JP2004308038A

JP2004308038A - Low-density wet type nonwoven fabric

Info

Publication number: JP2004308038A
Application number: JP2003100964A
Authority: JP
Inventors: Tomohiro Hayakawa; 友浩早川; Hisashi Nagi; 比佐志凪; Tomoyasu Sonedaka; 友康曽根高; Hideki Kamata; 英樹鎌田
Original assignee: Kuraray Co Ltd
Current assignee: Kuraray Co Ltd
Priority date: 2003-04-04
Filing date: 2003-04-04
Publication date: 2004-11-04

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a wet-type nonwoven fabric having excellent filling properties of a resin, etc., and high paper strength in spite of low density. <P>SOLUTION: The low-density wet-type nonwoven fabric has a fiber cross section of flat shape and comprises 3-40 mass% of a polyvinyl alcohol-based binder fiber in which A/B is ≥3 when the length of a long side is A and the thickness at the central part (1/2A) of the long side is B. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

【０００１】
【発明の属する技術分野】
本発明は、熱風乾燥方式のような高速乾燥や、マルチシリンダー方式のような低温乾燥等の非加圧方式の乾燥条件下でも高い強力を有し、しかも樹脂等の充填性が良好な低密度湿式不織布に関する。
【０００２】
【従来の技術】
低密度で空隙率の高い湿式不織布を製造する方法として、熱融着繊維による熱風乾燥方式での検討が行われているが、資源リサイクルの観点から再利用が困難な熱融着繊維の利用は敬遠されてきている。再利用の観点から有効なバインダーとして、熱水処理で容易にリサイクル可能なポリビニルアルコール（以下、ＰＶＡと略記する）系繊維は、その水溶性と強い接着性を有するという特徴を生かして、製紙用あるいは不織布用バインダー繊維として用いられている。ＰＶＡ系バインダー繊維の十分な接着性は、抄紙工程において繊維が分散した水中で膨潤し、乾燥工程の熱により十分溶解し、乾燥しながら結晶化することにより達成される。従来、ＰＶＡ系バインダー繊維を用いて紙または不織布を製造する場合、乾燥工程において熱ドラム方式のヤンキードライヤーが一般的に使用されている。ヤンキードライヤーは乾燥熱量が大きいため乾燥時にＰＶＡ系バインダー繊維が十分に溶解し、接着性を発現する。
【０００３】
しかし、用途によっては二次加工として樹脂等を含浸したり、あるいはソフト感を出すために嵩高さが要求される場合がある。このような場合にはヤンキードライヤー方式の乾燥を行った場合、乾燥時に加圧されるため、密度が高くなってしまい、嵩高さが発現しないという問題があった。加圧せず、しかも密度を上げず乾燥する方式として、エアースルードライヤー等が用いられるケースが増加してきているが、エアースルードライヤーを用いて乾燥した場合、エアースルードライヤーは乾燥時間が短く、また乾燥熱量が小さいため、従来のＰＶＡ系バインダー繊維では十分に溶解できず、その結果、十分な接着性を発現できないという問題があった。
上記問題点を解決するために、ＰＶＡ系樹脂のケン化度を低下させ、かつＰＶＡ系樹脂の重合度を低下させ溶解性を高める手法が提案されている（例えば、特許文献１、２参照。）。また低ケン化度のＰＶＡ系樹脂にカルボキシル基やスルホン酸基、シリル基、四級アンモニウム塩等のカチオン性基などイオン性の官能基を導入することにより、溶解性を向上させる手法がとられている（例えば、特許文献３、４、５、６参照。）。このように、バインダー繊維の接着性向上を達成するために、ＰＶＡ系樹脂の改質を中心とした検討がなされているが、実際のエアースルードライヤー等の加圧しない乾燥方式での紙力は不十分であり、目標とする低密度でありながら紙力のある紙または不織布は得られなかった。
【０００４】
【特許文献１】
特開昭５１−９６５３３号公報
【特許文献２】
特開昭５１−９６５３４号公報
【特許文献３】
特開昭６０−２３１８１６号公報
【特許文献４】
特開平４−１２６８１８号公報
【特許文献５】
特開昭５８−２２０８０６号公報
【特許文献６】
特開２００３−２７３２８号公報
【０００５】
【発明が解決しようとする課題】
