Mechanism of photoprotection of wool with formaldehyde and thiol derivatives Keith R Millington,* George Maurdev and Michael J Jones Commonwealth Scientific and Industrial Research Organisation, Materials Science and Engineering, PO Box 21, Belmont, Victoria 3216, Australia Email: keith.millington@csiro.au Received: 20 October 2008; Acccepted: 9 February 2009 The application of thiourea and formaldehyde offers significant reductions in photoyellowing for natural, bleached and fluorescent-brightened wools. We have examined the use of alternative thiol derivates to thiourea and shown that a mixture of N-acetylcysteine and formaldehyde is similarly effective. Photo-induced chemiluminescence studies on treated wool fabrics show that the intensity of chemiluminescence following a brief exposure to ultraviolet A radiation is reduced for wool treated with thiourea and formaldehyde, N-acetylcysteine–formaldehyde and thiourea dioxide–formaldehyde, but not for formaldehyde alone. This demonstrates that a lower population of free radicals is formed in irradiated thiol-derivative formaldehyde-treated wools because of free radical scavenging by the sulphur-containing species. This free radical scavenging results in reduced photoyellowing. The effect of thiourea and formaldehyde treatment on the intrinsic fluorescence of wool is significantly different to N-acetylcysteine– formaldehyde. Thiourea and formaldehyde quenches tryptophan fluorescence relative to untreated wool, whereas N-acetylcysteine–formaldehyde treatment results in an increase in intensity probably because of a reduction in disulphide quenching. Introduction The application of a mixture or pre-condensate of thiourea and formaldehyde (TUF) followed by thermal curing to improve the photostability of wool has been previously investigated in a number of studies [1–5]. The treatment was first developed for use on silk [6] and had some limited commercial use on wool treated with fluorescent whitening agents (FWAs) in the 1970s, but, unfortunately, the benefit of the treatment is significantly reduced following the first laundering cycle. Now there are concerns over the safety of thiourea, which has been shown to be a carcinogen [7]. Similar issues exist regarding the commercial use of formaldehyde [7], but it is still widely used in the production of phenolic, urea, melamine and acetal resins, which have wide use in the manufacture of adhesives and binders for the wood, plastics, textiles, leather and related industries. The mechanism of TUF photoprotection for natural wool involves quenching of singlet and triplet excited states in wool protein and an increased rate of photobleaching by visible light [8]. This quenching effect of TUF on wool is very similar to the effect of iodide ions on polyamide polymers. Heavy atom quenching of the intrinsic phosphorescence and fluorescence of polyamide by halide ions, in particular iodide, is of commercial importance and has led to the use of iodide as a light stabiliser for nylon [9,10]. In this work, we have explored the use of possible alternative thiol derivatives to thiourea in the TUF treatment and assessed their effects on the rate of photoyellowing of FWA-treated wool under wet and dry conditions. Thiourea contains a C=S bond and, strictly speaking, is not a thiol, but it can have thiol character as a result of tautomerisation (Scheme 1). Thiourea dioxide (TUD) is the S VI oxidation product of thiourea but is a strong reducing agent, as it is hydrolysed to the sulphinate anion. It is widely used for reductive bleaching of textiles as an alternative to sodium dithionite. Lewis [11] suggested that TUF treatment of wool works by releasing a sulphinic acid into the wool as the polymer photo-oxidises (Scheme 2). The sulphinic acid bleaches yellow chromophores present in the wool, which explains the apparent increase in the rate of photobleaching for TUD-treated wool. In an earlier study [12], we found that a rinse treatment with N-acetyl cysteine (NAC) and a metal chelator (oxalic acid) improved the photostability of FWA-treated wool, so it was of interest to compare the effects of NAC– formaldehyde (NACF) treatment and TUF. We have also examined the effects of these treatments on the intrinsic ultraviolet (UV) and visible fluorescence of wool. We have applied the photo-induced chemiluminescence (PICL) technique to determine the effect of these treat- ments on the population of hydroperoxides and peroxy free radicals formed in FWA-treated wool after exposure to UVA (320–400 nm) radiation. We have recently shown that fibrous proteins such as wool and feather keratin, silk and collagen all emit chemiluminescence when exposed to UV and visible light [13]. Our intention is to demon- strate that PICL can be used, at least qualitatively, to determine the effectiveness of potential photostabilisers for wool. Experimental Materials and treatments Thiourea, thiourea dioxide, N-acetyl cysteine and formaldehyde (37 wt% aqueous) were obtained from Sigma Aldrich (USA). NH 2 S NH 2 NH NH 2 SH NH 2 + NH 2 S – Scheme 1 doi: 10.1111/j.1478-4408.2009.00183.x ª 2009 The Authors. Journal compilation ª 2009 Society of Dyers and Colourists, Color. Technol., 125, 117–122 117