Polymer rheology 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]. 

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-... 

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). 

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... 

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. 

Rheology is the study of the deformation of materials. This includes the elastic deformation of solids such as metals as well as the viscous behavior of fluids such as water or oil. There is a wide range of materials that exhibit both a viscous and an elastic response to an applied force, and polymers fall into this group. One of the best ways to determine the viscous nature of these materials is with a capillary rheometer. 

Capillary rheometers of various types are used for determining the rheological properties of polymer melts as can be seen from Table 3.1. The principal feature is that these rheometers are capable of extruding polymer samples at different speeds through the capillary of appropriate size. They are broadly categorized as... 

Molten polymers are viscoelastic materials, and so study of their behaviour can be complex. Polymers are also non-ideal in behaviour, i.e. they do not follow the Newtonian liquid relationship of simple liquids like water, where shear-stress is proportional to shear strain rate. Unlike Newtonian liquids, polymers show viscosity changes with shear rate, mainly in a pseudoplastic manner. As shear rate increases there is a reduction in melt viscosity. This is true of both heat-softened plastics and rubbers. Other time-dependent effects will also arise with polymer compounds to complicate the rheological process behaviour. These may be viscosity reductions due to molecular-mass breakdown or physical effects due to thixotropic behaviour, or viscosity increases due to crosslinking/branching reactions or degradation. Generally these effects will be studied in rotational-type rheometers and the extrusion-type capillary rheometer. 

Figure 9.10 Bypass capillary rheometer, automatically monitoring polymer rheology during...

Capillary rheometers are the most widely used rheological instruments for polymer melts. They are, however, generally limited to rather high shear rates. Rotational rheometers can provide data at lower shear rates. Cone-plate and parallel disc instruments have been popular with thermoplastic melts. Pressurized instruments, such as biconical or Mooney shearing disc instruments, are used with elastomers to prevent slippage [39]. Sandwich rheometers are used at the lowest shear rates and shear stresses. 

The rheological data needed for constructing a rheogram are obtained on sophisticated scientific instruments, namely, rheogoniometers, capillary rheometers, mechanical spectrometers, and so forth. These instruments are very expensive and require trained operators. Thus, the collection of the necessary flow data is not always possible due to constraints of finance and the limited technical capabilities of most polymer processors. 

Figure 3.9 Plots of log rj versus log co (O) and log G versus log co (A) in oscillatory shear flow, and plots of log jj versus log y ( ) and log IVj versus log y (A) in steady-state shear flow for a commercial polystyrene at 200 °C. The data for jj and at low shear rates were obtained using the cone-and-plate fixture of a rotational-type rheometer, the data for jj and at high shear rates were obtained using a continuous-flow capillary rheometer, and the data for r and G were obtained using the paraUel-plate fixture of a rotational-type rheometer. Refer to Chapter 5 for details of the experimental methods employed to obtain the data. (Reprinted from Han, Rheology in Polymer Processing, Chapter 3. Copyright 1976, with permission from Elsevier.)...

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