Strain sweep tests

Strain Sweep Test Protocols for Nonlinear Viscoelasticity Investigations.826... 

It is clear that this data treatment is strictly valid providing the tested material exhibits linear viscoelastic behavior, i.e., that the measured torque remains always proportional to the applied strain. In other words, when the applied strain is sinusoidal, so must remain the measured torque. The RPA built-in data treatment does not check this y(o )/S (o)) proportionality but a strain sweep test is the usual manner to verify the strain amplitude range for constant complex torque reading at fixed frequency (and constant temperature). 

At sufficiently low strain, most polymer materials exhibit a linear viscoelastic response and, once the appropriate strain amplitude has been determined through a preliminary strain sweep test, valid frequency sweep tests can be performed. Filled mbber compounds however hardly exhibit a linear viscoelastic response when submitted to harmonic strains and the current practice consists in testing such materials at the lowest permitted strain for satisfactory reproducibility an approach that obviously provides apparent material properties, at best. From a fundamental point of view, for instance in terms of material sciences, such measurements have a limited meaning because theoretical relationships that relate material structure to properties have so far been established only in the linear viscoelastic domain. Nevertheless, experience proves that apparent test results can be well reproducible and related to a number of other viscoelastic effects, including certain processing phenomena. 

Strain Sweep Test Protocols eor Nonlinear Viscoelasticity Investigations... 

According to strain sweep test protocols described above, RPA-FT experiments and data treatment yield essentially two types of information, which reflects how the main torque component, i.e., r(l[Pg.829]

Because the duration for one measurement is very short (e.g., with a 1-Hz input, a cycle is completed in 1 sec), a dynamic test is suitable for gaining information in a short time frame or for monitoring time-dependent changes in gel network properties. When monitoring the gelation process at a fixed frequency, it usually takes a few hours for G to become approximately constant. The constancy can be judged by a constant value of G at a fixed frequency during a subsequent frequency or strain sweep test, which usually takes several minutes. 

A remarkable non-linear viscoelastic behaviour, known as the Payne effect was observed for NR/CNT nanocomposites. A sample with 10 phr of CNT displayed a great Payne effect. It was reported that, below 3.8 wt% of CNT, the Payne effect was not observable.It was reported as well a reduction of Payne effect for a composite with 9.1 wt% CNT, by increasing the temperature, from 298 K to 353 K. For this composite, the Payne effect was however much higher than for a composite with the same content of CB. It was commented that at this concentration CNT was above the percolation threshold. Remarkable Payne effect was observed also in IR based nanocomposites,from strain sweep tests performed in the shear mode it was noticed also below the percolation threshold of CNT (7.2 phr, see above) and it became much more evident above said threshold. 

Providing tests are performed at low strain amplitude, small enough for the complex modulus to exhibit no strain dependency, then dynamic testing yields in principle linear viscoelastic functions. This implies that, with an unknown material, a preliminary strain sweep test is performed in order to experimentally detect the maximum strain amplitude for a linear response to be observed [i.e. G lo, f(Y)]-As illustrated in Fig. 6 with data from Dick and Pawlowsky [20], such a requirement is practically never met within the available experimental window with filled rubber materials, whose linear region tends to move back to a lower and lower strain range as the filler content increases. 

Fig. 14 Strain sweep tests on a pure atactic polypropylene and an unfilled styrene-butadiene rubber compound using either an open-gap (VCOR) or a closed-cavity (RPA) torsional dynamic rheometer (authra- s experimental data)...

Fig. 15 Strain sweep tests on various SBR1500 based materials gum SBR is a sample cut from the bale compounds were prepared in a Haake mixer with Banbury rotors according to the following formulation (phr) SBR 1500 100 N330 Carbon Black 0, 30 or 50 Naphtenic Oil 5 Zinc oxide 5 Stearic acid 3 TMQ (trimethylquinoline, polymerized) 2 IPPD (N-isopropyl-N -phenyl-p-phenylene diamine) 1...

Table 1 Strain sweep tests experiments on SBR based materials SBR1500 gum...

Dynamic rheological properties of the melt are very sensitive to the molecular structure of a polymer. It is essential to perform dynamic strain sweep test before... 

The strain sweep test was perfonned [101] by means of a dynamic mechanical spectrometer (Rheometrics RDS 2) equipped with parallel plates. To avoid rapid drying during the test, the exposed edges of the specimen were covered with a thin layer of vaseline. It was noticed (Figure 14) that if the deformation was kept below 10%, both moduli remained relatively constant. 

Figure 14. Storage ( , circles) and loss ( , triangles) moduluses of a gluten -water mixture from a strain sweep test carried out at 28 (open symbols) and 43 °C (full symbols), and I rad s-1 frequency (modified from [101]).

Strain sweep tests (rubber process analyzer, updated for fourier transform rheometry) on molten virgin and carbon black filled poly(ethylene-co-butyl acrylate) composites and likely morphology in both the solid and molten states. 

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