JP5811839B2 - Paper-making surface sizing agent, paper-making surface coating liquid containing the same, and paper obtained by coating these - Google Patents

Description

The present invention relates to a surface sizing agent for papermaking, in particular, an emulsion-type surface sizing agent for papermaking, a surface coating liquid for papermaking using the same, and paper obtained by coating them.

In general, the emulsion type surface sizing agent for papermaking can be broadly classified into an aqueous solution type and an emulsion type, but the latter has a relatively low viscosity even when the solid content (non-volatile content) is set high, and has excellent handling properties. There is an advantage that the foamability of the coating liquid is relatively small.

  However, emulsion type surface sizing agents are unstable with respect to mechanical share, so when coated on paper with a coating method that applies a strong shearing force, such as a size press method, the emulsion is destroyed and wrinkles and foaming occur. There are problems such as occurrence of coating, uneven coating, etc., resulting in reduced operability and, as a result, reduced size effect. In particular, when hard water in which metal ions are dissolved, such as industrial water, is used.

  The following are known as means for improving the stability of the emulsion type surface sizing agent against the mechanical share.

  In Patent Document 1, a water-soluble copolymer containing (A) (1) a carboxyl group-containing unsaturated monomer and (2) a hydrophobic unsaturated monomer is used as a protective colloid. A paper surface sizing agent comprising an emulsion obtained by emulsion polymerization of a hydrophobic unsaturated monomer (B) in an aqueous medium is disclosed.

  In Patent Document 2, a surface obtained by polymerizing a nonionic hydrophilic unsaturated monomer, a hydrophobic unsaturated monomer, and optionally an anionic unsaturated monomer in the presence of an alkali salt of shellac. A sizing agent is disclosed.

  Although these surface sizing agents have improved stability against mechanical share and excellent size performance, when a coating liquid is actually prepared using this, it is used together with a water-soluble polymer such as oxidized starch. Due to the interaction, there is an adverse effect that the resulting coating liquid has a lot of foaming. Furthermore, the use of a polymer emulsifier has a problem of increasing the product cost.

  Patent Document 3 discloses an emulsion obtained by emulsion polymerization using styrenes, acrylamides, and acrylic esters as main components as a surface sizing agent having excellent mechanical stability without using the protective colloid. In the emulsion, although the foaming of the coating solution was small, in the paper drying process after coating on the paper, the emulsion did not sufficiently melt and spread, so that the size performance was not satisfactory.

JP-A-8-246391 JP-A-12-136499 JP 50-4308 A

The present invention provides an emulsion-type surface sizing agent that is excellent in size performance and mechanical stability, and has a small foaming (hereinafter referred to as low foaming property) of a coating solution when applied to paper. Objective.

  As a result of intensive studies to solve the above problems, the present applicant has found that styrenes, (meth) acrylamides, (meth) acrylic esters, without using polymer emulsifiers or alkali salts of natural resins as protective colloids. It has been found that an emulsion-type surface sizing agent for papermaking obtained by emulsion polymerization of a monomer component containing a specific amount in a specific ratio using a chain transfer agent and a surfactant can solve the above problems.

That is, the present invention provides an interface between a chain transfer agent and an unsaturated monomer containing 5 to 40% by weight of styrene, 10 to 50% by weight of (meth) acrylamides, and 35 to 85% by weight of (meth) acrylic acid esters. A surface sizing agent for papermaking comprising an emulsion obtained by emulsion polymerization using an activator , wherein the glass transition temperature (Tg) of the copolymer contained in the emulsion is 25 ° C. or less , The present invention relates to a surface coating solution for papermaking containing the same, and paper obtained by coating them.

The emulsion type sizing agent for papermaking of the present invention is excellent in mechanical stability while maintaining high size performance, so that it is difficult to cause wrinkles even in a coating method with a strong shearing force, and particularly wrinkles are likely to occur. It is suitable when diluted with hard water. In addition, the coating liquid obtained by adding another water-soluble polymer to the paper emulsion type sizing agent of the present invention is also characterized by low foaming.

