JPH07128906A - Electrophotographic toner - Google Patents

Abstract

PURPOSE:To obtain the toner which is negatively charged without use of a conventionally used charge control agent and which hardly causes offsetting without any silicone oil. CONSTITUTION:The resin to be used in this electrophotographic toner is a mixture of the polyester resin and styrene-acrylic resin. When the acid value of the polyester resin is controlled to (a) and the content of the polyester resin in the resin used in the toner to p%, 100<=a.p<=3500. In the inequality, (a) is milligrams of the potassium hydroxide necessary to neutralize 1 gram of the polyester resin, and (p) is the ratio in % of the weight of the resin used in the toner to that of the polyester resin.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to electrophotographic toner for laser printers, copying machines and the like.

[0002]

2. Description of the Related Art Various image forming methods utilizing electrostatic phenomenon in laser printers, copying machines and the like are described in US Pat. No. 2,297,691 and Japanese Patent Publication No. 42-23910. For example, a process of adsorbing a toner, which is a developer, on an electrostatic latent image formed on a photoreceptor, then transferring the toner to a transfer material, and then fixing the toner is performed. The toner is usually a powder made of a mixture of a resin and a pigment or a dye and various additives, and its particle size is about several μm to 20 μm. In order to obtain a clear developed image without fogging by using such particles, it is important that the toner has an appropriate charging property. As this method, various charge control agents have been used, and resins having a large positive charge have been used. As charge control agents, colored compounds such as triphenylmethane derivatives, nigrosine, and metal salts of monoazo dyes have been used for monochrome images, and metal salts of oxycarboxylic acids such as salicylic acid and various quaternary ammonium salts for color images. ，
Colorless or light-colored compounds such as amine compounds such as imidazole derivatives have been used. Of these, the ones for monochrome use dyes and pigments that have the ability to charge, so many types of structures with similar structures have been studied, so there are many types and toners with chargeability that are compatible with many devices are made. Became possible. Color-based ones need to be colorless or light-colored as compared with monochrome ones, so that structures similar to pigments and dyes cannot be applied as they are. In addition, there are very few types because the days since the product was commercialized are short and the research period is short. On the other hand, as a method for imparting a charging property to a resin, methods such as partial hydrolysis after polymerization and adjustment of the degree of polymerization have been attempted. At present, styrene-acrylic resin or polyester resin is mainly used as the toner resin. In addition, polystyrene resins, ABS resins, styrene-butadiene resins, polyalkylmethacrylate resins, epoxy resins, etc. have been investigated.

[0003]

When a resin is used as a resin for color toner, both have advantages and disadvantages. For example, styrene-acrylic resin can be obtained with a high degree of polymerization, and has a property that offset hardly occurs,
Fixability to OHP is low. Further, since the resin has a low charging property, a charging agent which gives a large charging amount is required when a large charging amount is required. Especially in the case of color, since a colorless or light-colored electrifying agent must be used, there are few kinds thereof, and when the dispersibility in the resin is taken into consideration, only a few are applicable. Currently, copiers mainly use forward rotation development, and the mainstream of transfer drums is OPC, so toners that are positively charged are often used. However, printers often use negatively charged toners because reversal development is the mainstream. It is used. A salicylic acid-based metal complex is currently used as a charging agent used for negatively charging toners for color, but it cannot be applied to a developing device that requires a large amount of charge to exceed the control range of this charging agent. Polystyrene resin, A
BS resin, styrene-butadiene resin, polyalkylmethacrylate resin, epoxy resin, etc. have the same problem. Therefore, polyester resin is often used as the resin for negatively charged toner for color.

Polyester resin is suitable for reversal development because it is charged relatively negatively as compared with styrene-acrylic resin. Moreover, since the surface after fixing is very flat, the light transmittance is very high, and it is advantageous over styrene-acrylic resin when making a color OHP. However, offset is likely to occur and it is necessary to apply silicone oil to the fixing roll. As a result, the image fixing portion of the device becomes large, which leads to an increase in size and cost of the device. In addition, it is necessary to control the amount of silicone oil to the minimum necessary.If it is too large, stains tend to adhere to the image, and the surface of the toner tends to scatter light, which makes it difficult to focus and the texture becomes unpleasant. Problems such as discomfort to the user occur.

