WO2012015205A1

WO2012015205A1 - Reflection sheet for backlight unit

Info

Publication number: WO2012015205A1
Application number: PCT/KR2011/005475
Authority: WO
Inventors: Hyo Gil Cha; Jong Sun Yoon; Sung Kwan Na
Original assignee: Skc Haas Display Film Company
Priority date: 2010-07-26
Filing date: 2011-07-25
Publication date: 2012-02-02
Also published as: TW201213977A; KR101158691B1; TWI468801B; KR20120010309A

Abstract

Provided is a reflection sheet for a backlight unit, including: a base film made of an opaque polyester film; a first coating layer formed by coating a first UV-curable coating composition on an upper surface of the base film, the first coating layer including particles having an average particle size of 3 to 50㎛; and a second coating layer formed by coating a second UV-hardenable coating composition on a lower surface of the base film, the second coating layer not including particles, wherein an overall thickness of the reflection sheet is 200 to 280㎛. The reflection sheet according to the present invention has a small thickness, and excellent durability and heat resistance.

Description

REFLECTION SHEET FOR BACKLIGHT UNIT

The present invention relates to a reflection sheet used in a backlight unit for LCD, and more particularly, to a reflection sheet for a thin film type backlight unit having excellent heat resistance.

A reflection sheet is a film that functions to reflect again the light, which is exited from a lower surface of a light guiding plate, toward the light guiding plate, to minimize the loss of light.

In recent, a thickness of a backlight unit has been decreased with the employment of an LED light source , and optical films including the reflection sheet are also required to have a small thickness. However, the LED light source has only 20% light efficiency and emits about 80% of its energy as heat. As a result, the heat remains inside of the backlight having a small thickness, which causes the optical film to creases. In order to solve this problem, a multilayer-structure reflection sheet, which is obtained by coating an adhesive agent on one surface of a reflection sheet, depositing a transparent polyester film thereon, and forming a heat-hardenable coating layer thereon, has been developed. However, this causes a problem in a manufacturing process due to a coating process of an adhesive agent and an increase in costs due to deposition of two sheets of films. Furthermore, an adhesive layer vulnerable to heat is heat-distorted due to remaining of heat for a long time, which causes occurrence of flexure or creases, resulting in image defects. Furthermore, an overall thickness of the sheet is increased to 300㎛ or more, due to the structure of sheets deposited by lamination.

An object of the present invention is to provide a reflection sheet without incuring flexure or creases caused by heat from a light source. Specifically, there is provided a reflection sheet having coating layers formed on both surfaces thereof using a UV coating composition in order to improve heat resistance while having an overall thickness of 200 to 280㎛.

Another object of the present invention is to provide a reflection sheet having one surface including particles (beads), in order to prevent an interface between a reflection sheet and dot print patterns or unevenness patterns (by laser processing), which are formed on a lower surface of a light guiding plate to exit light from a light guiding plate, from being rubbed, that is to say, in order to prevent scratch, which is cuased by contacting an upper surface of the reflection sheet and a lower surface of a light guiding plate with each other, from being generated on an interface between.

Still another object of the present invention is to provide a coating layer formed by a microgravure roll type coating method in ordr to form a uniform coating layer using a predetermined coating amount.

The present invention is directed to a thin film type reflection sheet having excellent heat resistance by forming coating layers on both surfaces using a UV-curable coating composition. In particular, the present invention is characterized in that a UV-curable coating composition necessarily containing particles is coated on an upper surface of a base film.

In one general aspect, as shown in FIG. 1, a reflection sheet for a backlight unit includes:

a base film made of an opaque polyester film;

a first coating layer formed by coating a first UV-curable coating composition on an upper surface of the base film, the first coating layer including particles having an average particle size of 3 to 50㎛; and

a second coating layer formed by coating a second UV-curable coating composition on a lower surface of the base film, the second coating layer not including particles,

wherein an overall thickness of the reflection sheet is 200 to 280㎛.

