WO2006052843A2

WO2006052843A2 - Media providing non-contacting formation of high contrast marks and method of use

Info

Publication number: WO2006052843A2
Application number: PCT/US2005/040202
Authority: WO
Inventors: Hailing Duan; Janet M. Carlock
Original assignee: Fuji Hunt Photographic Chemicals, Inc.
Priority date: 2004-11-05
Filing date: 2005-11-07
Publication date: 2006-05-18
Also published as: WO2006052843A3; WO2006052843A9

Abstract

A laser markable media that can provide superior mark quality with high contrast, high resolution, and a high degree of quality consistency, and that does not rely on physical damages to the material integrity on the exposed area. The laser markable media further provides a balanced performance between good media storage stability, also or heat resistance and optimum sensitivity to laser exposure. Also disclosed is a laser markable media that has a high degree of transparency to satisfy a wider range of application requirements than found in the prior art and a method of using the media.

Description

MEDIA PROVIDING NON-CONTACTING FORMATION OF HIGH CONTRAST MARKS AND METHOD OF USE

FIELD OF THE INVENTION

The present invention relates to a media for non-contact rapid marking that can use a focused beam of electromagnetic wave of specific frequencies and intensity to form high contrast and high resolution marks on a mark formation layer through an isolation layer. The markable media can exhibit high storage stability against heat, harmful chemical exposure and mechanical abrasion. The present invention also relates to a method of using the media.

DESCRIPTION OF THE RELATED ART Products and package labeling have become increasingly important in industry, especially for clearly visible, sharp, high contrast images. Pre-printed label or packaging materials are the predominant method for product and package labeling. However, when an application requires frequent information changes, such as individualized product identification, coding, production date'or expiration date marking, there is a need for rapid marking of variable contents, or on-the-fly and on- demand marking application.

Various printing technologies are used for such application, including direct thermal printing on self-adhesive labels, thermal dye-transfer printing, inkjet printing, embossing or stamping, among others. However, production throughput is often limited due to bottlenecks in the printing speed, particularly when physical contact with each product or label is necessary, such as thermal printing (either direct or dye transfer), drop-on-demand (DOD) type inkjet printing, embossing or stamping. In addition, since these marking technologies rely on physical contact, they are not suitable for marking on products with un-even surfaces. Thermal printing systems also have other disadvantages, such as dirt accumulation on the thermal head and wearing of the contacting surface, which degrades marking quality and readability.

For a non- impact high speed marking application, continuous inkjet (CIJ) technology is also frequently used. However, CIJ technology has problems of frequent nozzle clogging and VOC issues for solvent-based ink systems, or mark smearing problem for aqueous-based ink system, due to slow drying speed of the marks on non-absorbing surfaces, such as plastic films, metal or plastic containers, and the like. Another disadvantage of CIJ technology is its low resolution and low contrast in terms of marking quality. This especially becomes a problem for bar- code printing.

Methods are known in the art for non-contacting rapid marking using focused beams of electromagnetic wave of specific wavelengths and intensity, such as laser beams, which is commonly known as "laser marking". However, one key disadvantage of laser marking is that it requires strong interaction of the laser beam with the material to be marked, to yield significant color or density changes on unmarked areas. The difficulty is that many packaging materials, such as plastic films or containers, metal cans or glass bottles, either do not have sufficient interaction with laser beam (particularly with low power and/or long wavelength laser beams), or the interaction does not yield significant contrast change on the material to yield high quality marks, or in the case that the interaction is strong, it causes direct damages on the material itself.

To enhance the laser beam-material interaction, energy absorbing compounds have been proposed either to be dispersed into the packaging material to be marked on, or to be mixed into a coating composition which in turn is coated on the surface of the material to be marked on. Typical examples of such technology are inorganic based phyllosilicates, metal oxides and silicates compounds, such as talc, kaolin, sericite, mica or metal-oxide coated mica, titanium oxides, tin oxides, iron oxides, or oxides of Sb, As, Bi, Cu, Ga, Ge Si, and the like, as disclosed in US Patent Nos. 6884289, 6855910, 677683, 6727308, 6719837, 6693657, 6689205, 6545065, 6521688, 6444068, 6376577, 6291551, 6214917, 5977514, 5928780, 5866644, 5855969, 5576377, 5030551, Japanese Patent Publication Nos. 2003/277570, and World Patent Document No. WO 2004/050766, WO 01/00719 and WO 03/006558.

However, even with the enhanced interaction between laser beam and material, mark density or contrast are often too weak to become satisfactory commercial products, since it relies on charring or decomposition of the material to be marked on, to either form carbon-rich structures in the material as dark marks, or to generate trapped micro-bubbles (from decomposed material) to form foaming structure in the material as white marks. These mark formation mechanisms often yield poor quality marks because many polymer materials are difficult to carbonize without excessive burning, vaporizing, or complete decomposition, which causes damage to material integrity. Another disadvantage of relying on inorganic laser absorption substances to improve the problem of laser sensitivity is the haziness these additives bring into the material to be marked on, observed as a reduced transparency of the media material. Reduced transparency limits the use of laser markable materials to a narrower range of commercial applications.

To enhance mark contrast and color, it is known in the art that pigments of organo-metallic complexes, inorganic oxides or salts, or carbon black pigment could be used as additives to be dispersed into the packaging material, or to be mixed into a coating composition which, in turn, is coated on the surface of the material to be marked, hi addition, dual coating layers of contrast colors is also proposed, in which the top coating is to be evaporated (ablated) by the laser marking, and thus expose the bottom coating of contrast color. Typical examples of compounds used in these technologies include organo-metallic complex such as copper phthalocyanines, amine molybdate, or colored metal oxide and hydroxide, or metal phosphate/oxide mixed-phase pigments, sulfide and sulfide/selenium pigments, carbonate pigments, chromate and chromate/molybdate mixed-phase pigments, complex-salt pigments and silicate pigments, as disclosed in U.S. patents and U.S. published patent application nos. 2005/0032957, 6888095, 6855910, 6284184, 6207240, 6139614, 6022905, 5840791, 5667580, 5626966. 5576377, 4861620 and 4401992. However, major disadvantages of pigment-based laser marking formulation include the problem of the large particle size of the pigments relative to the desired substrate or coating thickness, and uneven distribution of these solid particles in the media. These problems result in uneven marks and coating coverage, or excessive burning in the marking areas causing damage to media integrity. In addition, some of the currently known marking pigments contain heavy metals that have environmental disadvantages. For laser marking based on the ablation approach, excessive releasing of ablated material or debris into the ambient environment is a significant disadvantage; not only are hazardous materials released into the environment, but also it requires frequent cleaning of the lens on the laser marking head to remove the accumulated fragments or debris released from the ablated marking material. Another disadvantage of the ablation approach is it requires a large laser energy dose, strong enough to completely vaporize the coated layer on the material to be marked. This either leads to slower marking speed which means lower productivity, or more equipment and operation spending for a higher powered laser system.

