A NEW NON-ISOTHERMAL RHEOLOGICAL CONSTITUTIVE EQUATION AND ITS APPLICATION TO INDUSTRIAL FILM BLOWING PROCESSES

Industrially important polymer processing operations are characterized by high deformation rates and rapid changes of temperature. In these non-isothermal flows of viscoelastic fluids the material response can be best understood with the help of a constitutive equation relating the stress to the history of deformation and temperature.; While previous studies have considered the effect of the deformation history, they have not adequately treated the contribution to the stress arising out of a non-uniform temperature field; temperature is generally assumed to be a parameter rather than a variable. In the present work, the dumbbell model of dilute polymer solutions was modified to account for both the deformation and the thermal effects. This was then extended to polymer melts in a phenomenological manner and used to explain qualitatively some unusual experimental results obtained by other researchers; in these experiments, which involved the stretching polymer samples at a constant stretch rate, it was found that the stress developed in a sample that was rapidly cooled, but always kept above a temperature T, was greater than the stress in a second sample that was deformed at the same stretch rate and a constant lower temperature T.; A pilot scale model for making polymeric films was set up. It was employed to blow a broad molecular weight distribution polystyrene, both isothermally and with cooling under ambient conditions. The stretch rates, stresses, and temperatures in the melt region were measured. The model parameters were calculated using the stress in the axial direction and these were then successfully compared with the model parameters obtained in steady shearing and transient uniaxial extension. The predictions for the circumferential direction stress were, however, lower than the experimentally measured values.; On the basis of the theoretical work done in this thesis, as supported by experiments on film blowing, one can conclude that under the non-isothermal conditions normally found in polymer processing operations, the thermally generated stresses may be a very significant fraction of the total stress in the material. This thermal contribution to the stress becomes increasingly important as the cooling rate increases, and must be taken into account in operations such as film blowing and fiber spinning where the cooling rates may be greater than 10,000(DEGREES)C/sec.