The research, led by Lukas Fliri of Aalto University and BOKU University together with colleagues at Aalto and VTT Technical Research Centre of Finland, focused on fibers spun with the Ioncell technology, a process that dissolves pre-hydrolyzed kraft birch pulp in a superbase-derived ionic liquid and dry-jet wet spins it into filaments. Unlike the native cellulose I allomorph found in wood and cotton, these regenerated fibers crystallize in the cellulose II form, the same allomorph that matters for carbon fiber production. Yet, as the authors point out, nearly all of the fundamental mechanistic data on cellulose pyrolysis has been gathered on cellulose I, while studies on cellulose II have mostly reported end-material properties without the underlying physics and chemistry. That gap made it difficult for the team to compare their own carbonization experiments with the literature, so they decided to build the reference dataset themselves.

The experimental design deliberately echoes a classic: the influential 1960s work by Tang and Bacon on the carbonization of Fortisan cellulose II fibers. Fliri and colleagues heated Ioncell fibers from room temperature up to 1100 degrees Celsius under helium in a simultaneous thermal analyzer coupled to a mass spectrometer, using heating rates spanning 2.5 to 40 kelvin per minute. Tiny samples of around 3.5 milligrams minimized heat and mass transfer distortions, and the evolved gases, water, carbon monoxide and carbon dioxide, were quantified through a calibration procedure based on calcium oxalate monohydrate. In parallel, batch carbonizations were carried out in a tubular furnace under nitrogen at nine target temperatures between 300 and 1100 degrees Celsius, with no thermostabilization pretreatment and no tension applied to the fibers, so that the intrinsic behavior of the material could be observed without process tricks that boost yield.

The thermal analysis revealed the familiar fingerprint of cellulose pyrolysis: a violent mass loss concentrated in a window of only 70 to 100 degrees, with onset temperatures between 288 and 322 degrees Celsius depending on heating rate. At the peak of this event, cellulose chains depolymerize and expel levoglucosan, the sugar anhydride that dominates the volatile stream, alongside furans, dehydrated sugars, water and carbon oxides. Crucially, the slower the heating, the more char survived. Char yield at 1100 degrees Celsius fell log-linearly as the heating rate rose, and the combined yields of water and carbon dioxide tracked the char yield almost perfectly. This is exactly what the two-competing-pathway model of cellulose pyrolysis predicts: long residence times below 300 degrees favor dehydration reactions that lock carbon into char precursors, while rapid heating funnels the material toward volatilization as levoglucosan instead.

The kinetic analysis added quantitative teeth to this picture. Using Friedman’s isoconversional method, the team found that the apparent activation energy of Ioncell fiber pyrolysis sits in a narrow band of roughly 190 to 220 kilojoules per mole up to about 80 percent conversion, consistent with a single dominant step: the rupture of the beta-O-4 glycosidic bond that links the glucose units of cellulose. Beyond that conversion level, the activation energy climbs sharply, signaling a shift from depolymerization to reactions within the solid carbonaceous residue. A first-order kinetic model, fitted with activation energies of 227 to 231 kilojoules per mole and pre-exponential factors matching literature values for cellulose I, described the mass loss curves well, but the authors caution that this is a phenomenological description of the main mass loss step, not proof of a simple one-step mechanism, and the parameters should not be extrapolated to isothermal treatments or temperatures above about 600 degrees.

One of the more surprising results concerns heat. The apparent heat of pyrolysis, measured by integrating the differential scanning calorimetry signal, came out between roughly 180 and 250 kilojoules per kilogram across all heating rates, in close agreement with decades of measurements on microcrystalline cellulose and filter paper. When the team ran control experiments on pure levoglucosan, the heat of its thermal evaporation matched the apparent heat of cellulose pyrolysis almost exactly. The implication is striking: the endothermic demand of cellulose pyrolysis is largely just the energy needed to evaporate levoglucosan. From a process engineering standpoint, this reaction heat is small compared with the roughly 650 kilojoules per kilogram of sensible heat required simply to bring the fibers up to their degradation temperature, a fact that matters for anyone designing industrial carbonization furnaces.

