JP4748698B2 - Method for producing polylactic acid foamable resin particles - Google Patents

Description

上式における樹脂粒子に含浸した二酸化炭素の重量は二酸化炭素含浸前後の樹脂粒子の重量差から求められ、樹脂粒子の重量測定は０．０００１ｇの位まで計測することとする。
本発明方法によって得られる発泡粒子は、二酸化炭素含浸量にもよるが、おおむね（Ｔｇ［℃］−６５）℃以下程度の結晶化の進まない温度条件下で且つ、おおむね５〜１００ｋｇｆ／ｃｍ²Ｇの圧力条件下にて保存することにより、好ましく保管することができる。
【００１９】
本発明で得られる発泡性粒子は、発泡粒子成形体用原料として用いられる。本発明による発泡性粒子を用いて発泡粒子成形体とするには、該発泡性粒子を加熱して発泡粒子とした後、この発泡粒子を型内に充填し、加熱し、融着させればよい。
発泡性粒子を発泡させる方法としては、その樹脂粒子を加熱軟化させて発泡させる方法が好ましく採用できる。即ち、二酸化炭素が含浸している発泡性粒子を加熱し、これを発泡させる。発泡させるための加熱媒体としては、水蒸気、加熱調整した空気や窒素等が挙げられるが、通常は水蒸気が用いられる。発泡性粒子を加熱し発泡させる方法としては、従来公知の方法が採用できるが、通常は密閉容器内に発泡性粒子を充填し水蒸気を導入して発泡させる。尚、密閉容器にはわずかに内部の圧力をリークさせる開孔弁が備わっていると、密閉容器内の空気が排除でき、密度が均一な発泡粒子が得られ易い。
二酸化炭素が含浸している樹脂粒子を加熱する際の温度、すなわち発泡温度は、通常、基材樹脂の（ガラス転移温度−３０℃）〜（ガラス転移温度＋６０℃）、好ましくは（ガラス転移温度−１０℃）〜（ガラス転移温度＋４０℃）である。発泡温度が前記範囲より低いと、十分な発泡が起こり難く、また前記範囲より高いと発泡粒子の独立気泡率が低下してしまい良好な成形性を示す発泡粒子が得られにくいといった問題が発生する。
【００２０】
本発明における発泡性粒子から得られる発泡粒子は、見かけ密度が０．０１５〜０．３ｇ／ｃｍ^３であり、０．０１５〜０．２ｇ／ｃｍ^３であることが好ましい。密度が前記範囲より大きい場合は、発泡粒子の密度のばらつきが大きくなり易く、型内にて加熱成形際の発泡粒子の膨張性、融着性にばらつきが生じ、その結果、得られる発泡粒子成形体の物性低下の虞がある。一方、該密度が前記範囲より小さい場合、発泡倍率が比較的高いために、成形収縮率が大きな成形体となる虞れがある。尚、得られた発泡粒子高温、高湿条件下を避けて加水分解しないような条件下で保存することが好ましい。
【００２１】
本明細書において発泡粒子の見かけ密度は、２３℃のエタノールの入ったメスシリンダーを用意し、該メスシリンダーに相対湿度５０％、２３℃、１ａｔｍの条件にて２日放置した５００個以上の発泡粒子（発泡粒子群の重量Ｗ１）を金網などを使用して沈めて、エタノール水位上昇分より読みとられる発泡粒子群の容積Ｖ１（ｃｍ³）にてメスシリンダーに入れた発泡粒子群の重量Ｗ１（ｇ）を割り算することにより求める（Ｗ１／Ｖ１）。
【００２２】
また、発泡性粒子から得られる発泡粒子の嵩密度は０．０１〜０．２ｇ／ｃｍ³であることが好ましい。
本明細書において発泡粒子の嵩密度は、空のメスシリンダーを用意し、該メスシリンダーに相対湿度５０％、２３℃、１ａｔｍの条件にて２日放置した５００個以上の発泡粒子（発泡粒子群の重量Ｗ２）を入れたときメスシリンダーの目盛りが示す容積（嵩体積）Ｖ２（ｃｍ³）にてメスシリンダーに入れた発泡粒子群の重量Ｗ２（ｇ）を割り算することにより求める（Ｗ２／Ｖ２）。
【００２３】
更に、該発泡粒子の平均気泡径は、１０〜５００μｍであり、好ましくは３０〜４００μｍである。該気泡径が前記範囲より小さいと、加熱成形時において膜強度が弱すぎるために破泡等が生じ、養生回復性の悪い成形体となる。また、該気泡膜厚が前記範囲より大きいと加熱発泡時において膜強度が強すぎるために、十分な膨張が生じず、表面平滑性の劣った成形体となってしまう。
本明細書において、発泡粒子の平均気泡径は、発泡粒子を略二分割し、その発泡粒子断面に存在する全ての気泡の最大径を求め、この操作を１０個以上の発泡粒子について行ない、求められた該最大径の算術平均値をもって平均気泡径とする。
【００２４】
発泡粒子成形体を製造するには、発泡粒子を型内に充填した後に、スチーム、熱風等の加熱媒体により該発泡粒子を加熱して成形を行うことが好ましい。
この加熱成形により発泡粒子は相互に融着し、一体となった発泡成形体を与える。この場合の成形用の型としては慣用の金型や特開２０００−１５７０８に記載の連続成形装置に使用されているスチールベルトが用いられる。また、加熱手段としては、通常スチームが用いられ、その加熱温度は発泡粒子表面が溶融する温度であればよい。
【００２５】
発泡粒子成形体を製造する場合、型内に供する発泡粒子に予め空気、窒素、二酸化炭素等の無機ガスにより気体を付与しておくことが好ましい。又、ブタン等の有機ガスも使用できる。気体を付与した発泡粒子を成形用発泡粒子として用いることにより、発泡粒子の成形時の発泡性、成形性、回復性が向上する。該気体は、好ましくは０．３〜４ｍｏｌ／（１０００ｇ発泡粒子）、更に好ましくは０．７〜４ｍｏｌ／（１０００ｇ発泡粒子）の範囲内で付与する。
【００２６】
尚、本明細書において、発泡粒子の気体量（ｍｏｌ／１０００ｇ発泡粒子）は以下のように求められる。
【数５】

前記式中の気体増加量（ｇ）は次のように求める。成形機に充填される気体を付与することにより内部圧力が高められた発泡粒子を５００個以上取り出して６０秒以内に相対湿度５０％、２３℃の大気圧下の恒温室に移動し、その恒温室内の秤に乗せ、該発泡粒子を取り出して１２０秒後の重量を読み取る。このときの重量をＱ（ｇ）とする。次に、該発泡粒子を相対湿度５０％、２３℃の大気圧下の同恒温室内にて２４０時間放置する。発泡粒子内の高い圧力の気体は時間の経過とともに気泡膜を透過して外部に抜け出すため発泡粒子の重量はそれに伴って減少し、２４０時間後では平衡に達しているため実質的にその重量は安定している。上記２４０時間後の該発泡粒子の重量を同恒温室内にて測定し、このときの重量をＳ（ｇ）とする。上記のいずれの重量も０．０００１ｇまで読み取るものとする。この測定で得られたＱ（ｇ）とＳ（ｇ）の差を（３）式中の気体増加量（ｇ）とする。また、発泡粒子成形体を得るための他の方法としては発泡性粒子を直接型内に充填して加熱成形する方法、即ち、発泡性粒子を発泡粒子とし該発泡粒子を成形するのではなく、発泡性粒子を型内にて発泡及び融着させて発泡粒子成形体を作る方法がある。この方法は前述の発泡性粒子を発泡粒子として、該発泡粒子を型内成形する方法と比較して成形体の生産性には優れるものの得られる成形体の密度のばらつきが発生する虞がある。