In another general aspect, as shown in FIG. 2, a reflection sheet for a backlight unit includes:

a base film made of an opaque polyester film;

a second coating layer formed by coating a second UV-curable coating composition on a lower surface of the base film, the second coating layer including particles having an average particle size of 3 to 50㎛; and

wherein an overall thickness of the reflection sheet is 200 to 280㎛.

In the present invention, the first UV-curable coating composition and the second UV-curable coating composition may be the same or different from each other.

Specifically, the first UV-curable coating composition used in the first coating layer may contain: 5 to 15 wt% of a urethane acrylate-based oligomer obtained by reacting polycarbonate diol and an aliphatic diisocyanate compound; 1 to 10 wt% of an acrylate monomer having an isocyanurate group, 5 to 10 wt% of hydroxy pivalic acid neopentyl glycol diacrylate, and 10 to 15 wt% of an acrylate-based monomer mixture of two or more selected from mono acrylate-based monomers, bifunctional acrylate-based monomers, trifunctional acrylate-based monomers, and multifunctional acrylate-based monomers, except the acrylate monomer having an isocyanurate group and the hydroxy pivalic acid neopentyl glycol diacrylate; 1 to 5 wt% of a light initiator; 5 to 15 wt% of particles having an average particle size of 3 to 50㎛; and 50 to 70 wt% of a solvent.

Further, the second UV-curable coating composition used in the second coating layer may contain: 5 to 15 wt% of a urethane acrylate-based oligomer obtained by reacting polycarbonate diol and an aliphatic diisocyanate compound; 1 to 10 wt% of an acrylate monomer having an isocyanurate group, 5 to 10 wt% of hydroxy pivalic acid neopentyl glycol diacrylate, and 10 to 15 wt% of an acrylate-based monomer mixture of two or more selected from mono acrylate-based monomers, bifunctional acrylate-based monomers, trifunctional acrylate-based monomers, and multifunctional acrylate-based monomers, except the acrylate monomer having an isocyanurate group and the hydroxy pivalic acid neopentyl glycol diacrylate; 1 to 5 wt% of a light initiator; and 50 to 70 wt% of a solvent.

Further, the second UV-curable coating composition may further include 5 to 15 wt% of particles having an average particle size of 3 to 50㎛, as necessary.

According to the present invention, coating layers may be formed on both surfaces of an opaque polyester base film in order to minimize flexure or creases due to heat while each overall thickness thereof is small. Here, in order to form a coating layer, which has no flexure or creases because of excellent resistance to heat from a light source, the study confirmed that it is most preferable to form the coating layer by using a UV-curable coating composition. The present inventors found out that, when a urethane acrylate-based oligomer obtained by reacting polycarbonate diol and an aliphatic diisocyanate compound, an acrylate monomer having an isocyanurate group, hydroxy pivalic acid neopentyl glycol diacrylate, and an acrylate-based monomer mixture except the acrylate monomer having an isocyanurate group and the hydroxy pivalic acid neopentyl glycol diacrylate are used as components of the UV-curable coating composition, a reflection sheet having excellent heat resistance, excellent abrasion resistance, and high reflectance can be provided, and thereby completing the present invention.

In addition, the present invention is characterized by uniformly coating the first coating layer and the second coating layer in a thickness of 3 to 20㎛. Here, if the coating thickness is less than 3㎛, hardness of resin is insufficient due to interruption of oxygen in air and heat resistant property of resin is insufficient. If the coating thickness is greater than 20㎛, a resin layer may become cracked or broken due to a large thickness thereof.

In addition, in order to prevent an interface from being rubbed due to frictional abrasion, which occurs between a light guiding plate and dot print patterns provided on a lower surface of the light guiding plate after lamination or unevenness patterns by laser processing, and an upper portion of the reflection sheet due to vibration, the first coating layer formed at an upper surface of the coating layers is made to contain particles while the particles have an average particle size of 3 to 50㎛, thereby minimizing frictional abrasion caused by vibration between two faces contacted with each other. Here, if the average particle size of the particles is less than 3㎛, the particles are buried in the resin layer, thereby minimizing an surface contact area by surface protrusion, resulting in insufficient slip property. If the average particle size of the particles is greater than 50㎛, the particles are too large as compared with the thickness of the resin layer, thereby separating the particles from the coating layer or damaging the lower surface of the light guiding plate due to excessive surface unevenness.

