Introduction
KEEPING the colors, design and price of a garment or fabricaside, what ultimately a customer generally considers to choose a particulartextile product in a retail shop is the handle and appearance of a garment.Practically everyone who examines a textile automatically touches it withtheir fingers to get an impression of the hand. Hence, almost all apparel andhome furnishing textiles are treated with softeners. Only a few specialtyfabrics do not receive a softener finish, consequently, it is easier to statewhich fabrics are not softened. These include wall coverings, carpeting andmost industrial textiles. Therefore, softening of textiles becomes animportant finishing process of many after treatment processes in a textilechemical processing industry. The hand of a fabric is a subjective sensationfelt by the skin when a textile fabric is touched with the finger tips andgently compressed. e perceived softness of a textile is the combination ofseveral measurable physical phenomena such as elasticity, compressibility andsmoothness.
Almost all the natural fibres, by providence arrangement,have some percentage of wax which makes fibre naturally soft, the classicalexample is cotton, the most widely used fibre. However, the presence of waxboth on the surface and on the bulk of fibre makes it resistant for wetting.Unfortunately, the lack of water absorbency makes the fibres unsuitable fordyeing and printing which are the primary objectives of a textile processingunit. Therefore, in order to make the fibre suitable for dyeing, variouspreparatory processes such as desizing, scouring, bleaching, etc. are carriedout, which actually remove the natural softening agents to make the fibres moreabsorbent. Therefore, generally after dyeing and printing the fabrics becomeharsh and stiff. Finishing with softeners can overcome this deficiency and evenimprove on the original suppleness. The softening treatments impart softhandle (supple, pliant, sleek and fluffy), smoothness and enhance flexibility,drape and pliability. Other properties improved by softeners include thefeeling of added fullness, antistatic properties and sew ability.
With chemical softeners, textiles can achieve an agreeable,soft hand and some smoothness. However, the disadvantages sometimes seen withchemical softeners include reduced crockfastness, yellowing of white goods,changes in hue of dyed goods and fabric structure slippage. Most softenersconsist of molecules with both a hydrophobic and a hydrophilic part. Therefore,they can be classified as surfactants (surface active agents) and are to befound concentrated at the fibre surfaces 1. Most softeners have lowwater solubility. Therefore softening products are usually sold as oil in wateremulsions containing 20-30% solids. The softener molecules typically contain along alkyl group, sometimes branched, of more than 16 and up to 22 carbonatoms, but most have 18 corresponding to the stearyl residue. Exceptions tothis molecular structure are the special categories of silicones, paraffins andpolyethylene softeners. About one-third of the softeners used in the textileindustry are silicone based as it imparts excellent soft hand combined withvarious other properties such as water repellency, superior smoothness, greasyfeel, excellent body, improved crease resistance, etc. The silicones wereactually first utilized by the textile industry primarily as lubricants infibre and fabric manufacture. Silicone softeners are also applied with permanentpress finishes to improve garment wear life and permanent press finishdurability 2. It can also be used with other finishing agents formultifunctional finishes, for example, it can be used in resin finishing oftextiles to have a soft wrinkle resistant fabric. Recently, by AmericosIndustries, silicone softeners are also formulated with special polymers toimpart a unique leather soft finish. This article, therefore, discusses thefundamental principles behind silicone finishing, various developments insilicones and their corresponding textileapplications. This paper includes the contribution from Americos in the fieldof silicone finishing of textiles.
Conclusion
Silicone finishing is becoming increasingly important in
textiles as it imparts a very unique soft handle with supple, pliant, sleek and
fluffy effect. It also enhances smoothness, flexibility, drape and pliability
of the fabric greatly. Manipulation of the basic silicone chemistry has
resulted in multifunctional finishes that not only impart the unique soft hand
of silicone but also the other essential properties such as crease resistance,
wrinkle resistance, leather soft effect, durability etc. Silicone
nanoemulsions, as it shows improved penetrability into the textile structures
produces favorable unusual soft hand and other properties that are obtained
with micro and macro emulsions. Americos has particularly shown its excellence
in producing the silicone nanoemulsions and special silicone softeners for
multifunctional finishes with its state-of-the-art manufacturing technology.
