JPS6411941B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a toner for developing electrostatic images in electrophotography, electrostatic recording, electrostatic printing, etc. Methods for developing electrostatic images can be roughly divided into liquid developing methods that use a wet developer in which colorants such as various pigments and dyes are finely dispersed in an insulating organic liquid, cascade methods, bristle brush methods, magnetic brush method,
There is a so-called dry developing method that uses a developing method such as an impression method or a powder cloud method, and uses a fine powder or powder particle developer called dry toner. Separately, reduced iron powder, etc. and toner particles in which dyes/pigments are dispersed in natural or synthetic resin are mixed and rubbed to impart a triboelectric charge to the toner particles.2
component-based toner and 1 containing magnetic particles such as triiron tetroxide along with dyes and pigments in the toner particles.
There are component-based toners. The present invention relates to a dry toner for developing electrostatic images used in the latter dry developing method. In general, dry toner fixing methods involve directly fusing and fixing a toner image obtained by development onto a photoconductive photoreceptor or electrostatic recording medium that holds an electrostatic charge image, or by developing and fixing the toner image. The toner image obtained by the method is transferred from a photoconductive photoreceptor or an electrostatic recording medium onto a transfer sheet such as paper, and then fused onto the transfer sheet. be. At that time, the toner image is fused either by contact with the solvent vapor of the toner resin or by direct or indirect heating of the toner image.Heating methods include electric furnaces such as nichrome heaters and ceramic heaters. There are two types of heating methods: a non-contact heating method using a heating roller, and a contact heating method using a heating roller. Generally, the fixing method using the heating roller is as follows:
The powder particle toner image on the fixing sheet such as paper is transferred to the surface of the heating roller, which has a built-in heat source such as a nichrome heater or a ceramic heater, and whose surface is formed or coated with a non-adhesive or releasable material. The toner is heated and fused by passing through the toner while being in pressure contact with the surface, thereby fixing the to-be-fixed sheet. In the above method, the surface of the heating roller and the toner image surface of the fixing sheet come into pressure contact, so the thermal efficiency when fusing the toner image onto the fixing sheet is extremely good, and the fixing can be carried out quickly. Because you can
In particular, it is extremely suitable for a transfer type electrophotographic copying apparatus aimed at high-speed copying. however,
In the heating fixing method, the surface of the fixing roller and the toner image come into pressure contact in a heated and molten state, so a part of the toner image adheres to the surface of the fixing roller and is transferred, and then transferred again onto the next sheet to be fixed. As a result, a so-called offset phenomenon occurs, resulting in stains on the sheet to be fixed. Conventionally, in order to prevent the toner from adhering to the surface of the fixing roller, for example, the roller surface is formed or coated with a material that is non-adhesive or has excellent releasability for toner images, such as fluorine-based resin, and the surface is In addition, a method has been used in which an anti-offset liquid such as silicone oil is supplied from the inside of the roller to the surface or from the outside of the roller to the surface to coat the roller surface with a thin film of the liquid. Although this method is effective in preventing toner offset, it generates an odor due to heating of the offset prevention liquid and requires a device to supply the offset prevention liquid onto the surface of the fixing roller. This has the disadvantage that the mechanism of the copying device becomes complicated and high precision is required to obtain stable results, making the copying device expensive. However, if offset prevention liquid is not supplied, toner adheres to the surface of the fixing roller, causing an offset phenomenon and dirt on the lower roller attached to the fixing roller. There was an inconvenience in that offset prevention liquid had to be supplied. On the other hand, even when using a fixing roller that does not supply an offset prevention liquid, it is possible to perform heating roller fixation without causing the toner offset phenomenon by imparting offset prevention ability to the toner itself. Proposed. For example, JP-A-50-133242, JP-A-55-
As described in Publication No. 6895, a so-called two-stage polymer resin with high dispersity and high weight average molecular weight (Mw)/number average molecular weight (Mn) is obtained by polymerizing a mixed solution of a polymerizable monomer and a high polymerization degree resin. Examples include toners for developing electrostatic images containing as a main component of a dry toner resin component. According to the above method, heating roller fixing can be performed without causing an offset phenomenon even when no offset prevention liquid is supplied to the surface of the fixing roller, so the mechanism of the fixing device can be simplified. At the same time, it is possible to improve the accuracy and stability of, for example, a high-speed copying machine incorporating such a fixing device.
