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. 相似文献
Summary: The rheological behavior of polyethylenes is mainly dominated by the molecular weight, the molecular weight distribution and by the type, the amount and the distribution of the chain branches. In this work a linear metallocene catalyzed polyethylene (m‐PE), a branched metallocene catalyzed polyethylene (m‐bPE), a conventional linear low density polyethylene (LLDPE) and a low density polyethylene (LDPE) have been investigated in order to compare their rheological behavior in shear and in elongational flow. The four samples have similar melt flow index and in particular a value typical of film blowing grade. The melt viscosity has been studied both in shear and in isothermal and non‐isothermal elongational flow. The most important features of the results are that in shear flow the m‐PE sample shows less pronounced non Newtonian behavior while in the elongational flow the behavior of m‐PE is very similar to that of the linear low density polyethylene: the narrower molecular weight distribution and the better homogeneity of the branching distribution are reasonably responsible for this behavior. Of course the most pronounced non‐linear behavior is shown, as expected, by the LDPE sample and by the branched metallocene sample. This similar behavior has to be attributed to the presence of branching. Similar comments hold in non‐isothermal elongational flow; the LDPE sample shows the highest values of the melt strength and the other two samples show very similar values. As for the breaking stretching ratio the opposite is true for LDPE while m‐PE and LLDPE show higher values. The transient isothermal elongational viscosity curves show that the branched samples show a strain hardening effect, while LLDPE and m‐PE samples present a linear behavior.
Dimensionless flow curves of different polyethylene samples. 相似文献
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. 相似文献
Melt strength (MS) and breaking stretching ratio (BSR) data relative to polyethylenes with different molecular structure (high density (HDPE), low density (LDPE), and linear low density (LLDPE)) are shown. HDPE and LLDPE samples show high breaking stretching ratio and low melt strength values. The LDPE samples exhibit very large melt strength values but low breaking stretching ratio values. For the last mentioned samples, differences are shown in the non-isothermal elongational behavior between samples polymerized with tubular and vessel technology. For all the samples, MS decreases with increasing melt index while BSR increases with melt index. 相似文献
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. 相似文献
The uniaxial extensional flow at 150°C of 11 linear low density polyethylenes (LLDPE) and one low density polyethylene was measured in a Rheometrics Extensional Rheometer. The presence of silicone oil did not affect the results. However, large effects of the molding time were observed. For specimens molded for 14 min, strain hardening was not observed for any gas-phase polymerized LLDPE. As the molding time was increased to 40 min, the strain hardening was quite apparent, the elongational viscosity nearly doubled, the equilibrium plateau vanished, and the maximum strain at break Increased by about 20 percent. Explanation for the molding time effects can be found in the concept of low entanglement density in the virgin gas-phase resins. The entanglement increases with time at temperatures above the melting point. The specimens molded for longer time show strain hardening. 相似文献
In this work we present an experimental study of shear and apparent elongational behavior of linear low-density (LLDPE) and low-density (LDPE) polyethylene blends by means of capillary rheometry. The characterization of these rheological properties is crucial in the design of a blend that combines the ease of processing of LDPE with the mechanical advantages of the LLDPE. Two different low-density polyethylenes and one common linear low-density polyethylene were used to prepare the blends. The results obtained indicate a strong sensitivity of the rheology of the blend to changes in the molecular weight of the LDPE employed. For the higher molecular weight LDPE, the shear viscosity of the blend was essentially equal to that of the LDPE homopolymer up to a concentration of 25% of LLDPE, whereas the apparent extensional viscosity was appreciably lower. For the lower molecular weight LDPE, the same trend was obtained regarding the shear viscosity, but in this case the apparent extensional viscosity of the blend was somewhat higher than that of the LDPE homopolymer. 相似文献
This paper investigates three aspects of linear-low-density polyethylene (LLDPE) rheological properties: shear viscosity variations with shear rate and temperature, tensile behavior determined with an extensiometer, and extrusion defects. The differences in shear viscosity variation with shear rate and temperature between LLDPE and LDPE (low-density polyethylene) are shown. These differences, attributed to wider molecular weight distribution and to long chain branching (LCB) in LDPE, involve different extrusion behaviors. The lack of LCB in LLDPE can be demonstrated by comparison of the measured Newtonian viscosity with the value of the same parameter calculated from molecular weight distribution and composition law of Newtonian viscosities. The lack of LCB leads to good melt extensibility, which is shown by tensile properties of polyethylene melts determined with a non-isothermal extensiometer. The melt fracture phenomenon is studied because it promotes surface defects on bubbles in film application. Extrudate distortions are examined at the laboratory extruder outlet. This test shows differences between LLDPE and LDPE, but also between some LLDPE samples. 相似文献
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. 相似文献
An environmental benign process, which uses supercritical carbon dioxide (ScCO2) as a processing aid, is developed in this work to prepare long chain branching polypropylene (LCB-PP). Results from the oscillatory shear rheology, melt elongational behavior and Fourier transformed infrared spectroscopy (FTIR) show that long chains have been linked as branches to the original linear PP chains using scCO2-assisted reactive extrusion in the presence of cumene hydroperoxide and 1,6-hexanediol diacrylate. Compared to the initial linear PP, the branched samples show higher storage modulus (G′) at low frequency, distinct strain hardening of elongational viscosity, lower melt flow rate, increased crystallization temperature and improvement of the melt strength. ScCO2 can improve the branching efficiency of modified PPs. The elastic response, melt strength and strain hardening parameter of the modified PPs increase with increasing scCO2 concentration, which is ascribed to scCO2 acting as a plasticizer for reducing PP viscosity and a carrier for active chemical species. 相似文献