Laser marking compositions, resin films, and laminates

Abstract

This laser marking composition contains: at least one (meth)acrylic resin; and a metal oxide including at least one metal selected from the group consisting of bismuth, antimony, molybdenum, copper, iron, nickel, chromium, zirconium, and neodymium. With respect to the entire structural units of the (meth)acrylic resin, the proportion of structural units derived from an acrylic acid alkyl ester including an alkyl group having 1-4 carbons is 55 mass% or more.

Description

Technical Field

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

Background Art

[0002] For various packages such as foods and pharmaceuticals, and various members such as electronic members, it is necessary to print variable information for traceability such as manufacturing lot numbers and manufacturing dates. As one of the marking methods for this purpose, a laser marking method may be used. In particular, a color-developing type laser marking method in which a resin or a pigment is discolored by a laser for marking has been used in various places in recent years because printing can be performed without generating odors or dust.

[0003] The following compositions are known as labels and inks for laser marking. For example, Patent Document 1 discloses an adhesive containing an adhesive resin (A) and a bismuth-based laser color former (B) as an adhesive capable of forming an adhesive layer with good contrast after printing and suppressing coloring. Further, Patent Document 2 discloses a laser marking ink composition sufficiently provided with laser printability (visibility), blocking resistance, adhesiveness, and laminate strength, containing a binder resin, a white pigment, and an organic solvent, wherein the binder resin contains a polyurethane resin and a cellulose derivative, the cellulose derivative is a lower acyl group-substituted cellulose derivative and / or a lower alkyl group-substituted cellulose derivative, and the white pigment is titanium oxide having an average particle diameter of 0.26 μm or less.

[0004] Patent Document 1: Japanese Patent No. 6292429 <000​​​​​​​​​​Resin compositions that can be blackened by laser irradiation utilize the reduction reaction of inorganic oxides caused by laser light to produce color. During this process, the heat generated by the reduction can cause carbonization of the resin and the generation of gases. As a result, the carbonization of the resin may spread more than intended, or the laminate may swell, making it easy for one-dimensional and two-dimensional codes to be misread. For example, the adhesive using a bismuth-based laser colorant disclosed in Patent Document 1 may cause deformation of the printed text due to the heat generated during printing, which may make it difficult to read the printed content. Furthermore, in the laser marking ink composition disclosed in Patent Document 2, the urethane resin is prone to carbonization, which can cause the resin surrounding the inorganic oxide to carbonize, making it difficult to read the printed content depending on the material. In addition, the (meth)acrylic copolymer listed as a comparative example in Patent Document 2 has methacrylic resin as its main component, which can lead to problems such as gas generation and swelling during printing. This disclosure is made in view of the above-mentioned conventional circumstances, and aims to provide a laser marking composition capable of forming a resin film that has excellent visibility and readability when printing one-dimensional or two-dimensional codes, and suppresses gas generation during printing, 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 at least one (meth)acrylic resin and a metal oxide containing at least one metal selected from the group consisting of bismuth, antimony, molybdenum, copper, iron, nickel, chromium, zirconium, and neodymium. A laser marking composition wherein the proportion of structural units derived from alkyl acrylate containing alkyl groups having 1 to 4 carbon atoms to the total structural units of the (meth)acrylic resin is 55% by mass or more. <2> The total proportion of structural units derived from methacrylic acid and structural units derived from alkyl methacrylate to the total structural units of the (meth)acrylic resin is less than 45% by mass. <1> The laser marking composition described above. <3> The total proportion of structural units derived from ethyl acrylate, methyl acrylate, and 2-hydroxyethyl acrylate to the total structural units of the (meth)acrylic resin is 20% by mass or more. <1> or <2> The laser marking composition described above. <4> The metal oxide includes a bismuth-containing compound. <1> ~ <3> A laser marking composition according to any one of the following items. <5> <1> ~ <4> A resin film made using the laser marking composition described in any one of the items. <6> <5> A laminate having the resin film described above. [Effects of the Invention]

[0007] According to this disclosure, it is possible to provide a laser marking composition capable of forming a resin film that has excellent visibility and readability when printing one-dimensional or two-dimensional codes, and that suppresses gas generation during printing, 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 embodiments of this disclosure are described in detail below. However, this disclosure is not limited to the embodiments described below. In the embodiments described below, the components (including elemental steps, etc.) are not essential unless otherwise specified. The same applies to numerical values ​​and their ranges, and do not limit this 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 in the laser marking composition or sample solution excluding the organic solvent.

