JP2013067076A - Method for manufacturing polytetrafluoroethylene porous membrane - Google Patents
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本発明は、ポリテトラフルオロエチレン多孔質膜の製造方法に関する。 The present invention relates to a method for producing a polytetrafluoroethylene porous membrane.
ポリテトラフルオロエチレン（ＰＴＦＥ）多孔質膜が、種々のフィルタ濾材に使用されている。ＰＴＦＥ多孔質膜は、ＰＴＦＥシートの延伸により形成され、延伸により生じた無数のＰＴＦＥフィブリルと当該フィブリル間の空隙とからなる多孔質構造を有する。また、ＰＴＦＥ多孔質膜は、フィブリル化していないＰＴＦＥの結節（ノード）を有することがあり、このような膜の表面を電子顕微鏡などで観察した場合、複数のＰＴＦＥフィブリルがノードに連結されている様子が確認される。 Polytetrafluoroethylene (PTFE) porous membranes are used in various filter media. The PTFE porous membrane is formed by stretching a PTFE sheet, and has a porous structure including innumerable PTFE fibrils generated by stretching and voids between the fibrils. Moreover, the PTFE porous membrane may have a PTFE nodule (node) that is not fibrillated. When the surface of such a membrane is observed with an electron microscope or the like, a plurality of PTFE fibrils are connected to the node. The situation is confirmed.
ところで、フィルタ濾材として通気性の向上は重要な課題の一つである。ＰＴＦＥ多孔質膜を備えるフィルタ濾材についても例外ではなく、当該濾材の通気性向上のために、高通気性のＰＴＦＥ多孔質膜の製造が試みられてきた。例えば、ＰＴＦＥシートに対する延伸の程度を大きく（延伸倍率を高く）して、フィブリル間の空隙を拡張させることで、得られるＰＴＦＥ多孔質膜の通気性向上が期待される。しかし、延伸倍率を高くするだけでは、ＰＴＦＥ多孔質膜の通気性が向上する一方で、当該膜が薄くなって強度が低下し、フィルタ濾材への使用に適さなくなることがある。また、延伸倍率以外の延伸条件、例えば延伸速度、延伸温度などによっては、延伸倍率が高くなるにつれて延伸時のフィブリル発生量が増大することで、却って通気性が低下する。このように、他の特性を大きく変化させることなく、高通気性のＰＴＦＥ多孔質膜を得ることは難しい。 By the way, improvement of air permeability as a filter medium is one of important issues. The filter medium having a PTFE porous membrane is no exception, and attempts have been made to produce a highly breathable PTFE porous membrane in order to improve the air permeability of the filter medium. For example, the air permeability of the obtained PTFE porous membrane is expected by increasing the degree of stretching of the PTFE sheet (increasing the stretching ratio) and expanding the gap between the fibrils. However, only increasing the draw ratio improves the air permeability of the PTFE porous membrane, while the membrane becomes thin and the strength decreases, and may not be suitable for use in a filter medium. In addition, depending on stretching conditions other than the stretching ratio, such as stretching speed and stretching temperature, the amount of fibrils generated during stretching increases as the stretching ratio increases, so that the air permeability decreases. Thus, it is difficult to obtain a highly breathable PTFE porous membrane without significantly changing other characteristics.
高通気性のＰＴＦＥ多孔質膜を開示する具体的な例として、特開平7-196831号公報には、ＰＴＦＥシートに対して、ＰＴＦＥの融点未満の温度で低速度および高倍率の延伸を行うことにより、高い気孔率を有する高通気性のＰＴＦＥ多孔質膜が得られることが記載されている。当該公報によれば、この多孔質膜は、平均寸法が直径１μｍの円より大きいノードを有さない。特開平7-196831号公報の技術では、ＰＴＦＥシートに対する強い延伸によって高通気性のＰＴＦＥ多孔質膜を得ており、得られる多孔質膜の厚さは、０．１〜１０μｍと非常に薄い（[0023]参照）。このため、ＰＴＦＥ多孔質膜としての強度は低いと考えられる。 As a specific example for disclosing a highly breathable PTFE porous membrane, Japanese Patent Application Laid-Open No. 7-196831 discloses that a PTFE sheet is stretched at a low speed and a high magnification at a temperature below the melting point of PTFE. Describes that a highly breathable PTFE porous membrane having a high porosity can be obtained. According to the publication, this porous film does not have nodes larger than a circle having an average dimension of 1 μm in diameter. In the technique of Japanese Patent Laid-Open No. 7-96831, a highly breathable PTFE porous membrane is obtained by strong stretching with respect to a PTFE sheet, and the thickness of the obtained porous membrane is as very thin as 0.1 to 10 μm ( [0023]). For this reason, it is thought that the strength as a PTFE porous membrane is low.
特許第2792354号公報には、ＰＴＦＥシートを半焼成の状態にした後に、少なくとも５０倍以上の面積延伸倍率で延伸し、さらにヒートセットを実施することにより、圧力損失が低い、高通気性のＰＴＦＥ多孔質膜が得られることが記載されている。当該公報によれば、この多孔質膜におけるノードの最大面積は２μｍ2以下である。特許第2792354号公報の技術は、圧力損失をできるだけ低くするために、非常に薄いＰＴＦＥ多孔質膜を得る技術であり、得られる多孔質膜膜の厚さは０．５〜１５μｍである（[0022]〜[0026]参照）。このため、ＰＴＦＥ多孔質膜としての強度は低いと考えられる。 In Japanese Patent No. 2792354, a PTFE sheet is made into a semi-fired state, then stretched at an area stretch ratio of at least 50 times, and further heat set, whereby a high pressure permeability PTFE with low pressure loss. It is described that a porous membrane can be obtained. According to the publication, the maximum area of nodes in this porous film is 2 μm 2 or less. The technology of Japanese Patent No. 2792354 is a technology for obtaining a very thin PTFE porous membrane in order to reduce the pressure loss as much as possible, and the thickness of the obtained porous membrane membrane is 0.5 to 15 μm ([[ [0022] to [0026]). For this reason, it is thought that the strength as a PTFE porous membrane is low.
