A Natural Frequency Approach to Study the Blown Film Helicoidal Stability of HDPE Bimodal Resins and Its Relation to Their Rheological Behavior

A quantitative study of the helicoidal bubble stability (HBS) of five different HDPE bimodal resins processed under the same conditions was conducted to determine their natural frequencies and how these are related to molecular weight distribution features and typical viscoelastic data. The natural frequency omega(n), of each resin was obtained by representing the HBS phenomenon as an undamped system under the influence of a harmonic force, whose solution required force balance and Fourier series analyses of video-recorded oscillations of the bubbles. It was found that omega(n) was directly related to the stability grading on these bimodal resins established by experimental technicians running the blown film line. The same trends were observed when comparing with the entanglement index, Mw(B)/Me, the characteristic retardation time t(67%) obtained from recovery compliance data, and the cross-over point frequency (COP). These trends were not linear, indicating perhaps the existence of a percolation threshold for omega(n) with respect to the entanglement index and to the mentioned rheological parameters. (C) 2015 Society of Plastics Engineers