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CHEMICAL SOFTENING
7.9 SILICONES The use of silicones (polyorganosiloxanes) in
textile finishing has grown steadily in volume and variety [8188]
in the half-century since the preparation of the first commercial
silicones. Polydimethylsiloxanes (PDMS) of varying viscosity soon
became widespread in the textile and numerous other industries.
Their excellent lubricating properties and chemical inertness gave
finishers a truly new material to work with. The traditional PDMS
[89] varieties are still widely used but are being replaced by a
newer complex range of silicone derivatives producing diverse
effects, from elastomers to supersofteners. The physical
presentation of silicones has steadily improved along with the
chemistry. Early silicones based on dimethylsiloxane were
invariably manufactured as white milky emulsions with droplet sizes
as large as 300 m. These had to be used with great care due to poor
stability and the danger of silicone spotting on fabrics. Modern
silicone emulsions have excellent mechanical and chemical
stability. Whilst coarser emulsions have their place, the finisher
can now choose from highly stable mechanical emulsions with
particle sizes down to 1 m or less. Such fine emulsion particles
are of the same order as the wavelength of visible light so that
the products have an opalescent or translucent appearance. However,
developments have not stopped and several types of silicone are now
available as microemulsions with particle sizes down to 50 nm or
less. Microemulsions take on the appearance of a clear solution and
apart from superb stability they have performance advantages as a
direct result of the finer particle size. In particular, the
improved penetration into microfibre fabric gives better internal
lubrication and improved handle. On natural fibres, particularly
cotton and cellulosics where fibrillation can take place, a similar
mechanism operates to give perceptibly improved handle. Silicone
chemistry is special in that it bridges inorganic (SiO, HSiO) and
organic chemistry (SiCH3, CH3SiR) in the same molecule. This is
borne out by the partially ionic nature of the SiO bond. The
silicon atom is slightly positive in nature so that the pendent
methyl groups are much more stable than in an organic molecule. Not
surprisingly, silicones applied to textiles have a characteristic
silicone-feel, particularly when applied at higher levels. It can
be argued that organics have a corresponding fatty-feel for which
humans have a slight preference. Silicone usage in finishes has in
part been successful as a result of skilful formulation of
silicones with organic finishes so that a synergy is achieved. The
outstanding developments in silicone chemistry have to some extent
been concerned with making silicones more organic yet retaining the
useful properties of the silicone origin. This is exemplified by
the recent introduction of aminofunctional silicones, which impart
a much more acceptable type of handle. The desirable properties of
silicones result from several unusual physical properties some of
which are listed below [90,91]. Silicones used for textile
treatment are invariably based on the polydimethylsiloxane
backbone. The presence of the methyl group has a strong bearing on
the properties of the finish. The presence of the methyl
substituent as opposed to, for example, phenyl groups is largely
responsible for most of the attributes of silicones: (1) Highly
flexible backbone: the SiO bonds rotate freely, 0 kJ mol1 bond
rotation energy; films are extremely flexible and lubricative;
SILICONES
283
(2) Very low surface tension: the low surface tension of 21 mN
m1 means that the film has little affinity for most organics and
reduces fibre-to-fibre cohesion, maximising fabric bulk; it is also
water-repellent; (3) High bond strength: the SiO bond is very
strong, 445 kJ mol1, compared to carbon carbon at 346 kJ mol1 and
carbonoxygen at 358 kJ mol1 silicones are therefore resistant to
breakdown at high temperatures; (4) Low glass transition
temperature: films remain flexible from 90 C to +200 C without any
step-change in physical properties; (5) Very low vapour pressure:
products above 50 cst have no volatiles and no odour whatsoever and
almost no weight loss on heating; (6) Chemically inert: silicones
are resistant to strong oxidative attack and only show weakness
under strong alkaline conditions at high temperature; they are also
resistant to UV and IR radiation and are physiologically benign;
(7) Highly compressible: treatments have positive effects on fibre,
yarn and fabric modulus; (8) Permeable to nitrogen and oxygen: this
is associated with comfort in wear; (9) Temperature-stable physical
properties: refractive index, surface tension, density and
viscosity change little with temperature; (10) High dielectric
stability and strength: widely used outside the textile industry in
electrical applications; (11) Highly surface-active: very good
spreading and film-forming properties on fibre surface; alkoxylated
silicones are excellent wetting agents. These factors undoubtedly
contribute to the desirable effects imparted by silicones but the
precise mechanism relating to softness and handle is not fully
understood.
