Rheology in Paints and Coatings – Essential Concepts

1. What is Rheology?
2. Why does rheology matter for paints & coatings?
3. Essential notions you need to know for your work
4. How to measure rheology properties?
5. How to adjust rheology?

What is Rheology?

In the paints and coatings industry, flow properties of the final formulation hold a prime importance. The manufacturing & storage of coatings, the process of application and film formation require a high degree of control on flow to achieve success and desired finish. The study of flow or deformation of materials when subjected to stress is called “Rheology”.

• In the case of solids, the material deforms elastically when stress is applied.
• In the case of liquids/fluids, when stress is applied, the material initiates flow, and the energy utilized disperses within the fluid as heat. (Goodwin & Hughes, 2000)

Rheology measurement allows to study structure-flow property relations or flow behavior of all materials. The measurements are useful to stimulate flow behaviors of paints and coatings under different processing and application conditions.

For example, a coating must be stable during storage, easy to apply and result in a smooth and defect-free surface finish. These aspects are connected closely with rheology resulting in desired product performance.

The term ‘rheology’ originates from Greek words ‘rheo’ translating as ‘flow’ and ‘logia’ meaning ‘the study of’. The word ‘Rheology’ was invented in the 1920s by Professor E. C. Bingham at Lafayette College in Indiana.

Why does rheology matter for paints & coatings?

In paints and coatings, rheology examines the behavior of flow and deformation properties during:

• Manufacturing (pigment dispersion, mill base preparation)
• Storage (pigment/particle settling, in-can properties)
• Application (brush, spray, roller), and
• Film formation (leveling, adhesion, texture).

It influences key characteristics such as vertical flow, leveling, gloss, adhesion, film thickness, covering power, spattering tendency, brush and roll resistance, sedimentation tendency and pigment stabilization of a formulation. During the coating production, rheology is useful to obtain optimal flow behavior to the mill base.

Manufacturing

During paint manufacturing, the rheology of the mill base matters a lot to prevent loss of energy during the dispersing stage (also called grinding process). Rheology modifiers are added during the production process mostly before the dispersing stage - type and amount adjusted to the dispersing equipment - to obtain optimal flow behavior to the mill base.

If the mill base is too thin, it will lead to a turbulent flow which leads to a great loss of the energy supplied during the dispersion process.

Initially, the low viscosity of the medium helps wet the solid particles. Then a higher viscosity is needed to well separate the particles from one another. This is when the rheology modifiers are added. The shear rate of a liquid under a dissolver is the medium range, so you will target thickeners able to raise the viscosity in this medium shear rate range.

How to know if your mill base has an optimal flow?

Under normal mixing speed, you should see a vortex in the mill base. It is often called the “donut effect” because of its shape (watch video below).


At the same time, having the optimum rheology can considerably improve shelf life by reducing the settling tendency of pigments and fillers in a formulation. Without adequate rheological control, those systems very often show syneresis effects. Let’s understand it in detail.

Storage

After achieving proper dispersion of your formulation ingredients, the last thing you want is that the particles sink to the bottom due to gravity and form an irreversible hard sediment. This phenomenon is called settling. During storage, the paint should have a viscosity sufficiently high so that heavy pigment particles do not settle down.


Sedimentation mainly occurs during the storage of a liquid system under low shear rate.


The building of a reversible physical network allows paint to behave as solid system and as soon as force is applied, this physical network breaks down and system behaves as a liquid. This arrangement of building a reversible 3D network is a concept to prevent sedimentation during storage. The physical network must be strong enough to resist the gravitational force during storage. On the other hand, it must be weak enough to be broken down as soon as shear is applied.

The minimum amount of shear stress needed to make a system flow is called the yield value of that system. If the applied stress is lower than the yield value, the system behaves as an elastic solid: the particles are frozen-in and they cannot go down under the influence of gravity.

Different rheology modifiers are used to create a reversible physical network and “freeze in” the particles at low shear to avoid sedimentation. Explore our exclusive selection guide on rheology modifiers to find the the chemistries that fit best with your system and thus, help you achieve great anti-settling properties.

