Pigmented Latex Coatings: Microstructure and Viscoelastic Mechanical Properties
Published: 2000
Publication Name: Nordic Pulp & Paper Research Journal
Publication URL: https://www.researchgate.net/publication/289088837_Pigmented_latex_coatings_Microstructure_and_viscoelastic_mechanical_properties
Abstract:
While much research has been performed on the optical properties and printing quality of paper coatings, the mechanical properties of pigmented latex coatings have received more limited attention. However, adequate mechanical properties of paper coatings are vital for use performance (e.g. calendering, printing degradation during printing operations, and pick). The objective of this study was to investigate microscopic and macroscopic mechanisms that determine the viscoelastic response of pigmented latex coating films. Pigmented coating layers were prepared with different microstructures by using three different pigment shapes (spheres, rhombs, and plates), and changing the pigment volume concentration. Styrene-butadiene latices with two degrees of carboxylation (acidic level 0.3% and 4.5%) were evaluated to determine its effect on performance. The viscoelastic material responses were determined by dynamic mechanical thermal analysis using tensile mode. The range of linear viscoelastic behavior was determined. Changes in viscoelastic response over entire pigment volume concentration range were fundamentally different for the three different pigment systems. Polystyrene plastic pigment below the latex glass transition exhibited lower storage modulus than the latex, hence the addition of plastic pigment did not lead to reinforcement. Above the latex glass transition temperature and below the pigment glass transition the addition of pigment did have a reinforcing effect. The addition of calcium carbonate pigment to latex resulted in a reinforcement effect over the entire pigment volume concentration range. For clay coatings a depression reaction was observed. This behavior was reversible for very high pigment volume concentrations. Below 80% pigment volume concentration, the depression behavior was irreversible, resulting in a common transition. For all pigments a decrease in tan Delta was seen with increasing pigmentation. Glass transition temperatures determined by dynamic mechanical thermal analysis were found consistently higher than measured by differential scanning calorimetry. Master curves were calculated by applying WLF-theory and time-temperature superposition for polystyrene pigment coatings.