**2. Extrusion applied in the manufacture of polymeric nanocomposites**

#### **2.1. Polymers and nanoparticles**

Thermoplastic polymers and nanoparticles are the main materials used to produce polymer nanocomposites by melt extrusion. Thermoplastic polymers include polyolefins, polyesters, and polyamides among other polymer families. On the other hand, the nanoparticles can be classified according to the number of dimensions in the nanometer range. Zero-dimensional (0D): it is defined as a particle that is measured within a nanoscale range, that is, less than 100 nm, among them are all the nanoparticles, for example ZnO, TiO<sup>2</sup> , etc. One-dimensional (1D) has two dimensions in this scale, such as nanotubes and nanofiber. Finally, two-dimensional (2D) is referred to nanoparticles, where one dimension is in the nanometer scale, for example graphene and nanodisks [5].

#### **2.2. Modification of nanoparticles**

Unlike particles of micro size, in the nanoparticles, the interparticle forces such as Van der Waals and electrostatic forces, as well as the magnetic attraction, become stronger, which results in the nanoparticles forming agglomerates, which are difficult to disperse individually 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 have been explored.

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

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.

Thermoplastic polymers and nanoparticles are the main materials used to produce polymer nanocomposites by melt extrusion. Thermoplastic polymers include polyolefins, polyesters, and polyamides among other polymer families. On the other hand, the nanoparticles can be classified according to the number of dimensions in the nanometer range. Zero-dimensional (0D): it is defined as a particle that is measured within a nanoscale range, that is, less than

(1D) has two dimensions in this scale, such as nanotubes and nanofiber. Finally, two-dimensional (2D) is referred to nanoparticles, where one dimension is in the nanometer scale, for

Unlike particles of micro size, in the nanoparticles, the interparticle forces such as Van der Waals and electrostatic forces, as well as the magnetic attraction, become stronger, which results in the nanoparticles forming agglomerates, which are difficult to disperse individually

, etc. One-dimensional

**2. Extrusion applied in the manufacture of polymeric** 

100 nm, among them are all the nanoparticles, for example ZnO, TiO<sup>2</sup>

can be scaled up at industrial level.

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**nanocomposites**

**2.1. Polymers and nanoparticles**

example graphene and nanodisks [5].

**2.2. Modification of nanoparticles**

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, widely used to modify graphene [11].

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 an initiator [13].

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 the polymer matrix.

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 was observed.

Mechanical agitation is a common technique for the dispersion of nanoparticles in liquid systems; however, after a while, the nanoparticles tend to agglomerate. To improve the efficiency of dispersion and exfoliation, the ultrasound waves have been applied to stir particles, taking as the separation of individualized nanoparticles results. Ultrasonication is an effective method to disperse CNT in liquids that have a low viscosity, such as water, acetone, and ethanol. In this stage of application of ultrasound, some power factors must be taken care of, for example, because in the case of nanotubes, it has been seen that ultrasound waves can induce defects such as the formation of amorphous carbon in the CNT [19]; recommendations have been made as a sonication of the low power bath to preserve the length and structure of the CNT [20].

After applying these methods to modify the nanoparticles aiming to improve their dispersion in the polymer, it is necessary to consider the conditions of the extrusion process where these nanoparticles are incorporated, since it can be in different modalities or extrusion conditions in order to avoid reagglomerations or degradations of the polymer.

#### **2.3. Manufacture of nanocomposites**

In general, the most used mixing methods for the preparation of nanocomposites are insitu polymerization, solution, and melt mixing. In the in-situ polymerization method, the nanoparticles are first dispersed in the liquid monomer (or a monomer solution), and from there, they are mixed to carry out the polymerization, which can be initiated by heat or by the diffusion of an initiator. In the solution method, the polymer is dissolved in a solvent, and the filler is dispersed in the same solution. The intercalated nanocomposite is obtained by removing the solvent by vaporization or precipitation [21]. Because both processes use a solvent, it is not practical at the industrial level. The melt mixing method takes advantage of the melt temperature of the polymer matrix, and in this way, it achieves the mixing with the nanoparticles. Within this method, one of the most striking is the melt extrusion process [22].

in solid form; it is heated until reaching the molten state and leaves the extruder in the latest state. In this case, the extruder acts as a pump, providing the necessary pressure to pass the

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**Figure 1.** (a) Basic diagram of an extruder, (b) evolution of the pressure along an extruder [25].

