Mass colouration

Fig. 3.114. The names, molecular masses, colour index numbers and structures of the azo dye used in this study. Reprinted with permission from M. Chen et al. [171].

Aldehydes, particularly a,/Tunsaturatcd ones, react readily at low temperatures with amines to give polymeric high molecular mass, coloured products of unknown structure, called melanoidins. Heterocyclic ring systems, such as pyridines, pyrazines, pyrroles, and imidazoles, have been shown to be present. Melanoidins usually contain 3 4% nitrogen. 

Structures 4.1 show some low-molecular-mass coloured compounds formed in model Maillard systems. The organic chemistry underlying colour in melanoidins was not clear until 1972, when a specific compound, 4a, was isolated by Severin and Kronig167 in 0.07% yield from a heated aqueous mixture of xylose and isopropylamine acetate. It is yellow with Alliax = 365 nm. When the amine was replaced by glycine or lysine, the yield dropped further. Similar results were obtained when ara-binose was substituted for xylose. Subsequent work led to the identification of related compounds (see 4b and 4c), in some of which the ring oxygen of the furfural is replaced by NR. 

Structures 4.1 The structures of some low-molecular-mass coloured compounds formed in model Maillard systems... 

Coloured starch (high molecular mass) coloured saccharides... 

A thread viscose produced by the methods described above is very lustrous and this is looked upon as an undesirable property. Matt yams are therefore made by adding approximately 2 per cent by weight of titanium dioxide to the viscose solution before spiiming. Mass coloured yarns are finding increasing application. An insoluble finely divided pigment is in-... 

Broda, J. (2004a). WAXS Investigations of The Mass Coloured Polypropylene Fibers. Fibres and Textiles in Eastern Europe, Vol.ll, pp. 95... 

RK Datta, K Sen, AK Sengupta, RS Gandhi. Influence of mass colouration on structure and properties of polypropylene feeder and draw textured yams. Proc World Textile Congress on Polypropylene in Textiles, Huddersfleld, 1996, pp 231-248. 

Plastics may be coloured using a mass colouration technique, or with specific polymers it may be possible to surface dye. Mass colouration is the method normally employed, and colourants are incorporated into the polymer along with other additives such as stabilisers or lubricants. 

Three crystalline compounds, one violet, one pale green, and one deep green in colour, all have the molecular formula CrClj. 6H2O. When equal masses of the three compounds are separately treated with an excess of aqueous silver nitrate at room temperature, the masses of white precipitate produced are in the ratio 3 2 1. Suggest an explanation for these results. 

Amino-4 -methylthiazole slowly decomposes on storage to a red viscous mass. It can be stored as the nitrate, which is readily deposited as pink crystals when dilute nitric acid is added to a cold ethanolic solution of the thiazole. The nitrate can be recrystallised from ethanol, although a faint pink colour persists. Alternatively, water can be added dropwise to a boiling suspension of the nitrate in acetone until the solution is just clear charcoal is now added and the solution, when boiled for a short time, filtered and cooled, deposits the colourless crystalline nitrate, m.p. 192-194° (immersed at 185°). The thiazole can be regenerated by decomposing the nitrate with aqueous sodium hydroxide, and extracting the free base with ether as before. 

Benzophenone. To o 5g.of benzophenone add 1 g. of naphthalene (as a solvent) and heat until completely molten. Add a small piece of sodium and heat again. The surface of the sodium becomes green, and the colour [which is that of the free radical, (C H5),C-ONa] spreads throughout the molten mass on strong heating. 

Transfer 30 g. of the hydrochloride to a 500 ml. separatory funnel, add 100 ml. of water and shake until a thin paste of uniform consistency is obtained add 10 per cent, aqueous sodium hydroxide solution in the cold with shaking until the whole mass has become bright green (the colour of the free base) and the mixture has an alkaUne reaction. Extract the free base by shaking with two 60 ml. portions of benzene (1). Dry the combined benzene extracts with a Uttle anhydrous potassium carbonate, and filter into a distiUing flask fitted with a water condenser. Distil off about half of the benzene, and pour the residual hot benzene solution into a beaker. Upon cooUng, the p-nitrosodimethylaniUne erystallises in deep green leaflets. Filter these off and dry them in the air. The yield of p-nitrosodimethylaniUne, m.p. 85°, from the hydrochloride is almost quantitative. 

Aurin tricarboxylic acid [4431-00-9] M 422.4, m 300". The acid is dissolved in aqueous NaOH, NaHS03 solution is added until the colour is discharged and then the tricarboxylic acid is ppted with HCI [Org Synth Coll Vol I 54 1947]. Do not extract the acid with hot water because it softens forming a viscous mass. Make a solution by dissolving in aqueous NH3. See Aluminon for the ammonium salt. 

Colouring is not usually restricted to the surface but is throughout the mass so that damage due to scratching and abrasion is less obvious than with coated metals. 

Of these materials zein, the maize protein, has been used for plastics on a small scale. It can be cross-linked by formaldehyde but curing times are very long. Complicated bleaching processes have led to the production of almost colourless samples in the laboratory but the process cannot readily be extended to large-scale operation. The cured product has a greater water resistance than casein. Proteins from soya bean, castor bean and blood have also been converted into plastic masses but each have the attendant dark colour. 

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