ADHESION OF FOOD POWDERS WITH NONELECTROSTATIC AND ELECTROSTATIC COATING

摘要

This study investigated the effects of powder resistivity, coating voltage, relative humidity (RH) and coating density on adhesion. Cocoa powder with a high resistivity (1.15 × 1013 Ωm) showed a stronger electrostatic adhesion than starch powder with a medium resistivity (2.56 × 1010 Ωm) and NaCl powder with a low resistivity (7.31 × 105 Ωm). The adhesion of starch and cocoa powders coated at 0, 40 and 95 kV increased with increasing voltage. The adhesion at 0 kV should be dominated by the van der Waals force, at 40 and 95 kV by the electrostatic image force and at high RH by capillary force. Theoretical calculations were in the correct range, but the assumptions in those calculations make them unreliable. For nonelectrostatic coating (0 kV), there was no significant change in adhesion when RH increased from 30 to 60%, while adhesion increased when RH increased from 60 to 80%. For electrostatic coating, the adhesion decreased when RH increased from 30 to 60%, but the adhesion at 80% RH was larger than the adhesion at 30 and 60% RH. For both nonelectrostatic and electrostatic coating, the adhesion force decreased as coating density increased to 1.0 mg/cm2, but there was no significant change from 1.0 mg/cm2 to 2.0 mg/cm2.