JP3673623B2 - Method for producing polyolefin microporous membrane - Google Patents
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Description
【０００１】
【発明の属する技術分野】
本発明は、超高分子量ポリオレフィンを含有するポリオレフィン組成物からなる微多孔膜を製造する方法に関し、特に高透過性ポリオレフィン微多孔膜の製造方法に関する。
【０００２】
【従来の技術及び発明が解決しようとする課題】
ポリオレフィン微多孔膜は、電池用セパレーター、電解コンデンサー用隔膜、各種フィルター、透湿防水衣料、逆浸透濾過膜、限外濾過膜、精密濾過膜等の各種用途に用いられている。
【０００３】
従来からポリオレフィンに有機媒体及び微粉末シリカ等の無機粉体を混合し溶融成形後、有機媒体及び無機粉体を抽出して微多孔膜を得る方法が知られているが、この方法では無機粉体を抽出する工程が必要であり、得られる微多孔膜の透過性は無機粉体の粒径によるところが大きく、制御が難しかった。
【０００４】
近年、重量平均分子量が７×105 以上の超高分子量ポリオレフィンを溶媒中で加熱溶解した溶液からゲル状シートを成形し、前記ゲル状シート中の溶媒量を脱不揮発性溶媒処理により調整し、次いで加熱延伸した後、残留溶媒を除去することにより、超高分子量ポリオレフィンの微多孔膜を製造する方法が種々提案されている（特開昭60-242035 号、同61-495132 号、同61-195133 号、同63-39602号、同63-273651 号）。
【０００５】
しかしながら上記方法においては、延伸法によって微細な孔を多数形成させるため、孔径が小さくかつ孔径分布が狭い微多孔膜を得ることができるが、比較的孔径が大きく、高い透過性を必要とする水処理や精密濾過膜等に好適なポリオレフィン微多孔膜が得られないという問題がある。
【０００６】
したがって本発明の目的は、比較的孔径が大きく、優れた透過性を有するポリオレフィン微多孔膜を効率的に製造する方法、特に微多孔膜の透過性を制御することができる製造方法を提供することである。
【０００７】
【課題を解決するための手段】
上記目的に鑑み鋭意研究の結果、本発明者らは、超高分子量成分を含有するポリオレフィン組成物の溶液を調製して、この溶液を押出機のダイリップより押し出した後、少なくとも 50 ℃／分の冷却速度で 90 ℃以下まで急冷してゲル状成形物を形成し、しかる後延伸することなく残留溶媒を除去すれば、優れた透過性を有するポリオレフィン微多孔膜を製造することができ、またポリオレフィン組成物と溶媒との配合割合を変えることによって得られる微多孔膜の透過性を制御することができることを発見し、本発明に想到した。
【０００８】
すなわち、本発明のポリオレフィン微多孔膜の製造方法は、重量平均分子量５×105 以上の超高分子量ポリオレフィン(A) と重量平均分子量５×105未満のポリオレフィン(B) の混合物で、(B) ／(A) の重量比が0.2〜20であるポリオレフィン組成物５〜35重量％、及び溶媒95〜65重量％からなる溶液を調製し、前記溶液をダイリップより押し出し、少なくとも 50 ℃／分の冷却速度で 90 ℃以下まで急冷して膜厚が10〜300μｍのゲル状成形物を形成し、しかる後残存する溶媒を除去し、乾燥することを特徴とする。
【０００９】
【発明の実施の形態】
以下、本発明を詳細に説明する。
【００１０】
[1] ポリオレフィン組成物
ポリオレフィン微多孔膜用の原料として、重量平均分子量５×105 以上の超高分子量ポリオレフィン(A) と、重量平均分子量５×105 未満のポリオレフィン(B) との混合物であるポリオレフィン組成物を使用する。
【００１１】
超高分子量ポリオレフィン(A) の重量平均分子量は５×105 以上であり、特に５×105 〜５×106 であるのが好ましい。超高分子量ポリオレフィンの重量平均分子量が５×105 未満であると、得られる微多孔膜の機械的強度が不十分である。このような超高分子量ポリオレフィンとしては、超高分子量ポリエチレンが好ましい。
【００１２】
ポリオレフィン(B) としては、エチレン、プロピレン、1-ブテン、4-メチル−ペンテン-1、1-ヘキセン等を重合した結晶性の単独重合体又は共重合体が挙げられ、なかでも高密度ポリエチレンが好ましい。ポリオレフィン(B) の重量平均分子量は５×105 未満であり、特に１×103 以上５×105 未満であるのが好ましい。
【００１３】
超高分子量ポリオレフィン(A) とポリオレフィン(B) との組成物は、両者をブレンドすることにより得ることができるが、多段重合により製造することもできる。多段重合法としてはリアクターブレンド法が挙げられる。この場合、オレフィンを同じ反応器内で多段重合（例えば二段重合）し、高分子量部分と低分子量部分とを連続的に製造する。
