Abstract:
Charge control agent whose active ingredient is a zinc compound of an alkylsalicylic acid derivative, the compound being a zincified alkylsalicylic acid derivative which is an alkylphenol derivative having a carboxyl group introduced thereto; Toner for developing electrostatic images, comprising said charge control agent, a coloring agent and a binder resin for toner; Charge control resin particles comprising solidified milled particles of a thermally kneaded mixture of at least said charge control agent and a resin.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a charge control agent capable of controlling the charge amount of a toner for developing electrostatic latent images in electrophotography, electrostatic recording, electrostatic printing and others, a manufacturing process thereof, charge control resin particles containing said charge control agent and a toner for developing electrostatic images. 
     2. Description of the Prior Art 
     In copying machines and other equipment based on electrophotography, various dry or wet toners containing a coloring agent, a fixing resin and other substances are used to visualize the electrostatic latent image formed on the photoreceptor having a photosensitive layer containing an inorganic or organic photoconductive substance. The chargeability of such toners is the most important factor in electrostatic latent image developing systems. Thus, to appropriately control the charge amount of toner, a charge control agent providing a positive or negative charge is often added to the toner. 
     Charge control agents providing a positive charge for toner in actual application include the nigrosine dyes disclosed in Japanese Patent Examined Publication No. SHO41-2427 and elsewhere. Charge control agents providing a negative charge for toner in actual application include the metal complex dyes disclosed in Japanese Patent Examined Publication Nos. SHO41-20153, SHO43-17955, SHO45-26478 and elsewhere. However, most of the charge control agents described above are structurally complex and unstable; for example, they are likely to be decomposed or deteriorated to lose their initial charge control performance when exposed to mechanical friction or impact, temperature or humidity changes, electric impact, light irradiation, etc. Also, many of such conventional charge control agents are colored so that they are unsuitable for use in full-color toners. 
     As a means for resolving these problems, Japanese Patent Laid-Open No. SHO62-145255, for example, discloses the containment of a metal salt of salicylic acid or a derivative thereof as a charge control agent. Japanese Patent Laid-Open No. SHO63-163374 proposes methods for determining the crystalline structures and crystalline diameters of metal salts of salicylic acid and derivatives thereof to be contained in the toner, with particular mention of zinc 3,5-di-tert-butylsalicylate. Although these substances are advantageous in that they can be used in color toners, their heat stability, uniform dispersibility in resin, or charge control performance, or charging rise speed, in particular, is insufficient so that they remain to be investigated further. 
     The present invention was developed in view of the aforementioned problems in the prior art and is intended to provide a charge control agent which possesses good heat resistance, good affinity for resin and good dispersibility in resin, which ensures a high speed of charging rise, which is excellent in charge control stability to temperature and humidity changes (environmental resistance) and charge control characteristic stability over time (storage stability), and which can be used in various chromatic or achromatic toners, a manufacturing process thereof, a toner for developing electrostatic images which exhibits a high speed of charging rise, which is excellent in charge stability to temperature and humidity changes (environmental resistance) and charge characteristic stability over time (storage stability), and which can be used as various chromatic or achromatic toners, and charge control resin particles used as a starting material for the production of said toner. 
     SUMMARY OF THE INVENTION 
     [A] Resolving the above problems, the charge control agent whose active ingredient is a zinc compound of an alkylsalicylic acid derivative, the compound being a zincified alkylsalicylic acid derivative (i.e. an alkylsalicylic acid derivative provided with zinc) which is an alkylphenol derivative having a carboxyl group introduced thereto. 
     Because this charge control agent possesses good heat resistance, good affinity for resin and good dispersibility in resin and ensures a high speed of charging rise, it is highly effective in increasing the speed in electrophotographic and other processes and improving the quality of initial images thereof, is excellent in environmental resistance and storage stability, and can be used in various chromatic or achromatic toners. 
     The aforementioned introduction of a carboxyl group to an alkylphenol derivative can be achieved by the Kolbe-Schmitt reaction. 
     The charge control agent of the present invention described above is preferably a charge control agent whose active ingredient is a zinc compound of 3,5-di-tert-butylsalicylic acid, the compound being a zincified 3,5-di-tert-butylsalicylic acid which is 2,4-di-tert-butylphenol having a carboxyl group introduced thereto by the Kolbe-Schmitt reaction. 
     The charge control agent of the present invention described above is preferably a charge control agent wherein the aforementioned zinc compound of an alkylsalicylic acid derivative is a zinc 3,5-di-tert-butylsalicylate represented by General Formula (1) below. 
                                
in Formula (1), t-Bu represents a tert-butyl group.
 
