Лако-красочные материалы — производство

The optimum coupling conditions depend on the nature of the diazo and coupling components used. The acidic diazonium salt solution is generally added to the so­lution of the coupling component. Because, in addition to acid derived from the diazotization process, additional acid is released during coupling, the optimum pH value must be adhered to by adding bases. Alkali metal hydroxides, sodium carbonate, sodium hydrogencarbonate, ammonia, calcium carbonate, magnesium oxide, etc., are used for this purpose. Sodium acetate or formate (weakly acid) or sodium phosphate (weakly alkaline) can also be added to buffer the acid. These are added to the reaction mixture before, during, or after combining the compo­nents. Because the pH value at the feed point of the diazonium salt solution is always different than that after thorough mixing has been completed, the type of stirrer and speed of stirring are also important in many instances. The sequence in which the two components are combined can also greatly influence the result.

The coupling reaction may be completed immediately after the components are mixed or after several hours. If the reaction requires a longer time, it is advi­sable to cool with ice and avoid exposure to bright light. To check whether excess diazonium compound is still present, a drop of reaction solution is spotted onto filter paper together with a component that couples readily (e. g., weakly alkaline H acid solution). If no coloration appears, coupling is complete. The presence of unconsumed coupling component can be determined by spotting with a solution of diazonium salt.

Attention must also be paid to the volume of coupling solution or suspension. With starting components of low solubility, the physical state is an important fac­tor. To achieve complete reaction of diazo or coupling components with low solubility, it is often necessary to ensure that the reactants are distributed as finely as possible. This is carried out, for example, by adding dispersing agents to the diazo component or to the coupling mixture or by adding emulsifiers during acid precipitation of the coupling component prior to azo coupling. In each case, the most favorable reaction conditions must be precisely established and, because of the various influences that result in undesirable side effects, they must be care­fully adhered to during manufacture.