JP3181060B2 - Surface sizing of cellulosic products - Google Patents

Abstract

The invention relates to a method of surface sizing cellulose-based products and to an aqueous sizing composition. The method comprises applying to the surface of cellulose-based products an aqueous dispersion of a copolymer obtainable by free-radical emulsion polymerization of a monomers mixture comprising at least one monomer selected from the group consisting of styrene and derivatives thereof; at least one monomer selected from the group consisting of esters of ethylenically unsaturated carboxylic acids and alkanols; and at least one monomer selected from the group consisting of ethylenically unsaturated carboxylic and sulfonic acids and salt thereof; optionally in combination with other ethylenically unsaturated copolymerizable monomers. The aqueous sizing composition comprises a dispersion of a copolymer obtainable by free-radical emulsion polymerization of such a monomers mixture and an electrolyte.

Description

Description: FIELD OF THE INVENTION The present invention relates to surface sizing of cellulosic products,
More particularly, it relates to a sizing method comprising applying an aqueous dispersion of an emulsion polymerized monomer to the surface of such a product. The invention further relates to an aqueous size composition containing a dispersion of the emulsion polymerized monomers.

BACKGROUND OF THE INVENTION In the art, paper, board and paperboard are known.
It is known to use polymer dispersions as sizing agents in the production of cellulosic products such as boards). Sizing agents slow or prevent the aqueous solution from absorbing and spreading into the sized product. The prior art discloses a wide variety of polymer dispersions, for example,
And those obtained by emulsion polymerization of ethylenically unsaturated monomers in the presence of protective colloids and / or emulsifiers that impart stability to the resulting dispersion. The dispersion can be used for surface sizing, including applying the dispersion to the surface of the product to be sized. Examples of monomers used for this purpose include styrene and alkyl (meth) acrylates. These surface sizes are examples of the most effective ones currently used and usually give a high sizing response.

For example, in addition to having the ability to produce cellulosic products with the desired degree of resistance to penetration by aqueous liquids such as printing inks and writing inks, the polymer dispersion used for surface sizing is Should also be informative. The surface size is usually applied to the surface of the cellulosic product in the size press. It is known to add an electrolyte to a size solution in a size press to prevent static electricity from increasing during subsequent coating, drying or conversion steps. However, even with a low concentration of electrolyte,
Experience has shown that the polymer particles contained in the size dispersion can cause agglomeration to form a precipitate. This leads to application problems such as difficulty administering the size dispersion, poor sizing performance, poor sizing of the paper, and the like. Therefore, for best results in surface sizing, it is desirable that the polymer dispersion have good stability in the presence of base and electrolyte. It is further beneficial for the application process that the polymer dispersion has low foaming properties and advantageous viscosity and flow properties.

Accordingly, it is an object of the present invention to provide an improved method for surface sizing cellulosic products. It is another object of the present invention to provide an aqueous dispersion containing emulsion polymerized monomers that provides improved surface sizing results. It is another object of the present invention to provide an aqueous size composition containing a dispersion of emulsion polymerized monomers that has beneficial utilization properties in surface sizing, especially high performance in the presence of electrolytes and bases. Other objects of the present invention will become clear hereinafter.

The present invention generally relates to a method for sizing a cellulosic product by applying to the surface of the product an aqueous sizing composition containing a dispersion of emulsion polymerized monomers,
And, it relates to an aqueous size composition containing such a dispersion system. More specifically, the method of the present invention comprises applying, to the surface of a cellulosic product, an aqueous dispersion of a copolymer obtained by free radical emulsion polymerization of a monomer mixture comprising the following (a) to (c): Including: (a) 70-99.5% by weight of (a1) at least one monomer selected from the group consisting of styrene and its derivatives, and (a2) at least one monomer selected from the group of esters of ethylenically unsaturated carboxylic acids and alkanols. (B) 0.5 to 15% by weight of at least one monomer selected from the group consisting of ethylenically unsaturated carboxylic acids and salts thereof, ethylenically unsaturated sulfonic acids and salts thereof, and (c) 0 to 15% by weight. 15% by weight of other ethylenically unsaturated copolymerizable monomers.

