bleaching of pigmented speciality animal fibres and wool

Bleaching of pigmented speciality animal fibres and wool

A Bereck Sergixhe Universitiit GH Wuppertal, Fachbereich 9 - Textilchemie, 042097 Wupprtal, Germany

Introduction cashmere or alpaca, and a solution of the problem can be

'The natural colour of animal fur is closely related to the character of the environment in which the animal lives, examples are the white winter fur of arctic animals, the grey-brown coat of animals living in forests and marshlands, and the yellow fur of creatures inhabiting sandy areas.' [l].

Even though most animals providing speciality fibres are domesticated and bred by ranching and farming, in many breeding areas, e.g. in South America and China [2], there seems to be only little control of their natural colou~ Chiefly because of the relatively large number of farmers, many of them running only a small flock of animals. Yet

even more elusive than it is in the case of wool. In all three cases mentioned above, especially (b) and

(c), bleaching of the pigmentation can help to solve the problem. On the other hand, pigmented fibre bleaching is the most delicate and risky wet operation during the whole processing of speciality animal fibres. Any mistake in controlling of the bleaching process can give rise to serious fibre damage. Most careful control of the bleaching parameters is therefore essential. It is therefore of vital importance to search for less damaging and safer processes; even minor improvements that yield better fibre quality should be regarded as valuable.

The natural pigments of animal fibres some of the major problems faced by the speciality fibres processing industry are caused by natural pigmentation, such as: (a) The colour of pigmented material is not uniform (b) The material available is of an undesired colour or too

(c) Single dark pigmented fibres occur in light coloured

In animal (and human) hair two types of pigment ocm: (a) Eumelanin: responsible for black, dark brown and

grey colours and commonly referred to as melanin (b) Pheomelanin: present in yellow, reddish-brown and

red hair (in acid extracts of red hair, yellow, orange, brown and violet components have been found [l]).

dark for pastel shade andor white articles

or white material.

Such single pigmented fibres can cause expensive imper- fections in light coloured wool products. Even with this fibre, being typically produced in a white colour on larger farms where breeding is carefully controlled, a decisive improvement of the situation cannot be expected in the near future. Today it can be said that wool lots completely free of dark fibres do not exist [3]. This applies even more for speciality animal fibres, such as white mohair,

The structure and biosynthesis of melanins are not exactly known. Eumelanin and pheomelanin are thought to be formed by different mechanisms, and differences in their chemical structures are indicated by their visible absorption spectra [4]. Eumelanin is formed by enzymatic (tyrosinase) oxidation of tyrosine and by polymerisation of several oxidation products [!XI. Figure 1 shows some of the possible oxidation reactions and products. The

HO Horcoz" HO \ NHp - Ox orcozH 0 0 NHp

ox N H p

L-Tyrosine L-Hydroxytyrosine L-Dopaquinone (3-hydroxyphenylalanine) (3.4dihydroxyphenylalanine = Dopa)

J ox O m C O p H % Im

0 ' N HO ~ C O Z H -

H H Leucodopachrome Dopachrome (red) 5,6-Dihydroxyindole Indole-5,6quinone

Figure 1 Oxidation products of tyrosine

REV. PROG. COLORATION VOLUME 24 1994 17

A \ / R'

Figure 2 Polymerisation of tyrosine oxidation products to form melanin [7]

OQ 0

o$o 0

H

OQQ 0

- 3 0 - O- /

0 H

Figure 3 Resonance structures of melanin

polymerisation seems to occur randomly, involving a random assortment of tyrosine derivatives. Some possible polymerisation reactions are shown in Figure 2 [7]. The resulting product is a highly crosslinked and highly con- jugated system (Figure 3), which absorbs radiation throughout the visible spectrum.

Pheomelanin is regarded as the generic name of possibly several different yellow-red pigments, such as trichosiderin, trichoxanthin, pyrrotricholes, trichochromes and gallo- pheomelanin, which have been reviewed by Stoves [l]. Some examples of the several suggested pigment formulae are shown in Figures 4 and 5. Figure 4 shows the yellow xanthommatin, obtained from tryptophan via 3-hydroxy- kynurenine, while Figure 5 illustrates the yellow trichochromes formed in a reaction between dopaquinone and cysteine, leading to a dihydrobenzothiazine which is believed to be a precursor of these pigments.