References
[1] S. D. Wolfgang, H. J. Peter J. "Chemical Finishing of Textiles", (2004) Woodhead Publishing.
[2] Welch, "Catalysts and processes for formaldehyde-free durable press finishing of cotton textiles with polycarboxylic acids" April 11, (1989) US patent 4,820,307.
[3] A.A. Vaidya, S.S. Trivedi, "Textile auxiliaries and finishing chemicals", (1975), R.C.Vora.
[4] M. Lewin, S. B.Sello, "Handbook of Fibre Science and Technology: Volume II, Chemical Processing of Fibres and Fabrics, Functional Finishes, Part B", (1984) CRC Press.
[5] J. G. Richard, C. Julian, A. Wataru, "Silicon-Containing Polymers: The Science and Technology of Their Synthesis and Applications", Published 2000 Springer.
[6] M. Koch, Pakistan Textile Journal, 55(5) (2006)59.
[7] L. Muncheul, N. Kenji, J. D. Seok, T. Takako, I. Toshihiko, M. Yukiino, W. Tomiji. Sen'i Gakkaishi 61(11) (2005) 309.
[8] X. Yang, C. Grosjean, Y. C. T ai, C. M. Ho, Sensors and Actuators A: Physical, 64(1) (1998) 101.
[9] Kendall, "Restructuring silicone rubber to produce fluid or grease", July 15, (1997) US Patent 5,648,419.
[10]Burrill, "Detergent foam control agents containing a silicone antifoam and a fatty alcohol", February 21, (1989) US Patent 4,806,266.
[11]Yang, "Stable emulsions containing amino polysiloxanes and silanes for treating fibers and fabrics", March 19, (1991) US Patent 5,000,861.
[12] K.O. Jang, M. Yeh, Textile Research Journal, 63(10) (1993)557.
[13] G.c. Johnson, S. Sterman, N.Y. Williamsville, "Use of modified epoxy silicones in treatment of textile fabrics", May 12, (1970), US Patent 3,511,699.
[14] Kimura, "Process for preparing epoxy-modified silicone resins", October 12, (1982), US Patent 4,354,013.
[15] Scheper, "Treatment of fabric articles with specific fabric care actives", May 11, (2004), US Patent 6,734,153.
[16] Friesen, "Siloxane-grafted membranes" October 31, (1989), US Patent 4,877,528.
[17] A. Ravve, "Principles of Polymer Chemistry", (2000) Springer.
[18] B. Berman, F. Flores, Dermatologic Surgery, 25 (6) (1999) 484.
[19] Pines, "Treatment of textile fabrics with epoxypolyoxyalkylene modified organosilicones", January 15, (1980), US Patent 4,184,004.
[20] C. M. Carr, "Chemistry of the Textiles Industry", (1995) Springer.
[21] S. Vijaya "Finishing of silicones", Dye chemical Pharmaceutical business news, special article, December (2005) 45.
[22] Glover, "High polymer content silicone emulsions", April 25, (1989), US Patent 4,824,877.
[23] T. Okada, Y. Ikada, Makromolekulare Chemie 192(8) (1991) 1705.
[24] Mueller, "Reactive silicone and/or fluorine containing hydrophilic prepolymers and polymers thereof", January 7, (1992), US Patent 5,079,319.
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Silicones
In the science of silicone finishing, a fundamental difference between silicon and silicone should be noted, in that silicon is an atom that lies below carbon atom in the periodic table, while the term silicone refers to artificial polymers based on a framework of altering silicon and oxygen (siloxane bonds). The electronic configuration of silicon is 1S2,2S2,2P6,3S2,3P2. The larger atomic radius of silicon atom makes the silicon-silicon single bond much less energetic due to which the silanes (SinH2n + 1) are much less stable than alkanes. The opposite, however, is true of silicon-oxygen bonds that are more energetic (about 22 Kcal/ mole) than the carbon oxygen bonds. Polysiloxanes, therefore, have recurring Si-O linkages in the backbones.
Why not silanes but polysiloxanes?
