EP2024187B1

EP2024187B1 - A printing process

Info

Publication number: EP2024187B1
Application number: EP07761811A
Authority: EP
Inventors: Nigel Anthony Caiger; Alexander Grant; Robin Mcmillan; Stewart Kessel
Original assignee: Sun Chemical Corp
Current assignee: Sun Chemical Corp
Priority date: 2006-05-05
Filing date: 2007-05-03
Publication date: 2012-01-11
Anticipated expiration: 2027-05-03
Also published as: WO2007131099A2; EP2024187A2; WO2007131099A3; ATE540821T1; GB0609011D0

Abstract

A process of ink jet printing onto an optical disc substrate is disclosed which consists of the steps of i) screen printing a radiation-curable primer onto the optical disc substrate to form a layer of wet primer, ii) ink jet printing a radiation- curable ink onto the wet primer layer, and iii) curing the primer and the ink simultaneously.

Description

The present invention relates to a printing process, in particular, to a process for printing on optical discs, to a primer composition for use in the process and to printed discs made using the process.
The term "optical discs" refers to optical information storage discs such as those designed to hold data which can be retrieved by means of a laser or other optical device and encompasses compact discs (CDs), digital versatile discs (DVDs) and the like. The term "radiation-curable" as used herein means that curing is induced by one or more types of radiation, such as ultraviolet light (UV) or electron beam (EB) radiation.
Radiation-curing ink jet printing has become a commercially accepted technology for producing graphic display and other products on a wide range of substrates, such as paper, cardboard, plastics and glass. The advantages of ink jet printing include being able to print direct from a computer, being able to economically print small print runs, being able to easily customise individual images, minimal down time between jobs and the robust performance of the printed product.
Printing optical discs has previously been carried out using offset, flexo, screen and pad printing. All those techniques have the disadvantage that they have high set up costs making them unsuitable for printing anything other than large print runs.
Ink jet printing directly onto optical discs has not previously enjoyed wide commercial success. The quality of ink jet printed images is dependent on the properties of the substrate. Optical discs are generally made of polycarbonate or acrylic materials and can be spin coated with an acrylicbased coating. These are poorly wetted by commercially available inks leading to poor print quality and/or poor adhesion of the cured ink to the disc.
One approach to enable ink jet printing onto optical discs is to provide pre-coated discs which have been treated with coatings that provide a surface suited for ink jet printing. Until now pre-coated CDs have only enjoyed limited success, for reasons of cost and because the coatings are insufficiently robust. For example, optical discs that have been spray coated with a silica based coating are available for ink jet printing using water based ink jet inks. Those discs have not been practical for printing large runs, have been expensive to produce and the quality of the printed image has been inferior to those printed on optical discs by offset or screen printing. Furthermore the resulting printed discs have not generally been found to be very hard wearing, the printed coating tending to scratch easily. Those discs also have been found to have poor water and humidity resistance. Thus the use of pre-coated optical discs for subsequent ink jet print has generally been limited to small scale print runs in niche markets.
US 6,846,541 describes a process in which an optical disc is screen printed with a white layer which is then UV-cured followed by screen printing of an ink receiving layer which is then UV-cured followed by printing of an aqueous ink jet ink to form an image which is then overcoated with a solvent based overcoating layer.
EP 1 154 418A2 discloses a process which involves screen printing a UV curable ink-receiving layer onto an optical disc and curing that layer and then ink jet printing an aqueous ink onto the cured layer.
US 5,920,329 discloses a process in which an optical disc is coated with a hydrophilic film and an aqueous printing ink is ink jet printed onto that hydrophilic ink-receiving layer.
EP 0 652 555A2 discloses a process in which an optical disc is coated with a UV-curable protective layer containing an absorbent filler. The protective layer is cured and a water-based or oil-based ink is applied to the cured layer using writing implements or an ink jet printer.
Accordingly, there remains a need for an ink jet printing process which produces images of acceptable quality.
The present invention provides a process of ink jet printing onto an optical disc substrate which comprises the steps of i) screen printing a radiation-curable primer onto the optical disc substrate to form a layer of wet primer, ii) ink jet printing a radiation-curable ink onto the wet primer layer, and iii) curing the primer and the ink simultaneously.
Thus, according to the invention, a CD, for example, may be printed by a process in which the CD blank is taken from a stack, a layer of primer is applied to the blank by screen printing, the primed blank is passed under the ink jet print head and an image is printed onto the wet primer layer. Because the ink is printed onto the top of the primer layer and does not come directly into contact with the optical disc substrate itself, the printer is not limited to the printing of special inks suitable for printing directly onto the surface of optical disc substrates. Preferably, the ink jet ink is applied using a single pass of a print head thus providing rapid printing.