A schematic overview of the possible routes for melanin

H

biosynthesis is given in Figure 6. Melanin is inherently attached to a protein matrix called melanoprotein. Its chemical composition differs greatly from that of keratin [8,9]. On the basis of experimental data, Hall and Wolfram have proposed that a chemically specific protein matrix is a prerequisite for the formation of melanin granules, irrespective of their animal origin. Thus melanoproteins isolated from oriental human h a poodle hair and squid had very S i a r compositions [S].

Both eumelanin and pheomelanin occur in form of b t e granules, which due to their high chemical resistance can easily be isolated from the fibre, e.g. by acid hydrolyhc, reductive or enzymatic degradation of the fibre keratin [9,10]. The isolated granules show a rod-like structure, having a length of 0.5-1.3 p and a width of 0.2A.45 p; the width/ length ratio varies between about 0.3 and 0.5 [ll]. The melanin content of pigmented fibres can be as high as 10% 191. The biological (including evolutionary and genetic) aspects of the occurrence and formation of different melanin types have recently been reviewed by Ryder [12].

Melanin granules can OCCUT in the cortex or in the cuticle (Figure 7). Observation of pigmented fibres by light microscopy reveals the discrete particulate nature of the natural pigment, as opposed to dark fibres homogeneously coloured by staining (e.g. by urine). Transmission electron microscopy allows a much more detailed view of the fibre and of the pigment distribution within it. The electron density of native melanin is higher than that of keratin, and this difference can be greatly increased by different staining techniques, because metal cations are preferably absorbed by melanin (Figures 7-9).

COzH

NHZ Dihvdrobenzothiazine derivative Xanthommalin formed from dopaquinone and cysteine Trichochromes

Figure 4 Structure of xanthommatin [ l ] Figure 5 Structure of yellow trichochromes and precursor [ l ]

18 REV. PROG. COLORATION VOLUME 24 1994

Tyrosine

[O] Tyrosinase I Dopaquinone

[o/ k c y s t e i n e

Dopachrome Cysteinyldopas

Benzothiazine metabolites J

lndole metabolites

Eumelanins Mixed-type melanins Pheomelanins

Figure 6 Schematic outline of pigment formation in melanocytes [4]

Figure 7 Segment of the cross-section of a pigmented wool fibre (photo I Kaplin. Deutsches Wollforschungsinstitut, 1982)

Figure 8 Cross-section of a cashmere fibre (photo K-H Phan, Deutsches Wollforschungsinstitut. 1989)

Figure 9 Cross-section of a vicuna fibre (photo K-H Phan, Deutsches Wollforschungsinstitut, 1989)

Pigment bleaching As mentioned above, the highly crosslinked melanin pigments are extremely stable against chemical attack, and

they are practically impervious to reducing Bgents. They can, however, be at least partially dissolved in alkali. The partial dissolution of melanin granules within the (wool) fibres during soda alkaline treatment can be demonstrated by transmission electron microscopy (Figure 10).

The only efficient way to decompose and decolorise melanin is a treatment with oxidising agents in alkaline media. Of course, under conditions that allow sufficient bleaching, substantial damage of the chemically far more sensitive fibre keratin is unavoidable [13].

The only widespread application of a conventional alkaline oxidative bleach of pigmented keratin fibres is the in vim bleaching of human hair, usually carried out by a hairdresser In most cases synergetic mixtures of hydrogen peroxide and other peroxides such as persulphates (so- called boosters) are used [14,15]. Human hair is much more stable than fine animal fibres, and the damage it sus- tains during bleaching is generally regarded as tolerable.

The mechanism of human hair bleaching was studied in detail by Wolfram et al., who proposed a two-step mechanism [14]. The melanin polymer disintegrates in the first stage, when solubilised melanin dye is formed. This step is a necessary prerequisite of the bleaching process, but it hardly affects the colour of hair, i.e. only the solubility-restraining crosslinks are eliminated and the chromophoric groups appear to be left virtually intact. The second decolorisation step may be considered as being of greater practical importance, and seems to be the slower process. Wolfram et al. established that the crosslinks are much more labile than the chromophores, and this has been strongly supported by experimental findings.