     As an example of the charge control agent of the present invention, there may be mentioned a charge control agent whose active ingredient is a crystalline zinc 3,5-di-tert-butylsalicylate represented by General Formula (1) above, and having major peaks of X-ray diffraction using the CuKα-characteristic X-ray (wavelength 1.541 Å) at Bragg angles 2θ of at least 5.7±0.2°, 6.4±0.2°, 6.7±0.2° and 15.4±0.2°. 
     [B] The aforementioned charge control agent of the present invention is preferably a charge control agent wherein the aforementioned zinc compound of an alkylsalicylic acid derivative is obtained by adding a solution and/or dispersion liquid of the aforementioned alkylsalicylic acid derivative to a solution containing a zinc provider to cause a reaction between the zinc provider and the alkylsalicylic acid derivative. 
     The solution and/or dispersion liquid of the alkylsalicylic acid derivative used in this case is preferably a solution of 3,5-di-tert-butylsalicylic acid in alkali. 
     The aforementioned zinc compound of an alkylsalicylic acid derivative is preferably a zinc 3,5-di-tert-butylsalicylate represented by General Formula (1) above. 
     As a preferable example of the charge control agent of the present invention, there may be mentioned a charge control agent whose active ingredient is a crystalline zinc 3,5-di-tert-butylsalicylate represented by General Formula (1) above, and having major peaks of X-ray diffraction using the CuKα-characteristic X-ray (wavelength 1.541 Å) at Bragg angles 2θ of at least 7.7±0.2° and 15.7±0.2°. 
     This zinc 3,5-di-tert-butylsalicylate preferably has major peaks of X-ray diffraction using the CuKα-characteristic X-ray (wavelength 1.541 Å) at Bragg angles 2θ of at least 5.2±0.2°, 6.7±0.2°, 7.7±0.2° and 15.7±0.2°. 
     It is desirable that the bulk density of zinc 3,5-di-tert-butylsalicylate in these charge control agents be 2 to 5 ml/g. Because the zinc 3,5-di-tert-butylsalicylate particles which constitute the active ingredient of the charge control agents have an appropriately low bulk density, they can be transported with an increased mass per unit volume so that the transportation cost can be reduced significantly, and when used in a toner, they improve the dispersibility in resins such as resins for toner, making it easy to uniformly disperse in the resin, increasing the speed of toner charging rise, and improving the environmental resistance and storage stability of the toner. 
     [C] The toner of the present invention for developing electrostatic images comprises at least any one of the charge control agent in [A] or [B] above, a coloring agent and a resin. 
     Because the toner of the present invention for developing electrostatic images is a toner wherein the charge control agent contained therein possesses good heat resistance, good affinity for resin and good dispersibility in resin and ensures a high speed of charging rise, it is highly effective in increasing the speed in electrophotographic and other processes and improving the quality of initial images thereof, is excellent in charge stability to temperature and humidity changes (environmental resistance) and charge characteristic stability over time (storage stability), and can be used in various chromatic or achromatic toners. 
     [D] The charge control resin particles of the present invention comprise solidified milled particles of a thermally kneaded mixture of at least any charge control agent described above and a resin. 
     The mixing ratio by weight of the charge control agent and resin may, for example, be 1:9 to 9:1. 
     The toner of the present invention for developing electrostatic images may also comprise the aforementioned charge control resin particles, a coloring agent and a binder resin for toner. 
     When preparing a toner for developing electrostatic images using the charge control resin particles of the present invention, a coloring agent and a binder resin for toner, the crystalline zinc 3,5-di-tert-butylsalicylate contained in the charge control resin particles is highly uniformly dispersed in the resin so that its charge control performance is improved. In particular, reverse charging in the initial stage of the frictional charging of the toner and charge variation over time can be prevented. 
     [E] The charge control agent of the present invention is preferably a charge control agent whose active ingredient is a zinc compound of an alkylsalicylic acid derivative which compound is prepared by zincifying a compound obtained via a step of introducing a carboxyl group to an alkylphenol derivative. This charge control agent whose active ingredient is a zinc compound of an alkylsalicylic acid derivative can be produced by zincifying a compound obtained via a step of introducing a carboxyl group to an alkylphenol derivative. 
     This charge control agent is of low impurity content and high purity. For this reason, it possesses high heat resistance and high charge providing performance, is small in the variation of the amount of providing charge, is excellent in environmental resistance and storage stability, and can be used in various chromatic or achromatic toners. In addition, because it is highly chargeable and ensures a high speed of charging rise, it is highly effective in increasing the speed in electrophotographic and other processes and improving the quality of initial images thereof. 
     The toner of the present invention for developing electrostatic images preferably comprises at least this charge control agent, a coloring agent and a resin. 
     This toner for developing electrostatic images is highly chargeable, is small in the variation of the amount of charge, is excellent in environmental resistance and storage stability, and can be used as various chromatic or achromatic toners. In addition, because it is highly chargeable and exhibits a high speed of charging rise, it is highly effective in increasing the speed in electrophotographic and other processes and improving the quality of initial images thereof. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows an X-ray diffraction chart of the charge control agent of Example 1. 
         FIG. 2  shows a liquid chromatography analysis chart of the charge control agent of Example 1. 
         FIG. 3  shows an X-ray diffraction chart of the charge control agent of Example 2. 
         FIG. 4  shows an X-ray diffraction chart of the charge control agent of Comparative Example 1. 
         FIG. 5  shows a liquid chromatography analysis chart of the charge control agent of Comparative Example 1. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Charge Control Agent 
     As manufacturing processes of an alkylsalicylic acid derivative, there may be mentioned a process wherein an alkylsalicylic acid derivative is synthesized by alkylating a salicylic acid derivative as described in (a) below, and a process wherein an alkylsalicylic acid derivative is synthesized by introducing a carboxyl group to an alkylphenol derivative as described in (b) below (Kolbe-Schmitt reaction). 
     (a) Alkylation Reaction of Salicylic Acid 
     For example, an alkylsalicylic acid is prepared by introducing an alkyl group to salicylic acid using an alkyl halide in the presence of a Lewis acid such as AlCl 3 . 
     
       
                 
         
             
             
         
      
     
     (b) Kolbe-Schmitt Reaction of Alkylphenol 
     For example, an alkylphenol sodium salt is prepared by mixing an alkylphenol with an aqueous solution of sodium hydroxide, heating the mixture, and removing water under reduced pressure. This alkylphenol sodium salt is reacted with carbon dioxide with heating to yield an alkylsalicylic acid sodium salt, which is treated in an aqueous solution of hydrochloric acid or sulfuric acid to precipitate salicylic acid. 
     