Radiation curable compositions typically shrink upon curing, as the various components cross-link to form a 3D-network of higher density than the original composition. That shrinkage presents a particular problem in the printing of optical discs because it can cause the disc to warp, thereby rendering it unusable or unreliable. As mentioned below, the shrinkage of the primer composition may be reduced by including in the primer components which are already at least partially polymerised, such as polymers or oligomers. However, including relatively high molecular weight components such as polymers or oligomers in the primer leads to a corresponding increase in viscosity, which in turn influences the method by which the primers can be applied to the discs. The present inventors have found that radiation curable primer compositions having a viscosity well suited to screen printing can give a desirably low degree of shrinkage which makes possible their use on optical discs.
The primer layer may extend over part or all of one face of the disc.
The primer is cured after the printing of the radiation-curable ink onto the primer layer, that is, the primer is screen printed onto the optical disc and, before the primer is cured, a radiation curable ink is printed onto the primer layer by ink jet printing. Both the primer and the ink jet ink are then cured simultaneously, that is, in the same step by the same radiation source. For example, where the ink is a UV-curing jet ink the primer is advantageously also UV-curable and can be cured by exposure to the same dose of radiation which is used to cure the ink.
It has been found that improved print qualities can often result from printing the ink jet ink onto the primer prior to curing of the primer layer. Factors that may lead to an enhanced print quality include i) better adherence of the ink jet ink to the wet (not fully cured) primer, ii) better spreading of the ink on the wet primer layer which has a higher surface energy than when cured, and iii) the primer being more receptive to the ink prior to curing. In particular, the appearance of lines as a result of the poor spreading of the ink droplets has been found to be lessened when ink jet printing occurs prior to curing the primer.
It will usually be desirable to minimise the time between the priming and printing stages in order to avoid contamination of the primer by dust and to avoid the need to store the primed discs. Advantageously, the printing step occurs no longer than 1 hour after the priming step, preferably no longer than 10 minutes. The primer and the radiation-curable ink are preferably applied to the optical disc substrate in-line, that is, the primer is applied as part of the printing process. In an in-line process, the printing will usually be carried out no longer than 1 minute after the priming. The embodiment of the invention in which the primed substrate is not cured before printing is particularly suited to in-line processing because the printing can take place almost immediately after the primer has been applied. This contrasts with known primer systems in which the primer must be dried after application and before printing, which entails extra process complexity, a need for storage space and delay.
In a favoured embodiment, the primer is applied to the optical disc substrate immediately before the substrate is carried into the print head of the printer. The primer composition and the ink will desirably be such that the ink droplets spread on the surface of the primer layer and coalesce to form a film. Advantageously, in the interval between printing of the ink onto the primer and curing of the ink and primer, a limited degree of mixing of the ink and the primer takes place. Such mixing is not in any way essential but where it takes place it may serve to enhance the adhesion of the cured ink film to the primer layer.
Advantageously, the disc is cleaned of dust, for example by a jet of air, prior to application of the primer.
The viscosity of the primer prior to application to the optical discs is suitable for screen printing. Advantageously, the viscosity of the primer is not less than 500 cP (centipoise), preferably not less than 800 cP, more preferably not less than 1000 cP and especially not less than 1200 cP at 25 °C. Primers having relatively high viscosities of 500 cP or higher and especially 1000 cP or higher have been found not to shrink significantly on curing, the level of shrinkage on curing being linked to the viscosity of the composition, higher viscosity primers generally displaying less shrinkage. Advantageously, the viscosity of the primer is not greater than 10000 cP, preferably not greater than 6000 cP, more preferably not greater than 5500 cP and especially not greater than 4500 cP at 25°C. For example, the viscosity of the primer may be in the range of from 500 cP to 6000 cP at 25 °C, such as from 1000 cP to 5500 cP at 25°C and preferably from 1200 cP to 4500 cP at 25 °C. Viscosities within the preferred ranges have typically been found to be suitable for screen printing. Primers of relatively low viscosities suitable for screen printing, for example of approximately 1200 cP, have been found to be suitable for "fast" printing applications where the lay down of the primer in a short timescale is required. Relatively high viscosities, for example of approximately 4500 cP, have been found to be more applicable for "slower" printing applications in which the speed of application of the primer layer is of lesser importance. The viscosity of the primer may be measured using a REL (Research Equipment (London) Ltd) cone and plate viscometer.
