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Cellulose-based fiber-reinforced composites are gaining attention for their eco-friendly attributes and cost-effectiveness. However, their application in high-strength domains remains limited due to the dominance of synthetic and inorganic fibers. This study explores the potential of composites utilizing “Ioncell fiber”, a unique cellulose fiber, in comparison to carbon, cellulosic, and glass fiber composites. Our findings reveal that Ioncell fiber composites exhibit earlier thermal degradation compared to carbon fiber composites according to thermogravimetric analysis (TGA). Analysis via scanning electron microscopy (SEM) highlights exceptional interaction between Ioncell fiber and bio-based epoxy, surpassing other fibers. Additionally, assessment of composite hydrophilicity or hydrophobicity through contact angle measurements reveals distinctive surface characteristics, with Ioncell exhibiting a contact angle of 80°, comparable to carbon fiber’s contact angle of 75°, while glass transition results demonstrate Ioncell fiber’s transformation closely resembling that of carbon fiber composites. Although Ioncell fiber exhibits lower strength (approximately 50 cN per tex) compared to carbon fiber (222 cN per tex), Ioncell composites demonstrate promising strength levels nearly half that of carbon fiber composites (approximately 230 MPa for Ioncell fiber composite compared to 500 MPa for carbon fiber composite). These results underscore the potential of Ioncell composites as sustainable alternatives to petroleum-based and synthetic fiber composites, thus contributing to a more environmentally sustainable future. [ABSTRACT FROM AUTHOR]

Activated carbon produced from biomass exhibits a high specific surface area due to the natural hierarchical porous structure of the precursor material. To reduce production costs of activated carbon, bio-waste materials receive more and more attention, which has led to a steep increase in the number of publications over the past decade. However, the characteristics of activated carbon are highly dependent on the properties of the precursor material used, making it difficult to draw assumptions about activation conditions for new precursor materials based on published work. Here, we introduce a Design of Experiment methodology with a Central Composite Design to better predict the properties of activated carbons from biomass. As a model precursor, we employ well-defined regenerated cellulose-based fibers which contain 25 wt.% chitosan as intrinsic dehydration catalyst and nitrogen donor. The use of the DoE methodology opens up the possibility to better identify the crucial dependencies between activation temperature and impregnation ratio on the yield, surface morphology, porosity and chemical composition of the activated carbon, independent of the used biomass. The use of DoE yields contour plots, which allows for more facile analysis on correlations between activation conditions and activated carbon properties, thus enabling its tailor-made manufacturing. [ABSTRACT FROM AUTHOR]

The performance of biobased carbon fiber (CF) can potentially be improved to a new level by enhancing its graphitization by including graphitic additives as structural templates in the precursor. Mixing these additives in the precursor spinning solution can influence the solution rheology and thus its spinning process and ensuing carbonization, though these effects are not well understood. Herein, we analyze the influence of carbon nanotube (CNT) and graphene oxide (GO) additives on the rheology of cellulose solutions in ionic liquid as well as the subsequent precursor and CF preparation. Addition of GO both thickened the solution and clearly increased its elastic (solid‐like) nature in comparison to pure cellulose solution in ionic liquid, while CNT only made the solution moderately more elastic. Still, solutions with both additives were spinnable into continuous precursor fibers, though the inclusion of GO somewhat disturbed cellulose alignment in the fiber, as observed through X‐ray diffraction. In addition, GO induced structural order development, observed as a decrease in the intensity and ratio between the Raman peaks at ~1300 cm−1 (related to disorder) and ~1600 cm−1 (related to sp2‐hybridized carbon in general). [ABSTRACT FROM AUTHOR]