       
                 
         
             
             
         
      
     
     The alkylsalicylic acid derivatives (e.g., 3,5-di-tert-butylsalicylic acid) produced by the two methods described above were reacted with a zinc provider to synthesize their zinc compounds, which were analyzed comparatively. As a result, it was shown that impurities contained in the alkylsalicylic acid derivatives synthesized by alkylating salicylic acid derivatives are present in small amounts in the finally synthesized zinc compounds of salicylic acid derivatives and adversely affect the charge-providing performance of the charge control agent. 
     The alkylsalicylic acid derivative in the charge control agent of the present invention is preferably a salicylic acid derivative of a benzene nucleus or naphthalene nucleus having an alkyl group (e.g., carbon number 1 to 18) as a substituent. The 3,5-di-tert-butylsalicylic acid produced from the starting material 2,4-di-tert-butylphenol by the Kolbe-Schmitt reaction, in particular, is preferred. 
     The zinc compound of an alkylsalicylic acid as the active ingredient in the charge control agent of the present invention is preferably a zinc 3,5-di-tert-butylsalicylate represented by General Formula (1) below. 
                                
in this formula, t-Bu represents a tert-butyl group.
 
     Although a zinc compound of an alkylsalicylic acid as the active ingredient in the charge control agent of the present invention can be produced by a reaction using any of an aqueous or organic solvent system, an aqueous reaction is preferred from the viewpoint of cost and safety. 
     In the case of an aqueous reaction, a zinc compound of an alkylsalicylic acid can be obtained using steps [1] to [4] below.
     [1] Step for dissolving a salicylic acid derivative in an alkaline aqueous solution.   [2] Step for dissolving a zinc provider in water.   [3] Reaction step wherein the aqueous solution of a zinc provider obtained in step [2] is added to the aqueous solution of a salicylic acid derivative obtained in step [1], while heating the latter, and the mixture is stirred with heating until the reaction is completed.   [4] Post-treatment step for filtering the reaction mixture obtained in step [3] and washing, drying and then milling the cake filtered off.   