[0011] <Laser marking composition> The laser marking composition of this disclosure contains at least one (meth)acrylic resin and a metal oxide containing at least one metal selected from the group consisting of bismuth, antimony, molybdenum, copper, iron, nickel, chromium, zirconium, and neodymium, wherein the proportion of structural units derived from alkyl acrylate containing alkyl groups having 1 to 4 carbon atoms to the total structural units of the (meth)acrylic resin is 55% by mass or more. The laser marking composition disclosed herein makes it possible to form a resin film that has excellent visibility and readability when printing one-dimensional or two-dimensional codes, and suppresses gas generation during printing. The reason for this is not clear, but it is presumed to be as follows. Comparing the structural units derived from alkyl acrylates that can be contained in (meth)acrylic resin with those derived from alkyl methacrylates, the difference lies in whether or not a methyl group is directly bonded to the carbon atoms constituting the main chain of the (meth)acrylic resin. Carbon atoms directly bonded to a methyl group become tertiary carbons. Decomposition of (meth)acrylic resin is more likely to occur by laser irradiation at locations where tertiary carbons constituting the main chain of the (meth)acrylic resin are present. If the (meth)acrylic resin contains a large amount of structural units derived from alkyl methacrylates, gases derived from decomposition products are more likely to be generated. In the present disclosure, since the proportion of the structural units derived from acrylic acid alkyl esters containing an alkyl group having 1 to 4 carbon atoms in all the structural units of the (meth)acrylic resin is 55% by mass or more, the proportion of tertiary carbon atoms among the carbon atoms constituting the main chain in the (meth)acrylic resin can be maintained relatively low, and it is considered that the generation of gas derived from decomposition products can be easily suppressed. Further, as the generation of gas is suppressed, the generation of swelling of the resin film made of the laser marking composition can be easily suppressed. Further, since the carbon number of the alkyl group contained in the structural unit derived from acrylic acid alkyl ester is 1 to 4, the glass transition temperature of the (meth)acrylic resin is unlikely to be low. Therefore, deformation of the resin film due to heat generated by the reduction of the inorganic oxide by laser irradiation can be easily suppressed. As a result, it is presumed that the visibility and the readability when printing a one-dimensional code or a two-dimensional code are improved.

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

[0013] ((Meth)acrylic resin) The laser marking composition of the present disclosure contains at least one kind of (meth)acrylic resin, and the proportion of the structural units derived from acrylic acid alkyl esters containing an alkyl group having 1 to 4 carbon atoms in all the structural units of the (meth)acrylic resin is 55% by mass or more. The proportion of the structural units derived from acrylic acid alkyl esters containing an alkyl group having 1 to 4 carbon atoms in all the structural units of the (meth)acrylic resin is preferably 56% by mass or more, more preferably 60% by mass or more, and still more preferably 90% by mass or more. The proportion of the structural units derived from acrylic acid alkyl esters containing an alkyl group having 1 to 4 carbon atoms in all the structural units of the (meth)acrylic resin may be 99% by mass or less. The proportion of the structural units derived from acrylic acid alkyl esters containing an alkyl group having 1 to 4 carbon atoms in all the structural units of the (meth)acrylic resin is preferably 55% to 99% by mass.

