Difference between revisions of "Coating"

Revision as of 13:27, 8 August 2016

General Description

Paper composed only of individual fibres has a relatively rough surface. Since this roughness has a negative effect on print quality, a large amount of paper and board is coated in order to attain a smooth surface for printing. The term 'coating' describes the application of a mixture of water, white pigments, binder, and various additives to one or both sides of the surface of the paper in order to create specific surface properties. Paper may be coated either with equipment that is an integral part of the paper machine (on-machine) or on separate coating equipment (off-machine). The coated sheet is first dried by infrared radiation, by hot air or by a combination of an infrared and a hot air dryer and finally a short steam-heated cylinder section.

The composition of the coating colour is largely determined by the demands made on the surface of the paper by the particular printing process. Coatings can be complex mixtures of ingredients and usually require preparation before use. The coating colour preparation is carried out in the 'coating kitchen' where the coating chemicals are mixed and adjusted. The finished coating colour is screened before entering the coating machine. Some raw materials may be received wet for easier handling and to avoid any dust problems. All coatings will contain some sort of binder, which is mainly carboxylated styrene-butadiene, styrene-acrylonitrile, or acrylic latexes. Starch may be used for added stiffness and will typically be cooked prior to use. Stearates (Ca and NH4) are used to provide lubrication during the calendering process and to provide surface texture. Pigments such as fine clay, talc, or calcium carbonate will be used, as will colours, brighteners and other low volume speciality chemicals.

Coating machines consist of an unwinder, a coating station or stations (coating application unit), a drying section (infrared, hot-air and heated cylinders) and a reel. For optimal operation, various control measurements are installed.

Nearly all coatings are water-based and applied with a roller, air knife, size press, blade and bar and more recently curtain colour coating systems. The objective is to apply a consistent and uniform thickness of coating material to the paper sheet. Every coating system needs a specific coating colour in which solid content and viscosity have to be settled for reaching the best runnability and paper quality results. Air knives work at about 30 – 50 % solids, others about 50 – 70 %. The coating slurry is generally recirculated through the system with constant filtering to remove fibres and other contamination in order to maintain the slurry quality. These filters will be cleaned at regular intervals thus generating a concentrated waste stream.

There are different grades of coated paper and boards. However, the classification of coated grades has not yet been standardised. Certain terms for describing the major types of coated paper and board are used throughout Europe. They are described below.

Art paper

The term refers to wood-free or paper containing only a small amount of wood that is coated on both sides. The weight of the coating exceeds 20 g/m2 per side, resulting in the complete coverage of the fibre structure of the paper. Up to three coats are applied to each side to obtain the high quality required. This paper is used for high-quality printed products.

Machine-coated papers

This grade includes both wood-free paper and wood-containing paper with a coat weight of up to 25 g/m2 per side. Higher weights are often obtained by applying a double coat. The coating formulations are normally simpler than those for art paper. Machine-coated papers are used mainly for printing purposes.

Light-weight coated (LWC) papers

These papers contain high amounts of wood fibres and have coat weights in the range of 5 – 12 g/m2 per side. They are mass-produced papers and are used for the production of catalogues, magazines, advertising pamphlets, etc. The weight ranges have been extended below these limits (ULWC = ultra-LWC) and also above them (MWC = medium-weight coated and HWC = heavy-weight coated). Today the heavy grades are often provided with a double coat and are, consequently, grouped with machine-coated grades.

Folding boxboard and chromoboard

In these grades of board, the white topliner is coated on the board machine. The remaining layers of board can consist of bleached or unbleached pulp or of paper for recycling. The coating weight is in the range of 12 – 33 g/m2 and the coat is normally comprised of a precoat and a topcoat. More recently, three coating layers have also been applied. The board is rarely coated on the reverse side. Depending on the operation of the coating kitchen, the coating application and the measures in place for recovery or separate treatment of effluents which contain coating colour, relevant releases to water can occur. In some applications, somewhat increased concentrations of volatile organic compounds (VOC) can be measured in the exhaust gases of the drying hood. Potential releases to water can be caused either by a spill of concentrated coating components (approximately 50 % consistency) from the coating kitchen or coater station or by a spill of diluted coating components from the washing water of tanks and piping. The concentrated discharges can be collected in tanks or mobile containers for solid waste treatment or for recovery of minerals as fillers, after removing the excess water by sedimentation. The diluted streams are collected in tanks, from where they are led to the effluent treatment. Usually, effluents from coating require a separate pretreatment in a flocculation plant. Otherwise, coating waste water may cause disturbances in the performance of the biological waste water treatment plant. In recent years, the membrane filtration technology has produced good results in the recycling of coating colour. Applications in Finland, Sweden, France and Germany recover coating colour back to the coating kitchen where it is proportioned to the fresh coating colour. Coated broke is normally repulped so that some of the coatings are returned to the water circuit.

Dyeing of paper

Coloured papers are obtained by dyeing the paper stock or the paper surface (size press, paper coating). Optically brightened papers can be produced in the same manner. Stock dyeing is the most widely used type of paper dyeing. Dyes, pigments, and optical brighteners are added either batch-wise in the pulper or mixing chest or introduced continuously into the stock flow. Continuous addition has the advantage of a shorter zone in the stock line that must be cleaned when the colour is changed. However, because of the lower contact time compared to batch addition, a lower colour yield is obtained for intensely coloured papers and more complex equipment is required for this dyeing process. When the surface of the paper is coloured in the size press, the dyes are added to the size press liquor. Surface dyeing has gained acceptance only in individual cases because uniform dyeing of the paper is difficult to achieve. However, this process has the advantage of the absence of dyes in the water circuits.

Surfaces of papers can also be coloured by coating. In normal coating the surface of the paper or board is covered with a pigment coating. In the case of coloured coatings, the starting material is the white coating mixture, and the desired shade is attained by adding a dispersion of an organic or inorganic pigment. Depending on the fibrous material to be dyed and the intended purpose of the paper, different types of pigments and dyes are used as basic dyes (cationic dyes), direct dyes, and acid dyes. Additionally, fixing agents and other additives are used to improve dye fixation and to obtain better dyeing results. Inorganic pigments or organic pigments (e.g. azo and phthalocyanine types) and carbon black are pigments used for paper dyeing.

The potential environmental impact of dyeing is mainly the releases to water. Especially in mills with several changes of tints or shades per day, the water circuits have to be cleaned after a certain time. Usually, the paper mills work in campaigns producing first the paler tints, changing step by step to the deeper tints. The colouration of the water then just has to be readjusted. However, when for instance deep green is reached, the water system has to be washed. The coloured waste water is sent to the external recipient via the waste water treatment plant. Several times per month, the piping is subjected to a chemical treatment to remove deposits and colour in the piping. In some mills, elemental chlorine and hypochlorite are used as cleaning agents.

Source: Best Available Techniques (BAT), Reference Document for the Production of Pulp, Paper and Board (2015)

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