JP4078592B2 - Method for producing electret filter media - Google Patents
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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１〜１０5ｐｐｍ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／ｍ2であり、好ましくは１０〜６０ｇ／ｍ2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】
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、１〜１０5ｐｐｍ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１０5ｐｐｍ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５０〜４００ｃｍ3／ｃｍ2／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／ｍ2のものでは、０．３〜２ＭＰａ、２０〜５０ｇ／ｍ2のものでは、０．６〜３ＭＰａ、５０〜１００ｇ／ｍ2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１２０ｃｍ3／ｃｍ2／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】
（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／ｍ2、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】
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]
BACKGROUND 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]
Examples of conventional techniques for electretizing a porous dielectric sheet include a direct current corona charging method and a water spray charging method.
[0003]
The direct current corona charging method is a method of electretization by causing positive or negative corona ions to collide with a porous dielectric sheet. However, in this method, charges are accumulated only in the surface layer portion of the porous dielectric sheet, that is, the portion exposed to corona ions. And, 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 drawback that the porous dielectric sheet is not sufficiently charged.
[0004]
Japanese National Patent Publication No. 9-501604 discloses a method of electretizing a water jet or water droplet by colliding with a porous dielectric sheet. There is a description that it is preferable to use higher-purity water such as distilled water or ion-exchanged water as the water that collides with the porous dielectric sheet. However, with such high-purity water, the dielectric sheet is highly advanced. It 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, there is a description that it is preferable to use higher-purity water such as distilled water or ion-exchanged water, but in such water, non-conductive The charging effect due to the contact between the sheet and the mixed solution is low. In addition, since a water-soluble organic solvent is mixed for the purpose of increasing 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 contained in the mixed solution. It becomes easy to leak. As a result, there is a drawback that the non-conductive sheet cannot be highly electretized.
[0006]
[Problems to be solved by the invention]
In view of the above-described conventional problems, the present invention provides a method for producing an electret filter medium that is sufficiently electretized to the inside of a porous dielectric sheet.
[0007]
[Means for Solving the Problems]
The present invention relates to 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 to allow the aqueous solution to pass through the porous dielectric sheet, and then dried. About. As a result of intensive studies, the present inventors have found that 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 electreted than when pure water is struck. Since the aqueous solution has relatively low permeability to the porous dielectric sheet and no outflow of accumulated charges on the porous dielectric sheet occurs, a high electretization can be realized.
[0008]
DETAILED DESCRIPTION OF THE INVENTION
Examples of porous dielectric sheets that can be treated in the present invention include fiber sheets (for example, woven fabrics, knitted fabrics, nonwoven fabrics, and composites thereof), porous films (for example, perforated films), foams, or the like. There is a complex. Preferably, it is an ultrafine fiber 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 object to be charged may be a single sheet or a structure in which a plurality of sheets are stacked. Further, in order to increase the sheet strength, a reinforcing material such as spunbond may be laminated and subjected to the aqueous solution injection treatment.
[0010]
The material of the porous dielectric sheet may be composed of one kind or a plurality of kinds, but it is preferable that at least one kind of material having a volume resistivity of 10 14 Ωcm or more is included from the viewpoint of charge retention. If the porous dielectric sheet is composed only of a material having a volume resistivity of less than 10 14 Ωcm, it is difficult for charges to accumulate and it cannot be highly electretized. Moreover, the problem that a charge lifetime will become extremely short arises. Specific examples of the material include polyolefin, polyester, polylactic acid, polycarbonate, polyvinyl chloride, and polyvinylidene chloride. 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 . An average fiber diameter is 1-20 micrometers, Preferably it is 1-10 micrometers.
[0012]
It is essential that the aqueous solution sprayed onto 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 a 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) of less than 3.0 is used, the porous dielectric sheet cannot be highly electretized. Specific examples of preferred organic or inorganic compounds include carboxylic acids, carboxylates, ammonia, ammonium salts, amines, carbonates, bicarbonates, hypochlorites, etc., particularly preferably at normal temperature and pressure. It is ammonia that is volatile. Surfactants and organic solvents not only increase the permeability of the aqueous solution to the porous dielectric sheet, but also form a film on the surface of the porous dielectric sheet to prevent high electrets of the porous dielectric sheet. 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 formula. In addition, the acid dissociation constant (Ka) mentioned here refers to the acid dissociation constant (Ka) in water under normal temperature and normal pressure conditions.
[0013]
[Expression 1]
The concentration of the organic and inorganic compounds in the aqueous solution varies depending on the compound, but is 1 to 10 5 ppm. If the concentration is less than 1 ppm, the electret is not highly converted and the effect is insufficient. Conversely, if it exceeds 10 5 ppm, the electrical conductivity of the aqueous solution increases and the charge accumulated in the non-woven fabric flows out. Cannot be electretized.