特許第3863183号公報には、ＰＴＦＥの融点以上の温度で行われるアモルファス固定の前後で、ＰＴＦＥシートに対する２つのタイプの延伸技術を組み合わせることにより、通気性と強度とを両立させたＰＴＦＥ多孔質膜が得られることが記載されている。当該公報によれば、この多孔質膜は、互いに平行に配列した非常に細長い（アスペクト比にして２５以上）ノードを有する。 Japanese Patent No. 3863183 discloses a porous PTFE membrane that achieves both air permeability and strength by combining two types of stretching techniques for PTFE sheets before and after amorphous fixation performed at a temperature equal to or higher than the melting point of PTFE. Is obtained. According to the publication, this porous film has very elongated nodes (in an aspect ratio of 25 or more) arranged in parallel to each other.
本発明は、これら従来の方法とは異なる、通気性と強度とを両立させたＰＴＦＥ多孔質膜の製造方法の提供を目的とする。 The object of the present invention is to provide a method for producing a porous PTFE membrane that has both air permeability and strength different from those of the conventional methods.
上述のように、従来、高通気性のＰＴＦＥ多孔質膜を得るために、ＰＴＦＥシートに対する延伸方法に着目がなされてきた。特許第3863183号公報に開示のように、通気性向上に伴う多孔質膜の強度低下を防ぎ、通気性と強度とを両立させたＰＴＦＥ多孔質膜を得ようとする場合においても同様である。これに対して本発明者らは、ＰＴＦＥシートの延伸に伴うＰＴＦＥフィブリルの発現状態を改めて検討した結果、従来より着目されてきたＰＴＦＥシートに対する延伸方法ではなく、ＰＴＦＥシート自体の形成方法に着目することによって、通気性と強度とを両立させたＰＴＦＥ多孔質膜が得られることを見出した。 As described above, conventionally, attention has been paid to a stretching method for a PTFE sheet in order to obtain a highly breathable PTFE porous membrane. As disclosed in Japanese Patent No. 3863183, the same applies to the case of obtaining a PTFE porous membrane that prevents deterioration of the strength of the porous membrane accompanying the improvement of air permeability and achieves both air permeability and strength. On the other hand, as a result of reconsidering the expression state of PTFE fibrils associated with the stretching of the PTFE sheet, the present inventors pay attention to the forming method of the PTFE sheet itself, not the stretching method for the PTFE sheet that has been attracting attention conventionally. Thus, it has been found that a PTFE porous membrane having both air permeability and strength can be obtained.
即ち、本発明のＰＴＦＥ多孔質膜の製造方法は、ＰＴＦＥ微粉末および液状潤滑剤の混合物から、前記混合物の押出成形を含む工程により、ＰＴＦＥシートを形成し、前記形成したシートを延伸して、延伸により生じたＰＴＦＥフィブリルおよび当該フィブリル間の空隙からなる多孔質構造を有するＰＴＦＥ多孔質膜を形成する方法であって、押出シリンダーに収容した前記混合物を、前記シリンダーに接続された押出ダイスから押し出すことによって、前記混合物の押出成形を行い、前記押出成形における、前記押出シリンダーの断面積Ａｉと、前記押出ダイスにおける前記混合物の流路の最小断面積Ａｏとの比Ａｉ／Ａｏが３０未満である、方法である。 That is, in the method for producing a porous PTFE membrane of the present invention, a PTFE sheet is formed from a mixture of PTFE fine powder and a liquid lubricant by a process including extrusion molding of the mixture, and the formed sheet is stretched. A method of forming a PTFE porous membrane having a porous structure composed of PTFE fibrils produced by stretching and voids between the fibrils, the mixture contained in an extrusion cylinder being extruded from an extrusion die connected to the cylinder The mixture is extruded, and the ratio Ai / Ao between the cross-sectional area Ai of the extrusion cylinder and the minimum cross-sectional area Ao of the flow path of the mixture in the extrusion die in the extrusion molding is less than 30. Is the way.
本発明の製造方法では、ＰＴＦＥ微粉末および液状潤滑剤の混合物から、当該混合物の押出成形を含む工程により、延伸後にＰＴＦＥ多孔質膜となるＰＴＦＥシートを形成する。ここで、押出成形を、押出シリンダーに収容した上記混合物を押出ダイスから押し出すことによって行い、その際、押出シリンダーの断面積Ａｉと、押出ダイスにおける混合物の流路の最小断面積Ａｏとの比Ａｉ／Ａｏを３０未満とする。押出シリンダー内の混合物が押出ダイスから押し出される際、当該混合物に対してＰＴＦＥ同士を結着させる力が働くが、比Ａｉ／Ａｏを３０未満とすることにより、その力が弱くなる。即ち、本発明の製造方法では、ＰＴＦＥ同士の結着力が低いＰＴＦＥシートを形成し、これを延伸する。これにより、通気性と強度とを両立させたＰＴＦＥ多孔質膜が得られる。なお、従来のＰＴＦＥ多孔質膜の製造方法では、延伸するＰＴＦＥシートを押出成形により形成する場合においても比Ａｉ／Ａｏは３０以上、通常８０〜１５０程度である。例えば、特表2000-513648号公報には、比Ａｉ／Ａｏが３０〜３００であり、７５〜１００が好ましいことが記載されている。特開昭59-152825号公報には、比Ａｉ／Ａｏが１１７（＝（１３０／１２）2）である例が記載されている。 In the production method of the present invention, a PTFE sheet that becomes a PTFE porous film after stretching is formed from a mixture of fine PTFE powder and a liquid lubricant by a process including extrusion molding of the mixture. Here, the extrusion molding is performed by extruding the mixture contained in the extrusion cylinder from the extrusion die, and at this time, the ratio Ai between the cross-sectional area Ai of the extrusion cylinder and the minimum cross-sectional area Ao of the flow path of the mixture in the extrusion die. / Ao is less than 30. When the mixture in the extrusion cylinder is extruded from the extrusion die, a force that binds the PTFE to the mixture acts on the mixture, but when the ratio Ai / Ao is less than 30, the force is weakened. That is, in the production method of the present invention, a PTFE sheet having a low binding force between PTFEs is formed and stretched. Thereby, the PTFE porous membrane which made air permeability and intensity | strength compatible is obtained. In the conventional method for producing a porous PTFE membrane, the ratio Ai / Ao is 30 or more, usually about 80 to 150, even when a stretched PTFE sheet is formed by extrusion molding. For example, JP 2000-513648 discloses that the ratio Ai / Ao is 30 to 300, and 75 to 100 is preferable. Japanese Patent Laid-Open No. 59-152825 describes an example in which the ratio Ai / Ao is 117 (= (130/12) 2 ).
