Origin of cloudiness in PET bottles modified with chain extenders for mechanical recycling

Abstract Melt viscosity improvers (MVs) are known as chain extenders that increase the intrinsic viscosity of recycled polyethylene terephthalate (PET) by forming covalent bonds with PET molecules. Experimental results showed that MVs accelerate epitaxial crystallization in a direction perpendicular to the orientation of MV agglomerates. Acting as nucleating agents, MVs promote crystallization under tension and the formation of light-scattering structures, resulting in cloudiness. Transmission electron microscopy confirmed the presence of MV agglomerates within PET bottles. Wide-angle X-ray diffraction further revealed that MVs altered the crystal orientation of PET by 90° compared to neat PET. Small-angle X-ray scattering during tensile tests indicated that MV-induced crystals within PET bottles were highly oriented perpendicular to the MV orientation direction, potentially contributing to enhanced mechanical drawability and toughness.