Work carried out with black karakul wool at DWI in Aachen led to similar conclusions [16]. Thus treatment with a sodium carbonate solution for 3 h resulted in a very dark brownblack extract, and electron microscopic investigations of the treated material showed that a substantial part of the melanin pigment was dissolved (Figure 10). However, there was practically no change in the hue of the fibre. Even a (conventional) alkaline bleaching with hydrogen peroxide affected fibre colour only to a minor extent, even though it destroyed most of the pigment granules (Figure 11).

The best chance for an efficient pigment bleaching with minimum fibre damage is provided by the use of metal catalysts in a mordanting step preceding peroxide

Figure 10 Cross-section of a black karakul wool fibre treated with 10 gA sodium carbonate (3 h. 70°C); luminosity ( V ) : 6.5 untreated [16]


Figure 11 Black karakul wool treated with hydrogen peroxide only; luminosity (V): 11.8 [16]

bleaching. In industrial practice iron(") salts are used as a mordant. The basic principle of the process is outlined in Figure 12 [17]. In the first step the pigmented fibre material is treated with a solution of iron@) sulphate, rinsed and finally bleached with hydrogen peroxide. The presence of iron([[) ions in the melanin pigment causes hydrogen peroxide to undergo radical decomposition, leading to oxidising species (Haber-Weiss mechanism) that are far more aggressive than the perhydroxy anion which is generally regarded as the bleaching agent under usual alkaline bleaching conditions. These radicals bring about not only a more complete disruption of the melanin polymer but are also very efficient in the decolorisation of the dye formed (Figure 13).

The method was first studied in scientific detail by Laxer and Whewell [18], who showed that iron is preferably absorbed by melanin and found a strong relationship between melanin content (colour) of the fibre and the uptake of iron from an iron sulphate solution. They also estimated that iron([[) ions are more firmly bound to melanin than to keratin. More recently, Giesen and Ziegler at DWI in Aachen extensively studied the absorption of different metal ions by pigmented keratin fibres [9,19].

conditions [20,21]. So optimisation work at DWI beginning in 1979 on the mordanthydrogen peroxide bleaching method was camed out with the aim of improving exist- ing practical bleaching processes [22-%]. All accessible formulations were tested. It was found that the great majority of the recipes caused substantial fibre damage and did not give sufficient bleaching effect. Therefore it seemed worthwhile to reinvestigate the whole process. The most important results of this work are summarised in the following sections [22-261.

Choice of catalyst A comparison of different metal salts that were candidates for bleaching catalysts showed that only iron(rr), iron(ni) and copper(n) ions had a sigruhcant catalytic effect under the conditions used (Table 1). Some other metal salts catalysed only fibre keratin decomposition, but had no influence at all on the bleaching of melanin.

Table 1 Catalytic influence of mordants on bleaching and on the decomposition of hydrogen peroxide [22]

Hydrogen peroxide decompositionb

Alkali (I m01-w x 103) solubility (%)a

Mordant in Mordant in Mordant YasC Merino Karakul Y e d solution' melaninad

Fe(ii) Fe(iii) CU(ll) CO(ll) Ni(ii) Mn(ii) Sn(ii) Zn(ii) Cr(iii) Ti(iii) Nil

23.4 37.3 48.3 0.35 1.17 0.59 25.0 67.1 71.3 0.43 2.78 0.73 30.8 93.6 92.3 3.32 5.10 2.15 11.2 21.6 20.3 0.00 7.67 7.38 9.7 38.2 39.9 0.06 0.50 0.13