Dye-based laser marking formulations can avoid the above disadvantages, and offer better marking quality with much higher contrast, even at a much lower laser energy dose. Dye based marking technology developed for conventional contacting thermal printing has been proposed for laser marking applications. For example, JP 2001-246860 discloses the use of a thermal recording material which contains a electron donor-type dye precursor and a urea-urethane-type developer, and U.S. Patent No. 5413629 discloses a method of preparing a laser markable material by using an ink which contains an electron donor-type electron donor-type dye precursor and an electron acceptor-type developer in the printing process.

However, these systems that rely on conventional direct thermal printing technology have disadvantages of poor long-term storage stability or heat resistance, due to the nature of the energy delivery means in direct thermal printing, which relies on contacting heat transfer to rapidly trigger color formation reaction near the contacting interface, and thus requires the reactive media changing color at a threshold temperature of about 80⁰C to about HO⁰C. On the other hand, for packaging and labeling applications, the media often requires wide tolerance over broad temperature ranges and with a long exposing period. In these applications, the long-term storage stability or heat resistance of direct thermal media are often not sufficient, and undesired fogging could result during storage or product transportation.

Another significant disadvantage of dye-based media relying on direct thermal printing technology is its susceptibility towards undesired chemical exposure, especially exposure to acid and base solutions or organic solvents.

However, for certain packaging and labeling applications, the coated substrate often requires strong resistance towards various chemical attacks. For example, in typical label printing, solvent based flexographic inks are frequently use, or in some cases a solvent-based primary coat on label films is applied to enhance the leveling and ink adhesion to the film. In both cases, organic solvents in these formulation often cause undesired color, opacity or density changes on above said imaging layer, due to destabilization of the dye-developer system.

Further, in conventional thermal printing media, electron donor-type dye precursor compounds and developer compounds are often ball-milled into fine solids with a wide range of particle-size distribution, which is dispersed into a resin media often having very different refractive indices. This leads to another disadvantage of such systems, that is, an undesired high level of haziness and low transparency of the marking media. Again, reduced transparency limits the use of laser markable media to a narrower range of commercial applications. Finally, solid dispersed dye precursor particles with a wide range of particle-size distributions could also lead to lower marking resolution.

To improve the media stability and enhance heat resistance of dye-based laser markable media, U.S. Patent No. 5691757 and Japanese patent JP3391000 disclose laser markable compositions using a high melting point developer, above 200°C, combined with inorganic laser absorption substances such as aluminum oxide and mica, to avoid losing marking sensitivity from using high melting point developer. Such combination leads to a very high mark formation threshold temperature, at least in the range of 200-250°C or even higher. One problem of this approach is the risk of decomposition of the polymer media during the high temperature marking process, and releasing of undesired chemical vapor as

"smoke", which is indeed frequently observed with those laser marking methods relying on charring of the material to be marked. In addition, for such a high temperature marking media, either higher powered laser marking equipment becomes necessary, or slower marking speed, and thus lower productivity, has to be accepted.

To prevent releasing of undesired chemical vapor, the idea of transparent "cover sheet" has been suggested in the prior art. U.S. Patent No. 5843547 discloses a method to make a multilayered laser markable label, in which at least one layer of transparent protective film material with a transparent adhesive composition is stacked and adhered to the top of a laser markable media, which is composed of a base layer film material on top of which is coated with a polymeric laser markable layer in which is dispersed 0.1% - 10% of colored pigments and inorganic metal salts, such as copper hydroxide phosphate. The laser marking process is applied through the transparent "cover sheet" to form marks in the underneath laser markable media. If desired by application, the top transparent "cover sheet" along with the transparent adhesive composition can be peeled off from the laser markable media after marking. Similar structures are disclosed in U.S. Patent No. 5340628 and Japanese patent 3391000, except that the laser markable layers are both relying on dye-based thermal printing technology instead of inorganic pigments, and in the case of Japanese patent 3391000, as already described above, a high melting point developer is used in conjunction with inorganic laser absorption additives. While the release of decomposed chemical vapor during laser marking can be prevented by the approaches in these prior arts, the disadvantage of the method disclosed in U.S. Patent No. 5843547 is its inorganic pigment based laser imaging media, which tends to have inferior mark quality, poor contrast and consistency, as compared to dye-based marking systems. The disadvantage of the approach disclosed in U.S. Patent No. 5340628 is its poor long-term storage stability or heat resistance which are inherited from its origin of conventional thermal imaging media. The disadvantage of the approach disclosed in Japanese patent 3391000 is its requirement of >200°C mark formation temperature, which could lead to decomposition of certain polymer materials used for the transparent "cover sheet" during high temperature marking process, releasing undesired chemical vapor; or at least it could introduce significant physical distortion to the marking media due to the residue thermal stress, since the mark formation temperature will be well above the glass transition temperature, T_g, of most of the polymer materials disclosed in that patent. Finally, all three approaches suffer from the disadvantages of high level of haziness described earlier, and thus reduced transparency of the mark formation media, common to all laser markable coatings containing solid dispersed species. It is accordingly noted that in the methods and composition of the prior art described above, it is very difficult to simultaneously achieve good mark quality, high contrast, high storage stability or heat resistance of a marking material, while at the same time maintaining good laser sensitivity and eliminating undesired chemical vapor release during marking process. Therefore, there is a need for an improvement to the prior art to achieve a better laser markable media configuration as well as a coating composition for a laser markable layer that provides a balance between good storage stability or heat resistance and optimum sensitivity to laser exposure, and at the same time provides superior mark quality, high contrast and zero releasing of undesired chemical vapors.

BRIEF DESCRIPTION OF THE DRAWINGS

Figures 1-5 are cross-sections of illustrative media of the invention.

DETAILED DESCRIPTION OF THE INVENTION

A first objective of the present invention is to provide a media that can be marked with a laser provide superior mark quality with high contrast, high resolution, and a high degree of quality consistency, and that does not rely on physical damage to the material integrity on the exposed area, such as ablation, charring, or trapping of gaseous bobbles released from chemical decomposition of coating ingredients.

A second objective of the present invention is to provide a media that has a balanced performance between good media storage stability or heat resistance and optimum sensitivity to laser exposure. A third objective of the present invention is to provide a laser markable media that have high degree of transparency to satisfy wider range of application needs. Yet, another objective of the present invention is to provide laser markable media configurations that do not release decomposed chemical vapors or debris during laser marking process, and that can isolate the mark formation layer from direct exposure to the environment, and therefore the mark formation layer is protected from direct mechanical abrasions or chemical attacks.

A further objective of the present invention is to provide a method of using the media.

