硼酸改性PAN热稳定化行为的研究

将硼酸直接加入聚丙烯腈(PAN)聚合液中,制成硼酸改性PAN膜,采用差示扫描量热(DSC)法、傅里叶变换红外光谱(FTIR)、元素分析等手段分析了硼酸对PAN热稳定化过程的影响。结果表明:硼酸改性提高了PAN的热稳定化反应活化能,并使放热峰值温度提高,当硼酸质量分数为2%时,活化能达271.27kJ/mol;硼酸含量越高,对环化反应的抑制作用越强,相对环化率越低;经过相同时间预氧化,改性PAN氧含量相对较低,硼酸改性阻碍了致密氧化层的快速形成,有利于减轻皮芯结构。     

Thermoanalytical Investigations on the Effect of Atmospheric Oxygen on HTPB resin

TG-DSC studies, carried out on hydroxy-terminated polybutadiene (HTPB) in air and nitrogen atmospheres show three transitions which give rise to (1) an exothermic DSC peak and mass gain in TG at 170°C–240°C, seen only in air (2) exothermic peak and low mass loss, both in air and nitrogen, due to depolymerization, cyclization and cross linking, and (3) final mass loss corresponding to pyrolysis in nitrogen and combustion in air, appearing respectively as an endothermic peak and as a sharp exothermic peak in the two atmospheres. The FTIR spectrum of the product isolated from TG after the mass gain step shows addition of oxygen to the butadiene back bone. Arrhenius activation parameters (E and A) were computed for the exothermic oxygen addition reaction. The Ozawa method refined by MKN two-term approximation for p(x) function gave results quite comparable to those from Kissinger method.     

水和弱酸对过氧化氢异丙苯热危险性的影响

为研究水和弱酸对过氧化氢异丙苯(CHP)热危险性的影响,利用C600微量热仪研究了CHP及CHP与不同含量水、不同含量弱酸、不同pH值弱酸的混合物在空气中受热分解的反应和放热特性,根据实验数据得出了相应的动力学和热力学参数,利用Semenov热爆炸模型计算出了自加速分解温度(SADT)。结果表明:少量的水和弱酸都能够使CHP的起始放热温度、最大放热温度、活化能降低;弱酸的pH值对CHP的热危险性也有影响,pH值越低,CHP的热危险性越大;计算得到CHP的SADT为84℃,通过对CHP及CHP与不同含量水、弱酸和不同pH值弱酸混合物SADT进行比较,发现少量的水和弱酸均能降低CHP的SADT,弱酸的pH值对SADT也有影响,pH值越低,CHP与弱酸混合物的SADT越小。     

硼酸改性对聚丙烯腈纤维预氧化的影响

通过在线监测聚丙烯腈(PAN)纤维的热收缩应变和应力,研究了硼酸改性对PAN纤维预氧化的影响。试验结果表明:随着硼酸溶液浓度的增加,PAN纤维化学反应的起始和结束温度向高温方向移动,化学收缩量降低,预氧化纤维体密度逐渐降低,硼酸对PAN纤维化学反应有抑制作用;硼酸对分子内环化和分子间交联反应的抑制可有助于预氧化纤维均质化程度的提高,并可防止PAN纤维表面在较高温度下被过度氧化。     

DSC法研究聚丙烯腈纤维的环化反应动力学

通过差示扫描量热(DSC)法研究了聚丙烯腈(PAN)纤维的热性能及其在不同升温速率下的环化反应动力学。结果表明:PAN纤维在氮气气氛中只发生环化反应,DSC曲线的放热峰尖而窄。根据Kissinger法得出其活化能为66.86 kJ/mol,反应级数为1。由DSC曲线及外推法得到升温速率为0时,其放热峰峰值为256.8℃。随着稳定化时间的增加,PAN纤维的环化度增大,纤维稳定化程度提高。     

Economic trade-offs in acrylic acid reactor design

The acrylic acid process using air oxidation of propylene presents many interesting design trade-offs, particularly in the design of the reactor. The desired and undesired reactions are highly exothermic and very temperature dependent (large activation energies), so a large flowrate of inert water is also fed to the reactor to act as a thermal sink. Propylene conversion increases with temperature and reactor size, but acrylic acid yield decreases with increasing temperature. The heat of reaction is removed by generaing steam, and the steam pressure is an important design optimization variable since it sets low limits on reactor temperature. Using low-pressure steam gives high acrylic acid yield and lower carbon dioxide generation but requires large reactors. Larger air flowrates increase reactor oxygen concentrations, which reduce reactor volume but increase air compression costs.This paper explores the effects of the many design trade-offs on capital investment, energy cost and product selectivity.     

