Physicochemical Properties Of Printing Inks

Printing inks can be categorized into water-based and solvent-based types based on their solvent composition. Water-based inks, primarily utilizing water-soluble dyes, account for over 80% of total demand but suffer from relatively poor lightfastness and water resistance; pigment-based inks are environmentally friendly yet more costly, though they are showing a growth trend.
Ink formulations consist of solvents, colorants, co-solvents, and additives. For instance, the basic composition of a specific inkjet ink includes: solvent-soluble pigment (C.I. Solvent Black 27) at 2.5–5.0%; solvents (methyl ethyl ketone and isopropanol, approx. 6:1 ratio) at 80–87%; trace amounts of co-solvents; plasticizer (dibutyl phthalate) at 3.0–5.8%; and film-forming agent (nitrocellulose) at 5.0–10.0%.
Key physicochemical parameters for inks include viscosity, density, pH value, surface tension, and electrical conductivity. For example, test results for a specific ink showed: viscosity of 2.8 × 10⁻³ Pa·s, density of 0.89 g/cm³, pH of 6.5, surface tension of 23.4 mN/m, and conductivity of 0.165 S/m. Detailed performance specifications include: static surface tension critical range of 35–72 mN/m; dynamic surface tension decay parameter (t₀.₅) ≤ 2 s; target contact angle ≤ 30°; dynamic viscosity range of 5–50 mPa·s; target pH range of 7.0–9.0; chromaticity coordinate deviation (ΔE) ≤ 1.0; initial drying time ≤ 30 s; and abrasion resistance (rub cycles) ≥ 1,000.
The chemical constituents of the ink must comply with environmental protection standards. Water-based inks must not contain intentionally added elements-such as lead, mercury, cadmium, nickel, and hexavalent chromium-or their compounds; furthermore, limits are imposed on harmful substances such as volatile organic compounds (VOCs), ammonia and its compounds, methanol, formaldehyde, phenol, and halogenated hydrocarbons. Requirements for harmful substance limits in water-based inks include VOCs ≤ 100 g/L and a total content of lead, cadmium, hexavalent chromium, and mercury ≤ 100 mg/kg . For solvent-based inks, the content of benzene-series solvents must be controlled.