20 Finishing agents – Water softeners, water repellents and optical brighteners

V. RameshBabu

Introduction to softeners

SOFTENERS have gained great importance in textile finishing; almost no piece of textile leaves the production facilities without being treated with a softener. This softening treatment is to give the textiles the desired handle, make further processing easier and improve the handling properties. A nice, soft handle is often the decisive criterion for buying a textile and is therefore of most vital importance for marketing many textiles. A Softener’s main purpose is to improve the aesthetic properties of textiles In most cases, the duration of the effect is limited since the products applied during the treatment are eliminated by subsequent washing; for this reason they must be applied in the final stage of the treatment.

Application of textile softeners in practise

As a general rule, the softening agents applied are hygroscopic or lubricating agents, which facilitate the fibre sliding within the fabric structure, thus granting easier deformation and creasing of the fabric. In most cases, the duration of the effect is limited since the products applied during the treatment are eliminated by subsequent washing; for this reason they must be applied in the final stage of the treatment. The most common softeners are below:

1. Non-ionic Softener
2. Anionic Softener
3. Cationic Surfactants
4. Silicone-Based Softeners
5. Reactive Softeners
6. Amphoteric softeners
7. Polyethylene-based softeners

Non-ionic Softeners:

Generally ethers and polyglycol esters, oxiethylates products, paraffins and fats. These softening agents are generally less efficient than anionic and cationic ones but they withstand the effects of hard waters, acid or basic environment and also in presence of cations and anions, therefore the normal fabric care conditions.

Anionic softeners: Anionic softeners are produced by the condensation of fatty acids. They have good characteristics as lubricating softening agents and give the fabric a full hand. They are unstable in hard water and acid environment. Anionic finishing agents have negatively charged hydrophilic group. The cellulose acquires negative charge when wetted out and attracts the positively charged hydrophobic group, with hydrophilic group orienting away from the fiber surface.

Anionic softeners are heat stable at normal textile processing temperature and compatible with other components of dye and bleach baths. They can easily be washed off and provide strong antistatic effects and good rewetting properties because their anionic groups are oriented outward and are surrounded by a thick hydration layer. They are often used for special applications, such as medical textiles, or in combination with anionic fluorescent brightening agents.

Cationic Surfactants:

Usually they are quaternary ammonium salts, amino-esters and amino amides; they are recommended for all types of fibre, and can be also applied with exhaustion process in acid environment (pH 4-5). These are the best softening agents and are also called molecular velveting. Agents because they form bonds with the cationic group on the surface of the fibre generally with negative electric potential. They can give some problem in presence of large anions, and they can cause dye toning, or a reduction in fastness to light values in the presence of direct and reactive dyes; they also have a high polluting charge as waste water (bactericides).

Silicone-based softeners: Silicones are macromolecules comprised of a polymer backbone of alternating Silicon and Oxygen atoms with organic groups attached to silicon. Silicone’s softening capability comes from the siloxane backbone’s flexibility and its freedom of rotation along the Si-O bonds.

They are insoluble in water, and therefore must be applied on fabrics after emulsification or dissolution in organic solvents. They feature quite good fastness to washing. They create a lubricating and moderately waterproof film on the surface and give fabrics a silky hand.

They provide very special unique hand, high lubricity, good sewability, elastic resilience, crease recovery, abrasion resistance and tear strength. They show good temperature stability and durability, with high degree of permanence for those products that form cross linked films and a range of properties from hydrophobic to hydrophilic. As per the required

properties the organoreactive group is modified and the results are achieved. Matex has a developed a complete range of silicone softeners like Diamino silicone(DAS), Reactive aminosilicone (RAS), Aminofunctional silicone (AFS), Organofunctional silicone (OFS), Premium aminofunctional silicone (PAS), polyether silicone (HYS) and epoxy silicone (NYS).

Reactive Softeners:

N-methylol derivatives of superior fatty amides or urea compounds replaced with fatty acids. The products have to be cross-linked and provide permanent softness and water repellency. Amphoteric softeners: Typical properties are good softening effects, low permanence to washing and high antistatic effects. They have fewer ecological problems than similar cationic products. Examples are betaine and the amine oxide type.

Polyethylene-based softeners: Polyethylene can be modified by air oxidation in the melt at high pressure to add hydrophilic character (mainly carboxylic acid group). Emulsification in the presence of alkali provides higher quality and more stable products. They show high lubricity that is not durable to dry cleaning. They are stable to extreme pH conditions and heat at normal textile processing condition, and compatible with resins and fluorescent brightening agents. They impart lubricity especially required for yarns. They are very strong in improving properties like tensile strength/tear strength, sewability, abrasion resistance and rubbing. Matsoft PE emulsion and Matsoft PEW emulsion belongs to this category.