10.0 42.7 37.8 0.04 1.14 0.27 8.6 38.0 40.3 0.01 0.09

10.9 24.5 42.0 0.03 0.12 0.20 10.0 37.6 37.3 0.56 0.46 0.84 11.3 61.2 60.8 0.00 0.46 0.37 11.0 30.0 32.0 0.00 0.067 0.136

a Bleaching of mordanled fibres b 3 h, 70"C, pH 8 , 6 g I-' sodium pyrophosphale c Luminosity measured on (originally black) karakul wool d Bleaching of isolated and mordanled melanin dispersion; 3 h. 70°C. pH 8,

Chemistry and technology of pigment bleaching A review of the literature and of industrial formulations revealed a broad variety of chemicals and treatment

Peroxide/mordanl 10 ml I-' hydrogen peroxide as in (35%), 6 g of I-' karakul sodium wool pyrophosphale

f Dispersion of mordanled melanin

d 0 (0 0 0

Mordantedl rinsed libre Pigmented

fibre @ @ a

Bleached fibre

Mordanted libre

sr 0 Fa 0 H

Figure 12 Principle of dark fibre bleaching [17]


Figure 13 Black karakul wool treated with hydrogen peroxide after pretreatment (mordanting) with iron(ii) sulphate; luminosity ( V ) : 38.8 [I61

Kinetic studies of peroxide decomposition led to the conclusion that these three metal ions plus cobalt@) had high catalytic activity. Even though the highest rate of peroxide decomposition was observed in the presence of cobalt, no bleaching occurred, apparently because peroxide was decomposed virtually completely within a few minutes.

The use of iron@) ions led to the same bleaching effect as iron@), but the absorption of the former was not as selective, resulting in heavily damaged fibre protein during bleaching. Therefore attempts were made to assess optimum conditions for bleaching using iron(e) as a mordant. This form of iron offers several advantages: its absorption is highly selective under a wide range of conditions, its salts are relatively cheap and the iron content tolerated in effluent water is comparatively high.

Mordanting Increasing the concentration of iron(i1) ions above 0.035 mol I-' did not significantly enhance the iron uptake by the pigmented fibres. In agreement with the findings of Giesen and Ziegler [19], over the pH range 3.0-3.5, a treatment time of 60 min and a temperature of 80°C were found to be appropriate. The presence of chelating agents, such as EDTA, critic acid, oxalic acid or tartaric acid impaired the selectivity of absorption.

One of the most important findings was that correct choice of the reducing agent, which has to be added to the mordanting bath in order to prevent iron(@ ions from being oxidised, is of decisive importance. Several commercial products were compared for their effective- ness as reducing and stabilising agents for iron(a) ions, for the bleaching effect achieved after subsequent oxidative bleaching and for any propensity to cause wool damage. The results of this work were later verified and quantified by Trollip et al. [27].

It transpired that hypophosphorous acid is an excellent stabiliser for iron(n) ions under mordanting conditions. More importantly, cystine suffered hardly any attack from hypophosphorous (or phosphorous) acid under conditions that favour disulphide scission induced by sulphurcontaining reducing agents ('Table 2) [28,29]. This fact has important practical consequences.

Rinsing Rinsing following the mordanting step proved to be critical with regard to selectivity and consequently to fibre damage. The iron(n)-melanin interaction is, as Laxer and Whewell postulated, apparently much stronger than the iron(r1)-keratin interaction. Thus, in addition to the selective absorption of iron, selective desorption becomes an important factor in optimising a pigment bleaching process. This fact offers a genuine opportunity to develop processes in which, if absorbed iron is completely removed from the fibre keratin, the attack of radicals will be localised exclusively at the melanin pigment, while the fibre keratin undergoes only a simple peroxide bleaching. This fact has only been noted occasionally hitherto [18,21]. The data in Table 3 demonstrate the influence of the rinsing process and of the reducing agent on the tensile strength and abrasion resistance of treated wool.

In bleaching experiments carried out with blends of white merino and black karakul wool, it could be shown that a normal cold rinse would remove virtually no iron from either type of fibre. During a hot rinse, however, using a 0.5 ml I-' hypophosphorous acid solution, the iron content of karakul wool decreased by 59% and that of the white merino wool (the iron uptake of which was much lower anyway) by 78%. The iron content of the melanin isolated from the karakul wool remained practically unchanged, even under these conditions. The process could not be improved by adding chelating agents to the rinsing bath. Whenever chelating effect was observed, bleaching efficiency decreased and the relationship between lightness.on the one hand and fibre damage on the other hand could not be influenced.

