41. ***What are the components of paint?***
A: The components of the coating include pigments, binders, and additives (such as dispersants, lubricants, hydration resistance agents, defoamers, etc.).
42. ***What is the role of pigments in paint?***
A: Pigments constitute the primary component of paint, accounting for approximately 70%-80%. They determine the fundamental printing performance of coated paper, including ink receptivity, smoothness, gloss, whiteness, and opacity.
43. ***What pigments are used in paint?***
A: Commonly used pigments include kaolin (China clay), calcium carbonate, talc, iron white, forged white, and precipitated barium sulfate. Their particle size is generally around 1-2 μm. Coating pigments are typically white inorganic pigments. The basic requirements for pigments include high whiteness and opacity, suitable and uniform particle size and shape, ease of dispersion in water, and good chemical stability.
44. ***What is the role of binders in paint?***
A: Binders in paint serve to bond pigment particles together and ensure a strong adhesion between the coating and the base paper. Without proper bonding, printing may fail due to issues such as powdering or hair dropping. Insufficient adhesion between pigment particles can cause powdering, while weak bonding between the coating and the base paper can result in hairing during printing. Binders also influence quality indicators such as ink receptivity and gloss of coated paper. Typically, the amount of binder accounts for about 10%-20% of the pigment content. Binders play a decisive role in the surface strength of coated printing paper.
45.*** What are the main types of binders used in paint?***
A: There are three commonly used types of binders: natural, naturally modified, and synthetic. Natural and naturally modified binders include starch, modified starch, casein, bean casein, carboxymethyl cellulose, and others. Synthetic binders include polyvinyl alcohol, styrene-butadiene latex, polyvinyl acetate emulsion, and polyacrylic acid vinegar emulsion. Among these, polyvinyl alcohol exhibits the strongest bonding capability, with its bonding strength equivalent to 2-2.5 times that of latex, 3 times that of dry acetate, and 3-4 times that of starch.
46. ***What are the auxiliary agents in coatings and their effects?***
A: Auxiliary agents are chemicals used to enhance the performance of coatings and meet specific requirements. These agents include dispersants (e.g., sodium hexametaphosphate) to improve pigment dispersion, water-resistant agents to enhance water resistance, flow agents to improve coating fluidity, and lubricants (e.g., zinc stearate) to enhance glazing effects during supercalendering. Other common additives include softening plasticizers (e.g., glycerin), defoamers (e.g., amyl alcohol or octanol), anti-mold agents (e.g., carbendazim), and pH regulators (e.g., ammonia or sodium hydroxide). The total amount of auxiliary agents is generally less than 5% of the paint.
47. ***What are the coating processes?***
A: Base paper coating can be divided into single-side coating and double-side coating. Single-side coating involves applying the coating to only one side of the base paper, such as in single-sided coated paper. Double-side coating applies the coating to both sides of the base paper, as seen in double-sided coated paper. Both single-sided and double-sided coated papers can be further categorized into single-coating (one layer) and double-coating (two layers). Double-coating is often used for producing coated printing paper with higher quality requirements and larger coating amounts. Double-coated paper offers better printability compared to single-coated paper, making it an effective method for improving the quality of coated paper.
48. ***How are coating machines classified?***
A: Coating operations are classified into internal and external types based on the relationship between the coating machine and the paper machine, and into single-coating and double-coating based on the number of coating applications. In-machine coating involves installing a coating machine on the paper machine to achieve continuous papermaking and coating. External coating uses independent coating machines, primarily roll coaters, air knife coaters, and blade coaters.
49. ***What are the characteristics of roll coating?***
A: Roll coaters apply a coating roller to the surface of the paper. The coating amount can be adjusted by varying the pressure between the metering rollers; increased pressure reduces the coating amount. Roll-type double-sided coaters have a simple structure, enabling simultaneous double-sided coating. As an in-machine coater, they offer suitable coating speeds and are less likely to produce coating streaks. However, they cannot achieve the coating thickness or smoothness of air knife coaters or the gloss of blade coaters.
50. ***What are the characteristics of air knife coating?***
A: Air knife coaters use pressurized jet air to adjust the coating amount and smooth the coating layer. This process is referred to as air knife coating. Advantages include achieving thicker coating amounts, more uniform coating layers, and reduced risk of paper breakage during coating, making it particularly suitable for pressure-sensitive coatings. However, this method requires coatings with appropriate concentration and roughness for uniformity and smoothness. Disadvantages include difficulty in coating high-solid-content materials and susceptibility to local blockages caused by dried coating, leading to coating marks.