A method for producing a regenerating developer, and a photosensitive resin composition suitable for the method.

Abstract

According to the present invention, a regenerated developing liquid is produced from a waste developing liquid for a water-developable flexographic printing original plate, the regenerated developing liquid having high performance in regard to a development speed and a problem of adhesion of aggregate to a surface of a printing plate. The present invention provides a method for producing a regenerated developing liquid from a waste developing liquid generated by the development of a water-developable flexographic printing original plate having a photosensitive resin layer formed from a photosensitive resin composition, the method comprising leaving the waste developing liquid in a tank without adding a coagulant to the waste developing liquid, allowing the photosensitive resin composition contained in the waste developing liquid to float and aggregate, discharging the floating aggregate from the tank, and obtaining the liquid remaining in the tank as the regenerated developing liquid, characterized in that the true density of the photosensitive resin composition before light curing is 0.91-0.97 g / cm3 and the photosensitive resin composition contains a water soluble compound in the ratio of 1.5% to 15% by mass.

Description

[Technical Field]

[0001] The present invention relates to a method for producing a high-performance regenerated developer from developing waste liquid generated by developing water-developable flexographic printing plates, and to a photosensitive resin composition for water-developable flexographic printing plates suitable for this method. [Background technology]

[0002] Flexographic printing plates generally have a structure in which a photosensitive resin layer formed from a photosensitive resin composition is provided on a support. The production of such flexographic printing plates is carried out, for example, by selectively exposing the photosensitive resin layer with ultraviolet light and then developing the exposed photosensitive resin layer with an aqueous developer. During development, brushing or other methods physically remove the photosensitive resin composition from the unexposed areas of the photosensitive resin layer, dispersing or dissolving it in the developer. Repeated development of the photosensitive resin layer using the same developer increases the concentration of the photosensitive resin composition dispersed in the developer, leading to a decrease in development speed or the aggregation of the dispersed photosensitive resin composition, forming aggregates. These aggregates reattach to the surface of the printing plate, degrading the quality of the plate surface. Therefore, it is necessary to discard the developer with a high concentration of the photosensitive resin composition and replace it with a new developer to resume plate production. However, repeatedly discarding used developer and replacing it with a new one is undesirable from the standpoint of environmental impact and manufacturing costs. Therefore, attempts are being made to remove the photosensitive resin composition from used developing solutions and reuse them.

[0003] Patent Document 1 discloses a method for separating the aggregated resin solids by agglomerating resin components dispersed in used developer solution with a coagulant. However, coagulants are expensive, and their use is undesirable from a cost standpoint. Furthermore, there is a fundamental problem that developer solutions to which coagulants have been added cannot be reused as developer solutions because the development speed is significantly reduced.

[0004] Furthermore, Patent Document 2 discloses a method for reusing a developer by agglomerating a photosensitive resin composition using a special dedicated dispersion filter, and then filtering the aggregates with another aggregate filter. However, this method requires the use of a special dedicated dispersion filter and an aggregate filter, which necessitates the cumbersome task of maintaining both filters.

[0005] Furthermore, Patent Document 3 discloses a method for producing a regenerated developer by removing undissolved resin components from the developer waste solution by treating it with a ceramic filter. This method requires only one filter. However, even with this method, frequent backwashing of the filter is necessary to prevent clogging, resulting in a significant maintenance burden.

[0006] To solve the problems of the prior art described above, the applicant proposed in Patent Document 4 a method in which developing waste liquid is left to stand in a tank for a long time, causing the photosensitive resin composition dispersed in the developing waste liquid to float and coagulate due to the difference in specific gravity with water, the main component of the developing waste liquid, and then the floating aggregate is discharged from the tank, with the liquid remaining in the tank being used as a regenerated developing solution. With this method, a regenerated developing solution with a low solid content concentration can be obtained with a high recovery rate using a simple method. However, in recent years, with the increasing resolution of printed materials, there has been a demand for a method of producing a regenerated developing solution with even higher performance in terms of developing speed and the problem of aggregates re-adhering to the surface of the printing plate. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Application Publication No. 06-194824 [Patent Document 2] Japanese Patent Publication No. 2011-232407 [Patent Document 3] International Publication No. 2018 / 061958 [Patent Document 4] PCT / JP2022 / 9590 [Overview of the project] [Problems that the invention aims to solve]

[0008] The present invention was conceived in view of the current state of the prior art described above, and its purpose is to provide a method for producing a regenerated developer from developing waste liquid generated by the development of a water-developable flexographic printing plate, which has high performance in terms of development speed and the problem of aggregates re-adhering to the surface of the printing plate, and a photosensitive resin composition for water-developable flexographic printing plates suitable for this method. [Means for solving the problem]

[0009] To achieve this objective, the inventors diligently studied photosensitive resin compositions used in printing plates, which are the target of development. As a result, they found that by using a photosensitive resin composition with a specific density range and including a water-soluble compound in a specific proportion within that range, a high solid content recovery rate can be achieved when producing a regenerated developer by the static method. Consequently, a regenerated developer with a high development speed and less problem of aggregates re-adhering to the surface of the printing plate can be obtained.

[0010] The present invention was completed based on the above findings and has the following configurations (1) to (6). (1) A method for producing a recycled developer from developing waste liquid generated by developing a water-developable flexographic printing plate having a photosensitive resin layer made of a photosensitive resin composition containing at least a hydrophobic polymer, a hydrophilic polymer, a photopolymerizable unsaturated compound, a photopolymerization initiator, and a plasticizer, A process in which the developing waste liquid is left to stand in a tank without adding a coagulant to the developing waste liquid, and the photosensitive resin composition contained in the developing waste liquid is allowed to float and coagulate, and A process of discharging floating aggregates from the tank and obtaining the liquid remaining in the tank as a regenerated developer. In a method including, The true density of the aforementioned photosensitive resin composition before photocuring is 0.91 to 0.97 g / cm³. 3The photosensitive resin composition contains a water-soluble compound in a proportion of 1.5 to 15% by mass. A method characterized by the following. (2) The true density of the photosensitive resin composition after photocuring is 0.93 to 0.99 g / cm³ 3 The method according to (1), characterized in that (3) The method according to (1), characterized in that the water-soluble compound is a water-soluble, photopolymerizable unsaturated compound selected from the group consisting of compounds having polyalkylene glycol in their compound structure, compounds containing a carboxyl group in the molecule, compounds containing a hydroxyl group in the molecule, compounds containing an amino group in the molecule, and mixtures thereof. (4) A method for producing a water-developable flexographic printing plate, characterized in that a regenerated developer obtained by the method described in any one of (1) to (3) is used as a developer. (5) A photosensitive resin composition for water-developable flexographic printing plates containing at least a hydrophobic polymer, a hydrophilic polymer, a photopolymerizable unsaturated compound, a photopolymerization initiator, and a plasticizer, wherein the true density of the photosensitive resin composition before photocuring is 0.91 to 0.97 g / cm³ 3 A photosensitive resin composition for water-developable flexographic printing plates, characterized in that the photosensitive resin composition contains a water-soluble compound in a proportion of 1.5 to 15% by mass. (6) A water-developable flexographic printing plate having a support and a photosensitive resin layer, wherein the photosensitive resin layer is made of the photosensitive resin composition for water-developable flexographic printing plates described in (5). [Effects of the Invention]

[0011] According to the method of the present invention, a photosensitive resin composition with a specific density is used, and a water-soluble compound is included in the photosensitive resin composition in a specific proportion. Therefore, a high-performance recycled developer with a high development rate and virtually no problem of aggregates re-adhering to the surface of the printing plate can be produced from the developing waste liquid generated by the development of water-developable flexographic printing plates. [Brief explanation of the drawing]

[0012] [Figure 1] An example of a developing apparatus for a water-developable flexographic printing original plate that can be used in the method of the present invention is shown. [Figure 2] An example of the shape of a discharge port installed on the side of a tank that can be used in the method of the present invention is shown. [Figure 3] Another example of the shape of a discharge port installed on the side of a tank that can be used in the method of the present invention is shown. [Figure 4] An example of a state where the lid of the discharge port installed on the side of the tank that can be used in the method of the present invention is closed is shown. [Figure 5] An example of a state where the lid of the discharge port installed on the side of the tank that can be used in the method of the present invention is open is shown.

Embodiments for Carrying Out the Invention

[0013] A method for producing a regenerated developing solution from the developing waste liquid of the water-developable flexographic printing original plate of the present invention, and the photosensitive resin composition used in the method will be described in detail below.

