JP7733604B2 - Modifier for inorganic fillers in papermaking - Google Patents

Description

The present invention relates to a modifier for inorganic fillers used in papermaking.

In general, inorganic fillers are sometimes added to paper made from cellulosic fibers. Fillers are added to improve optical properties, primarily whiteness and opacity, which tend to be insufficient with cellulosic fibers alone. In recent years, due to growing environmental awareness and efforts to reduce production costs, there have been efforts to reduce the amount of cellulosic fibers made from wood and instead increase the amount of filler used to increase the filler content in paper.

For example, Patent Document 1 proposes ^the use of precipitated calcium carbonate as a filler, which is composed of primary particles and/or aggregates thereof having an acicular or columnar shape and an average particle diameter of 4.0 to 15.0 μm and a BET specific surface area of 5.0 to 10 m 2 /g.

Furthermore, Patent Document 2 describes paper to which a filler is added, and proposes light calcium carbonate as the filler, which is formed by flocculating primary particles of spindle-shaped calcium carbonate and has a secondary particle size of 1 to 10 μm, a BET specific surface area of 8 to 20 m ² /g, and a pore volume of 1.5 to 3.5 cm ³ /g.

Furthermore, Patent Document 3 proposes reducing the drying load during paper production by using rosette-type calcium carbonate, which is made by agglomerating spindle-shaped primary particles of calcium carbonate into a burr-like shape, in combination with talc as a filler.

Furthermore, Patent Document 4 proposes that the use of a reaction product between a compound obtained by adding an alkylene oxide having 2 to 4 carbon atoms to the active hydrogen of a polyalkyleneimine having a weight-average molecular weight of 500 to 10,000 and a higher fatty acid having 12 to 24 carbon atoms and/or an ester compound of a higher fatty acid having 12 to 24 carbon atoms as a paper modifier can impart various added value, such as optical properties and flexibility, while preventing a significant decrease in paper strength.

Furthermore, Patent Document 5 proposes that a copolymer obtained by reacting a hydrophobic vinyl monomer such as styrene or a (meth)acrylic acid ester with a carboxyl group-containing vinyl monomer in a weight ratio of 30/70 to 90/10 can be used as a filler modifier for papermaking, thereby imparting excellent opacity improvement effects and suppressing print-through while maintaining paper strength.

JP 2013-060692 A International Publication No. WO2004/108597 JP 2012-172287 A Japanese Patent Application Laid-Open No. 2005-082949 Japanese Patent Application Laid-Open No. 2007-332512

When the amount of filler added to paper is increased, the optical properties improve, but the paper's strength, including tensile strength and tear strength, decreases. This is thought to be because inorganic fillers have poor affinity with pulp fibers and inhibit hydrogen bonding between the pulp fibers.

In light of the above situation, the objective of the present invention is to provide a technology that suppresses the decrease in strength of paper containing inorganic fillers.

As a result of extensive research, the present inventors have found that the above problems can be solved by modifying the inorganic filler by adding a specific polycarboxylic acid copolymer.
The present invention includes, but is not limited to, the following aspects.
[1] A modifier for inorganic fillers for papermaking, containing a polycarboxylic acid copolymer,
The polycarboxylic acid copolymer is
(1) Structural unit 1 derived from a monomer represented by the following formula 1:

[In the formula, R ¹ , R ² and R ³ each independently represent a hydrogen atom or an alkyl group having 1 to 3 carbon atoms; x represents an integer of 0 to 2; y represents 0 or 1; R ⁴ represents a hydrogen atom or a hydrocarbon group having 1 to 30 carbon atoms; R ⁵ O are the same or different and represent an oxyalkylene group having 2 to 18 carbon atoms; and n is the average number of moles of oxyalkylene groups added and represents a number of 1 to 100]
(2) Structural unit 2 derived from a monomer represented by the following formula 2:

[wherein R ⁶ , R ⁷ and R ⁸ each independently represent a hydrogen atom, a methyl group or —(CH ₂ )rCOOM ² ; M ¹ and M ² are the same or different and represent a hydrogen atom, an alkali metal, an alkaline earth metal, an ammonium group, an alkylammonium group or a substituted alkylammonium group; r is an integer of 0 to 2; and —(CH ₂ )rCOOM ² may form an anhydride with —COOM ¹ or another —(CH ₂ )rCOOM ² , but when an anhydride is formed, M ¹ and M ² are not present in those groups.]
wherein the weight ratio of structural unit 1 to structural unit 2 is 65/35 to 99/1.
[2] The weight average molecular weight (Mw) of the copolymer is 5000 to 100000, and the ratio of weight average molecular weight (Mw) to number average molecular weight (Mn) is Mw / Mn, which is 1.0 to 10.0. [1] The modifier according to.
[3] The modifier according to [1] or [2], which is used in an amount of 0.1 to 10 parts by weight per 100 parts by weight of the inorganic filler.
[4] The modifier according to [1] or [2], which is added to an inorganic filler before mixing with pulp.
[5] The modifier according to [1] or [2], which is used by being added to paper stock.
[6] The modifier according to [5], wherein the paper material contains 5 to 50 parts by weight of an inorganic filler per 100 parts by weight of pulp.

According to the present invention, it is possible to produce paper that contains inorganic fillers such as precipitated calcium carbonate but with reduced loss in strength. The details of why the present invention achieves such excellent effects are unclear, and the present invention is not bound by the following speculation, but it is believed that the copolymer of the present invention adsorbs to both the inorganic filler and the pulp fibers, causing the inorganic particles to adhere uniformly to the pulp fibers and strengthening interfiber bonds, thereby reducing the loss in paper strength that accompanies the internal addition of the inorganic filler.

