Conventional flame retardants

The principles needed to design a polymer of low flammability are reasonably well understood and have been systematized by Van Krevelen (5). A number of methods have been found for modifying the structure of an inherently flammable polymer to make it respond better to conventional flame retardant systems. For example, extensive work by Pearce et al. at Polytechnic (38, 39) has demonstrated that incorporation of certain ring systems such as phthalide or fluorenone structures into a polymer can greatly increase char and thus flame resistance. Pearce, et al. also showed that increased char formation from polystyrene could be achieved by the introduction of chloromethyl groups on the aromatic rings, along with the addition of antimony oxide or zinc oxide to provide a latent Friedel-Crafts catalyst. 

ABS/clay nanocomposites that are prepared by a direct melt intercalation technique without any conventional flame retardant, show an enhanced formation of char in the course of thermal degradation and thus exhibit an improved thermal stability (74). 

Bourbigot, S., Duquesne, S., Fontaine, G., Bellayer, S., Turf, T., and Samyn, F. 2008. Characterization and reaction to fire of polymer nanocomposites with and without conventional flame retardants. Mol. Cryst. Liq. Cryst. 486 325-339. 

Finally, carbon black did reduce burning rate somewhat, especially when high structure grades were used (Table X), but not sufficiently to act as a sole flame-retardant—only as an assistant to more conventional flame-retardant additives. The importance of char and free radicals in burning mechanisms (10) and the relationship of char and free radicals to carbon black should prompt further exploratory studies of this property—viz., carbon black s synergistic combination with conventional flame retardants. 

NEC has a target to phase out the use of all halogenated flame retardants by 2011. In 1999 the company launched a polycarbonate containing a silicone flame retardant that it claims to be far superior to conventional flame retardants and is neither phosphorus nor halogen based and can be recycled up to five times for the same purpose. 

Gilman, J.W. Kashiwagi, T. Polymer-layered silicate nanocomposites with conventional flame retardants. In Polymer-Clay Nanocomposites Pinnavaia, T.J., Beall, G.W., Eds. John Wiley Sons, 2000 193-206. 

Gilman JW, Kashiwagi T (2000) Polymer-Layered Silicate Nanocomposites with Conventional Flame Retardants. Polymer-Clay Nanocomposites, eds. TJ Pinnavaia, GW Beall, Wiley, Chichester... 

A conventional flame-retarded polyurethane foam produces a flake-like residue which is insufficient to protect the rear aluminium sheet (Fig. 3.155). 

Intumescents function because, under fire conditions, they foam to produce an insulating carbon char which both protects the substrate from high temperatures and allows only a small proportion of polymer to be involved in the fire. Heat release can be controlled because the surface of the char nearest the flame will ablate, so absorbing energy. This contrasts with conventional flame retardants that absorb energy by endothermic chemical decomposition or liberation of water, or by altering the polymer s surface chemistry to slow down oxygen access. 

Nord-Min is regarded as a fire barrier additive , not a flame retardant, in that it only functions once a fire has commenced. It is often, therefore, used in synergy with other, conventional flame retardants. The intercalation process, having utilised acids, is followed by neutralisation with either ammonia or caustic soda. Final washing to remove all traces of these various water-soluble reagents is necessary to avoid any interactions during processing in, for example, PU foams. 

NOR-1 as a synergist with brominated or phosphorus FRs can provide flame retardancy in moulded articles without antimony trioxide. V-2 formulations can be designed with lower levels of conventional flame retardants. It is easily melt processible and does not weaken the mechanical or physical properties of polyolefins, and also allows for safer use and recycling. 

Synergistic flame retardancy Nanocomposites have been demonstrated to reduce flammability, particularly through lowering peak heat release in cone calorimeter experiments. In combination with conventional flame retardants such as magnesium hydroxide or aluminum trihydrate, several polyolefin-based wire and cable products have been developed that incorporate 5% nanoday to reduce the use of conventional fire retardant agents and to improve physical properties [14, 15]. 

S. Bourbigot, S. Duquesne, G. Fontaine, S. BeUayer, T. Tnrf, F. Samyn, Characterization and reaction to fire of polymer nanocomposites with and withont conventional flame retardants. Molecular Crystals and Liqnid Crystals 486 (2008) 325/[1367]-339/[1381]. 

Nanofillers have already been mentioned. They can improve flame retardancy or else reduce the amount of conventional flame retardant needed. This is not just a cost question, because high levels of flame retardant often spoil the mechanical properties. 

Some HALS act co-operatively with conventional flame retardants and there are high molecular weight types that are said to act as heat stabilisers. 

DuPont-Toray, Ichimura Sangyo and Takayasu in Japan have incorporated aramid fibres in polymers instead of conventional flame retardants, using a special blending method that gives a uniform dispersion. Advantages include better tracking resistance and lower specific gravity than many other flame retarded compositions. 

Gihnan, J. W. and Kashiwagi, T., Polymer-layered silicate nanocomposites with conventional flame retardant . In Pinnavaia, T. J. and Beall, G. W. (eds.). Polymer-clay nanocomposites. New York John Wiley, 2000, pp. 193-206. 

Aluminum trihydrate (ATH) Martinal OL 104 LE from Albemarle was used as a conventional flame retardant. 

With few exceptions, nanocomposites do not self-extinguish until most of the fuel has been burnt. That is, they burn slowly but completely. Therefore, polymer/clay nanocomposites are unable to meet the fire safety standard of UL-94 and LOI tests when used alone. Improvements on LOI or UL-94 tests can be observed in some systems that combine clay and conventional flame retardants. In this section, the issue will be discussed briefly. 

Researchers have explored many combinations of clay with various halogenated non-halogenated flame retardants (FRs). This idea is to combine the barrier effect of the clay with the chemical action of conventional flame retardants. 

In conclusion, it is unfortunate that despite impressive flame retardancy results with clays, almost all the nanocomposites fail to pass the LOI or UL-94 test when the clays are used alone. When the clays are combined with conventional flame retardants, the results are usually more promising. 

In spite of the encouraging results obtained in polymer/LDH flame retardant nanocomposites, the use of LDHs alone is insufficient for ensuring adequate fire resistance to meet the required standards, such as LOI values and UL-94 test ratings, especially at low LDH concentrations. The combination of LDH with conventional flame retardants is an effective way to avoid this limitation. By this means, it is possible to reach the flame retardancy required by the market with a halogen-free, nontoxic flame retardant system and improved mechanical properties. There are also many issues concerning the synergy between LDH and conventional flame retardants. 

Talc is a naturally occurring magnesium silicate which is finding broad application as a filler in polyolefins. Apparently, it provides a moderate flame retardant effect, but because talc is inexpensive, it is used as a partial substitute for more expensive flame retardants. Fumed silica is used as a filler in epoxy resins for the encapsulation of electronic devices at a relatively high loading, np to 80 to 90 wt%. Because of the relatively small amount of combnstible resin, this composition can be flame retarded by the addition of a very small amonnt of a conventional flame retardant. It is not clear if the silica contribnles to the flame retardancy by any mechanism other than heat dispersion. Nanodis-persed clay, which is one of the main topics of this book, is an alnminosiUcate. The mechanism of its flame retardant action is discnssed in other chapters of the book. 

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