**3. Ultrasound-assisted extrusion applied to the manufacture of polymeric nanocomposites**

In the last decade, the application of ultrasound waves for the preparation of polymer nanocomposites by melt extrusion has shown a growing interest. It seems that the scientific interest is ten times larger than the industrial interest, since only 3 patents have been registered in comparison to 36 published articles, as can be seen in **Figure 6**. This gives us a perspective of the relevance that this technology has had in recent years. Several studies report the preparation of polymer nanocomposites by means of ultrasound-assisted extrusion, resulting in the break nanoparticle agglomerates as nanoclays, as well as improvements in the dispersion of nanoparticles in a polyamide 6 [28]. Another study reported an improvement in both rheological and mechanical properties after the ultrasonic treatment, where it is also shown that this change in properties is attributed to the decrease in the size of the clay agglomerates in HDPE [29].

**3.1. Fundamentals of ultrasound applied to the manufacture of nanocomposites**

equipment power, can reach frequencies up to 2 MHz [33].

nanocomposites (last 10 years).

Before delving into the subject, it is necessary to mention that sound is a mechanical wave that needs a medium for its propagation. This medium can be liquid, solid, or gas. The propagation of the sound according to the medium can be transverse and longitudinal, and this will depend on the direction in which the energy travels. The frequency of audible sound for humans is between 20 Hz and 20 kHz. That inaudible sound with values of frequencies above 20 kHz is known as ultrasound. The ultrasound of low power or high frequency corresponds to the sound of low amplitude (higher frequency) and is related to the physical effect of the medium on the wave and is in a range of 2–10 MHz; these frequencies are widely used in the medical area for obtaining images and chemical analysis. On the other hand, ultrasound of high power (low frequency), between 20 and 100 kHz, is used for cleaning, plastic welding, as well as for the area of sonochemistry, which with the development of high-performance

**Figure 6.** Production of articles and patents on ultrasound-assisted extrusion for the preparation of polymeric

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Chemical and physical effects of ultrasound in liquid systems are typically explained in terms of acoustic cavitation. The definition of cavitation is complicated. In some cases, acoustic cavitation is defined as an isothermal transition of the liquid-vapor phase limit in a fluid due to a decrease in pressure, as a response to the change below of the vapor pressure of the liquid, or when the temperature has risen above the boiling point [34]. In both cases, acoustic cavitation

Other authors argue that the application of ultrasound to extrusion has to be carried out in stages to favor the dispersion of the nanoparticles, as in the case of carbon nanotubes (CNT), where it has been found that the dispersion of CNT can be favored when using two stages of processing. The first is the preparation of a masterbach (concentrate method), which is then diluted in the polymer to increase the dispersion of CNT. It is generally accepted that the dispersion is improved due to the high voltage of cutting that acts on the agglomerates during the second stage [30]. The combination of the masterbach technique with assisted ultrasound has been an important improvement for the dispersion of nanoparticles in polymeric matrices, mainly tested in polymer-CNT systems [31]; in turn, it has also been shown that ultrasound can favor the hybridization of polymer chains on the nanoparticles [32].

**Figure 6.** Production of articles and patents on ultrasound-assisted extrusion for the preparation of polymeric nanocomposites (last 10 years).

#### **3.1. Fundamentals of ultrasound applied to the manufacture of nanocomposites**

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

In the last decade, the application of ultrasound waves for the preparation of polymer nanocomposites by melt extrusion has shown a growing interest. It seems that the scientific interest is ten times larger than the industrial interest, since only 3 patents have been registered in comparison to 36 published articles, as can be seen in **Figure 6**. This gives us a perspective of the relevance that this technology has had in recent years. Several studies report the preparation of polymer nanocomposites by means of ultrasound-assisted extrusion, resulting in the break nanoparticle agglomerates as nanoclays, as well as improvements in the dispersion of nanoparticles in a polyamide 6 [28]. Another study reported an improvement in both rheological and mechanical properties after the ultrasonic treatment, where it is also shown that this change in properties is attributed to the decrease in the size of the clay agglomerates in

Other authors argue that the application of ultrasound to extrusion has to be carried out in stages to favor the dispersion of the nanoparticles, as in the case of carbon nanotubes (CNT), where it has been found that the dispersion of CNT can be favored when using two stages of processing. The first is the preparation of a masterbach (concentrate method), which is then diluted in the polymer to increase the dispersion of CNT. It is generally accepted that the dispersion is improved due to the high voltage of cutting that acts on the agglomerates during the second stage [30]. The combination of the masterbach technique with assisted ultrasound has been an important improvement for the dispersion of nanoparticles in polymeric matrices, mainly tested in polymer-CNT systems [31]; in turn, it has also been shown that ultrasound

can favor the hybridization of polymer chains on the nanoparticles [32].

**3. Ultrasound-assisted extrusion applied to the manufacture of** 

ultrasound.

HDPE [29].

