DRY PIGMENT GRANULATE AND A METHOD FOR THE MANUFACTURE THEREOF

Abstract: The present invention relates to dry pigment granulates comprising hollow and/or solid pigments for paper coating, especially for fine paper and to a method for the manufacture thereof. The pigment granulate comprises 95-99.5 wt% of organic polymer pigment, 0-94.5 wt% of inorganic pigment and 0.5-5 wt% of a binder or a mixture of binders and the particle size of the granulate is 4-400 mu m.

Description: Dry pigment granulate and a method for the manufacture thereof The present invention relates to dry pigment granulates comprising organic hollow and/or solid pigments for paper coating, plastic mixtures, paints, coatings and printing inks, especially for fine paper and to a method for the manufacture thereof. Inorganic pigments such as clay, caolin, calcium carbonate and titanium dioxide are used in paper coatings to provide a surface with a desired appearance and printability. Organic pigments, commonly referred to as plastic pigments, are used as partial replacement for inorganic pigments to improve the optical print properties for coated paper and paper board. Two general classes of plastic pigments, e.g. solid bead and hollow sphere pigments, are used in the preparation of coatings for paper and paper board. Plastic pigments enhance the performance of pigmented coatings for paper and paper board in a variety of ways. In most cases they are used as additives where they are substituted for mineral pigments in the range of 3-20 % by dry weight of pigments. Plastic pigments enhance the optical performance of coatings by increasing sheet gloss, brightness and opacity. Plastic pigments are generally supplied as polymer particles dispersed in water or as latex where the solid content varies between 25 and 60 wt%. Water dispersions of plastic pigments are shipped and stored in tanks, drums or the like and during longterm storage settling can occur. Additionally, plastic pigments are not normally freeze and thaw stable, therefore freezing must be avoided. Long term storage may also cause microbial growth, such as bacterial and fungicidal growth, which has to be prevented by toxic additives. A further disadvantage of plastic pigment dispersions is the costs related to the transport of large volumes of water. The addition of plastic pigment dispersions with low solid contents will also increase the amount of water in paper coating formulations, which in turn will require a lot of heating energy to evaporate water from the coating and to dry the coated paper. EP-06146766 discloses a coated paper suitable for ink fusion thermal transfer and manufacture thereof. A coated paper giving excellent image reproduction when used in ink fusion transfer type thermal printers and copiers with also high gloss in both the non-image and image parts, is described. The uppermost layer of the recording layer of the paper comprises at least 100 parts by weight of a pigment and 10-50 parts by weight of a binder, and 30-70 wt% of the pigment consist of 2 synthetic silica having a specific surface area of 20-600 m /g. Synthetic silica is blended in the coating solution containing pigment. Suitable pigments are inorganic pigments and organic pigments, such as fine particles of styrene resins or acrylic resins, and they may be hollow particles of polystyrene or styrene methyl methacrylate copolymers. The binder may be a resin, emulsion, latex or natural high polymer and suitable binders are polyvinyl alcohol, starches, fibre derivatives such as carboxy methyl cellulose, styrene acrylic resins and the like. Additionally dyes or coloured pigments may be added to adjust the colour of the coating. The coating solution is manufactured by blending the components with each other. FI-970936 discloses a method for the preparation of hollow polymer particle latex useful in coating applications, by emulsion copolymerization. The method comprises the preparation of seed latex with a water-soluble initiator and an anionic surfactant, followed by the preparation of latex of a highly carboxylated copolymer on the seed latex, then further preparing an intermediate shell, then swelling the particles and finally preparing a hard shell on the swollen expanded particles and optionally preparing an external shell on the hard shell. When spray-dried, plastic pigments according to prior art become dusty powders with a particle size of 1-50 μm, which powders are difficult to handle and thus dry plastic pigments are not commonly used. Based on the above it can be seen that there exists a