Thermal degradation of Kevlar fiber by high‐resolution thermogravimetry

A novel high‐resolution thermogravimetry (TG) technique in a variable heating rate mode that maximizes resolution and minimizes the time required for TG experiments has been performed for evaluating the thermal degradation and its kinetics of Kevlar fiber in the temperature range ∼ 25–900°C. The degradation of Kevlar in nitrogen or air occurs in one step. The decomposition rate and char yield at 900°C are higher in air than in nitrogen, but the degradation temperature is higher in nitrogen than in air. The initial degradation temperature and maximal degradation rate for Kevlar are 520°C and 8.2%/min in air and 530°C and 3.5%/min in nitrogen. The different techniques for calculating the kinetic parameters are compared. The respective activation energy, order, and natural logarithm of preexponential factor of the degradation of Kevlar are achieved at average values of 133 kJ/mol (or 154 kJ/mol), 0.7 (or 1.1), and 16 min⁻¹ (or 20 min⁻¹) in air (or nitrogen). The technique based on the principle that the maximum weight loss rate is observed at the minimum heating rate gives thermal degradation results that were in excellent agreement with values determined by traditional TG experiments. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 565–571, 1999     

Cold‐crystallization kinetics of syndiotactic polystyrene

The kinetics of the isothermal and nonisothermal cold crystallization of syndiotactic polystyrene (s‐PS) were characterized with differential scanning calorimetry. A Johnson–Mehl–Avrami analysis of the isothermal experiments indicated that the cold crystallization of s‐PS at a constant temperature followed a diffusion‐controlled growth mode with a decreasing nucleation rate. Furthermore, the slow nucleation rate was the controlling step of the entire kinetic process. For nonisothermal cold‐crystallization kinetics, we used a simple model based on a combination of the well‐known Avrami and Ozawa models. The analysis revealed that, unlike for melt crystallization, the Avrami and Ozawa exponents were not equal. The activation energies for the isothermal and nonisothermal cold crystallizations of s‐PS were 792.0 and 148.62 kJ mol^?1, respectively, indicating that the smaller motion units in cold crystallization had a weaker temperature dependence than those in melt crystallization. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 3464–3470, 2003     

High‐resolution thermogravimetry of poly(4‐methyl‐1‐pentene)

Thermal degradation and kinetics of poly(4‐methyl‐1‐pentene) were investigated by nonisothermal high‐resolution thermogravimetry at a variable heating rate. Thermal degradation temperatures are higher, but the maximum degradation rates are lower in nitrogen than in air. The degradation process in nitrogen is quite different from that in air. The average activation energy and frequency factor of the first stage of thermal degradation for the poly(4‐methyl‐1‐pentene) are 2.4 and 2.8 times greater in air than those in nitrogen, respectively. Poly(4‐methyl‐1‐pentene) exhibits almost the same decomposition order of 2.0 and char yield of 14.3–14.5 wt % above 500°C in nitrogen and air. The isothermal lifetime was estimated based on the kinetic parameters of nonisothermal degradation and compared with the isothermal lifetime observed experimentally. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 2201–2207, 1999     

High‐Temperature Isothermal Oxidation of Ultra‐High Temperature Ceramics Using Thermal Gravimetric Analysis

Oxidation of ZrB₂ + SiC composites is investigated using isothermal measurements to study the effects of temperature, time, and gas flow on oxidation behavior and microstructural evolution. A test method called dynamic nonequilibrium thermal gravimetric analysis (DNE‐TGA), which eliminates oxidation during the heating ramp, has been developed to monitor mass change from the onset of an isothermal hold period (15 min) as a function temperature (1000°C–1600°C) and gas flow (50 and 200 mL/min). In comparing isothermal to nonisothermal TGA measurements, the scale thicknesses from isothermal tests are up to 4 times greater, indicating that oxidation kinetics are faster for isothermal testing, where the oxide scale thickness is 110 μm after 15 min at 1600°C in air. Isothermal oxidation followed parabolic kinetics with a mass gain that is temperature dependent from 1000°C–1600°C. The mass gain increased from ~5 to 45 g/m² and parabolic rate constants increased from 0.037 to 2.2 g²/m⁴·s over this temperature range. The effect of flow velocity on oxidation is not significant under the given laminar flow environment where the gas boundary layer is calculated to be 4 mm. These values are consistent with diffusion of oxygen through the glass‐ceramic surface layer as rate limiting.     

