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.
The 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.