Characterizing Powders for Coatings Processes
The high performance and low environmental impact of powder coatings make them attractive for a range of applications. Powder coating technology avoids the use of a solvent by fluidizing the fine powders used with air, encouraging them to behave in a liquid-like way. While this approach confers environmental advantage by avoiding the volatile organic emissions associated with solvent use, it presents the challenge of ensuring that the powder flows easily and smoothly during application. Demand for thinner films that necessitate the use of ever-smaller particles intensifies pressure on powder flowability, as interparticle forces of attraction increase dramatically with decreasing particle size.
Here we look at powder-handling issues pertinent to this important industrial sector, focusing particularly on storage, blending and powder fluidization. By considering problems typically encountered during operation we show how effective powder characterization can give insight and guidance for troubleshooting, providing knowledge for the optimal development, selection and use of a powder coating. The data presented in these examples were all generated using the FT4 Powder Rheometer from Freeman Technology, a universal powder tester that incorporates a range of complementary test programs for the measurement of bulk, shear and dynamic properties.
Powder Coating TechnologyPowder coating production involves the melt blending of polymers/resins with a range of additives, followed by extrusion and grinding of the extrudate to the required particle size. Powders are manufactured and delivered in batches to allow close matching of customer requirements with powder properties, although additives may still be incorporated immediately before application to change specific aspects of the final finish.
Coatings are applied to a substrate either in a fluidized bed or more commonly using spraying processes, the focus of this article. Prior to spraying, powder is fluidized in a hopper to ensure smooth, consistent flow to the gun. During spraying the powder becomes charged, encouraging adhesion to an earthed substrate. Any powder that does not adhere is recovered and recycled to significantly improve powder utilization levels, minimizing waste. The substrate is cured at high temperature to allow the discrete powder particles to fuse to a smooth, finished film.
Powder recycle is an important issue commonly responsible for problematic powder flow. Although the particle size properties of the virgin material are closely defined, attrition during the spraying process is likely to have an impact on the size and/or shape of the powder. Mixing of this recycled material with virgin material, in varying ratios, means that for most of the time the process is operating with material that has much less closely defined properties, often with a higher proportion of fines. The ratio of fresh to recycled material is, therefore, one of the variables commonly manipulated to address flow issues.
Factors Influencing Powder Flow PropertiesAlthough particle size and shape directly impact on powder flowability, these factors are far from being the only ones to influence behavior. Influencing factors can be categorized as either primary variables, properties of the particles that make up the powder, or secondary parameters, which relate to the system as a whole. Hardness, porosity and surface roughness are good examples of important primary variables, as are particle size and shape. Key secondary variables include moisture content, degree of segregation and, perhaps most critically, air content. With a change in air content alone a powder can switch from liquid- to solid-like behavior.
The number of variables affecting powder properties means that accurate sample characterization is a complex issue; flow behavior is heavily dependent on processing history as well as physical properties. Because many of the individual parameters influencing flowability can be so easily changed by processing, even two samples that initially contain identical particles can subsequently behave very differently.
This sets powder measurement apart from liquid or gas characterization and places demands on any methodology to ensure that the powder is analyzed in a known or consistent state. Failure to do so results in inconsistent data, a commonly encountered problem with many analyzers. With the FT4 rheometer, a conditioning step before each test program ensures that a precise volume of homogeneous, consistently packed powder is always used for analysis. This confers high reproducibility and repeatability, allowing the impact of both primary and secondary variables to be determined.
Quantifying Sensitivity to MoistureThe fine powders used in coatings processes are often sensitive to moisture uptake, which can have a deleterious affect on their flow performance. Storage humidity therefore requires careful control and the air used for fluidization must be dry. With dynamic testing, sensitivity to moisture can easily be explored by examining samples exposed to different levels of humidity.
Comparing BFE (Basic Flowability Energy – see sidebar Dynamic Measurement) for a dry sample with BFE for a range of powders having higher moisture contents allows the sensitivity of flow behavior to moisture content to be quantified. This information is particularly useful when defining conditions for a new product or when trying to rationalize a flow problem.
Assessing Blend HomogeneityMixing additives with a powder coating to impart phosphorescence, for example, requires the blending of a relatively small mass into the bulk powder before application. Dry blending and/or co-sieving of the feeds are possible options depending on the scale of operation. If the resulting mixture is not homogeneous then the properties of the final film will be compromised.
The highly differentiating nature of the basic BFE measurement makes it an excellent tool for assessing blending performance. By taking a series of samples from a blended bulk and comparing BFE values, the degree of homogeneity can be rapidly assessed. This allows determination of the impact on blending efficiency of factors such as blend time, loading procedure and vessel fill level, facilitating optimization of the blending process. Figure 1 shows results typical of poor blending.
