Principles of Coating Techniques for Paper and Board
Historically, papers used to be coated in much the same way as painting a wall: the color was applied and distributed with a brush, then dried and smoothed. The process has remained essentially unchanged to the present day, although it has undergone major improvements over time. Today, roll or jet applicators are em¬ployed instead of a brush, and the color layer is leveled out using either a pneu¬matic or mechanical system, such as a blade or a metering bar. The moist sheet then passes between infrared dryers before being conveyed to high-temperature air floats and/or drying cylinders.
In principle, coating can be divided into different phases:
. • Application of the coating color onto the base paper. Surplus of coating color is applied in several coating methods (existing coating systems are shown in Sec¬tion 7.10).
. • Metering of the coating (metering may take place before, during or after the application).
. • Drying of the coating.
. • Smoothing of the surface e. g. by supercalendering or soft calendering.
Pre-metering, or metering before application takes place, e. g., in the metered size press (MSP) and in the gate roll press. Simultaneous metering or metering and application take place at the same time in the short dwell time applicator (SDTA) and in the conventional size press. Post-metering, or metering after application takes place in the blade coater, in the rod coater, and in the air knife coater.
Figure 7.9 depicts a schematic arrangement of backing roll, roll applicator and doctortogether with the coating base. The applicator roll runs at a slower rate than the paper sheet and applies a generous amount of color (20–30 times in excess) to the base paper. After wetting the base paper, the waterline migrates into the coating base.
This penetration is assisted by hydrodynamic pressure building up in the nip as a result of the web speed. Consequently, the kinds of application and metering characterize the various coating methods. In both the metered size press (MSP) and in the gate roll press, metering takes place on the surface of a roll, and the coating film is then transferred onto the paper in the nip between two rolls.
Coating can be done double-sided, i. e., both sides of the paper are coated simul¬taneously (e. g., MSP), or on only one side at a time. Printing papers are usually coated similarly on both sides, and boards often on one side only.
Single coating means that only one coating layer is applied per paper side, in multiple coating several coating layers are applied per paper side. The most com¬mon multiple coating is double coating. So-called art papers may be triple-coated,
i. e., three coating layers per paper side.
If coating is done on-machine, then the coating will immediately follow the manufacturing of the base paper without intermediate reeling. Off-machine coat¬ing means that the paper is reeled after the paper machine and coating is done in a separate machine. Web defects may be removed during re-reeling.
During coating the following effects occur:
Liquid is removed from the color, which then adheres to the base paper. As the liquid phase strikes into the base paper, water containing dissolved substances and other color components (mainly binders, cobinders and additives) migrates into the base paper. Actually, filtration of the coating components takes place in the z-direction, i. e. perpendicular to the sheet plane. This anchors the color to the paper surface.
If color strike-in is too intense the coating layer may become depleted of binder. This may in turn give rise to undesirable phenomena like dusting or pick¬ing. If the color is dewatered extremely quickly, solids content and hence viscosity will increase.
This increases e. g. the blade pressure in blade coating, so the coating color needs dilution – a measure that may result in poorer surface quality. Another key phenomenon is the mechanical shear stress experienced by the color in the clearance between the stationary doctor and the paper sheet as it passes by. The shear rate D (rate of application of strain) on the color may be calculated from the velocity v of the paper and the clearance d between paper and doctor as follows:
D = v/d [s–1]
The clearance d is difficult to measure due to the flexible cover of the backing roll and the compressibility of the base paper. The shear rate D may attain a multiple of 106 s–1. This value plays a crucial role in coating color rheology and runnability. More recent analysis shows that at high speed, the above simple shear generation model may no longer be applicable. Boundary slip conditions can occur, leading to a pseudo-plug flow under the doctor nip initiated by the rapid change in shear rate upstream from the nip. Under these circumstances, the coating color behaves partially elastically, in contrast to the classical viscous flow described here.
Appropriate absorbency of the coating base paper, water retention (WR) of the coating color as well as its rheology or viscosity ensure good coating quality and runnability. These mechanisms take place within fractions of a second: at a coater speed of 1500 m min–1, e. g. the paper web reaches the doctor just 0.04 s after the moment of color contact. Under adverse drying conditions, binder may distribute nonuniformly into the coating surface. Along with local variations in surface po¬rosity caused by nonuniform coating consolidation during drying, this is a typical cause of print mottle as a result of irregular ink absorption.