**4.2 Mechanical properties**

Material tensile properties are important for both engineering and packaging applications, since they represent the ability of materials to withstand the load transferred in the longitudinal direction. Impact strength is essential to engineering applications, due to the need to bear high loads for very short periods of time. Thus, tensile properties are vital in evaluating bags or mooring ropes, while impact strength is a critical feature in recycled plastic fencing, furniture and automobile parts. Tensile properties, mean values and confidence intervals (Fisher's LSD test) are shown in **Figures 4**–**7**.

 With respect to elastic modulus (**Figure 4**), a comparison between PPr1, PPr3, and PP showed that elastic modulus, which is related to composite rigidity, tends to decrease with the addition of PPr, but this effect is only significant for the composite with 30%wt of PPr. However, when recycled filler (GP) was added to PP/ PPr blends, the modulus rose until the mean composite modulus values were equal to those of virgin PP. This finding suggests that the previous decline in stiffness observed in PP/PPr blends can be solved by adding glass powder.

*Study of the Technical Feasibility of the Use of Polypropylene Residue in Composites… DOI: http://dx.doi.org/10.5772/intechopen.81147* 

**Figure 4.**  *Elastic modulus of PP and PP/PPr/GP-base composites.* 

#### **Figure 5.**

*Stress x strain curves of composites samples with: (a) 0% and (b) 10% of PPMA.* 

By contrast, the improvement in elastic modulus was reversed when PPMA was added to PP/PPr/GP composites. This is significant because good cohesion between the matrix and GP, as shown in **Figure 3**, was expected to improve the

**Figure 7.**  *Impact strength of PP and PP/PPr/GP-base composites.* 

tmodulus of the composite. One reason that may explain the previous undesirable result is the high amount of PPMA used as plasticizer. In other words, the amount of PPMA exceeded what was needed to coat the particle surfaces, diffusing in the

*Study of the Technical Feasibility of the Use of Polypropylene Residue in Composites… DOI: http://dx.doi.org/10.5772/intechopen.81147* 

polymer matrix and influencing plasticizer properties or those of a third polymer component.

Yield stress is the maximum stress at which the material begins to exhibit permanent deformation. As the elastic limit shifts, the material does not return to its original dimensions after the applied stress is removed. This property is particularly important for automotive applications. The yield stress of composites is illustrated on median stress x strain curves (**Figure 5**), showing the PPMA effect.

 Yield properties (**Figure 6**) are generally in accordance with the modulus trend, that is, the higher the stiffness, the greater the stress and lower the strain at the yield point of the material. This behavior became evident when virgin PP was compared to PPr/GP composites. For products whose performance is highly dependent on tensile properties, the PPr3/GP sample remains the best option, considering elastic modulus, yield and environmental aspects, since the properties are very similar to those of neat PP even with the addition of 30% PPr and 5%wt of GP.

With respect to the automotive applications of plastic materials, acceptable impact strength is one of the requirements and, in the case of composites, this property is highly sensitive to particle/matrix debonding during mechanical energy dissipation. **Figure 7** shows the mean values and confidence intervals of impact strength.

 The PPr1/GP/PPMA sample showed somewhat better results compared to virgin PP, with a P-value of 0.0838, indicating no significant difference between results. The use of PPMA as a coupling agent for GP could be optimized to improve the impact strength of composites.

In the present study, the PPMA grade used exhibited a maleic anhydride level of 1.0% by weight, but other grades with higher levels and greater affinity to GP could be tested in future research.

Compared to virgin resin, blends of PP with PPr demonstrated poor impact properties (**Figure 7**). In addition to higher impurity levels, PPr is expected to show lower molecular weight than PP, and both factors can contribute to failure in recycled material blends. Fukuhara et al. [58] evaluated isotactic polypropylene with different molecular weight and observed less Izod impact strength in samples with lower molecular weights. Furthermore, any structural particularity in PPr able to influence PP crystalline morphology can modify mechanical properties. Xu et al. [59] studied the relationship between spherulite size and crystallinity in the impact strength of PP samples with several different nucleating agents. The authors reported that impact strength was primarily controlled by spherulite size for samples with low crystallinity. For high crystalline samples, crystallinity itself is the decisive factor in strength. The authors also observed that impact strength is greater in PP samples with small spherulites and lower crystallinity. Nevertheless, no clear relationship between the degree of crystallinity and impact strength of samples was observed in the present study (**Table 3**, **Figure 7**). As such, other factors, such as impurity content, may exert the greatest influence on impact results. Given that products such as furniture and automobile parts require high impact strength, suitable coupling agents should be added to recycled composites in order to enhance their properties.

### **5. Conclusions**

The greatest challenge to plastics in the automotive industry is in recycling. Some automotive manufacturers, such as Ford and Toyota, are recycling their vehicles plastics and reusing in the new vehicles, for example, old or damaged bumpers are recycled and reused in bumper reinforcement cores [60].

 According to the results presented, it can be concluded that the properties did not vary significantly as a function of composition. Since the objective was to produce lower cost composites (incorporating recycled PP and glass powder) and more sustainable materials without significant loss of properties, this result is within the parameters established. In other words, it was possible to recover post-consumer materials, replacing the virgin resin without significant loss of mechanical integrity in the final product. The addition of maleated polypropylene (PPMA) was shown to significantly improve the toughness of the material.

In conclusion, based on the properties analyzed and the sustainable appeal of the new products, the powder-based composites displayed potential for use in various applications in the automotive industry, replacing conventional materials.
