CROSS REFERENCE TO RELATED APPLICATIONS

This application is a 371 National Stage Application of PCT/EP2016/074159, filed Oct. 10, 2016. This application claims the benefit of European Application No. 15189614.9, filed Oct. 13, 2015, which is incorporated by reference herein in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to UV curable inkjet inks and printing methods therewith.

2. Description of the Related Art

Over the last years, offset and flexographic printing systems have increasingly been replaced by industrial inkjet printing systems due to their flexibility in use, e.g. variable data printing, and due to their enhanced reliability allowing their incorporation into production lines.

Radiation curable inkjet inks have been the preferred choice of ink because high quality images can be printed even on non-absorbing ink-receivers, such as plastic foils for food packaging.

Special UV curable inkjet inks have been developed that meet the migration limits for ink components when printing on food packaging. For example, EP 2053101 A (AGFA) discloses free radical curable liquids and inks for inkjet printing of food packaging materials comprising no initiator or otherwise one or more initiators selected from the group consisting of non-polymeric di- or multifunctional initiators, oligomeric initiators, polymeric initiators and polymerizable initiators and having a very specific polymerizable composition.

There has been some criticism that (meth)acrylates give off an unpleasant odour before and after curing, which can irritate the eyes, skin and respiratory organs.

In one approach, this criticism has been addressed by trying to replace the (meth)acrylates in the UV curable inkjet ink by other polymerizable compounds. For example, US 2013053469 A (DURST) discloses to manufacture inkjet inks by replacing (meth)acrylates by vinyl based monomers, such as vinyl esters, vinyl carbonates and vinyl carbamates. However, these monomers tend to cure much slower, causing both problems of migration and productivity loss of the printing process.

In another approach, the amount of (meth)acrylates in the UV curable inkjet ink was reduced by replacing it by water or organic solvents. For example, US 2012225968 A (SEIKO EPSON) discloses an aqueous UV curable inkjet ink including: a pigment; a water-soluble organic solvent; a surfactant; at least either of a urethane (meth)acrylate having a weight average molecular weight of 1,000 to 10,000 and a cross-linked urethane (meth)acrylate having a constitutional unit including the urethane (meth)acrylate; a compound having radical polymerizable groups; a photoradical polymerization initiator; and water. However, it was found that storage stability of such an inkjet ink was limited. An improvement for this storage stability, and also spreading of the ink on a substrate, is disclosed in US 2014002556 A (SEIKO EPSON) by including a specific polyether-modified silicone oil in the aqueous UV curable inkjet ink. However, oils do not evaporate and may deteriorate the substrate, e.g. a card board. The colorant used in both patents is a colour pigment dispersed in the aqueous phase as a self-dispersible pigment or with the aid of a polymeric dispersant.

US 2015111981 A (FU ET AL.) discloses a method of preparing UV-curable inkjet pigment ink for textile printing, in which mini-emulsion techniques are used to encapsulate monomer, photo initiator, thermal initiator and pigment in mini-emulsion particles to make a UV-curable pigment paste, and the UV-curable pigment paste is mixed with water-soluble co-solvent, surfactant and water to make a UV-curable pigment ink. It was found that to increase the monomer conversion rate in such a pigmented UV curable inkjet ink using a pigment paste that a thermal initiator had to be incorporated, which reduced the storage stability.

Hence there remains a need for improved UV curable inkjet inks that exhibit less bad odour or skin-sensitization, while maintaining good cure speed and storage stability.

SUMMARY OF THE INVENTION

In order to overcome the above problems, preferred embodiments of the present invention have been realised with an aqueous UV curable inkjet ink as defined below.

It was found that by replacing a large part of the polymerizable compounds by water and by incorporating the polymerizable compounds in capsules and performing UV curing in combination with polyfunctional thiol compounds that good storage stability and curing speed was obtained. Good physical properties, such as adhesion and scratch resistance on non-absorbing substrates, were observed.

It was found that aqueous UV curable inkjet inks were obtained have good curing speed and excellent storage stability when a self-dispersible pigment was used in the aqueous medium and especially when the colour pigment was incorporated in the core of the capsule. In the latter case this results in a mono-colloid system exhibiting higher storage stability than when the colorant was included in the aqueous medium.

By using free radical curable chemistry instead of cationically curable chemistry, the residual water from the aqueous medium after the jetting and drying did not reduce the curing speed.

Further objects and advantages of the invention will become apparent from the description hereinafter.