[0014] First, an example of a developing apparatus for a water-developable flexographic printing original plate that can be used in the method of the present invention is shown in FIG. 1. In FIG. 1, 1 is a developing tank, and the developing solution 2 used for developing the flexographic printing original plate is stored herein. 3 is a pipe, and the developing solution with an increased concentration of the photosensitive resin composition due to repeated development is transferred as developing waste liquid from the developing tank 1 to the stationary tank 4 through this pipe 3. FIG. 1 shows the state after the developing waste liquid has been left standing in the stationary tank 4, where the aggregates 5 float and gather at the upper part, forming a solid-liquid mixed phase containing the aggregates below the liquid surface of the developing waste liquid. 6 is a discharge port installed on the side of the stationary tank 4, and for example, the solid-liquid mixed phase containing the aggregates 5 is discharged from this discharge port 6 to the aggregate recovery tank 7. 8 is a pump, and the regenerated developing solution from which the aggregates 5 have been removed and the solid content concentration has decreased is returned to the developing tank 1 through the pipe 9 by this pump 8.

[0015] Figure 1 illustrates a configuration in which a settling tank is provided as a tank, and the developing waste liquid is transferred from the developing tank to the settling tank. By providing a settling tank separately from the developing tank in this way, the next developing process can be carried out while the tank is settling, and furthermore, there is the advantage that solid matter separated during settling does not accumulate on brushes or other parts immersed in the liquid in the developing tank. However, the present invention is not limited to this configuration, and for example, the developing tank may be configured in the same way as the settling tank, and the same processes carried out in the settling tank may be carried out in the developing tank. In the present invention, a settling tank or such a developing tank can be used as a tank for settling developing waste liquid when manufacturing a regenerated developing solution from the developing solution.

[0016] In the method for producing a regenerated developer of the present invention, the developing waste liquid used as a raw material is generated by developing a water-developable flexographic printing plate. The water-developable flexographic printing plate has a basic structure in which a photosensitive resin layer is provided on a support, the photosensitive resin layer being made of a photosensitive resin composition containing at least a hydrophobic polymer, a hydrophilic polymer, a photopolymerizable unsaturated compound, a photopolymerization initiator, and a plasticizer. In the present invention, the support is not particularly limited, and conventionally known supports can be used.

[0017] A key feature of the present invention is that the photosensitive resin composition constituting the photosensitive resin layer of the printing plate has a true density of 0.91 to 0.97 g / cm³ before photocuring. 3 The developer must contain a water-soluble compound in a proportion of 1.5 to 15% by mass. The developer used for developing water-developable flexographic printing plates is an aqueous developer with water as its main component. The true density (specific gravity) of water is 1.0 g / cm³ at room temperature. 3 Therefore, the true density of the photosensitive resin composition before photocuring is 0.91 to 0.97 g / cm³. 3This means that the uncured photosensitive resin composition that has not been exposed to light is lighter than water, which is the main component of the aqueous developer used for developing the printing original plate. Here, the developer waste liquid generated by developing the printing original plate contains particles of water-insoluble solids derived from the photosensitive resin composition in the uncured photosensitive resin layer of the unexposed portion removed from the printing plate. Therefore, when the developer waste liquid is allowed to stand, the particles of water-insoluble solids derived from the uncured photosensitive resin composition that was contained in the developer waste liquid float and aggregate due to the difference in specific gravity with water. By removing this floating aggregate, the developer waste liquid can be regenerated as a developer.

[0018] The upper limit of the true density of the photosensitive resin composition before photocuring is 0.97 g / cm 3 and it is necessary to be, preferably 0.96 g / cm 3 and more preferably 0.95 g / cm 3 If the true density before photocuring exceeds the above upper limit, the difference in specific gravity with water, which is the main component of the developer waste liquid, becomes small, making it difficult for the photosensitive resin composition to float and aggregate. This may increase the standing time of the developer waste liquid and make it impractical. In addition, the photosensitive resin composition that could not be recovered remains in the regenerated developer and may cause adhesion to the surface of the printing plate. On the other hand, the lower limit of the true density of the photosensitive resin composition before photocuring is 0.91 g / cm 3 and it is necessary to be, preferably 0.92 g / cm 3 and more preferably 0.93 g / cm 3 If the true density before photocuring is less than the above lower limit, the floating of the photosensitive resin composition is too fast, and the photosensitive resin composition may float and aggregate prematurely during the development of the printing original plate. There is a risk that the aggregate derived from the photosensitive resin composition removed by development may reattach to the surface of the printing plate.

[0019] In this invention, "true density" refers to a density where only the volume occupied by the substance itself is used for density calculation, and pores and internal voids are not included in the volume, making it a density suitable for photosensitive resin compositions. The true density can be measured by placing the photosensitive resin composition in a Gay-Lussac type pycnometer. The voids in the sample are completely degassed and replaced with liquid, and the relationship between weight and volume is calculated to determine the true density. The true density of a photosensitive resin composition can be controlled by adjusting the density of each component constituting the photosensitive resin composition (hydrophobic polymer, hydrophilic polymer, photopolymerizable unsaturated compound, photopolymerization initiator, and plasticizer, etc.) and the mixing ratio of each component. Specifically, the desired true density can be achieved by fine-tuning the average value of the densities calculated from the densities and percentages of each component constituting the photosensitive resin composition by comparing it with the measured true density value measured by the method described later in the examples.

[0020] In this invention, the true density "before" photocuring is within a specific range (0.91~0.97 g / cm³). 3 The specification states that the true density is (0.02 g / cm³), but this is a numerical range set taking into account the true density "after" photocuring. The inventors have found that when a printing plate is exposed to light and the photosensitive resin composition constituting the photosensitive resin layer in the exposed portion of the printing plate is photocured, the volume of the resin composition shrinks slightly, and accordingly the true density is slightly (0.02 g / cm³). 3 We found that it increases to a certain extent. In this invention, the upper limit of the true density "before" photocuring is set to "0.97 g / cm³". 3 The reason it's set to this value is that, with this value, the true density "after" photocuring will be "0.99 g / cm³". 3 This is because, as the specific gravity remains lower than that of water, which is the main component of the developer solution, water-insoluble solid particles in the developing waste solution derived from the resin composition of the photocuring part can be removed by floating and agglomerating upon standing.

[0021] Conventionally, the only water-insoluble solid components in developing wastewater that have been considered are those derived from un-photocured photosensitive resin compositions in the unexposed portions of the printing plate that are scraped off with a brush during development. In contrast, the present invention focuses on the fact that not only un-photocured photosensitive resin compositions in the unexposed portions of the printing plate, but also photocured photosensitive resin compositions in the exposed portions of the printing plate, can be scraped off with a brush during development and become included in the developing wastewater. Specifically, even if halftone dots are formed on the printing plate by photocuring due to exposure, those with insufficient three-dimensional shape due to insufficient photocuring can be scraped off from the plate by brushing during development and flow into the developing solution. In particular, in recent years, with the increasing resolution of printed materials, there has been a growing demand for the manufacture of printing plates with small halftone dots. However, it is difficult to form small halftone dots on the printing plate with high precision. As a result, weak halftone dots with insufficient three-dimensional shape due to photocuring are rubbed off from the plate by brushing during development and flow into the developer solution, where they exist as water-insoluble solids in the developer waste. Since these water-insoluble solids originate from the photosensitive resin composition *after* photocuring, their true density is greater than that of the water-insoluble solids originating from the photosensitive resin composition *before* photocuring, which has been the focus of conventional research, as described above. Therefore, in order to reliably remove the water-insoluble solids originating from the photosensitive resin composition *after* photocuring by allowing it to float and aggregate through standing, it is preferable to consider the true density *after* photocuring. The upper limit of the true density *before* photocuring as defined in this invention is 0.97 g / cm³. 3 The value was set taking this into consideration. The true density of the photosensitive resin composition "after" photocuring in this invention was measured using an evaluation sheet manufactured from the photosensitive resin composition, which had a photosensitive resin layer thickness of 500 μm with both sides covered with polyester film, and was exposed to ultraviolet light to a total exposure amount (integrated light amount) of 1000 mJ. The ultraviolet exposure amount can be measured with an ultraviolet integrated light meter.

