Thermal aging effects on LDPE greenhouse films: anisotropy and degradation

Abstract

This study investigates the diverse and directional effects caused by artificial thermal aging in unstabilized Low-Density Polyethylene films that are commonly used in Laghouat's Greenhouses in Algeria. The films were subjected to three different temperatures (50 °C, 60 °C, and 68 °C) for 18 months. After the exposure, the films underwent mechanical and thermal assessments in three different orientations: machine direction, 45° inclination direction, and transversal direction. Anisotropic tendency was observed, notably in the machine direction orientation, resulting in an increase in yield stress and Young's modulus with a decrease in ductility especially at higher exposure temperatures. Morphological changes and secondary crystallization were identified in the degradation process. The increase of crystallinity confirmed by Differencial Scanning Calorimetry indicated that structural transformations occur during prolonged aging. Calculations highlighted a significant increase in crystallinity degree, influencing mechanical characteristics and causing time-dependent stiffening. Moreover, the increased activation energy with thermal aging time at higher temperatures, revealed an increase in energy demand during the thermal degradation process. Considering elongation at break as a service life criterion, films at 68 °C exhibited notable losses, reaching 50% by the 12th month for all orientations. By the 18th month, machine direction orientation experienced an 88% loss, while inclination direction and transversal direction orientations showed 86% and 96%, respectively, marking critical degradation. This study provides concise insights into the significance heterogeneity and anisotropy induced by artificial thermal aging in unstabilized low-density polyethylene films, which is crucial for optimizing materials in Laghouat's Greenhouses.