本発明者らは上記問題点を解決すべく鋭意検討を重ねた結果、扁平断面形状のＰＶＡ系バインダー繊維を紙または不織布に使用して、バインダー繊維の表面積を向上させて接着面積を増加させることにより、熱風乾燥方式のような高速乾燥や、マルチシリンダー方式のような低温乾燥等の非加圧方式の乾燥条件下においても、低密度であるにもかかわらず高い紙力を発現させることが可能であることを見出した。
【０００６】
【課題を解決するための手段】
すなわち本発明は、繊維断面が扁平形状をしており、長辺の長さをＡ、長辺の中央部（１／２Ａ）の厚さをＢとしたときに、Ａ／Ｂ≧３であるＰＶＡ系バインダー繊維を３〜４０質量％含有してなる低密度湿式不織布であり、好ましくは湿式不織布の密度が厚み測定時荷重１２ｇ／ｃｍ^２の条件下で０．１５ｇ／ｃｍ^３以下、乾比引張強力が０．４Ｎ・ｍ／ｇ以上である上記の低密度湿式不織布に関するものである。
【０００７】
【発明の実施の形態】
ＰＶＡ系バインダー繊維の十分な接着性は、抄紙工程において繊維が分散した水中で膨潤し、乾燥工程の熱により十分溶解し、乾燥しながら結晶化することにより達成される。しかしながら、従来のＰＶＡ系バインダー繊維では加圧状態での乾燥では十分な紙力が得られるが、樹脂等を含浸して保持体として使用する場合やソフトさを出すために嵩高さが必要な非加圧方式の乾燥条件では溶解性が不足し十分な接着性を得ることは困難であった。従来の技術では溶解性の向上、すなわちその指標である結晶融解温度を低下させるために、前記したようにケン化度の低下や変性基導入による結晶サイズ低下を利用したのに対し、本発明で使用するＰＶＡ系バインダー繊維は、繊維を扁平形状とすることにより溶解性を大幅に向上させ、接着面積を増大することで紙力の向上を達成し、目標とする低密度湿式不織布を得ている。
【０００８】
具体的には本発明で使用するＰＶＡ系バインダー繊維は、扁平形状で扁平形状の長辺の長さをＡ、その長辺の中央部（１／２Ａ）の厚さをＢとした時にＡ／Ｂ≧３であるＰＶＡ系バインダー繊維の含有率が３〜４０質量％である低密度湿式不織布である。ここで、Ａ／Ｂ＜３の場合、乾燥時の溶解性が低下し、また主体繊維との接着面積が狭くなるため高い紙力を得ることができない。好ましくは５≦Ａ／Ｂ≦１５である。
【０００９】
本発明の湿式不織布中のＰＶＡ系バインダー繊維は全固形分に対して３〜４０質量％含有していることが必要である。湿式不織布中のＰＶＡ系バインダー繊維の含有量が３質量％よりも少ないと、繊維の構成本数が少ないためバインダーとして機能せず、したがって接着性が発現しなくなる。一方、湿式不織布中のＰＶＡ系バインダー繊維の含有量が４０質量％より多いと、バインダーが主体となるため、バインダーの収縮による湿式不織布の表面平滑性の低下や、不織布が硬くなる等の品位の低下を招く恐れがある。好ましくは５〜３０質量％、より好ましくは１０〜２５質量％である。
【００１０】
本発明のＰＶＡ系バインダー繊維に用いるＰＶＡ系樹脂の種類には特に制限はなく、例えば、低ケン化度ＰＶＡやカルボキシル基、スルホン酸基、エチレン基、シラン基、シラノール基、アミン基、アンモニウム基等を共重合しても何ら構わない。ただし３０℃水中における繊維の溶出量が２０％以下であることが好ましい。溶出量が２０％を超えると、歩留まり悪化によるコストアップ、白水（抄紙中に使用する水）への溶出による排水負荷の上昇や、紙にしたとき、溶出したＰＶＡの再付着により紙が硬くなる等の紙品位の低下が生じる場合がある。溶出量は好ましくは１０％以下、より好ましくは５％以下である。
【００１１】
本発明のＰＶＡ系バインダー繊維に用いるＰＶＡ系樹脂の重合度については、溶出量の面からは３００以上、一方、生産性、コストの面からは３０００以下であることが好ましい。より好ましくは８００〜２０００である。またＰＶＡのケン化度についてはＰＶＡの溶出の面から９５モル％以上であることが好ましい。ＰＶＡのケン化度が９５モル％よりも低いと、バインダー使用時のＰＶＡの溶出が著しく、歩留まりの低下および排水への溶出等の問題が発生したり、またバインダーとして使用された後も耐水性が極めて低く、特に湿潤条件でバインダー性能が著しく低下する。より好ましくはケン化度９６〜９９．９モル％の範囲である。
【００１２】
本発明のＰＶＡ系バインダー繊維は、上記したＰＶＡ系樹脂を水に対して８〜１８質量％溶解させ、該ポリマーに対して固化能を有する塩類の水溶液からなる凝固浴中の吐出させ繊維状とした後、２〜５倍湿延伸を行い、乾燥することにより得られる。水に溶解するＰＶＡ樹脂の濃度が８質量％より低い場合、ノズルから凝固浴中へ押出した際固化が遅くなり、単繊維同士が膠着して分散性が悪化し、そのため生産性が低下しコストアップとなる。一方、水に溶解するＰＶＡ系樹脂の濃度が１８質量％より高い場合、ＰＶＡポリマーが溶解した溶液の粘度が高くなり、紡糸が不可能となる。好ましくは１０〜１６質量％である。
【００１３】
固化能を有する塩類の水溶液としては、硫酸ナトリウム（芒硝）、硫酸アンモニウム、炭酸ナトリウム等が挙げられる。これら固化能を有する塩類の水溶液からなる凝固浴中に吐出させ繊維状とした後に湿延伸を行うが、湿延伸倍率が２倍より小さいと正常な紡糸が行えず、一方湿延伸倍率が５倍を超える延伸を行うと、ＰＶＡ分子の配向が著しく進行するため結晶融解温度が上昇し、得られた繊維は水に対する膨潤度が低下し、バインダーとして機能しなくなる。
【００１４】
また本発明において、ＰＶＡ系バインダー繊維の繊維断面を扁平形状とする場合の紙力以外の利点として、抄紙工程中での単糸分繊性も向上する。扁平形状であると凝固浴中で表面積が広いため固化が速く、通常の円ノズルでの紡糸原綿よりも紙用途で使用する場合に問題となる単糸同士の膠着が低減し、分散性の良好な紙原料が得られる。
