METHOD FOR MAKING ORGANIC PIGMENT

FIELD OF THE INVENTION

The invention relates to a method as defined in the preamble of claim 1 for making organic pigment, wherein the pigment is formed from a starting material of plant origin, and to a paper product as defined in the preamble of claim 18.

BACKGROUND OF THE INVENTION

Known from the prior art are different kinds of pigments and methods for making them. Pigments and fillers of both mineral and organic origin are known.

It is known from the prior art to form or- ganic pigments from starch-based materials and pure cellulose by chemical treatment steps. In these cases, the pigment is usually present in a mixture with some other material or in a dispersion. It is also known from the prior art to make organic pigments from oil- based materials.

When mineral fillers and coating pigments are replaced by materials of organic origin either partially or totally, the environmental load can be reduced. By using fillers or pigments of organic origin e.g. in the 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. Additionally, in typical cases, paper strength can be im- proved 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 oil price development, and high production costs. Also the waste treatment of

oil-based materials can be considered as one problem of making oil-based pigments.

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

OBJECTIVE OF THE INVENTION

The objective of the invention is to disclose a new type of method for making organic pigment from new types of organic starting materials. A further objective of the invention is to provide an easy and cost-effective production method for the industrial production of organic pigment.

SUMMARY OF THE INVENTION

The method and paper product according to the invention are characterized by what has been presented in the claims. The invention is based on a method for making organic pigment in such manner that the organic pigment is formed from a starting material of plant origin. According to the invention, the starting material is pulverized to a suitable particle size by mechani- cal grinding in at least two grinding steps to form the organic pigment.

The invention is specifically based on a method for making organic pigment from the starting material simply by mechanical grinding substantially without any complex chemical treatment.

In this context, the pigment refers to a material or substance or to a mixture thereof that provides an optical property or color or brightness in a carrier, product or the like or their surface. The pigment may be translucent or opaque. The pigment referred to herein may be used as a filler, coating agent, additive

or the like. In this context, the pigment does not refer to a mineral-based pigment.

In this context, grinding refers to any grinding, pulverizing, crushing or decomposition of a mate- rial to a desired particle size. The operation of the grinding devices, preferably mills, is typically based on applying pressure, on cutting, rubbing, compressing and/or on the effect of an impact provided by blowing, or on an equivalent operational principle. Most mills operate as a combination of many operational principles .

In one embodiment of the invention, the starting material is ground by a grinding method selected from the group comprising crushing-based grinding, at- trition-based grinding, cutting-based grinding, blasting-based grinding, wet grinding, dry grinding, grinding under pressure and combinations thereof. In one embodiment, the applied grinder is selected from the known grinding devices, such as an impact mill, air jet mill, sand grinder, pearl mill, ball mill, vibration mill, screw mill and combinations thereof. Each grinding step may be performed by the same grinding method or alternatively by different grinding methods. In one embodiment, the starting material is ground by air jet grinding, preferably by an air jet mill. Typically, in the air jet mill, air or steam which is led through the nozzles carries the material that is being pulverized. The high speed of the air or steam produces turbulent zones into which the solid particles are fed. The high kinetic energy of the air effects upon the particles impacting each other with sufficiently high force. Strong turbulence reduces the particle size as the particles impact and rub against each other and the mill wall. In one embodiment, the starting material is ground by Cryo-grinding. Cryo-

grinding refers to grinding which is performed at low temperatures, i.e. cold grinding.

In one embodiment of the invention, a grinding aid, e.g. a grinding mediator, which is harder than the starting material is used in the grinding. The grinding aid may be e.g. a mineral compound, lime concentrate, grinding ball or the like. Grinding aids allow reducing the mean size of the particles generated during grinding. In one embodiment, grinding aid is used in an amount of 10 to 30% of the total feed material in the grinding.

In one embodiment of the invention, grinding is performed in at least two steps by one or more devices. In one embodiment, grinding is performed in at least two steps, so that in each step the material is ground to a predetermined particle size, e.g. to coarse and fine fractions.

