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1. The quality of CF, their strength and modulus of elasticity in particular, can be increased by the following methods: reducing the porosity of the initial PAN fibres by selecting the optimum conditions for spinning, plasticization drawing, finishing, and drying; decreasing the nonuniformity of the fibre diameter due to suppression of deformation resonance during spinning by selecting the jet formation and hardening conditions; decreasing the fibril and crystallite size by reducing the precipitator and solvent concentration gradient in the precipitation zone (spinning into mild baths); creating optimum conditions for mesophase self-ordering of the material at 450-550°C during precarbonization; increasing the cohesive energy by increasing the density to 1.8-2.1 g/cm 3 . 2. Replacing convective tempering of PAN twists in thermooxidative treatment by conductive tempering reduces the treatment time by 3-4 times.Carbon fibres (CF) made from polyacrylonitrile (PAN) copolymers are the most widely used CF. This type of fibre has a set of properties (high strength and modulus of elasticity, dimensional stability, resistance to corrosion, low density) that predetermined their use in high-tech industries such as rocket-space, aviation and atomic, ship building, and production of high-quality sporting goods. However, the production volumes of these fibres is still comparatively small, 12-15,000 tons a year, which is due to imperfect technology and equipment that do not ensure the required level of quality indexes, production economy, and respect of environmental requirements. Some scientific premises and technical solutions on improving PAN CF manufacturing technology are examined below.As for the quality indexes, together with the CF manufacturing technology, many investigators [1] believe that the properties of the initial PAN fibre, particularly its defectiveness, degree of orientation, and microfibril structure are of determining importance here.Of the large number of defects characteristic of wet-spun PAN fibres, we distinguish the two that most strongly affect the quality of CF: porosity and nonuniformity of the filament diameter. The negative effect of porosity on the quality of CF is manifested in two ways. First, since the features of the structure of the initial PAN fibre are preserved in the structure of the CF, the porosity is also preserved, causing nonuniformity of internal stresses and brittleness in the CF. The second negative mechanism of the effect of porosity is the decrease in the thermal stability of PAN fibre, i.e., the lower value of the maximum attainable limiting temperature of thermal decomposition of the polymer. Pores serve as nuclei or centers of the onset of thermolysis of PAN fibre and do not allow attaining the temperature of 500-550°C in rapid heating required for mesophase rearrangement of the structure of the oxidized fibre during carbonization without intensive decomposition [2].The appearance of pores in PAN fibre is predetermined by the nature of wet spinning, where the volume of solve...

Polyaerylonitrile microfibres with a linear density of O. 02-0.10 tex can be manufactured by wet spinning into spinning baths with a low concentration of precipitant. When the jets of spinning solution come into contact n4th such a spinning bath, a concentration of precipitant lower than the threshold concentration where coagulation does not take place is established on the sulface of the fibre for a short time (0. 02-0.56 sec). The spun fbres have a liquid segment longer than the stressed part of the jet exposed to normal stresses. Fibres with a liquid segment can be drawn by 5-10 times, which allows fabricating microfibres with a linear density of O.02-O.lO tex having a strength of 45-80 cN/tex and elongation of 15-20%. Fibres spun into baths with a low concentration of precipitant have high polvsity, which could be attributed to formation of a liquid polymer phase in phase decomposition of the spinning solution.Chemical fibres with a filament linear density of 0.05-0.10 tex are assigned to a special group and designated as "microfibres" [1 ]. Microfibres can be used to fabricate thinner yarn and consequently higher quality textiles. Microfibre yarn increases the covering power, packing density, and elasticity of fabrics [2].The problem of fabricating polyester [3], polyamide [4], and dry-spun polyacrylonitrile (PAN) microfibres [2] has been examined in the literature. Some characteristics of fabrication of PAN microfibres by the wet method using dimethylforrnamide (DMF) and an