JP2004066027A - Production method of electret filter medium - Google Patents

Production method of electret filter medium Download PDF

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Publication number
JP2004066027A
JP2004066027A JP2002225213A JP2002225213A JP2004066027A JP 2004066027 A JP2004066027 A JP 2004066027A JP 2002225213 A JP2002225213 A JP 2002225213A JP 2002225213 A JP2002225213 A JP 2002225213A JP 2004066027 A JP2004066027 A JP 2004066027A
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Prior art keywords
porous dielectric
dielectric sheet
aqueous solution
filter medium
sheet
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JP2002225213A
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JP4078592B2 (en
Inventor
Tadao Masumori
増森 忠雄
Seiji Tokuda
徳田 省二
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Toyobo Co Ltd
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Toyobo Co Ltd
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Abstract

<P>PROBLEM TO BE SOLVED: To provide a production method of an electret filter medium of a porous dielectric sheet which is highly and sufficiently turned into an electret to its interior. <P>SOLUTION: An aqueous solution containing at least one organic or inorganic compound having an index (pKa) of acid dissociation of not less than 3.0 is jetted to the porous dielectric sheet with a pressure sufficient for the solution to pass through the sheet so as to turn the sheet into the electret to its interior sufficiently, and then the sheet is dried to produce the electret filter medium. <P>COPYRIGHT: (C)2004,JPO

Description

【0001】
【発明の属する技術分野】
本発明は、気体中の微粒子の捕捉に用いられるエレクトレット濾材の、改良された製造方法に関する。
【0002】
【従来の技術】
多孔性誘電体シートをエレクトレット化する従来の技術として、直流コロナ荷電法、水流噴霧荷電法等が挙げられる。
【0003】
直流コロナ荷電法は、正もしくは負のコロナイオンを多孔性誘電体シートに衝突させることによりエレクトレット化する方法である。しかしながら、この方法では、多孔性誘電体シートの表層部分、すなわち、コロナイオンに曝されている部分にのみ電荷が蓄積してしまう。そして、その蓄積電荷により形成される反発電界により、コロナイオンが多孔性誘電体シート内部まで浸透することができず、その結果、多孔性誘電体シート内部まで十分に荷電されないという欠点を有する。
【0004】
特表平9−501604号公報には、水の噴流または水滴流を多孔性誘電体シートに衝突させることによりエレクトレット化する方法が開示されている。多孔性誘電体シートに衝突させる水としては、蒸留水、イオン交換水といったより純度の高い水を使用するのが好ましいとの記載があるが、そのような高純度の水では誘電体シートを高度にエレクトレット化することはできない。
【0005】
特開2002−115177号公報には、非導電性シートに水と水溶性有機溶剤との混合溶液を付与し、次いで該非導電性シートを乾燥するエレクトレット加工品の製造方法について開示されている。水と水溶性有機溶剤との混合溶液において、水としては、蒸留水、イオン交換水といったより純度の高い水を使用するのが好ましいとの記載があるが、そのような水では、非導電性シートと混合溶液との接触による荷電効果は低い。また、非導電性シートへの浸透性を高める目的で水溶性有機溶剤を混合しているため、非導電性シートと混合溶液との接触角が小さくなり、シート上の蓄積電荷が混合溶液中に流出しやすくなる。その結果、非導電性シートを高度にエレクトレット化できないという欠点を有する。
