Dynamic Contact Angle Relaxation Kinetics in High Speed Blade Coating Systems
Dynamic contact angle relaxation rates dictate air entrainment and streak defect thresholds in high speed blade coating systems operating above 1200 m/min.
Dynamic Wetting Failure
Surface Interaction
Interfacial phenomenon where a liquid coating or ink fails to spread and adhere to a moving substrate. Dynamic wetting failure occurs when the speed of the application line exceeds the rate at which the liquid can displace air at the surface. This results in air entrainment and visible defects in the coating layer.
Kinetic Limitation
Rapid movement of the paper web through a coating nip creates a pressure gradient that resists liquid contact. Dynamic wetting failure is more common on non-porous or low energy surfaces like plastic films or highly sized boards. Surface tension of the liquid and the roughness of the substrate play primary roles in stabilizing the wetting line.
Adding surfactants to the coating mix can increase the critical speed by lowering the dynamic surface tension. The geometry of the application zone also dictates how the liquid air interface responds to increasing line speeds.
Production Impact
Uncoated spots and pinholes represent the most common visual evidence of this failure. These defects compromise the barrier properties of packaging and create uneven ink absorption during printing. Operators often must reduce machine speeds to regain a stable wetting front, which lowers the overall throughput of the converting line.
Precision monitoring of the web tension and the application angle helps to prevent the onset of air entrainment at high velocities.