Coffee stain on textiles. Mechanisms of staining and stain removal
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Coffee stains on textiles are mainly caused by the water-soluble and acidic colored substances in coffee. The acidic nature of coffee stain has been shown by ultraviolet and visible spectroscopy of coffee as a function of pH; ion-pair formation with a cationic surfactant and titration with Hyamine 1622 and a surfactant-specific electrode; and precipitation of the colored components in coffee with barium hydroxide as a barium salt. The permanence of coffee stains on textiles depends on the nature of the fibers. The affinity of coffee stain to fibers, indicated by resistance to detergency, increases in the order polyester<cotton<nylon. Coffee stain has little affinity to polyester fibers but adheres to cotton and even more firmly to nylon. The strong affinity to nylon and the pH dependence of staining suggest an ionic interaction of carboxyl and phenolic groups with amine end-groups in nylon. The ionic attraction is augmented by nonionic interactions that are enhanced by the oligomeric or polymeric nature of the staining substances. In accord with the dominantly acidic character of coffee stain, an alkaline medium is needed for the dislodgment of coffee stain from nylon fibers. Bleaching of coffee in solution with perborate, activated with sodiumn-nonanoyloxybenzenesulfonate, or Oxone® (DuPont, Wilmington, DE) obeys pseudo first-order kinetics. Oxone, a peroxysulfate triple salt, is a more powerful oxidant for coffee stain than sodium perborate, but its use is limited by the bleach fastness of dyes.
Key wordsBleaching of coffee stain coffee color coffee stain on textiles mechanisms of staining mechanisms of stain removal oxone sodium perborate stain resistance of nylon
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