Ink composition.

Field of the Invention

This invention relates to ink jet inks.

Background of the Invention

Inkjet printing is a technique that places a number of constraints on the nature of the ink that is used. These constraints include low viscosity (usually less than 10 cP at 25 ° C), sufficient surface tension that the jet can form the necessary large number of small droplets, and sufficient conductivity to direct the droplets onto a substrate to be printed as desired can. The ink must be a homogeneous liquid that is suitable for rapid conversion to a dry printed area on the substrate.

So far, the boundary conditions have been satisfactorily met by using ink jet inks with a binder, a colorant, a conductive component and, as a main component of the liquid phase, low molecular weight organic solvents. A mixture of different solvents can be used to achieve the desired combination of solubility and drying properties, but to achieve rapid drying these are highly flammable, volatile solvents such as methyl ethyl ketone and ethanol.

Known inkjet inks can be UV curable. The binder component is, for example, a prepolymer, usually of high functionality.

US-A-4,303,924 discloses an radiation curable ink jet ink which, in addition to a colorant and a conductive ingredient, has 0 to 90% of a monofunctional polymerizable monomer and 5 to 80% of a multifunctional polymerizable monomer. The colorant is a dye. The conductive component is oil-soluble. It is noted that 0% organic solvent can be present, but all of the examples given contain more than 10% of such a solvent which, together with the high trifunctional monomer content, results in a liquid phase in which the conductive component is soluble. If there was no solvent, the viscosity of the ink is unlikely to be sufficiently low for ink jet printing. The ink is likely to have to be used in an inert gas atmosphere, which is a considerable disadvantage.

The aim of the invention was to provide an inkjet ink which meets the specified boundary conditions, but which avoids the use of volatile, flammable, environmentally unfriendly solvents and can be used in ambient air.

Summary of the invention

An ink jet ink according to the invention has a colorant, a polar conductive component and, as the main component of at least the liquid phase, one or more polymerizable monomers in which the conductive component is soluble. The ink is also free of volatile organic solvent.

An ink according to the invention has a number of desirable properties and advantages, which are summarized below. Perhaps most importantly, because of the non-volatile monomer molecules, inks according to the invention are suitable for all types of ink jet printing. Such monomer molecules are thermally stable, non-flammable liquids with low viscosity and have low odor and low toxicity. These liquids are designed to replace all of the traditional volatile solvents and binders used in the known ink jet formulations. The ink can also be designed with these or a range of solvents and binders.

Description of the invention

An ink jet formulation according to the invention will usually have mixtures of monomers which have different degrees of functionality and contain combinations of mono-, di-, tri- and higher-functional material. Such substances can be cured using UV radiation, which is why the formulation can contain a photoinitiator and / or a photoactivator. In addition to a colorant and a conductive component, the formulation can also contain conventional additives such as stabilizers, surfactants and wetting agents.

With an inkjet printer in continuous operation, the unused solvent-free ink can be recycled and remains stable against heat, moisture and oxidation. During printing, the ink droplet is brought onto the substrate surface and converted to a dry film or dot by polymerizing the monomer molecules through the action of an external energy source directed towards the printing area near the printer. For example, the external energy source can be a UV light source. The UV light source initiates the polymerization process, which typically takes less than 5 msec. The preferably selected light source only emits UV-A light, for example at a wavelength of 315-400 nm, the necessary removal of the ozone formation occurring with UV-B or UV-C light sources becoming unnecessary.

The selection of substances is wide and will depend on the application and the desired properties. To clarify the principle, an ink jet formulation can be designed to provide great solvent resistance by incorporating a relatively large proportion of higher functional monomer to thereby, once cured, produce a highly cross-linked insoluble film.

A number of commercial monomers, such as those with acrylic, vinyl or epoxy functional groups, photoinitiators and photoactivators are available and suitable for use in ink jet formulation and capable of polymerizing by UV light. The reaction can proceed by addition polymerization; all reactants are converted to a final polymer binder, leaving no by-product or trace of liquid. This reaction can take place in two ways, either through a radical mechanism or through the formation of a cationic mechanism.

