**3. Injected PP nanocomposites**

In the last three decades, a large interest in nanocomposites was seen in both academic and industry fields [21], due to their potential improvement of properties with a low content of the second phase. In the case of nanocomposites, nanofiller dispersion and orientation are very determinant of mechanical and thermal properties. In theory, only well-dispersed and exfoliated (nanoclays) nanoparticles could lead to an effective improvement of composite performance. Most of commercial nanoparticles, such as nanoclay, are hydrophilic and have weak adhesion or interaction with a hydrophobic matrix as PP, leading to a nanocomposite with a poor dispersion. As a solution to this problem, some producers recommend using masterbatches (MBs), which include all compatibilizers needed to promote nanoclay dispersion and have the additional advantage of being easy to process and compatible with standard processing as injection molding. In fact, some authors have reported nanocomposite preparation by using MB [22–25]. However, these studies indicate that nanoparticles were not exfoliated but intercalated. The influence of the flow pattern during injection molding on the fracture and impact properties of complex injected PP/clay nanocomposites has been studied [22, 23]. Nanoparticles were mostly intercalated—even though they were chemically modified and compatibilized—and both fracture parameters (KIC and G) and impact toughness were determined by molecular and nanoparticle orientation induced by the flow pattern. Toughness mechanisms—as particle delamination or separation were active only in certain loading directions. It was stated before that nanoclay delamination or splitting is an effective toughness mechanism in nanocomposites [26]. In PP, craze-like bands are one of the main mechanisms responsible for matrix energy absorption during deformation. To activate this mechanism, free surfaces are necessary for craze bands to initiate and nanoclay delamination produces those surfaces. However, these crazes can initiate only at the pole of clay particles, i.e., only particles oriented at 45° or more to load direction can induce multiple crazing in tensile-loaded samples and subsequently act as a toughening mechanism. Besides, weld and flow lines produced during filling acted as defects in the presence of nanoparticles [22, 23]. For PP/nanoclay composites under tensile conditions, the amount of absorbed energy was lower at the weld line than away from it and in the flow direction. This is a clear example of how injection molding flow pattern affects the piece performance of injected nanocomposites.

As nanofiller dispersion and exfoliation are crucial, a great effort has been made to improve them by adding additives. However, it is also possible to improve dispersion and exfoliation by changing their processing characteristics. An example of this is shear-controlled orientation in injection molding (SCORIM), which is a not conventional injection molding technique based on a shear-controlled application to the molten polymer during holding stage. SCORIM involves the use of a conventional injection molding machine with a special device with two

**63**

*Polypropylene Blends and Composite: Processing-Morphology-Performance Relationship…*

**Skin orientation (A110 index) Percentage of crystallinity (%) J integral (N/mm)** 0.15 31 94.6 0.165 37 83.8 0.17 40 73 0.189 37.5 40.6 0.19 41 51.4 0.195 39 29.8 0.20 42 8.6

pistons that generate the shear stresses. It was reported that SCORIM improves the performance of injected parts by controlling their morphology [27, 28]. Significant improvements were found in both stiffness and tensile resistance, molecular and filler orientation, dimensional tolerance, esthetic appearance, and weld line elimination in PP [29] and in its nanocomposites [30–32]. It was demonstrated that SCORIM changes the morphology of PP nanocomposites, not only in terms of molecular and nanoclay orientation but also in crystal phases present in PP matrix: the shear stresses driven by SCORIM process induce the formation of γ phase in PP nanocomposites [24, 33]. SCORIM induces a thicker skin in nanocomposites, i.e., a larger proportion of orientated molecules and clay particles, which favors the sliding of macromolecules, improving the deformation capability. Meanwhile, γ polymorph induces a larger-scale plastic deformation compared with the common α phase. γ phase promotes tearing of PP ligaments leading to fibrillation which is a toughness mechanism [34]. All these morphology features—better molecular and particle orientation and γ polymorph presence—

