Flame Retardant Mechanism of Polymer-Clay Nanocomposites

FLAME RETARDANT MECHANISM OF POLYMER- CLAY NANOCOMPOSITES  

National Institute of Standards and Technology, Gaithersburg, Maryland 

For more than a decade potential environmental problems associated with organo-bromine flame retardant systems have motivated the search for non-halogen-based approaches to reduce polymer flammability. Initially, research focused on development of new phosphorus-based flame retardants, and numerous publications and patents have been issued in this area. Similarly motivated research has also produced nonhalogen flame retardant approaches based on other elements, such as boron and sihcon. At the same time, work on the use of additives, or flllers, with nanometer-scale primary particle sizes, produced polymer nanocomposites. These materials exhibit enhancement in a variety of physical properties at one-tenth the loading required when micrometer-size additives are used.  

Flame Retardant Polymer Nanocomposites, edited by Alexander B. Morgan and Charles A. Wilkie Copyright 2007 lohn Wiley Sons, Inc. 

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J.W. Gilman, Flame retardant mechanism of polymer-clay nanocomposites, in Flame Retardant Polymer Nanocomposites, A. Morgan and C. Wilkie (Eds.), Wiley-Interscience, Hoboken, NJ, 2007, p. 67. 

In this section we briefly review the initial development of clay nanocomposites and present initial work on their flammability properties. In the next section we review several studies aimed at determining the flame retardant mechanism of polymer-clay nanocomposites. This is not intended as an exhaustive review of all nanocomposite flanunability research instead, an attempt is made to present an objective view of the mechanism proposed for how clay nanocomposites reduce polymer flanunability, based on data from NIST and other research groups. 

Wang, J. Du, J. Zhu, J. Wilkie, C.A. An XPS study of the thermal degradation and flame retardant mechanism of polystyrene-clay nanocomposites. Polym. Degrad. Stab. 2002, 77, 249-252. 

The comprehensive flame retardation of polymer-clay nanocomposite materials was reported by Dr. Jeff Gilman and others at NIST [7]. They disclosed that both delaminated and intercalated nanoclays improve the flammability properties of polymer-layered silicate (clay) nanocomposites. In the study of the flame retardant effect of the nanodispersed clays, XRD and TEM analysis identified a nanoreinforced protective silicate/carbon-like high-performance char from the combustion residue that provided a physical mechanism of flammability control. The report also disclosed that The nanocomposite structure of the char appears to enhance the performance of the char layer. This char may act as an insulation and mass transport barrier showing the escape of the volatile products generated as the polymer decomposes. Cone calorimetry was used to study the flame retardation. The HRRs (heat release rates) of thermoplastic and thermoset polymer-layered silicate nanocomposites are reduced by 40% to 60% in delaminated or intercalated nanocomposites containing a silicate mass fraction of only 2% to 6%. On the basis of their expertise and experience in plastic flammability, they concluded that polymer-clay nanocomposites are very promising new flame-retarding polymers. In addition, they predict that the addition... 

It appears that the flame retardant mechanism for the polyamide/clay nanocomposites discussed in this chapter relates to the formation of a continuous protective carbonaceous char layer that acts as a heat shield. The mechanism is similar to that for other kinds of polymer/clay nanocomposites. 

Kashiwagi, T. Shields, J.R. Harris, R.H., Jr. Awad, W.A. Flame retardant mechanism of a pol3mia clay nanocomposite, in Proceedings of the 14th Conference on Advances in Flame Retardant Polymers, Stamford, CT, 2003. 

Little further activity is found in the literature until the advent of the importance of exfoliated layered clays in the dramatic enhancement of polymer mechanical performance at low concentrations was reported [2]. Subsequent systematic evaluations of the thermal stability of polymer-clay nanocomposites were initiated by Jeff Gilman s group at NIST and Emmanuel Giannelis group at Cornell, with remarkable results. This work led to a dramatic increase in scientific investigations focused on the structure-property relationships of polymer-clay nanocomposites to thermal stability and flame retardancy. 

An excellent review of the work on the flame retardancy of polymer nanocomposites was published in 2007 [3]. This chapter will focus on the evaluation of the proposed mechanisms for enhanced thermal stability of polymer-clay nanocomposites, the proposed relationships between enhanced thermal stability of polymer-clay nanocomposites and flame retardancy, and the synergies that develop between traditional flame retardants for polymers and polymer-clay nanocomposites. 

Because of the multifaceted features of clay as a nanoparticle, the benefits in polymer-clay nanocomposites range from increased mechanical performance, barrier performance, and thermal stability. A systems approach to the design of polymer-clay nanocomposites with excellent flame retardancy will provide superior solutions in relation to formulating existing flame retardants with polymer-clay nanocomposites. Providing surface modifications for the clay with higher thermal stability that will not compromise the mechanical and barrier performance of... 

Kashiwagi T (2004), Flame retardant mechanism of polyamide 6-clay nanocomposites . Polymer 45 881-891. 

In recent years, a new and emerging class of clay-filled polymers called Polymer-Clay Nanocomposites (PCN) was developed. Properties such as superior mechanical strength, reduction in weight, increased heat-resistance and flame retardancy, improved barrier properties against oxygen, carbon dioxide, ultraviolet light, moisture and volatiles, as well as conservation of flavor in drinks and beverages are achievable with these novel composites [17-21]. 

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