How to improve the opacity of paper?
February 21, 2023
Opacity is a paper property. It refers to how translucent or see through a sheet of paper is. A high opacity means that the paper is not very see-through and it is easy to read print on the page without the distractions of marking from the other side showing through,the opacity of a sheet of paper can be affected by its basis weight as well as any added filler or coating materials.
What are the factors that affect the the Opacity of paper?
- Raw material
The plant cell itself is colorless and transparent. The reason why the plant fiber is opaque is that it contains lignin, resin and other substances that make the fiber opaque and with white to dark brown.
The crystal type, particle size, size distribution, and decentralization performance of the pigment all affect the opaque of the paper.
The properties of the paint in paper are determined by many factors, such as quality balance, pigment retention rate, electrical and chemical properties, and scattered color, which is very important to maximize all properties.
- Impact of beating
The higher the compactness of the paper, the more air squeezed out of the paper, the less air medium in the paper layer, and the paper is nearly homogeneous, with less light refraction and easy to pass through the paper layer. The greater the transparency of the paper, so it can improve the opaque paper by reducing the degree of pulp.
Influence of paper forming
Use ceramic deterring board instead of table roll to improve the paper evenness and increase the opacity,the composite press replaces the ordinary press, and uses a controllable medium-high calender to improve the tightness of the paper sheet, reduce the pressure of the press and calender to improve the opacity.
Adopt polyvinyl acetate forming screen and white water recovery system to improve the retention rate of fine fibers and improve the opacity of paper.
With the improvement of the surface roughness of the paper, the light scattering increases, the rougher the paper surface, the more opaque the paper, and the mesh and blanket grain of the paper surface also increase the paper roughness. Therefore, the paper opacity increases. It can be assumed that the paper opacity is proportional to the weight of grams per square meter within the range of small change in the paper quantity.
What are the reasons for the increase in the opacity of paper?
Adding fillers with refractive index greater than cellulose in the paper increases the number of light scattering interfaces in the paper, that is, there are three different interfaces between fiber and air, between filler and fiber, and between filler and air. Among these three interfaces, the difference of refractive index between filler and air is large. Therefore, the light gets the maximum scattering at the interface between the filler and the air, resulting in an increase in opacity.
How fillers affect the opacity of paper?
The purpose of fillers in paper making is to make the paper obtain specific properties, such as whiteness, opacity, paper strength, loose thickness, smoothness, etc. At the same time, the addition of fillers makes the internal gap of the paper smaller and smaller, thus increasing the ink absorption of the paper and making the ink absorption more uniform.
Generally, the insoluble or slightly soluble minerals added into the slurry are about 10% - 25% in the printing paper. The amount of different paper added is different and may vary greatly.
First of all, we should know that the paper without filler is composed of fiber and air. Air exists in the pores between the fibers. Due to the different refractive index between the fibers and the air, when the light beam shines on the surface of the paper, that is, part of the light scatters on the interface between the fibers and the air, giving the paper a certain (low) opacity, which is reflected in the printing, mainly showing that it is very easy to print through.
In terms of improving the opacity of paper, we think that hardwood pulp should be used as much as possible from the raw material to reflect the good optical properties of hardwood pulp. At the same time, the amount of mechanical pulp should be appropriately increased, but the amount of this part of pulp is not easy to increase (within 20%).
The beating degree of hardwood pulp shall be reduced as much as possible under the condition that the paper machine is normal: the amount of filler shall be appropriately increased in the use of filler, and the opacity of paper sheet shall be significantly improved: In the use of dyes, the amount of blue and red dyes can be appropriately increased, which will effectively improve the opacity of the paper.
Reduce the wet water as much as possible in the paper machine operation, thicken the paper pages, and micro -rough. Under the circumstances, appropriately reduce the smoothness, reduce the tension of the body, and reduce the water of the paper pages. Paper uniformity and delicateness will also affect the opaque of the paper industry.