Powder Coatings

Considerable requirements are placed on paints and coatings, simply due to all the different applications and situations they are used in. The high degree of complexity is also a result of the many different materials that are processed – usually mixtures of polymers with catalysts and flow aids. To ensure smooth fabrication and operation, several material variables need to be present within the materials. This application report shows how some of Anton Paars’ instruments are used to analyze essential properties of powder coatings. Particle size is an important parameter for quality control and product quality which can be easily measured with laser diffraction. Powder rheology enables the characterization of powder behavior and can be easily combined with rheology to understand the curing behavior. Anton Paar’s instruments deliver information on your materials’ structure, behavior, and consistency. Any type of sample can be analyzed – from the incoming raw materials to intermediate products and the final paints, inks, and coatings.

Introduction

Powder coatings are an increasingly important technology with numerous applications. Initially designed to generate coatings displaying a greater resilience than comparable liquid-applied ones, the technique’s current rapid development is essentially driven by the need for more environment-friendly, solvent-free processes.[1] Coatings consist of complex mixtures of powders, including - but not limited to - systems that use two copolymers along with the catalysts (so-called hybrid coatings) or, as in the simpler case of polyethylene coatings, of polymer mixtures consisting of monomers and Ziegler-Natta catalysts.[2]

To cover and understand the whole life-cycle of powder coating demands the ability to measure and analyze the right properties at any step – from the incoming raw materials, across all intermediary steps like mixing, to the final and finished product. And the life-cycle does not end with the finished product – it is also transported, stored and finally applied onto a substrate. The right measurements make it possible to handle this complexity by measuring the properties needed to ensure high quality products, from the raw material to the final coating. Development of new products necessitates understanding and predicting the behavior during the manufacturing as well as the application process. Similarly, in quality control a consistent production and processing is needed to guarantee reliable and homogeneous product (and thereby paint and coating) quality.

The measurements covered in this application report focus on powder rheology, melt/curing rheology and particle sizing.

Powder rheology helps to understand the behavior of powders and can be used as quick quality control tool as well as for in-depth powder analysis. Anton Paar uses its sensitive MCR rheometers as a basis for powder rheology. The rheometers can be equipped with two different cells (powder shear and powder flow cell) which 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 processing steps can be simulated: starting from mixing to storage, over hopper discharge to fluidized bed reactor, pneumatic conveying and finally application (e.g. electrostatically-assisted spraying). With the modular setup of the rheometers the powder shear cell can be combined with an oven to control temperature and even humidity.

Melting and curing can be studied with rheology. Oscillatory tests enable deep structural and behavioral analysis, to obtain for example information about the deformation and the time-dependent behavior of samples. The modularity of the devices makes it possible to measure under precisely controlled temperatures to learn about the samples’ drying and curing behavior.

The particle size analyzer (PSA) is capable of measuring the particle size and particle size distribution of powder coating samples. Information about particle size is relevant for the optimization of production processes as well as the outer appearance of the final coating. For this reason a manufacturing plant would benefit of the PSA not only in the quality control but also in the development of new and efficient coatings.

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