Tracking carbonization in Ioncell cellulose II fibers: pyrolysis dynamics and development of the physicochemical properties

Numerous mechanistic studies on the transformations during cellulose pyrolysis and carbonization have emerged in different fields working on thermal valorization of cellulosic feedstocks. Yet, the complex and superimposing reactions are still not completely understood. The available fundamental data are mostly focused on the naturally occurring cellulose I allomorph. Instead, recent investigations on the pyrolysis and carbonization of the cellulose II allomorph—relevant to produce cellulose-based carbon fibers (CFs)—are more concerned with the obtainable material properties. This lack of systematic physicochemical characterization data in the literature caused issues and complicated comparisons in our work focused on the carbonization of Ioncell® regenerated cellulose II fibers. Thus, a systematic dataset on their pyrolysis behavior and carbonization characteristics in the temperature range from 300–1100 °C was gathered. The pyrolysis dynamics of Ioncell® fibers were investigated using simultaneous thermal analysis coupled to mass spectrometry at different heating rates in the range of 2.5–40 K/min. Higher amounts of char, water, and carbon dioxide were observed when decreasing the heating rate. This corroborated the two-competing-pathway model of cellulose carbonization. Isoconversional and first-order kinetic models describing the pyrolysis of Ioncell® fibers were developed and discussed considering the available literature on cellulose pyrolysis. Changes in fiber morphology, chemical composition, and nanotexture were characterized in samples drawn in the temperature range of 300–1100 °C. Remarkable correlations between carbonization temperature, morphological, chemical, and nanotextural properties were identified. Altogether, these help to establish a phenomenological model describing the transformation of Ioncell® cellulose fibers into carbon fibers.