The present invention relates to a modifier for inorganic fillers for papermaking. By adding the modifier of the present invention to inorganic fillers, it is possible to suppress a decrease in paper strength when the inorganic filler is internally added to paper.
Polycarboxylic acid copolymer
The modifier according to the present invention comprises a polycarboxylic acid copolymer containing at least structural unit 1 and structural unit 2, which will be described below. The weight ratio of structural unit 1 to structural unit 2 in the polycarboxylic acid copolymer according to the present invention is 65/35 to 99/1, preferably 70/30 to 97/3, more preferably 80/20 to 95/5, and may be 85/15 to 93/7. Here, the ratio of each structural unit contained in the copolymer basically corresponds to the charging ratio of each structural unit when synthesizing the copolymer. It is expected that having the structural ratio within this range makes it easier to uniformly disperse the inorganic filler as fine particles.

From the viewpoints of fluidity and workability, the weight-average molecular weight Mw of the copolymer is preferably 5,000 to 100,000. The lower limit of the weight-average molecular weight may be 10,000 or more or 15,000 or more, and the upper limit of the weight-average molecular weight may be 90,000 or less or 80,000 or less.

Regarding the molecular weight distribution of the copolymer, Mw/Mn is preferably 1.0 to 10.0, but may also be 1.2 to 8.0 or 1.5 to 6.0. Here, the weight-average molecular weight and other parameters can be measured by a known method using gel permeation chromatography (GPC) in terms of polyethylene glycol.

(Structural unit 1)
The copolymer according to the present invention comprises a structural unit 1 derived from a monomer represented by formula 1.

Here, R ¹ , R ² and R ³ each independently represent a hydrogen atom or an alkyl group having 1 to 3 carbon atoms, x represents a number from 0 to 2, y represents 0 or 1, R ⁵ O are the same or different and represent an oxyalkylene group having 2 to 18 carbon atoms, and n is the average number of moles of oxyalkylene groups added and represents a number from 1 to 100. R ⁴ represents a hydrogen atom or a hydrocarbon group having 1 to 30 carbon atoms.

R ¹ , R ² and R ³ each independently represent a hydrogen atom or an alkyl group having 1 to 3 carbon atoms.
R ⁵ O may be the same or different and represent an oxyalkylene group having 2 to 18 carbon atoms. Examples of the oxyalkylene group include an oxyethylene group (ethylene glycol unit), an oxypropylene group (propylene glycol unit), and an oxybutylene group (butylene glycol unit), with an oxyethylene group (ethylene glycol) and an oxypropylene group (propylene glycol) being preferred.

Here, "the same or different" means that when the general formula contains a plurality of R ⁵ O (when n is 2 or more), each R ⁵ O may be the same oxyalkylene group or different (two or more types) oxyalkylene groups. When the general formula contains a plurality of R ⁵ O, examples of the embodiment include an embodiment in which two or more oxyalkylene groups selected from the group consisting of oxyethylene groups (ethylene glycol units), oxypropylene groups (propylene glycol units) and oxybutylene groups (butylene glycol units) are mixed, and an embodiment in which oxyethylene groups (ethylene glycol units) and oxypropylene groups (propylene glycol units) are mixed, or an embodiment in which oxyethylene groups (ethylene glycol units) and oxybutylene groups (butylene glycol units) are mixed, is preferred, and an embodiment in which oxyethylene groups (ethylene glycol units) and oxypropylene groups (propylene glycol units) are mixed is more preferred. In an embodiment in which different oxyalkylene groups are mixed, the addition of two or more types of oxyalkylene groups may be block addition or random addition.

n is the average number of moles of oxyalkylene groups added and represents a number between 1 and 100. n is preferably between 1 and 50, more preferably between 1.5 and 40, and even more preferably between 2 and 30. The average number of moles added refers to the average number of moles of alkylene glycol units added to 1 mole of monomer.

The copolymer according to the present invention may contain only one type of structural unit 2, or a combination of two or more types. It is particularly preferable to contain two types of monomers with different average numbers of moles of oxyalkylene groups added, and it is even more preferable to contain a combination of a monomer in which the average number of moles of oxyalkylene groups added, n, is 1, and a monomer in which the average number of moles of oxyalkylene groups added, n, is greater than 1 and less than or equal to 100.

In a preferred embodiment, when n is 1, examples include compounds in which ^R4 is a hydrogen atom, such as 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, and 4-hydroxybutyl (meth)acrylate, with 2-hydroxyethyl (meth)acrylate being particularly preferred.

When a monomer in which n is 1 is used in combination with a monomer in which n is greater than 1 and 10 or less, the weight ratio of the two is preferably 1/99 to 99/1, more preferably 3/97 to 50/50, and even more preferably 5/95 to 30/70. This weight ratio corresponds to the weight ratio of the monomers charged when synthesizing the copolymer.

^R4 represents a hydrogen atom or a hydrocarbon group having 1 to 30 carbon atoms. The effects of the present invention are fully achieved when the number of carbon atoms is not too large. Therefore, ^R4 is preferably a hydrogen atom or a hydrocarbon group having 1 to 10 carbon atoms, more preferably a hydrogen atom or a hydrocarbon group having 1 to 5 carbon atoms, and most preferably a hydrogen atom or a methyl group.

