Bubble instabilities observed in film blowing using four different polyolefins are discussed: high-density polyethylene (HDPE), low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), and polypropylene (PP). Special attention is given to the effect of the frost line height on the bubble stability, an effect mostly ignored in the literature. A video-camera system was used to record the bubble shape and oscillations. In general, three forms of instabilities and combinations were observed: (1) axisymmetric periodic variations in the bubble diameter; (2) helical motions of the bubble; and (3) variations in the position of the solidifcation line. The four resins show different stability behaviors. The LDPE has the most stable operating space and the PP is the most unstable one. No correlation was observed between bubble stability and oscillato y shear rheological properties of the resins. Instability is enhanced by increasing take-up ratio, increasing blow-up ratio, and decreasing frost line height. Furthermore, for the LDPE, some operating points were not attainable and multiple steady states were, observed. Our results are in a poor agreement with the predictions Cain and Denn's 1988 analysis.

An extensive experimental study of the effects of material characteristics and processing parameters on the kinematics and dynamics of film blowing is presented. Three polyethylene resins, a high‐density polyethylene (HDPE), a low‐density polyethylene (LDPE), and a linear low‐density polyethylene (LLDPE) were investigated. The convergent flow analysis of Cogswell was used to characterize the elongational flow behavior of the polymers. Strain rates and pressure inside the bubble (Pi) have been determined over a wide range of film blowing conditions. Moreover, on‐line bubble temperature and birefringence measurements have been carried out along the length of the bubble. The experimental results reveal that the three polymers display different behaviors. The LLDPE requires the highest Pi value and the LDPE, the lowest. Consistent with this, the LLDPE shows the lowest in‐plane birefringence and the LDPE, the highest. Interactions between various process parameters affecting the Pi value are characterized. Bubble instability is correlated to the apparent uniaxial elongational viscosity and Pi. The most stable polymer (LDPE) has the highest elongational viscosity and requires the lowest Pi. Stresses have been calculated with the help of the birefringence and Pi data. The stress and strain rate data were used to calculate an apparent nonuniform biaxial elongational viscosity of the melts, but could not be correlated through any simple constitutive equation.

Blown film properties depend on the thermo-mechanical history experienced by molten polymer during biaxial deformation. In this study on-line birefringence measurements along the length of the bubble in film blowing of a linear low density polyethylene (LLDPE) were carried out in order to assess the stress level in the melt zone and total orientation in the solid zone. Bubble temperature measurements were carried out to find out the onset and the end of crystallization. Strain rates were also determined from bubble diameter and axial velocity measurements. We have focused on the effects of key processing parameters on the thermo-mechanical history of polymers. The relations between the birefringence and temperature profiles are described. The birefringence value is shown to be very small in the molten zone and increases rapidly as crystallization proceeds. The birefringence of the solidified film is strongly dominated by the crystalline phase contribution. Stresses in the molten blown film were calculated using the data of birefringence and pressure inside the bubble. The birefringence technique appears to be a promising but limited tool to determine stresses occurring in film blowing.

An experimental study of the effects of material characteristics and processing parameters on the orientation and mechanical properties of different polyethylene (PE) films is presented. Several counter-intuitive results were observed. For the linear low density PE (LLDPE) the machine direction (MD) and transverse direction (TD) modulus values decreased with increasing take-up-ratio (TUR). For the low density PE (LDPE) the moduli also decreased with increasing TUR at high blow-up-ratio (BUR). At low BUR, however, there was a considerable overall increase in the TD modulus as TUR increased while the MD modulus was not much affected as TUR increased. Upon increasing BUR, the TD modulus drastically decreased, while the MD modulus remained almost constant. The extrusion temperature and frost line height did not significantly influence the modulus of the films studied. For the LLDPE, it is shown that the modulus somewhat decreased with increasing polymer flow rate. Significant variations of birefringence along the TD and MD were observed in blown films. At low BUR, the in-plane birefringence of the LLDPE increased with TUR up to a maximum, before decreasing to negative values. No general correlation between the birefringence and tensile modulus data could be found. Based on our present and previous results, we believe that it is also unlikely that final film properties can be related to strain rates and stresses occurring in the film blowing zone by any simple correlation. Many processing variables and polymer structural factors affect the ultimate film properties. Any attempt to control film properties will have to take those into account.

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