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Title:
COATING COMPOSITION FOR FORMING LASER-MARKABLE MATERIAL HAVING HEAT AND HUMIDITY STABILITY
Document Type and Number:
WIPO Patent Application WO/2009/009066
Kind Code:
A1
Abstract:
A laser markable coating composition suitable for CO2 laser markable materials, including at least one color forming agent and at least one waterborne solvent-free polyurethane, wherein the at least one waterborne solvent-free polyurethane is selected from a polyester-based polyurethane, polyether based polyurethane, polycarbonate-based polyurethane, and castor oil-based polyurethane, and wherein the at least one waterborne solvent-free polyurethane constitutes at least 50 weight percent of the total organic resin binder weight.

Inventors:
WAN HAI-XING (US)
Application Number:
PCT/US2008/008416
Publication Date:
January 15, 2009
Filing Date:
July 09, 2008
Export Citation:
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Assignee:
FUJIFILM HUNT CHEMICALS U S A (US)
WAN HAI-XING (US)
International Classes:
G03C1/00; G03F7/00
Domestic Patent References:
WO2006063165A22006-06-15
Attorney, Agent or Firm:
LEE, Roger, H. (P.O. Box 1404Alexandria, Virginia, US)
Download PDF:
Claims:

Claims

We claim:

1. A laser markable coating composition suitable for CO 2 laser markable materials, comprising at least one color forming agent and at least one waterborne solvent- free polyurethane, wherein the at least one waterborne solvent-free polyurethane is selected from a polyester-based polyurethane, polyether-based polyurethane, polycarbonate-based polyurethane, and castor oil-based polyurethane, and wherein the at least one waterborne solvent-free polyurethane constitutes at least 50 weight percent of the total organic resin binder weight.

2. The laser markable coating composition according to claim 1 , wherein the total polyurethane binder is greater than about 80% of the total binder weight.

3. The laser markable coating composition according to claim 1 , wherein the binder comprises from about 5% to about 40% of the total solid weight of the coating composition.

4. The laser markable coating composition according to claim 1 , wherein the total polyurethane binder is greater than about 95% of the total binder weight.

5. The laser markable coating composition according to claim 1 containing at lease one color forming agent selected from a electron donor-type dye precursor and the electron acceptor-type developer.

6. The laser markable coating composition according to claim 5, wherein the electron donor-type dye precursor is a fluorene series compound.

7. The laser markable coating composition according to claim 5, wherein the electron donor-type dye precursor is a compound having a solubility of higher than about 10g/100g in ethyl acetate.

8. The laser markable coating composition according to claim 5, wherein the electron donor-type dye precursor is microencapsulated.

9. The laser markable coating composition according to claim 5, wherein the electron donor-type dye precursor particle has a glass transition temperature, Tg, of from 15O 0 C to about 190° C.

10. The laser markable coating composition according to claim 5, wherein the electron donor-type dye precursor particle has a particle size of from about 0.2 μm to about 2 μm.

1 1.The laser markable coating composition according to claim 5, wherein the electron acceptor-type developer is a metal salt of salicylate.

12. The laser markable coating composition according to claim 1 1 , wherein the electron acceptor-type developer is a zinc salicylate.

13. A laser markable material comprising at least one color forming layer and at least one assistant layer, wherein at least one waterborne solvent-free polyurethane is selected from a polyester-based polyurethane, polyether-based polyurethane, polycarbonate-based polyurethane, and castor oil-based polyurethane, and wherein the at least one waterborne solvent-free polyurethane constitutes at least

50 weight percent of the total organic resin binder weight.

14. The laser markable material according to claim 13, wherein the polyurethane is present in at least one layer selected from a color forming layer and an assistant layer.

15. The laser markable material according to claim 14, wherein the assistant layer is selected from a protective layer, an intermediate layer, and a primer coat layer.

16. The laser markable material according to claim 14, wherein the binder is waterborne and solvent-free polyurethane.

17. The laser markable material according to claim 14, wherein the solid binder weight comprises from about 10% to about 90% of total solid weight.

18. The laser markable material according to claim 14, wherein said polyurethane possesses at least 80% of total binder weight.

19. The laser markable material according to claim 14, wherein said polyurethane possesses at least 95% of total binder weight.

20. The laser markable material according to claim 17, wherein the binder comprises from about 30% to about 60% of the total solid weight.

Description:

Coating Composition for Forming Laser-Markable Material Having Heat and Humidity Stability

BACKGROUND

Products and package labeling has become increasingly important in industry, especially for clearly visible, sharp, high contrast marks as well as marks of color rather than just black and white images. Currently, printing, embossing, stamping, and label application are the predominant methods for product marking. However, when labeling requires frequent information changes, such as individualized product identification, coding, production date, or expiration date marking, a method for producing a rapid change in marking content, known within the printing industry as "on-demand printing" and "marking on-the-fly", is needed.

Laser beam marking is a growing area of great interest that can offer a clear advantage over conventional marking technologies in terms of marking speed, as well as the additional features of environmental friendliness, safety, and minimal maintenance. The principle of laser marking is that the laser beam is absorbed by heat sensitive materials and is converted to heat. Subsequently the heat sensitive material reacts to the heat to form a mark. One widely used laser marking method in industry is laser ablation. However, a key disadvantage of laser ablation is that it requires strong interaction of the marking substrate with the laser beam to yield significant color or density changes over unmarked area. On the other hand many packaging materials, such as plastic films or containers and glass bottles, do not have sufficient interaction with the laser beam, the interaction does not yield significant contrast change on the material, or the interaction causes damages to the substrate surface.

