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

7.2
Dimensional stability and shrink-resist
finishing for machine-washable wool
Changes in fabric dimensions during use can be a major defect and are often
the cause of unserviceability and consumer complaint. There are two types
of shrinkage (Saville, 2000): relaxation shrinkage and felting shrinkage.
Relaxation shrinkage is the first stage of shrinkage and occurs when the
stresses or strains introduced during the processing of textile materials are
relaxed in water, or water plus detergent. Felting shrinkage is non-reversible
and is caused by the progressive entanglement of wool fibres through
mechanical action during washing. This tendency of wool to felt prevents
the use of untreated wool materials as machine-washable textiles.
The physical structure of the scaly cuticle layer of the wool fibre is considered
to be the main reason for felting. The modification of the wool surface either
by degradative methods and/or by application of a polymer on the wool
surface is the major approach to achieve anti-felting properties of wool.
Electron scanning microscopy shows that wool fibre is covered by a thin
sheath of overlapping scales like the slates on a roof (
Fig. 7.1).
 These scales
are responsible for the felting and shrinkage that occurs during laundering.
This is due to the differential frictional effect (DFE) caused by the difference
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Wool finishing and the development of novel finishes
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in friction between 
µ
a
 (friction coefficient when rubbing in the against-scale
direction) and 
µ
w
 (friction coefficient when rubbing in the with-scale direction).
Finishers can make use of this property in a process known as milling to
achieve consolidation of the fabric and make it fuller and denser. On the
other hand, felting shrinkage can be detrimental to the performance of wool
fabrics. Shrink-resist finishing is required to give wool fabrics and garments
machine washability.
An understanding of the structure of the cuticle cells in the surface scale
of fibres is necessary for the development of successful shrink-resist finishes.
The wool cuticle is about 400 to 500 nm thick. The cuticle cells comprises an
endocuticle (120 to 180 nm thick), an exocuticle (150 to 200 nm thick) and
an outermost epicuticle layer (5 to 7 nm thick) (
Fig. 7.2).
 The epicuticle layer
contains 18-methyleicosanoic acid covalently bonded to the surface of wool
fibre via a thioester bond (called the F-layer) to form a hydrophobic barrier.
The exocuticle layer contains a high proportion of crosslinked disulphide
and isopeptide bonds, resulting in the resistance to attack from alkaline
agents and proteolytic enzymes. The endocuticle, a layer lying below the
exocuticle, has a relatively low crosslink density (3% half-cystine) and thus
is easily permeable and more susceptible than the exocuticle to chemical
attack (Feldtman et al., 1983; Mori and Inagaki, 2006).
Anti-felt or shrink-resist finish processes have been developed for decades
in order to obtain easy-care (machine-washable) wool products. Commercially
EHT = 12.00 kV
3 
µm
WD = 10 mm
Photo No. = 1147
10
µm
Mag = 1.75 KX
Detector = SEL
7.1 
Scanning electron micrograph of wool fibres.
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Advances in wool technology
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successful shrink-resist processes used by the textile industry in the past, and
technologies currently being developed, can be divided into four groups:
1.
Combination of oxidation and additive polymer processes.
2.
Additive polymer only processes.
3.
Plasma treatment followed by resin polymer or softener finishing.
4.
Enzymatic processes (
Section 7.6).
Many useful reviews of wool shrink-resist processes are available (McPhee
and Shaw, 1984; Byrne, 1996; Holme, 2000, 2007a). It is useful to distinguish
between shrink-resist treatments applied at loose fibre stage to either scoured
wool or to sliver, and to shrink-resist treatments applied at fabric or garment
stage. The loose fibre treatments rely on reducing the differential friction