In one embodiment of the invention, the starting material is preground in the first grinding step to a predetermined particle size before the second mechanical grinding step.

In one embodiment of the invention, the starting material is ground in more than two grinding steps. In one embodiment, the starting material is ground in 3 to 10 grinding steps.

In one embodiment of the invention, the starting material is pretreated before the first grinding step e.g. by classification, mechanically and/or chemically or in an equivalent manner. In one embodiment, the pretreatment is selected from the group comprising cooling, freezing, drying, chemicalization, pre- crushing, screening the starting material, washing the starting material, removing the impurities, heat treatment, biotechnical treatment, e.g. enzymatic or micro- bial treatment, ultrasound treatment and combinations

thereof. In one embodiment, the starting material is treated in more than one pretreatment steps.

In one embodiment of the invention, the starting material is treated between the grinding steps. In one embodiment, the treatment between the grinding steps is selected from the group comprising cooling, freezing, drying, chemicalization, heat treatment, biotechnical treatment, e.g. enzymatic or microbial treatment, ultrasound treatment and combina- tions thereof.

In one embodiment, a sorting or classifying device can be used in conjunction with the grinding before any one of the grinding steps, whereby the particles according to a predetermined particle size are conducted to the grinding.

In one embodiment, the starting material is dried before grinding or before a grinding step. In one embodiment, the starting material is cooled before grinding or before a grinding step. In one embodiment, the starting material is cooled to a temperature below 0°C before grinding and/or between grindings, and grinding is preferably performed at a temperature below 0 ⁰ C. In one embodiment, cooling is performed by liquid nitrogen. In one embodiment of the invention, the formed organic pigment is treated after grinding by modifying the surface properties of the pigment and thus improving the properties of the pigment in respect to different uses. In this context, surface modification may re- fer to chemical surface treatment of the pigment, to coating, covering, surface dispersing, dispersing in order to make slurry or to an equivalent modification treatment .

In one embodiment, the material is ground to a particle size of less than lOOμm, in one embodiment to a particle size of less than 50μm, and in one embodi-

ment to a particle size of less than 30μm to form the organic pigment .

In one embodiment, a classification device is arranged in conjunction with the grinding for classify- ing the particles generated during the grinding to form a desired particle size distribution for the application.

In one embodiment, the moisture level of the starting material is adjusted to affect the particle size of the organic pigment that is being formed.

In one embodiment of the invention, the starting material of plant origin is a wood-based material. In one embodiment, the wood-based material is selected from the group comprising wood bits, dust, sawdust, chips, damp wood, waste wood, fiber pulp, wood pulp, cellulose, mechanical pulp, derivatives thereof or the like or mixtures thereof.

In one embodiment of the invention, the starting material of plant origin is a fiber-based material. In one embodiment, the fiber-based material is selected from the group comprising graminaceous plants, herbaceous plants, cereals or the like or plant bits, various aqueous or sewage sludges, sludge pulps, fiber- based industrial waste flows, starch-based materials, derivatives thereof or the like or mixtures thereof. In one preferred embodiment, the stalk part of the plants is utilized as the fiber-based material.

In one embodiment, the organic pigment is formed from a mixture of starting materials of plant origin which contains various starting materials of plant origin.

The method according to the invention may be applied for use in making different kinds of pigments which are applicable in the paper industry in paper products, e.g. fine papers, newsprints, printing papers and soft tissues, in the cosmetics industry, e.g.

in dental pastes, in the paint industry, hygienic product industry, plastics industry, coating industry, composite product industry, board industry or in an equivalent application. Also, the organic pigment made according to the invention may be used to replace fibrous and/or abrasive constituents in some applications .

The pigment formed according to the invention may be used either alone or in a mixture with other substances in different kinds or pigment products, mixtures, compounds or the like.

Further, the invention is based on a paper product which is formed from paper and which contains organic pigment made by any one of the methods pre- sented above.

The organic pigment formed according to the invention may be used in papermaking and in a paper product as a surface treatment agent for paper, as a coating agent and/or paper filler or their component. 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, fiber 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 which is suitable for the purpose.

