JP2005299010A - Paper quality improving agent - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a paper quality improving agent for internal loading improving the bulk of a pulp sheet with slight lowering of strength or rigidity. <P>SOLUTION: The paper quality improving agent for internal loading comprises a copolymer containing a constituent unit derived from a monomer represented by general formula (a) (wherein, R<SP>1</SP>denotes an H or a methyl group; AO denotes a 2-4C oxyalkylene group; n is a number of 1-300 expressed in terms of average value; and X denotes an H or a 1-3C alkyl group) and a constituent unit derived from a monomer represented by general formula (b) (wherein, R<SP>2</SP>denotes an H or a methyl group; and M<SP>1</SP>denotes an H or a cation). The pulp sheet is obtained by loading the paper quality improving agent for internal loading into pulp. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an internally added paper quality improver useful for improving the paper quality of a pulp sheet, and a pulp sheet formed by adding the same.

  From the viewpoint of protecting the global environment, it is required to reduce the amount of pulp used. As a result, there is a demand for lighter paper and increased blending of waste paper pulp. However, the paper obtained simply by reducing the amount of pulp in the paper is inferior in quality due to a decrease in opacity due to the thin paper. Further, weight reduction that reduces the amount of pulp in the paper is not desirable because the strength of the paper is lowered, or the paper that requires rigidity such as paperboard is lowered in rigidity. On the other hand, when the blending ratio of the used paper pulp is increased, the paper thickness is lowered and the opacity is lowered due to a decrease in whiteness due to residual ink in the used paper pulp and a decrease in pulp fiber during the recycling process. Therefore, when the amount of pulp in the paper is reduced and the ratio of the used paper pulp is increased, the thickness of the resulting paper is reduced, the strength and rigidity are reduced, and the opacity and whiteness are further reduced.

  Various bulk improvement methods have been attempted in the past for the purpose of preventing thickness reduction due to weight reduction. For example, the production method for reducing the press pressure has a problem that the smoothness is lowered and the printability is inferior. In addition, a method of using a cross-linked pulp, mixing with synthetic fiber, filling a filler such as an inorganic substance between pulp fibers, and providing voids (Patent Document 1) can be mentioned, but pulp cannot be recycled. Or the smoothness of the paper is impaired. Moreover, specific alcohol and / or its polyoxyalkylene adduct (Patent Document 2) is known as a bulking agent for paper. Furthermore, in the fatty acid polyamide polyamine type marketed as a bulking agent, although the bulk is improved, the strength and rigidity of the paper are lowered and the paper performance is not sufficient. On the other hand, in order to improve opacity and whiteness, a method of adding a large amount (for example, 5 to 20% by weight) of an inorganic filler such as calcium carbonate, kaolin, or white carbon has been practiced in the art. However, simply adding a large amount of inorganic filler significantly increases the weight of the paper, making it impossible to reduce the weight of the paper.

Furthermore, Patent Document 3 discloses a paper for paper comprising polymer fine particles of an ethylenically unsaturated monomer or diene monomer having a cationic polyvinyl alcohol having a mercapto group as a dispersant as a paper internal additive. An internal additive is disclosed, and Patent Document 4 mainly discloses grafted starch obtained by graft copolymerization with a monomer containing (meth) acrylamide while maintaining the form of starch particles without gelatinizing the starch. Although the additive for papermaking used as a component is disclosed, these are improved to some extent with respect to the rigidity, but the bulk is hardly improved.
JP-A-5-230798 WO 98/3730 pamphlet JP-A-8-199079 Japanese Patent Laid-Open No. 11-302992

  An object of the present invention is to provide an internally added paper quality improver that improves the bulk of a pulp sheet and causes little reduction in strength and rigidity.

  The present invention relates to a structural unit derived from a monomer represented by general formula (a) (hereinafter referred to as monomer (a)) and a monomer represented by general formula (b) (hereinafter referred to as monomer ( An internal additive paper quality improver containing a copolymer containing a structural unit derived from b)) and a pulp sheet obtained by adding the internal additive paper quality improver to pulp are provided.

Wherein R ¹ is a hydrogen atom or a methyl group, AO is an oxyalkylene group having 2 to 4 carbon atoms, n is an average value of 1 to 300, X is a hydrogen atom or an alkyl group having 1 to 3 carbon atoms. The n AOs may be the same or different.)

(In the formula, R ² represents a hydrogen atom or a methyl group, and M ¹ represents a hydrogen atom or a cation.)

