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How to analyze paint and coating parameters

Background

High and diverse demands are placed on paints and coatings – simply because of the different applications and situations for which they are used. This high degree of complexity can also be traced back to the many different particulates and liquids that are processed. In order to develop new formulations, but also to ensure consistent quality during production, it is necessary to have a precise understanding of the materials and their behavior. This can be achieved by testing various paint parameters, such as particle size or flowability. This article presents a number of measuring methods which can be used to investigate paint and coating parameters in order to understand and predict their behavior.

Life cycle of paints and coatings

Figure 1 shows the typical life cycle of paints, coatings, and inks, starting from the raw material through the production steps to the finished product, its application, and the applied product. Depending on the step in the life cycle and the material, different properties influence what is happening inside the material. To ensure smooth processing in the subsequent steps and a controlled quality of the final product it is necessary to understand these material properties. The sections below describe a number of methods for measuring these coating parameters. As an additional dimension in these considerations, it is necessary to take into account that the materials can be in a liquid or dry form.

Figure 1: Life cycle of paints and coatings and the corresponding crucial measurement parameters

Figure 1: Life cycle of paints and coatings and the corresponding crucial measurement parameters

Investigating the behavior of powder/dry materials

Powders (e.g. pigments) are used as raw material and dispersed in solutions for liquid paints, coatings, and inks. There are also powder paints and coatings which are applied in dry form and then cured to form a smooth layer. Powders have multiple properties (like particle size) that influence how the material can be processed or applied and that in turn will also influence the properties of the applied paint or coating.

Particle size and particle size distribution

The particle size and particle size distribution of powders can be measured with laser diffraction(1). This technique is based on the observation and analysis of laser light which is diffracted from particles. The particle size and particle size distribution of paints and coatings are essential parameters for the quality control of incoming raw materials (to check that the powder particles are within specifications), but also during fabrication, e.g. milling and mixing of the paints and coatings. Knowing the particle size and particle size distribution of a powder (Figure 2) is also crucial for the final product, as these parameters will influence how the material can be applied onto a substrate, as well as the curing process and outer appearance of the final coating. For example:

  • Fine particles with narrow particle size distribution pack closer together, forming thin films that maintain the aesthetic properties and durability typical for a thicker film.
  • Larger particles tend to flow better and are relatively easy to control during application but curing processes require a longer time and higher temperatures.

From the calculated volume-weighted particle size distribution many parameters can be determined, including the principal D-values (D10, D50, D90, D[4,3]), the span, which gives an indication about the broadness of the distribution, as well as the percentage of particles in different size classes, which is important when checking the amount of fine and coarse fraction. In addition to particle size and particle size distribution, these analyzers can perform a pressure series to evaluate the effect of pressure on a powder’s fine fraction during manufacturing and the powder coating process.

Read more about these parameters.   

Figure 2: Particle size distribution of a powder coating (q3 density in red and q3 cumulative in gray)

Figure 2: Particle size distribution of a powder coating (q3 density in red and q3 cumulative in gray)

Powder rheology

Powder rheology is the characterization of powder behavior by simulating all processing conditions – from fluidized powder systems to consolidated powders. Ideally, powder rheology takes into account internal influences (e.g. particle size, particle shape) and external influences (e.g. humidity, temperature) to phenomologically characterize powder behavior. It can be used for R&D when creating new formulations e.g. to find the optimal processability or processing conditions, as well as for quality control during manufacturing(2).

Investigating powder rheology requires a rheometer with the relevant powder rheology accessories, e.g.:

  • A powder shear cell for investigating the flow behavior of consolidated powders and their time-dependent behavior
  • A powder flow cell for characterizing powder behavior under various realistic conditions, i.e. to simulate manufacturing processes

These two cells implemented on a rheometer make it possible to analyze powders in any state they are in during the process – from compacted and consolidated to fully fluidized. In this manner all steps of the process can be simulated. Mixing and storage processes can be analyzed as well as the flowability the powders have during hopper discharge. It is also possible to determine fluidization properties which are useful insights since the powders will typically experience fluidized bed reactors, pneumatic transport, and spraying through a nozzle (Figure 3). Basic fluidization properties are analyzed with pressure drop and deaeration measurements. More complex shear-rate-dependent measurements simulate, for example, kinks during pneumatic transport or spraying through a nozzle.

