In the paper making process, according to the different types of paper produced, some paper types need further surface treatment to meet higher surface performance requirements. The surface treatment of paper can generally be divided into surface sizing, calendering and other processes.

What is surface sizing?

• Surface sizing is a process to apply a uniform thin layer of adhesive on the surface of the paper, which aims to make the surface of the paper obtain hydrophobic and other special properties.

• Method of paper sizing: internal sizing and external sizing.

• In-machine sizing is generally carried out in the drying section of the paper machine.

• External sizing is to apply the unloaded paper roll on the special sizing device outside the paper machine.

What is the main function of surface sizing?

• Improve the surface properties of the paper;

• Improve the print ability of the paper;

• Reduce the difference between two sides of the paper;

• Improve the void structure and absorption of the paper. In addition, surface sizing can also improve the physical strength of the paper, such as the folding strength, bursting strength and tensile strength of the paper.

• Improve the gap structure and absorption of paper pages. In addition, the surface gel can also improve the physical strength of the paper pages, such as increasing the torture, brokenness and tension strength of the paper pages.

What are the factors affecting surface sizing?

Nature of paper pages

• Paper is a thin leaf with interlaced fibers on the plane. The paper made from softwood pulp has 2 million fibers arranged in 1 gram. The porosity of paper is one of the most basic properties of paper and is related to all other properties of paper.

Properties of sizing solution

• The key properties of solution used as sizing solution are surface tension, wetting property and viscosity. In addition,the contact time and solid material concentration with paper also affects greatly.

• Although the viscosity depends on the concentration and temperature, it has a great relationship with the permeability. The square root of the permeability viscosity is directly proportional. For example, in the case of sizing starch, the paper will curl when the viscosity changes with aging and the liquid absorption on both sides changes. If the concentration is high, the surface sizing effect will be improved.​

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• ​However, if it is excessive, the water content will increase after sizing and pressing, and the paper will be difficult to dry. Therefore, it is not ideal. The film forming property has a great relationship with the molecular condensation energy. Therefore, the molecular structure plays a significant role.

Drying and calendering

• The type and running speed of sizing and pressing are the same as that of drying, which greatly affect the completion. If the drying conditions are not appropriate, the film on the surface will be fuzzed or spot peeled off.

• The type and running speed of sizing and pressing are the same as that of drying, which greatly affect the completion. If the drying conditions are not appropriate, the film on the surface will be fuzzed or spot peeled off.

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• If it dries rapidly, the film on the surface will have cracks and checkers. If too much calendering, the film on the surface will break and penetrate into the paper discontinuously. Moreover, the surface sizing agent sometimes falls off.

Which paper needs to be applied for surface sizing?

• Due to the limitations of technical level and operating costs, surface sizing can only be used on special types of paper, such as banknote paper, security paper, sea paper, offset printing paper, whiteboard paper, striped kraft paper, etc. With the improvement of the technical level of the paper industry and the improvement of people's requirements for the quality of paper, some paper grades (newsprint, etc.) that did not need surface treatment have also begun to use surface sizing technology.