**polymeric nanocomposites**

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

170 Nanocomposites - Recent Evolutions

Before delving into the subject, it is necessary to mention that sound is a mechanical wave that needs a medium for its propagation. This medium can be liquid, solid, or gas. The propagation of the sound according to the medium can be transverse and longitudinal, and this will depend on the direction in which the energy travels. The frequency of audible sound for humans is between 20 Hz and 20 kHz. That inaudible sound with values of frequencies above 20 kHz is known as ultrasound. The ultrasound of low power or high frequency corresponds to the sound of low amplitude (higher frequency) and is related to the physical effect of the medium on the wave and is in a range of 2–10 MHz; these frequencies are widely used in the medical area for obtaining images and chemical analysis. On the other hand, ultrasound of high power (low frequency), between 20 and 100 kHz, is used for cleaning, plastic welding, as well as for the area of sonochemistry, which with the development of high-performance equipment power, can reach frequencies up to 2 MHz [33].

Chemical and physical effects of ultrasound in liquid systems are typically explained in terms of acoustic cavitation. The definition of cavitation is complicated. In some cases, acoustic cavitation is defined as an isothermal transition of the liquid-vapor phase limit in a fluid due to a decrease in pressure, as a response to the change below of the vapor pressure of the liquid, or when the temperature has risen above the boiling point [34]. In both cases, acoustic cavitation is presented as a response to the decrease in pressure due to the propagation of an acoustic wave. In other words, during the expansion and compression characteristic of ultrasound waves, there is a formation, growth, and the implosive collapse of bubbles. But how is this bubble formed? The nucleus theory states that any liquid contains intrinsically tiny spaces (cavitation nuclei) full of gases, which undergo a change of pressure to quickly grow to cavities and then to bubbles. However, this principle has evolved, and it is accepted that a nucleus is needed that originates cavitation. The formation of this core can occur in two ways: for pure homogeneous liquid that does not contain impurities or gas, cavities will form due to the effect that the acoustic pressure will have on the liquid called homogeneous nucleation. In real systems or practical experiments, it is thought that a heterogeneous nucleation occurs, in which the neighboring liquid molecules are broken because the liquid contains "weak sites," in the limits of the liquid and a solid or in the liquid-solid-gas interfaces, where cavitation can start more easily [35]. These cavitation cores generate bubbles that expand during the phase of rarefaction and collapse during the compression phase; stable and transient bubbles are formed [36]. Stable bubbles can remain oscillating during many cycles of acoustic pressure. On the other hand, transients generally exist for less than one cycle; during this cycle, they expand at least twice their original size and then collapse violently. It is said that the pressure and temperature inside the bubble increase to more than 1000 atm and 5000 K [37] during cavitation (**Figure 7**). The collapse of the bubble is a violent process which generates localized shock waves, which results in an effect on the liquid or solid.

double or single screw extruders with different arrangements in their mixing zones are used, at speeds ranging from 50 to 100 rpm, in order to improve the efficiency in the dispersion of nanoparticles, while temperature profiles vary according to the polymer-nanoparticle system. As for the treatment with ultrasound, a specially designed camera to contain a sonotrode is attached to the extruder, which in general according to the literature is usually made of titanium. This chamber has a controlled temperature and a nozzle to extract the molten nanocomposite. The sonotrode is connected to an ultrasonic generator, which operates at frequency intervals that can range from 10 to 100 kHz and with powers that can reach 1000 W. The data obtained are usually collected by means of an oscilloscope. **Figure 8** shows a system devel-

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*3.2.1. Technical characteristics of the design of the process of ultrasound-assisted extrusion*

Ultrasound has been applied to molten polymers as a very efficient way to reduce the resistance of the shaping channels by decreasing the viscosity of the polymers. The results showed that the application of ultrasound disturbs the convergent flow of molten polymer in the entrance zone and changes the flow patterns, which leads to lower elastic stresses, increasing the movement of the molecular chains, so that the elastic recovery is faster. Guo et al. [39] demonstrated significant changes in the properties of polymeric materials by applying ultrasound during the extrusion process and applying ultrasonic oscillations in the direction parallel to the polymer melt flow (**Figure 9**). Scientists at the University of Akron have applied longitudinal vibrations in the direction perpendicular to the direction of flow using two sonotrodes symmetrically in the nozzle during the extrusion double screw of polymer nanocomposites containing carbon nanotubes and polypropylene clays (Isayev et al. [30]; **Figure 10**). In addition to dispersing nanocomposites, the application of ultrasound to the polymer has resulted in an increase in crystallinity, the reduction of structural defects, and the

Ultrasound in the extrusion process has been used to improve the compatibility and dispersion of additives. The effects of ultrasound on polymers can be both physical and chemical.

oped by Ávila-Orta et al. [38].

improvement of mechanical properties.

**Figure 8.** Typical configuration of an extruder coupled with an ultrasound device.

#### **3.2. Preparation of nanocomposites by means of ultrasound-assisted extrusion**

The incorporation of ultrasound in melt processing methods requires, in its simplest form of a processing system or equipment, a sonotrode, and an ultrasonic wave generator. At present,

**Figure 7.** Acoustic cavitation phenomenon in Newtonian fluid.

double or single screw extruders with different arrangements in their mixing zones are used, at speeds ranging from 50 to 100 rpm, in order to improve the efficiency in the dispersion of nanoparticles, while temperature profiles vary according to the polymer-nanoparticle system. As for the treatment with ultrasound, a specially designed camera to contain a sonotrode is attached to the extruder, which in general according to the literature is usually made of titanium. This chamber has a controlled temperature and a nozzle to extract the molten nanocomposite. The sonotrode is connected to an ultrasonic generator, which operates at frequency intervals that can range from 10 to 100 kHz and with powers that can reach 1000 W. The data obtained are usually collected by means of an oscilloscope. **Figure 8** shows a system developed by Ávila-Orta et al. [38].

