A resin varnish for printing ink is produced by condensing phenol and formaldehyde
in an ink solvent to obtain an ink solvent solution of resole initial condensate,
after which the resole initial condensate and a rosin ester containing 30-60 wt
% non-aromatic conjugate resin acid and 20-60 wt % aromatic resin acid are reacted
at 100-250 C. in a system containing ink solvent or in a system containing
ink solvent and drying oil. A high-viscosity, high-molecular-weight ink resin is
produced efficiently by reacting, with resole initial condensate in ink solvent,
a rosin ester containing specified contents by percentage of conjugate acid and
aromatic resin acid. The resin varnish obtained has very few decomposition components
and nonuniformly gelled components, so it also is suitable for screen printing
ink and letterpress printing ink, in addition to offset printing ink.
