Impact of functionalised dispersing agents on the mechanical and viscoelastic properties of pigment coating

The effect of poly(acrylic acid) - poly(styrene sulphonic acid) (AA-SSA) and poly(acrylic acid) - poly(2-acrylamido-2-methylpropane sulphonic acid) (AA-AMPS) dispersing agents on the viscoelastic and mechanical properties of precipitated calcium carbonate-latex composites used was investigated. Four different formulations were prepared using carboxylated styrene butadiene (SBR) and styrene-acrylate (SA) latexes. Pore space was characterised using mercury porosimetry. The storage modulus and loss factor were evaluated through dynamic mechanical thermal analysis (DMTA) low frequency single cantilever bending mode. The ultimate tensile strength and the Young modulus were also measured. At low latex contents, storage modulus was found to be higher for SBR latex composites. At high latex content, the SA storage modulus was found to be higher. It is suggested that at lower latex content, the viscoelastic properties are function of physical microstructure, which at low latex is influenced by the latex glass transition temperature (Tg). Softer latex spreads more on the pigment surface providing higher stiffness, since pigments are held together by latex bridges. At higher latex content, the composite stiffness tends to be more dependent on the stiffness of the pure latex. The AA-SSA dispersant creates strong pigment-latex interfacial adhesion in dry composites, which is reflected in high elastic modulus and tensile strength. The AA-AMPS dispersant formulations had greater resistance to water. Due to the compatibility between the AMPS blocks and the SBR latex within the composite, higher storage modulus stability in water saturated composites is measured (at room temperature 56% of the storage modulus is preserved).