LASER-IMAGEABLE MARKING COMPOSITIONS

Field of the Invention

This invention relates to laser-imageable marking compositions. Background of the Invention

WO02/068205, WO02/074548, WO2004/043704 and WO2005/012442, and also

US2003/0186001 , US2003/0186000, US2006/0040217 and US2005/0032957 (the content of each of which is incorporated herein by reference), describe laser imaging and also materials that can be used for that purpose. Examples that are provided typically involve the use of high energy lasers.

There are many attractions in using non-contact near-IR sources, in particular diode lasers, to generate images from coatings for applications such as variable information packaging. Favourable attributes of diode lasers such as economy, portability and ease of use, are attractive for current needs in the packaging industry, such as in-store labelling.

The use of ink formulations that incorporate materials which absorb radiation from far-IR to mid-IR sources such as heat (~1 to 20 μm) and CO ₂ laser (-10 μm), allows the production of coatings that can generate a distinct coloured image on exposure to such wavelengths but not near-IR sources. The use of ink formulations that incorporate materials which absorb radiation from near-IR sources, such as diode lasers (~1 μm), allows the production of coatings that will generate a distinct coloured image on exposure to near, mid or far-IR irradiation.

WO2006/051309 describes a recording medium which is a colourless or transparent composition comprising a photo or thermally-sensitive-acid generator and a charge-delocalising compound, e.g. a basic compound capable of interacting with a cationic moiety, generated by stimulation of the acid-generator, resulting in a shift of the spectral absorption characteristics of the irradiated region from the non-visible to the visible region of the electromagnetic spectrum. In this manner, a coloured image can be formed from a colourless transparent starting material using photo or thermal stimuli. Summary of the Invention

The present invention is a composition that may be solvent-based and/or melt- processable, and can use UV (e.g. of 266, 355 or 405 nm), near-infrared (800-2000 nm) or CO ₂ laser imaging technology. A radiation-sensitive composition of the present invention comprises a mixture of a photoacid generator such as a tosylate of aliphatic

alcohols, alicylic alcohols or aromatic hydroxides, for producing an acid in response to electomagnetic radiation (UV, X-ray, E-beam or gamma) and at least one dye that is substantially insensitive to change in temperature and sensitive to changes in hydrogen concentration. By means of the invention, effective marking can be achieved, and in a variety of colours. The composition is typically initially colourless or transparent, and can be used to mark a substate or polymer matrix effectively, using non-visible radiation. Description of the Invention

In general terms, a composition of the invention can be formulated and used in known manner. Reference may be made to the patent specifications identified above.

A wide variety of dyes and pigments are suitable for use in the composition of the present invention. They may be initially coloured or colourless, soluble in organic or aqueous solvents and subject to one or more colour changes over a range of pH values. Dyes sensitive to changes in pH may be used singly, or in combination with each other or in combination with dyes relatively insensitive to changes in pH, to produce a broad range of colour changes.

Preferred acid-sensitive dyes for use in the present invention are described in "Dyestuffs and Chemicals for Carbonless Copy Paper" presented at Coating Conference (1983, San Francisco, CA pp 157-165) by Dyestuffs & Chemicals Division, Ciba-Geigy Corp Greenboro, NC. Useful classes of acid-sensitive dyes described in the publication as colour formers include fluorans, phthalide and leuco dyes. Leuco dyes are understood to be colourless in neutral or alkaline media, but become coloured when they react with an acidic or electron-accepting substance. Suitable examples include compounds such as triphenylmethanephthalide compounds, azaphthalide compounds, isoindolide phthalide compounds, vinylphthalide compounds, spiropyran compounds, rhodamine lactam compounds, lactone and dilactone compounds, benzoyl leuco methylene blue (BLMB), derivatives of bis-(p-di-alkylaminoaryl)methane, xanthenes, indolyls, auramines, chromenoindol compounds, pyrollo-pyrrole compounds, fluorene compounds, and fluoran and bisfluoran compounds, with fluoran compounds being preferred. Particularly preferred commercial leuco dye products include the Pergascript range produced by Ciba Speciality Chemicals, Basel, Switzerland and those by Yamada Chemical Co. Ltd, Kyoto, Japan.

