Types, properties and applications of surface sizing agents commonly used in papermaking

Surface sizing is a process in the processing of paper or cardboard, usually located at the end of the drying cadres of the paper machine, so that the paper page is not completely dry but has a certain strength when sprayed with a layer of glue, after subsequent drying on the surface of paper and cardboard to form a layer of adhesive film, so as to achieve the purpose of changing the surface performance of paper or cardboard.

Surface additives that improve page properties or increase water resistance of paper or board can be added by surface sizing. Originally, due to the limitation of technical level and paper requirements, surface sizing agent can only be used on special paper, such as: banknote paper, securities paper, Marine drawings, etc. With the improvement of the technical level of the paper industry, the surface sizing of paper has become a routine paper processing process, especially in recent years due to the popularity of printing, copying, fax on the surface properties of cultural paper, wrapping paper and corrugated paper, strength and water resistance put forward higher requirements, so the paper industry is constantly exploring new surface sizing technology.

1. The current surface sizing has the following advantages

1) Improve the printing performance of paper and cardboard;

2) The surface strength or water resistance of the paper can be improved by selecting different types of surface sizing agents;

3) Improve the physical strength of paper and cardboard;

4) Surface sizing can reduce the difference between two sides of the paper;

5) It is not affected by the water quality and water temperature of the paper, and the sizing effect is relatively stable;

6) Good glue retention effect, low sizing cost;

7) The use of internal sizing at the same time can make up for some defects of internal sizing.

2. Classification and function of surface sizing agent

According to the function of the surface sizing agent, it can be divided into water resistance and enhancement. Alkyl ketene dimer (AKD) and styrene maleic anhydride copolymer can be used as surface sizing agents to improve water resistance. Starch, carboxymethyl cellulose (CMC), polyethylene (PVA) alcohol and so on can be used as reinforcement.

The following describes the types and base characteristics of surface sizing agents commonly used in paper mills.

1) Starch and its modifiers

Starch is a kind of natural polymer compound, it is an important surface sizing agent and paper strengthening agent. Corn starch is widely used, and potato starch has a better effect. Natural raw starch has high viscosity, poor fluidity, easy to coagulate, easy to precipitate after dilution with water, and there are still great limitations in terms of cohesiveness and film formation, so modified starch is used in the sizing system.

The biggest advantage of using starch modification as a surface sizing agent is that it is rich in raw materials and cheap, which is very suitable for China's national conditions, and the surface enhancement effect is obvious, which can improve the printing effect. However, starch is a polymer water-soluble substance, which contains hydrophilic groups in its structure, and it is difficult to resist liquid penetration after film formation. This requires the use of modified starch and other surface sizing agents to meet the requirements of the paper. The most common type of surface sizing is oxidized starch.

2) Polyacrylamide (PAM)

As a surface sizing agent, PAM should be mixed with glyoxal, which can form a cross-linked network during the drying process. Due to the presence of trivalent aluminum ions and calcium ions in the paper, some of the -CONH: groups in PAM molecules are hydrolyzed into -COOH, and these metal polyvalent ions will produce ionic cross-linking bonds with the -COOH in PAM, thus making the paper water-resistant. PAM high price, good quality, suitable for use with cheap starch.

3) Polyvinyl Alcohol (PVA)

Polyvinyl alcohol is divided into many grades according to the degree of polymerization and hydrolysis. Generally used as surface sizing PVA polymerization degree of l000~2000, alcohol hydrolysis degree of 98% ~ 99%. The PVA produced in China is mainly 1798 for surface sizing, that is, the polymerization degree is l700 and the alcoholysis degree is 98%. After the PVA surface sizing paper, the fiber has a good adhesion after drying, good surface strength, PVA formed film has a good oil resistance, but the disadvantage is high viscosity, poor coating, easy to penetrate into the inside of the paper layer (only to improve the surface strength), usually in order to achieve the required surface sizing effect, it is inevitable to increase the amount. In order to solve this problem, the boric acid pretreatment technology can be used. The principle is that when the paper treated with boric acid is coated with PVA, boric acid and PVA can cross link to form a gel, which inhibits the excessive penetration of PVA into the inner layer of the paper. To achieve the same surface strength, 3% solution may be used if only PVA is used, but l % ~ L. 5% boric acid solution is used, and the concentration is enough as long as l %. At present, the application of starch polyvinyl alcohol as a surface application agent and paper mills are decreasing.