Paint & Coatings Industry - April 2008 - (Page 50)

This is Part 1 of an informational review of milling media. The intention is to provide a periodic update of various aspects and considerations when using ceramic media in micro-milling and dispersing applications. Milling Media Review – Part 1: Bead Density his article reviews the effects of bead density. It is not intended to be an absolute guide or a scientific paper, but rather to highlight important factors and potential consequences for bead mill operation. Figure 1 | Commonly used milling media. Glass Zircon ZirStar S.G. 2.5 g/cm3 S.G. 3.8 g/cm 3 S.G. 4.6 g/cm 3 B.D. 1.5 kg/l B.D. 2.3 kg/l B.D. 2.8 kg/l T Bead Densities Today there are numerous agitator mill designs used in microgrinding and dispersion applications. These mills use an ever-expanding range of milling media. Figure 1 details some of the more common types available. The choice of correct media is critical to efficient mill operation. An important factor in this regard is the density of the media itself. The following discussion reviews some of the main considerations and effects of “bead density”. Milling Actions Grinding, or de-agglomeration, is affected by impact, pressure and shear forces between grinding media and mill parts. Depending on the mill type, one of these forces can be the dominant grinding/dispersing action (Figure 2). In each situation there is a critical energy required to break the target particles. This “Stress Energy” (SE) is directly proportional to the bead density. Zirconia Steel S.G. 6.0 g/cm 3 S.G. 7.2 g/cm 3 B.D. 3.8 kg/l B.D. 4.3 kg/l SE = ρ.V2.d3, where ρ= bead density, V = rotational speed and d = bead diameter Figure 2 | Forces between grinding media and mill parts. Impact Ball: Ball Shear Ball: Ball Pressure Ball: Ball Impact Ball: Wall Pressure Ball: Wall Shear Ball: Wall An additional consideration is the number of critical incidents that occur. This is known as the “Stress Number” (SN). The Specific Milling Energy (Esp) needed to produce a certain mass of product is the energy supplied to the mill, related to the mass of solid particles processed. It is also proportional to the product of SE and SN.1 Esp ∞ SE.SN.EM.FE, where EM = energy supplied, FE = efficiency factor Figure 3 | Bead acceleration. Bead Effects Beads are accelerated by rotation of the machine’s discs or pegs, and the rate of acceleration (radial/ tangential) is directly proportional to the density of the bead (Figure 3). Additionally, beads encounter drag effects from the product itself, e.g., liquid density, viscosity and flow rate. The bead resistance to this drag is critical to the efficiency By Dr. Paul Hassall, Saint-Gobain ZirPro | SEPR, Le Pontet, France 50 APRIL 2008 | W W W . P C I M A G . C O M http://WWW.PCIMAG.COM

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Paint & Coatings Industry - April 2008 Contents Viewpoint Industry News Calendar of Events Company News Mergers and Acquisitions Names in the News Snapshots of Success – Automotive Applications Designing UV-Curable Materials for High-Temperature Optical Fiber Applications The Future of UV Coatings is Now! Milling Media Review Part 1 – Bead Density Effect Preparation and Properties of UV-Curable Polyurethane Acrylate Resins for Metal Surfaces Conquering Color Harmony High-Performance PU Crosslinkers for Two-Component Waterborne Applications Decorative Film Laminates How Long Will the Gilding Glitter? Gaining a Competitive Advantage Using Benchmarking Supplier Showcase Materials Watch Products and Literature Classifieds Advertiser Index Automotive Color Trends New England Museum Solves Century-Old Vanderbilt Mansion Mystery

Paint & Coatings Industry - April 2008

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