Classification and Characteristics of Paper Coating Lubricants (I)

1The function and classification of lubricant

The application of paper coating lubricants dates back to the beginning of this century. At that time, the adhesive for paper pigment coating was mainly animal glue or casein, and the solid content of the paint was very low. Although these adhesives have good adhesion and excellent water retention properties, the resulting films are brittle, so an objective need to add an additive that can improve the folding and bending resistance of coated paper and paperboard. These additives also improve the flow and leveling properties of the wet coating. This type of additive, which acts both as a wet coating and as a dry coating, was once considered a "plasticizer" additive, the lubricant discussed in this article.

The function of lubricants differs depending on the type of paper and the differences in the production habits of paper mills. The properties of the lubricant are sometimes evaluated by the fluidity of the coating and some properties of the coated paper (such as gloss, smoothness, oil absorption, surface strength, etc.). Some categories of lubricants have special functional properties, such as "viscosity adjustment characteristics", "improvement of dry friction resistance", "improvement of wet adhesion", "improvement of wet rub resistance", "ink luster and protection Permeability, "plasticity," "flexibility," and "improved gloss." The ideal lubricant should show the following properties:

(1) Lubricate the coating and improve its flow characteristics;
(2) ensure a smoother coating;
(3) improve the gloss of coated products;
(4) Improve the suitability of paper printing;
(5) reduce the coating cracks and peeling when the paper is folded;
(6) Reduce or eliminate powdering in supercalendering;

The use of mechanical calendering can finish the finishing of certain grades of paper without the need for additional processes. Chemicals used to apply lubricants are grouped into the following eight categories:

(1) Insoluble metal soap dispersions;
(2) Soluble soaps;
(3) Sulfated or sulfonated oils;
(4) Fatty acid esters;
(5) Wax, polyethylene or other hydrocarbon emulsions;
(6) aliphatic amines or amides (synthetic waxes);
(7) Polyethylene glycol and polyglycol ethers;
(8) Soybean lecithin "Oleic acid mixture.

Among the above types of lubricants, metal soap aqueous dispersions, wax emulsions, and calcium stearate high solids lubricant blends are the most commonly used lubricants in coating applications; sulfated or sulfonated mineral oils and some natural Or synthetic wax dispersions are mainly used in pigment-free systems, such as where paper or board surfaces require a single slip feature; soy lecithin “Oleic acid blends are currently the latest coating lubricant.

2 Composition and characteristics of major lubricants

2.1 Insoluble metal soap dispersions

Insoluble or low-solubility metal soaps are formed by the reaction of metal bases or oxides with fatty acids or soluble soaps. Alkali metal soaps include stearates, palmitates, ricinoleates, and the like of calcium, manganese, magnesium, aluminum, and zinc. Among them, calcium stearate has long occupied an important position in the field of lubricants.

Typical calcium stearate is prepared by making CaO into Ca(OH)2 by reaction with stearic acid from animal or marine organisms or other vegetable fats or oils. Calcium stearate can also be produced by precipitation of calcium hydroxide from soluble soaps such as sodium stearate. The calcium stearate produced by this method can be controlled in its size and shape (sheet crystals). Manufacturers of calcium stearate dispersions have improved their products and are now able to sell stable products with low viscosity, high solids content (57%), passed through 325 mesh screen, and screen residue less than "." 1%. Dispersions of calcium stearate are usually manufactured using high shear equipment. A 3%-5% nonionic or anionic surfactant is used as a dispersant in the finished product. The calcium stearate dispersion has maintained its position as the primary coating lubricant for many years. It is economical, practical, and shows excellent performance in anti-dust off powder.

2.2 Soluble soaps

Water-soluble soaps are produced by the reaction of high-soluble bases with fatty acids, including the reaction of metal hydroxides and fatty acids such as ammonium, sodium, and potassium. The most commonly used of these soaps are ammonium stearate, sodium stearate, and potassium oleate. Anhydrous solids of these compounds are commercially available. Ammonium stearate and potassium oleate can also be purchased as liquid dispersions with up to 33% solids. Since sodium stearate, which has a relatively low solid content, is a gel, this compound is usually used in a dry state. The melting point of soluble soaps is much lower than the melting point of insoluble soaps, since they form true solutions in water, they remain more dispersed throughout the coating when dried, and tend to follow the carrier into the base paper. Therefore, they have a more plasticizing effect on paints than the insoluble soaps do not have the role of anti-linting properties and surface lubricity. They increase the viscosity to a greater extent than insoluble soaps, especially in starch-bonded coatings. The use of water soluble soaps has gradually decreased with the emergence of low viscosity coating processes for blade coaters. However, they are still used in some roller coating applications. Some factories customarily use roller coaters as precoaters.

2.3 Sulfated and sulfonated oils

Such lubricants include sulphated oils formed by reacting unsaturated fatty acids or lipids with sulfuric acid, such as Turkish red oil (sulfated castor oil), sulfated red oil (sulfated oleic acid), cetyl sulfate oil, etc. . Sulfated oils are liquid at room temperature and are a very low melting point material. Accordingly, they are merely medium-effective coating lubricants and, like the water-soluble soaps, remain dispersed throughout the coating in the dry state and tend to migrate into the base paper. Its main role is plasticization and uniform coating effect. It is also sometimes used to reduce viscosity in certain protein adhesive coatings and full latex paints. These substances are all strong wetting agents and therefore their use is limited in many coating applications where resistance to wet rubbing is required.

(to be continued)