CA1210543A

CA1210543A - Aqueous dispersions, of a graft copolymer for paper coating

Info

Publication number: CA1210543A
Application number: CA000396256A
Authority: CA
Inventors: Dieter Rahlwes; Gottfried Wallpott; John Goossens; Ulrich Beck
Original assignee: Bayer AG
Current assignee: Bayer AG
Priority date: 1981-02-17
Filing date: 1982-02-15
Publication date: 1986-08-26
Also published as: JPS57153015A; ES8302029A1; ES509646A0; EP0058366B1; DE3105779A1; EP0058366A3; EP0058366A2; DE3272235D1

Abstract

AQUEOUS DISPERSIONS, THEIR PREPARATION
AND THEIR USE

Abstract of the Disclosure Aqueous dispersions having a solids content of from 35 to 65% by weight of a graft copolymer obtained from a graft substrate of an aqueous dispersion based on styrene, butadiene, isoprene, chloroprene, acrylonitrile, (meth)-acrylic acid-C1-C12-alkyl esters, (meth) acrylic acid, itaconic acid, fumaric acid, maleic acid, vinyl acetate and vinyl chloride and mixtures of two or more of the above mentioned compounds on which are grafted one or more hydrophilic monomers from the group of unsaturated carboxylic acid amides, vinyl pyrrolidiene and ethylene oxide, wherein up to 600/o of the hydrophilic monomers may be replaced by one or more hydrophobic monomers, are suitable as binders for improved paper coating compounds free from cobinders.

Le A 20 683

Description

~Zl~S43 This invention relates to aqueous dispersions of polymers with thereon hydrophilic monomers grafted and to a process for their preparation. These dispersions are suitable for the preparation of improved coating compositions for flat substrates, in particular for coated printing carriers.
Synthetic binders based on copolymers of acrylic acid esters, styrene, butadiene and unsaturated carbo~-ylic acids are nowadays widely used for the manufacture of coated paper used for printing. The copolymers are used in the form of aqueous dispersions. However, these binders only partly satisfy the stringent requirements of high-speed coating machines. Their viscosity characteristics, water retention, migration in the pigment matrix and resistance to picking o~ten need to be adjusted by the addition of so-called co-binders. The co-binders used may be, for example, copolymers o$ vinyl acetate and acryl-amide (German Offenlegungsschri$t No. 24 50 039). However, these co-binders are only able to improve individual characteristics but cannot provide an overall improvement in quality for the multiplicity of properties requ~ed.
German O$~enlegungsschrift No. 27 03 418 discloses pigmented paper coating compounds which, in addition to the usual binder consisting of a synthetic late~ based on polyacrylates, polymethacrylates, styrene-butadiene polymers of polymers of vinyl acetate in the form of aqueous dispersions, also contain a high molecular weight water-soluble polymer as co-binder. An essential feature of these paper-coating compounds is that the water-soluble polymer is prepared by polymerisation in a water-in-oil emulsion. This enables the viscosity characteristics~
water-retention and resistance to picking to be improved but the problem 9 till remains to obtain high- $ilm hardness together with good binding capacity, low tendency to migration in the pigment matri~ sufficient covering o~ the fibres, high opacity and, particularly Le A 20 683 t it ~Z~543 in LWC paper (low weight per unit area), little or no tendency of the printing ink to soa~ through the paper.
German Offenlegungsschrift No. 1 495 722 discloses a process for the preparation of graft polymers by the reaction of acrylamide with substantially amorphous olefine copolymers contain-ing tertiary carbon atoms in aqueous dispersion by radical activation, characterised in that the olefine copolymers containing tertiary carbon atoms are ternary unsaturated copolymers of (1) ethylene, (2) propylene or butene and (3) a diene or triene.
Although the products of this process have been recommended for refining paper, the disadvantages mentioned above are still inherent.
It has now been found that improved paper-coating com-pounds which do not require an additional co-binder may be obtained by using certain dispersions as binders.
This invention therefore provides an aqueous dispersion having a solids content of 35 to 65 % by weight of a graft copolymer, said graft copolymer having been obtained from a graft substrate of an aqueous dispersion polymer of at least one compound selected from the group consisting of styrene, butadiene, isoprene, chloroprene, acrylonitrile, ~methlacrylic acid-Cl-C12-alkyl esters, (meth)acrylic acid, itaconic acid, fumaric acid, maleic acid, ~inyl acetate and vinyl chloride, on to which graft substrate has been grated 0.5 to 15 ~ by weight, based on the graft substrate, of at least one hydrophilic mo~omer selected from the group co~sisti~g of acrylamide, methacrylamide, vi~yl pyrrolidone and ethylene oxide with up to 60 ~ by ~eight o said grafted hydrophilic monomer having been replaced by at least one ~, ~
~ -3-hydrophobic monomer selected from styrene, butadiene, isoprene and (meth)acrylic acid-Cl-Cl2-alkyl esters.
The graft substrates preferably consist of styrene/
butadiene copolymers containing 40 to 75% by weight of styrene, 21 to 59~ by weight of butadiene and l to 4% by weight of one or more unsaturated carboxylic acids. In another preferred embodiment, the graft substrates consist of copolymers containing 40 to 75% by weight of styrene, 21 to 59% by weight of acrylic acid-Cl-Cl2-alkyl esters and 1 to 4% by weight of one or more unsaturated carboxylic acids. In a particularly preferred embodiment, the graft substrates consist of copolymers containing 40 to 75% by weight of styrene, 21 to 59~ by weight of butyl acrylate and l to 4% by weight of unsaturated carboxylic acids.
The unsaturated carboxylic acids used in the preferred embodiments may be fumaric, itaconic, maleic or in particular (meth)acrylic acid.
According to the invention, from 0.5 to 15~ by weight, based on the graft substrate, of water-soluble or at least hydrophilic monomers are used for grafting, (meth)acrylamide being preferred. It is particularly preferred to use for grafting from 0.5 to 6% by weight of (meth)acrylamide, based on the polymer substrate.
The graft substrates are prepared in the form of aqueous dispersions by the conventional techniques of emulsion polymerisa-tion. These dispersions are used as starting substances for th~
subsequent grafting reaction which must be carried out undex certain conditions in order to obtain reproducible dispersions according to the invention.

