Thermomechanical analysis presentation

Thermomechanical analysis is one of the thermal analysis techniques

1. Thermomechanical Analysis (TMA) Inara Ahmadova

2. Content

3. 1) Introduction Thermomechanical Analysis or TMA is one of the most common techniques that measure dimensional changes of solid or liquid materials as a function of temperature, time and applied force. TMA is often used to measure the coefficients of thermal expansion (CTE), glass transition temperature ( Tg ) and compression modulus of a material by applying constant force at varying temperatures.

4. Many materials change their thermomechanical features during heating and cooling. For example, phase changes may occur due to softening thermal expansion . TMA analyzes provide valuable information about the content , structure , production conditions and application possibilities of different materials. The application areas of TMA devices range from quality control to research and development. Areas of usages are generally plastics and elastomers , paints , adhesives , ceramics , glass , metal and composite materials .

5. 2) Apparatus

6. Instrumentation is arranged such that a predetermined load is applied to the sample, almost invariably through a quartz rod . Quartz has the advantage of possessing a small coefficient of thermal expansion and its low heat conductivity allows the a linear variable displacement transducer (LVDT) to be spaced away from heat sources. In this instrument, the load platform is actually a balance arm arrangement, allowing the quartz rod and probe assembly weight to be counterbalanced . Most displacement transducers drift significantly if their ambient temperature changes. Figure 4 shows water circulation below the transducer to obviate the heat-rise problem . The area around the sample can usually be both heated (electrically) and cooled (cold nitrogen gas) and the sample temperature (sensed by a thermocouple ) has to be subject to programmer control.

7. 3) THE PARTS OF A T M A INSTRUMENT This is the most important part of the instrument. A predetermined load is applied to the sample via probe. There are three main types of the probe for TMA : Expansion/Compression Probe: It is used for the measurement of the deformation by the thermal expansion and the transition of the sample under the compressed force is applied. Penetration Probe: It is used for the measurement of the softening temperature . Tension Probe: It is used for the measurement of the thermal expansion and the thermal shrinkage of the sample such as the film and the fiber. The materials of probes are quartz glass, alumina, and metals. The choice is dependent on the temperature range and/or the measurement purpose. Probe

8. Linear Variable Displacement Transducer (LVDT) LVDT is a type of electrical transformer used for measuring linear displacement (position). LVDTs are inherently frictionless , they have a virtually infinite cycle life when properly used. They have been widely used in applications such as power turbines , nuclear reactors , aircraft and many others. These transducers have low hysteresis and excellent repeatability . LVDT operation doesn’t require an electrical contact between the moving part and coil assembly, but instead relies on the electromagnetic coupling . Current is driven through the primary coil at A , causing an induction current to be generated through the secondary coils at B.

9. T hermocouple The temperature in the system is measured by thermocouples . The position of thermocouple is important. The thermocouple for temperature measurement is located near the sample. For the formation of the thermocouple at least two metals should be joined together to form two junctions. The thermocouple cannot be formed if the two junctions are not formed.  Some of the elements used commonly for thermocouple are Copper, Iron, Platinum, Rhodium, Iridium etc.

10. 4 ) TMA PT1000 This system meets all TMA/DTMA requirements and is designed with reproducibility, accuracy and high-precision in mind . The device is capable of performing a range of thermo - mechanical experiments on samples of varying shapes and sizes and in a wide temperature range. The integrated Force Generator permits carrying out of both static and dynamic measurements. This unit is intended for use with composite materials, glass, polymers, ceramics and metals .

11. 5) Dynamic and load effects in TMA The primary application of TMA is in the detection of changes in the modulus of a material at major transitions, such as melting or glass transitions . The material thus becomes fluid, or at least viscoelastic , above the transition, which leads to major complications in interpreting the curves. The material will flow more under higher loads. This is illustrated schematically in Figure 6 for a penetration probe being used to detect a glass transition of a polymer. In this case, high stress (C) activates the molecular motion at lower temperatures and the penetration will normally accelerate as temperature increases . This can be empirically changed by choice of compression geometry (see Figure 5) which will produce radically different curves.

12. This dynamic softening will not follow the material's expansion as does the static strain. Figure 7 shows, how this technique can be used to follow the softening of amorphous poly(ethylene terephthalate ) (PET) as it passes Tg and its subsequent hardening due to crystallization . For comparison, the DSC trace for the material is also shown. The static TMA can be taken as the locus of the lowest points of the trace throughout. It is clear that because of the complexities of thermal expansion, viscoelastic deformation above Tg and volumetric contraction during crystallisation , the normal TMA trace defies simple analysis.

13. Many industrial applications of TMA are comparative and used for product quality control . This is the case for wax or fat blends where the compression geometry automatically provides greater resistance to flow in the semi-solid materials as they melt. A typical result is shown in Figure 9 where the melting transitions of each wax component are defined by a sharp softening of the sample. 6) Applications

14. A further example of an industrial application is shown in Figure 10 . TMA with a square-ended probe is used to assess the degradation in mechanical properties of a nylon component exposed to hot engine oil . Component B will be close to failure in service. The changes which are assessed as a combination of elevated Tg and reduced crystallinity , are due to oxidative changes on the nylon and a decrease in the equilibrium level of water .

15. Figure 11 shows data for a series of poly(ester) yarns with different degrees of frozen-in orientation . The higher the initial sample orientation , the more complex the thermal profile . For the highest birefringence sample ( η =0.05) there are three discrete relaxations , which allow the frozen-in strain to translate to an active stress. This would appear to be an under-utilized technique.

16. Another interesting application of TMA is a study of the thermomechanical properties of the polymers used as denture bases . The glass-transition temperatures of such polymers are important because there are considerable variations in temperature in the mouth. Hugget used TMA measurements to watch the effect of various modifications of the polymerization of methyl methacrylate on the of the resultant material . The factors expected to increase of a polymer are: the presence of groups in the backbone which increase the energy required for rotation , e.g . long side chains ; secondary bonding between chains e.g. hydrogen bonding ; crosslink