     When producing a zinc 3,5-di-tert-butylsalicylate by this method, it is possible to obtain a crystalline zinc 3,5-di-tert-butylsalicylate having major peaks of X-ray diffraction using the CuKα-characteristic X-ray at Bragg angles 2θ of at least 5.7±0.2°, 6.4±0.2°, 6.7±0.2° and 15.4±0.2°. 
     On the other hand, using processes [1], [2], [3]′ and [4] for the production of a zinc compound of a salicylic acid derivative, wherein step [3] above is replaced with reaction step [3]′ wherein the aqueous solution of a salicylic acid derivative obtained in step [1] is added to the aqueous solution of a zinc provider obtained in step [2], while heating the latter, and the mixture is stirred with heating until the reaction is completed, a zinc compound of a salicylic acid derivative can be obtained at high purity and high yield in a short time. The crystalline zinc compound thus obtained was found to exhibit good charge control performance and to be more suitable as a charge control agent. 
     Referring to 3,5-di-tert-butylsalicylic acid as an example salicylic acid derivative, it is possible to selectively synthesize zinc 3,5-di-tert-butylsalicylate by adding drop by drop a solution containing a zinc provider to a solution of 3,5-di-tert-butylsalicylic acid in alkali to cause the reaction. For example, a sufficient amount of an alkaline aqueous solution is added to 2 mol of 3,5-di-tert-butylsalicylic acid (produced from the starting material 2,4-di-tert-butylphenol by the Kolbe-Schmitt reaction) and dissolved with heating. Separately, an aqueous solution incorporating 1 mol of a zinc provider is prepared. While heating this aqueous solution, the aforementioned aqueous solution of 3,5-di-tert-butylsalicylic acid is added drop by drop, and the reaction is carried out with heating and pH adjustment; after completion of the reaction, the reaction mixture is filtered and the cake filtered off is washed with water and dried. Examples of zinc providers include, but are not limited to, zinc sulfate, zinc chloride and zinc acetate. 
     When producing a zinc 3,5-di-tert-butylsalicylate by this method, it is possible to obtain a crystalline zinc 3,5-di-tert-butylsalicylate having major peaks of X-ray diffraction using the CuKα-characteristic X-ray at Bragg angles 2θ of at least 5.2±0.2°, 6.7±0.2°, 7.7±0.2° and 15.7±0.2°. 
     Although the particle diameter of zinc 3,5-di-tert-butylsalicylate in the present invention is not subject to limitation, it is desirable that the average particle diameter be not more than 20 μm, preferably not more than 10 μm, from the viewpoint of the improvement of its dispersibility in resin. 
     The zinc compound of alkylsalicylic acid derivative (especially zinc 3,5-di-tert-butylsalicylate) obtained by steps [1], [2], [3]′ and [4] can be obtained as appropriately low bulk density particles of the zinc compound of alkylsalicylic acid derivative (e.g., 2 to 5 ml/g). For this reason, the dispersibility of the particles in resins such as resins for toner is improved, the particles is easy to uniformly disperse in the resin, and the particles can be transported with an increased mass per unit volume so that the transportation cost can be reduced significantly. 
     Charge Control Resin Particles 
     The charge control resin particles of the present invention can, for example, be obtained by mixing the charge control agent of the present invention and a resin (e.g., binder resin) at a given ratio, heating and kneading the mixture, and cooling, solidifying and milling the resin composition thus obtained. 
     The mixing ratio by weight of a charge control agent (e.g., the aforementioned crystalline zinc 3,5-di-tert-butylsalicylate) and a resin (e.g., binder resin) in the charge control resin particles of the present invention may preferably be 1:9 to 9:1, more preferably 3:7 to 7:3. 
     The charge control agent of the present invention surpasses conventional salicylic acid metal salts in dispersibility in resin. When preparing the charge control resin particles of the present invention by mixing the charge control agent of the present invention and a resin, and preparing a toner for developing electrostatic images by mixing these charge control resin particles with a coloring agent and a binder resin, the charge control agent of the present invention is more uniformly dispersed in the resin so that its charge control performance is improved. In particular, reverse charging in the initial stage of the frictional charging of the toner and charge variation over time can be prevented. 
     Examples of resins which can be used in the charge control resin particles of the present invention include the commonly known binder resins for toner described below. A resin preferably used in charge control-resin particles is a resin having a number-average molecular weight (Mn) of 2500 to 30000, more preferably a resin having a weight-average molecular weight (Mw)/number-average molecular weight (Mn) ratio in the range from 2 to 20. The resin in charge control resin particles and the resin used to prepare a toner using the same charge control resin particles may be identical or not. 
     The charge control resin particles of the present invention are, for example, produced as described below. Specifically, charge control resin particles can be obtained by thoroughly mixing at least a charge control agent and a resin in a Henschel mixer or another mechanical mixer, and subsequently kneading the mixture in a molten state under normal or increased pressure in a hot kneader such as a heat roll, a kneader or an extruder, cooling, solidifying and then milling the mixture. 
     The charge control agent used in the production of the charge control resin particles of the present invention may, for example, be in the form of a dry powder or a pre-drying aqueous pressed cake. When using a charge control agent in a dry powder form, a dispersion aid or an additive may be added to improve its dispersibility in resin. For example, water or any organic solvent may be used. Specifically, after the charge control agent of the present invention, a resin and water or any organic solvent are mixed in a Henschel mixer or another mechanical mixer, the mixture is charged and kneaded in a heat roll, a flasher, a kneader, an extruder, or the like. Alternatively, when using a flasher, a kneader, or the like, it is possible to charge and mix the charge control agent, resin and water or any organic solvent in a single step at one time. Subsequently, the mixture is kneaded in a molten state with heating under increased or normal pressure. The remaining water or solvent is then evaporated off under normal or reduced pressure to dry the mixture. The mixture is cooled, solidified and then milled to yield charge control resin particles. Although the aforementioned organic solvent may be a commonly known organic solvent, it is preferable to use a low-boiling highly volatile solvent such as ethanol, methanol, isopropanol or acetone. 
     Toner for Developing Electrostatic Images 
     The toner of the present invention for developing electrostatic latent images comprises the charge control agent of the present invention or the charge control resin particles of the present invention, a binder resin for toner, and a coloring agent. 
     The amount of charge control agent incorporated in the toner of the present invention for developing electrostatic images may be 0.1 to 10 parts by weight, preferably 0.5 to 5 parts by weight per 100 parts by weight of the binder resin for toner (or the sum of the binder resin for toner and the resin in the charge control resin particles). 
     Examples of resins which can be used in the toner of the present invention include resins that have traditionally been used as binder resins for toners. Specifically, there may be mentioned synthetic resins such as polystyrene resin, styrene-acrylic resin, styrene-butadiene resin, styrene-maleic resin, styrene-vinyl methyl ether resin, styrene-methacrylate copolymer, polyester resin, phenol resin and epoxy resin. These resins may be used singly or in blends of several kinds. Of these resins, those having a glass transition point of 50 to 75, a softening point of 80 to 150 and a number-average molecular weight of 1000 to 30000 are preferred, with greater preference given to those wherein the weight-average molecular weight/number-average molecular weight ratio is 2 to 50. 
     For preferable use of a binder resin for toner and a resin in charge control resin particles in a toner used for full-color imaging by subtractive color mixture or for OHP (overhead projectors) etc., the resin or binder resin is required to have special properties, for example, it should be transparent, substantially colorless (no tone damage occurs in the toner image), compatible with the charge control agent used, fluid under appropriate heat or pressure, and pulverizable. Examples of such resins for preferable use include polystyrene resin, acrylic resin, styrene-acrylic resin, styrene-methacrylate copolymer and polyester resin. A polyester resin or styrene-acrylic resin having an acid value of 1 to 50 mg KOH/g, in particular, is preferred. 
     The toner of the present invention may incorporate various known dyes and pigments as coloring agents. Examples of such dyes or pigments which can be used in color toners include carbon black, organic pigments such as quinophthalone, Hansa Yellow, Rhodamine 6G Lake, quinacridone, Rose Bengale, copper Phthalocyanine Blue and copper Phthalocyanine Green, various oil-soluble dyes or disperse dyes such as azo dyes, quinophthalone dyes, anthraquinone dyes, xanthene dyes, triphenylmethane dyes and phthalocyanine dyes, and dyes and pigments modified with higher fatty acids, synthetic resins, or the like. 
     The toner of the present invention for developing electrostatic images may incorporate the aforementioned coloring agents singly or in combination. Dyes and pigments having good spectral characteristics can be preferably used to prepare toners of the three primaries for full-color imaging. Chromatic monocolor toners may incorporate an appropriate combination of a pigment and dye of the same color tone, such as a rhodamine pigment and dye, a quinophthalone pigment and dye, or a phthalocyanine pigment and dye, as coloring agents. 
     Also, to improve toner quality, additives, e.g., anti-offset agents, fluidity-improving agents (e.g., various metal oxides such as silica, aluminum oxide and titanium oxides, magnesium fluoride, etc.) and cleaning aids (e.g., metal soaps of stearic acid etc.; various synthetic resin microparticles such as fluorine-series synthetic resin microparticles, silicone-series synthetic resin microparticles and styrene-(meth)acrylic synthetic resin microparticles), can be added internally or externally. 
     Anti-offset agents (releasing agents) used to improve toner fixability as described above include various waxes, particularly those having average molecular weights of 500 to 15000. Specifically, there can be used polyolefin type waxes such as low molecular polypropylene, polyethylene, oxidized polypropylene and oxidized polyethylene; and natural waxes such as carnauba wax, rice wax and montan wax. 
     The toner of the present invention for developing electrostatic images is, for example, produced as described below. A toner having an average particle size of 5 to 20 μm is obtained by thoroughly mixing a binder resin for toner and coloring agent a charge control agent or charge control resin particles as described above, and, if necessary, a magnetic material, a fluidizing agent and other additives, using a ball mill or another mechanical mixer, subsequently kneading the mixture in a molten state using a hot kneader such as a heat roll, a kneader or an extruder, cooling, solidifying and then pulverizing the mixture, and classifying the particles. 
     Other usable methods include the method in which the starting materials are dispersed in a binder resin solution and then spray dried, and the polymerizing toner production method in which a given set of starting materials are mixed in a monomer for binder resin to yield an emulsified suspension which is then polymerized to yield the desired toner (e.g., the method described in Japanese Patent Laid-Open No. HEI 1-260461 and Japanese Patent Laid-Open No. HE12-32365). When using the toner of the present invention as a two-component developer, development can be achieved by the two-component magnetic brush developing process or another process using the toner in mixture with a carrier powder. 
     Any known carrier can be used. Examples of the carrier include iron powder, nickel powder, ferrite powder and glass beads about 50 to 200 μm in particle diameter, and such materials as coated with acrylate copolymer, styrene-acrylate copolymer, silicone resin, polyamide resin, ethylene fluoride resin or the like. 
     When using the toner of the present invention as a one-component developer, a fine powder of a ferromagnetic material such as iron powder, nickel powder or ferrite powder may be added and dispersed in preparing the toner as described above. Examples of developing processes which can be used in this case include contact development and jumping development. 
     EXAMPLES 
     The present invention is hereinafter described in more detail by means of the following examples, but these are not to be construed as limitative on the present invention. In the description below, “part(s) by weight” are referred to as “part(s)” for short. 
     Examples 1 and 2 and Comparative Example 1 pertain to the production of a charge control agent. 
     Example 1 
     25.0 g of 3,5-di-tert-butylsalicylic acid (0.10 mol obtained from the starting material 2,4-di-tert-butylphenol by the Kolbe-Schmitt reaction) was dissolved in 200 ml of a 2% aqueous solution of NaOH and heated to about 70. Separately, 14.4 g (0.05 mol) of zinc sulfate heptahydrate was dissolved in 200 ml of water. This aqueous solution of zinc sulfate was added drop by drop to the aforementioned aqueous solution of 3,5-di-tert-butylsalicylic acid over a period of about 30 minutes. Subsequently, after the reaction was carried out at 70 to 80 for 2 hours, the reaction mixture was adjusted to a pH of 7.0±0.5 to complete the reaction. 
     This reaction solution was filtered while it remained hot, and the cake filtered off was washed with water and dried to yield 27.8 g of a white fine powder. The bulk density of this white powder was 5.2 ml/g. Analysis of the thus-obtained white powder using a powder X-ray diffraction apparatus detected major peaks of X-ray diffraction using the CuKα-characteristic X-ray [wavelength 1.541 Å] at Bragg angles 2θ of 5.7±0.2°, 6.4±0.2°, 6.7±0.2° and 15.4±0.2°. An X-ray diffraction chart is shown in  FIG. 1  and a liquid chromatography analysis chart in  FIG. 2 . 
     In the present and following Examples and Comparative Examples, X-ray diffraction analysis of crystalline zinc 3,5-di-tert-butylsalicylate was conducted using the apparatus shown below on the undermentioned condition. Instrumentation: MXP-18 X-ray diffraction apparatus (manufactured by MAC SCIENCE K.K.)
     Target: Cu   Wavelength: 1.5405 Å (CuKα1)   Voltage and current: 40.0 kV, 200 mA   Divergence slit: 1.0°   Receiving slit: 0.30 mm   Scattering slit: 1.0°   Scanning speed: 4.0 deg/min   

