From in-lab to on-line: Versatile options for particle size analysis of powder coatings using laser diffraction

Introduction

Understanding the particle size of powder coatings is important to ensure product quality and performance, and laser diffraction offers far more detail about size distributions compared to less sophisticated techniques. Off-line systems have long had a place in development and QC workflows because of their robustness, versatility and accuracy, and they remain a vital laboratory tool. However, for fast-moving process control situations, on-line systems offer considerable benefits. 

In this article, we show how an on-line system tailored to the needs of the powder coating industry can provide cost-effective, real-time information on particle sizes, facilitating swift decision-making on milling parameters while also minimizing wastage.

The importance of particle size of powder coatings

Powder coatings have long been a popular finishing technique for various industrial products and consumer goods, including automobiles. This is because they are easy to apply, avoid the waste and health issues from use of solvents, and because they impart an aesthetically pleasing, durable finish that can be tailored to the demands of the application. As a result, they continue to experience an increase in market share, aided by technological innovations such as ultra-thin coatings and special-effect finishes.

Powder coatings take the form of a thermoplastic or thermosetting polymer that is applied electrostatically to a metal surface using a spray gun, and then cured (using heat or UV) to partially melt the powder. Many producers of powder coatings operate on a batch basis, because this allows small product runs and a wide array of products to be produced in an economical manner with only a few mills. Profits are directly related to processing as many batches as possible while maintaining stringent quality controls, amongst which particle size is particularly crucial.

The most important properties influenced by particle size are: 

• Processability and application efficiency: The particle size directly influences the processability of the powder coating. Smaller particles have a larger overall surface area compared to larger particles, which improves electrostatic charging during application, and leads to better adhesion of the powder to the substrate.
• Surface smoothness: A fine and even particle distribution is crucial for the surface quality of the finished coating. Larger particles tend to create a rough surface, while smaller particles result in a smoother and more aesthetically pleasing surface. Furthermore, a uniform particle size ensures that the entire film is evenly distributed, and cures at the same time, resulting in a more uniform layer thickness and minimizing the formation of defects such as bubbles. 
  
• Physical strength and resistance: The particle size also influences the curing process of the powder coating. Smaller particles melt and flow together faster and more evenly, which leads to better network formation and thus higher mechanical strength and chemical resistance of the coating. Smoother coatings resulting from an even distribution of particle sizes also improve scratch-resistance and reduces susceptibility to dirt.
  
• Efficiency and material saving: With an even particle distribution, the powder is used more efficiently, which reduces overspray and therefore makes it easier to recover the excess powder. This leads to cost savings and a lower environmental impact, as less waste is generated.
• Method versatility: Depending on the area of application and type of substrate to be coated, the optimal particle size can vary. Smaller particles are advantageous for complex geometries and hard-to-reach areas because they can penetrate corners and recesses better. For applications that require a particularly thick coating, larger particles may be more suitable, as they make greater layer thicknesses possible.

Optimum particle sizes vary considerably depending on the application, but they typically have a Dv50 of 20–30 microns, although within the industry there is a trend towards the smaller particles essential for producing the thinner films that are now in demand.

Assessing and controlling particle size

It will be clear from the above discussion that assessing and controlling particle size is important for achieving an optimal powder coating product. Various methods can be used to achieve this, including air classifiers, grindometers and sieves. However, even when the labor-intensiveness of these methods is addressed by (semi-)automation, they retain the drawback that they only provide an approximate measure of particle size, with no detail on the particle size distribution.

Laser diffraction addresses this need by providing a much more rigorous way of determining particle size, with the additional benefits of ease of use, high sample throughput, and wide measuring range. The output from a laser diffraction measurement includes detailed information on the particle size distributions (PSDs), including multi-modal distributions and outliers – all essential for helping to ensure product quality.

What is laser diffraction?

Laser diffraction measures particle size distributions by measuring the angular variation in intensity of light scattered as a laser beam passes through a particulate sample. Large particles scatter light at small angles relative to the laser beam, while small particles scatter light at large angles. 

This angular scattering intensity data is then analyzed to calculate the size of the particles responsible for creating the scattering pattern, using the Mie theory of light scattering. The particle size distributions are then reported as a volume-equivalent sphere diameter.

Versatile off-line measurements of particle size

Laser diffraction is already routinely used in the powder coatings industry, most commonly within the lab as an off-line system. This is a versatile setup, allowing accurate, reliable size distributions to be determined for a variety of purposes, especially during product development and for final product quality control.

For such situations, Malvern Panalytical offers the Mastersizer 3000+, which is an ideal instrument for off-line measurement of particle sizes in the range of 0.01–3500 microns. This latest edition of the instrument offers additional support for routine analysis in the form of the Size Sure software feature, which reliably differentiates between steady-state and transient signals, and so enhances confidence in routine measurements. For powder coating applications, the Aero accessories for the Mastersizer 3000+ make it easy to ensure that solid powders are well-dispersed in an air stream for accurate measurement, avoiding the need to disperse samples in a liquid.

