JP3832539B2 - Magnetic recording medium - Google Patents

Description

【０１７７】
得られた紡錘状ヘマタイト粒子粉末を含む塗料を厚さ１２μｍのポリエチレンテレフタレートフィルム上にアプリケーターを用いて５５μｍの厚さに塗布し、次いで、乾燥させることにより非磁性下地層を形成した。非磁性下地層の厚みは３．５μｍであった。
【０１７８】
得られた非磁性下地層は、光沢が２０４％、表面粗度Ｒａが６．６ｎｍ、塗膜のヤング率（相対値）は１２４であった。
【０１７９】
＜磁気記録媒体の製造−磁気記録層の形成＞
鉄を主成分とする針状金属磁性粒子粉末（平均長軸径０．１１５μｍ、平均短軸径０．０１４５μｍ、軸比７．９、保磁力１９０９Ｏｅ、飽和磁化値１３３．８ｅｍｕ／ｇ、Ａｌ含有量（中心部１．２６重量％、表層部０．８４重量％、表面被覆０．７５重量％）、Ｎｄ含有量０．１２重量％、樹脂吸着強度８１．６％）１２ｇ、研磨剤（商品名：ＡＫＰ−３０、住友化学（株）製）１．２ｇ、カーボンブラック（商品名：＃３２５０Ｂ、三菱化成（株）製）０．３６ｇ、結合剤樹脂溶液（スルホン酸ナトリウム基を有する塩化ビニル−酢酸ビニル共重合樹脂３０重量％とシクロヘキサノン７０重量％）及びシクロヘキサノンとを混合して混合物（固形分率７８％）を得、この混合物を更にプラストミルで３０分間混練して混練物を得た。
【０１８０】
この混練物を１４０ｍｌガラス瓶に１．５ｍｍφガラスビーズ９５ｇ、結合剤樹脂溶液（スルホン酸ナトリウム基を有するポリウレタン樹脂３０重量％、溶剤（メチルエチルケトン：トルエン＝１：１）７０重量％）、シクロヘキサノン、メチルエチルケトン及びトルエンとともに添加し、ペイントシェーカーで６時間混合・分散を行った後、潤滑剤及び硬化剤を加え、更に、ペイントシェーカーで１５分間混合・分散して磁性塗料を得た。
【０１８１】
得られた磁性塗料の組成は下記の通りであった。

【０１８２】
上記磁性塗料を前記非磁性下地層の上にアプリケーターを用いて１５μｍの厚さに塗布した後、磁場中において配向・乾燥し、次いで、カレンダー処理を行った後、６０℃で２４時間硬化反応を行い０．５インチ幅にスリットして磁気テープを得た。磁気記録層の厚みは１．０μｍであった。
【０１８３】
得られた磁気テープは、保磁力値が１９８０Ｏｅ、角型比（Ｂｒ／Ｂｍ）が０．８８、光沢度が２２３％、表面粗度Ｒａが６．４ｎｍ、塗膜のヤング率（相対値）が１２８、線吸収係数が１．２４μｍ^−１、走行耐久性が２８．９分、すり傷特性がＡであり、磁気テープの耐腐蝕性を示す保磁力の変化率は４．６％、飽和磁束密度の変化率は２．７％であった。
【０１８４】
【作用】
本発明において最も重要な点は、平均長軸径０．００５〜０．３０μｍ、ＢＥＴ比表面積値３５〜１５０ｍ^２／ｇである針状ヘマタイト粒子粉末の水性懸濁液を酸濃度１．０Ｎ以上，ｐＨ値３．０以下，温度範囲２０〜１００℃の条件で酸による溶解処理を行い、該水性懸濁液中に存在する針状ヘマタイト粒子粉末全体量の５〜５０重量％を溶解させた後、残存する針状ヘマタイト粒子粉末を水洗して得られる針状ヘマタイト粒子粉末の水性懸濁液にアルカリ水溶液を添加してｐＨ値を１３以上に調製した後、８０〜１０３℃の温度範囲で加熱処理し、次いで、濾別、水洗、乾燥して得られた平均長軸径０．００４〜０．２９５μｍ、ＢＥＴ比表面積値３５．９〜２１２ｍ^２／ｇ、粉体ｐＨ値が８以上、且つ、可溶性ナトリウム塩の含有量がＮａ換算で３００ｐｐｍ以下、可溶性硫酸塩の含有量がＳＯ_４換算で１５０ｐｐｍ以下の針状ヘマタイト粒子粉末を非磁性下地層用の非磁性粒子粉末として使用した場合には、該結合剤樹脂中における分散性が優れていることに起因して、非磁性下地層の表面平滑性と基体の強度を向上させることができ、当該非磁性下地層の上にアルミニウムが存在している鉄を主成分とする針状金属磁性粒子粉末を用いた磁気記録層を設けた場合に、磁気記録層の光透過率が小さく、強度が大きいとともに、より表面平滑であって、且つ、より耐久性に優れている磁気記録媒体を得ることができるとともに、磁気記録層中に分散させているアルミニウムが存在している鉄を主成分とする金属磁性粒子粉末の腐蝕に伴う磁気特性の劣化を抑制することができるという事実である。
【０１８５】
非磁性下地層の表面平滑性と基体の強度を向上させることができた理由について、本発明者は、高密度針状ヘマタイト粒子相互を強固に架橋して凝集させる原因となっている可溶性ナトリウム塩や可溶性硫酸塩を十分水洗除去することができたことに起因して、凝集物が解きほぐされて、実質的に独立している粒子とすることができ、その結果、ビヒクル中における分散性が優れた針状ヘマタイト粒子粉末が得られることによるものと考えている。
【０１８６】
本発明に係る磁気記録媒体の表面平滑性が向上する理由について、本発明者は、本発明における針状ヘマタイト粒子の長軸径の幾何標準偏差値が１．５０以下であり、粗大な粒子や微細な粒子の存在が少ない粒子であること及びＢＥＴ比表面積値が３５．９〜２１２ｍ^２／ｇであり、粒子内部及び粒子表面に脱水孔が少ない粒子であることの相乗効果により、ビヒクル中での分散性が向上し、その結果、得られる非磁性下地層の表面平滑性が向上したためと考えている。
【０１８７】
また、本発明に係る磁気記録媒体の表面平滑性が優れているもう一つの理由として、本発明者は、針状ヘマタイト粒子相互を強固に架橋して凝集させる原因となっている可溶性ナトリウム塩や可溶性硫酸塩を十分水洗除去することができたことに起因して、凝集物が解きほぐされて、実質的に独立している粒子とすることができ、その結果、ビヒクル中における分散性が優れた針状ヘマタイト粒子が得られることによるものと考えている。
【０１８８】
本発明における針状ヘマタイト粒子粉末はビヒクル中における分散性が優れているという事実について、以下に説明する。
【０１８９】
出発原料として使用する針状ゲータイト粒子粉末は、前述した通り、各種製造法により製造されるが、いずれの方法においても針状ゲータイト粒子粉末を製造する主な原料が硫酸第一鉄である場合には当然反応母液中に硫酸塩〔ＳＯ_４ ^２−〕が多量に存在している。
【０１９０】
特に、酸性溶液中からゲータイト粒子を生成する場合には、同時に、Ｎａ_２ＳＯ_４等水可溶性硫酸塩を生じるとともに、反応母液にはＫ^＋、ＮＨ^４＋、Ｎａ^＋等アルカリ金属を含んでいるので、アルカリ金属や硫酸塩を含む沈澱を生じ易く、この沈澱はＲＦｅ_３（ＳＯ_４）（ＯＨ）_６（Ｒ＝Ｋ^＋、ＮＨ^４＋、Ｎａ^＋）で示される。これら沈澱物は難溶性の含硫鉄塩で常法による水洗によっては除去することができない。この難溶性塩はその後の加熱処理工程において可溶性ナトリウム塩や可溶性硫酸塩になるが、この可溶性ナトリウム塩や可溶性硫酸塩は、高密度化のための高温加熱処理工程において針状ヘマタイト粒子の形状の変形、粒子相互間の焼結を防止するために必須である焼結防止剤によって、針状ヘマタイト粒子相互を架橋しながら粒子内部及び粒子表面に強固に結合されることにより、針状ヘマタイト粒子相互間の凝集が一層強まる。その結果、殊に、粒子内部や凝集物内部に閉じ込められた可溶性硫酸塩や可溶性ナトリウム塩は、常法による水洗によって除去することが極めて困難となる。
【０１９１】
硫酸第一鉄と水酸化ナトリウムとを用いてアルカリ性水溶液中で針状ゲータイト粒子を生成する場合には、同時に生成される硫酸塩はＮａ_２ＳＯ_４であり、また、母液中にＮａＯＨが存在し、これらは共に可溶性であるため針状ゲータイト粒子を十分水洗すれば本質的にはＮａ_２ＳＯ_４及びＮａＯＨを除去できるはずである。しかし、一般には針状ゲータイト粒子の結晶性が小さいため、水洗効率が悪く、常法により水洗した場合、なお、粒子中に可溶性硫酸塩〔ＳＯ_４ ^２−〕、可溶性ナトリウム塩〔Ｎａ^＋〕等水可溶性分を含んでいる。そして、この水可溶性分は、前述した通り、焼結防止剤によって針状ヘマタイト粒子相互を架橋しながら粒子内部及び粒子表面に強固に結合されることにより、針状ヘマタイト粒子相互間の凝集が一層強まる。その結果、殊に、粒子内部や凝集物内部に閉じ込められた可溶性硫酸塩や可溶性ナトリウム塩は、常法による水洗によって除去することが極めて困難となる。
【０１９２】
上述した通り、可溶性ナトリウム塩や可溶性硫酸塩が焼結防止剤を介在して粒子内部や粒子表面及び凝集物内部に強く結合されている高密度ヘマタイト粒子は、湿式粉砕して粗粒をほぐした後、スラリーのｐＨ値を１３以上に調整し、８０℃以上の温度で加熱処理すると、アルカリ性水溶液が高密度ヘマタイト粒子の粒子内部まで十分浸透し、その結果、粒子内部や粒子表面及び凝集物内部に強く結合している焼結防止剤の結合力が徐々に弱まり、粒子内部や粒子表面及び凝集物内部から解離され、同時に水可溶性ナトリウム塩や水可溶性硫酸塩も水洗除去しやすくなるものと考えられる。
【０１９３】
また、磁気記録層中に分散されている鉄を主成分とする針状金属磁性粒子粉末の腐蝕に伴う磁気特性の劣化が抑制される理由について、本発明者は、非磁性下地層用針状ヘマタイト粒子粉末の金属の腐蝕を促進する可溶性ナトリウム塩や可溶性硫酸塩等の可溶性分が少ないこと及び粉体ｐＨ値が８以上と高いことに起因して、鉄を主成分とする針状金属磁性粒子粉末の腐蝕の進行を抑制できたものと考えている。
【０１９４】
事実、本発明者は、後出の実施例及び比較例に示す通り、湿式粉砕後の針状ヘマタイト粒子粉末を、ｐＨ値が１３未満のアルカリ性懸濁液中で８０℃以上の温度をかけて加熱処理した場合、湿式粉砕後の針状ヘマタイト粒子粉末を、ｐＨ値が１３以上のアルカリ性懸濁液中で８０℃未満の温度をかけて加熱処理した場合、針状ヘマタイト粒子粉末を湿式粉砕をすることなく粗粒を含んだままで、ｐＨ値１３以上のアルカリ性懸濁液中で８０℃以上の温度をかけて加熱処理した場合のいずれの場合にも、アルミニウムが存在している鉄を主成分とする金属磁性粒子粉末の腐蝕の進行を十分には抑制できないことから、可溶性分が少ないことと、粉体ｐＨ値が８以上であることの相乗効果により鉄を主成分とする金属磁性粒子粉末の腐蝕の進行が抑制できるという現象を確認している。
【０１９５】
また、本発明に係る磁気記録媒体の耐久性が向上した理由については未だ明らかではないが、本発明者は、アルミニウムが存在している鉄を主成分とする針状金属磁性粒子粉末を磁性粒子として用いたことに起因して、塗料中における磁性粒子に対する結合剤樹脂の吸着強度が高まり、その結果、磁気記録層中における磁性粒子や磁気記録層自体の非磁性下地層に対する密着度が高まったことによるものと考えている。
【０１９６】
【実施例】
次に、実施例並びに比較例を挙げる。
【０１９７】
＜針状ゲータイト粒子粉末の種類＞
出発原料１〜６
針状ヘマタイト粒子粉末の出発原料として、表１に示す特性を有する針状ゲータイト粒子粉末を準備した。
【０１９８】
【表１】

【０１９９】
＜低密度針状ヘマタイト粒子粉末の製造＞
実施例１〜７、比較例１〜６
出発原料である針状ゲータイト粒子粉末の種類、焼結防止剤の種類及び添加量、加熱脱水温度及び時間を種々変化させた以外は、前記本発明の実施の形態と同様にして、低密度針状ヘマタイト粒子粉末を得た。なお、比較例４で得られた粒子粉末は、針状ゲータイト粒子粉末である。
【０２００】
この時の主要製造条件を表２に、得られた低密度針状ヘマタイト粒子粉末の諸特性を表３に示す。
【０２０１】
【表２】

