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 aramid copolymer fibers. These fibers are made by the reaction of [5-amino-2-(p-aminophenyl)benzimidazole] (amidobenzimidazole, ABI) and p-phenylenediamine (p-PDA) with terephthaloyl chloride to form poly(p-phenylene-benzimidazole-terephthalamide-co-p-phenylene terephthalamide). In this study, we examine three different fibers, all of which are commercially produced materials obtained from an industry contact. One is a homopolymer fiber that is made by reacting ABI with p-phenylenediamine to form poly 5-amino-2-(p-aminophenyl)benzimidazole, or PBIA. The other two copolymer fibers examined in this study are expected to be random copolymers with different ratios of PBIA and PPTA linkages³. The relative ratios of these linkages could not be determined experimentally using solid-state nuclear magnetic resonance. These fibers are designated as PBIA-co-PPTA1, PBIA-co-PPTA3 to extend the designations used in a previous publication⁴. PBIA-co-PPTA3 was not previously studied, but has a similar structure. These fiber systems have also been the focus of several recently granted patents^5,6,7.

Superior ballistic resistance of body armor is predicated on the mechanical properties of the materials that comprise it, such as ultimate tensile strength and strain to failure^8,9,10. Significant efforts^11,12,13 have been focused on examining the long-term stability of polymeric fibers used in body armor by investigating detrimental changes in these mechanical properties after exposure to environmental conditions. The effect of environmental conditions on aramid copolymer fibers has not been the subject of a lot of research^3,4. One challenge to studying these materials is the difficulty in disentangling yarns for testing. Prior work by McDonough⁴ investigated a technique by which water was used to disentangle yarns prior to performing single fiber tensile testing. However, there was no complete understanding on whether the mechanical strength of the fibers was altered by this water exposure. An alternative to disentangling the fibers is to test the mechanical strength of the yarn bundle, however, this requires a large amount of material, and is considered to average the strength of the fibers in the yarn bundle, providing less specific information. The goal of this project is to examine the effect of elevated humidity and temperature on the mechanical properties of aramid copolymer fibers. Thus, it is essential to find an alternative solvent for coating removal and fiber disentanglement that will enable us to distinguish hydrolysis in the fibers due to the environmental exposure from that induced by sample preparation. The preparation of single fibers for testing is further complicated by their small size. In this work, we investigate several common solvents (water, methanol, and acetone) and select acetone as the best choice for the preparation of single fibers for testing. All fibers were rinsed with methanol before further testing. Fourier transform infrared (FTIR) spectroscopy is performed to determine if the coating dissolution and disentanglement step caused any chemical degradation in the material. The detailed video protocol showing the sample preparation steps of disentanglement, chemical analysis, and mechanical testing of copolymer aramid fibers is intended to assist other researchers in developing methodologies for performing similar studies of single fibers in their laboratories.

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 acetone, water, and methanol. Finally, acetone was selected as the best solvent for fiber separation based on the ease of detangling and the scanning electron microscopy (SEM) results (described later).
3. Immerse the fiber in 2 mL to 3 mL of the solvent in a labeled Petri dish and cover with the Petri dish lid.
4. Allow the yarns to soak in acetone for 30 min, then discard the solvent.
5. Repeat steps 1.3 to 1.4 at least two additional times and then allow the solvent to evaporate.
6. To remove any acetone residue and to aid in drying, immerse the sample in 2 mL to 3 mL of methanol.
7. Allow the yarns to soak in methanol at least 30 min.
8. Remove the yarn from the solvent and allow to dry for at least 24 h.

2. Analysis of Coating Dissolution Step by Scanning Electron Microscopy

1. Separate the individual fibers with tweezers, which are previously washed using different solvents from the yarn bundle, for analysis under a stereo microscope if necessary.
2. Mount the fibers on a stainless-steel stub (1 cm diameter) by adhering them with tweezers onto double-sided carbon tape.
3. Coat the fibers with a conductive material such as Au/Pd to mitigate the surface charging effects under the SEM.
4. Load the fiber samples into a scanning electron microscope and image them at 2 kV accelerating voltage and 50 pA – 100 pA electron current. Apply charge neutralization settings to counter charging effects where necessary.

3. Analysis of Coating Dissolution Step by Fourier Transform Infrared Spectroscopy

1. Cut approximately 30 mm to 40 mm of the washed yarn bundle.
2. Obtain an adhesive IR sample card and remove the protective backing.
3. While wearing gloves to protect the sample from contamination, slightly twist the fiber bundle to coalesce the sample for analysis and place the sample over the window in the card.
4. Prepare the FTIR for analysis according to the manufacturer's specifications. Turn on the purge gas, fill the detector with liquid nitrogen, and install the ATR accessory using the magnetic alignment plate in the sample compartment.
5. Program the parameters for number of scans and instrument resolution in the advanced measurement tab of the instrument software, in this case, 128 scans are averaged at a resolution of 4 cm^-1.
6. Clean the window of the ATR accessory with a low lint wipe and methanol.
7. Collect a background by pressing the collect background button in the basic measurement window of the software with the parameters selected in step 3.5.
8. Align the fiber sample over the window in the ATR accessory, using the microscope and video monitor to help position the fiber.
9. Collect a sample spectrum by pressing the collect sample button in the basic measurement window of the software using the parameters selected in step 3.5.
10. Repeat steps 3.6-3.9, collecting at least 3 spectra per sample until all samples have been analyzed.

