A Word about Vitrimers – A Thermoset That Can Act Like a Thermoplastic

#composites, #sustainability, #vitrimer, #recyclableresin, #biocomposite, #bioresins

I have been seeing articles in several different things that I read about a new class of polymers that is rather intriguing.  These polymers are called “vitrimers”.  Polymers that were processable by covalent bond exchange reactions (movable and reconfigurable covalent crosslinks) were developed initially in the 1990’s at the University of Illinois Urbana-Champaign (UIUC).  James Economy’s group was studying thermosets and their covalent bonding schemes to determine what their topology was – essentially how does the shape of the monomer have an effect on how it polymerizes – to see if they could make modifications to these shapes by heating or cooling or slightly changing the way they are bonded together.  This group initially came up with a class of polymers based on epoxies that when heated could move their crosslink covalent bonds which allowed them to stretch into new shapes or topologies and then hold that shape when they were cooled back down. 

From "New material: vitrimers promise to impact composites” Reinforced Plastics, Volume 62, Issue 3, Denis A. Kissounko et. al.

I thought that this pic told the story of the difference between thermoplastics and thermosets when deformed.  Where vitrimers stand is right in the middle between these two types of molecular topology and where the crosslinks are.  The green bond lines in the bottom molecule are the traditional covalent crosslinks that exist in a thermoset resin system.  What this demonstrates is that very similar molecular structures can be either thermoplastic or thermoset based on the actions of these little green covalent crosslink bonds.

The history of vitrimers is fairly typical of new resins in the composites business.  Even though James Economy’s team probably made the first polymer that could change its bonds around when heated and stressed – basically a vitrimer – credit for the discovery goes to a French researcher in 2011 by the name of Ludwik Leibler, probably because he came up with the name which stuck when he showed off what he had done.  Possibly also because he got quite a bit of commercial success out of his creation. 

So, looking at the pic above, vitrimers also have these green covalent bonds, but they are different than how they are arranged in a thermoset in that they can change how they are bonded when heated making them more flexible and less resistant to movement.  This makes them a thermoset that can act as a thermoplastic and be reformable using conventional thermoplastic molding machinery and processes.  And, interestingly, they can also be welded in much the same manner as thermoplastics and when they cool back down they have the same mechanical properties as a thermoset plastic. 

Effectively what is going on is that these reformable covalent bonds or what has been called bond-exchange reactions can happen quite rapidly when the polymer is heated above what may be called a “glass transition temperature” although that term may or may not apply to vitrimers, then when the polymer is cooled down these reactions where the crosslink bonds swap around happen extremely slowly to the point where they may not be perceptible for a very long time.

The wonderful thing about these polymers is that it appears that some smart chemists have figured out how to “vitrimerize” several different commercially available resin types, including epoxies.  And, on top of that, some of the plant based epoxies are even simpler to turn into useful vitrimers.  Epoxidized plant oils are apparently some of the best oils to use to make vitrimers.  And by plant oil, by far the most common plant oil used for bioplastics is epoxidized soy bean oil.  The stuff is ubiquitous, cheap, and has a fairly simple chemistry to use to make all sorts of bioplastics.  Others that are apparently fairly easy to use are vanillin type oils and things like our old friend lignin.  Talk about an abundant organic.

Since these epoxidized plant oil based vitrimers come from renewable resources, namely plant matter, the question then becomes what happens when you are done using them.  Can you turn your resin back into something that is either useful as a new plastic or potentially useful to soil bacteria to make more of the stuff that makes plants grow? 

In one article I came across (1) when looking at this very issue is from some researchers in China who were looking at vitrimers based on a lignin derived polyphenol BDEF (4,4’-methylenebis(2-propylphenol)) that has properties similar to our old friend and nemesis bisphenol-A (BPA).  BPA is of course the root of most aerospace epoxies, and unfortunately not pleasant on the environment or to humans exposed to it.  BDEF on the other hand is a derivative of lignin that can be easily epoxidized.  What these researchers did was to make up a batch of this lignin-based vitrimer resin, impregnate some carbon fiber fabric material (T300-1K – Toray of course) to create a prepreg and hand laid up some panels for test.  They got about 60% fiber volume fraction which is basically the aerospace standard for test panels of carbon fiber / epoxy composites. 

