JP7621646B2 - Interlayer strength improver and method for manufacturing laminated paper - Google Patents

Description

The present invention relates to an interlayer strength improver that has an excellent effect of improving interlayer strength. It also relates to a method for manufacturing laminated paper in which a spray liquid containing the interlayer strength improver is sprayed onto the surface of a wet paper.

High-basis-weight paperboard (coated white board, paper tube base paper, etc.) manufactured in the paper manufacturing industry is produced by making multiple paper layers and then combining them. However, if the interlayer strength between the layers of the combined paper is insufficient, problems such as delamination and blistering during printing can easily occur when the board is used as a paperboard product.

When paper layers are joined together, the interlayer strength of the joined paper is improved by spraying a liquid containing a compound that has the effect of improving interlayer strength onto the surface of the wet paper. Specific compounds include ungelatinized starch or ionic polyacrylamide, or both, and various technologies have been proposed to further increase the interlayer strength of joined paper.

For example, a method has been proposed in which a mixture of ungelatinized starch, ionic polyacrylamide, a water-soluble aluminum compound, and a water-soluble inorganic electrolyte is sprayed onto the surface of a wet paper and then the paper is made by combining the materials (Patent Document 1). Also, an interlayer strength improver made of a cationic polyacrylamide resin and an anionic acrylamide resin has been proposed (Patent Document 2). Still further, an interlayer strength improver made of a water-soluble polymer obtained by polymerizing a specific cationic monomer and an anionic monomer has been proposed (Patent Document 3).

However, these proposals are still not fully satisfactory in terms of the effect of improving interlayer strength, and in particular, they are not fully satisfactory in terms of the effect of improving the paper strength of paper types that require higher strength against the backdrop of increased waste paper recycling rates and declining waste paper quality.

On the other hand, a method has been disclosed in which a polyacrylamide resin consisting of a glyoxylic acid modified (meth)acrylamide and a cationic monomer is used as a papermaking additive (Patent Documents 4 and 5).

However, these inventions assume that the compound is added to pulp slurry, and there is no mention of the improvement in paper strength when used as an interlayer strength improver.

JP 2018-3231 A JP 2002-317393 A JP 2010-077567 A International Publication No. 2017/073355 International Publication No. WO 2012/164916

The present invention aims to provide an interlayer strength improving agent that has an excellent effect of improving interlayer strength, and a method for manufacturing laminated paper in which a spray liquid containing the agent is sprayed onto the surface of a wet paper.

As a result of extensive research into solving the above problems, the inventors discovered that spraying an interlayer strength improver made of a mixture of a specific cationic resin and a specific anionic acrylamide resin in a certain ratio onto wet paper efficiently adheres to the surface of the wet paper, resulting in excellent interlayer strength, and thus completing the present invention.

That is, the present invention, which is a means for solving the above problems,
<1> An interlayer strength improver comprising a cationic resin (I) containing, as monomer units, 50 to 100 mol % of a cationic vinyl monomer unit (a) containing at least one selected from the group consisting of a tertiary amino group and a quaternary ammonium salt, and 0 to 50 mol % of a (meth)acrylamide (b); and an anionic acrylamide-based resin (II) containing, as monomer units, 4 to 20 mol % of a vinyl monomer (c) represented by the following general formula (A), and 80 to 96 mol % of a (meth)acrylamide (b), wherein the ratio of the cationic resin (I) to the anionic acrylamide-based resin (II) is (I):(II)=10:90 to 40:60 in mass ratio;

General formula (A)

（式中Ｒ^１はＨ又はＣＨ_３を表し、Ｒ^２はＨ、Ｎａ、Ｋ又はＮＨ_４を表す）

(wherein ^R1 represents H or _CH3 , and ^R2 represents H, Na, K or _NH4 )

<2> The interlayer strength improver of <1>, in which the cationic vinyl monomer unit (a) containing at least one selected from the group consisting of a tertiary amino group and a quaternary ammonium salt is diallyldimethylammonium chloride.

<3> A method for producing a laminated paper, comprising spraying the interlayer strength improver according to <1> or <2> onto a surface of a wet paper and then laminating the wet paper;
It is.

By spraying it onto the surface of wet paper, an interlayer strength enhancer can be provided that has excellent interlayer strength enhancing effects.

The interlayer strength improver in the present invention must contain a cationic resin (I) having a specific monomer composition and an anionic acrylamide resin (II) having a specific monomer composition.

The cationic resin (I) contains, as monomer unit components, 50 to 100 mol % of cationic vinyl monomer units (a) containing at least one selected from the group consisting of tertiary amino groups and quaternary ammonium salts, and 0 to 50 mol % of (meth)acrylamides (b).

The cationic vinyl monomer unit (a) containing at least one selected from the group consisting of a tertiary amino group and a quaternary ammonium salt can be introduced by using at least one selected from the group consisting of a vinyl monomer containing a tertiary amino group and a vinyl monomer containing a quaternary ammonium salt as a component for homopolymerization or copolymerization.

