Prediction of elongation viscosity of polymer melts

Melt extension flow is a common flow pattern during polymer processing, such as entrance converging flow in die extrusion or runner injection of polymer melts from an extruder barrel, blow molding, blowing film and melt spinning. Extensional viscosity is one of the important characterizations of the flow characteristics for polymer fluids. A new extension viscosity equation was established based on White‐Metzner model, Vinogradov‐Malkin viscosity equation and a new relaxation time equation in the present paper. The melt elongation viscosities of metallocene linear low‐density polyethylene (mLLDPE) and polyvinyl butyral (PVB) resins at 130°C were estimated applying this viscosity equation, and the predictions were compared with the measured data of mLLDPE and PVB resins at 130°C reported from reference. The results showed that calculations were close to the experimental data. The parameters in this equation were easy to be determined and the equation was convenient to use for estimating the extension viscosity of polymer melts. POLYM. ENG. SCI., 2010. © 2010 Society of Plastics Engineers     

Effects of Extrusion Rate,Temperature, and Die Diameter on Melt Flow Properties During Capillary Flow of Low-Density-Polyethylene

The melt flow properties of a low-density polyethylene were measured at test temperatures varying from 140 to 170°C and in a wide range of extrusion rates by means of a capillary rheometer, to identify the influence of extrusion conditions (such as temperature, shear rate, and die diameter) on the melt flow behavior in the present paper. The results showed that the entry pressure drop increased nonlinearly with an increase of the piston speeds, and it decreased with an addition of the die diameter. The melt shear flow obeyed roughly the power law and the melt shear viscosity decreased approximately linearly with an increase of the true shear rates in a bi-logarithmic coordinate system. The dependence of the melt shear viscosity on temperature accorded approximately the Arrhenius expression. Under these experimental conditions, the entrance pressure drop increases as an exponential function with an addition of the channel contraction ratio.     

Elongation properties of polyethylene melts

The extensional rheological properties of three grades of polyethylene melts, low density polyethylene (LDPE), linear low density polyethylene (LLDPE), and high density polyethylene (HDPE) were measured using a melt spinning technique under the test conditions with temperature ranging from 150 to 210°C and extrusion rate varying from 11.25 to 22.50 mm s^?1. The results showed that the melt strength decreased with a rise of temperature while increased with an increase of extensional rate. With the rise of extensional strain rate and temperature, the melt extensional viscosity decreased. The extensional stress and viscosity reduced with increasing extrusion velocity when the temperature and extensional rate were constant. Moreover, the melt strength and extensional viscosity of the LDPE resin was the highest and the LLDPE was the lowest under the same experimental conditions. POLYM. ENG. SCI., 2011. © 2011 Society of Plastics Engineers     

Dichotomous behavior of polymer melts in isothermal melt spinning

Isothermal melt spinning experiments have been conducted using two polyethylene melts of low density (LDPE) and high density (HDPE) to produce steady state spinline profiles. The data revealed the threadline extensional viscosity exhibiting a contrasting picture : extension thickening behavior for LDPE and extension thinning one for HDPE. A White-Metzner model having a strain rate-dependent relaxation time was then found to be able to simulate this dichotomy in melt spinning fairly well: the fluids whose relaxation times have smaller strain rate-dependence can fit LDPE data with extension thickening extensional viscosity whereas the fluids whose relaxation times have larger strain rate-dependence can fit HDPE data with extension thinning extensional viscosity. This dichotomous nature of viscoelastic fluids is also believed to be able to explain other similar contrasting phenomena exhibited by polymer melts, such as vortex/no vortex in entry flows, cohesive/ductile fracture modes in extension, and more/less stable draw resonance than Newtonian fluids.     

Melt spinning flow behaviour of high-density polyethylene blended with low-density polyethylene

ABSTRACT

The melt spinning flow behaviour of a high-density polyethylene (HDPE) blended with a low-density polyethylene (LDPE) was studied using a melt spinning technique in temperature ranging from 160 to 200°C and die extrusion velocity varying from 9 to 36?mm?s^?1. The results showed that the melt apparent extension viscosity of the blends was higher than those of the LDPE and HDPE; the melt apparent extension viscosity decreased with increasing temperature; while the melt apparent extension viscosity increased with increasing extension strain rate when the extension strain rate was lower than 0.2?s^?1, and then decreased; the melt apparent extension viscosity reached up to a maximum value when extension strain rate was about 0.2?s^?1; the relationship between the melt apparent extension viscosity and the LDPE weight fraction did not follow the mixing rule.     

