Wool Yellowing Research Articles

Xanthoproteic reaction for the evaluation of wool antifelting treatments

The substances responsible for the yellowing of wool treated with nitric acid are two amino acid constituents of the fibre: tryptophan and tyrosine. Nitric acid penetrates the fibres and carries out electrophilic aromatic substitution on the two above‐mentioned amino acid residues, producing different colour yields. The intensity of yellowing depends in various ways on the treatment conditions (time, temperature, nitric acid concentration, agitation, and liquor ratio). Yellowing evaluation shows abnormal yellowing depending on acid concentration in the range 5.6–5.9 m. Working in this region makes it possible to use the chromatic reaction in order to show the damage done to wool fibres by the oxidising agents utilised in normal antifelting treatments. Wool damage by the oxidants is usually evaluated by dyeing methods based on different affinity of damaged fibres. By contrast, the xanthoproteic reaction yields chromogens as a function of the accessibility of tryptophan and tyrosine residues for the action of nitric acid on damaged fibres, and can be used for assessing the degree of antifelting treatment and its possible unevenness through the development on the treated wool of a yellow coloration more intense than on untreated wool.

Trace metals can affect hydroxyl radical production and yellowing of photo-irradiated wool

Previous studies identified that trace metals within the wool fibre are associated with the production of free radicals and yellowing in photo-irradiated wool, and that protein-bound redox-active transition metals, particularly iron and copper, can catalyse the production of hydroxyl radicals and oxidation of proteins in biological systems. This present study investigated the relationship between trace metal content, hydroxyl radical production, tensile strength and yellowing of photo-irradiated metal-doped wool fabric. Iron increased the production of hydroxyl radicals and phototendering, and both copper and iron increased the rate of photoyellowing. The tensile strength of iron-doped wool fabric decreased whereas the strength of copper-doped wool increased after artificial weathering while wet. Manganese reduced hydroxyl radical production and had little effect on photoyellowing. Antioxidants and chelating agents were added to quench hydroxyl radicals and sequester metal ions. Ascorbic acid, citric acid and ethylenediamine tetraacetic acid increased hydroxyl radical production whereas deferoxamine mesylate (desferal) and diethylenetriamine pentaacetic acid reduced production.

Studies on the Yellowing of Wool

Studies on the Yellowing of Wool

Sunlight exposure caused yellowing and increased mineral content in wool

This study determines how levels of various trace metals in wool and the colour of the fibre change as a result of sunlight exposure and treatment with chelating compounds during wool growth. Twenty-four yearling Merino sheep were clipped on the shoulders and rumps and fitted with sheep coats modified with transparent patches. Patches over the shoulder wool (one per sheep) were either polyethylene (PE) that transmits ultraviolet light or polyvinyl chloride (PVC) that excludes ultraviolet light. The rump wool on each sheep was treated either with a copper chelator treatment (kojic acid or methyl gentisate in aqueous alcohol) or aqueous alcohol only. For 12 of the sheep the rumps were exposed to sunlight through PE patches while rump wool on the other sheep was covered by the sheep coat. Wool was harvested after 11 weeks’ growth with yellowness (Y-Z) and individual mineral contents measured using the same clean wool sample. Sunlight exposure through PE patches caused a mean increase in Y-Z to 9.1 (shoulder) or 9.5–10.1 (rump) from a base level of 7.1–7.2 (shoulder) or 7.0–7.6 (rump) in wool protected by the sheep coat. In contrast, there was no significant change in Y-Z for the PVC patch (shoulder). Therefore, it appears that ultraviolet light damage caused the increased Y-Z. Most of the trace metals analysed increased in the shoulder wool exposed to sunlight but the paired differences for PVC were lower than PE. It appears that changes in fibre caused by sunlight exposure (especially ultraviolet light) facilitate adsorption of minerals from the environment, including the animal’s own suint. Application of the chelating compounds to the rump wool caused pronounced yellowing of the wool with Y-Z increase being most pronounced for kojic acid. Copper levels in the wool were reduced by kojic acid and methyl gentisate while calcium levels were increased by kojic acid and reduced by methyl gentisate. It is not clear from these findings whether minerals and copper in particular contribute to yellowing of wool. However, the different effects of sunlight and chelation on mineral contents in wool shown may well relate to alternative mechanisms of discoloration (i.e. photoyellowing versus bacterial).

