O o d h e a d p u b L i s h I n g L i m I t e d

7.6
Enzyme applications in wool finishing
Biotechnology has recently found application within the textile industry in
order to lessen the environmental impact of textile wet processes. The
restrictions on AOX in effluent have become part of the legislation in EU
countries. Even in other countries where there is no such legislation, there is
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Table 7.4 Flame-retardant finishes for wool
Trade name
Chemical constitution
Process and durability
Huntsman
Flovan
®
 CGN
Phosphorus and nitrogen-based
Non-durable
Pyrovatim
®
 PBS
compound, halogen free
Semi-durable
Thor Specialities Ltd.
Aflammit
® 
ZR
Potassium hexafluorozirconate
Applied by an exhaust process, durable to washing and dry
Aflammit
® 
TI
Potassium hexafluorotitanate
cleaning
Aflammit
® 
ZAL
Zirconium acetate solution
Used in combination with Aflammit
®
 ZR for low smoke emission
Aflammit
® 
WPA
Sodium salt of  tetrabromophthalic acid
Suitable for wool/nylon blend, durable to several washes at 40
°C,
used in combination with Aflammit
®
 ZR/TI for better performance
Firestop Chemicals Ltd
Noflan
Organophosporus flame retardant
Applied using pad–dry–cure method, durable to dry cleaning up
based on complex alkyl phosphonate
to 25 cycles
Schill & Seilacher
Flacavon
® 
R neu
An organic phosphorus and
Applied by padding, dipping and spraying methods and drying
nitrogen-containing compound
at 100
°C, durable to dry cleaning.
CHT R Beitlich GmbH
Apyrol ZFK
Potassium hexafluorozirconate
Applied by exhaustion method, durable to washing  and dry
cleaning
Apyrol ZAC
Zirconium acetate solution
Used in combination with Apyrol ZFK  for low smoke finishing
Avocet Dye & Chemical Co. Ltd
Cetaflam
® 
PHFZ
Potassium hexafluorozirconate
Applied by exhaustion process, fast to washing and dry cleaning
Cetaflam
® 
PHTI
Potassium hexafluorotitanate
Cetaflam
® 
ZAS
Zirconium acetate solution
Applied by exhaustion in combination with Cetaflam
® 
PHFZ to
give low-smoke FR finishes, durable to washing and dry
cleaning
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ool finishing and the development of novel finishes
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Cetaflam
® 
DTB Liquid Tetrabromophthalate derivative
Used in combination with the Zirpro
® 
process to reduce
after-flaming times and minimise heat release, applied by
exhaustion
Cetaflam DB WN 240
Blend of flame retardant components
Durable to repeated dry cleaning and standard neutral launderings
Rhodia Consumer
Amgard RD
Phosphorus and nitrogen-based
Applied by pad–dry, brushing, dipping or spraying technique,
compound
non-durable
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Advances in wool technology
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pressure on textile manufacturers and retailers to limit or even eliminate this
kind of contamination in the effluent. Because of such environmental concerns,
much research effort has gone into searching for environmentally friendly
processes for wool processing. The environmental benefits gained by using
enzymes as bio-catalysts in wool processes to replace harsh chemicals,
especially for wool shrink resistance, and simultaneously to improve dyeability,
handle and whiteness of wool are well recognised.
Enzymes are biological catalysts for specific chemical reactions and require
comparatively mild conditions. All enzymes are proteins and biodegradable.
The precise reaction specificity of an enzyme can be used for specific or
targeted textile finishing without causing undesirable effects. There are
thousands of enzymes available. Classification of enzymes is very important
for enzymes users as well as the scientific community. The Enzyme Commission
of the International Union of Biochemistry devised a rational system for
classification of enzymes. The classification is made by code numbers,
consisting of four digits separated by dots, on the basis of the total reaction
catalysed. The first digit shows the main class to which the enzyme belongs.
In this system, enzymes are divided into six groups, which are presented in
Table 7.5.
Most of enzymes used in the textile industry belong to Group 3, hydrolases.
This Group includes the amylases, cellulases, pectinases, catalyses and proteases
which are used for various textile applications such as desizing, bioscouring,
bio-polishing, bleach cleaning-up and wool shrink resistance. Proteases,
proteolytic enzymes or peptidases are general terms for enzymes which
catalyse the hydrolysis of certain peptide bonds in protein molecules (forming
the group EC 3.4.XX of hydrolases). These have been suggested for
incorporation in wool processing for improving scouring efficiency, handle
properties, imparting shrink resistance and low temperature dyeability.