  The internal additive paper quality improver of the present invention is added to the pulp slurry at the time of paper making to suppress a decrease in the strength and rigidity of the pulp sheet, or while improving the strength and rigidity of the pulp sheet, Can be improved. About the principle which the bulk of the pulp sheet of this invention improves, it is thought that a bulky pulp sheet is obtained by controlling the dispersion | distribution or partial aggregation state of a pulp with the copolymer concerning this invention. Usually, when a bulky pulp sheet is obtained, there are more voids between the pulps, and the strength of the paper is greatly reduced. However, in the case of the copolymer according to the present invention, the copolymer bonded between the fibers is As a result, the quality of the internally-added paper is such that there is little decrease in strength and rigidity despite the increase in bulk, or the strength and rigidity are improved despite the increase in bulk. An improver can be provided.

[Copolymer]
The copolymer according to the present invention includes a structural unit derived from the monomer (a) and a structural unit derived from the monomer (b) as essential structural units. Furthermore, the structural unit derived from the monomer (henceforth a monomer (c)) represented by general formula (c) can also be included.

(In the formula, R ³ , R ⁴ and R ⁵ are groups in which any one of them is represented by — (CH ₂ ) _m COOM ³ , and the remainder is a hydrogen atom or an alkyl group having 1 to 3 carbon atoms, m is a number from 0 to 2, M ² and M ³ each represent a hydrogen atom or a cation, and the —COOM ² group and — (CH ₂ ) _m COOM ³ group may form an anhydride.)
In the monomer (a), AO represents an oxyalkylene group having 2 to 4 carbon atoms, preferably an oxyalkylene group having 2 to 3 carbon atoms, and more preferably an oxyethylene group. n is a number from 1 to 300 indicating the average number of moles added of the oxyalkylene group, which prevents the copolymer from solidifying at room temperature, facilitates an increase in the solids concentration of the copolymer, and is added to the pulp slurry. 1 to 200 is preferable from the viewpoint of improving the handleability in the process. X represents a hydrogen atom or an alkyl group having 1 to 3 carbon atoms, preferably an alkyl group having 1 to 2 carbon atoms, and more preferably a methyl group.

  Monomer (a) includes, for example, a compound represented by general formula (I) (hereinafter referred to as compound (I)) and a compound represented by general formula (II) (hereinafter referred to as compound (II)). It can manufacture by making it esterify.

(In the formula, AO, n, X and R ¹ have the above-mentioned meanings.)
Examples of the compound (I) include methoxy (poly) ethylene glycol, methoxy (poly) propylene glycol, methoxy (poly) butylene glycol, ethoxy (poly) ethylene glycol, ethoxy (poly) propylene glycol, ethoxy (poly) butylene glycol, One-terminal alkyl-capped polyalkylene glycols such as propoxy (poly) ethylene glycol can be mentioned, and one or more of these can be used. Among these, those containing an ethylene glycol chain such as methoxy (poly) ethylene glycol are particularly preferable.

  In the esterification, the conversion rate of the compound (I) is preferably 95% or more, more preferably 98% or more from the viewpoint of performance. The esterification reaction is not particularly limited and can be performed by a known method using a known catalyst, a polymerization inhibitor, and, if necessary, a known solvent.

  A wide variety of known catalysts can be used as the catalyst used in the esterification reaction. Examples of such a catalyst include sulfuric acid, paratoluenesulfonic acid, methanesulfonic acid and the like. A well-known thing can be widely used also as a polymerization inhibitor used for esterification reaction. Examples of such a polymerization inhibitor include quinones such as hydroquinone and methoquinone; cuperone; phenothiazine and the like.

  The esterification reaction can be performed under reduced pressure or normal pressure, and can be performed in the presence or absence of a solvent. From the aspect of distilling off the water produced in the esterification reaction from the reactor, it is preferable to use a solvent that can be azeotroped with water. When performing the esterification reaction without solvent, an inert gas such as nitrogen is used. The product water can also be driven out through the reactor. Examples of such a solvent include aromatic compounds such as benzene, toluene and xylene; alicyclic compounds such as cyclohexane; and aliphatic hydrocarbons such as n-hexane.

In the monomer (b), examples of the cation represented by M ¹ include alkali metal ions, alkaline earth metal ions, ammonium ions, alkylammonium ions, substituted alkylammonium ions, and the like. Examples of the monomer (b) include acrylic acid, methacrylic acid, and salts thereof, and acrylates and methacrylates are preferable. These can use 1 type (s) or 2 or more types.

