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Rheological investigation of paints and coatings

Paints and coatings are used for a variety of purposes. Besides protecting surfaces against various environmental influences (e.g. UV radiation, air humidity, chemicals, etc.), they are used to decorate both interiors and exteriors, or for enhancing functions. From production to end use application, raw materials such as binders (resins), solvents, pigments, fillers, and additives should remain uniformly mixed. In addition, they need to remain stable during pumping processes and storage, as well as after application with brushes, rollers, spray guns, etc. Thus, rheological measurements of paints and coatings are crucial for evaluating their quality.

Rheological behavior of paints and coatings

The rheological properties of paints and coatings have an impact on their behavior in many situations: 

Pumping and flow behavior during production and application

The liquid raw materials and the finished paints and coatings are pumped through pipes during production and application. Knowledge of the flow behavior at different shear rates is important for designing the necessary equipment.

Behavior after coating, brushing, spraying, and rolling processes (leveling and sagging behavior, wet layer thickness)

After application, the correct leveling and sagging behavior of a paint or coating is essential for the end result. To ensure optimal leveling and avoid sagging, rheological parameters such as viscosity should be neither too high nor too low. Hence, the material’s time-dependent rheological behavior has to be balanced to get the desired result. These characteristics are often referred to as thixotropic behavior.

Separation and sedimentation behavior at rest (long-term storage stability)

If dispersed pigments and fillers do not remain in suspension, they will build up a layer of sediment at the bottom of the container. This will ultimately lead to inhomogeneity in the paint or coating.

Investigating paints and coatings using rheological tests

Rheological tests are useful to:

  • Calculate the amount of shear rate which will affect a paint or coating sample during application
  • Evaluate whether the viscosity values of a paint or coating will fit the requirements after application
  • Measure the long-term storage stability of paints and coatings 

Ballpoint pen inks

Ballpoint pens are the world’s most-used pens – millions are made and sold each day. They are beautifully simple devices that work by dispensing ink over a metal ball at the pen’s point; i.e. the ballpoint. Nonetheless, they don’t always perform as they should. Sometimes the ink stops coming out, and at other times ink builds up unevenly on the metal ball, leading to smudges on the page and on our fingers. 

These effects are influenced at least by a couple of factors. These include the precision of the ball and socket assembly as well as its tribological properties, and also the rheological and tribological behavior of the ink. For instance, if the ink’s viscosity decreases, either due to shear or temperature, more ink flows through the socket and onto the tip of the ball. Fatigue of the ball-and-socket mechanism can also lead to such increases or decreases in ink flow.

Tribological tests on ballpoint pen ink

Measuring the rheology of ballpoint inks can provide insight into their flow behavior under typical operating conditions. In addition, the friction and wear behavior of the ball-and-socket system can be studied by making tribology measurements. The graph below shows how two ink samples with different viscosities behave in a metal/ink/paper system, simulating the behaviour of ink going from a ballpoint pen onto paper.

For a metal/ink/paper system, the coefficient of friction is measured as a function of the sliding speed of the metal against the paper. The coefficient of friction reaches a minimum at the point when a fluid film forms between the surfaces. Ideally, this should happen at writing speed, and the film should be just thin enough to transport only enough ink for the typeface to stay clean, but thick enough to protect the ball and bracket from wear.

This test requires a rheometer equipped with a ball-on-three-plates setup. 

Fumed silica

Fumed silica is a very low density, high surface area material that is often used as a filler, a thickening agent, and a flowing agent, among others. The properties of fumed silica can be tailored to specific end uses by varying the raw material used as well as the process used to produce it. Thus, analysis and quality control are important for determining whether the product can fulfill its desired purpose. Powder rheology measurements can provide useful information about the properties of powders such as fumed silica. For example, the cohesion strength describes the binding forces between the powder particles, whereas deaeration measurements indicate how long air will remain in a powder sample. Deaeration is an important indicator of whether a powder can be easily transported by pneumatic conveying.

Rheological tests on fumed silica

Deaeration measurements can be carried out on a rheometer equipped with a powder cell. First, the powder is fully fluidized for a defined time period, then airflow is stopped and the pressure inside the powder cell is measured multiple times at very short intervals. When the signal reaches constant values, it is assumed that air has completely escaped from the powder sample.

This test requires a rheometer which can handle powder rheology.