Fluidization in the Feed HopperAeration Behavior and the Impact of Fines
The spray guns used to apply powder draw their feed from a hopper containing fluidized material. Ideally, within the hopper the powder should flow like a fluid, behaving much like a boiling liquid; inadequate fluidization causes slow and unsteady transport of the powder to the spray nozzle. Geysering, where air blows large holes through the surface of the powder, and a ‘dead bed’, no percolation of air through the material, are good examples of poor fluidization behavior. Other problems associated with operation of the hopper include stratification, a separation of fines and larger particles, and dusting, which is when fine powder is blown out of the hopper by the fluidizing air.1
Fines are a critical issue for each of these problems since they influence aeration, compaction and segregation behavior. As noted earlier, while the fines content of a virgin powder coating may be specified, the hopper contains a mixture of virgin and recycled material, which has probably undergone attrition. Virgin-to-recycle ratio is therefore commonly used as a lever for fines content control, to maintain flowability and surface finish quality. Dynamic investigations can be used both to quantify the likelihood of attrition and predict the impact of fines on flow properties.
Aeration and Fluidization Behavior
Aeration and fluidization behavior is a very important characteristic that is determined by measuring flow energy during aeration and eventual fluidization of the powder sample. The FT4 system uses a mass flow controller to deliver a known air flow velocity through a sinter at the base of the test vessel.
A typical aeration and fluidization characteristic is exhibited by material B, as shown in Figure 2. Important features are the initial response to aeration (slope), the sharpness of the bend in the characteristic curve and the energy required to create fluidization. For a typical test protocol fluidization energy will be around 10 mJ. The profiles in Figure 2 show a large difference in the fluidization energy of the two materials and indicate that material B is superior, entraining air easily and fluidizing over a relatively small air flow range.
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A series of BFE measurements, interspersed with agitation stages designed to promote attrition, allows quantification of the propensity of a powder to attrite. This is useful, basic information for assessing whether the material recycled from the spray booth is likely to be significantly different from virgin material. It can also be useful for rationalizing differences in process performance linked to a switch in coating.
Consolidation BehaviorConsolidation is caused by either vibration or the direct application of pressure, easily occurring therefore during transportation or storage, often unintentionally. Small levels of applied pressure can produce significant compaction, particularly for cohesive powders. For these materials even overnight storage in a hopper, when the powder is simply stored under its own weight, may result in compaction.
Consolidation data for materials A and B are shown in Figure 3. These results indicate that the flow energy of material B increases more markedly with compaction than the flow energy of material A, particularly if compaction is induced by vibration. Practically this behavior indicates that material B is likely to be more resistant to flow directly after transportation or storage, but will have superior properties when aerated/non-consolidated.
De-Aeration CharacteristicThe rate at which entrained air escapes from a powder bulk is another important powder characteristic. If air is retained then the powder will flow more easily, for longer, while the release of air radically changes flow properties allowing the powder to become consolidated and more dense. De-aeration characteristics therefore need to be optimized to suit the specific requirements of a process.
Figure 4 shows the de-aeration characteristics of materials A and B, highlighting the reluctance of A to release entrained air. These data are consistent with both the aeration and consolidation behavior of the materials. Material B picks up air easily, fluidizing well, however it also releases air easily, particularly when subjected to vibration, becoming compacted. The very high flow energy of material B, when consolidated by tapping, is directly attributable to the sample’s ability to release air. Sample A on the other hand both entrains and releases air much more slowly.
SegregationThe propensity of a sample to segregate or stratify can be quantified in the same way as attrition, by comparing flow energy measurements before and after steps are taken to encourage segregation. When using the FT4, segregation is promoted by subjecting the sample to specific cycles, with or without an air flow through the base sinter, during which fines will tend to migrate downward and large particles upward. The subsequent measurement of BFE and/or aerated/fluidized flow energy permits the identification and quantification of any segregation behavior, with re-homogenization of the sample and further testing confirming any findings. Samples from different parts of the bed may be taken for size analysis to provide further insight into segregation behavior.
An example of data from such a study is shown in Figure 4. This approach could be used to assess segregation as a function of virgin to recycle material ratio, which would provide useful guidance for avoiding stratification in the hopper.
The application of powder coatings is a demanding process in terms of its requirement for excellent powder flowability. The fluidization characteristics of these powders are particularly important, and instruments, such as the FT4 that have the accuracy and sensitivity needed to measure very small flow energies are therefore extremely valuable to the industry. These universal powder testers also permit the impact of other key process variables, such as moisture, consolidation, attrition and segregation, to be quantified. They are therefore an important tool for the optimization of powder coating processes, providing insight into the flow properties of a material and the impact on them of different unit operations. This data is invaluable when developing new formulations and troubleshooting existing processes.
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dynamic measurement, the flow properties of a powder are determined by
measuring the energy required to displace material in a certain pattern. This
energy is calculated from measurements of the force and torque acting on a
blade as it moves along a helical path, through a conditioned sample. A
downward traverse of the blade produces a bulldozing action within the sample
and a highly differentiating measure defined as basic flowability energy (BFE).
Upward testing on the other hand avoids compression and generates specific
energy (SE) data, which is useful for predicting the unconsolidated behavior of