【発明の詳細な説明】[Detailed description of the invention]
（産業上の利用分野）
本発明は経時変化の少ない寸法安定性にすぐれ
たポリエチレン−スチレン共重合体よりなる発泡
成形体を製造する方法に関する
（従来の技術）
ポリエチレンに易揮発性発泡剤を含有させた発
泡性ポリスチレン樹脂粒子を水蒸気等で加熱発泡
して得たポリスチレン発泡粒子を型内で再加熱し
て任意の発泡成形体とすることは広く知らている
が、この成形体は長期間に恒つて使用している
と、除々に収縮すると云う経時変化を示す。例え
ば10倍の発泡成形体では180日後において約3.3／
1000mmすなわち１ｍ当り3.3mmの収縮を示し、そ
の後も除々に収縮を続ける。本発明者らは、この
収縮が何の影響で発生するかについて研究したと
ころ発泡成形体中に残存する発泡剤、溶剤の逸散
が成形体の収縮に大きく影響することを見出し
た。しかしながら、ポリスチレンは易揮発性発泡
剤や溶剤との親和性に優れるため、仮に特開昭54
−158468号広報に示されている手段を講じてもポ
リスチレン発泡成形体より易揮発性発泡剤や溶剤
を急速に除去することが困難であり、長期間にわ
たつて寸法安定性を求めることは難しい。
他方、ポリスチレン−スチレン共重合体樹脂粒
子より得られた発泡成形体は成形後２〜３週間経
過すると、その後の長期にわたる寸法変化が少な
いことを見出した。すなわち、ポリエチレン−ス
チレン共重合体樹脂粒子からポリスチレンの場合
と同様の方法で得られた予備発泡粒子から作られ
た発泡成形体はポリスチレン発泡成形体に比して
経時寸法安定性にすぐれており、180日経過後に
おいて2.5／1000mm以内という収縮率を示す。
（解決すべき問題点）
本発明者らはさらに寸法安定性にすぐれたポリ
エチレン−スチレン共重合体の発泡成形体を得る
ために種々検討した結果、本発明を完成したもの
で、本発明の目的は長期間にわたる寸法安定性に
すぐれた発泡成形体を提供するにある。
（問題点を解決するための手段）
すなわち、本発明は易揮発性発泡剤を含有しる
ポリエチレン−スチレン共重合体樹脂を加熱して
予備発泡した発泡粒子を、型内に充填し、加熱し
て得られた発泡成形体を、40℃〜65℃の温度下で
４時間以上、好ましくは８時間以上、熟成するこ
とを特徴とする寸法安定性にすぐれた発泡成形体
の製造方法であつて、本発明の方法においては樹
脂としてポリエチレン−スチレン共重合体樹脂を
使用しているため熟成期間中に発泡成形体中に残
存している発泡剤や溶剤が容易に逸散し、これに
より発泡成形体の寸法が安定するものと解され
る。
本発明において使用するポリエチレン−スチレ
ン共重合体とはポリスチレン樹脂粒子にスチレン
モノマーとその重合触媒とを加えてポリエチレン
樹脂粒子中でスチレンを共重合して得られたもの
である。そして、本発明ではこのポリスチレン−
スチレン共重合体に発泡剤を含浸して発泡性ポリ
エチレン−スチレン共重合体樹脂粒子とするので
あり、発泡剤としてはプロパン、ブタン、ペンタ
ン、メチルクロライド、メチレンクロライド、ジ
クロロジフルオロメタン、ジクロロジフルオロエ
タン等の低沸点の炭化水素、ハロゲン化炭化水素
を単独若しは２種以上を混合したものである。
これらの発泡剤を含有したポリエチレン−スチ
レン共重合体樹脂粒子を、該粒子の軟化点以上の
温度、例えば水蒸気等の加熱媒体で加熱して予備
発泡粒子となし、この発泡粒子を成形金型内に充
填した後、再び前期加熱媒体により発泡成形体と
される。
本発明では上記予備発泡粒子を一旦室温で２日
〜14日、好ましくは３日以上放置した後、成形す
ることが好ましい。すなわち、予備発泡後１日以
内に整形すると整形後の収縮が著しく、長期にわ
たつて寸法安定性のすぐれた発泡成形体を得るこ
とができず、また、15日以後に成形すると成形時
における発泡粒子間の融着不良を生じ、所望の発
泡成形体を得ることができない。
次に、本発明の熟成条件として40℃未満では発
泡成形体中の残存発泡剤や溶剤の逸散が遅く、65
℃以上では熟成後の発泡性形体の収縮率のバラツ
キが大きいので好ましくなく、また、熟成期間と
して４日間未満では残存発泡剤や溶剤の逸散が不
十分であり結果的に長期寸法安定性に劣るのであ
り、したがつて、好ましくは８時間以上である。
かかる理由により本発明では熟成条件として40〜
65℃の温度下で４時間以上、好ましくは８時間以
上と規定するものである。
次に本発明を実施例を以つて具体的に説明す
る。
実施例 １
発泡剤としてブタンを含有したポリエチレン−
スチレン共重合体（ポリエチレン含有量40％）を
水蒸気加熱により発泡倍率10倍の予備発泡粒子を
得た。この予備発泡粒子を、室温で１週間放置
後、金型に充填してスチームで再加熱して発泡成
形体を得た。この発泡成形体を40℃で24時間熟成
した。熟成後の発泡成形体の金型に対する収縮率
は10〜11％であつた。そして、この発泡成形体
を、６カ月放置した後、発泡成形体の寸法の変化
率を測定したところ0.8〜1.0であつて殆ど寸法変
化が認められなかつた。
実施例 ２
実施例１と全く同様にして発泡成形体を得、こ
れを40℃で４時間熟成したところ金型に対する収
縮率は８〜10％であり、この発泡成形体の６カ月
後の寸法変化率は1.1〜1.6％であつて殆ど寸法変
化が認められなかつた。
比較例 １
実施例１と全く同様にして発泡成形体を得、こ
れを40℃で３時間熟成したところ金型に対する収
納率は６〜９％であり、この発泡成形体の６カ月
後の寸法変化率は1.3〜3.6％であつて寸法変化率
は極めて大であつた。
比較例 ２
発泡性重合体樹脂としてポリスチレンを使用し
た以外は実施例１と同様にして発泡成形体を得、
これを50℃で24時間放置したところ金型に対する
収縮率は２〜４％であり、この発泡成形体の６カ
月後の寸法変化率は2.4〜3.8％であつて寸法変化
率は極めて大であつた。
実施例３〜６及び比較例３〜４
実施例１と全く同様にして発泡成形体を作りこ
れを第１表に示す条件で熟成した後、発泡成形体
の金型寸法に対する収縮率および６カ月後の寸法
変化率をそれぞれ第１表に示す。
(Industrial Application Field) The present invention relates to a method for producing a foamed molded article made of a polyethylene-styrene copolymer that exhibits little change over time and has excellent dimensional stability. (Prior art) Polyethylene contains an easily volatile foaming agent. It is widely known that foamed polystyrene particles obtained by heating and foaming expanded polystyrene resin particles with water vapor or the like can be reheated in a mold to form any foam molded product. When used for a long time, it gradually shrinks over time. For example, for a 10x foamed product, after 180 days it is approximately 3.3/
It shows a contraction of 1000mm, or 3.3mm per 1m, and continues to gradually shrink after that. The inventors of the present invention conducted research into what causes this shrinkage, and found that the dissipation of the foaming agent and solvent remaining in the foam molded product greatly influences the shrinkage of the molded product. However, since polystyrene has excellent affinity with easily volatile blowing agents and solvents,
