SOLUBILITY PARAMETERS IN INK DESIGN
The Hansen Solubility Parameters position each molecule in the 3-dimensional ‘space of solubility’ based on 3 main characteristics: a dispersive component (D), a polar component (P), and a hydrogen bonding component (H). A solubility sphere can be defined for each molecule, from which the radius is a measure of its solubility.
The HSP’s can give insight into the compatibility or solubility of compounds and are frequently used by ChemStream in order to improve and speed up their inkjet ink development.
Figure 1: The Hansen solubility sphere of 1 molecule in the 3D Hansen solubility space
Since the surface of pigments is key to obtain a stable nano-dispersion, it is important to have insight into the pigment quality of similar pigments from different suppliers. Although the suppliers do not always mention the details about their work-up procedures and surface treatments of the pigments, a Hansen solubility study can give us more information.
In Figure 2 we depict the Hansen solubility spheres of 1 pigment type from 3 different suppliers.
From this study it was clear that pigment quality C did not have any H-bonding facility on its surface, while qualities A and B do have some H-bonding part at their surface that can act as an anchoring site for interacting with the pigmentophilic part of a dispersing agent. In practice indeed, when milling these pigments into an UV-curable nano-dispersion, we discovered that pigment quality C did not give us an acceptable dispersion while pigment qualities A and B gave better results. This case shows how we can use HSP for preselecting pigment types from suppliers upfront in order to reduce the total amount of work in the development of stable pigment nano-dispersions.
Figure 2: The Hansen solubility spheres of 1 type of pigment from 3 different suppliers
When developing inkjet inks, one needs to be sure that all the materials that are in contact with that ink will not suffer or disintegrate. Therefore it is very important to do ink/material compatibility testing of printhead components, tubings, ink bottles or cartridges. Usually these tests are done by soaking the materials in the ink for some time at an elevated temperature, and measuring their dimensions and weight before and after. On the other hand, if the composition of these materials is known, ChemStream uses the HSP’s for predicting the risk of incompatibility with the ink upfront.
Figure 3: The Hansen solubility spheres of PP and POM compared with some standard UV-curable monomers
Figure 3 shows us the position of polypropylene (PP) and polyoxymethylene (POM) in the Hansen solubility space with their solubility spheres. The blue dots represent some standard UV-curable monomers that are used in UV-curable inkjet development. One can easily see that the solubility sphere of PP has almost no overlap with the monomers, while the solubility sphere of POM comprises all monomers. This means that POM will be very sensitive to attack by these monomers, while PP will be much more resistant. These results provide ChemStream with knowledge that helps them to make faster and better choices in their ink development and process requirements.