Inkjet printing processes fall into two main types: continuous processes and drop-on-demand (DOD) processes. Continuous processes use electrically conductive inks to produce a stream of drops electrically charged ink that are deflected by an electric field to an appropriate location on a substrate. In DOD processes, individual drops of ink are expelled from the nozzle of a print head either by vibration of a piezoelectric actuator (in piezoelectric inkjet printing) or by heating the ink to form a bubble (in thermal inkjet printing, also known as bubblejet printing). Thermal inkjet printing has advantages over piezoelectric printing, with printers and print heads being lower cost and with the printing process being able to achieve better resolution.
Inkjet inks need to satisfy a number of requirements, including the following:                Viscosity must be appropriate. With DOD inks there are greater limitations on inks for thermal printing than for piezoelectric printing, with it generally being necessary for inks to have a viscosity of below 4 mPa·s at print head operating temperature (which is typically 40-50° C.), which usually equates to a viscosity of less than 10 mPa·s at a temperature of 25° C., to be capable of being thermally inkjet printed. In this specification, all viscosity values are at 25° C. unless otherwise specified.        The ink must not cause unacceptable levels of clogging or blockage of printing nozzles.        The ink must not result in build up of deposits on the ejection heaters of thermal inkjet print heads (a process known as kogation) to an unacceptable level during the working life of a print head.        The ink should be stable in storage, without settling out or coagulation of materials.        The resulting print needs to satisfy desired characteristics depending on the field of use, with possible relevant factors including waterfastness, scratch resistance, durability, lack of shrinkage, lack of cracking, flexibility, optical density (for coloured inks), uniformity of deposition.        
In addition inks suitable for use in thermal inkjet printing must also comprise a volatile solvent which vapourises in use to form bubbles of ink. Water is often chosen as such a solvent as it has a low viscosity and carries no ignition risk.
Radiation-curable inks are known and provide a greatly superior printing result than for non-curable inks. These typically comprise one or more monomers etc. curable in response to appropriate conditions, typically ultraviolet (UV), infra red (IR), microwave or heat.
Two main ways by which curing can be achieved are by a free radical mechanism and by a cationic mechanism. Curing by the free-radical mechanism suffers from a number of drawbacks, such as oxygen inhibition and poor adhesion on difficult (e.g. low surface energy) substrates, which the cationic mechanism does not suffer from.
Additionally curing by the cationic mechanism also offers a number of advantages including:                excellent adhesion to a variety of substrates (metals, plastics, glass and ceramics)        very good dimensional stability (low shrinkage)        low viscosity        excellent toughness        no oxygen inhibition and high curing rates in air        formable coatings        post cure effect beneficial for reaching full cure and optimum properties.        
However, it is known that water can prevent or inhibit the cationic curing process. Inkjet inks which are cationically curable are therefore typically free of water.
Other major practical difficulties can arise when formulating curable inks comprising water. For example, cationically- and free-radically-curable monomers and oligomers tend to have limited solubility in water, and may undergo undesirable phase separation unless care is taken.
WO 2006/056781 discloses curable inks for use in inkjet printing having a viscosity less than 100 mPas, which contain cycloaliphatic epoxy compounds, oxetane monomers, hydroxy-containing compounds and optionally an allyl/vinyl ether monomer, together with at least one cationic photoinitiator, among other ingredients. The exemplified inks have viscosities in the range 91.7 to 19.0 mPas at 25° C. The compositions are water-free.
US 2005/0171237 discloses a cationically curable composition, suitable for use in piezoelectric inkjet printing, comprising a low viscosity reactive resin comprising an oxetane and a cyloaliphatic epoxy resin, in combination with a higher viscosity reactive resin of similar composition. The compositions are substantially non-aqueous.
US 2005/0090580 discloses an ink for inkjet recording comprising oxetanes and aliphatic epoxy compounds, see paragraph 75). The compositions are curable by a cationic mechanism. It is generally desired that the composition is prepared so as not to contain any volatile component such as water and organic solvents.
US 2004/0166253 concerns inkjet inks suitable for use in piezoelectric inkjet printing, where the viscosity of the ink is reduced and has good performance under high humidity conditions. The composition comprises an alicyclic epoxy material, an oxetane and optionally a vinyl ether. The composition, does not contain water.