- ~2~S~3 The invention therefore also provides an improved paper coating compound which comprises an aqueous dispersion of pigment and as the sole binder therefor, a dispersion as deined above.
The polymerisation initiators used may be, for example, inorganic or organic peroxides. The grafting yield in the graft polymerisation according to the invention, that is to say the percentage of monomer which i~ chemically bound to the graft subs~rate, may be from 10 to 95%.
The grafting reaction is preferably carried out at a temperature of 70 to 100Co In one embodiment given here by way of examplel a 52%
styrene-butadiene latex is used as graft substrate to which 0.9 to ; ~ : 1.7% by weight of acrylamide, based on the graft substrate, with 0.9 to 1.7% by weight of water, based on the graft substra-te, are added at 95C, either all at once or in batch as with stirring.
The reaction is started by the addition of 0.1% of ammonium ~peroxydisulphat:e dissolved in 1% by weight of water, both quantities bas~d on the graft substrate, and the temperature is maintained at 95C with stirring until the reaction is completed.
Most of the paper-coating characteristics required including the mo~t important, can be simultaneously optimised by the aqueous dispersions according to the present invention so that better results are obtained than with the cobinder methods hitherto used~ The aqueous dispersions according to the invention are preferably used for the preparation of paper-coating compounds although other applications are also possible, e.g. the preparation of coating compounds for other substrates, such as pasteboard, cardboard, textiles and leather.

.

