Development of an in-line viscometer in an extrusion molding process

In this work, an in-line viscometer to measure the viscosity of polymer melts under extrusion molding processes was developed. The in-line viscometer contains a stress sensor and a shear rate sensor which were installed between the screw and the die of an extruder. In this way, the flow line after the screw cannot be changed, unlike the present in-line capillary rheometer which can change the diameter of the pipe of the flow line and hence influence the throughput. All data acquisition is done by a computer such that the melt viscosity can be calculated automatically. The shear-thinning behavior of a low-density polyethylene (LDPE) under three different temperatures is presented in all experiments. It is concluded that the melt viscosity can be effectively monitored. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 63: 919–924, 1997     

高分子材料动态成型过程中熔体非线性行为的研究

借助流变测量和连续介质理论,不依赖已有的本构关系,对平行叠加正弦振动条件下高分子熔体经毛细管的动态挤出过程进行了理论分析。以低密度聚乙烯(LDPE)为原材料,实验测量LDPE熔体在一定振动频率和振幅下毛细管入口压力、体积流量和挤出胀大的瞬态值,即可得到动态成型过程中高分子熔体剪切应力、剪切速率和表观粘度的变化规律:随振幅和频率的变化,LDPE熔体的表观粘度呈非线性变化趋势;在不同的振幅和频率下动态挤出LDPE熔体,跟稳态挤出时一样,壁面剪切应力与壁面剪切速率也成非线性比例关系。     

Closed loop viscosity control of reactive extrusion with an in-line rheometer

A novel in-line melt rheometer (ILR) measured viscosity to control ionomer neutralization during reactive extrusion. The ILR has the advantages of an in-line location to minimize measurement delay and a “partial Couette” flow geometry, which affords independent control of the shear rate. The reactive extrusion process for ethylene methacrylic acid (EMAA) ionomer neutralization was modeled as a first order plus dead time process. A comparison of the process responses to experimentally determined residence time distributions for the extruder showed that there was an appreciable inherent delay despite the advantage of the rheometer's in-line location. Proportional-integral and minimum variance control algorithms were implemented. The quality of control was generally good, but demonstrated the non-linearity of the neutralization process, suggesting that adaptive control techniques could improve control performance.     

Viscosity measurement in a pipe flow using a numeric recursion approach for in-line quality control

The use of a pipe as a sensor for measuring viscosity and shear rate would require a separated flow rate instrument and would result in a more complicated mechanism for in-line quality control. In this study, a method of applying numeric recursion to the pipe pressure drop to directly calculate the viscosity and the shear rate is proposed. This viscosity sensor was verified by using it to measure the viscosity and shear rate for low-density polyethylene. The viscosity and shear rate obtained by this approach differ little from those obtained by using a standard off-line capillary rheometer. Therefore, this viscosity measurement method is applicable for in-line quality control.     

Flow analysis and end corrections in falling coaxial cylinder viscometry

The assumptions involved in the analysis of flow of polymer melts in a closed end falling coaxial cylinder viscometer^(1–3) are examined and it is shown that true values of shear rate and shear stress at the wall of the moving cylinder can be obtained by assuming classical Poiseuille flow. Flow curves obtained with a variety of apparatus geometries, driving loads and polymers are consistent with those of other measurement techniques. This apparatus, which is a simple modification of a capillary extrusion rheometer, provides measurements of apparent viscosity at much lower shear rates than can be obtained with the former viscometers. A quantitative measurement of end correction effects is provided by the present analysis. End effects, which are formally similar to those in capillary extrusion, were measured for several polystyrenes and polyethylenes.     

Empirical viscosity model for polymers with power‐law flow behavior

The aim of this study is the creation of an empirical model that can be used to predict the viscosity of a semicrystalline polymer melt for polymer process control. We have developed an empirical viscosity equation from both analysis and experimentation that takes into consideration the effect of temperature on both the flow behavior and the consistency index in the power law. The analysis is based on previously published literature, whereas the experimental data have been obtained from both the literature and laboratory investigations. The coefficients of the empirical model for low‐density polyethylene and polypropylene have been obtained from the experimental data derived from capillary rheometer measurements in the laboratory. This empirical model is able to produce results matching the experimental data with remarkable accuracy. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 3045–3057, 2003     

聚合物动态挤出流变行为研究

本文论述聚合物材料毛细管动态流变行为的测量原理,介绍了自行研制成功的用于合物熔体挤出的毛细管动态流变仪。在该仪器上对ＬＤＰＥ进行了实验研究,发现熔体的粘度与振动源的频率、振幅呈非线性关系。在振动必场作用下ＬＤＰＥ熔体的粘度减小,随振动频率的变化有一最小值。这对矣合物动态塑化挤出工艺过程控制具有十分重要意义。     

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.     

