JP2004307662A - Method for producing crystalline polylactic acid-based resin foam - Google Patents

Abstract

An object of the present invention is to provide a crystalline polylactic acid-based resin foam using a polylactic acid-based resin that maintains the property of decomposing naturally and becoming harmless when abandoned, so as to obtain a uniform and finely foamed foam. Attempt to provide a manufacturing method.
SOLUTION: A crystalline polylactic acid resin obtained by copolymerizing a D-form and an L-form at a ratio of 2 to 13% and 98 to 87% by weight, respectively, is selected, and dimethyl ether and a hydrocarbon are used as foaming agents. Is extruded and foamed by using a mixture obtained by mixing at a ratio of 95/5 to 5/95 by weight.

Description

で表わされるＤ−乳酸及びＬ−乳酸を上記の割合で含んだものである。
【００１５】
ポリ乳酸系樹脂は、上述のように、Ｄ−体とＬ−体との比率によって非晶性のものと結晶性のものとがある。一般にＤ−体又はＬ−体が１００％近くを占めている樹脂は結晶性であり、Ｄ−体及びＬ−体のうちの一方が増すに従って樹脂は次第に非晶化となる傾向を示し、一方が２０％を越えると樹脂は完全に非晶性となる。また、結晶性のポリ乳酸系樹脂は、一般に融点が高く、普通１００℃以上の融点を示し、その融点は非晶性の樹脂の軟化点（ガラス転移点）よりも５０℃以上高い。従って、結晶性ポリ乳酸系樹脂は耐熱性の樹脂として使用することができる。
【００１６】
Ｄ−乳酸とＬ−乳酸とからなる共重合体のうち、両者がどのような割合で含まれている共重合体が、押出発泡を行うのに適しているかは知られていない。この発明者は、この点について検討した結果、Ｄ−体が２〜１３重量％、Ｌ−体が９８〜８７重量％含まれている共重合体が押出発泡を行うに適していることを見出した。そこで、この発明では、結晶性ポリ乳酸系樹脂として、Ｄ−体が２〜１３重量％でＬ−体が９８〜８７重量％の共重合体を用いることとしたのである。
【００１７】
上述のＤ−体とＬ−体との共重合割合は、ポリ乳酸系樹脂の製造時における原料の仕込量から特定することができる。また、得られたポリ乳酸系樹脂からも決定することができる。それは単量体のＤ−体又はＬ−体の場合と同様に、共重合体の比旋光度を測定することによって定めることができる。それは例えば、次のようにして測定することができる。
【００１８】
まず、問題のポリ乳酸系樹脂０．２ｇをクロロホルムに溶解し、２０ｍｌとする。旋光計（日本分光社製 ＤＩＰ−３７０）を使用して２５℃、セルの層長１００ｍｍでナトリウムスペクトルＤ線を照射して上記溶液の旋光度を測定し、この旋光度から下記（１）式にて比旋光度を求める。得られた比旋光度から下記（２）式又は（３）式によりＤ−体及びＬ−体の割合を算出する。
比旋光度（度）＝１００×α／〔ｌ×ｃ〕 （式１）
α：試料の旋光度測定値（度）
ｌ：セルの層長（ｍｍ）
ｃ：試料溶液の濃度（ｇ／ｍｌ）
Ｄ−体割合（％）＝（比旋光度＋１５６）×１００／３１２ （式２）
Ｌ−体割合（％）＝１００−（Ｄ−体割合） （式３）
【００１９】
この発明では、こうして決定された含有割合を基準にして、Ｄ−体が２〜１３重量％、Ｌ−体が９８〜８７重量％の割合で、共重合したポリ乳酸系樹脂を選んで使用する。その理由は、上述のように種々の組成のポリ乳酸系樹脂について押出発泡の実験を行って得られた結果に基づいている。すなわち、Ｄ−体が２重量％より少ないか又はＬ−体が９８重量％より多くなると、ポリ乳酸系樹脂は溶融状態から急激に結晶化するために、発泡に適した溶融粘度を樹脂に保持させることが困難となり、かりに発泡体ができても、得られた発泡体は脆性が強く、実使用する際に問題が生じるからである。また逆にＤ−体が１３重量％より多いか又はＬ−体が８７重量％より少なくなると、ポリ乳酸系樹脂は非晶化の傾向を示し、融点が低くなり実用に適したものとならないからである。そのうちでは、含有割合として２〜１０重量％のＤ−体と、９８〜９０重量％のＬ−体とを含むものが好ましく、さらに３〜９重量％のＤ−体と９７〜９１重量％のＬ−体を含むものが最も好ましい。
【００２０】
この発明では、上述の共重合割合のものの中で、とくにメルトマスフローレートが１〜１５ｇ／１０分のポリ乳酸系樹脂を用いることが好ましい。その中では５〜１０ｇ／１０分のメルトマスフローレートを示すものを用いることが好ましい。この場合、メルトマスフローレートはＪＩＳ Ｋ ７２１０が規定する方法に従い、１９０℃で２．１６ｋｇｆの荷重をかけて測定した値である。
【００２１】
結晶性ポリ乳酸系樹脂におけるＤ−体の含有割合と、メルトマスフローレートとは、何れも結晶性ポリ乳酸系樹脂の押出発泡の容易さに大きな影響を及ぼすものである。従って、結晶性ポリ乳酸系樹脂のメルトマスフローレートとＤ−体含有比率との比Ｗ、すなわち

の値を採ると、Ｗが０．３３〜３の範囲内にあるようにし、とりわけ０．５〜２の範囲内にあるようにすると、押出発泡がし易くなる。Ｗが０．３３より小さくなると、押出発泡時の負荷が大きくなり、良好な発泡体が安定して得られにくくなる。また、逆にＷが３より大きくなると、結晶性ポリ乳酸系樹脂の溶融粘度が温度に敏感になり過ぎて、発泡に適した溶融粘度をコントロールすることが困難になる。
【００２２】
この発明では、結晶性ポリ乳酸系樹脂を押出機に入れ、溶融した樹脂に発泡剤を含ませる。発泡剤としてはジメチルエーテルと炭化水素とを用いる。炭化水素は、例えばプロパン、ブタン、ペンタンを用いることができる。そのうちではブタンを用いることが好ましい。それは押出発泡させ易く、また気泡を均一に微細化することができるからである。ブタンとしては、ノルマルブタン、イソブタン、及びこれらの混合物の何れをも用いることができる。その中ではイソブタンが３０〜１００重量％でノルマルブタンが０〜７０重量％の混合物、とりわけイソブタンが５０〜１００重量％でノルマルブタンが０〜５０重量％の混合物を用いることが好ましい。それは、とくに気泡を微細化することができるからである。
【００２３】
ジメチルエーテルは押出機中で溶融した結晶性ポリ乳酸系樹脂に対し、優れた相溶性を示し、顕著な可塑化効果をもたらし、押出発泡において発泡性を著しく改善する。しかし、ジメチルエーテルは結晶性ポリ乳酸系樹脂に対して相溶性が良好過ぎるために気泡を大きくする傾向を示す。これに対し、炭化水素は結晶性ポリ乳酸系樹脂との相溶性があまり良好でないので、逆に発泡核剤として働き、気泡を微細化するように働く。また炭化水素は、結晶性ポリ乳酸系樹脂との相溶性が余り良くないので、炭化水素だけを発泡剤として使用すると、発泡体中の気泡膜が破れてスが発生する。ところが、ジメチルエーテルと炭化水素とを組み合わせて使用すると、上述の欠点が抑えられ長所が発揮されて良好な発泡体を得ることができる。
【００２４】
この発明では、ジメチルエーテルと炭化水素とを重量比で、９５／５ないし５／９５の割合で混合して用いる。このうちで好ましいのは９０／１０ないし１０／９０の割合であり、さらに好ましいのは８５／１５ないし１５／８５の割合である。ジメチルエーテル／炭化水素の割合が大きいと、すなわち炭化水素に比べてジメチルエーテルが多いと、炭化水素による気泡微細化の効果が充分に発揮されないで、気泡が粗く外観の劣った発泡体が得られる。逆に上記の割合が小さいと、樹脂が発泡に適した粘度を示す領域が狭くなり、従って高倍率に発泡させることが困難になるとともに、発泡体中にスが発生しやすくなる。ジメチルエーテルと炭化水素の合計使用量は、ポリ乳酸系樹脂１００重量部に対し０．５〜１５重量部であり、１〜１０重量部が好ましく、２〜８重量部がさらに好ましい。
【００２５】
ジメチルエーテルと炭化水素の混合物を注入するには、従来から発泡剤を注入する方法を採用することができる。すなわち、押出機のシリンダーに穿設した孔から溶融された樹脂中に混合物をポンプ等により圧入する。ジメチルエーテルと炭化水素とは、別々の孔から注入してもよいが、予め所定の割合に混合してのち１つの孔から注入することが好ましい。
【００２６】
結晶性ポリ乳酸系樹脂中に上述の混合物を能率よく均一に分散させるためには、上述のようにジメチルエーテルと炭化水素とを予め混合したのち樹脂中に注入するか、別々に注入する場合はジメチルエーテルを注入し、その後に炭化水素を注入するのが好ましい。また、樹脂中に上記の混合物を能率よく均一に分散させるには、ジメチルエーテルと炭化水素とを何れもそれぞれの臨界温度以上に加熱され且つ臨界圧力以上に加圧した状態で溶融された樹脂中に注入することが好ましい。