【００１４】
重量比(B) ／(A) は0.2 〜20であるのが好ましく、特に0.5 〜10であるのが好ましい。重量比(B) ／(A) が0.2 未満であると、溶液粘度が高くなりすぎるためにゲル状成形物の成形性が低下し、また得られるゲル状成形物の厚み方向の収縮が起きやすく、微多孔膜の透過性が低下する。一方、(B) ／(A) の重量比が20を超えると低分子量成分が多くなりすぎ、ゲル構造が緻密化するため、微多孔膜の透過性が低下する。
【００１５】
ポリオレフィン組成物の分子量分布（重量平均分子量Mw／数平均分子量Mn）は300 以下であるのが好ましく、５〜50であるのがより好ましい。分子量分布が300 を超えると、低分子量成分が多くなり、ゲル構造が緻密化するため、微多孔膜の透過性が低下する。
【００１６】
なおポリオレフィン組成物には、必要に応じて酸化防止剤、紫外線吸収剤、アンチブロッキング剤、顔料、染料、無機充填材等の各種添加剤を、本発明の目的を損なわない範囲で添加することができる。
【００１７】
[2] 製造方法
上記ポリオレフィン組成物を溶媒に加熱溶解することにより、溶液を調製する。この溶媒としては、ノナン、デカン、デカリン、p-キシレン、ウンデカン、ドデカン、流動パラフィン等の脂肪族又は環式の炭化水素、あるいは沸点がこれらに対応する鉱油留分等を用いることができる。この溶媒はダイリップからの押し出しの際に揮発しないので、「不揮発性溶媒」と言うことができる。
【００１８】
不揮発性溶媒の粘度としては25℃において30〜500 cSt であるのが好ましく、50〜200 cSt がより好ましい。25℃における粘度が30cSt 未満では、不均一なダイリップからの吐出を生じ、また500 cSt を超えると、脱不揮発性溶媒が困難となる。
【００１９】
ポリオレフィン組成物と不揮発性溶媒との配合割合は、両者の合計を100 重量％として、ポリオレフィン組成物が５〜35重量％、好ましくは10〜30重量％であり、不揮発性溶媒が95〜65重量％、好ましくは90〜70重量％である。ポリオレフィン組成物が５重量％未満では（不揮発性溶媒が95重量％を超えると）、ゲル状成形物（ゲル状シート）状に成形する際にダイス出口でスウェルやネックインが大きくなり、ゲル状シートの成形性及び自己支持性が低下する。一方、ポリオレフィン組成物が35重量％を超えると（不揮発性溶媒が65重量％未満では）、ゲル状シートの厚み方向の収縮が大きくなるため、空孔率が低く孔の径が小さい微多孔膜となる。またゲル状シートの成形性も低下する。この範囲内でポリオレフィン組成物と不揮発性溶媒との配合割合を変えることにより、得られる微多孔膜の透過性を制御することができる。
【００２０】
加熱溶解は、ポリオレフィン組成物を不揮発性溶媒中で完全に溶解する温度で撹拌しながら行うか、又は押出機中で均一混合して溶解する方法で行う。不揮発性溶媒中で撹拌しながら溶解する場合は、加熱温度はポリオレフィン組成物及び不揮発性溶媒の種類により異なるが、例えば超高分子量ポリエチレン／高密度ポリエチレン／流動パラフィンの場合には140 〜250 ℃とするのが好ましい。
【００２１】
押出機中で溶解する場合は、まず押出機に上述したポリオレフィン組成物を供給し、溶融混練する。溶融温度は超高分子量ポリオレフィンの種類によって異なるが、超高分子量ポリオレフィンの融点＋30℃〜＋100 ℃が好ましい。例えば超高分子量ポリエチレンの場合は160 〜230 ℃、特に170 〜200 ℃であるのが好ましい。溶融状態のポリオレフィン組成物に不揮発性溶媒を押出機の途中から供給する。
【００２２】
このようにして溶融混練したポリオレフィン組成物／不揮発性溶媒の加熱溶液を直接に又は別の押出機を介して、あるいは一旦冷却してペレット化した後再度押出機を介して、ダイリップから押し出す。ダイリップとしては、通常長方形の口金形状をしたシート用ダイリップを用いるが、二重円筒状の中空状ダイリップ、インフレーションダイリップ等も用いることができる。シート用ダイリップの場合、ダイリップのギャップは通常0.1 〜５mmであり、140 〜250 ℃に加熱する。加熱溶液の押し出し速度は20cm／分〜15ｍ／分であるのが好ましい。
【００２３】
このようにしてダイリップから押し出した加熱溶液は、急冷によりゲル状成形物（ゲル状シート）とする。急冷方法としては、冷風、冷却水、その他の冷却媒体に直接接触させる方法、冷媒で冷却したロールに接触させる方法等を用いることができる。
【００２４】
