In order to fully understand the behavior of rheology modifiers, the coating formulator uses a variety of viscosity measuring devices:

• Brookfield viscometer - low & medium shear   viscosity
• Stormer - medium shear rate viscosity
• ICI Cone & Plate - high shear
• Rheometer - low to high shear viscosity

Paints are non-Newtonian liquids. During application to a substrate they are subject to very high shear rates that lower viscosity.

The speed with which viscosity recovers following application (and removal of the shearing force) will directly influence the properties of the film.

Slow viscosity recovery - Good leveling, good for gloss, but may sag
Fast viscosity recovery - Reduced flow, no sag, but brush marking,
orange peel and reduced gloss.

For typical paints, the viscosity at high shear rate must be sufficiently low to allow application, but not so low that the film applied is too thin with insufficient covering power.

The application properties of aqueous coatings are dependent on the use of rheology modifiers to achieve the required balance of application properties for a given market.  Usually a combination of modifiers will be used to provide the best balance of container viscosity, application viscosity, anti-settling properties, spatter resistance, flow and leveling properties

Ciba® VISCALEX® Alkali Swellable Emulsions (ASE)
VISCALEX® ASEs are dispersions of insoluble acrylic polymers in water.  These polymers have a high percentage of acid groups distributed throughout their polymer chains. When these acid groups are neutralized, the salt that is formed is ‘hydrated’.  Depending on the concentration of acid groups, the molecular weight and degree of crosslinking, the salt either swells in aqueous solutions or becomes completely water soluble.

As the concentration of neutralized polymer in an aqueous formulation increases, the swollen polymer chains start to overlap, until they ‘tangle up’.  It is this overlapping and tangling which causes viscosity to increase.  Again, the concentration of acid groups, the molecular weight and degree of crosslinking of the polymer are important in determining the observed rheology and the efficiency of thickening.

Ciba® VISCALEX® Liquid Dispersion Polymers (LDP)
VISCALEX® LDPs are dispersions of water-soluble acrylic polymers in mineral spirit.  Because the polymers do not require neutralizing to become water ‘soluble’, they are dispersed in an organic carrier so are still ‘handlable’ at high concentrations.  The advantage of these types of materials, is that they can be used over a wide range of formulation pHs.  One disadvantage is the small level of retained solvent, which will compromise film formation in glossy coatings, hence, these products are more often used in water-based adhesive formulations and highly filled (high pvc) systems.

Ciba® RHEOVIS® Hydrophobically modified Alkali Swellable Emulsions (HASE)
Like the ASE products, these are dispersions of water-insoluble acrylic polymers in water. Again the acid groups on the polymer chains must be neutralized to make the polymer chains water ‘soluble’. These HASE products differ from VISCALEX® (ASE) in that, in addition to acid groups being distributed through the polymer chain, there are also long chain hydrophobic groups.

These ‘hydrophobic modifications’ can associate either with each other on an intra- or intermolecular basis (forming micelles just like surfactants) or in combination with other hydrophobic materials, including surfactants in a coating formulation, hence, these rheology modifiers are often referred to as ‘associative’.

The combination of associations – intramolecular, intermolecular and with the other hydrophobic elements in a formulation – have a significant effect on rheological properties.  When compared to ASE rheology modifiers, HASE modifiers are less ‘shear-thinning’ or conversely more ‘Newtonian’.