【００２７】
具体的に云えば、ジメチルエーテルは１２７℃の臨界温度と５．３ＭＰａの臨界圧力を示すから、１２７℃以上で５．３ＭＰａ以上の圧力下に溶融樹脂中に注入することが好ましい。また、炭化水素としてイソブタンを用いる場合には、イソブタンは１３５℃の臨界温度と３．６ＭＰａの臨界圧力を示すから、１３５℃以上で３．６ＭＰａ以上の圧力下に溶融樹脂中に注入することが好ましく、また炭化水素としてノルマルブタンを使用する場合には、ノルマルブタンは１５２℃の臨界温度と３．８ＭＰａの臨界圧力を示すから、１５２℃以上で３．８ＭＰａ以上の圧力下に溶融樹脂中に注入することが好ましい。従って、上記３種の混合発泡剤を使用する場合には、それぞれの臨界温度、臨界圧力のうちで最大の組合せである温度１５２℃、圧力５．３ＭＰａ以上で、溶融樹脂中に注入することが好ましい。
【００２８】
この発明で用いる押出機は、どのような形のものであってもよい。例えば、単軸押出機でも、二軸押出機でも、またこれらを連結したタンデム型の押出機でも用いることができる。押出機としては、結晶性ポリ乳酸系樹脂をよく混合するとともに溶融し、発泡剤を均一に含ませることができて、引き続き所定の温度に冷却して維持し、押し出すことができる能力を持ったものを用いる必要がある。
【００２９】
押出機としてタンデム型の押出機を用いる場合には、一段目の押出機で発泡剤を圧入後、発泡剤含有の溶融樹脂を二段目の押出機に連続的に供給するようにする。このとき、一段目の押出機と二段目の押出機とを接続している接続管内で、樹脂を融点以上の温度に維持するとともに、ジメチルエーテルと炭化水素との臨界温度及び臨界圧力のうち、高い方の温度及び圧力以上に維持して、発泡剤を樹脂中に分散させる。こうして発泡剤が分散された樹脂は、二段目の押出機内で全体が均一な組成及び温度を持つように攪拌される。
【００３０】
この発明では、発泡剤としてジメチルエーテルと炭化水素とを混合して用いることが必要とされるが、それ以外にこれに公知の発泡剤を混合して用いることができる。この場合に、公知の発泡剤はジメチルエーテルと炭化水素との合計量よりも少なくする。公知の発泡剤としては、ジニトロソペンタメチレンテトラミンのような樹脂の融点以上で分解してガスを発生する固体化合物や、塩化メチルのような加熱すると樹脂中で気化する液体や、二酸化炭素や窒素のような加圧下に樹脂に溶解される不活性な気体など、その何れをも使用することができる。
【００３１】
この発明では押出機内で上述の発泡剤を溶融樹脂に含有させたあとで、これを樹脂の融点以上の温度に保持したまま押し出すことができる。しかし、有利なのは、発泡剤含有の溶融樹脂を押出機内で引き続き冷却して、もとの樹脂の融点以下の温度にして、しかも発泡剤含有樹脂がまだ流動性を保っている間に、樹脂を押出機から押し出すことである。後者の温度での押し出しは、押出成形においては異常なことであるから、以下に説明する。
【００３２】
一般に、樹脂を押出発泡させる場合には、結晶性の樹脂では樹脂の融点以上の温度で押し出し、非晶性の樹脂では、樹脂のガラス転移点以上の温度で押し出すのが、常識に適っている。なぜならば、融点又はガラス転移点以下の温度で押し出そうとすると、樹脂が固化して押し出すことができなくなるからである。
【００３３】
事実、これまでは、例えば樹脂として融点が２５５℃の結晶性のポリエチレンテレフタレートを用いて、これを押出発泡させる場合には、樹脂温度を２７０〜２９０℃として押し出し、また樹脂として融点が１６０℃のポリプロピレンを押出発泡させる場合には、樹脂温度を１７０℃として押し出して来た。また、樹脂としてガラス転移点が９０〜９５℃の非晶性のポリスチレンを押出発泡させる場合には、樹脂温度を１２０〜１６０℃として来た。何れも、押出時の樹脂温度は、樹脂の融点又はガラス転移点よりも高い温度とされて来た。
【００３４】
ところが、この発明では、前述のように結晶性ポリ乳酸系樹脂を融点以下の温度として押し出すのであるから、これは異常なことである。融点以下の温度の中では、融点と融点より４０℃だけ低い温度との間で、押し出すこととなる。また、その中では、融点より１℃だけ低い温度と、融点より３０℃だけ低い温度との間にすることがさらに好ましく、また融点より２℃だけ低い温度と、融点より２０℃だけ低い温度との間にすることが最も好ましい。このような温度で押し出すことができる理由は、ジメチルエーテルと炭化水素とを含んだ結晶性ポリ乳酸系樹脂が、その融点以下に冷却されたとき、過冷却された状態に留まっていて、なお暫らくは流動性を保持しているからである。
【００３５】
溶融したジメチルエーテルと炭化水素含有のポリ乳酸系樹脂を一様に樹脂の融点以下に冷却するには、色々な方法を採ることができる。その方法は、例えば多数のピンを持ったスクリュー又は多数の切欠を持ったスクリューフライト付きのスクリューのような高い混練機能を持ったスクリューを使用して樹脂を混練しながら、押出機のバレルを冷却水などで冷却する方法である。
【００３６】
この発明は、こうして発泡剤含有の溶融樹脂を発泡に適した温度まで冷却する。そして、発泡剤含有樹脂がまだ流動性を保持している間に、樹脂を押出機の先端に付設した金型から押し出して発泡させる。このとき、金型を通る樹脂は、結晶性ポリ乳酸系樹脂の融点以上の温度であってもよいが、通常は融点と、融点より４０℃だけ低い温度との間の温度にされる。このとき樹脂は発泡を行うに適した溶融粘度を持っている。
【００３７】
この発明では、結晶性ポリ乳酸系樹脂の押出発泡成形性をさらに良くするために、結晶性ポリ乳酸系樹脂に高級脂肪酸アルコール性エステルを添加して使用することが好ましい。高級脂肪性エステルは、結晶性ポリ乳酸系樹脂の可塑剤として有効に作用し、押出発泡成形時の条件幅を広げるとともに、結晶性ポリ乳酸系樹脂と発泡剤、特に炭化水素との相溶化剤として有効に働き、発泡体中にスが発生することを抑える効果がある。また、高級脂肪酸アルコール性エステルは帯電防止剤としても有効に作用し、ジメチルエーテルや炭化水素の如き可燃性発泡剤を使用した際の発火事故等を防ぐ効果がある。
【００３８】
高級脂肪酸アルコール性エステルとは、高級脂肪酸と多価アルコールにより形成されるエステルを意味し、そのエステル１分子中に遊離の水酸基を１個以上含むものをいう。そして、分子中にエーテル結合を有しないものが特に好ましい。
【００３９】
上記多価アルコールとしては、例えば、グリセリン、ペンタエリスリット、ソルビット、ソルビタン、マンニット、マンニタン、ジペンタエリスリット、ジグリセリン等を用いることができる。
【００４０】
上記高級脂肪酸としては、例えば、カプリン酸、ラウリン酸、ミリスチン酸、パルミチン酸、ステアリン酸、パルミトレイン酸、オレイン酸、リノール酸等の飽和若しくは不飽和で炭素数が１０〜３０の１価の高級脂肪酸、又は、これらの高級脂肪酸が混在する牛脂肪酸、糠油脂肪酸、ヤシ油脂肪酸等を用いることができる。
【００４１】
上記高級脂肪酸アルコール性エステルとしては、例えば、ラウリン酸モノグリセライド、パルミチン酸モノグリセライド、ステアリン酸モノグリセライド、ペンタエリスリットモノラウレート、ジペンタエリスリットジステアレート、ソルビタンモノオレエート、ソルビタンセスキ糠油脂肪酸エステル、ソルビタンモノパリミテート、ソルビタンモノラウレート、ソルビタンモノステアレート、マンニタンモノオレエート、マンニタンモノラウレート等を用いることができる。その中ではステアリン酸モノグリセライドが好ましい。
【００４２】
高級脂肪酸アルコール性エステルの添加量は、少ないと、上述した効果が得られず、逆に多すぎると高級脂肪酸アルコール性エステル発泡体の表面に多く出てくるためベトツキが生じて好ましくなく、結晶性ポリ乳酸系樹脂１００重量部に対して０．０１〜１．０重量部が好ましく、０．０２〜０．５重量部がより好ましく、０．０３〜０・３重量部が特に好ましい。
【００４３】
この発明では、ポリ乳酸系樹脂を押出発泡させるにあたって、これまで他の樹脂を押出発泡させるにあたって用いられてきた種々の添加剤を使用することができる。例えば、これまで気泡調整剤として、タルク、炭酸カルシウム、棚砂、醐酸亜鉛、水酸化アルミニウム、シリカ、炭酸ナトリウム、炭酸カリウム、弗素樹脂粉末等を添加して来たが、この発明でもこれらを添加することができる。気泡調整剤としての添加量は、その種類によって異なるが、一般には結晶性ポリ乳酸系樹脂１００重量部に対して０．０１〜５重量部の範囲内にするのが適している。タルク、炭酸カルシウムなどを充填剤として使用する場合には、２０重量部程度まで添加することができる。
【００４４】
そのほか、添加剤としては、帯電防止剤、難燃剤、着色剤を用いることができる。帯電防止剤としては、例えば第４級アンモニウム塩化合物、脂肪酸エステル化合物及び脂肪酸アマイド化合物のような脂肪酸化好物を用いることができる。脂肪酸エステル化合物及び脂肪酸アマイド化合物の如き脂肪酸化合物を用いた場合にはポリ乳酸系樹脂の押出発泡成形性を一層良くすることができる。脂肪酸化合物の中では、分子量が１５０〜１０００、特には２００〜８００のものが好ましい。一般的な使用量は、結晶性ポリ乳酸系樹脂１００重量部に対して０．０３〜３．０重量部であり、好ましくは０．０５〜２．０重量部、さらに好ましくは０．１〜１．０重量部である。
【００４５】
