Enzyme applications in textile processing

Classification of dyeing and printing auxiliaries by function

Scheme 10.11 shows the transfer of an amino group using a transaminase

10.4.2 Enzyme applications in textile processing

Enzymes have traditionally been closely associated with the desizing of cellulosic fabrics. In recent years, however, the sphere of possible, if not actual, uses has widened considerably.

Further developments can reasonably be expected, particularly as much of this vigorous research is motivated by environmental concerns. An overview of this activity is given below.

Mercerisation, scouring and alkali boiling of cellulosic fibres

The traditional mercerising of cotton presents quite hostile conditions for enzymes. Hence it is not surprising that little use of enzymes has been reported, either in traditional mercerising or as an alternative means of obtaining similar effects. Cegarra [76] has concluded that, because of the strongly alkaline nature of mercerising solutions and the resultant transformation of the cellulose structure, it seems rather unlikely that enzymes will provide alternatives to alkali in the near future. Even so, it is pertinent to study the effects that mercerisation may have on any subsequent enzyme treatment. It has been shown [77]

that enzymatic hydrolysis is accelerated on cellulose that has been mercerised without tension compared with stretch mercerisation.

Possible uses of enzymes in scouring or alkali boiling offer somewhat more scope, although conditions can still be rather hostile as regards alkalinity and temperature. Nevertheless, there are some emergent signs. Various enzymes, such as cellulases, pectinases, lipases and proteases, have been compared [77], leading to the tentative conclusion that cellulases give the best results for the removal of impurities, together with slightly inferior whiteness, a similar loss in strength and less contaminated effluents, compared with the traditional alkaline scour. Use of either a pectinase or a pectinase/cellulase mixture for the removal of pectin from cotton has also been studied [78]. Effective removal using such enzymes was found at 40 °C and pH 4.5, giving a higher degree of whiteness than alkaline washing at the boil for the same degree of cellulose degradation. Cellulase can facilitate the alkaline scouring of viscose [79], enabling the concentration of alkali (36–60 g/l) traditionally used to be reduced by 5–10 g/l, and giving, moreover, a more uniform and consistent swelling process than when alkali is used alone. Another study [80] also demonstrated possibilities for using enzyme formulations in cotton scouring.

Desizing of cellulosic fabrics

The enzymatic desizing of cellulosic fabrics is a long-established standard process.

Amylolytic enzymes are used to convert any type of starch size into water-soluble products without affecting the cellulosic fibres. Using enzymes in their natural or modified state, products are available to allow desizing at 20–70 °C, 70–90 °C or 85–115 °C [81]. Cegarra [76] has intimated that, given the availability of such products, further studies are likely to be concentrated on formulations allowing simultaneous desizing and scouring in an alkaline medium, replacing the present two-stage process.

Nevertheless, research continues to explore improved desizing processes. Advantages have been claimed for lipases [82] and traditional amylase desizing can be improved with the help of a thermostable lipase, giving both technical and environmental advantages [83].

Cotton sized with poly (vinyl alcohol) (PVA) is generally desized in water at about 80 °C.

However, a mixture of two different PVA-degrading enzymes gives equivalent desizing at only 30–55 °C and pH 8.0. Extending the enzymatic treatment time to 4–6 hours (compared with one hour) resulted in minimum residual PVA [84]. Environmental benefits were also

found, since the PVA content in the liquid waste after desizing for four hours was negligible.

Advantages for oxidoreductases over amylolytic enzymes have been observed, since they break down lignin impurities and are effective over a wider range of temperature and of pH [85].

Bleaching of cellulosic fibres

The possibility of catalysing the action of hydrogen peroxide by enzymes is an interesting one, but the need to avoid fibre damage is critical and so such catalysis by a peroxidase is not currently practical. However, the careful use of glucose oxidase in conjunction with an enzymatic desizing process is reported [86] to permit the novel and eco-friendly use of starch-containing effluent liquors for subsequent bleaching. This allows the use of hydrogen peroxide as the oxidising agent, together with gluconic acid (10.28) which has outstanding sequestering properties and good biodegradability. Ecological and economic advantages are claimed, including minimising the effluent pollution load, reducing chemical consumption and processing under mild conditions.

After bleaching it is important to ensure that the fibre does not contain residual hydrogen peroxide since this can interfere with subsequent processes, particularly coloration. Certain enzymes, particularly catalases, used to eliminate peroxide are bio-friendly and time-saving [87], thus having significant advantages over traditional methods [88]. Such a technique can be used in either batchwise or continuous washing-off after bleaching to give rapid and complete decomposition of any residual peroxide [89].