7.9.1 Polydimethylsiloxanes Although the first siliconorganic
compounds were prepared in 1863, it was military demands in the
1940s that resulted in the large-scale introduction of
dimethylsiloxane polymers. Textile applications developed in the
1950s, where low-viscosity silicones were used as water-repellents.
It was known that high-viscosity silicone fluids imparted softness
but they were not economical until the development of emulsion
polymerisation in the 1960s. High viscosity silicone finishes were
used initially on synthetic fibres but soon became widespread
either alone or in formulations with organics. Structure 7.24 shows
the most basic type silicone used in textile finishing
polydimethylsiloxane (PDMS). Polydimethylsiloxanes are marketed as
white opaque emulsions containing up to 60% silicone oil. PDMS oils
can be low-viscosity, volatile materials at less than 1 cst to
greases with a viscosity of 1 000 000 cst. To produce softening,
the polymer viscosity must be over 10 000 cst. The higher viscosity
polymers are more difficult to formulate but give increased
softening effect so that 100 000 cst materials represent a good
compromise [92,93]. PDMS-based products have several
284
CHEMICAL SOFTENING
Structure of polydimethylsiloxane (PDMS)
CH3 CH3 Si CH3
CH3 O Si CH3
CH3 O Si CH3n
CH3
n can range from 0 to 2500
7.24
uses as finishes from simple winding aids, components in sewing
thread lubricant formulations and sewing lubricants for synthetics,
to stand-alone softeners. Delivery onto the fabric can be
problematic unless care is taken. Emulsions can break down under
the shear of the padding process, giving build-up on machinery and
silicone spots on fabrics that are difficult to remove. If the
correct formulation is selected and the use conditions are within
the design limits, excellent results can be achieved. For exhaust
application, cationic emulsions are available. The cationic
emulsifiers can impart a degree of softening. To some extent the
exhaust process is aided by dilution of the emulsion as it is added
to the treatment bath. The reduced concentration of emulsifier
results in instability of the oil droplets and a tendency to
deposit more easily onto the fabric. Build-up of silicone on the
insides of winches and jets is largely avoidable by choice of
product and good working practices. Heating the exhaust bath to
temperature before the silicone emulsion is added will reduce the
risk of the emulsion splitting at the heat exchanger. Pre-dilution
of the emulsion and slow addition to the bath from a side tank will
also help. Polydimethylsiloxane is very effective on synthetic
fibres where the surface area is low. Deposition of very small
quantities, as low as 0.1% of active softener, can eliminate fusing
of polyester during cutting and high-speed sewing and produces a
very noticeable improvement in handle and elasticity. Cotton fibre
requires larger amounts of silicone to achieve a particular effect,
due to its high specific surface. Nonetheless, silicones are
commonly used on cotton and cotton blends to good effect. PDMS
emulsions are particularly useful for application with easycare
resins. The non-ionic emulsions have excellent stability in the
high electrolyte concentrations and low pH of the resin bath. This
type of silicone is extremely stable under the severe conditions
required to cure crease-resist resins, typically 12 min at 180 C.