【００２７】
発泡粒子成形体の形状は特に制約されず、その形状は、例えば、容器状、板状、筒体状、柱状、シート状、ブロック状等の各種の形状であることができる。また、寸法安定性、表面平滑性において優れたものである。
発泡粒子成形体の密度（ｇ／ｃｍ³）は、好ましくは０．０１〜０．２ｇ／ｃｍ³のものであり、成形体の外形寸法から求められる体積ＶＭ（ｃｍ³）にて成形体重量ＷＭ（ｇ）を割り算する（ＷＭ／ＶＭ）ことにより求められる。
【００２８】
本発明において基材樹脂の融点及びガラス転移温度は ＪＩＳ Ｋ ７１２１−１９８７に準拠して測定する。
基材樹脂の融点は、示差走査熱量測定によって得られる第２回目のＤＳＣ曲線から得られる、ピーク頂点の温度である。
前記基材樹脂の示差走査熱量測定によって得られる第２回目のＤＳＣ曲線とは、基材樹脂１〜５ｍｇを、示差走査熱量計によって１０℃／分の昇温速度で２００℃まで昇温し（ここで、得られるＤＳＣ曲線を第１回目のＤＳＣ曲線という。）、次いで、２００℃から１０℃／分の降温速度で０℃まで降温する。その後、再度１０℃／分の昇温速度で２００℃まで昇温して得られるＤＳＣ曲線を第２回目のＤＳＣ曲線をいう。
また、該基材樹脂にピーク頂点の温度が２つ以上現れる場合には、最も高温度側のピーク頂点温度を融点とする。
【００２９】
基材樹脂のガラス転移温度は、示差走査熱量測定によって得られる第２回目のＤＳＣ曲線から得られる、各ベースラインの延長した直線から縦軸方向に等距離にある直線と、ガラス転移の階段状変化部分の曲線とが交わる点である中間点ガラス転移温度とする。
【００３０】
【実施例】
次に本発明を実施例によりさらに詳細に説明する。
【００３１】
実施例１〜４、比較例１〜３
結晶性ポリ乳酸（（株）島津製作所製ラクティ９０３０）とタルクとを押出機にて溶融混練した後、ストランド状に押出し、次いでこのストランドを約２５℃の水中で降温速度３０００℃／分以上で急冷固化させた後に切断して、直径約１．３ｍｍ、長さ約１．９ｍｍ、１個当たり約３ｍｇの樹脂粒子を得た。なお、タルクは２０００ｐｐｍとなるように添加した。次に、５Ｌの内容積を有するオートクレーブ内を表１に示す雰囲気温度に調整した後、この樹脂粒子１０００ｇを投入した。二酸化炭素を圧力調整弁を介してオートクレーブ内に圧入し、オートクレーブ内の圧力を表１に示す圧力に調整し、１５時間保持した。次に、オートクレーブ内の炭酸ガスを抜き出した後、発泡性樹脂粒子を取出した。この発泡性樹脂粒子の炭酸ガス（二酸化炭素）含浸量を表１に示す。この炭酸ガスが含浸した発泡性樹脂粒子を、密閉容器内に充填した後、水蒸気を導入して表１に示す温度に加熱し、膨張発泡した発泡粒子を得た。この発泡粒子の見かけ密度等を表１に示す。尚、上記実施例にて得られた発泡粒子の独立気泡率は各実施例において５回づつ独立気泡率の測定を行なったが、全て８０％以上のものであった。また、発泡粒子の見かけ密度のばらつきも小さいものであり、各実施例において５回づつ見かけ密度を測定したが表１に示した見かけ密度の±５％の範囲内に全てが納まっていた。本明細書において発泡粒子の独立気泡率は、ＡＳＴＭ Ｄ−２８５６−７０［１９７６再認定］の手順Ｃに準じて、嵩体積約２５ｃｍ^３の発泡粒子サンプルを使用して求められる値である。得られた発泡粒子を密閉容器内に充填し、二酸化炭素にて加圧し、表２に示す二酸化炭素を発泡粒子へ含浸させた後、２００×２５０×１０ｍｍの金型に充填し、表２に示す温度の水蒸気で加熱成形した。得られた成形体は３０℃で２４時間養生した。得られた発泡粒子成形体の密度を表２に示す。
【００３２】
参考例１
結晶性ポリ乳酸（（株）島津製作所製ラクティ９０３０）とタルクとを押出機にて溶融混練した後、ストランド状に押出し、次いでこのストランドを約２５℃の水中で降温速度３０００℃／分以上で急冷固化させた後に切断して、直径約１．３ｍｍ、長さ約１．９ｍｍ、１個当たり約３ｍｇの樹脂粒子を得た。なお、タルクは２０００ｐｐｍとなるように添加した。次に、５Ｌの内容積を有するオートクレーブ内を表１に示す雰囲気温度に調整した後、この樹脂粒子１０００ｇを投入した。二酸化炭素を圧力調整弁を介してオートクレーブ内に圧入し、オートクレーブ内の圧力を表１に示す圧力に調整し、１５時間保持した。次に、オートクレーブ内の炭酸ガスを抜き出した後、樹脂粒子を取出した。この樹脂粒子の炭酸ガス（二酸化炭素）含浸量を表１に示す。得られた炭酸ガス含浸発泡性粒子を２００×２５０×１０ｍｍの金型に充填し、表２に示す温度の水蒸気で加熱成形した。得られた成形体は３０℃で２４時間養生した。得られた発泡粒子成形体の密度を表２に示す。
【００３３】
【表１】

【００３４】
【表２】

【００３５】
尚、表２における成形体の融着性及び外観は以下の基準にて評価した。
（融着性）
○：成形体を折り曲げて破断させ、成形体破断面の観察したところ発泡粒子の材料破壊が確認できた。
×：成形体を金型内から空気圧を利用して取り出そうとしたところ、成形体を構成している発泡粒子同士の表面において界面剥離が起こり、成形体が崩れてしまった。
（外観）
○：表面平滑性、金型形状再現性に優れ良好なものであった。
×：成形体を金型から取り出すことができない程、発泡粒子相互の融着性に劣り、多数の表面凹凸が発泡粒子間に存在することを、金型が開かれた状態で確認できた。
参考までに表１に示したＣＯ₂含浸時の雰囲気温度、ＣＯ₂含浸量Ｘ及び−２．５Ｘ＋５５又は−２．７Ｘ＋５５の値を表３に示した。
【００３６】
【表３】

【００３７】
【発明の効果】
本発明の製造方法によれば、型内成形における、成形型形状再現性や発泡粒子融着性が良好な型内成形用ポリ乳酸発泡粒子を得るための発泡性に優れた発泡性樹脂粒子が得られる。本発明の製法にて得られた発泡性樹脂粒子を原料として得られる発泡粒子は、見かけ密度のばらつきが小さく、独立気泡率も高いものであり、また最終的に得られる発泡粒子成形体は、軽量性、寸法安定性、緩衝性及び機械的強度に優れ、緩衝材、包装資材等として好適に使用されると共に、生分解性を有しているためその後の廃棄処分が容易となるなどその産業的意義は多大である。
【図面の簡単な説明】
【図１】図１は実施例及び比較例で使用した樹脂粒子の発熱ピークを示すＤＳＣ曲線である。[0001]
BACKGROUND OF THE INVENTION
