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

Periodic Reporting for period 2 - WoCaFi (Unlocking the Entire Wood Matrix for the Next Generation of Carbon Fibers)

Reporting period: 2018-07-01 to 2019-12-31

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 average driving distances ranging from 10 000 km (Italy) to 14 000 km (Germany) per year (EU average: 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 aims 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.
• Find ways to activate the precursor filaments to reduce the energy requirements during the carbonization phase and thereby generate additional energy and costs savings.
In this first phase of the WoCaFi action, the research team focused on the preparation of multi-component precursor filaments and studied the interaction of different biopolymers with cellulose when embedded in a continuous, homogeneous matrix. The team consists of two doctoral candidates, one postdoctoral researcher and the PI.

Doctoral candidate 1 has prepared various filaments containing different amounts and different types of lignin. Systematic studies regarding the macromolecular properties of lignin and their effect on the spinnability were conducted. This included compositional analyses to establish a comprehensive mass balance. Lignin as comparably smaller biopolymer might get lost in the spin bath. To guarantee maximum retention in the filament, certain structural characteristics are required. In the next step, the interaction of the two polymers was studied with a hyphenated simultaneous TGA/DSC-MS system. (TGA - thermogravimetric analysis, DSC - differential scanning calorimetry, MS - mass spectrometry). Thorough processing of the analytical results showed not only the expected elevated carbon yield but also synergistic effects that increased the yield further. The student was then sent on a short research mission to 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. There, systematic carbonization trials were made with the precursor filaments prepared at Aalto University. The results also served as a basis to plan and design a continuous carbonization line that will be installed at Aalto University at the beginning of 2020.
The results of this first phase are currently summarized to be published in a peer-reviewed journal. In addition, our work led to a collaboration with a group working on enzymatically modified lignin. The respective results are also currently summarized in a joint manuscript and are about to be published.

Doctoral candidate 2 was focusing on strategies to increase the carbon yield of carbohydrates upon pyrolysis. Secondary biopolymers with functional groups were co-dissolved and spun together with cellulose. Heteroatoms present in the functional groups showed a pronounced catalytic effect and increased the carbon yield notably. They also affected the resulting carbon structure. After taking care to protect the intellectual property related to those findings we are now in the phase of publishing these results too.
Realizing that under certain carbonization conditions we can retain the heteroelements in the carbon structure, we also initiated a collateral research stream targeting bio-based carbon material for electrochemical applications. In an ongoing master thesis we are studying ways to increase the specific surface area while maintaining an even distribution of heteroelements in the carbon residue.

The postdoctoral researcher is a distinguished expert in diffraction analysis and has developed tailored WAXS/SAXS and Raman analyses to understand the structural features of the precursor and carbon fibres. A homogeneous solution of two or more constituents does not guarantee a continuous and homogeneous filament matrix. During the coagulation of a mixed polymer solution, phase separation can occur if the individual polymers are not sufficiently compatible. This results in a discontinuous structure with micro domains of the minor-share constituent. Inevitably, defects emerge in the carbon fiber, which will limit the mechanical properties. Systematic studies have been recently presented at the annual spring meeting of the American Chemical Society and are also summarized for publication.
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. With the results obtained so far, 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. A provisional patent application (application number FI20195042) was filed at the beginning of 2019, Moreover, collateral application fields were identified. To explore those in more detail, new collaborations with renowned experts have been established. Encouraging results were found within initial research activities and shall be further explored through a bilateral project with national funding.

The successful results so far have 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 facilitate the ambitious research targets of the final phase. The fundamentals to convert wood into precursor filaments have been established and we expect to accomplish this goal during the final phase.