Estimating realistic confidence intervals for the activation energy determined from thermoanalytical measurements

Thermogravimetric and thermo-kinetic analysis of sugarcane bagasse pith: a comparative evaluation with other sugarcane residues

In this study, thermogravimetric and thermo-kinetic analysis of sugarcane bagasse pith (S.B.P.) were performed using a robust suite of experiments and kinetic analyses, along with a comparative evaluation on the thermo-kinetic characteristics of two other major sugarcane residues, namely sugarcane straw (S.C.S.) and sugarcane bagasse (S.C.B.). The thermogravimetric analysis evaluated the pyrolysis behavior of these residues at different heating rates in a nitrogen atmosphere. The Kissinger, advanced non-linear isoconversional (ANIC), and Friedman methods were employed to obtain effective activation energies. Moreover, the compensation effect theory (CE) and combined kinetic analysis (CKA) were used to determine the pre-exponential factor and pyrolysis kinetic model. Friedman's method findings indicated that the average activation energies of S.C.S., S.C.B., and S.B.P. are 188, 170, and 151 kJ/mol, respectively. The results of the ANIC method under the integral step Δα = 0.01 were closely aligned with those of the Friedman method. The CKA and CE techniques estimated ln(f(α)Aα) with an average relative error below 0.7%. The pre-exponential factors of S.C.S., S.C.B., and S.B.P. were in the order of 1014, 1012, and 1011 (s-1), respectively. From a thermodynamic viewpoint, positive ∆G* and ∆H* results provide evidence for the non-spontaneous and endothermic nature of the pyrolysis process, indicating the occurrence of endergonic reactions.

Kinetics and Mechanism of Liquid-State Polymerization of 2,4-Hexadiyne-1,6-diyl bis-(p-toluenesulfonate) as Studied by Thermal Analysis

A detailed investigation of the liquid-state polymerization of diacetylenes by calorimetric (DSC) and spectroscopic (in situ EPR) thermal analysis techniques is performed. Isoconversional kinetic analysis of the calorimetric data reveals that liquid-state polymerization is governed by a well-defined rate-limiting step as evidenced by a nearly constant isoconversional activation energy. By comparison, solid-state polymerization demonstrates isoconversional activation energy that varies widely, signifying multistep kinetics behavior. Unlike the solid-state reaction that demonstrates an autocatalytic behavior, liquid-state polymerization follows a rather unusual zero-order reaction model as established by both DSC and EPR data. Both techniques have also determined strikingly similar Arrhenius parameters for liquid-state polymerization. Relative to the solid-state process, liquid-state polymerization results in quantitative elimination of the p-toluenesulfonate group and the formation of p-toluenesulfonic acid and a polymeric product of markedly different chemical and phase composition.

Kinetics of Polycycloaddition of Flexible α-Azide-ω-Alkynes Having Different Spacer Length

Two flexible α-azide-ω-alkynes differing in the length of the hydrocarbon spacers (C₈ vs. C₁₂) between functional groups are synthesized. Their bulk polymerization kinetics is studied by differential scanning calorimetry (DSC) and parameterized with the aid of isoconversional methodology. The monomer with a shorter hydrocarbon spacer has somewhat greater reactivity. The effect is traced to a moderate increase in the effective value of the preexponential factor that arises from the fact that the respective monomer has a higher initial molar concentration in itself. The techniques of GPC and NMR provide additional kinetic and mechanistic insights into the studied reaction.

The Kinetics of Formation of Microporous Polytriazine in Diphenyl Sulfone

This study highlights the value of nonisothermal kinetic methods in selecting temperature conditions for the isothermal preparation of microporous polymeric materials. A dicyanate ester is synthesized and the kinetics of its polymerization in diphenyl sulfone are studied by calorimetry under nonisothermal conditions. The kinetics are analyzed by a model-based approach, using the Kamal model, as well as by a model-free approach, using an advanced isoconversional method. Both approaches correctly predict the time to completion of polymerization at a given temperature. The material prepared independently at the predicted temperature is characterized by electron microscopy and CO₂ adsorption measurements and is confirmed to possess a microporous structure with a multimodal distribution of micropores with two major maxima at ~0.5 and 0.8 nm.

Inverse Modeling of Thermal Decomposition of Flame-Retardant PET Fiber with Model-Free Coupled with Particle Swarm Optimization Algorithm

The thermal decomposition model of flame-retardant polyethylene terephthalate (FRPET) fiber is essential for predicting its fire behavior and do relevant fire simulation. In this work, the thermal decomposition character of FRPET is investigated via thermogravimetric analysis at four heating rates. Two kinetic schemes are proposed based on the analysis of experimental data and model-free methods. The model-free methods (Friedman and advanced Vyazovkin methods) are employed to determine possible search range for particle swarm optimization algorithm with constriction factor (CFPSO). Thus, this coupled method could evaluate the kinetic parameters for two proposed schemes without initial guess. Both models could reasonably predict the experimental data with obtained parameters, and the second two-step consecutive model shows better performance. The performance of CFPSO on the second model is further compared with improved generalized simulated annealing algorithm, and CFPSO was found to be more effective. Furthermore, global sensitivity analysis was conducted via the Sobol method to investigate the influence of kinetic parameters for the second model on predicted results. The most influential parameters are ln A and E _α of the second reaction and reaction order n of the third reaction.

