JP3653151B2 - Image forming method and magnetic toner used therefor - Google Patents

Description

Ｎ：磁性トナーの個数平均粒子径（μｍ）
Ｍ：シリカ付着量（重量％）
また、磁性材を内包するトナー担持体上に規制部材によって磁性トナーの薄層を形成し、該トナー担持体とそれに対向して設けられた背面電極との間に直流電界を形成し、該トナー担持体と該背面電極に沿って設置された記録媒体との間にトナーの通過を制御する制御電極を設けてトナー担持体上の磁性トナーを画像パターンに応じて直接飛翔させる画像形成方法に使用される磁性トナーであって、該磁性トナーのコールターカウンター法による体積平均粒子径が６〜９μｍであり、且つ該磁性トナーの個数平均粒子径（Ｎ）と該磁性トナーの表面に付着している比表面積が１００〜１６０ｍ²／ｇである疎水性シリカ微粉末の付着量（Ｍ）とが下記式（１）を満足することを特徴とする磁性トナーを提案するものである。
【数４】

Ｎ：磁性トナーの個数平均粒子径（μｍ）
Ｍ：シリカ付着量（重量％）
【０００５】
以下、本発明の画像形成方法を図１に基づいて説明する。
図１において、８はホッパー、９は攪拌羽根であって、ホッパー８には磁性トナー４が貯溜され、攪拌羽根９によって攪拌される。そして、ホッパー８内の磁性トナー４は、内部に磁性材１を有するアルミニウム等からなるトナー担持体２の回転によって移動するようになっている。
トナー担持体２の周面の磁性トナー４はシリコーンゴムやウレタンゴム等の規制部材３によって薄層が形成され、該トナー担持体２とそれに対向して設けられた背面電極５との間に直流電界を形成し、該トナー担持体２と該背面電極５に沿って設置された転写紙等の記録媒体６との間に磁性トナー４の通過を制御する制御電極７を設けてトナー担持体２周面上の磁性トナー４を画像パターンに応じて直接記録媒体６に飛翔させ画像を形成するものである。
図１においては磁性トナー４をマイナス帯電とし、背面電極５および制御電極７をプラス帯電としているが、磁性トナー４をプラス帯電とし、背面電極５および制御電極７をマイナス帯電としてもよい。
【０００６】
本発明の画像形成方法は上記磁性トナーに特徴を有するものであって、該磁性トナーは、コールターカウンター法による体積平均粒子径が６〜９μｍである。体積平均粒子径が６μｍ未満ではトナー粒子同士の付着力が大きくなるため、トナー担持体上で均一なトナー層を形成するのが困難になり、また機内の排風などの影響を受けやすくなるため、機内汚れや制御電極へのトナー付着などが起こりやすくなる。また、体積平均粒子径が９μｍを越えると、個々のトナー粒子の質量が大きくなり、トナー飛翔時の慣性力が大きくなるため、記録媒体に飛翔した時点で飛び散りやすくなり、良好な画質が得られなくなる。
【０００７】
本発明は、その表面に比表面積が１００〜１６０ｍ²／ｇである疎水性シリカ微粉末が付着されている磁性トナーを使用する。
疎水性シリカ微粉末の比表面積が１００ｍ²／ｇ未満では、磁性トナーの流動性が不足し、トナー担持体上に均一なトナーの薄層を形成することができない。
また、比表面積の大きい疎水性シリカ微粉末では、シリカそのものあるいはシリカ被覆された磁性トナーの摩擦帯電量が高くなる傾向があるため、疎水性シリカ微粉末の比表面積が１６０ｍ²／ｇを越えた場合には、磁性トナーの摩擦帯電量が増加してトナー担持体への付着力が増加し、現像量の減少による画像濃度低下が発生する。さらに、その高帯電量の磁性トナーは制御電極を通過する際に電極部に付着しやすく、高帯電量の磁性トナーが付着した電極では正常な信号制御ができなくなるため画像不良が発生しやすくなる。
【０００８】
本発明では磁性トナーに付着している前述の比表面積を有する疎水性シリカ微粉末の付着量（Ｍ）と磁性トナーの個数平均粒子径（Ｎ）との関係が前記式（１）を満足するように該疎水性シリカ微粉末を付着する。
更に下記式（２）を満足するように疎水性シリカ微粉末を付着するのがより好ましい。
【数５】