     Example 2 
     14.4 g (0.05 mol) of zinc sulfate heptahydrate was dissolved in 200 ml of water. Separately, 25.0 g of 3,5-di-tert-butylsalicylic acid (0.10 mol obtained from the starting material 2,4-di-tert-butylphenol by the Kolbe-Schmitt reaction) was dissolved in 200 ml of a 2% aqueous solution of NaOH and heated to about 70. This aqueous solution of 3,5-di-tert-butylsalicylic acid was added drop by drop to the aforementioned aqueous solution of zinc sulfate over a period of about 30 minutes. Subsequently, after the reaction was carried out at 70 to 80 for 2 hours, the reaction mixture was adjusted to a pH of 7.0±0.5 to complete the reaction. 
     This reaction solution was filtered while it remained hot, and the cake filtered off was washed with water and dried to yield 27.9 g of a white fine powder. The bulk density of this white powder was 3.2 ml/g. Analysis of the thus-obtained white powder using a powder X-ray diffraction apparatus detected major peaks of X-ray diffraction using the CuKα-characteristic X-ray [wavelength 1.541 Å] at Bragg angles 2θ of 5.2±0.2°, 6.7±0.2°, 7.7±0.2° and 15.7±0.2°. An X-ray diffraction chart is shown in  FIG. 3 . 
     Comparative Example 1 
     25.0 g of 3,5-di-tert-butylsalicylic acid (0.10 mol obtained by butylation of salicylic acid) was dissolved in 200 ml of a 2% aqueous solution of NaOH and heated to about 70. Separately, 14.4 g (0.05 mol) of zinc sulfate heptahydrate was dissolved in 200 ml of water. This aqueous solution of zinc sulfate was added drop by drop to the aforementioned aqueous solution of 3,5-di-tert-butylsalicylic acid over a period of about 30 minutes. Subsequently, after the reaction was carried out at 70 to 80 for 2 hours, the reaction mixture was adjusted to a pH of 7.0±0.5 to complete the reaction. 
     This reaction solution was filtered while it remained hot, and the cake filtered off was washed with water and dried to yield 27.3 g of a white fine powder. The bulk density of this white powder was 6.9 ml/g. Analysis of the thus-obtained white powder using a powder X-ray diffraction apparatus detected major peaks of X-ray diffraction using the CuKα-characteristic X-ray [wavelength 1.541 Å] at Bragg angles 2θ of 5.7±0.2°, 6.4±0.2° and 15.4±0.2°. An X-ray diffraction chart is shown in  FIG. 4  and a liquid chromatography analysis chart in  FIG. 5 . 
     Examples 3 through 6 pertain to the production of charge control resin particles. 
     Example 3 
     
