Types and functions of surface sizing agents
1 Introduction
The purpose of sizing is to make paper or paperboard resist the diffusion and penetration of liquids (especially water and aqueous solutions). Surface sizing [1, 2] refers to the process of evenly applying appropriate sizing materials on the surface of wet paper after the wet paper web is dehydrated to a certain value in the drying section. In modern papermaking technology, surface sizing has become the main form of surface sizing treatment of paper sheets. Its function is not only limited to giving paper a certain liquid resistance. In some cases, it emphasizes its improvement of paper printing performance and paper surface performance. Therefore, surface sizing is also called surface modification or surface enhancement.
In recent years, with the development of paper surface sizing technology, many chemical companies have developed and produced chemicals suitable for paper surface sizing. This article will mainly introduce the types of surface sizing chemicals and their functions in different paper grades.
2 Types of surface sizing agents
2.1 Traditional surface sizing agents
Starch is the most commonly used carrier and the chemical with the largest amount used in sizing pressing. There are many literature reports on this aspect [3, 6]. Here we need to emphasize cationic starch and enzyme-converted starch. Cationic starch [7] can form ionic bonds with fibers, so it can remain on the fiber surface more during the process of paper recycling, reducing the COD of white water and benefiting the environment. Enzyme-converted starch [8] is a biologically modified starch. Its conversion result is similar to that of oxidized starch, both of which hydrolyze the long molecular chains of starch into short molecular chains. The preparation process of enzyme-converted starch is relatively simple and can be made and used immediately. The most prominent advantage of the more commonly used oxidized starch is its low cost of use, so it is increasingly favored by paper mills.
In addition to starch, PVA, CMC and sodium alginate [9] are sometimes also used as carriers in sizing presses. These chemicals have good film-forming properties and can seal the pores of paper.
2.2 Synthetic polymer surface sizing agents [10-14]
Synthetic polymer surface sizing agents play an extremely important role in the modern papermaking industry. Different from traditional in-stock sizing agents, they are specially designed for surface sizing and are the mainstream products of surface sizing agents at present. This type of surface sizing agent can be mainly divided into three types: ① water-soluble polymer surface sizing agents (SMA and SAA types); ② polymer aqueous dispersion surface sizing agents (SAE type); ③ polyurethane aqueous dispersion surface sizing agents (PUD type).
2.2.1 Water-soluble polymer surface sizing agents [15-18]
These water-soluble polymers are mainly ammonium salts, sodium salts or mixed salts of styrene-maleic anhydride copolymers (SMA type) and styrene-acrylic acid copolymers (S type). The product gradually emerged with the transition of paper machine systems to medium alkalinity. It is mainly used to overcome the excessive reduction of paper surface friction coefficient after the abuse of AKD in the pulp.
SMA and SAA are both anionic polymers. Their water solubility comes from the dissociation of carboxylates, so they cannot be used in low pH environments. The role of SMA and SAA products often requires the help of certain cationic substances. For example, the carboxylate ions in the long chain of the polymer are adsorbed on the surface of the paper with the help of A13+, while the hydrophobic styrene groups face the outside of the paper, thus giving the paper a certain degree of water resistance. SMA and SAA products also have a certain film-forming ability, which can change the air permeability of the paper and increase the friction coefficient of the base paper surface.
There are many factors that affect the performance of this type of product. The molecular weight of the polymer affects both the sizing effect and the film-forming ability of the product. The higher the molecular weight, the better the sizing effect and the stronger the film-forming ability. The type of salt will also affect the properties of the polymer. Generally speaking, ammonium salts have better sizing effects due to their easy dissociation, while sodium salts have stronger film-forming ability. Compared with other types of polymer surface sizing agents, this type of product will produce a lot of foam during use, which will affect the stability of the sizing pressing effect and limit its application in the papermaking industry.
2.2.2 Polymer water dispersion surface sizing agent
The chemical composition of this type of product is mainly styrene-acrylate copolymer. SAE products can be made into anionic or cationic types. Cationic products have strong fiber-binding ability and are mainly used in systems with more anionic substances, while anionic products are widely used in the production of multifunctional office paper.
The mechanism of action of SAE products is different from that of SMA and SAA products. They often cannot form films on the surface of paper sheets. Structurally, the center of the particles in the aqueous dispersion is a hydrophobic core, which is surrounded by a hydrophilic protective colloid. The hydrophobic core is usually weakly ionic or non-ionic. When the product is applied to the surface of the paper, the hydrophobic core is redistributed on the paper sheet or partially penetrates into the inside of the paper sheet due to heat, and finally combines with the fiber through hydrogen bonds, thereby giving the paper a certain degree of water resistance by changing the fiber surface energy.
The production process of SAE products is simple and environmentally friendly. It has good compatibility with other chemicals. It is not easy to produce foam during actual use. It has gradually become the mainstream product among synthetic surface sizing agents.
