EP0072176A1

EP0072176A1 - Toners for developing electrostatic image

Info

Publication number: EP0072176A1
Application number: EP82304081A
Authority: EP
Inventors: Hideki Murata; Kiyoshi Tamaki; Sadatugu Terada
Original assignee: Konica Minolta Inc
Current assignee: Konica Minolta Inc
Priority date: 1981-08-05
Filing date: 1982-08-02
Publication date: 1983-02-16
Also published as: DE3270107D1; EP0072176B1

Abstract

There is disclosed a toner for developing electrostatic image which comprises a polymer obtained by polymerization of a vinyl monomer in the presence of two or more polymerization initiators, said initiators having different half-life periods.

The toner for developing electrostatic image according to the present invention can be used for fixation with heating roller without production of offset phenomenon of the toner even when a fixing roller without any supply of an offset inhibiting liquid onto the surface thereof is employed.

Description

This invention relates to a toner for developing electrostatic image in electrophotography, electrostatic recording, electrostatic printing and the like.
Generally, methods for developing electrostatic image can be classified into the following two types:

Namely, a liquid development method using a wet developing agent having finely dispersed a coloring agent such as a pigment or a dye in an insulating organic liquid and the so-called dry development method using a finely divided particle or fine powder of a developer (called a dry toner) according to the cascade, fur brush, magnetic brush, impression, powder cloud or like manner. Moreover, dry toner powder developers can be generally classified into the two component type toner wherein magnetic particles of reduced iron powder are admixed and rubbed with toner particles dispersed in a natural or synthetic resin to give frictional electric charge to toner particles; and the one component type toner wherein magnetic particles of tri-iron tetroxide are involved with a pigment and a dye in toner particles.