Hereinafter, constitutions of the present invention will be described in more detail.

In the present invention, it is preferable to use an opaque polyester film having an opacity of 95% or greater as the base film. It is preferable to use an opaque polyester film having a glass transition temperature of 70 to 80℃, preferably 73 to 76℃, retaining heat-resistant property, humidity-resistant property, and yellowing-resistant property as well as excellent dimensional stability at the time of cutting a sheet, and containing excellent light reflecting characteristics due to a difference in the refractive index at an interface, which is caused by a layer of large quantity of micro-sized bubbles. Both a film type where inorganic particles of barium sulfate or titanium oxide are added in the film and a film type which includes organic particles having bad usability with a polyester film and a bubble layer formed by enabling an interface to be torn due to stretching during forming of the film may be applicable for the opaque polyester film.

In the present invention, the first coating layer is formed on the upper surface of the base film, and laminated on the light guiding plate. Therefore, particles are necessarily required in order to prevent the first coating layer from being rubbed due to lamination on the light guiding plate. As the particle, organic-based polymer beads made of hard acrylate, polystyrene, nylon, soft acrylate, silicone or the like may be used. Among them, soft acrylate, nylon, soft silicon or the like having excellent resistance to frictional abrasion due to vibration is preferable. The particles preferably have an average particle size of 3 to 50㎛. If the average particle size of the particles is less than 3㎛, the particles are buried in the resin layer, thereby minimizing a surface contact area by the surface protrusion, resulting in an insufficient slip property. If the average particle size of the particles is greater than 50㎛, the particles are too large as compared with the thickness of the resin layer, thereby separating the particles from the coating layer or damaging the lower surface of the light guiding plate due to excessive surface unevenness. A content of the particles is preferably in a range of 5 to 15wt% based on total weight of the UV-hadenable coating composition used to form the first coating layer. If the content of the particles is less than 5 wt%, a particle density is too small to provide unevenness on a surface, thereby falling to minimize occurrence of scratches due to frictional abrasion with a lower surface of a light guiding plate. If the content of the particles is greater than 15wt%, mechanical properties such as surface toughness or a heat-resistant property are poorly effective due to a relative shortage of UV resin.

In the present invention, a thickness (T2) of the first coating layer means a dry coating thickness not containing particles, as shown in FIG. 1. The particle is made to be necessarily protruded from the surface by using the particle 11 of which a size is larger than the coating thickness. The thickness (T2) of the first coating layer 10 is preferably 3 to 20㎛. If the coating thickness is less than 3㎛, the hardness of a resin is insufficient due to interruption of oxygen in air and a heat resistant property of resin is insufficient. If the coating thickness is greater than 20㎛, a resin layer may be cracked or broken due to a large thickness thereof.

The first UV-curable coating composition used in the first UV coating layer may contain: 5 to 15 wt% of a urethane acrylate-based oligomer obtained by reacting polycarbonate diol and an aliphatic diisocyanate compound; 1 to 10 wt% of an acrylate monomer having an isocyanurate group, 5 to 10 wt% of hydroxy pivalic acid neopentyl glycol diacrylate, and 10 to 15 wt% of an acrylate-based monomer mixture of two or more selected from mono acrylate-based monomers, bifunctional acrylate-based monomers, trifunctional acrylate-based monomers, and multifunctional acrylate-based monomers, except the acrylate monomer having an isocyanurate group and the hydroxy pivalic acid neopentyl glycol diacrylate; 1 to 5 wt% of a light initiator; 5 to 15 wt% of particles having an average particle size of 3 to 50㎛; and 50 to 70 wt% of a solvent.

In the present invention, the second coating layer 30 is formed on a lower surface of the base film, and minimizes the occurrence of flexure or creases due to heat by coating a resin having heat resistance like in the first coating layer. In a case where the coating layer is formed on only the upper surface of the base film, the resin is shrunken due to heat, and thus, the flexure of the coating layer may occur in a direction in which the resin is coated. Consequently, a symmetrical coating layer is formed on the counter surface, thereby maintaining a balance. The second coating layer may contain particles 31, as shown in FIG. 2, or may not contain particles 31, as shown in FIG. 1.

