Surface paper strength enhancer, coating liquid, and method for producing paper

Abstract

The present invention relates to: a surface paper strength enhancer containing a polymer (A) comprising, as reactive components, a (meth)acrylamide (a1), a (meth)acrylamide (a2) having a carboxy group, an ethylenically unsaturated monomer (a3) having a sulfo group, and a crosslinkable monomer (a4), wherein a solution of the surface paper strength enhancer having a non-volatile concentration of 20 wt% has a viscosity of 100-5,000 mPa·s at a temperature of 25°C; a coating liquid containing the surface paper strength enhancer; and a method for producing paper, the method comprising a step for applying the coating liquid to the surface of a base paper and then drying the coating liquid.

Description

Surface paper strength enhancer, coating liquid, and paper manufacturing method

[0001] The present invention relates to a surface paper strength agent, a coating liquid, and a method for producing paper.

[0002] In recent years, growing environmental awareness and demands for cost reductions have led to increased use of recycled pulp and the reduction in the weight of base paper itself. Therefore, simply adding strength enhancers to raw pulp makes it difficult to maintain paper strength due to the influence of impurities such as fine fibers and calcium ions contained in the paper. To compensate for this strength, surface strength enhancers such as starch and polyacrylamide are applied to the surface of the base paper. Furthermore, in surface coating, productivity has improved, and papermaking machines have become faster. Film-transfer size presses such as gate rolls and rod metering have been adopted instead of the traditional two-roll size press, requiring better penetration of the coating solution and a higher paper strength enhancement effect.

[0003] One way to achieve a high paper strength is to increase the molecular weight of the polyacrylamide, a paper strength enhancer. However, this makes it difficult for the surface strength enhancer to penetrate into the paper, resulting in insufficient paper strength. Therefore, an alternative approach is to focus on the composition of the surface strength enhancer, allowing it to interact with the aluminum compound contained in the base paper without impairing its permeability.

[0004] One such technique is known, for example, in Patent Document 1, which describes a surface strength agent containing a polymer of monomer components including (meth)acrylamide, N-substituted (meth)acrylamides, an unsaturated monomer having a sulfonic acid group, and an α,β-unsaturated dicarboxylic acid. In this technique, an aluminum compound contained in the base paper interacts with the surface strength agent to form a pseudo-polyion complex (PIC), which fixes the surface strength agent to the paper and provides a high paper strength enhancing effect. However, the effect is still insufficient.

[0005] Japanese Patent Application Laid-Open No. 2019-173257

[0006] An object of the present invention is to provide a surface paper strength agent having excellent paper strength enhancing effects.

[0007] The present inventors have conducted extensive research focusing on the composition of monomer components and the physical properties of polymers, and have found a solution to the above problems, thereby completing the present invention. That is, the present invention relates to the following surface paper strength agent, coating solution, and paper manufacturing method.

[0008] 1. A surface paper strength agent comprising a polymer (A) containing, as reactive components, (meth)acrylamide (a1), (meth)acrylamide having a carboxy group (a2), an ethylenically unsaturated monomer having a sulfo group (a3), and a crosslinkable monomer (a4), wherein the viscosity of a solution of the surface paper strength agent having a non-volatile content of 20% by weight at 25°C is 100 to 5,000 mPa·s.

[0009] 2. The surface paper strength agent according to the above item 1, wherein the reaction components further contain (a5) an ethylenically unsaturated monomer having a carboxy group other than component (a2).

[0010] 3. The surface paper strength agent according to the preceding paragraph 1, wherein, based on 100 mol% of all reaction components, the (a1) component is 72 to 97 mol%, the (a2) component is 0.01 to 15 mol%, the (a3) ​​component is 0.4 to 10 mol%, and the (a4) component is 0.001 to 8 mol%.

[0011] 4. The surface paper strength agent according to paragraph 1, wherein, when the maximum value of turbidity (unit: NTU) of an aqueous solution containing 1% by weight of the surface paper strength agent and 0.5% by weight of aluminum sulfate at a nonvolatile content of pH 5 to 8 is X and the minimum value is Y, the following relationships are satisfied: 10≦X≦200 and X−Y≦50.

[0012] 5. A coating liquid containing the surface paper strength agent according to any one of items 1 to 4 above.

