Although the ancient Egyptians already used inks 5000 years ago, inks and their properties have been continuously improved and refined over thousands of years. Nevertheless, the basic principle of tinta acqua (colored water) remains the same. Today, in addition to inks based on dissolved dyes, there are many which contain pigments in solid form. Unlike dyes, pigments do not dissolve in the medium but are present in dispersed form, e.g. as suspensions.
Pigment-based inks have an advantage over those containing dissolved dyes: after drying, they are light-stable and less sensitive towards water or other solvents. They also have higher color intensity. This is an important criterion for the ‘permanent document’ quality rating.
Dispersed pigments are a challenge for the manufacture of inks. The pigment particles can settle and clog fountain pen springs or printer tubings and capillaries. For this reason it is important to use pigments with suitably sized particles– generally a few up to several hundred nanometers.
Fig. 1: Functional principle of the IG-1000.
Static laser diffraction proven method
For the determination of particle size, static laser diffraction has long been a proven and accepted method. Static laser diffraction measures dispersed particles in a cuvette or solid particles in an air current. When the laser beam hits a particle, the light beam is diffracted and a diffraction grating is formed wherein the diffraction angle is mathematically proportional to the particle size. In principle: large particles generate small diffraction angles, small particles generate large diffraction angles.
Shimadzu’s SALD series for particle size determination, with its many basic models and dispersing units for solid and liquid samples, uses the measuring method described above. An extensive range of accessories and measuring cells allows a large variety of applications.
As with all measuring methods, static laser diffraction has its limits, here the determination of very small particles in the nano-range. For this purpose, Shimadzu has developed a new innovative technique combining static laser diffraction with dielectrophoresis.
Fig. 2: Particle size distribution curves of the inks.
Induced grating method
In the induced grating method, a special grating electrode is immersed in the particle dispersion. A laser is directed through this electrode. When a voltage is applied across the grating, the particles will move towards the voltage field of the grating. After a large number of particles have collected near the grating, a diffraction phenomenon can be observed and the light intensity is captured using a detector.
The actual measurement starts when the voltage is switched off: due to Brownian motion the particles diffuse back into the solution and the light intensity at the detector decreases.
The temporal process of the decrease in intensity of diffraction indicates particle size: small particles diffuse back into the dispersion faster than larger particles.
The advantage of the induced grating method lies in the calculation of the particle size distribution without the need for refractive index input as a measurement condition. In addition, moderate impurities caused by larger particles do not interfere with any measurement.
Fig. 3: Statistics over five measurements (black ink).
Fast and straightforward analysis
In order to guarantee error-free functioning of ink cartridges, all dyes must be measured: this includes cyan, yellow, magenta as well as black. The color ‘black’ strongly absorbs light and its measurement is therefore particularly challenging for overall determination. With the induced grating method it is possible to measure black pigments with high accuracy and reproducibility.
Sample pretreatment for ink cartridges is straightforward and simply requires the determination of a suitable concentration range for the particles. Dilution ranges with respect to the inks are between 1:2 and 1:50. The results speak for themselves: highly reproducible and stable values can be attained within a very short time (measuring time approximately 30 s.).