Structural and chemical transformations in thermooxidative stabilization of copolymeric polyacrylonitrile fibres

The processes that take place during thermooxidative stabilization in copolymeric polyacrylonitrile (PAN) fibres of varying composition were examined. It was shown that the primary, or relaxation, shrinkage of PAN fibres is a function of the conditions of their fabrication and can be reduced by annealing the fibres above the glass transition temperature. The chemical transformations of PAN fibres are accompanied by structural transformations, manifested as shrinkage or elongation (flow) of the fibre. Mechanisms of deformation processes are proposed. The schemes of the chemical transformations of the structure of the fibres during their thermooxidative stabilization were examined. The direction and rate of chemical transformations of the structure of the fibres are a function of the composition of the copolymers; these parameters can be assigned and regulated by altering the conditions of spinning the fibres in the spinning bath. All-Russian Scientific-Research Institute of Polymer Fibres, Mytishchi. Translated fromKhimicheskie Volokna, No. 6, pp. 14–18, November–December, 1999.     

A comparison of coagulation and gelation on the structures and stabilization behaviors of polyacrylonitrile fibers

The fiber spinning methods determine the formation of the physical structures of polyacrylonitrile (PAN) fibers which further affect stabilization reactions and the mechanical performances of the resultant carbon fibers. In this study, PAN fibers were prepared by both dry-jet gel spinning (g-PAN) and dry-jet wet spinning (w-PAN), and their stabilization behaviors were compared. While the stabilized w-PAN fibers show sheath-core structures, the stabilized g-PAN fibers exhibit relatively uniform stabilized structures along the radial direction. Additionally, the stabilization reactions of g-PAN fibers occur faster than that of w-PAN fibers, and the cyclization, oxidation, and crosslinking reaction activation energies of g-PAN fibers are lower than that of w-PAN fibers, respectively. Moreover, the carbon yield of g-PAN is higher than that of w-PAN fibers. We believe that above changes are possibly ascribed to the formation of different PAN sheath structures and oriented chain structures during dry-jet wet spinning and dry-jet gel spinning. It is concluded that gel spinning could significantly reduce the sheath-core difference of PAN fibers and the stabilized fibers as compared with wet spinning, which leads to a faster stabilization and more uniform stabilized structures. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 137, 48671.     

Kohlenstoffäden

Since a few years the development of carbon fibres from polyacrylonitrile (PAN) is concentrated in particular on improvements of the stabilization stage, i.e. of the early stage of the thermal degradation of PAN. The chemical and physical changes of PAN in the temperature range from 150 to 350°C influencing the properties of carbon fibres are discussed. According to the complex reaction process there exists a variety of possible technical procedures. In conformity with the actual knowledge it may be advantageous to substitute the usual stabilization treatment of PAN in air by a catalytic treatment in fluid reaction mediums. Nitroaromatic compounds in the presence of SnCI₄ adducts are most suited for this purpose. In this way a remarkable shortening of the stabilization time is realized without diminishing the quality of the resulting carbon fibres.     

The Role of Different Factors in Creation of the Structure of Stabilized Acrylic Fibres

Chemical aspects of transformations of PAN fibres in the stage of thermooxidative stabilization are examined with methods of thermal analysis. The regions of cyclization and oxidation reactions are determined for three PAN copolymers of different composition. It is shown that the cyclization and oxidation reactions appearing on the DSC curves in the form of unseparated exothermic peaks can be separated by selecting the conditions of preliminary heat treatment of the fibres. It was found that preliminary heating of PAN fibres to a certain temperature causes the formation of a structure that does not undergo oxidation on repeated heating. An explanation of this phenomenon is proposed. Loading within certain limits does not affect the course of structural transformations of the fibres during thermooxidative stabilization.     

Carbon fibres from a SO2 treated PAN precursor

The paper describes the mechanism of stabilization and conversion of PAN fibres preliminarily oxidized with SO₂ to carbon fibres. Elementary analysis, IR and mass spectrographic analysis are used to determine the development of PAN structure at various stages of stabilization. The effect of the stabilization conditions in continuous processing upon the tensile strength of carbon fibres has been shown. Shrinkage measurements during the stabilization and carbonization have been taken. Thus, a relationship between the tensile strength and shrinkage produced during the carbonization has been demonstrated.     

