Disentangling High Strength Copolymer Aramid Fibers to Enable the Determination of Their Mechanical Properties

Summary

The primary goal of the study is to develop a protocol to prepare consistent specimens for accurate mechanical testing of high strength copolymer aramid fibers, by removing a coating and disentangling the individual fiber strands without introducing significant chemical or physical degradation.

Abstract

Traditionally, soft body armor has been made from poly(p-phenylene terephthalamide) (PPTA) and ultra-high molecular weight polyethylene. However, to diversify the fiber choices in the United States body armor market, copolymer fibers based on the combination of 5-amino-2-(p-aminophenyl) benzimidazole (PBIA) and the more conventional PPTA were introduced. Little is known regarding the long-term stability of these fibers, but as condensation polymers, they are expected to have potential sensitivity to moisture and humidity. Therefore, characterizing the strength of the materials and understanding their vulnerability to environmental conditions is important for evaluating their use lifetime in safety applications. Ballistic resistance and other critical structural properties of these fibers are predicated on their strength. To accurately determine the strength of the individual fibers, it is necessary to disentangle them from the yarn without introducing any damage. Three aramid-based copolymer fibers were selected for the study. The fibers were washed with acetone followed by methanol to remove an organic coating that held the individual fibers in each yarn bundle together. This coating makes it difficult to separate single fibers from the yarn bundle for mechanical testing without damaging the fibers and affecting their strength. After washing, fourier transform infrared (FTIR) spectroscopy was performed on both washed and unwashed samples and the results were compared. This experiment has shown that there are no significant variations in the spectra of poly(p-phenylene-benzimidazole-terephthalamide-co-p-phenylene terephthalamide) (PBIA-co-PPTA1) and PBIA-co-PPTA3 after washing, and only a small variation in intensity for PBIA. This indicates that the acetone and methanol rinses are not adversely affecting the fibers and causing chemical degradation. Additionally, single fiber tensile testing was performed on the washed fibers to characterize their initial tensile strength and strain to failure, and compare those to other reported values. Iterative procedural development was necessary to find a successful method for performing tensile testing on these fibers.

Introduction

Currently, significant focus in the field of personal protection is on reducing the mass of the body armor needed for personal protection for law enforcement and military applications¹. Traditional armor designs have relied on materials like poly(p-phenylene terephthalamide) (PPTA), also known as aramid, and polyethylene to provide protection against ballistic threats². However, there is an interest in exploring different high strength fiber materials for their potential to reduce the weight of armor required to stop a specific ballistic threat. This has led to the exploration of alternative materials such as ar....

Protocol

1. Dissolution of Coating on Copolymer Fibers to Aid in Fiber Separation

1. Wearing appropriately selected chemically resistant gloves to prevent contamination of the fiber, cut 160 mm to 170 mm from each yarn bundle extracted using ceramic scissors or a fresh steel razor blade. Reserve the remainder of the yarn if needed for further analysis in a labeled container.
2. Knot or clamp the ends of the yarn to keep the yarn from tangling when immersed in the solvent.
   NOTE: For this study, solvents of wide ranging polarity (from the polarity series) were initially explored. Based on qualitative results, a more in-depth examination was conducted using....

Results

The copolymer aramid fibers studied here are difficult to separate from yarn bundles into individual fibers for testing. The fibers are entangled and coated with processing chemicals that make them very difficult to separate without damaging the fibers. Figure 3 shows the structural morphology of fibers within a yarn. Even as part of a larger bundle, the fiber surfaces show extensive roughness and tears that are likely caused by strong adhesion to adjacent fi.......

Discussion

The method described herein provides an alternate solvent-based protocol for removing coatings from aramid copolymer fibers without using water. Two previous studies^3,4 showed the evidence of hydrolysis in the fibers of this chemical composition, with exposure to water vapor or liquid water. Avoiding hydrolysis during the sample preparation is critical for the next phase of experiments where these sets of fibers will be examined for their susceptibility to ageing.......

Materials

Name	Company	Catalog Number	Comments
Stereo microscope	National	DC4-456H	Digital microscope
RSA-G2 Solids Analyzer	TA Instruments		Dynamic mechanical thermal analyzer used in transient tensile mode with Film Tension Clamp Accesory
Vertex 80	Bruker Optics		Fourier Transform Infrared spectrometer used to analyze results of washing protocol, equipped with mercury cadmium telluride (MCT) detector.
Durascope	Smiths Detection		Attenuated total reflectance accessory used to perform FTIR
Torque hex-end wrench	M.H.H. Engineering	Quickset Minor	Torque wrench
Methanol	J.T. Baker	9093-02	methanol solvent
Acetone	Fisher	A185-4	acetone solvent
Cyanoacrylate	Loctite		Super glue
FEI Helios 660 Dual Beam FIB/SEM	FEI Helios		Scanning electron microscope
Denton Desktop sputter coater			sputter coater
25 mm O.D. stainless steel washers with a 6.25 mm hole			25 mm O.D. stainless steel washers with a 6.25 mm hole
Silver behenate			Wide angle X-ray scattering (WAXS) standard
Xenocs Xeuss SAXS/WAXS small angle X-ray scattering system	Xenocs Xeuss		SAXS/WAXS small angle X-ray scattering system equipped with an X-ray video-rate imager for SAXS analysis with a minimum Q = 0.0045 Å-1, detector separate X-ray video-rate imager for WAXS analysis (up to about 45° 2θ) sample holder chamber.
Fit 2D software			Software to analyze WAXS data