###### Abstract  
Plastic mechanical recycling is an attractive method for reducing the amounts of waste plastics. However, the alterations in the mechanical properties (degradation) in recycled plastics is a limitation to the material's mechanical recycling. In this study, the mechanical recycling was enhanced by the addition of a \"molten resin reservoir\" unit at the end of the twin-screw extruder. Recycled polypropylene (RPP) obtained from a household was re-extruded with this developed extrusion unit. The tensile properties, type of crystalline, and conformation of polypropylene polymorphs were evaluated and compared for virgin polypropylene (VPP), recycled polypropylene (RPP) without extrusion (RPP-original), and RPP with extrusion by using a new type of extruder (RPP-extrusion). It could be found that the tensile properties of RPP-extrusion were improved, so as to be similar to those of VPP. In addition, the conformation of RPP-extrusion was similar to that of VPP by increasing the ratio between the helix and parallel band. This study succeeded in regenerating the tensile properties and inner structures in recycled PP, which could prolong the used lifetime and decrease the amount of waste from single-use plastic. polyproylene recycling; recycled plastic; household plastic waste; polypropylene; twin-screw extrusion; tensile properties; crystalline conformation +
Footnote †: journal: Applied Sciences  
emission, air emission with organic compounds, and waste production [5]. Mechanical recycling was carried out by the reprocessing of waste plastics with a mechanical method in order to produce new plastic products; however, the poor properties of the mechanically recycled products were an obstruction to the usage of mechanical recycling [6]. Extrusion is a mechanical process that is mainly used in plastic mechanical recycling. The degradation of recycled plastics in the extrusion process is caused by chain scission or the forming of cross-links [7,8]. From our previous work, it can be found that the degradation of plastics was not caused by chemical degradation but that the physical degradation was the main reason [9,10]. In addition, the suitable processing condition was able to improve the mechanical properties of plastics. In order to decrease the effect from heat and shear of the extrusion process on the properties of plastics, the new additional unit connected at the end of the twin-screw extruder was established, namely as a \"molten resin reservoir\" [11]. The molten resin reservoir unit was the blank space connected between the end of the screws and the die. This blank space was controlled by the temperature, same as for the extrusion process. However, there was no shear stress from the screw rotation. As a result, the molten plastic products were relaxed from the shear stress by being kept in the blank space of the molten resin reservoir for a short time (40-60 s). Then, the extrudate came out from the die and cooled down to room temperature by water. [PERSON] et al. [11] found that the elongation at break of unsorted recycled plastics extruded by the new type of twin-screw extruder with the additional molten resin reservoir unit was much more improved than for the conventional extrusion process. It is worth noting that the addition of the molten resin reservoir unit proved effective for the regeneration of properties in plastic products. This study is the first to compare the mechanical properties and inner structures of virgin polypropylene (VPP) and recycled polypropylene (RPP) extruded by using a twin-screw extruder with an additional molten resin reservoir unit. The tensile properties, crystalline type, inner structure conformation, and molecular weight distribution were characterized by a tensile test, X-ray diffraction, infrared microscope, and high-temperature gel permeation chromatography.