Nanoparticles layered silicates

In order to produce high-performance elastomeric materials, the incorporations of different types of nanoparticles such as layered silicates, layered double hydroxides, carbon nanotubes, and nanosilica into the elastomer matrix are now growing areas of rubber research. However, the reflection of the nano effect on the properties and performance can be realized only through a uniform and homogeneous good dispersion of filler particles in the rubber matrix. 

The 0-d nanoparticles can be nano-metal oxides (such as silica,1 titania,2 alumina3), nano-metal carbide,4 and polyhedral oligomeric silsesquioxanes (POSS),5 to name just a few the 1-d nanofibers can be carbon nanofiber,6 and carbon nanotubes (CNT),7 which could be single-wall CNTs (SWCNT) or multiwall CNTs (MWCNT) etc. the 2-d nano-layers include, but are not limited to, layered silicates,8 layered double hydroxides (LDH),9 layered zirconium phosphate,10 and layered titanates,11 etc. 3-d nano-networks are rarely used and thus examples are not provided here. 

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 numerous works dealing with the combination of nanoparticles and FR compounds, surface modifications of nanoparticles were only aimed to promote good dispersion of the nanoparticles into the polymer matrix (with intercalated or exfoliated morphologies for layered silicates as nanoparticles), even in the presence of the usual FRs, for example ammonium polyphosphate (APP) or magnesium hydroxide (MH). The initial aim was to combine the individual effects of each component to achieve strong synergistic effects. 

Moreover, the development of new strategies for surface modifications of nanoparticles with compounds having FR activity could provide a new field of research on FR systems. The use of novel phosphorus-, nitrogen-, or halogen-containing modifiers, instead of alkylammonium ions, for layered silicates seems promising. FR action conferred by the surface modifier can be combined with action due to composite morphology, particularly when the host polymer is a polymer blend instead of a pure polymer. 

This chapter develops at first the more frequent combinations of nanoparticles that concern layered silicates associated with phosphorus compounds, as well as metallic hydroxides and halogen compounds. The association of natural layered silicates with intumescent FR (IFR) systems represents one of the main contributions of the combined use of nanoparticles and FRs. Moreover, combinations of layered silicates with other phosphorus compounds have been studied and have led to significant improvements for fire retardancy. 

The growing interest in other categories of nanoparticles, such as synthetic anionic layered silicates, CNTs, nano-oxides or -hydroxides, metallic phosphates, etc., has materialized either through the study of combinations of those nanoparticles with layered silicates or with metal hydroxides or phosphorus FRs. Such combinations are also detailed in Section 12.3. Nevertheless, for some combinations, interpretations of the possible interactions between components are sometimes missing or not completely detailed. 

The FR properties of polymer-layered silicate nanocomposites have been studied for a wide range of polymers, especially for organomodified montmorillonites (OMMT) in thermoplastics. Depending on the nature of the polymer, the decomposition pathway and decomposition products may change.8 A major consequence of the introduction of modified clays is the formation or the enhancement of charred structure, caused by cross-linking processes possibly catalyzed by the nanoparticles. 

Since one objective of the combined use of organomodified layered silicates and FR compounds could be to replace effective FR systems containing halogenated FR, few studies mention the concomitant use of silicates, and more generally nanoparticles, with halogenated compounds. In some cases, comparisons have been made between those systems and the corresponding ones in which silicates are associated with other kind of FRs. All these studies have been carried out rather recently by research groups in countries where the use of brominated FR seems less undesirable than in Europe. 

Some other investigations have been carried out on combinations of oxide nanoparticles with FRs, mainly phosphorus FR, but also with organomodified layered silicates. 

New trends involve the use of nanoparticles in synthetic fibers. Polymer-layered silicates, nanotubes, and POSS have been successfully introduced in a number of textile fibers, mainly poly-amide-6, polypropylene, and polyester. Although they reduce the flammability of these fibers, but on their own are not effective enough to confer flame retardancy to a specified level. However, in presence of small amounts of selected conventional FRs (5-10 wt %), synergistic effect can be achieved. With this approach fibers having multifunctional properties can also be obtained, e.g., water repellency or antistatic properties along with fire retardancy. Most of the work in this area at present is on the lab scale and there is a potential to take this forward to a commercial scale. 

The last few years have seen the extensive use of nanoparticles because of the small size of the filler and the corresponding increase in the surface area, allowing to achieve the required mechanical properties at low filler loadings. Nanometer-scale particles including spherical particles such as silica or titanium dioxide generated in-situ by the sol-gel process (4-8), layered silicates (9-12), carbon (13) or clay fibers(14,15), single-wall or multiwall carbon nanotubes (16,17) have been shown to significantly enhance the physical and mechanical properties of rubber matrices. 