The effect of PDMS in an easy-care finish is to give remarkable
improvements in crease recovery, tear strength and softness, as
well as abrasion resistance and similar wear parameters. Typical
results obtained on 100% cotton woven fabric are given in Table
7.11. Silicones, and PDMS in particular, can be beneficial to the
colour of textiles. The smooth silicone film on the fabric surface
produces a brightening effect in the same manner as varnish on
wood. The effect is particularly noticeable on polyester, which has
a high refractive index compared to other fibres. Light arriving at
the surface of a material with a high refractive index is absorbed
less than by a low refractive index material. A lower portion of
the light is therefore absorbed by the dye inside the fibre. The
silicone film, which has a low refractive index, will
SILICONES
285
Table 7.11 Improvements in easy care properties of 100% cotton
woven fabric by addition of PDMS to resin system Resin only Crease
recovery angle warp plus weft Elmendorf tear strength warp way (kg)
Softness rating (1 to 14, where 1 is the best) 210 0.5 9 Resin +
PDMS 240 0.75 5
therefore couple more light into the fibre. This effect is
particularly useful for dyeing polyester microfibres where it helps
to obtain a deep black colour. Although this is not a softening
effect, the improved visual effect enhances the obvious handle
improvement. It is known by formulators that a small portion of
hydrophobic silicone included in an organic softener formulation
will give a small but useful reduction of hydrophobicity. The
effect is associated with an improved rate of water wicking into
the fabric and is particularly noticeable with the more polar
anionic softeners used for pad application. This improvement is
associated with greater comfort and better performance in the case
of towelling. Additions greater than about 2% silicone in a typical
anionic softener become ineffective [94,95]. Polydimethylsiloxanes
are not permanent on textiles but the higher-molecular-weight
materials in particular have a useful degree of durability on most
fibres.
7.9.2 Reactive polydimethylsiloxanes For waterproofing
applications the durability of PDMS is limited. Reactive
polydimethylsiloxanes were developed in the early 1970s in order to
produce more permanent finishes. In this case a proportion of the
methyl groups in the chain are replaced with hydrogen or a terminal
methyl is replaced with a silanol (Structures 7.25 and 7.26)
[96,135].Hydrogen siloxane Terminal silanol
CH3 CH3 Si CH3
CH3 O Si H 7.25
CH3 O Sin CH3
CH3 CH3 CH3
CH3 CH3 7.26
CH3 O SinCH3
HO Si O Si
OH
These first generation reactive silicones provide a good
permanent soft finish to many types of fabric and are important
additives to permanent press finishing formulations, water- and
showerproof finishes, machine wash wool treatments and as general
softeners and elastomers. Although the permanency on cotton can be
satisfactory, the repeated swelling by hydration and dehydration
during laundering weakens the attachment. They are generally used
with metallic catalysts. By altering the proportions of hydrogen
silane to silanol, several different finishing effects are
possible. A typical metallic catalyst is
286
CHEMICAL SOFTENING
tetrabutyl titanate, which has a secondary advantage of
improving the bonding to the textile substrate. The more elastic
effect is the result of cross-linking to form a three-dimensional
matrix. The silicone can be supplied as a silane and is readily
hydrolysed to give pendent hydroxy groups, which condense with the
aid of the organometallic catalyst to give an elastic gel matrix
with enhanced permanency. More effective results require
short-chain tri-functional crosslinkers to produce a highly
crosslinked rubber-like polymer with excellent elasticity (Scheme
7.10). This is particularly useful on knit goods in which
dimensional stability and snapback represent added value.
Trifunctional crosslinkers are typically hydrosilane (SiH), acyloxy
(SiOCOCH3) or ester (SiO2R). These can link three silicone
molecules to form a highly crosslinked gel network
[9799].Crosslinking of reactive silicones
H Si CH3 O CH3 SiScheme 7.10
OH O + H 2OCatalyst
Si CH3
O
+ H2
O OH + HO Si CH3
O
O CH3 + H2O
CH3 Si O Si
The epoxy functional silicones comprise an important subset in
the reactive polydimethylsiloxane range. These have the capability
to react with the hydroxy groups on cotton and cellulosics to give
permanent effects. Epoxy silicones (Structure 7.27) enhance
dimensional stability and can be better softeners than PDMS. They
are durable to many domestic laundering cycles and have very low
yellowing properties. Compatibility with easy-care resin baths is
very good and they are thermally stable at the cure temperatures.
Their permanency makes them ideal for this purpose. Unfortunately,
the softness attainable is well below the amino silicones.Epoxy
reactive polydimethylsiloxanes
CH3 (CH3)3Si O Si CH3 On
CH3 Si O Si(CH3)3m
(CH2)2 O CH2 CH CH2 O
7.27
7.9.3 Organo modified polydimethylsiloxanes The introduction of
side-groups other than hydrogen and hydroxy can result in dramatic
changes in properties. Some interesting effects can be produced for
example, polydimethylsiloxanes with long alkyl groups can result in
lubricants with glide properties for application to recording tapes
(Structure 7.28). Other uses are scratch resistant finishes and
fibre lubricants.
