PROCESS FOR FORMING A PIGMENT PRODUCT FROM A CELLULOSE DERIVATIVE

FIELD OF THE INVENTION The invention relates to the process for forming a pigment product from a cellulose derivative as defined in the preamble of claim 1, to the pigment product as defined in the preamble of claim 14 and to the use of a pigment product as defined in the pream- ble of claim 15.

BACKGROUND OF THE INVENTION

Known from the prior art are different kinds of cellulose derivatives and processes for their pro- duction.

Also known from the prior art are different kinds of pigments and fillers of mineral and organic origin and processes for their production. Known from the prior art is the formation of organic pigments from starch-based materials and pure cellulose. In these cases, the pigment is usually present in a mixture with some other material or in a dispersion. Also known from the prior art is the production of organic pigments from oil-based materials. As mineral fillers and coating pigments are replaced by materials of organic origin either partly or completely, the environmental load can be reduced. By using fillers or pigments of organic origin e.g. in papermaking, the recycling of paper is enhanced and the deinking sludge is easier to process further or dispose of. This allows the application of used paper e.g. in the production of biofuels. Also, in the typi ^¬ cal cases, the strength of paper can be improved by organic fillers, and smoothness of the paper surface

during calendering can be improved by organic pigments .

A problem with oil-based organic pigments is their price which follows the development of the oil price, and high production costs.

A problem with the known starch-based organic pigments and those formed from pure cellulose is often the complexity of their preparation due to many production steps and therefore the scarcity of industrial production.

OBJECTIVE OF THE INVENTION

The objective of the invention is to disclose a novel process for producing a pigment product from a cellulose derivative. Another objective of the invention is to provide an industrially applicable process for industrial production of a cellulose-based pigment product .

SUMMARY OF THE INVENTION

The process and the pigment product according to the invention are characterized by what has been presented in the claims.

The invention is based on a process for pro- ducing an organic pigment product from a cellulose derivative. According to the invention, a selected cellulose derivative containing at least one OH group is crosslinked by a hydroxyl-active crosslinker to form a substantially insoluble pigment product. The pigment product may be in the form of particles, a particle cluster, a dispersion or a foam.

The invention is specifically based on reprocessing cellulose and a derivative thereof to a higher degree of processing. In this context, a pigment product refers to a material or agent providing a substrate or a surface

with color or other optical property. The pigment may be translucent or opaque.

In one embodiment of the invention, the provided pigment product is pulverized to microparticles . In this context, microparticles refer to micro- or nanoscale particles in the size class of preferably less than 50μm, more preferably less than 10μm. The particles may be of any shape, e.g. spherical.

In one embodiment, the microparticles are formed substantially in connection with the production of the pigment product. In one embodiment, the microparticles are formed substantially after the production of the pigment product.

In this context, pulverization refers to any pulverization, grinding, crushing or disintegration of a material to a desired particle size. Operation of the pulverizing devices, preferably mills, is typically based on application of pressure, cutting, rubbing, compression and/or the effect of an impact provided by blowing, or an equivalent principle of operation. Most mills operate as a combination of many principles of operation.

Pulverization may be performed by any suitable process known per se. In one embodiment, the pig- ment product is pulverized by air jet pulverization, preferably an air jet mill. In one embodiment, the pulverization is performed as Cryo-pulverization. The pulverization may be performed as wet or dry pulverization. In one embodiment of the invention, the cellulose derivative is cellulose ether. In one embodiment, the cellulose derivative is hydroxy propyl cellulose (HPC) . In one embodiment, the cellulose derivative is selected from the group of hydroxy ethyl cellulose and carboxy methyl cellulose. In a preferred embodiment, the cellulose derivative is selected so as to be solu-

ble in water. In one embodiment, the cellulose derivative has substantially high molecular weight. The cellulose derivative may be a mixture of a number of cellulose compounds. The cellulose derivative may contain small amounts of compounds which are not cellulose- based.

In one embodiment of the invention, the crosslinker is selected from the group of divinyl sul- phones, dianhydrides, dialdehydes, formaldehydes, gly- oxal, diisocyanates, ethylene glycol, diglycidyl ether, oxalic and citric acids and inorganic salts, e.g. aluminum sulphate. The solubility of the cellulose derivative may be adjusted by crosslinkers . The crosslinkers are preferably selected so as to obtain a substantially insoluble pigment product, preferably insoluble in water, with a cellulose derivative.

