Optical Brightening Agents: Properties, Function, Mechanism and Usages

Last Updated on 25/03/2021

Optical Brightening Agents: Properties, Function, Mechanism and Usages

Md. Mosharaf Hossain,
Kiriti Kingkar Mondal & Tawhidul Islam
Dept. of Textile Engineering
Primeasia University, Dhaka

Abstract:
Man’s earliest endeavors to enhance the brightness of white objects in everyday use, especially clothing, led to the invention of various bleaching processes. KRAIS’S discovery in 1929 that the natural substance esculin could be applied to textiles to give whites of unprecedented brilliance led to the industrial development of optical brightening agents (OBAs) or fluorescent whitening agents (FWA’s). Fluorescent whiteners are colorless to weakly colored organic compounds that, in solution or applied to a substrate, absorb ultraviolet light and re-emit most of the absorbed energy as blue-violet light between 400 and 500nm. These compounds are included in commercial detergents formulation in order to increase whiteness and intensifying the colors. The aim of this work was to study several industrial situations related with color changes after domestic washing. In order to achieve that, the effect of optical brighteners on white and colored textiles was considered. The action of eight of the most known commercial detergents, on different colors, was tested. Among the selected washing agents, three of them were more specific products: a ‘color protector’, ‘for black and dark colors’ and ‘with active oxygen’. Fabrics with and without pre-brightening were tested. In all the cases, color fastness tests to washing (with ECE detergent and standard soaps) and to rubbing were done in order to conclude about the influence of these parameters on final results.

Keywords: Optical brightening agents, whitening effect, brightness, color fastness.

Introduction:
The use of optical brightening agents in the textile chemical processing is rapidly gaining globally recognition because of their non-toxic and eco-friendly characteristics with the increasingly important requirements for textile manufacturers to make the fabric white at a desired level in textile production. One kind of whitening treatment is anti – biotic. The appearance and handle of cotton knits can be significantly improved by treatment with cellulose and such treatment leads to improve brightness of dyed fabric.

What is Optical Brightening Agents?
Optical brightening agents (sometimes called Optical brighteners, fluorescent brightening agents (FBAs), or fluorescent whitening agents (FWAs), optical bleaches) are one type of compound which, when applied to a textile material, absorbs the short wavelength electromagnetic radiation (300-400 nm) which is invisible to the human eye, and converts it into visible light of longer wavelength between 400 and 500 nm, which is emitted either as violet, pure blue or greenish blue. Because the main use of these dyes in laundry detergents and textile finishing. Optical Brightener are generally found in domestic waste waters that have a component of laundry effluent. Its characteristic is the incident light to generate fluorescence excitation, so that the stained material was similar fluorite sparkling effect, to the naked eye to see the material is white, especially when added to a yellowish-white material. Fluorescent whitening agents may be used to whiten or brighten a textile or paper material. Further more, well all round fastness property and a good yield are also desired. In addition to this, different shades of whites are desired, as white shades are subject to fashion trends, they are requires to be used on a verity of finishing processes and they should be compatible with practically all chemicals and auxiliaries used at different stages.

History of Optical Brightening Agents:
Textile material (cotton, wool, linen and silk) and synthetic (mainly polyamide, polyester and polyacrylonitrille) are not completely white and effort have been made since ancient time to free from this yellowish ting. Bleaching in the sun, bluing and mater chemical bleaching of textile and other materials increased the brightness of the products and eliminated to a certain hue or the local impurity of the original or industrially treated material.

When optical brighteners first came up, they regarded as bleaching auxiliaries which enable short or milder bleach when used in very small quantities (approx 0.001 – 0.05%) they were also called as optical bleaching agents it could be improved with the help of horse chestnut extra acts. This is due to fact the inner back of the horse chestnut contains aesculin or esculinic acid, a glucoside which is derivative of coumain and which has ultra violet fluorescent. Then came the introduction of organic products based on Diaminostilbine sulphonic acid derivatives.

Historical Development of Optical Brightening Agents:
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 grayish yellow hue or the local impurity of the original or industrially treated material. When optical brightening agents 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 an esculin 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 Diaminostilbine sulphonic acid derivatives.

Chemistry of Optical Brightening Agents:
About 80% of all optical brightening agents (OBAs) produced are derived from stilbene derivatives, the latter absorbing in the ultra violet regions at (α) = 342 nm. All optical brighteners are dyestuffs, but in place of the chromophoric system which is the characteristic for dyes, it contains a fluorescening system and like a normal dye certain substituent 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 or less substantive derivatives of diaminostilbene disulphonic acid. The stilbene derivatives can be present in two isomeric forms, i.e. in the Cis configuration and in the Trans configuration. Optical brightening agents 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 trans form will not go on cotton and for this reason, the solutions 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. 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 fluoresce 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 depended 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 comparering different textile fabrics treated with different brighteners and processing approximately the same brightness difference in hue can be deleted, since the human eye is particularly sensitive to difference in whiteness. If an optically brightened fabric with radish 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 radish one in bright sunlight.