Extraction and characterisation of non‐scourable chromophores from discoloured fleece wool

The colour of scoured wool is a critical determinant of its value and quality, with the presence of significant non‐scourable discoloration severely limiting its use. In order to develop effective protocols for the removal or prevention of non‐scourable fleece wool discoloration, it is imperative that all significant contributing chromophores are characterised and their origin established. We describe the location, extraction and characterisation of chromophores from non‐scourable yellow fleece wool. Yellow discoloration was found to be located predominantly in the cuticular region of the wool fibre. Chromophoric compounds were extracted, isolated and characterised by tandem mass spectrometry, with five phenazine derivatives identified: phenazine, 1‐hydroxyphenazine, phenazine‐1‐carboxylic acid, pyocyanine and 1‐methoxyphenazine. Phenazines are brightly coloured pigments that are characteristic secondary metabolites of the bacterial genus Pseudomonas, a known ubiquitous component of the wool fleece microflora. The results of this research represent significant progress in our knowledge of wool discoloration, providing insight into both the chemical and microbial origin of non‐scourable wool yellowing.

Photoyellowing of wool. Part 1: Factors affecting photoyellowing and experimental techniques

The yellowing of wool exposed to sunlight is a serious commercial shortcoming compared to cotton and synthetic fibres, particularly when photostable brilliant whites and bright pastel shades are required. Part 1 of this review discusses the effects of light on wool keratin and the factors that affect the rate of photoyellowing, including oxidative bleaching, fluorescent whitening and the presence of moisture. The effect of variation of the wavelength of light, particularly attenuation of the ultraviolet wavelengths in sunlight by window glass that can result in photobleaching of wool is described. The experimental techniques that have been used to study the complex photochemistry involved in yellowing, and to identify the nature of the yellow chromophores formed, are also discussed. Part 2 of the review will focus on the photochemical mechanisms involved, and discusses potential ways for improving wool's photostability.

Inheritance of moisture content in greasy and clean wool and its relationship to other wool traits in Merinos

Moisture in greasy wool is associated with wool yellowing, fleece rot and fly strike, whereas the high moisture content of wool fabric is related to desired woollen fabric traits. This paper reports on a simple gravimetric method of determining greasy and clean wool moisture contents to determine fleece moisture heritability. The moisture index of greasy wool has a heritability of 0.37 ± 0.04 and phenotypic correlations of –0.31 ± 0.02 with yield, 0.69 ± 0.01 with suint index and 0.50 ± 0.01 with Methylene Blue absorption. The moisture index of greasy wool was strongly genetically correlated with yield (–0.48 ± 0.06), suint index (0.93 ± 0.03), Methylene Blue absorption (0.92 ± 0.03) but not significantly genetically correlated with clean wool moisture absorption (0.18 ± 044). Moisture absorption of clean wool has a very low heritability of 0.02 ± 0.03. The correlations between moisture index and suint index suggest that greasy fleece moisture index is primarily determined by suint index. The implications of variation in the moisture index of wool on fleece rot and fly strike are discussed.

Alkali‐promoted yellowing of wool. Yellow degradation products from a model for protein‐bound cystine

NN“‐Diacetyl‐L‐cystine bismethylamide reproduces faithfully the known alkali‐promoted chemistry of cystine in wool and becomes bright yellow in the process. It is suggested that two minor, labile, but intensely yellow components of the product mixture, N‐methylthiopyruvamide and one or more polysulphides, are analogues of the elusive pigments in the yellowing of wool under similar conditions.

Effect of External Lipids on the Process of Wool Yellowing

We have made an analysis of the lipids deposited on the surface of wool staple fibers divided into tip and base sections prior to investigation. The analysis is limited to a determination of free fatty acids and the ester fraction by gas chromatography and mass spectrography. The findings show that the changes in lipid composition, both quantitative and qualitative, depend on the location from which the lipids have been sampled.

The Yellowing and Bleaching of Wool

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.

A note on canary coloration of wool in Ghokla sheep as influenced by restricted frequency of access to water

Yellowing of wool as a function of physiological stress caused by nutritional deficiency in the feed of sheep (Das, 1965) though not confirmed by subsequent studies (Singh et al. 1977) evoked interest in the role of physiological stress in causing canary staining of wool. Restricted water intake as is generally the case with sheep in arid or even semiarid regions during summer, imposes considerable physiological stress. Furthermore, canary coloration is thought to be a sequel to an adaptive mechanism to hot and humid conditions in Indian sheep which have a greater reliance on cutaneous evaporation for dissipating body heat (Acharaya & Singh, 1976). It was, therefore, decided to study the influence, if any, of restricting access to water on the canary coloration of wool (non-scourable yellow coloration of the autumn clip) in the stain-susceptible Chokla sheep

The Formation of Carbonyl Groups during Irradiation of Wool and Its Relevance to Photoyellowing

