Masters Theses

Date of Award

5-1994

Degree Type

Thesis

Degree Name

Master of Science

Major

Polymer Engineering

Major Professor

Joseph E. Sprueiell

Committee Members

D.C. Bogue, R.S. Benson

Abstract

The present research consisted of two parts: i) Investigation of the Spunbonding process through mathematical modeling and ii) Experimental investigation of the role of molecular weight and its distribution in structure development during melt spinning of polypropylene. A mathematical model for the Reicofil Spunbonding process, developed earlier at the University of Tennessee, was applied to investigate the effects of various process and material variables on the resulting filament structure and properties. The material parameters studied were the polymer elongational viscosity and the kinetics of "Stress induced crystallization". The process parameters varied included the extrusion temperature and the temperature and flow rate of the cooling air. Some changes in the process itself, such as the length of the spinline and the shape of the drawdown venturi were also considered. A combination of more than one of the effects was tried in order to examine the possibility of producing a thinner fiber with higher final birefringence.

The model indicated that the elongational viscosity is a key factor in controlling the amount of drawdown, the temperature of onset of crystallization and the level of molecular orientation developed in the spunbonded filament. The kinetics of stress induced crystallization also has a major influence on the extent of undercooling before the crystallization begins but it has relatively little effect on the drawdown of the filament. Among the process parameters, the cooling air temperature and the air flow rate appear to be the most significant variables. Changes in the process, such as the shape of the drawdown venturi can also lead to significant changes in the resulting filament diameter and properties.

Melt spinning experiments were conducted on polypropylenes of various molecular weight and molecular weight distributions to study the structure development in the filament and also to obtain the respective apparent elongational viscosities. Three sets of polypropylene samples, each consisting of three samples with similar melt flow index but different polydispersities were examined. The on-line profiles of the spinline and the characteristics of the as-spun filaments give an indication of how the molecular weight and its distribution affect the structure and properties of the spun filaments. Filaments spun from samples of broad molecular weight distribution are found to develop higher crystallinity but lower birefringence and tensile strength than those spun from the samples of narrow molecular weight distribution(MWD). From the birefringence profiles it can be suggested that the crystallization takes place closer to the spinneret as the molecular weight distribution widens. The higher crystallinity of the broad MWD samples is attributed to stress induced crystallization due to the presence of long chain molecules in those samples. It was speculated that the long chain molecules are entangled, and, when elongated in the spinning process produce substantial molecular orientation and "row-nuclei" which cause crystallization to occur at higher temperatures and proceed to greater levels of crystallinity and crystalline perfection. The apparent elongational viscosities were obtained by the "inversion procedure". It was observed that the samples with narrow molecular weight distribution showed an Arrhenius type Newtonian behavior whereas the samples with higher polydispersities seemed to deviate from the Arrhenius type formulation. This may indicate that viscoelasticity plays an important role in those samples.

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