Albert Co

Draw resonance in film casting of viscoelastic fluids: A linear stability analysis

Abstract In order to understand the role of viscoelasticity on draw resonance in the isothermal film casting process, a steady state analysis and a linear stability analysis for three-dimensional flow disturbances have been conducted. The constitutive equation used is a modified convected Maxwell model, with shear-rate dependent viscosity and fluid characteristic time. The numerical results indicate that the flow is stable below a lower critical draw ratio and above an upper critical draw ratio. Shear thinning in viscosity reduces the lower critical draw ratio and somewhat increases the upper critical draw ratio—thereby enlarging the region of instability. Slower shear reduction in fluid characteristic time dramatically decreases the upper critical draw ratio but has no significant effect on the lower critical draw ratio; therefore, fluids with higher characteristic time are more stable.

Film casting of a modified Giesekus fluid : Stability analysis

The stability of isothermal film casting of polymeric fluids to infinitesimal and finite-amplitude disturbances was investigated using a modified Giesekus rheological model. The linear stability analysis indicates that extensional-thickening elongational viscosity enhances stability, whereas extensional-thinning elongational viscosity or shear-thinning shear viscosity reduces stability. An upper critical draw ratio is present when a sharp rise exists in the elongational viscosity curve. The stability limits predicted by the linear stability analysis for infinitesimal disturbances were validated by the nonlinear stability analysis for finite-amplitude disturbances. Also, the growth rate and frequency of oscillation predicted by the linear stability analysis were found to agree with those predicted by the nonlinear stability analysis for the initial growth of the disturbances. The results from the nonlinear stability analysis showed that just beyond the critical draw ratio, the sustained disturbance was sinusoidal. However, further beyond the critical draw ratio, the sustained disturbance consisted of narrow, sharp peaks alternating with wide, thin troughs.

Film casting of a modified Giesekus fluid: a steady-state analysis

Abstract A theoretical analysis of the isothermal steady-state film casting process is conducted. A modified Giesekus rheological model is used in the study. The coupled effects of the rheological model parameters on the velocity profile, the stress profiles, and the film tension are found to be related to the elongational viscosity curve. A positive (negative) slope of the elongational viscosity curve corresponds to a velocity profile above (below) the Newtonian profile. The stress profiles are related to both the magnitude and shape of the elongational viscosity curve. The normalized film tension and the elongational viscosity exhibit strikingly similar characteristics. At high deformation rates, fluids with higher chain extensibility have higher elongational viscosity and require higher film tension; fluids with larger mobility factor have lower elongational viscosity and require lower film tension.

Multilayer film casting of modified Giesekus fluids Part 1. Steady-state analysis

Abstract A steady-state analysis of the isothermal multilayer film casting process has been conducted. The effects of the rheological properties of each layer and the processing conditions on the fluid velocity and stress profiles and the film tension were studied. The fluids were modeled using a modified Giesekus model, which is capable of predicting a wide range of rheological behavior. The mechanics of multilayer film casting were found to depend on the shape and magnitude of the elongational viscosities of the fluid components at the deformation rates of the process. The layer with a much larger elongational viscosity dominates the overall flow behavior, even if its thickness fraction is small.

Multilayer film casting of modified Giesekus fluids Part 2. Linear stability analysis

Abstract A linear stability analysis of the multilayer film casting of polymeric fluids has been conducted. A modified Giesekus model was used to characterize the rheological behaviors of the fluids. The critical draw ratio at the onset of draw resonance was found to depend on the elongational and shear viscosities of the fluids. Extensional-thickening has a stabilizing effect, whereas shear-thinning and extensional-thinning have destabilizing effects. The critical draw ratios for bilayer films of various thickness fractions are bounded by those for single layer films of the two fluids. When the two fluids have a comparable elongational viscosity, the critical draw ratio at a given Deborah number varies linearly with thickness fraction. When one fluid has a much larger elongational viscosity, it dominates the flow and the critical draw ratios at most thickness fractions remain close to its critical draw ratio as a single layer film. When the dominating fluid exhibits extensional-thickening, a film with a certain thickness fraction has more than one critical draw ratio at a given Deborah number and may not exhibit draw resonance within some range of the Deborah number.

Film Casting of a Low Density Polyethylene Melt

Film casting is one of the major commercial film manufacturing processes. Although various investigators have studied the process, no comprehensive set of data is available. In this study, film casting experiments of a LDPE polymer melt are conducted. The rheological properties of the melt, the film tension, the velocity profile, and the film width profile due to necking in will be presented. The thickness profile of the solidified film and the edge bead profile will also be reported. These experimental data will be useful for process analysis and verification of film casting simulation.

Edge Effects in Film Casting of Molten Polymers

In most analyses of the film casting process, edge effects such as neck-in and edge beading are usually neglected. In this work, we investigated the significance of these effects and their dependence on the rheological properties of the melts, the draw ratio, and the extrusion rate. Two linear low-density polyethylene melts and a low-density polyethylene melt were considered. The rheological behaviors of these melts were characterized under shear and elongational flows. Streamlines from the die exit to the chill roll, velocity profiles, film tension, neck-in profiles, thickness profile of the solidified film, and edge bead thickness profile were examined.

Tension in multilayer film casting of polymer melts

Measurement of tension in molten films can be used to control the quality of film products. It is also critical to the analysis and the modeling of the film casting process. In this work, the film tension of three-layer film consisting of LDPE and LLDPE were measured using a non-contacting air nozzle system. The dependence of film tension on draw ratio and film thickness fraction was related to the rheological properties of the polymer melts. Oscillation of film tension was observed when draw resonance occurred. The frequency of thickness oscillation in the solidified film was found to be similar to that of the tension oscillation in the molten film.

Paper coating rheology

Publisher Summary This chapter discusses paper-coating rheology. With the advances in theological testing methods and computational tools, the understanding of the blade coating of paper is improved considerably. Coating of high solids gives the paper less chance to roughen upon contact with water and the coating layer does not sink into the paper web. The application of paper coating has a unique aspect in the sense that the clearance between the blade and the web is only one order of magnitude larger than the pigments in the suspension. However, high solids content naturally brings challenging rheological problems. The importance of water retention on the ability of a coating to operate at specific conditions is one significant step. However, paper coating still offers a number of challenges, as high speeds and high solids are desired. Still, the small scale between the blade and the web requires a better understanding of the small-scale microstructures that can form under the blade. A direct comparison of a complex fluid dynamics calculation and pilot scale results has yet to be accomplished. These issues should become clear because efforts are made to understand the role of rheology in the blade coating process.