Examples of the above-mentioned monomers include esters of unsaturated monocarboxylic acids such as (meth)acrylate (hereinafter, "(meth)acrylate" means "acrylate or methacrylate") with (poly)alkylene glycols such as (poly)ethylene glycol, (poly)ethylene (poly)propylene glycol, (poly)ethylene (poly)butylene glycol, methoxy(poly)ethylene glycol, methoxy(poly)ethylene (poly)propylene glycol, and methoxy(poly)ethylene (poly)butylene glycol.

Specific examples include (poly)alkylene glycol (meth)acrylates such as (poly)ethylene glycol (meth)acrylate, (poly)ethylene (poly)propylene glycol (meth)acrylate, (poly)ethylene (poly)butylene glycol (meth)acrylate, methoxy(poly)ethylene glycol (meth)acrylate, methoxy(poly)ethylene (poly)propylene glycol (meth)acrylate, and methoxy(poly)ethylene (poly)butylene glycol (meth)acrylate, as well as 4-hydroxybutyl (meth)acrylate and 2-hydroxyethyl (meth)acrylate. While one or a combination of two or more of these may be used as the monomer of Formula 2, it is preferable to use a (poly)alkylene glycol (meth)acrylate, and it is even more preferable to use a (poly)ethylene glycol (meth)acrylate or a methoxy(poly)ethylene glycol (meth)acrylate. When the monomer is a (poly)alkylene glycol (meth)acrylate, the average number of moles of (poly)alkylene glycol added is preferably 1 to 50. When the monomer n is 1 and is a (poly)alkylene glycol (meth)acrylate, the average number of moles of (poly)alkylene glycol added in the monomer is preferably 1. Furthermore, when n is greater than 1 and less than or equal to 10 and is a (poly)alkylene glycol (meth)acrylate, the average number of moles of (poly)alkylene glycol added in the monomer is preferably greater than 1 and less than or equal to 100, more preferably 1.5 to 50, and even more preferably 2 to 20.

The above-mentioned monomers can be produced by known methods without any particular limitations. Examples of such methods include esterifying an unsaturated monocarboxylic acid such as (meth)acrylic acid with a (poly)alkylene glycol such as (poly)ethylene glycol, (poly)ethylene (poly)propylene glycol, (poly)ethylene (poly)butylene glycol, methoxy(poly)ethylene glycol, methoxy(poly)ethylene (poly)propylene glycol, or methoxy(poly)ethylene (poly)butylene glycol.

(Structural unit 2)
The copolymer according to the present invention comprises a structural unit 2 derived from a monomer represented by formula 2.

Here, R ⁶ , R ⁷ , and R ⁸ each independently represent a hydrogen atom, a methyl group, or -(CH ₂ )rCOOM ² , and -(CH ₂ )rCOOM ² may form an anhydride with -COOM ¹ or another -(CH ₂ )rCOOM ² , but when an anhydride is formed, M ¹ and M ² are not present in those groups. M ¹ and M ² are the same or different and represent a hydrogen atom, an alkali metal, an alkaline earth metal, an ammonium group, an alkylammonium group, or a substituted alkylammonium group. r represents an integer of 0 to 2.

Examples of the above-mentioned monomers include unsaturated monocarboxylic acid monomers and unsaturated dicarboxylic acid monomers. Examples of unsaturated monocarboxylic acid monomers include acrylic acid, methacrylic acid, and crotonic acid, as well as their salts (e.g., monovalent metal salts, ammonium salts, and salts with organic amines). Examples of unsaturated dicarboxylic acid monomers include maleic acid, itaconic acid, citraconic acid, and fumaric acid, as well as their salts (e.g., monovalent metal salts, ammonium salts, and salts with organic amines), and their anhydrides.

The copolymer according to the present invention may contain only one type of structural unit 2, or may contain two or more types.
(Synthesis of copolymer)
The copolymer of the present invention can be produced by copolymerizing each of the predetermined monomers by a known method, such as polymerization in a solvent or bulk polymerization.

Solvents used in solvent polymerization include, for example, water; lower alcohols such as methyl alcohol, ethyl alcohol, and isopropyl alcohol; aromatic hydrocarbons such as benzene, toluene, and xylene; alicyclic or aliphatic hydrocarbons such as cyclohexane and n-hexane; esters such as ethyl acetate; and ketones such as acetone and methyl ethyl ketone. From the perspective of the solubility of the raw material monomers and the resulting copolymer, it is preferable to use one or more solvents selected from the group consisting of water and lower alcohols, with water being more preferred.

When copolymerization is carried out in a solvent, each monomer and polymerization initiator may be continuously added dropwise to a reaction vessel, or a mixture of each monomer and polymerization initiator may be continuously added dropwise to a reaction vessel. Alternatively, a solvent may be charged into a reaction vessel, and a mixture of the monomers and solvent and a polymerization initiator solution may be continuously added dropwise to the reaction vessel, or some or all of the monomers may be charged into a reaction vessel, and the polymerization initiator may be continuously added dropwise.

