Influence of latex surface properties on interactions with starch and carboxymethyl cellulose in coating colours

Coating colour structure and properties are to a large extent dependant on the behaviour of the latex binder and its interaction with other coating components. Added water-soluble polymers such as starch and carboxymethyl cellulose (CMC) in general have a marked effect on the stability of the dispersed latex particles. This paper presents a systematic study of how the distribution of latex in coating colours containing oxidised starch, amphoteric (both anionic and cationic groups) starch or CMC depends on latex parameters such as polymer particle size, charge and surfactant composition. A series of seven model styrene-butadiene latexes was investigated. Complex formation between surfactant and starch occurred when the hydrophobic part of the surfactant was an alkyl chain. None of the surfactants containing an aryl group showed any tendency to formation of starch-surfactant complexes at low ionic strengths. In that respect, both starch grades behaved similarly to each other. No interactions between surfactant and CMC were detected. For calcium carbonate based coating colours containing the amphoteric type of starch, low shear rate viscosity was substantially higher when a model latex containing the alkyl type of surfactant was used compared to any other investigated model latex of the same particle size and degree of carboxylation. Similar effects were observed for colours containing the oxidized starch. Coated papers were produced by means of a Helicoater. Gloss and ink requirement were affected by the starch-latex interactions. It was also found that CMC flocculated all model latexes, even though CMC was not adsorbed on the latex particles. The flocculation concentration was dependent on the particle size of the latex and the ionic strength of the suspension. Calcium carbonate based coating colours containing CMC were formulated with the model latexes. Measurements of coated layer properties (when drawn down on synthetic paper) indicated that the density was slightly lower when the non-ionic surfactant latex was used. However, since CMC destabilised all model latex grades, print properties did not alter systematically after a change of latex type, CMC level or CMC grade. An important conclusion was, even though clear colloid interaction effects were observed in model systems, that the impact of these effects are probably small for real coating colours, full scale coating runnability and print properties. This can be explained by the complexity of coating colours and the coating process.