One method to overcome this disadvantage is to coat the substrate with a specifically formulated liquid coating composition that can absorb the energy of a laser beam to yield visible marks on the coated substrate. The laser markable coating composition generally can contain color forming agents which include inorganic and organic pigments and dyes, as well as binders and other coating

composition additives such as surfactants, plasticizers, antifoaming agents, ultraviolet radiation absorbers, sensitizers, and other compounds know to the art. The binder used in a liquid coating composition is generally a natural or synthetic resin that has the major function of a film former. Binders that function as a film forming agent in a laser markable coating composition are known in the art and are described in many patents, for example US 5608429, US 5691757, and US 6210472. In addition to the function as a film former, binder resins have been used to obtain special effects in a laser markable coating composition. In US 6261348, for example, a coating composition is described which is free of dye or pigment compounds and which can generate white marks after being exposed to the laser beam. In an exemplary embodiment, not only is the binder resin a film former, but also a heat responsive material used to form a mark. US 613342 describes a coating composition that contains a combination of an opaque mix of resin and colorant. After receiving the exposure by a laser beam, the opacity of the resin fades and the color from the colorant becomes visible. The resins claimed in this patent have the dual functions of film forming and color masking.

Although the binder in a laser markable coating composition can function to form special effect marks, this same effect of forming a white mark can be a severe disadvantage, having an adverse effect that contributes to poor mark quality when used with a laser markable coating composition that contains color forming agents. For example, it has been shown that low mark density and poor color purity is produced when a laser markable material is marked with a CO 2 laser when the laser markable coating composition contains a polyvinyl alcohol as the binder resin in combination with black color forming agents. An opaque white mark is generated after being exposed with a CO 2 laser. The white mark generated by the binder resin combines with the desired mark color given by the incorporated color forming agents to cause a lower mark density and reduced color purity on the marked region of the laser markable material. One solution to solve this problem has been provided in WO 2006/063165 A2. It claims that a substituted or unsubstituted polyurethane compound can be used as the binder in the laser markable coating composition to promote improved mark density.

While use of a laser markable coating composition containing polyurethane as a binder can provide the benefit of producing marks of higher density as provided in WO 2006/063165 A2, it has been found that these binder resins initiated an unexpected reaction between the color formation agents, and generated stain when the coated material was stored in a hot and/or humid environment. Unfortunately, many packaged goods can be exposed to hot and/or humid conditions when being handled, transported, and stored. When this stain formation occurs, laser markable coatings will be of only limited use in the product and package labeling industry.

In view of the above requirements, a need exists to provide a laser markable coating composition containing binder resins that are inert to color formation as well as inert to the CO 2 laser beam. Thus the requirement exists in the art that the laser markable material coated with a laser markable coating composition which is free of stain formation under the hot and/or humid condition.

SUMMARY

Improved resins selected as binders for preparing a liquid coating composition for a laser markable material are provided. In the laser markable coating composition, the resin provides a media for suspending the color formation agents and other ingredients as well as being chemically and physically inert. This chemical and physical inertness can be effective to reduce or prevent unwanted white or opaque marks as well as minimize color formation reaction of the color forming agent. Minimizing the unwanted color formation reaction can provide reduced stain on the coated material which can be produced during storage in a hot and/or humid environment.

According to one aspect, a laser markable coating composition is provided comprising a binder resin which is commercially available in the market and inert to color formation. In this coating composition, the selected binder resin will not physically nor chemically affect the color forming system - an electron donor type of dye precursor and an electron acceptor type of developer. Thus, a stain free background of the laser markable material can be obtained.

According to another aspect, a laser markable material is provided comprising a color forming layer and assistant layers such as a protective layer in which the selected binder resin will not adversely affect the color formation agents in the laser markable layer to obtain a stain-free background for the laser markable material under a hot and/or humid condition.

DETAILED DESCRIPTION

Many natural and synthetic resins are known in the art which can be selected for use as a binder in a coating composition of laser markable material. Examples of useful binder resins include starch and modified derivatives, cellulose and modified derivatives, gelatin, casein, gum arabic, pectin, sodium alginate, silicate resin, polyvinyl alcohol, polyacrylic resin, epoxy, polystyrene, polyester, polyacrylic amide, styrene-acrylic acid copolymer, styrene-butadiene copolymer, ethylene-vinyl acetate copolymer, styrene-maleic anhydride copolymer, ethylene- maleic anhydride copolymer, isobutylene-maleic anhydride copolymer, polyvinyl pyrrolidone, ethylene-acrylic acid copolymer, vinyl acetate-acrylic acid copolymer, and others known in the art. Among these resins, polyvinyl alcohol, gelatin, and cellulose derivatives are particularly preferred for a laser markable coating composition.

When a requirement for the resin of the laser markable coating composition is that it should be free from the formation of unwanted white marks by CO 2 laser exposure when used as the film former, polyurethane and its derivatives are know in the art to be ideal compounds. For example, compounds claimed in WO 2006/063165 A2 include Macekote 9525 and Alberdingk U400N which are polyether-based polyurethanes, Alberdingk U2101VP which is a polyester-based polyurethane, Alberdingk U9152VP which is a polycarbonate-based polyurethane, and Alberdingk CUR21 , a castor oil-based polyurethane.

However, a laser markable material containing these binders can easily generated stain in a hot and/or humid environment. The stain may be caused by

an unexpected reaction between the binder and color formation agents which are suspended in the binders.

It is now discovered that the use of a waterborne, solvent-free polyurethane emulsion, suspension, or dispersion as the binder in an exemplary laser markable coating composition which comprise color formation agents generates minimal stain under hot and/or humid storage conditions. These waterborne, solvent-free polyurethane compounds offer similar CO 2 laser inertness to that of polyurethanes claimed in WO 2006/063165 A2. More importantly, minimal stain increase is achieved when the laser markable material is stored under hot and/or humid storage conditions. This offers a clear advantage over the prior art binders. Herein, the definition for "waterborne, solvent-free polyurethane compounds" are those polyurethane compounds that do not contain organic solvents and which contain only water or other inorganic compounds as the solvent to form an emulsion, suspension, or dispersion.

Specific examples of waterborne, solvent-free polyurethane or modified derivatives are emulsions, suspensions, or dispersions of polyester-based polyurethane, polyether-based polyurethane, polycarbonate-based polyurethane, castor oil-based polyurethane, or any combinations thereof.

