**2. Injected PP microcomposites**

The first attempt to obtain a good composite is to add a microparticle to the polymer matrix. Among PP microcomposites, fiber-reinforced plastics are a popular type of composites used in many engineering applications mainly because of their excellent capability to form complex shapes. These fibers—glass or carbon, stiff and elastic—generally increase both stiffness and strength of PP matrixes. Even though injection molding is currently the most used technique to process this kind of composites, there are some issues that directly link the processing with a nonuniform orientation of fibers and their breakage. In fact, there is a strong heterogeneity of the microstructure in terms of fiber orientation of injected parts: short-glass fibers use to tend parallel to the injection flow direction in the skin zones and highly angled with respect to flow direction in the core layer. Fiber orientation depends also on location along the piece (e.g., distance from injection points). There is a strong dependence of the macroscopic mechanical behavior on fiber orientation. In fact, when the average angle of fiber orientation varies in only a few degrees with respect to loading direction—corresponding with a change in the average value of the component of fiber orientation matrix with respect to loading direction of only a few hundredths the composite tensile strength varies by approximately 7.5% [14]. In addition to

**61**

**Figure 4.**

*SEM pictures of PP and PP/Al fracture surfaces.*

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

orientation, fiber length also dominates the tensile strength of injection-molded composites. In general, fiber breakage results in a decrease in tensile strength, so it is important to know how injection molding affects fiber breakage. An increase in shear (injection velocity, shear components, etc.) may cause an enhanced fiber and matrix orientation which would lead to higher tensile performance along the flow direction. However, it also causes a remarkable fiber breakage [15]. Experimental results indicate that an increase in injection velocity results in a decrease in ultimate tensile stress, due to the high fiber breakage. This effect is partly attenuated at low mold temperature, due to an increase of fiber orientation [3]. In case of complex parts that contain weld lines, the situation is even more complicated. The fibers are nonuniformly distributed in the regions around the defects, and there is also a distribution of glass fiber densities. All these features modify fracture behavior of injected pieces changing failure patterns, with crack pathways that follow stress

As it was stated before, not only all modifications are done to obtain a nanocomposite with improved mechanical performance, but also esthetic features are searched for some special applications. Composites of thermoplastic polymers with metallic fillers are an important group of engineering materials with a wide range of properties including electric and thermal conduction, high mechanical properties, and improved esthetic quality. Currently, metallic looking plastics replace metals by plastic in many applications, trying to achieve the quality and prestige of metals and adding value to products [16]. It is possible to obtain a metallic looking plastic part by adding metallic pigments. In this way, it is possible to eliminate postprocessing operations such as painting. Metallic pigments have different shapes and sizes. Particles with a flake shape promote the reflected light in a specular way increasing the metallic appearance of part surfaces [17]. In spite of the injection defects—as weld or flow lines—being known to affect pieces of esthetics, this could be improved by adjusting processing conditions [18, 19]. Melt temperature is one of the processing parameter that more influences esthetic of injected parts: higher melt temperatures decrease shrinkage and make weld lines wider and more diffuse [20]. In case of PP/aluminum composites, the presence of aluminum that increases thermal conductivity plus the inherent temperature gradient and shear stresses of the injection molding induces β-polymorph formation. This effect also depends on processing conditions; a higher melt temperature induces a higher β-phase content. At the same time, the mechanical performance of parts shows to be dependent on PP morphology, i.e., processing conditions. Quasi-static fracture performance also depends on the location of the samples. At weld line zone, PP/aluminum composite failed in a brittle way following the weld line. Fracture toughness of both PP and PP/aluminum is similar, indicating that weld lines are a predominant weak defect inside the injected parts. Away from weld lines, PP and PP/aluminum show a

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

concentrators developed during processing [4].

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

orientation, fiber length also dominates the tensile strength of injection-molded composites. In general, fiber breakage results in a decrease in tensile strength, so it is important to know how injection molding affects fiber breakage. An increase in shear (injection velocity, shear components, etc.) may cause an enhanced fiber and matrix orientation which would lead to higher tensile performance along the flow direction. However, it also causes a remarkable fiber breakage [15]. Experimental results indicate that an increase in injection velocity results in a decrease in ultimate tensile stress, due to the high fiber breakage. This effect is partly attenuated at low mold temperature, due to an increase of fiber orientation [3]. In case of complex parts that contain weld lines, the situation is even more complicated. The fibers are nonuniformly distributed in the regions around the defects, and there is also a distribution of glass fiber densities. All these features modify fracture behavior of injected pieces changing failure patterns, with crack pathways that follow stress concentrators developed during processing [4].

