FI67568C - GIPSPIGMENT ELLER FILMEDEL - Google Patents

Description

1 67568

Gypsum pigment or filler

The invention relates to a new type of gypsum pigment or filler having an average particle size of 0.5 to 5.

Of the coating pigments currently in use, kaolin is plate-like. Its average form factor, i.e. the ratio of the largest dimension to the smallest, is about 20-30. Talc is even more plate-like. When using such coating pigments, it is relatively easy to produce glossy coatings, which, however, have the disadvantage of too low ink absorption and often also poor brightness or opacity, as well as a tendency to bliste-ring phenomenon during the ink drying step. To remedy these shortcomings, small amounts of calcium carbonate are also often added to the glossy paper coating. However, the addition of carbonate lowers the gloss and adversely affects the stability of the coating paste. Carbonate also causes difficulties in the water system of a wood-containing paper mill, calcium carbonate reacts consuming aluminum sulphate, whereby fluctuations in the amounts of waste paper affect the pH of the water system.

Matte paper can in principle be produced by coating with the same kaolin-containing paste as glossy paper, but by treating in calendering so lightly that the desired smoothness is achieved, but not gloss. In such a paper, better brightness is desired. Interfering marks are also easily generated when handling paper.

Wet-ground calcium carbonate-based matt coating pigments having an average form factor of 1-3 are suitable for wood-free paper grades. When used in acidic papermaking conditions, the above problems arise.

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Dry-ground gypsum pigment for paper coating is known from FI application 750387. In such a gypsum pigment, the form factor varies between about 2-6 and higher dry matter contents can be achieved than with the gypsum pigment grades mentioned below. Such a pigment is well suited for the production of matte paper. It has not been possible to use it on glossy paper grades, since more than 40% of the amount of pigment in the gypsum coating results in a clear reduction in gloss and less than 40% in the paste containing kaolin is very difficult to stabilize. This is probably due to the finest fraction of pigment grade, with a particle size of less than 0.3, which is necessarily formed during dry milling and which cannot be removed due to flocculation. The production of a sufficiently fine pigment by dry grinding is also difficult because the gypsum easily loses its crystal water under the influence of the grinding energy, becoming a reactive hemihydrate.

U.S. Pat. No. 4,310,360 and SE Patent Application No. 8200586 disclose methods for preparing gypsum pigment by a wet milling process, for example from process gypsum to be crystallized in connection with the production of phosphoric acid. In these processes, milling is performed using 0.5-3.0% carboxymethylcellulose or combinations of carboxymethylcellulose and certain other polymers as milling aids in mills filled with microfill bodies of about 0.5-4 mm in diameter.

The pigment obtained by these methods has a form factor of about 15 or more. It is quite interesting in some of its features. Due to its plate-like particle shape, it is suitable for replacing kaolin in glossy paper grades if it is ground to a sufficiently fine, at least 70% less than 2 [mu] m, preferably at least 80% less than 2 [mu] m fineness. The particle size distribution of the pigment then becomes gentle. It has been found that such a plate-like gypsum pigment resulting from high shear grinding is difficult to mix with 3,67568 other pigments such as kaolin or talc due to its high specific surface area. Thus, for example, the preparation of a stable coating paste with kaolin in a pigment ratio of 75:25 (kaolin first mentioned) is almost impossible. In addition, the production of such a pigment is very difficult technically due to the high viscosity and thixotropic nature of the pigment slurry. At the same time, the high specific surface area results in a very high need for dispersants of the polyphosphate or polyacrylate type in addition to CMC in order for the pastes to be stable.

The coating pigment prepared according to the known method for low-quality papers is easier to apply in that the pigment is coarser and therefore easier to handle. Thus, it can be used, for example, in applications requiring good surface strength and wet abrasion resistance compared to conventional pigments with better results. However, miscibility with kaolin is not at the desired level. Another shortcoming is the relatively low ink absorption of the coating, which would be desired in some applications. The printability by the gravure printing method is also not good enough. This method cannot produce a gypsum pigment that would provide as good smoothness and uniformity to matte paper as is available with new types of calcium carbonate based coating pigment grades.

