What do you do when you want to have more than one functional layer on a given substrate? Oh yeah, and you want to lay each layer down at the same time… Welcome to simultaneous multilayer coating! A functional technique via slot die coating technology.
Let’s say you would like to have an optical film that enhances viewing, brightens the display, reduces glare, and doesn’t have static when being handled as a film. Usually one coating would not be capable of providing all this functionality in one coated liquid. In the past that would mean multiple passes through a coating station, multiple curing steps, loss of film with winding and rewinding, contamination from multiple process steps, and scrap due to handling. I have listed 4 functional fluids in the example above and with the right chemistry, the right process, and the right technical design you can find a solution to this quandary.
The only caveat with coating multiple fluids at the same time is that the technology must be pre-metered. The fluids are fed through separate channels and combine only after they have left the slot die cavity. Internal combination of these low viscosity fluids would lead to turbulent mixing. A common version of multilayer slot die coating is referred to as a slide die. Slide dies are manifolds stacked one on top of another so the fluid flows up and out for the slot cavity, then “slides” down the face of the die to the next cavity where the fluids stack and stack and stack until cascading onto the substrate below.
Slide coating is very familiar to those from the photographic film industry, but multilayer coating need not go the way of the analog camera. Coating multiple fluids at once can have significant advantages if applied properly. With these great advantages also come great considerations: complex rheology and flow dynamics, interlayer adhesion and surface tension, possible diffusion, edge effects, and fluid substrate interaction. Depending on the flow calculations of the fluid, curved or chamfered lip geometries may be required to reduce eddies from forming in the layered fluids. Vortices can cause mixing, which is not wanted in multilayer slide coating. If you thought product and process development engineers needed to iterate for a standard project – you haven’t seen anything yet! Developing the viscosity and surface tension combination that works appropriately for the separation lines of equipment and the combining fluids can take time and patience.
Another area of interest is coating two sides of the same substrate at the same time. This is a process technique that requires at least one TWOSD technique in combination with a traditional or advanced coating technique for the first layer. The trick is not limited to coating however, as the curing of a double-sided substrate is more complex than it seems. Whether you want to develop multi-functional coatings or dual-side products, it’s time to find a partner and work on the chemistry to get it right.