  The emulsion type surface sizing agent for papermaking of the present invention (hereinafter referred to as surface sizing agent) is 5 to 40% by weight of styrenes, 10 to 50% by weight of (meth) acrylamides, and 35 to 85% by weight of (meth) acrylic acid esters. It contains an emulsion obtained by emulsion polymerization of an unsaturated monomer containing 1% using a chain transfer agent and a surfactant.

  Examples of styrenes used in the present invention include styrene, α-methyl styrene, t-butyl styrene, dimethyl styrene, acetoxy styrene, hydroxy styrene, vinyl toluene, chlorovinyl toluene, and the like. These styrenes can be used alone or in combination of two or more. Among these styrenes, styrene is preferable from the viewpoint of size effect. The proportion of styrene used is usually 5 to 40% by weight, preferably 10 to 30% by weight, based on the total amount of unsaturated monomers. When the amount of styrene used is less than 5% by weight, the hydrophobicity of the emulsion is lowered and the size performance is lowered, and when it exceeds 40% by weight, the glass transition temperature (hereinafter referred to as Tg) of the emulsion becomes too high. In the paper drying process after the surface sizing agent is applied to the paper, the size effect is reduced due to insufficient dissolution and spreading of the emulsion.

Examples of (meth) acrylamides used in the present invention include acrylamide, methacrylamide, dimethylaminoethyl (meth) acrylamide, diethylaminoethyl (meth) acrylamide, dimethylaminopropyl (meth) acrylamide, and diethylaminopropyl (meth) acrylamide. . These (meth) acrylamides can be used alone or in combination of two or more. Among these (meth) acrylamides, (meth) acrylamide is preferable from the viewpoint of mechanical stability. The use ratio of (meth) acrylamides is usually 10 to 50% by weight, preferably 10 to 20% by weight, based on the total amount of unsaturated monomers.
If the use ratio of (meth) acrylamides is less than 10% by weight, the stability against mechanical shear is insufficient, and if it exceeds 50% by weight, the hydrophilicity of the emulsion increases and the size performance decreases.

(Meth) acrylates used in the present invention are methyl (meth) acrylate, ethyl (meth) acrylate, butyl (meth) acrylate, isobutyl (meth) acrylate, sec-butyl (meth) acrylate. , Tert-butyl (meth) acrylate, N-octyl (meth) acrylate, 2-ethylhexyl (meth) acrylate, decyl (meth) acrylate, dodecyl (meth) acrylate, hexadecyl (meth) acrylate, ( Examples include octadecyl (meth) acrylate, octadecenyl (meth) acrylate, icosyl (meth) acrylate, docosyl (meth) acrylate (cyclopentyl) methacrylate, and cyclohexyl (meth) acrylate. These (meth) acrylic esters can be used alone or in combination of two or more.
Among these (meth) acrylic esters, if one having a low Tg is selected, it is advantageous in that an emulsion having a low Tg described later can be easily obtained. From this viewpoint, ethyl acrylate, butyl acrylate, and 2-ethylhexyl (meth) acrylate are preferable. The use ratio of (meth) acrylic acid esters is usually 35 to 85% by weight, preferably 50 to 80% by weight, based on the total amount of unsaturated monomers. When the amount of (meth) acrylic ester used is less than 35% by weight, the Tg of the emulsion becomes too high, and when it exceeds 85% by weight, the hydrophobicity is lowered, and in any case, the size effect is lowered.

The unsaturated monomer composition used for the preparation of the emulsion is specifically 25 ° C. or less, preferably −40 so that the Tg of the copolymer obtained by polymerization is lower from the viewpoint of the size effect. It is desirable that the temperature be 0 ° C to 0 ° C. By lowering the Tg of the copolymer, in the drying step after the surface sizing agent (emulsion) is coated on the paper, the emulsion is likely to dissolve and spread on the paper, and a better sizing effect is exhibited. In the present invention, Tg is a calculated value calculated by the following FOX equation.
<FOX type>
1 / Tg = W1 / Tg1 + W2 / Tg2 + ... + Wi / Tgi + ... + Wn / Tgn
[In the above FOX formula, Tg of the homopolymer of each monomer constituting the polymer composed of n monomers is Tgi (K), and the mass fraction of each monomer is Wi, and (W1 + W2 + ... + Wi + ... Wn = 1). ]

As long as the emulsion of this invention satisfy | fills the said monomer composition, the other monomer copolymerizable with these can be used together as needed. As other monomers, nitrile monomers such as unsaturated carboxylic acids, α-olefins, and (meth) acrylonitrile can be used in combination.