The above objectives of the present invention have been achieved by configuring a laser markable media with the following key features: (1) the mark formation layer comprises at least one kind of electron donor dye precursor encapsulated or isolated by a polymer having a T_g of from about 120°C to about 190°C, wherein at least about 80% w/w of said dye precursor has a solubility of higher than lOg/lOOg of ethyl acetate and approximately 90% of the total volume of said dye precursor particles have a diameter of from about 0.2μm to about 5μm, and (2) the laser markable material is configured in such a way that the said mark formation layer is located behind a protective substrate or coating, through which the laser irradiation will be applied, and the said protective substrate material is significantly transparent to the wavelength of the laser intend to be used and having an on-set pyrolysis temperature of at least 200°C.

DETAILED DESCRIPTION OF THE INVENTION A. Composition of the mark formation layer

To achieve the first three objectives of the present invention, the composition of the mark formation layer comprises the following key elements: an electron donor dye precursor preferably micro-encapsulated within a polymer of specific T_g range, an electron acceptor compound which can react with the electron donor dye precursor to turn it into a dye with a strong absorption in the wavelength range of visible spectrum, and a polymer dispersion media in which both species are dispersed and coated in such way that they are in close proximity of reaction lengths from each other. Electron donor dye precursor

An electron donor dye precursor that can be preferably used in the present invention is not particularly limited as long as it is substantially colorless, and is preferably a colorless compound that has such a nature that it colors by donating an electron or by accepting a proton from an acid. A particularly preferred structural feature in the backbone of the electron donor dye precursor includes a ring structure which is subjected to ring opening reaction or cleavage in the case where it is in contact with an electron accepting compound. Typical examples of such structural feature are a lactone, a lactam, a saltone, or a spiropyran, among others. Examples of the electron donor dye precursor include a triphenylmethane phthalide series compound, a fluorane series compound, a phenothiazine series compound, an indolyl phthalide series compound, a leucoauramine series compound, a rhodamine lactam series compound, a triphenylmethane series compound, a triazene series compound, a spiropyran series compound, a fluorene series compound, a pyridine series compound, and a pyradine series compound.

Specific examples of the fluorane series compound include the compounds described in U.S. Patent Nos. 3624107, 3627787, 3641011, 3462828, 3681390, 3920510 and 3959571. Specific examples of the fluorene series compound include the compounds described in Japanese Patent Application No. 61-240989. Specific examples of the spiropyran series compound include the compounds described in U.S. Patent No. 3971808. Specific examples of the pyridine series and pyradine series compounds include the compounds described in U.S. Patent Nos. 3775424, 3853869 and 4246318.

Among the fluorane series, the compounds represented by following structural formula (1) are preferable because these can be incorporated into the microcapsules in very high concentration and hence can provide high mark density. Formula (1)

wherein Rl and R2 are each independently selected from hydrogen, C]-C₈ alkyl, unsubstituted or C₁-C₄ alkyl- or halogen-substituted C₄-C₇ cycloalkyl, unsubstituted phenyl or C₁-C₄ alkyl-, hydroxyl- or halogen-substituted phenyl, C₃-C₆ alkenyl, Ci- C₄ alkoxy, phenyl-Ci-C₄ alkyl, Ci-C₄ alkoxy-Ci-C₄ alkyl and 2-tetrahydrofuranyl, or Rl and R2 together with the linking nitrogen atom are an unsubstituted or Ci-C₄ alkyl-substituted pyrrolidino, piperidino, morpholino, thiomorpholino or piperazino ring.

Specific examples of the phthalide series compound include the compounds described in U.S. Patent Nos. Re. 23024, 3491111, 3491112, 3491116, and 3509174. Among the phthalide series, the compounds represented by following structural formula (2) are most preferable because it can be incorporated into the microcapsules at a very high concentration and can provide high mark density.

Formula (2)

Another preferred compound is represented by formula (3) which is as follows. Formula (3)

A preferable embodiment of the present invention is that the solubility of the said electron donor dye precursor is higher than about lOg/lOOg in ethyl acetate, more preferably is higher than about 15 g/100 g in ethyl acetate, and most preferably is higher than about 18 g/100 g in ethyl acetate.

A preferable embodiment of the present invention is that more than about 80% by weight of the electron donor dye precursors are compounds represented by structural formula (1) or formula (2), and a more preferable embodiment is that more than about 90% by weight are said compounds and a most preferable embodiment is that about 100% by weight are said compounds.

Micro-encapsulation It is preferred that the electron donor dye precursor in the composition of the present invention be used after being formed into a microcapsule, preferably via a surface polymerization process. Specific examples of such polymer capsule materials include polyurethane, polyurea, polyamide, polyester, polycarbonate, a urea- formaldehyde resin, a melamine resin, polystyrene, a styrene-methacrylate copolymer and a styrene-acrylate copolymer. Among these, polyurethane, polyurea, polyamide, polyester and polycarbonate are preferred, and polyurethane and polyurea are particularly preferred.

For example, in the case where polyurea is used as the capsule wall material, the microcapsule wall can be easily formed by reacting a polyisocyanate, such as diisocyanate, triisocyanate, tetraisocyanate or a polyisocyanate prepolymer, with a polyamine, such as diamine, triamine or tetramine, a prepolymer having two or more amino groups, piperazine or a derivative thereof, or a polyol, in the aqueous phase by the interface polymerization process.

A composite wall formed with polyurea and polyamide or a composite wall formed with polyurethane and polyamide can be prepared in such a manner that, for example, a polyisocyanate and a secondary substance for forming the capsule wall through reaction therewith (for example, an acid chloride, a polyamine or a polyol) are mixed with an aqueous solution of a water-soluble polymer (aqueous phase) or an oily medium to be encapsulated (oily phase), and subjected to emulsifϊcation and dispersion, followed by heating. The production process of the composite wall formed with polyurea and polyamide is described in detail in JP-A-58-66948, the contents of which are incorporated by reference. For additional detailed description of such process, refer to known published literatures, such as "Polyurethane Handbook" written by Keiji Iwata, and published by Nikkan Kogyo Shimbun, Ltd. (1987) and "Polyurethane Handbook" edited by Dr. Gϋnter Oertal, and published by Hanser Gardner Publications, Inc. (2^nd ed., 1993), the contents of which are incorporated by reference.