Dyeing of cellulosic fibres

Enzyme processing before, during or after dyeing is an active area of study. Enzyme pretreatment may have beneficial or adverse effects on subsequent coloration. The action of enzymes during coloration may improve the coloration process or provide a combined process, such as desizing/coloration or coloration/biofinishing. With the emergence of biofinishing techniques, it is important to know how such enzyme treatments are affected by any prior coloration process. Most of the published work deals with enzyme pretreatments or aftertreatments.

In one study [90], enzyme pretreatment increased colour yield without affecting fastness properties. However, pretreatment of cellulosic fibres with cellulase lowered the subsequent fixation of homobifunctional triazine reactive dyes but did not impair the fixation of other types of reactive dyes [91]. Another study suggested that the enhanced brightness of reactive dyeings was greater with triazine dyes than with vinylsulphone types when cotton was pretreated or aftertreated with cellulase [92].

The enzyme biofinishing of cotton after dyeing was found to be inhibited by direct or reactive dyes but not by vat dyes [93]. In another investigation of reactive dyes in this context, biofinishing was variously influenced by the type of dye–fibre bond, the type of chromogen, the presence of metal ion, the number of reactive groups per molecule and even by the dye application method [91]. Yet another study [94] showed that cellulase pretreatment boosts dye exhaustion and cellulase aftertreatment increases the apparent depth of the dyeing. Interactions between cellulase enzyme pre- or post-treatments and reactive or direct dyes have been studied by Buschle-Diller et al. [95], with the objectives of

elucidating the mechanism of enzymatic degradation and specifying optimum conditions for a combined dyeing/biofinishing process.

Denim washing

The practice of stonewashing of dyed cotton denim fabrics to give a ‘distressed’ or washed-down appearance was traditionally carried out with pumice stones. This was labour-intensive, time-consuming, caused abrasion of the fabric surface and created debris. Several authors have described how the stonewashed effect can be produced more advantageously using cellulase enzymes to partially or wholly replace the stones [76,96–101]. The enzymes provide a controllable means of surface attack of the fibres, thus bringing about the desired uneven appearance. Advantages claimed include savings in time and labour, much less fabric abrasion and no debris. The enzymes used are neutral or acidic cellulase preparations, which may contain endo-, exo- or beta-gluconases. The finisher can exploit the differing characteristics of acidic and neutral cellulases by employing washing procedures that take advantage of each type of formulation [98].

Biofinishing of cellulosic fabrics

Biofinishing, or ‘biopolishing’ as it is more popularly known, is similar to denim washing in its use of cellulase enzymes, although the effects intended are quite different. The process is designed to eliminate, by dissolution, the cellulosic fibrils projecting from the surface of the fabric. This treatment results in [76]:

– a cleaner, smoother surface – a softer, cooler feel

– improved resistance to pilling – brighter, sometimes deeper colours.

The precise effects obtained are dependent on the fabric quality, the type of cellulase enzyme and the application conditions, but no mechanical forces are involved in removal of the fibrils. The process has attracted considerable attention and is now one of the main methods of defibrillating lyocell fabrics [94,101–114]. Simultaneous treatment with cellulase and protease enzymes has been applied to the biofinishing of wool/cotton blends [115].

Acidic cellulases at pH 4.5–5.5 and 45–55 °C or neutral cellulases at pH 6–8 and 50–60 °C are effective in biofinishing [106,107]. Heavier fabrics and lower enzyme concentrations need longer treatment times but 30–60 minutes is a typical duration. The treatment is terminated by inactivating the enzyme, either by raising the pH to 10 or by increasing the temperature to 75 °C for 10–15 minutes. The process is usually monitored by assessing the weight loss of the fabric; a weight loss of 3–5% usually represents an adequately finished effect without excessive loss of fabric strength [107]. Dissolution of the cellulose involves depolymerisation as illustrated in Scheme 10.16 [107].

Wool processing

It is more difficult to control the enzymatic processing of wool. Hence there is a greater danger of fibre damage compared with cellulosic fibres. Since cellulose is a highly crystalline

OH OH

CH₂OH

OH OH

CH₂OH

OH OH

CH₂OH O

OH OH

CH₂OH

H₂O cellulase

enzyme

Scheme 10.16

material possessing only limited amorphous regions, it is relatively easy to restrict the action of enzymes to the surface of the fibre and to the amorphous material, thus leaving the strength of the fibre unchanged [116] In the case of wool, however, proteases and lipases catalyse the degradation of different components of the fibre. Proteases, having diffused into the interior of the fibre, hydrolyse parts of the endocuticle and proteins in the cell membrane complex. This is difficult to control and can lead to serious damage of the fibre.

SEM micrographs have shown the complete damage of wool fibres and released cortical cells characteristic of uncontrolled attack by protease enzymes [116].

