Rheology capillary rheometers

Understanding the melt rheology of rubber nanocomposites is crucial from the processing perspective. Bandyopadhyay et al. [37] have studied the melt flow behavior of rubber-silica hybrid nanocomposites in a capillary rheometer. 

Experimental polymer rheology data obtained in a capillary rheometer at different temperatures is used to determine the unknown coefficients in Equations 11 - 12. Multiple linear regression is used for parameter estimation. The values of these coefficients for three different polymers is shown in Table I. The polymer rheology is shown in Figures 2 - 4. 

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. 

There are a number of techniques that are used to measure polymer viscosity. For extrusion processes, capillary rheometers and cone and plate rheometers are the most commonly used devices. Both devices allow the rheologist to simultaneously measure the shear rate and the shear stress so that the viscosity may he calculated. These instruments and the analysis of the data are presented in the next sections. Only the minimum necessary mathematical development will he presented. The mathematical derivations are provided in Appendix A3. A more complete development of all pertinent rheological measurement functions for these rheometers are found elsewhere [9]. 

Appendix A3 Rheological Calculations for a Capillary Rheometer and for a Cone and Plate Rheometer... 

Rubber-based nanocomposites were also prepared from different nanofillers (other than nanoclays) like nanosilica etc. Bandyopadhyay et al. investigated the melt rheological behavior of ACM/silica and ENR/silica hybrid nanocomposites in a capillary rheometer [104]. TEOS was used as the precursor for silica. Both the rubbers were filled with 10, 30 and 50 wt% of tetraethoxysilane (TEOS). The shear viscosity showed marginal increment, even at higher nanosilica loading, for the rubber/silica nanocomposites. All the compositions displayed pseudoplastic behavior and obeyed the power law model within the experimental conditions. The... 

Melt flow rheology measurements were obtained on the MBAS polymer using an Instron capillary rheometer. The data reported were obtained using an 0.056-inch capillary, 90° included angle, with an L/D of 36. In Figure 5 the maximum shear stress (lb/in2) is plotted vs. the apparent shear rate (sec 1). The apparent viscosity (lb-sec/in2) vs. tem-... 

FIG. 15.46 Viscosity, 77, and first normal stress difference, Nh of Vectra 900 at 310 °C as functions of shear rate, according to Langelaan and Gotsis (1996). The first normal stress coefficient, Yi, is estimated from N, by the present author. ( ) Capillary rheometer ( ) and ( ) cone and plate rheometer ( ) complex viscosity rj (A) non-steady state values of the cone and plate rheometer. Courtesy Society of Rheology. 

We can see in Fig. 21 that, for the LDPE (FN 1010) above a given flow rate, bands of discontinuity appear in the birefringence pattern. These bands are observed parallel to the flow direction, at an intermediate position between centreline and die wall. Such "defects" may also be detected in the converging flow near the die entry (Fig. 22). For these flow conditions, the extrudate begins to exhibit distorded volume shapes, characteristic of melt fracture. By comparison with rheological studies carried out on a capillary rheometer, we may assume that these defects belong to the family referred to as "upstream instablities" in Chapter III.4. 

The rheological behaviour of the two polymers was determined using classical techniques of rheometry, already described in Chapter II. 1 (rotational and capillary rheometers for shear viscosity and first normal stress difference measurements CogsweU method for the elongational viscosity). 

Senouci, A. and Smith, AC. 1988. An experimental study of food melt rheology. I. Shear viscosity using a slit die viscometer and a capillary rheometer. Rheol. Acta 27 546-554. 

Among the many different classes of thermotropic polymers, only a limited number of polyesters based on aromatic ester type mesogenic units have been studied by rheological methods, beginning with the publication by Jackson and Kuhfuss of their work on the p-oxybenzoate modified polyethylene terephthalate, PET, copolymers. They prepared a series of copolyesters of p-hydroxybenzoic acid, HBA, and PET and measured the apparent melt viscosity of the copolymers as a function of their composition by use of a capillary rheometer. On inclusion of low levels of HBA into PET, the melt viscosity increased because of partial replacement of the more... 

Jerman and Baird recently conducted rheological studies on the same copolymers using an Instron capillary rheometer. They also measured die swell and entrance pressures. They observed that the viscosity of the 60 mole % HBA/PET copolymer was two orders of magnitude lower than that of PET when compared at the same temperature of 285 °C, which is similar to the results reported earlier by Jackson and Kuhfuss Die swell of the copolymers was highly temperature dependent. In general,... 

Figure 11.11 Shear viscosity rj and first normal stress difference as functions of shear rate y for Vectra B-950 at 300°C. The different symbols are for data acquired on different cone-and-plate and capillary rheometers. (From De Neve et al. 1993a, with permission of the Journal of Rheology.)...

As demonstrated previously (7), the ramping of rotor speeds can be used to obtain similar rheological information as is obtained via more traditional capillary rheometer methods. Specifically, it was shown that the following relation is applicable to torque and rotor speed data... 

The capillary rheometer is a valuable tool for predicting the processability of thermoplastic resins. This is done by measuring melt viscosities at shear rates and temperatures commonly encountered in extrusion and injection molding. This procedure is difficult and time consuming due to the complex nature of rheological measurements and analyses. An automated system for acquisition and analyses of capillary rheometer data has been developed to speed up and simplify this important analytical technique. 

Capillary rheometry and parallel-plate rheometry use the fact that wall slip will manifest itself as a geometry-dependent phenomenon. That is, wall slip will appear as a geometric effect on apparent rheological properties. In the capillary-rheometer technique, slip will manifest itself as an effect of capillary diameter ( )) on the shear stress (t, ). Wall slip in capillary rheology can be calculated from an analysis that involves the following ... 

Rheological and Tensile Properties. Melt rheological measurements were made on an Instron Capillary Rheometer (0.993" L X 0.05014" D) at a temperature of 200°C and at various shear rates corresponding to crosshead speeds of from 0.005 in./min to 20 in./min. Measurements were also made with an Instron TM Model (0.05034" D X 1.0074" L) at 200°C and at various shear rates corresponding to crosshead speeds of from 0.006 in./min to 10 in./min. 

In other words, apparent viscosity (as well as other apparent values in polymer rheology, snch as apparent shear rate and apparent shear stress) is a value calculated assuming Newtonian behavior and considering all pressure drops within the capillary (when using a capillary rheometer). A nonlinearity between shear rate and shear stress is typically observed for polymer melts. The fluid may behave like Newtonian at a very low shear rates to give a limiting viscosity iJq. 

Capillary Rheometer and an Extruder Are They in Agreement One can ask a reasonable question If rheology data are obtained in a capillary rheometer, are they applicable to an extruder To answer this question, at least for a specific set of conditions, a direct comparison was made [31]. It was found that the capillary rheometer and extruder are in good agreement for neat plastics (polystyrene and polypropylene), but extruder systematically measures lower viscosities in glass-fiber-filled plastics. 

Correct and imaginative use of suitable rheological equipment, such as the Monsanto Processability Tester, allows pertinent information to be obtained at lower cost than the classical trial-and-error approach. Furthermore, better cost efficiency can be achieved when interpreting capillary rheometer results in terms of processing and selecting the best operating conditions with respect to material flow properties. 

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