Spanish Patent 498334 discloses a process for processing natural gypsum into a paper coating pigment by a first step of dispersing the gypsum (0.5-100 μm) with anionic or nonionic surfactants of the dicarboxylic acid ester derivative type or a polyoxyethylated fatty alcohol ether or cellulose ether , in the second stage, grinding of the gypsum slurry thus obtained in a mill equipped with grinding pieces with an average diameter of 1.4-1.8 mm to a particle size of about 0.5-1.0 μm and in the third stage representing a plate-like or needle-like particle form, 8-68% 4 67568

Addition of a finely divided micropigment containing Al 2 O 3 to the gypsum pigment. This patent mentions in general terms the amount of additive in the dispersion of gypsum from 0.2 to 5% of the gypsum and the dry matter content from 40 to 80%.

Spanish Patent 498334 does not provide detailed information on usable chemicals, their dosage or dry matter content in the form of working examples. However, since the usefulness of the gypsum pigment must be ensured by the addition of a micropigment, it is obvious that the process has operated in a low viscosity range, giving the pigment a round shape, a form factor of about 1-4, and at the same time a steep particle size distribution. To correct the theology and coating properties of such a pigment, the addition of a micropigment is necessary.

FI patent application 803245 discloses a method for preparing a coating pigment slurry from calcium carbonate with a very high dry matter content, up to 80%. According to the publication, the result is obtained by grinding calcium carbonate at a low dry matter content without dispersants. The resulting pigment slurry with a steep particle size distribution is filtered and then reslurried with dispersants.

There are several publications on the effect of particle size distribution and particle size distribution of pigments and fillers on the packability of pigments and further on the properties of products (e.g. paper coatings). To correct for the particle size distribution, the excess particle size fraction is reclassified or the missing fraction is added. The compressibility is often changed in the desired direction also by adding a variety of pigments of various kinds.

In gypsum pigment fabrication experiments, it was found that a lower particle size distribution was obtained at a low dry matter content than when supplied in accordance with the aforementioned U.S. Patent 4,310,360 and SE Patent Application No. 5,67568,8200,586. On the other hand, without the grinding aids, the dry matter content of the ground, filtered and redispersed gypsum pigment was difficult to obtain even at the same level as with the said known methods.

It was now found, quite surprisingly, that when the dry matter content of gypsum in the slurry was adjusted by 2 to 22% units higher with dispersants than was found to be useful without dispersants, a new quality of gypsum pigment was obtained. there are no scaly pigment particles below about 0.3 microns as are known in the pigments of U.S. Pat. No. 4,310,360 and SE Patent Application No. 8200586. The length to thickness ratio of the new pigment grade particles is lower than that obtained by the above methods (about 15), but greater than an average of 2-3, which is approximately the low viscosity without excipients or the ratio obtained according to the method of ES patent 498334. The particle size distribution of the new pigment grade is between those obtained by known methods.

The invention will be described in more detail below with reference to the accompanying drawings, in which

Figure 1A shows a prior art sheet-like gypsum pigment in which the ratio of the length of the particles to the thickness is about 15-20. Figure IB shows a kaolin pigment. Figure 1C shows a gypsum pigment made at a low dry matter content without grinding aids, with a particle shape factor of about 1-3 and a steep particle size distribution. Figure ID shows a wet ground calcium carbonate pigment. Fig. IE shows a gypsum pigment obtained by dry grinding and Fig. 1F shows a gypsum pigment according to the invention. Figures 1A-F are 3000x magnification.

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Figure 2A shows a gypsum pigment according to the invention having a form factor of about 3-4 and Figure 2B a corresponding pigment having a form factor of about 4-6. Both are ground using 0.8% anionic surfactant and dispersing air in about 1-10% of the slurry volume. The dry matter content in Fig. 2A was 69% and the temperature was 70 ° C and in the case of 2B 72% and the temperature was 45 ° C. Fig. 2C shows wet powdered calcium carbonate and Fig. 2D a gypsum pigment according to the invention using 0.6% anionic surfactant and 0 , 2% carboxymethylcellulose to increase viscosity.

