Polymer nanocomposites nanoparticles, flame retardants

PP-g-MA) silicate nanocomposites and intercalated thermoset silicate nanocomposites for flame-retardant applications were characterised by XRD and TEM [333], XRD, TEM and FTIR were also used in the study of ID CdS nanoparticle-poly(vinyl acetate) nanorod composites prepared by hydrothermal polymerisation and simultaneous sulfidation [334], The CdS nanoparticles were well dispersed in the polymer nanorods. The intercalation of polyaniline (PANI)-DDBSA (dodecylbenzene-sulfonate) into the galleries of organo-montmorillonite (MMT) was confirmed by XRD, and significantly large 4-spacing expansions (13.3-29.6A) were observed for the nanocomposites [335],... 

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 %. 

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). 

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). 

Nanodispersed metal hydroxides have been proved as efficient flame retardants for polymeric materials. It has been shown [107] that the LOI obtained from EVA containing 50 wt% Mg(OH)2 increases from 24% to 38.3% when micrometric Mg(OH)2 (2-5 irm) is replaced with nanometric Mg(OH)2. The enhancement of EVA flame retardancy by nanosized Mg(OH)2 was attributed to the good dispersion of the nanoparticles, which leads to the formation of more compact and cohesive charred layers during the combustion test. Therefore, the nanodispersed LDH layers may also contribute to the flame retardancy of polymer/LDH nanocomposites. 

A new class of materials, called nanocomposites, can avoid the disadvantages of traditional flame retardant systems. Generally, the term nanocomposite describes a two-phase material with a suitable nanofiller (organoclay, nanoparticles, carbon nanotubes, etc.) dispersed in the polymer matrix at the nanometric scale [3,4]. 

Although the incorporation of microscale particles as fillers into polymers has been well explored scientifically, the decrease in size of particles to nanometers, and the simultaneous increase in interface area, results in extraordinary new material properties.In one such application, the flammability properties of polymers have been improved with the addition of nanoscale particles. These filled nanocomposites provide an attractive alternative to conventional flame retardants. At present, the most common approach to improving flammability is the use of layered silicates such as clays, as described in Chapter 3. However, there are many different shapes and types of nanoparticles. (Here, a nano scale particle is defined as having at least one dimension on the nanometer scale.) When all three dimensions are on the order of nanometers, we are dealing with true nanoparticles, such as spherical silica particles, having an aspect ratio of 1. Another type of nanoparticle has only one dimension on the nanometer scale. Such nanoscale... 

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. 

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