19.01.2023 by Martin Rosenschon

Thermal Analysis Can Also Be Dynamic

Characterization of Visco-Elastic Material Properties Using Dynamic-Mechanical Analysis

In the design process for a product or component, knowledge of the temperature-dependent properties of the materials used is of central importance. Winter tires, for example, consist of rubber mixtures that are specifically adapted for cold temperatures. This ensures optimum grip as well as abrasion properties and therefore safe driving.

Dynamic mechanical analysis (abbreviated: DMA) is a method that provides information on the elastic and viscous behavior of a material as a function of temperature and load frequency. A test sample is subjected to a defined, oscillating load and the resulting deformation is measured. The parameters storage modulus E', Viscous modulusThe complex modulus (viscous component), loss modulus, or G’’, is the “imaginary” part of the samples the overall complex modulus. This viscous component indicates the liquid like, or out of phase, response of the sample being measurement. loss modulus E'' and damping factor tan δ can be determined from the applied StressStress is defined as a level of force applied on a sample with a well-defined cross section. (Stress = force/area). Samples having a circular or rectangular cross section can be compressed or stretched. Elastic materials like rubber can be stretched up to 5 to 10 times their original length.stress σ, the resulting StrainStrain describes a deformation of a material, which is loaded mechanically by an external force or stress. Rubber compounds show creep properties, if a static load is applied.strain ε and their offset δ (see figure 1). The storage modulus E' represents the elastic reversible (spring-like) behavior and the loss modulus E' represents the viscous component or also the energy dissipation. The combination of both parameters is reflected in tan δ, which describes the damping properties.

Figure 1: Schematic principle of a DMA measurement

By using different sample holders, accessories and measuring methods, almost any material can be measured with the DMA, from liquid or viscous media to soft elastomers, and from unfilled and fiber-reinforced plastics to metals and ceramics.

Depending on the material, temperature and load, the characteristics of the viscoelastic properties vary greatly. At room temperature and low deformations, metals and their alloys are usually purely elastic, whereby polymers mostly show a mixed behavior of viscosity and elasticity. Polymers also have a so-called glass-transition temperature. At low temperatures, they are comparatively stiff and brittle: as the name suggests, glass-like. In the Glass Transition TemperatureThe glass transition is one of the most important properties of amorphous and semi-crystalline materials, e.g., inorganic glasses, amorphous metals, polymers, pharmaceuticals and food ingredients, etc., and describes the temperature region where the mechanical properties of the materials change from hard and brittle to more soft, deformable or rubbery.glass transition, the amorphous polymer chains can move toward each other and the viscous portion increases. After that, the material is in the entropy-elastic state and is – depending on the material – comparatively soft. Based on the direct change in mechanical properties, the glass transition can be clearly identified via dynamic mechanical analysis. In addition to DMA, it can also be determined using differential scanning calorimetry (abbreviated: DSC) based on the resulting change in heat capacity.             

However, DMA is the much more sensitive method in this regard and allows for the resolution of effects involving little or no thermal changes. Figure 2 shows the measurement of a sample made of polytetrafluoroethylene (PTFE), also known under the brand name Teflon®, using DSC (red, 10 K/min) and DMA (black, 1 Hz, 2 K/Min). The best-known example of the use of PTFE is the non-stick coating for pans, which is due to its high thermal and chemical resistance. However, it is also frequently used in medical applications or in tribological systems such as bearings.

Three effects can be seen in the DMA measurement. At -123°C (onset E'), the material shows a glass transition in storage modulus E' (solid line) which is attributable to the amorphous regions. Between 20°C and 40°C, PTFE has two closely spaced solid-solid transformations. In the DMA measurement – based on the test parameters – one effect can be seen at 29°C (onset E'). In the DSC curve (red), both transformations can be identified with peak temperatures at around 21°C and 31°C. Furthermore, a glass transition occurs at 113°C (onset E') in the DMA curve. While the solid-solid transformations can be clearly portrayed by the DSC, the glass transition temperatures in this case cannot be recorded using this method. Due to the low heat flows, these can only be measured using DMA. Since glass transitions originate from the amorphous part of the material, their measurement using differential scanning calorimetry is often difficult, especially for highly crystalline materials, and it requires the use of DMA.
 

Figure 2: Measurement of a PTFE sample using DMA (black) and DSC (red)

 

Whether high-strength or soft materials, high or low loads, NETZSCH offers the right DMA system for your application – starting with table-top devices providing dynamic forces in the double-digit Newton range up to high-force systems with loads of up to 1.5 kN. Depending on the device and setup, measurements can be carried out from -160°C up to 1500°C in frequency ranges from 0.0001 to 200 Hz.

The application of dynamic mechanical analysis can answer a large number of questions. The results allow for the best possible materials to be selected for specific operating temperatures and load cases, as in the example of winter tires. By including the frequency dependency, materials can also be evaluated with regard to their sound insulation in the human hearing range. Comparative measurements can be used to evaluate the influence on polymers of fillers such as glass fibers, additives and plasticizers, and recipes can be derived. On the basis of viscoelastic material characteristics, process parameters can also be analyzed, such as whether a resin fully hardens during processing. 
In addition, with suitable accessories, the influence of humidity on the material can be observed or the reaction of the material with liquid media (e.g., oil or solvents) can be examined. For this purpose, humidity generators or immersion baths are available for the DMA systems.   

This is only a handful of the many possible uses of DMA measurements. DMA devices usually have other measurement modes, such as RelaxationWhen a constant strain is applied to a rubber compound, the force necessary to maintain that strain is not constant but decreases with time; this behavior is known as stress relaxation. The process responsible for stress relaxation can be physical or chemical, and under normal conditions, both will occur at the same time. relaxation, CreepCreep describes a time and temperature dependent plastic deformation under a constant force. When a constant force is applied to a rubber compound, the initial deformation obtained due to the application of the force is not fixed. The deformation will increase with time.creep measurements and much more, which also expand the field of application.

Within the next few weeks, we would like to introduce you to a wide variety of application examples recorded with NETZSCH DMA devices in different application fields and inspire you for your future tasks and challenges. Stay tuned!