Investigation of the oscillating flow phenomenon in high density polyethylene

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The nature and causes of the oscillating flow phenomenon in high density polyethylene are examined in this report. Empirical equations are developed describing the onset (both stress, SSOF, and rate, SROF) of oscillating flow in terms of molecular structure. The SSOF is not constant, but is found to vary slightly with the molecular weight distribution. The SROF is shown to be predictable in terms of molecular weight distribution. The physics of the oscillations is critically examined, and the simple model of Okubo and Hori describing these oscillations has been successfully tested. The model involves alternating compression of the polymer in the capillary rheometer barrel and decompression after a material breakdown in the die, in which flow from the capillary exceeds that expected from the rheometer output for an incompressible material. The point for initiation for oscillating flow is positively identified to be in the die, not, as some have suggested, in the die entrance region. Examination of the critical shear stress for random ethylene–propylene rubbers suggests that the reason why linear polyethylene appears unique is that the critical stress may be unattainable in other polymers and that the magnitude of the effect tends toward being vanishingly small. A critical shear strain criterion seems to describe the variations in critical stress with ethylene–propylene copolymer composition, but the critical strain criterion appears to be an oversimplification in general.