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 2 450 039) 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-OS2703418 pigment-containing paper coating slips are known which, in addition to a synthetic latex commonly used as a binder, based on polyacrylates, polymethacrylates, styrene-butadiene polymers, or polymers of vinyl acetate in the form of aqueous dispersions as co-binders, are a high molecular weight, water-soluble polymer contain. 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 at night, high film hardness with good binding power at the same time; low tendency to migrate in the pigment matrix, particularly good fiber coverage, good opacity and, in particular with LWC papers (lower basis weight), to achieve less deflection of the printing ink into the paper.

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 of a copolymer based on two or more of the compounds styrene, butadiene, isoprene, (meth) acrylic acid-C1 -C12 alkyl esters and (meth) acrylic acid, on which one or more hydrophilic monomers from the series fumaric acid, ltakonic acid, maleic acid or (meth) acrylic acid amide. Vinylpyrrolidone and ethylene oxide are grafted on, it being possible for up to 60% of the hydrophilic monomers to be replaced by one or more hydrophobic monomers.

0008Suitable hydrophobic monomers are styrene, butadiene, isoprene, (meth) acrylic acid C1-C12 alkyl esters. The graft substrates preferably consist of styrene / butadiene copolymers with 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. 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.

0009According 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, preference being given to (meth) acrylamide. 0.5 to 6% by weight of (meth) acrylamide, based on the polymer substrate, is very particularly preferably grafted on.

0010The 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.

0011The 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 after the end of the reaction the starting solid content of the graft substrate is essentially retained. and carries out the grafting reaction by adding a conventional polymerization initiator.

0012Suitable polymerization initiators are, for. B. 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%.

0013The grafting reaction is preferably carried out at 70 to 100 ° C.

0014As 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, with 0.9 to 1.7% by weight of water at 95 ° C. based on the graft substrate, 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.

0015The 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 application are also conceivable and possible, such as, for. B. the production of coating compositions for other substrates such as cardboard, cardboard, textiles and leather.

0016The 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.

0017By 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 can be avoided, which with the usual use of z. B. polyvinyl alcohol can occur in coating colors.

0018The 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.

0019For 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.

0020As 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). According to the invention, 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.

0021Due 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

0022Carboxylated 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="EP0058366B1_D0001.tif" /></tables>

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

0023To 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="EP0058366B1_D0002.tif" /></tables>

0024The properties of the coating slips are listed in Table 3.

0025After calming and deaeration, the coating slips thus produced are applied using a scraper test coating machine. at a speed of 200 m / min .. on a wood-containing, bleached base paper made of 55% wood pulp and 45% bleached sulfite pulp (softwood). an ash content of 9.9% and a basis weight of 50 g / m2. on both sides with an application weight of 8-10 g / m² spread on each side (dry).

0026Even 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. The coated papers are then satined over a super calender (2 passes) and checked after air conditioning at 20 ° C and 65% relative humidity.

0027The test results are summarized in Table 4.<tables id="tabl0003" num="0003"><img file="EP0058366B1_D0003.tif" /></tables><tables id="tabl0004" num="0004"><img file="EP0058366B1_D0004.tif" /></tables>

c) evaluation

0028The 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 towards “mottling”.

0029Of particular note is the significant increase in the picking value.

d) test methods

0030The following test methods were used: (test climate, unless stated otherwise: 20 ° C, 65% relative humidity).<ul id="ul0002" list-style="none"><li>PH value :<ul id="ul0003" list-style="none"><li>electrometric according to DIN 53 785</li></ul></li><li>Solids content:<ul id="ul0004" list-style="none"><li>a) for latices: according to DIN 53 189</li><li>b) for coating slips: differential weighing after drying at 110 ° C</li></ul></li><li>Viscosity:<ul id="ul0005" list-style="none"><li>according to DIN 53 788</li></ul><ul id="ul0006" list-style="none"><li>a) Measuring device: Brookfield viscometer LVT, spindle 3 or 4, 60 rpm<ul id="ul0007" list-style="none"><li>Test temperature: 20 ° C</li></ul></li><li>b) Measuring device for recording rheograms: Rheomat 15, measuring systems AC, test temperature: 20 ° C</li></ul></li></ul>

0031Pouring point of the coating slips:<ul id="ul0008" list-style="none"><li>100 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.</li></ul>

0032The solids content determined in this way denotes the pour point.