Laser markable coating composition containing waterborne, solvent-free polyurethane as a binder

A laser markable coating composition comprising the exemplary waterborne, solvent-free polyurethane as binder can also comprise color-forming agents and auxiliary additives.

The exemplary color-forming agent is a colorless compound that becomes colored after the laser-facilitated reaction with another compound in the coating composition. The color-forming agent can also include a compound having a primary color that further changes color after reacting with another compound in a laser-marked material. A preferred example of color forming agents is a pair consisting of an electron donor-type dye precursor and an electron acceptor-type developer. This pair of colorless compounds reacts with each other to produce colored compound.

Examples of the electron donor-type 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.

Examples of the electron acceptor-type compound, which reacts with the electron donor-type dye precursor, include an acidic substance, such as activated bentonite, metal salt of salicylate, phenol compound, organic acid or its metallic salt, oxybenzoate, and others known in the art.

The auxiliary additives which can be incorporated into the coating composition can include: known coating additives, such as surfactants, anti-foam agents, plasticizers, rheological agents, biocides, antistatic agents, solvents, photoinitiator for radiation curing, and other compounds known in the art, additives for enhanced laser marking performance, such as heat transfer agents, melting agents, ultraviolet ray absorbing agents, antioxidants, as well as other compounds know in the art. Heat transfer agents absorb CO 2 laser emission at 943 cm "1 and convert it to heat. Examples can include mica, fumed silica, fumed alumina, and inorganic or organic compounds that have strong absorption in the range of 900 cm '1 to 1000 cm "1 . Melting agents may be contained in the laser- sensitive recording layer of the laser markable material in order to improve the laser responsiveness. Examples can include aromatic ether, thioether, ester.aliphatic amide, ureide, and other compounds know in the art. Ultraviolet ray absorbing agents include a benzophenone series ultraviolet ray absorbing agent, a benzotriazole series ultraviolet ray absorbing agent a salicylic acid series ultraviolet ray absorbing agent, a cyanoacrylate series ultraviolet ray absorbing agent, and an oxalic acid anilide series ultraviolet ray absorbing agent. Antioxidants include a hindered amine series antioxidant, a hindered phenol series antioxidant, an aniline series antioxidant and a quinoline series antioxidant, and other compounds know in the art.

One or more water-soluble resins which comprise the binder in the coating composition which also comprise the above-described color forming agents and auxiliary additives can be appropriately selected from a solvent-free polyurethane or its modified derivatives, such as the emulsion, suspension, or dispersion of polyester-based polyurethane, polyether-based polyurethane, polycarbonate- based polyurethane, castor oil-based polyurethane, or any combinations thereof. The emulsion, suspension, and dispersion of these polyurethane compounds must be waterborne and solvent-free.

A combination of the polyurethane compounds and other types of resins that are inert to the color forming agents in the laser markable coating composition, such as acrylic, epoxy, cellulose, and other resins known in the art, can be used to form a liquid laser markable coating composition depending on necessity. In order to avoid intensifying the white mark problem described above, it is preferred that about at least 50% or more of the total binder solid weight comprising the laser markable coating composition is the waterborne and/or solvent-free polyurethane and modified derivatives. In the total laser markable coating composition, it is preferable that the concentration of binder can be from about 0.1% to about 60%, more preferably from about 1 % to about 50% and most preferably from about 2% to about 40%,

The laser markable coating composition can be formulated as a single part composition that contains all color forming agents and necessary auxiliary additives. Alternately the laser markable coating composition can be designed to have multiple parts to obtain increased storage stability before being mixed and coated onto a substrate. A preferred embodiment for the binder compounds is the capability to be incorporated into any of the parts of the laser markable coating composition. Waterborne and/or solvent-free polyurethane and modified derivatives have been found to meet this requirement.

Laser markable coating composition containing microencapsulated dye and the solvent- free polyurethane binder

In order to obtain high mark density, the electron donor-type dye precursor in a laser markable coating composition containing microencapsulated dye and the solvent- free polyurethane binder preferably exists in a high concentration in the microcapsules. The compounds represented by general structural formula 1 are preferable because these can be incorporated into the microcapsules in very high concentration and can provide high mark density.

Formula 1

Where, Ri and R 2 , represent an alkyl group, such as a butyl group, a sec-butyl group, a ferf.-butyl group, a propyl group, an ethyl group, a methyl group; R 3 represents a hydrogen, or an alkyl group, such as a butyl group, a sec. -butyl group, a terf.-butyl group, a propyl group, an ethyl group, a methyl group; and R 4 represents an imino-benzene group or a hydrogen.

A preferable embodiment is that the solubility of the said electron donor-type dye precursor is higher than about 10g/100g of ethyl acetate, more preferably is higher than about 15 g/100 g of ethyl acetate, and most preferably is higher than about 18 g/100 g of ethyl acetate. The most preferable compound (Formula 2) is shown as below:

Formula 2

A preferable embodiment is that more than about 80% by weight of the electron donor-type dye precursors are of the compounds represented by structural formula 1 , and a more preferable embodiment is that more than about 90% by weight are the said compound and a most preferable embodiment is that about 100% by weight are the said compound.

There are many methods known in the art to incorporate the color forming agents into a laser markable coating composition. Several examples to incorporate the color forming agents are by dispersing the solid powder of color forming agents into the binder medium, dissolving the color forming agents in a solvent and add the solution of color forming agents into the binder medium, and microencapsulating the color forming agents and dispersing the encapsulated color forming agents into the binder medium.

A preferred method is micro-encapsulating the color forming agents and dispersing the encapsulated color forming agents into the binder medium. When micro-encapsulated and protected by the wall of capsule, the color forming agents are protected and a chemically stable coating composition can be achieved producing minimal color stain, especially under the storage conditions of high heat and/or humidity.

