Studies on blown film extrusion. III. Bubble instability

An experimental study has been carried out to investigate flow instabilities in blown film extrusion. Two types of flow instabilities were observed, depending on whether a bubble was under uniaxial or biaxial stretching. Under biaxial stretching, the phenomenon of a surface wave-type instability was observed, yielding wavy bubble shapes which very much resembled water waves at the free surface. Under uniaxial stretching, another type of instability, frequently referred to as draw resonance, was observed. It was also observed that, once draw resonance occurs, the amplitude and frequency of bubble diameter pulsing increased with stretch ratio. Quantitative information was obtained from a series of motion pictures taken of bubble diameter in both types of flow instability. It was observed further that an increase in extrusion melt temperature enhanced the severity of bubble instability.     

Mathematical and computational modeling of heat transfer and deformation in film blowing process

The blown‐film extrusion process was investigated both experimentally and theoretically. In experimental study, nonisothermal experiments were conducted using low‐density polyethylene. Rheological parameters were studied, considering the polymer melt as a power law fluid in nonisothermal conditions. Axial tension, bubble diameter, and film thickness at a variety of film extrusion conditions, that is, different flow rate, pressure difference across the film, and take‐up speeds were measured. In theoretical study, an analysis was employed to simulate the blown‐film extrusion process by setting up the force‐ and energy‐balance equations on the film bubble moving upward. Four nonlinear complex differential equations were integrated numerically, using an iterative backward shooting method and the fifth‐order Runge‐kutta technique. The program written, based on a mathematical model, predicts the bubble shape, temperature profile, and film thickness as a function of the distance along the machine axis. Furthermore, the model evaluates the elongational viscosity of LDPE in biaxial tension in terms of distance from die axis and take‐up speed. In this simulation, the total stress components in machine and the transverse directions were computed from the die exit up to the freeze line, the knowledge of which is necessary for evaluation of the elastic memory build up in heat‐shrinkable films. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 2115–2123, 2002     

Rheology-processing-property relationships in tubular blown film extrusion. I. High-pressure low-density polyethylene

Three different grades of high-pressure low-density polyethylene resin were used to establish relationships between tubular film blowability and the molecular parameters, namely, the molecular weight distribution (MWD) and the degree of long-chain branching (LCB), and also between the processing conditions and the mechanical properties of the tubular blown films produced. For the study, both the shearing and elongational flow properties of the resins were determined. During the tubular film blowing experiment we measured the freeze-line position, the tubular bubble diameter, the takeup speed, the axial tension, the pressure inside the tubular bubble, and the mass flow rate of the resin. The thickness of the tubular blown films was measured from the samples collected. In order to determine the tubular film blowability, we measured the maximum takeup speed at which the tubular blown bubble broke, for various blowup ratios. The measurements described above permitted us to calculate the tensile stresses at the freeze line, in both the machine and transverse directions, and they were found to be correlatable to the processing conditions employed. It has been found that the tubular film blowability is increased as the resin's MWD becomes narrower and the degree of LCB is less. It has been found further that a resin having lower elongational viscosity tends to give a greater draw-down ratio, indicating a better tubular film blowability. Finally, the tensile properties of the tubular blown films were found correlatable to the processing variables, namely, blowup and takeup ratios.     

Studies on blown film extrusion. II. Analysis of the deformation and heat transfer processes

Having investigated the elongational flow behavior of polymer melts (part I of this series), we have carried out both theoretical and experimental studies in order to better understand the deformation and heat transfer processes involved in blown film extrusion. For the experimental study, nonisothermal experiments were carried out, using high-density and low-density polyethylenes. Measurements were taken of the axial tension, bubble diameter, and film thickness at a series of extrusion conditions (i.e., flow rate, pressure difference across the film, and take-up speed). For the theoretical study, an analysis was carried out to simulate the blown-film extrusion process, by setting up the force- and energy-balance equations on the blown bubble moving upward. The approach taken in the theoretical study may be considered as an extension of the earlier work by Pearson and Petrie who considered the isothermal operation of Newtonian fluids. In the present study, However, we have considered the nonisothermal operation of power law fluids, whose rheological parameters were determined by an independent experimental study an described in part I of this series. Four highly nonlinear differential equations were solved numerically with the aid of the CDC 360 digital computer, using the fourth-order Runge-Kutta method. The mathematical model predicts the bubble shape, temperature profile, and film thickness as a function of the distance along the machine axis. Comparison is made of the experimentally observed bubble shapes with the theoretically predicted ones, showing a reasonable agreement.     