Textile materials like cotton or cotton/polyester blends are almost always pre-brightened when manufactured. This is because the printing and colors will be brighter and more attractive if applied to bright fabric. Moreover, the washing agents and commercial detergents available nowadays commonly have optical brighteners combined in them and while washing the fabric gets whiter. It is known that, in most cases, the presence of an optical brightener causes a decrease in the light fastness of a dyed fiber. These compounds can also have a direct photochemical effect on the fiber in the absence of dyes, as in the case of wool. Optically whitened wool will yellow on exposure to light much faster than untreated wool by a photocatalytic process. However, in colored textiles, sometimes a difference on hue is detected already in the first domestic wash, even in the case of solid colors. This undesirable effect has been a considerable obstacle for several textile industries, with clients and consumers becoming more and more demanding. Therefore, it’s important to study the effect in order to avoid, as much as possible, similar situations.

Commercial name of optical brightening agents in textile

Chemical Constitution of Optical Brighteners
Optical brightening agents are usually derivatives of:

• Triazine-stilbenes (di-, tetra- or hexa-sulfonated)
• Coumarins
• Imidazolines
• Diazoles
• Triazoles
• Benzoxazolines
• Biphenyl-stilbenes

Brighteners can be “boosted” by the addition of certain polyols like high molecular weight polyethylene glycol or polyvinyl alcohol. These additives increase the visible blue light emissions significantly. Brighteners can also be “quenched”. Too much use of brightener will often cause a greening effect as emissions start to show above the blue region in the visible spectrum. Besides the formation of cis isomer in stilbene-containing brighteners (only the trans isomer is optically active), continued exposure to UV-containing light will actually cleave the molecule and start the process of degradation.

Fluorescence of Optical Brighteners Mechanism
Brightening is neither bleaching nor blueing. Fluorescent colours will reflect more light than they can absorb from the visible range of the spectrum. Whiteness can also be increased by using substances which would give colourless solutions but were strongly fluorescent. Fluorescence is produced by the absorption of radiation having a high energy on the part of the molecule, which re-emits this radiation as a radiation of lower energy i.e. of longer wavelength, 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 structural principles. For example, Anthranilic acid has very strong blue violet fluorescence in its aqueous solution, but nevertheless unsuitable as a brightener.

Most of the brighteners will hardly fluoresce in powder form; their fluorescence will only appear in solution. There are some types, which will not fluoresce 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 dependent on the structure of the molecule but also on its condition. Whether a fluorescent substance is suitable as a 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 possessing approximately the same brightness, differences in hue can be detected, since the human eye is particularly sensitive to differences 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 than a reddish one in bright sunlight.

On the other hand, if both fabrics are seen side by side in a room at a distance of several yards from the window where there will be lower proportion of ultra violet light, the reddish shade will appear to be stronger. These strong variations will not be observed in the case of a neutral shade, i.e., in sunlight the neutral white shade appears slightly superior to the reddish and slightly inferior to the greenish shade, whereas the opposite effect is obtained in reduced day light. The same effect can be observed when comparing optically brightened material having a rough surface with such material which has a smooth surface. While an optically brightened smooth material of a reddish shade may appear equal in brilliance to that of a similar material with a greenish shade, it will be seen that in the case of a rough surface the reddish material will appear to be more brilliant.

Properties of Optical Brightening Agents

Whiteness & Brightness
To the trained observer, even bleached are whit textile material has a slight yellow tinge. This small amount of yellow can give the impression of slight soiling and may detract from their aesthetic appeal the presence of slight amount of blue gives the impression that the textile material is whiter. Before advent of OBAs. Improved whiteness was obtained using a laundry blue, which is a blue pigment.

The development of OBAs had meant that this slight addition of blue can be obtained through the light reflected by the OBAs in the presence of ultraviolet radiation. This makes white textile whiter and brighter. Colored textile materials tend to appear brighter. OBAs are present in most domestic but these are usually only suitable for cellulosic textile material.

Light Fastness
There is large variation in the light fastness rating of these compounds when applied to cellulosic and protein fibers their light fastness range 1 to 2, and in some instance may reach 3. It should be pointed out that this poor light fastness is not too important in the cause of cellulosic’s, since any loss of OBAs effect due to sunlight will be replaced in subsequent laundering with domestic detergent. Fluorescent brighteners on xylon can reach a light fastness rating 4 with selected OBAs, a rating as high as 7 for polyesters, and in this class of acrylic fibers a light fastness of about 4 – 5.

The poor overall light fastness of fluorescent brighteners is due to their continuous absorption & emission of light which result in their chemical degradation.

Washing Fastness
The washing fastness rating of fluorescent brighteners if about 3. The fair washing fastness of fluorescent brighteners is due to partly to their lack of substantively of textile material and their gradual degradation by exposure to sunlight. The fair washing fastness may not be noticeable in cellulose because of the presence OBAs in domestic detergents when fluorescent is used on other fibers, they are applied in the manufacturing situation and brighteners are chosen which will last the expected life of the textile article.

