A procedure for precise determination of thermal stabilization reactions in carbon fiber precursors

Thermal stabilization of polyacrylonitrile (PAN) fibers is an important step in production of carbon fibers. Understanding the onset and temperature range of the stabilization reactions is a key for adjusting processing parameters such as tension, stretching, etc. However, stabilization reactions are very complex and overlap. In order to separate the stabilization reactions, we combined the results of FTIR (Fourier Transform Infrared Spectroscopy), DSC (Differential Scanning Calorimetry), TGA (Thermogravimetry analysis), TMA (Thermomechanical analysis), and densitometry. It was shown that combination of DSC and TGA allowed separation of reactions regardless of the composition of initial PAN fibers. FTIR, TMA and densitometry results verified the validity of proposed method. Accordingly, three special and commercial grade PAN fibers with different chemical composition were studied. FTIR results indicated that during thermal stabilization of PAN fibers chemical reactions including cyclization, oxidation and dehydrogenation occurred in the fibers and a ladder polymer was formed. According to DSC and TGA curves, initiation temperature, temperature range and order of occurrence of these reactions were a function of chemical composition of initial fibers. In fibers containing itaconic acid plus methyl acrylate comonomers, oxidation reactions already started at 175 °C. Cyclization started above 210 °C, and reactions occurring above 250 °C were mainly dehydrogenation. In fibers containing only itaconic acid cyclization initiated above 210 °C, dehydrogenation started after 242 °C and oxidation occurred only after 284 °C. In fibers containing vinyl acetate comonomers, the initial reactions above 240 °C were attributed to cyclization. Oxidation occurred below 290 °C and dehydrogenation started above 290 °C.     

Mechanism and kinetics of the stabilization reactions of itaconic acid-modified polyacrylonitrile

The structural evolution and thermal behavior of polyacrylonitrile (PAN) homopolymer and copolymer [P(AN-IA)] containing about 1.5 mol% itaconic acid (IA) during stabilization in air were studied by Fourier transform infrared (FTIR) spectroscopy, differential scanning calorimetry (DSC) and thermogravimetry (TG). A new parameter Es=A1595cm⁻¹/A2243cm⁻¹ was defined to evaluate the extent of stabilization. The kinetic parameters, viz. activation energy (E_a) and pre-exponential factor (A) of the stabilization reactions were calculated by Kissinger method and Ozawa method. FTIR analysis indicated that the cyclization of nitrile groups was initiated at a lower temperature by the IA comonomer and the stabilization proceeded at a more moderate pace in P(AN-IA) than in PAN, while an IA additive was found to be decomposed and failed to initiate the cyclization at a lower temperature. The improvement effect of IA comonomer on the stabilization reactions was further confirmed by the dynamic thermal analysis and kinetic study.     

Equivalency of Kinetic Schemes: Causes and an Analysis of Some Model Fitting Algorithms

The same experimental data can often be equally well described by multiple mathematically equivalent kinetic schemes. In the present work, we investigate several model‐fitting algorithms and their ability to distinguish between mechanisms and derive the correct kinetic parameters for several different reaction classes involving consecutive reactions. We have conducted numerical experiments using synthetic experimental data for six classes of consecutive reactions involving different combinations of first‐ and second‐order processes. The synthetic data mimic time‐dependent absorption data as would be obtained from spectroscopic investigations of chemical kinetic processes. The connections between mathematically equivalent solutions are investigated, and analytical expressions describing these connections are derived. Ten optimization algorithms based on nonlinear least squares methods are compared in terms of their computational cost and frequency of convergence to global solutions. Performance is discussed, and a preferred method is recommended. A response surface visualization technique of projecting five‐dimensional data onto the three‐dimensional search space of the minimal function values is developed.     