4. Analysis of Fibers by Wide Angle X-Ray Scattering

1. While wearing nitrile gloves, cut approximately 25 mm of the yarn from the yarn spool using a razor blade.
2. Center each bundle of the yarn over the 6.25 mm inner hole of a 25 mm stainless steel washer.
3. Tape the yarn bundle to the washer to hold it in place using cellophane tape.
4. Repeat steps 4.1 to 4.3 for the other two types of yarn.
5. Tape the washers containing the yarn bundles to a stainless-steel sample holder block (which contains metallic rods for positioning) as shown in Figure 1. The fibers should be in the vertical configuration for analysis.
6. Mount a silver behenate control sample to the sample holder block in the same position as the washers.
7. Open the door to the instrument and mount the sample holder block to the analysis stage using the magnetic alignment system.
8. Close the door to the sample holder chamber and activate the vacuum pump to evacuate the sample analysis chamber. Monitor the vacuum gauge mounted next to the instrument until the vacuum reaches approximately 1600 Pa.
9. Open the instrument software, activate the beam, and perform a horizontal scan to determine the x-location of each sample on the sample holder.
10. After identifying x-location of each sample, perform a vertical scan to optimize the y-location to obtain the maximum signal intensity for each sample.
11. Once the x and y-locations are determined, begin the measurement by analyzing the silver behenate control sample to determine the distance between the sample and each detector.
12. Analyze the first fiber sample using a 10 min exposure time.
13. Repeat step 4.13 two additional times for a total scan time of 30 min.
    NOTE: This protocol is used instead of one long 30 min scan case because there are issues with the sample exposure to minimize wasted instrument time.
14. Average the 3 scans to obtain the final result using the average function in Fit 2D software.
15. Repeat steps 4.13-4.15 for each additional sample.

5. Yarn Disentanglement and Preparation for Tensile Testing

1. Obtain a 30 cm x 30 cm or larger transparent plastic board (polycarbonate sheets are used in these experiments) that can be placed on a dark background, or a dark plastic board of the same dimensions.
2. Cut pieces of low tack masking tape (approximately 10 mm 5 mm) and have them available for the following steps. Perform this step on a glass surface and cut the tape with a razor blade.
3. Tape both ends of a 20 mm gauge rectangular paper template to the plastic board so that it lies completely flat.
   NOTE: 20 mm is selected as the optimal gauge length for these tests based on previous work and the available jaw separation of the instrument.
4. Wearing nitrile gloves to prevent contamination, cut approximately 70 mm to 80 mm of rinsed yarn and place it on a glass slide or other clean surface (Figure 2a-b).
5. Using a stereo microscope to assist disentanglement, carefully remove a single fiber from the yarn using tweezers. Take care to avoid snagging or damaging the fiber during this process. Discard any fibers that are damaged (Figure 2c).
6. Place a single fiber on top of the paper template, making sure that the fiber is aligned with the markers on the template (Figure 2d-f).
7. Tape both ends of the fiber to the board. In order to improve the visibility of the fiber, put a dark background underneath the transparent plastic board or use a black plastic board. The fiber should lay straight and slightly taught across the template (Figure 2f).
8. Repeat steps 5.3 to 5.7 until approximately 35 to 45 fibers are mounted on separate paper templates for each type of fiber. In this case, there are three types of fibers: PBIA-co-PPTA1, PBIA, and PBIA-co-PPTA3.
9. Once all fibers are taped to the plastic board, add one small drop of cyanoacrylate adhesive to each end of the fiber aligned to the paper template. Leave 1 cm free of glue at the ends of the paper templates for gripping during tensile testing.
   NOTE: Cyanoacrylate was found to be the best adhesive for this material, an unsuccessful attempt with a 24 h cure epoxy is shown in the Representative Results.
10. Allow the adhesive to cure for at least 24 h before testing.

6. Single Fiber Tensile Testing

1. Determine the gage length and the rate of extension that provides the most consistent results for the specimen of interest. These parameters may be dictated by the amount of available sample and by the limitations of the experimental setup.
2. Prepare the instrument for testing by installing tensile grips and calibrating the gap.
3. Program the instrument to move the grips to provide a gap of 30 mm, which is the gage length selected based on the size of the paper template and the 10 mm space left at each end for the jaw.
4. Loosen the grip faces to create a gap for loading the paper template that contains the single fiber.
5. Move one of the samples prepared in step 5 to the instrument. Using gloved hands, a small spatula, and tweezers, feed the template through both grips, using the marks on the template to assist the placement. Make sure that the glue is outside of the grip area.
6. Gently align and close the top grip face, while still supporting the fiber so that it does not slide down.
7. Tighten the top and bottom screws with a torque wrench until the screws are just tight.
8. Repeat step 6.7 for the bottom screws.
9. Tighten the screws on the upper and lower grips using a torque wrench. Take care to tighten the screws in a cross pattern to balance the load on the fiber.
   NOTE: The appropriate torque to use may vary and must be experimentally determined. 30 cN·m was used in these experiments.
10. Trim both sides of the paper template with scissors.
11. Program the instrument to perform the tensile test at a constant rate of extension of 0.0125 mm/s, monitor the display and stop the test when the fiber has broken.
12. At the end of the test, remove the fiber from the grips by loosening the grip faces. Observe the break location and preserve the broken fiber in a labeled container for further analysis.
    NOTE: Fibers that break at the grip face are discarded from analysis as "jaw breaks" as described in ASTM D3822.
13. Return the gap to 30 mm and repeat steps 6.4-6.12 until all samples are tested.
14. Save the broken fiber fragments in the template for further microscopic analysis.

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