押し出された溶液の急冷は少なくとも50℃／分の冷却速度で90℃以下となるまで、好ましくは80℃以下となるまで行う。急冷により、膜内に残留する不揮発性溶媒は均一かつ微細な貫通孔状に分布するようになる。冷却速度が50℃／分未満では、得られる微多孔膜の孔径が小さい。徐冷すると残留不揮発性溶媒が独立泡状に存在するようになり、またポリオレフィン組成物の結晶化度も上昇するため、不揮発性溶媒が除去されにくくなる。
【００２５】
ゲル状成形物の厚さは10〜300 μｍであるのが好ましい。厚さが10μｍ未満では、ゲル状成形物の強度が低く、成形が困難となる。一方、厚さが300 μｍを超えると、自己支持性が発揮されず、得られた膜の空孔率が低下し、透過性も低くなり、さらに脱不揮発性溶媒が困難となる。
【００２６】
急冷により得られたゲル状成形物を易揮発性溶剤で洗浄することにより、残留不揮発性溶媒を除去する。洗浄用の易揮発性溶剤としては、ペンタン、ヘキサン、ヘプタン等の炭化水素、塩化メチレン、四塩炭素等の塩素化炭化水素、三フッ化エタン等のフッ素化炭化水素、ジエチルエーテル、ジオキサン等のエーテル類等を用いることができる。これらの易揮発性溶剤は不揮発性溶媒に応じて適宜選択することができ、単独もしくは混合して用いる。洗浄方法としては、ゲル状成形物を易揮発性溶剤に浸漬して抽出する方法、易揮発性溶剤をシャワーする方法、又はこれらの組合せ等を用いることができる。
【００２７】
上記洗浄は、成形物中の残留不揮発性溶媒が１重量％未満になるまで行う。その後易揮発性溶剤を乾燥する。乾燥した成形物は、ポリオレフィン組成物の結晶分散温度乃至融点の温度範囲内で熱固定するのが望ましい。
【００２８】
[3] ポリオレフィン微多孔膜
以上のようにして製造したポリオレフィン微多孔膜は、透気度が10〜170 秒／100 cc、空孔率が35〜95％、平均貫通孔径が0.1 〜0.5 μｍの高透過性膜である。またポリオレフィン微多孔膜の厚さは用途に応じて適宜選択することができるが、一般に５〜250 μｍであり、特に20〜200 μｍであるのが好ましい。
【００２９】
ポリオレフィン微多孔膜には、必要に応じてプラズマ照射、界面活性剤含浸、表面グラフト等の親水化処理等の表面修飾を施すことができる。
【００３０】
【実施例】
以下本発明を実施例により詳細に説明するが、本発明はこれらに限定されるものではない。
【００３１】
実施例１〜５、比較例２、３
重量平均分子量（Mw）が2.5 ×106 の超高分子量ポリエチレン（UHMWPE）と、重量平均分子量（Mw）が3.0 ×105 の高密度ポリエチレン（HDPE）とを、重量比（HDPE／UHMWPE）が４となるように混合することにより、ポリエチレン組成物（Mw／Mn＝14.2）を調製した。このポリエチレン組成物100 重量部に0.375 重量部の酸化防止剤をドライブレンドし、二軸押出機に投入した。この二軸押出機のサイドフィーダーから流動パラフィン（135 cSt ／25℃）を注入した。流動パラフィンの注入量は、ポリエチレン組成物の溶液濃度が表１に示す値（ただしポリエチレン組成物＋流動パラフィンを100 重量％とする。）となるようにした。混合物を200 ℃で溶融混練してポリエチレン組成物溶液とした。
【００３２】
得られたポリエチレン組成物溶液をロングリップＴダイ（ダイリップの開口度：0.6 mm、ダイリップの幅：550 mm）に供給し、３ｍ／分の速度で押し出し、直ちに60℃／分の冷却速度で40℃まで急冷し、表１に示す幅及び厚さのゲル状シートを得た。なおゲル状シートの引取速度は表１に示す厚さとなるように調整した。ゲル状シートを塩化メチレンで洗浄して残留流動パラフィンを抽出除去した後乾燥し、115 ℃で熱固定を行ってポリエチレン微多孔膜を得た。
【００３３】
ゲル状シートの成形性と脱不揮発性溶媒（洗浄）性の良否、及び得られたポリエチレン微多孔膜の物性の評価を以下の方法で行った。それぞれの結果を表１に示す。
【００３４】
(1) ゲル状シートへの成形性の評価
ゲル状シートへの成形性として、シート成形時のスウェル、ネックイン、メルトフラクチャー、吐出量の均一性及びシート表面の平滑性を目視により観察し、以下の基準で評価した。
○・・・すべての評価が良好。
△・・・一部の評価が不良。
×・・・すべて又は殆どの評価が不良。
【００３５】
(2) 脱不揮発性溶媒性の良否
ゲル状シートの脱不揮発性溶媒（洗浄）性の良否については、ゲル状シート（半透明）を100 mm×100 mmにカットし、型枠に固定したまま塩化メチレンに５分間浸漬し、自然乾燥したものを目視で観察し、以下の基準で評価した。なお完全に不揮発性溶媒を除去できたものは膜表面が乱反射によって白く見え、不揮発性溶媒が除去できなかったものは洗浄前と同様に半透明のままである。
○・・・白く見える。
△・・・やや半透明。
×・・・半透明。
【００３６】
(3) ポリエチレン微多孔膜の物性