There are no particular limitations on the polymerization initiator that can be used in the copolymerization. When copolymerization is performed in an aqueous solvent, examples include persulfates such as ammonium persulfate, sodium persulfate, and potassium persulfate; and water-soluble organic peroxides such as t-butyl hydroperoxide. In this case, accelerators such as sodium bisulfite and Mohr's salt may be used in combination. Furthermore, when copolymerization is performed in a solvent such as a lower alcohol, aromatic hydrocarbon, alicyclic or aliphatic hydrocarbon, ester, or ketone, examples of polymerization initiators that can be used include peroxides such as benzoyl peroxide and lauryl peroxide; hydroperoxides such as cumene peroxide; and aromatic azo compounds such as azobisisobutyronitrile. In this case, accelerators such as amine compounds may also be used in combination. Furthermore, when copolymerization is performed in a water-lower alcohol mixed solvent, the aforementioned polymerization initiators or combinations of polymerization initiators and accelerators can be appropriately selected and used. The polymerization temperature varies depending on polymerization conditions such as the solvent and polymerization initiator used, but is typically between 50 and 120°C.

In copolymerization, a chain transfer agent may be used to adjust the molecular weight, if necessary. Examples of chain transfer agents that can be used include known thiol compounds such as mercaptoethanol, thioglycerol, thioglycolic acid, 2-mercaptopropionic acid, 3-mercaptopropionic acid, thiomalic acid, octyl thioglycolate, and 2-mercaptoethanesulfonic acid; phosphorous acid, hypophosphorous acid, and their salts (e.g., sodium hypophosphite, potassium hypophosphite), sulfurous acid, hydrogen sulfite, dithionous acid, metabisulfite, and their salts (e.g., sodium sulfite, potassium sulfite, sodium hydrogen sulfite, potassium hydrogen sulfite, sodium dithionite, potassium dithionite, sodium metabisulfite, potassium metabisulfite); and lower oxides and salts thereof. These may be used alone or in combination. Furthermore, to adjust the molecular weight of the copolymer, a monomer with even higher chain transfer properties may be used in addition to the monomers listed above as the monomer for obtaining the copolymer. Examples of monomers with high chain transfer properties include (meth)allylsulfonic acid (salt) monomers. The blending ratio of such monomers in the copolymer is typically 20% by weight or less, and preferably 10% by weight or less.

When copolymerization is performed in an aqueous solvent to obtain a copolymer, the pH during polymerization typically becomes strongly acidic due to the influence of the monomer having an unsaturated bond, but this may be adjusted to an appropriate pH. If pH adjustment is required during polymerization, the pH can be adjusted using an acidic substance such as phosphoric acid, sulfuric acid, nitric acid, alkyl phosphoric acid, alkyl sulfuric acid, alkyl sulfonic acid, or (alkyl)benzenesulfonic acid. Among these acidic substances, phosphoric acid is preferred due to its pH buffering effect. However, to eliminate the instability of the ester bonds in ester-based monomers, polymerization is preferably performed at a pH of 2 to 7. There are no particular limitations on the alkaline substance that can be used to adjust the pH, but alkaline substances such as NaOH and Ca(OH) ₂ are commonly used. pH adjustment may be performed on a solution containing the monomers before polymerization, or on a solution containing the copolymer after polymerization. Alternatively, a portion of the alkaline substance may be added before polymerization to perform polymerization, and then the pH of the solution containing the copolymer may be further adjusted.

The modifier according to the present invention is used together with an inorganic filler for papermaking. The inorganic filler for papermaking is not particularly limited, but examples thereof include calcium carbonate such as heavy calcium carbonate and light calcium carbonate, kaolin such as engineered kaolin, calcined kaolin, and delaminated kaolin, calcium sulfite, gypsum, talc, white carbon, amorphous silica, diatomaceous earth, magnesium carbonate, titanium dioxide, aluminum hydroxide, calcium hydroxide, magnesium hydroxide, zinc hydroxide, and papermaking sludge. The average particle size of the inorganic filler is not particularly limited, but is preferably 0.1 to 10 μm, more preferably 0.5 to 7 μm, and even more preferably 1 to 5 μm.

In the present invention, an inorganic filler and a copolymer are used in combination. The amount of copolymer used is preferably 0.1 to 20 parts by weight, more preferably 0.5 to 15 parts by weight, and even more preferably 1 to 10 parts by weight, per 100 parts by weight of the inorganic filler.

In the present invention, the ash content of paper is preferably 3 to 50% by weight, more preferably 5 to 40% by weight, and may be 8 to 30% by weight or 10 to 20% by weight, from the viewpoint of improving the optical properties of the paper, mainly whiteness and opacity. The ash content of paper can be adjusted by increasing or decreasing the amount of filler blended into the paper stock, taking into account factors such as filler retention.

In the present invention, paper can be produced using organic fillers in addition to inorganic fillers such as calcium carbonate. Examples of organic fillers include urea-formalin resin, vinyl chloride resin, polystyrene resin, urea/formalin resin, melamine resin, styrene/butadiene copolymer resin, phenolic resin, and hollow plastic particles. There are no particular restrictions on the ratio of inorganic and organic fillers used, and it can be adjusted depending on the quality of the paper.

As an inorganic filler, calcium carbonate such as precipitated calcium carbonate is particularly preferred, but there are no particular restrictions on the calcium carbonate as long as it is suitable for papermaking applications. For example, calcium carbonate in the shape of needles, columns, spindles, spheres, cubes, rosettes, etc. can be used.

The precipitated calcium carbonate used in the present invention may be calcite-based or aragonite-based, and may be produced by any manufacturing method. There are no particular restrictions on the average particle size of the calcium carbonate, but it is preferably 0.1 to 10 μm, more preferably 0.5 to 7 μm, and even more preferably 1 to 5 μm.

