JP3111118B2 - Lithographic printing plate for scanning exposure - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

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BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a scanning lithographic printing plate utilizing a silver complex diffusion transfer method suitable for using laser light, especially semiconductor laser light as a light source.

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2. Description of the Related Art A lithographic printing plate in which a transferred silver image obtained by a silver complex salt diffusion transfer method can be used immediately as ink acceptability has already been disclosed in JP-B-48-30562, JP-A-53-21602, and JP-A-54-562. 103104, 56
And are well known.

According to a typical silver complex salt diffusion transfer method suitable for the method of making a lithographic printing plate, a support and an undercoat layer on which a halation is prevented, a silver halide emulsion layer, When the photosensitive material comprising the development nucleus layer is image-exposed and developed, the silver halide on which the latent image is formed becomes blackened silver in the emulsion layer. At the same time, the silver halide having no latent image formed thereon is dissolved by the action of the silver halide complexing agent contained in the developing solution and diffuses to the surface of the photosensitive material. The dissolved and diffused silver complex salt is deposited as a silver image on the physical development nuclei of the surface layer by the reducing action of the developing agent. After the development process, if necessary, a sensitization process is performed after the development process to enhance the ink acceptability of the obtained silver image, and the silver image is set in an offset printing machine, and the ink image is transferred to a printed material.

In general, in the practice of the silver salt diffusion transfer method, a method of mixing a developing agent into a material is widely known. Therefore, in such a material, a processing solution used in a development step is generally used. A so-called "alkaline activating solution" substantially free of a developing agent can be used, and this alkaline activating solution has an advantage that fatigue due to air oxidation is extremely small.

According to the above-mentioned conventional method, a silver halide emulsion layer is formed by a merocyanine dye, a cyanine dye, etc.
The spectrum was sensitized so as to have a photosensitive maximum in a green region near nm, and exposure for several seconds to several tens of seconds was given by a plate making camera using a normal light source such as a tungsten light source. However, in the above-mentioned conventional method, there is a limit even though the lithographic printing plate is originally excellent in sharpness and resolution. In addition, if a color print is to be obtained from a color original, not only the resolution is insufficient, but also the production of the printing plate and the plate making operation are complicated.

[0006] Today, as a method for solving the above-mentioned problem, a method of directly making a plate using a laser beam has been considered. For example, US Pat. No. 4,501,81
No. 1, JP-A-59-71055 and JP-A-59-7105
6, No. 60-61752, No. 60-75838, No. 60-100148, No. 61-114235, No. 63-47756, helium / neon laser, argon ion laser, semiconductor laser, light emitting diode, etc. A lithographic printing plate used for the above is disclosed.

However, as described in the above-mentioned patent specification, in a lithographic printing plate utilizing a silver complex salt diffusion transfer method, the surface physical development nucleus layer has a large effect on the spectral sensitization, so that the target laser The sensitivity to light is reduced, the storage stability is deteriorated, and as a result of forming a soft transferred silver image by silver complex salt diffusion transfer development, the sharpness, the resolving power is reduced, and further background smearing occurs,
Since a silver image is removed during printing and sufficient printing durability cannot be obtained, it has been difficult to obtain a target high-quality lithographic printing plate. Therefore, a sensitizing dye satisfying these requirements has not only a sufficiently high sensitivity at the wavelength of the laser beam, but also a good storage stability, and a high contrast transferable silver image can be formed. In addition, it must satisfy such conditions that there is no adverse effect such as background contamination, and that a strong transfer-precipitated silver which cannot be formed even during minute printing can be formed.

[0008] Such properties are also influenced by factors such as the halogen composition of the silver halide emulsion, the silver halide grain size, the type and degree of chemical sensitization. Emulsions mainly composed of silver chloride as silver halide have the advantage that diffusion transfer development occurs promptly, and that transfer precipitation silver used as an ink receiving portion is easily generated. However, in general, the sensitivity is low, and if chemical sensitization is performed sufficiently to achieve high sensitivity, the sensitivity to laser light will be lower.If diffusion transfer development is too fast, the print strength of the transferred silver will be reduced. Tend to be weak.