In the monomer (c), R ³ , R ⁴ and R ⁵ are groups in which any one of them is represented by — (CH ₂ ) _m COOM ³ , and the remainder is a hydrogen atom or a carbon number of 1 to 3 In which any one is —COOM ³ or —CH ₂ COOM ³ , and the remainder is preferably a hydrogen atom or a methyl group. Examples of the cation represented by M ² and M ³ include the same groups as those described above for M ¹ .

  Examples of the monomer (c) include maleic acid, fumaric acid, itaconic acid, citraconic acid, salts thereof or anhydrides thereof, and maleic acid, alkali metal salts of maleic acid, and maleic anhydride are preferable.

  When the monomer (b) and the monomer (c) are salts, the neutralizing agent used for forming the salt is not particularly limited as long as it ionizes a carboxyl group or the like, preferably , Alkali metal or alkaline earth metal hydroxides, and various amines are used, and the amount used is 0 to 1.1 with respect to 1 equivalent of the carboxyl group in the monomer (b) or (c). Equivalents are preferred. The neutralizing agent may be used as it is, but it may be diluted and dissolved in a very small amount of water. When the equivalent of the neutralizing agent exceeds 1.1 with respect to 1 equivalent of carboxyl group, the pH when the aqueous copolymer solution is added to the pulp slurry may become alkaline. Since improvement may not be expected, 1.1 or less is desirable.

  The copolymer of the present invention can be obtained by copolymerizing the monomer (a) and the monomer (b) and, if necessary, the monomer (c). The molar ratio of the monomer (a), the monomer (b) and the monomer (c) constituting the copolymer of the present invention is such that (a ) / [(B) + (c)] = 5/95 to 80/20, preferably 10/90 to 70/30. The molar ratio of the monomer (b) to the monomer (c) is such that the amount of unreacted or homopolymerized monomer (c) is reduced when the copolymer is produced. From the viewpoint of improving paper quality such as performance, the range of (b) / (c) = 70/30 to 100/0 is preferable, and the range of 80/20 to 100/0 is more preferable.

  The copolymerization method is not particularly limited, and may be radical polymerization or ionic polymerization. For example, a known polymerization method such as solution polymerization or bulk polymerization using a polymerization initiator can be employed. The polymerization method can be carried out either batchwise or continuously. In this case, a known solvent can be used as the solvent is used without any particular limitation. Examples of such a solvent include water; alcohols such as methyl alcohol, ethyl alcohol, and isopropyl alcohol; aromatic or aliphatic hydrocarbons such as benzene, toluene, xylene, cyclohexane, and n-heptane; esters such as ethyl acetate. Ketones such as acetone and methyl ethyl ketone, etc., but from the solubility of the monomer mixture and the copolymer obtained, one kind selected from the group consisting of water and lower alcohols having 1 to 4 carbon atoms or Two or more are preferably used.

  As a polymerization initiator, well-known things, such as an organic peroxide, an inorganic peroxide, a nitrile compound, an azo compound, a diazo compound, a sulfinic acid compound, can be used, and it is not specifically limited. Examples of such a polymerization initiator include persulfates such as ammonium persulfate, sodium persulfate, and potassium persulfate; hydrogen peroxide; azo compounds such as azobis-2-methylpropionamidine hydrochloride and azobisisobutyronitrile. A peroxide such as benzoyl peroxide, lauroyl peroxide, cumene hydroperoxide, and the like, and one or more of these can be used. Further, as a promoter, reducing agent such as sodium bisulfite, sodium sulfite, sodium pyrobisulfite, sodium formaldehyde sulfoxylate, ascorbic acid, etc .; one or more of amine compounds such as ethylenediamine, sodium ethylenediaminetetraacetate, glycine, etc. Can also be used together. The addition amount of the polymerization initiator is preferably 0.05 to 50 mol% with respect to the total of the monomer (a), the monomer (b) and the monomer (c).

  In the copolymerization reaction, a chain transfer agent can be added. Examples of the chain transfer agent include lower alkyl mercaptan, lower mercapto fatty acid, thioglycerin, thiomalic acid, 2-mercaptoethanol and the like.

  The polymerization temperature is appropriately determined depending on the polymerization method, the solvent, the polymerization initiator, the chain transfer agent, and the like to be used.

  The obtained copolymer can be deodorized as needed. In particular, when a thiol such as mercaptoethanol is used as a chain transfer agent, an unpleasant odor tends to remain in the polymer, and therefore, it is desirable to perform a deodorization treatment.

