Long-term stability of UHMWPE fibers Amanda L. Forster a, * , Aaron M. Forster b , Joannie W. Chin b , Jyun-Siang Peng b , Chiao-Chi Lin b , Sylvain Petit b , Kai-Li Kang a , Nick Paulter a , Michael A. Riley a , Kirk D. Rice a , Mohamad Al-Sheikhly c a Material Measurement Laboratory, National Institute of Standards and Technology, Gaithersburg, MD, USA b Engineering Laboratory, National Institute of Standards and Technology, Gaithersburg, MD, USA c Materials Science and Engineering Department, University of Maryland, College Park, MD, USA article info Article history: Received 3 October 2014 Received in revised form 22 January 2015 Accepted 27 January 2015 Available online 7 February 2015 Keywords: UHMWPE fibers Body armor Oxidation Activation energy Degradation Artificial aging abstract The performance of ultra-high molecular weight polyethylene (UHMWPE) fibers for ballistic protection is predicated on the development of a highly aligned molecular structure that allows the polymer to exhibit a superior strength in the axial direction of the fiber. However, even an ideal molecular structure will be subjected to degradation during use, which can reduce the high strength of these fibers, and impact their ability to protect the wearer. In this work, the long term stability of UHMWPE fibers are investigated and the activation energy for this mechanism was calculated to be approximately 140 kJ/mol, in agreement with previous reports. The inclusion of accelerated aging temperatures that encompass the alpha- relaxation temperature introduced physical effects in addition to oxidative degradation that compli- cate a simple explanation of the changes in properties. Assuming that the shift factors that were used in this analysis are correct, it would take approximately 36 years for the tensile strength of this UHMWPE yarn to fall by 30% at 43  C. Changes in the oxidation index of this material due to aging are also studied using Fourier Transform Infrared (FTIR) Spectroscopy, and no simple correlation between the retained strength and the oxidation index was found. Published by Elsevier Ltd. 1. Introduction Ultra high molecular weight polyethylene [1e4] (UHMWPE) is one of the two main types of fibers currently used in ballistic- resistant body armor. UHMWPE is a long-chain polyolefin with a molar mass between 3 million and 5 million. Its tensile strength is reported to be approximately 40% greater than PPTA fiber [5] due to its high crystallinity and highly oriented zig-zag sp 3 conformation. Polyethylene has no functional groups, resulting in superior chemical resistance as compared to other materials [6]. A well- publicized field failure in 2003 of a body armor based on the fi- ber poly(p-phenylene-2,6-benzobisoxazole), or PBO, has prompted our work to better understand the long term stability of other classes of fibers used in body armor when exposed to elevated temperatures [7e9]. Degradation of polyolefins initiates from thermal decompo- sition of hydroperoxides and peroxides produced due to defects, unsaturations, and other impurities introduced during process- ing [10]. The rate of degradation is dependent upon many factors such as the number of free radicals generated, the presence of scavenger compounds (e.g., antioxidants), the presence of oxy- gen, and the degree of crystallinity of the polymer. UHMWPE fibers used in soft body armor are protected from photo- oxidation by a protective fabric carrier, and exposure to ionizing radiation is not expected to occur during general use. The likely routes to initiate degradation are thermal exposure (from storage and wear) or mechanically-initiated degradation from routine use of the armor. Mechanical degradation initiates free radical formation and can occur at folds [11] due to move- ment of the armor on the wearer. Once these free radicals are generated they may follow the established free radical reactions such as hydrogen abstraction and hydroperoxide formation resulting in main chain scission, reduction of molecular weight, and oxidation. Each of these short-term exposures results in cumulative damage that may lead to unforeseen performance reductions in the armor over time. Mechanically-induced oxidation is not a commonly explored route for artificial aging of UHMWPE fibers since it can be difficult to quantify and control * Corresponding author. E-mail address: amanda.forster@nist.gov (A.L. Forster). Contents lists available at ScienceDirect Polymer Degradation and Stability journal homepage: www.elsevier.com/locate/polydegstab http://dx.doi.org/10.1016/j.polymdegradstab.2015.01.028 0141-3910/Published by Elsevier Ltd. Polymer Degradation and Stability 114 (2015) 45e51