[0015]
In the present invention, when the aqueous solution is sprayed onto the porous dielectric sheet, the sheet is placed on a net-like support having an air permeability of 50 to 400 cm 3 / cm 2 / sec, the aqueous solution is sprayed from above, and The lower part is preferably in a reduced pressure state. The air permeability is measured using a frangole type tester described in JIS-L1096. Specifically, the net-like support is a porous structure made of metal yarn or plastic yarn, and examples thereof include 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 ejected from a nozzle having a large number of orifices along the width direction of the sheet, which is installed several cm above the porous dielectric 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, when 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, 50 to 100 g / m 2 , 1 to 4 MPa Preferably there is. If the pressure is too high, pinholes will open in the porous dielectric sheet and the filtration performance will deteriorate. If the aqueous solution cannot sufficiently pass through the porous dielectric sheet due to the pressure being too low, the porous dielectric sheet cannot be highly electretized. The nozzle preferably has one or more rows of orifices having a diameter of 0.05 to 0.2 mm arranged at a pitch of 0.5 to 3 mm. Further, the jetting process can be continuously performed by making the mesh support movable and transporting the porous dielectric sheet in the longitudinal direction. Although the conveyance speed is not particularly limited, a preferable range is 1 to 100 m / min. Further, the optimum number of injections and the treatment surface (one side or both sides) are not particularly limited because they depend on the basis weight of the porous dielectric sheet and the average fiber diameter.
[0017]
At the same time as the injection of the aqueous solution, it is preferable that the lower portion of the mesh support is brought into a reduced pressure state using an exhaust blower or the like. The suction negative pressure is not particularly limited, but 200 to 2000 mmAq is preferable. 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]
Any conventionally known method can be used for the drying method after the aqueous solution is sprayed onto the porous dielectric sheet. For example, methods such as a hot air drying method, a vacuum drying method, and a natural drying method are applicable. Among these, 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 that does not cause the electret to disappear. Preferably it is 120 degrees C or less, More preferably, it is 100 degrees C or less, More preferably, it is good to set it as 80 degrees C or less. In addition, it is more preferable to remove excess water by nip roll, water absorption roll, suction suction or the like as preliminary drying before hot air drying.
[0019]
As a pretreatment of the porous dielectric sheet performed before the aqueous solution injection treatment, it is preferable to perform a direct current corona charging treatment. If electric charge is accumulated in the sheet surface layer in advance by performing DC corona charging treatment, only the inside of the sheet needs to be charged by the injection treatment, so it can be highly electretized in a short time with a small number of treatments. Can do.
[0020]
The porous dielectric sheet of the present invention preferably contains additives such as hindered phenol stabilizers, sulfur stabilizers, phosphorus stabilizers, fatty acid metal salts, crystal nucleating agents and the like. By containing these additives, the electret property of the porous dielectric sheet is dramatically improved. The content of these additives is 0.025 to 5 parts by weight, preferably 0.05 to 3 parts by weight, most preferably 0.1 to 1 part by weight, based on 100 parts by weight of the porous dielectric sheet. Part. If the content is small, the electretization effect is not sufficient. Conversely, even if the content is large, the effect is saturated and bleed out, which is not preferable.
[0021]
Hereinafter, the effects of the present invention will be described more specifically by way of examples. In the following, a melt-blown nonwoven fabric produced by a conventionally known method is used as a porous dielectric, but this is not a property of limiting the method of the present invention, and any design changes may be made in accordance with the purpose described above and below. Is also included in the technical scope of the present invention.
[0022]
(Aqueous solution injection treatment)
A porous dielectric sheet is placed on a reticulated support (96 mesh) with an air permeability of 120 cm 3 / cm 2 / sec, and an aqueous solution is injected at a pressure of 1 MPa from a nozzle with a diameter of 0.1 mmφ and a pitch of 1 mm located 2 cm above the nonwoven fabric. Processed. The base water was high-purity water obtained by subjecting general 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 body just below the nozzle was set to a reduced pressure state of 600 mmAq. This treatment was performed twice for 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 treatment)
A porous dielectric sheet is placed on a 0.5 mm thick silicon sheet laid on the ground electrode of an aluminum flat 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 obtained by placing an electret filter medium sample in a duct and controlling the linear velocity of the filter medium to be 10 cm / second, and reading the static pressure difference upstream and downstream of the electret filter medium with a pressure gauge. The particle collection efficiency E (%) was evaluated 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]
[Expression 2]
(Examples 1-2)
0.1 parts 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 blow method. The nonwoven fabric was subjected to a direct current corona charging treatment, and then an aqueous solution containing ammonia (pKa = 9.2) was used to carry out an injection treatment. In Example 1, an aqueous solution containing 5 ppm of ammonia was used, and in Example 2, an aqueous solution containing 10,000 ppm was used for the injection treatment. After drying, the particle collection efficiency and pressure loss were measured, and the QF value was calculated. The results are shown in Table 1.