In Fig. 3, curve A shows the particle size distribution of a gypsum pigment ground without additives in a low dry matter content and curve B according to SE patent application 8200586 ground to the same average particle size. When the grinding takes place according to the invention to the value of the shape factor 3-15, an optimal particle size distribution between curves A and B is obtained.

The ratio of the length to the thickness of the new pigment grade particles thus varies on average between 3 and 15 and is preferably 4-6. The formation of such a pigment requires an optimal ratio of crystal cutting and breaking forces in the refiner.

When producing a fine pigment for glossy paper grades, one works closer to the lower limit of the given dry matter content and closer to the upper limit in the production of coarser matte pigments and fillers in order to keep the shape of the pigment similar.

Grinding aids are used in the production of the new pigment. Different types of dispersants or surfactants are suitable for this purpose. However, these affect the dry matter content at which the optimal result is achieved.

Il 7 67568 When surfactant grinding aids are used, it has surprisingly been found that the method according to the invention only works when the additive or combination of additives is used to disperse the air in the gypsum slurry to be ground in fine bubbles. The conventionally calculated dry matter content is high in this case, about 67-75%. In the lower part of this dry matter range, the viscosity still falls below the optimum. In order to purchase the viscosity, it is then advantageous to lower the temperature of the sludge to be ground or to add, for example, low or medium-viscosity carboxymethylcellulose as a thickener, depending on the dry matter content used. The same result is also achieved at a lower dry matter content by using more thickener.

The fact that the gypsum pigment grade according to the invention has not previously been produced is probably due to the fact that a commercial grade coating pigment has generally been supplied with a dry matter content of at least 67% required for transportability and a high dry matter coating paste or, if possible, a low dry matter content has been tested. so much so that post-treatment of the pigment by filtration is possible without different process steps. The gypsum pigment according to the invention is obtained using conventional dispersants at a dry matter content of less than 67%, but so thick and viscous that it should be diluted or post-dispersed before the removal of excess water by filtration or centrifugation can take place effortlessly. When surfacing with surfactants, it has not previously been realized in gypsum grinding at high dry matter content to pay attention to the possibility of improving the flowability of gypsum slurry by dispersing air bubbles in the slurry so that its volume increases by about 1-10%.

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The invention is described in more detail below by way of examples.

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The synthetic gypsum used in the examples is derived from the production of phosphoric acid by the wet process. Before the experiments, the fairy-tale bicrystalline gypsum has been purified either by water washing alone or by classifying it as the finest fraction. The dispersion, grinding and screening of the ground gypsum have been performed with conventional equipment, such as those described in e.g. U.S. Patent No. 4,310,360.

Example 1 (Comparative experiment)

Application of the technique known from the grinding of calcium carbonate to gypsum:

Experiments were performed to grind synthetic gypsum according to the method described in FI application 803245 with the grinder described therein. It proved impossible to carry out grinding up to a particle size of about 70-85% below 2 [mu] m, so that the dry matter content of the slurry could have been made higher than 50-55% without dispersants. In addition, the mills used in the experiments were so exposed to stresses during grinding that production-scale grinding would not be possible. The ground pigment sludge solidified on entering the mill and was exceptionally dark in color. Thus, under these conditions, heavy mill wear also occurred.

Example 2 (Comparative experiment)

The gypsum was ground without grinding aids in a dry weight range of 45-50% suitable for continuous operation of the mill. Excess water was removed from the samples with a pressure filter. The solids content of the filter cakes was around 70% at best and the filter cakes were solid. When dispersing the filter cakes in water with substances found to be good in dispersing gypsum (CMC + polyacrylate + polyphosphate), approximately the same dry matter content (67-70%) was achieved as supplied under the previously described U.S. patent. Thus, the method does not improve the dry matter content of the gypsum slurry. The particle size distribution of the obtained gypsum pigment was found to be steep, and the ratio of the length of the particles 9 to the thickness was on average about 1-3. 67568

Example 3 (Comparative experiment)

The gypsum pigment according to Example 2 was further subjected to paper coating tests on a pilot machine by the blade method. About 68% of the dry matter content was ground using the dispersants described in SE patent application 8200586 as reference pigments. These gave coating pastes containing 10 parts of styrene-butadiene latex and 1.5 parts of CMC, which could be run at a dry matter content of 2-3%. Using control pastes, the papers also had a better gloss than the gypsum pigment prepared according to Example 1. The experiment shows that when dispersed without dispersants, gypsum does not yield pastable pastes at elevated dry matter content.

