Toner for developing electrostatic image and image forming method

A toner for developing an electrostatic image is composed by a binder resin, and a magnetic material and/or a colorant. The binder resin (a) comprises a styrene resin polymerized in the presence of a poly-functional polymerization initiator, (b) provides a molecular weight distribution on a GPC chromatogram showing a maximum (P1) in a molecular weight range of 3.5.times.10.sup.3 -5.times.10.sup.4 and a maximum (P2) or shoulder in a molecular weight range of at least 1.times.10.sup.5, and (c) contains 15 wt. % or less of a resin component in a molecular weight range of at most 3.times.10.sup.3. Further the toner contains at most 100 ppm of styrene and benzaldehyde.

FIELD OF THE INVENTION AND RELATED ART 
The present invention relates to a toner for use in electrophotography, 
electrostatic recording, etc., and an image forming method using the 
toner. 
Hitherto, a large number of electrophotographic processes have been known, 
inclusive of those disclosed in U.S. Pat. Nos. 2,297,691; 3,666,363; and 
4,071,361. In these processes, in general, an electrostatic latent image 
is formed on a photosensitive member comprising a photoconductive material 
by various means, then the latent image is developed with a toner, and the 
resultant toner image is, after being transferred onto a transfer material 
such as paper, as desired, fixed by various fixing methods to obtain a 
copy. The fixing methods may include a pressure fixing system of passing 
between at least two metal rollers, an oven fixing system of passing in a 
heated atmosphere given by an electric heater, a hot roller fixing system 
of passing hot rollers as the most popular system at present, and a fixing 
system using a film as disclosed in U.S. Pat. No. 5,149,941. 
In the heat-fixing system using such hot rollers, a sheet carrying a toner 
image to be fixed (hereinafter called "fixation sheet") is passed, while 
the surface of a hot roller having a releasability with the toner is 
caused to contact the toner image surface of the fixation sheet under 
pressure, to fix the toner image. In this method, as the hot roller and 
the toner image on the fixation sheet contact each other under a pressure, 
a very good heat efficiency is attained for melt-fixing the toner image 
onto the fixation sheet to afford quick fixation, so that the method is 
very effective in a high-speed electrophotographic copying machine. In 
this method, however, a toner image in a melted state is caused to contact 
a hot roller under pressure, so that there is observed a so-called offset 
phenomenon that a part of the toner image is attached and transferred to 
the hot roller and then transferred back to the fixation sheet to stain 
the fixation sheet. It has been regarded as one of the important 
conditions in the heat-fixing system to prevent the toner from sticking to 
the hot roller. 
In the hot-roller fixing, a relatively long time is required from turning 
on the power supply for the heater until the hot rollers are heated to a 
temperature suitable for fixation, thus requiring not a short time 
(waiting time) for providing a hard copy in office work. Thus, the waiting 
time causes a time loss and lowers the efficiency of the office work. 
Various proposals have been made in order to shorten the waiting time and 
improve the efficiency of fixation in the hot-roller fixing system. 
Regarding a binder resin in order to improve the fixability of a toner, it 
is required that the viscosity of the toner on melting is lowered to 
provide a large adhesion area with the fixation sheet, so that the glass 
transition temperature (Tg) and molecular weight of the binder resin for 
the toner are required to be lowered. 
If the Tg and molecular weight of the binder resin are simply lowered, the 
above-mentioned offset is liable to occur. In this way, the 
low-temperature fixability and anti-offset characteristic are generally 
contradictory, so that it is not easy to develop a toner satisfying these 
requirements simultaneously. 
As a binder resin satisfying the above requirements, Japanese Patent 
Publication (JP-B) 63-32182 and JP-B 63-32382 have proposed a binder resin 
having two peaks in a molecular weight distribution as measured by gel 
permeation chromatography (GPC). The binder resin has been designed to 
improve the fixability by its low-molecular weight component and improve 
the anti-offset characteristic by its high-molecular weight component, 
thus showing excellent performances. However, further lowering in 
molecular weight causes a decrease in developing performance of the 
resultant toner and an increase in volatile matter content within the 
toner. Accordingly, a toner having a further increased fixability and 
suitably used in the hot-roller fixing system while avoiding the above 
difficulty is desired. 
In recent years, the recording method using electrophotography has extended 
its applicability including office use and private or home use. For such 
use, if the toner retains a large residual monomer or solvent content, 
unpleasant odor is evolved at the time of image formation or fixing, so 
that a toner with a reduced residual monomer or solvent content is 
desired. 
In recent years, a contact charging means has been developed in place of a 
corona charging system to be used in electrophotographic apparatus so as 
to prevent occurrence of ozone under high voltage application for forming 
electrostatic images on a photosensitive member surface. In an 
electrophotographic apparatus using such contact charging means, the 
occurrence of ozone is almost prevented so that it becomes possible to 
omit an ozone filter. In this case of using no ozone filter, however, the 
problem of odor evolved from a developer is liable to occur noticeably. 
In order to reduce the residual monomer content in a toner, several methods 
may be employed, inclusive of the use of an increased amount of 
polymerization initiator for producing a binder resin, a prolonged period 
of distillation for removing the solvent under a reduced pressure after 
such such polymerization, or a high-temperature kneading under a reduced 
pressure for producing a toner. These methods for reducing the residual 
monomer content are accompanied by several difficulties. For example, a 
simple increase in polymerization initiator amount causes difficulties in 
molecular weight distribution of the resultant polymer, such as an 
increase in low-molecular weight component and broadening of peaks in the 
molecular weight distribution and can result in poor developing 
performances. The prolonged period of distillation under a reduced 
pressure for removing the solvent after the polymerization is accompanied 
by difficulties, such as a long-time occupation of the production 
apparatus increased energy consumption requiring a large heat energy and a 
lowering in molecular weight of the resin due to depolymerization 
(decomposition of the polymer). The high-temperature kneading under a 
reduced pressure for production of a toner is liable to cause a 
degradation of the other components of the toner, such as wax, thus 
resulting in, e.g., poor dispersibility of the components. 
A corona discharger has been conventionally used as a charging means in 
electrophotographic apparatus, etc. As is briefly mentioned hereinbefore, 
however, the corona discharger is accompanied by difficulties, such as 
necessity of applying a high voltage and occurrence of a large quantity of 
ozone. 
Accordingly, in recent years, it has been considered to use a contact 
charging means instead of a corona discharger. More specifically, such 
contact charging means may be constituted by an electroconductive roller, 
as a charging member, which is supplied with a voltage and is caused to 
contact a photosensitive-member as a member to be charged, thereby 
charging the photosensitive member surface to a prescribed potential. By 
using such a contact charging means, it is possible to use a lower voltage 
and reduce the amount of ozone generation in comparison with a corona 
discharger. 
In an image forming apparatus using a step of electrostatically 
transferring a toner image formed on a latent image-bearing member 
(photosensitive member) to a transfer-receiving medium in a sheet form, 
such as paper, it has been proposed to use a transfer device in the form 
of a roller, etc., supplied with a bias voltage to be pressed against a 
latent image-bearing member in the form of, e.g., a rotatable cylinder or 
an endless belt so as to pass a transfer-receiving medium therebetween and 
transfer a toner image on the latent image-bearing member onto the 
transfer-receiving medium as disclosed, e.g., in Japanese Laid-Open Patent 
Application (JP-A) 59-46664. 
In contrast with transfer means utilizing corona discharge, such a transfer 
device can enlarge an area of contact of the transfer-receiving medium 
onto the latent image-bearing member by regulating the pressure of the 
transfer roller exerted against the latent image-bearing member, thereby 
positively supporting the transfer-receiving medium under pressure at the 
transfer position. As a result, it is possible to reduce a synchronization 
failure due to a conveyer for transfer-receiving medium and minimize 
transfer deviation due to loop or curl of the transfer-receiving medium. 
Accordingly, it is possible to easily comply with the requirements of 
shorter conveying passage for transfer-receiving media and a smaller 
diameter of latent-image bearing member as required in compactization of 
such image forming apparatus in recent years. 
However, in the case of using contact charging means as described above, 
there arises a problem of charging failure unless a sufficient contact 
with the member to be charged is ensured. There is also a problem that, if 
a developer component remains on the surface of a photosensitive member at 
the abutting position exerting a certain pressure by the charging member 
against the photosensitive member, the developer component remaining on 
the surfaces of the charging member and the photosensitive member are 
stuck thereat, thus adversely affecting the latent image formation and 
developing. 
In the apparatus adopting abutting transfer disclosed in JP-A 59-46664, it 
is necessary to apply a certain pressure against the transfer device as a 
transfer current is supplied at the abutting position. When such an 
abutting pressure is applied, the pressure is also applied to the toner 
image on the latent image-bearing member, thus being liable to cause 
agglomeration. 