The lithographic printing plate for an infrared semiconductor laser has been developed to satisfy the above-mentioned various properties. However, a sensitizing dye used for enhancing the photosensitivity in the infrared region and a sensitizing dye using the same are used. The time-dependent change of the lithographic printing plate coated with the adjusted emulsion was remarkably large as compared with other types, and there was a problem in storability. Various methods have been proposed for improving the storage stability. For example, Japanese Patent Application Laid-Open Nos. 63-243943 and 6
4-13539, pp. 64-40947, 64-4
0949, same Hei 1-223463, same Hei 2-28735
7, the method described in the specification of JP-A-3-287273, etc. has been proposed, but it has not yet reached a sufficient level.

[0010]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a scanning lithographic printing plate having an excellent storability and using an infrared semiconductor laser as an exposure light source.

[0011]

The object of the present invention is to provide at least one undercoat layer, a silver halide emulsion layer and a physical development nucleus sequentially coated on a support, wherein the silver halide emulsion layer has a thickness of 700 nm or more. A lithographic plate for scanning exposure containing a sensitizing dye having a maximum spectral sensitivity in a long wavelength region, and an amount of a developing agent capable of developing the exposed silver halide in an alkali activating bath in the component. In the printing plate, the silver halide emulsion layer contains at least one mercapto compound represented by the general formula (1) and / or (2), and the content of the sensitizing dye is such that spectral sensitization by the sensitizing dye is suppressed. It is attained by adjusting the content so as to exceed the content to be maximized and to reduce the spectral sensitivity and to be 1.5 to 4 times the content to maximize the spectral sensitization.

In the formula, R1 and R2 each represent a hydrogen atom, an alkyl group, an aryl group, an aralkyl group, an alkenyl group, an amino group, an acylamide group or a sulfonamide group, and the sum of the carbon numbers of R1 and R2 is 3 or more.

In the formula, R3 represents an alkyl group or an aryl group having 3 or more carbon atoms.

As specific examples of the mercapto compound represented by the general formula (1) used in the present invention,
However, the present invention is of course not limited to these.

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The amount of the mercapto compound represented by the general formula (1) used in the present invention depends on the compound and the layer constitution, but is from 0.9 mmol to 180 mol per mol of silver.
Mmol, preferably in the range of 2.7 to 45 mmol.

Specific examples of the mercapto compound of the general formula represented by Chemical Formula 2 used in the present invention include Chemical Formulas 17 to 2
9, but the present invention is of course not limited to these.

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The addition amount of the mercapto compound represented by the general formula (2) used in the present invention depends on the compound and the layer structure, but is from 0.9 mmol to 180 mol per mol of silver.
Mmol, preferably in the range of 2.7 to 45 mmol.

The infrared sensitizing dye having a maximum spectral sensitivity in a long wavelength region of 700 nm or more used in the present invention is, for example, US Pat. Nos. 2,095,854 and 2,095,8.
No. 56, No. 2,955,939, No. 3,482
No. 978, No. 3,552,974, No. 3,57
No. 3,921, No. 3,582,344, No. 3,6
23,881 and the like.

Preferably, for example,
2, a dye represented by the general formula of Chemical formula 33 is used.

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In the formulas of Chemical Formulas 30, 31, 32, and 33,
Z1 and Z2 may be the same or different and each represents an atom group necessary to form a 5- or 6-membered nitrogen-containing heterocyclic ring. R1 and R2 may be the same or different and each represents an alkyl group or an alkenyl group. R3 represents an alkyl group, an alkenyl group, or an aryl group. R4 to R10 may be the same or different and each represents a hydrogen atom, a halogen atom, an alkyl group, an aryl group, or an alkoxy group. However, R6 and R7 or R8 and R9 can be linked to each other to form a 5- or 6-membered ring. Y is a sulfur atom, an oxygen atom,>
It represents NR13 (R13 is an alkyl group). X represents an acid anion. l, m, n, p and q are each 1 or 2
Represents Q represents a group of atoms necessary to form a 5- or 6-membered heterocyclic ring.

