Nanoadditives

POSS can be added nearly to all types of polymers. These are physically large with respect to polymer dimensions and are equivalent in size to most polymer segments and coUs. This has been the latest commercially available nanoadditive for the polymers. Hybrid Plastics in the United States are the global supplier of POSS. 

Structure and Properties of Carbon Nanotube and Other Nanoadditives.265... 

Provided in this chapter is an overview on the fundamentals of polymer nanocomposites, including structure, properties, and surface treatment of the nanoadditives, design of the modifiers, modification of the nanoadditives and structure of modified nanoadditives, synthesis and struc-ture/morphology of the polymer nanocomposites, and the effect of nanoadditives on thermal and fire performance of the matrix polymers and mechanism. Trends for the study of polymer nanocomposites are also provided. This covers all kinds of inorganic nanoadditives, but the primary focus is on clays (particularly on the silicate clays and the layered double hydroxides) and carbon nanotubes. The reader who needs to have more detailed information and/or a better picture about nanoadditives and their influence on the matrix polymers, particularly on the thermal and fire performance, may peruse some key reviews, books, and papers in this area, which are listed at the end of the chapter. 

The nanoadditives used for fabricating the PNs can be a vast number of inorganic nanomaterials that, in general, are classified into four categories according to their dimensionality as shown in... 

FIGURE 11.1 A schematic illustration of a general process of preparing PNs with clay as nanoadditive. 

The impact of the nanocomposite technology on polymers is huge, reflected in enhanced properties of the resulting PNs, such as enhanced mechanical, barrier, solvent-resistant, and ablation properties.12 The effect of nanocomposite technology on the thermal and fire performance of the polymers is primarily observed in two important parameters of the polymers (1) the onset temperature (7( ,nsct) in the thermogravimetric analysis (TGA) curve—representative of the thermal stability of the polymer, and (2) the peak heat release rate (peak HRR) in cone calorimetric analysis (CCA)—a reflection of the combustion behavior (the flammability) of the polymer. The Tonset will be increased and the peak HRR will be reduced for a variety of polymers when nanoscale dispersion of the nanoadditive is achieved in the polymer matrix. 

In this chapter, an overview of the fundamentals of PNs is described, according to the author s understanding and experience as well as support from numerous references and review articles. The content of this chapter covers all kinds of inorganic nanoadditives, but, because the most widely investigated and thus understood nanoadditives used to enhance the thermal and fire resistance of the polymers are clays (natural or synthetic) followed by the CNTs and colloidal particles, the focus of the chapter is primarily on clays, particularly on the silicate clays and LDHs, as well as the CNTs. This includes structure, properties, and surface treatment of the nanoadditives, design of the modifiers, synthesis, characterization of the structure/morphology, and thermal and fire... 

STRUCTURE AND PROPERTIES OF NANOADDITIVES 11.2.1 Structure and Properties of Clays... 

Clays, natural or synthetic, are the most widely investigated and understood nanoadditives used to enhance the flame retardancy of polymers through nanocomposite technology, because of their unique properties, particularly the ease of surface treatment and application in polymer matrices. Clay can be cationic and anionic materials, in accordance with the charge on the clay layers. In this chapter, the focus is on two kinds of clays montmorillonite (MMT), a naturally occurring cationic clay that belongs to the smectite group of silicates, and LDH, an anionic clay that does occur naturally but for which the synthetic form is more common. Other clays will also be mentioned as appropriate. 

Because of the unique combination of mechanical, electrical, and thermal properties, the CNTs have been excellent candidates to substitute or complement the conventional nanofillers in the fabrication of multifunctional PNs. The first PNs using CNTs as the nanoadditive was reported in 1994.20 By far, the CNTs have been the second most investigated nanoadditives to reduce the flammability of the polymers through nanocomposite technology. A difficulty of the application of the CNTs in polymers is the dispersion of CNTs in the matrix polymer, and the high cost of the CNTs is another problem. 