#### *3.2.1. Technical characteristics of the design of the process of ultrasound-assisted extrusion*

Ultrasound has been applied to molten polymers as a very efficient way to reduce the resistance of the shaping channels by decreasing the viscosity of the polymers. The results showed that the application of ultrasound disturbs the convergent flow of molten polymer in the entrance zone and changes the flow patterns, which leads to lower elastic stresses, increasing the movement of the molecular chains, so that the elastic recovery is faster. Guo et al. [39] demonstrated significant changes in the properties of polymeric materials by applying ultrasound during the extrusion process and applying ultrasonic oscillations in the direction parallel to the polymer melt flow (**Figure 9**). Scientists at the University of Akron have applied longitudinal vibrations in the direction perpendicular to the direction of flow using two sonotrodes symmetrically in the nozzle during the extrusion double screw of polymer nanocomposites containing carbon nanotubes and polypropylene clays (Isayev et al. [30]; **Figure 10**). In addition to dispersing nanocomposites, the application of ultrasound to the polymer has resulted in an increase in crystallinity, the reduction of structural defects, and the improvement of mechanical properties.

Ultrasound in the extrusion process has been used to improve the compatibility and dispersion of additives. The effects of ultrasound on polymers can be both physical and chemical.

**Figure 8.** Typical configuration of an extruder coupled with an ultrasound device.

**Figure 7.** Acoustic cavitation phenomenon in Newtonian fluid.

shock waves, which results in an effect on the liquid or solid.

172 Nanocomposites - Recent Evolutions

**3.2. Preparation of nanocomposites by means of ultrasound-assisted extrusion**

The incorporation of ultrasound in melt processing methods requires, in its simplest form of a processing system or equipment, a sonotrode, and an ultrasonic wave generator. At present,

is presented as a response to the decrease in pressure due to the propagation of an acoustic wave. In other words, during the expansion and compression characteristic of ultrasound waves, there is a formation, growth, and the implosive collapse of bubbles. But how is this bubble formed? The nucleus theory states that any liquid contains intrinsically tiny spaces (cavitation nuclei) full of gases, which undergo a change of pressure to quickly grow to cavities and then to bubbles. However, this principle has evolved, and it is accepted that a nucleus is needed that originates cavitation. The formation of this core can occur in two ways: for pure homogeneous liquid that does not contain impurities or gas, cavities will form due to the effect that the acoustic pressure will have on the liquid called homogeneous nucleation. In real systems or practical experiments, it is thought that a heterogeneous nucleation occurs, in which the neighboring liquid molecules are broken because the liquid contains "weak sites," in the limits of the liquid and a solid or in the liquid-solid-gas interfaces, where cavitation can start more easily [35]. These cavitation cores generate bubbles that expand during the phase of rarefaction and collapse during the compression phase; stable and transient bubbles are formed [36]. Stable bubbles can remain oscillating during many cycles of acoustic pressure. On the other hand, transients generally exist for less than one cycle; during this cycle, they expand at least twice their original size and then collapse violently. It is said that the pressure and temperature inside the bubble increase to more than 1000 atm and 5000 K [37] during cavitation (**Figure 7**). The collapse of the bubble is a violent process which generates localized

**Figure 9.** Schematic diagram of the ultrasonic irradiation extrusion system used by Guo et al. [39].

shown in **Figure 11**. It is important to mention that within the aspects that modify the dispersion of the nanoparticles is the intensity of the applied ultrasound, where it has been demonstrated that the power of the ultrasound is a function of the reduction in the size of the agglomerates of nanoparticles, which favors the dispersion. It has also been found that a good exfoliation and dispersion are improved at low extrusion rates in order to increase the time of the ultrasonic treatment [44]. However, high exposure times to ultrasonic vibrations produce a degradation of the polymeric material, that is, there is a breakdown of the

**Figure 11.** Some configurations of the process of extrusion assisted by ultrasound in molten polymers. (a) [40], (d) [41] ultrasound equipment placed at the exit of the extruder. (b) [42], (c) [43] ultrasound chamber along the extrusion

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*3.2.2. Characteristics and final properties of NCPs produced by ultrasound-assisted extrusion*

In the last two decades, the effect of ultrasound in the preparation of nanocomposites has been studied. In 2003, Isayev and Hong employed for the first time the ultrasonic vibration to prepare nanocomposites. This study reported that the application of ultrasound improves the dispersion and reduced size of silica agglomerate (0.3 μm). The viscosity of the ultrasonically treated mixtures was found to be higher than that of the silane-treated

**Table 1** summarizes information from publications involving the use of the ultrasonic treatment technology for the preparation of polymer nanocomposites. The information shows the polymer matrix studied, the nanoparticles, the focus of the study, and improved properties. It is evident that the most studied structures using ultrasound are clays and those based on

polymer chains, as demonstrated by means of rheological studies.

mixtures.

equipment.

carbon such as graphene and nanotubes.

**Figure 10.** Schematic diagram of the ultrasonic irradiation extrusion system used by scientists at the University of Akron, Isayev et al. [30].

