KR101652257B1 - Reduction method of Silver streak defects of polymethylmethacrylate resin for back light unit for in-plane switching mode liquid crystal display device - Google Patents

Abstract

The present invention relates to a silver strike defect of a polymethyl methacrylate resin for a backlight unit for a high-resolution IPS LCD having an excellent light transmittance and a yellow index suitable for a light guide plate, Of the invention. More specifically, methyl methacrylate homopolymer or methyl (meth) acrylate copolymer having a content of DMF of 40 ppm or less; And a phosphorus antioxidant of a specific structure; And a high alcohol. The composition has excellent optical properties and fluidity, and is excellent in thermal decomposition resistance, so that it is excellent in stability under high temperature injection conditions and can significantly reduce defects in silver strike, .

Description

Technical Field [0001] The present invention relates to a method for reducing silver strike defect in a polymethylmethacrylate resin for a horizontal electric field type liquid crystal display device backlight unit,

The present invention relates to a method for reducing silver streak defect of a polymethylmethacrylate resin for a horizontal electric field type liquid crystal display backlight unit, and more specifically, The present invention relates to a method for reducing defects in silver strike of a polymethylmethacrylate resin for a horizontal electric field type liquid crystal display (LCD) backlight unit capable of ejecting a thin light guide plate with excellent brightness, high fluidity, and excellent heat resistance.

Liquid crystal displays (LCDs) are the most popular display devices due to their excellent image quality, light weight, thinness and low power. Such a liquid crystal display device includes a horizontal electric field (In-Plane Switching), a twisted nematic (TN), and a vertical electric field. Among them, the IPS method is widely used in mobile, notebook, monitor and the like because of wide viewing angle, excellent contrast ratio, quick response speed compared to VA method, and wide viewing angle.

However, in spite of this advantage, the high-resolution IPS method has a problem in that the luminance is lowered due to the reduction of the aperture ratio, thereby changing the configuration of the color filter and causing the color coordinates of the panel to shift in a yellow color. A liquid crystal display device requires a backlight unit that emits light to a liquid crystal panel. The LED package of the backlight unit is changed to compensate for a yellow shifted panel. A series of processes that change the current LED package to a blue shift LED package with low yield all cause a rise in manufacturing cost and a decrease in production efficiency.

On the other hand, the polymethyl methacrylate (PMMA) resin used as the material of the light guide panel of the backlight unit is a polymer of methyl methacrylate (MMA), and it is higher in the trend of the display industry, It is very important to manufacture a polymethyl methacrylate composition having fluidity, excellent optical properties exhibiting low color coordinates due to use of high-resolution IPS panels, and thermal decomposition stability for reducing a defective rate at high temperature injection.

Such a polymethyl methacrylate resin can be produced by suspension polymerization, bulk polymerization, solution polymerization, etc. According to Japanese Patent No. 2000-26507 and No. 2752458, a polymethyl methacrylate resin is produced by continuous bulk polymerization However, the obtained polymethylmethacrylate resin can be used as an ordinary light guide plate for LCD, but other parts of the backlight unit such as an LED package are changed in an IPS-type panel requiring a low color coordinate due to a high color coordinate There is a drawback to doing so.

Korean Patent Laid-Open Publication No. 2013-0060655 discloses a method for polymerizing a polymethyl methacrylate resin using a methyl methacrylate monomer having a small amount of specific impurities to enhance optical properties. However, in the suspension polymerization, due to a dispersant, There is a limit to apply to repeated extrusion for injection or resin recycling.

Korean Patent No. 0689635 discloses a composition obtained by adding a crosslinking agent in the polymerization of methyl methacrylate resin. However, when the above-described methyl methacrylate resin is produced by a light-emitting device for LED light source by injection molding, fluidity is not sufficient. Therefore, it is difficult to exhibit sufficient effect for thinning of the light guide plate for the purpose of producing flow marks, It is difficult to obtain a light guide plate having satisfactory performance, such as generation of gas due to heat deterioration of the resin.