Examples of vinyl monomers containing a tertiary amino group include dialkylaminoalkyl (meth)acrylates such as dimethylaminoethyl (meth)acrylate, diethylaminoethyl (meth)acrylate, dimethylaminopropyl (meth)acrylate, and diethylaminopropyl (meth)acrylate; dialkylaminoalkyl (meth)acrylamides such as dimethylaminopropyl (meth)acrylamide and diethylaminopropyl (meth)acrylamide; di(meth)allylalkylamines such as di(meth)allylmethylamine and di(meth)allylethylamine; and inorganic acid salts such as hydrochlorides and sulfates of the vinyl monomers having a tertiary amino group, as well as organic acid salts such as formates and acetates of the vinyl monomers having a tertiary amino group.

Vinyl monomers containing a quaternary ammonium salt include those obtained by reacting a vinyl monomer having a tertiary amino group with a quaternizing agent. Examples of the quaternizing agent include alkyl halides such as methyl chloride and methyl bromide; aralkyl halides such as benzyl chloride and benzyl bromide; dimethyl sulfate, diethyl sulfate, epichlorohydrin, 3-chloro-2-hydroxypropyltrimethylammonium chloride, and glycidyltrialkylammonium chloride.

Among these, cationic vinyl monomer units containing quaternary ammonium salts are particularly preferred because they exhibit little change in cationicity due to pH and can stably maintain cationicity when sprayed, which is advantageous for improving interlayer strength, and diallyldimethylammonium chloride is more preferred from the viewpoint of improving interlayer strength. When cationic vinyl monomer units containing primary amino groups and secondary amino groups are used in the cationic resin (I), they are unsuitable because they tend to thicken or gel when mixed with the anionic acrylamide resin (II) to form an interlayer strength improver.

The proportion of cationic vinyl monomer (a) containing at least one selected from the group consisting of tertiary amino groups and quaternary ammonium salts must be within the range of 50 to 100 mol % of the total of components (a) and (b), which is 100 mol %. If it is less than 50 mol %, the interlayer strength improving effect as an interlayer strength improver will decrease.

(Meth)acrylamides (b) can be introduced by using acrylamide or methacrylamide as a copolymerization component. The acrylamide used as a copolymerization component can be in the form of a powder or an aqueous solution. The proportion of (meth)acrylamide units (b) in the components of the cationic resin (I) must be within the range of 0 to 50 mol % out of the total of 100 mol % of the components (a) and (b).

The components of the cationic resin (I) may contain other components as optional components in place of part of the components (a) and (b) as long as the effects of the present invention are not impaired. Examples of vinyl monomers and the like as other components include anionic vinyl monomers containing a carboxyl group. These may be used alone or in combination of two or more. The proportion of optional components in the components of the cationic resin (I) is preferably 10 mol % or less.

Anionic vinyl monomers containing a carboxyl group include acrylic acid, methacrylic acid, 2-(meth)acrylamido-N-glycolic acid, N-acryloylglycine, 3-acrylamidopropanoic acid, 4-acrylamidobutanoic acid, maleic acid, fumaric acid, itaconic acid, citraconic acid, and salts thereof.

When cationic resin (I) is used as a component of the interlayer strength improver of the present invention, a chain transfer agent (d) may be introduced as a component of cationic resin (I) in order to adjust the viscosity to an easy-to-spray level. From the viewpoint of the interlayer strength improvement effect, the chain transfer agent (d) is preferably used in an amount of 0 to 1.5 mol % relative to 100 mol % of the total of components (a) and (b) (including the optional components when used).

Examples of the chain transfer agent (d) include compounds having a thiol group, such as 2-mercaptoethanol, 3-mercaptopropionic acid, thioglycolic acid and its esters; phosphites, phosphites, hypophosphites, hypophosphites and their hydrates; sulfites, hydrogen sulfites, dithionites, metabisulfites and their hydrates; sodium salts of allylsulfonic acid and methallylsulfonic acid; (meth)allylsulfonic acid and its salts, such as alkali metal salts or ammonium salts, such as potassium salts; and isopropyl alcohol.

There are no particular restrictions on the molecular weight of the cationic resin (I), but a weight average molecular weight of 10,000 to 500,000 g/mol is preferred because it is easy to spray when used as an interlayer strength improver.

The anionic acrylamide resin (II) contains 4 to 20 mol % of a vinyl monomer (c) represented by the following general formula (A) and 80 to 96 mol % of (meth)acrylamides (b) as monomer unit components.

General formula (A)

（式中Ｒ^１はＨ又はＣＨ_３を表し、Ｒ^２はＨ、Ｎａ、Ｋ又はＮＨ_４を表す）

(wherein ^R1 represents H or _CH3 , and ^R2 represents H, Na, K or _NH4 )

Monomers that can form the structure represented by general formula (A) include 2-acrylamide-N-glycolic acid, 2-methacrylamide-N-glycolic acid, and their salts such as sodium, potassium, and ammonium. When these monomers are not used, glyoxylic acids can be used to introduce the structure represented by general formula (A) into the (meth)acrylamide polymer after copolymerization. Glyoxylic acids include glyoxylic acid and its salts such as sodium, potassium, and ammonium.