  The present invention relates to a method for producing polylactic acid expandable particles having microbial degradability. More specifically, it has excellent mold shape reproducibility and foamed particle fusion.Polylactic acid for in-mold moldingGet foam particlesforPolylactic acid foamability with good foamabilityresinThe present invention relates to a method for producing particles.
[0002]
[Prior art]
Foamed particle molded bodies made of resins such as polystyrene, polyethylene, and polypropylene are widely used for packaging cushioning materials, agricultural boxes, fish boxes, automobile members, building materials, civil engineering materials, and the like. However, since these foamed particle molded articles are hardly decomposed by microorganisms when left in a natural environment after use, there is a risk of causing environmental destruction problems due to dust scattering.
On the other hand, studies have been made on resins that can be decomposed by microorganisms. To date, for example, a biodegradable resin made of polylactic acid has been put to practical use as a surgical suture, and has been used for many years. In recent years, lactic acid, which is a raw material of polylactic acid, can be produced in large quantities and at low cost by fermentation using corn and the like as raw materials.
Therefore, there has been a demand for a foam made of polylactic acid that has a proven record in practicality, human safety and microbial degradability.
As prior art relating to foams made of polylactic acid, there are extrusions such as JP-T-5-508669, JP-A-4-304244, JP-A-5-139435, JP-A-5-140361, JP-A-9-263651, etc. Examples relating to foams and those relating to expanded particles such as JP-A-5-170965, JP-A-5-170966, JP-A-2000-136261 and the like can be mentioned.
In the prior art related to the above-mentioned polylactic acid foam, particularly those related to foamed particles, a foam having a desired shape can be obtained without being relatively restricted in shape, and the objects such as light weight, buffering property, and heat insulating properties can be obtained. Therefore, it is particularly promising as a practical one because it is easy to design physical properties according to the above.
However, the conventional foamed particle molded body made of polylactic acid is one in which foamable resin particles are filled in a mold and the resin particles are foamed with hot air and at the same time the particles are fused together. The adhesiveness was insufficient, and the mechanical properties were inferior. In order to solve such a problem, Japanese Patent Application Laid-Open No. 2000-136261 describes that the crystallinity of the expandable resin particles is adjusted to 0 to 20%. In the method of adjusting the crystallinity depending on the impregnation temperature of the agent, the desired expandable resin particles may not be obtained.
