KR20070008104A - Reflection sheet - Google Patents

Abstract

A reflective sheet is provided to suppress the loss of light and improve the reflectance, by forming the reflective sheet of polypropylene-based resin through supercritical fluid process. A reflective sheet has polypropylene-based resin as base resin. The reflective sheet contains fine bubbles having an average particle size of 50 micrometers or less. The reflective sheet has a thickness of 0.5 millimeters. The fine bubbles are formed by supercritical fluid foaming process. The reflective sheet further contains an ultraviolet absorbent or an ultraviolet stabilizer.

Description

Reflection Sheet {Reflection sheet}

The present invention relates to a reflective sheet, and more particularly to a reflective sheet for a backlight unit of a liquid crystal display.

Recently, liquid crystal displays, which are used as main components of TVs, desktop computers, notebooks, mobile phones, etc., use a liquid crystal layer of non-light-emitting materials, so a separate light source must be installed, and the side or rear side of the light source reflects the light emitted to the rear part. In order to install the reflective sheet. The reflected light is emitted to the front while passing through a diffusion plate, a light guide plate, a diffusion film, and a brightness improving sheet.

Here, the reflective sheet should have reflectivity to reflect the light of the light source lamp to improve the brightness to the extent that the liquid crystal screen does not flash and concealment so that devices, frames, and the like arranged on the rear surface of the backlight unit are not identified.

Conventionally, as the reflection sheet, a white pigment such as titanium oxide is dispersed in a sheet made of a synthetic resin, a metal deposition film is laminated on the surface, or a bubble is dispersed in order to scatter light in a sheet made of a synthetic resin.

However, when the white pigment is dispersed, the amount of pigment should be increased to suppress the leakage of light to the back side. Since the white pigment absorbs light of a specific wavelength, the amount of the addition increases the light loss and the reflectance is deteriorated. .

In addition, when the metal deposition treatment on the surface, there is a problem that the manufacturing cost is high first, and because the diffuse reflection is difficult to occur, there is a problem that the luminance stain occurs even in the slight distortion of the sheet. In the case of vapor deposition, there is a problem that high luminance cannot be obtained depending on the type of the metal, or the durability is insufficient, so that deterioration progresses rapidly, and thus the luminance decreases with time.

On the other hand, in order to form a bubble in the sheet | seat made of synthetic resin, the method of extending | stretching or foaming is used.

The "light reflector and light reflector" of Korean Patent No. 200200 is a light reflector in which a UV protective layer is laminated on at least one surface of a substrate to which an inorganic filler is added to a polyolefin resin. A light reflector is disclosed in which a light reflectance of 550 nm is 95%.

Japanese Patent Laid-Open No. 4-239540 discloses a porous film drawn from a polyester film containing 2 to 25% by weight of polyolefin and a porous film drawn from a porous film and a polyester film containing from 5 to 25% of inorganic particles. The white polyester film which laminated | stacked was disclosed. In this case, since the amount of the inorganic particles is small at 5 to 25% by weight, high reflectance cannot be obtained because the reflection of light by the inorganic particles is small and the air dispersion is insufficient.

Japanese Patent Application Laid-Open No. Hei 8-143692 also discloses a white reflective film in which two kinds of polyolefins and polystyrene resins are contained in a polyester resin, but high reflectance cannot be obtained in this case as well.

When a foaming method is used to form bubbles in a sheet of synthetic resin, a method of producing a foam using a chemical foaming agent or a physical foaming agent is known.

In general, the chemical foaming method is a method of mixing a raw material resin and a low molecular weight organic foaming agent which decomposes at a molding temperature to generate a gas, and heating the foaming agent to a temperature above the decomposition temperature of the foaming agent to perform foam molding. In this method, the generation of gas is sensitive to the decomposition temperature, and the decomposition temperature is also easily prepared because a foaming aid is added, and a foam having independent bubbles can be obtained.

The gas foaming method, which is a physical foaming method, is a method in which a low boiling point organic compound such as butane, pentane, and dichlorodifluoromethane is supplied to a place where a resin is melted by a molding machine, kneaded, and then released into a low pressure region to be molded. The low boiling point organic compound used in this method is characterized by having high solubility and excellent solubility because of its affinity for resin, and thus a high magnification foam.

However, the use of such a method increases the cost, and in the case of chemical foams, problems such as discoloration and odor of the foams occur due to decomposition residues of the foams remaining in the foams, and foams manufactured by the physical foaming method are flammable or toxic. It has the potential to cause air pollution problems. In addition, such a method has a limitation in producing fine bubbles at a level that can minimize the reflectance.

