Crystallization of isotactic polypropylene during processing

Author

Jianguo Zhou

Date of Award

8-1988

Degree Type

Thesis

Degree Name

Master of Science

Major

Polymer Engineering

Major Professor

Joseph E. Spruiell

Committee Members

Donald C. Bogue, Edward S. Clark

Abstract

The crystallization kinetics of three Exxon polypropylenes with different molecular weights and polydispersities were studied. The investigation included studies of crystallization rates of three polypropylene samples carried out under quiescent isothermal and nonisothermal crystallization conditions and in the melt spinning process. On-line measurements of diameter, temperature, tension and birefringence were carried out to follow the development of orientation and crystallization. A mathematical model of the melt spinning process which includes a description of the development of orientation and crystallization was used to help analyze and interpret the results.

An approximate crystallization rate equation applicable to both quiescent and molecular orientation induced crystallization was developed and was applied to characterize the crystallization rates for these polypropylenes. The crystallization rate constants were evaluated by comparison between experimental data and the results of mathematical modeling.

Elongational viscosity and heat transfer coefficient were calculated based on experimental data from on-line measurements carried out at low spinning speeds. Elongational viscosity, heat transfer coefficient and crystallization rate function provide key equations for mathematical modeling of the melt spinning process of polypropylene.

The results from this research indicate a large difference of behavior between the three resins studied. The higher molecular weight, 12 MFR resin, develops a high level of crystallinity and a monoclinic crystal structure at all spinning speeds studied. The 300 MFR resin develops a relative low level of crystallinity and a structure that contains primarily smectic phase. The structure of the 35 MFR resin varies widely with spinning conditions, from smectic phase to monoclinic phase with increasing take-up velocity. The differences in crystalline order results largely from differences in crystallization kinetics caused by the molecular orientation and consequent difference in the temperature at which the structure is developed during processing.

The development of crystallinity during processing is determined by the interaction of stress-induced crystallization rate and cooling rate. High cooling rate tends to reduce the fraction of the sample crystallized by reducing the amount of time available for crystallization in the temperature range where crystallization rate is significant. Increasing molecular orientation increases the crystallinity by increasing the crystallization rate K(T,f_a). This research indicates that crystallization kinetics during processing are largely controlled by the molecular orientation developed in the melt in the temperature range where crystallization can occur. This molecular orientation is in turn controlled by the resin's apparent elongational viscosity. Increasing the resin molecular weight tends to increase both its elongational viscosity and its crystallization rate during processing. The increase of crystallization rate during processing compared to the quiescent case increases rapidly with decrease of resin MFR.

Recommended Citation

Zhou, Jianguo, "Crystallization of isotactic polypropylene during processing. " Master's Thesis, University of Tennessee, 1988.
https://trace.tennessee.edu/utk_gradthes/13391