GB2501039B

GB2501039B - Printing ink

Info

Publication number: GB2501039B
Application number: GB1312930.9A
Authority: GB
Inventors: Gould Nigel; Ward Jeremy
Original assignee: Sericol Ltd
Current assignee: Sericol Ltd
Priority date: 2011-01-19
Filing date: 2012-01-19
Publication date: 2019-06-05
Anticipated expiration: 2032-01-19
Also published as: GB2501039A; WO2012098404A1; GB201312930D0

Description

Printing ink

The present invention relates to a printing ink, in particular to a screen printing ink.

In screen printing, an ink forming an image is supported on a mesh stretched across a frame. The ink is forced through openings in the mesh and onto the substrate by the action of squeegee which is drawn across the mesh. Once the ink has been transferred to the substrate it must dry within a reasonable amount of time which is dependent on the application. Inks suitable for application to a substrate using screen printing typically have a viscosity of 0.1 to 10 Pas (1 to 100 poise) at 25°C when measured under shear conditions encountered during the printing process.

The screen printing technique is suitable for many different types of drying processes. Two main ink chemistries are used: inks that dry by solvent or mobile liquid vehicle evaporation and inks that dry by exposure to ultraviolet radiation.

Screen printing inks that dry by solvent or mobile liquid vehicle evaporation are commonly formulated to contain a large proportion of a mobile liquid vehicle or solvent. Unfortunately, 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.

Screen printing inks that dry by exposure to ultraviolet radiation contain unsaturated organic compounds, termed monomers or oligomers, which polymerise by irradiation, typically UV radiation, 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.

However, difficulties arise in formulating inks which polymerise by irradiation but which provide printed images having both scratch and chemical resistance and high flexibility.

The high ink deposit that is provided by screen printing provides high colour strength and opacity. However, unless the degree of crosslinking that occurs during the curing process is controlled, films with very low flexibility can be produced. As the film becomes thicker the UV light which penetrates the film decreases in intensity resulting in a film which is cured at the surface but essentially uncured at the substrate/coating interface. The resulting stresses within the coating tend to show as a wrinkling of the surface that is easily removed, leaving still fluid material below.

The attenuation of light energy as it passes into, or through any material, is described by Beer-Lambert's Law:

where l0 is the intensity of incident energy, la is the intensity of the energy at a given depth, A is the absorbance of a coating at a given wavelength, and d is the depth from the surface.

Ideally, in order to minimise the stress in curing thick sections, the coating should be formulated to cure throughout the thickness at a relatively uniform rate as opposed to curing from the surface down.

The film thickness can be controlled to some extent by mesh selection but even with the finest possible meshes, the film weight deposited is still 5 microns. The high film weight typically produced means that it is not possible to get the required flexibility whilst retaining the chemical and scratch resistance properties.

There therefore exists a need for a screen printing ink that is capable of providing high flexibility and chemical and scratch resistance.

Accordingly, the present invention provides a screen printing ink comprising at least 25% by weight of organic solvent based on the total weight of the ink, a radiation-curable oligomer, a free-radical photoinitiator and 10% by weight or less of a dispersed pigment based on the total weight of the ink, wherein the ink comprises less than 5% by weight of water based on the total weight of the ink.

As previously discussed, film flexibility is partially a function of the film thickness. If the crosslink density of the film is fixed, the film flexibility observed decreases with increasing film thickness. The inventors of the present invention have noted that the inclusion of an organic solvent in the ink composition in combination with a radiation-curable oligomer and a free-radical photoinitiator allows the final dry film thickness to be controlled and hence the film flexibility to be improved whilst still enabling a high cross link density to be used. This approach produces films with high flexibility that still possess high film hardness and chemical resistant properties.

The present invention will now be described with reference to the accompanying drawings, in which:

Fig. 1 shows a test for assessing the flexibility and extensibility of the print as detailed in Example 1; and

Fig. 2 also shows a test for assessing the flexibility and extensibility of the print as detailed in Example 1.

The inks of the present invention comprise a modified ink binder system. The presence of a radiation-curable oligomer and a photoinitiator in the ink means that crosslinks can be formed in the dried ink film, leading to improved adhesion to a range of substrates and improved chemical and scratch resistance. The presence of at least 25% by weight of organic solvent means that the final dry film thickness can be controlled and hence the film flexibility can be improved, whilst still maintaining an improved chemical and scratch resistance.

