Laser marking compositions, resin films, and laminates

Abstract

To provide a laser marking composition capable of forming a colored layer with excellent appearance, color development, and adhesion. [Solution] The laser marking composition contains a resin component comprising at least one (meth)acrylic resin containing at least one of a hydroxyl group and a carboxyl group, an isocyanate crosslinking agent comprising an aliphatic isocyanate crosslinking agent, a color pigment, and a coloring agent, wherein the content of the isocyanate crosslinking agent is 0.5 to 10 parts by mass per 100 parts by mass of the total of the resin component, the color pigment, and the coloring agent.

Description

Technical Field

[0001] The present disclosure relates to a laser marking composition, a resin film, and a laminate.

Background Art

[0002] Conventionally, laser marking labels that irradiate laser light to mark various information are known as labels for displaying various information such as characters and two-dimensional codes. Laser marking labels include an etching type and a color-developing type. In particular, the color-developing type has an advantage in that there is less concern that the printing will disappear due to external release of dust, surface wear, or chemical dropping compared to the etching type.

[0003] For example, Patent Document 1 discloses a color-developing type laser marking label. In Patent Document 1, the composition of the second layer that develops color by laser light, particularly the type and content of the crosslinking agent, are limited. As specific types of the crosslinking agent, either (i) an aromatic aliphatic isocyanate-based crosslinking agent or (ii) a mixture of an aliphatic or alicyclic isocyanate-based crosslinking agent and an aromatic isocyanate-based crosslinking agent is selected. It is said that a laser marking label with good print readability and re-peelability can be obtained in this way.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] When a laser marking composition containing the crosslinking agent described in Patent Document 1 was used to manufacture a laminate, the short pot life of the laser marking composition resulted in poor productivity and sometimes yielded resin films with poor appearance. As a result, it was sometimes impossible to efficiently obtain laser-marked labels. Therefore, in order to improve the pot life, we tried using aliphatic or alicyclic isocyanate crosslinking agents instead of aromatic aliphatic isocyanate crosslinking agents and aromatic isocyanate crosslinking agents. This improved the pot life and resulted in labels with a good appearance. However, it was found that the color development was insufficient. Generally, when trying to improve color development, the goal is often to increase the amount of color-developing pigment added or to adjust the reduction rate of the color-developing pigment. However, if the amount of color-developing pigment added is increased, the color of the color-developing pigment itself may color the laser marking composition or the resin film obtained from the laser marking composition (for example, gray), which can result in a loss of contrast in the printed area after laser irradiation. Furthermore, when using aliphatic or alicyclic isocyanate crosslinking agents, the adhesion of the resin film formed using the laser marking composition to the substrate was sometimes poor. This disclosure is made in view of the above-mentioned conventional circumstances and aims to provide a laser marking composition capable of forming a colored layer with excellent appearance, color development, and adhesion, as well as a resin film and laminate using this laser marking composition. [Means for solving the problem]

[0006] The specific means for achieving the aforementioned objectives are as follows: <1> It contains a resin component comprising at least one (meth)acrylic resin containing at least one of a hydroxyl group and a carboxyl group, an isocyanate crosslinking agent comprising an aliphatic isocyanate crosslinking agent, a color pigment, and a coloring agent. A laser marking composition in which the isocyanate-based crosslinking agent is contained in an amount of 0.5 to 10 parts by mass relative to 100 parts by mass of the total of the resin component, the color pigment, and the coloring agent. <2> The proportion of the aliphatic isocyanate crosslinking agent in the isocyanate crosslinking agent is 95% by mass or more. <1> The laser marking composition described above. <3> The aliphatic isocyanate crosslinking agent includes an adduct of hexamethylene diisocyanate. <1> or <2> The laser marking composition described above. <4> The proportion of the hexamethylene diisocyanate adduct in the isocyanate-based crosslinking agent is 95% by mass or more. <3> The laser marking composition described above. <5> The proportion of the total of at least one (meth)acrylic resin containing at least one of the hydroxyl group and carboxyl group in the resin component is 70% by mass or more. <1> ~ <4> A laser marking composition according to any one of the following items. <6> <1> ~ <5> A resin film made using the laser marking composition described in any one of the items. <7> <6> A laminate having the resin film described above. [Effects of the Invention]

[0007] This disclosure provides a laser marking composition capable of forming a colored layer with excellent appearance, color development, and adhesion, as well as a resin film and laminate using this laser marking composition. [Brief explanation of the drawing]

[0008] [Figure 1] This figure schematically shows an example of a cross-sectional structure of a laminate according to one embodiment of the present disclosure. [Modes for carrying out the invention]

[0009] The present disclosure is described in detail below. However, the present disclosure is not limited to the following embodiments. In the following embodiments, the components (including elemental steps, etc.) are not essential unless otherwise explicitly stated. The same applies to numerical values ​​and their ranges, and do not limit the present disclosure.

[0010] In this disclosure, the term "process" includes not only processes that are independent of other processes, but also processes that cannot be clearly distinguished from other processes, provided that the purpose of such process is achieved. In this disclosure, the numerical range indicated using "~" includes the numbers before and after "~" as the minimum and maximum values, respectively. In numerical ranges described in stages within this disclosure, the upper or lower limit of one numerical range may be replaced with the upper or lower limit of another numerical range described in stages. Furthermore, in numerical ranges described within this disclosure, the upper or lower limit of that range may be replaced with the values ​​shown in the examples. In this disclosure, each component may contain multiple types of the corresponding substance. If multiple types of the substance corresponding to each component are present in the composition, the content or amount of each component means the total content or amount of the multiple types of substances present in the composition, unless otherwise specified. In this disclosure, each component may include multiple types of particles. If multiple types of particles corresponding to each component are present in the composition, the particle size of each component refers to the value for a mixture of such multiple types of particles present in the composition, unless otherwise specified. In this disclosure, the terms “layer” or “film” include cases where, when the region in which the layer or film exists is observed, it is formed not only over the entire region but also over only a portion of the region. In this disclosure, the term "lamination" refers to stacking layers, and two or more layers may be bonded together or detachable. In this disclosure, "(meth)acrylic" means at least one of acrylic and methacrylic, and "(meth)acrylate" means at least one of acrylate and methacrylate. In this disclosure, the average thickness of a layer or film is given as the arithmetic mean of measuring the thickness of five points on the layer or film in question. The thickness of a layer or film can be measured using a micrometer or the like. In this disclosure, if the thickness of a layer or film can be measured directly, it is measured using a micrometer. On the other hand, when measuring the thickness of a single layer or the total thickness of multiple layers, the measurement may be performed by observing the cross-section of the object to be measured using an electron microscope. In this disclosure, "solids" refers to the components of the laser marking composition or sample solution excluding the organic solvent.