The second UV-curable coating composition used in the second coating layer may contain: 5 to 15 wt% of a urethane acrylate-based oligomer obtained by reacting polycarbonate diol and an aliphatic diisocyanate compound; 1 to 10 wt% of an acrylate monomer having an isocyanurate group, 5 to 10 wt% of hydroxy pivalic acid neopentyl glycol diacrylate, and 10 to 15 wt% of an acrylate-based monomer mixture of two or more selected from mono acrylate-based monomers, bifunctional acrylate-based monomers, trifunctional acrylate-based monomers, and multifunctional acrylate-based monomers, except the acrylate monomer having an isocyanurate group and the hydroxy pivalic acid neopentyl glycol diacrylate; 1 to 5 wt% of a light initiator; and 50 to 70 wt% of a solvent.

A thickness (T3) of the second coating layer 30 is preferably 3 to 20㎛. If the coating thickness is less than 3㎛, the hardness of resin is insufficient due to interruption of oxygen in air and heat resistant property of resin is insufficient. If the coating thickness is greater than 20㎛, a resin layer may be cracked or broken due to a large thickness thereof.

In the present invention, a composition which is less yellowed by a light source and has excellent heat resistance and durability may be used as the UV-curable coating composition used in the first coating layer and the second coating layer, and a combination and a content range of components thereof may characterize the present invention.

In the present invention, the urethane acrylate-based oligomer is obtained by reacting polycarbonate diol and an aliphatic diisocyanate compound, which have excellent heat resistance, and a molecular weight thereof is preferably 1500 to 2000 g/mol. The polycarbonate diol has a structure in which hydroxyl groups are positioned at both ends of a polycarbonate structure, and abrasion resistance and heat resistance thereof are excellent by a soft feel effect. As the aliphatic diisocyanate compound, isophorone diisocyanate, hexamethylene diisocyanate, 4,4-dicyclohexylmethane diisocyanate, or the like may be used. Among them, isophorone diisocyanate is preferable since it has excellent heat resistance and is less yellowed. A content of the urethane acrylate-based oligomer is preferably 5 to 15 wt% based on total weight of the composition. If the content of the urethane acrylate-based oligomer is less than 5 wt%, surface hardness is not sufficient, and thus, stickness may remain after hardening. If the content of the urethane acrylate-based oligomer is greater than 15 wt%, a heat-resistant property itself becomes better but stiffness tends to decrease.

In the present invention, the acrylate monomer having an isocyanurate group is used to improve heat-resistant property and stiffness, and a specific example of the acrylate monomer may be tri(2-hydroxyethyl)isocyanurate triacrylate (THEICTA). A content of the acrylate monomer is preferably 1 to 10 wt%.

The hydroxy pivalic acid neopentyl glycol diacrylate is used to improve an adhesive property, and a content thereof is preferably 5 to 10 wt%.

In the present invention, in order to satisfy surface characteristics, an acrylate-based monomer mixture of two or more selected from mono acrylate-based monomers, bifunctional acrylate-based monomers, trifunctional acrylate-based monomers, and multifunctional acrylate-based monomers is together used as a monomer except the acrylate monomer having an isocyanurate group and the hydroxy pivalic acid neopentyl glycol diacrylate.

The acrylate-based monomer mixture is used to further improve UV hardenability. In the present invention, by using two or more selected from mono acrylate-based monomers, bifunctional acrylate-based monomers, trifunctional acrylate-based monomers, and multifunctional acrylate-based monomers, a coating composition capable of forming a uniform coating layer, which facilitate viscosity control and coating, can be prepared. A content of the acrylate-based monomer mixture is preferably 10 to 15 wt% based on total weight of the composition. If the content thereof is less than 10 wt%, viscosity attenuating effect is deteriorated and hardability is largely decreased. If the content thereof is greater than 15 wt%, durability including heat resistance is largely decreased.

