View From The Cab

By David Tollefson, Columnist

Being still a grower of corn, the number one crop in the whole USA as well as Minnesota, I’m all for new uses for corn other than animal feed, human food as well as fuel in our vehicles.

The most recent Minnesota Farm Guide has an interesting cover story with the above title, written by Andrea Johnson.

Under the main title, is a sub-title “Corn kernels/wood lignin could make strong compostable plastic.”

Speaking about lignin, it reminds us students from the St. Paul campus of the U of M years ago. We had a world-renowned dairy professor named W.E. Petersen, a native of Denmark, who had BS, MS, and PhD’s in 1916,1917, and 1928 respectively. He was a fascinating teacher, mostly just plain lecturing without notes or other teaching aids.

At the end of the class he would pull an old envelope or whatever he had for notes out of his pocket and declare, “well, I guess I got it all.”

Peterson would mention “lignin” occasionally, mostly in reference to the amount of fiber in alfalfa haylage, alfalfa hay, or corn silage.

So with that background, here’s the column:

Creating a high-quality compostable plastic using corn is the goal of Daniel Krajovic, graduate student at the University of Minnesota working in the Hillmyer Research Group. He’s taken on a lab project that studies the potential to combine corn kernels with wood lignin. The work is funded by the Minnesota Corn Research & Promotion Council.

Polylactic acid, or PLA, is a compostable, corn-based plastic with the potential to replace oil-based plastics, but its poor mechanical and thermal properties limit its application.

In the past, PLA’s properties have been improved by blending it with nondegradable rubbers, which complicates its end-of-life processing.

Krajovic has a different strategy. “My route to upgrading PLA is to chemically ‘join it at the hip’ with a different polymer, which can be derived from tree lignin,” he said. “So, the entire plastic can be bio-based and industrially compostable.”

Making plastic out of corn involves milling kernels into starch, which is then hydrolyzed with acids or enzymes to produce dextrose – a sugar. Bacterial fermentation of the dextrose produces lactic acid.

The lactic acid is further processed to produce a monomer (a molecule that can be bonded to other molecules), called lactide, which is then polymerized into PLA.

This process has been around since the late 1990s, Krajovic said. Minnesota’s own NatureWorks, LLC is one of the world-leading producers of PLA.

Corn PLA’s largest market is rigid packaging. That fits right in with Krajovic’s work at toughening corn PLA with wood lignin.

There is not yet a commercially viable process  to incorporate lignin into plastic, so he uses a nonrenewable source of a lignin-derivable monomer in the lab. He hopes that will change before he finishes his graduate work in another two and a half years.

To turn lignin into plastic, it must first be broken down into small parts. These parts then undergo several chemical reactions to transform into a liquid. Then, this liquid is reacted with lactide made from corn. The combination creates PLA with enhanced mechanical properties. After this product is purified, it is melt-pressed into testing specimens. These can be plastic films or plastic bars ready for mechanical and thermal testing.

From his experiments, Krajovic has shown that his corn/wood lignin PLA can hold its shape at temperatures greater than 275 degrees Fahrenheit.

“The lignin component also makes it far more ductile – so we can stretch it much farther before it breaks,” he said. “Additionally, this material is transparent, which is favorable for plastics in the rigid packaging space.  Most of the conventional bulk-blending methods produce opaque materials that you can’t see through.”

With its impressive combination of properties, Krajovic  envisions the corn/wood lignin PLA to eventually be used as storage bins, food containers, Tupperware or water bottles – with the crucial bonus of being industrially compostable.

As the synthetic process matures, he relates, there will be more opportunities to appeal to sustainability, including greener catalyst selection and reduced solvent use.

“It’s exciting work in the laboratory, and I’m grateful for the opportunity to support corn upcycling,” he said.

Krajovic doesn’t want to limit his perspectives to the lab bench, however. 

“I’m always looking to learn more about the real state of affairs in growing renewable feedstocks to be more conscious of stakeholders in my work,” he said. “And I’m so grateful for farmers’ skills and work ethic in sustainably growing corn. There wouldn’t be any of us researching PLA and using it to challenge oil-based plastics without you!”

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On a recent Tuesday (annual election day for the state’s townships), we had a very persistent southeast wind that blew most of the next night.

The next day I had at least three calls to come with my 6430 to widen driveways and township roads, and also pull out three different vehicles stuck in snowbanks on Blue Mounds township roads. One fellow whose driveway I widened said he’d lived there 22 years, and never seen anything quite like the persistent drifting from the northwest and southeast.

So I was glad to help out, rather than just sitting in my house looking out the windows.

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Please contact David Tollefson with thoughts or comments on this or future columns at: adtollef@hcinet.net