Thermal behavior of acrylonitrile carboxylic acid copolymers

Polyacrylonitrile (PAN) and copolymer of acrylonitrile–vinyl acids prepared by solution polymerization technique have been characterized by Differential Scanning Calorimetry (DSC) (under dynamic as well as isothermal conditions), themograviemetric analysis (TGA), and on‐line DSC‐FTIR spectroscopy. The DSC of copolymers was carried out at 5°C/min in nitrogen and air. In nitrogen atmosphere the DSC exotherm show a very sharp peak, whereas, in air atmosphere DSC exotherm is broad, and starts at a much lower temperature compared to what is observed in nitrogen atmosphere. The initiation temperature of PAN homopolymer is higher than that for the copolymers. For instance, the initiation temperature of PAN in air is 244°C, whereas, the onset of exothermic reaction is in the range of 172 to 218°C for acrylonitrile–vinyl acid copolymers. As the vinyl acid content increases the ΔH value reduces. The ΔH value of PAN in air was 7025 J/g, whereas, for P(AN‐AA) with 5.51 mol % of acid it was 3798 J/g. As the content of acrylic acid comonomer is increased to 17.51 mol % the value of ΔH decreases further to 1636 J/g. The same trend was observed with MAA and IA as well. DSC‐FTIR studies depict various chemical changes taking place during heat treatment of these copolymers. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 685–698, 2003     

Thermogravimetric and differential thermal analysis of cellulose,hemicellulose, and lignin

The thermal degradation of samples of cellulose, hemicellulose, and lignin have been investigated using the techniques of thermogravimetric analysis (TGA) and differential thermal analysis (DTA) between room temperature and 600°C. The results calculated from static and dynamic TGA indicated that the activation energy E for thermal degradation for different cellulosic, hemicellulose, and lignin samples is in the range 36–60, 15–26, and 13–19 kcal/mole, respectively. DTA of all the wood components studied showed an endothermic tendency around 100°C in an atmosphere of flowing nitrogen and stationary air. However, in the presence of flowing oxygen this endothermic effect was absent. In the active pyrolysis temperature range in flowing nitrogen and stationary air atmospheres, thermal degradation of Avicel cellulose occurred via a sharp endothermic and a sharp exothermic process, the endothermic nadir and exothermic peak being at 320° and 360°C, respectively. In the presence of oxygen, combustion of Avicel cellulose occurred via two sharp exothermic processes. DTA studies of different cellulose samples in the presence of air showed that the shape of the curve depends on the sources from which the samples were prepared as well as on the presence of noncellulosic impurities. Potassium xylan recorded a sharp exothermic peak at 290°C in a nitrogen atmosphere, and in a stationary air atmosphere it yielded an additional peak at 410°C, while in the presence of oxygen the curve showed two sharp exothermic peaks. DTA traces of periodate lignin in flowing nitrogen and air were the same and showed two exothermic peaks at 320° and 410°C, while in the presence of oxygen there were two exothermic peaks in the temperature range 200°–500°C.     

Effect of the nature and mole fraction of acidic comonomer on the stabilization of polyacrylonitrile

Differential scanning calorimetry (DSC) and thermogravimetric analysis measurements were used to study the influence of the acidic comonomers acrylic acid, methacrylic acid, and itaconic acid on the exothermic reactions occurring during the heat treatment of acrylonitrile copolymers under air atmosphere. The presence of these reactions was noticed in the DSC exotherms of the copolymers, which appear as doublets. These doublets were resolved into their constituent peaks, arising due to the occurrence of oxidative and cyclization reactions, and the area under the resolved peaks was considered as the extent of the reaction. The comonomers under study were found to enhance the secondary oxidation reactions and retard the extent of cyclization reactions. The degree of cyclization decreases abruptly with the increase in the comonomer content beyond ⋍ 2 mol %. © 1996 John Wiley & Sons, Inc.     