As the mono acrylate-based monomer, 2-phenoxy ethyl acrylate (2-PEA), (2-(2-ethoxyethoxy)ethyl acrylate (EOEOEA), acryloyl morpholine (ACMO), or the like may be used.

As the bifunctional acrylate-based monomer, dipropylene glycol diacrylate (DPGDA), tripropylene glycol diacrylate (TPGDA), butanediol diacrylate (BDDA), bisphenol A(ethoxylate) diacrylate (BPA(EO)DA), bisphenol A(ethoxylate) methyl diacrylate (BPA(EO)4MDA), or the like may be used.

As the trifunctional acrylate-based monomer, trimethylolpropane triacrylate, pentaerythritol triacrylate (PETA), trimethylpropane triacrylate (TMPTA), or the like may be used.

As the multifunctional acrylate monomer, dipentaerythritol hexaacrylate (DPHA), dipentaerythritol pentaacrylate (DPPA), pentaerythritol tetraacrylate (PETTA), or the like may be used.

In the present invention, a light initiator is necessarily contained in order to enable UV hardening, and any light initiator that can be generally used for UV hardening can be used without limitation as the light initiator of the present invention. More preferably, a mixture of one or more selected from 1-hydroxy-cyclohexyl-phenyl-ketone, 2-benzyl-2-N,N-dimethylamino-1-(4-morpholinophenyl)-1-butanone, 2-hydroxy-2-methyl-1-phenyl-propan-1-on, 4,4-diaminobenzophenone, 2-methyl-1〔4-(methylthio)phenyl〕-2-morpholinopropan-1-on, and a mixture of oxy-phenyl-acetic acid 2-(2-oxo-2-phenyl-acetoxy-ethoxy)-ethyl ester & oxy-phenyl-acetic acid 2-(2-hydroxy-ethoxy)-ethyl ester (I-754, Ciba Company).

The solvent in the present invention is used to control viscosity, thereby improving dispersibility of particles and precisely coating a thin film with a microthickness on a surfac of the base film. Any solvent that can dissolve the polyurethane acrylate-based oligomer and the acrylate-based monomer mixture may be used without limitation. Preferably, methylethyl ketone, toluene, or the like is used, and a mixture thereof may be used. A content of the solvent is preferably 50 to 70 wt%, and a coating composition having excellent viscosity of 10 to 200 cps (25℃), which exhibits within the above range of the solvent, can be prepared.

In the present invention, it is preferable to uniformly coat the first coating layer and the second layer by using a predetermined coating amount. Preferable example of coating method may include a method of coating using a microgravure roll, but is not limited thereto.

An irradiation amount of UV light for hardening the UV-curable coating composition used in the first coating layer and the second coating layer is 0.1 to 1J/㎠, and preferably 0.3 to 0.5J/㎠. A high voltage mercury lamp using a wavelength of 200 to 400 nm as a main wavelength may be used.

The reflection sheet according to the present invention has a small overall thickness and excellent heat resistance, thereby allowing a backlight unit for LCD to be manufactured to have a small overall thickness, and has an upper surface with unevenness, thereby preventing an interface with a lower surface of the light guiding plate from being rubbed due to frictional abrasion with the lower surface of the light guiding plate.

In addition, the reflection sheet according to the present invention is manufactured by only coating processing without using a lamination method, thereby minimizing the manufacturing costs.

The above and other objects, features and advantages of the present invention will become apparent from the following description of preferred embodiments given in conjunction with the accompanying drawings, in which:

FIG. 1 is a cross-sectional view of a reflection sheet according to a preferred embodiment of the present invention; and

FIG. 2is a cross-sectional view of a reflection sheet according to another preferred embodiment of the present invention.

[Detailed Description of Main Elements]

10: first coating layer

11, 31: particle

20: base film

30: second coating layer

T1: Overall thickness

T2: thickness of first coating layer

T3: thickness of second coating layer

Hereinafter, the present invention will be described in detail by examples, but the present invention is not limited to the following examples.

Hereinafter, physical properties were measured as follows.

