**1. Introduction**

The study of polymeric nanocomposites has grown during the last 10 years due to the remarkable properties that result from the combination of a continuous phase (polymer matrix) and a dispersed phase (nanoparticles), where at least one dimension of the dispersed phase is found in the scale of nanometric size (<100 nm). Such nanocomposites have attracted the attention of academic and industrial researchers due to their extraordinary electrical, thermal, chemical, and biological properties and potential applications in various sectors of the industry such as in the health sector, automotive industry, energy, construction, and food industry among

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

others [1, 2]. One of the most popular methods used to prepare such materials is melt extrusion, since it is a flexible and versatile process, which does not require the use of solvents and can be scaled up at industrial level.

and uniformly in the polymeric matrix; this implies obtaining compounds similar to conventional composites [6]. For this reason, various techniques in the modification of nanoparticles

Ultrasound-Assisted Melt Extrusion of Polymer Nanocomposites

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The modification of the surface, in general, decreases the surface energy of the nanoparticles, improving the affinity between the polymer matrix and the nanoparticles. Natural clays have a stratified structure bonded by strong covalent bonds [7], thus hindering its homogeneous dispersion in many polymers. Therefore, a surface modification is needed, and in this case, it is carried out through a cation exchange process, in which the sodium and calcium cations present in the inter-clay galleries are replaced by alkylammonium species, usually quaternary ammonium containing alkyl, phenyl, benzyl, and pyridyl groups [8]. In metal nanoparticles such as nanoparticles of silicon dioxide, titanium dioxide and aluminum oxide are surface modified with organosilane coupling agents [9], while in carbon-based nanoparticles, surface modification is carried out by covalent functionalization or not covalent. In CNT for example, functionalization of the covalent bond of functional entities can be performed at the ends of the tubes or on their side walls. This process can be carried out by reaction with some molecules with high chemical reactivity, such as fluorine [10]. A noncovalent functionalization is the π-π interactions with aromatic molecules, such as pyrene,

Another approach to modify the surfaces of nanoparticles is based on grafting synthetic polymers on the surface of the substrate, which improves the chemical functionality and alters the topology of the surface of the materials [12]. The graft can be done in two ways: (1) by means of obtaining a polymer with a reactive terminal group and subsequently it is grafted to the surface of the nanomaterial, and (2) the graft is made from the growth of the polymer from

In spite of all the available modifications for nanoparticles, sometimes they are not usually enough and it is necessary to look for alternative or previous methods to the extrusion process that helps us to de-agglomerate the nanoparticles and to reduce the size of these. One of these methods is mechanical milling by high-energy ball mill. The alteration of the solids by mechanical grinding gives rise not only to the fragmentation of the particles but also to structural changes, polymorphic transformations, variation of the properties of the surface, generation of defects, increases of reactivity, induction of chemical reactions, etc. [14]. Mechanical grinding has been applied in phyllosilicates, for example some studies have been carried out on kaolinite, pyrophyllite and some smectites, finding that grinding originates first a disordered phase of the mineral and later a more amorphous phase giving a structural destruction of the same, resulting in an exfoliation of the clay layers [15, 16]. It has also been successfully applied to carbon materials either to modify the morphology of carbon nanotubes or to introduce functional groups, which allow an improvement in dispersion and compatibility with

Another way is to carry out a premixing of the nanoparticles with the polymer using by calendaring. This method has been used in the exfoliation and dispersion of montmorillonite in a DEGBA epoxy resin [17], as well as in dispersion of multiple-wall carbon nanotubes in an epoxy resin [18]. In both cases, a better dispersion of the nanoparticles in the polymer matrix

have been explored.

widely used to modify graphene [11].

an initiator [13].

the polymer matrix.

was observed.

However, even with all these advantages, the lack of homogeneous dispersion of nanoparticles in the polymer matrix is still a problem with melt extrusion. An alternative to improve the dispersion is the application of ultrasound waves during the polymer processing in the molten state, named ultrasound-assisted extrusion. The first report of the use of ultrasound coupled in extrusion was made by Isayev et al. for processing vulcanized elastomers devulcanization [3]. These authors reported that the ultrasound waves have the ability to cause an incision in the C-S and S-S bonds of the crosslinked rubber, causing the breaking of the reticulated network and thereby achieving the devulcanization of the rubber. Later, it was applied to the study of polymer mixtures in the molten state [4], and in the last decade, this technology has been used for the preparation of polymer nanocomposites. Although it has been proven that this technology improves the dispersion of nanoparticles and that it has a great potential for application, the fundamentals for applying this technology in melt extrusion process are still not well understood. For example, the effects observed by the application of ultrasound have been explained on the basis of acoustic cavitation, treating the molten polymer as a Newtonian system; however, polymer cannot be considered as Newtonian fluids. For this reason, a general overview of the basic principles of ultrasound, the development and use of this technology in the preparation of polymeric nanocomposites in the molten state, and the mechanisms that have been proposed so far for the understanding of the phenomenon that generates the dispersion of the nanoparticles in the polymer is described below.