これまで、ＰＴＦＥ同士の結着力が低いＰＴＦＥシートを延伸しても、強度が確保されたＰＴＦＥ多孔質膜は得られないと考えられてきた。換言すれば、ＰＴＦＥ多孔質膜の強度を確保するために、押出成形時の比Ａｉ／Ａｏを大きくして、延伸前のＰＴＦＥシートにおけるＰＴＦＥ同士の結着力を向上させることが試みられてきた。しかし、本発明者らの検討によれば、ＰＴＦＥ多孔質膜の通気性を向上させる場合、ＰＴＦＥ同士の結着力が強い従来のＰＴＦＥシートでは、得られるＰＴＦＥ多孔質膜の強度が却って低下する。本発明者らは、敢えて比Ａｉ／Ａｏを小さく、３０未満とし、ＰＴＦＥ同士の結着力が低いＰＴＦＥシートを形成した後にこれを延伸して、通気性と強度とを両立させたＰＴＦＥ多孔質膜を実現した。比Ａｉ／Ａｏを３０未満として形成したＰＴＦＥシートを延伸した場合、そうではないＰＴＦＥシートを延伸した場合に比べて、延伸により生じるフィブリルの数が少なく、大きなノードが多数形成されるとともに、延伸によるシート厚の減少の程度が小さくなる。ＰＴＦＥフィブリル数の減少によって、ＰＴＦＥ多孔質膜の通気性が向上するとともに、大きなノードの形成およびシート厚の減少の程度が小さいことが、ＰＴＦＥ多孔質膜の強度の確保に寄与すると推定される。 Until now, it has been considered that a PTFE porous membrane with sufficient strength cannot be obtained even if a PTFE sheet having a low binding force between PTFEs is stretched. In other words, in order to ensure the strength of the PTFE porous membrane, it has been attempted to increase the ratio Ai / Ao at the time of extrusion molding to improve the binding force between PTFE in the PTFE sheet before stretching. However, according to the study by the present inventors, when the air permeability of the PTFE porous membrane is improved, the strength of the obtained PTFE porous membrane is decreased in the conventional PTFE sheet having a strong binding force between the PTFEs. The present inventors dared to make the ratio Ai / Ao small and less than 30, and after forming a PTFE sheet having a low binding strength between PTFE, the PTFE porous membrane was stretched to achieve both air permeability and strength Realized. When a PTFE sheet formed with a ratio Ai / Ao of less than 30 is stretched, the number of fibrils produced by stretching is smaller than that when a PTFE sheet that is not stretched, and many large nodes are formed. The degree of reduction in sheet thickness is reduced. It is presumed that the decrease in the number of PTFE fibrils improves the air permeability of the PTFE porous membrane, and the small degree of formation of large nodes and reduction in sheet thickness contributes to securing the strength of the PTFE porous membrane.
［ＰＴＦＥシートの形成］
ＰＴＦＥ微粉末は、押出成形を含む工程によりＰＴＦＥシートが形成できる限り特に限定されず、市販のＰＴＦＥファインパウダーを使用してもよい。市販のＰＴＦＥファインパウダーは、例えば、ダイキン工業製ポリフロンＦ−１０４、Ｆ−１０６、Ｆ−１０１ＨＥ、旭硝子製フルオンＣＤ−１２３、ＣＤ−１、ＣＤ−１４５、ＸＣＤ−８０９、ＣＤ−０１４、ＣＤ−１２６、三井・デュポンフロロケミカル製テフロン６−Ｊ、６５−Ｎ、６０１−Ａである。
[Formation of PTFE sheet]
The PTFE fine powder is not particularly limited as long as a PTFE sheet can be formed by a process including extrusion molding, and a commercially available PTFE fine powder may be used. Commercially available PTFE fine powders are, for example, Daikin Industries' Polyflon F-104, F-106, F-101HE, Asahi Glass Fluon CD-123, CD-1, CD-145, XCD-809, CD-014, CD- 126, Teflon 6-J, 65-N, 601-A manufactured by Mitsui DuPont Fluorochemicals.
液状潤滑剤の種類は、ＰＴＦＥ微粉末の表面を濡らすことが可能であり、ＰＴＦＥ微粉末と液状潤滑剤との混合物をシートに成形した後に、蒸発や抽出などの手段によって除去可能である限り、特に限定されない。液状潤滑剤は、例えば、流動パラフィン、ナフサ、ホワイトオイル、トルエン、キシレンなどの炭化水素類である。液状潤滑剤として、各種のアルコール、ケトン、エステル、フッ素系溶剤などを使用してもよい。 As long as the type of liquid lubricant can wet the surface of the PTFE fine powder and can be removed by means such as evaporation or extraction after the mixture of the PTFE fine powder and the liquid lubricant is formed into a sheet, There is no particular limitation. Examples of the liquid lubricant include hydrocarbons such as liquid paraffin, naphtha, white oil, toluene, and xylene. As the liquid lubricant, various alcohols, ketones, esters, fluorine-based solvents and the like may be used.
ＰＴＦＥ微粉末と液状潤滑剤との混合比は、ＰＴＦＥ微粉末および液状潤滑剤の種類あるいはＰＴＦＥ微粉末と液状潤滑剤との混合物からＰＴＦＥシートを形成する具体的な工程に応じて適宜選択すればよく、通常、ＰＴＦＥ微粉末１００重量部に対して、液状潤滑剤が５〜３５重量部程度である。形成されるＰＴＦＥシートにおけるＰＴＦＥ同士の結着をより弱める観点からは、混合比は２０〜３０重量部が好ましい。 The mixing ratio of the PTFE fine powder and the liquid lubricant may be appropriately selected according to the kind of the PTFE fine powder and the liquid lubricant or the specific process of forming the PTFE sheet from the mixture of the PTFE fine powder and the liquid lubricant. The liquid lubricant is usually about 5 to 35 parts by weight with respect to 100 parts by weight of the PTFE fine powder. From the viewpoint of further weakening the binding between PTFE in the formed PTFE sheet, the mixing ratio is preferably 20 to 30 parts by weight.