By “radiation-curable” is meant a material that polymerises or crosslinks when exposed to radiation, commonly ultraviolet light, in the presence of the photoinitiator.

The ink comprises an oligomer. The oligomer may possess different degrees of functionality, and a mixture including combinations of mono, di, tri and higher functionality oligomers may be used.

Radiation-curable oligomers suitable for use in the present invention comprise a backbone, for example a polyester, urethane, epoxy or polyether backbone, and one or more radiation polymerisable groups. The polymerisable group can be any group that is capable of polymerising upon exposure to radiation.

Typically, oligomers have a molecular weight of 600 to 4,000. Molecular weights can be calculated if the structure of the oligomer is known or molecular weights can be measured using gel permeation chromatography using polystyrene standards. Thus, for polymeric materials, number average molecular weights can be obtained using gel permeation chromatography and polystyrene standards.

The oligomer polymerises by free-radical polymerisation.

Suitable free-radical polymerisable monomers are well known in the art and include (meth)acrylates, α,β-unsaturated ethers, vinyl amides and mixtures thereof. Typically, monomers have a molecular weight of less than 600.

Monofunctional (meth)acrylate monomers are well known in the art and are preferably the esters of acrylic acid. Preferred examples include phenoxyethyl acrylate (PEA), cyclic TMP formal acrylate (CTFA), isobornyl acrylate (IBOA), tetrahydrofurfuryl acrylate (THFA), 2-(2-ethoxyethoxy)ethyl acrylate, octadecyl acrylate (ODA), tridecyl acrylate (TDA), isodecyl acrylate (IDA) and lauryl acrylate. PEA is particularly preferred.

Preferably, the ink comprises a multifunctional monomer or two or more multifunctional monomers. Preferably, the ink comprises a multifunctional (meth)acrylate monomer.

Suitable multifunctional (meth)acrylate monomers include di-, tri- and tetra- functional monomers. Examples of the multifunctional acrylate monomers that may be included in the screen printing inks include hexanediol diacrylate, trimethylolpropane triacrylate, pentaerythritol triacrylate, polyethyleneglycol diacrylate (for example tetraethyleneglycol diacrylate), dipropyleneglycol diacrylate, tri(propylene glycol) triacrylate, neopentylglycol diacrylate, bis(pentaerythritol) hexaacrylate, and the acrylate esters of ethoxylated or propoxylated glycols and polyols, for example, propoxylated neopentyl glycol diacrylate, ethoxylated trimethylolpropane triacrylate, and mixtures thereof.

Suitable multifunctional (meth)acrylate monomers also include esters of methacrylic acid (i.e. methacrylates), such as hexanediol dimethacrylate, trimethylolpropane trimethacrylate, triethyleneglycol dimethacrylate, diethyleneglycol dimethacrylate, ethyleneglycol dimethacrylate, 1,4-butanediol dimethacrylate. Mixtures of (meth)acrylates may also be used. (Meth)acrylate is intended herein to have its standard meaning, i.e. acrylate and/or methacrylate. Mono and multifunctional are also intended to have their standard meanings, i.e. one and two or more groups, respectively, which take part in the polymerisation reaction on curing. α,β-unsaturated ether monomers can polymerise by free-radical polymerisation and may be used in combination with one or more (meth)acrylate monomers. Examples are well known in the art and include vinyl ethers such as triethylene glycol divinyl ether, diethylene glycol divinyl ether, 1,4-cyclohexanedimethanol divinyl ether and ethylene glycol monovinyl ether. Mixtures of α,β-unsaturated ether monomers may be used. N-vinyl amides and N-(meth)acryloyl amines may also be used in the inks of the invention. N-vinyl amides are well-known monomers in the art and a detailed description is therefore not required. N-vinyl amides have a vinyl group attached to the nitrogen atom of an amide which may be further substituted in an analogous manner to the (meth)acrylate monomers. Preferred examples are N-vinyl caprolactam (NVC) and N-vinyl pyrrolidone (NVP). Similarly, N-acryloyl amines are also well-known in the art. N-acryloyl amines also have a vinyl group attached to an amide but via the carbonyl carbon atom and again may be further substituted in an analogous manner to the (meth)acrylate monomers. A preferred example is N-acryloylmorpholine (ACMO).

