11. ***What is the opacity of paper?***
A: Opacity refers to the extent to which paper resists the transmission of light, thereby preventing show-through. The opacity of paper depends on its light scattering ability, the number of light scattering interfaces within the paper (i.e., the number of independent particles), and the magnitude of the refractive index difference at these scattering interfaces. A higher number of light scattering interfaces, along with a greater difference in refractive indices between scattering interfaces, enhances light scattering ability and increases paper opacity.
12. ***How does filler affect the opacity of the paper?***
A: Unfilled paper consists of fibers and air. Air resides in the pores between fibers, and due to the differing refractive indices of fiber and air, some light is scattered at the fiber-air interface when a beam strikes the paper's surface. This scattering contributes to the paper's inherent (low) opacity, which in printing manifests as ease of ink penetration. Adding fillers with a refractive index greater than that of cellulose increases the number of light scattering interfaces within the paper. Specifically, three distinct interfaces are formed: fiber-air, filler-fiber, and filler-air. Among these, the refractive index difference between filler and air is significant, leading to maximum light scattering at the filler-air interface and thus enhancing paper opacity.
13. ***How does filler improve paper surface smoothness?***
A: The network formed by interwoven fibers inherently contains numerous pores and exhibits surface roughness. Adding fine particle fillers enhances the softness and plasticity of the paper, enabling more effective calendering treatment and thereby improving the smoothness of the paper after calendering.
14. ***What is the effect of filler on the strength of paper?***
A: The bond between fibers forms the basis of paper strength. Adding fillers reduces fiber-to-fiber bonding, and consequently, adding large amounts of filler significantly diminishes paper strength. In particular, tensile strength, folding endurance, and burst resistance are greatly reduced, while tear resistance decreases to a lesser extent. Therefore, the amount of filler used must be carefully controlled.
15. ***How does filler affect the thickness of the paper?***
A: Paper fibers have a density of approximately 1 g/cm³, whereas most fillers have densities ranging from 2.5 to 3.0 g/cm³. Thus, adding fillers reduces the bulk thickness of the paper. However, when the amount of added filler is small, bulk thickness may increase with increasing filler content, particularly for short-fiber papers.
16. ***How does filler affect the stiffness of the paper?***
A: Paper stiffness is a function of both its thickness and elastic modulus. To evaluate the impact of fillers on stiffness, one must consider their effects on bulk thickness and tensile strength. When the amount of filler is small, its addition does not significantly affect paper stiffness. However, as filler content increases, stiffness decreases substantially.
17. ***What is the role of color in paper?***
A: Even bleached pulp fibers exhibit slight yellow or gray hues due to lignin absorption of purple and blue light at wavelengths of 400-500 nm. Higher lignin content results in darker paper. To achieve higher whiteness, blue-purple or red-blue dyes are often added to bleached pulp. Colored paper production also requires dyeing.
18. ***What are the ingredients of colorants?***
A: Colorants include pigments and dyes used for dyeing and toning paper. Most pigments are inorganic substances and insoluble in water. Dyes can be natural or synthetic; synthetic dyes have largely replaced natural ones and are typically water-soluble or become so after chemical treatment.
19. ***Other chemical auxiliaries in paper accessories and their effects?***
A: To meet specific requirements for certain paper applications, various non-fiber additives are often incorporated into pulp or paper. These can be categorized as follows:
(1) Sizing agents: Additives such as urea-formaldehyde resin and phenolic resin enhance paper sizing.
(2) Dry strength agents: Substances like cationic starch and polyacrylamide improve dry strength.
(3) Retention aids: Compounds such as polyacrylamide, polyoxyethylene, and polyvinylimidazole reduce filler and fine fiber losses during papermaking.
(4) Defoamers: Agents like silicone oil, turpentine, tridecyl alcohol, tributyl phosphate, amyl alcohol, and octanol eliminate foam during papermaking.
(5) Water-resistant agents: Materials such as paraffin wax, metal soap, glyoxal, melamine resin, and dimethylamine ethyl acrylate enhance water resistance in specialized papers.
20. ***How can paper be easily classified?***
A: Paper is typically classified according to its use into cultural paper, industrial paper, household paper, and special-purpose paper. Cultural paper can further be divided into uncoated paper (e.g., newsprint, offset paper, electrostatic copy paper, writing paper) and coated paper (e.g., coated paper, calendar paper, matte paper, glass cardboard).