[0011] <Laser marking composition> The laser marking composition of this disclosure contains a resin component comprising at least one (meth)acrylic resin containing at least one of a hydroxyl group and a carboxyl group, an isocyanate crosslinking agent comprising an aliphatic isocyanate crosslinking agent, a color pigment, and a coloring agent, wherein the content of the isocyanate crosslinking agent is 0.5 to 10 parts by mass per 100 parts by mass of the total of the resin component, the color pigment, and the coloring agent. The laser marking composition disclosed herein makes it possible to form a colored layer with excellent appearance, color development, and adhesion. The reason for this is not clear, but it is presumed to be as follows. By using an aliphatic isocyanate crosslinking agent, the thickening of the laser marking composition over time is suppressed compared to when an aromatic or aromatic aliphatic isocyanate crosslinking agent is used. Furthermore, it allows for a greater margin in the amount of crosslinking agent added. As a result, it is presumed that the thickness of the colored layer formed by the laser marking composition becomes more uniform, improving the appearance. On the other hand, the inventors found that the relationship between the total content of resin components, color pigments, and colorants and the content of isocyanate-based crosslinking agents affects the color development and adhesion of the resin film. As a result of diligent research, they found that by setting the content of isocyanate-based crosslinking agents to 0.5 to 10 parts by mass per 100 parts by mass of the total of resin components, color pigments, and colorants, the decrease in color development and adhesion can be suppressed. From the above, it is presumed that the laser marking composition of this disclosure makes it possible to form a colored layer with excellent appearance, color development, and adhesion.

[0012] Hereinafter, each component constituting the laser marking composition of the present disclosure will be described.

[0013] (Resin component) The laser marking composition of the present disclosure contains at least one (meth)acrylic resin containing at least one of a hydroxyl group and a carboxyl group as a resin component, and may contain other resins as needed.

[0014] -(Meth)acrylic resin- The laser marking composition of the present disclosure contains at least one (meth)acrylic resin containing at least one of a hydroxyl group and a carboxyl group as a resin component. As the (meth)acrylic resin containing at least one of a hydroxyl group and a carboxyl group, a (meth)acrylic resin containing at least one of a structural unit derived from a (meth)acrylic monomer having a hydroxyl group and a structural unit derived from a (meth)acrylic monomer having a carboxyl group is preferable. The total proportion of the structural unit derived from the (meth)acrylic monomer having a hydroxyl group and the structural unit derived from the (meth)acrylic monomer having a carboxyl group in all the structural units of the (meth)acrylic resin is preferably 5% by mass or more, more preferably 8% by mass or more, from the viewpoint of the strength of the resin film. The total proportion of the structural unit derived from the (meth)acrylic monomer having a hydroxyl group and the structural unit derived from the (meth)acrylic monomer having a carboxyl group in all the structural units of the (meth)acrylic resin is preferably 15% by mass or less, more preferably 12% by mass or less. The total proportion of the structural unit derived from the (meth)acrylic monomer having a hydroxyl group and the structural unit derived from the (meth)acrylic monomer having a carboxyl group in all the structural units of the (meth)acrylic resin is preferably 5% by mass to 15% by mass.

[0015] When the laser marking composition of this disclosure contains one type of (meth)acrylic resin, the (meth)acrylic resin used is one in which the total proportion of structural units derived from (meth)acrylic monomers having hydroxyl groups and structural units derived from (meth)acrylic monomers having carboxyl groups in the total structural units of the (meth)acrylic resin satisfies the above conditions. If the laser marking composition of the present disclosure contains two or more types of (meth)acrylic resins, it is sufficient that the total proportion of structural units derived from (meth)acrylic monomers having hydroxyl groups and structural units derived from (meth)acrylic monomers having carboxyl groups in the total structural units of the (meth)acrylic resin as a whole satisfies the above conditions. In this case, two or more types of (meth)acrylic resins that satisfy the above conditions may be used in combination, or one or more types of (meth)acrylic resins that satisfy the above conditions may be used in combination with one or more types of (meth)acrylic resins that do not satisfy the above conditions, or two or more types of (meth)acrylic resins that do not satisfy the above conditions may be used in combination.

[0016] Specific examples of (meth)acrylic monomers having a hydroxyl group include 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, 3-methyl-3-hydroxybutyl (meth)acrylate, 1,3-dimethyl-3-hydroxybutyl (meth)acrylate, 2,2,4-trimethyl-3-hydroxypentyl (meth)acrylate, 2-ethyl-3-hydroxyhexyl (meth)acrylate, and polypropylene. Examples include ethylene glycol mono(meth)acrylate, polyethylene glycol mono(meth)acrylate, poly(ethylene glycol-propylene glycol) mono(meth)acrylate, and pentaerythritol tri(meth)acrylate. Among these, 2-hydroxyethyl (meth)acrylate is preferred, and 2-hydroxyethyl methacrylate is more preferred.

[0017] Specific examples of (meth)acrylic monomers having a carboxyl group include acrylic acid, methacrylic acid, crotonic acid, maleic anhydride, fumaric acid, itaconic acid, glutaconic acid, and citraconic acid. Among these, acrylic acid or methacrylic acid is preferred, and acrylic acid is more preferred.

[0018] (Meth)acrylic resin may contain structural units derived from derivatives of (meth)acrylic acid, such as alkyl (meth)acrylate esters. Alkyl (meth)acrylate esters may have functional groups other than hydroxyl groups and carboxyl groups, such as amino groups and glycidyl groups. Specific examples of alkyl (meth)acrylates include methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, isopropyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, t-butyl (meth)acrylate, n-hexyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, octyl (meth)acrylate, cyclohexyl (meth)acrylate, phenyl (meth)acrylate, benzyl (meth)acrylate, phenoxyethyl (meth)acrylate, isobornyl (meth)acrylate, 2-methoxyethyl (meth)acrylate, 2-ethoxyethyl (meth)acrylate, glycidyl (meth)acrylate, and tetrahydrofurfuryl (meth)acrylate. Among these, at least one of methyl (meth)acrylate and ethyl (meth)acrylate is preferred.

[0019] The (meth)acrylic resin may optionally contain structural units derived from other monomers such as vinyl acetate, vinyl ether, acrylonitrile, and styrene.

[0020] When the (meth)acrylic resin is a copolymer, the polymerization mode is not particularly limited and may be random copolymerization, alternating copolymerization, block copolymerization, or graft copolymerization.

[0021] The weight-average molecular weight (Mw) of the (meth)acrylic resin is preferably in the range of 5,000 to 1,000,000, more preferably in the range of 10,000 to 800,000, and even more preferably in the range of 100,000 to 750,000. If the weight-average molecular weight (Mw) of the (meth)acrylic resin is 5,000 or higher, the resin film tends to be less brittle. Also, if the weight-average molecular weight (Mw) of the (meth)acrylic resin is 1,000,000 or lower, the film-forming properties tend to be excellent. When the laser marking composition of this disclosure uses two or more (meth)acrylic resins in combination, it is preferable that the weight-average molecular weight (Mw) of the mixture of the two or more (meth)acrylic resins is within the above range.