need for plastic pigments which are easy to handle and to blend with other ingredients needed for applications, such as paper coating, and without the need to transport large amounts of water. An object of the invention is to provide dry pigment granulate comprising organic hollow and/or solid pigments for applications, such as paper coating, and especially for fine paper, coated paper board, plastic mixtures, paints, coatings and printing inks and to provide a method for the manufacture thereof. A further object of the invention is to provide dry pigment granulate comprising organic hollow and/or solid pigments, which can be easily handled at a paper mill, and a method for the manufacture thereof. Characteristic features of the dry pigment granulate comprising organic hollow and/or solid pigments, and of the method for the manufacture thereof are stated in the claims. The disadvantages of the dusty plastic pigments according to the state of the art can be avoided or significantly reduced by the dry pigment granulate comprising organic hollow and/or solid pigments, and by the method for the manufacture thereof according to the invention. It has bees noted that dry pigment granulate comprising organic hollow and/or solid pigments for paper coating and especially for fine paper can be manufactured by granulating pigment particles, preferably by spray-drying, with a suitable binder or a mixture of binders, and preferably with binders which also are commonly used in paper coating application. The binder binds the particles in to agglomerates, which are easy to handle in powder form without dusting problems. Suitable organic pigments for granulating are organic hollow or solid polymer pigments i.e. fine particles of styrene based polymers such as polystyrene, fine particles of acrylic resins such as polymethyl methacrylate, polyacrylate, polyacrylonitrile and fine particles of polyvinyl chloride or polycarbonates or copolymers thereof or combinations thereof. Also melamine-formaldehyde and urea- formaldehyde pigment particles may be used. Preferred organic pigments are hollow particles of polystyrene or styrene methyl methacrylate copolymers. When inorganic pigments are also incorporated in the pigment granulate, preferable organic pigments are polyacrylate, polystyrene and copolymers thereof and melamine-formaldehyde and urea-formaldehyde pigments. The binder may be a resin, an emulsion, a latex or a natural high polymer or a compound which preferably is used in paper coating applications. Examples of suitable binders are polyvinyl alcohol, polyvinylacetate, styrene-butadiene latex, polyethylene glycol, polyethylene wax, calcium stearate, cellulose derivatives, such as carboxy methyl cellulose, hydroxy ethyl cellulose and similar compounds. Carboxy methyl cellulose is especially suitable because it is used in paper manufacture as a thickener in sizing pastes, for improving retention and for binding and it is easy to handle. Styrene-butadiene latex, polyvinyl acetate and carboxymethyl cellulose are particularly suitable when inorganic pigments are also used. When dry white pigment granulate is desired also inorganic pigment is incorporated into the dry pigment granulate. Suitable inorganic pigments are caolin, calcium carbonate, (GCC and PCC), titanium dioxide and other white inorganic pigments. Dry white pigment granulates are specially suitable for paper coating, plastic mixtures, paints and printing inks. The particle size of the organic pigment in the dispersion to be granulated is typically 120-2000 nm. The pigment granulate comprises 5-99.5 wt% of organic polymer pigment, 0-94.5 wt% of inorganic pigment and 0.5-5 wt% of a binder or a mixture of binders. Additionally the granulate may contain other components such as whitening agents, antistatic agents and antibacterial agents. The particle size of the spray-dried granulate is between 4 and 400 μm, and preferably between 100 and 300 μm in the case no inorganic pigment is used. The particle size of the dry pigment granulate comprising organic hollow and/or solid particles and inorganic pigment or pigments is typically 4-60 μm, with an average of approx. 