Heterogeneous Film-Forming Latexes : Preparation - Morphology - Mechanical Properties

Abstract: This thesis describes the preparation of heterogeneous latexes, which may serve as binders in coating applications, and the characterization of these. The latexes were prepared in two-stage seeded emulsion polymerization. The seed latexes used were polystyrene, whose properties had been changed by copolymerization with methacrylic acid (MAA) or divinyl benzene. The second-stage polymers were copolymers of styrene-co-isoprene-co-MAA or styrene-co-butadiene-co-MAA with varying compositions. The monomers in the second stage of polymerization were added either as a batch, or as a semi-continuous operation during the whole of the second stage or during various periods. The hydrophilicity of the polymeric phases was found to affect the particle morphology to a great extent. It was possible to change the particle morphology from an inverted core-shell to a core-shell morphology simply by adding small amounts of MAA to the monomer mixture used in the second stage of polymerization. Crosslinking of the seed particles had an even stronger effect on the morphology development. Increased amounts of isoprene and MAA, as well as an increased phase ratio between the second-stage polymer and the seed was found to increase deviations between particle sizes obtained by TEM and QELS. Monomer swelling experiments showed that it was possible to estimate the monomer volume fraction at which the seed polymer glass transition temperature (Tg) and the so-called gel point occurred, through pressure measurements. When the pressure was monitored in the copolymerization of butadiene and styrene, the conversion at which crosslinking began was detected as an irregularity on the pressure curve plotted as a function of the second-stage monomer conversion. Films made from the heterogeneous latexes were found to consist of two phases with the hard seed particles remaining as particles within the films. Mechanical measurements on the films showed that films made from latexes with crosslinked seed particles having irregular shapes, had the highest mechanical strength due to inter-particle contacts. Films made from latexes with core-shell morphologies exhibited lower mechanical strength due to the smooth seed particle structures left in the film. When latexes with crosslinked seed particles of irregular shapes were used as binders in model paper coating applications, they increased the porosity, gave greater light scattering ability and higher coating gloss of the coating layer, compared with latexes having core shell morphologies, due to retarded shrinkage of the coating during drying.