An important specification parameter in the determination of zinc oxide (ZnO) quality is the 325 mesh or 44 micron (µm) sieve test. Testing for oversized zinc oxide particles is accomplished by passing the powder through a screen that has openings precisely calibrated in microns.
Figure 1 below shows a 325 U.S. mesh sieve. Each square in the sieve will allow anything less than 325 mesh (approximately 1/600th of an inch) to pass through. For the 325 mesh test, 25 to 100 grams of ZnO is placed on the sieve and then rinsed through with a tap water hose sprayer.
Zochem ZnO is submicronic, or less than 1 micron in particle size, which means that close to 100% of the ZnO should rinse through the screen when tested. Product that is trapped on the screen is known as “retains” or oversize particulate contamination (“+325’s”).
Why is it important to have low +325 mesh retains? If retains are excessive, performance problems can occur in many applications, including:
• Rubber – oversize hard particulates may cause inclusions, voids, and tears.
• Coatings – oversize particulates cause low Hegman grind (dispersion of particles) and visual defects in thin coatings.
• USP lotions – oversize hard particulates are rough on the skin.
• USP solutions – oversize particulates might not dissolve properly.
• Electronics – oversize particulates may cause insulation breakdown or be a conduit for shorting.
• Plastics – oversize hard particulates cause visual defects, voids, and tears in a thin film.
• Ceramics – oversize particulates cause visual defects in glazes.
While not desirable, +325 mesh retains are the product of a natural reaction (zinc vapor and oxygen) and process inefficiencies that cannot be completely eliminated.
Zochem approached this issue by challenging itself to reduce +325 mesh retains. Over several years, Zochem carefully studied +325 mesh retains using a scanning electronic microscope (SEM) with x-ray elemental spectra to carefully identify the particulates, and then examined its zinc oxide manufacturing process to determine their origins. Following a lengthy series of process adjustments the ZnO benefits were immediately apparent.
By engineering one idea and type of +325 particulate at a time, Zochem has now reduced its +325 mesh retains by a factor of 10. This is unprecedented, and by any measure, a phenomenal improvement!
Because of Zochem’s targeted particulates research and development, we have achieved the lowest +325 retains of any zinc oxide producer in the world. Zochem’s +325 lab testing results are typically 0.001% retained (compared to other ZnO producers of 0.01% to 0.05% typical +325 mesh retains result).
Zochem’s low +325 mesh retains is an important part of our pledge to provide exceptional zinc oxide product quality to our customers in all industries. If you think that your process might benefit from industry-lowest +325 mesh oversize particulates call Zochem or one of our distributors for samples.
For more information on this subject please contact John Stourac at jstourac@zochem.com, (312) 813-4620.