SynopsisExperiments were performed in a sliding plate rheometer with a high density polyethylene to determine the conditions for the onset of slip and the relationship between slip velocity and shear stress. It was found that melt slip occurs at a critical shear stress of approximately 0.09 MPa in both steady and transient shear tests. The effect of the presence of a layer of fluorocarbon at the interface on both the slip velocity and the critical shear stress for the onset of slip, was also studied. Exponential shear was used to study the effect of shear history on slip. Roth steady state and dynamic models for the slip velocity are proposed that are consistent with the experimental observations. Results of oscillatory shear experiments suggest that melt slip is a physicochemical process in which the polymer-wall interface undergoes continuous change during successive cycles.

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The viscoelastic properties of a model binary polymer blend exhibiting an upper critical solution temperature phase diagram were investigated by utilizing small amplitude oscillatory and steady shear measurements. A mixture of unentangled monodisperse polystyrene and poly(phenyl methyl siloxane), exhibiting Newtonian shear viscosity, was used, and its phase diagram was established by turbidity and dynamic light scattering measurements. In the miscible region, the concentration dependence of the viscosity was adequately described by a mixing rule accounting for the surface fractions instead of volume fractions. Near the phase separation temperature and far from the glass transition, critical concentration fluctuations dominated the linear viscoelastic response and were responsible for the observed thermorheological complexity. An appropriate quantitative account of these fluctuations resulted in the accurate rheological determination of both the binodal and spinodal temperatures, extending thus the applicability of the relevant procedure earlier applied to lower critical solution temperature blends involving higher molecular weight entangled polymers. In the phase separated regime, the normal stress of the dispersed phase undergoing spinodal decomposition followed a recent scaling proposed for molecular mixtures with large viscosity difference.

Experiments were performed in capillary and slit rheometers to determine the effects of the presence of fluoropolymers at the polymer-wall interface on wall slip and extrudate distortion of a molten high density polyethylene. Two fluoropolymers were used to modify the interface. It was found that one of these decreased slip compared to the clean interface while the other increased slip. However, both were found to improve extrudate appearance, thus indicating that both adhesion promoters and slip promoters may improve extrudate appearance. The mechanism underlying this rather surprising conclusion is also discussed.

The calendering and rolling processes are used in a wide variety of industries for the production of rolled sheets or films of specific thickness and final appearance. The acquired final sheet thickness depends mainly on the rheological properties of the material. Materials which have been used in the present study are foodstuff (such as mozzarella cheese and flour-water dough) used in food processing. These materials are Theologically viscoplastic, obeying the Herschel-Bulkley model. The results give the final sheet thickness and the torque as a function of the roll speed. Theoretical analysis based on the Lubrication Approximation Theory (LAT) shows that LAT is a good predictive tool for calendering, where the sheet thickness is very small compared with the roll size. However, in rolling where this is not true, LAT does not hold, and a 2-D analysis is necessary.