Synthesis and Polymerization Kinetics of Rigid Tricyanate Ester

A new rigid tricyanate ester consisting of seven conjugated aromatic units is synthesized, and its structure is confirmed by X-ray analysis. This ester undergoes thermally stimulated polymerization in a liquid state. Conventional and temperature-modulated differential scanning calorimetry techniques are employed to study the polymerization kinetics. A transition of polymerization from a kinetic- to a diffusion-controlled regime is detected. Kinetic analysis is performed by combining isoconversional and model-based computations. It demonstrates that polymerization in the kinetically controlled regime of the present monomer can be described as a quasi-single-step, auto-catalytic, process. The diffusion contribution is parameterized by the Fournier model. Kinetic analysis is complemented by characterization of thermal properties of the corresponding polymerization product by means of thermogravimetric and thermomechanical analyses. Overall, the obtained experimental results are consistent with our hypothesis about the relation between the rigidity and functionality of the cyanate ester monomer, on the one hand, and its reactivity and glass transition temperature of the corresponding polymer, on the other hand.

Solvent-induced changes in the reactivity of tricyanate esters undergoing thermal polymerization

The mechanism of thermally stimulated polymerization of tricyanate ester remains the same in solution as in the melt, but Arrhenius parameters of the rate-limiting reaction are significantly affected by solvation.

Polymerization Kinetics of Cyanate Ester Confined to Hydrophilic Nanopores of Silica Colloidal Crystals with Different Surface-Grafted Groups

This study investigates the kinetics of confined polymerization of bisphenol E cyanate ester in the nanopores of the three types of silica colloidal crystals that differ in the concentration and acidity of the surface-grafted proton-donor groups. In all three types of pores, the polymerization has released less heat and demonstrated a very similar significant acceleration as compared to the bulk process. Isoconversional kinetic analysis of the differential scanning calorimetry measurements has revealed that the confinement causes not only a dramatic change in the Arrhenius parameters, but also in the reaction model of the polymerization process. The obtained results have been explained by the active role of the silica surface that can adsorb the residual phenols and immobilize intermediate iminocarbonate products by reaction of the monomer molecules with the surface silanols. The observed acceleration has been quantified by introducing a new isoconversional-isothermal acceleration factor Zα,T that affords comparing the process rates at respectively identical conversions and temperatures. In accord with this factor, the confined polymerization is 15-30 times faster than that in bulk.

Study of Mechanical and Moisture Absorption Behavior of Epoxy/Cloisite-15A Nanocomposites Processed Using Twin Screw Extruder

This paper presents the effect of process parameters of twin screw extruder and addition of Cloisite-15A on mechanical, thermal and moisture barrier properties of epoxy/Cloisite-15A nanocomposites. Four lobed kneading blocks were used the in shearing zone of the extruder, based on their effectiveness in dispersing nanofillers in epoxy. Screw speeds from 100 min−1 to 400 min−1, number of passes up to 15, temperature from 5°C to 80°C and Cloisite-15A contents from 1 wt.% to 2.5 wt.% were considered for designing the L12 Orthogonal Array. Improvements in tensile strength, compression strength, flexural strength, impact strength, hardness and moisture diffusivity in the nanocomposites were 11.89%, 20.06%, 27.73%, 37.26%, 25.48% and 56.22% respectively, when compared to neat epoxy. The improvements were achieved for screw speed of 400 min–1, 5 passes through the extruder, processing temperature of 5°C and 2 wt.% of Cloisite-15A. Dispersion of Cloisite-15A in epoxy was studied by XRD, SEM and TEM. Thermal stability and moisture barrier properties were superior in the nanocomposites.

Solid-state polymerization of a novel cyanate ester based on 4-tert-butylcalix[6]arene

Solid-state polymerization of a cyclic cyanate ester shows zero-order kinetics and proceeds through cooperative breaking of CN bonds.

