## 3 Results and Discussion
### Thermo-Mechanical Properties
#### 3.2.1 Tensile Strength  
Tensile Strengths and Strain Behaviors of Recycled Polypropylene and Virgin Polypropylene  
The tensile strength of recycled PP at elevated temperature conditions of 24 \({}^{\circ}\)C, 40 \({}^{\circ}\)C, 60 \({}^{\circ}\)C, and 80 \({}^{\circ}\)C provides important data on the possible application temperature of recycled PP. The information which is shown in Figure 3a represents stress vs. strain curves for recycled PP at variable temperatures. Also, Figure 3b represents a comparison of tensile strengths between virgin PP and recycled PP at variable temperatures. From Figure 3a, it is shown that recycled PP's stress vs. strain relationships are linear at strains lower than the maximum strain. The stress vs. strain relationships depicts recycled PP to have characteristics of tough thermoplastic materials without yield point. Stress of recycled PP continuously decreased after yielding until fracture occurred and recorded a total increase in elongation at break of 244.4%. Due to the development of residual thermal stresses in the recycled PP structure during cooling down from the melting temperature in the hot compression molding, the yield strains of recycled PP decreased as the test temperature increased [46]. Furthermore, the decrease in tensile strength was because of transitions related to an increase in molecular mobility within the crystalline phase caused by reduced shear strength of the crystals at high temperatures [35]. It was also observed that recycled PP has a high elongation to failure because recycled PP is like a semi-crystalline material which has a high ability to absorb more energy before it fails [27; 47]. According to the study conducted by [PERSON] et al. [12], there is a reduction of the molecular weight with an increase in the number of processing cycles, which results in an increase in the flow activation energy of the PP samples. Also, the strain vs. stress relationship results showed that stress at break and strain at break of the PP samples decreased significantly after each processing cycle, which was attributed to lower cohesion in the polymer. The lower cohesion was because there was a decrease in molecular weight as the amount of reprocessing increased. The tensile strength of recycled PP was lower than that of virgin PP by 33.0-38.2%, as shown in Figure 3b. [PERSON] et al. [47] did a similar experiment of investigating how maximum tensile strength related to the amount of recycling and found out that maximum tensile strength decreased as the number of injection cycles increased. This phenomenon is because recycled PP comes from diverse sources with a collection of different grades and properties, e.g., a blend of two PP homopolymers with different molecular weights due to a different lifetime [35]. Furthermore, recycled PP undergoes natural  
Figure 3: (**a**) Stress vs. strain behavior for recycled PP and (**b**) Tensile strength for 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 Mallick [46]. degradation due to chain scission and thermo-mechanical degradation because of high temperature, oxidation, and shearing during processing [48]. Nevertheless, the mismatching of properties such as phase size, phase structure, and miscibility of components due to contamination by residues such as inorganic, organic, or biological residues can affect the thermo-mechanical properties of the polymers significantly [15; 31].