Influence of side‐chain structures on the viscoelasticity and elongation viscosity of polyethylene melts

Metallocene‐catalyzed, low‐density and linear low‐density polyethylenes with similar melt indexes were used to investigate how side‐chain structures influence the elongation viscosity and viscoelastic properties. The viscoelastic properties were determined with a rotation rheometer, while the elongation viscosities were acquired by using isothermal fiber spinning. The Phan‐Thien‐Tanner (PTT) model was also used to understand how the side‐chain structure affects the elongation behavior. Experimental results demonstrate that the log G′ vs. log G″ plot can qualitatively describe the effects of the side chain branch on the rheological properties of polyethylene melts. According to the results determined by the PTT model, low‐density polyethylene (LDPE) has low elongation viscosities at high strain rates. This low elongation viscosity can be attributed to the fact that LDPE has high shear thinning behavior. The long‐chain branching tends to increase entanglements, thereby enhancing the storage modulus, elongation viscosity and shear‐thinning behaviors. Uniform side‐chain distribution lowers the entanglements, which results in a low storage modulus, elongation viscosity and shear‐thinning behavior.     

A study of melt density of flowing linear polyethylene

Linear polyethylene was extruded from a capillary rheometer with the driving piston operated at fixed speed and at fixed pressure. Apparent viscosity and melt density were measured in both extrusion modes. Apparent density decreased at shear rates approaching the melt fracture region in fixed piston-speed operation. Flow of other polymer melts was essentially incompressible in fixed piston-speed operation, and all polymers exhibited incompressible flow in fixed-pressure extrusion. The oscillating portion of the flow curve of linear polyethylene reflects alternating periods in which the polymer exits faster and slower than the rate at which the advancing piston clears the rheometer reservoir. Linear polyethylene behaves differently from most other polymers in fixed piston-speed extrusion and during melt fracture because of the existence of a more extensive entanglement network in the melt. It is suggested that melt fracture in general results from a tensile failure of the entanglement network, which may occur at the die inlet and in the orifice.     

Entrance pressure drop studies of corn meal dough during extrusion cooking

The entrance pressure drop during extrusion cooking of corn grits was measured using a cylindrical die viscometer attached to a single screw extruder and compared with results obtained using low-density polyethylene (LDPE). The cylindrical die viscometer had a length to diameter ratio of 40 with half-entry angles of 30, 37.5, 45 and 90° with the horizontal. The entrance pressure drop at the die was measured as a function of extrusion temperature, product moisture content and the die entry angle. Results indicate that the flow behavior of corn grits and the entrance pressure drop were affected by product moisture content, process temperature and the shear history in the extruder. Entrance pressure drop also increased with wall shear stress for plastic melt, but for food biopolymer, the increase was observed provided shear history effects were minimized. Entrance correction increased with apparent shear rate for LDPE, but the reverse was true for corn meal. Using Cogswell's analysis, corn grits exhibit severe extension thinning behavior in entry flow.     

Note on the paper “a general equation in polycondensation”

The influence of temperature and shear rate on the flow behavior of high density polyethylene (HDPE) melts during extrusion has been investigated by using a Ceast Rheovis 2100 capillary rheometer which was manufactured by the Ceast Co. in Italy. It was found that the entry pressure drop for the samples was very low during extrusion, the relationship between shear stress and shear rate did not obey the power law strictly, and the phenomenon of shear-induced crystallization was easily produced when temperature was near the melting point of the samples, even though the shear rate was not high. The dependence of the shear viscosity for the samples on temperature can be described by using the Arrhenius equation. © 1993 John Wiley & Sons, Inc.     

Studies on blown film extrusion. I. Experimental determination of elongational viscosity

Blown-film extrusion experiments were carried out to investigate the elongational flow behavior of viscoelastic polymer melts at different melt temperatures. Materials chosen for study were high-density polyethylene, lowdensity polyethylene, and polypropylene. In the study, isothermal blown-film extrusion experiments were carried out in which the molten blown film traveled upward through a heated chamber of about 13 in. in length maintained at the same temperature as the melt. Axial tension was measured at the take-up roller, the axial profiles of bubble diameter were determined by a photographic technique, and, from the samples collected, the variation in the film thickness along the axial direction was found. These measurements were used later to determine the elongational viscosity, using the force balance equations. It was found, in the experiment, that a careful control of the pressure difference across the thin film permitted one to maintain the bubble diameter constant, and, therefore, depending on the choice of the extrusion conditions, either a uniaxial or biaxial elongational flow was made possible. The experimental results show that, depending on the materials, elongation rate, and melt temperature tested, the elongational viscosity may decrease or increase with elongation rate, and may also stay constant independent of elongation rate. It was observed that the data of elongational viscosity obtained under uniaxial stretching in blown film extrusion is consistent with the data of elongational viscosity obtained earlier by use of the melt-spinning operation.     

