JP2004199783A - Magnetic recording medium - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a magnetic recording medium excellent in dispersibility, film coating smoothness, electromagnetic conversion property, durability and long-term preservability, in which an MR head erodes little and which has slight influence on environment when incinerated. <P>SOLUTION: The magnetic recording medium has an underlayer containing inorganic powder and/or magnetic powder and a bonding agent formed on a non-magnetic support and at least one layer of a magnetic layer in which ferromagnetic fine powder and a bonding agent are dispersed on the underlayer. The bonding agent of the underlayer is obtained by using the following acrylic copolymer together with polyurethane resin which is obtained by reacting polyol compound having a cyclic structure and long-chain alkyl chain with organic diisocyanate and has a hydrophilic polar group. The acrylic copolymer contains as raw material a radical polymerizable monomer having nitrogen by 45 to 80 mass % to the sum totals of a monomer and (meta) acrylate monomer having alkyl (meta) acrylate and/or a benzene ring and is obtained by copolymerizing them and has a hydrophilic polar group in a molecule. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

【００２９】
上記ポリオールと反応させてポリウレタン樹脂を形成するために用いられるポリイソシアネートとしては、特に制限はなく通常に使用されているものを用いることができる。例えば、ヘキサメチレンジイソシアネート、トリジンジイソシアネート、イソホロンジイソシアネート、１，３−キシリレンジイソシアネート、１，４−キシリレンジイソシアネートシクロヘキサンジイソシアネート、トルイジンジイソシアネート、２，４−トリレンジイソシアネート、２，６−トリレンジイソシアネート、４，４’−ジフェニルメタンジイソシアネート、ｐ−フェニレンジイソシアネート、ｍ−フェニレンジイソシアネート、１，５−ナフタレンジイソシアネート、３，３−ジメチルフェニレンジイソシアネート等を挙げることができる。
【００３０】
ポリウレタン樹脂は上記アクリル系樹脂と同様の極性基を有することが好ましい。ポリウレタン樹脂に極性基を導入するには、極性基を有するポリエステルポリオール、ポリエーテルポリオール等の極性基含有ポリオールと、極性基を有しないポリエステルポリオール、ポリエーテルポリオール等のポリオールとジイソシアネートとから製造することができる。例えば、２価アルコール或いは２塩基酸の一部を極性基含有ジオールあるいは極性基含有２塩基酸に変えて製造できる。極性基含有ポリオールは、以下のポリオールの主鎖または側鎖に上記一般式で表される極性基を有するものである。
【００３１】
また、鎖延長剤としては、それ自体公知の物質、多価アルコール、脂肪族ポリアミン、脂環式ポリアミン、芳香族ポリアミン等が使用できる。なかでも分子量５０〜５００の多価アルコールが好ましい。５０未満では塗膜が脆くなるので耐久性が低下する。５００より多いと塗膜のＴｇが低下し、軟らかくなるので耐久性が低下する。
【００３２】
多価アルコールとしてはビスフェノールＡ、水素化ビスフェノールＡ、ビスフェノールＳ、ビスフェノールＰ及びこれらのエチレンオキド、プロピレンキシド付加物、シクロヘキサンジメタノール、シクロヘキサンジオール、ハイドロキノン、ビス（２-ヒドロキシエチル）テトラブロモビスフェノールＡ、ビス（２-ヒドロキシエチル）テトラブロモビスフェノールＳ、ビス（２-ヒドロキシエチル）テトラメチルビスフェノールＳ、ビス（２-ヒドロキシエチル）ジフェニルビスフェノールＳ、ビス（２-ヒドロキシエチル）ジフェニルビフェノール、ビス（２-ヒドロキシエチル）チオジフェノール、ビス（２-ヒドロキシエチル）ビスフェノールＦ、ビフェノール、ビスフェノールフルオレン、ビスフェノールフルオレンジヒドロキシエチルエーテル等の環状構造を有する短鎖ジオールが好ましい。
ビスフェノールＡ、水素化ビスフェノールＡ、ビスフェノールＳ、ビスフェノールＰ及びこれらのエチレンオキド、プロピレンキシド付加物、シクロヘキサンジメタノール、シクロヘキサンジオール等の芳香族、脂環族を有するジオールがさらに好ましい。
【００３３】
本発明で使用される極性基含有ポリウレタン系樹脂の平均分子量は５，０００〜１００，０００が好ましい。更には１０，０００〜５０，０００好ましい。５，０００未満では得られる磁性塗膜が脆くなり得られる磁性塗膜が脆くなるなど物理的強度が低下し磁気記録媒体の耐久性に影響を与える。分子量が１００，０００を超えると溶剤への溶解性が低下し、分散性が低下する。また、所定濃度における塗料粘度が高くなって作業性が著しく悪くなり取扱が困難となる。
【００３４】
本発明で使用される極性基含有ポリウレタン系樹脂のＯＨ基としては、分岐ＯＨ基を有することが硬化性、耐久性の面から好ましく、１分子当たり２個〜４０個が好ましく、１分子当たり３個〜２０個がさらに好ましい。
【００３５】
本発明で使用されるその他結合剤として、前述の窒素を有するラジカル重合性モノマー、アルキル（メタ）アクリレート、ベンゼン環を有する（メタ）アクリレートモノマーからなり親水性極性基を有するアクリル系樹脂および極性基含有ポリウレタン樹脂とともに、さらにこれらの合計量の等量以下の量で、その他の極性基を持つ結合剤を併用しても良い。併用できるその他樹脂としては、特に塩素含有の樹脂以外は特に制限はなく、従来から結合剤として使用されている公知の熱可塑性樹脂、熱硬化性樹脂、反応型樹脂およびこれらの混合物を使用することができる。熱可塑性樹脂としてはガラス転移温度が−１００〜１５０℃、数平均分子量が１，０００〜２００，０００、好ましくは１０，０００〜１００，０００ものである。具体的には、アクリロニトリル、スチレン、ブタジエン、エチレン、ビニルブチラール、ビニルアセタール、ビニルエーテル等を構成単位として含む重合体、共重合体、各種ゴム系樹脂がある。また、熱硬化性樹脂または反応型樹脂としてはフェノール樹脂、フェノキシ樹脂、エポキシ樹脂、尿素樹脂、メラミン樹脂、アルキド樹脂、アクリル系反応樹脂、ホルムアルデヒド樹脂、シリコーン樹脂、エポキシ−ポリアミド樹脂、ポリエステル樹脂とイソシアネートプレポリマーの混合物等が挙げられる。
【００３６】
II．磁性体
［強磁性金属粉末］
本発明に使用される強磁性金属粉末としては、Ｆｅを主成分とするもの（合金も含む）であれば、特に限定されないが、α−Ｆｅを主成分とする強磁性合金粉末が好ましい。これらの強磁性粉末には所定の原子以外にＡｌ、Ｓｉ、Ｓ、Ｓｃ、Ｃａ、Ｔｉ、Ｖ、Ｃｒ、Ｃｕ、Ｙ、Ｍｏ、Ｒｈ、Ｐｄ、Ａｇ、Ｓｎ、Ｓｂ、Ｔｅ、Ｂａ、Ｔａ、Ｗ、Ｒｅ、Ａｕ、Ｈｇ、Ｐｂ、Ｂｉ、Ｌａ、Ｃｅ、Ｐｒ、Ｎｄ、Ｐ、Ｃｏ、Ｍｎ、Ｚｎ、Ｎｉ、Ｓｒ、Ｂなどの原子を含んでもかまわない。Ａｌ、Ｓｉ、Ｃａ、Ｙ、Ｂａ、Ｌａ、Ｎｄ、Ｃｏ、Ｎｉ、Ｂの少なくとも１つがα−Ｆｅ以外に含まれるものが好ましく、特に、Ｃｏ，Ａｌ，Ｙが含まれるのが好ましい。さらに具体的には、ＣｏがＦｅに対して１０〜４０原子％、Ａｌが２〜２０原子％、Ｙが１〜１５原子％含まれるのが好ましい。
【００３７】
これらの強磁性粉末にはあとで述べる分散剤、潤滑剤、界面活性剤、帯電防止剤などで分散前にあらかじめ処理を行ってもかまわない。また、強磁性金属粉末が少量の水、水酸化物または酸化物を含むものなどであってもよい。
【００３８】
強磁性粉末の含水率は０．０１〜２％とするのが好ましい。結合剤の種類によって強磁性粉末の含水率は最適化するのが好ましい。
結晶子サイズは８〜２０ｎｍ、好ましくは１０〜１８ｎｍ、特に好ましくは１２〜１６ｎｍである。
結晶子サイズは、Ｘ線回折装置（理学電機製 ＲＩＮＴ２０００シリーズ）を使用し、線源ＣｕＫα１、管電圧５０ｋＶ、管電流３００ｍＡの条件で回折ピークの半値幅からＳｃｈｅｒｒｅｒ法により求めた平均値を用いた。
【００３９】
強磁性金属粉末の長軸長は０．０１〜０．１０μｍであり、好ましくは０．０３〜０．０９μｍ、特に好ましくは０．０５〜０．０８μｍである。本発明の磁性層に使用される強磁性粉末のＢＥＴ法による比表面積（Ｓ_BET）は３０ｍ²／ｇ以上５０ｍ²／ｇ未満が好ましく、３８〜４８ｍ²／ｇがさらに好ましい。これにより、良好な表面性と低いノイズの両立が可能となる。
該長軸径は、透過型電子顕微鏡写真を撮影し、その写真から強磁性粉末の短軸径と長軸径とを直接読みとる方法と画像解析装置カールツァイス社製ＩＢＡＳＳＩで透過型電子顕微鏡写真トレースして読みとる方法を併用して求められる。
【００４０】
強磁性粉末のｐＨは用いる結合剤との組合せにより最適化することが好ましい。その範囲は４〜１２であるが、好ましくは７〜１０である。強磁性粉末は必要に応じ、Ａｌ、Ｓｉ、Ｐまたはこれらの酸化物などで表面処理を施してもかまわない。その量は強磁性粉末に対し０．１〜１０％であり表面処理を施すと脂肪酸などの潤滑剤の吸着が１００ｍｇ／ｍ²以下になり好ましい。強磁性粉末には可溶性のＮａ、Ｃａ、Ｆｅ、Ｎｉ、Ｓｒなどの無機イオンを含む場合があるが２００ｐｐｍ以下であれば特に特性に影響を与える事は少ない。また、本発明に用いられる強磁性粉末は空孔が少ないほうが好ましくその値は２０容量％以下、さらに好ましくは５容量％以下である。
【００４１】
また形状については先に示した粒子サイズについての特性を満足すれば針状、粒状、米粒状あるいは板状いずれでもかまわないが、特に針状の強磁性粉末を使用することが好ましい。針状強磁性金属粉末の場合、針状比は４以上１２以下が好ましく、更に好ましくは５以上１２以下である。
強磁性金属粉末の抗磁力Ｈｃは好ましくは２，０００〜３０，０００Ｏｅであり、更に好ましくは２，１００〜２９，０００Ｏｅで、飽和磁束密度は好ましくは１，５００〜３，０００Ｇであり、更に好ましくは１，６００〜２，９００Ｇである。σｓは、好ましくは１４０〜１７０ｅｍｕ／ｇ、更に好ましくは１４５〜１６０ｅｍｕ／ｇである。
【００４２】
［強磁性六方晶フェライト粉末］
強磁性六方晶フェライト粉末の板径は、１０〜２００ｎｍが好ましい。特にトラック密度を上げるため磁気抵抗ヘッドで再生する場合、低ノイズにする必要があり、板径は４０ｎｍ以下が好ましいが、１０ｎｍ未満では熱揺らぎのため安定な磁化が望めない。２００ｎｍより大きいとノイズが高く、いずれも高密度磁気記録には向かない。
板状比（板径／板厚）は１〜１５が望ましい。好ましくは１〜７である。板状比が小さいと磁性層中の充填性は高くなり好ましいが、十分な配向性が得られない。１５より大きいと粒子間のスタッキングによりノイズが大きくなる。この粒子サイズ範囲のＢＥＴ法による比表面積は１０〜２００ｍ²／ｇを示す。比表面積は概ね粒子板径と板厚からの算術計算値と符合する。
粒子板径・板厚の分布は通常狭いほど好ましい。数値化は困難であるが、粒子ＴＥＭ写真より５００粒子を無作為に測定する事で比較できる。分布は正規分布ではない場合が多いが、計算して平均サイズに対する標準偏差で表すとσ／平均サイズ＝０．１〜２．０である。粒子サイズ分布をシャープにするには粒子生成反応系をできるだけ均一にすると共に、生成した粒子に分布改良処理を施すことも行われている。たとえば酸溶液中で超微細粒子を選別的に溶解する方法等も知られている。
【００４３】
磁性体で測定される抗磁力Ｈｃは５００ Ｏｅ〜５，０００ Ｏｅ程度まで作成できる。Ｈｃは高い方が高密度記録に有利であるが、記録ヘッドの能力で制限される。本発明ではＨｃは２，０００Ｏｅ〜３，０００Ｏｅ程度であるが、好ましくは２，２００ Ｏｅ以上、２，８００ Ｏｅ以下である。ヘッドの飽和磁化が１．４テスラーを越える場合は、２，０００ Ｏｅ以上にすることが好ましい。Ｈｃは粒子サイズ（板径・板厚）、含有元素の種類と量、元素の置換サイト、粒子生成反応条件等により制御できる。
【００４４】
飽和磁化σｓは４０ｅｍｕ／ｇ〜８０ｅｍｕ／ｇである。σｓは高い方が好ましいが微粒子になるほど小さくなる傾向がある。σｓ改良のためマグネトプランバイトフェライトにスピネルフェライトを複合すること、含有元素の種類と添加量の選択等が良く知られている。またＷ型六方晶フェライトを用いることも可能である。
磁性体を分散する際に磁性体粒子表面を分散媒、ポリマーに合った物質で処理することも行われている。
【００４５】
表面処理材は無機化合物、有機化合物が使用される。主な化合物としてはＳｉ、Ａｌ、Ｐ、等の化合物、各種シランカップリング剤、各種チタンカップリング剤が代表例である。量は磁性体に対して０．１〜１０％である。磁性体のｐＨも分散に重要である。通常４〜１２程度で分散媒、ポリマーにより最適値があるが、媒体の化学的安定性、保存性から６〜１１程度が選択される。磁性体に含まれる水分も分散に影響する。分散媒、ポリマーにより最適値があるが通常０．０１〜２．０％が選ばれる。
【００４６】
六方晶フェライトの製法としては、
（１）酸化バリウム・酸化鉄・鉄を置換する金属酸化物とガラス形成物質として酸化ホウ素等を所望のフェライト組成になるように混合した後溶融し、急冷して非晶質体とし、次いで再加熱処理した後、洗浄・粉砕してバリウムフェライト結晶粉末を得るガラス結晶化法。
（２）バリウムフェライト組成金属塩溶液をアルカリで中和し、副生成物を除去した後１００℃以上で液相加熱した後洗浄・乾燥・粉砕してバリウムフェライト結晶粉末を得る水熱反応法。
（３）バリウムフェライト組成金属塩溶液をアルカリで中和し、副生成物を除去した後乾燥し１１００℃以下で処理し、粉砕してバリウムフェライト結晶粉末を得る共沈法等があるが、本発明は製法を選ばない。
【００４７】
III．非磁性粉末
本発明の磁気記録媒体は、非磁性支持体上に結合剤と非磁性粉末からなる下層を有する。
下層に使用できる非磁性粉末は無機物質でも有機物質でもよい。また、カーボンブラック等も使用できる。無機物質としては、例えば金属、金属酸化物、金属炭酸塩、金属硫酸塩、金属窒化物、金属炭化物、金属硫化物等が挙げられる。具体的には二酸化チタン等のチタン酸化物、酸化セリウム、酸化スズ、酸化タングステン、ＺｎＯ、ＺｒＯ₂、ＳｉＯ₂、Ｃｒ₂Ｏ₃、α化率９０〜１００％のα−アルミナ、β−アルミナ、γ−アルミナ、α−酸化鉄、ゲータイト、コランダム、窒化珪素、チタンカーバイト、酸化マグネシウム、窒化ホウ素、２硫化モリブデン、酸化銅、ＭｇＣＯ₃、ＣａＣＯ₃、ＢａＣＯ₃、ＳｒＣＯ₃、ＢａＳＯ₄、炭化珪素、炭化チタン等が単独あるいは２種類以上の組み合わせで使用される。好ましいのは、α−酸化鉄、酸化チタンである。
【００４８】
非磁性粉末の形状は針状、球状、多面体状、板状のいずれでも良い。非磁性粉末の結晶子サイズ は０．００４μｍ〜１μｍが好ましく、０．０４μｍ〜０．１μｍが更に好ましい。０．００４μｍより小さいと分散が困難となる傾向がある。また１μｍより大きいと表面粗さが大きくなる傾向がある。