ＰＴＦＥ微粉末および液状潤滑剤の混合物（以下、単に「混合物」）の押出成形は、押出シリンダーに収容した混合物を、押出シリンダーに接続された押出ダイスから押し出すことにより行う。また、押出成形の際の、押出シリンダーの断面積Ａｉと、押出ダイスにおける混合物の流路の最小断面積Ａｏとの比Ａｉ／Ａｏを３０未満とする。これらの条件が満たされる限り、押出成形の具体的な手法は特に限定されない。押出成形は、典型的には、ラム押出成形またはペースト押出成形である。押出成形には、押出シリンダーと、当該シリンダーに接続された押出ダイスとを備える押出成形機を用いればよい。 Extrusion of a mixture of PTFE fine powder and liquid lubricant (hereinafter simply “mixture”) is performed by extruding a mixture contained in an extrusion cylinder from an extrusion die connected to the extrusion cylinder. Further, the ratio Ai / Ao between the cross-sectional area Ai of the extrusion cylinder and the minimum cross-sectional area Ao of the flow path of the mixture in the extrusion die at the time of extrusion molding is set to less than 30. As long as these conditions are satisfied, the specific method of extrusion is not particularly limited. Extrusion is typically ram extrusion or paste extrusion. For extrusion molding, an extrusion molding machine including an extrusion cylinder and an extrusion die connected to the cylinder may be used.
比Ａｉ／Ａｏは２５以下が好ましい。この場合、形成されるＰＴＦＥシートにおけるＰＴＦＥ同士の結着力がさらに低下し、最終的に得られたＰＴＦＥ多孔質膜における強度および通気性の両立がより確実となる。比Ａｉ／Ａｏの下限は、押出成形を含む工程によって、延伸に耐えられるＰＴＦＥシートが形成できる限り特に限定されず、ＰＴＦＥ微粒子および液状潤滑剤の種類ならびに両者の混合比によって異なるが、例えば５以上であり、７以上が好ましい。 The ratio Ai / Ao is preferably 25 or less. In this case, the binding force between the PTFEs in the formed PTFE sheet is further reduced, and the strength and the air permeability in the finally obtained PTFE porous membrane are more reliably ensured. The lower limit of the ratio Ai / Ao is not particularly limited as long as a PTFE sheet that can withstand stretching can be formed by a process including extrusion molding, and varies depending on the types of PTFE fine particles and the liquid lubricant and the mixing ratio of the two, for example, 5 or more And 7 or more is preferable.
押出ダイスにおける混合物の流路の断面積は、当該流路における混合物の進行方向に垂直な断面の面積である。 The cross-sectional area of the flow path of the mixture in the extrusion die is an area of a cross section perpendicular to the traveling direction of the mixture in the flow path.
混合物は、ＰＴＦＥ微粒子と液状潤滑剤とが単純に混合したスラリーあるいはペーストの状態で押出成形してもよいし、所定の形に予め圧力成形した状態で押出成形してもよい。 The mixture may be extruded in the form of a slurry or paste in which PTFE fine particles and a liquid lubricant are simply mixed, or may be extruded in a state in which the mixture is pressure-molded in a predetermined shape.
混合物は、任意の形状に押出成形すればよく、例えばシート状に成形してもよいし、丸棒などの棒状に成形してもよい。押出成形後は、必要に応じて、圧延などの手法を用いてＰＴＦＥシートとすればよい。シート状である混合物の押出成形体は、そのままＰＴＦＥシートとしても、必要に応じて、圧延などによる厚さの調整を経てＰＴＦＥシートとしてもよい。シート状以外の形状、例えば、棒状である混合物の押出成形体は、圧延などを経てＰＴＦＥシートとすればよい。 The mixture may be extruded into an arbitrary shape, for example, may be formed into a sheet shape, or may be formed into a rod shape such as a round bar. After extrusion molding, a PTFE sheet may be formed using a technique such as rolling as necessary. The extruded product of the mixture in the form of a sheet may be used as it is as a PTFE sheet, or as necessary, after adjusting the thickness by rolling or the like. Extrudates of a mixture other than a sheet shape, for example, a rod shape, may be formed into a PTFE sheet through rolling or the like.
本発明の製造方法において、押出成形を含む工程により形成したＰＴＦＥシートは、ＰＴＦＥ多孔質膜を得るための従来のＰＴＦＥシートに比べて、ＰＴＦＥ同士の結着力が弱い。本発明の製造方法のＰＴＦＥシートにおける面内方向の引張強度は、通常、０．５〜８ＭＰａである。通気性と強度との両立がより確実なＰＴＦＥ多孔質膜が得られる観点から、ＰＴＦＥシートにおける面内方向の引張強度は、１〜５ＭＰａが好ましい。 In the production method of the present invention, a PTFE sheet formed by a process including extrusion molding has a weaker binding force between PTFEs than a conventional PTFE sheet for obtaining a PTFE porous membrane. The tensile strength in the in-plane direction of the PTFE sheet of the production method of the present invention is usually 0.5 to 8 MPa. The tensile strength in the in-plane direction of the PTFE sheet is preferably 1 to 5 MPa from the viewpoint of obtaining a PTFE porous membrane with more reliable air permeability and strength.
［ＰＴＦＥシートの延伸］
本発明の製造方法では、上記のように形成したＰＴＦＥシートを延伸してＰＴＦＥ多孔質膜を得る。延伸方法は、ＰＴＦＥ多孔質膜が得られる限り、特に限定されない。例えば、ＰＴＦＥシートをＰＴＦＥの融点以下の温度で逐次二軸延伸すればよい。逐次二軸延伸の方向は特に限定されないが、例えば、一段目の延伸方向がＰＴＦＥシートの押出方向（ＭＤ方向。帯状のＰＴＦＥシートの場合は、通常、その長手方向）であり、二段目の延伸方向が、シート面内における一段目の延伸方向とは垂直な方向（ＴＤ方向。帯状のＰＴＦＥシートの場合は、通常、その幅方向）である。延伸は同時二軸延伸または一軸延伸であってもよい。延伸が逐次二軸延伸である場合、少なくとも一方の延伸が、ＰＴＦＥの融点以上の延伸であってもよい。
[Extension of PTFE sheet]
In the production method of the present invention, the PTFE sheet formed as described above is stretched to obtain a PTFE porous membrane. The stretching method is not particularly limited as long as a PTFE porous membrane can be obtained. For example, the PTFE sheet may be sequentially biaxially stretched at a temperature below the melting point of PTFE. Although the direction of sequential biaxial stretching is not particularly limited, for example, the first stretching direction is the extrusion direction of the PTFE sheet (MD direction; in the case of a strip-shaped PTFE sheet, usually the longitudinal direction), The stretching direction is a direction perpendicular to the first-stage stretching direction in the sheet plane (TD direction; in the case of a strip-shaped PTFE sheet, usually the width direction). Stretching may be simultaneous biaxial stretching or uniaxial stretching. When the stretching is sequential biaxial stretching, at least one of the stretching may be stretching not lower than the melting point of PTFE.
