Pigment chemistry, particle size, and surface treatment determine ink viscosity, thixotropy, tack, and transfer behavior on press. Smaller particles = more transparency and dot gain. Surface-treated grades reduce viscosity at high pigment loading. Always validate ink rheology with new pigment grades on test prints before commercial production.
Press operators see the consequences of pigment selection long before colorimeters do. The pigment particle size, surface treatment, and dispersion quality determine whether the ink runs cleanly, clogs nozzles, picks on offset rollers, or causes dot gain. Specifying the right grade for each printing process is critical.
How pigment affects ink rheology
Particle size: Finer particles increase ink viscosity at the same pigment loading because more particles per unit volume increases inter-particle interactions. d50 less than 0.2 micron typical for offset inks; coarser for screen printing.
Surface treatment: Surface-treated pigments have lower inter-particle attraction in the ink vehicle, reducing apparent viscosity at given loading. Allows higher pigment concentration without ink becoming too thick to run.
Pigment loading: More pigment = darker color but also higher viscosity, slower drying, more risk of misting on offset press. Standard process color inks: 15-20% pigment loading.
Pigment-vehicle compatibility: Mismatch creates ink that's stable in the can but breaks down on press as shear forces overcome dispersant stabilization. Always specify ink-grade pigments matched to your specific vehicle (oil, solvent, water).
By printing process
Offset (sheetfed and web): high tack, viscous oil-based inks. Pigment requirements: small particle size for high tinctorial strength, controlled rheology to prevent misting and roller pickup. Process colors: PB15:3, PR57:1 (or PR122 for premium magenta), PY12/13/14, carbon black.
Flexographic: low-viscosity solvent or water-based inks. Fast drying. Pigment requirements: very fine particle for clean dispersion through anilox cells, surface-treated grades for waterborne. Common: PR48:2, PB15:3, PY83, PG7.
Gravure: low-viscosity solvent or water-based inks transferred from etched cells. Pigment requirements: ultra-fine, narrow particle size distribution for clean cell release. Often supplied as flushed pigments.
Screen printing: high-viscosity, high-pigment-loading pastes. Pigment requirements: relatively coarser particles tolerated, good dispersion through screen mesh. Common: PR48:2, PB15:3, PY74, PG7. UV-curable variants available.
Digital inkjet: very low viscosity, ultra-fine pigment dispersions. Pigment requirements: d50 less than 0.3 micron, narrow PSD, stabilized for long-term inkjet ink shelf life. Specialty digital-grade pigments.
Common ink-rheology problems and pigment-related fixes
Misting on offset press: ink throwing droplets off the rollers. Cause: low ink tack, often from over-loaded pigment with poor stabilization. Fix: lower pigment loading, switch to surface-treated grade with better dispersant compatibility.
Streaking on flexo: uneven ink transfer creating streaks in printed image. Cause: poor pigment dispersion creating localized agglomerates. Fix: use finer-particle grade, longer dispersion mill time, or flushed pigment instead of dry powder.
Dot gain in offset: printed dots larger than expected. Cause: ink too thin (low viscosity), or too much pigment loading reducing tack. Fix: verify pigment-vehicle compatibility; reduce pigment loading or switch grade.
Ink lifting / picking: ink picks off paper on next pass. Cause: insufficient ink tack, often from over-extending pigment in vehicle. Fix: optimize pigment loading; ensure surface-treated grade is matched to vehicle.
Shade drift through ink shelf life: ink color shifts over weeks of storage. Cause: pigment flocculation as dispersant breaks down. Fix: use non-flocculating surface-treated grade (e.g. PB15:4 instead of PB15:3 for waterborne).
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About the Author
Bhargav Kanani is the Sales Director at Kanani Dyes Chem LLP, an ISO 9001/14001/45001/17025 certified manufacturer of organic pigments based in Gujarat, India. With deep expertise in pigment chemistry, manufacturing, and global B2B trade, he advises formulators across paints, coatings, plastics, inks, and specialty applications worldwide.