Table 7.5 The Enzyme Commission’s system of classification of enzymes and
assigning code numbers (Palmer, 2001)
First digit
Enzyme class
Type of reaction catalysed
1
Oxidoreductases
Oxidation/reduction reactions
2
Transferases
Transfer of an atom or group between two
molecules
3
Hydrolases
Hydrolysis reactions
4
Lyases
Removal of a group from substrate (not by
hydrolysis)
5
Isomerases
Isomerisation reactions
6
Ligases
The synthetic joining of two molecules, coupled
with the breakdown of pyrophosphate bond in a
nucleoside triphosphate
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Proteolytic enzymes can be divided into exopeptidases (which hydrolyse
terminal peptide bonds) and endopeptidases (which hydrolyse peptide bonds
inside the substrate molecule). Proteolytic enzymes can be further grouped
according to the chemical nature of the catalytic site. They are divided
among 13 sub-subclasses (
Table 7.6).
 Four distinct families are the serine
endopeptidases (such as chymotrypsin, trypsin and subtilisin), the cysteine
endopeptidases (such as papain), the aspartic endopeptidases (such as pepsin)
and the metalloendopeptidases (such as thermolysin). In particular, serine
endopeptidases have been extensively studied. The application of enzymes
in wool processing to replace harmful chemicals has been extensively studied
and developed in last decade, and has been recently reviewed by Heine and
Höcker (1995, 2001).
Early studies and more recent work (Moncrieff, 1953; Bishop et al., 1998;
Shen et al., 1999) have confirmed that the action of proteases with undamaged
wool is slow. This is due to the protective nature of the hydrophobic epicuticle
surface containing fatty acid molecules and highly crosslinked cuticle cell
components. However, once some of the cystine disulphide crosslinks in the
cuticle cells are broken, the rate of enzyme reaction is greatly increased.
Early claims suggested that proteases were unable to penetrate into wool
fibre even when the fibre is wet and swollen owing to the large size of the
enzyme molecules (Moncrieff, 1953). However, it was later found that during
treatment with proteolytic enzymes, enzyme attack occurred preferentially
at the highly swellable cell membrane complex by penetrating between cuticle
cells and then between cortical cells. Once the enzyme has diffused into the
membranes between the cells, the enzyme can rapidly disrupt the cell membrane
complex, and eventually damage the fibre if the reaction is prolonged. A
Table 7.6 The Enzyme Commission’s system of classification of
peptidases (NC-IUBMB, 2008)
Sub-subclass
Type of peptidase
Number of entries
3.4.11
Aminopeptidases
20
3.4.13
Dipeptidases
12
3.4.14
Dipeptidyl-peptidases and
9
tripeptidyl-peptidases
3.4.15
Peptidyl-dipeptidases
4
3.4.16
Serine-type carboxypeptidases
4
3.4.17
Metallocarboxypeptidases
20
3.4.18
Cysteine-type carboxypeptidase
1
3.4.19
Omega peptidases
9
3.4.21
Serine endopeptidases
98
3.4.22
Cysteine endopeptidases
54
3.4.23
Aspartic endopeptidases
38
3.4.24
Metalloendopeptidases
80
3.4.25
Threonine endopeptidase
1
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fluorescence microscopy study by Heine (1991) demonstrated the diffusion
of fluorescently labelled enzymes between the cuticle scales, through the
cell-membrane complex and into the cortical cells. Therefore it is difficult to
limit enzymatic degradation to the cuticle scales and to achieve machine-
washable wool without significant fibre damage (Heine et al., 2000).
Heine (Heine, 1991; Nolte et al., 1996) investigated the removal of lipids
from the wool fibre surface outer layer by treating grease-free wool with a
lipoprotein-lipase. It was suggested that the aliphatic-hydrocarbon content
of the cuticle surface was reduced by 20%, and this led to an improvement
in wettability. The lipoprotein-lipase-treated wool top also showed a reduced
felting capacity compared with the untreated top, but this effect did not
constitute shrink resistance according to the IWS standard.
