Brighteners, optical

Optical brighteners are organic substances which are used in plastics to correct discoloration or to increase whiteness and brightness. They absorb ultraviolet radiation below 300 nm and emit it as visible radiation below about 550 nm. The usage of optical brighteners in plastics is very small compared with their applications in other markets. This explains why one of the references cited below is about detergents. 

(1972). Optical brighteners, technology and applications, Noyes, New York. Lange, K. Robert (ed.) (1994). Detergents arid cleaners, a handbook for formulators, Hanser, Munich. 

Keywords brightness, white, hindered amine, benzophenone, benzo-triazole, heterocyclics, masterbatch. 

This book is about additives for plastics in this context, we shall treat paper and certain related materials as additives for resins. As will become apparent, there are also additives (such as pigments) in the paper used to make laminates, so this topic is relevant to the book s central theme in more them one way. 

Paper laminates are structures consisting of either thermosetting (the usual type), or thermoplastic resins, reinforced by a variety of fibrous materials. Such laminates are produced in a multilayer construction, either having several layers of the resin and fibrous material, or consisting of a single sheet of the reinforced resin, on top of a substrate such as particleboard or fibreboard. The end uses of the finished article can be divided into decorative applications and so-called industrial ones. 

The TLC of optical brighteners or bleaches, derived from 4,4 -diaminostilbene-2,2 -disulphonic acid, has been described by Theidel [86]. He was able to separate the cis and trans isomers on polyamide layers, using methanol-ammonium hydroxide-water (66 + 27 + 7) surprisingly the cis-compound migrated ahead of the trans-. The opposite order was found in TLC experiments of Latestak [50] on silica gel G, using the solvents n-propanol-5% sodium bicarbonate solution (66 + 33), n-butanol-pyridine-water (33 + 33 + 33) and amyl alcohol-p3n idine-25 % ammonium hydroxide (33 + 33 + 33). The substances were detected by observation in UV light. 

In daylight optical brighteners can thus compensate for the esthetically undesirable yellowish cast found in white industrial substrates, such as textiles, papers, or plastics. Furthermore, since a portion of the daylight spectrum not perceived by the eye is converted to visible light, the brightness of the material is enhanced to give a dazzling white. 

The first method of compensating for the yellowish cast was the use of a blue dye such as ultramarine or indigo. Bluing lowers the reflectance of the sample in the long-wavelength part of the visible spectrum (curve b). As a result, the sample takes on a neutral white appearance, but at the same time it loses brightness so that it looks grayer. 

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Optical amplifiers Optical bleaches Optical brighteners... 

Organic Reactions. Nitric acid is used extensively ia iadustry to nitrate aHphatic and aromatic compounds (21). In many iastances nitration requires the use of sulfuric acid as a dehydrating agent or catalyst the extent of nitration achieved depends on the concentration of nitric and sulfuric acids used. This is of iadustrial importance ia the manufacture of nitrobenzene and dinitrotoluene, which are iatermediates ia the manufacture of polyurethanes. Trinitrotoluene (TNT) is an explosive. Various isomers of mononitrotoluene are used to make optical brighteners, herbicides (qv), and iasecticides. Such nitrations are generally attributed to the presence of the nitronium ion, NO2, the concentration of which iacreases with acid strength (see Nitration). 

Papermaking additives can be categorized either as process additives or as functional additives. Process additives are materials that improve the operation of the paper machine, such as retention and drainage aids, biocides, dispersants, and defoamers they are primarily added at the wet end of the paper machine. Functional additives are materials that enhance or alter specific properties of the paper product, such as fillers (qv), sizing agents, dyes, optical brighteners, and wet- and dry-strength additives they may be added internally or to the surface of the sheet. 

Phenidone, l-phenyl-3-pyrazolidone (738) (B-76M140404), has been used as a nonstaining, high contrast photographic developer. New optical brighteners containing 2-pyrazolines (739) and pyrazoles (740) have been synthesized recently and their properties and applications reviewed (75AG(E)66s). 

Optical brighteners are colourless fluorescent dyestuffs that absorb in the near UV (350-390 nm) and fluoresce in the violet to blue region of the spectrum (425-445 nm). 

Stilbene, 4-[2-( 1,3,4-oxadiazol-2-yl)vinyl]-as optical brightener, 6, 446 Stilbenes... 

Some laundry detergents contain optical brighteners. These are fluorescent dyes that glow blue-white in ultraviolet light. The blue-white color makes yellowed fabrics appear white. 

Lawrence and Ducharme [489] have described a fast, simplified method for the detection of fluorescent whiteners in polymers, in which the polymer dissolution was applied directly to the thin layer. Also the separation of optical brighteners (Leucopur EGM, Azur 4, Azur 5, Hostalux ABC, Uvitex OB, Eastobrite OB) from plastics and migration into water and olive oil was studied by HPTLC on RP-18 silica using various mobile phase mixtures and UV detection [490]. 

A collection of UV spectra of plasticisers, fluorescent whitening agents (optical brighteners), UV absorbers, as well as of phenolic and aminic antioxidants was published by Hummel and Scholl [21]. UV absorbance data for isolated chromophores are listed elsewhere [22]. A general UV atlas of organic compounds is available [23]. 

Fluorescence is much more widely used for analysis than phosphorescence. Yet, the use of fluorescent detectors is limited to the restricted set of additives with fluorescent properties. Fluorescence detection is highly recommended for food analysis (e.g. vitamins), bioscience applications, and environmental analysis. As to poly-mer/additive analysis fluorescence and phosphorescence analysis of UV absorbers, optical brighteners, phenolic and aromatic amine antioxidants are most recurrent [25] with an extensive listing for 29 UVAs and AOs in an organic solvent medium at r.t. and 77 K by Kirkbright et al. [149]. 

Fluorescence in UV radiation is a frequently used method for detection of TLC spots, e.g. of Tinuvin 326 [42]. The fluorescence emitted by optical brighteners under UV light on a thin-layer plate has been utilised as a means of analysing these compounds [42]. On the whole, the use of fluorescence detection in poly-mer/additive analysis of extracts is certainly not overwhelming. Applied fluorescence has been described in a monograph [156]. 

OB, OB A Optical brightener, optical brightening agent PVI Pre-vulcanisation inhibitor... 

Descriptive terms such as fluorescent whitening agents , optical brighteners and optical bleaches have all been used for the products described in this chapter as FBAs. Many of these terms have validity and the term fluorescent brightening agents is preferred here only because it has been adopted in the indexes of Chemical Abstracts. 

Figure 16 ToF-SIMS images of total ion current, AKD (alkyl ketene dimer) size, optical brightener and hexosan distribution on multipurpose paper surface after coating with Au-Pd. Reproduced from Fardim and Holmbom [70], with permission from Elsevier. 2005.

Whiter whites Brighter brights You ve heard such claims made by many manufacturers of laundry detergents. The chemical structure of the additives called optical brighteners provides the essential factors that make these superior detergents possible. 

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