Capillary rheometer barrel

Figure 6.4.1 shows how an annular die can be installed in a capillary rheometer barrel. A screw extruder may also be used to feed a polymer melt to the annulus (Ehrmann and l finter, 1973 Ehrmann, 1976 Okubo and Hori, 1980). Using the notation given in Figure 6.4.1, in which k is the ratio of outer to inner cylinder radius, the equations for shear stress and shear rate become... 

Capillary rheometers are in the form of a barrel where the operator puts the polymer sample. After heating to equilibrate its temperature, the sample is pushed by a piston through a die at chosen rates. Various sizes and shapes of dies are available. Capillary rheometers measure the rheological properties under broad ranges of conditions of temperature, pressure, stress, strain and time, allowing the adoption of parameters near to those for processing. 

Capillary rheometers are used extensively to measure viscosity in the intermediate to high shear rate range. The rheometer has for all practical purposes a lower limit in viscosity measurement because of the plunger seals. These seals are shown on the bottom of the plunger in Fig. 3.16, and they induce a frictional resistance when they are pushed through the rheometer barrel. The piston force can be evaluated without polymer in the barrel, but it is always a source of error at low viscosities because of experimental variability. Moreover, barrel friction is one of the critical corrections that must be made when evaluating viscosity measurements... 

A new polymer was developed by the polymer scientists in a company. A sample has been provided to the process development laboratory to determine the viscosity of the polymer as a function of shear rate and temperature. The instrument available is an old capillary rheometer. The piston has a diameter of 9.525 mm, and a series of capillaries that fit the rheometer barrel have a diameter of 2.54 mm and lengths of 25.4, 50.8, 76.2, and 101.6 mm. The rheometer temperature was set at 270 °C. Shear viscosity data are needed to estimate process performance. 

A Rheometer Plunger B Hybrid O-Ring System C Rheometer Barrel D Capillary Die E Die Holding Assembly F Back Pressure Assembly G Piston... 

In order to facilitate rapid melt viscosity measurement and data analysis a modified Gflttfert capillary rheometer has been interfaced to a Hewlett-Packard data acquisition system. All test parameters (temperature, barrel pressure, etc) are monitored automatically and the data is stored on magnetic tape. After testing is complete, raw data is entered into an analysis program used to compute tables and draw plots of shear stress, shear rate, and apparent viscosity. Examples of the application of this system to commercial polymers are discussed. 

To calculate shear stress, shear rate and melt viscosity, the melt pressure and piston velocity must be monitored. The former is measured using a 0 to 20,000 psi pressure transducer mounted in the rheometer barrel just above the capillary. A linear potentiometer is used to monitor piston position during the test. Utilizing the piston displacement data and the HP real time clock, the piston velocity is calculated. 

In a capillary rheometer, such as the Monsanto Processability Tester, the extrusion pressure is measured, within the barrel, using a pressure transducer, located just above the die entrance. 

The particular ESM extruder was modified in such a manner that it can be used with a measuring barrel serving as a large capillary rheometer, whereby an auger in lieu of a pressure piston forces the material into the nozzle. This means that the pressure drop is measured analogously to the capillary rheometer. The results of this test produce the yield point and the apparent flow curve of the body, amongst other readings. 

A capillary rheometer is a rheometer, usually of the piston type, in which a thermoplastic is converted to a melt by heating in controlled conditions in the body (the cyhnder or barrel) of the apparatus and then extruded through a capillary die. 

This test simulates conditions occurring in the annular space between the tip of the screw flight and the extruder barrel [7, 9]. Plumb and Glaeser [10] developed a test method for filled elastomers based on a capillary rheometer. The specimen in this test is a cone-shaped torpedo in the flow channel. The flow conditions change in... 

Most capillary rheometers use round-hole dies with the pressure (or force) measured above the die in the barrel. The melt flow undergoes a large contraction as it moves from the barrel into the die and an expansion as it leaves the die to the atmosphere. These entrance and exit effects add an extra pressure drop (PJ to the measured pressure (P ,) in addition to the pressure drop due to flow through the die (AP). This will result in a viscosity measurement that is erroneously high due to the P component. 

Figure 8 shows a schematic of a typical capillary rheometer. The molten polymer is held in a cylindrical reservoir and heated to the desired temperature. A motor-driven piston is used to force the material down the barrel... 

The capillary dies must be very smooth, accurately machined, and hard. They are often made of tungsten carbide. Accuracy is especially critical. In a round-hole capillary, for example, the shear rate is a function of the radius cubed [see Eq. (7)]. A 1% error in the radius will result in a 3% error in the shear rate determination. Some dies have entry angles machined into their entrance to ease the abruptness of the transition from barrel to die and reduce the entrance pressure losses (see Bagley correction). Typical dies have diameters in the range of 0.5 to 2 mm, but specialty dies are available in almost any practical dimension. Depending on the piston speed and die selection, a wide range of shear rates can be achieved on capillary rheometers (see Fig. 9). 