これら非磁性粉末の平均粒径は０．００５〜２μｍが好ましく、０．０１〜０．２μｍが更に好ましい。必要に応じて平均粒径の異なる非磁性粉末を組合せたり、単独の非磁性粉末でも粒径分布を広くして同様の効果をもたせることもできる。０．００５μｍより小さいと分散が困難となる傾向があり、２μｍより大きいと表面粗さが大きくなる傾向がある。
【００４９】
非磁性粉末の比表面積は１〜１００ｍ²／ｇ、好ましくは５〜７０ｍ²／ｇ、更に好ましくは１０〜６５ｍ²／ｇである。１ｍ²／ｇより小さいと表面粗さが大きくなる傾向があり、１００ｍ²／ｇより大きいと所望の結合剤量で分散ができない等分散が困難となる傾向がある。
ジブチルフタレート（ＤＢＰ）を用いた吸油量は５〜１００ｍｌ／１００ｇ、好ましくは１０〜８０ｍｌ／１００ｇ、更に好ましくは２０〜６０ｍｌ／１００ｇである。比重は１〜１２、好ましくは３〜６である。
タップ密度は０．０５〜２ｇ／ｍｌ、好ましくは０．２〜１．５ｇ／ｍｌである。０．０５ｇ／ｍｌよりも小さいと飛散する粒子が多く操作が困難となる傾向がある。２ｇ／ｍｌよりも大きいと装置に固着し易くなり操作が困難となる傾向がある。
【００５０】
非磁性粉末のｐＨは２〜１１であることが好ましいが、ｐＨは６〜９が特に好ましい。ｐＨが２より小さいと高温、高湿下での摩擦係数が大きくなる傾向がある。またｐＨが１１より大きいと脂肪酸の遊離量が減少し、摩擦係数が大きくなる傾向がある。
非磁性粉末の含水率は０．１〜５質量％、好ましくは０．２〜３質量％、更に好ましくは０．３〜１．５質量％である。０．１質量％よりも小さいと分散が困難となる傾向がある。５質量％よりも大きいと分散後の塗料粘度が不安定となる傾向がある。
【００５１】
強熱減量は２０質量％以下であることが好ましく、強熱減量が小さいものが好ましい。
また、非磁性粉末が無機粉末である場合にはモース硬度は４以上、１０以下のものが好ましい。モース硬度が４より小さいと耐久性が確保できなくなる傾向がある。
非磁性粉末のステアリン酸吸着量は好ましくは１〜２０μｍｏｌ／ｍ²、更に好ましくは２〜１５μｍｏｌ／ｍ²である。
非磁性粉末の２５℃での水への湿潤熱は２００ｅｒｇ／ｃｍ²〜６００ｅｒｇ／ｃｍ²の範囲にあることが好ましい。また、この湿潤熱の範囲にある溶媒を使用することができる。１００〜４００℃での表面の水分子の量は１〜１０個／１００Åが適当である。水中での等電点のｐＨは３〜９の間にあることが好ましい。
【００５２】
これらの非磁性粉末の表面にはＡｌ₂Ｏ₃、ＳｉＯ₂、ＴｉＯ₂、ＺｒＯ₂、ＳｎＯ₂、Ｓｂ₂Ｏ₃、ＺｎＯで表面処理することが好ましい。特に分散性に好ましいのはＡｌ₂Ｏ₃、ＳｉＯ₂、ＴｉＯ₂、ＺｒＯ₂、であるが、更に好ましいのはＡｌ₂Ｏ₃、ＳｉＯ₂、ＺｒＯ₂である。これらは組み合わせて使用しても良いし、単独で用いることもできる。また、目的に応じて共沈させた表面処理層を用いても良いし、先ずアルミナで処理した後にその表層をシリカで処理する方法、またはその逆の方法を採ることもできる。また、表面処理層は目的に応じて多孔質層にしても構わないが、均質で密である方が一般には好ましい。
【００５３】
本発明の下層に用いられる非磁性粉末の具体的な例としては、昭和電工製ナノタイト、住友化学製ＨＩＴ−１００，ＺＡ−Ｇ１、戸田工業社製ＤＰＮ−２５０、ＤＰＮ−２５０ＢＸ、ＤＰＮ−２４５、ＤＰＮ−２７０ＢＸ、ＤＰＢ−５５０ＢＸ、ＤＰＮ−５５０ＲＸ 石原産業製酸化チタンＴＴＯ−５１Ｂ、ＴＴＯ−５５Ａ，ＴＴＯ−５５Ｂ、ＴＴＯ−５５Ｃ、ＴＴＯ−５５Ｓ、ＴＴＯ−５５Ｄ、ＳＮ−１００，ＭＪ−７、α−酸化鉄Ｅ２７０、Ｅ２７１、Ｅ３００、チタン工業製ＳＴＴ−４Ｄ、ＳＴＴ−３０Ｄ、ＳＴＴ−３０、ＳＴＴ−６５Ｃ、テイカ製ＭＴ−１００Ｓ、ＭＴ−１００Ｔ、ＭＴ−１５０Ｗ、ＭＴ−５００Ｂ、ＭＴ−６００Ｂ、ＭＴ−１００Ｆ、ＭＴ−５００ＨＤ。堺化学製ＦＩＮＥＸ−２５、ＢＦ−１、ＢＦ−１０、ＢＦ−２０、ＳＴ−Ｍ、同和鉱業製ＤＥＦＩＣ−Ｙ、ＤＥＦＩＣ−Ｒ、日本アエロジル製ＡＳ２ＢＭ、ＴｉＯ２Ｐ２５、宇部興産製１００Ａ、５００Ａ、チタン工業製Ｙ−ＬＯＰ及びそれを焼成したものが挙げられる。特に好ましい非磁性粉末は二酸化チタンとα−酸化鉄である。
【００５４】
下層には、非磁性粉末と共にカーボンブラックを混合することで表面電気抵抗（Ｒｓ）を下げることができ、光透過率を小さくすることができるとともに所望のマイクロビッカース硬度を得る事ができる。
下層のマイクロビッカース硬度は通常、２５〜６０ｋｇ／ｍｍ²、好ましくはヘッドあたりを調整するために、３０〜５０ｋｇ／ｍｍ²である。マイクロビッカース硬度は薄膜硬度計（日本電気製 ＨＭＡ−４００）を用いて、稜角８０度、先端半径０．１μｍのダイヤモンド製三角錐針を圧子先端に用いて測定することができる。
光透過率は一般に波長９００ｎｍ程度の赤外線の吸収が３％以下、たとえばＶＨＳ用磁気テープでは０．８％以下であることが規格化されている。このためにはゴム用ファーネス、ゴム用サーマル、カラー用ブラック、アセチレンブラック等を用いることができる。
【００５５】
本発明の下層に用いられるカーボンブラックの比表面積は１００〜５００ｍ²／ｇ、好ましくは１５０〜４００ｍ²／ｇ、ＤＢＰ吸油量は２０〜４００ｍｌ／１００ｇ、好ましくは３０〜２００ｍｌ／１００ｇである。カーボンブラックの粒子径は５〜８０ｎｍ、好ましく１０〜５０ｎｍ、さらに好ましくは１０〜４０ｎｍである。カーボンブラックのｐＨは２〜１０、含水率は０．１〜１０％、タップ密度は０．１〜１ｇ／ｍｌが好ましい。
本発明に用いられるカーボンブラックの具体的な例としてはキャボット社製、ＢＬＡＣＫＰＥＡＲＬＳ ２０００、１３００、１０００、９００、８００、８８０、７００、ＶＵＬＣＡＮ ＸＣ−７２、三菱化成工業社製、＃３０５０Ｂ、３１５０Ｂ、３２５０Ｂ、＃３７５０Ｂ、＃３９５０Ｂ、＃９５０、＃６５０Ｂ，＃９７０Ｂ、＃８５０Ｂ、ＭＡ−６００、コロンビアカーボン社製、ＣＯＮＤＵＣＴＥＸ ＳＣ、ＲＡＶＥＮ ８８００、８０００、７０００、５７５０、５２５０、３５００、２１００、２０００、１８００、１５００、１２５５、１２５０、アクゾー社製、ケッチェンブラックＥＣなどがあげられる。
【００５６】
カーボンブラックを分散剤などで表面処理したり、樹脂でグラフト化して使用しても、表面の一部をグラファイト化したものを使用してもかまわない。また、カーボンブラックを塗料に添加する前にあらかじめ結合剤で分散してもかまわない。これらのカーボンブラックは上記無機質粉末に対して５０質量％を越えない範囲、下層総質量の４０％を越えない範囲で使用できる。これらのカーボンブラックは単独、または組合せで使用することができる。本発明の下層で使用できるカーボンブラックは例えば「カーボンブラック便覧」カーボンブラック協会編」を参考にすることができる。
【００５７】
また下層には有機質粉末を目的に応じて、添加することもできる。例えば、アクリルスチレン系樹脂粉末、ベンゾグアナミン樹脂粉末、メラミン系樹脂粉末、フタロシアニン系顔料が挙げられるが、ポリオレフィン系樹脂粉末、ポリエステル系樹脂粉末、ポリアミド系樹脂粉末、ポリイミド系樹脂粉末、ポリフッ化エチレン樹脂も使用することができる。
【００５８】
ＩＶ．その他添加剤
本発明の磁気記録媒体において磁性層または下層には分散効果、潤滑効果、帯電防止効果、可塑効果などを付与するための添加剤を含有しても良い。これら添加剤としては二硫化モリブデン、二硫化タングステン、グラファイト、窒化ホウ素、フッ化黒鉛、シリコーンオイル、極性基を持つシリコーン、脂肪酸変性シリコーン、フッ素含有シリコーン、フッ素含有アルコール、フッ素含有エステル、ポリオレフィン、ポリグリコール、ポリフェニルエーテル、フェニルホスホン酸、ベンジルホスホン酸基、フェネチルホスホン酸、α−メチルベンジルホスホン酸、１−メチル−１−フェネチルホスホン酸、ジフェニルメチルホスホン酸、ビフェニルホスホン酸、ベンジルフェニルホスホン酸、α−クミルホスホン酸、トルイルホスホン酸、キシリルホスホン酸、エチルフェニルホスホン酸、クメニルホスホン酸、プロピルフェニルホスホン酸、ブチルフェニルホスホン酸、ヘプチルフェニルホスホン酸、オクチルフェニルホスホン酸、ノニルフェニルホスホン酸等の芳香族環含有有機ホスホン酸およびそのアルカリ金属塩、オクチルホスホン酸、２−エチルヘキシルホスホン酸、イソオクチルホスホン酸、（イソ）ノニルホスホン酸、（イソ）デシルホスホン酸、（イソ）ウンデシルホスホン酸、（イソ）ドデシルホスホン酸、（イソ）ヘキサデシルホスホン酸、（イソ）オクタデシルホスホン酸、（イソ）エイコシルホスホン酸等のアルキルホスホン酸およびそのアルカリ金属塩、燐酸フェニル、燐酸ベンジル、燐酸フェネチル、燐酸α−メチルベンジル、燐酸１−メチル−１−フェネチル、燐酸ジフェニルメチル、燐酸ビフェニル、燐酸ベンジルフェニル、燐酸α−クミル、燐酸トルイル、燐酸キシリル、燐酸エチルフェニル、燐酸クメニル、燐酸プロピルフェニル、燐酸ブチルフェニル、燐酸ヘプチルフェニル、燐酸オクチルフェニル、燐酸ノニルフェニル等の芳香族燐酸エステルおよびそのアルカリ金属塩、燐酸オクチル、燐酸２−エチルヘキシル、燐酸イソオクチル、燐酸（イソ）ノニル、燐酸（イソ）デシル、燐酸（イソ）ウンデシル、燐酸（イソ）ドデシル、燐酸（イソ）ヘキサデシル、燐酸（イソ）オクタデシル、燐酸（イソ）エイコシル等の燐酸アルキルエステルおよびそのアルカリ金属塩、アルキルスルホン酸エステルおよびそのアルカリ金属塩、フッ素含有アルキル硫酸エステルおよびそのアルカリ金属塩、ラウリン酸、ミリスチン酸、パルミチン酸、ステアリン酸、ベヘン酸、ステアリン酸ブチル、オレイン酸、リノール酸、リノレン酸、エライジン酸、エルカ酸、酸等の炭素数１０〜２４の不飽和結合を含んでも分岐していても良い一塩基性脂肪酸およびこれらの金属塩、または、ステアリン酸ブチル、ステアリン酸オクチル、ステアリン酸アミル、ステアリン酸イソオクチル、ミリスチン酸オクチル、ラウリル酸ブチル、ステアリン酸ブトキシエチル、アンヒドロソルビタンモノステアレート、アンヒドロソルビタンジステアレート、アンヒドロソルビタントリステアレート等の炭素数１０〜２４の不飽和結合を含んでも分岐していても良い一塩基性脂肪酸と炭素数２〜２２の不飽和結合を含んでも分岐していても良い１〜６価アルコール、炭素数１２〜２２の不飽和結合を含んでも分岐していても良いアルコキシアルコールまたはアルキレンオキサイド重合物のモノアルキルエーテルのいずれか一つとからなるモノ脂肪酸エステル、ジ脂肪酸エステルまたは多価脂肪酸エステル、炭素数２〜２２の脂肪酸アミド、炭素数８〜２２の脂肪族アミンなどが使用できる。また、上記炭化水素基以外にもニトロ基およびＦ、Ｃｌ、Ｂｒ、ＣＦ₃、ＣＣｌ₃、ＣＢｒ₃等の含ハロゲン炭化水素等炭化水素基以外の基が置換したアルキル基、アリール基、アラルキル基をもつものでも良い。
【００５９】
また、アルキレンオキサイド系、グリセリン系、グリシドール系、アルキルフエノールエチレンオキサイド付加体等のノニオン界面活性剤、環状アミン、エステルアミド、第四級アンモニウム塩類、ヒダントイン誘導体、複素環類、ホスホニウムまたはスルホニウム類等のカチオン系界面活性剤、カルボン酸、スルホン酸、硫酸エステル基等の酸性基を含むアニオン界面活性剤、アミノ酸類、アミノスルホン酸類、アミノアルコールの硫酸またはリン酸エステル類、アルキルベタイン型等の両性界面活性剤等も使用できる。これらの界面活性剤については、「界面活性剤便覧」（産業図書株式会社発行）に詳細に記載されている。これらの潤滑剤、帯電防止剤等は必ずしも純粋ではなく主成分以外に異性体、未反応物、副反応物、分解物、酸化物等の不純分が含まれても構わない。これらの不純分は３０質量％以下が好ましく、さらに好ましくは１０質量％以下である。
【００６０】
これらの具体例としては日本油脂社製：ＮＡＡ−１０２、ヒマシ油硬化脂肪酸、ＮＡＡ−４２、カチオンＳＡ、ナイミーンＬ−２０１、ノニオンＥ−２０８、アノンＢＦ、アノンＬＧ、竹本油脂社製：ＦＡＬ−２０５、ＦＡＬ−１２３、新日本理化社製：エヌジエルブＯＬ、信越化学社製：ＴＡ−３，ライオンアーマー社製：アーマイドＰ、ライオン社製、デュオミンＴＤＯ、日清製油社製：ＢＡ−４１Ｇ、三洋化成社製：プロフアン２０１２Ｅ、ニューポールＰＥ６１、イオネットＭＳ−４００等があげられる。
【００６１】
本発明で使用されるこれらの分散剤、潤滑剤、界面活性剤は下層、磁性層でその種類、量を必要に応じ使い分けることができる。例えば、無論ここに示した例のみに限られるものではないが、分散剤は極性基で吸着もしくは結合する性質を有しており、磁性層においては主に強磁性粉末の表面に、下層においては主に非磁性粉末の表面に前記の極性基で吸着もしくは結合し、一度吸着した有機燐化合物は金属あるいは金属化合物等の表面から脱着しがたいと推察される。従って、本発明の強磁性粉末表面あるいは非磁性粉末表面は、アルキル基、芳香族基等で被覆されたような状態になるので、強磁性粉末あるいは非磁性粉末の結合剤樹脂成分に対する親和性が向上し、さらに強磁性粉末あるいは非磁性粉末の分散安定性も改善される。また、潤滑剤としては遊離の状態で存在するため下層、磁性層で融点の異なる脂肪酸を用い表面へのにじみ出しを制御する、沸点や極性の異なるエステル類を用い表面へのにじみ出しを制御する、界面活性剤量を調節することで塗布の安定性を向上させる、潤滑剤の添加量を下層で多くして潤滑効果を向上させるなどが考えられる。
【００６２】
本発明で用いられる添加剤のすべてまたはその一部は、磁性層あるいは下層用塗布液の製造時のいずれの工程で添加してもよい。例えば、混練工程前に強磁性粉末と混合する場合、強磁性粉末と結合剤と溶剤による混練工程で添加する場合、分散工程で添加する場合、分散後に添加する場合、塗布直前に添加する場合などがある。
【００６３】
Ｖ．非磁性支持体
本発明の磁気記録媒体は、以上の材料により調製した塗布液を非磁性支持体上に塗布して下層あるいは磁性層を形成する。
本発明に用いることのできる非磁性支持体としては二軸延伸を行ったポリエチレンナフタレート、ポリエチレンテレフタレート、ポリアミド、ポリイミド、ポリアミドイミド、芳香族ポリアミド、ポリベンズオキシダゾール等の公知のものが使用できる。好ましくはポリエチレンナフタレート、芳香族ポリアミドである。これらの非磁性支持体はあらかじめコロナ放電、プラズマ処理、易接着処理、熱処理などを行っても良い。
【００６４】
また、本発明に用いることのできる非磁性支持体は中心線平均表面粗さがカットオフ値０．２５ｍｍにおいて０．１〜２０ｎｍ、好ましくは１〜１０ｎｍの範囲という優れた平滑性を有する表面であることが好ましい。また、これらの非磁性支持体は中心線平均表面粗さが小さいだけでなく１μ以上の粗大突起がないことが好ましい。
得られた支持体の算術平均粗さは（Ｒａ）の値［ＪＩＳ Ｂ０６６０−１９９８、ＩＳＯ ４２８７−１９９７］で０．１μｍ以下であることが、得られた磁気記録媒体のノイズが小さくなるので好ましい。
本発明の磁気記録媒体における非磁性支持体の好ましい厚みとしては３〜８０μｍである。
【００６５】
VI．バックコート層、下塗り層
本発明で用いる非磁性支持体の磁性塗料が塗布されていない面にバックコート層（バッキング層）が設けられていてもよい。バックコート層は、非磁性支持体の磁性塗料が塗布されていない面に、研磨材、帯電防止剤などの粒状成分と結合剤とを有機溶剤に分散したバックコート層形成塗料を塗布して設けられた層である。粒状成分として各種の無機顔料やカーボンブラックを使用することができ、また結合剤としてはニトロセルロース、フェノキシ樹脂、ポリウレタン等の樹脂を単独またはこれらを混合して使用することができる。本発明の非磁性支持体の磁性塗料およびバックコート層形成塗料の塗布面に接着剤層が設けられていてもよい。
【００６６】
また、本発明の磁気記録媒体においては、下塗り層を設けても良い。下塗り層を設けることによって支持体と磁性層又は下層との接着力を向上させることができる。下塗り層としては、溶剤への可溶性のポリエステル樹脂が使用される。下塗り層は厚さとして０．５μｍ以下のものが用いられる。
【００６７】
VII．製造方法
本発明の磁気記録媒体の製造方法は例えば、走行下にある非磁性支持体の表面に磁性塗布液を所定の膜厚となるように塗布する。ここで複数の磁性層塗布液を逐次あるいは同時に重層塗布してもよく、下層塗布液と磁性層塗布液とを逐次あるいは同時に重層塗布してもよい。上記磁性塗布液もしくは下層塗布液を塗布する塗布機としては、エアードクターコート、ブレードコート、ロッドコート、押出しコート、エアナイフコート、スクイズコート、含浸コート、リバースロールコート、トランスファーロールコート、グラビヤコート、キスコート、キャストコート、スプレイコート、スピンコート等が利用できる。
