Advancements in ink-jet printing technology have prompted a print revolution, from photo quality printing at home, to poster production and even carpet manufacture. Ink-jet printing is so versatile because the print head is not in contact with the surface - this allows for printing on a wide range of uneven surfaces. Improvements have also been made in the durability of inks, which can now be resistant to water and UV, increasing the range of applications for which ink-jet printed materials can be used.
Although the majority of ink-jet inks are dye-based molecular dispersions, pigmented inks are being used increasingly for applications which require durability and water resistance. Pigmented inks are produced in a two stage process. In the first stage the pigment is dispersed in a mobile phase. This 'pre-mix' is then milled to remove agglomerates and to reduce the primary particles to the optimum size, generally 200nm [1].
During this milling process it is important to monitor the particle size distribution. As well as tracking the primary particle size the detection of agglomerates or oversized particles is crucial. Even small numbers of oversized particles in the final ink can cause defects and color inconsistencies in the printed film as well as blockages in the print head.
In addition to tracking the milling run, it is important to monitor the stability of the final ink. If agglomerates are formed during storage then these will affect the printing process and quality when ink is used.
Laser diffraction instruments are commonly used to monitor milling processes. This technique obtains particle size information by recording the light scattered by particles illuminated by the laser beam. What is measured by the instrument is the scattered light energy as a function of angle. Light scattering is then related to particle size on the basic principle that large particles scatter light at low angles and small particles scatter light at high angles.
The measurements reported in this application note were made using a Mastersizer, which uses laser diffraction to measure particles in a wide size range (10nm to 3.5mm) within a single, rapid measurement. This capability to measure a broad size range makes the Mastersizer ideally suited to monitoring a milling process as both the mill feed and final product can be measured in the same set up, and in this case any agglomerates can be detected.
This application note covers a simple example of how the Mastersizer has been used to detect and quantify the presence of agglomerates during the production and storage of an ink-jet ink.
Measurements were made of an ink sample intentionally seeded with a known volume of oversized material. The seed material contains a significant mode of large material, between 0.5 and 5 microns in size, which makes up 20% of the total volume of the sample. The cumulative particle size distributions of the original ink and the seed material, used to simulate agglomerates, are shown in Figure 1. The ability of the Mastersizer to detect large particles was studied as the seed material was added to the original ink sample. The customer requirement was that the Mastersizer should be able to detect even the smallest volumes of oversized particles in the ink, such is the effect of any oversized particles on the performance of the ink.
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The seed material was added to the ink to give volumes of coarse material between 0.4% and 2%, as a percentage of the total volume of particles within the sample. Figure 2 shows the particle size distribution of the original ink, the seed material and the sample seeded with 2% coarse material. The additional coarse material in the 2% seed sample can clearly be detected from the size distribution. However, as the differences are subtle then a qualitative measure of the large particle content may be preferable, as it provides an easier way to compare samples.
In this example, the Dv99.5 was be used to monitor the volume of coarse material. The Dv99.5 is the diameter below which 99.5% of the volume particle particles are present within the size distribution. As this percentile is on the upper limit of the distribution, it provides a sensitive means for detecting agglomerates. Care must be taken when using percentiles at the upper end of the distribution, as the measurement reproducibility can be significantly affected by sampling errors. However, for small particles (<10μm) these errors are minimized.
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Figure 3 shows the Dv50 and the Dv99.5 for the original ink sample as well as samples containing coarse particle fractions up to 2%. This graph shows that the Dv50 is insensitive to the increasing volume of coarse material. However, the Dv99.5 increases from 0.6μm to 1.3μm, clearly tracking the introduction of increasing volumes of coarse material. This suggests that the Mastersizer provides a realistic method for coarse particle detection within the application.
The importance of the above capabilities can be shown in the application of the Mastersizer to detecting changes in the particle size of inks during storage. The ink dispersion must be stable to agglomeration over time, and this is tested during accelerated stability studies. Figure 4 shows an example of the changes which can be detected using laser diffraction when a formulation is unstable. This graph indicates that agglomeration has occurred during 3 days of storage, causing a broadening of the primary mode in the size distribution and the formation of an additional mode of large material at over 1μm in size. During printing, the presence of these large agglomerates would cause blockages in the print head.
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The presence of oversized material, or agglomerates, is an important parameter in a wide range of applications. In the case of pigmented inks, agglomeration increases the likelihood of printer nozzle blockages and can lead to defects in the printed film. This application note has demonstrated how the Mastersizer can be used to monitor the primary particle size of inks and can detect even very small volumes of large material. The system therefore provides a robust, sensitive method for both quality control and stability monitoring, allowing the large particle content in different sample batches to be compared quickly.
