USE OF INORGANIC PHOSPHATES IN FIRE RETARDED THERMOPLASTICS - A REVIEW. S. DUQUESNE, M. LE BRAS, S. BOURBIGOT and R. DELOBEL

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1 USE OF INORGANIC PHOSPHATES IN FIRE RETARDED THERMOPLASTICS - A REVIEW S. DUQUESNE, M. LE BRAS, S. BOURBIGOT and R. DELOBEL Laboratoire de Geênie des Procedes d'interactions Fluides Reèactifs - Materiaux, E.N.S.C.L., Universite des Sciences et Technologies de Lille, BP. 108, F Villeneuve d'ascq Cedex Abstract One of the principal classes of flame retardants used in organic polymers is the phosphorus-compounds. In particular, inorganic phosphates have flame retardant properties of interest in thermoplastics. Ammonium phosphoric acids salts were first recommended two centuries ago and are still used today. However, new legislation is emphasizing the development of new flame retardant additives which are environmentally friendly. This paper reviews the different kind of inorganic phosphates used as flame retardants. It includes the use of red phosphorus, ammonium phosphates. We will also describe intumescent systems and low melting phosphate glasses. INTRODUCTION In many fields such as electrical, electronic, transport or building, the use of organic polymers is restricted by their flammability which increases fire risk and fire hazard, i.e. fire consequences either on humans or on structures. Thus there is now an increasing demand for fire retardant systems (FR) capable of reducing both these risks. One possible approach is the use of phosphorus compounds whose effective flameretardant performance is well known 1-3. In particular, inorganic phosphates are widely used in fire retarded thermoplastics. Some of them have a long history but new products are still being developed. Indeed, they present high FR performances for generally, low loading and enable to preserve when compared with hydroxides-based systems, the physical properties of the polymeric matrix. Phosphorus Research Bulletin Vol. 10 (1999), 88

2 This paper presents a review of the use of inorganic phosphates in FR systems including red phosphorus, ammonium phosphates, P-based intumescent formulations and low melting phosphate glasses systems. RED PHOSPHORUS This allotropic form of phosphorus is relatively nontoxic and is not spontaneously flammable but is easily ignited. It is a polymeric form of phosphorus having thermal stability up to 450 Ž. Its high flameproofing efficiency was recognized in the sixties. Red phosphorus is highly efficient in oxygen or nitrogen containing polymers such as polyethylene terephtalate 4, polycarbonates, and polyamides because of its high percentage of P. Red phosphorus is also an highly effective flame retardant in polyolefins with a low additives loading (less than 10%). It appears to have multiple modes of action (condensed and gas phase mechanism). Levchik et al.5 investigate the action of red phosphorus in nylon 6. They demonstrate that it oxidizes during combustion and leads to the formation of phosphoric acid species. The nylon reacts with this species to form phosphate esters which are then thermally decomposed to form a char. Literature 6 also reports the contribution of gas phase mechanism in the mode of action of red phosphorus. In fact, phosphorous monoxide produced from the combustion of red phosphorus has been found in the flame which results in a dramatic reduction in flame strength. Red phosphorous has found commercial interest in polyamides for electrical and electronic components in Europe. However, because of its color and because of problems during processing, its importance as flame retardant tend to decrease for many years. AMMONIUM PHOSPHATES Ammonium (ortho)phosphoric salts were first recommended for flame retardancy of theatre curtains by Gay-Lussac 7 in 1821 and are still used. Advantages of these salts (monoarnmonium or diammonium orthophosphate or their mixture) are low Phosphorus Research Bulletin Vol. 10 (1999), 89

