Up In Flames

As I am on a chemistry placement for my chemistry degree, I feel I should dedicate this blog entry to the chemistry I’m doing this year!

DuPont Teijin Films (DTF, the company I’m working for this year) is a company which, unsurprisingly, designs and manufactures film; mainly based on polyethylene terephthalate, for use in all sorts of things. My main project so far has been working with flame retardant films, which, in my opinion, are quite important in this safety focused world. Of course, I can’t give you specifics of what we make at DuPont Teijin Films but what I can do is give you an overview of the general chemistry of how flame retardants work.

Hydrogen recombination and radical scavanging by a phosphorous based flame retardant. M is a third body species.

Hydrogen recombination and radical scavanging by a phosphorous based flame retardant. M is a third body species.

There are several different ways in which a flame retardant can prevent the spread of flames but first you’ll need to know about what actually happens to a polymer during combustion. A polymer, on combustion, will undergo various chain scissions (polymer chain breaks up) to form radical species – these scissions can either be at the end of the chain at the carboxylic acid group or random at any point in the chain. The radicals formed in chain scission can then propagate further breakdown, resulting in destruction of the polymer. Functional groups which have been added to the polymer to give desired properties can also be stripped off during a fire – this is known as chain stripping.

Many flame retardant additives used in the past included heavy metals (e.g. lead or halogens) but these can often be harmful or toxic and therefore safer and more environmentally friendly alternatives are sought after. A popular choice for the base of flame retardant additives is now phosphorous.

A standard PET based film after a burn test

A standard PET based film after a burn test.

Phosphorous flame retardants work in several ways. Firstly, at high temperatures in complete combustion, the phosphorous can oxidise to phosphorous pentoxide, P25, which can then hydrolyse into phosphoric acid. Phosphoric acid then acts to form a char on the surface of the film (a carbonous layer) which forms a barrier to any combustible gases formed by the polymer pyrolysis (breakdown) to the fire; thus reducing the fuel source of said fire. Phosphoric acid allows the cross linking necessary between phosphorous additive containing polymer chains to occur which results in the formation of the char barrier. This layer also protects the polymer from further breakdown to an extent by shielding it from some of the heat of the fire; the char also means that phosphorous based flame retardants tend not to produce any toxic gases during combustion as most of the phosphorous will be present within the char.

Besides char formation, phosphorous based flame retardants work in the gas-phase. Molecular phosphorous and phosphorous compounds can act as radical scavengers and promote hydrogen recombination, thus reducing propagation of the breakdown of the polymer and the fuel source available to the fire. This is one of the major benefits of phosphorous based flame retardants, as both charring and radical scavenging can occur at the same time.

A PET based film with flame retadant additive after a burn test.

A PET based film with flame retardant additive after a burn test.

One of the benefits of working on flame retardant films at DTF is that I get to test all the films we make, which basically means my boss and I spend a lot of time just setting film on fire to see how well it might hold up. These tests are completely controlled and our films are quite good so they tend not to burn very much – but we do compare the flame retardant films to standard ones, which are much more dramatic when they burn. I’ve even been on a trip to Warrington to a large testing site where they do much bigger and bolder flame tests, which was quite a sight I must say!

Claire Brodie

Photos property of DuPont Teijin Films
Main source: M. Rakotomalala, S. Wagner and M. Doring, Materials, 2010, 3, 4300.

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