Fire retardant polymers mechanical properties

Firstly there are forms of polymers, such as polytetrafluoroethylene, which are intrinsically fire retardant. The second types are rendered fire retardant by the inclusion of a suitable additive in the formulation. These include additives based on antimony, bromine, nitrogen, phosphorus and silicon. An essential requirement for fire retardant polymers used in enclosed spaces is that they do not release any toxic products upon combustion. Ffowever, antimony containing additives are going out of favour due to the release of toxic antimony volatiles upon combustion. The properties and mechanisms by which these polymers operate are discussed in Chapters 1 and 6. The third group of polymers consist of intumescent materials and these are being increasingly used as a means of imparting fire retardancy in polymers and this is discussed in Chapter 7. 

Key mechanical and electrical properties of intrinsically fire retardant polymers (i.e., LOI > 30%) are listed in Tables 6.10 and 6.11, respectively. 

Polymer-clay nanocomposites are characterized by improved thermal, mechanical, barrier, fire retardant, and optical properties compared to the matrix of conventional composites, commonly called particulate microcomposites, because of their unique phase morphology deriving from layer intercalation or exfoliation that maximizes interfacial contact between the organic and inorganic phases and enhances bulk properties [8]. 

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. 

Camino, G., Maffezzoli, A., Braglia, M., De Lazzaro, M., and Zammarano, M., Effect of hydroxides and hydroxycarbonate structure on fire retardant effectiveness and mechanical properties in ethylene-vinyl acetate copolymer, Polym. Deg. Stab., 74(3), 457-A64, December, 2001. 

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. 

Polymer clay nanocomposites have, for some time now, been the subject of extensive research into improving the properties of various matrices and clay types. It has been shown repeatedly that with the addition of organically modified clay to a polymer matrix, either in-situ (1) or by melt compounding (2), exfoliation of the clay platelets leads to vast improvements in fire retardation (2), gas barrier (4) and mechanical properties (5, 6) of nanocomposite materials, without significant increases in density or brittleness (7). There have been some studies on the effect of clay modification and melt processing conditions on the exfoliation in these nanocomposites as well as various studies focusing on their crystallisation behaviour (7-10). Polyamide-6 (PA-6)/montmorillonite (MMT) nanocomposites are the most widely studied polymer/clay system, however a systematic study relating the structure of the clay modification cation to the properties of the composite has yet to be reported. 

Additives are used for a wide variety of purposes, and may be classified as fillers, anti-oxidants, stabilisers, plasticisers, fire retardants, pigments and lubricants. Anti-oxidants and stabilisers are usually used in rather small quantities in order to prevent degradation of the polymer when it is exposed to air, light and heat the intention here is to maintain the properties of the polymer rather than to modify them. Fillers may be used either simply to produce a cheaper product or to improve the properties, in particular the mechanical properties. Lubricants may be used externally, to prevent adhesion of the polymer to the processing equipment, or internally, either to aid flow during processing or to reduce friction between the product and other materials. 

A premium family of fire retardants is based on reactive compounds that are able to enter into the polymer chain during the polymerization process (mainly polyesters, epoxides and polyurethanes), or monomers that contain fire-retardant groups (like bromo-styrene). Due to their relative physical stability (regarding diffusivity to the atmosphere), the advantage over additives is clear. In this case, it is also important to study the effects on mechanical and optical properties and mainly on weather resistance. 

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