【0006】
【発明が解決しようとする課題】
本発明は上記従来の問題点を鑑みて、多孔性誘電体シートにおいて、その内部まで十分に、かつ、高度にエレクトレット化されたエレクトレット濾材の製造方法を提供するものである。
【0007】
【課題を解決するための手段】
本発明は、多孔性誘電体シートに、有機もしくは無機化合物を含有する水溶液を、該水溶液が多孔性誘電体シート内を通過するのに十分な圧力で噴射させ、次いで乾燥するエレクトレット濾材の製造方法に関する。本発明者らは鋭意研究した結果、前記水溶液が酸解離指数(pKa)3.0以上の有機もしくは無機化合物を少なくとも一種類以上含有し、かつその濃度が1〜10ppmである場合、高純度の水を衝突させる時と比べて、多孔性誘電体シートがより高度にエレクトレット化されることを見出した。該水溶液では多孔性誘電体シートへの浸透性が比較的低く、多孔性誘電体シート上の蓄積電荷の流出が生じないため、高度なエレクトレット化が実現できるのである。
【0008】
【発明の実施の形態】
本発明で処理することのできる多孔性誘電体シートとしては、繊維シート(例えば、織物、編み物、不織布、及び、これらの複合体)、多孔フィルム(例えば、穴開きフィルム)、発泡体、或いはこれらの複合体などがある。好ましくは、メルトブロー法により作製された極細繊維不織布である。極細繊維不織布は繊維表面積が大きいため、粒子捕集効率が向上する。
【0009】
被荷電処理体としての多孔性誘電体シートは、一枚、あるいは、複数枚積層した構成であってもよい。またシート強度を高めるためにスパンボンド等の補強材を積層して水溶液噴射処理を施してもよい。
【0010】
多孔性誘電体シートの材質としては、一種類、あるいは、複数の種類から構成されてもよいが、電荷保持の点から体積抵抗率1014Ωcm以上の材質を少なくとも一種類以上含むことが好ましい。もし、該多孔性誘電体シートが体積抵抗率1014Ωcm未満の材質のみで構成されていれば、電荷が蓄積しにくく、高度にエレクトレット化することはできない。また、電荷寿命が極端に短くなってしまうという問題が生じる。具体的な材質としては、ポリオレフィン、ポリエステル、ポリ乳酸、ポリカーボネート、ポリ塩化ビニル、ポリ塩化ビニリデン等であるが、ポリオレフィンが好ましく、なかでもポリプロピレンが特に好ましい。
【0011】
本発明における多孔性誘電体シートが、ポリプロピレンからなるメルトブロー不織布の場合、目付は5〜100g/mであり、好ましくは10〜60g/mである。平均繊維径は1〜20μmであり、好ましくは1〜10μmである。
【0012】
多孔性誘電体シートに噴射する水溶液中には、酸解離指数(pKa)3.0以上の有機もしくは無機化合物が少なくとも一種類以上含有されていることが必須である。ここで多段階の解離平衡が存在する化合物の場合は、その最も小さい酸解離指数(pKa)が3.0以上であることが必須である。酸解離指数(pKa)が3.0より小さい化合物のみ含有する水溶液を使用した場合は、多孔性誘電体シートを高度にエレクトレット化することができない。好ましい有機もしくは無機化合物の具体例として、カルボン酸、カルボン酸塩、アンモニア、アンモニウム塩、アミン類、炭酸塩、炭酸水素塩、次亜塩素酸塩等が挙げられ、特に好ましくは、常温常圧で揮発性であるアンモニアである。なお、界面活性剤や有機溶剤は多孔性誘電体シートへの水溶液の浸透性を高めるだけでなく、多孔性誘電体シート表面に被膜を形成し、多孔性誘電体シートの高エレクトレット化を妨げるため、該水溶液中に含有されるべきではない。酸解離指数(pKa)とは酸解離定数(Ka)より以下の式に従って算出される。また、ここで言う酸解離定数(Ka)とは常温常圧の条件における水中での酸解離定数(Ka)のことを指す。
【0013】
【数1】

Figure 2004066027
【0014】
上記有機および無機化合物の水溶液中での濃度は、その化合物により異なるが、1〜10ppmである。1ppm以下であると高度にエレクトレット化されず効果が不十分であり、逆に10ppmよりも大きいと水溶液の導電率が大きくなって不織布に蓄積された電荷が流出してしまい、結果として高度にエレクトレット化することができない。
【0015】
本発明において多孔性誘電体シートに前記水溶液を噴射する場合、シートを通気度50〜400cm/cm/秒の網状支持体に載せ、この上方より水溶液を噴射するとともに、該網状支持体の下方を減圧状態とすることが好ましい。通気度はJIS−L1096に記載のフランジール形試験機を用いて測定される。網状支持体とは具体的には金属ヤーンやプラスチックヤーンの織物からなる多孔構造物であり、平織り、綾織り、朱子織りなどの織り形状が挙げられる。金属素材としてはステンレス、ブロンズ等、またプラスチック素材としてはポリプロピレン、ポリエステル、ポリウレタン、ナイロン、ポリフェニレンサルファイドなどがある。
【0016】