The influence of oxygen on the chemical reactions during stabilization of pan as carbon fiber precursor

DTA measurements were used for studies of the kinetics of the cyclization and oxidation of PAN during the thermal treatment in air and nitrogen. The cyclization is a first order reaction with an activation energy of about 30 kcal/mole in nitrogen and about 34 kcal/mole in air. For copolymer PAN (5% methylacrylate) lower activation energies were found. Oxygen promotes the initiation of the cyclization but does not catalyze the cyclization reaction itself. The sequence of cyclization and oxidation reactions is discussed in detail.     

Effect of electron beam irradiation on polyacrylonitrile precursor fibers and stabilization process

The electron beam was imposed on the polyacrylonitrile precursor fibers before the fibers were stabilized. The effect of electron beam irradiation on the chemical structure, transverse section, and surface morphology and thermal properties of the fibers in the process of stabilization was characterized by the use of Fourier transform infrared spectroscopy (FTIR), scanning electron microscope (SEM), and differential scanning calorimeter, respectively. A parameter η = I(C?N)/[I(C?N) + I(C?N)] was defined to evaluate the extent of cyclization in the stabilization process. The kinetic parameters, viz. activation energy (E) and pre‐exponential factor (A) of the stabilization reactions, were calculated by Kissinger method. FTIR analysis indicated that the cyclization of nitrile groups was initiated at room temperature by electron beam irradiation. The transformation of C?N groups to C?N ones was accelerated in the process of stabilization. The extent of cyclization of the stabilized fibers was increased. SEM analysis indicated that irradiation could also decrease the internal and surface defects of the stabilized fibers treated at 300°C. The activation energy of cyclization reaction was reduced from 302 to 280 kJ/mol and 260 kJ/mol through 100 and 200 kGy electron beam irradiation, respectively. The reaction temperature range was expanded, and the exothermic rate was slowed down in the process of stabilization, which was the reason why the stabilized fibers have improved cyclization degree and less internal defects. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Thermal properties of acrylonitrile/itaconic acid polymers in oxidative and nonoxidative atmospheres

Thermal stabilization of polyacrylonitrile (PAN) fibers is an indispensable process in the manufacture of carbon fibers. The effects of acidic comonomers on the thermal properties of PAN have attracted much attention because of their importance in the fibers spinning and heat treatment process. In this study, oxidative and nonoxidative atmospheres are adopted in differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) test to disclose the key effects of oxygen on the thermal properties of PAN/itaconic acid (IA) polymers. The DSC results under oxidative atmosphere are consistence to the reports by previous researchers: the exothermic curves of copolymers containing 0.6 wt % and 1.0 wt % IA exhibit lower initiation temperature and more broaden shapes than that of PAN homopolymer, indicating that IA facilitates both cyclization and oxidation reactions. However, copolymers containing the same content of IA shows no apparent improving effect on the thermal properties under inert atmosphere, which has not been mentioned in the published literature. TGA indicates that oxygen remarkably increases the thermal stability of AN/IA copolymers structure, and will bring high carbon yield in the eventual carbon fibers. The influential mechanisms of oxidative and nonoxidative atmospheres on thermal stabilization reactions of PAN were discussed. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Texture and Properties of Acrylonitrile–Ammonium Itaconate Copolymer Precursor Fibers and Carbon Fibers

