Optimizing fitting parameters in thermogravimetry

This study presents an alternative to simple estimation of parametric fitting models used in thermal analysis. The addressed problem consists in performing an alternative optimization method to fit thermal analysis curves, specifically TG curves and their first derivatives. This proposal consists in estimating the optimal parameters corresponding to fitting kinetic models applied to thermogravimetric (TG) curves, using evolutionary algorithms: differential evolution (DE), simulated annealing and covariance matrix adapting evolutionary strategy. This procedure does not need to include a vector with the initial values of the parameters, as is currently required. Despite their potential benefits, the application of these methods is by no means usual in the context of thermal analysis curve’s estimation. Simulated TG curves are obtained and fitted using a generalized logistic mixture model, where each logistic component represents a thermal degradation process. The simulation of TG curves in four different scenarios taking into account the extent of processes overlapping allows us to evaluate the final results and thus to validate the proposed procedure: two degradation processes non-overlapped simulated using two generalized logistics, two processes overlapped, four processes non-overlapped and four processes overlapped two by two. The mean square error function is chosen as objective function and the above algorithms have been applied separately and together, i.e., taking the final solution of the DE algorithm is the initial solution of the remaining. The results show that the evolutionary algorithms provide a good solution for adjusting simulated TG curves, better than that provided by traditional methods.     

Is the original Kissinger equation obsolete today?

The original Kissinger, Friedman, and modified Kissinger–Akahira–Sunose (KAS) methodologies were used to evaluate the apparent activation energy of theoretically simulated complex processes consisting of two overlapping independent JMA-like (Johnson–Mehl–Avrami-like) subprocesses. Three overlay series were studied, each representing one of the basic conceptual types of peak overlap. It was shown that, in the case of complex processes, both the tested isoconversion methods (KAS and Friedman) provide good qualitative information about the activation energies of the involved overlapping signals. However, from the quantitative point of view, the data are not easy to interpret, and deconvolution procedures are necessary for meaningful results to be obtained. On the other hand, in most cases, the apparent activation energy determined by the original Kissinger equation for the overlapped dominant peak corresponded very well to the true values determined for the sole processes. This suggests large robustness of the Kissinger methodology that even nowadays may be considered very advantageous and utilized in kinetic analysis of complex processes.     

Thermal behavior of Cd<Superscript>2+</Superscript> and Co<Superscript>2+</Superscript> phenyl-vinyl-phosphonates under non-isothermal condition

The thermal behavior of Cd²⁺ and Co²⁺ phenyl-vinyl-phosphonates was studied using two different experimental strategies: the coupled TG-EGA (FTIR) technique by decomposition in nitrogen respectively air, and the kinetic analysis of TG data obtained in dynamic air atmosphere at four heating rates. In nitrogen two decomposition steps were observed: the loss of crystallization water, respectively the decomposition of the phenyl-vinyl radical. In air, the same dehydration was observed as the first step, but the second one is a thermooxidation of the organic radical with formation of the pyrophosphoric anion. The kinetic analysis of the TG non-isothermal data was performed by the isoconversional methods suggested by Friedman and Flynn, Wall and Ozawa, as well as by the non-parametric (Sempere-Nomen) method. All processes put in evidence in TG curves exhibit strong changes of the activation energy values with the conversion degree, which mean that these processes are complex ones. Assuming that each of these processes consists in two steps, the application of non-parametric method leads to average values of the activation energy close to the average values of this parameter obtained by isoconversional methods.     

The role of thermal analysis in environmental protection

In the past industry has developed chemicals and products, optimized for the best suitable properties concerning different application fields. Now, ideas of environmental precaution are arising, on the one hand looking for reduction of materials flow to avoid or minimize the waste, on the other hand following the idea of process and product integrated environmental protection. That means to develop processes and products which are safer and more tolerant regarding the environment and its organisms.In this connection thermal analysis is a very successful tool for predicting the risk of burning processes or fire accidents. Thermal analysis in this context means the classical thermal analysis methods like DTA, TG, DSC and its couplings with gas analysis methods as well as the simulation of burning processes in different kind of furnaces with identification and quantification of the evolved gases.     

