printing inks
printing inks
This invention concerns printing inks for use in ink-jet printers. In particular, this invention concerns printing inks for use in ink-jet printers that are cured using ultraviolet radiation.
In ink-jet printing, minute droplets of black or coloured ink are ejected in a controlled manner from one or more reservoirs or printing heads through narrow nozzles on to a substrate which is moving relative to the reservoirs. The ejected ink forms an image on the substrate. For high-speed printing, the inks must flow rapidly from the printing heads, and, to ensure that this happens, they must have in use a low viscosity, typically below 50 mPas at 25° C. although in most applications the viscosity should be below 25 mPas. Typically, when ejected through the nozzles, the ink has a viscosity of 10.5 mPas at the jetting temperature which is usually elevated to about 40° C. (the ink might have a much higher viscosity at ambient temperature). The inks must also be resistant to drying or crusting in the reservoirs or nozzles. For these reasons, ink-jet inks for application at or near ambient temperatures are commonly formulated to contain a large proportion of a mobile liquid vehicle or solvent. In one common type of ink-jet ink this liquid is water—see for example the paper by Henry R. Kang in the Journal of Imaging Science, 35(3), pp. 179-188 (1991). In those systems, great effort must be made to ensure the inks do not dry in the head due to water evaporation. In another common type the liquid is a low-boiling solvent or mixture of solvents—see, for example, EP 0 314 403, EP 0 424 714 and GB9927247.8. Unfortunately, ink-jet inks that include a large proportion of water or solvent cannot be handled after printing until the inks have dried, either by evaporation of the solvent or its absorption into the substrate. This drying process is often slow and in many cases (for example, when printing on to a heat-sensitive substrate such as paper) cannot be accelerated.
Another type of ink-jet ink contains unsaturated organic compounds, termed monomers, which polymerise by irradiation, commonly with ultraviolet light, in the presence of a photoinitiator. This type of ink has the advantage that it is not necessary to evaporate the liquid phase to dry the print; instead the print is exposed to radiation to cure or harden it, a process which is more rapid than evaporation of solvent at moderate temperatures. These monomers may be acrylate or methacrylate esters, as is disclosed in EP 0540203B, U.S. Pat. No. 5,270,368 and in WO 97 31071. In such ink-jet inks it is necessary to use monomers possessing a low viscosity. In practice it is difficult to find (meth)acrylate monomers or combinations of (meth)acrylate monomers which do not give compositions with an unacceptably high viscosity for ink-jet printing. This is especially true if materials are selected with more than one polymerisable functional group in the molecule, though these monomers have the advantage of giving a more cross-linked, and therefore tougher, polymer after irradiation. Thus, there is a restriction on the selection of the higher molecular weight multifunctional (meth)acrylates which are generally more viscous but which, other things being equal, give after polymerisation more resistant films. In practice monofunctional acrylate monomers need to be used with multifunctional acrylate monomers to arrive at a suitably low viscosity for ink-jet printing. However, this tends to reduce the cure speed and decreases the cross-link density which affects properties such as adhesion and toughness.