2.2.3 Polyurethane aqueous dispersion [19]
The molecular structure of polyurethane is that many hydrophobic groups extend from a low molecular weight polymer skeleton. Cationic products can be directly adsorbed on the fiber, and anionic products also need the help of cationic substances to bind to the fiber. In the post-drying section, after the polyurethane molecules are heated, their hydrophobic groups can rotate around the molecular skeleton, so that the hydrophilic groups are parallel to the fiber surface and the hydrophobic groups face the outside of the paper surface. Compared with other synthetic polymer surface sizing agents, polyurethane products can give paper higher water resistance, and its effect is comparable to AKD and rosin glue. Unlike AKD, PUD products have no aging period, will not reduce the friction coefficient of the paper surface, and are not easy to hydrolyze. However, its relatively high cost of use also limits its promotion in the papermaking industry.
3 Improvement of paper properties by surface sizing
After surface sizing, many properties of paper will be improved [20-21]. Such as improved water resistance, improved surface strength and surface friction coefficient, reduced air permeability, etc. Surface sizing agents can be used in the production process of different types of paper, and their roles are also different.
3.1 Cultural paper
In addition to requiring water resistance, cultural paper also emphasizes printability and performance on the printing press. In recent years, with the popularity of inkjet printing and high-speed printing presses, the requirements for paper printability have become increasingly higher. Therefore, in the modern papermaking industry, a combination of surface sizing and internal sizing is often used to improve paper performance. Generally speaking, the contribution of surface sizing to paper sizing performance is relatively small, and it mainly improves the printability and runability of paper. The following explains the role of surface sizing agents according to different printing methods.
3.1.1 Inkjet printing
Inkjet printing is to form text or images by spraying ink droplets onto the printing medium. High-quality inkjet printing requires bright colors, clear image edges, no ink spots in the printing area, and fast ink drying. Since the ink used in inkjet printing is water-based, the water resistance of the paper surface is a key factor affecting printability. Surface sizing agents, especially SAE products, can improve inkjet printability by changing the surface energy of the paper and the contact angle with water. In general, after using surface sizing agents, the following properties of inkjet printing paper are improved:
① Inkjet printing density;
② Text edge fuzzing;
③ Paper bleeding. With the continuous advancement of inkjet printing technology, the printing speed is also increasing, which puts forward new requirements for the drying speed of ink on the paper surface. In order to meet this requirement, some advanced paper machines will add a small amount of pigment (3-5g/m2) at the sizing press to improve the paper’s ability to absorb ink.
3.1.2 Laser printing and electrostatic copying
Surface sizing agents can mainly improve the adhesion of paper to toner. Toner contains resin and pigment. The resin in the commonly used black toner is usually styrene-acrylic copolymer. Therefore, SAE surface sizing agents can improve the adhesion of paper to toner. There are many methods to evaluate the adhesion of toner. Among them, the commonly used method is to adhere a sticky tape to the printing area, peel off the tape under certain conditions, and compare the printing density before and after peeling to obtain the retained print density (RPD). The higher the retained print density, the better the adhesion of the toner.
3.1.3 Offset printing
The most concerned about the surface strength and water resistance of paper in offset printing. In printing, the surface strength of paper is often measured by the degree of hair loss and powder loss on the surface of the paper. The higher the surface strength of the paper, the less likely it is to hair loss and powder loss during printing, and the higher the printing efficiency. In offset printing, the paper must be kept moist. The better the water resistance of the paper, the higher the wet pull strength.
Starch is the most widely used surface enhancer, but when the surface strength of the paper needs to be further improved, a polymer surface sizing agent must be added. The water resistance of SMA and SAA surface sizing agents is limited and is generally not suitable for offset printing paper. SAE products can significantly improve the surface strength of paper. In addition, since it can change the surface energy of paper, it also increases the ink transfer rate during the printing process.
3.2 High-grade coated paper
The purpose of using surface sizing agents on this type of paper is to give the paper a certain degree of water resistance to prevent adhesives from penetrating into the interior of the paper during the coating process. For example, using surface sizing agents on the surface of carbonless copy paper can effectively prevent the penetration of microcapsules in the coating components and prevent the penetration of adhesives in the production of coated label paper. Compared with AKD, surface sizing agents do not require aging to play their role, which is particularly important for high-speed coated paper production lines.
The selection of surface sizing agents is mainly based on the coating treatment process and purpose of the paper. If heavy sizing is required, polyurethane surface sizing agents can be used, but the high water resistance of polyurethane also reduces the adhesion between the adhesive in the coating and the paper surface. The water resistance of SAE surface sizing agents is slightly poor, but it can ensure the adhesion between the coating and the paper.
3.3 Coated white board paper
A large amount of secondary fibers are used in the production process of coated white board paper, so there is a large amount of anionic garbage in the paper machine system, which makes the sizing efficiency in the pulp low. At this time, surface sizing is needed for sizing reinforcement. Cationic SAE surface sizing agents are widely used on this type of paper.
4. Conclusion
With the rapid development of the papermaking industry, surface sizing has also become an indispensable process in papermaking production. The surface addition of papermaking chemicals has also received more and more attention. However, even if the retention of chemicals in surface sizing is close to 100%, the final use effect of the paper is still affected by the quality of the base paper. Therefore, seeking a balance between wet end treatment and surface treatment is the most economical and effective method.
