FI109214B - Procedure for adding pigment to paper - Google Patents

Description

1 109214

A method for applying pigment to paper

The invention relates to a process for adding pigment to paper, wherein the pigment raw material is added to the paper, after which the pigment is formed.

In addition to cellulose-containing fibers, an important part of the composition, particularly of certain grades of paper, is constituted by the fillers used there, which can be very high in paper. Fillers have 10 effects on the appearance and technical properties of paper by improving the optical properties of the paper, such as increasing opacity and improving printability.

Fillers are usually inorganic and mineral based and consist of small particles that fit into the interstices of the paper fiber network. Fillers can be selected from a variety of inorganic minerals.

Increasing the filler proportion also reduces the cost of raw materials, 20 since the cost of fillers is usually lower than that of the fibrous material. In addition, the process technically the filler can facilitate dewatering in the early stages of web formation, but in this case their retention in the formed paper web is important.

In general, the filler is obtained from a filler supplier meeting certain quality requirements and with a suitable particle size distribution, and mixed with other pulp components, followed by a pulp of appropriate consistency from the headbox to the forming part, which begins to dewater the pulp to form a paper molding. In addition, there are inorganic fillers whose chemical structure enables them to be prepared from stock solutions in situ, for example in a paper mill. Such a filler is particularly calcium carbonate, and numerous examples of the use of precipitated calcium carbonate (PCC) as a filler for papers have been disclosed in the patent literature. The manufacture of PCC integrated in papermaking is disclosed in U.S. Patent No. 5,558,782, according to which fines in recirculating water can be separated by precipitation of calcium carbonate in finely-recycled water, and the finely-containing PCC 2 109214 can be used as a filler. In addition, the precipitation of calcium carbonate in an aqueous medium already containing the selected paper stock fibrous material has been of particular interest, and in particular the so-called lumen-loading technique has been the subject of research. For example, European Patent Publication No. 791685 discloses the in situ precipitation of calcium carbonate from calcium hydroxide with carbon dioxide into the pores and lumens of cellulosic fiber walls in a fluidized state. U.S. Pat. No. 5,223,090 discloses the carbon dioxide precipitation of calcium carbonate to 10 lumens of fibers in a pressurized refiner. U.S. Patents 50,96539 and 5,275,699, in turn, disclose the precipitation of calcium carbonate on fiber walls by treating the fibers in a first step with a solution of a soluble calcium salt, for example, calcium chloride and in a second step with a solution containing a carbonate anion.

15! U.S. Patent No. 5,679,920 discloses another in s / fi / precipitation method of calcium carbonate among the fibrous material. In this method, the slurry of the paper raw material fibers is combined with a calcium hydroxide slurry in a tubular reactor, and carbon dioxide gas is introduced into the pipe filing mixture 20.

International Publication WO 90/09483, to which e.g. Finnish Patent No. 96,336 and U.S. Patent No. 5,262,006, describe a method for removing dissolved gypsum in a recycled pulp by adding carbonate or bicarbonate anionic salt to a stock containing recycled pulp, e.g., sodium carbonate, wherein: : sodium is precipitated as a filler and soluble sodium sulfate is eliminated with water.

»30 Using recrystallized calcium sulfate as a filler in paper. U.S. Patent 4,801,355 may be cited as an example.

* · "* The in situ deposition of fillers on the fibrous material of the paper requires: of course, major changes to the pulp processing system of the paper mill. In addition, the provision of paper with fillers follows a conventional method: the filler and fibers are mixed together. the raw material of the paper is passed as a dilute suspension to the paper formation.

i 3 109214 i i I In paper coating, on the other hand, it is intended to precisely adjust the surface properties, for example for printing, by adding pigment to the surface of the paper web by means of a suitable application method. These methods are intended to completely cover the surface of the paper.

WO 00/03092 (p. 6, p. 12) discloses the transfer of calcium oxide to the surface of a paper web, e.g.

Conventional application methods use binders to provide pigment 15 to the surface of the paper web. Em. in WO 00/03092, which aims to use as little water as possible in coating and to implement the idea, e.g. in dry web, it is necessary to use carbon dioxide gas and to provide a carbon dioxide atmosphere somewhere along the web path.

20

The use of carbon dioxide to prevent the dissolution of low-pH calcium carbonate in the paper stock is known from Finnish patent 103520, which is the subject of International Publication WO99 / 35333.

It is an object of the present invention to provide a novel method for applying pigment to paper which provides new possibilities specifically for providing the paper surface with small amounts of pigment (usually less than 2 g / m 2). It is an object of the present invention to provide a process that does not require major changes in the papermaking process and which is particularly suited to the smoothing and coating of paper with pigment. To accomplish this, the process according to the invention is essentially characterized in that the pigment feedstock is fed in a soluble form to at least partially formed moving paper or web, and after feeding the feedstock is: under conditions which allow the pigment to precipitate.

