Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Processes of preparing a desired or intentional composition...
Reexamination Certificate
2003-07-23
2004-11-30
Szekely, Peter (Department: 1714)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Processes of preparing a desired or intentional composition...
C106S206100, C106S501100, C536S106000
Reexamination Certificate
active
06825252
ABSTRACT:
The present invention relates to pigmented coating compositions comprising starch dispersions as binders, to a novel process for their preparation and to the dispersions obtainable by that process.
Paper is composed of individual fibers and has thus a rough surface and pores whose size corresponds approximately to the fiber dimensions. Since the roughness has a negative effect on printing, most paper and board is coated to impart a smooth surface for printing. Coating of paper is a process by which a mixture of water, white pigments, binder and optionally various additives are applied to one or both sides of a paper sheet. Those pigmented paper coating compositions are also known as coating colors. The resulting covering and smoothening of the surface of the paper improves its glaze, gloss, brightness and above all, its printability in all conventional printing processes.
The properties of the coating colors must be adapted to the coating process and the intended use of the coated paper. The solids content greatly influences the runnability of the coating colors in the coating machine. It basically influences the viscosity of the color and its flow behavior at varying shear stress in the coating machine. The various components of the coating color determine the solids content attainable at a given viscosity. The highest possible solids content is aimed for to save drying energy. For most coating processes the water retention capacity of the coating color is also important; i.e. the ability of the coating color to retain water in spite of the sucking action of the coating base paper. If the water retention capacity of a coating color is too low, it may be too strongly dewatered between application and leveling.
Apart form water. the main components of a coating color are pigments and binders. The function of the binder is to bind the pigment particles to each other and to fix the coat to the base paper. Binders have a large influence on the coating and processing properties of the coating color. Binders are derived from natural raw materials or are prepared fully synthetically. In general, natural binders are used as additives for synthetic polymer binders. Solutions of starch derivatives are a common binder of natural origin. Moreover, the sodium salt of carboxymethyl cellulose (CMC) is the principal cellulose ether used as a naturally derived semi-synthetic co-binder in paper coating.
Whereas in the initial days of paper coating natural binders were used exclusively, in the 1940s the demand for synthetic binders in the form of polymer dispersions (latices) started increasing. These products made it possible for the first time to attain a high solids content at low viscosities, a prerequisite for modern high-speed coating machines. Styrene-butadiene dispersions are the most widely used latices worldwide. Acrylate dispersions are of comparable importance in Europe. These latices are usually mixed with co-binders, such as starch or CMC. A major disadvantage of synthetic and semisynthetic binders is their higher price in comparison to natural binders such as starch.
It was found by the present inventors that starch dispersions can be used as the binder in a pigmented coating composition. The coating composition of the present invention comprises a pigment and a starch dispersion of discrete crosslinked starch particles in an aqueous liquid.
Using the starch dispersions as a binder. storage stable coating compositions can be prepared at relatively high solids contents without significantly high viscosities. The pigmented coating compositions are preferably used as coating colors (pigmented paper coating) to coat paper and cardboard. But they can also be applied to other substrates such as for example plastic films. The pigmented coatings are preferably used to improve paper properties as a precoat or as a topcoat.
Preferably, the starch dispersions are used in the pigmented paper coating compositions as co-binders in combinations with synthetic binders, i.e. polymer latices such as styrene-butadiene dispersions and acrylate dispersions. However, due to the low viscosity of the coating compositions comprising the starch dispersions they may also be used as the sole binder. When the coating composition comprises the starch dispersion as the only binder it is often desirable to use the starch in higher amounts than a synthetic latex in order to obtain the same binding strength.
The pigmented coating compositions comprise the starches preferably in levels from 1 to 300 parts by weight of dry starch based on 100 parts by weight of pigments depending on the specific starch dispersion employed, the desired properties and whether the starch is used as co-binder or the sole binder. If the starch functions as a co-binder it is preferably used in amounts of from 1 to 10 parts by weight in combination with a binder, e.g. a latex, which is used in amounts of from 1 to 20 parts by weight; if the starch is the sole binder it is preferably used in amounts of from 8 to 300 parts by weight, all parts based on 100 parts of pigment.
Preferred pigments are clay, calcium carbonate, calcium sulfoaluminate (satin white), talc, titanium dioxide, barium sulfate, silica or plastic pigments based on polymer particles with a high glass transition temperature like polystyrene.
The pigmented coating composition may contain additional ingredients usually comprised in coating colors. Examples for such additives are foam control agents, optical brightening agents, rheology additives, wetting agents, crosslinking agents, surfactants, lubricants, dispersing agents and dyes.
The advantages of using the starch dispersions according to the present invention as binders in pigmented coating compositions are that higher amounts of starch can be used compared to conventional solutions of starch derivatives and that high solids formulations can be prepared without excessive viscosities and without viscosity instabilities as observed when conventional starch solutions are used in high amounts. The solids content of the pigmented coating compositions preferably ranges from 50 to 74% by weight, more preferably from 55 to 74% by weight and most preferably from 60 to 74% by weight. The pigmented coating compositions simultaneously show superior water retention capacity avoiding runnability deficiencies during coating. Properties of the paper coated with the pigmented coating compositions such as e.g. paper gloss, ink gloss, dry pick resistance, ink piling, ink set off, mottling and brightness are at least as good as with standard coatings if not better. The use of starch as co-binder or sole binder also means a cost advantage versus the use of synthetic binders and co-binders.
Starch can be formed into a dispersion by processing the starch using shear forces and simultaneously crosslinking. A mixture of starch and an aqueous liquid, preferably selected from water and a combination of water and alcohol is processed using shear forces, preferably in an extruder, in the presence of a crosslinker. A hydroxylic liquid, preferably selected from water, alcohol and a mixture of water and alcohol is then added to the mixture and the mixture is dispersed to obtain a dispersion of starch particles.
The starch is preferably native starch, but previously modified starch derivatives may be used as well. Preferred sources of native starch are corn, wheat, rice, potato, tapioca, and barley. The starch can be waxy starch. Starch derivatives which can be used are e.g. cationic and anionic starches, carboxylated starches, carboxy methylated starches, sulfated starches, phosphated starches, starch ethers like hydroxy alkylated starches, e.g. hydroxy ethylated and hydroxy propylated starches, oxidized starches containing carboxy or dialdehyde groups or hydrophobized starches like acetate esters, succinate ester, half-esters or phosphate esters and the like. In the process of preparing the starch dispersion starch granules or pregelatinized starch can be used as preferred starting material. Mixtures of starch with other biopolymers containing at lea
Annen Thomas Martin Stephan
Helbling André Marcel
Hills Michael Robert
Stollmaier Friederike Theresia
Ecosynthetix Inc.
Szekely Peter
Young & Thompson
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