金型としては、得ようとする発泡体の形状に対応した形のオリフィスを持ったものを用いることができる。シート状の発泡体を得る際にはサーキュラダイを使用し、ボード状の発泡体を得る際にはフラットダイ、或いはＴダイを使用する。発泡シートは熱成形により色々な形状に成形することができるので、結晶性ポリ乳酸系樹脂発泡体の形態としては好適である。
【００４６】
金型のオリフィスから押し出された後の結晶性ポリ乳酸系樹脂は、発泡した後で、従来のポリスチレンやポリオレフィン押出発泡成形時と同じような操作により、冷却しながら形を整えて発泡体とされる。発泡したシートを得ようとする場合には環状のオリフィスから樹脂を円筒状で押出発泡させ、押し出した筒状発泡体を冷却したマンドレルに沿って進行させつつ冷却し、その後冷却した筒状発泡体を軸線方向に切り開いて発泡したシートとする。
【００４７】
この発明では、結晶性ポリ乳酸系樹脂が融点と融点より４０℃だけ低い温度との間の温度で押し出される場合には、金型出口剪断速度を３００秒^{− １} 以上で押し出すことが望ましい。剪断速度が３００秒^{− １} より低いと、金型内で部分的に結晶化が進んで外観の悪い発泡体になってしまったり、金型内部で発泡が生じて外観や品質の悪い発泡体となってしまう。但し、剪断速度があまりにも速すぎると、具体的には１００００秒^{− １} より速いと溶融樹脂にフラクチャーが生じ外観及び品質の悪い発泡体となってしまう。品質及び外観ともに優れた発泡体を得るための好ましい剪断速度は４００〜８０００秒^{− １} であり、特に好ましくは５００〜６０００秒^{− １} である。本発明における剪断速度とは金型先端出口における剪断速度のことであり、一般に押出成形で用いられる数式を用いて算出する。
【００４８】
なお、上記の場合、サーキュラ金型の口金出口部分における剪断速度は、下記二重管での計算式により算出されたものである。
剪断速度（秒^{− １} ）＝６Ｑ／〔π（Ｌ_２ ^２ −Ｌ_１ ^２ ）（Ｌ_２ −Ｌ_１ ）〕
但し、Ｑは、体積押出量（ｃｍ^３ ／秒）であり、Ｌ_１ （ｃｍ）は（ｒ_０ −ｔ_０ ／２）であり、Ｌ_２ （ｃｍ）は（ｒ_０ ＋ｔ_０ ／２）である。ここで云うｒ_０ とは（金型出口部における外側金型の内径＋金型出口部における内側金型の外径）／２であり、ｔ_０ とは金型出口間隙のことである。
【００４９】
その際、質量押出量（ｇ／秒）から体積押出量（ｃｍ^３ ／秒）への変換に使用する結晶性ポリ乳酸系樹脂の密度は１．０ｇ／ｃｍ^３ とする。また、Ｔダイ又はフラットダイの場合は、平行溝の計算式を用いる。
剪断速度（秒^{− １} ）＝６Ｑ／Ｗｔ^２
但し、Ｑは、体積押出量（ｃｍ^３ ／秒）であり、Ｗ（ｃｍ）は金型出口平行溝の幅であり、ｔ（ｃｍ）は金型出口平行溝の間隔である。
【００５０】
３倍以上の高い倍率に発泡したシートを安定的につくるためには、結晶性ポリ乳酸系樹脂の発泡に適した金型を用いることが好ましい。具体的に云えば、金型は出口に向かってラッパ状に開いている押出孔（オリフィス）を持ったものを使用することが好ましい。このような形状の金型を使用することによって、流れ方向の波打ちが少なく、高い倍率に発泡した結晶性ポリ乳酸系樹脂発泡シートを安定的に得ることができる。また、熱成形に適した良好な結晶性ポリ乳酸系樹脂発泡シートを安定的に得るためには、金型口径と冷却したマンドレル外径との比率、マンドレル外径／金型口径は２〜５、更には２．５〜４．５とするのが好ましい。
【００５１】
この発明方法によれば、密度０．０３〜０．６０ｇ／ｃｍ^３ の一様に発泡した結晶性ポリ乳酸系樹脂発泡体が得られる。得られた結晶性ポリ乳酸系樹脂発泡体は、断熱材、緩衝材、工業用部材、建材、土木資材、農業用資材、食品包装材として使用することができる。特に、発泡シートとすると、熱成形機によってトレー等のいろいろな形状に熱成形することができ、食品容器、果実用トレー、梱包材等としてより広く使用することができる。成形用の発泡シートとしては、厚すぎると熱成形が困難であり、薄いとクッション性や断熱性に劣り、密度が大きすぎると断熱性に劣り、小さすぎると強度に劣るため、好ましい厚みは０．５〜５ｍｍ、更に好ましい厚みは０．７〜４ｍｍ、特に好ましい厚みは０．８〜３ｍｍである。また好ましい密度は０．０２〜０．４ｇ／ｃｍ^３ 、更に好ましい密度は０．０３〜０．３ｇ／ｃｍ^３ 、特に好ましい密度は０．０４〜０．２ｇ／ｃｍ^３ である。
【００５２】
【発明の効果】
この発明によれば、樹脂としてＤ−体とＬ−体とが、重量でそれぞれ２〜１３％及び９８〜８７％の割合で共重合してなる結晶性ポリ乳酸系樹脂を用いるので、遺棄分解性がよくて良好な機械的性質を持って実用に適した製品を得ることができる。また、この発明では、ジメチルエーテルと炭化水素とが重量比で９５／５ないし５／９５の割合で混合されてなる混合物を発泡剤として上記の樹脂に圧入し、得られた溶融樹脂を押出機から押し出して発泡させるので、発泡が容易であって、均一微細に発泡した発泡体を容易に得ることができる。また、発泡剤含有の溶融樹脂を押出機先端に取り付けた金型から、剪断速度３００秒^{− １} 以上で押し出すと、ジメチルエーテルと炭化水素とからなる発泡剤が結晶性ポリ乳酸系樹脂に対して有効に働くとともに、結晶性ポリ乳酸系樹脂に発泡に適した溶融粘度を与えることとなって、押し出された結晶性ポリ乳酸系樹脂は、３倍以上というような高い倍率で一様に発泡した良質の結晶性ポリ乳酸系樹脂発泡体を得ることができる。
【００５３】
【発明実施の態様】
以下に実施例と比較例とを挙げて、この発明のすぐれている所以を具体的に明らかにする。以下で単に部というのは重量部を表わしている。また、押出時の樹脂温度は、押出機樹脂出口部に設置したブレーカープレートの樹脂流路の中心に熱電対を固定して、この熱電対の示す温度をもって、これを示した。
【００５４】
なお、この発明でいう結晶性ポリ乳酸系樹脂の融点は、ＪＩＳ Ｋ７１２１：１９８７（プラスチックの転移温度測定方法）により測定した融解ピーク温度を基準にしている。
【００５５】
また、平均気泡径はＡＳＴＭ Ｄ２８４２−６９の試験方法に準拠し測定した。すなわち試験体を押出方向（ＭＤ）、押出方向に直交する方向（ＶＤ）に沿って切断し、それぞれの切断面のカット面外側（外周）より１／１０〜９／１０以上内側を走査型電子顕微鏡（株式会社日立製作所製Ｓ−３０００Ｎ）で２０倍（場合により２００倍）に拡大して撮影する。撮影した画像をＡ４用紙上に４画像づつ印刷し、夫々の方向に平行な任意の一直線上（長さ６０ｍｍ）にある気泡数から気泡の平均弦長（ｔ）を下記式により算出した。但し任意の直線はできる限り気泡が接点でのみ接しないようにした（接してしまう場合は気泡数に含める）。計測は６ヶ所とした。
平均弦長ｔ＝６０／（気泡数×写真の倍率）
そして次式により各方向における気泡径を算出した。
Ｄ＝ｔ／０．６１６
押出方向に沿って切断した面で、押出方向の気泡径と厚み方向（ＶＤ）の気泡径を測定し、押出方向に直交する方向に沿って切断した面で幅方向の気泡径と厚み方向の気泡径を測定する。
押出方向と幅方向は、６ヶ所の測定値を平均したものを押出方向の平均気泡径（Ｄ_{Ｍ Ｄ} ）、幅方向の平均気泡径（Ｄ_{Ｔ Ｄ} ）とし、厚み方向は１２ヶ所の平均を厚み方向の平均気泡径（Ｄ_{Ｖ Ｄ} ）とした。
さらにそれらの算術平均を平均気泡径とした。
平均気泡径（ｍｍ）＝（Ｄ_{Ｍ Ｄ} ＋Ｄ_{Ｔ Ｄ} ＋Ｄ_{Ｖ Ｄ} ）／３
また、試験片厚みが薄く、ＶＤ方向に６０ｍｍ長さ分の気泡数を数えられない場合は、測定可能な長さ分の気泡数を数えて６０ｍｍ分の気泡数に換算した。
【００５６】
また発泡シートの外観判定（気泡径の判定）として平均気泡径が大きいものは外観が良くないので平均気泡径が０．５ｍｍ未満のものを外観○、０．５ｍｍより大きいものを外観×とした。
【００５７】
密度についてはＪＩＳ Ｋ７２２２：１９９９「発泡プラスチック及びゴム−見掛け密度の測定」記載の方法で測定した。
【００５８】
発泡体におけるスの発生状況については、発泡体をＴＤ方向にカッターでカットし、その断面を目視により判定した。
【００５９】
【実施例１】
結晶性ポリ乳酸系樹脂として融点が１３２．９℃（ＪＩＳ Ｋ ７１２１）で、Ｄ−体割合が８．４重量％、Ｌ−体割合が９１．６重量％で、メルトマスフローレートが６．３ｇ／１０分、メルトマスフローレート（ｇ／１０分）／Ｄ−体割合（％）が０．７５のものを使用した。
【００６０】
上記樹脂１００重量部に気泡調整剤としてタルク１部、ステアリン酸モノグリセライドのマスターバッチ（前記ポリ乳酸系樹脂／ステアリン酸モノグリセライド（モノグリセライド含量：９５重量％以上）（重量比）＝４）３重量部（高級脂肪酸アルコール性エステル成分０．７５重量部）を混合し、この混合物を、一段目押出機の口径が５０ｍｍ、二段目押出機の口径が６５ｍｍであるタンデム式押出機に投入した。
【００６１】
押出機内で樹脂を初め１３５℃に加熱して樹脂を溶融し、その後樹脂温度を１７５℃まで上昇させ、この状態で、押出機のバレルの途中から溶融樹脂にジメチルエーテルを３重量部、ブタン（イソブタン）を３重量部の割合で圧入した。その際の圧入順序はジメチルエーテルをブタンより先に圧入した。引き続き、押出機内で溶融樹脂を冷却し、樹脂温度を１１６℃としてサーキュラ金型（出口口径６０ｍｍ、出口間隙０．７ｍｍ、外側金型開き角度３０度、内側金型開き角度３５度）から筒状の発泡体を３０ｋｇ／時、剪断速度５．４×１０^２ 秒^{− １} （押出量（Ｑ）＝８．３３ｃｍ^３ ／秒、ｒ_０ ＝２．９７１ｃｍ、出口間隙（ｔ_０ ）＝０．０７ｃｍ、Ｌ_２ ＝３．００６ｃｍ、Ｌ_１ ＝２．９３６ｃｍ）で押し出した。
