Publication of printing paper optical properties and testing

In recent years, the rapid development of China's paper industry, the total output of paper and paperboard, total consumption has ranked second in the world, and formed a complete set of standardized systems. Among them, the national standardization law, measurement law and product quality law are the three basic laws that must be followed in the production and sale of paper products.

China's paper industry standards include national standards, light industrial standards and corporate standards. As far as the application field is concerned, there are product standards, product performance test method standards, and product test environment atmospheric conditions standards, as well as paper quality supervision and inspection papermaking special measurement instruments Light Industry Department department measurement verification procedures. According to China's papermaking industry standardization system table, published printing paper is divided into two categories: non-coated paper and coated paper. Publishing of large-scale papers used in the printing industry, such as newsprint, offset printing, offset printing, letterpress printing, etc., are non-coated printing papers; coated papers, low-quantity coated papers, cast-coated papers, etc. are coated printing papers. class. The optical properties of the paper are based on the optical principle of the test. Whiteness, color, and opacity are the diffuse reflection characteristics of the paper. Gloss and print gloss are mirror reflection characteristics of the paper. This article focuses on the optical properties such as whiteness, chromaticity, opacity, etc., of published printing paper and its testing standards.

Optical properties
1. Whiteness
As we all know, white paper can truly and objectively reflect all the colors of printed graphics, improve the contrast and sharpness of the text, make the copy colorful, and achieve the effect of illustrations. The higher the paper whiteness, the more significant this effect is. However, the whiteness should not be too high, or the reflected light is strong, and the visual nerve stimulation is too strong and can easily cause visual fatigue. Therefore, the higher the whiteness of the printing paper is, the better. Moreover, the whiteness values ​​of printed papers for different purposes are not all the same. It is reported that from the perspective of protecting children’s eyesight, many children’s books use low-white paper, and some books even use beans. Green, light yellow writing paper. Heilongjiang Children's Publishing House also used bean green and light yellow writing paper; Anhui Children's Publishing House also reduced the whiteness of the supplementary book paper to 76% to 85%. The Ministry of Education stipulates that the whiteness of children's textbooks is 75% to 76%.

Although the printing and printing papers are basically white or nearly white, they all have color casts, some are bluish and some are reddish. The purpose is to make the visual judgments look more white, but it also varies from person to person. In any case, the paper supplied in the same batch should have the same whiteness, uniform color tone, and no obvious color difference, so as to avoid stratification in the color of cuts in binding printed volumes.

2. Opacity
The level of opacity of printing paper directly affects the print penetration situation of printed products. All kinds of printing paper must have sufficient opacity, otherwise, it will easily lead to printing failures.

3. Gloss
The gloss of prints is closely related to the specular reflection properties of paper. The print glossiness of a paper refers to the fact that it is printed on a paper sample with a predetermined standard glossy ink under specified conditions and the glossiness of the blotted area is measured after drying, expressed as a percentage. General paper gloss is high, the gloss of the printed matter is high, and the printed matter is vividly illustrated and colorful. Such as coated paper gloss and printing gloss minimum required 60% and 88%.

Paper optical performance standards
The national standard GB/T1910-1999 classifies newsprint products into excellent products, first-class products and qualified products.

1 brightness. Brightness is also called whiteness, and the standard specifies that the whiteness values ​​of the three grades are not less than 52%, 50%, and 48%, respectively;

2 tone reference value. It is stipulated that the three grades of products are based on L*≥80.0, a*=-2.0 to +2.0, and b*<10.0 as the basis for controlling the color tone of each plant.

3 color difference â–³E*. The color difference â–³E* of the same batch of paper should be no more than 2.5 and 3.0 respectively for excellent products and first-grade products, and there is no requirement for qualified products.

4 opacity value. The opacity values ​​of the 45g/m2 paper three grade products were all 86.0%; the paper 47g/m2, the excellent product was 88.0%, the first grade and the qualified product was 86.0%; the ≥49g/m2 paper, the premium product was 90.0%, The first-class goods and qualified products were 87.0%.

