Light exposure colourant

A white to light tan-coloured, amorphous or microcrystalline powder, which gradually darkens on exposure to light. M.p. 144° to 148°. 

Light exposure may cause either fading or darkening of both materials and colours. (Note that temperature/humidity also influences change.) Ink fade is normally checked by Xenontest type equipment (main supplier Heraeus) and is measured against the British wool scale. 

When packaging cured meat products it is important to keep the O2 and light exposure at a minimum. Moller et al. (2002) investigated the colour stability of cured ham under different packaging and storage conditions according to Table... 

Characters and Tests.—Light fawn-coloured powder, which changes colour by exposure to air metallic taste. The presence of free iodine is recognised by the preparation emitting the odour peculiar to this element, and by distilled water, after having been shaken with it and filtered, giving a blue colour (starch iodide) with cold aqueous solution of starch. 

The most sensitive indicator that evidence occurrence of wood degradation process under UV light exposure is the colour change of wood [24,45, 73-75]. The rapid colour differences evidenced after irradiation as primary consequences of the photo-oxidation phenomena can be useful for establishment a correlation between testing of accelerated and natural durability [76]. 

Ethyl iodide is a heavy liquid, of b.p. 72° and of d, 1 94 insoluble in water, When freshly distilled it is colourless, but on prolonged exposure to light it darkens in colour owing to the liberation of free iodine. Its chemical properties are almost identical with those of ethyl bromide given on pp. 102 and 103. 

Aniline when freshly distilled is a colourless liquid of b.p. 184° and d, 1025 on exposure to air and light, it develops a deep brown colour. It is an extremely important substance technically, being the starting point of many azo and other dyes. 

Iodobenzene, as usually prepared, is a very pale yellow liquid of b.p. 188°, and d, 1 83. The freshly distilled pure liquid i colourless, but soon redevelops the yellow colour on exposure to light. Iodobenzene is insoluble in water. 

Pure phenol is a colourless crystalline substance, having m.p. 43°, and b.p. 182° on exposure to air, it slowly sublimes, and on exposure to light, develops a pink colour. It has a characteristic odour, and a limited solubility in water. Phenol in dilute aqueous solution has strongly antiseptic properties, but the crystalline substance should not be allowed to come in contact with the skin, as it may cause severe blistering. 

Phenylhydrazine on exposure to light slowly darkens and eventually becomes deep red in colour salts of the base share this property but to a lesser degree, the sulphate and acetate (of the common salts) being most stable to light. Phenylhydrazine is largely used in organic chemistry to characterise aldehydes and ketones as their phenyl-hydrazones (pp. 342, 345), and carbohydrates as their osazones (pp. 136-140). It is readily reduced thus in the process of osazone formation some of the phenylhydrazine is reduced to aniline and ammonia. On the... 

Ultraviolet absorbers are a form of light screen which absorb primarily in the ultraviolet range. It is a requirement for most ultraviolet absorbers that absorption in the visible range should be negligible if this were not so the resultant instant colour formation could be worse than that experienced after prolonged exposure of the polymer. 

Struxine, C2iH3(,04N2, obtained by Schaefer from deteriorated nux-vomica seeds in about 0-1 per cent, yield, is regarded as a decomposition product of strychnine or brucine. It forms rhombic crystals from alcohol, is colourless, but becomes yellow on exposure to light and chars at 250°. It yields normal and acid salts, the latter only from excess of acid. With sulphuric acid it gives no coloration, but addition of potassium dichromate produces a yellow colour changing to green. 

Pigments for finishes are selected on the basis of their colour, but special attention must be paid to inertness in the chosen binder and stability and light fastness under the conditions of application and exposure. Flake pigments such as aluminium and micaceous iron oxide give finishes of lower moisture-vapour permeability than conventional pigments, and consequently contribute to better protection. 

Colour change Caused by exposure to light, the effect is greatest with hardwoods and is important in furniture, panelling and joinery any masked areas become blemishes. Strong sunshine bleaches most woods to a common brown colour. 

Bisphenol derivatives show very good antioxidant activity and are the best materials for light coloured articles. Selected materials are FDA approved. These derivatives have low volatility and no effect on cure rate and do not give a bloom. A slight pink discoloration can occur after prolonged exposure to light in white or light coloured products. 

Chemically carotenoids have conjugated double bonds that render them liable to oxidation. This tendency to oxidation can be diminished by adding antioxidants to the product. In the sort of product where natural colours are used suitable antioxidants would be tocopherols or ascorbic acid. Chemical antioxidants such butylated hydroxytoluene might be suitable technically but would not fit the image of an allnatural product. Ascorbic acid could be declared as vitamin C rather than as an antioxidant. Oxidation can be started by exposure to light and so this is best avoided. Carotenoids are generally stable to heat. The levels required can be as low as 10 ppm. P-Carotene is available as a nature identical form. 

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