Thermogravimetric Fourier Transform Infrared Spectroscopy of Hydrocarbon Fuel Residues

Although thermogravimetric analysis (TG) has become an indispensable tool for the analysis and characterization of materials, its scope is limited as no information is obtained about the qualitative aspects of the evolved gases during the thermal decomposition. For processes involving mass loss, a powerful technique to provide this missing information is Fourier transform infrared spectroscopy (FT-IR) in combination with TG. It supplies a comprehensive understanding of thermal events in a reliable and meaningful way as data are obtained from a single sample under the same conditions. The coupling TG/FT-IR is used in fuel analysis for the identification of residual volatiles, to determine their sequence of release and to resolve thermogravimetric curves. In this work, the usefulness of TG/FT-IR for characterizing middle distillate fuel residues is illustrated with some typical examples of recent application. A Bio-Rad FTS 25 FT-IR spectrometer coupled with a TA Instruments TGA 2950 thermogravimetric analyzer was used for data aquisition. The results obtained demonstrate the utility of this combined technique in determining the decomposition pathway of tarry materials at various stages of pyrolysis, thereby allowing new insights into the complex thermal behaviour of hydrocarbon residual systems. This revised version was published online in August 2006 with corrections to the Cover Date.     

Dependence of the preexponential factor on temperature

Summary The dependence of the preexponential factor on the temperature has been examined and the errors involved in the activation energy calculated from isothermal and non-isothermal methods without considering such dependence have been estimated. It has been shown that the error in the determination of the activation energy calculated ignoring the dependence of Aon Tcan be rather large and it is dependent on x=E/RT, but independent of the experimental method used. It has been also shown that the error introduced by omitting the dependence of the preexponential factor on the temperature is considerably larger than the error due to the Arrhenius integral approach used for carrying out the kinetic analysis of TG data.     

Determination of filler content for natural filler polymer composite by thermogravimetric analysis

Determination of filler content by thermogravimetric (TG) analysis is commonly utilized to investigate the effectiveness of processing methods for composite materials and to quantify the dispersion of filler within the matrix. However, the existing analysis method is not capable of accurately predicting the filler content for natural fiber composites for the case where thermal degradation of the filler and matrix occurs within similar temperature ranges. In the present study, the authors have proposed a generic equation for the determination of filler content which can be utilized for any given range of thermal degradation temperatures in natural filler polymer composites. Oil palm shell unsaturated polyester composites were selected to verify the proposed equation using the TG test with the results indicating good agreement between the estimated and experimental filler contents with a maximum error on the order of 10 %. The suggested technique provides a simple, yet generic, approach to determining the filler content of green or lignocellulose-based polymer composites by TG analysis.

     

TG studies of a composite solid rocket propellant based on HTPB-binder

Thermal decomposition kinetics of solid rocket propellants based on hydroxyl-terminated polybutadiene-HTPB binder was studied by applying the Arrhenius and Flynn-Wall-Ozawa's methods. The thermal decomposition data of the propellant samples were analyzed by thermogravimetric analysis (TG/DTG) at different heating rates in the temperature range of 300-1200 K. TG curves showed that the thermal degradation occurred in three main stages regardless of the plasticizer (DOA) raw material, the partial HTPB/IPDI binder and the total ammonium perchlorate decompositions. The kinetic parameters E _a (activation energy) and A (pre-exponential factor) and the compensation parameter (S _p) were determined. The apparent activation energies obtained from different methods showed a very good agreement. This revised version was published online in August 2006 with corrections to the Cover Date.     

Comparative kinetic study of decomposition of some diazepine derivatives under isothermal and non-isothermal conditions

Thermal analysis is one of the most widely used methods for studying the solid state of pharmaceutical substances. TG/DTG and DSC curves provide important information regarding the physical properties of the pharmaceutical compounds (stability, compatibility, polymorphism, kinetic analysis, phase transitions etc.). The purpose of a kinetic investigation is to calculate the kinetic parameters and the kinetic model for the studied process. The results are further used to predict the system’s behaviour in various circumstances. A kinetic study regarding the diazepam, nitrazepam and oxazepam thermal decomposition was performed, under non-isothermal and isothermal conditions and in a nitrogen atmosphere, for the temperature steps: 483, 498, 523, 538 and 553 K. The TG/DTG data were processed by three methods: isothermal model-fitting, Friedman’s isothermal-isoconversional and Nomen-Sempere non-parametric kinetics. In the model-fitting methods the kinetic triplets (f(α), A and E _a) that defines a single reaction step resulted in being at variance with the multi-step nature of diazepines decomposition. The model-free approach represented by isothermal and non-isothermal isoconversional methods, gave dependences of the activation energies on the extent of conversion. It is very difficult to obtain an accord with the similar data which resulted under non-isothermal conditions from a previous work. The careful treatment of the kinetic parameters obtained in different thermal conditions was confirmed to be necessary, as well as a different strategy of experimental data processing.     