【０２０２】
【表３】

【０２０３】
＜高密度針状ヘマタイト粒子粉末の製造＞
実施例８〜１４、比較例７〜１１
低密度針状ヘマタイト粒子粉末の種類、高密度化加熱処理の温度及び時間を種々変化させた以外は、前記本発明の実施の形態と同様にして、高密度針状ヘマタイト粒子粉末を得た。
【０２０４】
この時の主要製造条件を表４に、得られた高密度針状ヘマタイト粒子粉末の諸特性を表５に示す。
【０２０５】
【表４】

【０２０６】
【表５】

【０２０７】
＜針状ヘマタイト粒子粉末の酸による溶解処理＞
実施例１５〜２１、比較例１２〜１３
高密度針状ヘマタイト粒子粉末の種類、湿式粉砕の有無、酸濃度、スラリーのｐＨ値、加熱温度及び加熱時間を種々変化させた以外は、前記本発明の実施の形態と同様にして、針状ヘマタイト粒子粉末を得た。
【０２０８】
この時の主要製造条件を表６に、得られた針状ヘマタイト粒子粉末の諸特性を表７に示す。
【０２０９】
【表６】

【０２１０】
【表７】

【０２１１】
＜針状ヘマタイト粒子粉末のアルカリ性懸濁液の加熱処理＞
実施例２２〜２８、参考例１〜２
針状ヘマタイト粒子粉末の種類、アルカリ性懸濁液のｐＨ値、加熱温度及び加熱時間を種々変化させた以外は、前記本発明の実施の形態と同様にして針状ヘマタイト粒子粉末を得た。
【０２１２】
この時の主要製造条件を表８に、得られた針状ヘマタイト粒子粉末の諸特性を表９に示す。
【０２１３】
【表８】

【０２１４】
【表９】

【０２１５】
＜針状ヘマタイト粒子粉末の表面被覆処理＞
実施例２９
アルカリ性懸濁液中における加熱処理後にデカンテーション法により水洗して得られた実施例２２のｐＨ値が１０．５の水洗スラリーは、スラリー濃度が６８ｇ／ｌであった。この水洗スラリー５ｌを再度加熱して６０℃とし、該スラリーに１．０Ｎのアルミン酸ナトリウム水溶液１２６ｍｌ（針状ヘマタイト粒子に対しＡｌ換算で１．０重量％に相当する。）を加え、３０分間保持した後、酢酸水溶液を用いてｐＨ値を８．０に調整した。次いで、前記本発明の実施の形態と同様にして、濾別、水洗、乾燥、粉砕して粒子表面が被覆物により被覆されている粒子からなる針状ヘマタイト粒子粉末を得た。
【０２１６】
この時の主要製造条件を表１０に、得られた針状ヘマタイト粒子粉末の諸特性を表１１に示す。
【０２１７】
実施例３０〜３５
針状ヘマタイト粒子粉末の種類、表面処理物の種類及び添加量を種々変化させた以外は、実施例２９と同様にして、常法により粒子表面が被覆物により被覆されている粒子からなる針状ヘマタイト粒子粉末を得た。
【０２１８】
この時の主要製造条件を表１０に、得られた針状ヘマタイト粒子粉末の諸特性を表１１に示す。
【０２１９】
【表１０】

【０２２０】
【表１１】

【０２２１】
＜磁気記録媒体の製造−非磁性支持体上への非磁性下地層の形成＞
実施例３６〜４９、比較例１４〜２２、参考例３〜１１
実施例１５〜３５、比較例１、３、７〜１３及び参考例１、２で得られた針状ヘマタイト粒子粉末を用いて、前記本発明の実施の形態と同様にして、非磁性下地層を形成した。
【０２２２】
この時の主要製造条件及び諸特性を表１２乃び表１３に示す。
【０２２３】
【表１２】

【０２２４】
【表１３】

【０２２５】
＜磁気記録媒体の製造−磁気記録層の形成＞
磁気記録層の形成に用いた磁性粒子粉末とその諸特性を表１４に示す。
【０２２６】
【表１４】

【０２２７】
実施例５０〜６３、比較例２３〜３１、参考例１２〜２０
実施例３６〜４９、比較例１４〜２２及び参考例３〜１１で得られた非磁性下地層の種類、磁性粒子粉末の種類を種々変化させた以外は、前記本発明の実施の形態と同様にして、磁気記録媒体を製造した。
【０２２８】
この時の主要製造条件及び諸特性を表１５乃び表１６に示す。
【０２２９】
【表１５】