Shear viscosity of carbon black filled polypropylene at very low shear stresses

The shear viscosity of carbon black filled polypropylene with a range of different carbon blacks was investigated. This was accomplished using (a) a constant shear stress creep instrument, (b) a cone-plate rotational rheometer, (c) a capillary extrusion rheometer. It was found that stresses exist for these compounds below which there are only finite deformations and no steady flow, Much attention was given to measurements of creep at low stresses, especially in the neighborhood of yield values. The magnitudes of yield stresses obtained from such creep measurements are significantly lower than those obtained using standard extrapolation to zero shear rate of higher stress data from rotational instruments. The shear viscosity behavior near the yield value differs significantly depending upon the carbon black used. A high viscosity was observed (～10⁹ to 10¹⁰ Pa.s) plateau in some compounds.     

The influence of intermolecular hydrogen bonding on the flow behavior of polymer melts

The influence of hydrogen bonding on the flow behavior of polymer melts at high shear rate has been investigated using a capillary extrusion rheometer. The systems studied were copolymers of ethylene and acrylic or methacrylic acid. Hydrogen bonding was found to substantially enhance both flow activation energy and viscosity level, as well as the degree of dependence of viscosity on rate of shear. It was also found that hydrogen bonding does not influence the critical shear stress for onset of “melt fracture.” The data support the view that hydrogen bonds act effectively as temporary (quasi-) crosslinks during the short time scales of deformation involved in flow at high shear rates.     

New developments in melt rheology aid formulating rigid vinyl compounds with improved extrusion processing

This paper describes a new approach to the rheological characterization of PVC compounds using a capillary rheometer. Since the control of the thermal degradation rate of rigid PVC is very critical in extrusion, a new quantitative method of measuring the true degradation rate of PVC is proposed for formulating rigid vinyl compounds. The usefulness of this new technique is demonstrated in formulating PVC compounds with the lowest rate of degradation measured on the Kayeness capillary rheometer. This study shows a very good correlation between the capillary rheometer data and the extrusion melt temperature, melt viscosity, and the rate of degradation of PVC. The ideal melt temperature for extrusion is determined from the rheology data of each PVC compound to attain the longest run time. The study also shows how to measure the true melt temperature in extrusion, an important variable, to control the rate of degradation in processing. The results of this study show how the new rheology technique can be used as a practical tool for product development, quality control of the PVC compounds, and the development of optimum extrusion parameters.     

Influence of shearing history on the properties of polymer melt. II

The flow pattern of molten polymer through capillaries was studied by using a tracing method. An incompatible polymer was added to polypropylene as a tracer, and the influence of shear in a capillary rheometer on the tracer particle size as well as on the distribution of the particles in the extrudate in relation to flow was studied. It was found that the particle size varied inversely with shear rate of extrusion, capillary aspect ratio, polymer viscosity, and extrusion temperature. The flow was found to be of the telescopic type, and the tracer particle size was independent of position along the radius of the cylindrical extrudate. It is assumed that the supermolecular structure of a polymer melt is of the cluster type suggested by Busse, and it is postulated that spherical clusters of molecules in the melt are modified by shear analogously to the spherical particles of the incompatible tracer polymer. The large spectrum of melt properties obtainable, by various shearing treatments, from a polymer of constant molecular structure suggests that a wide range of supermolecular structures must be possible in the polymer melt.     

Rheological properties of a liquid crystalline copolyester

Rheological studies of an experimental liquid crystalline (LC) copolyester were carried out using a capillary rheometer and a cone and plate rheometer. Rheological characteristics of the polymer in the nematic state were observed. The nematic melt was found to be pseudoplastic and the degree of pseudoplasticity varied with shear rate. Melt viscosity was found to decrease with shear rate. Negative die swelling was observed at the exit of the capillary rheometer at temperatures marginally above the solid-nematic transition temperature of the polymer and was also found to be a function of shear rate. The dynamic mechanical properties of the polymer were studied as a function of temperature. The activation energies of flow and of dynamic mechanical deformation were calculated.     