More recently, Schumacher-Hamedat et al. reported that adding small amounts of NTA to the rinsing bath had an advantageous effect on the bleaching results [30]. Cegarra et al. studied the influence of pH on the sequester- ing efficiency of nitrilotriacetic, ethylenediamine tetra- methylphosphonic and oxyaminotrimethylenephos- phonic acids during alpaca bleaching [31].

Table 2 Disulphide bond cleavage by various reducing agents under typical mordanting contitions [27]

Cysteine content (pmol g-l)

Reducing agent Before After hot rinseb hot rinseb

Hypophosphorous acid, H,PO, 25 26 Phosphorous acid, H,PO, 26 24 Sodium hydrogen sulphite, NaHSO, 93 70 Sodium sulphite, Na$iO, 84 60 Sodium disulphite, Na2S20, 127 74

a 3 x I@ moi r1 reducing agent (approx. 2-3% on the mass of karakul in the case of the sulphur-based compounds). 90 min. 70°C. pH 3.0 (at start)

b 20rnin.80"C


Table 3 Influence of the reducing agent and rinsing on properties of selectively bleached wool [22]

Abrasion Reducing agent Yam bundle resistance in mordant Rinsing Bleaching tenacity wet (Martindale/ solution conditions conditions (cN terl) cycles)

Sodium Cold 1 min 7O0C,pH8 6.3 20 000 dithionite, 0.3 g I-l 80°C 2 x 10 rnin 7OoC, pH 8 6.4 28 500

Hypophosphorous Cold 1 rnin 70°C,pH8 7.1 27 500 acid (SO%), 1 g P1 80°C 2 x 10 rnin 70°C, pH 8 7.5 35 000

80°C 2 x 10 min' 70"C, pH 8 7.8 35 000 80°C 2 x 10 min 55%, pH 8.5 7.5 39 000 80°C 2 x 10 min' 55%, pH 8.5 7.9 45 000

a lg t-' hypophosphorous acid (50%) was added to first rinsing bath

Bleaching with iron catalysts The bleaching parameters (including the iron content of the mordanting bath) have been optimised by using central composite designs. The most important factors proved to be the bleaching temperature and the concentration of hydrogen peroxide. Conditions giving the best results with different fibre types are quoted below.

Tetrasodium pyrophosphate (diphosphate) was found to be the most suitable stabilising agent. The concentration of pyrophosphate (and of other salts, e.g. sodium chloride and sodium sulphate) present in the bleaching bath had a decisive influence on the bleaching process. When used at optimum concentrations (e.g. 10 g I-' pyrophosphate), they rendered a substantial degree of fibre protection.

Bleaching with catalysts other than iron salts Despite of the aquatic toxicity of cobalt(r1) and copper(i1) ions, and thus the expected limitations concerning possible practical usage, their superb catalytic activity encouraged workers to investigate the efficiency of these salts as bleaching catalysts [32,33]. In both cases processes that were, at least technically, feasible and that conferred some advantages compared with the iron mordanting method were developed: (a) No reducing agents during mordanting are needed (b) Air need not be excluded, and therefore there is no

limitation in the choice of bleaching machinery (c) Unpigmented wool is not discoloured [32] (d) Treatment times are much shorter [33].

However, for environmental reasons, industrial application of such processes seems currently most unlikely.

Afterbleaching with reducing agentti One recent development concerns aftertreatment (clearing) with reductive bleaching agents, a common measure when sulphur-containing reductive agents are used during mordanting. In a bleaching process as much as 60- 90% of the original hydrogen peroxide can remain unused at the end of the process. Marmer et al. developed a beauti- fully simple method for utilising this 'spare' peroxide; they

converted it into the strong reducing agent thiourea dioxide by adding thiourea to the liquor (34,351. They were able to show that conversion of the toxic thiourea added was fast and complete. The feasibility of the process has been proven by industrial trials.

