Cellulose derived Diels-Alder covalent adaptable networks: a route towards recyclable thermosets

Sammanfattning

Plastic materials have grown to become essential in our everyday lives but the usage of plastics is connected to two significant issues; the major part is produced from fossil-based resources and only a fraction of the materials is recycled. Thermosets are especially difficult to recycle due to their permanent crosslinks, which is suboptimal. An alternative approach for crosslinking emerging in the last decades, is to utilise dynamic covalent bonds that can be formed and broken reversibly under certain stimuli. This gives rise to covalent adaptable networks (CANs) which can be recycled more easily compared to traditional thermosets. The goal of this work was to functionalize microcrystalline cellulose (MCC) with furan moieties to enable reversible crosslinking via the Diels-Alder (DA) reaction. A material of this kind solves both issues of renewability of the raw material and recyclability, and therefore could contribute towards more sustainable plastics. In order to find an effective way to graft furan moieties onto MCC, thorough exploratory work was conducted, testing out numerous pathways experimentally. Eventually, an efficient route was successfully developed, synthesising a novel, fully bio-based cellulose ester with an estimated degree of substitution ranging between 0.5 and 2.5. This was done by first reacting furfuryl alcohol with succinic anhydride to form the corresponding acid ester. The carboxylic acid was then activated in-situ with 1,1'-carbonyldiimidazole before being reacted with cellulose homogeneously in DMAc/LiCl. The products were characterised with FTIR and solution-state NMR spectroscopy, as well as thermogravimetric analysis which showed a slightly lower thermal stability of the products compared to pristine MCC, but more importantly still high enough to handle temperatures where the reverse DA-reaction occurs. Initial testing of synthesising a CAN by combining the chemically modified cellulose with a flexible bismaleimide, showed that the DA reaction took place as desired. To complete the work in the future, the CAN synthesis will have to be developed further in order to make the material on larger scales, to then study the recyclability and characterise the final material properties.