WO2008009987A1

WO2008009987A1 - A printing ink

Info

Publication number: WO2008009987A1
Application number: PCT/GB2007/050377
Authority: WO
Inventors: Kevin Burns
Original assignee: Sericol Limited
Priority date: 2006-07-20
Filing date: 2007-07-04
Publication date: 2008-01-24
Also published as: GB0614431D0

Abstract

This invention relates to a printing ink and in particular inks for use in screen printing that are cured using ultraviolet radiation and are formulated to provide structure stability. The ink comprises one or more UV-curable (meth)acrylate monomers, one or more UV- cu rable multifunctional (meth)acrylate oligomers, fumed or precipitated silica, one or more free-radical photoinitiators, and one or more colouring agents, wherein the ink further comprises from 0.5 to 5 wt% of a passive acrylic resin.

Description

A Printing Ink

This invention relates to a printing ink and in particular inks for use in screen printing that are cured using ultraviolet radiation and are formulated to provide structure stability.

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. Screen-printing inks are commonly formulated to contain a large proportion of a mobile liquid vehicle or solvent and the ink dries by the evaporation of the 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.

Another type of ink contains unsaturated organic compounds, termed monomers, 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.

One application for screen-printing inks is in the provision of a base coat for optical media, such as DVDs and CDs. In the process for printing optical media, a base coat is applied to the optical medium. The base coat is applied to the optical media using an optical media press which contains small screens for applying the screen-printing ink. The base coat which is applied is typically white although other colours are sometimes used, e.g. black. The optical medium is printed with the base coat and then the base coat is cured. The base coat is then typically overprinted using an offset ink.

Optical media presses have a high throughput and the press will usually be left to run automatically. From time-to-time, the operator is required to stop the printing process, for example to adjust the offset printing parameters. When the press is stopped, the screen-printing ink which will inevitably be present on the screen has a tendency to seep through the screen into the printing area. Such seepage is to be avoided since it leads to the contamination of the printing area.

The level of seepage of a given screen-printing ink is determined by the "structure" of the ink. The term "structure" is well-known in the art and relates to the viscosity of the ink at low shear. In order to minimise or prevent seeping, a screen-printing ink will typically have a structure defined in terms of a viscosity of 20-80 Pas, more preferably 30-60 Pas and most preferably 40-55 Pas when measured at 25°C at a low shear. Low shear in this context is intended to mean a shear which is lower than the shear applied to the ink during printing. Typically, low shear will be a shear applied using a Brookfield RV6 viscometer running at 5 rpm.

hi order to provide the appropriate structure to a screen-printing ink for use in an automated optical media press, silica is added to the ink. The silica is a well-known component added to increase the viscosity of the ink. The silica exists as fine particles (typically ranging from 0.007 to 0.05 μm) which tend to link together by a combination of fusion and hydrogen bonding to form chain-like aggregates with high surface areas. The provision of such aggregates imparts structure to the ink. The silica is most commonly used as fumed (pyrogenic) silica although precipitated silica may also be used.

It is further known that the increased structure imparted by the silica may be further improved by incorporating a polar activator. A polar activator is a component which increases the strength of the hydrogen bonding by individual silica particles both to other silica particles and to other components of the ink. Such polar activators are known in the art and include ethylene diamine, ethylene glycol, glycerine, propylene glycol, polyoxyethylene sorbitol, aqueous KOH, water, dihexyl sodium sulfosuccinate and nonylphenoxypoly(ethylene-oxy) ethanols.

However, it has been found that many such polar activators have a tendency to attack the plasties material from which the optical medium is composed. Such a plasties material is typically polycarbonate. Thus, the amount of polar activator would ideally be reduced, although this would lead to a reduction in the structure of the ink. There remains, therefore, a requirement in the art for a screen-printing ink which possesses the required structure without containing materials which have a tendency to attack the substrate.

