Paper Coating : Derivatives of Natural Polymer Binders
In the initial phases of paper coating, i. e. the late 19th century till nearly the first half of the 20th Century, these binder types were used exclusively. All are hydro¬philic. The most widely employed binders of this type are starch and its derivatives with a consumption of approximately 800 000 tpa worldwide at present. Additional natural binder types are cellulose ethers such as carboxymethyl cellulose, in the United States, hydroxyethyl cellulose, soybean protein, and, to a very small extent, casein and alginates. Natural binders act as protective colloids that prevent the flocculation of the pigments; they increase the viscosity and water retention of coating colors and give the coat a higher stiffness. Natural binders have a relatively low water resistance (wet pick). They are mainly used in combination with syn¬thetic polymer binders and/or hardening agents (see Section 18.104.22.168.6).
Starch derivatives of various types are used in mixtures with other binders. For example, oxidized starches are usually employed together with polymer disper¬sions. Hydrolyzed starches exhibit high stability in solution, good binding power, and good flow behavior. Hydrolyzed, esterified starches exhibit good stability in solution, high binding power, and increased reactivity towards wet-strength ad¬ditives (hardening agents) such as urea- or melamine- formaldehyde resins. Hy¬drolyzed, etherified starches exhibit the same properties as the esterified deriva¬tives. However, in contrast to the esterified derivatives, they can be used at pH values above 8 without the risk of saponification. Starch derivatives that contain phosphate and amino groups are compatible with cationogenic substances such as satin white.
The phosphate groups react with multivalent metal ions, such as alu¬minum ions, which leads to a certain degree of water resistance. The amino groups react with aldehydes, which enhances the activity of hardening agents.
Cellulose Derivatives: Pure carboxymethyl cellulose (CMC) coats of 0.5–3 g m–2 increase the grease resistance and printability of paper. Depending on the use, various mixtures of low- and high-viscosity CMC are employed. Pigment-contain-ing CMC coats, which can be applied on the size press, contain up to 10 % semi-technical, low-viscosity CMC or salt-free, purified CMC. CMC is usually processed together with other natural or synthetic binders.
Above all, CMC increases the effectiveness of optical brighteners (see 22.214.171.124.5.5) and the water retention of the coating mixture. Water retention is so high that the addition of other binders that promote water retention in the coat is only necessary to a lesser extent. In general, the amounts of CMC employed are 0.3–1.5 % based on the pigment, and very low viscosity types are preferred. This gives an adequate coating color viscosity, even if the solids content of the coating color is high. Types of CMC that are soluble in cold water are rapidly becoming established because they do not require dissolving at elevated temperature. The presence of satin white pigment can cause strongly interfering coagulations in the coating color.
Casein has been used as a central binder type in the past, but nowadays is of only marginal importance in cast coatings. For its application it must be present in the dissolved state. It is dissolved by the addition of alkali (e. g., ammonia, sodium hydroxide, borax, or sodium carbonate) either separately in a cooker (up to 70 °C) or with the pigment in a kneader. The casein concentration is limited to ca. 20 % due to its high viscosity. The limit of processability is shifted to ca. 33 % by the addition of urea or dicyanodiamide, which reduces the viscosity and increases the storage stability of casein solutions. The mixing of casein solutions with pigments,