Synergistic Aspects of the Combination of Magnesium Hydroxide and

Downloaded by UNIV MASSACHUSETTS AMHERST on September 27, 2012 | http://pubs.acs.org Publication Date: November 24, 2005 | doi: 10.1021/bk-2006-0922.ch016

Chapter 16

Synergistic Aspects of the Combination of Magnesium Hydroxide and Ammonium Polyphosphate in Flame Retardancy of EthyleneVinyl Acetate Copolymer 1

1

2

Serge Bourbigot, Sophie Duquesne, Zakia Sébih , Sébastien Ségura , and René Delobel 2

1

2

Laboratoire Procédés d'Elaboration de Revêtements Fonctionnels (PERF), École Nationale Supérieure de Chimie de Lille (ENSCL), B.P 108, 59652 Villeneuve d'Ascq Cedex, France Centre de Recherche et d'Étude sur les Procédés d'Ignifugation des Matériaux (CREPIM), Parc de la Porte Nord, Rue Christophe Colomb, 62700 Bruay-la-Buissière, France 2

In this work, flame retardancy of ethylene-vinyl acetate copolymer ( E V A ) is investigated incorporating Mg(OH) (MDH) as flame retardant combined with ammonium polyphosphate (APP) as potential synergist. It is shown that A P P is a synergistic agent in E V A - M D H formulations in terms of LOI and cone calorimetry. The interactions between A P P and MDH are studied using Mg and P solid state N M R . The formation of magnesium phosphate stabilizing phosphorus in the system and it is proposed that the combination A P P / M D H provides a physical/thermal barrier protecting the substrate; this barrier is constituted of magnesium phosphate glass and MgO-like ceramic. The degradation of the polymeric matrix is slowed and the flow of flammable molecules which issue from the degradation of the polymer is reduced. 2

25

200

31

© 2006 American Chemical Society

In Fire and Polymers IV; Wilkie, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2005.

201

Introduction A flame retardant should inhibit or even suppress the combustion process. Depending on their nature, flame retardants can act chemically and/or physically in the solid, liquid or gas phase /. They interfere with combustion during a particular stage of this process, e.g. during heating, decomposition, ignition or flame spread. Metal hydroxides such as Al(OH) or Mg(OH) achieve their effect by decomposing endothermically with the release of water, which cools the substrate to a temperature below that required for sustaining the combustion process 2. The main effect here is to delay the ignition time 34. A ceramic-like protective layer is then formed (e.g., aluminum or magnesium oxide) which slows the degradation of the polymer and reduces the flow of flammables molecules 4. Ethylene-vinyl acetate copolymers (EVA) are widely used in low-voltage electrical wires 5; their major shortcoming remains their great flammability. One goal of this work is to reduce the flammability of EVA by the incorporation of flame retardants. This can be achieved using Mg(OH) (hereafter called MDH) but high loading must be used to get high limiting oxygen indexes (LOI) and V-0 rating in the UL-94 test 6. Previous studies demonstrated the benefit of using a combination of zinc borate with MDH in EVA 47. Zinc borate acts as a synergist in EVA-MDH formulations, e.g., LOI is increased by 40% and the total heat release in the cone calorimeter (external heat flux of 50 kW/m ) is decreased by 50% at the same loading of flame retardant (i.e. MDH/Zinc borate). It was postulated 8 that zinc borate degrades into boron oxide which plays the role of a binder in the protective MgO-based ceramic, reinforcing the protective effect of the ceramic layer. This mechanism suggests the formation of a glassy coating, and the idea in this work is to substitute zinc borate with another type of glass former: phosphates. Phosphates, such as ammonium polyphosphate (APP), are known to be both efficient flame retardants and glass formers (formation of phosphorus oxides at high temperature) /. The network structure of phosphate glasses is based upon a tetrahedral P 0 structural unit. The structural behavior of phosphate glasses is well suited for accommodation of various modifier cations (such as, for example, Mg ) which themselves may stabilize the network. So, it is expected that phosphate will combine these two effects to reinforce the action of MDH in EVA. In this paper, the action of phosphate in EVA-MDH system is examined using the usual fire testing protocols (cone calorimetry by oxygen consumption


3

2

2

2

4

2+


202 and limiting oxygen index (LOI)). The potential interactions between phosphate and MDH are investigated using thermal analyses and Mg and P solid state NMR. The role of phosphate in EVA-MDH formulation is then discussed. 25

31


Experimental EVA (Elastane E0119) containing 19 mol.-% vinyl acetate was supplied by ExxonMobil. MDH was a commercial grade (Magnifin H5 from Matinswerk), and APP was supplied by Clariant (Exolit AP422). Tablel gives the composition of the formulations. Table I. Composition of the FR formulations Formulation EVA-MDH EVA-MDH/APP (50-x/x) *x: percentage

MDH (wt.-%) 50 50-x*

EVA (wt.-%) 50 50

by weight ofAPP in the formulation,

0% 395°C), only one band, centered at -26 ppm, is observed suggesting the formation of magnesium phosphate material. In phosphate material, the network is made of P 0 tetrahedra that can be classified according to the number of bridging oxygen atoms per P 0 unit (Q ). The chemical shift at -26 ppm is evidence of the formation of Q middle groups containing two bridging and two non-bridging oxygen atoms. At 500°C, an additional broad band, centered at -33 ppm and at 0 ppm, appears suggesting the presence of Q branching groups (-33 ppm) and orthophosphate (0 ppm) due to thermooxidative degradation of the phosphate species. 4