Furthermore, the cost can also be reduced. However, in manufacturing, the two-stage polymer resin
As the number of steps increases, the viscosity of the mixture obtained by uniformly mixing high-polymerization resin with monomer and dissolving it increases, resulting in poor workability and difficulty controlling polymerization, which is a disadvantage in resin production. . Furthermore, unreacted monomers tend to remain in the resin even after polymerization is completed, and even if a dry toner is prepared using the resin, blocking properties, which are the characteristics of the toner itself,
There was a drawback in using the toner that the lower roller attached to the fixing roller was easily contaminated during long runs. Also, as stated in Special Publication No. 51-23354,
Examples include toners for developing electrostatic images containing a crosslinked resin as a main component. This method also imparts offset properties to the toner, simplifies the fixing device, and also improves workability during manufacturing, which is a drawback of two-stage polymer resins, and lower roller stains during long runs due to blocking. be done. However, toners for developing electrostatic images containing a crosslinked resin as a main component have a drawback of having a high softening point and poor fixability. As a result of intensive research, the inventors of the present invention have invented a new toner for developing electrostatic images that overcomes the above-mentioned drawbacks. That is, the first object of the present invention is to perform heating roller fixing without causing toner offset even when using a fixing roller that does not supply offset prevention liquid to the surface of the fixing roller. An object of the present invention is to provide a dry toner for developing electrostatic images. A second object of the present invention is to provide a dry toner for developing electrostatic images with good fixing properties. A third object of the present invention is to provide a dry toner for developing electrostatic images that does not cause blocking, has fluidity, and has storage stability. A fourth object of the present invention is to provide a dry toner for developing electrostatic images in which the lower roller attached to the fixing roller does not get dirty even during long runs. The above-mentioned purposes are directed to electrostatic charge fixing devices for heat roller fixing, which contain a vinyl resin as a main component obtained by suspension polymerization of vinyl monomers in the presence of a polymerization initiator and a crosslinking agent, and also contain a releasing substance. In toner for image development,
The polymerization initiator is composed of two or more types of polymerization initiators having different half-lives, and in the polymerization initiator,
It is characterized in that the ratio τ _A /τ _B of the half-lives τ A and τ _B at 70°C of the polymerization initiator A having the longest half-life and the polymerization initiator _B having the shortest half-life is 10 to 10 ⁶ This is achieved by a toner for developing an electrostatic image for fixing with a hot roller. In the present invention, if an appropriate polymerization temperature is set for polymerization initiators with different half-lives, a rapid initiation system will be obtained for the polymerization initiator with a short half-life, and a rapid initiation system will be obtained for the polymerization initiator with a long half-life. is a slow-onset system. Here, the active species concentration in the rapid initiation system [P ^* ]
is expressed by the following formula. [P ^* ]=[I ₀ ]−∫ ^t ₀ Rtdt : Polymerization initiation efficiency of the catalyst [I ₀ ] : Initial initiator concentration ∫ ^t ₀ Rtdt : Deactivated active species concentration In other words, in a rapid initiation system, the polymerization initiator Thermal decomposition of is saturated in the initial stage, the polymerization rate decreases with polymerization time, and all active species are deactivated before all monomers have reacted, so the saturation phenomenon of polymerization yield is observed early. The degree of polymerization of the resulting resin also decreases. On the other hand, the active species concentration [P ^* ] in the slow-starting system is expressed by the following formula. [P ^* ]=∫ ^t ₀ Ridt−∫ ^t ₀ Rtdt ∫ ^t ₀ Ridt: Concentration of the generated starting species In other words, in a slow initiation system, the steady radical concentration [P ^* ]s is reached at the beginning of the reaction, and after that Ri = Rt. Therefore, by appropriately combining a rapid initiation system and a slow initiation system, it is possible to control the active species concentration of the reaction initiator during the polymerization process, and as a result, it is possible to control the molecular weight distribution. becomes. Here, in producing a resin having good toner properties, a polymerization initiator A having the longest half-life,
