TG and DSC kinetics of thermal decomposition and crystallization processes

The simplen ^th order model equation combined with the Arrhenius approach of the temperature dependency of the reaction rate constant is widely used in thermal analysis. The new Mettler software package for thermal analysis, GraphWare TA72 allows to access a full model comprising the power law and the crystallization kinetics (AvramiErofe'ev). The kinetics of the following reactions are studied to illustrate some applications:

thermal decomposition of dissolved dibenzoylperoxide, (dynamic and isothermal DSC measurement)

crystallization of polyethylene terephthalate (PET) (isothermal DSC measurements).

The kinetic model applied and the accuracy of the kinetic data obtained are discussed by means of a comparison of a predicted behaviour with the kinetic data measured isothermally.     

Influence of particle size and kinetic parameters on tire pyrolysis

One of the most important parameters that can significantly affect the cost of the tire pyrolysis process is the time needed for thermal decomposition of the tire material. In this work, the influence of particle size and kinetics of thermal decomposition on the pyrolysis time was studied. The apparent kinetic parameters of tire thermal decomposition were estimated using three different approaches based on thermogravimetry measurements. In separate experiments, tire particles with different sizes were pyrolyzed under isothermal conditions in a laboratory flow reactor at different residence times of the particles in the reactor and the data recorded were employed in the determination of tire conversion during the thermal decomposition. A mathematical model of tire pyrolysis considering heat conduction in the tire particles was developed. All three types of estimated kinetic parameters were used to determine the conversion behavior at isothermal conditions and the results were compared with experimental data obtained. The model was used also to calculate the pyrolysis time in an industrial scale reactor at different temperatures and particle sizes.     

Kinetics of isothermal thermooxidative degradation of poly(vinyl chloride)/chlorinated polyethylene blends

The thermooxidative degradation of poly(vinyl chloride)/chlorinated polyethylene blends of different compositions was investigated by means of isothermal thermogravimetry in flowing atmosphere of synthetic air at temperatures 240–270 °C. The main degradation processes are dehydrochlorination of PVC and CPE. For calculation of the apparent activation energy and apparent pre-exponential factor two kinetic methods were used: isoconversional method and Prout–Tompkins method. True compensation dependency between Arrhenius parameters, obtained using Prout–Tompkins model, was found. Calculated kinetic parameters of isothermal thermooxidative degradation are close to those from non-isothermal degradation and confirm the assumption of the main degradation process in PVC/CPE blends.     

Kinetic study of the thermal decomposition of TEA<Superscript>+</Superscript>/SAPO-5

A silico alumino phosphate with AFI structure (SAPO-5) was prepared in a two-phase medium and characterized by XRD, followed by the addition of TEA⁺. The kinetics of the TEA⁺/SAPO-5 thermal decomposition reaction was studied by isothermal and dynamic thermogravimetry. Two kinetic models, D₃ and D₄ based on diffusion processes were found as best to fit the isothermal data. On the other hand, the best fit for the dynamic data is the F₁ first order reaction model. According to the apparent activation energy values, the use of the dynamic method indicates a higher temperature dependence than the isothermal method.     

Thermooxidative degradation of poly(vinyl chloride) and chlorinated polyethylene with different Ca/Zn carboxylates

The thermooxidative degradation of poly(vinyl chloride) (PVC), chlorinated polyethylene (CPE) and PVC/CPE blend 50/50 was investigated by means of dynamic and isothermal thermogravimetric analysis in the flowing atmosphere of air. To estimate the thermooxidative stability of the samples the characteristics of thermogravimetric (TG) curves were used. Kinetic parameters (the apparent activation energy E and preexponential factor Z) were calculated after isoconversional method for the first stage of dynamic degradation where dehydrochlorination (DHCl) of PVC and/or CPE is the main degradation reaction. Despite the chemical resemblance, the degradation mechanisms of CPE and PVC are different, as a consequence of differences in microregularity of the corresponding polymer chains. The addition of Ca/Zn carboxylates as well as the ratio of Ca and Zn carboxylates have considerably different influence on the investigated polymers. This revised version was published online in August 2006 with corrections to the Cover Date.     

Kinetics of the thermal decomposition of CaCo3 in Co2 and some observations on the kinetic compensation effect

The kinetics of the thermal decomposition of CaCO₃ in CO₂ were investigated using both dynamic and isothermal techniques. Values of apparent activation energies range from 200–1000 kcal mol^?1 depending upon sample size and heating rate. It is concluded that thermal transport rather than mass transport or chemical processes is rate determining. The results are compared with earlier work in O₂ and discussed in terms of “the kinetic compensation effect”, i.e., the reported linear relationship between the logarithm of the preexponential term and the activation energy derived from the Arrhenius equation.     