Further, in case where the surface of the latent image-bearing member is 
composed of a resin, the toner agglomerate contacting the latent 
image-bearing member is also liable to stick to the abutting surfaces of 
the latent image bearing member and the transfer device. 
When such phenomena occur, various difficulties can be encountered, such as 
a lack in a latent image formed and a transfer dropout, leading to 
formation of defective toner images. 
SUMMARY OF THE INVENTION 
A generic object of the present invention is to provide a toner for 
developing electrostatic images and an image forming method having solved 
the above-mentioned problems. 
A more specific object of the present invention is to provide a toner for 
developing electrostatic images showing excellent low-temperature 
fixability and anti-offset characteristic as well as excellent developing 
performance, which toner contains little volatile matter such as a 
residual monomer and is less liable to evolve an odor. 
Another object of the present invention is to provide an image forming 
method adopting a hot-roller fixing system capable of complying with 
shortening of the waiting time and higher speed of electrophotographic 
process. 
Another object of the present invention is to provide a toner for 
developing electrostatic images containing little volatile matter content, 
such as residual monomer, decomposition product, by-products, and residual 
solvent and having little odor. 
A further object of the present invention is to provide an image forming 
method accompanied with suppressed odor, such as ozone odor or toner odor. 
A further object of the present invention is to provide a toner for 
developing electrostatic images suitable for use in an image forming 
method adopting contact charging means and transfer means including an 
abutting transfer roller, which toner is free from sticking onto the 
surfaces of the charging means and the photosensitive member and can 
provide toner images of excellent image qualities continually for a long 
period. 
According to the present invention, there is provided a toner for 
developing an electrostatic image, comprising a binder resin, and a 
magnetic material and/or a colorant, wherein 
the binder resin (a) comprises a styrene resin polymerized in the presence 
of a poly-functional polymerization initiator, (b) provides a molecular 
weight distribution on a GPC chromatogram showing a maximum (P1) in a 
molecular weight range of 3.5.times.10.sup.3 -5.times.10.sup.4 and a 
maximum (P2) or shoulder in a molecular weight range of at least 
1.times.10.sup.5 and (c) contains 15 wt. % or less of a resin component in 
a molecular weight range of at most 3.times.10.sup.3 and 
the toner contains at most 100 ppm of styrene and benzaldehyde. 
According to another aspect of the present invention, there is provided an 
image forming method, comprising: 
charging an electrostatic latent image-bearing member by abutting a 
charging member supplied with a voltage to the electrostatic latent 
image-bearing member; 
exposing the charged electrostatic image-bearing member to light to form an 
electrostatic latent image thereon; 
developing the electrostatic latent image with the above-mentioned toner to 
form a toner image thereon; 
transferring the toner image onto a transfer-receiving material while 
pressing the transfer-receiving material by a transfer member supplied 
with a voltage against the toner image, and 
fixing the toner image transferred to the transfer-receiving material onto 
the transfer-receiving material by a hot roller having a core metal 
thickness of at most 1 mm. 
These and other objects, features and advantages of the present invention 
will become more apparent upon a consideration of the following 
description of the preferred embodiments of the present invention taken in 
conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION 
The binder resin in the toner according to the present invention is 
characterized by showing a molecular weight distribution on a chromatogram 
of GPC (gel permeation chromatography) showing a peak (maximum) in a 
molecular weight region of 3.5.times.10.sup.3 -5.times.10.sup.4 and a peak 
(maximum) or shoulder in a molecular weight region of at least 
1.times.10.sup.5. Herein, the shoulder means a point on a GPC chromatogram 
which provides an extreme point on a curve given by differentiating the 
GPC chromatogram. 
The maximum in the molecular weight region of 3.5.times.10.sup.3 
-5.times.10.sup.4 provides a toner showing a good fixability and a good 
pulverizability in a pulverization step for providing the toner, and the 
maximum or shoulder in the molecular weight region of at least 
1.times.10.sup.5 provides a good anti-offset characteristic. 
A resin component in the molecular weight range of at least 
1.times.10.sup.5 and providing a maximum or shoulder in the range provides 
a better anti-offset characteristic if it is contained in a larger 
proportion, but an excess thereof can hinder the fixing performance. In 
the molecular weight distribution according to GPC, the resin component in 
the molecular weight range of at least 1.times.10.sup.5 may preferably be 
5-50 wt. %, more preferably 10-50 wt. %. Below 5 wt. %, good anti-offset 
characteristic cannot be achieved in some cases, and it becomes difficult 
to prevent toner flowout through a cleaning member provided to a fixing 
device. On the other hand, in excess of 50 wt. %, the liability of 
impairing the fixability is pronounced. In order to enhance the 
anti-offset characteristic while retaining the fixability, it is preferred 
to shift the maximum or shoulder in the molecular weight distribution 
toward a higher-molecular weight side and provide the resin component with 
a higher molecular weight. 
In order to obtain such a high molecular weight resin component, it is 
preferred to effect polymerization in the presence of a polyfunctional 
polymerization initiator. The polyfunctional polymerization initiator may 
preferably have at least three functional groups generating radicals, more 
preferably four or more functional groups. According to our study, a very 
strong internal friction acts during melt-kneading for toner production, 
and a large shearing force is applied to the polymer, thus causing 
severance of the polymer components to provide a tendency of lowering the 
molecular weight of the high molecular weight component as a whole. As a 
result, the binder resin in the toner is liable to have a molecular weight 
distribution which is lower as a whole than that of the binder resin as a 
starting material of the toner, so that the molecular weight distribution 
of the binder resin as the toner material does not completely correspond 
to the anti-offset characteristic of the resultant toner. 
According to our study, if the number of radical-generating functional 
groups in the poly-functional polymerization initiator is increased, the 
molecular weight distribution of the binder resin as the toner material 
can be shifted to a higher molecular weight side and the molecular 
severance of the high-molecular weight component during melt-kneading for 
toner production cannot be readily caused, so that a resin component 
having a molecular weight of at most 3.times.10.sup.3 cannot be readily 
formed. As a result, the resultant toner is provided with a better 
anti-offset characteristic than a toner obtained by using a starting 
binder resin produced by using a polymerization initiator having less 
functional groups. It is also possible to reduce the residual monomer 
content in the starting binder resin by using a poly-functional 
polymerization initiator having a larger number of functional groups. 
The presence of a maximum in the molecular weight range of 
3.5.times.10.sup.3 -5.times.10.sup.4 in the GPC molecular weight of 
3.5.times.10.sup.3 -5.times.10.sup.4 in the GPC molecular weight 
distribution is preferred in view of the toner fixability and 
pulverizability in the pulverization step for toner production. The 
position of the maximum at a lower molecular weight side in the molecular 
weight distribution favors a lower temperature fixation. In view of 
anti-blocking characteristic, it is preferred that the maximum is present 
in the molecular weight region of 5.times.10.sup.3 5.times.10.sup.4. A 
low-molecular weight component having a molecular weight of below 
5.times.10.sup.3 is liable to adversely affect the developing performance, 
etc. If the maximum is present at a molecular weight below 
5.times.10.sup.3 and the amount of such a low-molecular weight component 
is increased, the anti-offset characteristic is adversely affected, and 
several difficulties are liable to be encountered, such as occurrence of 
blocking, occurrence of toner sticking onto the drum surface, and 
occurrence of melt-sticking onto the inside of toner production apparatus. 
Further, the toner can stick to a toner-carrying member (developing 
sleeve) or triboelectricity-imparting member (coating blade or coating 
roller) to lower the triboelectricity-imparting ability, thus impairing 
the developing performance. If the position of the lower molecular weight 
side maximum is shifted beyond a molecular weight of 5.times.10.sup.4 a 
poor fixability results. 
The component having a molecular weight of at most 5.times.10.sup.4 favors 
the fixability and may preferably occupy 30-95 wt. %, further preferably 
40-90 wt. %, in the molecular weight distribution. Below 30 wt. %, it is 
difficult to obtain good fixability, and poor pulverizability is liable to 
result in the pulverization step for toner production. On the other hand, 
in excess of 95 wt. %, it becomes difficult to obtain a sufficient 
anti-offset characteristic. 
The content of a low-molecular weight resin component providing a maximum 
in the molecular weight range of 3.5.times.10.sup.3 -5.times.10.sup.4 is 
increased in order to provide a good low-temperature fixability. 
Accordingly, the residual monomer content and residual by-products at the 
time of synthesizing the resin component greatly affect the residual 
monomer content and by-products in the total resin. 
If the reduction in residual monomer content in the toner is aimed at 
simply by increasing the polymerization initiator amount and controlling 
the production conditions so as to reduce the residual monomer content in 
the low-molecular weight resin component, the molecular weight 
distribution of the low-molecular weight resin component becomes broad and 
the content of a resin component having a molecular weight of at most 
3.times.10.sup.3 corresponding to the foot of the low-molecular weight 
component peak is increased, thus being liable to result in a low toner 
chargeability and a lowering in image density. 
