Study of oxidation properties and decomposition kinetics of three-dimensional (3-D) braided carbon fiber

The XRD, SEM, isothermal oxidation-weight loss and non-isothermal thermogravimetry (TG)-differential thermogravimetry (DTG) were used to study the oxidation properties and oxidation decomposition kinetics of three-dimensional (3-D) braided carbon fiber (abbreviated as fiber). The results showed that the non-isothermal oxidation process of fiber exhibited self-catalytic characteristic. The kinetic parameters and oxidation mechanism of fiber were studied through analyzing the TG and DTG data by differential and integral methods. The oxidation mechanism was random nucleation, the kinetic parameters were: lg A=10.299 min⁻¹; E_a=156.29 kJ mol⁻¹.     

Kinetics of Thermal Decomposition of Lithium Hexafluorophosphate

With on-line coupled thermo-gravimetric analyzer-Fourier transform infrared spectrometer technique, the thermal decomposition of lithium hexafluorophosphate (LiPF₆) and its gas evolution at inert environment (H₂O<10 ppm) were studied under both non-isothermal and isothermal conditions. The results showed that the LiPF₆ decomposition is a single-stage reaction with LiF as final residue and PF₅ as gas product. In addition, its decomposi-tion kinetics was determined as 2D phase boundary movement (cylindrical symmetry) under both non-isothermal and isothermal conditions. Furthermore, the activation energy of LiPF₆ decomposition was calculated as 104 and 92 kJ/mol for non-isothermal and isothermal con-ditions, respectively.     

DSC kinetics of the thermal decomposition of copper(II) oxalate by isoconversional and maximum rate (peak) methods

The copper(II) oxalate was synthesized, characterized using FT-IR and scanning electron microscopy and its non-isothermal decomposition was studied by differential scanning calorimetric at different heating rates. The kinetics of the thermal decomposition was investigated using different isoconversional and maximum rate (peak) methods viz. Kissinger–Akahira–Sunose (KAS), Tang, Starink^1.95, Starink^1.92, Flynn–Wall–Ozawa (FWO) and Bosewell. The activation energy values obtained from isoconversional methods of FWO and Bosewell are 0.9 and 3.0 %, respectively, higher than that obtained from other methods. The variation of activation energy, E _α with conversion function, α, established using these different methods were found to be similar. Compared to the FWO method, the KAS method offers a significant improvement in the accuracy of the E _a values. All but the Bosewell maximum rate (peak) methods yielded consistent values of E _α (~137 kJ mol^?1); however, the complexity of the thermal decomposition reaction can be identified only through isoconversional methods.     

Synthesis and kinetic analysis of isothermal and non-isothermal decomposition of ammonium dinitramide prills

Ammonium dinitramide (ADN) prills were prepared by emulsion crystallization and characterized by optical microscopic, thermogravimetric (TG) and differential scanning calorimetric (DSC) techniques. The isothermal and non-isothermal decomposition kinetics of ADN prills were studied by TG. The differential isoconversional method of Friedman (FR) and integral isoconversional method of Vyazovkin were used to investigate the dependence of activation energy (E _a) with conversion (α) and the results were compared with literature data. The dependence of activation energy was also derived from isothermal data. A strong dependence of E _a with α is observed for the ADN prills. All the methods showed an initial increase in E _a up to α=∼0.2 and later decreases over the rest of conversion. The apparent E _a values of FR method are higher than that of Vyazovkin method up to α=∼0.45. The calculated mean E _a values by FR, Vyazovkin and standard isoconversional method for α between 0.05 and 0.95 were 211.0, 203.9 and 156.9 kJ mol⁻¹, respectively.     

Kinetics of the thermal dehydration of trans-fluoroaquobis(ethylenediamine)chromium(III) tetracyanometallate(II) [metal(II) = Ni(II), Pd(II) and Pt(II)]

The solid phase thermal deaquation—anation of trans-[CrF(H₂O)(en)₂][M(CN)₄] (M = Ni, Pd, Pt; en = ethylenediamine) has been investigated by means of non-isothermal DSC and isothermal and non-isothermal TG measurements. The physical model for these reactions (nucleation, growth, diffusion or intermediates) has been found by comparison of the isothermal and non-isothermal TG data for all the principal g(α) expressions (0.2?α?0.8) and by the shape of the isothermal curves. The values found for activation energy are low (～ 130 kJ mol^?1 for the Ni compound, ～ 140 kJ mol^?1 for the Pd compound, and ～ 100 kJ mol^?1 for the Pt compound). These data permit the assignment of the deaquation—anation mechanism of the S_N1 type involving a square-base pyramid activated complex and elimination of water as Frenkel defects.     

