Chemistry
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Disentangling High Strength Copolymer Aramid Fibers to Enable the Determination of Their Mechanical Properties
Chapters
Summary September 1st, 2018
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.
Transcript
This method can help answer key questions in the single-fiber testing field about methodologies for dissolving coatings and disentangling challenging fiber samples. The main advantage of this technique is that the water method of coating dissolution has been replaced with a solvent-based method that is less damaging to the fibers. Generally, individuals new to this method will struggle because of the difficulty in manipulating single fibers.
Wearing the appropriate chemically resistant gloves for preventing contamination of the fiber, use a fresh steel razor blade to cut 160 to 170 millimeters from each extracted yarn bundle, reserving the remainder of the yarn in a labeled container for further analysis as appropriate. Secure the ends to keep the yarn from tangling, and immerse the fibers in two to three milliliters of solvent in a labeled Petri dish. Place the lid on the dish and allow the yarns to soak for 30 minutes.
After the third immersion, submerge the samples in two to three milliliters of methanol for at least 30 minutes. Then allow the yarn to dry for at least 24 hours. The next day, place a 30 by 30 centimeter or larger transparent plastic board onto a dark background of the same proportions.
And use a razor blade to cut five to 10 millimeter pieces of low-tack masking tape on a glass surface. Tape both ends of a 20-millimeter gauge rectangular paper template to the plastic board so that the template lies completely flat. And wearing nitrile gloves to prevent contamination, cut approximately 70 to 80 millimeters of the solvent-rinsed yarn.
Place the cut yarn on a clean surface, and using a stereo microscope, use tweezers to carefully remove a single fiber from the yarn. Place a single fiber on top of the paper template, making sure that the fiber is aligned with the markers on the template. And tape both ends of the fiber to the board.
Place a dark background under the plastic board to improve the visibility of the fiber. The fiber should lay straight and slightly taut across the template. When all of the samples have been mounted on a separate paper template for each type of fiber, add one small drop of cyanoacrylate adhesive to each end of the fibers, leaving one centimeter free of glue at the ends of the paper templates for gripping during the tensile testing.
After allowing the glue to dry for at least 24 hours, install the tensile grips and calibrate the tensile testing instrument to provide a 30-millimeter gap. Next, use gloves, a small spatula, and tweezers to feed a template through both of the grips, using the marks on the template to assist with the placement. Gently align and close the top grip face while keeping the fiber supported so it does not slide down, and use a torque wrench to tighten the screws on the upper and lower grips until the screws are just tight.
Then, use a torque wrench to tighten the screws securely in a cross-pattern, to balance the load of 30 centinewton meters on the fiber. After trimming both sides of the paper, program the tensile test instrument to deliver a constant 0.0125 millimeters per second extension rate, monitoring the display, and stopping the test when the fiber has broken. At the end of the test, loosen the grip faces to remove the fiber and note the break location.
Then, preserve the broken fiber in a labeled container for further analysis. Fourier Transform Infrared Spectroscopy reveals some intensity changes after washing, but no major changes in spectra indicative of chemical degradation. Nearly all of the samples affixed to the paper templates with the epoxy adhesive exhibit an uncharacteristic slipping behavior and jagged stress strain curves.
While fibers fixed with cyanoacrylate adhesive are predominantly devoid of sample slippage. In this representative experiment, 35 samples from each type of fiber were tested, between 15 to 26 of which, per type, were successful. Scanning electron microscopy imaging of the failed fibers indicated that all of the fibers underwent brittle fracture that resulted in fibrillation.
However, the diffractograms of the meridonial scattering also revealed the present of two major brag peaks at two theta angles of about 26 and 28 degrees, indicating a very low degree of crystallinity overall for each of the fiber types along the chain axis. While attempting this procedure, it's important to remember to handle the fibers very carefully to avoid damaging them while they're being disentangled. After its development, this technique paved the way for researchers in the field of single-fiber tensile testing to explore artificial aging of aramid copolymer fibers, which is the next step in our research study.
Don't forget that working with solvents such as acetone and methanol can be extremely hazardous and that precautions such as wearing gloves and working in a hood should always be taken while performing this procedure.
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