25 μm. The method for the manufacture of dry pigment granulate comprises spray-drying of a pigment particle dispersion having a dry solid content of 15-75 vol%, preferably 30-65 vol%. A binder as such or as a water solution is blended with the organic pigment particles and water in a high speed mixer and then the mixture is granulated by spraying using any suitable spray-drying apparatus known in the art. The binder or a mixture of binders are preferably dispensed in water to obtain a water solution containing 0.5-20 wt% of the binder, before blending with the pigment particles and spray-drying. Other additives, such as whitening agents, antistatic agents or antimicrobial agents may be incorporated in the water solution. In the case inorganic pigments are used the inorganic pigment is slurried in water (when solid form), the obtained slurry is blended with the organic pigment particle dispersion, desired additives are added and the mixture is spray-dried. Preferable inorganic pigments are caolin, calcium carbonate (GCC = ground calcium carbonate or PCC = precipitated calcium carbonate) and titanium dioxide. One of the most important applications of the invention is the use of the granulated pigment in coating of fine paper, in printing and for coated paper board, but the granulate according to the invention can also readily be used in coatings like paints, in thermoplastics, plastic mixtures, printing inks, in decorative paper applications and as fillers in concrete. An example of an equipment which is suitable for spray-drying of the granulates is provided in Figure 1. In Figure 1 is presented a spray-drying apparatus with A product inlet, B gas/steam, C air inlet, D compressed air inlet, E product outlet, F air outlet, G retained particles, and 1 represents feed tank, 2 nozzle atomizer, 3 drying chamber, 4 fluid bed, 5 cyclon separator, 6 filter and 7 secondary product inlet. The product is fed to the atomizer system and sprayed downwards in the drying chamber co-currently with hot drying air. The required moisture is reached in the fluid bed, wherefrom the powder is taken out. Primary drying air is directed into the drying chamber through an air distributor at a top. Secondary air enters through the bed and mixes with a primary air. Fine powder particles in the spent drying air are caught by the wet droplets in the atomization zone and move with these into the fluid bed. Any fine material which is not caught this way is conveyed with the exhaust air to the cyclone collector, where it is separated from the air and returned to the atomization zone or the fluid bed. The invention is illustrated in more detail with the following examples to which the scope of the invention is not limited. Example 1 Spray-drying granulating was performed by using spray-bed with a high pressure nozzle at the top, and a fluid bed at the air bottom. The fines were recirculated. 37.5 kg of hollow particle polyacrylate-polystyrene based polymer dispersion with a solid content of approx. 30 wt% and 4.2 kg of a 10 % polyvinyl alcohol - water solution were mixed a high speed mixer. The solution was then fed into the cyclon via the nozzle atomizer. The operating conditions were the following: Inlet temperature 152°C Outlet temperature 59 C Inlet to fluid bed 70°C Fluid bed 54°C Nozzle pressure 130 bar The total spraying time was 57 min. Granulated hollow polymer particles with the following characteristics where obtained at the product outlet: Particle size 240 μm Untapped density: 0.24 g/1, Tapped density: 0.27 g/1, Water content 0.9 % Example 2 Spray-drying granulating was performed in a similar way as was described in Example 1. 38.8 kg of hollow particle polyacrylate-polystyrene based polymer disspersion with a solid content of approx. 30 wt% and 1.5 kg of a 10 % carboxy methyl cellulose - water solution were mixed with a high speed mixer. The solution was then fed into the cyclon via the nozzle atomizer. The operating conditions were the following: Inlet temperature 165 O C, Outlet temperature 68 O C, Inlet to fluid bed 70 O C, Fluid bed 65°C Nozzle pressure 130 bar The total spraying time was 35 min. Granulated hollow polymer particles with the following characteristics were obtained at the product outlet: Particle size 280 μm Untapped density: 0.24 g/1 Water content 1.0 % Example 3 Dry pigment granulate comprising organic polymer pigment according to example 1 and inorganic pigment. 0.04 wt% of Dispex N40 (0.4 g) with respect to inorganic pigment was added to 1 litre of water, and the pH of the mixture was adjusted to alkaline (8.5) with NaOH. 600 g of calcium carbonate (PCC-40) was added to the mixture with vigorous agitation and a homogeneous slurry was obtained. Then 3 kg of organic polymer pigment and a sufficient amount of water were added to obtain a suitable dispersion for spray-drying. Example 4 Dry pigment granulate comprising organic polymer pigment according to example 1 and inorganic pigment. 