Addition effect of erbium(III) trifluoromethanesulfonate in the homopolymerization kinetics of a DGEBA resin

Solid bisphenol-A epoxy resin of medium molecular weight was cured using a Lewis acid initiator (erbium(III) trifluoromethanesulfonate) in three different proportions (0.5, 1 and 2 phr). A kinetic study was performed in a differential scanning calorimeter. The complete kinetic triplet was determined (activation energy, pre-exponential factor, and integral function of the degree of conversion) for each system. A kinetic analysis was performed with an integral isoconversional procedure (model-free), and the kinetic model was determined both with the Coats-Redfern method (the obtained isoconversional E value being accepted as the effective activation energy) and through the compensation effect. All the systems followed the same isothermal curing model simulated from non-isothermal ones. The “nucleation and growth” Avrami kinetic model A_3/2 has been proposed as the polymerization kinetic model. The addition of initiator accelerated the reaction having higher influence when low temperatures were applied.     

Model-free kinetics analysis of ZSM-5 catalyzed pyrolysis of waste LDPE

Both thermal and catalytic decomposition of waste LDPE sample is studied to understand the effect of catalyst (ZSM-5) on the decomposition behaviour. The nonlinear Vyazovkin model-free technique is applied to evaluate the quantitative information on variation of E_α with α for waste LDPE sample under both catalytic and noncatalytic nonisothermal conditions. The literature reported data on such variation under noncatalytic condition and effects of different catalysts on the LDPE sample are compared with the results of the present study.Results show that the optimum catalyst composition is around 20 wt.%, where the reduction in maximum decomposition temperature is around 70 °C. Presence of ZSM-5 shows similar reduction in maximum decomposition temperature as reported for Al-MCM-41 and MCM-41. Similar trend to literature reported data is observed for variation of E_α with α for LDPE under nonisothermal noncatalytic condition. ZSM-5 catalyzed decomposition of the LDPE sample in the present study indicates that E_α is strong and increasing function of α and consists of four steps. Cracking of large polymer fragments on the external surface of the catalyst, oligomerization, cyclization, and hydrogen transfer reactions inside the catalyst pores might be the possible reaction mechanisms involved during catalytic decomposition.     

Thermal degradation mechanism of highly filled nano-SiO2 and polybenzoxazine

Effects of high nano-SiO₂ loading (up to 30 mass%) on polybenzoxazine (PBA-a) thermal degradation kinetics have been investigated using nonisothermal thermogravimetric analysis (TG). The DTG curves revealed three stages of thermal decomposition process in the neat PBA-a, while the first peak at low temperature was absent in its nanocomposites. As a consequence, the maximum degradation temperature of the nanocomposites shifted significantly to higher temperature as a function of the nano-SiO₂ contents. Moreover, the degradation rate for every degradation stage was found to decrease with the increasing amount of the nano-SiO₂. From the kinetics analysis, dependence of activation energy (E _a) of the nanocomposites on conversion (α) suggests a complex reaction with the participation of at least two different mechanisms. From Coats–Redfern and integral master plot methods, the average E _a and pre-exponential factor (A) of the nanocomposites showed systematically higher value than that of the PBA-a, likely from the shielding effect of the nanoparticles. The main degradation mechanism of the PBA-a was determined to be a random nucleation type with one nucleus on the individual particle (F1 model), while that of the PBA-a nanocomposite was the best described by diffusion-controlled reaction (D3 model).     

