Effect of secondary coating on the performance of matt color inkjet paper
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First, the development status of matt color inkjet printing paper
With the rapid development of computer technology, digital imaging technology and network technology, computers are rapidly spreading, office automation is getting higher and higher, digital cameras and camera phones are increasing in sales, and inkjet printing technology is a special printing method. Its non-contact, low noise, high speed, low cost, good quality, no size restrictions, environmental protection and other advantages have been widely used. With the continuous development of the color inkjet printer market, its main consumables, color inkjet paper (also known as color spray paper), has attracted more and more attention from users. At present, the production of matt and matt coated paper in Western Europe accounts for about 50% of the coated paper market, and in the United States, matte paper also accounts for 20% of the output of coated printing paper; in the North American market, this trend is still Continue to grow. Matte coated paper and matt coated paper have become an important part of the coated paper market due to their low sheet gloss and high print gloss and excellent printability and readability. Nowadays, the non-glare inkjet paper used in China is basically imported from abroad, and the price is relatively high. Therefore, it is urgent to select high-quality and low-cost inkjet paper to replace foreign imported products.
However, the production of matt and matt coated papers is mainly achieved by a combination of pigments of different properties to achieve lower sheet gloss. This combination of pigments requires both a reduction in gloss and a high-quality paper-printed surface. One or two pigments are difficult to meet. Usually, a variety of pigments are used to match each other, sometimes as many as five. Production is extremely inconvenient and costly.
To reduce costs, it is necessary to change the coating formulation of the coated paper and to use a reasonable coating method without degrading the performance of the inkjet paper.
Second, the pigment used in the non-gloss inkjet paper
In the past, calcium carbonate and barium sulfate were used as pigments, but the effect was not good. Porcelain suppliers have also developed several kinds of porcelain clay for low-gloss coated paper, but the effect is not very satisfactory. Many pigment suppliers are now actively developing low-gloss coated paper-specific pigments to improve product quality and reduce operational complexity. Grinding calcium carbonate (GCC) has the advantages of good fluidity and high whiteness, but it is difficult to control its narrow particle size distribution during grinding, and the shape is irregular, which is not conducive to obtaining high gloss difference. In contrast, light calcium carbonate can control its crystal shape, narrow particle size distribution and average particle size according to its production process, and the whiteness is high, which overcomes the shortage of porcelain clay and ground calcium carbonate, so it can be considered to develop low gloss. Lightweight calcium carbonate for coated paper. Foreign countries have advanced technology in this area, and have developed spherical lightweight calcium carbonate (SCC) for low gloss coated paper.
Calcium carbonate is one of the commonly used coating pigments in the paper industry, and the cost is low, which can effectively improve the solid content of the coating and play a good viscosity-reducing effect. Light calcium carbonate (ie, precipitated calcium carbonate PCC) has a small particle size, a narrow particle size distribution, easy control of size and shape, high whiteness, strong hiding power, and remarkable application effect. However, the large amount of calcium carbonate will affect the glossiness and surface bonding strength of the paper, which affects the print quality. Calcium carbonate is generally not used alone, often with other pigments, generally with PCC instead of the right amount of silica, and the binder polyvinyl alcohol (PVA) for coating configuration.
Third, secondary coating
The so-called secondary coating is to pre-coat the generally low-cost pigment on the surface of the paper to produce a relatively flat base paper for high-quality pigment coating. This results in a smoother topcoat for better surface performance and more uniform ink absorption during printing.
In the case where the total coating amount is the same, the surface of the secondary coated paper remains smooth after printing, and the primary coated paper becomes rough due to fiber swelling. The post-print gloss of the secondary coated paper is also superior. After the coating amount of the secondary coated paper is increased, the overall quality before and after printing is improved. In the case of the same base paper, even if the total coating amount of the secondary coated paper is lower than that of the primary coated paper, the fiber coverage and smoothness are superior to the one-time coating, and the cost of the secondary coating material can also be reduced. Because the primer is made of inexpensive PCC, the quality is best mentioned when topping. In addition, the printing ink mainly stays on the surface layer of the secondary coated paper, thereby reducing the amount of ink, reducing the fiber swelling and reducing the drying requirements during printing.
Fourth, Influence of secondary coating on physical properties of color inkjet printing paper
1. Influence on paper opacity
Opacity refers to the degree of opacity when incident light is applied to the paper. The opacity of the paper is the main factor that determines whether the paper is printed after printing. (It can be seen from Fig. 1) that the secondary coating and the one coating are under the same solid content conditions, and the opacity is remarkably high.
2. Influence on paper whiteness
Whiteness is an important indicator for evaluating paper and board. The higher the whiteness of the paper, the brighter the printed graphic. The paper with high whiteness can reflect almost all the color light, making the printed ink color pleasing to the eye and the visual effect is good. Paper with low whiteness, because it only absorbs part of the color light, can not faithfully express the contrast between the light and dark parts, and easily cause color cast.
For coating inkjet paper, the factors affecting the whiteness of the paper are mainly the whiteness of the pigment, the amount of adhesive required, the light scattering coefficient and the light absorption coefficient. The higher the whiteness of the pigment, the smaller the amount of adhesive required, and the higher the whiteness of the coated paper. After the PCC is applied under the primer, the surface coating of silica and the same coating amount of PCC and the silica mixed pigment are the same, the whiteness is obviously higher than the latter, because the whiteness of the silica is higher than that of the PCC. The secondary coating is coated with silica, the surface system of the coated paper is mainly silica, and the primary coating is a mixed system of silica and PCC, and the whiteness is relatively low (as shown in Figure 2). , figure omitted).
3. Influence on smoothness
The particle size and degree of dispersion of the pigment affect the smoothness of the coated paper. The smaller the particle size of the pigment, the better the smoothness of the coated paper. However, if the pigment is not well dispersed, the pigment will reaggregate and the fluidity of the pigment dispersion will be deteriorated, which will affect the flatness of the paper surface. The smoothness will be measured by the air leakage method roughness meter, and the size will be expressed by the PPS value, and the PPS value. The lower the printing, the higher the smoothness of the print.
It can be seen that in the secondary coating, the paper can be made to have better smoothness.
4. Influence on paper cobb value
For inkjet paper, good absorbency is a prerequisite for obtaining high quality images. If the absorption of the coating is poor, the ink permeation drying time is slow, and the ink is easily splashed and beaded. However, if the absorption is too good, the ink penetrates quickly, which is easy to cause offset, the image gloss is low, and the image is illusory. There are many ways to measure the absorbency of paper and paperboard. We used the Cobb Absorption assay. Because color inkjet water is mostly water-based ink. Therefore, the water absorption value represents the absorption of the ink.
In the secondary coating, since good absorbability can be obtained, the ink of the secondary coating is excellent in absorbability with respect to the primary coating, and good printing performance can be obtained.
Fifth, the impact of secondary coating on the printing performance of color inkjet paper
Color density (also called optical density or density) refers to the characteristic measure of ink absorption when ink is printed onto paper to form a pattern or a handwriting. It is an important parameter to measure the quality of printed images, affecting the final visual effect, color gamut and color vividness. degree.
In summary, the secondary coating method is relatively better in physical properties and printing performance than in one coating, but in practical applications, it is required to be applied twice for secondary coating. Compared with the first coating, the workflow is increased, and the working efficiency is reduced. The practical application is to improve the efficiency, and it is inclined to use one coating under the premise of meeting the performance requirements.