Analysis of thermogravimetric weight loss data from the literature has indicated correlations between the overall energetics of decomposition and characteristics of the thermogravimetric curves for measurements made under nitrogen. Such correlations provide a means for comparing relative thermal stabilities of polymers from easily obtained weight loss data. ZUSAMMENFASSUNG:Eine Analyse der thermogravimetrischen Gewichtsverlust-Daten aus der Literatur hat eine Korrelation zwischen der gesamten Dekompositionsenergie und charakteristischen Eigenschaften der therrnogravimetrischen Kurven fiir Messungen unter Stickstoff ergeben. Diese Korrelation ermoglicht einen Vergleich der relativen thermischen Stabilitaten von Polymeren auf Grund einfach erhaltener Gewichtsverlustdaten.
synopsisThe bondability of the following polymers as a function of length of exposure to excited helium or oxygen was investigated: lowdensity polyethylene, highdensity polyethylene (two types), poly(4methyl-l-pentene), poly(viny1 fluoride), poly(vinylidene fluoride), FEP Teflon, poly(oxymethy1ene) copolymer, nylon 6, nylon 66, poly(ethy1ene terephthalate), and polystyrene. Generally, the bond strength increases rapidly initially and then remains nearly constant, perhaps decreasing in some cases a t long exposure times. A method is presented for calculating bond strength-versusexposure time curves. The calculated curves generally fit the data reasonably well. Polypropylene showed a rapid increase in bondability with exposure to excited oxygen. Helium was ineffective toward this polymer under normal conditions, but could produce good bond strength at higher temperatures.
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
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