The world has gone patch coating crazy! Batteries, pharmaceuticals, even adhesives. The interest in reducing waste and providing functional coating that can be used right off the coating station is very high. With this increased interest in intermittent coating, we also have to consider the technical challenges.
Every coating process has a start and stop. In patch coating, this occurs more often than at the beginning and end of a roll. Of course the steady-state continuous flow in between starting and stopping has been discussed at length and has its own issues to deal with. But what special issues are associated with the start and stop of a coating head? For this discussion, we will concentrate on slot die patch coating. Any shape can be coated-as long as it is a rectangle!
In the start-up and stop flow analysis, the challenge is to reduce waste and defects associated with the transition from fluid flow to not. The considerations of start-up include wetting of the fluid on the substrate, pump control of fluid dynamics, and physical position of the coating equipment to the substrate.
- Wetting: The ability of the fluid to displace air at the fluid/solid interface in coating is critical to reducing defects. If the surface tension of the fluid and the surface energy of the substrate are not compatible, the start of the coating bead may be delayed or jagged. This would lead to an improper “head’ of the coated patch. Surface energy modification may be required to produce a solid patch.
- Pump Control: There are many techniques for intermittent coating, with the most widespread technique using valve control of the fluid and physical movement of the coating head. The valve acts as the immediate start/stop of fluid flow, while the mechanical movement of the coating head breaks the wetted bead. The timing of the valve control with the mechanical movement can produce a good head or a poor start that includes poor bridging of the fluid, streaks, or a heavy “head.”
- Physical Position: Where the coating head sits in relation to the substrate can determine the output flow of the fluid “head.” As a liquid surface approaches a substrate, the liquid boundary layer has vapor molecules that begin to adsorb to the substrate surface. This adsorption forms a bridge when the concentration is high enough. If the bridge occurs with a concentration that is even across the coating width, the coating will be uniform for the “head” or beginning of the coated patch. If the concentrated vapor is too far away, the substrate surface too rough, or the concentration of the fluid fluctuates, the coating will create a curved coating “head” to the patch.
The considerations of stopping are similar to start-up, but need to consider fluid reaction to mechanical and rheologic behavior. The resultant “tail” of the coated patch are a function of the timing between the valve for fluid control, mechanical movement of the coating head, and wettability of the substrate by the fluid.
There is not just one way to coat patches of fluid onto a substrate, but as long as you are aware of the coating fundamentals, you will be successful in your intermittent coating operation.