Start-up pressure transients in a capillary rheometer

The “rise time” required to achieve a steady pressure reading in a capillary rheometer operated at constant piston speed can be very long, up to several hours under certain circumstances. This phenomenon can pose a serious problem in the measurement of melt viscosity, and it would be useful to be able to estimate the rise time in the planning of experiments. Based on experiments involving several types of polyethylene, we found that the rise time increases with L/D and the amount of polymer initially in the reservoir and decreases with diameter and piston speed. When the rise time is short, melt viscoelasticity contributes to the rise time, but when it is long, melt compressibility is the dominant factor. A model was developed for the latter case, and this was found to give an accurate prediction of the rise time, given the viscosity and compressibility. The model can also be used to determine the power-law parameters from the start-up pressure trace, P(t), for a single experiment. Alternatively, if the viscosity is known, the compressibility can be inferred from a single pressure trace.     

Thermal degradation of polypropylene in a capillary rheometer

Melt viscosity of a polypropylene (PP) resin was measured in a capillary rheometer between 220 and 260°C. The melt viscosity showed a power law behavior with strong shear rate dependence. The effects of temperature and shear rate on the degradation were studied in the rheometer by heating at 260 and 280°C, and extruding at shear rates up to 10000 sec ?1 . Melt flow index (MFI) of samples after shearing and heating treatment was measured to characterize the molecular weight change. An increase in MFI was found for PP sheared at high temperature. Heating for longer time also increased MFI. Increase of shear rate had a small effect on increasing MFI at 260°C but produced a larger effect at 280°C. A constant increment in MFI was observed in PP subjected to high temperature processing and was attributed to degradation due to oxygenated products.     

毛细管流变仪在聚烯烃加工中的应用

采用毛细管流变仪测定聚烯烃的流变行为,得到一系列剪切速率与剪切应力、剪切速率与剪切黏度的流变数据.利用该流变数据可进行专用料配方优化及加工条件筛选等工作,同时,还根据不同批次产品的流变曲线的偏离情况,监控产品质量。     

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。     

Determination of true flow curves from capillary rheometer data

The evaluation of polymer melt viscosity versus shear rate has been customarily done by a time-consuming graphical method, which corrects for non-Newtonian behavior and pressure losses at flow transitions. It is shown by mathematical analysis that the flow data in terms of applied pressure and apparent shear rate can be treated by a semigraphical method, in which calculation of the true flow curve can be done with a computer. Moreover, it is possible in some cases to program the data treatment entirely for computer calculation. Results obtained by the proposed method are in excellent agreement with those obtained by the older graphical method.     

Compressional effects in the capillary flow of polycarbonate

Recently, the characteristics for pressure corrections in capillary flow have been detailed. The apparent viscosity increases with increasing capillary shear rate for polystyrene and for poly(methyl methacrylate) have been previously explained using free volume theory. These general methods have been developed further in this work and applied to the non-Newtonian flow of a new system, the polycarbonate of bisphenol A. The pressure correction for up to 2 kilobars will be shown to linearize the capillary pressure drop versus the parameter L/D, capillary length over diameter. This correction eliminates the viscosity difference due to variations in L/D ratio. It is also observed that the zero-shear viscosity obtained by the extrapolation of the corrected capillary flow curves agrees well with new and independent data on the same polycarbonate obtained using a Weissenberg rheogoniometer. The flow data have been compared with theories and with earlier published data on the same polymer. The two sets of data are not concordant. These new and corrected shear-dependent data are, however, shown to be expressed qualitatively by the theory of Graessley, using the most probable molecular weight distribution.     

An automated high pressure capillary rheometer with hydraulic drive

A new capillary rheometer design is described, in which a hydraulic device is used to produce the pressure driving the sample fluid through capillaries. The device is fully automated and controlled by a PC unit. Various measuring modes can be preselected and are automatically executed. Some preliminary results with New tonian fluids and with polymer solutions show the functioning of the new rheometer.     

Rheological properties of carbon mixes at low shear rates using a capillary rheometer

A capillary rheometer is used to measure the rheological properties of green carbon mixes consisting of petroleum coke and carbon black as filler and pitch tar as binder. It has been shown that in the low shear rate range (0·3 to 2·0 sec^?1), the carbon mixes behave as Bingham body with yield stress of 6 × 10⁵ dyn/cm² and plastic viscosity of 2 × 10⁶ poise.     

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     

Steady flow and dynamic viscoelastic behavior of nylon 1313 using parallel‐plate rheometer and capillary rheometer

In this study, a novel nylon with long alkane segments (also called nylon 1313), which was synthesized using 1,13‐tridecanedioic acid in our laboratory, has been characterized. Different rheological behaviors of nylon 1313 have been presented using steady shear, creep recovery, and dynamic tests. The time‐temperature effects have also been investigated. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 1643–1651, 2005     

The application of a capillary extrusion rheometer to the determination of the flow characteristics of lard

A capillary extrusion rheometer was designed, and techniques were developed to obtain basic information on the flow behavior of lard. Since constant and adjustable shearing rates are a necessary feature for the study of the basic rheological behavior of a material, the rheometer was designed for use in conjunction with a tensile tester having a wide selection of crosshead rates. Techniques were developed for accurately measuring the flow variables and for treating the data. A comparison study on a Newtonian liquid sample proved the validity of the approach. Lard at 23.4C was tested, and its flow parameters (fluid-consistency index and flow-behavior index or degree of non-Newtonian behavior) were obtained. Presented at the AOCS Meeting, Cincinnati, October 1965. E. Utiliz. Res. Dev. Div., ARS, USDA.     