The production of cellulose-based textile fibers with high toughness is vital for extending the longevity and thus developing a sustainable textile industry by reducing the global burden of microplastics. This study presented strategies to improve fiber toughness by tuning spinneret geometries. Experimental studies were conducted by spinning with different spinneret geometries and measuring the mechanical and structural properties of the spun fibers. In addition, numerical simulation tools were used to better understand the effects of spinneret geometry. The altering parameters of the spinneret geometries were the capillary diameters D, the angle of the entry cone into the spinning capillary, and the ratio of capillary length to diameter L/D. The highest fiber toughness could be achieved at a capillary aspect ratio of 1 to 2. The obtained maximum fiber toughness was 93 MPa with a tensile strength of 60 cN/tex and a concomitant elongation of 16.5%. For these fiber properties, a 13 wt% solution of a high-purity pulp with higher viscosity in [DBNH][OAc] was spun into a 1.3 dtex fiber using a D100 spinneret with a capillary of 1:1 length/diameter and an entrance angle of 8°. It was noticeable that the microvoid orientations decreased almost linearly with increasing toughness of the fibers. The morphologies of the fibers were similar regardless of the spinneret geometries and the raw materials used in the spinning process. In summary, by modulating the spinneret geometries, Ioncell fibers obtained high toughness that have the potential to replace synthetic fibers. [ABSTRACT FROM AUTHOR]

The production of sustainable and high-performance fabrics requires high mechanical strength of the individual (staple) fibers. Although Ioncell fibers already exhibit higher fiber strength than commercial man-made cellulose fibers or cotton fibers, we further aimed to increase both strength and toughness to gradually approach synthetic fibers in these properties. Decisive factors for the achievable mechanical properties of the fibers were the pulp purity, the cellulose concentration in the spinning solution and length-to-diameter (L/D) ratio of the cylindrical part of the spinneret. The absence of low molecular weight fractions in combination with an increased average molecular weight had the highest impact on the achievement of both high strength and toughness. Using a spinneret with a high L/D ratio, it was possible to spin Ioncell fibers with a tensile strength of 925 MPa (61.5 cN/tex) and a modulus of toughness of 83.3 MPa (55.5 J/g). According to a fluid dynamic simulation, uniformly longer molecular cellulose chains in combination with a longer cylindrical capillary promoted an effective alignment of the cellulose molecules inside the spinneret capillary before entering the airgap, thus creating the conditions for a simultaneous increase in tensile strength and elongation i.e. toughness of the fiber. Mechanistically, high fiber toughness is caused by the structural parameters in longitudinal direction, in particular by a higher tilt angle, a longer periodicity of the lamellar plane and lower micro void orientation. In summary, we have developed lyocell-type fibers with high strength and toughness, which can potentially be used as a surrogate for synthetic fibers. [ABSTRACT FROM AUTHOR]

Cellulose can be dissolved with another biopolymer in a protic ionic liquid and spun into a bicomponent hybrid cellulose fiber using the Ioncell® technology. Inside the hybrid fibers, the biopolymers are mixed at the nanoscale, and the second biopolymer provides the produced hybrid fiber new functional properties that can be fine-tuned by controlling its share in the fiber. In the present work, we present a fast and quantitative thermoanalytical method for the compositional analysis of man-made hybrid cellulose fibers by using thermogravimetric analysis (TGA) in combination with chemometrics. First, we incorporated 0–46 wt.% of lignin or chitosan in the hybrid fibers. Then, we analyzed their thermal decomposition behavior in a TGA device following a simple, one-hour thermal treatment protocol. With an analogy to spectroscopy, we show that the derivative thermogram can be used as a predictor in a multivariate regression model for determining the share of lignin or chitosan in the cellulose hybrid fibers. The method generated cross validation errors in the range 1.5–2.1 wt.% for lignin and chitosan. In addition, we discuss how the multivariate regression outperforms more common modeling methods such as those based on thermogram deconvolution or on linear superposition of reference thermograms. Moreover, we highlight the versatility of this thermoanalytical method—which could be applied to a wide range of composite materials, provided that their components can be thermally resolved—and illustrate it with an additional example on the measurement of polyester content in cellulose and polyester fiber blends. The method could predict the polyester content in the cellulose-polyester fiber blends with a cross validation error of 1.94 wt.% in the range of 0–100 wt.%. Finally, we give a list of recommendations on good experimental and modeling practices for the readers who want to extend the application of this thermoanalytical method to other composite materials. [ABSTRACT FROM AUTHOR]

