EP0084808A1 - Paper coating composition - Google Patents

Abstract

1. Paper coating composition consisting of A) water B) a binder in the form of latex particles, consisting of a copolymer of alkyl esters of acrylic or methacrylic acid, an unsaturated carboxylic acid an other monomers which have an ethylene bond and at least one polar group, C) conventional fillers or pigments D) a dispersing agent E) and optionally other conventional additives characterised in that the binder B is made up of an emulsion polymer of 1) 45-94,5 % by weight of alkyl esters of acrylic and/or methacrylic acid, optionally in admixture with styrene 2) 3-20 % by weight of hydroxyalkyl ester of acrylic or methacrylic acid 3) 2-10 % by weight of acrylamide and/or methacrylamide 4) 0.5-5 % by weight of an unsaturated polymerisable carboxylic acid 5) an optionally up to 20 % by weight, based on the weight of the copolymer, of other monoolefinically unsaturated monomers and in that the aqueous dispersion of the emulsion polymer has a minimum film forming temperature of not higher than 508C.

Description

The invention relates to paper coating slips which essentially consist of an aqueous phase, a pigment dispersed therein and a binder. The invention particularly relates to a new binder for paper coating slips.

With the increasing spread of offset printing, coated offset papers are becoming increasingly important. With multicolor offset printing at high printing speeds, the quality of the paper coating is constantly increasing. When the printing cylinder rolls off the paper surface, high tensile forces act on the paper surface due to the toughness of the printing ink. Efforts are therefore made to give the paper surface a high structural strength perpendicular to the surface by means of a suitable line. Since moisture is transferred to the paper during the printing process, the high structural strength must be maintained even when wet. This property is called wet pick resistance.

When producing multicolor prints in one operation, it is necessary. that the moisture transferred to the unprinted areas quickly blows away, because otherwise the ink would be repelled by the water film remaining on the surface during the next printing run.

State of the art

Paper coating slips originally contained natural products such as starch or casein as binders. However, the wet pick resistance of the coatings produced with it is insufficient and moreover of fluctuating quality. Combinations of these natural products with dispersed synthetic binders have the same disadvantage.

In EP-A 14 904 combinations of two synthetic resins are proposed as binders for paper coating slips, one of which is soft and sticky and the other, also synthetic resin, as "cobinder" due to the content of units of a hydroxyalkyl ester of acrylic or methacrylic acid and an unsaturated acid is very hydrophilic. The subject of EP-A 15 517 is also a binder-cobinder combination of a soft, dispersed resin and a hydrophilic binder made of natural or synthetic polymers. The latter is first emulsified in an oil and incorporated into the coating slip in this form.

DE-A 2 933 765 is also based on the principle of binder and cobinder, the hydrophilic cobinder being partly water-soluble and partly chemically linked to the binder.

DE-A 3 035 179 discloses a dispersed synthetic resin which is said to be usable as the sole binder in paper coating slips. The resin is a two-stage emulsion polymer made from a diene, styrene, Acrylic esters, an unsaturated acid and optionally limited amounts of hydroxyalkyl esters. Since it is relatively hydrophobic, it can only be successfully used in practice in combination with hydrophilic natural substances such as starch or casein as a cobinder.

Task and solution

A synthetic resin is to be incorporated into a paper coating slip as a binder, which can be produced with constant properties, can be used as a sole binder and results in a paper coating of high wet pick resistance and high water absorption capacity. To achieve this object, the paper coating slip according to claim 1 is proposed.

The essential new component of the new composition contains the binder B in an amount of preferably about 5 to 25% by weight, based on the weight of the fillers C. The other constituents are customary in paper coating slips. They always contain water as a liquid vehicle and chalk, kaolin, heavy spar, titanium dioxide or other white or colored pigments as fillers. The proportion of fillers in the solids content of the coating slip is generally between 80 and 95% by weight. Other additives that can optionally be used are crosslinking agents, anti-foaming agents, thickening agents or stabilizers. Additional binders are generally not required, although it is of course possible, if not advantageous, to add further binders.

The dispersant (D) is a customary and necessary component of the binder dispersion, but also has the function in the compositions according to the invention to disperse the fillers.

Advantages of the invention

By completely dispensing with natural substances as binders or cobinders, the properties of the paper coating slips and the paper coatings produced from them are free from the fluctuations which are always associated with the use of organic natural substances. They can therefore always be processed in the same way and result in a paper line of consistently high quality. Another advantage is that this goal can be achieved with a single binder. This results in a reduced effort for the manufacturer of the paper coating slip in the storage of the raw materials and, due to the omission of a weighing and mixing process, also in the production itself. For the user, all problems that arise from inaccuracies in conventional multi-component binders are eliminated the setting of the mixing ratio of binder and cobinder.

The new binder gives the paper coating excellent water absorption and quick knocking as well as excellent wet picking resistance.

