Optical brighteners, optical brightening agents (OBAs), fluorescent brightening agents (FBAs), or fluorescent whitening agents (FWAs), are chemical compounds that absorb light in the ultraviolet and violet region (usually 340-370 nm) of the electromagnetic spectrum, and re-emit light in the blue region (typically 420-470 nm) through the phenomenon of fluorescence. These additives are often used to enhance the appearance of color of fabric and paper, causing a "whitening" effect; they make intrinsically yellow/orange materials look less so, by compensating the deficit in blue and purple light reflected by the material, with the blue and purple optical emission of the fluorophore.[1]
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The most common classes of compounds with this property are the stilbenes, e.g., 4,4′-diamino-2,2′-stilbenedisulfonic acid. Older, non-commercial fluorescent compounds include umbelliferone, which absorbs in the UV portion of the spectrum and re-emit it in the blue portion of the visible spectrum. A white surface treated with an optical brightener can emit more visible light than that which shines on it, making it appear brighter. The blue light emitted by the brightener compensates for the diminishing blue of the treated material and changes the hue away from yellow or brown and toward white.[2]
Approximately 400 brightener types are listed in the international Colour Index database,[4] but fewer than 90 are produced commercially, and only a handful are commercially important. The Colour Index Generic Names and Constitution Numbers can be assigned to a specific substance. However, some are duplicated, since manufacturers apply for the index number when they produce it. The global OBA production for paper, textiles, and detergents is dominated by just a few di- and tetra-sulfonated triazole-stilbenes and a di-sulfonated stilbene-biphenyl derivatives. The stilbene derivatives are subject to fading upon prolonged exposure to UV, due to the formation of optically inactive cis-stilbenes. They are also degraded by oxygen in air, like most dye colorants. All brighteners have extended conjugation and/or aromaticity, allowing for electron movement. Some non-stilbene brighteners are used in more permanent applications such as whitening synthetic fiber.
Brighteners can be "boosted" by the addition of certain polyols, such as high molecular weight polyethylene glycol or polyvinyl alcohol. These additives increase the visible blue light emissions significantly. Brighteners can also be "quenched". Excess brightener will often cause a greening effect as emissions start to show above the blue region in the visible spectrum.
Brighteners are commonly added to laundry detergents to make the clothes appear cleaner. Normally cleaned laundry appears yellowish, which consumers do not like.[2] Optical brighteners have replaced bluing which was formerly used to produce the same effect.
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.[5] 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".
End uses of optical brighteners include:
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From around to , chemical brighteners were used by many Chinese farmers to enhance the appearance of their white mushrooms. This illegal use was mostly eliminated by the Chinese Ministry of Agriculture.[6]
Optical brighteners are often referred to as fluorescent whitening agents. Essentially, they are large molecules composed of (excuse my Science) phenyl and heterocyclic groups with conjugated double bonds. Most optical brighteners used in consumer products such as laundry detergents are with stilbene derivatives (1) or biphenyl derivatives (2). Fun fact: Stilbene means “glittering” in Greek.
Before these fluorescent molecules were invented in the s, blue dyes were used in laundry detergents and textiles to add a little bit of blue hue to help offset the “yellowing” of white clothes and linens due to aging and staining. But those blue dyes only make laundry visually appear whiter, not brighter. The fluorescent optical brighteners work differently by absorbing invisible UV light and turn it into visible lights in blue color. Once deposited on laundry fibers, those fluorescent molecules make clothes and sheets appear to be both whiter and brighter by hiding the yellowing hue away from our visual detections. Optical brighteners have gained huge popularity in applications for laundry detergents, textiles, cosmetics, paper, and hair care products over the years. Nowadays, millions of pounds of optical brighteners are used and discharged as wastewater into our ecosystems every year.
There are more than 400 optical brighteners that have been made in history. Due to performance, health and environmental concerns, less than 50 of them are still in mass production for commercial uses. Those remaining have much more acceptable safety profiles in terms of their toxicity to the environment and human health. However, they remain as concerned chemicals in commercial applications such as laundry detergents for the following reasons:
Once discharged as wastewater, those optical brighteners escape from water treatment plants and get released into open or underground water. They biodegrade slowly and may end up accumulating in marine lives. Some optical brighteners are acutely harmful for aquatic organisms.
Under natural lighting, those optical brightener molecules slowly go through a photodegradation (the alteration of materials by light) process and break down to smaller pieces as metabolites. The data from a few research reports suggests that potential estrogen disruption and reproductive toxicity concerns from certain stilbene derivatives and their metabolites need to be further investigated.
The functionality of optical brighteners has NOTHING to do with laundry cleaning.
They are designed to dissolve in water and then absorb onto laundry fibers during the washing cycle. Resisting to be rinsed off, they stay on fibers in order to create an optical illusion to make clothes and linens appear whiter and brighter. Once absorbed and accumulated onto laundry fibers after rinsing and drying, those optical brighteners and related metabolites are in direct contact with our skin and may cause harm for some people with sensitive skin.