Weight loss isotherms

Analysis of weight loss isotherms displayed in Fig. 8 shows that the first step in the interaction between Nb02F and carbonates of other alkali metals is similar to the interaction described by Equation (11). However, compounds of the M(Nb04F form, where M = Na, K, Rb, Cs, were not found [85]. The instability of such compounds is related to the ionic radii of the alkali metals, which are greater than that of Nb5+, thus the ions are too large to occupy the octahedral cavities formed by the oxygen and fluorine ions. 

Typical weight-loss isotherms for samples of 57-// quartz in a 90 vol. % matrix of 17-20-// calcium carbonate can be seen in Fig. 1. All of the weight loss data are reported on the basis of normalized 500-mg initial sample weights. The large initial portion of the reaction evident in these curves was observed in all cases. The amount of the initial reaction was found to be extremely dependent on the experimental conditions and varied from approximately 20 to 80% of the total amount of reaction. The same data are shown in Fig. 2 in parabolic plots of the square of the weight loss versus time. In the latter stages of reaction a parabolic dependence is observed, indicating the... 

LARC-TPI is a linear thermoplastic PI which can be processed ia the imide form to produce large-area, void-free adhesive bonds. Mitsui Toatsu Chemicals, Inc., has obtained Hcense to produce this product commercially for appHcations such as adhesives, films, mol ding compounds, etc. These are thermooxidatively stable and show essentially no loss ia weight at 300°C ia air. Weight loss does not exceed 2—3% after isothermal aging ia air at 300°C for 550 h. 

C/min to 140°C, (3) hold for 2 hours, and (4) heat to 500°C at 3°/min. Oxygen was introduced at the time the temperature reached 140°C. The increase in temperature after the isothermal (140°C) region led to an increase in the rate of chemisorption, up to the temperature at which combustion (burn-off) becomes the dominant process resulting in rapid weight loss (ca. 270°C). 

Table I presents the results of "isothermal" simultaneous thermoanalytical (STA) runs, at 573 K and 773 K, for all three products. Similar data, at a fixed heating rate is shown in Table II. One of the crucial parameters is the temperature of maximum weight loss rate, corresponding to the time when dehydrochlorination of PVC starts becoming important. This temperature is close to 573 K in all cases. In fact, at a relatively fast heating rate, almost no decomposition occurs at temperatures under 563 K. If the materials are heated at 573 K for a prolonged period, complete dehydrochlorination takes place, but no further stages of PVC decomposition occur. None of the three materials investigated decomposes completely until a temperature of ca. 773 K is attained. Even then only a certain fraction of the entire mass of the samples is volatilised, due to the presence of inorganic fillers in their composition.

While the rate of decomposition in air was more rapid than in nitrogen, the kinetics of oxidative decomposition cannot be estimated reliably by isothermal weight loss because ofthe possibility of competing oxidative weight gain process. For this reason the kinetics were not estimated from the available data... 

A representative sample of many of the polyimide films that were produced were subjected to thermo-mechanical analysis (TMA), torsional braid analysis (TBA), thermal gravimetric analysis (TGA), infrared spectral analysis and weight loss on prolonged heating (e.g. isothermal studies), TABLE II. The softening temperature as measured by TMA and TBA are in general... 

Based on the assumption that there is no chemical interaction or any synergistic effect caused by physical changes in the blends, theoretical weight losses of the blends can be calculated, using the isothermal aging data of the individual components as well as their weight percents in the blends. The results are compared with the experimental data in Table IV (BCB/K-353) and Table V (BCB/Dicyanate). 

TGA. Weight-loss measurements for the electrical-electronic grade novolac epoxy were reported in our earlier work. For samples heated in N2 to 350°C no differences attributable to the presence of FR were observed. Isothermal measurements indicated a 20% weight loss for the unfilled molding compound after 12-days at 220°C. 

TGA data confirm transformation of the polyene structure into the carbon one. Thus isothermal heating at 200°C resulted in almost complete dehydrochlorination of the polymer, and intense weight loss occurs only at 300°C (Fig. 4.2, curve 2). 

Fig. 19. Structures of the bisbenzocyclobutene 41, the diacetylenes 42, 43 and the hismaleimide 44 used in the DSC and isothermal weight loss experiments described in Figs. 20-24...

The enhancement of thermal stability as measured by these isothermal aging experiments by the addition of bisbenzocyclobutenes to dienophilic thermosetting monomers has also been observed with bis activated acetylenes and dicyanates. The isothermal weight loss results obtained on a one to one mole... 

Diels-Alder mechanism. It was found that those polymers which had the least weight loss in the isothermal aging study were derived from those monomers which by DSC polymerized to the greatest extent by a Diels-Alder mechanism. It was inferred therefore that if similar considerations apply to the copolymers of bismaleimides and bisbenzocyclobutenes then these materials too might be the result of predominantly Diels-Alder type polymerizations. To the extent to which these conclusions are correct, the enhanced thermal stability of the bisbenzocyclobutene/bismaleimide copolymers is likely to be due to their being Diels-Alder polymers of some sort. 

Via DSC measurements it was shown that the new bis(vinylbenzyl)ethers can undergo a homopolymerization and a copolymerisation with BMI. In a BMI/-divinylbenzylether blend the Diels-Alder copolymerization is favoured over the divinylbenzylether homopolymerisation. The Tgs of the new copolymers are well in excess of 270 °C when the BMI/divinylbenzylether molar ratio is 1 1 (Table 9). Isothermal weight loss studies over a period of 4000 hours indicate that the BMI-MDA/divinylbenzylether copolymer is the most stable system of this family. 

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