【００１５】
本発明で用いられるＰＶＡ系バインダー繊維の単繊維の平均繊度に特に制限はないが、０．１〜１０ｄｔｅｘのものが好適に用いられる。平均繊度が０．１ｄｔｅｘよりも細い場合は製造が困難となり生産性が低下し、コストアップが問題となる。平均繊度が１０ｄｔｅｘよりも太くなると、接着性が低下するようになる。より好ましくは０．３〜５．０ｄｔｅｘ、さらに好ましくは０．４〜３．０ｄｔｅｘの範囲である。
本発明の繊維はあらゆる形態で使用することができ、例えばカットファイバー、フィラメントヤーン、紡績糸としてもかまわない。
【００１６】
本発明のＰＶＡ系バインダー繊維の繊維長については１〜１０ｍｍの範囲であることが好ましい。バインダー繊維の繊維長が１ｍｍ未満となると、切断が困難となり、一方１０ｍｍを越えると湿式抄造時の分散性が悪く抄紙原紙の製造が困難となり使用できない。好ましくは２〜８ｍｍ、より好ましくは３〜６ｍｍである。
【００１７】
本発明で用いられる主体繊維としては低密度となる様に、水素結合等による自己接着性のない、あるいは自己接着性の低い繊維が好適に用いられ、例えばポリアミド系、ＰＶＡ系、ポリ塩化ビニリデン系、ポリ塩化ビニル系、ポリエステル系、ポリアクリル系、ポリオレフィン系の合成繊維、ガラス繊維、アスベスト、金属繊維、炭素繊維等の無機繊維、天然繊維よりも自己接着性の低いセルロース系等の半合成繊維、再生繊維並びにこれらの混合物を使用することが可能である。
【００１８】
本発明において湿式不織布を製造するにあたっては、通常の湿式抄紙法に準じて行えばよい。例えば前述のＰＶＡ系バインダー繊維を含有するスラリーを公知の手段により離解処理、あるいは主体となる繊維と離解処理し分繊後に抄造を行うが、該スラリーにおける繊維分濃度を約１〜１０質量％程度にし、さらにチェストにて０．５〜５質量％となるように希釈し、抄紙機により抄紙する。用いられる抄き網は円網や短網等でよく、乾燥機は多筒タイプやスルードライヤータイプを使用した抄紙工程の乾燥温度９０〜２５０℃で行えばよい。勿論、加圧を低減し目標の密度とすることが可能ならばヤンキードライヤータイプの乾燥機を使用してもかまわない。
【００１９】
本発明で得られる湿式不織布はＪＩＳＰ８１１８試験法に準拠した方法にて、厚み測定時荷重１２ｇ／ｃｍ^２の条件下での測定した密度が０．１５ｇ／ｃｍ^３以下であることが好ましい。密度が０．１５ｇ／ｃｍ^３よりも大きい場合、嵩高さが発現せず、したがってソフト感が出ない。より好ましくは０．１ｇ／ｃｍ^３以下０．０３ｇ／ｃｍ^３以上である。
【００２０】
また本発明で得られる湿式不織布の乾比引張強力は０．４Ｎ・ｍ／ｇ以上であることが好ましい。乾比引張強力が０．４Ｎ・ｍ／ｇよりも低い場合、紙作製時や樹脂加工時の加工張力に耐えることができない。より好ましくは、１．０Ｎ・ｍ／ｇ以上１０Ｎ・ｍ／ｇ以下である。
【００２１】
本発明の繊維断面が扁平形状のＰＶＡ系バインダー繊維を３〜４０質量％含有させることにより、得られる湿式不織布は低密度にもかかわらず、高い紙力を発現させることが可能となる。
【００２２】
【実施例】
以下実施例により本発明を説明するが、本発明はこれら実施例により限定されるものではない。なお本発明において、ＰＶＡバインダー繊維の溶出量、膨潤度、断面形状、紙の密度、紙の乾紙力は以下の測定方法により測定されたものとする。
【００２３】
［ＰＶＡバインダー繊維のＰＶＡ溶出量％］
繊維中のＰＶＡ樹脂純分が１ｇとなるように換算量を秤量したのち、３０℃水１００ｍｌ中に浸漬し、液温を３０℃に保ったまま３０分静置する。静置後未溶解部分を除去した上澄み５０ｍｌを採取し、水蒸気浴上で蒸発乾固したのち、１０５℃の乾燥機中で４時間乾燥させ、乾燥後乾燥残分ａ（ｇ）を計量する。この乾燥残分ａにはＰＶＡと硫酸ナトリウム等の無機分が含まれるため、さらに５００〜８００℃でＰＶＡ成分が完全に消失するまで焼成する。焼成後、残分ｂ（ｇ）を測定し、下記式から溶出量を求めた。
溶出量（％）＝（ａ−ｂ）×２００
【００２４】
［ＰＶＡバインダー繊維の膨潤度％／ＰＶＡ］
繊維中のＰＶＡ樹脂純分が１ｇとなるように換算量を秤量したのち、３０℃水１００ｍｌ中に浸漬し、液温を３０℃に保ったまま３０分静置する。静置後繊維分を濾取して、３０００ｒｐｍの遠心脱水機で１０分間脱水し、脱水後の質量（Ｃ）を測定する。質量測定の終わったサンプルを１０５℃の熱風乾燥機中で４時間乾燥させた後、質量（Ｄ）を測定する。
膨潤度（％／ＰＶＡ）＝（Ｃ−Ｄ）／Ｄ×１００
【００２５】
［ＰＶＡバインダー繊維の断面形状］
走査型電子顕微鏡〔（株）日立製作所製〕により断面を撮影し、長辺（Ａ）と厚み（Ｂ）の比率Ａ／Ｂを算出した。
【００２６】
［紙の密度ｇ／ｃｍ^３］
厚み測定時の荷重を１２ｇ／ｃｍ^２とすること以外はＪＩＳＰ８１１８に準拠して測定し、算出した。
【００２７】
［紙の乾比引張強力Ｎ・ｍ／ｇ］
２３℃、５０％ＲＨ室内で２４時間調湿後、幅１５ｍｍ、長さ１７０ｍｍの試料を把持長さ１００ｍｍ、引張速度１００ｍｍ／分で測定した強力ＤＳ（Ｎ／ｍ）と試料の秤量Ｗ（ｇ／ｍ^２）により下記式にて算出する。
乾比引張強力（Ｎ・ｍ／ｇ）＝ＤＳ／Ｗ
【００２８】
［実施例１］