SILICONES Polydimethylsiloxane with long alkyl chains as super
lubricants
287
CH3 CnH2n+1 Si CH3 O
CH3 Si CH3 7.28 O
CH3 Sim
O CnH2n+1Alkyl chain e.g. n=18
CH3
Structure of n-alkyl ethylene oxide propylene oxide
polydimethylsiloxane
CH3 (CH3)3Si O Si CH3 Om
CH3 Si R
CH3 O Sin
O Si(CH3)3 R
R (CH2)2 O(C2H4O)(C3H6O) EO PO
R = C8 to C18 alkyl
R = H or alkyl
7.29
The n-alkyl polyethers with terminal alkyl groups are
particularly useful. Many such products have uses outside the
interest of finishers such as wetting agents and emulsifiers with
HLB values that can vary greatly. The low physiological activity
makes such emulsifiers particularly suitable for lotions and
sunscreens. Similarly, they are used in finish formulations
[100107]. In the compound shown in Structure 7.29, the variables m
and n and the alkyl chain length can be varied to produce
emulsifiers, demulsifiers, stabilisers, and lubricants. It is also
possible to introduce ionic end groups to PDMS substituents
[104,105]. This can be done in many ways, one of which is to start
with the addition of allyl glycidyl ether to a silane site on the
PDMS. This epoxy reactive intermediate product can then be
transformed into a range of anionic, cationic and amphoteric
materials with unique properties. The cationic compounds are of
particular interest to the finish formulator for preparation of
pseudo ionic delivery systems. They have much higher substantivity
for textile substrates than fully organic materials and form more
tenacious films. The overall preparation of ionic
organopolysiloxanes is shown in Scheme 7.11. These examples
illustrate the symphonic complexity of silicone chemistry, which
has a largely untapped potential to produce many interesting
products for textile finishing.
7.9.4 Aminofunctional siloxanes Aminofunctional siloxanes
represent the most active growth area in finishes since the 1980s.
Products in this category include microemulsions and
supersofteners. Their usefulness extends to every conceivable type
of textile substrate, reflecting the diversity of properties and
their pleasing tactile effects. They represent the best balance
between organic and silicone chemistry yet produced and bring out
the best characteristics of both. The basic building block is
288
CHEMICAL SOFTENING Preparation of ionic organopolysiloxanes
CH3 Si O Si O CH3n
CH3 Si Om
(CH2)2 O CH2 CH CH2 O O CH2 CH CH2 N(R)3 + CH3CO2Cationic
(a)
(b) Scheme 7.11
O CH2 CH CH2 N(R)2 OH CH2CO2Amphoteric
Aminofunctional siloxane structure
CH3 R Si CH3 O
CH3 Si CH3 Om
R' Si O (CH2)3 HN 7.30n
CH3 Si CH3 H H O R R and R' = CH3 OH OCH3
(CH2)2 N
polydimethylsiloxane with aminoethylaminopropyl side-groups.
Structure 7.30 [74] is by far the most common base for amino
silicone finishes [108,109]. This structure lends itself to wide
variation. The PDMS fraction, m, is usually in the range 50 to 2000
units. The amino-containing portion is usually in the range 1 to 20
units. Terminal groups on the PDMS can be CH3, OH or OCH3.
