Enhancing the Thermal Stability of Plasticized Polyvinyl Chloride Films Using Sustainable and High Dispersity Surface-Functionalized Layered Double Hydroxides

Abstract

Polyvinyl chloride (PVC) is a long-standing thermoplastic with excellent performance in chemical resistance, healthcare, construction, and packaging. However, this versatile thermoplastic has low thermal resistance during processing and service life. In this project, Layered Double Hydroxide (LDH) is used as an environmentally friendly thermal stabilizer in two stages: before surface modification and after surface treatment. The successful synthesis of MgAlZn-LDH structures with a constant molar ratio of Mg+Zn/Al=2 and three different molar ratios of Mg/Zn=0.5, 1, and 2 is confirmed by powder X-ray analysis. The plasticized PVC compounds show improved thermal stability for all PVC-LDH samples compared to the reference sample, with C-R2 (Mg/Zn=2) exhibiting the best thermal stability. However, EDS results reveal that LDHs have a nonuniform distribution in PVC with weak dispersity, indicating a tendency for LDHs as inorganic additives to agglomerate in PVC. To enhance the distribution of LDHs in PVC, the surface of LDH-2R is organo-treated with Oleic Acid (OA) as a surfactant derived from renewable resources. Different molar ratios of OA/LDH (3, 6, 9, 12 mmol) are used for the surface modification. Powder X-ray analysis shows that the modification process not only preserves the integrity of LDHs but also increases the crystal size with fewer defects. Additionally, XPS analysis confirms that the surface chemistry of LDHs changes after modification, leading to a decrease in surface energy and more stable structures. EDS maps demonstrate the uniform distribution and strong dispersity of the finest surface-modified LDH in PVC, indicating excellent compatibility between LDH and PVC in compounds. Furthermore, the MC-9OA PVC compound exhibits excellent static thermal stability compared to the reference compound. Discoloration tests reveal significant color stability in the modified compounds, with uniform color change indicating the uniform distribution of LDHs in PVC. Raman spectroscopy analysis illustrates the thermal stabilization of LDH in PVC degradation through the formation of metal chloride during the thermal decomposition of PVC. Modified compounds exhibit lower rates of metal chloride formation, demonstrating the significant impact of OA on the efficiency of LDHs in absorbing and neutralizing the HCl released during PVC thermal degradation.