JP7057072B2 - Copolymer latex for inkjet coated paper and the copolymer latex-containing composition - Google Patents

Description

The present invention relates to a copolymer latex for inkjet coated paper and a composition containing the copolymer latex. Specifically, it is a copolymer latex used as a binder for inkjet coated paper for water-based ink, and contains a copolymer latex having excellent water-based ink absorbability of inkjet coated paper and the copolymer lattice. It relates to a coating composition for inkjet coated paper.

In recent years, coated paper has been used in a large number of printed matter because of its high printing effect. Even in periodical publications such as quarterly and monthly papers, the number of cases where coated paper is used for all pages is increasing considerably. In particular, on-demand printing is becoming mainstream in the transmission of personal information such as direct mail and product catalogs in the mail order business. Specifically, on-demand printing such as electrophotographic method and inkjet method is utilized. There is.
Common offset printing paper coating compositions are made from a pigment dispersion in which a white pigment such as clay or calcium carbonate is dispersed in water, a binder for adhering and fixing the pigments to each other and the pigment to the base paper, and other additives. A water-based paint to be composed is used, and as a binder, a synthetic emulsion binder typified by a styrene-butadiene copolymer latex or a natural binder typified by starch or casein is used.
On the other hand, the coating composition used in the inkjet method (hereinafter referred to as a coating composition for inkjet coated paper) is a pigment dispersion liquid in which a porous pigment such as silica having excellent water absorption is dispersed in water, pigments and pigments. It is composed of a binder for adhering and fixing the pigment to the base paper, a cationic fixing agent for fixing the water-based ink on the paper surface, and other additives. As a binder, polyvinyl alcohol has excellent protective colloidal properties and hydrophilicity. Is commonly used. Further, in the coating composition for offset printing paper, the use of styrene-butadiene copolymer latex, which is a general synthetic emulsion binder, tends to be restricted because the absorbability of the water-based ink is inferior.

On the other hand, polyvinyl alcohol used in the coating composition for inkjet coated paper tends to have inferior surface strength of the coating layer as compared with the synthetic emulsion binder.

Inkjet printing is the process of flying a very small amount of ink to adhere to and fix it on the inkjet paper to form characters and images, and the absorbency of water-based ink into the coating layer forming the surface of the coated paper. Retention and fixability affect the blurring of characters and the sharpness of images. Further, it is known that the absorbency of the water-based ink also affects the drying of the ink on the surface after printing, and thus has a great influence on the printing workability. Therefore, there is a demand for an inkjet coated paper in which characters do not blur, a clear image can be obtained, and printing workability is good. In order to improve the above points, for example, Japanese Patent Application Laid-Open No. 2001-219646 (Patent Document 1) defines the ink absorption capacity of the ink receiving layer of an inkjet recording medium and the contact angle of the ink receiving layer with respect to the ink. Have been described. Further, WO2003 / 008198 (Patent Document 2) describes that the contact angle of the surface of the inkjet recording sheet with pure water is 90 ° or more and the contact angle with ink is 30 ° or less. However, the balance between the suitability for inkjet printing and the surface strength of the coating layer is still insufficient.

Japanese Unexamined Patent Publication No. 2001-219646

WO2003 / 008198

INDUSTRIAL APPLICABILITY The present invention is a synthetic emulsion binder whose use is limited due to its poor water-based ink absorbency in inkjet coated paper, and even when used in a coating composition for inkjet coated paper, the water-based ink absorbency into a coating layer. It is an object of the present invention to provide a copolymer latex having excellent surface strength of the coating layer and further excellent stability of the coating composition, and a composition containing the copolymer latex.

