Paints and coatings are used for a variety of purposes. In addition to protecting surfaces from various environmental influences (e.g. UV radiation, air humidity, chemicals, etc.), they are also used to decorate interiors and exteriors or to enhance functionality. Raw materials such as binders (resins), solvents, pigments, fillers and additives should remain homogeneously mixed from production to end-use application. Additionally, they need to remain stable during pumping and storage, and after application with brushes, rollers, Spray Guns, etc. Therefore, the importance of rheological measurements of paints and coatings for assessing their quality cannot be overstated.
In many cases, the rheological properties of paints and coatings influence their behavior:
During production and application, liquid raw materials and finished paints and coatings are pumped through pipelines. Understanding flow behavior at different shear rates is important for designing the necessary equipment.
The correct leveling and sagging behavior of a paint or coating after application cannot be overemphasized for the final result. To ensure proper leveling and avoid sagging, rheological parameters such as viscosity should be neither too high nor too low. Therefore, the rheological behavior of the material over time needs to be balanced to obtain the desired results. These properties are often referred to as thixotropic behavior.
If dispersed pigments and fillers are not kept in suspension, they will form a layer of sediment on the bottom of the container. This can eventually lead to unevenness in the paint or coating.
Rheological testing can be used for:
Calculates the amount that will affect the shear rate of a paint or coating sample during application
Evaluate whether the viscosity value of a paint or coating meets the requirements after application
Measuring long-term storage stability of paints and coatings
Paints and Coatings Typically Measured:
ballpoint pen ink
fumed silica
powder coating i
powder coating two
top coat
wall paint
Other materials and applications
Ballpoint pens are the most commonly used pens in the world - millions of them are produced and sold every day. They are very simple devices that work by dispensing ink on a metal ball at the tip of the pen. That is, a ballpoint pen. Still, they don't always perform the way they should. Sometimes the ink stops flowing, and sometimes the ink builds up unevenly on the metal ball, causing smudges on the page and on your fingers.
These effects are influenced by at least several factors. These include the precision of the ball and socket assembly and its tribological properties, as well as the rheological and tribological behavior of the ink. For example, if the viscosity of the ink decreases, whether due to shear or temperature, more ink will flow through the socket and onto the tip of the ball. Fatigue of the ball and socket mechanism can also cause this increase or decrease in ink flow.
Measuring the rheology of ballpoint pen inks can provide insight into their flow behavior under typical operating conditions. Furthermore, the friction and wear behavior of ball and socket systems can be studied by performing tribological measurements. The graph below shows how two ink samples with different viscosities behave in a metal/ink/paper system, simulating how ink behaves from a ballpoint pen to paper.
For metal/ink/paper systems, the coefficient of friction is measured as a function of the sliding velocity of the metal relative to the paper. The coefficient of friction reaches its minimum value when a fluid film forms between the surfaces. Desirably, this should happen at writing speeds, and the film should be thin enough to transmit just enough ink to keep the typeface clean, but thick enough to protect the ball and holder from abrasion.
This test requires a rheometer equipped with a three-ball setup.
Fumed silica is a very low density, high surface area material commonly used as a filler, thickener, flow agent, etc. The properties of fumed silica can be tailored to a specific end use by varying the raw material used and the process used to produce it. Therefore, analysis and quality control are important to determine whether a product will perform for its intended purpose. Powder rheology measurements can provide useful information about the properties of powders such as fumed silica. For example, cohesive strength describes the binding forces between powder particles, while outgassing measurements indicate how long air will remain in a powder sample. Degassing is an important indicator to measure whether the powder can be conveniently transported by pneumatic force.
Outgassing measurements can be performed on a rheometer equipped with a powder cell. First, the powder is fully fluidized for a defined period of time, then the gas flow is stopped and the pressure inside the powder cell is measured several times at very short intervals. When the signal reaches a constant value, it is assumed that the air has completely escaped from the powder sample.
This test requires a rheometer capable of handling powder rheology.