effect either by degrading the scale structure or by masking the scales. The
fabric treatments generally rely on at least a degree of interfibre bonding to
reduce interfibre movement.
Of the loose fibre treatments, the chlorine-Hercosett shrink-resist technology
has been the leading process for the continuous superwash treatment of wool
tops since its early development in the 1960s. In this procedure wool top is
first chlorinated using acid hypochlorite or chlorine gas (the Kroy process),
anti-chlorinated with sodium sulphite, neutralised and passed through a
Hercosett solution (a polyamide-epichlorohydrin polymer) followed by softener
application; all these steps are accomplished continuously in a suction drum
back-washer line. The chlorination treatment removes covalently bound lipids
from the wool surface and also oxidises surface cystine disulphides to cysteic
acid ( RSO
3
–
 residues); these dual effects create a more hydrophilic and
anionic fibre surface. The latter surface is highly receptive to the cationic
polymer, Hercosett, which ‘exhausts’ on to the surface of each individual
fibre and self-crosslinks on drying. The latter process ensures that Hercosett
polymer is fixed and garments made from the treated wool show long-lasting
shrink resistance in subsequent machine washing cycles (Lewis, 2005). The
Intercellular cement
(1% half-cystine)
Epicuticle
(12% half-cystine)
Exocuticle-A
(35% half-cystine)
Exocuticle-B
(15% half-cystine)
Endocuticle
(3% half-cystine)
7.2 
Schematic diagram of wool cuticle (Feldtman 
et al
., 1983).
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Wool finishing and the development of novel finishes
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AOX generated in the process arises from the use of chlorine as an oxidant
and also from the resin; however, low-AOX grades of Hercosett are available.
It is important to note that there are no free phenols in wool and the potential
for generation of highly toxic AOX species such as dioxins is low (Shaw,
1990).
Dichlorodicyanuric acid (DCCA) is a major alternative chlorination reagent
for imparting shrink resistance to wool. DCCA reacts to release hypochlorous
acid which acts as an oxidising agent (Veldsman and Swanepol, 1971; Levene
and Cohen, 1996; Cardamone and Yao, 2004; Cardamone et al., 2004a).
Although the chlorine-Hercosett processes impart excellent and robust
shrink resistance to wool tops, pressures caused by AOX legislation and
general concerns about the use of chlorine remain. New approaches are
highly desirable to remove the problem of AOX. Currently, effective alternative
resins are being developed. The ideal resin should be effective on wool top
as well as on fabric, be linked permanently to the fibre surface; its application
should be water based, any AOX contamination in the effluent must be
avoided, and characteristic wool properties must not be changed, e.g. avoidance
of yellowing.
Alternative commercial resins that have been proposed for oxidation/
resin shrink-resist finishing are Listrilan SR from Stephenson Speciality
Chemicals; Polymer DP5570 from Devan-PPT Chemicals, Ambergate, UK;
Lanaperm VPA from Clariant; and Beetle Resin PT763 from BIP, among
others. The resin Listrilan SR is applied in continuous shrink-resist processing
for wool tops from a suction drum backwash bowl immediately after rinsing
following the intermediate pre-treatment stages which include chlorination,
antichlorination, neutralisation and rinsing (Stephenson technical information).
The polymer, DP5570, is a reactive acrylic copolymer. It is claimed to be
able to compete economically with the Hercosett polymer, but offers additional
benefits such as non-yellowing, natural softness and control of hairiness/
facing/pilling that occurs during washing and has become a problem with the
standard process. The addition of low polymer solids is all that is required to
fully meet easy-care standards, with the benefit of a reduced polymerisation
temperature leading to energy saving (Benisek, 2006; Devan-PPT technical
data sheet). Other natural polymers such as collagen (Hesse et al., 1995b)