Preferred color forming agents comprising the laser markable coating composition are a pair of electron donor-type dye precursor and an electron acceptor-type developer. In an exemplary embodiment, either the dye precursor

or the developer or both can be micro-encapsulated. It is preferred that electron- donor type dye precursor is micro-encapsulated.

The micro-encapsulation process for the electron donor-type dye precursor is described in detail as follows:

For encapsulation, a surface polymerization process is particularly preferably employed, such that the electron donor-type dye precursor that becomes a core of the microcapsules is dissolved or dispersed in a hydrophobic organic solvent to prepare an oily phase, which is then mixed with an aqueous phase obtained by dissolving a water-soluble polymer in water, and is then subjected to emulsification and dispersion by using, for example, an homogenizer, followed by heating, so as to conduct a polymer-forming reaction at the interface of the oily droplets, whereby a microcapsule wall of a polymer substance is formed.

The reactants for forming the polymer substance are added to the interior of the oily droplets and/or the exterior of the oily droplets. Specific examples of the polymer substance include polyurethane, polyurea, polyamide, polyester, polycarbonate, a urea-formaldehyde resin, a melamine resin. 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 emulsification 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.

As the polyisocyanate compound, a compound having an isocyanate group of three or more functional groups is preferred, and a difunctional isocyanate compound may be used in combination therewith.

Specific examples thereof include a diisocyanate, such as xylene diisocyanate or a hydrogenated product thereof, hexamethylene diisocyanate or a hydrogenated product thereof, tolylene diisocyanate or a hydrogenated product thereof and isophorone diisocyanate, as the main component; a dimer or a trimer thereof (burette or isocyanaurate); a compound having polyfunctionality as an adduct product of a polyol, such as trimethylolpropane, and a difunctional isocyanate, such as xylylene diisocyanate; a compound of an adduct product of a polyol, such as trimethylolpropane, and a difunctional isocyanate, such as xylylene diisocyanate, having a polymer compound, such as polyether having an active hydrogen, such as polyoxyethylene oxide, introduced therein; and a formalin condensation product of benzeneisocyanate.

The compounds described in JP-A-62-212190, JP-A-4-26189, JP-A-5-317694 and Japanese Patent Application No. 8-268721 are preferably used. Specific examples of the polyol and/or the polyamine added to the aqueous phase and/or the oily phase as one constitutional component of the microcapsule wall through the reaction with the polyisocyanate include propylene glycol, glycerin, trimethylolpropane, triethanolamine, sorbitol and hexamethylenediamine. In the case where a polyol is added, a polyurethane wall is formed.

In an exemplary embodiment, conditions for the microencapsulation reaction are set so that at least about 90% of the total volume of said 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 thickness of the microcapsule wall is preferably between about 0.01 μm and about 0.3μm.

The microencapsulation reaction is also controlled so that the microcapsule wall has a Tg of from about 150 ° C to about 190 0 C 1 preferable from about 160 0 C to about 180°C, and most preferably from about 165 ° C to about 175 0 C.

In order to achieve the above-described features, it is preferable to set specific reaction conditions for microcapsule preparation. These conditions include emulsification process of the electron donor-type dye precursor, addition rates and amounts of the polyisocyanate and polyamine to form the microcapsule wall, as well as mixing and reaction temperature, time, and agitation. In the reaction, it is preferred to increase the reaction rate by either maintaining a high reaction temperature or by adding an appropriate polymerization catalyst.

The polyisocyanate, the polyol, the reaction catalyst and the polyamine for forming a part of the wall are described in detail in known literature, 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).

Particle size of the microcapsules in the suspension can be measured by diluting the suspension into aqueous solution and using laser scattering method based on Mie-scattering theory to measure the particle size and distribution. Typical equipment used for such measurement is Horiba's LA series, Beckman Coulter's LS series or Malvern Instruments' Mastersizer series.

The T g of the microcapsule wall can be measured by using conventional differential thermal analysis methods, such as DSC (Differential Scanning Calorimeters) or DDSC (Dynamic DSC) 1 which measures specific heat (C p ) change over different temperature ranges. Both a microcapsule-containing suspension and a blank suspension should be placed in the sample trays before measurement. Typical equipment used for such measurements are Perkin Elmer Diamond DSC, Sapphire DSC, HyperDSC™, or TA Instruments Q-series.

The microcapsule wall may further contain, depending on necessity, a metal- containing dye, a charge adjusting agent, such as nigrosin, and other arbitrary additive substances. These additives may be contained in the capsule wall during wall formation or 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 low temperature and having the quality of high coloring properties, it is preferred to use a plasticizer that is suitable for the polymer that is used as the wall material. The plasticizer preferably has a melting point of about 5O 0 C or more, and more preferably of about 12O 0 C or more. Among plasticizers, those in a solid state at ordinary temperature can be preferably selected.

For example, in the case where the wall material comprises polyurea or polyurethane, as a plasticizer a hydroxyl compound, a carbamate compound, an aromatic alkoxy compound, an organic sulfoneamide compound, an aliphatic amide compound, and an arylamide compound are preferably used.

As the hydrophobic organic solvent used for forming the core of the microcapsule by dissolving the electron donor-type dye precursor compound upon preparing the oily phase, an organic solvent having a boiling point of from about 100 to about 300 0 C is preferred.

Specific examples thereof 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. Among these, an ester compound is particularly preferably used from the standpoint of emulsification stability of the emulsion dispersion.

