A Review of Transition Metal-Based Flame Retardants - American

Chapter 19

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A Review of Transition Metal-Based Flame Retardants: Transition Metal Oxide/Salts, and Complexes Alexander B. Morgan Advanced Polymers Group, Multiscale-Composites and Polymers Division, University of Dayton Research Institute, Dayton, O H 45469-0160

This short review covers flame retardant chemistry other than the main classes of halogen, phosphorus, or metal hydroxide, and instead focuses on the transition metal element flame retardants that have shown some effectiveness in reducing polymer flammability. There appears to be a lot of promise in these flame retardant additives which utilize a wide range of chemistry and solid state structures to induce either char formation or highly effective vapor phase free radical inhibition for the burning plastic Not all of the materials reviewed in this paper are effective though by themselves, and this paper hopes to review some of the approaches and comment on why they may be promising new avenues for flame retardant research.

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© 2009 American Chemical Society

In Fire and Polymers V; Wilkie, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2009.

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The Periodic Table of Flame Retardants Flame retardant chemistry today for polymeric materials focuses on just a few elements of the periodic table. Group VII (halogen) is still the most commonly used, with bromine and chlorine being the main active species in organohalogen compounds use to flame retard plastics (7). Fluorine and iodine are also effective in flame retardancy, but each of these elements has drawbacks which prevent their ready use in flame retardant additives. In the case of Fluorine, the C-F bond is too strong to release fluorine into the gas phase for flame retardant work in plastics, but if the C-F compound is a gas, it makes an excellent flame extinguishing agent (Halon extinguishers). Iodine tends not to be used since the C-I bond is quite weak and organoiodine compounds are not very stable. Stability is important to a flame retardant since it will need to be ready to activate when fire occurs, and for passive methods of fire protection, such as flame retardant additives in plastics, it may be years or more before such a compound is needed to protect against an ignition source. Organoiodine compounds do not have this level of stability, especially when exposed to light. Phosphorus is another element commonly used to flame retard plastics, in both its red elemental form and in P(III)/P(V) oxidation states as both inorganic phosphate and organophosphorus compounds (2). Interesting, some other group V elements get used for flame retardancy, but only in specific structures. Nitrogen gets used in melamine (3,4) and a few other specialized compounds, as does antimony in the form of antimony oxide (1,5). Other elements such as M g (6), A l , Si (7), Β ( ο ι o=r H C V /*/ ο \

s

s

3

Tetramethyltin

\ /

CH CH

3

3

Tris(2,4 pentanediono Chromium (III)

CH

3

Tris(2,4 pentanediono Cobalt (III)

Figure 1. Organometallics and Metal Complexes use for methane flame inhibition and smoke/flame reduction in PVC



322 results for this system. Kaolin clay by itself was found to be the most effective at delaying time to ignition and reducing peak heat release rate, but when Kaolin clay was used in combination with the iron chelate treated clay in a nearly 1:1 ratio, the iron chelate enabled significant reductions in smoke release while keeping peak heat release rate low. However, this reduction in peak heat release rate by the iron chelate modified clay was still not as good as Kaolin alone. In a second example, similar polymer chelate chemistry to that used by the iron system was used to chelate cobalt and this chelate (Figure 2 middle) was used directly to flame retard polypropylene in combination with magnesium hydroxide (43). For this study, it was found that as there was a maximum level of cobalt chelate that could be added, as above this level the replacement of Mg(OH) with the chelate would result in increases in heat release and smoke production. Still, about 10 phr of the cobalt chelate in combination with 40 phr of Mg(OH) yielded a material with superior overall flammability performance as well as lowered smoke release and greatly lowered C O production. In a final example were iron, zinc, and cobalt chelates (Figure 2 bottom) that were used with kaolin clay. In this example, the iron chelate showed the best performance with dramatic delays in time to peak HRR, as well significant lowering of total HR and total smoke release (44). However, in none of these papers is a detailed mechanism described, although T G A data in each paper does suggest that the metal chelates do have an effect on the depolymerization kinetics of the polypropylene, but these results do not always correlate to the observed flammability behavior. For the example of Co, Zn, and Fe chelates used with Kaolin in polypropylene, the Co chelate delayed the peak mass loss of the polymer the most, and yet this did not translate into a delayed time to ignition or delayed time to peak H R R in the cone calorimeter. Instead the Co chelate was one of the poor performers in this system. Therefore there must be other chemistry ongoing in these systems which would explain the observed reductions in flammability and smoke release. 2