This invention is directed to a dry toner for development of electrostatic image to be employed for the dry development method.
In general, the method for fixing a dry toner is a method wherein the toner image formed by development is directly fused and fixed onto a photoconductive photosensitive material or an electrostatic recording material having the electrostatic image or another method wherein the toner image formed by development is transferred onto a transfer sheet such as paper from the photoconductive photosensitive material or electrostatic recording material having the said image and subsequently fused onto the transfer sheet. In this instance, fusion of toner image can be conducted either by contacting with a solvent vapor of the resin for toner or by direct or indirect heating to the toner image, wherein heating is effected by a non-contact heating manner with an electric furnace such as nichrom heater or ceramic heater or by a contact heating manner with heating roller.
Generally, the said fixing method using heating roller is effected by passing the powder toner particle image surface of the sheet to be fixed in contact with under pressure the surface of heating roller with a built-in heating source such as nichrom heater or ceramic heater, said surface being made of a non-tacky or releasing material, to fix the said image onto the said sheet with heat fusion. In the said method, an extremely high thermal efficiency, when toner image is to be fused onto the sheet to be fixed, can be obtained and a rapid fixation can be accomplished owing to pressure contact of the heating roller surface with the toner image surface of the sheet to be fixed.' Therefore, this method is highly suitable for an electrophotographic copying device according to transfer system for high speed copying. However, it has been sometimes observed in this heat fixing method that a part of the toner image is transferred with sticking and adhering onto the fixing roller surface and re-transferred onto the next sheet to be fixed (the so-called offset phenomenon), thereby producing stain over the said sheet, owing to pressure contact of the fixing roller surface with the toner image in a heat-fused state.
Hitherto, there has been proposed, for avoiding stick- ness and adhesion of the toner onto the fixing roller surface, a method wherein the roller surface is formed or coated with a material having a superior non-tacky or releasing property, e.g., a fluoroplastic and, simultaneously, the roller surface is coated with a thin liquid film by feeding an offset inhibiting liquid, e.g., silicone oil to the said surface from the inside of the said roller or from the outside of the said roller. This method is regarded as being effective with regard to inhibition of offset in toner, but has disadvantages that the offset inhibiting liquid when heated tends to produce odor, that a complicated mechanism in a copying device is required owing to the need of additional device to feed the offset inhibiting liquid to the fixing roller surface and that a copying device itself is expensive because of the required high precision for good stability results.
However, when the offset inhibiting liquid is not supplied, the toner is stuck and adhered onto the fixing roller surface to produce offset phenomenon and/or stain in the lower roller attached to the fixing roller and hence the offset inhibiting liquid is inconveniently to be supplied in despite of the above- depicted disadvantages.
On the other hand, there have been proposed the methods wherein offset inhibiting ability is given to the toner itself, for accomplishing fixation with heating roller without toner offset phenomenon even if a fixing roller is used without any supply of the offset inhibiting liquid. For instance, Japanese Patent Laid-Open Application No. 133242/1975 or Japanese Patent Published Application No. 6895/1980 discloses a toner for developing electrostatic image which contains as a main component for dry toner resin component the so-called two stage polymerization resin formed by polymerization of a mixture of a polymeric monomer with a highly polymerized resin and having a high dispersion degree, i.e., weight average molecular weight(Mw)/number average molecular weight(Mn). According to this invention, no offset phenomenon is seen and fixation with heating roller can be effected even if no offset inhibiting liquid is fed onto the surface of fixing roller and, therefore, mechanism of fixing device can be simplified and, for example, a high speed copying machine equipped with such a fixing device can show improved precision and stability with a reduced cost.
However, the said two stage polymerization resin has disadvantage in its preparation that increased number of steps is required for its preparation and the viscosity of a solution of a highly polymerized resin homogeneously dissolved in a monomer is increased, which leads to poor workability and difficult polymerization control. Moreover, the unreacted monomer tends to readily remain in the resulting resin at the end of polymerization. Also, when a dry toner is derived from the said resin, there is disadvantage in the use of the toner that there may easily occur blocking property, the characteristic of the toner, and stain on the lower roller attached to the fixing roller on long run.
As disclosed in Japanese Patent Published Application No. 23354/1976, there has been proposed the toner for developing electrostatic image containing as a main component a crosslinked resin. According to this method, offset property can be given to a toner, a fixing device is simplified and workability problem in preparation, blocking property and stain in the lower roller on long run can be improved. However, the toner for developing electrostatic image containing as a main component a crosslinked resin shows disadvantage of being a higher softening point and a reduced fixing property.
The present inventors have made earnest studies and, as a result, found out a new toner for developing electrostatic image which can eliminate the aforesaid disadvantages.
It is, accordingly, a primary object of this invention to provide a new dry toner for developing electrostatic image which can be used for fixation with heating roller without production of offset phenomenon of the toner even when a fixing roller without any supply of an offset inhibiting liquid onto the surface of said fixing roller is employed.
Another object of this invention is to provide a dry toner for developing electrostatic image having a good fixing property.
A further object of this invention is to provide a dry toner for developing electrostatic image which has a fluidity and a storage stability without any blocking.
A still further object of this invention is to provide a dry toner for development of electrostatic image without any stain in the lower roller attached to a fixing roller even on long run operation.
These objects can be accomplished by the present invention, which is directed to a toner for development of electrostatic image. The present toner comprises a polymer obtained by polymerization of a vinyl monomer in the presence of two or more polymerization initiators having different half-life periods.
As one embodiment of the present invention, the present toner may comprise as a main component the said vinyl polymer and at least one of releasing substances selected from a polyolefin, a polysiloxane, a fatty acid metal salt, a higher fatty acid, a higher alcohol, a higher fatty acid amide, a natural paraffin and a synthetic paraffin.
As another embodiment of the present invention, the present toner may comprise a polymer obtained by polymerization of a vinyl polymer in the presence of two or more polymerization initiators having different half-life periods and a crosslinking agent.
As still another embodiment of the present invention, the present toner may comprise as a main component the latter polymer and at least one of the above-defined releasing substances.
In accordance with the preferred embodiment of this invention, the vinyl resin for toner is applied as a main component for a dry toner, whose resin can be prepared by one stage polymerization of a vinyl monomer in the presence of a polymerization initiator from the initial stage of polymerization, said initiator having a ratio of the half-life period (_TA) of a longer half-life polymerization initiator A to the one (τ_B) of a shorter half-life polymerization initiator B at 70°C in the range of 10 to 10⁶.
In this invention, where a suitable polymerization temperature is set for polymerization initiators having different half-life periods, a rapid initiation system is prepared for the initiator having a shorter half-life period, which a slow initiation system is prepared for the initiator having a longer half-life period.
In this instance, a concentration of the active species [P] may be represented according to the following equation:

wherein f: Polymerization initiation efficiency of catalyst
[I_o]: Initial concentration of initiator
JoRtdt: Concentration of deactivated active species

