High Density Polyethylene Degradation Followed by Closed-loop Recycling

The objective of this work is to investigate the loss of performance undergone by a polymer during a long-term closed-loop recycling process. High density polyethylene (HDPE) is subjected up to 50 extrusion cycles under constant processing conditions. The effect of recycling is then determined by following its degradation with increasing number of generation. After selected cycles, the material is characterized in terms of physical (density, GPC), thermal (DSC, TGA), and mechanical (tension, flexion) properties. No significant change are observed in density DSC, and TGA tests. But for GPC, the weight average molecular weight (M-w) is found to decrease while the number average molecular weight (M-n) do not change significantly, thus leading to a decreasing polydispersity index. Intrinsic viscosity also decreases, while melt flow index (MFI) increases. From the tensile stress-strain curves, recycling seems to have no significant effect on Young's modulus (E-y), but a moderate increase of the strain at yield is observed followed by a slight decrease, while the stress at yield decreases. For the break-up conditions, stress and energy at break are found to increase significantly. Finally, three-point bending tests show that the flexural modulus (E-b) decreases with recycling. Overall, the recycling process leads to an important modification of the polymer's mechanical properties mainly due to chain scission.