Mechanism of softeners: Softeners provide their main effects on the surface of the fabrics. Small softener molecules, in addition, penetrate the fiber and provide an internal plasticization of the fiber forming polymer by reducing of the glass transition temperature.

Depending on the ionic nature of the softener molecule and the relative hydrophobicity of the fiber surface, cationic softeners orient themselves with their positively charged ends toward the partially negatively charged fabrics (zeta potential), creating a new surface of hydrophobic carbon chain that provide the characteristic excellent softening and lubricity seen with cationic softeners

Anionic softeners, on the other hand, orient themselves with their negatively charged ends repelled away from the negatively charged fiber surface. This leads to higher hydrophilicity, but less softening compared to cationic softeners.

The orientation of non-ionic softeners depends on the nature of the fiber surface, with the hydrophilic portion of the softener being attracted to hydrophilic surfaces and the hydrophobic portion being attracted to hydrophobic surface, thus imparting hydrophilicity or hydrophobicity.

Application methods: Softeners are mostly applied by forced application (padding, spraying) from relatively concentrated solutions, which transfers all of the liquor onto the fabric. In batch processing softeners are often applied by exhaustion from diluted baths on machines such as jet, overflow or winch. Here the exhaustion rate is relevant to ecological considerations of waste water loads. Applications by foam applicator by spray techniques are common in the case of made-up-garments. Thus it depends upon the substrate and the feasibility of adopting either of the processes.

Water Repellency:

Water Repellency is more difficult to define because various static and dynamic tests are used to measure water repellency. Generally speaking water repellent fabrics are those which resist being wetted by water, water drops will roll off the fabric.

A fabric’s resistance to water will depend on the nature of the fiber surface, the porosity of the fabric and the dynamic force behind the impacting water spray. The conditions of the test must be stated when specifying water repellency.

Water Repellent Fabrics have open pores and are permeable to air and water vapor. Water-repellent fabrics will permit the passage of liquid water once hydro-static pressure is high enough.

WATER REPELLANTS-Agents

• Hydrocarbon Hydrophobes
• Silicones
• Flurochemicals
• HYDROCARBONHYDROPHOBES

1. Paraffin Waxes

The oldest and most economical way to make a fabric water repellent is tocoat it with paraffin wax. Solvent solutions, molten coatings and wax emulsions are ways of applying wax to fabrics. Of these, wax emulsionsare the most convenient products for finishing fabrics. An important consideration in making water repellent wax emulsion is that theemulsifying system not detract from the hydrophobic character of paraffin. Paraffin wax melts and wicks into the fabric when the fabric isheated. This will cause most of the fibers to be covered with a thin layer of wax, especially those that are exposed to water, and the fabric will haveexcellent water repellent properties. The major disadvantage of wax water repellents is poor durability. Wax is easily abraded by mechanical actionand wax dissolves in dry cleaning fluids. It is also removed by laundryprocesses. A typical wax emulsion consists of paraffin wax as the hydrophobe, anemulsifying agent, an emulsion stabilizer (protective colloid) and analuminum or zirconium salt to deactivate the emulsifying agent when the fabric is heated.

The stability of wax dispersions and the durability of wax finishes havebeen increased by formulating polymers such as poly(vinyl alcohol), polyethylene and copolymers of stearyl acrylate-acrylic or methacrylicacids.

Fiber Reactive Hydrocarbon Hydrophobes:

N-MethylolStearamide

In an effort to improve the durability of hydrocarbon based water repellents, several approaches incorporating reactive groups have foundcommercial success. The simplest of these is N-methylolstearamide.Stearamide reacts with formaldehyde to form the N-methylol adduct. Thisadduct is water dispersible and either will react on curing with cellulose. Pyridinium Compounds

A variation of N-methylolstearamide is the pyridinium type water repellents. These were once very popular and used extensively as reactive type water repellent finishes. Toxicological considerations have curtailed the use of pyridinium-type water repellents.

Pyridinium type water repellents co-applied with fluorochemical repellentsresulted good,long-lasting water repellency. The finish was durable to fieldlaundry procedures and namedQuarpel by its inventors.

Resin Formers

The multiple reactive sites on methylolmelamines can be utilized for makingresin-forming water repellents.

Silicones

These products are stable to washing in water at 60°C with solvents. They canbe used to treat uniforms, raincoats and sportswear. These products are verypopular since they allow excellent waterproofing properties, optimum solidity (thatcan also be enhanced if combined with resins) and a very pleasant and soft hand(they are also used as softening agents.)They are suitable to be applied on all types of fibres. The highwater-repellent effect and the soft hand are due to the orientation toward theouter surface of methyl groups.