1) Physical Properties of UV-curable Coating Composition Before Hardening

- Viscosity: Viscosity of a sample was measured by using Brookfield VISCOMETER(model name: LVD-Ⅱ+) at 25℃.

- Solid content: After a sample was kept in an oven at 107℃ for 1 hour and taken out, solid content of the sample was measured by the following Formula 1.

[Formula 1]

Solid content (%) : [(container+ crude liquid) (container)/sample weight]x100

2) Physical Property Evaluation of Reflection Sheet Having Coating Layer

- adhesive strength evaluation

After a manufactured sheet was cut into 100 matrix structures within an area of 10 × 10 mm, a tape was attached thereon, and then vertical releasing was strongly performed. At this time, the number of matrixes that come away was recorded. The above adhesive strength evaluation was performed on the sample after completion of heat resistance evaluation as well.

- Reflectance Measurement

Reflectance of a manufactured sheet was measured. Here, reflectance was measured by using a reflectance measurement device (model: Ultra Scan PRO) at 550nm.

- Shrinkage Measurement

Shrinkage of a manufactured sheet was measured. A coated reflection sheet is cut to have 200mm length and 15mm width, which was then kept in an oven at 150℃ for 30 minutes, and length shrinkage change was measured. Here, MD (mechanical direction of film) shrinkage and TD (traverse direction of film) shrinkage, of the film, were measured respectively.

- Heat Resistance Evaluation

Reflection sheets were uniformly cut into A4 size, which were then kept in respective ovens at 90℃, 100℃, and 120℃ for 1 hour, and the curl and creases of the reflection sheets were measured.

i) A flexure state of each reflection sheet was evaluated while it was hanged at an upper end of each oven at each temperature.

ii) A flexure state of each reflection sheet was evaluated while it was fixed to edges of an upper surface of a glass plate at each temperature.

- Surface hardness

A pencil surface hardness of a reflection sheet was measured at the load of 200g by using a pencil hardness testing device (Model: KP-M5000M) A pencil produced by Mitsubishi company was used, and the test for pencil surface hardness was performed five times. If the number of scratches is two or more, the reflection sheet was determined to be defective.

[Preparation Examples 1 to 5]

Preparation of UV-curable coating composition

A light-hardenable resin composition was prepared by combining respective components at a weight ratio as shown in Table 1.

[Table 1] (Unit: wt%)

1) Urethane Acrylate: Urethane acrylate-based oligomer, having a molecular weight of 1800g/mol, which was obtained by reacting isophorone diisocyanate and polycarbonate diol at an equivalent ratio of 1:2, was used.

2) THEICTA: Tri(2-hydroxyethyl)isocyanurate triacrylate

3) TPGDA: Tripropylene glycol diacrylate

4) DPGDA: Dipropylene glycol diacrylate

5) BPA(EO)4MDA: Bisphenol A ethoxylate methyldiacrylate (ethoxylate (EO), content of 4 mole%)

6) HPNDA: Hydroxy pivalic acid neopentyl glycol diacrylate

7) PETA: pentaerythritol tetraacrylate

8) Particles: Soft acrylic resin beads (MX-1500, Soken Company) having an average particle size of 15㎛ were used.

9) Light initiator: 1-hydroxy-cyclohexyl-phenyl-ketone (I-184, Ciba Company)

Viscosities and solid contents of the coating compositions prepared in the preparation examples 1 to 7 were measured, and the results thus obtained were tabulated in Table 2.

[Table 2]

[Example 1]

A UV-curable coating composition prepared in Preparation example 2 was coated on an upper surface of a base film (SY70, 225㎛ PET film, SKC Company) by using a microgravure roll, and a UV-curable coating composition prepared in Preparation example 6 was coated on a lower surface of the base film by using the microgravure roll. Coating was performed while maintaining 10㎛ of coating thickness. Here, an aperture of the microgravure roll was 55Φ, 50/50 mesh gravure roll was used in order to uniformly increase the coating thickness, running speed and running speed rate were set to the optimum condition.