Preferred photoacid generators for use in the invention are esters or amides of aromatic or other organic sulfonic acids. These include tosylates, e.g. of aliphatic

alcohols, alicyclic alcohols, aromatic hydroxides, aliphatic amines, alicyclic amines, hetrocyclic amines or hetroaromatic amines. A cationic photoacid generator may be used, and examples are Cyracure and lrgacure compounds available from Dow.

The acid-generators may be one or a mixture of compounds. Thus, in one embodiment, a substrate that generates an acid is used in combination with an irradiation absorber, and the energy absorbed by the latter, e.g. on irradiation with a diode laser, triggers acid release. The released acid will in turn react with the dye precursor (colour former) to form a coloured complex (i.e. coloured mark). An acid may also be generated from combination of NIR absorber and functional polymer (e.g. polyvinyl chloride, polyvinyl acetate, polyvinyl tosylate), using energy from a diode laser. The acid formed in turn will react with the dye precursor to form a coloured complex (i.e. colour mark). By way of example, dye precursor (e.g. Pergascript) and NlR absorber (e.g. Lumogen available from BASF, Projet 825LDI available from Avecia) is incorporated into vinyl chloride lacquer and a colour change is caused by irradiation with energy from a diode laser (at 808 nm). Red, blue or black colours can be obtained by varying the dye precursor. The colour of the complex can be varied by varying the colour former. In principle, it should be possible to create any single colour by this process.

Both aqueous and non-aqueous carrier systems can be used. Non-aqueous systems are preferred. Particularly preferred carriers are ketone and ester solvents such as methyl ethyl ketone and ethyl acetate.

Any suitable binder may be used, and examples are known. Depending on the nature of the constituents, the composition may be an ink, coating composition or lacquer. Alternatively, a composition of the invention may be formulated in a melt- processable polymer; again, examples of suitable materials are known to those of ordinary skill in the art. Melt-extrusion and injection-molding give products in which no or minimal degradable or discolouration is seen.

Compositions such as coatings and inks may be formulated, by way of illustration, from a combination of non-ionic tosylate photoacid generator (e.g. hydroquinone ditosylate, phenyl tosylate) and different colour formers (e.g. Ciba Pergascripts) in a solvent-based binder system. When such a coating solution is applied to clear PP or PET film, the resulting transparent coating of reactive compositions can undergo an irreversible colour change on exposure to 254 nm UV lamp or a 266 or 355

nm laser. A range of compositions can be formulated, to give, from colourless, a black, blue, red, yellow or green colour change.

Compositions based on tosylate photoacid generators and Pergascript colour formers may not absorb at 405 nm and therefore do not undergo a colour change when exposed to irradiation at this wavelength. To avoid any such problem, a UV absorber or

405 nm sensitizer may be added. An example of a suitable UV absorber is parasol

1789.

Certain reactive compositions including a cationic photo-acid generator may not be daylight-stable (i.e. they colour readily). This stability can be improved by the addition of anti-oxidant. For example, the anti-oxidant Tinuvin 770 DF improves daylight handling. The higher the loading of anti-oxidant, the easier the handling, although more energy may be required for the composition to undergo a colour change.

A preferred additive is a compound of a cation such as a metal or ammonium ion with a metal oxyarion, e.g. ammonium octamolybdate (AOM). Another is an amine molybdate.

A coating composition of the invention may be coated on any suitable substrate, e.g. paper or a polymer film, such as a transparent polyester, or polyolefin film. In a particular embodiment, multi-colour imaging may be achieved by coating two sides of clear PET film (or another transparent substrate through which UV light cannot travel) with different photo-reactive compositions. Three different colours can be formed by the following process

(a) on exposure of side one to a UV source, colour one is formed;

(b) on exposure of side two to a UV source, colour two is formed;

(c) on exposure of both sides to a UV source, a composite, third colour is formed.