[0022] Next, we will explain another feature of the present invention: the use of a photosensitive resin composition containing a water-soluble compound in a proportion of 1.5 to 15% by mass as the photosensitive resin layer of the printing plate. As explained above, the true density before photocuring is 0.91 to 0.97 g / cm³. 3The characteristic of being "water-insoluble" is intended to address problems arising from "water-insoluble" compounds among the components of the photosensitive resin composition (re-adhesion of aggregates formed by the aggregation of "water-insoluble" compounds (solids) to the surface of the printing plate). On the other hand, the other characteristic is intended to address problems arising from "water-soluble" compounds among the components of the photosensitive resin composition. Many of the components of the photosensitive resin composition (hydrophobic polymers, hydrophilic polymers, photopolymerizable unsaturated compounds, photopolymerization initiators, and plasticizers, etc.) are "water-insoluble" compounds, but "water-soluble" compounds are also sometimes used. For example, "water-soluble" photopolymerizable unsaturated compounds are sometimes used to shorten the development time. The inventors have found that in printing plates obtained using a photosensitive resin composition containing a large amount of this "water-soluble" photopolymerizable unsaturated compound, a large amount of the photopolymerizable unsaturated compound remains in the unexposed areas of the photosensitive resin layer, meaning it has not been photocured, i.e., has not been photopolymerized, and remains "water-soluble." This uncured compound dissolves into the developer during development with an aqueous developer, reducing the development rate of the waste developer. Therefore, it is preferable not to include too much of the "water-soluble" photopolymerizable unsaturated compound in the photosensitive resin composition, in order to balance shortening the development time with increasing the development rate of the waste developer. In this invention, the limiting of the content of the water-soluble compound in the photosensitive resin composition to a specific range of 1.5 to 15% by mass is based on this finding. It should be noted that the "water-soluble" compound in the photosensitive resin composition is not limited to the above-mentioned photopolymerizable unsaturated compound. For example, some hydrophilic polymers also contain "water-soluble" components, and these hydrophilic polymers also cause similar problems to those of the "water-soluble" photopolymerizable unsaturated compound. Furthermore, commercially available "water-soluble" photopolymerizable unsaturated compounds may contain small amounts of unreacted products or partially reacted products that have not fully reacted. These unreacted and partially reacted products also cause the same problems as "water-soluble" photopolymerizable unsaturated compounds. Therefore, in this invention, these hydrophilic polymers, unreacted products, and partially reacted products are also included in the water-soluble compound, and their content is limited to a specific range.

[0023] In the present invention, the upper limit of the water-soluble compound content in the photosensitive resin composition is required to be 15% by mass, preferably 12% by mass, more preferably 10% by mass, even more preferably 7% by mass, and particularly preferably 5% by mass. If the water-soluble compound content in the photosensitive resin composition exceeds the above upper limit, a large amount of the water-soluble compound in the unexposed portion of the photosensitive resin composition may dissolve into the developer solution during development with an aqueous developer solution, potentially reducing the development rate of the development waste solution. On the other hand, the lower limit of the water-soluble compound content in the photosensitive resin composition is required to be 1.5% by mass, preferably 1.8% by mass, more preferably 2.0% by mass, and particularly preferably 2.2% by mass. If the water-soluble compound content in the photosensitive resin composition is below the above lower limit, the effect of shortening the development time of the printing plate may be inferior. The water-soluble compound content in a photosensitive resin composition can be controlled by adjusting the water-soluble compound content of each component constituting the photosensitive resin composition (hydrophobic polymer, hydrophilic polymer, photopolymerizable unsaturated compound, photopolymerization initiator, and plasticizer, etc.) and the blending ratio of each component. Specifically, the desired water-soluble compound content in the photosensitive resin composition can be achieved by fine-tuning the average value of the water-soluble compound content calculated from the water-soluble compound content of each component and the percentage of each component, compared with the measured value of the water-soluble compound content in the photosensitive resin composition measured by the method described later in the examples. If the water-soluble compound is a photopolymerizable unsaturated compound, any water-soluble photopolymerizable unsaturated compound conventionally used to shorten the development time can be used. Examples of such water-soluble, photopolymerizable unsaturated compounds include those having polyalkylene glycol in their compound structure, those containing a carboxyl group in their molecule, those containing a hydroxyl group in their molecule, those containing an amino group in their molecule, and mixtures thereof, all of which are selected from the group consisting of these.Specifically, examples of photopolymerizable unsaturated compounds having polyalkylene glycol in their compound structure include polyethylene glycol di(meth)acrylates such as diethylene glycol di(meth)acrylate, triethylene glycol di(meth)acrylate, and tetraethylene glycol di(meth)acrylate. Examples of photopolymerizable unsaturated compounds having a hydroxyl group in the molecule include polyhydric alcohol (meth)acrylates such as 2-hydroxyethyl(meth)acrylate, 2-hydroxypropyl(meth)acrylate, and glycerin mono(meth)acrylate. Examples of photopolymerizable unsaturated compounds containing an amino group in the molecule include N,N-dimethylaminoethyl methacrylate, dimethylaminoethyl acrylate, diethylaminoethyl methacrylate, and dimethylaminomethyl methacrylate. In this invention, "compound" includes not only high molecular weight polymers such as resins, but also low molecular weight oligomers and monomers. The "compound" of this invention is not limited to its form and includes liquids, semi-solids, and solids. Specifically, hydrophobic polymers, hydrophilic polymers, photopolymerizable unsaturated compounds, and plasticizers are considered "compounds" in this invention, and organic compounds added in addition to these components are also included as "compounds." In this invention, a water-soluble compound is a compound that dissolves in water, and water solubility refers to a compound that dissolves transparently when mixed with water at 25°C under stirring. The amount of water-soluble compounds in a photosensitive resin composition is the total amount of all water-soluble compounds contained in the photosensitive resin composition, and its content can be determined by measuring the mass of the dissolved compound in a water solubility test where the compound is mixed with water at 25°C under stirring, and calculating the ratio of the mass of the water-dissolved compound to the mass of the photosensitive resin composition used in the water solubility test. On the other hand, a "water-insoluble" compound is a compound that does not dissolve in water or becomes translucent or cloudy ("water-dispersible"). Specifically, "water-insoluble" refers to a compound that does not dissolve or becomes translucent or cloudy ("dispersible in water") when mixed with water at 25°C at a concentration of 10% by mass under stirring.If the compound dissolves clearly, filter it using a cylindrical funnel-shaped glass filter (pore size 16μ~40μ, manufactured by Shibata Scientific Co., Ltd.) to confirm that there are no insoluble compounds.

[0024] Based on the requirements that the photosensitive resin composition used in the method of the present invention must satisfy, each component of the photosensitive resin composition that can be used in the method of the present invention (hydrophobic polymer, hydrophilic polymer, photopolymerizable unsaturated compound, photopolymerization initiator, and plasticizer, etc.) will be described in detail below.

[0025] (Hydrophobic polymer) In the present invention, "hydrophobic polymer" refers to a polymer obtained by polymerizing a conjugated diene. Hydrophobic polymers, along with hydrophilic polymers, are major components of photosensitive resin compositions. Specifically, hydrophobic polymers include polymers obtained by polymerizing conjugated diene hydrocarbons, or copolymers obtained by copolymerizing conjugated diene hydrocarbons with monoolefin unsaturated compounds. Examples include butadiene polymers, isoprene polymers, chloroprene polymers, styrene-butadiene copolymers, styrene-butadiene-styrene copolymers, styrene-isoprene copolymers, styrene-isoprene-styrene copolymers, styrene-chloroprene copolymers, acrylonitrile-butadiene copolymers, acrylonitrile-isoprene copolymers, methyl methacrylate-butadiene copolymers, methyl methacrylate-isoprene copolymers, acrylonitrile-butadiene-styrene copolymers, and acrylonitrile-isoprene-styrene copolymers. Among these, butadiene polymers are preferably used from the viewpoint of their properties as flexographic printing plates, namely the rebound elasticity of the plate surface, the strength of the stretchable material, the hardness of the resin plate, the morphological stability when unexposed, and their availability. These hydrophobic polymers may be used individually or in combination of two or more. In the present invention, the hydrophobic polymer may be fine particles, and polymers obtained from water-dispersed latex can be used. Many different synthetic latexes are commercially available, so you can obtain the desired one from there as appropriate. When using water-dispersed latex, it may be added as is, or it may be added as a solid resin after drying to remove the water. The proportion of hydrophobic polymer in the photosensitive resin composition that forms the photosensitive resin layer is preferably in the range of 10 to 45% by mass.