Nonisothermal curing kinetics of a novel polymer containing phenylsilylene and propargyl–hexafluorobisphenol a units

A novel thermosetting polymer, poly[(phenylsilylene) propargyl–hexafluorobisphenol A] (PBAFS), with a new structure was synthesized. The structure of PBAFS and its cured resins were characterized by Fourier transform infrared spectra. During curing, a hydrosilylation reaction may occur between Si? H and C?C bonds and a Claisen rearrangement reaction of aryl propargyl ether led to formation of chromene, which immediately preceded polymerization on heating. The dynamic viscosity behavior was investigated by rheological experiment. Thermal stability of the cured PBAFS was also measured by Thermogravimetric analysis. The curing behavior of PBAFS was monitored by nonisothermal differential scanning calorimetry at different heating rates. The kinetic parameters and the kinetic model of the cure reaction were evaluated by Kissinger, Ozawa, and Friedman methods. The cure reaction of PBAFS was found nth‐order in nature and the prediction curves by Friedman method for nonisothermal curing reaction were in good agreement with the experimental curves. The isothermal curing time of PBAFS were predicted by Vyazovkin and model‐fitting methods from the nonisothermal kinetic parameters. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Crystallization kinetics for simulation of processing of various polyesters

The approach to determine crystallization kinetic parameters based on the DSC nonisothermal crystallization experiments is applied to poly(butylene terephthalate) (PBT) and poly(ethylene‐2,6‐naphthalate) (PEN). The differential form of the Nakamura equation and master curve approach are used. The isothermal induction times are obtained from nonisothermal induction times according to the concept of induction time index. The correction of temperature lag between the DSC furnace and the sample is incorporated. The corrected nonisothermal crystallization kinetic data is shifted with respect to an arbitrarily chosen reference temperature to obtain the master curve. By fitting the obtained master curve with the Hoffman‐Lauritzen equation, the model parameters for the crystallization rate constant are obtained. The relative crystallinity measured at different cooling and heating rates is described by these model parameters. © 2006 Wiley Periodicals, Inc. J Appl PolymSci 102: 2847–2855, 2006     

Nonisothermal crystallization behavior of poly(aryl ether ether ketone)

A study has been made of the crystallization behavior of poly(aryl ether ether ketone), PEEK, under nonisothermal conditions. A differential scanning calorimeter (DSC) was used to monitor the energetics of the crystallization process from the melt. For nonisothermal studies, the melt was crystallized by cooling at rates from 1°C/min to 10°C/min. A kinetic analysis based on the recently proposed model for nonisothermal crystallization kinetics to remedy the drawback of the Ozawa equation was applied. The Avrami exponent for the nonisothermal crystallization process was strikingly different from that of the isothermal process, which indicates different crystallization behaviors. The results agree with the morphological observation reported in the literature. This study shows that correct interpretation of the Avrami exponent provides valuable information about the crystal structure and its morphology.     

