This week I’m going to finish the polyester/epoxy resin part of the discussion of PFAS and microplastics by talking about vinyl ester resins. These resins are sort of a cross between epoxies and polyesters, because to make them you start with the precursor to an epoxy like bisphenol-A (called DGEBA – the diglycydil ether of bisphenol-A), or a phenolic like Novolac and make an ester out of it through the use of one of the acrylic acids. The result is dissolved in styrene at about 35-45% by weight and used as a composite resin. Vinyl ester resins are used in high performance boat building because they are nearly as strong as epoxies, they are also a bit tougher so they don’t crack over time, and they are more resistant to attack by seawater. So, you can make a lighter weight hull that has the same stiffness and strength as if you used a polyester resin. And vinyl esters as produced for the marine industry also will cure at room temperature, unlike most epoxies. And even though these resins are based on an epoxy or phenolic precursor, they are priced between the cost of common marine polyester and the cost of aerospace grade epoxy resin. Vinyl ester resins also flow more freely than either polyester or epoxy, so they are a bit easier to handle and use as well.
This is a pic from INEOS of a boat built using their AME™ line of epoxy vinyl ester resins. They have five different formulations of vinyl esters tailored to five different industries and applications, using different phenolic or bisphenol or even urethane backbones. All of them are manufactured in largely the same manner starting with an epoxy or urethane precursor and making an ester out of that using an acrylic acid (pic of structure below). The resultant compound is then dissolved in styrene to make a liquid resin with relatively low viscosity and good workability.
This should look familiar to you if you saw my post last week. This is what is called a “vinyl group” and it is what makes vinyl esters be like polyesters. The main difference being that this vinyl group is attached to a molecule like bisphenol-A instead of just a single benzene ring. So, it is the double bonded oxygen in this group that reacts to the ether end of the DGEBA epoxy precursor, or the urethane precursor to make the resin itself. And then this structure repeats to make long chains of this stuff that can be cross-linked with the styrene that it is dissolved in to make a solid resin. This is actually very similar chemistry to polyesters, except for the addition of the bisphenol-A or urethane structure in the midst of this resin which makes it more epoxy-like than polyester resin. And that’s also what makes this resin have mechanical properties sort of halfway between polyester and epoxy.
Acrylic acid itself is made by oxidizing propylene (3 carbon atoms with one of the carbon-carbon bonds being a double bond). Propylene is a common byproduct of the manufacture of both ethylene and gasoline, so it is part of the whole petrochemical infrastructure. It is also used in the manufacture of acrylic textile fabrics, so this chemistry is also part of the microfiber problem that I have talked about for a few weeks now.
It is also the same chemistry that makes the manufacture of both polyesters and epoxies produce toxic chemistries that hang around for a long time because they are very resistant to weathering, seawater, etc. And there are of course inevitable leaks of the reactants and the reaction byproducts into the local environment where these resins are manufactured, so the same precautions are applied in the manufacture of these resins as are followed both with polyesters and with epoxies.
These resins are more resistant to water uptake than polyesters or epoxies and are very resistant to saltwater attack (commonly called corrosion in the industry) and other environmental exposures. And, since they can be cured using UV light, once cured they are also resistant to UV exposure. This makes them very useful for a number of applications, like the marine industry, home built airplanes, large chemical storage tanks that can be left outside, and also for in-service chemical process tanks because they are very resistant to even strong acid attack.
Now we need to get back to some of the potential issues with vinyl ester resins – at least the chemistry of vinyl esters and the similarity of these resins to acrylic plastics. The only major difference between a vinyl ester resin as used in composites and an acrylic plastic (or fabric) is the incorporation of the epoxy of urethane grouping in the midst of the polymer chain. This is of course what makes these resins have their good mechanical properties. The resistance to water uptake, saltwater corrosion, and UV resistance come from the fact that the vinyl ester resins only cross-link at a limited number of sites along the chain rather than at every double bonded oxygen like polyester resins do. This also makes vinyl ester resins somewhat more flexible (tougher) than either epoxies or polyesters.
It does not, however, make their chemistry and the chemistry of their manufacture and use less toxic. The use of styrene as a diluent has the same issues that we talked about last week with polyesters. And an additional wrinkle with vinyl ester resins is that the catalyst that is the most common since it is the best at making the resin cross link is based on a cobalt oxides. Cobalt oxides have been banned in Europe for nearly 20 years at this point, and styrene is about to be banned as well, even though the boat building industry has significantly cleaned up their act since the 1970’s with regard to styrene. In the US we don’t have quite as many restrictions on the use of these chemistries as there are in Europe, but we are not that far behind.
Fortunately the composites industry has taken note of this and is in the process of developing not only plant-based sources for these chemicals, they are also developing different means of curing them so that they end up with a solid resin with similar mechanical properties and environmental resistance without the toxicity of cobalt oxides and styrene. Work in this area is focused on using different transition metals than cobalt to initiate the reactions, replacing the styrene with other similar compounds that do not have the neurotoxicity of styrene, and also using UV light to cure the resins rather than having to use any catalysts. All of this is possible but has not been actively pursued until recently because all of them had been deemed to be more expensive than the traditional ways of making these resins. Fortunately that is changing.
And this is also true of the textile industry, which is already making a transition to more natural fibers and non-toxic or plant-based chemistries to enable themselves to be called “green” and/or just to be environmentally responsible companies that want to be sustainable.
This and the fact that the composites industry is changing the way that they make vinyl ester resins and are even looking for replacements for styrene that are potentially plant-based is the good news in this whole line of chemistry. The fact that acrylic fabrics have started to get a bad reputation because they are not sustainable, including the fact that they have a tendency to pill more than wool or cotton or other natural fibers, is also good news since these fabrics are made using acrylonitrile which is a very similar chemistry to the precursors for vinyl ester resins.
That’s about it for this week. This is the next installment of this multi-part series on this topic. Next week I’m going to go back to talking more about sustainability and recycling of composites and will return to the PFAS and microplastics sometime later this year. There is some very good news on the circularity of fiberglass from wind turbine blades that I need to report on next week and I want to get back to this topic because there is a lot going on that needs to be highlighted.
As always, I hope everyone that reads these posts enjoys them as much as I enjoy writing them. I will post this first on my website – www.nedpatton.com – as 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.
My second book is now completely in the hands of my publisher. Most of you know that it 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. The title of the book, at least for now, is “Close the Circle, A Roadmap to Composite Materials Sustainability.” It truly is a roadmap which I hope that at least at some level the industry will follow. Only time will tell.
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 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’s a picture of the book.
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