Fiber‐reinforced composites based on natural fibers are promising alternatives for materials made of metal or synthetic polymers. However, the inherent inhomogeneity of natural fibers limits the quality of the respective composites. Man‐made cellulose fibers (MMCFs) prepared from cellulose solutions via wet or dry‐jet wet spinning processes can overcome these limitations. Herein, MMCFs are used to prepare single fiber epoxy composites and UD composites with 20, 30, 40, and 60 wt% fiber loads. The mechanical properties increase gradually with fiber loading. Young's modulus is improved three times while tensile strength doubles at a loading of 60 wt%. Raman spectroscopy is employed to follow conformational changes of the cellulose chains within the fibers upon mechanical deformation of the composites. The shift of the characteristic Raman band under strain indicates the deformation mechanisms in the fiber. Provided stress transfer occurs through the interface, it is a direct measure of the fiber‐matrix interaction, which is investigated herein. The shift rate of the 1095 cm−1 band decreases in single fiber composites compared to the neat fibers and continues to decrease as the fiber loading increased. [ABSTRACT FROM AUTHOR]

In this study, we propose a convenient method for testing the fibrillation tendency of man-made cellulosic fibres (MMCFs) and investigate the possibility to apply a commercial crosslinker for Tencel fibres on the ionic liquid-based regenerated cellulosic fibre (Ioncell fibre). The fibrillation tendency of various MMCFs including viscose, Modal, Tencel and Ioncell fibres were examined through wet abrasion by using ball bearing and blending methods. The fibrillation tests using a laboratory blender was found to be a superior method over the ball bearing method in terms of time and energy saving. The fibrillation tendency of the fibres highly depended on their cellulose molecular orientation and the treatment intensity (time, temperature and alkalinity) in the blender. This fibrillation method was also applied to discover the effect of the crosslinking on the fibrillation tendency of the fibres. The Ioncell fibre proved to be suitable for crosslinking treatment to reduce fibrillation using 1,3,5-triacryloyl-hexahydro-1,3,5-triazine (TAHT)—a commercial Tencel crosslinker. [ABSTRACT FROM AUTHOR]

Lyocell fibers have received increased attention during the recent years. This is due to their high potential to satisfy the rising market demand for cellulose-based textiles in a sustainable way. Typically, this technology adopts a dry-jet wet spinning process, which offers regenerated cellulose fibers of excellent mechanical properties. Compared to the widely exploited viscose process, the lyocell technology fosters an eco-friendly process employing green direct solvents that can be fully recovered with low environmental impact. N-methylmorpholine N-oxide (NMMO) is a widely known direct solvent that has proven its success in commercializing the lyocell process. Its regenerated cellulose fibers exhibit higher tenacities and chain orientation compared to viscose fibers. Recently, protic superbase-based ionic liquids (ILs) have also been found to be suitable solvents for lyocell-type fiber spinning. Similar to NMMO, fibers of high mechanical properties can be spun from the cellulose-IL solutions at lower spinning temperatures. In this article, we study the different aspects of producing regenerated cellulose fibers using NMMO and relevant superbase-based ILs. The selected ILs are 1,5-diazabicyclo[4.3.0]non-5-ene-1-ium acetate ([DBNH]OAc), 7-methyl-1,5,7-triazabicyclo[4.4.0] dec-5-enium acetate ([mTBDH]OAc) and 1,8-diazabicyclo[5.4.0]undec-7-enium acetate ([DBUH]OAc). All ILs were used to dissolve a 13 wt% (PHK) cellulose pulp. The study covers the fiber spinning process, including the rheological characterization of the various cellulose solutions. Moreover, we discuss the properties of the produced fibers such as mechanical performance, macromolecular properties and morphology. [ABSTRACT FROM AUTHOR]

