Principles of Coating

During the application of a coating, mechanical forces of various types are exerted. The amount of shear force directly affects the viscosity value for non-Newtonian fluids. Most coatings are subject to some degree of shear thinning when worked by mixing. As shear rate is increased, the viscosity drops, in some cases dramatically. 

As indicated, viscosity, the resistance to flow, is the key property describing the behavior of liquids subjected to forces such as mixing. Viscosity is simply the ratio of shear stress to the shear rate  

Thin or low viscosity liquids flow easily while high viscosity liquids move with considerable resistance. In the ideal or Newtonian case, viscosity is constant over any region of shear. However, very few liquids are truly Newtonian. Most liquids drop in viscosity as shear work is applied. This phenomena is known as shear thinning. A liquid can be affected by the amount of time that force is applied. A shear thinned liquid will tend to return to its initial viscosity over a period of time. Therefore, if viscosity is to be reported accurately, the time under shearing action and the time at rest must also be noted. 

The effect on the viscosity of a fluid varies from fluid to fluid as force is applied. These different effects are described below. 

Plastic fluids behave more like plastic solids until a specific minimum force is applied to overcome the yield point. Gels and ketchup are extreme examples. Once the yield point has been reached, the liquids begin to approach Newtonian behavior as shear rate is increased. Although plastic behavior is of no benefit to ketchup, it has some beneflt in paints. Actually, it is the yield point phenomenon that is of practical value as illustrated by no-drip paints. When the brush stroke force has been removed the paint s viscosity builds quickly until the flow stops. Dripping is prevented because the yield point exceeds the force of gravity. 

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Abstract The principles of coatings to either enhance reflectivity of mirrors or to enhance transmission of glass optics are described. Then the ion assisted deposition and ion beam sputtering techniques are addressed. Performances of these technique-sand their limitations are illustrated with the characteristics of the VIRGO mirrors coated at LMA. The importance of metrology is emphasized. 

Basic principles of coating thickness measurement with quartz oscillators... 

Principle of Coating in Turbine/Drum/Screw (Endless) or in Eluid... 

Fig. 7.9 Principle of coating color application (source Omya).

Coates, G. E. Green, M. L. H. Powell, P. Wade, K. 1977, Principles of OrganometalUc Chemistry, Chapman Hall London... 

The special design of the Latham bowl allows for a specific blood cell separation known as SURGE. This technique makes use of the principle of critical velocity. The Latham bowl is filled until the huffy coat, ie, layer of platelets and white cells, moves in front of the bowl optics. At this point the machine starts to recirculate plasma through the bowl at increasing rates. The smallest particles, ie, platelets, ate the first to leave the bowl. Their high number causes the effluent line to turn foggy. The optical density of the fluid in the effluent line is monitored by the line sensor. A special algorithm then determines when to open and close the appropriate valves, as well as the optimum recirculation rate. 

The principle of the safety match is also used in the pull-wire fuse lighter used to start a fuse train for the ignition of fireworks items or more frequently for blasting work. This is a reversed pull match whereby the striker material is coated on a pull wire, and the match head material is within a small metal cup in a cardboard tube. Pulling the coated wire vigorously out of the device ignites the match mixture in the tube for fire transfer to the tubular fuse train. 

Some of the principles of hot dipping are common to galvanising, tinning, terne coating and aluminising. 

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