Potential Applications of Nanocomposites for Flame Retardancy

Fire Materials Laboratory, Centre for Materials Research and Innovation, University of Bolton, Bolton, UK 

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

Since it is evident from discussions in previous chapters that nanodispersed, functionalized, largely inert particles such as clays and synthetic alternatives cannot promote sufficient flame retardant activity alone but only in the presence of more conventional flame retardants, their potential usefulness will be determined by their ease of processing and the manner in which they influence both process and end product. Essential issues to be considered and resolved are nanoparticle compatibility with the polymer matrix and other additives present, the ability to maintain a nanodispersion during all processing stages, their influence on rheology, and the possible compromise between effective concentrations levels and optimization of these. 

TABLE 11.1 Typical Characteristics of Commercial (Southern Clay Products) Clays 

Qay Treatment Modifier Concentration (meq/100 g clay) (i-Spacing (A) Density (g/cm ) Compatible Polymer Examples  

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Horrocks, A.R. and Kandola, B.K. 2007. Potential applications of nanocomposites for flame retardancy. InFlame Retardant Polymer Nanocomposites, Morgan, A.B. and Wilkie, C.A. (Eds.), Wiley-VCH, Verlag GmbH Co, KGaA, Hoboken, NJ, Chapter 11. 

Therefore, it appears that a combination of organoclays and conventional flame retardants possesses significant potential to be useful flame retardant systems. A more detailed understanding of the flame retardant mechanism by which such an additive combination exerts its positive effects may further improve its performance and safety and reduce overall additive loading and cost. Moreover, the development of feasible and relevant manufacturing methods based on the intercalated flame retardant clays described here which facilitate the dispersion of flame retardant additives and increase flame retardant efficiency foretells of a promising future for flame retardant polymer nanocomposite materials in everyday applications. 

Carbon nanotubes (CNTs) and carbon nanofibers (CNFs), due to their unique structure and properties, appear to offer quite promising potential for industrial application [236]. As prices decrease, they become increasingly affordable for use in polymer nanocomposites as structural materials in many large scale applications. In fact, three applications of multiwall CNT have been discussed recently first, antistatic or conductive materials [237] second, mechanically reinforced materials [238,239] and third, flame retarded materials [240,241]. The success of CNTs in the field of antistatic or conductive materials is based on the extraordinary electrical properties of CNTs and their special geometry, which enables percolation at very low concentrations of nanotubes in the polymer matrix [242]. 

A variety of discontinuous (short) functional fillers may be combined with thermoplastic or thermoset matrices to produce composites. The fillers may differ in shape (fibers, platelets, flakes, spheres, or irregulars), aspect ratio, and size. When the fully dispersed (exfoliated or deagglomerated) fillers are of nanoscale dimensions, the materials are known as nanocomposites. They differ from conventional microcomposites in that they contain a significant number of interfaces available for interactions between the intermixed phases. As a result of their unique properties, nanocomposites have great potential for applications involving polymer property modification utilizing low filler concentrations for minimum weight increase examples include mechanical, electrical, optical, and barrier properties improvement and enhanced flame retardancy. 

In light of the issues with natural clays, one likely trend is an increase in the use of synthetic clays, such as fiuorinated synthetic mica, magadiite, and layered double hydroxides (LDH). This last clay, since it has the potential to release water under fire conditions [much like Mg(OH)2 or Al(OH)3], may find even more use in flame retardant applications. Cost issues and limited sources for synthetic clays will slow the adaptation of these materials, so most of the work will probably be seen in the patent or open literature. More work will be seen for nanocomposites containing nanofillers, such as carbon nanotubes and nanofibers, and these will probably also be combined with additional fiame retardants. 

A trend that has already begun to arise is the use of multiple types of nanofillers in the same polymer to yield a multicomponent nanocomposite. Some workers have found that some types of nanofillers cannot bring all of the desired properties to the final material, so clays have been combined with multiwall carbon nanotubes to bring enhanced properties.The observation for most polymer additives is that they cannot be used for all applications in all polymers, and the same observation will surely be made about nanocomposites. A clay may be used to enhance the flammability performance, bnt it could also be combined with a conductive nanoflller to impart antistatic aspects or electrical conductivity in the final system. One potential way to look at the use of multiple nanoparticles is that each nanoparticle plays a complementary role in flammability reduction. For example, one could choose a clay for mass loss rate or fuel release reduction, but then use a colloidal particle to flu in the gaps between clay plates as the nanocomposite thermally decomposes. Perhaps even more useful, the colloidal particle could have catalytic or flame retardant properties that encourage... 

DPPES was grafted onto the surface of GNO by condensation to form a synergistic phosphorus/silicon-containing GNO flame retardant (DPPES-GNO). A series of nanocomposites that contained 0,1,5, and 10 wt% DPPES-GNO was prepared. XPS, FTIR, Raman spectroscopy, and TEM verified that DPPES covalently bonded to GNO. Furthermore, the addition of DPPES-GNO (up to 10 wt%) to neat epoxy significantly increased its thermal stability and improved the char yield and LOI by 42% and 80%, respectively. The phosphorus, silicon, and GNO layer stmctures of DPPES-GNO caused the continuous and insulating char layer to protect the inner polymer matrix. The approach that is described herein has great potential for the development of a novel synergistic phosphorus/silicon—GNO flame retardant with polymer nanocomposite applications. 

The main limitations of these biodegradable polymers towards their wider application are their relatively low thermal and mechanical resistance and limited gas barrier properties, which limit their access to certain industrial sectors, such as food packaging, in which their use would be justified when biodegradability is required. Nevertheless, the above drawbacks could be overcome by enhancing their properties through the use of filler and/or additives. In the last two decades, the addition of nanofillers to polymers has attracted great attention for the potentiality of these materials to improve a high number of polymer properties for example, polymer layered silicate nanocomposites, because of the nanometer size of the silicate sheets, exhibit, even at low filler content (1-5 wt%), markedly improved mechanical, thermal, barrier and flame retardant properties, in comparison to the unfilled matrix and to the more conventional microcomposites. ... 

The observation that polymer-clay nanocomposites have significantly lower peak heat release rates (PHRRs) when compared to the pure polymer [47] stimulated a dramatic effort focused on the evaluation of the flame-retardant potential of clay dispersed in polymer. The decrease in PHRR can be related directly to the decrease in the spread of fire from one combustible material to another. This affect is directly applicable to definition (1) above. Figures 8.1 and 8.2 contain a comparison of cone calorimeter results for a pure polymer and a nanocomposite of that polymer with montmorillonite. 

Polymer/sihca composite blends, not only improve the physical properties, snch as the mechanical properties and thermal properties of the materials, but they can also exhibit some unique properties that have attracted strong interest in many industries. Besides common plastics and rubber reinforcanent, many other potential and practical applications of this type of nanocomposites have been reported coatings, flame-retardant materials, optical devices, electronics and optical packaging materials, photo resist materials, photo-luminescent conducting film, per-vaporation membrane, ultra-permeable reverse-selective membranes, proton exchange membranes, grouting materials, sensors and materials for metal uptake, etc. As for the colloidal polymer/sihca nanocomposites with various morphologies, they usually exhibit enhanced, even novel, properties when compared with the traditional nanocomposites and have many potential applications in various areas. 

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