Powder coatings, shift

Fundamental studies by reflection angle infrared spectroscopy of the bonding of EME coupling agents to metal oxides reveal a significant shift in the carbonyl absorbance band when the coupling agent is applied as a very thin layer on a metal oxide. The shift is reproducible and the extent varies with the type of oxide. These results were obtained both by use of copper mirrors and from CuzO powder coated with very thin layers of model compounds. The compounds were not removable by isopropanol, a solvent for the bulk compound. The thiol absorbances of thin layers of model compounds were also found to decrease in relative intensity with time. This illustrates that a specific chemical interaction has occurred. 

For oxidized metal spheres, there is only one absorption band, near the vlo frequency of the coating material, as shown by the example of a Mg sphere with a MgO coating [298]. This is similar to the case of an ultrathin film on a metal plane substrate (Section 3.2). It follows that the usual SSR (Section 1.8.2) may also be applied to the surfaces of metal powders (see also the discussion in Section 3.9.4). Bamickel and Wokaun [314] reported that a dielectric coating shifts the resonance frequency of a metallic particle toward the red. Applying the MGEM dielectric function, Martin [315] calculated the reflection spectrum of an ensemble of S-pm Zn spheres coated with ZnO of variable thickness. He obtained the same qualitative result as Ruppin for the isolated particle [298], that in the limiting case of the absence of the core, the absorption maximum is at the Frohlich frequency. As the thickness of the coating decreases relative to the core radius, the surface-mode frequency increases and approaches vlo monotonicaUy. 

A very successful mounting and fittings manufacturer with annual sales totaling 80m identified their powder coating system as the major bottleneck. Only three shifts could be worked, more was impossible. 

Fig. 2. An automatic, in-line powder coating system, the Autocoat, based on the fluidised bed technique. It is capable of repetitively coating 15,000 articles per 8 hr. shift in polyethylene, PVC or nylon powder. (Courtesy of Plastic Coatings Ltd.)...

The main requirements for a coating are that it is easy to apply (i.e., low viscosity) and dries to a non-tacky finish within a reasonable time. Originally, this equated to the solution and evaporation of the solvent. However, because of environmental concerns, formulations have tended to shift the concentration of solvents from 80% to less than 20% (known as high solids coatings). Many industrial processes use polymer powders (of approximately 40 pm diameter),... 

Cork powder (50—100 g L ) and sodium silicate (10—15 g L ) are generally used for the mould coat, although oflier components are sometimes included, too, e.g., carboxy methyl cellulose (CMC). The cork suspension is sprayed onto the mould surface and heated to 80—90 °C. During casting, the cork powder gradually bums away, and a new coating is usually applied once or twice a shift. 

Fig. 9. A battery of rotational lining machines enable one man to line 300 cylinders per 8 hr shift. A specially formulated self-adhesive polyethylene powder is the coating material. (Courtesy of Plastic Coatings Ltd.)... 

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