and chitosan (Julia et al., 1998, 2000; Pascual and Julia, 2001; Roberts
and Wood, 2001) have been suggested as alternatives to the Hercosett
process.
Alternative oxidation agents to prepare the surface of the wool to allow
adhesion and spreading of the polymer have been widely investigated.
Potassium peroxymonosulphate (Caroat), the oxidative constituent of which
is permonosulphuric acid (H
2
SO
5
), has received the greatest attention. However
the extent of oxidation is much less and the level of shrink resistance is
sometimes inadequate, especially for low-twist yarns.
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An AOX-free continuous shrink-resist treatment for wool tops (the Perachem
process) has recently been developed at the University of Leeds, UK (Holme,
2007a; Lewis and Hawkes, 2007). This consists of a preliminary step in
which the surface energy of the wool fibres is increased by the partial removal
of the hydrophobic covalently bound surface lipids. This is achieved by
treatment with a nucleophilic agent (e.g. N-hexadecyltriammonium bromide)
under alkaline conditions. This step is followed by oxidation of the cystine
bonds in the A-layer of the exocuticle (see 
Fig. 7.2)
 and sulphitolysis with
sulphites to generate Bunte salt groups  (—SSO )
3
–
. These groups create a
negatively charged wool fibre surface and cause cuticle swelling which also
contributes to the overall shrink-resist effect of the treatment. The last step
is exhaustion of a cationic low-AOX Hercosett resin. The whole process is
carried out in a continuous six-bowl treatment. It is claimed that this patented
process results in a wool that is whiter and softer than that achieved by
conventional chlorine-Hercosett treatments while the dyeing properties remain
the same.
For treatment of wool fabrics, a number of additive treatments alone
using polymeric finishes for machine washable wool have been commercially
applied. Currently available synthetic polymers include the surface-active
Bunte salt polymer (Securlana from Cognis); silicone rubber with silicone-
modified polyurethane (Dicrylan 7702 from Huntsman Textile Effects, formerly
Ciba); reactive polysiloxane-based softener (Arristan 64 from CHT Group),
among others. The aim in this process is to achieve adequate adhesion of the
polymer to the fibre surface so that the interfibre bonds hold during the
washing process, but at the same time to avoid deterioration of the fabric
handle caused by the interfibre bonding.
Traditionally the most successful commercial polymer for fabric treatment
has been the Sirolan BAP process, in which a water-soluble bisulphite adduct
of a polyether polyisocyanate (Synthappret BAP) is mixed with a polyurethane
dispersion. Synthappret BAP is a self-crosslinking polymer containing
carbamoyl sulphonate groups that react readily under alkaline conditions
(Fig. 7.3).
 This mixture is made alkaline with sodium bicarbonate, padded
on to wool fabric, and the polymer crosslinked on the fabric surface by
stenter curing at elevated temperatures (150
°C) (Guise and Jackson, 1973;
Guise, 1977; Cook and Fleischfresser, 1985).
Bunte salt (—SSO
3
Na) polymer finishing for shrink-resistant wool has
been reported in a number of publications (Bell and Lewis, 1975; Lewis,
1977, 1982, 1999). Preparation of a Bunte salt polymer is based on esterification
of the relevant alcohol with chloroacetic acid, followed by reaction with
sodium thiosulphate. This results in the production of a Bunte salt polymer
with mono-, bi- and trifunctionality 
(Fig. 7.4).
 This polymer can be applied
by a pad–dry–heat cure procedure. The reaction between Bunte salt groups
and wool cysteine residues occurs at the curing stage to form polydisulphide
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Wool finishing and the development of novel finishes
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R—N==C==O  +  NaHSO
3
Isocyanate
O
||
R—N—C—SO
3
–
   Na
+
H
O
||
R—N—C—N—R  +  HSO
3
–
H
H
R—N==C==O  +  SO
3
–
+ OH
–
+ R—NH
2
O
||
R—N—C—SO
3
–
H
Bisulphite adduct
R—N==C==O  +  R—NH
2
O
||
R—N—C—N—R
H
H
7.3
 Chemical reactions of isocyanate and bisulphite adduct (Guise
and Jackson, 1973; Guise, 1977).
  