疎水性シリカ微粉末の付着量が式（１）の範囲よりも少ない場合には、磁性トナーの流動性が不足するためトナー担持体上へのトナー供給が不良となり、連続プリント時に画像濃度が不足する。また、逆に疎水性シリカ微粉末の付着量が式（１）の範囲よりも多い場合には、トナーから遊離して存在する遊離シリカが増加し、この遊離シリカが制御電極に選択的に付着するために、画像不良が発生しやすくなる。
なお、上式におけるシリカ付着量（重量％）とは、疎水性シリカ微粉末を表面に付着していないトナー粒子と疎水性シリカ微粉末との合計重量中、つまり磁性トナー全体の疎水性シリカ微粉末の重量割合を示しているものである。
【０００９】
本発明においては磁性トナーが負帯電性の場合、表面に付着させる疎水性シリカ微粉末のＰＨ値は７〜９であることが望ましい。ここでいうＰＨ値とは水：メタノール＝１：１溶液にシリカ微粉末を４重量％分散させた場合の溶液のＰＨ値である。ＰＨ値が７未満ではシリカ微粉末の帯電量がマイナス側に高くなり、磁性トナー全体の帯電量がマイナス側に高くなってしまうため、制御電極へのトナー付着による画像不良が発生しやすくなる。逆にＰＨ値が９を越えて高い場合には、負帯電性の磁性トナーの帯電量が低くなるため、トナー担持体上に均一なトナー層を形成できなくなる。このようなＰＨ値の疎水性シリカ微粉末は、トリメチルシリル基で表面処理したものが挙げられる。
なお、本発明でいう体積平均粒子径とは体積積算５０％径のことであり、個数平均粒子径とは個数積算５０％径であって、該個数平均粒子径も体積平均粒子径と同様にコールターカウンター法により測定した値である。
【００１０】
また、本発明の磁性トナーは、磁性粉の含有量が２０〜４０重量％であることが好ましい。磁性粉の含有量が２０重量％より少ないとトナー担持体上を良好に搬送供給されるだけの磁力が得られにくく、４０重量％より多いとトナー担持体上での搬送性は良好であるものの、トナー担持体との磁気的吸着力が大きいため良好な飛翔性が得られにくく、更に磁性トナーの比重が大きいため磁性トナーが飛翔した場合は慣性力が大きいために記録媒体上の画像部に飛び散りが発生しやすくなる。
【００１１】
本発明の磁性トナーは、少なくとも結着樹脂、磁性粉、帯電制御剤からなり、これらを熱ロール、ニーダー、エクストルーダー等の熱混練機によって混練した後、機械的な粉砕、分級によって得ることができる。また、懸濁重合法や乳化重合法等の重合法によって得ることもできる。
本発明の磁性トナーに使用される結着樹脂としては、ポリスチレン、ポリエチレン、ポリプロピレン、ビニル系樹脂、ポリアクリレート、ポリメタクリレート、スチレン−アクリル酸エステル系共重合体、ポリ塩化ビニリデン、ポリアクリロニトリル、ポリエーテル、ポリカーボネート、ポリエステル樹脂、エポキシ樹脂、セルロース系樹脂などが好適に使用される。
【００１２】
本発明の磁性トナーに使用される磁性粉としては、結晶学的にはスピネル、ペロブスカイト、六方晶、ガーネット、オルソフェライト構造を有するフェライトやマグネタイト等が適用される。該フェライトの構造は、ニッケル、亜鉛、マンガン、マグネシウム、銅、リチウム、バリウム、バナジウム、クロム、カルシウム等の酸化物と３価の鉄酸化物との焼結体である。磁性粉の形状は球状タイプ、六面体タイプ、八面体タイプが好適に使用される。
【００１３】
本発明の磁性トナーに使用される帯電制御剤としては、正極性としてはニグロシン染料や第４級アンモニウム塩、負極性としては含金属モノアゾ系染料等が使用できる。
【００１４】
本発明の磁性トナーにはカーボンブラックを含有するのが望ましい。使用されるカーボンブラックとしては、通常トナー用として公知のものが使用できるが、ＢＥＴ比表面積及び吸油量が大きい、いわゆる導電性グレードのカーボンブラックは少量の添加で有効なトナー帯電性調整機能が得られるため本発明の磁性トナーに好適に用いられる。
上記カーボンブラックとしては具体的には、その体積固有抵抗率が３０Ω・ｃｍ以下のものが望ましく、数平均粒子径、吸油量、ＰＨ等に制限なく使用でき、例えば市販品としては以下のものが挙げられる。すなわち、キャボット社製のＣＳＸ−９９やＶＵＬＣＡＮ ＸＣ−７２Ｒ、ライオン社製のケッチンブラックＥＣ、東海カーボン社製の＃４５００、＃５５００、＃２５５５、＃３８５５、電気化学工業社製のデンカブラック、三菱化学社製の＃２４００Ｂ等が挙げられる。
【００１５】
本発明においてトナー粒子に疎水性シリカ微粉末を添加し付着させる方法としては、攪拌羽根を有する三井三池工業社製のヘンシャルミキサー等の高速攪拌機など一般的に使用されている方法が好適に使用できる。
また、本発明における付着とはトナー粒子表面に疎水性シリカ微粉末がまぶし処理された状態または疎水性シリカ微粉末がトナー粒子表面に固定化された状態をいい、またこの両者の状態を同時に満足した状態をいうものである。
【００１６】
【実施例】
以下、実施例に基づき本発明を説明する。