The primer may be a clear varnish or lacquer. Alternatively, the primer may contain a pigment. In many cases, the primer will be colourless. However, in some cases it may be desirable to provide a coloured background colour for printing on by including in the primer a colorant such as a coloured pigment or dye. Preferably the primer comprises a white pigment, such as TiO₂, and an opaque white primer coating is provided. Where a white pigment is provided, the pigment preferably provides a very dense white. Preferably the pigment is present at a level of no less than 10% by weight and more preferably no less than 20% by weight of the total weight of the primer. Preferably, the pigment is present at a level of no greater than 60% by weight and more preferably no greater than 50% by weight of the total weight of the primer. TiO₂ has been found to be a particularly suitable pigment. The primer may comprise an agent to provide a matt finish to the coating surface, such as silica. The primer may comprise an antisettling aid to assist in providing an even distribution of pigment throughout the primer coating. The primer may comprise a flow agent, which may be beneficial in levelling the primer coating across the optical disc to achieve an even coating. Preferably the levelling flow agents are present at a level of at least 0.1% by weight and more preferably at least 0.5% by weight, for example from abut 1% to 2.5% by weight based on the total weight of the primer. One or more flow agent may also be present to improve the flow characteristics (rheology) of the primer so that it is suitable for screen printing. In particular the inclusion of silica has been found to enhance the screen-printability of the primer compositions. Preferably rheology improving flow agents comprise at least 0.1% by weight and more preferably at least 0.3% by weight, for example from about 0.5% to 1% by weight based on the total weight of the primer. The skilled person will be aware that further materials may be included in the primer composition to modify the properties of the primer, for example, the primer may comprise a stabiliser component and/or a filler component.
The primer composition may include a photoinitiator that initiates free-radical or cationic chain reactions on exposure to radiation, such as UV radiation.
In one embodiment, the primer is UV-curable and the process of the invention involves curing the primer by exposure to UV-light. UV-curable primers will, in general, comprise at least one photoinitiator, which may be a cationic photoinitiator or a free radical photoinitiator, as explained further below. In another embodiment, the primer is EB-curable and the process of the invention involves curing the primer by exposure to an electron beam. EB-curable primers do not require a photoinitiator, although one or more may be present.
In embodiments in which the primer is UV-curable, the photoinitiator(s) preferably is present at a level of no less than 1% by weight and more preferably no less than 2% by weight based on the total weight of the primer. Preferably any photoinitiator(s) is present at a level of no more than 20% by weight and more preferably no more than 10% by weight based on the total weight of the primer. When the primer comprises a pigment the primer is advantageously selected so that a deep cure level is achieved, that is, any photoinitiator(s) present are efficient at curing throughout the depth of the pigment-containing coating. Typical photoinitiators that may be used in a primer coating according to the invention include phosphine oxide photoinitiators that are particularly effective in curing white pigment-containing primers.
The primer is preferably 100% solids i.e. containing essentially no volatile component such as water or organic solvents. The primer will contain at least one component that can be cured by a radiation-induced reaction.
The radiation-curable components may be oligomeric or polymeric materials of relatively high number average molecular weight (MW) and/or are monomers having a relatively low number average MW of less than 500. The primer advantageously comprises at least one oligomer. Advantageously, that oligomer has a number average MW of greater than 500 and preferably greater than 1000. The primer advantageously comprises no less than 10% and preferably no less than 20% by weight of one or more oligomers based on the total weight of the primer. Advantageously, the primer comprises not more than 50% and preferably not more than 40% by weight of oligomer based on the total weight of the primer. For example, the oligomer may be present at from 1% to 60% and preferably from 20% to 40% by weight based on the total weight of the primer. In a preferred embodiment, the oligomers have a number average molecular weight of more than 500 and are present at a level of from 20% to 40% by weight based on the total weight of the primer. Preferred oligomers exhibit a low level of shrinkage on curing. Examples of oligomers that have been found to be particularly suitable for inclusion in the primers of the invention include polyester, epoxy, modified epoxy and urethane oligomers with urethane oligomers being especially preferred. The primers may include acrylate oligomers which have been found to give rapid cure times. The presence of oligomers in the primer composition has been found to reduce the level of shrinkage of the primer on curing. Oligomers tend to form fewer cross-links on curing than low molecular weight monomers, and therefore help to reduce the degree of shrinkage of the primer on curing. The application of primers which display a low level of shrinkage on curing is particularly important on optical discs in order to minimise warping of the discs.
A wide range of monomers may be used in the primer compositions of the invention with those that exhibit a low level of shrinkage on curing and high adhesion to the optical disc substrates being preferred. The monomers may be monofunctional or multifunctional (that is, having more than one polymerisable group). Advantageously, the primer includes both monofunctional and multifunctional monomers. The monomers may be present in an amount of from 10% to 60% by weight, preferably from 10% to 40% by weight and more preferably from 15% to 30% by weight, based on the total weight of the primer.
