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.

Low migration UV-curable ink for packaging films

US 2014/0285568 A1
Curable liquids and inkjet inks for food packaging applications
Agfa Graphics NV

The use of radiation curable ink jet inks is preferred for ink jet printing onto non-absorbing surface due to the rapid drop solidification and subsequent removal of the drying stage needed with solvent inks. Due to the demand for increased print speeds and image quality, UV curable ink jet inks are necessarily of relatively low viscosity, thus limiting the materials available. In addition, low viscosity curable liquids tend to be either toxic, irritating or both. This patent discusses the use of very low viscosity and relatively benign monomers containing both acrylate and vinyl ether functionality (discussed in a previous Agfa patent application EP 0097508 A), as applied to food packaging.

Even when using the safest of UV materials, there is some resistance to residual monomers coming in contact with food. In addition, photo initiators, UV stabilisers and colorants also present issues if allowed to move into foodstuffs. Extractable monomers can cause problems in two separate ways; set-off and migration. Set-off occurs in roll-to-roll printing where the printing front-side of a packaging material comes in contact with the unprinted back-side and unreacted monomers are “set-off” onto the back-side intended for food contact. Migration occurs when the unreacted monomers move through the packaging film to the covered foodstuff.

The solution to the former issue is to ensure the formulation cures fast enough and to a sufficient degree and hardness such that transfer set-off is impossible. This is also discussed in the previous Agfa patent above. The migration issue is also partially solved by this approach, but the first step in the ink jet printing process is the deposition of uncured monomers onto the packaging film. It is this monomer migration into the film, in the period between monomer/film contact and cure, that this patent addresses.

Popular packaging materials that show issues with monomer migration tend to be olefin base polymers such as polypropylene film. Due to the very low viscosity of some of the exemplified inks, the patent suggests that the monomers can easily penetrate into the substrate before they can be effectively cured. The proposed solution to this is the application of a primer coating that acts as a barrier to monomer migration. It is claimed that the key to this is the combination of a free-radical polymerisable monomer or oligomer, at least one diffusion hindered acetalisation catalyst and a diffusion hindered hydroxyl containing compound.

The diffusion hindered materials are simply low molecular weight species covalently bonded to high molecular weight polymer or fragments. The experimental describes the preparation of such a diffusion hindered UV sensitiser, shown below. This molecule has an additional benefit of having two acrylate groups available to react the UV active moiety into the cured network, ensuring zero migration.

Sept-14The experimental discloses the composition of a full CMYK ink jet designed to show no migration when cured and so no set off, but the core of the patent is the primer. This material consists of propoxylated neopentyl glycol diacrylate (67 wt%), triglycerol diacrylate (15 wt%), a phosphate ester of propylene glycol monoacrylate (5 wt%) and 11 wt% of various polymeric photoinitiators.

How the glycol and the phosphate ester (acetalisation catalyst) helps prevent monomer migration is not mentioned, but the results are impressive. Without a primer layer, monomer extractables measured through the back of the printed polypropylene film are up at around a few thousand parts per billion (ppb). With a comparative primer without the acetalisation catalyst the measured extractables were down to a few hundred ppb, and with this catalyst the results were at <10 ppb.