（１）平均重合度１７００、ケン化度９８．０モル％のＰＶＡ樹脂１４質量％の水溶液からなる紡糸原液を孔数４０００、縦３０μｍ×横１８０μｍの長方形のスリット型の紡糸口金より飽和硫酸ナトリウムからなる凝固浴中に吐出させ、第一ローラーで捲き取った後、４倍の湿延伸を行い、定長乾燥機中にて１２０℃、１０分間乾燥させ、繊度１．５ｄｔｅｘ、膨潤度１８２％、溶出量６．９％、断面形状Ａ／Ｂ＝６．３の扁平状ＰＶＡ繊維を得た。
（２）上記（１）の扁平状ＰＶＡ繊維を３ｍｍの繊維長にカットしたものをバインダー繊維として用い、繊維純分として２０質量部、主体繊維としてガラス繊維〔東レ（株）製、直径９μｍ、繊維長６ｍｍ〕を８０質量部混合し、均一に混合攪拌してスラリーを調製した。かかるスラリーを用いてＴＡＰＰＩ式抄紙機に供給して抄造した後、乾燥温度２１０℃のネット式エアースルードライヤーを用いて乾燥し、紙を製造した。結果を表１に示す。
【００２９】
［実施例２］
（１）実施例１と同じＰＶＡ樹脂を用いて実施例１と同様の条件にて繊維を製造し、繊度３．８ｄｔｅｘ、膨潤度１６２％、溶出量３．１％、断面形状Ａ／Ｂ＝１６．０の扁平状ＰＶＡ繊維を得た。
（２）上記（１）で得られた扁平状ＰＶＡ系繊維をバインダー繊維として用い、実施例１と同様の条件で紙を製造した。結果を表１に示す。
【００３０】
［実施例３］
紙の組成をバインダー繊維５質量部、主体繊維としてガラス繊維を９５質量部とする以外は実施例１と同様の条件にて紙を製造した。結果を表１に示す。
【００３１】
［実施例４］
（１）平均重合度１７００、ケン化度９８．０モル％、エチレン含有量５モル％のＰＶＡ樹脂を用いる以外は実施例１と同様の条件にて繊維を製造し、繊度１．５ｄｔｅｘ、膨潤度１５４％、溶出量２．３％、断面形状Ａ／Ｂ＝６．１の扁平状ＰＶＡ繊維を得た。
（２）上記（１）で得られた扁平状ＰＶＡ系繊維をバインダー繊維として用い、実施例１と同様の条件で紙を製造した。結果を表１に示す。
【００３２】
［実施例５］
紙に使用する主体繊維をポリエステル繊維〔（株）クラレ製「ＥＰ０４３×５」、０．５ｄｔｅｘ×５ｍｍ〕とし、抄紙時の乾燥温度を１７０℃にすること以外は実施例１と同様の条件にて紙を製造した。結果を表１に示す。
【００３３】
［実施例６］
紙に使用する主体繊維を炭素繊維〔東レ・デュポン（株）社製、０．６ｄｔｅｘ×６ｍｍ〕に変更する以外は実施例１と同様の条件にて紙を製造した。結果を表１に示す。
【００３４】
［比較例１］
（１）平均重合度１７００、ケン化度９９．９モル％のＰＶＡ樹脂が１４質量％の水溶液からなる紡糸原液を孔直径６０μｍ、孔数４０００の紡糸口金より飽和硫酸ナトリウムからなる凝固浴中に吐出させ、第一ローラーで捲き取った後、４倍の湿延伸を行い、定長乾燥機中にて１２０℃、１０分間乾燥させ、繊度１．０ｄｔｅｘ、膨潤度１４５％、溶出量１．０％、Ａ／Ｂ＝２．０の断面形状が繭型であるＰＶＡ繊維を得た。
（２）上記（１）で得られたＰＶＡ繊維をバインダー繊維として用い、実施例１と同様の条件にて紙を製造した。結果を表１に示す。
【００３５】
［比較例２］
（１）平均重合度１７００、ケン化度９８．０モル％のＰＶＡ樹脂１８質量％のＤＭＳＯ（ジメチルスルホキシド）溶液からなる紡糸原液を孔数２００００、孔径６０μｍの紡糸口金より吐出させ、３倍の湿延伸を行った後、定長乾燥機中にて１４０℃、１０分間乾燥させ、繊度２．２ｄｔｅｘ、膨潤度１７０％、溶出量３．３％、断面形状Ａ／Ｂ＝１．０の円型断面のＰＶＡ繊維を得た。
（２）上記（１）で得られた円型断面のＰＶＡ繊維をバインダー繊維として用い、実施例１と同様の条件にて紙を製造した。結果を表１に示す。
【００３６】
［比較例３］
紙の組成をバインダー繊維２質量部、ガラス繊維９８質量部とする以外は実施例１と同様の条件にて紙の製造を行った。結果を表１に示す。
【００３７】
【表１】

【００３８】
実施例１〜６で得られた紙は、低密度であるにもかかわらず、紙作製時や樹脂加工時の加工張力に耐えうる紙力のあるものであった。一方、比較例１、２で得られた紙はバインダー繊維の断面形状Ａ／Ｂ＜３であるため、バインダーが十分に溶解しておらず、紙力の弱いものであった。また比較例３はバインダー繊維の含有量が３質量％未満であるため紙力が弱く、紙作製時や樹脂加工時の加工張力に耐えうるものではなかった。
【００３９】
【発明の効果】
本発明の、繊維断面が扁平形状のＰＶＡ系バインダー繊維の含有率が３〜４０質量％である湿式不織布を用いることにより、加工性に問題のない紙力を有し、樹脂等の充填性が優れた低密度湿式不織布を得ることが可能となる。
【図面の簡単な説明】
【図１】扁平断面繊維の断面概略図[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention has high strength even under non-pressurized drying conditions such as high-speed drying such as hot air drying and low-temperature drying such as multi-cylinder drying, and has a low density with good resin filling property. The present invention relates to a wet nonwoven fabric.