According to the invention, paper and similar cellulosic products are provided by aqueous dispersions of fine particles of a copolymer containing a monomer mixture based on styrene, alkyl acrylate and carboxy and / or sulfo group-containing monomers in the form of emulsion polymerization. It has been found that surface sizing can provide such products having very high resistance to penetration by aqueous liquids. The present invention enables the production of cellulosic products with improved sizing effects compared to conventional methods using aqueous dispersions of copolymers based on styrene and alkyl acrylate. Surprisingly, carboxy and / or
Alternatively, it has been found that the improvement is achieved by adding a sulfo group-containing monomer. Thus, the present invention can provide the same sizing effect with a lower amount of polymer, thereby leading to cost savings and economic benefits. As used herein, the term "sizing" refers to the treatment of cellulosic products to achieve high resistance to penetration by aqueous liquids or high hydrophobicity.

The dispersions according to the invention are very effective in the presence of electrolytes and bases and have low foaming properties and advantageous viscosity and flow properties. Thereby, it is possible to produce cellulosic products having a high sizing reaction in a very advantageous manner. The improved performance seen when the present dispersion is used in the presence of an electrolyte offers significant utility benefits, thereby mitigating or reducing the problems associated with aggregation and precipitation seen with prior art dispersions. Can be removed.
The dispersions according to the invention can be prepared with low concentrations of inexpensive and readily available emulsifiers or dispersing aids, nevertheless this dispersion has a high stability in the presence of electrolytes over a wide pH range And performance. Thereby, essentially no additional stabilizers and / or protective colloids are used, which naturally yields further economic benefits.

The monomers of groups (a), (b) and (c) as defined herein have at least one ethylenically unsaturated bond which is used in the preparation of the dispersion and makes the monomer polymerizable by a free radical mechanism. .

The monomer of the group (a) contains at least one monomer (a1) selected from the group consisting of styrene and its derivatives. Styrene is preferred. Preferably, the styrene derivative is uncharged and suitable styrene derivatives included in group (a1) include α-methylstyrene and styrene substituted with C ₁ -C ₄ alkyl such as vinyl toluene, Other types of derivatives can be used, for example, halogen-substituted styrenes such as chlorostyrene.

The monomers of group (a) further contain at least one monomer (a2) selected from the group of esters of ethylenically unsaturated carboxylic acids and alkanols. Monoethylenically unsaturated esters are preferred. Esters are 3 ~
6, preferably a carboxylic acid having 3 to 4 carbon atoms,
And 1-18, suitably 1-8 and preferably 1-4.
Suitably derived from alkanols having a carbon atom of The alkanol is preferably a monohydric saturated alcohol. Suitable esters include well-esterified carboxylic acids such as monoesters of monocarboxylic acids such as alkyl acrylates and alkyl methacrylates, diesters of dicarboxylic acids such as dialkyl maleates and dialkyl fumarate, and trialkyl esters of aconitic acid. And the like. Among these, alkyl acrylate and alkyl methacrylate are preferred.

Suitable alkyl acrylates and methacrylates include those of C ₁ -C ₁₈ , suitably C ₁ -C ₈ and preferably C ₁ -C ₄ alkyl. Examples of suitable alkyl acrylates include methyl, ethyl, n-propyl, n
-Butyl, isobutyl, tert-butyl, neopentyl,
n-hexyl, cyclohexyl, 2-ethylhexyl,
Octyl, decyl, palmityl and stearyl acrylate are included. Butyl acrylate and mixtures containing butyl acrylate are particularly preferred. Examples of suitable alkyl methacrylates include methyl, isopropyl, n-butyl, isobutyl and tert-butyl methacrylate.

