Environmentally-Friendly Chemical and Physical Methods for Deweaving and Functionalization of Waste Cotton Fabric

As the demand for cellulosic fibers increases, the amount of cotton fabric waste produced also increases. Many methods currently available for recycling cellulose involve costly and potentially hazardous reagents such as ionic liquids and N-methylmorpholine-N-oxide [1]. We will show that mild physical and chemical treatments can deweave cotton fabric into fibers that can then be re-weaved or repurposed for many applications.<br/>Diagonally-cut cotton muslin fabric pieces of around 0.25 g were deweaved using a 40 mL solution of 0.5 M citric acid, 0.5 M sodium nitrate, or a 3:1 mixture of the two aforementioned solutions, being stirred at 600 rpm and 50°C. Samples were subsequently washed, with multiple rinses of vacuum filtration with deionized water (&gt;18mΩ/cm), and then vacuum dried. The solution was shown to successfully deweave material, however significant decreases in solution volume, pH, and fiber bleaching imply that the solution requires replenishment by adding fresh stock solution with each reuse. Around 93% of the mass of the untreated fabric was retained after deweaving, and suspended microfibers could be observed in the final solution.<br/>The untreated fabric and deweaved fibers were examined using Fourier-transform infrared (FTIR) spectroscopy, which indicates preservation of the β-glycosidic linkage (898 cm^-1) in cellulose as well as other peaks characteristic of cellulose. Comparison of the peak associated with the C=O stretching vibration (1727 cm^-1) shows that this peak is insignificant for the untreated cotton fabric and the fibers deweaved with sodium nitrate, but is clearly seen in the fibers deweaved with the 3:1 mixture and is even higher in the fibers deweaved with citric acid, suggesting that esterification between citric acid and cellulose occurred. This is supported by the results of a back titration experiment, using sodium hydroxide and then hydrochloric acid, performed on the citric acid-deweaved sample. After titration, the fiber surface exhibited a color change to that of the indicator's acidic form, which could result from citrate groups added during esterification.<br/>Tensile strength tests were conducted on individual fibers to characterize mechanical changes after deweaving. Compared to fibers from the untreated fabric, fibers deweaved with citric acid have a lower ultimate tensile strength (175 MPa compared to 404 MPa) but a greater elongation at rupture (5.8% compared to 4.6%). However, the untreated fibers snapped upon reaching the ultimate tensile strength, while the fiber strands within the deweaved fibers were gradually pulled apart, pointing to the decrease in friction between fiber strands. This is supported by previous results from X-ray computed tomography and scanning electron microscopy, which show that the fiber bundle is less tightly packed after deweaving. Negative zeta potential values (-3 to -6 mV) on the solution with suspended microfibers may be due to dissociation of carboxylic acid groups from esterification, further indicating the presence of interfiber repulsion that can increase the spacing between fibers. The arrangement of cellulose polymers within the strands of fibers, and thus the strength of individual strands, is likely preserved as the total crystallinity index and the lateral order index computed with FTIR peak heights [2] show no substantial changes. Therefore, the decrease in fiber strength is likely due to physical, rather than chemical, changes.<br/>Preliminary tests demonstrate the possibility of scaling up the deweaving procedure by using larger parallelogram-shaped pieces of fabric. Experiments are being conducted to quantify the degree of esterification, clarify the chemical basis for deweaving, retain and analyze the produced microfibers, and scale up the process.<br/><br/>[1] El Seoud, O. A., et. al, <i>Macromol. </i><i>Mater. Eng.</i>, <b>2020</b>, 305, 1900832<br/>[2] Nelson, M. L. and O’Connor, R. T., <i>J. Appl. Polym. Sci.</i>, <b>1964</b>, 8, 1325<br/><br/><i>We would like to thank the Morin Charitable Trust for funding.</i>