FI126543B - A process for the preparation of an aqueous pigment-containing cationic high solids dispersion, an aqueous pigment dispersion and its use - Google Patents

Description

A process for the preparation of an aqueous pigment-containing cationic high solids dispersion, an aqueous pigment dispersion and its use

The invention relates to the preparation of a cationic pigment, in particular to high aqueous solids dispersions. In particular, the present invention relates to a process according to the preamble of claim 1 for the preparation of an aqueous dispersion for example for the production of paper or board.

The invention also relates to an aqueous pigment-containing dispersion according to the preamble of claim 21 and to its use.

Technology background

The pigments used as fillers for paper and board are added as an aqueous dispersion to the pulp stock in the machine headbox. In most cases, good retention of pigments (attachment to pulp fibers) requires the use of retention agents. If the pigment used is Precipitated calcium carbonate (PCC) produced in the vicinity of a paper or board mill and obtained as a slurry of about 20-25% solids from the pigment manufacturing process, a separate retention agent may not be necessary since the surface charge of the PCC is attaches well to the anionic fiber network.

The transportation of dilute PCC sludge, ie about 20-25% solids, over longer distances cannot be considered for cost reasons. If the PCC application is located far from its manufacturing site, the PCC slurry is typically concentrated, for example, by centrifugation to a higher solids content, then the slurry is dispersed with an anionic dispersant and the resulting high solids aqueous dispersion is transported to the application. Alternatively, the dispersion may be dried to a dry anionic pigment. When the dispersion or aqueous slurry of the dry anionic pigment is added to the stock, a cationic retention agent must also be added.

Also, when ground calcium carbonate (GCC) is used, this will usually be delivered to the application in the form of a high solids dispersion with an anionic dispersant.

Alternatively, GCC is supplied to the application as an anionic dispersant stabilized pigment. Thus, the use of GCC also requires a cationic retention agent.

Purpose and Summary of the Invention

It is an object of the present invention to provide a stable, high solids aqueous dispersion of pigments for use in the paper and board industry, having a cationic surface of the pigment used as a filler and having good retention on negatively charged pulp fibers without a separate retention agent.

In particular, it is intended to provide a dispersion having such a good stability that no pigment precipitation occurs over a shelf life of several weeks, even though the viscosity of the dispersion is relatively low.

A further object is to provide a dispersion which when used for surface sizing of paper gives the paper good printability.

It is further intended to provide a dispersion which when used in paper coating gives the paper good optical properties, good color densities and rapid drying of the ink.

The invention is based on the idea that an aqueous dispersion of pigments can be prepared by a process in which either a dry pigment or an aqueous pigment slurry or paste having a pigment content of about 30-75% by weight is dispersed with a dispersant and an optional additive to form a stable dispersion. According to the invention, cationic modified starch is used as a dispersant. For example, water may be used as an additive.

The invention provides an aqueous high solids dispersion comprising pigments and a dispersant, characterized in that the dispersant is a cationic modified starch.

The new dispersion can be used as a filler for paper or board, where necessary, the dispersion is diluted with water and introduced into the headbox of a paper or board machine.

The dispersion can also be used for surface sizing of paper or board, whereby the dispersion is mixed with cationic starch suitable for conventional surface sizing, and the mixture is applied to a dried web.

A third use is for coating paper or board, whereby the dispersion is mixed with a coating paste which is applied to the dried web. More specifically, the method according to the invention is essentially characterized by what is stated in the characterizing part of claim 1.

The aqueous dispersion according to the invention is characterized in what is stated in the characterizing part of claim 21.

The uses according to the invention are characterized by what is claimed in claims 34-37.

The invention provides significant advantages. Thus, as shown in the results below, the use of cationic starch as a dispersing chemical provides excellent stability and flow properties. The results also show that when cationic pigments are used in combination with cationic surface starch, inkjet printability can be improved with respect to e.g. print sharpness. The dispersed dry matter has excellent opacity when dry.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 schematically illustrates alternative ways of dispersing pigments according to the method of the invention.

Figure 2 shows the imprint of inkjet printing on surface glued paper with different surface glues.

Description of Application Examples

As described above, in the new technology for preparing an aqueous dispersion of pigments, either a dry pigment or an aqueous pigment slurry or paste having a pigment content of about 30-75% by weight is dispersed with a dispersant and optionally an additive to form a stable, high solids aqueous dispersion. The cationic biopolymer is used as the dispersant. .