Two suitable types of curing reaction which are well known in the ink jet and other fields are free-radical curing and cationic curing. Suitable free-radically curable components for both the primer and the ink layers include ethylenically unsaturated monomers and oligomers such as acrylate and methacrylate components and vinyl components.
Acrylate monomers include multifunctional acrylate monomers such as hexanediol diacrylate, trimethylolpropane triacrylate, pentaerythritol triacrylate, polyethyleneglycol diacrylates (for example, tetraethyleneglycol diacrylate), dipropyleneglycol diacrylate, tri(propylene glycol) triacrylate, neopentylglycol diacrylate, bis(pentaerythritol) hexa-acrylate, and the acrylate esters of ethoxylated or propoxylated glycols and polyols, for example, propoxylated neopentyl glycol diacrylate, ethoxylated trimethylolpropane triacrylate. 1,6-hexanediol diacrylate (HDDA) has been found to be a particularly suitable monomer for inclusion in the primer compositions of the invention due to its good adhesion to optical disc substrates. Monofunctional acrylate monomers may be present such as the esters of acrylic acid, for example octyl acrylate, decyl acrylate, isobornyl acrylate, phenoxyethyl acrylate, tetrahydrofuryl acrylate, 2-(2-ethoxyethoxy) ethylacrylate.
Methacrylate monomers include hexanediol dimethacrylate, trimethylolpropane trimacrylate, triethyleneglycol dimethacrylate, diethyleneglycol dimethacylate, ethyleneglycol dimethacrylate, 1,4-butanediol dimethacrylate.
Suitable vinyl components include N-vinyl pyrolidones, N-vinyl caprolactams, vinyl ethers and styrenes. 1-Vinyl caprolactam (VCAP) has been found to be a particularly suitable vinyl monomer for inclusion in the primer compositions of the invention due to its good adhesion to optical disc substrates. Vinyl ether compounds include α,β-unsaturated ether monomers, such as triethylene glycol divinyl ether, diethylene glycol divinyl ether, 1,4-cyclohexanedimethanol divinyl ether and ethylene glycol monovinyl ether, as well as ethyl-1 propenyl ether, triethyleneglycol methyl propenyl ether, triethyleneglycol methyl vinyl ether and 2-cyclopenten-1-yl ether.
Suitable cationically curable components include oxygen-containing ring opening monomers and oligomers such as those comprising an oxetane ring or an oxirane ring. Oxirane species include cycloaliphatic oxiranes (also known as epoxides), such as 3,4-epoxy cyclohexyl methyl-3,4-epoxy cyclohexyl carboxylate and the glycidyl ethers of polyols. Oxiranes derived by the epoxidation of unsaturated materials may also be suitable, for example, epoxidised soybean oil, epoxidised polybutadiene or epoxidised alkenes. Oxetane species include mono-functional and multi-functional oxetanes, for example, 3-ethyl-3-hydroxymethyl-oxetane, bis[(1-ethyl-3-oxetanyl)methyl] ether, 3-ethyl-3-[2-ethylhexyloxy)methyl]oxetane, [1,4-bis(3-ethyl-3-oxetanylmethoxy)methyl]benzene and trimethylolpropane oxetane. Many further suitable materials are known to the skilled person.
The primer may be free-radically curable, cationically curable or it may be curable by a combination of the two mechanisms. Free-radical curing is preferred. Advantageously, the primer composition comprises components curable by a free-radical mechanism whilst being substantially free of components that are curable only by cationic mechanism. Some cationic systems generate acidic by-products on curing. Preferably, the primer does not generate any such acidic by-product on curing in order to avoid degradation of the disc surface.
In general, the primer should be such that the primer wets the optical disc substrate sufficiently well to form a film on the substrate when applied by screen printing. The primer adheres well to the disc material or spin coat, enabling the primer coating to adhere well to the disc. In one embodiment, the primer swells the disc material, thereby promoting good adhesion.
The invention further provides an apparatus suitable for printing an optical disc according to the process of the invention. Preferably the apparatus for printing optical discs includes a screen printing module for screen printing a primer coating onto the optical disc, an ink jet printing module for ink jet printing an image onto the primer coating and a source of radiation for curing the primer and the ink. Preferably, the screen-printed primer and the ink jet printing are applied in-line.
The invention also provides an optical disc produced by the process of the invention. For example, a printed optical disc comprising an optical disc substrate having on at least one surface a layer of cured primer being the cured product of a radiation-curable primer and, on the layer of cured primer, an image formed of cured jet ink. The layer of cured primer composition is desirably in the range of from 7 µm to 15 µm thick.
The invention also provides a screen-printable radiation-curable primer composition for use in the process of the invention comprising at least one radiation-curable component, the primer composition having a viscosity of from 1000 cP to 5500 cP, especially 1200 cP to 4500 cP, at 25 °C. The primer composition may comprise one or more photoinitiators. The primer composition may also comprise one or more passive or inert resins such as acrylic resins. The primer may include one or more pigments. Preferably the pigment is present at no less than 20% by weight, for example from 20% to 50% by weight, based on the total weight of the primer. In a preferred embodiment, the primer comprises at least 10% by weight, for example from 20% to 40% by weight, of oligomer based on the total weight of the primer composition.