0.04 wt% of Dispex N40 (0.12 g) with respect to inorganic pigment was added to 1 litre of water, and the pH of the mixture was adjusted to alkaline (8.5) with NaOH. 300 g of calcium carbonate (PCC-10) was added to the mixture with vigorous agitation and a homogeneous slurry was obtained. Then 6 kg of organic polymer pigment and a sufficient amount of water were added to obtain a suitable dispersion for spray-drying. Example 5 Dry pigment granulate comprising organic polymer pigment according to example 1 and inorganic pigment. 2 wt% of Dispex N40 (12 g) with respect to inorganic pigment was added to 1 litre of water. 600 g of caolin-40 was added to the mixture with agitation and a homogeneous slurry was obtained. Then 3 kg of organic polymer pigment and a sufficient amount of water were added to obtain a suitable dispersion for spray- drying. Example 6 Dry pigment granulate comprising organic polymer pigment according to example 1 and inorganic pigment. 2 wt% of Dispex N40 (6 g) with respect to inorganic pigment was added to 1 litre of water. 300 g of caolin- 10 was added to the mixture with agitation and a homogeneous slurry was obtained. Then 6 kg of organic polymer pigment and a sufficient amount of water were added to obtain a suitable dispersion for spray- drying. Example 7 Dry pigment granulate comprising organic polymer pigment according to example 1 and inorganic pigment. 600 g of titanium-dioxide -40 was added to 1 litre of water with agitation and a homogeneous slurry was obtained. Then 3 kg of organic polymer pigment was added and a dispersion was obtained for spray-drying. Example 8 Dry pigment granulate comprising organic polymer pigment according to example 1 and inorganic pigment. 300 g of titanium-dioxide -10 was added to 1 litre of water with agitation and a homogeneous slurry was obtained. Then 6 kg of orgamc polymer pigment was added and a dispersion for spray-drying was obtained The spray-drying of disperions from Examples 3-8 was performed at drying temperature of approx. 180 °C and at cyclone temperature of approx. 80 °C. The homogeneous granulate particles had a particle size of 4-60 μm, with an average of approx. 25 μm.

Claims: Claims 1. Dry pigment granulate, characterized in that it comprises 5-99.5 wt% of an organic polymer pigment, 0-94.5 wt% of an inorganic pigment and 0.5-5 wt% of a binder or a mixture of binders and the particle size of the granulate is 4-400 μm. 2. Dry pigment granulate according to claim 1, characterized in that the organic polymer is styrene based polymer, acrylic resin, polyvinyl alcohol, polyacrylonitrile, melamine-formaldehyde, urea- formaldehyde, polyacrylate, polycarbonate or a copolymer thereof and preferably a hollow particle polymer of styrene, styrene metyl methacrylate or polyacrylate. 3. Dry pigment granulate according to claim 1 or 2, characterized in that the inorganic pigment is caolin, calcium carbonate or titanium dioxide. 4. Dry pigment granulate according to any one of claims 1-3, characterized in that the binder is polyvinyl alcohol, polyethylene glycol, polyethylene wax, polyvinylasetate, styrene-butadiene latex or a cellulose derivative, preferably carboxy methyl cellulose, styrene-butadiene latex or polyvinylacetate. 5. Dry pigment granulate according to any one of claims 1-4, characterized in that the granulate comprises additives, preferably whitening agents, antistatic agents and antimicrobial agents. 6. Method for the manufacture of pigment granulate, characterized in that the method comprises followings steps: a) mixing pigment(s), binder(s), water and optionally additives into a dispersion, b) spray-drying the dispersion obtained from a). 7. Method for the manufacture of organic pigment granulate according to claim 6, characterized in that the solid content in the dispersion is 5-75 wt-%, preferably 30-65 wt%. 8. Use of a pigment granulate according to any one of claims 1-5 or obtained according to claim 6 or 7 in paper coating, in thermoplastics in printing inks, in decorative papers, in coatings, in paints in plastic mixtures, in coated paper boards and in concrete fillers.

Inventors: KAERNAE TOIVO, KATSARAS NIKITAS, KOSKINEN JUKKA, LAAMANEN HANNA, SKRIFVARS MIKAEL, KAERNAE, TOIVO, KATSARAS, NIKITAS, KOSKINEN, JUKKA, LAAMANEN, HANNA, SKRIFVARS, MIKAEL
Applicants: KAERNAE TOIVO, KATSARAS NIKITAS, KOSKINEN JUKKA, LAAMANEN HANNA, NESTE CHEMICALS OY, SKRIFVARS MIKAEL, KAERNAE, TOIVO, KATSARAS, NIKITAS, KOSKINEN, JUKKA, LAAMANEN, HANNA, SKRIFVARS, MIKAEL
Application year: 2000
Main classification: C08J3
IPC codes: C08J3, C09D7, C09D11, C09D133, D21H19