**5. Conclusions**

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

**Electrical resistivity (Ohm/cm) e-Painting efficiency GC (kJ/m2**

**)**

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

PP/CNT weld line 1.109 0.7 3.3 PP/CNT bulk 1.1010 0.6 8.4 Neat PP 1.1014 0.1 18.1

The use of two different reinforcements at the same time may expand the application field of PP composites by combining their properties. Hybrid nanocomposites made by a rigid filler and soft particles have attracted attention due to incorporation of both stiffness and higher energy absorption and elongation at break. The goal is to obtain an optimal balance between rigidity and impact resistance [43–45]. An example of these kinds of hybrid composites is a rubber/nanoclay/polypropylene nanocomposite, which may increase simultaneously stiffness and toughness of PP. A study about how injection molding flow pattern and inhomogeneities affect the morphology and performance of this hybrid nanocomposite at different locations in injected intricate moldings was reported in literature [46]. A noticeable morphological feature was found: rubber particles appear to be more elongated and oriented in flow direction in the skin of injected pieces, while they appear spherical shaped in the core. Regarding nanoclay, an orientation profile was found: there is a strong orientation of nanoparticles in flow direction in skin zone, while they are randomly distributed in the core. A scheme of these morphology features is shown in **Figure 5**. Surprisingly, there are no significant morphological differences between the zone near the injection point and the zone of the weld line. These morphological features have an important influence in mechanical performance of injected pieces. In fact, fracture features showed to be dependent on the morphology developed during processing: in the core—with spherical-shaped rubber particles and randomly oriented nanoclay—a cavitation process was seen accompanied by shear yielding; in the skin, with elongated-shaped rubber particles and strongly orientated nanoclay particles in flow direction, there were no signs of cavitation, and fracture surface was slightly rugged. It is known that size and shape of rubber particles play a key role in

**64**

**Figure 5.**

*Particle morphology feature scheme.*

to occur) (**Table 2**).

**Table 2.**

**4. Injected PP hybrids**

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

Through this chapter, it has been shown that PP composites' performance depends not only on their intrinsic properties but also on processing conditions. Processing of a two- or three-phase PP-based composite induces distinct morphologies and microstructures that depend on both processing conditions and phase interaction, i.e.:


These induced characteristics, such as crystallinity, crystalline phase, or phase morphology, will definitely affect final performance of processed pieces, including thermal, mechanical, and fracture behaviors. Moreover, if different types of reinforcements are added in a composite, it has been observed that not only the developed morphology during processing but also the interaction between reinforcements is crucial in the final performance of pieces.

All these features should be kept in mind when trying to use a composite, knowing that laboratory results should not be directly extrapolated to final processed pieces.