In the present invention, the conditions for the microencapsulation reaction are set so that at least about 90% of the total volume of said electron donor dye precursor particles have an average particle diameter of the microcapsules that are formed of between about 0.3 to about 12 μm, preferably between about 0.2μm and about 5μm, and most preferably between about 0.2 μm and about 2 μm. The microcapsule material and microencapsulation reaction are also carefully selected and controlled so that the microcapsule wall has a glass-transition temperature, T_g, of from about 120°C to about 190°C, preferably from about 150°C to about 180⁰C. The microcapsule wall may further contain, depending on necessity, a metal- containing dye, a charge adjusting agent, and other arbitrary additive substances. These additives may be contained in the capsule wall if added before or during wall formation or added at other arbitrary times as required. In order to adjust the charging property of the surface of the capsule wall, a monomer, such as a vinyl monomer, may be graft-polymerized depending on necessity. Furthermore, in order to make a microcapsule wall having excellent substance permeability at desired marking temperature and to obtain superior mark quality of high coloring effect, it is preferred to use a plasticizer that is suitable for the polymer of the chosen wall material. The plasticizer preferably has a melting point of about 12O⁰C or higher. For example, in the case where the wall material comprises polyurea or polyurethane, a hydroxyl compound, a carbamate compound, an aromatic alkoxy compound, an organic sulfonamide compound, an aliphatic amide compound, and an arylamide compound are preferably used as a plasticizer. The core of the microcapsule can be prepared by dissolving the electron donor dye precursor compound in a hydrophobic organic solvent having a boiling point of preferably from about 100 to about 300⁰C so as to form the oily phase. Specific examples of the solvent include an ester compound, dimethylnaphthalene, diethylnaphthalene, diisopropylnaphthalene, dimethylbiphenyl, diisopropyldiphenyl, diisobutylbiphenyl, 1 -methyl- 1 -dimethylphenyl-2-phenylmethane, 1 -ethyl- 1 - dimethylphenyl- 1 -phenylmethane, 1 -propyl- 1 -dimethylphenyl- 1 -phenylmethane, triarylmethane (such as tritoluylmethane or toluyldiphenylmethane), a terphenyl compound (such as terphenyl), an alkyl compound, an alkylated diphenyl ether (such as propyldiphenyl ether), hydrogenated terphenyl (such as hexahydroterphenyl) and diphenylterphenyl. These hydrophobic organic solvents may be used alone or in combinations of two or more.

In the case where the electron donor dye precursor to be encapsulated has poor solubility in the hydrophobic organic solvent, a low boiling point solvent having high solubility may additionally be used in combination. Preferred examples of the low boiling point solvent include ethyl acetate, isopropyl acetate, butyl acetate, and methylene chloride.

During microcapsule formation, water-soluble polymers are added to the aqueous phase of the reaction mixture to form a protective colloid in order to stabilize the emulsified dispersion. The type and addition amount of the water- soluble polymers are selected so that the viscosity of the coating composition of the present invention falls into a range of from about 5 centipoises (cps) to about 30 cps, preferably from about 10 cps to about 25 cps, and most preferably from about 10 cps to about 20 cps. Viscosity is measured using Brookfield Programmable DV-II+ viscometer with S21 small size spindle at 100-200 RPM. Regular RV series spindle may also be used depending on sample quantity.

The water-soluble polymer used to form the protective colloid can be appropriately selected from known anionic polymers, nonionic polymers and amphoteric polymers. The water-soluble polymer preferably has a solubility of 5% or more in water at the temperature at which the emulsification is to be conducted. Specific examples thereof include polyvinyl alcohol and a modified product thereof, polyacrylic amide and a derivative thereof, an ethylene-vinyl acetate copolymer, a styrene-maleic anhydride copolymer, an ethylene-maleic anhydride copolymer, an isobutylene-maleic anhydride copolymer, polyvinyl pyrrolidone, an ethylene-acrylic acid copolymer, a vinyl acetate-acrylic acid copolymer, a cellulose derivative, such as carboxymethyl cellulose and methyl cellulose, casein, gelatin, a starch derivative, gum arabic and sodium alginate. Among these, polyvinyl alcohol, gelatin, and a cellulose derivative are particularly preferred. In order to further uniformly emulsify and disperse the oily phase and the aqueous phase, a surface-active agent may be added into at least one of either the oily phase or the aqueous phase. The addition amount of the surface-active agent is preferably from about 0.1% to about 5%, and more preferably from about 0.5 to about 2%, based on the weight of the oily phase. In the case that the surface-active agent is added into the aqueous phase, appropriate selection should be given to those anionic or nonionic surface-active agents that do not cause precipitation or aggregation through interactions with the protective colloid. Preferred examples of such surface-active agent include sodium alkylbenzenesulfonate, sodium alkylsulfate, sodium dioctyl sulfosuccinate and a polyalkylene glycol (such as polyoxyethylene nonylphenyl ether).

Electron acceptor developer dispersion

The electron acceptor compound, which reacts with the electron donor dye precursor, may be used singly or in a combination of two or more. Examples of the electron acceptor compound include an acidic substance, such as a phenol compound, an organic acid or a metallic salt thereof and an oxybenzoate; specific examples thereof include the compounds described in JP-A-61-291183, the contents of what are incorporated by reference. Among these, a bisphenol compound is preferred from the standpoint of obtaining good coloring characteristics. Compositions of electron acceptor developers are disclosed in U.S. Patent No. 6797318 Example-1 as Developer Emulsion Dispersion, U.S. Patent No. 5409797 Example-1 as Emulsion Dispersion, and U.S. Patent No. 5691757 Example as Color Developer. The contents of the U.S. patents are incorporated by reference.

The electron acceptor compound may be used as a solid dispersion prepared in a sand mill with water-soluble polymers, organic bases, and other color formation aids or may be used as an emulsion dispersion by dissolution in a high boiling point organic solvent that is only slightly water-soluble or is water-insoluble, mixing with a polymer aqueous solution (aqueous phase) containing a surface-active agent and/or a water-soluble polymer as a protective colloid, followed by emulsification, for example, by a homogenizer. In this case, a low boiling point solvent may be used as a dissolving assistant depending on necessity. Furthermore, the electron acceptor compound and the organic base may be separately subjected to emulsion dispersion, and also may be dissolved in a high boiling point solvent after mixing, followed by conducting emulsion dispersion. The emulsion dispersion particle diameter is preferably about 1 μm or less. In this case, the high boiling point organic solvent used can be appropriately selected, for example, from the high boiling point oils described in JP-A-2-141279. Among these, the use of an ester compound is preferred from the standpoint of emulsion stability of the emulsion dispersion, and tricresyl phosphate is particularly preferred. The oils may be used as a mixture thereof and as a mixture with other oils. The water-soluble polymer contained as the protective colloid can be appropriately selected from known anionic polymers, nonionic polymers and amphoteric polymers. The water-soluble polymer preferably has a solubility of about 5% or more in water at a temperature at which the emulsification is to be conducted. Specific examples thereof include polyvinyl alcohol and a modified product thereof, polyacrylic amide and a derivative thereof, an ethylene-vinyl acetate copolymer, a styrene-maleic anhydride copolymer, an ethylene-maleic anhydride copolymer, an isobutylene-maleic anhydride copolymer, polyvinyl pyrrolidone, an ethylene-acrylic acid copolymer, a vinyl acetate-acrylic acid copolymer, a polyurethane, a polyether, a polyether based polyurethane copolymer, a styrene acrylic polymer, a polymer of acrylic or methacrylic acid and their derivative thereof, a polyester or a derivative thereof, a cellulose derivative, such as carboxymethyl cellulose and methyl cellulose, casein, gelatin, a starch derivative, gum arabic and sodium alginate. Among these, polyvinyl alcohol, gelatin, and a cellulose derivative are particularly preferred.