Metameric Effect of OBA
Metamerism is a normal phenomenon relating to how the human eye perceives color. It occurs when “two different color object have the same color appearance to normal human viewer under one light source ( metameric match ) but look different under another light source( metameric mismatch)” To a print marker, this means that the painstakingly precise color information applied to each print will be compromised whenever that print in viewed under a different light source. Thus, one primary goal of any print marker should be to avoid metamerism in order to validate the time spent on color management and to uphold the integrity of the reproduction. After all, what good is reproduction if it does? Now that we understand metamerism and why it should be avoided, how do OBAs fit into the picture? When OBAs are exposed to UV light, the treated paper appears brighter and whiter. When OBAs are not exposed to UV light (in the evening), the OBAs “lose activity” causing your eye to actually see the paper color without OBAs. This will look creamy or somewhat yellowed. This amount of “OBAs activity loss” will vary constantly depending upon how much exposure the paper has to UV light. Picture the lightening condition inside of an art gallery and how they will change depending upon the time of day. This will have subsequent effect on the art itself; your print could be illuminating the print. It a case like this, where there is a high UV component, inkjet papers that contain OBAs will strongly fluoresce and will appear bright white. However, in the evening when the same print is displayed with low or non – existent UV component (or incandescent tungsten illumination), the OBAs will not fluoresce, making the paper appear yellow, therefore causing your eyes to see the image color differently.

Fluorescence
The ability of a substance to absorb light at a specific wavelength and emit it at a higher wavelength is called fluorescence. Optical brighteners or fluorescent whitening agents are added to improve the appearance properties (brightness, whiteness etc.) of printing and writing papers (Heikkilä et al. 1998). When light strikes a fluorescent material, some of the electrons acquire energy and a portion of energy is converted into heat. Thus, the emitted light has lower energy as compared to the incident light, which results in longer wavelengths (Hubbe et al. 2008). In contrast, non-fluorescent material either completely absorbs and converts such energy to heat, or instantly releases the energy by emitting light at a wavelength equal to the incident light (Hubbe et al. 2008).

Desired Properties of Fluorescent Whitening Agents for Textiles Use:
Before selecting an optical brightener for textile application, we must look for following properties-

1. It should have good solubility, should not have its own color and good substantivity for the textile substrate under OBA application.
2. OBA’s should have good light as well as wet fastness properties.
3. Its rate of strike on the substarte.
4. Build up and exhaustion properties.
5. Requirement of electrolytes and its sensitivity towards different exhausting agents.
6. Effect of temperature on the exhaustion and build up properties.
7. Application pH range and sensitivity towards change in pH.
8. Effect of water hardness.
9. It should have good leveling and penetrating properties.
10. 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.
11. It should be compatible and stable with finishing chemicals, auxiliary and process such as heat and temperature.
12. It should be stable and fast to the common oxidative and reductive bleaching chemicals and bleaching systems.

Composition
Members of this class of diaminostilbene sulfonate derivatives are highly conjugated molecules having planar structure and anionic charge. All of these adjectives also apply to direct dyes. What makes this particular class of direct dyes different is that they absorb ultraviolet light and re-emit light in the blue region of the visible spectrum. Another traditional term for fluorescent whitening agents (FWAs) is “optical brightening agents” (OBAs). Three general types of FWAs are widely available. The type most often used by papermakers has four sulfonate groups (tetrasulfonated). It has intermediate solubility in water and it is readily retained on fibers, especially if alum or another cationic material is present. Hexasulfonated FWAs don’t retain as well when added at the wet end, but they may give higher optical efficiency when used at the size press due to less association between the molecules in the dried starch film. Disubstituted FWAs are sometimes used for specialty purposes, for instance when water bleed-fastness is critical.

Function
Increasing the white appearance of papers by absorbing invisible ultraviolet light and re-emitting it in the blue region of the visible spectrum. This strategy can compensate for a yellow tint of many types of pulps that have been bleached to moderate levels.

Classification of OBA
The classification of OBA can be either on the chemical structure of the brightener or on its method of application.

They can be classified in to two large groups-

1. Direct (substantive) brightener.
2. Disperse brightener.

1) Direct optical brightening agents are predominantly water-soluble substance used for the brightening of natural fibers and occasionally for synthetic material such as polyamide.

2) Disperse optical brightening agents are mainly water insoluble and as with disperse dyes they are applied either to colored from an aqueous dispersion on they can be used for mass coloration. They are used for synthetic materials such as polyamide polyester acetate.

From the chemical point of view, they are classified according to either chemical structure. Chemical optical brightening agents are classified in to derivatives of stlibene, coumarin, 1, 3 diphenyl pyrazoline, derivative of naphthalene dicarboxylic acid, derivatives of heterocyclic dicarboxylic acid, derivatives of cinnamic acid and substance belonging to other chemical system.

