{"id":16958,"date":"2024-04-10T06:00:24","date_gmt":"2024-04-10T12:00:24","guid":{"rendered":"https:\/\/www.ulprospector.com\/knowledge\/?p=16958"},"modified":"2024-03-18T09:26:58","modified_gmt":"2024-03-18T15:26:58","slug":"pc-rheology-understanding-and-control","status":"publish","type":"post","link":"https:\/\/ulprospector.ul.com\/16958\/pc-rheology-understanding-and-control\/","title":{"rendered":"Rheology – Understanding and Control"},"content":{"rendered":"

\"paintingThis article is the second in a series concerning rheology of paint and paint processes. The term rheology<\/strong><\/a> was first described by Professor Bingham and accepted by The American Society of Rheology when it was founded in 1929 and defined as follows:<\/p>\n

\u00a0 \u00a0 \u00a0 \u00a0 Rheology: <\/em><\/strong>The study of the deformation and flow of matter <\/em><\/strong><\/p>\n

The science of rheology<\/strong><\/a> provides the study of materials<\/a> 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<\/strong> is the measurement of a fluid\u2019s resistance to flow when subjected to <\/strong>shear<\/strong>.<\/strong><\/p>\n

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.<\/p>\n

Figure 1 \u2013 Single Point Viscosity Measurement<\/p>\n

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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<\/strong><\/a> is defined as a fluid where the shear stress is linearly proportional to the shear strain rate. A Thixotropic<\/strong><\/a> or Pseudoplastic fluid<\/strong><\/a> demonstrates a decrease in viscosity with increasing shear rate, whereas a Dilatant<\/strong> fluid provides an increase in viscosity with an increase in shear rate (Figure 2).<\/p>\n

Figure 2. Viscosity behavior of various types of fluids subjected to Shear Stress<\/p>\n

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Figure 3. For a thixotropic<\/a> 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.<\/p>\n

Schematic 1. Thixotropic Paint viscosity<\/p>\n

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Key Definitions<\/strong><\/p>\n

Shear Stress<\/u><\/em><\/strong> (<\/strong>\u03d2<\/strong>) – A type of stress that acts coplanar with cross section of material and is expressed as dyne\/cm2<\/sup><\/em><\/strong><\/p>\n

\u03d2<\/strong>\u00a0(shear stress<\/em>) =\u00a0F (force) \/ A (area)<\/em><\/strong><\/p>\n

Shear Rate<\/u><\/em><\/strong> (D) – Is expressed as sec-1<\/sup>or dyne-sec\/cm2<\/sup> or poise\u00a0 <\/em><\/strong><\/p>\n

D (shear rate) = V (velocity) \/\u00a0C\u00a0(thickness)<\/em><\/strong><\/p>\n

Young\u2019s Modulus<\/u><\/em><\/strong> (E) \u2013 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<\/em><\/strong><\/p>\n

E = <\/em><\/strong>s \/<\/em><\/strong>e<\/p>\n

Yield stress<\/u><\/em><\/strong> \u2013 Yield stress is defined as the minimum shear stress necessary to initiate flow at which a material begins to deform or flow<\/em><\/strong><\/p>\n

Figure 3 \u2013 Viscosity vs. Shear Rate requirements for application and settling processes<\/p>\n

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Figure 4 \u2013 Viscosity vs. Shear rate for multiple application methods<\/p>\n

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As Figure 3 and 4 illustrates for most paint applications, it is normally more desirable to have a paint that provides Pseudoplastic<\/a> (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<\/a> and waterborne<\/a> 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.<\/p>\n

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Explore the ULTRUS Portfolio supporting regulatory, supply chain and sustainability needs. Learn more here<\/a>!<\/h3>\n
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Multiple\u00a0rheology\/control<\/em><\/strong>\u00a0modifiers can be found using the\u00a0Prospector Search Engine<\/em><\/strong>\u00a0<\/a>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.<\/p>\n

The issues that impact rheology in paints include:<\/p>\n