The monomers of group (a) comprise from about 70 to 99.5% by weight, suitably at least about 80% by weight, preferably at least about 85% by weight and most preferably from about 90 to about 99% by weight, based on the weight of the monomers used in the polymerization. It can be used in an amount of% by weight. The monomers of group (a) may have a monomer (a1) to monomer (a2) weight ratio of about 10: 1 to about 1:10 and suitably about 7: 1.
To about 1: 3. In a preferred embodiment of the invention, the monomers of group (a1) make up at least 50% by weight of the monomers of group (a), and the preferred weight ratio of (a1) to (a2) is from about 5: 1 to about 1: 1. 1, most preferably about 4: 1 to 2: 1.

Monomers of group (b) include ethylenically unsaturated carboxylic acids and their salts and ethylenically unsaturated sulfonic acids and their salts, with monoethylenically unsaturated monomers being preferred. The monomers of group (b) thus have at least one carboxy or sulfo group in either free acid or salt form, ie, carboxylic acids, carboxylate salts,
Sulfonic acids and sulfonic acid salts. Generally, it is preferred to use the monomers in the form of the free acid. Carboxylates and sulfonates can be prepared from the corresponding acids by treating the monomers with a solution of a base such as sodium hydroxide, potassium hydroxide, ammonia or an amine, such that the fully or partially neutralized monomer is can get. Of these, ammonia is generally preferred.

C ₃ -C ₆ Examples of suitable carboxylic acid monomers, preferably include carboxylic acids C ₃ -C _4, such as acrylic acid, methacrylic acid and di - half esters such as maleic acid and fumaric acid and tricarboxylic acid a and diesters - C ₁ -C ₄ alkyl monoester of a monocarboxylic acid, maleic acid, dicarboxylic acids such as fumaric acid and itaconic acid, tricarboxylic acids such as aconitic acid and its C ₁ -C ₄ alkyl mono equal. Acid C ₂ -C ₁₀ is mentioned as an example of suitable sulphonic acid monomers, such as vinyl sulfonic acid (ethylene sulfonic acid), allyl sulfonic acid, styrene sulfonic acid, 2-sulfoethyl methacrylate,
2-acrylamidopropanesulfonic acid and 2-acrylamidoethanesulfonic acid. In general, carboxylic acids and their salts are preferred monomers of group (b). In particular, they are usually more readily available and cheaper than sulfonic acids.

The monomers of group (b) comprise from about 0.5 to 15% by weight, suitably at least about 1% to about 10%, preferably less than 7%, preferably in the range of about 1% to It can be used in an amount of 5% by weight.

In addition to the essential monomers of groups (a1), (a2) and (b), the dispersions are obtained in the form of emulsion-polymerized groups (c)
And other ethylenically unsaturated copolymerizable monomers. Examples of suitable monomers included in group (c) are olefins such as ethylene, propylene, 1-butene, isobutene, 1-hexene and 1-octene;
Aliphatic conjugated dienes such as 3-butadiene and isoprene,
Vinyl acetate, vinyl propionate, vinyl butyrate, vinyl carboxylic acids such as vinyl laurate and vinyl stearate, vinyl chloride, vinylidene chloride, acrylamide, methacrylamide, C ₁ -C ₁₈ alkyl acrylamides and C ₁ -C ₁₈ alkyl (Meth) acrylamides such as methacrylamide, as well as hydroxy-functional monomers.

