Doctoral Dissertations

Jenq-Der Tsou

Date of Award

6-1984

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Polymer Engineering

Major Professor

Donald C. Bouge

Committee Members

Edward S. Clark, J. E. Spruiell, John F. Fellers

Abstract

The dynamics of both steady and unsteady isothermal melt spinning have been studied, with emphasis on the effect of the die flow variables. Experimental results reported by earlier workers and supplemental data obtained in the present work are compared with theory and are in reasonable agreement.

An analysis of die flow effects must begin with consideration of the extrudate swelling (die swell) which occurs at the end of the die. This problem has been treated at various levels of rigor, ranging from finite-element solutions to one by Matsui and Bogue, which uses simplified kinematical assumptions. This analysis can be brought into agreement with some of the earlier theories and with experiment by the use of two adjustable parameters. This modified kinematical assumption also provides the initial stress conditions required in the subsequent analyses.

Next, one must consider the dynamics of steady spinning, especially as it is influenced by die flow (or die swell) effects. The experimental results show that the differential model used in the present work and an integral model can be used almost interchangeably to predict the dynamics of steady spinning, provided that one uses the same kinematics in the near die flow region.

Finally, the steady flow solution can be subjected to an infinitesimal (linear) disturbance and the stability of the spinline can be studied theoretically. This is the so-called "draw resonance" problem in which the spinline shows unbounded, periodic disturbances above some critical draw ratio. Certain unusual effects associated with the thinning behavior of polymer melts and the die flow conditions are thus clarified. Considered in terms of the parameter "a" (having to do with the Bogue-White deformation rate dependent relaxation time) and the stress ratio (the ratio of the first normal stress difference to the shear stress in the die flow), one finds that the behavior is quite different depending on whether "a" is less or greater than 1/√3.

In the existing analysis, there are two stable regions of spinning, one at low values of draw ratio and another at high values of draw ratio. Similar results come from the present analysis if a < 1/√3, although there is an effect of the stress ratio which cannot be predicted by the existing theories. Except in the second stable region at high draw ratios, it was shown that increasing the stress ratio makes the spinline less stable.

For a > 1/√3,however, the second stable region no longer exists; the critical draw ratio is determined by the maximum draw ratio attainable under any conditions (that is, from the steady spinning analysis). Beyond a certain maximum draw ratio, the velocity gradient goes to infinity and ductile failure occurs. Similarly, increasing the stress ratio results in lower critical draw ratios.

The agreement between theory and experiment is quite good for one of the two polypropylenes studied and is in at least qualitative agreement for the other, the first material having the narrower molecular weight distribution. A completely quantitative comparison is difficult to make because of the uncertainty in choosing the single relaxation time which is required by the theory.

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