[0026] (hydrophilic polymer) In the present invention, a "hydrophilic polymer" is a polymer containing units (monomer units) derived from unsaturated monomers and conjugated diene monomers, each containing at least one hydrophilic functional group such as a carboxylic acid, sulfonic acid, phosphoric acid, or polyalkylene glycol within its molecule. For ease of separation and recovery, the hydrophilic polymer is preferably water-dispersible. Furthermore, from a manufacturing standpoint, a hydrophilic polymer obtained by removing water from a water-dispersed latex is preferred. For example, a polymer obtained by removing water from a water-dispersed latex in which polymer particles obtained by emulsion polymerization using hydrophilic unsaturated monomers, units derived from conjugated diene monomers, and other monomers copolymerizable therewith are dispersed in water is a good example. As the hydrophilic functional group in the hydrophilic polymer, a carboxyl group is preferred from the viewpoint of developability. This carboxyl group may also be a metal salt such as sodium or potassium. From the viewpoint of ink resistance of the printing plate, the hydrophilic polymer may be internally crosslinked polymer particles, and the degree of gelation of the hydrophilic polymer, which is an indicator of internal crosslinking, is preferably 10 to 90% from the viewpoint of ease of floating and aggregation. The proportion of hydrophilic polymer in the photosensitive resin composition that forms the photosensitive resin layer is preferably in the range of 10 to 45% by mass.

[0027] The degree of gelation of the hydrophilic polymer, as used here, is defined by its insolubility in toluene. The specific measurement of the degree of gelation is performed as follows: In the case of water-dispersible latex, the latex solution is spread on a 100 μm thick PET film and dried at 100°C for 1 hour, and 1 gram of the hydrophilic polymer is taken out. The obtained hydrophilic polymer is immersed in a toluene solution at 25°C for 48 hours, the insoluble components are separated by a filter, and it is dried at 110°C for 2 hours, and the weight after drying is measured. The degree of gelation is measured by calculating the mass % by dividing the dried insoluble components by the weight before immersion in the toluene solution.

[0028] The hydrophilic polymer is preferably in particulate form, with an average particle size of 400 nm or less, and particularly preferably 150 nm or less. If the average particle size is too large, the water-developability of the resulting printing plate may decrease. As the hydrophilic polymer, a polymer obtained from a known water-dispersible emulsion may be used. Specific latexes include carboxyl group-containing butadiene latex described in JP-A-1-300246, JP-A-61-128243, and JP-A-6-194837, amino group-containing diene polymers described in JP-A-7-134411, and styrene-butadiene latex synthesized using a reactive emulsifier described in Japanese Patent No. 3836433. Among these, carboxyl group-containing butadiene latex and carboxyl group-containing styrene-butadiene latex are preferred in terms of water-developability and the physical properties of the printing plate.

[0029] (Photopolymerizable unsaturated compound) In the present invention, it is necessary to use a specific amount of a "water-soluble" photopolymerizable unsaturated compound, as described above, in order to balance the reduction of development time with the increase in the development rate of the developing waste liquid. However, in the present invention, it is not necessary to use only "water-soluble" photopolymerizable unsaturated compounds; it is preferable to use "water-insoluble" compounds in combination with "water-soluble" compounds. Conventionally known photopolymerizable unsaturated compounds used in printing plates can be used, and are not particularly limited, but examples include photopolymerizable oligomers and photopolymerizable monomers. These photopolymerizable unsaturated compounds may be used alone or in combination of two or more. The proportion of photopolymerizable unsaturated compounds in the photosensitive resin composition that forms the photosensitive resin layer is preferably in the range of 1 to 50% by mass.

[0030] Photopolymerizable oligomers are polymers in which ethylenically unsaturated groups are bonded to the terminals and / or side chains of a conjugated diene polymer, and have a number-average molecular weight of 1,000 or more and 10,000 or less. The conjugated diene polymer constituting the conjugated diene ethylenically polymer is composed of a homopolymer of a conjugated diene unsaturated compound or a copolymer of a conjugated diene unsaturated compound and a monoethylenically unsaturated compound. Examples of such copolymers include butadiene polymers, isoprene polymers, acrylonitrile-butadiene copolymers, acrylonitrile-isoprene copolymers, methyl methacrylate-isoprene copolymers, acrylonitrile-isoprene copolymers, methyl methacrylate-butadiene copolymers, acrylonitrile-butadiene-styrene copolymers, and the like. Of these, butadiene polymers, isoprene polymers, and acrylonitrile-butadiene copolymers are preferred in terms of rubber elasticity and photocurability, and butadiene polymers and isoprene polymers are particularly preferred.

[0031] A photopolymerizable monomer is a photopolymerizable compound having an ethylenically unsaturated group with a molecular weight of 500 or less, and is preferably a compound having at least two ethylenically unsaturated groups at its terminal and / or side chain. Examples of photopolymerizable monomers include polyfunctional photopolymerizable monomers having two or more photopolymerizable unsaturated groups and monofunctional photopolymerizable monomers having one photopolymerizable unsaturated group. Specific examples of photopolymerizable monomers include, for example, hexamethylene di(meth)acrylate, ethylene glycol (meth)acrylate, diethylene glycol di(meth)acrylate, 1,4-butanediol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, 1,9-nonanediol di(meth)acrylate, glycerin di(meth)acrylate, triethylene glycol di(meth)acrylate, PEG#400 di(meth)acrylate, 1,3-butanediol dimethacrylate, neopentyl glycol di(meth)acrylate, PEG#200 di(meth)acrylate, 1,3-butanediol di(meth)acrylate, neopentyl glycol di(meth)acrylate, 1,10-decanediol di(meth)acrylate, and ethylene oxide adduct of bisphenol A di(meth)acrylate. Examples include compounds having three ethylenically unsaturated groups, such as acrylate, trimethylolpropane tri(meth)acrylate, ethylene oxide-modified trimethylolpropane tri(meth)acrylate, Light Ester P-2M (manufactured by Kyoeisha Chemical Co., Ltd., trade name), oligopropylene glycol di(meth)acrylate, polytetramethylene glycol di(meth)acrylate, tricyclodecanedimethanol di(meth)acrylate, trimethylolpropane tri(meth)acrylate, glycerin tri(meth)acrylate, tri(meth)acrylate of a 3-mol ethylene oxide adduct of pentaerythritol, pentaerythritol tri(meth)acrylate, pentaerythritol tetra(meth)acrylate, and dipentaerythritol hexa(meth)acrylate. Among these, compounds having three ethylenically unsaturated groups are particularly preferred.

[0032] In the present invention, in order to increase the mechanical strength of the printing plate and improve its print resistance, a monofunctional photopolymerizable monomer having one photopolymerizable unsaturated group may be included in the photopolymerizable compound. Commercially available monofunctional photopolymerizable monomers can be used as such. Specifically, alkyl(meth)acrylates such as methyl(meth)acrylate, ethyl(meth)acrylate, propyl(meth)acrylate, butyl(meth)acrylate, isoamyl(meth)acrylate, 2-ethylhexyl(meth)acrylate, lauryl(meth)acrylate, stearyl(meth)acrylate, etc.; cycloalkyl(meth)acrylates such as cyclohexyl(meth)acrylate, chloroethyl(meth)acrylate, chloropropyl(meth)acrylate, etc.; halogenated alkyl(meth)acrylates such as chloroethyl(meth)acrylate, chloropropyl(meth)acrylate, etc.; alkoxyalkyl(meth)acrylates such as methoxyethyl(meth)acrylate, ethoxyethyl(meth)acrylate, butoxyethyl(meth)acrylate, etc.; 2-hydroxyethyl(meth)acrylate, 2-hydroxypropyl(meth)acrylate, 2-hydroxybutyl(meth)acrylate Examples of monofunctional photopolymerizable monomers having heterocyclic structures include methacrylates having hydroxyl groups such as 3-chloro-2-hydroxypropyl(meth)acrylate and β-hydroxy-β'-(meth)acryloyloxyethyl phthalate, methacrylates having amino groups such as dimethylaminoethyl methacrylate, cycloalkyl(meth)acrylates such as cyclohexyl(meth)acrylate, alkoxyalkyl(meth)acrylates such as methoxyethyl(meth)acrylate, ethoxyethyl(meth)acrylate, and butoxyethyl(meth)acrylate, phenoxyalkyl(meth)acrylates such as phenoxyethyl(meth)acrylate and nonylphenoxyethyl(meth)acrylate, isobornyl(meth)acrylate, and cyclic trimethylolpropaneformal(meth)acrylate. Among these, alkyl methacrylates are particularly preferred as monofunctional photopolymerizable monomers, and alkyl methacrylates having 8 to 18 carbon atoms and being linear are especially preferred.The content of these alkyl methacrylates is preferably 1 to 30 parts by mass, and particularly preferably 3 to 20 parts by mass, in the photosensitive resin composition obtained from the water-dispersed latex.

