Patent GB-808893-A (GB)
Title: Etching
IPC: C23F1/42
Filed: 1957-07-29
Abstract: 808,893. Etching. DOW CHEMICAL CO. July 29, 1957 [July 31, 1956], No. 24008/57. Class 100(2) An etching bath comprises a mixture of (a) an aqueous solution of nitric acid, (b) a wetting agent comprising succinic acid esters, petroleum sulphonates, saturated aliphatic acids, or esters of polyhydric alcohols and aliphatic acids, (c) 0.02-0.3 grams per litre of bath solution of a sul-. phur-containing compound selected from the group of sulphuric acid inorganic sulphates, and organic sulphates or sulphonates, the organic portion of the molecule consisting of phenyl radicals, alkyl radicals having up to eight carbon atoms, or alkylphenyl radicals the alkyl part having up to six carbon atoms, (d) a water-immiscible organic liquid substantially unreactive with the aqueous, nitric acid solution. (a) The nitric acid, measured as 42‹Baume is present as 3-15 per cent, preferably 5-11. per cent, by volume. (b) The wetting agent may be an ester of sulpho-succinic acid, preferably dioctyl sodium sulphosuccinate, with an aliphate alcohol having from four to twelve carbon atoms. For etching combination printing places (i.e. having half-tone and line areas), the alcohol should have from seven to nine carbon atoms. The proportion of wetting. agent is 2 to 10 grams per litre. (c) Suitable compounds are sulphuric acid, sodium. sulphate, sodium acid sulphate, magnesium. sulphate, aluminium sulphate, ammonium sulphate, diethyl sulphate, dimethyl sulphate, ethyl hydrogen sulphate, n-butyl sodium sulphonate, benzene sulphonic acid, sodium xylene sulphonate, sodium toluene sulphonate. The sulphur content of the solute ranges preferably from 0.03-0.18, and more particularly 0.04-0.095 grams per litre. (d) The organic liquid may be liquid alkyl benzenes or mixtures thereof, the alkyl portion comprising one or more groups, each group having up to four carbon atoms, up to a total of ten carbon atoms, e.g. mono-, di-, or tetraethyl benzenes, di- or tetra-methylbenzenes, or monobutylbenzenes. The liquid is present in 2-4.5 per cent, preferably 2.5-3.3 per cent, by volume. The bath may be conditioned by the addition of magnesium, 3- 5 grams per litre, or soluble nitrates, e.g. magnesium nitrate or sodium nitrate. The amount of conditioning agent is such that the molar ratio of hydrogen in the nitric acid to the total nitrate radical in the bath is 0.3-0.9, preferably 0.4-0.8. The bath is particularly suitable for etching combination printing plates. The temperature is 70-75 ‹F, and the etching time from 7¢ minutes for a fresh bath to 15 minutes for a used bath. If nitrates are added, the nitric acid concentration may be lower. The paddle speed is varied from 600-280 r.p.m., using higher speeds first so as to etch the half-tone areas to the desired depth of 0.0025-0.007 inches, then lower speeds to etch the line acids to the desired depth of 0.016 inches, the half-tone areas not being further etched at the slower speed. The plate may comprise mainly magnesium, in the form of an alloy containing 3 per cent aluminium, 1 per cent zinc, 0.06 per cent manganese, 0.025 per cent calcium, and minor traces of other impurities. The resist may be a polyvinyl alcohol photoresist. Examples of bath composition:- (1) Nitric acid 42‹Be, 16 litres; dioctyl succinate ester (a material comprising 75 per cent by weight of dioctyl-sodium sulphosuccinate, 20 per cent water, 5 per cent ethyl alcohol), 900 grams; diethylbenzene 3.3 litres; sulphuric acid (s.p.g. 1.84), 6 c.c., (2) Nitric acid, 16 litres; dioctyl succinate ester, 950 grams; diethyl benzene, 4.15 litres; anhydrous sodium sulphate, 50 grams. Specifications 703,814, 703,815, 704,824 705,004, 709,315 and 725,110 are referred to.