Examples of the ester compound include a phosphate, such as triphenyl phosphate, tricresyl phosphate, butyl phosphate, octyl phosphate or cresylphenyl phosphate; a phthalate, such as dibutyl phthalate, 2-ethylhexyl phthalate, ethyl phthalate, octyl phthalate or butylbenzyl phthalate; dioctyl tetrahydrophthalate; a benzoate, such as ethyl benzoate, propyl benzoate, butyl benzoate, isopentyl benzoate or benzyl benzoate; an abietate, such as ethyl abietate or benzyl abietate; dioctyl adipate; isodecyl succinate; dioctyl azelate; an oxalate, such as dibutyl oxalate or dipentyl oxalate; diethyl malonate; amaleate, such as dimethylmaleate, diethyl maleate ordibutyl maleate; tributyl citrate; a sorbate, such as methyl sorbate, ethyl sorbate or butyl sorbate; a sebacate, such as dibutyl sebacate or dioctyl sebacate; an ethylene glycol ester, such as a formic acid monoester or diester, a butyric acid monoester or diester, a lauric acid monoester or diester, a palmitic acid monoester or diester, a stearic acid monoester or diester, or an oleic acid monoester or diester; triacetin; diethyl carbonate; diphenyl carbonate; ethylene carbonate; propylene carbonate; and a borate, such as tributyl borate or tripentyl borate.

These hydrophobic organic solvents may be used alone or in combinations of two or more. Among these, tricresyl phosphate is preferably used, either singly or as a mixture with other solvents since it provides high emulsion stability.

In the case where the electron donor-type dye precursor to be encapsulated has poor solubility to 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.

In the case where the electron donor-type dye precursor compound is used in the laser-sensitive recording layer of the laser markable material, the content of the electron donor-type dye precursor is preferably from about 0.1 to about 5.0 g/m 2 , and more preferably from about 1.0 to about 4.0 g/m 2 .

When the content of the electron donor-type dye precursor is in the range of from about 0.1 to 5.0 g/m 2 , a sufficient coloring density can be obtained, and when the content is 5.0 g/m 2 or less, both a sufficient coloring density can be achieved while the transparency of the laser-sensitive recording layer can also be maintained.

During microcapsule formation, water-soluble resins are added to the aqueous phase of the reaction mixture as a binder in order to stabilize the emulsified dispersion and formed microcapsules, and the type and the addition amount of the water-soluble resins are selected so that the viscosity of the coating composition has a viscosity of from about 5 centipoise (cP) to about 30 cP, preferably from about 10 cP to about 25 cP, and most preferably from about 10 cP to about 20 cP. Viscosity is measured using Brookfield Programmable DV-II+ viscometer with small sample adapter plus a S21 spindle at 100-200 RPM. Regular RV series spindle may also be used depending on sample quantity.

The water-soluble resin composition used as the binder for microcapsule formation of the electron-donor dye precursor can also be appropriately selected from compounds which can include solvent free polyurethane or its modified derivatives, such as the emulsion, suspension, or dispersion of polyester based polyurethane, polyether based polyurethane, polycarbonate

based polyurethane, castor oil based polyurethane, or any combinations thereof. It is preferred that about at least 50% or more of the total binder solid weight comprising the electron-donor dye precursor microcapsule is the waterborne and/or solvent-free polyurethane and modified derivatives.

The mixing ratio of the oily phase to the aqueous phase (oily phase weight/aqueous phase weight) is preferably from about 0.02 to about 0.6, and more preferably from about 0.1 to about 0.4. When the mixing ratio is in the range of from 0.02 to 0.6, a suitable viscosity can be maintained. This provides both an improved productivity of use for coating the composition as well as optimized stability of the coating composition.

In order to further uniformly emulsify and disperse the oily phase and the aqueous phase, a surfactant may be added to at least one of the oily phase and the aqueous phase. As the surfactant, a known surfactant for emulsification may be used. The addition amount of the surfactant 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.

As the surfactant contained in the aqueous phase, one that does not cause precipitation or aggregation through an action with the binder can be used by appropriately selecting from anionic and nonionic surfactants. Preferred examples of the surface-active agent include sodium alkylbenzenesulfonate, sodium alkylsulfate, sodium dioctyl sulfosuccinate and a polyalkylene glycol (such as polyoxyethylene nonylphenyl ether).

The emulsification can be easily conducted by subjecting the oily phase containing the foregoing components and the aqueous phase containing the binder and the surfactant to a means that is generally used for fine particle emulsification, such as high speed agitation or ultrasonic wave dispersion by using a known emulsifying apparatus, such as a homogenizer, Manton

Gaulin, an ultrasonic wave disperser, a dissolver or a KADY mill. After the emulsification, it is preferred that the emulsion is heated to a temperature of

from 30 to 70° C for accelerating the capsule wall-forming reaction. During the reaction, it is preferred that water is added to the emulsion to decrease the probability of collision of the capsules or that sufficient agitation is conducted to prevent aggregation of the capsules.

A dispersion, emulsion, or suspension containing the said type of polyurethane or its modified derivatives for preventing aggregation may further be added during the reaction. Formation of a carbon dioxide gas is observed with progress of the reaction, and termination of the formation can be determined as completion of the capsule wall-forming reaction. In general, the reaction is conducted for several hours to obtain the objective microcapsules.

Laser markable coating composition containing electron acceptor-type developer and the solvent free polyurethane binder

The exemplary coating composition is used after mixing with an electron acceptor compound which acts as the developer for the laser marking coating composition. It is preferred that the liquid coating composition be provided separately from the developer in order to maintain the stability of the coating composition.

Specific examples of the electron acceptor-type compound, which reacts with the electron donor-type dye precursor, include but are not limited to an acidic substance, such as activated bentonite, metal salt of salicylate, phenol compound, organic acid or its metallic salt, and oxybenzoate.