【０２３０】
【表１６】

【０２３１】
【発明の効果】
本発明に係る磁気記録媒体は、非磁性下地層用非磁性粉末として針状ヘマタイト粒子粉末を用いた場合には、微粒子成分が少ないこと、及び、粉体ｐＨが８以上であって、可溶性塩の含有量が少ないことに起因してビヒクル中での分散性が優れているので強度と表面平滑性に優れている非磁性下地層を得ることができ、該非磁性下地層を用い、且つ、磁気記録層の磁性粒子粉末として所定量のＡｌを含有する鉄を主成分とする針状金属磁性粒子粉末を用いて磁気記録媒体とした場合には、光透過率が小さく、表面平滑性に優れ、強度が大きく、耐久性に優れ、且つ、磁気記録層中に分散されている鉄を主成分とする針状金属磁性粒子粉末の腐蝕に伴う磁気特性の劣化を抑制することができるので高密度磁気記録媒体として好適である。[0001]
BACKGROUND OF THE INVENTION
The present invention is accompanied by corrosion of acicular metal magnetic particle powders mainly composed of iron having a low light transmittance, smooth surface, high strength, excellent durability, and dispersed in a magnetic recording layer. Provided is a magnetic recording medium in which deterioration of magnetic characteristics is suppressed.
[0002]
[Prior art]
In recent years, as video recording and audio magnetic recording / reproducing devices have been recorded for a long time and reduced in size and weight, the performance of magnetic recording media such as magnetic tapes and magnetic disks has been improved, that is, higher recording density and higher output characteristics. In particular, there is an increasing demand for improvement of frequency characteristics and low noise.
[0003]
In order to improve these characteristics of the magnetic recording medium, various attempts have been made by those skilled in the art from the viewpoints of improving the performance of the magnetic particle powder and reducing the thickness of the magnetic layer.
[0004]
First, the performance enhancement of magnetic particle powder will be described.
[0005]
A characteristic of the magnetic particle powder suitable for satisfying the above requirements for the magnetic recording medium is that it has a high coercive force and a large saturation magnetization.
[0006]
In recent years, acicular metal magnetic particle powder mainly composed of iron obtained by heating and reducing acicular goethite particle powder or acicular hematite particle powder in reducing gas as magnetic particle powder suitable for high output and high density recording Is widely used.
[0007]
The acicular metal magnetic particle powder mainly composed of iron has high coercive force and large saturation magnetization, but the acicular metal magnetic particle powder mainly composed of iron used for the magnetic recording medium is Since they are very fine particles of 1 μm or less, especially about 0.01 to 0.3 μm, they are easily corroded and have deteriorated magnetic properties, and in particular, have the disadvantage of reducing saturation magnetization and coercive force. .
[0008]
Therefore, in order to maintain the characteristics of the magnetic recording medium using the acicular metal magnetic particle powder mainly composed of iron as the magnetic particle powder over a long period of time, the acicular metal magnetic particle powder mainly composed of iron There is a strong demand to suppress corrosion as much as possible.
[0009]
Next, the thinning of the magnetic recording layer will be described.
[0010]
Recently, the demand for higher image quality of video tapes is increasing, and the frequency of the carrier signal to be recorded is higher than that of conventional video tapes. That is, it has shifted to the short wavelength region, and as a result, the magnetization depth from the surface of the magnetic tape is remarkably shallow.
[0011]
In order to improve the high output characteristics of a magnetic recording medium, particularly the S / N ratio, with respect to a short wavelength signal, it is strongly required to reduce the thickness of the magnetic recording layer. This is the case, for example, “Development of Magnetic Materials and Highly Dispersed Magnetic Powder Technology” (1982), page 312 published by the General Technology Center Co., Ltd. The high output characteristics can be maintained with low noise with respect to the wavelength signal. For this purpose, both the coercive force Hc and the residual magnetization Br are required to be large and the coating film must be thinner. "..." as described.
[0012]
As the magnetic recording layer becomes thinner, several problems have arisen. First, there is a problem of smoothing and uneven thickness of the magnetic recording layer. In order to make the magnetic recording layer smooth and free of uneven thickness, the surface of the base film must also be smooth. This is the case, for example, published by the Engineering Information Center Publishing Department, "Magnetic Tape-Friction and Wear Occurrence Factors and Trouble Countermeasures for the Head Running System-Comprehensive Technical Documents" (hereinafter referred to as "Comprehensive Technical Documents") “The surface roughness of the magnetic layer after curing is strongly dependent on the surface roughness of the base (back surface roughness), and the two are almost proportional to each other. As the surface of the base is smoothened, the uniform and large head output is obtained and the S / N is improved.
[0013]
Secondly, the base film is also made thinner like the magnetic layer, and as a result, the strength of the base film has become a problem. This fact is, for example, the above-mentioned “Development of magnetic materials and high dispersion technology of magnetic powder” on page 77 “… ...... High density recording is a major theme imposed on the current magnetic tape. In order to shorten the length of the tape and miniaturize the cassette, it is important for long-time recording, which requires reducing the thickness of the film base. As the tape becomes thinner, the tape stiffness decreases sharply, making it difficult for the recorder to run smoothly.The thickness of the video tape has become thinner and thinner in both the longitudinal and width directions. The improvement is greatly desired.
[0014]
Furthermore, the demand for higher performance of magnetic recording media has not been limited recently, and the surface of the magnetic recording layer and the magnetic recording medium itself have been reduced along with the thinning of the magnetic recording layer and the thinning of the nonmagnetic support described above. Therefore, it is strongly required to improve the durability of the magnetic recording layer surface and the magnetic recording medium itself.
[0015]
This fact is disclosed in Japanese Patent Application Laid-Open No. 5-298679, “... Recently, with the development of magnetic recording, the demand for high image quality and high sound quality is increasing, and the improvement of electromagnetic conversion characteristics, especially the formation of fine particles of ferromagnetic powder. However, it is required to increase the density and further smooth the surface of the magnetic tape to reduce noise and increase C / N .... However, the magnetic layer and device during the running of the magnetic tape are required. As a result of the increase in the coefficient of friction for contact with the system, the magnetic layer of the magnetic recording medium tends to be damaged or peeled off after a short period of use, especially for video tapes where the video head and the magnetic recording medium are fast. Therefore, the ferromagnetic powder easily falls off from the magnetic layer and causes clogging of the magnetic head, and therefore, it is desired to improve the running durability of the magnetic layer of the magnetic recording medium. It is clear from ... "is described.
[0016]
By the way, determination of the end of the magnetic tape of a magnetic recording medium such as a video tape is currently performed by detecting a portion of the magnetic recording medium having a high light transmittance with a video deck. Magnetic recording media become difficult to detect when the light transmittance of the entire magnetic recording layer increases as the magnetic recording medium becomes thinner or the magnetic particle powder dispersed in the magnetic recording layer becomes ultrafine. In order to reduce the light transmittance by adding carbon black or the like to the layer, it is essential to add carbon black or the like to the magnetic recording layer in the current video tape.
[0017]
However, the addition of a large amount of nonmagnetic carbon black or the like not only inhibits high density recording, but also inhibits thinning. In order to further reduce the thickness of the magnetic tape by reducing the magnetization depth from the surface of the magnetic tape, it is strongly required to reduce the nonmagnetic particle powder such as carbon black added to the magnetic recording layer as much as possible. Yes.
[0018]
Therefore, there is a strong demand for a magnetic recording medium having a low light transmittance even if the amount of carbon black added to the magnetic recording layer is reduced.
[0019]
Along with the thinning of the magnetic recording layer and the thinning of the nonmagnetic support, a nonmagnetic particle powder such as hematite particles is bonded onto the nonmagnetic support such as a base film to form a magnetic recording layer. At least one nonmagnetic underlayer dispersed therein has been provided, and has already been put into practical use (Japanese Patent Publication No. 6-93297, Japanese Patent Laid-Open No. 62-159338, Japanese Patent Laid-Open No. 63-187418, JP-A-4-167225, JP-A-4-325915, JP-A-5-73882, JP-A-5-182177, JP-A-5-347017, JP-A-6-60362. Issue gazette).
[0020]
However, a magnetic recording medium in which a nonmagnetic underlayer in which nonmagnetic powder is dispersed in a binder resin is formed on a nonmagnetic support and a magnetic recording layer is provided on the nonmagnetic underlayer is improved in surface smoothness. There was a problem of poor durability.
This fact is disclosed in Japanese Patent Application Laid-Open No. Hei 5-182177. If the magnetic layer is provided as an upper layer after providing a non-magnetic thick undercoat layer on the surface of the support, the surface roughness of the support is reduced. Although the influence can be eliminated, there has been a problem that the wear and durability of the head are not improved, because conventionally, a thermosetting resin is used as a binder as a non-magnetic lower layer, so that the lower layer is cured and the magnetic layer It is considered that the friction between the head and the head and the contact with other members are performed in an unbuffered state, and the magnetic recording medium having such a lower layer is slightly inflexible. ... "as described.
[0021]
Therefore, as the magnetic recording layer is thinned and the nonmagnetic support is thinned, the light transmittance is small, the surface is smooth, the strength is high, the durability is excellent, and the magnetic recording layer is dispersed in the magnetic recording layer. Magnetic recording media in which deterioration of magnetic properties due to corrosion of acicular metal magnetic particle powders containing iron as a main component is suppressed are currently most demanded, but these properties are sufficiently satisfied. A magnetic recording medium has not yet been obtained.
[0022]
[Problems to be solved by the invention]
Therefore, the present invention provides corrosion of acicular metal magnetic particle powder mainly composed of iron having a small light transmittance, a smooth surface, high strength, excellent durability, and dispersed in a magnetic recording layer. An object of the present invention is to obtain a magnetic recording medium in which the deterioration of magnetic properties accompanying the above is suppressed.
[0023]
[Means for Solving the Problems]
The technical problem can be achieved by the present invention as follows.
[0024]
That is, the present invention relates to a nonmagnetic support, a nonmagnetic underlayer comprising a nonmagnetic particle powder and a binder resin formed on the nonmagnetic support, and a magnetic particle powder formed on the nonmagnetic underlayer. In a magnetic recording medium comprising a magnetic recording layer comprising a binder resin, the magnetic particle powder is an acicular metal magnetic particle powder mainly composed of iron in which 0.05 to 10% by weight of aluminum is present in terms of Al. And the non-magnetic particle powder has an average major axis diameter of 0.005 to 0.30 μm and a BET specific surface area value of 35 to 150 m.²An aqueous suspension of acicular hematite particles having a pH of 3.0 N, a pH value of 3.0 or less, and a temperature range of 20 to 100 ° C. After dissolving 5 to 50% by weight of the total amount of acicular hematite particles present therein, an aqueous alkaline solution is added to the aqueous suspension of acicular hematite particles obtained by washing the remaining acicular hematite particles with water. After adding and adjusting the pH value to 13 or more, heat treatment is carried out in a temperature range of 80 to 103 ° C., and then the average major axis diameter obtained by filtration, washing and drying is 0.004 to 0.295 μm, BET specific surface area value 35.9 to 212m²/ G, the powder pH value is 8 or more, the content of soluble sodium salt is 300 ppm or less in terms of Na, and the content of soluble sulfate is SO₄The magnetic recording medium is characterized by being acicular hematite particle powder of 150 ppm or less in terms of conversion.
[0025]
The present invention also provides a magnetic recording medium in which the surface of the acicular hematite particle powder is coated with at least one of aluminum hydroxide, aluminum oxide, silicon hydroxide, and silicon oxide. is there.
[0026]
The configuration of the present invention will be described in more detail as follows.
[0027]
First, the nonmagnetic underlayer in the present invention will be described.
[0028]
The nonmagnetic underlayer in the present invention is formed on a nonmagnetic support and is composed of acicular hematite particle powder and a binder resin.
[0029]
Nonmagnetic supports include polyethylene terephthalate, polyethylene, polypropylene, polycarbonate, polyethylene naphthalate, synthetic resin films such as polyamide, polyamideimide, and polyimide, which are currently widely used for magnetic recording media, metal foils such as aluminum and stainless steel, A board and various kinds of paper can be used, and the thickness varies depending on the material, but usually 1.0 to 300 μm, more preferably 2.0 to 200 μm is used. In the case of a magnetic disk, polyethylene terephthalate is usually used as a nonmagnetic support, and the thickness is usually 50 to 300 μm, preferably 60 to 200 μm. In the case of magnetic tape, the thickness varies depending on the material, and in the case of polyethylene terephthalate, it is usually 3 to 100 μm, preferably 4 to 20 μm. In the case of polyethylene naphthalate, it is usually 3 to 50 μm, preferably 4 to 20 μm. In this case, it is usually 2 to 10 μm, preferably 3 to 7 μm.
[0030]
The acicular hematite particle powder in the present invention has an average major axis diameter of 0.004 to 0.295 μm and a BET specific surface area value of 35.9 to 212 m.²/ G, powder pH value is 8 or more, soluble sodium salt is 300 ppm or less in terms of Na, soluble sulfate is SO₄Containing 150 ppm or less in terms of conversion.
[0031]
The particle shape of the acicular hematite particle powder in the present invention is acicular. Here, the term “needle shape” means not only the literal needle shape but also the spindle shape and the rice grain shape.
[0032]
When the average major axis diameter of the acicular hematite particle powder in the present invention exceeds 0.295 μm, the particle size is too large, which is not preferable because the surface smoothness of the coating film is impaired. When the average particle size is less than 0.004 μm, dispersion in the vehicle becomes difficult due to an increase in intermolecular force due to the formation of fine particles. Considering the dispersibility in the vehicle and the surface smoothness of the coating film, the average major axis diameter is preferably 0.008 to 0.275 μm, more preferably 0.016 to 0.245 μm.
[0033]
The average minor axis diameter of the acicular hematite particle powder in the present invention is preferably 0.002 to 0.147 μm. The reason why the lower limit value and the upper limit value of the average minor axis diameter of the acicular hematite particle powder in the present invention are determined is the same as the reason why the average major axis diameter is determined. Considering the dispersibility in the vehicle and the surface smoothness of the coating film, the average minor axis diameter is more preferably 0.004 to 0.137 μm, and still more preferably 0.008 to 0.122 μm.
[0034]
The axial ratio of the acicular hematite particle powder in the present invention (the ratio of the average major axis diameter to the average minor axis diameter, hereinafter simply referred to as “axis ratio”) is preferably 2 to 20. When the axial ratio is less than 2, it is difficult to obtain a coating film having sufficient strength. When the axial ratio exceeds 20, entanglement of particles in the vehicle increases, and dispersibility may deteriorate and viscosity may increase. Considering the dispersibility in the vehicle and the strength of the obtained coating film, the axial ratio is more preferably in the range of 3-10.
[0035]
The reason why the lower limit value and the upper limit value of the BET specific surface area value of the acicular hematite particle powder in the present invention are determined is the same as the reason why the upper limit value and the lower limit value of the average major axis diameter are determined. Considering the dispersibility in the vehicle and the surface smoothness of the coating film, the BET specific surface area value is 38 to 141.4 m.²/ G is preferred, more preferably 41-113.1 m.²/ G.
[0036]
The acicular hematite particle powder in the present invention preferably has a geometric standard deviation value of the major axis diameter of 1.50 or less. When it exceeds 1.50, it is not preferable because the existing coarse particles adversely affect the surface smoothness of the coating film. Considering the surface smoothness of the coating film, the geometric standard deviation value of the major axis diameter is more preferably 1.48 or less, and still more preferably 1.45 or less. Considering industrial productivity, the lower limit value of the geometric standard deviation value of the major axis diameter is 1.01.
[0037]
The acicular hematite particles in the present invention preferably have a high degree of densification, and the specific surface area S measured by the BET method._BETSurface area S calculated from the major axis and minor axis diameters measured from the values and particles shown in the electron micrograph_TEMRatio to the value (S_BET/ S_TEMValue), 0.5 to 2.5 is preferable.
[0038]
S_BET/ S_TEMWhen the value is less than 0.5, the densification of the needle-like matite particle powder has been achieved, but the particle diameter has increased due to the sintering between the particles and a sufficient surface. A coating film having smoothness cannot be obtained. S_BET/ S_TEMWhen the value exceeds 2.5, densification is not sufficient, and a large number of dewatering pores exist inside and on the surface of the particle, so that dispersibility in the vehicle becomes insufficient. Considering the dispersibility in the vehicle and the surface smoothness of the coating film, S_BET/ S_TEMThe value is preferably 0.7 to 2.0, more preferably 0.8 to 1.6.
[0039]
The powder pH value of the acicular hematite particle powder in the present invention is 8 or more. When the powder pH value is less than 8, the needle-like metal magnetic particle powder mainly composed of iron contained in the magnetic recording layer formed on the non-magnetic underlayer is gradually corroded, and the magnetic properties are reduced. Causes deterioration. Considering the corrosion prevention effect of the acicular metal magnetic particle powder containing iron as a main component, the powder pH value is preferably 8.5 to 11, more preferably the powder pH value is 9.0 to 10.5. .
[0040]
The content of the soluble sodium salt in the acicular hematite particle powder in the present invention is 300 ppm or less in terms of Na. When it exceeds 300 ppm, the acicular metal magnetic particle powder mainly composed of iron contained in the magnetic recording layer formed on the nonmagnetic underlayer is gradually corroded to cause deterioration of magnetic properties. In addition, the dispersion characteristics of the hematite particle powder in the vehicle are likely to be impaired, and the white flower phenomenon may occur in the storage state of the magnetic recording medium, particularly in a high humidity environment. Considering the corrosion prevention effect of the acicular metal magnetic particle powder containing iron as a main component, it is preferably 250 ppm or less, more preferably 200 ppm or less, and even more preferably 150 ppm or less. In consideration of industrial productivity, the lower limit is about 0.01 ppm.
[0041]
The content of soluble sulfate in acicular hematite particles in the present invention is SO.₄It is 150 ppm or less in terms of conversion. When it exceeds 150 ppm, the acicular metal magnetic particle powder mainly composed of iron contained in the magnetic recording layer formed on the nonmagnetic underlayer is gradually corroded to cause deterioration of magnetic properties. In addition, the dispersion characteristics of the hematite particle powder in the vehicle are likely to be impaired, and the white flower phenomenon may occur in the storage state of the magnetic recording medium, particularly in a high humidity environment. Considering the corrosion prevention effect of the acicular metal magnetic particle powder containing iron as a main component, it is preferably 100 ppm or less, more preferably 70 ppm or less, and even more preferably 50 ppm or less. In consideration of industrial productivity, the lower limit is about 0.01 ppm.
[0042]
The acicular hematite particle powder according to the present invention may have a surface of at least one compound of aluminum hydroxide, aluminum oxide, silicon hydroxide or silicon oxide (hereinafter referred to as aluminum hydroxide), if necessary. It may be coated with an object or the like). Needle-like hematite particles composed of needle-like hematite particles whose particle surfaces are coated with a coating have good affinity with the binder resin when dispersed in a vehicle, and a desired degree of dispersion can be easily obtained.
[0043]
The coating amount of aluminum hydroxide and the like is Al equivalent to the acicular hematite particle powder, SiO₂Conversion or Al conversion amount and SiO₂0.01 to 50% by weight in terms of the total amount of conversion is preferable. When the amount is less than 0.01% by weight, it is difficult to obtain the effect of improving dispersibility by coating. When the amount exceeds 50% by weight, the coating effect is saturated, and it is meaningless to coat more than necessary. Considering the dispersibility in the vehicle and industrial productivity, the coating amount of aluminum hydroxide and the like is more preferably 0.05 to 20% by weight.
[0044]
The acicular hematite particle powder coated with an aluminum hydroxide or the like in the present invention is approximately the same particle size as the acicular hematite particle powder in the present invention that is not coated with an aluminum hydroxide or the like, Axial ratio, BET specific surface area value, geometrical standard deviation value of major axis diameter, S_BET/ S_TEMValue, powder pH value, soluble sodium salt and soluble sulfate.
[0045]
Next, a method for producing acicular hematite particle powder in the present invention will be described.
[0046]
The acicular hematite particle powder in the present invention is an aqueous suspension of the acicular hematite particle powder obtained by washing the acicular hematite particle powder, which is the particle powder to be treated, with water after performing dissolution treatment with an acid under specific conditions. After adjusting the pH value to 13 or more by adding an alkaline aqueous solution to the liquid, it can be obtained by heat treatment in a temperature range of 80 to 103 ° C.
[0047]
Here, the acicular hematite particle powder as the particles to be treated can be obtained by various methods. For example, there are a method of directly generating acicular hematite particle powder by a wet method, a method of generating an acagenite (β-FeOOH) particle powder, and then heat dehydrating to obtain acicular hematite particle powder. As a production method, a needle-like goethite particle powder, which is a starting material of needle-like hematite particle powder, is produced by a wet method, and the obtained needle-like goethite particle powder is heated and dehydrated to obtain needle-like hematite particle powder. Is also industrially preferable.
[0048]
Therefore, first, a general method for producing acicular goethite particle powder, which is a starting material of acicular hematite particle powder as the treated particle powder, will be described.
[0049]
The acicular goethite particles, as will be described in detail later, are obtained by reacting with ferrous salt and either alkali hydroxide or alkali carbonate or mixed alkali of alkali hydroxide and alkali carbonate. It is obtained by aeration of an oxygen-containing gas such as air through a suspension containing a ferrous iron-containing precipitate such as iron carbonate to produce acicular goethite particles.
[0050]
As is well known, a typical basic reaction of acicular goethite particles includes (1) a suspension containing a ferrous hydroxide colloid obtained by adding an equivalent amount or more of an alkali hydroxide aqueous solution to a ferrous salt aqueous solution. a method of generating acicular goethite particles by aeration of oxygen-containing gas at a pH value of 11 or higher and a temperature of 80 ° C. or lower, and (2) a reaction between an aqueous ferrous salt solution and an aqueous alkali carbonate solution. FeCO obtained₃A method for producing goethite particles having a spindle shape by aeration of a suspension containing nitrobenzene, if necessary, followed by an oxygen-containing gas and an oxidation reaction; (3) ferrous salt aqueous solution and alkaline carbonate aqueous solution Aged suspension containing iron-containing precipitate obtained by reacting with alkali hydroxide is aged if necessary, and then oxygenated gas is passed through to conduct oxidation reaction to produce spindle-shaped goethite particles (4) An oxygen-containing gas is bubbled through a ferrous salt aqueous solution containing a ferrous hydroxide colloid obtained by adding less than an equivalent amount of an alkali hydroxide aqueous solution or an alkali carbonate aqueous solution to a ferrous salt aqueous solution. Oxidation reaction is performed to generate acicular goethite core particles, and then to the ferrous salt aqueous solution containing the acicular goethite core particles, Fe in the ferrous salt aqueous solution is added.²⁺A method of growing an acicular goethite core particle by aeration of an oxygen-containing gas after adding an alkali hydroxide aqueous solution of an equivalent amount or more to the above, (5) an aqueous alkali hydroxide solution or carbonic acid of less than an equivalent amount in the ferrous salt aqueous solution Acicular goethite core particles are generated by performing an oxidation reaction by bubbling an oxygen-containing gas into a ferrous salt aqueous solution containing ferrous hydroxide colloid obtained by adding an aqueous alkaline solution, and then generating the acicular goethite The ferrous salt aqueous solution containing the core particles is added to the Fe ferrous salt aqueous solution.²⁺A method of growing an acicular goethite core particle by aeration of an oxygen-containing gas after adding an alkali carbonate aqueous solution of an equivalent amount or more with respect to (6) an alkali hydroxide or alkali carbonate aqueous solution less than the equivalent of ferrous salt aqueous solution The needle-like goethite core particles are generated by conducting an oxidation reaction by aerating an oxygen-containing gas to an aqueous ferrous salt solution containing ferrous hydroxide colloid obtained by adding, and then in the acidic to neutral region There is a method of growing the acicular goethite core particles.
