As I talked about briefly last week, there is more to the story about the use of cellulose and lignin for development of sustainable composites. Interestingly enough, the researchers that decided to mix cellulose nanofibrils with lignin and wet spin fibers from them for subsequent carbonization weren’t initially intending these fibers for use as structural carbon fiber. They were looking for a replacement for copper wire and knew that carbon nanofibrils (basically carbon nanotubes) were more electrically conductive than copper. These little wrapped tubes of graphene conduct electricity with very little resistance.
What the researchers were faced with was the fact that it is very hard to make a long fiber just with these little tubes of graphene. They don’t stick together very well, and you can’t use them in any of the common fiber manufacturing processes. They don’t melt, so you can’t melt-spin them. They don’t dissolve in water, so you can’t wet-spin them either. And they also don’t dissolve in any organic solvents so you can’t make a slurry of them. And they don’t align well in any sort of liquid or meltable medium commonly used to make fibers. While it can be done, and some carbon fibers have been made using just these little tubes of graphene, their carbon content 22% - pretty low) and properties have rendered them not commercially viable. And, they also don’t conduct electricity because the graphene structure is obliterated in the carbonization step.
So, in the 2010’s up to the early 2020’s, several researchers worked on developing a carbon fiber precursor using a mixture of lignin and cellulose nanofibrils. Initially, researchers were looking for a conductive fiber with good structural properties. Work went on through the 2010s with a number of different additives to either lignin or cellulose with some success, but the resulting fiber was just not quite strong enough nor did it conduct electricity well enough.
In a paper in 2019 (1) a group from two Universities in Finland and one in the US – Western Michigan University, proposed using a mixture of lignin and cellulose in a hydrogel to wet-spin a fiber. A figure from that paper is above that graphically describes their process – that gloved hand is holding a resulting short fiber . This process is used in many fiber applications industrially, and is rather common and there is even industrial scale equipment available to make lots of this stuff. They were following along a path that had been trodden for a few years, but this paper does lay out a much simpler process for making these microfibers using ionic liquid (water solutions) rather than organic solvents like acetone.
And, in 2020, some researchers in Europe and Australia (2, 3) reported on successful production of carbon fiber using a mix of lignin and cellulose nanofibrils as a precursor fiber. While they and others were initially looking for a conductive fiber with acceptable structural properties, what they ended up with is a process for making carbon fiber that with much more effort can yield mechanical properties nearly as good as PAN-based carbon fiber.
They started with birch pulp – a tree that is quite common in Europe and is used extensively in the European wood products industry as a building material. So, the pulp has high availability, and is a good source of wood waste to start the process of converting that waste into usable carbon fiber. These researchers eventually concluded that this is a viable approach, and that using the right mixture of the cellulose nanofibrils and the available lignin in the waste wood pulp will eventually lead to a fully sustainable, high performance carbon fiber.
The resulting precursor fibers that these researchers came up with actually have a fairly good looking structure when you take micrographs of the cut fiber (pic below).This pic is of cross sections of the fiber where it has been cut.You can see a fairly regular structure if you look at these carefully, which is what you want to see for a precursor fiber that is to be carbonized into a structural carbon fiber.The precursor fibers made of PAN have a fairly similar look to the cross section micrographs.And these researchers have shown that the lignin content in these fibers makes them survive the high temperature carbonization part of the process that drives off everything but the carbon, with a much higher carbon content than just lignin or just cellulose fibers alone.
The bottom line here is that it appears that high performance structural carbon fiber using a mix of both lignin and cellulose from wood product and agricultural waste is on the horizon, and may, if we are all lucky, replace the petroleum based carbon fiber of today. This is very good news indeed.
That’s about enough for this week. I am going to be reminding everyone right here each week that I will be presenting one more paper toward the end of this year. This one, like the first one I presented this year, is about sustainability – a subject that all of you that have read my posts know is a passion of mine. This one will be at the Carbon Fiber Conference in Salt Lake City being put on by Composites World (https://www.carbonfiberevent.com/). In that presentation I will focus on current work in sustainability of carbon fiber in particular. I’m of course going to talk about new fibers and fiber precursors made from plants, so again, focusing on closing the circle.
So, to sign off, I am providing the pic of my book cover that I end this post with every week. And, the best place to get one is to go to my website and buy one. I will send you a signed copy for the same price you would get charged on Amazon, except that I charge $8 shipping. Anyway, here’s the link to get your signed copy: https://www.nedpatton.com/product-page/the-string-and-glue-of-our-world-signed-copy. Here’s a picture of the book, for those of you just tuning in.
(1) “Conductive Carbon Microfibers Derived from Wet-Spun Lignin/Nanocellulose Hydrogels”, Ling Wang, Mariko Ago, Maryam Borghei, Amal Ishaq, Anastassios C. Papageorgiou, Meri Lundahl, and Orlando J. Rojas, ACS Sustainable Chemical Engineering 2019, 7, 6013-6022.
(2) “Cellulose-lignin composite fibres as precursors for carbon fibres. Part 1 – Manufacturing and properties of precursor fibres”, Mikaela Trogen, Nguyen-Duc Le, Daisuke Sawada, Chamseddine Guizani, Tainise Vergara Lourençon, Leena Pitk¨anen, Herbert Sixta, Riddhi Shah, Hugh O’Neill, Mikhail Balakshin, Nolene Byrne b, Michael Hummel, Carbohydrate Polymers 252 (2021) 117133.
(3) “Cellulose-lignin composite fibres as precursors for carbon fibres. Part 2 – The impact of precursor properties on carbon fibers,” Nguyen-Duc Le, Mikaela Trogen, Yibo Ma, Russell J. Varley, Michael Hummel, Nolene Byrne, Carbohydrate Polymers 250 (2020) 116918.
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