More specifically, in the rapid initiation system, thermal decomposition of a polymerization initiator is saturated at the initial stage, a polymerization rate decreases as a polymerization time goes and all active species are deactivated prior to exhaustion of all monomers for reaction. Then, a saturated state of a polymerization yield is observed at an early stage and a polymerization degree of the resultant resin is also lowered.
On the other hand, a concentration of the active species [P] in the slow initiation system may be represented according to the following equation:
wherein, IoRIdt: Concentration of resultant initiation species
More specifically, in the slow initiation system, a constant radical concentration [P]s can be attained at an initial period of the reaction and thereafter RI = Rt is obtained.
Accordingly, it is possible to control concentrations of active species in initiators during polymerization by a suitable combination of the rapid initiation system with the slow initiation system and consequently it becomes feasible to control a molecular weight distribution.
For preparing a resin having a favourable toner property, a ratio of the half-life period of a polymerization initiator A having the longest half-life period to the one of a polymerization initiator B having the shortest half-life period at 70°C (τ_A/τ_B) is preferably 10 to 10⁶. When the ratio is less than 10, there is disadvantage that no resin having a favourable toner property can be obtained as the resulting resin has a dispersion degree, i.e., Mw/Mn, of less than 5. When the ratio is more than 10⁶, a rapid initiation system may occur abruptly and there are seen disadvantages of being difficult to control, of taking a too long time to complete the slow initiation system and the like.
The half-life period of the slow initiation type initiator in this invention is preferably 3 to 30 hours. When it is shorter than 3 hours, there can not be produced a toner resin having a good offset property, while it may disadvantageously take a too long time to complete polymerization when it is longer than 30 hours.
A polymerization temperature which may be usually applied in this invention is 40 to 100°C, preferably 60 to 90°C. The polymerization temperature may be varied during polymerization process provided that a ratio of the half-life period (_TA) of the longest half-life polymerization initiator A to the one (_TB) of the shortest half-life polymerization initiator B (τ_A/τ_B) at 70°C should be in the range of 10 to 10⁶. Since control for molecular weight distribution of the resulting vinyl resin may also depend upon the sort and added amount of the polymerization initiator, said control should be effected from a general point of view inculd- ing these factors.
As the polymerization initiators which may be employed in this invention, there may be employed any of oil- soluble initiators commonly used in the art. Representative examples thereof may include a peroxide initiator such as acetylcyclohexylsulfonyl peroxide, isobutyryl peroxide, diisopropyl peroxydicarbonate, di-2-ethylhexyl peroxydicarbonate, 2,4-dichlorobenzyl peroxide, t-butyl peroxypivalate, 3,5,5-trimethylhexanoyl peroxide, octanoyl peroxide, decanonyl peroxide, lauroyl peroxide, stearoyl peroxide, propionyl peroxide, succinic acid peroxide, acetyl peroxide, t-butylperoxy-2-ethylhexanoate, benzoyl peroxide, p-chorobezoyl peroxide, t-butylperoxyisobutyrate, t-butylperoxymaleic acid, t-butyl peroxylaurate, cyclohexanone peroxide, t-butyl peroxyisopropylcarbonate, 2,5-dimethyl-2,5-dibenzoylperoxyhexane, t-butyl peroxyacetate, t-butyl peroxybenzoate, diisobutyl- diperoxyphthalate, methyl ethyl ketone peroxide, dicumyl peroxide, 2,5-dimethyl-2,5-di-t-butylperoxy- hexane, t-butylhydroperoxide, di-t-butylperoxide, 2,5-dimethyl-2,5-di-t-butylperoxyhexane, diisopropylbenzene hydroperoxide, p-methanehydroperoxide, pinane- hydroperoxide, 2,5-dimethylhexane-2,5-dihydroperoxide, cumene hydroperoxide and the like; an azo initiator such as 2,2'-azobisisobutyronitrile, 1,1'-azobiscyclohexane-l-carbonitrile, 2,2'-azobis-4-methoxy-2,4-dimethylvaleronitrile, 2,2-azobis-2,4-dimethylvaleronitrile and the like.
As the rapid-initiation initiator, there may be preferably employed 2,2'-azobis-4-methoxy-2,4-dimethylvaleronitrile, 2,2'-azobis-2,4-dimethylvaleronitrile and the like, while benzoyl peroxide and the like may be mentioned as the slow-initiation initiator.
As the crosslinking agent which may be employed in this invention, there may be mainly employed a compound having two or more polymerizable double bonds; for example, an aromatic divinyl compound such a diethylenic carboxylic acid ester as ethylene glycol diacrylate, ethylene glycol dimethacrylate, trimethylolpropane triacrylate, allylmethacrylate, tetraethylene glycol dimethacrylate, 1,3-butanediol dimethacrylate and the like; other divinyl compound such as N,N-divinylaniline, dinivyl ether, divinyl sulfide, divinyl sulfone and the like; and a compound having 3 or more vinyl groups and these agents may be employed alone or in combination therewith. The crosslinking agent may be added in a range of 0.001 to 5%, upon the total weight of the monomer employed, and preferably 0.001 to 0.5%, as a too large amount thereof tends to increase a softening point to deteriorate its fixing ability.
'As the polymerization process which may be employed in this invention, there may be applied suspension polymerization, solution polymerization, bulk polymerization and the like and suspension polymerization is most suitable.
The term "vinyl resin" as used herein is meant to include a monopolymer of a vinyl monomer and a copolymer resin of a vinyl monomer with other copolymerizable monomer.
As the vinyl monomer which may be employed in this invention, there may be mentioned, a styrene for instance, styrene, a-methylstyrene, p-methylstyrene, p-tert-butylstyrene, p-methoxystyrene, p-phenylstyrene, 3,4-dichlorostyrene, an a-methyl unsaturated monocarboxylic acid ester for instance, methyl acrylate, ethyl acrylate, n-butyl acrylate, isobutyl acrylate, propyl acrylate, n-octyl acrylate, dodecyl acrylate, 2-ethylhexyl acrylate, stearyl acrylate, 2-chloroethyl acrylate, phenyl acrylate, methyl a-chloroacrylate, methyl methacrylate, ethyl methacrylate, propyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, n-octyl methacrylate, dodecyl methacrylate, 2-ethylhexyl methacrylate, stearyl methacrylate, phenyl methacrylate and the like.