Bleaching practice In Tables 4-14 bleaching parameters are given which lead to good results with different fibre types under industrial conditions [36]. Nevertheless, it is important to bear in mind that the inherent nature of these materials dictate that quality and behaviour can differ greatly from lot to lot, and therefore prior laboratory tests are always indispensable.

Suggested mordanting and rinsing conditions are quoted in Table 15, while the recommended bleaching conditions for different fibre types are summarised in %ble 4. In the case of karakul wool, bleaching at 70°C over a shorter time may be more economical. Although hypophosphorous acid is a costly product, its use during

Table 4 Suggested bleaching conditions for keratin fibres and feathers pigmented in the bulk' [36]

Hydrogen peroxide Temp. Time

Fibre type (35%) (ml I-I) pHb ("C) (rnin)

Karakul wool 25-45 8-8.5 50-60 45-180 Cashmere Alpaca Camel Yak

Rabbit 20-40 8-8.5 50-60 45-120 Chicken feathers

Goat 30-50 8.5 60-70 60-180 Human hair

a Sodium pyrophosphate 10 g I-', liquor ratio 15:l b Adjusted with ammonium hydroxide


Table 5 Properties of mordant bleached speciality fibres pigmented in the bulk 1361

Table 8 Properties of Selectively bleached wod tops, yams and knitted fabrics (same material as bleached tops)' 1231

Colour Alkali Cysteic solubility acid

Treatment X Y Z ("/.I (W

Cashmere Untreated 7 7 6 1 1 0.52 Bleach 1' 32 32 20 31 1.58 Bleach 2b 33 34 22

Camel Untreated 20 20 12 13 0.85 Bleach 1' 55 55 35 34 2.14 Bleach 2b 58 60 45

a Bleach 1 - hydrogen peroxide (35%) 30 ml F1, sodium pyrophosphate 10 g I-l, 1 h. 55%. pH 8.5

b Bleach 2 -As bleach 1 plus reducing afteflreatment with Blankit IN, 1 h, 60°C Comment Medium shades were obtained while alkali solubility values were relatively low

Table 6 Properties of mordant bleached pigmented alpaca yam 1361

Wash Perspiration Colour fastnessnab

Treatment X Y Z C W T C W T

Untreated 5.3 4.9 2.9 4 4-5 3r 4 4 3r Untreated+dyed* 3.6 3.5 2.5 5 4-5 3r 3 2-3 3r Bleach' 29 29 15 Bleach+dyed 1 1 1 1 7 4-5 4-5 3r 3 3-4 3r

a C - staining on cotton, W - staining on wool, T -change of shade 01 test material (r - redder)

b Wash fastness according to DIN 54014 but at 30°C c Perspiration fastness according to DIN 54020 (alkaline) d Dyeing with Lanasol Yellow 4G. Cibacron Pront Turquoise G and Lanasol Red

2G, all CGY and 0.3% 0.w.f. each e Bleach - hydrogen peroxide (35%) 24 ml I-l, sodium pyrophosphate

10 g V1, 1 h, W C , pH 8.5 Comment Cdour lastness of bleached materials was similar that of unbleached fibres

Table 7 Suggested conditions for the bleaching of single pigmented fibres in while material (selective bleaching)n 1361

Group 1 Group 2 conditionsb conditionsC

Hydrogen peroxide (35%) 10 ml 10 ml I-l

Time 120-1 80 min 45-90 min Temperature 70°C 70°C

Sodium pyrophosphate 10 g I-1 10 g I-'

PH 8.0-8.5 8.0-8.5

a Liquor ratio 15:1, mordanting and rinsing Conditions as in Table 15 b Group 1 fibres - White merino wool, cashmere c Group 2 libres - Crossbred, Lincoln, European wool types in general (coarser

wool). mohair, alpaca, rabbit, and goose. duck and chicken feathers Commenr Bleaching of single pigmented fibres in fine wools (merino and cashmere) takes longer than in the case of warser wool types

Top bundle Yam bundle tenacity tenacity Fabric

Colour (cN ter') (cN te r ' ) abrasion resistance (Martindale/

X Y Z Dry Wet Dry Wet cycles)

