#composites, #sustainability, #fibers, #resins, #bio-based, #natural_fiber, #plant-based, #resin_recycle, #sustainable, #recyclable_resin, #recycling, #vanillin, #lignin, #pasta_waste
We talked a bit about sustainable carbon fiber last week, so this week I need to provide another piece of the puzzle – sustainable resins. There is some exciting work being done in labs around the world to develop new formulations of epoxy resins and also methods of dissolving or disassociating epoxy resins so that they can be removed from the carbon or glass or other fiber. The goal is to recycle epoxy-based composites and potentially reconstitute what comes off of the fibers into epoxies so that they can be used again.
One such research project that is bearing fruit is being done at the National Renewable Energy Lab in Golden Colorado (1). This project was undertaken at NREL sponsored by DOE’s Vehicle Technologies Office, so it is directly tied to the burgeoning use of carbon / epoxy composites in cars and trucks, both of which are beginning to use carbon / epoxy composites in newer models.
What the NREL researchers did was to develop a newer formulation of epoxy from bio-based precursors and what is called an anhydride hardener that is easier to take apart when it is at the end of its life. They have introduced linkages in the epoxy backbone that are much more easily broken apart by a process called methylation. What this process does is to break some of the bonds in the epoxy, targeting mostly the cross-linking bonds that make the resin solid, and adding methyl groups (one carbon with three hydrogens) to each side of the cross-link bond freeing up the epoxy resin.
The neat trick that the NREL researchers came up with is that this process can happen at room temperature by using the right set of catalysts. What this means is that the carbon fiber that gets released from the epoxy is completely reusable since it hasn’t been damaged or degraded in any way. So, this is a completely recyclable and reusable p[lant-based epoxy resin.
There is another group of researchers from two universities in Italy that use waste flour from pasta factories. The waste flour was “epoxidized” using UV light and ozone along with some processing reported in a paper (2) to add oxygen atoms to the hydrocarbon chains in the waste flour. What they got was a high yield of usable epoxy resin that just needed the addition of a hardening agent to turn it into solid epoxy resin. And they used a plant-based hardener to do it. So, their epoxy is fully plant based. In addition, the formulation that they used had what they called a “cleavable amine” hardener based on Recyclamine™ from Connora Technologies. And, while the Italian researchers made a completely plant-based epoxy it’s mechanical properties were slightly inferior to typical commercial epoxies made using petroleum precursors. In their report, they identify areas of potential improvement in these properties, so the work continues until they perfect the process. And this report was from 2021, so it is feasible to think that within a very short span of years, we could be seeing high performance epoxy resins from pasta waste. But it probably wouldn’t make a very good mix with marinara sauce.
In another paper from researchers in the Forest Products industry in China and the School of Chemistry at the University of Birmingham in the UK (3), they used tung oil from forest products waste material to create a diglycidyl ether of bisphenol-A (our friend from the petroleum based epoxies). The hardener that they used created what is called a “vitrimer” which is a plastic that has thermoplastic properties at high temperatures, and thermoset properties at lower temperatures. This makes their epoxy easily recyclable because it can just be melted out of the carbon fibers at high temperature. Alternatively, using an ethanol-NaOH mixture (sodium hydroxide) the ester bonds can be broken and the resin dissolves leaving essentially a virgin carbon fiber. This process is also in its infancy and not only has not been scaled up yet, but they are still working out some of the mechanical and chemical property kinks in this resin system.
In yet another paper from researchers at the Eindhoven University of Technology in The Netherlands (4) published this year by the American Chemical Society, rather than using bisphenol-A, they used a vanillin type structure made from lignin (our glue friend in all plants) to create a monomer to replace bisphenol-A and made epoxy resins with their new vanillin structured backbone. This new resin has acetal, aldehyde, and oxirane groupings, and can be cured using the newer plant-based amine hardeners. I realize that all of the organic groupings are somewhat of a mystery, but it is good to think of them more as flavor compounds made by Mother Nature than anything. Vanillin structures we are all very familiar with because natural vanilla extract is used in nearly all baked goods. And acetal itself is an important flavoring compound in your favorite whisky. Here’s what it looks like. It is also called Poloxymethylene (POM). In this structure, there are commonly aromatic (benzene) rings attached where the figure below has R1 and R2.
This is really an exciting time for these resins, and since there is so much work going on both in the development of plant-based recyclable epoxies and plant-based precursors for carbon fiber, there is every possibility that we are on the verge of a complete revolution in the carbon / epoxy composites business. While the move from the petroleum-based resins and fiber precursors to entirely plant-based organics will take time, the motion in the industry is in the right direction.
That’s about enough for this week. I will be writing one more of these before the holiday break, and will be back in the second week of January. I need a break and I’m sure all of my readers need one as well, So, as usual, I need to sign off with the fact that my book is out and ready for anyone to purchase. 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. Get one for someone you know that is curious about these materials and give it to them as a gift. 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, for those of you just tuning in.
(2) Esposito Corcione, C.; Ferrari, F.; Striani, R.; Visconti, P.; Greco, A. Recycling of organic fraction of municipal solid waste as an innovative precursor for the production of bio-based epoxy monomers. Waste Manag. 2020, 109, 212–221.
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