Sunday, June 22, 2008

Addition Agents

Addition agents for brightening, hardening, grain refining, surface smoothing, increasing the limiting current density, and reducing trees are frequently added to the acid copper sulfate bath, but the use of a particular addition must be evaluated for each application, because undesirable characteristics can then be avoided. For example, many of the addition are generally higher when addition agent proposed result in embitterment of the plate. Deposition potentials greatly increased by adding gelatin (0.2 g/l) or glue. These additions result in grain refinement, but this is chiefly unidirectional, because the structure remains columnar and becomes more fibrous. Gelatin additions to the sulfate bath introduce porosity, organic inclusions both.

The effects of glycine, one of the amino acids formed by the hydrolysis of gelatin, are similar to the effects of gelatin. Cystine and methionine, like gelatine, are cathode-polarizing agents. On the other hand, dicarboxylic amino acids appeared more effective than monocarboxylic.

Phenolsulfonic acid is used in the electrotyping industry, but the results with it depend on the sulfonation and purification procedures. Deposits become harder and smoother after a bath has been electrolyzed or dummied for a short time following an addition of phenol or phenolsulfonic acid.

The smoothing and grain refining tendencies of addition agents are sometimes associated with the formation of complex ions with copper or of colloids at or near the cathode interface. Gelatin of glycine, for example, forms complex ions with copper and also exist in colloidal form. Particles of colloids arising form additions of selenious and arsenic oxides have been observed by ultramicroscopic examination to concentrate at the cathode.

With 0.14 to 0.42 mg/l selenious oxide and 0.0065 to 0.13 mg/l aresnious oxide, grain structure was refined, surfaces were smoothed, and density was improved. Nevertheless, the ability of a colloidal or ionic addition agent to improve the deposit cannot be conclusively predicted in advance on the basis of a general knowledge of its ionic or colloidal behavior in electrolytes. Thus, in a copper (II) nitrate bath colloidal addition agents were effective as cathode polarizers, but did not result in surface smoothing or grain refining.

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