Practicality of physical recycling method for waste polymer composites exampled via high density polyethylene/automobile shredder residue composites

With the booming of automobiles industry, the recycling of automobile shredder residue (ASR) with complicated constitutes gradually become an urgent issues owing to its difficulty for traditional recycling. Herein, solid‐state shear milling (S3M) technology which can control the domain size of the various polymers in blend was applied to overcome the mismatch of viscosity and poor compatibility of ASR and thus modify the processability to match practical manufacturing. Resultantly, the melt processing of ASR was achieved and a high‐density polyethylene (HDPE)/ASR composite was also prepared with more optimal mechanical properties (tensile strength: 53.2 MPa, impact strength: 36.9 kJ/m2), compared with commercially available recycled PE. This work not only provided a feasible application route for the recycling of ASR material, but also demonstrated the optional methodology for recycling polymer waste with complicated component via physical recycling way.

and considered to be of application value, because it can turn waste to chemical raw materials consisting of H 2 , CO 2 , CO, CH 4 and so forth. 16 Nevertheless, the cost of the above technologies are relatively high and most importantly, the process will generate a huge amount of CO 2 and increase carbon emissions, which is not in line with the serious carbon strategies being implemented by countries around the world. [17][18][19] Compared with chemical route, physical recycling methods are generally conducted by direct blending of the thermoplastic polymers and considered as a secondary one because of the difficulty in sorting. 20,21 The mismatch of the viscosity and poor compatibility can deteriorate the mechanical performance of the as-prepared recycled materials. 22,23 Moreover, the durability of recycled plastics produced by different physical recycling methods will be different, causing poor surface morphology and slight reduction in strength, heat resistance, moisture resistance and failure strain. [24][25][26][27][28] Fortunately, with the merging of mechanochemistry, the physical recycling method can retain the performance of waste polymers without degradation and deterioration. 29,30 Mechanochemistry can induce chemical reactions through the action of a force field, promote the formation of a stable compatible interface between incompatible systems, and at the same time reduce the size of phase domains to achieve homogenization. 22,23,31,32 Solid state shearing milling (S 3 M) technology is a typical mechanochemical method which realizes processing through the three-dimensional shearing via disc rotation. Three-dimensional shearing structure can not only provide strong friction, extrusion, but also provide strong shear and circumferential stress field effects, realizing physical recycling of tough and viscoelastic polymer materials. 7 In particular, S 3 M is able to operate in all solid state, without discharge of waste water. 6,33 For instance, Yang 34 prepare wheat straw and polylactic acid composite material via S 3 M technology to improve the compatibility and processability between incompatible matrices. Song 7 achieved ultra-fine crushing of ASR, and took it as a nucleating agent in rigid polyurethane foam. Unambiguously, S 3 M can transform the ASR into fillers in polymer processing industry. In this study, ultra-fine ASR powder fabricated by S 3 M technology is compounded into HDPE as raw materials rather than traditional fillers. Under the forced interaction between ASR and HDPE, a high-performance HDPE/ASR composite was explored. Two issues are emphatically studied: (1) the processing rheology of the HDPE/ASR composites and (2) the domain between ASR and HDPE. This study provides a simple and effective method for ASR recycling, and simultaneously demonstrates the practicality of physical route for waste polymer recycling.  The as-prepared HDPE/ASR composites were injection molded into dumbbell specimens with a fine neck dimension of 50 mm (length), 4 mm (width), 2 mm (thickness), and rectangular specimens with a dimension of 100 mm (length), 10 mm (width), 2 mm (thickness) by a Minijet injection molder (HAAKE Minijet, HAAKE Technology Co., LTD, Germany). The detailed processing parameters were summarized in Table 1. The prepared HDPE/ASR composites were named as ASR-X, where X represents the weight percentage of ASR.

Characterization
The mean diameter of ASR powder and HDPE/ASR composites was measured using a particle size analyzer instrument (Zetasizer NanoS90, Malvern Instruments Ltd, UK). The morphology of ASR powder and HDPE/ASR composites was observed by scanning electron microscope (Quanta X50 FEI Instrument, USA) with an accelerating voltage of 20 kV. The phase domain size of HDPE/ASR composites was characterized by a polarized light microscope (Linkam THMS600, Linkam Instruments, Germany). The HDPE/ASR composite material was cut into specimens with a thickness of 15 μm using an ultramicrotome, and then heated to 160 • C via a hot stage. The Nano Measurer software was used to measure the dispersion of ASR particles in the HDPE matrix by taking the distribution interval and the average value of 200 individual measurements. The crystallization and melting behaviors of the HDPE/ASR composites were investigated by a differential scanning calorimeter (Q20, TA Instrument, USA) in a nitrogen atmosphere at a rate of 10 • C min −1 from 50 to 220 • C. The thermal stability of the HDPE/ASR composites was tested by a thermo gravimetric analyzer (Q20, TA Instrument, USA) under a nitrogen atmosphere at a heating rate of 10 • C min −1 from room temperature to 600 • C. The mechanical property of the HDPE/ASR composite was measured using an electronic universal testing machine (L-10, Shenzhen Reger Instrument Co., LTD, China) according to GB/T 1040-2006, with 1kN test sensor. The rheological behavior of the HDPE/ASR composite was also investigated by dynamic shear rheometer (AR2000, TA Instrument, USA). Small amplitude oscillating shear scanning was conducted with 1% strain and frequency range from 5 × 10 −2 to 1 × 10 2 rad/s. The testing temperatures were set at 180, 190, and 200 • C, respectively.