得られたポリエチレン微多孔膜の物性を以下の方法で測定した。
▲１▼膜厚：断面を走査型電子顕微鏡により測定した。
▲２▼空孔率：重量法により測定した（単位は％）。
▲３▼透気度：JIS P 8117に準拠して測定した（単位は秒／100cc ）。
▲４▼平均孔径：窒素吸脱着方式の孔径測定機（COULTER POROMETER II、（株）日科機製）により測定した（単位はμｍ）。
【００３７】
実施例６、７
高密度ポリエチレン（HDPE）と超高分子量ポリエチレン（UHMWPE）との重量比（HDPE／UHMWPE）を表１に示す値とした以外は実施例１と同様にして、ポリエチレン微多孔膜を製造した。ゲル状シートの成形性、脱不揮発性溶媒性の良否及びポリエチレン微多孔膜の物性を実施例１と同様に評価した。結果を表１に示す。
【００３８】
実施例８
重量平均分子量（Mw）が5.0 ×106 の超高分子量ポリエチレン（UHMWPE）を配合した以外は実施例１と同様にしてポリエチレン微多孔膜を製造した。ゲル状シートの成形性、脱不揮発性溶媒性の良否及びポリエチレン微多孔膜の物性を実施例１と同様に評価した。結果を表１に示す。
【００３９】
比較例１
ゲル状シートを115 ℃の温度で５×５倍に同時二軸延伸した以外は実施例１と同様にしてポリエチレン微多孔膜を製造した。ゲル状シートの成形性、脱不揮発性溶媒性の良否及びポリエチレン微多孔膜の物性を実施例１と同様に評価した。結果を表１に示す。
【００４０】
比較例４
重量平均分子量（Mw）が2.5 ×106 の超高分子量ポリエチレン（UHMWPE）のみを用いた以外は実施例１と同様にして、ポリエチレン微多孔膜を製造した。ゲル状シートの成形性、脱不揮発性溶媒性の良否及びポリエチレン微多孔膜の物性を実施例１と同様に評価した。結果を表１に示す。
【００４１】
比較例５
重量平均分子量（Mw）が3.0 ×105 の高密度ポリエチレン（HDPE）のみを用いた以外は実施例１と同様にして、ポリエチレン微多孔膜を製造した。ゲル状シートの成形性、脱不揮発性溶媒性の良否及びポリエチレン微多孔膜の物性を実施例１と同様に評価した。結果を表１に示す。
【００４２】
比較例６
押し出したポリエチレン組成物溶液の冷却速度を30℃／分としてゲル状シートを成形した以外は実施例１と同様にして、ポリエチレン微多孔膜を製造した。ゲル状シートの成形性、脱不揮発性溶媒性の良否及びポリエチレン微多孔膜の物性を実施例１と同様に評価した。結果を表１に示す。
【００４３】
表１から明らかなように、実施例１〜８のポリエチレン微多孔膜は、比較的孔径が大きく、透過性に優れており、かつゲル状シートの成形性及び脱不揮発性溶媒性が良好であった。
【００４８】
【発明の効果】
以上詳述したように、本発明により重量平均分子量５×105 以上の超高分子量ポリオレフィン(A) と重量平均分子量５×105 未満のポリオレフィン(B) のポリオレフィン組成物（重量比(B) ／(A) が0.2 〜20である。）の溶液を調製し、前記溶液をダイリップより押し出し、少なくとも50℃／分の冷却速度で90℃以下まで急冷してゲル状成形物を形成し、しかる後残存する不揮発性溶媒を除去し、乾燥することにより製造されたポリオレフィン微多孔膜は、優れた透過性を有し、かつ透過性の制御が容易である。本発明の方法ではゲル状成形物の成形性及び脱不揮発性溶媒性が良好であるので、微多孔膜の製造効率が良好である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing a microporous membrane comprising a polyolefin composition containing ultrahigh molecular weight polyolefin, and more particularly to a method for producing a highly permeable polyolefin microporous membrane.
[0002]
[Prior art and problems to be solved by the invention]
Polyolefin microporous membranes are used in various applications such as battery separators, electrolytic capacitor membranes, various filters, moisture-permeable and waterproof clothing, reverse osmosis filtration membranes, ultrafiltration membranes, and microfiltration membranes.
