Surfactant Diffusion Rate Impact on Air Entrainment Thresholds in Curtain Coaters
Surfactant diffusion kinetics govern dynamic surface tension within millisecond curtain residence times, dictating high-speed air entrainment speed thresholds.
Coating Skip Defects
Surface Integrity
Non-uniform deposition of functional aqueous or solvent-based chemistry creates areas of bare substrate on paper or board. Coating skip defects arise during the application process when the liquid curtain or roll film fails to bridge microscopic surface irregularities or mechanical gaps. This phenomenon creates an imbalance in barrier performance, moisture sensitivity, and ink receptivity across the processed sheet.
Operational Variables
Machine direction speed fluctuations often trigger this condition when the viscosity of the fluid remains inconsistent relative to the line velocity. Air entrapment under the applicator blade also forces the fluid away from low points on the web surface, leaving islands of raw fibre exposed to the downstream environment. Proper metering settings prevent the excessive shearing that leads to irregular film splitting at the nip point.
Adjusting the contact angle of the doctor blade maintains the pressure required to force the fluid into the porous structure of the base stock, ensuring complete coverage. Production runs require precise synchronization between the pump delivery pressure and the web tension to hold the film thickness within a set tolerance of five microns. Any drift in ambient humidity alters the drying rate, which forces the fluid to pull back from certain areas before reaching the required depth of cure.
Structural Impact
Permanent loss of moisture barrier properties occurs wherever the substrate remains uncovered by the protective chemistry. Functional failure manifests as rapid water vapor transmission through the unsealed gaps, causing the packaging to lose structural rigidity in high humidity conditions. Final print quality drops because the ink absorbs differently into the raw fibres compared to the sealed sections, resulting in an uneven gloss finish.
Finished stock with these gaps presents a risk to the end user who relies on the consistent protection of a chemical layer against liquid penetration or environmental degradation.