Detection of Early Stages of Degradation on PPTA Fibers Through the Use of Positron Annihilation Lifetime Spectroscopy

• NCJ Number: 305394
• Author(s): Neylan Lopez-Perez,
• Date Published: August 2019
• Grant Number(s): 2016-R2-CX-0011
• Agencies: NIJ-Sponsored
• Publication Type: Thesis/Dissertation
• Award: Nanostructural Characterization of Ballistic Fiber Degradation

Annotation

This dissertation focuses on the effects of sonication, pH changes, and sweat on the free volume of poly (p-phenylene terephthalamide) (PPTA) fibers, presents results that demonstrate the fibers’ reduced performance when exposed to different degradation factors, and contributes to future studies of lifetime predictions of bulletproof vests and the development of the next generation of soft body armors.

Abstract

The author’s research for this dissertation focused on the effects of sonification, pH changes, and sweat on the free volume of poly (p-phenylene terephthalamide) (PPTA) fibers which are used in Twaron fibers and are one of the most commonly used fibers on soft body armors such as bulletproof vests. Free volume is defined as the unoccupied space between the polymer molecules and is responsible for characteristics such as diffusion and viscosity. The author used a non-destructive technique known as positron annihilation lifetime spectroscopy (PALS) to measure the free volume in PPTA. Changes in the free volume of fibers that had been degraded under different conditions were compared to their mechanical performance. Degradation in DI (deionized) water, pH 4 and pH 10 aqueous solutions was conducted for 10 weeks at 80^oC. Sweat degradation of PPTA fibers was also conducted for 10 weeks at 25^oC, 50^oC, and 100^oC. Fibers degraded in pH4 and sweat solutions had greater loss of mechanical performance and changes in the free volume. PALS was able to detect changes in the nanostructure of PPTA fibers at early stages of degradation. The author’s data was supported by mechanical tests and is complementary to other characterization techniques such as small angle X-ray scattering (SAXS). The author concluded that highly crystalline fibers such as PPTA are very resistant to chemical attacks and therefore they degrade at a slow rate, but accelerating their degradation through a combination of H2SO4 and temperature show that higher concentrations of acid initiate the formation of more free volume, leading to loss of mechanical performance. The author suggests that the combined use of PALS and mechanical testing can be used to measure small changes in materials’ physical structure and resulting effects on material performance.