Date of Award

12-2007

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Legacy Department

Chemical Engineering

Advisor

Harrison, Graham M

Committee Member

Ogale , Amod A

Committee Member

Hirt , Douglas E

Committee Member

Cox , Christopher L

Abstract

Film casting is a common industrial process used to produce polymeric films. During film casting, a polymer melt is extruded through a flat die before rapid cooling on a chill roll. The chill roll velocity is faster than the velocity at which the melt exits the die, thus the polymer melt is stretched and oriented in an extensional flow. The material properties and processing conditions have a significant impact on the process and the final thermal/mechanical properties of the film produced.
Optimization of industrial scale film casting processes is still greatly dependent on trial and error methods. Therefore, this work is motivated primarily by the vision of the Center for Advanced Engineering Fibers and Films (CAEFF) to provide industry partners with computer-aided simulation methods for the design and optimization of future film casting processes.
This work is unique in that it employs an integrated experimental and modeling research approach towards the investigation of the film casting process. Experimentally, we study the film development in the air-gap between the die and the chill roll, and the final film properties. The modeling uses parameters such that the physical conditions under which the film casting experiments are conducted are identical to the experiments. The model inputs are directly derived from the rheological and thermal characterization of the polymeric materials used in the experiments. Therefore, this work provides a comprehensive set of experimental data, coupled with some simulations, which contribute towards a more detailed understanding of the film casting process.
In this work, we experimentally investigate the impact of material properties (such as polymer viscosity) and process conditions (such as die temperature, draw ratio and air-gap length) on the film formation process in the region between the die exit and the chill roll. Experiments are conducted using polypropylene, and a full thermal and rheological characterization of these materials is used both to interpret the experimental results and to provide parameters for the subsequent simulations. The effect of secondary processing steps, such as uniaxial stretching, on film strength, orientation and crystallinity is also studied. Finally, the measured width, temperature and velocity profiles are compared to model predictions.
The machine direction (vx) and transverse direction (vy) velocity components are measured as a function of position in the air-gap. We believe that these are the first pointwise measurements of the vy velocity component in film casting using the LDV technique. The vy velocity component is a result of the film neck-in, and is seen to decrease from the film edges to the centerline. Calculated centerline strain rates are found to depend on the draw ratio due to the effects of the resistance to flow as the film cools near the chill roll and the tension applied to the film as draw ratio is increased.
An increase in the die temperature, or a decrease of the material molecular weight, causes an increase in film neck-in due to the reduced resistance to flow. Increasing the air-gap length also increases the neck-in. This is due to the reduction of the strain rate (Deborah number) as the air-gap length is increased.
Increasing the draw ratio results in an increase in the temperature drop in the air-gap region. This observation is due to improved heat transfer from the film as a result of the decrease in film thickness as draw ratio is increased. Temperature maps show a minimum in the temperature in the central portions of the film, as the chill roll is approached, due to the formation of edge beads at the film edges.
The Primary film samples produced on the take-up roll are found to possess the mesomorphic crystalline morphology of isotactic polypropylene. This is attributed to the quenching action of the chill roll. Increasing the draw ratio slightly increases the crystalline content of the Primary film due to the decrease in the film temperature at the chill roll. Uniaxial stretching increases the moduli of the film due to the increase in film orientation and the effect on the film crystalline morphology. The stretching conditions also impact the film properties.
The wealth of experimental data generated during the course of this work is employed in the verification of the CAEFF FISIM 2D Cast Film Model. The experimental results are used as an indicator of the success of the model in capturing the primary physical characteristics of the film casting process. Using model parameters obtained directly from the thermal and rheological characterization of the polymer materials, and identical process variables to the experiments, we find that the model captures the qualitative effect of draw ratio, die temperature and polymer viscosity on the film geometry. The model also predicts the temperature drop and velocity profiles in the web.

Share

COinS