The resin component having a molecular weight of at most 3.times.10.sup.3 
may preferably be at most 15%, more preferably at most 13%, further 
preferably at most 10%. 
It has been discovered preferable that the low-molecular weight resin 
component providing a maximum in the molecular weight range of 
3.5.times.10.sup.3 5.times.10.sup.4 is prepared by polymerization in the 
presence of at least two different polymerization initiators including a 
polymerization initiator A having a longer half-life and a polymerization 
initiator B having a shorter half-life and under a condition providing 
half-lives .tau..sub.A and .tau..sub.B, respectively, of the 
polymerization initiators at the polymerization temperature satisfying a 
ratio .tau..sub.A /.tau..sub.B of at least 1.5 and is used to constitute a 
toner for developing electrostatic images for accomplishing the above 
objects. 
More specifically, according to a result of our extensive study, when the 
low-molecular weight resin component providing a maximum in the molecular 
weight region of 3.5.times.10.sup.3 -5.times.10.sup.4 is produced by 
polymerization in the presence of at least two different polymerization 
initiators including the polymerization initiators A and B described 
above, it is easy to provide a peak in the molecular weight distribution 
showing a sharp low-molecular weight side than the maximum and thus 
providing a content of at most 15% of the component having a molecular 
weight of at most 3.times.10.sup.3 As a result it is possible to provide a 
toner with sufficient developing performance and fixing characteristic as 
well as a reduced residual monomer content and less odor. 
It is further preferred that the polymerization temperature for producing 
the low-molecular weight resin component is in the range of 
75.degree.-145.degree. C., and the polymerization initiator B having a 
shorter half-life shows a half-life .tau..sub.B at the polymerization 
temperature of at least 0.1 hour, further preferably 0.5-10 hours. 
The ratio .tau..sub.A /.tau..sub.B between the half-lives .tau..sub.A and 
.tau..sub.B of the polymerization initiator A having a longer half-life 
and the polymerization initiator B having a shorter half-life may 
preferably be in the range of 2 to 5.times.10.sup.3. 
It is further preferred to adopt such a combination that the polymerization 
temperature for producing the low-molecular weight resin component is in 
the range of 75.degree.-145.degree. C. and, at the polymerization 
temperature, the polymerization initiator B having a shorter half-life 
shows a half-life .tau..sub.B of 0.5-3 hours and the polymerization 
initiator A having a longer half-life shows a half-life .tau..sub.A of 2 
to 60 hours providing a ratio .tau..sub.A /.tau..sub.B of 2 to 
5.times.10.sup.2. 
The amounts of the polymerization initiators A and B and the ratio 
therebetween may be determined in view of the molecular weight 
distribution of the resultant low-molecular weight resin component, the 
kinds of monomers therefor and the production conditions. The total amount 
of the polymerization initiators A and B may preferably be 0.1-5 wt. parts 
per 100 wt. parts of the polymerizable monomer(s) for synthesizing the 
low-molecular weight resin component providing a maximum in the molecular 
weight range of 3.5.times.10.sup.3 -5.times.10.sup.4. The ratio of the 
polymerization initiator A/the polymerization initiator B may be in the 
range of 0.01-100, preferably 0.1-10. 
It is preferred to use a polymerizable vinyl monomer as the polymerizable 
monomer for providing the low-molecular weight resin component (i.e., a 
low-molecular weight vinyl resin). The thus-produced vinyl resin may 
preferably comprise a styrene resin, preferred examples of which may 
include styrene homopolymer, styrene-acrylate copolymer, and 
styrene-methacrylate copolymer. 
By using the above-described method, it is possible to reduce the residual 
monomer content in the toner. According to the present invention, the 
content of styrene monomer and benzaldehyde is required to be at most 100 
ppm in the toner. It is preferred that the benzaldehyde content is at most 
10 ppm as it evolves a peculiar and strong odor. The residual styrene 
monomer content may preferably be at most 50 ppm. In case where an acrylic 
monomer ((meth)acrylate or (meth)acrylic acid) is used as a comonomer, the 
residual acrylic monomer content may preferably be at most 30 ppm. 
By reducing the residual monomer and by-produced benzaldehyde contents, not 
only the occurrence of odor is suppressed during a copying operation but 
also the so-called filming or melt-sticking is also suppressed as the 
sticking of toner particles onto the photosensitive member becomes 
difficult. The filming or melt-sticking has been liable to occur 
particularly in an apparatus including a contact charging and/or a contact 
transfer device, but the phenomenon can be suppressed by reducing the 
residual monomer and benzaldehyde contents in the toner. 
We have also found that the reduced residual monomer content leads to 
reduction of toner particles and silica powder attached to a transfer 
roller and reduction of free silica accumulated on a stay in a developing 
device. 
The reason for the above phenomena has not been fully clarified as yet. 
However, it may be considered that the residual monomer contributes to 
isolation of the silica powder from the toner particles, and the reduction 
of the residual monomer has led to the isolation of the silica powder. 
The determination of the residual monomer and benzaldehyde content may be 
performed by gas chromatography, e.g., in the following manner. 
2.55 mg of N,N-dimethylformamide (DMF) is used as the internal standard and 
100 ml of acetone is added thereto to form a solvent containing the 
internal standard. Then, 400 mg of a toner sample is dissolved in a 
portion of the solvent to form a 10 ml solution. The solution is then 
subjected to 30 min. of ultrasonic vibration, followed by 1 hour of 
standing and filtration through a 0.5 .mu.m-filter. Then, 4 .mu.l of the 
sample solution is injected to a gas chromatograph. 
The conditions for the gas chromatography may include the following. 
Capillary column (30 m.times.0.249 mm, internal surface thereof being 
coated with a 0.25 .mu.m-thick layer of a separating agent (DBWAX, mfd. by 
J & W Scientific, U.S.A.)) 
Detector: FID (flame ionization detector), nitrogen pressure: 0.45 
kg/cm.sup.2. 
Injection temp.: 200.degree. C., Detector temp.: 200.degree. C. The column 
temperature is raised from 50.degree. C. at a rate of 5.degree. C./min. 
for 30 min. 
Preparation of calibration curve. 
Standard samples obtained by adding the objective monomer in varying 
amounts into the DMF-acetone solution in the same amount as the sample 
solution are similarly subjected to gas chromatography to determine the 
weight ratio/areal ratio between the monomer and the internal standard DMF 
with respect to the standard samples containing varying amounts of the 
objective monomer. 
Alternatively, the measurement by gas chromatography may be performed in a 
similar manner as above by using toluene as the internal standard and 
tetra-hydrofuran as the solvent. 
The polymerization initiator used in the present invention may be an 
ordinary oil-soluble initiator, examples of which may include: peroxide 
initiators, such as acetylcyclohexylsulfonyl peroxide, isobutyryl 
peroxide, diisopropyl peroxydicarbonate, 2-ethylhexyl peroxydicarbonate, 
2,4-dichlorobenzoyl peroxide, t-butyl peroxypivarate, 
3,5,5-trimethylhexanoyl peroxide, octanoyl peroxide, octanoyl peroxide, 
decanoyl peroxide, lauroyl peroxide, stearoyl peroxide, propionyl 
peroxide, succinic acid peroxide, acetyl peroxide, t-butyl 
peroxy-2-ethylhexanoate, benzoyl peroxide, parachlorobezoyl peroxide, 
t-butyl peroxyisobutyrate, t-butyl peroxymaleic acid, t-butyl 
peroxylaurate, cyclohexanone peroxide, t-butyl-peroxyisopropylcarbonate, 
2,5-dimethyl-2,5-dibenzoyl-peroxyhexane, t-butyl peroxyacetate, t-butyl 
peroxybenzoate, diisobutyl diperoxyphthalate, methyl ethyl ketone 
peroxide, dicumyl peroxide, 2,5-dimethyl-2,5-di-t-butyl-peroxyhexane, 
t-butyl cumyl peroxide, t-butyl hydroperoxide, di-t-butyl peroxide, 
2,5-dimethyl-2,5-di-t-butylperoxyhexane, diisopropylbenzene hydroperoxide, 
paramenthane hydroperoxide, pinane hydroperoxide, 
2,5-dimethylhexane-2,5-dihydroperoxide, and cumeme hydroperoxide; and 
azo-type initiators, such as 2,2'-azobisisobutyronitrile, 
1,1'-azobiscyclohexane-1-carbonitrile, 
2,2'-azobis-4-methoxy-2,4-dimethyl-valeronitrile, and 
2,2'-azobis-2,4-dimethylvaleronitrile. These initiators may be used 
singly, or in combination of two or more species, or in combination with a 
poly-functional radical polymerization initiator, as desired. 
In order to produce a high-molecular weight styrene resin component which 
is preferably used in the present invention, it is preferred to use a 
poly-functional polymerization initiator, by which it is possible to 
produce a higher molecular weight styrene resin component providing a 
satisfactory anti-offset characteristic. 