The kinetics of the dissolution and swelling of different cellulose fibers in the ionic liquid 1-ethyl-3-methylimidazolium acetate ([EMIM][OAc]) was studied by varying solvent power and temperature. Natural fiber, flax, and man-made fibers, Cordenka and Lyocell-type (Ioncell) were used with one Ioncell fiber containing lignin and hemicelluloses. Through the addition of water, the solvent power was modified from very good (neat ionic liquid), to moderate (with 5 wt% water) and weak (15 wt% water). The temperature was varied to correlate the fiber dissolution rate with the solvent viscosity. All fibers were characterized by chemical composition, crystallinity, molecular weight distribution and dynamic vapor sorption. It was demonstrated that while the rate of fiber dissolution in neat ionic liquid depends on fiber accessibility and solvent viscosity, the water-induced decreased solvent power dominates the general fiber behavior. Flax appeared to be the most "sensitive" to the solvent power due to its hierarchical structure. The fastest dissolution or swelling was recorded for Ioncell and the slowest for Cordenka. [ABSTRACT FROM AUTHOR]

In this paper, we report new results related to the development of a novel regenerated cellulose fiber process of the Lyocell type, denoted Ioncell™, and characterized by the use of a powerful direct cellulose solvent, 1,5-diaza- bicyclo[4.3.0]non-5-enium acetate ([DBNH][OAc]) a superbase-based ionic liquid (IL). The focus of this work is on the effects of air gap conditioning (AGC) during the dry-jet wet spinning operation. The installation of an AGC system on the spinning line led to significant improvements of the fiber properties. The fiber titer variation decreased significantly, and the fiber toughness increased by approximately 50% when controlling the temperature and the relative humidity in the airgap using a convective air flow. The presence of water vapor in the air stream was a determinant factor for the improvement of the fiber elongation. The interaction of water vapor with the spinning dope was investigated using dynamic vapor sorption. The diffusion coefficient of water vapor inside the dope could be identified from those experiments and used in a numerical simulation model of the heat and water vapor transfer in the air gap between the spinning dope and the surrounding air. The experimental and simulation results suggest that dope convective cooling and surface hydration lead to a higher fiber toughness. [ABSTRACT FROM AUTHOR]

The successful recycling of colored textile waste and reuse of respective dyes would represent a major milestone of global efforts to reduce the environmental impact of the textile industry. The chemical upcycling of dyed pre- and postconsumer cotton waste is promoted by studying the spinability and color fastness of seven vat and reactive dyes (i.e. Indanthren Blue BC 3%, Indanthren Red FBB coll, Indanthren Brilliant Green FBB coll, Levafix Brilliant Red E-4BA, Levafix Blue E-GRN gran, Remazol Brilliant Blue R spec, and Remazol Black B 133%) during dry-jet wet spinning. Apart from the fabrics dyed with Levafix Brilliant Red E-4BA, all samples dissolved in 1,5-diazabicyclo[4.3.0]non-5-ene actetate, a superbase based ionic liquid, and could be converted to new colored man-made cellulose fibers. It was found that there is a clear discrepancy between the recyclability of dyed pre- and postconsumer cotton waste, resulting in significantly higher fiber properties up to tenacities of 59.8 cN/tex and elongations of 13.1% in case of the latter. All recycled fibers displayed a noticeable color change in the CIELab space (ΔE = 8.8–25.6) throughout the spinning process. Despite these deviations, almost all fibers and demo fabrics produced thereof exhibited bright colors that can be reused in textile industry. Only Remazol Black B 133% did not sufficiently translate to the new textile product. The wash and rubbing fastness of the fabrics knitted from the regenerated fibers was superior to the dyed waste fabrics mainly because of the homogenous distribution of the dyes along the fiber cross-section. [ABSTRACT FROM AUTHOR]

