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

7.3
Stain-repellent and stain-release finishing
In recent years, stain-repellent and stain-release treatments have become
popular for apparel, upholstery and floor coverings. These markets are
continuing to grow. Industrial uniforms and military uniforms are other market
areas where stain-repellent/releases are desirable. The terms ‘soil’ and ‘stain’
‘repellence’, ‘block’ or ‘protection’ and ‘release’ have always been used in
textile industries but sometimes their meanings are ambiguous, and they
may be interchangeable. Thiry (2005) describes a soil as the medium that
goes on a textile substrate. It may or may not create a stain. A stain is the
deposit from the soil on the fabric. Therefore, successful stain-repellent and
stain-release finishes should (a) cause the fabric to repel the soiling substances
and thus prevent staining, (b) prevent stains from forming weak bonding to
fibres and (c) allow easier removal of stains during domestic washing.
There are a variety of treatments available to protect fabrics from stains
and maintain a good appearance during use. Three mechanisms are involved:
1.
Stain repellence:
• reducing the surface energy of the fibre to such an extent that both
aqueous and oily substances (polar and non-polar liquids) are repelled,
also making the fabric surface difficult to wet, thus preventing the
spread of contaminants;
• chemical modification of the fibre surface to confer a negative charge
which will repel most of the anionic colorant acids used in drinks.
Chemical modification may also be used to block sites for the
adherence of contaminants, while retaining the hydrophilic
characteristics of fibres.
2.
Stain release: treatments to improve fibre wetting, lower the adherence
of contaminants and allow them to be transferred to detergents used in
washing.
3.
Dual stain repellence and stain release: surface treatment to provide
dual stain repellent and release properties as a flip-flop mechanism for
fabric from dry to wet washing.
Stain-repellent and stain-release finishing have been discussed and reviewed
by Thiry (2005), Holme (2003), Schindler and Hauser (2004a) and Baldwinson
(1990).
Wool carpets are required to be protected against accidental staining due
to spillages of drinks and foods. The predominant source of stains from food
and drinks are anionic colorant acids. The stain-repellent treatment can block
the charged protonated amine groups in the fibre and thus achieve repulsion
of anionic stains. One major group of ‘stain blockers’ is the sulphonated acid
dyes used to block the dye sites in wool. Holme (2003) described a fibre-
reactive dichlorotriazine compound containing a sulphonate group as stain-
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Wool finishing and the development of novel finishes
159
repellent agent covalently bound to the wool, and the mechanism of action
for further improvement in stain-blocking performance by applying with a
Thiotan-type resist compound.
Recently Trichromatic Carpet Inc. of Canada and Canesis Network Ltd
developed a new stain-protection product, Lanashield, for use on wool and
wool blends. Lanashield, an anionic stainblocker, provides wool carpets
with durable resistance to a wide range of coloured anionic stains from soft
drinks and cordials. It is reported that Lanashield stain protection does not
cause yellowing of wool and is durable to shampooing.
Wool fibres have a naturally low surface energy, because the very thin
layer of covalently bonded lipids make the surface hydrophobic. This means
that wool fibres possess a natural water repellence, allowing time to remove
liquid spills before they can cause permanent staining. However, most oil-
based soils are difficult to remove; these tend to stick to the fibres mainly by
non-polar bonding, a predominantly hydrophobic interaction. Therefore both
water and oil-based soil-repellent treatment is essential for wool. Application
of a fluorochemical finish can reduce the surface energy of wool fibre and
further enhance water and oil-based stain resistance. Fluorochemical repellent
finishes for textiles are now available that provide improved durability to
washing and dry cleaning. The chemical structure of a fluorochemical is
typically based upon a polyacrylate. The perfluorinated side-chains of the
