JP2024060128A - Photocurable resin composition - Google Patents

Abstract

[Problem] To provide a photocurable resin composition for transparent-processed paper, which does not produce a film that is too hard after curing and which shows little discoloration even when exposed to ultraviolet light. [Solution] This is a resin composition for transparent-processed paper that is obtained by impregnating absorbent paper and then curing with active energy rays, and is a photocurable resin composition that contains an alkylene oxide-modified multifunctional (meth)acrylate and a photopolymerization initiator, and is characterized in that the (meth)acrylate is non-aromatic and the average number of alkylene oxide-modified units added is 9 to 20. [Selected Drawing] None

Description

The present invention relates to a photocurable resin composition for transparent paper that is impregnated into absorbent paper and then cured by exposure to active energy rays.

Conventionally, one technique for impregnating paper with resin to make it translucent is to impregnate the paper with a photo-UV curable ink to make the paper transparent. For example, a solventless composition containing a UV-polymerizable composition containing a polymerizable vinyl oligomer and a photo-polymerizable vinyl monomer, and an oxidative polymerization composition has been proposed as a composition to be impregnated into the paper (Patent Document 1). However, this formulation is not a fast-curing type, but contains an oxidizing component that does not cure with UV light, and cures gradually, which causes problems such as changes in transparency over time, smearing of printed matter during printing, and blocking when the paper is stacked after printing.

As a fast-curing composition that can address these issues, the applicant has previously invented a resin composition for transparent-processed paper with a viscosity of 500 to 3,000 mPa·s that contains a bifunctional EO-modified bisphenol A diacrylate, a polyfunctional acrylic monomer, and a photopolymerization initiator (Patent Document 2). This composition has good permeability into paper and a very fast curing speed, making it an extremely excellent invention as a photocurable resin composition for transparent-processed paper. However, if the composition is continuously exposed to light such as ultraviolet light after curing, it may yellow over time, leaving room for improvement.

Patent No. 3282019 Patent No. 6793485

The object of the present invention is to provide a photocurable resin composition for transparent paper that is applied to opaque envelopes and the like, cured with active energy rays such as ultraviolet light, and that processes a part or the entire paper into a translucent state so that the underlying printed matter or pattern can be seen through, and that does not produce an excessively hard film after curing and that does not discolor much even when exposed to ultraviolet light.

In order to achieve the above object, the invention of claim 1 provides a photocurable resin composition for transparent paper, which is obtained by impregnating absorbent paper and then curing with active energy rays, and which contains an alkylene oxide-modified polyfunctional (meth)acrylate (A) and a photopolymerization initiator (B), and is characterized in that (A) is non-aromatic and the average number of alkylene oxide modifications added is 9 to 20.

The invention of claim 2 provides the photocurable resin composition according to claim 1, characterized in that (A) is ethylene oxide-modified trimethylolpropane triacrylate.

The invention of claim 3 provides a photocurable resin composition according to either claim 1 or 2, characterized in that the blending ratio of (A) to the total solid content is 50 to 98% by weight.

The photocurable resin composition of the present invention is useful as a fast-curing photocurable resin composition for transparent paper, as the film formed after curing is not too hard and discoloration is minimal even when exposed to ultraviolet light. It can be applied to opaque envelopes and the like to make part or all of the paper translucent so that the underlying printed matter or pattern can be seen through.

The composition of the present invention is composed of an alkylene oxide-modified polyfunctional (meth)acrylate (A) and a photopolymerization initiator (B). In this specification, (meth)acrylate includes both acrylate and methacrylate. Poly(meth)acrylate is a general term for polyfunctional (meth)acrylates such as di(meth)acrylate, tri(meth)acrylate, and tetra(meth)acrylate.

The alkylene oxide-modified polyfunctional (meth)acrylate (A) used in the present invention is a non-aromatic compound having multiple acryloyloxy groups, and is a major component of the cured film. Alkylene oxide-modified means that the skeleton has a block structure of alkylene oxide, such as ethylene oxide (hereinafter referred to as EO), propylene oxide (hereinafter referred to as PO), and butylene oxide (hereinafter referred to as BO). The average number of alkylene oxide-modified units is 9 to 20, and preferably 9 to 15. If it is less than 9, discoloration over time and flexibility may decrease, and if it exceeds 20, the viscosity may increase and penetration into paper may decrease.

Examples of (A) include EO-modified glycerin poly(meth)acrylate, EO-modified diglycerin poly(meth)acrylate, EO-modified trimethylolpropane tri(meth)acrylate, EO-modified pentaerythritol poly(meth)acrylate, EO-modified dipentaerythritol poly(meth)acrylate, and PO-modified and BO-modified products thereof, which may be used alone or in combination of two or more. Among these, trifunctional to hexafunctional ones are preferred in terms of the balance between curability and cure shrinkage, and trifunctional to tetrafunctional ones are even more preferred. Furthermore, EO-modified ones are preferred in terms of viscosity, curability, and availability, and EO-modified trimethylolpropane triacrylate is particularly preferred.

