Latex-based aqueous UV-curable inks

EP 2 960 306 A1
Aqueous radiation curable inkjet inks
Agfa Graphics NV

UV curable ink jet inks lacking solvents have been used extensively in industrial applications, because of their reliability and because they adhere to a multitude of substrates. One disadvantage of these “100%” solids UV curable ink jet inks is that relatively thick layers are printed, compared to aqueous ink jet inks which generally contain less than 20% – and usually less than 10% – solids. Thin layers generally have better flexibility than thick layers and so a low solids UV curable formulation would potentially be advantageous. However in most other aspects, low solids aqueous latex ink jet inks tend to exhibit an inferior performance compared to UV curable inkjet inks.

A latex is a stable colloidal dispersion of natural or synthetic polymer particles in an aqueous medium. Synthetic latexes can be made by polymerising monomers, generally (meth)acrylates, that have been emulsified with surfactants, and such materials have been frequently used in ink jet inks. Acrylate based polymeric particles often have a relatively high minimum film formation temperature (MFT), which prevents the use of temperature sensitive substrates for latex ink jet inks. The MFT tends to be very closely related to the glass transition temperature (Tg) of the latex polymer and so thermosensitive substrates could feasibly be used if the Tg of the latex ink was low, below 0C for example. Unfortunately, such low Tg latexes tend to show poor jetting properties and blocked nozzles as the latex can easily film form in the nozzle.

This patent suggests that low MFT latex dispersions can be jetted successfully if fully water insoluble (meth)acrylate monomers are mixed into this latex. It is believed that this liquid polymerisable compound rapidly migrates into the surface of the hydrophobic polymer particles, forming a barrier and protecting them against agglomerating and fusing together. This is suggested to be particularly advantageous for low MFT (<0C) polyurethane particles which, without the exemplified monomers, can only be reliably used in ink jet inks at very low levels when high levels (>10 wt%) would need to be used to confer the required physical properties to the final printed film.

The experimental section discusses this approach in detail. One example took 100g of a 42 wt% solids polyurethane dispersion (Bayhydrol UV XP 2689) and added this to 100g of water. To this was added 5.3g of the monomer dipropyleneglycol diacrylate (DPPA) and stirred for 20 minutes at 800 rpm using a Disperlux mixer. This mixture was then filtered and used in an ink jet ink at 23-35 wt% along with 20 wt% of a colour pigment dispersion (Diamond D75C – a 15 wt% dispersion of C.I. Pigment Blue 15:3 in water), humectant (10 wt% 2-pyrrolidone and 20 wt% 1,2-hexane diol), photo initiator (1.1 wt% Irgacure 500) and water. Comparative inks were also formulated using only the polyurethane dispersion without the addition of the UV curable monomer or photo initiator.

These inks were evaluated on a Dimatix DMP2831 ink jet printer (10pl head) with a cartridge temperature set to 24C, at a firing frequency of 5 kHz and a firing voltage of 20-25V. The results show that at the higher levels of polyurethane dispersion, only the exemplified monomermodified dispersions show good ink jet performance, whereas the unmodified inks would not jet.

Further inks were formulated using this dispersion mix with an aim to investigate various photo initiators. Three groups of photo initiator were tested, water soluble, water insoluble and acrylated photo initiators. These inks were tested by applying them on polyester film, drying at 60C for 3 minutes then curing using a Fusion VPS/1600 lamp at 20 m/min. HPLC was used to test for the presence of any extractable photo initiator from the cure films. The results clearly show that the reactive acrylated photo initiators resulted in far less PI leaching from the cured film. No data is presented regarding mechanical properties.

Careful manufacturing process for improved latex ink

US 2012/0232215 A1 
Single batch latex ink compositions and methods 
Hewlett Packard Development Company, L.P.

The addition of polymers to a pigmented ink jet ink will generally improve printed image robustness at the expense of a higher ink viscosity which, depending on the printhead technology used, may thus preclude the use of such materials. As frequently discussed here, polymer encapsulated pigments potentially offer the best of both worlds without any compromise in the ink’s viscosity.

However, even with the best encapsulated pigments, as with everything, there is still some compromise and that comes in the form of a more involved manufacturing process, and it is this point that the patent hopes to address, with a relatively simple approach.

The example used (4.6 wt%) of the monomers styrene, hexamethylacrylate and acrylic acid at a ratio of 40:55:5, and added 0.1 wt% of azobisisobutyronitrile (AIBN) and 2.3 wt% of a pigment powder (BASF D7086 cyan). This paste was then added to water (92.3 wt%) in which had been dissolved sodium dodecyl sulphate (0.7 wt%). This was then mixed at high shear in a microfluidiser at 10,000 psi for 15 minutes to produce the dispersed pigment emulsion.

This emulsion was then charged into a reaction vessel and degassed under argon before heating to 80C for two hours. After this time a portion of 0.1 wt% of potassium persulfate, dissolved in a small amount of water, was added over the course of three hours, with a further 3 hours to ensure full reaction of the monomers. The resultant dispersion was allowed to cool, neutralised with potassium hydroxide and filtered.

The main point exemplified in the text is that the addition of the thermal initiator over time is critical, as without this approach the particle size is generally significantly below 250 nm and unsuitable for ink jet, presumably due to viscosity issues. Thus by starting polymerisation with a relatively low number of initiator species, then the number of polymer particles nucleated will be relatively low and these particles will then grow during the subsequent polymerisation and further nucleation will be minimal. In comparison, if all the initiator is added at the start of the polymerisation, then there will be significantly higher polymer particle nucleation resulting in more, smaller particles in the final ink.