For many years, the standard for measuring particle size in the paint industry has been the Hegman gage or similar instruments. The problem with these instruments is that they are subjective, and readings from one person to the next can vary widely. These instruments also only give you information concerning the largest particles in the batch and very little information concerning the mean particle size.

For analyzing particle size, the latest arrival in the marketplace is the HPAS-2000. This system uses a standard light microscope, digital camera, capture card and Hockmeyer Equipment Corp.'s proprietary MicroPart software for accurate particle distribution information. The HPAS technology is not new technology. This technology has been used by companies who supply the market with standards for calibrating laser particle analyzers and by law-enforcement agencies for use in fingerprint, hair and fiber analysis. In law enforcement, the repeatability of a system is very important.

Image analysis is used for QC in the automotive industry, steel industry and many other industries in the world. The reason this technology has not been more widely used is because computer technology and digital imaging technology have been too expensive to make these types of instruments a viable resource. Now that the price of the faster computers and the megapixel digital cameras are in a range that most companies can afford, this type of system has found its place in the particle size analysis market.

The HPAS system is simple to operate and does not have some of the drawbacks the other systems have. It works with much lower dilution ratios than the other systems, usually 100:1 or less. In some cases, no dilution at all is required. Simply apply the full strength sample to a slide with a thin film applicator. If dilution is required, use solvents or resins found in the formula of the product being analyzed. The use of resins is encouraged in the dilution process to increase viscosity and slow evaporation of the sample while at the same time reducing the risk of shocking the formula and causing re-agglomeration.

The HPAS system has the ability to measure any type of pigment or particle, including carbon black and transparent particles. It has algorithms that, when enabled in the parameters, will enhance the edges of any transparent particle, allowing the particle to be analyzed as one particle instead of multiple particles. The system can analyze dry powders right out of the bag by simply mixing the powder with a compatible liquid to separate the loose agglomerates. In most cases, a 100:1 dilution of dry powder and vegetable oil works well.

Cleanup of the system after an analysis means putting the slide in the trash. The changeover between dry powders and liquid dispersions is once again putting the slide in the trash; this means you can track any dispersion from the bag to the finished packaging with only minimal waste.

Other Size-Measurement Tools

In the past, particle size analysis has been achieved by several other types of measurement tools. One such method, the sieve method, uses a given volume of product washed through a screen. The remaining material on the screen is then weighed to determine if an acceptable grind has been achieved. This type of measurement takes a long time to accomplish and generates significant waste material that will eventually have to be reclaimed or disposed.

There are sedimentary-type instruments on the market that separate the different particle sizes by the use of gravity. The largest particles settle first then the next size and so on. This type of measurement is time consuming, and the product is diluted to decrease the settling time, which generates waste.

Additionally, there are laser or light-scattering type systems. These systems use lasers and receptors to determine the volume of particles as they are ultrasonically bombarded and pumped past the laser beam. The reason for using the ultrasonic step in the analysis is to keep the particles separated during the analysis process.

The problem with this step is that, in most cases, by ultrasonically bombarding the pigments in their very highly diluted state, agglomerates that were not broken down in the milling or dispersing process break down. This produces a false reading showing a smaller particle size distribution. If this step were removed from the analysis process, the highly diluted particles would re-agglomerate. The reason for the re-agglomeration is that by diluting at such high ratios with water or solvent, most of the resin in the sample is washed off the pigment particles. In doing so, this exposes the positive and negative charges found on each particle, allowing flocculation to occur. This will give a false reading showing a larger particle size distribution.

Many manufacturers who have used laser systems have indicated that they have difficulty with transparent particles producing a phenomenon called backscatter. This is where the light passes through the particle, instead of reflecting off the particle, and then splits in many different directions hitting many receptors at the same time causing false readings. There are also special systems for measuring carbon black, which absorbs light. Laser systems are expensive and very sensitive to changes in variables such as diluents, refractive indices and operator interface, which make the repeatability of these systems questionable.

HPAS-2000

The HPAS-2000 is very simple in both concept and use. It uses a standard light microscope fitted with a black and white or color digital camera and a black and white or color capture card to give the operator live video of the sample on the slide (within the visible wave length of light). The operator adjusts the light intensity and focus on the microscope and freezes the image; once the image is frozen the analysis begins.

Figure 1 shows carbon black pigment. At the top right is the mean monitor. This chart monitors the mean particle size vs. the number of images used to generate the histogram. As more images are accumulated, the mean particle size is graphed and when there is no significant change in the graph, you have a representative sample of the batch. During the time the image is live, there is a light meter displayed on the right-hand side of the image, allowing the operator to adjust the light intensity of each image while it is live. This light meter is used to control the light intensity or to keep the light intensity the same from the first image to the last image of the analysis. When the "Freeze" button in the upper left hand corner of the screen is clicked, the screen image is frozen and ready for analysis.