Examples of poly-functional polymerization initiator usable in the present 
invention may include: di-functional radical polymerization initiators, 
such as 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane, 
1,1-bis(t-butylperoxy)cyclohexane, 
1,4-bis(t-butyl-peroxycarbonyl)cyclohexane, 2,2-bis(t-butylperoxy)octane, 
n-butyl-4,4-bis(t-butylperoxy)valerate, 2,2-bis(t-butylperoxy)butane, 
1,3-bis(t-butylperoxyisopropyl)benzene, 
2,5-dimethyl-2,5-di(t-butylperoxy)hexane, 
2,5-dimethyl-2,5-di(t-butylperoxy)hexane, 
2,5-dimethyl-2,5-di(benzoylperoxy)hexane, di-t-butyl-diperoxyisophthalate, 
2,2-bis(4,4-di-t-butylperoxycyclohexyl)propane, di-t-butyl 
peroxy-.alpha.-methylsuccinate, di-t-butyl peroxydimethylglutarate, 
di-t-butyl peroxyhexahydroterephthalate, di-t-butyl peroxyazelate, 
2,5-dimethyl-2,5-di(t-butylperoxy)hexane, diethylene 
glycol-bis(t-butylperoxycarbonate), and di-t-butyl peroxytrimethyladipate; 
tri-functional radical polymerization initiators, such as 
tris(t-butylperoxy)triazine, and vinyl-tris(t-butylperoxy)silane; and 
other poly-functional radical polymerization initiators, such as 
2,2-bis(4,4-di-t-butylperoxycyclohexyl)propane, copolymerizates of 
t-butylperoxyallylcarbonate (e.g., "Hyper B" and "Hyper G" series 
available from Nippon Yushi K.K.), and copolymerizates of 
t-butylperoxymaleic acid. 
These poly-functional polymerization initiators may be used singly or in 
combination of two or more species, or in combination with a 
mono-functional polymerization initiator, as desired. The poly-functional 
polymerization initiator may be used in a proportion of 0.01-5 wt. %, 
preferably 0.05-3 wt. %, of the monomer(s) giving a high-molecular weight 
styrene resin providing a maximum or shoulder in the molecular weight 
region of at least 1.times.10.sup.5. 
The molecular weight (distribution) of a binder resin may be measured based 
on a chromatogram obtained by GPC (gel permeation chromatography) in the 
following manner. 
In the GPC apparatus, a column is stabilized in a heat chamber at 
40.degree. C., tetrahydrofuran (THF) solvent is caused to flow through the 
column at that temperature at a rate of 1 ml/min., and 50-200 .mu.l of a 
GPC sample solution adjusted at a concentration of 0.05-0.1 wt. % is 
injected. The identification of sample molecular weight and its molecular 
weight distribution is performed based on a calibration curve obtained by 
using several monodisperse polystyrene samples and having a logarithmic 
scale of molecular weight versus count number. The standard polystyrene 
samples for preparation of a calibration curve may be available from, 
e.g., Pressure Chemical Co. or Toso K.K. It is appropriate to use at least 
10 standard polystyrene samples inclusive of those having molecular 
weights of, e.g., 6.times.10.sup.2, 2.1.times.10.sup.3, 4.times.10.sup.3, 
1.75.times.10.sup.4, 5.1.times.10.sup.4, 1.1.times.10.sup.5, 
3.9.times.10.sup.5, 8.6.times.10.sup.5, 2.times.10.sup.6 and 
4.48.times.10.sup.6. The detector may be an RI (refractive index) 
detector. For accurate measurement, it is appropriate to constitute the 
column as a combination of several commercially available polystyrene gel 
columns in order to effect accurate measurement in the molecular weight 
range of 10.sup.3 -4.times.10.sup.6. A preferred example thereof may be a 
combination of .mu.-styragel 500, 10.sup.3, 10.sup.4 and 10.sup.5 
available from Waters Co; a combination of Shodex KF-801, 802, 803, 804 
and 805 available from Showa Denko K.K.; or a combination of TSK gel 
G1000H, G2000H, G2500H, G3000H, G4000H, G5000H, G6000H, G7000H, and GMH 
available from Toso K.K. 
In the present invention, the weight percentage of each resin component in 
the total binder resin may for example be obtained in the following 
manner. 
The weight percentage of the resin component in the molecular weight range 
of at most 3.times.10.sup.3 is measured as an areal percentage of a 
component in a molecular weight range of 4.times.10.sup.2 
-3.times.10.sup.3 with respect to the total area of component in a 
molecular weight range of 4.times.10.sup.2 or larger, respectively based 
on the GPC chromatogram of the binder resin (THF-soluble). In case where 
any THF-insoluble is present, the weight percentage of the respective 
components measured with respect to the THF-soluble within the toner 
binder resin is corrected by taking the THF-insoluble into consideration. 
For example, the above-obtained areal percentage of the component in the 
molecular weight range of 4.times.10.sup.2 -3.times.10.sup.3 is multiplied 
by the THF-soluble percentage for correction in order to obtain the weight 
percentage of the component in the total binder resin. 
An example of such a GPC chromatogram is shown in FIG. 2. 
The binder resin of the present invention may preferably comprise a styrene 
polymer or a styrene copolymer. Herein, the styrene polymer means a 
polymer (homopolymer or copolymer) of only one or more of styrene-type 
monomers, i.e., styrene and its derivatives, and the styrene copolymer 
means a copolymer of a styrene-type monomer and another comonomer. 
More specifically, examples of the styrene-type monomer may include: 
styrene; and styrene derivatives, such as o-methylstyrene, 
m-methylstyrene, p-methylstyrene, p-methoxystyrene, p-phenylstyrene, 
p-chlorostyrene, 3,4-dichlorostyrene, p-ethylstyrene, 2,4-dimethylstyrene, 
p-n-butylstyrene, p-tertbutylstyrene, p-n-hexylstyrene, p-n-octylstyrene, 
p-n-nonylstyrene, p-n-decylstyrene, and p-n-dodecylstyrene. 
Examples of the comonomer for providing the styrene copolymer may include 
ethylenically unsaturated monoolefins, such as ethylene, propylene, 
butylene, and isobutylene; unsaturated polyenes, such as butadiene; 
halogenated vinyls, such as vinyl chloride, vinylidene chloride, vinyl 
bromide, and vinyl fluoride; vinyl esters, such as vinyl acetate, vinyl 
propionate, and vinyl benzoate; methacrylates, such as methyl 
methacrylate, ethyl methacrylate, propyl methacrylate, n-butyl 
methacrylate, isobutyl methacrylate, n-octyl methacrylate, dodecyl 
methacrylate, 2-ethylhexyl methacrylate, stearyl methacrylate, phenyl 
methacrylate, dimethylaminoethyl methacrylate, and diethylaminoethyl 
methacrylate; acrylates, such as methyl acrylate, ethyl acrylate, n-butyl 
acrylate, isobutyl acrylate, propyl acrylate, n-octyl acrylate, dodecyl 
acrylate, 2-ethylhexyl acrylate, stearyl acrylate, 2-chloroethyl acrylate, 
and phenyl acrylate, vinyl ethers, such as vinyl methyl ether, vinyl ethyl 
ether, and vinyl isobutyl ether; vinyl ketones, such as vinyl methyl 
ketone, vinyl hexyl ketone, and methyl isopropenyl ketone; N-vinyl 
compounds, such as N-vinylpyrrole, N-vinylcarbazole, N-vinylindole, and 
N-vinyl pyrrolidone; vinylnaphthalenes; acrylic acid derivatives or 
methacrylic acid derivatives, such as acrylonitrile, methacryronitrile, 
and acrylamide; esters of .alpha.,.beta.-unsaturated acids and diesters of 
dibasic acids. These comonomers may be used singly or in combination of 
two or more species. 
The binder resin used in the present invention can include a crosslinking 
structure, as desired, obtained by using a crosslinking monomer, examples 
of which are enumerated hereinbelow. 
Aromatic divinyl compounds, such as divinylbenzene and divinylnaphthalene; 
diacrylate compounds connected with an alkyl chain, such as ethylene 
glycol diacrylate, 1,3-butylene glycol diacrylate, 1,4-butanediol 
diacrylate, 1,5-pentanediol diacrylate, 1,6-hexanediol diacrylate, and 
neopentyl glycol diacrylate, and compounds obtained by substituting 
methacrylate groups for the acrylate groups in the above compounds; 
diacrylate compounds connected with an alkyl chain including an ether 
bond, such as diethylene glycol diacrylate, triethylene glycol diacrylate, 
tetraethylene glycol diacrylate, polyethylene glycol #400 diacrylate, 
polyethylene glycol #600 diacrylate, dipropylene glycol diacrylate and 
compounds obtained by substituting methacrylate groups for the acrylate 
groups in the above compounds; diacrylate compounds connected with a chain 
including an aromatic group and an ether bond, such as 
polyoxyethylene(2)-2,2-bis(4-hydroxyphenyl)propanediacrylate, 
polyoxyethylene(4)-2,2-bis(4-hydroxyphenyl)propanediacrylate, and 
compounds obtained by substituting methacrylate groups for the acrylate 
groups in the above compounds; and polyester-type diacrylate compounds, 
such as one known by a trade name of MANDA (available from Nihon Kayaku 
K.K.). Polyfunctional crosslinking agents, such as pentaerythritol 
triacrylate, trimethylethane triacrylate, tetramethylolmethane 
tetracrylate, oligoester acrylate, and compounds obtained by substituting 
methacrylate groups for the acrylate groups in the above compounds; 
triallyl cyanurate and triallyl trimellitate. 
