Coating color

The term coating slip (also known as coating slip) is common in the paper industry and refers to paints consisting of pigments , binders and additives that are applied (coated) to the paper surface with special coating machines to refine the surface of the paper. These papers are referred to as "coated papers" and are characterized by better printability and feel .

Historical development

Coated papers were already known in ancient China and Arabia. Here we used starch and minerals , which were painted by hand on the paper surface (Handpapiermacherei). Coating colors were first applied to paper with a roller in Dresden in 1866 and then rubbed and smoothed with brushes. Further development in Germany was driven by the colored paper mill in Aschaffenburg and the Scheufelen (1892) and Zanders (1895) paper mills . The technical advancement of the online coating machines led to machines with a width of over 10 meters and a running speed of over 3000 m / min.

Objectives of surface finishing

Improvement of the printability
Achieving a smoother, more homogeneous surface
wettability adapted to the printing process
optimal ink absorption / ink holding capacity
better overall opacity
Gloss (if required)
Better visual and / or haptic appearance (higher quality paper)
Increase in basis weight and density

Line styles

Starch screed (surface sizing with a starch solution)
Pigment coating (pigment-containing coating color)

Composition of coating colors

The main components of coating colors are:

water
Pigments (usually mineral white pigments or white minerals ),
Dispersants for pigments (usually polyacrylates )
Binders (usually synthetic acrylate copolymers)
Strength
Thickeners (usually methyl cellulose derivatives or acrylate copolymers)
Additives to regulate viscosity and water retention
Aid to reduce wet abrasion
Shading dyes, for example optical brighteners
Anti-foaming agents
Biocides

The solids content in a coating color is around 65-70% by weight, of which almost 90% is pigments. The proportion of binder is 10–15% by weight (based on the dry substance), all other additives are added in low concentrations (less than 1% by weight). The choice of pigments is of particular importance because they form the actual "body" of the line. The choice of pigments depends on the quality requirements of the coated papers, for example:

white
opacity
smoothness
shine
Pick resistance
Printability

As a rule, the focus is on printability and economy. To achieve optimum quality, the pigment and binder (e.g. hard or soft) must be matched to one another. When choosing the pigments, the most important criteria are the particle size (0.1 to 5 µm), the particle shape (platelets, spheres, needles), the whiteness and the need for binding agents. Mainly calcium carbonates , kaolins and talc are used. Satin white ( calcium aluminate sulfate ) or calcined clays are used as special pigments with special effects such as gloss and feel . Because coating pigments are cheaper than cellulose , and because recycling papers can also be upgraded with coating pigments, the demand for coating pigments has grown enormously. In Europe alone, over 10 million tonnes are used each year.

Varieties of coated papers

Base papers can be wood-containing or wood-free (see paper). Coated paper types are mainly art paper (letterpress), magazine printing paper (magazines) and folding box, while newsprint or office copier paper, for example, is not or only minimally coated. The quality requirements of the line depend primarily on the printing process ( offset printing / gravure printing ). The "coating weight" (application weight) can be 10 to 30 g / m² per side (top or bottom) and make up up to 50% of the paper weight. Depending on the coat weight, coated papers are divided into different classes:

With minimal lines of less than 5 g / m² one speaks of surface-pigmented papers.
ULWC papers ( ultra light weight coated paper ) are in the range of 5–6 g / m² coat weight per side
LWC paper ( light weight ), 7–12 g / m² per side
MWC papers ( medium weight ), 10–15 g / m² per side
HWC papers ( heavy weight ) up to 20 g / m² per side
Art paper 20–30 g / m² per side
Folding box board 15–30 g / m² per side

Technique of painting

When painting paper and cardboard, a distinction is made between

Blade painting: Here, the coating color is applied in excess to the paper and then the excess is removed again with a blade, so that an absolutely flat surface is created. The blade (a sharp metal blade) can also be replaced by a rotating rod (with or without grooves) the so-called roll squeegee or an air stream (air knife)
Film press: Here the paper is passed through a gap between two rollers, whereby the coating color is transferred to the paper. The coating color was previously metered onto the rollers as a "film" using a roller doctor blade. This film is then transferred from the roller to the paper web in the gap. It is possible to apply different coating colors to the front or back (respective roller). Film presses are used to apply not only coating colors, but also starch solutions (glue) to the paper.

In the size press , the paper is also fed through a gap between two rollers, whereby the glue (starch solution) is not pre-dosed onto the rollers, but is located in an open sump between the two rollers through which the paper web runs.

Cast coating: Here the coating color is applied to the paper and then passed around a large-format, highly polished, chrome-plated and steam-heated cylinder. The coating color takes on the surface structure of the cylinder (smoothness) and is dried at the same time.

Curtain coating : With curtain coating, the coating color is dosed through a gap (nozzle) onto the horizontally moving paper web underneath in the form of a curtain. The advantage of this coating method is that it works without contact (without a roller or doctor blade) and therefore causes fewer breaks in the paper web. Due to the process, it is very demanding in terms of the chemical / physical properties of the coating color (viscosity, air content, temperature, etc.) and mechanical engineering (aerodynamics of the paper web, synchronism of the entire process engineering machine chain and its peripherals, etc.).

Spray coating : The coating color is sprayed onto the passing paper through a large number of nozzles. This process could not establish itself on the market because it is mechanically demanding (clogging of the nozzles, ambient spray mist), and the line image does not meet the requirements.

Types of order

Rollers and rollers (e.g. film coating but also for some blade coatings)
Slot nozzles (e.g. curtain painting, but also with some blade lines)
Sump (e.g. size press)
Spray nozzles (spray coater)

Types of stroke

Equalizing line: Here, the excess of the coating color applied to the paper is scraped off (scraped off) from the paper surface by a squeegee, so that a layer of coating color of varying thickness remains on the paper, which evens out (equalizes) the unevenness of the paper surface. This leads to a smoother surface of the paper but to a non-uniform appearance of the same, since no uniform coating color layer covers the surface and the base paper can show through in places. This group includes painting with a rigid or flexible blade, roller squeegee or air knife.
Contour coating: Here, a layer of coating color is applied to the paper, which lies like a film on the paper according to the paper surface and follows the unevenness of the surface. A good coverage of the base paper is achieved here, since the coating color film is equally thick everywhere, but the surface of the paper remains uneven. Film and size presses, curtain coaters and spray coaters belong to the group of contour lines.

literature

Paper dictionary; Eds. Lothar Göttsching and Casimir Katz; Gernsbach 1999; ISBN 3-88640-080-8