Bleaching Process for Full Whites and OBA application | Desiredproperties of Good OBA | Mechanism of Fluorescent Whitening

Introduction

The coloring matter ,whether it is natural or present as a contaminant in the fiber is generally decolorized by different bleaching methods. However the appearance of the textile substrate is somewhat creamish after the bleaching, therefore chemical treatments are become necessary to neutralize the yellow tint of the textile fibers.

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 substrate is more than the incident light , due to which the object appears brighter.

Desired properties of Good OBA
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.  

1. OBA’s should have good light as well as wet fastness properties.
2. Its rate of strike on the substarte.
3. Build up and exhaustion properties.
4. Requirement of electrolytes and its sensitivity towards different exhausting agents.
5. Effect of temperature on the exhaustion and build up properties.
6. Application pH range and sensitivity towards change in pH.
7. It should have good leveling and penetrating properties.
8. 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.
9. it should be compatible and stable with finishing chemicals, auxiliary and process such as heat and temperature.
10. It should be stable and fast to the common oxidative and reductive bleaching chemicals and bleaching systems.

Chemical Constitution of Optical Brighteners
Optical brighteners 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.

Mechanism of Fluorescent Whitening

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.