Shrinkage Mechanisms in Thermooxidative Stabilization of Acrylic Fibres

Shrinkage processes in two types of acrylic fibres in heating in air in different regimes in the 50-250°C temperature region were investigated by a thermomechanical method. The mechanisms of the shrinkage were revealed in heating—cooling cycles and its partial reversibility and relation with the cyclization reaction was demonstrated. The irreversible part of shrinkage of the fibers is due to cyclization and relaxation of the macromolecules, while reversible shrinkage is due to the entropic elasticity of the polymer macromolecules.     

Effect of γ-radiation on the structure and thermal properties of polyacrylonitrile fibres

-Radiation significantly affects both the initial structure and the thermal properties of PAN fibres. The following are probably the most important results of radiation exposure: the temperature of the beginning of cyclization decreases; the exothermic nature of cyclization decreases so that the weight losses in the region of the m.p. decrease, indicating destructive processes in the polymer chain; when irradiated samples are heated, intermolecular cross-links form with the participation of oxygen; the duration of oxidation of the fibres before a given density level is attained is reduced significantly.St. Petersburg University of Design and Technology. Translated from Khimicheskie Volokna, No. 3, pp. 18–21, May–June, 1995.     

Correlation of chemical shrinkage of polyacrylonitrile fibres with kinetics of cyclization

It has been established that the most important step in the production of carbon fibres from polyacrylonitrile (PAN) precursor fibres is the oxidative thermal treatment applied. During this treatment, physical phenomena and chemical reactions take place accompanied by the shrinkage of the fibres, which has a physical or chemical origin, depending on the nitrile cyclization reactions. The aim of the present study is to establish a correlation between the chemical shrinkage of PAN fibres and the kinetics of cyclization reactions. Based on the isothermal treatment of PAN fibres, we developed a method in order to distinguish between physical and chemical shrinkages. The onset time for the chemical shrinkage follows a relationship with temperature. Chemical shrinkage versus cyclization time data were fitted with the exponential rise to the maximum of the curves. Furthermore, the cyclization kinetics was studied using differential scanning calorimetry and the kinetic parameters determined were identical to those calculated from the chemical shrinkage, demonstrating that the latter is directly related to the kinetics of the cyclization reactions. It was therefore concluded that according to the method developed to distinguish the physical from the chemical shrinkages: (1) there exists a certain onset time for a given treatment temperature to trigger the chemical shrinkage; (2) cyclization reactions do not start below a limiting temperature of 168 °C; (3) at 340 °C, the temperature where the cyclization reactions are completed, the maximum shrinkage is 24%; and (4) the oxidized PAN fibres contain mainly ladder polymer structures with approximately symmetrical sequences connected in angled positions. Copyright © 2007 Society of Chemical Industry     

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.     

Shrinkage of highly oriented gel-spun polymeric fibres

Summary The melting and shrinking process of gel-spun hot-drawn UHMWPE fibres has been investigated. Instead of being superheated, the highly oriented fibres start to melt at the fibre ends and shear bands, at a temperature near the equilibrium melting temperature of polyethylene. For draw ratios of 40, the fibre transforms into a ribbon shape upon melting. This change in shape could be prevented by constrained heating before shrinkage. Ribbon-like melting was also observed for gel-spun hot-drawn poly(L-lactide) fibres.     

Electron Paramagnetic Resonance in Investigation of Polymer Fibres. A Review

The results obtained with EPR in studying polymer fibres suggest the following conclusions: EPR offers broad opportunities for studying the causes of the decrease in the strength of polymer fibres on exposure to temperature and light, and stabilization of the strength properties in irradiation (sterilization) of medical fibre materials allows tracing the causes of the decrease in or preservation of the therapeutic properties of native and immobilized enzymes; EPR with stable nitroxyl radicals (spin-probe method) allows investigating the kinetics of formation of new phases in polymer—solvent systems and the effect of the conditions of fabricating the fibres on the quantitative ratio between dense and loose packing of macromolecules in the amorphous structure of polymers; EPR allows identifying complexation of some metal ions with different functional groups in polymers.     

