Improving the photostability of whitened wool by applying an anti‐oxidant and metal chelator rinse

Abstract

The combination of a water‐soluble anti‐oxidant with a metal‐chelating agent improved the wet and dry photostability of wool treated with a fluorescent whitening agent (FWA) significantly when used as a rinse treatment after FWA application. The most effective anti‐oxidants identified were l‐ascorbic acid and N‐acetyl cysteine. Oxalic acid was the most effective metal chelator, capable of limiting visible yellowing under both wet and dry conditions, and its use as a rinse treatment on FWA‐treated wool also improved its initial whiteness. Although rinse treatments containing l‐ascorbic acid provided a high level of photoprotection when wool samples were examined immediately after exposure to light, it was unsatisfactory due to the slow reaction of its oxidation product dehydroascorbic acid with free amino groups in wool to produce pink chromophores after standing for a few days. These pink chromophores slowly turned yellow over several weeks.

UV damage to hair and the effect of antioxidants and metal chelators

“…The behaviour of NAC in showing negligible free radical photogeneration is in agreement with previous work on wool, which showed it to confer a high level of resistance to photoyellowing when applied as a rinse treatment (with oxalic acid) to optically brightened wool . NAC is a powerful antioxidant with several pharmaceutical applications, including use as an mucolytic agent and as the antidote for overdoses of the analgesic paracetamol (acetaminophen) .…”

“…White hair treated with ascorbic acid (AA) underwent a slow colour change through pink to yellow, which is why there is a large difference in the untreated data for AA-treated hair between Tables I and III. A similar colour change has been observed previously for AA-treated wool [27], and is due to the reaction of the major oxidation product of AA with amino acid residues in the keratin protein. Ascorbic acid would therefore be unsuitable as an additive to protect white or grey hair against photoyellowing, and we cannot use this method to assess its efficacy as a protective treatment.…”

Wool: Structure, Properties, and Processing

“…The use of antioxidants to protect wool from free radical damage that leads to yellowing has been investigated, but no commercial solution has yet been developed. A combination of an antioxidant (N-acetylcysteine) and a metal chelator (oxalic acid), applied as a rinse treatment, was highly effective in decreasing the rate of photoyellowing of FWA-treated wool (170). The treatment was, however, not substantive to the fibers, and the benefits were lost on laundering.…”

Improving the whiteness and photostability of wool

“…Rinsing FWA-treated wool with low concentrations of an antioxidant such as N-acetylcysteine (NAC) was highly effective against photoyellowing when combined with a metal chelator such as oxalic acid, and provided much better photoprotection than the antioxidant alone, particularly under wet conditions (Millington, 2006a). Unfortunately the benefits of the rinse treatment are lost on laundering, and efforts to identify a substantive antioxidant/metal chelator combination that could be applied during wet finishing and provide effective protection against photoyellowing under both wet and dry conditions have so far been unsuccessful.…”