[0003]
Conventionally, a method is known in which an inorganic medium such as an organic medium and fine powder silica is mixed with polyolefin and melt-molded, and then the organic medium and the inorganic powder are extracted to obtain a microporous film. A process for extracting the body is required, and the permeability of the resulting microporous membrane is largely dependent on the particle size of the inorganic powder, and is difficult to control.
[0004]
In recent years, a gel sheet is formed from a solution obtained by heating and dissolving an ultrahigh molecular weight polyolefin having a weight average molecular weight of 7 × 10 5 or more in a solvent, and the amount of the solvent in the gel sheet is adjusted by a non-volatile solvent treatment. Next, various methods have been proposed for producing a microporous membrane of ultra-high molecular weight polyolefin by removing the residual solvent after heating and stretching (Japanese Patent Laid-Open Nos. 60-242035, 61-495132, 61-61). 195133, 63-39602, 63-273651).
[0005]
However, in the above method, since a large number of fine pores are formed by the stretching method, it is possible to obtain a microporous membrane having a small pore size and a narrow pore size distribution. However, water having a relatively large pore size and requiring high permeability can be obtained. There is a problem that a polyolefin microporous membrane suitable for processing, microfiltration membranes, etc. cannot be obtained.
[0006]
Accordingly, an object of the present invention is to provide a method for efficiently producing a polyolefin microporous membrane having a relatively large pore size and excellent permeability, particularly a production method capable of controlling the permeability of the microporous membrane. It is.
[0007]
[Means for Solving the Problems]
As a result of diligent research in view of the above object, the present inventors prepared a solution of a polyolefin composition containing an ultrahigh molecular weight component, and after extruding this solution from the die lip of an extruder, at least 50 ° C./min. By rapidly cooling to 90 ° C. or lower at a cooling rate to form a gel-like molded article, and then removing the residual solvent without stretching, a polyolefin microporous membrane having excellent permeability can be produced. It was discovered that the permeability of the microporous membrane obtained by changing the blending ratio of the composition and the solvent can be controlled, and the present invention has been conceived.
[0008]
That is, the method for producing a polyolefin microporous membrane of the present invention is a mixture of an ultrahigh molecular weight polyolefin (A) having a weight average molecular weight of 5 × 10 5 or more and a polyolefin (B) having a weight average molecular weight of less than 5 × 10 5 , (B ) / (A) is prepared by preparing a solution comprising 5 to 35% by weight of a polyolefin composition having a weight ratio of 0.2 to 20 and 95 to 65% by weight of a solvent, and extruding the solution from a die lip, and at least 50 ° C./min. A gel-like molded product having a film thickness of 10 to 300 μm is formed by rapid cooling to 90 ° C. or less at a cooling rate, and then the remaining solvent is removed and dried.
[0009]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be described in detail.
[0010]
[1] Polyolefin composition As a raw material for a polyolefin microporous membrane, a mixture of an ultrahigh molecular weight polyolefin (A) having a weight average molecular weight of 5 × 10 5 or more and a polyolefin (B) having a weight average molecular weight of less than 5 × 10 5 A polyolefin composition is used.
[0011]
The weight average molecular weight of the ultra-high molecular weight polyolefin (A) is 5 × 10 5 or more, and preferably 5 × 10 5 to 5 × 10 6 . When the weight average molecular weight of the ultrahigh molecular weight polyolefin is less than 5 × 10 5 , the resulting microporous membrane has insufficient mechanical strength. As such ultra high molecular weight polyolefin, ultra high molecular weight polyethylene is preferable.
[0012]
Examples of the polyolefin (B) include crystalline homopolymers or copolymers obtained by polymerizing ethylene, propylene, 1-butene, 4-methyl-pentene-1, 1-hexene, etc. preferable. The weight average molecular weight of the polyolefin (B) is less than 5 × 10 5 , particularly preferably 1 × 10 3 or more and less than 5 × 10 5 .
[0013]
The composition of the ultrahigh molecular weight polyolefin (A) and the polyolefin (B) can be obtained by blending both, but can also be produced by multistage polymerization. An example of the multistage polymerization method is a reactor blend method. In this case, the olefin is subjected to multistage polymerization (for example, two-stage polymerization) in the same reactor to continuously produce a high molecular weight portion and a low molecular weight portion.
[0014]
The weight ratio (B) / (A) is preferably 0.2 to 20, and more preferably 0.5 to 10. If the weight ratio (B) / (A) is less than 0.2, the solution viscosity becomes too high, so that the moldability of the gel-like molded product is lowered, and shrinkage in the thickness direction of the resulting gel-like molded product is likely to occur. The permeability of the microporous membrane is reduced. On the other hand, when the weight ratio of (B) / (A) exceeds 20, the amount of low molecular weight components becomes excessive and the gel structure becomes dense, so that the permeability of the microporous membrane is lowered.