The present invention is as follows.
[1] 5 to 30% by weight of the structural unit derived from the aliphatic conjugated diene-based monomer, 0.1 to 10% by weight of the structural unit derived from the ethylene-based unsaturated carboxylic acid monomer, and 60 other structural units. A copolymer latex for inkjet coated paper containing up to 94.9% by weight, the latex film I prepared by the following method has a contact angle I of 40 ° or less with respect to ion-exchanged water, and the latex prepared by the following method. The inkjet coated paper is characterized in that the contact angle II of the film II with respect to the ion-exchanged water is 60 ° or more, and the difference between the contact angles of the latex film I and the latex film II with respect to the ion-exchanged water is 30 ° or more. For copolymer latex.
(How to make Latex Film I)
A copolymer latex (solid content 48%) is applied onto a PET film using # 8 wire-bar, dried in a hot air circulation oven set at 200 ° C. for 1 minute, and then taken out.
(How to make Latex Film II)
A copolymer latex (solid content 48%) is applied onto a PET film using # 8 wire-bar, dried in a hot air circulation oven set at 120 ° C. for 1 minute, and then taken out.
[2] The copolymer latex for inkjet coated paper according to the above [1], wherein the minimum film forming temperature (MFT) of the copolymer latex is 50 ° C. or higher.
[3] The copolymer latex for inkjet coated paper according to any one of the above [1] to [2], wherein the gel content is 80% by weight or more.
[4] The copolymer latex for inkjet coated paper according to any one of the above [1] to [3], wherein the average particle size is 120 nm or more.
[5] The inkjet coating according to any one of the above [1] to [4], wherein the content of the anionic emulsifier contained in the copolymer latex (solid content) is 0.1% by weight or more. Copolymer latex for paper.

By using the copolymer latex for inkjet coated paper of the present invention, since the water-based ink absorbency is excellent, characters do not blur, a clear image can be obtained, and an inkjet coated paper having excellent surface strength can be obtained. Further, the coating composition using the copolymer latex for inkjet coated paper of the present invention can be obtained with a small amount of agglomerates.

The copolymer latex of the present invention contains a structural unit derived from an aliphatic conjugated diene-based monomer, a structural unit derived from an ethylene-based unsaturated carboxylic acid monomer, and other structural units.

The structural unit derived from the aliphatic conjugated diene-based monomer is a structural unit formed by polymerizing the aliphatic conjugated diene-based monomer. Examples of the aliphatic conjugated diene-based monomer include 1,3-butadiene, 2-methyl-1,3-butadiene, 2,3-dimethyl-1,3-butadiene, 2-chlor-1,3-butadiene, and substitutions. Examples thereof include linear conjugated pentadiene, substitution and side chain conjugated hexadiene, and one or more of these can be used. In particular, the use of 1,3-butadiene is preferable.

The structural unit derived from the ethylene-based unsaturated carboxylic acid monomer is a structural unit formed by polymerizing the ethylene-based unsaturated carboxylic acid monomer. Examples of the ethylene-based unsaturated carboxylic acid monomer include monobasic acids such as acrylic acid, methacrylic acid, crotonic acid, maleic acid, fumaric acid, and itaconic acid (anhydride), and one of these is used. Alternatively, two or more types can be used.

Examples of the monomer corresponding to other structural units include an alkenyl aromatic monomer, a vinyl cyanide monomer, an unsaturated carboxylic acid alkyl ester monomer, and an unsaturated single amount containing a hydroxyalkyl group. Examples include the body and unsaturated carboxylic acid amide-based monomers.

Examples of the alkenyl aromatic monomer include styrene, α-methylstyrene, methyl-α-methylstyrene, vinyltoluene, divinylbenzene and the like. These can be used alone or in combination of two or more. In this embodiment, it is industrially easy to manufacture, and from the viewpoint of availability and cost, it is particularly preferable to use styrene.

Examples of the vinyl cyanide-based monomer include monomers such as acrylonitrile, methacrylonitrile, α-chloroacrylonitrile, and α-ethylacrylonitrile. These can be used alone or in combination of two or more. In the present embodiment, acrylonitrile or methacrylonitrile is particularly preferable from the viewpoint of easy industrial production, availability and cost.

Examples of unsaturated carboxylic acid alkyl ester-based monomers include methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, butyl acrylate, glycidyl methacrylate, dimethyl fumarate, diethyl fumarate, dimethyl maleate, diethyl maleate, and dimethyl itaconate. , Monomethyl fumarate, monoethyl fumarate, 2-ethylhexyl acrylate and the like. These can be used alone or in combination of two or more. In the present embodiment, it is industrially easy to manufacture, and from the viewpoint of availability and cost, the use of methyl methacrylate is particularly preferable.

Examples of the unsaturated monomer containing a hydroxyalkyl group include β-hydroxyethyl acrylate, β-hydroxyethyl methacrylate, hydroxypropyl acrylate, hydroxypropyl methacrylate, hydroxybutyl acrylate, hydroxybutyl methacrylate, and 3-chloro-2-hydroxypropyl. Examples thereof include methacrylate, di- (ethylene glycol) maleate, di- (ethylene glycol) itaconate, 2-hydroxyethyl maleate, bis (2-hydroxyethyl) maleate, and 2-hydroxyethylmethyl fumarate. These can be used alone or in combination of two or more.