In case of carbon nanotubes (CNTs), it is important not only to attain a good dispersion but also to obtain an interconnected network morphology (above the percolation threshold) to lead an improvement in composites' performance. This morphology depends on nanotube orientation, dispersion, and distribution [35]. It is known that injection-molded parts have a higher percolation threshold than compressed ones due to the morphology and orientation induced by processing [36–38]. Moreover, weld lines could make particle dispersion and orientation even more complex for this kind of materials. It was reported that PP-/CNTinjected parts presented also agglomerates and an isotropic morphology induced by flow pattern during injection [39]. Also, an orientation profile of CNT trough molding thickness has been seen near to the injection point. CNT particles in the skin zone are oriented parallel to the flow front, while they tend to align transversally in the core zone. This has also been observed for fiber-reinforced polymers [40, 41]. In weld line region particularly, it was reported that agglomerates are more diffuse with a random orientation of CNT in both skin and core zones [42]. This heterogeneous orientation induces different fracture mechanisms in the pieces, weld line zone being the weakest part of injected pieces. Agglomerates act also as defects diminishing fracture energy of nanocomposites, when compared with pure PP. As a result of this particle orientation, electrical conductivity—both AC and DC—also changes along injected pieces: at the weld line region, there is an increase in conductivity values due to the more efficient conductive filler distribution. In this example, it can be clearly seen that morphology developed during injection molding is a crucial feature which

*DOI: http://dx.doi.org/10.5772/intechopen.85634*

*Orientation, percentage of crystallinity, and fracture energy values [34].*

**Table 1.**

improved PP/nanoclay toughness (**Table 1**).

*Polypropylene Blends and Composite: Processing-Morphology-Performance Relationship… DOI: http://dx.doi.org/10.5772/intechopen.85634*


**Table 1.**

*Polypropylene - Polymerization and Characterization of Mechanical and Thermal Properties*

for neat PP and 16.01 MJ/m3

the piece performance of injected nanocomposites.

As nanofiller dispersion and exfoliation are crucial, a great effort has been made to improve them by adding additives. However, it is also possible to improve dispersion and exfoliation by changing their processing characteristics. An example of this is shear-controlled orientation in injection molding (SCORIM), which is a not conventional injection molding technique based on a shear-controlled application to the molten polymer during holding stage. SCORIM involves the use of a conventional injection molding machine with a special device with two

**3. Injected PP nanocomposites**

similar fracture behavior characterized by a nonlinearity of stress vs. strain curves with a crack stable propagation and large plastic deformation. However, differences in the propagation mode between PP and PP/aluminum parts were found. In fact, specific plastic work *wp* (specific energy absorption per unit volume) is equal to

indicate that much more energy is involved in the propagation of a crack in the PP of PP/Al samples than in the PP of neat PP samples. The occurrence of β-phase in the composite promotes matrix fibrillation and makes PP of PP/aluminum parts to

In the last three decades, a large interest in nanocomposites was seen in both academic and industry fields [21], due to their potential improvement of properties with a low content of the second phase. In the case of nanocomposites, nanofiller dispersion and orientation are very determinant of mechanical and thermal properties. In theory, only well-dispersed and exfoliated (nanoclays) nanoparticles could lead to an effective improvement of composite performance. Most of commercial nanoparticles, such as nanoclay, are hydrophilic and have weak adhesion or interaction with a hydrophobic matrix as PP, leading to a nanocomposite with a poor dispersion. As a solution to this problem, some producers recommend using masterbatches (MBs), which include all compatibilizers needed to promote nanoclay dispersion and have the additional advantage of being easy to process and compatible with standard processing as injection molding. In fact, some authors have reported nanocomposite preparation by using MB [22–25]. However, these studies indicate that nanoparticles were not exfoliated but intercalated. The influence of the flow pattern during injection molding on the fracture and impact properties of complex injected PP/clay nanocomposites has been studied [22, 23]. Nanoparticles were mostly intercalated—even though they were chemically modified and compatibilized—and both fracture parameters (KIC and G) and impact toughness were determined by molecular and nanoparticle orientation induced by the flow pattern. Toughness mechanisms—as particle delamination or separation were active only in certain loading directions. It was stated before that nanoclay delamination or splitting is an effective toughness mechanism in nanocomposites [26]. In PP, craze-like bands are one of the main mechanisms responsible for matrix energy absorption during deformation. To activate this mechanism, free surfaces are necessary for craze bands to initiate and nanoclay delamination produces those surfaces. However, these crazes can initiate only at the pole of clay particles, i.e., only particles oriented at 45° or more to load direction can induce multiple crazing in tensile-loaded samples and subsequently act as a toughening mechanism. Besides, weld and flow lines produced during filling acted as defects in the presence of nanoparticles [22, 23]. For PP/nanoclay composites under tensile conditions, the amount of absorbed energy was lower at the weld line than away from it and in the flow direction. This is a clear example of how injection molding flow pattern affects

