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Rheology in Paints and Coatings – Essential Concepts

SpecialChem – Mar 11, 2020

TAGS:  Rheology Modifiers / Thickeners 

Rheology in Paints and Coatings

  1. What is Rheology?
  2. General Classification of Flow Behavior
  3. Measuring Rheological Properties
  4. Achieving Required Rheology in Paints and Coatings

Flow properties are important in the paints and coatings industry and the study of flow is called rheology. The manufacturing of coatings as well as the process of application and film formation require a high degree of control on flow to achieve success and desired finish.

Rheology examines the behavior of flow and deformation properties of the coating during these stages. Sedimentation, sprayability, sagging, brushing resistance and adhesion are some of the important characteristics influenced by the rheology of a formulation. Thus, the commercial viability of paints and coatings formulation largely depends on having the correct rheology for the application.

When formulating a high-performance paint or coating system, it is essential to understand the background of rheology technology and functional additives to get complete process efficiency.

Let's discuss these concepts in detail...



What is Rheology?


Rheology is the study of flow or deformation of materials when subjected to stress.

  • 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)

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.


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.


Role of Shear Stress, Shear Rate and Viscosity


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, a part of rheology, can be obtained from rheological measurements. Viscosity is a measurement of the fluid’s resistance to flow and 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.
Schematic plot of shear rate vs shear rate
Schematic Plot of Shear Stress vs. Shear Rate for Newtonian (Solid Line) and Non-Newtonian Fluids in a Simple Shear Flow.
Source: Researchgate.net


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.

Viscosity Flow Curves (Source: Malvern Instruments Limited, UK)
Viscosity Flow Curves
(Source: Malvern Instruments Limited, UK)

Newtonian Fluids: The viscosity is invariable with the shear rate or shear stress. 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.

Viscosity Curves for the Description of Rheological Behavior (Source: Universität Stuttgart)
Viscosity Curves for the Description of Rheological Behavior
(Source: Universität Stuttgart)


Measuring Rheological 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.


Types of Rheometers


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.

Shearing a Fluid Between Two Parallel Plates (Source: Massachusetts Institute of Technology)
Shearing a Fluid Between Two Parallel Plates
(Source: Massachusetts Institute of Technology)


The most common measuring systems are:

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

Common Measuring System for Rotational Rheometers
Common Measuring System for Rotational Rheometers


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.

Testing Rheology  Viscosity in Practice PB


Achieving Required Rheology in Paints and Coatings


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 Main Category
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. In addition to rheology modifiers, the rheology of paints and coatings is determined by:



Rheological Additives for Water-borne Coatings


The high interdependency of the individual coating raw materials like binders, surfactants or pigments requires a lot of effort to achieve the ideal rheological profile in water-borne coatings.

Common rheological additives or thickeners for water-borne paints and coatings include:

  • Cellulose derivatives
  • Acrylic thickeners
  • Clays
  • Hydrophobically modified ethoxylated urethanes (HEUR)
  • Hydrophobically modified alkali swellable emulsions (HASE)

The thickeners can be divided into two thickening mechanisms.

  • Cellulose derivatives, acrylic thickeners, and clays have a thickening effect of the aqueous phase.
  • HASE and HEUR thickeners have an associative mechanism providing viscosity by interacting with other ingredients of the coating formulation.

HASE, HEUR and Acrylic Thickeners


HASE and HEUR thickeners could be used to obtain shear forces during production. However, they have an associative mechanism. Therefore, they should be added in the formulation after the pigments and resins have been dispersed.

For brush and roll applications, the HASE and HEUR thickeners are of choice for:

  • Brush and roll applications
  • Obtaining flow, levelling and proper film build

For spray applications the lower modified HASE and HEUR types can be used but better results are to be expected with pure acrylic thickeners since these provide lower viscosity at higher shear assuring proper atomization.

Acrylic and HASE type thickeners demand a pH between 8 and 10 to assure proper activation and reproducibility.


Cellulose Derivatives and Clay-based Thickeners


Cellulose derivatives and clay-based thickeners are suitable for storage and transport. These thickeners provide high viscosity at low shear rates and are best to obtain sag resistance. Cellulose derivatives have the best water retention capacity and ideal for water-based coatings used on porous substrates.

The combinations of HEUR or HASE type thickeners with cellulose derivatives or clay thickeners could provide a better balance between flow/leveling and sag resistance.

Considering the above-mentioned aspects, a general categorization of rheology additives for WB systems can be represented as:

Main Category Sub-Category Suitable Rheological Additives
Production Dissolving Cellulose derivatives
Bead Milling Cellulose derivatives
Mixing HASE, HEUR
Storage & transport Storage, time effects Cellulose derivatives, Clays, Acrylics
Storage, temperature effects Cellulose derivatives, Clays, Acrylics
Transportation, time effects Clays
Transportation, temperature effects Clays
Application Roller HASE, HEUR
Brush HASE, HEUR
Airless spray Acrylic
Conventional spray Acrylic
Industrial rolling, founding, dipping HASE, HEUR
Drying/curing Physical drying Acrylic
Polymerization (2K/Auto-oxidation) Cellulose derivatives, Clays
Polymerization at elevated temperature Cellulose derivatives, Clays


Rheological Additives for Solvent-borne Coatings


Unlike water-borne coatings, the flow properties can be regulated via the molecular weight of the dissolved binder solventborne formulations. Rheology modifiers are sometimes added to solvent-based systems to render certain application properties. Common thickening additives for solvent-based paints and coatings include:

  • Organoclays and Organo waxes: Suitable for a wide range of applications. They have good heat resistance and offer thixotropic flow with excellent sag resistance. However, they provide less thixotropy than organics and poor leveling.
  • Hydrogenated castor oils: When used as rheology additives, hydrogenated castor oil offers strong shear thinning, leveling and sag resistance. But seeding, temperature control and need of adequate shear and dwell time are some of the disadvantages associated with these additives.
  • Fumed silicas: Being Chemically inert, they are difficult to disperse in solvent-based medium.


Coatings Professionals Stay Alert!


Make the most out of performance characteristics by selecting the most suitable rheology additive for your formulation. Get hands on tips on how to select the best suitable rheology modifier for your paint or coating system.

Rheology Modifiers Selection for Paints and Coatings



References


  1. https://core.ac.uk/download/pdf/37377051.pdf
  2. http://wernerblank.com/pdfiles/rheology.pdf
  3. https://people.clarkson.edu/~skrishna/DHR_Rheology_Theory.pdf
  4. https://www.academia.edu/35683142/Rheology_Additives_Rheology_Additives_Content
  5. https://onlinelibrary.wiley.com/doi/abs/10.1002/9780470027318.a0611
  6. http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.517.6595&rep=rep1&type=pdf
  7. https://www.aca.fi/wp-content/uploads/2018/10/Rheology-Seminar_ACA-Systems_brochure.pdf 
  8. The Rheology Handbook: For Users of Rotational and Oscillatory Rheometers by Thomas G. Mezger
  9. European Coatings Handbook by Thomas Brock, Michael Groteklaes, Peter Mischke
  10. Rheology: Concepts, Methods and Applications by Aleksandr I︠A︡kovlevich Malkin, Alexander Ya. Malkin, Avraam I. Isayev

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