In order to solve this problem, US Patent No. 4473665 discloses a manufacturing method for obtaining a foamed molded product which is uniformly dispersed in a fine cell having a diameter of 2 ~ 25㎛. In this method, first, the inert gas is impregnated under pressure until it is saturated in the thermoplastic sheet, heated to the glass transition temperature of the thermoplastic resin, and depressurized to produce a cell nucleus by supersaturating the gas impregnated in the resin, thereby quenching the cell. To control growth. In this way, a fine foam having a large number of cells can be obtained, but since the inert gas has low affinity with the resin, it requires 10 hours to completely impregnate the gas in the resin, which leads to a problem of low productivity.

Accordingly, the inventors of the present invention have confirmed that the present invention can provide a reflective sheet having a high reflectivity capable of uniformly forming a large number of fine bubbles and freely controlling the bubble diameter.

Accordingly, an object of the present invention is to provide a reflective sheet having excellent productivity and having excellent productivity by uniformly forming a plurality of fine bubbles.

Reflective sheet of the present invention for achieving the above object is characterized by using a polypropylene-based resin as the base resin, containing a micro bubble having an average particle diameter of 50㎛ or less.

The reflective sheet is characterized in that the thickness is 0.5 ~ 1.5㎜.

The reflective sheet is characterized in that the specific gravity is 0.2 ~ 0.8.

The microbubbles of the reflective sheet are formed using a supercritical fluid technology.

The reflective sheet may further include an inorganic filler of 40 parts by weight or less based on 100 parts by weight of the base resin.

The inorganic filler is at least one selected from barium sulfate, calcium sulfate, magnesium sulfate, barium carbonate, calcium carbonate, magnesium chloride, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, zinc oxide, titanium dioxide, alumina, silica, talc, zeolite. It is done.

The inorganic filler is characterized in that the average particle diameter is 10㎛ or less.

The reflective sheet may further include an ultraviolet stabilizer or an oxidation stabilizer as other additives.

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

The reflective sheet of the present invention uses a polypropylene resin as a base resin as a white synthetic resin capable of foaming. Polypropylene resin is a very light plastic with a specific gravity of about 0.9, and has a wide range of applications due to its stiffness, impact resistance, electrical properties, water resistance, and chemical resistance, and biaxially stretched polypropylene resin is used for film, carpet, container, and weak parts. Mainly used in such fields. Recently, composites of rubbers and filters are being used for large industrial parts such as automobile bumpers, other automobile parts, and panels.

The polypropylene-based resin of the present invention is a propylene homopolymer, syndiotactic, isotactic, and polypropylene and propylene having various constituents as main components, and binary, ternary, and quaternary random aerials, which are copolymers with other monomers. Copolymers or block copolymers can be used.

Herein, the microbubbles may have an average particle diameter of 50 μm or less. When the average particle diameter is 50 μm or less, incident light does not penetrate into the interior of the reflective sheet, and the reflectance increases because the number of diffuse reflections at the bubble interface increases significantly.

Meanwhile, the reflective sheet of the present invention preferably has a specific gravity of 0.2 to 0.8. When the specific gravity is 0.2 or more, it is preferable to increase the content of microbubbles while maintaining the appropriate rigidity for post-processing and use as the reflective sheet. The microbubble may be included as much as it can increase the reflectance while reducing the light loss due to light absorption in the non-foamed portion or light transmission due to the transparency of the reflective sheet.

The reflective sheet of the present invention is preferably such that the thickness is 0.5mm or more. By setting it to 0.5 mm or more, it is easy to handle at the time of post-processing, and it is excellent in shape maintainability. In addition, less light leaks and the reflectance increases. In addition, when used in the backlight unit for LCD, it is desirable to meet the trend of thinning of the liquid crystal display with a thickness of 1.5 mm or less.

In addition, in order to form microbubbles in the present invention, it is preferable to use a foaming technique using a supercritical fluid, and in particular, microbubbles of 50 µm or less can be formed. More preferably, the supercritical fluid used here is a supercritical fluid of N ₂ or CO ₂ .

In order to further improve the reflection efficiency, the reflective sheet of the present invention preferably further includes an inorganic filler.

The inorganic filler may include at least one selected from barium sulfate, calcium sulfate, magnesium sulfate, barium carbonate, calcium carbonate, magnesium chloride, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, titanium dioxide, alumina, silica, talc, zeolite, and the like. . In particular, it is preferable to use barium sulfate or calcium carbonate which is excellent in dispersibility and optical properties with polypropylene. It is preferable that the inorganic filler has an average particle diameter of 10 μm or less, because the average particle diameter of more than 10 μm exhibits a disadvantage in that the surface properties decrease. On the other hand, it is preferable that the addition amount is 40 weight part or less with respect to 100 weight part of base resins, and when it exceeds 40 weight part, there exists a problem that surface characteristics fall similarly.

