FI112797B - Coating element for paper, cardboard and similar fibrous webs, comprises pigment-binder structure or particle granule having rotationally symmetrical shape - Google Patents

Abstract

A coating element consists of pigment-binder structure or particle granule. The particle granule has rotationally symmetrical shape and comprises inner and crust parts. The inner part has lesser density than the crust part. The particle granule comprises pigment particles, binder, and optionally additives. The surface layer of the granule has layer(s) of pigment particles. Independent claims are also included for (A) a method of preparing the coating element by forming an aqueous slurry containing pigment particles, binder and optionally additives, and drying the slurry in specified conditions where the pigment particles are bound to one another through a binding agent and form rotationally symmetrical particle granules; (B) a coated fibrous web containing a web from lignocellulose-based raw material and a coating layer on one surface of the web; and (C) a method of coating a fibrous web by spreading particle granules on surface(s) of the web, and subjecting the coated web to mechanical treatment to flatten the granules.

Description

, 112797

Granule made of plate-like particles

The present invention relates to a granule according to the preamble of claim 1. Such a granule comprises a granular structure obtained from sheet-like mineral particles and binders. The granule structure can be used, for example, to cover a paper web or to fill spaces between fibers of a fibrous web (filler application). The granule can also be used as a porous ingredient in other matrices P-10, such as permanent polymers, elastomers or resins. It can also be used in intermediate coatings of various coatings to give special properties.

The use of different fillers has many different purposes. The filler may give the matrix special properties such as strength, or it may make the structure economical, since the cost of a filler is generally cheaper than the surrounding matrix material. The disadvantage of using mineral fillers is the weight gain resulting from their use. Low density fillers can be used to minimize this disadvantage, but the best solution for other properties may not always be achieved. Various chemically or thermally structured pigments are available on the market which consist of solid particles which are bonded to one another either chemically by an inorganic or organic binder or: thermally, e.g., by rebellion. Commercial so-called the rebated kaolins are formed of lamellar particles which are thermally bonded to agglomerates.

The particles form disorganized structures that make them usable. ; 25 in water slurries (poor rheology, very dilatant at higher solids).

When used, naturally occurring minerals have a characteristic particle size distribution. This distribution can be controlled by mechanical processing to influence, for example, the theology of the aqueous solution of the particles, or the degree of compression of the final product. Generally, a steep distribution is more difficult than a gentle distribution and imposes greater demands on the process technology.

2 112797 When used as a filler in a fibrous web, its binding is promoted by the use of a suitable polymer that provides bonding between the particle surfaces and the fiber matrix. In the polymer matrix, the filler may be surface-treated with a suitable agent to optimally adhere the particles to the polymer matrix.

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There are significant drawbacks to known solutions. Thus, when bulky particle particles are used as a filler or fiber pigment coating pigment, the weight of the filled or coated product is increased because of the high density of the bulk particles. This may have a detrimental effect on product use or economy. If the same properties could be achieved with a lower density pigment, it would be of great economic advantage.

The rheology of plate-like particles imposes certain restrictions on the use of particles. As the aspect ratio of the particles increases, the maximum solids content of the water slurry is lower. Such slurries have dilatant properties which is detrimental to the coating event.

The porosity of the coating layer has a significant effect on the optical properties of the layer. Air pores trapped between the particles and the matrix behave in an opaque manner, thus reducing the transmission of light. This is particularly the case with paper webs. From the viewpoint of the optical and printing properties of the coating, it would be advantageous if the finest part of the particle distribution remained in the coating surface layer and: formed a smooth, finely divided printing substrate. A coarser portion, on the other hand, would bring bulk to the coating layer. In practice, there is no such distribution: without much arrangement, this optimum coating structure is now achieved by coating the web twice, first with a coarser pigment and finally with a finer pigment. This increases costs both in terms of investment and operating costs.

When the plate-like particles are used in the coating of the fibrous web, and the commonly used blade coating methods are used as the coating method, it has been found that the plate-like particles under the blade are oriented mainly in the direction of the web. In this case, no significant amount of air pores are formed between them. Indeed, in order to increase the porosity of the coating structure, a second pigment of less sheet shape is generally used in the coating compositions to give the structure more porosity.

In connection with the fiber web coating process, the swelling of the fibers occurs due to the presence of water in the coating and the surface roughening due to shrinkage of the coating structure, whereby the coated dried surface generally always requires a smoothing treatment (calendering).

The object of the present invention is to eliminate the drawbacks of the known solutions and to provide a completely new kind of granule suitable for e.g. filling and coating of fibrous webs.

Granules suitable as color pigments for building materials are described in DE-15 application 196 32 928. Granules consist primarily of spherical particles such as titanium dioxide. There is no mention in the publication of the particle size distribution of the pigments or the internal structure of the granules.

In our previous Fl patent application 20000929 representing PL 2 § 2 ·. 20 mom, referred to as pigment particles with: ·. granule, but the application does not mention the particle size distribution of the pigment particles.