The proportion of vinyl monomer (c) represented by general formula (A) must be within the range of 4 to 20 mol % out of the total of 100 mol % of components (b) and (c). If it is less than 4 mol %, the interlayer strength improving effect as an interlayer strength improver will decrease, and if it exceeds 20 mol, it will become less easily diluted and dispersible in water when used as an interlayer strength improver.

The proportion of (meth)acrylamide units (b) in the anionic acrylamide resin (II) must be within the range of 80 to 96 mol % out of a total of 100 mol % of components (b) and (c). If it is less than 80 mol % or exceeds 96 mol %, the interlayer strength improving effect as an interlayer strength improver is reduced.

As an optional component of the anionic acrylamide resin (II), a cationic vinyl monomer containing at least one selected from the group consisting of a secondary amino group, a tertiary amino group, and a quaternary ammonium salt, an anionic vinyl monomer containing a carboxyl group, or other vinyl monomer may be used in place of a portion of the components (b) and (c) as long as the effects of the present invention are not impaired. These may be used alone or in combination of two or more. The proportion of optional components in the components of the anionic acrylamide resin (II) is preferably 10 mol% or less.

Examples of vinyl monomers containing a secondary amino group include diallylamine, inorganic acid salts such as diallylamine hydrochloride and sulfate, and organic acid salts such as formate and acetate.

Examples of vinyl monomers containing a tertiary amino group include dialkylaminoalkyl (meth)acrylates such as dimethylaminoethyl (meth)acrylate, diethylaminoethyl (meth)acrylate, dimethylaminopropyl (meth)acrylate, and diethylaminopropyl (meth)acrylate; dialkylaminoalkyl (meth)acrylamides such as dimethylaminopropyl (meth)acrylamide and diethylaminopropyl (meth)acrylamide; di(meth)allylalkylamines such as di(meth)allylmethylamine and di(meth)allylethylamine; and inorganic acid salts such as hydrochlorides and sulfates, or organic acid salts such as formates and acetates, of the vinyl monomers having a tertiary amino group described above.

Vinyl monomers containing a quaternary ammonium salt include those obtained by reacting a vinyl monomer having a tertiary amino group with a quaternizing agent. Examples of the quaternizing agent include alkyl halides such as methyl chloride and methyl bromide; aralkyl halides such as benzyl chloride and benzyl bromide; dimethyl sulfate, diethyl sulfate, epichlorohydrin, 3-chloro-2-hydroxypropyltrimethylammonium chloride, and glycidyltrialkylammonium chloride.

Examples of the anionic vinyl monomer containing a carboxyl group include the anionic vinyl monomer containing a carboxyl group listed for cationic resin (I).

Other vinyl monomers include N-substituted (meth)acrylamides such as N-ethyl(meth)acrylamide and N,N-dimethyl(meth)acrylamide; di(meth)acrylates such as ethylene glycol di(meth)acrylate and diethylene glycol di(meth)acrylate; bis(meth)acrylamides such as N,N'-methylenebis(meth)acrylamide and N,N'-ethylenebis(meth)acrylamide; divinyl esters such as divinyl adipate and divinyl sebacate; components that act as crosslinking agents such as water-soluble aziridinyl compounds, water-soluble multifunctional epoxy compounds, and silicon-based compounds; or (meth)allyls such as (meth)allylsulfonic acid or its salts, (meth)allyl alcohol, and (meth)allylamine; and nonionic vinyl monomers such as (meth)acrylic acid esters, (meth)acrylonitrile, styrene, styrene derivatives, vinyl acetate, vinyl propionate, and methyl vinyl ether.

When an anionic acrylamide resin (II) is used as a component of the interlayer strength improver of the present invention, the chain transfer agent (d) mentioned as being usable for the cationic resin (I) can be introduced as a component of the anionic acrylamide resin (II) in order to adjust the viscosity to one that is easy to spray. From the viewpoint of dilution dispersibility in water and interlayer strength improvement effect, the chain transfer agent (d) is preferably used in an amount of 0.1 to 3 mol % relative to 100 mol % of the total of the components (b) and (c) (including the optional components when used). As the chain transfer agent (d) used in the anionic acrylamide resin (II), sodium methallylsulfonate is preferred in terms of ease of viscosity adjustment and industrial availability.

There are no particular restrictions on the molecular weight of the anionic acrylamide resin (II), but a weight average molecular weight of 100,000 to 5,000,000 g/mol is preferred because it has a high effect of improving interlayer strength.

The polymerization initiator used to synthesize the cationic resin (I) and the anionic acrylamide resin (II) is not particularly limited, and any known polymerization initiator can be used. Examples include persulfates such as sodium persulfate, potassium persulfate, and ammonium persulfate, peroxides such as hydrogen peroxide, benzoyl peroxide, tert-butyl hydroperoxide, and di-tert-butyl peroxide, bromates such as sodium bromate and potassium bromate, perborates such as sodium perborate, potassium perborate, and ammonium perborate, percarbonates such as sodium percarbonate, potassium percarbonate, and ammonium percarbonate, and perphosphates such as sodium perphosphate, potassium perphosphate, and ammonium perphosphate. In this case, they can be used alone, or can be used in combination with a reducing agent as a redox polymerization initiator.