Examples of the unsaturated carboxylic acid amide-based monomer include acrylamide, methacrylamide, N-methylolacrylamide, N-methylolmethacrylamide, N, N-dimethylacrylamide and the like. These can be used alone or in combination of two or more.

In addition to the above-mentioned monomers, any monomer used in ordinary emulsion polymerization such as ethylene, propylene, vinyl acetate, vinyl propionate, vinyl chloride and vinylidene chloride can be used.

The structural unit constituting the copolymer latex of the present invention is a structural unit derived from an aliphatic conjugated diene-based monomer of 5 to 30% by weight, and a structural unit derived from an ethylene-based unsaturated carboxylic acid monomer. It contains 1 to 10% by weight, and other structural units of 60 to 94.9% by weight.

The structural unit derived from the aliphatic conjugated diene-based monomer needs to be contained in an amount of 5 to 30% by weight. If it is less than 5% by weight, the surface strength of the coating layer tends to be inferior, and if it exceeds 30% by weight, the film-forming property of the copolymer latex is improved, and as a result, the water-based ink absorption to the coating layer tends to be inferior. be. It is preferably 7 to 27% by weight.

The structural unit derived from the ethylene-based unsaturated carboxylic acid monomer needs to be contained in an amount of 0.1 to 10% by weight. If it is less than 0.1% by weight, the hydrophilicity tends to be inferior, and if it exceeds 10% by weight, the miscibility with the cation fixing agent when miscible as a coating composition tends to be inferior. It is preferably 0.2 to 6% by weight.

Other structural units should be contained in an amount of 60-94.9% by weight. If it is less than 60% by weight, the water-based ink absorbability and the miscibility with the cation fixing agent when miscible as a coating composition tend to be inferior, and if it exceeds 94.9% by weight, the surface strength of the coating layer is inferior. There is a tendency. It is preferably 67 to 92.8% by weight.

The copolymer latex of the present invention can be obtained, for example, by an emulsion polymerization method.

When carrying out emulsion polymerization, in addition to the above-mentioned monomers, an emulsifier (surfactant), a polymerization initiator, and, if necessary, a chain transfer agent, a reducing agent and the like can be blended.

Examples of the emulsifier (surfactant) include anions such as sulfate ester salts of higher alcohols, alkylbenzene sulfonates, alkyldiphenyl ether sulfonates, aliphatic sulfonates, aliphatic carboxylates, and sulfate ester salts of nonionic emulsifiers. One or more nonionic emulsifiers such as a sex emulsifier, an alkyl ester type of polyethylene glycol, an alkyl phenyl ether type, and an alkyl ether type can be used in combination. Above all, the anionic emulsifier is preferably contained in an amount of 0.1% by weight or more, more preferably 0.3% by weight or more, based on the copolymer latex (solid content). When it is contained in an amount of 0.1% by weight or more, the water-based ink absorbency tends to be good. The upper limit is preferably 3.0% by weight or less, and more preferably 1.0% by weight or less. If it exceeds 3.0% by weight, the copolymer latex tends to foam, and the surface strength of the coating layer tends to be inferior.

Examples of the polymerization initiator include water-soluble polymerization initiators such as lithium persulfate, potassium persulfate, sodium persulfate, and ammonium persulfate; cumene hydroperoxide, benzoyl peroxide, t-butyl hydroperoxide, and acetyl peroxide. , Diisopropylbenzenehydroperoxide, oil-soluble polymerization initiators such as 1,1,3,3-tetramethylbutylhydroperoxide. These can be used alone or in combination of two or more. In particular, it is preferable to select from potassium persulfate, sodium persulfate, cumene hydroperoxide, and t-butyl hydroperoxide. The blending amount of the polymerization initiator is not particularly limited, but is appropriately adjusted in consideration of the combination of the monomer composition, the pH of the polymerization reaction system, other additives and the like.