The ratio τ _A /τ B of half-life τ _A and τ _B at 70°C of polymerization initiator B, which has the _shortest half-life, is 10 to 10 ⁶ , and when it is smaller than 10, the dispersity of the resulting resin , the weight average molecular weight (Mw)/number average molecular weight (Mn) is less than 5, which has the disadvantage that good toner properties cannot be obtained.On the other hand, when it is greater than ¹⁰⁶ , the rapid start system is It has the disadvantages that it occurs suddenly, making it difficult to control, and that it takes too much time to complete the slow onset system. The half-life of the slow-starting initiator in the present invention is 3
Time to 30 hours is preferred; if it is shorter than 3 hours, a toner resin with good offset properties cannot be obtained, and if it is longer than 30 hours, it takes too long to complete the polymerization. have The polymerization temperature usually used in the present invention is 40~
The temperature is 100°C, preferably 60°C to 90°C, and the ratio of half-lives τ A and τ _B at 70°C of polymerization initiator A, which has the longest half-life, and polymerization initiator _{B, which has the shortest half-life, τ A /} τ _B is 10
The polymerization temperature may be changed during the polymerization process as long as it is within the range of ~ ¹⁰⁶ . Furthermore, in addition to these factors, the type and amount of polymerization initiator added are also involved in controlling the molecular weight distribution of the resulting vinyl resin, so these should be comprehensively controlled. As the polymerization initiator used in the present invention, a common oil-soluble initiator is used. Typical specific examples include, for example, acetylcyclohexylsulfonyl peroxide, isobutyryl peroxide,
Diisopropyl peroxydicarbonate, di-
2-ethylexyl peroxydicarbonate,
2,4-dichlorobenzoyl peroxide, t
-Butyl peroxypivalate, 3,5,5-trimethylhexanonyl peroxide, octanoyl peroxide, decanonyl peroxide, lauroyl peroxide, stearoyl peroxide, propionyl peroxide, succinic acid peroxide, acetyl peroxide oxide, t-butyl peroxy-2-ethylhexanoate, benzoyl peroxide,
Parachlorobenzoyl peroxide, t-butyl peroxyisobutyrate, t-butyl peroxymale acid, t-butyl peroxylaurate, cyclohexanone peroxide,
t-Butylperoxyisopropyl carbonate, 2,5-dimethyl-2,5-dibenzoylperoxyhexane, t-butylperoxyacetate, t-butylperoxybenzoate, diisobutyl diperoxyphthalate, methyl ethyl ketone peroxide, dicumyl Peroxide, 2,5-dimethyl-2,5-butyl peroxyhexane, t-butylcumyl peroxide,
t-Butyl hydroperoxide, di-t-butyl peroxide, 2,5-dimethyl-2,5-di-t-butyl peroxyhexane, diisopropylbenzene hydroperoxide, paramenthane hydroperoxide, vinylane hydroperoxide, 2 , 5-dimethylhexane-2,5-dihydroperoxide, peroxide-based initiators such as cumene hydroperoxide, 2,2'-azohisisobutyronitrile, 1,1'-azobiscyclohexane 1-carbonitrile, 2,2′-Azobis-
Examples include azo initiators such as 4-methoxy-2,4-dimethylvaleronitrile and 2,2'-azobis-2,4-dimethylvaleronitrile. Preferably used as a rapid initiation system initiator is 2,2'-azobis-4-methoxy-2,
Examples include 4-dimethylvaleronitrile and 2,2'-azobis-2,4-dimethylvaleronitrile, while examples of slow-starting initiators include benzoyl peroxide. The crosslinking agent used in the present invention is mainly a compound having two or more polymerizable double bonds, such as aromatic divinyl compounds such as divinylbenzene, divinylnaphthalene, and their derivatives, and ethylene glycol. diacrylate, ethylene glycol dimethacrylate,
Diethylene carboxylic acid esters such as trimethylolpropane triacrylate, allyl methacrylate, tetraethylene glycol dimethacrylate, 1,3-butanediol dimethacrylate, N,N, divinylaniline, divinyl ether, divinyl sulfide, divinyl sulfone, etc. All divinyl compounds and compounds having three or more vinyl groups are selected singly or in mixtures and are added in an amount ranging from 0.001 to 5% based on the total monomer weight. If the amount added is too large, the softening point will increase and the fixing properties will deteriorate, so it is preferably kept at about 0.001 to 0.5%. The vinyl resin referred to in the present invention includes not only a homopolymer of a vinyl monomer but also a copolymer resin of a vinyl monomer and another monomer copolymerizable with the vinyl monomer. The vinyl monomers used in the present invention include styrene, α-methylstyrene, p-methylstyrene, p-terr-butylstyrene, p-methoxystyrene, p-phenylstyrene, 3,4