[0014] When the laser marking composition of the present disclosure contains one type of (meth)acrylic resin, as long as it is a (meth)acrylic resin that satisfies the above conditions, it may be a homopolymer composed of structural units derived from a single (meth)acrylic monomer, or it may be a copolymer composed of structural units derived from two or more types of (meth)acrylic monomers. In addition, when the laser marking composition of the present disclosure contains two or more types of (meth)acrylic resins, as long as the ratio of the structural units derived from alkyl acrylates containing an alkyl group having 1 to 4 carbon atoms in all the structural units contained in the two or more types of (meth)acrylic resins is 55% by mass or more, two or more types of homopolymers having different structural units may be used in combination, or at least one homopolymer and at least one copolymer may be used in combination, or two or more types of copolymers having different structural units may be used in combination. Further, when the laser marking composition of the present disclosure contains two or more types of (meth)acrylic resins, at least one (meth)acrylic resin in which the ratio of the structural units derived from alkyl acrylates containing an alkyl group having 1 to 4 carbon atoms in all the structural units of the (meth)acrylic resin is 55% by mass or more and at least one (meth)acrylic resin in which the ratio is less than 55% by mass may be used in combination.

[0015] Here, the (meth)acrylic monomer means at least one of derivatives of acrylic acid such as acrylic acid and alkyl acrylates, and derivatives of methacrylic acid such as methacrylic acid and alkyl methacrylates. The derivatives of acrylic acid and methacrylic acid may have substituents such as a hydroxyl group, an amino group, a carboxyl group, and a glycidyl group. In addition, other monomers other than the (meth)acrylic monomer may be used for the (meth)acrylic resin.

[0016] Specific examples of (meth)acrylic monomers include methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, i-propyl (meth)acrylate, n-butyl (meth)acrylate, i-butyl (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.

[0017] 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, polypropylene glycol mono(meth)acrylate, polyethylene glycol mono(meth)acrylate, poly(ethylene glycol-propylene glycol) mono(meth)acrylate, and pentaerythritol tri(meth)acrylate.

[0018] Other monomers include crotonic acid, maleic anhydride, fumaric acid, itaconic acid, glutaconic acid, and citraconic acid, which contain a carboxyl group. Other monomers that do not contain a carboxyl group include vinyl acetate, vinyl ether, acrylonitrile, and styrene.

[0019] Preferred alkyl acrylates containing alkyl groups with 1 to 4 carbon atoms include ethyl acrylate, methyl acrylate, n-butyl acrylate, i-butyl acrylate, t-butyl acrylate, and other butyl acrylates, as well as 2-hydroxyethyl acrylate.

[0020] In this disclosure, in some embodiments, the total proportion of structural units derived from ethyl acrylate, methyl acrylate, and 2-hydroxyethyl acrylate to the total structural units of the (meth)acrylic resin is preferably 20% by mass or more, more preferably 30% by mass or more, even more preferably 40% by mass or more, and particularly preferably 65% ​​by mass or more. The total proportion of structural units derived from ethyl acrylate, methyl acrylate, and 2-hydroxyethyl acrylate to the total structural units of the (meth)acrylic resin may be 99% by mass or less. The total proportion of structural units derived from ethyl acrylate, methyl acrylate, and 2-hydroxyethyl acrylate to the total structural units of the (meth)acrylic resin is preferably 20% by mass to 99% by mass. Ethyl acrylate, methyl acrylate, and 2-hydroxyethyl acrylate have high glass transition temperatures when homopolymerized. Therefore, in (meth)acrylic resins, the main chain of the (meth)acrylic resin is less likely to move even if heat is generated by laser irradiation in the structural units derived from ethyl acrylate, methyl acrylate, and 2-hydroxyethyl acrylate. As a result, it tends to be easier to print with high precision. Furthermore, in other embodiments of this disclosure, the total proportion of structural units derived from ethyl acrylate, methyl acrylate, and 2-hydroxyethyl acrylate to the total structural units of the (meth)acrylic resin may be 1% by mass or less.

[0021] In this disclosure, the total proportion of structural units derived from methacrylic acid and structural units derived from alkyl methacrylate to the total structural units of the (meth)acrylic resin is preferably less than 45% by mass, more preferably 40% by mass or less, even more preferably 35% by mass or less, and particularly preferably 5% by mass or less. The total proportion of structural units derived from methacrylic acid and structural units derived from alkyl methacrylate to the total structural units of the (meth)acrylic resin may be 0% by mass or more. The total proportion of structural units derived from methacrylic acid and structural units derived from alkyl methacrylate to the total structural units of the (meth)acrylic resin is preferably 0% by mass or more and less than 45% by mass. If the total proportion of structural units derived from methacrylic acid and alkyl methacrylate to the total structural units of the (meth)acrylic resin is less than 45% by mass, the generation of gases derived from decomposition products that may occur during the decomposition of the (meth)acrylic resin tends to be more easily suppressed.