[0051]
It should be noted that during the formation reaction of goethite particles, different elements such as Ni, Zn, P, and Si that are usually added to improve various characteristics such as the major axis diameter, minor axis diameter, and axial ratio of the particles are added. There is no problem.
[0052]
The acicular goethite particle powder as a starting material in the present invention usually has an average major axis diameter of 0.005 to 0.4 μm, an average minor axis diameter of 0.0025 to 0.2 μm, and an axial ratio of 2 to 2. 20, Geometric standard deviation value of major axis diameter is 1.70 or less, BET specific surface area value is 50-250m²/ G, powder pH value is 3 to 9, soluble sodium salt is 300 to 1500 ppm in terms of Na, soluble sulfate is SO₄Containing 150 to 3000 ppm in terms of conversion.
[0053]
Next, a method for producing acicular hematite particle powder as the treated particle powder will be described.
[0054]
The acicular hematite particle powder that is the particle powder to be treated in the present invention can be obtained by subjecting the acicular goethite particle powder, which is a starting material, to heat dehydration treatment in a temperature range of 550 to 850 ° C.
[0055]
When the temperature of the heat dehydration treatment is lower than 550 ° C., the densification is insufficient, and there are many dehydration holes inside and on the surface of the acicular hematite particles. The particle shape is not maintained due to the progress of dissolution from the dehydration holes, etc., and the dispersibility of the obtained acicular hematite particle powder in the vehicle becomes insufficient. It is difficult to obtain. When the temperature exceeds 850 ° C., the density of the needle-like matite particles is sufficiently increased. However, since sintering occurs between the particles and the particles, the particle diameter increases, and similarly, the coating film having a smooth surface is obtained. It is difficult to obtain. The upper limit of the heat dehydration temperature is preferably 800 ° C.
[0056]
The acicular hematite particle powder as the treated particle powder in the present invention is obtained by subjecting the acicular goethite particle powder to low-temperature heat dehydration treatment in a temperature range of 250 to 500 ° C. to obtain a low-density acicular hematite particle powder, The low-density acicular hematite particle powder is preferably a densified acicular hematite particle powder obtained by subjecting the low-density acicular hematite particle powder to a high-temperature heat treatment in a temperature range of 550 to 850 ° C.
[0057]
A low temperature heating dehydration temperature of less than 250 ° C. is not preferable because a long time is required for the dehydration reaction. When the low temperature heating dehydration temperature exceeds 500 ° C., the dehydration reaction may occur rapidly, and the shape of the particles may be easily broken, or sintering between the particles may be caused. Low density acicular hematite particles obtained by low temperature heat dehydration treatment are obtained from acicular goethite particles.₂O is dehydrated and consists of low-density particles having many dewatering holes, and the BET specific surface area value is about 1.2 to 2 times that of the acicular goethite particle powder as a starting material.
[0058]
The obtained low density acicular hematite particle powder has an average major axis diameter of 0.005 to 0.30 μm, an average minor axis diameter of 0.0025 to 0.15 μm, and an axial ratio of 2 to 20, Geometric standard deviation value of major axis diameter is 1.70 or less, BET specific surface area value is 70-350m²/ G, S_BET/ S_TEMThe value is 2.6 or more, the powder pH value is 3 to 9, the soluble sodium salt is 500 to 3000 ppm in terms of Na, and the soluble sulfate is SO.₄Containing 300 to 4000 ppm in terms of conversion.
[0059]
Subsequently, the low-density hematite particle powder is heat-treated at a temperature of 550 to 850 ° C. to obtain a densified needle-like hematite particle powder. When the high-temperature heat treatment temperature is less than 550 ° C., the densification is insufficient, and there are a large number of dehydration holes inside and on the particle surface of the hematite particles. The particle shape is not maintained due to the progress of dissolution, etc., and the dispersibility of the obtained acicular hematite particle powder in the vehicle becomes insufficient, and it is difficult to obtain a smooth coating film when a nonmagnetic underlayer is formed. . When the temperature exceeds 850 ° C., the density of the hematite particles is sufficiently increased. However, since the particles and the particles are sintered with each other, the particle diameter is increased, and similarly, it is difficult to obtain a coating film having a smooth surface. . The upper limit of the heat dehydration temperature is preferably 800 ° C.
[0060]
The obtained high-density needle-like hematite particles usually have an average major axis diameter of 0.005 to 0.30 μm, an average minor axis diameter of 0.0025 to 0.15 μm, and an axial ratio of 2 to 20, Geometric standard deviation value of major axis diameter is 1.70 or less, BET specific surface area value is 35 to 150 m²/ G, S_BET/ S_TEMThe value is 0.5 to 2.5, the powder pH value is 2.5 to 9, the soluble sodium salt is 500 to 4000 ppm in terms of Na, and the soluble sulfate is SO.₄It contains 300 to 5000 ppm in terms of conversion.
[0061]
The acicular hematite particle powder in the present invention is preferably preliminarily coated with a sintering inhibitor prior to heat dehydration treatment or high temperature heat treatment at a temperature range of 550 to 850 ° C. The coating treatment with the sintering inhibitor is an aqueous suspension containing acicular goethite particle powder as starting material particle powder or low-density acicular hematite particle powder obtained by low-temperature heat dehydration treatment at a temperature range of 250 to 500 ° C. Add anti-sintering agent, mix and stir, filter, wash and dry.
Yes.
[0062]
Examples of the sintering inhibitor include commonly used phosphorus compounds such as sodium hexametaphosphate, polyphosphoric acid, and orthophosphoric acid, No. 3 water glass, sodium orthosilicate, sodium metasilicate, silicon compounds such as colloidal silica, and boric acid. Use aluminum compounds such as boron compounds, aluminum acetate, aluminum sulfate, aluminum chloride, aluminum nitrate, alkali aluminates such as sodium aluminate, aluminum compounds such as alumina sol and aluminum hydroxide, and titanium compounds such as titanium oxysulfate. be able to.
[0063]
The amount of anti-sintering agent present on the surface of the acicular goethite particle powder depends on various conditions such as the type and amount of anti-sintering agent, pH value in alkaline aqueous solution and heat treatment temperature, but the total weight of particles The content is 0.05 to 10% by weight.
[0064]
Next, a method for producing acicular hematite particle powder in the present invention will be described.
[0065]
The acicular hematite particle powder in the present invention is obtained by subjecting an aqueous suspension of acicular hematite particle powder as a treated particle powder to an acid concentration of 1.0 N or more, a pH value of 3.0 or less, and a temperature range of 20 to 100 ° C. The acicular hematite obtained by dissolving 5 to 50% by weight of the total amount of acicular hematite particles present in the aqueous suspension and then washing the remaining acicular hematite particles with water. An aqueous alkali solution is added to an aqueous suspension of particle powder to adjust the pH value to 13 or more, and then heat treatment is performed in a temperature range of 80 to 103 ° C.
[0066]
First, the acicular hematite particle powder as the treated particle powder in the present invention will be described.
[0067]
The particle shape of the acicular hematite particle powder as the treated particle powder is acicular. Here, the term “needle shape” means not only the literal needle shape but also the spindle shape and the rice grain shape.
[0068]
The acicular hematite particle powder as the particle powder to be treated has an average major axis diameter of 0.005 to 0.3 μm, and a BET specific surface area value of 35 to 150 m.²/ G.
[0069]
When the average major axis diameter of the acicular hematite particle powder as the particle powder to be treated exceeds 0.3 μm, the particle size of the acicular hematite particle powder in the present invention obtained after the treatment is too large. Since surface smoothness is impaired, it is not preferable. When the average particle size is less than 0.005 μm, dispersion in the vehicle becomes difficult due to an increase in intermolecular force caused by atomization of the acicular hematite particles obtained in the present invention after the above treatment. Considering the dispersibility in the vehicle and the surface smoothness of the coating film, the average major axis diameter is preferably 0.02 to 0.25 μm.
[0070]
The reason why the lower limit value and the upper limit value of the BET specific surface area value of the acicular hematite particle powder as the treated particle powder are determined is the same as the reason why the upper limit value and the lower limit value of the average major axis diameter are determined. Considering the dispersibility in the vehicle and the surface smoothness of the coating film, the BET specific surface area value is preferably 37 to 135 m.²/ G.
[0071]
The acicular hematite particle powder as the treated particle powder has an average minor axis diameter of 0.0025 to 0.15 μm, a geometric standard deviation value of major axis diameter of 1.70 or less, S_BET/ S_TEMThe value is 0.5 to 2.5, the powder pH value is 2.5 to 9, the soluble sodium salt is 500 to 4000 ppm in terms of Na, and the soluble sulfate is SO.₄It is preferably 300 to 5000 ppm or less in terms of conversion.
[0072]
Next, the dissolution treatment with acid of the acicular hematite particles as the particles to be treated will be described.
[0073]
The acicular hematite particle powder as the particle powder to be treated is subjected to a dissolution treatment with an acid, and then coarsely pulverized in advance to loosen the coarse particles, and then slurried, and then wet pulverized to further refine the coarse particles. It is preferable to loosen it. The wet pulverization may be performed using a ball mill, a sand grinder, a colloid mill, or the like so that at least coarse particles of secondary aggregated particles of 44 μm or more are eliminated. The degree of wet pulverization is 10% or less, preferably 5% or less, and more preferably 0% for coarse particles of 44 μm or more. If coarse particles of 44 μm or more remain in excess of 10%, it is difficult to obtain the effect of dissolution treatment with an acid in the next step.
[0074]
The concentration of the acicular hematite particle powder in the aqueous suspension used for the dissolution treatment with an acid is preferably 1 to 500 g / l, and more preferably 10 to 250 g / l. If it is less than 1 g / l, the amount of treatment per treatment unit is too small, which is not industrially preferable. When it exceeds 500 g / l, it is difficult to perform uniform dissolution treatment.
[0075]
As the acid used for the dissolution treatment with an acid, any of sulfuric acid, hydrochloric acid, nitric acid, sulfurous acid, chloric acid, perchloric acid, and oxalic acid can be used. In the case of performing the treatment at a high temperature or considering the corrosion, deterioration, economic efficiency, etc. of the container in which the dissolution treatment is performed, sulfuric acid is preferable.
[0076]
The acid concentration in the dissolution treatment with an acid is 1.0 N or more, preferably 1.2 N or more, more preferably 1.5 N or more. If it is less than 1.0 N, it takes a very long time to dissolve the acicular hematite particle powder, which is industrially disadvantageous.
[0077]
The initial pH value in the acid dissolution treatment is a pH value of 3.0 or less, preferably a pH value of 2.0 or less, more preferably a pH value of 1.0 or less. Considering dissolution time and the like, a pH value of 1.0 or less is suitable. When the pH value exceeds 3.0, it takes a very long time to dissolve the acicular hematite particle powder, which is industrially disadvantageous.
[0078]
The temperature range of the aqueous suspension in the dissolution treatment with an acid is 20 to 100 ° C, preferably 50 to 100 ° C, more preferably 70 to 100 ° C. When the temperature is lower than 20 ° C., it takes a very long time to dissolve the needle-like hematite particle powder, which is industrially disadvantageous. When the temperature exceeds 100 ° C., dissolution of the particles proceeds rapidly, making it difficult to control, and an apparatus such as an autoclave is required, which is not industrially preferable.
[0079]
In the case where the average major axis diameter of the acicular hematite particle powder as the particle powder to be treated is 0.05 to 0.30 μm, which is a relatively large region, the conditions for the dissolution treatment are hard conditions such as acid concentration. It is preferable to carry out at 1.5N or more, pH value 1.0 or less, and temperature range 70-100 degreeC, and in the case of 0.005-0.05 micrometer which is an area | region where an average major axis diameter is comparatively small, The conditions are preferably soft under conditions such as an acid concentration of 1.0 to 1.5 N, a pH value of 1.0 to 3.0, and a temperature range of 20 to 70 ° C.
[0080]
The dissolution treatment with an acid is carried out until 5 to 50% by weight, preferably 10 to 45% by weight, more preferably 15 to 40% by weight of the total amount of acicular hematite particle powder, which is the particle powder to be treated, is dissolved. When the amount is less than 5% by weight, the fine particle component is not sufficiently removed by dissolution. When the amount exceeds 50% by weight, the entire particle powder becomes fine particles, and the loss due to dissolution is large, which is industrially preferable. Absent.
[0081]
In addition, the aqueous solution of the iron salt dissolved by the dissolution treatment with acid is separated from the slurry by filtration and used as a ferrous salt raw material for the production of acicular goethite particle powder from the viewpoint of resource recycling. Can do.
[0082]
Next, heat treatment in an alkaline suspension of acicular hematite particle powder as a particle to be treated will be described.
[0083]
In the heat treatment of the acicular hematite particle powder in the alkaline suspension, the aqueous suspension of the remaining acicular hematite particle powder after the dissolution treatment with acid is filtered and washed, and the cake obtained is dispersed again in water. It is preferable to leave it as an aqueous suspension of the acicular hematite particles, or as an aqueous suspension of the needle-like hematite particles powder that is washed with water by decantation. .
[0084]
The concentration of the acicular hematite particle powder in the alkaline suspension used for the heat treatment of the alkaline suspension is preferably 50 to 250 g / l.
[0085]
As the alkaline aqueous solution, an aqueous solution of an alkali hydroxide such as sodium hydroxide, potassium hydroxide, calcium hydroxide or the like can be used.
[0086]
The pH value in the alkaline suspension containing acicular hematite particle powder is 13 or more. When the pH is less than 13, solid cross-linking due to the sintering inhibitor existing on the particle surface of the acicular hematite particle powder cannot be effectively removed, and soluble sodium salt present in the particle surface and on the particle surface is soluble. Unable to wash out sulfates. The upper limit is a pH value of 14. Needle-like hematite particles The effect of removing solid cross-linking due to the sintering inhibitor present on the particle surface and washing out soluble sodium salts, soluble sulfates, etc., and also during the heat treatment of alkaline suspension Considering the cleaning effect for removing alkali such as sodium attached to the particle surface of the hematite particle powder, the pH value is preferably in the range of 13.1 to 13.8.
[0087]
The heating temperature of the alkaline suspension containing the acicular hematite particle powder having a pH value of 13 or higher is preferably 80 ° C. or higher, more preferably 90 ° C. or higher. When the temperature is lower than 80 ° C., it is difficult to effectively remove the solid cross-linking caused by the sintering inhibitor present on the surface of the acicular hematite particle powder. The upper limit of the heating temperature is preferably 103 ° C, more preferably 100 ° C. When the temperature exceeds 103 ° C., the solid cross-linking can be effectively removed, but an autoclave or the like is necessary, or the liquid to be treated is boiled under normal pressure, which is industrially disadvantageous.
[0088]
Needle-like hematite particles powder that has been heat-treated in an alkaline suspension is separated by filtration and washed with water in the usual manner, so that soluble sodium salts, sulfates, and alkaline suspensions washed out from the inside of the needle-like hematite particles and the particle surface During the heat treatment of the liquid, alkali such as sodium attached to the surface of the needle-like hematite particles is removed and then dried.
[0089]
As a water washing method, if a method commonly used in industry such as a method of washing by decantation, a method of washing by a dilution method using a filter thickener, a method of washing by passing water through a filter press, etc. is used. Good.
[0090]
In addition, if the soluble sodium salt and soluble sulfate contained inside the acicular hematite particle powder are washed out with water, the acicular hematite particle powder is used in subsequent steps, for example, in the coating treatment step described later. Even if soluble sodium salts or soluble sulfates adhere to the surface of the particles, they can be easily removed by washing with water.
[00091]
Next, the surface coating treatment of the acicular hematite particle powder coated with the aluminum hydroxide or the like in the present invention is carried out to an aqueous suspension obtained by dispersing the acicular hematite particle powder in the present invention in an aqueous solution. The aluminum compound, the silicon compound or both of the compounds are mixed and stirred, or if necessary, the pH value is adjusted after mixing and stirring, whereby the aluminum hydroxide is added to the surface of the acicular hematite particles. The product, aluminum oxide, silicon hydroxide and silicon oxide may be coated, followed by filtration, washing with water, drying and grinding. If necessary, a deaeration / consolidation process may be further performed.
[00092]
As the aluminum compound and silicon compound used for the surface coating treatment, the same aluminum compound and silicon compound used as the above-mentioned sintering inhibitor can be used.
[0093]
The addition amount of the aluminum compound is 0.01 to 50% by weight in terms of Al with respect to the acicular hematite particle powder. If it is less than 0.01% by weight, the dispersion in the vehicle is insufficient, and if it exceeds 50% by weight, the coating effect is saturated, so there is no point in adding more than necessary.
[0094]
The amount of silicon compound added is SiO to acicular hematite particle powder.₂It is 0.01 to 50% by weight in terms of conversion. If it is less than 0.01% by weight, the dispersion in the vehicle is insufficient, and if it exceeds 50% by weight, the coating effect is saturated, so there is no point in adding more than necessary.
[0095]
When an aluminum compound and a silicon compound are used in combination, the equivalent amount of Al and SiO₂The total amount with the converted amount is preferably 0.01 to 50% by weight.
[0096]
As binder resins, vinyl chloride-vinyl acetate copolymer, urethane resin, vinyl chloride-vinyl acetate-maleic acid copolymer, urethane elastomer, butadiene-acrylonitrile copolymer, which are currently widely used in the production of magnetic recording media. For example, synthetic rubber resins such as coalescence, polyvinyl butyral, nitrocellulose, polyester resins, polybutadiene, epoxy resins, polyamide resins, polyisocyanates, electron beam curable acrylic urethane resins, and mixtures thereof can be used. In addition, each binder resin has —OH, —COOH, —SO.₃M, -OPO₂M₂, -NH₂And the like (where M is H, Na, K). Considering the dispersibility of the particles, -COOH, -SO as polar groups₃Binder resins containing M are preferred.
[0097]
The mixing ratio of the acicular hematite particle powder and the binder resin in the present invention is 5 to 2000 parts by weight, preferably 100 to 1000 parts by weight of the acicular hematite particle powder with respect to 100 parts by weight of the binder resin.
[0098]
The coating thickness of the nonmagnetic underlayer formed on the nonmagnetic support in the present invention is in the range of 0.2 to 10.0 μm. When it is less than 0.2 μm, it is difficult to improve the surface roughness of the nonmagnetic support, and the strength tends to be insufficient. Considering the thinning of the magnetic recording medium and the strength of the coating film, the coating film thickness is more preferably 0.5 to 5.0 μm.
[0099]
Note that the nonmagnetic underlayer may contain a lubricant, an abrasive, an antistatic agent, etc. used in the production of a normal magnetic recording medium, if necessary.
[0100]
The nonmagnetic underlayer using acicular hematite particle powder in the present invention in which the particle surface is not coated with the aluminum hydroxide or the like has a coating film glossiness of 187 to 300%, preferably 193 to 300%. More preferably, the coating film has a surface roughness Ra of 0.5 to 8.5 nm, preferably 0.5 to 8.0 nm, more preferably 0.5 to 7.5 nm. The rate (relative value) is 119 to 160, preferably 120 to 160.
[0101]
The nonmagnetic underlayer using acicular hematite particle powder whose particle surface is coated with aluminum hydroxide or the like has a coating film glossiness of 190 to 300%, preferably 195 to 300%, more preferably 197. ~ 300%, surface roughness Ra of coating film is 0.5 to 8.3 nm, preferably 0.5 to 7.8 nm, more preferably 0.5 to 7.3 nm, Young's modulus (relative value) of coating film Is 120-160, preferably 123-160.
[0102]
Next, the magnetic recording medium according to the present invention will be described.
[0103]
The magnetic recording medium according to the present invention is a magnetic recording medium comprising a metal magnetic particle powder mainly composed of iron in which aluminum is present and a binder resin on a nonmagnetic underlayer formed on a nonmagnetic support. And a recording layer.
[0104]
The acicular metal magnetic particle powder containing iron as a main component in the present invention contains 0.05 to 10% by weight of aluminum in terms of Al.
[0105]
When the abundance of aluminum is less than 0.05% by weight in terms of Al, the resin adsorption strength in the vehicle of the acicular metal magnetic particle powder containing iron as a main component is not sufficient, and the dispersion becomes difficult. Therefore, it is impossible to obtain a magnetic recording medium having excellent durability. If it exceeds 10% by weight, the effect of improving the durability of the magnetic recording medium is recognized, but the effect is saturated and it does not make sense to exist more than necessary. In addition, the increase in aluminum, which is a nonmagnetic component, impairs the magnetic properties of acicular metal magnetic particles mainly composed of iron.
[0106]
The location of aluminum is almost uniform from the center to the surface when it is contained only in the central part of the particles of acicular metal magnetic particle powder containing iron as the main component, or when contained only in the surface layer part. Any of the above cases may be used, or a coating layer may be formed on the surface of the particles, and further, these various positions may be combined. If the surface of the magnetic recording layer and the durability of the magnetic recording medium are taken into account, the iron that consists of particles in which aluminum is contained almost uniformly from the center to the surface and a coating layer is formed on the particle surface. An acicular metal magnetic particle powder having a main component is preferred.
[0107]
As is well known, the acicular metal magnetic particle powder mainly composed of iron containing aluminum inside the particle is obtained by variously changing the addition timing of the aluminum compound in the above-described acetic acid goethite particle formation reaction step. In addition, an acicular goethite particle powder containing aluminum at a desired position inside the particle is obtained, and the acicular goethite particle powder or the acicular goethite particle powder is heated and dehydrated at a desired position inside the particle. It can be obtained by heating and reducing acicular hematite particle powder containing aluminum in a temperature range of 300 to 500 ° C.
[0108]