【００６２】
筒状の発泡体は、冷却水で冷却された外形１７０ｍｍのマンドレルで冷却された後、カッターで切断し、シート状の発泡体とし、巻取機によりロール状に巻き取った。得られた発泡シートは均一に発泡しており、密度が０．０６４ｇ／ｃｍ^３ 、厚みが２．０ｍｍ、平均気泡径が０．３６ｍｍで均一微細な気泡を有し、スの発生がなく、外観に優れていた。
【００６３】
【実施例２】
結晶性ポリ乳酸系樹脂として融点が１４２．４℃で、Ｄ−体割合が４．４重量％、Ｌ−体割合が９５．６重量％で、メルトマスフローレートが７．５ｇ／１０分、メルトマスフローレート（ｇ／１０分）／Ｄ−体割合（％）が１．７のものを使用した。
【００６４】
上記樹脂１００重量部に気泡調整剤としてタルク１重量部、実施例１で使用したものと同じステアリン酸モノグリセライドのマスターバッチ３重量部を混合し、この混合物を実施例１で使用した押出機に投入した。押出機内で樹脂を初め１６５℃に加熱して樹脂を溶融し、その後樹脂温度を１８０℃まで上昇させ、この状態で、押出機のバレルの途中から溶融樹脂にジメチルエーテルを４重量部、ブタン（ノルマルブタン／イソブタン＝３５／６５）を１重量部の割合で圧入した。引き続き、押出機内で溶融樹脂を冷却し、樹脂温度を１２９℃とした以外は実施例１と同様にして発泡シートを製造した。
【００６５】
得られた発泡体は均一に発泡しており、密度が０．０７９ｇ／ｃｍ^３ 、厚みが１．６ｍｍ、平均気泡径が０．４０ｍｍで均一微細な気泡を有し、スの発生がなく、外観に優れていた。
【００６６】
【実施例３】
ジメチルエーテルを２重量部、ブタン（イソブタン）を４重量部の割合で圧入し、樹脂温度を１１７℃とした以外は実施例１と同様にして発泡シートを製造した。
【００６７】
得られた発泡体は均一に発泡しており、密度が０．０６７ｇ／ｃｍ^３ 、厚みが１．９ｍｍ、平均気泡径が０．３２ｍｍで均一微細な気泡を有し、スの発生がなく、外観に優れていた。
【００６８】
【実施例４】
ジメチルエーテルを５重量部、ブタン（イソブタン）を１重量部の割合で圧入し、樹脂温度を１１４℃とした以外は実施例１と同様にして発泡シートを製造した。
【００６９】
得られた発泡シートは均一に発泡しており、密度が０．０６２ｇ／ｃｍ^３ 、厚みが２．０５ｍｍ、平均気泡径が０．４４ｍｍで均一微細な気泡を有し、スの発生がなく、外観に優れていた。
【００７０】
【比較例１】
ジメチルエーテルを６重量部圧入し、樹脂温度を１１３℃とした以外は実施例１と同様にして発泡シートを製造した。
【００７１】
得られた発泡シートは密度が０．０６０ｇ／ｃｍ^３ 、厚みが２．１ｍｍ、平均気泡径が０．５６ｍｍで、スの発生はないが、外観に劣っていた。
【００７２】
【比較例２】
ブタン（イソブタン）を５重量部圧入し、樹脂温度を１２３℃とした以外は実施例１と同様にして発泡シートを製造した。
【００７３】
得られた発泡シートは密度が０．１２ｇ／ｃｍ^３ 、厚みが１．３ｍｍでスが多数発生していた。
【００７４】
【比較例３】
サーキュラ金型の出口間隙を１．０ｍｍ（剪断速度２．７×１０^２ 秒^{− １} ）とした以外は実施例２と同様にして発泡シートを製造しようとしたが、金型内で内部発泡が生じ良好な発泡シートが得られなかった。また内部発泡をなくそうと金型温度を下げている途中で溶融樹脂が結晶化し、押出圧力が急上昇し、インターロックにより押出機が停止したためサンプルが得られなかった。
【００７５】
【比較例４】
１７２．２℃で、Ｄ−体割合が１．６重量％、Ｌ−体割合が９８．４重量％で、メルトマスフローレートが５．４ｇ／１０分、メルトマスフローレート（ｇ／１０分）／Ｄ−体割合（％）が３．４のものを使用した。
【００７６】
上記樹脂１００重量部に気泡調整剤としてタルク１重量部、実施例１で使用したものと同じステアリン酸モノグリセライドのマスターバッチ３重量部を混合し、この混合物を実施例１で使用した押出機に投入した。押出機内で樹脂を初め１７５℃に加熱して樹脂を溶融し、その後樹脂温度を２１０℃まで上昇させ、この状態で、押出機のバレルの途中から溶融樹脂にジメチルエーテルを４重量部、ブタン（ノルマルブタン／イソブタン＝３５／６５）を１重量部の割合で圧入した。引き続き、押出機内で溶融樹脂を冷却し、樹脂温度を１５３℃として実施例１で使用した金型から押し出したが、金型から溶融樹脂が断続的に噴出し、安定した状態で発泡シートをつくることができなかった。
【００７７】
以上の実施例及び比較例で得られた結果を表にまとめると、表１のとおりとなる。
【００７８】
【表１】

[0001]
[Industrial applications]
The present invention relates to a method for producing a crystalline polylactic acid-based resin foam.
[0002]
[Prior art]
The polylactic acid-based resin is a resin obtained by polymerizing naturally occurring lactic acid. The polylactic acid-based resin is attracting attention as a resin having so-called abandoned degradability, which is decomposed by microorganisms existing in nature. In addition, the polylactic acid-based resin has excellent mechanical properties at room temperature due to its composition. Therefore, polylactic acid-based resins are attracting attention as environmentally friendly resins having practical utility.
[0003]
Lactic acid contains an asymmetric carbon atom in the molecule. For this reason, lactic acid exhibits optical activity. Therefore, there are three types of lactic acid: D-form, L-form, and racemic form (equivalent mixture of D-form and L-form). For this reason, polylactic acid obtained by polymerizing lactic acid produces polymers having various properties depending on the mixing ratio of these three types and the difference in the polymerization method. In fact, polylactic acid has a variety of different properties, from crystalline to amorphous, and has different melting points or softening points.
[0004]
In general, when heated, the polylactic acid-based resin softens or melts and immediately becomes a low-viscosity liquid. Liquids of low viscosity cannot retain air bubbles therein. Therefore, the temperature range in which the polylactic acid-based resin exhibits a melt viscosity suitable for foaming when heated is extremely narrow. For this reason, it is difficult for the polylactic acid-based resin to be heated to develop a state suitable for foaming, and therefore, is considered to be a resin that is difficult to foam. Among them, crystalline polylactic acid-based resins are considered to be resins having a distinct melting point and having a sharp melting point below the melting point, so that foaming is particularly difficult.