The carbonyl contents of wool fabrics have been determined by colorimetric analysis after partial acid hydrolysis. Irradiation of dry wool fabrics with simulated sunlight causes an increase in carbonyl content. The rate of production is approximately the same for unbleached, bleached, and fluorescently whitened fabrics, even though these fabrics yellow at markedly different rates. In contrast, irradiation of the same fabrics in water results in few additional carbonyl groups, although extensive yellowing occurs. Thus it seems unlikely that carbonyl groups are directly responsible for yellowing. However, the observation that hydrothermal yellowing of dry-irradiated wool is accompanied by a loss in carbonyl content leads us to suggest that carbonyl groups could be involved indirectly in yellowing. The keto acids present in partial acid hydrolysates of irradiated wool have been identified by reductive amination with sodium 3H-borohydride in ammonia solution, a reaction which converts keto acids to the corresponding 3H-amino acids. The formation of radiolabeled alanine, glycine, proline, serine, threonine, glutamic acid, and tyrosine provides evidence for the photo-oxidation of a number of amino acid residues during irradiation of wool.

50—PRIMARY REACTIONS IN THE PHOTOYELLOWING OF WOOL KERATIN

After a study of the photochemistry of wool keratin, a mechanism is proposed for the reactions that lead to the sunlight yellowing of wool. The involvement of singlet-excited oxygen is confirmed by both direct and indirect methods.

The Yellowing of Greasy Wool in an Alkaline Medium

The Yellowing of Greasy Wool in an Alkaline Medium

A Theory of the Yellowing and Bleaching of Wool

A Theory of the Yellowing and Bleaching of Wool

Fluorescent Whitening Agents

Sulfonated aromatic aldehydes strongly sensitize the yellowing of wool by simulated sunlight, especially for exposure in the wet state. Compounds of this class are formed when stilbene-type fluorescent whitening agents are irradiated, either in solution or on wool. The possibility that these sulfonated benzaldehyde derivatives are responsible for the accelerated photoyellowing of fluorescently whitened wool is discussed. It is concluded that they may be important in the photoyellowing of whitened fabrics in the wet state, although they play little part in the photoyellowing of dry fabrics.

COMMUNICATION: IMPROVEMENT OF THE NATURAL FLAME-RESISTANCE OF WOOL. PART I: METAL-COMPLEX APPLICATIONS

Negatively charged titanium and zirconium complexes with α-hydroxy carboxylic acids and fluorides can be exhausted on the positively charged wool fibre in acid conditions, the result being a significant improvement in the natural flame-resistance of wool that is fast to washing and dry-cleaning. The complexes with carboxylic acids are exhausted on the wool fibre at the boil, whereas the fluoride complexes can be effectively exhausted at lower temperatures. The fluoride complexes can also be applied by a pad-batch-rinse-dry technique. Titanium complexes are more effective than the zirconium ones, probably because of better penetration of the fibre with the smaller titanium complexes. However, titanium complexes cause yellowing of wool, which increases with light exposure. Zirconium complexes do not affect the shade of wool and are fast to light.

Use of 2,4‐Dihydroxybenzophenone‐2‘‐ammonium Sulphonate to Prevent the Yellowing of Wool by Ultraviolet Radiation

The use of 2,4‐dihydroxybenzophenone‐2′‐ammonium sulphonate (das) to prevent the yellowing of wool by ultraviolet radiation is discussed. The optimum conditions of application of this absorber to wool have been determined. The Yellowness Index, a measure of the degree of yellowing, was used to determine the effect of the concentration of das on its protective action against ultraviolet radiation. Exposure in a Xenotest apparatus indicated that initially bleaching occurred, followed by yellowing, which decreased with increase in the concentration of das. The washing and dry‐cleaning fastness properties of das on wool were assessed and the effect of dye on these properties was also determined, das has poor washing fastness, but this can be improved by mordanting the wool before treatment with das. This absorber does not significantly improve the light fastness of acid dyes on wool.

Studies in Wool Yellowing

The results of irradiations of brightened and unbrightened wools in the wavelength range 300–600 nm show that the brightener accelerates both yellowing and free radical formation in the region where maximum excitation of fluorescence occurs (350–410 nm). Radical-decay studies have enabled comparisons of the radicals produced at 310 and 365 nm to be made.

Studies in Wool Yellowing

Wool and silk fabrics which have been exposed to light of wavelengths present in sunlight undergo further changes in yellowness on treatment with water. Depending upon the wavelengths of the initial radiation, wetting may cause either bleaching or yellowing. For wool fabrics irradiated at 310 nm, wetting initially produces bleaching followed by yellowing, whereas fabrics irradiated at 365 nm are either yellowed or bleached depending on whether or not the sample is fluorescent-brightened. At wavelengths greater than 390 nm, only yellowing is observed. The effect of oxygen on the photoyellowing of wet fluorescent-brightened wool and silk has been examined.