Some physical changes induced by ultrasound in polymer systems are the dispersion of loads and other base components. Several systems have been developed, where good results of nanoparticle dispersion are obtained. Different ways of feeding and positions of the ultrasound along the zones of the extruder have been tested, aiming to find out the behavior of the nanoparticles in the matrix depending on the type of configuration. Some configurations of ultrasound-assisted extrusion of equipment that have been patented are

**Figure 11.** Some configurations of the process of extrusion assisted by ultrasound in molten polymers. (a) [40], (d) [41] ultrasound equipment placed at the exit of the extruder. (b) [42], (c) [43] ultrasound chamber along the extrusion equipment.

shown in **Figure 11**. It is important to mention that within the aspects that modify the dispersion of the nanoparticles is the intensity of the applied ultrasound, where it has been demonstrated that the power of the ultrasound is a function of the reduction in the size of the agglomerates of nanoparticles, which favors the dispersion. It has also been found that a good exfoliation and dispersion are improved at low extrusion rates in order to increase the time of the ultrasonic treatment [44]. However, high exposure times to ultrasonic vibrations produce a degradation of the polymeric material, that is, there is a breakdown of the polymer chains, as demonstrated by means of rheological studies.

#### *3.2.2. Characteristics and final properties of NCPs produced by ultrasound-assisted extrusion*

Some physical changes induced by ultrasound in polymer systems are the dispersion of loads and other base components. Several systems have been developed, where good results of nanoparticle dispersion are obtained. Different ways of feeding and positions of the ultrasound along the zones of the extruder have been tested, aiming to find out the behavior of the nanoparticles in the matrix depending on the type of configuration. Some configurations of ultrasound-assisted extrusion of equipment that have been patented are

**Figure 10.** Schematic diagram of the ultrasonic irradiation extrusion system used by scientists at the University of Akron,

**Figure 9.** Schematic diagram of the ultrasonic irradiation extrusion system used by Guo et al. [39].

Isayev et al. [30].

174 Nanocomposites - Recent Evolutions

In the last two decades, the effect of ultrasound in the preparation of nanocomposites has been studied. In 2003, Isayev and Hong employed for the first time the ultrasonic vibration to prepare nanocomposites. This study reported that the application of ultrasound improves the dispersion and reduced size of silica agglomerate (0.3 μm). The viscosity of the ultrasonically treated mixtures was found to be higher than that of the silane-treated mixtures.

**Table 1** summarizes information from publications involving the use of the ultrasonic treatment technology for the preparation of polymer nanocomposites. The information shows the polymer matrix studied, the nanoparticles, the focus of the study, and improved properties. It is evident that the most studied structures using ultrasound are clays and those based on carbon such as graphene and nanotubes.


They found that the waves of ultrasound improve the compatibility between PP and PS and breakup of the clay agglomerates and as a result exfoliated the clay layers in the PP/PS matrix [64]. Similar observations were made for PP/clay nanocomposites. Two methods for the fabrication of polypropylene/clay nanocomposites are compared. In the first approach, a twostage process was implemented. First, the nanocomposites were prepared using a co-rotating twin-screw extruder followed by a single-screw extruder, in which the ultrasound was implemented. In the second method, a single-stage process was used. In addition, two regimens of feeding were used in the process. In both processes, it was observed that the ultrasound generates a degradation of the polymer matrix and intercalation/exfoliation of clay; however, the single-stage process led to a minor polymer degradation [65]. Li et al. [28] prepared polyamide 6/montmorillonite nanocomposites by using a conventional and an ultrasonic extrusion technology. The results showed that the elongation at break and impact strength of the ultrasonicated nanocomposites increase due to the improved dispersion of montmorillonite and decreased size of spherulites [28]. Other works have focused on the preparation of clay nanocomposites with different polymer matrix as a HDPE and LLDPE. For example, Niknezhad and Isayev [59] applied ultrasound continuous method for the production of films polymer/clay nanocomposites. In this process, compounding, ultrasonic treatment, and film casting were combined in a single-step process. It has been found that the effect of the dispersion of the clay depends on the amplitude of ultrasound used, affecting the crystallinity and the mechanical properties of the material, as well as the permeability to gases [59]. On the other hand, the application of ultrasound irradiation and maleic anhydride (MA) addition, during the preparation of PP/Clay nanocomposites in a twin screw extruder, showed to have a very significant effect on the simultaneous grafting of MA onto the PP chains and in the exfoliation/dispersion of the clay. The tensile modulus increased with ultrasound intensity, and an opposite effect occurs with elongation, which decreases with the applied ultrasound [58]. As for carbon-based nanocomposites, polyetherimide (PEI) systems with 20% carbon nanofibers (CNF) have been studied. It was established that ultrasound with high power is effective in obtaining relatively more homogeneous dispersion with improved electrical and thermal conductivity in the CNF/PEI nanocomposites in comparison with extruded untreated ones. An increase in Young's modulus was observed while retaining tensile strength up to 15% of CNF [54]. In another study, it was mentioned that the effect of ultrasound on the rheological, electrical, morphological, and mechanical properties of the Polyetherimide (PEI) matrix with multiple-wall carbon nanotubes (MWCNT) has been carried out from 1 to 10% by weight. In ultrasound-treated nanocomposites, an increase in viscosity and storage module was observed. As for the mechanical properties, the authors conclude that there is a relationship between the content of MWCNT and the application of ultrasound because the Young module and the resistance showed an increase by using 5 and 10% load. The authors also notice that working amplitudes are important factor to improve the dispersion. Rheological and electrical percolations were found between 1 and 2% in load weight of MWCNT. The observed effect is attributed to the fact that the ultrasound breaks the agglomerates of MWCNT improving its dispersion, which affects to a greater degree the rheology of the material than to the electrical conductivity [30]. Blanco et al. [53] mention that ultrasonic vibration has a significant effect on the conductivity of PA/MWCNT systems; in these nanocomposites, the percolation rate is reduced from 7 to 3 wt% when ultrasound is applied. This is attributed to a better