Specific examples of Z1 and Z2 include thiazole, benzothiazole, naphtho [1,2-a] thiazole, naphtho [2,1-a] thiazole, selenazole,
Benzoselenazole, naphtho [1,2-a] selenazole, naphtho [2,1-a] selenazole, oxazole, benzoxazole, naphtho [1,2-a] oxazole, naphtho [2,1-a] oxazole, 2 -Quinoline, 4-quinoline, 3,3-dialkylindolenine, imidazole, benzimidazole, naphtho [1,
2-a] Nitrogen-containing heterocycles such as imidazole and pyridine can be exemplified. These heterocycles include an alkyl group (eg, methyl, ethyl, butyl, trifluoromethyl, etc.), an aryl group (eg, phenyl, tolyl, etc.), a hydroxy group, an alkoxy group (eg, methoxy, ethoxy, butoxy, etc.), A carboxy group, an alkoxycarbonyl group (eg, methoxycarbonyl, ethoxycarbonyl, etc.), a halogen atom (eg, fluorine, chlorine, bromine,
Iodine), aralkyl group (eg, benzyl, phenethyl, etc.), cyano group, alkenyl group (eg, allyl, etc.)
May have one or more substituents.

In R1 and R2, examples of the alkyl group include lower alkyl groups such as methyl, ethyl and butyl, hydroxyalkyl groups such as β-hydroxyethyl and γ-hydroxypropyl, and alkoxy groups such as β-methoxyethyl and γ-methoxypropyl. Alkyl group, β-acetoxyethyl,
acyloxyalkyl groups such as γ-acetoxypropyl and β-benzoyloxyethyl; alkoxycarbonylalkyl groups such as methoxycarbonylmethyl; sulfoalkyl groups such as β-sulfoethyl, γ-sulfopropyl and δ-sulfobutyl; benzyl, phenethyl and sulfobenzyl And alkenyl groups such as aralkyl groups and the like.

Examples of R3 include the alkyl groups and alkenyl groups described above for R1 and R2, and aryl groups such as phenyl, tolyl, methoxyphenyl, chlorophenyl, and naphthyl.

R4 to R10 represent a hydrogen atom, a halogen atom (for example, fluorine, chlorine, bromine, iodine), R1, R2
An alkyl group, an aryl group or R1,
An alkoxy group having an alkyl as described for R2 (ie, an OR1 group), and R6 and R7 or R8 and R9
The 5- or 6-membered ring formed by and may be substituted with a lower alkyl group or the like. R11 and R12 are R1, R2
Represents an alkyl group or an aryl group as described in
And R12 can be linked to each other to form a 5- or 6-membered ring. R13 is an alkyl group as described for R1 and R2.

Examples of the acid anion X include alkyl sulfate ions such as methyl sulfate and ethyl sulfate, thiocyanate ions, toluene sulfonate ions, halogen ions such as chlorine, bromine and iodine, and perchlorate ions. It does not exist when a betaine-like structure is used.

Specific examples of the sensitizing dye used in the present invention are shown below.

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The addition amount of the sensitizing dye used in the present invention is as follows.
Depending on the type of sensitizing dye or the type of silver halide photographic emulsion, etc., the sensitizing dye content exceeds the content that maximizes the spectral sensitization by the sensitizing dye, is the region where the spectral sensitivity decreases, And it can be used in a range of 1.5 times or more and 4 times or less the content that maximizes spectral sensitization. If it is out of this range, the preservability will deteriorate.

In the present invention, to add a sensitizing dye to a silver halide emulsion, methanol, isopropanol,
It can be added as a solution of a single or mixed solvent such as pyridine, dimethylformamide and water. It can also be added to the emulsion using ultrasonic dispersion. Further, U.S. Patent Nos. 3,482,981 and 3,585,1
Nos. 95, 3,469,987 and 2,91
The method described in US Pat. No. 2,343 can also be used.

In the present invention, the component contains a developing agent in such an amount that the exposed silver halide can be developed in an alkali activating bath. The developing agents used in the present invention include dihydroxybenzenes such as hydroquinone, methylhydroquinone and catechol, 1-phenyl-3-pyrazolidone, 1-phenyl-4-methyl-
3-pyrazolidone, 1-phenyl-4,4-dimethyl-
1-phenyl- such as 3-pyrazolidone and 1-phenyl-4-hydroxymethyl-4-methyl-3-pyrazolidone
3-pyrazolidones and the like. The amount of the developing agent of the present invention depends on various factors of the silver halide emulsion, for example, the amount of silver,
Although depending on the type of the developing agent such as the amount of the binder, the amount of dihydroxybenzenes is 0.0045 mol / m ^{2 to} 0.0
A range of 18 mol / m ² is preferable, and 1-phenyl-3-
Pyrazolidones are 1/20 of dihydroxybenzenes.
It is in the range of 1/2 weight.