[0022] In this disclosure, the weight-average molecular weight (Mw) of the (meth)acrylic resin is a value measured by the following method. Specifically, it is measured according to (1) to (3) below. (1) Apply a solution of (meth)acrylic resin to release paper and dry it at 100°C for 1 minute to obtain a film-like (meth)acrylic resin. (2) Using the film-like (meth)acrylic resin obtained in (1) above and tetrahydrofuran, a sample solution with a solid content concentration of 0.2% by mass is obtained. (3) Using gel permeation chromatography (GPC), the weight-average molecular weight (Mw) of the (meth)acrylic resin is measured as a standard polystyrene equivalent under the following conditions.

[0023] ~Conditions~ Measurement device: High-speed GPC (Model number: HLC-8220 GPC, manufactured by Tosoh Corporation) Detector: Differential Refractometer (RI) (integrated into HLC-8220, manufactured by Tosoh Corporation) Columns: Four TSK-GEL GMHXL columns (Tosoh Corporation) connected in series. Column temperature: 40℃ Eluent: Tetrahydrofuran Sample concentration: 0.2% by mass Injection volume: 100μL Flow rate: 0.6mL / min

[0024] The glass transition temperature Tg of (meth)acrylic resin is preferably -20°C or higher, more preferably 0°C or higher, and even more preferably 10°C or higher, in order to suppress deformation of the printed area due to heat and gas during printing and to enable accurate printing of one-dimensional and two-dimensional codes. The glass transition temperature Tg of (meth)acrylic resin may be 100°C or lower from the viewpoint of good workability of the resin film and less brittleness. The glass transition temperature Tg of (meth)acrylic resin is preferably -20°C to 100°C. The glass transition temperature (Tg) of (meth)acrylic resin is determined by measuring the temperature using a differential scanning calorimetry (DSC) (e.g., EXSTAR6000, manufactured by Seiko Instruments Corporation) in a nitrogen atmosphere with a 10 mg sample and a heating rate of 10°C / min, and finding the inflection point of the resulting DSC curve. If two or more inflection points are observed in the DSC curve using a differential scanning calorimetry (DSC), the temperature at the inflection point with the highest temperature is taken as the glass transition temperature (Tg) of the (meth)acrylic resin.

[0025] Furthermore, if the structural units constituting the (meth)acrylic resin are known, the Tg of the (meth)acrylic resin may be the value obtained by converting the absolute temperature (K) calculated by the following formula to Celsius temperature (°C).

[0026]

number

[0027] In the formula, Tg1, Tg2, ... and Tg n m1, m2, ..., and m n These represent the mole fractions of each monomer.

[0028] Furthermore, "glass transition temperature expressed as the absolute temperature (K) of a homopolymer" refers to the glass transition temperature expressed as the absolute temperature (K) of a homopolymer produced by polymerizing the monomers individually. The glass transition temperature of a homopolymer can be measured by the method described above using a differential scanning calorimetry (DSC).

[0029] The glass transition temperatures (expressed in Celsius temperature (°C)) of representative monomers are as follows: methyl acrylate is 10°C, ethyl acrylate is -22°C, n-butyl acrylate is -54°C, 2-ethylhexyl acrylate is -70°C, 2-hydroxyethyl acrylate is -15°C, 4-hydroxybutyl acrylate is -80°C, t-butyl acrylate is 43°C, vinyl acetate is 32°C, acrylic acid is 106°C, methyl methacrylate is 105°C, and 2-hydroxyethyl methacrylate is 85°C. For example, by using these representative monomers, it is possible to adjust the aforementioned glass transition temperatures as appropriate. For monomers other than those mentioned above, the "glass transition temperature when used as a homopolymer" is based on the values ​​listed in the Polymer Handbook (4th edition, Wiley-Interscience; hereafter the same). If the value is not listed in the Polymer Handbook, the glass transition temperature of the homopolymer obtained by the measurement method described above is used. Furthermore, absolute temperature (K) can be converted to Celsius temperature (°C) by subtracting 273 from it, and Celsius temperature (°C) can be converted back to absolute temperature (K) by adding 273 to it.

[0030] When two or more types of (meth)acrylic resins are used in combination, it is preferable that the glass transition temperature Tg of the (meth)acrylic resin exhibiting the highest glass transition temperature Tg is within the above range.

[0031] The method for producing (meth)acrylic resin is not particularly limited, and it can be produced by polymerizing monomers using methods such as solution polymerization, emulsion polymerization, and suspension polymerization. However, when preparing the laser marking composition after producing the (meth)acrylic resin, solution polymerization is preferred because the processing steps are relatively simple and can be carried out in a short time.

[0032] Solution polymerization generally involves placing a predetermined organic solvent, monomer, polymerization initiator, and a chain transfer agent (if necessary) into a polymerization tank and heating the mixture for several hours under a nitrogen atmosphere or at the reflux temperature of the organic solvent while stirring. The weight-average molecular weight of the (meth)acrylic resin can be adjusted to the desired value by controlling the reaction temperature, reaction time, amount of solvent, and type and amount of catalyst.

[0033] Organic solvents used in the polymerization reaction of (meth)acrylic resins include aromatic hydrocarbon compounds, aliphatic or alicyclic hydrocarbon compounds, ester compounds, ketone compounds, glycol ether compounds, and alcohol compounds. These organic solvents may be used individually or in mixtures of two or more.

[0034] More specifically, organic solvents used in polymerization reactions include aromatic hydrocarbon organic solvents such as benzene, toluene, ethylbenzene, n-propylbenzene, t-butylbenzene, o-xylene, m-xylene, p-xylene, tetralin, decalin, and aromatic naphtha; aliphatic or alicyclic hydrocarbon organic solvents such as n-hexane, n-heptane, n-octane, isooctane, n-decane, dipentene, petroleum spirits, petroleum naphtha, and turpentine oil; ester organic solvents such as ethyl acetate, n-butyl acetate, n-amyl acetate, 2-hydroxyethyl acetate, 2-butoxyethyl acetate, 3-methoxybutyl acetate, and methyl benzoate; acetone, methyl ester Examples include ketone-based organic solvents such as tyl ketone, methyl isobutyl ketone, isophorone, cyclohexanone, and methylcyclohexanone; glycol ether-based organic solvents such as ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, and diethylene glycol monobutyl ether; and alcohol-based organic solvents such as methyl alcohol, ethyl alcohol, n-propyl alcohol, isopropyl alcohol, n-butyl alcohol, isobutyl alcohol, s-butyl alcohol, and t-butyl alcohol.

[0035] Examples of polymerization initiators include organic peroxides and azo compounds that can be used in conventional polymerization methods.

[0036] The content of (meth)acrylic resin in the solid content of the laser marking composition is preferably 15% to 99.5% by mass, more preferably 20% to 99% by mass, and even more preferably 40% to 98.5% by mass. When the (meth)acrylic resin content is 15% to 99.5% by mass, the heat resistance of the printed area tends to improve.