Effects of the stabilization temperature on the structure and properties of polyacrylonitrile-based stabilized electrospun nanofiber microyarns

Polyacrylonitrile (PAN) precursor nanofiber microyarns were successfully synthesized using a modified electrospinning method, and then were stabilized at different stabilization temperatures. Effects of stabilization temperature on the structure, thermal, and mechanical properties of stabilized microyarns were studied by FTIR, XRD, DSC, TG, and mechanical behavior test. It was found that when the stabilization temperature was above 250 °C, cyclization and dehydrogenation reactions were basically completed, and PAN crystal microstructure was almost completely destroyed, as well as the thermally stable ladder structure was formed. The results also showed that the strength of the stabilized microyarns increased to a threshold value and started to reduce thereafter as the stabilization temperature increased.     

Decoration of electrospun polyacrylonitrile nanofibers with ZnO nanoparticles and their application for removal of Pb ions from waste water

The adsorption behavior of lead (II) from aqueous solutions utilizing ZnO/polyacrylonitrile (PAN) nanofibers was studied. ZnO/PAN nanofibers were prepared by electrospinning method. The changes of the parameters of adsorbent amount, pH, contact time, and temperature were tested in the adsorption experiments. The adsorption was well described by the Langmuir adsorption isotherm model. The thermodynamic parameters indicate that the adsorption process is exothermic. The dynamic behavior of the lead (II) ions adsorption by PAN/ZnO nanofibers was well described by the pseudo-second-order kinetic model. The adsorbent can be regenerated by suitable desorption processes for multiple uses without significant loss of its adsorption capacity.     

FT-IR study of the stabilization reaction of polyacrylonitrile in the production of carbon fibers

The stabilized fibers prepared by heating PAN and a PAN copolymer in air and under reduced pressure have been examined by FT-IR spectroscopy in order to determine their chemical structures. Three kinds of reactions, cyclization, dehydrogenation, and oxygen uptake are found to occur almost simultaneously in the stabilization process in air by digital difference spectrum method. The effect of the comonomer is confirmed to accelate the dehydrogenation reaction and also the formation of acridone ring in the thermal stabilization in air by kinetic study.     

非等温法研究TGDDM/DDS体系固化反应动力学

采用DSC对4,4′-四缩水甘油基二氨基二苯基甲烷(TGDDM)和3,3′-二氨基二苯基砜(DDS)体系的固化反应动力学进行了研究.分别通过n级反应法和Malek的最大概然机理函数法确定了固化反应机理函数,求解了固化反应动力学参数,得到了固化反应动力学模型.结果表明,通过Kissinger,Crane方法求解动力学参数所得到的n级反应模型与实验值差别较大;而采用Malek方法判别机理,表明该固化反应按照自催化反应机理进行,实验得到的DSC曲线与模型计算所得到的曲线吻合的较好,所确立的模型在5~20K/min的升温速率下能较好地描述TGDDM/DDS体系的固化反应过程,并为工艺参数的选择和工艺窗口的优化提供了理论依据.     

Thermal behavior of polyacrylonitrile polymers synthesized under different conditions and comonomer compositions

Polyacrylonitrile (PAN) polymers are used as precursors for carbon fiber production. This process requires an oxidative stabilization step, which can be studied by differential scanning calorimetry (DSC). In this sense, thermal behavior of PAN based terpolymers by different polymerization processes, compositions and itaconic acid concentrations in the reaction media were investigated. The obtained results showed that the addition of itaconic acid and methyl acrylate as comonomers resulted a lower heat flow during the process comparing to the PAN homopolymer. It suggested that these comonomers aid the oxidative stabilization stage for all studied process. The redox system polymerization at 40°C resulted in a lower heat flow. Itaconic acid decreases slightly initial and peak temperatures of the terpolymer and heat flow until concentration of 3y. The cyclization temperature decreases when MAis incorporated into the terpolymer compared to the MMA terpolymer and increases when MAA is the acidic monomer. Among terpolymers the AN/MA/AA polymer showed the best thermal behavior for carbon fiber producing.     