polyacrylate are oriented to point directly away from the fibre surface and
hence the —(CF
2
)
n
—CF
3
 terminal groups form a low-energy repellent surface
(Fig. 7.7).
 However, in commercial practice there are various monomers that
can be incorporated to produce a synergistic improvement for special end
use performance that may be required (Holme, 2003). There are a wide
variety of components that can be combined in the final active product
formulation, such as dimethyl siloxane for softness of handle or ethylene
oxide for hydrophilicity and stain-release properties. The durability of
fluorochemical finishes can be improved by incorporating a crosslinkable
monomer or other functional groups into the fluorochemical polymeric
‘backbone’.
CH
2
CH
2
CH
2
CH
2
CF
3
     CF
2  
n       
CH
2  2 
O   C    CH
O
CF
3
     CF
2  
n       
CH
2  2 
O   C    CH
O
CF
3
     CF
2  
n       
CH
2  2 
O   C    CH
O
7.7
 A fluorochemical polymer molecule.
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Advances in wool technology
160
In order to achieve dual stain-repellent and stain-release properties, unique
polymers for textile finishing have been developed by incorporating both
highly hydrophilic polymer segments and highly fluorinated polymer segments
within a single chain molecule, so-called ‘hybrid copolymers’. An example
of a successful hybrid copolymer based on a conventional fluorochemical-
based block and a hydrophilic block is shown in 
Fig. 7.8.
 This functions
effectively as a hydrophobic stain repellent in air under dry conditions and
also as an effective stain release during the laundering process. A ‘flip-flop’
mechanism of the dual stain-repellent and release properties has been suggested.
The hydrophilic blocks are shielded by the fluorocarbon segments when dry,
presenting a repellent surface. After immersion in the wash bath, the hydrophilic
blocks swell and reverse the interfacial characteristics of the surface, yielding
the hydrophilic surface necessary for oily soil release (Schindler and Hauser,
2004b). During drying, a re-orientation of the copolymer occurs and the
stain-repellent fluorinated polymer parts re-assert their dominance at the
fabric/air interface.
Perfluorooctane sulphonate (PFOS), used as a polymerisation aid in the
production of fluorinated resins and occurring in trace amounts as a by-
product of the manufacturing process, has, however, raised environmental
concerns. PFOS has been found to be a persistent organic pollutant and
bioaccumulates in wildlife and humans. The problem with different
perfluorochemicals, including a large number of PFOS-related compounds
and PFOA (perfluorooctanoid acid) compounds, is that the chemicals may
degrade in the environment to PFOS and PFOA respectively, but no further
degradation of PFOS or PFOA will occur. With increasing attention focused
on environmental issues relating to the sources and pathways of human
CH  CH
2
      or          N     CH
2  2
X is:
B is the bridging group
Y is:         CH
2
 CH
2
O  
n
      or        O   SiO  
n
      or        NH  C   O   CH
2
  CH
2
O 
n
  C    NH
CH
3
CH
3
O
O
R
F
R
F
O
O
C==O
C==O
—X—B—Y—B—X—
where R
F
 is:
CH
2 2
    CH
2
 
n
 CF
3
   or       CH
2 2
  N  SO
2
  CH
2
 
n
 CF
3
R
7.8
 Hybrid copolymer with fluorine-based hydrophobic and
hydrophilic blocks.
© 2009 Woodhead Publishing Limited

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Wool finishing and the development of novel finishes
161
exposure to PFOA, alternatives for PFOA-free stain-repellent polymers or
reformulated fluorochemicals have been developed in recent years.
Nanotechnology increasingly attracts worldwide attention and has potential
for application on wool textiles by provision of a range of novel properties,
such as water repellence, stain resistance and self-cleaning. The ‘lotus effect’
has been applied to textiles to enable self-cleaning by creation of a
superhydrophobic surface on fabrics. Wool has a natural hydrophobic surface
with a micro-scale of roughness due to the cuticle scales. Application of
nanoparticles onto the fibre surface can bring multi-level roughness to enhance
the hydrophobicity of wool fabrics. Nanotechnology approaches to create
superhydrophobic surfaces on wool have recently been reported (Zhang et