[0002]
[Prior art]
As a method of manufacturing a wet nonwoven fabric having a low density and a high porosity, a hot-air drying method using hot-melt fibers has been studied, but the use of hot-melt fibers that are difficult to reuse from the viewpoint of resource recycling is It has been shunned. As an effective binder from the viewpoint of reuse, polyvinyl alcohol (hereinafter abbreviated as PVA) fibers, which can be easily recycled by hot water treatment, are characterized by their water-solubility and strong adhesiveness, and are used for papermaking. Alternatively, it is used as a binder fiber for nonwoven fabric. Sufficient adhesiveness of the PVA-based binder fiber is achieved by swelling in water in which the fiber is dispersed in the papermaking process, sufficiently dissolving by the heat of the drying process, and crystallizing while drying. Conventionally, when producing paper or nonwoven fabric using a PVA-based binder fiber, a Yankee dryer of a heat drum type is generally used in a drying step. Since the Yankee dryer has a large amount of drying heat, the PVA-based binder fibers are sufficiently dissolved during drying, and exhibit adhesiveness.
[0003]
However, depending on the application, bulkiness may be required in order to impregnate resin or the like as secondary processing, or to give a soft feeling. In such a case, when the drying by the Yankee dryer method is performed, since the pressure is applied during the drying, the density is increased, and there is a problem that the bulkiness is not developed. As a method of drying without increasing pressure and increasing the density, cases where an air-through dryer is used are increasing, but when drying with an air-through dryer, the drying time of the air-through dryer is short, and Since the amount of drying heat is small, conventional PVA-based binder fibers cannot be sufficiently dissolved, and as a result, there has been a problem that sufficient adhesiveness cannot be exhibited.
In order to solve the above problems, there has been proposed a method of reducing the degree of saponification of the PVA-based resin and reducing the degree of polymerization of the PVA-based resin to enhance the solubility (for example, see Patent Documents 1 and 2). ). In addition, a method of improving solubility by introducing an ionic functional group such as a carboxyl group, a sulfonic acid group, a silyl group, or a cationic group such as a quaternary ammonium salt into a PVA-based resin having a low saponification degree has been adopted. (For example, see Patent Documents 3, 4, 5, and 6). As described above, in order to achieve an improvement in the adhesiveness of the binder fiber, studies have been made mainly on the modification of the PVA-based resin, but the paper strength in a non-pressurized drying method such as an actual air-through drier is considered. Insufficient, low-strength papers or nonwovens with the desired low density were not obtained.
[0004]
[Patent Document 1]
JP-A-51-96533 [Patent Document 2]
JP-A-51-96534 [Patent Document 3]
JP-A-60-231816 [Patent Document 4]
JP-A-4-126818 [Patent Document 5]
JP-A-58-220806 [Patent Document 6]
JP 2003-27328 A [0005]
[Problems to be solved by the invention]
The present inventors have made intensive studies to solve the above problems, and as a result, using a PVA-based binder fiber having a flat cross section for paper or nonwoven fabric, to increase the surface area of the binder fiber and increase the bonding area. Even under low-pressure drying conditions such as high-speed drying such as hot-air drying and low-temperature drying such as multi-cylinder drying, high paper strength can be achieved despite low density. Was found.
[0006]
[Means for Solving the Problems]
That is, in the present invention, A / B ≧ 3 when the fiber cross section has a flat shape and the length of the long side is A, and the thickness of the central portion (1 / 2A) of the long side is B. a low density wet-laid nonwoven fabric for a PVA-based binder fiber comprising 3 to 40 wt%, preferably wet-laid nonwoven fabric having a density of under the conditions of a thickness measured under load 12g / cm ² 0.15g / cm ³ or less, dry ratio The present invention relates to the low-density wet nonwoven fabric having a tensile strength of 0.4 Nm / g or more.
[0007]
BEST MODE FOR CARRYING OUT THE INVENTION
Sufficient adhesiveness of the PVA-based binder fiber is achieved by swelling in water in which the fiber is dispersed in the papermaking process, sufficiently dissolving by the heat of the drying process, and crystallizing while drying. However, although the conventional PVA-based binder fiber can obtain sufficient paper strength by drying under pressure, it can be used as a holding body by impregnating with a resin or the like, or a non-bulk material that requires bulkiness for softness. Under the drying conditions of the pressure method, the solubility was insufficient and it was difficult to obtain sufficient adhesiveness. In the prior art, in order to improve the solubility, that is, to lower the crystal melting temperature, which is an indicator thereof, a decrease in the degree of saponification and a decrease in the crystal size due to the introduction of a modifying group were used as described above. The PVA-based binder fiber used has a significantly improved solubility by forming the fiber into a flat shape, and achieves an improvement in paper strength by increasing the bonding area, thereby obtaining a target low-density wet nonwoven fabric. .