Co-pyrolysis behaviors and kinetics of plastics–biomass blends through thermogravimetric analysis

Co-pyrolysis behaviors of plastics–biomass blends were investigated using a thermogravimetric (TG) analysis from room temperature to 873 K with a heating rate of 5–40 K min^?1 in an inert atmosphere. The selected biomass sample was sawdust of pine wood (WS). Polyvinyl chloride (PVC), low-density polyethylene (LDPE), high-density polyethylene (HDPE), and polypropylene (PP) were selected as plastic samples. The difference of mass loss between experimental and theoretical ones (calculated as arithmetic sums of those from each separated component) was used as a criterion of synergetic effect. The experimental results indicated that a significant synergetic effect existed during the high-temperature region of plastics and WS co-pyrolysis process, specially, the dehydrochlorination reaction of PVC and the degradation of hemicellulose and cellulose in the WS during the co-pyrolysis process showed synergetic effect, as well as the reaction of plastics (LDPE, HDPE, and PP) and WS. Based on the TG data with different heating rates, the kinetics parameters, especially activation energy, were calculated using the Friedman method. The activation energy of plastics, WS, and their blends were from 92.8 to 359.5 kJ mol^?1. The activation energy of the PVC–WS blends was at a range of 180.2–254.5 kJ mol^?1 in the second stages. The activation energies range of LDPE–WS, HDPE–WS, and PP–WS blends were 164.5–229.6, 213.2–234.3, and 198.4–263.6 kJ mol^?1, respectively.     

Cyclo dehydration reaction of polyhydrazides. II. Kinetic parameters obtained from isothermal thermogravimetry

The kinetics of the thermal conversion reaction of poly-(1,3-phenyl-1,4-phenyl)-hydrazide into poly-(1,3-phenyl-1,4-phenyl)-1,3,4-oxadiazole have been studied with isothermal thermogravimetry in continuation of a study with nonisothermal thermogravimetry described in a previous paper. Although the isothermal measurements are much more time-consuming, they provide some new information and insight about the cyclo dehydration reaction of the polyhydrazide. The physical state of the sample, rubbery or glassy, seems to influence the kinetics considerably. The kinetic parameters determined with the isothermal method for the polymer in its glassy state agree well with the parameters derived from the previously reported nonisothermal measurements, while the kinetic parameters for the expected rubbery state differ considerably. The morphological state or the history of the polymer has also a considerable influence on the kinetics of the isothermal conversion process. The powder form of the polymer has a much lower isothermal conversion rate than the film form.     

Isothermal and non-isothermal pyrolysis kinetics of Kapton polyimide

The kinetics involved in the thermal decomposition of Kapton^® polyimide 100HN under nitrogen atmosphere were studied by applying various fitting techniques to the isothermal and non-isothermal gravimetric data. The correlation of the reaction mechanism fitting, the analytical model fitting and the isoconversional method to these data was examined in relation to the kinetic parameters and the kinetic predictions. The mechanisms for solid-state reactions fit the isothermal data very well but result in highly uncertain values for the kinetic parameters when applied to the non-isothermal data. Isoconversional methods show that the apparent activation energy depends on the extent of conversion but do not provide information for the reaction order and the pre-exponential factor. Three single heating-rate analytical models by Coats-Redfern, MacCallum-Tanner and van Krevelen were analysed using the non-isothermal data. A multi-heating rate model is proposed and its validity is compared to the single-heating rate models on the basis of kinetic predictions.     

Thermolysis of polyethylene hydroperoxides in the melt, Part 12: Mechanisms and kinetics of thermolysis

The thermolysis of polyethylene hydroperoxides is attributed to the reaction of two hydroperoxide groups. This bimolecular reaction appears as a first-order reaction with the mean values of the hydroperoxide concentrations that can be used for the experimental verification of the kinetics. In low molecular mass liquids and solutions these findings would be irreconcilable. However, in polymer melts, this contradiction is more apparent than real. It is a consequence of the heterogeneous kinetics valid in polymer melts. The bimolecular reaction involves the decomposition of pairs of hydroperoxide groups that are relatively close in the elementary oxidation volumes. By diffusion these hydroperoxide groups can come close enough for reaction. From the chemical point of view the decomposition is a bimolecular reaction. However, from the kinetic point of view it is a first-order reaction of the hydroperoxide pairs. The dependency of the first-order rate on the initial hydroperoxide concentration is explained by the heterogeneous kinetics. The activation energy of the overall process can be related to the sum of the activation energies pertaining to the chemical reaction and to the diffusion process.     

