Operating window of solution casting, part I: Newtonian fluids

Abstract

An experimental study was carried out to investigate the fluid mechanics of solution casting. Newtonian glycerol solutions were used as test fluids and were casted on the polyethylene terephthalate films. The focus of this study is on the evaluation of the operating window, i.e., a region for stable and uniform processing. Different types of defects such as stable or unstable poolings, vibrating edges, and air entrainment outside the operating windows were observed. The effects of two different start-up approaches on the operating window were studied. One of the key operating parameters is the maximum casting velocity V max for stable operation. The fluid viscosity is the most critical parameter on V max . It was found that if the gap between the slot die exit and the moving film substrate is smaller than 600 lm, V max might go down and then go up as the fluid viscosity increases. On the other hand, if the gap is larger than 600 lm, V max will decrease as the fluid viscosity increases. Competition of two different types of defects, i.e., unstable pooling and air entrainment, can decide V max, as evidenced by the observation on the fluid motion and dynamic contact angles of different cases. The importance of different forces on V max was also analyzed.

Coupled fluid‐particle modeling of a slot die coating system

“…The coating gap is usually 2–10 times final film thickness, and the coating liquid fills the coating gap under the downstream side . Geometrical parameters used in the simulation of slot die coating process are taken from previous experimental studies and presented in Table together with the particle characteristics. Two slot dies with geometrical parameters presented in Table are simulated (case A and case B), to study the effect of feed slot gap/coating gap ratio.…”

“…There are a number of experimental studies on the flow behavior in slot dies . Glycerin/water solution has been widely used in experiments as a Newtonian test fluid for coating, and the operating window for this coating solution is defined in several studies . Experimentally, the flow visualization techniques are used to study the shape and contact angles of upstream and downstream meniscus; just before and after the coating defects appear on the coating beam …”

Operating window of solution casting. II. Non‐Newtonian fluids

“…The slot die used in this study was the same as that used in a previous work . The casting solution emanated from the slot die and was deposited on a moving poly(ethylene terephthalate) substrate.…”

“…Huang et al studied the fluid mechanics of Newtonian fluids in solution casting. They determined the stable operating windows in terms of the maximum operating speed as functions of the material properties and operating variables.…”