Researchers uncover how to make better polymers from pulp and paper waste

Millions of tons of lignin—a biopolymer that strengthens the cell walls of plants—are produced each year by the pulp and paper industry. Although lignin production is widescale, lignin is considered a waste product with no commercial application due to its complexity as a non-uniform material. Incorporation of lignin into commercially important polymers remains a challenge in both pure research and applied industrial environments; yet if we could understand and use this material in a predictable manner, it would have a myriad of applications. Fortunately, a 2018 study in ACS Sustainable Chemistry & Engineering has revealed how to prepare and characterize different copolymers of lignin and polylactide (PLA).

https://www.youtube.com/watch?v=McCNMLE8Mw0

The study conducted by Dr. Parisa Mehrkhodavandi’s group at the University of British Columbia is the first systematic study exploring the relationship between synthetic method, structure, and other properties of lignin-PLA copolymers. Using two major strategies—“grafting to” and “grafting from”—they prepared three types of polymers: linear PLAs, cyclic PLAs, and star-shaped lignin-PLA copolymers.

Linear PLA, cyclic PLA, and star-shaped lignin-PLA copolymers were synthesized by three methods. Source: ACS Sustainable Chemistry & Engineering.

According to Dr. Mehrkhodavandi, this research is an example of “taking something that is not useful, and valorizing—or adding value—to it.” Although lignin is currently burned as a source of fuel, Dr. Mehrkhodavandi says that “The problem is there are a lot of aromatic components. You burn it, and you produce a lot of carcinogenic substances, so this is a terrible situation.”

Their results showed that using a greater amount of lignin results in the formation of more lignin-PLA copolymers and minimizes the formation of polymerized lactide. However, they also discovered that the average molecular weight of PLA segments in the products was higher than expected, which indicates that not all of the lignin used in each reaction was incorporated.

Lignin produced by an industrial kraft pulping process. Source: Domtar.

Spools of polylactide (PLA) plastic used for 3D printing. Source: 3D Insider.

The researchers chose to reproduce and re-characterize the products of three synthetic routes previously used in the literature. Of these routes, two were “graft-from” methods while the third was “graft-to”. Although the third method was the most successful method in terms of lignin incorporation, it was the least “green”. Due to the high activity, low cost, and tolerance of water commonly found in bioderived materials, indium catalysts have become the hallmark of Dr. Mehrkhodavandi’s research with successful applications across a myriad of polymerization reactions.

According to Dr. Mehrkhodavandi, some polymers whose syntheses are reported in the literature have not been fully characterized and actually consist of a crude mixture of different products. “As chemists, we want to know what it is that we’ve made,” she says. “It bugs me as a chemist to not know what it is that I’ve made.” The researchers used a broad range of techniques that revealed structural information that was previously unknown. One such technique was NMR spectroscopy, which allowed assignment of relative stereochemistry for various copolymers, a vital factor in a polymer’s physical properties. Other techniques used such as mass spectrometry and infrared spectroscopy elucidated additional structural information essential to understanding the complex relationship between the synthetic method used and the resulting polymer structure.

Thermogravimetric analysis showed that some unreacted lignin was removed after purification. Source: ACS Sustainable Chemistry & Engineering.

The researchers also used thermogravimetric analysis—in which the mass is measured as the temperature increases—to check for unreacted lignin in the purified products which could cause error in determination of melt properties. To characterize how different polymers flow or melt, the researchers used a technique called melt rheology. They found that compared to linear PLAs, cyclic PLAs and star-shaped lignin-PLA copolymers have greater fluidity, a measure of how freely a substance may flow, and that cyclic polymers are less deformable than stars.

Finally, the researchers also performed rheological and mechanical measurements on blends of PLA and lignin-PLA copolymers. They discovered that while the elastic response of these blends initially improves as the concentration of lignin-PLA copolymers is increased, increasing the concentration past 10% does not improve elastic response, but does result in a sharp loss of thermal stability.

Because the lignin-PLA copolymers made in this study can be processed at relatively low temperatures, the researchers believe that lignin—a cheap and renewable type of biomass— may in the future be useful for materials applied in agricultural, packaging, and biomedical industries. However, one major challenge lies in the fact that lignin, especially from pulp and paper waste, is a complex non-homogenous mixture with inconsistent reactions and products.

Furthermore, Dr. Mehrkhodavandi says that, “part of what we were hoping to do was to […] take lignin and see if we could make it miscible with a polymer like PLA” since “the problem with lignin is that it’s not a very miscible material.” While the commercial application of lignin remains difficult, the Mehrkhodavandi group’s research is making such an endeavour a possibility in the future. In the future, Dr. Mehrkhodavandi hopes to secure funding to continue research into lignin-PLA polymers.

— Jessica Li, Jonah Adelman, Chenyu Jiang, Kevin Yau

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