Preferably, the pigment to be dispersed is in the form of a dry pigment, an aqueous pigment slurry, or an aqueous pigment paste.

In one embodiment, a pigment is introduced for dispersion, the charge state of which is not converted to an anion between pigment preparation and dispersion. This makes it possible to completely avoid the use of discrete anionic chemicals, such as polyacrylates.

In many applications, the pigment should have a good dry opacity.

In one embodiment, which fulfills the above dry condition requirement, the calcium carbonate pigment has a surface area of about 5-100 m2 / g, particularly about 10-50 m2 / g. Porous calcium carbonate materials suitable for this as well as other applications include, for example, those consisting of a calcium carbonate material whose particles are composed of calcium carbonate agglomerates or clusters. These are described, for example, in EP 0 944 551, whereby precipitated calcium carbonate crystals (PCC crystals) are in the form of multicore, typically regular, usually spherical clusters containing a plurality of interconnected, in particular spherical, calcium carbonate particles having a particle size of about 40-400 nm, typically about 40-200 nm.

The clusters disclosed in that patent have abundant internal reflecting surfaces which improve the opacity of the pigment and filler.

The pigment can be used as such, but it is of course possible to use structured pigments, i.e. calcium carbonate combined with light scattering pigments. This increases the diffused reflection of the light, increasing the likelihood of the light beam colliding with the reflected surface. EP 0 944 551 also discloses a process for the preparation of porous pigments, in this case calcium carbonate clusters. The publication is incorporated herein by reference.

The cationic biopolymer used in the preparation is preferably a cationic modified, substituted starch or a cationic sorbitol or maltodextrin.

As a specific example, suitable modified starch comprises cationic nitrogen-containing substituents such as amine or amide groups.

The modified starch has a nitrogen content of, for example, about 0.1 to 2% by weight of its dry weight.

In a particularly preferred embodiment, the degree of substitution of the modified starch is 0.1 to 0.2, especially about 0.16.

The technology can be applied to various pigments, especially mineral pigments. Examples include calcium carbonate such as Precipitated Calcium Carbonate (PCC) or Ground Calcium Carbonate (GCC); talc; plaster; titanium Dioxide; and kaolin.

Particularly preferably, the present solution is applied to PCC, especially to PCC that has been dried after the PCC manufacturing process.

In another preferred embodiment, the solution is applied to a PCC aqueous slurry or paste obtained by filtering dilute PCC aqueous slurry from the PCC manufacturing process.

In a third embodiment, the pigment is dry PCC or GCC or an aqueous slurry or paste of PCC or GCC having a pigment content of 30-60% by weight and the dry pigment or pigment slurry or paste is stabilized with an anionic dispersant.

In one embodiment, the aqueous dispersion has a pigment content of about 30-75% by weight, preferably 50-60% by weight, most preferably about 57% by weight, and - the dispersant in the aqueous dispersion has a concentration of 0.2-5.0% by weight, preferably 0.5-3% , 0% by weight, most preferably 0.7-1.5% by weight, especially about 0.7% by weight.

It is possible for the dry pigment or the aqueous pigment slurry or paste, the dispersant and, if necessary, the water to be introduced into a dispersing device, which is typically a disintegrating device operating on a back-mill principle.

The resulting dispersion can be used as a filler for paper or cardboard, where necessary, the dispersion is diluted with water and introduced into the headbox of a paper or board machine.

The dispersion can also be used as a filler for paper or board without the use of a separate retention agent.

In one embodiment, the dispersion is used for surface sizing of paper or board, whereby the dispersion is mixed with cationic starch suitable for conventional surface sizing, and the mixture is applied to a dried web.

The dispersion can also be mixed with a coating paste which is applied to the dried web.

The pigment dispersion can be carried out in various ways, for example, according to the schematic process shown in Figure 1.

As indicated above, the aqueous dispersion is prepared without the addition of a polyelectrolyte such as polyacrylate before, during or after dispersion. Thus, in one embodiment, a dispersion having a substantially polyelectrolyte-free aqueous phase is prepared. For example, the polyacrylate content is less than 0.1%, preferably less than 0.01%, especially less than 0.001%, of the dry content of the dispersion.

