Coated paper is one of the fastest growing varieties in the paper industry in China in the past 10 years. Coated paper is used as a printing substrate, and it is a high-grade product in the category of cultural printing paper. It is a paper made by coating and finishing the printing base paper. In recent years, low-quantity coated papers that have emerged abroad have attracted a lot of interest from commercial printing, publishing and printing users because of their low basis weight (total amount not exceeding 60 g/m2), low cost, and good printing performance. , magazines, newspaper ads inserts, supermarket merchandise catalogs and other uses for printing.
In the past 10 years, the annual increase rate of coated paper consumption for coated paper and mechanical pulp in China was 18.8% and 25%, respectively. It is not difficult to predict that with the continuous adjustment and development of China’s economy and the improvement of people’s living standards, the use of coated paper in China will rise rapidly. Therefore, in the printing of coated papers, it is necessary to have a deep understanding of the printability of coated papers, especially the improvement of the printability of coated papers.
First, the basic situation of coated paper
The coated paper is coated on the surface of the base paper with a coating solution mainly composed of an auxiliary solvent such as a pigment, an adhesive, a tackifier, a fluorescent agent, a lubricant, etc., and dried and then subjected to a smoothing treatment. Excellent performance.
Pigment coated paper is the largest and most important category of coated paper. The pigment-coated paper is mainly composed of white pigments such as kaolin, calcium carbonate, barium sulfate, and titanium dioxide, plus a variety of adhesives and coatings formulated with various chemical adjuvants as required, and the coating is uniformly coated in a certain amount. Made of cardboard. Offset printing coated paper, commonly known as copperplate paper. The name of the copperplate paper has been long-standing. The material used to make the letterpress screen version was originally heavy metal copper. This type of printing plate is particularly suitable for printing such coated paper, so it is called copperplate paper. This kind of paper is a high-grade printing paper made by coating and finishing the base paper. Offset printing coated paper is used for printing various high-grade prints, such as calendars, pictorials, pictures, illustrations, high-end trademarks, product specifications, and so on.
Second, the printability of coated paper
The printability of paper includes workability and quality adaptability. Operational suitability mainly affects the printing efficiency, and quality suitability mainly affects the printing effect. As far as the operational suitability of paper is concerned, it is closely related to the tensile strength, toughness, flatness, and deformability of the paper. Closely related to quality suitability are paper whiteness, opacity, smoothness, surface strength, dimensional stability, and ink absorbency. It is customary to refer to the print quality applicability of paper as a narrow sense of printability.
Compressibility
When the paper is under certain pressure, it will be slightly compressed. After removing the excess force, the degree of paper returning to the original state is called compressibility. Compressibility is related to the size of the pressure. Paper compressibility is particularly important for printing. Because when printing, the paper is subjected to a certain pressure so that the ink is transferred from the plate to the paper. The paper with high compressibility can make the graphic image of the printing plate have sufficient and good elastic contact with the paper surface when the printing pressurization is appropriate, so that the graphic on the printing plate can be well transferred to the paper. Coated paper is generally less compressible due to the presence of the coating. The thicker the coating, the lower the compressibility. Under normal circumstances, coated paper with a large amount of coating has poor printability. Low coated paper due to low coating (applied around 6g/m2 per side), and the base paper contains mechanical pulp with good compressibility (usually the ratio is ≥25%), plus the use of soft calendering finishing, compressibility Coated paper is good, so its printing performance is good. In addition, the paper with good compressibility can also reduce the requirements for the smoothness of the paper and the flatness of the text surface.
2. Surface strength
The surface strength of paper is an important printability index. It refers to the degree of binding between paper fibrous tissue, fillers, and sizing, which determines the surface's ability to resist abrasion, powder loss, and lint resistance when printed.
For coated paper, the coating particles must adhere well to each other, the coating and the paper basically require adhesive bonding, and the paper-based fibers should have sufficient bonding strength. Otherwise, when the paper is brought into contact with the ink film on the plate at a relatively fast speed under the corresponding printing pressure, the ink film will peel off the paint particles and fine fibers, causing so-called hair loss and powder dropping, resulting in a paste plate . In offset printing coated paper, there is also the phenomenon of powder falling and hair loss due to the high viscosity of the offset ink used.
3. Whiteness
Coated paper, like other printed paper, has different requirements for whiteness depending on the purpose of the print. White paper can improve the contrast of text ink, and truly and objectively reflect the full color of the printed image, making the copy colorful and achieve the effect of illustration. The higher the paper whiteness, the more significant this effect is. However, the higher the whiteness of the paper, the stronger the reflected light energy and the greater the stimulation of the optic nerve. Inappropriate high brightness causes visual fatigue and is particularly disadvantageous for adolescents in their growing years. From the point of view of human physiological characteristics and usage, the whiteness value of printed paper for different purposes should not be the same, and the printed paper should have a suitable whiteness, rather than the higher the whiteness, the better.
4. Opacity
Opacity is also an important performance indicator for printing paper. The level of opacity directly affects the print penetration of printed products. Most printing papers need to have enough opacity, otherwise it will lead to after-printing. However, there are also individual patterns that need to be printed to show the effect of printing. This is a different story.
For the low-quantitative coated paper with double-sided printing, it is more practical to use printing opacity for evaluation because of its low basis weight and low coating amount. The opacity of a low basis weight coated paper is related to the opacity of the base paper, the coating formulation, and the hiding of the coating. (to be continued)