3 HO—C H —OH + 
H C—COOH
HO—C—COOH
H C—COOH
12
24
2
2


–3H
2
O
  
H C—CO—O—C H —OH
HO—C—CO—O—C H —OH
H C—CO—O—C H —OH
2
12
24
12
24
2
12
24


+3ClCH
2
COOH
–3H
2
O
  
H C—CO—O—C H —O—CO—CH Cl
HO—C—CO—O—C H —O—CO—CH Cl
H C—CO—O—C H —O—CO—CH Cl
2
12
24
2
12
24
2
2
12
24
2


+3Na
2
SO
2
O
3
  
H C—CO—O—C H —O—CO—CH —SSO Na
HO—C—CO—O—C H —O—CO—CH —SSO Na
H C—CO—O—C H —O—CO—CH —SSO Ha
2
12
24
2
3
–
+
12
24
2
3
–
+
2
12
24
2
3
–
+


7.4
 Preparation of a Bunte salt polymer with trifunctionality
(reproduced with permission from Lewis, 1999).
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Advances in wool technology
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crosslinked polymer films at the fibre surface as well as ‘spot welding’
fibre–fibre bonding and polymer–fibre bonding, resulting in the shrink-resist
effect (Fig. 7.5). The pad–batch application method can be achieved by
adding sulphites or bisulphites as catalysts. The preparation of Bunte salt-
terminated surface-active agents and their curing mechanism are discussed
in detail by Lewis (1982, 1999). Recently Cognis has launched a water-
soluble Bunte salt polymer, Securlana, for shrink-resist finishing of wool
garments by an ‘exhaust’ process. This system uses magnesium chloride to
C==O
3 HC—CH
2
—SH
NH
Wool cysteine residue
+
SSO
3
–
 Na
+
SSO
3
–
 Na
+
SSO
3
–
 Na
+
Bunte salt polymer
S—S—CH
2
—CH
S—S—CH
2
—CH—C - - - -
C==O
NH
NH
O
+    3NaSO
3
S—S—
S—S—
S—S
S—S
S—S
Fully crosslinked polymer
Wool cystine (reformed)
Disulphide rearrangement
CH—CH
2
—S—S
NH
C==O
+
HC—CH
2
—S—S—CH
2
—CH
C==O
NH
C==O
NH
7.5
 Curing reaction of Bunte salt polymer with wool (reproduced with
permission from Lewis, 1982).
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Wool finishing and the development of novel finishes
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promote polymer exhaustion at 40–50
°C; when the exhaust is nearly complete,
ammonium hydroxide is added to bring about crosslinking (Lewis, 2005;
Cognis technical data sheet).
Aqueous silicone-based polymer systems are also produced, but silicone
polymer does not adhere strongly to wool. A previous study (Cook, 1984)
claimed that the level of shrink-resistance conferred on wool by an aqueous
silicone–polymer emulsion could be dramatically improved by adding a
small amount of Synthappret BAP. The treatment also increased the smoothness
and softness of the treated fabrics. More recently Wang et al. (2005) have
introduced a novel bisulphite adduct of siloxane-modified aqueous
polyurethane. The introduction of polysilicone segments into a polyurethane
aqueous system can significantly improve the handle of the treated fabrics.
Fabrics treated using this process are reported to possess good shrink resistance
and handle, with the additional benefit of permanency of these effects.
Ciba Speciality Chemicals (now Huntsman Textile Effects) have introduced
an aqueous solution of modified polyurethane and modified polydimethyl
siloxane (Dicrylan WSR) for polymer shrink-resist finishing and soft handle
for wool fabrics. Further development of the process led to a novel product,
Dicrylan 7702, a combination of crosslinkable silicone rubber with silicone-
modified polyurethane which can react on the wool fibre when a special
metal-free catalyst (Phobotone Catalyst 7639) is used. This finish allows
tumble-drying after washing, but this performance is not achievable using
silicone rubber alone (Holme, 2007a; Ciba technical data sheet).
It is widely believed that the use of plasma treatment for the production
of shrink-resistant wool will become an alternative to the existing processes.
This technology has been attracting worldwide attention because it is eco-
friendly, uses a lower volume of water and the effluent is AOX-free (Wool
Record, 2004). Modification of the wool surface by plasma treatment has
been the subject of a number of publications (Gregorski and Pavlath, 1980;
Bradley et al., 1992; Byrne and Godau, 1995; Rakowski, 1995; Zuchairah