  Specific examples of the unsaturated carboxylic acid include acrylic acid, (anhydrous) maleic acid (referred to as maleic anhydride or maleic acid, hereinafter the same applies when referred to as “anhydrous”), maleic acid neutralized salt, maleic acid. Acid half ester, maleic acid half ester neutralized salt, (anhydrous) itaconic acid, itaconic acid neutralized salt, itaconic acid half ester, itaconic acid half ester neutralized salt, (anhydrous) citraconic acid, citraconic acid neutralized salt, citracone Acid half ester, citraconic acid half ester neutralized salt, and the like can be mentioned, and one kind can be used alone, or two or more kinds can be used in combination. Among these, considering the size effect, at least one selected from the group consisting of maleic acid, maleic acid neutralized salt, maleic acid half ester, and maleic acid half ester neutralized salt is preferable.

The neutralizing agent used to form the neutralized salt includes alkali metals (sodium hydroxide, potassium hydroxide, etc.), ammonia (ammonia, ammonium carbonate, etc.), aliphatic amines having about 1 to 12 carbon atoms. (Monomethylamine, dimethylamine, trimethylamine, monoethylamine, diethylamine, monobutylamine, etc.), alicyclic amines (cyclohexylamine, etc.), aromatic amines (aniline, etc.) and the like. Or 2 or more types can be used.

The half ester is any of the unsaturated carboxylic acids (limited to those capable of forming an ester bond) and an alkyl group having about 1 to 18 carbon atoms (linear, branched or cyclic). Or an alcohol having an esterification reaction. Specific examples of the alkyl group include, for example, methyl, ethyl, n-propyl, i-propyl, n-butyl, i-butyl, t-butyl, neopentyl, n-hexyl, and cyclohexyl. Group, 2-ethylhexyl group, decyl group, palmityl group, stearyl group and the like.

As the α-olefins, various known ones can be used without particular limitation, and these may be any of linear, branched, or cyclic forms, and the number of constituent carbon atoms is 5 It may be about ˜22. Specific examples include branched α-olefins [diisobutylene (refers to 2,4,4-trimethylpentene-1 and / or 2,4,4-trimethylpentene-2), 3-methyl-1-butene, 3- Methyl-1-pentene, 4-methyl-1-pentene, etc.], a linear α-olefin having about 5 to 22 carbon atoms, preferably 12 to 18 [1-hexene, 1-octene, 1-decene, 1-dodecene, 1-tetradecene, 1-hexadecene, 1-octadecene, 1-eicocene, 1-tetracocene, 1-triacontene, etc.], cyclic α-olefin [cyclohexene, methylcyclohexene, vinylcyclohexane, 4-vinylcyclohexene, cyclopentene , Methylcyclopentene, etc.] and the like can be used alone or in combination of two or more. Kill. Among these, taking into consideration the polymerizability with the unsaturated carboxylic acids, the dispersibility of the emulsion particles, the size effect, etc., branched α-olefins and / or linear α-olefins are particularly diisobutylene and / or Alternatively, a linear α-olefin having about 5 to 22 carbon atoms, preferably 12 to 18 carbon atoms is preferable.

The emulsion of the present invention is obtained by emulsion polymerization of the unsaturated monomer using a chain transfer agent and a surfactant.
The chain transfer agent used in the present invention contributes to the emulsification dispersibility of the emulsion, and the viscosity of the resulting surface sizing agent can be in an appropriate range.

As a chain transfer agent used by this invention, a well-known thing can be especially used without a restriction | limiting. Preferably, for example, non-polymerized thiols such as ethanethiol and propanethiol or neutralized salts thereof, thiolic acids such as thioglycolic acid, thiomalic acid, dimethyldithiocarbamic acid or neutralized salts thereof, and second such as isopropanol Water-soluble chain transfer agent (solubility in water at 25 ° C. of 0.1 g / 100 g or more) such as secondary alcohols such as secondary alcohols, hypophosphites such as sodium hypophosphite, and compounds of the following general formula (1) Is mentioned.