Rheological properties of HDPE/ASR composites
As shown in Figure 1A-C, HDPE/ASR composites exhibited a typical shear thinning phenomenon, which is a typical viscoelastic characteristic of pseudoplastic fluid. 35 At the same shear rate, the higher amount of ASR induced lower apparent viscosity. Moreover, with increasing scanning frequency, the storage modulus (G') and loss modulus (Gε) showed an upward trend, and G' and Gε decreased with the increase of ASR content. ASR can reduce the viscosity and elasticity of the melt due to two aspects. First, the ASR components are processed by S 3 M, the molecular chains are broken by the strong shearing effect causing weakened intermolecular entanglement; at the same time, as demonstrated by the absence of plateau, there is no strong inter-and intramolecular force between ASR indicating rigid network was not generated during processing. 36

Morphology of HDPE/ASR composites
The surface of HDPE/ASR composites prepared by injection molding was smooth ( Figure 2) and the prepared test specimens did not experience warping and deformation. Although all sample exhibited a ductile fracture, the cross sections of ASR-10 and ASR-20 composites were smooth, while those of ASR-30, ASR-40, and ASR-50 were rough with defect void structure. This phenomenon is mainly caused by the complex components of ASR, which includes polar components such as PU and PA. When the content of ASR was low, S 3 M can minimize the domain size to achieve partially compatible. However once ASR content increased gradually, the particle collision probability of polar component content was unneglectable, causing obvious agglomeration and poor interface. These defects are easy to develop into cracks under the action of stress, and eventually lead to material failure. 37 Phase domain refers to the size of the dispersed phase (ASR particles) in the polymer matrix (HDPE). Controlling the size of the dispersed phase domain is one of the key factors to prepare the material with excellent performance in multicomponent composites. 38,39 As shown in Figure 3, ASR particles were dispersed in the HDPE matrix. The average phase domain size of ASR-10 composites is about 8.97 μm, while that of ASR-50 composites increased to 11.87 μm, an increase of about 32.33%. Generally speaking, the phase interface with larger domain size is more likely to become the stress concentration point, which leads to material failure more easily.

Crystalline information of HDPE/ASR composites
Crystallinity is an important factor affecting the properties of composites. Generally, the reduction of crystallinity will seriously affect the mechanical properties and thermal properties of polymer materials. Figure 4A shows the DSC curves of different HDPE/ASR composites and pure HDPE. For quantitative analysis, the crystallinity (X c ) of various composites was calculated according to the following equation 40 : where ΔH f represent the melting enthalpy, ΔH ASR represent the enthalpy of HDPE in ASR(5.2 J/g), 7 P ASR represent the percentage of ASR in composites, and the standard enthalpy (ΔH 0 ) of HDPE is 293.0 J/g. 41 As summarized in Table 2, X c decreased from 55.60% to 26.76% with increasing ASR content. Unambiguously, the addition of ASR caused a significant decrease in the crystallinity of HDPE matrix, because the increased content of ASR. The rather complicated constitutes including elastomer can prevent the movement of molecular chains folding back and forth into lattice, and thus the nucleus can hardly form and grow. In order to further study the thermal decomposition performance of HDPE/ASR composites, thermogravimetric analysis (TG) was used to characterize different HDPE/ASR composites. As shown in Figure 4B and Table 3, the thermal decomposition interval of pure HDPE material is mainly 450-500 • C, and the residual carbon rate is 0.1%. The thermal decomposition interval of HDPE/ASR composite is relatively wide, which was about 250-500 • C. The decomposition can be attributed to the rater complicated constituents of ASR, such as PVC (250-320 • C), PE, PP (320--500 • C) and PA. 7 Owing to the existence of inorganic fillers, HDPE/ASR composites with higher ASR content have higher thermal decomposition residue content. Heat resistance index (T HRI = 0.49 × [T 5 + 0.6 × (T 30 -T 5 )]. T 5 , T 30 , and T 50 represent the corresponding decomposition temperatures of 5, 30, and 50 wt % weight loss, respectively.) can also quantitatively reflect the thermal   Figure S1 in Supporting Information). Nevertheless, the surface and mechanical performance of HDPE/ASR composite are still better than those of regenerated PE, making it suitable for practical manufacturing. (Figure S2 in Supporting Information).

CONCLUSION
In this study, HDPE/ASR composites were prepared by thermoplastic processing method. Under the optimal content condition (ASR content is 20 wt%), the domain size of the HDPE/ASR composite was controlled at 10.2 μm making it possible for the melting processing of ASR based blend or composites. Moreover, the tensile strength, impact strength, and bending strength of ASR/ HDPE composites were 53.2 MPa, 23.8 MPa, and 36.9 kJ/m 2 , respectively, which were even higher than that of commercially available recycled PE composites. This work not only provided a feasible application route for the recycling of ASR material, but also demonstrated the optional methodology for recycling polymer waste with complicated component via physical recycling way.

ACKNOWLEDGMENTS
This work is financiered by the National Key Research and Development Program of China (2019YFC1908205) and Natural Science Foundation of Sichuan Province (2022NSFSC0387).

CONFLICT OF INTEREST STATEMENT
Authors have no conflict of interest relevant to this article.

DATA AVAILABILITY STATEMENT
The data that support the findings of this study are available from the corresponding author upon reasonable request.