Mechanism and kinetics of thermal degradation of insulating materials developed from cellulose fiber and fire retardants

The mechanism and kinetics of thermal degradation of materials developed from cellulose fiber and synergetic fire retardant or expandable graphite have been investigated using thermogravimetric analysis. The model-free methods such as Kissinger–Akahira–Sunose (KAS), Friedman, and Flynn–Wall–Ozawa (FWO) were applied to measure apparent activation energy (E_α). The increased E_α indicated a greater thermal stability because of the formation of a thermally stable char, and the decreased E_α after the increasing region related to the catalytic reaction of the fire retardants, which revealed that the pyrolysis of fire retardant-containing cellulosic materials through more complex and multi-step kinetics. The Friedman method can be considered as the best method to evaluate the E_α of fire-retarded cellulose thermal insulation compared with the KAS and FWO methods. A master-plots method such as the Criado method was used to determine the possible degradation mechanisms. The degradation of cellulose thermal insulation without a fire retardant is governed by a D3 diffusion process when the conversion value is below 0.6, but the materials containing synergetic fire retardant and expandable graphite fire retardant may have a complicated reaction mechanism that fits several proposed theoretical models in different conversion ranges. Gases released during the thermal degradation were identified by pyrolysis–gas chromatography/mass spectrometry. Fire retardants could catalyze the dehydration of cellulosic thermal insulating materials at a lower temperature and facilitate the generation of furfural and levoglucosenone, thus promoting the formation of char. These results provide useful information to understand the pyrolysis and fire retardancy mechanism of fire-retarded cellulose thermal insulation.

     

Pyrolysis kinetics of elephant grass pretreated biomasses

The Elephant Grass (Pennisetum purpureum Schum) was pretreated by two independent processes, through washing with hot water (W-EG) and acid solution (AW-EG) to improve its energy properties to apply it in a thermochemical process conversion into fuel. The biomasses were analyzed by proximate and ultimate analysis; and the pyrolysis kinetics, before and after pretreatments, were evaluated by the apparent activation energy (E _a) for decomposition in the temperature range of greater volatile matter through the Model-free kinetics using thermogravimetric analysis data. The kinetics of the microcrystalline cellulose Avicel PH-101 was performed to evaluate the E _a result of pure cellulose. The pretreatments were efficient in increasing the volatile matter and heating value, decreasing moisture and ash content, and improving its energetic power to the application in fast pyrolysis process for bio-oil production. The TG results have shown that the reduction in ash content facilitates the pyrolysis process, increasing the volatile matter and decreasing the apparent activation energy required to biomasses degradation, due to less diffusional resistances to heat and mass transfer of W-EG and AW-EG. The Avicel PH-101 showed the highest value of apparent activated energy (E _a = 276.2 kJ mol^?1) which could be explained by its crystallinity, suggesting that crystalline cellulose regions are less accessible to heat diffusion than amorphous regions, requiring more energy to its degradation.     

Application of model-fitting and model-free kinetics to the study of non-isothermal dehydration of equilibrium swollen poly (acrylic acid) hydrogel: Thermogravimetric analysis

The dehydration kinetics of equilibrium swollen poly (acrylic acid) hydrogel is analyzed by both model-fitting and model-free approaches. The conventional model-fitting approach assuming a fixed mechanism throughout the reaction and extract a single values of the apparent activation energy (E_a) and pre-exponential factor (A) and was found to be too simplistic. The values of Arrhenius parameters obtained in such a way are in fact an average that does not reflect changes in the reaction mechanism and kinetics with the extent of conversion. The model-free approach allows for a change of mechanism and activation energy, E_a, during the course of a reaction and is therefore more realistic. The complexity of the dehydration of poly (acrylic acid) hydrogel is illustrated by the dependence of E_a and A on the extent of conversion, α (0.05 ≤ α ≤ 0.98). Under non-isothermal conditions, E_a decreases with α for 0 ≤ α ≤ 0.50, followed by an approximately constant value of E_a during further dehydration. For 0 ≤ α ≤ 0.50, dehydration is complex, which probably involving a combination of several processes. In the constant-E_a region, non-isothermal dehydration follows the three-dimensional phase boundary model (R3). The complex hydrogen-bond pattern in poly (acrylic acid) hydrogel is probably responsible for the observed dehydration behavior. An existence of compensation effect is accepted and explanation of compensation effect appearance during the hydrogel dehydration is suggested.     