Thus, for example, one side of clear PET film is coated with a red colour- changing composition and the other with a blue colour-changing composition. On exposure of side one to energy from a 266 nm laser, a red mark is obtained. Exposure of side two to the laser gives a blue mark. When both sides are exposed to the laser, a purple mark is obtained.

In another example, the film is coated on opposite sides with a yellow colour- changing composition and a blue colour-changing composition. On exposure of side one to energy from a 266 nm laser, a yellow mark is obtained. Exposure of side two to

the laser gives a blue mark. When both sides are exposed to the laser, a green mark is obtained.

A full colour image can be created, e.g. using a 266 nm laser, from photosensitive compositions of this invention by the following process: (a) black colour pixels of the image are created with 266 nm laser from an

AOM-based composition coated on paper board;

(b) blue colour pixels of the image are created with 266 nm laser from a blue composition coated on clear PP or PET film;

(c) red colour pixels of the image are created with 266 nm laser from a red composition coated on clear PP or PET film; and

(d) yellow colour pixels of the image are created with 266 nm laser from a yellow composition coated on clear PP or PET film.

When the different colour pixels created individually are superimposed, a full colour image is obtained. The following Examples illustrate the invention. Examples 1 to 17 are tosylate- based coating compositions, while Examples 18 to 22 use a cationic photoacid generator. Examples 23 to 31 use an anti-oxidant, AOM or molybdate. Examples 32 and 33 are of melt-extrudable compositions. Examples 34 to 40 are of coatings incorporating a NIR absorber. Example 41 is of an aqueous composition. Example 1

Into stirred methyl ethyl ketone (7.5 g), acrylic binder Elvacite 2028 (2.5 g) was added portionwise. After the addition was complete, the mixture was stirred at room temperature until complete dissolution was achieved. To the resulting solution hydroquinone ditosylate (0.25 g) and Pergascript Blue SRB (0.25 g) were added and the mixture was stirred at room temperature until complete dissolution was achieved. The resulting clear solution was applied to clear polypropylene (PP) film and polyester (PET) film using a K-bar. Transparent films were formed which, when exposed to 254 nm UV lamp, changed irreversibly from colourless to blue. The colourless to blue image was also achieved on irradiation with a 355 or 266 nm laser. Examples 2 to 5

Example 1 was repeated but, instead of Pergascript Blue SRB, the following were used: (2) Pergascript Black 1-2R (0.25 g), (3) Pergascript Red 1-6 (0.25 g), (4) Yamada Yellow Y726 (0.25 g) and (5) a combination of Pergascript Blue SRB (0.15 g) and Yamada Tellow Y726 (0.15 g). Irriversible colour changes to black, red, yellow and

green were observed; the yellow and green images were also seen in Examples 4 and 5 when using a 405 nm laser. Examples 6 to 10

The procedures and results of Examples 1 to 5 were repeated, using polyvinyl butyral binder (2.5 g) instead of Elvacite. Examples 11 to 15

The procedures and results of Examples 1 to 5 were repeated, using ethyl acetate (7.5 g) and phenyl tosylate (0.5 g) instead of MEK and hydroquinone ditosylate, 0.5 g instead of 0.25 g of each leuco dye component in Examples 11 to 14 and, in Example 15, 0.25 g Yamade Yellow Y726 and 0.25 g Pergascript Blue SRB. Examples 16 and 17

The procedures and results of Examples 1 and 11 were repeated, except that Parsol 1789 (0.5 g) was added with the hydroquinone ditosylate or phenyl tosylate and Pergascript dye. The colour change was also observed on imaging with a 405 nm laser. Examples 18 to 22

The procedures and results of Examples 1 to 5 were repeated, except that Cyracure 6992 (0.6 g) was used instead of hydroquinone ditosylate; the amount of the leuco dye in Examples 18 to 21 was 0.3 g. The colour change was also observed on imaging with a 405 nm laser. Examples 23 to 27

The procedures and results of Examples 18 to 22 were repeated, except that Tinuvin 770 DF (0.1 g) was added with the Cyracure and leuco dye or combination. Examples 28 and 29

The procedures of Examples 1 and 3 were repeated, except that ammonium octamolybdate (AOM) (3.0 g) was added to the solution comprising solvent, binder, tosylate and dye. The mixture was stirred until uniform dispersion is achieved. The coating was applied to PP film and PET films and paper using a K-bar.

