Aqueous dispersions, their preparation and use.

0001The invention relates to aqueous dispersions of polymers with grafted hydrophilic monomers and a process for their preparation. These dispersions are suitable for the production of improved coating compositions for flat substrates, in particular for coated printing media.

0002In the production of coated papers which are suitable for printing, synthetic binders based on copolymers of acrylic esters, styrene, butadiene and unsaturated carboxylic acids are largely used today, the copolymers being used in the form of aqueous dispersions. These binders only partially meet the high requirements on the fast-running coating machines. The viscosity behavior, the water retention, the migration behavior in the pigment matrix and the pick resistance often have to be adjusted by adding other components, so-called co-binders. For example, copolymers of vinyl acetate and acrylamide (DE-OS 24 50 039) are used as co-binders. However, only individual properties are improved, but not a simultaneous quality improvement for the majority of the required properties.

0003From DE-OS 27 03 418 pigment-containing paper coating slips are known which, in addition to a synthetic latex usually used as a binder, based on polyacrylates, polymethacrylates, styrene-butadiene polymers or polymers of vinyl acetate in the form of aqueous dispersions as a co-binder, have a high molecular weight, contain water-soluble polymer. The production of the water-soluble polymer by polymerization in a water-in-oil emulsion is essential for these paper coating slips. This improves the viscosity behavior, the water retention and the pick resistance. However, it remains a problem that high film hardness combined with good binding capacity, low tendency to migrate in the pigment matrix, particularly good fiber coverage, good opacity and, in particular with LWC papers (lower basis weight), prevent the ink from striking away into the paper to reach.

0004Furthermore, from DE-OS 1 495 722 a process for the preparation of graft polymers by reacting acrylamide with essentially amorphous olefin copolymers containing tertiary carbon atoms in aqueous dispersion by means of radical activation is known, which is characterized in that ternary olefin copolymers containing tertiary carbon atoms are used. unsaturated copolymers of (1) ethylene, (2) propylene or butene; and (3) a diene or triene.

0005Although the process products for paper finishing are recommended, the disadvantages mentioned above cannot be avoided.

0006It has now been found that, with certain dispersions, improved paper coating slips are obtained as binders, which do not require another co-binder.

0007The invention therefore relates to aqueous dispersions with a solids content of 35 to 65% by weight of a graft copolymer from a graft substrate from 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 compounds mentioned, to which one or more hydrophilic monomers from the series of unsaturated carboxamides, vinylpyrrolidone and ethylene oxide are grafted, it being possible for up to 60% of the hydrophilic monomers to be replaced by one or more hydrophobic monomers.

0008Suitable unsaturated carboxamides are derived, for example, from fumaric acid, itaconic acid, maleic acid and (meth) acrylic acid.

0009Suitable hydrophobic monomers are styrene, butadiene, isoprene, (meth) acrylic acid-C₁-C₁₂-Alkyl ester. Preferably best the graft substrates of styrene / butadiene copolymers around 40 to 75 wt .-% styrene, 21-59 wt .-% of butadiene and 1 to 4 wt .-% of one or more unsaturated carboxylic acids. In a further preferred embodiment, the graft substrates consist of copolymers with 40 to 75% by weight of styrene, 21 to 59% by weight of acrylic acid-C₁-C₁₂Alkyl esters and 1 to 4% by weight of one or more unsaturated carboxylic acids. In a particularly preferred form, the graft substrates consist of copolymers with 40 to 75% by weight of styrene, 21 to 59% by weight of butyl acrylate and 1 to 4% by weight of unsaturated carboxylic acids. In the preferred embodiments, unsaturated carboxylic acids are fumaric, itaconic, maleic and in particular (meth) acrylic acid.

0010According to the invention, 0.5 to 15% by weight, based on the graft substrate, of water-soluble or at least hydrophilic monomers are used for the grafting, with (meth) acrylamide being preferred. 0.5 to 6% by weight of (meth) acrylamide, based on the polymer substrate, is very particularly preferably grafted on.

0011The graft substrates are produced in the form of aqueous dispersions according to the generally known techniques of emulsion polymerization. These dispersions serve as starting substances for the subsequent grafting reaction, during the implementation of which certain conditions must be observed so that the dispersions according to the invention can be obtained reproducibly.