These crosslinking agents may preferably be used in a proportion of about 
0.01-5 wt. parts, particularly about 0.03-3 wt. parts, per 100 wt. parts 
of the other vinyl monomer components. 
In the present invention, the high-molecular weight styrene resin 
polymerized in the presence of a poly-functional polymerization initiator 
and the low-molecular weight resin component providing a maximum in the 
molecular weight range of 3.5.times.10.sup.3 -5.times.10.sup.4 may be 
blended in a weight ratio of 10-70:90-30, preferably 20-60:80-40. 
The above-described binder resin can be blended with another resinous 
compound as described below in an amount less than the binder resin within 
an extent not adversely affecting the effect of the present invention. 
Examples of such another resinous compound may include: silicone resin, 
polyester, polyurethane, epoxy resin, polyvinyl butyral, rosin, modified 
rosin, terpene resin, phenolic resin, aliphatic or alicyclic hydrocarbon 
resin such as low-molecular polyethylene or low-molecular weight 
polypropylene, aromatic petroleum resin, chlorinated paraffin, paraffin 
wax, etc. 
The binder resin used in the present invention may be obtained through 
polymerization, such as bulk polymerization, solution polymerization, 
suspension polymerization, or emulsion polymerization. 
The toner for developing electrostatic images according to the present 
invention can further contain a charge control agent, as desired, for 
further stabilizing the chargeability. 
Charge control agents known in the art at present may include the 
following. 
Examples of the negative charge control agent may include: organic metal 
complexes and chelate compounds inclusive of monoazo metal complexes 
acetylacetone metal complexes, and organometal complexes of aromatic 
hydroxycarboxylic acids and aromatic dicarboxylic acids. Other examples 
may include: aromatic hydroxycarboxylic acids, aromatic mono- and 
poly-carboxylic acids, and their metal salts, arthydrides and esters, and 
phenol derivatives, such as bisphenols. 
Examples of the positive charge control agents may include: nigrosine and 
modified products thereof with aliphatic acid metal salts, etc,, onium 
salts inclusive of quarternary ammonium salts, such as tributylbenzyl 
ammonium 1-hydroxy-4-naphtholsulfonate and tetrabutylammonium 
tetrafluoroborate, and their homologous inclusive of phosphonium salts, 
and lake pigments thereof; triphenylmethane dyes and lake pigments thereof 
(the laking agents including, e.g., phosphotungstic acid, phosphomolybdic 
acid, phosphotungsticmolybdic acid, tannic acid, lauric acid, gallic acid, 
ferricyanates, and ferrocyanates); higher aliphatic acid metal salts; 
acetylacetone metal complexes; diorganotin oxides, such as dibutyltin 
oxide, dioctyltin oxide and dicyclohexyltin oxide; and diorganotin 
borates, such as dibutyltin borate, dioctyltin borate and dicyclohexyltin 
borate. These may be used singly or in mixture of two or more species. 
Among these, nigrosine compounds and organic quarternary ammonium salts 
are particularly preferred. 
It is preferred to use the toner according to the present invention 
together with silica fine powder blended therewith in order to improve the 
charge stability, developing characteristic and fluidity. 
The silica fine powder used in the present invention provides good results 
if it has a specific surface area of 30 m.sup.2 /g or larger, preferably 
50-400 m.sup.2 /g, as measured by nitrogen adsorption according to the BET 
method. The silica fine powder may be added in a proportion of 0.01-8 wt. 
parts, preferably 0.1-5 wt. parts, per 100 wt. parts of the toner. 
For the purpose of being provided with hydrophobicity and/or controlled 
chargeability, the silica fine powder may well have been treated with a 
treating agent, such as silicone varnish, modified silicone varnish, 
silicone oil, modified silicone oil, silane coupling agent, silane 
coupling agent having functional group or other organic silicon compounds. 
It is also preferred to use two or more treating agents in combination. 
Other additives may be added as desired, inclusive of: a lubricant, such as 
polytetrafluoroethylene, zinc stearate or polyvinylidene fluoride, of 
which polyvinylidene fluoride is preferred; an abrasive, such as cerium 
oxide, silicon carbide or strontium titanate, of which strontium titanate 
is preferred; a flowability-imparting agent, such as titanium oxide, 
aluminum oxide, hydrophobic titanium oxide or hydrophobic aluminum oxide, 
of which a hydrophobic one is preferred; an anti-caking agent, and an 
electroconductivity-imparting agent, such as carbon black, zinc oxide, 
antimony oxide, or tin oxide. It is also possible to use a small amount of 
white or black fine particles having a polarity opposite to that of the 
toner particles as a development characteristic improver. 
It is also a preferred embodiment of the present invention to incorporate 
within the toner a waxy substance, such as low-molecular weight 
polyethylene, low-molecular weight polypropylene, microcrystalline wax, 
carnauba wax, sasol wax, or paraffin wax in an amount of 0.5-10 wt. parts 
per 100 wt. parts of the binder resin. 
The toner according to the present invention can be mixed with carrier 
powder to be used as a two-component developer. In this instance, the 
toner and the carrier powder may be mixed with each other so as to provide 
a toner concentration of 0.1-50 wt. %, preferably 0.5-10 wt. %, further 
preferably 3-10 wt. %. 
The carrier used for this purpose may be a known one, examples of which may 
include: powder having magnetism, such as iron powder, ferrite powder, and 
nickel powder and carriers obtained by coating these powders with a resin, 
such as a fluorine-containing resin, a vinyl resin or a silicone resin. 
The toner according to the present invention can be constituted as a 
magnetic toner containing a magnetic material in its particles. In this 
case, the magnetic material also functions as a colorant. Examples of the 
magnetic material may include: iron oxide, such as magnetite, hematite, 
and ferrite; metals, such as iron, cobalt and nickel, and alloys of these 
metals with other metals, such as aluminum, cobalt, copper, lead, 
magnesium, tin, zinc, antimony, beryllium, bismuth, cadmium, calcium, 
manganese, selenium, titanium, tungsten and vanadium; and mixtures of 
these materials. 
The magnetic material may have an average particle size of 0.1-2 .mu.m, 
preferably 0.1-0.5 .mu.m. The magnetic material may preferably show 
magnetic properties under application of 10 kilo-Oersted, inclusive of: a 
coercive force of 20-150 Oersted, a saturation magnetization of 50-200 
emu/g, and a residual magnetization of 2-20 emu/g. The magnetic material 
may be contained in the toner in a proportion of about 20-200 wt. parts, 
preferably 40-150 wt. parts, per 100 wt. parts of the resin component. 
The toner according to the present invention can contain a colorant which 
may be an appropriate pigment or dye. Examples of the pigment may include: 
carbon black, aniline black, acetylene black, Naphthol Yellow, Hansa 
Yellow, Rhodamine Lake, Alizarin Lake, red iron oxide, Phthalocyanine 
Blue, and Indanthrene Blue. These pigments are used in an amount 
sufficient to provide a required optical density of the fixed images, and 
may be added in a proportion of 0.1-20 wt. parts, preferably 2-10 wt. 
parts, per 100 wt. parts of the binder resin. Examples of the dye may 
include: azo dyes, anthraquinone dyes, xanthene dyes, and methine dyes, 
which may be added in a proportion of 0.1-20 wt. parts, preferably 1-10 
wt. parts, per 100 wt. parts of the binder resin. 
The toner according to the present invention may be prepared through a 
process including: sufficiently blending the binder resin, the wax, a 
metal salt or metal complex, a colorant, such as pigment, dye and/or a 
magnetic material, and an optional charge control agent and other 
additives, as desired, by means of a blender such as a Henschel mixer or a 
ball mill, melting and kneading the blend by means of hot kneading means, 
such as hot rollers, a kneader or an extruder to cause melting of the 
resinous materials and disperse or dissolve the magnetic material, pigment 
or dye therein, and cooling and solidifying the kneaded product, followed 
by pulverization and classification. 
The thus obtained toner may be further blended with other external 
additives, as desired, sufficiently by means of a mixer such as a Henschel 
mixer to provide a toner for developing electrostatic images. 