Generally, the silicone treatment of textile consists of treating them with silicone polymer emulsions but not with the silane monomers which may also impart the softness and water repellent characteristics. The reason being is that the silane monomers, during treatment, liberate hazardous chemicals, for instance treatment of textiles with chloromethylsilanes liberates hydrochloric acid 3. Attempts have been made to improve the treatment of textiles with silane monomers, using of ammonia to absorb the hydrochloric acid in the case of chloromethylsilanes. The attempts which have been patented are replacement of chlorine with acetoxy, alkoxy, amino, or isocyanate groups 4. However, none of these has resulted in a practical process. Therefore, silanes are not used as such for textile finishing but are converted to polysiloxanes, which can be applied to textiles as solutions in organic solvents or as aqueous emulsions.
Polysiloxanes
The chemistry and technology of polysiloxanes have been the interest of many researchers and manufacturers as it finds very wide applications 5, the silicones used for textile applications are polymers with a -O-Si-O- backbone. According to proper chemical nomenclature, these polymers are polysiloxanes
Engineering of silicone oxygen of siloxane bonds (fig. 1) with organic substitutes results in various kinds silicone polymers 6. Accordingly, the substituents R can be a hydrogen, hydroxyl, alkyl, aryl, or alkoxyl group. The substituents in the polymer chain can be all of the same kind or can be different. However, for textile applications, R is usually either methyl or hydrogen groups and are the most important of the organic substituents used in commercial silicones, the vast majorities of which are polydimethylsiloxanes (PMDS). Polysiloxane is a mixture of inorganic and organic substances. Because of their "inorganic-organic" structure and the flexibility of the siloxane bonds, silicones have the following unique properties,
» Thermal/ oxidative stability,
» Low temperature flow ability,
» Low viscosity change with temperature,
» High compressibility,
» Low surface tension (spreadability), and
» Low fire hazard
They also have good electrical insulating (dielectric properties) characteristics and water repellency (hydrophobicity) which are maintained over a wide range of temperatures 7. As a result of having combined unique properties, these semi-inorganic materials i.e. polysiloxanes become industrially important and find applications in many diverse markets, such as aerospace, automotive, construction, electrical, electronics, medical materials, performance chemicals and coatings, personal care and textiles 8. Fluids and greases, emulsions, rubber products and resins are some of the materials based on silicone technology 9.
Textile applications
Silicones have broad utility in textile processing and finishing; most of the products for this industry are based on PMDS technology. The applications for silicones vary widely and include antifoam for fabric and carpet dyeing, print paste softeners, fabric finishes and coatings 10. In fabric processing, silicone antifoams are often used to maximize the efficiency of the scouring baths, washing/dyeing and bleaching options. They serve as fibre lubricants for spinning, winding and slashing. Various types of silicones are commonly used as softeners, wetting agents and water repellents. In sewing operations, silicone thread lubricants are essential to meet the demands of industrial high-speed sewing machinery. Silicones also have many uses in nonwoven applications such as binders, additives for wet-laid processes.
Some silicones are supplied as neat fluids, while others are in the form of emulsions or room temperature curing elastomers. PMDS is extremely versatile, and can be modified to formulate a wide range of products with tailored hydrophobicity and durability, used to modify the feel and appearance of fabrics, or to improve processing.
Type: durable and nondurable / reactivity
Aminosilicones are for durable finish. Polydimethylsiloxane is a non durable finish 11. Polydimethylsiloxane with its terminal reactive hydroxyl groups is a conventional, may be semi-durable finish. Polymethylhydrogensiloxanes acts as a reactive as well as cross-linking agent and hence is responsible for producing durable finish with the blend of polydimethylsiloxane. Aminofunctional silicones and other organo-functional silicones contain groups such as amino, substituted amino, epoxide, or alcohol groups attached to the polymer backbone. Therefore, they offer durable, soft and lively hand and a slight increase in wrinkle recovery and flat appearance. The presence of silane coupling also plays a major role in enhancing the durability 12.
Silicone softeners include both polydimethylsiloxane polymers as well as a wide range of organo-modified polydimethylsiloxanes. Performance enhancing additives and finishes based on PMDS technology can be nonreactive (Fig. 2a), conventional reactive (Fig. 2b) or the organo-functional materials (Fig. 2c). With these silicones, one of the advantage to the processors is that the ability to derive mort~ than one benefit from a single product.
