Dynamic Mechanical Analysis DMA

With the establishment of our own dynamic mechanical analysis, we now have a complete range of classic thermal analysis methods in our laboratory. Thermogravimetry (TGA) and differential scanning calorimetry (DSC) have been used successfully in our company for many years.

Elastomers are viscoelastic materials and combine viscous and elastic properties. These properties can be ideally measured in dynamic tests – and that is what DMA is for.

The DMA measures quantitatively and qualitatively:

• Viscoelastic behaviour and damping properties at various defined deformations or frequencies
• Loss and storage module for various defined deformations or frequencies
• Flow and relaxation behaviour …
  all dependent on temperature (-100°C to 600°C)

We can measure almost all elastomer samples, regardless of whether they are finished parts, test plates or damaged parts.
Among others, standards such as ISO 6721-1:2011 apply to DMA.

The DSC measures heat flows as a function of temperature; the change in enthalpy as a function of temperature,
the TGA measures the weight of compounds in the same way.

At DMA, we have several options:

DMA measurements and parameters:

• Distance; different amplitudes
• frequencies
• Force, static or dynamic
• Storage and loss module
• Loss angle (tan δ)

All sizes are measured within a very small tolerance range:
Force max. 18N; ± 10 -5 N
Displacement resolution 1nm
Frequency (sinusoidal) up to max. 200Hz
We can measure in a temperature range from -100°C to the end of the polymer’s existence range. Cooling is provided by a compressed air cooler.

What are the requirements for the samples to be measured?
Almost none!
In tensile mode, the usual tensile test specimens (S3A, S3, etc.) shortened in the clamping area are used.
For compression and bending modes, plane-parallel samples from test plates or finished parts are required.
In both cases, we can set the required sample thicknesses with our splitting machine.

The measurement times per test are generally longer than in TGA or DSC. DMA has a larger sample chamber volume and poor heat transfer to the sample, so small cooling and heating rates are used.

We will start with the following equipment:

• Cantilever (fixed bending, single or double)
• towing mode
• Pressure mode
  This mode allows most measurements to be performed on polymers.

There are roughly two different modes of operation for the DMA:

I. Creep tests under constant load or constant deformation at constant or linearly falling or rising temperatures; the resulting deformations are then measured under constant load or the changing force required is measured under constant deformation.
II. Dynamic measurements at different frequencies or amplitudes (temperatures as above)

I)

Here, the residual tensile deformation and tensile stress relaxation were determined at different temperatures on the same sample. The DMA not only provides the corresponding displacement information (blue y-axis on the right) but also records the force drop at constant deformation (black, y-axis on the left).
This also works in pressure mode, whereby measurements of the compression set can be taken even on the smallest particles.

The percentage deformation (-20%) is shown in black on the left y-axis, the temperature in red on the first right y-axis, and the force required for the deformation in blue on the second right y-axis.
Here, a DVR was measured in accordance with VW PV 3307 at 150°C of 10.8% – with a total measurement time of just over 4 hours.
(When temperatures rise and fall, the temperature is shown on a separate y-axis (red); the measurement time is simply shown on the x-axis.)

What is shown here at 150°C can also be measured in cold conditions as an alternative to the cold DVR or TR10, except that we are not limited to any sample geometries and can also measure very small particles.

Data of this type can also be measured:

II)

Let’s take a look at what we can do with dynamic measurements.
Classically, the curves obtained here look like this:

Two of the several possible measurement data are plotted here above the temperature: storage modulus as an expression of the elastic component and the loss angle as an expression of the ratio of storage and loss modulus. As viscoelastic materials show a dependence on the deformation rate, the data for an EPDM material is shown here for 3 different frequencies. Even in a frequency range of two powers of ten, the glass transition determined via tan delta changes by 12°K. You can also see the increase in storage modulus with frequency (green)
Such a multi-frequency measurement can be carried out in one run.
The non-linear storage and loss moduli can be used to obtain the data required for finite element analyses for material calculations.
The tan delta is a measure of the elastic component of the material. If it is small, the material has high elasticity; if it is high, it has high damping. This dimensionless value indicates the elasticity of a material with a high degree of accuracy; we can measure it with an accuracy of 10-4.

Why tan delta?

If a polymer, which consists of long molecular chains, is deformed, the chains react to the force applied with a slight time delay by reorienting. This mobility is temperature-dependent. At very low temperatures, the mobility is frozen: the glass state. As temperatures rise, mobility increases. The chains can follow very slow deformations well by reorientation. With very fast deformations, there is no longer enough time and the material appears to harden. The tan delta and the glass transition increase with increasing frequency.

Illustration of the phase shift between the acting deformation and the reaction of the viscoelastic sample:

Further questions:
Bernd Sprenger (bernd.sprenger(at)o-ring-prueflabor.de)