Thermal Analysis of Polymeric Materials

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4.6.3 Standardization and Technique

Thermogravimetry needs a check of the accuracy of temperature, mass, and time measurements. Practically all thermobalances are capable of producing good data with only infrequent checks of the calibration via a standard mass. Since changes in volume of the sample take place, a buoyancy correction should be done routinely. The mass, m, of the displaced gas can easily be calculated from the ideal gas law (m = pM V/RT).

The packing of the sample is of importance if gases are evolved during the experiment because they may seriously affect the equilibrium. Questions of gas flow and convection effects should be addressed, and, if needed, eliminated by proper baffling. Noise in a thermogravimetric curve can often be attributed to irregular convection currents.

To establish the true sample temperature is difficult since in most cases the temperature sensor is removed from the sample, in contrast to the principles of thermometry (see Sect. 4.1). In addition, sample masses are often larger than those used in a DSC, so that the sample may develop excessive internal temperature gradients (see Sect. 4.3). All these problems are aggravated because heat transfer in thermogravimetry is usually across an air or inert gas gap from the furnace, and an additional separation exists between sample and temperature sensor. Further temperature imbalances may occur during transitions. Chemical reactions, as are frequently studied by thermogravimetry, have heat effects typically 10 to 100 times greater than heat effects during phase transition.

This short summary leads to the conclusion that the mass axis, customarily drawn as the ordinate, is much better defined than the temperature abscissa. As in DSC, thus, careful calibration of temperature is necessary. For a simple calibration, one might suggest to check a standard material for its weight loss under reproducible conditions of sample mass, packing, heating rate, sample holder configuration, and atmosphere type, flow, and pressure. Efforts to establish international standards, however, were abandoned when it was found that it was not possible to fix all these instrumental and experimental variables satisfactorily. One has to develop one’s own standards of weight loss under carefully controlled conditions to check the reproducibility of the equipment. The thermogravimetry of calcium oxalate, described in Figs. 4.194–196, below, may give a standard trace.

The most successful temperature calibration is obtained by the analysis of a ferromagnetic material. A pellet of the reference sample is placed within the field of a magnet so that the resulting magnetic force adds to or subtracts from the gravity effect on the thermobalance. At the Curie temperature of the standard, the magnetic force vanishes, and an equivalent mass increase or decrease is registered by the instrument. The thermogravimetry curve at the top of Fig. 4.186 shows such an experiment with increasing and decreasing temperature, T. As in other thermal analysis curves, various characteristic temperatures, T1, T2, and T3, can be chosen and compared between laboratories. Certified reference materials for thermogravimetry are available from the National Institute for Standards and Technology. A typical calibration curve is shown at the bottom in Fig. 4.186. The three characteristic calibration points are marked on the mass trace and on the temperature curve for

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Fig. 4.186

heating and cooling experiments. As with the reporting of data for DSC, thermogravimetry is much in need of standard practices. The ICTAC recommendations are listed together with the recommendations for the differential thermal analysis in Appendix 9. One should read these recommendations every time before one writes a research report.

Time-calibration, as discussed in Sect. 4.1, poses little problems. Any good watch or stop watch, as well as the internal clocks of computers, are usually adequate for the relatively slow heating rates used in thermogravimetry. For standard of time, see Fig. 4.3.

4.6.4 Decomposition

An analysis of black praseodymium dioxide, PrO2, in a variety of atmospheres is shown in Fig. 4.187 as an example of decomposition reactions. The measurements were carried out with a Mettler Thermoanalyzer as described in Figs. 4.178–180 with a gas-flow rate of 10 L h 1. The TGA-curves are recalculated in terms of the chemical composition. The left two curves show the large shifts of the temperatures of oxygen loss in the presence of reducing atmospheres (H2 and CO). The reaction goes to the yellow Pr2O3 almost without formation of intermediates. In a vacuum, PrO2 is much more stable and several intermediates can be identified. The compounds are listed on the right-hand side of the graph. The Pr7O12, with 1.714 oxygen atoms per praseodymium atom is particularly stable. Note also the little maximum at the beginning of the decomposition trace in a vacuum. It is caused by the recoil of the leaving oxygen. The stability of Pr7O12 is increased in oxygen, nitrogen, or air. The oxygen effect is expected, since it will directly influence the chemical equilibrium. The influence of N2, which is not a reaction partner, is caused by the reduction of the rate of diffusion

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Fig. 4.187

out of the sample of the oxygen that is generated in the reaction. This self-generated oxygen atmosphere retards the decomposition. The decomposition temperature is, thus, dependent on pressure, sample packing, and sample geometry, making the TGA qualitative (quantitative in mass determination only, not in time and temperature).

