#20 from R&D Innovator Volume 2, Number 1          January 1993

Plastic from Trees
by John Meister, Ph.D.

Dr. Meister is an associate professor of chemistry at the University of Detroit.

I didn't know what to expect when I started trying to alter lignin, the polymer that binds cellulose fibers together and gives trees their sturdy structure.  I certainly didn't believe the work would lead to a new and important type of chemistry.   

But from previous work with lignin in the Phillips Petroleum Company research labs, I knew it comprised the second largest mass of polymer made on this planet.  I also knew that, while nature is very good at making lignin as a stiffening agent for trees, humans are not so good at finding uses for it.  Most of the lignin separated from trees during paper production is burned or dumped; its complicated chemistry is wasted.  Since I had just completed a successful research effort to convert starch into water-treatment chemicals, I felt I might be able to find new applications for lignin.

With my optimistic blinders firmly affixed, I set out to build a side chain made from a monomer called 2-propenamide, then bind it to lignin with a reaction called grafting.  I hoped the resulting compound could be used as a water-thickening agent. 

Unfortunately, no one knew how to graft effectively to lignin.  From a long review of the literature, I decided to break a bond on lignin with cerium ion and have that broken bond attack one of the monomer molecules.  This would leave a broken bond at the end of the monomer and cause the reaction to attack other monomers.  Ideally, a chain would then form on the lignin, one monomer at a time.

To begin this project, I gave two undergraduates a list of chemicals and recipes to try out various reactions.  For one reaction, we borrowed a bottle of the solvent dioxane.  Although dioxane is usually contaminated with peroxides, we didn't bother to purify it since we figured peroxides might actually help our reaction.  

The dioxane reaction looked very promising as it thickened and formed a water-soluble solid. (Unlike lignin itself, grafted lignin should be water-soluble.)  Furthermore, the mass of solid recovered was close to the sum of the lignin plus the monomer.  I was excited by these results and put in extra hours in the lab.

A Non-repeating Reaction

We spent the summer running reactions and developing tests for the products.  Our data continued to indicate that we were making grafted lignin.  But when the summer ended, I could no longer repeat the reaction.

I wish I'd applied the fundamental rule of trouble-shooting:  "Identify and investigate what changed between the successful process and the unsuccessful process."  That would have explained the sudden failure much quicker.

Instead, I wasted time checking the purity of the key reagents, the cleanliness of our equipment, and the quality of the nitrogen gas we used to blanket the reaction.  When I finally checked whether the dioxane solvent was causing the failure, I realized we'd lost critical information.  The borrowed bottle of dioxane had been exhausted just as summer ended, when the reaction failed and the undergraduates returned to class.

Although we wondered if peroxides in the original dioxane had promoted the reaction, weeks of irradiation and oxygen saturation experiments, all intended to produce peroxided dioxane, yielded no new grafted lignin.  When I finally talked to the people who'd supplied the original dioxane, I learned it had been dried over calcium chloride.  The contamination of the solvent with a chloride salt should not have changed our reaction product, as chloride salts are very stable and play virtually no role in initiating this type of polymerization. 

But chemistry is an experimental science, so we went back to the lab and tried the reaction again, this time with chloride ion added to the irradiated, peroxided dioxane.  Eureka!  The grafting reaction worked.

As we began a frantic rush to answer the seemingly endless questions about this new reaction, one question seemed paramount:  "Are we really grafting onto lignin?" 

Normal lignin is a complex tangle of chains, and I assumed that we had added chains of polymerized propenamide to the lignin.  But it was incredibly difficult to analyze this new material.   To find the one bond in 10 million that distinguishes lignin grafted to a polymeric side chain from lignin molecules mixed with polymer molecules is to find a needle in a haystack.  Yet we had to prove the identity of our product before we could fairly claim to have produced it.

We spent the next two years developing analytical methods that proved we'd made grafted lignin.  In 1984, we patented the material and method and published the results.  This led to a government grant that allowed us to pick up our research pace.  The grant meant I would receive a summer salary, a welcome change from the previous five years.

The Material May be Useful

As we investigated the properties of grafted lignin, a series of inventions began to flow from the lab.  Unlike lignin, the modified material was water-soluble and had the properties of a water-thickening agent.  It has potential to purify water for industrial processes, dewater sewage sludge, and serve as a drilling-mud additive for oil wells.

I certainly have enjoyed the interest among government agencies and corporations in my findings, not to mention their research support.  But I am enjoying even more the fact that other laboratories are currently working to develop my work into useful products. 

Although these new materials were triumphs in their own right, each time I described these grafted lignins, I had to explain that the starting material was derived from wood.  That got me wondering if we could graft to lignin while it's still in the wood.  Although I read a great deal about the structure and composition of wood while pondering that question, the direct route to the answer led right through the lab!  

We prepared a series of reactions and, after a year of experiments to identify trends and prove synthesis, showed that we could graft lignin directly in pieces of wood.  We then used styrene instead of 2-propenamide as the monomer.  That discovery, in 1989, led to a group of recyclable composites based on plastic and wood.  These can be heat-molded into any shape and are based on a renewable material.

While there have been other reports of altering wood to make it compatible with plastic, our reaction seems to be the most efficient and allows the wood to bind with many types of plastics.  My finding of a method to "plasticize" wood is attracting even more attention than the original grafted lignin discovery.  The material is lighter than normal plastics, and it could make humidity-tolerant furniture.  It will be fun to see how this discovery develops.

A Rock in the Pond

An invention is like a rock hitting a pond—its waves radiate in all directions.  In chemistry, an inventor can ride one of these waves.  A new reaction can carry us and a bunch of new molecules to a host of applications. 

But there's an irony here.  When I look back, I realize that 12 years of breakthroughs all originated with the borrowing of a contaminated bottle of solvent.  I shudder at the thought of trying to develop another grafting reaction for lignin because they are hard to produce.  Yet I don't dwell on the fact that all of our developments depended on one fortuitous "borrowing."  I can't—I have a wave to ride.