Fire Retardant Mechanisms in Polymers

B. Schartel, M. Bartholmai, and U. Knoll, Some comments on the main fire retardancy mechanisms in polymer nanocomposites, Polym. Adv. Technol., 2006, 17 772-777. 

Flame-retardant additives are capable of significant reduction in the ha2ard from unwanted fires, and techniques are now available to quantify these improvements. Combined with an understanding of fire-retardant mechanisms, polymer-retardant interactions, and reuse technology, formulations optimi2ed for pubHc benefit and manufacturing practicaUty can be selected. 

Kaspersma, J. Doumen, C. Munro, S. Prins, A. M. Fire retardant mechanism of aliphatic bromine compounds in polystyrene and polypropylene, Polymer Degradation and Stability, 2002, 77(2), 325-331. 

Camino, G. Duquesne, S. Delobel, R. et al. 2001. Mechanism of expandable graphite fire retardant action in polyurethanes. In Fire and Polymers. Materials and Solutions for Hazard Prevention, Nelson, G.L., Wilkie, C.A., Eds. ACS Symposium Series 797 American Chemical Society Washington, 2001 pp. 90-109. 

In this paper we report the use of some phosphine oxides, phosphonic acids, and phosphinic acids to impart fire retardant properties to polymers. In addition, we postulate a mechanism by which these compounds behave as flame retardant agents. 

The fire-retardant mechanism associated with nanoclays has recently been studied and is likely to involve the formation of a ceramic skin which catalyzes char formation by thermal dehydrogenation of the host polymer to produce a conjugated polyene structure. " The nanocomposite structure present in the resulting char appears to enhance the performance of the char through reinforcement of the char layer. These effects would explain the apparent fire-retardant synergy observed when nanoclays are incorporated into polymer formulations containing condensed phase fire-retardant systems, including coated fillers. 

The mechanism of burning for polymers is believed to take place through thermal pyrolysis of the solid plastic to produce gases that act as fuel for the fire (45). Fire retardants work in both the condensed and the vapor phase to interrupt melting of the polymer and burning of the gases. Triaryl phosphates function well in the vapor phase. Alkyl aryl phosphates are believed to decompose in the flame front to form polyphosphoric acid, which stays in the condensed phase to form char, which reduces flammability and smoke evolution (46. 47). 

Fillers are typically used to enhance specific properties of polymers, and the polymer/ nanocomposites based on nanoclays have gained attention because of their ability to improve the mechanical, thermal, barrier, and fire-retardant properties of polymers [3]. Nanosized fillers have been introduced in a wide spectrum of applications ranging from providing photocatalyst activation and conductivity to improve melting... 

In epoxy resin, the combination of ATH and phosphonium-modified clay additives showed superposition or even synergetic behavior for nearly all fire retardancy properties. Schartel et al. suggested that the presence of ATH resulted in an increase in residues and a small decrease in effective heat of combustion because of dilution of the pyrolysis products [24], Both fire retardancy mechanisms have their primary source in the conversion of ATH into aluminum oxide, which increased the residues, and water, which diluted and cooled the flame zone. In addition, the presence of organophosphorus decreased the effective heat of combustion through a gas phase. Most of the phosphorus was liberated during polymer pyrolysis and influenced the Are behavior through flame inhibition. 

Camino G, Duquesne S, Delobel R, Eling B, Lindsay C, Roels T (2001) Mechanism of expandable graphite fire retardant action in polyurethanes. In Nelson GL, Wilkie CA (eds) Fire and polymers. ACS symposium series, vol 797. American Chemical Society, Washington, pp 90-109... 

Specific aspects of barrier formation were discussed above. A silicate or sihcate-char surface layer acting as a barrier for heat and mass transport is probably the main general fire retardancy mechanism of all layered-silicate nanocomposites. Most sources claim that this mechanism is responsible for the strongly improved performance in a cone calorimeter test. In particular, the strong reduction in PHRR is used to propose that layered silicates are the most promising approach for fire retardancy of polymers. However, the barrier effects and their influences on cone calorimeter results are not described in detail, so that the specific characteristics of these mechanisms are unclear. 

Nanoparticles added to thermoplastic polymers improve the mechanical properties, increase Tg and enhance fire retardancy. Nanoparticles in UPRs bring similar effects [217]. Montmorillonite (MMT) was used to obtain polymer nanocomposites [218]. That general approach was applied to the UPR-layered MMT nanocomposites. The structure of the nanocomposites was investigated by XRD and TEM. Organophilic MMT was applied dodecylammonium-bromide treated MMT was used. A UPR synthesized... 

Literatures are available on POSS-polymer composites synthesized from different thermoplastics [71-74]. These composites are lightweight and show good fire retardancy, thermal stability, and mechanical reinforcement. Literatures on POSS-rubber composites are yet to come in a big way. 

Aryloxyphosphazene copolymers can also confer fireproof properties to flammable materials when blended. Dieck [591] have used the copolymers III, and IV containing small amounts of reactive unsaturated groups to prepare blends with compatible organic polymers crosslinkable by the same mechanism which crosslinks the polyphosphazene, e.g. ethylene-propylene and butadiene-acrylonitrile copolymers, poly(vinyl chloride), unsaturated urethane rubber. These blends were used to prepare foams exhibiting excellent fire retardance and producing low smoke levels or no smoke when heated in an open flame. Oxygen index values of 27-56 were obtained. 

Functionalized polymers are of interest in a variety of applications including but not limited to fire retardants, selective sorption resins, chromatography media, controlled release devices and phase transfer catalysts. This research has been conducted in an effort to functionalize a polymer with a variety of different reactive sites for use in membrane applications. These membranes are to be used for the specific separation and removal of metal ions of interest. A porous support was used to obtain membranes of a specified thickness with the desired mechanical stability. The monomer employed in this study was vinylbenzyl chloride, and it was lightly crosslinked with divinylbenzene in a photopolymerization. Specific ligands incorporated into the membrane film include dimethyl phosphonate esters, isopropyl phosphonate esters, phosphonic acid, and triethyl ammonium chloride groups. Most of the functionalization reactions were conducted with the solid membrane and liquid reactants, however, the vinylbenzyl chloride monomer was transformed to vinylbenzyl triethyl ammonium chloride prior to polymerization in some cases. The reaction conditions and analysis tools for uniformly derivatizing the crosslinked vinylbenzyl chloride / divinyl benzene films are presented in detail. 

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