Chain transfer agents include, for example, alkyl mercaptans such as n-hexyl mercaptan, n-octyl mercaptan, t-octyl mercaptan, n-dodecyl mercaptan, t-dodecyl mercaptan, and n-stearyl mercaptan; dimethylxanthogen disulfide, and diisopropyl. Xanthogen compounds such as xanthogen dysulfide; thiuram compounds such as tetramethylthium disulfide, tetraethylthiuram disulfide, and tetramethylthium monosulfide; Allyl compounds such as allyl alcohols; halogenated hydrocarbon compounds such as dichloromethane, dibromomethane, and carbon tetrabromide; vinyl ethers such as α-benzyloxystyrene, α-benzyloxyacrylonitrile, and α-benzyloxyacrylamide. Chain transfer agents such as triphenylethane, pentaphenylethane, achlorein, metaacrolein, thioglycolic acid, thioalinic acid, 2-ethylhexylthioglycolate, turpinolene, and α-methylstyrene dimer. These can be used alone or in combination of two or more. The blending amount of the chain transfer agent can be appropriately adjusted in consideration of the combination of other additives and the like.

Reducing agents include, for example, reducing saccharides such as dextrose and saccharose; amines such as dimethylaniline and triethanolamine; carboxylic acids such as L-ascorbic acid, erythorbic acid, tartrate acid, and citrate and salts thereof; sulfite. Examples thereof include salts, hydrogen sulfites, pyrosulfites, nitionates, nithionates, thiosulfates, formaldehyde sulfonates, benzaldehyde sulfonates and the like. In particular, it is preferable to select from L-ascorbic acid and erythorbic acid. The blending amount of the reducing agent can be appropriately adjusted in consideration of the combination of other additives and the like.

Further, in the emulsified polymerization, saturated hydrocarbons such as pentane, hexane, heptane, octane, cyclohexane and cycloheptene, and non-saturated hydrocarbons such as penten, hexene, heptene, cyclopentene, cyclohexene, cycloheptene, 4-methylcyclohexene and 1-methylcyclohexene are not used. Hydrocarbon compounds such as saturated hydrocarbons, aromatic hydrocarbons such as benzene, toluene and xylene can be used. In particular, cyclohexene and toluene, which have a moderately low boiling point and are easily recovered and reused by steam distillation after the completion of polymerization, are suitable from the viewpoint of environmental problems, although they are different from the object of the present invention.

Furthermore, known additives such as oxygen supplements, chelating agents, and dispersants may be used as needed, and the types and amounts of these are not particularly limited, and appropriate amounts can be used as appropriate. Furthermore, known additives such as antifoaming agents, antiaging agents, preservatives, antibacterial agents, flame retardants, and ultraviolet absorbers may be used, and the types and amounts of these are not particularly limited, and appropriate amounts are used as appropriate. Can be done.
Further, the copolymer latex of the present invention can be appropriately blended with other latex depending on the purpose of use thereof.

The temperature at the time of emulsion polymerization is preferably set in the range of 30 to 100 ° C., more preferably in the range of 40 to 85 ° C. from the viewpoint of the pressure in the tank and productivity in consideration of safety.

Examples of the method of adding the monomer component and other components at the time of emulsion polymerization include a batch addition method, a split addition method, a continuous addition method, and a power feed method. Above all, it is preferable to adopt a continuous addition method (hereinafter, may be referred to as “continuous addition”). Furthermore, the linkage may be performed a plurality of times.

The reaction time of the emulsion polymerization is, for example, preferably 1 to 15 hours, more preferably 2 to 10 hours from the viewpoint of productivity.

Further, in the emulsion polymerization, it is preferable to confirm that the polymer conversion rate exceeds 97% and terminate the reaction. In this way, a copolymer latex is obtained. The polymer conversion rate can be calculated from the amount of solid content or the amount of heat obtained by cooling the polymerization tank.

From the viewpoint of dispersion stability, the obtained copolymer latex is preferably adjusted in pH to 5 to 9.5 with ammonia, potassium hydroxide, sodium hydroxide or the like, and is preferably 5.5 to 8.5. It is more preferable to adjust to.

Further, it is preferable that the unreacted monomer and other low boiling point compounds are removed from the obtained copolymer latex by a method such as hot vacuum distillation.

The copolymer latex of the present invention needs to have a contact angle I of the latex film I prepared by the following method with respect to ion-exchanged water of 40 ° or less. When the contact angle I exceeds 40 °, the water-based ink absorbency tends to be inferior. It is preferably 30 ° or less.
(How to make Latex Film I)
A copolymer latex (solid content 48%) is applied onto a PET film using # 8 wire-bar, dried in a hot air circulation oven set at 200 ° C. for 1 minute, and then taken out.

