Scope

This guide covers the significance of referencing the techniques used whenever specifying the particle size distribution of a coating powder.

Referenced Documents

D1921 Test Methods for Particle Size (Sieve Analysis) of Plastic Materials

D3451 Guide for Testing Coating Powders and Powder Coatings

Significance and Use

4.1 This guide describes the need to specify the measuring technique used whenever quoting the particle size distribution of a coating powder.

4.2 This guide is for use by manufacturers of coating powders and by specifiers for process control and product acceptance.

Particle Size of Coating Powders

5.1 The size of the particles comprising a coating powder plays a critical role in the fluidization, application, and reclamation of the powder, and in the final appearance of the coated part. Coating powders are comprised of particles of widely differing sizes, from as low as about 1 μm to as high as about 150 μm. Collectively, the individual particles form a size distribution, defined by the percentages of particles present of a given size or within a given size range. There are generally few particles at the low and high ends of the distribution, the majority being in the 25 to 65-μm range. The distribution can be described by an actual plot of the particle size distribution, or by numerical attributes of the distribution, such as the calculated values of its mean, median, mode, and span. The mean represents the average particle size (the sum of all the particle sizes divided by the number of particles). The median represents a size such that half the particles are larger than it and half the particles are smaller than it. The mode represents the most frequently occurring particle size. For all coating powders these three figures are numerically different. The span is an indication of the width of the particle size distribution. Referring to Table A1.1, the span is calculated by subtracting the d10 from the d90 and then dividing by the d50 or median particle size.

5.2 The particle size distribution is generally chosen by the coating powder manufacturer from knowledge of the application technique, the required cured film thickness, surface appearance, and performance. Once the desired particle size distribution has been selected, it needs to be monitored to ensure consistency from batch to batch and, indeed, within each batch. Occasionally the coating powder applicator may specify the particle size from knowledge of the specific application equipment or customer requirements, or both. 

5.3 It is important for all involved to understand that the numerical data comprising a particle size distribution are significantly dependent on the technique used to obtain them. It is, therefore, of little use to quote or specify a particle size distribution, and even less a single particle size, without also defining the technique used to obtain that measurement, or, if a single size, whether it is, for example, the mean, median or modal value.

6. Measurement of Particle Size

6.1 There are a wide variety of instruments currently available for measuring the particle size distributions of coating powders. Actual sieving, such as described in Test Methods D1921, where the percentage weight of coating powder retained on sieves of known mesh size is measured, is relatively inexpensive and direct. It is, however, significantly slower than indirect measurement techniques, such as laser scattering and electrolytic conductivity, such as described in Guide D3451. With indirect measurement techniques, a secondary effect, induced by the presence of the coating powder particles, is measured, such as changes in light scattering or in the conductivity of an electrolyte. These effects are analyzed using a specific theoretical algorithm, unique to the measurement technique, and the particle size distribution calculated that would cause the measured changes. Various other statistical data on the distributions, such as the mean, the median, the mode, and the span are also often automatically calculated. 

6.2 Secondary measurement techniques make assumptions such as the measured particles being spherical, and do not acknowledge the fractured, randomized shapes the particles actually possess. Others require the preparation of a suspension of the particles in a liquid, which could alter the physical state of particle agglomerates present in the dry state. Even the required processing for dry powder measurement techniques could mechanically break up larger particles or agglomerates into smaller ones, or both.

6.3 Thus not only can the theoretical algorithms for the measuring techniques be quite different, but each measurement technique can cause the particle size distribution to change during sample preparation or the measurement process itself, or both. This simply serves to emphasize that once a measurement technique has been selected, there is still need for consistency in all aspects of its operation.

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