Accordingly, the present invention provides a screen-printing ink comprising one or more UV-curable (meth)acrylate monomers, one or more UV-curable multifunctional (meth)acrylate oligomers, fumed or precipitated silica, one or more free-radical photo initiators, and optionally one or more colouring agents, wherein the ink further comprises from 0.5 to 5 wt% of a passive acrylic resin.

The ink of the present invention therefore reduces the amount of polar activator by incorporating a small quantity of a passive acrylic resin. The ink may thus be formulated without the presence of a polar activator which in turn reduces or eliminates degradation of the substrate.

The present invention will now be described with reference to the drawing in which the Fig. shows a graph of viscosity against storage time for inks of the present invention and a comparative ink.

The screen-printing ink of the present invention dries primarily by curing, i.e. by the polymerisation of the monomers present, as discussed hereinabove, and hence is a curable ink. The ink does not, therefore, require the presence of water or a volatile organic solvent to effect drying of the ink, although the presence of such components may be tolerated. Preferably, however, the ink-jet ink of the present invention is substantially free of water and volatile organic solvents. The ink of the present invention contains one or more UV-curable (meth)acrylate monomers. The monomer may be multifunctional or monofunctional or a mixture thereof. (Meth) aery late 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. Such materials are well-known in the art.

Examples of the multifunctional acrylate monomers which may be included in the ink of the present invention 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. Particularly preferred are difunctional acrylates with a molecular weight greater than 200.

In addition, suitable multifunctional acrylate monomers include esters of methacrylic acid (i.e. methacrylates), such as hexanediol dimethacrylate, trimethylolpropane trimethacrylate, triethyleneglycol dimethacrylate, diethyleneglycol dimethacrylate, ethyleneglycol dimethacrylate, 1 ,4-butanedioI dimethacrylate.

It is possible to modify further the properties, particularly the cure speed and viscosity, of the ink by inclusion of monofunctional monomers.

The monofunctional (meth)acrylate monomers are also 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

Mixtures of (meth) acrylates may also be used. The total amount of the (meth)acrylate monomer present in the formulation is preferably at least 10 wt%, more preferably at least 15 wt% and most preferably at least 20 wt%, based on the total weight of the ink, and preferably no more than 60 wt%, more preferably no more than 45 wt%, more preferably no more than 30 wt% based on the total weight of the ink.

The ink also contains a UV-curable (meth)acrylate oligomer. The oligomer is preferably a UV-curable (meth)acrylate having a molecular weight of from 500 to 5,000. The degree of functionality of the oligomer determines the degree of crosslinking and hence the properties of the cured ink, particularly a balance between hardness and flexibility. The oligomer is multifunctional meaning that it contains more than one UV-reactive functional group per molecule. The degree of functionality is preferably from 2 to 6.

UV-curable oligomers of this type are well known in the art. A preferred example is bisphenol A epoxy acrylate having a weight average molecular weight of 900. The ink of the present invention may be a blend of oligomers.

The ink preferably contains 5 to 20 wt% and most preferably 10 to 15 % wt% of UV- curable oligomers based on the total weight of the ink.

The ink also contains fumed or precipitated silica, but preferably fumed silica. The silica is a well-known component added to increase the viscosity of the ink and hence imparts structure to the ink. The silica is typically a hydrophilic silica and is preferably untreated with hydrophobising compounds. The surface area of the silica affects the structure and a silica having a higher surface area will tend to provide a greater structure. The surface area is balanced during the formulation of the ink with amount of silica and the amount of other components in the ink to provide the required structure. A preferred silica is a hydrophilic fumed silica having a surface area of 200 m²/g. Examples of suitable silicas include Aerosil® 200 from Degussa and Cabosil® M-5 from Cabot. The silica is preferably present at 0.5 to 3 wt%, more preferably 1 to 2 wt% based on the total weight of the ink. The ink of the present invention also contains from 0.5 to 5 wt% of a passive acrylic resin. Passive resins are resins which do not enter into the UV curing process, i.e. the resin is substantially free of functional groups which polymerise under the curing conditions to which the ink is exposed. The resin includes (meth)acrylate monomers although other co-monomers may be included in the resin. Preferably the resin contains 0-15 wt% of other co-monomers. The resin preferably has a weight average molecular weight from 20,000 to 100,000, more preferably 25,000 to 60,000 and most preferably 30,000 to 40,000. A particularly preferred resin is a poly(methyl methacrylate / n-butyl methacrylate) resin having a weight average molecular weight of 34,000 available as Elvacite® 2013 from Lucite International.