[0013] 6. A method for producing paper, comprising the step of applying the coating liquid according to the preceding paragraph 5 to the surface of base paper and drying it.

[0014] 7. The method for producing paper according to the preceding paragraph 6, wherein the base paper contains an aluminum compound.

[0015] The paper strength agent of the present invention exhibits excellent paper strength enhancing effects when made into paper.

[0016] Turbidity graph of aluminum sulfate aqueous solution, and mixture of surface paper strength agent and aluminum sulfate of Example 3 and Comparative Example 2

[0017] The paper strength agent of the present invention comprises a polymer (A) (hereinafter referred to as polymer (A)) containing, as reactive components, (meth)acrylamide (a1) (hereinafter referred to as component (a1)), (meth)acrylamide (a2) having a carboxy group (hereinafter referred to as component (a2)), an ethylenically unsaturated monomer (a3) ​​having a sulfo group (hereinafter referred to as component (a3)), and a crosslinkable monomer (a4) (hereinafter referred to as component (a4)). Hereinafter, (meth)acrylic means methacrylic and / or acrylic, (meth)acrylate means methacrylate and / or acrylate, and ethylenically unsaturated means having one or more carbon-carbon double bonds and / or carbon-carbon triple bonds in the molecule (the same applies hereinafter).

[0018] The component (a1) is methacrylamide or acrylamide, which may be used alone or in combination of two or more.

[0019] The molar ratio of the component (a1) is preferably 72 to 97 mol %, more preferably 75 to 97 mol %, and even more preferably 80 to 95 mol %, with the total of all reaction components being 100 mol %.

[0020] Component (a2) is a (meth)acrylamide having a carboxy group. The carboxy group is a group represented by —COOH. By using component (a2), the surface paper strength agent is more likely to interact with the aluminum compound contained in the base paper, resulting in a high paper strength enhancing effect.

[0021] Examples of component (a2) include (meth)acrylamidoalkanoic acids such as N-(meth)acryloylglycine (2-((meth)acryloylamino)acetic acid), 3-(meth)acrylamidopropionic acid, 4-(meth)acrylamidobutanoic acid, 5-(meth)acrylamidopentanoic acid, and 6-(meth)acrylamidohexanoic acid; (meth)acrylamidoalkylalkanoic acids such as 3-(meth)acrylamido-2-methylbutanoic acid, 3-(meth)acrylamido-3-methylbutanoic acid, and 4-(meth)acrylamido-3,3-dimethylhexanoic acid; and 2-(meth)acrylamido-N-glycolic acid. These may be used alone or in combination of two or more. Of these, 2-(meth)acrylamido-N-glycolic acid is preferred because it has a hydroxy group in addition to a carboxy group that is likely to interact with the aluminum compound contained in the base paper.

[0022] The component (a2) may be prepared by adding one or more selected from monoalkanoic acids, alkylmonoalkanoic acids, alkanedicarboxylic acids, hydroxycarboxylic acids, aldehydealkanoic acids, and salts thereof to the reaction components, and reacting the resulting mixture with the component (a1), and the resulting mixture may be used as is.

[0023] Examples of monoalkanoic acids include monoalkanoic acids such as acetic acid, propionic acid, butanoic acid, pentanoic acid, and hexanoic acid. Examples of alkylmonoalkanoic acids include 2-methylbutanoic acid, 3-methylbutanoic acid, and 3,3-dimethylhexanoic acid. Examples of alkanedicarboxylic acids include succinic acid and oxalic acid. Examples of hydroxycarboxylic acids include citric acid and glycolic acid. Examples of aldehydecarboxylic acids include glyoxylic acid. Examples of the salts thereof include alkali metal salts such as lithium, sodium, and potassium of the monoalkanoic acids, alkylmonoalkanoic acids, alkanedicarboxylic acids, hydroxycarboxylic acids, and aldehydealkanoic acids; alkaline earth metal salts such as magnesium salts and calcium salts; ammonium salts such as ammonia; and organic amine salts such as trimethylamine, triethylamine, trimethanolamine, and triethanolamine. These may be used alone or in combination of two or more.