上記の配合比からなる原料をスーパーミキサーで混合し、加圧ニーダーで１２０℃で熱溶融混練後、ジェットミルで粉砕し、その後乾式気流分級機で分級し、分級品に対し疎水性シリカ微粉末（トリメチルシリル基による疎水化処理品、比表面積１２０ｍ²/ｇ、ＰＨ値８．２）をシリカ付着量（Ｍ）が１．５重量％となるようヘンシェルミキサーで混合して体積平均粒子径が７．４μｍ、個数平均粒子径（Ｎ）が６．１μｍの本発明の磁性トナーを得た。
【００１７】

上記の配合比からなる原料をスーパーミキサーで混合し、加圧ニーダーで１２０℃で熱溶融混練後、ジェットミルで粉砕し、その後乾式気流分級機で分級し、分級品に対し疎水性シリカ微粉末（トリメチルシリル基による疎水化処理品、比表面積１４０ｍ²/ｇ、ＰＨ値７．６）をシリカ付着量（Ｍ）が１．５重量％となるようヘンシェルミキサーで混合して体積平均粒子径が８．１μｍ、個数平均粒子径（Ｎ）が６．８μｍの本発明の磁性トナーを得た。
【００１８】
＜比較例１＞
体積平均粒子径が５．５μｍ、個数平均粒子径（Ｎ）が４．４μｍ及びシリカ付着量（Ｍ）が３．０重量％となるように調製した以外は実施例１と同様にして比較例１の磁性トナーを得た。
＜比較例２＞
体積平均粒子径が９．７μｍ、個数平均粒子径（Ｎ）が７．５μｍ及びシリカ付着量（Ｍ）が１．０重量％となるように調製した以外は実施例１と同様にして比較例２の磁性トナーを得た。
【００１９】
＜比較例３＞
実施例１の溶融混練物をジェットミルで粉砕し、その後乾式気流分級機で分級し、分級品に疎水性シリカ微粉末（トリメチルシリル基による疎水化処理品、比表面積８０ｍ²／ｇ、ＰＨ値８．５）をシリカ付着量（Ｍ）が１．５重量％となるようヘンシェルミキサーで混合して体積平均粒子径が７．８μｍ、個数平均粒子径（Ｎ）が６．２μｍの比較例３の磁性トナーを得た。
＜比較例４＞
実施例１の溶融混練物をジェットミルで粉砕し、その後乾式気流分級機で分級し、分級品に疎水性シリカ微粉末（トリメチルシリル基による疎水化処理品、比表面積１８０ｍ²／ｇ、ＰＨ値７．２）をシリカ付着量（Ｍ）が１．５重量％となるようヘンシェルミキサーで混合して体積平均粒子径が７．７μｍ、個数平均粒子径（Ｎ）が６．２μｍの比較例４の磁性トナーを得た。
【００２０】
＜比較例５＞
実施例１の溶融混練物をジェットミルで粉砕し、その後乾式気流分級機で分級し、分級品に疎水性シリカ微粉末（トリメチルシリル基による疎水化処理品、比表面積１２０ｍ²／ｇ、ＰＨ値８．２）をシリカ付着量（Ｍ）が１．０重量％となるようヘンシェルミキサーで混合して体積平均粒子径が７．５μｍ、個数平均粒子径（Ｎ）が６．０μｍの比較例５の磁性トナーを得た。
＜比較例６＞
実施例１の溶融混練物をジェットミルで粉砕し、その後乾式気流分級機で分級し、分級品に疎水性シリカ微粉末（トリメチルシリル基による疎水化処理品、比表面積１２０ｍ²／ｇ、ＰＨ値８．２）をシリカ付着量（Ｍ）が２．５重量％となるようヘンシェルミキサーで混合して体積平均粒子径が７．６μｍ、個数平均粒子径（Ｎ）が６．２μｍの比較例６の磁性トナーを得た。
【００２１】
実施例１〜２及び比較例１〜６の磁性トナーについて、別紙図１の画像形成装置を用いて１，０００枚までの転写紙への印字テストを行った。画像形成装置の条件は以下の通りである。
トナー担持体印加電圧：＋１００Ｖ
制御電極印加電圧：＋２５０Ｖ（画像信号部）
背面電極印加電圧：＋１８００Ｖ
印字テストの結果と各磁性トナーの物理特性を表１に示す。
なお、表１において、画像濃度はマクベス社製のマクベス反射濃度計ＲＤ−９１４で測定し、画質は転写紙上の画像を目視で次の基準により評価したものである。
○・・・・画質に問題がなく良好である
×・・・・文字周辺に磁性トナーの飛び散りあり
×△・・・スジ状の濃度ムラが発生
××・・・スジ状の画像ヌケが発生
【００２２】
【表１】