-Dichlorostyrene, methyl acrylate, ethyl acrylate, n-butyl acrylate, isobutyl acrylate, propyl acrylate, n-acrylate
-Octyl, dodecyl acrylate, acrylic acid 2
-Ethylhexyl, stearyl acrylate, 2-chloroethyl acrylate, phenyl acrylate,
α-methyl chloroacrylate, methyl methacrylate, ethyl methacrylate, propyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, n-octyl methacrylate, dodecyl methacrylate, 2-ethylhexyl methacrylate,
Examples include stearyl methacrylate and phenyl methacrylate. Other monomers used to form vinyl polymers include ethylenically unsaturated monoolefins such as ethylene, propylene, butylene, and isobutylene, vinyl chloride, vinyl bromide, vinyl fluoride, vinyl acetate, Vinyl esters such as vinyl propionate, vinyl benzoate, vinyl butyrate, vinyl benzoate, vinyl formate, vinyl caproate, vinyl ethers such as vinyl methyl ether, vinyl ethyl ether, vinyl isobutyl ether, vinyl methyl ketone, vinyl hexyl ketone, etc. Examples include vinyl ketones. In the electrostatic image developing toner according to the present invention, any suitable pigment or dye can be used as a colorant. For example, carbon black (including prepolymerized or grafted carbon black), nigrosine dye, aniline blue, calco oil blue, chrome yellow, ultramarine blue, Dupont oil red, quinoline yellow, methylene blue chloride, phthalocyanine blue, malachite green. Oxalates, lamp blacks, rose pengals and mixtures thereof are used and are contained in the toner in sufficient amounts to color the toner so that it can be developed to form a visible image. The anti-offset agent used in the present invention is selected from releasable substances that melt at a temperature lower than the softening point of the binder constituting the toner and have low surface energy. Examples of offset preventive agents suitable for the toner of the present invention include polyolefins, fatty acid metal salts, fatty acid esters, partially saponified fatty acid esters, higher fatty acids, higher alcohols, liquid or solid paraffin waxes, amide waxes, polyhydric alcohol esters, silicone varnish or polysiloxane,
Examples include aliphatic fluorocarbons.
Such offset inhibitors may be used alone or in combination of two or more. The polyolefin is, for example, a resin such as polypropylene, polyethylene, or polybutene, and has a softening point of 80 to 180°C, preferably 100°C when measured by the ring and ball method specified in JIS Standard 2531-1960.
~160℃. For example, the fatty acid metal salts include metal salts of maleic acid and zinc, magnesium, calcium, etc.; metal salts of stearic acid and zinc, cadmium, barium, lead, iron, nickel, cobalt, copper, aluminum, magnesium, etc.; Basic lead stearate; metal salts of oleic acid with zinc, magnesium, iron, cobalt, copper, lead, calcium, etc.; metal salts of palmitic acid with aluminum, calcium, etc.; lead caprylate; lead caproate; linoleic acid Examples include metal salts of ricinoleic acid and zinc, cobalt, etc.; calcium ricinoleate; metal salts of ricinoleic acid and zinc, cadmium, etc., and mixtures thereof. Examples of the fatty acid ester include ethyl maleate, butyl maleate, methyl stearate, butyl stearate, cetyl palmitate, and ethylene glycol montanate. Examples of the partially saponified fatty acid ester include partially saponified calcium esters of montanic acid. Examples of the higher fatty acids include dodecanoic acid, lauric acid, myristic acid, palmitic acid, stearic acid, oleic acid, linoleic acid, ricinoleic acid, arachidic acid, behenic acid, lignoceric acid, ceracoleic acid, and mixtures thereof. I can do it. Examples of the higher alcohol include dodecyl alcohol, lauryl alcohol, myristyl alcohol, palmityl alcohol, stearyl alcohol, aracyl alcohol, and behenyl alcohol. Examples of the paraffin wax include natural paraffin, microwax, synthetic paraffin, and chlorinated hydrocarbons. As the amide wax,
For example, stearic acid amide, oleic acid amide,
Examples include palmitic acid amide, lauric acid amide, behenic acid amide, methylene bis stearamide, ethylene bis stearamide, and the like. Examples of the polyhydric alcohol ester include glycerin stearate, glycerin ricinolate, glycerin monobehenate, sorbitan monostearate, propylene glycol monostearate,
Examples include sorbitan triolate. Examples of the silicone varnish or polysiloxane include methyl silicone varnish, phenyl silicone varnish, and dimethylpolysiloxane.