纤维素溶液吹膜过程中拉伸黏度的测定

通过非等温吹塑薄膜挤出实验研究了纤维素的NMMO(N-甲基吗啉-N-氧化物，亦称N-甲基氧化吗啉)溶液在不同工艺条件下的拉伸流动行为。讨论和推导了挤出过程中的理论数学模型，测定了纤维素溶液在加工过程中膜泡半径和膜厚随加工方向的变化，并对沿加工方向上拉伸速率和表观拉伸黏度的变化进行了探讨。研究表明：质量份数为9％的纤维素溶液在加工过程中拉伸黏度受拉伸速率与温度的综合影响，沿加工方向呈现先微降，再渐升的趋势；在加工过程中随着牵引速度和膜泡内外压力差(△P)的增大，膜泡的厚度变小，拉伸速率变大，拉伸黏度变小。     

Studies on melt spinning. V. Elongational viscosity and spinnability of two-phase systems

Isothermal melt spinning experiments were carried out to investigate the elongational flow behavior and spinnability of polymer blends and a calcium carbonate-filled polypropylene. Blends chosen for study were mixtures of polystyrene (Dow Chemical, Styron 678) with high-density polyethylene (Union Carbide, DMDJ 4309), and mixtures of polystyrene (Dow Chemical, Styron 686) with high-density polyethylene (Union Carbide, DMDJ 4309). For the study, measurements were taken of the thread diameter by photographic techniques and of the thread tension by means of a Saxl tensiometer. These measurements were later used to determine the elongational viscosity of the material investigated. It was found that, in all the blends and filled systems investigated, the elongational viscosity decreases with elongation rate and that the relationship between the elongational viscosity and blending ratio is very complex. An attempt is made to offer explanations of the observed complicated relationship with the aid of microphotographs of fiber samples, which display the complexity of fiber morphology in two-phase systems. It was also found that there exists some correlation between the elongational viscosity and the maximum stretch ratio which may be considered as representing fiber spinnability.     

Transient elongational viscosity of LLDPE/LDPE blends and its relevance to bubble stability in the film blowing process

Transient elongational viscosity of linear low density polyethylene (LLDPE) and its blends with 10% and 20% of low density polyethylene (LDPE) was measured at two temperatures by a constant strain rate elongational rheometer. In addition, the performance of the blends in the film blowing process was assessed in terms of bubble stability at two processing temperatures. An operating window for stable bubble production was determined. The elongational viscosity measurements on blends revealed stronger strain hardening characteristics at a higher temperature of testing. These results correlate favorably with findings from a bubble stability investigation where it was found that the size of the operating window for stable bubble production increased with increasing extrusion temperature. This work seems to indicate that increasing processing temperature during the film blowing of LLDPE-rich blends could lead to a processability improvement of these blends as far as bubble stability is concerned.     

The use of melt strength of low density polyethylene as a processability measure in film blowing extrusion

The melt strength method is analyzed in terms of its ability to predict the axial stress levels found in tubular film extrusion of low density polyethylene. For certain values of the capillary L/D, the level of axial stresses and average Newtonian viscosity found in the filament stretching and film blowing are comparable at the same draw ratio. The elongation to break of the finished film was found to be primarily determined by the axial stress during the film blowing, at constant frost line height.     

Comparison between a linear and a branched low-density polyethylene

Two low-density polyethylenes, a linear low-pressure (LLDPE) and a branched high-pressure (LDPE), have been compared. Their shear and extensional behavior and melt fracture phenomena have been investigated, and some mechanical and optical properties of their blown films have been measured. The rheological analysis showed major differences between the samples, both in shear viscosity and in elongational viscosity. The LLDPE exhibited two types of melt fracture, the first of which—a fine scale extrudate roughness—was not shown by the LDPE and appeared at a very low shear rate. The concomitance in LLDPE of a high shear viscosity and a low elongational viscosity and the presence of melt fracture at low shear rate resulted in its more difficult processing into film. The mechanical properties of the LLDPE film approached those of high-density polyethylene while the optical characteristics were in the range of LDPE. Such a coexistence of properties makes LLDPE an interesting material for film production.     