水溶液の噴射は多孔性誘電体シートの数cm上方に設置した、シートの幅方向に沿って多数のオリフィスを有するノズルより、水溶液が該シートを通過するのに十分な圧力で噴射する。通過するのに十分な圧力は、多孔性誘電体シートの目付によって異なる。例えば、目付が5〜20g/mのものでは、0.3〜2MPa、20〜50g/mのものでは、0.6〜3MPa、50〜100g/mのものでは、1〜4MPaであることが好ましい。圧力が高すぎると、多孔性誘電体シートにピンホールが開き、濾過性能が低下してしまう。また圧力が低すぎることが原因で多孔性誘電体シート内を水溶液が十分に通過することができなければ、多孔性誘電体シートを高度にエレクトレット化することができない。ノズルは直径0.05〜0.2mmのオリフィスをピッチ0.5〜3mmで1列あるいは複数列配置したものが好ましい。また網状支持体を可動とし、多孔性誘電体シートをその長手方向に搬送させることにより噴射処理を連続的に行うことが出来る。その搬送速度は特に限定されないが、好ましい範囲を挙げると1〜100m/分である。また最適な噴射回数や処理面(片面か両面か)は多孔性誘電体シートの目付や平均繊維径に依存するため特に限定されない。
【0017】
また水溶液の噴射と同時に、網状支持体の下方を、排気ブロアー等を用いて減圧状態とすることが好ましい。吸引負圧は特に限定されないが、200〜2000mmAqが好適である。減圧状態にすると、多孔性誘電体シート内を水溶液が十分に通過でき、多孔性誘電体シートを高度にエレクトレット化することができる。
【0018】
多孔性誘電体シートに水溶液噴射処理した後の乾燥方法については、従来公知の方法がいずれも使用可能である。例えば、熱風乾燥法、真空乾燥法、自然乾燥法等の方法が適用可能である。これらのうちでも熱風乾燥法は、連続処理が可能であるため好ましい。熱風乾燥法の場合、乾燥温度としてはエレクトレットを消失させない程度の温度にする必要がある。好ましくは120℃以下、より好ましくは100℃以下、さらに好ましくは80℃以下にするのがよい。また、熱風乾燥前に、予備乾燥として、ニップロール、吸水ロール、サクション吸引等によって過剰な水分を取り除いておくと、より好ましい。
【0019】
水溶液噴射処理を施す前に行う多孔性誘電体シートの前処理として、直流コロナ荷電処理を行うことが好ましい。直流コロナ荷電処理を行うことによって予めシート表層部分に電荷を蓄積させておけば、噴射処理によりシート内部のみを荷電すればよいため、少ない処理回数で短時間に、かつ、高度にエレクトレット化することができる。
【0020】
本発明の多孔性誘電体シートには、ヒンダードフェノール系安定剤、硫黄系安定剤、リン系安定剤、脂肪酸金属塩、結晶核剤等の添加剤が含有されることが好ましい。これらの添加剤を含有することにより、多孔性誘電体シートのエレクトレット性が飛躍的に向上する。これらの添加剤の含有量は、多孔性誘電体シート100重量部に対して、0.025〜5重量部であり、好ましくは0.05〜3重量部、最も好ましくは0.1〜1重量部である。含有量が少ないとエレクトレット化効果が十分ではなく、逆に含有量が多くても効果は飽和し、ブリードアウトするため好ましくない。
【0021】
以下、実施例によって本発明の作用効果をより具体的に示す。下記では多孔性誘電体として従来公知の方法で製造されるメルトブローン不織布を使用したが、これは本発明方法を限定する性質のものではなく、前・後記の趣旨に沿って設計変更することはいずれも本発明の技術的範囲に含まれるものである。
【0022】
(水溶液噴射処理)
多孔性誘電体シートを通気度120cm/cm/秒の網状支持体(96メッシュ)に載せ、不織布の上方2cmに位置する直径0.1mmφ、ピッチ1mmのノズルから、1MPaの圧力で水溶液噴射処理を行った。なおベースとなる水は一般的な水道水を二段の逆浸透膜処理、次いでイオン交換膜処理を施した高純度の水とした。支持体の搬送速度を10m/分とし、ノズル直下の網状体の下方を600mmAqの減圧状態とした。この処理をシートの表裏について各2回ずつ行った。その後このシートを70℃の熱風オーブン中に1分間滞留させて乾燥した。
【0023】
(直流コロナ荷電処理)
多孔性誘電体シートを、アルミ平板の接地極上に敷いた厚み0.5mmのシリコンシート上に置き、多孔性誘電体シート上方1cmに設置した針状電極を用いて+15kVの直流高電圧を10秒間印加した。
【0024】
(濾過特性の評価)
圧力損失(PD)は、エレクトレット濾材試料をダクト内に設置し、濾材通過線速度が10cm/秒になるようコントロールし、エレクトレット濾材上流、下流の静圧差を圧力計で読み取り求めた。また粒子捕集効率E(%)の評価は粒子径0.3μmのDOP粒子を用い、10cm/秒にて行った。圧力損失PD(mmAq)と粒子捕集効率E(%)を用いて、数2より濾材品質係数QFを算出した。
【0025】
【数2】
Figure 2004066027
【0026】
(実施例1〜2)
メルトフローインデックス1000のポリプロピレン樹脂100重量部に対してIrganox1010を0.1重量部配合し、メルトブロー法により目付30g/m、平均繊維径2.5μmのメルトブロー不織布を作製した。この不織布に直流コロナ荷電処理を施した後、次いで、アンモニア(pKa=9.2)を含有する水溶液を使用し、噴射処理を行った。実施例1では、アンモニア5ppmを含有する水溶液、実施例2では10000ppmを含有する水溶液を、それぞれ使用して噴射処理を行った。乾燥後、粒子捕集効率、圧力損失を測定し、QF値を算出した。結果を表1に示す。