熔融纺丝工艺制备碳纤维原丝

聚丙烯腈基碳纤维的制备主要采用溶液纺丝方法,生产过程需要溶剂回收,工艺流程长,因此制造成本高。笔者主要介绍了聚丙烯腈基碳纤维的制备工艺概况,特别介绍了熔融纺丝路线制备聚丙烯腈原丝的方法。利用共聚改性、增塑改性、纺丝后处理等方法,可以制备聚丙烯腈基碳纤维,并提出了由熔融纺丝制备聚丙烯腈基碳纤维的可行路线。     

Experimental study of the structure of polyacrylonitrile gel fibres

The structure of PAN gel fibres spun by the water-thiocyanate method is investigated. It is shown that in identical spinning conditions, the packing density of the structural elements in gel fibres based on poly(AN-co-MA-co-ItA)terpolymer is lower than in gel fibres based on poly(AN-co-MA-co-AMPS). A structural model of PAN gel fibre is composed and substantiated. It is shown that the lower packaging density of the structural elements in PAN gel fibres is responsible for the higher sorption power with respect to different inorganic and organic compounds and higher rate polymer-analog transformations.Translated from Khimicheskie Volokna, No. 5, pp. 40–45, September–October, 2004.     

Modern carbon fibres from polyacrylonitrile (PAN)-polyheteroaromatics with preferred orientation

This contribution presents the mechanical data of high performance carbon fibres currently available on the market, based on experimental studies on stabilization and pyrolysis of PAN (SAF) from Courtaulds. The effects of stabilization and carbonization temperatures on the properties as well as on the ultra- and crystalline structures of the fibres are shown in more detail.     

Mechanisms of Transformation of Acrylic Fibres During Thermooxidative Stabilization

The shrinkage of polyacrylonitrile fibres comprising 93% acrylonitrile, 5.7% methyl acrylate, and 1.3% itaconic acid was investigated by a thermomechanical method in heating them from room temperature to 300°C. It was shown that the irreversible shrinkage of the fibres during their first heating is due to a decrease in the stresses accumulated during spinning. The shrinkage observed in repeated heating of the fibres is caused by two processes: 1) by unfolding of segments of PAN macromolecules in the amorphous constituent of the structure of the polymer with an increase in the temperature (thermoelastic effect); this part of the shrinkage is reversible in heating-cooling cycles; 2) by cyclization of the polymer units due to polymerization of CN groups; this is the irreversible part of the shrinkage. In the 200-300°C temperature region, relaxation processes caused by irreversible unfolding of the macromolecules of the melted ordered structure of the fibre also contribute to the shrinkage. Mechanisms of the shrinkage processes in a wide range of temperatures are proposed. Shrinkage caused by physical processes of transformation of the structure of the fibre and chemical processes of formation of heterocycles is distinguished. It was shown that the kinetics of cyclization of PAN fibres can be investigated during their thermooxidative stabilization as one of the stages of fabrication of carbon fibres from PAN based on thermomechanical data.     

Status and Prospects for Development of Polyacrylonitrile Fibre Production. A Review

Sixty years of experience in manufacture of fibres from acrylonitrile polymers and copolymers suggest favorable prospects for the development of this kind of textile stock. The characteristics of the primary and supermolecular structure of this class of fibre-forming polymers, the ability to widely vary the methods of converting them into the viscous-flow state, and the methods of spinning, orientational drawing, modification and thermal fixation treatments form the technological base for creation of PAN fibres with defined properties. The basic trends in scientific and technical progress in development of production (volumes and assortment) of PAN fibres are: increasing the unit capacities for production of large-tonnage assortments of fibres while minimizing power and materials consumption; creating multivariant production lines to manufacture fibres for special applications and with specific performance characteristics; increasing the economy of production while minimizing environmental pressure. The important prospects for expanding the areas of application of PAN fibres are due to the reactivity of the functional groups in the polymer substrate. Industrial implementation of technologies based on polymer-analog transformations will allow creating new kinds of materials for use in industry, agriculture, and construction and for data collection, storage, and transfer.     

不同共聚单体与丙烯腈的共聚合及其表征

采用四种共聚单体衣康酸(IA)、丙烯酸甲酯(MA)、丙烯酰胺(AM)、甲基丙烯酸甲酯(MMA)分别与丙烯腈(AN)进行自由基溶液共聚合,讨论了不同共聚单体对聚合反应动力学及所得纺丝原液的粘度的影响,对聚合物的热性能进行了DSC分析,考察了不同共聚单体对聚丙烯腈原丝热性能的影响。结果表明:AN/IA体系随IA含量的增加其反应速率明显下降;与其它单体相比AM更可能有效地降低聚丙烯腈原丝的预氧化温度,缓和放热,而衣康酸次之。同时,IA含量过大,会导致原丝预氧化时的降解。     

Effect of surface-active substances on the fibre-forming process and properties of nitron fibre

Conclusions The effect of surface-active agents on the surface tension of polyacrylonitrile (PAN) spinning solutions has been studied.It has been found that introducing SAS into the precipitation bath increases fibre-formation stability. The use of SAS as modifying additives to the spinning solution and into the precipitation bath leads to an increase in the uniformity of spun fibre properties.The possibility of reducing the linear density of Nitron fibres on introducing SAS into the spinning solution and the precipitation bath has been demonstrated.Translated from Khimicheskie Volokna, No. 3, pp. 24–26, May–June, 1985.     

