Paint & Coatings Industry - March 2008 - (Page 64)

From Rocks to Nanos ver many years the continuous development of wet grinding mills has resulted in finer and finer particle size reduction with lower process times. It is well established that small-diameter grinding media (beads) produce finer product particle sizes. Bead sizes down to 0.05 mm are now available. The inherent high-density ceramic material used results in smaller media with higher kinetic energy potential. However, these can only be used under certain conditions. The bead mill mechanical specification has to be engineered correctly, with particular reference to the separator type (gap/screen) and other rotating/ Figure 1 | PSD history by time. O 4.0 Volume Frequency Percent 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 0.0 0.1 0.2 0.4 0.5 Particle Diameter (μm) 80 20 40 60 Sample stationary element tolerances (seals, bead eliminators, grinding zone limits, etc.). Product viscosity is also an important factor as small beads typically have less mass, which in turn can result in a fall in bead velocity under certain rheological conditions. Most horizontal/vertical mill mechanical aspects within the grinding chamber are not suitable for maintaining constant bead velocity; the grinding zones are typically too far apart. Most horizontal mills are unable to accommodate very small beads for prolonged usage, as eventual mechanical break down (bead entry into separators, seal housing, etc.) and separator blockage will occur. In addition, most bead mills generate the necessary grinding action in a fairly random manner, and cylinders must be packed with a large volume of beads. Generally, horizontal mills give unstable grinding media control as there is a tendency for the media to move continually toward the separator/outlet. In the case of a vertical mill, there is an uneven bead packing density, heavier at the bottom and lighter at the top. In order to maximize the efficiency of these mills, a media charge of up to 70–90% is required. Of course, efficient separation of the product and grinding media is essential, and large volumes of media will result in high process temperatures and pressures, separator damage, and mill wear (cylinders and agitators). The agitation, by either discs or pegs, imparts shear and attrition forces to the bead mass, which in turn transfers some of this energy into the grinding action. However, most of this energy is lost in frictional heat build-up and it is necessary to incorporate an efficient cooling system. Controlling and managing this otherwise random bead action becomes paramount in order to use the energy being consumed more efficiently. Whilst most mills will produce, in time, a dispersion where the majority of particles fall into the By John Allen, Manager Director | CDC Microtron Ltd., Warrington, Cheshire, U.K. 64 MARCH 2008 | W W W . P C I M A G . C O M http://www.pcimag.com

Table of Contents Feed for the Digital Edition of Paint & Coatings Industry - March 2008

Viewpoint Industry News Calendar of Events Company News Mergers and Acquisitions Names in the News Marketplace Quarterly Stock Watch Reaching Strict VOC Requirements With Outstanding Durability for Industrial Maintenance and Marine Coatings A Unique Fungicide for Wood-Filled Plastic deSigns of the Times: Slacking Off! From Rocks to Nanos Shear-Free Dispersion Process Fluorinated Oxetane Oligomers as Versatile Intermediates for Polymer Modification The Effects of TPO Composition on Adhesion and Proposed Chemical Mechanism Supplier Showcase Color Management Materials Watch Products Classifieds Advertiser Index

Paint & Coatings Industry - March 2008

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