[0015]
The molecular weight distribution (weight average molecular weight Mw / number average molecular weight Mn) of the polyolefin composition is preferably 300 or less, more preferably 5 to 50. When the molecular weight distribution exceeds 300, low molecular weight components increase and the gel structure becomes dense, so that the permeability of the microporous membrane is lowered.
[0016]
In addition, various additives such as antioxidants, ultraviolet absorbers, antiblocking agents, pigments, dyes, inorganic fillers, and the like can be added to the polyolefin composition as long as they do not impair the purpose of the present invention. it can.
[0017]
[2] Production method A solution is prepared by heating and dissolving the polyolefin composition in a solvent. Examples of the solvent include aliphatic or cyclic hydrocarbons such as nonane, decane, decalin, p-xylene, undecane, dodecane, and liquid paraffin, or mineral oil fractions having boiling points corresponding to these. Since this solvent does not volatilize during extrusion from the die lip, it can be referred to as a “nonvolatile solvent”.
[0018]
The viscosity of the non-volatile solvent is preferably 30 to 500 cSt at 25 ° C., more preferably 50 to 200 cSt. When the viscosity at 25 ° C. is less than 30 cSt, non-uniform discharge from the die lip occurs, and when it exceeds 500 cSt, it becomes difficult to remove a non-volatile solvent.
[0019]
The blending ratio of the polyolefin composition and the nonvolatile solvent is 5 to 35% by weight, preferably 10 to 30% by weight of the polyolefin composition, and 95 to 65% by weight of the nonvolatile solvent, where the total of both is 100% by weight. %, Preferably 90 to 70% by weight. If the polyolefin composition is less than 5% by weight (if the non-volatile solvent exceeds 95% by weight), the swell and neck-in will increase at the outlet of the die when forming into a gel-like molded product (gel-like sheet). Sheet formability and self-supporting properties are reduced. On the other hand, if the polyolefin composition exceeds 35% by weight (if the non-volatile solvent is less than 65% by weight), the shrinkage in the thickness direction of the gel-like sheet increases, so that the microporous membrane with low porosity and small pore diameter It becomes. Moreover, the moldability of a gel-like sheet also falls. By changing the blending ratio of the polyolefin composition and the non-volatile solvent within this range, the permeability of the resulting microporous membrane can be controlled.
[0020]
The dissolution by heating is performed with stirring at a temperature at which the polyolefin composition is completely dissolved in the non-volatile solvent, or by a method in which the polyolefin composition is uniformly mixed and dissolved in an extruder. When dissolving with stirring in a non-volatile solvent, the heating temperature varies depending on the type of polyolefin composition and non-volatile solvent. For example, in the case of ultrahigh molecular weight polyethylene / high density polyethylene / liquid paraffin, 140 to 250 ° C. It is preferable to do this.
[0021]
When melt | dissolving in an extruder, the polyolefin composition mentioned above is first supplied to an extruder, and it melt-kneads. The melting temperature varies depending on the type of the ultrahigh molecular weight polyolefin, but the melting point of the ultrahigh molecular weight polyolefin is preferably + 30 ° C. to + 100 ° C. For example, in the case of ultra high molecular weight polyethylene, the temperature is preferably 160 to 230 ° C, particularly 170 to 200 ° C. A nonvolatile solvent is supplied to the molten polyolefin composition from the middle of the extruder.
[0022]
The heated solution of the polyolefin composition / nonvolatile solvent melt-kneaded in this way is directly or via another extruder, or once cooled and pelletized, and then extruded from the die lip again via the extruder. As the die lip, a sheet die lip having a generally rectangular base shape is used, but a double cylindrical hollow die lip, an inflation die lip, or the like can also be used. In the case of a die lip for a sheet, the gap of the die lip is usually 0.1 to 5 mm and heated to 140 to 250 ° C. The extrusion rate of the heated solution is preferably 20 cm / min to 15 m / min.
[0023]
The heated solution extruded from the die lip in this way is made into a gel-like molded product (gel-like sheet) by rapid cooling. As the rapid cooling method, a method of directly contacting cold air, cooling water, or other cooling medium, a method of contacting a roll cooled by a refrigerant, or the like can be used.
[0024]
The extruded solution is rapidly cooled at a cooling rate of at least 50 ° C./min until it becomes 90 ° C. or lower, preferably 80 ° C. or lower. By rapid cooling, the non-volatile solvent remaining in the film is distributed in the form of uniform and fine through holes. When the cooling rate is less than 50 ° C./min, the pore diameter of the resulting microporous membrane is small. When it is slowly cooled, the residual non-volatile solvent will be present in the form of closed bubbles, and the crystallinity of the polyolefin composition will also increase, so that the non-volatile solvent will be difficult to remove.