Acrylonitrile–ammonium itaconate copolymer {P[AN-co-(NH₄)₂IA]} was fabricated by free-radical solution copolymerization of acrylonitrile (AN) and ammonium itaconate [(NH₄)₂IA]. The copolymer was confected into a spinning solution (dope) and spun into precursor fibers. Then the precursor fibers were converted to carbon fibers by stabilization and carbonization processes. Another copolymer of acrylonitrile–itaconic acid [P(AN-co-IA)] and its resultant fibers were studied in contrast. Due to preferable hydrophilicity and facilitating cyclization reaction in stabilization process, P[AN-co-(NH₄)₂IA] fibers could increase the draw-ratio in the spinning process and in the stabilization process to attenuate some property-limiting facts in precursor fibers and carbon fibers. It has been found P[AN-co-(NH₄)₂IA] precursor fibers gave a 30% improvement in tenacity over P(AN-co-IA) precursor fibers. Results for P[AN-co-(NH₄)₂IA] precursor fibers carbonized at 1400°C ultimately showed a 26.7% increase in carbon fiber tensile strength over P(AN-co-IA) based carbon fibers. Characterization analysis using scanning electron microscope (SEM), transmission electron microscope (TEM) and high resolution transmission electron microscope (HRTEM) manifested that these improvements are due to fewer surface defects, better interior morphology, higher degree of orientation and graphitization.     

Effects of an itaconic acid comonomer on the structural evolution and thermal behaviors of polyacrylonitrile used for polyacrylonitrile‐based carbon fibers

The influence of an acid comonomer on the structural evolution and thermal behaviors of a polyacrylonitrile (PAN) terpolymer containing about 4.19 wt % methyl methacrylate and 0.98 wt % itaconic acid (IA) during a heating treatment were studied by Fourier transform infrared (FTIR) spectroscopy, differential scanning calorimetry (DSC), and thermogravimetry (TG) and compared with the PAN homopolymer. From the results of FTIR spectroscopy and DSC, we found that the presence of the IA comonomer promoted cyclization reactions at a faster rate and broadened the exothermic peak; this eased the concentration of heat release. By comparing the results of the TG data, we found that the presence of the IA comonomer promoted dehydrogenation reactions and the formation of a large quantity of better ladder structures. In additional, the better ladder structures that formed were easier to convert from ladder structures into graphitelike structures. The activation energy of random scissions at about 300°C was calculated by the Kissinger method. The results show that the presence of the IA comonomer inhibited the random scission reactions to some extent. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40834.     

AIBN对P(AN-co-IA)共聚物性能影响的研究

主要研究了丙烯腈(AN)与衣康酸(LA)发生共聚反应时,不同偶氮二异丁腈(AIBN)引发剂用量对聚合产物的分子质量、分子立构及热性能的影响。研究结果表明:丙烯腈聚合产物的分子链构型符合无规聚合的概率论分布规律,但聚合物分子中全同立构体的比例随AIBN加入量的增大而增加;通过研究聚合物在氮气氛围中的热降解,发现共聚物在受热分解时热量的变化随引发剂用量的增加而趋于平缓。     

热解气氛对油漆稀料和塑料共热解特性的影响

为研究不同热解气氛对油漆稀料与聚对苯二甲酸乙二醇酯（PET）共热解特性的变化规律，采用同步热分析仪，根据预实验结果，测定在升温速率为10 ℃/min时，不同热解气氛（空气和氮气）条件下油漆稀料和PET塑料共热解过程中失重（TG）、焓变（DSC）的变化，并采用Coats?Redfern法对样品进行热动力学分析。结果表明，样品在氮气气氛下的热解过程存在明显的滞后现象，质量损失率为82.2 %；在空气气氛中更有利于油漆稀料和PET塑料的共热解反应中间产物的检出；PET塑料及其燃烧残留物在空气中的平均活化能均小于氮气中的值，但混合物在空气中400~475 ℃温度间活化能更低；500 ℃后混合物达到二次燃烧条件。     

Effect of boric acid and heat treatment for the formation of poly(vinyl alcohol)/iodine complex films iodinated at solution before casting