         
         Polyester resin [DIACRON ER561 (trade name), produced by Mitsubishi Chemical Corporation] . . . 50 parts 
         Charge control agent (zinc 3,5-di-tert-butylsalicylate obtained in Example 1) . . . 50 parts Methanol . . . 10 parts 
       
    
     The above ingredients were charged in a kneader and mixed for 15 minutes to moisten the polyester resin powder and charge control agent with the methanol, after which this mixture was kneaded in a molten state while the methanol was evaporated by gradual heating. After cooling, this mixture was further kneaded with a pair of heat rolls and cooled, and then roughly crushed in a vibration mill to yield charge control resin particles 1. 
     Example 4 
     Charge control resin particles 2 were prepared in the same manner as Example 3, except that the zinc 3,5-di-tert-butylsalicylate obtained in Example 1 was replaced with the zinc 3,5-di-tert-butylsalicylate obtained in Example 2. 
     Example 5 
     
         
         Styrene-acrylic copolymer resin [ALMATEX CPR600B (trade name), produced by Mitsui Chemicals, Inc.] . . . 70 parts 
         Charge control agent (zinc 3,5-di-tert-butylsalicylate obtained in Example 2) . . . 30 parts 
       
    
     The above ingredients were uniformly pre-mixed in a Henschel mixer, after which this mixture was kneaded in a molten state using a kneader. After cooling, this mixture was further kneaded with a pair of heat rolls and cooled, and then roughly crushed in a vibration mill to yield charge control resin particles 3. 
     Example 6 
     Charge control resin particles 4 were prepared in the same manner as Example 5, except that the zinc 3,5-di-tert-butylsalicylate obtained in Example 1 was replaced with the zinc 3,5-di-tert-butylsalicylate obtained in Example 2. 
     Examples 7 through 13 and Comparative Example 2 pertain to a toner for developing electrostatic images. 
     Example 7 
     
         
         Polyester resin [DIACRON ER561 (trade name), produced by Mitsubishi Chemical Corporation] . . . 100 parts 
         Carbon black [MA-100 (trade name), produced by Mitsubishi Chemical Corporation] . . . 6 parts Low polymer polypropylene [VISCOL 550-P (trade name), produced by Sanyo Chemical Industries, Ltdt.] . . . 5 parts 
         Charge control agent (zinc 3,5-di-tert-butylsalicylate obtained in Example 2) . . . 1 part 
       
    
     The above ingredients were uniformly pre-mixed using a high-speed mixer. This premix was then kneaded in a molten state using an extruder, cooled and thereafter roughly milled using a vibration mill. The rough milling product obtained was finely pulverized using an air jet mill equipped with a classifier to yield a negatively chargeable black toner 10 to 20 μm in particle diameter. Five parts of this toner was admixed with 95 parts of an iron powder carrier [TEFV 200/300 (trade name), produced by Powdertech Co., Ltd.) to yield a developer. 
     This developer was measured in a polyethylene bottle. This bottle was rotated under standard conditions (20-60% RH) in a ball mill at a rotation rate of 100 rpm to stir and charge the developer, and changes over time in the charge amount of the developer were determined. The results of the determination of changes over time in the charge amount are shown in Table 1. 
     Separately, this developer was measured in a polyethylene bottle. This bottle was rotated in each of a low-temperature low-humidity (5-30% RH) atmosphere and a high-temperature high-humidity (35-90% RH) atmosphere in a ball mill at a rotation rate of 100 rpm for 10 minutes to stir and charge the developer, and changes over time in the charge amount of the developer in each case were determined. The results of the determination of the environmental stability of the charge amount are shown in Table 2. 
     Changes Over Time in Charge Amount 
     
       
         
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Time (min) 
                 1 
                 3 
                 5 
                 10 
                 15 
                 30 
               
               
                   
               
             
             
               
                 Charge amount (−μC/g) 
                 36.0 
                 37.3 
                 38.5 
                 39.3 
                 39.5 
                 39.8 
               
               
                   
               
             
          
         
       
     
     Environmental Stability 
     
       
         
               
               
               
             
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 5° C. - 30% RH 
                 35° C. - 90% RH 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Charge amount (−μC/g) 
                 40.0 
                 39.5 
               
               
                   
               
             
          
         
       
     
     When this developer was used to form toner images using a commercial copying machine (selenium drum type), fogging-free high-quality images with good thin line reproducibility and good charge stability and sustainability without image density reduction were obtained. The offset phenomenon was never observed. 
     Comparative Example 2 
     A negatively chargeable black toner 10 to 20 μm in particle diameter and a developer were prepared in the same manner as Example 7, except that the zinc 3,5-di-tert-butylsalicylate obtained in Example 2 was replaced with the zinc 3,5-di-tert-butylsalicylate obtained in Comparative Example 1 (prepared from 3,5-di-tert-butylsalicylic acid obtained by butylation of salicylic acid). Using this developer, changes over time in charge amount and the environmental stability of charge amount were determined in the same manner as Example 7. The results are shown in Tables 3 and 4, respectively. 
     Changes Over Time in Charge Amount 
     
       
         
               
               
               
               
               
               
               
             
           
               
                 TABLE 3 
               
               
                   
               
               
                 Time (min) 
                 1 
                 3 
                 5 
                 10 
                 15 
                 30 
               
               
                   
               
             
             
               
                 Charge amount (−μC/g) 
                 30.5 
                 32.9 
                 34.5 
                 37.3 
                 40.0 
                 41.5 
               
               
                   
               
             
          
         
       
     
     Environmental Stability 
     
       
         
               
               
               
             
               
               
               
             
           
               
