Self-extinguishing polymer-clay nanocomposites

SEONGCHAN PACK AND MIRIAM H. RAFAILOVICh Samsung CheU Industries, Inc. 

Because of environmental concerns regarding the use of halogenated FR derivatives [17], the demand for an alternative FR to replace the halogenated FR formulations has increased. One set of candidates is a group of aryl phosphates, whose chemical structures and applications for flame retardancy have been reviewed extensively in the literature [18, 19]. Some commonly used aryl phosphates and their properties are listed in Table 10.1. 

Recently, Pack et al. have demonstrated that this problem can be overcome when RDP is directly absorbed onto unfunctionalized sodium clay surfaces [10]. They showed that the interactions between RDP and clays were sufficiently strong so that the RDP was able to penetrate the galleries between clay platelets, allowing the clays to exfoliate or intercalate in a large class of matrices. This method of functionalization was shown to be simpler and more cost-effective than those that functionalized the clays with di-tallow molecules, and in certain cases the same synergistic effects were observed with halogenated FR. 

In contrast to the Cloisite clays, addition of phosphate-coated clays was able to improve the flame retardant response in certain classes of polymers. For example, addition of 5% RDP-coated clays to PC was able to achieve a UL-94 VI rating, whereas the addition of Cloisite 20A failed. Similarly, addition of 10% RDP-coated clays to HIPS achieved a rating of VI, whereas no effect was observed with Cloisite clays. 

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Self-extinguishing polymer-clay nanocomposites Table 10.2 dooi of nanocomposites with FR and Cloisite 20A... 

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. 

Recently, new approaches on flame retardancy deal often with nanofillers and in this section some examples of improvements of fire behavior of polymeric foams obtained by use of nanoclays or nanofibers will be shown. Much more details on flame retardancy of polymeric nanocomposite may be found elsewhere as for example in the book edited by A. B. Morgan and C. A. Wilkie105 or in scientific review.106 Polymer nanocomposites have enhanced char formation and showed significant decrease of PHRR and peak of mass loss rate (PMLR). In most cases the carbonaceous char yield was limited to few weight %, due to the low level of clays addition, and consequently the total HRR was not affected significantly. Hence, for polymer nanocomposites alone, where no additional flame-retardant is used, once the nanocomposite ignites, it burns slowly but does not self-extinguish... 

Thermosetting nanocomposites exhibit a reduced rate of heat release compared to neat polymer. However, the approach to nanocomposites itself is not sufficient to comply with the actual fire test standards. For this reason, traditional flame retardants are currently used in combination with nanofillers, and researchers are focusing on the individuation of synergistic systems. As an alternative to the most common cationic clays, anionic clays show improved performance in terms of flame retardancy. Epoxy nanocomposites based on anionic clay exhibit unique self-extinguishing behavior in a UL-94 horizontal burning test never observed before in a pure nanocomposite. The formation of a continnous intu-mescent ceramic layer on the surface of a polymer during combustion reduces the heat release rate to a higher extent than do montmorillonite nanocomposites. 

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