Paper Coating Thickeners
The main function of thickeners is to adjust the viscosity of the coating color to the desired level and to impart the necessary degree of water retention (Table 3.5). Thickeners must be able to interact strongly with water molecules if they are to increase the water retention of coating colors. They also need to interact with other ingredients of coating formulations, especially pigments, in order to display a thickening effect. The nature and strength of these interactions depend on the chemical composition of the polymer.
Thickeners also need to display pronounced pseudoplastic flow. High runnability depends on a combination of these features. Most synthetic thickeners are supplied in the form of aqueous, acidic dispersions (which are often erroneously referred to as “emulsions” in the literature) or alka¬line solutions of synthetic polymers. Some are supplied in the form of true water-in-oil emulsions, but they are much less common.
The main products used as thickeners are CMC, PVOH (both already described in Section 18.104.22.168.4.1), acrylic copolymers and associative thickeners. The acrylic copolymers are nonionic monomers and acrylic acid or methacrylic acid. The non¬ionic monomers are mainly esters of acrylic acid (principally methyl acrylate and ethyl acrylate) and methacrylic acid (principally methyl methacrylate), and acryla¬mide. The monomers that are selected need to be fairly hydrophilic and polar in Table 3.5 Main requirements of thickeners order that the polymer is able to dissolve in aqueous coating colors and interact with other polar coating ingredients such as pigments. Strongly hydrophobic mon¬omers such as styrene, butadiene, and ethylene are hardly ever used because they interfere with the interaction between the polymer and water molecules.
Poly-acrylic acid can also be used as a thickener in its dissociated form, i. e., as the sodium or ammonium salt, if its molar mass is high enough. Polymers have to be able to dissolve before they can exert a thickening effect. Most natural products have to be heated and converted to make them capable of being dissolved, whereas acrylic copolymers are soluble on account of the alkali ions contained in the coat¬ing color. When addition of alkali increases the pH of the dispersion, the carboxyl groups dissociate and donate a proton, which causes them to become anionic. The formation of anionic charges along the polymer chain causes it to stretch owing to mutual repulsion, and water molecules are attracted to the polymer chain and become attached to it. The dispersed thickener particles then dissolve, which al¬lows them to unfold their effects.
It only takes a few minutes to dissolve them completely. Nevertheless, it is important to ensure that sufficient alkali is available in the coating color because the thickener dispersion consumes alkali and the stability and viscosity of the coating color can suffer if the pH is allowed to drop. Another important point to be considered is the sensitivity of the dissolved poly¬mer to electrolytes. Polyvalent cations such as Ca2+, Mg2+, Al3+, and Fe3+ can have a detrimental effect on the performance of these products by occupying the sites of anionic charge on the polymer.
The interaction between the polymer and water has a pronounced effect on the viscosity of the aqueous phase as well as on the water retention of the coating color. Like all hydrocolloids, synthetic thickeners bind a large number of water molecules along their polymer chains, with the result that their diameter and volume in¬crease and they occupy a greater space in the aqueous phase (Fig. 3.14). Apart from this hydrodynamic mechanism, there are a number of other mechanisms by which the thickeners are able to restrict the mobility of the aqueous phase. The high molar mass and the high degree of structural order cause a large increase in the volume of the dissolved polymers under hydrodynamic forces. The anionic charges along the length of the polymer also have the effect of stiffening the poly¬mer chain by causing it to unfurl and stretch out because the charges repel each other. The mobility of the aqueous phase is lower because of the stiffness and extension of individual polymer chains and the crosslinking between different polymer chains.
The maximum degree of internal crosslinking is obtained as the result of the associative interaction between the hydrophobic side chains (see Sec¬tion 22.214.171.124.4.3). The common feature of all these effects is that the mobility of the aqueous phase is reduced owing to intramolecular and intermolecular crosslink¬ing and the viscosity is increased due to the increase in the volume of the polymer. The pronounced thickening effect of thickeners is the result of their interaction with pigment particles. Acrylic and cellulosic thickeners have a high affinity for pigments because the polar functional groups of the thickener molecule are at¬tracted by the polar surfaces of the pigment. The polymer chains are adsorbed on the surface of the pigment particles and bind them together by means of a bridg¬ing mechanism, leading to a higher degree of crosslinking within the entire sys¬tem. The thickening effect, i. e., the increase in viscosity at low shear, is principally the result of the thickener forming “bridges” by means of adsorption.