[0003]
[Problems to be solved by the invention]
  Therefore, the present invention has good mold shape reproducibility and foamed particle fusion property.Polylactic acid for in-mold moldingGet foam particlesforPolylactic acid foaming property with excellent foaming propertyresinIt is an object of the present invention to provide a method for producing particles.
[0004]
[Means for Solving the Problems]
  In order to solve the above problems, the present inventors have intensively studied a method for producing polylactic acid foamable particles. As a result, in a resin mainly composed of polylactic acid, carbon dioxide is used as a foaming agent. Good mold shape reproducibility and foamed particle fusing property when the relationship between the amount of impregnation of the particles and the ambient temperature during impregnation of the foaming agent is satisfied.For in-mold moldingThe present inventors have found that the polylactic acid foamable particles can be obtained and can be obtained as polylactic acid foamable particles having excellent foamability. That is, according to the present invention, the following method is provided.
(1)Apparent density of 0.015 to 0.3 g / cm ³ A method for producing polylactic acid foamable resin particles for obtaining in-mold polylactic acid foamed particles having an average cell diameter of 10 to 500 μm,Polylactic acid is the main component, and the calorific value in differential scanning calorimetry is 15~ 50J /gCarbon dioxide is added to the resin particles at an ambient temperature [° C.] that satisfies the following formula (1).2.5-20% by weightPolylactic acid foaming property characterized by impregnationresinParticle production method.
[Expression 2]
  3 <atmosphere temperature [° C.] ≦ −2.5X + 55 (1) [wherein X represents an impregnation amount (% by weight) of carbon dioxide into the resin particles.]
[0005]
DETAILED DESCRIPTION OF THE INVENTION
  In the present invention,To obtain polylactic acid foamed particles for in-mold moldingUse,Foaming properties made of a resin based on polylactic acidresinParticles (hereinafter also simply referred to as expandable particles) are produced by impregnating carbon dioxide as a foaming agent into resin particles made from a base resin mainly composed of polylactic acid resin. The polylactic acid which is the main component of the base resin refers to a polymer containing 50% by weight or more of a lactic acid component in terms of the weight of the monomer to be polymerized. This includes, for example, (1) a polymer of lactic acid, (2) a copolymer of lactic acid and another aliphatic hydroxycarboxylic acid, and (3) a lactic acid, an aliphatic polyhydric alcohol and an aliphatic polycarboxylic acid. Copolymers, (4) copolymers of lactic acid and other aliphatic polyvalent carboxylic acids, (5) mixtures of combinations of any of (1) to (4) above, and the like are included.
[0006]
In the present invention, a biodegradable aliphatic polyester containing at least 35 mol% of an aliphatic ester component can be mixed with polylactic acid. The aliphatic polyester in this case includes a hydroxy acid polycondensate, a ring-opening polymer of lactone, a polycondensate of an aliphatic polyhydric alcohol and an aliphatic polycarboxylic acid, and the like. Examples of the hydroxy acid polycondensate include polycondensates of hydroxybutyric acid, and examples of the ring-opening polymer of lactone include polycaprolactone. A polycondensation product of an aliphatic polyhydric alcohol and an aliphatic polycarboxylic acid. Examples of the condensate include polybutylene succinate, polybutylene succinate adipate, poly (butylene adipate / terephthalate) and the like.
The mixing ratio of the biodegradable aliphatic polyester to the polylactic acid is 50% by weight or less, preferably 5 to 30% by weight, based on the total amount of both.
[0007]
In the present invention, it is preferable to use a resin mainly composed of polylactic acid having a melting point of 130 to 180 ° C, preferably 140 to 180 ° C.
[0008]
Specific examples of the lactic acid polymer include L-lactic acid, D-lactic acid, DL-lactic acid or their cyclic dimer L-lactide, D-lactide, DL-lactide, or a mixture thereof. A polymer can be mentioned.
[0009]
Specific examples of the method for producing polylactic acid include, for example, a method of direct dehydration polycondensation using lactic acid or a mixture of lactic acid and aliphatic hydroxycarboxylic acid as a raw material (for example, as shown in US Pat. No. 5,310,865). Manufacturing method), ring-opening polymerization method for polymerizing cyclic dimer (lactide) of lactic acid (for example, manufacturing method disclosed in US Pat. No. 2,758,987), cyclic of lactic acid and aliphatic hydroxycarboxylic acid Dimer, for example, ring-opening polymerization method for polymerizing lactide or glycolide and ε-caprolactone in the presence of a catalyst (for example, production method disclosed in US Pat. No. 4,057,537), lactic acid and aliphatic A method of directly dehydrating polycondensation of a mixture of a dihydric alcohol and an aliphatic dibasic acid (for example, a production method disclosed in US Pat. No. 5,428,126), polylactic acid and an aliphatic dibasic acid A method of condensing an alcohol, an aliphatic dibasic acid and a polymer in the presence of an organic solvent (for example, a production method disclosed in European Patent Publication No. 071880 A2), a dehydration polycondensation reaction in the presence of a catalyst with lactic acid. When producing a polyester polymer by carrying out, a method of performing solid phase polymerization in at least a part of the steps can be exemplified, but the production method is not particularly limited. In addition, a small amount of an aliphatic polyhydric alcohol such as glycerin, an aliphatic polybasic acid such as butanetetracarboxylic acid, a polyhydric alcohol such as a polysaccharide may be coexisted and copolymerized. The molecular weight may be increased by using a binder (polymer chain extender) such as a polyisocyanate compound.
[0010]
The polylactic acid used in the present invention has a melting point particularly required by the method described later among the above-described resins mainly composed of polylactic acid.