Basic Types of Whiteners
Basic class types of brighteners include:

1. Triazine-stilbenes (di-, tetra- or hexa-sulfonated)
2. Coumarins
3. Imidazolines
4. Diazoles
5. Triazoles
6. Benzoxazolines
7. Biphenyl-stilbenes

Factors Influencing Whitening Process
The factors influencing whitening process are as follows:

1. pH of the bath
2. Temperature of the bath, and
3. Time required for the process.

Mechanism of action
Absorption (A) of light quanta by the brightener molecules induces transitions from the singlet ground state S0 to vibrational levels of the electronically excited singlet states (S1).

Brighteners in the S1 state are deactivated by several routes. Fluorescence results from radiative transitions to vibrational levels of the ground state (F). Deactivation processes competing with fluorescence are mainly non-radiative deactivation to the S0 state (IC) and non-radiative transition to the triplet state (intersystemcrossing, ISC).

The efficiency of fluorescence is measured by the quantum yield:

……………………………………….Number of quanta emitted
The quantum yield (Φ) = …………………………………..……………….
……………………………………….Number of quanta absorbed

It is determined by the relative rates of fluorescence emission and the competing processes. When fixed in solid substrates, brighteners fluoresce with high quantum yields.

Method of whitening agent
There are two methods, which are generally used for this purpose, by using a blue tinting agent, which absorbs the yellow part of the light and reflected light appears to be of bluish tint. The total light reflected by this mean is less than the total incident light.

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 nonionic OBA’s are exhaust on all synthetics.

Measurement of Whiteness and Evaluation of OBA
OBA are evaluated in the same way as dyes. Their concentration in powder and liquid form is determined by subjective (visual) comparison of the samples in daylight or under an ultra violet lamp, by titration with cetylpyridiniumchloride or spectrophotometrically against a standard of known concentration. Objects can only be seen as colored objects when they are illuminated by light. Since light is an electromagnetic radiation, which is either absorbed or reflected by the object which appears, colored due to the action of electromagnetic radiation on the human eye. Fluorescers or optical brighteners, as they are also called, are to improve the whiteness of textiles. Objective measurement of the whiteness and of the change in whiteness can be accomplished by color measurement, since different hues of whiteness can be measured like any other color.

To assist in a correct interpretation of such measurements, knowledge of the fundamentals of colorimetry is required, that is color measurement comprising the systematic compilation and evaluation of physical data supplied by suitable measurements. The wavelengths of the colors perceived in the range from violet {400- 430 nm} {shorter wavelengths are called ultra violet}, to 430- 485 nm blue, 485 to 570nm green, 570- 585 nm yellow, 585 to 610 nm orange, and above 610 nm red. There are many cases in practice where it is interesting to know the degree to which an optically brightened fabric has been bleached, as it is often important to know the original whiteness and the increase in whiteness achieved by the optical brightener. Besides, it is often interesting to know whether a shading dyestuff was used in addition to the brightener to enhance the whiteness. It is necessary to use a special spectrophotometer for the measurement of optically brightened specimens. Although the reflectance curve will not be in accordance with actual conditions up to a wave length of approximately 420nm (i.e. in the excitation range of the brightener), the reflectance curve of the substrate will be reproduced accurately at higher wavelengths where fluorescence is no longer observed.

Storage
OBAs are extremely sensitive to light and should not be left standing unprotected for long. Though standard solutions may be kept in dark for several days, fresh ones are to be prepared during application.

Exhaustion of OBAs
The exhaustion of fluorescent brighteners is dependent on several factors:

1. Nature of textile goods
2. Temperature
3. Addition of salt
4. Liquor ratio
5. Concentration of fluorescent brightener
6. pH of bath

Application of Fluorescent Brightening Agent (FBA)
The FBA can be divided into three application categories.

1. Anionic: containing sulfonate group used for Cotton, Wool and Nylon.
2. Cationic: used for mostly Polyacrylonittrile fibers.
3. Nonionic: used for PET, Acetate, Polyacrylonittrile and Polyamide.

FBA are applied by exhaust or padding process and then fixed by heat treatment.

FBA for Cellulogic Fiber
Tiponal BV, 2B, GS, RBN, 4BM. Applied by exhaustion or padding method. Exhausts at pH 8-11 or higher. Salt should be used for exhausting Tiponal BV on the fiber. Tiponal 4BM is suitable for application by the padding technique. Blancophor CL is intending for whitening cellulogic fiber. It is stable to Hydrochloride and Chloride Bleach bath, so that it may be applied while bleaching with these bleaching agents. Fluorite BW stables alkaline hydrogen peroxide and per salt bleach and hence can be applied during bleaching. It can be applied by exhaust dyeing method. Since it has affinity for cellulogic fiber. Uvitex CK stable to hard water.

FBA Polyester Fiber
Fluorite XMF intended for PES. Available in past form. Applied at 100c with carrier. Thermosol used at high temperature pad-dry-bake process. Palanil white G liquid. Blancophor EBL of Bayer AG. Uvitex SEB-nonionic, micro fore aqueous dispersion all used in this purpose.