The radiation-curable oligomers have free-radical polymerisable groups, preferably (meth)acrylate groups. Acrylate functional oligomers are preferred. Multifunctional oligomers are also preferred and multifunctional (meth)acrylate oligomers are most preferred.

Multifunctional oligomer is intended to have its standard meaning in the art, where the oligomer comprises two or more radical polymerisable groups, preferably three or more, more preferably four or more. Oligomers comprising six polymerisable groups are particularly preferred. Preferably the multifunctional oligomer comprises two to six polymerisable groups.

The oligomer preferably comprises a urethane backbone.

Particularly preferred radiation-curable oligomers are urethane (meth)acrylate oligomers as these have excellent adhesion and elongation properties. Most preferred are di-, tri-, or even higher functional urethane acrylates, however, use of higher functional acrylates may lead to the formation of undesirably brittle films.

Other suitable examples of radiation-curable oligomers include epoxy based materials such as bisphenol A epoxy acrylates and epoxy novolac acrylates, and polyester based materials, which have fast cure speeds and provide cured films with good solvent resistance.

The radiation-curable oligomer used in the preferred inks of the invention cures upon exposure to radiation in the presence of a photoinitiator to form a crosslinked, solid film. The resulting film has good adhesion to substrates and good solvent resistance. Any radiation-curable oligomer that is compatible with the remaining ink components and that is capable of curing to form a crosslinked, solid film is suitable for use in the ink of the present invention. Thus, the ink formulator is able to select from a wide range of suitable oligomers.

In a preferred embodiment, the ink optionally comprises a monomer with a molecular weight of less than 600. Preferably, the ink optionally comprises a (meth)acrylate monomer with a molecular weight of less than 600.

In one embodiment the ink comprises 50 to 100%, or 75 to 100% by weight of free-radical curable oligomer and 0 to 25% by weight of free-radical curable monomer, based on the total weight of radiation-curable material present in the ink.

Preferably the ink comprises less than 20% by weight of a monomer (e.g. (meth)acrylates) with a molecular weight of less than 600 based on the total weight of the ink, or less than 10% by weight, more preferably less than 5% by weight.

Preferably the ink comprises less than 20% by weight of a (meth)acrylate monomer with a molecular weight of less than 600 based on the total weight of the ink, or less than 10% by weight, more preferably less than 5% by weight.

In an alternative embodiment of the invention the ink further comprises a radiation-curable material which is capable of polymerising by cationic polymerisation. Suitable materials include, oxetanes, cycloaliphatic epoxides, bisphenol A epoxides, epoxy novolacs and the like. The radiation-curable material according to this embodiment may comprise a mixture of cationically curable monomer and oligomer. For example, the radiation-curable material may comprise a mixture of an epoxide oligomer and an oxetane monomer.

The radiation-curable oligomer is preferably present in the composition in an amount of 2% to 75% by weight, based on the total weight of the ink, more preferably 2 to 45% by weight, more preferably 5 to 35% by weight, more preferably 8 to 25% by weight, and most preferably 10% to 25% by weight.

The free-radical photoinitiator can be selected from any of those known in the art. For example, benzophenone, 1-hydroxycyclohexyl phenyl ketone, 1- [4-(2-Hydroxyethoxy)-phenyl]-2-hydroxy-2-m ethyl-1 -propane-1 -one, 2- benzyl-2-dimethylamino-(4-morpholinophenyl)butan-1-one, iso propyl thioxanthone, benzil dimethylketal, bis(2,6-dimethylbenzoyl)-2,4,4-trimethylpentylphosphine oxide or mixtures thereof. Such photoinitiators are known and commercially available such as, for example, under the trade names Irgacure and Darocur (from Ciba) and Lucerin (from BASF).

In the case of a cationically curable system, any suitable cationic initiator can be used, for example sulphonium or iodonium based systems. Non limiting examples include Rhodorsil PI 2074 from Rhodia; MC AA, MC BB, MC CC, MC CC PF, MC SD from Siber Hegner; UV9380c from Alfa Chemicals; Uvacure 1590 from UCB Chemicals; and Esacure 1064 from Lamberti spa.