Papp, S., and I. Dekany. 2002. Growth of Pd nanoparticles on layer silicates hydropho-bized with alkyl chains in ethanol-tetrahydrofuran mixtures. Coll. Polym. Sci. 280 956-962. 

Polymer materials, modified with layered silicate nanoparticles, have some significant advantages. For example, introducing Na-montmorrillonite into polymer matrix increases initial modulus, tensile strength, thermal stability and fire resistance, reduces gas permeability rate of material. 

Nanoparticles or nanofillers are collective terms for modified layered silicates (organoclay), graphite nanoflakes, carbon nanotubes, and a number of materials dispersed in the polymer matrix, when the particles size is in order of nanometers (one thousands of micron), or tens of nanometers. A plastic filled with nanoparticles, typically in the range of 2-10% (w/w) is called a nanocomposite. 

The nanoscopic fillers, as mentioned above, have at least one characteristic length that is of the order of nanometers. Uniform dispersion of these nanoscopically sized particles or nanoelements can lead to ultra-large interfacial area between the constituents (approaching 700 m /cm in dispersions of layered silicates in polymers) and also to ultrasmall distance between the nanoelements (approaching molecular dimensions at extremely low loadings of the nanoparticles). 

Inorganic nanoflllers such as clays or ceramics may improve mechanical properties and dielectric properties. An abundant literature has been devoted to layered silicates for applications in the biomedical domain, hydroxyapatite (HAp e.g., nanoparticles of 300 nm in Figure 13.1a) might be of interest. Ferroelectric ceramics are attractive for their high dielectric permittivity and electroactive properties. As an example, BaTiOa particles with d 700 nm are shown in Figure 13.1b. Conductive nanoparticles should induce electrical conductivity in polymeric matrices, but to preserve the mechanical properties, small amount should be used. Consequently, there is great interest in conductive nanotubes [i.e., carbon nanotubes (CNTs)], which exhibit the highest... 

Mechanical Properties Toyota Central Research Laboratories in Japan was the first to obtain significant mechanical improvement of a PA matrix by adding as little as about 2 wt% of montmorillonite (MMT) [Kojima et ah, 1993 Usuki et ah, 1993 Okada and Usuki, 2006], Improvement in the mechanical properties on the vitreous and rubbery plateau by layered silicate nanoparticles depends on several factors, including clay surface modification, polymer chemistry, processing method, level of exfoliation, and clay orientation. In this section we present an overview of the influence of these factors on the dynamic mechanical properties of PLSN. 

The final section Part IV is concerned with physical properties of polymeric nanocomposites (PNCs). Two types of nanoparticles, leading to two different characters and applicabilities of PNC, are discussed layered silicates (with natural or synthetic clays), used in structural-type PNCs and the others used in functional PNCs. Sender et al. in Chapter 13 describe the performance of PNCs with acicular ferroelectric particles producing PNCs with good electroactive (dc conductivity) and mechanical properties. In Chapter 15, Nicolais and Carotenuto focus on metal clusters in polymeric matrices, which combine optical transparency with magnetism, luminescence. Ultraviolet-visible absorption, thermochromism, and so on. 

Inorganic or organic nanoparticles have been incorporated to enhance the mechanical, barrier and thermal properties of PLA. Over the past few years, various nanomaterials have been investigated for reinforcing PLA, including layered silicates, carbon nanotube, hydroxyapaite, layered titanate, aluminum hydroxide, etc. 

A further group of nanocomposites are the well-known bentonites, revitalized at the end of the 1980s by Toyota Research. Bentonites are swellable, three-layer silicates consisting mainly of montmorillonite. The new organophilic bentonite Nanofil product family from Siid Chemie AG is based partly on the Bavarian calcium bentonites, which are activated by acid leaching [see Table 27 [77]], and do without cationic exchange. (Author s remark With a mean particle size of 4 pm, these are by no means nanoparticles ). 

Due to recent improvements in visualization of nanoscopic material with enhanced resolution, huge advances in the characterization technology have been widespread in all the nanotechnology disciplines, compared to just about 20 years ago. A few examples of nanoscopic materials that are less than Ipm are illustrated in Figure 1.2. Current efforts in the nanoparticle—polymer field go into very small metallic and semiconductor nanoparticles with 1—2nm diameter, and use of micron-sized fillers (e.g., layered silicates) (Polymer-Nanoparticle Composites Part 1 (Nanotechnology), 2010). 

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