[0022] The total proportion of structural units derived from monomers containing a carboxyl group in their molecule, such as acrylic acid, methacrylic acid, and other monomers containing a carboxyl group, to the total structural units of the (meth)acrylic resin is preferably 20% by mass or less, more preferably 10% by mass or less, and even more preferably 5% by mass or less. The total proportion of structural units derived from monomers containing a carboxyl group in their molecule to the total structural units of the (meth)acrylic resin may be 0.5% by mass or more. The total proportion of structural units derived from monomers containing a carboxyl group in their molecule to the total structural units of the (meth)acrylic resin is preferably 0.5% by mass to 20% by mass. When the total proportion of structural units derived from monomers containing carboxyl groups in the molecule to the total structural units of (meth)acrylic resin is 20% by mass or less, it is possible to suppress the attenuation of absorbance caused by the reaction of metal oxides reduced by laser irradiation with carboxyl groups, and visibility tends to improve.

[0023] 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.

[0024] 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.

[0025] 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.

[0026] ~Conditions~ Measurement device: High-speed GPC (Model number: HLC-8220 GPC, Tosoh Corporation) Detector: Differential Refractometer (RI) (integrated into HLC-8220, 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

[0027] 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 Inc.) 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 defined as the glass transition temperature (Tg) of the (meth)acrylic resin.

[0028] 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).

[0029]

number

[0030] In the formula, Tg1, Tg2, ... and Tg n m1, m2, ..., and m n These represent the mole fractions of each monomer.

[0031] 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).

[0032] 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.

[0033] 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.

[0034] 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.

[0035] 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.

[0036] 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.

[0037] More specifically, organic solvents used in polymerization reactions include, for example, aromatic hydrocarbon organic solvents represented by benzene, toluene, ethylbenzene, n-propylbenzene, t-butylbenzene, o-xylene, m-xylene, p-xylene, tetralin, decalin, and aromatic naphtha; aliphatic or alicyclic hydrocarbon organic solvents represented by n-hexane, n-heptane, n-octane, i-octane, n-decane, dipentene, petroleum spirits, petroleum naphtha, and turpentine oil; ester organic solvents represented by 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-i-butyl 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, i-propyl alcohol, n-butyl alcohol, i-butyl alcohol, s-butyl alcohol, and t-butyl alcohol.

[0038] Examples of polymerization initiators include organic peroxides and azo compounds that can be used in conventional polymerization methods.

[0039] (Meth)acrylic resin may be a commercially available product. Examples of commercially available (meth)acrylic resins include KP-1876E (product name: Nissetsu®, manufactured by Nippon Carbide Industries Co., Ltd.) and H-4002 (manufactured by Negami Industries Co., Ltd.).

[0040] 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.

[0041] (Metal oxides) The laser marking compositions of this disclosure contain a metal oxide comprising at least one metal selected from the group consisting of bismuth, antimony, molybdenum, copper, iron, nickel, chromium, zirconium, and neodymium. The metal oxide functions as a coloring pigment. Among these, bismuth-containing compounds are preferred due to their excellent blackness during color development, and bismuth(III) oxide (Bi2O3) is more preferred. In this case, metal oxides with many oxygen vacancies are preferred to improve laser printability.

[0042] 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: A 5 mL 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) from 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 average particle size value used is the volume average.

[0043] The content of metal oxide 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 metal oxide content is 0.2% by mass or more, the laser marking tends to develop color appropriately and the readability of the laser marking area tends to be good. If the metal oxide content is 50.0% by mass or less, dust generation during laser marking is suppressed, which tends to result in good readability of the laser marking area.