[0024] The molar ratio of component (a2) is preferably 0.01 to 15 mol%, more preferably 0.05 to 12 mol%, and even more preferably 0.1 to 10 mol%, based on 100 mol% of all reactive components, because this allows the surface strength agent to interact with the aluminum compound contained in the base paper, making it easier to exhibit a high paper strength enhancing effect, and also reduces the cost of the surface strength agent.

[0025] The component (a3) ​​is an ethylenically unsaturated monomer having a sulfo group. The sulfo group is —SO ３ The use of component (a3) ​​makes it possible to obtain a surface strength agent with a predetermined viscosity, which facilitates penetration into the interior of the base paper when coated, and also facilitates the exertion of a high paper strength enhancing effect.

[0026] Examples of component (a3) ​​include vinyl sulfonic acid, methallylsulfonic acid, and p-styrenesulfonic acid. These components (a3) ​​may be used in the form of salts such as alkali metal salts (e.g., sodium, potassium) or ammonium salts. These may be used alone or in combination of two or more. Among these, methallylsulfonic acid or a salt thereof is preferred, and sodium methallylsulfonate is more preferred.

[0027] The molar ratio of component (a3) ​​is preferably 0.4 to 10 mol%, more preferably 0.6 to 8 mol%, and even more preferably 0.8 to 5 mol%, based on 100 mol% of all reactive components, in order to obtain a surface paper strength agent having a predetermined viscosity and to facilitate the exertion of a high paper strength enhancing effect.

[0028] The component (a4) is a crosslinkable monomer.

[0029] Examples of the component (a4) include N-alkyl(meth)acrylamides such as N-methyl(meth)acrylamide, N-ethyl(meth)acrylamide, N-isopropyl(meth)acrylamide, and N-t-butyl(meth)acrylamide; N,N-dialkyl(meth)acrylamides such as N,N-dimethyl(meth)acrylamide, N,N-diethyl(meth)acrylamide, and N,N-diisopropyl(meth)acrylamide; N,N'-alkylenebis(meth)acrylamides such as N,N'-methylenebis(meth)acrylamide and N,N'-ethylenebis(meth)acrylamide; crosslinking monomers having a triallyl group such as triallyl isocyanurate, triallyl trimellitate, triallylamine, and triallyl(meth)acrylamide; Examples include triazines having a (meth)acryloyl group, such as 1,3,5-tri(meth)acryloyl-1,3,5-triazine and 1,3,5-tri(meth)acryloylhexahydro-1,3,5-triazine. These may be used alone or in combination of two or more.

[0030] Among these, N,N-dialkyl(meth)acrylamide and N,N'-alkylenebis(meth)acrylamide are preferred, and N,N-dimethyl(meth)acrylamide and N,N'-methylenebis(meth)acrylamide are more preferred.

[0031] The molar ratio of the component (a4), relative to 100 mol% of all reactive components, is preferably 0.001 to 8 mol%, more preferably 0.003 to 7 mol%, and even more preferably 0.005 to 5.5 mol%, from the viewpoint of suppressing excessive crosslinking reaction and making it easier to reduce the occurrence of gelation.

[0032] The reaction components may further contain an ethylenically unsaturated monomer having a carboxy group other than the component (a2) (hereinafter referred to as the component (a5)).

[0033] Examples of component (a5) include ethylenically unsaturated monocarboxylic acids such as (meth)acrylic acid, 3-butenoic acid, 4-pentenoic acid, 5-hexenoic acid, and 4-methylhexene; ethylenically unsaturated dicarboxylic acids such as itaconic acid, fumaric acid, maleic acid, citraconic acid, and muconic acid; ethylenically unsaturated monocarboxylic anhydrides such as (meth)acrylic acid anhydride; and ethylenically unsaturated dicarboxylic anhydrides such as itaconic anhydride, maleic anhydride, and citraconic anhydride. These components (a5) may be used as salts, such as alkali metal salts (e.g., lithium, sodium, and potassium); alkaline earth metal salts (e.g., magnesium salts and calcium salts); ammonium salts (e.g., ammonia); and organic amine salts (e.g., trimethylamine, triethylamine, trimethanolamine, and triethanolamine). These may be used alone or in combination of two or more. Among these, (meth)acrylic acid, (meth)acrylic anhydride, itaconic acid, and itaconic anhydride are preferred.

