FI105200B - Kaolin-based coating agent and filler and a process for its preparation - Google Patents

Description

, 105200

Kaolin-based coating and filler and process for its preparation

The present invention relates to a kaolin-based coating and filler according to the preamble of claim 1.

The invention also relates to a process according to the preamble of claim 5 for the preparation of such a coating and filler. According to the method, kaolin and PCC are mixed in an aqueous phase to form a kaolin / PCC aqueous slurry. The invention also relates to the use of such an aqueous slurry.

Filling and coating pigments are used in papermaking. The reasons are economic and technical; reasonably priced mineral pigments can replace some of the expensive fiber material while improving the printing properties of the paper, such as opacity, brightness 15 and gloss. Commonly used filler and coating pigments are e.g. kaolin and calcium carbonate. Kaolin, which is a sheet-like or lamellar aluminum silicate mineral in particle form, is prepared from natural kaolin by purification and fractionation. Calcium carbonate can be derived from natural minerals or synthetic. The latter is prepared by a precipitation reaction, which is called doped calcium carbonate · PCC.

Kaolin has valuable filler properties which are important especially in paper technology.

• «'. Thus, it improves the optical properties of paper, such as gloss, light scattering and brightness. Kaolin is essentially dust-free, stays well on paper (good adhesion, or retention), and in most cases is fairly uniform.

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Kaolin is also used as a coating pigment. The main purpose of the coating is to cover • · · = .. 1; base paper fiber network with a pigment-binder blend that improves the printability of the paper. % of the coating paste, which is why the effect of the coating pigment kaolin on the properties of the paper is greater than that of kaolin used as a filler. , typically <88.

PCC provides good lightness and opacity to paper, whether it is used as a filler or as a pigment for coating. However, PCC has the disadvantage that its gloss and retention are worse than that of kaolin. PCC has a brightness of 94 and is bluish (kaolin is yellowish).

It is an object of the invention to provide a product which combines the good properties of kaolin and PCC while avoiding the disadvantages of these.

It is previously known to use kaolin and PCC simultaneously in coating pastes and kaolin and PCC separately as fillers in paper. To prepare the coating pastes, kaolin and PCC as well as any other components such as dispersants, latex resins, starch, etc., are mixed together in the aqueous phase to form an aqueous slurry. This results in a physical mixture of kaolin and PCC and other components. With a 1: 1 weight ratio of kaolin to PCC, such a blend enhances the brightness of the kaolin paste used in paper coating by up to 2 units. Kaolin alloying does not: 20% significantly improve PCC particle relaxation.

The present invention is based on the idea that, instead of forming a "physical mixture", PCC particles are attached to kaolin particles; kaolin is highly coated

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• · · with small PCC particles that adhere predominantly to the anionic surfaces of kaolin.

• * · 25 The kaolin based coating and filler contains from 10 to 90% by weight, preferably from about 30 to. ···. 70% by weight of kaolin and 90-10% by weight, preferably about 70-30% by weight, of PCC.

·· · To obtain such a product, kaolin and PCC are mixed in an aqueous phase having a pH of 1 · · · · · · · · · · · · · · · ·, (1), being maintained at an alkaline pH of about 8 to 10, whereby the PCC particles disintegrate. To 40-60 nm particles capable of binding to kaolin particles • 1 «.1. 30 physical forces.

More particularly, the coating and filler according to the invention is essentially characterized in what is set forth in the characterizing part of claim 1.

The method according to the invention, in turn, is characterized by what is stated in the characterizing part of claim 5.

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The use according to the invention is characterized by what is claimed in claim 15.

The invention provides considerable advantages. Thus, the kaolin coated with PCC 10 according to the invention gives kaolin the brightness and opacity of PCC, while kaolin improves the retention and gloss of PCC. With a 1: 1 weight ratio of kaolin to PCC, the brightness of the kaolin paste increases by 2 to 4 units.

Other details and advantages of the invention will be apparent from the following detailed description, with reference to the accompanying microscope photographs.

Figure 1 shows particles of precipitated calcium carbonate, most of which are smaller than 0.2 μΐη, Figure 2 shows kaolin particles, and Figure 3 shows kaolin particles coated by the process of the invention.