The method according to the invention pro- vides considerable advantages compared to the prior art .

The invention provides an industrially applicable, simple and affordable way of making organic pigment. The method according to the invention is easy and simple to carry out as a production process.

The method according to the invention provides an organic pigment product with a lower grammage than in the previously known pigments. The organic pigment according to the invention provides cost sav- ings e.g. in transportation expenses due to the lower grammage, and in the papermaking in the form of a lower calendering pressure and reduced wearing in the equipment parts, thanks to the softer composition of the pigment . In addition, one advantage of the organic pigment according to the invention is recyclability . Consequently, waste problems resulting from the use of the pigment can be reduced. Various materials of plant origin, forest industry waste, recycling materials and/or materials which were previously considered as reject products can be used as the starting materials for the organic pigment according to the invention. Furthermore, compared to the previously known oil- based pigments, the production costs of the pigment according to the invention are lower.

DETAILED DESCRIPTION OF THE INVENTION

The invention will be described in more de- tail with the aid of the accompanying examples.

Example 1

In this example, different grinding combinations for making organic pigment used in a paper prod- uct were tested. The pigment was formed from birch dust.

In the first grinding test, the starting material was ground in two grinding steps, wherein the first step was pregrinding. In the second grinding test, the starting material was preground, and the preground product was

subjected to a cooling treatment before the second grinding step. In the third grinding test, heat treatment was applied as the treatment step after pregrind- ing, and then the second grinding step was performed. In the third grinding test, the starting material was preground, followed by a cooling treatment, and grinding mediators were applied in the second actual grinding step to enhance the grinding effect. In the fifth grinding test, grinding was performed in two steps, and grinding mediators were applied in the second step.

In the sixth grinding test, the starting material was pretreated by drying and ground after pre- treatment in two steps. Based on the grinding tests, one could conclude that grinding in at least two steps provides extremely good quality organic pigment for use in a paper product.

Example 2

In this example, organic pigment was formed from birch dust with the average particle size of about 230μm. Feeding moisture of the wood was arranged to be about 20%. Crushing under a pressure roller was applied as pregrinding, so that the particle size of the starting material could be halved before the second actual grinding step.

Dust that was utilized as the starting mate- rial was mechanically ground by Cryo-grinding without aids to the particle size class d50 of less than llμm in one step. Before grinding, the dust was cooled to a temperature of below 0°C by feeding liquid nitrogen, and cold dust was pulverized by grinding. Particles generated in conjunction with grinding were classified with a classifier.

When white-colored and hard lime concentrate with a particle size of about 100 to 200μm was used as aid in an amount of about 20% of the fed material in the grinding performed in the above-mentioned manner, the pigment particle size class d50 of less than 8μm was obtained. The average particle size was 10.14μm. The following particle size distribution was obtained for the ground product: dlθ% = 2.87 d30% = 5.34 d50% = 7.86 d70% = 11.51 d90% = 19.43

Example 3

In this example, organic pigment was formed from spruce cutter chips with feeding moisture of about 30%.

The starting material was first reduced by a hammer mill and then cooled and ground by an air jet mill in one step. The rotational speed of the classifier was lβOOOrpm.

The following particle size distribution was obtained for the ground product: dlθ% = 2.26μm d30% = 6.04 d50% = 8.83 d70% = 16.42 d90% = 21.98

Example 4

In this example, organic pigment was formed from birch dust.

The starting material was first reduced by a hammer mill and then cooled at 110 ⁰ C and ground by an

air jet mill. The rotational speed of the classifier was 20000rpm.

The following particle size distribution was obtained for the ground product: dlθ% = 2.35μm d30% = 3.98 d50% = 5.69 d70% = 8.13 d90% = 13.85

Based on the tests, it was concluded that organic pigments could be formed from starting materials of plant origin by multi-step grinding. An easily industrially applicable way of making organic pigment was discovered.

The method according to the invention is suitable in different embodiments for use in making the most different kinds of organic pigments.

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.