Specific examples thereof include a bisphenol compound, such as 2,2-bis(4'- hydroxyphenyl)propane (generic name: bisphenol A) 1 2,2-bis(4- hydroxyphenyl)pentane, 2,2-bis(4'-hydroxy-3\5'-dichlorophenyl)propane 1 1 ,1- bis(4'-hydroxyphenyl)cyclohexane, 2,2-bis(4'-hydroxyphenyl) hexane, 1 ,1-bis(4'- hydroxyphenyl)propane, 1 ,1-bis(4'-hydroxyphenyl)butane, 1 ,1-bis(4'- hydroxyphenyl)pentane, 1 ,1-bis(4'-hydroxyphenyl)hexane, 1 ,1-bis (4 1 - hydroxyphenyl)heptane, 1 ,1-bis(4'-hydroxyphenyl) octane, 1 ,1-bis(4'- hydroxyphenyl)-2-methylpentane, 1 ,1-bis(4'-hydroxypenyl)-2-ethylhexane, 1 ,1- bis(4'-hydroxyphenyl)dodecane, 1 ,4-bis(p-hydroxyphenylcumyl)benzene, 1 ,3-

bis(p-hydroxyphenylcymy1)benzene, bis(p-hydroxyphenyl) sulfone, bis(3-allyl-4- hydroxyphenyl)sulfone and bis(p-hydroxyphenyl)acetic acid benzyl ester; a salicylic acid derivative, such as 3,5-di-.alpha.-methylbenzylsalicylic acid, 3,5-di- tert-butylsalicylic acid, 3-.alpha.-.alpha.-dimethylbenzylsalicylic acid and 4-(.beta.- p-methoxyphenoxyethoxy)salicylic acid; a polyvalent metallic salt thereof (in particular, a zinc salt and an aluminum salt are preferred); an oxybenzoate, such as p-hydroxybenzoic acid benzyl ester, p-hydroxybenzoic acid 2-ethylhexyl ester and .beta.-resorcinic acid 2-phenxyethyl ester; and a phenol compound, such as p-phenylphenol, 3,5-diphenylphenol, cumylphenol, 4-hydroxy-4'- phenoxydiphenylsulfone.

Among these, the metal salt of salicylate is preferred, for instance, zinc salicylate. Good coloring characteristics can be achieved by using this type of developer.

In addition, the electron acceptor-type compounds may be used singly or in a combination of two or more.

The electron acceptor-type 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 the waterborne and solvent-free polyurethane and its modified derivatives as the binder (aqueous phase), 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 subjecting to 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 waterborne resins contained as the binder can be appropriately selected from the compounds: solvent-free polyurethane or its modified derivatives, such as the emulsion or suspension or dispersion of polyester based polyurethane, polyether based polyurethane, polycarbonate based polyurethane, castor oil based polyurethane, or any combinations thereof.

Using only the solvent-free polyurethane or its modified derivatives as the binder in a laser markable coating composition is preferred for making laser markable material. A combination of polyurethane compounds and other known binder resins that are inert to color forming agents in the laser markable coating composition, such as acrylic, epoxy, cellulose, and others known in the art., can be a selected as required for the marking of laser markable material. The preferred concentration of the solvent-free polyurethane and its modified derivatives is about 50% or more of the total binder quantity within the liquid coating composition. This concentration is necessary in order to minimize the formation of white marks when used in combination with other binder compounds outside.

Mixing ratio of the oily phase to the aqueous phase (oily phase weight/aqueous phase weight) is preferably from 0.02 to 0.6, and more preferably from 0.1 to 0.4. 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.

Compositions of electron acceptor-type developers are disclosed in US 6,797,318 Example-1 as Developer Emulsion Dispersion, US 5,409,797 Example-1 as Emulsion Dispersion, and US 5,691 ,757 Example as Color Developer.

Preparation of mixed coating dispersion comprising a combination of the coating composition containing a microencapsulated electron donor-type dye precursor and the coating composition containing an electron acceptor-type developer

In order to prepare a material for laser marking from the coating composition, the coating compositions are mixed together to prepare a mixed coating dispersion which is subsequently coated on a substrate for use as a laser markable coating layer for laser marking.

In this process, the mix ratio of the 2 coating compositions is such that the ratio of total weight of electron donor-type dye precursors and the total weight of the developers is between from about 1 :0.5 to about 1 :30, preferably from about 1 :1 to about 1 :1.5.

In order to effectively and uniformly produce a laser markable coating material from the laser markable coating composition, as well as to maintain the strength of the coated film, additional binder resins and auxiliary additives can be used. In this composition also, the binder resin can be the waterborne and/or solvent-free polyurethane and its modified derivatives. The auxiliary additives can be surfactants, anti-foam agents, plasticizers, rheological agents, biocides, antistatic agents, water, cross linking agents, and other compounds know in the art.

Auxiliary additives for better laser marking performance may also be used, such as heat transfer agents, melting agents, ultraviolet ray absorbing agents, antioxidants, and other additives known in the art.

In addition, to coat a substrate using the mixed coating dispersion to prepare a laser marking material, a known coating method applied to an aqueous or organic solvent series coating composition is used for coating the laser markable coating composition on a support.

Assistant layers to form a laser markable material

The laser markable material may further comprise, on the support, assistant layers such as a protective layer, an intermediate layer, an undercoating layer called primer, and others known in the art.

The protective layer is located on top of the laser markable material and is commonly known as a topcoat. The function of the topcoat layer is to provide protection for the laser-recording layer against physical damage such as rubbing, to protect against moisture attack, to strengthen the resistance against instant heat impact, and to block attack from ultraviolet radiation, heat, and humidity.

Intermediate layers may also be applied on the laser markable material. They function to prevent inter-mixing of the layers and also for blocking a gas (such as oxygen) that is harmful to image preservation properties.

One or more undercoating layers may be applied on the support before coating the laser markable coating composition. These can include a light reflection preventing layer or other necessary functional layers necessary to improve the adhesion of the said layers to the support.

In order to avoid the described white mark problem and the stain problem caused by un-wanted color forming reaction between the color forming agents, it has now also been discovered that the binder in the coating composition of the assistant layers in a laser markable material can also be appropriately selected from a solvent-free polyurethane or its modified compounds, such as the emulsion, suspension or dispersion of polyester-based polyurethane, polyether-based polyurethane, polycarbonate-based polyurethane, castor oil-based polyurethane, or any combinations thereof.