20 !! I Theoretical background of the invention is briefly as follows: * * · * · · · ·. The other surface of the kaolin shale particles consists of silicate oxygen groups and is · · · charged (-) or anionically charged. One surface has aluminum-hydroxy-25 silyl groups. At a pH of 5 to 7, this surface has a positive (+) charge, i.e. it is cationic. PCC particles, in turn, have an anionic charge (-) and their charge is pH dependent. At pH 8.5, the PCC has a Z potential of 3 mV and at pH 6 a Z potential of> 25 mV. Similarly, at pH 10, the Z potential is> 25 mv. According to the invention: ['] is carried out at a pH of from about 8 to about 10, preferably from about 8 to about 9, and particularly preferably from about 8.4 to about 8.6.

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In the context of the invention, it has been found that when the PCC has a low Z potential and when 4,105,200 particles have just crystallized, the internal bonding forces of the calcium carbonate crystals are low. In this case, the PCC particles readily decompose in the aqueous phase into smaller particles, 40 to 60 nm in diameter, typically 50 nm, and associate with the positively charged Al-OH sites on kaolin. This results in the formation of an kaolin particle in the aqueous phase, the other surface of which is covered with small PCC particles. Usually, the PCC particles are predominantly (= at least 60%) attached to negatively charged, anionic surfaces of kaolin. Typically, PCC is attached to at least 90%, up to 95%, of the anionic surfaces of kaolin alone.

The coating and filler material (i.e. coating and filler pigment) provided by the invention contains 10 to 90, preferably about 30 to 70% by weight of kaolin and 90 to 10, preferably about 70 to 30% by weight of PCC. After preparation, it is in the form of an aqueous slurry having a dry solids (Kap) content of 5 to 95, preferably 40 to 80, particularly preferably about 65 to 75%. It may be used as such as a filler for paper or for the preparation of a composition for paper coating. The coating pigment-water slurry typically has a Kap of about 60-80, e.g. 70%, and the filler slurry has a Kap of about 40-60%, typically about 50%. The aqueous slurry optionally contains other components for use in pigment pastes and filler compositions, such as 0.01 to 10% by weight of a polyelectrolyte, e.g. polyacrylic acid or a derivative thereof. If desired, the kaolin / PCC aqueous slurry may be dried to form a powder product.

«« * · *. In the process of the invention, kaolin and PCC are mixed in an aqueous phase at a pH of 8 to 10 to form an aqueous kaolin / PCC slurry. Suitable bases or acids, respectively, may be used to adjust the pH to · · ·. Preferred bases include hydroxides and carbonates of alkali metals, particularly sodium hydroxide and sodium carbonate. Of the acids, mineral acids and organic acids are preferred, with phosphoric acid being particularly preferred.

··· «·. '' ·. According to the method, kaolin and PCC can be added in an optional order or «· .1. 30 simultaneously into the aqueous phase. According to a preferred alternative, the PCC is first mixed with water to form a PCC aqueous slurry having a solids content of from about 5% to about 95%. The pH of the aqueous slurry is adjusted to a value of 8-10, preferably a value of 8-9, after which kaolin is added with stirring. Since kaolin reacts acidically in the aqueous phase, the pH is adjusted during stirring with e.g. sodium hydroxide or carbonate.

In order to contact kaolin and PCC as well as possible, the aqueous slurry 5 is stirred vigorously during the addition of the second component (e.g. kaolin). According to a preferred embodiment, the aqueous slurry is agitated by a blow mixer. For purposes of this application, "impact mixer" means a device in which rotating peripheries have plate-like collision surfaces in which, with excessive shear (after impact), the fluid and suspension flow from the inner blade 10 to the outer peripheral blade blades, caused by differences in speed and / or direction of rotation of successive blade vanes. The device is used here so that liquid and slurry are always supplied to it with a lower volume flow capacity than is taken out of it. In such a device, the plates are either radial or slightly inclined in the direction of rotation. The ability of a percussion mixer to perform homogeneous mixing and shear as a result of shocks between a solid and a solid percussion surface is, by probabilities, incomparably more homogeneous than with other types of mixer. Typically, more than 95% of the material flow will receive nearly 100% of the maximum impact energy and only 5% to 10% of the material will receive less than 60% of the maximum impact energy. This corresponds to the turbulence intensity of the mix. the dispersion is present in all so-called "Dissolver" types. Many times higher in high-shear mixers. Due to the above-mentioned reasons, also the aggregates *, * and flocs formed are re-disaggregated almost completely in said percussion mixer compared to a conventional mixer.

• · ·% * 25. ···. Kaolin is used in the invention as a conventional pigment or filler grade product which. * * ·. is produced from natural kaolin by purification and fractionation.

• · • # · · · «%.« ··. PCC can be prepared by any known technique by causticization or carbonation. Known solutions are described, for example, in U.S. Patent Nos. 4,824,654 and • · /// ,. 5,364,610; FI application 942815; and DE publication Ί 759 551.