Mixing ratio of the oily phase to the aqueous phase is preferably from 0.02 to 0.6, and more preferably from 0.1 to 0.4 by weight. When the mixing ratio is in the range of from 0.02 to 0.6, a suitable viscosity can be maintained, and thus the production adequacy and stability of the coating composition become excellent.

Other Additives

The other components in the mark formation layer are not particularly limited and can be appropriately selected depending on necessity, and examples thereof include known laser absorption enhancing additives, melting agents, known

UV absorbing agents, and known antioxidants.

Selection of laser absorption enhancing additives should depend on the type of laser used and its emitting wavelength. For example, if a CO₂ laser is used, the emitting wavelength should be in the range of 9.4μm to 10.6μm, depending on design. In such a case, mica, SiO₂, Al_xO_y, CaSiO₃, kaolin or their mixed salts or oxides may be used as laser absorption enhancing additives. If a YAG laser is used, the emitting wavelength should be about 1.06μm. hi such case, oxides or salts of Ti,

Fe, Sn, Sb, As, Bi, Cu, Ga, and Ge may be used. In either case, the addition amount and particle size or morphology should be selected in such way that the addition of these additives does not substantially affect the transparency of the mark formation layer.

A melting agent may be contained in the mark formation layer in order to accelerate dye formation reaction. Examples of melting agents include an aromatic ether, a thioether, an ester, an aliphatic amide and an ureide. Specific examples thereof are described in JP-A-58-57989, JP-A-58-87094, JP-A-61-58789, JP-A-62- 109681, JP-A-62- 132674, JP-A-63-151478, JP-A-63-235961, JP-A-2-184489 and JP-A-2-215585.

Preferred examples of the UV absorbing agent include a benzophenone series, a benzotriazole series, a salicylic acid series, a cyanoacrylate series and an oxalic acid anilide series. Specific examples thereof are described in JP-A-47-10537, JP-A-58-111942, JP-A-58-212844, JP-A-59-19945, JP-A-59-46646, JP-A-59-109055, JP-A-63-53544, JP-B-36-10466, JP-B-42-26187, JP-B-48-30492, JP-B-48-31255, JP-B-48-41572, JP-B-48-54965, JP-B-50- 10726, and U.S. Patent Nos. 2719086, 3707375, 3754919 and 4220711. Preferred examples of the antioxidant include a hindered amine series, a hindered phenol series, an aniline series and a quinoline series. Specific examples thereof are described in JP-A-59-155090, JP-A-60- 107383, JP-A-60-107384, JP-A-61-137770, JP-A-61-139481 and JP-A-61-160287.

The coating amount of the other components is preferably from about 0.05 to about 1.0 g/m², and more preferably from about 0.1 to about 0.4 g/m². The other components may be added either inside the microcapsules or outside the microcapsules, or in the dispersion of the electron acceptor compounds of the composition of the present invention.

Composing the mark formation layer

In order to obtain a coating composition for the mark formation layer of the present invention, the above key components may be mixed uniformly and dispersed within a selected polymer media (binder). In this process, the mix ratio of the coating composition of the present invention is such that the ratio of total weight of electron donor dye precursors and that of the electron acceptor compounds is between from about 1:0.5 to about 1:30, preferably from about 1 :1 to about 1:10.

The amount of the electron donor dye precursor in the said mark formation layer is preferably in the range of from about 0.1 to 5.0 g/m². hi this range, both a sufficient coloring density can be achieved and the transparency of the laser- sensitive recording layer can also be maintained. More preferably, the amount of the electron donor dye precursor is from about 1.0 to about 4.0 g/m². In the preparation of the mark formation layer, both the water-soluble polymer used as the protective colloid when preparing for the electron donor dye precursor composition or its microcapsule composition and the water-soluble polymer used as the protective colloid when preparing the electron acceptor dispersion of this invention function as the binder of the mark formation layer. Adding and mixing another binder separately from the above protective colloids is also possible. Preferably, water soluble polymers are generally used, and examples thereof include polyvinyl alcohol, hydroxyethyl cellulose, hydroxypropyl cellulose, epichlorohydrin-modified polyamide, ethylene-maleic anhydride copolymer, styrene-maleic anhydride copolymer, isobutylene-maleic salicylic anhydride copolymer, polyacrylic amide, methylol-modified polyacrylamide, casein and gelatin.

In order to impart water resistance to the binder, a water resisting agent may be added thereto, and an emulsion of a hydrophobic polymer, specifically a styrene- butadiene rubber latex, a styrene acrylic polymer, a acrylic or methacrylic series polymer or a copolymer and their derivative thereof, a polyester or a copolymer thereof, may be added thereto.

In order to safely and uniformly coat the mark formation layer, and to maintain the strength of the coated film, the mark formation layer of the present invention may further contain methyl cellulose, carboxymethyl cellulose, carboxyl- modified polyvinyl alcohol, polystyrene or a copolymer thereof, polyether, polyurethane resin or a derivative thereof, polyether based polyurethane copolymer, polyethylene or a copolymer thereof, epoxy resin, polyamide resin, polyvinyl butyral resin or starch compounds. In order to coat a substrate with the mixed coating dispersion to prepare a mark formation layer, a known coating method suitable for aqueous or organic solvent series coating composition is used.

B. Configuration of the laser markable media Isolation layer

The isolation layer of the laser markable media of the present invention is defined as the medium between the mark formation layer and the laser irradiation source. It can be a supporting sheet on which the mark formation layer is coated, or a coating layer on top of the mark formation layer. The isolation layer and the mark formation layer can be in tight contact through coating or pressure lamination, or in a close proximity through an adhesive layer. In the latter case, the adhesive material should satisfy the same transmittance criteria of the isolation material defined below. The benefits of this isolation medium are: a) block the releasing of undesired chemical vapor resulting from decomposition of the materials in the mark formation layer during laser marking process, b) protect the mark formation layer from mechanical abrasion as well as chemical attack, including harmful gases in the atmosphere, such as O₂, O₃ and SO₂, which tend to accelerate mark fading, background fogging, or yellowing over long period of storage.

Depending on the type of laser selected for the application and the intent for which the laser markable media of the present invention is to be used, the isolation material should be substantially transparent to the specified wavelength of the laser selected. Preferably, the transmittance of the isolation layer is at least about 70% or higher, more preferably about 80% or higher, and most preferably about 90% or higher. Higher transmittance at the specific wavelength of the selected laser ensures minimum attenuation of the delivered laser energy at the mark formation layer, and thus enables a maximum achievable marking speed for at a given laser power. A second benefit of higher transmittance at the specific wavelength of the selected laser is that heat generation within the isolation media, which could induce undesired thermal stress of the material and cause physical distortion, is minimized.

