subject: Laser Marking And Printing Ink Therefor [print this page] Laser marking and printing ink therefor Laser marking and printing ink therefor
A non-destructive laser marking method including irradiating laser light on a thermosensitive color-forming layer formed by printing with a printing ink containing a leuco dye as a color former and an acidic substance as a color developer, and at least one background color formation inhibitor selected from the group consisting of a water-soluble amino acid, an ammonium salt of an inorganic acid, a pH buffer, and water. Laser marking can be achieved with low energy and at high speed while minimizing background color formation to provide a high-contrast color image.
Marking of information on a label, etc. is generally effected by direct printing inks. The printing is usually carried out by pad printing or screen printing.
However, in the method of removing an offset printing ink by laser light irradiation, a contrast between the paper substrate and the printing ink is required, and clear marking cannot be achieved with a lightly colored label. Further, removal of a sheetfet offest ink, which is nothing less than destruction of the material, causes roughening of the boundary between irradiated and non-irradiated areas, resulting in a failure of obtaining precise marking. It has therefore been suggested to utilize non-destructive color development by laser light irradiation on a label comprising a paper substrate, etc., i.e., thermosensitive color formation. We can find in literature various examples of application of the thermosensitive color formation system, the term "JP-A" as used herein means an "unexamined published Japanese patent application"), but no cases has been reported in which the system is put to practical use.
It turned out, however, that the above-mentioned printing inks sometimes suffers from color formation during ink preparation (dark color formation) or after printing (background color formation) (the dark color formation will hereinafter be included in background color formation) due to the fact that both the leuco dye as a color former and an acidic substance as a color developer exist in the same system. There has still been a need of further improvement for practical use.