In one embodiment of the invention, the crosslinker is used in an amount of 4 to 15% by weight of the amount of cellulose derivative. In one embodiment of the invention, the crosslinking reaction is activated by at least one activator selected from the group of heat, pH, catalyst, drying and radiation. In one embodiment, the catalyst is an aluminum sulphate catalyst. In one embodiment, it is possible to use metal catalysts, such as inorganic salts of aluminum, magnesium and zinc, as the catalyst. In an alternative embodiment, it is possible to use acid catalysts as the catalyst, in particular for dialdehyde and glyoxal crosslinkers. In one embodiment of the invention, the cel-

■ lulose derivative is dissolved prior to crosslinking in a selected solvent, in one preferred embodiment in water. Any suitable solvent may be used as the solvent. The combination of the cellulose derivative and the solvent is in the form of a solution, sludge, suspension or dispersion. In one embodiment, the solid

content in the solution of the cellulose derivative or the like is less than 10% by weight.

In one embodiment, the crosslinker may be in liquid form and dissolved in a suitable solvent, e.g. water or ethanol.

In one embodiment of the invention, a surface-active agent is added to the cellulose derivative prior to crosslinking. The surface-active agent may be selected from the group of dispersing agents, non- ionic and anionic tensides . In one embodiment, other suitable additive required by the use is added to the cellulose derivative.

In one embodiment of the invention, a preferably light-scattering porous foam structure of the pigment product is formed by foaming, e.g. heavy stirring. In a preferred embodiment, the resultant foam structure is stabilized, e.g. by crossbinding. In one embodiment, the crosslinker is added in the step of foaming the cellulose derivative. In one embodiment, the crosslinker is added in the step of stabilizing the structure. In one embodiment, the crosslinker is added to the cellulose derivative prior to foaming. In one embodiment, the cellulose derivative is heated to provide the foam. The foaming may be provided chemi- cally and/or mechanically.

In one embodiment of the invention, the resultant pigment product is dried. In one embodiment, drying is performed at a temperature of 70 to 100 ⁰ C, more preferably at a temperature of about 80 to 9O ⁰ C. Crosslinking may be completed during drying. During drying of the pigment product, if not earlier, the crosslinking generates between the polymer chains bonds which are insoluble in water.

In one preferred embodiment, the pulveriza- tion is performed after drying.

In one embodiment of the invention, the resultant pigment product is cured by heat.

The invention is also based on the pigment product formed in a manner described above and on the use thereof. The provided pigment product may be used in the surface treatment, such as coating and covering, of a substrate, e.g. paper, and/or as a filler in a substrate, such as paper.

The pigment product and process for the pro- duction thereof according to the invention are applicable in the paper industry, e.g. in fine papers, newsprints, printing papers and soft tissues, in the cosmetics industry, e.g. in toothpastes, in the paint industry, hygiene product industry, plastics industry, coating industry, composite product industry, sheeting industry or in an equivalent application.

In this context, paper refers to any fiber- based paper or fiber product or the like. The paper may have been made from chemical pulp, mechanical pulp, chemimechanical pulp, recycled pulp, and/or mixtures thereof or the like. The paper may contain suitable fillers and additives and different kinds of surface treatment and coating agents. The paper may be in the form of a web or a sheet, or in other form suit- able for the purpose.

The pigment product formed according to the invention may be used either alone or in a mixture with other agents in different kinds or pigments, pigment precursors, mixtures, compounds or the like. The pigment product or particles thereof formed according to the invention may be modified e.g. by crosslinking and functionalizing in the manner required by the use. The product is provided with different functional properties. Properties of the product may be tailored in terms of e.g. opacity, lightness, printing properties and oil absorption. The product may

also be utilized as an additive or auxiliary agent in different uses.

The invention provides an excellent industrially applicable way of producing an organic pigment product .

The process according to the invention provides a porous pigment product with lower specific gravity than in the previously known mineral-based pigments . Another advantage of the pigment product according to the invention is recyclability. This permits the reduction of waste problems.

The pigment product according to the invention is applicable in novel paper applications in which low specific gravity of the raw materials provides the end product, such as paper, with new functional properties and improves cost-efficiency in the product supply chain from the producer to the customer.

DETAILED DESCRIPTION OF THE INVENTION

The invention is described in more detail by the following example.

Example 1

In this example, crosslinking a commercial hydroxy propyl cellulose (HPC) and production of the pigment product according to the invention were stud- ied.

