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ALL ABOUT OPTICAL BRIGHTENING AGENTS :

ESCON INTERNATIONAL (WWW.ESCONINDIA.COM)

(MFRS OF RINO BRAND POLYESTER WHITENING AGENTS)

Fluorescent brighteners and optical whitening agents

white textile articles become yellowish on storage. This undesired effect can

be removed as follows:

(1) By using chemical bleaching agent such as hypochlorite or peroxide. In this

method, there are chances of spoiling colored goods and damage the fiber.

(2) By using small amount of blue coloring matter, which absorbs yellow light

and due to this yellowed fabric appears white.

(3) By using fluorescent compound which absorbs ultra violet light and converts

the energy into visible light of higher wavelength. In this way, a yellow

appearance can be corrected by the emission of a corresponding amount of

blue-violet light by the fluorescent compound. The effectiveness of fluorescent

agent depends on the presence of ultraviolet light in the illuminant.

The first use of fluorescent compound to whiten textile materials was,

described by Krais in 1929, but it has been commercialized only in 1940. Many

brightening agents have been described in patent literature, and used commercially.

They are sold in different names such as Blankophor (FBY), Calcofluor (ACY),

Fluolite (ICI), Leucophor (S), Photine (HWL), Pontamine White (DUP), Tinopal (GY),and Uvitex (Cac,

Ciba). As a class, they are termed fluorescent or optical whitening or brightening agents.

By far the greatest use of brighteners is in detergents, and almost every

commercial detergent contains one or more brighteners, in the proportion 0.05% to 0.3%. Brighteners are

also used in textile processing and the manufacture of paper.

They are also used in plastics, waxes, polishes, cosmetics and in hair rinse. They are also used in the

manufacture of synthetic fiber of all types.

For fluorescent substances to act as brightening or whitening agents, the

emitted light must be essentially blue light so that it effectively neutralizes the normal pale yellow or

cream color of so called white materials. In practice the dominant wavelength of the emitted light must be

around 450mμ.

Optical brightening is based on an addition of light, whereas the bluing

method achieves its white effect through the removal of light. The optical brighteners should fulfill the

following two requirements.

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(i) It should be optically colorless on the substrate and

(ii) It should not absorb in the visible part of the spectrum.

Since the fluorescent light of an optical brightener is itself colored, i.e. blue

to violet or, sometimes, a bluish-green cast.

The overall effect given by a whitening agent therefore depends on a number

of factors.

(i) Its intrinsic effectiveness as a fluorescent.

(ii) Its spectral absorption and emission characteristics.

(iii) The ultra-violet content of the viewing light.

(iv) The self-color of the substrate.

(v) The concentration on the substrate, which will of course depend on the

substantively of the whitener for the substrate, and on the method of

application.

(vi) The physical form of the whitener on the substrate: the shade given by a

fluorescent on nylon, for example, is normally much more violet than the

shade on cotton.

Chemical Constitution

Stilbene derivatives

Most water soluble brighteners for the textile materials are stilbene derivatives, and bistraizinyl

derivatives of 4,4'-diaminostilbene-2,2' disulphonic acid are of special interest. One of the early

brighteners manufactured by IG was 4, 4' bis (phenyluereido) stilbene 2, 2' disulphonic acid, obtained by

reaction of 4, 4' diaminostilbene - 2, 2' disulphonic acid with phenylisocyanate in aqueous medium. It was

marketed as Blankophar R. (I).

The usefulness of this product is limited by its instability in a boiling bath.

Present day commercial products are of the same general types as Blankophor B with varing substituents

on the tiazine ring.

In a similar manner a number of other optical brightening agents of this type (CC/DAS) can be prepared.

The principal effects of these variations are changes in solubility, affinity, acid

fastness, etc. The class of bistriazinyl compounds, in solution, is not fast toward

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hypochlorite; some compounds, however, show a certain amount of stability after

application to the fiber. The bistraizinyl brightners are employed principally on

cellulosics, such as cotton or paper. Some products also show affinity for nylon at the weakly alkaline pH

of most of the commercial detergents.

By coupling diazotised 4-aminostilbene-supphonic acid with 2-aminoaphathalene-6-sulphonic acid and

oxidising the product with alkaline hypochlorite a compound is obtained with structure (III; X=SO3Na,

Y=SO3Na); it can be added to a detergent as a brightner and has an advantage over many other stilbene

derivatives in that it remains effective in presence of bleaching agents.