[0005]
Therefore, attempts have been made to modify the polylactic acid-based resin to facilitate foaming. For example, JP-A-2000-44716 proposes that lactic acid is copolymerized with a polysaccharide such as acetyl cellulose to obtain a polylactic acid-based resin. However, such resins not only require complicated processes and special equipment to copolymerize lactic acid with polysaccharides, but also the resulting resin is a polylactic acid-based resin's inherent destruction There is a problem that the sex is impaired.
[0006]
Japanese Patent Application Laid-Open No. 2002-155197 proposes a method of mitigating a sharp decrease in viscosity near the melting point of a resin by including an isocyanate compound as a crosslinking agent in a biodegradable resin composition. However, when an isocyanate compound is included, there is a problem in that the obtained resin is impaired in the inherent decomposability of the polylactic acid-based resin.
[0007]
[Problems to be solved by the invention]
The present invention provides a crystalline polylactic acid-based resin that is made of a polylactic acid-based resin that maintains the property of decomposing naturally and becomes harmless when abandoned, so that a foam having uniform and fine cells can be obtained. It is intended to provide a method for producing a foam.
[0008]
[Means for solving the problem]
This inventor tried extrusion foaming by adding various additives to the crystalline polylactic acid-based resin. As a result, when a crystalline polylactic acid-based resin of a specific composition is selected and extruded and foamed using dimethyl ether and a hydrocarbon in combination as a foaming agent, high-quality foaming can be achieved without impairing the destructive degradability of the polylactic acid-based resin. I found that I got a body.
[0009]
As described above, crystalline polylactic acid-based resin has been considered to be a resin having a low melt viscosity and difficult to foam, and so far, attempts have been made exclusively to improve the melt viscosity. Little was considered. However, as a result of examining the relationship between the composition of the crystalline polylactic acid-based resin and the foaming agent, the present inventors found that the crystalline polylactic acid-based resin obtained by copolymerizing a D-form and an L-form at a specific ratio. In contrast, when extrusion foaming is performed using a mixture in which dimethyl ether and hydrocarbon are mixed at a specific ratio as a foaming agent, a melt viscosity suitable for foaming is unexpectedly obtained, and a good quality is easily obtained. It was found that a foam was obtained. The present invention has been completed based on such findings.
[0010]
As a result of further experiments, the inventor found that, in performing the above-mentioned extrusion foaming, when the resin was extruded at a specific shear rate from a mold attached to the tip of the extruder, it was easier and more reliable to obtain a good-quality foam. It was found that it could be obtained. Further, they have found that a high-quality foam can be obtained more easily by extrusion foaming by adding a higher fatty acid alcoholic ester such as stearic acid monoglyceride to the above-mentioned crystalline polylactic acid-based resin. The present invention has been completed based on such findings.
[0011]
In the present invention, a crystalline polylactic acid resin obtained by copolymerizing a D-form and an L-form at a ratio of 2 to 13% and 98 to 87% by weight, respectively, is heated in an extruder and melted. A mixture obtained by mixing dimethyl ether and hydrocarbon as a foaming agent at a weight ratio of 95/5 to 5/95 as a foaming agent is injected into the resin, and the obtained molten resin is extruded from an extruder and foamed. The present invention provides a method for producing a crystalline polylactic acid-based resin foam.
[0012]
Of the foams, sheet-like foams are generally difficult to obtain uniformly and finely foamed, but the present invention is useful in that a good-quality sheet-like foam can be easily obtained. It is. That is, among the foams, a foam other than the sheet-like foam, for example, a rod-like foam, can be relatively easily foamed at a high magnification and of good quality. However, it is difficult to obtain a high-magnification sheet-shaped foam, but such a sheet-shaped foam can be easily obtained according to the method of the present invention.
[0013]
In general, a polylactic acid-based resin is produced by polymerizing L-lactic acid and / or D-lactic acid, or by ring-opening polymerization of L-lactide and / or D-lactide and / or DL-lactide. In the present invention, a polylactic acid-based resin produced by any of these methods can be used.
[0014]
The polylactic acid-based resin that can be used in the present invention is a crystalline polylactic acid obtained by copolymerizing D-form and L-form at a ratio of 2 to 13% and 98 to 87% by weight, respectively. It is a system resin. As the resin, in addition to the resin consisting of only D-lactic acid and L-lactic acid, a resin in which another hydroxycarboxylic acid is further copolymerized can be used. In short, the resin has the formula

It contains D-lactic acid and L-lactic acid represented by the above-mentioned ratios.
[0015]
As described above, the polylactic acid-based resin is classified into an amorphous resin and a crystalline resin depending on the ratio between the D-form and the L-form. Generally, a resin in which the D-form or the L-form occupies nearly 100% is crystalline, and as one of the D-form and the L-form increases, the resin tends to gradually become amorphous. Exceeds 20%, the resin becomes completely amorphous. A crystalline polylactic acid resin generally has a high melting point and usually shows a melting point of 100 ° C. or higher, and the melting point is 50 ° C. or higher than the softening point (glass transition point) of the amorphous resin. Therefore, the crystalline polylactic acid-based resin can be used as a heat-resistant resin.
[0016]
It is not known what proportion of the copolymer composed of D-lactic acid and L-lactic acid is suitable for extrusion foaming. As a result of studying this point, the present inventor has found that a copolymer containing 2 to 13% by weight of the D-form and 98 to 87% by weight of the L-form is suitable for extrusion foaming. Was. Therefore, in the present invention, a copolymer having a D-form of 2 to 13% by weight and an L-form of 98 to 87% by weight is used as the crystalline polylactic acid resin.
[0017]
The copolymerization ratio of the D-form and the L-form described above can be specified from the amount of raw materials charged during the production of the polylactic acid-based resin. It can also be determined from the obtained polylactic acid-based resin. It can be determined by measuring the specific rotation of the copolymer, as in the case of the D- or L-form of the monomer. It can be measured, for example, as follows.
[0018]
First, 0.2 g of the polylactic acid resin in question is dissolved in chloroform to make 20 ml. Using a polarimeter (DIP-370, manufactured by JASCO Corporation), the solution was irradiated with sodium spectrum D line at 25 ° C. at a layer length of 100 mm, and the optical rotation of the solution was measured. From the optical rotation, the following formula (1) was obtained. Find the specific rotation in. From the obtained specific rotation, the ratio of the D-form and the L-form is calculated by the following equation (2) or (3).
Specific rotation (degree) = 100 × α / [l × c] (Equation 1)
α: Optical rotation measured value (degree) of sample
l: cell layer length (mm)
c: concentration of sample solution (g / ml)
D-body ratio (%) = (specific rotation + 156) × 100/312 (formula 2)
L-body ratio (%) = 100− (D-body ratio) (Formula 3)
[0019]
In the present invention, the copolymerized polylactic acid-based resin is selected and used at a ratio of 2 to 13% by weight of the D-form and 98 to 87% by weight of the L-form based on the content ratio thus determined. . The reason for this is based on the results obtained by conducting experiments on extrusion foaming of polylactic acid-based resins having various compositions as described above. That is, when the D-form is less than 2% by weight or the L-form is more than 98% by weight, the polylactic acid-based resin rapidly crystallizes from a molten state, so that the resin has a melt viscosity suitable for foaming. This is because it is difficult to form the foam, and even if a foam is formed, the obtained foam is highly brittle and causes a problem when actually used. Conversely, if the D-form is more than 13% by weight or the L-form is less than 87% by weight, the polylactic acid-based resin tends to be amorphous, and has a low melting point, making it unsuitable for practical use. It is. Among them, those containing 2 to 10% by weight of the D-form and 98 to 90% by weight of the L-form are preferable, and 3 to 9% by weight of the D-form and 97 to 91% by weight. Those containing the L-form are most preferred.
[0020]
In the present invention, it is preferable to use a polylactic acid-based resin having a melt mass flow rate of 1 to 15 g / 10 min. Among them, those having a melt mass flow rate of 5 to 10 g / 10 minutes are preferably used. In this case, the melt mass flow rate is a value measured by applying a load of 2.16 kgf at 190 ° C. in accordance with the method specified by JIS K 7210.
[0021]
Both the content ratio of the D-form in the crystalline polylactic acid-based resin and the melt mass flow rate greatly affect the ease of extrusion foaming of the crystalline polylactic acid-based resin. Therefore, the ratio W between the melt mass flow rate of the crystalline polylactic acid-based resin and the D-form content ratio, that is,

When the value of is adopted, W is in the range of 0.33 to 3, especially when it is in the range of 0.5 to 2, extrusion foaming becomes easy. When W is less than 0.33, the load during extrusion foaming increases, and it becomes difficult to obtain a good foam in a stable manner. Conversely, if W is greater than 3, the melt viscosity of the crystalline polylactic acid-based resin becomes too sensitive to temperature, making it difficult to control the melt viscosity suitable for foaming.
[0022]
In the present invention, a crystalline polylactic acid-based resin is put into an extruder, and a blowing agent is contained in the molten resin. Dimethyl ether and hydrocarbon are used as the blowing agent. As the hydrocarbon, for example, propane, butane, and pentane can be used. Among them, it is preferable to use butane. This is because it is easy to extrude and foam, and the bubbles can be uniformly fined. As butane, any of normal butane, isobutane, and a mixture thereof can be used. Among them, a mixture of 30 to 100% by weight of isobutane and 0 to 70% by weight of normal butane, particularly a mixture of 50 to 100% by weight of isobutane and 0 to 50% by weight of normal butane is preferably used. This is because air bubbles can be particularly reduced.
