Systhesis Ultramarine pigment

BACKGROUND
The ultramarine blue pigment is the synthetic form of the semi-precious stone lapis lazuli. For centuries, a blue pigment has been prepared from this stone. This very high-quality pigment was, however, very expensive and so its chemical synthesis was sought. It was only in 1828 that J. B. Guimet discovered the ultramarine blue pigment synthesis process.

The current synthesis process for the ultramarine blue pigment, based on the process discovered by Guimet, consists of heating a mixture of meta-kaolin, sodium carbonate, sulfur and a reducer. These reagents are mixed in the desired proportions, then are calcined in furnaces. The temperature cycle is comprised of several steps: heating to 800° C. in the absence of air, followed by an oxidation step at a lower temperature.

This process produces ultramarine blue pigment of good quality, but presents several disadvantages:

1. the high temperatures and the duration of synthesis make this process costly in energy;
2. during heating in the absence of air and during cooling, a large quantity of gas is released, namely sulfur dioxide and carbon dioxide; SO2must be treated in order to adhere to local environmental protection standards, which increases the cost of production;
3.  the raw pigment wash water has a high COD (chemical oxygen demand) due to the presence of residual polysulfides and sodium thiosulfate. To adhere to local environmental protection standards, this water must be treated before being discharged;
4. the quality of the blue pigment synthesized varies as a function of the various parameters and as a function of the temperature cycle, in particular during oxidation.

Shortly after the discovery of the synthesis of the ultramarine blue pigment, pink and green pigment having the same structure as ultramarine blue pigment and also containing sulfur species were synthesized.

Ultramarine green was synthesized by the calcination, in the absence of air, of raw materials similar to those used for the manufacture of ultramarine blue. During its production, problems were encountered related to the discharge of the raw pigment wash water which contains a large quantity of polysulfides.

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Ultramarine Pigments for plastic and masterbatch

The main contaminant which may be present in a plastic masterbatch based on Ultramarine pigment is sulphide. There are two possible reasons for its presence.

Firstly, Ultramarine pigments contain sulphur chromophores, plus small quantities of unreacted sulphur. These unreacted species are volatile at plastics processing temperatures and are released as sulphides. Normally the amount of free sulphur present even in the general purpose grades manufactured by Holliday Pigments is not enough to cause any objectionable taint or odour as it is removed along with the other volatiles such as moisture during the production process.

However, food contact plastics are especially sensitive to taint and for these applications even well processed standard grades may not be pure enough. For that reason Holliday Pigments Premier BC ranges for severely demanding food contact applications are free from unreacted sulphur and superior performance is ensured by improving the specification for volatiles to 0.05% maximum at 150°c (standard grades are tested at 105°c) and the un-reacted sulphur to 20ppm maximum (standard grades 200ppm). After thorough independent testing of bottle caps coloured with this grade and immersed in pure mineral water, no evidence of taint in the water could be detected.

Secondly, Ultramarine is sensitive to attack by acids which release sulphides. Such acids may be present as breakdown products of the polymer (such as PVC) or in some foods (sparkling mineral water and fruit juices). While the polymer may protect standard grades of Ultramarine against such weak acids for a period, it is much better to use an Ultramarine which has its own protective coating to resist such attack. 

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