There are a number of industries where clean coating is required – medical, pharmaceutical, optical, printed electronics and energy storage devices to name a few. There are a number of areas that a converting and coating manufacturer needs to concentrate on to develop a clean product. These include the raw materials (substrate and fluid), the backing roll, the coating head, but most importantly the fluid delivery system.
When you are concerned about cleanliness of a finished product, the best place to start is at the beginning. Make sure the dirt and debris does not even start in the system. This means making sure the fluid entering the delivery system is clean. This may mean raw material inspection or possibly filtering the fluid prior to coating. But making sure the fluid delivery system has clean fluid at the beginning of the process is not everything that needs to be considered.
At its most basic, the fluid delivery system has a tank for holding the fluid and a pump that feeds the coating head. Additional fluid delivery system features outside of the tank and pump can include agitation for the tank, filtering after the pump, de-gassing, temperature control, online injection and static mixers prior to the coating head. With each additional connection comes an opportunity for debris or air to infiltrate the system and cause coating defects.
The trick is to have the least number of connections along the tank to coating head path while providing the functionality required for the fluid handling. Air can enter the system at a seal and cause bubble defects that can disrupt coating or cause coating defects. Once air is in the system, it is difficult to remove. There are three main ways to remove gas if air is present: lower the total pressure, raise the fluid temperature (to increase the vapor pressure and have the air rise faster) or avoid spaces where air can enter the system.
So once air is in the system, how can they be removed? This is described by Stoke’s law-
VB = bubble rise velocity
d = bubble diameter
g = acceleration of gravity
ρ1 = liquid density
ρg = gas density
μ = liquid viscosity
Where the time required for a bubble to be removed by rising through the fluid is defined by-
t = time
H = liquid thickness
What these equations tell us, is that a larger diameter bubble releases faster and smaller bubbles take longer to escape the system. In addition, a larger vessel with these smaller bubbles (especially in high viscosity liquids) will require the longest times of all. So to encourage bubble removal, spread the fluid into a thin film. Casting the fluid over an incline plane will encourage bubble removal and allow for cleaner coating.
In addition to air, general cleanliness is still a concern. Whether it is a concern with cross contamination or lot-to-lot repeatability, cleaning the fluid delivery system is critical to clean coating. Utilizing sanitary fittings and avoiding crevices and welded joints will help maintain clean surfaces.
Commonly, operators prefer to shear particles off surfaces instead of utilizing mechanical force. To properly shear particles off with a fluid, the fluid should have polar or non-polar characteristics to match the chemical character of the particle to be sheared. In all cases, the time to clean the fluid delivery system should be minimized to maximize operational up time.
When coating products for clean industries, keep these concepts in mind. Fluid preparation and delivery can enhance your coating process if proper measures are taken up front. Dedicating fluid delivery systems for a specific product will also save valuable production time.
Mark’s first introduction to the world of slot die coating was in 1996, when he was learning the technology at 3M. Today, Mark and Tim Marion own and operate Coating Tech Slot Dies, which supplies precision coating equipment. In addition, CTSD provides education and consultation for improved coating efficiencies and increased coating capability. Mark is a contributing editor to several industry trade journals and a frequent technical presenter at both domestic and international industry events.