~Z:~543 The process according to the invention not only ena~les many of the praperties to be optimised simul-taneously but also provides the possibility of e~cellent controlled optimisation of specific characteristics of 5 coatings, such as their compressibility~ microporosity, colour absorption, smoothness and binding power.
It was surprisingly found that when the aqueous dispersions, i.e. the modified latices according to the present invention, are used for 10 the manufacture of lightweight printing paper, the latex does not pass through the paper, covering of the fibres is improved and a homogeneous~ uniform coating matrix is obtained. The opacity, degree of whiteness and smoothness are thereby improved and uniform and 15 rapid printing dye uptake are obtained together with an e~ually good or improved binding power. The further advantage of the binder according to the invention is that the irreversible thickening which is liable to occur in the substances hitherto used, e.g. polyvinyl ? 20 alcohol in staining colours, is now prevented.
Compared with the state of the art, the latices modified according to the invention provide more bulky and consistently more microporous coatings. In offset printing, this ensures rapid and unifor~ uptake 25 of colour, which is particularly advantageous for wet_ in-wet multicolour printing.
For intaglio printing, latices modified according to the invention provide high compressibïlity and smoothness of the coating with less calendering work. The coating 30 structure described provides high and rapid colour-absorption and high printing-gloss~
There ~s a demand for stiffer paper both for offset printing and for intaglio printing. It has h~therto been -~mpossible to produce stiff paper because the use Le A 20 683 ~2~543 o~ latices with hard films was accompanied by a marked deterioration in the printing characteristics, i.e.
uneven porosity, reduced compressi~ility and missing aots. However, due to the special structure of the coating, latices with hard ~ilms modified according to the invention enable stiff paper to preserve all the characteristics necessary for intaglio printing so that it is possible for the first time to use stif~
paper in the intaglio printing process.
Due to the uniform embodiment o~ the modified binder in the coating matri~ and its high binding power, the proportion of binder in the coating compound may be reduced, whereby a further improvement in the printing characteristics is obtained.
Example 1 a) Preparation of the ~raft latex_ Carbo~ylated styrene-butadiene late~ containing 600~ styrene, 36~ butadiene and 4% (meth) acrylic acid and having solids content of 50%, the usual film hardness of 50 to 60 Shore A and a high pigment binding power is used as graft substrate which is reaoted with a) 2% acrylamide = gra~t latex A and b) 2~ methacrylamide = graft late~ B, the percentages being based on the grait substrate.
The latex characteristics are entered in Table 1 below.
Table 1 Late~ A B Untreated graft substrate Solid content % 50 50 50 PE 5.0 5.3 5.2 Viscosity mPa.s 210 140 95 Film hardnessSh A 82-71 80-~9 56-48 b) Preparation of the coating compound and method of carryin~ out the coatin~ test A paper-coating compound suitable ~or the ma~ufacture of paper for of~set printing îs prepared by dispersing Le A 20 683 il2~5~3 100 g of china clay in an approximately 650/o aqueous slurry using 0.2 g of ~mmoniacal water ànd 0.3 g o~
sodium polyphosphate. Th0 latices described under a) are added to the homogeneous suspension, in each case in a quantity amounting to 10~ (polymer solid content, based on the clay). The PH is adjusted to 8.5 with ammoniacal water after the final solids content of the staining colour has been adjusted to 55% with water, The compositions are shown in Table 2.
Table 2 Composition of paper-coatin~ _ompounds (~) Experiment _1 2 3 Water for dispersion I 53.8 -15 Water for final adjustment 1 26.2 Sodium polyphosphate I O.3 China clay Dinkie A 1 100 Graft latex A 20 - ~
Graft latex B - 20 t 20 Graft substrate - - 20 pH I 8.5 Solids content 1 55 The properties of the coating compQunds are shown 25 in Table 3 below.
When the coating compounds prepared as described above have been paci~ied and freed from air, they are applied to both sides of a bleached body paper of 55~
mechanical wood pulp and 45~ bleached sulphite cellulose 30 (pine wood) having an ash content of 9.9~ and a weight of 50 g/m2, using an experimental doctor blad0 spread coating machine at a speed of 200 m/min.~ the dry weight of the coating applied being 8 to 10 g/m on each side~
When the coating compounds with grafted late~ are used, no coagulate or deposit is formed either on the doctor blade or on the counter roller even aYter the machine Le A 20 683 .