【００２３】
表１から明らかなように本発明の磁性トナーは、１，０００枚後の画像濃度及び画質共に実用上何等問題の無い結果であった。これに対し、比較例１、比較例３〜６の磁性トナーは１，０００枚後の画像濃度が低く問題があり、比較例１〜４及び比較例６の磁性トナーは画質が悪く実用上問題となる結果であった。
【発明の効果】
以上説明したように本発明の磁性トナーは、その粒子径及び疎水性シリカ微粉末の付着量を特定の値とすることにより、トナー飛翔方法の画像形成方法において、多数プリント後も画像濃度が低下することなく画質が良好な画像が得られるものである。
【図面の簡単な説明】
【図１】図１は本発明の画像形成方法を用いた装置の概念図である。
【符号の説明】
１ 磁性材
２ トナー担持体
３ 規制部材
４ 磁性トナー
５ 背面電極
６ 記録媒体
７ 制御電極
８ ホッパー
９ 攪拌羽根[0001]
[Industrial application fields]
The present invention relates to an image forming method for forming an image by causing a magnetic toner to fly directly to a recording medium by a direct current electric field, and a magnetic toner used therefor.
[0002]
[Prior art]
Conventionally used electrophotography is a method in which a photoconductive photoconductor is uniformly charged, and an image portion (or non-image portion) is exposed to form a latent image, and the latent image is developed by developing means. Then, it is transferred to a transfer recording medium such as paper and fixed by fixing means to obtain an image.
Therefore, the conventional electrophotographic method has a large number of steps until an image is obtained in this way, and accordingly, the number of parts increases, and there is a problem that the apparatus becomes complicated and large.
In view of the problems of the conventional electrophotographic method, an image forming method represented by Swedish Patent No. 8704883 has been proposed as a method that can solve this problem.
In this image forming method, a direct current electric field is formed between a toner carrier and a back electrode provided opposite to the toner carrier, and a toner is provided between the toner carrier and a recording medium installed along the back electrode. An image forming method (hereinafter, this image forming method is referred to as a toner flying method) in which an electric means (control electrode) for controlling the passage of the toner is provided and the toner on the toner carrier is directly caused to fly according to the image pattern. This toner flying method does not require a photoreceptor in electrophotography, and has the advantage that steps such as charging, transfer, peeling, and photoreceptor cleaning can be omitted.
[0003]
[Problems to be solved by the invention]
Thus, the toner flying method has various advantages. However, a conventional magnetic toner having a volume average particle diameter of about 10 μm and a number average particle diameter of about 8 μm used in a two-component development method using a carrier is used with hydrophobic silica attached to the surface thereof. However, since the amount added is about 0.1 to 0.4% by weight and is less than the range of the present invention, there is a problem that a uniform toner layer cannot be formed on the toner carrier. It was. On the other hand, magnetic toner is also used in a developing method in which toner is caused to fly on the surface of a photoconductor by applying a DC and AC developing voltage. In this method, the higher the fluidity and triboelectric charge of the magnetic toner, the better the developability. Therefore, a hydrophobic silica having a specific surface area exceeding 160 m ² / g is used. Therefore, when used in the toner flying method as in the present invention, there is a problem that highly charged toner or free silica tends to adhere to the control electrode. As described above, it is known that some problems arise when the conventional magnetic toner is used as it is for the toner flying method. A particular problem is that the magnetic toner adheres to the periphery of the control electrode that controls the passage of the magnetic toner, and the flying of the magnetic toner is restricted by the electrostatic charge of the adhered magnetic toner, so that a desired image pattern cannot be obtained. Or the function of the control electrode is lowered and the image density is lowered.
[0004]
[Means for Solving the Problems]
The present invention has been made to solve the problems in the toner flying method. A thin layer of magnetic toner is formed by a regulating member on a toner carrier including a magnetic material, and the toner carrier and the toner carrier are opposed to the toner carrier. And a control electrode for controlling the passage of toner between the toner carrier and a recording medium installed along the back electrode. An image forming method for directly flying the above magnetic toner according to an image pattern, wherein the magnetic toner has a volume average particle diameter of 6 to 9 μm by Coulter counter method, and the number average particle diameter (N image of) a specific surface area that is adhered to the surface of the magnetic toner 100~160m hydrophobic silica fine powder adhering amount is ² / g (M) and is characterized by satisfying the following formula (1) Forming method is to propose.
[Equation 3]