Preferably the free-radical photoinitiator is present in an amount of 1 to 20% by weight, preferably 4 to 10% by weight, based on the total weight of the ink.

The ink of the invention comprises an organic solvent. The organic solvent is in the form of a liquid at ambient temperatures and is capable of acting as a carrier for the remaining components of the ink. The organic solvent component of the ink of the invention may be a single solvent or a mixture of two or more solvents. As with known solvent-based screen printing inks, the organic solvent used in the ink of the present invention is required to evaporate from the printed ink, typically on heating, in order to allow the ink to dry. The solvent can be selected from any solvent commonly used in the printing industry, such as glycol ethers, organic carbonates, lactones, glycol ether esters, alcohols, ketones, esters and pyrrolidones.

The organic solvent is preferably present in an amount of at least 25% by weight, more preferably at least 40% by weight, and more preferably at least 50% by weight, for example 25 to 85% by weight, or 25% to 80% by weight based on the total weight of the ink. In a particularly preferred embodiment the organic solvent is present in an amount of at least 50% by weight, for example 50 to 85%, or 50% to 75% by weight based on the total weight of the ink.

Known solvent-based screen printing inks dry solely by solvent evaporation with no crosslinking or polymerisation occurring. The film produced therefore has limited chemical resistance properties. In order to improve resistance of prints to common solvents such as alcohols and petrol, binder materials that have limited solubility in these solvents are added to the ink. The binder is typically in solid form at 25°C so that a solid printed film is produced when solvent is evaporated from the ink. Suitable binders such as vinyl chloride copolymer resins generally have poor solubility in all but the strongest of solvents such as cyclohexanone, which is classified as “harmful” and has a strong odour. In order to solubilise the binder, these solvents are generally added to the ink.

The ink of the present invention includes radiation-curable material that cures as the ink dries and it is not therefore necessary to include a binder in the ink in order to provide a printed film having improved solvent resistance. In one embodiment of the invention the organic solvent is not therefore required to solubilise a binder such as a vinyl chloride copolymer resin, which means that the ink formulator has more freedom when selecting a suitable solvent or solvent mixture. A major factor to consider when selecting the one or more solvents to be used in accordance with the present invention is the resulting volatility of the ink when using such solvents, this being particularly so because of the ‘open’ nature of the screen printing process, which freely allows evaporation; there is a large amount of wet ink that is being processed (e.g. being spread over the screen, forced through the screen, applied to the substrate) on or above the substrate. Inks should not be so volatile that the ink undesirably thickens during the printing process, nor should they clog or block the printing screen (also due to thickening or solidifying in the holes in the mesh) if the print process is temporarily stopped. Thus, the inks need to be ‘screen stable’. Therefore, preferably solvents with lower volatilities are selected for making screen stable inks. Using a standard of N-butyl acetate having an evaporation rate of 100, preferably solvents having an evaporation rate of 70 or less are selected, more preferably of 50 or less and even more preferably of 30 or less. Examples of solvents may include those having an evaporation rate of 10 or less. The solvent selection may be affected by the printing environment, for example, when printing in a hot country, solvents that are less volatile (having a lower evaporation rate number) are selected.

Other important factors in selecting solvents include the solvents compatibility with the oligomers used and health and safety considerations.

In one embodiment the organic solvent is a low toxicity and/or a low odour solvent. Solvents that have been given VOC exempt status by the United States Environmental Protection Agency or European Council are also preferred here.

In another embodiment solvents can be selected from glycol ethers and organic carbonates and mixtures thereof. Cyclic carbonates such as propylene carbonate and mixtures of propylene carbonate and one or more glycol ethers being specific examples.

Alternative preferred solvents include lactones, which have been found to improve adhesion of the ink to PVC substrates. Mixtures of lactones and one or more glycol ethers, and mixtures of lactones, one or more glycol ethers and one or more organic carbonates are particularly preferred. Mixtures of gamma butyrolactone and one or more glycol ethers, and mixtures of gamma butyrolactone, one or more glycol ethers and propylene carbonate are particularly preferred.

In another embodiment of the invention, dibasic esters and/or bio-solvents may be used.