In a preferred embodiment, the aqueous dispersion obtained contains from about 0.1% to about 2.0% by weight, particularly from about 0.5% to about 0.99% by weight, of a cationic starch derivative on a dry weight basis.

For example, an aqueous dispersion having a Z potential at pH 9.5 of at least +1 mY in 20% solids is prepared using the new technology.

Particularly preferred embodiments are considered with reference to the drawings.

Preparation of dry cationic pigment and preparation of cationic pigment dispersion as a two step process

A slurry of 24% solids from pigment (PCC) manufacture is fed through a conduit 11 to a centrifuge 1 for dewatering. A centrifugal PCC slurry having a solids content of about 50% is removed from the centrifuge. The paste is fed along the tube 12 to the dryer 4, and the product 5 leaving the dryer 13 through the tube 13 is about 100% dry cationic pigment (PCC) which can be delivered as such to the application. At the application site or at the pigment manufacturer, the dry cationic pigment can be slurried in the desired amount of water, supplied with a dispersant, and dispersed. The product provides a cationic PCC dispersion with a solids content typically of about 50%.

Dispersion of the anionic pigment in water with cationic modified starch into a cationic pigment dispersion PCC slurry having a solids content of 24% is fed to a centrifuge 1 for dewatering. The PCC slurry removed from the centrifuge via conduit 16 typically contains about 50% solids and is conducted to a dispersant 3 to which an anionic dispersant is added.

The dispersed slurry is led by tube 17 to a dryer 4 where it is dried. The product obtained, the dried anionic pigment (PCC), is a commercial product known per se.

For example, the dry anionic pigment prepared in a manner known per se can be converted, for example, either at the point of use of the pigment or at the pigment manufacturer, into an aqueous dispersion of a high solid cationic pigment. The dry anionic pigment is slurried in the desired amount of water, a dispersant is added thereto and dispersed. The product provides a cationic PCC dispersion with a solids content typically of about 50%.

Although the anionic PCC pigment itself is a useful paper filler, converting the pigment to cationic as described above is advantageous because the cationic pigment has good retention and does not necessarily require the use of a retention agent.

Ground calcium carbonate (GCC) is also supplied as a dry anionic pigment or an anionic pigment dispersion. These products can also be converted to cationic as described above.

The following non-limiting examples illustrate the new technology.

Sludge preparation example

A low solids PCC slurry was prepared which was filtered to a high solids paste. This was dispersed using cationic modified starch as the dispersant (Vector 20157, Roquette). The viscosity of the prepared cationic, high dry solids PCC aqueous dispersion was 430 cP. The product was stored in an IBC container for 4 weeks. After storage, it was found that the high solids cationic aqueous dispersion of PCC had not settled and the viscosity of the slurry was measured as 100cP.

Example 1

Effect of pigment content and dispersant content on dispersion viscosity and stability.

Dry PCC was used as the pigment in the experiment. The pigment was dispersed in water using cationic modified starch as the dispersant (Vector® 20157; Roquette). Viscosity was determined on a Brookfield rotary viscometer at 100 rpm (Br100) and 50 rpm (Br50). The measure of viscosity is mPa.s The precipitation tendency of the dispersion was measured by pouring the dispersion into a 100ml volumetric flask and observing the increase (mm) in height of the clear water column as a function of 2, 7 and 14 days.

The results are shown in Table 1. The table shows that a dispersion having a pigment content of 57% and a dispersant content of 0.7% has good stability (low settling and no change as a function of time) and viscosity within a suitable range.

table 1

Table 2 shows the viscosity of the dispersion as a function of time. The pigment was dispersed in water using cationic modified starch as the dispersant (Vector® 20157; Roquette). Viscosity was determined with a Brookfield rotary viscometer as above. The dispersion having a pigment content of 57.5% and a dispersant content of 0.7% has a suitable viscosity which does not appreciably change with time.

Table 2

Tables 1 and 2 show that the modified starch used gives the dispersion good stability and good flow properties.

