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.

4) CMC

CMC is a white powder, granular or fibrous substance, odorless, tasteless, non-toxic, is a good surface sizing agent. CMC generally has a viscosity below 300mPa.S and a degree of substitution between 0.6 and 0.75, and is more suitable for use as a surface sizing agent.

Compared with PVA, carboxymethyl cellulose (CMC) has higher advantages as a surface sizing agent:

(1) It has excellent film formation and whole film transfer performance, and can form a good closure and oil resistance on the surface of paper or cardboard.

(2) Due to the poor film formation of modified starch, it is impossible to form a continuous and complete film on the surface of the paper, and will form a "accumulation" on the surface of the paper and block the pores, resulting in spots when printing, therefore, starch with CMC, can largely overcome this shortcoming.

(3) CMC has good dispersion, it can fully disperse other rubber particles to form a single and stable system, and can effectively coagulate and precipitate in a certain period of time.

(4) If the appropriate amount of CMC glue into the surface sizing, you can form a continuous and complete and flexible film on the surface of the paper, this film has a moisture control function, can prevent paper or cardboard in the printing process warping deformation and eliminate the size error when overprinting.

(5) Due to the excellent bonding ability of CMC, it strengthens the combination between the fiber and the fiber on the surface of the paper or board, and between the fiber and the filler, thus effectively improving the surface strength of the paper. And make the surface of the paper (board) to obtain a good ink acceptance, improve the gloss of the paper, so that the printed text image is shiny and beautiful, but also reduce ink consumption. CMC film strength is large, oil resistance, but high viscosity, so the use of medium relative molecular weight, substitution degree of 0.5~0.7mol/g CMC, coated paper rupture strength and printing gloss is good.

5) Modified AKD

Modified AKD is mainly used in combination with starch on cardboard, which can improve the water resistance of cardboard and greatly reduce the cost of paper under the condition of relatively small dosage. The AKD curing problem that was previously worried has basically been solved, like ordinary high-strength corrugated or cow leather box board, the full curing period can also be completed within 2 hours. This modified AKD plus surface sizing starch method is also applied to ordinary cultural paper to completely replace in-pulp sizing. Add to the surface of the paper, so that not only can reduce the amount of AKD, but also shorten the aging time of AKD.

6) Copolymers

(1) Styrene-Maleic anhydride Polymer (SMA)

SMA is an alternate copolymer of styrene and maleic anhydride, a light amber colored mucus with a solid content of 25% to 40%, which can be infinitely diluted in water. Mainly used for coating, securities paper, greaseproof paper, carbon paper, copy paper, etc., can improve the wear resistance of paper, strength, oil absorption.

(2) Styrene-Acrylate Copolymer (SAE)

The copolymer (SAE) of styrene and acrylate prepared by emulsion polymer is usually used in latex form. At present, most of the SAE surface sizing agents used are aqueous anionic polymers, which will cause emulsion precipitation and have adverse effects on sizing because of AB+ and Ca2+ in the water during use. In order to improve the water resistance of the emulsion after film formation, it is generally necessary to add a small amount of surfactant, such as oxidized starch with better effect. Or directly add a small amount of aluminum sulfate to improve its sizing performance.

(3) Styrene-Acrylate-Maleic anhydride monoester copolymer (SAM)

Styrene-acrylate-maleic anhydride monoester copolymer (SAM) is a medium molecular weight product with random copolymerization of styrene, acrylic acid and maleic anhydride, and the effect is better than SMA. This will be the technical development direction of sheet paper surface sizing.

(4) Polyurethane aqueous dispersion (PU)

Polyurethane (PU) water dispersion is a new type of surface sizing agent, which can produce higher sizing degree with less dosage and improve paper performance. Mainly used for coated paper and offset paper, the application cost is high.

7) Animal glue

Animal glue is a protein extracted from the bones, skin or tendons of animals. High-grade skin glue and bone glue light and transparent, also known as gelatin. Both skin glue and bone glue can be used for surface sizing, but the most suitable for high-grade paper is skin glue, compared with bone glue, skin glue viscosity, covering ability and film formation are better than bone glue, can produce a stronger film, and therefore can give greater strength to the paper. Due to the high price of animal glue, it is mostly limited to the surface sizing of high-grade paper. When using animal glue, it is first fully swollen in cold water, and then dissolved in hot water of about 50 to 60 degrees, and a glue with a concentration of 4% to l0% is made in stirring. If necessary, a small amount of organic acids or bases can be added to improve solubility. In order to reduce its viscosity, an appropriate amount of aluminum sulfate can be added. Adding 2% ~ 3% soap to animal glue (the amount of animal glue) can improve the color, opacity and gloss of paper; Adding a small amount of sodium chloride, glycerin, non-drying oil, sodium acetate, etc., can also improve the kneading resistance of paper. When animal glue and starch are mixed, 0.25% ~ l.0% soap (for the amount of animal glue) can be added to prevent the dispersion of animal glue starch.

Our company's newly developed water-soluble chitosan solid surface size agent, originated from Japan's deep-sea rich Marine natural organic raw materials, with the help of modern Marine bioengineering technology of new biological agents. It uses starch as the carrier, combines various substances in starch with its unique physical properties, completes the reaction in the paper page gluing drying instantaneously, forms a strong and ductile network structure layer, thus giving the paper excellent surface strength, and has the noble reputation of surface sizing agent.

First, product characteristics

1, the product is solid, to solve the transportation distance, packaging and inventory;

2, the product is weak anionic, with whitening agent, AKD surface sizing compatibility;

3, the product is water-soluble chitosan sizing agent, can be miscible with water at any rate, the product fills the domestic gap;

4, the product is molecular level, ultra-low dosage, the maximum dosage of 200g/ ton paper, is the traditional surface sizing agent dosage 1/20;

5, the product is environmentally friendly, food grade, non-toxic, tasteless, no VOC;

6, the product is reactive, and starch groups in the drying part to complete the reaction, the internal bond strength and interlayer bond strength is better than polyvinyl alcohol, styrene and C film sizing agent, with excellent surface strength;

7, the product and starch to form a chemical film, reduce paper permeability, excellent moisture resistance;

8, the PH value of the solution is weak alkaline, and the mixture of starch (acidity) is neutral, preventing equipment corrosion.