Requirements of Coated Papers for the Printing Process
Printed matters are used for advertising, news and information, transactions, edu¬cation, and entertainment. Here they are in competition with other communica¬tion media, such as radio and television, and the internet. Printing on packaging paper and board has the functions of advertising, delivering information, and showing data such as barcodes. So paper and board have to maintain their com¬petitiveness through continuous product development strongly supported by coat¬ing technology as a prerequisite for excellent print quality.
Printing Methods and Printing Ink Properties
For the various printing methods and the press constructions the major press variables are paper feed, paper take-off, and drying. Table 7.1 shows the major classes in different types of printing for different types of presses with the primary product categories for each type of press. Offset printing methods comprise news¬paper offset, heatset offset and sheet fed offset. Newspaper offset printing today is mainly coldset offset which needs no drying capacity in the presses. Web fed maga¬zine offset is heatset. Here drying is done by hot air. The type of paper feed relates to the scale of printing and the paper or board basis weight. Large scale printing and printing on low basis weight substrates is web fed.
The type of drying correlates to the demands that the printing method places on the ink properties and the desired quality level of the final product. Inks that contain a large proportion of solvent such as gravure, flexographic, and heatset offset inks are typically dried through evaporating the solvent by air flow. Depend¬ing on the air temperature, drying may be done by “hot air” as in heatset offset, “warm air” as in gravure, or “cold air” as in flexographic printing. The drying mechanism can also be a function of the ink, as in oxidizing and polymerizing inks. Polymerizing inks are mainly used in sheet fed offset, but it is likely that their use in web fed printing will increase. News inks do not really dry during the one day lifetime of newspapers.
The steps during printing include transfer of the ink to the plate, its transfer to the receiving substrate or paper, and print drying. Mechanical printing uses the rotary principle. This means that printing ink is transferred to the paper in a nip between two rotating cylinders (Fig. 7.3.). Ink transfer between flat surfaces or a flat surface and a roller has also been used in the past. As the achievable speed is less than in rotary printing this kind of printing has become obsolete.
In offset printing the ink is transferred to the paper from the printing plate using a separate transfer cylinder – the offset or blanket cylinder. The characteristic of an offset plate is, that the printing areas (image areas) and nonprinting areas (non¬image areas) are in the same plane. The printing and nonprinting areas are gen¬erated via the different wetting properties of two coatings on the printing plate (Fig. 7.4.). Before applying the ink, the printing plate must be evenly wetted with the fountain solution. The printing areas attract the fatty ink and the nonprinting areas attract the water.
In letterpress, flexography, and hot stamping, the printing and nonprinting areas form a geometrical profile in the z-direction of the plate surface (Fig. 7.5.). The printing areas are above the mean depth of the profile, and the nonprinting areas are below. Letterpress and flexography (“flexo”) differ in the characteristics of the plates and the inks. In flexo, the plate profile is higher, and its polymeric material is softer than in letterpress. Flexographic inks are two orders of magnitude less viscous than letterpress inks. Under nip pressure, a softer plate conforms more easily and achieves more contact with a rough printing substrate. This process is necessary for ink transfer actually to occur. Squeezing of the inks toward the edge of dots or characters allows one to recognize flexography and letterpress.
In the gravure technique, cells – a negative relief engraved or etched in a copper layer on the printing cylinder – form the printing area. For better durability, copper cylinders receive a subsequent chrome plating. Figure 7.6 shows that different levels of darkness result in halftone image printing by varying the area of the cells or by varying the depth of the cells. The two principles of gravure – area modula¬tion and depth modulation – are shown in a magnified cross direction view of the printing cylinder.
Electronic printing – also synonymous with digital printing – consists of non¬impact processes. The printing methods differ considerably in the principles they employ. Electrophotography is the most mature black-and-white nonimpact tech¬nology. In color technology photocopying was the impetus for early development. Today high end color electrophotography challenges sheet-fed offset printing. At the low end, higher complexity prevents color electrophotography from competing with color ink-jet. The inks in electrophotographic printing are so called toners, which are commonly powdered materials.
The particle size is typically 5–10 mm with a narrow size distribution. With an increase in resolution, particle sizes had to be decreased. Approximately, the particle size is a fifth of a pixel size. Liquid toners are applied when high resolution is required. Toner particles dispersed in a carrier such as a kerosene-like solvent can be smaller than 1 mm. After transfer to the paper, the carrier phase becomes superfluous.
Toner particles consist primarily of pigment to impart color, thermoplastic resins to bind the pigment to the paper, and charging additives to enhance the charging capacity. Powdered toners are trans¬ferred by adhesive, electrical, and magnetic forces.
In ink-jet printing, a drop is the basic imaging element. Its principle is the gen¬eration of ink drops from a continuous stream of liquid emanating from a nozzle. It needs very small nozzles through which the ink emerges in order to reach high resolution. The ink should not clog the nozzles or form a crust when in the stand¬by state. In thermal DOD (drop on demand) ink-jet, rapid thermal gradients in the heating period impose high demands on the thermal stability of the inks. A charac¬teristic of ink-jet inks compared with inks in other printing methods is that they are very fluid. This is especially true for the continuous stream ink-jet method where rapid drop formation requires viscosity near 1 mPa s. In thermal jetting, viscosity is commonly less than 5 mPa s, 10 mPa s is the upper limit.
