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Unlocking the Entire Wood Matrix for the Next Generation of Carbon Fibers

Periodic Reporting for period 4 - WoCaFi (Unlocking the Entire Wood Matrix for the Next Generation of Carbon Fibers)

Reporting period: 2021-07-01 to 2022-03-31

Our modern societies are built on the mobility of the individual. Travelling to and from work, trips for leisure and shopping, or journeys to holiday destinations are integral part of our lives. The most popular means of transportation today is the private car, with an EU-average driving distance of 12 000 km/a. Households in the EU have on average 1.4 cars, which equals to almost 0.7 cars per adult. In 2016, the global car production has surpassed the 70 million mark for the first time. This numbers make it clear that irrespective of whether and when the mobility of our society will shift to electric vehicles, solutions are needed to increase the fuel and energy efficiency of transportation to mitigate effects of climate change and account for the continuous depletion of fossil fuels. One key strategy is to reduce the weight of the vehicle by replacing metal components through lightweight composite materials. In particular carbon fiber reinforced composite (CFRC) elements offer significant weight reduction while maintaining the strength and safety properties. Carbon fibers are still predominantly produced from polyacrylonitrile (PAN) precursor filaments and remain an expensive commodity. For this reason, CFRCs are mostly found in high-end applications such as space- and aircrafts or low-volume products like sports and leisure equipment. However, they remained unattractive for products, which require bulk amounts at considerably reduced costs like the automotive sector.
Despite numerous initiatives worldwide to develop low-cost CFs from alternative precursor materials, no viable solutions have yet emerged. Biopolymers such as cellulose or lignin as renewable precursor for carbon fibers are experiencing a renaissance because respective precursor filaments can be produced substantially cheaper than state-of-the art PAN filaments. However, those bio-based CFs still suffer from two distinct limitations: (i) the strength properties are still not on the level of steel; (ii) refining wood to isolate cellulose pulp and lignin requires processes that add to the costs of the precursor filament and render the price of the resulting CFs still too high.

WoCaFi’s aim was to overcome those hurdles by turning wood in its entirety into high-quality continuous filaments to be converted into low-cost bio-based carbon fibers. Instead of separating the constituents of wood and processing them in isolated form (pure cellulose or lignin fibers) or combining wood pulp with technical lignin, wood is only mildly pretreated and dissolved directly into a special solvent to be spun into filaments. In addition, less energy input is expected for the carbonization phase, reducing the overall costs even further. Thus, a new low-price category of CFs is envisioned, which is suitable for all applications with property requirements in the mid-range.

The overall objectives of WoCaFi are:
• Produce multi-component filaments that contain two or more biopolymers homogeneously distributed across the fiber matrix.
• Elucidate the interaction of different biopolymers (cellulose-lignin, cellulose-hemicellulose, cellulose-chitosan) during pyrolysis and identify synergistic effects that increase the carbon yield and promote the formation of the carbon network.
• Build on the above knowledge-base to convert the entire wood matrix into high-quality precursor filaments and turn them into fully-biobased CFs.
WoCaFi was structured in two research pillars that provided the foundation for the final two objectives. Pillar 1 focused on the preparation of carbon fibers from mixtures of cellulose and lignin. Different types of lignin were blended successfully with pulp cellulose to produce high-quality precursor filaments which were then carbonized together with Deakin University, Australia. Deakin is working at the forefront of carbon fiber research. Their campus in Geelong hosts Carbon Nexus, a world-leading CF research institute that is co-owned by Deakin University, Victoria State Government, and the Australian Government. The results of this collaboration were published in six peer-reviewed articles. These studies also served as a basis to plan and design a continuous carbonization line that will be installed at Aalto University in 2022.
Pillar 2 focused on strategies to increase the carbon yield after carbonization by using additives such as the natural polymer chitosan. We could show that chitosan can act as a catalyst to change the pyrolysis mechanism, increasing the carbon yield significantly. The studies were conducted in collaboration with Tohoku University, Japan, providing world-leading expertise regarding the pyrolysis mechanisms of biomass and resulted in three joint peer-reviewed publications. Motivated by the results with chitosan, keratin sourced from waste chicken feathers was also co-dissolved with cellulose and spun into filaments. After successful carbonization the resulting bio-carbon material proved suitable for electrochemical applications such as energy storage in supercapacitors.
Objective 1 focused on the solubilization of the entire wood matrix to spin precursor filaments with cellulose, hemicellulose and lignin. After mild pre-treatment high-quality filaments could be produced and were carbonized offline. The results confirmed the central hypothesis of WoCaFi and are currently summarized to be published soon.
Objective 2 was to carbonize all mixed-polymer precursor fibers both off- and online. Online carbonization was performed at Carbon Nexus until COVID-19-related restrictions and closures prevented a further collaboration. Nevertheless, continuous carbonization was possible through collaboration with other partners. Currently, Aalto University is installing its own continuous carbonization line to continue the research started in WoCaFi.
Although cellulose-lignin composite filaments have been described already earlier, systematic studies were still lacking. Especially structural analysis and an understanding of the distribution of two and more polymers in a fiber matrix were missing entirely. Through WoCaFi the requirements for the raw material could already be refined, which is extremely valuable when trying to advance closer to an actual business case. In addition, new research collaborations were initiated to develop strategies that allow for sustainable modification and activation of the feedstock to provide raw material tailored for carbon fiber production.
The addition of non-wood based biopolymers with heteroatoms has yielded surprisingly good precursor filaments. The resulting composite filaments demonstrated autocatalytic effects towards dehydration during pyrolysis and led to a substantially higher carbon yield. This has direct effects on the cost structure of the final product. Moreover, collateral application fields were identified.
The successful results motivated the Academy of Finland to support the newly established activities in the field of biobased carbon fibers. They have granted investment funding for a continuous carbonization line for “the acquisition, establishment or strengthening of nationally significant research infrastructures that promote scientific research”. Based on the experience gained at Carbon Nexus, we have designed a highly flexible research line, which will allow us to study various biopolymers as precursors for fully bio-based carbon fibers.
From tree to carbon fiber (photo credit: Valeria Azovskaya)