An embodiment of the image forming method according to the present 
invention will be described with reference to FIG. 1 illustrating an 
apparatus therefor. 
The apparatus includes a rotating drum-type electrostatic image-bearing 
member (hereinafter referred to as "photosensitive member") 1, which 
basically comprises an electroconductive substrate 1b of, e.g., aluminum 
and a photoconductive layer 1a disposed on the outer surface thereof and 
rotates at a prescribed peripheral speed (process speed) in a clockwise 
direction as illustrated on the drawing. 
In this embodiment, the photosensitive member comprises an organic 
photoconductor (OPC) and is constituted as a photosensitive drum having an 
outer diameter of 30 mm. 
The apparatus also includes a charging roller 2 which comprises a metal 
core 2b and an electroconductive elastomer layer 2a disposed on the outer 
surface thereof. The charging roller 2 is pressed against the 
photosensitive member 1 at a certain pressing force and rotates following 
the rotation of the photosensitive member 1. The charging roller 2 is 
supplied with a voltage from a charging bias supply 3 and thereby changes 
the surface of the photosensitive member 1 to a prescribed polarity and 
potential. Then, the photosensitive member is illuminated with imagewise 
exposure light 4 to form an electrostatic latent image thereon, which is 
then developed into a toner image by a developing device 5. 
In this embodiment, the charging roller 2 has an outer diameter of 16 mm, 
and the electroconductive rubber 2a comprises styrene-butadiene rubber 
(SBR) surfaced with a resin principally comprising a nylon resin. The 
charging roller 2 has a hardness of 64 degrees (ASKER-C). A prescribed 
voltage is applied to the core metal 2b of the charging roller 2 from a DC 
power supply 3 which can be superposed with an AC voltage. 
The charging roller 2 may be abutted against the photosensitive member 1 at 
a pressure of 5-500 g/cm.sup.2, preferably 10-100 g/cm, and supplied with 
a DC voltage of 200 V to 1.5 kV in terms of an absolute value. 
The AC voltage need not be superposed but, when used, may preferably be 
adjusted to a peak-to-peak voltage of 500-5000 V and a frequency of 
50-3000 Hz. 
On the charging roller 2, a portion of the toner or external additive to 
the toner having slipped by a cleaning blade 9 is liable to be deposited 
and accumulated, thus resulting in charging irregularity and fog. 
Accordingly, the charging roller 2 may preferably be equipped with a 
cleaning mechanism 2c which contacts the charging roller 2 at a 
penetration of preferably at least 0.5 mm only in operation thereof. 
The photosensitive drum 1 after the transfer of a toner image is generally 
cleaned by a cleaning member such as a cleaning blade or roller to remove 
the residual toner or other dirt, thus resulting in a clean surface to be 
again subjected to image formation thereon. 
Such a cleaning operation may be performed in parallel and simultaneously 
with the charging, developing and/or transfer operation in 
electrophotography. 
The apparatus further includes a transfer roller 6 which basically 
comprises an electroconductive elastomer layer 6a surfacing a core metal 
6b. The transfer roller 6 is pressed against the photosensitive member 1 
at a certain pressing force and rotated at a peripheral speed which is 
equal to or different from that of the photosensitive member. A 
transfer-receiving material 8 is conveyed between the photosensitive 
member 1 and the transfer roller 6 and simultaneously therewith the 
transfer roller 6 is supplied with a bias voltage of a polarity opposite 
to that of the toner from a transfer bias voltage supply 7, whereby the 
toner image on the photosensitive member is transferred onto the surface 
of the transfer-receiving material. 
In this embodiment, the transfer roller 6 has an outer diameter of 16 mm, 
and the electroconductive elastomer layer 6a comprises foamed 
ethylene-propylene-diene terpolymer (EPDM). The transfer roller 6 has a 
hardness of 30 degrees (ASKER-C). The transfer roller 6 may preferably be 
abutted against the photosensitive member 1 at a pressure of at least 0.5 
g/cm, more preferably 3-100 g/cm, in terms of a linear pressure [g/cm] (=a 
total force applied to the transfer roller [g]/length of abutment [cm]). 
The transfer roller 6 may be supplied with a DC voltage of 3.5-7.0 KV in 
terms of an absolute value. 
On the transfer roller 6, a portion of the toner and toner additive 
scattered from the developing device to the photosensitive member or 
having slipped by the cleaning blade 9 is liable to be deposited and 
accumulated, thus resulting in difficulties, such as transfer dropout, 
transfer failure or transfer irregularity. 
Therefore, it is preferred to apply a voltage of a polarity opposite to 
that used for transfer to the transfer-receiving member 8 in order to 
clean the transfer roller. The DC voltage applied for the purpose may 
preferably be 3.5-7 kV in terms of an absolute value. 
The transfer roller 6 may further preferably be equipped with a cleaning 
mechanism 6c. 
Then, the transfer-receiving material 8 carrying a toner image is conveyed 
to a fixing device 11 which basically comprises a heating roller 11a 
enclosing a halogen heater and an elastomeric pressure roller 11b pressed 
against the heating roller 11a. Being passed between the rollers 11a and 
11b, the toner image is fixed onto the transfer-receiving material and is 
outputted as an image product. 
More specifically, referring to FIG. 2, the heating roller 11a of the 
heating device comprises a core metal 14 within which a heater 12 is 
enclosed, and the core metal 14 is surfaced with a resin layer 13. The 
pressure roller 11b comprises a core metal 16 surfaced with an elastomer 
layer 15. 
In the image forming method according to the present invention, it is 
preferred to minimize the heat capacity of the heating roller 11a so as to 
shorten the waiting time, and the core metal 14 of the heating roller 11a 
may preferably be formed in a thickness of at most 1 mm. The core metal 14 
comprises any metal or alloy as far as appropriate strength and stability 
are ensured but may preferably comprise carbon steel. 
The heating roller 11a may preferably be coated with a layer 13 of a resin 
showing good releasability, examples of which may include 
fluorine-containing resin, silicone resin and amide resin. 
The fixing roller may preferably be equipped with a cleaning mechanism, 
such as a cleaning web or a cleaning pad, of which a cleaning web 11c as 
shown is preferred. 
The image forming method according to the present invention may suitably be 
applied in a system wherein a transfer-receiving material is passed 
between the rollers 11a and 11b at a speed (process speed) of at least 150 
mm/sec (corresponding to a feed rate of 22.5 sheets of A4 vertical 
size/min.). 
In this embodiment, the surface of the photosensitive member 1 is cleaned 
with a cleaning mechanism 9 provided with a cleaning blade pressed in a 
counter direction against the photosensitive member to remove the dirt 
such as residual toner after the transfer, and is charged-removed by a 
discharging exposure device 10 to be subjected to repetitive image 
formation. 
Hereinbelow, the present invention will be described more specifically 
based on Examples which however should not be construed to limit the scope 
of the invention in any way. In the following description, "part(s)" used 
to describe a formulation are all by weight. 
Synthesis Example 1 
A four-necked flask (polymerization vessel) equipped with a 
nitrogen-introducing pipe, a condenser, a stirrer and a thermometer was 
charged with 200 parts of deionized water, 80 parts of styrene, 20 parts 
of n-butyl acrylate and 0.40 part of tetra-functional 
1,4-bis(t-butylperoxycarbonyl)cyclohexane (HTP) as a poly-functional 
polymerization initiator, and the content was subjected to 25 hours of 
suspension polymerization at 90.degree. C. Thereafter, the product was 
cooled, washed with water and dried to obtain a high-molecular weight 
polymer, which is referred to as binder resin A and provided a molecular 
weight distribution by GPC showing a peak (P2) at a molecular weight of 
5.1.times.10.sup.5. 
Then, the above polymerization vessel was charged with 800 parts of xylene 
and heated under nitrogen stream and stirring to 140.degree. C. At the 
temperature, a mixture of 84 parts of styrene, 16 parts of butyl acrylate, 
0.9 part of di-t-butyl peroxide (DTBP, half life=1.6 hours at 140.degree. 
C.) as a polymerization initiator B, and 0.2 part of p-methane 
hydroperoxide (half-life=5.0 hours at 140.degree. C.), was added dropwise 
in two hours by using a continuous dropping device, followed by 4 hours of 
polymerization to obtain a solution of a low-molecular weight polymer 
(binder resin B) which provided a molecular weight distribution by GPC 
showing a peak (P1) at a molecular weight of 1.0.times.10.sup.4. 
Into the above polymer solution (containing 70 parts of the binder resin 
B), 30 parts of binder resin A and 4 parts of low-molecular weight 
polypropylene (weight-average molecular weight (Mw)=about 10.sup.4) were 
dissolved and mixed in 4 hours under sufficient stirring at 100.degree. 
C., followed by removal of the solvent by vacuum distillation (about 20 
mmHg, about 40.degree. C., for 24 hours) and 1 hour of heating at 
80.degree. C. under vacuum (about 20 mmHg), to obtain toner binder resin 
I. 
