Paper making and fiber liberation – Processes and products – With coating after drying
Reexamination Certificate
2002-01-09
2004-05-18
Chin, Peter (Department: 1731)
Paper making and fiber liberation
Processes and products
With coating after drying
C162S168300, C162S184000, C106S214200, C106S217600, C106S217900, C106S287250
Reexamination Certificate
active
06736936
ABSTRACT:
This invention relates to pigment coating compositions useful in preparing coated paper, board and other cellulosic materials, particularly rheologically enhanced coating compositions. The invention also relates to coated articles prepared from said coating compositions and further relates to novel polymers which may be used in said coating compositions.
It is known to apply pigment coating compositions to the surface of formed paper or board, for instance to improve the printability, gloss and optical characteristics. This pigment coating composition is known as coating colour. Typically a coating colour is applied as an aqueous dispersion comprising a blend of pigment(s) with binder.
Generally a coating colour composition comprises one or more pigments, a fluorescent whitening agent (FWA), a binder, a rheology modification agent and optionally other chemical agents. The pigment is usually a white inorganic particulate material, for instance calcium carbonate or kaolin, and normally comprises at least 75%, often at least 85% by weight of the dispersed solids in the coating colour composition. Fluorescent whitening agents (FWA), also known as optical brightening agents (OBA) enhance the light reflectance qualities, thus improving the whiteness and brightness of the coated sheet. Binder is present to affix the pigment to the coated paper or board sheet and is normally an adhesive polymeric material. The binder may be an aqueous latex comprising dispersed particles of water insoluble adhesive polymer. Alternatively the binder may be an aqueous composition, comprising water soluble starch. It is also possible that the binder comprises both aqueous latex polymer and starch. The rheology of the coating colour composition is normally adjusted to suit the specific application.
Typically a coating colour composition is applied to the surface of the paper or board sheet by means of a coating applicator, which may be a bar or a blade. Blade coaters are popular because they can coat at speeds exceeding 1,200 m/min (3,930 ft/min) and use coating solids up to 70%. In addition, the blade creates an even distribution of coating over the surface of the paper or board sheet and excess coating is removed to leave a smooth flat coated surface. It is important that the coating has a smooth flat surface in order to improve printability.
It is normally necessary to adjust the rheology to within quite narrow parameters in order to attain the most appropriate rheology for application. Since the application of the coating colour composition normally involves exposing it to high shear, it important that the coated composition exhibits the correct flow properties and water retention properties. For this reason it is standard practice to incorporate rheology modifiers into the coating colour composition. The water retention capability of a coating colour composition relates to the ability of the composition to hold the water. The release of water into the paper or board sheet must be relatively slow in order for the surface of the coating to be smoothed. Rapid penetration of the water into paper or board sheet would result in excessive amounts of water being removed from the coating prior to reaching the blade, which would impair the ability of the blade to adequately smooth the coating. Additionally, the excessive release of the water into the sheet can lead to an unequal distribution of binder, often known as binder migration. Typically the binder migration occurs through the z-direction of the coating. The consequence of binder migration is an impairment of the surface properties.
Various polymeric materials have been used for the purpose of improving the rheology and water retention characteristics of coating colour compositions and coatings therefrom. It is well known to apply natural polymers such as sodium carboxymethyl cellulose, hydroxyethyl cellulose, methyl cellulose and sodium alginate. In addition various synthetic polymers including polyvinyl alcohol and acrylic polymers have also been proposed for this purpose. For instance U.S. Pat. No. 4,423,118 describes a coating colour composition comprising as a thickener a copolymer of an ethylenically unsaturated carboxylic acid, an ethylenically unsaturated amide and a hydrophobic monomer having limited solubility in water. The preferred copolymers comprise between 30 and 97 weight % acrylic acid, 1 to 50 weight % acrylamide and 2 to 70 weight % acrylonitrile.
In achieving optimum flow and water retention characteristics for a coating colour composition, the coated paper or board sheet can suffer the disadvantage of impaired optical properties. It would thus be desirable to provide a coating colour composition that still exhibits optimum rheological properties, with a high degree of water retention and yet the coated paper or paper board sheet has improved optical properties.
According to the invention a coating colour composition is provided, comprising an aqueous dispersion of,
(a) a pigment,
(b) a binder,
(c) a fluorescent whitening agent and
(d) a water-soluble polymer formed from a water-soluble ethylenically unsaturated monomer or monomer blend,
characterised in that the water-soluble polymer consists essentially of,
(i) 90 to 100 mole % hydrophilic essentially nonionic repeating units and,
(ii) 0 to 10 mole % anionic repeating units and has an average molecular weight of between 50,000 and 500,000.
The pigments may be an inorganic material such as calcium carbonate, kaolin clay, aluminium or magnesium silicates, such as china clay, barium sulphate, satin white, titanium dioxide, talcum, gypsum and muscovite mica. Alternatively the pigment may be a polymeric plastic pigment comprising microspheres, for instance 0.1-1.0 &mgr;m diameter, that are either hollow or solid. Such polymeric plastic pigments may be based on polystyrene, with the polymer sometimes having butadiene or acrylic components. Preferably however, the pigment is an inorganic compound, more preferably calcium carbonate or clays such as kaolin or blends of calcium carbonate with clay. The choice of pigment or blend of pigments is usually determined by the particular application. For instance the pigment component single coatings for low coat weight (LWC) papers may consist solely of clay. It may also be desirable to apply multiple layers of coating to the surface of paper or board. The first coatings (pre-coat) may desirably consist of 100% calcium carbonate as the pigment component, whereas for the top coats it may be desirable to use blends of calcium carbonate and clay as the pigment component. The pigment is normally provided as an aqueous dispersion of usually at least 40 or 50% solids. Preferably the pigment dispersion is at least 60 or 70% solids and can be up to 80%. A particularly preferred pigment dispersion comprises between 70 and 72% calcium carbonate.
In order to assist the stability of the pigment dispersion it is sometimes desirable to include a dispersant. The dispersant can be for instance a surfactant although more preferably the dispersant is a polymeric dispersant, for instance a relatively low molecular weight water-soluble anionic polymer. Particularly preferred are sodium polyacrylates of molecular weight in the range 1,000 to 6,000 for instance as described in EP-B-129329.
The pigment would normally constitute at least 75% by weight, for instance at least 85 or 90% by weight of the total solids present in coating colour composition.
The binder may be for instance an aqueous latex polymer dispersion based on copolymers of butadiene/styrene, acrylonitrile/butadiene/styrene, acrylic acid esters, acrylic acid and esters/styrene/acrylonitrile, ethylene/vinyl chloride, and ethylene/vinyl acetate; or homopolymers, such as polyvinyl chloride, polyvinylidene chloride, polyethylene and polyvinyl acetate or polyurethanes. The binder dispersions may be prepared by aqueous emulsion polymerisation. Preferred binders consist of styrene/butyl acrylate or styrene/butadiene/acrylic acid copolymers or styrene/butadiene rubbers. Other polymer latexes which may be used in
Cockcroft Robert
Weston Philip Mark
Chin Peter
Ciba Specialty Chemicals Water Treatments Ltd.
Hug Eric
Mansfield Kevin T.
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