Thermal behaviour, compatibility study and decomposition kinetics of glimepiride under isothermal and non-isothermal conditions

In the present work, the thermal decomposition of glimepiride (sulfonylurea hypoglycemic agent) was studied using differential scanning calorimetry (DSC) and thermogravimetry/derivative thermogravimetry (TG/DTG). Isothermal and non-isothermal methods were employed to determine kinetic data of decomposition process. The physical chemical properties and compatibilities of several commonly used pharmaceutical excipients (glycolate starch, microcrystalline cellulose, stearate, lactose and Plasdone®) with glimepiride were evaluated using thermoanalytical methods. The 1:1 physical mixtures of these excipients with glimepiride showed physical interaction of the drug with Mg stearate, lactose and Plasdone®. On the other hand, IR results did not evidence any chemical modifications. From isothermal experiments, activation energy (E _a) can be obtained from slope of lnt vs. 1/T at a constant conversion level. The average value of this energy was 123 kJ mol^–1. For non-isothermal method E _a can be obtained from plot of logarithms of heating rates, as a function of inverse of temperature, resulting a value of 157 and 150 kJ mol^–1, respectively, in air and N₂ atmosphere, from the first stage of thermal decomposition.     

Thermal decomposition of sodium propoxides

Sodium alkoxides, namely, sodium n-propoxide and sodium iso-propoxide were synthesized and characterized by various analytical techniques. Thermal decomposition of these compounds was studied under isothermal and non-isothermal conditions using a thermogravimetric analyzer coupled with mass spectrometer. The onset temperatures of decomposition of sodium n-propoxide and sodium iso-propoxide were found to be 590 and 545 K, respectively. These sodium alkoxides form gaseous products of saturated and unsaturated hydrocarbons and leave sodium carbonate, sodium hydroxide, and free carbon as the decomposition residue. Activation energy, E _a, and pre-exponential factor, A, for the decomposition reactions were deduced from the TG data by model-free (iso-conversion) method. The E _a for the decomposition of sodium n-propoxide and sodium iso-propoxide, derived from isothermal experiments are 162.2 ± 3.1 and 141.7 ± 5.3 kJ mol^?1, respectively. The values obtained from the non-isothermal experiments are 147.7 ± 6.8 and 133.6 ± 4.1 kJ mol^?1, respectively, for the decomposition of sodium n-propoxide and sodium iso-propoxide.     

Thermal decomposition kinetics of lanthanum and neodymium bromide complexes with glycine or alanine

Four complexes of rare earth bromides with amino acids, REBr₃·3L·3H₂O (RE=La, Nd;L=glycine or alanine) were prepared and characterized by means of chemical analysis, elemental analysis, molar conductivity, thermogravimetry, IR spectra and X-ray diffraction. Their thermal decomposition kinetics from ambient temperature to 500°C were studied by means of TG-DTG techniques under non-isothermal conditions. The kinetic parameters (activation energyE and pre-exponential constantA) and the most probable mechanisms of thermal decomposition were obtained by using combined differential and integral methods. The thermal decomposition processes of these complexes are distinguished as being of two different types, depending mainly on the nature of the amino acid.     

Thermal properties of drug polymorphs: A case study with felodipine form I and form IV

Thermal decomposition kinetics and solution thermodynamics of two polymorphs, felodipine form I and form IV, were investigated. The thermal decomposition kinetics of the polymorphs, containing thermal decomposing mechanism and the kinetic parameters were studied under non-isothermal conditions using Popescu method, and the shelf life was simply calculated. The results showed that nucleation and growth (n = 3/4) of Avremi–Erofeev equation is the most probable mechanism function for form I, and the integral form is G(α) = [−ln(1 − α)]^3/4; while the Mample Powel law (n = 1) is the most probable mechanism function for form IV, corresponding to G(α) = α. Notably, the individual amorphous phases of crystal felodipine form I and IV were obtained after the heating–cooling cycle of DSC tests, which were identified by TMDSC and FT-IR measurements. As the interim products before the collapse, it can be inferred that different amorphous intermediates may be the determinant for different thermal decomposition mechanisms of crystal forms I and IV. The solubility data and solution thermodynamic parameters, including enthalpy, entropy and Gibbs free energy have also been calculated by Van’t Hoff equation in ethanol aqueous. The results illustrated the polymorphic pair is enantiotropic with the transition temperature of 322.23 K and the conversion is driven by entropy.     