【0027】
(実施例3)
実施例1〜2と同様のメルトブロー不織布に直流コロナ荷電処理を行い、次いで、炭酸水素ナトリウム(pKa=6.3)5ppmを含有する水溶液の噴射処理を行った。乾燥後、粒子捕集効率、圧力損失を測定し、QF値を算出した。結果を表1に示す。
【0028】
(実施例4)
実施例1〜2と同様のメルトブロー不織布に直流コロナ荷電処理を行い、次いで、酢酸ナトリウム(pKa=4.5)5ppmを含有する水溶液の噴射処理を行った。乾燥後、粒子捕集効率、圧力損失を測定し、QF値を算出した。結果を表1に示す。
【0029】
(比較例1)
実施例1〜2と同様のメルトブロー不織布に直流コロナ荷電処理を行い、次いで、何も添加していないベース水の噴射処理を行った。乾燥後、粒子捕集効率、圧力損失を測定し、QF値を算出した。結果を表1に示す。
【0030】
(比較例2)
実施例1〜2と同様のメルトブロー不織布に直流コロナ荷電処理を行い、次いで、クエン酸(pKa=2.8)5ppmを含有する水溶液の噴射処理を行った。乾燥後、粒子捕集効率、圧力損失を測定し、QF値を算出した。結果を表1に示す。
【0031】
(比較例3)
実施例1〜2と同様のメルトブロー不織布に直流コロナ荷電処理を行い、次いで、アンモニア(pKa=9.2)濃度200000ppmを含有する水溶液の噴射処理を行った。乾燥後、粒子捕集効率、圧力損失を測定し、QF値を算出した。結果を表1に示す。
【0032】
(比較例4)
実施例1〜2と同様のメルトブロー不織布に直流コロナ荷電処理を行い、次いで、アンモニア(pKa=9.2)0.5ppmを含有する水溶液の噴射処理を行った。乾燥後、粒子捕集効率、圧力損失を測定し、QF値を算出した。結果を表1に示す。
【0033】
(比較例5)
実施例1〜2と同様のメルトブロー不織布に直流コロナ荷電処理を行い、次いで、イソプロピルアルコール20重量%を含有水溶液の噴射処理を行った。乾燥後、粒子捕集効率、圧力損失を測定し、QF値を算出した。結果を表1に示す。
【0034】
(比較例6)
実施例1〜2と同様のメルトブロー不織布に直流コロナ荷電処理を行い、その後、粒子捕集効率、圧力損失を測定し、QF値を算出した。結果を表1に示す。
【0035】
【表1】
Figure 2004066027
【0036】
表1より明らかなように、実施例では何れも高い粒子捕集効率を示した。これに対して化合物を配合しない水を噴射した比較例1、pKaが3.0以下のクエン酸を添加した比較例2、高濃度および低濃度のアンモニア水溶液を噴射した比較例3、4では、直流コロナ荷電処理と同等もしくはこれより低い粒子捕集効率およびQFに留まり、高度にエレクトレット化されていない。またポリプロピレンとの親和性の高いイソプロピルアルコールを多量に含んだ水溶液を使用した場合(比較例5)、エレクトレット性は著しく低下することがわかった。
【0037】
【発明の効果】
本発明によれば、多孔性誘電体シートの内部まで十分に、かつ高度にエレクトレット化することができ、低圧損ながら高い粒子捕集効率を有するエレクトレット濾材を得ることができる。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an improved method for producing an electret filter medium used for capturing fine particles in a gas.
[0002]
[Prior art]
As a conventional technique for converting a porous dielectric sheet into an electret, a direct current corona charging method, a water spray charging method, and the like can be given.
[0003]
The direct current corona charging method is a method of forming electrets by colliding positive or negative corona ions with a porous dielectric sheet. However, according to this method, charges are accumulated only in the surface layer portion of the porous dielectric sheet, that is, in the portion exposed to corona ions. Further, due to the repulsive electric field formed by the accumulated charges, corona ions cannot penetrate into the porous dielectric sheet, and as a result, there is a disadvantage that the inside of the porous dielectric sheet is not sufficiently charged.