0012The invention therefore furthermore relates to a process for the preparation of aqueous dispersions with a solids content of 35 to 65% by weight of a graft copolymer from a graft substrate as described above in the form of an aqueous dispersion with a solids content of 35 to 65% by weight, characterized in that 0.5 to 15% by weight, based on the graft substrate, of hydrophilic monomers which can be replaced by up to 60% by weight by hydrophobic monomers, together with about 0.5 to 15% by weight of water, based on the graft substrate, are added so that the starting solid content of the graft substrate is essentially retained after the end of the reaction, and the grafting reaction is carried out by adding a customary polymerization initiator.

0013Suitable polymerization initiators are, for example, inorganic and organic peroxides. In the graft polymerization according to the invention, the graft yield, that is the percentage of the monomer which is chemically bound to the graft substrate, can be between 10 and 95%.

0014The grafting reaction is preferably at 70 to 100 °_C. carried out.

0015As an example of an embodiment, a 52% styrene-butadiene latex is used as the graft substrate, to which 0.9 to 1.7% by weight of acrylamide, based on the graft substrate, is 0.9 to 1.7% by weight at 95.degree. Water, based on the graft substrate, is added in one portion or in batches with stirring. The reaction is started by adding 0.1% ammonium peroxodisulfate dissolved in 1% by weight of water, both based on the graft substrate, and kept at 95 ° C. with stirring until the reaction has ended.

0016The aqueous dispersions according to the present invention allow the most important and most of the required paper coating properties to be optimized at the same time in a manner which leads to better results than in the case of previous co-binder processes. The aqueous dispersions according to the invention are preferably used for the production of paper coating slips, but other areas of use are also conceivable and possible, such as, for example the production of coating compositions for other substrates such as cardboard, cardboard, textiles and leather.

0017The method according to the invention enables both the simultaneous optimization of many properties and the targeted, excellent optimization of special coating properties, such as compressibility, micro-porosity, color absorption, smoothness and binding capacity.

0018By using the aqueous dispersions, ie the modified latices according to the present invention, it is surprisingly found in the production of printing paper with a low basis weight that the latex does not penetrate through the paper, that the fiber coverage is improved and that a homogeneous, uniform coating matrix is ​​formed. This improves the opacity, the degree of whiteness and the smoothness, and enables uniform and rapid ink absorption with the same or improved binding ability achieved. The further advantage of the binder according to the invention is that irreversible thickening is avoided, which can occur with the usual use of, for example, polyvinyl alcohol in coating colors.

0019The latices modified according to the invention bring about more voluminous and uniformly microporous lines than in the prior art. In offset printing, this requires rapid, even ink acceptance, which offers particular advantages in wet-on-wet multi-color printing.

0020For gravure printing, the latices modified according to the invention bring about a high compressibility and smoothness of the coating with reduced calender work. Due to the line structure described, a high rapid ink acceptance and high printing gloss is achieved.

0021As with offset papers, there is also a desire for stiffer papers in gravure printing. Until now, it was not possible to produce stiff papers because the use of latices with hard films resulted in a significant deterioration in the printing properties: uneven porosity, reduced compressibility, poor expression (missing dots). The latices with hard film modified according to the invention make it possible for the first time to print stiff papers using the gravure printing process, while maintaining the properties required for gravure printing.

0022Due to the uniform incorporation of the modified binder in the line matrix, the binder content in the line mass can be reduced due to its high binding capacity, which ultimately leads to a further improvement in the printability properties.

example 1

a) Production of the graft latex

0023Carboxylated styrene-butadiene latex with 60% styrene, 36% butadiene and 4% (meth) acrylic acid and a solids content of 50%, a usual film hardness of 50-60 ° Sh A and high pigment binding capacity as a graft substrate is reacted with:<ul id="ul0001" list-style="none"><li>a) 2% acrylamide = graft latex A</li><li>b) 2% methacrylamide = graft latex B,</li></ul>the percentages refer to graft substrate. The latex properties are listed in Table 1.<tables id="tabl0001" num="0001"><img file="EP0058366A2_D0001.tif" /></tables>

b) Preparation of the coating slip and execution of the coating test

0024To prepare a paper coating slip which is suitable for the production of paper for offset printing, 100 g of china clay are dispersed in about 65% aqueous slurry using 0.2 g of ammonia water and 0.3 g of sodium polyphosphate. 10% (polymer solids, based on clay) of the latices described under a) are added to the homogeneous suspension. With ammonia water, the pH is adjusted to 8.5 after the final solids content of the coating color has been adjusted to 55% with water. The respective composition is shown in Table 2.<tables id="tabl0002" num="0002"><img file="EP0058366A2_D0002.tif" /></tables>

0025The properties of the coating slips are listed in Table 3.

