Facile Antimicrobial and Flame-Retardant Treatment on Denim Clothing Using Citric Acid: Influence of Concentration and Application Method

Considering the popularity and the need to improve properties of denim, citric acid (CA), as a safe, readily available, economical, and environmentally friendly compound, was used to impart antibacterial and flame-retardant properties to denim. For this purpose, the effects of CA and sodium dihydrogen phosphate (SDHP) concentration as catalyst, as well as application methods, including pad-dry-cure and exhaustion, were investigated on the antimicrobial activity of the samples against the Gram-positive bacterium Staphylococcus aureus and the Gram-negative bacterium Escherichia coli. The sample treated with 5% CA and 3% catalyst using the pad-dry-cure method demonstrated the best performance and the maximum reduction in bacterial growth. Since the CA in the exhaustion method had no reasonable antibacterial effects, subsequent heat treatment, including drying at 80 °C and curing at 130 °C, was performed. The sample treated with 10% CA and 6% catalyst exhibited the maximum antibacterial effect through the exhaustion/curing method. The treated fabric exhibited a slower burning rate, reduced flame spread, and self-extinguishing properties, indicating successful flame-retardant properties. Moreover, the physico-mechanical properties of the sample treated by the pad-dry-cure method were adversely affected, showing decreased air and water vapor permeability, abrasion resistance, tensile strength, and bending length. To enhance the fabric properties, silicone softener was added to improve the handle and flexibility. However, while the softener improved the fabric comfort and drape, it negatively impacted the antibacterial results, reducing their effectiveness. On the other hand, silicone softener enhances the flame-retardant properties of the fabric, increasing its resistance to ignition and flame spread. To comprehensively evaluate the effects of the treatment, field-emission scanning electron microscopy (FESEM), thermogravimetric analysis (TGA), and Fourier-transform infrared spectroscopy (FTIR) were performed. These analyses confirmed the chemical interactions between CA, catalyst, and the denim, as well as changes in thermal stability and surface morphology. Additionally, the fire resistance of the treated samples was examined after repeated washing, confirming the durability and persistence of the flame-retardant effect. The results highlight the effective role of CA as an antimicrobial and flame-retardant agent for denim, making it a promising multifunctional treatment for improving the performance of denim fabrics. However, post-treatment with silicon softener may be required to optimize the functionality and comfort.