Aminofunctional siloxanes as such lie within these bounds. The
outstanding softening properties of the aminofunctional siloxanes
can in part be explained by the presence of the amine
functionality. The polymer is cationic and is therefore strongly
oriented at the fibre surface. This more ordered deposition coupled
with the characteristics of the silicone leads to supersoftening
effects. This type of handle is not defined however, it is
recognised as having the softness of the most effective cationics
but with added bulk and resilience. Although these products can be
applied from mild alkaline baths, they are normally applied at pH 6
or below. If the acidity is reduced towards pH 4 the amino groups
become more strongly positively charged and the rate of exhaustion
increases. The adhesion to the fibre also increases giving good
durability. Aminofunctional polysiloxanes can be applied by pad or
exhaust methods. They are usually supplied as microemulsions and
therefore have excellent stability to shear, temperature and water
hardness. They also have good compatibility with easy-care
resin/catalyst baths. Microemulsions have a much lower tendency to
deposit on machine parts and shear breakdown on padding rollers is
negligible. Typical application levels of aminofunctional siloxanes
range from 0.25% to 1% based on the weight of fabric.
SILICONES
289
Microemulsions of aminofunctional polysiloxanes are particularly
effective on microfibre [110]. The fine particle size allows the
emulsion to penetrate into the closely packed yarn structure giving
internal lubrication and softening [111113]. This is illustrated in
Figure 7.7, where a small section of a 76 dtex 144 filament yarn is
shown. Each filament is 0.53 dtex and 7 m in diameter.100 nm
microemulsion
1 m diameter mechanical emulsion
144 filament microfibre yarn
5 m diameter mechanical emulsion
Figure 7.7 Microemulsion penetration into 76 dtex 144 filament
microfibre polyester yarn
The improved distribution on the true fibre surface results in
easily demonstrated improvements in the properties of microfibre
fabrics [114]. Microfibres are found in diverse markets and
aminofunctional silicones can be selected which optimise the often
conflicting requirements of different end-uses (Table 7.12).
Microfibre yarns contain many filaments and consequently have a
much larger surface area than conventional yarns. Filament denier
ranges from 1 denier to less than 0.5 denier for normal fabrics,
but very fine fibres as fine as 0.01 denier are used in special
non-wovens [114118]. Abrasion resistance and tear strength are
improved by epoxy reactive silicones. Wrinkle recovery is optimised
with low amino content silicones in combination with more strongly
crosslinking types. Softness is undoubtedly optimised with high
amino content products but yellowing and water control may be
compromised. Absorbency can be maximised with polyether modified
silicones, whereas water-repellency is improved with strongly
reactive silicones [119123]. Aminofunctional polysiloxanes, which
produce the greatest softening effects, have a tendency to cause
yellowing as the amino content increases [93,94,98,108]. Good
whiteness can be achieved by careful selection of a polymer with a
maximum amine value consistent with an acceptable degree of
yellowing. The yellowing behaviour is a result of the nitrogens
ability to form coloured azo and azoxy compounds. The presence of
metal catalysts and prolonged exposure to high
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CHEMICAL SOFTENING
Table 7.12 Finishing demands on microfibre fabrics End-use
Outerwear Garment types Slacks, dresses, shirts, blouses, skirts,
jackets Finish requirements Supersoft, excellent drape, easy-care,
light-weight, dimensional stability, comfort Water-repellent,
airpermeable, wind-tight, light-weight, soft, easy-care,
dimensional stability Softness, dye fastness, light fastness,
drape, easy-care, low soil, low fogging
Sportswear
Raincoats, anoraks, ski jackets sailing wear track suits, sweat
suits Sleeping bags, tents, shades, workwear, filters, car
upholstery
Technical
temperature are the major factors. Aminofunctional silicones for
use with easy-care resins should therefore be selected with care.
For exhaust application followed by a normal drying regime,
particularly on coloureds, a full supersoft finish can be easily
achieved. On brilliant white fabrics there is a possibility of
high-amine-value, strongly cationic finishes causing yellowing of
the fluorescent whiteners, particularly at very low pH. The
relationship between amine value, aminoethylaminopropyl content and
yellowing is shown in Figure 7.8, for typical aminofunctional
siloxanes. The amine value is determined by titration and is quoted
as millilitres of molar hydrochloric acid required to neutralise
the amine contained in one gram of silicone polymer.
0.8
Amine value
0.6
Moderate yellowing
0.4
Low yellowing
0.2
Very good whiteness
0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Aminoethylaminopropyl function/mol %
Figure 7.8 Relationship between amine value and
aminoethylaminopropyl content and tendency to cause yellowing
