Coatings, pigments

Modern coating is performed at very high speeds, and the fastest coating operations at the moment are carried out at speeds approaching 1600 metres per minute. The rheology and flow behaviour of the dispersions at these speeds is therefore of paramount importance in the coating process. 

The binders vary quite widely—the most common being starch, soy protein and latexes in conjunction with other soluble polymers. Styrene-butadiene latexes have been the most popular but ethylene-vinyl acetate binders are also used. The method of polymer synthesis provides a way of modifying the properties of the latex. For example, adjustment of the ratio of styrene butadiene in the co-polymer gives rise to different degrees of softness or hardness. This property has a profound influence on the quality of the coating. It is also possible to co-polymerise monomers so as to introduce, for example, carboxy groups on to the surface of the latex particle which in turn assist in 

The other important function of the binder is its effect upon the rheological properties of the coating mix. Starch, which is widely used, is unsuitable for use in its unmodified form because its solution viscosity is generally too high and also because of the problem of retrogradation. It is usually modified by reducing its molecular weight by either oxidative or hydrolytic (sometimes enzymatic) procedures. 

The flow behaviour of aqueous coating dispersions, because of their high pigment and binder content, is often complex. They have viscosities which are not independent of the shear rate and are therefore non-Newtonian. Shear thickening (when the viscosity of the dispersion increases with shear rate) and shear thinning or pseudoplastic behaviour (when the viscosity decreases with shear rate), may 

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Titanium dioxide pigment coated with pentaerythritol, trimethylolpropane, or trim ethyl ol eth an e exhibits improved dispersion characteristics when used in paint or plastics formulations. The polyol is generally added at levels of 0.1—0.5% (96). 

Pigment-coated printing papers usually are required to have high brightness to achieve contrast between the printed and unprinted areas. The coatings frequendy have a glossy surface. There also is a demand for duU-coated papers and for dull papers upon which glossy ink films can be printed. 

Pigmented Coating Processes for Paper and Board, TAPPI Press BookNo. 28, Technical Association of the Pulp and Paper Industry, Atianta, Ga., 1962. 

Advantages of the electron beam processor are its abiUty to penetrate thick and highly pigmented coatings. It is used to cross-link reactive unsaturated polymers, nonreactive thermoplastic polymers, iasulation, and wire-cable covetings (4,9,16) (see Insulation, electric-wire and cable COVERDIGS). 

The exceUent adhesion to primed films of polyester combined with good dielectric properties and good surface properties makes the vinyhdene chloride copolymers very suitable as binders for iron oxide pigmented coatings for magnetic tapes (168—170). They perform very weU in audio, video, and computer tapes. 

Strontium. Strontium 2-ethyIhexanoate [2457-02-5] is a candidate to replace lead in lead-free paints. It functions well under adverse weather conditions and promotes through drying. Outdoor performance of low pigmented coatings like stains may be effected negatively by using strontium instead of zirconium. 

Alternatively, the same coatings can be cured by electrons from an electron accelerator without the use of photoinitiators. Electrons from a 150-600 kV accelerator are energetic enough to create free radicals on impact with the polymer molecules and curing ensues. Clear and pigmented coatings can be cured. Electron accelerators are extremely expensive, but are cheap to run. 

Figure 14. Ti02 anatase pigmented coating, chalk rating 4, general view (2)...

Pigmented Coatings Cured with Visible Light... 

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