In addition, in June 2002, the China Press Association Material Supply Committee formulated the “China Newsprint (User) Quality Standards” with reference to GB/T 1910-1999 and the newsprint manufacturer’s standards for newsprint production. Among them, the technical indicators related to optical performance It is stipulated that the whiteness of newsprint with a basis weight of 45g/m2 and 48.8g/m2 is ≥56.0% and the opacity is ≥90.0%.

Coated paper
According to the national standard GB/T1033-1995, coated paper is divided into A, BI, B-II, and C. The whiteness value is 80.0% except for C, which is 80.0%. The opaqueness value varies depending on the amount of quantitation, 70-90 g/m2 is not less than 85.0%, 90-130 g/m2 is not less than 90.0%, and the paper specification greater than 130 g/m2 is not less than 95.0%. The same opacity value is the same for different quantification products. The national standard has no specific requirements for the color value of coated paper, but the color of the same batch of paper should not be significantly different.

3. Offset printing paper

The publishing and printing industry calls offset printing paper double-tape paper, which is commonly known as Daolin paper in the trade, and is specifically used for multi-color offset printing. The light industry standard QB/T1012-1991 classifies it into three grades: A, B, and C. The specified whiteness values ​​are not less than 87.0%, 82.0%, and 77.0%, respectively, and the opacities vary depending on the amount of quantification. 60g/m2, 70g/m2, 80g/m2, 90g/m2, 100g/m2, 120g/m2, and 150g/g M2 offset paper opacity values, A and other products were 84.0%, 86.0%, 87.0%, 89.0%, 91.0%, 93.0% and 95.0%, B and other products such as C and A and other products should be lower. There is no specific requirement for color, only the color of each batch of paper should be consistent and there should be no significant difference.

4. Offset book paper

The light industry standard QB/T1211-1991 stipulates that offset papers are classified into A, B, and C grades. The whiteness values ​​are A: 70.0% to 75.0%, B: ≥ 60.0%, and C: ≥ 55.0%. The opacity value of the three grades of the product shall be not less than 78.0% for 52g/m2, not less than 80.0% for 60g/m2, not less than 82.0% for 70g/m2 paper, A, and not less than 80.0% for B and C grades. The color should be uniform and there should be no significant difference in each batch of paper.

5. Low weight coated paper
Also known as lightweight coated paper, LWC paper, there is no unified national standard or industry standard. Finished product inspection is generally performed according to enterprise standards or contract standards. Since this type of paper is suitable for color printing and has a low shipping cost, the printing cost has been greatly reduced and it has been widely used internationally. The light-coated paper produced by some paper-making enterprises such as Shandong Huatai, Tranlin, Chenming, Caohe Silver Dove, Yueyang Paper and Guangzhou Paper has not only met domestic demand, but also has some exports. It is reported that in the new national standards for coated printing papers that are being formulated, relevant specifications and specifications for low-quantity coated papers will be set.

In the method standards for publishing optical properties of printing papers: National Standard GB/T7973 "Determination of diffuse reflectance factors for pulp, paper and paperboard (diffusion/vertical method)", GBT7974 "Determination of paper and paperboard whiteness (Diffuse) "Radio/Vertical Method", GB/T7975 "Determination of Paper and Cardboard Color (Diffuse/Vertical Method)" and GB/T8940.1 "Paper and Cardboard Whiteness Measurement Method (45/0 Directional Reflection Method)" Promulgated and Implemented It has been more than 10 years. The promulgation and implementation of these method standards are of great significance for the detection of uniform paper performance, improvement of paper optical performance testing techniques, popularization of theoretical knowledge of colorimetry, and guidance of production. These standards apply to optical performance tests of published printing papers related to diffuse reflection factors.

Source: Printing Technology

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