Of useful desirability is the formulation of a protective topcoat coating composition comprising the waterborne, solvent-free polyurethane emulsions, suspensions, and dispersions to provide necessary protection for the color forming layer in a laser markable material. The protective coating composition not only provides the desired protection, but also eases the concern of the white mark problem and the stain problem that affect the quality of a laser marked material.

Unlike the laser markable coating composition comprising the color forming components, it is preferred in an exemplary embodiment to use only the inventive waterborne, solvent-free polyurethane and its modified derivatives as the binder components in an assistant coating composition layer. Preferably the quantity of

binder for the protective topcoat and other assistant layers comprises from about 10% to about 90% of the total solid weight of the protective and other assistant coating compositions. More preferably the inventive binder composition is from about 20% to about 70%, and most preferably is from about 30% to about 60% of the total solid weight of the coating composition.

In addition to the binder, there are other auxiliary additives in the assistant and protective coating compositions. The auxiliary additives can be regular coating additives, such as surfactants, anti-foam agents, plasticizers, rheological agents, biocides, antistatic agents, water, hardening agents, cross linking agents, and other additives know in the art. Other auxiliary additives that promote the quality and performance of the laser markable materials can be also incorporated into the protective coating composition, such as heat transfer agent, ultraviolet ray absorbing agent, and others know in the art.

The coating composition of the protective layer preferably has 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 material is maintained.

EXAMPLES

Exemplary embodiments will now be illustrated in details with reference to Examples, but the present invention should not be construed as limited thereto.

Example 1

This experiment is designed to compare exemplary solvent-free polyurethane compounds with the disclosed polyurethane compounds of WO 2006/063165 A2 in terms of performance against white mark formation.

8 polyurethane dispersions listed in Table 1-1 are tested with polyvinyl alcohol as the comparison. A blank glass slide is used as a reference.

Table 1-1:

Experimental procedure:

a) Coat the tested sample solution on a glass slide of 1" x 4" with the K Control Coater (RK Print Coat Instruments, Ltd.). No. 7 coating bar is used to give the film thickness of 80 micrometer when wet.

b) The coated sample solution is allowed to dry overnight under ambient condition.

c) Expose all the coated samples with Domino S100 laser maker (Domino Amjet, Inc.) under a matrix exposure.

A matrix exposure consisting of 70 of the same mark, the letter "M", was applied onto each of the coated samples, using a Domino S-100 CO 2 laser marker with a f=80mm lens, which provides 35mm X 35mm marking field and a spot size of from about 250 to about 280 μm. The design of the test marking matrix is such that each row consists of 7 characters, with increasing laser power output from 26.5% to 100% (5.2W→19.6W from left to right), and 20% power increment between neighboring characters, and each column consists of 10 characters, with increasing marking speed from 1300 bits/mS to 9500 bits/mS (from bottom to top), and 20% speed increment between neighboring characters.

d) A picture is taken for the CO 2 laser matrix exposed sample on a black background.

e) Number of white "M" letters and degree of whiteness of the letter are compared to judge the sample that has minimum response to CO 2 laser energy. Less number of "M" letter indicates less interaction with CO 2 laser beam.

The experimental results are shown in Figures 1A, 1 B and 1 C. As can be seen from such figures, the solvent-free polyurethane and its modified derivatives are inert to the CO 2 laser energy as well as other polyurethane compounds. The inertness to CO 2 laser energy is the property of polyurethane compounds in general. This

property offers good performance to resist white marking caused by CO 2 laser exposure.

Example 2

This experiment is designed to determine the stability of a laser markable material under hot and/or humid conditions and how stability is affected by incorporating the solvent-free polyurethane and its modified derivatives as a binder into a laser markable coating composition. In this experiment, the inventive polyurethane and modified derivatives are used as a binder in making: Part A - the coating composition containing the micro-encapsulated dye precursor; and Part B - the coating composition containing the electron acceptor-type developer. The inventive polyurethane and modified derivatives that are not solvent-free are used in this experiment as a reference binder to compare the experimental results. Below is the procedure to conduct the experiment.

Experiment Procedure:

1) Making Part A - coating composition containing micro-encapsulated dye precursor

13.3 g of electron donor-type dye precursor (Trade Name: PSD-184, Nippon Soda) 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 0 C, and then cooled down to 45"C. 12.6 g of capsule wall material (Trade

Name: D-140N, Mitsui Takeda Chemical Co., Ltd.) was added into the ethyl acetate solution.

The above ethyl acetate solution was added in 53 g of 6%w/w solution of the said polyurethane dispersion, and emulsified with a homogenizer at 15,000 rpm for 5 minutes.

8Og of water and 0.75 g of tetraethylenepentamine were added and mixed with a stirrer at 400 rpm for 4 hours for encapsulation reaction.

Part A is completed at this step. The coating composition is named A.

After the reaction was completed, the particle size distribution of the encapsulated electron donor-type dye precursor particles and the viscosity of the liquid coating composition were measured with Beckman Coulter's LS- 100Q particle size analyzer and Brookfield Programmable DV-II+ viscometer with S21 small size spindle at 100-200 RPM.

The T g of the microcapsule wall is measured by using (Perkin Elmer's Diamond DSC, Sapphire DSC 1 HyperDSC™, or TA Instruments' Q-series). A blank suspension without microcapsule is prepared under the same conditions as reference sample. Both the microcapsule containing suspension and the blank suspension are placed in the sample trays before measurement.

Table 2-1 lists the polyurethane and its modified derivative as the replacement of polyvinyl alcohol as a binder in Part A:

Table 2-1 :

The quantity of each binder sample is adjusted to maintain equivalent solid content.

2) Making Part B - coating composition containing the electron acceptor-type developer

4.2g of an UV light absorbing agent (Trade Name: Tinuvin 328, Ciba Geigy), 1.0 g of tricresylphosphate, and 36.4g of developer (Trade Name: RO54,

Sanko Chemicals) were added in 16.Og of ethyl acetate, and dissolved by heating up to 70°C. This ethyl acetate solution was added in the below described aqueous solution and dispersed with a homogenizer at 12,000 rpm for 5 minutes. Aqueous solution for emulsified developer dispersion

Water 68.4g

7.4%w/w solution of test polyurethane dispersion 75.5g

Surfactant A (Trade Name: W-502, Waco Pure Chemical Industries) 11.2g Surfactant B (Trade Name: NEOPELEX G-15, Kao) 11.2g Part B is completed at this step. The coating composition is named B.