An example of a primer composition according to the invention will now be described for the purpose of illustration only.

Component	Wt %
Urethane oligomer - CN 982/388	29
HDDA monomer	12
N-vinyl caprolactam monomer	9.5
Lucerin TPO	4.0
Iracure 907	1
Silicone anti-foam additive	1.8
TiO₂ white pigment	42
Silica	0.7

CN 982/388 is an aliphatic urethane diacrylate oligomer available from Cray Valley / Sartomer.
Lucerin TPO is a UV-curable phosphine oxide photoinitiator available from BASF.
Iracure 907 is a UV-curable α-aminoketone photoinitiator available from Ciba Specialty Chemicals.

This primer composition was screen printed onto a CD disc and subsequently ink jet printed with a uv-curable jet ink. The primer and ink were cured together with a medium pressure curing lamp.

Claims

A process of ink jet printing onto an optical disc substrate which comprises the steps of i) screen printing a radiation-curable primer onto the optical disc substrate to form a layer of wet primer, ii) ink jet printing a radiation-curable ink onto the wet primer layer, and iii) curing the primer and the ink simultaneously.
A process as claimed in claim 1 in which the ink is UV-curable.
A process as claimed in claim 1 or claim 2 in which the primer and the ink are printed onto the substrate in-line.
A process as claimed in any preceding claim in which the primer is UV-curable.
A process as claimed in any preceding claim in which the primer is cured by a free-radical mechanism.
A process as claimed in any preceding claim in which the viscosity of the primer is in the range of from 500 to 6000 cP at 25 °C.
A process as claimed in any preceding claim in which the primer includes at least one pigment.
A process as claimed in claim 7 in which the primer comprises no less than 20% pigment by weight based on the total weight of the primer.
A process as claimed in any preceding claim in which the primer includes at least one oligomer having a number average molecular weight of more than 500.
A process as claimed in any preceding claim in which the primer comprises from 20% to 40% of oligomer by weight based on the total weight of the primer.
An apparatus for printing optical discs including a screen printing module for screen printing a radiation curable primer coating onto the optical disc to form a layer of wet primer, an ink jet printing module for ink jet printing a radiation-curable ink to form an image on the primer coating and a source of radiation, the apparatus being configured so that the ink jet printing module prints the radiation curable ink onto the wet primer layer and the Source of radiation cures the primer and the ink simultaneously.
An apparatus as claimed in claim 11, wherein the screen printing module and the ink jet printing module are in-line.
An optical disc produced by the process of any one of claims 1 to 10 being a printed optical disc comprising an optical disc substrate having on at least one surface a layer of cured primer being the cured product of a radiation-curable primer and, on the layer of cured primer, an.image formed of cured jet ink.