The next step is setting the threshold (see Figure 2). By setting the threshold, the operator is telling the computer which pixels in the image are particles and which pixels are background light. This is accomplished by dividing the gray scale into 255 divisions. Zero on the gray scale is Black, 255 on the gray scale is White. By setting the threshold the operator is telling the computer, for example, that the pixels to be measured are between 0 and 175 on the gray scale, and the background light is all the pixels between 176 and 255 on the gray scale. Figure 2 shows that the software is asking the operator to set the threshold. Once the threshold is set, the operator clicks on the OK button and the computer takes over for the segmentation step.

The segmentation step, depicted in Figure 3, is where all of the pixels that represent particles are segmented or separated from each other by overlaying each particle with its own color. This identifies individual particles. Once each particle has been identified, the computer uses a pre-calibrated measurement to give each particle a projected area. The projected area of each particle is used to give an equivalent diameter. The equivalent diameter is then plotted in a histogram showing number of particles on the Y axis and the equivalent diameter on the X axis of the histogram.

These steps are repeated and the data is accumulated from multiple snapshots to produce one histogram. After each snapshot, the data is averaged with all of the previously collected data to update the histogram. This may all sound complicated but, to the operator, it is nothing more than moving the slide around on the microscope, freezing the image and hitting the analyze button - the computer does the rest. Once multiple snapshots have been accumulated, the mean particle size and the standard deviation have become stable numbers, the histogram is titled, and the data is dumped into a Microsoft(r) Excel spreadsheet. This allows the data to be viewed by anyone in the company who has Excel on their computer. In addition, any of the snapshots can be saved as .tiff files to be viewed on most image viewing programs or web browsers.

Figure 4 is an example of a comparison between a standard and a sample from a batch. The latest version of the software gives the operator the ability to save a standard and recall it for comparison to the batch in process. The summary box in the upper left corner shows the mean and standard deviation measurements for the number, area and volume distributions. It also contains information on number of particles measured and number of images taken to accumulate those particles. The settings box holds the key to the repeatability of the HPAS-2000 system containing every variable used to analyze the product.

The operator is supplied with all the same data on the log scale. We also give percentiles for number, area and volume distributions (Figure 5).

The ability to view live images, collect size data and store this information in standard operating software such as Excel is very important to any company that is interested in particle size distribution. For a company that still uses the Hegman or equivalent gages, this technology allows them to predict physical properties of their products by using information generated on the entire particle distribution (within the range of visible light) instead of just the largest particle size found on the grind gage.

In a QC environment, this means that the operator can make a slide, measure the particle distribution, and compare it to a saved standard by having the standard histogram overlaid on the batch histogram and determine whether the dispersion process should continue or whether the process was finished. This streamlined QC process can replace steps such as reading the batch sample on the grind gage; making the sample into a finished product by adding additional letdown resin and solvent; doing a drawdown on opacity charts beside the standard; drying the charts in an oven or under special lights to cure the drawdown; and reading the drawdown with gloss meters, colorimeters, or other instruments to determine if the batch is up to the specification of the standard.

This process may have to occur 2-3 times during the milling or dispersion process because the grind gage indicates that the grind has been achieved but the color development, transparency, gloss, or opacity have not yet been achieved. With the HPAS system, when the mean particle size has reached that of the standard all of the other physical properties of the batch should match those of the standard. This will allow the milling or dispersion operator to know whether to continue processing the batch within about five minutes of taking the sample. Other systems require as much as an hour or more to make the same determination. In addition to improved QC, the company will receive a tremendous savings in time for the QC lab and for the operators on the floor.

Another way the HPAS system can save time is by checking incoming raw materials. The QC lab could get a pre-shipment of a given pigment, mill it to the specification of the standard, and check the sample against the standard for all the physical properties needed. The customer can then decide whether that particular lot of pigment has the capability to reach the standard. If not, the customer can request another lot of pigment. The customer will be able to e-mail images and Excel spreadsheets showing the manufacturer that the pigment was milled or dispersed to the same average particle size as the standard but was weak in color or did not match some of the other physical characteristics of the standard.

The HPAS system can be networked. This means that one company with multiple plants can have one standard for a given product, instead of each plant having its own standard for that particular product. Some of our customers with multiple systems have developed a database of standard histograms. When a particular batch is being made, the QC department pulls up the standard for that product from the database. By using the same standard histogram for all of the different locations, one of the variables of the dispersion and milling process- particle distribution - is removed.

Each HPAS system is a turnkey system that includes the microscope, camera, computer, capture card, Hockmeyer's MicroPart(r) software, monitor, keyboard and printer. The operating system is Microsoft Windows NT(r) and Office 2000(r) software, PC Anywhere(tm) from Symantec(tm), and other software are included. The HPAS system is designed to be a particle size analyzer but it is also serves as a powerful desktop computer to be used for any number of daily tasks when not being used as a particle analysis system.

In addition, with the use of PC Anywhere, technical support and training is just a click away. The computer comes with a network card installed so that all the customer needs to do is connect to the network, go online and connect to Hockmeyer's technical support staff. This saves the customer time and money on training and technical support, allowing it to take place on the spot without travel costs.

For more information on particle size analysis, contact Hockmeyer Equipment Corp., 6 Kitty Hawk Lane, Elizabeth City, NC 27909; phone 252/338.4705; fax 252/338.6540; visit www.hockmeyer.com; or e-mail sales@hockmeyer.com.