The reflective sheet of the present invention may further include an ultraviolet absorber and an ultraviolet stabilizer as a light stabilizer. Examples of the ultraviolet absorber include 2-hydroxy-4-methoxy-2'-carboxybenzophenone, 2,4-dihydroxybenzophenone, 2-hydroxy-4-methoxybenzophenone and 2-hydroxy- 4-n-octoxybenzophenone, 2,2 ', 4,4'-tetrahydroxybenzophenone, 4-dodecyloxy-2-hydroxybenzophenone, bis (5-benzoyl-4-hydroxy-2 Benzophenone compounds such as -methoxyphenyl) methane and 2,2'-dihydroxy-4-methoxybenzophenone, and 2-2'-methylenebis [4- (1,1,3,3-tetramethyl Butyl-6- (2H-benzotriazol-2-yl) phenol], 2- (2'-hydroxy-5'-methylphenyl) benzotriazole, 2- (2'-hydroxy-3 ', 5' Benzotriazole compounds such as -di-tert-butylphenyl) -5-chlorobenzotriazole and the like.

In addition to the ultraviolet absorber, it is preferable to supplement the ultraviolet stabilizer in order to suppress deterioration of the resin. As a ultraviolet stabilizer, a hindered amine stabilizer etc. are mentioned.

In addition, the reflective sheet of the present invention may further include an antioxidant, although the primary antioxidants such as hindered phenol derivatives and aromatic amine derivatives are effective as the antioxidant, but phosphorus derivatives showing an effect on high temperature heat resistance or It is preferable to use together secondary antioxidants, such as a sulfur type | system | group derivative. The primary antioxidant acts as a radical scavenger to destabilize the polymer by deodorizing free radicals generated in the polymer material, and the secondary antioxidant acts as a peroxide decomposer and acts as a secondary antioxidant. will be.

The amount of each of the above-described ultraviolet stabilizer, ultraviolet absorber and antioxidant is preferably 1 weight or less per 100 parts by weight of the base resin.

The manufacturing method using the supercritical fluid foaming technique as a method of manufacturing the reflective sheet of the present invention will be briefly described as follows. The following description corresponds to a method of manufacturing the reflective sheet of the present invention, and the scope of the present invention is not limited thereto.

Firstly melting the base resin of the reflective sheet of the present invention; Injecting a supercritical fluid into the molten base resin; A third step of kneading the supercritical fluid with the molten base resin; A fourth step of foaming the kneaded supercritical fluid in the base resin; And a fifth step of extruding the expanded molten base resin.

In the reflective sheet of the present invention, an inorganic filler or a light stabilizer, an antioxidant, or the like may be further included as necessary. In this case, it is preferable to inject and mix each component as necessary in the first step. Each component is as described above.

Hereinafter, examples of the present invention will be described in more detail, but the scope of the present invention is not limited to these examples.

Compositions and composition ratios of Examples 1 to 6 and Comparative Examples 1 to 9 are shown in Table 1 below.

Supercritical fluid technology, chemical foaming technology and stretching technology of the following manufacturing method is prepared by the following method, respectively.

(1) Supercritical Fluid Technology

0.2 part by weight of a benzophenone ultraviolet absorber and 0.1 part by weight of a phenolic antioxidant were added to 100 parts by weight of a homopolypropylene resin having a melt index of 0.5 g / 10 min (200 ° C., 4 kg _f ) in an extruder, and melted at 200 ° C. After the addition, N ₂ gas of about 1% of the supercritical fluid state was introduced, and the supercritical fluid and the base resin introduced were kneaded using a screw at 165 ° C. Thereafter, the supercritical fluid is made into a foamed state just before extrusion and passed through a pressing roll to form a reflective sheet.

When calcium carbonate having an average particle size of 1.5 µm was added, the above polypropylene resin and additives were added to the extruder together with calcium carbonate.

(2) chemical foaming technology

100 parts by weight of a homopolypropylene resin having a melt index of 0.5 g / 10 min (200 ° C., 4 kg _f ) and 0.2 part by weight of 2,5-dimethyl-2,5-di (t-butylperoxy) hexane as an organic peroxide 1 part by weight of divinylbenzene, which is a functional monomer, and 0.1 part by weight of a phenolic antioxidant were fed to a 65 mm tandem extruder at 50 kg / h, and plasticized at 200 ° C. in a first stage extruder (diameter 65 mm), and then isoformed as a blowing agent. 5 parts by weight of butane was pressed against 100 parts by weight of the resin, and the resin temperature was cooled to 150 ° C. in a second stage extruder and passed through the pressing roll and the guide roll to obtain a sheet having a thickness of 2 mm.

(3) stretching technology

Calcium carbonate and an additive were mixed with 100 parts by weight of the homopolypropylene resin having a melt index of 0.5 g / 10 min (200 ° C., 4 kg _f ) to obtain a resin composition. The obtained resin composition was processed into pellets using a twin screw extruder, and then melt-extruded at 230 ° C. using an extruder equipped with a T die to obtain an unstretched sheet. The sheet thus obtained was uniaxially stretched at a draw ratio of 6.5 times between the preheat roll and the stretch roll heated to 85 ° C to obtain a sheet.

