Levelling agents

Both -hydroxybenzaldehyde and its methyl ether, -methoxybenzaldehyde [123-11-5] (p-anisaldehyde) have found extensive use in electroplating. The most widespread appHcation has been in alkaline bright zinc plating, both in non-cyanide (77) and in cyanide-containing (78) baths. The aldehydes act as both brightening and leveling agents. 

Modern solutions fall mainly into three types (a) the plain cyanide bath which contains typically 20-25 g/1 of copper cyanide, 25-30 g/1 total sodium cyanide (6.2 g/1 free sodium cyanide), and is operated at 21-38 C and 110-160 A/m (b) the Rochelle copper bath to which is added 35-50g/1 of Rochelle salt and which is used at 66 C at up to 645 A/m and (c) the high-efficiency cyanide baths which may contain up to 125 g/1 of copper cyanide, 6-11 g/1 of free sodium or potassium cyanide, 15-30 g/1 of sodium or potassium hydroxide, and are operated at up to 6-9A/dm and 65-90 C. Most bright cyanide copper baths are of the high-efficiency type and, in addition, contain one or more of the many patented brightening and levelling agents available. Periodic reverse (p.r.) current is also sometimes used to produce smoother deposits. 

M) and at low current levels yields relatively smooth surfaces, even in the absence of leveling agents. 

As mentioned in Table 8.1, amphoteric surfactants contain both an anionic and a cationic group. In acidic media they tend to behave as cationic agents and in alkaline media as anionic agents. Somewhere between these extremes lies what is known as the isoelectric point (not necessarily, or even commonly, at pH 7), at which the anionic and cationic properties are counterbalanced. At this point the molecule is said to be zwitterionic and its surfactant properties and solubility tend to be at their lowest. These products have acquired a degree of importance as auxiliaries in certain ways [20-25], particularly as levelling agents in the application of reactive dyes to wool. 

Since levelling agents are invariably surfactants, they may be anionic, cationic, nonionic or amphoteric in nature. Sometimes combinations of these are used. The chemical structure of commercial products is seldom revealed, however hence only general principles can be covered here. The main mechanisms by which levelling agents operate [337-341] are as follows ... 

Many, but not all, levelling agents promote migration of dye in addition to retarding dyeing, such agents will obviously be a further aid to level dyeing. In some cases, however,... 

Table 10.36 Levelling agent types and their uses...

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