For the above reasons, it has been earnestly desired to introduce a toner resin which has a negative chargeability and is less likely to cause offset even without silicone oil.

[0006]

Means for Solving the Problems As a result of studying the above-mentioned problems, the present inventors have found that the resin used is a mixture of a polyester resin and another resin, and the acid value of the polyester resin to be mixed and the ratio of addition are within a certain range. The present invention has been accomplished by finding a toner whose chargeability is controlled and which is less likely to cause an offset by using the specified toner resin. That is, the means are as follows.

In the electrophotographic toner, the resin used is a mixture of resins having different chemical structures with different repeating units, and at least one of the resins in the mixture is polyester and the acid of the polyester resin is When the value is a and the ratio of the content of the polyester resin in the resin mixture used for the toner is p%, a and p
The toner for electrophotography is characterized in that the product of is in the range represented by the following formula.

[0008]

## EQU00007 ## 100.ltoreq.a.p.ltoreq.3500 where a is the number of mg of potassium hydroxide required to neutralize 1 g of the polyester resin, and p is the polyester resin in the weight of the resin used for the toner. The percentage of the weight of is shown in%.

The acid value of the polyester generally shows a larger value as the number of carboxyl groups present at the ends of the long chain formed by polymerization increases. Further, when there is a residue having a tendency to show acidity such as a sulfonyl group or an aromatic hydroxyl group in addition to the carboxyl group, the value may be larger. When the repeating units are the same, the acid value is almost inversely proportional to the degree of polymerization.
The inventors of the present invention have found that the larger the acid value, the more negatively charged the resin is, and when the repeating units between ester bonds are the same, the acid value and the charge amount have a substantially linear relationship. When a polyester carrier having an acid value of about 10 was charged with a ferrite carrier, it was about 30 to 60 μC / g, although it varied depending on the types of the polyester resin and the carrier. When a styrene-acrylic resin having an acid value of 5.2 was mixed as a resin other than this resin and the polyester resin, the charge amount became almost as follows.

P (%) Charge amount (μC / g) 20 Approx. 8 to 18 40 Approx. 16 to 36 60 Approx. 24 to 54 80 Approx. 32 to 72 100 Approx. 40 to 90 The charge amount is measured by a blow-off method. A measuring device was used. Further, p is the percentage of the weight of the polyester resin in the weight of the resin used for the toner, and the styrene-acrylic resin and the polyester resin are heated, melted and kneaded, and then coarsely pulverized and finely pulverized. It was obtained by the classification operation. From the above results, it is understood that a proportional relationship is substantially established between the polyester content and the charge amount.

However, at the time of toner preparation, particles of one of the styrene-acrylic resin and the polyester resin (particle size is 4 to 20 μm) are prepared in advance by the above method, and fine particles of the other resin are surface-modified. When it is made by a method of fixing with a quality device, it does not always have a linear relationship. P when fine particles are polyester
Even if is small, the amount of charge tends to be large.

As the acid value of the styrene-acrylic resin increases, the negative chargeability tends to increase, but not so much as the polyester resin. In the case of styrene-acrylic resin, it may be easier to absorb moisture when trying to increase the acid value.Therefore, rather than controlling the charging property by the mixing ratio of styrene-acrylic resin and acid value, the mixing ratio of polyester resin and acid value may be adjusted. Controlling the charging property tends to produce better results.

By the way, the more the carboxyl group of the polyester resin, the lower the compatibility with other resins tends to be. Further, the compatibility varies depending on the structure of the repeating unit of the polyester, and in the case of a styrene-acrylic resin synthesized in a state where the proportion of styrene in the monomer is high, an aromatic unit such as a benzene ring exists in the repeating unit of the polyester. However, in the case of a styrene-acrylic resin synthesized in the state where the proportion of acrylic acid ester in the monomer is high, although there are many methylene units in the repeating unit of the polyester, The compatibility tends to be higher.

The transparency tends to improve as the amount of polyester resin increases, but it may decrease when the amount of carboxyl groups increases. Regarding the resin mixed with the polyester resin, the transparency tends to improve as the content of acrylate ester increases in the case of styrene-acrylic resin. Alkyl acrylate resins have relatively high transparency in the visible region, but they have low compatibility with some polyester resins and tend to have reduced transparency. When one of these resin mixtures having low transparency was observed with a microscope (observation magnification was 50 times), a sea-island structure was observed.