[0034] The molar ratio of component (a5) is preferably 10 mol % or less, more preferably 5 mol % or less, and even more preferably 3 mol % or less, based on 100 mol % of all reactive components, in order to ensure a sufficient paper strength enhancing effect of the surface paper strength agent.

[0035] The reaction components may further contain a monomer (a6) (hereinafter referred to as component (a6)) other than the components (a1) to (a5).

[0036] Examples of the component (a6) include ethylenically unsaturated monomers having a secondary amino group, such as diallylamine; ethylenically unsaturated monomers having a tertiary amino group, such as (meth)acrylates having a tertiary amino group, such as N,N-dimethylaminoethyl (meth)acrylate and N,N-diethylaminoethyl (meth)acrylate, and (meth)acrylamides having a tertiary amino group, such as N,N-dimethylaminopropyl (meth)acrylamide and N,N-diethylaminopropyl (meth)acrylamide; quaternized salts obtained by reacting the polymerizable monomers having a secondary amino group or the polymerizable monomers having a tertiary amino group with a quaternizing agent (examples of the quaternized salts include inorganic acid salts such as hydrochlorides and sulfates; and organic acid salts such as acetates. Examples of the quaternizing agent include methyl chloride, benzyl chloride, dimethyl sulfate, and epichlorohydrin); monomers having an aromatic ring, such as styrene, α-methylstyrene, and vinyltoluene; Examples of suitable acrylates include alkyl (meth)acrylates such as methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, n-butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, and cyclohexyl (meth)acrylate; vinyl carboxylates such as vinyl acetate and vinyl propionate; nitriles such as acrylonitrile; mercaptans such as 2-mercaptoethanol and n-dodecyl mercaptan; alcohols such as ethanol, isopropyl alcohol, and n-pentyl alcohol; aromatic compounds such as α-methylstyrene dimer, ethylbenzene, isopropylbenzene, and cumene; and carbon tetrachloride. These may be used alone or in combination of two or more.

[0037] The molar ratio of the component (a6) is preferably 10 mol % or less, more preferably 5 mol % or less, and even more preferably 3 mol % or less, with the total of all reaction components being 100 mol %.

[0038] In addition, in the production of a (meth)acrylamide polymer, additives such as inorganic acids such as hydrochloric acid, sulfuric acid, phosphoric acid, etc.; inorganic acid salts such as sodium sulfate, potassium sulfate, ammonium sulfate, etc.; inorganic bases such as sodium hydroxide, potassium hydroxide, calcium hydroxide, etc.; starches such as oxidized starch, starch phosphate, urea phosphate esterified starch, etherified starch, crosslinked starch, APS-modified starch, enzyme-modified starch, cationized starch, amphoteric starch, etc.; polyvinyl alcohol, urea, antifoaming agents, antioxidants, preservatives, disinfectants, etc. may be added alone or in combination of two or more.

[0039] For example, the polymer (A) of the present invention can be prepared by adding a solution of a polymerization initiator to a solution containing the components (a1) to (a4), and optionally the components (a5) to (a6), and additives, which has been placed in a solvent in advance in a reaction apparatus, followed by reaction. The polymerization initiator and solvent can also be added as appropriate during or after the reaction.

[0040] Examples of the solvent include water and organic solvents, which may be used alone or in combination of two or more.

[0041] Examples of water include tap water, ultrapure water, pure water, ion-exchanged water, hard water, soft water, industrial water, etc. These may be used alone or in combination of two or more.

[0042] Examples of organic solvents include alcohols such as methanol, ethanol, n-propyl alcohol, isopropyl alcohol, n-butyl alcohol, sec-butyl alcohol, t-butyl alcohol, isobutyl alcohol, n-hexyl alcohol, n-octyl alcohol, ethylene glycol, propylene glycol, diethylene glycol, triethylene glycol, and diacetone alcohol; and ethers such as ethylene glycol monobutyl ether, propylene glycol monomethyl ether, and propylene glycol monoethyl ether. These may be used alone or in combination of two or more.

[0043] Of these, water is preferred from the viewpoint of sufficiently dissolving and dispersing the components (a1) to (a6).

[0044] The amount of solvent used is adjusted so that the non-volatile concentration of all reaction components is 10 to 40% by weight, preferably 15 to 35% by weight.