Powder coatings are an emission-free alternative to liquid coatings as they do not contain any solvents. Typically, powder coatings are applied electrostatically. Afterwards, a thin film is formed by melting individual powder paint particles in an oven. The time and temperature required for film formation are very important as both parameters strongly influence the size of the production equipment and further influence the process cost. Therefore, the goal is to develop powder coatings that require low temperature and short time film formation.
To determine the curing behavior of powder coatings, temperature tests can be performed with an oscillatory rheometer. The oscillatory test simultaneously determines the viscous behavior described by the loss modulus G" and the elastic behavior represented by the storage modulus G'. Thus, the curing behavior can be determined as a time-dependent behavior at a constant test temperature (isothermal test) or Temperature-dependent behavior over a specific temperature range.
This test requires a rheometer equipped with a Peltier temperature control system.
This is just one of the rheological studies commonly used in the automotive industry.
Powder coating is an evolving technology, originally developed to create more resilient coatings. However, powder coatings are also gaining popularity for their more environmentally friendly, solvent-free process. Processing involves several steps, such as fluidization and/or pneumatic conveying, which are strongly influenced by powder rheology. Thus, rheological measurements can provide an informative picture about the quality of the particles and whether the powder is suitable for powder coatings. The powder needs to have the right melt rheology (viscosity and solidification behavior), but at the same time it needs to be fluidizable and have good air retention for easy transport. One way to improve powder flowability and fluidization is to add small amounts of fumed silica .
Powder quality can be checked with a rheometer equipped with a powder cell; for example, by taking a pressure drop measurement, which shows the flow rate required for complete fluidization of the sample – see figure below. The suitability of powders for pneumatic conveying can also be determined by degassing measurements with the powder unit. Viscosity measurements can also be made under various shear conditions, which can be a good indicator of possible difficulties during transportation.
This test requires a rheometer capable of handling powder rheology.
The top coat is usually the last coat. In most cases it is used to give surfaces a shiny look and to protect them from weather conditions and other influences. In order not to sag or develop unwanted brush marks after application, the finish needs to recover its structure in due time; it cannot recover either too slowly or too quickly. An important quality factor for topcoats is therefore the time-dependence of structure regeneration, which in turn affects surface leveling and sag behavior. In some cases, this structural recovery can be described using the rheological term thixotropy.
In rheology, thixotropic behavior is defined as a reduction in the structural strength of a sample during a testing interval with a constant shear load, followed by a complete regeneration of the structure during a subsequent resting interval. This behavior can be measured in a rotational or oscillatory test by performing a three-compartment thixotropy test. The test program simulates the application process using the following three measurement intervals:
Rest Intervals: Evaluate structures at rest by presetting very low shear loads.
Load intervals: Evaluation of structural decomposition behavior during application under constant high shear loads.
Structural Restoration Interval: Structural regrowth is assessed over time after application. The preset measurement conditions are exactly the same as the rest interval.
This test requires a rheometer equipped with a Peltier temperature control system.
This is just one of the rheological studies commonly used in the automotive industry.
The quality of paint is important both in production and application. A satisfactory wall paint needs to be stirrable, mixable, dispersible as well as pumpable and flowable. Depending on the application, wall coverings need to be spreadable, brushable, rollerable, pourable or sprayable. Another quality factor is the surface leveling and sagging behavior of the paint. Satisfied, the internal structure should recover within the correct time period. There should also be sufficient time for degassing during this time. A smooth, shiny and even surface free from drips or splashes is often required.
The number of rheological tests available has been steadily increasing, especially for R&D users as well as quality and process control users. Rheometers can be used to evaluate phenomena such as yield point when determining the strength of structures at rest, shear-thinning behavior in flow regimes, and thixotropy to analyze recovery of internal structures over time after application.
Most coatings exhibit shear-thinning flow behavior with a decrease in viscosity as the shear rate increases. Therefore, the faster you stir, the lower the resulting viscosity. This behavior can be measured with a spin test.
This test requires a rheometer with a Peltier temperature control system.
See here for more information on the following topics:
Determining the Flow Behavior of Coarse Dispersions
Rheology of Coatings: Determination of Yield Point with the MCR 72
Modern Rheological Measurement Methods for Coating Technology