If the number average molecular weight of the polyester resin used is too small, the flatness of the image surface tends to be lowered. The acid value tends to decrease as the number average molecular weight increases, although it depends on the repeating unit of the chemical structure. Therefore, the number average molecular weight is preferably in the range of 1600 to 20000. If the weight average molecular weight is too small, offset tends to occur, and if it is too large, the Tm of the resin becomes unclear. Therefore, the weight average molecular weight is 2
It is desirable that the range is from 20 to 20. Further, it is necessary that the Tg of the polyester resin does not undergo modification such as fusion in a general environment. Therefore, the temperature is preferably at least 55 ° C or higher. If Tg is too high, Tm tends to be high. Therefore, in order to set Tm to an appropriate value, Tg
Is preferably 70 ° C. or lower.

Examples of the resin mixed with the polyester resin include styrene-acrylic resin, epoxy resin, allylic acrylate resin, allylic acrylate-acrylic copolymer, styrene-butadiene copolymer and ABS resin. Of these, styrene-acrylic resin is easy to use because it is easy to match the thermal characteristics such as Tg and Tm. If the number average molecular weight of the styrene-acrylic resin is too small, the flatness of the image surface tends to decrease. Further, although it depends on the repeating unit of the chemical structure, when the number average molecular weight increases, melting by heat tends to be difficult to proceed. Therefore, the number average molecular weight is preferably in the range of 2000 to 40,000. If the weight average molecular weight / number average molecular weight is too small, offset tends to occur, and if it is too large, the Tm of the resin becomes unclear. Therefore, the weight average molecular weight / number average molecular weight is preferably in the range of 2 to 40. Further, it is necessary that the Tg of the styrene-acrylic resin does not undergo modification such as fusion in a general environment.
Therefore, the temperature is preferably at least 55 ° C or higher.
More preferably, the temperature is 60 ° C. or higher. If Tg is too high, Tm tends to be high. So T
Tg is preferably 70 ° C. or lower in order to set m to an appropriate value.

In the present invention, the polyester resin is used as one of the factors for controlling the charge amount of the toner, but in addition to this, a conventional charge control agent is added to try to match the charge state of the toner with an apparatus. There are no restrictions. For example, if the mutual compatibility of the resins to be mixed is low, a sea-island structure may be formed after mixing. This may reduce the transmittance after development, but in this case, it may be possible to improve the transmittance by changing the mixing ratio of the polyester resin. However, changing the mixing ratio of the polyester resin may deviate from the required charge amount. In this case, use the conventionally used charging agent to ensure the required charge amount. can do.

As the colorant, carbon black may be added in the case of black toner, and a colorant corresponding to the intended color may be added in the case of color toner.

The mixing ratio (polymerization ratio) of the resin, the colorant, and the charge control agent varies depending on the developing machine to be applied and the coloring intensity of the colorant, but is approximately as follows.

Resin: Colorant: Charge control agent = 80 to 99:
1-10: 0-10 In a similar application of the present invention, a method of swelling a polyester resin into styrene and an acrylate ester (that is, a raw material of a styrene-acrylic resin) and then polymerizing the same (JP-A-4-106102
No. 4,106,103), etc., other types of resin are fixed to the resin particles of the core by a suspension polymerization method or the like. But,
The present invention is a completely different invention from the above application because the toner is formed by a simple mixing method in which the already polymerized resin is kneaded using a mixer or a kneader.

A method in which a polyester resin is mixed with a vinyl-based monomer and a divinyl-based monomer (that is, a raw material such as polyethylene or styrene-acrylic resin) and then polymerized (Japanese Patent Application Laid-Open No. HEI-1)
Others, such as a suspension polymerization method, are used to fix other types of resins to polyester resins. However, again, the present invention is a completely different invention from the above application because the toner is formed by a simple mixing method of only kneading the already polymerized resin.

The resin of the present invention is said to be widely used in the future, not only in the two-component developing system which is mainly used at present (a system in which toner is charged by frictional charging generated by mixing and stirring toner and carrier). There is no limitation on the adoption of a one-component developing method (a method of charging the toner by frictional charging generated by stirring the toner with a brush or the like instead of the carrier).