Thermal and thermooxidative degradation of engineering thermoplastics and life estimation

This investigation deals with the thermal or thermooxidative degradation behavior of three engineering polymers [e.g., poly(ethylene terephthalate) (PET), poly(ether sulfone) (PES), and poly(ether ether ketone) (PEEK)] by using thermogravimetry‐coupled mass spectrometry (TG‐MS) analysis. The experiments were conducted both in argon and in air separately to study the changes in the degradation pattern of the polymers under varied sample environments. The samples were subjected to a programmed heating rate of 10°C/min and a temperature range from ambient to 800°C. For all these polymers, the decomposition rate, percentage weight loss, and the nature of the evolved gases were found to vary while changing the environment from argon to air. Methods of nonisothermal kinetic analysis, proposed by Flynn and Wall, and the shelf life estimation, proposed by Toop, have been described. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 1737–1748, 2004     

Structure and high‐resolution thermogravimetry of liquid‐crystalline copoly(p‐oxybenzoate‐ethylene terephthalate‐p‐benzamide)

Thermotropic liquid‐crystalline copoly(ester‐amide)s consisting of three units of p‐oxybenzoate (B), ethylene terephthalate (E) and p‐benzamide (A) were studied by proton nuclear magnetic resonance at 200 and 400 MHz, wide‐angle X‐ray diffraction, and high‐resolution thermogravimetry to ascertain their molecular and supermolecular structures, thermostability and kinetics parameters of thermal decomposition in both nitrogen and air. The assignments of all resonance peaks of [¹H]NMR spectra for the copoly(ester‐amide)s are given and the characteristics of X‐ray equatorial and meridional scans are discussed. Overall activation energy data of the first major decomposition have been evaluated through three calculating techniques. The thermal degradation occurs in three steps in nitrogen and air. The degradation temperatures are higher than 447 °C in nitrogen and 440 °C in air and increase with increasing B‐unit content at a fixed A‐unit content of 5 mol%. The temperatures at the first maximum weight‐loss rate are higher than 455 °C in nitrogen and 445 °C in air and also increase with an increase in B‐unit content. The first maximum weight‐loss rates range between 11.1 and 14.5%min⁻¹ in nitrogen and between 11.9 and 13.5%min⁻¹ in air. The char yields at 500 °C in both nitrogen and air range from 45.8 to 54.3 wt% and increase with increasing B‐unit content. But the char yields at 800 °C in nitrogen and air are quite irregular with the variation of copolymer composition and testing atmosphere. The activation energy and Ln (pre‐exponential factor) for the first major decomposition are usually higher in nitrogen than in air and increase slightly with an increase in B‐unit content at a given A‐unit content of 5 mol%. The activation energy, decomposition order, and Ln (pre‐exponential factor) of the thermal degradation for the copoly(ester‐amide)s in two testing atmospheres, are situated in the ranges of 210–292 kJmol⁻¹, 2.0–2.8, 33–46 min⁻¹, respectively. The three kinetic parameters of the thermal degradation for the aromatic copoly(ester‐amide)s obtained by high‐resolution thermogravimetry at a variable heating rate are almost the same as those by traditional thermogravimetry at constant heating rate, suggesting good applicability of kinetic methods developed for constant heating rate to the variable heating‐rate method. These results indicate that the copoly(ester‐amide)s exhibit high thermostability. The isothermal decomposition kinetics of the copoly(ester‐amide)s at 450 and 420 °C are also discussed and compared with the results obtained based on non‐isothermal high‐resolution thermogravimetry. © 1999 Society of Chemical Industry     

Kinetics of oil generation from Colorado oil shale

Isothermal and nonisothermal methods have been used to investigate the kinetics of oil generation during decomposition of 91.7 ml/kg (22 U.S. gal/short ton) Colorado oil shale. The result from the nonisothermal method gives an apparent activation energy of 219.4 kJ/mol and a frequency factor of 2.81 × 10¹³s^?1. Furthermore, the process is found to be first-order to within experimental error. These results compare favourably with isothermal data reported here and in the literature. The results show the reliability and convenience of nonisothermal kinetic experiments in studying oil-shale decomposition reactions. The principal advantages are short-term experiments and the lack of initial heat-up periods. Moreover, nonisothermal experiments more accurately simulate actual conditions of above-ground and in situ oil-shale retorting. These kinetics are ‘effective’ values and can only properly be used to describe the macroscopic oil-production process rather than the complex microchemistry.     