(In the formula, R represents hydrogen or a methyl group, n represents an integer of 1 to 4, and Z represents hydrogen or an alkali metal atom.) Specific examples of the compound of the general formula (1) include allyl sulfonic acid and allyl. Examples include sodium sulfonate, methallyl sulfonic acid, sodium methallyl sulfonate, and ammonium methallyl sulfonate. Among these water-soluble chain transfer agents, those having a (meth) allyl sulfonic acid group represented by the general formula (1) are more preferable in terms of enhancing the emulsifying dispersibility of the emulsion and being excellent in size effect.

Other than water-soluble substances such as t-dodecyl mercaptan, n-dodecyl mercaptan, n-octyl mercaptan, bromotrichloromethane, 2-mercaptobenzothiazole and the like can also be used as a chain transfer agent.
The amount of chain transfer agent used is usually about 0.01 to 5% by weight, more preferably 1.5 to 3.0% by weight, based on the total amount of unsaturated monomers. When the amount of the chain transfer agent used is less than 0.01% by weight, the emulsifying dispersibility is insufficient, and when it exceeds 5% by weight, a sufficient size effect cannot be obtained.

As the surfactant used in the present invention, known anionic, cationic, and nonionic surfactants can be used, and polymeric emulsifiers obtained by copolymerizing various monomer components, shellac, etc. Those derived from natural resins are excluded. A surface sizing agent obtained using a polymer emulsifier or one derived from a natural resin is not suitable for the surfactant in the present invention because it tends to cause foaming when mixed with oxidized starch or the like to form a coating solution. .
Examples of anionic surfactants include alkyl sulfates, alkyl sulfate esters, alkyl benzene sulfonates, polyoxyethylene alkyl ether sulfates, polyoxyethylene alkyl phenyl ether sulfates, dialkyl sulfosuccinate esters, alkyl sulfonates, Polyoxyethylene alkyl ether sulfosuccinate, polyoxyethylene styryl phenyl ether sulfosuccinate ester salt, polyoxyethylene alkyl ether phosphate ester, α-olefin sulfonate, naphthalene sulfonate formalin condensate, polyoxyethylene alkyl phenyl ether Examples thereof include sulfate ester salts.
Examples of cationic surfactants include alkyltrimethylammonium chloride, alkyldimethylethylammonium ethyl sulfate, dialkyldimethylammonium chloride, and alkylnimethylbenzylammonium chloride. Nonionic surfactants include polyethylene glycol, polyoxyethylene alkyl ether, Examples include polyoxyethylene alkyl phenyl ether, polyoxyethylene styryl phenyl ether, polyoxyethylene sorbitan fatty acid ester and the like. As the surfactant used in the present invention, from the viewpoint of improving the mechanical stability and size effect of the obtained emulsion, at least one anionic property selected from alkyl sulfate ester salts and polyoxyethylene alkyl ether sulfate ester salts is used. A surfactant or a cationic surfactant having a quaternary ammonium salt such as alkyltrimethylammonium chloride is preferred. As the usage-amount of surfactant, it is about 0.1 to 10 weight% normally with respect to the total amount of an unsaturated monomer, Preferably it is 0.5 to 5 weight%. If the amount of the surfactant used is less than 0.1% by weight, a stable emulsion cannot be formed, and if it exceeds 5% by weight, the coating liquid tends to foam.

Examples of the radical polymerization initiator used in the present invention include inorganic peroxides [hydrogen peroxide, ammonium persulfate, potassium persulfate, etc.], organic peroxides [t-butylperoxybenzoate, dicumyl peroxide, etc. , Lauryl peroxide, etc.], azo compounds [2,2′-azobisisobutyronitrile, dimethyl-2,2′-azobisisobutyrate, etc.] and the like. More than seeds can be used. Moreover, the usage-amount is about 0.01-10 weight part normally when the total amount of an unsaturated monomer is 100 weight part, Preferably it is 0.5-3 weight part.