Kinetic Study on Pyrolysis of Extracted Oil Palm Fiber. Isothermal and non-isothermal conditions

Pyrolysis of extracted oil palm fibers under isothermal and non-isothermal conditions was carried out in a thermogravimetric analyzer. Isothermal curves showed that increasing pyrolysis temperature resulted in a faster pyrolysis and a higher conversion of oil palm fibers into gaseous products. Raw material sizes (below 1.0 mm) had insignificant effects on the isothermal pyrolysis, but the fibers with a size fraction of 1.0 to 2.0 mm resulted in a lesser conversion. Two-step reactions were found in the non-isothermal pyrolysis as evidenced by the presence of two peaks in the derivative thermogravimetry curves. Raw material sizes had no obvious effects on the temperature at which the maximum rate of pyrolysis occurred, but affected the rate of sample mass loss. For the low and high temperature regimes, a three-dimensional diffusion mechanism and a first-order of reaction mechanism respectively were used to describe the non-isothermal pyrolysis kinetics of extracted oil palm fibers. This revised version was published online in July 2006 with corrections to the Cover Date.     

Kinetics of Thermal Dissociation of Ammonium Metavanadate

The solid products of thermal decomposition of ammonium metavanadate can be used as catalysts in many important processes, and a knowledge of the dynamics of these processes is therefore essential. The thermal dissociation of ammonium metavanadate was studied under non-isothermal conditions in air atmosphere. This process occurred in three steps under the applied experimental conditions, and was associated with the elimination of ammonia and water below 330°C and with the formation of nitrogen oxides above 330°C. The kinetics of particular stages of (NH₄)₂OV₂0₅ decomposition was evaluated from the dynamic mass loss data by means of the integral method, with applycation of the Coats and Redfern approximation. The first stage of decomposition to ammonium hexavanadate is governed by a random nucleation model, the second step by a three-dimensional diffusion or contracting volume model, and the last stage again by a random nucleation model. The apparent activation energies found for the particular stages were 144.97, 378.31 or 184.40 and 260.65 kJ mol^?1, respectively.     

Synthesis and cure kinetics of diphenyl(diphenylethynyl)silane monomer

Diphenyl(diphenylethynyl)silane ((ph–C≡C)₂–Si–ph₂) (DPDPES) was synthesized by the Grignard reaction. The corresponding isothermal and non-isothermal cure kinetics of DPDPES were analyzed by using differential scanning calorimetry (DSC), and the molecular structure was characterized by H-NMR. The results showed that all the cure curves were typically sigmoid shape and cure reactions could be described by an autocatalytic kinetic model by isothermal DSC. The kinetic data, for example, activation energy (E) and frequency factor (A), were 119.22 kJ/mol and 4.67 × 10⁷ (s^?1), respectively. The non-isothermal DSC analyses showed that E and A were 162.12 kJ/mol and 1.32 × 10⁹ (s^?1), respectively, and the reaction order was 0.94. Based on the research work of this paper, it can be said that the cure reaction of DPDPES monomer was of autocatalytic and diffusion-controlled characteristics, and the effect of the diffusion was more evident at low temperature. The cure reaction of DPDPES was a first-order kinetic reaction.     

Preparation and characterization of microcapsulated red phosphorus and its multi-step thermal oxidation processes based on kinetic approach

Aluminium hydroxide/melamine–formaldehyde resin microcapsulated red phosphorus (MRP) was successfully prepared by two-step processes. The microcapsulated red phosphorus was characterized with Fouriertransform infrared spectroscopy (FTIR) and scanning electron microscope (SEM). Meanwhile its water absorption, thermostability were also determined. The results show that the MRP exhibited lower water absorption and higher thermostability compared with red phosphorus (RP) itself. Moreover, the thermal oxidative decomposition kinetics of MRP was investigated by TG/DTG and DTA in air atmosphere using non-isothermal experiments. The results show that the MRP’s decomposition consisted of two steps. And the apparent activation energies E_α was determined by applying both the Ozawa–Flynn–Wall (OFW) and Kissinger, Akahira and Sunose (KAS) methods. It was found that the dependence of E_α on α is complex. Both of the steps in this study fitted Sestak–Berggren (SB) model in overall reaction controlled kinetics and the corresponding model parameters, n, m, A were obtained. The simulated curves were fitted to experimental curves by plotting dα/dt vs. temperature at different heating rates.