Temperature dependence of processes during oxidation of PAN fibres

The 3000-PAN-filament tows have been stretched by a constant axial load during heating in air at a constant rate of . The extension of the tow begins about the glass-transition temperature (70°C). Temperature dependence of the fibre diameter, width of X-ray reflection of the PAN, and endothermic effect occurring at low temperature (below 240°C), suggest that at 130°C begins a molecular rearrangement. IR spectra of the oxidized fibres and gas evolution suggest that near this temperature (at about 145°C) begins cyclization, but the cyclized fraction of the fibres remains very low up to over 220°C, unless a prolonged oxidation at this temperature is carried out. TGA and DTA results suggest that at about 240°C begins rapid cyclization, which is at about 320°C followed by partial degradation of the fibres. In these processes the fibres lose about 26% of their original weight. Final degradation of the fibres begins at about 400°C.     

Improving Carbon Fibre Production Technology

The quality of carbon fibres, their strength and modulus of elasticity in particular, can be increased by the following methods: decreasing the porosity of the initial PAN fibres by selecting the optimum conditions for spinning, plasticizing drawing, finishing, and drying; decreasing the inhomogeneity of the diameters of PAN fibres by suppressing deformation resonance during spinning by selecting conditions of jet formation and precipitation that do not allow formation of bulbs in the jets or prevent their deformation; decreasing fibril and crystallite size by reducing the precipitant and solvent concentration gradient in the precipitation zone (spinning in mild baths); creating optimum conditions for interphase self-ordering of the polymer at 450-550°C during precarbonization; increasing the cohesive energy by increasing the density to 1.75-2.10 g/cm³. Substitution of convective tempering of PAN twists in thermooxidative treatment by conductive (contact) treatment decreases the duration of the treatment by 3-4 times and improves the environmental conditions. Use of reflecting screens in high-temperature furnaces reduces heat losses by 4-5 times and decreases consumption of cooling water. The progress made in the technology and creation of high-capacity equipment suggests that the cost of high-strength carbon fibre will decrease to $10-15/kg in the future in the condition of large-scale production.     

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.     

Monitoring and control of fabrication of fibres from a polymer melt based on the fiber temperature (review)

The literature on use of the fibre temperature parameter for monitoring and control of fabrication of fibres from a polymer melt, in particular, cooling of the spun fibre and thermoplasticization drawing, was reviewed. A thin jet of air passed through the bundle of spun fibres has effective characteristics which can be used to monitor and control the fibre cooling conditions, to increase the intensity of cooling of the fibre, and to improve the fibre quality indexes. The efficiency of controlling fibre drawing with the Amur system increases if the temperature of the fibre at the outlet of the thermoplasticizer is measured instead of the thermoplasticizer temperature. A direct regulator using a bimetallic plate can be used instead of the Amur system; this allows increasing the stability of the fibre temperature during drawing by 3–5 times. The characteristic sections of deviation of the fibre temperature during spinning and the increase in its temperature during cold drawing bear information on the process and fibre parameters, but require additional studies to determine the important parameters and their correlations. The informativeness of the fibre temperature parameter increases if it is examined in association with the other properties of the fibre and the operating parameters of the process. A realization algorithm is possible in terms of the sign of the deviation of the properties of the fibres and the parameters of the process. Most technical solutions can be implemented with standard thermistors and thermocouples. The proposed information can be used by specialists in chemical fibres for automatic monitoring and control, optimization of the process, and improvement of the equipment.Barnaul'sk OKB Avtomatiki Corp. Translated from Khimicheskie Volokna, No. 2, pp. 50–54, March–April, 1995.     

Physicochemical principles in dyeing freshly spun polyacrylonitrile fibres and films with mixtures of cationic dyes

Conclusions Two process stages may be discerned in the technology of dyeing freshly spun PAN fibres with cationic dyes and mixtures of them, depending on the temperature. In the first stage, at 20–50°C, the dye rapidly diffuses into the freshly spun gel and is sorbed in its submicroscopic volume. The second stage — bonding of the dye with formation of heteropolar bonds — takes place during the process of drying and heat-setting the fibre. The nonequilibrium sorption of dyes in the first stage is characterized by the instantaneous values of the affinity of the freshly formed polymer for the dyeing solution, _ins ⁰ .It is possible to determine the compatibility of cationic dyes in mixtures in dyeing freshly spun PAN gels from the _ins ⁰ values. For compatible components, these figures should not exceed 0.8–1.4 kJ/mole.Freshly spun PAN fibres containing various acid groups up to 1.5% by wt., and prepared by various technologies, have practically an identical affinity for cationic dyes.Translated from Khimicheskie Volokna, No. 4, pp. 39–41, July–August, 1986.     