これらについては例えば株式会社総合技術センター発行の「最新コーティング技術」（昭和５８年５月３１日）を参考にできる。本発明の磁気記録媒体に適用する場合、塗布する装置、方法の例として以下のものを提案できる。
【００６８】
磁性層塗布液の塗布で一般的に適用されるグラビア、ロール、ブレード、エクストルージョン等の塗布装置により、まず下層を塗布し、下層が未乾燥の状態のうちに、
（１）特公平１-４６１８６号公報、特開昭６０-２３８１７９号公報、特開平２-２６５６７２号公報等に開示されているような支持体加圧型エクストルージョン塗布装置により、上層を塗布する。
（２）特開昭６３-８８０８０号公報、特開平２-１７９７１号公報、特開平２-２６５６７２号公報に開示されているような塗布液通液スリットを２個有する一つの塗布ヘッドにより上下層をほぼ同時に塗布する。
（３）特開平２-１７４９６５号公報に開示されているようなバックアップロール付きのエクストルージョン塗布装置により、上下層をほぼ同時に塗布する。
【００６９】
本発明の磁気記録媒体の磁性層の厚みは用いるヘッドの飽和磁化量やヘッドギャップ長、記録信号の帯域により最適化されるものであるが、一般には０．０１μｍ以上０．１０μｍ以下であり、好ましくは０．０２μｍ以上０．０８μｍ以下であり、更に好ましくは０．０３μｍ以上０．０８μｍ以下である。磁性層を異なる磁気特性を有する２層以上に分離してもかまわず、公知の重層磁性層に関する構成が適用できる。
この極薄層の磁性層を安定的に塗布するには支持体上に無機粉末を含有する下層を介在させて、その上に磁性層をウエット・オン・ウエットで塗布することが望ましい。
【００７０】
磁性層塗布液の塗布層は、磁気テープの場合磁性層塗布液の塗布層中に含まれる強磁性粉末にコバルト磁石やソレノイドを用いて長手方向に磁場配向処理を施す。ディスクの場合、配向装置を用いず無配向でも十分に等方的な配向性が得られることもあるが、コバルト磁石を斜めに交互に配置すること、ソレノイドで交流磁場を印加するなど公知のランダム配向装置を用いることが好ましい。等方的な配向とは強磁性金属微粉末の場合、一般的には面内２次元ランダムが好ましいが、垂直成分をもたせて３次元ランダムとすることもできる。六方晶フェライトの場合は一般的に面内および垂直方向の３次元ランダムになりやすいが、面内２次元ランダムとすることも可能である。また異極対向磁石など公知の方法を用い、垂直配向とすることで円周方向に等方的な磁気特性を付与することもできる。特に高密度記録を行う場合は垂直配向が好ましい。また、スピンコートを用い円周配向してもよい。
【００７１】
乾燥風の温度、風量、塗布速度を制御することで塗膜の乾燥位置を制御できる様にすることが好ましく、塗布速度は２０ｍ／分〜１，０００ｍ／分、乾燥風の温度は６０℃以上が好ましい、また磁石ゾーンに入る前に適度の予備乾燥を行なうこともできる。
【００７２】
乾燥された後、塗布層に表面平滑化処理を施す。表面平滑化処理には、例えばスーパーカレンダーロールなどが利用される。表面平滑化処理を行うことにより、乾燥時の溶剤の除去によって生じた空孔が消滅し磁性層中の強磁性粉末の充填率が向上するので、電磁変換特性の高い磁気記録媒体を得ることができる。
カレンダー処理ロールとしてはエポキシ、ポリイミド、ポリアミド、ポリアミドイミド等の耐熱性プラスチックロールを使用する。また金属ロールで処理することもできる。
本発明の磁気記録媒体は、表面の中心線平均粗さが、カットオフ値０．２５ｍｍにおいて０．１〜４ｎｍ、好ましくは１〜３ｎｍの範囲という極めて優れた平滑性を有する表面であることが好ましい。その方法として、例えば上述したように特定の強磁性粉末と結合剤を選んで形成した磁性層を上記カレンダー処理を施すことにより行われる。
【００７３】
カレンダー処理条件としては、カレンダーロールの温度を６０〜１００℃、好ましくは７０〜１００℃、さらに好ましくは８０〜１００℃の範囲である。カレンダーロールの圧力は１００〜５００ｋｇ／ｃｍ、好ましくは２００〜４５０ｋｇ／ｃｍ、さらに好ましくは３００〜４００ｋｇ／ｃｍの範囲の条件で作動させることによって行われる。得られた磁気記録媒体は、裁断機などを使用して所望の大きさに裁断して使用することができる。
【００７４】
【実施例】
結合剤として使用するアクリル系樹脂、ポリウレタン樹脂の合成例及びこれらの樹脂を用いた具体的実施例ならびに比較例をあげるが、本発明はこの実施例に限定されるものではない。なお実施例中の「部」の表示は特に断らない限り「質量部」を示す。
【００７５】
［合成例ＡＣ−１］
＜アクリル系樹脂ＡＣ−１の合成＞
撹拌機、コンデンサー、温度計及び窒素ガス導入口を備えた重合容器に、脱イオン水３６０部、過硫酸カリウム５部、炭酸ソーダ１．６部を仕込み窒素置換後５７℃に昇温した。一方、あらかじめ脱イオン水３９０部、メチルメタクリレート１６５部、ベンジルメタクリレート５０部、Ｎ-ビニルピロリドン２７５部、２−ヒドロキシメチルメタクリレート１０部、ラウリル硫酸ナトリウム１０部をホモミキサーで混合乳化したものを上記重合容器中へ８時間を要して均一に滴下させ、さらに５７℃で２時間反応させ重合を完結した後、メタノール５００部、硫酸ナトリウム５０部を添加しポリマーを析出させた。析出したポリマーをメタノール５，０００部で２回、次いで脱イオン水５，０００部で４回洗浄し、ろ過、乾燥してアクリル系樹脂ＡＣ−１を得た。スルホン酸ナトリウム基含有量は６．５×１０^-5ｅｑ／ｇであった。
【００７６】
［合成例ＡＣ−２〜５、ａｃ-６、７］
アクリル系樹脂ＡＣ−２〜５、ａｃ-６、７の合成
合成例ＡＣ−１と同様にして表１に示される単量体の種類・量比（質量％）で共重合し、同様の方法で処理してアクリル系樹脂を得た。
【００７７】
【表１】

【００７８】
［合成例ＰＵＡ−１］
ポリウレタン樹脂ＰＵＡ−１の合成
温度計、撹拌機、還流式冷却器を具備し予め窒素置換した容器に、表２に示した組成のポリオールとをシクロヘキサノン３０％溶液に窒素気流下６０℃で溶解した。次いで触媒としてジブチルスズジラウレート６０ｐｐｍを加え更に１５分間溶解した。更に表２に示したイソシアネ−ト化合物を加え６時間加熱反応し、ポリウレタン樹脂ＰＵＡ−１を得た。
【００７９】
［合成例ＰＵＡ−２、ａ−１、２］
ポリウレタン樹脂ＰＵＡ−２、ａ−１、２の合成
合成例ＰＵＡ−１と同様にして、表２に示される鎖延長剤、有機ジイソシアネート化合物を用いてポリウレタン樹脂ＰＵＡ−２、ａ−１、２を得た。
【００８０】
【表２】

【００８１】
［実施例１］
＜上層磁性塗料液の調整＞
強磁性針状金属粉末 １００部
組成：Ｆｅ／Ｃｏ／Ａｌ／Ｙ＝６８／２０／７／５、
表面処理剤：Ａｌ₂Ｏ₃，Ｙ₂Ｏ₃，Ｈｃ：２５００Ｏｅ、
結晶子サイズ：１４０Å， 長軸径：０．０８μｍ、
針状比：６，ＢＥＴ比表面積：４６ｍ²／ｇ，σs：１５０ｅｍｕ／ｇ
アクリル系樹脂ＡＣ−１（表１に記載） ６部
ポリウレタン樹脂ＰＵＡ−１（表２に記載） １２部
フェニルホスホン酸 ３部
α−Ａｌ₂Ｏ₃（粒子サイズ０．１５μｍ） ２部
カーボンブラック（粒子サイズ ２０ｎｍ） ２部
シクロヘキサノン １１０部
メチルエチルケトン １００部
トルエン １００部
ブチルステアレート ２部
ステアリン酸 １部
【００８２】

【００８３】
上記上層用磁性塗料および下層用非磁性塗料組成物のそれぞれについて、各成分をオープンニーダーで６０分間混練した後、サンドミルで１２０分間分散した。得られた分散液に３官能性低分子量ポリイソシアネート化合物（日本ポリウレタン製 コロネート３０４１）を６部加え、さらに２０分間撹拌混合した後、１μｍの平均孔径を有するフィルターを用いて濾過し、磁性塗料および非磁性塗料を調製した。
さらに上記非磁性塗料を乾燥後の厚さが１．８μｍになるように塗布し、さらにその直後に磁性塗料を乾燥後の厚さが０．０８μｍになるように同時重層塗布した。両層が未乾燥の状態で３，０００ガウスの磁石で磁場配向を行ない、さらに乾燥後、金属ロールのみから構成される７段のカレンダーで速度１００ｍ／ｍｉｎ、線圧３００ｋｇ／ｃｍ、温度９０℃で表面平滑化処理を行なった後、７０℃２４時間加熱硬化処理を行い、３．８ｍｍ幅にスリットし磁気テープを作成した。
【００８４】
［実施例２〜６］
アクリル系樹脂およびポリウレタン樹脂を表３に示したように変更し、実施例１と同様の方法で実施例２〜６を作成した。
［比較例１〜６］
アクリル系樹脂、ポリウレタン樹脂および磁性体を表３に示したように変更し、実施例１と同様の方法で比較例１〜６を作成した。なお、表３中のＭＲ１１０、Ｎ−２３０４はそれぞれ市販の極性基含有塩化ビニル系樹脂（日本ゼオン製）、極性基含有ポリウレタン樹脂（日本ポリウレタン製）を示す。
【００８５】
【表３】

【００８６】
〔測定方法〕
（１）磁性層表面粗さＲａ
デジタルオプチカルプロフィメーター（ＷＹＫＯ製）を用いたる光干渉法により、カットオフ０．２５ｍｍの条件で中心線平均粗さをＲａとした。
（２）５００パス走行後ヘッド汚れ
ＤＤＳドライブでサンプルテープを全長５００パス繰り返し走行させ、走行後のヘッド汚れを観察した。
汚れが見られたものを×、見られなかったものを○とした。
（３）ヘッド腐食
ＭＲヘッドの金属部材であるパーマロイを磁性層に接触させた状態で６０゜C９０％ＲＨ環境に２週間放置したのち、パーマロイ部材の表面を光学顕微鏡で観察し、表面の変化を調べた。
腐食のような変色が見られたものを×、見られなかったものを○とした。
【００８７】
［実施例７］
＜上層磁性塗料液の調整＞
強磁性板状六方晶フェライト粉末 １００部
組成（モル比）：Ｂａ／Ｆｅ／Ｃｏ／Ｚｎ＝１／９.１／０.２／０.８、
Ｈｃ：２４５０Ｏｅ、板径：２６ｎｍ、板状比：４
ＢＥＴ比表面積：５０ｍ²／ｇ、σs：６０ｅｍｕ／ｇ
アクリル系樹脂ＡＣ−２（表１に記載） ６部
ポリウレタン樹脂ＰＵＡ−１（表２に記載） １２部
フェニルホスホン酸 ３部
α−Ａｌ₂Ｏ₃（粒子サイズ０．１５μｍ） ２部
カーボンブラック（粒子サイズ ２０ｎｍ） ２部
シクロヘキサノン １１０部
メチルエチルケトン １００部
トルエン １００部
ブチルステアレート ２部
ステアリン酸 １部
【００８８】
上記上層用磁性塗料の各成分を実施例１と同様の方法で磁性塗料を調製した。さらに実施例１と同様の非磁性塗料を乾燥後の厚さが１.５μｍになるように塗布し、さらにその直後に上記磁性塗料を乾燥後の厚さが０.０８μｍになるように、厚さ６２μｍで中心面平均粗さが３μｍのポリエチレンテレフタレート支持体上に同時重層塗布を行った。両層がまだ湿潤状態にあるうちに周波数５０Ｈｚ、磁場強度２５０ガウスおよび周波数５０Ｈｚ、磁場強度１２０ガウスの２つの磁場強度交流磁場発生装置の中を通過させランダム配向処理を行った。乾燥後、７段のカレンダーで温度９０℃、線圧３００ｋｇ／ｃｍにて処理を行い、３．７インチに打ち抜き、表面研磨処理を施した後、ライナーが内側に設置済みの３.７インチのＺｉｐ−ｄｉｓｋカートリッジに入れ、所定の機構部品を付加し、３.７インチフレキシブルディスクを得た。
【００８９】
［実施例８〜１２］
アクリル系樹脂及びポリウレタン樹脂を表４に示したように変更し、実施例７と同様の方法で実施例８〜１２を作成した。
［比較例７〜１２］
アクリル系樹脂、ポリウレタン樹脂および磁性体を表４に示したように変更し、実施例７と同様の方法で比較例７〜１２を作成した。なお、表４中のＭＲ１１０、Ｎ−２３０４はそれぞれ市販の極性基含有塩化ビニル系樹脂（日本ゼオン製）、極性基含有ポリウレタン樹脂（日本ポリウレタン製）を示す。
【００９０】
【表４】

【００９１】
〔測定方法〕
（１）磁性層表面粗さＲａ：磁気テープと同様の方法で測定した。
（２）ＯＴＭ摺動後ヘッド汚れ：Ｚｉｐドライブでディスク状サンプルをＯＴＭ（Ｏｎｅ Ｔｒａｃｋ Ｍｕｌｔｉｐａｓｓ）摺動させた後のヘッド汚れを観察した。条件は、回転数３６００ｒｐｍ、２００ｈｒとした。
（３）ヘッド腐食：テープ状媒体と同様の方法で測定した。
【００９２】
【発明の効果】
本発明によれば、塗膜強度が向上し、磁性層表面が削れにくくなり、ヘッド汚れが少なくなり、走行耐久性に優れる磁気記録媒体を提供できる。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a magnetic recording medium in which a nonmagnetic powder or a lower layer in which magnetic powder and a binder are dispersed is provided on a nonmagnetic support, and further provided with a magnetic layer.
[0002]
[Prior art]
Conventionally, a high-strength, high-dispersibility polyurethane has been used as a binder for magnetic recording media, but it has been common to use a vinyl chloride resin in combination. The basic dispersibility of the magnetic material and non-magnetic powder was sufficient with polyurethane alone, but a vinyl chloride resin containing a polar group was used for the purpose of adjusting the viscosity of the coating solution and adjusting the physical properties of the tape.
However, when used in data media that require high capacity, high speed and high reliability in recent years, extremely small amounts of hydrochloric acid gas generated from PVC may adversely affect not only tape storage but also head corrosion. I understand that.
In particular, MR (Magnet Resistance) heads used in data recording systems for computers use metal thin films, and there is a concern about deterioration of characteristics due to corrosion.
Further, when the tape is stored for a long period of time, the fatty acid generated by alteration of the material in the tape by hydrochloric acid gas, for example, hydrolysis of the ester lubricant may migrate and precipitate on the surface of the magnetic layer and crystallize. Since the recording density is improved as compared with the prior art, the influence of even smaller foreign matters is increasing.
[0003]