[0023]
Dimethyl ether exhibits excellent compatibility with the crystalline polylactic acid-based resin melted in the extruder, brings about a remarkable plasticizing effect, and significantly improves foamability in extrusion foaming. However, dimethyl ether has a tendency to increase bubbles because the compatibility with the crystalline polylactic acid resin is too good. On the other hand, hydrocarbons do not have very good compatibility with the crystalline polylactic acid-based resin, and conversely work as a foam nucleating agent and work to make air bubbles fine. In addition, since hydrocarbons do not have very good compatibility with the crystalline polylactic acid-based resin, when only hydrocarbons are used as the foaming agent, the foam film in the foam is broken and foam is generated. However, when dimethyl ether and a hydrocarbon are used in combination, the above-mentioned drawbacks are suppressed, the advantages are exhibited, and a good foam can be obtained.
[0024]
In the present invention, dimethyl ether and hydrocarbon are mixed and used at a weight ratio of 95/5 to 5/95. Among these, a ratio of 90/10 to 10/90 is preferable, and a ratio of 85/15 to 15/85 is more preferable. When the ratio of dimethyl ether / hydrocarbon is large, that is, when the amount of dimethyl ether is larger than that of hydrocarbon, the effect of the hydrocarbon to make the bubbles fine is not sufficiently exhibited, and a foam having coarse cells and poor appearance is obtained. Conversely, if the above ratio is small, the region where the resin exhibits a viscosity suitable for foaming becomes narrow, so that it becomes difficult to foam the resin at a high magnification, and it becomes easy for foam to be generated in the foam. The total amount of dimethyl ether and hydrocarbon used is 0.5 to 15 parts by weight, preferably 1 to 10 parts by weight, more preferably 2 to 8 parts by weight based on 100 parts by weight of the polylactic acid-based resin.
[0025]
To inject a mixture of dimethyl ether and a hydrocarbon, a method of injecting a blowing agent can be conventionally used. That is, the mixture is pressed into the molten resin through a hole formed in the cylinder of the extruder by a pump or the like. The dimethyl ether and the hydrocarbon may be injected from separate holes, but it is preferable that the dimethyl ether and the hydrocarbon are mixed in a predetermined ratio in advance and then injected from one hole.
[0026]
In order to efficiently and uniformly disperse the above mixture in the crystalline polylactic acid-based resin, dimethyl ether and hydrocarbon are previously mixed as described above and then injected into the resin, or dimethyl ether is injected when separately injected. , Followed by hydrocarbons. In order to efficiently and uniformly disperse the above mixture in the resin, both dimethyl ether and hydrocarbon are heated to their respective critical temperatures or higher, and are melted in a resin melted in a state of being pressed to a critical pressure or higher. Injection is preferred.
[0027]
More specifically, dimethyl ether has a critical temperature of 127 ° C. and a critical pressure of 5.3 MPa, so it is preferable to inject the dimethyl ether into the molten resin at a temperature of 127 ° C. or more and a pressure of 5.3 MPa or more. When isobutane is used as a hydrocarbon, since isobutane exhibits a critical temperature of 135 ° C. and a critical pressure of 3.6 MPa, it can be injected into the molten resin at a temperature of 135 ° C. or more and a pressure of 3.6 MPa or more. Preferably, when normal butane is used as the hydrocarbon, normal butane has a critical temperature of 152 ° C. and a critical pressure of 3.8 MPa. Injection is preferred. Therefore, when using the above-mentioned three types of mixed foaming agents, it is possible to inject into the molten resin at a temperature of 152 ° C. and a pressure of 5.3 MPa or more, which are the maximum combination of the respective critical temperatures and critical pressures. preferable.
[0028]
The extruder used in the present invention may be of any shape. For example, a single-screw extruder, a twin-screw extruder, or a tandem extruder in which these are connected can be used. The extruder has the ability to mix and melt the crystalline polylactic acid resin well, to be able to uniformly contain the foaming agent, and to continuously cool and maintain at a predetermined temperature and extrude. You need to use something.
[0029]
When a tandem-type extruder is used as the extruder, the foaming agent is press-fitted in the first-stage extruder, and then the molten resin containing the foaming agent is continuously supplied to the second-stage extruder. At this time, in the connecting pipe connecting the first-stage extruder and the second-stage extruder, while maintaining the resin at a temperature equal to or higher than the melting point, of the critical temperature and critical pressure of dimethyl ether and hydrocarbon, Maintain at or above the higher temperature and pressure to disperse the blowing agent in the resin. The resin in which the foaming agent is dispersed is stirred in the second extruder so that the whole has a uniform composition and temperature.
[0030]
In the present invention, it is necessary to use a mixture of dimethyl ether and a hydrocarbon as a foaming agent. In addition to this, a known foaming agent can be mixed and used. In this case, the known blowing agent is smaller than the total amount of dimethyl ether and hydrocarbon. Known foaming agents include solid compounds that decompose above the melting point of the resin to generate gas, such as dinitrosopentamethylenetetramine, liquids that evaporate in the resin when heated, such as methyl chloride, carbon dioxide and nitrogen. Any of them, such as an inert gas which is dissolved in a resin under pressure as described above, can be used.
[0031]
According to the present invention, after the above-described foaming agent is contained in the molten resin in the extruder, the molten resin can be extruded while being maintained at a temperature equal to or higher than the melting point of the resin. Advantageously, however, the blowing agent-containing molten resin is subsequently cooled in the extruder to a temperature below the melting point of the original resin, and while the blowing agent-containing resin is still flowable, the resin is cooled. Extruding from an extruder. Extrusion at the latter temperature is abnormal in extrusion molding, and will be described below.
[0032]
In general, when a resin is extruded and foamed, it is common sense to extrude a crystalline resin at a temperature equal to or higher than the melting point of the resin and, in an amorphous resin, extrude the resin at a temperature equal to or higher than the glass transition point of the resin. . This is because if the resin is extruded at a temperature lower than the melting point or the glass transition point, the resin is solidified and cannot be extruded.
[0033]
In fact, hitherto, for example, when using crystalline polyethylene terephthalate having a melting point of 255 ° C. as a resin and extruding and foaming the resin, the resin is extruded at a resin temperature of 270 to 290 ° C., and the resin has a melting point of 160 ° C. When the polypropylene was extruded and foamed, the resin was extruded at a resin temperature of 170 ° C. In the case where amorphous polystyrene having a glass transition point of 90 to 95 ° C. is extruded and foamed as a resin, the resin temperature is set to 120 to 160 ° C. In any case, the resin temperature at the time of extrusion has been set to a temperature higher than the melting point or the glass transition point of the resin.
[0034]
However, in the present invention, since the crystalline polylactic acid-based resin is extruded at a temperature lower than the melting point as described above, this is unusual. At temperatures below the melting point, the material is extruded between the melting point and a temperature 40 ° C. lower than the melting point. It is more preferable that the temperature be between 1 ° C. lower than the melting point and 30 ° C. lower than the melting point, and that the temperature is 2 ° C. lower than the melting point and 20 ° C. lower than the melting point. Most preferably, it is between. The reason that it can be extruded at such a temperature is that when the crystalline polylactic acid-based resin containing dimethyl ether and hydrocarbons is cooled below its melting point, it remains in a supercooled state, and for a while Is because of maintaining fluidity.
[0035]
Various methods can be employed to uniformly cool the melted dimethyl ether and hydrocarbon-containing polylactic acid-based resin below the melting point of the resin. The method cools the barrel of the extruder while kneading the resin using a screw with a high kneading function, such as a screw with many pins or a screw with a screw flight with many notches. This is a method of cooling with water or the like.
[0036]
The present invention thus cools the blowing agent-containing molten resin to a temperature suitable for foaming. Then, while the foaming agent-containing resin still retains fluidity, the resin is extruded from a mold attached to the tip of the extruder and foamed. At this time, the temperature of the resin passing through the mold may be higher than or equal to the melting point of the crystalline polylactic acid resin, but is usually set to a temperature between the melting point and a temperature lower by 40 ° C. than the melting point. At this time, the resin has a melt viscosity suitable for foaming.
[0037]
In this invention, in order to further improve the extrusion foaming moldability of the crystalline polylactic acid-based resin, it is preferable to use the crystalline polylactic acid-based resin by adding a higher fatty acid alcoholic ester to the resin. The higher fatty ester effectively acts as a plasticizer for the crystalline polylactic acid resin, broadening the range of conditions during extrusion foam molding, and a compatibilizer for the crystalline polylactic acid resin and a foaming agent, particularly a hydrocarbon. And effectively suppresses the generation of dust in the foam. The higher fatty acid alcoholic ester also effectively acts as an antistatic agent, and has an effect of preventing a fire accident or the like when a flammable blowing agent such as dimethyl ether or hydrocarbon is used.
[0038]
The higher fatty acid alcoholic ester means an ester formed by a higher fatty acid and a polyhydric alcohol, and refers to an ester containing one or more free hydroxyl groups in one molecule of the ester. Those having no ether bond in the molecule are particularly preferable.