[0045] Examples of polymerization initiators include persulfates such as ammonium persulfate, potassium persulfate, and sodium persulfate; azo compounds such as 2,2'-azobis(2-amidinopropane) hydrochloride and 2,2'-azobis[2(2-imidazolin-2-yl)propane] hydrochloride; and hydrogen peroxide. These may be used alone or in combination. Among these, ammonium persulfate, potassium persulfate, and 2,2'-azobis(2-amidinopropane) hydrochloride are preferred from the viewpoint of ensuring sufficient progress of solution polymerization. The polymerization initiator can be added in a single batch, in divided batches, or by continuous dropwise addition, as appropriate. The content of the polymerization initiator is typically about 0.001 to 5 parts by weight, and preferably about 0.01 to 2 parts by weight, per 100 parts by weight of all reaction components.

[0046] Examples of the polymerization method include a method using only a dropping polymerization method, a method using only a simultaneous polymerization method (charging a monomer mixture all at once), and a method combining a simultaneous polymerization method and a dropping polymerization method.

[0047] As for reaction conditions, the temperature is preferably 40 to 100° C., more preferably 50 to 100° C., and the reaction time is preferably 1 to 10 hours, more preferably 1 to 8 hours.

[0048] The surface paper strength agent of the present invention may contain additives such as antifoaming agents, preservatives, chelating agents, water-soluble aluminum compounds, and urea; and starches such as oxidized starch, phosphated starch, APS-modified starch, enzyme-modified starch, cationized starch, and amphoteric starch.

[0049] The physical properties of the surface strength agent of the present invention include a viscosity of 100 to 5,000 mPa·s at 25°C of a solution of the surface strength agent with a non-volatile content of 20% by weight. Note that the viscosity here is a value measured with a B-type viscometer (Brookfield viscometer). By exhibiting this viscosity, the surface strength agent is more likely to exhibit excellent paper strength enhancing effects. From the same perspective, the viscosity is preferably 150 to 4,900 mPa·s, and more preferably 200 to 4,800 mPa·s.

[0050] As another physical property, a specific turbidity (unit: NTU) value at a pH of 5 to 8 of an aqueous solution containing 1% by weight of the surface paper strength agent and 0.5% by weight of aluminum sulfate in non-volatile concentration is used.

[0051] Turbidity is the degree of cloudiness, and is a value obtained by measuring scattered light at 180 degrees using 900 nm infrared light using an ANALITE NEPHELOMER 152 (manufactured by McVan Instruments). The measured value is a relative evaluation value to a standard substance (formazin standard solution 400 NTU, manufactured by Wako Pure Chemical Industries, Ltd.).

[0052] The turbidity of the present invention is measured using an aqueous solution containing a surface strength agent and aluminum sulfate at predetermined concentrations. The maximum (X) and minimum (Y) values ​​are read from the turbidity values ​​in the pH range of 5 to 8, and the maximum (X) and the difference between the maximum and minimum values ​​(X-Y) are used as physical property values. The aqueous solution may be prepared by mixing the surface strength agent with an aluminum sulfate aqueous solution that has been diluted to a predetermined concentration with ion-exchanged water, or by mixing the surface strength agent and aluminum sulfate, each diluted to a predetermined concentration with ion-exchanged water. Regarding the turbidity value, in the case of an aluminum sulfate aqueous solution alone, increasing the pH promotes polymerization of the aluminum compound, resulting in high turbidity values ​​between pH 5 and 8, and maintaining turbidity up to pH 10 (Figure 1 shows the change in turbidity in an aluminum sulfate aqueous solution with a nonvolatile content of 0.5 wt %). On the other hand, when the surface paper strength agent is mixed with aluminum sulfate, the polymerization of the aluminum compound is suppressed, and the turbidity value is smaller than that of aluminum sulfate alone. As a result, it is presumed that the surface paper strength agent of the present invention is more likely to exhibit a high paper strength enhancing effect.

[0053] The maximum value of the turbidity (X) is preferably 10≦X≦200 (maximum value of 10 or more and 200 or less), more preferably 25≦X≦190 (maximum value of 25 or more and 190 or less), and even more preferably 35≦X≦180 (maximum value of 35 or more and 180 or less), because the surface paper strength agent is likely to exhibit an excellent paper strength enhancing effect.

