Extraction Process of Color Powder and Cotton Screen Printing with Natural Dyes from Longan Leaf Powder

Longan leaves, an abundant agricultural waste in Thailand, were utilized to produce natural powdered dye for sustainable screen printing on cotton fabric. The leaves were boiled (1 kg in 5 L water at 100 °C for 2 h), filtered, dried with maltodextrin (15 g/L), and ground to obtain a reddish-brown powder with a particle size of 15.381 μm and a yield of 20.53%. The dye was mixed with starch paste at 70:30, 80:20, and 90:10 ratios and applied to cotton fabric using screen-printing at 120 and 150 °C for 5 and 10 min. FTIR analysis confirmed the presence of hydroxyl and carbonyl functional groups. Postmordanting with alum and ferrous sulfate (5 and 10 g/L), with or without tannic acid pretreatment, was conducted. Results showed that a higher dye ratio and temperature increased color strength, with a <i>K</i>/<i>S</i> value of 1.21 and <i>L</i>* value of 78.63. Pretreatment with tannic acid significantly enhanced mordant binding, especially with 10 g/L ferrous sulfate, producing a dark blackish-brown tone (<i>K</i>/<i>S</i> = 6.56). Colorfastness tests showed ratings of 5 for washing and dry cleaning, and 4-4.5 for water and perspiration. Despite slightly higher costs, the dyeing process is biodegradable and reduces chemical pollution, making longan leaf dye a promising alternative for eco-conscious textile printing. This study is among the first to transform longan leavesan underutilized agricultural wasteinto powdered natural dyes for cotton screen printing. Unlike previous research focusing on microbial pigments or conventional plant sources, this work introduces a sustainable and regionally sourced alternative with enhanced color performance and fastness properties.