The binder (B)

is a latex produced by emulsion polymerization from the monomer components specified in claim 1 under (1) to (5).

The main components are alkyl esters of acrylic and / or methacrylic acid, which preferably contain 1 to 12 carbon atoms in the alkyl radical, optionally in a mixture with styrene. However, compositions without styrene are preferred. It is advantageous if at least some of the ester alkyl radicals contain 4 or more carbon atoms and if a certain proportion of methyl or ethyl esters is still present. Finally, it is advantageous if the esters are derived at least in part from acrylic acid. A preferred composition of the ester components (1) consists of methyl methacrylate as the hardening component and ethyl or in particular butyl acrylate as the softening component.

The quantitative ratio of hardening and softening monomer components - taking into account components (2) to (5) - is determined so that the minimum film formation temperature (MFT according to DIN 53787) of the binder latex is not above 50 ° C and preferably below 40 ° C. This is generally achieved by the composition of component (1) from lower methacrylic acid esters with 1 to 4 carbon atoms in the ester alkyl radical and / or styrene on the one hand and from higher methacrylic acid esters and / or acrylic acid esters on the other.

The hydroxyalkyl esters (2) of acrylic or methacrylic acid used are preferably those having 2 to 5 carbon atoms in the hydroxyalkyl radical. Hydroxyethyl, 2-hydroxypropyl and 4-hydroxybutyl esters are preferred, especially those of acrylic acid. The absorbency of the paper coating increases with an increasing proportion of the hydroxy esters, but the wet pick resistance exceeds a maximum at a proportion of about 10 to 15%, so that proportions above 20% are unsuitable.

Acrylic and / or methacrylamide (3) is contained as a second hydrophilic component in addition to the hydroxy ester in the emulsion polymer. Neither of the two hydrophilic components alone allows the advantageous combination of properties of high wet pick resistance and high suction strength of the paper coating produced therewith. Methacrylamide is particularly effective. If the unsubstituted amides are also preferred, the corresponding N-alkyl-substituted amides, for example with C _1-4 -alkyl radicals, can optionally also be used.

As unsaturated polymerizable carboxylic acid (4), acrylic or methacrylic acid is preferably involved in the structure of the emulsion polymer. The preferred proportion is 1 to 3% by weight. The carboxyl groups can optionally be neutralized before, during or after the polymerization with a suitable base, such as ammonia, organic amines or alkali metal hydroxide. The preferred pH of the coating slip is 7.5 to 11.

A further monomer component (5) is not necessary in order to achieve the objectives of the invention, however, limited amounts of other, radically polymerizable monomers different from components (1) to (4) can often be incorporated into the emulsion polymer without disadvantage. Examples of such comonomers, which sometimes even have an advantageous effect, are acrylonitrile and methacrylonitrile, but these monomers are mostly avoided because of the residual monomer content which is difficult to avoid. Other useful monoethylenically unsaturated comonomers are vinyl pyrrolidone, vinyl chloride, vinylidene chloride, vinyl acetate or other vinyl esters or monoolefins. Polyunsaturated monomers, e.g. Glycol diacrylates are preferably not used because they cause crosslinking, reduce water absorption and can interfere with film formation. Insofar as they are present as an impurity in the hydroxyester used, their proportion in the total polymer should be so low that it has no adverse influence, i.e. usually less than 0.5% by weight, preferably less than 0.05% by weight.

The emulsion polymer can be prepared by customary methods of emulsion polymerization in the aqueous phase using water-soluble free radical formers, such as peroxodisulfates, hydrogen peroxide or water-soluble redox systems, and - preferably anionic and / or non-Bibnic - emulsifiers. The particle size of the latex particles is preferably below 300 nm (weight average). It is known that the particle size can be controlled most precisely if the batch polymerization is carried out batchwise carried out by the monomer or emulsion feed process and uses an amount of emulsifier in the aqueous receiver which is close to the critical micelle concentration, or if the batch latex method is used batchwise. These methods are well known to those skilled in the art.

The dispersant (D)

can consist of several components. As a rule, an anionic and / or nonionic emulsifier is present which has already been used to produce and stabilize the binder latex. Further dispersants are required to disperse the pigments. Examples include polyacrylic acid or its water-soluble salts with K values between 10 and 35, or polyphosphates. In addition, cellulose or starch derivatives can be present. They essentially serve for thickening in order to set the desired viscosity of the coating slip.