Unsulphonated agents such as (III; X=CN, Y=H) can be applied as brighteners

for synthetic fibres or plastic materials; other such as (III; X-SO2OPh or SO2N (alkyl)2 Y=H) are said to

be suitable for similar purposes and also for application to oils, fats and waxes. Cationic groups such as

SO2NHC2H4NMe2 can be used to confer affinity for polyacrylonitrile fibers.

Derivatives of dibenzothiophene-5, 5-dioxide

A further group of brighteners, which has been studied primarily by American Cyanamid, was found in

the derivatives of 3, 7- diaminodibenzothiophene-2, 8-disulfonic acid 5, 5 dioxide IV. These compounds

are relatively weak in comparison with stilbene derivatives and give a greenish fluorescence. However,

they do show good fastness to hypochlorite.

Azoles (derivatives of 5-membered-ring heterocycles)

Monoazoles

The group based on compound V arose from efforts to find hypochlorite stables

compounds with neutral fluorescence, and was patented by Geigy. With warer solubilising group these

types of compounds are suited to brightening cellulosic

materials or nylon from soap and detergent baths. Water insoluble derivatives of this family, e.g.

compounds having sulfamyl, arysulfnate, or nitrile groups, are suitable for brightening synthetic fibers

and resins.

Blankophor WT, has been proposed as a brightner in the dyeing of wool from an acid bath. This

compound is not effective in the presence of soap or synthetic detergents.

Bisazoles

Bisnaphotriazzolyl compounds of structure VI are obtained by coupling

tetrazotised amino compounds with 2 moles of a naphthyamine derivative

coupling in ortho-position, followed by oxidation. These products have good

hypochlorite stability. Compounds of structure VII are combinations of azoles

with napthotriazoles.

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These compounds posses good fastness to hypochlorite and show mostly a redviolet fluorescence.

Coumarin Derivatives

The optical brightening of textile fibers by fluorescent compounds was mentioned by Krais. By treatment

of flax with esculin, a glucoside of esculeting (VIII) a brightening effect was achieved, but this effect was

not fast to washing and light. Later the use of β-menthylumbelliferone (IX) and similar compounds as

brightners for textiles and soap was patented. As an improvement over β-menthylumbelliferone, 7-

aminocoumarin were proposed. These latter are used for brightening wool and nylon either in soap

powders or detergents or as salts under acid dyeing conditions.

A further development of the coumarin group consists in the use of derivatives of 3-phenyl-7-

aminocoumarin (x). These compounds displace the hue towards more neutral shades and, at the same

time, significantly improve the lightfastness. Brightners of this type are suitable by synthetic fibres and

plastics.

Coumarin derivatives substituted at position 3 by an aryl radical and 7 by a group such as ureido or a

substituted triazinylamino group Fig. XI, XII are of special

interest in that they can be applied to fibers of cellulose, wool, polyanide, polyurethane, cellulose acetate

of polyacrylonitrile by dyeing processes or

to inert fibers such as polyesters by incorporation in the melt. They have good

fastness to light and impart a neutral white appearance to treated fibers.

Derivatives of 6-membered-ring heterocycles

A further class of brighteners is derived from pyrazine. These compounds may be employed for

brightening wool and various synthetic fibers from a weakly acid to neutral bath or from soaps and

synthetic detergents.

Derivatives of pyrazoline

Knorr discovered this Class of optical brightening agents. It displays intensive blue fluorescence and high

affinity and substantivity for the fibers. Optical brightening agents based on this group are used mainly

for the surface brightening of polyamide, acetate and polyacrylonitrile. They are unstable against

oxidants.

Finishing of Commercial Optical Brighteners

Optical brighteners may be obtained as powders, pastes, liquid water-soluble

forms or stable dispersions.

Pastes

Pastes are prepared from wet cake with various additives such as sodium

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chloride, sodium sulphate, urea, etc. in a mixer.

Powder

The powder form is produced from above paste and dried in spray drier. The powder are produced either

diluted i.e. with additives or as concentrates where the content of additive is low or completely absent.

Instant finish (easily water soluble)

Lately "instant" finishing has been used in the preparation of powders. It

involves converting poorly wettable powders of optical whitening agents into easily wettable or soluble

material, which is also non-powdering during addition. For achieving the "instant" finish, optical

brightening agent wetted powder is submitted to a mechanical treatment during which individual powder

particles are brought into contact, stick together agglomerate to fine beads, which dried in spray drier.

Liquid forms

the most common liquid forms involve the preparation directly from the final

product.