Magnetic recording media that use an acrylic copolymer instead of a vinyl chloride resin as a binder for a magnetic layer have been proposed (see Patent Documents 1 to 4). However, these magnetic recording media have a problem that the dispersibility of the magnetic powder and the durability of the magnetic recording medium are poor.
[0004]
A magnetic recording medium using a polyurethane having a glass transition temperature of 100 ° C. or higher obtained by polymerizing a diol compound having a cyclic structure and a long alkyl chain and a diisocyanate compound as a binder of a nonmagnetic layer has also been proposed ( (See Patent Document 5). When such a polyurethane is used, improvement in durability can be expected, but when used alone, sufficient dispersibility cannot be ensured and it has been necessary to use it together with a vinyl chloride resin.
[0005]
It consists of a polyurethane having a cyclic structure and a long chain alkyl chain and a diisocyanate compound and a glass transition temperature of 100 ° C. or higher, a (meth) acrylate unit containing a benzene ring, and a radically polymerizable unit containing nitrogen. Although a magnetic recording medium in which an acrylic copolymer having a ionic polar group is used in combination with dechlorination has been proposed, it has been found that the acrylic resin has low compatibility with the polyurethane. When such an acrylic resin with low compatibility is used, it is easy to form a structure in which the acrylic resin phase and the urethane phase are separated in the medium, and the acrylic resin phase with relatively low mechanical strength is damaged when the medium is running. It was found to have the following disadvantages. In addition, since the use of a vinyl chloride resin is possible, head corrosion due to dehydrochlorination reaction occurs, and there is a great concern that harmful substances such as dioxins are generated when incinerated, which also has a great impact on the environment.
[0006]
[Patent Document 1]
JP-A-5-20668
[Patent Document 2]
JP-A-6-4853
[Patent Document 3]
Japanese Patent Laid-Open No. 7-169038
[Patent Document 4]
JP-A-8-180366
[Patent Document 5]
JP-A-11-96539
[0007]
[Problems to be solved by the invention]
An object of the present invention is to provide a magnetic recording medium that is excellent in dispersibility, coating film smoothness, electromagnetic conversion characteristics, durability, and long-term storage, has little MR head corrosion, and has a small environmental impact during incineration. is there.
[0008]
[Means for Solving the Problems]
In the present invention, a polyurethane resin obtained by polymerizing a diol compound having a cyclic structure and a long alkyl chain and a diisocyanate compound is used, and an acrylic resin having improved compatibility with the polyurethane is used in combination, whereby durability is improved. It has improved dramatically and the latest high-density magnetic recording media has been able to satisfy quality at an extremely high level. Further, since no vinyl chloride resin is used, it is possible to provide a magnetic recording medium that suppresses head corrosion and has a low environmental impact.
[0009]
The object of the present invention has been solved by the following (1) to (5).
(1) Magnetic recording having a magnetic layer in which a lower layer containing inorganic powder and / or magnetic powder and a binder is provided on a nonmagnetic support, and at least one ferromagnetic fine powder and a binder are dispersed thereon. In the medium, the binder in the lower layer is a polyurethane resin obtained by reacting the following acrylic copolymer and a polyol compound having a cyclic structure and a long alkyl chain with an organic diisocyanate and having a hydrophilic polar group A magnetic recording medium characterized by using the above.
Acrylic copolymer: 45 to 80% by mass of a radically polymerizable monomer having nitrogen as a raw material with respect to the total amount of monomers, including a (meth) acrylate monomer having an alkyl (meth) acrylate and / or a benzene ring An acrylic copolymer obtained by copolymerizing these and having a hydrophilic polar group in the molecule.
(2) Magnetic recording having a nonmagnetic support and a lower layer containing inorganic powder and / or magnetic powder and a binder, and further having a magnetic layer in which at least one ferromagnetic fine powder and a binder are dispersed. In the medium, the binder in the magnetic layer is a polyurethane obtained by reacting the following acrylic copolymer and a polyol compound having a cyclic structure and a long alkyl chain with an organic diisocyanate, and having a hydrophilic polar group The magnetic recording medium according to (1), wherein a resin is used in combination.
Acrylic copolymer: 45 to 80% by mass of a radically polymerizable monomer having nitrogen as a raw material with respect to the total amount of monomers, including a (meth) acrylate monomer having an alkyl (meth) acrylate and / or a benzene ring An acrylic copolymer obtained by copolymerizing these and having a hydrophilic polar group in the molecule.
(3) The magnetic recording medium according to (1) or (2), wherein the acrylic copolymer has a sulfonate group as a hydrophilic polar group.
(4) The magnetic recording medium according to any one of (1) to (3), wherein the content of the polymerizable monomer having a hydrophilic polar group is 0.3 to 6% by mass.
(5) The magnetic recording medium according to any one of (1) to (4), wherein the hydrophilic polar group is an acrylic copolymer introduced by a persulfate used as a polymerization initiator.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
I. Binder
In the magnetic recording medium of the present invention, the binder used to form the lower layer is composed of a radically polymerizable unit containing nitrogen, an alkyl (meth) acrylate, and a (meth) acrylate monomer having a benzene ring, and hydrophilic polarity. An acrylic resin having a group and a polyol compound having a cyclic structure and a long alkyl chain are reacted with an organic diisocyanate, and a polyurethane resin having a hydrophilic polar group is used in combination. In addition, (meth) acrylate is a general term for acrylate and methacrylate.
[0011]
(1) Acrylic copolymer
The radically polymerizable unit containing nitrogen used in the present invention includes (meth) acrylamide, diacetone (meth) acrylamide, N-methylol (meth) acrylamide, N-methoxymethyl (meth) acrylamide, and N-ethoxymethyl (meth). Acrylamide, N-butoxymethyl (meth) acrylamide, dimethylaminopropyl (meth) acrylamide, N, N-dimethylacrylamide, dimethylaminomethyl (meth) acrylate, dimethylaminoethyl (meth) acrylate, diethylaminomethyl (meth) acrylate, diethylamino Ethyl (meth) acrylate, dimethylaminopropyl (meth) acrylate, N-vinyl-2-pyrrolidone, N-vinylcarbazole, 2-vinyl-4,6-diamino-5-triazine, 2 Vinylpyridine, 4-vinylpyridine, maleimide, N- phenylmaleimide and acrylonitrile. In addition, examples of the radically polymerizable unit having a heterocyclic ring containing nitrogen and oxygen in the ring include (meth) acryloylmorpholine, morpholinoethyl (meth) acrylate, N-vinyloxazolidone and the like. These may be used alone or in combination of two or more.
