DETAILED DESCRIPTION OF THE INVENTION Phenol resins suitable for use in the color developing ink of the present invention are proton releasing type phenol resins generally known to this art.
Specifically, they are phenol-formaldehyde copolymers, so-called novolak resins, and phenol-acethylene copolymers.
Specific examples of suitable phenol resins are disclosed in U.
S.
Pat.
Nos.
3,455,721, 3,516,845 and 3,649,352.
Examples of these copolymers include p-phenylphenol-formaldehyde copolymer, p-fluorophenol-formaldehyde copolymer, p-chlorophenol-formaldehyde copolymer, p-bromophenol-formaldehyde copolymer, p-iodophenol-formaldehyde copolymer, p-nitrophenol-formaldehyde copolymer, p-carboxyphenol-formaldehyde copolymer, o-carboxyphenol-formaldehyde copolymer, p-alkoxycarbonylphenols-formaldehyde copolymers, p-aroylphenol-formaldehyde copolymer, p-lower alkoxyphenol-formaldehyde copolymers, p-alkyl(C.
sub.
1 to C.
sub.
12)phenol-formaldehyde copolymer (e.
g.
, p-methylphenol, p-ethylphenol, p-n-propylphenol, p-isopropylphenol, p-n-amylphenol, p-isoamylphenol, p-cyclohexylphenol, p-1,1-dimethyl-n-propylphenol, p-n-hexylphenol, p-isohexylphenol, p-1,1-dimethyl-n-butylphenol, p-1,2-dimethyl-n-butylphenol, p-n-heptylphenol, p-isoheptylphenol, p-5,5-dimethyl-n-amylphenol, p-1,1-dimethyl-n-amylphenol, p-n-octylphenol, p-1,1,3,3-tetramethylbutylphenol, p-isooctylphenol, p-n-nonylphenol, p-isononylphenol, p-1,1,3,3-tetramethylbutylphenol, p-n-decylphenol, p-isodecylphenol, p-n-undecylphenol, p-isoundecylphenol, p-n-dodecylphenol and the like formaldehyde copolymers), copolymers of isomers of the above-described p-alkylphenols with formaldehyde, and copolymers of the mixture of two or more of the above-described alkylphenols and the isomers thereof with formaldehyde.
Preferred copolymers are copolymers of a p-substituted phenol wherein the p-substituent is a halogen atom, a phenyl group, an alkyl group, a nitro group, a carboxy group, an alkoxy group, an aroyl group or an alkoxycarbonyl group.
More specifically, p-substituents, such as chlorine, a phenyl group and a C
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