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**Table 1.** Summary of the experimental results of some reviewed publications involving the application of extrusion assisted by ultrasound for preparation of polymer nanocomposites, last 10 years.

Regarding the use of ultrasound in the preparation of nanocomposites with clay, improvements in degree of clay dispersion have been found. Kim and co-workers made use of the ultrasonic-assisted continuous extrusion process to the preparation of polypropylene (PP) and polystyrene (PS) nanocomposites with 3% loading of organophilic montmorillonite clay. They found that the waves of ultrasound improve the compatibility between PP and PS and breakup of the clay agglomerates and as a result exfoliated the clay layers in the PP/PS matrix [64]. Similar observations were made for PP/clay nanocomposites. Two methods for the fabrication of polypropylene/clay nanocomposites are compared. In the first approach, a twostage process was implemented. First, the nanocomposites were prepared using a co-rotating twin-screw extruder followed by a single-screw extruder, in which the ultrasound was implemented. In the second method, a single-stage process was used. In addition, two regimens of feeding were used in the process. In both processes, it was observed that the ultrasound generates a degradation of the polymer matrix and intercalation/exfoliation of clay; however, the single-stage process led to a minor polymer degradation [65]. Li et al. [28] prepared polyamide 6/montmorillonite nanocomposites by using a conventional and an ultrasonic extrusion technology. The results showed that the elongation at break and impact strength of the ultrasonicated nanocomposites increase due to the improved dispersion of montmorillonite and decreased size of spherulites [28]. Other works have focused on the preparation of clay nanocomposites with different polymer matrix as a HDPE and LLDPE. For example, Niknezhad and Isayev [59] applied ultrasound continuous method for the production of films polymer/clay nanocomposites. In this process, compounding, ultrasonic treatment, and film casting were combined in a single-step process. It has been found that the effect of the dispersion of the clay depends on the amplitude of ultrasound used, affecting the crystallinity and the mechanical properties of the material, as well as the permeability to gases [59]. On the other hand, the application of ultrasound irradiation and maleic anhydride (MA) addition, during the preparation of PP/Clay nanocomposites in a twin screw extruder, showed to have a very significant effect on the simultaneous grafting of MA onto the PP chains and in the exfoliation/dispersion of the clay. The tensile modulus increased with ultrasound intensity, and an opposite effect occurs with elongation, which decreases with the applied ultrasound [58].

As for carbon-based nanocomposites, polyetherimide (PEI) systems with 20% carbon nanofibers (CNF) have been studied. It was established that ultrasound with high power is effective in obtaining relatively more homogeneous dispersion with improved electrical and thermal conductivity in the CNF/PEI nanocomposites in comparison with extruded untreated ones. An increase in Young's modulus was observed while retaining tensile strength up to 15% of CNF [54]. In another study, it was mentioned that the effect of ultrasound on the rheological, electrical, morphological, and mechanical properties of the Polyetherimide (PEI) matrix with multiple-wall carbon nanotubes (MWCNT) has been carried out from 1 to 10% by weight. In ultrasound-treated nanocomposites, an increase in viscosity and storage module was observed. As for the mechanical properties, the authors conclude that there is a relationship between the content of MWCNT and the application of ultrasound because the Young module and the resistance showed an increase by using 5 and 10% load. The authors also notice that working amplitudes are important factor to improve the dispersion. Rheological and electrical percolations were found between 1 and 2% in load weight of MWCNT. The observed effect is attributed to the fact that the ultrasound breaks the agglomerates of MWCNT improving its dispersion, which affects to a greater degree the rheology of the material than to the electrical conductivity [30]. Blanco et al. [53] mention that ultrasonic vibration has a significant effect on the conductivity of PA/MWCNT systems; in these nanocomposites, the percolation rate is reduced from 7 to 3 wt% when ultrasound is applied. This is attributed to a better

Regarding the use of ultrasound in the preparation of nanocomposites with clay, improvements in degree of clay dispersion have been found. Kim and co-workers made use of the ultrasonic-assisted continuous extrusion process to the preparation of polypropylene (PP) and polystyrene (PS) nanocomposites with 3% loading of organophilic montmorillonite clay.

**Table 1.** Summary of the experimental results of some reviewed publications involving the application of extrusion

**Polymer nanocomposite Focus of the study Property improvement(s) Reference**

Improvement electrical and thermal conductivity, Young's modulus. Storage modulus and complex viscosity generally increased.

Improved dispersion, elongation at break, Young's modulus and tensile strength. The abrasion resistance was improved at certain amplitudes at low CNT loadings only.