Example 2

Surface bonded paper with inkjet printability

A cationic PCC pigment dispersion made from a semi-dry filter cake using modified starch (Vector® 20157) as a dispersant was mixed with conventional cationic starch for surface sizing. Three experiments were performed, i.e. 1) a control in which the surface was treated with cationic surface starch alone; 2) adding anionic pigment to the cationic surface starch; and 3) adding a cationic pigment to the cationic surface starch. The blends were applied to the surface of the paper and subjected to inkjet printing. The prints of the experiments are shown in Figure 2. It is seen that the sharpest prints were obtained in Experiment # 3 using a cationic pigment. This indicates that the use of a cationic pigment in paper surface adhesive improves inkjet printability by preventing ink from spreading and blending with other inks.

Example 3 Inkjet Printability of Coated Paper

A cationic PCC pigment dispersion made from a semi-dry filter cake using modified starch (Vector® 20157) as a dispersing agent was mixed with the coating color. The results were compared with the titanium dioxide pigment commonly used. Three experiments were performed. As a control (Experiment 1), a coating paste containing 15% titanium dioxide was used. In Test 2, half of the titanium dioxide was replaced in Test # 2. 3 was completely replaced by titanium dioxide with the cationic PCC pigment described above. The papers were coated, the inkjet printing was performed, and the coated papers and prints were tested. The results are shown in Table 3.

Table 3

The improvement in fluorescence (i.e., the difference in brightness measured with and without UV light) is due to the reduced titanium dioxide content of the coating color. The optical density of the coating colors with the colors Blue, Magenta and Yellow increased significantly with cure PCC. Ink drying accelerated with cationic PCC. The lightness of the white paper pressed against the printed surface was less than that of the control.

The invention is not intended to be limited to the exemplary embodiments set forth above. The invention is intended to be extensively applied within the scope of the following claims.

Claims (38)