[0033] (Photopolymerization initiator) Photopolymerization initiators are incorporated into photosensitive resin compositions to enable photopolymerization of photopolymerizable unsaturated compounds. Any photopolymerization initiator capable of polymerizing polymerizable carbon-carbon unsaturated groups by light can be used. Among these, those that have the function of generating radicals by self-decomposition or hydrogen abstraction upon light absorption are preferably used. Examples include benzoin alkyl ethers, benzophenones, anthraquinones, benzyls, acetophenones, and diacetyls. Two or more of these polymerization initiators may be mixed, and it is particularly preferable to include a compound consisting of two types, benzyl alkyl ketals and benzophenones, which have different photosensitivity. The proportion of photopolymerization initiator in the photosensitive resin composition that forms the photosensitive resin layer is preferably in the range of 0.5 to 3.0% by mass.

[0034] (Plasticizer) Plasticizers are added to photosensitive resin compositions to impart flexibility to the photosensitive resin composition. Conventional plasticizers used in printing plates can be used, but plasticizers that are liquid at room temperature and are water-insoluble or water-dispersible are preferred. Preferred plasticizers are polymers having repeating units derived from conjugated diene compounds, and polymers having hydrophilic groups in at least one type of repeating unit are also preferred. Examples of hydrophilic groups include carboxyl groups, sulfonic acid groups, hydroxyl groups, and amino groups, with carboxyl groups being preferred. Specific examples of conjugated diene compounds include butadiene, isoprene, and styrene, and copolymers thereof are also acceptable. Specifically, examples include liquid polybutadiene, liquid polyisoprene, liquid styrene-butadiene copolymers, polybutadiene having carboxyl groups in its side chains or terminals, styrene-butadiene copolymers having carboxyl groups in its side chains or terminals, polybutadiene having carboxyl groups in its side chains or terminals, and isoprene having carboxyl groups in its side chains or terminals.

[0035] Polybutadiene having a carboxyl group in the side chain or terminal, styrene-butadiene copolymer having a carboxyl group in the side chain or terminal, and polyisoprene having a carboxyl group in the side chain or terminal may have the carboxyl groups in the polymer metal-chlorinated. In particular, in the method of the present invention, development is performed with an aqueous developer, so metal chlorination is preferred. As the metal salt, alkali metal salts such as lithium, sodium, and potassium are preferred.

[0036] In addition to polymers having repeating units derived from the conjugated diene compounds mentioned above, other plasticizers may be used in combination with hydrocarbon oils such as paraffin oil, ester compounds such as adipic acid esters, sebatic acid esters and phthalic acid esters, polyesters such as adipic acid-based polyesters, and phosphoric acid compounds such as phosphate esters. The proportion of plasticizer in the photosensitive resin composition that forms the photosensitive resin layer is preferably in the range of 3 to 20% by mass.

[0037] In addition to the hydrophobic polymer, hydrophilic polymer, photopolymerizable unsaturated compound, plasticizer, and photopolymerization initiator described above, the photosensitive resin composition of the present invention may optionally contain additives such as hydrophilic compounds, ultraviolet absorbers, surface tension modifiers, thermal polymerization inhibitors, dyes, pigments, fragrances, or antioxidants.

[0038] In the photosensitive resin composition used in the method of the present invention, from the viewpoint of kneading in an extruder, it is preferable that the total content of the hydrophobic polymer and hydrophilic polymer is 40 to 95% by mass, the content of the photopolymerizable unsaturated compound is 5 to 35% by mass, and the content of the photopolymerization initiator is 0.1 to 10% by mass. Furthermore, from the viewpoint of water developability, the content ratio of the hydrophilic polymer is preferably 40 to 90% by mass when the total content of the hydrophobic polymer and hydrophilic polymer is 100% by mass.

[0039] Next, the developer solution that forms the basis of the developing waste liquid in the method of the present invention will be described. The developer solution used for developing water-developable flexographic printing plates is an aqueous developer solution mainly composed of water, and more preferably contains water, an alkaline compound such as sodium hydroxide or sodium carbonate, and a surfactant. The alkaline compound is added to bring the pH of the developer solution into the alkaline range (preferably 8.0 to 11.0). The surfactant is added to promote the dispersion of the photosensitive resin composition in water and to maintain the dispersed state. The concentration of the surfactant in the developer solution is 0.5 to 10% by mass. The developer solution may also contain organic solvents, etc., as long as they do not reduce the solubility of the alkaline compound or surfactant. The temperature of the developer solution is preferably 25°C to 50°C, and particularly preferably 35°C to 45°C.

[0040] Suitable surfactants include sodium alkylnaphthalene sulfonate and sodium alkylbenzene sulfonate. Other usable surfactants include anionic surfactants containing carboxylates, sulfates, sulfonates, and phosphates; nonionic surfactants containing polyethylene glycol chains, polyhydric alcohol derivatives, and sorbitan derivatives; cationic surfactants containing primary to tertiary amine salts and quaternary ammonium salts; and amphoteric surfactants containing amino acid-type and betaine-type hydrophilic groups.

[0041] The method for developing a water-developable flexographic printing plate is not particularly limited as long as it is a method that can wash away the photosensitive resin composition in the un-photocured areas of the printing plate using an aqueous developer and form a relief image. Examples include a method of rubbing with a brush using the developer, or a method of rubbing away the photosensitive composition in the un-photocured areas using a combination of spraying and brushing. For the brush, the diameter of the brush filaments, the diameter of the holes, and the pitch spacing of the holes can be appropriately selected. Specifically, a pitch spacing of 6 to 10 mm, a hole diameter of bristle bundles of 5 to 8 mm, and a filament diameter of 160 to 200 μm are preferred. Generally, the developer is repeatedly used in the above-described development method until the concentration of the photosensitive resin composition exceeds a certain level and the development speed decreases, and thereafter it is referred to as developing waste liquid. The developing waste liquid is then allowed to stand to separate the photosensitive resin composition. In this invention, the developer liquid before this standing separation is referred to as developing waste liquid. The present invention relates to a method for producing a recycled developer from the waste developer solution generated in this manner. This method will be described in detail below.

[0042] In the method of the present invention, the developing waste liquid is left to stand in a tank 4 as shown in Figure 1 without the addition of a coagulant. The developing waste liquid contains a high concentration of a photosensitive resin composition. As described above, this photosensitive resin composition is controlled to have a true density (specific gravity) of less than 1.0, and is lighter than water, which is the main component of the aqueous developer used for developing water-developable flexographic printing plates. Therefore, when left to stand, the photosensitive resin composition contained in the developing waste liquid floats to the surface due to the difference in specific gravity and moves in close proximity to each other spatially. The photosensitive resin composition that had already partially coagulated in the developing waste liquid then coagulates further to form aggregates 5. As a result, in the tank 4, as shown in Figure 1, a separated state is achieved in which the aggregates 5 exist just below the liquid surface of the developing waste liquid, and the other components constituting the developing waste liquid (aqueous developer mainly composed of water) exist below it. The aggregates 5 located just below the liquid surface of the developing waste liquid do not actually exist as aggregates 5 alone, but as a solid-liquid mixed phase in which the aggregates 5 and the aqueous developing solution are mixed. To facilitate the removal of the aggregates, the tank 4 can be provided with an outlet 6 on its side for discharging the aggregates 5. In this invention, the aggregates 5 can be efficiently removed by discharging the solid-liquid mixed phase containing the aggregates 5 formed below the liquid surface of the developing waste liquid, for example, from the outlet 6 of the tank 4, thereby obtaining a regenerated developing solution with a low solid content concentration at a high recovery rate. In this invention, the method of removing the aggregates is not limited to discharge from the outlet on the side of the tank. For example, although the removal efficiency is lower, it is also possible to provide an outlet on the bottom of the tank and discharge from this outlet, to inject water into the tank from the top to cause an overflow, or to scoop up and collect the aggregates with a ladle.

[0043] When allowing developing waste liquid to stand in a tank, it is important to leave it still for a certain period of time without stirring or other disturbances. The standing time is preferably 5 hours or more, and more preferably 8 hours or more. There is no particular upper limit to the standing time, but 24 hours is sufficient. By allowing sufficient standing time in this way, most of the photosensitive resin composition contained in the developing waste liquid will float and aggregate due to the difference in specific gravity, and a solid-liquid mixed phase containing the aggregates can be reliably formed below the liquid surface of the developing waste liquid.

[0044] The tank shape is preferably a roughly rectangular prism, a roughly cubic shape, or a roughly cylindrical shape, with a roughly rectangular prism or a roughly cylindrical shape being preferred because they have a larger surface area ratio. A roughly rectangular prism is particularly preferred because it allows for effective use of space within the device and facilitates discharge from the outlet.

