foam sheet from the surface to the center of the foam sheet.
[0011]
FIG. 3 is a copy of a cell photo at the bottom of a container in which this secondary foamed sheet is molded with a mold. Similar to FIG. 2 except that the cells are flattened by molding, and between the large cells having a diameter of 0.2 to 0.5 mm (average 0.35 mm), fine cells having a diameter of 0.1 mm or less are formed from the surface of the foam sheet. A large amount is present throughout the foam layer up to the center.
[0012]
The cells found in the extruded foam sheet increased in the thickness direction due to secondary foaming. On the other hand, new cells that were not seen in the extruded foam sheet were newly generated at the cell-cell interface and on the surface of the foam sheet. I understand the situation. The fact that it was not observed in the extruded foam sheet described here means that it was not observed in at least 50 times magnification observation.
[0013]
The smaller the cell diameter of the large cell seen from the extruded foam sheet, the better the surface of the foam sheet becomes and the lower the transparency. However, the process of increasing the nucleating agent in order to reduce the size of the cell often causes problems in manufacturing such as sheet punching and die clogging, and the generation of fine cells makes the surface of the foam sheet beautiful. Since the transparency is also lowered, it is not necessary to make it particularly small, and the average cell diameter may be 0.2 to 0.5 mm. The average cell diameter shown here is a microphotograph of a cross section cut along the sheet flow direction (extrusion direction: MD direction), and the maximum cell diameter of all cells excluding fine cells of 0.1 mm or less. Is measured and averaged.
[0014]
The generated fine cells vary depending on the cell diameter of the extruded foam sheet, the type and amount of the foaming agent, the sheet cooling conditions when forming the extruded foam sheet, etc., but the effect of the heating conditions during molding changes. Is particularly prominent. It is made to foam to a thickness of 1.7 times or more, preferably 2.0 times or more of the thickness of the foamed sheet before heating by heating at the time of molding. When foaming heating is less than 1.7 times, fine cells are hardly generated. The cell diameter of the generated fine cells is 1/3 or less of the average cell diameter of large cells from the extruded foam sheet, and preferably 0.1 mm or less. Although it is desirable that the cell diameter is small and occurs in multiple layers in the thickness direction, if it is 1/3 or less of the average cell diameter of the cells from the extruded foam sheet, there is an effect in improving the surface properties of the molded container. When the thickness is 1 mm or less, the transparency lowering effect is also exhibited.
[0015]
A thin-walled container is preferable for exhibiting the effect of reducing the transparency of the present invention, and a thinner sidewall is preferable for reducing the bulkiness of the molded container. However, if the thickness is too thin, it is difficult to ensure moldability and the strength of the container is lowered. Therefore, the side wall thickness of the molded container is preferably 0.7 to 1.5 mm. In order to obtain a container having this side wall thickness, it is appropriate that the thickness of the extruded foam sheet before molding and heating is 0.6 to 1.3 mm. Moreover, although the higher one is preferable when ensuring the intensity | strength of a shaping | molding container, the one where a magnification is low is good in order to suppress the thickness of a shaping | molding container, and 2.5-5 times is preferable.
[0016]
Styrenic resin foam sheet in which fine cells are generated between cells by heating during molding is polystyrene homopolymer resin, (meth) acrylic acid, maleic anhydride, acrylonitrile, acrylic (meth) based on styrene. A copolymer resin of vinyl monomers such as acrylate and diene monomers such as butadiene and isoprene, and a polystyrene resin composed of a mixed resin of these resins and polyphenylene oxide, and the like, if necessary. A resin obtained by mixing a plasticizer such as liquid paraffin and a lubricant such as ethylene bisstearyl amide into an extruded foam sheet using a general-purpose extrusion foaming facility, and one or both sides of this extruded foam sheet as required. Styrene resin film, olefin resin film, polyester resin film Refers to those obtained by laminating directly or via an adhesive layer such as various films such as vinyl acetate-based resin film.
[0017]
In forming an extruded foam sheet, for example, calcium carbonate, barium sulfate, silica, titanium oxide, clay, aluminum oxide, bentonite, diatomaceous earth, talc, etc. can be used as a nucleating agent as a nucleating agent. Of these, particularly preferably, talc is selected, and a chemical foaming agent such as a combination of an inorganic carbon dioxide generator and a weak acid, azodicarbonamide, azobisformamide, azobisisobutyronitrile, N, Particularly preferred is N'-dinitropentatetramine, a baking soda-citric acid-based chemical foaming agent, etc., preferably a baking soda-citric acid-based chemical foaming agent is selected and mixed with the resin, and kneaded in an extruder to melt the resin. . Thereafter, lower hydrocarbon, particularly preferably butane, is injected as a foaming agent, cooled to an appropriate foaming temperature, and extruded and foamed through a die. The effect of the chemical foaming agent in this process is not clear, but it can hardly be selected as a nucleating agent at the time of extrusion foaming, and conditions under which fine cells are not formed on the extruded foam sheet can be easily selected.
[0018]
Both the inner and outer surfaces of the sheet extruded and foamed through a die are rapidly cooled by blowing cooling air at 20 to 40 ° C. immediately after the die is ejected. The degree of cooling is preferably blown with 0.7 to 1.4 m ³ of air per kg of discharged resin. By cooling the surface, fine cells are generated on the surface of the secondary foamed sheet by secondary foaming by heating, which is effective in generating fine cells uniformly from the surface of the foamed sheet to the center. When the air ring amount is less than 0.7 m ³ per kg of discharged resin, the amount of fine cells generated on the surface is small, and when sprayed over 1.4 m ³ , the take-up resistance of the sheet is strong and rubs against the mandrel surface. This may cause problems due to generation of powder and streaks.
