Frequency-Dependent Measurements of Filled Elastomer Blend & Master Curve Construction - METTLER TOLEDO

Frequency-Dependent Measurements of Filled Elastomer Blend & Master Curve Construction

Purpose

The measurement of a filled elastomer blend using frequency-dependent DMA

 

Sample

The filled NR/SBR elastomer blend used for the DMA heating experiments in section 4.5.5. Temperature-dependent DMA measurements of filled elastomer blends is measured. The sample contains 25.5% NR, 7.2% SBR and 53.3% carbon black.

 

Conditions

Measuring cell: DMA/SDTA861e with shear sample holder

Sample preparation: Cylinder of 5-mm diameter punched out from a 0.9-mm thick film and mounted in the shear sample holder with predeformation

DMA measurement: The sample was measured isothermally in the frequency range 10-3 Hz to 103 Hz. The measurement temperature was increased in steps of about 10 K between -50 °C and 0 °C. The displacement amplitude was set to 3 Pm to make sure the sample was always in the linear range


Interpretation

The diagram shows the shear modulus and tan 𝛿 of an isothermal measurement at -48°C. To shorten the measurement time, 8 values per frequency decade were measured in the frequency range below 10Hz. At higher frequencies, 40 values per frequency decade were measured. 

The temperature-dependent experiments in Section 4.5.5 showed that the second small glass transition of SBR could only be observed with difficulty. The frequency-dependent measurement at -48°C however gives a clear indication of this second glass transition. If a separate relaxation range occurs, peaks with linearly sloped sides are expected in the log-log display of the G" and tan G' curves. In the low frequency side of the tan G peak, a deviation from this behavior can be seen at frequencies less than 4*10-2 Hz. This is due to the overlap with a second, small relaxation range namely the glass transition of the SBR. The measured curves were used to construct a master curve, whereby G' and G" were shifted simultaneously to achieve the best degree of overlap for both curves.  




It is not possible to construct an optimum master curve over the entire frequency range with this sample because the two relaxation ranges exhibit different temperature dependencies. This is a clear indication that the time-temperature superposition principle does not hold true. There can be many reasons for this such as the overlap of several relaxation ranges with different temperature dependencies, changes in the sample due to a reaction or crystallization, nonlinear behavior due to a displacement amplitude that is too large, as well as inadequate temperature stability during the measurement. In this particular case, the experimental conditions were thoroughly checked. Since the sample is stable, the deviation from the superposition principle is due to an overlap of relaxation ranges. 

 

Conclusions

Direct frequency-dependent measurements over a large frequency range allow a detailed analysis of the relaxation behavior and comprehensive information to be obtained on mechanical properties even with relatively complex materials. Checking the validity of the time-temperature superposition principle using master curve construction is a very sensitive way to detect the overlap of different relaxation processes. This can, for example, be caused by phase separation, crystallization and other inhomogeneities. 

Frequency-Dependent Measurements of a Filled Elastomer Blend and Master Curve Construction | Thermal Analysis Application No.HB479  | Application published in METTLER TOLEDO TA Application Handbook Elastomers Volume 2