Application to Fabrics

Silicone finishes are applied to fabrics either from an organic solvent or from water as an emulsion. When cationic emulsifiers are used to make anemulsion, the finish may be applied by exhaustion since the negative fiber surface charges attract positively charged particles. Generally however, silicone water repellents are co applied with a durable press finish. Durablepress resins enhance the durability of the water-repellent finish. Siliconerepellents are also used to make upholstered furniture stain repellent. Chlorinated solvent solutions are sprayed onto upholstery by the retailer asa customer option. The fabric is resistant to water borne stains such ascoffee and soft drinks.

Advantages and Disadvantages

Silicone water repellents are durable to washing and dry-cleaning. Durability is brought about by the formation of a sheath of finish around the fiber.If the sheath cracks, durability is lost. Adsorption of hydrophilic substancesfound in dry cleaning and laundry products also impair water repellency. Silicones are more durable than wax repellents but less durable thanfluorochemical finishes. Silicones are more expensive than wax repellentsand less expensive than fluorochemical repellents. Silicone finishes resistwater borne stains but not oil borne stains. Fabric hand can be made softand pliable.

Fluorinated products

They feature an excellent fastness to washing in water and solvents. They aresuitable for treating uniforms, raincoats and sportswear, tablecloths andprotection clothes (after washing they must be ironed to recover their originaleffect). These products allow excellent waterproofing results also with oil-repellency properties. The application is carried out by padding the fabric, whichis then dried at 100-120°C.

REPELLENT FINISHING WITH FLUOROCHEMICALS

The oil and water repellent features of fluorochemical polymers lead tofinishes applicable in two consumer product areas, durable rainwear fabrics and stain/soil resistant products. For rainwear products, superior durability to repeated laundering and drycleaning

Is the major advantage. For stain and soil resistance, the plus features are the fluorochemical’sability to prevent oils from penetrating into the fabric or from soils stickingto the fiber surface. Most fabric stains are caused by liquids depositingcolouring matter on the fabric. Water borne stains can be held out bysilicone water repellents; however, oil based stains can only be repelled bythe low surface energy of closely packed fluorocarbon tails. For textilesthat cannot be laundered, e.g. upholstery fabrics and carpets, stain and soil repellency is an important consumer plus. For fabrics that can belaundered or dry cleaned, stain removal is more important than stainprevention.

Optical Brighteners

Numerous materials especially textiles, both classical {Cotton, wool, linen and silk} and synthetic {mainly polyamide, polyester and polyacrylonitrile }, are not completely white and efforts have been made since ancient times to free from their yellowish tinges. Bleaching in the sun, blueing and later chemical bleaching of textile and other materials increased the brightness of the products and eliminated to a certain extent the yellowish tinge to greyish yellow hue or the local impurity of the original or industrially treated material. When Optical brighteners first came up they were regarded as bleaching auxiliaries, which enabled a shorter or a milder bleach when used in very small quantities {Approximately 0.001 to 0.05% }. They were also called as Optical Bleaching Agents. Cotton and linen bleachers knew 200 years ago the effect of bleaching could be improved with the help of horse chestnut extracts. This is due to the fact the inner bark of the horse chestnut contains aesculin or esculinic acid, a glucoside which is a derivative of coumarin and which has ultra violet fluorescence. Scientist recommended aesculin for improving the whiteness on the basis of theoretical considerations. An aqueous solution of aesculin proved more suitable, but had two major draw backs. Firstly it was not fast to washing and secondly aesculin on the fiber was very sensitive to light. Then came the introduction of organic products based on Diaminostilbinesulphonic acid derivatives.

Classification of Optical Brightening Agents:

The classification of of OBA can be based either on the chemical structure of the brightener or on its method of application. They can be broadly classified primarily in to two large groups. Direct {Substantive} brighteners and Disperse brighteners. Direct optical brightening agents are predominantly water soluble substances used for the brightening of natural fibers and occasionally for synthetic materials such as polyamide. Disperse optical brightening agents are mainly water insoluble and as with disperse dyes they are applied either to colored from an aqueous dispersion or they can be used for mass colouration. They are used for synthetic materials such as polyamide, polyester, acetate and occasionally on paper. From the chemical point of view they are classified according to their chemical structure. Chemical optical brightening agents are classified in to derivatives of stilbene, coumarin, 1,3 diphenylpyrazoline, derivatives of naphthalene dicarboxylic acid, derivatives of heterocyclic dicarboxylic acids, derivatives of cinnamic acid and substances belonging to other chemical systems.