The enzyme kinetics do not only rely on the concentration of the reaction
substrate (fibre), reaction temperature and pH, but also on the diffusion of
enzyme to and into the solid phase of the fibre and the diffusion of the
reaction products out of the solid phase into the solution. The complex
structure of the wool fibre is important in the reaction. In addition, buffer
and surfactants could be other important factors affecting the activities of
enzymes. It is well known that proteolytic enzymes are compatible with non-
ionic surfactants, which are widely recommended to be used in the enzymatic
treatments. A recent study (Zhang et al., 2006) has shown that the activity of
the protease Esperase towards wool can be promoted by an ethoxylated alkyl
phosphate anionic surfactant. Therefore, this anionic surfactant can provide
additional benefits as an alternative choice to the widely used non-ionic
surfactants. This could lead to the development of a more efficient enzymatic
scouring process. Use of a buffer can maintain the optimum pH for protease
activity during the enzymatic treatment, but it is found that different buffer
systems and their ionic strengths have different effects on the activity of
proteolytic enzymes. This is due to the interference of buffer cations and
anions with the conformation of enzymes and their biological reactions.
An early study (Moncrieff, 1953) showed that the attack on wool fibres
by proteolytic enzymes is variable. Enzymes appear to attack weathered
fibres more rapidly and therefore are inclined to attack the tips of fibres more
than the root ends. Irregularity of damage of fibres by enzymes can be
clearly shown by scanning electron microscopy.
Bishop  et al. (1998) reported that carefully controlled treatments with
proteolytic enzymes can reduce the buckling load and collapse energy of
wool yarns. These treatments were shown to improve the softness and reduce
the subjectively perceived prickle of wool fabric knitted from the treated
yarns. Combination of chlorination and treatments with proteolytic enzyme
was also reported to improve handle properties, especially for coarse wool
and mohair fibres, as well as improving fibre whiteness without causing any
real damage to the wool (Holme, 2006c).
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In the past decade, scientists have made a great effort to use different
wool pre-treatments prior to enzyme treatment in order to limit enzymatic
degradation to the cuticle scales and thus to achieve machine-washable wool
without causing significant fibre damage. Different combinations of enzymes,
oxidants/reductants and polymers have been proposed to improve felting
and shrink resistance of wool (Levene and Shakkour, 1995; Levene et al.,
1996; Jovancic et al., 1998; Cardamone et al., 2006). In some of the early
enzyme finishing processes, wool was pre-treated by chlorine or hydrogen
peroxide prior to incubating the fibres with proteases. Oxidative treatments
of wool can disrupt disulphide bonds and open up the wool fibre surface
assisting enzymatic attack on the cuticle. On the other hand, oxidative pre-
treatment probably induces a partial removal of the fatty acid barrier from
the epicuticle, which confers hydrophilicity to wool (Cardamone et al., 2005).
Consequently the enzymatic attack on the cuticle can be selectively activated.
It has been claimed that a combination of chlorination with chlorine gas or
dichloroisocyanuric acid and subsequent enzyme treatments with a protease
such as papain showed that pre-oxidation will limit enzymatic attack to the
cuticular layer, resulting in the enzymatic descaling of wool fibres to enhance
not only lustre but also shrink resistance (Moncrieff, 1953; Levene and
Shakkour, 1995). However, the process might cause fabric yellowing.
Recently a two-step process which combines bleaching, shrinkage prevention
and biopolishing was suggested as a way to make wool feel silky smooth.
This involved a pre-treatment using hydrogen peroxide enhanced by
dicyandiamide and stabilised by gluconic acid for powerful oxidation, and
followed by enzyme treatment with proteases in the presence of sodium
sulphite in triethanolamine buffer. Benefits claimed were a high level of
whiteness, the removal of protruding fibre ends for fabric smoothness and
shrink-resistance (Cardamone et al., 2004b, 2005).