ＰＴＦＥシートの延伸には、公知の延伸装置を使用できる。 A known stretching apparatus can be used for stretching the PTFE sheet.
液状潤滑剤は、延伸を行う前に、加熱あるいは抽出などの手法により、ＰＴＦＥシートから除去することが好ましい。 The liquid lubricant is preferably removed from the PTFE sheet by a technique such as heating or extraction before stretching.
ＰＴＦＥシートの延伸倍率、延伸速度、延伸温度は、得たいＰＴＦＥ多孔質膜の特性に応じて任意に設定できる。ＰＴＦＥシートを逐次二軸延伸する場合、１段目の延伸温度は、１５０〜４００℃が好ましく、２００〜３８０℃がより好ましい。１段目の延伸倍率は、１〜８倍が好ましい。１段目の延伸倍率が過度に高くなると、得られたＰＴＦＥ多孔質膜の強度が低下することがある。２段目の延伸温度は、４０〜４００℃が好ましく、４０〜３８０℃がより好ましい。２段目の延伸倍率は、４〜２０倍が好ましく、４〜１５倍がより好ましい。２段目の延伸倍率が過度に高くなると、１段目の延伸で形成されたフィブリルが破壊されて、得られたＰＴＦＥ多孔質膜の強度が低下することがある。 The stretching ratio, stretching speed, and stretching temperature of the PTFE sheet can be arbitrarily set according to the characteristics of the PTFE porous membrane to be obtained. When the PTFE sheet is sequentially biaxially stretched, the first stage stretching temperature is preferably 150 to 400 ° C, more preferably 200 to 380 ° C. The first stage draw ratio is preferably 1 to 8 times. If the stretch ratio of the first stage is excessively high, the strength of the obtained PTFE porous membrane may be lowered. The second stage stretching temperature is preferably 40 to 400 ° C, more preferably 40 to 380 ° C. The second stage draw ratio is preferably 4 to 20 times, and more preferably 4 to 15 times. If the stretch ratio of the second stage is excessively high, the fibrils formed by the first stage of stretching may be destroyed, and the strength of the obtained PTFE porous membrane may be lowered.
本発明の製造方法では、通気性と強度とを両立させたＰＴＦＥ多孔質膜を形成できる。得られたＰＴＦＥ多孔質膜の通気性は、ＪＩＳ Ｐ８１１７に準拠して測定したガーレー通気度から換算した通気度（ｃｍ3／（秒・ｃｍ2）：単位面積、単位秒あたりにＰＴＦＥ多孔質膜を透過する気体の体積）にして、例えば、５ｃｍ3／（秒・ｃｍ2）以上であり、比Ａｉ／Ａｏなど、ＰＴＦＥ多孔質膜の製造条件によっては、７ｃｍ3／（秒・ｃｍ2）以上、さらには１０ｃｍ3／（秒・ｃｍ2）以上となる。得られたＰＴＦＥ多孔質膜の面内方向の引張強度（ＭＰａ）は、例えば、０．９ＭＰａ以上であり、比Ａｉ／Ａｏなど、ＰＴＦＥ多孔質膜の製造条件によっては、１．２ＭＰａ以上となる。 In the production method of the present invention, a porous PTFE membrane having both air permeability and strength can be formed. The air permeability of the obtained PTFE porous membrane was measured by air permeability (cm 3 / (second · cm 2 ) converted from Gurley air permeability measured in accordance with JIS P8117: unit area, PTFE porous membrane per unit second. by volume of gas) that transmits, for example, a 5 cm 3 / (s · cm 2) or more, such as the ratio Ai / Ao, is the manufacturing conditions of the PTFE porous film, 7 cm 3 / (s · cm 2) In addition, it is 10 cm 3 / (second · cm 2 ) or more. The tensile strength (MPa) in the in-plane direction of the obtained PTFE porous membrane is, for example, 0.9 MPa or more, and is 1.2 MPa or more depending on the production conditions of the PTFE porous membrane, such as the ratio Ai / Ao. .
本発明の製造方法では、ＰＴＦＥ同士が弱く結着したＰＴＦＥシートに対して延伸を行うことで、厚いながらも高い通気度を有するＰＴＦＥ多孔質膜を形成できる。本発明の製造方法では、例えば、厚さが８０μｍ以上であり、ＪＩＳ Ｐ８１１７に準拠して測定したガーレー通気度から換算した通気度が５ｃｍ3／（秒・ｃｍ2）以上のＰＴＦＥ多孔質膜を形成できる。 In the production method of the present invention, a PTFE porous membrane having a high air permeability can be formed by stretching the PTFE sheet in which PTFEs are weakly bound to each other. In the production method of the present invention, for example, a PTFE porous membrane having a thickness of 80 μm or more and an air permeability calculated from Gurley air permeability measured in accordance with JIS P8117 is 5 cm 3 / (sec · cm 2 ) or more. Can be formed.
また、本発明の製造方法では、このようなＰＴＦＥ多孔質膜を軽度の延伸により実現できる。例えば、ＰＴＦＥシートに対して面積延伸倍率が１６〜５０倍の延伸を行うことにより、上述した厚さおよび通気度を有するＰＴＦＥ多孔質膜を形成できる。 In the production method of the present invention, such a porous PTFE membrane can be realized by mild stretching. For example, a PTFE porous membrane having the above-described thickness and air permeability can be formed by stretching the PTFE sheet with an area stretching ratio of 16 to 50 times.
ＰＴＦＥシートの延伸の途中あるいは延伸後に、ＰＴＦＥの融点以上の温度にＰＴＦＥシートを加熱して、当該シートを焼成してもよい。 During or after the stretching of the PTFE sheet, the sheet may be fired by heating the PTFE sheet to a temperature equal to or higher than the melting point of PTFE.