5~3 has been in operation for a considerable time, and no coating compound if sound to pass through the-p~per to the counter roller. ~he coated paper is subse~uently satinised by two passages through super glazing rollers, and the paper is tested after it has been air conditioned at 20C and 650/o relative humidity.
The results of the tests are summarized in Table 4 Table 3 Properties of the paper coatin~ compounds Experiment 1 2 3 Solids c~ntent (%) 54.9 55.1 54.8 pH 8.4 8.5 8.4 ~iscosity (mPa,s) Brookfield (immediately) 594 670 390 Brookfield (after 24 hours) 682 743 430 Rheomat at D = 3.5 . lol 320 342 376 a~ D = 1.103 42 44 49 Setting point oh 76.8 75.2 79.1 Water retention capacity slightly consid-(comparison with ungrafted improved erably late~) improved Table 4 Coatin~ characteristics Experiment _ _ 1 2 3 Application oi coating (g/m on each side) I _ 10 Thickness (mm) o,o75 0.079 o.o6g Water absorption (g/m2) according to Cobb (60 sec.) 21 22 20.5 Smoothness according to Bekk. (sec./ml) 1110 1190 640 Le A 20 683 ~21~:~5~3 Table 4 (Cont.) Roughness according to Bendtseen (ml/min) 25 22 35 Bending strength un~er i~p2cts according to Schlenker, 40 62 58 47 IGT picking resistance (cm/sec.) 124 120 116 E and N wiping test (/0) 28 26 22 Intaglio printing sample pring 1 2 3 Micro contour test 1 2 4 Gloss (%) 2~ 25 22 Opacity (/0) 93.9 92.6 91.1 Degree of whiteness (/0) 82.3 82.4 81.6 Fibre covering (o,b) 78.0 7~.6 69.7 Dynamic sheet stiffness 0.16 0.16 0.14 c) Assessment The coated paper samples from ~2~periments 1 and 2~ in which the coated compounds contained the graft latices according to the invention, show significant improvements in hardness of the coating, opacity, printing character-istics and covering of the fibres compared with thepaper coated with conventional latex which has not been grafted (Experiment 3). The gloss was also somewhat increased. The setting point of these coating compounds `
was reduced so that the coating can be rapidly immobilised before the drying process proper and migration of the binder can be suppressed. This enabled the tendency to mottling to be also significantly reduced.
The marked increase in picking-resistance should be 3 particularly mentioned.
d) Test methods .
The ~ollowing test methods were employed:
(atmosphere under which tests were carried out, unless otherwise indicated: 20C, 65~6 relati~e humidity) Le A 20 683 -~2~5~3 Electrometric according to Solid content: a) for latices: according to b) for coating compounds: diff-erence weighing after drying at llO~C
Viscosity: According to DIN 53 788 a) measuring instrument: Brookfield viscosimeter LVT~ spindle 3 or 4, 60 revs/min Test temperature: 20C
b) measuring instrument for taking rheographs: Rheomat 15, measur-ing systems A-C, test temperature 20C.
Settin~ uoint of the coatin~ compounds:
100 g o~ the coating compound are poured on an unglazed clay plate which has been dried under con-stant conditions. As soon as the shine due to moisture has disappeared from the surface of the coating compound, a sample is remo~ed to determine the solids contents.
The solids content in percent determined by this method defines the setting point.
Water~ LL~ Y--bL~_ 10 cc of the coating compound are spread-coated over a sample area of the paper to be coated, using a small doctor blade~ height of gap 0.2 mm, at a constant ~ coating speed for a few tenths of a second. Water penetrating the Le A 20 683 5~3 - 12 _ ~aper from the coating compound causes the relative humidity of the air to`rise on the underside of the pape~, The relationship o~ this moisture increase with time is continously registered - by means of a hygroscopic humidity meter. The greater the increase in moisture per unit time, the lower is the water retention capacity of the coating compound. Humidity meter:
VAISALA (equipment HN 11) measurements in air conditioned chamber ( 650/o relative humity).
Application wei~ht: Di~ferences in weight per unit area between air conditioned samples o~ coated and uncoated paper (according to DIN 53 111).
Thick_ess: Determination according to DIN 53 105 Water absorption accordin~ to Coob:
Determination according to DIN 53 132 60 sec.
25 Smoothness accordin~ to Bekk:
Determination according to Rou~hness accordin~ to.Bendtsen:
Determination according to Fluxural strength according to Schlenker:
Determination according to DIN 53 121 bending angle 40~, Le A 20 683 ~lZ~5~3 Dynamic sheet stiffness: Test method according to Kodak-Pathe DIN 5~ 123 Pickin~ resistance: Determination according to Tappi standard T 499 Su 64 using the IGT instrument, Model AC2, and a crushing stress of 35 kg/cm . The picking stain used was low viscosity IGT
picking test stain.
10 K 1 N test: Colour absorption of coated (printing ink absorption) paper determined by the usual Tappi routine control method RC 19, exposure time 3 min.
The K I N value is the differ-ence in degree of whiteness between untreated paper and paper treat~d with test ink.
Measuring instrument: Elrepho, Filter R 457. Testing colour:
K + N testing ink of K ~ N
laboratories Inc., Melrose Park Micro contour test: Test method ~or assessing the uniformity of the coating, procedure similar to the K I N
~5 wiping test but time of e posure to test only 2 minutes. Test colour: blue test colour 3811 of Lorilleux Druck~arben Gmb~, Ratingen. The assessment is made ~isually, using a five-stage assessment scale ~rom Grade 1 for an absolutely homogeneous test surface free ~rom stipples, gaps and streaks, to Grade 5 ~or a very turbulent, uneven sur~ace with many faults.
Le A 20 683 ~l2~543 printin test: ~ntaglio printing sample test on sample printing machine, system Dr. Durner.
The printing results were assessed visually ana graded according to sharp~ess of the raster rep-roduction, uni~ormity of colour absorption, freedom from missing dots, shining through and the like. 3 Grades were used:
1 = very good

