1.The influence of water-based ink viscosity on the surface coloring force of kraft paper
 
The viscosity of water-based ink is 28.7±0.5s. Water-based ink has the best coloring power for surface printing of kraft paper. The sample viscosity of 19.6±0.5s water-based ink has the worst coloring power.
 
When the viscosity of the water-based ink is low, the force between the pigment particles, connecting materials and related additives is small, and the fluidity is good, but the reduction of adhesion will lead to the ink adhering to the surface of the paper less during printing, and the surface of the kraft paper has further penetration and absorption after printing. When the viscosity is low, the pigment particles are easier to enter the paper, so the coloring power is reduced. When the viscosity is too large, the cohesion of the ink is large, affecting the adsorption of the paint particles and the paper surface, resulting in a low number of paint particles transferred to the paper surface, thereby reducing the coloring power. The viscosity of the formula balances the force between the components in the experiment, so the ink coloring power is good.
 
 
2.The influence of water-based ink viscosity on the surface gloss of kraft paper
 
When the viscosity of water-based ink is 23.2±0.5s, the gloss is the best, and the viscosity is 28.7±0.5s.
 
The gloss of the paper reflects the surface properties of the paper, which is the ability of the paper surface to display images, and the gloss of the paper is related to specular reflection. This experiment focuses on the influence of ink viscosity on gloss. The number of pigment particles and the film forming properties of the connecting materials are the main factors. When the viscosity is low, the cohesion of the pigment particles is reduced, which affects the film formation of the ink on the surface of the paper. Because the connecting material can quickly penetrate into the paper through the capillary tube, the film-forming component is reduced, more pigment particles gather on the surface of the kraft paper, and the gloss is lower after printing. When the viscosity is too high, although the film-forming component of the ink increases, due to the reduction of the printing speed, the flatness of the ink film is poor, and the gloss will also be affected.
 
 
3.The influence of water-based ink viscosity on the surface adhesion of kraft paper
 
The viscosity of water-based ink has great influence on the surface adhesion of kraft paper. The adhesion level of water-based ink on the surface of kraft paper under different viscosity, ink adhesion refers to the bond and attraction between the water-based ink and the surface of the substrate through physical or chemical action, that is, pigments, resins, fibers, fillers and glues. Factors affecting ink adhesion include mechanical bonding, adsorption, chemical bonding and diffusion of ink and substrate surface. The complex pores between kraft paper are the main factors that determine the adhesion of the ink. Due to the large viscosity, it helps to form a force between the paint particles and the paper surface, thus strengthening the combination of ink and paper, and the adhesion is large.
 
There are many factors that affect the printing effect of kraft paper. By controlling the paper surface structure, pigment particles, resin type, preparation process and printing conditions, the coloring force, gloss and adhesion of water-based inks with different viscosities were compared to indicate the printing effect of kraft paper. The viscosity of 23.2±0.5s and 28.7±0.5s water-based ink printing coloring power and gloss are the best; The viscosity of 28.7±0.5s and 31.3±0.5s water-based inks have the best adhesion. Considering the printing speed, printing cost and printing quality, the optimal viscosity of water-based ink is 28.7±0.5s.