Untreated 70 75 67 10.5 9.5 12.0 9.2 1 1 500 Bleached 70 75 66 10.3 9.2 11.6 8.2 9 500

a Average fibre diameter 23.7 wm, yam count 67 tex (1 5 N m), cover factor of knitted fabrics 4.0

Comment Wooldamage is comparablewith thedamagecausedbya conventional alkaline peroxide bleach

Table 9 Properties of selectively bleached woven wool fabrics in industrial production 1231

Bundle tenacity (cN te r l ) Abrasion

Fabric resistance Fabric weight Selective (Martindale/ structure (g m*) bleaching Dry Wet cycles)

Plain 122 No 10.5 7.5 20 000 weave Yes 9.9 6.6 14 000

Panama 348 No 8.4 6.6 10 000 Yes 8.4 6.2 9 500

Twill 273 No 9.9 7.9 28 000 Yes 9.5 6.2 22 000

Comment See Table 8

Table 10 Properties of selectively bleached uphdstery fabrics (industrial production) [23]

Selective Fabric bleaching

Abrasion Fabric resistance' Recovery weight (W (W (g m-2)

Pile weight (9 m-?

Mohair No plush Yes

No Yes

Epingl6 No (pure wool) Yes

2.1 89 613 0.8 92 638 1 .o 91 712 1.3 93 733 3.2 94 524 2.3 94 536

31 7 327 334 350

Comment Pile recovery became better during bleaching probably as a result of fabric shrinkage

mordanting is certainly advantageous with the more valuable fine animal fibres.

Tables 5-11 show some results obtained under practical conditions in industrial trials; they refer to so-called 'selective bleaching', i.e. to the bleaching of single pigmented fibres in white material. As mentioned above, the


Table 11 Properties of selectively bleached fibres and feathers (361

Abrasion Bundle Alkali resistance tenancity wet solubility (Martindale/

Material (cN terl) ("N cycles)

Alpaca fabric Untreated 11.5 13 1 1 000 Bleach l a 10.2 38 9 500

Alpac&ml Untreated 10.4 14 10 5OOc Bleach l a 9.4 7 8 oooc Goose feathers Untreated 8 000 Bleach 2d 18 000

a Bleach 1 - hydrogen peroxide (35%) 10 ml I-l, sodium pyrophosphate

b 60:40 top blend c Knitted fabric d Bleach 2 - as bleach 1 but 55°C Comment Bleaching conditions were always appropriate for complete bleaching 01 pigmented libre

10 g I-l, 1 h. 70°C. pH 8.C-8.5

Table 12 Influence of formaldehyde on properties of mordant-bleached wool with formaldehyde added to mordanting hatha [36]

White woolb Black karakul wool

Form- Yarn aldehyde bundle (30%) Alkali tenacity Abrasion Alkali added solubility wet resistance solubility (9 1-l) (W (cN tex-I) (Martindale) (%) Y

Untreated 16 7.1 51 16 6 0 49 6.9 31 73 51 2 40 6.3 33 55 48 5 31 6.0 36 38 45 7.5 26 5.8 36 34 43 10 24 6.1 36 30 44 15 21 5.9 36 28 44

effect (e.g. improving mechanical properties) attributable to formaldehyde crosslinking (Tables 12-14) [22,36]. This work gave the following results.

Formaldehyde reacts with wool under mordanting (1 h, 80°C, pH 3) and bleaching (3 h, 70°C, pH 8) conditions. The application of formaldehyde in the bleaching bath gives rise to less wool damage but at the same time to reduced bleaching efficiency, because, as expected, hydrogen peroxide is reduced by formaldehyde. There was no sign of bleaching by performic acid, which can be formed during the reaction of formaldehyde with hydrogen peroxide. When formaldehyde was added to the mordanting bath, fabric abrasion resistance and wet tensile strength (measured on yam bundles) were increased (compared with the strength of corresponding samples treated without formaldehyde), but only when sulphur-containing reducing agents were used. With phosphorous acid or hypophosphorous acid reducing agents the formaldehyde had no effect on tensile strength, probably because the loss of strength during bleaching was relatively low anyway, and there were not many split disulphide bonds to 'repair' or substitute. However, a slight increase in abrasion resistance also occurred in this case (Table 13).