Suitable monofunctional monomers that cure through the radical mechanism contain vinyl compounds and (meth) acrylic acid esters. Particular examples are octyl acrylate, decyl acrylate, nonylphenol ethoxylate acrylate, N-vinyl pyrollidone, ethyl diglycol acrylate, isobornyl acrylate, ethyl hexyl acrylate, lauryl acrylate, butanediol monoacrylate, b-carboxyethyl acrylate, isobutyl acrylate and hydroxypropyl methacrylate, polypropylene methacrylate, polypropylene methacrylate, and polypropylene methacrylate.

Suitable monofunctional monomers that cure through a cationic mechanism include vinyl ethers, monofunctional cycloaliphatic epoxies, α-epoxides and isodecylgycidyl ethers.

It is preferred that some monofunctional monomer be present in the new ink because such substances can dissolve polar conductive substances and have low viscosity. However, such substances will usually not be the only polymerizable component, since some crosslinking is desired so that this polymerization will quickly result in a dry ink. The amount of monofunctional monomer in the formulation can be up to 70% by weight, for example 25 to 60%.

In order to create balanced properties, the new ink will contain, almost unchanged, somewhat difunctional material, for example in an amount of up to 70% by weight, preferably 20 to 60%, most preferably 30 to 50%, of the formulation. Lower levels of difunctional monomer are associated with longer ink drying times.

Suitable difunctional monomers which harden by a radical mechanism contain difunctional (meth) acrylic acid esters, for example hexanediol di (meth) acrylate, tetraethylene glycol diacrylate, tripropylene glycol diacrylate, butanediol laacrylate, polyethylene glycol diacrylate and triethylene glycol dimethacrylate. Suitable difunctional monomers that cure through a cationic mechanism include triethylene glycol divinyl ether, 1,4-cyclohexanedimethanol divinyl ether, butanediol diglycidyl ether and difunctional cycloaliphatic epoxy resins.

Trifunctional monomers that can be used in the invention contain ethoxylated trimethylolpropane triacrylate: If present (for the reasons given above), tri- or higher functional components will usually have up to 10% by weight of the formulation.

Di- and higher functional monomers are usually not solvents for polar conductive components. The low-functionality monomers have at least the main constituent of the liquid phase. The total content of polymerizable monomers in the ink will usually be 50 to 95% by weight, e.g. at least 70% and often at least 80%, of the formulation.

Suitable photoinitiators, especially for radical hardening, contain 2-hydroxy-2-methyl-1-phenylpropan-1-one, acrylic ketones, 1-hydroxycyclohexylphenyl ketone, 2-methyl-1- [4- (methylthio) phenyl] -2-morpholinylpropanone, 2,2-Dlmethoxy-1,2-diphenylethan-1-one, benzophenone, isopropylthioxanthone and p-phenylbenzophenone. A photoinitiator suitable for the cationic curing process is a triarylsulphonium hexafluoroantimonate salt.

Suitable photoactivators and photosynergists contain ethyl4- (dimethylamino) benzoate, N-methyldiethanolamine and 2-ethylhexyldimethylaminobenzoate. Such substances are usually only required for radical hardening.

The colorant choice for a monomer-based ink jet ink is important, but it is possible to produce either a dye-based monomer ink of the invention or a dispersion-pigmented monomer-based ink. The use of pigment can provide faster curing, which leads to faster printing, and improved solvent resistance.

A range of colors are available, including black. The problem with black UV curable inks is the strong light absorption that prevents successful curing or polymerization. A black ink according to the invention may have a mixture of dyes of different colors that allow UV light to penetrate at a discrete wavelength and initiate the polymerization to produce a black colored film.

Suitable colorants contain carbon black pigment, titanium dioxide dye, ink jet dyes which contain metal azo complex dye substances and mixtures of colored dye substances. The colorant can be in dispersion, if necessary, in the form of particles coated with a material, for example a polymer, which is compatible with the components of the liquid phase.