What is necessary is just to perform the manufacturing method of the emulsion (surface sizing agent) of this invention by the well-known emulsion polymerization method. For example, the unsaturated monomer, chain transfer agent, surfactant and water can be added to a predetermined reaction vessel, heated to a predetermined temperature with stirring, and then produced by dropping a radical polymerization initiator. it can. Further, the unsaturated monomer may be dropped together with a radical polymerization initiator after adding a chain transfer agent, a surfactant and water to a predetermined reaction vessel. The concentration of the unsaturated monomer in the reaction system is usually about 5 to 60% by weight, and the reaction may be carried out at a temperature of about 40 to 100 ° C., preferably 50 to 90 ° C. for about 1 to 10 hours.

Examples of the radical polymerization initiator used in the present invention include inorganic peroxides such as hydrogen peroxide, potassium persulfate, and ammonium persulfate, water-soluble radical polymerization initiators such as an aqueous azo-based initiator, and the persulfates. And a redox polymerization initiator in the form of a combination of a reducing agent such as sodium hydrogen sulfite and sodium thiosulfate. The initiator is used in an amount of about 0.05 to 5% by weight, preferably 0.1 to 3% by weight, based on the total amount of unsaturated monomers.

  The emulsion thus obtained has a non-volatile concentration of usually 15 to 30% by weight, a pH of usually 8.0 to 10.0, and a particle size of usually about 50 to 300 nm, preferably 70 to 110 nm. “Particle diameter” refers to the average primary particle diameter of emulsion particles, and is a value measured by a dynamic light scattering method. If the particle size becomes too large, the size effect tends to decrease.

The surface sizing agent of the present invention can be used by directly coating the surface of the base paper as the surface sizing agent, but various additives such as carboxy used mainly as a paper strength enhancer. Celluloses such as methylcellulose, water-soluble polymers such as polyvinyl alcohols, polyacrylamides, sodium alginate, anti-slip agents, antiseptics, rust preventives, pH adjusters, antifoaming agents (silicon antifoaming agents, etc.), Add a thickener, filler, antioxidant, waterproofing agent, film-forming aid, pigment, dye, etc., dissolve in water, dilute to prepare a coating solution, and apply it to the base paper surface. Can be used as a paper coating surface coating solution. The amount of surface sizing agent (emulsion) used in the coating liquid is a non-volatile concentration, usually about 0.01 to 2% by weight, and the amount of water-soluble polymer used is a non-volatile concentration, usually 0.001 to 1. About 1% by weight.

The coating means is not particularly limited, and examples thereof include various means such as a size press method, a gate roll method, a bar coater method, a calendar method, and a spray method. Since the surface sizing agent for papermaking of the present invention is excellent in mechanical stability, it can be used in various coating machines such as a two-roll size press coating method, a rod metalling size press coating method, and a gate roll coating method. . The coating amount (in terms of nonvolatile content) is usually 0.001 to 2 g / m2, preferably 0.01 to 0.5 g / m2.

The base paper is not particularly limited, and for example, uncoated paper made from wood cellulose fibers can be used. Examples of the papermaking pulp as a raw material for the base paper include chemical pulps such as LBKP and NBKP, mechanical pulps such as GP and TMP, and other waste paper pulps. Further, the base paper may contain chemicals such as an internal sizing agent, a filler, and a paper strength agent.

  The obtained paper includes recording paper (form paper, PPC paper, thermal recording base paper, pressure sensitive recording base paper, etc.), coated paper (art paper, cast coated paper, high quality coated paper, etc.), packaging paper (craft paper, pure white roll) Paper, etc.], western paper [note paper, book paper, printing paper, newsprint paper, etc.], paperboard [manila ball, white ball, chip ball, liner, etc.].

Hereinafter, the present invention will be specifically described with reference examples and comparative examples, but the present invention is not limited to these examples. In each example, all parts and% are based on weight unless otherwise specified.
In each example, the viscosity is a value measured by a B-type viscometer (manufactured by Toki Sangyo Co., Ltd.), and the particle size is a laser particle size analyzer (product name “LPA-3000 / 3100”, Otsuka Electronics Co., Ltd.). Measured value).