As the other monomer which may be employed for the preparation of the vinyl polymer, there may be mentioned, for example, an ethylenic unsaturated monoolefin such as ethylene, propylene, butylene, isobutylene and the like; a vinyl halide such as vinyl chloride, vinyl bromide, vinyl fluoride, a vinyl ester such as vinyl acetate, vinyl propionate, vinyl benzoate, vinyl butyrate, vinyl benzoate, vinyl formate, vinyl caproate and the like; a vinyl ether such as vinyl methyl ether, vinyl ethyl ether, vinyl isobutyl ether and the like; a vinyl ketone such as vinyl methyl ketone, vinyl hexyl ketone and the like; and the like.
In the toner for developing electrostatic image according to this invention, there may be employed any suitable pigments or dyes as a coloring agent. For example, carbon black (including pre-polymerized or grafted carbon black), Nigrosine dyes (C.I. No. 50415B), Aniline Blue (C.I. No. 50405), Chalcoil Blue (C.I. No. Azoec Blue 3), Chrome Yellow (C.I. No. 14090), Ultramarine Blue, (C.I. No. 77103), Du Pont Oil Red (C.I. No. 26105), Quinoline Yellow (C.I. No. 470051), Methylene Blue Chloride (C.I. No. 52015), Phthalocyanine Blue (C.I. No. 74160), Malachite Green Oxalate (C.I. No. 42000), Lamp Black (C.I. No. 77266), Rose Bengal (C.I. No. 45435) and a mixture thereof. This agent is incorporated into a toner in a sufficient amount to color the toner so as to form a visible image when developed.
The offset inhibition agent which may be employed in this invention can be selected from releasing substances having a lower fusing point than a softening point of the binder forming a toner and a low surface energy.
As the offset inhibition agent suitable for the present toner, there may be mentioned, for example, a polyolefin, a fatty acid metal salt, a fatty acid ester, a partially saponified fatty acid ester, a higher fatty acid, a higher alcohol, a liquid or solid paraffin wax, an amide-type wax, a polyhydric alcohol ester, a silicone vanish or polysiloxane, an aliphatic fluorocarbon and the like. Also, there may be employed these agents alone or in admixture with two or more.
As the said polyolefin, there may be mentioned those - polyolefins having a softening point (measured according to the ring and ball method prescribed in JIS standard 2531 - 1960) of 80 to 180°C, preferably 100 to 160°C, such as polypropylene, polyethylene, polybutene and the like. As the said fatty acid metal salt, there may be mentioned, for example, maleic acid salts with zinc, magnesium, calcium and the like; stearic acid salts with zinc, cadmium, barium, lead, iron, nickel, cobalt, copper, aluminum, magnesium and the like; dibasic stearic acid lead salt; oleic acid salts with zinc, magnesium, iron, cobalt, copper, lead, calcium and the like; palmitic acid salts with aluminum, calcium and the like; caprylic acid lead salt; caproic acid lead salt; linolic acid salts with zinc, cobalt and the like; ricinolic acid calcium; ricinoleinic acid salts with zinc, cadmium and the like; and a mixture thereof. As the said fatty acid ester, there may be mentioned, for example, ethyl maleate, butyl maleate, methyl stearate, butyl stearate, cetyl palmitate, montanic acid ethylene glycol ester and the like. As the said partially saponified fatty acid ester, there may be mentioned, for example, calcium-partially saponified product of montanic acid ester and the like.
As the said higher fatty acid, there may be mentioned, for example, dodecanoic acid, lauric acid, myristic acid, palmitic acid, stearic acid, oleic acid, linolic acid, ricinolic acid, arachinic acid, behenic acid, lignoceric acid, selacholeic acid, and the like and a mixture thereof. As the said higher alcohol, there may be mentioned, for example, dodecyl alcohol, lauryl alcohol, myristyl alcohol, palmityl alcohol, stearyl alcohol, arachyl alcohol, behenyl alcohol and the like. As the said paraffin wax, there may be mentioned, for example, natural paraffin, micro wax, synthetic parattin, chlorinated hydrocarbon and the like. As the said amide-type wax, there may be mentioned, for example, stearic amide, oleic amide, palmitic amide, lauryl amide, behenic amide, methylenebisstearoamide, ethylenebisstearoamide and the like. As the said polyhydric alcohol ester, there may be mentioned, for example, glycerol stearate, glycerol ricinolate, glycerol monobehenate, sorbitan monostearate, propylene glycol monostearate, sorbitan trioleate and the like. As the said silicone varnish or polysiloxane, there may be mentioned, for example, methyl silicone varnish, phenyl silicone varnish, dimethylpolysiloxane and the like. As the said aliphatic fluorocarbon, there may be mentioned, for example, a low polymeric compound such as ethylene tetrafluoride or propylene hexafluoride; a fluorine-containing surface active agent as disclosed in Japanese Patent Laid-Open Application No. 124428/1978 and the like.
An amount of the offset inhibition agent comprising such releasing substance to be incorporated into the toner is 0.1 to 10% by weight, preferably 1 to 5% by weight. In case of too small amount, offset phenomenon tends to occur, while toner fluidity becomes poor with decreased cleanability in case of too large amount.
Moreover, the present toner for developing electrostatic image may include, if necessary, a variety of other toner additives, e.g., an electric charge controlling agent, a plasticizer and the like.
Also, there may be employed in this invention any useful magnetic coloring agent for the toner of mono- component developer. As the magnetic coloring agent, there may be desirably employed any of those substances which can be strongly magnetized along magnetic field and, preferably, has a black color, a good dispersibility in resin with chemical stability and an easy availability of the finely divided particle form with a particle size of less than lµ. Particularly, there is most preferably employed magnetite (tri-iron tetroxide). Representative examples of magnetic or magnetizable materials may include a metal such as cobalt, iron, nickel; an alloy or mixture of such metal as aluminum, cobalt, steel, lead, magnesium, nickel, tin, zinc, antimony, beryllium, bismuth, cadmium, calcium, manganese, selenium, titanium, tungsten, vanadium; a metal compound containing a metal oxide such as aluminum oxide, iron oxide, copper oxide, nickel oxide, zinc oxide, titanium oxide, magnesium oxide; a refractory nitride such as vanadium nitride, chromium nitride; a carbide such as tungsten carbide, silica carbide; ferrite; and a mixture thereof. An amount of such a strongly magnetic substance to be incorporated into a toner is desirably of about 30 to 300 parts by weight, particularly preferably, 50 to 200 parts by weight upon 100 parts by weight of resin components.
This invention will be more concretely illustrated by way of the following examples, but embodiments of this invention is not intented to be limited thereto. In these examples, all parts are by weight unless otherwise indicated.