Increased the exfoliation and dispersion of GNPs on the polymer. Thermal and conductivity properties were

Improved clay dispersion compared with non-treated ones, intercalated-exfoliated structures was found. Young's modulus enhanced and increase viscosity in most

increased.

studies.

process.

Improves nanoclay dispersion, which results in an enhancement of the reinforcement of the fillers and decreases the viscosity of the composites during the

Improve strength and elongation of the composites at break, ultrasound-induced homogeneous dispersion of nanoparticles in the polymeric matrix

Composite viscosity decreased as the ultrasound intensity and the filler content

decreased.

[30, 45–54]

[55, 56]

[47]

[57–60]

[61]

[62]

[63]

thermal, electrical, mechanical and rheological properties.

the ultrasonic amplitude and concentration on CNFs (morphology), rheology, electrical resistivity, abrasion and mechanical properties.

vibration on exfoliation, and dispersion of GNP's in the

dispersion, morphology, mechanical and rheological

dispersion and mechanical properties of polymer matrix

morphology, as well as the rheological and mechanical properties of the composites

Effects of the ultrasound intensity, experimental temperature, filler content, and particle size on the composite viscosity

MWCNT Effect of ultrasound on

CNF Effects of the variation of

GNP Effects of the ultrasound

polymer matrix.

Cloisite 20A Effects of ultrasound on clay

properties.

Sepiolite Effect of ultrasound on

Nanosilica Effects of ultrasound on the

assisted by ultrasound for preparation of polymer nanocomposites, last 10 years.

Carbon nanoparticles

176 Nanocomposites - Recent Evolutions

Ceramic nanoparticles

Other nanoparticles Flash aluminum

flake pigments (FAFP)

dispersion of nanotubes in the matrix, resulting in an increase of three orders of magnitude in the electrical resistivity for the system PA6/MWCNT at 7 wt%. These authors concluded that the application of ultrasound improves the processability of the material and that it is possible to reduce the percentage of nanotubes in the preparation of nanocomposites with conductive properties without affecting thermal properties [53]. Ávila-Orta et al. [51] used polypropylenes with different flow rates (MFI) and 10% multiwall carbon nanotubes for the preparation of nanocomposites. Four different fabrication methods based on melt extrusion were used. In the first method, melt extrusion fabrication without ultrasound assistance was used. In the second and third methods, an ultrasound probe attached to a hot chamber located at the exit of the die was used to subject the sample to fixed frequency and variable frequency, respectively. In the fourth method, the carbon nanotubes were treated in a fluidized air-bed with an ultrasound probe before being used in the fabrication of nanocomposites. It was found that the MFI decreases regardless of the method used in processing, the same is not the case with the other properties. For example, as to the size of agglomerates, the smallest value was found using PP of MFI = 2.5 using variable ultrasound frequency in processing; in this sample, it was found a lower surface/agglomerate ratio and a higher value of electrical charge (1040 V) [51]. A similar study showed that the electrical properties in nanocomposites of PP/ MWCNT with different values of MFI of the polymer matrix depend on the methods used in the ultrasound-assisted extrusion because the ultrasound waves decrease the agglomerates of nanotubes producing conductive materials and static dissipators with a negative dielectric constant [66].

surface will generate microjets of fluid of high velocity, directed toward the solid surface. The impact of microjets of fluid on the solid surface causes localized erosion. In addition to this effect, we have the formation of shock waves, inducing effects such as breaking aggregates

**Figure 12.** Comparison between the dynamics of the induced ultrasound of a Newtonian fluid and a non-Newtonian.

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Researchers have tried to explain the phenomenon of the dispersion of nanoparticles in polymeric matrices when using ultrasound in molten state. In this context, Zhong et al. argue that the propagation of the ultrasonic wave in a material generates waves of oscillatory pressure and induces the expansion and contraction of bubbles in the polymeric matrix that leads to a possible rupture of the agglomerates of nanoparticles, which would give place to a better dispersion. A small amount of bubbles usually dissolves or is trapped in the polymer that melts during extrusion [70]. In polymeric compounds, the particles are easily present in the form of porous agglomerates that introduce more gaps in the system. The existence of bubbles in the nanocomposites decreases the speed of the ultrasound and therefore the energy consumption. Based on experimental observation, a possible cavitation mechanism is suggested, depicted in **Figure 13**. The cavitation of bubbles in compounds can occur by internal and external cavitation mechanisms. The cavitation of the outer bubble could remove the particles from the primary agglomerates (**Figure 13a**), while the cavitation of the inner bubble would break the agglomerates from the inside (**Figure 13b**). One or both of these mechanisms would lead to better dispersion seen after

Espinoza-González proposed [72] a mechanism based on mechanochemistry to explain the physical and chemical effects of ultrasound in polymer matrices, as well as for the dispersion mechanism of nanostructures. This mechanochemical mechanism is mainly based on the deformation or stress experienced by the chemical bonds during the vibration movement. The generated vibration movement causes the appearance of different fatigue points along the polymer chain called nodes, in which the greatest deformation occurs between the links of the chain, reducing the energy of link dissociation leading to the activation of multiple

of particles [69].

ultrasonic treatment [71].

reaction mechanisms, degradation, or chain extension.