Even if the measures shown in Publication No. 158468 are taken, it is difficult to rapidly remove easily volatile foaming agents and solvents from polystyrene foam moldings, and it is difficult to obtain dimensional stability over a long period of time. . On the other hand, it has been found that a foamed molded article obtained from polystyrene-styrene copolymer resin particles exhibits little dimensional change over a long period of time after 2 to 3 weeks have passed after molding. That is, a foam molded product made from pre-expanded particles obtained from polyethylene-styrene copolymer resin particles in the same manner as polystyrene has superior dimensional stability over time compared to a polystyrene foam molded product. After 180 days, the shrinkage rate is within 2.5/1000mm. (Problems to be Solved) The present inventors have completed the present invention as a result of various studies in order to obtain a polyethylene-styrene copolymer foam molded product with further excellent dimensional stability. The object of the present invention is to provide a foamed molded article with excellent long-term dimensional stability. (Means for Solving the Problems) That is, the present invention involves filling a mold with foamed particles pre-foamed by heating a polyethylene-styrene copolymer resin containing an easily volatile foaming agent, and heating the foamed particles. A method for producing a foamed molded product with excellent dimensional stability, characterized in that the foamed molded product obtained is aged at a temperature of 40°C to 65°C for 4 hours or more, preferably 8 hours or more. In the method of the present invention, since a polyethylene-styrene copolymer resin is used as the resin, the foaming agent and solvent remaining in the foam molded product easily dissipate during the aging period, and as a result, the foam molding It is understood that the body dimensions are stable. The polyethylene-styrene copolymer used in the present invention is obtained by adding a styrene monomer and its polymerization catalyst to polystyrene resin particles and copolymerizing styrene in the polyethylene resin particles. In the present invention, this polystyrene-
The styrene copolymer is impregnated with a blowing agent to form expandable polyethylene-styrene copolymer resin particles, and the blowing agents include propane, butane, pentane, methyl chloride, methylene chloride, dichlorodifluoromethane, dichlorodifluoroethane, etc. Low-boiling hydrocarbons and halogenated hydrocarbons may be used alone or in combination of two or more. The polyethylene-styrene copolymer resin particles containing these blowing agents are heated to a temperature higher than the softening point of the particles, for example, with a heating medium such as water vapor, to form pre-expanded particles, and the expanded particles are placed in a mold. After filling the foam, it is again made into a foamed molded product using the heating medium. In the present invention, it is preferable to leave the pre-expanded particles at room temperature for 2 to 14 days, preferably for 3 days or more, and then mold them. In other words, if it is shaped within 1 day after pre-foaming, the shrinkage after shaping will be significant, making it impossible to obtain a foamed molded product with excellent dimensional stability over a long period of time, and if it is shaped after 15 days, the foaming during molding will occur. Poor fusion between particles occurs, making it impossible to obtain a desired foamed molded product. Next, as the aging condition of the present invention, if the temperature is lower than 40°C, the residual blowing agent and solvent in the foam molded product will dissipate slowly;
℃ or higher is undesirable because the shrinkage rate of the foamable form after aging will vary widely, and if the aging period is less than 4 days, the residual blowing agent and solvent will not be sufficiently dissipated, resulting in poor long-term dimensional stability. Therefore, it is preferably 8 hours or more.
