Mechanism of photoprotection of wool with formaldehyde and thiol derivatives

Abstract

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.