Application of global kinetic models to HMX beta-delta transition and cookoff processes

The reduction of the number of reactions in kinetic models for both the HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine) beta-delta phase transition and thermal cookoff provides an attractive alternative to traditional multi-stage kinetic models due to reduced calibration effort requirements. In this study, we use the LLNL code ALE3D to provide calibrated kinetic parameters for a two-reaction bidirectional beta-delta HMX phase transition model based on Sandia instrumented thermal ignition (SITI) and scaled thermal explosion (STEX) temperature history curves, and a Prout-Tompkins cookoff model based on one-dimensional time to explosion (ODTX) data. Results show that the two-reaction bidirectional beta-delta transition model presented here agrees as well with STEX and SITI temperature history curves as a reversible four-reaction Arrhenius model yet requires an order of magnitude less computational effort. In addition, a single-reaction Prout-Tompkins model calibrated to ODTX data provides better agreement with ODTX data than a traditional multistep Arrhenius model and can contain up to 90% fewer chemistry-limited time steps for low-temperature ODTX simulations. Manual calibration methods for the Prout-Tompkins kinetics provide much better agreement with ODTX experimental data than parameters derived from differential scanning calorimetry (DSC) measurements at atmospheric pressure. The predicted surface temperature at explosion for STEX cookoff simulations is a weak function of the cookoff model used, and a reduction of up to 15% of chemistry-limited time steps can be achieved by neglecting the beta-delta transition for this type of simulation. Finally, the inclusion of the beta-delta transition model in the overall kinetics model can affect the predicted time to explosion by 1% for the traditional multistep Arrhenius approach, and up to 11% using a Prout-Tompkins cookoff model.     

Study of the clay effect on crude oil combustion by thermogravimetry and differential scanning calorimetry

Thermogravimetry (TG) and differential scanning calorimetry (DSC) were used to study the effect of sand, silica and kaolinite on crude oil combustion. Three distinct regions, namely distillation and two combustion/cracking regions were observed on all TG curves. Thermogravimetric curves were analyzed using an Arrhenius-type kinetic model and a ratio method to obtain kinetic parameters. Activation energy and reaction order were obtained from this analysis. The reaction order seemed to be insensitive to the presence of granular materials. However, a significant reduction of activation energy was caused by addition of kaolinite to the crude oil, indicating that the kaolinite had a catalytic and surface area effect on crude oil combustion/cracking reactions.     

A novel methodology for designing thermal processes in order to optimize stabilization of polyacrylonitrile (PAN) fibers

The aim of this work is to describe a novel methodology for optimizing the stabilization of polyacrylonitrile (PAN) fibers, through designing of proper thermal treatment. The methodology is based on a set of design rules and the procedure for implementing them, utilizing the time‐temperature‐transition (TTT) and the maximum permittable stress (max.stress) plots. The proposed approach is implemented in order to optimize the stabilization of commercial PAN fibers, resulting in a series of multistage thermal treatments. The changes of both physical and chemical structures of PAN during the progress of the multistage treatments were investigated and showed that the fibers were progressively converted into completely stabilized material; this gradual transformation permitted improvement of fiber annealing and minimized the effect of the decomposition reactions. The proposed methodology can be universally applied for achieving the global optimum of the stabilization process for any PAN precursor.     

取向聚对苯二甲酸乙二酯纤维的非等温结晶动力学

取向聚对苯二甲酸乙二酯纤维的非等温结晶动力学张志英，赵家森（天津纺织工学院材料科学系天津３００１６０）关键词取向高聚物，聚对苯二甲酸乙二酯，非等温结晶，结晶动力学研究高聚物结晶动力学常用的等温方法有光透射法、密度法、显微镜法、Ｘ－射线衍射法、差示扫描...     

Square‐Wave Voltammetry of Cathodic Stripping Reactions. Diagnostic Criteria,Redox Kinetic Measurements,and Analytical Applications

A comparative study of different types of cathodic stripping reactions under conditions of square‐wave voltammetry is presented. Cathodic stripping processes involving reactions of second order as well as reactions coupled by adsorption of the reacting ligand are analyzed The inherent parameters, controlling the overall voltammetric behavior of each cathodic stripping electrode reaction are derived. The criteria for qualitative distinguishing of each mechanism are established as well as a methodology for redox kinetic measurements is proposed. The influence of the parameters of the excitation signal on the properties of the voltammetric response is analyzed in order to find optimal conditions for analytical application. The theoretical results are illustrated by the experiments with a series of uracil derivatives.     