Examples of reducing agents include sulfites, hydrogen sulfites, organic amines such as N,N,N',N'-tetramethylethylenediamine, azo compounds such as 2,2'-azobis-2-amidinopropane hydrochloride, and reducing sugars such as aldose. Azo compounds such as azobisisobutyronitrile, 2,2'-azobis-2-amidinopropane hydrochloride, 2,2'-azobis-2,4-dimethylvaleronitrile, 4,4'-azobis-4-cyanovaleric acid and salts thereof can also be used. Two or more of these initiators may be used in combination.

The method for producing the cationic resin (I) and the anionic acrylamide resin (II) is not particularly limited, and various conventionally known methods can be used. For example, in an inert gas atmosphere such as nitrogen gas, the above-mentioned monomer and water as a solvent (an organic solvent can also be used in combination if necessary), and a chain transfer agent and a crosslinking agent if necessary, are charged into a reaction vessel equipped with a stirrer and a thermometer, and the pH is adjusted if necessary with a pH adjuster such as an acid such as sulfuric acid or hydrochloric acid or an alkali such as sodium hydroxide, potassium hydroxide, or ammonia. A polymerization initiator is then added, and the reaction is carried out at a reaction temperature of 20 to 90°C for 1 to 15 hours to obtain the desired cationic resin (I) and anionic acrylamide resin (II). If necessary, the monomer, water, chain transfer agent, crosslinking agent, pH adjuster, and polymerization initiator can be polymerized while being dropped into the reaction vessel.

When introducing the structure of general formula (A) by reacting glyoxylic acid with a (meth)acrylamide copolymer, glyoxylic acid may be charged and reacted at the same time as the (meth)acrylamide copolymer is produced, or glyoxylic acid may be added to the (meth)acrylamide copolymer produced and reacted. There are no particular restrictions on the reaction conditions, but the reaction is usually carried out at a reaction temperature of 20 to 90°C and for a reaction time of about 30 minutes to 10 hours.

An interlayer strength improver can be produced by mixing cationic resin (I) and anionic acrylamide resin (II). The mass ratio of cationic resin (I) to anionic acrylamide resin (II) must be within the range of (I):(II) = 10:90 to 40:60. If the mass ratio of cationic resin (I) is less than 10%, the interlayer strength improving effect as an interlayer strength improver decreases, and if it exceeds 40%, not only will the interlayer strength improving effect as an interlayer strength improver reach a plateau, but it will also be economically disadvantageous.

The method for mixing the cationic resin (I) and the anionic acrylamide-based resin (II) is not particularly specified, but examples thereof include a method of mixing the cationic resin (I) and the anionic acrylamide-based resin (II), a method of polymerizing and mixing monomers constituting the anionic acrylamide-based resin (II) in the presence of the cationic resin (I),
A method can be used in which the monomers constituting the cationic resin (I) are polymerized and mixed in the presence of the anionic acrylamide resin (II).

The interlaminar strength improver containing the cationic resin (I) and the anionic acrylamide resin (II) is usually supplied in the form of an aqueous solution. There are no particular restrictions on the concentration of the cationic resin (I) and the anionic acrylamide resin (II) contained in the interlaminar strength improver, but from the viewpoints of transportation costs and handling, a concentration of 10 to 40 mass% is preferred.

There are no particular restrictions on the viscosity of the interlayer strength improver, but from the viewpoint of ease of handling, it is preferable that the B-type viscosity at the above concentration is about 10 to 15,000 mPa·s (25°C). There are no particular restrictions on the pH of the interlayer strength improver, but it is preferable that it is 2.5 to 7.0, since this makes it difficult for hydrolysis to occur in the monomer units that make up the cationic resin (I) and the anionic acrylamide resin (II).

In the manufacturing method of the laminated paper of the present invention, the interlayer strength improver is sprayed onto the surface of the wet paper and then the paper is laminated. Note that other papermaking additives such as starch, modified starch, casein, and CMC, which have traditionally been used as interlayer spray agents, can also be used in combination as necessary with the interlayer strength improver of the present invention in order to achieve the physical properties required for each type of paper.

The concentration of the dilute solution of the interlayer strength improver is 0.01 to 8% by mass, preferably 0.1 to 5% by mass, in terms of solid content. The viscosity of the diluted interlayer spray agent when used is about 1 to 50 mPa·s, preferably 1 to 10 mPa·s (25°C), and the pH is 2.0 to 8.0. There is no particular restriction on the amount of spray application, but it is usually 0.1 to 5 g/ ^m2 in terms of solid content per one paper layer.

In the manufacturing method of the combined paper of the present invention, the pulp used as the raw material for the paper includes bleached or unbleached chemical pulp such as kraft pulp and sulfite pulp; bleached or unbleached high-yield pulp such as groundwood pulp, mechanical pulp, and thermomechanical pulp; and waste paper pulp such as recycled newspapers, recycled magazines, recycled corrugated cardboard, and deinked recycled paper. These pulps are diluted with water to prepare a slurry, which is then used to make paper. Either an acidic or neutral pulp slurry may be used.

