Introduction
In spray coating applications, fine powder, often a polymer, is drawn from a storage container, fluidized, and then sprayed through a charged nozzle onto a substrate. It is crucial that the powder fluidizes effectively and consistently, as the formation of agglomerates can block the nozzle, disrupt the charging of particles, and result in poor adhesion or clumping on the substrate. Additionally, ensuring a smooth flow from the storage container is essential, as inconsistent flow into the fluidization chamber can lead to suboptimal fluidization.
Identifying and quantifying the powder properties that correlate with optimal process performance allows for the optimization of new formulations without the need to run extensive samples through the entire process. This approach can save significant time and raw materials, reduce costs, and minimize waste from rejected products.
Variations in Process Performance and Product Quality
Three samples of a polymer powder were used in a spray coating application with a corona charging system. Sample A demonstrated good performance, flowing smoothly through the nozzle and adhering well to the substrate. Sample B showed acceptable behavior, but Sample C performed poorly, causing nozzle blockages and subsequently falling off the substrate during transportation to the kiln. Despite these differences, particle size analysis revealed that all three powders had the same D50 and size distribution.
Samples from the three batches were analyzed with an FT4 Powder Rheometer®. Clear and repeatable variations were identified among the samples in several tests. These differences helped explain the variations in performance and allowed for future batches to be screened before being introduced into the process.
Test Results
Sample A produced the highest Basic Flowability Energy (BFE) and Specific Energy (SE) of the three samples, indicating greater cohesion and particle-particle interlocking. In contrast, Sample C had the lowest BFE and SE, suggesting that a certain level of inter-particular cohesion is necessary to form a uniform coating on the substrate— a criterion that Sample C failed to meet.
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Bulk Testing: Compressibility
Sample C had the highest compressibility of the samples, suggesting a higher proclivity to compact under forced flow conditions, like when the powder is taken from the storage vessel and transferred into the fluidization chamber. This increased compressibility is likely to promote the formation of agglomerates, which can hinder both the spraying and charging processes in the nozzle.
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Bulk Testing: Permeability
Sample A produced the lowest pressure drop across the powder bed, suggesting that it has the highest permeability. This indicates that it will be the most free-flowing under conveyance and that when fluidized, it is likely to flow more readily within an airstream. Sample C displayed the least permeability, producing the highest pressure drop in the powder bed. This is likely to generate more erratic, pulsatile flow into the fluidization chamber and inconsistent flow of the fluidized mass.
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Shear Cell Testing
No variation was found during shear cell testing, as the measured shear stress values for all three samples were nearly identical, with a relative standard deviation (RSD) of 2.5 %. The lack of correlation with process performance suggests that the highly consolidated, low-flow conditions of the shear cell test do not accurately reflect the behavior of these powders in the dynamic, aerated environment typical of fluidization operations.
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Conclusion
The FT4’s multivariate approach has revealed clear and consistent differences between the three powder samples in terms of dynamic and bulk properties, which correlate closely with their in-process performance. The findings also demonstrate that Shear Cell testing alone does not reliably represent powder behavior in this process, due to the different stress and flow regimes involved.
Among the three samples, Sample A exhibited the highest BFE, SE, and permeability, along with the lowest compressibility. This indicates that a degree of cohesion is needed to form an even coating, but the proclivity to agglomeration and erratic flow is problematic to the process.
Sample C, with the lowest BFE and permeability and the highest compressibility, is most sensitive to compaction throughout conveyance to the fluidization chamber. This leads to agglomerate formation that can block the nozzle and cause inconsistent charging.
Powder flowability is not an inherent material property but is instead about the ability of a powder to flow in a desired manner within specific equipment. Successful processing requires a good match between the powder and the process, and it is not uncommon for a powder to perform well in one process but poorly in another.
This underscores the need for multiple characterization methodologies to fully understand powder behavior across various operations. Instead of relying on single-number characterizations, the FT4’s multivariate approach models a variety of unit operations, enabling a direct examination of how a powder responds to various process and environmental conditions.
This information has been sourced, reviewed and adapted from materials provided by Micromeritics Instrument Corporation.
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