[0037] -Other resins- The laser marking compositions of this disclosure may contain, as a resin component, at least one other resin other than a (meth)acrylic resin containing at least one of a hydroxyl group and a carboxyl group. When the laser marking composition of this disclosure contains other resins as a resin component, the total proportion of at least one (meth)acrylic resin containing at least one of a hydroxyl group and a carboxyl group in the resin component is preferably 70% by mass or more, more preferably 80% by mass or more, and even more preferably 90% by mass or more. The total proportion of at least one (meth)acrylic resin containing at least one of a hydroxyl group and a carboxyl group in the resin component may be 99% by mass or less. The total proportion of at least one (meth)acrylic resin containing at least one of a hydroxyl group and a carboxyl group in the resin component is preferably 70% by mass to 99% by mass.

[0038] Other resins include those used to disperse coloring pigments or colorants (vehicles). Specific examples of other resins include acrylic resins and urethane resins.

[0039] (Crosslinking agent) The laser marking compositions of this disclosure contain an isocyanate crosslinking agent, which includes an aliphatic isocyanate crosslinking agent. The isocyanate crosslinking agent can react with at least one of the hydroxyl groups and carboxyl groups contained in the (meth)acrylic resin. In this disclosure, "isocyanate-based crosslinking agent" means a compound having two or more isocyanate groups in one molecule (a so-called polyisocyanate compound). "Aliphatic polyisocyanate compounds" include, for example, aliphatic polyisocyanate compounds, polymers of aliphatic polyisocyanate compounds (dimers (biuret), trimers (isocyanurate), or pentamers), and adduct compounds of aliphatic polyisocyanate compounds and polyol compounds [e.g., trimethylolpropane (TMP); the same applies hereinafter]. Specific examples of aliphatic polyisocyanate compounds include hexamethylene diisocyanate (HMDI), pentamethylene diisocyanate (PDI), tetramethylene diisocyanate, trimethylhexamethylene diisocyanate, and lysine diisocyanate.

[0040] Other isocyanate crosslinking agents besides aliphatic polyisocyanate compounds may be used in combination as the isocyanate crosslinking agent. Examples of other isocyanate crosslinking agents include alicyclic polyisocyanate compounds, aromatic polyisocyanate compounds, and aromatic aliphatic polyisocyanate compounds.

[0041] Alicyclic polyisocyanate compounds include alicyclic polyisocyanate compounds, polymers of alicyclic polyisocyanate compounds (dimers (biuret), trimers (isocyanurate), or pentamers), and adduct compounds of alicyclic polyisocyanate compounds and polyol compounds. Specific examples of alicyclic polyisocyanate compounds include isophorone diisocyanate (IPDI), hydrogenated tolylene diisocyanate, hydrogenated xylylene diisocyanate, hydrogenated 4,4'-diphenylmethane diisocyanate, and 4,4'-dicyclohexylmethane diisocyanate.

[0042] "Aromatic polyisocyanate compounds" include, for example, aromatic polyisocyanate compounds, polymers of aromatic polyisocyanate compounds (dimers (biuret), trimers (isocyanurate), or pentamers), and adduct compounds of aromatic polyisocyanate compounds and polyol compounds. Specific examples of aromatic polyisocyanate compounds include tolylene diisocyanate (TDI) and 4,4'-diphenylmethane diisocyanate.

[0043] In this disclosure, "aromatic aliphatic polyisocyanate compounds" are intended to be compounds having a structure in which an isocyanate group and an aromatic ring are bonded via an alkylene group in the molecule. Aromatic aliphatic polyisocyanate compounds include, for example, aromatic aliphatic polyisocyanate compounds, polymers of aromatic aliphatic polyisocyanate compounds (dimers (biuret), trimers (isocyanurate), or pentamers), and adduct compounds of aromatic aliphatic polyisocyanate compounds and polyol compounds. Examples of such aromatic aliphatic polyisocyanate compounds include o-xylenediisocyanate (XDI), m-xylenediisocyanate (XDI), and p-xylenediisocyanate (XDI).

[0044] The proportion of aliphatic isocyanate crosslinking agents in the isocyanate crosslinking agents is preferably 95% by mass or more, more preferably 97% by mass or more, and even more preferably 99% by mass or more. All isocyanate crosslinking agents may be aliphatic isocyanate crosslinking agents. The aliphatic isocyanate crosslinking agent preferably contains the adduct of hexamethylene diisocyanate. The proportion of the hexamethylene diisocyanate adduct in the isocyanate crosslinking agent is preferably 95% by mass or more, more preferably 97% by mass or more, and even more preferably 99% by mass or more. All of the isocyanate crosslinking agent may be the adduct of hexamethylene diisocyanate.

[0045] The content of isocyanate-based crosslinking agent per 100 parts by mass of the total resin component, coloring pigment, and colorant is set to 0.5 parts by mass to 10 parts by mass. If the content of isocyanate-based crosslinking agent per 100 parts by mass of the total resin component, coloring pigment, and colorant is 0.5 parts by mass or more, adhesion of the resin film formed by the laser marking composition of this disclosure to the adherend tends to be ensured. If the content of isocyanate-based crosslinking agent per 100 parts by mass of the total resin component, coloring pigment, and colorant is 10 parts by mass or less, the color development of the resin film formed by the laser marking composition of this disclosure tends to be improved. The content of isocyanate-based crosslinking agent per 100 parts by mass of the total of resin components, color pigments, and colorants is preferably 1.5 to 8 parts by mass, and more preferably 2 to 7.5 parts by mass.

[0046] (Coloring pigments) The laser marking compositions of this disclosure contain a color-developing pigment. The color-developing pigment is not particularly limited as long as it is a material that can exhibit black coloration upon laser irradiation. Preferably, the color-developing pigment is a metal oxide containing at least one metal selected from the group consisting of bismuth, antimony, molybdenum, copper, iron, nickel, chromium, zirconium, and neodymium. Among these, bismuth-containing compounds are preferred due to their excellent blackness when coloring, and bismuth(III) oxide (Bi2O3) is more preferred. In this case, laser printing Metal oxides with many oxygen vacancies are preferred to improve the properties of the material.

[0047] The volume-average particle size of the metal oxide is not particularly limited, but is preferably 0.05 μm to 30 μm, more preferably 0.1 μm to 15 μm, and even more preferably 0.3 μm to 1.5 μm. When the volume-average particle size of the metal oxide is 0.05 μm or larger, the metal oxide tends to absorb laser light and generate heat more easily, which tends to improve the color development during printing. On the other hand, when the volume-average particle size of the metal oxide is 30 μm or smaller, the dispersibility during film formation tends to be better. The volume-average particle size of the metal oxide refers to the value measured by the laser diffraction / light scattering method. The specific method for laser diffraction / light scattering is as follows: 5 mL of an aqueous dispersion of metal oxide is collected using a Pasteur pipette into a glass cell measuring 5 mm x 65 mm x 80 mm square, and this is placed in a laser diffraction / light scattering particle size distribution analyzer (for example, LA-960A (product name) manufactured by Horiba, Ltd.). After adjusting the concentration of the aqueous dispersion of metal oxide so that the transmittance of the laser light (red) is 80% to 90%, the average particle size of the metal oxide particles in the aqueous dispersion is determined by computer processing of the results measured under conditions of a measurement temperature of 25°C ± 1°C. The volume average value of the average particle size is used.

