1. 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
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 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