An extruder must have a system for feeding the material, a system for melting-plasticizing it, a pumping and pressurizing system (which usually generates a mixing effect), and finally, a device for forming the molten material. **Figure 1a** shows a basic scheme of an extruder [25]. Depending on the pressure that is exerted along the barrel or extrusion barrel, three main zones can be identified as indicated in **Figure 1b**. The feeding zone is the closest to the feeding of the material, where it is gradually compacted at a certain speed. The transition zone is a zone of intermediate compression of the material where the fusion takes place, in addition to which the air that could be trapped in the same escapes by means of the feed hopper. Finally, there is the dosing zone, in which the molten material is homogenized and pressurized to exit

One of the most important parts in this process is the screw and the barrel, since they contribute to carry out the functions of transporting, heating, melting, and mixing the material. For this reason, the stability of the process and the quality of the final product depend to a great extent on the screw design. The screw consists of a long cylinder surrounded by a helical fillet (**Figure 2**). The most important parameters to design it are: the length (L), diameter (D), the angle of the propeller (θ), and the thread pitch (w). When only one screw is used, the machine is called single-screw extruder, while when two screws are used, the term double-screw or twin-screw extruder is used. The mixing is highly dependent on the number of screws and its configuration. In the case of twin-screw extruders, the screws can co-rotate or counter-rotate

polymer through the nozzle.

*2.3.2. Screw configuration*

the extruder by means of the forming nozzle [26].

Melt extrusion is a continuous process that consists of passing a material in the molten state through a profile or given geometry. The preparation of a large variety of polymeric nanocomposites has been reported through this process from varying the polymer matrix to modifying the geometry and size of the nanoparticles to be used. It has been observed that the size and dispersion of the nanoparticles in the polymer are related to the improvement in the properties of the final nanocomposite. A great challenge in the preparation of polymeric nanocomposites is to achieve the homogeneous dispersion of the nanoparticles in the polymer matrix, knowing that a complete dispersion of the nanoparticles allows a greater matrix-nanoparticle interaction responsible for the improved properties in the final material [23].

#### *2.3.1. Melt extrusion*

In a broad definition, the extrusion process refers to any transformation operation in which a molten material is forced to traverse a nozzle to produce an article of constant cross section and in principle of indefinite length [24]. From the point of view of plastics, melt extrusion is clearly one of the most important processes of transformation, where the polymer is fed

**Figure 1.** (a) Basic diagram of an extruder, (b) evolution of the pressure along an extruder [25].

in solid form; it is heated until reaching the molten state and leaves the extruder in the latest state. In this case, the extruder acts as a pump, providing the necessary pressure to pass the polymer through the nozzle.

An extruder must have a system for feeding the material, a system for melting-plasticizing it, a pumping and pressurizing system (which usually generates a mixing effect), and finally, a device for forming the molten material. **Figure 1a** shows a basic scheme of an extruder [25]. Depending on the pressure that is exerted along the barrel or extrusion barrel, three main zones can be identified as indicated in **Figure 1b**. The feeding zone is the closest to the feeding of the material, where it is gradually compacted at a certain speed. The transition zone is a zone of intermediate compression of the material where the fusion takes place, in addition to which the air that could be trapped in the same escapes by means of the feed hopper. Finally, there is the dosing zone, in which the molten material is homogenized and pressurized to exit the extruder by means of the forming nozzle [26].