In the method for producing the combined paper of the present invention, the pulp slurry may contain appropriate amounts of fillers such as clay, kaolin, calcium carbonate, barium sulfate, titanium oxide, etc., and papermaking internal additives such as dry strength agents, wet strength agents, sizing agents, size fixing agents, antifoaming agents, pH adjusters, dyes, fluorescent brightening agents, retention aids, etc. The paper produced usually has a basis weight of ^about 10 to 400 g/m2.

The present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited to these examples. In the following, "parts" and "%" mean parts by mass and % by mass, respectively, unless otherwise specified.

<Measurement of molecular weight>
The weight average molecular weights of the cationic resin (I) and the anionic acrylamide resin (II) were measured by a GPC-MALLS method using a multi-angle light scattering detector connected to GPC under the following measurement conditions.
HPLC: Agilent 1100 series Column: SHODEX SB806MHQ manufactured by Showa Denko K.K.
Eluent: Phosphate buffer solution (pH 3) containing sodium nitrate
Flow rate: 1.0 ml/min Detector 1: Wyatt Technology Co., Ltd. Multi-angle light scattering detector DAWN
Detector 2: Differential refractive index detector RI-101 manufactured by Showa Denko K.K.

<Synthesis of Cationic Resin (I)>
[Synthesis Example 1]
A 1-liter four-neck flask equipped with a stirrer, a thermometer, a reflux condenser and a nitrogen inlet tube was charged with 103.47 parts of 65% diallyldimethylammonium chloride aqueous solution (corresponding to 50.06 mol% when the total of components (a) and (b) is 100 mol%) and 410.00 parts of ion-exchanged water, and the temperature was raised to 80° C. under a nitrogen gas atmosphere. Next, a monomer aqueous solution containing 141.82 parts of 50% acrylamide aqueous solution (corresponding to 49.94 mol% when the total of components (a) and (b) is 100 mol%), 0.25 parts of sodium hypophosphite monohydrate (corresponding to 0.12 mol% when the total of components (a) and (b) is 100 mol%), and 23.00 parts of ion-exchanged water, and a polymerization initiator aqueous solution containing 2.74 parts of ammonium persulfate and 80.00 parts of ion-exchanged water were added dropwise over 6 hours through separate lines. During the dropping, the flask was appropriately cooled or heated so that the temperature inside the flask was 85° C. After the dropping was completed, the temperature was kept at 85° C. for another hour, and then 35.00 parts of ion-exchanged water was added to cool the reaction solution. After cooling to 25° C., a 25% aqueous sodium hydroxide solution was added to the reaction solution, and the pH was adjusted to 4.0 to obtain a cationic resin (I-1) with a solid content of 25%. The proportion (mol%) of monomer units constituting the obtained cationic resin (I-1), solid content, viscosity, pH, and molecular weight are shown in Table 1.

[Synthesis Example 2, Comparative Synthesis Example 1]
Cationic resins (I-2, I-6) were obtained in the same manner as in Synthesis Example 1, except that the ratio of the monomer units constituting the cationic resin (I-1) was changed as shown in Table 1. The solid content, viscosity, pH, and molecular weight of the cationic resins (I-2, I-6) are shown in Table 1.

[Synthesis Example 3]
A 1-liter four-neck flask equipped with a stirrer, a thermometer, a reflux condenser and a nitrogen inlet tube was charged with 373.08 parts of a 65% aqueous solution of diallyldimethylammonium chloride and 410.00 parts of ion-exchanged water, and the temperature was raised to 80° C. under a nitrogen gas atmosphere. Next, an aqueous polymerization initiator solution containing 3.08 parts of ammonium persulfate and 100.00 parts of ion-exchanged water was added dropwise over a period of 7 hours through separate lines. During the dropwise addition, the flask was appropriately cooled or heated so that the temperature in the flask was 85° C. After the dropwise addition was completed, the temperature was maintained at 85° C. for another hour, and then 105.00 parts of ion-exchanged water was added to cool the mixture. After cooling to 25° C., a 25% aqueous solution of sodium hydroxide was added to the reaction solution, and the pH was adjusted to 4.0 to obtain a cationic resin (I-3) with a solid content of 25%. Table 1 shows the proportion (mol%) of monomer units constituting the obtained cationic resin (I-3), solid content, viscosity, pH, and molecular weight.

[Synthesis Example 4]
A 1-liter four-neck flask equipped with a stirrer, a thermometer, a reflux condenser and a nitrogen inlet tube was charged with 220.09 parts of dimethylaminoethyl methacrylate, 1.55 parts of sodium methallylsulfonate (corresponding to 0.70 mol% relative to the total of 100 mol% of the components (a) and (b)), and 420.00 parts of ion-exchanged water, and then 228.82 parts of a 30% aqueous sulfuric acid solution was added to adjust the pH to 3.0, and the temperature was raised to 80°C under a nitrogen gas atmosphere. Next, an aqueous polymerization initiator solution containing 1.60 parts of ammonium persulfate and 40.00 parts of ion-exchanged water was added dropwise over a period of 2 hours through separate lines. During the dropwise addition, the flask was appropriately cooled or heated so that the temperature inside the flask was 85°C. After the dropwise addition was completed, the temperature was maintained at 85°C for another hour, and then 60.00 parts of ion-exchanged water was added and cooled to obtain a cationic resin (I-4) with a solid content of 30%. The proportion (mol %) of monomer units constituting the resulting cationic resin (I-4), the solid content, viscosity, pH, and molecular weight are shown in Table 1.