The acicular metal magnetic particle powder mainly composed of iron whose particle surface is coated with aluminum is composed of acicular goethite particle powder whose particle surface is coated with an aluminum compound such as an oxide or hydroxide of aluminum. The needle-like hematite particle powder whose particle surface obtained by heating and dehydrating the needle-like goethite particle powder is coated with an aluminum compound such as an oxide or hydroxide of aluminum is heated and reduced in a temperature range of 300 to 500 ° C. Can be obtained.
[0109]
The acicular metal magnetic particle powder containing iron as a main component in the present invention is a particle containing 50 to 99% by weight, preferably 60 to 95% by weight of iron, and if necessary, Co other than iron and Al, Ni, P, Si, B, Nd, La, Y, etc. may be contained in an amount of about 0.05 to 10% by weight. When the magnetic recording medium according to the present invention is manufactured by using acicular metal magnetic particle powder mainly composed of iron in which Al and rare earth metal such as Nd, La, and Y are present, it is more durable. A magnetic recording layer and a magnetic recording medium excellent in properties can be obtained. In particular, acicular metal magnetic particle powder mainly composed of iron in which Al and Nd are present is most preferable.
[0110]
The acicular metal magnetic particle powder containing iron as a main component in the present invention has an average major axis diameter of 0.01 to 0.50 μm, preferably 0.03 to 0.30 μm, and an average minor axis diameter of 0.00. 0007 to 0.17 μm, preferably 0.003 to 0.10 μm, particles having an axial ratio of 3 or more, preferably 5 or more, and considering the dispersibility in the vehicle, the upper limit of the axial ratio Is 15, preferably 10.
[0111]
The shape of the particles is acicular. Here, the needle shape means not only a literal needle shape but also a spindle shape or a rice grain shape.
[0112]
In consideration of characteristics such as high density recording, the coercive force is preferably 900 to 3500 Oe, more preferably 1000 to 3500 Oe, and the saturation magnetization 90-170 emu / g is preferable, More preferably, it is 100-170 emu / g.
[0113]
The acicular metal magnetic particles mainly composed of iron in the present invention have a resin adsorption strength of 65% or more, preferably 68% or more, more preferably 70% or more.
[0114]
As the binder resin used for forming the magnetic recording layer, the binder resin used for forming the nonmagnetic underlayer can be used.
[0115]
The blending ratio of the magnetic particle powder and the binder resin in the magnetic recording layer is 200 to 2000 parts by weight, preferably 300 to 1500 parts by weight with respect to 100 parts by weight of the binder resin.
[0116]
The coating thickness of the magnetic recording layer provided on the nonmagnetic underlayer is in the range of 0.01 to 5.0 μm. If the thickness is less than 0.01 μm, uniform coating is difficult and phenomena such as uneven coating tend to occur, which is not preferable. If it exceeds 5.0 μm, it is difficult to obtain desired electromagnetic characteristics due to the influence of the demagnetizing field. Preferably it is the range of 0.05-1.0 micrometer.
[0117]
The magnetic recording layer may contain a lubricant, an abrasive, an antistatic agent and the like that are usually used as necessary.
[0118]
The magnetic recording medium according to the present invention has a coercive force of 900 to 900 when the acicular hematite particle powder according to the present invention that is not coated with aluminum hydroxide is used as the nonmagnetic particle powder for the nonmagnetic underlayer. 3500 Oe, preferably 1000 to 3500 Oe, squareness ratio (residual magnetic flux density Br / saturated magnetic flux density Bm) is 0.86 to 0.95, preferably 0.87 to 0.95, and the glossiness of the coating film is 195 to 300% The surface roughness Ra of the coating film is 8.5 nm or less, preferably 0.5 to 8.3 nm, more preferably 0.5 to 8.0 nm, and the linear absorption coefficient of the coating film is 1. 10-2.00 μm^-1, Preferably 1.20 to 2.00 μm^-1The Young's modulus (relative value) of the coating film is 120 to 160, preferably 122 to 160. Of the durability, the running durability is 18 minutes or more, preferably 20 minutes or more, more preferably 22 minutes or more, and scratches. The characteristic is B or A, preferably A, and the corrosivity indicated by the change rate (%) of the coercive force is 10.0% or less, preferably 9.5% or less, and the change rate (%) of the saturation magnetic flux density. The corrosivity is 10.0% or less, preferably 9.5% or less.
[0119]
The magnetic recording medium according to the present invention has a coercive force of 900 to 900 when the acicular hematite particle powder according to the present invention coated with an aluminum hydroxide or the like is used as the nonmagnetic particle powder for the nonmagnetic underlayer. 3500 Oe, preferably 1000 to 3500 Oe, squareness ratio (residual magnetic flux density Br / saturated magnetic flux density Bm) is 0.86 to 0.95, preferably 0.87 to 0.95, and the glossiness of the coating film is 200 to 300%. The coating film surface roughness Ra is 8.3 nm or less, preferably 0.5 to 8.0 nm, more preferably 0.5 to 7.8 nm, and the linear absorption coefficient of the coating film is 1. 10-2.00 μm^-1, Preferably 1.20 to 2.00 μm^-1The Young's modulus (relative value) of the coating film is 122 to 160, preferably 124 to 160. Of the durability, the running durability is 20 minutes or more, preferably 22 minutes or more, more preferably 24 minutes or more, and scratches. The characteristic is B or A, preferably A, and the corrosivity indicated by the change rate (%) of the coercive force is 10.0% or less, preferably 9.5% or less, and the change rate (%) of the saturation magnetic flux density. The corrosivity is 10.0% or less, preferably 9.5% or less.
[0120]
Next, a method for producing the nonmagnetic underlayer in the present invention will be described.
[0121]
The nonmagnetic underlayer in the present invention is formed by applying a nonmagnetic paint containing the acicular hematite particle powder, the binder resin and the solvent in the present invention on a nonmagnetic support to form a coating film, and then drying. Is obtained.
[0122]
As the solvent, it is possible to use methyl ethyl ketone, toluene, cyclohexanone, methyl isobutyl ketone, tetrahydrofuran and a mixture thereof, which are currently widely used for magnetic recording media.
[0123]
The usage-amount of a solvent is 50-1000 weight part in the total amount with respect to 100 weight part of acicular hematite particle powder. If it is less than 50 parts by weight, the viscosity becomes too high when it is used as a non-magnetic paint, making application difficult. If it exceeds 1000 parts by weight, the volatilization amount of the solvent when forming the coating film becomes too large, which is industrially disadvantageous.
[0124]
Next, a method for manufacturing a magnetic recording medium according to the present invention will be described.
[0125]
The magnetic recording medium according to the present invention comprises a nonmagnetic underlayer formed on a nonmagnetic support and a metal magnetic particle powder mainly composed of iron in which aluminum is present, a binder resin, and a solvent. It is obtained by applying a magnetic coating composition to form a coating film and then drying to form a magnetic recording layer.
[0126]
As the solvent used in the production of the magnetic paint, the solvent used for forming the nonmagnetic underlayer can be used.
[0127]
The total amount of the solvent used is 65 to 1000 parts by weight with respect to 100 parts by weight of the metal magnetic particle powder mainly composed of iron in which aluminum is present. If it is less than 65 parts by weight, the viscosity becomes too high when it is used as a magnetic coating material, making application difficult. When it exceeds 1000 parts by weight, the volatilization amount of the solvent at the time of forming the coating film becomes too large, which is industrially disadvantageous.
[0128]
DETAILED DESCRIPTION OF THE INVENTION
A typical embodiment of the present invention is as follows.
[0129]
For the remaining amount of the sieve, the slurry concentration after wet pulverization is obtained separately, and an amount of slurry equivalent to 100 g of solid content is passed through a 325 mesh (aperture 44 μm) sieve to quantify the amount of solid content remaining in the sieve. Sought by.
[0130]
The average major axis diameter and average minor axis diameter of the particles are the major axis diameter and minor axis diameter of about 350 particles shown in the photograph obtained by enlarging the electron micrograph (× 30000) four times in the vertical and horizontal directions, respectively. Each was measured and indicated by the average value.
[0131]
The axial ratio is shown as the ratio of the average major axis diameter to the average minor axis diameter.
[0132]
The geometric standard deviation value of the major axis diameter of the particle was indicated by a value obtained by the following method. That is, the value measured for the major axis diameter of the particles shown in the above enlarged photograph is calculated on the basis of the actual major axis diameter and the number of particles calculated from the measured value, and the horizontal axis on the lognormal probability paper according to a statistical method. The major axis diameter of the particles is plotted on the vertical axis, and the cumulative number of particles belonging to each of the predetermined major axis diameter sections (under the integrated sieve) is plotted in percentage on the vertical axis. Then, the value of the major axis diameter corresponding to the number of particles of 50% and 84.13% is read from this graph, and the geometrical standard deviation value = major axis diameter under integrated fluid under 84.13% / under integrated fluid under 50. The value was calculated according to the major axis diameter (geometric mean diameter) in%. The closer the geometric standard deviation value is to 1, the better the particle size distribution of the major axis diameter of the particles.
[0133]
The specific surface area value was indicated by a value measured by the BET method.
[0134]
As described above, the degree of densification of the acicular hematite particles is S_BET/ S_TEMIndicated by value. Where S_BETThe value is a specific surface area value measured by the BET method. S_TEMThe value is a value calculated according to the following equation assuming that the particles are rectangular parallelepiped using the average major axis diameter lcm and the average minor axis diameter wcm of the particles measured from the electron micrograph.
[0135]
S_TEM(M²/ G) = [(4lw + 2w²) / (Lw²・ Ρ_p)] × 10^-4(However, ρ_pIs the true specific gravity of hematite, 5.2 g / cm³Was used. )
[0136]
The amount of each of Al, Si, P, Nd, etc. present in the inside or the surface of the acicular hematite particle powder or the acicular metal magnetic particle powder containing iron as a main component is “fluorescent X-ray analyzer 3063M type”. (Manufactured by Rigaku Denki Kogyo Co., Ltd.) and measured according to JIS K0119 “General Rules for Fluorescence X-ray Analysis”.
[0137]
The pH value of the powder was measured by weighing 5 g of a sample into a 300 ml Erlenmeyer flask, adding 100 ml of boiled pure water, heating and holding the boiled state for about 5 minutes, then plugging it and letting it cool to room temperature. After adding water corresponding to the above, stoppered again, shaken for 1 minute, allowed to stand for 5 minutes, and then measured the pH of the obtained supernatant according to JIS Z 8802-7. It was.
[0138]
The content of the soluble sodium salt and the content of the soluble sulfate in the acicular hematite particle powder was determined by changing the supernatant prepared for the measurement of the powder pH value as No. Filter using 5C filter paper and add Na in the filtrate.⁺And SO₄ ^2-Was measured using an inductively coupled plasma optical emission spectrometer (manufactured by Seiko Instruments Inc.).
[0139]
The viscosity of the paint is measured by using an E-type viscometer EMD-R (manufactured by Tokyo Keiki Co., Ltd.), and the shear rate D = 1.92 sec.^-1It was shown by the value in.
[0140]
The resin adsorption strength indicates the adsorption strength of the resin adsorbed on the acicular metal magnetic particle powder containing iron as a main component, and the closer the value T% obtained by the following method is to 100, the more the resin is on the surface of the particle. It is strongly adsorbed on the surface.
[0141]
First, the resin adsorption amount Wa is obtained.
[0142]
20 g of particle powder to be measured and 56 g of a mixed solvent (27.0 g of methyl ethyl ketone, 16.2 g of toluene, 10.8 g of cyclohexanone) in which 2 g of vinyl chloride resin having a sodium sulfonate group are dissolved are placed in a 100 ml polybin together with 120 g of 3 mmφ steel beads. Mix and disperse in a paint shaker for 60 minutes.
[0143]
Next, 50 g of this coating composition is taken out, put into a 50 ml settling tube, and centrifuged at 10000 rpm for 15 minutes to separate the solid portion and the solvent portion. Then, the resin solid content concentration contained in the solvent portion is quantified by a gravimetric method, and the amount of resin present in the solid portion is obtained by subtraction from the charged resin amount, and this is the resin adsorption amount Wa (mg / g) to the particles. And
[0144]
Next, only the solid portion separated earlier was taken out into a 100 ml tall beaker, and 50 g of a mixed solvent (methyl ethyl ketone 25.0 g, toluene 15.0 g, cyclohexanone 10.0 g) was added thereto, followed by ultrasonic dispersion for 15 minutes. After making it into a suspended state, it is placed in a 50 ml settling tube and centrifuged at 10,000 rpm for 15 minutes to separate the solid portion and the solvent portion. Then, by measuring the resin solid content concentration in the solvent portion, the amount of resin extracted into the solvent phase out of the resin adsorbed on the particle surface is quantified.
[0145]
Further, the operation from taking out the whole amount of the solid part only into a 100 ml tall beaker to determining the amount of the resin dissolved in the solvent phase was repeated twice, and the total amount of extracted resin We in the solvent phase total three times (mg / G), and the value obtained according to the following formula was defined as the resin adsorption strength T (%).
[0146]
T (%) = [(Wa-We) / Wa] × 100
[0147]
The glossiness of the coating surface of the nonmagnetic underlayer and magnetic recording layer was determined by measuring the 45 ° glossiness of the coating using “Gloss Meter UGV-5D” (manufactured by Suga Test Instruments Co., Ltd.).
[0148]
For the surface roughness Ra, the center line average roughness Ra of the coating film was measured using “Surfcom-575A” (manufactured by Tokyo Seimitsu Co., Ltd.).
[0149]
Regarding the durability of the magnetic recording medium, the following running durability and scratch characteristics were evaluated.
[0150]
Running durability was evaluated based on the actual moving time at a load of 200 gw and a relative speed of the head and the tape of 16 m / s using “Media Duburity Tester MDT-3000” (manufactured by Steinberg Associates). Indicates that the running durability is good.
[0151]
For the scratch characteristics, the surface of the tape after running was observed with a microscope, the presence or absence of the scratch was visually evaluated, and the following four grades were evaluated.
A: No scratch
B: Some scratches
C: Scratch
D: Severe scratches
[0152]
The strength of the coating film was determined by measuring the Young's modulus of the coating film using “Autograph” (manufactured by Shimadzu Corporation). The Young's modulus was expressed as a relative value with a commercially available video tape “AV T-120 (manufactured by Victor Company of Japan)”. It shows that the intensity | strength of a coating film is so favorable that a relative value is high.
[0153]
The magnetic properties of the needle-shaped metal magnetic particle powder containing iron as a main component and the magnetic recording medium are measured using an “oscillating sample magnetometer VSM-3S-15” (manufactured by Toei Kogyo Co., Ltd.) and applied to an external magnetic field of 10 KOe. Measured.
[0154]
The change over time in the magnetic characteristics of the magnetic recording medium accompanying the corrosion of the needle-shaped metal magnetic particle powder containing iron as the main component in the magnetic recording layer is 14 days in an environment of a temperature of 60 ° C. and a relative humidity of 90%. The coercive force value and the saturation magnetic flux density value before and after being allowed to stand were measured, and the value obtained by dividing the amount of change by the value before leaving was indicated as a change rate as a percentage.
[0155]
The degree of light transmission is indicated by a linear absorption coefficient calculated by inserting the value of light transmittance measured for a magnetic recording medium using a “self-recording spectrophotometer UV-2100” (manufactured by Shimadzu Corporation) into the following equation. It was. The larger the value of the linear absorption coefficient, the harder it is to transmit light. In measuring the value of light transmittance, the same nonmagnetic support as the nonmagnetic support used in the magnetic recording medium was used as a blank.
[0156]
Linear absorption coefficient (μm^-1) = [Ln (1 / t)] / FT
t: Light transmittance at λ = 900 nm (−)
FT: Thickness (μm) of coating layer of film used for measurement (total thickness of nonmagnetic underlayer and magnetic recording layer)
[0157]
The thickness of each of the nonmagnetic support, the nonmagnetic underlayer and the magnetic recording layer constituting the magnetic recording medium was measured by the following method.
That is, first, the film thickness (A) of the nonmagnetic support is measured using a digital electronic micrometer K351C (manufactured by Anritsu Electric Co., Ltd.). Next, the thickness (B) of the nonmagnetic support and the nonmagnetic underlayer formed on the nonmagnetic support (the sum of the thickness of the nonmagnetic support and the nonmagnetic underlayer) was measured in the same manner. To do. Further, the thickness (C) of the magnetic recording medium obtained by forming the magnetic recording layer on the nonmagnetic underlayer (the sum of the thickness of the nonmagnetic support, the thickness of the nonmagnetic underlayer, and the thickness of the magnetic recording layer). ) Is measured in the same manner. The thickness of the nonmagnetic underlayer is indicated by B-A, and the thickness of the magnetic recording layer is indicated by CB.
[0158]
<Production of spindle-shaped hematite particle powder>
Spindle-shaped goethite particle powder (average major axis diameter 0.171 μm, average minor axis diameter 0.0213 μm, obtained by the above-mentioned method (2) for producing goethite particles using ferrous sulfate aqueous solution and sodium carbonate aqueous solution, Axial ratio 8.0, long axis diameter geometric standard deviation value 1.34, BET specific surface area value 151.6m²/ G, powder pH value 5.8, soluble sodium salt content (Na equivalent 721 ppm, soluble sulfate content (SO₄1200 g (converted 1121 ppm) was suspended in water to form a slurry, and the solid content concentration was adjusted to 8 g / l. 150 l of this slurry was heated to a temperature of 60 ° C., and 0.1N NaOH aqueous solution was added to adjust the pH value of the slurry to 10.0.
[0159]
Next, 36 g of No. 3 water glass was gradually added to the alkaline slurry as a sintering inhibitor. After the addition was completed, aging was performed for 60 minutes. Next, a 0.1N acetic acid solution was added to the slurry to adjust the pH value of the slurry to 6.0. Thereafter, filtration, washing, drying, and pulverization were performed by a conventional method to obtain spindle-shaped goethite particle powder in which a silicon oxide was coated on the particle surface. Silicon content is SiO₂It was 0.78% by weight in terms of conversion.
[0160]
1000 g of the obtained spindle-shaped goethite particle powder was put into a stainless steel rotary furnace and subjected to heat dehydration treatment at 350 ° C. for 40 minutes in the air while being rotated to obtain a low-density spindle-shaped hematite particle powder.
[0161]
The resulting low density spindle-shaped hematite particle powder has an average major axis diameter of 0.145 μm, an average minor axis diameter of 0.0193 μm, an axial ratio of 7.5, and a major axis diameter geometric standard deviation value of 1.34, BET specific surface area (S_BET) Is 176.5m²/ G, S indicating the degree of densification_BET/ S_TEMThe value is 4.15, the powder pH value is 6.3, the content of soluble sodium salt is 1717 ppm in terms of Na, and the soluble sulfate is SO₄1011ppm in terms of conversion, silicon content is SiO₂It was 0.85% by weight in terms of conversion.
[0162]
Next, 850 g of the low-density spindle-shaped hematite particle powder was put into a ceramic rotary furnace, and heat-treated at 610 ° C. for 30 minutes in the air while being driven to rotate, thereby sealing the dehydration holes.
[0163]
The densified spindle-shaped hematite particle powder has an average major axis diameter of 0.137 μm, an average minor axis diameter of 0.0190 μm, an axial ratio of 7.2, a geometric standard deviation value of major axis diameter of 1.35, BET Specific surface area (S_BET) Is 55.3m²/ G, ratio S indicating the degree of densification_BET/ S_TEMThe value is 1.28, the powder pH value is 5.3, the content of soluble sodium salt is 2626 ppm in terms of Na, and the soluble sulfate is SO₄3104ppm in conversion, silicon content is SiO₂It was 0.85% by weight in terms of conversion.
[0164]
After 800 g of the obtained densified spindle-shaped hematite particle powder was coarsely pulverized in advance with a Nara type pulverizer, it was put into 4.7 l of pure water, and then used with a homomixer (manufactured by Special Machine Industries Co., Ltd.). Peptified for a minute.
[0165]
Next, the slurry of the densified spindle-shaped hematite particle powder was mixed and dispersed for 3 hours at a shaft rotation speed of 2000 rpm while circulating through a horizontal SGM (Dispamat SL: manufactured by ESC Adchem). The screen residue at 325 mesh (aperture 44 μm) of the spindle-shaped hematite particle powder in the slurry was 0%.
[0166]
<Dissolution treatment of spindle-shaped hematite particle powder with acid>
Water was added to the resulting slurry of densified spindle-shaped hematite particle powder to adjust the concentration of the slurry to 100 g / l, and then 7 l of the slurry was sampled. While stirring the collected slurry, 70% by weight sulfuric acid aqueous solution was added to adjust the acid concentration of the slurry to 1.3N. The pH value of the slurry at this time was 0.65. Next, the slurry is heated with stirring to a temperature of 80 ° C., held at that temperature for 5 hours for dissolution treatment, and 29.7 of the total amount of spindle-shaped hematite particles present in the liquid. % By weight was dissolved.
[0167]
Next, this slurry was separated by filtration to separate a filtrate (an acidic aqueous solution of iron sulfate).
After separating the filtrate from the slurry, it was washed with water by a decantation method to obtain a water-washed slurry having a pH value of 5.0. The slurry concentration at this point was confirmed to be 68 g / l.
[0168]
Here, a part of the obtained water-washed slurry is separated and filtered using a Buchner funnel, and pure water is passed through, and the filtrate is washed with water until the conductivity becomes 30 μs or less. After drying, the mixture was pulverized to obtain spindle-shaped hematite particle powder.
[0169]
The obtained spindle-shaped hematite particle powder has an average major axis diameter of 0.131 μm, an average minor axis diameter of 0.0181 μm, an axial ratio of 7.2, a major axis diameter geometric standard deviation value of 1.35, and a BET ratio. Surface area value (S_BET) Is 58.1m²/ G, S indicating the degree of densification_BET/ S_TEMThe value is 1.28, the powder pH value is 4.8, the content of soluble sodium salt is 138 ppm in terms of Na, and the content of soluble sulfate is SO₄It was 436 ppm in terms of conversion.
[0170]
<Heat treatment of alkaline suspension of spindle-shaped hematite particles>
Water was added to the water-washed slurry of the spindle-shaped hematite particle powder after the dissolution treatment with the acid to a concentration of 50 g / l, and 5 l of the slurry was collected. While stirring the slurry, 6N NaOH aqueous solution was added to adjust the pH value of the slurry to 13.6. Next, this slurry was heated with stirring to a temperature of 95 ° C. and held at that temperature for 3 hours.
[0171]
Next, this slurry was washed with water by a decantation method to obtain a slurry having a pH value of 10.5. For the sake of accuracy, the slurry concentration at this point was confirmed to be 98 g / l.
[0172]
Next, 1 l of the obtained water washing slurry was filtered using a Buchner funnel, and pure water was passed through, and the filtrate was washed with water until the electric conductivity of the filtrate became 30 μs or less. Thus, the intended spindle-shaped hematite particle powder was obtained.
[0173]
The obtained spindle-shaped hematite particle powder has an average major axis diameter of 0.131 μm, an average minor axis diameter of 0.0181 μm, an axial ratio of 7.2, a major axis diameter geometric standard deviation value of 1.35, and a BET ratio. Surface area value (S_BET) Is 57.8m²/ G, S indicating the degree of densification_BET/ S_TEMThe value is 1.27, the powder pH value is 9.3, the content of soluble sodium salt is 99 ppm in terms of Na, and the content of soluble sulfate is SO₄It was 21 ppm in terms of conversion.
[0174]
<Manufacture of magnetic recording medium-formation of nonmagnetic underlayer on nonmagnetic support>
12 g of the spindle-shaped hematite particle powder obtained above, a binder resin solution (30% by weight of vinyl chloride-vinyl acetate copolymer resin having sodium sulfonate group and 70% by weight of cyclohexanone) and cyclohexanone are mixed to obtain a mixture (solid The mixture was further kneaded with a plast mill for 30 minutes to obtain a kneaded product.
[0175]
This kneaded product is placed in a 140 ml glass bottle with 95 g of 1.5 mmφ glass beads, a binder resin solution (30% by weight of a polyurethane resin having a sodium sulfonate group, 70% by weight of a solvent (methyl ethyl ketone: toluene = 1: 1)), cyclohexanone, methyl ethyl ketone and It was added together with toluene and mixed and dispersed for 6 hours with a paint shaker to obtain a coating composition. The paint viscosity at this time was 384 cP.
[0176]
The composition of the coating material containing the obtained spindle-shaped hematite particle powder was as follows.