Two types of commercial hydroxy propyl cellulose were used in the tests in a 50/50 mixture; a) Klucel H (M _w = 1150000, solution = 1% in water, viscosity = 1275 to 3500mPas) and b) Klucel M (M _w = 850000, solution = 2% in water, viscosity = 3500 to 7500mPas) .

The hydroxy propyl celluloses were crosslinked in an aqueous solution with glyoxal . Samples were prepared in portions of 10 to 5Og. The aqueous solution was prepared by first mixing 5Og of HPC powder mixture in hot water of a temperature of about 50 to 60 ⁰ C, with the solid content of 8% by weight. The homogenized dispersion was diluted in cold water (about 5 to 10 ⁰ C, pH 4 to 5) under constant stirring so that the final solid content of the resultant aque- ous solution was 5% by weight. Glyoxal solution (40% by weight in water) , a few drops of a surface-active agent (TISKO, Farmos) and an aluminum sulphate catalyst were added to the solution at room temperature under heavy stirring in a blade type mixer so as to ensure sufficient access of air to the resultant product. Glyoxal was added in an amount of 10% by weight of the amount of HPC and the catalyst was added in an amount of 5% by weight of the amount of HPC. The resultant product was in a foam form. The resultant foam product was dried at 90°C in an oven so as to remove water and obtain a foam structure which was insoluble in water. The short dwell time of the product in contact with the aqueous phase which was removed reduced film formation and softening in the structure of the foam product, improving the quality properties of the product.

Highly substituted high molecular weight hydroxy propyl cellulose is a non-ionic derivative which is soluble in cold water and forms a gelatinous medium at an elevated temperature of the solution and is resoluble during cooling. A reaction with a hydroxyl- active glyoxal crosslinker produces a stable foam structure which is insoluble in water.

Dried HPC-glyoxal foam structures were pul- verized to microparticles in an air jet pulverizer with a classifier speed of lOOOOrpm. Particle sizes of

the microparticles were in the range of 2 to lOOμm, and on average 8 to 25μm.

It was discovered that with smaller amounts of cellulose derivative it was possible to use the crosslinker in an amount of 4 to 5% by weight of the amount of HPC. A relatively high solution viscosity of the cellulose derivative had a positive effect on foaming.

In the tests it was discovered that the re- sultant pigment product is applicable for use both as a pigment and as a filler.

Example 2

In this example, crosslinking a commercial hydroxy propyl cellulose (HPC) and production of the pigment product according to the invention were studied.

Commercial hydroxy propyl cellulose Klucel G (Mw = 370000, solution = 2% in water, viscosity = 125 to 450mPas) was used in the tests.

The hydroxy propyl cellulose was crosslinked in aqueous solutions with methyl diphenyl diisocyanate (MDI) with different concentrations of the crosslin- ker. Samples were prepared in portions of 5 to 1Og. The aqueous solution was prepared by first mixing 50/1Og of HPC powder in hot water at a temperature of about 50 to 60 ⁰ C, with a solid content of 8% by weight. The homogenized dispersion was diluted in cold water (about 5 to 10 ⁰ C, pH 7) under constant stirring so that the final solid content of the resultant aqueous solution was 6% by weight. The MDI solution (about 30% by weight in ethanol) in an amount of either 50, 30 or 10% by weight of the amount of HPC and a few drops of a surface-active agent (TISKO, Farmos) were added to the solution at room temperature under heavy

stirring in a blade type mixer so as to ensure sufficient access of air to the resultant product. The resultant product was in a foam form.

The resultant foam products were dried at 90 ² C in an oven so as to remove water and obtain a foam structure which was insoluble in water. Reaction of the MDI in the presence of moisture produced active bubble formation in connection with drying which was preserved as fine-divided porosity in the dried foam structure. In all applied concentrations of the MDI the dried foam structures were opaquely white, rigidly fragile and porous, which improved the quality properties of the product. A reaction of HPC with a hy- droxyl-active diisocyanate crosslinker produces a sta- ble foam structure which is insoluble in water and which could be ground to particles.

By liquid immersion process the refractive index of the particles in the HPC-MDI foam structure was estimated to be in the range of 1.50 to 1.55, be- ing thus higher than that of the equivalent untreated HPC (1.45 to 1.50) .

The process according to the invention is applicable in different embodiments for use in producing the most different kinds of pigment products.

The invention is not limited merely to the example referred to above; instead, many variations are possible within the scope of the inventive idea defined by the claims .