31

25

4

4

n

2

3



209

| — i — i — i — i — | — i — i — i — i — | — i — i — i — i — | — i — i — i — i — | — i — i — i — i — | — i — i — i — i — | — i — i — i — r

10

0

-10

-20

-30

-40

-50

ppm 31

Figure 8. MAS-DD P NMR of MDH/APP (50/50 (wt/wt)) versus the temperature of treatment

Figure 9 shows the interactions between APP and MDH upon thermal degradation. The destabilization of the system occurs first between 250°C and 550°C. According to the discussion above, this can be assigned to ammonia and water evolution because of an acid/base reaction between MDH and APP (250°C-350°C) and because of the condensation of phosphate and the formation of magnesium phosphate. At higher temperatures, the stabilization of the system is due to the formation of a stable magnesium phosphate material. Note that formation of magnesium phosphate was confirmed by X-ray diffraction and further characterization will be published in a separate paper. At T < 360°C, the interactions between APP and MDH create the destabilization of APP. There is formation of polyphosphoric acid (PPA), which is able to solubilize MDH. It permits reactions between PPA and MDH (and/or MgO resulting from the degradation of MDH) to make magnesium phosphate. At this stage, short -O-P-O-Mg-O-P- chains are formed 13. As was mentioned above, no MgO was detected. An explanation of this might be that (i) MgO formed reacts immediately with APP to form magnesium phosphate (not detectable by Mg NMR) or (ii) MgO surfaces adsorb water to form MDH. At T > 360°C, there is formation of magnesium phosphate and of MgO. Magnesium phosphate grows around MgO grains and a protective cementitious matrix develops. 25



210

Figure 9. Curve of mass difference of MDH/APP (50/50 (wt/wt)) between the experimental TG curve and the calculated one (calculation made by linear combination of the experimental TG curves of the pure components) (air flow, heating rate = 10°C/min)


211 To summarize, heating a mixture of MDH and APP yields the formation of magnesium phosphate material where the Mg cation acts as a network modifier. The formation of this material prevents the volatilization of phosphorus oxides which remain as a protective layer in EVA-MDH/APP.


Conclusion In this work, we have shown that APP is a synergistic agent in EVA-MDH formulations in terms of LOI and cone calorimetry. The study of the interactions between MDH and APP shows that the presence of MDH prevents the volatilization of phosphorus oxides. The formation of magnesium phosphate material is suggested and might explain the high thermal stability of the protective layer. In the system EVA-MDH containing 5 wt% APP (best formulation), it is proposed that the layer consists of a magnesium phosphate glass stabilizing phosphorus and in an MgO-like ceramic. This combination provides a physical/thermal barrier protecting the substrate. The degradation of the polymeric matrix is slowed and the flow of flammable molecules which issue from the degradation of the polymer is reduced.

Acknowledgment The authors are indebted to Mister Pankewitch from CREPIM for skilful experimental assistance in cone calorimeter experiments. NMR experiments were made in the common research center of the University of Lille, Mister Bertrand Revel is acknowledged for helpful discussion and experimental assistance.

References 1. 2. 3. 4.

5. 6.

Bourbigot, S.; Le Bras, M. Flammability Handbook; Troitzsch, J., Ed.; Hanser Verlag Pub.: Munich, 2003, pp 133-157. Hornsby, P.R.; Watson, C.L. Plastic and Rubber Processing and Applications, 1986, 6, 169. Bourbigot, S.; Le Bras, M.; Leeuwendal, R., Shen, K.K.; Schubert, D. Polym. Deg. & Stab., 1999, 64, 419. Bourbigot, S.; Carpentier, F.; Le Bras, M.; Fernandez, C. Polymer Additives; Al-Malaika, S.; Golovoy, A., Wilkie, C.A., Eds.; Blackwell Science Pub.: London, 2001, pp 271-292. Ray I., and Khastgir D. J. Appl. Polym. Sci., 1994, 53, 297 Rothon R.N. Particulate-Filled Polymer Composites, : Particulate Fillers used as Flame Retardant; Rothon, R.N., Ed.; Longman: Harlow, 1995, Chapter 6.


212 7. 8.

9.


10. 11.

12. 13.

Carpentier, F.; Bourbigot, S; Le Bras, M.; Delobel, R.; Foulon, M. Polym. Deg. & Stab., 2000, 69, 83. Bourbigot, S.; Carpentier, F.; Le Bras, M. Fire and Polymer, ACS Symp. Ser. N° 797; Gordon, N., Wilkie, C.A., Eds.; American Chemical Society: Washington, DC, 2001; pp. 173-185. MacKenzie, K. J. D. ; Meinhold, R. H. American Mineralogist, 1994, 79, 43. Amoureux, J.P.; Fernandez, C.; Carpentier, L.; and Cochon, E. Phys. Stat. Sol., 1992, 132, 461. Camino, G., and Luda, M.P. Fire retardancy of polymers: the use of intumescence; Le Bras, M.; Camino, G.; Bourbigot, S.; Delobel, R., Eds; Royal Society of Chemistry: Cambridge, 1998; pp 48-63. Fayon, F.; Massiot, D.; Suzuya, K.; Price, D.L. J. Non-Cryst. Solids, 2001, 283, 88. Soudée, E.; Péra, J. Cement Concrete Res., 2000, 30, 315-321.


Synergistic Aspects of the Combination of Magnesium Hydroxide and

Recommend Documents