[0027]
(Example 3)
The same melt blown nonwoven fabric as in Examples 1 and 2 was subjected to a direct current corona charging treatment, and then an aqueous solution containing 5 ppm of sodium hydrogen carbonate (pKa = 6.3) was subjected to an injection treatment. After drying, the particle collection efficiency and pressure loss were measured, and the QF value was calculated. The results are shown in Table 1.
[0028]
Example 4
The same melt blown nonwoven fabric as in Examples 1 and 2 was subjected to a direct current corona charging treatment, and then an aqueous solution containing 5 ppm of sodium acetate (pKa = 4.5) was sprayed. After drying, the particle collection efficiency and pressure loss were measured, and the QF value was calculated. The results are shown in Table 1.
[0029]
(Comparative Example 1)
The same melt blown nonwoven fabric as in Examples 1 and 2 was subjected to a direct current corona charging treatment, and then a base water injection treatment to which nothing was added was performed. After drying, the particle collection efficiency and pressure loss were measured, and the QF value was calculated. The results are shown in Table 1.
[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, and then an aqueous solution containing 5 ppm of citric acid (pKa = 2.8) was subjected to an injection treatment. After drying, the particle collection efficiency and pressure loss were measured, and the QF value was calculated. The results are shown in Table 1.
[0031]
(Comparative Example 3)
The same melt blown nonwoven fabric as in Examples 1 and 2 was subjected to a direct current corona charging treatment, and then an aqueous solution containing an ammonia (pKa = 9.2) concentration of 200,000 ppm was subjected to an injection treatment. After drying, the particle collection efficiency and pressure loss were measured, and the QF value was calculated. The results are shown in Table 1.
[0032]
(Comparative Example 4)
The same melt blown nonwoven fabric as in Examples 1 and 2 was subjected to a direct current corona charging treatment, and then an aqueous solution containing 0.5 ppm of ammonia (pKa = 9.2). After drying, the particle collection efficiency and pressure loss were measured, and the QF value was calculated. The results are shown in Table 1.
[0033]
(Comparative Example 5)
The same melt blown nonwoven fabric as in Examples 1 and 2 was subjected to a direct current corona charging treatment, and then an aqueous solution containing 20% by weight of isopropyl alcohol was jetted. After drying, the particle collection efficiency and pressure loss were measured, and the QF value was calculated. The results are shown in Table 1.
[0034]
(Comparative Example 6)
The same melt blown nonwoven fabric as in Examples 1 and 2 was subjected to DC corona charging treatment, and then the particle collection efficiency and pressure loss were measured, and the QF value was calculated. The results are shown in Table 1.
[0035]
[Table 1]
As is clear from Table 1, all the examples showed 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 concentration and low concentration aqueous ammonia solutions were injected, The particle collection efficiency and QF are equal to or lower than those of the direct current corona charge treatment, and are not highly electretized. In addition, it was found that when an aqueous solution containing a large amount of isopropyl alcohol having a high affinity with polypropylene was used (Comparative Example 5), the electret property was significantly lowered.
[0037]
【The invention's effect】
ADVANTAGE OF THE INVENTION According to this invention, the electret filter medium which can fully and highly electretize to the inside of a porous dielectric sheet | seat, and has high particle | grain collection efficiency, while low-pressure loss can be obtained.
Claims (5)
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| PT2222908E (en) | 2007-12-06 | 2013-03-28 | 3M Innovative Properties Co | Electret webs with charge-enhancing additives |
| KR101577089B1 (en) | 2008-06-02 | 2015-12-11 | 쓰리엠 이노베이티브 프로퍼티즈 컴파니 | Electret webs with charge-enhancing additives |
| US7765698B2 (en) * | 2008-06-02 | 2010-08-03 | 3M Innovative Properties Company | Method of making electret articles based on zeta potential |
| JP5088453B2 (en) * | 2009-12-10 | 2012-12-05 | 日立化成工業株式会社 | CMP polishing liquid, substrate polishing method, and electronic component |
| JP2011142284A (en) * | 2009-12-10 | 2011-07-21 | Hitachi Chem Co Ltd | Cmp polishing liquid, method of polishing substrate, and electronic component |
| JP6034022B2 (en) * | 2011-12-27 | 2016-11-30 | 旭化成株式会社 | Nonwoven laminate |
| JP6565166B2 (en) * | 2014-10-30 | 2019-08-28 | 東洋紡株式会社 | Manufacturing method of electret filter media |
| 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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