[0008]
Specifically, the PVA-based binder fiber used in the present invention has a flat shape and a flat shape having a long side length of A and a center portion (1 / 2A) of the long side having a thickness of B / A. It is a low-density wet nonwoven fabric in which the content of PVA-based binder fibers with B ≧ 3 is 3 to 40% by mass. Here, when A / B <3, high paper strength cannot be obtained because the solubility at the time of drying is reduced and the bonding area with the main fiber is small. Preferably, 5 ≦ A / B ≦ 15.
[0009]
It is necessary that the PVA-based binder fiber in the wet nonwoven fabric of the present invention contains 3 to 40% by mass based on the total solid content. When the content of the PVA-based binder fiber in the wet nonwoven fabric is less than 3% by mass, the number of fibers constituting the fiber is so small that it does not function as a binder, so that the adhesiveness is not exhibited. On the other hand, if the content of the PVA-based binder fiber in the wet nonwoven fabric is more than 40% by mass, the binder is mainly used, so that the surface smoothness of the wet nonwoven fabric decreases due to the shrinkage of the binder and the nonwoven fabric becomes harder. There is a risk of lowering. Preferably it is 5 to 30% by mass, more preferably 10 to 25% by mass.
[0010]
The type of PVA-based resin used for the PVA-based binder fiber of the present invention is not particularly limited, and examples thereof include a low saponification degree PVA, a carboxyl group, a sulfonic group, an ethylene group, a silane group, a silanol group, an amine group, and an ammonium group. And the like may be copolymerized. However, it is preferable that the elution amount of the fiber in the water at 30 ° C. is 20% or less. When the amount of elution exceeds 20%, the cost increases due to a decrease in yield, the drainage load increases due to elution into white water (water used in papermaking), and when the paper is formed, the paper becomes hard due to the re-adhesion of the eluted PVA. Etc., the paper quality may deteriorate. The elution amount is preferably 10% or less, more preferably 5% or less.
[0011]
The degree of polymerization of the PVA-based resin used for the PVA-based binder fiber of the present invention is preferably 300 or more from the viewpoint of elution amount, and 3000 or less from the viewpoint of productivity and cost. More preferably, it is 800 to 2000. The degree of saponification of PVA is preferably at least 95 mol% from the viewpoint of PVA elution. When the saponification degree of PVA is lower than 95 mol%, the elution of PVA at the time of use of the binder is remarkable, causing problems such as a decrease in yield and elution to drainage, and water resistance even after being used as a binder. Is extremely low, and particularly under wet conditions, the binder performance is significantly reduced. The saponification degree is more preferably in the range of 96 to 99.9 mol%.
[0012]
The PVA-based binder fiber of the present invention has a fibrous form in which the above-mentioned PVA-based resin is dissolved in water in an amount of 8 to 18% by mass and discharged into a coagulation bath comprising an aqueous solution of a salt capable of solidifying the polymer. After that, it is obtained by performing 2 to 5 times wet stretching and drying. When the concentration of the PVA resin dissolved in water is lower than 8% by mass, solidification becomes slow when extruded from a nozzle into a coagulation bath, and the single fibers adhere to each other to deteriorate dispersibility, thereby lowering productivity and cost. Be up. On the other hand, when the concentration of the PVA-based resin dissolved in water is higher than 18% by mass, the viscosity of the solution in which the PVA polymer is dissolved increases, and spinning becomes impossible. Preferably it is 10 to 16% by mass.
[0013]
Examples of the aqueous solution of a salt having a solidifying ability include sodium sulfate (Glauber's salt), ammonium sulfate, sodium carbonate, and the like. Wet drawing is performed after discharging into a coagulation bath composed of an aqueous solution of a salt having such a solidifying ability to form a fiber. If the wet drawing ratio is smaller than 2, normal spinning cannot be performed, while the wet drawing ratio is 5 times. When the stretching is performed in excess of the above, the crystal melting temperature rises because the orientation of the PVA molecules progresses remarkably, and the obtained fiber has a reduced degree of swelling in water and does not function as a binder.
[0014]
Further, in the present invention, as an advantage other than the paper strength when the fiber cross section of the PVA-based binder fiber has a flat shape, the single-fiber separation property in the paper making process is also improved. The flat shape has a large surface area in the coagulation bath, so it solidifies quickly, and reduces the sticking between single yarns, which is a problem when used in paper applications, compared to the raw cotton spun with a normal circular nozzle, and has good dispersibility. Raw material is obtained.
[0015]
The average fineness of the single fibers of the PVA-based binder fibers used in the present invention is not particularly limited, but those having 0.1 to 10 dtex are preferably used. When the average fineness is smaller than 0.1 dtex, the production becomes difficult, the productivity is reduced, and the cost is increased. When the average fineness is larger than 10 dtex, the adhesiveness is reduced. The range is more preferably 0.3 to 5.0 dtex, and even more preferably 0.4 to 3.0 dtex.
The fibers of the invention can be used in any form, for example cut fibers, filament yarns, spun yarns.
[0016]
The fiber length of the PVA-based binder fiber of the present invention is preferably in the range of 1 to 10 mm. If the fiber length of the binder fiber is less than 1 mm, cutting becomes difficult. On the other hand, if it exceeds 10 mm, the dispersibility during wet papermaking is poor, and the production of papermaking base paper becomes difficult and cannot be used. Preferably it is 2-8 mm, more preferably 3-6 mm.