[0044] (Crosslinking agent) The laser marking compositions of this disclosure may contain a crosslinking agent to improve the strength of the resin film. Examples of crosslinking agents include isocyanate-based crosslinking agents, aluminum chelate-based crosslinking agents, epoxy-based crosslinking agents, and the like. When the laser marking composition contains a crosslinking agent, the (meth)acrylic resin preferably contains structural units derived from (meth)acrylic monomers having hydroxyl groups or other monomers containing carboxyl groups. When a laser marking composition contains a crosslinking agent, the amount of the crosslinking agent is preferably 0.1 to 10 equivalents relative to the total amount of hydroxyl and carboxyl groups of the (meth)acrylic resin. A crosslinking agent content of 0.1 equivalents or more suppresses molecular activity and improves printing accuracy. A crosslinking agent content of 10 equivalents or less suppresses discoloration of the (meth)acrylic resin. A crosslinking agent content of 0.3 to 3.0 equivalents is even more preferable because it facilitates film formation.

[0045] (White pigment) The laser marking composition disclosed herein may contain a white pigment 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, titanium dioxide-coated mica may be included because it reflects transmitted laser light, increasing the efficiency of the reduction reaction of metal oxides and 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 20 μm. The volume-average particle size of the white pigment refers to the value measured by laser diffraction / light scattering.

[0046] When the laser marking composition of this disclosure contains a white pigment, 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 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 is 50% by mass or less, a decrease in the color development of the metal oxide tends to be prevented.

[0047] (urethane resin) The laser marking composition disclosed herein may contain a urethane resin to improve printability when printing on the surface of a resin film. The inclusion of a urethane resin in the laser marking composition improves the adhesion of the printed layer formed on the surface of the resin film. The type of urethane resin is not particularly limited, and conventionally known urethane resins such as polycarbonate-based urethane resins, polyester-based urethane resins, and polyether-based urethane resins can be used. The urethane resin may be used alone or in combination of two or more types. When the laser marking composition of this disclosure contains a urethane resin, the content of the urethane resin in the solid content of the laser marking composition is preferably 2% to 75% by mass, more preferably 5% to 20% by mass, and even more preferably 10% to 15% by mass, from the viewpoint of improving printability. By setting the content of the urethane resin in the solid content of the laser marking composition to 75% by mass or less, laser printability can be maintained. By setting the content of the urethane resin in the solid content of the laser marking composition to 20% by mass or less, lamination suitability can be maintained.

[0048] Commercially available urethane resins can be used. Examples of commercially available urethane resins include, for example, "NE-8836 (polycarbonate type)", "NE-8811 (polycarbonate type)", "NE-8850 (polycarbonate type)" [all manufactured by Dainichi Seika Kogyo Co., Ltd.], as well as "Superflex 420 (polycarbonate type)", "Superflex 460 (polycarbonate type)", "Superflex 210 (polyester type)" [all manufactured by Daiichi Kogyo Seiyaku Co., Ltd.], "Pandex T-5275 (polyester type)", "Pandex T-9280 (polycarbonate type)", "Pandex T-9290 (polycarbonate type)", "Pandex T-1190 (polyester type)", and "Pandex T-8190 (polyether type)" [all manufactured by DIC Covestro Polymer Co., Ltd.].

[0049] (Filler) The laser marking composition of this disclosure may contain a filler to improve printability when printing on the surface of a resin film. The inclusion of a filler in the laser marking composition improves the slipperiness on the surface of the resin film, thereby improving operability when printing on the surface of the resin film and resulting in good printability. As fillers, known fillers such as inorganic particles like silica particles, and resin particles like acrylic beads and melamine beads can be used. Fillers may be used individually or in combination of two or more types.

[0050] The volume-average particle size of the filler is not particularly limited, but from the viewpoint of improving lubricity, it is preferably 0.5 μm to 25 μm, more preferably 1 μm to 15 μm, and even more preferably 2 μm to 10 μm. The volume-average particle size of the filler is measured by the same method as the volume-average particle size of the metal oxide described above. If the laser marking composition of this disclosure contains a filler, the filler content in the solid content of the laser marking composition is preferably 0.2% to 30.0% by mass, more preferably 0.5% to 20% by mass, and even more preferably 2% to 10% by mass, from the viewpoint of improving lubricity.