The amount of (A) is preferably 50 to 98% by weight, more preferably 60 to 95% by weight, and particularly preferably 70 to 93% by weight, based on the total solid components. By making it 50% by weight or more, a film that sufficiently suppresses discoloration over time can be formed, and by making it 98% by weight or less, sufficient hardening properties and impregnation into paper can be ensured.

The photopolymerization initiator (B) used in the present invention generates radicals when irradiated with ultraviolet light or an electron beam, and these radicals trigger the polymerization reaction. General-purpose photopolymerization initiators such as benzyl ketal, acetophenone, and phosphine oxide types can be used. By arbitrarily selecting the light absorption wavelength of the polymerization initiator, it is possible to impart curability over a wide wavelength range from the ultraviolet region to the visible light region. Specifically, benzyl ketals include 2.2-dimethoxy-1.2-diphenylethan-1-one, α-hydroxyacetophenones include 1-[4-(2-hydroxyethoxy)-phenyl]-2-hydroxy-2-methyl-1-propan-1-one and 2-hydroxy-1-{4-[4-(2-hydroxy-2-methyl-propionyl)-benzyl]-phenyl}-2-methyl-propan-1-one, α-aminoacetophenones include 2-methyl-1-(4-methylthiophenyl)-2-morpholinopropan-1-one, and acylphosphine oxides include 2.4.6-trimethylbenzoyl-diphenyl-phosphine oxide and bis(2.4.6-trimethylbenzoyl)-phenylphosphine oxide, which can be used alone or in combination of two or more.

Among the (B), it is preferable to use an α-hydroxyacetophenone type (c1) that is resistant to yellowing, or an α-aminoacetophenone type (c2) that has very high reactivity, and it is even more preferable to use (c1) and (c2) in combination. Commercially available products of (c1) include Omnirad 127D, 184, and 2959 (product names: manufactured by IGM Resins), and commercially available products of (c2) include Omnirad 907 and 379 (product names: manufactured by IGM Resins).

The amount of (B) is preferably 2 to 15 parts by weight, more preferably 3 to 12 parts by weight, and particularly preferably 5 to 10 parts by weight, per 100 parts by weight of the radically polymerizable component in the composition. When (b1) and (b2) are used in combination, the weight ratio of (b1):(b2) is preferably 3:2 to 1:3. By using this range, discoloration over time can be suppressed and good curability can be ensured.

When adjusting the viscosity of the composition of the present invention, it is preferable to use a reactive diluent rather than an organic solvent, in order to eliminate the need for a drying process and to reduce the environmental impact of solvent evaporation. By using a reactive diluent, it is possible to promote penetration into paper and adjust curing properties. In addition, since the composition may be dried in a drying oven after application to paper, it is preferable for the diluent to be low volatile from a safety standpoint.

As the reactive diluent, a polyfunctional (meth)acrylate compound (average number of additions of 8 or less when modified with alkylene oxide) is preferred in terms of good compatibility with (A) and curability. Examples of bifunctional (meth)acrylates include ethylene glycol di(meth)acrylate, propylene glycol di(meth)acrylate, polyethylene glycol di(meth)acrylate, polypropylene glycol di(meth)acrylate, polytetramethylene glycol di(meth)acrylate, 1.4-butanediol di(meth)acrylate, dimethylol tricyclodecane diacrylate, and dicyclopentanyl diacrylate. Examples of trifunctional (meth)acrylates include trimethylolpropane tri(meth)acrylate, tris(2-acryloxyethyl)isocyanurate, ε-caprolactone modified tris(2-acryloxyethyl)isocyanurate, pentaerythritol tri(meth)acrylate, and dipentaerythritol tri(meth)acrylate.

Furthermore, examples of tetrafunctional or higher (meth)acrylates include pentaerythritol tetra(meth)acrylate, dipentaerythritol tetra(meth)acrylate, dipentaerythritol penta(meth)acrylate, dipentaerythritol hexa(meth)acrylate, and caprolactone-modified dipentaerythritol hexa(meth)acrylate.

Among these, bifunctional (meth)acrylates are preferred in terms of the balance between curability and flexibility, and polyether backbone and C2-C10 aliphatic or alicyclic (meth)acrylates are also preferred. Examples include polyethylene glycol di(meth)acrylate with 8 or less EO additions and dimethylol-tricyclodecane diacrylate, which can be used alone or in combination of two or more types.