Preferred monomers included in group (c) are hydroxy-functional monomers, ie, monomers having at least one hydroxyl group. The monomers of group (c) include one or more hydroxy-functional monomers and one or more hydroxy-functional monomers and other ethylenically unsaturated copolymerizable monomers (eg, any monomer of group (c) as defined above). It may consist of a combination of Generally suitable hydroxy-functional monomers are hydroxyalkyl esters of monoethylenically unsaturated carboxylic acids and N
- is selected from alkanolamides, wherein the N- alkanol group and a hydroxyalkyl group may suitably 1 to 18 carbon atoms and preferably have C ₁ -C _4. Suitable hydroxy-functional monomers include hydroxyalkyl acrylates such as 2-hydroxyethyl acrylate and 3-hydroxypropyl acrylate, hydroxyalkyl methacrylates such as 2-hydroxyethyl methacrylate and 3-hydroxypropyl methacrylate, and N-methylol acrylamides such as N-methylol acrylamide. -Alkanol acrylamide, and N- such as N-methylol methacrylamide
Alkanol methacrylamide. Among these monomers, hydroxyalkyl acrylate and hydroxyalkyl methacrylate are preferred.

The monomers of group (c) may comprise up to about 15% by weight, suitably from 0.5% to about 10%, preferably up to about 5%, and a preferred range from 1% to about 5%, based on the weight of the monomers used in the polymerization. (A) +
The sum of the percentages of (b) + (c) is 100. When using group (c) monomers containing at least one hydroxy-functional monomer, the amount is suitably from 0.5 to 5% by weight and preferably from 1.0 to 5.0% by weight, based on the weight of the monomers used for the polymerization. .

The dispersion according to the invention can be prepared by copolymerizing the monomers of groups (a), (b) and (c) in a conventional manner. Emulsion polymerization processes are known in the art,
Encyclopedia of Polymer Science and Engineering, reference 6
Volume, Emulsion Polymerization, pages 1-51, John Wiley & Sons, I
nc., 1986 is included herein. The polymerization is suitably started in an aqueous layer to which the monomers, emulsifier and free radical initiator are added in any order, in the absence of oxygen and under an inert gas atmosphere, such as nitrogen. Emulsion polymerization, with stirring
Suitably occurs between 20 ° C and 100 ° C, preferably between 60 ° C and 90 ° C.

Suitable free-radical polymerization initiators are all capable of initiating free-radical polymerization, for example, conventional thermal initiators such as potassium and ammonium persulfates, organic and hydroperoxides, and peroxides There are hydrogen and redox systems such as iron (II) / peroxide, iron (II) / persulfate, peroxide / metabisulfate and persulfate / metasulfate. Chain transfer agents, such as conventional alkyl mercaptans and alkanethiols, are suitably used in the polymerization process to adjust the molecular weight of the copolymer in a known manner.

The polymerization is carried out as a batch process or in the form of a feed process.
Alternatively, they can be performed in combination. Suitably, some of the monomers used are first polymerized, and then the remaining monomers are fed continuously or in several steps to the polymerization zone, optionally together with additional free radical initiators.

The polymer dispersion preferably comprises the following steps: (i) initial polymerization of at least one monomer of group (a), and (ii) subsequent polymerization of monomers of groups (a), (b) and (c) Is obtained by

In this preferred embodiment, the initial polymerization step (i) is carried out substantially without monomers of groups (b) and (c) and at least a part of the total weight of the monomers used of group (a) is Present in step (ii). Group (a) used in the subsequent polymerization step (ii)
The amount of monomer can be from 5% to 95% and suitably from 10% to 75% by weight, based on the total weight of group (a) monomer used in the entire polymerization process. According to embodiments of the present invention using a polymer dispersion prepared by this initial and subsequent polymerization step, a well-defined core / shell structure (the core is more hydrophobic and the shell is less hydrophobic, ie, more hydrophilic) Is obtained. The degree of core / shell structure obtained will depend, inter alia, on the type of monomers used and their weight ratios and the manner in which the monomers and initiators are introduced into the polymerization zone, as those skilled in the art will readily recognize. After completion of the polymerization, the average particle diameter of the copolymer microparticles in the resulting dispersion is typically less than about 400 nm. Suitably, the average particle size is between about 40 and 200 nm, preferably between about 50 nm and 100 nm. The glass transition temperature Tg of the resulting suitable copolymer is from about 20C to about 80C, preferably from about 40C to about 70C.