Other oxidising agents used as a pre-treatment for wool are peroxy-
monosulphuric acid, peracetic acid and potassium permanganate. Alkaline
and reducing agents such as bisulphite can alternatively be used to open
disulphide crosslink bonds in the cuticle scales to make the fibre more
susceptible to enzymatic attack. These pre-treatments have been reported to
enhance the activities of proteases and improve the efficiency of proteases in
conferring anti-felting and shrink resistance of wool (Levene and Shakkour,
1995; Levene et al., 1996; El-Sayed et al., 2001). However, these processes
caused severe fibre damage when shrink resistance was reaching adequate
levels for machine-washable wool. Recently, Lenting et al. (2005, 2007)
used the addition of a high concentration of sodium salt in the peroxide pre-
treatment for restricting the oxidative reaction to the surface scale of wool
fibres. This resulted in an improvement in the susceptibility of the outer
surface protein layer to subsequent proteolytic hydrolysis. This agreed with
an earlier study which suggested that the presence of high concentrations of
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salt reduced the extent of swelling of the fibre and reduced the rate of
diffusion of the oxidant through the cuticle to the cortex. This led to preferential
attack on the cuticle (Maclaren and Milligan, 1981) and wool shrink resistance
was claimed without substantial loss of fibre tensile properties.
Jovancic et al. (2001) demonstrated that incorporating an enzyme in the
alkaline peroxide treatment bath enhanced wool wettability and improved
the effectiveness of subsequently applied chitosan biopolymer in achieving
a degree of shrink resistance.
A two-step treatment consisting of a low-temperature plasma treatment
(LPT) and subsequent enzymatic treatment by proteases has also been
investigated to achieve wool shrink resistance (Dybdal et al., 2001; Jovancic
et al., 2003). X-ray photoelectron spectroscopy (XPS) analysis revealed that
the LPT /enzymatic treatment completely removed the outermost lipid layer
(the F-layer) of the epicuticle. However, in order to avoid the excessive
damage of wool fibres, the subsequent enzymatic treatment was mostly used
to remove fibres protruding from the surface of the fabric and thus increased
its softness.
Although there is currently considerable interest in the use of enzymes to
achieve a shrink-resist finishing effect on wool, it is apparent that the results
of enzymatic treatments, especially with proteases, can be unpredictable and
may sometimes lead to unacceptable degradation of the fibre (see Fig. 7.11).
EHT = 20.00 kV
3 
µm
WD = 31 mm
Photo No. 2330
Mag = 1.50 K X
Detector = SE1
7.11
 Scanning electron micrograph of wool fibres treated with
proteases.
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Wool finishing and the development of novel finishes
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A new approach (Cavaco-Paulo and Silva, 2003; Silva et al., 2004, 2005,
2006; Smith et al., 2008) has increased the size of proteases to try to limit the
enzymatic degradation of wool fibre to their cuticle scales. The increase in
the size of the proteases was achieved by covalently attaching them to soluble
Eudragit polymer (
Fig. 7.12).
 Chemical modification of proteases with Eudragit
improved the thermal stability of the enzymes as well as enzyme recycleability
due to soluble–insoluble reversibility of the Eudragit polymer attached to the
enzyme.
Shen et al. (2007) have published the results from the bulk trials on wool
fabrics carried out using the modified proteases. It was demonstrated the
modification of the protease enabled the reaction of the enzyme with wool to
be controlled, so that less degradation of the wool occurred than in similar
treatments with the unmodified protease. An anti-felting effect has been
achieved without any significant weight loss being caused by the modified
protease during the treatment. This novel enzymatic process leads to
environmentally friendly production of machine-washable wool.
There is increasing interest in use of the protein-crosslinking enzymes
transglutaminases (EC2.3.1.13) for surface modification of wool fibres.
Transglutaminases belong to the class of transferases and act by a different
mechanism than hydrolases. These enzymes can catalyse the post-translational
modification of proteins by the formation of isopeptide bonds. This occurs
either through protein crosslinking via epsilon-(gamma-glutamyl)lysine bonds
7.12
 Illustration of protease immobilisation on soluble polymer and
their enzymatic degradation of wool fibres limited to the cuticle
scales.
Immobilised
polymer
Protease
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Advances in wool technology
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or through incorporation of primary amines at selected peptide-bound glutamine
residues (Griffin et al., 2002a). This crosslinking leads to increased protein
stability and increased resistance to chemical and proteolytic degradation. In
the enzymatic treatment of wool, transglutaminases can be used for protein
crosslinking within the fibres to compensate for the reduction of tensile
strength and degradation of wool after treatment with an oxidative agent
(e.g. chlorination for shrinkage prevention), a reducing agent (sodium sulphite)
and/or a protease.
It is also reported that treatment of wool with transglutaminases can decrease
the tendency to felting of wool without causing any negative effect of stiff
and harsh handle (McDevitt and Winkler, 1998; Griffin et al., 2002b; Cortez