Mechanism of polyacrylonitrile fibre spinning by the thiocyanate method

Conclusions 1. The process of spinning PAN fibres by the thiocyanate method into baths with various precipitating powers has been studied. The mildness and severity of the baths have been varied by varying the NaSCN concentration from 12 to 24%, and also varying the temperature from 5 to 20°C.2. A dependence of the threshold precipitation concentration on polymer content of the spinning solution has been shown. At a polymer content of 13.5%, the threshold precipitant (water) concentration at which precipitation begins is 63%.3. At NaSCN concentrations above 22–24% in the precipitation bath, a precipitant concentration is established on the surface of the forming fibre at the moment of contact between the spinning solution and the precipitation bath which is below the threshold value, and precipitation begins after a certain time, during the course of which, as a result of diffusion, the precipitant concentration is raised to the threshold level.4. On softening the spinning conditions by increasing the NaSCN concentration in the precipitation bath up to a certain limit, a fibre is formed with a more perfect fibril structure, which ensures obtaining a stronger carbon fibre.5. Extreme softening of the spinning conditions leads to an increase in fibre porosity. The suggestion is advanced that this phenomenon is connected with a change in the mechanism of spinning solution phase breakdown where, along with polymer precipitation in the form of a solid phase, partial precipitation in the form of a liquid phase takes place.Translated from Khimicheskie Volokna, No. 5, pp. 3–7, September–October, 1993.     

Dilatometric and thermomechanical studies of aromatic polyamide fibres

The deformation of SVM aromatic polyamide fibres in the temperature range from room temperature to 400°C was investigated by thermomechanical analysis and dilatometry. In the first heating, fibres which had not been thermally drawn were characterized by self-elongation along the axis of orientation and transverse shrinkage, while thermally drawn fibres were characterized by shrinkage in two directions at temperatures below 200°C and self-elongation at higher temperatures. Self-elongation and transverse expansion were observed on repeated heating for both types of fibres in the investigated temperature region.Translated from Khimicheskie Volokna, No. 1, pp. 38–40, January–February, 1995.     

Preparation of polyester fibre from polyethylene terephthalate modified with caprolactam

Conclusions A process for preparation of modified Lavsan fibre using caprolactam as the modifier has been proposed and has been checked out on the pilot plant of the Mogilevsk Khimvolokno Industrial Association.The introduction of pentamethylene units into the polyethylene terephthalate macromolecules increases the ability of the fibre to sorb disperse dyes. The modified fibre can be dyed at 100°C without carriers, to a tone of medium intensity.In technological processing of the modified polyester fibre, a decrease in thread defectiveness and breakage in the processes of spinning and weaving is observed. In physico-mechanical indices, the thread and finished semi-wool fabric conform to first grade material.The authors express their thanks to coworkers of the Minsk worsted combine for processing the experimental lot of modified polyester.Translated from Khimicheskie Volokna, No. 3, pp. 37–38, May–June, 1989.     

Carbonization of polyacrylonitrile fibre modified with polyphosphate

Thermal transformations of PAN fibres modified with polyphosphate (PP) were investigated. It was found that PP affects cyclization and persists in the structure of the carbon fibre. The decrease in ordering of the structure of the carbon fibre with an increase in the concentration of PP in the fibre decreases the strength and modulus of elasticity with a simultaneous increase in the electric conductivity of the modified fibre. Translated from Khimicheskie Volokna, No. 4, pp. 40–41, July–August, 1998.     