[Synthesis Example 5]
A 1-liter four-neck flask equipped with a stirrer, a thermometer, a reflux condenser and a nitrogen inlet tube was charged with 374.99 parts of a 77% aqueous solution of dimethylaminoethyl methacrylate methyl chloride quaternary compound, 1.55 parts of sodium methallylsulfonate (corresponding to 0.70 mol% when the total of the components (a) and (b) is 100 mol%), and 495.00 parts of ion-exchanged water. 1.00 parts of citric anhydride was added to adjust the pH to 2.5, and the temperature was raised to 80°C under a nitrogen gas atmosphere. Next, an aqueous polymerization initiator solution containing 1.60 parts of ammonium persulfate and 40.00 parts of ion-exchanged water was added dropwise over a period of 2 hours through separate lines. During the dropwise addition, the flask was appropriately cooled or heated so that the temperature in the flask was 85°C. After the dropwise addition was completed, the temperature was maintained at 85°C for another hour, and then 60.00 parts of ion-exchanged water was added and cooled to obtain a cationic resin (I-5) with a solid content of 30%. The proportion (mol %) of monomer units constituting the resulting cationic resin (I-5), the solid content, viscosity, pH, and molecular weight are shown in Table 1.

The abbreviations in Table 1 are as follows.
AAm: Acrylamide
DADMAC: Diallyldimethylammonium chloride
DM: Dimethylaminoethyl methacrylate
DMC: Dimethylaminoethyl methacrylate methyl chloride quaternary
SMAS: Sodium methallyl sulfonate
SPH: Sodium hypophosphite monohydrate
-: Not used

<Synthesis of anionic acrylamide resin (II)>
[Synthesis Example 6]
A 1-liter four-neck flask equipped with a stirrer, a thermometer, a reflux condenser, and a nitrogen inlet tube was charged with 16.04 parts of 2-acrylamide-N-glycolic acid (corresponding to 4.00 mol % when the total of components (b) and (c) is 100 mol %), 156.94 parts of a 50% aqueous acrylamide solution (corresponding to 46.00 mol % when the total of components (b) and (c) is 100 mol %), 2.28 parts of sodium methallylsulfonate (corresponding to 0.60 mol % when the total of components (b) and (c) is 100 mol %), and 390.00 parts of ion-exchanged water as monomers [1], and the temperature was raised to 80 ° C. under a nitrogen gas atmosphere. Next, 0.11 parts of ammonium persulfate was added to start the reaction. During the reaction, the flask was appropriately cooled or heated so that the temperature inside the flask was 85 ° C. One hour after the start of the reaction, an aqueous monomer solution containing 16.04 parts of 2-acrylamide-N-glycolic acid (corresponding to 4.00 mol % when the total of components (b) and (c) is 100 mol %), 156.94 parts of a 50% aqueous acrylamide solution (corresponding to 46.00 mol % when the total of components (b) and (c) is 100 mol %), 1.52 parts of sodium methallylsulfonate (corresponding to 0.40 mol % with respect to the total of components (b) and (c)), and 90.00 parts of ion-exchanged water, and an aqueous polymerization initiator solution containing 0.33 parts of ammonium persulfate and 45.00 parts of ion-exchanged water were added dropwise over one hour via separate lines as monomers [2]. During the dropwise addition, the flask was appropriately cooled or heated so that the temperature inside the flask was 85° C. After the dropwise addition, the mixture was maintained at 85° C. for an additional hour, and then 90.00 parts of ion-exchanged water was added and cooled to obtain an anionic acrylamide resin (II-1) having a solid content of 20%. The proportion (mol %) of monomer units constituting the obtained anionic acrylamide resin (II-1), solid content, viscosity, pH, and molecular weight are shown in Table 2.

[Synthesis Examples 7 to 10, Comparative Synthesis Examples 2 to 5]
Anionic acrylamide resins (II-2 to II-5, II-7 to II-10) were obtained in the same manner as in Synthesis Example 6, except that the compositions of monomers [1] and [2] were changed as shown in Table 2. The solid content, viscosity, pH, and molecular weight of the obtained anionic acrylamide resins are shown in Table 2.

