Mechanical properties of extensively recycled high density polyethylene

Abstract

In the plastics industry it has been common practice to mechanically recycle waste material arising from a production. However, mechanical recycling affect material mechanical properties and consequently quality of the end products; therefore it needs to be quantified. Mechanical recycling of high density polyethylene (HDPE) was simulated by one-hundred consecutive extrusions. After every cycle, portion of material was removed for the purpose of characterization. Solid mechanical properties of the material were characterized in terms of hardness and modulus measured with nanoindentation. Furthermore, shear creep compliance was measured to characterize the materials’ time-dependent mechanical properties in solid state. In addition, differential scanning calorimetry (DSC) measurements were performed in order to study structural changes through the degree of crystallinity. The results on hardness and modulus show deterioration of the material mechanical properties through the process of repeating recycling. This becomes increasingly evident after 10th extrusion. Similarly, shear creep compliance measurements show an unfavourable effect of mechanical recycling on the time-dependent mechanical properties. In this case, evident changes are visible in particularly after the 30th extrusion. After 100th recycling material mechanical properties reduces for about 20%. All those changes are well supported by changes of the degree of crystallinity.