Effects of Thermal Aging on Molar Mass of Ultra-High Molar Mass Polyethylene Fibers

Abstract

Ultra-high molar mass polyethylene (UHMMPE) is commonly used for ballistic-resistant body armor applications due to the superior strength of the fibers fabricated from this material combined with its low density. However, polymeric materials are susceptible to thermally induced degradation during storage and use, which can reduce the high strength of these fibers, and, thus, negatively impact their ballistic resistance. The objective of this work is to advance the field of lightweight and soft UHMMPE inserts used in various types of ballistic resistant-body armor via elucidating the mechanisms of chemical degradation and evaluating this chemical degradation, as well as the corresponding physical changes, of the UHMMPE fibers upon thermal aging. This is the first comprehensive study on thermally aged UHMMPE fibers that measures their decrease in the average molar mass via high-temperature size exclusion chromatography (HT-SEC) analysis. The decrease in the molar mass was further supported by the presence of carbon-centered free radicals in the polyethylene that was detected using electron paramagnetic resonance (EPR) spectroscopy. These carbon-centered radicals result from a cascade of thermo-oxidative reactions that ultimately induce C𾩗C ruptures along the backbone of the polymer. Changes in the crystalline morphology of the UHMMPE fibers were also observed through wide-angle X-ray diffraction (WAXS), showing an increase in the amorphous regions, which promotes oxygen diffusion into the material, specifically through these areas. This increase in the amorphous fraction of the highly oriented polyethylene fibers has a synergistic effect with the thermo-oxidative degradation processes and contributes significantly to the decrease in their molar mass.

Thermo-Oxidative Aging Effect on Charge Transport in Polypropylene/Ultra-High Molecular Weight Polyethylene Nanocomposites

“…In the PP/UHMWPE/MgO nanocomposite, a broad band below 700 cm −1 is attributed to the magnesium-oxygen bending vibration, as reported in previous studies [48,49]. In addition, the effects of thermo-aging reactions on the molar mass changes in PP and UHMWPE were observed using gel permeation chromatography (GPC), as reported elsewhere [44,45]. PP aged between 95 • C and 125 • C exhibited a 5% drop in molar mass after 1500 h of aging [44].…”

“…However, when aging PP and its composites at temperatures closer to the 𝑇 of PP, i.e., 140 °C [24,43], the recrystallization of PP structures resulted in an increase of 3-4 °C in 𝑇 and a 6% increase in crystallinity. In addition, the effects of thermo-aging reactions on the molar mass changes in PP and UHMWPE were observed using gel permeation chromatography (GPC), as reported elsewhere [44,45]. PP aged between 95 °C and 125 °C exhibited a 5% drop in molar mass after 1500 h of aging [44].…”

“…A significant drop in molar mass occurred when PP was aged at 135 • C, reducing the time to failure compared to the lower thermal aging temperatures [44]. In contrast, when subjected to thermal aging at 115 • C for 1344 h, UHMWPE displayed a significant drop in molar mass, of around 70% [45]. This suggests that, when PP/UHMWPEbased samples are aged at 120 • C, the UHMWPE phase is more likely to break before the PP phase.…”

Effect of Aging on Unidirectional Composite Laminate Polyethylene for Body Armor

“…The degradation of UHMMPE fibers and the subsequent decrease in their mechanical properties under normal use conditions, involving elevated temperature, humidity, or some combination thereof, can occur through a well-documented process known as thermal oxidation [ 2 , 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 , 23 ]. Elevated temperatures can induce C–C bond scissions along the backbone of a UHMMPE chain, primarily in the amorphous regions of the polymer [ 24 ].…”