[0051] (Other ingredients) The laser marking compositions of this disclosure may contain other resins and various additives, to the extent that they do not impair visibility, readability when printing one-dimensional or two-dimensional codes, and the effect of suppressing gas generation during printing. 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 coating. If the laser marking composition contains a crosslinking agent, the resin film may be cured. Methods for curing the resin film include drying with hot air, heating with a heating device such as an oven or hot plate, etc. 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 present 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 a third layer that is adhesive, provided as needed, stacked in this order. Alternatively, the first layer that transmits laser light and the second layer that is adhesive and 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 laminate of this disclosure, having the resin film of this disclosure, tends to suppress gas generation in the second layer during laser marking. Consequently, odor generation is suppressed. Furthermore, visibility and readability when printing one-dimensional or two-dimensional codes tend to improve.

[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 metal oxide in the area of ​​the second layer 20 irradiated with laser light changes color, 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. In this way, a laser marking label of the type in which a resin layer containing a metal oxide is contained inside the film and the resin layer changes color by laser irradiation is sometimes specifically referred to as an internally colored 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. [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.

[0070] 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 10 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.

[0071] 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.

[0072] 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.

[0073] 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 metal oxide present in the second layer 20, which tends to improve color development. 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.

[0074] [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.

[0075] 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.

[0076] 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.

[0077] [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.

[0078] The second layer formation step may be a step of applying a laser marking composition to be used for the second layer 20 to one side of the substrate film as the first layer 10, and curing it as necessary 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.

[0079] 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.

[0080] 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.

[0081] [Other embodiments] The laminates of this disclosure are not limited to laminate 1 applied to a laser marking label having a three-layer structure having a first layer, a second layer, and a third layer. The laminates of this disclosure may be laminates consisting of a second layer and a third layer without a first layer, or laminates having a second layer, a third layer, and other layers without a first layer, or laminates having a first layer, a second layer, a third layer, and other layers.

[0082] Other layers include coloring layers, printing layers, and easy-adhesion layers. A colored layer, for example, is placed between the second and third layers to give the entire laminate color, pattern, etc. The addition of a colored layer improves the aesthetic appeal of the laminate. The colored layer may include a layer containing a resin and a coloring agent. The resin contained in the colored layer is not particularly limited and may include resins similar to those used in the first layer. The coloring agent contained in the colored layer is not particularly limited and may include pigments, dyes, etc. The thickness of the colored layer is not particularly limited, and can range from 1 μm to 50 μm, for example. The colored layer may be formed by applying a resin composition for colored layer formation to the surface of the second layer facing the third layer, or it may be formed separately and then bonded to the surface of the second layer facing the third layer. When bonding the colored layer to the surface of the second layer, an adhesive layer may be further provided between the colored layer and the second layer.

[0083] The printed layer is, for example, a layer provided between the second and third layers and formed by a printing press. Specifically, for example, a resin composition containing resin, colorants, solvents, etc., is applied to the surface of a layer adjacent to the printed layer in accordance with the desired pattern, characters, etc., and the printed layer is formed by drying, curing, etc., as necessary. Providing a printed layer improves the design of the laminate. The printed layer may be provided only on a part of the laminate surface direction, or it may be provided on the entire surface. Printing methods include inkjet printing, screen printing, gravure printing, and flexographic printing.

[0084] The printed layer is formed, for example, by printing on the surface of the second layer that is on the side facing the third layer. If the laminate has a colored layer, the printed layer may be provided, for example, between the second layer and the colored layer, and may be formed by printing on the colored layer side of the second layer, or by printing on the second layer side of the colored layer. When forming a printed layer by printing on the surface of the second layer, it is preferable that the laser marking composition used to form the second layer contains at least one of the aforementioned urethane resin and filler. [Examples]

[0085] The present disclosure will be described in more detail below based on examples, but the present invention is not limited to these examples.