The polymer dispersion suitably contains at least one emulsifier or dispersing aid, the amount of which is between 0.25 and 20% by weight, preferably between 0.5 and 1%, based on the weight of the monomers used for the polymerization.
0% and most preferably 0.75-5% by weight.
Anionic, cationic, amphoteric and nonionic emulsifiers can be used, and are preferably anionic emulsifiers, for example of the type commonly used for anionic dispersions or emulsions. Suitable anionic emulsifiers include sulfuric acid,
It can be chosen from sulfonic acids, ether sulfates, alkyl and alkyl aryl phosphates and ether phosphates, and dialkyl sulfosuccinates, suitably in the form of alkyl metal salts or ammonium salts, such as sodium lauryl sulfate, sodium lauryl sulfonate And sodium dodecylbenzenesulfonate. Polyacrylic acid and its salts can also be used. The nonionic emulsifier can be selected from ethoxylated fatty alcohols, fatty acids, alkyl phenols or fatty acid amides, ethoxylated or non-ethoxylated glycerol esters, and sorbitan esters of fatty acids. The dispersion may contain other additives, such as preservatives, optical brighteners, defoamers,
And protective colloids. Such additives should preferably be non-ionic or anionic. Protective colloids that can be used include water-soluble cellulose derivatives, starch derivatives, gelatin, guar gum, xanthan gum, and polyvinyl alcohol.

The dispersion has high stability in the presence of bases and electrolytes and for long-term storage, even when prepared in the absence of low concentrations of emulsifiers and also of protective colloids. Thus, the present polymer dispersions can be prepared in the absence of protective colloids, such as starch and their derivatives, which are commonly used at high concentrations in the polymerization process.

The dispersion according to the invention can have a solids content of 0.01 to 60% by weight, suitably 10 to 50% by weight, based on the aqueous dispersion. The high solids dispersion can, of course, be diluted with water or mixed with an aqueous solution prior to use, for example an aqueous solution of starch and / or a derivative thereof, and a copolymer solid suitable for surface sizing. The amount is about 0.
It is in the range of 01 to 10.0% by weight.

The method of sizing according to the present invention comprises applying an aqueous polymer dispersion to the surface of cellulosic products such as paper, cardboard and paperboard, and the conventional surface sizing of polymer dispersions as well as starch and derivatives thereof. It can be applied together with the components that have been used for.
The application of the dispersion to the cellulosic surface is usually effected by a size press. Of course, the dispersion can be applied to the cellulosic surface by spraying or dipping. Products treated with the polymer dispersion usually dry at elevated temperatures. Suitably the amount of dispersion applied to the surface is calculated as 0.
It is from 0.05 to 5% by weight, preferably from 0.1 to 1% by weight.

In surface sizing, electrolytes that may be added to the aqueous sizing composition in the size press include inorganic electrolytes, preferably salts of alkali metals such as sodium chloride and sodium sulfate. Usually, the electrolyte content is from 0.1 to 2% by weight and suitably from about 0.3 to 1% by weight, based on the size composition. The aqueous size composition according to the present invention contains the aqueous polymer dispersion defined above and an electrolyte. The size composition can have a copolymer solids content of about 0.01 to 10.0% by weight. Starch and / or its derivatives may also be present in the size composition, a suitable solids content of the starch-based material is about 1-10% by weight, preferably 3-8% by weight. The present invention therefore relates to an aqueous size composition as further defined in the claims.

The invention will be further described in the following examples, which are not intended to limit the invention. All parts and percentages are by weight unless otherwise indicated.