Comparative Synthesis Example 1. 
The same polymerization vessel as used in Synthesis Example 1 was charged 
with 800 parts of xylene and heated under nitrogen stream and stirring to 
140.degree. C. At the temperature, a mixture of 84 parts of styrene, 16 
parts of butyl acrylate and 1.0 part of di-t-butyl peroxide (DTBP) as a 
polymerization initiator was added dropwise in 2 hours by using a 
continuous dropping device, followed by 4 hours of polymerization to 
obtain a solution of a low-molecular weight polymer (binder resin C) which 
provided a molecular weight distribution by GPC showing a peak (P1) at a 
molecular weight of 1.0.times.10.sup.4. 
With the above polymer solution, (containing 70 parts of the binder resin 
C), 30 parts of the binder resin A and 4 parts of the low-molecular weight 
polypropylene were mixed in the same manner as in Synthesis Example 1 to 
obtain a toner binder resin II. 
The toner binder resin II was further treated under vacuum at 80.degree. C. 
for 2 hours to obtain a toner binder resin III. 
Comparative Synthesis Example 2 
A binder resin E was prepared in the same manner as in Comparative Example 
1 except that the amount of di-t-butyl peroxide (DTBP, polymerization 
initiator) was increased to 1.5 parts. The binder resin E showed a peak 
(P1) at 0.9.times.10.sup.4. In the same manner as in Synthesis Example 1, 
30 parts of the binder resin A and 4 parts of the low-molecular weight 
polypropylene were mixed with the polymer solution containing 70 parts of 
the binder resin E, followed by removal of the solvent to obtain a toner 
binder resin IV. 
Comparative Synthesis Example 3 
A binder resin F was prepared through polymerization in the same manner as 
in preparation of the binder resin A in Synthesis Example 1 except that 
the polymerization initiator was changed to 0.2 part of 
2,2'-azobis(2,4-dimethylvaleronitrile) and the suspension polymerization 
was performed for 9 hours at 80.degree. C. The binder resin F provided a 
molecular weight distribution by GPC showing a peak (P2) at a molecular 
weight of 2.5.times.10.sup.5. 
Then, a solution of the binder resin B was prepared in the same manner as 
in Synthesis Example 1, and the solution containing 70 parts of the 
binder resin B was mixed with 30 parts of the binder resin F and 4 parts 
of the low-molecular weight polypropylene and treated under vacuum in a 
similar manner to obtain a toner binder resin V. 
Synthesis Example 2 
A binder resin G was prepared through polymerization in the same manner as 
in preparation of the binder resin A in Synthesis Example 1 except that 
the polymerization initiator was changed to 0.4 part of 
tris(t-butylperoxy)triazine and the suspension polymerization was 
performed for 8 hours at 80.degree. C. The binder resin G provided a 
molecular weight distribution by GPC showing a peak (P2) at a molecular 
weight of 6.0.times.10.sup.5. 
Then, a binder resin H was prepared through polymerization in the same 
manner as in preparation of the binder resin B in Synthesis Example 1 
except that a mixture of 85 parts of styrene, 15 parts of butyl acrylate, 
4.0 parts of DTBP (polymerization initiator B) and 1.0 part of 
2,5-dimethylhexane 2,5-dihydroperoxide (half-life at 140.degree. C.=30 
hours, polymerization initiator A) was added into 800 parts of xylene in 2 
hours, followed by 8 hours of polymerization. The binder resin H showed a 
peak (P1) at a molecular weight of 0.4.times.10.sup.4. 
Then, similarly as in Synthesis Example 1, 30 parts of the binder resin G 
and 4 parts of the low-molecular weight polypropylene were mixed with the 
polymer solution containing 70 parts of the binder resin H, followed by 
removal of the solvent to obtain a toner binder resin VI. 
Synthesis Example 3 
A four-necked flask (polymerization vessel) equipped with a 
nitrogen-introducing pipe, a condenser, a stirrer and a thermometer was 
charged with 200 parts of deionized water, 80 parts of styrene, 20 parts 
of n-butyl acrylate and 0.13 part of 
2,2-bis(4,4-tert-butylperoxycyclohexyl)propane as a poly-functional 
polymerization initiator, and the content was subjected to 25 hours of 
suspension polymerization at 90.degree. C. Thereafter, the product was 
cooled, washed with water and dried to obtain a high-molecular weight 
polymer, which is referred to as binder resin A and provided a molecular 
weight distribution by GPC showing a peak (P2) at a molecular weight of 
8.0.times.10.sup.5. 
Then, similarly as in Synthesis Example 1, a solution containing the binder 
resin B was prepared, and 30 parts of the binder resin J and 4 parts of 
the low-molecular weight polypropylene were mixed with the polymer 
solution containing 70 parts of the binder resin H, followed by removal of 
the solvent to obtain a toner binder resin VII. 
Comparative Synthesis Example 4 
______________________________________ 
Styrene 76 part(s) 
Butyl acrylate 23 part(s) 
Divinylbenzene 0.3 part(s) 
Di-tert-butyl peroxide 
0.8 part(s) 
______________________________________ 
The above ingredients were added dropwise in 4 hours to 200 parts of xylene 
heated to the reflux temperature, followed by 4 hours of polymerization 
under reflux of the xylene (138.degree.-144.degree. C.) to complete the 
polymerization and removal of the xylene by raising the temperature up to 
200.degree. C. under vacuum, thereby to obtain a resin VIII. 
Comparative Synthesis Example 5 
______________________________________ 
Styrene 73 part(s) 
Butyl acrylate 24 part(s) 
Di-tert-butyl peroxide 
0.8 part(s) 
______________________________________ 
The above ingredients were added dropwise in 2 hours to 200 parts of xylene 
heated 80.degree.-90.degree. C., followed by 6 hours of polymerization 
under reflux of the xylene to complete the polymerization and removal of 
the xylene by raising the temperature up to 200.degree. C. under vacuum, 
thereby to obtain a resin IX. 
EXAMPLE 1 
______________________________________ 
Binder resin VII 100 part(s) 
Triiron tetroxide 90 part(s) 
(average particle 
size = 0.2 .mu.m) 
Nigrosine 2 part(s) 
______________________________________ 
The above ingredients were sufficiently blended in a blender and 
melt-kneaded through a twin-screw kneading extruder set at 80.degree. C. 
The kneaded product was cooled, coarsely crushed by a cutter mill, finely 
pulverized by a pulverizer using jet air and classified by a 
multi-division classifier utilizing the Coanda effect, to obtain black 
fine powder (magnetic toner) having a weight-average particle size of 8.5 
.mu.m. 
100 parts of the resultant black fine powder (magnetic toner) and 0.6 part 
of positively chargeable hydrophobic dry-process silica were blended in a 
Henschel mixer to obtain a positively chargeable magnetic toner carrying 
the silica powder attached to the toner particles. 
0.2 g of the thus-obtained magnetic toner was dissolved in 20 ml of THF 
(tetrahydrofuran) and the solution was filtered to remove the magnetic 
material and other insoluble matter. Substantially no styrene resin 
remained insoluble in THF. The solution was subjected to GPC (gel 
permeation chromatography) measurement. The resultant GPC chromatogram is 
shown in FIG. 2 and the results are summarized in Table 1 appearing 
hereinafter. 
A succession image formation test of 20000 sheets of A4-size plain paper 
were performed by using an image forming apparatus shown in FIG. 1 under 
the following set of conditions: 
[Charging roller] 
abutting pressure against the photosensitive member: 50 g/cm, 
applied voltage: -1400 volts (DC) 
[Transfer roller] 
abutting pressure against the photosensitive member: 20 g/cm 
applied voltage: -6000 volts (DC) 
peripheral speed difference with the photosensitive member: 0 
Abutting pressure of the cleaning blade: 20 g/cm 
[Developing bias voltage] 
AC: Vpp=1300 volts, Vf=1800 Hz 
DC: Vdc=-210 volts 
Distance between the photosensitive member and the developer-carrying 
member: 300 .mu.m 
Hot fixing roller: comprising a 0.8 mm-thick core metal cylinder of carbon 
steel coated with a PTFE layer and containing two halogen lamps inside 
thereof, and the external surface thereof being regulated to a prescribed 
temperature (approximately 160.degree.-200.degree. C.). 
Pressure roller: comprising a 1 mm-thick core metal cylinder of carbon 
steel coated with a 2 mm-thick silicone rubber layer. 
Process speed: 200 mm/sec. 
As a result, even after copying of the 20000 sheets, images having 
excellent image qualities and high densities were obtained. 
Table 2 appearing hereinafter summarizes the results of evaluation of image 
density, fog, filming, dirt on the transfer roller, amount of silica 
accumulated on the stay of the developing device, residual monomer content 
in the toner and odor at the time of fixing. The odor at the time of 
fixing was evaluated as a relative test by 3 panelists. 