Powder coatings I

Powder coatings are an emission-free alternative to coatings in liquid form as they do not contain any solvents. Usually, a powder coating is applied electrostatically. Afterwards, a film is formed by melting the individual powder coating particles in an oven. The time and temperature required for the film formation are of great importance, as both parameters strongly influence the dimensions of the production plants and furthermore affect the process costs. Thus, the aim is to develop powder coatings which require a low temperature and short time for film formation.

Rheological tests on powder coatings

In order to determine the curing behavior of powder coatings, a temperature test with an oscillatory rheometer can be performed. An oscillatory test simultaneously determines both viscous behavior, described by the loss modulus G”, and elastic behavior, represented by the storage modulus G'. Thus, curing behavior can either be determined as the time-dependent behavior at a constant test temperature (isothermal test) or as temperature-dependent behavior within a certain temperature range.

This test requires a rheometer equipped with a Peltier temperature control system.

This is just one of the rheological investigations typically used in the automotive industry.

Powder coatings II

Powder coatings are a growing technology that was initially developed to create more resilient coatings; however, powder coatings have also become popular for their more environmentally friendly, solvent-free processes. Processing involves several steps, such as fluidization and/or pneumatic conveying, which are strongly influenced by the powder’s rheology. Thus, rheological measurements can provide an informative picture of particle quality and about whether a powder is suitable for a powder coating. The powder must have a suitable melt rheology (viscosity and curing behavior), but at the same time it must be fluidizable and have good air-retention so that it is transportable. One way to enhance the flowability and fluidization of a powder is to add a small amount of fumed silica.

Rheological tests on powder coatings

The powder quality can be examined by using a rheometer equipped with a powder cell; for example, by carrying out pressure-drop measurements, which show the flow rate that is required to fully fluidize a sample — see the graph below. The suitability of a powder for pneumatic transport can also be determined with the powder cell through deaeration measurements. It is also possible to make viscosity measurements under various shearing conditions, which can give a good indication of where difficulties may arise during the transport process.

This test requires a rheometer which can handle powder rheology.

Top coatings

A top coating is often the final layer. In most cases it is applied to make surfaces look shiny and to protect them against weather conditions and other influences. In order not to sag off the surface or to show undesired brush marks after application, a top coating has to recover its structure in just the right time; its recovery should be neither too slow nor too fast. Therefore, an important quality factor of top coatings is the time-dependence of the structural regeneration, which in turn influences surface leveling and sagging behavior. This structural recovery can be described in certain cases using the rheological term thixotropy.

Rheological tests on top coatings

In rheology, thixotropic behavior is defined as the reduction of a sample’s structural strength during a test interval with a constant shear load, and the complete regeneration of the structure during the subsequent interval at rest. This behavior can be measured in rotational or oscillatory tests, by performing a three-interval thixotropy test. The test procedure simulates the application process with the three following measuring intervals:

  1. Rest interval: Evaluation of the structure at rest by presetting a very low shear load.
  2. Load interval: Evaluation of the structural decomposition behavior during application under a constant high-shear load.
  3. Structure recovery interval: Evaluation of the structural regeneration over time after application. The preset measuring conditions are exactly the same as in the rest interval.

This test requires a rheometer equipped with a Peltier temperature control system.

This is just one of the rheological investigations typically used in the automotive industry.

Wall paints

The quality of paints is important in both production and application. Ideal wall paints must be stirrable, mixable, dispersable as well as pumpable and flowable. Depending on the application, wall paints have to be spreadable, brushable, rollable, pourable, or sprayable. Another quality factor is the paint’s surface leveling and sagging behavior once it is applied; ideally, the internal structure should recover in exactly the right time period. During this period there should also be enough time for deaeration. A smooth, glossy, and homogenous surface with no droplets or splashes is generally required.

Rheological tests on wall paints

The number of available rheological tests has been steadily increasing, especially for users in research and development as well as for quality and process control. A rheometer can be used to evaluate phenomena such as the yield point when determining the structure strength at rest, shear-thinning behavior in the flowing state, and thixotropy when analyzing the time-dependent recovery of the internal structure after application.

Most paints show shear-thinning flow behavior with a decrease in viscosity when the shear rate is increased. Therefore, the faster you stir, the lower the resulting viscosity is. This behavior can be measured by rotational tests.

This test requires a rheometer equipped with a Peltier temperature control system.