  The copolymer obtained by the above production method can be used as the paper quality improver of the present invention even in the acid form, but can be neutralized with an alkaline substance to form a salt as necessary. Examples of such alkaline substances include alkali metal or alkaline earth metal hydroxides; inorganic salts such as alkali metal or alkaline earth metal carbonates; ammonia; mono, di, and trialkyls (2 to 8 carbon atoms). Examples thereof include organic amines such as amine, mono, di, trialkanol (carbon number 2 to 8) amine.

  Weight average molecular weight of the copolymer of the present invention [gel permeation chromatography method, converted into polyethylene glycol, column: G4000PWXL + G2500PWXL (manufactured by Tosoh Corporation), eluent: 0.2 M phosphate buffer / acetonitrile = 7 / 3 (volume ratio), the same applies hereinafter) is preferably 10,000 to 200,000, more preferably 20,000 to 100,000, in order to obtain sufficient dispersibility as a paper quality improver.

[Internal paper quality improver]
The internal paper quality improver of the present invention contains the copolymer according to the present invention as described above, and further includes, for example, known surfactants, colorants, additives, etc. with the desired effects in the present invention. It can contain in the range which expression is not inhibited. The internal additive paper quality improver of the present invention can be prepared by mixing components such as the surfactant, colorant and additive in advance, or prepared by mixing when producing a copolymer. It can also be a thing.

  In addition to the copolymer according to the present invention, the internally added paper quality improver of the present invention can also contain other polymers as appropriate within a range that does not impair the desired effect of the present invention. .

  The content of the copolymer according to the present invention in the internally added paper quality improver of the present invention is preferably 1 to 100% by weight, more preferably 10 to 100% by weight in terms of solid content. The solid content concentration was measured with an infrared moisture meter (Infrared Moisture Determination Balance, FD-240, manufactured by Kett) under heating conditions of 150 ° C. for 20 minutes.

[Pulp sheet]
As a method for producing a pulp sheet in the present invention, a known method is used. By using the paper quality improver of the present invention, a pulp sheet having the paper quality improver added therein can be obtained. The addition amount of the internal additive paper quality improver of the present invention is preferably 0.05 parts by weight or more from the viewpoint of the paper quality improvement effect with respect to 100 parts by weight of pulp in terms of solid content, from the viewpoint of maintaining the original performance of the pulp sheet. 20 parts by weight or less is preferable. More preferably, it is 0.1-10 weight part. Even if such a small amount is added, the pulp sheet improves the bulk of the pulp sheet, and there is little decrease in strength and rigidity despite the increase in bulk, or a pulp sheet that improves in strength and rigidity despite the increase in bulk. Can be obtained.

  The internal additive paper quality improver of the present invention is added to the pulp before any paper making process (internal addition). It is added before the papermaking process where a thin liquid of pulp raw material is filtered to form a paper layer as it travels over the wire mesh, and is used as a pulper, refiner, etc., a disintegrator, a beating machine, a machine chest, a head box, and white water. Although it may be added to a tank such as a tank or piping connected to these facilities, a place where it can be uniformly blended with a pulp raw material, such as a refiner, a machine chest, or a head box, is desirable.

  In the production of the pulp sheet in the present invention, a sizing agent, a filler, a yield improver, a drainage improver, a paper strength improver, and the like, which are used in general paper making, may be added. In particular, in order for the paper quality improver of the present invention to exhibit its function, it is important to fix to pulp, and therefore, an agent that promotes fixing can be used. Examples of such agents include aluminum sulfate, cationized starch, compounds having an acrylamide group, polyethyleneimine, and the like. The amount of the agent that promotes fixing is preferably 0.001 to 5 parts by weight with respect to 100 parts by weight of the pulp.

  The pulp sheet obtained by using the internally added paper quality improver of the present invention has a bulk up rate of 2% or more, a paper strength maintenance rate of 85% or more, and a stiffness maintenance rate of 85% or more compared to the additive-free sheet. Preferably there is.

Furthermore, when the internal additive paper quality improver of the present invention is used in combination with a neutral sizing agent (alkyl ketene dimer), the size performance is the same or improved as compared with the neutral sizing agent added system alone.
The pulp sheet obtained using the internally added paper quality improver of the present invention is a newspaper paper in the item classification described in the Paper Pulp Technology Handbook (issued by the Paper Pulp Technology Association, pages 455-460, 1992), It is suitably used for non-coated printing paper, finely coated printing paper, coated printing paper, information paper, cardboard paper, white paperboard, packaging paper, or other paper or paperboard.