Application

There are numerous properties of your final system that depend on rheology. These include:

• Flow and leveling
• Sag resistance
• Brush ability
• Film thickness
• Adhesion
• Opacity

It is important to note that all aspects of paint flow, including stirring, pumping, transferring, sagging, and application involve shearing actions. During application like spraying or brushing, we are mainly in the high shear rate range.

Therefore, the commercial viability of paints and coatings formulation largely depends on having the correct rheology at every stage. When formulating a high-performance paint or coating system, it is essential to understand the background of rheology and role of functional additives to get complete process efficiency.

Essential Notions You Need to Know for Your Work

While understanding the rheology of any formulation, clarity around the core fundamentals enables efficient management of the rheological properties of your product. Let’s begin by understanding some fundamentals terms.

Viscosity, Sheer Rate, Sheer Stress

What is viscosity? Viscosity and rheology are often confused, and they are significantly different concepts. While rheology is the study of flow, viscosity, a part of rheology, can be obtained from rheological measurements. Viscosity is a measurement of the fluid’s resistance to flow and a function of shear rate or stress, with time and temperature dependence.

In rheological measurements, the strain rate is related to the measurement of the deformation and stress is related to the measurement of the force required to deform a material.

• Shear stress (τ; Pa), one of the main parameters studied in rheology, is the force per unit area that a fluid requires to start flowing.
• Shear rate (γ; s^-1) describes the velocity gradient. It is the rate at which a fluid is sheared or “worked” during flow.

Viscosity is obtained as a value when shear stress is divided by the shear rate. Viscosity is not a discrete value rather it depends on the conditions of measurements. It is a function of shear rate i.e. different viscosities are measured at different shear rates.

Over the lifetime, paints are exposed to a wide range of shear rates and shear environments. There is always a minimum stress which is required to initiate or maintain the flow which is estimated using yield stress property.

General Classification of Flow Behavior

The mechanical-rheological behavior of fluid can be determined by analyzing the flow behavior. Flow behavior further assists in the categorization of coatings. There are three general flow behaviors that include:

• Newtonian
• Shear thinning (pseudoplastic)
• Shear thickening (Dilatent)

These common fluid flow behaviors also indicate the viscosity of a material and its dependence on the shear rate.


Pseudo-plasticity: It is a form of structural viscosity, illustrated by a decrease in viscosity as shear rate is increased. Most paints and varnishes show some degree of pseudo plastic (shear thinning) flow behavior.

Newtonian Fluids: The viscosity is invariable with the shear rate or shear stress. Newtonian flow defines a system where viscosity is at indicated pressure and temperature constant regardless of applied shear rate and time. Therefore, a single viscosity measurement will give a true value for the viscosity; the rate of shear is directly proportional to the shearing force. Examples for Newton liquids are water and pure solvents. The typical Newtonian fluids include water, simple hydrocarbons and dilute colloidal dispersions.

Non-Newtonian Fluids: The viscosity varies as a function of the applied shear rate or shear stress. The fluid viscosity is both pressure and temperature-dependent and viscosity increases with increased pressure and decreased temperature.

Thixotropic Liquids: Exhibits a time-dependent response to shear strain rate over a longer period than that associated with changes in the shear strain rate. Thixotropic fluids are generally dispersions. When these fluids are at rest, they construct an intermolecular system of forces and turn the fluid into a solid, thus, increasing the viscosity.


If you wish to have more clarity around the core fundamentals for efficient management of the rheological profile of your product, it may be wise to take first our popular Rheology & Viscosity Made Easy course before moving ahead.

How to measure rheology properties?

Rheometry is an experimental technique employed to determine the rheological properties of fluids, including paints and coatings. Several rheometric tests can be performed to determine flow properties and viscoelastic properties depending on the type of rheometer being used and its capabilities.

Rheometers are used to measure the rheological properties of fluids, including both viscosity and viscoelasticity of fluids, semi-solids and solids.