N: Number average particle diameter of magnetic toner (μm)
M: Silica adhesion amount (% by weight)
In addition, a thin layer of magnetic toner is formed on the toner carrier containing the magnetic material by a regulating member, and a direct current electric field is formed between the toner carrier and the back electrode provided facing the toner carrier. A control electrode for controlling the passage of toner is provided between the carrier and the recording medium installed along the back electrode, and used for an image forming method in which the magnetic toner on the toner carrier is directly ejected according to the image pattern. The magnetic toner has a volume average particle diameter of 6 to 9 μm by the Coulter counter method, and is attached to the number average particle diameter (N) of the magnetic toner and the surface of the magnetic toner. The present invention proposes a magnetic toner characterized in that the amount (M) of hydrophobic silica fine powder having a specific surface area of 100 to 160 m ² / g satisfies the following formula (1).
[Expression 4]

N: Number average particle diameter of magnetic toner (μm)
M: Silica adhesion amount (% by weight)
[0005]
Hereinafter, the image forming method of the present invention will be described with reference to FIG.
In FIG. 1, reference numeral 8 denotes a hopper, and 9 denotes a stirring blade. The magnetic toner 4 is stored in the hopper 8 and stirred by the stirring blade 9. The magnetic toner 4 in the hopper 8 is moved by the rotation of the toner carrier 2 made of aluminum or the like having the magnetic material 1 therein.
The magnetic toner 4 on the peripheral surface of the toner carrier 2 is formed with a thin layer by a regulating member 3 such as silicone rubber or urethane rubber, and a direct current is applied between the toner carrier 2 and a back electrode 5 provided facing the toner carrier 2. A control electrode 7 that controls the passage of the magnetic toner 4 is provided between the toner carrier 2 and a recording medium 6 such as transfer paper installed along the back electrode 5 to form a boundary. The magnetic toner 4 on the peripheral surface is directly jumped to the recording medium 6 according to the image pattern to form an image.
In FIG. 1, the magnetic toner 4 is negatively charged and the back electrode 5 and the control electrode 7 are positively charged. However, the magnetic toner 4 may be positively charged and the back electrode 5 and the control electrode 7 may be negatively charged.
[0006]
The image forming method of the present invention is characterized by the above magnetic toner, and the magnetic toner has a volume average particle diameter of 6 to 9 μm by Coulter counter method. If the volume average particle diameter is less than 6 μm, the adhesion force between the toner particles becomes large, so that it becomes difficult to form a uniform toner layer on the toner carrier, and it is easy to be affected by exhaust air in the machine. In-machine contamination and toner adhesion to the control electrode are likely to occur. Also, if the volume average particle diameter exceeds 9 μm, the mass of individual toner particles increases and the inertial force at the time of toner flight increases, so that it tends to scatter when it flies to the recording medium, and good image quality is obtained. Disappear.
[0007]
In the present invention, a magnetic toner having a hydrophobic silica fine powder having a specific surface area of 100 to 160 m ² / g attached to the surface thereof is used.
If the specific surface area of the hydrophobic silica fine powder is less than 100 m ² / g, the fluidity of the magnetic toner is insufficient, and a uniform toner thin layer cannot be formed on the toner carrier.
In addition, the hydrophobic silica fine powder having a large specific surface area tends to increase the triboelectric charge amount of the silica itself or the silica-coated magnetic toner, so that the specific surface area of the hydrophobic silica fine powder exceeds 160 m ² / g. In this case, the triboelectric charge amount of the magnetic toner increases, the adhesion force to the toner carrier increases, and the image density decreases due to the decrease in the development amount. Furthermore, the magnetic toner with the high charge amount is likely to adhere to the electrode portion when passing through the control electrode, and normal signal control cannot be performed with the electrode to which the magnetic toner with the high charge amount is attached, so that an image defect is likely to occur. .
[0008]
In the present invention, the relationship between the adhesion amount (M) of the hydrophobic silica fine powder having the above-mentioned specific surface area adhering to the magnetic toner and the number average particle diameter (N) of the magnetic toner satisfies the above formula (1). The hydrophobic silica fine powder is adhered as follows.
Further, it is more preferable to attach hydrophobic silica fine powder so as to satisfy the following formula (2).
[Equation 5]