In summary, the application of ultrasound in the preparation of nanocomposites by extrusion generates an increase in some properties of great importance. However, the mechanisms by which ultrasound helps in the dispersion of nanoparticles is not known with precision, which is a significant aspect and would help to improve and create innovative methodologies aimed at the implementation of more specific nanocomposites.

#### **3.3. Models of the mechanism of dispersion of NPs in ultrasound-assisted extrusion**

The effect of ultrasound on fluids and Newtonian systems has been explained in terms of acoustic cavitation. This process, as mentioned above, involves at least three stages: nucleation, bubble growth, and the implosive collapse of the same, propitiating stable and transient acoustic cavitation events that are the cause of the effect of ultrasound. However, these physical or chemical effects will not be presented if ultrasound-led energy is less than the cavitation threshold [67]. In non-Newtonian fluids, the bubble in polymer solution implodes less violently compared to a Newtonian fluid such as water (**Figure 12**), which makes the impact of the liquid jet on the limit very small or even null. In particular, the dynamics of the collapse of bubbles near a solid boundary appears to be a critical problem in the dispersion of nanostructures in liquid systems, since the impact of the liquid jet on the surface of the agglomerates is considered mechanism dominant for the reduction of agglomerate size during acoustic cavitation [68].

If a solid is within the sample, the cavitation is given in a different way, due to the liquid– solid interface. An accepted explanation is that the cavitation that takes place near the solid

dispersion of nanotubes in the matrix, resulting in an increase of three orders of magnitude in the electrical resistivity for the system PA6/MWCNT at 7 wt%. These authors concluded that the application of ultrasound improves the processability of the material and that it is possible to reduce the percentage of nanotubes in the preparation of nanocomposites with conductive properties without affecting thermal properties [53]. Ávila-Orta et al. [51] used polypropylenes with different flow rates (MFI) and 10% multiwall carbon nanotubes for the preparation of nanocomposites. Four different fabrication methods based on melt extrusion were used. In the first method, melt extrusion fabrication without ultrasound assistance was used. In the second and third methods, an ultrasound probe attached to a hot chamber located at the exit of the die was used to subject the sample to fixed frequency and variable frequency, respectively. In the fourth method, the carbon nanotubes were treated in a fluidized air-bed with an ultrasound probe before being used in the fabrication of nanocomposites. It was found that the MFI decreases regardless of the method used in processing, the same is not the case with the other properties. For example, as to the size of agglomerates, the smallest value was found using PP of MFI = 2.5 using variable ultrasound frequency in processing; in this sample, it was found a lower surface/agglomerate ratio and a higher value of electrical charge (1040 V) [51]. A similar study showed that the electrical properties in nanocomposites of PP/ MWCNT with different values of MFI of the polymer matrix depend on the methods used in the ultrasound-assisted extrusion because the ultrasound waves decrease the agglomerates of nanotubes producing conductive materials and static dissipators with a negative dielectric

In summary, the application of ultrasound in the preparation of nanocomposites by extrusion generates an increase in some properties of great importance. However, the mechanisms by which ultrasound helps in the dispersion of nanoparticles is not known with precision, which is a significant aspect and would help to improve and create innovative methodologies aimed

**3.3. Models of the mechanism of dispersion of NPs in ultrasound-assisted extrusion**

The effect of ultrasound on fluids and Newtonian systems has been explained in terms of acoustic cavitation. This process, as mentioned above, involves at least three stages: nucleation, bubble growth, and the implosive collapse of the same, propitiating stable and transient acoustic cavitation events that are the cause of the effect of ultrasound. However, these physical or chemical effects will not be presented if ultrasound-led energy is less than the cavitation threshold [67]. In non-Newtonian fluids, the bubble in polymer solution implodes less violently compared to a Newtonian fluid such as water (**Figure 12**), which makes the impact of the liquid jet on the limit very small or even null. In particular, the dynamics of the collapse of bubbles near a solid boundary appears to be a critical problem in the dispersion of nanostructures in liquid systems, since the impact of the liquid jet on the surface of the agglomerates is considered mechanism dominant for the reduction of agglomerate size dur-

If a solid is within the sample, the cavitation is given in a different way, due to the liquid– solid interface. An accepted explanation is that the cavitation that takes place near the solid

at the implementation of more specific nanocomposites.

constant [66].

178 Nanocomposites - Recent Evolutions

ing acoustic cavitation [68].

**Figure 12.** Comparison between the dynamics of the induced ultrasound of a Newtonian fluid and a non-Newtonian.

surface will generate microjets of fluid of high velocity, directed toward the solid surface. The impact of microjets of fluid on the solid surface causes localized erosion. In addition to this effect, we have the formation of shock waves, inducing effects such as breaking aggregates of particles [69].

Researchers have tried to explain the phenomenon of the dispersion of nanoparticles in polymeric matrices when using ultrasound in molten state. In this context, Zhong et al. argue that the propagation of the ultrasonic wave in a material generates waves of oscillatory pressure and induces the expansion and contraction of bubbles in the polymeric matrix that leads to a possible rupture of the agglomerates of nanoparticles, which would give place to a better dispersion. A small amount of bubbles usually dissolves or is trapped in the polymer that melts during extrusion [70]. In polymeric compounds, the particles are easily present in the form of porous agglomerates that introduce more gaps in the system. The existence of bubbles in the nanocomposites decreases the speed of the ultrasound and therefore the energy consumption. Based on experimental observation, a possible cavitation mechanism is suggested, depicted in **Figure 13**. The cavitation of bubbles in compounds can occur by internal and external cavitation mechanisms. The cavitation of the outer bubble could remove the particles from the primary agglomerates (**Figure 13a**), while the cavitation of the inner bubble would break the agglomerates from the inside (**Figure 13b**). One or both of these mechanisms would lead to better dispersion seen after ultrasonic treatment [71].