[0019]
The styrene resin foam sheet thus obtained is heated at room temperature for about one week or more at room temperature, and then heated with a heater using a general-purpose sheet molding machine, 1.7 times or more of the sheet thickness before heating, Preferably, foaming is performed at a ratio of 2.0 times or more, and molding is performed by match molding using a male / female fitting mold and vacuum molding or pressure molding using one female or male mold.
[0020]
The molded container thus obtained has a low bulk and a sharp-shaped container is obtained, and a beautiful container with high opacity is obtained. In addition, a container having excellent strength and brittleness can be obtained.
[0021]
【Example】
EXAMPLES The present invention will be specifically described below with reference to examples, but the present invention is not limited to these examples.
(Examples 1 and 2)
Talc (manufactured by Matsumura Sangyo Co., Ltd.) mixed with 10% magnesium stearate as a nucleating agent in commercially available styrene resin 685 manufactured by A & M Co., and a baking soda-citric acid type chemical foaming agent (Daiblow No. 2 manufactured by Dainichi Seika Co., Ltd.) Are mixed into the general-purpose foam sheet extruder (manufactured by Toshiba Machine Co., Ltd.), kneaded and melted in the extruder, and then mixed with 85% isobutane / normal butane 15%, and then foamed. After cooling to an appropriate temperature, it was extruded from a circular die at a discharge rate of 240 Kg / H. 25 ° C. air was blown onto the inner surface of the sheet extruded from the die at 4 m ³ / Min and the outer surface was blown at 3 m ³ / Min to rapidly cool the surface. During the extrusion of the foamed sheet, there was no yield loss because there was no sheet perforation or clogging of dies due to aggregates.
[0022]
The properties of the obtained extruded foamed sheet are as shown in the table, and an extruded foamed sheet having a thickness of over 1 mm and a foaming ratio of about 2.8 times was obtained by fine adjustment of the butane press-fitting amount. FIG. 1 (copy of photomicrograph) shows the cell state after 2 weeks of curing of the extruded foam sheet obtained on the basis of Example 1. There are a few small cells with a diameter of about 0.1 mm on the surface, but the other parts are filled with cells with a diameter of 0.2 to 0.4 mm (average 0.32 mm). Fine cells were not seen in the part. The cell state of Example 2 was the same as that of Example 1 except that the number of cells became smaller as a whole.
[0023]
These extruded foamed sheets were heated for 24 seconds in a heater heating furnace having an atmospheric temperature of 150 ° C. by a commercially available single-molding machine (Charanger VAS) manufactured by Semba Iron Works, Inc., and secondary foamed about 2.3 times. The cell state of the secondary foamed sheet of Example 1 is shown in FIG. 2 (copy of micrograph) (Note: magnification is different from FIGS. 1, 3 and 4). As shown in the figure, a large number of fine cells were generated between the large cells formed during extrusion foaming from the surface of the foam sheet to the central portion.
[0024]
The heated secondary foamed sheet was immediately formed into a boxed lunch container (185 mm × 230 mm × 25 mmH) with a male and female fitting mold. In both the sheets of Examples 1 and 2, the molding elongation of the partition portion was satisfactory, and the moldability was good, the sharp surface was dense, and a beautiful container was obtained. The side wall of the container is not restricted by a mold, but the thickness of both sheets is about 1.3 mm, and the stacked height of the container is a commercially available polystyrene foam sheet (M-170 manufactured by Sun Polymer Co., Ltd.). It was 1/2 of the container molded with the same mold. FIG. 3 (Example 1) and FIG. 4 (Example 2) show the cell state of the foam sheet layer at the bottom of the lunch box. It can be seen that cells having a size of 0.1 mm or less are generated up to the center of the foam sheet layer between large cells having an average cell diameter of 0.35 mm (FIG. 3) and 0.32 mm (FIG. 4).
The transparency of the extruded foam sheet, the secondary foam sheet, and the bottom of the molded product of Examples 1 and 2 was measured with TRANSMITTANCE METER manufactured by Toyo Seiki. As shown in Table 1, the transparency is high in the extruded foam sheet stage where fine cells are not generated, but the secondary foamed product and molded product in which the fine cells are generated have a transparency of 1/2 or less of the extrusion foamed anti-reflective property. You can see that there are fewer things to see.
(Comparative Example-1)
As shown in Table 1, Comparative Example 1 was performed under the same conditions as in Example 2 except that no chemical foaming agent was added, and the results of the same evaluation as in Example 1 are also shown in Table 1.
The problems such as the yield loss of the extrusion holding of the foamed sheet and poor molding elongation were not particularly seen as in the examples.
[0025]
A cell photograph of the foam layer of the molded container is shown in FIG. Some fine cells of 0.1 mm or less were found on the surface layer of the container, and the cells were smaller than Example 2 centering on the surface layer portion, but no fine cells were observed in the central portion of the foamed sheet layer. The surface of the obtained container is slightly rougher than that of Example 2, and the surface has a glittering luster and is transparent and unfavorable.
[0026]
The transparency of the sheet is lower than the example at the extruded foam sheet stage due to the effect of the cell diameter being slightly smaller, but the decrease in transparency is low because there are almost no fine cells even when heated secondary foaming, which is clearly lower than the example The result was high in transparency.
[0027]
[Table 1]