Process Requirements

Equipment Used:
Three types of Thies machines are used for whitening process. These are:

1. Mini-soft
2. Eco-soft
3. Luft-roto

Accessories Used in Whitening
Key accessories used in whitening process are:

• Mixing tank
• Machine Tank
• Bohme meter
• pH meter
• Eye protecting glass
• Hand gloves
• Sewing machine
• Hand lifter for carrying the batch, and
• Gum boot.

Materials / Chemicals Used
Following materials/chemicals are used in the whitening process:

• Water
• Steam
• Compressed Air
• Wetting agent
• Detergent
• Sequestering agent
• Acid
• Anti-creasing agent
• Optical Brightener for Cotton & Polyester
• Caustic Soda,
• Hydrogen per Oxide, and
• Stabilizer, etc.

Operation Procedure

Whitening Process for 100% Cotton
The sequence for whitening process for 100% cotton fabric is as follows:

1. Take water in bath at required level.
2. Add Detergent, Sequestering Agent, Anti-creasing Agent, Stabilizer.
3. Circulate the fabric for 5 minutes at 50^oC.
4. Add Caustic Soda and circulate for 5 minutes at 50⁰C.
5. Add Hydrogen per Oxide and circulate for 5 minutes at 50^oC.
6. Add Optical Brightener agents and raise the temperature at 100⁰C.
7. Continue circulating the fabric for 50 minutes at constant temperature (100⁰C).
8. Circulate the fabric and decrease the temperature at 60⁰C.
9. Check the shade with approved shade.
10. Rinse and Drain.
11. Add Acetic acid and circulate for 10 minutes at 50⁰C.
12. Drain.
13. Unload.

Graphical Representation of Whitening Process of 100% Cotton
The whitening process for 100% Cotton can be graphically presented as below:

Whitening Process for Polyester-Cotton Blended Fabric
The sequence of Whitening Process for Polyester-Cotton Blended fabric is as follows:

1. Take water in bath at required level
2. Add Detergent, Sequestering Agent, Anti-creasing Agent, Stabilizer
3. Circulate the fabric for 5 minutes at 50⁰C
4. Add Caustic Soda and circulate for 5 minutes at 50⁰C
5. Add Hydrogen per Oxide and circulate for 5 minutes at 50⁰C
6. Add Optical Brightener agent and raise the temperature at 110⁰C
7. Continue circulating the fabric for 30 minutes at constant temperature (110⁰C)
8. Circulate the fabric and decrease the temperature at 600C
9. Check the shade with approved shade
10. Rinse and Drain
11. Circulate the fabric for 10 minutes at 80⁰C
12. Drain
13. Add Acetic acid and circulate for 10 minutes at 50⁰C
14. Drain
15. Unload

Graphical Representation of Whitening Process for Polyester Cotton Blended Fabric:
The whitening process for Polyester Cotton blended fabric can be graphically presented as below:

OBA Removal
A simple OBA stripping technique given below can be adopted in jiggers very efficiently and you can strip off almost 98% of OBA.

Recipe and method of process flow given below may be used as new hint.

a) Wetting the fabric:
Soap – 1 gm/liter, Soda ash 1 gm/liter – treat the fabric at 70°C for 30 minutes.

b) Oxidative Bleaching:
Potassium Permanganate (KMnO4) – 0.5% wof at cold for 30 minutes – to the same bath add Hydrochloric acid (35% conc.), 3 times the quantity of KMnO4 continue run for another 30 minutes. Then raise the temperature to 50°C and run for 30 minutes.

Drain the bath – cold wash – 30 minutes.

c) Reductive Bleaching:
Run 10 minutes with Caustic soda 5 gms/liter and then add 5 gms/liter of Hydros (Sodium Hydro Sulphite) at room temperature and run for 10 minutes. Then slowly raise the temperature to 60°C. During this process the brown coloration you have got would be converted to light beige like shade. Now do one thorough cold wash.

d) Oxalic Acid Treatment:
Treat with 2 gms/liter of Oxalic acid at room temperature for 30 minutes or 2 ends followed by a cold wash.

e) Soda ash Neutralization:
Treat with 2 gms/liter of soda ash at room temperature for 30 minutes and again do a cold wash.

f) Acetic acid Neutralization:
Treat the fabric with 1 gm/liter of acetic acid for 10 minutes at cold. Check the pH and let it be 6.

Now you will wonder to see thorough removal of OBA.