[0012]
These nitrogen-containing radically polymerizable units are 45 to 80% by mass, preferably 50 to 70% by mass, based on the total amount of monomers. If it is less than 45% by mass or exceeds 80% by mass, the compatibility with the polyurethane resin having a specific structure used in the present invention is deteriorated.
[0013]
Specific examples of the alkyl (meth) acrylate used in the present invention and the (meth) acrylate unit having a benzene ring include the following.
Alkyl (meth) acrylate units include methyl (meth) acrylate, ethyl (meth) acrylate, propyl (meth) acrylate, butyl (meth) acrylate, isobutyl (meth) acrylate, pentyl (meth) acrylate, hexyl (meth) acrylate Octyl (meth) acrylate, 2-ethylhexyl (meth) acrylate, lauryl (meth) acrylate, stearyl (meth) acrylate, cyclohexyl (meth) acrylate, and the like.
[0014]
Examples of the (meth) acrylate unit having a benzene ring include benzyl (meth) acrylate, phenoxyethyl (meth) acrylate, phenoxypolyethylene glycol (meth) acrylate, and nonylphenol ethylene oxide adduct (meth) acrylate. As the (meth) acrylate unit having a benzene ring, benzyl (meth) acrylate and phenoxyethyl (meth) acrylate are particularly preferable.
[0015]
10-45 mass% is preferable with respect to the total amount of a monomer, and, as for the total amount of the alkyl (meth) acrylate and the (meth) acrylate which has a benzene ring, 15-40 mass% is more preferable. If it is less than 10% by mass, the solubility of the copolymer in organic solvents such as ketones and esters is hindered. If it exceeds 45% by mass, the composition ratio of radically polymerizable units containing nitrogen relatively decreases, and the compatibility with the polyurethane resin having a specific structure used in combination with the present invention deteriorates.
[0016]
Other copolymerizable units that can be copolymerized include allyl glycidyl ether, methyl vinyl ether, ethyl vinyl ether, isobutyl vinyl ether, n-butyl vinyl ether, 2-ethylhexyl vinyl ether, n-octyl vinyl ether, lauryl vinyl ether, cetyl vinyl ether, stearyl. Alkyl vinyl ethers such as vinyl ether, alkoxyalkyl (meth) acrylates such as methoxyethyl (meth) acrylate, butoxyethyl (meth) acrylate, glycidyl (meth) acrylate, vinyl acetate, vinyl propionate, maleic anhydride, acrylonitrile, chloride Examples include vinylidene and glycidyl (meth) acrylate. When other copolymerizable units are included, the content is preferably 26% by mass or less. If this range is exceeded, the mechanical properties and dispersibility of the entire resin may decrease.
[0017]
The acrylic resin used in the present invention has a hydrophilic polar group for the purpose of improving the dispersibility of the nonmagnetic powder. The hydrophilic polar group is not particularly limited as long as it has a function of improving dispersibility. _Three M, -PO (OM) _Three , -COOM (M represents a hydrogen atom, an alkali metal or an ammonium salt), an amino group, or a quaternary ammonium base, particularly -SO _Three M is preferable because of its excellent dispersibility.
The preferred polar group content is 1 × 10 ^-6 ~ 50x10 ^-Five eq / g, more preferably 5 × 10 ^-6 ~ 20x10 ^-Five eq / g-binder. 1 × 10 ^-6 If it is less than eq / g, no effect is obtained, and 50 × 10 ^-Five If it exceeds eq / g, the viscosity of the coating becomes high, the workability is remarkably deteriorated, and handling becomes difficult. Two or more polar groups may be introduced, for example, —SO _Three In addition to M, -COOM may be introduced.
[0018]
The method for introducing a polar group of the polar group-containing acrylic resin used in the present invention includes, for example, an alkyl (meth) acrylate unit, a (meth) acrylate unit having a benzene ring, and a radically polymerizable unit containing nitrogen, which is hydrophilic. It can be synthesized by adding the polar group to an acrylic resin containing no polar group by reaction.
Specifically -SO _Three When M is introduced into an acrylic resin, first, an alkyl (meth) acrylate unit, a (meth) acrylate unit having a benzene ring, a radically polymerizable unit containing nitrogen, and a copolymerizable compound having a glycidyl group and, if necessary, The other compounds copolymerizable with these are copolymerized, and at the same time as the copolymerization or after obtaining a copolymer, -SO _Three It is obtained by reacting with a compound having M. Examples of the copolymerizable compound for introducing a glycidyl group include glycidyl (meth) acrylate and glycidyl vinyl ether. These may be used alone or in combination of two or more.
[0019]
Moreover, what copolymerized the copolymerizable polar group containing compound with the (meth) acrylate unit, the radically polymerizable unit containing nitrogen, the ether compound type | system | group radically polymerizable unit, and the other copolymerizable compound may be used. As the copolymerizable polar group-containing compound, a copolymerizable compound containing the polar group can be used. For example, -SO _Three Examples of copolymerizable compounds for introducing M include unsaturated hydrocarbon sulfonic acids such as 2-acrylamido-2-methylpropanesulfonic acid, vinylsulfonic acid, (meth) acrylsulfonic acid, p-styrenesulfonic acid, and the like. , Sulfoalkyl esters of (meth) acrylic acid such as (meth) acrylic acid sulfoethyl ester, (meth) acrylic acid sulfopropyl ester, and salts thereof. -SO _Three When it is necessary to introduce -COOM in addition to M, a copolymerizable compound containing -COOM, specifically (meth) acrylic acid, maleic acid and salts thereof can be used.
The amount of the polar group-containing compound that can be copolymerized is preferably 0.3 to 6% by mass, more preferably 1.0 to 5% by mass, based on the total amount of monomers.
[0020]
Further, in the production of the copolymer, the polar group is a method of copolymerizing a monomer mixture using a polar group-containing radical polymerization initiator, a monomer in the presence of a chain transfer agent having a polar group at one end. A method of copolymerizing the mixture may be used.
The polar group-containing radical polymerization initiator is preferably a persulfate, and examples thereof include ammonium persulfate, potassium persulfate, and sodium persulfate. Although the usage-amount of these radical polymerization initiators should just be 1-10 mass% with respect to the total amount of a monomer, Preferably it is 1-5 mass%.
The chain transfer agent having the above polar group at one end is not particularly limited as long as it is capable of chain transfer in the polymerization reaction and has a polar group at one end, but a halogenated compound having a polar group at one end, mercapto Examples thereof include compounds and diphenylpicrylhydrazine.
[0021]
Specific examples of the halogenated compound include 2-chloroethanesulfonic acid, sodium 2-chloroethanesulfonate, 4-chlorophenyl sulfoxide, 4-chlorobenzenesulfonamide, p-chlorobenzenesulfonic acid, sodium p-chlorobenzenesulfonate, 2-bromoethane. Examples include sodium sulfonate and sodium 4- (bromomethyl) benzenesulfonate, and sodium 2-chloroethanesulfonate and sodium p-chlorobenzenesulfonate are preferable.
[0022]
The mercapto compound is preferably 2-mercaptoethanesulfonic acid (salt), 3-mercapto-1,2-propanediol, mercaptoacetic acid (salt), 2-mercapto-5-benzimidazolesulfonic acid (salt), 3-mercapto. 2-Butanol, 2-mercaptobutanol, 3-mercapto-2-propanol, N (2-mercaptopropyl) glycine, ammonium thioglycolate or β-mercaptoethylamine hydrochloride are used.
These chain transfer agents having a polar group at one end can be used singly or in combination of two or more. The chain transfer agent having a polar group at one end particularly preferably used is 2-mercaptoethanesulfonic acid (salt) having a strong polarity. The amount of these chain transfer agents used may be 0.1 to 10% by mass, preferably 0.2 to 5% by mass, based on the total amount of monomers.
[0023]
Further, as the hydrophilic polar group of the present invention, -SO _Three It is also preferable to have a hydroxyl group together with M or the like. Examples of copolymerizable hydroxyl group-containing units include hydroxyethyl (meth) acrylate, hydroxypropyl (meth) acrylate, polyethylene glycol mono (meth) acrylate, polypropylene glycol mono (meth) acrylate, polyethylene glycol polypropylene glycol mono (meth) Hydroxyalkyl (meth) acrylates such as acrylate, glycerol mono (meth) acrylate, 3-chloro-2-hydroxypropyl (meth) acrylate, vinyl ethers such as hydroxyethyl vinyl ether, hydroxypropyl vinyl ether, hydroxybutyl vinyl ether, hydroxyethyl mono (Meth) allyl ether, hydroxypropyl mono (meth) allyl ether, hydroxybutyl mono (meth) ary (Meth) allyl ethers such as ether, diethylene glycol mono (meth) allyl ether, dipropylene glycol mono (meth) allyl ether, glycerin mono (meth) allyl ether, 3-chloro-2-hydroxypropyl (meth) allyl ether, (Meth) allyl alcohol etc. are mentioned. The vinyl alcohol unit can be introduced by copolymerizing vinyl acetate and saponifying with caustic alkali in a solvent. The amount of the monomer having a hydroxyl group is preferably 5 to 30% by mass in the total monomers.
[0024]
The number average molecular weight of the acrylic resin containing a radically polymerizable monomer having nitrogen and an alkyl (meth) acrylate and / or a (meth) acrylate monomer having a benzene ring and having a hydrophilic polar group is 1,000 to 200,000. preferable. More preferably, it is 1,000 to 100,000. If it is less than 1,000, the resulting magnetic coating film becomes brittle and the resulting magnetic coating film becomes brittle, so that the physical strength is lowered and the durability of the magnetic tape or the like is also affected. If it exceeds 200,000, the viscosity of the paint at a predetermined concentration becomes high, the workability is remarkably deteriorated, and handling becomes difficult.
[0025]
In order to polymerize the polymerization reaction system containing the polymerizable compounds and the chain transfer agent, known polymerization methods such as suspension polymerization, emulsion polymerization, solution polymerization and the like can be used. Among these polymerization methods, it is preferable to use suspension polymerization or emulsion polymerization with good dry workability from the viewpoint that the obtained acrylic copolymer can be easily stored in a solid state having high storage stability. It is preferred to use emulsion polymerization. The polymerization conditions vary depending on the polymerizable compound used, the polymerization initiator, the type of chain transfer agent, etc., but generally in an autoclave, the temperature is about 50 to 80 ° C., and the gauge pressure is about 4.0 to 1.0 MPa. The time is preferably about 5 to 30 hours. Polymerization is preferably performed in an atmosphere of a gas inert to the reaction from the viewpoint of ease of reaction control. Examples of such a gas include nitrogen and argon, and nitrogen is preferably used from the viewpoint of economy. In the polymerization, other components may be added to the polymerization reaction system in addition to the above components. Examples of such components include emulsifiers, electrolytes, polymer protective colloids, and the like.
[0026]
(2) Polyurethane resin
As the polyurethane resin used in combination with the acrylic copolymer, a polyurethane resin having a hydrophilic polar group obtained by reacting a polyol compound having a cyclic structure and a long alkyl chain with an organic diisocyanate is used.
[0027]
As the polyol having a cyclic structure and a long alkyl chain, a dimer diol represented by the structure of the following formula obtained by hydrogenating and reducing dimer acid is preferable.
The molecular weight of the diol having a cyclic structure and a long alkyl chain is preferably 500 to 1,000. If it is less than 500, the diisocyanate content is substantially increased and the urethane group concentration is increased, so that the solvent solubility is lowered. If it exceeds 1,000, the coating strength decreases.
The content of the diol having a cyclic structure and a long alkyl chain is preferably 5% by mass or more, more preferably 10 to 40% by mass in the polyurethane resin.
Formula 1 shows the structure of dimer diol.
[0028]
[Chemical 1]

[0029]
There is no restriction | limiting in particular as polyisocyanate used in order to make it react with the said polyol and form a polyurethane resin, What is used normally can be used. For example, hexamethylene diisocyanate, tolidine diisocyanate, isophorone diisocyanate, 1,3-xylylene diisocyanate, 1,4-xylylene diisocyanate cyclohexane diisocyanate, toluidine diisocyanate, 2,4-tolylene diisocyanate, 2,6-tolylene diisocyanate, 4, , 4'-diphenylmethane diisocyanate, p-phenylene diisocyanate, m-phenylene diisocyanate, 1,5-naphthalene diisocyanate, 3,3-dimethylphenylene diisocyanate, and the like.
[0030]
The polyurethane resin preferably has the same polar group as that of the acrylic resin. In order to introduce a polar group into a polyurethane resin, a polar group-containing polyol such as a polyester polyol or polyether polyol having a polar group, a polyol such as a polyester polyol or polyether polyol having no polar group, and a diisocyanate are used. Can do. For example, it can be produced by changing a part of a dihydric alcohol or dibasic acid to a polar group-containing diol or polar group-containing dibasic acid. The polar group-containing polyol has a polar group represented by the above general formula in the main chain or side chain of the following polyol.
[0031]
In addition, as the chain extender, a substance known per se, a polyhydric alcohol, an aliphatic polyamine, an alicyclic polyamine, an aromatic polyamine, or the like can be used. Of these, polyhydric alcohols having a molecular weight of 50 to 500 are preferred. If it is less than 50, since the coating film becomes brittle, durability is lowered. If it exceeds 500, the Tg of the coating film decreases and becomes soft, so the durability decreases.
[0032]
Polyhydric alcohols include bisphenol A, hydrogenated bisphenol A, bisphenol S, bisphenol P and their ethylene oxide, propylene oxide adducts, cyclohexanedimethanol, cyclohexanediol, hydroquinone, bis (2-hydroxyethyl) tetrabromobisphenol A, bis (2-hydroxyethyl) tetrabromobisphenol S, bis (2-hydroxyethyl) tetramethylbisphenol S, bis (2-hydroxyethyl) diphenylbisphenol S, bis (2-hydroxyethyl) diphenylbiphenol, bis (2-hydroxyethyl) ) Thiodiphenol, bis (2-hydroxyethyl) bisphenol F, biphenol, bisphenol fluorene, bisphenol fluor orange hydroxyethyl ester Short chain diols having a cyclic structure such as ether are preferred.
Bisphenol A, hydrogenated bisphenol A, bisphenol S, bisphenol P, and these ethylene oxides, propylene oxide adducts, cyclohexanedimethanol, cyclohexanediol and other diols having aromatic and alicyclic groups are more preferable.