Dibasic esters are known solvents in the art. They can be described as di(Ci-C4 alkyl) esters of a saturated aliphatic dicarboxylic acid having 3 to 8 carbon atoms having following general formula:

in which A represents (CH2)-i-6, and R1 and R2 may be the same or different and represent Ci-04 alkyl which may be a linear or branched alkyl radical having 1 to 4 carbon atoms, preferably methyl or ethyl, and most preferably methyl. Mixtures of dibasic esters can be used.

Bio-solvents, or solvent replacements from biological sources, have the potential to reduce dramatically the amount of environmentally-polluting VOCs released in to the atmosphere and have the further advantage that they are sustainable. Moreover, new methods of production

of bio-solvents derived from biological feedstocks are being discovered, which allow biosolvent production at lower cost and higher purity.

Examples of bio-solvents include soy methyl ester, lactate esters, polyhydroxyalkanoates, terpenes and non-linear alcohols, and D-limonene. Soy methyl ester is prepared from soy. The fatty acid ester is produced by esterification of soy oil with methanol. Lactate esters preferably use fermentation-derived lactic acid which is reacted with methanol and/or ethanol to produce the ester. An example is ethyl lactate which is derived from corn (a renewable source) and is approved by the FDA for use as a food additive. Polyhydroxyalkanoates are linear polyesters which are derived from fermentation of sugars or lipids. Terpenes and non linear alcohol may be derived from corn cobs/rice hulls. An example is D-limonene which may be extracted from citrus rinds.

Other solvents may be included in the organic solvent component. A possible source of other solvents can be derived from the way in which the colouring agent is introduced into the screen printing ink formulation. The colouring agent can be prepared in the form of a pigment dispersion in a solvent, e.g. 2-ethylhexyl acetate, however, screen printing pigment dispersions do not normally contain solvent.

The ink is substantially free of water, although some water will typically be absorbed by the ink from the air or be present as impurities in the components of the inks, and such levels are tolerated. The ink comprises less than 5% by weight of water, more preferably less than 2% by weight of water and most preferably less than 1% by weight of water, based on the total weight of the ink.

Inks of the invention comprise a dispersed pigment. Commercially available examples are sold under the trade-names Paliotol (available from BASF pic), Cinquasia, Irgalite (both available from Ciba Speciality Chemicals) and Hostaperm (available from Clariant UK). The pigment may be of any desired colour such as, for example, Pigment Yellow 13, Pigment Yellow 83, Pigment Red 9, Pigment Red 184, Pigment Blue 15:3, Pigment Green 7, Pigment Violet 19, Pigment Black 7. Especially useful are black and the colours required for trichromatic process printing. Mixtures of pigments may be used.

In one aspect of the invention the following pigments are preferred. Cyan: phthalocyanine pigments such as Phthalocyanine blue 15.4. Yellow: azo pigments such as Pigment yellow 120, Pigment yellow 151 and Pigment yellow 155. Magenta: quinacridone pigments, such as Pigment violet 19 or mixed crystal quinacridones such as Cromophtal Jet magenta 2BC and Cinquasia RT-355D. Black: carbon black pigments such as Pigment black 7.

The dispersed pigment is present in an amount of 10 weight% or less, more preferably 8 weight% or less and most preferably 2 to 5% by weight, based on the total weight of the ink.

The ink can optionally contain an inert resin, such as a thermoplastic resin. The thermoplastic resin does not include reactive groups that are able to crosslink on exposure to radiation. In other words, thermoplastic resin is not a radiation-curable material. Suitable materials have molecular weights ranging from 10,000 to 100,000 as determined by GPC with polystyrene standards. The thermoplastic resin can be selected from epoxy, polyester, vinyl or (meth)acrylate resins, for example. Methacrylate copolymers are preferred. When present, the ink can comprise 1 to 5% by weight of thermoplastic resin, based on the total weight of the ink. The thermoplastic resin increases the viscosity of the ink film prior to curing, leading to improved print definition. The thermoplastic resin also decreases the glass transition temperature of the cured ink, giving greater film flexibility for applications such as vehicle side application.

In a preferred embodiment, the ink of the invention comprises at least 25% by weight of organic solvent based on the total weight of the ink, wherein the organic solvent is selected from glycol ethers, organic carbonates, ketones, lactones and mixtures thereof.