Table 2-2 lists the polyurethane and its modified derivative as the replacement of polyvinyl alcohol as a binder in Part B:

Table 2-2:

The quantity of each binder sample is adjusted to maintain equivalent solid content

3) Make a laser markable coating composition pot solution by mixing Part A and Part B

Each of Part A samples and Part B samples were mixed as a pair of Aj+Bj. The mixing ratio is as below:

Part A 5.04 g

Part B 19.13 g

Deionized Water 6.40 g

To make coating pot solution 30.57 g

The coating pot solution made from Aj+Bj is named Tj

4) Coat the coating pot solution on PET film

Each of the above mixture was coated at 15ml/m 2 on a film of A4 size and 75μm thickness PET which was preliminarily coated with SBR lutex and gelatin, and the following laser marking was applied after drying.

Coating was done with the K Control Coater (RK Print Coat Instruments, Ltd.). No. 3 coating bar is used to give the film thickness of 24 micrometer when wet.

5) Store the coated samples in a humidity chamber under the condition of 80 0 C and 70% R. H. for 7 days. Keep a fresh coated sample of each test (Tj ) for comparison.

6) Measure and compare the density of the tested samples with a densitometer (X-Rite 310).

The experimental results are summarized below:

As shown from the comparison result, the exemplary solvent-free polyurethane and its modified derivatives have significantly reduced stain increase compared to other polyurethane compounds that are solvent-based or that contain a solvent. The laser markable material comprising the solvent-free polyurethane and its modified derivatives provide low stain after storage in a hot and/or humid environment.

Example 3

This experiment is designed to demonstrate how the stability of a laser markable material under a hot and/or humid condition is affected by incorporating the solvent- free polyurethane and its modified derivatives as a binder into a protective topcoat layer of a laser markable material. In this experiment, the polyurethane and its

modified derivatives comprise the binder of a protective coating composition. The non-inventive polyurethane and modified derivatives that are not solvent-free are used in this experiment as a reference binders for comparison to the experimental results. Below is the procedure to conduct the experiment

Experiment Procedure:

1 ) Make a laser markable composition and coat it on PET film

Making Part A - coating composition containing micro-encapsulated dye precursor

13.3 g of electron donor-type dye precursor (Trade Name: PSD-184, Nippon Soda) 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. 12.6 g of capsule wall material (Trade Name: D-140N, Mitsui Takeda Chemical Co., Ltd.) was added into the ethyl acetate solution.

The above ethyl acetate solution was added in 53 g of 6%w/w polyvinyl alcohol aqueous solution (Trade Name: Kurary Poval MP-103, Kuraray Co., Ltd.) and emulsified with a homogenizer at 15,000 rpm for 5 minutes.

8Og of water and 0.75 g of tetraethylenepentamine were added and mixed with a stirrer at 400 rpm for 4 hours for encapsulation reaction.

Part A is completed at this step.

Making Part B - coating composition containing the electron acceptor-type developer

4.2g of an UV light absorbing agent (Trade Name: Tinuvin 328, Ciba Geigy), 1.0 g of tricresylphosphate, and 36.4g of developer (Trade

Name: RO54, Sanko Chemicals) were added in 16.Og of ethyl acetate, and dissolved by heating up to 70°C. This ethyl acetate solution was added in the below described aqueous solution and dispersed with a homogenizer at 12,000 rpm for 5 minutes.

Aqueous solution for emulsified developer dispersion

Water 68.4g

15%w/w Poly-vinylalcohol

(Trade Name: Poval PVA205, Kurary Co., Ltd.) 19.8g

8%w/w Poly-vinylalcohol (Trade Name: Poval PVA217, Kurary Co., Ltd.) 55.7g

Surfactant A (Trade Name: W-502, Waco Pure Chemical Industries) 1 1.2g

Surfactant B (Trade Name: NEOPELEX G-15, Kao) 1 1.2g

Part B is completed at this step.

Make coating pot solution by mixing Part A and Part B

The mixing ratio is as below:

Part A 5.04 g

Part B 19.13 g

Deionized Water 6.40 g

To make coating pot solution 30.57 g

Coat the coating pot solution on PET film

The above mixture was coated at a film of A4 size and 75μm thickness PET which was preliminarily coated with SBR latex and gelatin, and the following laser marking was applied after drying.

Coating was done with the K Control Coater (RK Print Coat Instruments, Ltd.). No. 2 coating bar is used to give the film thickness of 12 micrometer when wet.

2) Make the protective coating composition

Add the compounds listed in Table 3-1 one by one. Add the next ingredient after the previous one is fully dissolved or dispersed.

Table 3-1

Table 3-2 lists the said polyurethane and its modified derivative to be tested as a binder in the protective coating composition:

Table 3-2:

The quantity of each binder sample is adjusted to maintain equivalent solid content.

3) Coat the protective coating composition on the color forming layer

The coated film made at Step 1) was coated with the protective coating composition made above.

Coating was done with the K Control Coater (RK Print Coat Instruments, Ltd.). No. 2 coating bar is used to give the film thickness of 12 micrometer when wet.

4) Store the coated samples in a humidity chamber under the condition of 80 0 C and 70% R. H. for 7 days. Keep a fresh coated sample of each test (Tj ) for comparison.

5) Measure and compare the density of the tested samples with a densitometer (X-Rite 310).

The experimental results are summarized as below:

As shown in the table, when using the solvent-free polyurethane dispersions as the binder in the protective coating composition for a laser markable material, a low stain background increase can be obtained after storing the material in a hot and/or humid environment.