4 109214 j Raw material in solution form can be sprayed or applied by any suitable method on the surface of a paper web in motion. The conditions under which the raw material is precipitated as a pigment may be conditions of the paper web which may or may not be suitable at the time of addition. Such conditions that cause precipitation may be temperature, pH, or ions added to the paper web. In addition, it is possible that, when spraying the solution into the shower, conditions are created which promote pigment precipitation, e.g. by spraying another solution.

10

The pigment is compounded at a stage where the basis weight of the paper has already been determined and water has been removed from the formed or formed dough. Precipitation can be carried out on a paper machine at a convenient location or on a paper finishing machine to finish paper with a frame-structure. The delay between feeding the raw material solution into the web or toward the web when precipitation is induced is as short as 0-1.0 s, whereby the pigment is better retained on the web surface.

The invention allows a novel "in s / fu" pigment deposition to be carried out in such a way that the pigment remains on one side of the web on which the raw material is added in soluble form. This results in filling the web pores and smoothing the web surface. surface quality, which influences printing properties, etc. The process differs from the coating in that the pigment particles are not ready to be applied to the surface of the web and the method can be applied in very small amounts (less than 2 g / m 2). This is because the pigment particles deposited on an in situ principle on a moving paper web adhere well to the fibers.

Pigment raw material in soluble form, as used herein, means a substance which, in solution form, contains any component of the pigment (in the case of salts, at least one of its ions) or contains all its components. In practice, the precipitation is carried out by adding the soluble salt, usually an inorganic salt containing at least one component of the pigment, to the moving paper web, whereupon the precipitation can take place by two principles: s 109214 (iia) or b) changing the paper web conditions after the addition such that the pigment precipitates.

In the latter alternative, the conditions may be deliberately altered precisely with a view to the precipitation of the pigment, or they will become suitable due to the usual process of making or finishing paper.

10

Alternatively, the precipitation of the pigment can be started already when its raw material is fed in solution form per paper web, i.e. the solution to be sprayed per paper web is already subjected to precipitation conditions in the air, e.g. by feeding another substance into the shower.

15

Conditions that can influence pigment precipitation include pH, temperature and / or the concentration or quality of ions from another source. Several factors can be adjusted simultaneously. The choice of precipitation methods depends on the pigment itself and its precipitation chemistry. By adjusting the conditions to the point of addition or after the addition point, many inorganic pigments can be precipitated, which will be described below.

<* :: The doped inorganic pigment is a salt. Salt precipitation; The '25 can be carried out in two ways: 1) The aqueous salt solution is added to the paper web at a certain concentration, and the precipitation takes place under the conditions of the web which give the salt from a dissolved form to a solid form.

30 ·; 2) An aqueous solution containing one ion of salt at a given concentration is added to the paper web, and precipitation occurs in Taina by the effect of a different concentration of another ion of different quality; tuksesta. In the case of salts, these are anions and 35 cations which form soluble salts separately but together produce a water insoluble salt.

6 109214 i

The invention will now be described with reference to certain pigments and their precipitation patterns. However, the invention is not limited to these excipients.

i | According to the invention, suitable salts are dissolved in a soluble form in the aqueous phase and treated with this water in neutral or acidic paper either on a paper machine or in paper finishing, such as a supercalender. This | then the salts are precipitated on the surface of the paper under certain conditions (temperature, pH and / or solubility product, i.e. ion concentration). With the Liu-10 paper machine, the salt can be added to a moving continuous paper web by damping rollers or brush rollers or with water from a spray dampening device. In turn, a post-processing machine, such as a supercalender, can be used for moistening or steaming or also for brushing.

In the following, the precipitation of gypsum, calcium carbonate and aluminum hydroxide is investigated.

Gypsum 20 It is known that the behavior of gypsum (calcium sulfate) is very different compared to other pigments. Under normal conditions, gypsum always dissolves in the aqueous phase at about 2 g / l. The solubility of gypsum is highly dependent on the system temperature and to some extent on the pH (at pH less than 2 gypsum: precipitation begins). The present invention utilizes g 25 solubility of gypsum vs. high temperature precipitation at the usual pH values used in papermaking. Gypsum is added to less than 2 g / l, e.g. Thereafter, during the heat treatment of the paper, which may be part of the papermaking process itself (e.g., hot calendering), the surface temperature of the paper rises so high that calcium sulfate precipitates. The temperature is preferably at least 150 ° C. The gypsum can be dissolved in moistening water, which is added after the drying section before hot calendering. The delay from wetting to the hot - calender roll nip is as short as possible, 0 to 1.0 s.

'35: Likewise, in a post-processing machine such as a supercalender, dissolved gypsum may be added to the paper, which is discharged from the roll and driven to a calender where the precipitation occurs at a temperature of 7109214. This application requires high supercalender temperatures, at or above 150 ° C. At the above temperature, the solubility of the gypsum is only about 500 ppm, so that the yield of the gypsum moves between 60% and 75% (from soluble form to solid). To make the gypsum settle on the surface of 5 papers, the aqueous solution is added before the calender nip and the delay from wetting to the hot calender roll nip is as short as 0-1.0 s.

