Capillary melt viscosities

Fig. 3. Capillary melt viscosity, where (°) = 300° C, (a)= Courtesy of The Dow Chemical Co.

Figure 15.7 Capillary melt viscosity of SPS versus other thermoplastics.

Piston Cylinder (Extrusion). Pressure-driven piston cylinder capillary viscometers, ie, extmsion rheometers (Fig. 25), are used primarily to measure the melt viscosity of polymers and other viscous materials (21,47,49,50). A reservoir is connected to a capillary tube, and molten polymer or another material is extmded through the capillary by means of a piston to which a constant force is appHed. Viscosity can be determined from the volumetric flow rate and the pressure drop along the capillary. The basic method and test conditions for a number of thermoplastics are described in ASTM D1238. Melt viscoelasticity can influence the results (160). 

In the capillary method, the time required for a liquid to flow through a capillary tube is determined. The melt under investigation flows with a constant rate through a tube with a small, definite cross-sectional area, such as a cylindrical capillary. The viscosity can be measured in an absolute way from the pressure drop. This method can yield the most reliable absolute data, the viscosity being given by a modified Hagen-Poiseuille equation ... 

Melt viscosities were measured using an Instron capillary melt rheometer (Model 3210) using a 0.050-in. diameter capillary (L D = 40 1). Corrected viscosities were calculated in the conventional manner. In all cases, samples were preheated for 7 min prior to data acquisition. 

A similar variety of samples was tested for thermal stability by capillary rheometry and TGA. Figure 6.3 shows the viscosity-shear rate dependence for PCTFE homopolymers and one copolymer (Alcon 3000). All materials, save one, showed virtually identical viscosity relationships despite large changes in inherent viscosity. Only the polymers from runs initiated by fluorochemical peroxides (FCP) showed a dependence of molecular weight (as measured by inherent viscosity) upon melt viscosity. 

A number of capillary viscometers or rheometers have been employed to measure melt viscosity. In some sense, these operate on a principle similar to the simple observation of a trapped bubble moving from the bottom of a shampoo bottle when it is turned upside down. The more viscous the shampoo, the longer it takes for the bubble to move through the shampoo. 

Two main types of viscometers are suitable for the determination of the viscosity of a polymer melt The rotation viscometer (Couette viscometer, cone-plate viscometer) and the capillary viscometer or capillary extrusiometer. The latter are especially suitable for laboratory use since they are relatively easy to handle and are also applicable in the case of high shear rates. With the capillary extrusiometer the measure of fluidity is not expressed in terms of the melt viscosity q but as the amount of material extruded in a given time (10 min). The amount of ex-trudate per unit of time is called the melt index or melt flow index i (MFI). It is also necessary to specify the temperature and the shearing stress or load. Thus MFI/2 (190 °C)=9.2 g/10 min means that at 190 °C and 2 kg load, 9.2 g of poly-... 

A number of instruments are based on the extmsion principle, including slit flow and normal capillary flow (Table 6). These instruments are useful when large numbers of quality control or other melt viscosity test measurements are needed for batches of a single material or similar materials. When melt viscosities of a wide range of materials must be measured, rotational viscometers are preferable. Extmsion rheometers have been applied to other materials with some success with adhesives and coatings (10,161). 

A third evaluation which can be applied to good effect to describe the processability of the polymer is a variable-load melt viscosity measurement. A precaution here is to conduct this test last, on the scraps left over from other physical testing, since the temperature in the rheometer may degrade the precious material irreversibly. A variable-load capillary rheometer simulates extrusion and may thereby provide the strand for evaluation of qualitative... 

The behavior of melt viscosity of sulfur-dicyclopentadiene solutions is of obvious interest from the point of sprayable coatings. The melt viscosity behavior has been reported recently, but only qualitatively and over a narrow range of compositions (18). The viscosity of sulfur measured by the capillary method by Bacon and Fanelli (19, 20) is considered to be the best (21). Recently, however, the viscosity of sulfur has been measured by an apparatus containing an electric motor and a rotating cylinder (22). Viscosity of the sulfur-DCP solutions are measured here with the help of a Brookfield synchro-lectric viscometer, which is of the later kind. Viscosity measurements have been carried out to follow the copolymerization reaction and to analyze the viscosity behavior. 

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 melt viscosity of LCPs is sensitive to thermal and mechanical histories. Quite often, instrumental influences are important in the value of viscosity measured. For example, the viscosity of HBA/HNA copolyesters are dependent on the die diameter in capillary flow (59). LCP melts or solutions are very efficiently oriented in extensional flows, and as a result, the influence of the extensional stresses at the entrance to a capillary influence the shear flow in the capillary to a much greater extent than is usually found with non-LC polymers. 

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. 

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. 

Melt viscosity measurements involve monitoring the pressures produced when molten polymer is forced through a capillary at various shear rates. Viscosities are calculated from capillary rheometer data using the following equations... 

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. 

Figure 3. Melt viscosity of PVC containing polymer 3 at 100-140 °C in a capillary rheometer.

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... 

Figure 4. Melt viscosity-glass transition relationships for plasticized S-PS (1.78 mol %) samples based on various levels of DOP and glycerol (r = 2 X 105 dyn/cm2 220°C 1" X 0.05" capillary (D) DOP (A)...

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