[0033]
The average molecular weight of the polar group-containing polyurethane resin used in the present invention is preferably 5,000 to 100,000. Furthermore, 10,000 to 50,000 are preferable. If it is less than 5,000, the resulting magnetic coating film becomes brittle and the resulting magnetic coating film becomes brittle, so that the physical strength is lowered and the durability of the magnetic recording medium is affected. When the molecular weight exceeds 100,000, the solubility in a solvent is lowered and the dispersibility is lowered. In addition, the viscosity of the paint at a predetermined concentration becomes high, the workability is remarkably deteriorated, and handling becomes difficult.
[0034]
The OH group of the polar group-containing polyurethane resin used in the present invention preferably has a branched OH group from the viewpoint of curability and durability, preferably 2 to 40 per molecule, preferably 3 per molecule. More preferably, 20 to 20.
[0035]
Other binders used in the present invention include the above-mentioned radically polymerizable monomer having nitrogen, alkyl (meth) acrylate, acrylic resin having a polar polar group and a polar group comprising a (meth) acrylate monomer having a benzene ring. A binder having another polar group may be used in combination with the contained polyurethane resin in an amount equal to or less than the total amount thereof. Other resins that can be used in combination are not particularly limited except for chlorine-containing resins, and use of known thermoplastic resins, thermosetting resins, reactive resins and mixtures thereof that have been conventionally used as binders. Can do. The thermoplastic resin has a glass transition temperature of −100 to 150 ° C. and a number average molecular weight of 1,000 to 200,000, preferably 10,000 to 100,000. Specifically, there are polymers, copolymers and various rubber resins containing acrylonitrile, styrene, butadiene, ethylene, vinyl butyral, vinyl acetal, vinyl ether and the like as structural units. Thermosetting resins or reactive resins include phenol resins, phenoxy resins, epoxy resins, urea resins, melamine resins, alkyd resins, acrylic reaction resins, formaldehyde resins, silicone resins, epoxy-polyamide resins, polyester resins and isocyanates. Examples thereof include a mixture of prepolymers.
[0036]
II. Magnetic material
[Ferromagnetic metal powder]
The ferromagnetic metal powder used in the present invention is not particularly limited as long as it contains Fe as a main component (including an alloy), but a ferromagnetic alloy powder containing α-Fe as a main component is preferable. These ferromagnetic powders include Al, Si, S, Sc, Ca, Ti, V, Cr, Cu, Y, Mo, Rh, Pd, Ag, Sn, Sb, Te, Ba, Ta, other than predetermined atoms. It may contain atoms such as W, Re, Au, Hg, Pb, Bi, La, Ce, Pr, Nd, P, Co, Mn, Zn, Ni, Sr, and B. It is preferable that at least one of Al, Si, Ca, Y, Ba, La, Nd, Co, Ni, and B is included in addition to α-Fe, and it is particularly preferable that Co, Al, and Y are included. More specifically, it is preferable that Co is contained in an amount of 10 to 40 atomic%, Fe is contained in an amount of 2 to 20 atomic%, and Y is contained in an amount of 1 to 15 atomic%.
[0037]
These ferromagnetic powders may be treated in advance with a dispersant, lubricant, surfactant, antistatic agent, etc. described later before dispersion. Further, the ferromagnetic metal powder may contain a small amount of water, hydroxide or oxide.
[0038]
The moisture content of the ferromagnetic powder is preferably 0.01-2%. It is preferable to optimize the moisture content of the ferromagnetic powder depending on the type of binder.
The crystallite size is 8 to 20 nm, preferably 10 to 18 nm, particularly preferably 12 to 16 nm.
As the crystallite size, an X-ray diffractometer (RINT2000 series manufactured by Rigaku Corporation) was used, and an average value obtained by the Scherrer method from the half-value width of the diffraction peak under the conditions of the radiation source CuKα1, the tube voltage 50 kV, and the tube current 300 mA was used. .
[0039]
The long axis length of the ferromagnetic metal powder is 0.01 to 0.10 μm, preferably 0.03 to 0.09 μm, and particularly preferably 0.05 to 0.08 μm. Specific surface area (S) of the ferromagnetic powder used in the magnetic layer of the present invention by the BET method _BET ) Is 30m ² / G or more 50m ² / G is preferable, 38 to 48 m ² / G is more preferable. This makes it possible to achieve both good surface properties and low noise.
The major axis diameter is obtained by taking a transmission electron micrograph and directly reading the minor axis diameter and the major axis diameter of the ferromagnetic powder from the photograph and tracing the transmission electron micrograph with the image analysis apparatus IBASSI manufactured by Carl Zeiss. It is required to use the reading method together.
[0040]
The pH of the ferromagnetic powder is preferably optimized depending on the combination with the binder used. The range is 4-12, preferably 7-10. The ferromagnetic powder may be surface-treated with Al, Si, P, or an oxide thereof as required. The amount is 0.1 to 10% with respect to the ferromagnetic powder, and when surface treatment is performed, the adsorption of a lubricant such as a fatty acid is 100 mg / m. ² The following is preferable. The ferromagnetic powder may contain soluble inorganic ions such as Na, Ca, Fe, Ni, and Sr. However, if it is 200 ppm or less, it does not particularly affect the characteristics. The ferromagnetic powder used in the present invention preferably has fewer pores, and its value is 20% by volume or less, and more preferably 5% by volume or less.
[0041]
In addition, the shape may be any of acicular, granular, rice granular, or plate-like shape as long as the above-described characteristics regarding the particle size are satisfied, but it is particularly preferable to use acicular ferromagnetic powder. In the case of acicular ferromagnetic metal powder, the acicular ratio is preferably 4 or more and 12 or less, more preferably 5 or more and 12 or less.
The coercive force Hc of the ferromagnetic metal powder is preferably 2,000 to 30,000 Oe, more preferably 2,100 to 29,000 Oe, and the saturation magnetic flux density is preferably 1,500 to 3,000 G. Preferably it is 1,600-2,900G. σs is preferably 140 to 170 emu / g, and more preferably 145 to 160 emu / g.
[0042]
[Ferromagnetic hexagonal ferrite powder]
The plate diameter of the ferromagnetic hexagonal ferrite powder is preferably 10 to 200 nm. In particular, when reproducing with a magnetoresistive head in order to increase the track density, it is necessary to reduce the noise, and the plate diameter is preferably 40 nm or less, but if it is less than 10 nm, stable magnetization cannot be expected due to thermal fluctuation. If it is larger than 200 nm, the noise is high and none is suitable for high-density magnetic recording.
The plate ratio (plate diameter / plate thickness) is preferably 1-15. Preferably it is 1-7. When the plate ratio is small, the filling property in the magnetic layer is preferably high, but sufficient orientation cannot be obtained. When it is larger than 15, noise increases due to stacking between particles. The specific surface area of this particle size range by the BET method is 10 to 200 m. ² / G. The specific surface area roughly agrees with the arithmetic calculation value from the particle plate diameter and plate thickness.
The distribution of the particle plate diameter and plate thickness is generally preferably as narrow as possible. Although it is difficult to digitize, it can be compared by randomly measuring 500 particles from a particle TEM photograph. In many cases, the distribution is not a normal distribution, but when calculated and expressed as a standard deviation with respect to the average size, σ / average size = 0.1 to 2.0. In order to sharpen the particle size distribution, the particle generation reaction system is made as uniform as possible, and the generated particles are subjected to a distribution improvement process. For example, a method of selectively dissolving ultrafine particles in an acid solution is also known.
[0043]
The coercive force Hc measured with a magnetic material can be made up to about 500 Oe to 5,000 Oe. A higher Hc is advantageous for high-density recording, but is limited by the capacity of the recording head. In the present invention, Hc is about 2,000 Oe to 3,000 Oe, preferably 2,200 Oe or more and 2,800 Oe or less. When the saturation magnetization of the head exceeds 1.4 Tesler, it is preferable to set it to 2,000 Oe or more. Hc can be controlled by the particle size (plate diameter / plate thickness), the type and amount of the contained element, the substitution site of the element, the particle generation reaction conditions, and the like.
[0044]
The saturation magnetization σs is 40 emu / g to 80 emu / g. σs is preferably higher, but tends to be smaller as the particles become finer. In order to improve σs, it is well known to combine spinel ferrite with magnetoplumbite ferrite and to select the type and amount of elements contained. It is also possible to use W-type hexagonal ferrite.
When the magnetic material is dispersed, the surface of the magnetic material particles is also treated with a material suitable for the dispersion medium and the polymer.
[0045]
As the surface treatment material, an inorganic compound or an organic compound is used. Typical examples of the main compound include compounds such as Si, Al, and P, various silane coupling agents, and various titanium coupling agents. The amount is 0.1 to 10% with respect to the magnetic substance. The pH of the magnetic material is also important for dispersion. Usually, about 4 to 12 has optimum values depending on the dispersion medium and polymer, but about 6 to 11 is selected from the chemical stability and storage stability of the medium. Water contained in the magnetic material also affects the dispersion. Although there is an optimum value depending on the dispersion medium and the polymer, 0.01 to 2.0% is usually selected.
[0046]
As a manufacturing method of hexagonal ferrite,
(1) Barium oxide, iron oxide, a metal oxide replacing iron, and boron oxide as a glass forming substance are mixed so as to have a desired ferrite composition, melted, rapidly cooled to an amorphous body, and then recrystallized. A glass crystallization method in which barium ferrite crystal powder is obtained by washing and grinding after heat treatment.
(2) A hydrothermal reaction method in which a barium ferrite composition metal salt solution is neutralized with an alkali to remove by-products, liquid phase is heated at 100 ° C. or higher, washed, dried and ground to obtain a barium ferrite crystal powder.
(3) The barium ferrite composition metal salt solution is neutralized with alkali, removed by-products, dried, treated at 1100 ° C. or less, and pulverized to obtain barium ferrite crystal powder. Invention does not choose a manufacturing method.
[0047]
III. Non-magnetic powder
The magnetic recording medium of the present invention has a lower layer made of a binder and a nonmagnetic powder on a nonmagnetic support.
The nonmagnetic powder that can be used for the lower layer may be an inorganic substance or an organic substance. Carbon black or the like can also be used. Examples of the inorganic substance include metals, metal oxides, metal carbonates, metal sulfates, metal nitrides, metal carbides, metal sulfides, and the like. Specifically, titanium oxide such as titanium dioxide, cerium oxide, tin oxide, tungsten oxide, ZnO, ZrO ₂ , SiO ₂ , Cr ₂ O _Three Α-alumina, β-alumina, γ-alumina, α-iron oxide, goethite, corundum, silicon nitride, titanium carbide, magnesium oxide, boron nitride, molybdenum disulfide, copper oxide, MgCO _Three , CaCO _Three , BaCO _Three , SrCO _Three , BaSO _Four Silicon carbide, titanium carbide or the like is used alone or in combination of two or more. Preferable are α-iron oxide and titanium oxide.
[0048]
The nonmagnetic powder may be acicular, spherical, polyhedral, or plate-shaped. The crystallite size of the nonmagnetic powder is preferably 0.004 μm to 1 μm, and more preferably 0.04 μm to 0.1 μm. If it is smaller than 0.004 μm, dispersion tends to be difficult. If it is larger than 1 μm, the surface roughness tends to increase.
The average particle size of these nonmagnetic powders is preferably 0.005 to 2 μm, more preferably 0.01 to 0.2 μm. If necessary, nonmagnetic powders having different average particle diameters can be combined, or even a single nonmagnetic powder can have the same effect by widening the particle size distribution. If it is smaller than 0.005 μm, dispersion tends to be difficult, and if it is larger than 2 μm, the surface roughness tends to increase.
[0049]
Specific surface area of non-magnetic powder is 1-100m ² / G, preferably 5 to 70 m ² / G, more preferably 10 to 65 m ² / G. 1m ² If it is smaller than / g, the surface roughness tends to increase, and 100 m ² If it is larger than / g, it tends to be difficult to disperse such that the desired amount of the binder cannot be dispersed.
The oil absorption using dibutyl phthalate (DBP) is 5 to 100 ml / 100 g, preferably 10 to 80 ml / 100 g, and more preferably 20 to 60 ml / 100 g. The specific gravity is 1 to 12, preferably 3 to 6.
The tap density is 0.05 to 2 g / ml, preferably 0.2 to 1.5 g / ml. If it is less than 0.05 g / ml, there are many particles that scatter, and the operation tends to be difficult. When it is larger than 2 g / ml, it tends to be fixed to the apparatus and the operation tends to be difficult.
[0050]
The pH of the nonmagnetic powder is preferably 2 to 11, but the pH is particularly preferably 6 to 9. When the pH is less than 2, the friction coefficient at high temperature and high humidity tends to increase. On the other hand, if the pH is higher than 11, the liberated amount of fatty acid tends to decrease and the friction coefficient tends to increase.
The water content of the nonmagnetic powder is 0.1 to 5% by mass, preferably 0.2 to 3% by mass, and more preferably 0.3 to 1.5% by mass. If it is less than 0.1% by mass, dispersion tends to be difficult. When it is larger than 5% by mass, the viscosity of the paint after dispersion tends to be unstable.
[0051]
The ignition loss is preferably 20% by mass or less, and the ignition loss is preferably small.
When the nonmagnetic powder is an inorganic powder, the Mohs hardness is preferably 4 or more and 10 or less. If the Mohs hardness is less than 4, the durability tends to be unable to be secured.
The stearic acid adsorption amount of the nonmagnetic powder is preferably 1 to 20 μmol / m ² And more preferably 2 to 15 μmol / m ² It is.
The heat of wetting of nonmagnetic powder into water at 25 ° C. is 200 erg / cm ² ~ 600 erg / cm ² It is preferable that it exists in the range. Moreover, the solvent which exists in the range of this heat of wetting can be used. The amount of water molecules on the surface at 100 to 400 ° C. is suitably 1 to 10 / 100Å. The pH of the isoelectric point in water is preferably between 3 and 9.
[0052]
The surface of these nonmagnetic powders is Al. ₂ O _Three , SiO ₂ TiO ₂ , ZrO ₂ , SnO ₂ , Sb ₂ O _Three It is preferable to perform surface treatment with ZnO. Particularly preferred for dispersibility is Al. ₂ O _Three , SiO ₂ TiO ₂ , ZrO ₂ More preferred is Al. ₂ O _Three , SiO ₂ , ZrO ₂ It is. These may be used in combination or may be used alone. Further, a surface-treated layer co-precipitated according to the purpose may be used, or a method of first treating with alumina and then treating the surface layer with silica, or vice versa. The surface treatment layer may be a porous layer depending on the purpose, but it is generally preferable that the surface treatment layer is homogeneous and dense.