It has been found that not all of the conductive salts used in the conventional ink jet formulations are suitable for the purposes of this invention because they can catalyze the breakdown of the photoinitiator and initiate the polymerization. However, satisfactory polar salts, including potassium thiocyanate (potassium rhodanide) (most preferred) have been found. Other suitable conductive salts include lithium nitrate, lithium nitrate trihydrate, ammonium thiocyanate and dimethylamine hydrochloride.

Stabilizers such as triethanolamine have been shown to prevent the photoinitiator from breaking down. An example of another stabilizer that can be used is ethylene glycol. Common formulation wetting agents, such as industrial surfactants based on organic solvents, can be included in the formulation.

More specifically, a general purpose or solvent resistant ink jet ink that can be cured by a radical mechanism using a 150 mm (6-inch) medium pressure mercury arc lamp operating at 12 W / mm (300 W / in) and at a wavelength of about 200- 300 nm works, or equivalent, has 0.5 to 5% conductive salt, 1 to 10% photoinitiator, 0.5% photoactivator, 0.05 to 5% colorant and 0 to 2% additives, the ratio being up to 50% monofunctional monomer, up to 50% difunctional monomer and 0 to 10% trifunctional monomer (the percentages are based on the weight of the ink formulation).

A similar formulation that cures under a UV-A light source (wavelength 315-400 nm) contains no photoactivator.

Another similar formulation, which also contains no photoactivator or functional monomer, is curable by a cationic mechanism using the medium pressure mercury arc lamp or equivalent (200-300 nm) or a UV-A source.

Another general purpose and solvent resistant ink jet ink, which is hardenable by a combination of a cationic and radical mechanism using the medium pressure mercury arc lamp or equivalent (200-300 nm), has 1 to 6% photoinitiator for radical polymerization, 1 to 15% photoinitiator for cationic polymerization, 0.5 to 5% conductive salt, 0.5 to 5% colorant and 0 to 2% additives, the ratio being up to 30% of each of the four types of monomers, that is, mono- and difunctional monomers, respectively, which cure through a radical or cationic mechanism.

In brief, the new ink can provide the following advantages and properties, among others: solvent-free (no volatile solvents); not flammable; no change in viscosity due to loss of solvent; no drying out; no solvent extraction or monitoring; low toxicity; slight smell; Resistance to solvents up to MilSpec 202F; General purpose coding and marking ink; short curing times; high conductivity; ie at least 70 mΩ / mm; resistant to increased temperature, moisture and oxidation; stable in the presence of conductive salts; can be black; UV-A (or UV-B / UV-C) light activated; no ozone formation; Viscosity less than 10, e.g. 5-6 cP at 25 ° C; allows fast continuous printing, for example at 1.5-2 m / sec.

The following examples illustrate the invention. All quantities are given in parts by weight.

example 1

60 parts of KSCN and 160 parts of 2,4,6-trimethylbenzoyldiphenylphosphine oxide were added to 1600 parts of N-vinyl-2-pyrrolidinone and mixed until dissolved. Hexanediol diacrylate (1000 parts) was added quickly with stirring.

200 parts of a pigment-in-polymer-in-monomer dispersion were prepared separately by dissolving 32 parts of polyvinyl butyral in 128 parts of N-vinyl-2-pyrrolidinone and adding 40 parts of carbon black and, after predispersion, the dispersion to a particle size of was not ground smaller than 1 µm. The mixture obtained above was added to this dispersion with stirring. The product was filtered to remove particles larger than 1 µm. A homogeneous ink jet ink which was curable under UV-A or UV-B or UV-C light was obtained.

Example 2

60 parts of KSCN were added to 1600 parts of N-vinyl-2-pyrrolidinone and mixed until dissolved. The isopropylthioxanthone photoinitiator (40 parts) and the ethyl diaminobenzoate activator (120 parts) were then added and mixed until dissolved. Hexanediol diacrylate (1000 parts) was quickly added with stirring.

The mixture was added to 200 parts of the same pigment-in-polymer-in-monomer dispersion as in Example 1 with stirring. The product was filtered to remove particles larger than 1 µm. A homogeneous ink jet ink curable under UV-C light was obtained.