TECHNICAL GUIDE · AQUEOUS INKJET FORMULATION
How to Evaluate Viscosity, Conductivity, pH and Surface Tension in Inkjet Dispersions
Turn four routine measurements into a useful formulation decision. Understand what they reveal about a water-based pigment dispersion, how to compare results fairly, and what still needs validation in the finished ink.
Evaluate the colourant first, then qualify the complete ink
A useful evaluation combines viscosity at a stated temperature and shear condition, conductivity with a stated temperature basis, calibrated pH measurement, and clearly defined surface-tension measurement. Compare the properties before and after formulation and storage, then confirm filtration and printing.
A ready-to-use aqueous pigment dispersion is ready for incorporation into an ink formulation—not necessarily for loading directly into a printer. Humectants, binder and wetting agents change the fluid. The colourant specification and the finished-ink specification serve different purposes.
| Property | Reporting unit | Main evaluation question | Essential context |
|---|---|---|---|
| Viscosity | mPa·s, numerically equal to cP | Does the fluid flow consistently through the intended formulation and operating conditions? | Temperature, instrument, geometry or measuring system, shear condition and pigment content |
| Electrical conductivity | µS/cm or mS/cm; 1 mS/cm = 1,000 µS/cm | Is the ionic environment consistent, and does it change after adding ingredients? | Temperature or compensation basis, calibration, cell and dilution status |
| pH | No unit | Is the formulation within its established chemical and material-compatibility window? | Sample temperature, calibrated electrode, sample preparation and reading stability |
| Surface tension | mN/m, numerically equal to dyn/cm | Are interfacial behaviour, drop formation and substrate wetting suitable? | Static or dynamic method, temperature, equilibration or surface age, and clean measuring surfaces |
Lower is not automatically better. The target is a reproducible fluid suited to the pigment architecture, complete recipe, printhead and application.
Measurement methods: references, not one universal inkjet standard
International methods provide measurement frameworks, not one acceptance range for every nano pigment dispersion. Confirm method scope and instrument suitability for the sample. AIC's TDS selects rotational viscosity, Wilhelmy plate tension and glass-electrode pH references for future QC alignment; historical values have not been retrospectively certified under them.
1. Viscosity: document temperature and shear, not just the number
ASTM D7867-13(2025)e1 addresses rotational viscosity as a function of temperature for paints, inks and related liquids. Record geometry, temperature and shear rate or rotational speed. Use equipment that resolves the low viscosity reliably; results near its lower torque limit deserve scrutiny.
Agree a reference temperature for batch comparisons and also measure the finished ink at jetting temperature. Where shear dependence matters, examine a flow curve. Do not compare different methods or temperatures as though they were interchangeable.
For context, Ricoh's MH5420/5440 specification lists viscosity of 10–12 mPa·s and surface tension of 28–35 mN/m. These are published values for that head family, not universal DTF/DTG limits or an AIC dispersion specification. Confirm the measurement and operating conditions with the manufacturer for your actual system.
2. Conductivity: compare the same temperature basis
Conductivity is useful for consistency checks but does not identify individual ions. ISO 7888:1985 addresses water measurements, not pigment concentrates; any adaptation requires validation. EPA Method 120.1 illustrates KCl calibration and temperature control for water-related samples.
Use an appropriate calibrated cell, avoid bubbles, and state the actual temperature and any compensation basis. Do not assume a generic compensation coefficient suits every pigment vehicle. Ion type, concentration and pH also influence particle interactions, as explained in Malvern Panalytical's zeta-potential guidance. Conductivity alone cannot establish stability.
3. pH: control the chemical environment without disturbing the dispersion
ISO 19396-1:2025 provides a glass-membrane sensor method for polymer dispersions and coating materials. Select a suitable electrode, calibrate with buffers bracketing the expected reading, and record temperature. Keep the electrode clean and check reading stability.
State whether the sample is measured as supplied or diluted: dilution changes the chemical environment. After adjusting pH, recheck viscosity, particle distribution and filterability. A corrected pH number alone does not demonstrate compatibility. Report storage changes in pH units, not percentages.
4. Surface tension: distinguish equilibrium from rapid surface creation
ASTM D1331-20(2024), Method C uses the Wilhelmy plate for surface tension. Document temperature and equilibration, and prevent contamination of the plate and liquid surface. This is the static method selected in AIC's TDS for QC alignment.
Static tension describes an equilibrated interface; dynamic tension is measured at a defined surface age. Rapid inkjet surface creation makes the distinction important. KRÜSS's inkjet guidance discusses these measurements and wetting. Report surface age for bubble-pressure measurements rather than equating them with plate results.
Rump and colleagues' surfactant research also shows why idle-time and drop-formation tests matter. It explains mechanisms in particular fluids, not AIC product performance. Validate the finished ink by drop watching and substrate printing, not tension alone.
AIC-Magenta Red 1015: reading an actual batch example
AIC-Magenta Red 1015, carboxylic PR122, lot 26122-C-065 illustrates batch reporting. These manufacturer-supplied results describe this particular lot, rather than literature-derived values or a specification for every AIC grade.
| Measurement | Reported result | How to use it |
|---|---|---|
| Solids | 15 wt% | Read viscosity and other properties in the context of this supplied concentration. |
| Viscosity | 1.64 mPa·s | A dispersion-level batch result, not a jetting specification for the complete ink. |
| Surface tension | 68.4 mN/m | The colourant's supplied result; wetting agents and the finished vehicle can change it. |
| pH | 7.89 | A batch reference to compare using a consistent electrode and sample procedure. |
| Electrical conductivity | 1,357 µS/cm | A batch reference; establish the measurement-temperature basis before strict comparisons. |
The lot also reports D50 = 132 nm, D90 = 181 nm, D97 = 214 nm and filtration times of 8 s, 21 s and 8 s. See the COA example and test conditions, PR122 particle-size report and D50, D90 and D99 guide.
The supplied data do not fully identify each measurement's temperature and instrument settings. We do not substitute assumed conditions or literature values. Obtain the laboratory procedure and current COA for a strict acceptance comparison. A TDS describes a grade; a COA records a batch.
Compare property drift after formulation and storage
Repeat the measurements after adding the intended humectant, binder and wetting package. For AIC's stated accelerated screen, 60 °C for 7 days, agree container and handling conditions. Re-equilibrate the aged sample to the original measurement temperature; record sediment and redispersibility too.
For positive-valued properties such as viscosity, report relative drift as |aged − initial| ÷ initial × 100%, alongside the raw values. Use pH-unit differences for pH. Specify any mixing before measurement.
Seven days at 60 °C does not establish a room-temperature shelf life without validated correlation. Add real-time storage, circulation and printer tests; acceptable viscosity drift alone cannot demonstrate nozzle reliability.
A practical qualification checklist for ink makers
- Define the sample: pigment CI, model, lot, surface chemistry, loading and dispersion versus finished ink.
- Agree methods: temperature, equipment, calibration, shear and surface-age conditions.
- Build a baseline: four liquid properties plus particle distribution and filterability.
- Check formulation and aging: track changes under matching measurement conditions.
- Print: evaluate drops, satellites, idle/restart and substrate performance; add fastness tests for textile inks.
Explore the AIC colour catalogue and surface-modification technology to select an inkjet colorant for DTF, DTG, roll-to-roll or single-pass development.
Frequently asked questions
Should the dispersion have the same surface tension as the finished ink?
Not necessarily. The ink vehicle changes interfacial behaviour. Qualify the final formulation against its printing requirements.
Can published papers supply missing AIC batch values?
No. Literature supports methods and mechanisms; the actual sample's measurements establish its properties.