The amount of the reactive diluent in the total solid content is preferably 50% by weight or less, more preferably 40% by weight or less, and particularly preferably 30% by weight or less. By making the amount 50% by weight or less, discoloration over time can be sufficiently suppressed, and the partial whitening phenomenon known as "white blur" after UV curing can be suppressed.

The photocurable resin composition of the present invention may contain various additives such as ultraviolet absorbers, leveling agents, defoamers, wetting agents, antiblocking agents, antioxidants, flame retardants, and polymerization inhibitors, as necessary, to the extent that the performance is not impaired.

The ultraviolet absorber (hereinafter referred to as UVA) is a radical chain initiation inhibitor that has an absorption band in the region of harmful ultraviolet rays with high energy, and can improve weather resistance, such as suppressing yellowing. Examples include benzotriazole-based, triazine-based, and benzophenone-based agents, and can be used alone or in combination of two or more types. Among these, hydroxyphenyltriazine-based agents, which can strongly absorb the long wavelength portion of ultraviolet rays, are preferred.

The amount of UVA blended is preferably 0.05 to 3.0% by weight, more preferably 0.1 to 1.0% by weight, based on the total solid content. By using this range, sufficient ultraviolet light absorption can be ensured. Commercially available products include Tinuvin 477 (product name: BASF Japan, hydroxyphenyl triazine type).

The leveling agent is blended to equalize the surface tension difference on the coating surface. By adding a leveling agent, the film thickness after application to the paper becomes uniform, and as a result, the amount of penetration into the paper can be made uniform, stabilizing the transparency after UV curing. Examples include acrylic, vinyl, and silicone-based agents. Of these, silicone-based agents that have siloxane bonds in the molecule and have a high ability to reduce surface tension are suitable. A commercially available product is BYK-UV3570 (product name: BYK-Chemie, polyester-modified polydimethylsiloxane with multiple acrylic functional groups).

The amount of the leveling agent is preferably 0.05% to 3.0% by weight, more preferably 0.07% to 1.0% by weight, based on the total solid components. By using an amount of 0.05% by weight or more, sufficient leveling properties can be achieved to ensure uniform transparency, and by using an amount of 3.0% by weight or less, an adequately good appearance can be ensured without excessive use.

The viscosity of the photocurable resin composition of the present invention is preferably 50 to 500 mPa·s at 25°C, and more preferably 100 to 400 mPa·s. By making it 50 mPa·s or more, bleeding after impregnation of the substrate can be suppressed and the amount of penetration can be made uniform, ensuring a sufficient appearance, while by making it 3,000 mPa·s or less, stable penetration into the substrate can be ensured and sufficient transparency can be obtained.

The photocurable resin composition of the present invention can be applied to a substrate such as paper by any known application method, such as gravure printing, flexographic printing, screen printing, roll coating, bar coating, or blade coating. The amount of coating can be any amount depending on the type of paper, but in general, a wet thickness of about 30 μm to 50 μm is recommended in order for the paper to have sufficient transparency.

After applying the photocurable resin composition of the present invention, it is preferable to increase the temperature in a drying oven to reduce the viscosity and promote the penetration of the resin into the substrate. Specific drying conditions include a drying time of about 60 seconds in a hot air drying oven set at a temperature of 80°C to 100°C. Under these conditions, no special equipment such as a far-infrared irradiation device is required, and the temperature of the paper surface is below 100°C, making it a working condition that does not pose any safety issues, and a transparent appearance can be stably obtained.

When the photocurable resin composition of the present invention is cured by ultraviolet light, a known light source such as a high pressure mercury lamp, a metal halide lamp, a xenon lamp, or an electrodeless discharge lamp is used, and curing is carried out by irradiating an integrated light amount of, for example, 50 to 1,000 mJ/ ^cm2 .

The present invention will be described in more detail below with reference to examples and comparative examples, but these are concrete examples and are not intended to be limiting. Unless otherwise specified, measurements were performed at room temperature of 25°C and relative humidity of 65%. The blend amounts are expressed as parts by weight in terms of solid content.

Example 1
The resin composition of Example 1 was prepared by mixing M3150 (product name: TMP(EO)15TA ^* , manufactured by Miwon) as (A), Omnirad 184 (product name: manufactured by IGM Resins) as (b1), and Omnirad 907 (product name: manufactured by IGM Resins) as (b2) in the formulation shown in Table 1 in a light-shielding bottle and stirring and degassing until uniformly dissolved.
*TMP(EO)TA means (EO) modified trimethylolpropane triacrylate, and the number after (EO) indicates the number of (EO) added. The same applies when the above is (PO).