The amount of calcium carbonate added to the paper stock is preferably 2 to 30% by weight of the pulp, taking into account factors such as opacity. Furthermore, the filler content in the paper (ash content in the paper) should preferably be 10% by weight or more, and 40% by weight or less.

In the present invention, there are no particular restrictions on the amount of calcium carbonate used, but when used as an internal filler, it is preferable to use 10 parts by weight or more of calcium carbonate per 100 parts by weight of the internal filler used, more preferably 50 parts by weight or more, and even more preferably 70 parts by weight or more.

According to the present invention, paper containing inorganic fillers such as calcium carbonate can effectively suppress a decrease in strength. The uses of paper in this invention are not particularly limited, and examples of paper include various types of base paper for coating, newsprint, high-quality paper, medium-quality paper, electrophotographic transfer paper, inkjet paper, thermal paper, pressure-sensitive paper, kraft paper, pressure-sensitive recording paper, packaging paper, paper containers, cardboard, wallpaper, fiberboard, photographic base paper, impregnated base paper, and flame-retardant paper. In one embodiment, the paper of the present invention is suitable as printing paper for various printing methods such as offset printing and gravure printing.

The basis weight of the paper according to the present invention is not particularly limited, but can be, for example, in the range of about 30 to 650 g/m ² , or may be in the range of 35 to 200 g/m ² , 40 to 120 g/m ² , or 45 to 80 g/m ^2. The paper according to the present invention can also be made into thick paper such as multi-layer paperboard or card exceeding the above range.

Pulp used as a papermaking raw material may include, for example, commonly used bleached chemical pulps such as bleached hardwood kraft pulp (LBKP) and bleached softwood kraft pulp (NBKP); unbleached pulps such as unbleached hardwood kraft pulp (LUKP) and unbleached softwood kraft pulp (NUKP); mechanical pulps such as groundwood pulp (GP), pressure groundwood pulp (PGW), refiner groundwood pulp (RGP), and thermomechanical pulp (TMP); deinked recycled paper pulp (DIP), and broke. Furthermore, one or more types of pulp fibers obtained from non-wood fiber sources such as kenaf, synthetic pulp, and inorganic fibers may also be blended into the base paper. Mechanical pulp and DIP can be bleached as needed, and the degree of bleaching can be adjusted as desired.

In addition to pulp and fillers, papermaking chemicals may also be used in the stock as needed. Examples of internal papermaking chemicals include internal sizing agents, retention aids, drainage aids, paper strength agents, dyes, fluorescent dyes, and bulking agents. Specific examples of internal sizing agents include alkyl ketene dimers, alkenyl succinic anhydride sizing agents, styrene-acrylic sizing agents, higher fatty acid sizing agents, petroleum resin sizing agents, and rosin sizing agents. Specific examples of retention aids, drainage aids, and paper strength agents include polyvalent metal compounds such as aluminum (specifically, aluminum sulfate, aluminum chloride, sodium aluminate, basic aluminum compounds, etc.), various starches, cellulose nanofibers, cellulose derivatives such as carboxymethyl cellulose, hydroxyethyl cellulose, and methyl cellulose, polyacrylamides, urea resins, polyamide/polyamine resins, polyethyleneimine, polyamines, polyvinyl alcohol, and polyethylene oxide.

When adding filler to pulp raw materials in the present invention, it is preferable to add the filler while thoroughly stirring the pulp raw materials. Examples of locations where the filler can be added include the machine chest inlet and the fan pump suction port.

There are no particular restrictions on papermaking conditions, and commercial-scale papermaking machines such as Fourdrinier papermaking machines, twin-wire papermaking machines, on-top papermaking machines, cylinder papermaking machines, and short-wire papermaking machines can be used, and an appropriate method can be selected depending on the purpose. Any method can be used for papermaking, including acidic, neutral, and weakly alkaline papermaking. However, since calcium carbonate dissolves in the acidic range, if calcium carbonate is added internally, it is desirable to make the paper in a neutral to weakly alkaline range.

Surface treatment agents may be applied to paper to improve surface strength, water resistance, and ink receptivity. While there are no particular limitations on the type of surface treatment agent, preferred are starches such as raw starch, oxidized starch, esterified starch, cationized starch, and self-modified starch produced by thermochemical or enzymatic modification in paper mills using acetylated tapioca starch as a raw material, as well as modified starches such as aldehyde starch and hydroxyethylated starch. It is also possible to use in combination with cellulose derivatives such as carboxymethyl cellulose, hydroxyethyl cellulose, methyl cellulose, and cellulose nanofiber; modified alcohols such as polyacrylamide, polyvinyl alcohol, carboxyl-modified polyvinyl alcohol, and acetoacetylated polyvinyl alcohol; styrene-butadiene copolymers, polyvinyl acetate, vinyl chloride-vinyl acetate copolymers, polyvinyl chloride, polyvinylidene chloride, and polyacrylic esters. Furthermore, to improve sizing properties, surface sizing agents such as styrene-based sizing agents, olefin-based sizing agents, acrylate-based sizing agents, styrene-acrylic sizing agents, and cationic sizing agents may also be used in combination. In addition, in the present invention, various auxiliary agents typically blended into clear coatings, such as dispersants, thickeners, water retention agents, defoamers, water-resistant agents, colorants, and conductive agents, may be used as needed.

