Reversibly photochromic printing inks

DETAILED DESCRIPTION OF THE INVENTION In the following description, abbreviations will be used for the sake of shortness.
"Photochromic" or "photodye" means a photochromic compound.
"Photodye-stuff" means an organic pigment-like particulate photochromic composition where the photochromic is surrounded by a protecting polymer, according to the invention.
"Chemical hardening" or "chemical drying" means the polymerization, curing or crosslinking of printing ink binder precursors.
"Physical drying" means solidification by the mere evaporation of solvents.
Furthermore, the term "photochromism" or "photochromic condition" as used herein, is meant to define the capability to change the color, generally from colorless to any visible coloration, after exposure for at most about 5 minutes to an energy radiation source having a wavelength comprised from about 200 to about 1100 nm.
The change of absorption behavior may occur instantaneously on exposure to the ideal wavelength of the particular photochromic reaction or after overcoming any activation phase.
The term "color" or "coloration" is to be understood as the overall absorption characteristic within the same range of wavelengths of the electromagnetic spectrum.
The color must be reversible and should last for a time period comprised from picoseconds up to about 30 minutes.
Reversibility, i.
e.
color cancellation, may be accelerated by heating or by exposure to a distinct wavelength radiation different from that of the first coloration.
The photochromics which can be used in the printing inks of this invention, are generally already known substances selected from three main classes A, B, and C hereinafter described.
Class A encompasses spiro-indolino-naphthoxazines as disclosed in U.
S.
Pat.
Nos.
3,562,172, 4,342,668, 4,440,672 and 4,699,473.
In these compounds, basically the same photocoloration reaction is involved as in the photochromics of class C.
Class A further comprises spirobenzopyranes as described in Japanese patent publication no.
61-1004, published 13 Jan