[0054] The difference (X-Y) between the maximum and minimum turbidity values ​​is preferably X-Y≦50 (the difference between the maximum and minimum values ​​is 50 or less), more preferably 3≦X-Y≦45 (the difference between the maximum and minimum values ​​is 3 or more and 45 or less), and even more preferably 5≦X-Y≦40 (the difference between the maximum and minimum values ​​is 5 or more and 40 or less), because this makes it easier for the surface paper strength agent to exhibit an excellent paper strength enhancing effect.

[0055] The non-volatile content of the surface paper strength agent is preferably 10 to 40% by weight, more preferably 15 to 30% by weight.

[0056] The coating solution of the present invention contains a surface strength agent, and the undiluted solution of the surface strength agent may be used as is, but it is preferable to dilute it with water or the like to a solids concentration of 0.1 to 15% by weight in order to enable the coating solution to be applied uniformly to the surface of the base paper and to facilitate penetration into the interior.

[0057] As for the physical properties of the coating liquid of the present invention, for example, the viscosity of an aqueous solution having a solids concentration of 5% by weight at 50° C. as measured with a Brookfield viscometer is usually 50 mPa·s or less, preferably 40 mPa·s or less.

[0058] The coating liquid of the present invention may also contain various known additives as needed, such as rosin-based sizing agents, alkyl ketene dimer-based sizing agents, polymer-based sizing agents, and other sizing agents; starches such as oxidized starch, phosphated starch, APS-modified starch, enzyme-modified starch, cationized starch, and amphoteric starch; celluloses such as carboxymethyl cellulose; paper strength agents such as water-soluble polymers such as polyvinyl alcohols and sodium alginate; anti-slip agents, preservatives, rust inhibitors, pH adjusters, antifoaming agents, thickeners, fillers, antioxidants, waterproofing agents, film-forming aids, pigments, and dyes.

[0059] The method for producing coated paper of the present invention comprises the steps of applying the coating liquid of the present invention to the surface of base paper and drying it.

[0060] Examples of base paper include PPC paper, inkjet recording paper, form paper, paperboard, liner, corrugating medium, newsprint, coated base paper, and thermal paper. Among these, application to paperboard is preferred because the surface paper strength agent penetrates well into the base paper and provides excellent paper strength enhancing effects. The base paper may also contain various chemicals such as fillers, internal sizing agents, and paper strength enhancers. However, in order to achieve the paper strength enhancing effect of the present invention, it is preferred to include an aluminum compound such as aluminum sulfate or polyaluminum chloride in the base paper. This can be achieved by adding an aluminum compound to the raw pulp that constitutes the base paper, or by coating the surface of the resulting base paper with an aluminum compound.

[0061] When the aluminum compound is internally added, the addition rate is preferably about 0.3 to 7% by weight, more preferably 0.5 to 5% by weight, based on the solid content weight of the raw pulp, from the viewpoint of interaction with the aluminum compound contained in the base paper when the coating liquid is applied. Also, from the same viewpoint, the coating amount when it is applied to the surface is preferably 0.05 to 5 g / m ２ The preferred range is 0.1 to 4 g / m ２ A degree is more preferable.

[0062] Examples of coaters used to coat the base paper include bar coaters, knife coaters, air knife coaters, calendars, gate roll coaters, blade coaters, two-roll size presses, film presses, and rod metering machines.

[0063] The coating amount (solid content) of the coating liquid is 0.001 to 3 g / m from the viewpoint of exhibiting an excellent paper strength enhancing effect. ２ The preferred range is 0.005 to 2 g / m ２ A degree is more preferable.

[0064] Conditions for drying the coated paper include a temperature of preferably 50 to 150° C., more preferably 60 to 140° C., and a drying time of preferably 1 to 10 minutes, more preferably 2 to 8 minutes.

[0065] The present invention will be described below with reference to examples, but the present invention is not limited thereto. In the examples and comparative examples, parts and percentages are by weight unless otherwise specified.