• [DBNH] [OAc] enables the separation of cotton and polyester in textile blends. • Cotton polyester blends can be used to spin textile grade MMCFs. • PET can be recovered by a simple filtration procedure. • PET slightly degrades in a [DBNH] [OAc] dispersion at elevated temperatures. The creation of a circular economy for cellulose based textile waste is supported by the development of an upcycling method for cotton polyester blended waste garments. We present a separation procedure for cotton and polyester using [DBNH] [OAc], a superbase based ionic liquid, which allows the selective dissolution of the cellulose component. After the removal of PET, the resulting solution could be employed to dry-jet wet spin textile grade cellulose fibers down to the microfiber range (0.75–2.95 dtex) with breaking tenacities (27–48 cN/tex) and elongations (7–9%) comparable to commercial Lyocell fibers made from high-purity dissolving pulp. The treatment time in [DBNH] [OAc] was found to reduce the tensile properties (<52%) and the molar mass distribution (<51%) of PET under certain processing conditions. [ABSTRACT FROM AUTHOR]

The viscoelastic properties of cellulose fibers play an important role in chemical recycling of textiles. Here we discriminated the intrinsic viscosity of cotton roll towels and bed linens using near-infrared imaging spectroscopy and supervised pattern recognition. The classification results showed training and test set accuracies of 84–97% and indicated that the relevant spectral features were related to water, cellulose, and cellulose crystallinity. We hypothesized that the decreasing intrinsic viscosity of cotton was associated with changes in cellulose crystallinity and water adsorption, which was supported by additional X-ray and sorption measurements. These results are important as they indicate the potential to non-invasively estimate the degree of polymerization and the suitability of different cotton materials for chemical recycling. We propose that changes in the degree of polymerization and cellulose crystallinity could be used as an indicator of the chemical quality of cellulose fibers, which would have wider impacts for textile recycling. [Display omitted] [ABSTRACT FROM AUTHOR]

Here, the carbonization of two Lyocell type regenerated cellulose fibres is reported. Commercially available Lyocell as well as the experimental Lyocell type fibre known as Ioncell-F spun from the ionic liquid 1,5-diazabicyclo[4.3.0]non-5-ene-1-ium acetate ([DBNH]OAc) is investigated, which supports higher draw ratio and thus improves precursor mechanical properties. Lyocell fibres are known to have improved mechanical properties over other regenerated cellulose fibres and are therefore considered to be better carbon fibre precursor candidates. The Lyocell fibres used in this study are carbonized utilizing a scaled down identical replica of an in use carbon fibre line. The importance of this is the ability to assess the performance of the Lyocell fibres under more realistic continuous carbonization processing conditions. The tensile properties, morphology, and chemical composition of all fibres are determined. It is shown that by changing the carbonization temperature and atmosphere fibres with different mechanical properties and diameter can be produced. Elemental analysis confirms that each fibre has a carbon content of ≥90%. [ABSTRACT FROM AUTHOR]

Ioncell-F, a recently developed process for the production of man-made cellulosic fibers from ionic liquid solutions by dry-jet wet spinning, is presented as an alternative to the viscose and N-methylmorpholine N-oxide (NMMO)-based Lyocell processes. The ionic liquid 1,5-diazabicyclo[4.3.0]non-5-ene acetate was identified as excellent cellulose solvent allowing for a rapid dissolution at moderate temperatures and subsequent shaping into continuous filaments. The highly oriented cellulose fibers obtained upon coagulation in cold water exhibited superior tenacity, exceeding that of commercial viscose and NMMO-based Lyocell (Tencel®) fibers. The respective staple fibers, which have been converted into two-ply yarn by ring spinning technology, presented very high tenacity. Furthermore, the Ioncell yarn showed very good behavior during the knitting and weaving processes, reflecting the quality of the produced yarn. The successfully knitted and woven garments from the Ioncell yarn demonstrate the suitability of this particular ionic liquid for the production of man-made cellulosic fibers and thus give a promising outlook for the future of the Ioncell-F process. [ABSTRACT FROM AUTHOR]