超高分子量聚乙烯气辅挤出影响因素的数值模拟及分析

以超高分子量聚乙烯的圆形轴对称气辅口模挤出为研究对象,在采用Polyflow软件对气辅口模挤出时的等温流动进行数值模拟之后,就入口流率、松弛时间以及零剪切黏度等物性和工艺参数对挤出胀大、速度分布、口模压降和熔体外表面上剪切速率的影响进行了数值模拟和分析。分析表明:气辅挤出是克服超高分子量聚乙烯传统挤出时面临一系列困难的有效加工方式。     

Melt rheology of nanometre‐calcium‐carbonate‐filled acrylonitrile–butadiene–styrene (ABS) copolymer composites during capillary extrusion

The melt flow properties during capillary extrusion of nanometre‐calcium‐carbonate‐filled acrylonitrile–butadiene–styrene (ABS) copolymer composites were measured by using a Rosand rheometer to identify the effects of the filler content and operation conditions on the rheological behaviour of the sample melts. The experiments were conducted under the following test conditions: temperature varied from 220 to 240 °C and shear rate ranged from 10 to 10⁴ s^?1. The filler volume fractions were 0, 10, 20, 30, 40 and 50%. The results showed that the shear flow did not strictly obey the power law under the test conditions, and that the entry pressure drop (ΔP_en) and the extension stress (σ_e) in entry flow increased nonlinearly, while the melt shear viscosity (η_s) and extension viscosity (η_e) decreased with increasing the wall shear stress (τ_w) at constant test temperature. The dependence of the melt shear viscosity on the test temperature was approximately consistent with the Arrhenius expression at fixed τ_w. When τ_w was constant, η_s and η_e increased while ΔP_en and σ_e decreased with the addition of the filler volume fraction. © 2002 Society of Chemical Industry     

Melt flow behavior in capillary extrusion of nanosized calcium carbonate‐filled poly(L‐lactic acid) biocomposites

Nanosized calcium carbonate (nano‐CaCO₃)‐filled poly‐L ‐lactide (PLLA) biocomposites were compounded by using a twin‐screw extruder. The melt flow behavior of the composites, including their entry pressure drop, melt shear flow curves, and melt shear viscosity were measured through a capillary rheometer operated at a temperature range of 170–200°C and shear rates of 50–10³ s^?1. The entry pressure drop showed a nonlinear increase with increasing shear stress and reached a minimum for the filler weight fraction of 2% owing to the “bearing effect” of the nanometer particles in the polymer matrix melt. The melt shear flow roughly followed the power law, while the effect of temperature on the melt shear viscosity was estimated by using the Arrhenius equation. Hence, adding a small amount of nano‐CaCO₃ into the PLLA could improve the melt flow behavior of the composite. POLYM. ENG. SCI., 52:1839–1844, 2012. © 2012 Society of Plastics Engineers     

Analysis of melt spinning master-curves of low density polyethylene

The extensional rheological properties of a low density polyethylene (LDPE) melt were studied by using melt spinning technique. Based on the extension properties of the LDPE melt under experimental conditions, the melt spinning master-curves were plotted by introducing scaling factor b and the draw ratio λ. The scaling factor b shows the combination effects on the preorientation before extension, the unwrapping and orientation of molecular chains during extension. Several linear relationships between b and temperature, log b and the logarithm of extrusion flow rate v₀ (log v₀) have been investigated. By using the values of b and the reference curve, the extension viscosity curves with different temperature and extrusion flow rate could be calculated, thus the measurement range of melt spinning technique would be extended effectively. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

The flow of polymer melts through the clearance over a barrier flight in extruders

The flow of polymer melts through the clearance over a barrier flight in extruders involves high, rapidly changing shear rates. Polymer melts, being viscoelastic, are expected to exhibit a high elasticity when they flow through the clearance, so the flow through the clearance may not be predictable or stable. The flow through the clearance over a barrier flight was investigated using a shear refining (SR) module connected to an extruder. Three polymers with different melt properties were tested: branched low density polyethylene (BLDPE), high‐density polyethylene (HDPE), and polystyrene (PS). The measured drag flow rate through the clearance was found to be equal to the prediction for a purely viscous fluid, which gives a linear velocity profile in the clearance. At the threshold rotor speed of the SR module whereupon the predicted drag flow rate through the clearance is the same as the extruder output rate, the melt pressures at the inlet and the outlet of the SR module were nearly equal and stable. Below the threshold rotor speed, the inlet pressure was higher than the outlet pressure. Above the threshold rotor speed, the inlet pressure was nearly zero and the outlet pressure fluctuated. The magnitude of the pressure fluctuation increased with increasing rotor speed and decreased with increasing melt temperature. HDPE, which had a higher melt elasticity, showed more pressure fluctuation than BLDPE and PS. The pressure fluctuation probably results from the flow instability through the clearance caused by the melt elasticity.     