[0004]
Japanese Patent Publication No. Hei 9-501604 discloses a method of forming an electret by colliding a jet of water or a stream of water droplets with a porous dielectric sheet. It is described that it is preferable to use higher-purity water such as distilled water or ion-exchanged water as water to be made to collide with the porous dielectric sheet. Cannot be electretized.
[0005]
Japanese Patent Application Laid-Open No. 2002-115177 discloses a method for producing an electret processed product in which a mixed solution of water and a water-soluble organic solvent is applied to a non-conductive sheet, and then the non-conductive sheet is dried. In a mixed solution of water and a water-soluble organic solvent, as the water, there is a description that it is preferable to use water of higher purity such as distilled water or ion-exchanged water. The charging effect due to the contact between the sheet and the mixed solution is low. In addition, since the water-soluble organic solvent is mixed for the purpose of enhancing the permeability to the non-conductive sheet, the contact angle between the non-conductive sheet and the mixed solution is reduced, and the accumulated charge on the sheet is reduced to the mixed solution. It is easy to leak. As a result, there is a disadvantage that the non-conductive sheet cannot be highly electretized.
[0006]
[Problems to be solved by the invention]
The present invention has been made in view of the above-mentioned conventional problems, and provides a method for manufacturing an electret filter medium in a porous dielectric sheet that is sufficiently electretized to the inside thereof and is highly electret.
[0007]
[Means for Solving the Problems]
The present invention provides a method for producing an electret filter medium in which an aqueous solution containing an organic or inorganic compound is sprayed on a porous dielectric sheet at a pressure sufficient for the aqueous solution to pass through the porous dielectric sheet, and then dried. About. The present inventors have conducted intensive studies. As a result, when the aqueous solution contains at least one organic or inorganic compound having an acid dissociation index (pKa) of 3.0 or more and the concentration thereof is 1 to 10 5 ppm, It has been found that the porous dielectric sheet is more highly electretized as compared with the case of colliding with pure water. The aqueous solution has relatively low permeability to the porous dielectric sheet, and does not cause outflow of accumulated charges on the porous dielectric sheet, so that a high-level electret can be realized.
[0008]
BEST MODE FOR CARRYING OUT THE INVENTION
Examples of the porous dielectric sheet that can be treated in the present invention include a fiber sheet (for example, a woven fabric, a knitted fabric, a nonwoven fabric, and a composite thereof), a porous film (for example, a perforated film), a foam, or a foamed material. And the like. Preferably, it is a microfiber nonwoven fabric produced by a melt blow method. Since the ultrafine fiber nonwoven fabric has a large fiber surface area, the particle collection efficiency is improved.
[0009]
The porous dielectric sheet as the charged body may have a configuration of one sheet or a stack of a plurality of sheets. Further, in order to increase the sheet strength, a reinforcing material such as a spun bond may be laminated and subjected to an aqueous solution spraying treatment.
[0010]
The material of the porous dielectric sheet may be composed of one kind or a plurality of kinds, but it is preferable to include at least one kind of material having a volume resistivity of 10 14 Ωcm or more from the viewpoint of charge retention. If the porous dielectric sheet is made of only a material having a volume resistivity of less than 10 14 Ωcm, it is difficult to accumulate electric charges and cannot be highly electretized. Further, there is a problem that the charge life is extremely shortened. Specific materials include polyolefin, polyester, polylactic acid, polycarbonate, polyvinyl chloride, polyvinylidene chloride, and the like. Polyolefin is preferable, and polypropylene is particularly preferable.
[0011]
When the porous dielectric sheet in the present invention is a melt blown nonwoven fabric made of polypropylene, the basis weight is 5 to 100 g / m 2 , preferably 10 to 60 g / m 2 . The average fiber diameter is from 1 to 20 μm, preferably from 1 to 10 μm.
