Printing Inks Chemicals

Offset inks use hard, high-tack resins: rosin-modified phenolics, alkyd varnishes, and petroleum resins for sheet-fed; mineral oil-gelled varnishes for heatset web. Gravure inks use nitrocellulose, polyurethane, or acrylic resins dissolved in fast-evaporating solvents. Flexo inks use acrylic emulsions or solventborne polyamide resins. UV inks use acrylated oligomers (epoxy acrylate, urethane acrylate) cured by radical photoinitiators.

Tack is measured with an inkometer (Tack-O-Scope) at standardized RPM and temperature (32°C/200 RPM typical). Sheet-fed offset inks run at tack 10–18; heatset web inks at 6–12. Tack is controlled by: adjusting resin concentration, adding tack reducer (petroleum distillate, mineral oil), or using softer resins. Ink train sequence is highest tack first down to lowest to prevent back-trapping.

Set-off (ink transfer from printed sheet to the back of the next sheet in the pile) is controlled with hard waxes (PE, PTFE, carnauba), anti-set-off spray powder (starch, CaCO3), and fast-setting varnish vehicles. Blocking (adhesion between printed substrates under pressure/heat) is prevented with hard wax additives, slip agents, and ensuring full ink cure before rewinding. PTFE and polyamide waxes give the best combination of slip and anti-blocking.