[0086] [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; alkyl acrylate monomer having an alkyl group with 1 to 4 carbon atoms], 21.0 parts by mass of methyl methacrylate [MMA; alkyl methacrylate monomer], and 14.0 parts by mass of 2-hydroxyethyl methacrylate [2HEMA; alkyl methacrylate monomer having a hydroxyl group] was prepared in a separate container. 20.0% by mass of this prepared monomer mixture was charged into 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. In this context, "solid content concentration" refers to the mass ratio of (meth)acrylic resin to the (meth)acrylic resin solution. Table 1 also lists the weight-average molecular weight (Mw), glass transition temperature (Tg) of the (meth)acrylic resin in polymerization example 1, as well as the percentage of structural units derived from alkyl acrylates containing alkyl groups with 1 to 4 carbon atoms (A, mass%), the total percentage of structural units derived from ethyl acrylate, methyl acrylate, and 2-hydroxyethyl acrylate (A-1, mass%), and the total percentage of structural units derived from alkyl methacrylate (B, mass%), out of the total structural units of the (meth)acrylic resin. The weight-average molecular weight of the (meth)acrylic resin solution is the value measured by 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 by the above formula to Celsius temperature (°C).

[0087] [Polymerization Example 2 - Synthesis of Polymer 10] In the synthesis of Polymer 1, the (meth)acrylic resin solutions of Polymerization Examples 2 to 10 were obtained in the same manner as in Polymerization Example 1, except that the monomers listed in Table 1 were used. In Table 1, MA is methyl acrylate (alkyl acrylate monomer having an alkyl group with 1 to 4 carbon atoms), BA is butyl acrylate (alkyl acrylate monomer having an alkyl group with 1 to 4 carbon atoms), and 2HEA is 2-hydroxyethyl acrylate (1 to 4 carbon atoms). nBMA represents n-butyl methacrylate (alkyl acrylate monomer having an alkyl group having a hydroxyl group), 2EHA represents 2-ethylhexyl acrylate (alkyl acrylate monomer), and AA represents acrylic acid.

[0088] [Table 1]

[0089] [Examples 1-16 and Comparative Examples 1-3] The components listed in Table 2 were blended in the proportions (parts by mass) listed in Table 2, and the mixture was prepared with ethyl acetate to obtain the laser marking compositions of Examples 1 to 16 and Comparative Examples 1 to 3, with a solid content concentration of 20% by mass. In addition, Table 2 shows the proportion of structural units derived from alkyl acrylates containing alkyl groups with 1 to 4 carbon atoms (A, by mass%), the total proportion of structural units derived from ethyl acrylate, methyl acrylate, and 2-hydroxyethyl acrylate (A-1, by mass%), and the total proportion of structural units derived from alkyl methacrylate (B, by mass%), out of the total structural units of the (meth)acrylic resin. In Table 2, Polymerization Examples 1-10 refer to the solid content of (meth)acrylic resin. Also, in Table 2, "white pigment" refers to the solid content of titanium dioxide pigment. The details of each component listed in Table 2 are as follows: • Color pigment 1: Bismuth oxide-based color pigment (42-970A, TOMATEC Corporation) • Color pigment 2: Iron oxide-based color pigment (NX-512 Yellow, pigment manufactured by Dainichi Seika Kogyo Co., Ltd., 56% by mass of iron oxide) • White pigment: NBK-967 White (58.5% by mass of titanium dioxide pigment, 7% by mass of cellulose acetate butyrate, methyl isobutyl ketone, Nikko Bix Co., Ltd.)

[0090] A 50 μm thick PET film (surface layer) was subjected to corona treatment on both sides. A laser marking composition was applied to one side of the PET film so that the film thickness after drying was as shown in Table 2. The laser marking layer (coloring layer) was formed by drying at 70°C for 3 minutes and then at 150°C for 3 minutes. 100 parts by mass of acrylic resin PE-121 (manufactured by Nippon Carbide Industries Co., Ltd.) was mixed with 0.53 parts by mass of crosslinking agent CK-401 (manufactured by Nippon Carbide Industries Co., Ltd.), and after mixing with ethyl acetate to an appropriate viscosity, the mixture was coated to a thickness of 20 μm onto release-treated PET (75E0010GT, manufactured by Fujimori Industries Co., Ltd.), and heated at 100°C for 1 minute to form an adhesive layer on the release-treated PET. The adhesive side of this adhesive layer was bonded to the laser marking layer to obtain the laser marking laminates for each example and comparative example. The following evaluations were performed on the resulting laminate for laser marking.