In Example 28, the coated samples, when exposed to a 254 nm UV lamp, changed irreversibly from white to blue. The image changed from white to blue with 355 and 266 nm lasers. With a CO ₂ laser, the sample imaged to afford a black mark on white background.

In Example 29, the coated samples, when exposed to a 254 nm UV lamp, changed irreversibly from off-white to red. The image changed from off-white to red with 355 and 266 nm lasers. With a CO ₂ laser, the sample imaged to afford a brown mark on

white background. Examples 30 and 31

The procedures of Examples 1 and 3 were repeated, except that bis(ethylhexyl)amine octamolybdate (1.5 g) was added with the hydroquinone ditosylate and leuco dye. In Example 30, the PET transparent film, when exposed to a 254 nm UV lamp, changed irreversibly from yellow to blue. The image changed from pale to blue with 355 and 266 nm lasers. With a CO ₂ laser, the samples imaged to afford a blue mark on a transparent pale yellow background.

In Example 31 , the transparent film, when exposed to a 254 nm UV lamp, changed irreversibly from colourless to red. The image changed from pale yellow to red with 355 and 266 nm lasers. With a CO ₂ laser, the samples imaged to afford a brown mark on a transparent pale yellow background. Example 32

Amaster batch was prepared from LLDPE (1200 g), Pergascript black I-2R (120 g) and hydroquinone ditosylate (120 g) on a two-roll mill at 160 ⁰ C, pressed into a thin film plaque and granulated.

To the specified resin an appropriate quantity of the master batch was added and compounded on a two-roll mill at 160 ^Q C. The compound was then pressed into a thin plaque and granulated.

Clear transparent thin films were extruded from the master batch at 190°C. A black mark was obtained when the films were exposed to a UV light source or to a 380 nm Excimer laser using the parameters presented in Table 1.

Table 1

Example 33

The procedure of Example 31 was repeated, but using Pergascript Blue 1-2G (120 g) instead of Pergascript Black 1-2R. Blue instead of black marks were obtained.

Examples 34 to 36

The procedures of Examples 18 to 20 were repeated, except that Project 830LD1 (0.04 g) was added with the Cyracure and leuco dye. Green solutions and transparent green coatings were obtained. Exposure to energy from a diode laser respectively afforded a blue, black or red mark on a green background. Example 37

Into stirred methyl ethyl ketone (8.5 g), vinyl chloride resin UCAR-VAGD (1.5 g) was added portionwise. After the addition was complete, the mixture was stirred at room temperature until complete dissolution was achieved. To the resulting solution, Pergascript SRB blue (0.3 g) and Projet 825LDI (0.04 g) were added and the mixture was stirred at room temperature until complete dissolution is achieved. The resulting green solution was applied to PP and PET films with K-bar. The resulting transparent green coating, on exposure to energy from a diode laser, afforded a blue mark on a green background. Example 38

The procedure of Example 37 was repeated, except that Pergascript 16B red (0.3 g) was used instead of Pergascript SRB blue. Exposure to energy from a diode laser afforded a red mark on a green background. Examples 39 and 40 The procedures and results of Examples 34 and 35 were repeated, except that lrgacure 250 (0.6 g) and Project Lunogen IR 788 (0.03 g) were used instead of the Cycacure and Project 830 LD1. Example 41

Pergascript IR Black (3 g), ProJet 820LDI (0.5 g) and Cyracure 6992 (6 g) were added to UH-5000 (an aqueous acrylic-urethane dispersion, ex-Scott-Bader, 80 g) and Octafoam 235 (ex. Octel, 1 g). The formulation was milled for 15 minutes and then drawn down on to PET film with a K bar. The resulting green coating afforded a black mark on a green background.