[0023]

Function The details of the polyester resin changing the chargeability of the toner resin are unknown. However, there is a proportional relationship between the acid value of the polyester and the absolute value of the charge amount, and the acid value is proportional to the number of acidic residues such as carboxyl groups at the ends of the polyester main chain, and these acidic residues are generally Since it is an electron-withdrawing group, it is presumed that these acidic residues may be negatively charged as toner particles by attracting electrons on the carrier surface during the contact charging process.

[0024]

EXAMPLES The present invention will be described with reference to examples.

(Example 1) Styrene-acrylic resin (No. 19155 manufactured by Hitachi Chemical Co., Ltd.) 85 parts by weight Polyester resin (FC-159 manufactured by Mitsubishi Rayon (acid value = 6.8)) 15 parts by weight C.I. I. Pigment Blue 15: 5 parts by weight Polypropylene: 2 parts by weight The above materials are put into a Henschel mixer, mixed, and kneaded by heating with a kneader, and then ground with a hammer mill. After repeating this operation three times, it was pulverized by a jet pulverizer and classified by an air stream classifier to obtain a blue toner having an average particle size of 10 μm. At this time
-The value of p is 102. It was -7.8 [mu] C / g when measured with a blow-off type charge amount measuring device.
As will be described later, in the case where the resin is only styrene-acrylic resin, when the charge amount is measured with this measuring device, the absolute value is 0 to 2 μ.
C / g. This shows that the addition of the polyester resin changed the charge amount of the resin.

1 part by weight of this toner was mixed with 15 parts by weight of carrier particles having an average particle size of 60 μm to obtain a two-component developer.
When this developer was tested with a commercially available laser printer,
The image was clear even after 1,000 copies.

From the above, it was confirmed that the toner of the present invention can be used as a toner for electrophotography.

Comparative Example 1 A toner was manufactured in the same manner as in Example 1 except that polyester resin was excluded from the toner material used in Example 1. When this was measured by a blow-off type charge amount measuring device in the same manner as in Example 1, it was found to be 1.5 μm.
It was C / g. 1 part by weight of this toner and an average particle size of 60
15 parts by weight of carrier particles of μm were mixed to obtain a two-component developer. When this developer was tested with a commercially available laser printer, it did not print at all.

From the above, it was found that when the polyester was not mixed with the resin, the resin particles could not be used as an electrophotographic toner unless a charge control agent was added.

(Example 2) 2,2-bis (4-hydroxyphenylene) propane (23 parts by weight) was stirred while n-
Add to butanol (200 parts by weight). (Stirring is continued until the end of the reaction.) Sodium hydroxide (8 parts by weight) dissolved in water (30 parts by weight) was added thereto, and the mixture was heated to 80 ° C. After the temperature of the reaction solution reaches 80 ° C., 2-bromoethanol (30 parts by weight) is added. Then at 80 ℃ 2
After stirring for an hour, cool to room temperature. After acidifying the reaction solution with 5% hydrochloric acid, water, n-butanol and 2-bromoethanol were removed by an evaporator, and the remaining solid was mixed with ethyl acetate and n-.
It was recrystallized from hexane to convert the two hydroxyl groups (-OH) of 2,2-bis (4-hydroxyphenylene) propane to -OC.
A compound modified with H ₂ CH ₂ OH was obtained. Next, this compound (8 parts by weight) and fumaric acid (3 parts by weight) were mixed with xylene (50 parts by weight).
(Parts by weight) and heated at 120 ° C. for 3 hours with stirring. During heating, dry nitrogen gas is allowed to flow in the reaction vessel in order to carry the water generated during the reaction out of the reaction system. After cooling, xylene was removed by an evaporator, the remaining solid was dissolved in ethyl acetate (20 parts by weight), this was added dropwise to stirring methanol (200 parts by weight), and the precipitated solid was collected by filtration. This is again dissolved in ethyl acetate (20 parts by weight), then dropped into stirring methanol (200 parts by weight), and the precipitated solid is collected by filtration. Thus, a polyester resin having the following structural formula was obtained.

[0031]

## STR1 ## HO- by _{(COCH = CHCOOCH 2 CH 2 OC} 6 H 4 C (CH 3) 2 C 6 H 4 OCH 2 CH 2 O) n-H Note 36.0 was measured acid value of the resin there were.

Next, a method for producing a toner using this resin will be described.