Liquid forms are solutions of optical brightening agents which are completely

miscible with water. Both soluble and insoluble brightening agents are used in such

preparations, and they are brought into solution either by dissolution in a solvent, or by chemical

processes, in which the brightening agent is converted into the salt of a soluble quaternary base. The

second type of liquid form is prepared most frequently from water-soluble derivatives of 4, 4'-

diaminostilbene-2, 2' -disulphonic acid. A paste or the dry product is acidified and the acid paste is dried

and suspended in an excess of amine and required concentration adjusted by water and quaternisation is

carried out with the hydrotropic substance.

A hydrotropic substance monoglycols diglycols or triglycols, glycerol, various

sugars, sulphite mother liquors, easily water-soluble amides, mono-alkanolamines,

dialkanolamines, trialkanolamines, urea and urethanes are used.

The liquid forms contain 10 to 40% active substance and 25 to 60%

hydrotropic substances; based on the weight of solid brightening agent.

Hydrotropic substances used in Optical Brightening Agent

Commercial Name Hydrotropic Substances

Albaphan CBS flussig deithylene glycol

Blankophor flussig urea, ammonia

Blankophor BBU flussig urea, ammonia

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Blankophor BBH flussig urea, ammonia

Blankophor BE flussig urea, diethylene glycol

Blankophor BA flussig urea, ammonia

Blankophor RA flussig urea, ammonia, diethylene glycol

Blankophor CI flussig oxyethylated phenol

Celumyl CSP flussig ethanolamine, diethylene glycol

Fluolite MP liquid diethylene glycol, ethanolamine

Leukophor AC flussig diethylene glycol, tamol

Leukophor BS flussig urea, ammonia

Tmopal CH 3632 diethanolamine

Tinopal UP liquid ethanolamine, ethylene glycol

Tmopal 2BF flussig diethanolamine

Tmopal 4BM flussig triethanolamine, urea

Tmopal LAT flussig oxyethylated phenol

Uvitex PRS fluss, konz triethanolamine, urea

Stable Dispersions

Stable dispersion of optical whitening agent is prepared by utilizing suitable

non-ionic dispersing agent. Disperse optical brightening agents are produced mainly or the brightening of

polyester fibers, cellulose acetate and polyacrylonitrile. The aqueous dispersion of the optical brightener

can be tinted with a blue dye. The additional blue coloration of the dispersion brings about an increase in

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brightening effect up to maximum 30%.

Evaluation and testing

The evaluation of fluorescent brightening agents and assessment of their practical performance are of

prime importance to the user of these agents. The main analytical tests applied to fluorescent brightening

agents are designed to determine the strength of fluorescent activity either in its own right or

comparatively against a known product and to establish the type of product and, if possible its identity.

Active strength of fluorescent brighteners

As all fluorescent brightening agents operate by absorption of ultraviolet light,

which can be used to measure the active fluorescent strength of a product. The

extinction coefficient of a solution of the fluorescent brightening agent at wavelengths from 200 to 400

nm is determined by spectrophotometer. It can, however, be a measure of active strength of a product, as

the peak absorption at “between” 345 to 365 run is a direct measure of its active fluorescent strength. For

cellulose fluorescent brightening agents this is usually around 350 nm and the absorption at this peak is

used to calculate the extinction coefficient through a 1 cm cell at a concentration of 1% weight/volume

solution, usually expressed at E (1%1cm). This value is used extensively in evaluation and

standardization, as a direct measure of the fluorescent activity.

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To establish the type of product, thin-layer chromatography is perhaps right

instrument.

Uses

the major consuming industries for optical whitener are as follows:

Detergent mixtures 40%

Paper 30%

Synthetic fibers and plastics 05%

Textiles 25%

Detergent Brighteners

Today scarcely a detergent exists which does not contain some cellulose brightener. One of the principal

requirements of such a product is that it has satisfactory affinity in the presence of detergent. The

brightening agent should have satisfactory build up in multiple washing, but should not discolor the

textile. In addition, the product must be chemically stable to the other components of detergents.

Detergent brighteners should have adequate light fastness. The washed textiles must not become

discolored in light. Certain brighteners can brighten the detergent itself, so that the agent appears pure

white.

Brighteners for the textile industry

Textile finishing requires brighteners of very good solubility and substantively.