0026After calming and deaeration, the coating slips thus produced are applied to a wood-containing, bleached base paper made from 55% wood pulp and 45% bleached sulfite pulp (coniferous wood), an ash content of 9, using a scraper test coating machine at a speed of 200 m / min. 9% and. a basis weight of 50 g / m², both sides with an application weight of 8-10 g / m². spread on each side (dry).

0027Even after a longer period of time, there is no coagulum or coating on the coating slips with grafted latex, neither on the scraper nor on the counter roller. Likewise, no streaking of the coating slip on the counter roller can be determined. D. The coated papers are then satined over a super calender (2 passes) and checked after air conditioning at 20 ° C and 65% relative humidity.

0028The test results are summarized in Table 4.<tables id="tabl0003" num="0003"><img file="EP0058366A2_D0003.tif" /></tables><tables id="tabl0004" num="0004"><img file="EP0058366A2_D0004.tif" /></tables><tables id="tabl0005" num="0005"><img file="EP0058366A2_D0005.tif" /></tables>

c) evaluation

0029The coated papers from tests 1 and 2, the coating slips of which contained the graft latices according to the invention, show markedly improved coating hardness, opacity, printability properties and high fiber coverage compared to paper coated with conventional, non-grafted latex (test 3). The gloss was also increased somewhat. The pour point of these coating slips was reduced so that an early coat immobilization before the actual drying is possible and the migration of the binder can be suppressed. This also significantly reduced the tendency to "mottling".

0030Of particular note is the significant increase in the picking value.

d) test methods

0031The following test methods were used: (test climate, unless otherwise stated: 20 ° C, 65% relative humidity)

0032pH value: electrometric according to DIN 53 785

0033Solids content: a) for latices: according to DIN 53 189 b) for coating slips: differential weighing after drying at 110 ° C

0034Viscosity: according to DIN 53 788 a) Measuring device: Brookfield viscometer LVT, spindle 3 or 4, 60 rpm testing temperature: 20 ° C b) Measuring device for recording rheograms: Rheomat 15, measuring systems AC, testing temperature: 20 ° C

Pouring point of the coating slips:

0035100 g of the coating slip are poured onto an unglazed clay plate which has dried under constant conditions. As soon as the moisture level has disappeared from the surface of the coating slip, a sample is taken to determine the solids content.

0036The solids content determined in this way denotes the pour point.

Water retention:

003710th cm³ the coating slip is spread with a small squeegee, slot height 0.2 mm, within a few tenths of a second - constant coating speed - over a sample surface of the paper to be coated. The water from the coating slip that penetrates the paper increases the relative humidity on the underside of the paper. This increase in moisture as a function of time is continuously recorded with a hygroscopic moisture sensor. The greater the increase in moisture per time, the poorer the water retention capacity of the coating slip. Humidity sensor: VAISALA (equipment HM 11) measurements in the climatic room (65% of the relative air humidity).

0038Order weight:. Basis weight difference between air-conditioned samples of coated and uncoated paper (according to DIN 53 111).

0039Thickness: Determination according to DIN 53 105

Water intake according to Coob:

0040Determination according to DIN 53 132, 60 sec.

0041Bekk smoothness: Determination according to DIN 53 107

Bendtsen roughness:

0042Determination according to DIN 53 108

Flexural strength according to Schlenker:

0043Determination according to DIN 53 121, bending angle 40 °.

Dynamic arch stiffness:

0044Test method according to Kodak-Pathe, DIN 5-3 123

0045Pick resistance: Determination according to Tappi standard T 499 Su 64 with the IGT device, type AC2, and a compressive stress of 35 kg / cm². The IGT pick test ink of low viscosity serves as the pick color.