[0012]
It is essential that the aqueous solution sprayed on the porous dielectric sheet contains at least one organic or inorganic compound having an acid dissociation index (pKa) of 3.0 or more. Here, in the case of a compound having multi-stage dissociation equilibrium, it is essential that the smallest acid dissociation index (pKa) is 3.0 or more. When an aqueous solution containing only a compound having an acid dissociation index (pKa) smaller than 3.0 is used, the porous dielectric sheet cannot be highly electretized. Specific examples of preferred organic or inorganic compounds include carboxylic acids, carboxylate salts, ammonia, ammonium salts, amines, carbonates, hydrogencarbonates, hypochlorites, etc., and particularly preferably at room temperature and normal pressure. Ammonia, which is volatile. Surfactants and organic solvents not only increase the permeability of the aqueous solution into the porous dielectric sheet, but also form a coating on the surface of the porous dielectric sheet and prevent the porous dielectric sheet from becoming highly electret. , Should not be contained in the aqueous solution. The acid dissociation index (pKa) is calculated from the acid dissociation constant (Ka) according to the following equation. The term “acid dissociation constant (Ka)” as used herein refers to the acid dissociation constant (Ka) in water at normal temperature and normal pressure.
[0013]
(Equation 1)
Figure 2004066027
[0014]
Concentration in the aqueous solution of the organic and inorganic compounds varies depending the compound is 1 to 10 5 ppm. If it is 1 ppm or less, the effect is insufficient because it is not highly electretized. Conversely, if it is more than 10 5 ppm, the electric conductivity of the aqueous solution increases and the electric charge accumulated in the nonwoven fabric flows out. Cannot be electretized.
[0015]
When the aqueous solution is sprayed on the porous dielectric sheet in the present invention, the sheet is placed on a mesh support having an air permeability of 50 to 400 cm 3 / cm 2 / sec. It is preferable that the lower part is in a reduced pressure state. The air permeability is measured using a flanger type tester described in JIS-L1096. The mesh support is specifically a porous structure made of a woven metal or plastic yarn, and has a woven shape such as plain weave, twill weave, and satin weave. Examples of the metal material include stainless steel and bronze, and examples of the plastic material include polypropylene, polyester, polyurethane, nylon, and polyphenylene sulfide.
[0016]
The aqueous solution is sprayed from a nozzle, which is provided several cm above the porous dielectric sheet and has a number of orifices along the width of the sheet, at a pressure sufficient for the aqueous solution to pass through the sheet. The pressure sufficient to pass depends on the basis weight of the porous dielectric sheet. For example, if the basis weight is 5 to 20 g / m 2 , 0.3 to 2 MPa, 20 to 50 g / m 2 , 0.6 to 3 MPa, and 50 to 100 g / m 2 , 1 to 4 MPa Preferably, there is. If the pressure is too high, a pinhole will open in the porous dielectric sheet, and the filtration performance will decrease. If the aqueous solution cannot sufficiently pass through the porous dielectric sheet due to too low pressure, the porous dielectric sheet cannot be highly electretized. The nozzle is preferably one in which orifices having a diameter of 0.05 to 0.2 mm are arranged in one or more rows at a pitch of 0.5 to 3 mm. In addition, by making the net-like support movable, and by transporting the porous dielectric sheet in the longitudinal direction, the spraying process can be continuously performed. The transport speed is not particularly limited, but a preferred range is 1 to 100 m / min. Further, the optimum number of times of spraying and the processing surface (one side or both sides) are not particularly limited because they depend on the basis weight and average fiber diameter of the porous dielectric sheet.
[0017]
In addition, it is preferable that the lower part of the mesh support is reduced in pressure using an exhaust blower or the like at the same time as the injection of the aqueous solution. The suction negative pressure is not particularly limited, but is preferably 200 to 2000 mmAq. When the pressure is reduced, the aqueous solution can sufficiently pass through the porous dielectric sheet, and the porous dielectric sheet can be highly electretized.
[0018]
As a drying method after the aqueous dielectric material is sprayed on the porous dielectric sheet, any conventionally known method can be used. For example, a method such as a hot air drying method, a vacuum drying method, and a natural drying method can be applied. Among them, the hot air drying method is preferable because continuous processing is possible. In the case of the hot air drying method, the drying temperature needs to be a temperature at which the electret is not lost. The temperature is preferably 120 ° C. or lower, more preferably 100 ° C. or lower, and further preferably 80 ° C. or lower. It is more preferable to remove excess water by nip rolls, water absorption rolls, suction suction, or the like as preliminary drying before hot air drying.
[0019]
It is preferable to perform a DC corona charging process as a pre-process of the porous dielectric sheet performed before performing the aqueous solution spraying process. If the electric charge is accumulated in the surface layer of the sheet in advance by performing the DC corona charging process, only the inside of the sheet needs to be charged by the spraying process. Can be.
