Dynamic Surface Tension Measurement in Liquid Curtain Coating Systems
Dynamic surface tension at millisecond surface ages dictates liquid curtain stability and dynamic wetting limits in paperboard coating operations.
Liquid Curtain Coating
Premetered Deposition
Non-contact methods for applying fluid layers to a moving substrate involve creating a continuous falling sheet of material. In liquid curtain coating, the fluid is pumped through a die and allowed to fall under gravity until it hits the web below. This technique allows for the simultaneous application of multiple layers without the use of blades or rollers.
Dynamic Stability
The integrity of the falling curtain depends on the balance between the flow rate and the physical properties of the liquid. Surface tension and viscosity must be controlled to prevent the curtain from breaking into droplets or oscillating. If the web is moving faster than the curtain, the liquid is stretched upon impact, which can improve the uniformity of the layer.
This process is highly sensitive to air currents and vibrations, which can cause ripples in the falling sheet. Engineers use enclosures and vacuum boxes to stabilize the air around the application zone.
Layer Uniformity
Accuracy in the thickness of the coating is achieved by controlling the pump speed and the width of the die gap. Because the process is premetered, all the fluid that leaves the die is deposited on the paper, resulting in very low waste. This method is particularly effective for applying functional barriers to corrugated board or recycled paper.
High-quality finishes are produced by maintaining a steady flow of liquid throughout the production run.