Melt viscosity, capillary rheometer measurement

Melt viscosity, or flow, is typically measured using extrusion plas-tometers (or melt indexers), capillary rheometers, and parallel plate rheometers. The extrusion plastometer measures the flow of a polymer melt under conditions specified by ASTM standard D 1238. This test yields a single, low-shear-rate value which is typically used to specify resins. Capillary rheometers determine viscosity over a range of shear rates in channel flow. While they are subject to error, these rheometers are still the only means of measuring viscosity at high shear rates (typically -y > 1000 s i). Parallel-plate rheometers also measure viscosity over a range of shear rates, but the maximum allowable shear rate is about 100 s i. 

Fig. 2 shows the flow curve for the neat exact 5361 at loot). The instability problem of the metallocene based polymers with narrow molecular distribution is well known. Fig. 3 shows the photographs of the extrudate samples with varying Dechlorane concentration collected during the capillary rheometers measurement. It is interesting to see that while the severe instabilities such as slip-stick and gross-melt fracture were observed at the shear rate from 177.8 s to 3162.2 s for the neat Exact resin, the severe instability appeared at the shear rate between 177.8 s and 562.2 s but disappeared at the shear rate above 1000.1 s for Exact/10% Dechlorane suspension. The shark-skin like instabilities were observed above the Dechlorane concentration of 20% and the shear rate at which the instability started to appear was decreased as the Dechlorane concentration was increased. Since the viscosity of the Dechlorane-filled systems was higher than that of the neat resin at all rates of shear, the instabilities are expected to develop the melt fracture at even lower shear rates. The shear viscosity vs. shear rate relationships measured with plate-plate rheometers and capillary rheometers are shown in Fig. 4. In this figure it is seen that both sets of data are reasonably matched. It is observed that at low shear rate range the viscosity increment due to the increase in the filler concentration is more pronounced than that at high shear rate. Both plate-plate and capillary measurements were carried out with constant shear rate (CSR) mode. While the capillary rheometer could accurately follow the preset shear rate values the plate-plate rheometer couldn t keep up with the preset shear rate values. Above two observations are due to the yield stress developed at low shear rate. At low shear rate particle-particle interaction dominates the flow phenomena and the yield stress was observed. At high shear rate hydrodynamic effect dominates the flow phenomena.

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). 

J The viscosity characteristics of a polymer melt are measured using both a capillary rheometer and a cone and plate viscometer at the same temperature. The capillary is 2.0 mm diameter and 32.0 mm long. For volumetric flow rates of 70 x 10 m /s and 200 x 10 m /s, the pressures measured just before the entry to the capillary are 3.9 MN/m and 5.7 MN/m, respectively. 

The shear viscosity, especially as measured with capillary rheometers characterized by high shear rates, is hardly sensitive to material structure since the investigator usually has to deal with the substantially destroyed structure in the molten sample. Melt stretching experiments would normally provide much more information [33]. 

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 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. 

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... 

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-... 

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 ... 

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... 

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... 

All variables in these equations are fixed except for piston velocity and melt pressure which are measured experimentally. An HP 3052A data acquisition system is used to acquire raw data from a Gflttfert capillary rheometer and to calculate and plot viscosity data. 

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. 

Similar to that of PTFE, the molecular weight of PFA cannot be measured by conventional techniques. An indirect factor called melt flow rate (MFR), also called melt flow index (MFI), is used, which is defined as the amount of polymer melt that would flow through a capillary rheometer at a given temperature under a defined load (usually, grams in 10 min). It is inversely proportional to viscosity viscosity is directly proportional to molecular weight of the polymer. 

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... 

There are five principal ASTM standards related to melt flow rates of thermoplastics. All of them deal with capillary rheometers one way or another. The capillary rheometer is the most common device for measuring viscosity of polymer melts. In deriving the viscosity relations, the following important assumptions should be taken into consideration ... 

Quality control of thermoplastic materials has relied predominantly on viscosity measurements, either in the melt or in solution. This is also the case for polymer blends. For example, QC of PPS/PPE Die PPS) blends (prepared by reactive processing) relies on measurements of density and melt viscosity in a capillary rheometer. Viscosity control before processing allows to assess incoming materials variability and suit-... 

Perhaps the most important factor to a process engineer in predicting extrusion or molding behavior is melt viscosity. Several methods are used to obtain the viscosity of polymer solutions and melts experimentally as a function of shear rate [19]. Instruments for making such measurements must necessarily accomplish two things (1) the fluid must be sheared at measurable rates, and (2) the stress developed must be known. Two kinds of instruments having simple geometry and wide use a rotational viscometer and capillary or extrusion rheometer. 

A systematic study of the basic rheological properties for a wide variety of polypropylene melts has been made by Minoshima et al. [89]. These authors measured shear viscosities at low shear rates in a Rheomatrics mechanical spectrophotometer and at high rates in an Instron capillary rheometer. The principal normal stress difference, Ni, was measured in the mechanical spectrophotometer with a cone and plate device. The elongational viscosity, of special importance to fiber formation, was measured in an apparatus built by Ide and White [90],... 

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