[0039]
As the polyhydric alcohol, for example, glycerin, pentaerythrit, sorbit, sorbitan, mannitol, mannitane, dipentaerythrit, diglycerin and the like can be used.
[0040]
Examples of the higher fatty acids include, for example, monovalent higher or higher unsaturated fatty acids having 10 to 30 carbon atoms such as capric acid, lauric acid, myristic acid, palmitic acid, stearic acid, palmitoleic acid, oleic acid, and linoleic acid. Alternatively, bovine fatty acids, bran oil fatty acids, coconut oil fatty acids, etc. in which these higher fatty acids are mixed can be used.
[0041]
Examples of the higher fatty acid alcoholic ester include lauric acid monoglyceride, palmitic acid monoglyceride, stearic acid monoglyceride, pentaerythrit monolaurate, dipentaerythrit distearate, sorbitan monooleate, sorbitan sesqui bran oil fatty acid ester, sorbitan Monopalmitate, sorbitan monolaurate, sorbitan monostearate, mannitan monooleate, mannitan monolaurate and the like can be used. Among them, stearic acid monoglyceride is preferable.
[0042]
If the amount of the higher fatty acid alcoholic ester is too small, the above-mentioned effects cannot be obtained. Conversely, if the amount is too large, a large amount of the higher fatty acid alcoholic ester comes out on the surface of the foamed foam, which is not preferable because of stickiness. The amount is preferably 0.01 to 1.0 part by weight, more preferably 0.02 to 0.5 part by weight, and particularly preferably 0.03 to 0.3 part by weight based on 100 parts by weight of the polylactic acid-based resin.
[0043]
In the present invention, when extruding and foaming a polylactic acid-based resin, it is possible to use various additives that have been used in extrusion foaming of other resins. For example, talc, calcium carbonate, shelf sand, zinc hydroxide, aluminum hydroxide, silica, sodium carbonate, potassium carbonate, fluororesin powder, etc. have been added as foam control agents so far. Can be added. The amount added as the cell regulator varies depending on the type thereof, but it is generally suitable to be in the range of 0.01 to 5 parts by weight based on 100 parts by weight of the crystalline polylactic acid-based resin. When talc, calcium carbonate or the like is used as a filler, it can be added up to about 20 parts by weight.
[0044]
In addition, antistatic agents, flame retardants, and coloring agents can be used as additives. As the antistatic agent, for example, fatty acid compounds such as quaternary ammonium salt compounds, fatty acid ester compounds and fatty acid amide compounds can be used. When a fatty acid compound such as a fatty acid ester compound and a fatty acid amide compound is used, the extrusion foaming moldability of the polylactic acid-based resin can be further improved. Among the fatty acid compounds, those having a molecular weight of 150 to 1,000, particularly preferably 200 to 800 are preferable. A general use amount is 0.03 to 3.0 parts by weight, preferably 0.05 to 2.0 parts by weight, and more preferably 0.1 to 3.0 parts by weight based on 100 parts by weight of the crystalline polylactic acid-based resin. 1.0 part by weight.
[0045]
A mold having an orifice corresponding to the shape of the foam to be obtained can be used as the mold. A circular die is used to obtain a sheet-like foam, and a flat die or a T-die is used to obtain a board-like foam. Since the foamed sheet can be formed into various shapes by thermoforming, it is suitable as a form of a crystalline polylactic acid-based resin foam.
[0046]
After being extruded from the mold orifice, the crystalline polylactic acid-based resin is foamed and then cooled and shaped into a foam by the same operation as in conventional polystyrene or polyolefin extrusion foaming. You. When trying to obtain a foamed sheet, the resin is extruded and foamed in a cylindrical shape from an annular orifice, the extruded tubular foam is cooled while advancing along a cooled mandrel, and then the cooled tubular foam Is opened in the axial direction to form a foamed sheet.
[0047]
In the present invention, when the crystalline polylactic acid-based resin is extruded at a temperature between the melting point and a temperature lower by 40 ° C. than the melting point, the mold exit shear rate is set to 300 seconds.^-1It is desirable to extrude the above. Shear rate 300 seconds^-1If the temperature is lower, crystallization partially proceeds in the mold, resulting in a foam having poor appearance, or foaming occurs in the mold, resulting in foam having poor appearance and quality. However, if the shear rate is too high, specifically, 10,000 seconds^-1If it is faster, fracture occurs in the molten resin, resulting in a foam having poor appearance and quality. The preferred shear rate for obtaining a foam having excellent quality and appearance is 400 to 8000 seconds.^-1And particularly preferably 500 to 6000 seconds.^-1It is. The shear rate in the present invention refers to the shear rate at the exit of the die tip, and is calculated using a formula generally used in extrusion molding.
[0048]
In the above case, the shear rate at the outlet of the die of the circular mold is calculated by the following double pipe calculation formula.
Shear rate (sec^-1) = 6Q / [π (L₂ ²-L₁ ²) (L₂-L₁)]
However, Q is a volume extrusion amount (cm³  / Sec) and L₁  (Cm) is (r₀-T₀  / 2) and L₂  (Cm) is (r₀+ T₀/ 2). R here₀Is (the inner diameter of the outer mold at the exit of the mold + the outer diameter of the inner mold at the exit of the mold) / 2, and t is₀  Is the mold exit gap.
[0049]
At that time, the mass extrusion rate (g / sec) to the volume extrusion rate (cm³  / S) is 1.0 g / cm.³  And In the case of a T die or a flat die, a parallel groove calculation formula is used.
Shear rate (sec^-1) = 6Q / Wt²
However, Q is a volume extrusion amount (cm³  / Sec), W (cm) is the width of the mold exit parallel groove, and t (cm) is the interval of the mold exit parallel groove.
[0050]
In order to stably produce a sheet foamed at a high magnification of three times or more, it is preferable to use a mold suitable for foaming a crystalline polylactic acid-based resin. Specifically, it is preferable to use a mold having an extrusion hole (orifice) that opens in a trumpet shape toward the outlet. By using a mold having such a shape, it is possible to stably obtain a crystalline polylactic acid-based resin foamed sheet which has less waving in the flow direction and is foamed at a high magnification. Further, in order to stably obtain a good crystalline polylactic acid-based resin foam sheet suitable for thermoforming, the ratio of the mold diameter to the cooled mandrel outer diameter, and the mandrel outer diameter / die diameter is 2 to 5 , And more preferably 2.5 to 4.5.
[0051]
According to the method of the present invention, the density is 0.03 to 0.60 g / cm.³  Of a crystalline polylactic acid-based resin foam uniformly foamed. The obtained crystalline polylactic acid-based resin foam can be used as a heat insulating material, a cushioning material, an industrial member, a building material, a civil engineering material, an agricultural material, and a food packaging material. In particular, when a foamed sheet is used, it can be thermoformed into various shapes such as trays by a thermoforming machine, and can be used more widely as food containers, fruit trays, packing materials, and the like. As a foamed sheet for molding, if it is too thick, thermoforming is difficult, if it is too thin, it is inferior in cushioning and heat insulating properties, if it is too large, it is inferior in heat insulating properties, and if it is too small, it is inferior in strength. 0.5 to 5 mm, more preferably 0.7 to 4 mm, particularly preferably 0.8 to 3 mm. The preferred density is 0.02 to 0.4 g / cm.³  More preferably, the density is 0.03 to 0.3 g / cm.³, A particularly preferred density is 0.04 to 0.2 g / cm³  It is.
[0052]
【The invention's effect】
According to the present invention, the crystalline polylactic acid resin obtained by copolymerizing the D-form and the L-form at a ratio of 2 to 13% and 98 to 87% by weight is used as the resin. A product suitable for practical use with good properties and good mechanical properties can be obtained. Further, in the present invention, a mixture obtained by mixing dimethyl ether and hydrocarbon at a ratio of 95/5 to 5/95 by weight is press-fitted into the above resin as a foaming agent, and the obtained molten resin is extruded from an extruder. Since it is extruded and foamed, foaming is easy, and a foam finely and uniformly foamed can be easily obtained. Further, a shearing speed of 300 seconds was obtained from a mold in which a molten resin containing a foaming agent was attached to the tip of the extruder.^-1When extruded as described above, the blowing agent composed of dimethyl ether and a hydrocarbon works effectively on the crystalline polylactic acid-based resin, and gives the crystalline polylactic acid-based resin a melt viscosity suitable for foaming. The obtained crystalline polylactic acid-based resin can provide a high-quality crystalline polylactic acid-based resin foam which is uniformly foamed at a high magnification such as three times or more.
[0053]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the advantages of the present invention will be specifically described with reference to Examples and Comparative Examples. Hereinafter, simply “parts” means “parts by weight”. The resin temperature at the time of extrusion was indicated by a thermocouple fixed to the center of the resin flow path of the breaker plate installed at the resin outlet of the extruder, and the temperature indicated by the thermocouple.
[0054]
The melting point of the crystalline polylactic acid resin referred to in the present invention is based on the melting peak temperature measured according to JIS K7121: 1987 (method for measuring the transition temperature of plastic).
[0055]
The average cell diameter was measured according to the test method of ASTM D2842-69. That is, the test specimen is cut along the extrusion direction (MD) and the direction perpendicular to the extrusion direction (VD), and the inside of each cut surface is 1/10 to 9/10 or more from the outside (outer periphery) of the cut surface. Images are taken with a microscope (S-3000N manufactured by Hitachi, Ltd.) at a magnification of 20 times (200 times in some cases). The photographed images were printed on an A4 sheet by four images, and the average chord length (t) of the bubbles was calculated by the following equation from the number of bubbles on an arbitrary straight line (length: 60 mm) parallel to each direction. However, the arbitrary straight line is set so that the bubbles do not contact only at the contact points as much as possible (if they do, the bubbles are included in the number of bubbles). The measurement was made at six locations.