[Synthesis Example 11]
A 1-liter four-neck flask equipped with a stirrer, a thermometer, a reflux condenser and a nitrogen inlet tube was charged with 15.40 parts of a 50% aqueous solution of glyoxylic acid (corresponding to 4.00 mol% when the total of the components (b) and (c) is 100 mol%), 170.02 parts of a 50% aqueous solution of acrylamide (corresponding to 46.00 mol% when the total of the components (b) and (c) is 100 mol%), 2.47 parts of sodium methallylsulfonate (corresponding to 0.60 mol% when the total of the components (b) and (c) is 100 mol%), and 380.00 parts of ion-exchanged water as monomers [1], and the pH was adjusted to 9.0 by adding 16.64 parts of a 25% aqueous solution of sodium hydroxide. Next, the temperature was raised to 80°C under a nitrogen gas atmosphere, and 0.12 parts of ammonium persulfate was added to start the reaction. During the reaction, the flask was appropriately cooled or heated so that the temperature inside the flask was 85°C. One hour after the start of the reaction, 15.40 parts of a 50% aqueous solution of glyoxylic acid (corresponding to 4.00 mol% when the total of the components (b) and (c) is 100 mol%), 170.02 parts of a 50% aqueous solution of acrylamide (corresponding to 46.00 mol% when the total of the components (b) and (c) is 100 mol%), 1.64 parts of sodium methallylsulfonate (corresponding to 0.40 mol% when the total of the components (b) and (c) is 100 mol%), and 90.00 parts of ion-exchanged water were mixed and further 16.64 parts of a 25% aqueous solution of sodium hydroxide was added to adjust the pH to 9.0, and an aqueous solution of a polymerization initiator, which was a mixture of 0.36 parts of ammonium persulfate and 45.00 parts of ion-exchanged water, was added dropwise over one hour via separate lines. During the dropwise addition, the flask was appropriately cooled or heated so that the temperature inside the flask was 85°C. After the dropwise addition, the mixture was kept at 85° C. for an additional hour, and then 85.00 parts of ion-exchanged water was added and cooled to obtain an anionic polyacrylamide resin (II-6) having a solid content of 20%. The proportion (mol %) of monomer units constituting the obtained anionic acrylamide resin (II-6), solid content, viscosity, pH, and molecular weight are shown in Table 2.

The abbreviations in Table 2 are as follows.
AAm: Acrylamide
AGA: 2-acrylamido-N-glycolic acid
Gly: Glyoxylic acid
IA: Itaconic acid
AA: Acrylic acid
SMAS: Sodium methallyl sulfonate

[Example 1]
<Adjustment of Interlaminar Strength Improver>
In a 200 mL beaker, 36.00 parts of cationic resin (I-1) with a solid content of 25%, 105.00 parts of anionic acrylamide resin (II-1) with a solid content of 20%, and 9.00 parts of ion-exchanged water were added, and the mixture was stirred and mixed so that the mass ratio of the solid contents was (I-1):(II-1) = 30:70, thereby obtaining an interlayer strength improver with a solid content of 20%. The proportion (mass%) of the resin constituting the obtained interlayer strength improver, the solid content, the viscosity, and the pH are shown in Table 3.

<Production of combined paper>
To a pulp slurry of 3% concentration made of recycled cardboard with a Canadian Standard Freeness (CSF) of 300 mL, 0.8% aluminum sulfate and 0.8% amphoteric polyacrylamide-based paper strength agent (product name "DS4421", manufactured by Seiko PMC Corporation) were added under stirring. Next, the pulp slurry was diluted to a concentration of 0.25% with water in which calcium sulfate had been dissolved and the electrical conductivity adjusted to 150 mS/m, and paper was made using a Noble & Wood sheet machine to produce two wet papers with a basis weight of 100 g/ ^m2 and a moisture content of 90%. An interlayer strength improver was diluted to an interlayer strength improver solids concentration of 0.5% and sprayed onto one surface of one of the wet papers so that the coating amount of the interlayer strength improver solids was 0.4 g/ ^m2. The other wet paper was then pressed onto the coated surface and dried in a drum dryer at 80°C for 100 seconds and at 120°C for 120 seconds to obtain a laminated paper.

<Measurement of interlaminar strength>
The resulting laminated paper was conditioned for 24 hours under conditions of 23° C. and 50% RH (relative humidity), and then the interlayer peel strength (T-peel strength: JIS P8139) was measured. The results are shown in Table 3.

[Examples 2 to 12, Comparative Examples 1 to 7]
An interlayer strength improver was obtained in the same manner as in Example 1, except that the cationic resin (I) and the anionic acrylamide resin (II) were changed as shown in Table 3. The proportion (mass%) of the resin constituting the obtained interlayer strength improver, the solid content, viscosity, and pH are shown in Table 3. The same operation as in Example 1 was carried out to prepare a laminated paper sprayed with the interlayer strength improver, and the interlayer peel strength was evaluated in the same manner as in Example 1. The results are shown in Table 3.

