The Yellowing and Bleaching of Wool

Abstract

The yellowing of wool is a complex phenomenon which is induced by a variety of agencies such as light, heat and aqueous chemical reactions.Yellowing of wool by light depends critically on the wavelength distribution of the light (u. v. region), humidity and the type of prebleach given to the wool; if fluorescent whitening agents (FWAs) are employed in the bleaching process then the sensitivity to photo‐yellowing is drastically increased. The copious amount of work carried out to overcome this complex situation will be summarised and the important role of the aromatic indole amino acid, tryptophan, will be detailed.In contrast to photoyellowing, exposure of wool to visible blue light (maximal effect 420–450 nm) promotes photobleaching of the yellow pigments, giving rise to complaints of colour change, especially in wool products dyed to pastel colours.To remove natural yellowness, chemical bleaching of wool is usually carried out by using hydrogen peroxide, the usage of which should be carefully controlled because of its fibre damaging characteristics. Optimisation of wool peroxide bleaching procedures has therefore been the subject of much work and these studies will be reviewed. Bleaching of wool with reducing agents is often practised either as an alternative to peroxide bleaching or more usually as an aftertreatment of peroxide‐bleached wool to improve the whiteness and stability to light.Trade practice in the field of wool bleaching has recently been critically reviewed; the results and recommendations of this assessment will be given.More recently, significant progress has been made in the field of selective dark fibre bleaching and in the bleaching of heavily pigmented wools (e.g. karakul) using methods based on ferrous ion mordanting.