Cture, causing thethe deteriorationthe the therirreversible changes inside the polymer structure, causing deterioration of of thermal, mechanical, and physical functionality on the recycledrecycled components [149,150]. During mal, mechanical, and physical performance of the supplies [149,150]. In the course of mechanical recycling, two competing degradation mechanisms (-)-Cedrene MedChemExpress happen: random random chain and mechanical recycling, two competing degradation mechanisms happen: chain scission scischainand chain crosslinking (2-Cyanopyrimidine Technical Information Figure five) [151,152]. chain scission isscission may be the procedure of sion crosslinking (Figure 5) [151,152]. Random Random chain the course of action of breaking bonds within the polymer backbonebackbone chain, leading towards the formation offree radicals. breaking bonds within the polymer chain, top towards the formation of reactive reactive no cost Chain crosslinking occurs when free radicals react, forming aforming a amongst polymer radicals. Chain crosslinking happens when free of charge radicals react, crosslink crosslink involving chains to chains to type astructure.structure. polymer type a network networkFigure five. Degradation mechanisms: (a) random chain scission and (b) crosslinking. Reproduced Figure five. Degradation mechanisms: (a) random chain scission and (b) crosslinking. Reproduced with permission [18]. with permission [18].Energies 2021, 14,9 ofChain scission is regarded to become the dominant mechanism and outcomes in a reduce in the polymer molecular weight and an increase in polydispersity displaying the presence of various chain lengths [122]. The presence of chain crosslinking, even so, increases the molecular weight because of the formation of longer chains and crosslinking [152]. The extent of degradation is dependent upon numerous things: the amount of re-processing cycles, polymer chemical structure, thermal-oxidative stability on the polymer, as well as the reprocessing situations [128,15254]. By way of example, Nait-Ali et al. [155] studied the influence of oxygen concentration on this competition amongst chain scission and chain crosslinking. They concluded that a well-oxygenated environment favours chain scission whilst a lowoxygenated environment provokes chain crosslinking. The presence of oxygen leads to the formation of oxygenated functional groups on the polymer chain, for instance ketones, which influence the final performance. HDPE, LDPE, and PP have been discovered to possess distinct degradation behaviours through mechanical reprocessing (Figure 6) [154]. HDPE and LDPE have higher thermal stability, can be subjected to a high number of extrusion cycles before degradation, and commonly undergo chain scission and chain branching/crosslinking. Chain scission has been shown to become the dominant degradation mechanism in HDPE by Abad et al. [156], additional supported by Pinherio et al. [152], who each studied HDPE subjected to 5 extrusion cycles. Even so, Oblak et al. [157] subjected HDPE to one hundred consecutive extrusion cycles at 22070 C and identified that the chain scission was dominant up to the 30th extrusion cycle but upon further boost, chain branching dominated. Sooner or later, crosslinking occurred just after the 60th cycle as determined by way of the melt flow index (MFI), rheological behaviour, and gas permeation chromatography (GPC). Jin et al. [158] identified that when virgin LDPE (vLDPE) was subjected to 100 extrusion cycles at 240 C to simulate the recycling method, chain scission and crosslinking occurred simultaneously, determined by rheological and MFI measurements. Nonetheless, even though bo.