[Brief description of the drawings]
BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a photomicrograph (electrophotography) showing a cross-sectional cell state in the sheet flow direction after curing an extruded foam sheet obtained in Example 1 for 2 weeks.
FIG. 2 is a photomicrograph (electrophotography) showing a cross-sectional cell state in the flow direction of a secondary foamed sheet obtained by heating and secondary foaming the extruded foamed sheet obtained in Example 1.
FIG. 3 is a photocopy of a micrograph showing a cell state in the flow direction of the foam sheet layer at the bottom of the lunch box formed from the extruded foam sheet obtained in Example 1.
FIG. 4 is a photocopy of a micrograph showing the cell state of the foam sheet layer in the flow direction at the bottom of the lunch box formed from the extruded foam sheet obtained in Example 2.
FIG. 5 is a photocopy of a photomicrograph showing the cross-sectional cell state in the flow direction of the foam sheet layer at the bottom of the lunch box formed from the extruded foam sheet of Comparative Example 1;

Claims

It consists of a styrene-based resin foam sheet having a composite cell structure in which fine cells of 1/3 or less of the average cell diameter of large cells exist between large cells, and the average cell diameter of the large cells is 0.2. A styrenic resin characterized in that the cell diameter of the fine cell is 0.1 mm or less, and the fine cell exists over the entire foamed layer from the surface to the center of the styrenic resin foamed sheet. Foamed sheet molding container.

Add talc as nucleating agent to styrene resin, butane and baking soda-citric acid chemical foaming agent as lower hydrocarbon blowing agent , and extrude foam through a die to obtain styrene resin foam sheet, after curing, heat with heater And a fine cell having a cell diameter of not more than 1/3 of the average cell diameter of the large cell and a cell diameter of not more than 0.1 mm between the large cells having an average cell diameter of 0.2 to 0.5 mm. styrene which the microcellular with present is the styrene-based resin foam sheet having a composite cell structure present throughout the foam layer from the surface of the styrenic resin foam sheet to the central portion, characterized by shaping it in a mold Method for forming a molded resin foam sheet container.

It consists of a styrene-based resin foam sheet having a composite cell structure in which fine cells of 1/3 or less of the average cell diameter of large cells exist between large cells, and the average cell diameter of the large cells is 0.2. A styrenic resin characterized in that the cell diameter of the fine cell is 0.1 mm or less, and the fine cell exists over the entire foamed layer from the surface to the center of the styrenic resin foamed sheet. Foam sheet.