It is also possible to increase the precipitation of gypsum in the paper by adding an excess of calcium or sulfate ions to the already sprayed aqueous phase containing the dissolved sulfate on the paper, i.e. the ratio of ionic strength to the solubility can be adjusted.

I A good substance for this is sodium sulfate (a source of sulfate ions), but also calcium carbonate or calcium bicarbonate (a source of calcium ions).

In the method used, many types of solid gypsum in solid form can be used as starting materials, since the particle shape of the raw gypsum to be dissolved does not affect the gypsum deposited on the surface of the paper web.

! 20

Aluminum hydroxide i

The alum, or aluminum sulphate, is dissolved in the aqueous phase to give a trivalent aluminum ion and a sulphate ion. Then the solution is. 25 on the acid side. This can be done with the paper machine wetting water as well as with a paper finishing machine such as a super calender wetting ··; ' through. Subsequently, alkaline water is added to the web, resulting in a web pH

v \: is allowed to rise and free aluminum forms aluminum hydroxide in the neutral region. Al (OH) 3 is a very light-colored, sheet-like pigment with a high specific surface area. On the other hand, if the web is already in a neutral position or at a pH suitable for precipitation, it may not be necessary to change the pH with water anymore, i.e. there are already conditions favorable for precipitation in the web. This depends on the buffer capacity of the web. · '··. * PH.

35

For example, after the drying section, the humidifying - ..: water may be applied to the web, and prior to the hot calender, the alkaline **. water to the web.

8 109214 i i | j

In addition, it is possible to add alkaline water already in the air to a jet of alumina, i.e. practically the two solutions are sprayed together.

5 Calcium carbonate

The means for precipitating CaCO 3 in the paper is to add a solution containing Ca ion, and immediately after the addition, a moistening water containing soda or sodium bicarbonate. Hereby, Ca-ions and carbon-10-sodium ions cause precipitation to occur.

The solution containing Ca ions may be derived from gypsum. It is then possible to improve the yield by precipitating at least a portion of the dissolved calcium added to the paper web as calcium carbonate when calcium sulphate is doped from the molten form by means of temperature.

The precipitated salts or pigments have the following effects on paper quality: the pigment is better obtained on the paper surface 20 - increases the brightness, increases the opacity (absorption and scattering), decreases the surface roughness, improves the surface of the paper (improves printability) for they dissolve in the aqueous phase and when precipitated, * 25 they may not form their former crystal form. When using gypsum, for example, raw gypsum works well, as long as the gypsum is sufficiently clean.

a particular advantage of gypsum is that its solubility is pH independent in the pH ranges used in papermaking, whereby it functions in both neutral and acidic systems.

In particular, the invention is suitable for use in a paper preparation, manufacturing and finishing process in which the paper surface is subjected to a certain pressure, e.g. Calendering may also provide a high temperature suitable for the surface of the web if this is necessary to effect precipitation. This may be any one or more nip calendars utilizing one or more hot rolls. The heat can be applied to the surface of the web in other ways, e.g. by infrared heating.

The invention is not limited only to certain points in the papermaking line and paper finishing. Precipitation may also be accomplished by spraying or spraying a press portion or a drying portion onto a surface of the web containing a pigment-containing solution.

i

Claims (12)

1. A process for adding pigment to paper, in which method pigment tear is added to paper, after which the pigment is formed, characterized in that the pigment tear is measured in soluble form to an at least partially shaped, moving paper web or against the paper web, and The pigment is brought in such circumstances that the pigment can be precipitated. 10 2. A method according to claim 1, characterized in that after the tear has been fed to the paper web, the circumstances in which the pigment precipitates are arranged in the paper web. Method according to claim 1, characterized in that, at the time of feeding, the paper web 15 already has such circumstances that the pigment precipitates. Method according to claim 2 or 3, characterized in that the pigment tear is added as a dissolved site, and by the circumstances in the web 20, the solubility of the salt added in the web is affected in such a way that the dissolved salt precipitates as a pigment on the surface of the web. 5. A process according to claim 4, characterized in that the conditions comprise at least any of the following: temperature, pH or ionic concentration of the dissolved salt. 6. A process according to claim 2 or 3, characterized in that the pigment tear is added in the form in which a first ion, one of the pigment's components, is in the dissolved state, and through a second ion the pigment-forming salt containing at least said the first ion and said second ion are deposited on the surface of the web. Process according to any of the preceding claims, characterized in that the pigment precipitated is at least one of the following: gypsum, aluminum hydroxide or calcium carbonate. Process according to any of the preceding claims, characterized in that the pigment is precipitated on a paper machine by adding FI 109215B Batch: B0207014 Date: 01/07/2002 Number of pages: 40 Previous document: FI 109214B Next document: FI 109216B