## 1 Introduction
There is also the possibility of mixing raw rubber with recycled, and adding compatibilizers/coupling agents such as ethylene vinyl acetate (EVA), maleic-anhydride-grafted polyethylene (MAPE), and ethylene-propylene-diene monomer (EPDM) [13; 14]. Over the last few years, increasing demands for TPE have been observed in the construction, automotive, and building industries because of their easy processing and wide range of properties which can be adjusted by the relative contents of the thermoplastic and elastomer phases. Although several reports have been published on standard processes like extrusion [15] and injection [16] and compression molding [17], very few works have been published on rotomolding. Rotational molding is a low-stress/shear process producing stress-free and seamless hollow parts such as containers, tanks, toys, medical equipment, and several similar products. Rotational molding can also produce parts over a wide range of sizes: from doll parts (less than 1 cm\({}^{3}\)) up to large tanks (more than 20,000 L). Complex shapes with excellent detail reproduction can be made due to simple mold fabrication, but complex structures such as fiber-reinforced, double-skinned products and foamed materials can also be processed. Furthermore, rotational molding provides more design freedom than any other plastics-generating process [18]. Unlike other processing methods, such as injection molding or blow molding, a specific feature of rotomolding is that there is no need to apply pressure [19]. The material melts and forms in the same mold. Also, there is no need to move the material from one part to another part of the machine. This means that low shear rates are applied on the polymer [20]. Nevertheless, the thermal efficiency (heat transfer) of the process is low, leading to \begin{table}
\begin{tabular}{c c c} \hline
**Component** & **Passenger Cars** & **Trucks/Buses** \\ \hline Rubber/elastomer & 41–48 & 41–45 \\ Carbon black & 22–28 & 20–28 \\ Metal/steel & 13–16 & 20–27 \\ Textile & 4–6 & 0–10 \\ Additives & 10–12 & 7–10 \\ \hline \end{tabular}
\end{table} 
Table 1: Composition (wt %) of different tire structures [6]. long cycles (close to 60 min). In the case of solid polymers, the size of the material has to be reduced to a powder form before processing, and the flow characteristics of these powders have to be good enough to achieve high-quality moldings. As a result, there are relatively few materials appropriate for rotational molding. Increased cost related to the grinding process is observed for most of these materials. In addition, only a few polymers can be used for rotational molding since the materials require special properties such as low viscosity and high thermal stability [21]. This is the main reason why polyethylene (PE) in its several forms is used to produce rotomolded parts. Today, low-density polyethylene (LDPE), high-density polyethylene (HDPE), linear low-density polyethylene (LLDPE), linear medium-density polyethylene (LMDPE), and medium-density polyethylene (MDPE) constitute close to 90% of all polymers which are rotationally molded [22]. The main reasons are PE's low cost and excellent resistance to processing (time and temperature). Nevertheless, there is a need to develop other materials for different applications where a balance between strength and ductility is needed. This is where TPE are of interest with controlled rubber content in a specific matrix. To produce rotomolded parts, an important parameter, seldom discussed in the literature, is the speed ratio, which must be carefully selected according to the mold shape. In rotational molding, the speed ratio is defined as follows [22]. \[\text{Speed ratio}=\frac{\text{Major axis rpm}}{(\text{Minor axis rpm}- \text{Major axis rpm})} \tag{1}\]  
Normally, the speed ratio for symmetric shapes like spheres or cubes is 4 to 1 (major to minor axis rpm speed) [23]. Once the rotation speed ratio is specified, the next main parameter is the mold rotational speed.