Brian Milligan

Fluorescent whitening agents—a survey (1974-82)

Abstract This survey presents a comprehensive coverage of the literature on fluorescent whitening agents for the period 1974 to mid-1982. It is intended to serve researchers in the field by providing an up-to-date summary of the published literature on the synthesis, textile applications and analysis of fluorescent whitening agents. The survey does not cover the use of fluorescent whitening agents as additives for soaps and detergents. The latter part of the survey describes the light fading reactions of five technologically important classes of fluorescent whitening agent. Recent progress in understanding the photochemical processes that are responsible for fading, and the factors that influence the light stability of fluorescent whitening agents in a polymeric environment, are summarized.

The Role of Thiol Groups in the Setting of Wool

Evidence is presented for a mechanism of setting in which the know n conformational changes in the poly peptide structure involve disulfide bond rearrangement via thiol disul fide interchange. The new arrangement of disulfide bonds helps to stabilize the new structure if reversion is inhibited by preventing further interchange. Thiol and disulfide analyses of wools before and after setting support this hypothesis. We have found no evidence for the formation of stabilizing cross-links other than disulfide bonds. This hypothesis accounts for the fact that the rate of setting in boiling water is enhanced by the addition of reducing agents, such as bisulfite or thioglycollate, which generate extra thiol groups It also explains why wool in which the natural thiol groups have been destroyed cannot be set in water but can be set using reducing agents. The occurrence of thiol disulfide interchange has also been demonstrated during the release of set. Release is accelerated by thiol-producing reagents and retarded by con ditions which inhibit interchange. A higher level of set may therefore be obtained by blocking or oxidizing free thiol groups in the set fibers, particularly after short setting times. Cross-linking reagents are no more effective than monofunctional blocking reagents for this purpose.

Studies on Wool Yellowing Part III: Sunlight Yellowing

The yellowing of wool fabrics caused by exposure to sunlight differs from that produced by some artificial light sources. Atlhough the initial pH of the fabric has little effect on the rate of sunlight yellowing in the pH range 4-10, mild treatment with alkali subsequent to yellowing increases the discoloration. The effects that bleaching, shrinkproofing, and other chemical treatments have on yellowing by subsequent exposure to sunlight are discussed.

Solubility as a Criterion of Cross-Linking in Wool

Three commonly used solubility tests have been applied to wools treated with various mono- and bifunctional active esters. Treatment with the bifunctional esters invariably leads to large reductions in solubility. Those monofunctional esters which introduce bulky groups into wool also decrease its solubility. However, bifunctional reagents reduce the solubility to a greater extent than monofunctional reagents of similar size, particularly with the performic acid-ammonia and urea-thioglycollate tests. The urea- bisulfite test, on the other hand, is so sensitive in detecting chemical modification that it differentiates poorly between the introduction of bulky groups and cross links. Whereas none of these tests can be used to prove the presence of cross links unequivocally, the former two can be used, in conjunction with other tests, as a qualitative criterion of cross-linking.

Evaluation of the Effects of Temperature and UV-Absorber Treatments on the Photodegradation of Wool

A simple controlled-temperature irradiator is described, which permits fabric samples to be irradiated at air temperatures in the range 35°C to 85°C. Photodegradation of wool fabrics, as assessed by measurement of breaking loads, tear strengths, and yel lowness indexes, increases rapidly as the temperature of irradiation is raised, and occurs 3 to 4 times faster at 75°C than at 35°C. A procedure is described for assessing the level of photoprotection conferred on wool by treatment with a UV absorber of the 2-hydroxybenzophenone type. Consistent lifetime improvement factors are ob tained for most levels of photodegradation, and for exposure either to the artificial light source or to sunlight through window glass.

Studies in Wool Yellowing: Part VII: Processes for Retarding Yellowing by Sunlight

The use of thiourea-formaldehyde resin for retarding the rate of yellow ing of wool by sunlight has been examined. This method provides considerable protection against yellowing, particularly of wet fabrics. It has been found that treatment with thiourea- formaldehyde mixtures. which does not give rise to resin, provides similar protection against yellowing. This modification has the advantages that the handle and color of the wool are unaffected, As for the resin treatment, initial rinsing reduces the pro tective effect, although this is not further reduced by subsequent washing.

Some Dyes Solubilized by Thiolsufate Groups

Four thiolsulfato azo dyes (DSSO3Na) and three of the corresponding sulfonate - dyes (DSO 3Na) have been prepared and their dyeing behavior on wool studied. Three of the thiolsulfates were found to be unexpectedly good dyes, especially in regard to washfastness. The thiolsulfate dyes under certain conditions of application undergo partial decomposition, so that some dye becomes bound covalently to the wool and some is converted to the symmetrical dye disulfide (DSSD).

The Sites of Reaction of Wool with Formaldehyde

A method for the characterization of the products of the reaction between wool and formaldehyde is described. The method involves the use of a mild procedure to hydrolyze the formaldehyde-treated wool using digestion with proteolytic enzymes, and subsequent chromatographic separation of the resultant amino acids. Lysine and glutamine side chains have been identified as two of the sites of reaction.

Effect of Acylation on the Setting of Wool

The acylation of wool fabrics with a range of active esters decreases the ability of the wool to set, whether in the presence or absence of a setting agent. Amino, thiol, hydroxyl and phenolic groups are all modified during acylation, but our results show that the decrease in settability can be attributed mainly to the modification of amino groups and to the effect that this has on the overall electrical charge on the fiber. The partial modification of thiol groups causes some decrease in settability, but only when setting is carried out in the absence of a reducing agent. Treatments with bifunctional acylating agents decrease settability further by introducing cross links between protein chains.

Fluorescent Whitening Agents Part V: The Use of Reducing Agents for Retarding the Yellowing of Fluorescently Whitened Wool by Sunlight

Immersion of fluorescently whitened wool in aqueous solutions of reducing agents during exposure to simulated sunlight has been shown to retard or prevent photoyellowing. The most effective reducing agents of the wide range examined were thioglycolic acid, sodium sulfite, and trishydroxymethylphosphine. Less photodecomposition of trytophan and histidine residues occurs during irradiation of either whitened or unwhitened wool in solutions of these reagents than in water, which may explain, in part, the role of reducing agents in inhibiting photoyellowing. In contrast, a study of bleached wool containing the bis(triazinylamino)stilbene whitener (I) showed that reducing agents do not affect signifi icantly the course or extent of photodecomposition of the whitener. Reducing agents were less effective inhihitors of yellowing when padded onto wool and dried than when used in aqueous solution. However, treatment of fluorescently whitened wool with tetrakishydroxymethylphosphonium chloride (THPC) by padding was found to provide good protection against yellowing in sunlight, especially for exposure in the wet state. Unfortunately the protective effect was not very durable to washing. The sulfonated triphenyl phosphine (II) was substantive to wool but provided good protection against yellowing only at high concentrations. Two water-soluble antioxidants of the hindered phenol type were also tested but were found to be ineffective.