Styrene-acrylic resin (No. 19155 manufactured by Hitachi Chemical Co., Ltd.) 85 parts by weight Polyester resin (acid value = 36.0) obtained by the above method 15 parts by weight C.I. I. Pigment Blue 15: 5 parts by weight Polypropylene: 2 parts by weight The above materials are put in a Henschel mixer, mixed, kneaded by heating with a kneader, and then ground with a hammer mill. After repeating this operation three times, it was pulverized by a jet pulverizer and classified by an air stream classifier to obtain a blue toner having an average particle size of 10 μm. At this time
-The value of p is 540. This was measured by a blow-off type charge amount measuring device and found to be -41.2 μC / g.

1 part by weight of this toner was mixed with 15 parts by weight of carrier particles having an average particle size of 60 μm to obtain a two-component developer.
When this developer was tested with a commercially available laser printer,
The image was clear even after 1,000 copies.

From the above, it was confirmed that the toner of the present invention can be used as a toner for electrophotography.

(Example 3) 2,2-bis (4-hydroxyphenylene) propane (23 parts by weight) and fumaric acid (1
2 parts by weight) is added to xylene (100 parts by weight), and the mixture is heated with stirring at 120 ° C. for 3 hours. During heating, dry nitrogen gas is allowed to flow in the reaction vessel in order to carry the water generated during the reaction out of the reaction system. After cooling, xylene was removed by an evaporator, the remaining solid was dissolved in ethyl acetate (50 parts by weight), this was added dropwise to stirring methanol (500 parts by weight), and the precipitated solid was collected by filtration. This is again dissolved in ethyl acetate (50 parts by weight) and then added dropwise to stirring methanol (500 parts by weight) to collect the precipitated solid by filtration. Thus, a polyester resin having the following structural formula was obtained.

[0037]

Embedded image HO- was _{(COCH = CHCOOC 6 H 4 C} (CH 3) 2 C 6 H 4 O) n-H Note was measured acid value of the resin 45.6.

Next, a method for producing a toner using this resin will be described.

Styrene-acrylic resin (No. 19155 manufactured by Hitachi Chemical Co., Ltd.) 70 parts by weight Polyester resin obtained by the above method (acid value = 45.6) 30 parts by weight C.I. I. Pigment Blue 15: 5 parts by weight Polypropylene: 2 parts by weight The above materials are put in a Henschel mixer, mixed, kneaded by heating with a kneader, and then ground with a hammer mill. After repeating this operation three times, it was pulverized by a jet pulverizer and classified by an air stream classifier to obtain a blue toner having an average particle size of 10 μm. At this time
-The value of p is 1368. This was measured by a blow-off type charge amount measuring device and found to be -101.3 μC / g.

1 part by weight of this toner was mixed with 15 parts by weight of carrier particles having an average particle size of 60 μm to obtain a two-component developer.
When this developer was tested with a commercially available laser printer,
The image was clear even after 1,000 copies.

From the above, it was confirmed that the toner of the present invention can be used as a toner for electrophotography.

Example 4 Styrene-acrylic resin (No. 19155 manufactured by Hitachi Chemical Co., Ltd.) 25 parts by weight Polyester resin obtained in Example 3 (acid value = 45.6) 75 parts by weight C.I. I. Pigment Blue 15: 5 parts by weight Polypropylene: 2 parts by weight The above materials are put in a Henschel mixer, mixed, kneaded by heating with a kneader, and then ground with a hammer mill. After repeating this operation three times, it was pulverized by a jet pulverizer and classified by an air stream classifier to obtain a blue toner having an average particle size of 10 μm. At this time
-The value of p is 3420. This was measured to be −253.2 μC / g by a blow-off type charge amount measuring device.

1 part by weight of this toner was mixed with 15 parts by weight of carrier particles having an average particle size of 60 μm to obtain a two-component developer.
When this developer was tested with a commercially available laser printer,
The image was clear even after 1,000 copies.

This toner did not cause offset even if silicone oil was not applied to the fixing roll.

From the above, it was confirmed that the toner of the present invention can be used as a toner for electrophotography.