0033Water retention:<ul id="ul0009" list-style="none"><li>10th 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 test 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).</li></ul>

0034Order weight:<ul id="ul0010" list-style="none"><li>Basis weight difference between air-conditioned samples of coated and uncoated paper (according to DIN 53 111).</li></ul>

0035Thickness:<ul id="ul0011" list-style="none"><li>Determination according to DIN 53105</li></ul>

0036Water intake according to Coob:<ul id="ul0012" list-style="none"><li>Determination according to DIN 53 132, 60 sec.</li></ul>

0037Smoothness according to Bekk:<ul id="ul0013" list-style="none"><li>Determination according to DIN 53 107</li></ul>

0038Bendtsen roughness:<ul id="ul0014" list-style="none"><li>Determination according to DIN 53 108</li></ul>

0039Flexural strength according to Schlenker:<ul id="ul0015" list-style="none"><li>Determination according to DIN 53 121, bending angle 40 °.</li></ul>

0040Dynamic bow stiffness:<ul id="ul0016" list-style="none"><li>Test method according to Kodak-Pathe, DIN 53 123</li></ul>

0041Pick resistance:<ul id="ul0017" list-style="none"><li>Determination according to Tappi standard T 499 Su 64 with the IGT device, type AC2, and a compressive stress of 35 kg / cm^2. The IGT pick test ink of low viscosity serves as the pick color.</li></ul>K + N test: (ink recording)

0042Determination of the color absorption of coated papers usually according to the Tappi routine control method RC 19, exposure time: 3 min.

0043The K + N value is the difference in the degree of whiteness between untreated and test color treated paper. Measuring device: Elrepho, filter R 457 Test color: K + N-Testing Ink. From K + N-Laboratories Inc., Melrose Park

0044Micro contour test:<ul id="ul0018" list-style="none"><li>Test method for assessing line uniformity, similar to K + N wipe test. Exposure time only 2 minutes, test color: blue test color 3811 from Lorilleux Druckfarben GmbH, Ratingen. The assessment is carried out visually on 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, faulty rest area.</li></ul>

0045Pressure test:<ul id="ul0019" list-style="none"><li>Gravure trial printing on the trial printing machine System Dr. Dürner. The print result was checked visually and rated according to the sharpness of the raster reproduction, uniformity of the color pick-up, closed printout, printouts and the like. 3 classes were defined:<ul id="ul0020" list-style="none"><li>1 = very ut,</li><li>2nd = good,</li><li>3rd = bad.</li></ul></li></ul>

0046Shine :<ul id="ul0021" list-style="none"><li>Determination of gloss according to DIN 67530 against comparison standard at a measuring angle of 60 °, measuring device: long reflectometer.</li></ul>

0047Opacity:<ul id="ul0022" list-style="none"><li>Determination according to DIN 53 146</li></ul>

0048Whiteness:<ul id="ul0023" list-style="none"><li>Measurement with remission photometer</li><li>Elrepho, filter R 457</li></ul>

0049Fiber coverage:<ul id="ul0024" list-style="none"><li>The coating color is spread on a black cardboard (application weight: 8-10 g_/m² 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.</li></ul>

0050A further assessment of the fiber coverage consists in the following method:<ul id="ul0025" list-style="none"><li>The 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.</li></ul>

Example 2

a) Production of the graft latex

0051As described in Example 1 under a), a styrene-butadiene latex as in Example 1, with a higher film hardness of approximately 80 ° Sh A, and somewhat reduced pigment binding capacity<ul id="ul0026" list-style="none"><li>2nd % Acrylamide = graft latex C</li></ul>

grafted on.

0052The latex properties are listed in Table 5.<tables id="tabl0005" num="0005"><img file="EP0058366B1_D0005.tif" /></tables>

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

0053Takes 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.

0054In 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).

0055The 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="tabl0006" num="0006"><img file="EP0058366B1_D0006.tif" /></tables><tables id="tabl0007" num="0007"><img file="EP0058366B1_D0007.tif" /></tables><tables id="tabl0008" num="0008"><img file="EP0058366B1_D0008.tif" /></tables>

c) evaluation

0056As 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 gravure printing in the LWC range.

Example 3

0057A relatively hard styrene-butadiene latex with a film hardness of approximately 95 ° Sh A and low pigment binding capacity is grafted according to Example 1a with 1.5% acrylamide = graft latex D.

0058This example is intended to show the significant increase in pigment binding capacity due to the treatment according to the invention.

0059As in Example 1, a coating test was carried out with Pfropflatex D and the coated paper was tested.

0060The following tables list latex properties, coating color composition and data, and test results.

0061This experiment confirms the possibility of increasing the binding capacity of a paper coating latex by treating this latex according to the invention.<tables id="tabl0009" num="0009"><img file="EP0058366B1_D0009.tif" /></tables><tables id="tabl0010" num="0010"><img file="EP0058366B1_D0010.tif" /></tables><tables id="tabl0011" num="0011"><img file="EP0058366B1_D0011.tif" /></tables><tables id="tabl0012" num="0012"><img file="EP0058366B1_D0012.tif" /></tables>