Application of TA and Kinetic Study to Compatibility and Stability Problems in Some Commercial Drugs. Remarks on statistical data

A thermal analysis and kinetic study on decomposition processes of some commercial drugs have been carried out to find their thermal stability. DSC/TG curves of some commercial drugs were compared with those of their active components, the excipients, the active component/excipient and the excipient/excipient mixtures. A kinetic study was carried out using both isothermal and dynamic TG curves. Both active components and commercial drugs tested show a first order decomposition mechanism. The kinetic data showed that excipients cause a decrease of the kinetic stability of the active components. Statistical analysis allowed us to select reliable kinetic parameters related to decomposition processes. This procedure showed that the values obtained by extrapolation,outside the temperature range where the processes occurred must be used with caution. Indeed half-time and shelf-time values, commonly used at room temperature, seemed to be unrealistic. This revised version was published online in July 2006 with corrections to the Cover Date.     

Thermal Stability of Prednisone Drug and Tablets

TG and DSC data were used to determine the thermal parameters of prednisone drug and tablets. Two formulations of prednisone 20 mg were analysed in the form of tablets. The TG curves of prednisone drug and tablets A and B displayed six, eight and seven thermal decomposition processes, respectively. Analysis of the DSC data pointed to chemical interactions between prednisone drug and the excipients of tablets A and B, suggested by alterations in the melting temperature of prednisone. The analysis revealed that prednisone drug is more stable than tablets A and B. This revised version was published online in July 2006 with corrections to the Cover Date.     

Application of Thermal Analysis to the Study of Some Waste Agricultural Products for the Preparation of Active Carbons

TG, DTG and DTA methods were used for the investigation of some waste agricultural products, such as grape seeds, walnut shells, plum and peach stones, which can serve as raw materials for the production of active carbons. It was demonstrated that thermo analytical methods are appropriate to study the thermal characteristics of the above wastes and the data obtained can be applied to the technological processes of active carbon preparation. This revised version was published online in August 2006 with corrections to the Cover Date.     

Thermal Characterization of Materials Using Evolved Gas Analysis

Thermal analysis combined with evolved gas analysis has been used for some time. Thermogravimetry (TG) coupled with Fourier transform infrared (FTIR) spectroscopy(TG/FTIR), Thermogravimetry (TG) coupled with mass spectrometry (TG/MS), and Thermogravimetry (TG) coupled with GC/MS offers structural identification of compounds evolving during thermal processes. These evolved gas analysis (EGA) techniques allow to evaluate the chemical pathway of the degradation reaction by determining the decomposition products. In this paper the TG/FTIR, TG/MS, and Pyrolysis/GC-MS systems will be described and their applications in the study of several materials will be discussed, including the analysis of the degradation mechanisms of organically modified clays, polymers, and coal blends. This revised version was published online in July 2006 with corrections to the Cover Date.     

Kinetic study of thermal processing of glycerol by thermogravimetry

Over the past years, the production of biodiesel has significantly increased in Brazil due to its obligatory use in the composition of diesel for vehicle use. As a result, in the most ordinary processes, a hundred thousand tons of glycerol is produced as by-product per 1 billion liters of biodiesel. Glycerol has already been widely studied. Nonetheless, the quantity produced today demands new proposals for uses, such as a fuel. In this aim, the authors studied the kinetics of the thermal processing of glycerol. In this research, thermogravimetry (TG), derivative thermogravimetry (DTG), and differential thermal analysis (DTA) were used to provide the experimental data. Kinetic parameters were calculated by Kissinger method for the global process observed during the heating of the samples from the room temperature up to 600 °C, both in open and in sealed crucibles (with a little hole). Kinetic data were also determined at different isoconversion conditions during heating, by applying Ozawa–Flynn–Wall and Blazejowski methods to TG data. Results show that glycerol heated from 30 to 600 °C, under normal pressure, does not experience simple volatilization. The activation energies calculated at different conversion degrees by these methods show that only volatilization occurs when the mass loss of glycerol is lower than 40% and that for higher conversion degrees, partial thermal decomposition and/or dissociation of glycerol are occurring as well. These facts are also confirmed by the volatilization enthalpies estimated using another method developed by Blazejowski based on Van’t Hoff equation.     

