Requirements of Base Papers/Board for Coating
Coated papers are classified as wood-containing or mechanical papers, and fine papers, which are also called woodfree, or WF papers. Wood-containing papers are made of a fiber furnish such as groundwood or other mechanical pulps. Recycled fibers (RCF) are also currently an important and steadily growing raw material for coated base papers. Woodfree papers are made of a fiber furnish of only chemical pulp, or only very little mechanical pulp (no more than 10 % of the fiber material). Typical mechanical coated papers are LWC papers (light weight coated) and ULWC papers (ultra light weight coated), MWC papers (medium weight coated) and HWC papers (high weight coated). Art printing papers are typical woodfree coated paper for the highest printing quality (Fig. 7.8).
Coated folding boxboard (FBB) is
an example of coated board, which is used in selling packages for goods. In North America and Japan there exists a classification system that is based on the bright¬ness of coated papers.
The base paper in coating must ensure trouble-free running of the coater, pro¬vide an optimal basis for paper finishing, and form a base for fixing the coating layers demanded by the end users. The lower the basis weight the more important are the properties of the base paper. Generally, the best coated paper surface is achieved with the best and most uniform base paper. Variations in formation, absorption, thickness, moisture, and roughness of the base paper have a great influence on the properties and the uniformity of the coating layer.
Nonuniform coat weight distribution is the most important factor in generating uneven print image, called mottling.
Further critical properties are paper web profiles in the machine and cross ma¬chine directions as regards basis weight, filler distribution, caliper and moisture.
The paper web has to be free from faults, holes and impurities, and have low fiber roughening potential (for web offset grades), low porosity with a uniform pore distribution, high smoothness, opacity and brightness, minimal two-sidedness, sufficient tensile and tear strength, as well as sufficient stiffness. Stiffness is re¬lated to the bulk of the coated paper. Stiffness especially becomes a critical property with low basis weights, high filler content and high coat weight, and in sheet printing. By choosing coarse, bulky fibers, careful wet pressing and by calendering as little as possible, bulk can be improved. In sheet-fed printing, the stiffness in the cross machine direction is more critical. This can be influenced with fiber furnish and by controlling fiber orientation with the jet:wire ratio in web forming.
Good internal bond strength is required both in heat-set web-offset (HSWO) and sheet-fed offset. If the internal bond strength is good in HSWO, the blistering temperature increases. In this printing method, printing ink is dried by external heat, while at the same time the remaining water in the base paper starts to evapo¬rate. If the internal strength and formation are poor, steam pressure inside the paper web can rise locally causing bubbles, which is called “blistering”. No crack¬ing in the fold is very important in high-quality printed jobs with colorful printing.
The cracking tendency is higher with stiff mechanical pulp fibers (e. g. TMP) than with chemical pulp fibers, but it remains a problem in WF papers – especially those with high basis weights. Folding properties can be affected by pulp refining, by pulp and pigment types, and by the amount of starch used in the coating formulations.
The base sheet of board mostly consists of several layers (multiply), therefore sufficient ply bond and crack fold are additional important parameters, besides a uniform and bright surface, to achieve a good quality of coated board.
Specific Base Paper Properties Affecting Coating
Base paper characteristics have a strong effect on the quality of coated paper and board and process economy. The main properties are as follows:
Strength Properties: To guarantee a good runnability of paper web in coating different strength properties are needed. Because base paper is wetted in coating units, it requires good temporary wet strength. Wet tensile strength, in principle, is measured using similar methods as for measuring tensile strength, but the sample is prepared differently. Other strength properties related to coating are tear strength and edge tearing resistance as well as bursting strength
Basis weight, caliper, and moisture profiles in the cross and machine directions: These have a strong influence on the uniformity of coating, e. g., calendering very often controls caliper, which means that in the cross direction some parts of the web are pressed more than others to reach a uniform caliper profile. These more compressed parts will differ in porosity and smoothness of base paper, which affects the coating amount.
Caliper before coating can be measured on-line for control. Basis weight variations usually cause caliper variations, they can be seen as streaks in the machine direction.
Moisture variation will affect coating amounts with risks of wrinkles if moisture variations across the web are high. High mois¬ture before coating can affect coating color penetration and the gloss of coated paper and, subsequently, ink settling in printing.
Porosity: If porosity is very high, absorption of coating is also high and coating weight can increase dramatically and drying problems may also occur.
Formation: This property refers to small-scale basis weight variations in paper. There are areas that are denser and less absorbent to the coating than other areas. To reach uniform coating penetration, base paper requires even formation, other¬wise mottling may occur in printing.
Smoothness: Smooth base paper gives a uniform and closed coated surface. The various coating processes have slightly different smoothness requirements. Air knife coaters require a relatively smooth surface because, in that method, coating is applied in a layer of rather uniform thickness, regardless of the base paper contour. The blade coaters’ coating film fills the cavities and leaves the high spots uncovered.