Further, it is necessary that the contact angle II of the latex film II prepared by the following method with respect to the ion-exchanged water is 60 ° or more, and the difference between the contact angles of the latex film I and the latex film II with the ion-exchanged water is 30 ° or more. .. When the difference in contact angles is less than 30 °, the film-forming property of the latex film is improved, and as a result, the water-based ink absorption into the coating layer tends to be inferior. It is preferably 35 ° or more.
(How to make Latex Film II)
A copolymer latex (solid content 48%) is applied onto a PET film using # 8 wire-bar, dried in a hot air circulation oven set at 120 ° C. for 1 minute, and then taken out.

The contact angle specified in the present invention is adjusted by the composition of the copolymer latex, the average particle size, the gel content, the amount and addition method of the ethylene-based unsaturated carboxylic acid monomer, and the type and amount of the emulsifier used. Can be done. For example, increasing the amount of emulsifier tends to lower the contact angles I and II, and lowering the average particle size and gel content of the copolymer latex tends to lower the contact angles II.

The copolymer latex of the present invention preferably has an average particle size of 120 nm or more. If the average particle size is less than 120 nm, the dispersion stability of latex tends to be inferior. Further, it also affects the film-forming property of the latex film, and when the average particle size is less than 120 nm, the film-forming property is improved, so that the water-based ink absorbency tends to decrease. The upper limit is preferably 400 nm or less, and more preferably 300 nm or less. If it exceeds 400 nm, the surface strength of the coating layer tends to be inferior.

The copolymer latex of the present invention preferably has a gel content of 80% by weight or more. If the gel content is less than 80% by weight, the surface strength of the coating layer and the water-based ink absorbency tend to decrease. The gel content of the copolymer latex is measured, for example, by the method of Examples described later.

The copolymer latex of the present invention preferably has a minimum film forming temperature (MFT) of 50 ° C. or higher. When the MFT is less than 50 ° C., the surface strength of the coating layer is improved, but the water-based ink absorbency tends to be lowered. The upper limit is preferably 90 ° C. or lower. If the temperature exceeds 90 ° C., the surface strength of the coating layer tends to be inferior. The MFT of the copolymer latex is measured according to JIS K6828-2.

The coating composition for inkjet coated paper containing the copolymer latex of the present invention generally uses silica as a pigment, but other pigments include, for example, calcium carbonate, kaolin clay, talc, and the like. Inorganic pigments such as barium sulfate, titanium oxide, aluminum hydroxide, zinc oxide and satin white, or organic pigments such as polystyrene latex can be used alone or in admixture.
Further, it is preferable to use 3 to 300 parts by weight (solid content) of the copolymer latex in the coating composition with respect to 100 parts by weight (solid content) of the pigment. If the content of the copolymer latex is 3 parts by weight or less, the pigment cannot be sufficiently adhered, which is not preferable, and if it exceeds 300 parts by weight, the air permeability of the coating layer and the water-based ink absorbency are deteriorated, which is not preferable.

Further, if necessary, polyvinyl alcohol, a natural binder such as modified starch such as starch, cationized starch, oxidized starch, or esterified starch, or a water-soluble synthetic binder such as carboxymethyl cellulose may be used. Further, synthetic latex such as polyvinyl acetate latex and acrylic latex may be used in combination with the copolymer latex of the present invention.
Further, boric acid or the like can be used as a cross-linking agent for polyvinyl alcohol.

When preparing the coating composition for inkjet coated paper, a cationic fixing agent can be added, and the cationic fixing agent includes a cationic polymer such as polyamine, polyethyleneimine, and polyacrylic amine, and a quaternary fixing agent. Known fixing agents such as ammonium salts can be used.

Further, as a method for applying the coating composition to the base paper, any coating machine such as an air knife coater, a curtain coater, a blade coater, a roll coater, and a bar coater may be used.

Hereinafter, the present invention will be described in more detail with reference to examples, but the present invention is not limited to these examples as long as the gist of the present invention is not exceeded. In the examples, the portion indicating the ratio and% are based on the weight standard unless otherwise specified. The evaluation of various physical properties in the examples was based on the following method.

