SynopsisAn experimental study of the influence of carbon black loading, particle size, and structure on the extrusion characteristics of polybutadiene and butadienestyrene copolymer synthetic rubber is described. The development of extrudate distortion and its mechanisms are considered. The viscosity and die swell have been related to black loading, particle size, and structure. Generally, it is found that black surface area and structure acts to increase viscosity and decrease elastic memory. This situation is analyzed in terms of the theory of nonlinear viscoelasticity. Two mechanistic theories are described which may explain this behavior. One theory is based on the continuum mechanics analysis of suspensions of particles in viscoelastic media. The rheological behavior of the black compounds is explained in terms of the increased severity of deformation in the polymer matrix surrounding the particle agglomerates. The second theory is based on the view of an entanglement network containing black particles. Polymer chains may be adsorbed onto the surface giving rise to increased entanglement densities.

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A comprehensive study of the effect of material, geometric, and operating variables on the onset of melt flow instabilities in extrusion was made. Primary objectives were to compare such melt instabilities for a variety of materials and to correlate instability criteria with independent rheological parameters. Seven polymer melts were studied: two polyethylenes, polypropylene, polystyrene, two polybutadienes, and an SBR copolymer. Two distinct extrusion apparatuses were used: a die fed by a screw extruder system and a die fed by an Instron rheometer. The variables studied were temperature, entry cone angle, length/diameter ratio of the die, and flow rate. Independent rheological measurements were performed. These consisted of the evaluation of normal stresses on the Weissenberg rheogoniometer, entrance pressure drop measurements on the Instron rheometer, and viscosity measurements on both instruments and the screw extruder. In a companion study, flow visualization studies were made in the die entry region of the Instron rheometer, for most of the above materials and also a solution of polyisobutylene and glycerine. Distinctively different patterns were observed for the various materials, ranging from severe channeling with corner eddies to nonchanneling flow. Interrelationships between the onset of instabilities with other measurements were noted. In comparing a variety of materials, a decrease in the critical shear rate is generally associated with an increased tendency of the material to channel, an increased normal stress/shear stress ratio and an increased entrance pressure drop/shear stress ratio. It is argued that the instability phenomena are strongly associated with the elastic properties of the materials.

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