[0048] The content of the color-developing pigment in the solid content of the laser marking composition is preferably 0.2% to 50.0% by mass, more preferably 0.5% to 25.0% by mass, and even more preferably 1.0% to 5.0% by mass. If the color-developing pigment content is 0.2% by mass or more, the color develops appropriately during laser marking, and the readability of the laser-marked area tends to be good. If the color-developing pigment content is 50.0% by mass or less, dust generation during laser marking is suppressed, and the readability of the laser-marked area tends to be good. The color-developing pigment may be used in a dispersed state by a vehicle component such as acrylic resin. Furthermore, the color-developing pigment may contain additives such as dispersants.

[0049] (Coloring agent) The laser marking composition of this disclosure contains a coloring agent. The inclusion of a coloring agent in the laser marking composition makes it possible to impart any desired color to the resin film formed using the laser marking composition. The laser marking composition disclosed herein may contain a white pigment as a coloring agent in order to further improve visibility by increasing the contrast between the black color of the printed area and the white color of the non-printed area. Various inorganic pigments can be used as white pigments. For example, white pigments such as titanium dioxide (TiO2), titanium dioxide-coated mica, zinc oxide (zinc oxide), basic lead sulfate, zinc sulfide, and antimony oxide can be used. Alternatively, barium sulfate, barium carbonate, precipitated calcium carbonate, diatomaceous earth, talc, clay, basic magnesium carbonate, and alumina white may also be used as white pigments. Among these, titanium dioxide (TiO2) is preferred as a white pigment because of its excellent whiteness. Furthermore, the above-mentioned white pigments and aluminum may also be included, as they can reflect transmitted laser light and increase the efficiency of the reduction reaction of the color-developing pigment, thereby improving color development. The volume-average particle size of the white pigment is not particularly limited, but is preferably 0.01 μm to 50 μm, more preferably 0.05 μm to 30 μm, and even more preferably 0.1 μm to 15 μm. The volume-average particle size of the white pigment refers to the value measured by laser diffraction / light scattering.

[0050] If the laser marking composition of this disclosure contains a coloring agent, the amount of the coloring agent in the solid content of the laser marking composition is appropriately set depending on the type of coloring agent. When the laser marking composition of this disclosure contains a white pigment as a coloring agent, the content of the white pigment in the solid content of the laser marking composition is preferably 0.01% to 50% by mass, more preferably 0.1% to 30% by mass, and even more preferably 1% to 20% by mass. If the content of the white pigment relative to the solid content of the laser marking composition is 0.01% by mass or more, the reduction efficiency of the color-developing pigment can be improved, and visibility tends to be further improved. If the content of the white pigment relative to the solid content of the laser marking composition is 50% by mass or less, a decrease in the color development of the color-developing pigment tends to be prevented. The coloring agent may be used in a dispersed state by a vehicle component such as acrylic resin. Furthermore, the coloring agent may contain additives such as dispersants.

[0051] (Other ingredients) The laser marking compositions of this disclosure may contain various additives, to the extent that they do not impair the appearance and color development effects. Such additives include, for example, dispersants, light stabilizers, heat stabilizers, plasticizers, tackifiers, fillers, and colorants.

[0052] (Organic solvents) The laser marking compositions of this disclosure may contain organic solvents to improve coating workability. The organic solvent is not particularly limited as long as it dissolves or disperses the various components contained in the laser marking composition. Examples of organic solvents include alcohol-based organic solvents such as methanol, ethanol, n-propanol, isopropanol, and butanol; ketone-based organic solvents such as acetone, methyl ethyl ketone, and methyl isobutyl ketone; ester-based organic solvents such as methyl acetate, ethyl acetate, propyl acetate, and butyl acetate; aliphatic hydrocarbon-based organic solvents such as n-hexane, n-heptane, and n-octane; alicyclic hydrocarbon-based organic solvents such as cyclohexane, methylcyclohexane, ethylcyclohexane, cycloheptane, and cyclooctane; and aromatic hydrocarbon-based organic solvents such as toluene and xylene. One type of organic solvent may be used alone, or two or more types may be used in combination.

[0053] If the laser marking composition of this disclosure contains an organic solvent, the content of the organic solvent contained in the laser marking composition is preferably 40% to 90% by mass.

[0054] <Resin film> The resin film of this disclosure is made using the laser marking composition of this disclosure. The method for producing a resin film using the laser marking composition of this disclosure is not particularly limited, and the resin film can be formed by known methods using a single-layer T-die extruder, a multi-layer T-die extruder, a calendering machine, etc. Furthermore, a resin film can be formed by applying the laser marking composition of this disclosure, which contains an organic solvent, to one side of a substrate film described later and drying it. Examples of such application methods include screen printing, gravure printing, bar coating, knife coating, roll coating, comma coating, blade coating, die coating, and spray painting. The formed resin film may be cured. The laser marking composition of this disclosure contains an isocyanate-based crosslinking agent, which may cause the resin film to harden. Methods for hardening the resin film include drying with hot air, heating with a heating device such as an oven or hot plate, etc. The resin film of this disclosure may include the laser marking composition of this disclosure, or it may be a cured film of the laser marking composition of this disclosure. The average thickness of the resin film is not particularly limited and may be, for example, 2 μm to 100 μm.

[0055] <Laminate> The laminate of the present disclosure comprises the resin film of the present disclosure. The laminate of the disclosure may be a laminate used for laser marking labels. The layer configuration of the laminate is not particularly limited, and may consist of a first layer that transmits laser light, a second layer that changes color when exposed to laser light, and an optional third layer that is adhesive, stacked in this order. Alternatively, the first layer that transmits laser light and the adhesive second layer that changes color when exposed to laser light may be stacked in this order. When the laminate has such a configuration, it is preferable to use the resin film of the present disclosure as the second layer.

[0056] The laminates of this disclosure tend to have excellent appearance and color development due to having the resin film of this disclosure. In addition, they tend to have improved readability when one-dimensional or two-dimensional codes are printed on them.

[0057] The following describes the application of the laminate of this disclosure to a three-layer laser marking label with reference to Figure 1. Figure 1 is a schematic diagram showing an example of the cross-sectional structure of a laminate 1 according to one embodiment of this disclosure. As shown in Figure 1, the laminate 1 has a first layer 10, a second layer 20, and a third layer 30, and the first layer 10, second layer 20, and third layer 30 are stacked in this order. The second layer 20 is in contact with the first layer 10.

[0058] Here, we will explain laser marking on the laminate 1. First, laser light is irradiated onto the laminate 1 from the first layer 10 side. The irradiated laser light passes through the first layer 10 and acts on the second layer 20. Since the second layer 20 is formed from the resin film of this disclosure, the color-developing pigment develops color in the area of ​​the second layer 20 irradiated with laser light, and the resin carbonizes due to the heat of the laser light. The colored and carbonized areas in the second layer 20 become the printed areas in the laser marking label. The printed areas are the areas in the second layer 20 that have turned black. A laser marking label of this type, which contains a resin layer with a color-developing pigment inside the film and causes the resin layer to develop color by laser irradiation, is sometimes specifically referred to as an internal color-developing type laser marking label. In this disclosure, "laser marking" is not limited to the act of writing meaningful information such as letters or symbols onto the laminate 1, but rather refers to any act of coloring at least a portion of the second layer 20 of the laminate 1 by irradiating it with laser light.

