In short, yes they do. So, why talk about whether or not they can burn. That’s a good question, and one that gets asked every time that a composite design is proposed, especially for the military. That’s because the long answer is far more complex than a yes or no.
I used this picture from the recent Japan Airlines mishap in Japan to show that yes, in fact, composites can burn. They are after all organic materials in that the resins that hold the fibers together are hydrocarbon based and made from refined petroleum.
And, like I also said, the answer to this is more complex than a simple explanation. The flammability of composites is a very complex issue that doesn’t have one answer. That is at least partly because of the beauty of composites – there’s an infinite number of ways to put fiber and resin together to make them, as well as a very large number of different fibers and resins to choose from. Some composite materials are fairly easy to light on fire, like your common fiberglass boat with polyester resin. And some, like those made with Nomex fiber, are hard enough to light on fire that they are used in the fire resistant clothing that firemen wear when they run into a burning house.
There is a good reason that the flammability of composites has once again come into focus as a critical area to get right. The imminent embrace of electric vehicles as passenger cars throughout the world will demand it. As some of you may know, auto makers have increasingly turned to composites to save weight – and thereby fuel – in cars and light trucks. And, as we move to an electric vehicle world, the chance of battery fires only increases with each crash that occurs on the freeway, or even on city streets for that matter. Lithium Ion batteries are notorious for catching on fire when they experience the kind of damage that can occur in a bad traffic accident. And, if the electric vehicle is largely composite, like some newer bodies on electric cars, they will catch on fire as well. This poses a tremendous safety hazard not only for the occupants of the burning vehicle, but also for the car the crashed into it and to the first responders trying to get the victims of the accident to safety.
From Composites World (https://www.compositesworld.com/articles/measuring-and-improving-fire-resistance-in-composites)
There are of course NIST and FAA standards for assessing the flammability of composites and are used by different parts of the composites industry. These standards are lengthy and fairly complex, so I am only going to mention that they exist and that there are standard ways to demonstrate flammability and toxicity of the smoke that comes from burning composites.
I thought that I would make it a bit simpler for folks to understand here since this is a newsletter and not an engineering document. Mostly what is flammable in composites are the resins. While some organic fibers like polyester and ultrahigh molecular weight polyethylene (Spectra, Dyneema) are flammable, others like Kevlar and Nomex are not. And none of the inorganic fibers, carbon fiber included, are flammable. I’m going to focus here on the resins and talk a little about general classes and the flammability of each class of resins. And, I’m ranking them from most to least flammable here.
First are typical thermoplastics like polyethylene, PES, PEEK, PEK, etc. These are fairly flammable since they are thermoplastics, have a melting point that is far below that of thermosets, and can evolve a flammable gas at a fairly low temperature. They also do not have a high limiting oxygen index (LOI). This index is essentially a measure of the minimum oxygen concentration required for resins to catch fire. Thermoplastic LOI is usually fairly low, meaning it doesn’t take much oxygen around for them to burst into flame.
A little higher on the LOI index are polyesters and vinyl esters. This is because of the styrene backbone in these resins, which is rich in carbon and hydrogen just ready to take up oxygen and at the right temperature, burst into flame. This is the resin used in most boats and surfboards, so don’t get that board too close to the fireplace.
While standard epoxies are next in line, their discussion is a bit more interesting. So, I want to talk about the high LOI end of the resin flammability spectrum first – phenolic and furan resins. These resins have very high to non-existent melting points – they don’t melt. And they also have a tendency to char on their surface before they burn which puts an insulating and isolating layer of carbon on the surface of the resin so that the flame can’t spread. But, they are difficult to process, tend to be brittle, and often end up with porosity because of the difficulty in processing which produces a crappy surface finish. So, they aren’t suitable for use in a car or light truck body.
Standard epoxies lie between the polyesters and phenolics in flammability. That, however, is not the entire answer, because there are many formulations of epoxy resins and they cover the watershed in flammability performance. Room temperature setting epoxies like the West System marine epoxies tend to be lower in the LOI index – more flammable – than are the high temperature setting, more advanced epoxy resins used in aerospace. But, even aerospace epoxies will catch fire under the right conditions like having an aircraft engine catch on fire because of a major jet fuel leak that spews jet fuel all over the fuselage of the airplane (pic at the top of this post).
In the past, there have been halogenated fire retardants incorporated mostly into epoxy resins, but those got banned in the same way that halogenated refrigerants got banned. They are horrible atmospheric warming gases because they deplete the ozone layer in the upper stratosphere that is keeping harmful solar radiation away from us delicate humans and our ecosystems.
Other things like aluminum trihydroxide and other minerals that can retard flame spread or add to the carbon char at a surface and extinguish the flame have also been used for various applications. There is always a trade-off, however, because addition of a mineral dust (particulate) to the composite in general causes a loss in strength or stiffness of the material, and also adds weight to the design.
One other alternative is to use a coating that either is fire retardant or resistant, or has what is called intumescence. These last coatings will char just like phenolics do and the carbon that is left on the surface insulates the composite under it and keeps the composite from evolving a flammable gas as the resin decomposes because of the fire.
So, like I said, composites do burn, and yes, the answer can be simple. But the answer to that is also complex. There are so many different ways to make a composite material that what is done now in industry, especially aerospace and defense, is to add low flammability to the material requirements right along with stiffness, strength, density, etc. to any design specification for a composite part or system. This, as usual, puts the onus on the composites designer or fabricator to demonstrate that their part in their material has the fire resistance that is required based on the stated requirements.
This is the world I have lived in for the past 40 years or so, and I’m used to it now. In fact, I have written these sorts of requirement specifications because sometimes I’m the customer that needs this thing and I want someone to make it for me the way it needs to be made, and to meet all of the requirements – including flammability – that the thing needs to meet.
That’s about it for this week. And again, this newsletter was written on my keyboard using my fingers and my brain, not AI. And, finally, I need to remind everyone that my book is 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. 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.
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