#### *2.3.2. Screw configuration*

Mechanical agitation is a common technique for the dispersion of nanoparticles in liquid systems; however, after a while, the nanoparticles tend to agglomerate. To improve the efficiency of dispersion and exfoliation, the ultrasound waves have been applied to stir particles, taking as the separation of individualized nanoparticles results. Ultrasonication is an effective method to disperse CNT in liquids that have a low viscosity, such as water, acetone, and ethanol. In this stage of application of ultrasound, some power factors must be taken care of, for example, because in the case of nanotubes, it has been seen that ultrasound waves can induce defects such as the formation of amorphous carbon in the CNT [19]; recommendations have been made as a sonication of the low power bath to preserve the length and structure of

After applying these methods to modify the nanoparticles aiming to improve their dispersion in the polymer, it is necessary to consider the conditions of the extrusion process where these nanoparticles are incorporated, since it can be in different modalities or extrusion conditions

In general, the most used mixing methods for the preparation of nanocomposites are insitu polymerization, solution, and melt mixing. In the in-situ polymerization method, the nanoparticles are first dispersed in the liquid monomer (or a monomer solution), and from there, they are mixed to carry out the polymerization, which can be initiated by heat or by the diffusion of an initiator. In the solution method, the polymer is dissolved in a solvent, and the filler is dispersed in the same solution. The intercalated nanocomposite is obtained by removing the solvent by vaporization or precipitation [21]. Because both processes use a solvent, it is not practical at the industrial level. The melt mixing method takes advantage of the melt temperature of the polymer matrix, and in this way, it achieves the mixing with the nanoparticles. Within this method, one of the most striking is the melt extrusion process [22]. Melt extrusion is a continuous process that consists of passing a material in the molten state through a profile or given geometry. The preparation of a large variety of polymeric nanocomposites has been reported through this process from varying the polymer matrix to modifying the geometry and size of the nanoparticles to be used. It has been observed that the size and dispersion of the nanoparticles in the polymer are related to the improvement in the properties of the final nanocomposite. A great challenge in the preparation of polymeric nanocomposites is to achieve the homogeneous dispersion of the nanoparticles in the polymer matrix, knowing that a complete dispersion of the nanoparticles allows a greater matrix-nanoparticle

in order to avoid reagglomerations or degradations of the polymer.

interaction responsible for the improved properties in the final material [23].

In a broad definition, the extrusion process refers to any transformation operation in which a molten material is forced to traverse a nozzle to produce an article of constant cross section and in principle of indefinite length [24]. From the point of view of plastics, melt extrusion is clearly one of the most important processes of transformation, where the polymer is fed

the CNT [20].

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*2.3.1. Melt extrusion*

**2.3. Manufacture of nanocomposites**

One of the most important parts in this process is the screw and the barrel, since they contribute to carry out the functions of transporting, heating, melting, and mixing the material. For this reason, the stability of the process and the quality of the final product depend to a great extent on the screw design. The screw consists of a long cylinder surrounded by a helical fillet (**Figure 2**). The most important parameters to design it are: the length (L), diameter (D), the angle of the propeller (θ), and the thread pitch (w). When only one screw is used, the machine is called single-screw extruder, while when two screws are used, the term double-screw or twin-screw extruder is used. The mixing is highly dependent on the number of screws and its configuration. In the case of twin-screw extruders, the screws can co-rotate or counter-rotate

**Figure 2.** Screw of an extruder [25].

and have different degrees of interpenetration. In **Figure 3**, some possible variants are shown. The advantages of its use include a good mixing and degassing capacity, as well as good control of the residence time and its distribution. Some disadvantages of these extruders are their price superior to that of the single screw and the fact that their performance is difficult to predict.

It is important to mention that the selection of a twin-screw extruder to a single-screw extruder depends mainly on the efficient transport as a function of the friction of the material with the barrel and the extrusion screw. In a single screw-extruder, a high level of friction material/ barrel and a low level in the screw provide a high carry per revolution. On the other hand, a poor carry per revolution will result in a low level of friction in the barrel and a high level in the screw. In addition, the amount of friction between the metal of the barrel or the screw and the performance of the extruder can change caused by a decrease in temperature. These troubles are minimized in a twin-screw extruder, where the interlock between the screws forms very close chambers, carrying the material forward [27].

#### *2.3.3. Screw configuration modification*

One aspect of great relevance is the definitive choice of the number and geometric design of the areas of the screw; this decision depends not only on the design of the nozzle and the expected flow rates but also on the melting characteristics of the polymer, its rheological behavior, and the speed of the screw. A simple screw, of three zones, is usually defined according to the number of turns of the propeller in the areas of feeding, compression, and dosing. An example of different screw configurations is shown in **Figure 4**.