[0011]
  In the method for producing expandable particles according to the present invention, as described above, a resin mainly composed of polylactic acid having a melting point is used, the resin is heated to a melting point or higher, and then the molten resin is used. It is important to make the resin particles rapidly cooled below the glass transition temperature, and further, the atmospheric temperature conditions when impregnating the resin particles with oxygen dioxide, which is a foaming agent for imparting foaming ability, impregnation with the foaming agent Adjustment of the quantity conditions is important. In the present invention, in order to preferably produce expandable particles, first, resin particles are made from a base resin. For example, after the resin particles are heated and melted and kneaded at a temperature higher than the temperature at which the resin is sufficiently melted by an extruder, the resin particles are extruded into strands, and then rapidly cooled by submerging the strand-shaped extrudate. By cutting to an appropriate length or by quenching the strand after it has been cut to an appropriate length or simultaneously with the cutting.TheThe calorific value in differential scanning calorimetry is 15~ 50J /gYou can get things. In addition, as a method for producing resin particles from a base resin, the base resin is heated to a temperature at which the resin is sufficiently melted by an extruder, melted and kneaded, and then extruded into a plate shape or a lump shape. After quenching with a cooling press, a mist cooling device or the like, the cooling resin can be crushed or broken into a lattice shape. The weight per resin particle obtained from the base resin is 0.05 to 10 mg, preferably 1 to 4 mg. When the particle weight is smaller than the above range, it becomes difficult to produce the resin particles. The shape of the resin particles is not particularly limited, and may be various shapes such as a spherical shape and a rod shape in addition to a columnar shape (pellet shape).
[0012]
  In the present invention, in order to obtain a plate-like base resin or a block base resin for obtaining resin particles as described above or to obtain resin particles by extrusion into a strand, polylactic acid in a molten state is used. When the main resin is cooled and solidified, the cooling is performed by rapid cooling as described above.Nau. The cooling rate in this case is approximately 2000 ° C./min or more, preferably 3000 ° C./min or more, and the upper limit is not particularly limited, but is usually about 10000 ° C./min. The degree of crystallinity of the resin particles obtained is preferably 20% or less, more preferably 10% or less, and the lower limit is not particularly limited, but is usually about 0%. In the present specification, the crystallinity of the resin particles is measured using an X-ray diffraction apparatus manufactured by Rigaku Denki Kogyo Co., Ltd., and is determined from the ratio between the crystal peak area and the peak total area of the obtained chart. Obtained by the above quenching treatmentTheResin particles are 15 in the DSC curve in differential scanning calorimetry.~ 50J /gExothermic peak is shown. That is, the calorific value in the differential scanning calorimetry is 15~ 50J /gResin particles contain crystalline polylactic acid, which has a melting point as a main component, and is rapidly quenched.WasRefers to things. Heat generation amount of resin particlesIs 20-45 J / g is preferredNo. The calorific value of the resin particlesIsIt is the calorific value based on crystallization of polylactic acid in the temperature rising process at 2 ° C./min. In this specification, the calorific value in differential scanning calorimetry is measured in accordance with JIS K7122-1987, and 1 to 4 mg of resin particles are increased to 200 ° C. by a differential scanning calorimeter at a rate of 2 ° C./min. It is calculated | required from the exothermic peak area of the DSC curve obtained when it warms.
[0013]
In the step of melt-kneading the base resin with an extruder as described above and extruding it into a strand or the like, in the case where the base resin has hygroscopicity, it is preferable to dry the base resin in advance. When a resin containing a large amount of moisture is put into an extruder, bubbles that adversely affect the uniformity of the foamed foam's bubbles when it is foamed into the resin particles, or melt kneaded with an extruder There is a risk that the physical properties of the base resin will deteriorate and the melt flow rate (MFR) will become extremely large.
In order to suppress the deterioration of the resin, it is also possible to employ a method of removing moisture from the base resin by vacuum suction using an extruder with a vent port.
The upper limit temperature of the extrusion temperature condition is also set so that the MFR of the base resin does not become extremely large.
[0014]
The base resin may be colored by adding a coloring pigment or dye such as black, gray, brown, blue, or green. If colored resin particles obtained from a colored base resin are used, colored foamed particles and molded bodies can be obtained.
Examples of the colorant include organic and inorganic pigments and dyes. As such pigments and dyes, various conventionally known pigments can be used.
Moreover, inorganic substances, such as a talc, a calcium carbonate, a borax, zinc borate, aluminum hydroxide, can be previously added to base resin as a bubble regulator, for example. When additives such as color pigments, dyes or inorganic substances are added to the base resin, the additives can be kneaded into the base resin as they are. It is preferable to make it and knead it with the base resin. Although the addition amount of the color pigment or dye varies depending on the color of the color, it is usually preferably 0.001 to 5 parts by weight with respect to 100 parts by weight of the base resin. By adding an inorganic substance to the base resin, an effect of improving the expansion ratio can be obtained.
In the present invention, additives such as flame retardants, antistatic agents, weathering agents and thickeners can be mixed.
[0015]
Assuming that the product is discarded after use, it is not preferable to add a high concentration of additives such as pigments and bubble regulators.
The obtained resin particles are preferably stored in an environment where hydrolysis does not proceed by avoiding high temperature and high humidity conditions.
[0016]
Next, the resin particles obtained as described above are impregnated with carbon dioxide as a foaming agent to form expandable particles.
As a method of impregnating the resin particles with carbon dioxide in this case, a method of obtaining foamable particles by impregnating the resin particles with carbon dioxide in a sealed container can be preferably employed. As another method, resin particles are dispersed in a dispersion medium in the presence of carbon dioxide in a sealed container, and the contents are stirred while adjusting the temperature, and the particles are impregnated with carbon dioxide. Can also be used.