The device used to apply the surface treatment coating liquid can be a known size press device, such as a two-roll type, three-roll type, gate roll type, or film transfer type. Film transfer types involve forming a wet coating film on an applicator roll and then transferring the coating film to the surface of the base paper. Examples include a transfer roll coater and a rod metering size press coater. Coating can also be performed using a coater (applicator) such as a curtain coater, spray coater, or blade coater.

Although it is possible to apply the surface treatment coating liquid to the paper at 0.1 to 5.0 g/ ^m2 per side using these coating devices and then dry and finish the paper with a dryer, it is preferable to treat the paper with a calender such as a machine calender, soft calender or shoe calender to improve printability. The surface treatment liquid may also be applied to both sides.

In the present invention, coated paper can also be produced that has at least one coating layer whose main components are a pigment and an adhesive. According to the present invention, the strength of the paper is improved, resulting in excellent blister resistance and good sizing properties, resulting in coated paper with excellent surface smoothness.

In one embodiment, a base paper coated with a coating liquid containing a surface treatment agent can be used to produce pigment-coated paper. This surface treatment improves the surface strength of the base paper or has the effect of cleaning foreign matter from the surface of the base paper, thereby suppressing the occurrence of streaks and other problems. However, in the case of on-machine coaters, which produce base paper, apply a coating layer primarily composed of pigment and adhesive, and then dry it, surface treatment is sometimes omitted to avoid limitations in drying capacity and the risk of paper breakage.

The coated base paper can also be subjected to a smoothing finish treatment using a machine calender, soft calender, or the like.
The pigment used in the coating layer is not particularly limited, and may be one or more pigments commonly used in the field of coated paper, such as heavy calcium carbonate, light calcium carbonate (whether of the present invention or not), calcium sulfite, gypsum, talc, kaolin, engineered kaolin, calcined kaolin, white carbon, amorphous silica, delaminated kaolin, diatomaceous earth, magnesium carbonate, titanium dioxide, aluminum hydroxide, calcium hydroxide, magnesium hydroxide, zinc hydroxide, inorganic fillers such as recycled inorganic particles from papermaking sludge and deinking froth, and organic fillers such as urea-formalin resin, vinyl chloride resin, polystyrene resin, urea/formalin resin, melamine resin, styrene/butadiene copolymer resin, phenolic resin, and hollow plastic particles. The mixing ratio can be adjusted depending on the paper quality and is not particularly limited.

The pigment coating solution can use adhesives typically used in the coated paper field, or various surface treatment agents used in uncoated paper. These adhesives can also be used alone or in combination. There are no particular restrictions on the amount of adhesive used, but it is generally 1 to 50 parts by weight, preferably 5 to 30 parts by weight, per 100 parts by weight of pigment.

In addition, various auxiliary agents such as colored dyes, colored pigments, fluorescent whitening dyes, thickeners, water retention agents, antioxidants, anti-aging agents, conductive inducers, antifoaming agents, UV absorbers, dispersants, pH adjusters, release agents, water-resistant agents, and water-repellent agents can be blended as needed.

The solids concentration of the pigment coating liquid can be selected from the range of 25 to 80% by mass, and in consideration of adjustment of the coating amount and operability, the range of 50 to 70% by mass is preferable.
There is no particular restriction on the number of coating layers provided on the coated base paper, whether they are one layer or two or more layers. In the case of multiple layers, they do not all need to be identical, and can be adjusted appropriately depending on the required quality level. There is also no particular restriction on the amount of coating applied to the coating layer, and it can be adjusted depending on the white paper quality and printing quality of the coated paper, but it is generally about 0.5 to 40.0 g/ ^m2 per side.

When applying the coating layer in the present invention, various coating devices commonly used in the field of coated paper manufacturing, such as air knife coaters, various blade coaters, gate roll coaters, roll coaters, die coaters, curtain coaters, and spray coaters, can be used as appropriate.

After the pigment coating liquid is applied to the base paper, the coating layer is dried to obtain the coated paper. The drying method can be selected from any of the usual methods, such as a steam heater, gas heater, infrared heater, electric heater, hot air heater, microwave, or cylinder dryer.

The coated paper obtained in this manner may be passed through various known and commonly used finishing devices, such as a super calender, gloss calender, soft calender, or matte calender, to give the product a finishing finish.

The present invention will be described in detail below using specific examples, but the present invention is not limited to the specific examples below. Unless otherwise specified in this specification, concentrations and other information are based on weight, and numerical ranges include their endpoints.

Experiment 1: Preparation and evaluation of modifier 1-1. Sample 1
291 parts of water and 7.5 parts of polyethylene glycol monoallyl ether (average number of moles of ethylene oxide added: 10) were charged into a glass reaction vessel equipped with a thermometer, a stirrer, a reflux device, a nitrogen inlet tube, and a dropping device, and the reaction vessel was purged with nitrogen while stirring. After heating to 100°C under a nitrogen atmosphere, the temperature was maintained at 100°C. An aqueous monomer solution containing 3.9 parts of methacrylic acid, 0.072 parts of acrylic acid, 22 parts of methoxypolyethylene glycol methacrylate (average number of moles of ethylene oxide added: 25), 36 parts of 2-hydroxypropyl acrylate, and 33.5 parts of water was added dropwise to the reaction vessel over 2 hours, and a mixture of 1.2 parts of ammonium persulfate and 44 parts of water was added dropwise to the reaction vessel over 2 hours. After the dropwise addition, the temperature was maintained at 100°C and the reaction was continued for another hour, yielding an aqueous copolymer solution with a concentration of 40%.

