Light Fastness of Optical Brighteners

Introduction

Optical brighteners (sometimes called optical bleaches or fluorescent whitening agents) are fluorescent white dyes that absorb ultraviolet region (340 – 370 nm), light of electromagnetic region emit back visible blue light region (420 – 470 nm). This property makes optical brightener effective at masking any yellowing that may be present in cotton fabric. 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. They are used on a variety of finishing processes and should be compatible with practically all chemicals and auxiliaries used at different stages.

Furthermore, all round fastness property and good yield are the desired properties expected from optical brightness. In addition to this, different shades of whites are desired, as white shades are subject to fashion trends.

However light fastness is a prime concern and requirement which customers are demanding is on higher side ie 4-5 .

Most of the white goods are exposed to sunlight as drying also takes place in sunlight in few regions as well as buyers are demanding extra Light fastness.

Light fastness:

There is a large variation in the light fastness rating of these compounds, and when applied to cellulosic fibres and protein fibres 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 an important aspect in the cause of cellulosic, since any loss of OBAs effect due to sunlight will be replaced in subsequent laundering with domestic detergent. Fluorescent brighteners on nylon can reach a light fastness rating 4 with selected OBAs, a rating as high as 7 for polyesters, and in this class of acrylic fibres a light fastness of about 4 – 5.

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

Chemical structure of stilbene OBA’s & light fastness

Textile material (cotton, wool, linen and silk) and synthetic (mainly polyamide, polyester and polyacrylonitrille) are not completely white and have yellowish hue. Bleaching in the sun, blueing and matter chemical bleaching of textile and other materials increased the brightness of the products and eliminated certain hues or the local impurity of the original or industrially treated materials. Initially optical brighteners were 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.

Stilbene, is a diarylethene, i.e., a hydrocarbon consisting of a trans- ethene double bond substituted with a phenyl group on both carbon atoms of the double bond. The name stilbene was derived from the Greek word stilbos, which means shining. (Source: Wikipedia, the free encyclopedia) .

Nearly 80% of all OBAs produced are derived from stilbene 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 fibre on which it is applied. In this manner brighteners which are suitable for cotton are more are less substantive derivatives of diaminostilbene disulphonic acid. This stilbene 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 trans form 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.

Structure of diamino stilbene sulphonic acid 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 fibre. Apart from this the product must meet certain demands in respect of properties such as fastness to washing and light, etc.

In the above context due to rapid changes in the isomeric systems of Stilbene OBAs when exposed to light, light fastness properties are greatly affected and as mentioned in beginning the ratings do not exceed 3-3.5.

Precautions in processing

Although these are inherent property of FWAs few measures that can be suggested for improving the Light Fastness are

1. Using right type of optical brightener : Use of a high substantive product so that most of the OBA used in application gets fixed to the fibre and less gets physically held.

2. Thorough washing after the process to remove any loosely held OBA, as this part affects the Light Fastness rating.

3. Avoid drying directly in sunlight and dry preferably away from the sunlight.