UV irradiation amount after coating was measured by using a high voltage mercury lamp of 300mJ/㎠. Physical properties of the reflection sheet thus obtained were measured, and the measured results were tabulated in Table 3.

[Example 2]

The reflection sheet was manufacture by the same method as Example 1 except that the UV-curable coating composition prepared in Preparation example 3 was used as the coating composition to be coated on the upper surface of the base film. Physical properties of the reflection sheet thus obtained were measured, and the measured results were tabulated in Table 3.

[Example 3]

The reflection sheet was manufactured by the same method as Example 1 except that the UV-curable coating composition prepared in Preparation example 4 was used as the coating composition to be coated on the upper surface of the base film. Physical properties of the reflection sheet thus obtained were measured, and the measured results were tabulated in Table 3.

[Example 4]

The reflection sheet was manufactured by the same method as Example 1 except that the UV-curable coating composition prepared in Preparation example 5 was used as the coating composition to be coated on the upper surface of the base film. Physical properties of the reflection sheet thus obtained were measured, and the measured results were tabulated in Table 3.

[Example 5]

The UV-curable coating composition prepared in Preparation example 2 was coated on an upper surface of a base film (SY70, 225㎛ PET film, SKC Company) by using a microgravure roll, and the UV-curable coating composition prepared in Preparation example 3 was coated on a lower surface of the base film by using the microgravure roll. Coating was performed while maintaining 10㎛ of coating thickness. Here, an aperture of the microgravure roll was 55Φ, 50/50 mesh gravure roll was used in order to uniformly increase the coating thickness, running speed and running speed rate were set to the optimum condition.

[Example 6]

The UV-curable coating composition prepared in Preparation example 7 was coated on an upper surface of a base film (SY70, 225㎛ PET film, SKC Company) by using a microgravure roll, and the UV-curable coating composition prepared in Preparation example 7 was coated on a lower surface of the base film by using the microgravure roll. Coating was performed while maintaining 10㎛ of coating thickness. Here, an aperture of the microgravure roll was 55Φ, 50/50 mesh gravure roll was used in order to uniformly increase the coating thickness, running speed and running speed rate were set to the optimum condition

[Comparative example 1]

Physical properties of a base film (SY70, 225㎛ PET film, SKC Company) of which both surfaces are not coated with coating layers in Example 1 were measured, and the measured results are tabulated in Table 3.

[Comparative example 2]

Physical properties of a RL713K product (SKC-Haas Company), which is a laminated reflection sheet commercially available, were measured, and the measured results were tabulated in Table 3. The RL713K product has a structure where a polyester-based adhesive layer is formed on a base film (SY70, 225㎛ opaque polyethylene terephthalate film, SKC Company), a transparent polyethylene terephthalate film (V5200, SKC Company) of 100㎛ is deposited thereon, and a hard coating layer is formed thereon.

[Comparative example 3]

The reflection sheet was manufacture by the same method as Example 1 except that the UV-curable coating composition prepared in Preparation example 1 was used as the coating composition to be coated on the upper surface of the base film. Physical properties of the reflection sheet thus obtained were measured, and the measured results were tabulated in Table 3.

〔Table 3〕

It can be seen from Table 3 that Examples 1 to 6 are similar to Comparative example 1 in view of reflectance and better than Comparative example 2 in view of shrinkage, and also exhibits good flexure state at the time of heat resistance evaluation and good surface hardness after hardening. Comparative example 3, which corresponds to a case where hydroxy pivalic acid neopentyl glycol diacrylate is not contained, showed deteriorated heat resistance.

