Improving the dyeability of cotton with tannin-rich natural dye through pretreatment with whey protein isolate

• Cotton was colored with a natural dye extracted from Xylocarpus granatum bark.
• The dyeability of cotton was improved by pretreatment with whey protein isolate.
• Insoluble tannin–protein complexes may enhance the dyeability of pretreated fabric.
• Mostly good to very good fastness properties were achieved for treated fabrics.
• UV protection properties were enhanced by the presence of dye or protein.

Cotton fabrics were pretreated with whey protein isolate (WPI) to improve their dyeability toward a tannin-rich natural dye obtained from Xylocarpus granatum bark. Comprehensive studies were conducted on these fabrics assessing their dyeability, color fastness, physical characteristics, and ultraviolet protection level. All dyed samples were reddish-brown with a significant enhancement in the sample color strength (K/S) being observed for the pretreated samples. This is due to insoluble complex formation between WPI and tannin, stabilized through hydrogen bonding and hydrophobic interactions. Dyeing at elevated temperatures and with longer times resulted in higher color strength values. Acidic pH values favor dye absorption with pH 5 yielding the highest color strength, probably due to tannin forming complexes with proteins most efficiently near the protein isoelectric point. Padding cotton with WPI at only 0.35% solid content afforded dyed fabrics having an approximately 50% increase in the K/S value in comparison to untreated fabrics. The color strength of the WPI-treated cotton was enhanced relative to that treated with soy protein isolate (SPI), under the same dyeing conditions. For protein-treated cotton fabrics, except for color fastness to light and crocking, all other fastness properties (washing, water, sea water, perspiration and hot pressing) were rated as good to excellent. After protein pretreatment, the tensile strength and tearing strength of fabrics were slightly reduced whereas a small increase in fabric stiffness was obtained. Measurements of the ultraviolet protection factor (UPF) indicated better fabric shielding against ultraviolet radiation on treatment with either natural dye or WPI.