The market for cellulosic fiber based food packaging applications is growing together with the importance of improving the thermal durability of these fibers. To shed light on this, we investigated the role of residual lignin in pulp on the thermal stability of refined pulp sheets. The unbleached, oxygen delignified, and fully bleached pulp sheets were studied after four separate refining degrees. Comparison by Gurley air resistance, Bendtsen porosity, and the oxygen transmission rate tests showed that lignin containing sheets had better air and oxygen barrier properties than fully bleached sheets. Sheet density and light scattering coefficient measurements further confirmed that the lignin containing pulps underwent more intense fibrillation upon refining that changed the barrier properties of the sheets. Thermal treatments (at 225 °C, 20 and 60 min, in water vapor atmospheres of 1 and 75 v/v %) were applied to determine the thermal durability of the sheets. The results revealed that the residual lignin in pulps improved the thermal stability of the pulp sheets in the hot humid conditions. This effect was systematically studied by tensile strength, brightness, and light absorption coefficient measurements. The intrinsic viscosity results support the findings and suggest that lignin is able to hinder the thermal degradation of pulp polysaccharides. In spite of the fact that lignin is known to enhance the thermal yellowing of paper, no significant discoloration of the pulp sheets containing residual lignin was observed in the hot humid conditions (75 v/v %). Our results support the idea of lignin strengthening the thermal durability of paper. [ABSTRACT FROM AUTHOR]

This study addresses in detail the role of water in the consecutive steps of film forming: cellulose dissolution, regeneration and drying, and its impact on the mechanical properties of the cellulose film. Prehydrolysis kraft (PHK) pulp was subjected to hydrothermal treatment (HT) prior to its dissolution in 1-ethyl-3-methylimidazolium acetate [emim]OAc. After the treatment, the DP of the pulp was 440 and the polydispersity index was as low as 3.1. HT-pretreated pulp was dissolved in [emim]OAc with a water content of 2, 7, 14 and 21 wt%. The pulp dissolution was completed within 15 min regardless of the water content in the solvent. The rheological behavior, one of the key properties in film formation, was determined at varied temperature and cellulose concentration. Cellulose films were prepared from solutions with cellulose concentrations of 8, 12 and 16 wt% at a temperature of 90 °C. The impact of water on the macromolecular, morphological and mechanical properties of the transparent films prepared was thoroughly studied. With a very low polydispersity of the cellulose chain, the films prepared revealed high strength ranging from 87 to 106.5 MPa at elongation from 10 to 50% in conditioned state. [ABSTRACT FROM AUTHOR]

Pulp of high cellulose content, alsoknown as dissolving pulp,is needed for many purposes, including the production of cellulosicfibers and films. Paper-grade pulp, which is rich in hemicellulose,could be a cheap source but must be refined. Hitherto, hemicelluloseextraction procedures suffered from a loss of cellulose and the non-recoverabilityof unaltered hemicelluloses. Herein, an environmentally benign fractionationconcept is presented, using mixtures of a cosolvent (water, ethanol,or acetone) and the cellulose dissolving ionic liquid 1-ethyl-3-methylimidazoliumacetate (EMIM OAc). This cosolvent addition was monitored using Kamlet–Taftparameters, and appropriate stirring conditions (3 h at 60 °C)were maintained. This allowed the fractionation of a paper-grade kraftpulp into a separated cellulose and a regenerated hemicellulose fraction.Both of these exhibited high levels of purity, without any yield lossesor depolymerization. Thus, this process represents an ecologicallyand economically efficient alternative in producing dissolving pulpof highest purity. [ABSTRACT FROM AUTHOR]