  In the following production examples and examples, “%” and “part” represent “% by weight” and “part by weight” unless otherwise specified.

Production Example 1
A glass reactor equipped with a thermometer, stirrer, dropping funnel, nitrogen inlet tube, and reflux condenser was charged with 2000 parts of water, the inside of the reactor was purged with nitrogen under stirring, and the temperature was raised to 80 ° C. in a nitrogen atmosphere. did. As monomer (a), methoxypolyethylene glycol monomethacrylate [ethylene oxide 9 mol adduct, trade name: ester M-9G, manufactured by Shin-Nakamura Chemical Co., Ltd., hereinafter abbreviated as MPEGMM (9)] 1872 parts, single amount Using 517 parts of methacrylic acid as the body (b), an aqueous solution obtained by adding 400 parts of a 30% aqueous sodium hydroxide solution and 1000 parts of water to these monomers, 4 hours, 19.5 parts of 2-mercaptoethanol and 200 parts of water An aqueous solution in which 24 parts of ammonium persulfate was dissolved in 250 parts of water was dropped into the reactor in 5 hours. And after completion | finish of dripping, 90 degreeC was maintained for further 2 hours. Then, it neutralized with 30% sodium hydroxide aqueous solution, and the copolymer (1) with a weight average molecular weight 30,000 was obtained.

Production Examples 2-7
Using monomers (a), monomers (b), and monomers (c) shown in Table 1, copolymers (2) to (7) were obtained in the same manner as in Production Example 1. Table 1 shows the results of the weight average molecular weight (Mw), solid content concentration, and pH of the obtained copolymers (1) to (7).

note)
* 1: Monomer type and monomer (a): MPEGMM (4) is methoxypolyethylene glycol monomethacrylate (trade name: Ester M-4G, manufactured by NK Shin-Nakamura Chemical Co., Ltd., 4 moles of ethylene oxide added. ), MPEGMM (n) other than MPEGMM (9) and MPEGMM (4) is methoxypolyethylene glycol monomethacrylate having an ethylene oxide addition mole number of n moles produced according to the method described in JP-A-11-71152, MPEGMA (9) is a methoxypolyethylene glycol monoacrylate having 9 moles of ethylene oxide added, produced according to the method described in the publication.
Monomer (b): MAA is an abbreviation for methacrylic acid (manufactured by Mitsubishi Rayon Co., Ltd.), and AA is an abbreviation for acrylic acid (manufactured by Toagosei Co., Ltd.).
Monomer (c): MAn is an abbreviation for maleic anhydride (manufactured by Wako Pure Chemical Industries, Ltd.).
* 2: 2-ME is an abbreviation for 2-mercaptoethanol (chain transfer agent), and is expressed in mol% with respect to the total monomer.

Examples 1-9 and Comparative Examples 1-7
<Pulp raw material>
As a pulp raw material, virgin pulp was used in which LBKP (hardwood bleached pulp) was disaggregated and beaten with a beater at 25 ° C. to give a 1% LBKP slurry. The Canadian standard freeness (JIS P 8121) of this product was 450 ml.

<Paper making method>
The virgin pulp slurry was weighed so that the pulp basis weight of the pulp sheet after papermaking was 70 g / m ² ± 1 g / m ² and adjusted to pH 4.5 with aluminum sulfate (Example 6 and Comparative Examples 1 to 7 were No addition of aluminum sulfate). Next, as shown in Tables 2 and 3, the internal additive paper quality improver (copolymers (1) to (7)) or comparative additive of the present invention is added in an effective amount of 0.5 to 1 per 100 parts of pulp. Internally added, fixing agent (cat 308) and sizing agent (AKD) were added in amounts as shown in Table 2, if necessary, paper was made with 80-mesh wire (area 625 cm ² ) using a square tappi paper machine, and wet sheet Got. Two sheets of filter paper (270 mm square) having a basis weight of 320 g / m ² were stacked on the wet sheet, and a coating plate was further stacked thereon to perform coaching, and then the wet sheet was taken out. Next, the wet sheet was sandwiched between the two filter papers and pressed at 55 g / cm ² for 5 minutes. After pressing, it was dried for 2 minutes at 105 ° C. using a mirror dryer. The dried pulp sheet was conditioned at 23 ° C. and 50% humidity for 1 day, and then the paper bulk up rate, paper strength, stiffness, and sizing degree were measured by the following methods. The papermaking is 5 sheets each, and the measured value is an average of 10 times / paper. The results are shown in Tables 2 and 3.