Three types of enzyme may be selected for the treatment of protein fibres [76,99]:

(1) Proteases, which can be classified as either peptidases or proteinases. These cleave polypeptide chains eventually into their component amino acids. Peptidases can be further classified as endopeptidases (which act on the main-chain amido groups along the polypeptide molecule) or as exopeptidases (which act only at terminal amino acid residues).

(2) Lipases, which mainly hydrolyse fatty esters, especially triglyceride esters of fatty acids.

(3) Lipoprotein lipases, which act on the lipoproteic bonds of lipoproteins (combinations of proteins with fatty ester molecules), thus breaching the hydrophobic barriers formed by these compounds.

The most widely used of these types are the proteases, but the others may be useful in some circumstances. A characteristic feature is that individual enzymes are highly specific in their action, so that although one protease may yield the required effect, another may fail to do so.

Bleaching of wool

A serine protease that is stable to hydrogen peroxide and is active in an alkaline medium has been found and marketed [76,117,118]. In fact this enzyme becomes more active with

increasing concentration of peroxide. This enzyme increases the whiteness of wool directly by decolorising the natural yellowish hue of the fibres. Hence, depending on the degree of whiteness required, this enzyme can be used either alone or in combination with hydrogen peroxide to effect the bleaching of wool. Serine protease can also be applied with bleaching agents that operate by a reductive mechanism.

Carbonising of wool

The traditional method of carbonising with sulphuric acid is environmentally undesirable and can easily lead to fibre damage. Hence it is not surprising that research has been directed towards alternatives in which enzymes are used to remove the cellulosic impurities from wool. Cellulases and lignases are mainly used but others have been proposed [116]:

(1) Removal of plant impurities by hydrolases, lyases or oxidoreductases.

(2) Cellulolytic and pectinolytic enzymes used to reduce the amount of sulphuric acid required.

(3) Incubation of wool with cellulases facilitated subsequent removal of burr with no chemical or physical damage to the wool.

(4) Application of a mixture of cellulases, pectinases and lignases, again without damage to the wool.

Dyeing of wool

The effects of enzyme treatments on the subsequent dyeability of wool have been evaluated.

One investigation included both chlorinated and unchlorinated wool [119]. Wool was treated with a protease at 50 °C and pH 7.5, followed by dyeing with CI Reactive Reds 28 and 116. The enzyme-treated wool showed more rapid dyeing and higher absorption with no effect on fastness. These effects were greater on the chlorinated wool than on the unchlorinated control. Alternatively, the enzyme-treated wool could be dyed at a lower temperature. The effect of pretreatment with a neutral protease on dyeing with acid dyes has also been examined [90,120], increased colour yields again being observed. It is essential, of course, to determine whether the economies of increased yield or lower dyeing temperature exceed the additional cost of enzyme treatment, and whether the durability of the wool is adversely affected.

Shrink-resist finishing of wool

This is an area of considerable research activity, comparable with the enzymatic stonewashing and biopolishing of cotton. However, there has been less success in translating this research into commercial processes. Evidently, the technical use of enzymes for wool fabrics will not become widespread for another five to ten years [121]. Since certain enzymes can remove cuticular scales from wool fibres, it is not surprising that they are of interest for shrink-resist finishing, either alone or in combination with traditional chlorination or resin-application processes. Interest in this area is acute, because of the environmental disadvantages of chlorination procedures. These yield absorbable organohalogen (AOX) by-products, which accumulate in the effluent and ultimately may give rise to toxicity problems in the food chain if taken up by aquatic organisms [116]. Hence there is considerable

commercial potential for an enzymatic descaling process that could wholly or even partially replace chlorination. The critical factor is to achieve the optimal degree of descaling reproducibility, with minimal effect on fibre strength.

Ideally, the anti-felting effect should be achieved using ‘soft chemistry’ without application of a synthetic resin and the entire process should be environmentally innocuous, producing no harmful substances [116,122]. This ideal has yet to be attained. It was originally thought that the large protease molecule would not be able to penetrate the fibre cuticle. If so, attack would be limited, as with chlorination, to the cuticular scales with only minor deterioration in mechanical properties attributable to damage in the interior [123]

This proved to be too simplistic a viewpoint, however, as some proteases even attack the highly swellable cell membrane complex preferentially, possibly penetrating this region by channelling beneath the cuticular scales [123–125]. Moreover, microscopic examination has indicated that enzymatic action on wool is not uniform, some fibres remaining practically intact whilst others are damaged considerably [126]. Most anti-felting investigations have been carried out with proteases but other types have also been examined, e.g. a protein disulphide isomerase which rearranges the disulphide bonds of cystine residues [127] and transglutaminase which introduces new crosslinks into the keratin structure [128].