#### *2.3.4. Mixing lines*

Most plastics need a previous stage of mixing before processing. Sometimes, it requires only extensive mixing, where the components of the formulation are mixed superficially and is made in fast mixers, and in other, intensive mixing of the different components of the formulation is necessary and is usually carried out in extruders. In some cases, both are necessary, extensive mixing prior to intensive. The use of twine-screw extruders is common in mixing lines. The configuration of the line is determined, among other things, by the type of additives to be combined in the extrusion. These lines usually have pelletizers at the extruder exit to obtain the material in pellet form. When additives or abrasive fillers have to be mixed with

the polymer, the polymer is usually added in the first feed hopper, and the filler is added once the plastic has melted, thereby reducing wear of the extruder caused by the filling. With large amounts of filler, the melt often has a large amount of air, steam, or gases, and so the extruder

**Figure 3.** Possible arrangement of the spindles in the twin screw extruders; (a) rotation against rotary and (b) rotary

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must have a vent zone (**Figure 5**).

**Figure 4.** Examples of different types of screw [25].

rotation; different degrees of interpenetration of the screws [25].

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**Figure 3.** Possible arrangement of the spindles in the twin screw extruders; (a) rotation against rotary and (b) rotary rotation; different degrees of interpenetration of the screws [25].

**Figure 4.** Examples of different types of screw [25].

and have different degrees of interpenetration. In **Figure 3**, some possible variants are shown. The advantages of its use include a good mixing and degassing capacity, as well as good control of the residence time and its distribution. Some disadvantages of these extruders are their price superior to that of the single screw and the fact that their performance is difficult

It is important to mention that the selection of a twin-screw extruder to a single-screw extruder depends mainly on the efficient transport as a function of the friction of the material with the barrel and the extrusion screw. In a single screw-extruder, a high level of friction material/ barrel and a low level in the screw provide a high carry per revolution. On the other hand, a poor carry per revolution will result in a low level of friction in the barrel and a high level in the screw. In addition, the amount of friction between the metal of the barrel or the screw and the performance of the extruder can change caused by a decrease in temperature. These troubles are minimized in a twin-screw extruder, where the interlock between the screws

One aspect of great relevance is the definitive choice of the number and geometric design of the areas of the screw; this decision depends not only on the design of the nozzle and the expected flow rates but also on the melting characteristics of the polymer, its rheological behavior, and the speed of the screw. A simple screw, of three zones, is usually defined according to the number of turns of the propeller in the areas of feeding, compression, and

Most plastics need a previous stage of mixing before processing. Sometimes, it requires only extensive mixing, where the components of the formulation are mixed superficially and is made in fast mixers, and in other, intensive mixing of the different components of the formulation is necessary and is usually carried out in extruders. In some cases, both are necessary, extensive mixing prior to intensive. The use of twine-screw extruders is common in mixing lines. The configuration of the line is determined, among other things, by the type of additives to be combined in the extrusion. These lines usually have pelletizers at the extruder exit to obtain the material in pellet form. When additives or abrasive fillers have to be mixed with

forms very close chambers, carrying the material forward [27].

dosing. An example of different screw configurations is shown in **Figure 4**.

*2.3.3. Screw configuration modification*

*2.3.4. Mixing lines*

to predict.

**Figure 2.** Screw of an extruder [25].

168 Nanocomposites - Recent Evolutions

the polymer, the polymer is usually added in the first feed hopper, and the filler is added once the plastic has melted, thereby reducing wear of the extruder caused by the filling. With large amounts of filler, the melt often has a large amount of air, steam, or gases, and so the extruder must have a vent zone (**Figure 5**).

**Figure 5.** Typical mixing line [25].

The characteristics of the melt extrusion process both in the selection and configuration of the screw type, as well as the feeding of the materials, affect the pre-dispersion of the nanoparticles, since a homogeneous predispersion will improve the dispersion efficiency when using ultrasound.
