Compressive yield stress at equilibrium, Py(øeq) Using Compressive Rheology to Understand Dispersion Stability Huai Nyin (Grace) Yow and Simon Biggs Institute of Particle Science and Engineering, University of Leeds, Leeds LS2 9JT Introduction For a consistent and smooth print, it is crucial to have a predictable and “stable” colloidal ink throughout the printing process. However, due to complex ink formulation, colloidal dispersions can be easily destabilized by addition of:- Electrolyte, which compromises the electrical double layer and/or dehydrates the polymeric shell, Polymer, which can induce depletion flocculation or cause bridging between particles, “Bad” solvent that compresses the double layer or is incompatible with the stabilizing polymer. Compressive Rheology Compressive rheology characterizes the particle network strength when the particle dispersion reaches an equilibrium height that is able to counter-balance the applied external force. This particle network strength is termed as compressive yield stress, Py(ø) and it is governed by:- Strength of inter-particle interaction forces Local volume concentration of particles Shear history during aggregation External force Compressive yield stress, Py(ø) Ref: Buscall and White. J. Chem. Soc. Faraday Trans 1. 1987. 83. 873. Objectives To understand the effect of temperature and electrolyte concentration on dispersion stability To correlate dispersion stability and particle morphology with compressive rheology Experimental Technique Theory of Compressive Yield Stress 1) Equipment Compressive yield stress at equilibrium, Py(øeq) Supernatant Falling zone Consolidating zone H0 H(t) LUMiSizer Centrifugal Sedimentation Centrifugal field used to induce de-mixing of particle dispersion Acceleration of centrifugal field in steps to probe particle bed compaction Stage 1 – bed formation 1,000 rpm for 8 hours Stage 2 – bed compression +500 rpm every 1.1 hours Up to 4,000 rpm Equilibrium particle volume fraction, øeq Where Δ = density difference between liquid and particles, g = centrifugal acceleration at bottom of bed, ø = particle volume fraction, H = centrifugation height and L = radial distance from centrifuge centre to bottom of bed Compressive Yield Stress Results Stable particle dispersion forms incompressible particle bed. This is because particles have already achieved close packing arrangement upon compression. 1) Acceleration effect on sedimentation height Increasing salt concentration 60°C 25°C Open particle bed, from aggregated particles, compresses under higher force. This is due to collapse of interstitial voids, allowing closer particle packing upon further compression. 2) Particle system Dispersion of poly(ethylene glycol) – stabilized polystyrene particles in water 880 nm @ 5wt% solids Depiction of thermal sensitivity Around 50°C with decrease of particle size After 60°C particle instability noted Bed Sedimentation Results Increasing salt concentration 25°C Increasing salt concentration 60°C 2) Particle morphology based on compressive rheology Increasing salt concentration 25°C Increasing salt concentration 60°C Particle destabilization instigated by addition of electrolyte (KCl) and temperature effects. Dual destabilization mechanism Low KCl concentration induces compression of electrical double layer. High KCl concentration reduces polymer solvency in dispersion medium. Temperature increase exacerbates particle destabilization. This is due to breaking of hydrogen bonding needed for polymer solvency. Synthesis In DEP At 60°C, aggregated particles have ‘sticky’ hard morphology. No room for additional compression, hence bed compression is governed by degree of particle aggregation. At 25°C, stable particles have core/shell structure. This allows compression of polymeric shell to reach similar final bed compression. EPSRC, UK CPEG, University of Leeds Cognis UK (for PEGMA stabiliser donation) Acknowledgement