2

Summary and Future Directions From the examples presented in this paper, it is clear that transition metal materials, whether they be oxides, salts, organometallics or metal chelate complexes, can serve as flame retardants for polymers. Therefore we can draw the following conclusions about this very broad class of potential flame retardants: •

The use of transition metal compounds as flame retardants is not universal, and one must look at the thermal decomposition chemistry of the polymer to be flame retarded, as well as the form of the metal and how it can interact with that specific polymer decomposition chemistry.


323 •


•

Depending upon the form of the transition metal compound, it can have either vapor phase flame retardancy (inhibition of radical formation/ recombination) or condensed phase flame retardancy (char formation). Both types of flame retardant mechanisms appear to be catalytic in nature. The transition metal compounds are most effective in combination with other flame retardants, and when used in small amounts.

These three points are however, all we can really say about these materials. While the vapor phase chemistry for some transition metal compounds has been worked out for controlled hydrocarbon flames, what the chemistry would be in the presence of a more complex polymer decomposition fuel source is not as clear. Further, how exactly each of these transition metals catalyzes char formation through crosslinking is in most cases supposed; it is not proven through isolation of catalyst cycle intermediates or through spectroscopic analysis. One can argue that this is for good reason, since measuring chemical pathways in the condensed phase of a polymer while it is on fire is a very difficult task, and to date only post-fire analysis has served to help identify what reactions may have occurred between polymer and transition metal. Still, there is a lot of support for the hypothesis that all of these transition metal compounds work in a catalytic manner to form graphitic types of chars which further resist pyrolysis and flame damage. A recent paper showed that multiwall carbon nanotubes could be grown from combusted polypropylene in the presence of activated nickel oxides, hydroxides, and carbonates on a zeolite catalyst or organically treated clay support (45). In this paper, studies of the how the catalysts would work in converting ethylene, methane, and acetylene into nanotubes indicated that the metal had to be reduced in situ to its zero valent state to form these nanotube structures. Indeed, another paper where an organically treated clay was partly exchanged with FeCl and C u C l found that enhanced char formation was found when this clay was put into a polymer and the polymer exposed to flame. This paper also posited that the metals needed to be in a particular oxidation state to induce the observed additional carbon formation which was not pyrolyzed away by the flame (46). These two examples, as well as the other examples, make it clear that the metal has to be in the right form and coordination with other atoms (either chelates or oxide supports) to do catalytic char-formation chemistry, and therefore this presents an area for fundamental surface science and catalyst work. Research in determining what transition metals can rapidly convert hydrocarbons to graphite could yield very promising new flame retardant materials for modern material use. These materials may be even more promising from an environmental perspective since they likely would be useful in low loadings which could make recycling of the plastic easier. 3

2

There is much to be learned about the use of transition metals in flame retardancy of polymeric materials. The fundamental chemical pathways of flame


In Fire and Polymers V; Wilkie, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2009. Crosslinked Cobalt Polyamide Chelate

τ

Iron P o l y m e r C h e l a t e o n K a o l i n



Figure 2. Metal Chelates Used in Polypropylene Flame Retardancy

Metal Polyamide/Polyacrylonitrile Chelates



326 retardancy need to be understood, as well as what chemical structure the metal atom needs to be in to induce the formation of graphitic char. Practical use of the material to pass flame retardant regulatory tests still needs to be done, as well as addressing all the other issues that comes with a new technology in the very regulatory-driven field of flame retardancy. Still, even with these unknowns there is a great deal of potential success in this part of the periodic table for new flame retardant chemistry to be developed, and it is hoped that flame retardant scientists, catalyst chemists, and inorganic/organometallic chemists could come together for such a project to find the new environmentally flame retardant additives that society is asking for.

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A Review of Transition Metal-Based Flame Retardants - American

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