Average chord length t = 60 / (number of bubbles x magnification of photograph)
Then, the bubble diameter in each direction was calculated by the following equation.
D = t / 0.616
Measure the bubble diameter in the extrusion direction and the bubble diameter in the thickness direction (VD) on the surface cut along the extrusion direction, and measure the bubble diameter in the width direction and the thickness direction on the surface cut along the direction perpendicular to the extrusion direction. Measure the bubble diameter.
In the extrusion direction and the width direction, the average of the measured values at the six locations was used to calculate the average bubble diameter (D) in the extrusion direction._MD), Average cell diameter in the width direction (D_TD), And the average of 12 locations in the thickness direction is the average cell diameter (D_{V D}).
Further, their arithmetic average was defined as the average bubble diameter.
Average bubble diameter (mm) = (D_MD+ D_TD+ D_{V D}) / 3
In the case where the thickness of the test piece was small and the number of bubbles for a length of 60 mm in the VD direction could not be counted, the number of bubbles for a measurable length was counted and converted to the number of bubbles for 60 mm.
[0056]
In addition, as the appearance judgment (judgment of cell diameter) of the foamed sheet, those having a large average cell diameter are not good in appearance. .
[0057]
The density was measured by the method described in JIS K7222: 1999 "Foamed plastic and rubber-Measurement of apparent density".
[0058]
Regarding the state of generation of foam in the foam, the foam was cut in the TD direction with a cutter, and the cross section was visually determined.
[0059]
Embodiment 1
The crystalline polylactic acid resin has a melting point of 132.9 ° C. (JIS K 7121), a D-form ratio of 8.4% by weight, an L-form ratio of 91.6% by weight, and a melt mass flow rate of 6.3 g. The melt mass flow rate (g / 10 minutes) / D-isomer ratio (%) was 0.75.
[0060]
100 parts by weight of the resin, 1 part of talc as a cell regulator, and 3 parts by weight of a masterbatch of monoglyceride stearic acid (the polylactic acid-based resin / monoglyceride stearate (monoglyceride content: 95% by weight or more) (weight ratio) = 4) (weight ratio) The higher fatty acid alcoholic ester component (0.75 parts by weight) was mixed, and this mixture was charged into a tandem-type extruder in which the diameter of the first-stage extruder was 50 mm and the diameter of the second-stage extruder was 65 mm.
[0061]
The resin is first heated to 135 ° C. in the extruder to melt the resin, and then the resin temperature is raised to 175 ° C. In this state, 3 parts by weight of dimethyl ether is added to the molten resin from the middle of the extruder barrel, butane (isobutane) ) At 3 parts by weight. In that case, dimethyl ether was injected before butane. Subsequently, the molten resin is cooled in an extruder, and the resin temperature is set to 116 ° C. to form a circular mold (outlet diameter: 60 mm, exit gap: 0.7 mm, outer mold opening angle: 30 °, inner mold opening angle: 35 °). Foam of 30 kg / h, shear rate 5.4 × 10²  Second^-1(Extruded amount (Q) = 8.33 cm³/ Sec, r₀  = 2.971 cm, exit gap (t₀  ) = 0.07 cm, L₂  = 3.006 cm, L₁  = 2.936 cm).
[0062]
The cylindrical foam was cooled with a mandrel having an outer shape of 170 mm cooled with cooling water, and then cut with a cutter to form a sheet-like foam, which was wound into a roll by a winder. The obtained foamed sheet is uniformly foamed and has a density of 0.064 g / cm.³  The thickness was 2.0 mm, the average bubble diameter was 0.36 mm, and uniform fine bubbles were formed.
[0063]
Embodiment 2
The crystalline polylactic acid resin has a melting point of 142.4 ° C., a D-form ratio of 4.4% by weight, an L-form ratio of 95.6% by weight, a melt mass flow rate of 7.5 g / 10 min, and a melt Mass flow rate (g / 10 min) / D-isomer ratio (%) of 1.7 was used.
[0064]
1 part by weight of talc as a cell regulator and 3 parts by weight of the same master batch of stearic acid monoglyceride as used in Example 1 were mixed with 100 parts by weight of the resin, and the mixture was charged into the extruder used in Example 1. did. The resin is first heated to 165 ° C. in the extruder to melt the resin, and then the resin temperature is increased to 180 ° C. In this state, 4 parts by weight of dimethyl ether is added to the molten resin from the middle of the barrel of the extruder, and butane (normal (Butane / isobutane = 35/65) at a rate of 1 part by weight. Subsequently, the molten resin was cooled in the extruder, and a foamed sheet was produced in the same manner as in Example 1 except that the resin temperature was set to 129 ° C.
[0065]
The resulting foam is uniformly foamed and has a density of 0.079 g / cm³  The thickness was 1.6 mm, the average cell diameter was 0.40 mm, and uniform fine cells were formed.
[0066]
Embodiment 3
A foamed sheet was produced in the same manner as in Example 1 except that dimethyl ether was press-fitted at a ratio of 2 parts by weight and butane (isobutane) was pressed at a ratio of 4 parts by weight, and the resin temperature was changed to 117 ° C.
[0067]
The obtained foam is foamed uniformly and has a density of 0.067 g / cm.³  The thickness was 1.9 mm, the average cell diameter was 0.32 mm, and uniform and fine cells were formed.
[0068]
Embodiment 4
A foamed sheet was produced in the same manner as in Example 1 except that 5 parts by weight of dimethyl ether and 1 part by weight of butane (isobutane) were press-fitted and the resin temperature was changed to 114 ° C.
[0069]
The obtained foamed sheet is foamed uniformly and has a density of 0.062 g / cm.³  The thickness was 2.05 mm, the average cell diameter was 0.44 mm, and the cells had uniformly fine cells.
[0070]
[Comparative Example 1]
A foamed sheet was produced in the same manner as in Example 1 except that 6 parts by weight of dimethyl ether was injected and the resin temperature was set to 113 ° C.
[0071]
The obtained foamed sheet has a density of 0.060 g / cm.³  The thickness was 2.1 mm, the average bubble diameter was 0.56 mm, and no dust was generated, but the appearance was poor.
[0072]
[Comparative Example 2]
A foamed sheet was produced in the same manner as in Example 1 except that 5 parts by weight of butane (isobutane) was press-fitted and the resin temperature was set to 123 ° C.
[0073]
The obtained foamed sheet has a density of 0.12 g / cm.³  And the thickness was 1.3 mm, and a lot of dust was generated.
[0074]
[Comparative Example 3]
The exit gap of the circular mold is set to 1.0 mm (shear speed 2.7 × 10²  Second^-1), But an attempt was made to produce a foamed sheet in the same manner as in Example 2. However, internal foaming occurred in the mold and a good foamed sheet could not be obtained. Further, the molten resin crystallized while the mold temperature was being lowered to eliminate internal foaming, the extrusion pressure rapidly increased, and the extruder was stopped by the interlock, so that no sample was obtained.
[0075]
[Comparative Example 4]
At 172.2 ° C., the D-form ratio was 1.6% by weight, the L-form ratio was 98.4% by weight, the melt mass flow rate was 5.4 g / 10 min, and the melt mass flow rate (g / 10 min) / Those having a D-isomer ratio (%) of 3.4 were used.
[0076]
To 100 parts by weight of the above resin, 1 part by weight of talc as a cell regulator and 3 parts by weight of the same master batch of stearic acid monoglyceride as used in Example 1 were mixed, and this mixture was charged into the extruder used in Example 1 did. The resin is first heated to 175 ° C. in the extruder to melt the resin, and then the resin temperature is raised to 210 ° C. In this state, 4 parts by weight of dimethyl ether is added to the molten resin from the middle of the barrel of the extruder, and butane (normal (Butane / isobutane = 35/65) at a ratio of 1 part by weight. Subsequently, the molten resin was cooled in the extruder and extruded from the mold used in Example 1 at a resin temperature of 153 ° C., but the molten resin was intermittently ejected from the mold to form a foamed sheet in a stable state. I couldn't do that.
[0077]
Table 1 summarizes the results obtained in the above Examples and Comparative Examples in a table.
[0078]
[Table 1]

Claims (4)

A crystalline polylactic acid-based resin obtained by copolymerizing the D-form and the L-form at a ratio of 2 to 13% and 98 to 87% by weight, respectively, is heated in an extruder, and a blowing agent is added to the molten resin. Wherein a mixture of dimethyl ether and hydrocarbon in a weight ratio of 95/5 to 5/95 is press-fitted, and the obtained molten resin is extruded from an extruder and foamed. Method for producing conductive polylactic acid-based resin foam. The method for producing a crystalline polylactic acid-based resin foam according to claim 1, wherein a higher fatty acid alcoholic ester is added to the crystalline polylactic acid-based resin. 3. The method for producing a crystalline polylactic acid-based resin foam according to claim 1, wherein the foam is extruded at a shear rate of 300 to 10000 sec ^{− 1} from a mold attached to the tip of the extruder. 4. The method for producing a crystalline polylactic acid-based resin foam sheet for thermoforming according to any one of claims 1 to 3, wherein the molten resin is extruded in a sheet shape from an extruder.