Addition of a reactive diluent to a catalyzed epoxy-anhydride system. I. Influence on the cure kinetics

The effect of a reactive diluent (RD) on the kinetics of the curing of an epoxy resin, based on diglycidyl ether of bisphenol A (DGEBA), with a carboxylic anhvdride derived from methyl-tetrahydrophthalic anhydride (MTHPA) catalyzed by a tertiary amine has been studied. The reactive diluent was a low-viscosity aliphatic diglycidyl ether, and the compositions per 100 parts by weight (pbw) of DGEBA were 10, 30, and 50 pbw of RD with the stoichiometric quantity of MTHPA and 1 pbw of catalyst. The curing kinetics was monitored by differential scanning calorimetry (DSC), and the kinetic parameters were determined from the nonisothermal DSC curves by the method described by Málek. The kinetic analysis suggests that the two-parameter autocatalytic model is the more appropriate to describe the kinetics of the curing reaction of this epoxy-anhydride system. The kinetic parameters thus derived satisfactorily simulate both the nonisothermal DSC curves and the isothermal conversion-time plots. Increasing the RD content leads to a small increase in both the nonisothermal and the isothermal heats of curing and has a slight effect on the kinetic parameters E, ln A, m, and n, and, consequently, on the overall reactivity of the system. On the other hand, the increase of the RD content significantly affects the structure of the crosslinked epoxy. It is confirmed that the introduction of aliphatic chains in the structure of the epoxy increases the mobility of the segmental chains in the glass transition region. The consequence of this chemical modification is a decrease of the glass transition temperature, T_g. © 1996 John Wiley & Sons, Inc.     

Crystallization behavior of poly(ether ether ketone ketone)

The melting behavior of semicrystalline poly(ether ether ketone ketone) (PEEKK) has been studied by differential scanning calorimetry (DSC). When PEEKK is annealed from the amorphous state, it usually shows two melting peaks. The upper melting peaks arise first, and the lower melting peaks are developed later. The upper melting peaks shown in the DSC thermogram are the combination (addition) of three parts: initial crystal formed before scanning; reorganization; and melting-recrystallization of lower melting peaks in the DSC scanning period. In the study of isothermal crystallization kinetics, the Avrami equation was used to analyze the primary process of the isothermal crystallization; the Avrami constant, n, is about 2 for PEEKK from the melt and 1.5 for PEEKK from the glass state. According to the Lauritzen-Hoffman equation, the kinetic parameter of PEEKK from the melt is 851.5 K; the crystallization kinetic parameter of PEEKK is higher than that of PEEK, and suggests the crystallizability of PEEKK is less than that of PEEK. The study of crystallization on PEEKK under nonisothermal conditions is also reported for cooling rates from 2.5°C/min to 40°C/min, and the nonisothermal condition was studied by Mandelkern analysis. The results show the nonisothermal crystallization is different from the isothermal crystallization. © 1996 John Wiley & Sons, Inc.     

Quiescent polymer crystallization: Modelling and measurements

The problem of predicting nonisothermal crystallization kinetics based on isothermal data is considered, with reference to the difficulties involved, both experimental and theoretical. The kinetic model used is the differential form of the Nakamura equation which is an extension of the Avrami equation so as to apply to nonisothermal crystallization. Nonisothermal induction times are obtained from isothermal induction times according to the concept of induction time index. The theory of Hoffman Lauritzen is used to extrapolate the limited isothermal crystallization rate data. Good agreement between DSC (differential scanning calorimetry) nonisothermal crystallinity results and model predictions is obtained for our own data on poly(ethylene terephthalate) (PET) and some literature data on nylon-6, if the temperature lag between the sample and the DSC furnace is taken into account. The advantages of the present approach in process modeling are pointed out. Quenching experiments have also been performed in which PET slabs are allowed to cool and crystallize from the melt under quiescent conditions. The resulting crystallinity distributions in the thickness direction are measured and predicted by using kinetic parameter values obtained from isothermal DSC measurements alone.     