In addition, the isolation layer material should have an on-set pyrolysis temperature that is well above the mark formation temperature. This will ensure that no decomposition of the isolation material occurs during the marking process, and thus no undesired chemical vapor is released. In the case that the electron donor dye precursor is encapsulated, the T_g of the microcapsulation material of the present invention should be controlled within a range such that it is well below the on-set pyrolysis temperature of the isolation material. In the case that the electron donor dye precursor and the electron acceptor compound are separated by other dispersing means, either the glass-transition temperature or the melting point of the dispersing or separation media should be chosen to be well below the on-set pyrolysis temperature of the isolation material. In either case, the preferable on-set pyrolysis temperature of the isolation material of the present invention should be at least about 200⁰C, more preferably about 25O⁰C and above.

It is not necessary that the isolation material of the invention be transparent in the wavelength range of the visible spectrum (about 400-700nm), depending on the application requirements. For most applications, a transparent isolation material in the wavelength range of visible spectrum is preferred, which will give a visible mark that is protected by the isolation layer from mechanical abrasion as well as chemical attack. Suitable isolation materials include polymer films or coating compositions, examples of which include, but are not limited to, a polyolefin film, such as polypropylene, polyethylene, or biaxially oriented polypropylene (BOPP), a polyester film, such as polyethylene terephthalate or polybutylene terephthalate, a cellulose triacetate film, a polylactide film, a polysulfone film, a polystyrene film, a polyether etherketone film, a polymethylpentene film, a nylon film, and coating compositions based on polyurethane resin or polyurethane copolymer, such as urethane-acrylate copolymer and polyether polyurethane copolymer, polyamide resin, epichlorohydrin-modified polyamide, polyacrylates, poly(meth)acrylates or derivatives thereof, core-shell acrylic latex, polyacrylic amide, styrene acrylic polymer polystyrene or a copolymer thereof, such as styrene-maleic anhydride copolymer and styrene-butadiene rubber latex, epoxy resin, ethylene-maleic anhydride copolymer, isobutylene-maleic salicylic anhydride copolymer, polycarbonate, polyester or a copolymer thereof, polyether, polyether based polyethylene or a copolymer thereof, polyvinyl butyral resin, methyl cellulose, carboxymethyl cellulose, and polyvinyl pyrrolidone. For CO₂ laser markable media, polyolefin films and coating formula based on polyurethane or polyurethane copolymer resins are preferred materials for the isolation layer of the present invention. It is understand that not all of the materials in the above list that are suitable for all the emitting wavelengths of the types of lasers listed in the following section describing laser marking equipment. Support layer

The support layer of the laser markable media of the present invention is defined as the substrate layer on which the mark formation layer is coated. In the case that the mark formation layer is coated onto the isolation layer described above, the support layer thus becomes the isolation layer. In other cases, the support layer is behind the mark formation layer, furthest from the direction of the incident laser beam.

In order to obtain a transparent laser-markable material, a transparent support with a wavelength range within the visible spectrum is preferably used in the present invention. Examples of the transparent support include, but are not limited to, synthetic polymer materials, examples of which include a polyester film, such as polyethyleneterephthalate or polybutyleneterephthalate, a cellulose triacetate film, a polylactide film, a polysulfone film, a polystyrene film, a polyether etherketone film, a polymethylpentene film, a Nylon film, a polyolefin film, such as polypropylene, polyethylene, or BOPP, and polyacrylates, poly(meth)acrylates, urethane acrylates, polycarbonate, polystyrene, and epoxy which can be used singly or in a combination of two or more by lamination.

Other Layers The laser-markable material of the present invention may further comprise, on the support, other layers, such as a primer layer, an adhesive layer followed with a releasing liner. The primer layer may be provided on the support before coating the mark formation layer, in order to improve the adhesion of the mark formation layer to the support. Depending on the application requirement, an adhesive layer and, if needed, a releasing liner may be coated/laminated on the opposite side of the support from the mark formation layer, to form a laser markable self-adhesive media.

As a primer layer, an acrylate copolymer, polyvinylidene chloride, styrene- butadiene rubber (SBR), or an aqueous polyester can be used, and the thickness of the layer is preferably from 0.05 to 0.5 μm. There are cases where, upon coating the mark formation layer onto the primer layer, the primer layer is swollen by the water content in the composition of the mark formation layer, which could deteriorate the mark quality in the mark formation layer. Therefore it is preferred that the primer layer is hardened with a hardening agent, such as a dialdehyde compound, e.g., glutaraldehyde or 2,3-dihydroxy-l,4-dioxane, and boric acid. These may be used singly or in a combination of two or more.

The addition amount of the hardening agent is appropriately determined depending on the material of the primer layer and selected from the range of from 0.2 to 3.0% by weight corresponding to a desired degree of hardening. The layer preferably also includes a fine particle substance having a refractive index of from about 1.45 to about 1.75, from the standpoint that the transparency of the laser- markable media is maintained.

Formation of the laser markable media

The laser-markable media of the present invention can be preferably produced by the process described below, but it is not limited thereto.

The production process of a laser-markable media of the present invention includes the steps of: coating the primer layer (if it is used) onto the support, coating a mark formation layer onto the primer layer (if it is used) on the support; and in the case that the support layer is not the isolation layer, coating an isolation layer on top of the mark formation layer, hi the case that the support layer also serves as the isolation layer, the primer layer may optionally be coated on both sides of the support, to facilitate additional printing on the opposite side of the mark formation layer. Depending on necessity, other layers are also formed.

In the production process of the laser-markable media of the present invention, in the case that the support layer is not the isolation layer, the mark formation layer and the isolation layer may be optionally coated simultaneously, and in this case, the coating compositions of the mark formation layer and the isolation layer are subjected to multilayer coating, whereby the mark formation layer and the isolation layer can be simultaneously formed. The technology of multilayer simultaneous coating is particularly suitable, in the case that the mark formation layer is further comprised of separate layers of electron donor dye precursor dispersion and dispersion of electron acceptor compounds. Alternatively, the laser-markable media of the present invention may be coated sequentially with known coating methods, in the following order: the primer layer, the mark formation layer, and the isolation layer. Examples of these coating methods include, but are not limit to, a blade coating method, an air knife coating method, a gravure coating method, a roll coating method, a spray coating method, a dip coating method and a bar coating method.

Various configurations of the laser-markable media of the present invention are illustrated below in the spirit of this invention, but not limit thereto. In the embodiment shown in Fig. 1 , the mark formation layer 1 is sandwiched between the support 2 and the isolation layer 3, which may be coated or laminated onto the mark formation layer. The mark formation layer comprises the electron donor dye precursor 4 encapsulated by capsule wall 5 and the electron acceptor compound 6, both dispersed in a same polymer medium 7 in close proximity of reaction length, but are prevented from direct contact by the capsule wall and the polymer of the media, when the laser markable material is under ambient temperature below the T_g of the polymers. When the energy is delivered into the mark formation layer via a laser beam 8, and the medium temperature is raised beyond the T_g, of the capsule wall, the capsule wall expands and opens, which leads to direct contact between the two compounds through migration or diffusion, and the dye precursor is turned into dye. Volatile compounds in the mark formation layer generated during the marking process are kept underneath the isolation layer. The result is that no undesired chemicals are released.