aqueous solution of sodium thiocyanate are examined in the present article.PAN fibres of the cotton type with a linear density of 0.13-0.17 tex are widely manufactured. The linear density can be increased further in principle by decreasing the diameter of the spinneret openings, decreasing the concentration of polymer in the spinning solution, or increasing spinneret drawing by increasing the speed at which the fibre comes out of the spinneret. The first two directions have technical and economic limitations. Spinning through spinnerets with a hole diameter of less than 0.05-0.07 nun is impossible due to the high viscosity of the spinning solutions, 30-40 Pa.sec. Using a polymer with a concentration below 12% is not economical due to the high cost of regenerating the solvent, Fabrication of thin fibres by increasing spinneret drawing due to an increase in the exit speed of the spun fibre from the spinneret is most rational. Solving the problem of fabricating PAN microfibres with the wet method thus essentially consists of searching for spinning conditions where high spinneret drawing is possible.The curve of the maximum fibre exit speed (a)max) as a function of the concentration of solvent in the spinning bath for spinning by the thiocyanate (curve 1) and dimethylformamide (curve 2) methods is shown in Fig. 1. In spinning by the thiocyanate method, the spinning solution used had a viscosity of 35 Pa. sec and contained 12.5% polymer with a molecular weight of 6-104, obtained by continuous polymerization in a 51.5% aqueous solution of NaS...

Polyacrylonitrile (PAN) has been widely used for a relatively long time for fabrication of fibres. There are many articles and monographs on very different aspects of this polymer and the fibres made from it. The structure and formation of PAN as a function of the fabrication conditions have also been studied repeatedly and in detail, in particular, especially in detail in [1, 2] and monograph [3]. Detailed studies were usually conducted using only one aprotic solvent --dimethylformamide (DMF). At the same time, in addition to DMF, another aprotic solvent --dimethylacetamide (DMAA) --can be used as a solvent for PAN. Despite the fact that these two solvents belong to the same class and that their great similarity is noted in the literature, there are also data on differences in their properties. Convincing data on the important difference in the values of the second virial coefficient (A2), a quantitative measure of the thermodynamic affinity between polyaner and solvent, for PAN in DMF and DMAA solvents, are cited in [4]. For DMAA A9 has a value of 6.1.10 -4 cm 3.mole/e 2 -4 3 2 9 ". .for DMF and 2.5.10 cm .mole/g . This means that DMAA has a lower thermodynamic affimty for PAN than DMF and thus lower solvent power. In consideration of these concrete and other data, the question of the structure of PAN fibres obtained from solutions in DMAA merits separate examination. The results of a study of this question are presented in the present article.Finished PAN fibres spun by the wet method from highly concentrated solutions in DMAA containing a lyophilic salt were investigated. The concentration of solvent in the aqueous spinning bath varied within a wide range --from 40 to 86 wt. %. The structure was studied with a JSM-840A scanning electron microscope by determining the shape and overall character of the structure of the cross section and the morphology of the surface of the fibres.The photomicrographs of the cross section and surface of fibres fabricated with variation of the ratio of spinning bath components are shown in Fig. 1. The shape of the cross section and morphology of the surface of PAN fibres very obviously change with a change in the composition of the spinning bath.For a 45 and 63% concentration of solvent in the spinning bath, the cross section of the fibres is clearly bean-shaped (Fig. la, b). The surface morphology can be characterized as striated, with a furrow width of tenths of a micrometer ( Fig. la', b').Increasing the concentration of solvent in the spinning bath to 70% causes small changes in the bean shape of the cross section of the fibres (Fig. lc). No significant changes were observed in the surface morphology (Fig. lc').The shape of the fibre cross section is totally different with a spinning bath solvent:precipitator ratio of 75:25 (Fig. ld), 82:18 (e), and 85:15 (f). The shape of the fibre cross section in Fig. ld-f is almost romld. As for the surface morphology (Fig. ld', e'), it does not change significantly with respect to the striated morphology in comparison to Fig. la'-c'. Ch...