When the amount of the hydrophobic silica fine powder adhering is less than the range of the formula (1), the magnetic toner is insufficient in fluidity, resulting in poor toner supply on the toner carrier and insufficient image density during continuous printing. To do. On the contrary, when the amount of the hydrophobic silica fine powder adhering is larger than the range of the formula (1), the free silica existing free from the toner increases, and this free silica selectively adheres to the control electrode. Therefore, image defects are likely to occur.
The silica adhesion amount (% by weight) in the above formula is the total weight of the toner particles not having the hydrophobic silica fine powder adhered to the surface and the hydrophobic silica fine powder, that is, the hydrophobic silica fine weight of the entire magnetic toner. It shows the weight ratio of the powder.
[0009]
In the present invention, when the magnetic toner is negatively charged, the PH value of the hydrophobic silica fine powder adhered to the surface is preferably 7-9. Here, the PH value is a pH value of a solution in which 4% by weight of silica fine powder is dispersed in a water: methanol = 1: 1 solution. When the PH value is less than 7, the charge amount of the silica fine powder is increased to the minus side, and the charge amount of the entire magnetic toner is increased to the minus side, so that an image defect due to toner adhesion to the control electrode is likely to occur. On the other hand, when the PH value exceeds 9 and the charge amount of the negatively chargeable magnetic toner is low, a uniform toner layer cannot be formed on the toner carrier. Examples of such a hydrophobic silica fine powder having a PH value include those treated with a trimethylsilyl group.
In the present invention, the volume average particle diameter means a 50% cumulative volume diameter, the number average particle diameter means a 50% cumulative diameter, and the number average particle diameter is the same as the volume average particle diameter. It is a value measured by the Coulter counter method.
[0010]
The magnetic toner of the present invention preferably has a magnetic powder content of 20 to 40% by weight. If the content of the magnetic powder is less than 20% by weight, it is difficult to obtain a magnetic force sufficient to be transported and supplied on the toner carrier, while if it exceeds 40% by weight, the transportability on the toner carrier is good. In addition, since the magnetic attraction force with the toner carrier is large, it is difficult to obtain a good flying property, and furthermore, since the magnetic toner has a large specific gravity, the inertial force is large when the magnetic toner is flying. Spattering is likely to occur.
[0011]
The magnetic toner of the present invention comprises at least a binder resin, magnetic powder, and a charge control agent, and these are kneaded by a thermal kneader such as a hot roll, a kneader, and an extruder, and then obtained by mechanical pulverization and classification. it can. It can also be obtained by a polymerization method such as a suspension polymerization method or an emulsion polymerization method.
Examples of the binder resin used in the magnetic toner of the present invention include polystyrene, polyethylene, polypropylene, vinyl resins, polyacrylates, polymethacrylates, styrene-acrylate copolymers, polyvinylidene chloride, polyacrylonitrile, polyethers. Polycarbonate, polyester resin, epoxy resin, cellulose resin and the like are preferably used.
[0012]
As the magnetic powder used in the magnetic toner of the present invention, spinel, perovskite, hexagonal crystal, garnet, ferrite having an orthoferrite structure, magnetite, or the like is applied crystallographically. The structure of the ferrite is a sintered body of an oxide such as nickel, zinc, manganese, magnesium, copper, lithium, barium, vanadium, chromium, calcium and a trivalent iron oxide. As the shape of the magnetic powder, a spherical type, a hexahedral type, and an octahedral type are preferably used.
[0013]
As the charge control agent used in the magnetic toner of the present invention, a nigrosine dye or a quaternary ammonium salt can be used for the positive polarity, and a metal-containing monoazo dye can be used for the negative polarity.
[0014]
The magnetic toner of the present invention preferably contains carbon black. As the carbon black used, those known for toners can be used. However, a so-called conductive grade carbon black having a large BET specific surface area and an oil absorption amount can provide an effective toner chargeability adjusting function with a small amount of addition. Therefore, it is preferably used for the magnetic toner of the present invention.
Specifically, the carbon black preferably has a volume resistivity of 30 Ω · cm or less, and can be used without limitation on the number average particle diameter, oil absorption, PH, etc. For example, the following are commercially available products: Can be mentioned. That is, CSX-99 and VULCAN XC-72R manufactured by Cabot, Ketchin Black EC manufactured by Lion, # 4500, # 5500, # 2555, # 3855 manufactured by Tokai Carbon, Denka Black manufactured by Denki Kagaku Kogyo, Mitsubishi Examples thereof include # 2400B manufactured by Kagakusha.
[0015]
In the present invention, as a method for adding and adhering the hydrophobic silica fine powder to the toner particles, a generally used method such as a high-speed stirrer such as a Hensial mixer manufactured by Mitsui Miike Kogyo Co., Ltd. having a stirring blade is preferably used. it can.
Further, the adhesion in the present invention refers to a state where the hydrophobic silica fine powder is applied to the toner particle surface or a state where the hydrophobic silica fine powder is fixed to the toner particle surface, and both of these conditions are satisfied simultaneously. It means the state that was done.
[0016]
【Example】
Hereinafter, the present invention will be described based on examples.