Dynamic rheological behavior of polypropylene melts with pulsatile pressure flow in a dynamic capillary rheometer

A self‐made dynamic capillary rheometer (DCR) was designed to investigate the dynamic viscoelastic characteristic of polypropylene (PP) melt during the pulsatile pressure extrusion. A vibration force field was parallel superposed upon steady shear flow in this DCR by means of a vibration driven piston. During the pulsatile pressure extruding process in DCR, the PP melt displayed apparent viscoelasticity. The experiment results proved the pressure pulsatile extrusion could reduce the viscosity of polymer melts effectively. The phase difference between the shear stress and the shear rate decreased with the superposed vibration. But, at large amplitude conditions, the viscosity has an increasing tendency. This maybe illuminated that large amplitude could be harmful for the vibration‐assistant polymer processing. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 1834–1838, 2006     

振动力场下聚合物熔体流变行为的响应与分析 Ⅰ.毛细管压力降和熔体表观粘度

在自行设计的恒速型毛细管动态流变装置上 ,对聚合物熔体进行动态挤出实验。借助已建立的振动力场下聚合物熔体流变行为的表征公式 ,分别计算振动力场下聚合物熔体在毛细管壁处的剪切应力、剪切速率和表观粘度。与稳态挤出时相比 ,引入振动力场后 ,发现毛细管压力降、表观粘度均显著降低 ,且随着振动频率和振幅的改变呈非线性变化趋势 ,作者对此进行了深入分析。     

EPDM橡胶的流变特性实验研究

为了研究橡胶熔体流变性能对其加工成型的影响，利用毛细管流变仪对三元乙丙橡胶（EPDM）的流变特性进行了实验研究。实验结果表明：EPDM橡胶在毛细管挤出时，剪切速率对剪切应力、剪切粘度和挤出胀大的影响最大；挤出温度对三者有一定的影响；在长径比相同时，毛细管半径对剪切应力和剪切粘度几乎没有影响，但对挤出胀大影响较大。     

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.     

Measurement and modeling of flow and mechanical properties of ASR-filled polymer composites

Screw extrusion process was used to evaluate the flow properties of ASR/LDPE composite whose rheological properties cannot be obtained from the conventional capillary rheometer measurements. The results show that the viscosity of the mixture increases with increasing automobile shredder residue (ASR) content. Although there exists significant scatter in the results because of the non-homogeneity of the mixtures, the result can be used to roughly predict the flow properties of the materials. Thus, based on linear regression fits of the flow data, a second order polynomial relationship was established between the viscosity of ASR/LDPE composite and ASR concentration. HDPE was used in the mechanical tests. The results also show that the tensile modulus can be predicted, accurately, using a conventional composite model.     

Shear viscosity of rubber modified thermoplastics: Dynamically vulcanized thermoplastic elastomers and ABS resins at very low stress

The shear viscosity of PP-EPDM dynamically vulcanized thermoplastic elastomers and ABS resins were investigated using (a) a constant shear stress creep instrument, (b) a rotational rheometer, and (c) a capillary extrusion rheometer. It was found that stresses exist for some of these materials below which they exhibit only finite deformations. This indicates they exhibit yield values. Much attention was given to measurements of creep at low stresses especially in the neighborhood of yield values. The magnitudes of the yield stresses obtained from these low stress measurements are significantly lower than those obtained using standard extrapolation to zero shear rate of higher stresses from rotational instruments. We also observed a high shear viscosity (～10⁹ PA s) plateau in the materials of high rubber content. We contrasted our results to those found in the literature.     

氟聚合物加工助剂在PP-R中的应用研究

采用毛细管流变学实验，研究了氟聚合物加工助剂（FPA）在无规共聚聚丙烯（PP-R）挤出中的作用。结果表明：FPA可以降低PP-R的表观粘度10%以上，从而降低了挤出压力。增加挤出量，并可消除机头口模的积粒现象。提高PP-R管材的表面光洁度。