Example 1
In a reaction vessel equipped with a stirrer, a cooling tube, two dropping funnels, a nitrogen introduction tube and a thermometer, an anionic surfactant (trade name: “Hitenol LA10”, manufactured by Daiichi Kogyo Seiyaku Co., Ltd.) 370 parts of soft water, 20 parts of styrene, 60 parts of 2-ethylhexyl acrylate, 20 parts of acrylamide, and 1.5 parts of sodium methacryl sulfonate were heated and stirred in a nitrogen atmosphere and raised to 75 ° C. Next, 2.3 parts of ammonium persulfate was charged and kept warm for 2 hours to complete the emulsion polymerization. Thereafter, the obtained composition was diluted with water so that the non-volatile content of the copolymer was 20%, and the pH was adjusted to 8.2 with a 10% aqueous sodium hydroxide solution. The emulsion thus obtained had a measured viscosity at pH 8.2, 25 ° C. of 68 mPa · s, and a light scattering particle diameter of 98 nm. The composition and physical properties of the emulsion are shown in Table 1.

Example 2-8
A sizing agent for papermaking was produced in the same manner as in Example 1 except that the type of monomer used and the amount used were changed as shown in Table 1.

Comparative Example 1
A reaction vessel equipped with a stirrer, a cooling tube, two dropping funnels, a nitrogen introducing tube and a thermometer was charged with 50 parts of styrene, 50 parts of 80% acrylic acid, 50 parts of soft water, 50 parts of isopropyl alcohol, and 4 parts of ammonium persulfate. The mixture was heated and stirred under a nitrogen atmosphere and raised to 80. Further, the system was kept warm for 5 hours to complete the polymerization. Thereafter, 21 parts of 48% sodium hydroxide and 133 parts of soft water were added for neutralization, and isopropyl alcohol was distilled off. The obtained copolymer was dispersed with 12 parts of 25% aqueous ammonia and diluted with water to a concentration of 20%. Thus, a water-soluble polymer having a viscosity measurement value of 75 mPa · s at pH 8.3 and 25 ° C. was obtained. This was used as a polymer emulsifier in the next synthesis.
In a reaction vessel equipped with a stirrer, a cooling tube, two dropping funnels, a nitrogen introduction tube and a thermometer, an anionic surfactant (trade name: “Hitenol LA10”, manufactured by Daiichi Kogyo Seiyaku Co., Ltd.) ,
370 parts of soft water, 20 parts of styrene, 60 parts of 2-ethylhexyl acrylate, 20 parts of acrylamide, 1.5 parts of sodium methacryl sulfonate, and 20 parts of the polymer emulsifier were heated and stirred in a nitrogen atmosphere and raised to 75 ° C. Next, 2.3 parts of ammonium persulfate was charged and kept warm for 2 hours to complete the emulsion polymerization. Thereafter, the obtained composition was diluted with water so that the nonvolatile content concentration of the copolymer was 20%, and the pH was adjusted to 9.1 with a 10% aqueous sodium hydroxide solution. The emulsion thus obtained had a measured viscosity at pH 8.5, 25 ° C. of 158 mPa · s and a light scattering particle diameter of 105 nm. The composition and physical properties of the emulsion are shown in Table 1.

Comparative Examples 2-6
Emulsions were produced in the same manner as in Synthesis Example 1 except that the types of monomers used and the amounts used were changed as shown in Table 1.

In Table 1, each symbol is as follows.
SMAS: sodium methallyl sulfonate AM: acrylamide St: styrene 2EHA: 2-ethylhexyl acrylate BA: butyl acrylate
(I): Polyoxyethylene alkyl ether sulfate ester (trade name: Hightenol LA10, manufactured by Daiichi Kogyo Seiyaku Co., Ltd.)
(II): Alkyl sulfate ester salt (trade name: Monogen Y-100, manufactured by Daiichi Kogyo Seiyaku Co., Ltd.) (III): Lauryltrimethylammonium salt (trade name: Catiogen TML, manufactured by Daiichi Kogyo Seiyaku Co., Ltd.)
(III): Lauryltrimethylammonium salt (trade name: Catiogen TML, manufactured by Daiichi Kogyo Seiyaku Co., Ltd.)
(IV): Polyoxyalkylene tridecyl ether (trade name: Neugen TDX-50, manufactured by Daiichi Kogyo Seiyaku Co., Ltd.)

