**4. Injected PP hybrids**

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

**65**

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

toughness mechanisms [47–50]. Spherical particles favor stress concentration which induces several absorption mechanisms—craze, shear yielding—while elongated ones are not able to produce stress fields needed to promote toughness mechanisms [50]. The non-visibility of the weld line seen previously in the morphological analysis is in concordance with fracture results: weld lines did not act as stress raisers and did not

Another kind of a hybrid composite which combines both reinforcement and energy consumption promotion is composites reinforced with both glass and cellulose fibers. Kahl et al. studied the synergetic effect of those two fibers in hybrid injected PP-based composites. They found that fiber orientation depends not only on the flow pattern but also on the amount of cellulose fibers present in the hybrid composite. There is a general trend of both kinds of fibers to orientate parallel to flow direction. A higher cellulose fiber content in the hybrid composite decreases the orientation of both fibers. Besides, short fibers tend to align following flow direction much more than longer ones [51]. There are some other examples of works in which fiber interactions govern the morphology of hybrid injected pieces. Gamze et al. found that mechanical performance of injected carbon nanotubes/glass fiber PP hybrid composites depends on fiber interaction in the matrix. The simultaneous usage of carbon nanotubes and glass fibers increases the system polarity, leading to a better dispersion of carbon nanotubes, with the subsequent effect in the final

In summary, there are several evidences indicating that not only the developed morphology during processing but also the interaction between reinforcements is

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

• The same composite would develop different morphologies or microstructures

• Different composites processed with the same conditions would also develop

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 rein-

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.

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

introduce an alternative crack path.

performance of pieces [52].

**5. Conclusions**

interaction, i.e.:

crucial in the final performance of injected pieces.

if processed with different conditions.

different morphologies or microstructures.

forcements is crucial in the final performance of pieces.

**Figure 5.** *Particle morphology feature scheme.*

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

toughness mechanisms [47–50]. Spherical particles favor stress concentration which induces several absorption mechanisms—craze, shear yielding—while elongated ones are not able to produce stress fields needed to promote toughness mechanisms [50]. The non-visibility of the weld line seen previously in the morphological analysis is in concordance with fracture results: weld lines did not act as stress raisers and did not introduce an alternative crack path.

Another kind of a hybrid composite which combines both reinforcement and energy consumption promotion is composites reinforced with both glass and cellulose fibers. Kahl et al. studied the synergetic effect of those two fibers in hybrid injected PP-based composites. They found that fiber orientation depends not only on the flow pattern but also on the amount of cellulose fibers present in the hybrid composite. There is a general trend of both kinds of fibers to orientate parallel to flow direction. A higher cellulose fiber content in the hybrid composite decreases the orientation of both fibers. Besides, short fibers tend to align following flow direction much more than longer ones [51]. There are some other examples of works in which fiber interactions govern the morphology of hybrid injected pieces. Gamze et al. found that mechanical performance of injected carbon nanotubes/glass fiber PP hybrid composites depends on fiber interaction in the matrix. The simultaneous usage of carbon nanotubes and glass fibers increases the system polarity, leading to a better dispersion of carbon nanotubes, with the subsequent effect in the final performance of pieces [52].

In summary, there are several evidences indicating that not only the developed morphology during processing but also the interaction between reinforcements is crucial in the final performance of injected pieces.