The production of the polymethylmethacrylate resin for a light guide plate using suspension polymerization additionally includes an additive such as an antioxidant and / or a heat stabilizer to prevent resin carbonization and yellowing depending on processing conditions of compression / injection. Known antioxidants include phenol-based, sulfur-based, or phosphorus-based antioxidants. Japanese Unexamined Patent Application Publication No. 2008-076764 and Japanese Patent Application Laid-Open No. 2010-051938 disclose the addition of an antioxidant to improve the thermal stability of methyl (meth) However, the phenol-based or sulfur-based antioxidant is unsuitable for the transparent methyl (meth) acrylate resin which requires high optical properties due to coloring of the antioxidant itself or its oxide. The phosphorus antioxidant is free of the above-mentioned coloring matter and is excellent in efficiency, and is applied to a methyl methacrylate resin. Korean Patent Registration No. 2004-0072227 discloses a resin composition containing a phosphorus-based antioxidant, The effect on physical properties is disclosed. However, the effect on the blue shift color coordinate for application of a high resolution IPS type LCD panel is not disclosed.

Thus, in order to manufacture a high-quality IPS LCD and a thin-type injection-molded light guide plate without raising the manufacturing cost by exhibiting a sufficiently low color coordinate to compensate for a yellow shifted panel while having optical properties of excellent light transmittance and yellowness, There is a problem to be solved in order to develop a resin having an excellent heat resistant decomposition property and a low silver streak defect rate even at a high injection temperature.

Japanese Patent Application Laid-Open No. 2000-26507 Japanese Patent No. 2752458 Korean Patent Publication No. 2013-0060655 Korea Patent No. 0689635 Japanese Laid-Open Patent Application No. 2008-076764 Japanese Patent Laid-Open No. 2010-051938 Korean Patent No. 2004-0072227

It is an object of the present invention to provide a method for reducing the silver strike defect of a polymethylmethacrylate resin which can be used as a thin extruded light guide plate for an IPS LCD. More specifically, since it has excellent optical properties and low color coordinates compared to conventional optical PMMA, it is possible to use a conventional LED package with a high yield as a backlight light source, thereby enabling cost reduction and production efficiency increase, The present invention provides a method of significantly reducing the silver strike defect of a polymethyl methacrylate resin by including a methyl methacrylate polymer or copolymer containing 40 ppm or less of dimethyl furan (DMF) and a phosphorus antioxidant and a lubricant will be.

The present invention relates to a polymer of methyl methacrylate (MMA) having a content of dimethylfuran (DMF) of 40 ppm or less or a methyl methacrylate (MMA) monomer having a content of dimethylfuran (DMF) of 40 ppm or less and methacrylate or acrylate The present invention relates to a method for reducing silver streak defect of a polymethyl methacrylate resin containing a phosphorus-based antioxidant and a higher alcohol, which is composed of one or two or more monomers,

[Chemical Formula 1]

( _Wherein R ¹ is any one selected from C _8-22 alkyl or C _8-22 alkenyl).

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

It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit or scope of the present invention as defined by the following claims. And a description of the known function and configuration will be omitted.

The dimethylfuran (DMF) is one of the impurities generated during the synthesis of methyl methacrylate (MMA). When a heat stabilizer or an antioxidant or the like is added to a copolymer of (meth) acrylate with a single polymer using methylmethacrylate (MMA) containing 40 ppm or less of dimethylfuran (DMF), improvement in light transmittance and yellowing (DMF) contained in the methyl methacrylate (MMA) is more than 40 ppm, whereas when the heat stabilizer or the antioxidant is added, the light transmittance And the effect of reducing the yellowness (YI) is weakened.

One of the other characteristics of the polymethylmethacrylate resin for IPS LCD backlight unit of the present invention for exhibiting excellent optical properties is one or two or more selected from the phosphorus-based antioxidants of the above formula (1).

When a backlight unit using a low-cost, high-yield LED package used in the manufacture of LCDs other than the light guide plate manufactured by injection molding a suspension polymerization resin composition containing the antioxidant of the present invention in a specific amount or more is applied to a high resolution IPS LCD, There is a feature that does not increase.

