I had a conversation recently with Replas, a company that uses recycled soft plastics to make picnic tables, park benches and bollards. They have been making this product for 20 years here in Australia but lately they have had a spate of incidents in which their product has burned and they wish to explore cheap ways to reduce the flammability of their product.
Diethyl ethylphosphonate I am reminded of my time working at Applied Polymers, where I had a project that involved flammability testing of rigid foam polyurethane. In this case the formula included diethyl ethylphosphonate (DEEP) as a flame retardant. I altered the levels of DEEP and tested the flammability of the products by cutting the foams into a wafer biscuit shape that was placed on a wire mesh and moved into a laminar flame. The flame was a mixture of air and butane, with the butane coming straight from a BBQ bottle.
Years later I have not worked with polymers for some time but have just carried out my PhD in pure chemistry. I am generally acquainted with the principle that the more C—H bonds are present, the more flammable a substance will be. I have learned that perfluorohexane and it's homologues are non combustible as they lack C—H bonds. I have read someplace that the C--F bond is so stable that it does not undergo cission during combustion. One time during my PhD I decided to test the paradigm of combustion relying on C--H bonds by attempting to ignite a solvent called formamide. I found that formamide was absolutely impossible to ignite at room temperature.
Perfluorohexane and Formamide
I learned during my Honours year that cellulose is less combustible than lignin. This is why when burning white paper not a lot of heat is given off and the flame is unimpressive and diffusive. High lignin newsprint on the other hand, which is cheaper anyway but turns yellow within one year, gives a much better flame and is more suitable for starting a fire. Lignin has perhaps two carbons for each oxygen, which makes it far more hydrocarbony than cellulose which has one carbon per oxygen.
 |  Lignin |
Some time ago I obtained a book called "Flames, their structure, radiation and temperature" by A. G. Gaydon and H. G. Wolfhard. I hadn't yet had time to read it and thought it was going to be about explosions, flames in house fires and perhaps largely a treatise of mathematical formulae. To my pleasant surprise, it is actually written with the PhD chemist in mind. I have the 3rd edition from 1970. The authors include the following paragraph in their preface:
 | "Our aim is to present as clearly as possible the underlying physical processes occurring in flames. We fully realise, of course, the need for quantitative measurements, but have avoided purely mathematical discussion; indeed we have little enthusiasm for abstract mathematical treatments of combustion, these usually involving many unknown and often unknowable parameters." |
The topic of chapter 2 is premixed flames. Here we learn that the outer flame on a Bunsen burner is not the burning of the hydrocarbon/air mixture, but is actually a diffuse flame that burns due to CO and H2, which are given off by the combustion of the fuel in the centre flame. The two flames can be separated by a glass tube known as a Smithells separator. The length of this separator is important as if it is long enough, the outer flame will no longer burn due to the gases being at a lower temperature. From this, we can take that there is some flammability in other organic bonds, albeit far less. Another section of the book describes the possibility of flames from ammonia. The flammability of the H-H bond is particularly well know, less well known is the flammability of C-O and N-H bonds. Another section of the book describes flames from halogen compounds' interaction with hydrocarbons.
The CO flame has been studied at length and is discussed in the book. It has a spontaneous combustion temperature of 609 in air and 588 in oxygen. Its heat is combustion is lower than that of alkanes, but it also has a smaller lower limit of combustion of around 10%. See image.
I remember being told in high school chemistry that the top of the inner flame is the hottest part of the Bunsen flame. The chart shows that propane and butane have a hotter flame than CO. It also shows that higher alkanes such as heptane and octane give an even hotter flame. However, these produce a flame with a higher degree of carbon zero, which gives a more luminous flame but leave a black solid residue. This is because the fuel to oxidant ratio for these hydrocarbons does not give an equal stoichiometric ratio. The lower alkanes are said to give a more clean burning flame as they produce only the oxidation product CO2; an odourless, colourless gas. Hence we have: Higher alkanes = hotter but dirtier flame.
I remember being told in high school chemistry that the top of the inner flame is the hottest part of the Bunsen flame. The chart shows that propane and butane have a hotter flame than CO. It also shows that higher alkanes such as heptane and octane give an even hotter flame. However, these produce a flame with a higher degree of carbon zero, which gives a more luminous flame but leave a black solid residue. This is because the fuel to oxidant ratio for these hydrocarbons does not give an equal stoichiometric ratio. The lower alkanes are said to give a more clean burning flame as they produce only the oxidation product CO2; an odourless, colourless gas. Hence we have: Higher alkanes = hotter but dirtier flame.
It is also possible to get flames from nirates and nitrites (p340), which shows that N-O bonds can also contribute to overall flammability of a substance.
Another point to note is that some mixtures of hydrocarbons with air or oxygen are too rich for a flame to propagate and they give a better flame when nitrogen gas is added to the mixture. Thus we have a situation where N2 is not at all flammable itself but its addition to a mixture it can increase overall flammability.
C—H bonds of course have more energy in them compared to their combustion products, but another part of their flammability comes from the readiness of C—H rich compounds to enter the vapour phase and thus create a combustible mixture with air. A diffusion flame at microgravity shows a lack of yellow, which seems to indicate that only the C—H bonds are combusting, but with not enough heat given off to combust the CO and H2.
Diffusion flame on Earth and on the International Space Station.