The raw materials having the above blending ratio are mixed with a super mixer, heated and kneaded at 120 ° C. with a pressure kneader, pulverized with a jet mill, and then classified with a dry air classifier. (A hydrophobized product with a trimethylsilyl group, a specific surface area of 120 m ² / g, a PH value of 8.2) was mixed with a Henschel mixer so that the silica adhesion amount (M) was 1.5% by weight, and the volume average particle size was 7 A magnetic toner of the present invention having a diameter of 0.4 μm and a number average particle diameter (N) of 6.1 μm was obtained.
[0017]

The raw materials having the above blending ratio are mixed with a super mixer, heated and kneaded at 120 ° C. with a pressure kneader, pulverized with a jet mill, and then classified with a dry air classifier. (A hydrophobized product with a trimethylsilyl group, a specific surface area of 140 m ² / g, a PH value of 7.6) was mixed with a Henschel mixer so that the silica adhesion amount (M) was 1.5% by weight, and the volume average particle size was 8 Thus, the magnetic toner of the present invention having a number average particle diameter (N) of 6.8 μm was obtained.
[0018]
<Comparative Example 1>
Comparative Example as in Example 1 except that the volume average particle size was 5.5 μm, the number average particle size (N) was 4.4 μm, and the silica adhesion amount (M) was 3.0 wt%. 1 magnetic toner was obtained.
<Comparative example 2>
Comparative Example as in Example 1 except that the volume average particle size was 9.7 μm, the number average particle size (N) was 7.5 μm, and the silica adhesion amount (M) was 1.0 wt%. 2 magnetic toner was obtained.
[0019]
<Comparative Example 3>
The melt-kneaded product of Example 1 was pulverized with a jet mill, and then classified with a dry air classifier. The classified product was hydrophobic silica fine powder (hydrophobized product with trimethylsilyl group, specific surface area 80 m ² / g, PH value 8 5) was mixed with a Henschel mixer so that the silica adhesion amount (M) was 1.5% by weight, and the volume average particle diameter was 7.8 μm and the number average particle diameter (N) was 6.2 μm. A magnetic toner was obtained.
<Comparative example 4>
The melt-kneaded product of Example 1 was pulverized with a jet mill, and then classified with a dry air classifier. The classified product was hydrophobic silica fine powder (hydrophobized product with trimethylsilyl group, specific surface area 180 m ² / g, PH value 7 .2) is mixed with a Henschel mixer so that the silica adhesion amount (M) is 1.5% by weight, and the volume average particle size is 7.7 μm and the number average particle size (N) is 6.2 μm. A magnetic toner was obtained.
[0020]
<Comparative Example 5>
The melt-kneaded product of Example 1 was pulverized with a jet mill, and then classified with a dry air classifier. The classified product was hydrophobic silica fine powder (hydrophobized product with trimethylsilyl group, specific surface area 120 m ² / g, PH value 8 .2) was mixed with a Henschel mixer so that the silica adhesion amount (M) was 1.0% by weight, and the volume average particle diameter was 7.5 μm and the number average particle diameter (N) was 6.0 μm. A magnetic toner was obtained.
<Comparative Example 6>
The melt-kneaded product of Example 1 was pulverized with a jet mill, and then classified with a dry air classifier. The classified product was hydrophobic silica fine powder (hydrophobized with trimethylsilyl group, specific surface area 120 m ² / g, PH value 8 .2) was mixed with a Henschel mixer so that the silica adhesion amount (M) was 2.5% by weight, and the volume average particle diameter was 7.6 μm and the number average particle diameter (N) was 6.2 μm. A magnetic toner was obtained.
[0021]
The magnetic toners of Examples 1 and 2 and Comparative Examples 1 to 6 were subjected to a print test on up to 1,000 transfer sheets using the image forming apparatus shown in FIG. The conditions of the image forming apparatus are as follows.
Toner carrier applied voltage: + 100V
Control electrode applied voltage: + 250V (image signal part)
Back electrode applied voltage: + 1800V
Table 1 shows the results of the printing test and the physical characteristics of each magnetic toner.
In Table 1, the image density was measured with a Macbeth reflection densitometer RD-914 manufactured by Macbeth, and the image quality was evaluated by visually observing the image on the transfer paper according to the following criteria.
○ ···· No problem in image quality and good × · · · Magnetic toner scattering around characters × △ · · · · · · · · · · · · · · · · · · · · · · · · · [0022]
[Table 1]