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The invention is based on the finding that when the crystalline form of the plate-like pig-; A suitable binder and possibly other excipients are added to the carrier solution of the menthol, such as kaolin, and the mixture is led to a process for forming droplets in the gas stream, whereupon the solvent phase of the droplets evaporates to form granules of particles and binder. Such conditions are obtained, for example, in connection with spray drying. The choice of carrier is application specific, the most common being water.

As the liquid in the droplets evaporates, it can be observed that the plate-like particles on the surface are aligned with the droplet surface, thereby minimizing surface energy. The phenomenon is caused by the force exerted by the water on the particles of the meniscus. Under the same force, the surface of the granule is evenly balanced as much as rotationally symmetrical, e.g. It has been found that the fine particles adhere to the surface of the droplet to form its surface layer. On the other hand, the movement caused by the evaporation of the water 5 also introduces a binder into the surface of the droplet. After drying, the droplet has formed a particle and binder granule, characterized by a layer of lamellar particles on the surface of the granule, in which the planar surfaces of the particles are oriented parallel to the spherical surface. The layer is enriched with binder. The granule still has a 10 porous center formed of disordered lamellar particles which in turn are impoverished with respect to the binder.

Not less than 60% <5 µm, not more than 95% <1 µm in size distribution of lamellar particles.

More specifically, the solution according to the invention is essentially characterized by what is stated in the characterizing part of claim 1.

The invention provides significant advantages. So when making granules. . For example, kaolin with a density of 2.7 g / cm 3 is used as a particle and latex with a solid density of 1.0 g / cm 3 as a binder is typically only about 60%, i.e. 1, of the granule of the invention. 6 g / cm 3. Thus, a favorable size distribution and controlled compression have been achieved. Granular Opticians'. it is possible to determine the properties already at the manufacturing stage.

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As the granule density is substantially lower than that of the bulk particles of its constituent parts, it has now surprisingly been found that such granule can advantageously be used as a low density filler or as a fiber web lining. As the coating, the granule binder should be selected such that plastic deformation is possible during calendering with the aid of pressure and / or temperature.

The granule according to the invention can be provided with a coating in which the surface layer of the granule forms the final coating surface layer. The plate-like 5,117,997 particles are already parallel to the surface, and with a slight smoothing calendering, a smooth printing base can be achieved.

The rheology of the spherical granules in the aqueous mixture is very good, whereby the volume concentration of the top-5 mixture may be high.

Coarse particles can also be used as granule building materials, which properties as such would not be suitable for producing a surface layer of the printing surface, but because they are surrounded by finely divided particles in the interior of the granules 10, their use provides an economic advantage. On the other hand, a very fine particle size of less than 0.3 µm can cause disadvantages in conventional coating applications, inter alia by filling the spaces between the particles and thereby reducing the porosity of the coating. These finely divided particles can be bonded to the surface layer of the granule, whereby their partial use is possible without these disadvantages. Generally speaking, when granules are used, particle size distribution is not as important as if the pigment were used as part of a conventional coating process.

Some of the aqueous phase that would conventionally be evaporated by the drying section of the coating machine: 20 is already evaporated during the granule making process, in which:! : heavier investments in paper industry equipment can increase capacity; '· And, as a whole, there is considerable economic advantage.

: The invention will now be explored in greater detail by means of a detailed description with reference to the accompanying drawings.

': Figure 1 is a schematic representation of a cross-section of a granule according to the invention; Figure 2 shows an electron microscope image of a granule according to the invention; Figure 3 shows an electron microscope image of a granule surface; granule.

As stated above, the particle granule according to the invention consists of sheet-like pigment particles and a binder. These lamellar particles have at least one planar surface, most often planar surfaces. The invention utilizes plate-like pigments as further described below. The surface layer of the resulting granule consists of a layer having at least one layer of pigment particles which are at least substantially positioned with their planar surface parallel to the surface of the granule and the plane surfaces of the pigment particles. Most of the pigments, i.e. at least 50%, preferably at least 70% and particularly preferably at least 90%, are disposed on the surface of the granule such that their long side planar surface is 15 surface of the surface of the granule. way.

Figure 1 is a schematic representation of the granule structure. Reference numeral 1 denotes plate-like particles arranged parallel to the surface. Reference numeral 2 denotes a porous surface portion enriched with organic excipients and fine. . 20 particle fraction. Reference numeral 3 denotes a pore-like central portion in which the particles are randomly oriented.

,, ..: Inside the granule, the proportion of binder is smaller than its surface area. Typical optionally, the binder comprises from about 0.5% to about 3% by weight of pigment in the interior, preferably; 25 about 1%. Indeed, the binder is concentrated on the surface of the granule, where it is present in an amount of 6 to 60% by weight, preferably about 50% by weight of the pigment.