Examples of the aliphatic fluorocarbons include low polymer compounds of tetrafluoroethylene and hexafluoropropylene, and fluorine-containing surfactants described in JP-A-53-124428. The content of the anti-offset agent made of this releasing substance in the toner is 0.1 to 10% by weight, preferably 1 to 5% by weight. If so, the fluidity of the toner deteriorates and the cleaning performance deteriorates. The toner for developing an electrostatic image according to the present invention may contain various other toner additives such as a charge control agent and a plasticizer, if necessary. Also used in the present invention are magnetic colorants useful for toners for one-component developers. Such a magnetic colorant is a substance that is strongly magnetized in the direction of a magnetic field, preferably black, well dispersed in the resin, and chemically stable, and furthermore, a fine particle with a particle size of 1μ or less. It is desirable that a substance can be easily obtained, and magnetite (triiron tetroxide) is particularly preferred. Typical magnetic or magnetizable materials include metals such as cobalt, iron, and nickel; aluminum, cobalt, steel,
Alloys and mixtures of metals such as lead, magnesium, nickel, tin, zinc, antimony, beryllium, bismuth, cadmium, calcium, manganese, selenium, titanium, tungsten, vanadium; aluminum oxide, iron oxide, copper oxide, nickel oxide , metal compounds including metal oxides such as zinc oxide, titanium oxide and magnesium oxide; refractory titides such as vanadium titride, chromium titide; carbides such as tungsten carbide and silica carbide; ferrites and mixtures thereof. etc. can be used and obtained. The amount of these ferromagnetic substances contained in the toner is preferably about 30 to 300 parts by weight per 100 parts by weight of the resin component. Particularly preferably, the amount is 50 to 200 parts by weight per 100 parts by weight of the resin component. Next, the present invention will be illustrated by Examples, but the embodiments of the present invention are not limited thereto. In the examples, "parts" are expressed in "parts by weight" unless otherwise specified. Example 1 0.1 part of benzoyl peroxide, 2,2'-azobis-(2,4-dimethylvaleronitrile)
1.5 parts to 85 parts of styrene, 15 parts of n-butyl acrylate, and ethylene glycol dimethacrylate.
A polymeric composition was prepared by dissolving in a mixed solution consisting of 0.3 parts. On the other hand, nitrogen introduction pipe, cooling pipe, dropping funnel,
Take 1 part of tricalcium phosphate and 0.01 part of sodium dodecylbenzenesulfonate as a dispersion stabilizer into a four-necked flask equipped with a stirring bar, add distilled water, and add the polymer composition while stirring from a dropping funnel. dripped. After completion of the dropwise addition, the temperature was raised to 80°C, stirring was continued for 20 hours, and polymerization was carried out in a one-step polymerization process. After the polymerization was completed, the mixture was treated with hydrochloric acid, filtered, washed with water, and dried to obtain a vinyl resin for toner (styrene-acrylic copolymer resin). This will be referred to as resin sample [A]. Next, for 100 parts of the resin sample [A] of the present invention, 10 parts of carbon black [Mitsubishi Carbon Black #30, manufactured by Mitsubishi Chemical Industries, Ltd.] and 10 parts of oil black were added.
Low molecular weight polypropylene [Viscol
After adding and mixing 3 parts of 550P manufactured by Sanyo Chemical Industries, Ltd.
