DSC Study of Oxidation Induction Periods

A theory of the evaluation of kinetic parameters of induction periods for non-isothermal processes is outlined and a method to obtain the parameters from non-isothermal differential scanning calorimetry measurements, based on the dependence of onset temperature of oxidation peak on heating rate, is presented. The applicability of the method is demonstrated on the study of oxidation induction periods of edible oils and polyolefines. In all cases, the parameters of an Arrhenius-like equation describing the temperature dependence of induction period have been obtained. It is shown that the method gives the parameters not affected by oxygen diffusion which are transferable to be used in modelling the non-isothermal induction periods where the effects of diffusion, heat transfer and evolution of reaction heat are explicitly involved. A method of estimating the residual stability after a thermooxidative stress of the material is suggested. This revised version was published online in July 2006 with corrections to the Cover Date.     

低密度聚乙烯/乙丙烯三元共聚(LDPE/EPO)共混体系的结晶动力学

用DSC方法研究了LDPE/EPO共混体系的等温及非等温结晶动力学,对LDPE/EPO共混体系的等温结晶动力学研究表明,共混物是三维生长的异相成核,共混物在各个结晶温度下的结晶过程都是以方式K_g(Ⅱ)进行的.采用联系Avrami方程和Ozawa方程导出的新非等温结晶动力学方程,处理了LDPE/EPO共混体系,得到了非等温结晶过程的一些基本参数,新方程很好地描述了此共混体系的非等温结晶动力学过程.     

Measurement and modeling of peroxides half-life: A thermo-kinetic approach

Half-life values of organic peroxides at elevated temperature conditions are important in characterizing the reactivity and are often available in literature or through vendors. However, there is often lack of details/accuracy on methods used to obtain these values, as well as differences in methods across vendors and publications, thus resulting in discrepant reactivity profile. To address this, a method involving calorimetric experiment and thermo-kinetic modeling was developed. The current approach was applied on five peroxides samples to obtain kinetic parameters and estimate their half-life in the temperature range of interest. The measurements were performed by DSC under non-isothermal conditions on the dilute peroxide solutions (∼0.12 M in mineral oil) and the data were kinetically treated according to three model-based and one model-free kinetic equations. A very good agreement was found between the half-life calculated by all kinetic methods, but significant differences were noticed with the kinetic parameters reported in literature. Additionally, the obtained half-life results, based on non-isothermal measurements developed kinetic models, were validated through isothermal calorimetric testing. Given the accuracy and robustness of our results, the current method can be applied to estimate half-life of organic peroxides at elevated temperature conditions.     

Polymer crystallization

Differential scanning calorimetry (dual furnace, null-balance, DSC) and optical microscopy (OM) have been used to study the isothermal crystallization kinetics of poly(oxymethylene)-POM. The non-isothermal crystallization of the same material has also been studied by optical microscopy. A very controversial problem is whether the isothermal kinetic parameters may be applied to describe the non-isothermal crystallization. The results show that the kinetic spherulite growth parameters obtained by non-isothermal optical microscopy are, within the experimental errors involved, the same as those obtained by isothermal optical microscopy or isothermal DSC. The importance of this finding is highlighted.     

DSC and FTIR study of the gamma radiation effect on cis-1,4-polyisoprene

The effect of γ-radiation on the cis-1,4-polyisoprene in the presence of oxygen is investigated by ATR-FTIR technique and non-isothermal DSC measurements. FTIR measurements have shown that the formation of hydroperoxides, ketones, alcohols and/or ethers is apparent already at lower, 20–50 kGy, doses of γ-radiation and it increases significantly with the exposure time. Besides, lactones, anhydrides, peresters, carboxylic acids, and esters are formed, too. Spectral changes in the region of C=C conjugated double bonds indicate a formation of shorter polyene structures and aromatic rings. Kinetic parameters describing the temperature dependence of the induction period have been obtained from DSC measurements using the isoconversional method. Residual stabilities have been calculated in order to characterize the gamma radiation effect on polyisoprene thermooxidative stability. Both methods proved that doses lower than 50 kGy do not cause severe changes in polymer properties.     

Thermokinetic parameters and thermal hazard evaluation for three organic peroxides by DSC and TAM III

The thermokinetic parameters were investigated for cumene hydroperoxide (CHP), di-tert-butyl peroxide (DTBP), and tert-butyl peroxybenzoate (TBPB) by non-isothermal kinetic model and isothermal kinetic model by differential scanning calorimetry (DSC) and thermal activity monitor III (TAM III), respectively. The objective was to investigate the activation energy (E _a) of CHP, DTBP, and TBPB applied non-isothermal well-known kinetic equation to evaluate the thermokinetic parameters by DSC. We employed TAM III to assess the thermokinetic parameters of three liquid organic peroxides, obtained thermal runaway data, and then used the Arrhenius plot to obtain the E _a of liquid organic peroxides at various isothermal temperatures. In contrast, the results of non-isothermal kinetic algorithm and isothermal kinetic algorithm were acquired from a highly accurate procedure for receiving information on thermal decomposition characteristics and reaction hazard.     

