Characterising and Killing the Exciting Rheology of Concentrated Suspensions with Novel Oligomeric Dispersants
Kodak has a long history of enabling both individuals and businesses to produce images, usually in hard-copy form. These images invariably contain polymers, particles (usually nano-sized) and surfactants.
Kodak has a long history of enabling both individuals and businesses to produce images, usually in hard-copy form. These images invariably contain polymers, particles (usually nano-sized) and surfactants. The polymers act as binders and rheology modifiers, the particles as colorants or to modify mechanical properties, while surfactants stabilize bulk and dispersed interfaces or are used to control wetting processes. The various components are combined in a liquid medium (preferably water!), at as high a concentration as possible, and then deposited in a precise manner on a substrate, which is usually a polymer film. Silver halide-based photographic coatings used to provide the overwhelming majority of Kodak’s business. Nowadays, digital printing has become more important, and inkjet printing in particular is a key technology in Kodak’s present and for its future (Figure 1).
Figure 1. Nanotechnology is important for inkjet printing – nanopigments in inks and nanoparticles for the coatings on inkjet papers.
One of the main digital printing applications of thin-film coatings is the production of thin, ink-receiving layers for inkjet printer papers. Such coatings usually have high internal porosity in order that surfaces are touch-dry immediately after printing. The porosity is usually achieved with coatings comprising anisotropic or porous particles with small quantities of polymeric binder and can be multilayered in composition (Figure 2).
Figure 2. Electron micrograph of two layers of porous coatings for inkjet papers: lower layer contains anisotropic and pre-aggregated particles, upper layer contains smaller, pre-aggregated particles.
Suspensions of such anisotropic and/or strongly interacting particles can give challenging – and fascinating - rheology when formulated at high concentration. However, rheological behaviour that is fun to measure in the lab may not be welcome in any manufacturing environment and particularly not one that uses high-speed, thin-layer coating. The industrial scientist faces the challenge of keeping manufacturing engineers happy by designing formulations of concentrated suspensions that meet the requirements of the manufacturing processes (i.e. the challenge of acting as a rheological “killjoy”). However, the initial recipes arriving in the rheology lab do exhibit fascinating properties, such as jamming or shear thickening and there is satisfaction to be gained from keeping these phenomena away from the engineers (we could rephrase this to: engineers are less happy than we are to meet samples with interesting properties).
In our laboratory, we have characterized and then “tamed” the challenging behaviour exhibited by suspensions of cationic boehmite (Catapal 200) or anionic fumed silica in order to increase flowability (remove elasticity, reduce viscosity and eliminate dependence on storage and shear history) while increasing concentration. Addition of commercially available dispersants – small molecules and polymers – did not give sufficiently well-behaved rheology. Our solution was to design and make oligomeric functional anchor buoy dispersants, and these did the trick.
The dispersants are a combination of a simple, inexpensive oligomeric buoy that is soluble in the continuous liquid phase (typically 25-30 acrylamide units for water). The anchor group is specific to the surface of interest: acidic, basic or hydrophobic for cationic, anionic or hydrophobic surfaces, respectively. Figure 3 shows the rheological behaviour of Catapal 200 suspensions at 20°C after gentle loading into the rheometer. The red curves are the viscosity on increasing stress for the first time, the green curves are the values on the second increase in stress, and the blue curves show the behaviour on a subsequent decrease in stress. In the absence of dispersant, the first exposure to high shear leads to irreversible thickening and either of the thickened states (blue or green curves) can thereafter be obtained repeatedly, dependent on shear history. In the presence of a dispersant (mercaptosuccinic acid anchor with an oligomeric buoy 27 acrylamide groups), the viscosity is reduced and no shear thickening occurs.
Figure 3. Flow curves (viscosity as a function of stress) for suspensions of Catapal 200 in the absence (lines) and presence (lines + symbols) of dispersant MSA(A)27.
The oligomeric anchor-buoy chemistry enables dispersants to be designed for a wide range of particles and fluid media. The synthetic route to these versatile materials is relatively straightforward. These proprietary dispersants have been used to stabilise anionic, cationic and hydrophobic particles and the benefits have been seen at macroscopic surfaces (surface tension and elasticity, foaming) and dispersed surfaces (small particle size, eliminate flocculation, and rheology control).
This work was a collaboration with Kodak scientists Trevor Wear (dispersant maker) Alan Pitt (dispersant designer) and John Hone (fellow colloid scientist).