Organo-modified polydimethylsiloxanes, particularly epoxy modified, were found to offer a significant improvement over conventional unreactive silicones 13, 14. The improvement was in terms of both a greater degree of softening and good durability of polymers to laundering. Amino-functional polydimethylsiloxanes softeners were found to have the same advantages as reactive silicones. Two additional benefits were found with aminofunctional silicone softeners. Knit fabrics became more elastic, with better stretch recovery.
. .
The softener also additionally delivered antistatic benefits and wrinkling resistance 15. These two benefits and the fact that the amino functional silicone are readily adsorbed from dilute solutions onto cotton fabrics in conjunction with traditional cationic softeners lead to their use in rinse cycle softeners in the middle to late 1980s. One of the features shared by many silicone materials is effectiveness at very low concentrations. Very small amounts (0.1 to 1.0% by weight) are usually required to achieve the desired properties, which can improve the cost efficiency of textile operations and help ensure a minimum of environmental impact.
Therefore, in spite of high cost, amine silicones did bring a consumer-perceptible new dimension to rinse cycle fabric softeners.
The reactivity of polydimethylsiloxanes can be increased by mixing with polymethylhydrogensiloxanes. The Si-H bond is hydrolyzed to -Si-OH, which can condense with another Si-OH group or a -Si-H group and forms cross links. However, hydrolysis produces hydrogen, which may create a fire hazard and a storage problem.
The Si-H bond hydrolyzes rapidly in an alkaline or strongly acidic medium but can be stabilized with certain organic additives in an aqueous medium buffered at pH 3-4. Oxidation of -Si-H groups by atmospheric oxygen or oxidizing agents can produce -SiOH groups, which can also contribute to eventual cross-linking of the finish on the fabric.
Polymethylhydrogensiloxanes produce a hard brittle film on fibres and the finish has a harsh handle. They are, therefore, not used alone, but in admixture with polydimethylsiloxanes, which act as plasticizers and improve the handle of the finished fabric. However polymethylhydrogen siloxanes can produce a highly water-repellent finish with a soft handle when crosslinked on cotton in the presence of organic peroxides.
The water repellency of "silicones" on synthetic
fabrics, especially those made of filament fibres, is fairly resistant to
laundering and dry-cleaning with pure solvents. The loss of water repellency
during dry cleaning is caused mainly by adsorption of hydrophilic substances,
such as detergents, and to a lesser extent by dissolution of the silicone
finish in the solvent. The durability of silicone finishes on cellulosic
fabric is impaired by swelling of cotton fibres during laundering. The
expansion of the fibres ruptures the silicone film essential for water repellency.
Since the repellent polysiloxane film does not melt and flow, the cracks in the
ruptured film cannot be sealed and the initial repellency restored by heating.
Although attempts have been made to form a covalent bond between the
polysiloxane and cellulose fibres, the -Si-O-CELL bond is yet not stable to
hydrolysis. Apart from the type of reactive groups, the viscosity and the
adsorption mechanism of the softener, as well as treatment conditions such as
curing temperature, are crucial factors affecting the performance properties
of the treated fabrics 12.
Method of preparation of silicones
The hydrophobicity of silicones was discovered at General Electric by Patnode, who observed that paper treated with chloromethylsilanes become water repellent when exposed to moist air. Preparations of such substituted chlorosilanes Rn - SiCL4-n where R is usually a methyl or phenyl group and n = 0, 1, 2, or 3 is the first step in the manufacture of silicones. There are two methods available for the preparation of chlorosilanes.
In the first, direct process, the starting materials can be prepared through hydrolyses of alkyl or arylsilicone halides. Organosilicone halides, in turn, are made commercially by heating alkyl or aryl halides with silicone at 250 to 289C (Fig. 3a) 17. Copper catalyzes this reaction. The second method is the Grignard method (Fig. 3b) in which a given chlorosilanes or a mixture is reacted with methyl or phenyl magnesium halide to yield the desired more highly alkylated or arylated chlorosilanes.