A process for preparing a pigment slurry wherein the inorganic pigment is dispersed in water in the presence of a dispersant, characterized in that the inorganic pigment comprises a cationic calcium carbonate dispersed with a cationic biopolymer to produce an aqueous slurry having a sol For 2 weeks, preferably at least 4 weeks. Process according to Claim 1, characterized in that the pigment to be dispersed is in the form of a dry pigment, an aqueous pigment slurry or an aqueous pigment paste. 3. A process according to claim 1 or 2, characterized in that a pigment is introduced into the dispersion, the charge state of which is not altered between the preparation of the pigment and the dispersion. Process according to any one of the preceding claims, characterized in that the surface area of the 2 2 calcium carbonate pigment is about 5 to 100 m / g, in particular about 10 to 50 m / g. A method according to any one of the preceding claims, characterized in that the pigment comprises calcium carbonate and another pigment, especially light scattering agent, such as titanium dioxide, kaolin, gypsum, silicate-based material, talc or various calcium carbonates. Process according to one of Claims 1 to 4, characterized in that the pigment consists of calcium carbonate. Process according to Claim 6, characterized in that the pigment consists of a porous calcium carbonate material, in particular of a calcium carbonate material whose particles are composed of calcium carbonate agglomerates. Process according to any one of the preceding claims, characterized in that the pigment comprises or consists of precipitated calcium carbonate. Process according to any one of the preceding claims, characterized in that the cationic biopolymer is a cationic modified, substituted starch or a cationic sorbitol or maltodextrin. Process according to claim 9, characterized in that the modified, substituted starch comprises cationic nitrogen-containing substituents such as amine or amide groups. The process according to claim 10, characterized in that the modified starch has a nitrogen content of about 0.1 to 2% by weight on a dry weight basis. Process according to any one of claims 9 to 11, characterized in that the degree of substitution of the modified starch is 0.1 to 0.2, preferably about 0.16. A process according to any one of the preceding claims, characterized in that the pigment is precipitated calcium carbonate which, after the process of making the precipitated calcium carbonate, is dried. A process according to any one of claims 1 to 12, characterized in that the pigment is an aqueous slurry or paste of precipitated calcium carbonate obtained by filtration of a dilute aqueous slurry of precipitated calcium carbonate resulting from the process of manufacturing the precipitated calcium carbonate. Process according to one of the preceding claims, characterized in that the aqueous dispersion is prepared without the addition of a polyelectrolyte, such as polyacrylate, before, during or after the dispersion. Process according to any one of the preceding claims, characterized in that a dispersion having an aqueous phase which is substantially free of polyacrylate, in particular a polyacrylate content of less than 0.1%, preferably less than 0.01%, in particular less than 0.001%, is prepared . Process according to any one of the preceding claims, characterized in that it contains from about 0.1 to about 2.0% by weight, in particular from about 0.5 to about 0.99% by weight, of a cationic starch derivative on a dry basis. Process according to any one of the preceding claims, characterized in that - the pigment content of the aqueous dispersion is about 30-75% by weight, preferably 50-60% by weight, most preferably about 57% by weight, and - the dispersant in the aqueous dispersion is 0.2-5.0 by weight, preferably from 0.5 to 3.0% by weight, most preferably from 0.7 to 1.5% by weight, in particular about 0.7% by weight. A method according to any one of the preceding claims, characterized in that the dry pigment or aqueous pigment slurry or paste; the dispersant and, if necessary, water are introduced into the dispersant, which is typically a disintegrating device operating on a back-mill principle. Process according to any one of the preceding claims, characterized in that an aqueous dispersion having a Z potential at pH 9.5 of 20% solids is at least +1 mV is prepared. An aqueous dispersion comprising an inorganic pigment and a dispersant, characterized in that the inorganic pigment comprises a cationic calcium carbonate, the dispersant comprises a cationic biopolymer and the aqueous slurry has a solids content of about 30 to 75% by weight, wherein said aqueous dispersion remains stable for at least . Aqueous dispersion according to claim 21, characterized in that the aqueous phase of the aqueous slurry is substantially acrylate-free, in particular having a polyacrylate content of less than 0.1%, preferably less than 0.01%, preferably less than 0.001%, of the dry content of the dispersion. An aqueous dispersion according to claim 21 or 22, characterized in that the aqueous slurry has a Z potential of at least +1 mV at a pH of 9.5 in 20% solids. An aqueous dispersion according to any one of claims 21 to 23, characterized in that the surface area of the 2 2 calcium carbonate pigment is about 5-100 m / g, in particular about 10-50 m / g. An aqueous dispersion according to any one of claims 21 to 24, characterized in that the pigment comprises calcium carbonate as well as another pigment, especially light scattering such as titanium dioxide, kaolin, gypsum, silicate-based material, talc or various calcium carbonates. An aqueous dispersion according to any one of claims 21 to 25, characterized in that the pigment consists of calcium carbonate. An aqueous dispersion according to claim 26, characterized in that the pigment consists of a porous calcium carbonate material, in particular of a calcium carbonate material whose particles are composed of calcium carbonate agglomerates. An aqueous dispersion according to any one of claims 21 to 27, characterized in that the pigment comprises or consists of precipitated calcium carbonate. An aqueous dispersion according to any one of claims 21 to 28, characterized in that the modified substituted starch comprises cationic nitrogen-containing substituents such as amine or amide groups. The aqueous dispersion according to claim 29, characterized in that the modified starch has a nitrogen content of from about 0.1% to about 2% by weight of its dry weight. An aqueous dispersion according to any one of claims 29 to 30, characterized in that the degree of substitution of the modified starch is 0.1 to 0.2, preferably about 0.16. An aqueous dispersion according to any one of claims 21 to 31, characterized in that the pigment is precipitated calcium carbonate, which after drying the precipitated calcium carbonate is dried. Dispersion according to one of claims 21 to 32, characterized in that - the pigment content of the aqueous dispersion is about 30-60% by weight, preferably 50-60% by weight, most preferably about 57% by weight, and - the dispersant in the aqueous dispersion is 0.2-5 , 0% by weight, preferably 0.5-3.0% by weight, most preferably 0.7-1.5% by weight, especially about 0.7% by weight based on the dry matter. Use of the dispersion according to any one of claims 21 to 33 as a filler for paper or board, wherein the dispersion, if necessary, is diluted with water and introduced into the headbox of a paper or board machine. Use according to claim 34 as a filler for paper or board without the use of a separate retention agent. Use of the dispersion according to any one of claims 21 to 33 for surface sizing of paper or board, wherein the dispersion is mixed with a cationic starch suitable for conventional surface sizing, and the mixture is applied to a dried web. Use of the dispersion according to any one of claims 21 to 33 for coating paper or cardboard, wherein the dispersion is mixed with a coating paste which is applied to a dried web. Use of the dispersion according to any one of claims 21 to 33 as a pigment or filler in inkjet paper. claim