More specifically, the light guide plate produced from the polymethyl methacrylate resin composition containing the phosphorus-based antioxidant of the present invention comprises a methyl methacrylate resin, a phenol antioxidant, a sulfur-based antioxidant, and a benzene ring Unlike a light guide plate made of a suspension polymerized methyl methacrylate resin using a phenoxy antioxidant, the remarkably low color coordinates enough to offset the high color coordinates of the high resolution IPS Even if the same LED package is applied, the same color coordinates can be displayed.

In the above formula (1), it is preferable that R ¹ is selected from C _8-22 alkyl or C _8-22 alkenyl, since the above hydrocarbon chain acts as a lubricant, thereby improving workability in injection molding have.

Examples of preferred phosphorus antioxidants of the present invention include, but are not limited to, titanyl pentaerythritol diphosphite, ditridecyl pentaerythritol diphosphite, dipentadecyl pentaerythritol diphosphite, distearyl pentaerythritol diphosphite, Dodecenyl pentaerythritol diphosphite, and dioctadecenyl pentaerythritol diphosphite. It is preferable, but not limited, to use one or more selected from these.

The content of the phosphorus antioxidant is 80 to 99% by weight of a polymer of methyl methacrylate (MMA) having a dimethylfuran (DMF) content of 40 ppm or less or a methyl methacrylate (MMA) monomer having a dimethylfuran (DMF) content of 40 ppm or less, And 0.07 to 0.25 parts by weight of a phosphorus-based antioxidant of the above formula (1), based on 100 parts by weight of a copolymer of 20 to 1% by weight of one or more monomers selected from the group consisting of methacrylate and acrylate Preferably 0.9 to 0.15 part by weight. When the phosphorus antioxidant is less than 0.07 parts by weight, it is difficult to expect a desired improvement in the color coordinates. When the phosphorus antioxidant is more than 0.25 part by weight, there is a problem that the luminance is lowered, silver is increased, and productivity is lowered due to mold cleaning.

The methyl methacrylate resin obtainable by the present invention was a high alcohol. The content of the higher alcohol is 80 to 99% by weight of a polymer of methyl methacrylate having a dimethylfuran (DMF) content of 40 ppm or less or a methyl methacrylate (MMA) monomer having a dimethylfuran (DMF) content of 40 ppm or less, Based on 100 parts by weight of a copolymer of 20 to 1% by weight of one or two or more monomers selected from the group consisting of acrylate, methacrylate, and acrylate, the total amount of the phosphorus-based antioxidant is preferably 0.1 to 0.3 parts by weight, More preferably 0.15 to 0.22 parts by weight. When the total amount of the phosphorus antioxidant and the high alcohol is less than 0.1 part by weight, the processed shear causes the degradation of heat resistance due to additives having a low molecular weight when the amount exceeds 0.3 part by weight, There is a problem of. The higher alcohol may be, for example, an alcohol having an alkyl group of 12 or more carbon atoms, preferably an alkyl alcohol having 12 to 20 carbon atoms, but is not limited thereto. Specific examples thereof include cetyl alcohol and stearyl alcohol. Such higher alcohols can be added at any stage of resin production such as suspension polymerization or extrusion processing. It has been found that one of the surprisingly obtainable effects by using the above-mentioned higher alcohol is that it has an effect of improving the heat resistance of the resin and reducing the generation of silver strike at high temperature injection.

On the other hand, the content ratio of the methyl methacrylate copolymer used as the transparent base resin used in the present invention is 80 to 99% by weight of methyl methacrylate and 20 to 1% of one or more monomers selected from methacrylate or acrylate based monomers % By weight. The copolymer having the above-mentioned content ratio is characterized by showing excellent heat resistance, light transmittance and low yellowness due to a large amount of gas generated during injection molding. When the amount of one or more monomers selected from the above-mentioned methacrylate-based or acrylate-based monomers is more than 20% by weight, the polymerization stability is lowered and the heat resistance is decreased due to the decrease of the glass transition temperature (Tg).

The methacrylate-based and acrylate-based monomers mean alkyl acrylates or derivatives thereof having an alkyl group of 1 to 8 carbon atoms, and examples thereof include methyl (meth) acrylate, ethyl (meth) acrylate, (Meth) acrylate, n-butyl (meth) acrylate, t-butyl (meth) acrylate and oligomers thereof. Most preferably, But is not limited thereto.

