Penetration, thermomechanical analysis

One of the more recently exploited forms of thermal analysis is the group of techniques known as thermomechanical analysis (TMA). These techniques are based on the measurement of mechanical properties such as expansion, contraction, extension or penetration of materials as a function of temperature. TMA curves obtained in this way are characteristic of the sample. The technique has obvious practical value in the study and assessment of the mechanical properties of materials. Measurements over the temperature range - 100°C to 1000°C may be made. Figure 11.19 shows a study of a polymeric material based upon linear expansion measurements. 

Thermomechanical Analysis (TMA). Thermomechanical analysis (TMA) measures shape stability of a material at elevated temperatures by physically penetrating it with a metal rod. A schematic diagram of TMA equipment is shown in Fig. 2.23. In TMA, the test specimen s temperature is raised at a constant rate, the sample is placed inside the measuring device, and a rod with a specified weight is placed on top of it. To allow for measurements at low temperatures, the sample, oven, and rod can be cooled with liquid nitrogen. 

In thermomechanical analysis (TMA) the deformation of the sample under stress is monitored against time or temperature while the temperature increases or decreases proportionally to time. Changes are detected by mechanical, optical, or electrical transducers. The stress may be a compression, penetration, tension, flexure, or torsion. Generally the instruments are also able to measure the sample dimensions, a technique called thermodilatometry. The stress F/A) expressed in N/m or Pa may be a normal tensile stress cr, a tangential shearing stress x, or a pressure change Ap the force applied is F and A is the area. 

Thermomechanical analysis allows the calculation of thermal expansivity from the same data set as used to calculate the Tg. Since many materials are used in contact with a dissimilar material in the final product, knowing the rate and amount of thermal expansion helps design around mismatches that can cause failure in the final product. These data are only available when the Tg is collected by thermal expansion, not by the flexure or penetration method. This is in many ways the simplest or most essential form of TMA measurement. A sample is prepared with parallel top and bottom surfaces and is allowed to expand under minimal load (normally 5mN or less, ideally OmN) as it is slowly heated and/or cooled. The CTE is calculated by ... 

Thermomechanical analysis methods are used in geometries more commonly associated with traditional mechanical testing to increase sensitivity or to mimic other tests. The most common of these are the flexural and penetration modes. Flexure studies involve loading a thin beam, often a splinter of material, with a constant load of lOOmN or more and heating until... 

Thermomechanical analysis in the penetration mode has been used to check the effect of plasticizers, solvents and moisture on the softening temperature of three cellulose derivatives used as film coating agents [26]. Table 11 lists the... 

Thermomechanical Analysis. A thermomechanical analyzer (Perkln-Elmer TMS 2) was used to measure glass transition temperature (Tg) and coefficient of thermal expansion (a ) of completely cured samples In the penetration and expansion modes respectively. Experimental conditions are listed In Table I. 

Thermomechanical analysis TMA Distance change Mechanical changes (expansion, contraction, penetration)... 

This system is expanded to the thermomechanical analysis capability which requires the measurement of length in a penetration, expansion, or extension mode. The basis of this analysis has been discussed in the literature. The relationships of the thermal expansivity a and the length 1 of a sample with respect to temperature and tensile force f are expressed as follows... 

Dilatometry and thermomechanical analysis (TMA) are also techniques used to monitor the thermal behavior of fibers. They both employ a sensitive probe in contact with the surface of the sample, and the thermal transitions are detected either by a change in volume or modulus of the sample, respectively. In the latter case, the probe necessarily penetrates the sample surface. A variable transformer records the voltage output that is directly proportional to the degree of displacement of the probe during a thermally induced transition. TMA is a more sensitive technique than either DTA or DSC for detecting thermal transitions. 

There are several other thermal analysis techniques. In thermomechanical analysis (TMA), mechanical changes are monitored versus temperature. Expansion and penetration characteristics or stress-strain behavior can be studied. In dynamic mechanical analysis (DMA), the variations with temperature of various moduli are determined, and this information is further used to obtain fundamental information such as transition temperatures. In thermogravimefric analysis (TGA), weight changes as a function of temperature or time (at some elevated temperature) are followed. This information is used to assess thermal stability and decomposition behavior. 

In thermomechanical analysis (TMA), the change in mechanical properties is measured as a function of temperature and/or time. A probe in contact with the sample moves as the sample undergoes dimensional changes. The movement of the probe is measured with an LVDT. The sample deformations that can be measured are compression, penetration, extension, and flexure or bending. 

Thermomechanical analysis (TMA) measures the deformation of a material contacted hy a mechanical prohe, as a function of a controlled temperature program, or time at constant temperature. TMA experiments are generally conducted imder static loading with a variety of probe configurations in expansion, compression, penetration, tension, or flexime. In addition, various attachments are available to allow the instrument to operate in special modes, such as stress relaxation, creep, tensile loading of films and fibers, flexural loading, parallel-plate rheometry, and volume dilatometry. The type of probe used determines the mode of operation of the instrument, the manner in which stress is apphed to the sample, and the amount of that stress. 

Thermomechanical analysis (TMA) has been used for softening measurements of polymers and the measurement of the amount of probe penetration into a polymer at particular applied forces as a function of temperature. This technique allows evaluations of and the evaluation of dimensional properties over the temperature range of use or under actual accelerated conditioning cycles (plots of temperature versus compression (mm)). 

Thermomechanical Analysis. The equipment used in thermomechanical analysis (TMA) is similar in principle to that for TD, but provision is made for applying various types of load to the specimen, so that penetration, extension, and flexure can be measured. This approach to analyzing such modes of deformation is illustrated schematically in Figure 15. The technique finds most use in polymer studies, as in the determination of glass-transition and softening temperatures for thin films and shrinkage characteristics of fibers. 

Thermal expansion coefficients are determined by thermomechanical analysis using a dilatation-penetration probe for adhesive pastes and an extension probe for self-standing films. The output of the thermal analyzer equipped with a dilatation probe is a curve plotting the variation of adhesive thickness as a function of the temperature. 

Thermomechanical Analysis. A schematic representation of a t3q)ical thermomechanical analyzer (tma) device is shown in Figure 13. Several such devices are available commercially. The probe and Ivdt assembly are similar to those of the dilatometer described above. The commercial units are generally fitted with a selection of sample probes so that besides thermal expansivity, sample penetration, softening point, and heat distortion under load can also be measured among other quantities. 

A series of polystyrene molecular weight standards available from the Pressure Chemical Company have been studied by Keinath et al. via thermomechanical analysis. AflYs ranged from nominal molecular weight 2,200 up to 7,200,000. Smooth glassy samples, contained in DSC pans, were prepared by fusing the as-received powdered samples. Tlje weighted probe of the DuPont 943 TMA unit was placed onto the surface of the sample and the entire assembly was heated at S C/min from room temperamre up to a point where the probe had penetrated through the whole thickness of the sample. 

---