The planographic printing plate which is the object of the present invention contains gelatin, and the layer containing the same can be an undercoat layer, an emulsion layer, or a physical development nucleus layer. These gelatin-containing layers can be hardened with a gelatin hardener.
Gelatin hardeners include, for example, inorganic compounds such as chrome alum, aldehydes such as formalin, glyoxal, malealdehyde and glutaraldehyde, N-methylol compounds such as urea and ethylene urea, mucochloric acid, 2,3 Aldehydes such as dihydroxy-1,4-dioxane, 2,4-dichloro-6-hydroxy-
Compounds having an active halogen such as S-triazine salt, 2,4-dihydroxy-6-chloro-S-triazine salt, divinyl sulfone, divinyl ketone, N, N, N
-Tricloyl hexahydrotriazine, compounds having two or more ethyleneimino groups or epoxy groups as active three-membered rings in a molecule, one of various compounds such as dialdehyde starch as a polymer hardener or Two or more can be used.

The hardener can be added to all layers or to some or only one layer.
Of course, in the case of two-layer simultaneous application, the diffusible hardener can be added to only one of the layers. The addition method can be added at the time of emulsion production or in-line at the time of coating.

The gelatin in the gelatin-containing layer of the present invention is partially formed of water-soluble gelatin, starch, dextrin, albumin, sodium alginate, hydroxyethylcellulose, gum arabic, polyvinyl alcohol, polyvinylpyrrolidone, carboxymethylcellulose, polyacrylamide, and styrene. It can be substituted with one or more hydrophilic polymers such as maleic anhydride copolymer and polyvinyl methyl ether-maleic anhydride copolymer. Further, an aqueous vinyl polymer dispersion (latex) can be added to the gelatin layer.

The polymeric binder in the undercoat layer is generally 0.5
More preferably to 10 g / m ² is 1 to 6 g / m ^2. The undercoat layer may contain pigments such as carbon black, dyes and the like for the purpose of preventing halation. Further, a photographic additive such as a developing agent can be included. The undercoat layer is disclosed in
8-5503, 48-100203, and 49-1
6507.

The silver halide emulsion layer includes, for example, silver chloride,
It consists of silver bromide, silver chlorobromide, and those containing silver iodide. The silver halide crystals may contain a heavy metal salt such as a rhodium salt, an iridium salt, a palladium salt, a ruthenium salt, a nickel salt, a platinum salt, and the like, and the addition amount is 10 ^-8 to 10 ^-3 per mol of silver halide. Is a mole. The crystal form of the silver halide is not particularly limited, and may be cubic to tetradecahedral grains, or core-shell or tabular grains. The silver halide crystal may be a monodisperse or polydisperse crystal,
Its average particle size is in the range of 0.2-0.8 μm. One preferred example is a monodispersed or polydispersed crystal containing 80 mol% or more of silver chloride, which contains a rhodium salt or an iridium salt.

The silver halide emulsion can be sensitized in various ways when it is manufactured or coated. It is preferred to chemically sensitize by methods well known in the art such as, for example, with sodium thiosulfate, alkyl thioureas, or with gold compounds such as rhodium gold, gold chloride, or a combination of both.

The surface layer existing above the emulsion layer contains physical development nuclei. As physical development nuclei, silver, antimony, bismuth, cadmium, cobalt, lead, nickel, palladium, rhodium, gold, platinum and other metal colloid fine particles, and sulfides, polysulfides, selenides of these metals, or those It may be a mixture or a mixed crystal. The physical development nucleus may or may not contain a hydrophilic binder. It may contain a copolymer of imidazole and acrylamide, a copolymer of acrylic acid and acrylamide, a hydrophilic polymer such as polyvinyl alcohol or an oligomer thereof, and the content thereof is 0.5 g / m2.
It is preferably ² or less. Further, the physical development nucleus layer may contain a developing agent such as hydroquinone, methylhydroquinone, or catechol, or a known hardener such as formalin or dichloro-s-triazine.