Also, the copolymer of the monomers is characterized by being produced by suspension polymerization. Bulk polymerization. However, due to the high flowability of the polymethylmethacrylate resin for thin injection during polymerization in the same equipment, the productivity is reduced to about 2/3 ~ 1/2 as compared with extrusion or other low fluidity products In this case, the obtained polymethyl methacrylate resin has a disadvantage that it can be applied to a light guiding plate by changing other parts of a backlight unit such as an LED package in an IPS type panel requiring a low color coordinate due to a high color coordinate.

The polymerization initiator used in the polymerization method is not particularly limited as a typical initiator used in Free Radical Polymerization, but it is preferable to use a peroxide compound or an azo compound, And the content of the initiator relative to 100 parts by weight of the monomer mixture is preferably 0.03 to 0.5 part by weight based on 100 parts by weight of the monomer mixture.

The number of alkyl groups of the chain transfer agent introduced to adjust the molecular weight of the polymethylmethacrylate of the present invention optimally can be used without limitation, but it is preferable that the number of carbon atoms is 1 to 12, the number of carbon atoms having one thiol functional group Alkyl mercaptan series or polythiol mercaptan series having two or more thiol functional groups, and preferable examples thereof include iso-propyl mercaptan, n-butyl mercaptan, t -Butyl mercaptan, n-decyl mercaptan, n-dodecyl mercaptan, hydroxyethyl mercaptan and n-octyl mercaptan, and still more preferred examples thereof include n-octyl mercaptan, but there is no particular limitation thereto .

The chain transfer agent is characterized by improving the thermal stability. The content of the chain transfer agent is preferably 0.2 to 0.6 parts by weight, more preferably 0.3 to 0.5 parts by weight based on 100 parts by weight of the monomer mixture, By weight. When the content of the chain transfer agent is less than 0.2 parts by weight, the molecular weight becomes excessively large, so that it is impossible to secure flowability for injection of the thin film light guide plate. When the content of the chain transfer agent exceeds 0.6 parts by weight, the molecular weight of the obtained resin becomes too small, There arises a problem that the physical properties of the resin as the resin are not effectively manifested.

The resin composition of the present invention may further contain additives as required. Such additives are preferably selected from the group consisting of ultraviolet stabilizers, light stabilizers, organic pigments, dyes, dispersants, reducing agents, and mixtures thereof, but are not limited thereto.

When the resin composition of the present invention was used to measure a long-side light transmittance and a yellowness of 150 mm in length, 80 mm in width, 3 mm in thickness and 3 mm in length, the light transmittance and the yellowness index (YI) Preferably 1.7 or less. The resin composition of the present invention which can exhibit such optical properties can be used for manufacturing an LCD light guide plate by injection molding.

 When the test piece is measured using a luminance meter, the color coordinates of the light guide plate suitable for manufacturing a high-resolution IPS LCD to which a conventional high-yield LED package is applied to the backlight unit is preferably in the range of 0.2013 to 0.0011.

The thickness of the light guide plate for LCD according to the present invention is preferably 0.4 to 1 mm, and the polymethyl (meth) acrylate resin composition of the present invention may be used alone or in combination as a light guide plate for a backlight unit for IPS LCD.

According to the present invention, a methyl methacrylate monomer having a dimethylfuran (DMF) content of 40 ppm or less and a phosphorus-based antioxidant and a higher alcohol of the above formula (1) are excellent in fluidity and thermal decomposition resistance, It is not only excellent in optical properties but also can be usefully used for a backlight unit for a high resolution IPS LCD due to its low color coordinate. Specifically, it has a characteristic of being able to attain an optical effect having a low color coordinate and a high long-wavelength transmittance and a long-light yellowing degree at the same time, and also has an unexpected effect that the silver strike occurrence rate can be lowered .

Hereinafter, the present invention will be described in more detail with reference to examples and comparative examples. It is to be understood by those skilled in the art that these embodiments are merely illustrative of the present invention and that the scope of the present invention is not limited by these embodiments.