A latex film is prepared in an atmosphere where the gel content of the copolymer latex is measured at a temperature of 40 ° C. and a humidity of 85%. After that, about 1 g of the latex film is weighed to make Xg. This is placed in 400 ml of toluene and impregnated for 48 hours. Then, this is filtered through a weighed 300 mesh stainless wire mesh, and then toluene is evaporated and dried, the mesh weight is subtracted from the dried weight, and the dried weight of the sample is weighed to Yg.
Gel content (%) = Y / X × 100

Measurement of Average Particle Size of Copolymer Latex The average particle size of the copolymer latex was measured by a dynamic light scattering method. FPAR-1000 (manufactured by Otsuka Electronics Co., Ltd.) was used for the measurement.

Measurement of contact angle of copolymer latex Copolymer latex is applied on PET film using # 8ware-bar (manufactured by RD SPECIALTIES), and in a hot air circulation oven, the following I and II, respectively. A latex film was prepared by the method, and after standing in an environment of 23 ° C. and 50% RH for 24 hours or more, measurement was carried out under the conditions of 23 ° C. and 50% RH. Contact angles were measured using a Fibro 1100 DAT compliant with TAPPI T558 & ASTM D-5725. 4 μl of ion-exchanged water was added dropwise, and the contact angle after 2 seconds was measured.
(How to make Latex Film I)
A copolymer latex (solid content 48%) is applied onto a PET film using # 8 wire-bar, dried in a hot air circulation oven set at 200 ° C. for 1 minute, and then taken out.
(How to make Latex Film II)
A copolymer latex (solid content 48%) is applied onto a PET film using # 8 wire-bar, dried in a hot air circulation oven set at 120 ° C. for 1 minute, and then taken out.

Evaluation of water-based ink absorption into the coated layer A halftone dot pattern is printed on each coated paper sample using an inkjet printer (MX-883 manufactured by Canon Inc.), and the degree of ink bleeding of the printed pattern is judged with the naked eye. The relative visual evaluation was performed from the 5th grade (excellent) to the 1st grade (inferior).

Evaluation of surface strength of coated paper The degree of adhesion of the coated layer to the cellophane tape when cellophane tape (Cellotape (registered trademark) CT405AP manufactured by Nichiban Co., Ltd.) is attached to the surface of the coated layer and slowly peeled off is visually observed. Was judged by, and a relative visual evaluation was performed from the 5th grade (excellent) to the 1st grade (inferior).

Evaluation of miscibility stability of the coating composition with a cation fixing agent <br /> The amount of agglomerates in the coating composition for inkjet coated paper was measured and used as an index.
200 g of the coating composition was collected as a sample and filtered through a 200 mesh stainless wire mesh. After drying the agglomerates remaining on the wire mesh, the weight was measured, and the ratio to the sample (in terms of solid content) was determined and used as the agglomerate generation rate. The incidence was evaluated in the following four stages.
◎: Less than 0.01% (very good)
○: 0.01 to less than 0.05% (good)
△: 0.05 to less than 0.1% (slightly inferior)
×: 0.1% or more (inferior)

100 parts of polymerized water and 0.9 part of potassium persulfate were charged into a polymerization reactor made of synthetic pressure resistant copolymer latex (A, J), and after sufficient stirring, each monomer and other compounds shown in Table 1 and others were charged. The compound was added and polymerization was started at 70 ° C., and the polymerization was terminated when the final polymerization conversion rate exceeded 97%.
Next, the pH of the obtained copolymer latex was adjusted to 8 with sodium hydroxide, and unreacted monomers and other low boiling point compounds were removed by distillation under reduced pressure to obtain copolymer latexes A and J. Obtained.
Synthesis of copolymer latex (BI)
95 parts of polymerized water and 1.2 parts of potassium persulfate were charged into a pressure-resistant polymerization reactor, and after sufficiently stirring, each of the monomers in the first stage shown in Table 1 and other compounds were added to bring the temperature to 70 ° C. Polymerization was started. When the polymerization conversion rate exceeds 70%, the first-stage monomers and other compounds are added to carry out the polymerization at 70 ° C., and when the final conversion rate exceeds 97%, the polymerization is terminated. bottom.
Next, the pH of the obtained copolymer latex was adjusted to 8 with sodium hydroxide, and unreacted monomers and other low boiling point compounds were removed by distillation under reduced pressure to obtain copolymer latex B to I. Obtained.