The majority of published work on extrusion behaviour deals with compounded stock. Those papers reporting work on raw rubbers have usually been on the use of capillary rheometers to determine extrusion properties at higher shear rates than are possible with Mooney viscometers. Capillary rheometers are, in principle, quite simple to use, and the application of electronic, minicomputer and laser technology has reduced the operation and data analysis to a routine task. There are no standard ASTM or other test procedures, but under a specific set of conditions, once a material is characterized, the data can be used as standard for comparison of all subsequent batches. It is readily possible to characterize a raw rubber by an extrusion experiment to determine the viscosity/shear rate curve, extrudate swell, and stress relaxation.Both Sezna and Karg have shown how the Monsanto Processability Tester (MPT), a modified, computerized extrusion rheometer, can be used in predicting mixing behaviour. The MPT (shown schematically in Fig. 7) is a most versatile instrument. It has a larger than conventional barrel for minimal pressure drop in the barrel, a pressure transducer at the entrance to the orifice, a microprocessor system, and a laser device for... 

The reasons for the continuing use of melt flow indexers are low cost (about 10% the price of a capillary rheometer), simplicity and speed of use. Improved PID heating controls and more precise linear barrel-bore machined from a nitride-hardened steel or specially engineered ceramic has led, with recent models, to more reproducible results. Melt indexers can be obtained in a range... 

In a typical capillary rheometer, one has a temperature-controlled barrel into which the test material (usually in powder or pellet form) is packed. Directly downstream of the barrel is a cylindrical die with known length and radius. A piston is programmed to force the molten material through the die at a constant rate. A pressure transducer located near the die entry records the pressure drop. The capillary rheometer is widely employed and has been analyzed in detail (see Macosko [25], who gives a thorough discussion). The expression for the shear viscosity is... 

This development involves the extrusion of a compacted powder billet prepared and extruded below the melting point. The powder is loaded into the 3/8" diameter barrel of an Instron capillary rheometer that is plugged at one end. A moderate pressure ( 0.23 GPa) is applied to compact the powder into a coherent billet. This billet is then extruded in the conventional manner giving a translucent coherent strand. 

The capillary rheometer (Fig. 3.20) is commonly used to obtain r] at high shear rates. Basically the device consists of a barrel for melting the polymer and a plunger that pushes the melt through the capillary. The data obtained from this device consist of the pressure required to push the melt through the capillary and the volumetric flow rate (plunger speed and cross-sectional area). Two corrections are applied to this data. 

A Sieglaff-McKelvey capillary rheometer (Tinius-Olsen Testing Machine Company) was used for compression with the barrel plugged and for shear with an appropriate capillary. This rheometer is driven with nitrogen gas pressure applied to a piston, which exerts pressure on the material in the barrel. A barrel of 6.35 mm radius was chosen for this experiment. A predetermined amount of sample was charged and preheated in the barrel for 5 minutes before pressure was applied. Measurements were made at 100 °C and 150 C. 

The movement of material in the barrel of a capillary rheometer was used to observe the wall slip [6]. SBR 1500 (E-SBR) and its compounds are used with a flow marker as illustrated in Figure 8.9. 

The basic geometry of a capillary rheometer is similar to that of a melt indexer. Molten polymer in a heated barrel is extruded through a capillary die under the influence of pressure exerted by a piston. Either the force applied to the piston or its rate of travel is controllable over a wide range, as are the temperature of the polymer and the dimensions of the capillary. The relationship between the force apphed to the piston and its rate of travel reflects the response of the melt viscosity to the applied shear stress. In modem instmments, experimental control and final calculations are handled by computer. Additional refinements come in the form of interchangeable capillary dies of various lengths and diameters, the use of pressure transducers, and precisely controlled piston rates. Slit flow rheometers are also available but are far less common than capillary rheometers. The theory and practical aspects of capillary flow are extensively covered in the works cited in the bibliography. 

White and Dee carried out flow visualization studies for the injection molding of polyethylene and polystyrene melts into an end-gated rectangular mold [28]. Experiments were conducted under isothermal conditions and also for situations where the mold temperature was below the polymer glass transition temperature or the melting point, as appropriate. The apparatus used was a modified capillary rheometer in which, instead of a capillary die, a combined-nozzle mold assembly was attached to the barrel, as shown in Figure 15.19. The mold could be heated to... 

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