本発明の製造方法により得られたＰＴＦＥ多孔質膜は、フィルタ濾材をはじめとする様々な用途に使用できる。本発明の製造方法により得られたＰＴＦＥ多孔質膜をフィルタ濾材に使用した場合、通気性および強度が両立したフィルタ濾材となる。このようなフィルタ濾材は、特に、空調用または掃除機用のエアフィルタ、集塵機用バグフィルタ、ベントフィルタなどの通気膜の用途に好適である。 The PTFE porous membrane obtained by the production method of the present invention can be used for various applications including filter media. When the PTFE porous membrane obtained by the production method of the present invention is used as a filter medium, the filter medium is compatible with air permeability and strength. Such a filter medium is particularly suitable for use in air-permeable membranes such as air filters for air conditioning or vacuum cleaners, bag filters for dust collectors, and vent filters.
以下、実施例により、本発明をさらに詳細に説明する。本発明は、以下の実施例に限定されない。 Hereinafter, the present invention will be described in more detail with reference to examples. The present invention is not limited to the following examples.
本実施例では６種類のＰＴＦＥ多孔質膜を作製し（実施例４種類、比較例２種類）、作製したＰＴＦＥ多孔質膜の通気量、強度（面内方向の引張強度）および厚さを評価した。 In this example, six types of PTFE porous membranes were prepared (four types of examples and two types of comparative examples), and the air permeability, strength (tensile strength in the in-plane direction) and thickness of the prepared PTFE porous membranes were evaluated. did.
ＰＴＦＥ多孔質膜に対する特性の評価方法を示す。 The evaluation method of the characteristic with respect to a PTFE porous membrane is shown.
［通気量］
作製したＰＴＦＥ多孔質膜の３箇所を、それぞれ４５ｍｍφの円形に打ち抜き、得られたサンプルのガーレー通気度（秒／１００ｍＬ）を、ＪＩＳ Ｐ８１１７に準拠して自動ガーレー式デンソメーターにより測定した。測定した３つのサンプルのガーレー通気度の平均値を算出し、それを単位秒・単位面積あたりの透過気体体積（ｃｍ3／（秒・ｃｍ2））に換算して、ＰＴＦＥ多孔質膜の通気量とした。
[Air flow rate]
Three portions of the produced porous PTFE membrane were each punched into a 45 mmφ circle, and the Gurley air permeability (seconds / 100 mL) of the obtained sample was measured with an automatic Gurley densometer in accordance with JIS P8117. The average value of the measured Gurley air permeability of the three samples was calculated, converted into the permeated gas volume per unit second and unit area (cm 3 / (second · cm 2 )), and the ventilation of the PTFE porous membrane The amount.
［引張強度］
作製したＰＴＦＥ多孔質膜をその長手方向（ＭＤ方向）に沿って幅１０ｍｍで切断して得たサンプルと、その幅方向（ＴＤ方向）に沿って幅１０ｍｍで切断して得たサンプルとを準備した。各々のサンプルに対して、テンシロン（オリエンテック製、UTM-III-100）を用いた引張試験を実施し、その引張強度（ＭＰａ）を求めた。各サンプルは、１つのＰＴＦＥ多孔質膜に対して切断する場所を変えて３つ準備し、計６つのサンプルの平均値を、ＰＴＦＥ多孔質膜の引張強度値とした。引張試験の条件は、チャック間距離が２０ｍｍ、引張速度が２００ｍｍ／分、測定温度が２５℃、とした。
[Tensile strength]
A sample obtained by cutting the produced PTFE porous membrane with a width of 10 mm along the longitudinal direction (MD direction) and a sample obtained by cutting with a width of 10 mm along the width direction (TD direction) are prepared. did. Each sample was subjected to a tensile test using Tensilon (manufactured by Orientec, UTM-III-100) to determine its tensile strength (MPa). Each sample was prepared by changing the place to be cut with respect to one PTFE porous membrane, and the average value of a total of six samples was taken as the tensile strength value of the PTFE porous membrane. The tensile test conditions were a chuck distance of 20 mm, a tensile speed of 200 mm / min, and a measurement temperature of 25 ° C.
［厚さ］
作製したＰＴＦＥ多孔質膜の厚さは、デジタル式マイクロメーター（最小測定値：０．１μｍ）を用いて、測定場所を変えた５点の平均値により評価した。
[thickness]
The thickness of the produced PTFE porous membrane was evaluated by using a digital micrometer (minimum measurement value: 0.1 μm) based on an average value of five points with different measurement locations.
（実施例１）
ＰＴＦＥファインパウダー（旭フルオロポリマー製、フルオンＣＤ−１４５）１００重量部と、液状潤滑剤として炭化水素系溶媒（ジャパンエナジー製ＮＳクリーン２２０）２０重量部とを均一に混合し、ＰＴＦＥファインパウダーと液状潤滑剤との混合物であるＰＴＦＥペーストを形成した。次に、形成したＰＴＦＥペーストを、断面が円形である成形管（内径約４０ｍｍ、長さ約４００ｍｍ）を用いて圧力成形して、押出成形に使用する円柱状の予備成形体を得た。圧力成形の圧力は１ＭＰａ、温度は４０℃、時間は１０分とした。次に、得られた予備成形体を、スリット状の吐出口を有する押出ダイスが接続された押出シリンダー（円筒形、内径４ｃｍ）に収容し、当該ダイスから押し出して、ＰＴＦＥシートを得た。押出シリンダーの断面積Ａｉ（＝π×（内径／２）2）と、押出ダイスにおけるＰＴＦＥペーストの流路の最小断面積Ａｏとの比Ａｉ／Ａｏは２５とした。押出温度は４０℃、押出速度は６０ｍｍ／分とした。次に、得られたＰＴＦＥシートを、一対のロールを用いて、厚さ０．２ｍｍとなるように圧延した後、１２０℃で乾燥して液状潤滑剤を除去した。液状潤滑剤除去後のＰＴＦＥシートの引張強度を、ＰＴＦＥ多孔質膜に対する引張強度と同様の手法により測定したところ、２．５ＭＰａであった。
Example 1
100 parts by weight of PTFE fine powder (Asahi Fluoropolymer, Fullon CD-145) and 20 parts by weight of a hydrocarbon solvent (Japan Energy NS Clean 220) as a liquid lubricant are uniformly mixed, and PTFE fine powder and liquid are mixed. A PTFE paste that was a mixture with a lubricant was formed. Next, the formed PTFE paste was subjected to pressure molding using a molding tube (inner diameter: about 40 mm, length: about 400 mm) having a circular cross section to obtain a columnar preform used for extrusion molding. The pressure for pressure molding was 1 MPa, the temperature was 40 ° C., and the time was 10 minutes. Next, the obtained preform was accommodated in an extrusion cylinder (cylindrical shape, inner diameter: 4 cm) to which an extrusion die having a slit-like discharge port was connected, and extruded from the die to obtain a PTFE sheet. The ratio Ai / Ao between the cross-sectional area Ai (= π × (inner diameter / 2) 2 ) of the extrusion cylinder and the minimum cross-sectional area Ao of the flow path of the PTFE paste in the extrusion die was 25. The extrusion temperature was 40 ° C., and the extrusion speed was 60 mm / min. Next, after rolling the obtained PTFE sheet so that it might become thickness 0.2mm using a pair of roll, it dried at 120 degreeC and removed the liquid lubricant. The tensile strength of the PTFE sheet after removal of the liquid lubricant was measured by the same method as the tensile strength for the PTFE porous membrane, and was 2.5 MPa.