The resulting copolymer was analyzed by gel permeation chromatography (GPC), and found to have a weight average molecular weight of 21,000 and an Mw/Mn ratio of 2.42 under the following analytical conditions:
Measurement equipment: Tosoh Corporation Columns used: Shodex Column OH-pak SB-806HQ, SB-804HQ, SB-802.5HQ
Eluent: 0.05 mM sodium nitrate/acetonitrile 8/2 (v/v)
・Standard substance: polyethylene glycol (manufactured by Tosoh)
・Detector: Differential refractometer (manufactured by Tosoh)
1-2. Sample 2
254 parts of water was charged into a glass reaction vessel equipped with a thermometer, a stirrer, a reflux device, a nitrogen inlet tube, and a dropping device, and the reaction vessel was purged with nitrogen under stirring. After heating to 100 ° C under a nitrogen atmosphere, the temperature was maintained at 100 ° C., and an aqueous monomer solution containing 215 parts of methoxypolyethylene glycol methacrylate (MPEG-MA, average number of moles of ethylene oxide added: 13.5), 3.1 parts of 3-mercaptopionic acid, 33 parts of methacrylic acid (MAA), and 42 parts of water was added dropwise over 2 hours. At the same time, a mixture of 2.7 parts of ammonium persulfate and 38 parts of water was added dropwise to the reaction vessel over 2.5 hours. After the reaction was continued for another hour while maintaining the temperature at 100 ° C., the mixture was cooled to 70 ° C., and neutralized to pH 7 with sodium hydroxide while adding water, to obtain an aqueous copolymer solution with a concentration of 43%. Analysis by GPC revealed that the copolymer had a weight average molecular weight of 12,000 and Mw/Mn of 1.62.

1-3. Sample 3
Into a glass reaction vessel equipped with a thermometer, a stirrer, a reflux device, a nitrogen inlet tube, and a dropping device, 218 parts of water and 13.9 parts of polyethylene glycol monoallyl ether (average number of moles of ethylene oxide added: 10) were charged, and the reaction vessel was purged with nitrogen while stirring. After heating to 100°C under a nitrogen atmosphere, the temperature was maintained at 100°C. A monomer aqueous solution containing 7.2 parts of methacrylic acid, 0.07 parts of acrylic acid, 41 parts of methoxypolyethylene glycol methacrylate (average number of moles of ethylene oxide added: 25), 67.2 parts of 2-hydroxypropyl acrylate, and 43.5 parts of water was added dropwise over 2 hours, while simultaneously adding a mixture of 0.01 parts of ammonium persulfate and 37.7 parts of water dropwise to the reaction vessel over 2.5 hours. The reaction was continued for another hour while maintaining the temperature at 100°C, yielding an aqueous solution of copolymer with a concentration of 30%. Analysis by GPC revealed that the copolymer had a weight average molecular weight of 73,000 and Mw/Mn of 6.55.

1-4. Sample 4 (Comparative Example)
A glass reactor equipped with a thermometer, a stirrer, a reflux device, a nitrogen inlet tube, and a dropping device was charged with 315 parts of water, and the reactor was purged with nitrogen while stirring. After heating to 100°C under a nitrogen atmosphere, a monomer aqueous solution containing 325 parts of 2-hydroxypropyl acrylate and 216 parts of water, and a mixture of 2.85 parts of ammonium persulfate and 77 parts of water were each added dropwise to the reactor over 2 hours while maintaining the temperature at 100°C. The reaction was continued for another hour while maintaining the temperature at 100°C, yielding a 40% aqueous polymer solution. Analysis using GPC revealed that the weight-average molecular weight of the polymer was 11,000 and Mw/Mn was 3.16.

1-5. Sample 5 (Comparative Example)
A glass reactor equipped with a thermometer, a stirrer, a reflux device, a nitrogen inlet tube, and a dropping device was charged with 198 parts of water, and the reactor was purged with nitrogen under stirring. The temperature was raised to 100 ° C. under a nitrogen atmosphere, and then the temperature was maintained at 100 ° C. A monomer aqueous solution containing 72 parts of acrylic acid, 70 parts of a 31% aqueous NaOH solution, and 54 parts of water, and a mixture of 4 parts of ammonium persulfate and 37 parts of water were each added dropwise over 2 hours. The reaction was continued for another hour while maintaining the temperature at 100 ° C., yielding an aqueous solution of a 36% homopolymer. Analysis using GPC revealed that the weight average molecular weight of the polymer was 14,000 and Mw / Mn was 1.71.

Experiment 2: Production and evaluation of inorganic filler-containing paper The polymer synthesized in Experiment 1 was added as a modifier to light calcium carbonate (average particle size 0.49 μm, acicular light calcium carbonate) produced in-house to prepare an aqueous slurry containing an inorganic filler. The weight ratio of the light calcium carbonate (inorganic filler) to the modifier was 100:1.

The above inorganic filler slurry was added to a pulp slurry containing bleached hardwood kraft pulp (LBKP, CSF: 370 ml) and bleached softwood kraft pulp (NBKP, CSF: 510 ml) in a weight ratio of 90:10. Then, 200 ppm of cationic polyacrylamide (Hakuto, Percol 3045) and 0.1% of anionic inorganic pigment (Heimo, Heimonite-01) based on the pulp solids were added and stirred to prepare a paper stock (paper stock solids: approximately 0.55%).