[0045] The discharge port located on the side of the tank is preferably approximately circular, approximately square, approximately rectangular, or slit-shaped, and among these, a rectangle with its longitudinal direction in the width direction of the tank is particularly preferred in terms of ease of separation, as shown in Figure 2. Also, as shown in Figure 3, there may be multiple slit shapes in the height direction. The height of the discharge port is preferably 1 cm to 10 cm, and more preferably 4 cm to 8 cm. If it is smaller, aggregates are more likely to clog the discharge port, and if it is larger, the wastewater volume increases and the recovery rate may decrease. Furthermore, if the tank has the shape of an approximately rectangular parallelepiped or approximately cube, the maximum width of the discharge port opening is preferably 50% or more of the maximum width of the tank, and more preferably 85% or more. Note that if the tank has the shape of an approximately rectangular parallelepiped or approximately cube, the maximum width of the tank refers to the larger of the depth and the width. Furthermore, if the tank has the shape of an approximately cylindrical shape, the circumferential length of the discharge port is preferably 1 / 8 or more of the circumference of the cross-sectional shape of the tank in the horizontal plane. By setting the maximum width and circumferential length of the discharge port opening as described above, discharge can be made smoother.

[0046] It is preferable that the discharge port located on the side of the tank is provided with an openable and closable lid, as shown in Figures 4 and 5. When the liquid level of the developing waste liquid is above the discharge port, opening the lid from a closed position allows for easy separation and discharge of the aggregates 5 floating just below the liquid level of the developing waste liquid. The lid may be shaped to match the discharge port, or it may be a plate-like structure that slides. The lid can be opened and closed manually or electrically.

[0047] The tank's outlet is located on the side of the tank. This allows for easy separation of aggregates floating just below the surface of the developing waste liquid. The lowest point of the outlet opening is preferably at a height of 50% or more of the tank's depth, more preferably at 55-90% of the tank's depth. If it is located at a height less than 50%, the difference between the outlet and the developing waste liquid's surface becomes too large, increasing the drainage volume, causing the aggregates floating just below the liquid surface to break down, and raising the solid content concentration in the regenerated developing solution.

[0048] In the method of the present invention, two discharge methods can be employed depending on the relationship between the liquid level of the developed waste liquid after standing and the position of the discharge port. First, if the liquid level of the developed waste liquid in the tank after standing is higher than the lowest position of the discharge port opening, the discharge port can be opened to separate and discharge the aggregates floating just below the liquid level of the developed waste liquid from the discharge port. Preferably, the liquid level of the developed waste liquid in the tank at the start of discharge is 3% or more, more preferably 5% or more, and even more preferably 7% or more, than the lowest position of the discharge port opening. If this height difference is less than the above lower limit, there is a risk that the aggregates floating just below the liquid level of the developed waste liquid will not be sufficiently discharged and will remain in the tank. Furthermore, the upper limit of this height difference is preferably 30% or less, more preferably 20% or less, and even more preferably 15% or less. If this height difference exceeds the above upper limit, the drainage volume will increase, and there is a risk that the aggregates floating just below the liquid level of the developed waste liquid will break down, causing the solid content concentration in the regenerated developer solution to increase.

[0049] If the lowest point of the outlet opening is at a height of 50% or more of the tank depth, and the liquid level of the developing waste liquid in the tank at the start of discharge is at least 3% of the tank depth higher than the lowest point of the outlet opening, then the tank depth is such that, for example, the liquid level of the developing waste liquid in the tank at the start of discharge is at least 53% of the tank depth. In such a case, for example, the explanation above regarding the preferred range of liquid level of the developing waste liquid in the tank at the start of discharge still holds true.

[0050] On the other hand, if the liquid level of the waste developer in the tank after standing is below the lowest point of the outlet opening, it cannot be discharged as is. In this case, standing allows the photosensitive resin composition contained in the waste developer to float and coagulate, forming a solid-liquid mixed phase containing the coagulated material below the liquid level of the waste developer. Then, water or developer is transferred to the tank to raise the liquid level above the outlet. By opening the outlet in this state, the coagulated material floating just below the liquid level of the waste developer can be separated and discharged from the outlet. When transferring water, it is desirable to add surfactants or pH adjusters, as this may dilute the regenerated developer and reduce the development speed. In this case as well, the same conditions apply to the liquid level of the waste developer in the tank at the start of discharge as when the liquid level of the waste developer in the tank after standing is higher than the lowest point of the outlet opening.

[0051] In either case, when opening the outlet from the tank to discharge the aggregates floating just below the surface of the developing waste liquid, water may be injected towards the outlet from a nozzle installed on the wall opposite the outlet to wash away the aggregates. Alternatively, the aggregates floating just below the surface of the developing waste liquid may be sucked out from the outlet. Possible methods of suction include using a pump from the piping connected to the outlet or using a wet / dry vacuum cleaner. Washing or sucking the aggregates away makes the discharge faster and more efficient.

[0052] The developing waste liquid used as a raw material for manufacturing the regenerated developing solution preferably has a solid content concentration of 5.0% by mass or more, and more preferably 6.0% by mass or more, in terms of the ease with which aggregates are formed. This solid content is mainly compounds derived from water-insoluble photosensitive resin composition components contained in the developing waste liquid.

[0053] When a separate tank is provided for storing developing waste liquid, the solid-liquid mixture containing aggregates can be separated and discharged from the tank. The liquid remaining in the tank can then be reused as regenerated developer, either directly or indirectly, by returning it to the developing tank. Indirectly, for example, a separate tank for regenerated developer can be provided to temporarily store the regenerated developer, which can then be returned to the developing machine as needed. Directly, the regenerated developer can be returned to the developing or rinsing section of the developing machine.

[0054] According to the method of the present invention, a photosensitive resin composition with a specific density is used, and a water-soluble compound is included in the photosensitive resin composition in a specific proportion. This allows for the production of a high-performance regenerated developer with a high development speed and virtually no problem of aggregates re-adhering to the surface of the printing plate. In particular, the regenerated developer produced by the method of the present invention has a low solid content concentration of 3.5% by mass or less, preferably 3.0% by mass or less, because condensates are efficiently removed, while maintaining sufficient developability. This solid content mainly consists of compounds derived from water-insoluble photosensitive resin composition components contained in the regenerated developer. Therefore, the regenerated developer obtained by the method of the present invention can be used without problems as a substitute for ordinary developers in the production of water-developable flexographic printing plates. In addition, since the solid content in the regenerated developer can be reduced to a very low level in the method of the present invention, it is not particularly necessary to separate aggregates with a filter. However, if it is desired to obtain a regenerated developer with a lower solid content more stably in continuous operation, it may be combined with filtration using a simple filter such as a nonwoven fabric. [Examples]

[0055] The effects of the method for producing the regenerating developer of the present invention are shown in the following examples, but the present invention is not limited to these examples. In the examples and comparative examples, parts and percentages are given on a mass basis unless otherwise specified. Furthermore, the measurements and performance evaluations in the examples and comparative examples were carried out according to the following procedure.

[0056] <True density of photosensitive resin composition before and after photocuring> The true density of the photosensitive resin composition before photocuring was determined by placing a sample of the photosensitive resin composition in a Gay-Lussac type pycnometer, completely degassing and replacing the voids in the sample with liquid, and calculating the relationship between its weight and volume. On the other hand, the true density of the photosensitive resin composition after photocuring was determined by preparing an evaluation sheet from the photosensitive resin composition with a photosensitive resin layer thickness of 500 μm, covered on both sides with polyester film, exposing this evaluation sheet to ultraviolet light exposure (cumulative light amount) of 1000 mJ, placing this sample in a Gay-Lussac type pycnometer, completely degassing and replacing the voids in the sample with liquid, and calculating the relationship between its weight and volume.

[0057] <Content of water-soluble compounds in photosensitive resin composition> The content of water-soluble compounds in the photosensitive resin composition was determined by performing a water solubility test in which the photosensitive resin composition was mixed with water at 25°C under stirring, and calculating according to the following formula. In the formula, the mass of water-soluble compounds remaining after drying is the mass of solids remaining after drying the solution from the solubility test. Water-soluble compound content (%) = [(Mass of water-soluble compound remaining after drying) / (Mass of photosensitive resin composition subjected to water solubility test)] × 100

[0058] <Solid content concentration in developing waste liquid> Ten grams of the developer waste liquid, from which aggregates had been removed, was placed in a petri dish and subjected to vacuum drying. The non-volatile content after drying was expressed as mass %, and this value was defined as the solid content concentration in the developer waste liquid.