Figure 1: The Mastersizer 3000+ system for off-line laser diffraction measurement of particle size, showing the Aero S accessory for efficient preparation of powder coating samples. 

However, using traditional off-line laser diffraction analysis to optimize process control presents difficulties, because of the time needed to take a sample, analyze it, and interpret the results generated. Such processes, because they are manual and frequently repeated, can also be prone to operator error, and moreover, the process of stopping and starting the milling process after every measurement results in significant amounts of waste material. A different approach to size determination is therefore needed – and this is where on-line monitoring presents an appealing solution.

Application example: Off-line laser diffraction

To illustrate off-line analysis of a powder coating, the Mastersizer 3000 was used with the Aero S dry dispersion accessory to determine the PSDs for two mixed polymer samples. As can be seen in the plot below, the Dv50 values are similar, but the second sample extends to both smaller and larger particle sizes, resulting in markedly different Dv10 and Dv90 values. This increased range (polydispersity) may improve the particle packing on the coating surface, reducing air inclusions and improving the mechanical properties of the finished product – insights that would not be available using less sophisticated techniques.

Figure 2: Particle size distributions for two mixed polymer powder coatings, acquired off-line using the Mastersizer 3000, with data table. To download the full application note, visit our website.

A tailored package for on-line monitoring of powder coatings

A fully automated on-line system, by enabling the consequences of a change in milling parameters to be visualized in real-time, would inherently overcome the problems of off-line monitoring for powder coatings. It would also help to avoid the familiar problem of accidentally over-milling a sample (‘over-grind’), and the associated negative consequences for energy consumption, powder flowability and product wastage.

However, facilities for manufacturing powder coatings typically contain 10–15 lines, and purchasing a full-featured on-line system for each milling line would be prohibitively expensive. One possibility is for a single on-line system to serve multiple milling lines, but:

• Ensuring fail-safe sample handling over multiple-metre distances would be complicated, requiring costly customization of sample lines
• There would be risks around cross-contamination of batches, requiring the scrapping of large amounts of material
• With an increasing number of lines for each optical system, it becomes less easy to achieve true real-time monitoring. 

Therefore, using a single analytical system for each milling line is unavoidable in order to achieve a setup that is economic in the long-term.

At Malvern Panalytical, we are keenly aware both of the challenges of process control at powder coating facilities, but also of the potential advantages of on-line laser diffraction. Therefore, We’ve developed a tailored on-line monitoring package specifically for powder-coating customers, which is closely based on our existing Insitec instrument for on-line monitoring of dry powders. Insitec has already been well-validated in challenging industrial applications over many years, where it is valued for providing >95% reliability and minimal downtime.

Figure 3: An Insitec system for on-line laser diffraction measurement of particle size, which is now available as an optimized, cost-effective package for process control in powder coating applications.

To offset some of the upfront costs associated with having a single on-line system for each milling line, each Insitec system features two-mode operation. The system is first used continuously during batch startup to move rapidly into the desired size range. Once this is established and a steady-state is reached, it can then be switched to intermittent measurements. In both modes, the amount of sample going to waste is reduced, providing an overall productivity improvement of up to 5%, and corresponding cost savings.

Further advantages come from integrated barcode readers, which can read the sample specification and provide automated alerts when the sample size goes outside the desired range. The Insitec system also uses Malvern Link II software, allowing control system integration and remote access to laser diffraction results. And when a batch is completed, the system calculates the average particle size, allowing QC labels to be printed that can be affixed to the palette confirming the particle size, providing extra assurance to the customer.

Application example: On-line laser diffraction

The image below displays a typical interface for on-line analysis of a powder coating. The “Start” button will trigger a continuous measurement for a given time (typically 3–5 min), shown by the open circles. In this mode, the system can be quickly brought within specification (between the red and orange lines). After this, the system automatically switches to intermittent measurements (typically every 10 min), shown by the filled circles. During this time, if a result falls outside of the tolerance zone, an alarm is raised to stop the material dosing. The right-hand panel displays the sample metrics, and also the sample information, which can be automatically populated by use of a barcode reader. 

Figure 4: Typical software panel for the on-line analysis of a powder coating sample using the two-mode Insitec system developed for the powder coatings industry. For more information about Insitec, visit our website.

Conclusion

In the powder coatings industry, off-line laser diffraction analysis – because of its inherent flexibility – will always have an important role to play in serving the needs of product development and product QC teams. However, with greater need to streamline and monitor manufacturing processes to improve efficiency, reduce energy costs and eliminate wastage, there is growing interest in also using on-line monitoring systems for process control.

Up to now, such systems have not been economical to install across multiple lines, but this has been resolved with our Insitec on-line system dedicated to the needs of particle sizing on particle milling lines. Providing cost-effective ‘plug-and-play’ capability for an individual milling line, this enables machine operators and plant managers to monitor particle sizing continuously during the initial stages of a milling run, followed by discontinuous monitoring to minimize wastage and keep costs down.

As a result, whether in-lab or at-line, laser diffraction offers a solution for those needing precise, reliable measurement of particle sizes of powder coatings – and hence for maximizing product quality while keeping costs down in a competitive marketplace.