Learn more about powder rheology, the classification of powder properties, and powder behavior.

Figure 3: Apparent powder viscosity in a sub-fluidized (gray) and fluidized (red) state in dependence of shear rate

Figure 3: Apparent powder viscosity in a sub-fluidized (gray) and fluidized (red) state in dependence of shear rate

Surface area

The surface area of powder particles has a large impact on how they interact with electrostatic charge and liquids, and therefore their suitability as functional coatings. For powder particles the surface area is not only a function of particle size, but also of shape, surface roughness, and porosity. The extent of the surface of pigments and fillers determines how much dispersant is required in paint, coating, and ink formulations. Determining the surface area and pore size of powder particles requires the gas adsorption technique and use of the BET method. Compounds with a high surface area can be analyzed using gas adsorption with nitrogen gas, either applying the vacuum-volumetric technique or dynamic flow method. Materials with a low surface area often require gas adsorption measurement with krypton gas as this provides increased sensitivity.

True density and tap density

The true density or skeletal density of the components of a coating powder needs to be known in order to calculate the theoretical powder coating density and non-volatile film density. This can be compared to the density of the coating powder blend of resin, pigments, fillers, and additives – as blended, after storage, and after curing. Any differences in density of the blend can indicate subtle differences in formulation, such as resin-to-pigment ratio. Analysis of true density (skeletal density) is performed using a gas pycnometer which measures solid volume by gas displacement.

Tap density is determined by repeatedly tapping a powder column and analyzing the relative volume change. It is carried out to obtain the powder volume filling and packing characteristics. Automatic tapped density tests can be carried out on single components or blends.

Figure 4: Powder bulk density as function of tapping of two nominally similar but distinct powder batches

Figure 4: Powder bulk density as function of tapping of two nominally similar but distinct powder batches

Investigating the behavior of dispersions/liquids

Dispersions and suspensions are mixtures of liquids with particulate matter, in the case of paints and coatings these particulates are powder pigments and other additives. With liquid paint and coatings knowledge of the flow behavior is important for controlling and optimizing coating application. Before mixing the coating, it is also an essential step to characterize the raw materials to check their material properties.

Rheology and viscometry

Rheology and viscometry are used to analyze the flow behavior, curing, and drying behavior of paints, coatings, and inks(3). A more extensive article can be found here.

Particle size and zeta potential

The particle size of the particles in the dispersion directly relates to properties such as appearance and uniformity of the coating layer. This coating parameter can be determined using a particle size analyzer based on the laser diffraction technique.

The zeta potential can be measured to analyze formulation stability and to avoid aggregation of dispersed particles. High zeta potential values indicate a stable formulation and prevent agglomeration of particles and inconsistent coating performance. These results, as well as particle size, can be determined on a particle analyzer using dynamic and electrophoretic light scattering (DLS and ELS).

Summary

A large number of different coating parameters influence the behavior of the material during the manufacturing process and the quality of the paint or coating applied. Particle size and particle size distribution are crucial parameters for quality control, while powder rheology is used to characterize powder behavior during the entire life cycle of manufacturing, storage, and application. The surface area of the particles provides information on how much dispersant is needed in the formulations, and the skeletal density can be used to calculate powder coating and non-volatile film density. Viscometry and rheology are used to determine factors like pumpability and spreadability as well as drying and curing behavior. All these properties need to be determined for quality control in production, and also for the development of new formulations.

References

1. Xu, R. (2002). Particle Characterization: Light Scattering Methods. Dordrecht: Springer Netherlands, pp 111–181.
2. Schütz, D., et al. (2018). “A multi-method approach to quality control illustrated on the industrial powder coating process”. Chemical Engineering Research and Design, 139, pp 136–143.
3. Mezger, T.G. (2017). Applied Rheology. 4th edition.