Bimodal Hard/Soft Latex Blends

Abstract: Polymer dispersions are employed in a wide variety of applications as film formers. Especially in the coating-, paint-, pharmaceutical and hygiene industry polymer dispersions are to a large extent used as barriers, controlled-release agents and adhesives. Control and steering of the polymerization process has resulted in the production of core-shell structures with soft and hard (low glass transition temperature (Tg) and high Tg) constituents and molecular modifications to increase the adhesion to substrates by incorporating for instance acrylic acid groups. However, there is also a need for new materials that combine the properties of two or more polymeric or/and inorganic materials, which is not easily achieved by polymerizing to a single dispersion. The most natural way to achieve this is the blending of two dispersions. This provides the possibility to fine-tune the final properties of the film material. The focus of this thesis was to study the effect of composition, particle size and particle size ratio (soft particle diameter/hard particle diameter) on the mechanical viscoelastic film properties and morphology of hard/soft latex blends. Viscoelastic properties were determined both in the solid and in the melt state. The results were compared with theoretical predictions based on self-consistent mechanical modeling. Furthermore, the effect of the addition of a varying amount of silica nanoparticles on the viscoelasic properties, morphology and water permeability of the well-characterized hard/soft latex system was studied. A new empirical equation was established for the characterization of the modulus as a function of volume fraction and particle size ratio. The particle size ratio was shown to have an effect on the film forming properties as well as on the dynamic mechanical properties of the latex films. With increasing volume fraction of hard particles a modulus enhancement was obtained in the temperature range in between the individual Tg's of the neat polymeric materials. By introducing silica nanoparticles in the hard/soft latex blends the aggregation of these silica particles enhanced the dynamic modulus further. Pukanszky's model, originally derived for filled polymers and polymer blends, was shown to be a very useful tool for the evaluation of the yield stress of hard/soft latex blends.