Use of DSC and TG for identification and quantification of the dosage form

Thermal analysis techniques, DSC and TG can advantageously be used in quality control of drug products.The methods are commonly used in preformulation for the study of polymorphism and for the study of the interactions drug substance-excipients, since these physical interactions can be the basis of the dosage form performance.For routine control of the drug products, DSC and TG methods which are quick, which require only few mg of the samples and which are automated, are very attractive for routine analysis of drug products. A single scan can give several qualitative and quantitative informations.DSC offer analytical possibilities only if the drug substance and the excipients do not have physical interactions or limited interactions (e.g. eutectic behaviour). About twenty marketed products have been analyzed by DSC and TG. In most of them identification of drug substance is easy. Several excipients could be identified in a tablet. Quantitations are demonstrated for some drug substances and excipients. DSC purity calculations have been applied to acetyl salicylic acid, paracetamol, cimetidine, pindolol, ibuprofen.     

The use of thermal analysis methods for conservation state determination of historical and/or cultural objects manufactured from lime tree wood

The TG, DTG and DSC methods were used for investigation of the thermo-oxidative degradation in static air atmosphere and oxygen flow of some sorts of lime tree wood (recent lime tree woods with different preparations, old lime tree woods extracted from some Romanian historical and/or cultural objects). At the progressive heating in the mentioned atmospheres, all the investigated materials exhibit three successive processes, associated with dehydration and two complex thermo-oxidative processes. Each analyzed material has a characteristic thermogram (TG, DTG and/or DSC curve) that can be considered a material “fingerprint”. It was pointed out that the following non-isothermal parameters can be used for distinction between a new and old lime tree wood: mass loss in the first process of thermo-oxidation, ratio between the mass losses in the first and the second processes of thermo-oxidation, the maximum rate of the first process of thermo-oxidation. Consequently, the certification of a patrimonial object manufactured from lime tree wood could be performed by applying the thermal analysis methods.     

Kinetic studies on the thermal decomposition of phosphate-doped sodium oxalate

The thermal decomposition kinetics of sodium oxalate (Na₂C₂O₄) has been studied as a function of concentration of dopant, phosphate, at five different temperatures in the range 783–803 K under isothermal conditions by thermogravimetry (TG). The TG data were subjected to both model-fitting and model-free kinetic methods of analysis. The model-fitting analysis of the TG data of all the samples shows that no single kinetic model describes the whole α versus t curve with a single rate constant throughout the decomposition reaction. Separate kinetic analysis shows that Prout–Tompkins model best describes the acceleratory stage of the decomposition, while the decay region is best fitted with the contracting cylinder model. Activation energy values were evaluated by both model-fitting and model-free kinetic methods. The observed results favour a diffusion-controlled mechanism for the thermal decomposition of sodium oxalate.     

Interrelations between soil respiration and its thermal stability

Biological transformation of organic matter in soil is a crucial factor affecting the global carbon cycle. In order to understand these complex processes, soils must be investigated by a combination of various methods. This study compares the dynamics of biological mineralization of soil organic matter (SOM) determined via CO₂ evolution during an 80-day laboratory incubation with their thermo-oxidative stability determined by thermogravimetry (TG). Thirty-three soil samples, originating from a wide range of geological and vegetation conditions from various German national parks were studied. The results showed a correlation between the amount and rate of respired CO₂ and thermal mass losses of air-dried, conditioned soils occurring around 100?°C with linear coefficients of determination up to R ²?=?0.85. Further, correlation of soil respiration with thermal mass losses around 260?°C confirmed previous observations. The comparison of TG profiles from incubated and non-incubated soils underlined the importance of thermal mass losses in these two temperature intervals. Incubated soils had reduced thermal mass losses above 240?°C and conversely an increased mass loss at 100?C120?°C. Furthermore, the accurate determination of soil properties by TG such as soil organic carbon content was confirmed, and it was shown that it can be applied to a wider range of carbon contents as was previously thought. It was concluded that results of thermal analysis could be a helpful starting point for estimation of soil respiration and for development of methods revealing processes in soils.