Polypropylene stabilization by hindered phenols - Kinetic aspects

This article deals with polypropylene stabilization by hindered 2,6-di-tert-butylphenols. Two aspects are mainly considered: the influence of stabilizer structure, of its concentration and temperature on induction period duration through a literature compilation completed by results obtained on PP samples stabilized by Irganox 1010 in conditions in which physical loss was negligible. Results show that the induction period duration is almost proportional to the phenol concentration and that the proportionality ratio is almost independent of the stabilizer structure in the investigated phenol family. A unique set of kinetic parameters can be therefore used to model the kinetic behaviour of all the family members. The kinetic approach can be more or less complex depending on the number of secondary processes taken into account. The results of simulations indicate that a two steps process allows generation of a kinetic behaviour in good agreement with experimental trends regarding effects of both stabilizer concentration and temperature on induction time.     

Thermokinetic analysis of two-step curing reactions in melt: Part I. Investigation of low molecular model systems

Kinetic investigations of thermo-controlled two-step reactions between an uretdione cross-linker and OH-groups are undertaken. Non-isothermal DSC measurements are chosen for the determination of the kinetic parameters (E, A, n) using of THERMOKINETICS software. The system can be described very accurately with a four-step kinetic model of consecutive reactions. On the basis of the kinetic model and parameters the realization of the first reaction step, which is the formation of the allophanate network is simulated. Finally, the result of the calculated progress of the reaction is evaluated by FT-IR and isothermal DSC measurements.     

Application of direct nonlinear regression from single TG curve to compounds undergoing simple and complex thermal decompositions

Nonlinear regression of the basic kinetic equation was used to determine kinetic parameters of simple and complex decomposition processes related to some pesticides and pharmaceutical substances. The method for this calculation was carried out using a single TG curve. These values were compared with those obtained with model‐fitting and model‐free methods. Nonlinear regression method is able to define a single mechanism for compounds undergoing one or few steps of decomposition processes. For compounds that undergo many steps of decomposition processes, only the mechanisms of some of them can be hypothesized. © 2003 Wiley Periodicals, Inc. Int J Chem Kinet 35: 611–622, 2003     

Thermal studies on beryllium titanyl/zirconyl oxalates

Beryllium titanyl oxalate tetrahydrate and beryllium zirconyl oxalate tetrahydrate were prepared in aqueous medium and characterized by elemental analyses, magnetic susceptibility measurements and IR spectral studies. The thermal behaviour of these compounds under non-isothermal conditions was investigated by thermogravimetric, derivative thermogravimetric and differential scanning calorimetric (DSC) techniques. The intermediates obtained at the end of the various thermal decomposition steps were identified on the basis of elemental analyses and IR spectral studies. The decomposition proceeds through three major steps, viz, dehydration of the hydrate, decomposition of the oxalate to carbonate and decomposition of the carbonate to oxide. The graphical method of Coats and Redfern was employed to calculate kinetic parameters such as apparent activation energy and order of reaction. Heats of reaction for the different decomposition steps were calculated from the DSC curves.     

Studies on Kinetics of Chlorination Reaction of Polyethylene

Through kinetic method, the reaction mechanism and the rate equations of chlorination of polyethylene are suggested in this paper. The rate of chlorination is second order with respect to the concentration of methylene remained and chlorine, respectively. Apparent changes in crystallinity during chlorination processes were determinated by differential scanning calorimetry(DSC). The mathematical model relating crystallinety with kinetic parameters and function of sequence length distribution are derived.     

ON A DIFFERENTIAL EQUATION FOR KINETICS OF NON- ISOTHERMAL CRYSTALLIZATION

A new differential equation was derived from the modified first-order kinetic model to describe the polymer crystallization processes. The crystallization experiments were carried out by means of DSC. Poly (ethylene terephthalate) resins were selected as the samples containing different catalysts. The relationships between the parameters obtained from the known Avrami equation and from one in the present paper were discussed. A method for applying the equation to determine the kinetic parameters from a constant heating and a constant cooling curve was proposed.