Determination of steady-state elongational viscosity for rubber compounds using bell-mouthed dies

A bell-mouthed die geometry was designed to cause convergent flow at a constant, uniform, elongational strain rate. An equation was derived, which showed that steady-state elongational viscosity could be calculated from a plot of pressure drop due to elongation against a simple function of die length. To obtain values of pressure drop due to elongation, it was necessary to correct the total pressure drop measured across the bell-mouthed dies for the contribution from shear occurring near the die wall. For this purpose, a simplified shape for the bell-mouthed dies was assumed, comprising several parallel sided segments. Applying a formula to pressure drop data measured across straight dies corresponding to these segments gave an estimate of the pressure drop due to shear across the bell-mouthed dies. Pressure drops due to elongation were determined by subtracting the pressure drop due to shear from the total pressure drop measured across the bell-mouthed dies. Measurements were also carried out with lubrication to validate the shear correction method. The results indicate that for the compound used in this study, a combination of bell-mouthed and straightsided dies can be used in a conventional capillary rheometer to determine steady-state elongational viscosity. An elongational viscosity of 190 kPa s at 90°C and at a strain rate of 10 s⁻¹ was determined for a simple styrene-butadiene rubber compound. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 1139–1150, 1997     

An in‐process ultrasonic approach to investigating the relaxation of orientation and disorientation of polymer melts

The orientation and relaxation behaviors of a low‐density polyethylene melt and polypropylene melts with different melt indices undergoing a shear flow in a restricted channel were investigated by using ultrasound. A capillary rheometer was used to force the polymer melt through a slit die equipped with pressure, temperature, and ultrasound sensors, and the variation of ultrasound velocity traversing the melt was measured. Experimental results revealed that due to different mechanisms involved, the relaxations of orientation and disorientation processes show different dependences of ultrasound velocity on shear rate, temperature, and melt index. POLYM. ENG. SCI., 2008. Published 2008 Society of Plastics Engineers     

茂金属聚乙烯(MPE)/线性低密度聚乙烯(LLDPE)共混熔体的流变学

研究了不同比例共混的茂金属聚乙烯 (MPE)和线性低密度聚乙烯 (LLDPE)熔体的流变学行为 ,讨论了共混物组成、剪切速率和剪切应力以及温度对熔体流变曲线、熔体粘度的影响 ,为MPE的共混改性提供了理论依据。结果表明 :随着LLDPE含量的增加 ,共混熔体的粘度降低 ,转变应力和非牛顿指数减小 ,粘流活化能升高 ,MPE的流动性和加工性能得到改善。     

挤出过程中HDPE流动特性及结晶行为的研究

$ 用毛细管流变仪,考察了挤出过程中HDPE熔体的流变行为。实验发现,当温度接近试样熔点时,熔体在较低的表观剪切速率(γ_a)下,易产生流动诱导结晶现象,表现为入口压力降(或剪切应力)突然增大;当γ_a大于发生该现象的临界值后,试样的流动服从幂律;试样熔体粘度对温度的依赖关系符合Arrhenius方程。     

Experimental studies on extrudate swell behavior of PS and LLDPE melts in single and dual capillary dies

Extrudate swell behavior of polystyrene (PS) and linear low‐density polyethylene (LLDPE) melts was investigated using a constant shear rate capillary rheometer. Two capillary dies with different design configurations were used, one being a single flow channel and the other being a dual flow channel. A number of extrudate swell related parameters were examined, and used to explain the discrepancies in the extrudate swell results obtained from the single and dual flow channel dies, the parameters including output rate and output rate ratio, power law index, wall shear rate, wall shear stress, melt residence time, pressure drop induced temperature rise, flow channel position relative to the barrel centerline, and the flow patterns. It was found in this work that the power law index (n value) was the main parameter to determine the output rate ratio and the extrudate swell between the large and small holes for the dual flow channel die: the greater the n value the lower the output rate ratio and thus decreased extrudate swell ratio. The differences in the extrudate swell ratio and flow properties for PS and LLDPE melts resulted from the output rate ratio and the molecular chain structure, respectively. The extrudate swell was observed to increase with wall shear rate. The discrepancies in the extrudate swell results from single and dual dies for a given shear rate were caused by differences in the flow patterns in the barrel and die, and the change in the melt velocities flowing from the barrel and in the die to the die exit. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 1713–1722, 2003