Claims

A reflection sheet for a backlight unit, comprising:

a base film made of an opaque polyester film;

a first coating layer formed by coating a first UV-curable coating composition on an upper surface of the base film, the first coating layer including particles having an average particle size of 3 to 50㎛; and

a second coating layer formed by coating a second UV-curable coating composition on a lower surface of the base film, the second coating layer not including particles,

wherein an overall thickness of the reflection sheet is 200 to 280㎛.
The reflection sheet for a backlight unit of claim 1, wherein the second coating layer includes particles having an average particle of 3 to 50㎛.
The reflection sheet for a backlight unit of claim 1, wherein the first coating layer has a thickness of 3 to 20㎛.
The reflection sheet for a backlight unit of claim 1, wherein the second coating layer has a thickness of 3 to 20㎛.
The reflection sheet for a backlight unit of claim 1, wherein the first UV-curable coating composition contains: 5 to 15 wt% of a urethane acrylate-based oligomer obtained by reacting polycarbonate diol and an aliphatic diisocyanate compound; 1 to 10 wt% of an acrylate monomer having an isocyanurate group, 5 to 10 wt% of hydroxy pivalic acid neopentyl glycol diacrylate, and 10 to 15 wt% of an acrylate-based monomer mixture of two or more selected from mono acrylate-based monomers, bifunctional acrylate-based monomers, trifunctional acrylate-based monomers, and multifunctional acrylate-based monomers, except the acrylate monomer having an isocyanurate group and the hydroxy pivalic acid neopentyl glycol diacrylate; 1 to 5 wt% of a light initiator; 5 to 15 wt% of particles having an average particle size of 3 to 50㎛; and 50 to 70 wt% of a solvent.
The reflection sheet for a backlight unit of claim 1, wherein the second UV-curable coating composition contains: 5 to 15 wt% of a urethane acrylate-based oligomer obtained by reacting polycarbonate diol and an aliphatic diisocyanate compound; 1 to 10 wt% of an acrylate monomer having an isocyanurate group, 5 to 10 wt% of hydroxy pivalic acid neopentyl glycol diacrylate, and 10 to 15 wt% of an acrylate-based monomer mixture of two or more selected from mono acrylate-based monomers, bifunctional acrylate-based monomers, trifunctional acrylate-based monomers, and multifunctional acrylate-based monomers, except the acrylate monomer having an isocyanurate group and the hydroxy pivalic acid neopentyl glycol diacrylate; 1 to 5 wt% of a light initiator; and 50 to 70 wt% of a solvent.
The reflection sheet for a backlight unit of claim 6, wherein the second UV-curable coating composition further includes 5 to 15 wt% of particles having an average particle size of 3 to 50㎛.
The reflection sheet for a backlight unit of claim 5 or 6, wherein the acrylate-based monomer mixture is a mixture of two or more selected from 2-phenoxy ethylacrylate, 2-(2-ethoxyethoxy) ethylacrylate, acryloyl morpholine, dipropylene glycol diacrylate, tripropylene glycol diacrylate, butanediol diacrylate, bisphenol A ethoxylate diacrylate, bisphenol A ethoxylate methyldiacrylate, trimethylolpropane triacrylate, pentaerythritol triacrylate, trimethylpropane triacrylate, dipentaerythritol hexaacrylate, dipentaerythritol pentaacrylate, and pentaerythritol tetraacrylate.
The reflection sheet for a backlight unit of claim 5 or 6, wherein the aliphatic diisocyanate compound is selected from isophorone diisocyanate, hexamethylene diisocyanate, and 4,4-dicyclohexylmethane diisocyanate.
The reflection sheet for a backlight unit of claim 5 or 6, wherein the light initiator is a mixture type of 1-hydroxy-cyclohexyl-phenyl-ketone, 2-benzyl-2-N,N-dimethylamino-1-(4-morpholinophenyl)-1-butanone, 2-hydroxy-2-methyl-1-phenyl-propan-1-on, 4,4-diaminobenzophenone, 2-methyl-1〔4-(methylthio)phenyl〕-2-morpholinopropan-1-on, and a mixture of oxy-phenyl-acetic acid 2-(2-oxo-2-phenyl-acetoxy-ethoxy)-ethyl ester, and oxy-phenyl-acetic acid 2-(2-hydroxy-ethoxy)-ethyl ester.
The reflection sheet for a backlight unit of claim 1 or 2, wherein the particle is a polymeric particle selected from hard acrylate, polystyrene, nylon, soft acrylate, and silicone.
The reflection sheet for a backlight unit of claim 1 or 2, wherein the first coating layer and the second coating layer are coated by using a microgravure roll.