GB1350930A - Concealed printing - Google Patents

Abstract

1350930 Invisible inks and developers therefor A B DICK CO 19 April 1971 [16 March 1970] 24037/71 Headings C4A and C5W [Also in Division B6] In a method of preparing and using a copy sheet of self-instructional or self-examinational material a visible image is formed on a sheet, which image comprises subject matter and a number of defined areas for receiving a multiple choice response for the subject matter. A concealed image comprising a Lewis acid is formed (before or after the visible image) in at least one of the defined areas. At least one of the areas is then contacted with a solid marking material containing a leuco dye intermediate effective to give an intense colour on contact with the Lewis acid. The Lewis acid may be an organic compound having one or more organic groups selected from carboxyl groups, ketone groups, quinone groups, ester groups, acid anhydride groups, nitro groups, cyano groups, halogen groups, substituted amine groups, oxime groups, imide groups and diazo groups. Alternatively the acid may be an inorganic material selected from inorganic acids, heteropolyacids and salts thereof. Among suitable acids there are named 2,4-dihydroxybenzene acid, gallic acid, 2,4- dihydroxy acetophenone, diphenyl carbazide, antipyrine, citric acid, phthalic acid, acid clays, phosphotungstic acid, tricarboxylic acids, bromosuccinic acid, salicylic acid, succinic acid and benzenephosphoric acid. The leuco-dye intermediate is preferably an N-bis(p-dialkylaminoaryl) methyl heterocyclic compound. Numerous examples of other suitable acids and leuco dye intermediates are given. The marking material may be a solid crayon or a coated paper, in which the leuco dye intermediate may be present in a composition also comprising a wax and a plasticizer. Specified waxes include paraffin and carnanba wax and polyalkylene glycols, the leuco dye being used in an amount 0À5-20 p.b.w. per 100 p.b.w. wax. For a paper coating the wax is dissolved in a solvent, e.g. an oil. The Lewis acid may be dissolved or dispersed in an ink, Example 3 being an ink comprising butanol, glycerine, water and citric acid, and Example 4 butanol, glycerine, water, gum arabic, starch, preservative and phthalic acid.