Neither of the above methods of synthesis yields any given chlorosilanes free from the other members of its series. For example chloromethylsilanes are hydrolyzed by water to silanols (Fig. 4), which condense spontaneously to siloxanes. Chlorotrimethylsilane yields hexamethyldisiloxane.
Dichlorodimethylsilane yields, depending on reaction conditions, 20 - 50% cyclic siloxanes and 80 - 50% linear polydimethylsiloxanes. Trichloromethylsilane yields cross-linked polymethylsiloxanes. Condensation reactions can occur between -SiOH and -SiH groups, if present, and between two -SiOH groups. In the presence of peroxides or upon irradiation, two -SiCH3 groups can also undergo a condensation reaction. The hydrolysis and condensation reactions of chloromethylsilanes or chlorohydrogensilanes are, therefore, more complex than those shown above and hence the separation of the reaction products requires highly efficient fractional distillation columns - a difficult processing step in the manufacture of silicone
The polymerized silicones are prepared by hydrolyzing a
known mixture of pure substituted chlorosilanes with water, washing the
hydrolysate free from hydrochloric acid and further polymerizing the neutral
hydrolysate to yield the desired product. The silicone fluids are prepared from
mono - and difunctional chlorosilanes so as to get end-blocked linear polymers.
Silicone: for softening
As the uniqueness of silicone finishing has been established in the previous sections, this section expands further on silicones as softeners. As the soft touch, supple feel and appearance, etc. are of significantly important for a customer, softening of textiles has become an important finishing operation. Silicone emulsions, especially, are capable of bestowing significant benefits in this regard. Therefore, silicone softeners are becoming extremely important because of their very good softness and greater wash permanence compared to other softeners. The mechanism of softening by silicone treatment is due to flexible film formation. The reduced energy required for bond rotation makes the siloxane backbone more flexible. Therefore, polysiloxanes form a flexible film on fibres and yarns and hence reduces the interfibre and inter yarn friction. Thus the silicone finishing of textile produce an exceptional soft handle combined with other properties such as superior smoothness, greasy feel, excellent body, improved crease resistance, etc.
The silicone molecules can produce a wide range of hand
variations, from dry to oily to resilient, and are also used for such purposes.
The extent of the effect depends on the degree to which the molecules are cross
linked. The responsibility from the finisher side is to select the appropriate
silicone softener from the vast range available in the textile auxiliaries
sector. Silicones particularly as softener are expected to confer the
following.
» Soft supple hand
» Improved sewability
» Improved tear strength
» Improved crocking fastness
» Outstanding hydrophobic or hydrophilic properties
» Higher crease recovery angle
» Improved wash permanence
» Very good anti-pilling properties
» Antistatic properties
» High
effectiveness and process stability
Silicone softeners include both polydimethylsiloxane polymers as well as a wide range of organo-modified pol ydimethylsiloxanes. Polydimethylsiloxanes, polymethylhydrogen-siloxanes or blend of these two fluids are generally used as softeners. Silicone softeners are also applied with permanent press finishes to improve garment wear life and permanent press finish durability 5. Organo-modified polydimethylsiloxanes, particularly epoxy modified, were found to offer a significant improvement over conventional unreactive silicones 19. The improvement was in terms of both a greater degree of softening and good durability of polymers to laundering.
Of the silicone softeners available, perhaps the most
common in current industrial usage and likely to be the best is the amino
functional silicone softeners (Fig. 5) 20. These materials offer a
range of handles depending on the relative size of x and the ratio of x:y. They
may be supplied as surfactantstabilized emulsions in water, either mechanical
or microemulsions. Mechanical emulsions contain large droplets which tend to
coalesce on the fabric, giving surface effects. The microemulsions, of much
smaller droplet size, will tend to migrate into the yarn and give an overall
softness to the whole structure. The aminosilicones may give a relatively dry
handle where the x:y ratio is high, and a typically greasy handle where the x:y
ratio is low.