<Preparation of coating solution>
For each of the surface sizing agents of Examples 1 to 8 and Comparative Examples 1 to 6, oxidized starch (trade name “Oji Ace A” manufactured by Oji Constarch Co., Ltd.), which was gelatinized at 18%, and water By mixing, a coating solution was prepared in which the non-volatile content of the oxidized starch was about 10.0% and the non-volatile content of the emulsion was about 1.5%. Each of the obtained coating liquids was subjected to an evaluation test for the size effect and the foamability and mechanical stability of the coating liquid by the following methods. The results are shown in Table 2.

<Size effect>
Using a gate roll coating machine (manufactured by SMT Co., Ltd.), each coating solution was applied to neutral medium paper (basis weight 54 g / m 2, drop size 9 seconds). Next, each processed paper was dried with a rotary dryer (105 ° C., 1 minute) to obtain a test paper. The drop size of the obtained test paper was measured by the following method to evaluate the size effect.

<Measurement of drop size>
Using each test paper obtained by the above method, Japan TAPPI No. Based on the method according to No. 33, 1 μl of water was dropped on the test paper surface, the time until the water droplet was absorbed by the paper surface was measured, and the drop size (seconds) was measured. The larger the value, the better the size effect. The results are shown in Table 2.

<Foaming test>
After adding 50 ppm of silicon-based antifoaming agent (trade name “DKQ1-071” manufactured by Toray Dow Corning Co., Ltd.) to each coating liquid, heating to 50 ° C. and treating with a home mixer for 2 minutes The amount of increase in the liquid level immediately after the treatment was measured. The results are shown in Table 2.

<Measurement of mechanical stability (hard water)>
Each coating solution was diluted 2-fold with water having a hardness of 40 ° dH to have a composition in which the non-volatile content of the oxidized starch was 1.25% and the non-volatile content of the emulsion was 0.05%. Subsequently, the coating liquid of the said composition was circulated for 1 hour in a 2 roll size press, and generation | occurrence | production of the aggregate and the stain | pollution | contamination of two rolls were visually evaluated on the following references | standards. The results are shown in Table 2.
A: Extremely excellent in mechanical stability (no agglomerates can be confirmed and there is no dirt on the roll)
○: Excellent in mechanical stability (Agglomerates cannot be confirmed, but the roll is slightly stained)
Δ: Slightly inferior in mechanical stability (a small amount of agglomerates can be confirmed and the roll becomes dirty)
X: Mechanical stability is inferior (a large amount of agglomerates can be confirmed, and the roll becomes extremely dirty)

Claims (9)

A chain transfer agent and a surfactant are used for unsaturated monomers containing 5 to 40% by weight of styrene, 10 to 50% by weight of (meth) acrylamides, and 35 to 85% by weight of (meth) acrylic acid esters. A surface sizing agent for papermaking , comprising an emulsion obtained by emulsion polymerization , wherein the glass transition temperature (Tg) of the copolymer contained in the emulsion is 25 ° C. or lower . The surface sizing agent for papermaking according to claim 1, wherein the (meth) acrylic acid ester is at least one selected from ethyl acrylate, butyl acrylate, and 2-ethylhexyl (meth) acrylate. The surface sizing agent for papermaking according to claim 1 or 2 , wherein the surfactant is at least one anionic surfactant selected from the group consisting of an alkyl sulfate ester salt and a polyoxyethylene alkyl ether sulfate ester salt. The surface sizing agent for papermaking according to any one of claims 1 to 3 , wherein the surfactant is a cationic surfactant having a quaternary ammonium salt. The paper transfer surface sizing agent according to any one of claims 1 to 4 , wherein the chain transfer agent is a water-soluble chain transfer agent. The surface sizing agent for papermaking according to claim 5 , wherein the water-soluble chain transfer agent is a compound represented by the following general formula (1).
(In the formula, R represents hydrogen or a methyl group, n represents an integer of 1 to 4, and Z represents hydrogen or an alkali metal atom.) The surface sizing agent for papermaking according to any one of claims 1 to 6 , wherein the emulsion has a particle size of 50 to 300 nm. The papermaking surface coating solution comprising a papermaking surface sizing agent and a water-soluble polymer according to any one of claims 1-7. A paper obtained by coating the surface sizing agent for papermaking according to any one of claims 1 to 7 or the surface coating liquid for papermaking according to claim 8 .