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However, according to a preferred embodiment of the invention, a uniformly sized, finely divided calcium carbonate is used, the preparation process of which is further described below: The carbonate ions μιη. The reaction between calcium hydroxide and carbonate ions is allowed to proceed without substantially stirring the reaction mixture until the desired calcium carbonate particle size 10 is achieved. The reaction is then quenched after a preselected time by applying strong stirring forces to the reaction mixture. The length of time selected depends on the size and shape of the particles to be produced.

PCCs already suitable for coating paper with a particle size or particle size of 15 0,2 μιη <0 <0,4 μιη are then prepared by carbonating calcium hydroxide having a particle size <3 μιη. In order to prevent the slaked lime crystals from growing too much, it is preferable to make contact between the slaked lime and the soda and / or carbon dioxide as soon as possible after the slaking of the calcium oxide. It is particularly advantageous to carry out the contact of slaked lime and soda in which the crystal embryos are formed in a fast rotating mixer which produces higher turbulent mixing and shear forces. For example, a percussion mixer is suitable for this use. The first contact of slaked lime and soda ·· * 'is also referred to in this application as a "premix" or "" premix ". After premixing, the crystal embryos are allowed to grow undisturbed until they are of the desired size. By not substantially stirring the reaction mixture (or "interfering"), it means that the reaction mixture is not subjected to turbulent agitation. In practice, the reaction is carried out in a reaction tank fed to the reaction mixture from above and in which the ··· um-free calcium carbonate-containing gelled precipitate decreases during the reaction time ··· ,,!,; inside the container towards the bottom, whereby laminar flows occur in the reaction mixture.

•. · '*. However, these do not interfere with the growth of crystal embryos.

«* 4 4 4. '. 30: After the selected time, the carbonation reaction is terminated by stirring the mixture so efficiently that at least some of the calcium carbonate crystals formed in the reaction mixture are broken. Preferably, the cutting is performed with an efficient mixer using strong turbulence, shocks and shearing. For this purpose too, it is particularly advantageous to use a percussion mixer.

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It takes about 15 to 35 minutes, typically about 20 to 30 minutes, to make a calcium carbonate product used as a filler in paper. It has been shown that disturbing the mixture before said time has elapsed and before the crystals have grown to 0.2-0.4 µm, yields abundant crystals <0.2 μιη, which are not advantageous for the production of paper pigments. .

For decomposing the PCC agglomerates, PCC particles having an average particle size of up to 200-400 nm and less than 6 months, preferably less than about 4 months, and preferably less than about 15 months, are used in the aqueous phase. A dispersant is added to the aqueous phase to promote dispersion.

The following example further illustrates the invention:

Example «I« tl I «;;; Preparation of PCC: • «·

CaO 0 10 mm was pre-milled with a tumble cone crusher <2mm 0. Extinguishing was performed with a * * · ATREX mixer, temperature 75-80 ° C, 12% solids. Ca (OH) j was allowed to run for 25 h in a tank for about 20 h. Subsequently, 1 HCO 3 was dissolved in water above 30 ° C to a ca. 32% saturated solution. To start the causticization process, the ingredients were mixed together in an ATREX mixer. The reaction was then allowed to proceed proceed substantially without mixing. The temperature was maintained at <30 ° C throughout the reaction. To prepare the coating PCC grade, the reaction was stopped after 15-25 minutes. · * * 30 starting with high-efficiency ATREX mixing to achieve the desired endpoint (0.2-0.4 μηι 0) by homogenizing the mixture when the crystals generated in the gel were dispersed in the formed NaOH. and the loose agglomerates in the gel are disintegrated.

PCC having a particle size of 100-400 nm was obtained. Figure 1 shows the fine structure of PCC. PCC up to 6 months of age was slurried in water to form a slurry of 40-60% solids. About 1% of the dispersant (polyacrylic acid) was added to the slurry.

To this slurry was then added kaolin powder (Comalco, Australia) with vigorous stirring with a turbulent impact mixer. The microscopic image of kaolin is shown in Figure 2. The pH of the aqueous slurry was maintained between 8-9, preferably about 8.4-8.6. H3PO4 acid was used for regulation.

As a result, an aqueous slurry of coated kaolin was obtained, and a microscopic photograph was taken. This is illustrated in Figure 3. In the figure, the PCC particles appear as circular particles of about 50 nm adhered to the kaolin strips.

When the dry solids content of the PCC slurry was 50%, 100 kg of water-slurry 66.7 kg of kaolin powder were added to give 166.7 kg of a kaolin / PCC slurry with a Kap of 70%.

