## 3 Results and Discussion
### Thermo-Mechanical Properties
#### 3.2.4 Hardness  
Hardness is the degree of permanent deformation of a material under a force applied to it using a durometer. Figure 8 displays the analyzed results of Shore D hardness for recycled PS compared to HIPS 7240 and GPPS 1540 as a function of test temperatures. Similarly, hardness for recycled PP was compared to virgin PP. Hardness is always related to the crystallinity of the polymer such that amorphous polymers are harder than crystalline polymers. Figure 8: Hardness 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]. A decreasing tendency of hardness for the polymers tested was observed as the temperature increased from 24 \({}^{\circ}\)C to 80 \({}^{\circ}\)C. Polymers consist of long-chain molecules in which the degree of entanglement varies with the length and exact shape of the polymer molecule. As the temperature increased, molecular mobility of polymers also increases, causing the polymer to become soft elastic hence decrease in hardness [55]. Recycled PS had a higher hardness than HIPS 7240 by 2.2-5.1%. A similar experiment was done by [PERSON] et al. [56] and showed that hardness for HIPS decreased with time because butadiene rubber in HIPS tends to embrittle due to weathering. GPPS 1540 recorded a higher hardness than that of recycled PS by 11.4-13.2% because GPPS 1540 is an amorphous polymer with brittle characteristics as observed from the stress-strain curves (Figure 4). Amorphous polymers have characteristics of high hardness due to their molecules which are irregularly arranged. When the molecules are irregularly arranged and bulky, an amorphous polymer is produced which consists of enmeshed chains of polymer which are disordered. Typically, such polymers are optically transparent, hard, and often fragile. Therefore, there is a tendency of materials such as GPPS 1540 with high tensile strength and modulus, as shown in Figures 5 and 6, respectively, to show higher hardness as shown in Figure 8. The high mechanical properties are also directly related to their crystallinity [27; 57]. Recycled PP had improved hardness compared to that of virgin PP by 11.7-26.5% because recycled PP could have less flexibility matrix than virgin PP as explained by [PERSON] et al. [58]. The results were contrary to the explanations given by [PERSON] et al. [59] that thermal degradation due to chain scission of virgin PP contributes cracks and hence lowers the hardness of recycled PP. Table 2 shows the summarized, analyzed results obtained from the thermo-mechanical characterization of the polymers (recycled PS, HIPS 7240, GPPS 1540, recycled PP). However, the thermo-mechanical properties of virgin PP were obtained from a literature review. mechanical properties of the polymers as a function of temperature was studied. From the DSC thermographs, the glass transition temperatures and melting temperatures for the studied recycled polymers were determined. It was found that Young's modulus, tensile strength, and hardness for recycled PP, virgin PP, recycled PS, HIPS 7240, and GPPS 1540 decreased as temperature increased. However, toughness for the recycled PP, virgin PP, recycled PS, and HIPS 7240 increased as the temperature increased, differing from GPPS whose toughness was maintained. HIPS 7240 and recycled PS showed characteristics of semi-crystalline and tough thermoplastic materials with a yield point while GPPS 1540 showed characteristics of an amorphous, brittle, and highly transparent material. On the other hand, recycled PP showed characteristics of a semi-crystalline and tough thermoplastic material without a yield point. The distinct characteristics of GPPS 1540 of being amorphous in terms of crystallinity made it unfit to be directly compared with recycled PS since it had a much higher Young's modulus, tensile strength, hardness, and very low toughness compared to that of HIPS 7240.