Melt in a roll mill, grind, grind in a jet mill,
It was classified to produce toner. This will be referred to as toner sample-1. Example 2 0.1 part of benzoyl peroxide, 1 part of 2,2'-azobis(2,4-dimethylvaleronitrile)
parts, 0.3 parts of 2,2'-azobis4-methoxy-2,4-dimethylvaleronitrile to 85 parts of styrene, n
- A polymerization composition was prepared by dissolving in a mixed solution consisting of 15 parts of butyl acrylate and 0.3 parts of benzene at a polymerization temperature of 80°C. Hereinafter, a styrene-acrylic copolymer resin (referred to as resin sample [B]) was produced in the same manner as in Example 1, and 100 parts of resin sample [B], 10 parts of carbon black, 1 part of oil black BW, and 2 parts of stearic acid were prepared. After adding and mixing parts, a toner was prepared. This will be referred to as toner sample-2. Example 3 1.0 part of 2,2'-azobisisobutyronitrile,
0.1 part of cyclohexanone peroxide, 85 parts of styrene, 15 parts of 2-ethylhexyl acrylate, and 0.1 part of trimethylolpropane triacrylate.
A polymeric composition was prepared by dissolving in a mixed solution consisting of 1. Below, Example 1 except that the polymerization temperature was 90°C
A resin (referred to as resin sample [C]) was produced in the same manner as above, and after adding and mixing 100 parts of resin sample [C], 10 parts of carbon black, 1 part of oil black BW, and 1 part of sodium oleate, it was made into a toner. . This will be referred to as toner sample-3. Example 4 Triiron tetroxide powder “Mapico Black BL-500” [Titan Kogyo Co., Ltd.] was added to 100 parts of resin sample [A].
After adding and mixing 67 parts of methylene blue chloride (charge control agent), 3 parts of low molecular weight polypropylene [Viscol 550P manufactured by Sanyo Chemical Industries, Ltd.], melted with a Banbury mixer, kneaded, crushed with a jet mill, and classified. A one-component magnetic toner was produced. This will be referred to as toner sample-4. Comparative Example 1 0.1 part of lauroyl peroxide and 1.5 parts of octanoyl peroxide were dissolved in a mixed solution consisting of 85 parts of styrene, 15 parts of n-butyl acrylate, and 0.3 parts of ethylene glycol dimethacrylate,
A polymeric composition was prepared. Hereinafter, a resin (referred to as resin sample [D]) was produced in the same manner as in Example 1 except that the polymerization temperature was set to 70°C.
100 parts, carbon black 10 parts, oil black
After adding and mixing 1 part of BW and 3 parts of low molecular weight polyolefin, a toner was prepared. This is referred to as comparative toner sample-1. Comparative Example 2 A copolymer resin was prepared by a two-stage polymerization method as follows. 85 parts of styrene, 15 parts of n-butyl acrylate, and 1 part of azobisisobutyronitrile were suspended and dispersed in 200 parts of distilled water containing 1 part of tricalcium phosphate and 0.01 part of sodium dodecylbenzenesulfonate, and the mixture was heated to 75°C. Polymerization was performed to obtain a resin with a high degree of polymerization. Next, 10 parts of the high polymerization resin and 3 parts of azobisisobutyronitrile were dissolved in 85 parts of styrene, 15 parts of n-butyl acrylate, and 0.1 part of ethylene glycol dimethacrylate, and 1 part of tricalcium phosphate was dissolved in 1 part of tricalcium phosphate. 75 parts, and suspended in 200 parts of distilled water containing 0.01 part of sodium dodecylbenzenesulfonate.
Polymerization was carried out at ℃. From the resin (referred to as resin sample [E]), 100 parts of resin sample [E], 10 parts of carbon black, 1 part of oil black BW, and 3 parts of low molecular weight polyolefin were prepared in the same manner as in Example 1.
The resulting toner is referred to as Comparative Toner Sample-2. The comparative test results of these toner samples are
They are summarized in the table.

[Table] From the above results, it can be seen that the toner of the present invention has better offset properties, fixing properties, and blocking properties, and less dirt on the lower roller during long runs, compared to conventional toners.

Claims (1)

[Scope of Claims] 1. Heat roller fixing device containing a vinyl resin as a main component obtained by suspension polymerization of a vinyl monomer in the presence of a polymerization initiator and a crosslinking agent, and containing a releasing substance. In the toner for developing an electrostatic image, the polymerization initiator is composed of two or more types of polymerization initiators having different half-lives, and among the polymerization initiators, polymerization initiator A has the longest half-life and polymerization initiator A has the shortest half-life. Half-life of polymerization initiator B at 70°C τ _A・τ _B
A toner for developing an electrostatic image for heat roller fixing, characterized in that the ratio τ _A /τ _B is 10 to 10 ⁶ .