Crystallization kinetics and mathematical modeling of linear low density polyethylenes on a melt spinline

Author

James L. Lambach

Date of Award

3-1988

Degree Type

Thesis

Degree Name

Master of Science

Major

Polymer Engineering

Major Professor

J. E. Spruiell

Committee Members

D.C. Bogue, J.F. Fellars

Abstract

A series of melt spinning experiments on three linear low density polyethylenes, LLDPEs, (melt indices of 4, 26 and 40) and a single high density polyethylene, HDPE, (10 melt index) were completed. The data were analyzed towards the purpose of better uncjerstanding the effects of orientation on the crystallization kinetics and structure development of polyethylene in a dynamic processing environment.

Spinning speeds ranged from approximately 500 to 2000 m/min and mass flow rates of 2 and 4 g/min were used. Dynamic spinline measurements were made (diameter, bire fringence, temperature and tension), and the as-spun fibers were characterized with regard to orientation, crystallinity, diameter and birefringence. The LLDPEs crystallized at about 10°C less than the HDPE, both on the spinline and in quiescent cooling studies.

The LLDPEs also exhibited significantly lower caxis orientation at a given spinning speed, or stress level. Regardless of spinning speed, all fibers exhibited b-axis (growth) orientation perpendicular to the fiber axis. For the two more viscous resins c-axis orientation increased with spinning speed. For the less viscous resins characteristic values of f_a and f _b were measured that did not change significantly from one spinning speed to the next. For f_c remaining constant with increasing spinning speed it was concluded that orientation was not affected by spinline stress, and it was proposed that radial temperature gradients caused the b-axis orientation.

Analyzing the spinline data using the Continuous Cooling Transformation approach showed that the two more viscous resins crystallized at significantly higher temperatures than would be expected by cooling effects alone, and spinline stresses at the onset of crystallization were consistently higher than for the two resins where stress did not enhance crystallization temperature. This result was consistent with the orientation data.

Modeling of the polyethylene spinline was successful for the two low viscosity LLDPEs. For the more viscous HDPE and LLDPE the model could not accurately predict the diameter profile and onset of crystallization at higher spinning speeds. Oversimplifications in some of the modeling equations were assumed responsible for the inaccuracies. It was recommended that several more spinning conditions be tested for the LLDPEs, and that a new constitutive equation for predicting reduction in amorphous orientation during crystallizaton be added to the model.

Recommended Citation

Lambach, James L., "Crystallization kinetics and mathematical modeling of linear low density polyethylenes on a melt spinline. " Master's Thesis, University of Tennessee, 1988.
https://trace.tennessee.edu/utk_gradthes/13257