[0017]
As the main fiber used in the present invention, a fiber having no self-adhesion due to hydrogen bonding or the like or a low self-adhesion is preferably used so as to have a low density. For example, polyamide-based, PVA-based, polyvinylidene chloride-based fibers are used. , Polyvinyl chloride-based, polyester-based, polyacryl-based, polyolefin-based synthetic fibers, inorganic fibers such as glass fibers, asbestos, metal fibers, and carbon fibers; and semi-synthetic fibers such as cellulose-based fibers having lower self-adhesiveness than natural fibers. It is possible to use recycled fibers as well as mixtures thereof.
[0018]
In producing a wet nonwoven fabric in the present invention, a conventional wet papermaking method may be employed. For example, the slurry containing the above-mentioned PVA-based binder fiber is defibrated by a known means, or defibrated with a main fiber, and then subjected to fiber separation, followed by papermaking. The fiber concentration in the slurry is about 1 to 10% by mass. Then, it is diluted with a chest so as to have a concentration of 0.5 to 5% by mass, and paper is made by a paper machine. The net used may be a circular net or a short net, and the dryer may be a drying machine at a drying temperature of 90 to 250 ° C. in a paper making process using a multi-cylinder type or a through dryer type. Of course, a Yankee dryer type dryer may be used if the pressure can be reduced to achieve the target density.
[0019]
The wet nonwoven fabric obtained by the present invention preferably has a density of 0.15 g / cm ³ or less measured under a load of 12 g / cm ² at the time of thickness measurement by a method based on JIS P8118 test method. When the density is larger than 0.15 g / cm ³ , no bulkiness is exhibited, and therefore a soft feeling is not obtained. More preferably, it is not more than 0.1 g / cm ^{3 and not} less than 0.03 g / cm ³ .
[0020]
The dry specific tensile strength of the wet nonwoven fabric obtained in the present invention is preferably 0.4 N · m / g or more. If the dry specific tensile strength is lower than 0.4 N · m / g, it cannot withstand the processing tension during paper production or resin processing. More preferably, it is 1.0 N · m / g or more and 10 N · m / g or less.
[0021]
When the fiber cross section of the present invention contains 3 to 40% by mass of the PVA-based binder fiber having a flat cross section, the obtained wet nonwoven fabric can exhibit high paper strength despite its low density.
[0022]
【Example】
Hereinafter, the present invention will be described with reference to examples, but the present invention is not limited to these examples. In the present invention, the amount of PVA binder fiber eluted, the degree of swelling, the cross-sectional shape, the density of the paper, and the dry paper strength of the paper are measured by the following measurement methods.
[0023]
[PVA elution amount of PVA binder fiber%]
After weighing the converted amount so that the PVA resin pure content in the fiber becomes 1 g, the fiber is immersed in 100 ml of water at 30 ° C., and left standing for 30 minutes while keeping the liquid temperature at 30 ° C. After standing, 50 ml of the supernatant from which the undissolved portion has been removed is collected, evaporated to dryness on a steam bath, dried in a dryer at 105 ° C. for 4 hours, and the dried residue a (g) is weighed after drying. Since the dried residue a contains PVA and inorganic components such as sodium sulfate, it is further fired at 500 to 800 ° C. until the PVA component is completely eliminated. After baking, the residue b (g) was measured, and the amount of elution was determined from the following equation.
Elution amount (%) = (ab) × 200
[0024]
[Swelling degree of PVA binder fiber% / PVA]
After weighing the converted amount so that the PVA resin pure content in the fiber becomes 1 g, the fiber is immersed in 100 ml of water at 30 ° C., and left standing for 30 minutes while keeping the liquid temperature at 30 ° C. After standing, the fiber content is collected by filtration, dehydrated by a centrifugal dehydrator at 3000 rpm for 10 minutes, and the mass after dehydration (C) is measured. After the sample whose mass has been measured is dried in a hot-air dryer at 105 ° C. for 4 hours, the mass (D) is measured.
Swelling degree (% / PVA) = (C−D) / D × 100
[0025]
[Cross-sectional shape of PVA binder fiber]
The cross section was photographed with a scanning electron microscope (manufactured by Hitachi, Ltd.), and the ratio A / B of the long side (A) to the thickness (B) was calculated.
[0026]
[Paper density g / cm ³ ]
The thickness was measured and calculated in accordance with JIS P8118 except that the load at the time of thickness measurement was set to 12 g / cm ² .
[0027]
[Dry specific tensile strength of paper Nm / g]
After conditioning for 24 hours in a room at 23 ° C. and 50% RH, a sample having a width of 15 mm and a length of 170 mm was measured at a gripping length of 100 mm and a pulling speed of 100 mm / min. / M ² ) is calculated by the following equation.
Dry specific tensile strength (Nm / g) = DS / W
[0028]
[Example 1]
(1) Saturated sodium sulfate was fed from a spinning dope comprising an aqueous solution of 14% by mass of a PVA resin having an average degree of polymerization of 1700 and a saponification degree of 98.0 mol% from a rectangular slit type spinneret having 4,000 holes and a length of 30 μm × 180 μm. And then wound up with a first roller, and then stretched by a factor of 4 and dried in a constant-length drier at 120 ° C. for 10 minutes, with a fineness of 1.5 dtex and a swelling degree of 182%. Thus, a flat PVA fiber having an elution amount of 6.9% and a cross-sectional shape A / B of 6.3 was obtained.