This study examined the role of boric acid and the effect of heat treatment on PVA‐iodine polarizing films prepared in the solution state before casting (IBC) of PVA/iodine/boric acid films. The films were prepared by casting aqueous solutions of 10 wt % poly(vinyl alcohol) (PVA) containing boric acid with 0, 0.1, 0.3, and 0.5 mol/l of I₂/KI aqueous solution, and I₂/KI(1 : 2) with 5 wt % of PVA. The effect of boric acid and heat treatment on the durability of the IBC PVA polarizing sheet films was investigated by UV–vis absorption spectroscopy. Boric acid was found to be essential for the complex formation in PVA/iodine solutions at relatively low I₂/KI concentrations and high temperatures. The strength of the complex peak at ∼ 600 nm in UV–vis absorption spectra increased with increasing boric acid concentration. With increasing heating temperature over 90°C the intensity of the peak at 600 nm corresponding to the complex decreased due to the evaporation of I₂ decomposed from I₅⁻, but the peak at 355 nm corresponding to free I₂·I₃⁻ was remained unchanged. From heat treatment at 150°C, the intensity of the peak at 600 nm decreased but the intensity of the complex peak (600 nm) of the sample with 0.5 mol/l boric acid was unaffected. The transmittance and degree of polarization for the films increased and decreased with increasing heat treatment time under heat and a humid atmosphere, respectively. However, this tendency decreased with increasing boric acid concentration and heat treatment. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Effect of fatty acid positional distribution and triacylglycerol composition on lipid by-products formation during heat treatment: III—Cyclic fatty acid monomers study

The influence of isomeric triacylglycerols (TG) containing 18:3n-3 and 18:2n-6 on the formation of cyclic fatty acid monomers (CFAM) after heat treatment was assessed. Diacid TG, containing linoleic acid (L) or linolenic acid (α-Ln) along with palmitic acid (P), and positioned either in the central position (PLP and PLnP, respectively), or in one of the two outer positions (PPL and PPLn, respectively) were synthesized. Monoacid TG of trilinolein and trilinolenin mixed with tripalmitin were also prepared. The CFAM formed after heating were analyzed after total hydrogenation. The results obtained with the model TG were compared to another model consisting of a canola oil and its randomized counterpart. In diacid TG, the location of α-Ln in the central position of the TG molecule (PLnP) greatly enhanced the formation of the CFAM upon heating at temperatures below 240°C. On the other hand, 18:3n-3 in monoacid TG (LnLnLn) was highly resistant to CFAM formation within the same range of temperatures (180–220°C). The TG structure, more than the TG composition, seemed to explain the differences in the CFAM formation between the original canola oil and its interesterified counterpart. Like α-Ln, 18:2n-6 was more prone to cyclization when attached at the central position of the model TG. Conversely, the influence of the TG composition on the cyclization rate was less important for L than for α-Ln. It was concluded that positioning the C18 polyunsaturated fatty acid in the central position of TG rendered the oils much more sensitive to the cyclization reaction upon heat treatment.     

Effect of Heating Rates on the Synthesis of Al2O3–SiC Composites by the Self-Propagating High-Temperature Synthesis (SHS) Technique

Various aspects of in situ formation of Al₂O₃–SiC composites by the self-propagating high-temperature synthesis (SHS) technique have been investigated using thermal analyses (TG/DTA) of a powder mixture (4Al, 3SiO₂, 3C) and pellets in an argon atmosphere at different heating rates. Both the reaction initiation and peak temperatures are found to increase with the heating rates. At lower heating rates, the powder samples do not reveal any exothermic peak possibly because of poor reactivity and sluggish exothermic reaction. The appearance of exothermic peaks in the DTA plots after melting of aluminum indicates reduction of silica by liquid aluminum. Conversion of aluminum is found to decrease marginally with an increase in heating rates. The apparent activation energy of the process compares well with the interdiffusion activation energy of silicon and oxygen, indicating that oxygen diffusion in Si formed at the reaction front may be the rate-controlling factor for this SHS process. From SEM studies it appears that the formation of SiC whiskers is through liquid-phase mass transfer.     

Estimation of butadiene in vulcanized BR and SBR by thermographic analysis

A method for the determination of butadiene in vulcanized polybutadiene (BR) and styrene–butadiene rubber (SBR) has been proposed. The method is based on determining the area under the exothermic peak (around 380°C) of the differential scanning calorimetry thermograph in nitrogen. The exotherm area was found to be linearly related to the butadiene content of the polymers and was unaffected by vulcanization or by loading with carbon black. This approach supplements and extends the existing methods of BR determination, which depend upon the estimation of styrene content. The existing evidence indicates that the exothermic reaction is due to cyclization of BR. The energy of activation calculated from the Arrhenius plot is 32–35 kcal/mole; it does not change with vulcanization, loading, or nature of the polymer.