Example 1

In a mixture of 85 parts of styrene and 15 parts of n-butyl acrylate were dissolved 0.1 part of benzoyl peroxide and 1.0 part of 2,2'-azobis(2,4-dimethylvaleronitrile) to form a polymerization composition. In a 1 Z four-neck flask equipped with a nitrogen inlet tube, a condenser, a dropping funnel and a stirrer were placed 1 part of calcium phosphate and 0.01 part of sodium dodecylbenzenesulfonate, distilled water was added thereto and then the said composition was added dropwise thereto with stirring from the dropping funnel. After completion of the dropwise addition, a temperature was raised to 80°C and a continuous stirring was conducted for 20 hours to conduct polymerization through a one step polymerization procedure. After completion of the polymerization, treatment with hydrochloric acid was conducted, and filtration, water washing and drying gave a vinyl resin (a styrene-acrylic copolymer resin) for toner. This resin was designated as Resin Sample A.
Thereafter, 100 parts of the present Resin Sample A were admixed with 10 parts of carbon black (Mitsubishi Carbon Black #30, manufacture by Mitsubishi Chemical Industries Ltd.) and 1 part of "Oil Black BW" (a charge controlling agent, tradename of Orient Kagaku K. K.), fused and kneaded with a roll mill and then ground and classified with a jet mill to prepare a toner. This toner is designated as Toner Sample 1.