[0053]
Specific examples of non-magnetic powders used in the lower layer of the present invention include Showa Denko Nanotite, Sumitomo Chemical HIT-100, ZA-G1, Toda Kogyo DPN-250, DPN-250BX, DPN-245, DPN-270BX, DPB-550BX, DPN-550RX Ishihara Sangyo Titanium oxide TTO-51B, TTO-55A, TTO-55B, TTO-55C, TTO-55S, TTO-55D, SN-100, MJ-7, α- Iron oxide E270, E271, E300, STT-4D, STT-30D, STT-30, STT-65C, manufactured by Titanium Industry, MT-100S, Teika MT-100S, MT-100T, MT-150W, MT-500B, MT-600B, MT -100F, MT-500HD. FINEX-25, BF-1, BF-10, BF-20, ST-M, Dowa Mining DEFIC-Y, DEFIC-R, Nippon Aerosil AS2BM, TiO2P25, Ube Industries 100A, 500A, Titanium Industry Y-LOP manufactured and what baked it are mentioned. Particularly preferred nonmagnetic powders are titanium dioxide and α-iron oxide.
[0054]
In the lower layer, by mixing carbon black together with nonmagnetic powder, the surface electrical resistance (Rs) can be lowered, the light transmittance can be reduced, and the desired micro Vickers hardness can be obtained.
The micro Vickers hardness of the lower layer is usually 25-60 kg / mm ² , Preferably 30-50kg / mm to adjust per head ² It is. The micro Vickers hardness can be measured using a thin film hardness meter (HMA-400 manufactured by NEC) using a diamond triangular pyramid needle having a ridge angle of 80 degrees and a tip radius of 0.1 μm at the tip of the indenter.
It is standardized that the light transmittance is generally 3% or less for absorption of infrared rays having a wavelength of about 900 nm, for example, 0.8% or less for a VHS magnetic tape. For this purpose, rubber furnace, rubber thermal, color black, acetylene black and the like can be used.
[0055]
The specific surface area of carbon black used for the lower layer of the present invention is 100 to 500 m. ² / G, preferably 150-400m ² / G, DBP oil absorption is 20 to 400 ml / 100 g, preferably 30 to 200 ml / 100 g. The particle size of carbon black is 5 to 80 nm, preferably 10 to 50 nm, and more preferably 10 to 40 nm. Carbon black preferably has a pH of 2 to 10, a water content of 0.1 to 10%, and a tap density of 0.1 to 1 g / ml.
Specific examples of carbon black used in the present invention include Cabot Corporation, BLACKPEARLS 2000, 1300, 1000, 900, 800, 880, 700, VULCAN XC-72, Mitsubishi Chemical Industries, Ltd., # 3050B, 3150B, 3250B. # 3750B, # 3950B, # 950, # 650B, # 970B, # 850B, MA-600, manufactured by Columbia Carbon Co., CONDUCTEX SC, RAVEN 8800, 8000, 7000, 5750, 5250, 3500, 2100, 2000, 1800, 1500, 1255, 1250, manufactured by Akzo, Ketjen Black EC and the like.
[0056]
Carbon black may be surface-treated with a dispersant or the like, or may be used after being grafted with a resin, or may be obtained by graphitizing a part of the surface. Moreover, before adding carbon black to a coating material, you may disperse | distribute with a binder beforehand. These carbon blacks can be used in a range not exceeding 50% by mass and not exceeding 40% of the total mass of the lower layer with respect to the inorganic powder. These carbon blacks can be used alone or in combination. Carbon black that can be used in the lower layer of the present invention can be referred to, for example, “Carbon Black Handbook” edited by Carbon Black Association.
[0057]
Further, an organic powder can be added to the lower layer depending on the purpose. Examples include acrylic styrene resin powder, benzoguanamine resin powder, melamine resin powder, and phthalocyanine pigment, but polyolefin resin powder, polyester resin powder, polyamide resin powder, polyimide resin powder, and polyfluorinated ethylene resin are also included. Can be used.
[0058]
IV. Other additives
In the magnetic recording medium of the present invention, the magnetic layer or the lower layer may contain additives for imparting a dispersion effect, a lubrication effect, an antistatic effect, a plasticizing effect, and the like. These additives include molybdenum disulfide, tungsten disulfide, graphite, boron nitride, graphite fluoride, silicone oil, silicone with polar groups, fatty acid-modified silicone, fluorine-containing silicone, fluorine-containing alcohol, fluorine-containing ester, polyolefin, poly Glycol, polyphenyl ether, phenylphosphonic acid, benzylphosphonic acid group, phenethylphosphonic acid, α-methylbenzylphosphonic acid, 1-methyl-1-phenethylphosphonic acid, diphenylmethylphosphonic acid, biphenylphosphonic acid, benzylphenylphosphonic acid, α -Cumylphosphonic acid, toluylphosphonic acid, xylylphosphonic acid, ethylphenylphosphonic acid, cumenylphosphonic acid, propylphenylphosphonic acid, butylphenylphosphonic acid, heptylphenylphosphoric acid Aromatic ring-containing organic phosphonic acids such as sulfonic acid, octylphenylphosphonic acid, nonylphenylphosphonic acid and alkali metal salts thereof, octylphosphonic acid, 2-ethylhexylphosphonic acid, isooctylphosphonic acid, (iso) nonylphosphonic acid, ( Alkylphosphonic acids such as iso) decylphosphonic acid, (iso) undecylphosphonic acid, (iso) dodecylphosphonic acid, (iso) hexadecylphosphonic acid, (iso) octadecylphosphonic acid, (iso) eicosylphosphonic acid and the like Alkali metal salts, phenyl phosphate, benzyl phosphate, phenethyl phosphate, α-methylbenzyl phosphate, 1-methyl-1-phenethyl phosphate, diphenylmethyl phosphate, biphenyl phosphate, benzylphenyl phosphate, α-cumyl phosphate, toluyl phosphate, xylyl phosphate, Ethylphenyl phosphate, phosphorus Aromatic phosphoric esters such as cumenyl, propylphenyl phosphate, butylphenyl phosphate, heptylphenyl phosphate, octylphenyl phosphate, nonylphenyl phosphate, and alkali metal salts thereof, octyl phosphate, 2-ethylhexyl phosphate, isooctyl phosphate, (iso) nonyl phosphate Alkyl phosphates such as (iso) decyl phosphate, (iso) undecyl phosphate, (iso) dodecyl phosphate, (iso) hexadecyl phosphate, (iso) octadecyl phosphate, eicosyl phosphate and alkali metal salts thereof, alkylsulfonic acid Esters and alkali metal salts thereof, fluorine-containing alkyl sulfates and alkali metal salts thereof, lauric acid, myristic acid, palmitic acid, stearic acid, behenic acid, butyl stearate, oleic acid, linoleic acid, linolenic acid, Eleiji Monobasic fatty acids which may contain or branch an unsaturated bond having 10 to 24 carbon atoms such as acid, erucic acid, acid and the like, and metal salts thereof, or butyl stearate, octyl stearate, amyl stearate, Contains 10 to 24 carbon unsaturated bonds such as isooctyl stearate, octyl myristate, butyl laurate, butoxyethyl stearate, anhydrosorbitan monostearate, anhydrosorbitan distearate, anhydrosorbitan tristearate However, it may be branched even if it contains a monobasic fatty acid that may be branched, an unsaturated bond having 2 to 22 carbon atoms or a 1 to 6-valent alcohol that may be branched, or an unsaturated bond having 12 to 22 carbon atoms. Monoalkyl ether of alkoxy alcohol or alkylene oxide polymer which may be Mono-fatty acid esters consisting of any one of, di-fatty acid esters or polyvalent fatty acid esters, fatty acid amides having 2 to 22 carbon atoms, and aliphatic amines having 8 to 22 carbon atoms can be used. In addition to the above hydrocarbon groups, nitro groups and F, Cl, Br, CF _Three , CCl _Three , CBr _Three It may have an alkyl group, an aryl group, or an aralkyl group substituted with a group other than a hydrocarbon group such as a halogen-containing hydrocarbon.
[0059]
In addition, nonionic surfactants such as alkylene oxide, glycerin, glycidol, and alkylphenol ethylene oxide adducts, cyclic amines, ester amides, quaternary ammonium salts, hydantoin derivatives, heterocycles, phosphonium or sulfoniums, etc. Amphoteric interfaces such as cationic surfactants, anionic surfactants containing acidic groups such as carboxylic acid, sulfonic acid, and sulfate ester groups, amino acids, aminosulfonic acids, sulfuric or phosphate esters of aminoalcohols, and alkylbetaines An activator or the like can also be used. These surfactants are described in detail in “Surfactant Handbook” (published by Sangyo Tosho Co., Ltd.). These lubricants, antistatic agents, and the like are not necessarily pure, and may contain impurities such as isomers, unreacted materials, side reaction products, decomposed products, and oxides in addition to the main components. These impurities are preferably 30% by mass or less, more preferably 10% by mass or less.
[0060]
Specific examples of these include: Nippon Oil & Fats Corporation: NAA-102, castor oil hardened fatty acid, NAA-42, cationic SA, Naimine L-201, Nonion E-208, Anon BF, Anon LG, Takemoto Yushi Co., Ltd .: FAL- 205, FAL-123, manufactured by Shin Nippon Chemical Co., Ltd .: NJ Lube OL, manufactured by Shin-Etsu Chemical Co., Ltd .: TA-3, manufactured by Lion Armor Co., Ltd .: Armido P, manufactured by Lion Co., Ltd., Duomin TDO, manufactured by Nisshin Oil Industries: BA-41G, Sanyo Kasei Co., Ltd .: Profan 2012E, New Pole PE61, Ionette MS-400, and the like.
[0061]
These dispersants, lubricants, and surfactants used in the present invention can be used properly in the lower layer and the magnetic layer as needed. For example, of course, the dispersant is not limited to the example shown here, but the dispersant has a property of adsorbing or bonding with a polar group, and in the magnetic layer, mainly on the surface of the ferromagnetic powder, and in the lower layer. It is presumed that the organic phosphorus compound adsorbed or bonded mainly to the surface of the nonmagnetic powder with the polar group and once adsorbed is difficult to desorb from the surface of the metal or metal compound. Accordingly, since the surface of the ferromagnetic powder or nonmagnetic powder of the present invention is coated with an alkyl group, aromatic group or the like, the affinity of the ferromagnetic powder or nonmagnetic powder for the binder resin component is high. In addition, the dispersion stability of the ferromagnetic powder or non-magnetic powder is also improved. In addition, since the lubricant exists in a free state, the lower layer and the magnetic layer use fatty acids having different melting points to control the bleeding to the surface, and esters having different boiling points and polarities to control the bleeding to the surface. It is conceivable to improve the stability of coating by adjusting the amount of the surfactant and to increase the amount of lubricant added in the lower layer to improve the lubricating effect.
[0062]
All or part of the additives used in the present invention may be added in any step during the production of the magnetic layer or lower layer coating solution. For example, when mixing with a ferromagnetic powder before the kneading step, when adding at a kneading step with a ferromagnetic powder, a binder and a solvent, when adding at a dispersing step, when adding after dispersing, when adding just before coating, etc. There is.
[0063]
V. Non-magnetic support
In the magnetic recording medium of the present invention, a lower layer or a magnetic layer is formed by applying a coating solution prepared from the above materials onto a nonmagnetic support.
As the nonmagnetic support that can be used in the present invention, known ones such as biaxially stretched polyethylene naphthalate, polyethylene terephthalate, polyamide, polyimide, polyamideimide, aromatic polyamide, and polybenzoxydazole can be used. Polyethylene naphthalate and aromatic polyamide are preferred. These nonmagnetic supports may be subjected in advance to corona discharge, plasma treatment, easy adhesion treatment, heat treatment and the like.
[0064]
The nonmagnetic support that can be used in the present invention has a surface having excellent smoothness with a centerline average surface roughness in the range of 0.1 to 20 nm, preferably 1 to 10 nm at a cutoff value of 0.25 mm. Preferably there is. These non-magnetic supports preferably have not only a small center line average surface roughness but also no coarse protrusions of 1 μm or more.
The arithmetic average roughness of the obtained support is preferably 0.1 μm or less in terms of (Ra) [JIS B0660-1998, ISO 4287-1997] because noise of the obtained magnetic recording medium is reduced. .
The preferred thickness of the nonmagnetic support in the magnetic recording medium of the present invention is 3 to 80 μm.
[0065]
VI. Backcoat layer, undercoat layer
A back coat layer (backing layer) may be provided on the surface of the nonmagnetic support used in the present invention on which the magnetic paint is not applied. The back coat layer is provided by applying a back coat layer forming paint in which particulate components such as abrasives and antistatic agents and a binder are dispersed in an organic solvent on the surface of the nonmagnetic support on which the magnetic paint is not applied. Layer. Various inorganic pigments and carbon black can be used as the particulate component, and as the binder, resins such as nitrocellulose, phenoxy resin, and polyurethane can be used alone or as a mixture thereof. An adhesive layer may be provided on the application surface of the magnetic coating material and the backcoat layer forming coating material of the nonmagnetic support of the present invention.
[0066]
In the magnetic recording medium of the present invention, an undercoat layer may be provided. By providing the undercoat layer, the adhesive force between the support and the magnetic layer or the lower layer can be improved. As the undercoat layer, a solvent-soluble polyester resin is used. The undercoat layer has a thickness of 0.5 μm or less.
[0067]
VII. Production method
In the method for producing a magnetic recording medium of the present invention, for example, a magnetic coating solution is applied to the surface of a nonmagnetic support under running so as to have a predetermined film thickness. Here, a plurality of magnetic layer coating solutions may be applied sequentially or simultaneously, and a lower layer coating solution and a magnetic layer coating solution may be applied sequentially or simultaneously. As the coating machine for applying the above magnetic coating solution or lower layer coating solution, air doctor coat, blade coat, rod coat, extrusion coat, air knife coat, squeeze coat, impregnation coat, reverse roll coat, transfer roll coat, gravure coat, kiss coat Cast coat, spray coat, spin coat, etc. can be used.
For example, “Latest coating technology” (May 31, 1983) issued by the General Technology Center Co., Ltd. can be referred to. When applied to the magnetic recording medium of the present invention, the following can be proposed as an example of a coating apparatus and method.
[0068]
First, the lower layer is applied by a coating apparatus such as gravure, roll, blade, and extrusion, which is generally applied in the application of the magnetic layer coating solution.
(1) The upper layer is applied by a support pressure type extrusion coating apparatus as disclosed in JP-B-1-46186, JP-A-60-238179, JP-A-2-265672, and the like.