[0177]
The obtained coating material containing spindle-shaped hematite particle powder was applied on a polyethylene terephthalate film having a thickness of 12 μm to a thickness of 55 μm using an applicator, and then dried to form a nonmagnetic underlayer. The thickness of the nonmagnetic underlayer was 3.5 μm.
[0178]
The obtained nonmagnetic underlayer had a gloss of 204%, a surface roughness Ra of 6.6 nm, and a Young's modulus (relative value) of the coating film of 124.
[0179]
<Manufacture of magnetic recording medium-formation of magnetic recording layer>
Needle-like metal magnetic particle powder mainly composed of iron (average major axis diameter 0.115 μm, average minor axis diameter 0.0145 μm, axial ratio 7.9, coercive force 1909 Oe, saturation magnetization 133.8 emu / g, Al content Amount (center part 1.26% by weight, surface layer part 0.84% by weight, surface coating 0.75% by weight), Nd content 0.12% by weight, resin adsorption strength 81.6%) 12 g, abrasive (product Name: AKP-30, manufactured by Sumitomo Chemical Co., Ltd.) 1.2 g, carbon black (trade name: # 3250B, manufactured by Mitsubishi Kasei Co., Ltd.) 0.36 g, binder resin solution (vinyl chloride having a sodium sulfonate group) -30% by weight of vinyl acetate copolymer resin and 70% by weight of cyclohexanone) and cyclohexanone were mixed to obtain a mixture (solid content ratio 78%), and this mixture was further kneaded with a plastmill for 30 minutes to obtain a kneaded product.
[0180]
This kneaded product is placed in a 140 ml glass bottle with 95 g of 1.5 mmφ glass beads, a binder resin solution (30% by weight of a polyurethane resin having a sodium sulfonate group, 70% by weight of a solvent (methyl ethyl ketone: toluene = 1: 1)), cyclohexanone, methyl ethyl ketone and After adding with toluene and mixing and dispersing for 6 hours with a paint shaker, a lubricant and a curing agent were added, and further mixed and dispersed for 15 minutes with a paint shaker to obtain a magnetic paint.
[0181]
The composition of the obtained magnetic paint was as follows.