Example 2.

In a mixture of 85 parts of styrene and 15 parts of n-butyl acrylate were dissolved 0.1 part of benzoyl peroxide, 0.5 part of 2,2'-azobis-(2,4-dimethylvaleronitrile) and 0.3 part of 2,2'-azobis-4-methoxy-2,4-dimethylvaleronitrile and a polymerized composition was formed at a polymerization temperature of 80°C.
Then, a styrene-acrylic copolymer resin was prepared and formed into a toner in the same manner as in Example 1. This toner is designated as Toner Sample 2.

Example 3.

In a mixture of 85 parts of styrene and 15 parts of 2-ethylhexyl acrylate were dissolved 1.0 part of 2,2'-azobisisobutyronitrile and 0.1 part of cyclohexanone peroxide to form a polymerization composition. By the same procedures as in Example 1 except that a polymerization temperature was 90°C, a resin was prepared and formed into a toner. This toner is designated as Toner Sample 3.

Example 4.

With 67 parts of tri-iron tetroxide powder "Mapicoblack BL-500" (manufactured by Titan Kogyo K. K.) and 0.3 part of methylene blue-chloride (a charge controlling agent) were admixed 100 parts of the Resin Sample A and then fused and kneaded with Banbury mixer and ground and classified with a jet mill to form a uni-component magnetic toner. This toner is designated as Toner Sample 4.

Comparative Example 1.

In a mixture of 85 parts of styrene and 15 parts of n-butyl acrylate were dissolved 0.5 part of lauroyl peroxide and 0.5 part of octanoyl peroxide to form a polymerization composition. By the same procedures as in Example 1 except that a polymerization temperature was 60°C, a resin was prepared and formed into a toner. This toner is designated as Comparative Toner Sample 1.

Comparative Example 2.

A copolymer resin was prepared according to the double-stage process.
In 200 parts of distilled water containing 1 part of calcium phosphate and 0.01 part of sodium dodecylbenzenesulfonate were suspended and dispersed 85 parts of styrene, 15 parts of n-butyl acrylate and 1 part of azobisisobutyronitrile and polymerization was effected at 75°C to give a resin with a high polymerization degree. Subsequently, 10 parts of the said highly polymerized resin and 3 parts of azobisisobutyronitrile were dissolved in 85 parts of styrene and 15 parts of n-butyl acrylate, suspended and dispersed in 200 parts of distilled water containing 1 part of calcium phosphate and 0.01 part of sodium dodecylbenzenesulfonate and then polymerization was effected at 75°C. The resin thus formed was made into a toner in the same manner as in Example 1. This toner is designated as Comparative Toner Sample 2.
Test results of the toner samples prepared as above are summarized in Table 1.
(Note)

a) A benzene solution of the polymerization initiator at 0.1 mol/2 was prepared and sealed into a glass tube previously purged with nitrogen and then dipped into a thermostat set at 70°C to effect thermal decomposition. A half-life period was determined and τ_A/τ_B calculated. The half-life period within 10 minutes was determined by extrapolation upon found values at several points.
b) Offset production was investigated using the fixing apparatus of U-Bix V (manufactured by Konishiroku Photo Ind. Co., Ltd.) at a roller temperature of 230°C and a feeding speed of 60 mm/sec.
c) Softening point of each toner sample was shown as a parameter for fixation.
d) Stain production of the under roller after long-run of 30,000 sheets was shown.
e) Blocking of toner grains when placed in a thermostat at 60°C over 2 hours was shown.

It can be clearly seen from the above results that the toner according to this invention is excellent in offset, fixing and blocking properties as compared with the prior art toner and also shows less stain of under roller in long-run.
Moreover, the double-stage process for preparing the resin as shown in the Comparative Example 2 provided a more reduced yield as compared with the present process and also a problem in its workability.

Example 5.

