Thermomechanical analysis applications

Thermal expansion and contraction are reversible effects of temperature which may be very important in some applications. Usually expansion is measured using thermomechanical analysis (TMA) (see ISO 11359-2 [4]). 

Dynamic properties are more relevant than the more usual quasi-static stress-strain tests for any application where the dynamic response is important. For example, the dynamic modulus at low strain may not undergo the same proportionate change as the quasi-static tensile modulus. Dynamic properties are not measured as frequently as they should be simply because of high apparatus costs. However, the introduction of dynamic thermomechanical analysis (DMTA) has greatly widened the availability of dynamic property measurement. 

The thermal characterisation of elastomers has recently been reviewed by Sircar [28] from which it appears that DSC followed by TG/DTG are the most popular thermal analysis techniques for elastomer applications. The TG/differential thermal gravimetry (DTG) method remains the method of choice for compositional analysis of uncured and cured elastomer compounds. Sircar s comprehensive review [28] was based on single thermal methods (TG, DSC, differential thermal analysis (DTA), thermomechanical analysis (TMA), DMA) and excluded combined (TG-DSC, TG-DTA) and simultaneous (TG-fourier transform infrared (TG-FTIR), TG-mass spectroscopy (TG-MS)) techniques. In this chapter the emphasis is on those multiple and hyphenated thermogravimetric analysis techniques which have had an impact on the characterisation of elastomers. The review is based mainly on Chemical Abstracts records corresponding to the keywords elastomers, thermogravimetry, differential scanning calorimetry, differential thermal analysis, infrared and mass spectrometry over the period 1979-1999. Table 1.1 contains the references to the various combined techniques. 

Thermogravimetry (TG), differential thermal analysis (DTA), and differential scanning calorimetry (DSC) are the most frequently used techniques in lignin chemistry, although thermomechanical analysis (TMA) has also been used effectively in the analysis of thermal properties of lignin (Goring 1963). In this section, the principles of TG, DTA, and DSC, and their application to lignin are described. 

For thermomechanical analysis (TMA), applicable only to dry fibers, a load of 0.005 g/den. was applied to the fiber and a scan rate of 20°C/mln was used. An X-Y recorder was used to record fiber length as a function of temperature. 

Two examples of the use of localised thermal analysis are provided in order to illustrate the generic applications of this approach. Figure 8 shows localised thermomechanical analysis of the surface of the multi-layer film in Figure 4. Measurements were made at points within this image describing the bulk polymer, the central gas-barrier layer and the thin tie-layer between this and the bulk film. The melting transition temperatures are consistent with high density polyethylene, poly(ethylene-co-vinyl alcohol) and medium density polyethylene for the bulk, gas barrier and tie layers, respectively [101],... 

Thermal analysis methods can be broadly defined as analytical techniques that study the behaviour of materials as a function of temperature [1]. These are rapidly expanding in both breadth (number of thermal analysis-associated techniques) and in depth (increased applications). Conventional thermal analysis techniques include DSC, DTA, TGA, thermomechanical analysis, and dynamic mechanical analysis (DMA). Thermal analysis of a material can be either destructive or non-destructive, but in almost all cases subtle and dramatic changes accompany the introduction of thermal energy. Thermal analysis can offer advantages over other analytical techniques including variability with respect to application of thermal energy (step-wise, cyclic, continuous, etc.), small sample size, the material can be in any solid form - gel, liquid, glass, solid, ease of variability and control of sample preparation, ease and variability of atmosphere, it is relatively rapid, and instrumentation is moderately priced. Most often, thermal analysis data are used in conjunction with results from other techniques. 

Figure 18.48 Application of thermomechanical analysis to thermal stress analysis of polyolefin films (as received and cold drawn) Source TA Instruments, New Castle, DE, USA)...

Information on standard methods for the determination of the properties of polymers is reviewed in Table 4.1. General reviews of the determination of thermal properties have been reported by several workers [1-6]. These include application of methods such as dynamic mechanical analysis [5], thermomechanical analysis [5], differential scanning calorimetry [4], thermogravimetric analysis [6], and Fourier transform infrared spectroscopy [4], in addition to those discussed below. 

Figure 4.6 shows an application of thermomechanical analysis to the characterization of a composite material, that is, an epoxy printed circuit board material. The Tg is readily determined from this curve. 

This system measures dimensional changes as a function of temperature. The dimensional behavior of a material can be determined precisely and rapidly with small samples in any form— powder, pellet, film, fiber, or as a molded part. The parameters measured by thermomechanical analysis are the coefficient of linear thermal expansion, the glass-transition temperature (see Figs. 9-10 and 9-11), softening characteristics, and the degree of cure. Other applications of TMA include the taking of compliance and modulus measurements and the determination of deflection temperature under load. 

Thermomechanical analysis instruments are ideally suited to measure creep. In these experiments the increase in strain is measured with time following the application of a constant stress to the sample, followed by the recovery of the strain when the stress is removed. Figure 4.26 shows a typical TMA creep-recovery curve. In these experiments, an instantaneous compression or tensile stress is applied to the sample, and the time-dependent strain is measured at constant temperature. During the loading cycle, the resultant creep curve... 

Menard, K. P. (1996), Perkin-Elmer Thermal Analysis Application Note Thermomechanical Analysis Basics, Part I. 

Riga, A. T. and Colhns, E. (1991), Material Characterization by Thermomechanical Analysis Industrial Applications, ASTM STP1136, pp. 71-83. 

Table 2.5 summarises the main applications of thermal analysis and combined techniques for polymeric materials. Of these, thermomechanical analysis (TMA) and dynamic mechanical analysis (DMA) provide only physical properties of a very specific nature and yield very little chemical information. DMA was used to study the interaction of fillers with rubber host systems [40]. Thermomechanical analysis (TMA) measures the dimensional changes of a sample as a function of temperature. Relevant applications are reported for on-line TMA-MS cfr. Chp. 2.1.5) uTMA offers opportunities cfr. Chp. 2.1.6.1). The primary TA techniques for certifying product quality are DSC and TG (Table 2.6). Specific tests for which these techniques are used in quality testing vary depending upon the type of material and industry. Applications of modulated temperature programme are (i) study of kinetics (ii) AC calorimetry (Hi) separation of sample responses (in conjunction with deconvolution algorithms) and (iv) microthermal analysis.

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