materials Article Effect of Thermal Aging on the Mechanical Properties of High Tenacity Polyester Yarn Tsegaye Sh. Lemmi 1 , Marcin Barburski 1, * , Adam Kabzi ´ nski 2 and Krzysztof Frukacz 2   Citation: Lemmi, T.S.; Barburski, M.; Kabzi ´ nski, A.; Frukacz, K. Effect of Thermal Aging on the Mechanical Properties of High Tenacity Polyester Yarn. Materials 2021, 14, 1666. https://doi.org/10.3390/ma14071666 Academic Editor: Gabriele Milani Received: 23 February 2021 Accepted: 24 March 2021 Published: 28 March 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Faculty of Material Technologies and Textile Design, Institute of Architecture of Textiles, Lodz University of Technology, 90-924 Lodz, Poland;Tsegaye.lemmi@dokt.p.lodz.pl 2 Sempertrans Belchatów Sp. z o.o., 97-427 Rogowiec, Poland; Adam.Kabzinski@semperitgroup.com (A.K.); Krzysztof.Frukacz@semperitgroup.com (K.F.) * Correspondence: marcin.barburski@p.lodz.pl Abstract: Textile materials produced from a high tenacity industrial polyester fiber are most widely used in the mechanical rubber goods industry to reinforce conveyor belts, tire cords, and hoses. Reinforcement of textile rubber undergoes a vulcanization process to adhere the textile materials with the rubber and to enhance the physio-mechanical properties of the product. The vulcanization process has an influence on the textile material being used as a reinforcement. In this work, the ef- fects of aging temperature and time on the high tenacity polyester yarn’s mechanical and surface structural properties were investigated. An experiment was carried out on a pre-activated high tenacity polyester yarn of different linear densities, by aging the yarn specimens under various aging temperatures of 140, 160, 200, and 220 ◦ C for six, twelve, and thirty-five minutes of aging time. The tensile properties and surface structural change in the yarns pre- and post-aging were studied. The investigation illustrates that aging time and temperature influence the surface structure of the fiber, tenacity, and elongation properties of the yarn. Compared to unaged yarn, an almost five times higher percentage of elongation was obtained for the samples aged at 220 ◦ C for 6 min, while the lowest tenacity was obtained for the sample subjected to aging under 220 ◦ C for 35 min. Keywords: thermal aging; high tenacity; polyester fiber; mechanical properties; industrial yarn 1. Introduction Polyester fiber is the most widely used synthetic fiber in several areas of application [1]. As per a report published by the textile exchange in 2019, polyester fiber had 51.5 percent of the total global fiber production market share in 2018 [2]. The use of polyester fiber ranges from apparel and household textiles, to industrial and special textiles [3]. This work only focuses on industrial polyester yarns. Based on the fiber’s morphological structure and properties, four primary industrial polyester yarns are commercially available. These yarns are high tenacity (HT) polyester yarn, high-modulus low-shrinkage (HMLS) polyester yarn, low-shrinkage (LS) polyester yarn, and super low-shrinkage (SLS) polyester yarn [4]. The availability of polyester (poly(ethylene terephthalate)) in a semi-crystalline or fully amorphous form attracts the attention of many researchers, and numerous studies have been conducted on the proper- ties of poly(ethylene terephthalate) (PET). Samui et al. [4] studied the relationship between the morphology and the properties of industrial polyester yarns. The study shows that amorphous orientation is the key structural parameter that has an influence on the essential properties of the yarn. Additionally, in another article, Samui et al. [5] studied the relation between the dynamic and static properties of different types of industrial polyester yarns, with their structure and end applications. Martin et al. [6] investigated the effect of the physical aging of semi-crystalline poly(ethylene terephthalate) on the degree of crystallinity. Jing and Shanyuan [7] analyzed the modulus–strain curve of poly(ethylene terephtha- late) and polyamide yarns in relation to physical structure change. Farhoodi et al. [8] Materials 2021, 14, 1666. https://doi.org/10.3390/ma14071666 https://www.mdpi.com/journal/materials