Now You Know Whitening Agent: Properties, Function, Mechanism and Usages (Part-4)

Whitening Agent: Properties, Function, Mechanism and Usages (Part-4)

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

Previous Part

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.

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-

Direct (substantive) brightener.
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:

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

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

pH of the bath
Temperature of the bath, and
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 .