0046K + N test: Determination of the color uptake of (ink-coated papers usually uptake) according to the Tappi routine control method RC 19, exposure time: 3 min. The K + N value is the difference in the degree of whiteness between untreated and treated with test ink Paper. Measuring device: Elrepho, filter R 457 test color: K + N-Testing Ink, from K + N-Laboratories Inc., Melrose Park

0047Micro contour test: Test method for assessing the line uniformity, similar to the K + N wipe test, but exposure time only 2 minutes. Test color: blue test color 3811 from Lorilleux Druckfarben GmbH, Ratingen. The assessment is carried out visually using a five-stage rating scale, from grade 1 for an absolutely homogeneous, speck, hole and veil-free test area to grade 5 for a very restless, uneven, defect-rich remaining area.

0048Printing test: Gravure test printing on system Dr. Dürner. The print result was checked visually and evaluated according to the sharpness of the raster reproduction, uniformity of the color pick-up, closed printout, show-through and the like. Three classes were defined for this:<ul id="ul0002" list-style="none"><li>1 = very good,</li><li>2nd = g_ut,</li><li>3rd = bad.</li></ul>

0049Gloss: determination of gloss according to DIN 67 530 against comparison standard at a measuring angle of 60 °, measuring device: long reflectometer

0050Opacity: determination according to DIN 53 146

0051Whiteness: Measurement with Elrepho remission photometer, filter R 457

0052Fiber covering: The coating color is spread on a black cardboard (application weight: 8-10 g / m^e.g. dry) and carefully dried. The whiteness (Elrepho, filter R 457) of the coated cardboard side is then measured. The higher the line covers the black background, the higher it is.

0053A further assessment of the fiber coverage consists in the following method:

0054The coated cardboard surface is scanned with a very narrowly focused beam of a spectrophotometer and the remission is recorded continuously over a length of the cardboard of approximately 10 cm. According to stochastic evaluation, the resulting fluctuations in remission result in a measure of the uniformity of the fiber coverage. Cross section of the illuminating beam 0.2 x 2 mm², Rectangular aperture.

Example 2

a) Production of the graft latex

0055As described in Example 1 under a), a styrene-butadiene latex as in Example 1, with a higher film hardness of approx. 80 ° Sh A, and slightly reduced pigment binding capacity, are grafted with 2% acrylamide graft latex C.

0056The latex properties are listed in Table 5.

0057<tables id="tabl0006" num="0006"><img file="EP0058366A2_D0006.tif" /></tables>Production of the coating slip and execution of the coating test

0058Takes place under the same conditions as described in Example 1 under b). In addition, tests (tests 6 and 7) were carried out with a binder content of 5% under otherwise identical conditions to test the printability in the gravure printing process. The compositions are described in Table 6, the properties of the coating slips in Table 7.

0059In these tests, too, no coagulum or coating on the scraper occurred, even after a long period. There was also no bleeding through of the coating color (coating of the counter roller).

0060The papers coated on both sides were then satined over a supercalender and checked after air conditioning. The coating properties are summarized in Table 8.<tables id="tabl0007" num="0007"><img file="EP0058366A2_D0007.tif" /></tables><tables id="tabl0008" num="0008"><img file="EP0058366A2_D0008.tif" /></tables><tables id="tabl0009" num="0009"><img file="EP0058366A2_D0009.tif" /></tables>

c) evaluation

0061As in Example 1, the experiments in this example, when using latex modified according to the invention, even with a higher film hardness, lead to an improvement in the coating properties and thus to very good printing properties, including gravure printing. In this way it is possible to produce paper with a low basis weight but high sheet stiffness, which is particularly suitable for rotogravure printing in the LWC area.

Example 3

0062A relatively hard styrene-butadiene latex with a film hardness of approx. 95 ° Sh A and a low pigment binding capacity is grafted according to Example 1a

00631.5% acrylamide = graft latex D

0064This example is intended to show the significant increase in pigment binding capacity due to the treatment according to the invention.

0065As in Example 1, a coating test was carried out with Pfropflatex D and the coated paper was tested.

0066The following tables list latex properties, coating color composition and data, and test results.

0067This experiment confirms the possibility of increasing the binding capacity of a paper coating latex by treating this latex according to the invention.<tables id="tabl0010" num="0010"><img file="EP0058366A2_D0010.tif" /></tables><tables id="tabl0011" num="0011"><img file="EP0058366A2_D0011.tif" /></tables><tables id="tabl0012" num="0012"><img file="EP0058366A2_D0012.tif" /></tables><tables id="tabl0013" num="0013"><img file="EP0058366A2_D0013.tif" /></tables>