4 Thermomechanical Analysis (TMA) Startseite（4热机械分析(TMA)Startseite）.pdfVIP

172 4 Thermomechanical Analysis 4 Thermomechanical Analysis (TMA) 4.1 Principles of TMA 4.1.1 Introduction A dilatometer is used to determine the linear thermal expansion of a solid as a function of temperature. Unlike the classical methods in which the experimental setup is kept as free of forces as possible, in thermomechanical analysis, a constant, usually small load acts on the specimen [1]. The measured expansion of the specimen can be used to determine the coef- ficient of linear thermal expansion α. The 1st heating phase yields information about the actual state of the specimen, including its thermal and mechanical history. When thermoplastics soften, especially above the glass transition, orientations and stresses may relax, as a result of which postcrystallization and recrystallization processes may occur. On the other hand, the specimen may deform under the applied test load. To determine the coefficient of expansion as a material characteristic, the material must not undergo irreversible changes, such as postcrystallization, postpoly- merization, relaxation of orientation or internal stress and so forth during a 2nd heating phase that has followed controlled cooling. In addition, the anisotropy of the molded part must be taken into account; it is advisable to measure in the x-, y- and z-axes. The behavior of plastics on exposure to heat is described in detail in Section 1.1.4. Coefficient of linear thermal expansion can serve as a material characteristic only where material behavior is reversible. The coefficient of linear thermal expansion may be recorded as the mean α (∆T) or differential α(T) and is calculated in accordance with DIN 53 752 [2], ISO 113591 Part 1 and 2