[0066] The following reaction components and polymerization initiators are abbreviated as follows: AM: acrylamide AGA: 2-acrylamido-N-glycolic acid Gly: glyoxylic acid SMAS: sodium methallylsulfonate DMAA: N,N-dimethylacrylamide MBAA: N,N'-methylenebisacrylamide IA: itaconic acid AA: acrylic acid APS: ammonium persulfate

[0067] <Viscosity> Polymer (A) was diluted with ion-exchanged water to a nonvolatile concentration of 20%, and the temperature was adjusted to 25°C, after which the viscosity was measured using a B-type viscometer (manufactured by Toki Sangyo Co., Ltd.).

[0068] Example 1 523 parts of ion-exchanged water was placed in a reactor equipped with a stirrer, a thermometer, a reflux condenser, a nitrogen gas inlet tube, and two dropping funnels. After removing oxygen from the reaction system through nitrogen gas, the system was heated to 85 ° C. Separately, 771.6 parts of a 50% AM aqueous solution, 0.8 parts of AGA, 26.9 parts of SMAS, 5.6 parts of DMAA, and 0.9 parts of MBAA were placed in the dropping funnel (1), and 299 parts of ion-exchanged water was added so that the nonvolatile content in the dropping funnel (1) was 38%. Furthermore, 1.5 parts of APS and 150 parts of ion-exchanged water were placed in the dropping funnel (2). Next, the contents were added dropwise to the reactor from the dropping funnels (1) and (2) over 3 hours. After completion of the dropwise addition, 0.5 parts of APS and 10 parts of ion-exchanged water were added, and when the estimated viscosity measured with a Brookfield viscometer reached approximately 4,000 mPa s (temperature 25°C, converted into a nonvolatile content of 20%), the polymerization was terminated and the mixture was cooled to obtain Polymer (A-1). The viscosity of the obtained Polymer (A-1) is shown in Table 1 (the same applies hereinafter).

[0069] Examples 2 to 9, Examples 11 to 14, Comparative Examples 1 to 6 Polymers (A-2) to (A-9), polymers (A-11) to (A-14), and polymers (B-1) to (B-6) were obtained in the same manner as in Example 1, except that the reaction components and amounts used were changed as shown in Table 1. Note that in Comparative Example 3 (polymer (B-3)) and Comparative Example 6 (polymer (B-6)), gelation occurred during polymerization, and therefore the following evaluations were not performed.

[0070] Example 10 523 parts of ion-exchanged water were placed in a reaction apparatus similar to that used in Example 1, and oxygen was removed from the reaction system by passing nitrogen gas through it. The reaction system was then heated to 85°C. Separately, 680.4 parts of a 50% aqueous AM solution, 24.6 parts of a 50% aqueous Gly solution, 26.3 parts of SMAS, 41.1 parts of DMAA, and 0.2 parts of MBAA were placed in a dropping funnel (1), along with 333 parts of ion-exchanged water so that the nonvolatile content in the dropping funnel (1) was 38%. Furthermore, 1.5 parts of APS and 150 parts of ion-exchanged water were placed in a dropping funnel (2). Next, the contents were added dropwise to the reaction apparatus from the dropping funnels (1) and (2) over a period of 3 hours. After completion of the dropwise addition, 0.5 parts of APS and 10 parts of ion-exchanged water were added, and when the estimated viscosity measured with a Brookfield viscometer reached about 4,900 mPa s (temperature 25°C, converted into a nonvolatile content of 20%), the polymerization was terminated and the mixture was cooled to obtain a polymer (A-10).

[0071] <Preparation of Aqueous Solution for Turbidity Measurement> 10 parts of aluminum sulfate 14-18 hydrate (manufactured by Wako Pure Chemical Industries, Ltd.) and 990 parts of ion-exchanged water were mixed in advance to prepare an aluminum sulfate aqueous solution with a solids concentration of 1%. Next, 5 parts of polymer (A-1) (non-volatile content: 1 part) and 45 parts of ion-exchanged water were mixed, and 50 parts of the aluminum sulfate aqueous solution was added to prepare an aqueous solution for turbidity measurement (non-volatile content concentration - polymer (A-1): 1%, aluminum sulfate: 0.5%). Aqueous solutions were also prepared in the same manner for polymers (A-2) to (A-14), polymer (B-1), polymer (B-2), polymer (B-4), and polymer (B-5).