Properties of Coating Layer Surface versus Printing Method
The composition of the coating color is largely determined by the demands put on the paper surface by the particular printing process. In the press room a faultless runnability of the paper is demanded and the publisher asks for an excellent print quality (printability).
Offset printing requires an especially well bound coat because considerable forces in the z-direction (surface strength/pick strength plus internal bond) are exerted by the highly viscous inks at the high printing speeds of today. To meet these strength demands a high amount of coating binder is necessary. At the same time, the coating must be sufficiently porous to permit fast, controlled absorption of the printing ink without reducing the print gloss.
Two further requirements are high water resistance, and sufficiently fast absorption of the fountain solution. In gen¬eral, the coated paper must have adequate stiffness, resistance to blistering, good dimensional stability, high brightness, no or low yellowing tendency, and good aging resistance. The coated and printed paper has to be without waviness, mot¬tling and ink strike-through. Additional important printability properties are high print gloss, even halftones, detailed tonal gradation, low ink consumption (high density), sharp dots (controlled dot gain), fast ink drying for immediate process¬ing, controlled water/ink interference and low fiber roughening by the moistening and the following drying process.
The importance of letterpress printing today is very much reduced by the cold-set offset process, whereby most of the newspapers are printed. Conventional news¬papers are uncoated papers, based on recovered and deinked printing papers and/ or mechanical pulps. However, a coated surface of such a newspaper or another base paper made from a relatively cheap furnish opens new applications for the cold-set web-offset process with significant improved print quality and cost-per-formance (Fig. 7.7).
Flexographic printing requires good ink absorbency, even fulltones without pat¬tern, high dot definition (no “shadow”), fast ink drying and good ink adhesion.
In rotogravure printing, the tensile forces between the printing cylinder and the surface of the paper are very small, as solvent-containing, low viscous inks are used. So rotogravure printing requires only very little pick strength of the paper
surface. For this reason, only a relative low amount of coating binder is needed. For rotogravure printing good paper compressibility, a high degree of smoothness, and adequate receptivity for the ink solvent are important.
Further important criteria for paper and board grades used for packaging in¬clude low odor, flexibility, fold crack stability, glue-ability, and grease resistance. Coated specialty papers require specific properties, such as alkali resistance (la¬bels), washability (wallpaper), and solvent resistance (silicone release paper).
In electronic printing the runnability of a paper is critical, because of the elec¬tricity involved. The electrical conductivity of the paper should be sufficient so that excessive accumulation of electricity on the paper does not occur despite charge removal conductors in printing devices.
Accumulation of charges causes sheets to adhere to each other leading to jamming. Image quality requires electrical re¬sistivity in the paper, because the paper must hold the electrical field during trans¬fer of the toner. These two factors determine the tolerance range for paper re¬sistivity. All toner particles should be transferred to the paper from the photo-conductor to prevent accumulation after the erasure step. Accumulation would cause a ghost image on the next print. With the rise in moisture content, resistivity drops exponentially.
If the paper is too conductive, the toner spreads sideways causing loss in detail. The moisture content of paper is critical, as is friction. Friction must be within a specific range since the transportation occurs commonly by friction. The thermal conditions in fusing determine the requirements on the dimensional stability of the paper and the strength of the surface. A lack of dimen¬sional stability can cause curl, cockle, and image depletion in duplex printing.
Softening of the paper surface in fusing may cause adhesion on the counter roll, loosening particles from the surface and leading to accumulation on the rollers of the printer. Good toner adhesion requires surface-chemical compatibility of paper and toner. With an increase in the resolution of printing and the related decrease in toner size, the smoothness requirements of the paper surface become increas¬ingly important.
In ink-jet printing, ink and paper interact very strongly. Because of the fluidity of the inks and their high velocity of impact on the paper, ink-jet papers require characteristics that are matched with the inks and the drop volumes. This is espe¬cially true at the higher end of the quality level. Ink-jet papers must be smooth so that drops impacting on the paper spread evenly. Sufficient and even porosity composed of small pores is necessary to absorb the solvent quickly and to counter¬act the spreading tendency. Dyes are trapped close to the surface of the paper.
This can be aided by cationic dye fixatives in the paper which capture the anionic dyes and allow the colorless vehicle to be absorbed into the paper or coating. The use of aqueous inks places demands on the dimensional stability of ink-jet papers. Cock¬ling and curling tendencies are critical factors. The impact of drops on paper causes enlargement in area, typically by a factor of two. This, as well as the need to decrease drop size, constrain efforts to raise resolution, although the commercially implemented levels keep going up and have exceeded 1000 dpi.
Printed pixels are most visible in light tones where they are far apart. The use of more than three colored inks helps to reduce the solvent that is transferred to the paper in multi¬color printing. This means supplementing the three process colors cyan, magenta, and yellow with additional color inks such as orange and green which also brings the advantage of a wider color gamut. Further requirements for ink-jet printability are fast ink drying, good tonal stability (no color shifting), no bleeding and no feathering.