Rheology for Coatings and Inks: A Practical Primer

Water and simple oils are Newtonian fluids: their viscosity is independent of the applied shear rate and can be described by a single value. Paints, inks, varnishes and pigment dispersions, by contrast, behave non-Newtonian almost without exception. Most are shear thinning: thick and structured at rest, markedly more fluid when stirred or applied. This is not a defect but a property the formulator designs deliberately.

A statement such as "viscosity 2500 mPa·s" is therefore meaningless unless the shear rate and temperature of the measurement are given. The same paint may show 50,000 mPa·s at 0.1 s⁻¹ and 150 mPa·s at 1000 s⁻¹. The flow curve, viscosity plotted against shear rate, summarises the behaviour of the material across the whole application process rather than at one point, and a rotational rheometer obtains it in a single measurement over a wide shear-rate range.

Each stage in the life of a paint or ink corresponds to a different shear-rate region. Storage and sedimentation occur at 10⁻³ to 10⁻¹ s⁻¹, levelling and sagging on the surface at roughly 10⁻² to 1 s⁻¹. Brush and roller application generate 10² to 10³ s⁻¹, while spraying, flexographic and offset printing reach very high shear rates of 10⁴ to 10⁵ s⁻¹. A single measurement point represents only one of these stages.

A practical approach is to read the flow curve in three regions. Low-shear viscosity governs storage stability and sag resistance, the mid-range governs brush-mark levelling and flow-out, and high-shear viscosity determines ease of application, film build and spray atomisation. A good formulation balances high viscosity at low shear against low viscosity at high shear; two paints may give the same value at 1000 s⁻¹ yet differ tenfold at 0.1 s⁻¹ and behave completely differently on the shelf.

For inks, extensional flow also matters: the ink film splitting from an anilox roll or an offset blanket experiences stretching, and misting or filament formation is linked to that behaviour. Rotational measurements do not capture it fully, but evaluated together with high-shear viscosity and the viscoelastic structure they provide strong indications.

Shear thinning describes how viscosity changes with shear rate; thixotropy adds the dimension of time, describing how quickly the structure rebuilds once shear is removed. For a paint this distinction is critical: the paint leaving the brush must be low in viscosity so that brush marks level out, but if the structure recovers too slowly it sags on vertical surfaces, and if it recovers too quickly levelling is poor. Correct thixotropy is a matter of balance.

The standard way to measure that balance is the three-interval thixotropy test (3ITT). In the first interval the initial structure is recorded at low shear rate (or low-amplitude oscillation), in the second a high shear rate simulating application is applied, and in the third the material returns to low shear while the recovery of viscosity (or of the modulus G′) over time is followed. The time taken to regain fifty or ninety per cent of the initial value gives a concrete number for comparing two formulations.

Older approaches such as the "thixotropic index" (the ratio of viscosities at two speeds) or the area of a hysteresis loop are still used for quick comparison, but because they contain no time information they are poor predictors of application behaviour. If a product sags or leaves brush marks, the answer is almost always found in the third interval of the 3ITT data.

Yield stress is the minimum stress that must be exceeded before a material begins to flow. It is the quantity that directly governs pigment settling, phase separation in the can and sagging on vertical surfaces: if the stress produced by the weight of a heavy pigment particle remains below the yield stress, the particle stays suspended. Yield stress is measured by methods such as a controlled stress ramp or constant low-rate shear, and the method used should be stated in the report, since different methods give different values.

Oscillatory tests interrogate the material without making it flow, using small-amplitude sinusoidal deformation. The storage modulus G′ represents the elastic (solid-like) structure and the loss modulus G″ the viscous (liquid-like) behaviour. In an amplitude sweep, the linear viscoelastic region where G′ stays constant shows how robust the structure is, and the point where G′ and G″ cross marks the transition to flow. A paint at rest with G′ > G″ possesses a network structure and will remain stable in storage.

A frequency sweep shows how the same structure behaves on different time scales and helps distinguish the type of polymeric binder or rheology modifier involved (cellulose ethers, HEUR, HASE, organoclay). A temperature ramp follows curing, gelation and the effects of storage temperature. Together these tests build a "structural fingerprint" that a single viscosity value can never provide.

Rheological data becomes valuable when it is matched to an application defect. Sagging points to insufficient low-shear viscosity or slow structure recovery; poor levelling to recovery that is too fast or a yield stress that is too high; settling in storage to a low yield stress or a G′ that declines with time; and spray-gun clogging to viscosity that does not fall far enough at high shear. Every defect has a counterpart on the curve, and the correction begins with adding the right rheology modifier in the right amount.

For batch comparison we recommend defining the trio of flow curve, 3ITT and amplitude sweep as the standard acceptance criterion rather than a single viscosity point. This trio catches a drift in a raw-material batch or inadequate dispersion in production before the product reaches the field. When rheological data is evaluated alongside surface appearance measurements such as gloss, haze and DOI, the effect of levelling defects on final film quality becomes directly visible.

NAKTEM plans flow curve, 3ITT, yield stress and oscillatory measurements on the same sample for coating and ink formulations and interprets the results in the language of the application process. Standard drawdowns and prints prepared with a printing proofer allow the rheological data to be matched to real transfer and appearance behaviour, so that a change in a modifier or a raw-material lot can be traced from the flow curve to the finished film.