Mitigation of the Color Generated During Mechanical Recycling of PET/MXD6 blends

• Published in: Polymer degradation and stability Vol. 194; p. 109748
• Main Authors: Jalilian, Mohammadali, Joshi, Anup S., Coleman, Maria R., Kim, Yong-Wah, Lawrence, Joseph G.
• Format: Journal Article
• Language: English
• Published: London Elsevier Ltd 01.12.2021; Elsevier BV
• Subjects: Chemical properties; Color; Color generation; Colorimetry; Imines; Mechanical properties; Mechanical recycling; Mixtures; MXD6; NMR; Nuclear magnetic resonance; Packaging; PET; Polyethylene terephthalate; Polymer blends; Recycling; Scavenging; Terephthalic acid; Tomography; Xylene; MXD6; NMR; Mechanical recycling; PET; Color generation
• ISSN: 0141-3910; 1873-2321
• DOI: 10.1016/j.polymdegradstab.2021.109748

•Color forming conjugated imine structures present in the PET/MXD6 blend was confirmed•A new method to mitigate the color generation by adding small amounts of TPA•Yellowness Index was reduced by 21% in the blend•Yellowness Index was reduced by 41% in the recycled products after two melt histories High oxygen barrier in Poly(ethylene terephthalate) (PET), a widely used packaging polyester, can be achieved by blending poly(m-xylene adipamide) (MXD6), a commercially available high-barrier polymer, with PET. This blend is used industrially to address the increasing demand for longer shelf-life PET packaging. However, the impact of color generation in PET/MXD6 blends during mechanical recycling on recycled PET (rPET) flake is an ongoing challenge to the PET recycling industry. This work demonstrates the effect of multiple recycles on the physical and chemical properties of PET/MXD6 blends. Through 2D NMR correlation spectroscopy, structure of color forming conjugated imines present in the PET/MXD6 blends was confirmed for the first time. Building on the prior art, a modified processing method is proposed to mitigate the color generation in the blend by scavenging the MXD6’s amine end groups by adding small amounts of terephthalic acid (TPA) in the melt. Colorimetric evaluation showed that the incorporation of 0.5% (w/w) TPA in the blends reduced the Yellowness Index by 21% in the blend and by 41% in the recycled products after two melt histories, due to lower conjugated imine concentration in the blended samples. This work demonstrates a potential pathway to reduce color generation typically observed in PET-MXD6 blends in order to reduce the need for separating MXD6 in the recycling processes and the costs associates with it.