On the other hand, the average particle diameter of the bubble formed in the above-mentioned reflective sheet is calculated | required by taking the SEM photograph of a cross section, and measuring and averaging the particle diameter of the bubble contained in a fixed cross-sectional area.

Specific gravity is measured by the submerged method, and measurement conditions are carried out at a temperature of 23 ° C. and a relative humidity of 65%. Thickness is calculated | required using a vernier caliper.

Reflectance is measured in the wavelength range of 380 ~ 780nm by using a spectrophotometer (UV spectrophotometer, Varian) and reflectance of the wavelength of 550nm as a representative value.

division PP Calcium carbonate UV absorbers Antioxidant Manufacturing method Bubble particle diameter (㎛) Seat weight Sheet thickness (mm) Reflectance (%) Supercritical fluid Chemical foaming Stretch Example 1 100 10 0.2 0.1 ○ × × 17 0.6 0.5 93 Example 2 100 0 0.2 0.1 ○ × × 20 0.6 One 95 Example 3 100 10 0.2 0.1 ○ × × 18 0.6 One 98 Example 4 100 20 0.2 0.1 ○ × × 19 0.6 One 98 Example 5 100 10 0.2 0.1 ○ × × 20 0.6 1.5 98 Example 6 100 10 0.2 0.1 ○ × × 19 0.6 2 99 Comparative Example 1 100 0 0.2 0.1 × ○ × 251 0.4 One 74 Comparative Example 2 100 10 0.2 0.1 × ○ × 250 0.4 One 74 Comparative Example 3 100 20 0.2 0.1 × ○ × 248 0.4 One 77 Comparative Example 4 100 0 0.2 0.1 × × ○ 15 0.5 0.3 91 Comparative Example 5 100 10 0.2 0.1 × × ○ 18 0.5 0.3 93 Comparative Example 6 100 20 0.2 0.1 × × ○ 16 0.5 0.3 94 Comparative Example 7 100 0 0.2 0.1 ○ × × 50 0.5 One 94 Comparative Example 8 100 0 0.2 0.1 ○ × × 77 0.4 One 92 Comparative Example 9 100 0 0.2 0.1 ○ × × 89 0.2 One 89 * PP: Homopolypropylene (SK Co., Ltd., H900F, MI: 0.5 (200 ℃ / 5㎏ _f )) * UV absorber: Benzophenone UV absorber * Antioxidant: Phenolic antioxidant * Calcium carbonate: Heavy calcium carbonate (Siraishi calcium, Sophton 1800, average particle diameter 1.5㎛)

As a result of the experiment, in Examples 1 to 6 it was possible to obtain a good high reflectivity of the reflective sheet, it can be seen that the increase in the amount of calcium carbonate and the thickness of the reflective sheet increases the slight reflectance increase effect.

On the other hand, in Comparative Examples 1 to 3 it can be seen that by applying a general chemical foaming method the size of the bubble particle size is relatively large, showing a low reflectance.

Comparative Examples 4 to 6 can be seen that a relatively low reflectance compared to Examples 1 to 3 as a film generated by applying the stretching technique bubbles. This is judged to be due to insufficient bubble dispersion and insufficient thickness.

In Comparative Examples 7 to 9, it can be seen that as the bubble diameter increases, the reflectance decreases.

As described above, the present invention may provide a reflective sheet for a backlight unit, in which a plurality of fine bubbles are uniformly formed to improve reflectance.

Claims (9)

Including a polypropylene resin as a base resin, Reflective sheet, characterized in that the micro-bubbles with an average particle diameter of 50㎛ or less is formed. The method of claim 1, The reflective sheet is a reflective sheet, characterized in that the thickness of 0.5mm or more. The method of claim 1, The reflective sheet is a reflective sheet, characterized in that the specific gravity is 0.2 ~ 0.8. The method of claim 1, The microbubble is a reflective sheet, characterized in that formed by supercritical fluid foam. The method of claim 4, wherein The supercritical fluid is a reflective sheet, characterized in that the supercritical fluid of N ₂ or CO ₂ . The method of claim 1, The reflective sheet further comprises an inorganic filler of 40 parts by weight or less with respect to 100 parts by weight of the base resin. The method of claim 6, The inorganic filler is at least one selected from barium sulfate, calcium sulfate, magnesium sulfate, barium carbonate, calcium carbonate, magnesium chloride, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, zinc oxide, titanium dioxide, alumina, silica, talc, zeolite. Reflective sheet. The method of claim 6, The inorganic filler is a reflection sheet, characterized in that the average particle diameter is 10㎛ or less. The method of claim 1, The reflective sheet further comprises a UV absorber or UV stabilizer.