Examples 2 to 7
In addition to the materials used in Example 1, M3190 (trade name: Miwon Corporation, TMP(EO)9TA) was used as (A), EM-228 (trade name: Choko Materials Industry Co., Ltd., polyethylene glycol diacrylate, EO addition number 7) and DCP-A (trade name: Kyoeisha Chemical Co., Ltd., dimethylol tricyclodecane diacrylate) were used as monomers, BYK-UV3570 (trade name: BYK-Chemie Corporation) was used as a leveling agent, and Tinuvin 477 (trade name: BASF Japan) was used as UVA. The resin compositions of Examples 2 to 7 were prepared by mixing and degassing the materials in the formulations shown in Table 1 in a light-shielding bottle until uniformly dissolved.

Comparative Examples 1 to 7
In addition to the materials used in the examples, M300 (trade name: Miwon Co., Ltd., TMPTA), M3130 (trade name: Miwon Co., Ltd., TMP(EO)3TA), M3160 (trade name: Miwon Co., Ltd., TMP(EO)6TA), M360 (trade name: Miwon Co., Ltd., TMP(PO)3TA), M240 (trade name: Miwon Co., Ltd., bisphenol A diacrylate, EO addition number 4), HBPE-4 (trade name: Daiichi Pharmaceutical Co., Ltd., hydrogenated bisphenol A diacrylate, EO addition number 4), and M4004 (trade name: Miwon Co., Ltd., pentaerythritol tetraacrylate, EO addition number 5) were used as acrylate monomers, and the mixtures shown in Table 2 were placed in a light-shielding bottle and stirred and degassed until uniformly dissolved, to prepare the resin compositions of Comparative Examples 1 to 7.

Table 1

Table 2

The evaluation method was as follows:

Measurement sample <br/> The resin composition was applied to copy paper PPC Paper High White (product name: manufactured by Otsuka Shokai, 68 g/ ^m2 ) using a #20 bar coater, and cured by UV irradiation using a high-pressure mercury lamp IGLANTAGE ECS-4011GX manufactured by I-Graphics Co., Ltd., with an illuminance of 100 mW/ ^cm2 and 100 mJ/ ^cm2 .

Viscosity: Using a Toki Sangyo cone-plate viscometer RC-550, measurements were taken at 25±1°C with a cone angle of 3° R17.65 at a rotation speed of 100 rpm for viscosities of 200 mPa·s or less, 50 rpm for viscosities between 200 and 800 mPa·s, and 10 rpm for viscosities over 800 mPa·s. Viscosities between 50 and 500 mPa·s were marked as ○, and all others were marked as ×.

Total light transmittance: Measured in accordance with JIS K7361-1 using a Hazeguard made by Toyo Seiki Seisakusho, with 60% or more rated as ○ and less than 60% rated as ×.

b ^* value: Using a Spectrophotometer SD6000 manufactured by Nippon Denshoku Industries Co., Ltd., the above-mentioned measurement sample was measured immediately after curing and after being left to stand under sunlight for 6 hours. The evaluation was carried out as follows: if the difference (Δb ^* value) was less than 5, it was rated as ◎; if it was between 5 and 8, it was rated as ○; if it was more than 8, it was rated as ×.

Flexibility: The coating was applied twice using a #20 bar coater and cured under the same conditions as above to prepare the measurement sample. When wrapped around a tube with a diameter of 20 mm, samples that did not crack were marked with an O, and samples that did crack were marked with an X.

The evaluation results are shown in Tables 3 and 4.
Table 3

Table 4

The photocurable resin compositions of the Examples were evaluated to be good in terms of viscosity, total light transmittance, Δb ^* value, and flexibility.

On the other hand, Comparative Example 1, which used TMPTA with no alkylene oxide modification, had poor flexibility, and Comparative Examples 2 to 4, which used TMPTA with an alkylene oxide modification addition number of 3 to 6, had poor Δb ^* values. Furthermore, Comparative Examples 5 and 6, which are bisphenol-based and have 4 EO modifications, had high viscosity and poor Δb ^* values, and Comparative Example 7, which is 4 functional and has 5 EO modifications, also had a poor Δb ^* value, and none of them were suitable for the present invention.

Claims (3)

This photocurable resin composition is for transparent paper, and is obtained by permeating absorbent paper and curing it with active energy rays. It contains an alkylene oxide-modified polyfunctional (meth)acrylate (A) and a photopolymerization initiator (B), and is characterized in that (A) is non-aromatic and the average number of alkylene oxide modifications added is 9 to 20. The photocurable resin composition according to claim 1, characterized in that (A) is ethylene oxide-modified trimethylolpropane triacrylate. 3. The photocurable resin composition according to claim 1, wherein the blending ratio of said (A) to the total solid content is 50 to 98% by weight.