[0020]
The porous dielectric sheet of the present invention preferably contains additives such as a hindered phenol-based stabilizer, a sulfur-based stabilizer, a phosphorus-based stabilizer, a fatty acid metal salt, and a crystal nucleating agent. By containing these additives, the electret properties of the porous dielectric sheet are dramatically improved. The content of these additives is 0.025 to 5 parts by weight, preferably 0.05 to 3 parts by weight, and most preferably 0.1 to 1 part by weight based on 100 parts by weight of the porous dielectric sheet. Department. If the content is small, the effect of forming electrets is not sufficient. Conversely, if the content is large, the effect is saturated and bleed out is not preferable.
[0021]
Hereinafter, the operation and effect of the present invention will be described more specifically with reference to examples. In the following, a melt-blown non-woven fabric manufactured by a conventionally known method was used as a porous dielectric, but this is not of a nature limiting the method of the present invention, and any design change in accordance with the spirits of the above and below may be used. Are also included in the technical scope of the present invention.
[0022]
(Aqueous solution injection treatment)
The porous dielectric sheet is placed on a mesh support (96 mesh) having an air permeability of 120 cm 3 / cm 2 / sec, and an aqueous solution is sprayed from a nozzle having a diameter of 0.1 mm and a pitch of 1 mm located 2 cm above the nonwoven fabric at a pressure of 1 MPa. Processing was performed. The base water was high-purity water obtained by subjecting common tap water to two-stage reverse osmosis membrane treatment and then ion-exchange membrane treatment. The conveying speed of the support was set to 10 m / min, and the lower part of the mesh immediately below the nozzle was reduced to 600 mmAq. This process was performed twice on each of the front and back sides of the sheet. Thereafter, the sheet was kept in a hot air oven at 70 ° C. for 1 minute and dried.
[0023]
(DC corona charging process)
The porous dielectric sheet is placed on a 0.5 mm-thick silicon sheet laid on a ground electrode of an aluminum plate, and a DC high voltage of +15 kV is applied for 10 seconds using a needle electrode placed 1 cm above the porous dielectric sheet. Applied.
[0024]
(Evaluation of filtration characteristics)
The pressure loss (PD) was determined by placing an electret filter medium sample in a duct, controlling the linear velocity of the filter medium passage to 10 cm / sec, and reading the static pressure difference upstream and downstream of the electret filter medium with a pressure gauge. The evaluation of the particle collection efficiency E (%) was performed at 10 cm / sec using DOP particles having a particle diameter of 0.3 μm. Using the pressure loss PD (mmAq) and the particle collection efficiency E (%), the filter medium quality factor QF was calculated from Equation 2.
[0025]
(Equation 2)
Figure 2004066027
[0026]
(Examples 1 and 2)
0.1 part by weight of Irganox 1010 was blended with 100 parts by weight of a polypropylene resin having a melt flow index of 1000, and a melt-blown nonwoven fabric having a basis weight of 30 g / m 2 and an average fiber diameter of 2.5 μm was prepared by a melt-blowing method. After subjecting this nonwoven fabric to a DC corona charging treatment, a spray treatment was then performed using an aqueous solution containing ammonia (pKa = 9.2). In Example 1, the injection treatment was performed using an aqueous solution containing 5 ppm of ammonia, and in Example 2, an aqueous solution containing 10000 ppm was used. After drying, the particle collection efficiency and the pressure loss were measured, and the QF value was calculated. Table 1 shows the results.
[0027]
(Example 3)
The same melt-blown nonwoven fabric as in Examples 1 and 2 was subjected to DC corona charging treatment, and then subjected to injection treatment of an aqueous solution containing 5 ppm of sodium hydrogen carbonate (pKa = 6.3). After drying, the particle collection efficiency and the pressure loss were measured, and the QF value was calculated. Table 1 shows the results.
[0028]
(Example 4)
The same melt-blown nonwoven fabric as in Examples 1 and 2 was subjected to DC corona charging treatment, and then subjected to injection treatment of an aqueous solution containing 5 ppm of sodium acetate (pKa = 4.5). After drying, the particle collection efficiency and the pressure loss were measured, and the QF value was calculated. Table 1 shows the results.
[0029]
(Comparative Example 1)
The same melt blown nonwoven fabric as in Examples 1 and 2 was subjected to a DC corona charging treatment, and then a base water injection treatment without any addition was performed. After drying, the particle collection efficiency and the pressure loss were measured, and the QF value was calculated. Table 1 shows the results.
[0030]
(Comparative Example 2)
The same melt-blown nonwoven fabric as in Examples 1 and 2 was subjected to a direct current corona charging treatment, followed by a spray treatment of an aqueous solution containing 5 ppm of citric acid (pKa = 2.8). After drying, the particle collection efficiency and the pressure loss were measured, and the QF value was calculated. Table 1 shows the results.
