Capillary type rheometer

Table 5. Results from shear degradation measurements in two different rheometers (capillary, Couette-type) compared with model predictions of narrowly distributed polystyrene samples in toluene... 

There are roughly three classes of viscosimeters available the capillary viscosimeter, the rotational viscosimeter, and the falling-ball viscosimeter. Both the capillary and the rotational viscosimeters are built in different versions that allow for the exact determination of the viscosity in well-defined flow fields. Especially rotational viscosimeters allow the exact adjustment of a constant flow profile, thus are available in high precision and expensive versions as rotational rheometers. Capillary viscosimeters are the best compromise between the exact determination of viscosity and a well-priced measurement device, and are therefore the most commonly found type of viscosimeters. Both rotational and capillary viscosimeters are available in simple and inexpensive versions as Brookfield viscosimeters and flow... 

Chapter 3 deals with rheometry which is the method of measurement of the various rheological parameters described in Chapter 2. The rheometers may be of Ae rotational type or the capillary type for shear flows and the shear free t)rpe for extensional flows. 

Rheometers used for determining the material functions of thermoplastic melts can be divided into two broad categories (1) rotational type and (2) capillary type. Furfiier, subdivisions are possible and Gese are shown in Table 3.2. In what follows, only those rheometers which are ptqmlarly used for riieological... 

A capillary rheometer is another type of instmment, in which the uncured mbber is extmded through a small orifice and the change in dimensions of the extmdate is measured with a laser [2]. This instmment generates high shear rates, compared to Mooney rheometer. The capillary rheometer can thus represent flow of compounds on mbber processing machinery, such as injection molds. 

An Ostwald viscometer is similar to an Ubbelohde-type rheometer except that it is simpler in design and is less expensive. A schematic of an Ostwald viscometer is shown in Fig 3.6(b). It is characterized by a lower bulb that acts as a solution reservoir. A solution of known polymer concentration is placed in the lower bulb. A single capillary tube in which the measurement is taken is connected to the bottom of the bulb and to two small bulbs at the top of the capillary. Fluid is forced from the lower bulb through the capillary into the two small bulbs attached to the top of the capillary. There is a line between the two bulbs and at the exit of the lower bulb. The fluid is then allowed to drain back into the lower bulb through the capillary, and the time for the fluid to travel between the two lines is recorded. The time, if there were no end effects, is proportional to the kinematic viscosity (/j/p). 

Figure 3.6 Solution capillary rheometers a) Ubbelohde-type rheometer (courtesy of Cannon Instrument Company, USA), and b) a schematic of an Ostwald viscometer...

Since pressure driven viscometers employ non-homogeneous flows, they can only measure steady shear functions such as viscosity, 77(7). However, they are widely used because they are relatively inexpensive to build and simple to operate. Despite their simplicity, long capillary viscometers give the most accurate viscosity data available. Another major advantage is that the capillary rheometer has no free surfaces in the test region, unlike other types of rheometers such as the cone and plate rheometers, which we will discuss in the next section. When the strain rate dependent viscosity of polymer melts is measured, capillary rheometers may provide the only satisfactory method of obtaining such data at shear rates... 

Many of the comments in the previous chapter about the selection of grade, additives and mixing before moulding apply equally in preparation for extrusion. It is important of course that the material should be appropriate for the purpose, uniform, dry, and free from contamination. It should be tested for flow and while many tests have been devised for this it is convenient to classify them as either for low or high rates of shear. The main terms used in such testing ( viscosity , shear rate , shear strain , etc.) are defined in words and expressed as formulae in ISO 472, and it is not necessary to repeat them here. Viscosity may be regarded as the resistance to flow or the internal friction in a polymer melt and often will be measured by means of a capillary rheometer, in which shear flow occurs with flow of this type—one of the most important with polymer melts—when shearing force is applied one layer of melt flows over another in a sense that could be described as the relationship between two variables—shear rate and shear stress.1 In the capillary rheometer the relationship between the measurements is true only if certain assumptions are made, the most important of which are ... 

The effect of the type of impact modifier on the melt flow of a PVC window profile formulation as a function of shear rates encountered during extrusion was investigated and the relationship between the melt flow and mechanical properties of the profiles evaluated. A Rheoplast Capillary Rheometer with a pre-shearing device was employed to investigate the melt viscoelastic properties of the formulations and the performance of the formulations in terms of post-extrusion shrinkage, surface gloss and enthalpy relaxation discussed. 2 refs. 

A variety of laboratory instruments have been used to measure the viscosity of polymer melts and solutions. The most common types are the coaxial cylinder, cone-and-plate, and capillary viscometers. Figure 11 -28 shows a typical flow curve for a thermoplastic melt of a moderate molecular weight polymer, along with representative shear rate ranges for cone-and-plate and capillary rheometers. The last viscometer type, which bears a superficial resemblance to the orifice in an extruder or injection molder, is the most widely used and will be the only type considered in this nonspecialized text. 

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... 

The most common measuring systems used for the effects of shear rate on the chemo-viscosity of thermosets have been the parallel-plate rheometer and the capillary rheometer however, the choice of rheometer is dependent on the type of system to be studied. The advantages and limitations of these systems are presented in Table 4.6. 

In terms of melt viscosity, the Hg cation resulted in such a high melt viscosity that a coherent strand did not emanate from the capillary rheometer at the lowest shear rate (0.88 sec"1). Six cations, Mg, Ca, Co, Li, Ba, and Na, gave very high and virtually identical melt viscosities. All of these materials were melt-fractured at the shear rate of the viscosity measurement (0.88 sec 1). This lack of distinction in cation type is most likely attributable to the departure of these highly viscous materials from laminar flow. Nevertheless, it is quite clear that these materials are very... 

The standard aims at description of the material behavior that is more likely to occnr in factory processing compare to that obtained with a piston-type capillary rheometer (ASTM D 3835). 

A comparatively simple system is to have a tension and reeling system as an addition to a capillary rheometer. An example of this type [35] is also shown in Fig. 16. The disadvantage of this method is that the measurement is not made under isothermal conditions. 

Figure 26 Cross sectional view of a piston type capillary rheometer.

In [31] a correlation is foimd between the Mooney viscosity and the apparent viscosity, obtained by means of a capillary viscometer. The investigations were carried out on both rheometers with 6 types of rubbers, namely NR crepe sheets (natural mbber), SMR-20 (Standard Malaysian Rubber, i.e., type natural rubber), SBR (styrene-butadiene rubber), NBR (acrylonitrile-butadiene rubber), MQ (silicone rubber), HR 268 R (butyl rubber) and their compounds. The following equation was obtained ... 

Since, for >. = 0 to oo, the quantity in the square bracket ranges from 1.00 to 1.18, the drop deformability D 0.55k. Thus, a small deformation of Newtonian drops in Newtonian matrix varies linearly with the capillary number. This proportionality was indeed demonstrated in Couette-type rheometer for a series of com syrup/silicon oil emulsions (Elemans 1989). 

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