                   
                 TABLE 4 
               
               
                   
                   
               
               
                   
                 5° C. - 30% RH 
                 35° C. - 90% RH 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Charge amount (−μC/g) 
                 41.4 
                 36.7 
               
               
                   
               
             
          
         
       
     
     Example 8 
     
         
         Polyester resin [DIACRON ER561 (trade name), produced by Mitsubishi Chemical Corporation] . . . 100 parts 
         Oil-soluble magenta dye [OIL PINK #312 (trade name). produced by Orient Chemical Industries, Ltd.] . . . 6 parts 
         Low polymer polypropylene [VISCOL 550-P (trade name), produced by Sanyo Chemical Industries. Ltd.] . . . 5 parts 
         Charge control agent (zinc 3,5-di-tert-butylsalicylate obtained in Example 1) . . . 1 part 
       
    
     The above ingredients were treated in the same manner as Example 7 to yield a negatively chargeable magenta toner 10 to 20 μm in particle diameter and a developer. Using this developer, changes over time in charge amount and the environmental stability of charge amount were determined in the same manner as Example 7. The results are shown in Tables 5 and 6, respectively. 
     Changes Over Time in Charge Amount 
     
       
         
               
               
               
               
               
               
               
             
           
               
                 TABLE 5 
               
               
                   
               
               
                 Time (min) 
                 1 
                 3 
                 5 
                 10 
                 15 
                 30 
               
               
                   
               
             
             
               
                 Charge amount (−μC/g) 
                 35.8 
                 36.8 
                 39.4 
                 39.6 
                 39.7 
                 39.5 
               
               
                   
               
             
          
         
       
     
     Environmental Stability 
     
       
         
               
               
               
             
               
               
               
             
           
               
                   
                 TABLE 6 
               
               
                   
                   
               
               
                   
                 5° C. - 30% RH 
                 35° C. - 90% RH 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Charge amount (−μC/g) 
                 39.6 
                 38.6 
               
               
                   
               
             
          
         
       
     
     When this developer was used to form toner images using a commercial copying machine (selenium drum type), fogging-free vivid magenta images with good thin line reproducibility, excellent spectral characteristics, and transparency suitable for color blending by superposing were obtained. 
     Example 9 
     
         
         Polyester resin [DIACRON ER561 (trade name), produced by Mitsubishi Chemical Corporation] . . . 100 parts 
         Phthalocyanine dye [VALIFAST BLUE 2606 (trade name), produced by Orient Chemical Industries, Ltd.] . . . 6 parts 
         Low polymer polypropylene [VISCOL 550-P (trade name), produced by Sanyo Chemical industries. Ltd.] . . . 5 parts 
         Charge control agent (zinc 3,5-di-tert-butylsalicylate obtained in Example 2) . . . 1 part 
       
    
     The above ingredients were treated in the same manner as Example 7 to yield a negatively chargeable cyan toner 10 to 20 μm in particle diameter and a developer. Using this developer, changes over time in charge amount and the environmental stability of charge amount were determined in the same manner as Example 7. The results are shown in Tables 7 and 8, respectively. 
     Changes Over Time in Charge Amount 
     
       
         
               
               
               
               
               
               
               
             
           
               
                 TABLE 7 
               
               
                   
               
               
                 Time (min) 
                 1 
                 3 
                 5 
                 10 
                 15 
                 30 
               
               
                   
               
             
             
               
                 Charge amount (−μC/g) 
                 36.3 
                 37.5 
                 38.4 
                 39.4 
                 39.5 
                 40.1 
               
               
                   
               
             
          
         
       
     
     Environmental Stability 
     
       
         
               
               
               
             
               
               
               
             
           
               
                   
                 TABLE 8 
               
               
                   
                   
               
               
                   
                 5° C. - 30% RH 
                 35° C. - 90% RH 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Charge amount (−μC/g) 
                 40.2 
                 39.8 
               
               
                   
               
             
          
         
       
     
     When this developer was used to form toner images using a commercial copying machine (selenium drum type), fogging-free vivid cyan images with good thin line reproducibility, excellent spectral characteristics, and transparency suitable for color blending by superposing were obtained. 
     Example 10 
     
         
         Polyester resin [DIACRON ER561 (trade name), produced by Mitsubishi Chemical Corporation] . . . 100 parts 
         Quinophthalone dye [KASEYAN YELLOW E-3GL (trade name), produced by Nippon Kayaku Co., Ltd.] . . . 6 parts 
         Low polymer polypropylene [VISCOL 550-P (trade name), produced by Sanyo Chemical Industries, Ltd.] . . . 5 parts 
         Charge control agent (zinc 3,5-di-tert-butylsalicylate obtained in Example 2) . . . 1 part 
       
    
     The above ingredients were treated in the same manner as Example 7 to yield a negatively chargeable yellow toner 10 to 20 μm in particle diameter and a developer. Using this developer, changes over time in charge amount and the environmental stability of charge amount were determined in the same manner as Example 7. The results are shown in Tables 9 and 10, respectively. 
     Changes Over Time in Charge Amount 
     
       
         
               
               
               
               
               
               
               
             
           
               
                 TABLE 9 
               
               
                   
               
               
                 Time (min) 
                 1 
                 3 
                 5 
                 10 
                 15 
                 30 
               
               
                   
               
             
             
               
                 Charge amount (−μC/g) 
                 36.0 
                 37.8 
                 39.5 
                 39.5 
                 39.5 
                 39.6 
               
               
                   
               
             
          
         
       
     
     Environmental Stability 
     
       
         
               
               
               
             
               
               
               
             
           
               
                   
                 TABLE 10 
               
               
                   
                   
               
               
                   
                 5° C. - 30% RH 
                 35° C. - 90% RH 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Charge amount (−μC/g) 
                 39.8 
                 39.0 
               
               
                   
               
             
          
         
       
     