Elongational viscosity in the isothermal melt spinning of polypropylene

The elongational viscosity of polypropylene has been investigated by isothermal melt spinning, carried out over a range of experimental conditions. The filament diameter and the elongational force were measured for running filaments and the relationship between elongational viscosity and elongational strain rate reported. The elongational viscosity was observed to decrease in the vicinity of the spinneret and then remained constant before increasing along the thread line. An increase in elongational viscosity did not occur within the isothermal zone until the elongational flow was fully developed. The onset of an increase in elongational viscosity was determined from the constant total elongational strain. The degree of molecular orientation was also studied by birefringence measurements and was investigated as a function of elongational stress. At a high elongational stress, the relation between birefringence and elongational stress departed from linearity and exhibited a rapid increase which can be related to the increase in elongational viscosity.     

Importance of elongational properties of polymer melts for film blowing and thermoforming

Two polyethylene and two polypropylene melts were characterized in uniaxial elongational flow. They exhibit significant differences with respect to strain hardening. For the polypropylenes it was shown that the elongational behavior found in uniaxial elongation is qualitatively reflected in biaxial deformation too. From the polyethylenes, films were blown using laboratory equipment, and the polypropylenes were processed into beakers by thermoforming. For both materials it could be shown that strain hardening is of advantage for the geometrical uniformity of the processed items. POLYM. ENG. SCI., 46: 1190–1195, 2006. © 2006 Society of Plastics Engineers     

Elongational flow properties of low-density polyethylene and linear low-density polyethylene from nonisothermal melt spinning experiments

Low-density polyethylene (LDPE) and also linear low-density polyethylene (LLDPE) resins can be characterized by the degree of strain hardening and down-gaging during elongation. A new method for the determination of the apparent elongational flow characteristics is presented. In a small scale apparatus, a molten monofilament is stretched under nonisothermal conditions similar to those found in tubular film extrusion. Measurement of resistance to elongational flow and apparent elongational strain rates permit the comparison of the process-ability of different resins under specified conditions. The effect of melt temperature and extension ratio are examined. The importance of the molecular structure of both LDPE and LLDPE resins on these properties is also outlined.     

Study of kinematics and dynamics of film blowing of different polyethylenes

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 (P_i) 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 P_i 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 P_i value are characterized. Bubble instability is correlated to the apparent uniaxial elongational viscosity and P_i. The most stable polymer (LDPE) has the highest elongational viscosity and requires the lowest P_i. Stresses have been calculated with the help of the birefringence and P_i 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.     

Molecular,rheological, and crystalline properties of low‐density polyethylene in blown film extrusion

The molecular weight and its distribution, degree of long chain branching and cooling rate strongly influence crystallinity during processing, which in turn determines the processability and the ultimate properties of the blown film. Generally a decrease in the number of branches and molecular weight of the polymer and the cooling rate results in an increase of the crystallinity. Length of the main chain and extent of branching in low‐density polyethylene (LDPE) are also factors that affect melt rheology and film crystallinity. Long chain branched polyethylene is suitable in the blown film process due to its better melt strength for bubble stability. The objective of this article is to describe the effect of molecular properties (e.g. molecular weight and its distribution, degree of long chain branching etc) of LDPE on film crystallinity at different cooling rates of blown film extrusion. Two different grades of LDPE were selected to investigate molecular characteristics, crystallinity, and rheology. The resins were processed in a blown film extrusion pilot plant using four different cooling rates. Molecular, rheological, and crystalline properties of the resins were key parameters considered in this study. POLYM. ENG. SCI., 47:1983–1991, 2007. © 2007 Society of Plastics Engineers     

Blown film extrusion of poly(lactic acid) without melt strength enhancers

Processing strategies were developed to manufacture poly(lactic acid) (PLA) blown films without melt strength enhancers (MSEs). The effects of processing temperature on PLA's melt properties (shear and elongational viscosities), PLA grades, and other processing conditions [ratio of take‐up roller to extruder's rotational screw speeds or processing speed ratio (PSR) and internal air pressures] on film's blow‐up ratio were examined. Experimental results indicate that extrusion‐blown amorphous and semicrystalline PLA films can be successfully manufactured without MSEs by controlling melt rheology through processing temperature and other extrusion processing conditions. PLA processed at lower extrusion temperature had higher melt viscosities, which favored the formation of stable films depending on the PSR and internal air pressure used. Inappropriate control of PSR and internal air pressure led to unstable films with various processing defects such as melt sag, bubble dancing, or draw resonance, irrespective of the lower extrusion processing temperature. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45212.     

