Keeping your head

Developments in inkjet technology have prompted a print revolution - from home printing of photos to poster production and even carpet manufacture. However, pigment particle size must be closely controlled to avoid nozzle blockages and maintain print quality. Laser diffraction-based particle size analysis is just the thing says Anne Virden

SINCE the print head is not required to be in contact with the receiving medium, inkjet methods can be used to print on all manner of uneven surfaces. Now, developments in pigment based inks have also improved the durability of the final product, making the print more waterproof and UV resistant.

While many inkjet inks are dye-based molecular dispersions, pigmented inks are becoming more commonplace, especially where durability and water resistance are required. Their production is a two-stage process, which begins with the dispersion of the pigment in a mobile phase. This ‘pre-mix’ is then milled to reduce particle size and break up any strongly-bound aggregates. Once created, the final ink dispersion must remain stable over long periods of storage.

The presence of small numbers of oversized particles in the final product can lead to nozzle blockages, defect formation within the printed film and inconsistent colour densities. Pigment particle size must also be small, 200nm or less. 

Laser diffraction-based particle size analysis is commonly applied to monitoring the size of pigment-based inkjet inks during milling. Using the right instrument at this stage, the size of both the primary ink particles and any large agglomerates can be determined in the same measurement.

The measurements described here were made using the Mastersizer 2000 (Malvern Instruments), which measures particles in the size range from 0.02 to 2000 microns without the need for any changes to the optical set-up. This makes it highly suitable for monitoring the particle size distribution of the milled pigment and at the same time easily detecting any oversized particles in the dispersion. The following example illustrates a simple method for quantifying coarse particle content.

Measurements were made of the size distributions of inks seeded with coarse

Figure 1: Undersize particle size distributions for the ink samples used in this study.

particles. Figure 1 shows the primary ink dispersion and the seed material used to simulate the appearance of agglomerates. The seed contains a significant mode of larger particles in the 0.5 – 5µm range, representing approximately 20% of the volume of particles present. The customer requirement in this case was for the laser diffraction system to detect these coarse particles as the seed material was added to the primary dispersion.

Figure 2 shows the response of the system after addition of the seed material to the primary dispersion, such that the percentage of coarse particles added accounted for approximately 2% of the total volume of particles in the sample. As can be seen, a proportional change in the particle size distribution is able to be observed, even though the coarse particle fraction is very small.

Inspecting particle size distributions will show whether or not large material is

Figure 2: Comparison of the particle size distribution reported after the addition of 2% by volume of the coarse particles with those for the primary dispersion and the seed material.

present, the differences between samples may be subtle, as is shown here. A quantitative method to determine the large particle content is preferable and can simplify comparisons between samples.

In this case, it was decided to monitor the change in the Dv99.5 – this is the particle size below which 99.5% of the distribution lies; for example if the Dv99.5 of a sample is 1.2 microns, 99.5% of the volume of material has a diameter of 1.2µm or less. This percentile is on the upper limit of the size distribution, and therefore provides a good means of detecting the presence of agglomerates. Care must be taken when using such percentiles, as the measurement reproducibility will be very much dependant on the sampling efficiency. However, sampling errors are minimised when the particle size is small (<10 µm).

Figure 3 shows the median particle size (Dv50) and Dv99.5 for the primary ink

Figure 3: Dv50 and Dv99.5 measured during the seeding of the primary ink sample.

dispersion and samples containing increasing coarse particle fractions up to 2%. From the graph it can be seen that the Dv50 remains relatively unaffected by the increased volume of coarse particles. However, the Dv99.5 increases from 0.6μm to 1.3μm, clearly detecting the introduction of the large particles. The change in Dv99.5 is almost linear with concentration, suggesting that monitoring this parameter would provide a reasonable method for detecting the presence of oversized material.

Once an ink dispersion has been created it is important that it remains stable over long periods of time. Figure 4  shows an example of the effects of storage for three days on the particle size of an inkjet formulation. The graph indicates that agglomeration has occurred during storage, causing a broadening of the sub-micron mode and the formation of large particles above 1µm in size. These large agglomerates would cause blockage of the inkjet printer head during operation.

The presence of oversized particles is an important parameter in the production

Figure 4: Particle size distributions recorded following milling, and after three days storage

of a wide variety of materials, illustrated here by the influence of large particles on the functionality of inkjet inks. In pigmented inks, oversized particles can increase the likelihood of printer nozzle block and can also lead to defect formation within the printed film.

The data presented in this article demonstrate the sensitivity of the Mastersizer 2000 to the presence of even very small volumes of large particles in a sample. The amount of oversized material present can be quantified easily using the technique, making comparisons between samples much quicker and easier.

The effect of storage time on particle size is also critical to the properties of many materials. The data also illustrate how particle size information can be used to monitor formulation stability. The large dynamic range offered by the Mastersizer 2000 makes it suitable for the detection of both primary particles and large, agglomerated material.