As an inorganic mineral, most unmodified nanoadditives are strongly hydrophilic and are generally compatible and miscible only with a few hydrophilic polymers, for instance, clay can only be made into PNs with polyethylene oxide),27 poly(vinyl alcohol),28 and a few other water soluble polymers. Most polymers are hydrophobic and thus they are neither compatible nor miscible with the unmodified nanoadditives, leading to an inability to achieve a PN with a good nanodispersion in most cases. Therefore, for most nanoadditives that have been used to prepare the PNs, an important and necessary feature is their surface treatment that provides compatibility to the nanoadditives and enables them to be uniformly dispersed (and/or separated into single nanoparticles) in the polymer matrix. 

Surface treatment of clays The surface treatment of the nanoadditives helps establish an interface with the polymer matrix and hence enhance the compatibility and/or the miscibility of the nanoadditive with the polymer matrix. In terms of the cationic silicate clay, such as MMT, the interface is... 

An advantage of the PNs is the strong interaction between the polymer matrix and the nanoadditives because of the nanoscale dispersion of the nanoadditives in the polymer matrix. As a result, the PNs exhibit unique properties that are not shared by their microscale counterparts—conventional polymeric composites.70 However, the PNs are not easy to obtain. Simple physical mixing of a polymer with nanoadditives does not result in a PN but rather one obtains a more conventional composite with poor mechanical and thermal properties because of phase separation and, hence, the poor physical interaction between the matrix polymer and the nanoadditives. 

To successfully achieve the PNs, the first problem encountered is how to incorporate the nanoadditive into the polymer matrix. Not only is it necessary to break up the agglomerates to much... 

Solvent blending Solvent blending, also called solution intercalation in the case of clay and other nanolayers, involves both dispersing the nanoadditive and dissolving the matrix polymer in a solvent or a solvent mixture. Three parameters have been considered to be important, particularly for clays, in choosing the surface treatment of the nanoadditives with this process The structure of the modifier, its miscibility with the polymer, and its thermal stability. The miscibility of the modifier here has two meanings miscibility with both the final polymer and the solvent chosen to dissolve the polymer. The modifier structure and its miscibility are perhaps more important than the thermal... 

In the case of PNs containing nanoadditives other than nanolayers, such as CNTs and POSS, the terms above are not used. They also show similar observations as those seen for the clay in the polymer matrix Aggregated to a large extent (immiscible or partially immiscible) or a few nanoadditive nanoparticles or separated into single nanoparticle, and all of these may coexist in the same PN. 

XRD is easily used but does not directly provide the dispersion information while TEM is an expensive and difficult method to use. With TEM, however, direct information, although qualitative, about the dispersion of clay (and other nanoadditives) in the matrix polymer can be obtained. The XRD method is usually used together with the TEM technique to characterize the morphology of the resulting PCNs. Attempts have been made to quantify the information that has been obtained through TEM technique,78 but it seems to be not so successful in current practice, particularly for describing the dispersion of clays. 

The loading of the nanoadditive is another factor affecting the thermal stability of the PNs. A significant enhancement is observed at a low loading of the nanoadditives above this loading,... 

The nanodispersed nanoadditives usually show enhanced fire performance and CCA has been the most powerful tool in analyzing the flammability of the PNs. In most cases, the PNs, as seen in Figure 11.20, show a significantly reduced peak HRR in the CCA curve. More examples of this are seen in PA-6/clay nanocomposite, which shows a 63% reduction in the peak HRR at 5% loading (Figure 11.2898 in which the heat release rate as a function of time for pure PA-6 and its clay nanocomposites is shown) and in poly(ethylene-co-vinyl acetate) (EVA)/clay nanocomposite,99 which shows a reduction of the peak HRR at about 50% at 5% organoclay loading. 

Like the thermal stability, however, the magnitude of the reduction in the flammability of the PNs is strongly dependent on the polymer matrices and the nanoadditives. Examples of matrix polymer dependence is illustrated with MMT-based PCNs The PCNs based on PS,6a EVA,99 and PA-6,98 etc. show a significant reduction of the peak HRR in the CCA curve (40%-70% at a clay loading about 5%) ... 

The CNTs can surpass the clays and other nanoadditives as effective flame-retardant additives, as reflected in the lower loading of the CNTs than the other nanoadditives needed to enhance the thermal and fire resistance.100 For instance, the results for PMMA in the presence of different nanoadditives are seen in Figure 11.29 in which the relationship of mass loss rate (MLR) and loading of... 

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