PCC accounted for 42.8% of dry matter and 57.2% of kaolin.

: '.20 t «;;;' From the pictures it can be estimated that there are kaolin particles per square millimeter ***. 200,000 - 400,000, of which about half are PCC coated. There are also PCC groups at the ends of the lamellar • · kaolin particles. Kaolin brightness improved by about 4 ISO »i *, ···. units to nearly 92. In the comparative test, the brightness of 50 PCC / 50 kaolin paste prepared by mixing at an acid pH of 2 · · · · 25 was 88 units of kaolin brightness.

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It was also found that the opacity of kaolin was improved when the kaolin was coated with PCC.

• • 9, ..,: Apparently, the PCC particles on the kaolin surface cause small air inclusions which «·. * * ·. reduce the passage of light, improving opacity.

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Gloss is defined as the ratio of the beam of light emitted to the surface of the paper and reflected from it.

«« 105200 9

The gloss is mainly influenced by the plateau (shape factor) of kaolin. The sheet requirement is limited by the requirement of porosity in the manufacture of paper. PCC 50 nm 0 + kaolin, the effect is to increase the porosity and enable the use of a more flattened kaolin. The PCC 50 Nm0 fills the spacing of the kaolin particles while continuing to flatten while being 5 porous portions and does not prevent water evaporation. PCC 50 Nm0 particles have no effect on the reflection of visible light having a wavelength of 400-750 Nm and a scattering particle size of> 200 Nm0.

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Claims (17)

1. Kaolin based coating and filler, characterized in that it contains 10 - 90% wt%, preferably about 30 - 70 wt%, kaolin and 90-10% wt%, preferably about 70-30 wt%, precipitated calcium carbonate (PCC), with at least 90% of the PCC particles being fixed to the anion surfaces of the kaolin. Coating and filler according to claim 1, characterized in that Coating and filler according to claim 1 or 2, characterized in that it is in the form of a water slurry whose concentration of dry material is 5 to 95, preferably 40 to 80, most preferably in about 65 to 75%. 15 4. Coating and filler according to claim 3, characterized in that the water sludge contains 0.01 - 10% by weight of a polyelectrolyte, such as polyacrylic acid or a derivative thereof. 20 Process for the preparation of a kaolin-based coating and filler: according to claim 1, wherein the process - the kaolin and the PCC are mixed together in aqueous phase to form a kaolin / PCC water sludge, characterized in that • ♦ · * · ί · * 25 - The pH of the aqueous phase is installed during the mixing of 8 - 10. • «o • · · 6. A process according to claim 5, characterized in that PCC is first mixed in water to form a saturated PCC sludge, whose concentration of dry material is about 5 - 95%, and the kaolin is added to this water sludge at a pH of 8 - 9, preferably at a pH of about 8.4 - 8.6. 14 105200 7. A process according to claim 5 or 6, characterized in that a kaolin / PCC water sludge is produced, the concentration of dry matter being 40-80%. Process according to any of claims 5-7, characterized in that a slurried kaolin-based coating and filler is produced which contains about 30-70% by weight kaolin and about 70-30% by weight PCC, wherein the PCC particles are mainly fixed to the anion surfaces of the kaolin. Process according to any of claims 5-8, characterized in that the water slurry is intensively mixed with the addition of kaolin. Process according to Claim 9, characterized in that the water sludge is mixed with a percussion chamber. 15 Method according to any of claims 5-10, characterized in that PCC particles are used, whose average particle size is at most 200-400 nm and which is less than 6 months old. The particle size of the PCC particle is mainly 40 - 60 nm. Method according to claim 11, characterized in that the PCC particles are less than about 4 months old, preferably about 3 months old. 13. A process according to any of the preceding claims, characterized in that dispersants are added during the aqueous phase. • · • · · * · ”'25 14. A process according to any of claims 5 to 13, characterized in that the kaolin / PCC water sludge is dried to form a powdered product. tl »• · l '.'. Use of kaolin / PCC water sludge prepared according to any one of claims 5 - 13 as filler as a seed in paper or for the preparation of a composition intended for coating paper. , 5 105200 Use according to claim 15, characterized in that water sludge, whose concentration of dry material is about 60 - 80%, is used for the preparation of the composition intended for coating paper. Use according to claim 15, characterized in that, as a filler for paper, water sludge is used, the concentration of dry matter being about 40 - 60%. • i i • <f I «• · • · · t · · • · 4 • ·« • 1 · • · «« · · • 1 • · «• · · • · • · · ·