Comparative evaluation of thermal decomposition behavior and thermal stability of powdered ammonium nitrate under different atmosphere conditions

In order to analyze the thermal decomposition characteristics of ammonium nitrate (AN), its thermal behavior and stability under different conditions are studied, including different atmospheres, heating rates and gas flow rates. The evolved decomposition gases of AN in air and nitrogen are analyzed with a quadrupole mass spectrometer. Thermal stability of AN at different heating rates and gas flow rates are studied by differential scanning calorimetry, thermogravimetric analysis, paired comparison method and safety parameter evaluation. Experimental results show that the major evolved decomposition gases in air are H₂O, NH₃, N₂O, NO, NO₂ and HNO₃, while in nitrogen, H₂O, NH₃, NO and HNO₃ are major components. Compared with nitrogen atmosphere, lower initial and end temperatures, higher heat flux and broader reaction temperature range are obtained in air. Meanwhile, higher air gas flow rate tends to achieve lower reaction temperature and to reduce thermal stability of AN. Self-accelerating decomposition temperature of AN in air is much lower than that in nitrogen. It is considered that thermostability of AN is influenced by atmosphere, heating rate and gas flow rate, thus changes of boundary conditions will influence its thermostability, which is helpful to its safe production, storage, transportation and utilization.

Isoconversion effective activation energy profiles by variable temperature diffuse reflection infrared spectroscopy

Thermal process characterization based on calculating effective activation energies from variable temperature diffuse reflection infrared spectroscopy (VT-DRIFTS) measurements is demonstrated. Experimental factors that affect the accuracies of activation energy values are outlined. Infrared radiation scattering efficiency, thermal conductivity, and inertness towards chemical reactions are factors that should be considered when selecting an appropriate diluent for preparing samples. The Kubelka-Munk representation is superior to apparent absorbance when baseline variations in spectra measured at different temperatures can be minimized. Variable-temperature infrared spectral features, such as integrated absorption band area, can be used to compute isoconversion effective activation energies, provided that measured quantities are proportional to species concentrations.

Thermal Denaturation of Collagen Analyzed by Isoconversional Method

An isoconversional method is proposed to be used for evaluating activation energy of protein denaturation. Applied to DSC data on collagen denaturation, the method yields an activation energy that decreases throughout the process. The Lumry-Eyring model gives an explanation for this decrease and affords estimates for the enthalpy of the reversible step and the activation energy of the irreversible step of denaturation. The reversible unfolding is detectable by multi-frequency temperature-modulated DSC.

Effect of Substituents in Aromatic Amines on the Activation Energy of Epoxy−Amine Reaction

Differential scanning calorimetry has been used to study the kinetics of epoxy-amine curing reaction between diglycidyl ether of 4,4'-bisphenol and aromatic amines with different electron-withdrawing/-donating substituents. The substituents include -NO2, -CN, -OCH3, -OH, and -CH3 groups. An advanced isoconversional method has been employed to determine the effective activation energy of the respective processes. An attempt has been made to link the experimental values with the results of quantum chemistry calculations. It has been found that regardless of the electron-withdrawing/-donating properties the presence of a substituent of a large negative charge in the ortho position causes an increase in the activation energy to approximately 100 kJ mol-1 from the normally observed values of 50-60 kJ mol-1.

Explorative Kinetic Study on the Thermal Degradation of Five Wood Species for Applications in the Archaeological Field

Differential scanning calorimetry (DSC) and thermogravimetric analysis (TG) were performed on wood samples of different essences (fir, chestnut, poplar, linden and oak) before consolidation. A kinetic analysis was applied on the two-steps decomposition processes occurring in all wood samples using either the multiheating rates Kissinger equation and the isoconversional Ozawa-Flynn-Wall method that enables the variation of activation energy to be determined as a function of the degree of reaction. Taking into account both decomposition temperature and activation energy for the first degradation step oak seems to be the less stable sample. The comparison of DSC curves performed in air with those in oxygen enables to consider the role of the partial pressure of oxygen in the mechanisms of both decompositions.

Hard to swallow dry: kinetics and mechanism of the anhydrous thermal decomposition of acetylsalicylic acid

The methods of thermal analysis and mass spectrometry have been used to study the kinetics and mechanism of the anhydrous thermal decomposition of acetylsalicylic acid. Both thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) show that decomposition occurs in two steps. Mass-spectrometric analysis of the residue left after the first decomposition step (approximately equal to 60% mass loss) suggests that in the condensed phase, acetylsalicylic acid decomposes by first forming linear oligomers that are further converted into cyclic oligomers. Model-free isoconversional kinetic analysis of TGA traces has been used to determine global activation energies as a function of the extent of reaction. This method of analysis has also been used to make kinetic predictions of shelf life at ambient temperatures (20-40 degrees C) under anhydrous conditions for acetylsalicylic acid. Our estimate of a shelf life of 876 days (approximately equal to 2.4 years) for 5% decomposition at 30 degrees C is in good agreement with shelf lives of 2-3 years that are stamped on over-the-counter aspirin bottles. Hence, this approach can be used to systematically study the factors that determine the decomposition kinetics of aspirin and may be used for express screening of pharmaceuticals in order to identify those with desirable thermal stabilities.