Strategies for Use
Some of the critical factors in FWA use include (a) retention, (b) quenching, (c) competition with other UV-absorbers, and (d) metamerism. The strategies for adding and retaining FWAs to the wet end of a paper machine are very similar to those used with direct dyes. For instance, FWA retention can be increased by sequential addition of alum to the pulp stream, either before or after the whitener. But there is a key difference; highly charged cationic polyelectrolytes easily can destroy the fluorescent character of the molecule. The effect is called “quenching.” (The effect is related, but much more problematic, than the tendency of the highly cationic polymers to modify the hue of certain direct dyes.) Both lignin and titanium dioxide are potent absorbers of ultraviolet light. For this reason, internal addition of FWAs to high-yield furnish or to furnish that contains TiO2 is likely to be ineffective. This is one of the reasons why it is common for papermakers to add all or most of the FWA at the size press. The idea is that light first encounters material nearer to the surface of the paper, and this is where much of the size-press formulation ends up. Metamerism refers to the phenomenon that two objects may appear to have identical color when viewed under a certain type of illumination, but the same objects might not match under a different lighting. This is a very common occurrence in the case of samples that contain different levels of activity of fluorescent whitening agents.

Common uses
Brighteners are commonly added to laundry detergents to replace whitening agents removed during washing and to make the clothes appear cleaner. Optical brighteners have replaced bluing which was formerly used to produce the same effect. Some brighteners can cause allergic reactions when in contact with skin, depending on the individual.

Brighteners are used in many papers, especially high brightness papers, resulting in their strongly fluorescent appearance under UV illumination. Paper brightness is typically measured at 457 nm, well within the fluorescent activity range of brighteners. Paper used for banknotes does not contain optical brighteners, so a common method for detecting counterfeit notes is to check for fluorescence.

Optical brighteners have also found use in cosmetics. One application is to formulas for washing and conditioning grey or blonde hair, where the brightener can not only increase the luminance and sparkle of the hair, but can also correct dull, yellowish discoloration without darkening the hair. Some advanced face and eye powders contain optical brightener microspheres that brighten shadowed or dark areas of the skin, such as “tired eyes”.

A side effect of textile optical whitening is to make the treated fabrics more visible with Night Vision Devices than non-treated ones. This may or may not be desirable for military or other applications. Optically brightened paper is often not useful in exacting photographic or art applications, since the whiteness decreases with time.

End uses of optical brighteners include:

1. Detergent whitener (instead of bluing agents)
2. Paper brightening (internal or in a coating)
3. Fiber whitening (internal, added to polymer melts)
4. Textile whitening (external, added to fabric finishes)
5. Color-correcting or brightening additive in advanced cosmetic formulas (shampoos, conditioners, eye makeup)

Role of Optical Brightening Agents in Textile Wet Processing:
The operation of whitening, i.e., bleaching or brightening, is concerned with the preparation of fabrics whose commercial value is dependent on the highest possible whiteness. In bleaching, textile process houses are concerned with the removal of colored impurities or their conversion into colorless substances. In chemical bleaching, impurities are oxidized or reduced to colorless products. Physical bleaching involves the introduction of a complementary color whereby the undesired color is made invisible to the eye in an optical manner, e.g., in bluing the yellow cast of substrates such as textiles, paper, sugar, etc. is eliminated by means of blue or blue-violet dyes. Through color compensation the treated product appears whiter to the eye; however, it is actually grayer than the untreated material.

With the aid of Optical brightening agents (OBAs), also referred to as fluorescent whitening agents, optical compensation of the yellow cast may be obtained. The yellow cast is produced by the absorption of short-wavelength light (violet-to-blue). With OBAs, this lost light is in part replaced; thus, a complete white is attained without loss of light. This additional light is produced by the whitener by means of fluorescence. Fluorescent whitening agents absorb the invisible UV portion of the daylight spectrum and convert this energy into the longer-wavelength visible portion of the spectrum, i.e., into blue to blue-violet light. Fluorescent whitening, therefore, is based on the addition of light, whereas the bluing method achieves its white effect through the removal of light.

A fluorescent whitener should be optically colorless on the substrate, and should not absorb in the visible part of the spectrum. In the application of OBAs, it is possible to replace the light lost through absorption, thereby attaining a neutral, complete white. Further, through the use of excess whitener, still more UV radiation can be converted into visible light, so that the whitest white is made more sparkling. Since the fluorescent light of a fluorescent whitener is itself colored, i.e., blue-to-violet, the use of excess whitener always gives either a blue-to-violet or a bluish green cast.

Many chemical compounds have been described in the literature as fluorescent compounds that provide a suitable whitening effect. Collectively these materials are aromatic or heterocyclic compounds; many of them contain condensed ring systems. An important feature of these compounds is the presence of an uninterrupted chain of conjugated double bonds, the number of which is dependent on substituents as well as the planarity of the fluorescent part of the molecule. Almost all of these compounds are derivatives of stilbene or 4,4-diaminostilbene; biphenyl; 5-membered heterocycles such as triazoles, oxazoles, imidazoles, etc. or 6-membered heterocycles, e.g. coumarins, naphthalimide, s-triazine, etc.

Textile substrates of natural or synthetic fibers are contaminated in the raw state by substances of varying degrees of yellowness. Bleaching is required to remove the yellowish cast. Chemical bleaching agents destroy the yellow coloring matter in fibers. However, even if bleaching processes are carried to the technically acceptable limits of damage to the fibers, they never succeed in completely removing this intrinsic color. To produce the color white, it is necessary to dye with a fluorescent whitener.