2 = good

3 = poor.
Gloss- Determination of the gloss 1~
according to DIN 67 530 against - a comparison standard at a measuring angle of 60~.
Measuring instrument: Lange Reflectometer.
Opacity: Determination according to Degree of whiteness: Determination with remission photometer Elrepho, Filter R 457.
25 Fibre covering: The coating stain is painted on black cardboard (application 8-10 g/m2 dry) and carefully dried. The degr~e of w~iteness (Elrepho, Filter R 457) of the 3 painted side of the cardboard is then measured. The more the coating covers the black surface, the higher is the degree of whiteness - ~ 35 Le A 20 683 The ~ollowing is another method for assessing the covering of fibre:
~ ~ery concentrated beam of a spectral photometer is swept over the coated cardboard surface and the remission is continuously recorded over a length of cardboard of about 10 cm. A measure of the uniformity o~ fibre covering is obtained by stochastic assessment of the fluctuations in remission. Cross-section of the illuminating beam:
O ~ 2 . 2 ~ rectangular shutter.
10 E~ample 2 a) Preparation of the ~raft late~
As described under a) of Example 19 2% acrylamide =
gra~t latex C are grafted on a styrene-butadiene latex similar to that of E~ample 1 but with a greater film 15 hardness of ca. 80 Shore A, a slightly lower pigment binding power.
The properties of the late~ are entered in Table 5, below Table 5 20 Late~ C untreated _ _ _ _ ~ra~t substrate Solids content % 50 50 pH 5~3 5.1 ~ Viscosity mPa.s 180 90 25 Film hardness Sh A 90-79 88-61 b) Preparation of the coating compound and method of carryin~ out ,the coating experiment The same conditions w~re employed as described in Example 1 b). In addition, e~periments were carried out (E~periments 6 and 7) using a binder content of 5~ under otherwise identical conditions to test the printing ch&racteristics in the intaglio printing process.
The compositions are shown in Table 6 below, the properties 35 of the coating compounds in Table 7 below.
No coagulate or deposit on the doctor blade was formed in these e~periments even after a considerable operation time. The coating stain did not seep through the paper (no deposit on counter roller).
Le A 20 683 ~Z1~5~3 The samples of paper coated on both sides were subsequently gla~ed over superglazing rollers and tested after they had been air conditioned. The properties of the coatings are summeri~ed in ~able 8 below.
Table 6 Composition of Paper coating compounds in ~
Experiment 4 5 ~ 7 Water for dispersion 1 53.~ -Water for final adjustment ~ 26.2 27.1 10 Sodium polyphosphate 1 0.3 China clay l 100 Graft latex C 20 - 10 Graft substrate - 20 - 10 pH 8., ~ -15 Solids content (%) ~ 55 Table 7 Properties of paper coating compounds E~periment 4 5 6 7 20 Solids content (%) 55.2 55.1 55.1 54.9 pH 8.5 8.6 8.4 8.5 Viscosity (mPa.s) Brookfield, immediately385 110 234 105 Brookfield after 24 hours 445 170 310 180 Rheomat at D = 3.5.101 472 165 345 160 at D = 1.103 38 27 43 25 Setting point (%) 76.2 79.8 75.4 77.6 Water retention capacity slightly slightly 3 (compared with latex improved improved which has not been grafted) .