Mostly bistriazinyl derivatives of 4, 4'-diminostillbene 2, 2'-disulfonic acid are

preferred. Brightening agents must possess definite stability in combined processes. The light fastness of

textile brighteners should be as high as possible. It is important again that the brightener does not

decompose to light in form colored by-products.

The optical brightener should possess following properties to give best results.

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(i) Substantively for the fiber

(ii) Rate of strike

(iii) Build-up shade

(iv) Sensitivity to electrolyte

(v) Effect of temperature

(vi) Effect of pH of bath.

Textile can be divided as follows:

(a) Natural fibers, cellulose and protein fibers

(b) Synthetic fibers

(c) Mixed fiber blends

Natural Fibers

These are dealt with under two groupings

(a) Cellulose fibers

(b) Wool fibers

Brightener for cellulose

The brightening of cellulose fibers constitutes the most important use of

optical brighteners.

Cellulose yams in the form of crops, cheeses and beams can all be treated

with fluorescent brightening agents on package dyeing machines. The selection of

brightener and method of operation are of major importance in obtaining level

brightening. It is important with prepared packages, such as cops, cheeses and

beams, that the preparation should produce the most stable and permeable

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construction.

Brighteners with good penetrating properties and of medium substantively are

required to obtain even brightening for cellulose. These are two methods that can be applied:

(a) Two-bath method, in which the brightener is applied to the fabric initially, then

dried and subsequently resin-finished.

(b) One-bath method, in which the brightener is applied in conjunction with the resin finish by inclusion

in the actual resin finishing bath.

It is important to take into consideration the resin and catalyst combination.

The solution of fluorescent brightener should never be mixed with a strong catalyst

solution.

Brighteners for wool

The introduction of fluorescent brightening agents that can be applied to wool

enhances the whiteness but cannot achieve the brilliance of fluorescent.

The most common brighteners for application to wool are a select range of

dastriazinc derivatives with certain pyrazolene derivatives. The application is

invariably in conjunction with hydrosulphite, either alone or in the stabilized form.

Certain brighteners yield excellent results under these mildly acid conditions, while

others require an acid addition to exhaust them effectively onto the fiber.

Brighteners for synthetic fibers

Synthetic fiber can be divided as:

(i) Cellulose acetate

(ii) Polyamide fibers

(iii) Polyester fibers

(iv) Polyacrylonitrile fibers

(v) Acrylic fibers

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Brighteners for Cellulose Acetate

Cellulose acetate show no affinity for the water-soluble cellulose brighteners

normally applied to cellulose, though they have some affinity for the soluble coumarin type. Their main

affinity if for the disperse dye type of brightener.

Acetate fiber can be brightened by a large number of compounds, such as

(aminocoumarin); (derivatives of 7-amino-3-phylecoumarin); (pyrazines);

(pyrazolines), provided the derivatives are water-insoluble; (bis (benzazoyl)

ethylenes). Brighteners for cellulose acetate can also be used in combination with

light-duty detergents.

Brighteners for polyamide fibers

Polyamide fibers and fabrics are produced from polyamide-6 and -66 type.

Fluorescent brighteners of the acid-dyeing coumarin type and of the stilbene-triazine types are widely

used, as well as the disperse type of synthetic-fiber brighteners.

In general, the easily water soluble products are preferred in textile applications, but aqueous dispersions

of difficulty soluble compound are also employed. Water-soluble anionic brighteners are applied very

much like acid wool dyes. Polyamide brightening in the spinning mass ("Dope Dyeing") requires

particularly heat and reduction stable products.

Mainly das-triazine are the most widely used fluorescent brighteners for

polyamide-6 and -66 fibers. Most brighteners give better results when applied from a bath containing a

reducing agent which in itself is of advantage by acting as a mild bleach.

Brighteners for polyester fibers

As a rule, polyester fibers require disperse dye-type fluorescent brighteners

due to their hydrophobic nature. These brightener particles penetrate into the fiber in a state of molecular

dispersion and they are held in the fiber not by polar affinity but by Van der Waals forces. Polyester

fibers show only very little swelling in water, ionic processes are not assumed to be of importance for the

movement of brightener in the polyester fiber. The fitness of dispersion of the brightener particles is

stabilized by the addition of dispersing agents during manufacturing.

They are all water-insoluble products which usually must be applied in conjunction with a carrier. The

maximum in brightness and fastness is achieved only by means of a final heat treatment. It is of

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fundamental importance for the brightener

to remain stable at the relatively high condensation temperature. Effective polyester brighteners are

compounds such as (biscbenzoxazoly) ethylenes; (naphthotriazolyl stilbenes), and (derivative of 7-amino-

3-phenylcoumarin).

