31. ***Why is paper coating necessary?***
A: After calendering, paper fibers exhibit a certain degree of tightness but still lack sufficient smoothness for high-grade fine printing. Uncoated paper has poor surface uniformity, resulting in uneven ink colors, low gloss, and inability to restore small dots, leading to layer loss, reduced image field uniformity, and compromised print quality. Therefore, fine printing paper requires coating to further enhance surface performance.
32. ***What is the purpose of paper coating?***
A: The purpose of paper coating is to cover the uneven and porous surface of ordinary paper with a layer of fine particles that have good ink absorption, achieving improved uniformity and smoothness. Coating also enhances paper gloss, stability, and opacity, with these characteristics increasing as the coating amount increases.
33. ***What are the types of printing coated paper?***
A: Printing coatings can generally be divided into ordinary coated paper and special coated paper. Ordinary coated paper is produced using standard coating methods, followed by drying and calendering, and finally cut or rerolled into flat or web paper (e.g., coated paper, matte paper, light-coated paper). Special coated paper refers to printing coated paper made using specialized coatings or processing methods, such as cast-coated paper (glassine board), embossed paper, and matte-coated paper.
34. ***What is the production process of printing coated paper?***
A: The production process of printing coated paper is as follows:
Base paper selection → Coating preparation → Coater application → Drying → Calendering or surface finishing → Slitting or rewinding.
35. ***How is the coating process classified?***
A: Coating operations are classified based on the relationship between the coater and the paper machine into machine-integrated and off-machine types, and based on the number of coatings into single and double coatings. Machine-integrated coaters are installed on the paper machine, enabling continuous paper and coating processes. Double-coated paper exhibits better printability than single-coated paper, improving coated paper quality. Coating operations typically use roller coaters, air-knife coaters, or blade coaters.
36. ***What are the drying methods for coated paper?***
A: Unlike uncoated paper, coated paper cannot be dried directly using dryers due to the wet state of the coating, which could cause paint adhesion to the dryer surface. Infrared drying or hot-air drying should be used initially, followed by contact drying with dryers once a certain degree of dryness is achieved.
37. ***What is the effect of supercalendering on coated paper?***
A: After coating, the base paper surface still contains fine irregularities and lacks gloss, failing to meet printing smoothness requirements. Therefore, surface supercalendering is necessary. Supercalendering involves passing the paper between alternating metal and paper rolls under pressure and friction to produce a shiny finish. Matte-coated paper either skips supercalendering or undergoes only slight calendering.
38. ***What are the requirements for base paper in coated paper production?***
A: Base paper quality significantly impacts coated paper performance. Poor control can affect coated paper properties. Regarding pulp composition, coated base paper predominantly uses sulfate wood pulp, with 70% hardwood pulp and the remainder softwood pulp. For lightweight, high-quality coated base paper, the proportion of softwood pulp should increase. In terms of strength, coated base paper requires high tensile and surface strength, achieved through pulp composition adjustments, increased beating degree, and surface sizing measures. Surface properties should avoid excessive roughness or smoothness, with moderate absorption and uniformity across the base paper's vertical and horizontal directions. Uniformity refers to consistency in weight, thickness, moisture, fiber organization, and smoothness.
39. ***What characteristics should coating possess?***
A: Coating for paper must exhibit good coating adaptability and printability. Adaptability ensures uniform and smooth coating layers, requiring even dispersion of components and appropriate concentration, viscosity, and fluidity for the coater type. Printability refers to the coating's ability to accept ink, ensuring good printing results.
40. ***What requirements must coating meet?***
A: After combining with the base paper, the coating must achieve the surface strength required for printing without powder shedding during printing; exhibit good ink acceptability for uniform ink transfer to the coating surface; possess high smoothness to enable slight extension during supercalendering and significantly improve smoothness; achieve high whiteness and gloss; and provide high opacity to hide or compensate for base paper defects. Additionally, different grades of coated paper have varying coating requirements, with higher grades demanding stricter standards.