Contents lists available at ScienceDirect Polymer Testing journal homepage: www.elsevier.com/locate/polytest Material Properties Anisotropy evolution of low density polyethylene greenhouse covering films during their service life M.I. Babaghayou a , Abdel-Hamid I. Mourad b,*,1 , Vicente Lorenzo c , S.F. Chabira a , M. Sebaa a a Ammar Telidji University, Mechanical Laboratory, Laghouat, PO BOX 37, G 03000, Algeria b United Arab Emirate University, Mechanical Engineering Department, Al Ain, United Arab Emirates c G.I. “POLímeros: Caracterización y Aplicaciones”, Universidad Politécnica de Madrid, Spain ARTICLE INFO Keywords: LDPE greenhouse film Photoageing Anisotropy of the properties HALS additives ABSTRACT The primary focus of this work is to investigate the effect of service life photoageing on the anisotropy of the properties of the low density polyethylene greenhouse covering films. As these films are usually susceptible to mechanical loads in any direction during installation and service, the evaluation of these properties in different directions is of great importance for the greenhouse design and integrity. Unstabilized and stabilized blown extruded single layer films were exposed to natural weathering over several months. The chemical evolution of photooxidation products was evaluated using infrared spectroscopy, the structural evolution was monitored by X-ray diffraction and optical techniques, and the mechanical properties were measured in machine, transverse and intermediate directions. The results indicate that the photoageing significantly increases the film crystal- linity, the crystal thickness and the optical birefringence. These structural changes do not only affect the me- chanical properties of the film but also the mechanical anisotropy. 1. Introduction The most common greenhouse covering material is low density polyethylene, LDPE, films. Due to harsh environmental service life conditions of high temperature, solar radiation, oxygen, chemical pol- lution, rain, wind, sand wind, ozone, dust, etc., these films are sus- ceptible to being photoxidized and experiencing structural changes related to chemocrystallization. As a consequence of these structural changes, the films become stiffer but also less ductile [1–4]. Also, it has been found that, after sun exposure, the structure and the mechanical properties of exposed and unexposed faces of the greenhouse covering films are different, i.e., that, after weathering, the films become het- erogeneous [4]. In addition, the film is usually subjected to complex and sustained applied loading conditions such as dead weight and wind action. Therefore, all parts of the structure of the greenhouse, including the covering film, should be produced, designed and constructed to safely support multiaxial applied loads without exceeding the design stresses. Furthermore, greenhouse covering films are anisotropic because a considerable fraction of them is produced by blown extrusion. The anisotropy of blown extruded films of polyolefins (including LDPE, linear low density polyethylene, polypropylene and some blends of these polymers), that is closely related to the processing parameters, is usually studied by means of structural (X-ray diffraction, transmission electronic microscopy, infrared dichroism) and mechanical character- ization techniques [5–12]. The results prove that the tensile moduli, yield strength, tensile strength, elongation at break, impact strength and tear propagation resistance of the films depend in a complex way on the extrusion conditions. At this point, it is interesting to note that the anisotropy of these films is not a static structural parameter but it changes over the service life of these products as a consequence of the photoxidation of PE [9,13,14]. Even more, it has been proved that anisotropy and weathering phenomena are interrelated because me- chanical stresses can affect the chain scission and crosslinking rates of the PE macromolecules and modify the oxygen diffusion rate [13,14], and both structural and chemical changes due to photooxidation de- termine the mechanical properties of the films [4,9,13–15]. In our previous work [4], the time evolution of the heterogeneity in the film characteristics due to photooxidation were addressed. Here, wider consideration is given to the effects of weathering on the aniso- tropy of properties. The influence of the natural weathering (e.g., solar light) and the effect of the use of stabilizing additives on several mi- crostructural parameters and on the mechanical behavior of blown extruded both unstabilized and stabilized LDPE films was investigated. https://doi.org/10.1016/j.polymertesting.2018.01.007 Received 3 December 2017; Accepted 9 January 2018 * Corresponding author. 1 On leave from Mechanical Design Department, Faculty of Engineering, Mataria, Helwan University, Cairo, Egypt. E-mail address: ahmourad@uaeu.ac.ae (A.-H.I. Mourad). Polymer Testing 66 (2018) 146–154 0142-9418/ © 2018 Elsevier Ltd. All rights reserved. T