Isothermal and nonisothermal crystallization kinetics of poly(ϵ‐caprolactone)

With increasing environmental awareness, evaluating the potential of biopolymers as a substitute for traditional materials has been of great interest. Crystallization kinetics provides fundamental knowledge required for evaluation, playing vital role in determining the final properties of the product. In this study, the isothermal and nonisothermal crystallization kinetics of poly(?‐caprolactone) (PCL) were investigated with the help of various models. The Avrami model best described the isothermal crystallization kinetics, suggesting three‐dimensional spherulitic growth, which was in agreement with the morphology studies; whereas the Liu model fit well under nonisothermal crystallization conditions. The failure of the Kissinger model to determine the activation energy was overcome with the Friedman model. The kinetic crystallizability determined by the Ziabacki model indicated a higher crystallization ability of PCL at lower cooling rates. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Captive sample reactor for kinetic studies of coal pyrolysis and hydropyrolysis on short time scales

An apparatus has been designed for kinetic studies of coal devolatilization under closely controlled heating rates, temperatures, and pressures. The reactor system employs a wire-mesh pyrolysis furnace driven by an electronically-controlled power supply to ensure uniform heating rates and precise crossover to a variable length isothermal period; a rapid quenching scheme permits resolution of kinetic processes in 0.1 s intervals, extending the sensitivity of kinetic measurements from the initial stage of pyrolysis through to completion of primary devolatilization. Time-resolved kinetic studies on a bituminous coal in vacuo (13 Pa) and under helium (0.22 MPa) illustrate the operation of the reactor.     

Kinetics of isothermal and nonisothermal crystallization of nylon 1313

The crystallization process of a new polyamide, nylon 1313, from the melt has been thoroughly investigated under isothermal and nonisothermal conditions. During isothermal crystallization, relative crystallinity develops in accordance with the Avrami equation with the exponent n ≈ 2 based on DSC analysis. Under nonisothermal conditions, several different analysis methods were used to elucidate the crystallization process. The Avrami exponent n is greater in the isothermal crystallization process, indicating that the mode of nucleation and the growth of the nonisothermal crystallization for nylon 1313 are more complicated, and that the nucleation mode might include both homogeneous and heterogeneous nucleation simultaneously. The calculated activation energy is 214.25 kJ/mol for isothermal crystallization by Arrhenius form and 135.1 kJ/mol for nonisothermal crystallization by Kissinger method, respectively. In addition, the crystallization ability of nylon 1313 was assessed by using the kinetic crystallizability parameters G. Based on this parameter, the crystallizability of many different polymers was compared theoretically. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 1415–1422, 2007     

Effect of nano-nucleating agent addition on the isothermal and nonisothermal crystallization kinetics of isotactic polypropylene

Using differential scanning calorimetry (DSC) technique, a comparative study has been made of the isothermal and nonisothermal crystallization kinetics of nonnucleated isotactic polypropylene (iPP) and of nucleated iPP with 0.5 wt% of single-walled carbon nanotubes (SWCNTs) as a nucleating agent. The Avrami exponents (n) of iPP and nucleated iPP are close to 3.0 for isothermal crystallization. These results indicate that the addition of nucleating agents did not change the crystallization growth patterns of the neat polymer and that crystal growth was heterogeneous three-dimensional spherulitic. The results show that the addition of SWCNTs can shorten the crystallization half-time (t _1/2) and increase the crystallization rate of iPP. In the nonisothermal crystallization process, the Ozawa model failed to describe the crystallization behavior of nucleated iPP. The Cazé–Chuah model successfully described the nonisothermal crystallization process of iPP and its nanocomposite. A kinetic treatment based on the Ziabicki theory is presented to describe the kinetic crystallizability, in order to characterize the nonisothermal crystallization kinetics of iPP and nucleated iPP. Polarized light microscopy (PLM) experiments reveal that SWCNTs served as nucleating sites, resulting in a decrease of the spherulite size.     