Study on Apparent Kinetics of the Reaction between Sulfuric and Hydriodic Acids in the Iodine–Sulfur Process

The iodine–sulfur (IS) thermochemical process for hydrogen production is one of the most promising approaches in using high‐temperature process heat supplied by a nuclear reactor. This process includes three reactions that form a closed cycle: the Bunsen reaction, in which iodine, water, and sulfur dioxide react to form sulfuric acid and hydriodic acid (HI); HI decomposition; and sulfuric acid decomposition. However, the side reactions between H₂SO₄ and HI may disturb the operation of the IS closed cycle. For optimal process conditions, the reaction kinetics between H₂SO₄ and HI should be examined. In this work, a preliminary kinetic study was conducted. Using the initial reaction rate method, the kinetic parameters of the reaction between sulfuric acid and HI, such as the apparent reaction orders and rate constant were determined. For I^?, the apparent reaction order was approximately 1.77, whereas the orders for H⁺ and SO₄^2? were 7.78 and 1.29, respectively. The apparent rate constant at 85 ± 1°C was approximately 2.949 × 10^?11 min^?1 (mol/L)^?9.84. The H⁺ concentration had more significant influence on the reaction rate than those of SO₄^2? and I^?. Such basic data provide useful information for related process design and further kinetics study.     

Eutectoid decomposition of intermetallic CuZr

The eutectoid decomposition of intermetallic CuZr under isothermal and nonisothermal conditions was studied by differential scanning calorimetry, electrical resistivity measurements, X-ray diffraction analysis, optical microscopy, and dilatometry. The kinetic diagram of the eutectoid decomposition of intermetallic CuZr was determined for the first time. The rate of the eutectoid decomposition of the CuZr phase was found to be one to two orders of magnitude lower than that in many other copper-based systems. The kinetics of the eutectoid decomposition under isothermal conditions was investigated, and the kinetic parameters of the decomposition were determined (the activation energy E_a = 395 ± 24 kJ/mol and the Avrami constant n = 3.3 ± 0.1).     

Temperature influence on the malonic acid decomposition in the Belousov-Zhabotinsky reaction

The kinetic investigations of the malonic acid decomposition (8.00 × 10⁻³ mol dm⁻³ ≤ [CH₂(COOH)₂]₀ ≤ 4.30 × 10⁻² mol dm⁻³) in the Belousov-Zhabotinsky (BZ) system in the presence of bromate, bromide, sulfuric acid and cerium sulfate, were performed in the isothermal closed well stirred reactor at different temperatures (25.0°C ≤ T ≤ 45.0°C). The formal kinetics of the overall BZ reaction, and particularly kinetics in characteristic periods of BZ reaction, based on the analyses of the bromide oscillograms, was accomplished. The evolution as well as the rate constants and the apparent activation energies of the reactions, which exist in the preoscillatory and oscillatory periods, are also successfully calculated by numerical simulations. Simulations are based on the model including the Br₂O species. The article is published in the original.     

Kinetics of thermal decomposition of manganese(ii) oxalate

The kinetics of manganese(II) oxalate thermal decomposition in the helium atmosphere was studied on the basis of isothermal measurements in the temperature range from 608 to 623 K. Manganese(II) oxide, MnO, was found to be the final product of reaction. The Avrami-Erofeev kinetic equation was used to describe all the experimental data in the range of decomposition degrees from 0.1 to 0.9. The determined activation energy equals 184.7 kJ mol^-1 with standard deviation ±5.2 kJ mol^-1. The estimated value of parameter n is 1.9 with standard deviation ±0.01 what suggests that the rate limiting step of MnC₂O₄ decomposition is the nucleation of new MnO phase and that the rate of nuclei growth is rising during decomposition. This revised version was published online in July 2006 with corrections to the Cover Date.     