The screen printing ink has a viscosity of 0.1 to 10 Pas (1 to 100 poise), preferably 0.2 to 5 Pas (2 to 50 poise), more preferably 0.5 to 3 Pas (5 to 30 poise) at 25°C. Viscosity may be determined by an ICI Rotothinner which operates with a fixed spindle and fixed speed, such as Sheen Instruments Digital Rotothinner 455N, with 0-60 Poise range.

Other components of types known in the art may be present in the ink to improve the properties or performance. These components may be, for example, surfactants, defoamers, dispersants, synergists for the photoinitiator, stabilisers against deterioration by heat or light, reodorants, flow or slip aids, biocides and identifying tracers.

In a preferred embodiment, the ink comprises an inorganic extender powder, such as talc, clay silicas and fumed silicas.

In a preferred embodiment of the invention the surface tension of the ink is controlled by the addition of one or more surface active materials such as commercially available surfactants. Adjustment of the surface tension of the ink allows control of the surface wetting of the ink on various substrates, for example, plastic substrates.

The present invention also provides an ink set comprising at least one of the inks as defined hereinabove. Preferably, the ink set comprises a cyan ink, a magenta ink, a yellow ink and a black ink (a so-called trichromatic set), wherein at least one of the inks is an ink according to the present invention. Preferably all of the inks in the ink set are inks according to the present invention. The inks in a trichromatic set are for halftone printing and can be used to produce a wide range of secondary colours and tones by overlaying the printed images on white substrate.

The ink set of the present invention can optionally include one or more of green ink, an orange ink and a violet ink. These colours further extend the gamut of colours that can be produced. Violet and orange inks are preferred, most preferred is orange ink.

The ink set of the present invention can optionally include a white ink. White ink can be used in two ways. When printing onto a transparent substrate, white ink can be printed over the image such that the image can be viewed from the reverse. Alternatively, white ink can be used to print a base coat onto a coloured substrate before the image is printed.

In a most preferred embodiment of the invention, the ink set comprises a plurality of colours that can be intermixed to achieve a wide range of tones and shades. A preferred ink set comprises one or more of the following components (base colours):

Yellow (green shade)

Yellow (red shade)

Orange

Red (blue shade)

Red (yellow shade)

Magenta

Blue

Violet

Green

White

Black

Where it is essential that a printed colour is an exact match to a standard, for example, a particular colour pantone, such as a corporate colour, the inks of the invention can be intermixed to produce the exact colour.

To give additional protection to a printed image, a clear ink of the present invention may be applied over the printed image.

The ink set of the invention can optionally include one or more metallic effect inks. The use of metallic colours such as silver is becoming increasing popular in advertising images, for example.

Conventional solvent-based metallic inks can produce very bright metallic effects. The metallic pigments are in the form of flakes or platelets and these are randomly orientated in the undried liquid ink. In the case of solvent-containing inks, the flakes can align parallel to the print surface as the ink film thickness reduces as a result of solvent loss in the drying process. The alignment of metallic pigment flakes parallel with the print surface results in good reflectivity and metallic lustre. However, the films produced can often have very poor rub properties, which means that the pigment can be easily removed from the print surface. UV cured metallic inks generally have better rub properties but are often dull in appearance because the metallic pigment flakes do not have time to align during the rapid UV curing process.

Metallic inks of the present invention overcome these problems because the inks dry in two stages, as discussed below. During the solvent evaporation step the metallic flakes have time to align, allowing a bright metallic effect to be produced in the final image. However, the UV curing stage yields a rub-resistant film.

The ink may be prepared by known methods such as stirring with a high-speed water-cooled stirrer, or milling on a horizontal bead-mill.

The present invention also provides a method comprising: (i) screen printing a screen printing ink or ink set of the present invention on to a substrate; (ii) evaporating at least a portion of the solvent from the printed ink; and (iii) exposing the printed ink to actinic radiation to cure the ink.

Preferred features of the inks that can be used according to this preferred embodiment are as described above for the inks of the invention.

Conventional screen printing techniques as discussed in the introduction are used in the method of the present invention.

The nature of the substrate is not limited and includes any substrate which may be subjected to screen printing such as glass, metals, plastics and paper. Flexible substrates may be used, especially flexible substrates used for the graphic printing industry. Non limiting examples include, polyesters, fabric meshes, vinyl substrates, paper and the like. The inks of the present invention are particularly suited for printing onto self adhesive vinyl and banner grade PVC substrates. Suitable substrates include styrene, polyester, PolyCarb (a polycarbonate), BannerPVC (a PVC) and VIVAK (a polyethylene terephthalate glycol modified).

