Crystallisation Behaviour and Mechanical Properties
There is a potentially misleading idea that during the recycling processes there is a continuous deterioration on mechanical properties of the recycled plastics. However, after repetitive reprocessing for a low numbers of cycles, Young’s modulus and yield stress of the recycled PP increase due to augmentation of crystallinity (da Costa et al. 2007). Ha and Kim (2012) successfully applied recycled PP as refrigerator plastics due to the comparable mechanical properties with virgin PP, and obtained about 50% cost-merit.
Aurrekoetxea et al. (2001) mimicked procedures of recycling PP through repetitively injection moulding for several cycles. With the increase of recycling cycles, the crystallinity increased from 44.5% (at the first cycle) to 48.5% (at the sixth cycle), and remained at 48.5% until the tenth cycle. When the recycling increased from one cycle to six cycles, the Young’s modulus grew from 1700 to 2000 MPa, and the yield stress rose from 34.8 to 36.4 MPa, because of the increase of crystallinity. However, the elongation at break decreased from 66% to 45%, and fracture toughness decreased from 2.24 to 1.98 MPa m1/2 due to the decrease of molar mass and tie-molecule density. They explained that the repetitively reprocessing breaks the molecular chain and allows the strained or entangled macromolecules to be released. The crystallisation is then developed by the rearrangement of these freed macromolecules segments. The crystalline structures hindered rotations of molecular segments, leading to the increase of stiffness.