Preferably the ink contains 0.6 wt% or more of the passive acrylic resin; and preferably the ink contains 3 wt% or less, more preferably 2 wt% or less and most preferably 1 wt% or less of the passive acrylic resin based on the total weight of the ink.

Since the ink contains the passive (meth)acrylate resin, other polar activators are not required. Although small amounts could be tolerated the ink is preferably substantially free of such components. The polar activators which most degrade polycarbonate substrates are volatile amines, that is amines having a molecular weight of less than about 200, and hence the ink of the present invention is preferably substantially free of volatile amines. In another embodiment, the ink is substantially free of reactive polar activators. Reactive polar activators are polar activators which react with the substrate causing degradation. By substantially free is meant that the amount is sufficiently low as to avoid a noticeable degradation of the substrate.

In addition to the components described hereinabove, the ink includes a free-radical photoinitiator which, under UV irradiation, initiates the polymerisation of the monomers and oligomers. Preferred free-radical photoinitiators include benzophenone, 1-hydroxycyclohexyl phenyl ketone, 2-benzyl-2-dimethylamino-(4- morpholinophenyl)butan-l-one, 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, Darocur (from Ciba) and Lucerin (from BASF). The wavelength of the UV radiation and the nature of the photoinitiator system used must of course coincide.

Preferably the photoinitiator is present from 1 to 20% by weight, preferably from 4 to 10% by weight, based on the total weight of the ink.

The ink of the present invention includes a colouring agent, which may be either dissolved or dispersed in the liquid medium of the ink. Preferably the colouring agent is a dispersible pigment, hi most cases the ink of the present invention is a white ink. In such a case the preferred colouring agent is titanium dioxide. However, other pigments maybe used, e.g. Pigment Black 7.

The total proportion of pigment present is preferably from 20 to 60 wt%, more preferably from 40 to 50 wt%, based on the total weight of the ink.

The ink of the present invention preferably has a viscosity of 0.1 to 10 Pas (1 to 100 poise) at 25⁰C under printing conditions. Viscosity may be determined using a Brookfield RV6 running at 5 rpm.

The inks of the invention may be prepared by known methods such as, for example, stirring with a high-speed water-cooled stirrer, or milling on a horizontal bead-mill.

The present invention also provides a method of printing comprising screen printing above-described ink on to a substrate and curing the ink. Preferably the substrate is an optical medium, e.g. a DVD or CD. Preferably the method further comprises curing the screen-printing ink and subsequently over-printing the ink with an off-set ink. Although the ink of the present invention is termed an ink, this term applies equally to any screen-printed coating composition containing the required components.

Example

The invention will now be described, by way of example, with reference to the following example (parts given are by weight). Ink formulation of the invention

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

Component %wt

Bisphenol A epoxy acrylate MW 900 8.20

Polyester resin acid no. 12-20 mg KOH/g 7.30

PMMA/PBMA acrylic resin MW 34,000 0.70

Titanium dioxide pigment 43.49

Micronised calcium carbonate filler 5.30 Hydrophilic fumed silica surface area 200 rn²/g 1.20

Hexane diol diacrylate 19.10

Tripropylene glycol diacrylate 3.50

Isodecyl acrylate 2.90 Diphenyl (trimethyl benzoyl) phosphine oxide 3.41

Hydroxy cyclohexyl phenyl ketone 0.80

B enzophenone 1.20 2-Methyl- 1 -[4-(methylthio)phenyl]-2- morpholinopropan- 1 -one 1 ,00