Each coating layer such as an undercoat layer, a silver halide emulsion layer and a physical development nucleus layer may contain some of anionic, cationic or neutral surfactants as coating aids. , An antifoggant, a matting agent, a thickener, an antistatic agent and the like.

As the support of the lithographic printing plate of the present invention,
Paper, a synthetic or semi-synthetic polymer film, a metal plate of aluminum, iron, or the like can be used as long as it can withstand lithographic printing. One or both surfaces of the support may be coated with one or more polymer films or metal thin films. The surface of these supports can be subjected to a surface treatment in order to improve the adhesion to the coating layer.

Particularly preferably used supports are paper coated on both sides or one side with a polyolefin polymer, a polyester film, a polyester film having a hydrophilic surface, an aluminum plate subjected to a surface treatment, and the like.
These supports may contain pigments for preventing halation and solid fine particles for improving the surface properties. The support may be light-transmissive so that backside exposure is possible.

The developing solution used in the present invention includes an alkaline substance such as sodium hydroxide, potassium hydroxide,
Lithium hydroxide, tribasic sodium phosphate, etc., sulfites as preservatives, silver halide solvents such as thiosulfate,
Thiocyanates, cyclic imides, 2-mercaptobenzoic acid, amines and the like, thickeners such as hydroxyethylcellulose, carboxymethylcellulose and the like, antifoggants,
For example, potassium bromide, a compound described in JP-A-47-26201, a developing agent such as hydroquinones, catechol, 1-phenyl-3-pyrazolidone, a developing modifier,
For example, it can contain a polyoxyalkylene compound, an onium compound, and the like. Further, as the developing solution, a compound or the like described in U.S. Pat. No. 3,776,728 which improves the ink running of the surface silver layer can be used.

The surface silver layer after development of the lithographic printing plate of the present invention can be converted to ink receptivity or enhanced in receptivity with any known surface treating agent. Examples of such a processing solution include JP-B-48-29723 and U.S. Pat.
21, 559 and the like. The printing method, the desensitizing solution, the humidifying solution, and the like to be used can be a commonly known method. Hereinafter, the present invention will be described with reference to Examples, but it should be understood that the present invention is not limited thereto.

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【Example】

Example 1 A matte layer containing silica particles having an average particle size of 5 μm was provided on one side of a polyethylene terephthalate base having a thickness of 100 μm, and carbon black and phenidone were provided on the other side. Undercoat layer containing silica powder with an average particle size of 7 (adjusted to pH 4.5)
And after chemical sensitization with hypo and gold compounds,
With 7.56 mmol per mole of silver halide and 2-
Silver halide emulsion layer containing mercaptobenzoic acid (pH
4.5).

The silver halide emulsion was dye-sensitized by adding the compound shown in Table 1 below per mol of silver halide and adding 67.9 mmol per mol of silver after addition of the dye. .

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[Table 1]

The gelatin for the undercoat layer was applied at 3.0 g / m ² , the gelatin for the emulsion layer was applied at 1.0 g · m ² , and the silver halide was converted to silver nitrate at 1.0 g / m ² . The undercoat layer and the emulsion layer were formed by using formalin as a hardener and gelatin with respect to gelatin.
It is contained in an amount of 0 mg / g gelatin.

After drying and heating at 50 ° C. for 2 days, Example 1 of JP-A-57-86835 was applied on this emulsion layer.
Was prepared in the same manner as above, and a lithographic printing plate was prepared by applying a liquid added to the nucleus coating liquid at a concentration of 0.67 g / m ² .

Each of the lithographic printing plates was 50 ° C. and 80%
R. H. And those that were not heated for 3 days.

[0110] filter that transmits infrared light having a wavelength of 780nm to the light source, and with a wedge of density difference 0.15 exposed with 10 ^-5 seconds in close contact with each lithographic printing plate, sensitometric this Sample.

An original including a fine line image was printed on the lithographic printing plate with a laser diode scanner LINOTYPE 200 which emits light at 780 nm, and this was used as a printing sample.

Each image-exposed printing plate was developed with the following DTR developer at 30 ° C. for 15 seconds.

