Nanoparticles flame retardant mechanisms

The polymer performance and production efficiency can be enhanced as a function of the basic features of the reinforcement fillers. In the attempt to achieve fillers with increased performance, the following features must be monitored density, flame retardancy, mechanical resistance, thermal conductivity, and magnetic properties. Nanoparticles of carbides, nitrides, and carbonitrides can be used to reinforce polymer matrix nanocomposites with desirable thermal conductivity. However, current trends in the design of these materials reveal that is not enough to choose a wellperforming material for each component of the heat dissipation path. In addition, careful attention must be paid to the manner in which these materials interact with each other. A filler that conducts heat well but does not wet the matrix may lead to poor results compared to a lower conductivity filler that does wet the matrix. In other words, a major fact that leads to interfacial resistance is faulty physical contact between filler and matrix, which primarily depends on surface wettability (Han and Fina2011). 

It is of interest to determine the flame retardant effectiveness of shapes or types of nanoparticles other than layered silicates, to find what shape or type of nanoparticle is most effective for improving the flammability properties of commodity polymers. In this chapter, flammability properties of nanocomposites containing nanoscale oxides such as nanoscale silica particles and metal oxides, polyhedral oligomeric silsesquioxanes (POSSs), and carbon-based nanoparticles such as graphite, single-walled carbon nanotubes (SWNTs), multiwalled carbon nanotubes (MWNTs), and carbon nanofibers (CNFs) are described and a flame retardant mechanism of these nanoparticles is discussed. 

The results obtained by Kuila et al. and Acharya et al. [63,64] from the EVA elastomer blended with lamellar-like Mg-Al layered double hydroxide (LDH) nanoparticles demonstrate that MH nanocrystals possess higher flame-retardant efficiency and mechanical reinforcing effect by comparison with common micrometer grade MH particles. Kar and Bhowmick [65] have developed MgO nanoparticles and have investigated their effect as cure activator for halogenated mbber. The results as shown in Table 4.2 are promising. 

For more than a decade, numerous research studies have been carried out on the flame-retardant properties conferred by nanoparticles and mainly by organo-modified layered silicates (OMLS). Earlier work at Cornell University and National Institute of Standards and Technology in the United States showed that nanocomposites containing OMLS reduced polymer flammability and enhanced the formation of carbonaceous residue (char).14 Owing to a strong increase in polymer viscosity, impairing processability, and also due to the breakdown of ultimate mechanical properties, the acceptable rate of incorporation for nanoparticles to improve flame retardancy is generally restricted to less than 10 wt %. 

The different flame-retardant (FR) mechanisms of action of current nanoparticles, such as layered silicates, carbon nanotubes (CNTs), and nano-oxides or -hydroxides, according to their nature and interfacial modifications, are relatively well known and detailed in numerous works.5 13 These mechanisms are rather different from those exhibited by usual FRs and correspond mainly to the following physical, physicochemical, or chemical actions ... 

In case of other fillers, the nanofillers can introduce new functionality into the polymer, e.g. electrical conductivity in case of carbon based nanoparticles, barrier properties in case of platelet like nanofillers (nanoclay, expanded graphite), enhancement of mechanical properties, enhanced flame retardancy, and many others. 

Addition of small amount of nanofillers may improve the properties of mbber and thermoplastics. In the polymer industry, polymer-filler nanocomposites are a promising class of material that offers the possibility of developing new hybrid materials with desired set of properties. Properties of mbbers and thermoplastics which have shown substantial improvements due to the incorporation of nanoparticles, are mechanical properties, decreased permeability to gases, water and hydrocarbons, thermal stability and heat distortion temperature, flame retardancy and reduced smoke emissions, chemical resistance, surface appearance, electrical and thermal conductivity, optical clarity in comparison to conventionally filled polymers [107]. 

Nowadays, ordered inorganic/organic PNs with a finely tuned structure have displaced a lot of traditional composite materials in a variety of applications because the intimate interactions between components can provide enhancement of the bulk polymer properties (i.e., mechanical and barrier properties, thermal stabihty, flame retardancy, and abrasion resistance). The reinforcing nanoparticle/ polymer adhesion is of primarily importance, as it tunes the final properties of the nanocomposite. Polymer/clay nanocomposites (PCNs) meet this demand due to the platelet-type dispersion of the clay filler in the organic matrix [1]. 

The accumulation of clay at the surface acts thus as a barrier which limits heat transfers and reduces the release of combustible volatiles into the flame. A substantial decrease in the peak heat release rate of the nanocomposite (25 to 50%) can be achieved compared to the neat polymer (Bourbigot et al, 2006). However, this effect is very dependent on the quality of dispersion of the nanoparticles within the host matrix, and a high degree of exfoliation is usually targeted in order to maximize both the mechanical and fire properties (Hackman and Hollaway, 2006). Other types of nanoparticles, such as silica (Si02), titanium dioxide (Ti02), carbon nanotubes or silesquioxane, have also proven to have significant flame-retardant properties (Laoutid et al., 2009). 

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