Comparative Example 2 Styrene-acrylic resin (No. 19155 manufactured by Hitachi Chemical Co., Ltd.) 20 parts by weight Polyester resin obtained in Example 3 (acid value = 45.6) 80 parts by weight C.I. I. Pigment Blue 15: 5 parts by weight Polypropylene: 2 parts by weight The above materials are put in a Henschel mixer, mixed, kneaded by heating with a kneader, and then ground with a hammer mill. After repeating this operation three times, it was pulverized by a jet pulverizer and classified by an air stream classifier to obtain a blue toner having an average particle size of 10 μm. At this time
-The value of p is 3648. This was measured by a blow-off type charge amount measuring device and found to be 269.3 μC / g.

1 part by weight of this toner was mixed with 15 parts by weight of carrier particles having an average particle size of 60 μm to obtain a two-component developer.
When this developer was tested with a commercially available laser printer,
The image of one copy was clear. However, offset was generated unless silicone oil was applied to the fixing roll. Therefore, when the image was developed without silicone oil, the images developed before the second sheet were thinly developed.

From the above, it was found that the toner of the present invention can be used as a toner for electrophotography, but offset occurs unless silicone oil is used.

(Example 5) Styrene-acrylic resin (No. 19602 manufactured by Hitachi Chemical) 85 parts by weight Polyester resin (FC-159 manufactured by Mitsubishi Rayon (acid value = 6.8)) 15 parts by weight C.I. I. Pigment Blue 15: 5 parts by weight Polypropylene: 2 parts by weight The above materials are put in a Henschel mixer, mixed, kneaded by heating with a kneader, and then ground with a hammer mill. After repeating this operation three times, it was pulverized by a jet pulverizer and classified by an air stream classifier to obtain a blue toner having an average particle size of 10 μm. At this time
The value of p is 102. It was -7.8 μC / g when measured with a blow-off type charge amount measuring device.

1 part by weight of this toner was mixed with 15 parts by weight of carrier particles having an average particle size of 60 μm to obtain a two-component developer.
This developer was tested on a commercial color copier and found to be 10
The image was clear even after 00 copies. Further, when the paper on which the image was output was left at 60 ° C., no change in color tone was observed even after 100 hours, and it was a bright blue color.

From the above, it was confirmed that the toner of the present invention can be used as an electrophotographic toner even if the type of styrene-acrylic resin is changed.

[0052]

EFFECTS OF THE INVENTION By using the resin having the composition described in the present invention, it is possible to obtain a toner having a negative charging property without using a charging agent which is usually used, and being less likely to cause an offset even without silicone oil. It became possible to obtain.

Claims (6)

[Claims] 1. In an electrophotographic toner, the resin used is a mixture of resins having chemical structures having different repeating units, and at least one resin of the resins in the mixture is polyester, and When the acid value is a and the ratio of the content of the polyester resin in the resin mixture used for the toner is p%, the product of a and p is in the range represented by the following formula: Photographic toner. ## EQU1 ## 100 ≦ a · p ≦ 3500 where a is the number of mg of potassium hydroxide required to neutralize 1 g of the polyester resin, and p is the weight of the resin mixture used for the toner. The weight ratio of the polyester resin is shown in%. 2. A polyester resin having a number average molecular weight of Mn,
An electrophotographic toner characterized in that both Mn and Mw / Mn are in a range represented by the following formula when the weight average molecular weight is Mw. ## EQU2 ## 1600 ≦ Mn ≦ 20000 2 ≦ Mw / Mn ≦ 20 3. The glass transition temperature of polyester resin is Tg.
And Tg is in the range represented by the following formula: [Formula 3] 55 ° C ≤ Tg ≤ 70 ° C 4. One of the resins other than polyester in the resin mixture is styrene-acrylic resin, and the content ratio of the polyester resin in the resin mixture used for the toner is p%, and the styrene-acrylic resin. An electrophotographic toner, wherein the sum of p and q is in a range represented by the following formula, when the resin content ratio is q%. [Equation 4] 3 ≦ p + q ≦ 100 5. When the number average molecular weight of the styrene-acrylic resin is Mn 'and the weight average molecular weight is Mw', Mn 'and Mw' / Mn 'are both in the ranges represented by the following formulas. Toner for electrophotography. ## EQU5 ## 2000 ≦ Mn ′ ≦ 40000 2 ≦ Mw ′ / Mn ′ ≦ 40 6. A toner for electrophotography, wherein Tg 'is in a range represented by the following formula, where Tg' is a glass transition temperature of styrene-acrylic resin. [Equation 6] 60 ° C ≤ Tg '≤ 70 ° C