The made up enough test panels that they could do material property characterization testing as well as some testing of the flexibility of their vitrimer concoctions.  They heated up a couple of the cured panels and showed that they could easily form them at a temperature of 220 degrees C.  And their vitrimer resin also self-healed some little cracks that they introduced with a razor blade when they heated up the panels to 200 degrees C. 

Then to demonstrate chemical recyclability, they dissolved the resin in ethylene glycol, a very common solvent, and were able to completely dissolve their cured resin from the carbon fiber leaving the original T300 fabric very nearly pristine. 

This appears to me to be just the beginning of a revolution in the high performance composites industry.  As more companies start selling their vitrimer based composites, we may see that the majors not only take an interest in this but will be bringing some vitrimer based high performance resin systems to the high end composites marketplace. 

One company that is already beginning to hawk their wares in this space is Mallinda of Denver, Colorado.  Mallinda was founded by Philip Taynton with a classmate of his Chris Kaffer.  These two were grad students at the University of Colorado Boulder Campus where Taynton was getting his Ph.D. in chemistry and Kaffer was getting his MBA.  Sound familiar?  Ph.D. scientist teams up with MBA right out of school and found a successful startup? 

While this very commonly happens at UC Berkeley, and then the company moves to a lower cost area to scale up, Mallinda did the opposite, they moved to Berkeley in 2016 into a technology incubator that Berkeley had stood up and worked to fine tune their resin system until they had a commercially viable product.  What you see above is what came of that endeavor, and they are well on their way.  They have been able to secure funding from NSF, the State of Colorado, DOE, and a Series A investment from SABIC Ventures.  That is some pretty serious stuff for a couple of grad students, so I would like to wish them well.  I’m pretty certain that we will hear a lot more from this company in the next few years because it appears that they are well on their way, and along the way have developed a world class set of business connections to boot. 

So, that’s it for this week’s post.  As always, I hope everyone that reads these posts enjoys them as much as I enjoy writing them.  And I hope people who are interested find something they can use in their lives or at least some ideas that they might be able to put into practice.  At least I hope that these make people think a bit about sustainability and some of the major issues looming before us.  And if you have any of the normal orthopedic issues that come with aging if you have had an active lifestyle, stay tuned to this newsletter and I will be giving you some more info in future posts.  I even have a surgery date – the last day in March this year.  Wish me luck.  I will have a new hip joint on April Fools Day!! 

I will post this first on my updated website – www.nedpatton.com – and then 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.  And that is especially true of the companies and research institutions that I mention in these posts.  The more we communicate the message the better we will be able to effect the changes in the industry that are needed. 

My second book, which will be out sometime next year, is a roadmap to a circular and sustainable business model for the industry which I hope that at least at some level the industry will follow.  Only time will tell.  At least McFarland announced it in their Fall Catalog.  And this time it is under a bit different category – Science and Technology.  Maybe it will get noticed – as always that is just a crap shoot.  One good thing that happened just today is that I got the proofs back for the book.  Now I’m going through them and making all of the little changes that need to be made to the book and building the index to the book.  Then when I’m done with that it goes to production and printing.  In other words – Coming Soon to a Bookstore Near You!!

Just so that everyone is reminded, I’ve included the approved cover at the end of this post.  Let me know whether or not you like the cover.  Hopefully people will like it enough and will be interested enough in composites sustainability that they will buy it.  And of course I hope that they read it and get engaged.  We need all the help we can get. 

Last but not least, I still need to plug my first book.  “The String and Glue of our World” 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 August of 2023 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 for an unsigned one, except that I have to charge for 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 are pictures of the covers of both books.