Le A 20 683 .
.. .

~Z;~tj5~3 Table 8 Coatin~ ProPerties Experiment 4 5 6 7 Coating application (g/m2 5 ~n each side 10 ~hic~ness (mm) 0.075 o.o68 0.077 0~067 Water absorption (g/m2) according to Cobb (60 sec.) 21 19.5 24 22.5 Smoothness according to `
10 Bekk (sec./ml) 1100 680 980 640 Roughness according to Bend~sen ~ml/min) 24 32 25 35 Bending strength under impacts according to Schlenker, 40 40 38 29 26 15 IGT picking resistance (cm/sec) 116 98 35 27 K & N wiping test (/) 27 23 32 28 Intaglio printing . sample print - - 1 3 20 Micro contour test 1 2 1 3 Gloss (~0) 30 27 38 34 Opacity (/0) 93.2 92.7 92.0 91.6 Degree of whiteness (/0) 83.7 83.5 84.0 83.8 Fibre covering (%) 78.4 73.3 80.0 75.g dyn. sheet strength 0.17 0.14 0.13 0.11 c) Assessment As described in E~ample 1, the experiments carried out in this E~ample also result in improvement of the coating properties when late~ modified according to the invention 3 is used, even when ~ilm hardness is increased, so that very good properties for the purposes of printing process-es are obtained, including intaglio printing. A paper Le A 20 683 which is light in weight but stiff can thus be obtained, which is particularly suitable for intaglio printing in the LWC range, Example 3 A relatively hard styrene-butadiene late~ having a film hardness of about 95 Shore A and low pigment binding power is grafted according to Example 1 a) with 1. 5% acrylamide = graft late~ D.
This e~ample is provided to illustrate the marked increase in binding power obtained by the treatment according to the invention.
A coating egperiment using graft latex D was carried out as described in Example 1 and t~e coated paper was tested, The following Tables show the lateæ properties, coating stain composition and data and test results.
This e2periment con~irms the possibility of increasing the binding power of a paper-coating latex by treating this latex in accordance with the invention.
Table 9 Latex D Untreated graft substrate Solid content (%) 50 50 pH 5.3 5.1 Viscosity mPaOs 140 82 Film hardness Sh A 97_95 89-86 Table 10 Composition of paper coatin~ compounds in Experiment _ 8 9 30 Water for dispersion 53.8 Water for final adjustment 1 26.~ - J
Sodium polyphosphate 1 0.3 China clay Dinkie A 1 100 Graft lateg D 20 35 Gra~t substrate - 20 pH I _~.5 Solid content (/0) . 1 55 -Le A 20 683 ~lZ~543 Table 11 Properties of paper-coatin~ compounds Experiment 8 9 Solids content (%) 55 .054 . 9 5 pH 8.4 8.5 Viscosity (mPa.s) Brookfield, immediately 185 90 Brookfield, after 24 hours 265 110 Rheomat 10 at D = 3.5 . 101 `210 85 at D = 1.103 26 21 Setting point (/0) 78.2 78.6 Water-retention capacity as (comparison with ungrafted comparison 15 latex) Table 12 -Properties of_coatin~
E~periment - 2 8 9 20 Coating application (g/m! on each side ¦ 10 -Thickness (mm) 0.75 0.75 Water absorption (g/m ) according to Cobb (60 sec.) 21.5 21 25 Smoothness according to Bekk.
(sec~/ml) 660 590 Roughness according to Bendtsen (ml/min.) 26 27 Bending strength under ~cts accor ~ g to 5chlenker, 4~ 62 59 IGT picking resistance (cm/sec.) 85 27 K & N wiping test (%) 26 28 Intaglio printing sample print 2 3 Micro contour test 2 3 Le A 20 683 ~L2~S~3 _ 20 -Gloss (o,b) 26 26 Opacity (/0) 92.5 92.6 De~ree cf whi~eness (/0) 81.8 81.9 Co~ering of fibres (%) 71.1 69.7 dynamic stiffness of sheets 0.19 0.18 Le A 20 683