Espinoza-González proposed [72] a mechanism based on mechanochemistry to explain the physical and chemical effects of ultrasound in polymer matrices, as well as for the dispersion mechanism of nanostructures. This mechanochemical mechanism is mainly based on the deformation or stress experienced by the chemical bonds during the vibration movement. The generated vibration movement causes the appearance of different fatigue points along the polymer chain called nodes, in which the greatest deformation occurs between the links of the chain, reducing the energy of link dissociation leading to the activation of multiple reaction mechanisms, degradation, or chain extension.

system as robust as ultrasound-assisted extrusion, it is necessary to simplify the system, so that the dispersion phenomenon can be analyzed from the simplest possible perspective. For example, studies can be carried out in batch systems of polymer melts and nanoparticles.

Ultrasound-Assisted Melt Extrusion of Polymer Nanocomposites

http://dx.doi.org/10.5772/intechopen.80216

181

In the last decade, the use of the ultrasonic assisted extrusion process has been used in the preparation of polymeric nanocomposites. This process has shown improvements in the dispersion of nanoparticles in the polymer matrix, which has led academics to make improvements in the design to achieve a greater effect on the properties of the final compound. Although the technique of ultrasound is known, it has not been possible to clearly explain the mechanisms of its action in polymer-nanoparticle systems, where despite the efforts made to achieve an adequate understanding of how the dispersion of nanoparticles occurs, it is still insufficient for the polymer nanocomposite theory to explain this phenomenon, and this limits the application of ultrasound in the manufacture of nanocomposites with specific properties. However, the large number of satisfactory results obtained in scientific articles on the novel properties and innovations that are made in patents on equipment and processing of nanocomposites provides a broad perspective of the evolution of this technology and its potential applications.

The authors acknowledge the financial support of CIQA through grant 6438 (2018), and of

, Diana Agüero-Valdez<sup>1</sup>

, José M. Mata-Padilla1

,

and

CONACyT through grants 294030 (LANIAUTO) and 296395 is greatly appreciated.

\*, Pablo González-Morones<sup>1</sup>

\*Address all correspondence to: carlos.avila@ciqa.edu.mx

, Juan G. Martínez-Colunga<sup>1</sup>

1 Centro de Investigación en Química Aplicada (CIQA), Saltillo, Coahuila, México

2 CONACYT—Unidad de Materiales, Centro de Investigación Científica de Yucatán (CICY),

**4. Conclusions**

**Acknowledgements**

**Conflict of interest**

**Author details**

Carlos A. Ávila-Orta<sup>1</sup>

Alain González-Sánchez<sup>1</sup>

Mérida, Yucatán, México

Víctor J. Cruz-Delgado<sup>2</sup>

The authors declare no conflict of interest.

**Figure 13.** Phenomenon of internal acoustic cavitation (a) and external (b) in polymeric nanocomposites according to Zhong et al. [71].

#### **3.4. Relevant aspects to consider in the dispersion mechanisms of NPs by ultrasound-assisted extrusion**

Ultrasound-assisted extrusion process turns out to be a very promising technology and that in the last 10 years has shown great advances in its application to the elaboration of polymeric nanocomposites. However, the mechanism to achieve the deagglomeration of nanoparticles and their dispersion in the polymer remains unknown. The phenomenon of acoustic cavitation is mainly proposed for the explanation of bubble dynamics, but it is possible to find in the literature and as mentioned above that the strict notion of cavitation is an isothermal transition of the liquid–vapor phase limit in a fluid of a single component, due to a decrease in pressure [34, 35, 73]. In other words, the cohesion between the fluid particles is overcomed by an externally applied stress, which causes the homogeneous nucleation of the vapor. Based on this argument and answering the initial question of the text on the phenomenon by which the dispersion of nanoparticles in polymeric systems results, the phenomenon of acoustic cavitation is questionable, since it is not enough to be able to explain the dispersion of nanoparticles during the ultrasound-assisted extrusion process to produce nanocomposites, since there is no phase change and also due to the viscoelastic characteristics of the polymer matrices that imply higher cutting efforts, which would hinder the formation of bubbles. However, it has been proven that in polymeric solutions, there is poor bubble formation due to cavitation effects [74]. On the other hand, there is also the idea that ultrasound causes vibrational effects on the polymer related to relaxation times at the chain level [75], which could help explain that the effects of friction in the polymer are the main causes of the dispersion of nanoparticles in the formation of nanocomposites.

A number of researches are still necessary to understand the effects of the different parameters (power, amplitude, and working frequencies) of the ultrasound waves in the preparation of nanocomposites, as well as the mechanism of action. To understand the nature of a system as robust as ultrasound-assisted extrusion, it is necessary to simplify the system, so that the dispersion phenomenon can be analyzed from the simplest possible perspective. For example, studies can be carried out in batch systems of polymer melts and nanoparticles.
