Thermomechanical methods, applications

The above-mentioned method of deformation calorimetry has found a rather wide application. Modifications of the original design were constructed 72-75) and applied for investigating the thermomechanical behaviour of polymers and polymer composites. At the same time, the commercial Calvet-type calorimeters has been used in thermomechanical experiments on rubbers not only in the uniaxial mode 76-78 but also in torsion 79 80). Thus, deformation calorimetry has proved to be quite adequate in terms of sensitivity, specificity, rapidity and reliability and therefore seems to be the most promising experimental method of thermomechanical type. 

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. 

Figure 7 shows CO2 evolution from the PET films with different thermomechanical histories and consequent differences in film thickness, ciystallinity and molecular orientation (9). In all three cases strong CO2 generation was observed and could be measured with good repeatability. Similar measurements have been made on poly(8-caprolactone) (PCL) and on PCL/PVC blends (10). These measurements on different polyesters demonstrate the applicability of the CO2 method to materials for which carbonyl absorption measurement is not very suitable. 

Despite the disadvantages, the method has found general application with minor adjustments. In another variant, the specimens are heated under load to a certain holding temperature and deformation Is measured after removal from the furnace. Torsion methods are employed for more detailed investigation of the thermomechanical properties of refractories (e.g. Hennicke and Tomsu, 1970 Spicak, 1971 Staron, 1975). 

Selection of the most suitable cross-linking method (peroxide, silane, or radiation) that gives the material the thermomechanical properties adequate for the specific application. 

Reading, M. and Haines, P.J., Thermomechanical, dynamic mechanical and associated methods, in Thermal Methods of Analysis Principles, Applications and Problems, Haines, P.J., Ed., Blackie Academic and Professional, Glasgow, U.K., 1995. 

Some examples of applications of neutron and synchrotron radiation diffraction applied to the determination of residual stresses in various industrial and technological components have been presented. The reliability of the technique has been shown, being able to determine residual stresses induced by various thermomechanical treatments, such as shrink-fit joints, welds and surface treatments in automotive and aerospace materials. It has been shown how, by this method, it is possible to determine stresses both in the coating and in the substrate of plasma-spray deposed hydroxyapatite layers on Ti alloy for biomedical applications. 

The following examples are used to demonstrate the broad range of applications of thermomechanical and thermoelectrical measurements. Many of the applications are not typical of the routine types of tests described earlier, but are placed here to show the diversity of these techniques in characterising materials and provide short case studies which present the methods in the context of addressing particular problems. 

The description of thermomechanical and thermoelectrical measurements in a modest chapter such as this is an ambitious exercise. The author has attempted to cover a wide range of methods and applications with the intention of illustrating the diversity of this field whilst emphasising the relationships between the static techniques (such as TMA and TSCA) and the dynamic techniques (dynamic force TMA, DMA and DETA). With the exception of TMA, these methods are often promoted as some of the more advanced thermal analysis techniques. It is hoped that the preceding pages help to dispel this myth without belittling their ability to measure useful properties. 

In this book, it is intended to provide the reader with useful and comprehensive experimental data and models for the design and application of FRP composites at elevated temperatures and fire conditions. The progressive changes that occur in material states and the corresponding progressive changes in the thermophysical and thermomechanical properties of FRP composites due to thermal exposure will be discussed. It will be demonstrated how thermophysical and thermomechanical properties can be incorporated into heat transfer theory and structural theory. The thermal and mechanical responses of FRP composites and structures subjected to hours of reahstic fire conditions will be described and validated on the full-scale structural level. Concepts and methods to determine the time-to-failure of polymer composites and structures in fire will be presented, as well as the post-fire behavior and fire protection techniques. 

The second, obvious application is studying polvmer-filler interactions in filled and reinforced composites. Some data are presented for wollastonite and quartz filled Bisphenol-A based and cycloaliphatic epoxies [6]. These data show, that Tg shifts observed by different relaxation methods (dielectric spectroscopy, DSC, thermomechanical measurements) are not necessarily the same (Table 1.), they depend on the effective frequency, changes in activation energy have also to be taken into account. Correlations between Tg shift and polymer adsorption can be understood using Lipatov s theory [7]. Positive Tg shift usually indicates strong adhesion, while negative Tg shift can be explained by the fact that the adsorbed polymer layer forms a looser structure than that of the bulk material. If both the neat resins and their composites are studied dielectrically, the origin of the low-... 

Tailored characterization methods for the SME were also developed for biomedical applications, such as for stents. A mechanical key characteristic for vascular stents is to withstand the compressive radial stresses over the lifetime of the application, i.e., maintain desirable thermomechanical characteristics with respect to recovery and deployment [63]. In a study on this topic, SME characterization methods were applied to a shape-memory stent from polymer networks, synthesized via photopolymerization of fert-butyl acrylate and PEG dimethacrylate [72]. The free recovery response of polymer stents at body temperature was studied as a function of Tg, crosslinking density, geometrical perforation, and deformation temperature. 

In addition, an adjustment to the specific sample geometries in various applications is needed. There are a number of crucial aspects for a successful translation of SMP technology into industrial applications, such as a standardization of the different methods described for quantification of the shape-memory properties. The recently reported 3-D thermomechanical constitutive model assuming active and frozen phases, representing the multiphase character of thermoplastic SMPs can be an especially fruitful approach for the future development of finite element models for prediction of the thermomechanical behavior. 

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. 

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. 

Physical modification involves thermal treatments such as plasma or nonthermal treatments like application of electric discharge, ultrasound, ultraviolet, or high-frequency cold plasma to the fiber surface. Stmctural and surface properties of the fibers are changed by these treatments, which result in improved mechanical bonding to polymers. These treatments are apphed to separate the fiber bundles into individual filaments and modify the fiber surface for more compatibility with the matrix in the composite [6]. If separation of the fiber bundles is desired, methods like steam explosion and thermomechanical processing are adopted. Methods like plasma (thermal) treatment, dielectric barrier techniques, or corona discharge (nonthermal) treatments (CDT) are anployed to modify the fiber surface. 

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