Calorimetric and rheokinetic analyses merged to capture crystallization kinetics in polyamide/clay nanocomposites: Revisiting predictability of models

Crystallization kinetics of polymer/clay systems was the subject of numerous investigations, but still there are some ambiguities in understanding thermal behavior of such systems under isothermal and nonisothermal circumstances. In this work, isothermal rheokinetic and nonisothermal calorimetric analyses are combined to demonstrate crystallization kinetics of polyamide6/nanoclay (PA6/NC) nanocomposites. As the main outcome of this work, we detected different regimes of crystallization and compared them by both isothermal dynamic rheometry and nonisothermal differential scanning calorimetry (DSC), which has not been simultaneously addressed yet. A novel analysis, somehow different from the common ones, is used to convert the storage modulus data to crystallinity values leading to more reasonable Avrami parameters in isothermal crystallization. It was found based on isothermal rheokinetic studies that increase of NC content and shear rate are responsible for erratic behavior of Avrami exponent and crystallization rates. Optimistically, however, isothermal crystallization by rheometer was confirmed by DSC. Nonisothermal calorimetric evaluations suggested an accelerated crystallization of PA6 upon increasing NC content and cooling rate. The crystallization behavior was quantified applying Ozawa (r² between 0.070 and 0.975), and combinatorial Avrami–Ozawa (r² between 0.984 and 0.998) models, where the latter appeared more appropriate for demonstration of nonisothermal crystallization of PA6/NC nanocomposites. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46364.     

A novel imidazole derivative curing agent for epoxy resin: Synthesis,characterization, and cure kinetic

A novel imidazole derivative (named as EMI‐g‐BGE) was synthesized through the reaction of 2‐ethyl‐4‐methyl imidazole (EMI) and butyl glycidyl ether (BGE) and characterized by elemental analysis, FTIR spectroscopy, and ¹H NMR spectroscopy. The curing kinetic of diglycidyl ether of bisphenol A (DGEBA) epoxy resin with EMI‐g‐BGE as curing agent was studied by nonisothermal DSC technique at different heating rates. Dynamic DSC scans indicated that EMI‐g‐BGE was an effective curing agent of epoxy resin. The apparent activation energy E_a was 71.8 kJ mol^?1 calculated through Kissinger method, and the kinetic parameters were determined by Málek method for the kinetic analysis of the thermal treatment obtained by DSC measurement. A two‐parameter (m, n) autocatalytic model (?esták‐Berggren equation) was found to be the most adequate selected kinetic model. In addition, the predicted curves from the kinetic model fit well with the nonisothermal DSC thermogram. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Thermo‐oxidative decomposition kinetics of elastomeric composites based on styrene‐(ethylene‐butylene)‐styrene triblock copolymer and organomontmorillonite

The elastomeric nanocomposites based on organomontmorillonite (OMMT) and styrene‐(ethylene‐butylene)‐styrene (SEBS) thermoplastic elastomer were prepared by melt processing using maleic anhydride grafted SEBS (SEBS‐g‐MA) as compatibilizer. Thermo‐oxidative decomposition behavior of the neat components and the nanocomposites were investigated using thermogravimertic analysis (TGA) in air atmosphere. The isoconversional method is employed to study the kinetics of thermo‐oxidative degradation. The heating modes and the composition of nanocomposites were found to affect the kinetic parameters (E_a, lnA and n). The E_a and lnA values of SEBS, OMMT, and their composites are much higher under dynamic heating than under isothermal heating. The reaction order (n) of OMMT was lower than those of SEBS and their composites. The obtained TG profiles and calculated kinetic parameters indicated that the incorporation of OMMT into SEBS significantly improved the thermal stability both under dynamic heating and under isothermal heating. The simultaneously obtained DSC data showed that the enthalpy of thermal decomposition decreased with OMMT loading. No significant change in the nonisothermal and isothermal stability of the nanocomposites with addition of SEBS‐g‐MA. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011