次に、液状潤滑剤除去後のＰＴＦＥシートを、二軸延伸機を用いて逐次二軸延伸した。逐次二軸延伸における一段目の延伸は、ＰＴＦＥシートの押出方向（ＭＤ方向）に、延伸温度２８０℃、延伸倍率４倍で行った。二段目の延伸は、ＰＴＦＥシートの幅方向（ＴＤ方向）に、延伸温度３００℃、延伸倍率４倍で行った。次に、延伸後のＰＴＦＥシートを、焼成時に収縮が起こらないように枠に固定した後、３８０℃に保持した乾燥機中に６０秒間投入して焼成し、ＰＴＦＥ多孔質膜を得た。 Next, the PTFE sheet after removal of the liquid lubricant was sequentially biaxially stretched using a biaxial stretching machine. The first-stage stretching in the sequential biaxial stretching was performed in the extrusion direction (MD direction) of the PTFE sheet at a stretching temperature of 280 ° C. and a stretching ratio of 4 times. The second stretching was performed in the width direction (TD direction) of the PTFE sheet at a stretching temperature of 300 ° C. and a stretching ratio of 4 times. Next, the stretched PTFE sheet was fixed to a frame so that shrinkage did not occur during firing, and then placed in a dryer maintained at 380 ° C. for 60 seconds to be fired to obtain a PTFE porous membrane.
得られたＰＴＦＥ多孔質膜の通気量、引張強度および厚さを、以下の表１に示す。 The air permeability, tensile strength, and thickness of the obtained PTFE porous membrane are shown in Table 1 below.
（実施例２）
ＰＴＦＥファインパウダーとして、フルオンＣＤ−１４５の代わりにダイキン工業製ポリフロンＦ−１０４を用いた以外は、実施例１と同様にして、ＰＴＦＥ多孔質膜を得た。得られたＰＴＦＥ多孔質膜の通気量、引張強度および厚さを、以下の表１に示す。また、得られたＰＴＦＥ多孔質膜の表面の走査型電子顕微鏡（ＳＥＭ）像を図１に示す。なお、液状潤滑剤を除去した後、逐次二軸延伸を行う前のＰＴＦＥシートの引張強度は、３．５ＭＰａであった。
(Example 2)
A PTFE porous membrane was obtained in the same manner as in Example 1 except that, as PTFE fine powder, Polyflon F-104 manufactured by Daikin Industries, Ltd. was used instead of Fullon CD-145. The air permeability, tensile strength, and thickness of the obtained PTFE porous membrane are shown in Table 1 below. Moreover, the scanning electron microscope (SEM) image of the surface of the obtained PTFE porous membrane is shown in FIG. After removing the liquid lubricant, the tensile strength of the PTFE sheet before sequential biaxial stretching was 3.5 MPa.
（実施例３）
逐次二軸延伸における一段目の延伸倍率を８倍、二段目の延伸倍率を１５倍とした以外は、実施例２と同様にして、ＰＴＦＥ多孔質膜を得た。得られたＰＴＦＥ多孔質膜の通気量、引張強度および厚さを、以下の表１に示す。また、得られたＰＴＦＥ多孔質膜の表面のＳＥＭ像を図２に示す。
(Example 3)
A PTFE porous membrane was obtained in the same manner as in Example 2 except that the stretching ratio of the first stage in sequential biaxial stretching was 8 times and the stretching ratio of the second stage was 15 times. The air permeability, tensile strength, and thickness of the obtained PTFE porous membrane are shown in Table 1 below. Moreover, the SEM image of the surface of the obtained PTFE porous membrane is shown in FIG.
（実施例４）
押出成形時における比Ａｉ／Ａｏを１０とした以外は、実施例２と同様にして、ＰＴＦＥ多孔質膜を得た。得られたＰＴＦＥ多孔質膜の通気量、引張強度および厚さを、以下の表１に示す。なお、液状潤滑剤を除去した後、逐次二軸延伸を行う前のＰＴＦＥシートの引張強度は、１．２ＭＰａであった。
Example 4
A PTFE porous membrane was obtained in the same manner as in Example 2 except that the ratio Ai / Ao at the time of extrusion was set to 10. The air permeability, tensile strength, and thickness of the obtained PTFE porous membrane are shown in Table 1 below. After removing the liquid lubricant, the tensile strength of the PTFE sheet before sequential biaxial stretching was 1.2 MPa.
（実施例５）
逐次二軸延伸における一段目の延伸温度を３８０℃、二段目の延伸温度を３８０℃とし、焼成を行わなかった以外は、実施例３と同様にして、ＰＴＦＥ多孔質膜を得た。得られたＰＴＦＥ多孔質膜の通気量、引張強度および厚さを、以下の表１に示す。
(Example 5)
A PTFE porous membrane was obtained in the same manner as in Example 3 except that the first-stage stretching temperature in sequential biaxial stretching was 380 ° C., the second-stage stretching temperature was 380 ° C., and no firing was performed. The air permeability, tensile strength, and thickness of the obtained PTFE porous membrane are shown in Table 1 below.
（比較例１）
押出成形時における比Ａｉ／Ａｏを３５とした以外は、実施例２と同様にして、ＰＴＦＥ多孔質膜を得た。得られたＰＴＦＥ多孔質膜の通気量、引張強度および厚さを、以下の表１に示す。また、得られたＰＴＦＥ多孔質膜の表面のＳＥＭ像を図３に示す。なお、液状潤滑剤を除去した後、逐次二軸延伸を行う前のＰＴＦＥシートの引張強度は、８．５ＭＰａであった。
(Comparative Example 1)
A PTFE porous membrane was obtained in the same manner as in Example 2 except that the ratio Ai / Ao at the time of extrusion was set to 35. The air permeability, tensile strength, and thickness of the obtained PTFE porous membrane are shown in Table 1 below. Moreover, the SEM image of the surface of the obtained PTFE porous membrane is shown in FIG. In addition, after removing the liquid lubricant, the tensile strength of the PTFE sheet before sequential biaxial stretching was 8.5 MPa.