The ink of the present invention comprises a radiation-curable component and therefore requires curing of the radiation-curable component upon exposure to actinic radiation. The source of actinic radiation can be any source of actinic radiation that is suitable for curing radiation-curable inks but is preferably a UV source. Suitable UV sources include mercury discharge lamps, fluorescent tubes, light emitting diodes (LEDs), flash lamps and combinations thereof. One or more mercury discharge lamps, fluorescent tubes, or flash lamps may be used as the radiation source. When LEDs are used, these are preferably provided as an array of multiple LEDs.

The present invention further provides a substrate having the ink or ink set as defined hereinabove printed thereon.

The invention will now be described with reference to the following example (parts given are by weight), which is not intended to be limiting.

Example

An ink formulation of the present invention was prepared by combining the following components:

Viscosity at 25°C by ICI Rotothinner = 2.5 Pas (25 poise).

The ink was screen printed onto a Mactac Imagin JT5929 P (self-adhesive vinyl substrate) using a 140 PW screen. The wet film was oven dried for 3 minutes at 60°C before being UV cured by passing the print through a conveyorised drier running at 10 m/min. The drier was fitted with one 80 W/cm2 medium pressure mercury lamp.

The cured print was tested for solvent resistance by rubbing with a soft cloth soaked in isopropyl alcohol. The number of double rubs being required to break through to the substrate being noted (maximum 100 double rubs).

The flexibility and extensibility of the print was assessed using the test detailed hereinbelow.

Equipment 1 large aluminium panel (30 x 30 cm) 2 small aluminium panels (5x10 cm)

Infrared thermometer gun

Hot air gun

Felt-edged squeege

The dried prints were cut into 15 cm x 6 cm lengths. The backing was removed from the prepared print sample and a 1 cm strip was adhered to the large aluminium panel at line one as shown in Fig. 1. A 1 cm strip at the opposite end was adhered to one of the small aluminium panels and the second small aluminium panel is held on top to grasp the print firmly, see Fig. 2.

The print sample was heated to 60°C with the hot air gun, and elongated to reach line 3, see for example print B of Fig. 1. The squeegee is used to smooth the stretched print onto the surface of the panel.

Results

Solvent resistance

Flexibility and Extensibility

Claims

1. A screen printing ink comprising at least 25% by weight of organic solvent based on the total weight of the ink, a radiation-curable oligomer, a free-radical photoinitiator and 10% by weight or less of a dispersed pigment based on the total weight of the ink, wherein the ink comprises less than 5% by weight of water based on the total weight of the ink.

2. The ink as claimed in claim 1, wherein the solvent is selected from glycol ethers, organic carbonates, ketones, lactones and mixtures thereof.

3. The ink as claimed in any preceding claim, wherein the radiation-curable oligomer is present in an amount of 2% to 75% by weight based on the total weight of the ink.

4. The ink as claimed in any preceding claim, wherein the radiation-curable oligomer is a multifunctional (meth)acrylate oligomer.

5. The ink as claimed in claim 4, wherein the radiation-curable oligomer comprises a polyester, urethane, epoxy or polyether backbone.

6. The ink as claimed in claim 5, wherein the radiation-curable oligomer comprises a urethane (meth)acrylate oligomer.

7. The ink as claimed in claims 4 to 6, wherein the radiation-curable oligomer has a molecular weight of 600 to 4,000.

8. The ink as claimed in any preceding claim, wherein the ink comprises less than 20% of a monomer with a molecular weight of less than 600, based on the total weight of the ink.

9. The ink as claimed in any preceding claim further comprising an inert resin.

10. An ink set wherein at least one of the inks is an ink as claimed in any one of claims 1 to 9.

11. A substrate having the ink as claimed in any of claims 1 to 9 or the ink set as claimed in claim 10 printed thereon.

12. A screen printing method comprising: (i) screen printing the screen printing ink as claimed in any one of claims 1 to 9 or the ink set as claimed in claim 10 on to a substrate; (ii) evaporating at least a portion of the solvent from the printed ink; and (iii) exposing the printed ink to actinic radiation to cure the ink.