Q&A about printing paper (128-138)

128. What is the gloss of paper?
A: Gloss is used to indicate a surface optical property of a paper. It reflects the gloss of the paper and the quality of the print, but also reflects the ability of the paper surface to display images. Theoretically, the glossiness is defined as the closeness of the paper mirror reflection ability and the full mirror reflection ability.

129. What kind of gloss?
A: Glossiness is divided into white paper glossiness and printing glossiness. Glossiness is usually referred to as white paper glossiness. It is the specular reflectance of paper under a certain angle and the mirror of standard black glass under the same angle. The reflectivity ratio is expressed. That is, the specular reflectance of the standard black glass is defined as 100%, and it is recorded as 100 degrees. The specular reflectance of the paper is the percentage of the standard black glass specular reflectance, and is recorded as how many degrees.

130. What are the ways to express gloss?
A: Although the glossiness is related to the specular reflection on the surface of the object, it is also affected by the physiological and psychological state of the observer. Therefore, it cannot be simply characterized by the physical measurement of specular reflection. Among the six existing expression methods, specular gloss and contrast glossiness are suitable for the printing and papermaking industries. The former is the ratio of the surface reflection (S) to the incident (I), as shown in Figure 1a; Also known as contrast gloss, is the ratio of the amount of specular reflection (S) of the object to the total amount of reflected light (D), as shown in Figure 1b.

131. How to measure gloss?
A: When measuring the gloss of paper with a gloss meter, it should be noted that the selected light has different angles of incidence and the results are different. The level of gloss depends not only on the surface characteristics of the object but also on the measurement angle. Specific steps are as follows:
(1) Prepare the sample. According to national standards "take paper and paperboard samples," the provisions of sampling, processing, and testing under standard conditions, drawing five samples, and paper samples to avoid watermarks, spots and other visible paper disease.
(2) Calibrate the instrument.
(3) Perform the measurement. Each time a piece of sample is inserted, the glossiness value is read, and the vertical and horizontal directions of the positive and negative sides are measured for each piece of the sample, and the average value of the two directions is taken as the gloss value. After each measurement, one instrument calibration is performed. After five measurements, the average value is the glossiness.

132. What is the relationship between gloss and apparent smoothness?
A: The decisive factor in determining the gloss of paper is the smoothness of the paper. Uneven paper surface, will reflect the incident light to all angles, the performance of low gloss; the smoother the paper, the smaller the proportion of reflected light diffusion, the higher the gloss.

133. What are the methods to increase paper gloss?
A: After the paper is coated, fine particles of the coating fill the depressions on the surface of the base paper, so that the smoothness of the paper is improved and the gloss is also increased. After super calendering, the fine unevenness of the paper surface is optically smooth (depressed). The gap is less than 1/16 of the incident light wavelength, and the gloss will increase dramatically. The influence of the coating amount on the glossiness is also very important. When the coating amount is low enough to mask unevenness on the surface of the original paper, no further supercalendering treatment can significantly improve the glossiness. Therefore, the use of coating and supercalendering processes, and proper control of the amount of coating is an effective way to increase paper gloss.
134. What is the printing gloss?
Answer: Printing gloss refers to the glossiness of paper after printing. In general, the higher the gloss of the paper, the higher the glossiness of the prints printed with it, and it can be said that the gloss of the white paper and the gloss of the prints are in principle uniform.

135. What are the factors affecting the print glossiness?
A: In addition to the glossiness of white paper, the factors that affect the glossiness of printing are the absorption of ink by paper, the speed of penetration of solvents, and the speed of ink conjunctiva. Therefore, it is necessary to obtain a good print glossiness. Consider various factors.

136. What is the paper whiteness? What is the standard of whiteness?
A: The whiteness of paper refers to the degree of whiteness of the paper. From a technical point of view, it contains various complex factors that are a function of the total spectral reflectance of the paper, the distribution of the illumination energy, the observation conditions, and the characteristics of the observer. The whiteness in the paper industry is the reflectivity of most papers for the blue light (457 nm wavelength) in the spectrum, while in Europe and North America it is the reflectivity of the green light in the paper (the dominant wavelength is 557 nm). Therefore, the physical measurement has different brightness standards and

There is still a difference in perceived whiteness.

137. What are the requirements for whiteness of various types of paper?
A: Whiteness is an important index for evaluating white paper. The whiteness requirements of various papers are related to their actual use. The printed book papers used in printed texts do not need to have a high degree of whiteness, and are generally suitable for about 70%; Offset printing paper and coated paper must have a high degree of whiteness, generally should be more than 80%, or even higher, so as to ensure the color of the colorful prints.

138. What are the factors that affect paper whiteness? How to improve paper whiteness?
A: The factors that affect the whiteness of paper are the wavelength of dyes, the whiteness of fillers, dyes, and coatings. To increase the whiteness of the paper, the most important thing is to use a pulp with a higher whiteness, that is, to strengthen the bleaching of the pulp; secondly, to select a filler, the higher the whiteness of the filler than the pulp, the greater the amount of paper added. The higher the whiteness. The whiteness of the coated printing paper is determined by the whiteness of the base paper, the whiteness of the coating, and the coating amount. Whether it is coated or non-coated paper, it is possible to increase the whiteness of the paper by adding fluorescent whitening agents to the pulp or coating; in the production of non-coated paper, the glue is often added, twisted and added. Filled with a small amount of product blue, lake blue or magenta, can achieve the purpose of whitening.

- "Printing Technology - Equipment and Special Printing"