[0031]
(Comparative Example 3)
The same melt-blown nonwoven fabric as in Examples 1 and 2 was subjected to DC corona charging treatment, and then sprayed with an aqueous solution containing an ammonia (pKa = 9.2) concentration of 200000 ppm. After drying, the particle collection efficiency and the pressure loss were measured, and the QF value was calculated. Table 1 shows the results.
[0032]
(Comparative Example 4)
The same melt-blown nonwoven fabric as in Examples 1 and 2 was subjected to DC corona charging, and then sprayed with an aqueous solution containing 0.5 ppm of ammonia (pKa = 9.2). After drying, the particle collection efficiency and the pressure loss were measured, and the QF value was calculated. Table 1 shows the results.
[0033]
(Comparative Example 5)
The same melt-blown nonwoven fabric as in Examples 1 and 2 was subjected to DC corona charging treatment, and then subjected to injection treatment of an aqueous solution containing 20% by weight of isopropyl alcohol. After drying, the particle collection efficiency and the pressure loss were measured, and the QF value was calculated. Table 1 shows the results.
[0034]
(Comparative Example 6)
DC corona charging treatment was performed on the same melt-blown nonwoven fabric as in Examples 1 and 2, then the particle collection efficiency and pressure loss were measured, and the QF value was calculated. Table 1 shows the results.
[0035]
[Table 1]
Figure 2004066027
[0036]
As is clear from Table 1, all of the examples exhibited high particle collection efficiency. On the other hand, in Comparative Example 1 in which water containing no compound was injected, Comparative Example 2 in which citric acid having a pKa of 3.0 or less was added, and Comparative Examples 3 and 4 in which high- and low-concentration aqueous ammonia solutions were injected, The particle collection efficiency and QF are equal to or lower than those of the DC corona charging treatment, and the particles are not highly electretized. It was also found that when an aqueous solution containing a large amount of isopropyl alcohol having a high affinity for polypropylene was used (Comparative Example 5), the electret property was significantly reduced.
[0037]
【The invention's effect】
ADVANTAGE OF THE INVENTION According to this invention, it can fully and highly electretize to the inside of a porous dielectric sheet, and can obtain the electret filter medium which has high particle collection efficiency despite low pressure loss.

Claims (6)

多孔性誘電体シートに、酸解離指数(pKa)3.0以上の有機もしくは無機化合物を少なくとも一種類以上含有する水溶液を、該水溶液が多孔性誘電体シート内を通過するのに十分な圧力で噴射させ、次いで乾燥することを特徴とするエレクトレット濾材の製造方法。An aqueous solution containing at least one organic or inorganic compound having an acid dissociation index (pKa) of 3.0 or more is applied to a porous dielectric sheet at a pressure sufficient for the aqueous solution to pass through the porous dielectric sheet. A method for producing an electret filter medium, which comprises spraying and then drying. 前記水溶液中の、酸解離指数(pKa)3.0以上の有機もしくは無機化合物の濃度が1〜10ppmであることを特徴とする請求項1に記載のエレクトレット濾材の製造方法。Electret filter media manufacturing method according to claim 1, characterized in that in the aqueous solution, the concentration of the acid dissociation exponent (pKa) 3.0 or more organic or inorganic compound is 1 to 10 5 ppm. 水溶液噴射よりも前に、多孔性誘電体シートに直流コロナ荷電処理を施すことを特徴とする請求項1乃至2のいずれかに記載のエレクトレット濾材の製造方法。3. The method for producing an electret filter medium according to claim 1, wherein a DC corona charging process is performed on the porous dielectric sheet before the aqueous solution is injected. 前記多孔性誘電体シートがメルトブロー不織布であることを特徴とする請求項1乃至3のいずれかに記載のエレクトレット濾材の製造方法。The method for producing an electret filter medium according to any one of claims 1 to 3, wherein the porous dielectric sheet is a melt-blown nonwoven fabric. 前記多孔性誘電体シートが1014Ωcmの材質を含有することを特徴とする請求項1乃至4のいずれかに記載のエレクトレット濾材の製造方法。The method for producing an electret filter medium according to any one of claims 1 to 4, wherein the porous dielectric sheet contains a material of 10 14 Ωcm. 前記多孔性誘電体シートがポリプロピレンを含有することを特徴とする請求項1乃至5のいずれかに記載のエレクトレット濾材の製造方法。The method for producing an electret filter medium according to any one of claims 1 to 5, wherein the porous dielectric sheet contains polypropylene.
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JP2017094250A (en) * 2015-11-20 2017-06-01 東洋紡株式会社 Electret filter medium, filter using the same, method of manufacturing the same

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