(2) A flat PVA fiber cut into a fiber length of 3 mm, which is cut into a fiber length of 3 mm, as a binder fiber, 20 parts by mass as a pure fiber, and a glass fiber as a main fiber [manufactured by Toray Industries, Inc., 9 μm in diameter, [Fiber length: 6 mm] was mixed in an amount of 80 parts by mass and uniformly mixed and stirred to prepare a slurry. The slurry was supplied to a TAPPI paper machine to form a paper, and then dried using a net-type air-through dryer at a drying temperature of 210 ° C. to produce paper. Table 1 shows the results.
[0029]
[Example 2]
(1) A fiber was produced using the same PVA resin as in Example 1 under the same conditions as in Example 1, and had a fineness of 3.8 dtex, a swelling degree of 162%, an elution amount of 3.1%, and a cross-sectional shape A / B = 16.0 flat PVA fibers were obtained.
(2) Paper was produced under the same conditions as in Example 1 using the flat PVA-based fibers obtained in (1) above as binder fibers. Table 1 shows the results.
[0030]
[Example 3]
A paper was produced under the same conditions as in Example 1 except that the composition of the paper was 5 parts by mass of the binder fiber and 95 parts by mass of the glass fiber as the main fiber. Table 1 shows the results.
[0031]
[Example 4]
(1) A fiber was produced under the same conditions as in Example 1 except that a PVA resin having an average degree of polymerization of 1700, a degree of saponification of 98.0 mol%, and an ethylene content of 5 mol% was used, a fineness of 1.5 dtex, and swelling. A flat PVA fiber having a degree of 154%, a dissolution amount of 2.3%, and a cross-sectional shape A / B of 6.1 was obtained.
(2) Paper was produced under the same conditions as in Example 1 using the flat PVA-based fibers obtained in (1) above as binder fibers. Table 1 shows the results.
[0032]
[Example 5]
The same conditions as in Example 1 were adopted except that the main fiber used for the paper was polyester fiber (“EP043 × 5”, 0.5 dtex × 5 mm, manufactured by Kuraray Co., Ltd.), and the drying temperature during papermaking was 170 ° C. Made paper. Table 1 shows the results.
[0033]
[Example 6]
Paper was manufactured under the same conditions as in Example 1 except that the main fiber used for the paper was changed to carbon fiber (0.6 dtex × 6 mm, manufactured by Du Pont-Toray Co., Ltd.). Table 1 shows the results.
[0034]
[Comparative Example 1]
(1) An undiluted spinning solution composed of an aqueous solution of 14% by mass of a PVA resin having an average degree of polymerization of 1700 and a saponification degree of 99.9 mol% was placed in a coagulation bath composed of saturated sodium sulfate from a spinneret having a pore diameter of 60 μm and a pore number of 4000. After discharging and winding up with the first roller, the paper is stretched by a factor of 4 and dried in a constant-length dryer at 120 ° C. for 10 minutes. The fineness is 1.0 dtex, the swelling degree is 145%, and the elution amount is 1.0. %, A / B = 2.0, a cocoon-shaped PVA fiber was obtained.
(2) Paper was manufactured under the same conditions as in Example 1 using the PVA fiber obtained in (1) above as a binder fiber. Table 1 shows the results.
[0035]
[Comparative Example 2]
(1) A spinning solution comprising a DMSO (dimethylsulfoxide) solution containing 18% by mass of a PVA resin having an average degree of polymerization of 1700 and a saponification degree of 98.0 mol% is discharged from a spinneret having a number of holes of 20,000 and a diameter of 60 μm, and is tripled. After performing wet stretching, it is dried in a constant-length dryer at 140 ° C. for 10 minutes, and is a circle having a fineness of 2.2 dtex, a degree of swelling of 170%, an elution amount of 3.3%, and a cross-sectional shape A / B = 1.0. A PVA fiber having a mold section was obtained.
(2) A paper was manufactured under the same conditions as in Example 1 using the PVA fiber having a circular cross section obtained in (1) above as a binder fiber. Table 1 shows the results.
[0036]
[Comparative Example 3]
A paper was produced under the same conditions as in Example 1 except that the composition of the paper was changed to 2 parts by mass of binder fiber and 98 parts by mass of glass fiber. Table 1 shows the results.
[0037]
[Table 1]

[0038]
The papers obtained in Examples 1 to 6 had sufficient paper strength to withstand the processing tension during paper production and resin processing, despite their low density. On the other hand, in the papers obtained in Comparative Examples 1 and 2, the cross-sectional shape of the binder fiber was A / B <3, so that the binder was not sufficiently dissolved and the paper strength was weak. In Comparative Example 3, since the content of the binder fiber was less than 3% by mass, the paper strength was weak, and it was not able to withstand the processing tension during paper production or resin processing.
[0039]
【The invention's effect】
By using the wet nonwoven fabric of the present invention in which the content of the PVA-based binder fiber having a flat cross section of the fiber is 3 to 40% by mass, the paper has a paper strength with no problem in processability, and the filling property of resin and the like is improved. An excellent low-density wet nonwoven fabric can be obtained.
[Brief description of the drawings]
FIG. 1 is a schematic cross-sectional view of a flat cross-section fiber.

Claims

Polyvinyl alcohol-based binder fiber having A / B ≧ 3, where the fiber cross section has a flat shape and the length of the long side is A, and the thickness of the central part (1 / 2A) of the long side is B. Is a low-density wet nonwoven fabric containing 3 to 40% by mass.

0.15 g / cm ³ or less under the conditions density thickness measurement load at 12 g / cm ² of wet-laid nonwoven fabric, dry ratio tensile strength is 0.4 N · m / g or more claims 1 low-density wet-laid nonwoven fabric as claimed.