<Transfer developing solution> Water 700 ml Sodium hydroxide 18 g Potassium hydroxide 7 g Sodium sulfite 50 g 2-mercaptobenzoic acid 1 g Uracil 10 g 2-methylaminoethanol 30 ml 5-phenyl-2-mercapto 0.1 g-1,3,3 4-Oxadiazole Potassium bromide 1g Add water to make the total volume 1L

After the development, the original plate was passed between two squeezing rollers to remove excess developer, and immediately treated with a neutralizing solution having the following composition at 25 ° C. for 20 seconds. Removed and dried at 50 ° C.

<Neutralizing solution> Water 600 ml Citric acid 10 g Sodium citrate 35 g Colloidal silica (20% solution) 5 ml Ethylene glycol 5 ml Add water to make 1 liter.

The printing durability was determined by attaching each printing sample to one sheet and mounting the sheet on an offset printing machine.
The following desensitizing solution was applied all over the plate surface, and printing was performed using the following humidifying solution.

<Desensitizing liquid> Water 600 ml Isopropyl alcohol 400 ml Ethylene glycol 50 g 3-mercapto-4-acetamido-5-n-heptyl-1,2,4-triazole 1 g

<Humidifier> o-phosphoric acid 10 g nickel nitrate 5 g sodium nitrite 5 g ethylene glycol 100 g colloidal silica (20% solution) 28 g Water is added to make 2 l.

The printing machine used was RYOBI3200.

Table 2 shows the relative sensitivity where the sensitivity of the unheated lithographic printing plate a is 100.

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[Table 2]

It is understood from Table 2 that the more the dye was added in excess, the less the change in sensitivity before and after heating.

As a result of examining the printing durability, it was found that there was no significant difference between the lithographic printing plate comparisons a and a to f. For g, the printing durability was weak.

Example 2 The procedure of Example 1 was repeated, except that sensitizing dye conversion 49 was used in place of sensitizing dye conversion 46 in Example 1. The same results as in Example 1 were obtained.

Example 3 The procedure of Example 1 was repeated, except that sensitizing dye 57 was used instead of sensitizing dye 46. The same results as in Example 1 were obtained.

Example 4 The procedure of Example 1 was repeated, except that Chemical Formula 8 was used instead of Chemical Formula 7 and Chemical Formula 20 was used instead of Chemical Formula 17. The same results as in Example 1 were obtained.

Example 5 Comparison a, d, and e of Example 1 was carried out except that 2-mercaptobenzoic acid was used instead of Chemical formulas 7 and 17, and the results as shown in Table 3 were obtained.

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[Table 3]

From the results shown in Table 3, it can be seen that sensitivity was not obtained even when a mercapto compound other than Chemical Formulas 1 and 2 was used, and that the photographic storage stability was not improved.

[0130]

According to the present invention, in a direct plate making method for a semiconductor laser diode / scanner system, a lithographic printing plate can be obtained by a silver salt diffusion transfer method which does not decrease the printing durability and has good storability. It became so.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-63-174045 (JP, A) JP-A-2-287357 (JP, A) JP-A-64-40947 (JP, A) JP-A-63-1 198064 (JP, A) JP-A-63-47773 (JP, A) JP-A-6-175366 (JP, A) JP-B-48-29723 (JP, B1) (58) Fields investigated (Int. ⁷ , DB name) G03F 7/07 G03C 8/06 501

Claims (1)

(57) [Claims] At least one undercoat layer, a silver halide emulsion layer and a physical development nucleus are sequentially coated on a support, and the silver halide emulsion layer has a maximum spectral sensitivity in a long wavelength region of 700 nm or more. A lithographic printing plate for scanning exposure containing a sensitizing dye having an amount of a developing agent capable of developing the exposed silver halide in an alkali activating bath in the constituent element. The emulsion layer contains at least one mercapto compound represented by the general formula (1) and / or the chemical formula (2), and the sensitizing dye content exceeds the content that maximizes spectral sensitization by the sensitizing dye; A lithographic printing plate for scanning exposure, wherein the lithographic printing plate is a region where the spectral sensitivity is reduced and the content is 1.5 times or more and 4 times or less the content that maximizes the spectral sensitization. Embedded image (Wherein, R1 and R2 represent a hydrogen atom, an alkyl group, an aryl group, an aralkyl group, an alkenyl group, an amino group, an acylamide group, and a sulfonamide group, respectively.
And the sum of the carbon numbers of R2 is 3 or more. ) (In the formula, R3 represents an alkyl group or an aryl group having 3 or more carbon atoms.)