Then, the above image. forming apparatus was remodeled by removing the 
fixing device and used to form unfixed images on plain paper sheets. On 
the other hand, the removed fixing device was used as an external fixing 
device of a variable temperature-type to effect a fixing test and an 
offset test of the unfixed images. 
The external fixing device was adjusted to have a nip of 4.0 mm and a 
process speed of 200 mm/sec. The fixing test was performed at various 
controlled temperatures in the temperature range of 
100.degree.-250.degree. C. at a temperature increment of 5.degree. C. so 
as to fix the unfixed images. The resultant fixed images were rubbed with 
a lens cleaning paper ("Dasper", mfd. by Ozu Paper Co., Ltd.) under a load 
of 50 g/cm.sup.2, and the lowermost temperature giving a decrease in image 
density after the rubbing of at most 2% was taken as a fixing initiation 
temperature. As a result, the fixing initiation temperature was as low as 
170.degree. C. and the offset initiation temperature was as high as 
250.degree. C., thus showing an excellent anti-offset characteristic. 
Then, the image forming apparatus was used to continuously form 100 sheets 
of A3-size images and then left standing for 30 sec., followed by forming 
5 sheets of A3-size solid white images. From the degree of soiling or dirt 
on both sides of the copied sheets, the offset toner flowout 
characteristic was evaluated whereby no dirt was observed on either side, 
thus showing excellent offset toner flowout-preventing characteristic. 
The above magnetic toner was left standing for two weeks at 50.degree. C. 
in a drier to evaluate the anti-blocking characteristic, whereby 
absolutely no problem was conceived in this regard. 
These results are summarized in Table 3. 
EXAMPLE 2 
A toner was prepared and evaluated in the same manner as in Example 1 
except that the binder resin I was used instead of the binder resin VII. 
The results are shown in Tables 1-3. 
EXAMPLE 3 
A toner was prepared and evaluated in the same manner as in Example 1 
except that the binder resin VI was used instead of the binder resin VII. 
The results are shown in Tables 1-3. 
EXAMPLE 4 
The toner prepared in Example 1 was evaluated by image formation in a 
commercially available copying machine ("NP-2020", mfd. by Canon K.K.). 
From the outset, excellent images having a high image density of 1.40 were 
obtained. Even after 20.times.10.sup.4 sheets of successive copying, 
excellent images showing an image density of 1.37 and free from fog or 
filming were obtained. Little odor was evolved. 
Comparative Examples 1-6 
Toners were prepared and evaluated in the same manner as in Example 1 
except that the binder resin VII was replaced by the binder resin II, III, 
IV, V, VIII and IX, respectively. 
The results are shown in Tables 1-3. 
TABLE 1 
__________________________________________________________________________ 
GPC peak 
of material GPC peak 
resin Proportion of 
of toner 
Proportion of 
Toner (.times.10.sup.4) 
M.W. .ltoreq. 3000 in 
resin (.times.10.sup.4) 
M.W. .ltoreq. 3000 in 
resin P1 P2 
material resin (%) 
P1 P2 toner resin (%) 
__________________________________________________________________________ 
Ex. 
1 VII 1.0 80 
10.0 1.0 
56 9.9 
2 I 1.0 51 
10.7 1.0 
32 12.9 
3 VI 0.40 
60 
12.8 0.45 
38 13.7 
Comp. 
Ex. 
1 II 1.0 51 
13.2 0.9 
32 14.5 
2 III 1.0 51 
13.2 0.9 
30 14.5 
3 IV 0.9 51 
17.8 0.9 
31 18.6 
4 V 1.0 25 
10.7 0.9 
17 13.3 
5 VIII 
5.0 -- 
12.3 7.0 
120 12.5 
6 IX 8.0 -- 
12.0 7.8 
-- 12.4 
__________________________________________________________________________ 
TABLE 2 
__________________________________________________________________________ 
Images after Dirt on 
Silica 
Initial images 
2 .times. 10.sup.4 sheets 
transfer 
accumulation 
Residual monomer (ppm) 
Density Fog 
Density 
Fog 
Filming 
roller 
on stay 
Styrene 
Acrylate 
Benzaldehyde 
Odor 
__________________________________________________________________________ 
Ex. 
1 1.42 .smallcircle. 
1.42 .smallcircle. 
.smallcircle. 
.smallcircle. 
.smallcircle. 
19 10&gt; -- .smallcircle. 
2 1.42 .smallcircle. 
1.41 .smallcircle. 
.smallcircle. 
.smallcircle. 
.smallcircle. 
23 10&gt; -- .smallcircle. 
3 1.40 .smallcircle. 
1.40 .smallcircle. 
.smallcircle. 
.smallcircle. 
.smallcircle..DELTA. 
54 10&gt; -- .smallcircle. 
Comp. 
Ex. 
1 1.33 .smallcircle..DELTA. 
1.25 .DELTA. 
.DELTA. 
.DELTA. 
.DELTA. 
350 50 20 .DELTA.x 
2 1.34 .smallcircle..DELTA. 
1.27 .smallcircle..DELTA. 
.smallcircle..DELTA. 
.smallcircle..DELTA. 
.DELTA. 
160 20 10 .DELTA. 
3 1.27 .DELTA. 
1.19 .DELTA.x 
.DELTA.x 
.DELTA. 
.DELTA. 
130 25 10 .DELTA. 
4 1.40 .smallcircle. 
1.38 .smallcircle. 
.smallcircle. 
.smallcircle. 
.smallcircle. 
25 10&gt; -- .smallcircle. 
5 1.32 .smallcircle..DELTA. 
1.35 .smallcircle..DELTA. 
.smallcircle. 
.DELTA. 
.DELTA. 
230 40 20 .DELTA.x 
6 1.33 .smallcircle..DELTA. 
1.37 .smallcircle..DELTA. 
x .DELTA. 
.DELTA. 
420 60 20 .DELTA.x 
__________________________________________________________________________ 
Evaluation standards of respective items in Table 2 are as follows: 
Fog 
.smallcircle.: no fog 
.smallcircle..DELTA.: Slight fog 
.DELTA.: Conspicuous fog 
.DELTA..times.: Conspicuous and severe fog 
Filming 
.smallcircle.: Not occurred 
.DELTA.: Slightly occurred 
.times.: Occurred 
Dirt on transfer roller 
.smallcircle.: None or very slight 
.smallcircle..DELTA.: A little dirt 
.DELTA.: Substantial dirt 
Silica accumulation on stay (of developing device) 
.smallcircle.: Very little 
.smallcircle..DELTA.: Little 
.DELTA.: Relatively much 
Odor (at the time of copying) 
.smallcircle.: Good 
.DELTA.: Slight odor 
.DELTA..times.: Odor 
Residual monomer 
-.smallcircle.: Not detected 
10&gt;: Less than 10 ppm 
TABLE 3 
______________________________________ 
Fixing Offset Image dirt 
initiation initiation by offset 
temp. (.degree.C.) 
temp. (.degree.C.) 
toner flowout 
Blocking 
______________________________________ 
Ex. 
1 170 250 .smallcircle. 
.smallcircle. 
2 170 250 .DELTA. .smallcircle. 
3 160 240 .smallcircle..DELTA. 
.smallcircle..DELTA. 
Comp. 
Ex. 
1 170 250 .DELTA. .smallcircle..DELTA. 
2 170 250 .DELTA. .smallcircle. 
3 160 230 .DELTA. .DELTA.x 
4 170 230 .DELTA.x .smallcircle. 
5 200 250 .smallcircle. 
.smallcircle. 
6 180 200 .DELTA.x .DELTA. 
______________________________________ 
Evaluation standards of items in Table 3 are as follows: 
Image dirt due to offset toner flowout 
.smallcircle.: Substantially no 
.smallcircle..DELTA.: Slight dirt 
.DELTA.: Some dirt 
.DELTA..times.: Substantial dirt 
Blocking 
.smallcircle.: Utterly no 
.smallcircle..DELTA.: Substantially no 
.smallcircle..DELTA.: Some 
.DELTA..times.: Substantial agglomeration 
As described above, the toner according to the present invention containing 
a specific binder resin and only little residual monomer show excellent 
low-temperature fixability as well as excellent anti-offset 
characteristic. Further, when used in an image forming method adopting a 
hot roller fixation system using a hot roller having a core metal 
thickness of at most 1 mm, the toner realizes shorter waiting time and 
higher speed electrophotographic process. 
Further, the toner according to the present invention contains little 
volatile matter such as residual monomer, thus evolving little odor at the 
time of copying. 
Further, by using the above image forming method utilizing the toner, it is 
possible to provide an image forming apparatus evolving little toner odor. 
Further, the toner is less sticky onto the charging member and 
photosensitive member and provides images with excellent image qualities 
and high image densities and free from fog for a long period.