Optical brighteners and its mechanism

Nearly 80% of all OBAs produced are derived from stil-bene derivatives, the latter absorbency in the ultra violet regions at ( ) = 342 nm. All the OBAs are dyestuffs, but in place of the chromophoric system which is the characteristic for dyes, it contains a fluorescing system, and like a normal dye certain substituents which promote the affinity depending on the type of fiber on which it is applied. In this manner brighteners which are suitable for cotton are more are less substantive derivatives of diaminostilbenedisulphonic acid. This stil-bene derivative can be present in two isomeric forms, i.e. in the Cis configuration and in the trans configuration. Optical brighteners in the Trans form can be made both in the powder and Liquid form. The Cis form, which is rapidly formed under the action of light from the transform will not go on cotton and for this reason, the solution of this whitener is protected against light. Many of the optical brighteners are derived from the heterocyclic compounds containing nitrogen atoms. Fluorescence is produced by the absorption of radiation having a high energy on the part of the molecule, which re – emits this radiation of lower energy i.e. of longer wave length, the difference in energy being transformed in to kinetic energy. To enable a molecule to fulfill this function, it must be built according to certain structure principles. Anthranilic acid is the organic compound with the formula C6H4(NH2)COOH. This amino acid is white solid when pure, although commercial samples may appear yellow. The molecule consists of a benzene ring with two adjacent functional groups, a carboxylic acid and an amine. For example Anthranilic acid has very strong blue violet fluorescence in the aqueous solution, but nevertheless unsuitable as a brightener. Most of the brightener will hardly fluoresce in powder form; their fluorescence will only appear in solution. There are some types, which will not fluorescence in solution and will only show this property after they have been applied on the fiber. Thus, it can be concluded that fluorescence is not only depends on the structure of the molecule, but also on its condition. Whether a fluorescent substance is suitable as brightener can only be determined after it has been applied to the textile fiber. Apart from this the product must meet certain demands in respect of properties such as fastness to washing and light, etc. On comparing different textile fabrics treated with different brighteners and processing approximately the same brightness difference in hue can be detected, since the human eye is particularly sensitive to difference in whiteness. If an optically brightened fabric with reddish white shade is compared with another fabric having a greenish white shade both of which appear to be equally brilliant, if viewed in daylight which is incident from a northerly direction, it will be seen that the greenish shade will appear more brilliant then the reddish one in bright sunlight.

By using fluorescent optical brightening agents:

The OBA s (optical brightening agents) are most widely used in textiles, paper, detergents and plastics. The optical brightening effect is obtained by the addition of light, which means that the amount of light reflected by the Fluorescent Whitening Agents (also called optical brightener) absorb high energy radiation in the ultraviolet to violet region (330nm-380nm) on the part of characteristic molecules and emit lower energy radiation in blue region in visible spectrum (400nm-450nm), which yields the counteracting the yellowing appearance. FWA should be transparent on the substrate and should not absorb the visible region of the spectrum. The OBAs are effective only when the incident light has a significance proportion (such as daylight) of UV rays. When material treated with OBAs are exposed to UV black light source, it glows in the dark. Anionic OBA’s exhaust on cotton, wool and silk.Cationic OBA’s exhaust on acrylic and certain polyesters and non-ionic OBA’s are exhaust on all synthetics.

Desired Properties of Fluorescent Whitening Agents for Textiles Use:

Before selecting an optical brightener for textile application we must look for following properties,

• It should have good solubility, should not have its own color and good substantivity for the textile substrate under OBA application.
• OBA’s should have good light as well as wet fastness properties.
• Its rate of strike on the substarte.
• Build up and exhaustion properties.
• Requirement of electrolytes and its sensitivity towards different exhausting agents.
• Effect of temperature on the exhaustion and build up properties.
• Application pH range and sensitivity towards change in pH.
• Effect of water hardness.
• It should have good leveling and penetrating properties.
• Should not decompose to colored products on exposure to atmospheric conditions as well as storage, and it should not absorb light in the visible region.
• It should be compatible and stable with finishing chemicals, auxiliary and process such as heat and temperature.
• It should be stable and fast to the common oxidative and reductive bleaching chemicals and bleaching systems.

REFERENCES and URLs

1. Broadbent D.A., “Basic Principles of Colouration”, Society of Dyers & Colourists, 2001.
2. Karmakar S.R., “Chemical Technology in the pretreatment processing of textiles”, Textile Science & Technology, Elsevier Publication, 1999.
3. Bhagwat R.S “Handbook of Textile Processing”, Colour Publication, Mumbai, 1999.
4. T.L.Vigo, “Textile Processing and Properties”, Elsevier, New York, 1994.