Types of Forces. Importance of All Forces Decreases with Size

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1 Types of Forces Volume / Mass: Gravity Volume d 3 Surface: Friction Surface Energy Van der Waals Charge capacity Surface d 2 Importance of All Forces Decreases with Size Volume forces Surface Forces 1 meter 1 micron Size (meters) California Polytechnic State University 1

2 Balance of Forces Different Between the Macro and Micro Surface Volume Volume Surface Macro - 1 meter Micro - 1 micron Important Properties of Small Particles Particle size and shape Surface Area Surface Energy Friction and Bulk Flow Packing and Density Surface Charge Interaction with light Absorption Scattering California Polytechnic State University 2

3 Particle Size and Shape Discussion will include particles with sizes from the tenths to tens of microns. Three distinct particle shapes: Needle / acicular Flat / platy Round / blocky Surface area Depends on particle size and shape. Increases linearly with particle size decrease. Typical values as a function of size: Area (m 2 /g) * 1 mm microns micron 3 10 nm 300 * Assumes density of 2 g/cc California Polytechnic State University 3

4 Surface Areas are Additive For mixtures of two different sized particles, surface area is simply the weighted average of the two. Example: Ti 2 with 3% Al 2 3 SA Ti 2 = 6 m 2 /g SA Al 2 3 = 300 m 2 /g SA Mixture = (0.97) 6 + (0.03) 300 = 14.8 m 2 /g rigin of Surface Energy California Polytechnic State University 4

5 Magnitude of Surface Energy Depends on two things: Amount of surface Chemical state of the surface atoms Types of Chemical Interactions Strongest Covalent Hydrogen H Weakest Van der Waals California Polytechnic State University 5

6 Consequences of Surface Energy Particles are attracted to other surfaces. Particles stick together Particles stick to bin walls, etc. Particles are difficult to disperse in liquids Van der Waal s force equation: F attraction W = Constant ( r1 r2 ) ( ) r1 + 2 = ( W ) 2 L r r 1 r 2 L Friction and Bulk Flow Friction is a surface phenomenon: proportional to amount of contact between two surfaces (i. e. surface area). proportional to strength of contact between two surfaces (i. e. surface energy). Friction is a very significant force for small particles Because of friction, the flow of particles (moving particles past one another) is difficult. California Polytechnic State University 6

7 Friction between Small Particles Small particles: are very light have high surface areas ne particle can hold up 1,000,000 others! Wall Pigment Bed Particle Packing Depends on particle shape Round particles pack the most densely acicular (rod-like) particles pack least densely Depends on particle size Large particles pack the most densely Small particles pack least densely California Polytechnic State University 7

8 Effect of Particle Shape on Packing Effect of Particle Size on Packing Material Particle Size True Density Bulk Density Void Fraction Si 2 Sand 1,000 μ 2.6 g/cc 1.6 g/cc 38% Ti μ 4.1 g/cc 0.8 g/cc 80% fumed Si μ 2.2 g/cc 0.05 g/cc 98% Packing Void Fraction 100% 80% 60% 40% 20% 0% Particle Size (microns) California Polytechnic State University 8

9 + Formation of Surface Charge in Slurries - H + H H + H H H2 Acid (Low ph) Positive Charge Base (High ph) Negative Charge Surface Potential (mv) 60.0 Acidic Solid 40.0 Ti 2 Zn Basic Solid 20.0 ph California Polytechnic State University 9

10 Some Solids are Both Acidic and Basic - - Si Si Si Si H H H2 + Al Al Al Al Si H Si H 2 + Si H Si - - Some Solids are Both Acidic and Basic Negative Faces Positive Edges California Polytechnic State University 10

11 Kaolinite House of Cards Size and Shape of Fillers & Pigments Material Size, μm Shape Titanium dioxide Calcium Carbonate Ground Precipitate Clays Ground Calcined Silica Diatomaceous Talc Mica block block block, aggregates plate plate irregular plate and fiber flake California Polytechnic State University 11

12 Titanium Dioxide Pigments Chalking: loose pigment particles form on the surface from the erosion of the binder as a result of photodegradation. Type I Anatase Type II Rutile Type III Rutile Type IV Rutile Product Name Ti 2 min.% Chalking Surface treatment LW 94 free none R medium resistant Al 2 3 R-900, R medium resistant Si 2 +Al 2 3 R medium resistant Si 2 +Al 2 3 Complete encapsulation to protect Ti2 from UV free radical reaction Effect of Filler & Binder on Gloss 60 Degree Gloss Dolomite / CaC3 BaS4 / CaC3 Mica / CaC3 Talc / CaC3 Kaogloss 90 / CaC3 EG-44 / CaC3 Control - CaC3 Epoxy / Cymel Urethane Acrylic Acrylic Vinyl Acrylic PVAc Control - EVCL Gloss Value California Polytechnic State University 12

13 Effect of Filler & Binder on Gloss 85 Degree Gloss Dolomite / CaC3 BaS4 / CaC3 Mica / CaC3 Talc / CaC3 Kaogloss 90 / CaC3 EG-44 / CaC3 Control - CaC3 Epoxy / Cymel Urethane Acrylic Acrylic Vinyl Acrylic PVAc Control - EVCL Gloss Value California Polytechnic State University 13