[0091] [Printability] Using a FAYb laser marker LP-Z130 (manufactured by Panasonic Corporation), a 15mm square filled pattern was printed on the surface layer of a laser marking laminate by irradiating it with laser light under the conditions of output (printing intensity) 25%, pulse period 50Hz, line width 0.07mm, and 2000mm / second. Afterward, the laser marking laminate was attached to a glass plate, and the color difference between the laminate itself and the printed area was measured using a colorimeter (product name "Spectrophotometer CM-3600A", manufactured by Konica Minolta, Inc.) with an opacity test paper specified in JIS K 5600-4-1:1999 placed on the back of the glass side. * ab was calculated. The results are shown in Table 2. ΔE * If ab is 5 or greater, it is practically acceptable. ΔE * The larger the value of ab, the better the visibility.

[0092] [2D barcode readability] Using a FAYb laser marker LP-Z130 (manufactured by Panasonic Corporation), 4mm square and 8mm square two-dimensional codes were printed on the surface layer of a laminate by irradiating it with laser light under the following conditions: output (print intensity) of 20%, 30%, and 50%, pulse period of 50Hz, line width of 0.07mm, and speed of 2000mm / second. Subsequently, a code reader (product name SR-H60W, manufactured by Keyence Corporation) was used to perform 100 reading tests, and the results were evaluated according to the following criteria. A rating of B or higher indicates that the product is practically acceptable. S: The success rate for reading 8mm square objects is over 80%. A: The success rate for reading 8mm square objects is 50% or higher. B: The success rate for reading 4mm square objects is 90%. C: The success rate for reading 4mm square objects is 50% or higher. The success rate for reading 4mm square objects (D) is less than 50%.

[0093] [Fukure] Using a FAYb laser marker LP-Z130 (manufactured by Panasonic Corporation), laser light was irradiated onto the surface layer of a laminate to print 4mm square and 8mm square two-dimensional codes under the following conditions: output (print intensity) of 20%, 30%, and 50%, pulse period of 50Hz, line width of 0.07mm, and speed of 2000mm / second. During this process, the occurrence of blistering between the release-treated PET and adhesive layers was observed visually and by touch, and evaluated according to the following criteria. A rating of B or higher indicates that the product is practically acceptable. The less blistering occurs, the more gas is suppressed during printing. A: It doesn't swell even at 50% print intensity. B: It swells at 50% print intensity. C: It swells regardless of print intensity.

[0094] [Table 2]

[0095] From the evaluation results shown in Table 2, it can be seen that the laser marking laminate having a color-developing layer (resin film) obtained from the laser marking composition of the example achieves a higher level of visibility, readability, and suppression of gas generation compared to the laser marking laminate having a color-developing layer (resin film) obtained from the laser marking composition of the comparative example.

[0096] The disclosure of Japanese Patent Application No. 2022-159099, filed on 30 September 2022, is incorporated herein by reference in its entirety. All documents, patent applications, and technical standards described herein are incorporated herein to the same extent as if each individual document, patent application, and technical standard were specifically and individually noted as being incorporated by reference. [Explanation of Symbols]

[0097] 1. Laminate 10 First layer (surface layer) 20. Second layer (color development layer) 30 Third layer (adhesive layer)

Claims

1. It contains at least one (meth)acrylic resin and a metal oxide containing at least one metal selected from the group consisting of bismuth, antimony, molybdenum, copper, iron, nickel, chromium, zirconium, and neodymium. The proportion of structural units derived from alkyl acrylates containing alkyl groups having 1 to 4 carbon atoms in the total structural units of the (meth)acrylic resin is 55% by mass or more. A laser marking composition wherein the proportion of structural units derived from methyl acrylate to the total structural units of the (meth)acrylic resin is 20% by mass or more.

2. The laser marking composition according to claim 1, wherein the total proportion of structural units derived from methacrylic acid and structural units derived from alkyl methacrylate to the total structural units of the (meth)acrylic resin is less than 45% by mass.

3. The laser marking composition according to claim 1, wherein the metal oxide comprises a bismuth-containing compound.

4. A resin film made using the laser marking composition according to any one of claims 1 to 3.

5. A laminate having a resin film as described in claim 4.

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