For this reason, in the present invention, the ripening conditions are
It is defined as 4 hours or more, preferably 8 hours or more at a temperature of 65°C. Next, the present invention will be specifically explained using examples. Example 1 Polyethylene containing butane as a blowing agent
Styrene copolymer (polyethylene content: 40%) was heated with steam to obtain pre-expanded particles with an expansion ratio of 10 times. The pre-expanded particles were left at room temperature for one week, then filled into a mold and reheated with steam to obtain a foamed molded article. This foam molded product was aged at 40°C for 24 hours. The shrinkage rate of the foam molded product after aging with respect to the mold was 10 to 11%. After this foamed molded product was left to stand for 6 months, the rate of change in dimension of the foamed molded product was measured, and it was 0.8 to 1.0, with almost no dimensional change observed. Example 2 A foamed molded product was obtained in exactly the same manner as in Example 1, and when it was aged at 40°C for 4 hours, the shrinkage rate with respect to the mold was 8 to 10%, and the dimensions of this foamed molded product after 6 months were The rate of change was 1.1 to 1.6%, and almost no dimensional change was observed. Comparative Example 1 A foamed molded product was obtained in exactly the same manner as in Example 1, and when it was aged at 40°C for 3 hours, the filling rate with respect to the mold was 6 to 9%, and the dimensions of this foamed molded product after 6 months were The rate of change was 1.3 to 3.6%, and the rate of dimensional change was extremely large. Comparative Example 2 A foam molded product was obtained in the same manner as in Example 1 except that polystyrene was used as the foamable polymer resin,
When this was left at 50℃ for 24 hours, the shrinkage rate with respect to the mold was 2 to 4%, and the dimensional change rate of this foam molded product after 6 months was 2.4 to 3.8%, which was extremely large. It was hot. Examples 3 to 6 and Comparative Examples 3 to 4 A foam molded product was made in exactly the same manner as in Example 1, and after ripening under the conditions shown in Table 1, the shrinkage rate of the foam molded product with respect to the mold dimensions and 6 months were determined. The subsequent dimensional change rates are shown in Table 1.
【表】【table】
【表】
実施例１〜６より明らかなように、40℃〜65℃
で４時間以上（好ましくは８時間以上）熟成する
ことにより発泡成形体の長期経時寸法変化率を小
さくすることが可能になり、180日を経ても寸法
収縮率は1.6／1000以内である。また比較例４の
場合は熟成後の金型に対する寸法収縮率にバラツ
キが発生し、比較例３の場合は６カ月後の寸法変
化率は大であつた。
（効果）
以上述べたように、本発明はポリエチレン−ス
チレン共重合体樹脂よりなる発泡体成形体を40℃
〜65℃の温度下で４時間以上熟成することにより
発泡成形体は成形時における寸法収縮は大きい
が、熟成後収縮率のバラツキを小さくすると共に
発泡成形体の経時における寸法収縮率を小さくす
ることが出来るため金型設計が容易になる等の効
果を奏する。更に本発明で得られた発泡成形体は
長期間にわたつて寸法変化が少ないので、従来の
発泡成形体のようにロボツトトレイに部品を収納
し作業する際に部品がとれなくなることがなく、
容易に取れるので、ロボツト作業の効率が向上す
る利点もある。[Table] As is clear from Examples 1 to 6, 40℃ to 65℃
By aging for 4 hours or more (preferably 8 hours or more), it is possible to reduce the long-term dimensional change rate of the foamed molded product, and even after 180 days, the dimensional shrinkage rate is within 1.6/1000. Further, in the case of Comparative Example 4, variations occurred in the dimensional shrinkage rate with respect to the mold after aging, and in the case of Comparative Example 3, the dimensional change rate after 6 months was large. (Effects) As described above, the present invention allows molded foams made of polyethylene-styrene copolymer resin to be heated at 40°C.
By aging at a temperature of ~65°C for 4 hours or more, the foamed molded product has a large dimensional shrinkage during molding, but it is possible to reduce the variation in the shrinkage rate after aging and to reduce the dimensional shrinkage rate of the foamed molded product over time. This has the effect of making mold design easier. Furthermore, the foamed molded product obtained by the present invention has little dimensional change over a long period of time, so unlike conventional foamed molded products, parts do not become unremovable when stored in a robot tray and worked.
Since it can be easily removed, it also has the advantage of improving the efficiency of robot work.