Example 1 An aqueous dispersion according to the invention was prepared as follows: In a stirred reactor equipped with a stirrer, reflux condenser and addition means, 1 part of sodium alkylsulfonate and 50 parts of deionized water were added. First, the reactor contents are heated to 80 DEG C. and 20% of a mixture (I) containing 25 parts of styrene, 8 parts of n-butyl acrylate and 0.2 parts of n-dodecanethiol are charged with nitrogen gas to the reactor. Followed by purging. Copolymerization was initiated by the addition of a 0.1 part aqueous solution of potassium persulfate in deionized water. After 15 minutes, the addition of the remainder of mixture (I) and mixture (II) containing 0.2 parts of potassium persulfate in deionized water was started. These mixtures are
Mixture (I) was continuously fed to the reactor for 100 minutes and mixture (II) for 120 minutes. 45 minutes after the start of the addition of the mixtures (I) and (II), 0.9 part of acrylic acid (2.9% by weight, based on the weight of the monomers used for the polymerization) and 2-
Addition of mixture (III) containing 0.9 parts of hydroxyethyl acrylate in deionized water was started and continued for 55 minutes. After complete addition of the monomers, the reactor contents were stirred at 80 ° C. for a further 60 minutes, then cooled to room temperature and the dispersion was adjusted to pH 6.5 by addition of ammonium hydroxide. The dispersion of the obtained copolymer particles (hereinafter, referred to as dispersion 1) had a solid content of about 37% and an average particle diameter of about 65 nm as measured by a Malvern Zetasizer 3 method.

Example 2 A dispersion used according to the invention and a dispersion used for comparative purposes were prepared according to the procedure of Example 1. However, the monomers of the mixture (III) are replaced by the monomers and amounts shown in Table I (% by weight, based on the total weight of the monomers used in the polymerization)
Replaced with

Example 3 Dispersion 1 of Example 1 was used for surface sizing of paper according to the method of the present invention, and the resulting paper was tested for liquid penetration properties. A comparison was made with an aqueous dispersion of a copolymer prepared by emulsion polymerization of styrene and n-butyl acrylate in the presence of starch as a protective colloid (hereinafter referred to as Dispersion 9).

A paper having a basis weight (basic weight) of 80 g / m ² was treated with the diluted dispersion (about 0.2% by weight solids), passed through a two roll size press, and dried at 105 ° C. in a drum dryer.

The sizing response of the paper was measured by the Cobb method. Table II below shows Cob measured according to TAPPI standard T 441 OS-63.
Indicates the b value.

As is evident from Table II, the paper treated with Dispersion 1 according to the invention has significantly lower Cobb values, compared to the paper treated with Dispersion 9 used for comparative purposes, at the corresponding polymer dose,
Therefore, it showed a high level of sizing.

Example 4 The dispersions 1 to 8 according to Example 2 were tested and their surface sizing efficiency was evaluated. A test was also conducted on a dispersion containing an electrolyte, which was prepared by adding an aqueous sodium chloride solution to the dispersion and then diluting with water. The sodium chloride content was 2% by weight based on the dispersion.

The paper was treated according to Example 3 by applying 0.20% by weight of dry polymer. The sizing response of the paper, i.e., hydrophobicity, was measured using the Hercules Size Test (HST) with test solution no. 2 (1% formic acid) at 80% reflectance. Table of results
Shown in III.

From this table it is clear that the dispersion according to the invention did not have any adverse effects due to the presence of the electrolyte. However, the comparative dispersion essentially lost sizing performance due to the addition of sodium chloride.

Example 5 The performance of the dispersion at various pH values was tested by preparing the dispersion according to Example 1 and adding various amounts of ammonium hydroxide. The paper was surface sized and evaluated as in Example 3.

From these results, it can be seen that the dispersion used according to the invention has a pH
Is essentially insensitive to changes in the range from 4 to 9.

