The estimated total annual production capacity of all carbon fibers (CF) in the world in 2014 is 104,600 t (in 2017 136,500 t). Growing demand for CF stimulated an active growth in their production and an annual increase in capacity by 10% until 2020 [1
The main advantages of carbon fibers are high flexibility, tensile strength and stiffness, adsorption, thermal stability, low coefficient of thermal expansion (CTE), etc. Due to these properties, CF and composites based on them are widely used in various industrial sectors. For example, in construction, CF are used for external reinforcement of load-bearing structures. In aviation, automotive, and green energy (wind turbines) industries, CF are used to produce low weight parts. In the nuclear industry, CF are not interchangeable in the construction of reactors, as insulation, where the key properties are resistant to high temperatures and pressure and radioactive radiation [2
High modulus and cord fibers are used, as a rule, as cellulosic precursors of CF. Such cellulosic fibers are produced on an industrial scale by an environmentally hazardous viscose process. The exception is Hyosung Company (Seoul, South Korea). They use an environmentally friendly NMMO process for obtaining new Lyocell cord fibers (hydrated cellulose fibers classified as Lyocell fibers by the international committee BISFA (Bureau International pour la Standardisation des Fibres Artificielles), Brussels, Belgium) [5
]. In this process, N
-oxide (NMMO) is used as a direct solvent of cellulose. The production of Lyocell cord fibers proceeds according to two processes. The traditional process includes the preliminary stage of slurry preparation (cellulose suspension in an aqueous solution of NMMO) and the process of direct interaction of NMMO monohydrate (water content 13.3%) with cellulose under conditions of heating and intensive mixing [6
]. It should be noted that three crystalline hydrates of NMMO have been identified and their dissolving ability increases with decreasing water content in them [8
]. The second process in many respects intersects with the original “solid-phase” method [9
], in which a solvent with a water content of 13.3% or less is used, which increases significantly the concentration of polymer in the solution, up to 45%. Orcel®
fibers obtained through solid-phase presolutions (“solid-phase” method), characterized by high crystallinity and orientation, which provide them with high mechanical properties (strength and Young’s modulus) [11
The main properties that carbon fibers obtained from cellulose precursors must have in order to create general-purpose structural materials are high strength and stiffness, a controlled coefficient of thermal expansion and a high thermal conductivity. Existing cellulose-based CF are produced only with viscose precursors. The existing new NMMO process, alternatively to viscose, is applied for textile and cord fiber production. The work on the creation of CF based on Lyocell precursors is mainly at the research stage. In this regard, it is extremely important to analyze the state and level of data known from the literature on the structural features of Lyocell precursors, their properties and thermolysis conditions.
The structure of potential precursor fibers is defined already at the stage of obtaining solutions and is further determined by the conditions of their formation [12
]. Thus, a change in the nature of the coagulation bath affects not only the structure of the formed material, but also the morphology of the obtained samples [13
]. For cellulose solutions in NMMO, such transformations are described in detail for coagulation into aqueous solutions of NMMO [15
] and alcohols [18
]. Along with a change in the chemical composition of the baths, their temperature was also varied in [18
], which made it possible to change the structure of the spun fibers. It is important to note that if the formed film (fiber) after coagulation into alcohol passes through a second bath with a different liquid, its structure and properties change [21
When producing CF, cellulose precursors are subjected to heat treatment up to 1500 or 2400 °C (higher temperatures make it possible to improve the special properties of CF) [23
]. To increase the values of the carbon yield and achieve good mechanical characteristics of the obtained CF, as in the case of viscose precursors, it is proposed to impregnate Lyocell fibers with pyrolysis catalysts and flame retardants [24
]. Traditional impregnations are ammonium phosphates, zinc and calcium chlorides, Lewis acids, etc. [25
]. Silicon-based compounds became an alternative to these impregnations [26
], interest to which arose already in the mid-70s. Thus, in [29
], the importance of using silicon-containing compounds in the process of obtaining CF from cellulose precursors was noted, which made it possible to obtain graphitized fibers with a mechanical strength of more than 1 GPa.
An important parameter of the CF is the coefficient of thermal expansion. The values of CTE depend not only on the type of precursor, but also on the method and conditions of its thermal processing into the CF. For precursors obtained in the absence of orientation during thermolysis, the maximum CTE values are realized in the direction of the fiber axis and the minimum in the transverse direction. The anisotropic structure realized when the samples are drawn during thermolysis leads to a change in these values to the opposite [31
]. The achievement of high mechanical characteristics of CF is associated with solving a number of issues: the choice of a precursor [32
], the conditions of spinning, structural and morphological properties, and ending with the modes of thermal processing in CF [33
]. Though the effective interconnection of these problems has not yet been identified, numerous results of studies of the mechanical characteristics of CF presented in the literature can be divided into two groups. The first group includes papers with unsatisfactory strength characteristics of CF [24
], and the second presents conflicting results. In one of the first papers devoted to the production of CF from Lyocell fibers, strength values equal to 1 GPa, and elastic modulus values of 100 GPa were achieved [34
]. Such high mechanical values are ambiguously correlated with X-ray diffraction data: in diffractograms, the angular position of the 002 reflection maximum does not reach 25° (2θ), and its intensity is very low. In [35
], the mechanical properties of CF obtained from viscose and Lyocell precursors are compared after impregnation with the same flame retardants and carbonization in the same temperature–time regime. The strength of Lyocell fibers is almost two times higher compared to viscose precursors, while the crystallinity for Lyocell is 40% higher than that of viscose fibers. After heat treatment of these precursors under the same conditions, the difference between the strength values of CF is leveled [35
]. Therefore, we can conclude that the strength and high crystallinity of the precursors are not limiting structural parameters that determine the high strength properties of CF.
The above data set one of the most actual and most important tasks in the problem of creating high-strength CF based on Lyocell fiber—searching for the ways of directional regulation of the structure of cellulose fiber precursors. In this regard, the aim of this research is to study the structural and morphological features of Lyocell precursors obtained from solutions in NMMO. When replacing water precipitation baths with more thermodynamically soft alcohol precipitation baths at different temperatures, were studied their mechanical and thermal properties and the relationship between the structure of the obtained precursors and properties were established.