Degradation depth profiles and fracture of UV exposed polycarbonate

Abstract

Photodegradation depth profiles have been determined in 3 mm thick polycarbonate injection moulded bars using molecular mass measurements and an attempt made to relate the results to the fracture behaviour in uniaxial tensile tests. Unstabilised samples were compared with similar bars containing a commercial stabilising system. Although the presence of stabiliser restricted molecular mass degradation to a thin surface layer, with little degradation indicated beyond 0·1 mm from the exposed surface, the bars made from stabilised polymer embrittled at least as quickly as those made from unstabilised material, for which molecular mass degradation was evident at least up to 0·4 mm from the exposed surface. The as moulded compressive residual stresses near the surface of unstabilised polycarbonate bars increased in magnitude on UV exposure during the period in which embrittlement developed and may have partially countered the eects of molecular degradation, whereas little varition in the residual stress distribution with exposure was observed in stabilised polycarbonate.     

Rheological properties of carbon mixes at very low shear rates using a capillary rheometer—III

A capillary rheometer has been used to determine the rheological properties of carbon mixes consisting of petroleum coke as filler and coal tar pitch as binder. It is shown that carbon mixes behave as Bingham materials with definite yield stresses. The yield stress was found to be a general property of all carbon mixes and its value is independent of the size of the capillary die. It was also seen that extrusion at low shear rates through big diameter capillaries results in slipflow. The slipflow occurs when the applied stress is somewhat below the yield stress, the material then extrudes without the formation of a core which otherwise would cause cracks in the product after baking. This explains why big diameter rods are generally extruded at very low speeds in usual manufacturing process. The modified Buckingham-Reiner equation has been applied to the case of slipflow and the values of yield stress and plastic viscosity calculated. These values agree well with those obtained from the consistency curve.     

Measurement of dynamic capillary pressure and viscosity via the multi-sample micro-slit rheometer

We develop two direct methods to simultaneously measure the dynamic capillary pressure and the viscosity of fluids by application of differential forces during flow into micro-channels. In the first method, a series of external pressures is applied in conjunction with the dynamic capillary pressure and a “Bagley analysis” is applied to the flow front velocity, and in the second, we utilize differential gravitational forces. By explicitly measuring the dynamic capillary pressure, the measurement window of the recently developed multi-sample micro-slit rheometer is extended to the regime where capillary forces are significant. These measurement methods will be useful in understanding filling flows encountered in diverse areas such as microfluidics, oil recovery and biological transport.     

Determination of the pressure dependence of polymer melt viscosity using a combination of oscillatory and capillary rheometer

For an accurate simulation of a high-pressure injection molding process by using the CAE software, it is important to understand the pressure sensitivity of a polymer's melt viscosity. The current work describes a method for the determination of the pressure dependence parameter D₃ of the Cross-WLF model. It uses a combined rheological technique using both dynamic and capillary rheometers. Three grades of polycarbonate homopolymers were studied in this work and their complex viscosities were measured using a dynamic shear rheometer. The dynamic data were used to obtain six out of the seven parameters of the Cross-WLF, except D₃. A capillary rheometer fitted with a counter pressure chamber was further used to characterize the pressure dependence of the zero shear viscosity and to determine the D₃ parameter. Finally, the derived parameters were validated by carrying out injection molding with a box tool and comparing the actual pressure profiles with simulation results using the Autodesk® MoldFlow® software. The validation results indicated that actual pressure profiles from the simulation were found to be less than 10% than that of injection molding. POLYM. ENG. SCI., 60:517–523, 2020. © 2019 Society of Plastics Engineers     

Wall slip and compressibility like effects in capillary rheometer tests on complex polymer systems

Abstract

Using an appropriate set of capillary dies, the applicability of the Mooney wall slip method has been investigated with several filled compounds. Inconsistent results were obtained, for example, ‘negative’ slip velocity. With respect to data measured in practical capillary rheometry, a model was developed for treating (barrel) pressure data versus die length/diameter ratio at fixed applied apparent shear rate, i.e. the so called Bagley plots. A very simple equation was obtained

P _meas = 2P _ends - 1/β ln [exp (P _ends β) - 4σ_w0 β L/D ]

which yields the wall shear stress at zero pressure σ_w0 , the ends pressure loss P _ends , and a factor β when fitted to experimental data by non-linear regression. Experimental results show that the factor β accounts for both wall slip and compressibility like effects. Negative β values indicate dominating wall slip effects, while positive values demonstrate compressibility like effects. Slip velocity is thus pressure dependent and consequently the combination of wall slip and a pressure dependent viscosity can mask the expected gap dependence in the analysis by Mooney.     

Relationship between torque and capillary rheometer thermal stability measurements for PVC compounds

A method is described to predict the time-temperature behavior and hence thermal stability of PVC melts in a Brabender Plasticorder torque rheometer from basic rheological data(flow and thermal stability observations) obtained on an Instron capillary rheormeter. The predicted and experimentally determined values of the Brabender thermal stability are shown to be in good agreement.     

Flow visualization and extrudate swell of natural rubber in a capillary rheometer: Effect of die/barrel system

An investigation was carried out to examine the effect of die/barrel system on the flow patterns and extrudate swell of natural rubber in the barrel of a capillary rheometer, using a colored tracer as the visualization technique. The capillary rheometer used in this work had two dies located along the barrel, which is novel in rheometer design. The flow of the rubber in the upper barrel was dependent on the piston/barrel action and changed with piston displacement, whereas the complexity of the flow in the lower barrel was dependent not only on the piston displacement, but also on the geometry of the upper die design. The flow patterns that developed in the whole barrel were independent of the die located at the bottom of the barrel. In addition, the change in extrudate swell was associated with the flow occurring in the barrel, residence time, elastic characteristic, and the temperature rise during the flow. It was concluded that the general style of the flow patterns of natural rubber was greatly dependent on the die geometry that the material had previously moved past. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 2525–2533, 2001