| Summary: | 博士 === 國立清華大學 === 化學工程學系 === 101 === In this research, the effect of hydrodynamics, rheology and polymer structures on the solvent casting of Newtonian and non-Newtonian fluids have been investigated. The main focus of the present study is on the evaluation of the operating window, i.e., a region for stable and uniform processing. The experimental aqueous solutions were Newtonian fluids, such as glycerin and corn syrup with viscosities of 110~10500 mPa•s and Non-Newtionian fluids, such as polyacryamid (PAA) and carboxymethyl cellulose (CMC).
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 Vmax for stable operation. The fluid viscosity is the most critical parameter on Vmax. It was found that if the gap between the slot die exit and the moving film substrate is smaller than 600μm, Vmax may go down and then go up as the fluid viscosity increases. On the other hand, if the gap is larger than 600μm, Vmax will decrease as the fluid viscosity increases. It found that casting defects of glycerin solutions were determined by critical maxium capillary number Camax(=μVmax/σ, Vmax is maxium casting speed, μ is viscosity, σ is surface tension). If Camax>1, the defect of casting is air entrainment, otherwise is unstable pooling. Competition of two different types of defects, i.e., unstable pooling and air entrainment can decide Vmax, as evidenced by the observation on the fluid motion and dynamic contact angles of different cases. The importance of different forces on Vmax was also analyzed.
The surrface tension of corn syrup solution is 72 mN/m more than 63 mN/m of glycerin. According to other literatures, lower capillary number can improve the Vmax . We found the same result and critical Camax with glycerin solutions.
We also investigated PAA of Boger fluids (elastic fluid with the constant viscosity) to understand the effect of elasticity on solvent casting process. It shows that the operating windows and Vmax of non-Newtonian solutions are higher than the Newtonian fluids. Especially, in high viscous force (Camax>1), the Vmax could raise significantly, because the elastic force offsets the viscous force. But higher elastic force will reduce the stability of coating beads.
The CMC of viscoelastic fluids have shear thinning and elasticity to affect the solvent casting process. Because of shear thinning, the Vmax of CMC solutions are higher than PAA. However, there are similar critical capillary numbers as PAA solution. If Camax>1, the Vmax could increase greatly, and decrase as the fluid elasticity increase. We also studied on the effect of polymer structures such as difference degree of substitution (DS) and molecular weight.
|
|---|