[0059] The following describes each layer of the laminate, using laminate 1 according to one embodiment of this disclosure as an example.

[0060] [First layer 10] The first layer 10 is a layer that transmits laser light. In this disclosure, the first layer 10 may be referred to as the surface layer.

[0061] As the first layer 10, an optically transparent film is used. In this disclosure, "optically transparent" means, for example, that the transmittance of laser light is 50% or more and the transmittance of visible light is 80% or more. If the transmittance of visible light in the first layer 10 is sufficiently high, when the laminate 1 after laser marking is viewed from the first layer 10 side, the lower layer, the second layer 20, can be clearly seen through the first layer 10. The transmittance of laser light and visible light of the substrate film can be measured, for example, using a known spectrophotometer.

[0062] The resin used as the material for the base film as the first layer 10 may be either a thermoplastic resin or a thermosetting resin. More specifically, resins used as the material for the base film as the first layer 10 include, for example, (meth)acrylic copolymers, vinyl butyral resins, polyvinyl chloride resins, fluororesins, polyester resins, polystyrene resins, and thermoplastic polyurethane resins (TPU). These resins have excellent transparency, heat resistance, and handling properties. These resins may be used individually or in combination of two or more types.

[0063] Among the resins mentioned above, polyester resins are particularly suitable as the material for the base film because they can sufficiently transmit laser light and have good handling and heat resistance. By making the base film as the first layer 10 a polyester resin, the versatility of the laminate 1 can be increased and fine laser marking can be achieved.

[0064] From the viewpoint of suppressing deformation due to heat during laser marking, the polyester resin is preferably an aromatic ester resin. From the viewpoint of suppressing deformation due to heat during laser irradiation, the aromatic ester resin is more preferably a transparent resin.

[0065] Examples of aromatic ester resins include polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polycyclohexylene dimethylene terephthalate, and polyethylene naphthalate (PEN). Among these, from the aforementioned viewpoint, polyethylene terephthalate is more preferable as the aromatic ester resin.

[0066] There are no particular restrictions on the thickness of the first layer 10, but a thicker layer is preferable from the viewpoint of chemical resistance and abrasion resistance. The upper limit of the thickness of the first layer 10 can be set as appropriate from the viewpoint of workability and cost. For example, from the viewpoint of good workability (e.g., handling) when bonding the laminate 1 to the substrate, the thickness of the first layer 10 is preferably in the range of 10 μm to 200 μm.

[0067] Furthermore, the resin used as the material for the base film as the first layer 10 may contain various additives, to the extent that they do not impair print readability and adhesion. Such additives include, for example, dispersants, light stabilizers, heat stabilizers, plasticizers, fillers, and colorants. Furthermore, the first layer 10 may be subjected to corona treatment on the side having the second layer 20, or an easy-adhesion layer may be provided.

[0068] [Second layer 20] The second layer 20 is colored by laser light. In this disclosure, the second layer 20 may be referred to as the color-developing layer 20. The second layer 20 is composed of the resin film of this disclosure.

[0069] The thickness of the second layer 20 is not particularly restricted, but is preferably 2 μm to 100 μm, more preferably 10 μm to 70 μm, and even more preferably 15 μm to 50 μm. If the thickness of the second layer 20 is 2 μm or more, the printing can be clearly recognized. Furthermore, if the thickness of the second layer 20 is 15 μm or more, the resistance to laser light penetration and printability are improved. Furthermore, if the thickness of the second layer 20 is 100 μm or less, the productivity of the second layer 20 is improved.

[0070] [Third layer 30] The third layer 30 is adhesive. In this disclosure, the third layer 30 may be referred to as the adhesive layer 30.

[0071] The adhesive used in the third layer 30 should be able to adhere to a substrate such as a resin plate, metal plate, or glass plate, and should also be able to be peeled off from the substrate. Specifically, the adhesive strength of the adhesive used in the third layer 30 is preferably 0.1 N / 25 mm to 40 N / 25 mm, and more preferably 0.3 N / 25 mm to 30 N / 25 mm. If the adhesive strength of the adhesive is 0.1 N / 25 mm or more, good adhesion to the substrate can be obtained. Furthermore, if the adhesive strength of the adhesive is 40 N / 25 mm or less, the peelability of the adhesive will be good. The adhesive strength of the adhesive is measured by attaching a 25 mm wide laminate to an aluminum plate with a load of 2 kg, leaving it at 23°C for 24 hours, and then peeling the laminate from the aluminum plate at a peeling angle of 180°, a peeling speed of 300 mm / min, and a measurement temperature of 23°C.

[0072] The third layer 30 is made of a resin composition. Examples of resin compositions used in the third layer 30 include (meth)acrylic adhesives, silicone adhesives, and synthetic rubber adhesives, and a (meth)acrylic adhesive is more preferable from the viewpoint of improving the adhesion between the second layer 20 and the third layer 30.

[0073] There are no particular restrictions on the thickness of the third layer 30, but it is preferably in the range of 5 μm to 100 μm. If the thickness of the third layer 30 is within the above range, the workability (e.g., handling) when bonding the laminate 1 to the adherend will be improved.

[0074] Furthermore, the resin composition used in the third layer 30 may contain various additives, to the extent that they do not impair print readability and adhesion. Examples of such additives include dispersants, light stabilizers, heat stabilizers, plasticizers, tackifiers, fillers, and colorants. Metal oxide pigments are preferred as the coloring agent used in the third layer 30. Using metal oxide pigments tends to increase the opacity of the substrate and reduce laser penetration. Furthermore, the laser light is reflected by the metal oxide pigment, increasing the efficiency of the reduction reaction of the color-developing pigment present in the second layer 20, and as a result, the color development tends to improve. Examples of metal oxide pigments include, but are not limited to, metal oxides containing at least one metal selected from the group consisting of titanium, molybdenum, copper, iron, nickel, chromium, zirconium, and neodymium.

[0075] [Laser marking method for laminate 1] Laser marking on the laminate 1 can be performed by irradiating the laminate 1 with laser light from the first layer 10 side.

[0076] Lasers used for laser marking include, for example, near-infrared lasers with a wavelength of around 1000 nm; YVO4 lasers, YAG lasers, and fiber lasers. UV lasers with a wavelength of 300 nm to 400 nm can also be used.

[0077] Laser marking on the laminate 1 is usually performed before attaching the laminate 1 to the substrate. It is also possible to perform laser marking after attaching the laminate 1 to the substrate, but in this case, it is preferable that the laminate 1 has sufficient penetration resistance so as not to damage the substrate to which the laminate 1 is attached by laser irradiation.

[0078] [Method for manufacturing laminate 1] The laminate 1 can be manufactured by forming the first layer 10, the second layer 20, and the third layer 30 in that order. For example, the laminate 1 can be manufactured by a manufacturing method that includes at least a second layer forming step of forming the second layer 20 on one side of the first layer 10, and a third layer forming step of forming the third layer 30 on the side of the second layer 20 that is not in contact with the first layer 10 after the second layer forming step.