Example 4

Gypsum pigment was prepared under various conditions, with the shape factor of the pigment varying from about 1 to 20. The samples were diluted into pumpable slurries which were centrifuged as dry as possible (20 min at a circumferential speed of 2990 m / min). The results are shown in Table 1.

table 1

Conditions for pigment Particle length In dry grinding of the precipitate, the ratio to thickness is the content of the substance,% approx. 50% of the dry matter approx. 1-3 66 content (KAP), no excipients 72% KAP, anionic 2-4 84 surfactant 0.8% 67568 I .

65% KAP, anionic surfactant 1%, CMC 1% 2-10 84 Pigment according to U.S. Pat. No. 4,310,360 approx. 20 78

The experiment shows that grinding according to the invention under conditions in which the dry matter content and viscosity are optimal gives a pigment which is more compact in terms of dewatering. The process of the invention can be utilized in the preparation of commercially available pigment sludge from a slurry or dry pigment with a dry matter content of less than 67%. In pilot-scale experiments, a decanter-type centrifuge or pressure filter has proven to be useful water separation devices.

Example 5 (Comparative experiment)

Purified phosphogypsum was ground according to U.S. Patent 4,310,360 at 68% dry matter content to about 40% less than 2 microns in particle size using 1.5% low viscosity CMC by weight of dry gypsum as a grinding aid. The length to width ratio of the particles obtained ranged from 15 to 20. The pigment was tested for coating special board. Important product properties of board quality include e.g. matte surface, good washability and gravure printability. In the coating paste, one half of the kaolin normally used was replaced with gypsum. Latex was used as a binder in 21% of the Pigment. From comparable samples, e.g. the following values (Table 2).

Example 6

Purified phosphogypsum was ground according to U.S. Patent 4,310,360 to a particle size of less than about 2 microns at a dry matter content of 58% using 1.5% low viscosity carboxymethylcellulose as the drying aid in the amount of dry gypsum. A pigment was obtained in which the ratio of the longest dimension of the plate-like particles to the shortest was on average about 15. The pigment was tested in a coating paste in which half of the pigment was kaolin and the binder content was 21%. The pigment according to the invention was prepared in a dry matter content of 58% by grinding to the same degree of fineness using 0.25% sodium hexametaphosphate from the amount of gypsum as a grinding aid. From this pigment, the ratio of the length of the particles to the thickness averaging about 2-6, a similar paste was prepared as from the previous one. Kaolin was added to the gypsum slurry dry to achieve the dry matter content of the paste required for blade coating. The comparison paste was also a paste containing only kaolin.

The coating test was performed on a pilot machine for special board, the important product properties of which are matte, gravure and wet abrasion strength. The results shown in Table 2 were obtained.

Table 2

Change compared to kaolin comparison,%

Feature The pigment according to the invention of U.S. Pat. No. 4,310,360

Brightness +0 +0.2

Opacity -1.8 +0.9

Ink absorption, K & N -21 +128 Oil absorption time, Patra +30 -91

Sileys, Fogra -5.8 -25

Gravure printing +10 +50

Gloss +0 -16

Surface strength, IGT +59 +43

Washing test slight improvement significant improvement

Example 7

In the coating of the board product according to Example 6, gypsum pigment 12,67568 made by the method of SE patent application 8200586 (particle length to thickness ratio approx. 20) and according to the invention (said ratio averaging about 3-4) were prepared at 69% dry matter content using 0.85 as excipient. % anionic surfactant (Na-dioctyl sulfosuccinate). The gypsum was dispersed before grinding until it was found that the gypsum sieve fluffed and its volume increased by about 1-10%. Using both pigments, pastes containing 75 parts of gypsum and 25 parts of kaolin and 20 parts of latex were made for a coating test. The results according to Table 3 were obtained.

