Paper Coating Binders
Binders are the second most abundant component in the coating color after the pigments with a total amount of approximately 3 V 106 t of dry substances. Many different binder types are used (Fig. 3.13). The most important aims for the appli¬cation of binders in coating colors are the binding of pigment particles to base paper, the binding of pigment particles to each other, partial filling of voids be¬tween pigment particles (porous coating structure) and affecting the viscosity and water retention of the coating color.
An ideal binder can be characterized by good binding power, good water reten¬tion properties, ease of mixing or dissolving in water, general ease of handling, good compatibility with other coating components, low or desired effect on the viscosity of the coating color, good mechanical and chemical durability, good opti¬cal and mechanical properties, nonodorous and harmless to health, low tendency to foaming, resistant to bacteria, constant quality properties, low price and good availability. Again, as with pigments, there is no single binder, which can meet all these requirements. Latexes meet many of them; however, they often need a co-binder or thickener to adjust the rheology and water retention to the desired level.
There are three kinds of binders in coating, the (main) binder, the co-binder and the sole-binder. A sole-binder is a single binder that alone can perform all the desired binder functions in a coating. Usually the binder systems consist of a combination of two binders, in which the (main) binder is responsible for the binding function. The co-binder is used to affect the rheology and water retention properties of the coating color. Its dosage is smaller than that of the main binder.
The binders can be classified by their origin and solubility in water:
.• Soluble in water:
.– Cellulose derivatives: ethers e. g. carboxy methyl cellulose
.– Polyvinyl alcohol (PVA, PVOH)
.• Insoluble in water:
.– Carboxylated styrene butadiene latexes (XSB Latex)
.– Styrene acrylate latexes (SA Latex)
.– Polyvinyl acetate latexes (PVAc Latex)
Water-soluble binders give better water retention for the coating layer than latexes that are not soluble in water. They also affect the rheological properties of the coating colors, making them more viscous, and also shear thinning (pseudo-plas-tic) and thixotropic. Some synthetic co-binders have a similar effect. Carboxylation of SB-Latex means incorporation of small amounts of unsaturated carboxylic acids such as acrylic acid, methacrylic acid, maleic acid, to improve considerably the compatability of these highly hydrophobic polymers with the other coating components.