[0079] The second layer formation step may be a step of applying a laser marking composition used for the second layer 20 to one side of the base film, which is the first layer 10, and curing it to form the second layer 20. The method for forming the second layer 20 may be the same as the method for manufacturing a resin film described above.

[0080] The third layer formation step may be a step of applying the resin composition used for the third layer 30 to the side of the second layer 20 that is not in contact with the first layer 10 after the second layer formation step, and curing it to form the third layer 30. In another embodiment, the third layer formation step may be a step of applying the resin composition used for the third layer 30, curing it to form the third layer 30, and then bonding the third layer 30 to the side of the second layer 20 that is not in contact with the first layer 10 after the second layer formation step. The resin composition used for the third layer 30 is as described in the "Third Layer 30" section. In the manufacturing method of the laminate 1, the method of applying the resin composition used for the third layer 30 and the method of curing the resin composition used for the third layer 30 can also be carried out by known application and curing methods as described above.

[0081] In the method for manufacturing the laminate 1, a first layer formation step for forming the first layer 10 may be included before the second layer formation step, if necessary. [Examples]

[0082] The present disclosure will be described in more detail below based on examples, but the present disclosure is not limited to these examples.

[0083] [Polymerization Example 1] 70.0 parts by mass of ethyl acetate [organic solvent] was charged into the reaction vessel of a reaction apparatus equipped with a stirrer, reflux condenser, sequential dropper, and thermometer. In addition, 100.0 parts by mass of a monomer mixture consisting of 65.0 parts by mass of ethyl acrylate [EA], 21.0 parts by mass of methyl methacrylate [MMA], and 14.0 parts by mass of 2-hydroxyethyl methacrylate [2HEMA] was prepared in a separate container. 20.0% by mass of this prepared monomer mixture was placed in the reaction vessel, and then heated and refluxed at reflux temperature for 10 minutes. Next, under reflux conditions, the remaining 80.0% by mass of the monomer mixture, 50.0 parts by mass of ethyl acetate, and 0.026 parts by mass of 2,2'-azobisisobutyronitrile [AIBN; polymerization initiator] were sequentially added dropwise to the reaction vessel over 120 minutes. After the addition was complete, the reaction was allowed to continue for another 150 minutes to complete the reaction. The solution after the reaction was diluted with ethyl acetate to a solid content concentration of 35.0% by mass to obtain the (meth)acrylic resin solution of Polymerization Example 1. The (meth)acrylic resin of Polymerization Example 1 is referred to as Example 1 in Table 1. In this context, "solid content concentration" refers to the mass ratio of (meth)acrylic resin to the (meth)acrylic resin solution. The weight-average molecular weight (Mw) of the (meth)acrylic resin in polymerization example 1 was 230,000, and the glass transition temperature (Tg) was 7.8°C. The weight-average molecular weight of the (meth)acrylic resin was measured using the method described above. The glass transition temperature Tg of the (meth)acrylic resin is the value obtained by converting the absolute temperature (K) calculated using the above formula to Celsius temperature (°C).

[0084] [Polymerization Example 2] A monomer mixture consisting of 62.5 parts by mass of methyl methacrylate [MMA], 29.5 parts by mass of ethyl acrylate [EA], 7.0 parts by mass of 2-hydroxyethyl methacrylate [2HEMA], and 1.0 part by mass of acrylic acid [AA] was used to obtain the (meth)acrylic resin solution of Polymerization Example 2 in the same manner as in Polymerization Example 1, except that the amount of polymerization initiator, reaction time, reaction temperature, etc., were appropriately changed. The (meth)acrylic resin of Polymerization Example 2 is referred to as Example 2 in Table 1. The weight-average molecular weight (Mw) of the (meth)acrylic resin in polymerization example 2 was 170,000, and the glass transition temperature (Tg) was 55°C.

[0085] [Polymerization Example 3] Using the same monomer mixture as in Polymerization Example 1, the process was carried out in the same manner as in Polymerization Example 1, except that the amount of polymerization initiator, reaction time, reaction temperature, etc., were appropriately changed to obtain the (meth)acrylic resin solution of Polymerization Example 3. The (meth)acrylic resin of Polymerization Example 3 is referred to as Example 3 in Table 1. The weight-average molecular weight (Mw) of the (meth)acrylic resin in polymerization example 3 was 120,000, and the glass transition temperature (Tg) was 7.8°C.

[0086] [Polymerization Example 4] A monomer mixture consisting of 40.0 parts by mass of methyl methacrylate [MMA], 33.5 parts by mass of n-butyl methacrylate [nBMA], 13.5 parts by mass of butyl acrylate [BA], 13.0 parts by mass of 2-hydroxyethyl methacrylate [2HEMA], and 0.5 parts by mass of acrylic acid [AA] was prepared in 100.5 parts by mass. The process was carried out in the same manner as in Polymerization Example 1, except that the amount of polymerization initiator, reaction time, reaction temperature, etc. were appropriately changed to obtain the (meth)acrylic resin solution of Polymerization Example 4. The (meth)acrylic resin of Polymerization Example 4 is referred to as Example 4 in Table 1. The weight-average molecular weight (Mw) of the (meth)acrylic resin in polymerization example 4 was 10,000, and the glass transition temperature (Tg) was 44°C.

[0087] [Examples 1-11 and Comparative Examples 1-9] The (meth)acrylic resins from polymerization examples 1-4, crosslinking agents 1-3, coloring pigments, and colorants (white pigments) were blended in the amounts shown in Table 1, and the mixture was prepared with ethyl acetate to obtain the laser marking compositions of Examples 1-11 and Comparative Examples 1-9, with a solid content concentration of 33% by mass. In Table 1, the "Content" column for the crosslinking agent represents the content (by mass) of isocyanate-based crosslinking agent per 100 parts by mass of the total of resin components, color pigments, and colorants. In Table 1, the column for "Percentage of (meth)acrylic resin in the resin components" represents the total percentage of (meth)acrylic resin containing at least one of hydroxyl groups and carboxyl groups in the resin components. Details of the crosslinking agents 1-3, coloring pigments, and white pigments are as follows. • Crosslinking agent 1: Duranate E405-70B (adduct of hexamethylene diisocyanate (HMDI) and trimethylolpropane (TMP)), manufactured by Asahi Kasei Corporation. • Crosslinking agent 2: Coronate HK (isocyanurate derivative of HMDI), manufactured by Tosoh Corporation. • Crosslinking agent 3: Takenate D-131N (isocyanurate of XDI), manufactured by Mitsui Chemicals, Inc. • Color pigment 1: 42-970A (bismuth oxide pigment, manufactured by TOMATEC), vehicle resin content: 50% by mass • Coloring agent 1: NX-501 White (titanium dioxide pigment, manufactured by Dainichi Seika Kogyo Co., Ltd.), Vehicle resin content: 20.1% by mass