Glycerine 0.70

Silicone oil flow additive 300 cSt 0.60 hi-can stabiliser 0.60

100.00

The ink is designated internal code no. 300S. Comparative ink formulation

An ink formulation which does not contain a passive acrylic resin was prepared by combining the following components:

Component % wt

Bisphenol A epoxy acrylate MW 900 6.20 Amine-modified bisphenol A epoxy acrylate MW 550 5.00

Polyester resin acid no. 12-20 mg KOH/g 8.00 Amine acrylate oligomer viscosity 3000-5000 mPas @ 2O⁰C 4.85

Titanium dioxide pigment 42.89

Micronised calcium carbonate filler 4.70 Hydrophilic fumed silica surface area 200m²/g 0.25

Hexane diol diacrylate monomer 17.30

Tripropylene glycol diacrylate 4,70 Diphenyl (trimethyl benzoyl) phosphine oxide 2.41

Hydroxy cyclohexyl phenyl ketone 0.80

Benzophenone 1.20

Glycerine 0.70

Silicone oil flow additive 300 cSt 0.50

In-can stabiliser 0.50

100.00

The ink is designated internal code no. URA46.

Stability testing

The above inks were tested for storage stability at elevated temperature (40⁰C) and at room temperature. Fig. 1 shows a graph of the viscosities of the samples stored at 4O⁰C. The graph shows a commercially available ink (URA46) which does not contain any passive acrylate resin. The remaining five inks represented by internal code 3008 contain a passive acrylate resin.

The five inks having internal code 3008 all have the same formulation as outlined hereinabove. The inks were prepared either on a Silverson rotor-stator or using a triple roll mill (TRM). The TRM tends to provide a better pigment dispersion on account of the higher shear involved. Both techniques are well-known in the art. The inks labeled in the Fig. as "1st Silverson", "2nd Silverson" and "3rd Silverson" are three batches of the same ink prepared using the same technique. Similarly, "1st TRM" and "2nd TRM" are two batches of the same ink prepared using the same TRM technique.

As may be seen from the graph, the inks of the present invention lose viscosity at a significantly lower rate than the comparative ink.

The inks have also been stored at room temperature and the results are shown in Table 1.

Table 1. Stability of inks over time.

^: Viscosity was determined using a Brookfield RV6, 5 rpm at 25⁰C.

Claims

1. A screen-printing ink comprising one or more UV-curable (meth)acrylate monomers, one or more UV-curable multifunctional (meth)acrylate oligomers, fumed or precipitated silica, one or more free-radical photoinitiators, and one or more colouring agents, wherein the ink further comprises from 0.5 to 5 wt% of a passive acrylic resin.

2. An ink as claimed in claim 1, wherein the ink is a white or black ink.

3. An ink as claimed in claims 1 or 2, wherein the ink has a viscosity of 20-80 Pas at 25°C measured at low shear.

4. An ink as claimed in any preceding claim, wherein the ink comprises 0.6 wt% or more of the passive acrylic resin.

5. An ink as claimed in any preceding claim, wherein the comprises 3 wt% or less of the passive acrylic resin

6. An ink as claimed in claim 7, wherein the ink comprises 2 wt% or less of the passive acrylic resin.

7. An ink as claimed in claim 8, wherein the ink comprises 1 wt% or less of the passive acrylic resin.

8. An ink as claimed in any preceding claim, wherein the ink is substantially free of volatile amines.

9. An ink as claimed in any preceding claim, wherein the ink is substantially free of reactive polar activators.

10. A method of printing, comprising screen printing the ink as claimed in any preceding claim on to a substrate and curing the ink.

1 1. A method as claimed in claim 10, wherein the substrate is an optical medium. i/. A metnoα as claimed in claim 11, wnerein me optical medium is selected lrom a DVD or CD.

13. A method as claimed in any of claims 10 to 12, wherein the method further comprises curing the screen-printing ink and subsequently over-printing the ink with an off-set ink.