Structure,properties, and applications of polyacrylonitrile/carbon nanotube (CNT) fibers at low CNT loading

Bi-component, polyacrylonitrile (PAN)/carbon nanotube (CNT) fibers were processed, at different core-sheath area ratios, by gel spinning. A percolated CNT network at 10 wt% CNT in the sheath enhanced electrical conductivity as compared to the neat PAN fiber, while PAN polymer in the core contributed to the good mechanical properties. Fibers with relatively thin sheath allowed overall CNT loading as low as 3.7 wt% to be made with good electrical conductivity, and PAN stabilization by Joule heating was demonstrated. Such fibers with combined good mechanical properties and electrical conductivity can also potentially be used for electrical heating of fabrics, for making smart textiles, and for electromagnetic interference shielding.     

Spinning hollow fibres from polyacrylonitrile into degassed baths

Conclusions Changes in the properties of water as a result of degassing by heating have been examined.Degassing the water which is used as a precipitant in spinning hollow fibres of PAN permits one to exert a directed action on their properties.Translated from Khimicheskie Volokna, No. 1, pp. 15–17, January–February, 1989.     

聚丙烯腈纤维预氧化前处理研究进展

综述了前人在聚丙烯腈原丝预氧化前处理方面的研究成果,介绍了前处理的作用机理及作用效果。研究发现,聚丙烯腈原丝的改性处理可显著提高纤维的结构和性能,并影响原丝在预氧化过程中的热力学和动力学,进而影响最终碳纤维的质量。     

Swelling of pitch-based carbon fibres during activation in carbon dioxide

Isotropic pitch-based carbon fibres are activated by carbon dioxide and by steam at a temperature ranging from 850 to 1000 °C. In both cases, a significant increase in micropore volume, as measured by physical adsorption of N₂ and CO₂, is found. Fibre diameter steadily decreases during gasification by steam. In contrast, an intermediate diameter increase is observed during activation by CO₂. The interval of burn-off corresponding to fibre swelling depends on the gasification temperature. Phenolic resin-based fibres do not show swelling which indicates that it is specific to pitch-based fibres activated by CO₂. Oxygen is taken up during swelling on the incompletely stabilized part of the isotropic pitch-based carbon fibres. The origin of swelling and its effect on microporosity are discussed in relationship to fibre stabilization during activation.     

Effect of activation on boron nitride coating on carbon fiber

Boron nitride (BN) thin coating has been formed on the surface of chemically activated polyacrylonitrile (PAN) carbon fibers by dip coating method. The chemical activation of PAN fibers was carried out by two different chemicals, i.e. nitric acid (HNO₃) and silver nitrate (AgNO₃) solution. The chemical activation changes the surface properties, e.g. surface area and surface microstructure of the carbon fibers. These surface modifications ultimately influence properties of boron nitride coating on carbon fibers. The boron nitride coating on carbon fibers showed better crystallinity, strength and oxidation resistance when carbon fibers were activated by HNO₃. This improvement in strength and oxidation resistance is attributed to better crystallinity of boron nitride coating on HNO₃ activated PAN fibers.     

Fabrication of carbon fibres from wet-spun commercial polyacrylonitrile fibres

The possibility of fabricating carbon fibre from commercial PAN fibres for textile applications was demonstrated. It is necessary to make some changes in the temperature—time conditions for total completion of the thermal stabilization process. Better strength of the carbon fibres, equal to 1401 MPa, was attained in conducting thermal stabilization cycle B. __________ Translated from Khimicheskie Volokna, No. 5, pp. 31–33, September–October, 2006.     

Change in structure of polypropylene during processes of fibre spinning by the high-speed method

Conclusions Conditions for preparing polypropylene fibres from polymers of various structural modifications, orientations, and crystallinities at spinning speeds of 1000–5000 m/min have been studied experimentally.The investigated fibres are characterized by a high degree of crystallinity and orientation of the crystalline regions.Depending on fibre spinning speed, a transition is observed through definite morphological structures — from spherulitic to a typical fibrillar structure, which is formed at very high spinning speeds.Translated from Khimicheskie Volokna, No. 6, pp. 47–49, November–December, 1989.     

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