Next, using this paper stock as a raw material, handsheet paper containing an inorganic filler was produced in accordance with JIS P8222 using a circular handsheet machine (basis weight: about 60 g/m ² ).
The properties of the produced handsheets were measured based on the following criteria.
Basis weight: JIS P8124
Paper thickness: JIS P8118
Density: JIS P8118
Ash content: JIS P8251
Bursting strength: JIS P8112
Tear strength: JIS P8116
Number of folding times: JIS P8115
Air permeability: JAPAN TAPPI No. 5
Smoothness: JIS P8119
Whiteness: JIS P8148
Opacity: JIS P8149

As is clear from the above results, adding the modifier of the present invention to the inorganic filler beforehand suppressed a decrease in paper strength, such as burst strength index and tear strength index, in inorganic filler-added paper. In particular, when the modifiers of Samples 1 to 3 were added to the inorganic filler, the tear strength index of the paper was significantly improved compared to when no modifiers were added (Sample 2-1) or when a homopolymer was added (Sample 2-5). As can be seen from the above results, adding the modifier of the present invention to the inorganic filler not only suppresses a decrease in paper strength, but is also expected to improve paper strength.

Experiment 3: Production and evaluation of inorganic filler-containing paper The modifier synthesized in Experiment 1 was added to light calcium carbonate (Tamapearl TP121-6S, manufactured by Okutama Kogyo Co., Ltd., average particle size: 1.8 μm, spindle-shaped light calcium carbonate) to prepare an aqueous slurry containing an inorganic filler. The weight ratio of the light calcium carbonate (inorganic filler) to the modifier was 100:1.

Next, the inorganic filler slurry and aluminum sulfate were added to a pulp slurry consisting of 100% bleached hardwood kraft pulp (LBKP, CSF: 360 ml) to prepare a paper stock. The amount of aluminum sulfate (aluminum sulfate) added was 0.5% of the total solids content of the pulp slurry.

Using this paper stock as a raw material, handsheets were made using a circular handsheet machine (basis weight: approximately 73 g/ ^m2 ). The handsheets were made in accordance with JIS P 8222, but the wet paper after press dewatering was dried using a laboratory-scale cylinder dryer, rather than by the method of contacting the paper with a metal plate as specified in JIS P 8222.

The tear strength index of the paper produced in this manner was measured in the same manner as in Experiment 2. When the modifier of the present invention was added, the strength of the paper was significantly improved.

Experiment 4: Evaluation of fixation of modifier to inorganic filler The fixation of the modifier according to the present invention to inorganic filler was evaluated. Specifically, an aqueous slurry (concentration 20%) was prepared by mixing light calcium carbonate (PC-35, average particle size 50 μm, manufactured by Sankyo Seifun) and modifier (Sample 1 or 5) in a weight ratio of 200:1. The aqueous slurry was stirred for 5 minutes at 400 rpm using a stirring device (ZZ-1200, manufactured by Tokyo Rikakikai). The stirred aqueous slurry was then centrifuged (15 minutes at 10,000 rpm) using a centrifuge (H-2000B, manufactured by Kokusan).

The modifier contained in the supernatant solution after separation was quantified using GPC under the conditions described above, and the amount of modifier fixed to the precipitated precipitated calcium carbonate was calculated.

When the fixation to inorganic fillers was evaluated, it was revealed that the modifier of the present invention (Sample 1) had significantly better fixation to inorganic particles than Sample 5, which was a comparative example.

Claims (6)

A modifier for inorganic fillers for papermaking, comprising a polycarboxylic acid copolymer,
The polycarboxylic acid copolymer is
(1) Structural unit 1 derived from a monomer represented by the following formula 1:
[In the formula, R ¹ , R ² and R ³ each independently represent a hydrogen atom or an alkyl group having 1 to 3 carbon atoms; x represents an integer of 0 to 2; y represents 0 or 1; R ⁴ represents a hydrogen atom or a hydrocarbon group having 1 to 30 carbon atoms; R ⁵ O are the same or different and represent an oxyalkylene group having 2 to 18 carbon atoms; and n is the average number of moles of oxyalkylene groups added and represents a number of 1 to 100]
(2) Structural unit 2 derived from a monomer represented by the following formula 2:
[wherein R ⁶ , R ⁷ and R ⁸ each independently represent a hydrogen atom, a methyl group or —(CH ₂ )rCOOM ² ; M ¹ and M ² are the same or different and represent a hydrogen atom, an alkali metal, an alkaline earth metal, an ammonium group, an alkylammonium group or a substituted alkylammonium group; r is an integer of 0 to 2; and —(CH ₂ )rCOOM ² may form an anhydride with —COOM ¹ or another —(CH ₂ )rCOOM ² , but when an anhydride is formed, M ¹ and M ² are not present in those groups.]
wherein the weight ratio of structural unit 1 to structural unit 2 is 65/35 to 99/1. The modifier according to claim 1, wherein the copolymer has a weight-average molecular weight (Mw) of 5,000 to 100,000, and the ratio of weight-average molecular weight (Mw) to number-average molecular weight (Mn), Mw/Mn, is 1.0 to 10.0. The modifier according to claim 1 or 2 is used in an amount of 0.1 to 10 parts by weight per 100 parts by weight of inorganic filler. The modifier according to claim 1 or 2 is added to an inorganic filler before mixing with pulp. The modifier according to claim 1 or 2, which is used by adding it to paper stock. The modifier according to claim 5, wherein the paper stock contains 5 to 50 parts by weight of an inorganic filler per 100 parts by weight of pulp.