A neck‐in model in extrusion lamination process

In this study, the experiment of the extrusion lamination process using high‐pressure process low‐density polyethylene (LDPE) was performed. The nonisothermal viscoelastic simulation of the extrusion lamination experiment was also carried out. The simulation results were in good agreement with the experimental data within wide range of take up velocity and air gap length. We developed the theoretical model based on force balance and deformation type of a film to predict the neck‐in behavior in the extrusion lamination or cast film process. It was suggested from the neck‐in model that the neck‐in correlates with the ratio of planar to uniaxial elongational viscosity. It was confirmed that the neck‐in model could predict the film edge shape and neck‐in properly for conventional LDPE. POLYM. ENG. SCI., 2010. © 2009 Society of Plastics Engineers     

Rheological effects of polyethylenes in film blowing

The aim of this work is to correlate the rheological properties and processability of various polyethylenes during the film‐blowing process. The effect of rheology on the kinematics and dynamics of film blowing for five different polyethylene resins has been extensively studied using a fully instrumented laboratory unit. The complex viscosity, shear viscosity, uniaxial elongational viscosity, and non‐uniform biaxial elongational viscosity, as well as the strain rates and stresses during film blowing, have been determined and correlated to the bubble stability. G′ versus G″ plots were found to be virtually independent of temperature for all polymers investigated. The more elastic polymers (larger G′ values) were found to be more stable in film blowing. Also, the more stable polymer melts were found to be those possessing larger elongational properties.     

Rheology-processing-property relationships in tubular blown film extrusion. II. Low-pressure low-density polyethylene

An experimental investigation was undertaken to establish rheology-processing-property relationships in the tubular blown film extrusion of low-pressure low-density polyethylene (LP-LDPE). For the study, three commercial LP-LDPE resins, each from a different resin manufacturer, were used in producing tubular films, by employing the apparatus described in Paper I of this series. Both molecular and rheological characterizations of the resin were conducted, enabling us to interpret the tubular film blowing characteristics of the resins. Correlations were obtained between the processing variables (namely, blowup and takeup ratios) and the tensile properties of the films. The tubular film blowing characteristics of LP-LDPE and HP-LDPE resins are compared. Differences in the rheological properties (namely, elongational viscosity) of the two types of resin are used in explaining the experimentally observed differences in their tubular film blowability.     

Experimental study of elongational flow and failure of polymer melts

A new and simple instrument for measurement of elongational flow response of polymer melts in constant uniaxial extension rate experiments is described. Quantitative stress development data are presented for a series of low-density polyethylene (LDPE), high-density polyethylene (HDPE), polystyrene (PS), polypropylene (PP), and poly(methyl methacrylate) (PMMA) melts. For small elongation rate E, linear viscoelastic behavior was observed; while for large E, LDPE and PS showed exponential stress growth, while HDPE and PP showed only linear stress growth. Stress relaxation experiments were carried out for several of the same melts in the instrument. Elongation to break and mechanisms of filament failure were studied. HDPE and PP have a tendency to neck and exhibit ductile failure, while at high E, LDPE and PS seem to show cohesive fracture. The elongational flow stress response data were compared to predictions of nonlinear viscoelastic fluid theory, specifically the Bogue-White formulation. The qualitative differences in responses of the melts studied were explained in terms of different dependences of the effective relaxation times on deformation rate and, more specifically, on values of the a parameter in the theory.     

扩链改性对聚对苯二甲酸乙二酯挤出吹膜成型的影响

通过添加多官能团环氧扩链剂对聚对苯二甲酸乙二酯(PET)进行扩链改性,然后用改性PET进行吹膜成型,研究了扩链剂含量对PET特性黏度、膜泡稳定性及薄膜力学性能和透明度的影响。结果表明,扩链剂的添加显著提升了PET的特性黏度;随着扩链剂含量的增加,薄膜膜泡稳定性以及表面质量、最大吹胀比、拉伸强度得到显著改善,断裂伸长率呈先增大后减小的趋势。当扩链剂质量分数为0.7%时,PET的特性黏度由纯PET的0.71 d L/g上升至0.94 d L/g,最大吹胀比则由1.9提高至4.5,横纵向拉伸强度分别为61.7 MPa和64.4 MPa,横纵向断裂伸长率达到最高,分别为12%和12.45%,较扩链剂质量分数为0.2%时提高了105.5%以及125.1%,而透光率仅下降1.4%,得到了性能较优的吹塑薄膜制品。