OBAs used in textiles can be divided into three categories

1. Products containing sulfonic acid groups, corresponding to acid dyes, for cotton, wool, and polyamides;
2. Cationic whiteners that behave in the same way as basic dyes, for polyacrylonitrile fibers; and
3. Whiteners containing no solubilizing groups, corresponding to disperse dyes, for polyester and secondary acetate fibers.

This is not a strict division since nonionic OBAs can whiten polyacrylonitrile and polyamide, and certain cationic OBAs produce effects on polyester. For dyeing fiber blends such as viscose–polyamide, polyamide–Spandex, or polyester–cotton, only compatible OBAs may be used that do not interfere with one another or have any detrimental effect on fastness properties.

Before treatment with optical brighteners, due care should be taken during pretreatment, scouring and removal of sizing chemicals. Otherwise these will hinder the effect of OBA onto the substrate and may result in uneven brightening effect on the fabric.

In an exhaust procedure the fluorescent whitener is exhausted from a long liquor onto the substrate until an approximate equilibrium is reached between whitener in the bath and whitener on the substrate. In this procedure the equilibrium is biased primarily toward OBA on the substrate, ie, the highest possible degree of exhaustion is desired. Exhaust procedures are used for loose stock, yarns, woven fabrics, and knit goods which give poor or unsatisfactory results in padding processes, and for garments and garment parts.

Padding methods, i.e. application from short liquors, are increasingly important for whitening piece-goods. Woven fabrics or knit goods are passed in an unfolded, open-width state through a small trough charged with treatment liquor containing OBA and subsequently between squeeze rollers to express the liquor to a precisely defined liquor pick-up.

In conjunction with the increased use of synthetic fibers and blends of synthetic and natural fibers, and the modernization of application processes which has taken place simultaneously, the technique of textile whitening has been improved considerably.

In case of polyester fibers brighteners are applied in the form of fine aqueous dispersions. The brightener particles penetrate into the fiber in a state of molecular dispersion and they are held in the fiber by Vander waals forces. A part of finely dispersed particle forms a molecular solution in the dye-bath. Some of the finely dispersed particles may diffuse directly into the fiber. In addition to the dispersion and diffusion of the brightener, stability is also of vital importance.

During the drying and, if required, the heat treatment that follows, the fluorescent whitener is fixed on the substrate. OBAs used in padding procedures must have low substantivity during the padding operation. This is an important prerequisite for level whitening with no tailing.

Furthermore, fluorescent whiteners are applied in combination with bleaching and finishing steps. Fluorescent whiteners used in such processes must be stable and should not interfere with the operation. Whitening in combination with the finishing process is used primarily for woven fabrics of cellulosic fibers and their blends with synthetic fibers.

We at Matex offer Optical brightening agents under the brand name Megawhite. Highest brightness and best fastness can be achieved by using Megawhite series of brighteners. It meets all the requirements with regard to whiteness, hues and fastness properties.

Despite the difficulties faced by OBA producers and customers alike in the midst of the current global economic downturn there is still room for innovative producers to develop and to introduce onto the market new products offering more cost-effective, safe and environmentally sound solutions for the customer. There is every reason to believe that OBAs have a bright future.

The Impact on Human Health:

(1) Had no stimulation to the skin
After years of animal and human experiments show that: even the direct skin contacts of fluorescent whitening agent CBS pure product, no irritation to the skin, does not cause skin allergies.

Shen Yongjia Professor prepared “fluorescent whitening agents,” a book that: fluorescent whitening agent will not be absorbed by the skin. Even if the fluorescent whitening agent CBS in the use of the process may have a small amount of adhesion on the skin, and there is no human skin reaction, and through washing daily activities (e.g., take a shower, wash) can easily be completely washed off, percutaneous absorption not.

Therefore, the skin damage and washing liquid contact will not add CBS.

(2) Had no adverse effect on wound healing
Published in 1994, “the German dermatology” magazine “of fluorescent whitening agent toxicological properties” in an article pointed out that, even if is in direct contact with the wound with textile material containing fluorescent whitening agent, nor on wound healing have adverse effects on human skin, and is not caused by pathological changes.

(3) Metabolism:
Fluorescent whitening agent CBS is a water soluble, soon completely excreted by normal metabolism.

Germany Georg Thieme Publishing House of the “environmental quality and safety” supplementary fourth volume “of fluorescent whitening agent” (`Environmental Quality and Safety` Supplement Volume IV `Fluorescent Whitening Agents`) pointed out in one book, by study of metabolism in mice, in the high-dose fed detergent with fluorescent whitening agent CBS, the vast majority of brightening agent can quickly through the intestinal tract discharge, are not absorbed in the intestine. The blood, liver, kidney, brain, muscle and fat were not fluorescent brightener residue, which will not cause accumulation. So, in daily life, even a small amount of fluorescent whitening agent CBS enters the human body, will be through the normal metabolism excreted very quickly.