Following the same procedures as in the Example 1, there was prepared the Resin Sample A.
With 100 parts of the Sample A thus prepared were then admixed 10 parts of carbon black (Mitsubishi Carbon Black #30, manufactured by Mitsubishi Chemical Industries Ltd.), 1 part of "Oil Black BW" (a charge controlling agent, tradename of Orient Kagaku K. K.) and 3 parts of a low molecular polypropylene (Viscol 550P, manufactured by Sanyo Chemical Industries, Ltd.) as an offset inhibiting agent comprising the present parting substance and fused and kneaded with a roll mill and then ground and classified with a jet mill to prepare a toner. This toner is designated as Toner Sample 5.

Example 6.

Following the same procedures as in the above Example 2, there was prepared the polymerized composition and then a styrene-acrylic copolymer resin. This resin is designated as Resin Sample B.
A mixture of 100 parts of the Resin Sample B, 10 parts of carbon black, 1 part of Oil Black BW and 2 parts of stearic acid was formed into a toner. This toner is designated as Toner Sample 6.

Example 7.

Following the same procedures as in the above Example 3, there was prepared the resin. This resin is designated as Resin Sample C.
A mixture of 100 parts of the Resin Sample C, 10 parts of carbon black, 1 part of Oil Black BW and 1 part of sodium oleate was formed into a toner. This toner is designated as Toner Sample 7.

Example 8.

Following the same procedures as in the above Example 4 except that 3 parts of a low molecular polypropylene (Biscol 550P, manufactured by Sanyo Chemical Industries, Ltd.) were additionally employed, there was prepared a uni-component magnetic toner. This toner is designated as Toner Sample 8.

Comparative Example 3.

Following the same procedures as in the above Comparative Example 1, there was prepared a resin. This resin is designated as Resin Sample D.
A mixture of 100 parts of the Sample D, 10 parts of carbon black, 1 part of Oil Black BW and 3 parts of a low molecular polyolefin was formed into a toner. This toner is designated as Comparative Toner Sample 3.

Comparative Example 4.

Following the same procedures as in the above Comparative Example 2, there was prepared a resin, which is designated as Resin Sample E.
Following the same manner as in the above Example 5, 100 parts of the Resin Sample E, 10 parts of carbon black, 1 part of Oil Black BW1 and 3 parts of a low molecular polyolefin were admixed to form a toner, which is designated as Toner Sample 4.
Test results of the toner samples prepared as above are summarized in Table 2.

Example 9.

_'Following the same procedures as in the above Example 1 except that 0.3 part of ethylene glycol dimethacrylate was additionally employed, there was prepared a vinyl resin (a styrene-acrylic copolymer resin), which is designated as Resin Sample F.
Then, a toner was prepared according to the same procedures as in the above Example 1 except that 100 parts of the Resin Sample F were employed. This toner is designated as Toner Sample 9.

Example 10.

Following the same procedures as in the above Example 2 except that 0.3 part of divinyl benzene was additionally employed, there was prepared a toner, which is designated as Toner Sample 10.

Example 11.

Following the same procedures as in the above Example 3 except that 0.1 part of trimethylolpropane triacrylate was additionally employed, there was prepared a toner, which is designated as Toner Sample 11.

Example 12.

Following the same procedures as in the above Example 3 except that 100 parts of the Resin Sample F were employed, there was prepared a uni-component magnetic toner. This toner is designated as Toner Sample 12.

Comparative Example 5.

Following the same procedures as in the above Comparative Example 1 except that 1.5 parts of octanoyl peroxide and 0.3 part of ethylene glycol methacrylate were employed, there was prepared a toner. This toner is Comparative Toner Sample 5.

Comparative Example 6.

In a mixture of 85 parts of styrene, 15 parts of n-butyl acrylate and 0.5 part of ethylene glycol dimethacrylate were dissolved 2.0 parts of benzoyl peroxide to form a polymerization composition. Thereafter, the same procedures as in the above Example 1 were repeated to form a toner. This toner is designated as Comparative Toner Sample 6.

Comparative Example 7.

Following the same procedures as in the above Comparative Example 6 except that 0.1 part of ethylene glycol dimethacrylate was employed, there was prepared a toner. This toner is designated as Comparative Toner Sample 7.
Test results of the toner samples prepared as above are summarized in Table 3.

Example 13.

Following the same procedures as in the above Example 9, there was prepared the Resin Sample F.
Thereafter, the same procedures as in the above Example 5 were repeated except that 100 parts of the Resin Sample F were employed instead of the Resin Sample A, thereby forming a toner. This toner is designated as Toner Sample 13.

Example 14.

Following the same procedures as in the above Example 10, there was prepared a resin, which is designated as Resin Sample G.
Thereafter, the same procedures as in the above Example 6 were repeated except that 100 parts of the Resin Sample G were employed, thereby forming a toner, which is designated as Toner Sample 14.

