Tensile rheometer

Fig. 7.14. Standard arrangements used for the characterization of shear flows (left) and extensional flows (right). Shear viscosities can be derived from the torque measured in a cone-and-plate rheometer the primary normal stress difference is deduced from the axial force. Elongational viscosities follow from the tensile force required for the drawing of a molten fibre in the tensile rheometer, as monitored by a leaf spring...

The properties of melts under extensional flows can be studied with the aid of a tensile rheometer. The basic experimental arrangement is sketched on the right-hand side of Fig. 9.16. A cylindrical rod of polymer melt, usually floating on a liquid substrate, is drawn by two pairs of ribbed rollers. One of the rollers is mounted on a leaf spring, so that the force required for the... 

Figure 10.23 Sketch showing the principle of operation of the Munstedt tensile rheometer (MTR). The cylindrical sample is glued to metal plates, one of which is mounted on a force transducer while the other is coupled to a linear actuator system. The sample and transducer are immersed in a vertical oil bath. Sketch courtesy of Jen Tiang.

The Mtinstedt tensile rheometer (MTR) is an end-separation device in which the sample is stretched vertically in a cylindrical oil bath [ 199]. This is an improved version of the universal extensional rheometer described by Mtinstedt etal. [177]. The basic idea is illustrated in Fig. 10.23. The specimen is fastened by an adhesive to small metal plates, one of which is attached to a force transducer at the bottom of the bath, and the other is coupled to a pull rod that is vertically displaced by a toothed belt driven by a motor. This instrument has been used for a number of important studies [ 158,191 ]. The MTR can reach strain rates of 5 s and can be used for creep measurements. The temperature is limited to about 220 °C because of the physical properties of the silicone oil used to fill the bath. 

The Rheo-Tex rheometer is an inexpensive, automated instmment using load cell technology to measure indentation and creep. Available software calculates hardness/softness, brittleness, plasticity, and tensile strength. This instmment is particularly valuable for measurements on foods and personal care products. 

In all of the rheometer testing of the uncured compounds, the commercial silica AZ showed the highest values with the B1 and B3 samples having the highest values among the B-series silica samples. The Mooney viscosity at 100°C increases as the number of particles in the aggregates increases. The same compounds were cured and tested, measuring tensile properties, tear resistance. 

FIG. 15.23 Extensional rheometer, designed by Miinstedt (1979). A servo control system is used to maintain a specified extensional rate of strain or a specified tensile stress. Courtesy Society of Rheology. For a modern version, see Miinstedt et al., 1998. 

The maximum strain rate (e < Is1) for either extensional rheometer is often very slow compared with those of fabrication. Fortunately, time-temperature superposition approaches work well for SAN copolymers, and permit the elevation of the reduced strain rates kaj to those comparable to fabrication. Typical extensional rheology data for a SAN copolymer (h>an = 0.264, Mw = 7 kg/mol,Mw/Mn = 2.8) are illustrated in Figure 13.5 after time-temperature superposition to a reference temperature of 170°C [63]. The tensile stress growth coefficient rj (k, t) was measured at discrete times t during the startup of uniaxial extensional flow. Data points are marked with individual symbols (o) and terminate at the tensile break point at longest time t. Isothermal data points are connected by solid curves. Data were collected at selected k between 0.0167 and 0.0840 s-1 and at temperatures between 130 and 180 °C. Also illustrated in Figure 13.5 (dashed line) is a shear flow curve from a dynamic experiment displayed in a special format (3 versus or1) as suggested by Trouton [64]. The superposition of the low-strain rate data from two types (shear and extensional flow) of rheometers is an important validation of the reliability of both data sets. 

A few rheometers are available for measurement of equi-biaxial and planar extensional properties polymer melts [62,65,66]. The additional experimental challenges associated with these more complicated flows often preclude their use. In practice, these melt rheological properties are often first estimated from decomposing a shear flow curve into a relaxation spectrum and predicting the properties with a constitutive model appropriate for the extensional flow [54-57]. Predictions may be improved at higher strains with damping factors estimated from either a simple shear or uniaxial extensional flow. The limiting tensile strain or stress at the melt break point are not well predicted by this simple approach. 

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. 

NOTE Tensile, tear, hardness, and flex life characteristics were determined with ASTM D412, D2240, D624, and D430, respectively. Cure time was determined with a Monsanto rheometer. Model R-100. Swelling characteristics of the elastomer were measured by weight difference after soaking the square shape (1 x 1 x 0.2 cm) In hexane for 28 hours at room temperature. 

DMA was performed by a rheometer ARES G2 from TA-instruments in torsion mode with a heating rate of 5 K/min and a frequency of 1 rad/s (6.28 Hz) as well with a dynamic mechanical spectrometer EPLEXOR 150N from GABO QUALIMETER Testanlagen GmbH, Germany in tensile mode with a heating rate of 3 K/ min and a frequency of 1 Hz. 

Figure 11a shows mechanical properties as a function of shear rate for rods extruded in a capillary rheometer. Mechanical properties, such as tensile modulus, are only 1.5 to 2X those of PET. They are essentially independent of shear rate. 5... 

In bromobutyl/chlorobutyl rubber blends, both elastomers have the polyisobutylene backbone and halogen reactive functionality. These polymers, being molecularly miscible, constitute an ideal system for co-vulcanization. Bromobutyl and chloro-butyl can be used interchangeably without significant effect on state of cure as measured by extension modulus, tensile strength, and cure rheometer torque development. Bromobutyl will increase the cure rate of a blend with chlorobutyl. However, where bromobutyl is the major part of the blends, chlorobutyl does not reduce scorch tendencies because the more reactive halogen unit can dominate. 

Extensiometer n. A rheometer for measuring the extensional flow properties of molten polymers. In one early form, the Cogswell rheometer, useful at tensile viscosities over lO Pa/s, unidirectional tensile force was exerted on a polymer rod by a dead-weight acting through a cam and pulley. As the cam rotated, the moment arm exerted... 

Plastics—Determination of Dynamic Mechanical Properties. Part 10 Complex Shear Viscosity Using a Parallel Plate Oscillatory Rheometer Plastics—Determination of Tensile-Impact Strength... 

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