## 3 Results and Discussion
### Thermo-Mechanical Properties
#### 3.2.3. Toughness  
Toughness is one of the main parameters that affect the performance of polymers in different engineering applications [51]. The results obtained for the toughness of HIPS 7240 and GPPS 1540 was compared to recycled PS. Also, the recycled PP's results were compared to virgin PP as a function of temperature, as shown in Figure 7.  
The results indicated that the amount of energy absorbed by recycled PS, HIPS 7240, recycled PP, and virgin PP increased as temperature increased from 24 \({}^{\circ}\)C to 80 \({}^{\circ}\)C because, at low temperatures, the thermoplastic materials are more brittle, causing the impact toughness to be small. At higher temperatures, the polymers are more ductile, and consequently, the impact toughness becomes higher. However, GPPS 1540 had exceptionally low toughness with no change as temperature increased due to its brittleness characteristics.  
Figure 6: [PERSON]’s modulus vs. temperature behavior for HIPS 7240, GPPS 1540, recycled PS, recycled PP, and virgin PP at 24 \({}^{\circ}\)C, 40 \({}^{\circ}\)C, 60 \({}^{\circ}\)C, and 80 \({}^{\circ}\)C. Data for virgin PP was obtained from [PERSON] et al. [46].  
Figure 7: Toughness vs. temperature behavior for HIPS 7240, GPPS 1540, recycled PS, recycled PP, and virgin PP at 24 \({}^{\circ}\)C, 40 \({}^{\circ}\)C, 60 \({}^{\circ}\)C, and 80 \({}^{\circ}\)C. Data for virgin PP was obtained from [PERSON] et al. [52].  
The toughness for recycled PS during impact was higher than that of HIPS 7240 by 21.3-44.0%. The higher impact strength for the recycled PS is caused by a reduction in molecular weight of the polymers due to chain scission, thermal stress, ionizing radiation during different recycling conditions, and chemical ageing of the polymer as a result of oxidative ageing and hydrolytic ageing. The presence of lower molecular weight chains between macromolecules is likely to act as a plasticizer, resulting in increased impact strength [39; 53]. However, in the thermo-oxidative degradation of HIPS during lifetime and recycling processes, the polybutadiene phase undergoes chemical changes that result in the polymer's mechanical failure [54].  
Due to its high fragility, virgin PP has low impact properties at low temperatures due to the intrinsic low \(T_{g}\) crystallinity, which is one of the main concerns with frequent use of this material [51]. As the temperature increased beyond 40 \({}^{\circ}\)C, recycled PP did not break because recycled PP was becoming ductile as the temperature increased, which caused molecular movement, thus requiring more energy to break it. According to the research done by [PERSON] et al. [35], the flexibility of a polymer depends basically on the rotation ability of its segments due to the mobility of the molecules within the crystalline phase. However, crystalline structures in semi-crystalline polymers (PP) impede such rotations; therefore, in its amorphous condition, the crystalline material is considerably more rigid than the plastic equivalent [27]. The results further showed that toughness for recycled PP was higher than that of virgin PP by 11.0-215.7%, which was similar to the results obtained by [PERSON] et al. [47] in which it was shown that impact energy increased as the number of injection cycles increased. The higher toughness of recycled PP than virgin PP could be due to the condition of a decrease of the molecular weight during recycling which increases the mobility and the ability of the chains to fold, facilitating thick lamellae formation and a high degree of crystallinity (elastomeric phase). The thick molecules were able to cross over to more than one crystal plate and tie the whole structure together hence increase the toughness. Matrix characteristics are, therefore, the main variables that determine the toughness of recycled PP [16; 42].