The PrO2 analysis can also serve as an example of the influence of heating rates. The bottom graph in Fig. 4.187 is a comparison of the PrO2 decomposition in O2 at constant heating rate to quasi-isothermal runs. The left-hand curve was obtained by stopping the heating as soon as a mass loss was detected until one practically reached equilibrium. The changes in decomposition temperatures are substantial, as much as 50 100 K.

A number of decompositions of polymers are also described in Sect. 3.4 (Fig. 3.49). It is possible not only to use thermogravimetry to measure the thermal stability and life-time (see Sect. 4.6.6), but also to identify polymers by their degradation parameters such as temperature, degradation products, char residue, evolved gases, differences in reaction with O2 and other gases, etc.

4.6.5 Coupled Thermogravimetry and Differential Thermal Analysis

The next examples illustrate the greater detail that can be obtained when thermogravimetry and DTA are combined. The experiments can be carried out either simultaneously or successively. Figures 4.188–191 are copies of pages from the Atlas of Thermoanalytical Curves. The measurements were made with the Derivatograph depicted in Fig. 4.185 and carried out at different heating rates and with varying masses. The derivative of the sample mass, dm/dt is also shown in the graphs and labeled DTGA. If DTGA and DTA identify both the same transition or reaction, the plots of dm/dt and T versus T are similar.

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the salicylic acid. Melting is followed by slow evaporation. In fact, it can be seen from the sensitive DTGA trace that before the crystals melt, some of the mass has already been lost by sublimation. After the evaporation endotherm, the exotherm 3 is visible. It indicates the oxidation of the remaining salicylic acid. The smaller sample mass, measured at a somewhat slower heating rate, was fully evaporated before the oxidation started, as can be seen from the missing DTA peak 3 in the thinline curve. This example shows the higher power of interpretation that is possible if DTA and TGA are available simultaneously. Full information would require chemical analysis of the gases evolved from the sample and an attempt at the elucidation of the kinetics.

Figures 4.190 and 4.191 show the analysis of the hexahydrate of praseodymium nitrate. The DTA and DTGA curves are complicated. From TGA, one can see the stages of the reaction. The first step, 1, is the melting endotherm, showing only in the DTA trace. Then water is lost, giving even larger endotherms and the corresponding steps in the TGA. The praseodymium nitrate is stable up to about 675 K, where it undergoes an endothermic reaction to praseodymium oxynitrate, losing N2O5. The reaction is given by the peaks labeled 4. The last step shown is 5, the change to the oxide, listed as Pr6O11. From Fig. 4.187 it can be deduced, however, that Pr6O11 is not stable in air beyond 725 K. It slowly goes to Pr7O12, reached at about 975 K. Checking the mass loss, one finds that Pr6O11 should occur at a 60.8% mass loss, while Pr7O12 shows a slightly larger loss of 61.4%.

Fig. 4.190

Figures 4.192 and 4.193 represent a series of curves obtained with a Mettler Thermoanalyzer similar to the one shown in Fig. 4.181. Each graph represents two TGA traces, TG1 and TG2, of low and high sensitivity, and a DTA curve. The top of Fig. 4.192 is of a kaolinite from South Carolina. Between 700 and 1000 K, water is

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Fig. 4.191

Fig. 4.192

released, to give a mixture of aluminum oxide and silicon dioxide. At about 1275 K, mullite, which has a formula composition of 3 Al2O3 and 2 SiO2, crystallizes. The excess of SiO2 remains as cristoballite. The mullite is the characteristic material in porcelain ware. Since during this crystallization, there is no mass lost, it could not have been studied by thermogravimetry alone.

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The bottom of Fig. 4.192 provides information on iron(III) hydroxide, an amorphous, brown, gel-like substance. Loss of water takes place over the whole temperature range up to 1,075 K. No distinct intermediate hydrates occur until hematite, Fe2O3, is obtained. The crystallization exotherm of the hematite is visible in the DTA curve at about 775 K.