Aminofunctional
silicone fluids are much more effective at imparting hand properties than
either methyl oils or silicones with carboxyl or epoxy functions. This
exceptional property stems from the fact that the partly protonated amino
groups of the softener molecule interact with the negatively charged cotton
fibres. Hence, the amino functional silicones are readily adsorbed from dilute
solutions onto cotton fabrics in conjunction with traditional cationic organic
softeners lead to their use in rinse cycle softeners. Two additional benefits
were found with aminofunctional silicone softeners. Knit fabrics became more
elastic, with better stretch recovery. The softener also additionally delivered
antistatic benefits and wrinkling resistance 9. All these fabric
property improvements exhibited durability to repeated launderings. In addition
the ability to blend these aminofunctional silicones with organic softeners and
retain performance properties allow the creation of softener blends with
optimum cost - performance parameters. Thus a polyester cotton blend fabric
showed improvement in durable press rating, wrinkle recovery and tear strength
even when half the amino functional silicone softener was replaced by an organic
softener, which however caused a loss in stretch/ recovery performance of
cotton knit fabric after five washes.
Microemulsions are generally aminosilicones. Epoxy silicones can also be used as microemulsions but softness is not as good as amino-micro silicone emulsions. Microsilicone emulsions give a permanent feel to the fabric with a high degree of softness 21. Amino functional silicones have shown good softening ability. Increased tear strength, greater abrasion resistance and improved wrinkle recovery are seen on polyester cotton blends and 100% cotton woven fabrics treated with aminofunctional silicones along with a durable press resin.
Silicones for multifunctional finish
Since the curing conditions of the polysiloxanes are similar to durable press cross-linking treatments with methylol compounds, polysiloxanes can be applied with a durable press finish. The durable press resins enhance the durability of the "silicone" finish. Wrinkle free finishes are renowned for the substantial quantities of glyoxal crosslinker and catalysts employed. Since the finishing process occurs in an acid milieu, silicone softeners are expected to be stable in saline solution and acids, and to be resistant to 'shear. Not many silicone softeners on the market can meet these demands.
Americos is one of the leading manufacturers of softeners both silicones based and non-silicones based for various textile substrates to impart soft handle along with various other special properties. Particularly, Americos silicone softeners are engineered to impart multifunctional properties. For example, Americos FX-30 is silicone based softener specially engineered with polyurethane (PU) polymers for white fabrics. It imparts excellent and durable softness to the fabric along with a special supple feel due to the presence of PU polymers. In addition, it also enhances the whiteness of the treated fabric. Americos also has commercialized a range of multifunctional silicone softeners such as Americos Rubrisoft FL, Americos Leather Soft 750 and Americos Rabroxil5011 which are produced using unique silicone polymer and combination of special polymers. Americos Rubrisoft FL gives acme bouncy effect commonly understood as stretch or rubbery effect. The finish is durable and it improves wrinkle recovery property fabric significantly along with softening. Similarly Americos Leather Soft 750 and Americos Rabrom 5011 impart a bulky, soft hand and non yellowing leather like effect. They also provide protection against harmful UV-B rays. Further, they increase the strength significantly and improve crease recovery property.
Micro/macro/nano emulsions
There are vast differences between the conventional microemulsions that made history in the textiles industry around 20 years ago, and the quite recent generation of macroemulsions. The primary difference is the panicle size of the silicones. These may be up to 80 nm in microemulsions, whereas they are at least 120 nm in macroemulsions.
Microemulsions are generally easier to make, but they also have their disadvantages. Adding more of them produces only a very limited increase in their softening effect. As more is added, the resultant higher proportion of emulsifier leads to hand saturation and may even cause it to deteriorate. With macroemulsions, however, this effect if it occurs at all - only becomes noticeable at much higher application rates. Unlike microemulsions, macroemulsions provide more resilience, smoothness and a fuller hand 22. However, its mechanical application properties depend crucially on the quality of the macroemulsions. Poorly emulsified emulsions bring more disadvantages than advantages. To optimize the desired parameters, s11ch as panicle size distribution, homogeneity and shear stability, the right processing technology is needed.