The impregnation of the resin particles with carbon dioxide is usually 5 to 100 kgf / cm2 of carbon dioxide in a sealed container in which the resin particles are placed.²It is carried out by press-fitting so as to be in the pressure range of G.
[0017]
  When the resin particles are impregnated with carbon dioxide according to the present invention, the ambient temperature (° C.) needs to satisfy the following formula. The ambient temperature is the temperature of the gas in the resin particle atmosphere when resin particles are put in a closed container without using a dispersion medium and impregnated with carbon dioxide, and the resin particles are placed in the closed container together with the dispersion medium. When carbon dioxide is impregnated, the temperature of the dispersion medium is used.
[Equation 3]
  3 <atmospheric temperature [° C.] ≦ −2.5X + 55 (1)
  In the above formula, X represents an impregnation amount [wt%] of carbon dioxide into the resin particles. If the atmospheric temperature is less than 3 ° C., there may be a problem in equipment for industrial production, and carbon dioxide impregnation equipment at low temperatures and expandable particle storage equipment can be used. When foaming particles impregnated with foam are foamed, the closed-cell ratio of the resulting foamed particles may be reduced, and the variation in apparent density may be large. It is difficult to obtain expanded particles having good properties. On the other hand, if the ambient temperature exceeds (−2.5X + 55), polylactic acid with high crystallinity cannot be expected to improve the expansion ratio due to the extreme progress of crystallization, and expanded particles with a low apparent density may not be obtained. There is. In addition, when the obtained expanded particles are to be heat-molded in a mold, the expandability of the expanded particles and the fusibility between the expanded particles may be reduced, and a good expanded particle molded body may not be obtained. . In addition, it is preferable that the atmospheric temperature at the time of impregnating the resin particles with carbon dioxide is (−2.7X + 55) [° C.] or less, particularly (−3.0X + 55) [° C.] or less. In the present invention, in particular, the carbon dioxide impregnation amount X in the resin particles is 2.5 to 20% by weight.AndFurther, it is preferably 2.5 to 17% by weight. If the impregnation amount is less than 2.5% by weight, the resin particles may not be sufficiently foamed. On the other hand, if the impregnation amount exceeds 20% by weight, the obtained foamed particles may be formed during in-mold molding. There is a possibility that the expansibility and the fusing property may be insufficient. This is presumably because the crystallization of the resin particles easily proceeds. In addition, the pressure of carbon dioxide in the resin particle atmosphere in the carbon dioxide impregnation step for the resin particles varies depending on the expansion ratio of the target expanded particles, but is usually 5 to 100 kgf / cm.²G and the impregnation time is 10 minutes to 24 hours.
[0018]
In this specification, the impregnation amount (% by weight) of carbon dioxide is obtained by the following formula.
[Expression 4]

The weight of the carbon dioxide impregnated in the resin particles in the above equation is obtained from the difference in weight of the resin particles before and after the carbon dioxide impregnation, and the weight measurement of the resin particles is measured to the order of 0.0001 g.
Although the expanded particles obtained by the method of the present invention depend on the amount of carbon dioxide impregnated, they are generally under a temperature condition where crystallization does not proceed to about (Tg [° C.] − 65) ° C. or less, and generally 5 to 100 kgf / cm.²By storing under the pressure condition of G, it can be preferably stored.
[0019]
The expandable particles obtained in the present invention are used as a raw material for expanded particle molded bodies. In order to obtain a foamed particle molded body using the foamable particles according to the present invention, after the foamable particles are heated to form foamed particles, the foamed particles are filled in a mold, heated, and fused. Good.
As a method for foaming the expandable particles, a method in which the resin particles are softened by heating and foamed can be preferably employed. That is, expandable particles impregnated with carbon dioxide are heated and foamed. Examples of the heating medium for foaming include water vapor, heat-adjusted air and nitrogen, and water vapor is usually used. As a method for heating and foaming the expandable particles, a conventionally known method can be adopted. Usually, the expandable particles are filled in a sealed container, and steam is introduced to cause foaming. If the airtight container is provided with an opening valve that slightly leaks the internal pressure, the air in the airtight container can be eliminated, and foam particles having a uniform density can be easily obtained.
The temperature at which the resin particles impregnated with carbon dioxide are heated, that is, the foaming temperature is usually (glass transition temperature-30 ° C.) to (glass transition temperature + 60 ° C.), preferably (glass transition temperature) of the base resin. −10 ° C.) to (glass transition temperature + 40 ° C.). When the foaming temperature is lower than the above range, sufficient foaming hardly occurs, and when the foaming temperature is higher than the above range, the closed cell ratio of the foamed particles is lowered and it is difficult to obtain foamed particles exhibiting good moldability. .
[0020]
  The expanded particles obtained from the expandable particles in the present invention have an apparent density of 0.015 to 0.3 g / cm.³InThe0.015 to 0.2 g / cm³BePreferGood. If the density is larger than the above range, the variation in the density of the expanded particles tends to be large, and the expansion and fusion properties of the expanded particles during the heat molding in the mold vary, and as a result, the resulting expanded particle molding There is a risk of deterioration of physical properties of the body. On the other hand, when the density is smaller than the above range, the foaming ratio is relatively high, so that there is a possibility that the molded article has a large molding shrinkage rate. In addition, it is preferable to preserve | save on the conditions which are not hydrolyzed avoiding the obtained foamed particle high temperature and high-humidity conditions.
[0021]
In the present specification, the apparent density of the expanded particles is a graduated cylinder containing ethanol at 23 ° C., and more than 500 expanded particles left in the graduated cylinder for 2 days under the conditions of relative humidity 50%, 23 ° C. and 1 atm. Particles (weight W1 of the expanded particle group) are sunk using a wire mesh or the like, and the volume V1 (cm^Three) By dividing the weight W1 (g) of the expanded particle group placed in the graduated cylinder (W1 / V1).