Properties of textile materials based on polifen fibres

Conclusions In processing man-made fibres in pure form or in mixtures with natural fibres, it is recommended to reduce the intensity of carding by increasing the separation between working parts of the carding machines and reducing their speed.To eliminate a large amount of fly in carding, one should carry out the technological preparation of the Polifen and wool fibre using heating and picking machines.The materials have a low air permeability and high efficiency when used as filtering elements.Translated from Khimicheskie Volokna, No. 4, pp. 41–42, July–August, 1986.     

A study of pyrolysis of phenolic resin reinforced with carbon fibres and oxidized pan fibres

Pyrolysis of composites prepared from a phenol-formaldehyde resin and carbon fibres (CFRC) or oxidized PAN fibres (OFRC) has been investigated up to 1270 K. Two temperature regions can be distinguished in the pyrolysis of the CFRC: below 770 K the carbon fibres hold the composite structure resisting the tendency of the resin to produce composite shrinkage; above 770 K the CFRC volume and porosity abruptly increase due to a weakening of fibre/resin adhesion. During the pyrolysis of the OFRC a chemical reaction at the fibre/resin interface apparently takes place, beginning at 570 K and producing important changes in the material at 870–970 K. Somewhat smaller weight losses and markedly lesser shrinkage of the pyrolysed composite with respect to its separately treated components have been found. The fibres and the resin coalesce into a product having relatively high density and low porosity, the latter mainly consisting of narrow pores. Hydrogen content of the product is lower than that of the separately treated constituents. The reflections of both the resin and the oxidized PAN fibres are seen only in the X-ray patterns of the OFRC treated at low temperature, but above 670 K only the reflection due to the fibres appears. The interlayer spacing of the fibres in the composite is greater than in the fibres alone treated under the same conditions. According to the IR spectra, the CN groups and aromatic CH groups disappear from the composite at lower temperature than from its components treated separately.The existence of the reaction between the resin and the oxidized PAN fibres, leading to the destruction of the fibre materials, means that the co-carbonization product at 1270 K is not a composite and will not have the necessary mechanical properties. Therefore, this study must continue to look for fibres and matrices which are compatible during co-carbonization to 1270 K.     

Thermomechanical and chemical properties of cross-linked polyacrylonitrile fibres

Conclusions 1. We have studied the thermomechanical and chemical properties of PAN fibres, cross-linked by treating with hydrazine hydrate, semicarbazide, and thiosemicarbazide.2. Cross-linked PAN fibres have increased heat resistances and thermal stabilities.3. Cross-linked PAN fibres do not dissolve in any known organic solvents and are resistant to hot mineral acids and alkalies.Third communication on: Deformation and thermomechanical properties of cross-linked polyacrylonitrile fibres.All-Union Synthetic Fibre Research Institute (VNIIV). Translated from Khimicheskie Volokna, No. 2, pp. 49–50, March–April, 1969.     

Fabrication of oriented polyester fibre during high-speed spinning

The effect of different high-speed spinning parameters using a heating zone on the properties of the fibre was demonstrated. An anomaly in the properties of the fibres was detected for spinning with a speed greater than 5000 m/min.All-Russian Scientific-Research Institute of Synthetic Fibres, Tver'. Translated from Khimicheskie Volokna, No. 6, pp. 37–38, November–December, 1993.     

Carbon Fibres with Specific Physical and Physicochemical Properties Based on Hydrated Cellulose and Polyacrylonitrile Precursors. A Review

A wide assortment of carbon fibres (CF) made from readily available hydrated cellulose (HC) and PAN staple fibres and twists, textile and other fibre materials, including heat and chemical-resistant composites, has been developed. The use of these materials is based on their unique thermal, physical, physicochemical, and biological properties. The precursor fibres have their own characteristics and the properties of the CF and CFM made from them are correspondingly different. The economic aspects of their fabrication differ slightly, and CF made from PAN perhaps now have some advantages. However, in the near future, the price ratio will vary in support of cellulose fibres since products made from crude oil, natural gas, and coal are gradually becoming more expensive. The economics of small-tonnage fibres with original specific properties should be taken into consideration based on the efficiency of their use. The uniqueness of these fibres gives those which are irreplaceable and most effective the right to life. In this respect, all of the CF and CFM examined are important and promising. Far from all of the possible applications of these fibres have been discovered and implemented, which makes them even more promising for the future.