This article is the second in a series concerning rheology of paint and paint processes. The term rheology was first described by Professor Bingham and accepted by The American Society of Rheology when it was founded in 1929 and defined as follows:
Rheology: The study of the deformation and flow of matter
The science of rheology provides the study of materials including those such as bulk solids as well as fluids. Solids are comprised of a large collection of very small particles which may also be moving or flowing. Prior to delving more deeply into the study of rheology, we need to define the terms used in more detail in order to enhance our understanding. Viscosity is the measurement of a fluid’s resistance to flow when subjected to shear.
A single point viscosity measurement using a viscometer (figure 1) or a viscosity cup will not provide the information necessary to determine how a paint will perform for multiple paint related properties including pigment dispersion, material transfer (pouring and pumping), application properties and resistance to hard settling.
Figure 1 – Single Point Viscosity Measurement
Multiple viscosity readings are necessary over a range of shear rates to determine what adjustments may be necessary to optimize pigment dispersion, paint transfer during handling and application as well as the flow and leveling of the applied paint. A Newtonian fluid is defined as a fluid where the shear stress is linearly proportional to the shear strain rate. A Thixotropic or Pseudoplastic fluid demonstrates a decrease in viscosity with increasing shear rate, whereas a Dilatant fluid provides an increase in viscosity with an increase in shear rate (Figure 2).
Figure 2. Viscosity behavior of various types of fluids subjected to Shear Stress
Figure 3. For a thixotropic paint shows the viscosity after shearing is lower than that of the same unsheared paint. The degree of divergence provides an estimate of the of the degree of thixotropy.
Schematic 1. Thixotropic Paint viscosity
Key Definitions
Shear Stress (ϒ) – A type of stress that acts coplanar with cross section of material and is expressed as dyne/cm2
ϒ (shear stress) = F (force) / A (area)
Shear Rate (D) – Is expressed as sec-1or dyne-sec/cm2 or poise
D (shear rate) = V (velocity) / C (thickness)
Young’s Modulus (E) – Expressed in pressure and is the ability of a material to withstand changes in length when under length wise tension and is sometimes referred to as elasticity. Uniaxial stress / Strain
E = s /e
Yield stress – Yield stress is defined as the minimum shear stress necessary to initiate flow at which a material begins to deform or flow
Figure 3 – Viscosity vs. Shear Rate requirements for application and settling processes
Figure 4 – Viscosity vs. Shear rate for multiple application methods
As Figure 3 and 4 illustrates for most paint applications, it is normally more desirable to have a paint that provides Pseudoplastic (shear thinning) behavior to enable viscosity reduction in the presence of shear in the application process (spray, roll, brush, reverse roll coat). However, there are additional variables present for both solvent-borne and waterborne paints. For solvent-borne paints, flow and leveling are very dependent on the evaporation rate of the solvents and temperature, ambient cure rate etc. For waterborne paints the initial appearance of the applied paint (sag resistance, ambient cure rate) is more dependent on relative humidity.
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Multiple rheology/control modifiers can be found using the Prospector Search Engine and are available to modify waterborne and solvent-borne paints to adjust application properties as well as for resistance to hard setting. There are multiple ingredients and variables that influence rheology in a coating formulation.
The issues that impact rheology in paints include:
- Coating ingredients
- Binders (solution versus latex or dispersion)
- Pigments (oil absorption)
- Filler pigments and extenders (oil Absorption)
- Pigment dispersants (selection based on type of pigment)
- Surfactants
- Amines amount and type (waterborne paints)
- Cosolvent
- Customization of rheological properties
- Criteria for rheology modification and selection
- Flow and leveling agents
- Surfactants
- Other additives
The viscosity of latex paints tends to exhibit excessive shear thinning behavior and is dependent on multiple compositional factors as listed above. For latex paints, when the viscosity at high shear rates is adjusted for proper application, the viscosity at low shear rates for proper leveling tends to be high. This is the reason why the leveling of latex paints tends to be poorer than that of solvent-borne paints. This is most pronounced at higher gloss levels. Accordingly, to counteract this phenomena, associative thickeners are used. In simple terminology, associative thickeners can be defined as a water-soluble polymer containing multiple hydrophobic groups.
Some common thixotropes and their incorporation include:
Rheology control in solvent-borne paints:
- Organo clay – Added during pigment dispersion step. Organo clay is made from phyllosilicates (types of clay) with quaternary amines covalently bonded to the clay surface. Probably the most common thixatrope used in solvent-borne paints.
- Hydrogenated castor wax – Added to mill base while cooling/heat activated
- Polyamide wax – Added to mill base while cooling/heat activated or can be preactivated and added during letdown.
- Fumed silica – Added during letdown
Rheology control agents for waterborne coatings include:
- Cellulosics – Derived from cotton, wood and paper and act by forming entangled chains to increase viscosity
- Hydroxyethyl cellulose
- Carboxyl functional cellulose
- Methyl cellulose
- Polyamides
- Synthetic clay
- Colloidal silica (have a high surface area to
- Polyacrylates
Associative thickener types for waterborne coatings include:
- HEUR (Hydrophobically Modified Ethoxylated Urethanes)
- HASE (Hydrophobically-Modified Alkali-Swellable Emulsions)
- HMEC (Hydrophobically-Modified Hydroxy Ethyl Cellulose)
- HEURASE – Hydrophobically Modified Ethoxylated Urethane Alkali Swellable Emulsion)
In summary rheology plays a major role in providing a paint that offers ease of pigment dispersion, good fluid transfer, acceptable application properties and long-term resistance to hard settling. Additional information concerning rheological materials can be found using Prospector’s search engine for key words such as rheology, thixotropy, flow and thickener.
Resources
Prospector Knowledge Center and Search Engine
Wikipedia
Organic Coatings, Science and Technology, Third Edition, Wiley, Wicks e.al. 2007
Organic Coatings, Science and Technology, Fourth Edition, Wiley, Jones e.al. 2017
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