[0182]
The magnetic paint is applied on the non-magnetic underlayer to a thickness of 15 μm using an applicator, then oriented and dried in a magnetic field, and then subjected to a calendar treatment, followed by a curing reaction at 60 ° C. for 24 hours. And slitting to 0.5 inch width to obtain a magnetic tape. The thickness of the magnetic recording layer was 1.0 μm.
[0183]
The obtained magnetic tape had a coercive force value of 1980 Oe, a squareness ratio (Br / Bm) of 0.88, a glossiness of 223%, a surface roughness Ra of 6.4 nm, and a Young's modulus (relative value) of the coating film. Is 128, linear absorption coefficient is 1.24 μm^-1The running durability was 28.9 minutes, the scratch property was A, the change rate of the coercive force indicating the corrosion resistance of the magnetic tape was 4.6%, and the change rate of the saturation magnetic flux density was 2.7%. It was.
[0184]
[Action]
The most important points in the present invention are an average major axis diameter of 0.005 to 0.30 μm, and a BET specific surface area value of 35 to 150 m.²An aqueous suspension of acicular hematite particle powders having an acid concentration of 1.0 N or more, a pH value of 3.0 or less, and a temperature range of 20 to 100 ° C. After dissolving 5 to 50% by weight of the total amount of acicular hematite particles present therein, an aqueous alkaline solution is added to the aqueous suspension of acicular hematite particles obtained by washing the remaining acicular hematite particles with water. After adding and adjusting the pH value to 13 or more, heat treatment is carried out in a temperature range of 80 to 103 ° C., and then the average major axis diameter obtained by filtration, washing and drying is 0.004 to 0.295 μm, BET specific surface area value 35.9 to 212m²/ G, the powder pH value is 8 or more, the content of soluble sodium salt is 300 ppm or less in terms of Na, and the content of soluble sulfate is SO₄When a needle-like hematite particle powder of 150 ppm or less in terms of conversion is used as the non-magnetic particle powder for the non-magnetic under layer, the non-magnetic under layer is attributed to the excellent dispersibility in the binder resin. The surface smoothness and the strength of the substrate can be improved, and a magnetic recording layer using acicular metal magnetic particle powder mainly composed of iron in which aluminum is present is provided on the nonmagnetic underlayer. In this case, the magnetic recording layer can have a low light transmittance, a high strength, a smoother surface, and a more durable magnetic recording medium, and can be dispersed in the magnetic recording layer. This is a fact that it is possible to suppress the deterioration of the magnetic properties accompanying the corrosion of the metal magnetic particle powder containing iron as a main component in which the aluminum being present is present.
[0185]
Regarding the reason why the surface smoothness of the nonmagnetic underlayer and the strength of the substrate could be improved, the present inventor suggested that the soluble sodium salt causes the high density acicular hematite particles to be firmly crosslinked and aggregated. And the soluble sulfate can be sufficiently washed away with water, so that the agglomerates can be unraveled into substantially independent particles, so that the dispersibility in the vehicle is improved. This is considered to be due to the fact that excellent acicular hematite particle powder is obtained.
[0186]
Regarding the reason why the surface smoothness of the magnetic recording medium according to the present invention is improved, the present inventor has obtained that the geometric standard deviation value of the major axis diameter of the acicular hematite particles in the present invention is 1.50 or less, It is a particle with a small presence of fine particles and a BET specific surface area value of 35.9 to 212 m²/ G, and due to the synergistic effect of particles with few dehydration pores inside and on the particle surface, the dispersibility in the vehicle is improved, and as a result, the surface smoothness of the resulting nonmagnetic underlayer is improved. I believe.
[0187]
Further, as another reason why the surface smoothness of the magnetic recording medium according to the present invention is excellent, the present inventor has obtained a soluble sodium salt that causes the needle-like hematite particles to be firmly crosslinked and aggregated. Due to the fact that the soluble sulfate was sufficiently washed and removed, the agglomerates can be unraveled into substantially independent particles, resulting in excellent dispersibility in the vehicle. This is thought to be due to the obtained needle-like hematite particles.
[0188]
The fact that the acicular hematite particle powder in the present invention is excellent in dispersibility in the vehicle will be described below.
[0189]
The needle-like goethite particle powder used as the starting material is produced by various production methods as described above. In any method, when the main raw material for producing the needle-like goethite particle powder is ferrous sulfate. Naturally, sulfate [SO₄ ^2-] In large quantities.
[0190]
In particular, when producing goethite particles from an acidic solution, Na₂SO₄This produces iso-water soluble sulfate and the reaction mother liquor has K⁺, NH⁴⁺, Na⁺Since it contains an alkali metal such as an alkali metal, a precipitate containing an alkali metal or sulfate is likely to be formed.₃(SO₄) (OH)₆(R = K⁺, NH⁴⁺, Na⁺). These precipitates are sparingly soluble sulfur-containing iron salts and cannot be removed by conventional water washing. This hardly soluble salt becomes a soluble sodium salt or soluble sulfate in the subsequent heat treatment step, but this soluble sodium salt or soluble sulfate is in the form of needle-shaped hematite particles in the high temperature heat treatment step for densification. The anti-sintering agent, which is essential to prevent deformation and sintering between particles, is firmly bonded to the inside and the surface of the particles while cross-linking the needle-like hematite particles. Aggregation between them is further strengthened. As a result, in particular, soluble sulfates and soluble sodium salts trapped inside the particles and aggregates are extremely difficult to remove by washing with water by a conventional method.
[0191]
When producing acicular goethite particles in an alkaline aqueous solution using ferrous sulfate and sodium hydroxide, the sulfate produced simultaneously is Na₂SO₄In addition, NaOH exists in the mother liquor, and both of them are soluble, so if the needle-like goethite particles are sufficiently washed with water, essentially Na₂SO₄And should be able to remove NaOH. However, since the crystallinity of needle-like goethite particles is generally low, the washing efficiency is poor. When washing with water by a conventional method, soluble sulfate [SO₄ ^2-], Soluble sodium salt [Na⁺] Contains water soluble content. As described above, this water-soluble component is firmly bonded to the inside and the surface of the particles while cross-linking the acicular hematite particles with the sintering inhibitor, thereby further aggregating the acicular hematite particles. Strengthen. As a result, in particular, soluble sulfates and soluble sodium salts trapped inside the particles and aggregates are extremely difficult to remove by washing with water by a conventional method.
[0192]
As described above, high-density hematite particles in which soluble sodium salt and soluble sulfate are strongly bonded to the inside of the particle, the surface of the particle, and the inside of the aggregate via an anti-sintering agent are wet-ground to loosen the coarse particles. After that, when the pH value of the slurry is adjusted to 13 or more and heat treatment is performed at a temperature of 80 ° C. or more, the alkaline aqueous solution sufficiently penetrates into the particles of the high-density hematite particles. It is thought that the binding force of the sintering inhibitor that is strongly bonded to the surface gradually weakens and dissociates from the inside of the particles, the surface of the particles, and the inside of the aggregates, and at the same time, water-soluble sodium salts and water-soluble sulfates can be easily washed and removed. It is done.
[0193]
In addition, regarding the reason why the deterioration of magnetic properties due to corrosion of acicular metal magnetic particle powder mainly composed of iron dispersed in the magnetic recording layer is suppressed, the present inventor Acicular metal magnetism mainly composed of iron due to low solubility of soluble sodium salts and sulfates that promote metal corrosion of hematite particles and high powder pH value of 8 or higher It is thought that the progress of the corrosion of the particle powder could be suppressed.
[0194]
In fact, as shown in the examples and comparative examples described later, the inventor applied the needle-like hematite particles after wet pulverization to a temperature of 80 ° C. or higher in an alkaline suspension having a pH value of less than 13. When heat-treated, needle-shaped hematite particles after wet pulverization are heat-treated in an alkaline suspension having a pH value of 13 or higher by applying a temperature of less than 80 ° C. The main component is iron in which aluminum is present in any case where heat treatment is performed by applying a temperature of 80 ° C. or higher in an alkaline suspension having a pH value of 13 or higher without containing coarse particles. The metal magnetic particle powder containing iron as a main component due to the synergistic effect of having a low soluble content and having a powder pH value of 8 or more because the progress of the corrosion of the metal magnetic particle powder can not be sufficiently suppressed. The progress of corrosion It has confirmed the phenomenon of being able to control.
[0195]
Further, although the reason why the durability of the magnetic recording medium according to the present invention is improved is not yet clear, the present inventor has obtained acicular metal magnetic particle powder mainly composed of iron in which aluminum is present as magnetic particles. As a result, the adhesion strength of the binder resin to the magnetic particles in the coating increased, and as a result, the adhesion of the magnetic particles in the magnetic recording layer and the nonmagnetic underlayer of the magnetic recording layer itself increased. I think it is due to this.
[0196]
【Example】
Next, examples and comparative examples are given.
[0197]
<Types of acicular goethite particle powder>
Starting materials 1-6
As a starting material for acicular hematite particles, acicular goethite particles having the characteristics shown in Table 1 were prepared.
[0198]
[Table 1]

