Specific Flame Retardant Mechanisms

Chlorinated aromatic products are relatively stable and therefore not very efficient, but chlorinated aliphatic and cycloaliphatic flame retardants are well 

FIGURE 1.1 Dependence of total flaming time of polypropylene measured in a UL-94 test on bromine content for an aliphatic brominated flame retardant and an aromatic brominated flame retardant. (From Ref. 23, cop)Tight 2001, Routledge/Taylor Francis Group, with permission.) 

It is generally accepted that the main mechanism of flame retardant action of halogenated flame retardants is in the gas phase, and it is primarily the chemical mode of action. The reaction begins with the abstraction of halogen radical from the flame retardant. This halogen immediately abstracts hydrogen from either the flame retardant additive or the polymer. An example of such a sequence of reactions, with the participation of bromine and an aliphatic polymer, is 

Atomic hydrogen and hydroxyl radicals are very important for sustaining com-bnstion. The hydrogen radical is responsible for the chain-branching free-radical reactions in the flame [reaction (1.7)], whereas the hydroxyl radical is responsible for the oxidation of CO to CO2 [reaction (1.8)], which is a highly exothermic reaction and is responsible for the larger part of the heat generation in the flame. 

In some other reactions, the more reactive radicals (H, OH, CHb ) are replaced by the less active Br radicals. If Br meets H in the presence of a neutral molecule (third body), HBr is regenerated. It has been found by spectroscopy that the introduction of halogen-containing inhibitors into the flame clearly reduces the concentration of H, OH, and HCO radicals, whereas there is an increase in the content of the diradicals C2 and soot. As the concentration of inhibitor is increased, the flame temperature decreases. Small additions of halogen inhibitors (on the order of a few mol%) can reduce the rate of flame propagation up to 10-fold and have a marked effect on the ignition limits. On the other hand, halogens accelerate the formation of soot in the flame. 

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Flame retardants or flame retardancy mechanisms, respectively, influence different fire properties quite differently, and, what is more, show different effectiveness in different fire scenarios, and thus fire tests. In extreme cases, flame retardancy with respect to a specific fire property or specific test is achieved with little or no improvement in performance in another fire property or fire test. This fundamental understanding in fire science sometimes may be overlooked in materials development, but is worth addressing. The influence on different fire risks and the dependency of effectiveness on the scenario addressed is discussed subsequently based on the fire retardancy mechanisms accompanying charring and barrier formation. 

However, most of the information, is still largely empirical. There are still not enough physico-chemical data such as the heat of comhustion of specific plastics (with and without flame-retardants) the rate of evolution of this heat the nature of the pyrolysis products or the effect specific additives may have on the course of degradation of the polymer. These are all features, which deserve far more systematic attention if a full understanding of flame-retardant mechanisms is to he reached. 

A schematic summary of the mechanism of action of flame retardants is given in Figure 17.3. The relationship between condensed phase and gas phase processes is indicated by referring to specific flame retardants. The influence of both chemical and physical processes should also be noted. 

Aramid Fibers. Aromatic polyamide fibers exhibiting a range of mechanical properties are available from several manufacturers, perhaps the best known being Du Pont s proprietary fiber Kevlar. These fibers possess many unique properties, such as high specific tensile strength and modulus (see Fig. 4). Aramid fibers have good chemical resistance to water, hydrocarbons, and solvents. They also show excellent flame retardant characteristics (see High PERFORMANCE fibers Polyamdes). 

The dynamic mechanical behavior of most homogeneous and heterogeneous solid and molten polymeric systems or composite formulations can be determined by DMA. These polymeric systems may contain chemical additives, including fillers, reinforcements, stabilizers, plasticizers, flame retardants, impact modifiers, processing aids, and other chemical additives, which are added to the polymeric system to impart specific functional properties and which could affect the process-ability and performance. 

Their unique properties predestine them for both very specific applications and broad use in the field of polymer composites. They not only enhance mechanical properties but also electrical and thermal properties, act as flame retardants, etc. Thus their positives can be successfully exploited from simple or advanced polymer matrix reinforcement, through electronic devices, sensors and actuators, to electrorheological fluids, to name just the most important applications. 

PVC-U formulations have low flammability due to the chlorine content. The addition of plasticiser in PVC-P formulations necessitates the use of flame retardant and smoke suppressant additives. These additives are known as functional fillers and a correct balance is necessary to achieve all the end-use specification requirements. They are predominately used in cable, conveyer belting and roofing membrane formulations to give resistance to fire initiation and propagation. It is also important to reduce dripping in a fire situation and that as little smoke as possible is generated. Antimony trioxide has been used extensively, usually in combination with phosphate ester plasticisers, giving excellent fire performance and mechanical properties. 

Antimony pentoxide is an alternative to antimony trioxide. It finds applications in semi-transparent materials and dark colors because of its low tinting strength. As with antimony trioxide, antimony pentoxide must be used together with halogen-containing compounds to function as a flame retardant (sec discussion under antimony trioxide). The other advantages of antimony pentoxide include its refractive index which is closer to most materials, its very small particle size, its high specific surface area, and its substantially lower density. Because of its small particle size, its is frequently used in the textile industry since its addition has only a small effect on color or on mechanical properties. Production of fine-denier fibers requires a stable dispersion and a small particle size filler. The flame retardancy of laminates is also improved with antimony pentoxide because small particles are easier to incorporate in the interfiber spaces. 

Specifically, PVC blends with polyethylene, polypropylene, or polystyrene could offer significant potential. PVC offers rigidity combined with flammability resistance. In essence, PVC offers the promise to be the lowest cost method to flame retard these polymers. The processing temperatures for the polyolefins and polystyrene are within the critical range for PVC. In fact, addition of the polyolefins to PVC should enhance its ability to be extruded and injected molded. PVC has been utilized in blends with functional styrenics (ABS and styrene-maleic anhydride co-and terpolymers) as well as PMMA offering the key advantage of improved flame resistance. Reactive extrusion concepts applied to PVC blends with polyolefins and polystyrene appear to be a facile method for compatibilization should the proper chemical modifications be found. He et al. [1997] noted the use of solid-state chlorinated polyethylene as a compatibilizer for PVC/LLDPE blends with a significant improvement in mechanical properties. A recent treatise [Datta and Lohse,... 

Commercial polyolefins often contain additives such as colorants, flame retardants, antioxidants, light stabilizers, nucleating agents, antistatic agents, lubricants (microcrystalline waxes, hydrocarbon waxes, stearic acid, and metal stearates), and so on. These additives aid the processing and fabrication of products from polyolefins. Detailed treatments about specific polyolefins, polymerization systems/ mechanism/processes, structures, properties, processing, and applications may be found in References 2-9. 

The demands on a flame retardant and thermoplastics formulated with such agents are manifold. The flame retardant should provide a durable flame-retarding effect by the addition of only small quantities of the additive it should be as cheap as possible and the manner of incorporation should be easy it should be nontoxic and should not produce fire effluents with increased toxicity it should not decompose at the processing temperatures it should not volatilize and smell and the mechanical, optical, and physical properties of the thermoplastics should be affected as little as possible. It is understandable that these far-ranging demands cannot be satisfied by only one flame retardant and for all thermoplastics with their manifold applications. One is therefore forced to seek the optimum flame retarding formulation for each thermoplastic and the specific application. Examples of typical formulations of several flame-retarded thermoplastics are given in Appendix A8. 

Special fillers and additives can influence mechanical properties, especially for improvement in dimensional stability, but they are mainly used to confer specific properties such as flame retardancy, ultraviolet (UV) stability, or electrical conductivity (Table 3.9). 

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