Granules can be prepared from a slurry having typically less than 50% by volume of sheet-like particulate raw material, but most commonly 20 to 30% by volume. The choice of the raw material is made according to the intended use, for example, high-brightness pigments such as kaolin or talc are used to coat the fiber web. Other applications may include other sheet materials such as mica, graphite or the like. The slurry contains a binder soluble or in dispersible form in the carrier used and a dispersant, if necessary. Commonly used latexes such as styrene-butadiene latexes or acrylic or vinyl acetate copolymer latexes are suitable as binder for most applications. The organic phase micelles of such latexes typically have a size of 0.1.0.2 µm. The solids and binder content of the slurry affect the structure of the granules formed.

The medium is especially water. However, for specific applications, organic solvents or mixtures of water and organic solvents may be used. Of these organic solvents, mention may be made in particular of alcohols such as ethanol and n- and i-propanol.

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The dimensions of the granule and the plate-like particle used for it have a certain optimum. The particles must be able to settle on the surface of the granule, with a suitable maximum particle level dimension of less than 0.1 times the diameter of the resulting granule.

15 Granules can be made with a spray dryer whose operating parameters can be used to adjust the size and structure of the granule formed. The average size of granules in commonly used dryers is always less than 1 mm, more generally 2 to 500 µm. Granules suitable for coating applications have a size of 10 to 50 µm, whereby commonly used coating kaolins-20 and having a particle size distribution of 98% <2 µm can be used as constituents or coarser than that; : '; or alternatively this finer product. However, according to the invention, the particle size distribution of the plate-like particles is at least 60% <5 µm, up to 95% <1 µm.

:; The method also makes it possible to use mixtures of particles having different distributions.

The granule dries in three steps: 1. The liquid film, which initially completely or nearly completely covers the surface, evaporates until the particles are exposed.

2. The liquid continues to evaporate from the interparticle region, the granule shrinks due to surface tension forces, the particles displace water from the interior of the granule, and water flows towards the surface.

112797 8 3. The particles can no longer shrink, the liquid front retracts between the particles and the rest of the water is removed by evaporation from the inside of the granule.

The plate-like particles minimize their free surface energy by aligning their planar-5 tan with the spherical granular surface. The average size of the surface-forming particles is smaller than that of the granule as a whole. This is due to the fact that small particles are able to turn more readily to the surface, and that, in the second drying step, water transferred to the surface carries the small particles to the surface between the large ones.

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Production of granules is also possible using other drying methods in which the droplet is free to dry and forms the granule structure of the invention.

In the example, an aqueous slurry having a dry solids content of 33.4% was prepared from commercial filler kaolin (Intramax, ed. Imerys Minerals). 5% by weight of vinyl acetate acrylate copolymer latex (Acronal, ed. BASF) was added to the slurry as a binder. The mixture was run into a pilot spray dryer. Figure 2 shows the resulting granule having a diameter of 60 to 70 µm. Figure 3, taken from the surface of the granule, shows that a large part of the surface consists of particles having a size <2 µm.

The structure of the split-cut pigment granule is shown in Figure 4, which corresponds to the principal view of Figure 1i.

The granule according to the invention is plastically deformable by pressure and / or temperature, as shown in Figure 5, which shows the structure of the granular layer applied to the glass surface after it has been calendered.

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Claims (10)

1. Particle granules consisting of disc-shaped pigment particles and of binder and in which the surface layer of the granulate consists of at least one layer of pigment particles which are arranged essentially so that their planar surfaces are parallel to the surface of the granules and the planar surfaces are closest to each other. the same plane, and the binder is concentrated to the surface layer of the granulate, characterized in that the size distribution of the disc-shaped particles is such that at least 60% is less than 5 µm and at most 95% less than 1 µm. 10 Granules according to claim 1, characterized in that between the pigment particles of the surface layer there are adhesives and pores. Granules according to claim 1 or 2, characterized in that the ratio between the surface of the disc-shaped pigment particles and the surface of the binder and the pores between the particles is 0.3..60: 1, preferably 0.5..4: 1. Granules according to any one of claims 1-3, characterized in that the inner part of the granulate is formed of a partially unorganized part of the disc-shaped pigment particles, in such a way that it mainly contains pigment particles, pores and small binder. ....: 5. Granules according to any one of claims 1-4, characterized in that 15 - 60%; . preferably 30 - 45% of its volume is pores. :: 25 Granules according to any one of claims 1-5, characterized in that the disc-shaped pigment particles are typically silicate minerals, in which the cations are alkali metals and / or alkaline earth metals. : 30 Granules according to any one of claims 1-6, characterized in that the binder is an organic binder which is soluble or dispersed in the liquid phase. 112797 Granules according to any one of claims 1-7, characterized in that the proportion of binder in the interior of the granulate is 0.5 to 3% by weight of the amount of pigment, preferably to about 1%. 9. Granules according to any one of claims 1-8, characterized in that the binder is concentrated to the surface portion of the granulate, the amount thereof contained in 6-60% by weight of the weight of the pigment, preferably about 50%. Granules according to any one of claims 1-9, characterized in that it is plastically moldable under the influence of pressure and / or temperature. I 1 (1 «; · * <» »> · * <* i * i I t 1 · '· 1. i» »