Preparation and Evaluation of Coating Compositions Copolymer latexes A to J were used to prepare coating compositions for inkjet coated paper according to the formulation shown below. Polyethyleneimine was used as the cationic fixing agent.
Formulation of coating composition
Colloidal silica (average primary particle size 20 nm) 100 parts Cationic fixing agent (solid content) 3 parts Copolymer latex (solid content) 30 parts ...・ ・ ・ ・ ・ ・ ・
Solid content concentration 9%

Preparation and Evaluation of Coated Paper Apply the above coating composition to the coated base paper (basis weight 55 g / m2) using a wire bar so that the coating amount per side is 15 g / m2 in terms of solid content. Then, it was dried at 150 ° C. for 3 minutes to obtain an inkjet coated paper. The obtained inkjet coated paper was subjected to each test and evaluated, and the results are shown in Table 1.

Each component in Table 1 is indicated by the following abbreviations.
(Monomer)
BDE: Butadiene STY: Styrene MMA: Methylmethacrylate ACN: Acrylonitrile HEA: Hydroxyethyl acrylate IA: Itaconic acid FA: Fumaric acid AA: Acrylic acid (other compounds)
EML1: Polyoxyethylene lauryl ether (Made by Kao Corporation, Emulgen 109P) (nonionic emulsifier)
EML2: Sodium dodecylbenzene sulfonate (Neoperex G-15, manufactured by Kao Corporation) (anionic emulsifier)
CHX: Cyclohexene (hydrocarbon compound)
α-MSD: α-methylstyrene dimer (chain transfer agent)
TDM: t-dodecyl mercaptan (chain transfer agent)

As shown in Table 1, the copolymer latexes A to F according to the present invention are all excellent in water-based ink absorption and surface strength of the obtained inkjet coated paper, and further excellent in stability of the coating composition. there were.

In Comparative Example 1, since the structural unit derived from the aliphatic conjugated diene-based monomer exceeds 30% by weight, the water-based ink absorbency is inferior.
In Comparative Examples 2 and 3, since the contact angle was not specified, the water-based ink absorbency was inferior.
In Comparative Example 4, the ethylene-based unsaturated carboxylic acid monomer exceeded 10% by weight, and the miscibility stability was inferior.

As described above, the copolymer latex for inkjet coated paper of the present invention has excellent mixing stability when used as a coated composition, and further has excellent water-based ink absorption into the coated layer, so that characters are blurred. First, a clear image can be obtained, and an inkjet coated paper having excellent surface strength can be obtained. Therefore, it can be suitably used as a binder component of a coating composition for inkjet coated paper.

Claims (3)

5 to 30% by weight of structural units derived from aliphatic conjugated diene-based monomers, 0.1 to 10% by weight of structural units derived from ethylene-based unsaturated carboxylic acid monomers, and 60 to 94% by weight of other structural units. A copolymer latex for inkjet coated paper containing 9% by weight, and as other structural units, a structural unit derived from an alkenyl aromatic monomer and a structural unit derived from a vinyl cyanide-based monomer. And structural units derived from unsaturated carboxylic acid alkyl ester-based monomers, the gel content is 80% by weight or more, the average particle size is 120 nm or more and 300 nm or less, and the anionic emulsifier and / or the nonionic emulsifier is contained. The amount is 0.1 parts by weight or more and 1 part by weight or less with respect to 100 parts by weight of the copolymer latex (solid content), and the contact angle I of the latex film I prepared by the following method with respect to ion-exchanged water is 40 ° or less. The contact angle II of the latex film II prepared by the following method with respect to the ion-exchanged water is 60 ° or more, and the difference between the contact angles of the latex film I and the latex film II with the ion-exchanged water is 30 ° or more. A characteristic copolymer latex for inkjet coated paper.
(How to make Latex Film I)
A copolymer latex (solid content 48%) is applied onto a PET film using # 8 wire-bar, dried in a hot air circulation oven set at 200 ° C. for 1 minute, and then taken out.
(How to make Latex Film II)
A copolymer latex (solid content 48%) is applied onto a PET film using # 8 wire-bar, dried in a hot air circulation oven set at 120 ° C. for 1 minute, and then taken out. The copolymer latex for inkjet coated paper according to claim 1, wherein the minimum film-forming temperature (MFT) of the copolymer latex is 50 ° C. or higher. The copolymer weight for inkjet coated paper according to any one of claims 1 and 2, wherein the content of the anionic emulsifier contained in the copolymer latex (solid content) is 0.1% by weight or more. Combined latex.