## 3 Results and Discussion  
### Thermal Properties  
The thermal behavior of the recycled polymers and virgin polymers were probed via DSC to determine glass transition temperature (\(T_{g}\)) and melting temperature (\(T_{m}\)). Glass transition temperature and melting temperature are determined to compare any modifications due to recycling or blending. Figure 1 predicts \(T_{g}\) of GPPS 1540, HIPS 7240, and recycled PS. Also, Figure 2 predicts \(T_{m}\) of virgin polypropylene (VPP) and recycled PP. Thermo-mechanical properties of the materials such as hardness, toughness, tensile strength, and Young's modulus depend on the values of \(T_{g}\) and \(T_{m}\)[36]. Also, \(T_{g}\) of polymers is essential in order to create the best methods of processing the materials into finished products and to foresee performance during the lifetime of the product [37]. Figure 1: Differential scanning calorimetry (DSC) plot for high impact polystyrene (HIPS) 7240, general purpose polystyrene (GPPS) 1540, and recycled polystyrene (PS). Figure 2: DSC plot for virgin polypropylene (PP) and recycled PP. The DSC analysis of GPPS 1540, HIPS 7240, and recycled PS showed similar thermographs where only one temperature corresponding to the glass transition temperature was observed. Virgin polystyrenes (GPPS 1540 and HIPS 7240) had a \(T_{g}\) of 93 \({}^{\circ}\)C and 95 \({}^{\circ}\)C, respectively. [PERSON] et al. [28] did a similar experiment for both GPPS and HIPS and got a common \(T_{g}\) of 95 \({}^{\circ}\)C. The presence of phenyl groups and amorphous structure in GPPS 1540 and HIPS 7240 is responsible for the relatively high glass transition temperature without having a sharp melting point [38]. On the other hand, recycled PS had a \(T_{g}\) of 97 \({}^{\circ}\)C. The higher \(T_{g}\) of recycled PS than virgin polystyrenes (GPPS 1540 and HIPS 7240) was attributed to increased crosslinking and change of the chemical structure of the polybutadiene phase which is influenced by contaminants, embrittlement, and thermo-oxidation on both polybutadiene and PS during lifetime and reprocessing [39; 40; 41]. In addition, the crystallinity of recycled polymers increases as a result of molecular weight reduction and entanglements of chains from several thermal reprocessing cycles [12]. It was observed that recycled PP and virgin PP had similar thermographs. Also, only single melting peaks of 149 \({}^{\circ}\)C and 171 \({}^{\circ}\)C for recycled PP and virgin PP, respectively, were observed. It is well known that PP is a semi-crystalline and polymorphic material with several crystal modifications including at least monoclinic (\(\alpha\)), hexagonal (\(\beta\)), and orthorhombic (\(\gamma\)), etc. The monoclinic \(\alpha\) form is the most prevalent and stable modification observed in all types of solution-crystallized PP samples and also in most melt-crystallized specimens. In this case, the melting peaks of 149 \({}^{\circ}\)C and 171 \({}^{\circ}\)C suggest that recycled PP and virgin PP are the endothermic peaks of \(\alpha\) crystals of PP [15; 17; 42]. [PERSON] et al. [26] and [PERSON] et al. [43] did a similar experiment and determined the melting point of virgin PP to be 166.3 \({}^{\circ}\)C and 163.3 \({}^{\circ}\)C, respectively. The \(T_{g}\) for recycled PP was \(\sim\)12.9% lower than that of virgin PP due to several factors such as reprocessing conditions, thermal history factors, the presence of crystals which are less perfect, chain degradation which occurs during melt processing, and other changes in crystals [43; 44]. The wide melting peak for recycled PP and virgin PP as shown in Figure 2 indicate that different crystalline structures coexist and that the distributions of the crystallite block length are wide in both the soft (amorphous, atactic PP) and hard (crystalline, isotactic PP) phases. The broad peak is also due to the melting of the crystallized phase made by the stereo-regular sequences co-crystallization [5].