[0022]
The bulk density of the expanded particles obtained from the expandable particles is 0.01 to 0.2 g / cm.^ThreeIt is preferable that
In the present specification, the bulk density of the foamed particles is determined by preparing an empty graduated cylinder and leaving it in the graduated cylinder at a relative humidity of 50%, 23 ° C., and 1 atm for 2 days. The weight (volume) V2 (cm^Three) By dividing the weight W2 (g) of the expanded particle group placed in the graduated cylinder (W2 / V2).
[0023]
Further, the average cell diameter of the expanded particles is 10 to 500 μm, preferably 30 to 400 μm. When the bubble diameter is smaller than the above range, the film strength is too weak at the time of heat forming, and thus bubble breakage occurs, resulting in a molded article having poor curing recovery. On the other hand, if the bubble film thickness is larger than the above range, the film strength is too strong at the time of heating and foaming, so that sufficient expansion does not occur and the molded article has poor surface smoothness.
In the present specification, the average cell diameter of the expanded particles is determined by dividing the expanded particles substantially in half, obtaining the maximum diameter of all the bubbles existing in the expanded particle cross section, and performing this operation for 10 or more expanded particles. The arithmetic average value of the obtained maximum diameter is defined as the average bubble diameter.
[0024]
In order to produce a foamed particle molded body, it is preferable to perform molding by filling the foamed particles in a mold and then heating the foamed particles with a heating medium such as steam or hot air.
By this heat molding, the expanded particles are fused to each other to give an integrated expanded molded body. As a mold for molding in this case, a conventional mold or a steel belt used in a continuous molding apparatus described in JP-A-2000-15708 is used. As the heating means, steam is usually used, and the heating temperature may be a temperature at which the surface of the expanded particles melts.
[0025]
In the case of producing a foamed particle molded body, it is preferable to give a gas to the foamed particles provided in the mold by using an inorganic gas such as air, nitrogen, carbon dioxide or the like in advance. An organic gas such as butane can also be used. By using the foamed particles provided with gas as the foamed particles for molding, the foamability, moldability, and recoverability at the time of molding the foamed particles are improved. The gas is preferably applied in the range of 0.3 to 4 mol / (1000 g expanded particles), more preferably 0.7 to 4 mol / (1000 g expanded particles).
[0026]
  In addition, in this specification, the gas amount (mol / 1000g expanded particle) of an expanded particle is calculated | required as follows.
[Equation 5]

  The amount of gas increase (g) in the above formula is determined as follows. 500 or more foamed particles whose internal pressure has been increased by applying a gas filled in the molding machine are taken out and moved to a constant temperature room under an atmospheric pressure of 23% and a relative humidity of 50% within 60 seconds. Place on a balance in the room, take out the foamed particles and read the weight after 120 seconds. The weight at this time is defined as Q (g). Next, the foamed particles are left for 240 hours in the same constant temperature room at a relative humidity of 50% and an atmospheric pressure of 23 ° C. Since the high-pressure gas in the expanded particles permeates the bubble membrane and escapes to the outside as time passes, the weight of the expanded particles decreases accordingly, and after 240 hours, the weight has reached equilibrium. stable. The weight of the expanded particles after 240 hours is measured in the same constant temperature room, and the weight at this time is defined as S (g). Any of the above weights shall be read up to 0.0001 g. The difference between Q (g) and S (g) obtained by this measurement is defined as the gas increase amount (g) in the equation (3). Further, as another method for obtaining a foamed particle molded body, a method in which foamable particles are directly filled in a mold and heat-molded, that is, the foamable particles are not foamed particles, and the foamed particles are not molded. Method for producing foamed particle molded body by foaming and fusing foamable particles in a moldButis there. This method is beforePredicateAlthough the foamable particles are used as foamed particles and the foamed particles are excellent in productivity as compared with the method of molding the foamed particles in the mold, there is a risk that the density of the resulting molded product may vary.
[0027]
The shape of the foamed particle molded body is not particularly limited, and the shape can be various shapes such as a container shape, a plate shape, a cylindrical shape, a column shape, a sheet shape, and a block shape. Moreover, it is excellent in dimensional stability and surface smoothness.
Density of foamed particle compact (g / cm^Three) Is preferably 0.01 to 0.2 g / cm^ThreeVolume VM (cm^Three) To divide the compact weight WM (g) (WM / VM).
[0028]
In the present invention, the melting point and glass transition temperature of the base resin are measured according to JIS K 7121-1987.
The melting point of the base resin is the peak apex temperature obtained from the second DSC curve obtained by differential scanning calorimetry.
The second DSC curve obtained by differential scanning calorimetry of the base resin is that the base resin 1-5 mg is heated to 200 ° C. at a temperature increase rate of 10 ° C./min by a differential scanning calorimeter ( Here, the obtained DSC curve is referred to as a first DSC curve.) Next, the temperature is lowered from 200 ° C. to 0 ° C. at a temperature lowering rate of 10 ° C./min. Thereafter, the DSC curve obtained by raising the temperature again to 200 ° C. at a rate of temperature increase of 10 ° C./min is the second DSC curve.
Further, when two or more peak apex temperatures appear in the base resin, the peak apex temperature on the highest temperature side is taken as the melting point.
[0029]
  The glass transition temperature of the base resin is obtained from the second DSC curve obtained by differential scanning calorimetry, a straight line that is equidistant from the extended straight line of each baseline in the vertical axis direction, and a step shape of the glass transition. The point where the curve of the change part intersectsIsThe midpoint glass transition temperature.
[0030]
【Example】
Next, the present invention will be described in more detail with reference to examples.
