Effect of Relative Humidity on Coating Efficiency in Nonelectrostatic and Electrostatic Coating

Abstract

Fifteen food powders were coated on aluminum targets at 0, +25, and -25 kV using corona electrostatic coating at 20% to 80% relative humidity (RH). The effect of RH on 3 losses, that is, targeting loss, coating loss, and transportation loss, which contribute to coating efficiency, was studied. RH had no effect on targeting loss in either nonelectrostatic or electrostatic coating. In nonelectrostatic coating, increasing RH increased coating loss for powders with particle size > or =297 microm, but had no effect on powders < or =227 microm. Large powders were free-flowing and clumped with increasing RH, and then rolled off the targets, resulting in high coating loss. RH had no effect on nonelectrostatic transportation loss for all powders, except for very high absolute humidity when capillary forces dominated. Electrostatic charging efficiency and powder resistivity decreased with increasing RH. Electrostatic coating loss for salts increased with increasing RH. At high RH, powder resistivity decreases, increasing the charge decay rate, which decreases electrostatic adhesion. Electrostatic coating loss for powders other than salts, whose resistivities are much higher than salts, was not affected by RH until 80% RH. Electrostatic transportation loss for powders other than proteins was not affected by RH. There was no significant difference between positive and negative electrostatic transfer efficiency and adhesion, except for transfer efficiency of soy protein and pork gelatin, whose high positive tribocharging values cause higher positive electrostatic transfer efficiency. A 20% to 60% RH is recommended for both nonelectrostatic and electrostatic coating.

The influence of particle size on separation and dustiness in powder mixtures during nonelectrostatic and electrostatic coating

“…Coarse powders fall faster than fine powders because of gravity force and miss the target, leading to separation [20]. Greater transfer efficiency for coarse powders than fine powders due to gravity force was also found by others [8,13,19]. Not only differences in targeting loss cause separation, but also the differences in adhesion loss cause separation, particularly when the mixture is coated electrostatically [15].…”

Effects of Particle Size on Mixtures During Nonelectrostatic and Electrostatic Powder Coating

“…3 and 4). As particle size increases, targeting loss decreases during nonelectrostatic coating on a conveyor belt (Xu and Barringer 2008). Coarse particles fall onto a horizontal target rather than remain in the air due to gravitational force, while fine powders are more likely to be carried away by the airflow, resulting in more coating loss of the fine powders (Ricks et al.…”

“…Coarse particles fall onto a horizontal target rather than remain in the air due to gravitational force, while fine powders are more likely to be carried away by the airflow, resulting in more coating loss of the fine powders (Ricks et al. 2002; Mayr and Barringer 2006; Xu and Barringer 2008). The higher targeting loss of fine particles caused a change in proportions on the targets in both the NaCl and starch mixtures.…”

Electrostatic Coating Technologies for Food Processing

“…Powder resistivity is also dependent on relative humidity. As the relative humidity increases, powder resistivity decreases, resulting in lower electrostatic adhesion (Xu & Barringer 2008). …”