[0088] [evaluation] The obtained laser marking compositions were evaluated for pot life, color development, and adhesion using the following method. <Pot Life> The viscosity of a laser marking composition that is easy to apply is approximately 1000 mPa·s to 2000 mPa·s. Furthermore, if the laser marking composition thickens too quickly, the coating appearance may deteriorate. According to the inventors' findings, there is a correlation between pot life and the appearance of the colored layer; colored layers formed with laser marking compositions that have a poor pot life tend to have a poor appearance. Therefore, instead of evaluating the appearance, this study confirmed the degree of viscosity increase of the laser marking composition to verify that there were no problems with the pot life of the laser marking composition. (viscosity measurement) The initial viscosity of the laser marking composition (adjusted to 1000 mPa·s to 2000 mPa·s) and the viscosity after storage at 25°C for 8 hours were measured. Viscosity was measured using a TV-10 viscometer (manufactured by Toki Sangyo Co., Ltd.) at 25°C and 10 rpm. The viscosity increase rate was calculated based on the viscosity after 8 hours of storage and the initial viscosity, and the pot life was evaluated based on the following criteria. The results are shown in Table 1. A: The viscosity increase is less than 1.5 times. B: The viscosity increase is 1.5 times or more but less than 1.8 times. C: The viscosity is increased by 1.8 times or more.

[0089] <Color development> A polyethylene terephthalate (PET) film (surface layer, manufactured by Toyobo Co., Ltd., product name "A4360", average thickness 50 μm) was subjected to corona treatment on both sides. A laser marking composition was applied to one side of the surface layer so that the film thickness after drying was 30 μm. The laser marking layer (coloring layer) was formed by drying at 70°C for 3 minutes and then at 150°C for 3 minutes. The film was then cured for one week at 23°C and 55% RH to prepare evaluation samples. Printing was performed by irradiating the surface layer of the evaluation sample with laser light. Equipment: LP-Z130 (FAYb laser, manufactured by Panasonic Devices SUNX) Design: 15mm square solid Laser power: 3.75 (overall condition 7.5 x correction 50%) Scanning speed: 1500mm / s (overall conditions 1500mm / s x 100% compensation) Print pulse period: 20 μs (overall condition 20 μs × correction 100%) Line width: 0.07mm The following method was used to measure the color of the solid color image. A Konica Minolta CM-3600A spectrophotometer was used to measure the color of the surface layer on the day of printing using the SCE (Specular Reflectance Removal) method. Both the solid print area and the unprinted area were measured three times each, and the average of these measurements was taken as the respective color value. The viewing angle was set to 10°, and a D65 light source was used. A standard white plate (calibrated by NIST) was placed in the background during measurement. The difference ΔL between the color value of the solid print area and the color value of the unprinted area was calculated. The results are shown in Table 1.

[0090] <Adhesion> An adhesive layer was bonded to the coated surface of the laser marking composition of the aforementioned evaluation sample to obtain an evaluation sample with adhesive. The adhesive layer was prepared by coating a release-treated PET with an adhesive solution prepared by mixing 100 parts by mass of acrylic adhesive PE-121 (manufactured by Nippon Carbide Industries Co., Ltd.), 0.53 parts by mass of crosslinking agent CK-401 (manufactured by Nippon Carbide Industries Co., Ltd.), and ethyl acetate to an appropriate viscosity, and then drying it. Next, the PET film on the adhesive-coated evaluation sample was peeled off, and the other side of the adhesive layer was attached to an aluminum plate. After standing in a 50°C environment for 24 hours, the sample was removed and left in a 23°C, 55% humidity environment for 24 hours or more. Subsequently, rapid peeling was performed (the aluminum plate was held down with the left hand, and the adhesive-coated evaluation sample was forcefully peeled off with the right hand). The adhesion between the surface layer and the colored layer was visually confirmed and evaluated according to the following criteria. The results obtained are shown in Table 1. A: No peeling. B: Some peeling is visible, but it does not affect practical use. C: Peeling is observed throughout.

[0091] [Table 1]

[0092] From Table 1, the following can be seen: In Comparative Examples 1-3, where crosslinking agent 1, an aliphatic isocyanate-based crosslinking agent, was used and the content of the isocyanate-based crosslinking agent exceeded 10 parts by mass relative to 100 parts by mass of the total resin component, color pigment, and colorant, the color development of the colored layer was inferior. In Comparative Example 4, where crosslinking agent 1, an aliphatic isocyanate-based crosslinking agent, was used and the content of the isocyanate-based crosslinking agent was less than 0.5 parts by mass per 100 parts by mass of the total resin component, color pigment, and colorant, the adhesion of the colored layer was poor. Comparative Example 5, a laser marking composition using crosslinking agent 2, which is an aliphatic isocyanate crosslinking agent, and in which the content of the isocyanate crosslinking agent exceeds 10 parts by mass per 100 parts by mass of the total of the resin component, color pigment, and colorant, had a short pot life and poor appearance of the colored layer. In Comparative Example 6, which used crosslinking agent 2, an aliphatic isocyanate-based crosslinking agent, and in which the content of the isocyanate-based crosslinking agent was less than 0.5 parts by mass per 100 parts by mass of the total resin component, color pigment, and colorant, the adhesion of the colored layer was poor. The laser marking compositions of Comparative Examples 7-9, which used crosslinking agent 3, an aromatic isocyanate-based crosslinking agent, exhibited short pot life and poor appearance of the colored layer. Furthermore, Comparative Example 9, which had the lowest content of the isocyanate-based crosslinking agent among Comparative Examples 7-9, showed poor adhesion of the colored layer. On the other hand, the colored layer formed using the laser marking composition of the example exhibited excellent appearance, color development, and adhesion. [Explanation of symbols]

[0093] 1. Laminate 10 First layer (surface layer) 20. Second layer (color development layer) 30 Third layer (adhesive layer)

Claims

1. It contains a resin component comprising at least one (meth)acrylic resin containing at least one of a hydroxyl group and a carboxyl group, an isocyanate crosslinking agent comprising an aliphatic isocyanate crosslinking agent, a color pigment, and a coloring agent. A laser marking composition in which the isocyanate-based crosslinking agent is contained in an amount of 0.5 to 10 parts by mass relative to 100 parts by mass of the total of the resin component, the color pigment, and the coloring agent.

2. The laser marking composition according to claim 1, wherein the proportion of the aliphatic isocyanate crosslinking agent in the isocyanate crosslinking agent is 95% by mass or more.

3. The laser marking composition according to claim 1, wherein the aliphatic isocyanate crosslinking agent comprises an adduct of hexamethylene diisocyanate.

4. The laser marking composition according to claim 3, wherein the proportion of the hexamethylene diisocyanate adduct in the isocyanate crosslinking agent is 95% by mass or more.

5. The laser marking composition according to claim 1, wherein the total proportion of at least one (meth)acrylic resin containing at least one of the hydroxyl group and the carboxyl group in the resin component is 70% by mass or more.

6. A resin film made using the laser marking composition according to any one of claims 1 to 5.

7. A laminate having a resin film as described in claim 6.

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