The batch carbonization experiments then traced how the fibers’ properties evolve with the final heat treatment temperature. Yields dropped steeply between 300 and 500 degrees and then plateaued between 6.8 and 9.4 percent by weight. Elemental analysis showed hydrogen and oxygen draining away as carbon accumulated, and a van Krevelen plot revealed two distinct linear regimes: below about 400 degrees the hydrogen-to-carbon and oxygen-to-carbon ratios follow a slope of roughly two hydrogens per oxygen, the signature of dehydration, while above that temperature decarboxylation and decarbonylation take over. Notably, oxygen loss largely stops above 600 degrees, and X-ray photoelectron spectroscopy confirmed that thermally stable oxygen functionalities remain embedded in the carbon even at 1000 degrees. These trapped oxygen atoms, the authors argue, act like reticulation agents, promoting five- and seven-membered rings that curve and distort the growing aromatic network and prevent the formation of a graphite-like structure, a key reason cellulose-derived carbons are non-graphitizing.

Morphology told a parallel story. The round cross-section of the Ioncell filaments survived carbonization intact, but the diameter shrank by roughly 60 percent by the time the fibers reached 600 degrees, after which further shrinkage was minimal. Fiber diameter correlated linearly with carbon content and log-linearly with carbonization yield, and the team interprets the shrinkage as the surface degradation of oriented cellulose microfibrils followed by their coalescence into denser cross-linked furanic and polyaromatic structures. Raman spectroscopy completed the picture: a polyaromatic network was already detectable at 400 degrees, and the ratio of the disorder-induced D band to the graphitic G band jumped from 0.59 to 0.93 between 600 and 700 degrees, then climbed more slowly toward 1100 degrees. The D-to-G intensity ratio correlated linearly with the hydrogen-to-carbon ratio above 600 degrees, indicating that large aromatic domains grow through dehydrogenation and radical condensation of smaller ring systems. Estimated in-plane aromatic cluster sizes grew from about 1.0 to 1.4 nanometers between 600 and 800 degrees and then stalled.

Perhaps the most consequential conclusion is what the study did not find. Older carbon fiber literature, still frequently cited, holds that the cellulose II crystal structure follows a special carbonization mechanism tied to the recondensation of four-carbon intermediates. The new data show no evidence of such a peculiarity: the pyrolysis behavior of Ioncell fibers matched the classic two-pathway models, and the high-temperature evolution agreed with general models for non-graphitizable carbons. The authors also observed an unexplained systematic mass loss above 800 degrees, but only at the slowest heating rate of 2.5 kelvin per minute, a phenomenon not previously reported that they flag for further confirmation. By identifying the temperature windows where dehydration, volatilization, dehydrogenation and aromatization each dominate, the dataset gives carbon fiber researchers a practical map for optimizing thermostabilization and carbonization schedules, and a common reference point for comparing cellulose II precursors of every kind. For a field trying to make carbon fiber cheap, renewable and strong enough for the mass market, that map may prove as valuable as the fibers themselves.

Subject of Research: Pyrolysis dynamics and physicochemical property development during the carbonization of Ioncell regenerated cellulose II fibers for carbon fiber production

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

Article References: Fliri, L., Yildirim, Z., Meinander, K., Guizani, C., & Hummel, M. (2026). Tracking carbonization in Ioncell cellulose II fibers: pyrolysis dynamics and development of the physicochemical properties. Journal of Materials Science. https://doi.org/10.1007/s10853-026-13716-6

Image Credits: AI Generated

DOI: 10.1007/s10853-026-13716-6

Keywords: carbon fibers, cellulose II, Ioncell, pyrolysis, carbonization, levoglucosan, char yield, Raman spectroscopy, thermogravimetric analysis, biobased materials, turbostratic carbon, kinetic modeling