Example 15.

Following the same procedures as in the above Example 11, there was prepared a resin, which is designated as Resin Sample H.
Thereafter, the same procedures as in the above Example 7 were repeated except that 100 parts of the Resin Sample H were employed, thereby forming a toner, which is designated as Toner Sample 15.

Example 16.

Following the same procedures as in the above Example 8 except that 100 parts of the Resin Sample F prepared in the above Example 13 were employed instead of the Resin Sample A, there was prepared a toner. This toner is designated as Toner Sample 16.

Comparative Example 8.

Following the same procedures as in the above Comparative Example 5, there was prepared a resin, which is designated as Resin Sample I.
Following the same procedures as in the above Comparative Example 3 except that 100 parts of the Resin Sample I was employed instead of the Resin Sample D, there was prepared a toner. This toner is designated as Toner Sample 8.

Comparative Example 9.

Following the same procedures as in the above Comparative Example 4 except that 0.1 part of ethylene glycol dimethacrylate were additionally employed, there was prepared a toner, which is designated as Comparative Toner Sample 9.
Test results of the toner samples prepared as above are summarized in Table 4.

Claims

1. A toner for developing electrostatic image which comprises a polymer obtained by polymerization of a vinyl monomer in the presence of two or more polymerization initiators, said initiators having different half-life periods.

2. A toner according to Claim 1 wherein a ratio of the half-life period of a longer half-life polymerization initiator (T_A) to the one of a shorter half-life polymerization initiator (_TB) is 10 to 10⁶.

3. A toner according to Claim 2 wherein said half-life period of a longer half-life polymerization initiator (_TA) is 3 to 30 hours.

4. A toner according to Claim 1 wherein said polymer is derived from an α,β-ethylenic unsaturated monomer.

5. A toner according to Claim 1 wherein said α,β-ethylenic unsaturated monomer is a styrene or an a-methylenic unsaturated monocarboxylic acid ester.

6. A toner according to Claim 4 wherein said polymer is a copolymer of a styrene with an a-methylenic unsaturated monocarboxylic acid ester.

7. A toner according to Claim 4 wherein said polymer is a copolymer of a-methylenic unsaturated monocarboxylic acid esters.

8. A toner according to Claim 1 wherein said polymer is obtained by polymerization in the further presence of a crosslinking agent.

9. A toner according to Claim 8 wherein a ratio of the half-life period of a longer half-life polymerization initiator (_TA) to the one of a shorter half-life polymerization initiator (_TB) at 70°C is 10 to 10⁶.

10. A toner according to Claim 9 wherein said half-life period of a longer half-life polymerization ilitiator (_TA) is 3 to 30 hours.

11. A toner according to Claim 8 wherein said crosslinking agent is a compound containing at least two vinyl groups.

12. A toner according to Claim 1 which comprises further a releasing agent.

13. A toner according to Claim 12 wherein a ratio of the half-life period of a longer half-life polymerization initiator (_TA) to the one of a shorter half-life polymerization initiator (_TB) at 70°C is 10 to 10⁶.

14. A toner according to Claim 13 wherein said half-life period of a longer half-life polymerization initiator (_TA) is 3 to 30 hours.

15. A toner according to Claim 12 wherein said releasing agent is a polyolefin, a metal salt of a fatty acid, an ester of a fatty acid, a partially saponified fatty acid ester, a higher fatty acid, a higher alcohol, a paraffin wax, a higher fatty acid amide, a fatty acid glyceride, a silicone varnish, a polysiloxane or an aliphatic fluorocarbon.

16. A toner according to Claim 15 wherein said polyolefin has a softening point of 80 _% 180°C.

17. A toner according to Claim 15 wherein the fatty acid moiety of said fatty acid metal salt, fatty acid ester, partially saponified fatty acid ester, higher fatty acid, higher fatty acid amide or fatty acid glyceride has 8 ~ 30 carbon atoms.

18. A toner according to Claim 15 wherein said higher alcohol has 8 ~ 30 carbon atoms.

19. A toner according to Claim 15 wherein the metal atom of said fatty acid metal salt is zinc, magnesium, calcium, cadmium, barium, lead, iron, nickel, cobalt, copper or aluminum.

20. A toner according to Claim 12 wherein said polymer is obtained by polymerization in the further presence of a crosslinking agent.

21. A toner according to Claim 1 which comprises further a coloring agent.

22. A toner according to Claim 21 wherein said coloring agent is carbon black.

23. A toner according to Claim 1 which comprises further a magnetic substance.