Ink-Jet Printing of Multi-Phase Formulations

Abstract

Ink-jet formulations are tailored for specific applications to give high performance in storage, during jetting, as drops impact the substrate and during evaporation; high performance of the final product is also targeted. This thesis explores the ink-jet printing of multi-phase formulations and their potential applications.
First, phase-separating inks were investigated. Formulations based upon binary mixtures of partially miscible liquids were explored: the minor component in suitable formulations was present initially below its miscibility limit and increased in concentration during evaporation until the mixture passed the binodal. Aqueous solutions of di(propylene glycol) methyl ether acetate (DPGMEA) phase separated after jetting: the new oil-rich phase formed at the contact line where evaporative flux is greatest. Phase-selective patterning was demonstrated using sodium oxalate and benzoic acid, which partitioned into opposite phases. Decane-in-methanol solutions phase separated throughout the drop but the high volatility of methanol did not allow composition gradients to equilibrate; phase selective patterning is not possible for this mixture. A quantitative criterion for the observation of phase separation during evaporation was developed and may be calculated from reference data.
Second, the delivery of high-molecular-weight (MW) polymers via emulsions was investigated. The ink-jet printing of high-(MW) polymers in solution is non-trivial: first, concentrated solutions are too viscous for print heads. Second, high strain rates during printing causes chain degradation. Third, high strain rates cause polymers to undergo the coil-stretch transition and introduce non-Newtonian jetting dynamics: long-lived elastic filaments develop that delay drop breakoff and decelerate the main drop. Emulsions shield polymers from high strain rates during printing through the interfacial tension and Gibbs elasticity of the dispersed phase droplets; strain occurs only in the polymer-free continuous phase. The optimised model formulation contained 3.8 %wt polystyrene (Mn = 419 kDa) overall; polystyrene was dissolved in methyl benzoate and dispersed throughout an aqueous solution of sodium dodecylsulphate. During evaporation on the substrate, the dispersed phase coalesced to give an even polystyrene deposit with the shape of a spherical cap. The emulsion increased the maximum printable concentration of the polymer by a factor of 15 and long-lived elastic filaments were not formed during jetting. A variety of discontinuous phase solvents were trialled: nozzle clogging was more frequent with toluene and anisole, whilst diethyl phthalate did not evaporate on the substrate. A neutrally buoyant discontinuous phase is advantageous.