3 required. These salts are used in large amount for non-durable flame retarded materials such as paper, fabrics... The mechanism of fire retardant action of mono- and diammonium phosphates on polystyrene has been investigated by Kishore et al.8 : flame retardancy is obtained via char formation. The proposed mechanism consists in 5 successive steps : hydroperoxide formation, decomposition of hydroperoxyde, ester formation via a reaction with polyphosphoric acid, dehydration of the ester forming double bonds and then char formation from the unsaturated substrate. Recently, our Laboratory has proposed that addition of ammonium polyphosphate (APP) in polyamide-6 during processing 9 or polyurethane (during the resin cure during processing of thermoplastic polymers 11 ) allows to obtains FR materials. Most particularly, the use of polyurethane/app systems as FR coating of foamed polymeric systems is studied. Moreover, a recent study 12 presents the fire retardancy of binary metal-ammonium phosphates (BMAPs; M: La, K, Ba, Sr, Zn, Bi, Ti, Mn, Ni, or Co) in polyamide-6. An advantage of these compounds for this application is their high thermal stability. In fact, polyamide is processed at high temperature, which limits the use of flame-retardants with low thermal stability. The mode of action of binary metal-ammonium phosphates is a condensed phase mechanism. Polyphosphoric acid produced from the combustion of BMAPs catalyses the decomposition of PA-6. However, it occurs reaction between BMAPs and PA-6 which leads to the formation of a char. or INTUMESCENT SYSTEMS Intumescent systems form on heating a cellular charred layer which insulates the underlying materials slowing down heat and mass transfer between the flame and the materials. The development of such systems began fifty years ago, with the preparation of intumescent coatings13. More recently, intumescent additives have been used for flame retardancy to polymeric materials14 '15 Monoammonium orthophosphate (MAP) and Ammonium Polyphosphate (APP) (the long chain of APP is obtained by heating Phosphorus Research Bulletin Vol. 10 (1999), 90

4 less condensed phosphates in presence of urea or in an ammonium atmosphere 16 ) are components of respectively intumescent paints (or varnish) and thermoplastic materials. Intumescence is a special case of a condensed phase mechanism. Intumescent systems consist generally of three active components : an acid source (such as APP), a carbonsupplying compounds (polyols such as pentaerythritol or carbonizing polymers such as polyamides or polyurethanes) and a blowing agent. Depending on the nature of the organic polymer, it is not necessary to include all the components to produce intumescence. When heated, the acid source, for example APP, liberates polyphosphoric acid which catalyzes the dehydration of the polyol producing esters mixtures. Then it occurs a carbonization process which leads to a carbon-rich residue. The spumific agent (APP evolving NH3 can play this role) decomposes to yield gaseous products which cause the char to swell. Additional crosslinking reactions stabilize the residue. The material resulting from the degradation of an intumescent formulation is an heterogeneous material composed of trapped gaseous products in a phosphocarbonaceous cellular material. Synergistic agents may be added in intumescent formulations. For example, the addition of zeolite improves considerably the flameproofing efficiency of a system PP/APP/PER 17. Moreover, P-N synergism is observed inthe phosphorylation of cellulose and it exists several reports on the literature on the P-Br synergism 19. In particular, the mixture APP and hexabromocyclododecane gives fire retardant properties of interest in polyacrylonitrile. New systems containing a char forming polymer have been recently developed for thermoplastics 11,20,21 For instance, the system PP/PUR/APP allows an increase of the Limiting Oxygen Index which corresponds to the oxygencontent in the atmosphere maintaining the combustion after ignition from 18 vol.-% (for the virgin polymer) to 32 vol.-%. PHOPHATE GLASSES Low melting phosphate glasses can function as effective intumescent flame retardants for organic polymers 22. Low melting glasses can be defined as vitreous materials which Phosohorus Research Bulletin Vol. 10 (1999). 91