<Bulk up rate>
The tightness is obtained according to JIS P8118, and the bulk up rate is calculated by the following equation. The smaller the tension is, the higher the bulk is, and the difference of 0.02 in the tension is sufficiently recognized as a significant difference. The larger the bulk up rate, the higher the bulk, and the difference in the bulk up rate of 2% is sufficiently recognized as a significant difference.

Bulk up rate (%) = [(1 / tensity of paper containing agent-added-1 / tensity of paper without additive) / (1 / tensity of paper without additive)] × 100
<Paper strength maintenance rate>
The burst strength (JIS P8112 method) is obtained and calculated by the following formula. The greater the paper strength maintenance rate, the higher the paper strength, and the difference in paper strength maintenance rate of 5% is sufficiently recognized as a significant difference.

Paper strength maintenance rate (%) = (Burst strength of paper containing agent / burst strength of paper without additive) × 100
<Stiffness maintenance factor>
Clark stiffness (according to JIS P8143 method) is obtained and calculated by the following equation. The greater the stiffness maintenance factor, the higher the stiffness, and the difference of 5% stiffness maintenance rate is fully recognized as a significant difference.

Rigidity retention rate (%) = (Clark stiffness of paper with added agent / Clark stiffness of paper without additive) × 100
<Size>
Measurement was performed based on the Stekk human method (JIS P8122).

* 1: Solid content per 100 parts of pulp
* 2: Additives ・ cat308: Cationized starch, manufactured by Nissho Chemical Co., Ltd. ・ AKD: Alkyl ketene dimer, Cylein S-94, manufactured by Kao Corporation ・ Polyacrylamide: Polystron 356, manufactured by Arakawa Chemical -PVA (GL-05): Polyvinyl alcohol, partial saponification, polymerization degree 500, manufactured by Nippon Synthetic Chemical Co., Ltd.-CMC (FT-3): Carboxymethylcellulose, manufactured by Nippon Paper Chemicals Co., Ltd.-Lauryl alcohol ethylene oxide 6 Mole adduct: Emulgen 106, manufactured by Kao Corporation From the results shown in Tables 2 and 3, the internal additive paper quality improver containing the copolymer of the present invention was added internally into the pulp slurry during paper making. A pulp sheet that has little decrease in strength and rigidity despite its improvement, or that has improved strength and rigidity despite an increase in bulk. It has been found.

Further, it can be seen that the size performance of the internally added paper quality improver of the present invention is equal to or improved by using in combination with a neutral sizing agent (alkyl ketene dimer) as compared with a neutral sizing agent added system alone.

Claims (5)

A structural unit derived from a monomer represented by general formula (a) (hereinafter referred to as monomer (a)) and a monomer represented by general formula (b) (hereinafter referred to as monomer (b)) An internal additive paper quality improver comprising a copolymer containing a derived structural unit.

Wherein R ¹ is a hydrogen atom or a methyl group, AO is an oxyalkylene group having 2 to 4 carbon atoms, n is an average value of 1 to 300, X is a hydrogen atom or an alkyl group having 1 to 3 carbon atoms. The n AOs may be the same or different.)

(In the formula, R ² represents a hydrogen atom or a methyl group, and M ¹ represents a hydrogen atom or a cation.) The internal additive paper quality improver according to claim 1, wherein the copolymer further comprises a structural unit derived from a monomer represented by formula (c) (hereinafter referred to as monomer (c)).

(In the formula, R ³ , R ⁴ and R ⁵ are groups in which any one of them is represented by — (CH ₂ ) _m COOM ³ , and the remainder is a hydrogen atom or an alkyl group having 1 to 3 carbon atoms, m represents a number of 0 to 2, M ² and M ³ each represents a hydrogen atom or a cation, and the —COOM ² group and — (CH ₂ ) _m COOM ³ group may form an anhydride.)   The molar ratio of the monomer (a), the monomer (b) and the monomer (c) constituting the copolymer is (a) / [(b) + (c)] = 5 / 95-80. The internal paper quality improver according to claim 1 or 2, wherein the ratio is / 20 and (b) / (c) = 70/30 to 100/0.   A pulp sheet obtained by adding the internally added paper quality improver according to claim 1 to pulp. The pulp sheet according to claim 4, wherein the amount of the internally added paper quality improver is 0.05 to 20 parts by weight in terms of solid content with respect to 100 parts by weight of the pulp.