Table 3

Property Change with the pigment according to the invention relative to the pigment according to SE patent application 8200586,%

Brightness +1.9

Opacity +3.2 K&N +59 Oil absorption time -64

Sileys, Fogra +29

Gravure printability +28

Gloss -10

Wash test slight improvement

Example 8

Gypsum and talc blend pastes were made using a commercial coating talc slurry, gypsum ground at a high dry matter content (67%) with 1.2% CMC, post-dispersed with 0.4% polyacrylate, and ground at low pH without excipients, then filtered and finally 67 to a dry matter content of 1.2% CMC and 0.4% polyacrylate-dispersed gypsum. The ratio of the length of the former gypsum particles to the thickness was over 15, the latter about 2-3. 80:20 (talc: gypsum) pastes were made using 10% latex and a further 1.2% low viscosity

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i3 67568 CMC based on the total amount of pigments. When using gypsum ground at a high dry matter content, an additional 0.3% of polyacrylate dispersant had to be added to the paste.

The viscosity (Brookfield) was 1520 mP. The pasta was stable for a couple of days in a 45 C oven. A similar paste was made from ground dry gypsum. The additional need for polyacrylate was only 0.1%. The viscosity of the paste was 1260 mP at the same dry matter content. The paste was stable at 45 ° C after a further 4 months.

The experiment shows that the miscibility of the gypsum pigment with talc improves as the shape factor of the pigment decreases and the particle size distribution steepens. The same applies to miscibility with pigments other than talc. The pigments according to the invention behave in terms of stability between the extreme cases of this example.

Example 9

Gypsum was dispersed in water at a dry matter content of about 72% using Na-dioctyl sulfosuccinate as an excipient in 1% of the amount of gypsum so that air was dispersed in the feed in fine bubbles. The slurry was then ground to a fineness of coating pigment and dried either as such or centrifuged to a dry matter content of more than 80%. After crushing the formed lumps, the dried samples could be easily dispersed to the original particle size. For comparison, the procedure was similar for gypsum pigments prepared according to U.S. Patent 4,310,360 and SE Patent Application 8200586. When these were dried, crushed and redispersed into aqueous slurries with a Diaf disperser, it proved impossible to achieve the original pigment fineness.

By particle size is meant herein the particle size below which 50% by weight of the pigment remains. The average shape factor is the ratio of the longest dimension (= length) to the shortest (= thickness) of the 14 67568 particles representing the most common particle shape of the sample, viewed from a microscopic image taken at 10,000 times magnification. Due to the roughness of the latter method, the order of magnitude of the shape factor is usually indicated, in which case, for example, the particle shape range 3-4 means more particles of the same shape than e.g. 3-6.

Claims (6)

15 67568 Gypsum pigment or filler having an average particle size of between 0.5 and 5, characterized in that it has a substantially total particle size range of more than 0.3 and the ratio of the longest dimension of the particles to the shortest is on average 3 to 15, preferably 4-6. A process for preparing a pigment according to claim 1, characterized in that the aqueous gypsum slurry is ground to a particle size of 0.5 to 5 μm in the presence of dispersants at a substantially higher dry matter content than is found useful in a grinding test without dispersants. Process according to Claim 2, characterized in that the grinding is carried out at a dry matter content 3 to 22% higher than that found to be useful without dispersants. Process according to Claim 1 or 2, characterized in that so much air is dispersed in the gypsum slurry containing surfactants with a dry matter content of 67 to 75% before grinding that the volume of the slurry increases by about 1 to 10%. . Process according to one of Claims 2 to 4, characterized in that the dry matter content of the slurry is increased after grinding by centrifugation or filtration, preferably by pressure filtration, and the filter cake containing more than 67 dry matter obtained by filtration or centrifugation is redispersed in a slurry form using a gypsum dispersant. structure. Process according to one of Claims 2 to 4, characterized in that the dry matter content of the slurry is increased after grinding by centrifugation or filtration, and the filter cake containing more than 75% dry matter is dried.