[0059] <Solid content concentration in regenerating developer> Ten grams of the regenerated developer, from which aggregates had been removed, was placed in a petri dish and subjected to vacuum drying. The non-volatile content after drying was expressed as mass%, and this value was defined as the solid content concentration in the regenerated developer.

[0060] <Solid content removal rate> The solid content removal rate was calculated from the solid content concentration in the developing waste solution and the solid content concentration in the regenerated developing solution according to the following formula. Solid content removal rate (%) = [(Solid content concentration in developing waste solution) - (Solid content concentration in recycled developing solution)] / (Solid content concentration in developing waste solution) × 100

[0061] <Development speed> For each example and comparative example, the printing plates produced were developed using new developer (i.e., never used for developing the original plate) and the recycled developer produced in each example and comparative example. The distance the printing plate could be washed in the depth direction in 10 minutes was measured. Development using new developer was to investigate the effect of shortening the development time by including water-soluble compounds in the photosensitive resin composition. A higher development speed with new developer indicates a shorter development time. On the other hand, development using recycled developer was to investigate the degree of reduction in the development speed of the waste developer due to water-soluble compounds leached into the developer during development with new developer. A higher development speed with recycled developer indicates that fewer water-soluble compounds leached into the developer, and therefore a lower degree of reduction in the development speed of the waste developer. A development speed of 0.5 mm / 10 minutes or higher is considered good.

[0062] <Image Reproducibility> Development was performed using the regenerated developer prepared in each example and comparative example. The diameter of the smallest independent point reproduced on the developed printing plate was measured and evaluated according to the following criteria. ○: Successfully reproduces a diameter of 200 μm. △: Successfully reproduces a diameter of 300 μm. ×: Failed to reproduce a diameter of 400 μm.

[0063] <Presence or absence of aggregates adhering to the surface of the printing plate> Development was carried out using the regenerated developer prepared in each example and comparative example, and the presence or absence of aggregates adhering to the surface of the resulting printing plate was visually determined and evaluated according to the following criteria. ○: No adhesion to the printing surface ×: Adhesion present on the printing surface.

[0064] Details of each component used in the photosensitive resin compositions of the examples and comparative examples are as follows. [Hydrophobic Polymerization] A1: Butadiene latex (Nipol LX111A2, manufactured by Nippon Zeon Co., Ltd., non-volatile content 54.0%) A2: Acrylonitrile-butadiene rubber emulsion (Nipol 1562, manufactured by Nippon Zeon Co., Ltd.: emulsion with 33.5% by mass of bound AN and a concentration of 41% by weight) A3: Styrene-butadiene latex (Nalstar SR-101, manufactured by Nippon A&L: gelation degree 95%, average particle size 0.13 μm, non-volatile content 46%) A4: Styrene-butadiene block copolymer (Kraton DX410, manufactured by Kraton: Styrene content 18%, Butadiene content 82%)

[0065] [Hydrophilic polymer] B1: Carboxylated nitrile butadiene latex (Sciatex NA-106, manufactured by Nippon A&L: gelation degree 45%, average particle size 0.14 μm, solids content 50%) B2: Carboxylated styrene-butadiene latex (Nalstar SR-101, manufactured by Nippon A&L: gelation degree 95%, average particle size 0.24 μm, solid content concentration 52%) B3: Carboxylated styrene-butadiene latex (Nalstar SR-112, manufactured by Nippon A&L: gelation degree 70%, average particle size 0.13 μm, solid content concentration 50%) B4: Carboxylated styrene-butadiene latex (Nalstar SR-140, manufactured by Nippon A&L: gelation degree 60%, average particle size 0.15 μm, solid content concentration 48%) B5: Emulsion polymerization solution of carboxy-modified styrene-butadiene polymer prepared by the following manufacturing method B6: Urea urethane-based hydrophilic polymer solution prepared by the following manufacturing method [Method for producing hydrophilic polymer B5] A monomer mixture consisting of 57 parts by mass of butadiene, 15 parts by mass of styrene, 20 parts by mass of ethyl acrylate, 3 parts by mass of methacrylic acid, and 5 parts by mass of acrylic acid was polymerized according to the production example in Japanese Patent No. 3836433, except for the composition ratio. After the polymerization reaction was completed, it was cooled. Next, the resulting copolymer latex was adjusted to a pH of 7 with sodium hydroxide, and unreacted monomers were removed by steam stripping, and the mixture was filtered through a 200-mesh wire mesh. In this way, the solid content concentration was finally adjusted to 40% by mass to obtain an emulsion polymerization solution of hydrophilic polymer B5. [Method for producing hydrophilic polymer B6] 60.0 parts by mass of hexamethylene diisocyanate, 31.0 parts by mass of dimethylolpropionic acid, 15.0 parts by mass of polytetramethylene glycol (molecular weight 850), and 3.0 parts by mass of di-n-butyltin dilaurate were dissolved in 150 parts by mass of tetrahydrofuran. This solution was placed in a 1-liter flask equipped with a stirrer and heated to 65°C while continuing to stir, and the reaction was continued for 2 hours. In a separate container, a solution of 92.0 parts by mass of acrylonitrile-butadiene oligomer having an amino group at the end (HycarATBN 1300×16, manufactured by Ube Industries, Ltd.) dissolved in 135 parts by mass of tetrahydrofuran was added to the 1-liter flask at room temperature while stirring. Tetrahydrofuran was removed from this polymer solution under reduced pressure by distillation, and the polymer was dried under reduced pressure to obtain a polymer with a number average molecular weight of 8500. Next, to a solution prepared by dissolving 25 parts by mass of this polymer in 33 parts by mass of tetrahydrofuran, an aqueous solution prepared by dissolving 0.6 parts by mass of lithium hydroxide and 1.4 parts by mass of magnesium acetate in 42 parts by mass of ion-exchanged water was added while stirring at room temperature to obtain urea urethane-based hydrophilic polymer B6 with a solid content concentration of 25% by mass.

[0066] [Photopolymerizable unsaturated compound] C1: Polybutadiene-terminated diacrylate (BAC-45, manufactured by Osaka Organic Industries Co., Ltd.) C2:3-methyl-1,5-pentanediol diacrylate (manufactured by Tokyo Chemical Industry Co., Ltd.) C3:1,10-decanediaacrylate (product code A-DOD-N, manufactured by Shin-Nakamura Chemical Industry Co., Ltd.) C4: Tricyclodecane dimethanol diacrylate C5: Polyethylene glycol #200 diacrelate (A-200, manufactured by Shin-Nakamura Chemical Co., Ltd.) C6: Glycerol dimethacrylate (manufactured by Tokyo Chemical Industry Co., Ltd.) C7: Stearyl acrylate (manufactured by Osaka Organic Industry Co., Ltd.) C8: Dimethylaminoethyl methacrylate (manufactured by Mitsubishi Gas Chemical Industries) C9: Glycerin monoacrylate (Bremmer GLM, manufactured by NOF Corporation)

[0067] [Plasticizer] D1: Liquid butadiene (LBR-361, manufactured by Kuraray) D2: Liquid styrene-butadiene (L-SBR-841, manufactured by Kuraray)

[0068] [Photopolymerization initiator] Benzyldimethyl ketal (manufactured by Tokyo Chemical Industry Co., Ltd.)

[0069] [Other additives] Heat stabilizer: 4-methoxyphenol (manufactured by Tokyo Chemical Industry Co., Ltd.) UV absorber: Chinuvin 326 (manufactured by Tokyo Chemical Industry Co., Ltd.)

[0070] (Example 1) Preparation of photosensitive resin composition A dope was prepared by mixing 27.5 parts by mass of butadiene latex (A1), a hydrophobic polymer, 24 parts by mass of carboxy-modified nitrile butadiene latex (B1), a hydrophilic polymer, 17.5 parts by mass of polybutadiene-terminated diacrylate (C1), a photopolymerizable oligomer, 12 parts by mass of 1,10-decanediaacrylate (C3), a photopolymerizable monomer, 12 parts by mass of glycerol dimethacrylate (C6), a photopolymerizable monomer, 5 parts by mass of liquid butadiene (D1), a plasticizer, 1.5 parts by weight of a photopolymerization initiator, 0.2 parts by mass of a heat stabilizer, and 0.01 parts by mass of an ultraviolet absorber in a container. The resulting dope was then introduced into a heating and kneading apparatus, and the solvent was removed under reduced pressure to obtain a photosensitive resin composition.