Fig. 6: Schematic representation of diffusion of micro and nano silicone emulsion droplets in cotton fibre
With the help of extreme shear technology, Americos emerges out with silicone nanoemulsions which have their unique penetrability it; the fabric and fibre structure. Thus it results in excellent softening properties. The schematic diagram shown in figure 6 depicts the penetration of silicone nanoemulsion droplets inside the structure of cotton. The cotton fibre is made of fibrillar structure and hence it has porous structure. The droplet size of nanoemulsion is so small that it can penetrate the micro and nanostructures very well compared to the droplets of microemulsions. Therefore, using Americos nanoemulsions, it is possible to obtain special softening effect. Americos Nanosoft 1140, Americos Nanosoft 1180, Americos Nanosoft 950 I and Americos Nano soft 2000 are the various silicone nanoemulsions currently Americos sells in the market for various textiles. These silicone nanoemulsions impart durable softness, crease resistance, oily and greasy soft hand, suppleness and limpness, excellent body, superior smoothness, and surface levelness.
Silicone finishing and water repellency
In addition to imparting soft feeling, silicone finishing, in general, imparting water repellent property to the textiles. Such water repellency property is provided by methyl group which are oriented and attached to the fibre surface by silicone links. The silicones are mostly built up of polymethylhydrogen siloxane and polydimethyl siloxane. The first one is reactive and is generally used as water-repellent mostly along with the second one. The problem with the reactive low molecular weight polysiloxanes is that they are liable to undergo further polymerization by lengthening of the -Si-O- chain and also by crosslinking of adjacent -Si-O- chains. This is undesirable in textile application and is prevented by previously replacing the end hydrogen atoms by more inert substituents like methyl groups.
Advantages of silicone water repellents include a high degree of water repellency at relatively low (0.5 - 1 % owf) concentrations, very soft fabric hand, improved sewability and shape retention, and improved appearance and feel of pile fabrics. Some modified silicone repellents can be exhaust applied (to pressure-sensitive fabrics). However, some of the limitations with the silicone repellents are increased pilling and seam slippage, reduced repellency if excessive amounts are applied (for example silicone double layer with polar outside, Fig. 7), only moderate durability to laundering (through hydrolysis of siloxane and rupture of the film by strong cellulose fibre swelling) and dry cleaning (adsorption of surfactants), and no oil and soil repellency. The silicone finish may enhance the attraction of hydrophobic dirt.
However, while silicone finishing is considered only for
enhancing the fabric handle, the hydrophobic property imparted as a result of
silicone finishing becomes a problem rather than to be an advantage. Attempts
have been thus made to enhance the hydrophilicity of the silicone treated
fabrics. Okada et al.23 have grafted acrylamide over the
silicone treated surface in order to render it a hydrophilic property. The
authors have first treated with corona discharge in air to introduce peroxides
onto the surface. These peroxides were further used to graft the acrylamide.
Likewise there are many attempts made to enhance the hydrophilicity of the
silicone treated textiles. However, Americos with its special R&D
efforts have discovered special silicone softeners which not only enhance
fabric handle properties but also preserve the hydrophilicity. Americos SF
1402 and Amerisoft Sil HL- 40 AS are the two special hydrophilic
silicone softeners that are currently in the market. They impart very good
hydrophilicity, durable softness and excellent shear stability. They are also
compatible with cationic and nonionic softeners and are stable with resin
applications.
Silicones containing amide groups: non-yellowing
One limitation with the aminofunctional silicone is that the amino group which is responsible for many unique properties also results in a propensity to yellowing, particularly during curing or drying, and the likelihood of yellowing increases with increasing amino content. Therefore, silicones containing not an amino group but an amide group (Fig. 8) were developed 20, 24. The benefits of these softeners are that they are essentially nonyellowing, and that the handle is very dry when compared to even the low-amine aminosilicones.
When low-yellowing silicone softeners are needed, amino fluids with low amine number are generally preferred. They tend to be used for white goods and pale shades. The higher the amine number of the aminoethyl-aminopropyl fluid, the more yellowing can be expected.
One corollary of this is that the hand becomes softer. Americos Silsoft 1140 and Americos Silky top are some of the prominent silicone softeners of which Americos Silkytop is a blend of cationic and silicone softener. It has especially nonyellowing character and it can be used for cotton, polyester and their blends. It imparts cotton garment excellent supple, soft and brilliant look. It does not affect dye fastness rather it increases the shade depth.
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