In another embodiment of the present invention shown in Figure 2, the electron donor dye precursor 4 and electron acceptor compound 6 are dispersed and coated into two distinct layers of polymer medium T and 7" (which can be the same or different material) isolated by an optional 3^rd polymer spacing layer 9, having a glass transition temperature T_g similar to that of the capsulation wall above, and additional laser absorption enhancing additive 10 may optionally be dispersed into either this spacing layer alone, or also into the electron acceptor layer. In this embodiment, when the energy of the incident laser beam is absorbed by the sensitizing agents in exposed areas, the spacing polymer is melted or softened locally, enabling cross-layer diffusion and a reaction between the electron donor dye precursor and the electron acceptor to form marks 11. This arrangement enhances the heat stability of the laser markable media, to prevent undesired interaction between the electron donor dye precursor and electron acceptor, forming fog in unmarked areas. In yet another embodiment shown in Figure 3, the laser markable media has the same configuration as in Figure 1. However, the laser beam 8 is irradiated from the support side (based on the definition, this support layer 12 now becomes an isolation layer), which in substantially transparent to the wavelength of the laser beam, but substantially non-transparent in the wavelength range of visible spectrum. On the other hand, the isolation layer 13 is substantially transparent in the wavelength range of visible spectrum, and thus the marks formed in the mark formation layer 1 become visible from the back side. Optionally, an adhesive layer (not shown) may be coated on the other side of the support / isolation layer 12, which, of necessity, must also be substantially transparent to the wavelength of the laser beam.

In a variation of the above embodiment of Figure 4, both isolation layers 14 and 15 are substantially transparent in the wavelength range of visible spectrum. However the isolation layer 14 is also substantially transparent to laser beam 8' with emission wavelength λ(l), and the isolation layer 15 is also substantially transparent to laser beam 8" with emission wavelength λ(2), where λ(l) and λ(2) may or may not be the same and the two isolation layers may or may not be significantly transparent to both λ(l) and λ(2), if they are different. The two isolation layer may also be both rigid or flexible or one rigid one flexible, made from different materials. In Figure 4, the encapsulated electron donor dye precursor 4 and the electron acceptor compound are located in polymer medium 7 of the mark formation layer 1 which further includes particles of laser absorption additive 16.

In this way, the marks may be formed by marking beams of the same or different frequencies from both sides. The formed marks in this embodiment are therefore resistant to chemical attacks and mechanical abrasions from both sides, hi addition, since the marking beam energy is absorbed only in the mark formation layer, which is sandwiched between two isolation layers, thus there is no release of decomposed chemicals or vaporized ingredients into the atmosphere during the marking process.

In yet another embodiment shown in Fig. 5, the mark formation layer 1 also serves as an adhesive layer on isolation layer/support 16. Both the encapsulated electron donor dye precursor 4 and the electron acceptor compound 6 are dispersed in an adhesive medium 17. The laser markable media of this embodiment may be adhered onto a product packaging surface, and then marked with a laser beam 8, or the reverse.

Laser marking equipment

The laser markable media of the present invention may be marked with a CO₂ laser, a YAG laser, a solid laser such as a ruby laser, or a diode laser such as, but not limited to, InGaAsP and GaAs. Among them, a 5-20W CW CO₂ laser in the emitting wavelength range of 9.4-10.6μm is preferred. A preferred laser marking system is one in which a Galvonometer beam steering technology that allows computer to control the beam with one or more rotating mirrors in X or X/Y-axes is used. Both Vector and Raster scanning schemes may be used depending on the application. Preferably the combination of laser beam quality, f-β lens quality, and focal distance will allow the marking spot- size at the focal plane to be below about 300 micron, more preferably to be below about 100 micron.

EXAMPLE

Examples of the various embodiments of the present invention are given below, but the invention should not be construed as being limited thereto.

[Preparation of liquid dispersion (A) containing an encapsulated electron donor dye precursor]

13.3 g of electron donor dye precursor represented by Formula (1), where Rl is C4H9 and R2 is C₂H₅, and 0.47 g of an UV light absorbing agent (trade name: Tinuvin P, Ciba Geigy Corp.) were added in 20 g of ethyl acetate and dissolved by heating up to 70°C, and then cooled down to 45°C. 12.6g of diisocyanate compound (trade name: Takenate D-140N, Mitsui Takeda Chemical Co., Ltd.) was added into the ethyl acetate solution. The above ethyl acetate solution was then added into 53 g of 6%w/w polyvinyl alcohol aqueous solution (trade name: Kuraray Poval MP- 103, Kuraray Co., Ltd.) and emulsified with a homogenizer at 15,000 rpm for 5 minutes. Finally, an amine solution of 9Og water and 0.5 g of tetraethylenepentamine were gradually added into the above mixture while agitating at 400 rpm for 4 hours to conduct an encapsulation reaction.

After the reaction was completed, the particle size distribution of the encapsulated electron donor dye precursor particles was measured with a Beckman Coulter's LS-100Q particle size analyzer, the viscosity of the liquid coating composition was measured with a Brookfield Programmable DV-II+ viscometer with S21 small size spindle at 100-200 RPM, and the T_g of the microcapsule wall was measured by using a Perkin Elmer's Diamond DSC with a blank suspension without microcapsule as reference. The following results were obtained: viscosity of the liquid dispersion = 18 cps, wherein 99% (volume) of the microcapsules have particle-size between 0.2-2μm, and the microcapsule wall Tg = 156°C.

[Preparation of liquid dispersion (B) containing an electron acceptor compound] 4.2g of an UV light absorbing agent (trade name: Tinuvin 328, Ciba Geigy),

1.0 g of tricresylphosphate, and 36.4g of an electron acceptor compound (compound 301 of U.S. Patent No. 6,797,318) were added in 16.Og of ethyl acetate, and dissolved by heating up to 7O⁰C. This ethyl acetate solution was added into the following aqueous solution and dispersed with a homogenizer at 12,000 rpm for 5 minutes.

Aqueous solution for emulsified dispersion (B)

Water 68.4g

15%w/w Poly-vinylalcohol (trade name: Poval PVA205, Kuraray Co.,Ltd.) 19.8g 8%w/w Poly-vinylalcohol (trade name: Poval PVA217, Kuraray Co.,Ltd.) 55.7g

Surfactant A (note = Compound 401 of USP 6,797,318) 11.2g

Surfactant B (note = Compound 401 of USP 6,797,318) 11.2g [Preparation of a mixed coating composition for coating the mark formation layer] The above dispersion (A) and dispersion (B) were mixed as follows. Dispersion (A) 8.9g Dispersion (B) 33g

[Coating the mark formation layer onto a support]

The above coating composition was coated onto a 175μm thick A4 size transparent PET film at ~10μm coating thickness with a bar coater, followed with about 3 minutes drying at 6O⁰C. The PET film had been preliminarily coated with SBR latex and gelatin mixture as primer.

