Quiescent and Oriented Crystallization Behavior of Linear Low Density Polyethylenes

A series of isothermal and non-isothermal crystallization experiments were performed on nine linear low density polyethylenes and one high density polyethylene. In addition, melt spinning experiments were performed on three linear low density polyethylenes in order to gain a better understanding of their spinline behavior and the effect of stress on their crystallization behavior.

Isothermal experiments showed that crystallization rates were dependent on branching of the polymer molecules. Resins with extensive branching had crystallization rates which were much lower than linear polyethylene. This behavior was also observed in non-isothermal crystallization experiments.

Half time analysis was performed on the isothermal data to obtain coefficients which were used to model non-isothermal crystallization kinetics. Predicted behavior was in reasonably good agreement with experimental results at lower levels of crystallinity when crystallization isotherms were superposable. Non-linear regression was also used to obtain crystallization coefficients.

On-line diameter, temperature, birefringence, and tension data were collected for three LLDPE's at different spinning conditions. These resins had similar melt indices but quite different spinline behavior. The spinline data was analyzed using the Continuous Cooling Transformation approach. These resins exhibited crystallization rates which were higher than could be obtained from cooling effects alone. A comparison of stress-enhanced crystallization rates as influenced by branching was inconclusive due to the presence of both a changing cooling rate and stress. Modeling of the spinline was somewhat successful in that general behavior could be predicted; however, high accuracy could not be obtained due to uncertainties in certain parameters.