[Comparative Example 8]
A 1-liter four-neck flask equipped with a stirrer, a thermometer, a reflux condenser, and a nitrogen inlet tube was charged with 92.17 parts of a 65% aqueous diallyldimethylammonium chloride solution (corresponding to 17.50 mol % when the total of the components (a) to (c) is 100 mol %) and 410.00 parts of ion-exchanged water, and the temperature was raised to 80° C. under a nitrogen gas atmosphere. Next, an aqueous monomer solution containing 23.35 parts of 2-acrylamide-N-glycolic acid (corresponding to 6.60 mol % when the total of the components (a) to (c) is 100 mol %), 228.48 parts of a 50% aqueous acrylamide solution (corresponding to 75.90 mol % when the total of the components (a) to (c) is 100 mol %), 2.78 parts of sodium methallylsulfonate (corresponding to 0.83 mol % when the total of the components (a) to (c) is 100 mol %), and 115.00 parts of ion-exchanged water, and an aqueous polymerization initiator solution containing 0.97 parts of ammonium persulfate and 40.00 parts of ion-exchanged water were added dropwise over a period of 3 hours via separate lines. During the dropwise addition, the flask was appropriately cooled or heated so that the temperature inside the flask was 85° C. After the dropwise addition was completed and the mixture was maintained at 85° C. for an additional hour, 90.00 parts of ion-exchanged water was added and cooled to obtain an interlayer strength improver with a solids content of 20% and the same constituent components and molar amounts as in Example 3. The proportion (mol %) of monomer units constituting the obtained interlayer strength improver, solids content, viscosity, pH, and molecular weight are shown in Table 3.
The same procedure as in Example 1 was carried out to prepare a laminated paper sprayed with the interlayer strength improving agent, and the interlayer peel strength was evaluated in the same manner as in Example 1. The results are shown in Table 3.

[Comparative Example 9] (Synthesis Example 1 of JP-A-2002-317393)
Into a 1-liter four-neck flask equipped with a stirrer, a thermometer, a reflux condenser, and a nitrogen gas inlet tube, 409.1 g of ion-exchanged water and 28.2 g (11.7 mol %) of 65% diallyldimethylammonium chloride were charged, and the temperature was raised to 90° C. Under nitrogen gas introduction, a mixed solution of 30 g of ion-exchanged water, 122.3 g (88.3 mol %) of a 50% aqueous acrylamide solution, 1.2 g of 5% sodium methallylsulfonate, 3.77 g of 0.5% triacrylformal, and 1.1 g of 20% ammonium persulfate was dropped over 2 hours to carry out a polymerization reaction, thereby forming a cationic resin containing diallyldimethylammonium chloride and acrylamide as monomer units.
After the dropping was completed, a mixed solution of 30g of water, 122.3g (93.8 mol%) of 50% aqueous acrylamide solution, 7.4g (6.2 mol%) of itaconic acid, 1.2g (0.04 mol%) of 5% sodium methallylsulfonate, 3.77g of 0.5% triacrylformal, and 1.1g of 20% ammonium persulfate was dropped over 2 hours, and the mixture was kept at 90°C for another 2 hours to carry out the reaction, thereby obtaining an interlayer strength improver consisting of a cationic resin and an anionic acrylamide resin with a solid content of 15%. The solid content, viscosity, and pH of the obtained interlayer strength improver are shown in Table 3.
The same procedure as in Example 1 was carried out to prepare a laminated paper sprayed with the interlayer strength improving agent, and the interlayer peel strength was evaluated in the same manner as in Example 1. The results are shown in Table 3.

(*1) The constituent monomer composition of the interlayer strength improver used was the same as that of the interlayer strength improver used in Example 3, but it differed from the interlayer strength improver of Example 3 in that all of the constituent monomers were copolymerized.
(*2) Synthesis Example 1 of JP-A-2002-317393

Compared to Comparative Examples 1 to 7, which do not meet the ratio of the components of the interlayer strength enhancer of the present invention, Examples 1 to 12 are found to have a superior effect of improving interlayer strength.

In addition, it can be seen that the interlayer strength improver obtained by copolymerizing the components cationic resin (I) and anionic acrylamide resin (II) as in Comparative Example 8 has a poorer interlayer strength improving effect than Example 3, which mixes cationic resin (I) and anionic acrylamide resin (II).

Comparative Example 9, which uses an interlayer strength improver synthesized with reference to Manufacturing Example 1 described in JP 2002-317393 A, shows a poorer interlayer strength improving effect than Examples 1 to 12 of the present invention.

Claims (3)

An interlayer strength improving agent comprising: a cationic resin (I) containing, as monomer units, 50 to 100 mol % of cationic vinyl monomer units (a) containing at least one member selected from the group consisting of a tertiary amino group and a quaternary ammonium salt, and 0 to 50 mol % of (meth)acrylamides (b); and an anionic acrylamide-based resin (II) containing, as monomer units, 4 to 20 mol % of a vinyl monomer (c) represented by the following general formula (A), and 80 to 96 mol % of (meth)acrylamides (b), wherein the ratio of the cationic resin (I) to the anionic acrylamide-based resin (II) is (I):(II)=10:90 to 40:60 by mass ratio.
General formula (A)

(wherein ^R1 represents H or _CH3 , and ^R2 represents H, Na, K or _NH4 ) The interlayer strength improver according to claim 1, wherein the cationic vinyl monomer unit (a) containing at least one selected from the group consisting of a tertiary amino group and a quaternary ammonium salt is diallyldimethylammonium chloride. A method for producing laminated paper, comprising spraying the interlayer strength improver according to claim 1 or 2 onto the surface of wet paper and then laminating the paper.