Particle Size Measurement and Powder Coatings

Powder coatings, substances which are decorative, protective, or both, are formed by applying a coating powder to a substrate, after which the coating is fused into a continuous film by applying radiant energy or heat. Coating powders are a mixture of finely divided organic polymer particles that usually contain fillers, pigments, and other additives which remain finely divided during storage under the right conditions. Powder coatings, unlike liquid coatings that may contain volatile organic solvents, can achieve similar or better characteristics of corrosion resistance, quality, and durability. Compared to liquid coatings, powder coatings have reduced production costs because this process is highly efficient, needing less labor and energy. Since powder coatings contain no volatile organic solvents, they eliminate organic solvent emissions and reduce waste disposal.

A variety of factors influence the performance of a powder coating. Powder particle size can have a major impact on many properties, including coating characteristics, handling, charging, and delivery. Depending on application requirements of object shape, thickness, and ambient conditions, the powder coating formulations may vary.

The Process

The powder coating process starts by applying a charge to an object's surface, usually by an electrical ground. First, the powder coating is given a charge which is opposite to the charge applied to the object. The powder is then directed toward the object in an air stream under pressure. When the powder particle gets closer to the charged object, electrostatic attraction controls and attaches the powder to the item. Initial handling, recycling, and recovery of the powder can lead to an increase in fines, which can deposit in nozzles or pumps during normal processing. These deposits can lead to uneven delivery under application pressure, resulting in blotchy or uneven films. Production and deposition of fines, caused by particle attrition due to high pressure in the process, can shut down the process completely.

The overall charge profile of the powder can also be influenced by the amount of fines, because fines can be charged more efficiently. When there are insufficient fines, wraparound can be reduced because an electrostatic field can easily influence the fines. Since coarse particles have greater mass, they have a lower charge/mass ratio. This reduces the efficiency after the electrical field is applied around the object, resulting in marginal or incomplete coating. The fines have a high charge/mass ratio. This leads to a thinner film thickness because the buildup of coating is limited to the electrical field, which reduces as film thickness increases.

The Problem

Particle size strongly affects film thickness because fines generate thin initial powder coats, while coarse particles generate thick initial powder coats. Therefore, the heated coats melt at different rates and decrease the final coating smoothness. This phenomenon is generally called "orange peel" in the coatings industry. The orange peel can also be linked to the electrostatic method or rheological factors of the powder that are highly dependent on particle size.

The Solution

It is very clear that the significance of particle size to powder processing should not be underestimated. Microtrac® particle size analyzers are able to measure powder coatings in the dry state. In general, the powder coatings are smaller than 250 µm, or 60 mesh, U.S. The SRA150 or SRA200 analyzer can be used, depending on the overall particle size requirements. The SRA150 analyzer can measure particles as small as 0.7 µm and can be used in applications where fines smaller than 4 µm are essential. The benefit of dry measurement of the coatings is that material can be used as a dry powder. However, in order to avoid biasing the reported particle size to fine sizes, low-energy dry-dispersion systems should be used. This can lead to recycling of the recovered powder or inadvertent milling changes.

The data for a powder coating can be depicted numerically or graphically, as shown below. In order to improve the capability of laser diffraction measurement, the data can be stored in the computer for later recovery. Instead of the cumbersome text files, which are used by many particle size analyzers, the data is stored in true dBasecompatible format. This makes data manipulation and data recovery processes significantly easier.

Besides easy data recovery, users can also carry out more advanced data analysis using standard programs, such as Microsoft Excel, dBase, Lotus 1-2-3, or FoxPro.

A proprietary algorithm used in Microtrac particle analyzers enables direct conversion of scattered light into a volume distribution. The patented silicon detector array, manufactured by Honeywell and specifically used in Microtrac analyzers, makes the measurement fast, easy, and precise. Production performed under ISO 9000 certification ensures the highest consistency and quality of all components to customers. With expert knowledge and 25 years of laser diffraction experience in all aspects of particle size, computer technology, and electronics, Microtrac assures the best possible technical support to its customers.

Microtrac S3000 with Vibratory or Linear Dry Powder Feeder

Microtrac S3000 with Vibratory or Linear Dry Powder Feeder

Capabilities

  • Measures multi-modal distributions without curve fitting or assumptions
  • Includes dry powder vibratory feeder for typical particle size distributions
  • Complete database management capability, which can be exported to all popular spreadsheets
  • Simplicity of operation
  • Modular, convenient design
  • Portable and small
  • Optional linear dry feeder for wide particle size distributions

This information has been sourced, reviewed and adapted from materials provided by Microtrac.

For more information on this source, please visit Microtrac.

Ask A Question

Do you have a question you'd like to ask regarding this article?

Leave your feedback
Submit