Kinetics of crystallization of metallic glasses studied by non-isothermal and isothermal DSC

A method for describing the lengths of induction periods at linear-heating measurements, is employed for the study of induction periods in the crystallisation of metallic glasses. For Fe₇₅Si₁₅B₁₀ glass, close values of the related kinetic parameters were obtained from isothermal and nonisothermal measurements. On the basis of the results obtained, the absence of induction period in the first crystallisation step of Al₉₀Fe₇Nb₃ glass in the isothermal DSC measurement has been elucidated.     

Estimation of the curing rate of acrylamide used as a consolidant in heritage sandstone conservation

An investigation of the curing (polymerisation) rate of acrylamide was carried out using isothermal and non-isothermal DSC in order to estimate the time for complete conversion of monomer at ambient temperatures. The non-isothermal data were used to model the rate using integral isoconversional and incremental isoconversional kinetic methods. Applying the equations for integral isoconversional methods and extrapolating to ambient temperatures resulted in non-sensical conversion–time curves, where the time estimated decreased for increasing degree of conversion to be reached. This odd behaviour was attributed to the incorrectness of the integration where the kinetic parameters (e.g. the activation energy) are a function of conversion. The problem was addressed by applying incremental methods which provided more reasonable results as the integration is carried out over small conversion increments where the kinetic parameters are assumed to be constant. Estimates of the conversion were compared to isothermal measurements and, although isothermal DSC produced significant variability in the data, extrapolated estimates from non-isothermal kinetic analysis produced, at best, an upper boundary for the estimation of the time to reach a fixed degree of conversion.     

A simple and precise linear integral method for isoconversional data

A simple and precise linear integral method to evaluate the activation energy dependence on the extent of conversion has been proposed. The method leads to consistent results with those from differential and integral non-linear procedure (Vyazovkin method). Moreover, the new procedure yields the pre-exponential factor and the kinetic model. The method was evaluated from isothermal, non-isothermal and non-linear non-isothermal data (CRTA).     

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.     

Simulation of isothermal cure of A powder coating

The curing of a thermosetting powder coating was studied by means of differential scanning calorimetry (DSC). The isothermal cure was simulated by non-isothermal experiments. The results of the simulation were compared with experimental isothermal data. From non-isothermal isoconversional procedures (free model), it was concluded that these permit simulation of the isothermal cure but do not enable us to determine the complete kinetic triplet (A preexponential factor, E activation energy, f(a) and/or g(a) function of conversion). Non-isothermal procedures based on a single heating rate or on master curves present difficulties for determination of all the kinetic parameters, due to the compensation effect between preexponential factor and activation energy. The kinetic triplet can be determined by a combination of various non-isothermal methods or by using experimental isothermal data in addition to non-isothermal data. This revised version was published online in July 2006 with corrections to the Cover Date.     

Methods of non-isothermal kinetic analysis by DSC curves: An investigation on lattice reorganization in copper

Various analytical methods of non-isothermal kinetics have been applied to DSC curves to determine the kinetic parameters ruling the lattice reorganization (primary recrystallization) of prestrained pure copper.The techniques by Borchardt and Daniels, and Rogers and Smith have been modified to make them suitable for complex phenomena other than nth-order reactions.The results supplied by the new formulas are consistent with the data previously obtained under isothermal conditions. The same applies to the other methods tested, provided an accurate temperature calibration be made to correct the thermal lag of the calorimetric sensors.Such a calibration is of prime importance when DSC peak shifts are analyzed as a function of heating rate.     

Transformation of dynamic DSC results into isothermal data for the curing kinetics study of the resol resins

Kinetics of thermosetting polymers curing is difficult to study by isothermal methods based on the differential scanning calorimetry (DSC) technique. The difficulty is due to the low sensitivity of the equipment for total reaction heat measurements during high temperature process. The aim of this paper is to display the equivalence between a dynamic model, the Ozawa method, and an isothermal isoconversional fit, which allows predicting the isothermal behavior of the resol resins cure through dynamic runs by DSC. In this work, lignin–phenol–formaldehyde and commercial phenol–formaldehyde resol resins were employed. In addition, the isothermal kinetic parameters for both resins were performed by means of transformation of the data obtained from the dynamic Ozawa method.     