Fluorescent brighteners are applied to polyester fibers by exhaust processes

or by pad. Thermosol processes, the choice being dependent on the characteristics

of the material and the machinery available in the works.

The stability of the brightener dispersion within the fiber is of vital importance.

To ensure that the stability of the dispersion is maintained it is necessary:

(a) To dilute the dispersion with water at approximately 40"C just before it is

required.

(b) To control the build-up of temperature of the liquor.

(c) That any additional dispersing agent should be compatible and have protective

colloid effect.

(d) To avoid any auxiliaries that a cloud point, especially for high temperature

applications.

(e) To use no electrolyte and adjust pH with acetic acid; and

(f) Preferably to maintain acidity of the bath between pH 4 and pH 6.

Brighteners for Acrylic fibers

Acrylic fibers are extremely temperature-sensitive and, for each type of fiber,

there is a temperature (known as the glass transition temperature). For most acrylic fibers, the glass

transition temperature is in the region of 80-90"C. The majority of fluorescent brighteners for acrylic

fibers are cationic in reaction. Any acrylic fiber cannot bind a larger number of cationic brightener

molecules, than the total number of the fiber's anionic groups.

Brighteners for Polyacrylonitrile fibers

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Basic brighteners are suitable for brightening polyacrylonitrile fibers. The

basicity of the brightener can be attained through either external amino groups or

heterocyclic rings of basic character.

Multi-fiber brightening

ideally, a laundry detergent should brighten all washable fibers; no such

formulation has been achieved practically. The diaminostilbene dissulfonate

brighteners show about 80% exhaustion on cotton; they have no affinity for cellulose acetate. Viscose

rayon and wash 'n' wear cotton usually show lowered affinity for these brighteners. While nylon presents

no problems, polyester even in cotton blends-is still

a challenge; so are acrylic and spandex fibers, with polypropylene still to make its appearance. The

development of a brightener for anyone of these fibers is a formidable task. The development of a single

brightener suitable for all fibers is highly improbable because optimum conditions for brightening one

fiber will not necessarily be satisfactory for another fiber.

Paper Brightener

most papers are brightened by addition of brightener both to the pulp and to

the surface coatings. The exact proportion varied with the type and quality of the

paper. Besides satisfactory exhaust at low temperatures, good paper brighteners also requires good acid

and alum stability as well as compatibility with the paper

fillers. Good affinity to the pulp is also needed, because and unabsorbed whitener is lost in the effluent or

white water from the screen.

Fine papers require relatively less brightener in the pulp, as the pulp is of a

better quality. Fine papers, which require a surface treatment with white pigments

and synthetic resin, are brightened by an after-treatment.

Brighteners for plastics

Plastics are brightened in the melt. The brightener must withstand special conditions; for example, it must

have stability to polymerisation catalysts (peroxides); sublimation fastness; and the highest possible light

fastness. The brightening agent must not

migrate to the surface and, by so doing, cause, "blooming". Good plastics brighteners, which are

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especially suited to polyvinyl chloride, are such as (bisbenzoxazolyl) (ethylenes) and (derivatives of 7-

amin-3-phenylcoumarin). In addition to polyvinyl chloride, plastics based on polystryene, polyethylene,

polypropylene, polyacrylates and polymemthacrylates and cellulose acetate are also brightened.

Brighteners for Cosmetic Preparations

The use of brighteners for cosmetic preparations, such as creams, salves,

lipsticks, etc. has been proposed, but so significant commercial usage has as yet

developed. Naphthotriazolyl stilbene derivatives may be used for this purpose.

Brighteners for Miscellaneous Application

In addition to the textile, detergent and paper industries, optical brighteners

are also used in various other branches of industry. Such as for the brightening of

feathers, fast, gelatin, wood shavings and sawdust, for the brightening of paints,

leather, furs and straw. The photographic industry makes extensive use of optical

brightening agents. They are also added into developers for colored photographs

where they improve the colored on the one hand and slow down the fading of color photographs on the

other. They are added to lubricants to increase their

fluorescence.

Biological aspects

Brighteners have no detrimental effect on bacteria. The recent reports of Snyder, Neutomm Glashoof and

co-workers indicate that brighteners in general use are not hazardous. Brighteners do not affect the

appearance of water, not the taste, at 1ppm.

Dr. Himadri Panda

&

Dr. (Mrs.) Rakhshinda Panda

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