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int. Cl. ⁷ , DB name) D21H 11/00-27/42

Claims (14)

(57) [Claims] 1. A method of surface sizing a cellulosic product by applying a size composition containing an aqueous dispersion of a copolymer obtained by free radical emulsion polymerization of a monomer mixture to the surface of the cellulosic product, Characterized in that the monomer mixture comprises: (a) 70 to 99.5% by weight of (a1) at least one monomer selected from the group consisting of styrene and its derivatives, and (a2) ethylenically unsaturated At least one monomer selected from the group consisting of esters of carboxylic acids and alkanols; (b) from 0.5 to 15% by weight of the group consisting of ethylenically unsaturated carboxylic acids and their salts and ethylenically unsaturated sulfonic acids and their salts At least one monomer selected, and (c) from 0 to 15% by weight of another ethylenically unsaturated copolymerizable monomer. - 2. The method according to claim 1, wherein the size composition containing the aqueous copolymer dispersion is applied to the surface of the cellulosic product by means of a size press. 3. A monomer mixture comprising about 90 to about 99% by weight of group (a) monomer, about 1 to about 5% by weight of group (b) monomer and 0 to about 5% by weight of group (c) monomer.
The method according to claim 1, comprising from about 5% by weight. 4. The group (a1) monomer is styrene, the group (a2) monomer is selected from C ₁ -C ₄ alkyl acrylates, C ₁ -C ₄ alkyl methacrylates and mixtures thereof, and the group (b) 3.) The monomer according to claim 1, wherein the monomer is an ethylenically unsaturated carboxylic acid selected from the group consisting of acrylic acid, methacrylic acid, maleic acid, fumaric acid, itaconic acid, aconitic acid and mixtures thereof. Method according to 3. 5. The process as claimed in claim 1, wherein the monomers of group (a) are used in a weight ratio of (a1) to (a2) of from 5: 1 to 1: 1. Such a method. 6. A process according to claim 1, wherein the monomer mixture contains about 1 to 5% by weight of monomers of group (c). 7. The process according to claim 1, wherein the monomers of group (c) contain at least one monomer having a hydroxyl group. 8. The group (a1) monomer is styrene, the group (a2) monomer is butyl acrylate, the group (b) monomer is acrylic acid or methacrylic acid, and the group (c) When the monomer is 2-
The method according to claim 1, wherein the method is hydroxyethyl acrylate. 9. The polymerization of said monomer mixture comprises the following steps: (i) an initial polymerization of at least one monomer of group (a); and (ii) a monomer of group (a), (b) and (c). 9. The method according to claim 1, which is carried out by a subsequent polymerization. 10. The method according to claim 1, wherein the cellulosic products include paper, cardboard and paperboard. 11. An aqueous size composition comprising: (a) 70 to 99.5% by weight of (a1) at least one monomer selected from the group consisting of styrene and its derivatives; and (a2) an ethylenically unsaturated carboxylic acid. At least one monomer selected from the group of esters of acids and alkanols; (b) 0.5 to 15% by weight selected from the group consisting of ethylenically unsaturated carboxylic acids and their salts and ethylenically unsaturated sulfonic acids and their salts. An aqueous dispersion (A) of a copolymer obtained by free-radical emulsion polymerization of a monomer mixture containing at least one monomer and (c) from 0 to 15% by weight of another ethylenically unsaturated copolymerizable monomer; It is characterized by containing an inorganic electrolyte (B), and starch and / or its derivative (C). 12. A monomer mixture, (a) (a1) styrene and (a2) C ₁ -C ₄ alkyl acrylates and C ₁ -C at least one monomer selected from _alkyl methacrylates of about 90 to about 99 wt% Wherein the weight ratio of monomer (a1) to monomer (a2) is 5: 1 to 1: 1, (b) about 1 to about 5% by weight of acrylic acid or methacrylic acid, and (c) at least one of 12. The size composition according to claim 11, characterized in that it contains 0 to about 5% by weight of another ethylenically unsaturated copolymerizable monomer containing monomer. 13. The size composition according to claim 11, wherein the content of the electrolyte is 0.1 to 2% by weight. 14. The size composition according to claim 13, wherein the electrolyte is sodium chloride or sodium sulfate.