[0199]
<Production of low density acicular hematite particle powder>
Examples 1-7, Comparative Examples 1-6
The low density needle is the same as the embodiment of the present invention except that the kind of needle-like goethite particle powder as a starting material, the kind and addition amount of the sintering inhibitor, the heat dehydration temperature and the time are variously changed. -Like hematite particle powder was obtained. In addition, the particle powder obtained in Comparative Example 4 is acicular goethite particle powder.
[0200]
The main production conditions at this time are shown in Table 2, and various characteristics of the obtained low density acicular hematite particles are shown in Table 3.
[0201]
[Table 2]

[0202]
[Table 3]

[0203]
<Production of high density acicular hematite particle powder>
Examples 8-14, Comparative Examples 7-11
A high-density needle-like hematite particle powder was obtained in the same manner as in the embodiment of the present invention except that the kind of the low-density needle-like hematite particle powder, the temperature and time of the densification heat treatment were variously changed.
[0204]
The main production conditions at this time are shown in Table 4, and various characteristics of the obtained high-density needle-like hematite particles are shown in Table 5.
[0205]
[Table 4]

[0206]
[Table 5]

[0207]
<Dissolution treatment of acicular hematite particle powder with acid>
Examples 15-21, Comparative Examples 12-13
Except for various changes in the type of high-density needle-shaped hematite particle powder, the presence or absence of wet grinding, the acid concentration, the pH value of the slurry, the heating temperature and the heating time, the needle-like shape A hematite particle powder was obtained.
[0208]
The main production conditions at this time are shown in Table 6, and various properties of the obtained acicular hematite particles are shown in Table 7.
[0209]
[Table 6]

[0210]
[Table 7]

[0211]
<Heat treatment of alkaline suspension of acicular hematite particle powder>
Examples 22-28, Reference Examples 1-2
Acicular hematite particle powder was obtained in the same manner as in the embodiment of the present invention except that the kind of acicular hematite particle powder, the pH value of the alkaline suspension, the heating temperature and the heating time were variously changed.
[0212]
The main production conditions at this time are shown in Table 8, and various characteristics of the obtained acicular hematite particle powder are shown in Table 9.
[0213]
[Table 8]

[0214]
[Table 9]

[0215]
<Surface coating treatment of acicular hematite particle powder>
Example 29
The water-washed slurry having a pH value of 10.5 in Example 22 obtained by washing with water by decantation after the heat treatment in the alkaline suspension had a slurry concentration of 68 g / l. 5 l of this water washing slurry was heated again to 60 ° C., and 126 ml of 1.0 N sodium aluminate aqueous solution (corresponding to 1.0% by weight in terms of Al with respect to acicular hematite particles) was added to the slurry, and 30 minutes was added. After being held, the pH value was adjusted to 8.0 using an acetic acid aqueous solution. Next, in the same manner as in the above-described embodiment of the present invention, needle-like hematite particle powder composed of particles whose particle surfaces were coated with a coating was obtained by filtration, washing with water, drying and pulverization.
[0216]
Table 10 shows the main production conditions at this time, and Table 11 shows various characteristics of the obtained acicular hematite particle powder.
[0217]
Examples 30-35
Except for changing the kind of acicular hematite particle powder, the kind of surface-treated product, and the amount added, the same as in Example 29, needle-like particles composed of particles whose particle surfaces are coated with a coating by a conventional method. A hematite particle powder was obtained.
[0218]
Table 10 shows the main production conditions at this time, and Table 11 shows various characteristics of the obtained acicular hematite particle powder.
[0219]
[Table 10]

[0220]
[Table 11]

[0221]
<Manufacture of magnetic recording medium-formation of nonmagnetic underlayer on nonmagnetic support>
Examples 36-49, Comparative Examples 14-22, Reference Examples 3-11
Using the acicular hematite particle powders obtained in Examples 15 to 35, Comparative Examples 1, 3, 7 to 13 and Reference Examples 1 and 2, in the same manner as in the embodiment of the present invention, a nonmagnetic underlayer Formed.
[0222]
Tables 12 and 13 show the main manufacturing conditions and various characteristics at this time.
[0223]
[Table 12]

[0224]
[Table 13]

[0225]
<Manufacture of magnetic recording medium-formation of magnetic recording layer>
Table 14 shows the magnetic particle powders used for forming the magnetic recording layer and various properties thereof.
[0226]
[Table 14]

[0227]
Examples 50-63, Comparative Examples 23-31, Reference Examples 12-20
Except that various types of nonmagnetic underlayers and magnetic particle powders obtained in Examples 36 to 49, Comparative Examples 14 to 22 and Reference Examples 3 to 11 were changed, the same as in the above embodiment of the present invention. Thus, a magnetic recording medium was manufactured.
[0228]
Tables 15 and 16 show the main manufacturing conditions and various characteristics at this time.
[0229]
[Table 15]

[0230]
[Table 16]

[0231]
【The invention's effect】
In the magnetic recording medium according to the present invention, when acicular hematite particles are used as the nonmagnetic powder for the nonmagnetic underlayer, the content of the fine particles is small, the powder pH is 8 or more, and the soluble salt Due to its low content, the dispersibility in the vehicle is excellent, so that a nonmagnetic underlayer excellent in strength and surface smoothness can be obtained. When the magnetic recording medium is a magnetic recording medium using a needle-shaped metallic magnetic particle powder mainly composed of iron containing a predetermined amount of Al as the magnetic particle powder of the recording layer, the light transmittance is small and the surface smoothness is excellent. High strength, high durability, and high-density magnetism because it can suppress the deterioration of magnetic properties due to corrosion of acicular metal magnetic particle powder mainly composed of iron dispersed in the magnetic recording layer It is suitable as a recording medium.

Claims (2)

Nonmagnetic support, nonmagnetic underlayer comprising nonmagnetic particle powder and binder resin formed on nonmagnetic support, and magnetic particle powder and binder resin formed on nonmagnetic underlayer In the magnetic recording medium comprising a magnetic recording layer, the magnetic particle powder is an acicular metal magnetic particle powder mainly composed of iron in which 0.05 to 10% by weight of aluminum is present in terms of Al, and Nonmagnetic particle powder is an aqueous suspension of acicular hematite particle powder having an average major axis diameter of 0.005 to 0.30 μm and a BET specific surface area value of 35 to 150 m ² / g. 3.0 or less, a dissolution treatment with an acid under the conditions of a temperature range of 20 to 100 ° C., 5 to 50% by weight of the total amount of acicular hematite particles present in the aqueous suspension is dissolved, and then remains Acicular hematite particle powder After adding an alkaline aqueous solution to an aqueous suspension of needle-like hematite particles obtained by washing with water to adjust the pH value to 13 or more, heat treatment is carried out at a temperature range of 80 to 103 ° C., followed by filtration, The average major axis diameter obtained by washing and drying is 0.004 to 0.295 μm, the BET specific surface area value is 35.9 to 212 m ² / g, the powder pH value is 8 or more, and the content of the soluble sodium salt is A magnetic recording medium comprising acicular hematite particles having a concentration of 300 ppm or less in terms of Na and a soluble sulfate content of 150 ppm or less in terms of SO ₄ . The surface of particles of acicular hematite particles for a nonmagnetic underlayer of a magnetic recording medium is coated with at least one of aluminum hydroxide, aluminum oxide, silicon hydroxide, or silicon oxide. The magnetic recording medium according to 1.