[0031]
Example 14,Comparative Examples 1-3
  Crystalline polylactic acid (Lacty 9030 manufactured by Shimadzu Corporation) and talc are melt-kneaded in an extruder and then extruded into a strand. The strand is then cooled in water at about 25 ° C. at a cooling rate of 3000 ° C./min or more. After quenching and solidifying, it was cut to obtain resin particles having a diameter of about 1.3 mm, a length of about 1.9 mm, and about 3 mg per particle. In addition, talc was added so that it might become 2000 ppm. Next, after the inside of the autoclave having an internal volume of 5 L was adjusted to the atmospheric temperature shown in Table 1, 1000 g of the resin particles were charged. Carbon dioxide was injected into the autoclave through the pressure control valve, the pressure in the autoclave was adjusted to the pressure shown in Table 1, and held for 15 hours. Next, after extracting the carbon dioxide in the autoclave,EffervescentResin particles were removed. thisEffervescentTable 1 shows the amount of resin particles impregnated with carbon dioxide (carbon dioxide). Impregnated with carbon dioxideEffervescentAfter filling the resin particles in a sealed container, water vapor was introduced and heated to the temperature shown in Table 1 to obtain expanded and expanded foam particles. Table 1 shows the apparent density and the like of the expanded particles. In addition, the closed cell ratio of the expanded particles obtained in the above-mentioned examples was measured every 5 times in each example, but all were 80% or more. In addition, the variation in the apparent density of the expanded particles was small, and the apparent density was measured five times in each example, but all were within the range of ± 5% of the apparent density shown in Table 1. In the present specification, the closed cell ratio of the expanded particles is approximately 25 cm in bulk volume according to the procedure C of ASTM D-2856-70 [1976 Recertification].³It is a value calculated | required using the expanded particle sample. The obtained foamed particles were filled in a sealed container, pressurized with carbon dioxide, impregnated with carbon dioxide as shown in Table 2, and then filled into a 200 × 250 × 10 mm mold. Heat-molded with steam at the indicated temperature. The obtained molded body was cured at 30 ° C. for 24 hours. The density of the obtained foamed particle molded body is shown in Table 2.
[0032]
Reference example 1
  Crystalline polylactic acid (Lacty 9030 manufactured by Shimadzu Corporation) and talc are melt-kneaded in an extruder and then extruded into a strand. The strand is then cooled in water at about 25 ° C. at a cooling rate of 3000 ° C./min or more. After quenching and solidifying, it was cut to obtain resin particles having a diameter of about 1.3 mm, a length of about 1.9 mm, and about 3 mg per particle. In addition, talc was added so that it might become 2000 ppm. Next, after the inside of the autoclave having an internal volume of 5 L was adjusted to the atmospheric temperature shown in Table 1, 1000 g of the resin particles were charged. Carbon dioxide was injected into the autoclave through the pressure control valve, the pressure in the autoclave was adjusted to the pressure shown in Table 1, and held for 15 hours. Next, after removing carbon dioxide in the autoclave, resin particles were taken out. Table 1 shows the carbon dioxide (carbon dioxide) impregnation amount of the resin particles. The obtained carbon dioxide-impregnated foamable particles were filled in a 200 × 250 × 10 mm mold and heat-molded with water vapor at the temperature shown in Table 2. The obtained molded body was cured at 30 ° C. for 24 hours. The density of the obtained foamed particle molded body is shown in Table 2.
[0033]
[Table 1]

[0034]
[Table 2]

[0035]
In addition, the fusing property and appearance of the molded body in Table 2 were evaluated according to the following criteria.
(Fusability)
○: The molded body was bent and broken, and the fracture surface of the molded body was observed, and material destruction of the expanded particles was confirmed.
X: When trying to take out the molded body from the mold using air pressure, interfacial peeling occurred on the surfaces of the foam particles constituting the molded body, and the molded body collapsed.
(appearance)
Good: Excellent surface smoothness and mold shape reproducibility.
X: It was confirmed that the molded product was inferior to each other so that the molded body could not be taken out of the mold, and a large number of surface irregularities existed between the expanded particles in a state where the mold was opened.
For reference, the CO shown in Table 1₂Atmospheric temperature during impregnation, CO₂The impregnation amount X and the value of -2.5X + 55 or -2.7X + 55 are shown in Table 3.
[0036]
[Table 3]

[0037]
【The invention's effect】
  According to the production method of the present invention,In-mold moldingMold shapeReproducibility andExpanded particle fusionButGoodFor in-mold moldingPolyLactic acidGet foam particlesforExcellent foamabilityresinParticles are obtained. Foamability obtained by the production method of the present inventionresinFoamed particles obtained using particles as raw materials have a small apparent density variation and a high closed cell ratio, and the finally obtained foamed particle molded body is light weight, dimensional stability, buffer properties and mechanical properties. The industrial significance is great, such as being excellent in strength, being suitably used as a cushioning material, packaging material, etc., and having biodegradability, so that subsequent disposal becomes easy.
[Brief description of the drawings]
FIG. 1 is a DSC curve showing an exothermic peak of resin particles used in Examples and Comparative Examples.

Claims (1)

A method for producing polylactic acid foamable resin particles for obtaining in-mold polylactic acid foamed particles having an apparent density of 0.015 to 0.3 g / cm ³ and an average cell diameter of 10 to 500 μm,
The polylactic acid as a main component, the resin particles of the heat generation amount is 15 to 50 J / g in differential scanning calorimetry, at carbon dioxide following formula (1) satisfies ambient temperature [℃] to a 2.5 to 20 A method for producing polylactic acid foamable resin particles, which is impregnated by weight% .
[Expression 1]
3 <atmospheric temperature [° C.] ≦ −2.5X + 55 (1)
[Wherein X represents the amount of carbon dioxide impregnated into resin particles (% by weight) ]

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