Whenever formaldehyde was used in combination with sulphur-containing reducing agents such as sodium bisulphite, the bleaching effect became much worse; the white wool was discoloured and a reductive aftertreatment (clearing) was necessary (Table 13). Also, in other cases the presence of formaldehyde in the mordanting bath led to reduced bleaching effects (Table 12). Aftertreating bleached goods with formaldehyde (10 ml l-I HCHO (35%), 60 min, 8OT, pH 3 (HC02H)) did not have any significant effect on mechanical strength.

Table 14 shows results obtained with cashmere and alpaca, which demonstrate that it is possible to avoid the

a Mordanting as in Table 15, bleaching - hydrogen peroxide (34%) 10 ml I-l,

b 300 g m-* fabric sodium pyrophosphale 10 g I-l, 3 h. 70°C, pH 8.5, liquor ratio 5:l

Table 13 Effect of formaldehyde on properties of selectively bleached WooP (221

occurrence of these fibres is much more frequent in mohair and alpaca than in high-quality white wool. An appropriate comment is added at the end of each table.

Use of formaldehyde in selective bleaching Most processes for the bleaching of pigmented fibres involve the use of formaldehyde 1201. One of the most frequent reasons advanced for the use of a crosslinking agent is supposedly to lower alkali solubility. Investig- ations have been carried out into the possible protective

Bundle Abrasion Form- tenacity resistance aldehyde wet Yellowness (Martindale/ (37%) (ml I-') (cN ter') (DIN G) cycles)

Untreated 7.1 28.0 51 000 Treatment l b 0 6.9 25.7 31 000

10 7.1 25.6 34 000 Treatment 2c 0 4.9 22.2 18 000

10 7.0 50.3 29 000

a Mordanling - iron(ii) sulphate heptahydrate 5 g 1-l. reducing agent as indicated, 60 min, 8O"C, pH 3; rinsing - 2 x 10 min, 80°C; bleaching - hydrogen peroxide (35%), 25 ml I-l, sodium pyrophosphate. 5 g 3 h, W0C, pH 8.5

b Reducing agent In mordanting bath - phosphorous acid (SO%), 2 ml I-1 c Reducing agent in mordantlng bath - sodium bisulphlle. 2 g 1-l


Table 14 Influence of formaldehyde in mordanting bath on properties of mordant bleached cashmere and alpaca' 1361

Form- Yam Abrasion Colour aldehyde bundle resistance Alkali Cysteic (30%) wet (Martindalel solubility acid (g I-l) (cN tex-') cycles) (%) (Yo) x Y z

Cashmere fabric Untreated 7.1 12 000 16 0.44 7 6 6 Bleachb 0 5.5 11 000 41 3.40 60 63 45 Bleachb 7.5 5.4 11 000 28 3.50 59 61 45

Alpaca yam Untreated 10.1 BleachC 0 6.9 BleachC 10 7.3

11 23 17

5 5 3 32 31 15 31 30 15

a Mordanting as Table 15 b Hydrogen peroxide (35%) 35 ml I-', sodium pyrophosphate 10 g I-', 3 h, W'C, pH 8.5 c Hydrogen peroxide (35%) 25 ml I-l, sodium pyrophosphate 10 g I-', 3 h, W C , pH 8.5

Table 15 Suggested mordanting and rinsing conditions for pigmented keratin fibres and feathers 1361

Mordanting Rinsing

Iron(ii) sulphate heptahydrate Hypophosphonus acid (50%)

10 g I-' 3-4 ml I-'

Formic acid TO pH 3-3.5 Temp. ("C) 80 80 Time (min) 60 20

use of formaldehyde in pigment bleaching when a mild reducing agent such as hypophosphorous acid is present.

* * *

A substantial part of this paper is based on a plenary lecture presented by the author at the 2nd International Symposium on Speciality Animal Fibres in Aachen [36].

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bleaching of pigmented speciality animal fibres and wool

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