[0071] Creating flexographic printing plates The photosensitive resin composition was sandwiched between a support film, which consisted of a polyethylene terephthalate film with a polyester adhesive layer coated on top of a 125 μm thick polyethylene terephthalate film, and a cover film, which consisted of the same polyethylene terephthalate film with an anti-tack layer (polyvinyl alcohol) coated on top of a cover film. At this time, the adhesive layer and the anti-tack layer were in contact with the photosensitive resin composition. Next, the film was laminated at 100°C using a heat press to obtain an A2 size flexographic printing plate consisting of a polyethylene terephthalate film support, adhesive layer, photosensitive resin layer, anti-tack layer, and cover film. The total thickness of the plate was 1.7 mm.

[0072] Next, the obtained flexographic printing plate was exposed (image area ratio 50%). Using the exposed flexographic printing plate, development was repeatedly performed under the following conditions to obtain a developed waste liquid with a solid content concentration of 5.0% by mass. <Developing equipment> The developing apparatus used had the structure shown in Figure 1. <Developer> For the developer, sodium oleate was measured out to make up 1% of the total developer solution and dissolved in tap water. This solution was then heated to a developer temperature of 40°C before use. <Developing Brush> For the developing brush, a support plate made of 10 mm thick PVC resin plywood was used, with nylon brushes with a diameter of 150 μm embedded in it under the following conditions. Hair bundle implantation hole diameter: 20mm Hair bundle implantation density: 120 hairs / hole

[0073] Next, the obtained waste developer solution was transferred from the developing tank 1 in Figure 1 to the standing tank 4, where it was left to stand for 5 hours. This allowed the photosensitive resin composition contained in the waste developer solution to float and coagulate, forming a solid-liquid mixed phase containing the aggregates 5 below the liquid surface of the waste developer solution. The volume of waste developer solution transferred to the standing tank 4 was 45 liters, and the liquid level of the waste developer solution in the standing tank 4 immediately after transfer was 75% of the depth of the standing tank 4.

[0074] The stationary tank 4 is a roughly rectangular parallelepiped with dimensions of 43 cm wide x 76 cm deep x 19 cm high, and a rectangular outlet 6 with dimensions of 5 cm high x 70 cm long is installed on one side, as shown in Figure 2. The outlet 6 is positioned such that the lowest point of its rectangular opening is at 70% of the depth of the stationary tank 4. The outlet 6 has a lid, the top of which is fixed to the wall by a fixed shaft, and the lid can be opened and closed around the fixed shaft, as shown in Figures 4 and 5.

[0075] After standing, the lid of outlet 6 was opened, allowing the solid-liquid mixed phase containing the aggregates 5 to be discharged from outlet 6 and collected in the aggregate recovery tank 7. Subsequently, the liquid remaining in the standing tank 4 was recovered as regenerated developer. The recovered regenerated developer amounted to 35 liters. Using this regenerated developer, the development speed, image reproducibility, and the presence or absence of aggregate adhesion to the printing plate surface were evaluated. The evaluation results are shown in Table 1.

[0076] Examples 2-16, Comparative Examples 1-4 A flexographic printing plate was prepared in the same manner as in Example 1, except that the mixing ratios of each component in the photosensitive resin composition constituting the photosensitive resin layer were changed as shown in Tables 1 and 2, and evaluated in the same manner as in Example 1. The evaluation results are shown in Tables 1 and 2.

[0077] [Table 1]

[0078] [Table 2]

[0079] As can be seen from Tables 1 and 2, Examples 1 to 16, which satisfy the requirements of the present invention, are excellent in all aspects of evaluation, including solid content removal rate, development speed, image reproducibility, and the presence or absence of aggregates adhering to the surface of the printing plate. On the other hand, in Comparative Example 1, where the true density of the photosensitive resin composition before photocuring was too low, the photosensitive resin composition floated and aggregated early during the development of the printing plate, and aggregates derived from the photosensitive resin composition removed by development re-adhered to the surface of the printing plate. In Comparative Example 2, where the true density of the photosensitive resin composition before photocuring was too high, the difference in specific gravity with water, the main component of the developing waste liquid, became small, making it difficult for the photosensitive resin composition to float and aggregate. The 5-hour standing time for the developing waste liquid was insufficient to remove the aggregates, making it impractical. In addition, the photosensitive resin composition that could not be recovered remained in the recycled developing solution and adhered to the surface of the printing plate. In Comparative Example 3, where the content of water-soluble compounds in the photosensitive resin composition was too low, the development speed with new developing solution was low, and the effect of shortening the development time of the printing plate was inferior. Furthermore, the development speed of the regenerated developer was also low. In Comparative Example 4, where the proportion of water-soluble compounds in the photosensitive resin composition was too high, a large amount of water-soluble compounds in the unexposed areas of the photosensitive resin composition leached into the developer when developed with an aqueous developer, significantly reducing the development speed of the regenerated developer. [Industrial applicability]

[0080] According to the method of the present invention, a photosensitive resin composition with a specific density is used, and a water-soluble compound is included in the photosensitive resin composition in a specific proportion. Therefore, a high-performance recycled developer with a high development rate and virtually no problem of aggregates re-adhering to the surface of the printing plate can be produced from the developing waste liquid generated by the development of water-developable flexographic printing plates. Accordingly, the present invention is extremely useful. [Explanation of Symbols]

[0081] 1 developer tank 2. Developer 3 Piping 4. Static tank 5 Aggregates 6 Outlet 7. Aggregated material recovery tank 8 pumps 9 Piping

Claims

1. A method for producing a recycled developer from developing waste liquid generated by developing a water-developable flexographic printing plate having a photosensitive resin layer made of a photosensitive resin composition containing at least a hydrophobic polymer, a hydrophilic polymer, a photopolymerizable unsaturated compound, a photopolymerization initiator, and a plasticizer, A process in which the developing waste liquid is left to stand in a tank without adding a coagulant to the developing waste liquid, and the photosensitive resin composition contained in the developing waste liquid is allowed to float and coagulate, and A process of discharging floating aggregates from the tank and obtaining the liquid remaining in the tank as a regenerated developer. In a method including, The true density of the photosensitive resin composition before photocuring is 0.91 to 0.97 g / cm³. ３ The following conditions must be met: the true density is measured without fine grinding the photosensitive resin composition; the photosensitive resin composition contains a water-soluble compound in a proportion of 1.5 to 15% by mass; the developer used for developing a water-developable flexographic printing plate is water containing an alkaline compound and / or a surfactant, and the developer optionally contains an organic solvent; the developing waste liquid has a solid content concentration of 5.0% by mass or more; and the water-soluble compound is a water-soluble photopolymerizable unsaturated compound and optionally a hydrophilic polymer, and the water-soluble photopolymerizable unsaturated compound is selected from the group consisting of compounds having polyalkylene glycol in their compound structure, compounds containing a carboxyl group in the molecule, compounds containing a hydroxyl group in the molecule, compounds containing an amino group in the molecule, and mixtures thereof. A method characterized by the following.

2. The true density of the photosensitive resin composition after photocuring is 0.93 to 0.99 g / cm³. ３ The method according to claim 1, characterized in that the true density is measured without finely grinding the photosensitive resin composition.

3. A method for producing a water-developable flexographic printing plate, characterized in that the regenerated developer obtained by the method described in claim 1 or 2 is used as a developer.

4. A photosensitive resin composition for water-developable flexographic printing plates for use in a method for producing the regenerating developer described in Claim 1, wherein the photosensitive resin composition contains at least a hydrophobic polymer, a hydrophilic polymer, a photopolymerizable unsaturated compound, a photopolymerization initiator, and a plasticizer, and the true density of the photosensitive resin composition before photocuring is 0.91 to 0.97 g / cm³ ３ A photosensitive resin composition for water-developable flexographic printing plates, characterized in that: the true density is measured without fine grinding the photosensitive resin composition; the photosensitive resin composition contains a water-soluble compound in a proportion of 1.5 to 15% by mass; and the water-soluble compound is a water-soluble photopolymerizable unsaturated compound and optionally a hydrophilic polymer, and the water-soluble photopolymerizable unsaturated compound is selected from the group consisting of compounds having polyalkylene glycol in their compound structure, compounds containing a carboxyl group in the molecule, compounds containing a hydroxyl group in the molecule, compounds containing an amino group in the molecule, and mixtures thereof.

5. A water-developable flexographic printing plate for use in a method for producing the regenerating developer described in Claim 1, wherein the printing plate comprises a support and a photosensitive resin layer, and the photosensitive resin layer is made of the photosensitive resin composition for water-developable flexographic printing plates described in Claim 4.

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