Synthetic Latex Binders in Paper Coating
The ongoing development of coating equipment and applicators in connection with the increasing production speed (actual process up to 2.000 m min–1; pilot coaters up to 3.300 m min–1) has forced the development of new binder types, which meet the changed requirements better. In Europe the first styrene-butyl acryl¬ate dispersion (SBA) was polymerized by Badische Anilin- und Soda-Fabrik, today BASF, in 1929. In the following years a partial substitution of the natural polymer binders began and from the1950s onwards an extensive and continuous growth in market demand has been observed.
The corresponding pioneer work in the USA took place in Dow Chemicals, where, from 1944 onwards, the production and market introduction of carboxylated styrene-butadiene latexes (XSB) started. Polvinyl acetate latexes (PVAc) as a coating binder were first researched and introduced to the market in 1955, also in the USA, where a higher proportion than in other world regions is still used. The actual worldwide market demand for these three main types of synthetic coating binders comes to approximately 2 V 106 t p. a. as dry material with approximately 75 % XSB-, 15 % SA-, 7 % PVAc- and 3 % other latexes.
These synthetic latex binders made it possible for the first time to attain a high solid content at low viscosity, a prerequisite for modern high-speed coating machines. In addition, polymer dispersions give the coat a higher water resistance, better flexibility, higher gloss, and better printability. The products employed today are aqueous dispersions of polymers, usually stabilized with anionic emulsifiers. The solid content of the dispersions is generally ca. 50 %. Very often, the polymers are copolymers of several monomers, e. g., styrene, butadiene, acrylic esters, vinyl acetate, and acrylonitrile. Apart from these main monomers small amounts of auxiliary monomers, such as acrylamide, acrylic acid, maleic acid, and methacryla¬mine are also added to modify the dispersion properties.
Styrene-butadiene dispersions lead to varying film hardness, depending on the proportion of styrene used. Approximately equal proportions of styrene and buta¬diene result in binders which provide a relatively soft film and a very good pigment binding capacity. Disadvantages are the odor of the dispersion and the tendency of the films to yellow when exposed to light. Acrylate dispersions are specialties and of high importance for impressive prints. Butyl acrylate is mainly copolymerized with styrene or vinyl acetate. The ratio of the soft component (butyl acrylate) to the hard one (styrene or vinyl acetate) determines the application characteristics of the dispersion. In general, acrylate dispersions have an excellent brightness and age¬ing resistance and are less odorous. Apart from these two most important groups of polymer dispersions, vinyl acetate homo- or copolymers have gained acceptance in paper coating plants, especially in the USA.
These products generally have a lower binding power, but provide very hard and porous coats, and have an excellent ageing resistance. Polymer dispersions based on methacrylates and copolymers of vinyl acetate and ethylene are less important in paper coating.
In recent years the requirements for coating binders have become more and more diverse. Changes in raw materials for paper production and/or production process conditions as well as new requirements for paper characteristics and print¬ing technologies have forced the development of tailor-made binders with very spe¬cific property profiles. Such products are not generally different from XSB- or SBA-latexes, as regards the principles of physics and chemistry. But with the know-how of correlation and influence factors of monomers, functional monomers, process auxiliaries, functional additives, process parameters and the interactions with dif¬ferent pigments, together with a flexible polymerization unit, it is possible to opti¬mize, very flexibly and quickly, a binder in respect of binding power, stiffness, print evenness, print gloss, blister resistance, low yellowing and coater runnabilty.
Apart from the two-component systems, binder + co-binder, synthetic sole binders which do not require a co-binder have been available since the early 1960s. Syn¬thetic binders are low-viscosity aqueous dispersions which usually do not influ¬ence properties such as viscosity and water retention that are important for the flow behavior of the coating. For this reason, these “multipurpose dispersions” are used alone in only a few cases. They are usually mixed with co-binders, which are responsible for adjusting the flow properties of the coating color. On addition of alkali, these products develop the required viscosity and water retention, but retain their dispersion form.
Special binders of this type have become popular, especially for illustration paper that is produced in large amounts and used in rotogravure or web offset printing. Table 3.2 lists the various binder systems, their influence on the production of coating colors, the rheological properties of the coating color, and the coating properties.