5 can be mixed with engineering thermoplastic polymers via conventional plastics compounding techniques. The addition of glasses in polymer allows the formation, during combustion, of a glassy resistant barrier which reduces the flame spread and limits markedly the heat transfer. The glass may act as a binder in the char which leads to improve the physical properties of the char. Moreover, phosphate compounds are known to be a chemically active source of phosphorous. The molten phosphate glass may provide an active source of phosphorous. For example, these glasses have shown to be highly efficient as flame retardant in rigid PVC samples 23. SUMMARY In this paper, the different classes of inorganic phosphates used as flame retardant additives have been reviewed. Red phosphorus has an effective fire retardant action in several kinds of polymer but it presents limits because of its color ind of security. Ammonium phosphates and intumescent systems, which are environmentally friendly, seem to be an interesting solution to substitute the halogen-based fire retardant additives, which are associated with health and environmental problems. We also reported original path to improve flame retardancy of polymer such as binary metal ammonium phosphate or low melting glasses. Inorganic phosphates have a long history in flame retardancy of polymers but they are still actual subjects of research and development. REFERENCES 1 A.M. Aaronson, ACS Symp. Ser., 486 (Phosphorus Chem.), 218 (1992) 2 E.D. Weil, S. Levchik, M. Ravey and W. Zhu, paper for the International Conference on Phosphorous Chemistry, Cincinnati (1998) 3 Kirk-Othmer Encyclopedia of Chemical Technology 4th Ed., 10, 976 (1994) 4 A. Granzow, R.G. Ferrillo and A; Wilson, J. Appl. Polym. Sci., 21, 1687 (1977) 5 G.F. Levchik, S.V. Levchik, G. Camino and E.D. Weil, Proceedings on the sixth European Meeting on Fire Retardancy of Polymeric Materials, Lille, France (1997) 6 E.N. Peters, J. Appl. Polym. Sci., 24, 1457 (1979) Phosphorus Research Bulletin Vol. 10 (1999), 92

6 7 JL. Gay Lussac, Annales de chime, 18(2), 211 (1821) 8 K. Kishore and K. Mohandas, Combustion and Flame, 43, 145 (1981) 9 C. Siat, S. Bourbigot and M. Le Bras, Recent Advances in Flame Retardancy of Polymeric Materials (Volume 7), M. Lewin ed., BCC Co. Pub., Norwalk, (1997). 10 S. Duquesne, Diplomes d'etudes Appronfondies, Universite de Lille (1998) 11 M. Bugajny, M. Le Bras, S. Bourbigot and R; Delobel, Polym. Deg. & Stab., 64(1), 157 (1999). 12 G.F. Levchik, S.V. Levchik, A.F. Selevich, A.I. Lesnikovich, A.V. Lutsko, L. Costa, Proceedings on the sixth European Meeting on Fire Retardancy of Polymeric Materials, Lille, France (1997) 13 H.L. Vandersall, J. Fire and Flammability, 2, 97 (1971) 14 G. Montaudo, E. Scamporino and D. Vitalini, J. Polym. Sci. : Polym. Chem., 21, 3361 (1983) 15 G. Camino, Actes du premier Colloque Franco hone l'ignifugation des Polymeres ed. J. Mane Saint Denis France, 36 (1985) 16 C.Y. Shen, N.E. Stahiheber and D.R. Dyroff, J. Amer. Chem. Soc., 91(1), 62 (1969) 17 S. Bourbigot, M. Le Bras, R. Delobel, P. Breant and J.M. Tremillon, Polymer Degradation and Stability, 54, 275 (1996) 18 M. Lewin, Fire Retardancy of Polymers. The use of Intumescence, edited by M. Le Bras, G. Camino, S. Bourbigot and R. Delobel, Cambridge (UK) : Royal Chem. Soc:, 203 (1998) 19 A. Ballistreri, G. Montaudo, C. Puglisi, E. Scamporino and D. Vitallini, J. Appl. Polym. Sci., 28, 1743 (1983) 20 M. Bugajny, M. Le Bras and S. Bourbigot, J. Fire Sci., submitted 21 S. Bourbigot, C. Siat and M. Le Bras, Recent Advances in Flame Retardancy of Polymeric Materials (Volume 8), M. Lewin ed., BCC Co. Pub., Norwalk, (1998). 22 C.J. Quinn and G.H. Beall, Proceedings of the Conference on Recent Advances in Flame Retardancy of Polymeric Materials, Stanford, Connecticut, 4, 62 (1993) 23 R.E. Myers and E. Licursi, J. Fire Sci., 3, 415 (1985) Phosphorus Research Bulletin Vol. 10 (1999), 93

6. Intumescent Flame Retardants

6. Intumescent Flame Retardants 6. ntumescent Flame Retardants ntumescence is an interesting phenomenon. The French verb tumere means "to swell". The Latin equivalent tumescere can be translated as "to swell up". Therefore tumid or tumescent