Knowledge of particle sizes and the size distribution of a powder system is a prerequisite for most production and processing operations. Particle size and size distribution have a significant effect on the mechanical strength, density, electrical and thermal properties of the finished object. Significant production losses can be incurred due to high rejection rates if size and size distribution of powders being used in a process are not adequately controlled. In most instances powder suppliers provide size and size distribution information, but that information needs to be checked for quality control purposes. Also, the validity of the supplied size information needs to be evaluated if the powder batch has been divided into smaller batches based upon the user's requirements. In certain processing operations, powder size and size distribution may have to be monitored at different stages of a processing operation for suitable process control.

Powder size and size distribution can be determined using numerous  commercially available instruments, or using instruments designed for operations in very specific environments. Some of these instruments can be used in an on-line mode of operation (i.e., integrated into the manufacturing process) or can be used off-line. The number of instruments that can be used in both modes, with minimal adjustments is steadily increasing. Some manufacturers now provide accessories that enable off-line instruments to be adapted to on-line operations. Most instruments operate in a batch mode of analysis. There are few instruments that can be used in a continuous mode.  Instruments can be used for analysis of dry powders and powders dispersed in suspension. Whenever possible, it is desirable to analyze powders dispersed in a state similar to that in which they will be used. Instruments are available that enable analysis over a fairly narrow size distribution or over a very broad size range. The choices available for particle size determination instruments can be quite confusing. Table 1 illustrates some of the existing and emerging techniques for size analysis, the physical principle on which the technique is based, and the general sample concentration and size range over which these techniques are applicable.

Particle Size Analysis Instruments Based on Different Physical Principles

Factors to be considered before selecting an instrument for size and size distribution analysis include, but are not limited to:

1. the amount of sample available;

2. the desired number of points on the size distribution (for data interpretation and/or resolution);

3. number and frequency of analyses required (laboratory, fast-response on on-line methods);

4. extent of human intervention and degree of operator skill required;

5. cost of instrument and other accessories, such as those needed for sample preparation, evaluation of data, etc.;

6. degree of automation required.

It is important to understand that as different instruments are based on different physical principles, there are bound to be differences in the size results obtained  from these instruments. Furthermore, even when using instruments based on the same physical principle, manufacturers' use of proprietary algorithms, components and variations in adaptations of the same basic physical principle can give rise to significant variations in the measured size. Thus, comparison of size and size distribution results from different instruments should be conducted with extreme caution. Suitable protocols can be developed for comparison of size results from different instruments. The development of such protocols would require extensive analysis of standard, well characterized powders similar to that of interest. The analysis would have to be conducted over a broad size range on all the instruments to be used, and under conditions similar to those under which the powder of interest would be analyzed. Statistical considerations for deviations in particle shape would have to be factored into the development of such protocols.

Given these considerations, numerous users prefer to consider particle size and size distribution results as relative measurements, where results from one run can be compared with those from another, obtained on the same instrument under similar conditions. The use of particle size and size distribution data as relative measurements is very significant when monitoring size distribution as a process control indicator. In such instances the magnitude and/or direction of change is indicative of changes in the manufacturing process, which can significantly impact on the quality of the product. Often, users define acceptable limits for these fluctuations and monitor size and/or size distribution data to ensure that it lies within these limits. In these applications the precision of the measurement is of far greater interest than the accuracy. Absolute measurements can be conducted with some degree of reliability when using techniques such as microscopy-based analysis. In these cases, the particles being counted for size analysis can be visually examined. However, in such a case the reliability of the absolute measurement can be affected by the number of particles that are counted, the representative nature of the particles included in the analysis, the shape of the particles, the state of dispersion and the specimen preparation technique followed.

The use of primary and secondary standards for calibration can significantly improve the reliability of analysis. Standards for calibration can also be developed from products used by the analyst on a regular basis. In these instances it is critical to compare the material used as the standard against a primary standard to ensure tractability. The development of protocols for sample preparation, analysis and data interpretation particular to the material system and/or the instrument is a good practice. Reliable and reproducible size analysis can be conducted by thoroughly understanding the instrument manufacturer's instructions for instrument operation. However, the experience gained by the operator through experimentation with different material systems and analysis under different operating conditions is invaluable in understanding the limitations and capabilities of the instrument. A schematic flowchart of the various stages involved in size analysis is illustrated in Figure 1.

Flowchart of General Procedures Followed for Particle Size and Size Distribution Analysis

Effective communication between powder manufacturers/suppliers and customers is a very critical factor in the development of robust size measurement procedures and protocols. This communication includes information about the raw material used for powder manufacture, procedures used for powder production, instruments and instrumental parameters used for analysis, information about sample preparation and powder characteristics. Establishing mutually acceptable protocols and procedures engenders a level of trust that can drastically reduce the amount of time and resources that needs to be devoted for analysis of each powder lot. Incorporating appropriate audits  also enable tracking performance and conformity to the agreed upon protocol. Such procedures are now also required for ISO 9000 certification.