Degradation of polymers by thermal gravimetric techniques

Abstract

hermogravimetric analysis (TGA) has come into wide T use for rapidly determining the thermal stability of substances. Essentially, by this method one measures weight loss as a function of temperature under controlled environmental conditions. Modern thermobalances are designed to produce a continuous record of weight as a function of temperature. Of course, information can be obtained by carrying out weighing operations manually. However, in order to r SPRING TRANSDUCER AKMATURE WEIGHT CALIBRATOR DAMPER VACUUM OL R CRUCIBLE CONTROLLING THERMOCOUPLE SAMPLE THERMOCOUPLE Figure I . Diagram of the design of the thermo-grav.POLYMER ENGINEERING A N D SCIENCE, JULY, 1965 insure that no features of the weight loss-temperature curve are overlooked, automatic continuous recording of weight and temperature are recommended.A commercial instrument that has been used by the present authors is shown schematically in Figure 1 ( 2 , 3 ) . The method used in this case is based upon an enclosed spring as the weight sensor, the extension of the spring being sensed electrically through the effect this has on the position of an iron core within a differential transformer. Several linear heating rates, as well as provision for maintenance of constant temperature, are incorporated in the furnace controls.Oftentimes pyrolysis occurs by a multi-step mechanism, with overlapping of temperature ranges for the various steps, so that the resulting irregular weight-temperature curves are very difficult to analyze. However, in many cases, including many polymer decompositions, the trace follows a simpler path which is characteristic of many decompositions. The sample weight decreases slowly as reaction begins, then drops rapidly over a relatively narrow temperature range and finally levels off as the reactant is exhausted. Figure 2 shows a primary thermogram for Teflon.The shapes of the curves in cases such as illustrated in Figure 2 are determined by the kinetic parameters of the pyrolysis, such as reaction order, frequency factor, and energy of activation. These values can be of importance in the elucidation of the mechanisms involved in polymer degradation (4) . TGA MethodsSome reported methods for obtaining kinetic parameters from such TGA traces involved lengthy equations and simplifying assumptions (5-7). Recently several exact methods have been proposed. These methods are based upon the assumption that the Arrhenius equation is valid for the degradation process.The relation of Freeman (8) may be written A log R, = n A log w, -(E/2,303R) A (1/T),where Rt is the rate of reaction, n is reaction order, E is 135

Thermal decomposition of mercerized linter cellulose and its acetates obtained from a homogeneous reaction

“…The values of these parameters are of major importance in the identification of the mechanisms involved in polymer decomposition [27,28] and as estimates to the thermal stability of polymers.…”

Thermal properties of cellulose acetate and its complexes with some transition metals

“…Thermogravimetric analyses, TGA and DTA, are widely used to investigate the thermal decomposition of polymers and to determine some kinetic parameters, e. g. the rate of decomposition, k, the reaction order, n, and the activation energy, E a 1) . The values of these parameters can be of major importance in the elucidation of the mechanisms involved in polymer degradation 2) and in the estimation of the thermal stability of polymers.…”

“…Tab. 2. The values of the start temperature (T onset ) and end temperatare (T outset ) of the main decomposition, the activation energy (DE), and the electronegativity of the cation of cellulose triacetate and its complexes.…”

Electrothermal lifetime prediction of polyimide wire insulation with application to aircraft

“…TG analysis is a common polymer degradation tool that measures the degree of degradation, i.e., weight loss, with respect to time, t, and temperature, T. 16 It has long been recognized as a means by which life testing can be conducted quickly and accurately, without the need for expensive, long-term aging studies. 10 The degree of degradation, a, varies from 0 (no weight loss) to 1 (complete weight loss).…”