20 common questions in papermaking(4)

16. What is the role of adhesives in coatings?

A: The role of the adhesive in the coating is to bond the pigment particles to each other, so that the coating and the base paper are firmly bonded, otherwise, failures will occur during printing. If the adhesion between the pigment particles is not strong, the phenomenon of powder and hair loss will occur; if the adhesion between the coating and the base paper is not strong, there will be a hair pulling phenomenon during printing; the adhesive will also affect the coating paper. Quality indicators such as ink receptivity and gloss. The amount of adhesive generally accounts for about 10% to 20% of the pigment. Adhesives play a decisive role in the surface strength of coated printing papers.


17. What are the main adhesives used in coatings?

Answer: There are three kinds of commonly used adhesives: natural, natural denatured and synthetic. Natural and natural modified starch, modified starch, casein, soybean casein, carboxymethyl cellulose, etc. Synthetic adhesives include polyvinyl alcohol, styrene-butadiene latex, polyvinyl acetate emulsion, polyacrylate emulsion, etc. Among them, polyvinyl alcohol has the strongest cohesive force, which is equivalent to 2-2.5 times that of latex, 3 times that of dry acetate, and 3 to 4 times that of starch.



18. What are the auxiliary agents and their functions in coatings?

Answer: Auxiliary agents are auxiliary chemicals used to improve the performance of coatings and coatings to meet certain special requirements. There are many types of adjuvants, but the total amount is very small, generally not more than 5% of the coating. For example, dispersing agent is to improve the dispersibility of pigments, and sodium hexametaphosphate is commonly used; water-resistant agent is to improve the water resistance of the coating; fluidizing agent is to improve the fluidity of the coating; lubricant is to improve the glazing of super calendering Effect. Commonly used additives are: lubricant zinc stearate, softening plasticizer glycerin, defoaming agent amyl alcohol or octanol, antifungal agent carbendazim, pH adjuster ammonia water and sodium hydroxide, etc.


19. What are the classifications of coating machines?

Answer: The coating operation is divided into internal type and external type according to the relationship between the coating machine and the paper machine, and is divided into single-layer coating and double-layer coating according to the number of coating times. In-machine coating is that a coating machine is installed on the paper machine, so that papermaking and coating are carried out continuously; external coating is that the coating machine and the paper machine are independent, and the coating machine used for coating operation mainly has rollers. There are three types of coater, air knife coater and blade coater.


20. What are the structural characteristics of paper?

Answer: Paper is a non-homogeneous material with complex structure. Its structural characteristics can be summarized as follows:

(1) It has a variety of elements: fibers, fillers, sizing materials and pigments, among which fiber is the most basic element of paper structure. The types and processing methods of fiber raw materials are different, and the structure and properties of paper are also different. Fillers, rubber materials, etc. also have performance differences due to different varieties.

(2) The distribution of fibers, fillers, sizing materials and pigments in the three directions of X-Y-Z of the paper sheet is anisotropic, which is manifested in different arrangement directions of fibers, different fiber distributions of different sizes, as well as fillers, sizing materials, and pigments. different from the distribution of air content, etc. The anisotropy of this structure is mainly determined by the method of papermaking and the equipment used.

(3) Most papers are two-sided, that is, the properties of the two sides of the paper are different.

(4) Paper has a pore structure. The porous structure (porosity) of paper determines the air permeability, moisture absorption, absorption and compressibility of paper, which is the basis for paper to absorb liquid substances such as water and ink.

(5) The combination of fibers, fillers, sizing materials and pigments determines the mechanical strength of the paper and the binding force with other substances.

Post time: Jul-19-2022