Rheometers provide information about the material’s:

• Viscosity – A material’s resistance to deformation and a function of shear rate or stress, with time and temperature dependence.
• Viscoelasticity – A property of a material exhibiting both viscous and elastic character when undergoing deformation. The measurements of G’, G”, tan δ with respect to time, temperature, frequency and stress/strain are important for the characterization of a material.

Rotational Rheometers

Rotational rheometers are the most common ones used to determine the linear flow behavior of liquids.

In rotational rheometers, the samples are loaded between two plates, or other similar geometry, such as cone and plate or alternatively a cup and bob system. Afterward, torque to the top plate is applied to exert rotational shear stress on the material and the resulting strain or strain rate (shear rate) is measured.



The most common measuring systems are:

1. Parallel Plate
2. Cone and Plate
3. Cup and Bobs

Other Notable Rheometers

Other types of rheometers depending on the geometry of applied stress include:

• Shear Rheometers – Deal with shear stress
• Extensional Rheometers – Apply extensional stress
• Capillary Rheometers – Measure non-linear shear thinning region at high shear rates. A high-shear, controlled-stress capillary rheometer consists of a heated barrel and a piston that drives molten material through a calibrated die, applying pressure either at a constant speed or a constant shear rate.

When compared to capillary rheometers, rotational rheometers offer many advantages, such as:

• Allowing the use of small samples of products.
• Providing a continuous measure of the rate of deformation and tension of shear, and a wider range of strain rate.
• Enabling an adequate analysis of time-dependent behavior.

Viscometers for Viscosity Measurements

Other viscometer types and methods used to measure the flow behavior of formulations include:

• Brookfield viscometer: Covers low to medium shear range
• Krebs Stormer viscometer (KU): Covers medium shear range
• ICI Cone and Plate viscometer (ICI): Covers high shear range

Brookfield and Krebs Stormer viscometers are simple to operate and often used to measure the viscosity at a given temperature and specific shear conditions. A Brookfield viscometer measures the torque required to rotate a spindle in a fluid. By changing speeds and spindles, a variety of viscosity ranges can be measured.

The selection of most appropriate measuring method for paint rheology is much linked to the purpose:

• An indicative method of the in-can appearance and on-spot application viscosity
• QC purposes
• A sophisticated method for precise rheology-measurement

Now when you are well aware of rheology basics and testing technique, you might want to go deeper to see which test method is most suited for you.

How to adjust rheology?

Rheology influences in-can properties, paint texture, application properties and film properties. Therefore, rheology modifiers or thickeners are used to achieve targeted stability and flow in paints and coatings formulations. These additives can influence production properties, enhance anti-settling properties during storage as well as improve sag resistance and film formation process.

Coating properties influenced by rheological additives at different stages are discussed below:

Rheological Aspect	Different Stages of Coating
Obtain shear forces, viscosity	Production Dissolving Bead Milling Mixing
Anti-settling	Storage and transport Time effects Temperature effects
Viscosity, flow, leveling, sag	Application Roller Brush Airless spray Conventional spray Industrial rolling, founding, dipping
Viscosity, surface defects	Drying/curing Physical drying Polymerization (2K/Auto-oxidation) Polymerization at elevated temperature

There is a broad range of commercially available rheology modifiers in our Coatings Database, used to modify properties of paints and coatings systems.

But the key question here is, with the availability of a wide range of rheology modifiers how you can find the one best suited to your formulation needs. Check out our exclusive guide curated to ease the process of rheology modifiers selection – whether you are working with waterborne or solvent-based system.



Do not forget, in addition to rheology modifiers, the rheology of paints and coatings is determined by:

• Solvent content
• Resin solubility
• Binder (chemistry, molecular weight, etc.)
• Pigments, fillers, and extender level
• Additives, such as wetting agents, dispersing agents, film-forming aids and many more.

Improve the Performance of Coatings with Practical & Advanced Rheological Guidance

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Rheology Modifiers/ Thickeners for Paints, Coatings and Inks

View a wide range of rheology modifiers available today, analyze technical data of each product, get technical assistance or request samples.