Kinetic analysis of thermal decomposition in liquid and solid state of 3-nitro and 4-nitro-benzaldehyde-2,4-dinitrophenylhydrazones

A kinetic study on the thermal decomposition of 3-nitro and 4-nitro-benzaldehyde-2,4-dinitrophenylhydrazones was carried out. The isothermal and dynamic differential scanning calorimetric curves were recorded for solids and melts, respectively. The standard isoconversional analysis of the obtained curves from both isothermal and nonisothermal analysis suggests an autocatalytic decomposition mechanism. This mechanism is also supported by the temperature dependence of the observed induction periods. Based on the results of the model-free method from nonisothermal regime, the kinetic model was derived and the kinetic parameters were obtained by means of a multivariate nonlinear regression.     

Kinetic Analysis of Non-isothermal DSC Data. Computer-aided test of its applicability

The applicability of the kinetic analysis of data obtained by non-isothermal differential scanning calorimetry (DSC) is discussed. The Johnson-Mehl-Avrami (JMA) model was used for the computer simulation of DSC traces subsequently analysed by common methods of kinetic analysis of non-isothermal data. For the temperature-independent kinetic exponent n of the JMA equation, the kinetic analysis was shown to provide correct results, e.g. a correct kinetic model and apparent activation energy. On the other hand, for the temperature-dependent kinetic exponent, there is a great possibility of erroneous determination of the correct kinetic model and apparent activation energy, especially at higher heating rates. Since the temperature dependence of n cannot be determined on the basis of non-isothermal DSC experiments, conclusions must be drawn with appropriate caution. This revised version was published online in August 2006 with corrections to the Cover Date.     

Crystallization kinetics of poly(glycolic acid-alt-6-aminohexanoic acid)

The crystallization behavior of a new regular poly(ester amide) constituted by glycolic acid and 6-aminohexanoic acid units under both isothermal and non-isothermal conditions is studied. Differential scanning calorimetry (DSC) is used to monitor bulk crystallization, and subsequently Avrami and Ozawa analyses are applied. A three-dimensional spherulitic growth from heterogeneous nuclei is deduced for isothermal crystallization, whereas higher exponents are obtained for non-isothermal crystallization when an Avrami equation is applied. However, modifications of the Ozawa methodology indicate a crystallization mechanism similar to that of the isothermal process.The maximum crystallization rate is deduced to take place at a temperature close to 91 °C by considering experimental data and theoretical equations with adjusted parameters. The equilibrium melting temperature is determined to be 168 °C by the characteristic Hoffman-Weeks plot. One crystallization regime is detected by using the Lauritzen-Hoffman kinetic theory for isothermal crystallization and also with an isoconversional method applied for non-isothermal crystallization. Activation energy of molecular transport and nucleation constant are close to 1500 cal/mol and 1.81 × 10⁵ K², respectively. Crystal morphology, nucleation, and spherulitic growth rates are also investigated with hot-stage optical microscopy (HSOM).     

Unified isothermal and non-isothermal modelling of neat PEEK crystallization

A differential generalized Avrami’s law is used to model crystallization kinetic of PEEK in considering that PEEK crystallization results from the contribution of two distinct mechanisms. The form of this equation allows to predict with good accuracy both isothermal and non-isothermal crystallization kinetics. Nevertheless, isothermal model parameters are not entirely satisfactory for predicting non-isothermal crystallization and the identification of kinetic parameters is needed for both isothermal and non-isothermal cases. The results show that the Avrami exponents and Arrhenius activation energies remain constant for both conditions and therefore suggest that these parameters are only material dependent. On the other hand, the other kinetic parameters depend on the crystallization condition and vary with temperature and/or cooling rate.     

Investigation on the thermal stability of nitroguanidine by TG/DSC-MS-FTIR and multivariate non-linear regression

Thermal behavior of nitroguanidine (NQ) has been investigated by TG/DSC-MS-FTIR simultaneous analysis performed under both isothermal and nonisothermal conditions. The isothermal test at 230 °C indicated that the release of gas products can be divided into several stages. The processing of the non-isothermal data, namely 5, 10, 15, and 20 K/min, was performed by using Netzsch Thermokinetics. The dependence of the activation energy evaluated by Friedman’s isoconversional method on the conversion degree shows that the investigated process is complex one, and can be divided into three parts. The mechanism of the process and the corresponding kinetic parameters were determined by Multivariate Non-linear Regression Program. The kinetic results was used to simulate the thermal decomposition of NQ under isothermal condition at 210 °C. The simulated curve is in agreement with the tested curve. The obtained results were also used for prediction of the thermal lifetime of NQ corresponding to a certain temperature.