[0025]
The thickness of the gel-like molded product is preferably 10 to 300 μm. When the thickness is less than 10 μm, the strength of the gel-like molded product is low and molding becomes difficult. On the other hand, when the thickness exceeds 300 μm, the self-supporting property is not exhibited, the porosity of the obtained film is lowered, the permeability is lowered, and further, the non-nonvolatile solvent becomes difficult.
[0026]
The gel-like molded product obtained by the rapid cooling is washed with a readily volatile solvent, thereby removing the remaining nonvolatile solvent. Easily volatile solvents for cleaning include hydrocarbons such as pentane, hexane and heptane, chlorinated hydrocarbons such as methylene chloride and tetrasalt carbon, fluorinated hydrocarbons such as ethane trifluoride, diethyl ether and dioxane. Ethers and the like can be used. These easily volatile solvents can be appropriately selected according to the nonvolatile solvent, and are used alone or in combination. As a cleaning method, a method of immersing and extracting a gel-like molded product in a volatile solvent, a method of showering a volatile solvent, a combination thereof, or the like can be used.
[0027]
The washing is performed until the residual nonvolatile solvent in the molded product is less than 1% by weight. Thereafter, the readily volatile solvent is dried. The dried molded product is preferably heat-set within the temperature range of the crystal dispersion temperature to the melting point of the polyolefin composition.
[0028]
[3] Polyolefin microporous membrane manufactured as described above has an air permeability of 10 to 170 seconds / 100 cc, a porosity of 35 to 95%, and an average through-hole diameter of 0.1 to 0.5 μm. It is a highly permeable membrane. The thickness of the polyolefin microporous membrane can be appropriately selected according to the use, but is generally 5 to 250 μm, and particularly preferably 20 to 200 μm.
[0029]
The polyolefin microporous membrane can be subjected to surface modification such as plasma irradiation, surfactant impregnation, and hydrophilic treatment such as surface grafting as necessary.
[0030]
【Example】
Hereinafter, the present invention will be described in detail by way of examples, but the present invention is not limited thereto.
[0031]
Examples 1-5, Comparative Examples 2, 3
Ultra high molecular weight polyethylene (UHMWPE) with a weight average molecular weight (Mw) of 2.5 × 10 6 and high density polyethylene (HDPE) with a weight average molecular weight (Mw) of 3.0 × 10 5 , and the weight ratio (HDPE / UHMWPE) A polyethylene composition (Mw / Mn = 14.2) was prepared by mixing so as to be 4. To 100 parts by weight of this polyethylene composition, 0.375 parts by weight of an antioxidant was dry blended and charged into a twin screw extruder. Liquid paraffin (135 cSt / 25 ° C.) was injected from the side feeder of this twin-screw extruder. The injection amount of liquid paraffin was such that the solution concentration of the polyethylene composition was the value shown in Table 1 (however, the polyethylene composition + liquid paraffin was 100% by weight). The mixture was melt-kneaded at 200 ° C. to obtain a polyethylene composition solution.
[0032]
The obtained polyethylene composition solution was supplied to a long grip T die (die lip opening degree: 0.6 mm, die lip width: 550 mm), extruded at a rate of 3 m / min, and immediately cooled at a cooling rate of 60 ° C./min. The gel-like sheet having the width and thickness shown in Table 1 was obtained by rapid cooling to ° C. The take-up speed of the gel sheet was adjusted so as to have the thickness shown in Table 1. The gel-like sheet was washed with methylene chloride to extract and remove residual liquid paraffin, dried, and heat-set at 115 ° C. to obtain a polyethylene microporous membrane.
[0033]
The gel sheet was evaluated for its moldability and non-volatile solvent (cleaning) properties, and the properties of the obtained polyethylene microporous membrane were evaluated by the following methods. The results are shown in Table 1.
[0034]
(1) Evaluation of formability to a gel-like sheet As formability to a gel-like sheet, swell, neck-in, melt fracture, uniformity of discharge amount and smoothness of the sheet surface were visually observed during sheet molding, Evaluation was made according to the following criteria.
○ ... All evaluations are good.
Δ: Some evaluations are poor.
X: All or most evaluations are poor.
[0035]
(2) Non-nonvolatile solvent property of the gel sheet Regarding the non-volatile solvent (cleaning) property of the gel sheet, the gel sheet (translucent) is cut into 100 mm x 100 mm and fixed to the mold What was immersed in methylene chloride for 5 minutes and naturally dried was visually observed and evaluated according to the following criteria. When the non-volatile solvent was completely removed, the film surface appeared white due to irregular reflection, and when the non-volatile solvent could not be removed, it remained translucent as before cleaning.
○ ... looks white.
Δ: Slightly translucent.
X: Translucent.
[0036]
(3) Physical properties of polyethylene microporous membrane The physical properties of the obtained polyethylene microporous membrane were measured by the following methods.
