Switching the Crystallization Priority Component upon Fast-Cooling PE/PP Recycled Blend and Its CaCO ₃ Composite

Isotactic polypropylene (PP) commonly raises crystallization prior to polyethylene (PE) due to its higher melting point upon cooling at the cooling rate around 10 K/min (∼0.17 K/s) in conventional DSC measurements. We observed that the priority component of crystallization switches to PE in the recycled PE/PP (∼1/1) blend once the cooling rates are higher than the critical 15 K/s in Flash DSC measurements. In addition, adding the fillers of calcium carbonate particles (18.8% mass) mainly accelerates PP crystallization and shifts up the critical cooling rate to 35 K/s. Since both critical cooling rates (15 and 35 K/s) are accessible to the industrial processing of plastic injection molding (30–40 K/s in the core and much higher in the skin), our observations are practically meaningful for plastic recycling with the dominant components of PE and PP.