Open-system nonequilibrium steady state: statistical thermodynamics, fluctuations, and chemical oscillations

Gibbsian equilibrium statistical thermodynamics is the theoretical foundation for isothermal, closed chemical, and biochemical reaction systems. This theory, however, is not applicable to most biochemical reactions in living cells, which exhibit a range of interesting phenomena such as free energy transduction, temporal and spatial complexity, and kinetic proofreading. In this article, a nonequilibrium statistical thermodynamic theory based on stochastic kinetics is introduced, mainly through a series of examples: single-molecule enzyme kinetics, nonlinear chemical oscillation, molecular motor, biochemical switch, and specificity amplification. The case studies illustrate an emerging theory for the isothermal nonequilibrium steady state of open systems.     

A kinetic expression for the pyrolytic decomposition of polytetrafluoroethylene

Despite the fact that the thermal decomposition of polytetrafluoroethylene has been extensively studied over the past six decades, some inconsistencies regarding the kinetic parameters, e.g. the order of the reaction, remain. Representative kinetic data are essential for practical purposes such as reactor design and scaling. In general the literature data refer to homogeneous bulk heating, whereas the case of the non-homogeneous heating of a single particle has not received attention. Data (reaction rate and pre-exponential factor) applicable to this latter case were experimentally determined from isothermal thermogravimetric analyses of the depolymerisation reaction of PTFE. The kinetic data obtained on coarse granules (800-1000 μm) are reported here. The rate law is consistent with a shrinking particle kinetic model, with chemical kinetics controlling phase-boundary movement. The mass loss rate is directly proportional to surface area. A rate law applicable to this case, and useable for geometries of arbitrary shape, is derived.     

Kinetics of Direct Reduction of Chrome Iron Ore

The kinetics of direct reduction of artificial chrome iron ore was studied by isothermal and non-isothermal methods. In the initial, middle and final periods, the reaction is controlled by nucleation and growth, a phase boundary reaction, and diffusion, respectively. In the main reaction region, the kinetic equation is 1–(1–)^1/3=kt and the apparent activation energy is 270 kJ mol^–1. The kinetic mechanisms found with the isothermal and non-isothermal methods do not differ, and the activation energy values are approximately the same. However, the non-isothermal method can demonstrate the kinetic process completely.This revised version was published online in November 2005 with corrections to the Cover Date.     

碳前驱体CH3ArCH2NH2的热解性能及动力学研究

通过密闭压力容器法、常压DSC、高压DSC及紫外分光光度定量分析法等实验手段,对液相沉积法制碳／碳复合材料用碳前驱体CH3ArCH2NH2的热裂解行为进行了研究,获得不同温度、不同压力下该碳前驱体的热分解温度和残碳率,用等温动力学和非等温动力学方法获得了热裂解反应的表观活化能,实验结果表明,常压热裂解温度大约为530．15－556．55K,1－3MPa的高压范围内的热裂解温度大约在618．34－675．49K,密闭压力容器中的残碳率为56．23％,常压下的残碳率为28．96％－36．47％,而高压下残碳率可达59．11％,根据基辛格等方法获得了等温条件下和非等温条件下热裂解反应的表观活化能Ea分别为206．78kJ/mol和183．93kJ/mol, 反应级数N~1.     

基于热重红外联用分析的PE、PS、PVC热解机理研究

利用TGA-FTIR联用技术考察了PE、PS、PVC三种典型塑料的热解特性。结果表明,热稳定性从弱到强依次为PVC、PS、PE。PE热解反应过程为典型的一段式反应,红外光谱分析结果表明,PE热解过程为无规则断链形式,生成产物成分复杂,且随热解过程而改变,开始以饱和烃基团为主,中后期以烯烃基团为主,同时有少量炔烃;PS热解过程同样为一段式反应,红外光谱显示主要热解产物为苯乙烯单体,说明热解过程主要是苯乙烯的解聚过程;PVC热解过程较为复杂,主要分为脱氯阶段和共轭多烯重构阶段,红外光谱结果表明,产物中有芳香族化合物。脱氯过程和共轭多烯重构、环化过程在时间和空间上有重合,给二噁英类污染物的生成制造了可能。     

TiO2薄膜光催化气相甲醇制氢

 在气相连续流动装置中以TiO2薄膜为光催化剂，对甲醇的脱氢反应进行了研究．考察了空速、光照时间和反应温度对甲醇转化率的影响以及产氢量与添加的水蒸气量和甲醇浓度的关系，并将TiO2薄膜催化剂的光催化活性与TiO2纳米粉体催化剂进行了比较．反应动力学研究表明，光催化甲醇脱氢反应为一级反应，其活化能为8．64kJ／mol．探讨了该反应的机理．