Mechanically strong all-cellulose composites are very attractive in the terms of fully bio-based and bio-degradable materials. Unidirectional flax-based all-cellulose composites are prepared via facile room-temperature impregnation with an ionic liquid, 1-ethyl-3-methyl imidazolium acetate. To determine the optimal processing conditions, the kinetics of flax dissolution in this solvent is first studied using optical microscopy. Composite morphology, crystallinity, density, the volume fraction of cellulose II and tensile properties are investigated, indicating that flax dissolution should be within certain limits. On the one hand, the amount of cellulose II formed through dissolution and coagulation should be high enough to "fuse" flax fibers, resulting in a density increase. On the other hand, only the surface layer of the fibers should be dissolved to maintain the strength provided by the inner secondary layer and avoid a detrimental decrease in crystallinity. The highest Young's modulus and strength, 10.1 GPa and 151.3 MPa, respectively, are obtained with a crystallinity of 43% and 20 vol% of cellulose II. [ABSTRACT FROM AUTHOR]

Cellulose in solution can be assembled into textile fibers by wet-spinning (Viscose etc.) or dry-jet wet spinning (Lyocell, Ioncell etc.), which leads to significant differences in the mechanical properties of fibers. We use scanning X-ray microdiffraction (SXM) to reveal regenerated fibers having a "skin-core" morphology. The "core" region comprises microfibrils (MFs) with ~100 nm in diameter. The cellulose forms elementary fibrils having a ribbon-like cross sectional shape of about 6 × 2 nm, which are packed into MFs. Our SXM studies demonstrate that MFs within Ioncell fibers are composed of elementary fibrils with homogeneous morphologies. Furthermore, the stacking of cellulose molecular sheets within elementary fibrils of Viscose fibers is preferentially along the 010 direction, while those of Ioncell fibers preferably stack in the 1–10 direction. The better structural regularities and distinct morphologies of elementary fibrils give Ioncell fibers enhanced mechanical properties and a wet strength far superior to those of Viscose fibers. [Display omitted] [ABSTRACT FROM AUTHOR]

Low temperature pre-treatments increase the char yield during cellulose carbonization. Although this effect is mostly understood on a kinetic basis, the formed chemical structures leave room for scrutiny. In ongoing ambitions to enhance the char yield of bioderived precursors, the thereby occurring chemistry was reinvestigated. A set of isothermal heating protocols ranging from 150° to 250°C was applied to man-made Ioncell® cellulose fibers and to Avicel® PH-101 microcrystalline cellulose. The prepared cellulosic samples were examined by solution state NMR using a tetra- n -butyl phosphonium acetate ([P 4444 ][OAc]): DMSO- d 6 (1:4 wt%) electrolyte as dissolving medium. Complementary, IR spectroscopy, size-exclusion chromatography (SEC) and thermogravimetric analysis coupled with mass spectroscopy (TGA-MS) measurements were performed. The NMR spectra evidenced the formation of levoglucosan end capped moieties as being the first and major occurring reaction during low temperature pre-treatments. In contrast to other mechanistic proposals, no signs for carbonyl or alkene functionalities were discernible in the cellulosic material soluble in the NMR electrolyte, even after treatment at 250 °C for several hours. Thermal cross-linking was observed in SEC already at temperatures not known to significantly influence the overall char yield. In the solution state analytics only partial solubility was observed, owing to the formation of a reluctant fraction previously described as "thermostable condensed phase". This resulted in analytical blind spots and led to discrepancies between NMR results and FTIR spectra in which carbonyl and alkene vibrations were clearly discernible. Those discrepancies might also imply the co-existence of different fractions in the early stages of cellulose pyrolysis. [Display omitted] • Application of solution state analytics to heat pretreated cellulose fibers. • Evidence for levoglucosan end capped cellulose structures by solution state NMR. • Identification of crosslinked polysaccharide fractions by GPC. • Formation of a thermostable condensed phase as important carbonization intermediate. [ABSTRACT FROM AUTHOR]