(2) Upper and lower layers by one coating head having two coating liquid passage slits as disclosed in JP-A-63-88080, JP-A-2-17971, and JP-A-2-265672. Are applied almost simultaneously.
(3) The upper and lower layers are applied almost simultaneously by an extrusion coating apparatus with a backup roll as disclosed in JP-A-2-174965.
[0069]
The thickness of the magnetic layer of the magnetic recording medium of the present invention is optimized by the saturation magnetization amount of the head used, the head gap length, and the band of the recording signal, but is generally 0.01 μm or more and 0.10 μm or less, Preferably they are 0.02 micrometer or more and 0.08 micrometer or less, More preferably, they are 0.03 micrometer or more and 0.08 micrometers or less. The magnetic layer may be separated into two or more layers having different magnetic characteristics, and a configuration related to a known multilayer magnetic layer can be applied.
In order to stably apply the ultrathin magnetic layer, it is desirable to interpose a lower layer containing an inorganic powder on the support, and apply the magnetic layer thereon by wet-on-wet.
[0070]
In the case of a magnetic tape, the magnetic layer coating liquid coating layer is obtained by subjecting the ferromagnetic powder contained in the magnetic layer coating liquid coating layer to a magnetic field orientation treatment in the longitudinal direction using a cobalt magnet or a solenoid. In the case of a disk, a sufficiently isotropic orientation may be obtained even without non-orientation without using an orientation device, but known random methods such as alternately arranging cobalt magnets obliquely and applying an alternating magnetic field with a solenoid. It is preferable to use an alignment device. In the case of a ferromagnetic metal fine powder, the isotropic orientation is generally preferably in-plane two-dimensional random, but can also be three-dimensional random with a vertical component. In the case of hexagonal ferrite, in general, it tends to be in-plane and vertical three-dimensional random, but in-plane two-dimensional random is also possible. Further, isotropic magnetic characteristics can be imparted in the circumferential direction by using a well-known method such as a counter-polarized magnet and making it vertically oriented. In particular, when performing high density recording, vertical alignment is preferable. Further, circumferential orientation may be performed using spin coating.
[0071]
It is preferable that the drying position of the coating film can be controlled by controlling the temperature, air volume, and coating speed of the drying air, the coating speed is 20 m / min to 1,000 m / min, and the temperature of the drying air is 60 ° C. or higher. It is also preferable that moderate pre-drying can be performed before entering the magnet zone.
[0072]
After being dried, the coating layer is subjected to a surface smoothing treatment. For the surface smoothing process, for example, a super calendar roll or the like is used. By performing the surface smoothing treatment, voids generated by removing the solvent during drying disappear and the filling rate of the ferromagnetic powder in the magnetic layer is improved, so that a magnetic recording medium having high electromagnetic conversion characteristics can be obtained. it can.
As the calendering roll, a heat-resistant plastic roll such as epoxy, polyimide, polyamide, polyamideimide or the like is used. Moreover, it can also process with a metal roll.
The magnetic recording medium of the present invention has a surface having extremely excellent smoothness with a center line average roughness of 0.1 to 4 nm, preferably 1 to 3 nm, at a cutoff value of 0.25 mm. preferable. As the method, for example, as described above, a magnetic layer formed by selecting a specific ferromagnetic powder and a binder is subjected to the above calender treatment.
[0073]
As the calendering conditions, the temperature of the calender roll is in the range of 60 to 100 ° C, preferably 70 to 100 ° C, more preferably 80 to 100 ° C. The pressure of the calendar roll is 100 to 500 kg / cm, preferably 200 to 450 kg / cm, more preferably 300 to 400 kg / cm. The obtained magnetic recording medium can be cut into a desired size using a cutting machine or the like.
[0074]
【Example】
Synthetic examples of acrylic resins and polyurethane resins used as binders, and specific examples and comparative examples using these resins are given, but the present invention is not limited to these examples. In addition, unless otherwise indicated, the display of "part" in an Example shows a "mass part".
[0075]
[Synthesis Example AC-1]
<Synthesis of acrylic resin AC-1>
A polymerization vessel equipped with a stirrer, a condenser, a thermometer and a nitrogen gas inlet was charged with 360 parts of deionized water, 5 parts of potassium persulfate and 1.6 parts of sodium carbonate, and the temperature was raised to 57 ° C. after nitrogen substitution. Meanwhile, 390 parts of deionized water, 165 parts of methyl methacrylate, 50 parts of benzyl methacrylate, 275 parts of N-vinylpyrrolidone, 10 parts of 2-hydroxymethyl methacrylate and 10 parts of sodium lauryl sulfate were mixed and emulsified with a homomixer in advance. It took 8 hours to drop uniformly into the container, and further reacted at 57 ° C. for 2 hours to complete the polymerization, and then 500 parts of methanol and 50 parts of sodium sulfate were added to precipitate the polymer. The precipitated polymer was washed twice with 5,000 parts of methanol and then four times with 5,000 parts of deionized water, filtered and dried to obtain acrylic resin AC-1. Sodium sulfonate group content is 6.5 × 10 ^-Five eq / g.
[0076]
[Synthesis Examples AC-2 to 5, ac-6, 7]
Synthesis of acrylic resins AC-2-5, ac-6, 7
It copolymerized by the kind and quantity ratio (mass%) of the monomer shown by Table 1 like the synthesis example AC-1, and processed by the same method, and obtained acrylic resin.
[0077]
[Table 1]

[0078]
[Synthesis Example PUA-1]
Synthesis of polyurethane resin PUA-1
A polyol having a composition shown in Table 2 was dissolved in a 30% cyclohexanone solution at 60 ° C. under a nitrogen stream in a container equipped with a thermometer, a stirrer, and a reflux condenser and previously purged with nitrogen. Next, 60 ppm of dibutyltin dilaurate was added as a catalyst and further dissolved for 15 minutes. Furthermore, the isocyanate compound shown in Table 2 was added and heated for 6 hours to obtain a polyurethane resin PUA-1.
[0079]
[Synthesis Example PUA-2, a-1, 2]
Synthesis of polyurethane resins PUA-2, a-1 and 2
In the same manner as in Synthesis Example PUA-1, polyurethane resins PUA-2, a-1 and 2 were obtained using a chain extender and an organic diisocyanate compound shown in Table 2.
[0080]
[Table 2]

[0081]
[Example 1]
<Adjustment of upper layer magnetic coating liquid>
100 parts of ferromagnetic needle metal powder
Composition: Fe / Co / Al / Y = 68/20/7/5,
Surface treatment agent: Al ₂ O _Three , Y ₂ O _Three , Hc: 2500 Oe,
Crystallite size: 140 mm, major axis diameter: 0.08 μm,
Needle ratio: 6, BET specific surface area: 46m ² / G, σs: 150 emu / g
Acrylic resin AC-1 (described in Table 1) 6 parts
Polyurethane resin PUA-1 (described in Table 2) 12 parts
Phenylphosphonic acid 3 parts
α-Al ₂ O _Three (Particle size 0.15 μm) 2 parts
Carbon black (particle size 20nm) 2 parts
110 parts of cyclohexanone
100 parts of methyl ethyl ketone
100 parts of toluene
Butyl stearate 2 parts
1 part of stearic acid
[0082]

[0083]
For each of the magnetic coating composition for the upper layer and the nonmagnetic coating composition for the lower layer, each component was kneaded for 60 minutes with an open kneader and then dispersed for 120 minutes with a sand mill. To the obtained dispersion, 6 parts of a trifunctional low molecular weight polyisocyanate compound (Coronate 3041 manufactured by Nippon Polyurethane) was added and stirred for 20 minutes, followed by filtration using a filter having an average pore size of 1 μm, A non-magnetic paint was prepared.
Further, the nonmagnetic coating material was applied so that the thickness after drying was 1.8 μm, and immediately thereafter, the magnetic coating material was simultaneously applied in multiple layers so that the thickness after drying was 0.08 μm. With both layers undried, magnetic field orientation was performed with a 3,000 gauss magnet, and after drying, the speed was 100 m / min, linear pressure 300 kg / cm, temperature 90 ° C. with a seven-stage calendar composed of only metal rolls. After performing the surface smoothing treatment at 70 ° C., a heat curing treatment was performed at 70 ° C. for 24 hours, and slitting to a width of 3.8 mm to prepare a magnetic tape.
[0084]
[Examples 2 to 6]
The acrylic resin and the polyurethane resin were changed as shown in Table 3, and Examples 2 to 6 were prepared in the same manner as in Example 1.
[Comparative Examples 1-6]
The acrylic resin, polyurethane resin and magnetic material were changed as shown in Table 3, and Comparative Examples 1 to 6 were prepared in the same manner as in Example 1. MR110 and N-2304 in Table 3 indicate a commercially available polar group-containing vinyl chloride resin (manufactured by ZEON Corporation) and a polar group-containing polyurethane resin (manufactured by Nippon Polyurethane), respectively.
[0085]
[Table 3]

[0086]
〔Measuring method〕
(1) Magnetic layer surface roughness Ra
The center line average roughness was Ra by a light interference method using a digital optical profilometer (manufactured by WYKO) under the condition of a cutoff of 0.25 mm.
(2) Head dirt after running 500 passes
The sample tape was repeatedly run with a total length of 500 passes with a DDS drive, and head contamination after running was observed.
The case where the stain was seen was marked with ×, and the case where the stain was not seen was marked with ○.
(3) Head corrosion
The permalloy, which is a metal member of the MR head, was left in a 60 ° C. 90% RH environment for 2 weeks in contact with the magnetic layer.
The case where discoloration such as corrosion was seen was marked with ×, and the case where color change was not seen was marked with ○.
[0087]
[Example 7]
<Adjustment of upper layer magnetic coating liquid>
100 parts of ferromagnetic plate-shaped hexagonal ferrite powder
Composition (molar ratio): Ba / Fe / Co / Zn = 1 / 9.1 / 0.2 / 0.8
Hc: 2450 Oe, plate diameter: 26 nm, plate ratio: 4
BET specific surface area: 50 m ² / G, σs: 60 emu / g
Acrylic resin AC-2 (described in Table 1) 6 parts
Polyurethane resin PUA-1 (described in Table 2) 12 parts
Phenylphosphonic acid 3 parts
α-Al ₂ O _Three (Particle size 0.15 μm) 2 parts
Carbon black (particle size 20nm) 2 parts
110 parts of cyclohexanone
100 parts of methyl ethyl ketone
100 parts of toluene
Butyl stearate 2 parts
1 part of stearic acid
[0088]
A magnetic paint was prepared in the same manner as in Example 1 for each component of the magnetic paint for the upper layer. Further, the same non-magnetic paint as in Example 1 was applied so that the thickness after drying was 1.5 μm, and immediately after that, the magnetic paint was thickened so that the thickness after drying was 0.08 μm. Simultaneous multilayer coating was performed on a polyethylene terephthalate support having a thickness of 62 μm and a center surface average roughness of 3 μm. While both layers were still wet, they were passed through two AC magnetic field generators having a frequency of 50 Hz, a magnetic field strength of 250 gauss, a frequency of 50 Hz, and a magnetic field strength of 120 gauss to perform random orientation treatment. After drying, it was treated at a temperature of 90 ° C. and a linear pressure of 300 kg / cm with a seven-stage calendar, punched to 3.7 inches, surface-polished, and then a 3.7-inch liner installed on the inside. It was put into a Zip-disk cartridge, and predetermined mechanical parts were added to obtain a 3.7 inch flexible disk.
[0089]
[Examples 8 to 12]
The acrylic resin and the polyurethane resin were changed as shown in Table 4, and Examples 8 to 12 were prepared in the same manner as in Example 7.
[Comparative Examples 7-12]
The acrylic resin, polyurethane resin and magnetic material were changed as shown in Table 4, and Comparative Examples 7 to 12 were prepared in the same manner as in Example 7. MR110 and N-2304 in Table 4 indicate a commercially available polar group-containing vinyl chloride resin (manufactured by ZEON Corporation) and a polar group-containing polyurethane resin (manufactured by Nippon Polyurethane), respectively.
[0090]
[Table 4]

[0091]
〔Measuring method〕
(1) Magnetic layer surface roughness Ra: Measured by the same method as for magnetic tape.
(2) Head dirt after OTM sliding: Head dirt after a disk-like sample was slid OTM (One Track Multipass) with a Zip drive was observed. The conditions were a rotational speed of 3600 rpm and 200 hours.
(3) Head corrosion: Measured by the same method as for the tape-shaped medium.
[0092]
【The invention's effect】
According to the present invention, it is possible to provide a magnetic recording medium with improved coating film strength, less magnetic layer surface scraping, less head contamination, and excellent running durability.

Claims (5)

In a magnetic recording medium having a magnetic layer in which a lower layer containing an inorganic powder and / or magnetic powder and a binder is provided on a nonmagnetic support and at least one ferromagnetic fine powder and a binder are dispersed thereon. The binder in the lower layer was obtained by reacting the following acrylic copolymer and a polyol compound having a cyclic structure and a long-chain alkyl chain with an organic diisocyanate, and used in combination with a polyurethane resin having a hydrophilic polar group. A magnetic recording medium characterized by the above.
Acrylic copolymer: 45 to 80% by mass of a radically polymerizable monomer having nitrogen as a raw material with respect to the total amount of monomers, including a (meth) acrylate monomer having an alkyl (meth) acrylate and / or a benzene ring An acrylic copolymer obtained by copolymerizing these and having a hydrophilic polar group in the molecule. In a magnetic recording medium having a magnetic layer in which a lower layer containing an inorganic powder and / or magnetic powder and a binder is provided on a nonmagnetic support and at least one ferromagnetic fine powder and a binder are dispersed thereon. The binder in the magnetic layer is obtained by reacting the following acrylic copolymer and a polyol compound having a cyclic structure and a long alkyl chain with an organic diisocyanate, and a polyurethane resin having a hydrophilic polar group. The magnetic recording medium according to claim 1.
Acrylic copolymer: 45 to 80% by mass of a radically polymerizable monomer having nitrogen as a raw material with respect to the total amount of monomers, including a (meth) acrylate monomer having an alkyl (meth) acrylate and / or a benzene ring An acrylic copolymer obtained by copolymerizing these and having a hydrophilic polar group in the molecule. The magnetic recording medium according to claim 1, wherein the acrylic copolymer has a sulfonate group as a hydrophilic polar group. The magnetic recording medium according to any one of claims 1 to 3, wherein the content of the polymerizable monomer having a hydrophilic polar group is 0.3 to 6% by mass. The magnetic recording medium according to any one of claims 1 to 4, which is an acrylic copolymer having a hydrophilic polar group introduced by a persulfate used as a polymerization initiator.