[Measurement and evaluation of physical properties]

One) Long light Light transmittance (Long Transmittance, LTT ), Long light  Long Yellow Index, LYI ) And Color coordinates ( Wy )

The resin composition was injection molded to produce a molded product having a length of 150 mm, a width of 80 mm, and a thickness of 3 mm. The light transmittance (%) and the yellowness degree at a wavelength of 380 to 780 nm in the longitudinal direction, which is a long optical path, were measured by a spectrophotometer according to ASTM D1003-00, E313-05 method.

The color coordinate measurement was performed by measuring brightness and color coordinates at nine locations using a luminance meter using an edge-type LED light source, and taking a center color coordinate value.

2) Liquidity (MI)

The fluidity of the pellets was expressed by the weight (g / 10 min) of the resin flowing out during 10 minutes measured by ASTM D1238 method at 230 캜 under 3.8 kg.

The flowability was evaluated to be sufficient if the flowability was 10 or more. The flowability measurement values for the respective Examples and Comparative Examples are shown in Table 1 below.

3) Pyrolysis stability

The resin composition pellets were injected under the following conditions to stabilize the cylinder temperature, and then 40 injection molded articles each having a length of 150 mm, a width of 80 mm and a thickness of 3 mm were successively injection molded. When the molded article was observed with naked eyes and a silver strike was confirmed, it was judged that a silver strike occurred. The occurrence rate (%) of silver streaks is good when it is 50% or less, and the values are shown in Table 1 below.

Injection Molding Machine: Hydraulic Injection Molding Machine (LG Siltron LGH170D)

Injection pressure: 170 kgf / cm ²

Injection mold: vertical 150mm width 80mm thickness 3mm, 2cavity, no vent line

Cylinder temperature: 280 ° C

Mold temperature: 50 ℃

Molding cycle: 80 seconds

[Example 1]

To 100 parts by weight of a monomer mixture containing 92% by weight of methyl methacrylate having a content of dimethylfuran (DMF) of 4 ppm and 8% by weight of methyl acrylate, 0.1 part by weight of AIBN as an initiator, and 0.45 parts by weight of normal octyl mercaptan (NOM) were mixed and stirred for 30 minutes. To 100 parts by weight of the monomer were added 200 parts by weight of water, 0.9 part by weight of a salt of a copolymer of methyl methacrylate-methacrylic acid as a suspension, sodium dihydrogen phosphate and disodium hydrogen phosphate as a buffer salt in a weight ratio of 2: 1, and 0.15 parts by weight in total. After polymerization at a reaction temperature of 80 ° C for 70 minutes, additional polymerization was carried out at 100 ° C for 30 minutes in order to remove the residual monomer. The polymerized beads were washed and dried, and then 0.07 part by weight of phosphorus-based antioxidant distearyl pentaerythritol diphosphite (S-6180) and 0.1 part by weight of stearyl alcohol were added to 100 parts by weight of the resin, followed by extrusion molding . Table 1 shows the results of optical properties such as light transmittance, yellowness, and color coordinates, and fluidity and silver streak occurrence rate.

[Example 2]

A DMF content of 20 ppm, and 0.1 parts by weight of distearyl pentaerythritol diphosphite as a phosphorus antioxidant.

[Example 3]

0.4 parts by weight of NOM as a chain transfer agent, 0.09 part by weight of phosphorus antioxidant distearyl pentaerythritol diphosphite, and 0.06 part by weight of stearyl alcohol were added.

[Example 4]

Except that 5 weight% of methyl acrylate, 0.5 weight part of the content, and 0.15 weight part of didodecyl pentaerythritol diphosphite as the phosphorus antioxidant were added.

[Comparative Example 1]

The procedure of Example 1 was repeated except that the phosphorus-based antioxidant of the present invention was not used.

[Comparative Example 2]

Except that the DMF content was 60 ppm and that 0.1 weight parts of phosphorus-based antioxidant distearyl pentaerythritol diphosphite was used.

[Comparative Example 3]

A monomer mixture prepared by mixing 92% by weight of methyl methacrylate having a DMF content of 4 ppm and 8% by weight of methyl acrylate and 0.012 parts by weight of AIBN and 0.43 part by weight of NOM was mixed with 100 parts by weight of the monomer mixture, Instead of the suspension polymerization, the polymerization was carried out in a fully mixed-type reactor controlled at a polymerization temperature of 140 ° C while being continuously supplied with stirring. The average residence time was 35 minutes. The reaction mixture was continuously withdrawn from the reactor, heated to 200 DEG C in a preheater, volatile components such as unreacted monomers were removed at 250 DEG C using a vented extruder with a vent, 0.15 parts by weight of stearyl alcohol was added thereto.