Protease activity towards wool can be increased by addition of sodium sulphite or bisulphite, either with the enzyme treatment or as a pretreatment [122]. Pretreatment with oxidising agents may also increase the effect of certain enzymes; hydrogen peroxide, dry chlorine, peracetic or performic acid, wet chlorination, potassium permanganate and peroxymono-sulphuric acid (H₂SO₅) have been used in this way [122]. Sulphite reduction increases proteolytic activity by cleavage of cystine disulphide bonds in the cuticle to form thiosulphonic acid groups, a reaction known as sulphitolysis [122,129,130]. When preceded by oxidative treatment, the action of sulphite yields electron-withdrawing sulphonic acid groups in the sulphur-rich cuticular layers, selectively activating the nucleophilic degradative reaction catalysed by the protease and thus preferentially directing the enzyme action to the cuticle [122,129,131]. Not all proteases are activated by sulphite, however [126].

Although the present situation and the way ahead appear uncertain, it is clear that enzyme treatment alone does not fulfil the technical requirements for shrink-resist finishing.

Even with enzyme treatment, some degree of chlorination (with the attendant AOX problems) and/or application of a resin will still be required. Two-stage or even three-stage processes have been proposed [116]:

(1) (i) Treatment with permanganate; (ii) proteolytic enzyme treatment; This gave complete descaling.

(2) (i) Treatment with papain (protease), monoethanolamine hydrosulphite and urea; (ii) treatment with dichloroisocyanuric acid; (iii) a second enzymatic treatment.

(3) (i) A combined protease treatment; (ii) wet chlorination or oxidative treatment (using sodium hypochlorite and potassium permanganate); (iii) application of a polymer.

(4) (i) Enzyme treatment; (ii) treatment in saturated steam.

(5) (i) Enzyme treatment; (ii) high-frequency radiation.

(6) The Schoeller Superwash 2000 process [132]: (i) so-called ‘black box’ pretreatment;

(ii) enzyme treatment; (iii) application of a low-AOX polyamide resin.

Most enzyme treatments of wool are carried out at about 50 °C for 30–60 minutes. The amount of enzyme required depends on the specific enzyme type and its commercial

strength. Optimal pH also depends on the enzyme type. In a study of sixteen commercial proteases for which the optimal pH varied from 3 to 10.5 [122], it was found that only papain (optimal pH 6.5–7) and alkaline proteases conferred shrink-resistance on sulphite-treated wool and these tended to cause too much fibre damage. It is thus clearly apparent that in this area of enzyme activity there is still scope for further development to meet the desired targets.

Biofinishing of wool

Enzymes can be used to modify the surface of wool fibres in order to improve lustre, softness, smoothness or ‘warmth’ of the fabric. Since such processes involve attack on the cuticular scales of the fibre, there is clearly a resemblance to shrink-resist treatments and similar methods are used [116]:

(1) (i) Treatment with potassium permanganate, ammonium sulphate, acetic acid and bisulphite; (ii) treatment with a proteolytic enzyme.

(2) Descaling by application of a heat-resistant neutral protease to confer a cashmere-like feel.

(3) Combined use of dichloroisocyanurate and a proteolytic enzyme.

(4) Complete removal of degraded or damaged portions of the wool (not merely the cuticle) using: (i) protease treatment; (ii) formic acid rinse and application of a softener.

(5) (i) Treatment with dichloroisocyanurate; (ii) neutralising and incubating with papain;

(iii) steaming at 100 °C.

Only empirical tests have so far been carried out, however [76]. In a detailed but small-scale study, various options were examined for the sequence: (i) oxidative treatment; (ii) protease treatment; (iii) application of softener, including exhaust or pad application [133].

The following products were examined:

Oxidising agents

(a) dichloroisocyanuric acid, (b) potassium peroxymonosulphate,

Enzymes

Papain and four protease formulations that varied from neutral to alkaline as regards optimal pH for activity.

Softeners

(a) a weakly cationic softener,

(b) a cationic silicone micro-emulsion,

The results varied widely:

– descaling: from none to full – fibre damage: from none to severe

– strength loss: from -6% (i.e. a slight increase in strength) to +30%

– Whiteness Index (original value -2): from -1.9 to +25.8.

Irrespective of the descaling effect, development of a ‘soft lustre’ depends on application of a softener. These experiments were positive in demonstrating the possibility of descaling the fibres and in perceptibly improving lustre under mild conditions. In particular, it was shown that papain is effective at the remarkably low concentration of 50 mg/l, showing a high degree of specificity after chlorination.

Degumming and desizing of silk

The use of enzymes in silk degumming or desizing is well-established [76,99,134–136]. In a study of eight enzymes under optimised conditions [137], weight losses of 24 ± 3% were observed in most cases but trypsin and pepsin gave extremely poor results. Increasing the treatment time at the optimal concentration of enzyme gave no further significant weight loss. There was no significant strength loss in the case of degummed silk and lustre was

In document Colorants and Auxiliaries Vol 2 (Page 81-90)