(4) Fluorescent whitening agent CBS had no teratogenic, carcinogenic free:
The acute toxicity and the research on various animal two-year chronic toxicity in mice experimental study demonstrated that: CBS belongs to the non-toxic substances, carcinogenic, teratogenic, no no no mutagenicity.

The Detergent Association (A.I.S.E.) test report “risk assessment of effects of domestic detergent ingredients to human body and environment of fluorescent whitening agent FWA-5” of fluorescent whitening agent CBS (also called FWA-5) toxicological research results show that: the fluorescent brightener CBS without light carcinogenic reaction.

German Environment Ministry in 1983 solemnly declare, fluorescent brightener CBS has no sensitization and teratogenicity.

The Impact on the Environment

(1) CBS degradation of high
CBS used in detergent is not high, after rinsing dilution, sewage by a general treatment, CBS can be reduced by more than 95-, and finally into the environment in small quantities. Even if a few into surface water (such as river), is also very rapid light degradation, photo degradation products is also very easy to two degradation, namely microbial eaten decompose into carbon dioxide, has no influence on the environment material.

(2) Had no effect on the food chain
CBS is a water-soluble fluorescent whitening agent, the fish after ingestion, not stranded in biological tissues and organs. Laboratory studies have shown that fish, even in high concentration CBS under the conditions of life after a period of time, out of this environment for two days, CBS accumulation in fish body will not.

The results showed the ecology research based on long, CBS has no harm to the environment.

August 2, 2011 afternoon, China Cleaning Products Industry Association held a “detergent with fluorescent whitening agent security” experts will meet the media in Beijing China hall of science and technology. The conference invited experts are engaged in toxicology, medicine, fluorescent whitening agent, washing supplies, textile dyes, research in the domestic well-known professor and researcher. Experts from the professional point of view, the fluorescent whitening agent of the authority of interpretation, and draw the same conclusion: safe detergent fluorescent whitening agents on human, friendly to the environment, is the auxiliary function cannot replace of cleaning products.

Fluorescent whitening agent research experts, East China University of Science and Technology doctoral tutor Professor Shen Yongjia said, fluorescent whitening agent is a kind of optical conditioner. The principle of its role is the invisible ultraviolet light into blue light visible to the naked eye, and Huang Guangxiang stack, fabric itself to emit white light, soft, pleasing whitening effect, has been widely used in many fields of textile, papermaking, detergents, plastics etc. Professor Shen said, “fluorescent whitening agent in clothes coated with a layer of white stuff to cover the stains” is ridiculous, because the fluorescent whitening agent and water, are colorless and transparent, cannot cover the stains. We add it to the washing liquid is because it has any product cannot replace color conditioning.

Toxicologists, Tongji Medical College of Huazhong University of Science and Technology of occupational and Environmental Health Sciences, Professor Wu Zhigang stressed, detergent for acute toxicity grade and salt fluorescent whitening agent, no teratogenic, carcinogenic, mutagenic no, no stimulation to the skin, will not cause skin allergies, not absorbed through skin, no adverse effects on wound healing. Even if eating, do not accumulate in the body, soon completely excreted by normal metabolism. In addition, he also mentioned the detergent with fluorescent whitening agent of environmental safety without harm, easy degradation, not to accumulate in organisms. Laundry detergent with fluorescent whitening agent in the world also have been in use for 40 years, its security has been widely recognized by many scientists, government departments and authorities.

Conclusion:
At first conclusion we can say that most of the typical commercial detergents of domestic use appear to contain a considerable amount of optical brightening agents. The effect of optical brighteners on colored textiles appears to be different depending on color, that is to say, on the corresponding region of the spectrum. It seems to be more visible on the violet-blue colors and almost insignificant on orange and red. However, even in these colors a slight effect can be observed in case of lower fastness. The lighter the color is more intense the optical brightener’s effect can be, even in colors with a good fastness.

Dark colors presenting lower fastness to washing showed only a small difference on hue.

References:

1. Chemistry of the textile industry (edited by c.m. carr).
2. Fluorescent brightening agent by r. Williamson.
3. Fluorescent whitening agents (merrow technical library, textile technology series).
4. Fluorescent whitening agents by a.k. sarkar.
5. Textile preparation and dyeing by a.k.r choudhury.
6. Chemistry & technology of fabric preparation & finishing (dr. Charles tomasino).
7. The complete book on natural dyes & pigment by nirr board of consultants & engineers.
8. Optical brighteners effect on white and colored textiles (by m. Fatima esteves, a. cyrne de noronha, r. marques marinho).
9. Volume 1: reference book on chemical engineering by dr. sen.
10. Chemical technology in the pretreatment processes of textile by s.r. karmakar.

Mazharul Islam Kiron

Founder & Editor of Textile Learner. He is a Textile Consultant, Blogger & Entrepreneur. He is working as a textile consultant in several local and international companies. He is also a contributor of Wikipedia.

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