consume more energy before break than pure PP injected parts (**Figure 4**).

for the PP of PP/Al composite. These values

**62**

4.51 MJ/m3

*Orientation, percentage of crystallinity, and fracture energy values [34].*

pistons that generate the shear stresses. It was reported that SCORIM improves the performance of injected parts by controlling their morphology [27, 28]. Significant improvements were found in both stiffness and tensile resistance, molecular and filler orientation, dimensional tolerance, esthetic appearance, and weld line elimination in PP [29] and in its nanocomposites [30–32]. It was demonstrated that SCORIM changes the morphology of PP nanocomposites, not only in terms of molecular and nanoclay orientation but also in crystal phases present in PP matrix: the shear stresses driven by SCORIM process induce the formation of γ phase in PP nanocomposites [24, 33]. SCORIM induces a thicker skin in nanocomposites, i.e., a larger proportion of orientated molecules and clay particles, which favors the sliding of macromolecules, improving the deformation capability. Meanwhile, γ polymorph induces a larger-scale plastic deformation compared with the common α phase. γ phase promotes tearing of PP ligaments leading to fibrillation which is a toughness mechanism [34]. All these morphology features—better molecular and particle orientation and γ polymorph presence improved PP/nanoclay toughness (**Table 1**).

In case of carbon nanotubes (CNTs), it is important not only to attain a good dispersion but also to obtain an interconnected network morphology (above the percolation threshold) to lead an improvement in composites' performance. This morphology depends on nanotube orientation, dispersion, and distribution [35]. It is known that injection-molded parts have a higher percolation threshold than compressed ones due to the morphology and orientation induced by processing [36–38]. Moreover, weld lines could make particle dispersion and orientation even more complex for this kind of materials. It was reported that PP-/CNTinjected parts presented also agglomerates and an isotropic morphology induced by flow pattern during injection [39]. Also, an orientation profile of CNT trough molding thickness has been seen near to the injection point. CNT particles in the skin zone are oriented parallel to the flow front, while they tend to align transversally in the core zone. This has also been observed for fiber-reinforced polymers [40, 41]. In weld line region particularly, it was reported that agglomerates are more diffuse with a random orientation of CNT in both skin and core zones [42]. This heterogeneous orientation induces different fracture mechanisms in the pieces, weld line zone being the weakest part of injected pieces. Agglomerates act also as defects diminishing fracture energy of nanocomposites, when compared with pure PP. As a result of this particle orientation, electrical conductivity—both AC and DC—also changes along injected pieces: at the weld line region, there is an increase in conductivity values due to the more efficient conductive filler distribution. In this example, it can be clearly seen that morphology developed during injection molding is a crucial feature which


**Table 2.**

*Electrical, e-painting efficiency, and fracture energy values [42].*

determines piece performance. A 3D interconnected CNT network is optimal to obtain good electrical conductivity values, but it is not favorable for obtaining a good fracture performance (since it inhibits alternative toughness mechanisms to occur) (**Table 2**).