As it was stated before, not only all modifications are done to obtain a nanocomposite with improved mechanical performance, but also esthetic features are searched for some special applications. Composites of thermoplastic polymers with metallic fillers are an important group of engineering materials with a wide range of properties including electric and thermal conduction, high mechanical properties, and improved esthetic quality. Currently, metallic looking plastics replace metals by plastic in many applications, trying to achieve the quality and prestige of metals and adding value to products [16]. It is possible to obtain a metallic looking plastic part by adding metallic pigments. In this way, it is possible to eliminate postprocessing operations such as painting. Metallic pigments have different shapes and sizes. Particles with a flake shape promote the reflected light in a specular way increasing the metallic appearance of part surfaces [17]. In spite of the injection defects—as weld or flow lines—being known to affect pieces of esthetics, this could be improved by adjusting processing conditions [18, 19]. Melt temperature is one of the processing parameter that more influences esthetic of injected parts: higher melt temperatures decrease shrinkage and make weld lines wider and more diffuse [20]. In case of PP/aluminum composites, the presence of aluminum that increases thermal conductivity plus the inherent temperature gradient and shear stresses of the injection molding induces β-polymorph formation. This effect also depends on processing conditions; a higher melt temperature induces a higher β-phase content. At the same time, the mechanical performance of parts shows to be dependent on PP morphology, i.e., processing conditions. Quasi-static fracture performance also depends on the location of the samples. At weld line zone, PP/aluminum composite failed in a brittle way following the weld line. Fracture toughness of both PP and PP/aluminum is similar, indicating that weld lines are a predominant weak defect inside the injected parts. Away from weld lines, PP and PP/aluminum show a

**Figure 4.** *SEM pictures of PP and PP/Al fracture surfaces.*

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

microstructure, which can be seen by polarized optical microscopy (PLM), as in

morphology determines the mechanical performance of injected PP parts.

microstructure and therefore in performance, are summarized.

**2. Injected PP microcomposites**

*Skin-core microstructure seen by polarized optical microscopy.*

The number of observed "layers" in the microstructure depends on the resolution of the experimental technique used. A simple analysis considers a three-layer model (two external skins and an inner core) [11–13], but other layers may be also observed (two external skins, two sub-skin regions, two shear zones, and an inner core). The intrinsic molecular nature of the polymer together with this layer

Besides, adding a second component—particles or additives—into a PP matrix may also change its crystalline structure, i.e., may produce changes in injected piece

Through this chapter, the relationship between processing and performance is reviewed for injected PP composites. The combined effect of the molding process and the fillers on the properties of the polymer composites is reviewed. Also, the effects of the occurrence of inhomogeneities, such as weld lines or flow lines in

The first attempt to obtain a good composite is to add a microparticle to the polymer matrix. Among PP microcomposites, fiber-reinforced plastics are a popular type of composites used in many engineering applications mainly because of their excellent capability to form complex shapes. These fibers—glass or carbon, stiff and elastic—generally increase both stiffness and strength of PP matrixes. Even though injection molding is currently the most used technique to process this kind of composites, there are some issues that directly link the processing with a nonuniform orientation of fibers and their breakage. In fact, there is a strong heterogeneity of the microstructure in terms of fiber orientation of injected parts: short-glass fibers use to tend parallel to the injection flow direction in the skin zones and highly angled with respect to flow direction in the core layer. Fiber orientation depends also on location along the piece (e.g., distance from injection points). There is a strong dependence of the macroscopic mechanical behavior on fiber orientation. In fact, when the average angle of fiber orientation varies in only a few degrees with respect to loading direction—corresponding with a change in the average value of the component of fiber orientation matrix with respect to loading direction of only a few hundredths the composite tensile strength varies by approximately 7.5% [14]. In addition to

**60**

**Figure 3**.

**Figure 3.**

performance.

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 4.51 MJ/m3 for neat PP and 16.01 MJ/m3 for the PP of PP/Al composite. These values 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 consume more energy before break than pure PP injected parts (**Figure 4**).