（比較例２）
逐次二軸延伸における一段目の延伸倍率を１０倍、二段目の延伸倍率を２０倍とした以外は、比較例１と同様にして、ＰＴＦＥ多孔質膜を得た。得られたＰＴＦＥ多孔質膜の通気量、引張強度および厚さを、以下の表１に示す。また、得られたＰＴＦＥ多孔質膜の表面のＳＥＭ像を図４に示す。
(Comparative Example 2)
A PTFE porous membrane was obtained in the same manner as in Comparative Example 1 except that the stretching ratio of the first stage in sequential biaxial stretching was 10 times and the stretching ratio of the second stage was 20 times. The air permeability, tensile strength, and thickness of the obtained PTFE porous membrane are shown in Table 1 below. Moreover, the SEM image of the surface of the obtained PTFE porous membrane is shown in FIG.
表１に示すように実施例１〜５では、比較例１〜２に比べて、通気量および引張強度がともに向上し、通気性と強度とを両立させたＰＴＦＥ多孔質膜が得られた。なお、実施例２と比較例１とでは、ＰＴＦＥ多孔質膜の製造条件のうち比Ａｉ／Ａｏのみが異なるが、比較例１に比べて実施例２では、厚いＰＴＦＥ多孔質膜、即ち、延伸による厚さの減少の程度が小さいＰＴＦＥ多孔質膜となった。比Ａｉ／Ａｏを３０未満とすることにより、比較例１などの従来のＰＴＦＥ多孔質膜とは厚さ方向の構造が異なるＰＴＦＥ多孔質膜が得られている可能性がある。 As shown in Table 1, in Examples 1 to 5, both the air flow rate and the tensile strength were improved as compared with Comparative Examples 1 and 2, and a PTFE porous membrane having both air permeability and strength was obtained. Note that Example 2 and Comparative Example 1 differ only in the ratio Ai / Ao among the production conditions of the PTFE porous membrane, but in Example 2 compared to Comparative Example 1, the thick PTFE porous membrane, ie, stretched As a result, the thickness of the PTFE porous membrane was small. By setting the ratio Ai / Ao to be less than 30, there is a possibility that a PTFE porous membrane having a structure in the thickness direction different from that of the conventional PTFE porous membrane such as Comparative Example 1 is obtained.
また、図１、２に示すように、実施例２、３のＰＴＦＥ多孔質膜では、方形、円形または楕円形の形状を有する比較的大きなノードが多数形成されていた。このノードのアスペクト比は、図１、２から明らかなように小さく、そのサイズは、長軸方向の長さにして１〜３０μｍ程度、典型的には３〜１０μｍ程度であった。これに対して、図３、４に示すように、比較例１、２のＰＴＦＥ多孔質膜では明瞭なノードが観察されなかった。また、比較例１、２に比べて実施例２、３のＰＴＦＥ多孔質膜では、フィブリルの径およびフィブリル間の間隔が大きく、延伸による厚さの減少の程度が小さいことを含め、これらの膜構造の違いが、通気性と強度との両立に寄与していると考えられる。なお、図１〜４において、各図の上下方向がＰＴＦＥ多孔質膜のＭＤ方向である。 As shown in FIGS. 1 and 2, in the PTFE porous membranes of Examples 2 and 3, a large number of relatively large nodes having a square, circular, or elliptical shape were formed. The aspect ratio of this node is small as apparent from FIGS. 1 and 2, and the size is about 1 to 30 μm, typically about 3 to 10 μm in terms of the length in the major axis direction. On the other hand, as shown in FIGS. 3 and 4, no clear nodes were observed in the porous PTFE membranes of Comparative Examples 1 and 2. Further, in the PTFE porous membranes of Examples 2 and 3 as compared with Comparative Examples 1 and 2, these membranes include that the fibril diameter and the interval between the fibrils are large, and the degree of thickness reduction due to stretching is small. It is considered that the difference in structure contributes to the balance between air permeability and strength. 1 to 4, the vertical direction of each figure is the MD direction of the PTFE porous membrane.
本発明のＰＴＦＥ多孔質膜の製造方法によれば、通気性と強度とを両立させたＰＴＦＥ多孔質膜が得られる。このようなＰＴＦＥ多孔質膜は、被濾過気体に含まれる異物を除去するフィルター濾材および当該濾材を備えるフィルターユニットなど、様々な用途に好適に使用できる。 According to the method for producing a porous PTFE membrane of the present invention, a porous PTFE membrane having both air permeability and strength can be obtained. Such a PTFE porous membrane can be suitably used for various applications such as a filter medium for removing foreign substances contained in the gas to be filtered and a filter unit including the filter medium.
Claims (4)
前記形成したシートを延伸して、延伸により生じたＰＴＦＥフィブリルおよび当該フィブリル間の空隙からなる多孔質構造を有するＰＴＦＥ多孔質膜を形成する、ポリテトラフルオロエチレン多孔質膜の製造方法であって、
押出シリンダーに収容した前記混合物を、前記シリンダーに接続された押出ダイスから押し出すことによって、前記混合物の押出成形を行い、
前記押出成形における、前記押出シリンダーの断面積Ａｉと、前記押出ダイスにおける前記混合物の流路の最小断面積Ａｏとの比Ａｉ／Ａｏが３０未満である、ポリテトラフルオロエチレン多孔質膜の製造方法。 A PTFE sheet is formed from a mixture of polytetrafluoroethylene (PTFE) fine powder and liquid lubricant by a process including extrusion molding of the mixture,
Stretching the formed sheet to form a PTFE porous membrane having a porous structure composed of PTFE fibrils generated by stretching and voids between the fibrils, and a method for producing a polytetrafluoroethylene porous membrane,
Extruding the mixture contained in an extrusion cylinder by extruding it from an extrusion die connected to the cylinder,
A method for producing a polytetrafluoroethylene porous membrane, wherein a ratio Ai / Ao between a cross-sectional area Ai of the extrusion cylinder and a minimum cross-sectional area Ao of a flow path of the mixture in the extrusion die is less than 30 in the extrusion molding .
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