## 1 Introduction
However, HIPS and other types of polystyrenes, such as expanded polystyrene (EPS) are accepted for recycling [18,19,20]. HIPS is a copolymer of styrene and acrylonitrile, toughened with polybutadiene and shows the right balance between elasticity and rigidity. The incorporation of a rubber phase into glassy polymers is acknowledged as an efficient technique to enhance the toughness of polymer products. HIPS is widely employed in various fields including housewares, packaging, automobiles, electrical appliances, and light-duty industrial components due to its low cost, ease of processing, versatility, coloring, relative thermal stability, and good rigidity [21,22,20]. PS and PP are among the five most widely used thermoplastic polymers in terms of share of the market [10,12,23]. HIPS and PP were discovered to be the primary components of the plastics fraction of waste [24,25]. Therefore, recycling of PP and PS has attracted considerable attention in recent years due to a decreased supply of petroleum resources, increased environmental concerns, increased need for sustainability, and energy security. Various strategies for recycling programs for PP have been developed. Fortunately, a large proportion of PP residues are recovered using industrial recycling and recycled PP can be used in various ways, such as industrial parts, composite matrix, reinforcements in the construction industry, etc. [3,14]. Several scholars have researched and studied changes in the mechanical and rheological characteristics of PP after repeated injection molding cycles. Molecular weight variations, the polymer's chemical structure, and some mechanical properties at room temperature have been analyzed as a result of degradation due to continuous processingoperations. However, limited research studies have been conducted and published on the effect of recycling activities on the thermo-mechanical properties of PP as a function of temperature. Also, despite the availability of PS recycling technology, recycled polystyrenes' market is characterized by relatively decreasing demand, overcapacity, and underuse due to limited knowledge on the effect of recycling on thermo-mechanical properties [19; 26; 27; 28; 29]. The use of recycled polymers instead of virgin polymers for industrial purposes is one of the most promising techniques for lowering the environmental impact and the expenses associated with scrapping components [14]. However, intensive melting due to prolonged exposure to high temperatures during the reprocessing process (melt degradation), ageing due to harmful effects of the environment, long-term heat ageing, and chain scission during service life of the polymers affect the thermo-mechanical properties of the recycled polymers. Furthermore, polymers are subjected to physical stresses (mechanical forces, heat, radiation), and chemical changes due to chemical deteriogens (humidity, oxygen, atmospheric pollutants). The conditions cause structural deterioration of the polymeric components [2; 27; 30]. Also, as a side effect to consider, contamination by residues such as inorganic, organic, or biological residues cause deterioration of the polymers during recycling. Besides, the mechanical recycling of polymers introduces further problems owing to the complexity of the morphological structures that occur between the polymeric components [31; 32; 33; 3]. Based on the various mechanisms involved in the life cycle of polymers, the changes significantly alter the mechanisms of stabilization, mechanical, thermal, and rheological properties of recycled polymers [14; 33; 34]. The thermoplastic nature of these recycled thermoplastics raises essential questions about mechanical performance at high temperatures and low strain rates [35]. Therefore, manufacturing industries have not widely adopted the recycled polypropylene (rec PP), and recycled polystyrene (rec PS) obtained from various sources such as industrial wastes, environmental damping etc.