With pure liquids the viscosity coefficient is a constant, Newtons equation:

η =

η = Coefficient of Viscosity (pronounced ‘ETA’)

Stress is the frictional force operating per unit area. Rate of shear is the variation in velocity of the moving liquid with depth of liquid. If we imagine a flowing river, at any instant the water moving fastest is at the surface, while the water in contact with the river bed is not moving at all. Considering the layers in-between, the nearer we get to the top the faster the water is flowing.

The constant viscosity coefficient indicating that, as the stress increases, the rate of shear does so at the same rate and vice versa. This is also true for polymer solutions, but if the polymer has an insoluble material in it the equation may be true no longer. Since most paints contain undissolved material it becomes important to quote viscosity in poise, temperature and either rate of shear or stress rate.

Ideal particle dispersion consists of completely uniform distribution of isolated individual particles throughout the liquid, but all the particles are in motion, fast moving liquid molecules and slower moving polymer molecules and gravity.

Should suitable parts of the surface of two particles come into contact, inter attractions may be strong enough to prevent the particles from separating by their own motion and a cage like network or structure may form.

Such a structure might give the appearance in the can of being very viscous and jelly-like, but once the paint is stirred the structure breaks up and the particles separate. Should the stirring stop, the 'thixotropy' (as it is known) will begin again and the viscosity rise.

If the viscosity increases when measured at low shear rates, over time the paint is said to be thixotropic and if the viscosity decreases as the rate of shear increases, the paint is said to be pseudo-plastic. If there is a minimum stress required before any flow can occur at all, the viscosity behavior is said to be plastic.

All paints show non-Newtonian viscosity to some degree which can be most beneficial. Thus, the viscous, even jelly-like paint does not show hard pigment settlement, applies easily under the shearing action of, for example, a brush or spraygun and does not 'sag'.

Because the viscosity rises as soon as the paint is still (or nearly so) on the object being coated, non-Newtonian viscosity may prove extremely difficult to adjust to obtain reasonable flowout of application marks. Also, if the effect is obtained by pigment flocculation, the perfectly uniform pigment distribution, which gives the maximum light scattering and absorption, will be lost and opacity will fall.

If the level of pigment in paint is high, the very bulk of the pigment in the paint will cause non-Newtonian viscosity. The close packing of the particles makes some flocculation inevitable and the viscosity will be high, simply because the particles impede each other’s movements.

A further phenomenon dilatancy can occur at very high pigment volumes. Here, the particles pack together so tightly (normally because of the shape of the particles that on stirring, the viscosity rises and the paint appears to set solid. When stirring ceases, fluidity returns.

Dispersion of a few per cent of very fine particle silica, SiO₂ (diameter about 0.015 µm), produces a largely pseudo-Elastic effect, the surface forces and very large surface area (e.g. 190-460m² per gram) are responsible.

Fine particle synthetic aluminum silicates (A120 .2Sio2.2H20) Bentonite which is added to impart non-Newtonian behavior to water paints. It has a plate-like particle and will swell to several times its dry volume by absorption of water or organic hydrocarbons. Between silicate layers, Bentones will gel organic liquids or impart non-Newtonian viscosity to paints containing organic solvents, due to attractive forces between the particles leading to structure throughout the paint.

Resinous thickeners can be formulated by balancing attractive chemical groups and solubilities within resin structures, by use of solvents. Microgels thicken paints by forming flocculated network structures within the resin, or colloidal resins from strong hydrogen bonding form microgel structures which thicken the paints.