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. An aqueous dispersion having a solids content of 35 to 65% by weight of a graft copolymer, said graft copolymer having been obtained from a graft substrate of an aqueous dispersion polymer of at least one compound selected from the group consisting of styrene, butadiene, isoprene, chloroprene, acrylonitrile, (meth)acrylic acid-C1-C12-alkyl esters, (meth)acrylic acid, itaconic acid, fumaric acid, maleic acid, vinyl acetate and vinyl chloride, on to which graft substrate has been grafted 0.5 to 15 by weight, based on the graft substrate, of at least one hydrophilic monomer selected from the group consisting of acryl-amide, methacrylamide, vinyl pyrrolidone and ethylene oxide with up to 60% by weight of said grafted hydrophilic monomer having been replaced by at least one hydrophobic monomer selected from styrene, butadiene, isoprene and (meth)acrylic acid-C1-C12-alkyl esters.

2. A dispersion according to claim 1, in which the graft substrate consists of a styrene/butadiene copolymer containing 40 to 75% by weight of styrene, 21 to 59% by weight of butadiene and 1 to 4% by weight of one or more unsaturated carboxylic acids.

3. A dispersion according to claim 1, in which the graft substrate consists of a copolymer containing 40 to 75% by weight of styrene, 21 to 59% by weight of acrylic acid-C1-C12-alkyl ester and 1 to 4% by weight of one or more unsaturated carboxylic acids.

4. A dispersion according to claim 1, in which the graft substrate consists of a copolymer containing 40 to 75% by weight of styrene, 21 to 59% by weight of butyl acrylate and 1 to 4% by weight of one or more unsaturated carboxylic acids.

5. A dispersion according to claim 2, 3 or 4, in which the unsaturated carboxylic acid used is acrylic acid or methacryl-acid.

6 . A dispersion according to claim 1, in which 0.5 to 6% by weight, based on the graft substrate, of acrylamide or methaeryl-amide is used as hydrophilic monomer.

7. A process for the preparation of an aqueous dispersion according to claim 1, characterised in that hydrophilic monomer, together with any monomer used, is admixed with about 0.5 to 15% by weight of water, based on the graft substrate and the grafting reaction is carried out by the addition of a polymerisation initiator.

8. An improved paper coating compound which comprises an aqueous dispersion of pigment and, as the sole binder therefor, a dispersion according to claim 1.