Figure 4.193 illustrates in its top graph the fast, explosive decomposition of ammonium picrate. At 475 K the fast, exothermic reaction takes place. The study of explosives by thermogravimetry and DTA is possible because of the small sample masses that can be used. An important branch of thermal analysis is thus the study of chemical stability of compounds that are used industrially.

Fig. 4.193

The bottom example in Fig. 4.193 represents the thermal decomposition of a silver carbonate, Ag2CO3, in helium. At about 400 550 K, the carbonate loses carbon dioxide and changes into the oxide Ag2O. A second, smaller loss of mass begins at a temperature of 675 K. Both mass losses are accompanied by an endotherm in the DTA trace, meaning that the reactions are entropy driven. The final decomposition product is metallic silver.

4.6.6 Applications of Thermogravimetry

Three detailed applications of thermogravimetry are described with more quantitative interpretations, i.e., efforts are made to develop information on the kinetics and equilibrium. The calcium oxalate/carbonate decomposition is treated first. The lithium hydrogen phosphate polymerization has been discussed above as a stepreaction in Sect. 3.1 (Figs. 3.16–22). Finally, the method and some examples of lifetime determinations based on TGA are shown at the end of this section.

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reaction is superimposed. The recorded mass-changes agree well with the assumed overall reaction. The temperatures for the change from one plateau to the other are, however, variable.

For the analysis of the data of the first step of the reaction with a nonisothermal kinetics one can make use of the Freeman-Carroll method described in Sect. 3.6.6 with Fig. 3.87. In such an attempt, one finds for different heating rates, different activation energies and an order of about one for the reaction, which does not fit the assumed kinetics. The plot of the data is shown in Fig. 4.195, together with the results of the Freeman-Carroll analysis. Comparing the heating-rate-dependent data with isothermal data on the same thermogravimetric balance, one finds different values for both, n and the activation energy. This is a clear indication that the assumptions of the analysis using the Freeman-Carroll plot are not satisfied.

Fig. 4.195

It is even more difficult to analyze the second step, where the DTA in Fig. 4.194 already shows a two-stage reaction. In the literature, one may find suggestions that the reaction is purely exothermic, as well as suggestions that the reaction is purely endothermic. Thus, even in often-analyzed systems, surprises are still possible.

The carbonate decomposition, step 3, has been studied separately. Well-defined CaCO3 crystals were analyzed by electron microscopy, surface area measurement, and particle counts. The TGA data of Fig. 4.196 show qualitatively the effects of sample mass and heating rate. Three of five sets of data were analyzed with the FreemanCarroll method. Again, there were large changes in the kinetic parameters with sample mass and heating rate, indicating that such analysis is not permissible. To get more information on the kinetics, isothermal runs were made using a Cahn balance with a controlled furnace and otherwise similar reaction conditions. Isothermal data were collected between 1 and 32 mg and temperatures between 600 and 700 K. Using

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Fig. 4.196

18 different functions to represent the data, it was found that the equation in Fig. 4.196 gives the best fit. This equation can be justified if one assumes that the mass loss is governed by the contracting geometry of the particles.

Lifetime prediction is an applied technique, which is frequently needed in industry to find out the probable performance of a new material. The philosophy of lifetime prediction is to identify the critical reaction which limits the life of a material, then to measure its kinetics quantitatively at high temperature where the reaction is fast. Finally, using proper kinetic expressions, one extrapolates the kinetics to the much longer reaction times expected at the lower temperatures at which the sample will be in service. Naturally, the reverse process, extrapolating the kinetics to higher temperatures, could also be carried out to find shorter lifetimes—as for example, for ablation processes.

In the present discussion it is tacitly assumed that the thermal analysis technique identifies the proper life-determining reaction and that the detailed chemistry and physics of the various failure mechanisms are as assumed. The example materials chosen for the discussion of lifetime determination are linear macromolecules. The example technique is mass loss, despite the fact that the useful life of a material may have ended long before a loss of mass is detected. If, for example, the material fails due to embrittlement caused by cross-linking, there would be no mass loss; only the determination of the glass transition by DSC or DMA at different times could help in such case.

There are two common methods of kinetic analysis based on the kinetics equations derived in Sects. 3.1 and 3.2. The first method is the steady-state parameter-jump method. As illustrated in Fig. 4.197, the rate of loss of mass is recorded while jumping between two temperatures, T1 and T2. At each jump time, ti, the rate of loss of mass is extrapolated from each direction to ti, so that one obtains two rates at