(1) Film thickness: The cross section was measured with a scanning electron microscope.
(2) Porosity: Measured by weight method (unit:%).
(3) Air permeability: Measured according to JIS P 8117 (unit: seconds / 100 cc).
(4) Average pore diameter: Measured with a nitrogen adsorption / desorption type pore diameter measuring device (COULTER POROMETER II, manufactured by Nikka Kikai Co., Ltd.) (unit: μm).
[0037]
Examples 6 and 7
A polyethylene microporous membrane was produced in the same manner as in Example 1 except that the weight ratio (HDPE / UHMWPE) of high density polyethylene (HDPE) to ultrahigh molecular weight polyethylene (UHMWPE) was changed to the values shown in Table 1. The formability of the gel sheet, the non-volatile solvent property, and the physical properties of the polyethylene microporous film were evaluated in the same manner as in Example 1. The results are shown in Table 1.
[0038]
Example 8
A polyethylene microporous membrane was produced in the same manner as in Example 1 except that ultra high molecular weight polyethylene (UHMWPE) having a weight average molecular weight (Mw) of 5.0 × 10 6 was blended. The formability of the gel sheet, the non-volatile solvent property, and the physical properties of the polyethylene microporous film were evaluated in the same manner as in Example 1. The results are shown in Table 1.
[0039]
Comparative Example 1
A polyethylene microporous membrane was produced in the same manner as in Example 1 except that the gel sheet was simultaneously biaxially stretched 5 × 5 times at a temperature of 115 ° C. The formability of the gel sheet, the non-volatile solvent property, and the physical properties of the polyethylene microporous film were evaluated in the same manner as in Example 1. The results are shown in Table 1.
[0040]
Comparative Example 4
A polyethylene microporous membrane was produced in the same manner as in Example 1 except that only ultrahigh molecular weight polyethylene (UHMWPE) having a weight average molecular weight (Mw) of 2.5 × 10 6 was used. The formability of the gel sheet, the non-volatile solvent property, and the physical properties of the polyethylene microporous film were evaluated in the same manner as in Example 1. The results are shown in Table 1.
[0041]
Comparative Example 5
A polyethylene microporous membrane was produced in the same manner as in Example 1 except that only high-density polyethylene (HDPE) having a weight average molecular weight (Mw) of 3.0 × 10 5 was used. The formability of the gel sheet, the non-volatile solvent property, and the physical properties of the polyethylene microporous film were evaluated in the same manner as in Example 1. The results are shown in Table 1.
[0042]
Comparative Example 6
A polyethylene microporous membrane was produced in the same manner as in Example 1 except that the gel sheet was molded at a cooling rate of the extruded polyethylene composition solution of 30 ° C./min. The formability of the gel sheet, the non-volatile solvent property, and the physical properties of the polyethylene microporous film were evaluated in the same manner as in Example 1. The results are shown in Table 1.
[0043]
As is clear from Table 1, the polyethylene microporous membranes of Examples 1 to 8 had a relatively large pore size, excellent permeability, and good gel sheet formability and non-volatile solvent resistance. It was.
[0048]
【The invention's effect】
As described in detail above, according to the present invention, a polyolefin composition comprising an ultrahigh molecular weight polyolefin (A) having a weight average molecular weight of 5 × 10 5 or more and a polyolefin (B) having a weight average molecular weight of less than 5 × 10 5 (weight ratio (B) / (A) is 0.2 to 20), and the solution is extruded from a die lip and rapidly cooled to 90 ° C. or lower at a cooling rate of at least 50 ° C./minute to form a gel-like molded product. The polyolefin microporous membrane produced by removing the remaining non-volatile solvent and drying is excellent in permeability and easy to control the permeability. In the method of the present invention, since the moldability and the non-volatile solvent property of the gel-like molded product are good, the production efficiency of the microporous film is good.
Claims (2)
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP23902797A JP3673623B2 (en) | 1996-08-21 | 1997-08-20 | Method for producing polyolefin microporous membrane |
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| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP23851596 | 1996-08-21 | ||
| JP8-238515 | 1996-08-21 | ||
| JP23902797A JP3673623B2 (en) | 1996-08-21 | 1997-08-20 | Method for producing polyolefin microporous membrane |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| JPH10114834A JPH10114834A (en) | 1998-05-06 |
| JP3673623B2 true JP3673623B2 (en) | 2005-07-20 |
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| JP4746830B2 (en) * | 2003-09-05 | 2011-08-10 | 東レ東燃機能膜合同会社 | Method for producing thermoplastic microporous membrane |
| JP5255298B2 (en) * | 2008-01-30 | 2013-08-07 | 帝人株式会社 | Laminated reflective sheet of porous film and metal sheet |
| JP2009204682A (en) * | 2008-02-26 | 2009-09-10 | Teijin Ltd | Heat dissipation reflective sheet |
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