[Complete the laser markable media and mark the media with a CO₂ laser marker] The above sheet was divided into three equal portions. One portion (invention) was pressure laminated with a 50μm transparent polyethylene (PE) film on top of the coated mark formation layer, another portion (invention) was further coated with a clear core-shell type acrylic latex dispersion (trade name: Rhoplex Multilobe 200) on top of the coated mark formation layer, and the last portion remains without further treatment (comparison). A Domino S 100 1 OW CO₂ laser marker with an emitting wavelength of

10.3μm and 80mm f-θ lens was used. The marking condition was set at "Mark- Speed" = 8000 bits/ms and "Laser on CO₂" = 200μs. After turning on the laser marker, sharp and high contrast marks were generated on all three samples. However, the comparison sample without an isolation layer showed smoke release during the marking process, while the two samples of the invention did not. Further, when using a microscope to observe the surface of the samples where marks formed, the comparison sample without an isolation layer shows clear damage on the surface of the coating, while the two samples of the invention did not. Rubbing tests also show that the comparative sample had much more severe surface damage on the media.

Claims

We claim:

1. A media that can generate a human-readable or machine-readable mark under the irradiation of a focused beam of electromagnetic wave of specific wavelength and intensity, said media comprising:

(a) a mark formation layer comprising at least one electron donor dye precursor and at least one electron acceptor compound which reacts with said electron donor dye precursor upon contact at an elevated temperature to form a colored dye, wherein said electron donor dye precursor is separated from said electron acceptor compound in the mark formation layer by either encapsulating said dye precursor within a polymer having a glass transition temperature, T_g, of from about 120°C to about 190°C, or by dispersing said electron donor dye precursor and said electron acceptor compound into two distinct sub-layers isolated by a third polymer spacing sub-layer having a glass transition temperature, T_g, or a melting point, T_m, of from about 12O⁰C to about 190° C, and wherein at least about 90% of the total volume of said electron donor dye precursor, when encapsulated, has a diameter from about 0.2 μm to about 5 μm; and

(b) an isolation layer which is substantially transparent at the wavelength of the focused beam irradiation source, and which has an on-set pyrolysis temperature of at least 200⁰C.

2. The media of claim 1 wherein said mark formation layer forms readable marks upon exposure to a focused beam irradiation source having a wavelength in the range from about 230 run to about 11 μm.

3. The media of claim 1 wherein said mark formation layer forms readable marks upon exposure to a focused beam irradiation source having a wavelength in the range from about 900 run to about 11 μm.

4. The media of claim 1 wherein said isolation layer has an on-set pyrolysis temperature of at least 250⁰C.

5. The media of claim 1 wherein said isolation layer has a transmittance level of at least 70% at the emitting wavelength of the focused beam of electromagnetic wave.

6. The media of claim 1 wherein said isolation layer has a transmittance level of at least 90% at the emitting wavelength of the focused beam of electromagnetic wave.

7. The media of claim 1 wherein said isolation layer has a transmittance level of at least 97% at the emitting wavelength of the focused beam of electromagnetic wave.

8. The media of claim 1 wherein said mark formation layer and said isolation layer are contacted through an adhesive layer which does not have substantial absorption at the emitting wavelength of the focused beam of electromagnetic wave.

9. The media of claim 1 wherein said electron donor dye precursor has the following structure:

where, Rl is -CH(CH₃)C₂H₅ and R2 is -C₂H₅.

10. The media of claim 1 wherein said electron donor dye precursor has the following structure:

11. The media of claim 1 wherein said electron donor dye precursor has the following structure:

12. The media of claim 1 wherein at least about 90% (by volume) of said electron donor dye precursor, when encapsulated, have a particle diameter from about 0.2 μm to about 2 μm.

13. The media of claim 1 wherein said electron acceptor compound in said mark formation layer is in the form of particles wherein at least about 90% (by volume) of the particles have a diameter from about 0.1 μm to about 3 μm.

14. The media of claim 1 wherein said electron acceptor compound in said mark formation layer is in the form of particles wherein at least about 90% (by volume) of the particles have a diameter from about 0.1 μm to below 2 μm.

15. The media of claim 1 wherein the ratio of the total weight of said electron donor dye precursor to the total weight of said electron acceptor compound in said mark formation layer is from about 1:0.5 to about 1:30.

16. The media of claim 1 wherein the ratio of the total weight of said electron donor dye precursor to the total weight of said electron acceptor compound in said mark formation layer is from about 1:1 to about 1 :10.

17. The media of claim 1 wherein said electron donor dye precursor is encapsulated in a polymer material having a glass transition temperature T_g of between 150°C and 190⁰C and comprising at least one polyurethane.

18. The media of claim 1 wherein said electron donor dye precursor and said electron acceptor compound are separated by a polymer spacing sub-layer having a glass transition temperature, T_g, or a melting point, T_m, of from about 150°C to about l90°C.

19. The media of claim 1 wherein said electron donor dye precursor is encapsulated, and said electron acceptor compound and said encapsulated electron donor dye precursor are dispersed in an adhesive medium.

20. The media of claim 1 wherein said mark formation layer further comprises at least one absorption enhancing additive that absorbs at the wavelength of the focused beam.

21. The media of claim 20 wherein said at least one absorption enhancing additive does not have substantial absorption in the wavelength range of the visible spectrum and wherein when the absorption enhancing additive is in the form of solid particles, the solid are dispersed in said mark formation layer wherein 90%

(by volume) of said solid particles have diameters below 5 μm.

22. The media of claim 21 wherein when the absorption enhancing additive is in the form of solid particles 90% (by volume) of said solid particles have diameters below 0.5 μm.

23. The media of claim 1 further comprising a support on which said mark formation layer is coated wherein said mark formation layer is between said support and said isolation layer.

24. The media of claim 23 wherein at least one of said isolation layer and said support is substantially transparent in the wavelength range of visible spectrum.

25. A method of generating a human-readable or machine-readable mark comprising exposing a media to irradiation of a focused beam of electromagnetic wave of specific wavelength and intensity, said media comprising:

(a) a mark formation layer comprising at least one electron donor dye precursor and at least one electron acceptor compound wherein said electron donor dye precursor is separated from said electron acceptor compound in the mark formation layer by either encapsulating said dye precursor within a polymer having a glass transition temperature, T_g, of from about 120°C to about

190°C, or by dispersing said electron donor dye precursor and said electron acceptor compound into two distinct sub-layers isolated by a third polymer spacing sub-layer having a glass transition temperature, T_g, or a melting point, T_m, of from about 120°C to about 190°C, and wherein at least about 90% of the total volume of said electron donor dye precursor, when encapsulated, has a diameter from about 0.2 μm to about 5 μm; and

(b) an isolation layer which is substantially transparent at the wavelength of the focused beam irradiation source, and which has an on-set pyrolysis temperature of at least 200⁰C,

wherein the media is exposed to said focused beam through the isolation layer and said focused beam causes formation of a colored dye that provides the human-readable or machine-readable mark in the mark formation layer.