example 1

0.5 g of ammonium peroxodisulfate and 0.4 g of a technical emulsifier, consisting of a reaction product of tri-isobutylphenol and 7 moles of ethylene oxide, are sulfated in a Witt'schen pot (2 liters) with reflux condenser, stirrer and feed vessel at 80 ° C. and converted into the sodium salt, in 400 g of dist. Water dissolved. In this solution, a mixture of 430 g of methyl methacrylate, 400 g of butyl acrylate, 100 g of 2-hydroxyethyl acrylate, 50 g of methacrylic acid amide, 20 g of methacrylic acid, 9.35 g of the abovementioned enulsifier, 1.5 g of the above initiator and 615 g of dist. Water-produced emulsion added dropwise at 80 ° C. After the end of the feed, the batch is kept at 80 ° C. for a further 2 hours, cooled to 50 ° C. and adjusted to pH 7.5 with NH ₃ solution. A mixture consisting of 15 g of a reaction product of 1 mol of isononylphenol with 7 mol of ethylene oxide and 42.9 g of a 35% reaction product of 1 mol of isononylphenol and 100 mol of ethylene oxide is then added. After cooling to room temperature, it is filtered through a fine-mesh sieve made of stainless steel.

The dispersion has a solids content of approx. 50% by weight and an average particle diameter of 154 nm. MFT: 19 ° C.

Example 2

The procedure is repeated, but 480 g of methyl methacrylate are used (instead of 430 g) and no methacrylic acid amide is used.

A dispersion having a solids content of 50% by weight and an average particle diameter of 154 nm is obtained. MFT: 21 ° C.

Example 3

The procedure of Example 1 is repeated, but 500 g of butyl acrylate are used (instead of 400 g) and no 2-hydroxyethyl acrylate is used. A dispersion with a solids content of

50% by weight with an average particle diameter of 136 nm.

MFT: 24 ° C.

Example 4

• 430 g Methylmethacrylat
• 300 g Butylacrylat
• 200 g 4-Hydroxybutylacrylat
• 50 g Methacrylsäureamid
• 20 g Methacrylsäure
• 9,35 g Umsetzungsprodukt aus Tri-isobutylphenol und 7 Mol Äthylenoxid, das sulfatiert und in das Na-Salz übergeführt ist.
• 1,5 g Ammoniumperoxodisulfat und
• 615 g dest. Wasser

The procedure is as in Example 1, but the feed consists of an emulsion with the following weights:

• 430 g methyl methacrylate
• 300 g butyl acrylate
• 200 g of 4-hydroxybutyl acrylate
• 50 g methacrylic acid amide
• 20 g methacrylic acid
• 9.35 g of reaction product from tri-isobutylphenol and 7 mol of ethylene oxide, which is sulfated and converted into the Na salt.
• 1.5 g of ammonium peroxodisulfate and
• 615 g dist. water

The dispersion has a solids content of 49.9% and an average particle diameter of 136 nm. MFT: 24 ° C. For the production of paper coating slips

For offset papers, 200 parts of china clay and 200 parts of chalk are dispersed in a dispersing unit with the help of 2 parts (solid) of a conventional dispersant based on low molecular weight polyacrylic acid (e.g. ®ROHAGIT SL 252) in 250 parts of water.

For this purpose, 120 parts of the binders described in the preceding examples are added with stirring and the pH of the coating slip is adjusted to 8.5 with NaOH. The wet pick resistance of the papers coated with these materials was checked with the PRÜFBAU multi-purpose test printing machine with pre-moistening. In this method, a strip of the paper to be tested is moistened with a mixture of water and isopropanol (80/20) using a rubber roller and then printed with a pick test ink.

Test conditions:

Results

No plucking ("dry plucking") was found in any of the coating colors in the non-moistened area. There were also no signs of rejection of the printing ink due to the pre-moistening. Significant differences were found when assessing the build-up of lines on the printing form (building-up occurs when the line comes off as a result of the pre-moistening and is transferred to the printing form during subsequent printing).

See table for evaluation of printing results.

Claims (4)

1. Paper coating slip consisting of A) water B) a binder in the form of latex particles C) usual fillers D) a dispersant E) optionally other customary additives
characterized,
that the binder B from an emulsion polymer 1) 45-94.5% by weight of alkyl esters of acrylic or / and methacrylic acid, optionally in a mixture with styrene 2) 3-20% by weight of a hydroxyalkyl ester of acrylic or methacrylic acid 3) 2-10% by weight of acrylic and / or methacrylamide or their N-alkyl derivatives 4) 0.5-5% by weight of an unsaturated polymerizable carboxylic acid 5) optionally up to 20% by weight, based in each case on the weight of the copolymer, of other monoolefinically unsaturated monomers
is built up and that the aqueous dispersion of the emulsion polymer has a minimum film formation temperature not above 50 ° C. 2. Paper coating slip according to claim 1, characterized in that the alkyl esters involved in the construction of the emulsion polymer (1) at least partially. Carry alkyl residues with at least 4 carbon atoms. 3. Paper coating slip according to claims 1 or 2, characterized in that the alkyl esters involved in the construction of the emulsion polymer (1) are at least partially. derived from acrylic acid. 4. paper coating slip according to claims 1 to 3, characterized by an average particle size of the binder (B) of at most 300 nm.