[0072] <Turbidity> The aqueous solution was adjusted to a temperature of 25°C and stirred at 500 rpm with a stirrer. Using a turbidity meter (instrument name: "ANALITE NEPHELOMETER 152", manufactured by McVan Instruments), a 1% aqueous sodium hydroxide solution was gradually added dropwise to increase the pH, or a 1% aqueous sulfuric acid solution was added dropwise to decrease the pH, so that the pH changed by 0.1. From the resulting turbidity graph, the maximum (X) and minimum (Y) turbidity values ​​at pHs from 5 to 8 were read, and the difference between the maximum and minimum values ​​was calculated. If the turbidity value was unstable, the value was waited until it stabilized, and the value at which it stabilized was used as the turbidity value. The turbidity value was a relative value when a formazin standard solution (400 NTU, manufactured by Wako Pure Chemical Industries, Ltd.) was used as the standard substance, and was indicated by measuring scattered light at 180 degrees using 900 nm infrared light. 1 shows a graph of turbidity when the surface strength agents of Example 3 and Comparative Example 2 were used. The results are shown in Table 1.

[0073]

[0074] <Preparation of Coating Liquid> Polymers (A-1) to (A-14), polymer (B-1), polymer (B-2), polymer (B-4) and polymer (B-5) were diluted with ion-exchanged water to a non-volatile content of 5%, to prepare coating liquids.

[0075] <Preparation of base paper> Recycled corrugated cardboard was beaten in a Niagara beater, and the raw pulp was adjusted to a Canadian Standard Freeness (C.S.F) of 350 ml. Aluminum sulfate 14-18 hydrate was added to the raw pulp at 1.0% of the weight of the non-volatile content of the raw pulp to obtain a slurry. The papermaking pH at this time was 6.5. The slurry was then dewatered in a tapping sheet machine and spun at 5 kg / cm. ２ After pressing for 2 minutes at 105°C, the paper was dried in a rotary dryer for 4 minutes and then conditioned at a temperature of 23°C and a humidity of 50% for 24 hours to prepare a base paper.

[0076] Evaluation Examples 1 to 14, Comparative Evaluation Examples 1 to 4 Using a bar coater, each coating liquid preheated to 50°C was applied to both sides of the base paper. The coated paper was then dried for 1 minute in a rotary drum dryer at 105°C to obtain coated paper. In this evaluation, the coating liquid adhesion amount (non-volatile content) was 1.5 g / m ２ The coating was carried out so that the result was as follows.

[0077] <Burst Strength> Using each coated paper, the specific burst strength (kPa m ２ The results are shown in Table 2.

[0078] <Compression strength> Using each coated paper, the specific compressive strength (N m ２ The results are shown in Table 2.

[0079]

Claims

1. A surface paper strength enhancer comprising a polymer (A) containing as reactive components (a1), (a2) a (meth)acrylamide having a carboxyl group, (a3) ​​an ethylenically unsaturated monomer having a sulfo group, and (a4) a crosslinkable monomer, wherein the viscosity of a solution of the surface paper strength enhancer having a non-volatile content of 20% by weight at 25°C is 100 to 5,000 mPa·s.

2. The surface paper strength agent according to claim 1, wherein the reaction components further contain an ethylenically unsaturated monomer (a5) having a carboxy group other than component (a2).

3. The surface paper strength agent according to claim 1, in which the total amount of the reaction components is taken as 100 mol %, and in which the amount of the (a1) component is 72 to 97 mol %, the amount of the (a2) component is 0.01 to 15 mol %, the amount of the (a3) ​​component is 0.4 to 10 mol %, and the amount of the (a4) component is 0.001 to 8 mol %.

4. The surface paper strength agent according to claim 1, wherein the maximum value of turbidity (unit: NTU) at pH 5 to 8 of an aqueous solution containing 1% by weight of the surface paper strength agent and 0.5% by weight of aluminum sulfate in non-volatile concentration is X and the minimum value is Y, and the agent satisfies 10≦X≦200 and X-Y≦50.

5. A coating solution containing the surface strength agent according to any one of claims 1 to 4.

6. A method for producing paper, comprising the steps of applying the coating liquid according to claim 5 to a surface of base paper and drying it.

7. The method of claim 6, wherein the base paper contains an aluminum compound.

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