## 1 Introduction  
Polymers are categorized into thermoplastics, thermosets, and elastomers according to their thermal and mechanical properties. Scientists prefer polymer thermoplastics more than thermosets because of their low production cycle, lower processing costs, and high thermoplastic separability [1; 2]. Global polymer production has increased enormously to more than 330 million tons over the last five decades because polymers are widely used in industry [3; 4; 5; 6]. In 2015, there were around 322 million tons produced worldwide, which was about 90 times higher than polymers produced in 1960. It was also reported that in the European Union, 58 million tons of polymers were produced and utilized, and only around 30% were recycled [3; 4]. As a result of the growth in polymer production and utilization, large quantities of polymer waste are generated [7; 8]. Polymers guarantee high durability, low density,high resistance-to-weight ratio, and low cost properties. However, many types of polymers cause long-term environmental pollution due to low biodegradability and chemical reactivity in the natural environment. The polymers break into smaller fragments termed as micro or macro-plastics that have a considerable set of ecosystem impacts and may harm animal and human health. Furthermore, other toxic compounds are released slowly into the soil, water, and air under various conditions [8,9]. Rapid growth of polymer use coincided with an increase in what is commonly called the green movement. The awareness of the environment means that the industry needs to be more concerned with waste disposal management. Also, there has been a need to increase the use of recycled polymers due to growing public concern for environmental degradation and strong government incentives to increase the rate of recycling of polymer wastes [7,10]. Apart from recycling, the disposal of waste polymers has been managed by incineration, reuse, and disposal. Disposal and incineration are methods which create the environmental problems of depleting landfill space and releasing massive amounts of carbon dioxide into the atmosphere, respectively. Recycling is the preferred option in reducing waste polymers since it makes it possible to reduce the volume of waste dumped in the environment in a much more environmental-friendly way, save energy, and reduce the need for natural resources because most polymer materials are obtained from gas and oil [7,11]. There are two methods of recycling, namely: chemical recycling and mechanical recycling. While these methods aim for the same objectives, they differ significantly. Mechanical recycling is when a polymer is ground, reprocessed, and re-granulated without changing the fundamental structure of the polymer to produce new polymer components. On the other hand, chemical recycling is the process where the polymer waste is converted back into its basic hydrocarbon/oil components which can be used as raw materials for production of new polymers using appropriate chemical solvents. Therefore, mechanical recycling is the most appropriate method of recycling from an industrial point of perspective because it is low cost, reliable, and an environmentally friendly process [12,13,14]. Polypropylene (PP) is a semi-crystalline and polymorphic thermoplastic with several crystal modifications such as hexagonal (\(\beta\)), orthorhombic (\(\gamma\)), monoclinic (\(\alpha\)), etc. PP has been widely used in various fields of engineering, such as medical equipment, automotive, packaging products, etc., because of its cost which is relatively low with excellent properties such as good processing performance, low density, resistance to moisture, high resistance to ageing, good recyclability, excellent chemical and corrosion resistance, and high thermal stability [15,16,17,3]. Polystyrene (PS) is an aromatic polymer produced from the styrene monomer. There are two main polystyrene groups: General purpose polystyrene (GPPS) and high impact polystyrene (HIPS).