[Comparative Example 4]

The content of DMF is 20 ppm, and the antioxidant is tris [2,4-di-t-butylphenyl] phosphite (tris [2,4-di-tert -butylphenyl] phosphite) was added in an amount of 0.15 part by weight.

[Comparative Example 5]

The antioxidant is added to the phenolic antioxidant 2- [1- (2-hydroxy-3,5-di-t-pentylphenyl) ethyl] -4,6-di- Except that 0.15 parts by weight of phenyl acrylate (2- [1- (2-Hydroxy-3,5-di-tert-pentylphenyl) ethyl] -4,6-di-tert-pentylphenylacrylate was added. The same procedure was followed.

[Comparative Example 6]

0.05 part by weight of distearyl pentaerythritol diphosphite, and 0.01 part by weight of stearyl alcohol were added to the reaction mixture.

[Table 1] Measurement results for Examples and Comparative Examples of the present invention

Claims (4)

(MMA) monomer having a dimethylfuran (DMF) content of not more than 40 ppm or 80 to 99% by weight of a methyl methacrylate (MMA) monomer having a dimethylfuran (DMF) content of 40 ppm or less and a methacrylate or acrylate And 20 to 1% by weight of one or more monomers selected from the group consisting of polyvinyl alcohol and polyvinyl alcohol, to obtain a polymethyl methacrylate resin, and
Extruding a polymethylmethacrylate resin composition comprising a polymethylmethacrylate resin, a phosphorus-based antioxidant of Formula 1, and an alkyl alcohol of 12 to 20 carbon atoms,
Wherein the content of the alkyl alcohol and the phosphorus antioxidant satisfy the following formulas (1) and (2).
[Chemical Formula 1]

( _Wherein R ¹ is any one selected from C _8-22 alkyl or C _8-22 alkenyl).
0.1? Alkyl alcohol + phosphorus antioxidant? 0.3 [Formula 1]
0.7? Phosphorus antioxidant / alkyl alcohol? 1.5 [Formula 2]
Wherein the alkyl alcohol is a part of the alkyl alcohol having 12 to 20 carbon atoms added to 100 parts by weight of the resin, and the phosphorus-based antioxidant is a mixture of 100 parts by weight of the resin Parts by weight of the phosphorus antioxidant). The method according to claim 1,
The content of the phosphorus antioxidant is 80 to 99% by weight of a polymer of methyl methacrylate (MMA) having a dimethylfuran (DMF) content of 40 ppm or less or a methyl methacrylate (MMA) monomer having a dimethylfuran (DMF) content of 40 ppm or less, Based on 100 parts by weight of a copolymer of 20 to 1% by weight of one or more monomers selected from methacrylate or acrylate,
0.07 to 0.25 parts by weight of a phosphorus-based antioxidant comprising the following formula (1).
[Chemical Formula 1]

( _Wherein R ¹ is any one selected from C _8-22 alkyl or C _8-22 alkenyl). The method according to claim 1,
The methacrylate-based or acrylate-based monomer is at least one monomer selected from the group consisting of methyl acrylate, ethyl (meth) acrylate, propyl (meth) acrylate, n-butyl (meth) acrylate, Wherein the polymethylmethacrylate resin is one or more selected from the group consisting of polyvinyl chloride, The method according to claim 1,
The phosphorus antioxidant may be at least one selected from the group consisting of didodecyl pentaerythritol diphosphite, ditridecyl pentaerythritol diphosphite, dipentadecyl pentaerythritol diphosphite, distearyl pentaerythritol diphosphite, dioctenyl pentaerythritol diphosphite, Wherein the silver streak defect ratio of the polymethylmethacrylate resin is at least one selected from the group consisting of cerium, pentaerythritol, cyanopentaerythritol diphosphite, and dioctadecenyl pentaerythritol diphosphite.