     When this developer was used to form toner images using a commercial copying machine (selenium drum type), fogging-free vivid yellow images with good thin line reproducibility, excellent spectral characteristics, and transparency suitable for color blending by superposing were obtained. 
     Example 11 
     
         
         Styrene-acrylic copolymer resin [ALMATEX CPR600B (trade name), produced by Mitsui Chemicals, Inc.] . . . 100 parts 
         Carbon black [MA-100 (trade name), produced by Mitsubishi Chemical Corporation] . . . 6 parts 
         Low polymer polypropylene [VISCOL 550-P (trade name), produced by Sanyo Chemical Industries, Ltd.] . . . 5 parts 
         Charge control agent (zinc 3,5-di-tert-butylsalicylate obtained in Example 2) . . . 1 part 
       
    
     The above ingredients were treated in the same manner as Example 7 to yield a negatively chargeable black toner 10 to 20 μm in particle diameter and a developer. Using this developer, changes over time in charge amount and the environmental stability of charge amount were determined in the same manner as Example 7. The results are shown in Tables 11 and 12, respectively. 
     Changes Over Time in Charge Amount 
     
       
         
               
               
               
               
               
               
               
             
           
               
                 TABLE 11 
               
               
                   
               
               
                 Time (min) 
                 1 
                 3 
                 5 
                 10 
                 15 
                 30 
               
               
                   
               
             
             
               
                 Charge amount (−μC/g) 
                 23.1 
                 25.2 
                 26.6 
                 27.0 
                 27.2 
                 27.6 
               
               
                   
               
             
          
         
       
     
     Environmental Stability 
     
       
         
               
               
               
             
               
               
               
             
           
               
                   
                 TABLE 12 
               
               
                   
                   
               
               
                   
                 5° C. - 30% RH 
                 35° C. - 90% RH 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Charge amount (−μC/g) 
                 27.6 
                 27.1 
               
               
                   
               
             
          
         
       
     
     When this developer was used to form toner images using a commercial copying machine (selenium drum type), fogging-free high-quality images with good thin line reproducibility and good charge stability and sustainability without image density reduction were obtained. The offset phenomenon was never observed. 
     Example 12 
     
         
         Polyester resin [DIACRON ER561 (trade name), produced by Mitsubishi Chemical Corporation] . . . 100 parts 
         Carbon black [MA-100 (trade name), produced by Mitsubishi Chemical Corporation] . . . 6 parts Low polymer polypropylene [VISCOL 550-P (trade name), produced by Sanyo Chemical Industries, Ltd.] . . . 5 parts 
         Charge control resin particles 2 obtained in Example 4 . . . 2 parts 
       
    
     The above ingredients were treated in the same manner as Example 7 to yield a negatively chargeable black toner 10 to 20 μm in particle diameter and a developer. Using this developer, changes over time in charge amount and the environmental stability of charge amount were determined in the same manner as Example 7. The results are shown in Tables 13 and 14, respectively. 
     Changes Over Time in Charge Amount 
     
       
         
               
               
               
               
               
               
               
             
           
               
                 TABLE 13 
               
               
                   
               
               
                 Time (min) 
                 1 
                 3 
                 5 
                 10 
                 15 
                 30 
               
               
                   
               
             
             
               
                 Charge amount (−μC/g) 
                 38.2 
                 39.0 
                 40.0 
                 39.7 
                 39.8 
                 40.1 
               
               
                   
               
             
          
         
       
     
     Environmental Stability 
     
       
         
               
               
               
             
               
               
               
             
           
               
                   
                 TABLE 14 
               
               
                   
                   
               
               
                   
                 5° C. - 30% RH 
                 35° C. - 90% RH 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Charge amount (−μC/g) 
                 40.0 
                 39.8 
               
               
                   
               
             
          
         
       
     
     When this developer was used to form toner images using a commercial copying machine (selenium drum type), fogging-free high-quality images with good thin line reproducibility and good charge stability and sustainability without image density reduction were obtained. 
     The offset phenomenon was never observed. 
     Example 13 
     
         
         Styrene-acrylic copolymer resin [ALMATEX CPR600B (trade name), produced by Mitsui Chemicals, Inc.] . . . 100 parts 
         Carbon black [MA-100 (trade name), produced by Mitsubishi Chemical Corporation] . . . 6 parts Low polymer polypropylene [VISCOL 550-P (trade name), produced by Sanyo Chemical Industries. Ltd.] . . . 5 parts 
         Charge control resin particles 4 obtained in Example 6 . . . 3.5 parts 
       
    
     The above ingredients were treated in the same manner as Example 7 to yield a negatively chargeable black toner 10 to 20 μm in particle diameter and a developer. Using this developer, changes over time in charge amount and the environmental stability of charge amount were determined in the same manner as Example 7. The results are shown in Tables 15 and 16, respectively. 
     Changes Over Time in Charge Amount 
     
       
         
               
               
               
               
               
               
               
             
           
               
                 TABLE 15 
               
               
                   
               
               
                 Time (min) 
                 1 
                 3 
                 5 
                 10 
                 15 
                 30 
               
               
                   
               
             
             
               
                 Charge amount (−μC/g) 
                 25.5 
                 26.5 
                 27.0 
                 27.9 
                 27.6 
                 27.8 
               
               
                   
               
             
          
         
       
     
     Environmental Stability 
     
       
         
               
               
               
             
               
               
               
             
           
               
                   
                 TABLE 16 
               
               
                   
                   
               
               
                   
                 5° C. - 30% RH 
                 35° C. - 90% RH 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Charge amount (−μC/g) 
                 27.9 
                 27.5 
               
               
                   
               
             
          
         
       
     
     When this developer was used to form toner images using a commercial copying machine (selenium drum type), fogging-free high-quality images with good thin line reproducibility and good charge stability and sustainability without image density reduction were obtained. The offset phenomenon was never observed.