Pearl milling

Temperatures of up to 100°C and higher may be observed as P.R.57 1 is dispersed in offset printing inks, especially in web offset inks by means of modern dispersion equipment. This is especially true for agitating pearl mills. Printing inks and prints may undergo color shifts if they are exposed to these conditions. The process, which is reversible, is caused by release of the water of crystallization from the pigment. 

Heating the leuco form in an aqueous alkaline medium in the presence of air, possibly also in the presence of a surfactant, affords the pigment form. Oxidation may also be achieved with sodium-m-nitrobenzenesulfonate. Another alternative is to precipitate the vat acid from the salt of the leuco form and to subsequently oxidize the product. It is also possible to mill the suspension of the leuco form together with aqueous sodium hydroxide/sodium dithionite in air, using a pearl mill. 

A higher quality a-modification with enhanced tinctorial strength and transparency is prepared from the leuco form of bromoisoviolanthrone. This intermediate in turn is manufactured by vatting crude bromoisoviolanthrone with sodium dithionite/aqueous sodium hydroxide. The product is separated and oxidized in an aqueous alkaline medium in the presence of surfactants. Application of shearing forces, preferably by means of a sand or pearl mill, and maintaining a temperature of 50°C produces improved pigment quality [26]. 

During the production of paint, it is important to keep control of the temperature. Due to the high shear forces a substantial amount of heat is produced especially in the pearl mill. Some ingredients in the paint are sensitive to heat, e.g. solvents which will have the risk to flash off. Therefore additional cooling is normally needed in this operation. Figure 9 shows a flow diagram of paint production in a factoiy. 

Binder Absorption, Smear Point, and Yield Point [1.78]. Smear point and yield point are used to determine the binder needed to formulate a suitable millbase for grinding by dissolvers, roll mills, ball mills, attritors, sand mills, and pearl mills. The amount of binder is given in volume or mass units. Apparatus rough glass plate, spatula with steel blade. 

Mills can be distinguished between the commonly used horizontal classic types and the more effective modern versions based on a stirring action. The latter are also known as pearl mills and are used for more difficult to disperse pigments. The traditional hammer mills are being phased out because of their unsafe operation. 

There are number of pearl mills available on the market, ranging from laboratory-scale to industrial-scale volumes. The ability of large-scale production is an essential prerequisite for the introduction of a product to the market. One advantage of pearl mills, apart from their low cost, is their ability in scaling up. 

Because of their small particle size and their resulting large surface area, pigment blacks are classed as pigments, which are more difficult to disperse. Depending on the supplied form, pigment blacks can be dispersed with all grinding machines, for example sand mill, pearl mill, ball mill, triple roll mill and others. 

Figure 1 Cross-section through a pearl mill. (Modified from Ref. 6.)...

The possibility of large-scale production is the prerequisite for the introduction of a product to the market. NanoCrysials are produced by pearl milling. Large-... 

Nanosuspensions are produced by using high-shear media mills or pearl mills. Impact created between the milling media and drug produces ample energy for the breakdown of the small-sized drug molecules into narKDsized particles... 

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