There is an article in the latest edition of Science (23 August 2024, pp854-860) where the NREL researchers that developed their plant-based epoxy resin fabricated a 9 meter wind turbine blade using glass fiber, carbon fiber, and balsa wood with their PECAN-15 resin and tested it. What they found was that the PECAN resin outperformed the traditional epoxies in nearly every category. This especially includes resistance to the environment (UV in particular), resistance to creep, and subsequent resistance to fatigue. Surface cracks are fairly rare with this new resin whereas that is the way most epoxies weather – creep and surface cracks due to UV degradation from the sunlight.
NREL Wind Turbine Blade Made with PECAN Resin
I have written about this before, but now that NREL’s efforts have been reported in Science, it seemed like a good time to add a bit more detail to the story and let everyone know what a breakthrough this really is.
PECAN stands for PolyEster Covalently Adaptable Network. However, it is not a polyester resin per se, it is an epoxy-anhydride formulation based on a readily available plant sugar – sorbitol. That is just the start of what sets this particular resin apart from its petroleum-based cousin which is most commonly made using propylene. And just so that everyone knows what this means, I need to add in a little graphic to this and talk about it a bit. I have to apologize here because I need to delve a little bit into the organic chemistry of this just to make a point.
What is shown below is a comparison of the molecular formulas of these two precursors in their typical organic chemistry format – with how all of the atoms are bonded together to make the molecules. Also on this graphic is one of the two molecules that makes a bisphenol-A based epoxy, epichlorohydrin. This is basically intended to show that these two epoxy resins have very different chemistry.
As you can see, propylene has three carbons as does epichlorohydrin, which is why it is used extensively in the manufacture of epoxies from petrochemical precursors. And, if you look at the epichlorohydrin you see the chlorine atom stuck out there at the end of all of this. This is what reacts so easily with the bisphenol-A to make the basics of petroleum-based epoxies. That is a chemistry that is quite a mouthful – diglycidyl ether of bisphenol-A or DGEBA. I don’t expect anyone to be able to remember that, or even understand the organic chemistry, what I do want to point out though is that epichlorohydrin has a chlorine atom and DGEBA does not. That means that the chlorine is stripped off when DGEBA is made. So, the chemistry that makes bisphenol-A based epoxies is toxic, and there is a lot of hazardous waste chemistry that needs to be properly disposed of when making standard epoxies.
That is not the case for this new PECAN resin. The chemistry to make it is not toxic. An additional benefit is that it takes a lot less energy to make this stuff than it does standard epoxies. And, on top of that, the precursor, sorbitol, is easily derived from what is left over from corn once the corn is removed from the stalk. It is called corn stover, and it is a very rich source of sorbitol. The pic to the left is a small thimble of the ground up corn stover that NREL used to make their PECAN resin.
And, I have talked here about how difficult it is to get the standard epoxies apart once they are cured into a solid resin. This is of course what makes them be able to weather pretty much anything that nature can throw at them. This, again, is not the case with the PECAN resin. It was formulated with these covalent networks that are similar to the way Mother Nature arranges things so that they stick together until they don’t have to stick together any more. The PECAN resin is as impervious to weather and environmental damage as standard epoxies, and in some cases even more so. But the covalent networks are relatively easy to take apart through a process called methanolysis. What this is really is the immersion of the resin in a methanol bath where some potassium bicarbonate is added as a catalyst. They were able to take the PECAN resin completely apart in this solvent bath in 7 days at room temperature. At elevated temperature this reaction would probably go faster, but the NREL researchers wanted to demonstrate that it could be taken apart if it is just left in the methanol / potassium bicarbonate solution for long enough.
So, NREL has solved the persistent and ever present problem of what to do with wind turbine blades at the end of their useful life.
When we add the plant-based carbon fiber that NREL developed about 10 years ago, it appears that NREL has a solution for the future that is truly circular. Using the plant-based carbon fiber and the PECAN resin, they have demonstrated that they can recycle both of these materials up to three times. What that means in years is that these materials are good for at least 60 years, and maybe more. Wind turbine blades typically last about 20 years, but with the environmental resistance of the PECAN resins, as well as their resistance to creep and surface cracking, they may in the future be able to get as much as 25 years or more out of wind turbine blades. While some may call this wishful thinking, it is not out of the realm of the possible for this to be the case.
To sum up here, it appears that NREL has completely cracked the code to making a completely sustainable and circular plant-based carbon fiber / epoxy system that is less expensive and easier / cheaper to make than traditional petroleum based carbon fiber composite systems.
Finally, I do need to say that since this was all developed at taxpayer expense in a Department of Energy laboratory, it is readily licensable to any composite materials manufacturer. The plant-based acrylonitrile for carbon fiber has already been licensed to Mars Materials, a small startup in McLean, Virginia that is organized as a public benefit corporation.
It will not be long before both of these materials find their way into some of the more mainstream companies like Hexcel, Toray, Teijin, Syensqo, just to name a few.
At least it’s a really good start.
That’s about it for this week. I hope everyone that reads these posts enjoys them as much as I enjoy writing them. As usual I will post this first on my website – www.nedpatton.com – as well as on LinkedIn. And if anyone wants to provide comments to this, I welcome them with open arms. Comments, criticisms, etc. are all quite welcome. I really do want to engage in a conversation with all of you about composites because we can learn so much from each other as long as we share our own perspectives.
I also wanted to let everyone know that I have finished the first draft of my second book. This one is about what I have been writing in these newsletters for the last 6 months or so – sustainability of composites and a path to the future that does not include using fossil fuels for either the raw materials or the process energy to make composites. Stay tuned to this space and I will let everyone know about my progress as I try to find a publisher. Hopefully that will not take too long, and there is the possibility that McFarland will pick this one up as well. I sent them a message about a week ago, so fingers crossed.
Finally, I still need to plug my first book, so here’s the plug. The book pretty much covers the watershed in composites, starting with a brief history of composites, then introducing the Periodic Table and why Carbon is such an important and interesting element. The book was published and made available last August, and is available both on Amazon and from McFarland Books – my publisher. However, 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. And as usual, here’s a picture of the book.
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