Lignin Readies for a New Role

Lignin produced as a byproduct by the pulp and paper industry now has limited value, mainly as a fuel at pulp mills.  However, a research team from Oak Ridge National Laboratory (ORNL), Oak Ridge, Tenn., has developed a process that transforms the lignin into a thermoplastic, potentially making the byproduct much more valuable.
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"Our work addresses a pathway to utilize lignin as a sustainable, renewable resource material for synthesis of thermoplastics that are recyclable," says Amit Naskar, a member of the Department of Energy laboratory's Material Science and Technology Division who headed the project.

The team altered the byproduct lignin to create larger lignin molecules by fractionation with methanol or by crosslinking with formaldehyde. This created a crosslinked rubber-like material that can be processed like plastics. More details can be found in a recent article in Green Chemistry.

Earlier work on lignin-based plastics used biomass material that had undergone harsh chemical treatment, resulting in a significantly degraded version of native lignin.

The ORNL team currently is investigating lignin thermoplastics with various types of soft segments using different functional groups or other sustainable polymers and will reveal their findings soon.

"Our concept was simple: bind a soft polymer chain with lignin hard molecules. You can select any soft segment and corresponding chemistry. Some of those are well known in the art. Bottom line is it should produce a multiphase mixture of hard and soft segments. We tried to use artificially increased molecular weight of lignin segment and it seems that has a good impact on properties of the rubbery materials," says Naskar.

The researchers also are working to make other types of materials from lignin. "It may take a few years to produce various value-added products from lignin," adds Naskar.

One key challenge the team faces is optimizing the synthesis technique. This includes looking at the optimal lignin quality, type of lignin, lignin/soft segment feed ratio, time and temperature of reaction, etc.

"Lignin is heterogeneous material. Although basic chemistry remains the same, depending on raw lignin source, the quality of the final material may vary. Lignin from hardwood and softwood behave differently. A lot more molecular understanding of lignin needs to be established particularly when someone attempts to heat and react it with another macromolecule," explains Naskar.

Another potential issue — processability of the material may vary depending on thermal history. "Compared to other synthetic material, this material is thermally less stable and we are trying to address this," he adds.

Naskar hopes within the next year lignin suppliers or other industrial firms will consider investing in the technology for scale-up trials. A similar material also can be made from lignin produced in biorefineries.