[0023]
As can be seen from Table 1, the magnetic toner of the present invention had no practical problems with respect to image density and image quality after 1,000 sheets. On the other hand, the magnetic toners of Comparative Examples 1 and 3 to 6 have a problem that the image density after 1,000 sheets is low, and the magnetic toners of Comparative Examples 1 to 4 and Comparative Example 6 have poor image quality and have practical problems. The result was.
【The invention's effect】
As described above, the magnetic toner according to the present invention reduces the image density even after a large number of prints in the image formation method of the toner flying method by setting the particle size and the amount of hydrophobic silica fine powder adhering to specific values. Thus, an image with good image quality can be obtained.
[Brief description of the drawings]
FIG. 1 is a conceptual diagram of an apparatus using an image forming method of the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Magnetic material 2 Toner carrier 3 Restriction member 4 Magnetic toner 5 Back electrode 6 Recording medium 7 Control electrode 8 Hopper 9 Stirring blade

Claims (4)

A thin layer of magnetic toner is formed by a restricting member on a toner carrier that contains a magnetic material, and a direct current electric field is formed between the toner carrier and a back electrode provided opposite to the toner carrier. An image forming method in which a control electrode for controlling the passage of toner is provided between the recording medium and a recording medium installed along the back electrode, and the magnetic toner on the toner carrier is directly ejected according to the image pattern, The volume average particle diameter of the magnetic toner by Coulter counter method is 6 to 9 μm, and the number average particle diameter (N) of the magnetic toner and the specific surface area attached to the surface of the magnetic toner are 100 to 160 m ² / An image forming method, wherein the adhesion amount (M) of the hydrophobic silica fine powder as g satisfies the following formula (1).

N: Number average particle diameter of magnetic toner (μm)
M: Silica adhesion amount (% by weight) The image forming method according to claim 2, wherein the hydrophobic silica fine powder has a PH value of 7 to 9. 4. The image forming method according to claim 3, wherein the surface of the hydrophobic silica fine powder is hydrophobized with a trimethylsilyl group. A thin layer of magnetic toner is formed by a restricting member on a toner carrier that contains a magnetic material, and a direct current electric field is formed between the toner carrier and a back electrode provided opposite to the toner carrier. Is used in an image forming method in which a control electrode for controlling the passage of toner is provided between the toner and a recording medium installed along the back electrode, and the magnetic toner on the toner carrier is directly ejected according to the image pattern. A magnetic toner having a volume average particle diameter of 6 to 9 μm according to the Coulter counter method of the magnetic toner, and a number average particle diameter (N) of the magnetic toner and a specific surface area attached to the surface of the magnetic toner. A magnetic toner characterized in that the adhering amount (M) of hydrophobic silica fine powder having a particle size of 100 to 160 m ² / g satisfies the following formula (1).

N: Number average particle diameter of magnetic toner (μm)
M: Silica adhesion amount (% by weight)

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